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Natural Sciences Doctoral Theses
2019 Investigation of chemical kinetics of pyrite ore roasting for production of sulphuric acid
Hiji, Morris
Hiji, M. (2019). Investigation of chemical kinetics of pyrite ore roasting for production of sulphuric acid http://hdl.handle.net/20.500.12661/1956 Downloaded from UDOM Institutional Repository at The University of Dodoma, an open access institutional repository. INVESTIGATION OF CHEMICAL KINETICS OF PYRITE
ORE ROASTING FOR PRODUCTION OF SULPHURIC
ACID
MORRIS HIJI
DOCTOR OF PHILOSOPHY
THE UNIVERSITY OF DODOMA
OCTOBER, 2019
INVESTIGATION OF CHEMICAL KINETICS OF PYRITE ORE
ROASTING FOR PRODUCTION OF SULPHURIC ACID
BY
MORRIS HIJI
A THESIS SUBMITTED IN FULFILMENT OF THE
REQUIREMENTS FOR THE DOCTOR OF PHILOSOPHY
THE UNIVERSITY OF DODOMA
OCTOBER, 2019
DECLARATION
AND
COPYRIGHT
I, MORRIS HIJI, declare that this thesis is my own original work and that it has not been presented and will not be presented to any University for a similar or any other degree award.
Signature ------
No part of this thesis may be reproduced, stored in any retrieval system, or transmitted in any form or by any means without prior written permission of the author or the University of Dodoma in that behalf. If transformed for publication in any other format shall be acknowledged that, this work has been transmitted for degree award at the University of
Dodoma.
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CERTIFICATION
The undersigned certify that they have read and hereby recommend for an acceptance by the University of Dodoma a thesis entitled: Investigation of chemical kinetics of pyrite ore roasting for production of sulphuric acid in fulfilments of the requirements for the degree of doctor of philosophy of the University of Dodoma.
Prof. SAID A. VUAI
Signature: Date
(SUPERVISOR)
Prof. JUSTIN W. NTALIKWA
Signature: Date
(SUPERVISOR)
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ACKNOWLEDGEMENT
I would like to express my deepest appreciation to all who helped me to complete this study. First of all, I have to appreciate the guidance given by my supervisors
Prof. S.A. Vuai and Prof. J.W. Ntalikwa. I am indebted for their valuable intellectual input, mentoring and encouragement in achieving the required task. Thank you very much!
I am very grateful to University of Dodoma Administration for financing this study and providing moral support. I would like to mention few of the staff: Prof A.M. Ame, the former Director of Graduate Studies and the current Deputy Vice Chancellor - Planning,
Finance and Administration; Ms. S.I.A. Sawasawa, Director of Human Resources and
Administration and Dr. M.I. Hamis, the Director of Graduate Studies.
Furthermore, I am very grateful to crucial role performed by the staff of Geological Survey of Tanzania, for providing necessary materials and permission to use their equipment such as XRF, AAS and CS analyser machines. I would also like to mention few of them: Prof.
A. Mruma, Chief Executive Officer; Ms. A.K. Rutaihwa, former Director of Mineral
Resources Laboratory and Export Permit Directorate and managers Mr. H.I. Gombera and
Mr. M. Makongoro.
I would like also to express my sincere gratitude to the College of Natural and
Mathematical Sciences of University of Dodoma, particularly the chemistry laboratory. I got useful technical assistance and cooperation during laboratory work from all technicians and scientists.
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I will not be fair enough if I could not mention the contribution of the following:
Mr. H. Mditi, the former Resident Mines Officer of Merelani and staff in general for supplying the pyrite samples used in this study. I would also like to mention metallurgist engineer, Mr. B. Semboja for his valuable expertise in the area of extractive metallurgy.
Last but not least, I express my sincerely appreciation to the College of Earth Sciences,
School of Mines and Petroleum Engineering and Department of Mining and Mineral
Processing Engineering. I would like to mention former college principal,
Prof. W.S. Mwegoha and dean, Dr. G. G. Kombe for providing valuable comments and feedback which helped me to shape the proposal. As host, they assisted me in many ways.
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DEDICATION
I am real indebted to my family members for full support I got from them through the entire-turbulent PhD journey. I convey special dedication to my wife Happiness; she showed enormous courage when study was complex and uncertain. I dedicate this work to my lovely children: Daniel, Abigail, Agatha and Anna-Paula who was born when I was fighting in the jungle. May God bless them! I can only show my heartfelt dedication by bible verse:
Psalm 116
1 I love the LORD, for he heard my voice;
he heard my cry for mercy.
2 Because he turned his ear to me,
I will call on him as long as I live.
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ABSTRACT
The large-scale production of mine wastes and their secure disposal has been a problem of global importance. In this work, the mine waste from Merelani, crude overburden pyrite ore was converted into a value added chemical, sulphuric acid. XRF, XRD, AAS and modified ASTM D-2492 tests were used to investigate the mineral and chemical composition of the ore. It was revealed that the ore comprised about 60% w/w of mineral pyrite (FeS2). The evaluation of XRD pattern shows that the pyrite is of good quality according to the International Centre for Diffraction Data (ICDD).
The elucidation of chemical kinetics of roasting of the ore to form a key precursor in the production of sulphuric acid, sulphur dioxide, revealed pseudo first order kinetic with respect to solid reactant in gas-solid system and optimum roasting temperature of 750° C.
The activation energy (Ea) value of 15 kJ/mol was deduced from Arrhenius equation suggesting that the roasting reaction was controlled by diffusion of oxygen through the ash layer to the reacting surface.
The main gaseous product of roasting crude pyrite ore was then converted into sulphuric acid, which apart from providing proper mitigation to the environment, but also serves as a social˗economic income to local people. The grade of sulphuric acid obtained was 40% w/w which is sufficient for battery acid use.
The techno-economic evaluation of a small-scale plant of converting crude pyrite into sulphuric acid was done with the aid of SuperPro Designer simulator. The plant involves five major sections namely: size reduction, pyrite roasting, gas cleaning, formation of NO gas, formation of NO2 gas and acid chamber. Both the capital investment and operating cost
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were estimated by standard engineering cost estimation methods in evaluating chemical process economic viability. The economic indicators showed that the project is feasible with estimated value of NPV (3,503,000 USD), IRR 18.05% and PBP of about 3 years.
Sensitivity analysis results were comparable with theory where the unit production cost decrease; the NPV and IRR increase with increasing plant production scale of maximum limit of 1.4.
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TABLE OF CONTENTS
Page
CERTIFICATION ...... ii
ACKNOWLEDGEMENT ...... iii
DEDICATION ...... v
ABSTRACT ...... vi
LIST OF TABLES ...... xiii
LIST OF FIGURES ...... xv
LIST OF PUBLICATIONS ...... xvii
LIST OF ACRONYMS AND ABBREVIATIONS ...... xx
CHAPTER ONE ...... 1
INTRODUCTION ...... 1
1.1 General Overview ...... 1
1.2 Problem Statement ...... 10
1.3 Objectives ...... 11
1.4 Hypothesis ...... 12
1.5 Significance of the Study...... 12
CHAPTER TWO ...... 13
LITERATURE REVIEW ...... 13
2.1 Production of Sulphuric Acid ...... 13
2.1.1 Introduction ...... 13
2.1.2 Raw materials...... 14
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2.1.3 Grades of sulphuric acid...... 19
2.1.4 Analytical and titrimetric methods ...... 22
2.1.5 Qualitative methods ...... 23
2.1.6 Determination of forms of sulphur ...... 24
2.1.7 Methods for production of sulphuric acid ...... 25
2.2 Chemical Kinetics ...... 30
2.2.1 Reaction mechanism ...... 31
2.2.2 Rate Law ...... 32
2.2.3 Analysis of rate data ...... 34
2.2.4 Method of excess ...... 38
2.2.5 Variables affecting the rate of reaction ...... 39
2.2.6 Experimental methods for studying kinetics of reactions ...... 51
2.3 Extractive Metallurgy of Pyrite Roasting ...... 53
2.3.1 Pyrometallurgical processes ...... 53
2.3.2 Objectives of roasting ...... 55
2.3.3 Thermodynamics of roasting and Kellog Diagram ...... 56
2.3.4 Factors affecting pyrite roasting ...... 57
2.3.5 Solid interface ...... 59
2.3.6 Formation of solid reaction products ...... 61
2.3.7 Homogenous and heterogenous reaction ...... 62
2.4 Techno-economical viability and environmental feasibility of the process ...... 67
2.4.1 Project costing ...... 67
2.4.2 Economic indicators ...... 69
2.4.3 Cash Flow Analysis ...... 73
2.4.4 Model and simulation ...... 74
2.4.5 Sensitivity analysis ...... 75
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2.5 Pyrite roasting technology ...... 76
2.5.1 Dead roasting ...... 81
2.5.2 Partial roasting ...... 82
2.5.3 Two-stage roasting ...... 82
2.5.4 Sulphating roasting ...... 83
2.5.5 Sulphating decomposition ...... 84
2.5.6 Direct reduction ...... 84
CHAPTER THREE ...... 86
MATERIALS AND METHODS ...... 86
3.1 Study Sites ...... 86
3.2 Methodology ...... 88
3.3 Chemical and Mineralogical Characterisation of Pyrite Ore ...... 88
3.3.1 Sample preparation ...... 88
3.3.2 Chemical characterisation of pyrite ore ...... 89
3.3.3 Mineralogical characterisation of pyrite ore ...... 90
3.4 Chemical kinetics of conversion of pyrite ore to sulphur dioxide ...... 92
3.4.1 Acquisation of kinetic data ...... 94
3.4.2 Analysis of kinetic data ...... 95
3.4.3 Evaluation of RSM and optimisation ...... 96
3.5 Conversion of pyrite ore to sulphuric acid ...... 97
3.5.1 Quantitative analysis of produced sulphuric acid ...... 98
3.5.2 Qualitative analysis of produced sulphuric acid ...... 100
3.6 Establishment of techno-economical viability of the process ...... 101
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CHAPTER FOUR ...... 103
RESULTS AND DISCUSSION ...... 103
4.1 Chemical analysis ...... 103
4.2 Mineralogical analysis ...... 105
4.3 Pyritic sulphur analysis ...... 110
4.4 Chemical kinetic results ...... 112
4.5 Production of sulphuric acid ...... 124
4.5.1 Quantity of produced sulphuric acid ...... 124
4.5.2 Quality of produced sulphuric acid ...... 126
4.6 Techno-economic analysis ...... 127
4.6.1 Process description ...... 127
4.6.2 Assumptions and limitations on process simulation and plant design ...... 131
4.6.3 Simulation results ...... 131
4.6.4 Viability of the Project ...... 143
4.6.5 Sensitivity Analysis ...... 144
4.7 Environmental aspects of sulphuric acid production ...... 146
4.7.1 Handling of raw materials ...... 146
4.7.2 Processing ...... 146
4.7.3 Product handling ...... 147
4.7.4 Prerequisite environmental mitigation for production of sulphuric acid ...... 148
CHAPTER FIVE ...... 150
CONCLUSION AND RECOMMENDATIONS ...... 150
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5.1 Conclusion ...... 150
5.2 Recommendations ...... 151
REFERENCES ...... 152
APPENDICES ...... 173
Appendix A: Dissolution of Rock-ore Samples for AAS Analysis ...... 173
Appendix B: Elements Quantified by XRF ...... 175
Appendix C: Sulphur Content (%) ...... 176
Appendix D: Kinetic Tool ...... 185
Appendix E: Titration Tool ...... 192
Appendix F: Cash Flow Analysis ...... 196
Appendix G: Major Equipment Specification and Fob Cost ...... 197
Appendix H: Component Flowrates (kg/ Batch) ...... 198
Appendix I: CS30000G Furnace and Pyrite Sample in Crucibles ...... 199
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LIST OF TABLES
Table 2.1: Composition of various types of pyrites ...... 17
Table 2.2 : Grades of sulphuric acid ...... 20
Table 2.3: Sulphuric Acid Density Table ...... 21
Table 2.4: Expression of reaction rate ...... 31
Table 2.5: Concentration versus time data ...... 36
Table 2.6: Two Point Method ...... 37
Table 2.7: Roasting Process Technologies ...... 79
Table 3.1: Represantive Data of Merelani-Tanzanite Deposit Samples ...... 88
Table 3.2: Parameters for Optimisation ...... 93
Table 3.3: CCD Internal Input Information ...... 93
Table 3.4: Experimental Design ...... 94
Table 3.5: Titration Data ...... 100
Table 4.1: Chemical Composition of Sample A ...... 103
Table 4.2: Chemical Composition of Sample B ...... 104
Table 4.3: AAS Results for the Total Iron in the Pyrite Ore ...... 111
Table 4.4: AAS Results for Iron in the Sulphate Sulphur ...... 111
Table 4.5: Pyritic Sulphur Analysis ...... 111
Table 4.6: Variation of Sulphur Content with Time and Particle Size ...... 114
Table 4.7: Results from Kinetic Tool ...... 117
Table 4.8: Experimental Design Table ...... 120
Table 4.9: Response Surface Regression: Order Versus T, PS ...... 120
Table 4.10: Analysis of Variance for Order...... 121
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Table 4.11: Recommended Optimum Conditions ...... 122
Table 4.12: Calculated Rate Constant Values ...... 122
Table 4.13: Results from Titration Tool ...... 125
Table 4.14: Installed Fluidised Bed Roaster ...... 130
Table 4.15: Major Finding from Process Simulator ...... 133
Table 4.16: Fixed Capital Estimate Summary ...... 134
Table 4.17: Profitability Analysis ...... 135
Table 4.18: Overall Process Data ...... 137
Table 4.19: Equipment Capacity Utilisation ...... 137
Table 4.20: Overall Materials Balances ...... 140
Table 4.21: Raw Materials Cost ...... 141
Table 4.22: Component Balance and Stream Report ...... 142
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LIST OF FIGURES
Figure 1.1: Global Production of Sulphuric Acid ...... 1
Figure 1.2: Worldwide Consumption of Sulphuric acid ...... 2
Figure 1.3: Importation cost of H2SO4 in Tanzania ...... 4
Figure 1.4: Isothermal Phase Diagram of Me-S-O system ...... 7
Figure 1.5: Effect of Temperature on Roasting Equilibria in the System Me-S-O ...... 8
Figure 2.1: Block diagram for production of SO2 from pyrite ...... 29
Figure 2.2: Modified Lead Chamber Process ...... 30
Figure 2.3: Data analysis by integral method ...... 35
Figure 2.4: Plot for differential method ...... 36
Figure 2.5: Three Point Method ...... 37
Figure 2.6a: Endothermic Reaction Van’t Hoff ...... 45
Figure 2.6b: Exothermic Reaction Van’t Hoff ...... 45
Figure 2.7: Temperature-dependency of the reaction rate ...... 50
Figure 2.8: Behavior of reacting solid particles ...... 64
Figure 2.9: Mechanism of Pyrite Roasting ...... 66
Figure 2.10: Cash Flow Curve ...... 73
Figure 2.11: Stationery Fluid Bed (SFB) ...... 77
Figure 2.12: Circulating Fluid Bed (CFB) ...... 78
Figure 3.1: The Geological Map of Merelani ...... 87
Figure 3.2: Classification of Constituent Types by Analyte Level ...... 90
Figure 3.3: Experimental Setup for Production of Sulphuric Acid ...... 97
Figure 3.4: Production of Sulphuric Acid ...... 98
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Figure3.5: Proposed Block Diagram of Producing Sulphuric Acid from Crude Pyrite
Ore from Merelani ...... 102
Figure 4.1: XRD Pattern of the Crude Pyrite from Merelani ...... 107
Figure 4.2: XRD Pattern of the Coal-derived Pyrite ...... 108
Figure 4.3: Image result for JCPDS card for pyrite ...... 109
Figure 4.4: Pyrite Crystalline Shape ...... 110
Figure 4.5: Sulphur Content versus Time for Various Temperature ...... 115
Figure 4.6: Mechanistic Suggestion for the SO2 Released by Thermal Decomposition of
Pyrite ...... 116
Figure 4.7a: Differential and Integral Method for Determination of Chemical
Reaction Order ...... 118
Figure 4.7b: Differential and Integral Method for Determination of Chemical
Reaction Order ...... 119
Figure 4.8: Rate Constants Versus Temperature ...... 123
Figure 4.9: Presence and Absence of Brown Ring Test ...... 127
Figure 4.10: Proposed PFD for Production of Sulphuric Acid from Pyrite Ore ...... 129
Figure 4.11: Operating Cost ...... 136
Figure 4.12: Gantt Chart ...... 138
Figure 4.13: Equipment Utilisation Chart ...... 139
Figure 4.14: Cash Flow Curve ...... 144
Figure 4.15 IRR as Function of Plant Production Scale ...... 145
Figure 4.16 Relationship Between Unit Production Cost and Plant Production Scale
...... 145
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LIST OF PUBLICATIONS
1. Journal Papers
1.1 Hiji, M. F., Vuai, S. A. and Ntalikwa, J. W. (2018). Roasting Kinetics of
Crude Pyrite as Acid Mine Drainage Remediation Option: A Case Study of
Merelani. Applied Journal of Environmental Engineering Science, 4(4), 4-4.
Journal detail: Applied Journal of Environmental Engineering Science
(Appl. J. Envir. Eng. Sci) is a free of charge open access journal that publishes
original research articles as well as review articles in all areas of physical
chemistry. It was founded in 2015 by Professor Rachid SALGHI, University
Ibn Zohr, Agadir Morocco. This journal is published by the University Ibn
Zohr, Agadir Morocco. The journal is under evaluation by Scopus and
Thomson Reuters (ISI). It is also abstracted and indexed by DOAJ, Google
Scholar, Scientific Indexing Services (SIS), Science Library Index, Turkish
Education Index and International Society for Research Activity (ISRA)
Journal-Impact-Factor (JIF) (ISRA-JIF).
1.2 Hiji, M. F., Ntalikwa, J. W., and Vuai, S. A. (2014). Producing sulphuric acid
in Tanzania and potential sources: A review. American Journal of Chemistry
and Applications, 1(4), 40-44.
Journal detail: American Journal of Chemistry and Applications is a peer
reviewed journal of high quality devoted to the publication of original research
papers from Chemistry and their broad range of applications. It covers (but not
limited to) all application problems of modern chemistry, including the
structure of inorganic and organic compounds, kinetics and mechanisms of
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chemical reactions, problems of chemical processes and apparatus, borderline
problems of chemistry, and applied research.
ISSN Print: 2381-4462
ISSN Online: 2381-4535
2. Conference/Seminar Abstracts and Papers
2.1 M. F. Hiji, S.A. Vuai, J.W. Ntalikwa (2018). Studying of the Chemical
Kinetics of Non-Catalytic Solid-Gas Systems with Respect to Solid Reactant.
2nd Edition of Global Conference on Catalysis, Chemical Engineering and
Technology (CAT-2018). Rome, Italy, 13-15 September 2018.
Conference detail: The abstract has been accepted for Oral Presentation under
“Chemical Kinetics and Catalysis” Session. The theme of the conference is
“Accentuate Innovations and Emerging Novel Research in catalysis and
Chemical Engineering”. The conference will provide a forum for academia and
industry to disseminate their latest innovations and practices. The focus is on
present state-of-the art new ideas, practical considerations and novel
applications in Catalysis and Chemical Engineering.
2.2 M. F. Hiji, S.A. Vuai, J.W. Ntalikwa (2018). Roasting Kinetics of Crude Pyrite
as Acid Mine Drainage Remediation Option: A Case Study of Merelani. In the
6th National Science, Technology and Innovation (STI) Conference 2018.
Mlimani City, Dar es Salaam, Tanzania, 4-6 July 2018.
xviii
Conference detail: The abstract was granted to be orally presented during 6th
STI conference organised by Tanzania Commission of Science and Technology
(COSTECH) with a theme "Towards Industrialization: The Role of Science,
Technology and Innovation".
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LIST OF ACRONYMS AND ABBREVIATIONS
%Cacid Percent Acid
∆H Heat of reaction a Stochiometric coefficient with respect to species A
A Surface area
AAS Atomic Absorption Spectroscopy
Adj MS Adjusted Mean of Squares
Adj SS Adjusted Sum of Squares
AMD Acidified Mine Drainage
ANOVA Analysis of Variance
ASTM American Society of Testing and Materials b Stochiometric coefficient with respect to species B
BHP Broken Hill Prospecting
C Concentration
CA Concentration of species A
Cacid Molarity of H2SO4
CB Concentration of species B
CCD Central Composite Design
CF Cash Flow
CFR Cash Flow Analysis Report
CNaOH Molarity of NaOH
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CoES College of Earth Sciences
COSTECH Commission of Science and Technology
Cp Heat capacity at constant pressure
CP Coarse Particles
CS Carbon -Sulphur
DF Degree of Freedom
DoE Design of Experiments
Ea Activation energy
EER Economic Evaluation Report
EIR Environmental Impact Report
EMS Emissions Report
EN European Union Quality Standard
EQR Equipment Report
ER Emission Report
FP Fine Particles
GEMP Guidebook for Evaluating Mining Project
GHGs Greenhouse Gases
GST Geological Survey of Tanzania
ICDD International Centre for Diffraction Data
ID Identification Number
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IRR Internal Rate of Return
JCPDS Joint Committee on Powder Diffraction Standards k Rate constant ko Frequency factor
LOD Limit of Detection
LE Lighter Elements
M Molarity
Me Metal
MeO Metallic Oxide
MeS Metallic Sulphide
MeSO4 Metallic Sulphate
Mp Molecular weight of solid product
MR Molecular weight of solid reactant
MSS Minitab Statistical Software
ND Not Detected
NOx Nitrogen Oxides
NPV Net Present Value
PBP Pay-Back Period
PDF Powder Diffraction File
PFD Process Flow Diagram
PS Particle Size
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r Reaction rate
R Gas constant
RMM Relative Molar Mass
Rp Radius of Sphere
RSM Response Surface Methodology
SCM Shrinking Core Model
STI Science, Technology and Innovation t Reaction time
T Reaction temperature
TAR Throughput Analysis Report
TPY Tonnes Per Year
TST Transition˗State Theory
USCOTA U.S. Congress Office of Technology Assessment
Vacid Volume of H2SO4
VFP Very Fine Particles
VNaOH Volume of NaOH w Weight of solid sample at time t w/w Ratio of weight by weight
XRD X-Ray Diffraction
XRF X-Ray Fluorescence
Z Porosity
α Stochiometric coefficient with respect to pyrite
β Stochiometric coefficient with respect to air
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γ Stochiometric factor
ρp Density of solid product
ρR Density of solid reactant
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CHAPTER ONE
INTRODUCTION
1.1 General Overview
Sulphuric acid has wide variety of uses in the manufacturing of large number of chemicals such as in phosphate fertiliser, explosives, glue, wood preservatives, lead˗acid batteries, detergents, plastics, man˗made fibers, dyes, drugs, fluorine chemicals, pigment production, steel pickling, non˗ferrous metals extraction and in the process of petroleum refining (Kiss, et al., 2010; Jones, 2006; Williams, 2003). Large quantities of sulphuric acid are produced each year than any other manufactured chemical because almost all consumer goods need it at some stage in their production.
Worldwide production of sulphuric acid was about 121 million tonnes in 1977, 200 million tonnes in 2000 and surpassed 2030.4 million tonnes in 2012 and about half of this production was used in fertiliser production (Sidana, 2016; Shakhashiri, 2007; Jones, 2006). The demand of sulphuric acid is still higher than the production. The gap between the produced and demand of sulphuric acid is about 50,000 TPY (Figure 1.1).
250,000
200,000
150,000 100,000 50,000 0 1985 1988 1991 1994 1997 2000
Annual Capacity Production
Figure 1.1: Global Production of Sulphuric Acid (in thousand of metric tonnes; source: www.basf.com)
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Due to its myriad uses, the consumption of sulphuric acid has been cited as an indicator of the general state of a nation’s economy, although many other indicators (such as energy consuption) might today be regarded as more important (Müller, 2000a). Figure 1.2 shows the consumption of sulphuric acid which complies with the country’s economic strength where United States of America (USA) and Asia are the top consumers (Kiss et al., 2010). Production of sulphuric acid is a breakthrough in industrial chemistry and a workhorse chemical of industrial world (Hiji et al., 2014).
Canada Mexico Others Japan Latin America F-USSR Europe Africa U.S. Asia 0.0 5.0 10.0 15.0 20.0 25.0 30.0 Share (percent)
Figure 1.2: Worldwide Consumption of Sulphuric acid (source: Kiss, et al., 2010)
There are two main available processes for production of sulphuric acid on large scale i.e.
Lead Chamber Process and Contact Process. The Lead Chamber Process is the older of the two processes and its acid grade produced is between 62% and 78% w/w (Shakhashiri,
2007). The process is also economically viable in small scale. Contact Process is currently the most commonly used method in terms of quality and quantity to meet the industrial requirement with acid concentration up to 98% w/w (NolaNZor, 2017; Davenport and
King, 2007).
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The primary production route of sulphuric acid is by burning elemental sulphur, which accounts for a half of global supply. Pyrite roasting contributes one third of global supply and the rest, is contributed by base metal smelting (Chepushtanova and Luganov, 2007).
Although burning of elemental sulphur is the main technology for the production of sulphuric acid, roasting of pyrite can be an alternative process once pyrite is available
(Habashi, 2006). The initial cost of producing sulphuric acid from pyrite is higher than burning of sulphur, however the operating cost is about half of that of the former and therefore the investment return exceeds that of sulphur burning within six years (Mutler and Warren, 2009; Runkel and Sturm, 2009). The technology of producing sulphuric acid by using pyrite roasting is currently being operated successfully in several countries such as Mali, China, Turkey, Spain and Finland (Runkel and Sturm, 2009).
Producing sulphuric acid by using locally available raw materials will make a meaningful contribution in social-economic development in Tanzania. According to United Nations data, about 3,111 tonnes of sulphuric acid were imported to Tanzania with a total price of one million USD in 2013 (UNdata, 2013). This is a significant amount of money with indispensable customers. Figure 1.3 shows the cost of importing sulphuric acid which is almost doubled within seven years. The current major use of sulphuric acid is in steel industry, mining industry, laboratory works, lead˗acid batteries, plastic industry and detergent production. The use of sulphuric acid is expected to be multiplied as the
Government is bidding to move the country into a middle state economy in 2025. This is done by attracting new investements in manufacturing industries such as textile, fertiliser, oil and gas, value addition products, pharmaceuticals, food products, iron and steel, chemical plants and even strengthening science subjects teaching in schools.
3
900,000 800,000 700,000 600,000 500,000 400,000 Cost (USD) 300,000 200,000 100,000 0 2004 2005 2006 2007 2008 2009 2010 2011 2012 Year
Figure 1.3: Importation cost of H2SO4 in Tanzania (Source: www.indexmundi.com)
The presence of pyrite and sulphide ore in Tanzania is evident and enormous. Pyrite from
Samena is economically attractive and a promising one. Samena is located about 5 km from Geita town. It is situated in one of several patches of Nyanzian rocks which occur in the extensive granitic complex lying south and east of Lake Victoria. Nyanzian rocks are the host of gold mineralisation of both reef and disseminated types such as sulphide impregnations (Minde, 1988; Britton, 1976). The preliminary geological data showed that
Samena Pyrite Ore contains up to 47% sulphur, negligible arsenic mineable sulphide˗rich material to a depth of 300 m, corresponding to 6200 metric tonnes of sulphur per metre of depth. This is a huge source of sulphur which has potential for producing one million ton- mole of sulphuric acid (Minde, 1988; Britton, 1976).
Another attractive source is crude pyrite; this is overburden or waste rocks that must be removed or excavated to allow access to the metallic ore deposit. Mine wastes are byproducts which are unwanted and have no economic value. In most mining projects, the quantity of overburden is enormous. The strip ratio (the ratio between overburden to the
4
quantity of mineral ore) is usually greater than one (GEMP, 2017). Significant amount of crude pyrite can be obtained from Merelani in Simanjiro district (Harris et. al, 2014).
In addition, large reserve of sulphide-gold ore is available in Kahama, Shinyanga region and Bulyanhulu in Geita region. Two well established Tanzania mine companies are running in those two areas. The export of 51600 TPY of concentrates containing gold, copper and silver per annum is done (Gordon, 2017; Morcombe, 2017). There is significant amount of sulphur in the concentrate, about 30%, which is another useful source of sulphur to be considered.
Like other raw materials, conversion of pyrite into a more useful product such as sulphuric acid involves several reactions. Thermodynamic and kinetic information are vital for performing chemical reactions. Thermodynamic parameters such as Gibb’s free energy and enthalpy of the reaction can provide information about feasibility of the chemical reaction and energy requirement of a process (either endothermic or exothermic reaction).
Thermodynamic information of chemical reactions is normally obtained from literatures such as Perry’s Chemical Engineering handbook. However, thermodynamic parameters cannot answer how a reaction occurs, how fast it will take place and the reaction mechanism. This kind of questions can be answered by studying chemical kinetics
(Fogler, 1986; Levenspiel, 1999). The chemical kinetics has direct influence on engineering: such as productivity and the factors responsible for its increase. The subject of kinetics is of great economic and technical importance. For instance, for a slow reaction, productivity will be low and consequently the Rate of Return of Investment
(IRR) will also be low. Factors that govern a chemical reaction are crucial and therefore should be known.
5
The chemical kinetics parameters are normally obtained experimentally. During roasting process, the physical, chemical and mineralogical properties of the ore constituents are altered to an extent determined by roasting conditions such as temperature, partial pressure of oxidising gas, feed compositions and grain size (Aylmore and de Klerk, 2013).
The roasting rate of pyrite to sulphur dioxide is strongly dependent on the roasting kinetics, which are largely function of temperature, grain size and mineralogical composition of pyrite (Runkel and Sturm, 2009; Amankwah, et al.,. 2005; La Brooy et al.,
1994). The reaction temperature and grain size of pyrite can be controlled while mineralogical composition of pyrite is site specific. As a result, the chemical kinetic parameters of pyrite roasting will differ from one sample to another.
The reported works on pyrite roasting studied its kinetics in terms of mechanism by sequential elementary reactions from pyrite into pyrrhotite (Fe7S8), magnetite (Fe3O4) and hematite (Fe2O3) (Cocic et al., 2007; Prasad et al., 1985). They only reported appropriate time and temperature for complete oxidation of pyrite to hematite.
However, they did not show the slowest reaction which is the rate determining step. Such kind of information could show kinetic parameters such as rate constant and order of the reaction which are crucial for optimising the process and performing the techno-economic viability of the process. Prasad (1985) only concluded that hematite is obtained after 30 min of the reaction (Equation 1.1) while Müller (2000a) only reported the sequence according to Equation 1.2 with optimum temperature range for complete reaction at
800° C to 940° C .
6
roasted at 610 °C FeS2 Fe7S8 Fe3O4 Fe2O3 ...... (1.1) in air
Fe S / FeS FeO ...... (1.2) FeS2 7 8 Fe2O3
Other factors such as atmosphere (oxygen partial presurre) during roasting can affect the formation of maximum amount of sulphur dioxide due to formation of byproduct iron sulphate. To guarantee high production of sulphur dioxide, excess oxygen of 13 to 14 volumes percent is normally used (Müller, 2000a). The roasting conditions can be elaborated by predominance area diagrams (Figure 1.4 and 1.5) which shows that roasting products are partial metallic sulphide (MeS), metallic oxide (MeO), metallic sulphate
(MeSO4) and metal (Me). Temperature and partial pressure of oxygen are crucial on the formation of desired product (Equation 1.2).
Figure 1.4: Isothermal Phase Diagram of Me-S-O system (Source: Sokic et al., 2008).
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Figure 1.5: Effect of Temperature on Roasting Equilibria in the System Me-S-O (Source: Rosenqvist, 2004).
Production of sulphuric acid can be a driving force in several areas particularly fertiliser industry, acid battery technologies, pharmaceutical industry, nickel and uranium mines.
As part of an effort to increase her agricultural production, Tanzania, along with a consortium from Germany, Denmark and Pakistan will build a 3 billion USD fertilizer plant. The plant will be operational in 2021 with production capacity of 3,800 TPD
(Azania Post Reporter, 2017). The factory will use natural gas to produce fertiliser. It will be built in southern Tanzania near big offshore gas reserves. Natural gas can be used for the industrial production of ammonia, a key fertiliser intermediate. Another key intermediate is phosphoric acid which is obtained by reacting phosphate rock with sulphuric acid. Note that agricultural fertilisers represent the largest single application for sulphuric acid, accounting up to 65% of its usage (Kiss, et al., 2010). There is also a potential of producing gypsum as byproduct during phosphoric acid production.
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The current plans in investing pharmaceutical companies in Tanzania will require vast quantities and reliable supply of sulphuric acid. M-Pharmaceutical Company is one of them; it is a 55 billion pharmaceutical plant that is expected to start production in the next
18 months. It is a joint venture by Tanzanian, IPP chairman, Reginald Mengi and
Dr. Nagesh Bhandari, an Indian Surgeon (Lamtey, 2018). Aga Khan also plans to set up a
20 million USD (about Sh45 billion) pharmaceutical plant as part of his contribution to the Government’s industrialisation drive (Kamagi, 2018).
Tanzania has found huge deposits of uranium in Mkuju River at Namtumbo District which is located in Lindi region 470 km southwest of Dar es Salaam. It represents one of the largest reserves in Tanzania, having estimated reserves of 182.1 million tonnes of ore grading 0.025% uranium. The production rate estimated per year is 12 million pounds of uranium oxide (U3O8) (Mohammed and Mazunga, 2013). Sulphuric acid has high solubility with uranyl sulfate complexes which makes it a desirable leaching agent for extraction of uranium from its ores (Huang et al., 2017).
Another useful project which is going to use large volumes of sulphuric acid is Kabanga nickel project. Kabanga nickel exploration and mining is found 130 km south west of
Lake Victoria in the Ngara district of the Kagera region in Tanzania. The exploration program has so far placed 9.7 million tonnes at 2.37% nickel into the indicated resource category, with an additional 30.3 million tonnes at 2.8% nickel in the estimated inferred resource category (Evans et al., 2000). Sulphuric acid is the most desired leaching agent of nickel and is normally needed in large volumes in nickel mines process (BHP, 2013).
In addition, the use of heat recovery equipment in pyrite roasting-sulphuric acid plants can produce electricity (Ashar and Golwalkar, 2013a). In Australia, similar operating plants
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produce up to 24MW (BHP, 2013) which is connected to national grid. On cogeration of power due to processing of pyrite to produce carbon˗neutral electricity and about 10MW power from a 1,000 TPD plant (Ashar and Golwalkar, 2013a)
The techno-economical feasibility of producing sulphuric acid is based on the key precursor, sulphur dioxide. The optimum condition of this precursor depends on several factors such as chemical and mineralogical composition of the pyrite ore, roasting temperature and particle size of pyrite. These factors are site specific. This study will determine kinetic parameters of roasting of pyrite ore that will enable the optimisation of sulphuric acid production from local available raw materials. The kinetic parameters will also be used in a model that evaluates techno-economical viability and environmental feasibility of the process of producing sulphuric acid.
1.2 Problem Statement
The knowledge of chemical kinetics is key to the economic success or failure of a chemical plant. The chemical kinetics of roasting pyrite ore to sulphur dioxide depends on temperature, grain size, mineralogical and chemical composition of pyrite (Runkel and
Sturm, 2009). Wide range of pyrite roasting temperature depending on the nature of pyrite ore are reported such as 610° C by Prasad (1985) and a range of 800° C to 940° C by
Müller (2000a).
The information about the chemical kinetics of pyrite roasting from Tanzania is still missing. The chemical kinetic parameters such as rate constant, reaction order, optimum time and temperature are crucial for techno˗economical evaluation and environmental feasibility of producing the key intermediate, sulphur dioxide, and further conversion into sulphuric acid. In addition, some of these parameters are not yet reported such as reaction
10
order, which is very important, because it allows predictions to be made about how fast reactions will respond on changing concentrations.
The field of chemical kinetics of pyrite is broad. There are few publications in the area of pyrometallurgy of pyrite roasting; this is because the emphasis is placed on leading method of producing sulphur dioxide by burning of elemental sulphur.
There is evidence of the presence of deposit of pyrite in Tanzania (Harris et. al, 2015;
Minde, 1988; Britton, 1976). The purpose of this work is to study the chemical kinetics of pyrite ore roasting; to optimise the conditions for producing sulphuric acid from local available pyrite and to perform economic viability of the proposed method.
1.3 Objectives
The main objective is to study the chemical kinetics of roasting pyrite ore from Merelani for production of sulphuric acid.
Specific objectives are:
1. To establish chemical and mineralogical characteristics of pyrite ore from
Merelani;
2. To determine the effect of temperature, particle size and concentration on
conversion of pyrite ore to sulphur dioxide;
3. To develop the appropriate protocol of converting pyritic sulphur in the ore into
sulphuric acid and
4. To establish techno-economical viability and environmental feasibility of the
process of producing sulphuric acid.
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1.4 Hypothesis
The study is based on the following hypothesis:
The mineral composition, particle size and reaction temperature of pyrite ore affect the chemical kinetic of pyrite roasting such that producing sulphur dioxide and ultimate sulphuric acid from it will require specific conditions and techno-economic approach.
1.5 Significance of the Study
In this study the chemical and mineralogical characteristics and chemical kinetic parameters of Merelani’s pyrite ore roasting were elucidated. This enables utilisation of mine wastes, particularly crude pyrite overburden which provides a social-economic development and a proper environmental mitigation of Acid Mine Drainage (AMD).
In addition, the model of techno-economical viability and environmental feasibility of producing sulphuric acid was studied. This is expected to raise awareness of researchers and policy makers to see the potential of investing in sulphuric acid plant, pyrite roaster, and smelter which will contribute positively to Tanzanian economy.
Apart from that, two multipurpose computer tools were developed. Firstly, standard tool for analysing chemical reaction kinetic data, which will be useful for teaching purposes: particularly for setting questions and answers for chemical reaction course and even for chemical reactor design use. Secondly, tool which deals with titration experiments is useful for marking and assessing students laboratory reports related to titration and any other titrimetric work.
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CHAPTER TWO
LITERATURE REVIEW
2.1 Production of Sulphuric Acid
2.1.1 Introduction
Pure sulphuric acid is a colorless, odorless, oily liquid. It freezes at 10.5° C and boils at
338° C. It has a strong affinity for water and is sometimes used as a drying agent. It can be used to chemically remove water from many compounds. It dehydrates sucrose (table sugar) leaving a spongy black mass of carbon and diluted sulphuric acid. Concentrated sulphuric acid reacts similarly with skin, paper, and other animal and plant matter. When it is mixed with water, a violent and highly exothermic reaction occurs. Therefore, concentrated sulphuric acid must be diluted by adding the acid slowly to cold water while the mixture is stirred to dissipate the heat (Shakhashiri, 2007).
The discovery of sulphuric acid is credited to the 8th century by Arabian chemist and alchemist, Jabir ibn Hayyan (Geber). Then in 17th century, Johann Glauber prepared sulphuric acid by burning sulphur together with saltpetre (potassium nitrate) in the presence of steam. Decomposition of saltpetre followed by oxidation produces SO3, which combines with water to produce sulphuric acid (Ashar and Golwalkar, 2013b)
The breakthrough was obtained in 1746 at Birmingham, when John Roebuck produced sulphuric acid in lead-lined chambers. This Lead Chamber Process was the first large scale production. In this process, the oxides of nitrogen (as nitrosy compounds) were used as homogenous catalyst for the oxidation of SO2. John Roebuck’s sulphuric acid was only about 35-40%. The further revamped of this process enabled to reach the concentration of
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78% w/w which was not high enough for many commercial scales (Jones, 2006; Ashar and Golwalkar, 2013b).
Another useful milestone was introduced in 1831 by the British vinegar merchant
Peregrine Phillips when patented a far more economical process called Contact Process. In this process, SO2 produced by either combustion of sulphur or roasting of pyrite in air, is combined with additional air and passed over a platinum catalyst at high temperature, where it combines with oxygen from the air to produce SO3. The catalyst platinum was replaced by vanadium pentoxide (V2O5) which was less toxic and expensive. Another revamped was the introduction of double Contact Process which reduced SO2 emissions
(Jones, 2006).
2.1.2 Raw materials
Sulphuric acid is manufactured from SO2. The primary raw materials for producing this key intermediate are by combustion of elemental sulphur or by oxidation of sulphides.
Since most carbonaceous or hydrocarbons fuels and the ore of many non-ferrous metals contain sulphudic sulphur, vast volumes are also produced involuntary in large fuel- consuming installations, such as power stations and in non-ferrous metal smelters (Müller,
2000b).
Sulphur dioxide is manufactured industrially from the following raw materials
(Shakhashiri, 2007) :
i) Elemental sulphur;
ii) Pyrite;
iii) Hydrogen sulphide containing waste gases;
iv) Sulphide ores of non-ferrous metals;
v) Waste sulphuric acid and sulphates;
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vi) Gypsum and anhydride;
vii) Flue gases from the combustion of sulphurous fossil fuels.
Sulphur occurs as native in the vicinity of volcanoes and hot springs. It is widely distributed in nature as pyrite (FeS2), galena (PbS), sphalerite (ZnS), cinnabar (HgS), stibnite (Sb2S3), gypsum (CaSO4. 2H2O), Epsomite (MgSO4. 7H2O), celestite (SrSO4), glauberite (NaSO4.CaSO4), etc. Sulphur also occurs in natural gas and petroleum crudes and must be removed from these products. Formerly this was done chemically, which wasted the sulphur. New processes now permit recovery, and these sources promise to be very important (Lide, 2000).
Today, elemental sulphur used for SO2 production is considered not only as a raw material but also, to an increasing extent, as an energy source. Modern sulphur˗burning plants are normally connected to a turbo-alternator combination for electrical power production, these are known as cogeneration units (Ashar and Golwalkar, 2013a).
Currently, pyrite and other iron sulphide ores still constitute in several countries, an important raw material basis for SO2 and subsequently for sulphuric acid manufacturing.
The most important iron sulphide minerals are pyrite (FeS2), pyrrhotite (Fe7S8), chalcopyrite (CuFeS2), sphalerite (ZnS) and galena (PbS) (Baba et al., 2011). Pyrite occurs in varying purity as sedimentary rocks, as minor constituents in coal deposits and in varying proportions in pyritic ores non-ferrous metals sulphide such as copper, zinc and lead. It is also in lesser extent in cobalt, nickel, manganese, bismuth, silver and gold. In some cases, pyrite may also contain arsenic, antimony, tellurium, thallium and as well as fluorides and chlorides (Müller, 2000a). Pyrite is one of the most abundant sulphide minerals at the Earth's surface and represents an important reservoir for iron and sulfur within the Earth's crust (Wolfe et al., 2007).
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Pyrite can be obtained as crude, beneficiated and flotation. Flotation pyrites are obtained in markable quantities as a byproduct of non˗ferrous metal sulphide beneficiation plants.
The sulphur content of pyrite concentrates may be as high as 50%. Average analyses of pyrite ores of different origins are presented in Table 2.1. Depending on the grain size, pyrites are grouped as flotation pyrites (grain size < 0.1 mm), fine pyrites (0˗6 mm), course pyrites (1˗12 mm) and lump pyrites (10˗100 mm).
In Japan, all pyrite ores are processed within the country, however, other countries such as
Russia, Spain and Norway are still export pyrite as raw materials for sulphuric acid production (Müller, 2000a).
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Table 2.1: Composition of various types of pyrites (Source: Müller, 2000a)
Country Location Grade Typical content, wt% S Fe Zn Cu Pb As Germany Meggen flotation 43 ̶ 48 37 ̶ 40 0.4 ̶ 1.2 0.03 ̶ 0.1 0.4 ̶ 0.6 0.06 Cyprus Kalavassos flotation 45 ̶ 48 40 ̶ 44 0.1 0.2 ̶ 0.3 0.1 0.01 Limni flotation 44 ̶ 46 37 ̶ 40 0.1 0.2 ̶ 0.3 0.1 Finland Autokumpu flotation 43 ̶ 48 40 ̶ 44 0.1 ̶ 1.1 0.2 ̶ 0.5 0.3 0.03 ̶ 0.06 Greece Kassandra flotation 47 ̶ 49 41 ̶ 44 0.6 ̶ 1.5 0.1 ̶ 0.3 0.5 ̶ 0.7 0.5 ̶ 1.8 Italy Maremma flotation 48 ̶ 50 44 ̶ 45 0.09 0.02 0.02 0.045 Yugoslavia Bor flotation 44 ̶ 49 41 ̶ 43 0.04 ̶ 0.1 0.25 ̶ 0.6 0.05 ̶ 0.06 0.03 ̶ 0.07 Norway Orkla fine 41 ̶ 46 37 ̶ 40 1.6 ̶ 2.0 0.2 ̶ 2.7 0.04 ̶ 0.05 0.04 ̶ 0.08 Foldall flotation 48 ̶ 50 44 ̶ 45 0.07 ̶ 0.08 0.06 ̶ 0.07 1.2 ̶ 1.4 0.03 ̶ 0.05 Portugal Aljustrel fine 45 ̶ 47 39 ̶ 40 2.0 ̶ 3.0 0.65 ̶ 1.33 1.0 ̶ 1.1 0.4 ̶ 0.5 Romania Baia Sprie fine 44 ̶ 45 37 ̶ 40 4.0 ̶ 5.0 0.6 ̶ 0.7 0.05 ̶ 0.06 0.03 ̶ 0.05 Sweden Boliden flotation 49 ̶ 50 45 ̶ 46 0.2 ̶ 0.4 0.04 ̶ 0.15 0.04 ̶ 0.05 0.05 ̶ 0.2 Spain Rio Tinto flotation 48 ̶ 49 43 ̶ 44 0.08 ̶ 1.0 0.15 ̶ 0.4 0.08 ̶ 1.0 0.26 ̶ 0.4 Tharsis flotation 46 ̶ 48 41 ̶ 43 1.3 ̶ 1.6 0.7 ̶ 1.1 0.5 ̶ 0.6 0.4 ̶ 0.5 San Telmo fine 45 ̶ 47 40 ̶ 41 1.0 ̶ 2.6 1.9 ̶ 2.5 0.05 ̶ 0.15 0.1 ̶ 0.2 Russia flotation 46 ̶ 48 39 ̶ 41 0.1 ̶ 0.2 0.9 ̶ 1.0 0.1 ̶ 0.2 0.05 ̶ 0.1
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In Tanzania, the country is rich in pyrite and other iron sulphide ores. The potential of utilising pyrite for social development of country is not yet established. There is no research or published work in this area. Pyrite is not considered as sovereign mineral wealth. There are three types of pyrite/iron sulphide ores in Tanzania namely as flotation; mineral; and crude. Concentrate or flotation pyrites are obtained from refractory ores where gold and silver are embedded in pyrites (FeS2) and chalcopyrites (CuFeS2).
Another promising future is extraction of mineral pyrite from Samena. There is large reserve of pyrite mineral free of arsenic and potential for producing up to one million tonne-mole of sulphuric acid (Britton, 1976). In most countries, sulphide concentrates are the only available forms of sulphide ores existing. However, Tanzania is rich to the extent that pyrite can be mined as a sole mineral of interest.
Crude-overburden pyrite is another potential source of sulphide ores that is available in significant amount in Tanzania. This is overburden (waste rock) which can be obtained from Merelani in Simanjiro district. As similar to previous forms of sulphide ores present in Tanzania, is not significantly mentioned in the literature. From the foregoing evidence, there is a need to consider sulphur and particularly pyrite as sovereign mineral wealth in
Tanzania.
Special attention should be given to sulphide-rich wastes such as in Merelani, because there is great danger of acidified mine drainage (AMD). AMD is one of the biggest environmental problems associated with the mining of sulphide-rich mineral deposits such as pyrite (FeS2 ) together with less minor chalcopyrite (CuFeS2), arsenopyrite
(FeAsS), sphalerite (ZnS) and galena (PbS) (Olías and Nieto, 2015). AMD is the result of the exposure of sulphides to air, water and micro-organisms and involve complex
18
processes governed by a combination of physical, chemical and biological factors (Amos et al., 2015; Simate and Ndlovu, 2014; Hudson-Edwards et al., 2011).
The control and mitigation of AMD is considered to be one of the major environmental challenges facing the mining industry worldwide. The estimated costs for total worldwide associated with the current and future remediation of AMD are approximately 100 billion
USD (Hudson-Edwards, 2011).
To achieve sustainable growth, human kind ideally requires the products of mining to be extracted without negative environmental impact. It is crucial to find a way to prevent, minimize, reuse and recycle the mining wastes. Reuse and recycle of mine wastes will provide beneficial application or conversion of wastes into valuable products. Multidisciplinary significant research efforts on mine wastes focuses on understanding their character, stability, impact, remediation and reuse are of enormous importance (Hudson-Edwards, 2011; Lottermoser, 2011; Younger and Wolkersdorfer, 2004).
The study on utilisation of crude-overburden pyrite from Merelani will provide not only social-economic development but also proper environmental mitigation of AMD.
2.1.3 Grades of sulphuric acid
The molarity of concentrated sulphuric acid is 18M with 98% grade and it is stable in storage. Other concentrations are used for different purposes. Some common concentrations are shown in Table 2.2. There is useful correlation between density and grade (Table 2.3). By knowing the density, the grade can be straightly read from the table and hence minimising the long method of its determination via titration.
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Table 2.2 : Grades of sulphuric acid (Source: Ashar and Golwalkar, 2013c)
Mass fraction Density Concentration Common 3 3 H2SO4 (kg/dm ) (mol/ dm ) name dilute sulphuric 10% 1.07 ≈1 acid battery acid (used in lead–acid 29–32% 1.25–1.28 4.2–5 batteries) chamber acid 62–70% 1.52–1.60 9.6–11.5 (fertilizer acid) tower acid 78–80% 1.70–1.73 13.5–14 (glover acid) concentrated 98% 1.84 ≈18 sulphuric acid
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Table 2.3: Sulphuric Acid Density Table (source: Lide, 2000)
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2.1.4 Analytical and titrimetric methods
Titration experiments are widely used in analytical chemistry to determine quantitatively unknown concentration of acids, bases, oxidants, reductants, metal ions, proteins and many other speciess. Titrations are based on a reaction between the analyte and a standard reagent known as the titrant. The reaction is of known and reproducible stoichimetry such as Equation 2.1 where the moles of base NaOH are twice that of H2SO4. In volumetric titrations, measuring the volume of a solution of known concentration is needed to react completely with the analyte.
+ Na SO (aq) 2H O (l) H2SO4(aq) 2NaOH(aq) 2 4 + 2 ...... (2.1)
Like all titrations, neutralisation depends on a chemical reaction of the analyte with a standard reagent. In all types of acid/base titrations, the most common is the titration of a strong acid such as hydrochloric or sulphuric acid with a strong base such as sodium hydroxide. The other common types are such as weak acid/strong base, weak base/strong acid. In all titrations, end point should be identified with aid of a chemical indicator or an instrumental method.
2.1.4.1 Determination of density of acid (g/mL)
Density of acid is easily found by measuring the weight of a fixed volume of acid and calculated it by formula mass divide by volume (Equation 2.2).
푚푎푠푠 (푔) 퐷푒푛푠𝑖푡푦 (푔⁄푚퐿) = … … … … … … … … … … … . … … … … … … … (2.2) 푣표푙푢푚푒 (푚퐿)
22
2.1.4.2 Determination of uknown molar concentration (M) of H2SO4
Assume a solution of sulphuric acid with unknown concentration. Pour exactly measured volume of sulphuric acid (Vacid) into a beaker and add few drops of phenolphthalein solution (chemical indicator for identification of completion reaction of neutralisation reaction). Solution will be colourless, as phenolphthalein becames pink only in basic solutions. Now use burette to read slowly adds NaOH solution (titrant) of known molar concentration (CNAOH). pH slowly goes up. Once all sulphuric acid becomes neutralised, one excess drop of strong base is enough to rapidly change pH of solution and change its colour to pink. When the colour of solution changes is a sign that all acid present has been neutralised, that titration end point has been reached. Using burette scale, one may read volume of the titrant used (VNaOH) (Mustoe, 2002). Then the molarity of unknown acid,
Cacid is determined by Equation 2.3.
퐶푁푎푂퐻푉푁푎푂퐻 퐶푎푐푖푑 = … … … … … … … … … … … … … … … … … … … … … … … … … (2.3) 2 푉푎푐푖푑
2.1.4.3 Percentage concentration (%) of acid
Once the density and molar concentration are known, the percentage (Equation 2.4) is calculated from the fraction of the weight of theoretically acid molecules in 1000 g solution (molarity x RMM) divided by the actual weight of 1000 g acid (density x 1000)
(Skoog et al., 2013)
(푅푀푀 표푓 퐴푐𝑖푑 푥 푚표푙푎푟𝑖푡푦) 푥 100 % 퐶 = … … … … … … … … … … … … … . . (2.4) 푎푐푖푑 퐷푒푛푠𝑖푡푦 푥 1000 푚퐿
2.1.5 Qualitative methods
Sulphuric acid has acidic character to litmus paper. When a solution of barium nitrate is added to the acid, white precipitation (cloudiness) of barium sulphate will be formed
23
(Equation 2.5). This is characteristic test of sulphuric acid and soluble sulphates
(Williams, 2003).
BaSO + 2 HNO ...... (2.5) H2SO4 + Ba(NO3)2 4 3
2.1.6 Determination of forms of sulphur
Section 2.1.2 shows that pyrite can occur as a minor constituents in coal deposits. This form of sulphur is normally analysed by ASTM D-2492, designated for identifying type of sulphur present in coal. It is necessary to know the total amount and forms of sulphur present in coal for effective control of oxide of sulphur and efficient as fuel (Speight,
2015). The scenario is similar to pyrite ore and the method was employed in this work.
There are three types of sulphur associated with pyrite ore:
i) Inorganic sulphur or sulphate sulphur: the sulphates of metals such as CaSO4
(gypsum) and FeSO4;
ii) Pyritic sulphur (FeS2): it occurs in two crystalline habits, pyrite (cubic) and
marcasite (orthorhombic). It is difficult to distinguish one from another and the
former being more common and
iii) Organic bound sulphur.
Organic sulphur and pyritic sulphur account for almost all the sulphur present in the pyrite ore. The sulphate sulphur is usually less than 1 % except in weathered pyrite ore. The methods for determination of the various forms of sulphur are based on the fact that sulphate sulphur is soluble in dilute hydrochloric acid solution, where as pyritic and organic sulphur is not attacked. In addition, dilute nitric acid dissolves sulphate and pyritic sulphur quantitatively and attacks organic sulphur only slightly.
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Pyrite sulphur is determined accurately by noting the quantity of iron extracted by dilute hydrochloric acid and subtracting this from the iron extracted by nitric acid, the difference being iron present as pyrite iron (FeS2), from which the equivalent quantity of pyrite sulphur can be calculated according to Equation 2.6.
32.06 % 푝푦푟𝑖푡푒 푠푢푙푝ℎ푢푟 = % 푝푦푟𝑖푡푒 𝑖푟표푛 × 2 × … … … … … … … … … … … . . (2.6) 55.85 Where 2 x 32.06/55.85 is the ratio of sulphur to iron in pyrite.
Organic sulphur is obtained by subtracting the combined percentages of sulphate sulphur and pyrite sulphur from total sulphur determined by x-Ray Fluorescence (XRF) method.
XRF has been used extensively for determining total sulphur in coal. The economy and speed of such XRF methods when used for multiple determinations (e.g. Al, Si, Ca, Mg,
Fe, K, P and S) is better than any other method for coal analysis (Speight, 2015).
The American Society for Testing and Materials (ASTM), designates this method as D-
2492 where the sulphate and pyritic sulphur are determined directly and the organic sulphur is taken as the difference between the total sulphur and the sum of the sulphate sulphur and pyritic sulphur. The organic sulphur is determined by Equation 2.7 (Speight,
2015; Calkins, 1994; Shimp et al., 1977).
푂푟푔푎푛𝑖푐 푠푢푙푝ℎ푢푟 = 푇표푡푎푙 푠푢푙푝ℎ푢푟 − (푠푢푙푝ℎ푎푡푒 푠푢푙푝ℎ푢푟 + 푝푦푟𝑖푡푒 푠푢푙푝ℎ푢푟) … … . (2.7)
2.1.7 Methods for production of sulphuric acid
There are three large scale methods for producing sulphuric acid namely, Contact Process,
Wet Process and Lead Chamber Process. Each of these methods has its merits and demerits. However, only Contact Process and Lead Chamber Process are the methods that
25
can either use sulphur or pyrite as source of SO2 (Shakhashiri, 2007; Jones, 2006). Wet
Process is a method of recovering sulphur from off gases to produce commercial grade concentrated sulphuric acid. The potential industries for this method are refinery and petrochemical industry, metallurgy industry, coal based industry (coking and gasification), power industry and sulphuric acid industry. With the growing sensitivity to atmospheric pollution, the recovery of sulphur from the waste gases is required. Starting with a low SO2 concentration (0.2 to 6%) can produce strong 98% (w/w) of sulphuric acid
(Jones, 2006; Kristiansen, 2005). However, the method is not feasible in Tanzania and
Africa in general due to lack of potential industries that are capable of implementing this process.
2.1.7.1 Contact Process
Contact Process is the currently leading method for producing sulphuric acid in terms of quality and quantity to meet the industrial requirement (Jones, 2006). The method is much superior to former Lead Chamber Process. SO2 is catalytically converted into SO3 in a reversible reaction. According to Le Chatelier's Principle, to favour the equilibrium to shift to the product side, it is operated either in high pressure or in high concentration of reactants.
To achieve high concentration of reactants, it is easy and cheap because oxygen which is among the reactants can be obtained in air in any required amounts. To operate in high pressure, it is not advantageous. The catalysts used (platinum or vanadium pentoxide
(V2O5) are expensive and platinum is poisonous due to possibility of formation of arsenic.
Although high quality (concentration of around 98%) and quantity of sulphuric acid can be obtained, the high price of catalyst, high operating temperature and pressure during formation of sulphur trioxide are disadvantages. Not only that, this method is only
26
feasible in large scale production. For this reason, implementing it in Tanzania is a challenge. Equations 2.8 to 2.10 show key reactions in the production of sulphuric acid by contact process. Equation 2.11 could replace Equation 2.10. Unfortunately, the reaction is highly exothermic such that the product is acid in the vapour form; the condensation is slow and expensive.
SO ...... (2.8) S + O2 2 ∆ H= -300 kJ/mol
T=400-450 oC
2SO ...... (2.9) 2SO2 + O2 3 ∆ H= -300 kJ/mol P= 2 atm
H S O (l)+ H O(l) ..(2.10) SO3+H2SO4(l) 2 2 7 2 2H2SO4 ( l ) ∆ H= -1660 kJ/mol
H SO (l) ...... (2.11) SO3+ H2O(l) 2 4 ∆ H= -88 kJ/mol
2.1.7.2 Lead Chamber Process
This was the first industrial method to produce sulphuric acid in large quantity (Khan,
2012; Sam, 2012). The method uses nitrogen dioxide (NO2) as oxygen carrying catalyst for the conversion of SO2 to SO3 in a packed chamber. The main advantage is that, once
NO2 is produced, it donates oxygen to become nitrogen oxide (NO) and NO takes oxygen from air to form NO2. It is easy, continuous and cheaper process. The concentration of acid produced by this method is less than 80% (w/w). This is the major limitation of the process. However, over 60% of the world production of sulphuric acid is used to make
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fertilisers (Shakhashiri, 2007), which needs only 65% (w/w) acid. Therefore this method seems to be more preferable in developing countries like Tanzania, whereby an acid with concentration of about 80% (w/w) suffices the agriculture and energy sectors.
Lead Chamber Process can be modified in four steps to produce sulphuric acid starting with pyrite (FeS2). The steps are formation of SO2 by roasting of pyrite; formation of NO
(oxygen carrier); conversion to NO2 and formation of sulphuric acid represented by
Equations 2.12 to 2.15 respectively (Jones, 1950).
Fe O + 4 SO ...... (2.12) 2FeS2+ 5.5 O2 2 3 2 ∆ H= -1660 kJ/mol
3Cu(NO ) +4H O +2NO H= -113.46 kJ/mol ...... (2.13) 3Cu+ 8HNO3 3 2 2 ∆
2NO ∆ H=-131kJ/mol ...... (2.14) 2NO+ 2O2 2
H SO (l)+N O H= -719.6 kJ/mol ...... (2.15) SO2(g)+ H2O(g)+NO2(g) 2 4 ∆
Equation 2.13 is formed by combining gaseous products in Equations 2.12, 2.13 and 2.14.
In Equation 2.14, NO takes an atom of O from the air to form NO2, and at once gives it up to the H2SO3 (H2O + SO2), making sulphuric acid, and again goes through the same operation of taking up elemental ‘O’ and passing it along. It is continuous process of reproducing NO. Figure 2.1 shows the process of converting pyrite to clean SO2 which involve four steps (Jones, 1950).
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Water Conc. H2SO4 Air
Pyrite burner Dust filter Pyrites Washing tower Drying
Pyrite Cinder Effluents Clean SO2
Figure 2.1: Block diagram for production of SO2 from pyrite
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Pyrite cinder is a byproduct during roasting of pyrites, and it is mainly iron oxide (Fe2O3).
Gas entering dehydration must be dust free to avoid plugging downstream SO2-oxidation catalyst. Dust is removed from the gas by a series of electrostatic precipitation and scrubbing steps (Jones, 2006). Drying is important to avoid unexpected formation of sulphuric acid that would affect performance of catalyst. Block diagram for a modified
Lead Chamber Process is shown in Figure 2.2.
Water
Figure 2.2: Modified Lead Chamber Process
2.2 Chemical Kinetics
Chemical kinetics is the study of the rate and mechanism by which one chemical species is converted to another. Rate can be defined as a change in a quantity with respect to time
(Equation 2.16). For a chemical reaction, rate can be defined as the mass in moles, of a product or reactant produced or consumed per unit time. There several ways of expressing reaction rate as shown in Table 2.4.
푐ℎ푎푛푔푒 𝑖푛 푞푢푎푛푡𝑖푡푦 푞푢푎푛푡𝑖푡푦푓푖푛푎푙 − 푞푢푎푛푡𝑖푡푦푖푛푖푡푖푎푙 ∆푞 푟푎푡푒 = = = … … … … … … (2.16) 푐ℎ푎푛푔푒 𝑖푛 푡𝑖푚푒 푡푓푖푛푎푙 − 푡푖푛푖푡푖푎푙 ∆푡
30
Table 2.4: Expression of reaction rate Property Rate equation Concentration ∆푐표푛푐푒푛푡푟푎푡𝑖표푛 푟푎푡푒 = ∆푡𝑖푚푒 Mass ∆푚푎푠푠 푟푎푡푒 = ∆푡𝑖푚푒 Volume ∆푣표푙푢푚푒 푟푎푡푒 = ∆푡𝑖푚푒 Pressure ∆푝푟푒푠푠푢푟푒 푟푎푡푒 = ∆푡𝑖푚푒 Colour ∆푎푏푠표푟푏푎푛푐푒 푟푎푡푒 = ∆푡𝑖푚푒 pH ∆푝퐻 푟푎푡푒 = ∆푡𝑖푚푒 Electrical conduction ∆푐표푛푑푢푐푡𝑖표푛 푟푎푡푒 = ∆푡𝑖푚푒
There are several ways of presenting reaction rate by selecting one reaction component, i for consideration: if the rate of change in number of moles of this component due to reaction is: dNi/dt, then the rate of reaction is defined as follows:
(Based on unit volume of reacting fluid):
1 dN moles i formed r i ...... (2.17) i V dt volume of fluid time
(Based on unit mass of solid in fluid-solid systems):
' 1 dNi moles i formed r i ...... (2.18) W dt mass of solid time
Equation 2.18 will be very useful in this work because pyrite roasting is a good example of fluid-solid system.
2.2.1 Reaction mechanism
Reaction mechanism is the sequence of individual chemical events whose overall result produces the observed reaction. Reaction mechanism can be understood during the study of chemical kinetics.The slowest step determines the chemical kinetics of the whole
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process and therefore it is called the rate-determing step. This contributes to the understanding of a reaction and overall kinetics of the reaction.
2.2.2 Rate Law
As explained in section 2.2.1; rate equation can be developed from rate-determining step.
However, initial rate method which utilise information from Table 2.4 and Equations 2.16,
2.17 and 2.18 is more practical method of determining rate equation. Rate law is the utilisation of initial value method. The rate law expresses the relationship between the rate of a reaction and the rate constant and the concentrations of the reactants raised to some powers. Consider Equation 2.19, rate law is written according to reaction of A and B to form a product.
푥 푦 푟 = 푘퐶퐴 퐶퐵 … … … … … … … … … … … … … … … … … … … … … … … … … … … … (2.19)
Where k is the rate law constant and x and y are order of reactions determined experimentally, and do not depend on stoichiometric coefficients from balanced chemical equation. The challenge is how to determine the rate parameters; the rate constant and order of reaction. There are two ways used to deduce the rate law:
i) Rate-determing step: the use of stoichiometric coefficient in slowest elementary
step as explained in section 2.2.1. The sequence of elementary reactions for pyrite
roasting is clearly known as shown by Equations 2.19 to 2.22 (Müller, 2000b).
2FeS+S (g) ...... (2.20) 2FeS2 2 ∆ H= 293 kJ/mol
2SO ...... (2.21) S2(g) + 2 O2 2 ∆ H= -723 kJ/mol
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2FeO+ 2 SO H= -948 kJ/mol ...... (2.22) 2FeS2 + 3 O2 2 ∆
Fe O ...... (2.23) 2FeO+ 0.5 O2 2 3 ∆ H= -282 kJ/mol
Although the elementary reactions behind pyrite roasting is already proposed, none
of the researches reported rate-determing step which could propose the key kinetic
parameters. Prasad (1985) and Müller (2000a) only reported time and temperature
for completing overall reaction (Equation 2.12).
Equation 2.12 also shows the reaction is highly exothermic although the roasting is
performed in high temperature between 500 and 1000° C . The elementary
reaction (Equation 2.20) shows the justification that during roasting some reactions
need application of heat. So this concludes that application of heat is crucial during
pyrite roasting although the overall reaction is highly exothermic;
ii) Experimental data: the measurement of intermediate in section 2.2.2 (i) is normally
complex and much tricked. The intermediates normally appeared in a very short
time and advanced equipment is needed to notice. However, actual data collection
can be a much better choice. There are usually two techniques of kinetic data
acquisition: concentration-time measurements in a batch reactor and concentration
measurements in a differential reactor.
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2.2.3 Analysis of rate data
After acquisition of kinetic data, the analysis involves two procedures namely; Integral
Method and Differential Method.
2.2.3.1 Integral Method
The Integral Method uses a trial-and-error procedure to find reaction order. The method is used most often when the reaction order is known and it is desired to evaluate the specific reaction rate constants at different temperatures to determine the activation energy.
Procedures for this method are:
1) Guess a particular form of rate equation;
2) Do appropriate integrations and mathematical manipulations;
3) Predict that: the plot of concentration function versus time should yield a straight
line;
4) Plot the data;
5) If a reasonably good straight line is obtained then the rate equation satisfactorily
fits the data.
Figure 2.3 shows trial and error method to determine order of the reaction. If the order assumed is correct, the appropriate plot of the concentration˗time data should be linear. The method is easy to use and is recommended for testing specific mechanisms or relatively simple rate expressions or when the data are so scattered that cannot being reliably in finding the derivatives needed in the differential method.
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Figure 2.3: Data analysis by integral method
2.2.3.2 Differential method
In the differential method, the reaction order and the rate constant are obtained by numerically differentiating the concentration versus time data. It is applicable where the rate is essentially a function of the concentration of only one reactant. Consider for example a reaction (Equation 2.24), with aid of rate law; rate equation is developed as according to Equation 2.25. By taking the logarithm of both sides, Equation 2.26 can be obtained. From Figure 2.4, the rate parameters α and k can be easily obtained from the plot.
A products………………………………………………………….……..……(2.24)
−푑퐶 −푟 = 퐴 = 푘 퐶훼 … … … … … … … … … … … … … … … … … … … . . … … … … … … … (2.25) 퐴 푑푡 퐴
dC A ln ln k ln CA...... (2.26) dt
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ln
slope=
ln CA
Figure 2.4: Plot for differential method
The challenge is how to get the values of dC/dt since the data collected are in the form of table 2.5. There are two common methods: Two Point Differention Method and Three Point Differention Method. Two Point Differention Method is useful for few data while the latter needs large amount of data for accuracy. Table 2.6 shows determination of dC/dt with aid of Two Point Differention Method. It is simple method and data are faster determined. Procedure for determination of dC/dt by Three Point Differention Method is clearly shown by Figure 2.5. The steps are much complex compared with the former method however it can be simplied by developing computer tools that will incorporate all crucial and complex equations.
Table 2.5: Concentration versus time data
Time (min) t0 t1 t2 t3 t4 t5 3 Con (mol/dm ) CA0 CA1 CA2 CA3 CA4 CA5
Differential Method is useful in more complicated situations and it can be used to develop or build up a rate equation to fit the data however it requires more accurate or larger amounts of data.
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Table 2.6: Two Point Method
ti Ci t C C/ t dC/dt
t1 C1 (dC/dt)1
t2-t1 C2-C1 (C/ t )2
t2 C2 (dC/dt)2
t3-t2 C3-C2 (C/ t )3
t3 C3 (dC/dt)3
t4-t3 C4-C3 (C/ t )4
t4 C4 (dC/dt)4
t5-t4 C5-C4 (C/ t )5
t5 C5
dC 3C 4C C Initial point : A A0 A1 A2 dt t0 2t dC C C Interior point s : A A2 A0 dt t1 2t dC C C A A3 A1 dt 2t t 2
dCA CA4 CA2 dt t3 2t
dCA CA5 CA3 dt t 4 2t
dC C 4C 3C Last point : A A3 A4 A5 dt t5 2t
Figure 2.5: Three Point Method
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2.2.4 Method of excess
In a situation with more than one reactant, partial analysis of the rate equation is needed.
Method of excess is useful to fullfill the required task. Consider the irreversible reaction,
Equation 2.27 with a rate equation represented by Equation 2.28. According to method of excess, there are two types of experiments, Excess B and Excess A.
A + B products …………………………………………………….…………….(2.27)
-rA = kCA CB …………………………………………………………………..….(2.28)
For Excess B experiments: CB remains unchanged during the reaction (CBo) and rate equation is in the form of Equation 2.29.
-rA = kCA CB = kCBo CA = k'CA ………………………………………..….(2.29)
In similar scenario for Excess A experiments: CA remains unchanged during the reaction and corresponding rate equation is depicted by Equation 2.30.
-rA = k''CB …………………………………………………..…….(2.30)
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Once and are determined, k can be calculated from the measurement of -rA at known concentration of A and B as shown by Equation 2.31.
3 --1 k= -rA /CA CB [dm /mol] /s………………………………….(2.31)
When method of excess is extended to pyrite roasting (Equation 2.12), oxygen is obvious in excess so it takes the shape of Equation 2.29 where A and B are FeS2 and O2 respectively and -rA is the rate of disappearance of A. The objective is to determine the order of reaction, α and rate constant, k. Since B is in excess, and then β is equal to zero order. The order obtained will be pseudo with respect to [A] which is in the units of weight (W) due to the use of definition of rate law based on unit mass of solid in fluid- solid systems (Equation 2.18). This enables to develop the equation under investigation as
Equation 2.32. The chemical kinetic parameter to be determined are k and .
-rA = k W …………………………………………………..……………………..(2.32)
2.2.5 Variables affecting the rate of reaction
Many variables affect rate of reaction such as concentration, pressure, temperature, particle size, catalyst, light etc. In homogenous systems, the temperature, pressure, composition are obvious variables as shown in Equation 2.33. In heterogeneous systems more than one phase is involved, hence the problem becomes more complex. Equation
2.34 shows additional variables in heterogeneous systems. Material may have to move from one phase to another, a good example is in the burning of charcoal, the diffusion of oxygen through the gas film surrounding the particle, and through the ash layer at the
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surface of the particle can play an important role in limiting the rate of reaction. In addition, the rate of heat transfer may also become a factor.
ri = f(temperature, pressure, composition)…………………………..………(2.33)
ri = f(rate of mass transfer, rate of heat transfer)……………………………...(2.34)
2.2.5.1 Concentration-dependent term of rate equation
Rate of reaction can not be directly measured; instead the measurement of concentration of a reactant or volume of gaseous product is normally done. There is a direct correlation between rate of reaction and concentration which shows whether the rate is zero order, first order, second order etc.
In elementary reactions, the the rate equation corresponds to a stoichiometric equation.
Consider a reaction (Equation 2.35) to elementary reaction, the corresponding reaction rate with respect to A will be represented by Equation 2.36.
aA + bB rR …………………………………………………………….………..………….(2.35)
푎 푏 −푟퐴 = 푘퐶퐴 퐶퐵 … … … … … … … … … … … … … … … … … … … … … … … … … … … (2.36)
The number of collisions of molecules A with B is proportional to the rate of reaction. But at a given temperature, the number of collisions is proportional to the concentrations of reactants in the mixture (CA, CB). Values a and b are empirically given rate expression called order of reaction. Equation 2.36 has ath order with respect to A and bth order with respect to B. For elementary reaction, a and b related to the stoichiometric coefficients, which represent the order of the reaction. However, in most of practical situation, the
40
effect of changing concentration is not related to stoichiometric coefficients and order of reaction needs not to be an integer. Such kind of reactions is called non˗elementary reactions.
2.2.5.2 Temperature-dependent term of rate equation
For many reactions and particularly elementary reactions, the rate expression can be written as a product of a temperature-dependent term and a composition-dependent term
(or concentration-dependent term) as shown in Equation 2.37.
r f (temperature). f (composition) k. f (composition)...... (2.37) i 1 2 2
For such reactions the temperature˗dependent term, the reaction rate constant, k has been explained by various theories. Such theories include:
i) Temperature-dependency from Arrhenius Law:
Comparison of theories (Collision Theory and Transition-state Theory), shows that
Arrhenius Law (Equation 2.38) is a good approximation to the temperature-dependency of both theories and is therefore in most cases the temperature-dependent term, the reaction rate constant, k can be well represented by Arrhenius Law. (Levenspiel, 1999; Fogler,
1986 ).
−퐸푎 푘 = 푘표푒 푅푇 … … … … … … … … … … … … … … … … … … … … … … … … … … … (2.38)
Where: ko = Frequency factor (same units as rate constant)
Ea = Activation Energy of the reaction (kJ/mol)
R = Gas constant (kJ/mol.K)
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T = Temperature (K)
NOTE: Units of ko and k depend on overall reaction order.
When the mechanism of reaction is known and whether it is elementary or not, then the prediction of the frequency factor and activation energy terms of the rate constant can be done.
ii) Temperature-dependency from Thermodynamics:
The importance of thermodynamic parameters such as Gibb’s free energy and enthalpy of the reaction for determination of feasibility of the chemical reaction and energy requirement of a process (either endothermic or exothermic reaction) have been well explained in section 1.1. In general there are two classes of thermodynamic properties namely: extensive and intensive property. Extensive property is one which depends on the mass of the system such as volume, internal energy and entropy. While intensive property is independent on the total mass of the system such as temperature and pressure. This kind of variable can always be calculated in terms of other intensive variables, for instance, the pressure of a gas or liquid can always be expressed as a function of its temperature and specific volume (Balmer, 2011; Çengel and Boles, 2008; Moran, 1999).
Kichhffo’s law of thermodynamics (Equation 2.39) shows the effect of temperature which is an intensive variable on heat of reaction. The same reaction, when carried at different temperatures, the enthalpies of reaction are also different.
푇1 ∆퐻 = ∫ 퐶푝푑푇 … … … … … … … … … … … … … … … … … … … … … … … … … … (2.39) 푇2
Where: Cp = Heat capacity at constant pressure.
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When Cp is greater than zero, the enthalpy will increase with increasing temperature, where as if it is less than zero, the enthalpy will decrease with increasing temperature. The weakness of this postulate is that, Cp itself might be temperature dependent (not necessarily a constant) in which case without knowing the functional dependence of Cp on
T, it might be difficult to predict beforehand how ∆H will change with T. Another limitation of (Equation 2.39) is that it can only be applied to small temperature changes
(less than 100 K), because over a larger temperature change, the heat capacity is not constant. Variation of Cp versus T is shown by Equation 2.40. This Equation can be incorporated into the integral in Equation 2.39 and solved . The empirical values for temperature dependence of Cp such as a1, b1 and c1 for some of elements and compounds can be obtained from literatures such as Perry’s Chemical Engineering handbook and other physical chemistry handbooks (Fleming, 2019a; Climent, et al., 2006; Mathot,
1984).
c 퐶 = 푎 + b T + 1 … … … … … … … … … … … … … … … … … … … … … … … … (2.40) 푝 1 1 T2
Another important postulate of temperature-dependency from thermodynamics was explained by Van’t Hoff law ( Equation 2.42), which applied only on elementary reversible reactions such as Equation 2.41 (( Horn, et al., 2001;Levenspiel, 1999).
k1 A R, H r ……………………………………………………………….(2.41) k2
d(ln K) H r ……….………………………………………………….....(2.42) dT RT 2
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Because K = Keq = k1/k2 for this reaction, we can then rewrite the Van’t Hoff relationship as:
d(ln k ) d(ln k ) H 1 2 r dT dT RT 2 ………………………………………………..(2.43)
2 The fact that the difference in derivative is equal to ∆Hr/RT suggests that each derivative alone is equal to a term of that form, or:
d(ln k ) E d(ln k ) E 1 1 and 2 2 …………………………….(2.44) dT RT 2 dT RT 2
Where:
E1 E2 Hr ………………………………..………………………….(2.45)
If the energy terms are assumed to be temperature-independent, Equation 2.44 above can be integrated to give Arrhenius equation (Levenspiel, 1999). In addition the plots of ln
Keq versus 1/T developed from Van’t Hoff (Figure 2.6) are similar to those from
Arrhenius equation.
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Figure 2.6a: Endothermic Reaction Van’t Hoff (Source: Fleming, 2019b)
Figure 2.6b: Exothermic Reaction Van’t Hoff (Source: Fleming, 2019b)
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iii) Temperature-dependency from Collision Theory:
Atoms, molecules or ions must collide before they react. Atoms must be close together in proper orientation in order to form chemical bonds. The postulate of collision theory is a powerful theory which explains the effects of physical state, temperature and concentration on reaction rates (Chang, 2005). The collision theory is based on the following:
1. The rate of reaction is proportional to the rate of reactant’s collisions.
2. The reacting species must collide in a proper orientation that allows contact
between the atoms that will become bonded together to form the product.
3. The collision must occur with adequate energy to permit mutual penetration of the
reacting species’ valence shells so that the electrons can arrange and form new
bonds.
According to the collision theory, there are two ways to increase reaction rate: increase the frequency of collisions, and increase the fraction of collisions that are effective. If you speed up the particles by increasing temperature, there is a greater likelihood that they will bump into one another, and the reaction rate goes up. The total energy within the particles is also increased; so when they do collide, they do so with more energy, increasing the fraction of effective collisions.
In addition, collision must take place in proper orientation in order for the reaction to proceed. However every reaction requires a certain amount of energy for it to proceed in the forward direction, yielding an appropriate activated complex along the way. Without adequate energy, proper orientation can fail to form the reaction product.
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Both postulates of the collision theory of reaction rates are accommodated in the
Arrhenius equation. The frequency factor, ko is related to the rate at which collisions having the correct orientation occur. The exponential term, e-Ea/RT is related to the fraction of collisions providing adequate energy to overcome the activation barrier of the reaction.
At one extreme, the system does not contain enough energy for collisions to overcome the activation barrier. In such cases, no reaction occurs. At the other extreme, the system has so much energy that every collision with the correct orientation can overcome the activation barrier, causing the reaction to proceed. In such cases, the reaction is nearly instantaneous (Atkins and De Paula, 2011).
iv) Temperature-dependency from Transition˗State Theory:
Transition˗State Theory (TST) describes the rate of elementary reactions on molecular scale. The theory was formulated to explain bimolecular reactions based on the relationship between kinetics and thermodynamics (Patra and Samantray, 2010; Gupta,
2007; McNaught and Wilkson, 1997).
TST states that: Atoms and molecules can collide and combine to form an unstable, high energy complex. When the molecules fall out of this high energy state, they may do so as new and different molecules, or in their original states. The energy required to reach the activated state must be available if the molecules are to change into something new
(Dambrowitz and Kuznicki, 2010).
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TST is considered as paradigm for interpretation of the rates of chemical processes as well as their dependence on temperature, medium, structure, and other parameters (Bartsch, et al., 2008; Arnaut et al., 2006). The basic assumption of TST is the existence of activated state (activated complex), which is formed via the activation of reactants, i.e. the collision between reactant molecules does not form the product of reaction directly. It can be summarised that chemical reaction (or any other activated process proceeds as two-stage process:
1. Formation of activated complex;
2. Decomposition of activated complex into the products of reaction.
The mathematical explanations on temperature-dependency from Collision Theory are as follows:
Consider a forward elementary reaction of a reversible reaction (Equation 2.46)
k1 A B AB, H r ……………………………………………………….(2.46) k2
So for the forward reaction, and similarly for the reverse reaction of Equation 2.46, k can be approximately as:
* H1 / RT k1 T e
* H2 / RT k2 T e ……………………………………………………………..(2.47)
Where:
* * H1 H2 Hr ……………………………………………..…………..(2.48)
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However, for liquids and solids (Levenspiel, 1999):
E H * RT and for gases : …………………………………………..(2.49) E H * (molecularity 1)RT
* With this definition, the difference between E and ∆H is small and of the order RT; hence
TST predicts approximately that:
k T eE / RT …………………………………………………………………(2.50)
Equation 2.50 shows relationship between kinetics and thermodynamics bimolecular reactions based on TST. TST is mostly applicable for bimolecular reactions and it also fails for some reactions at high temperatures due to the more complex motions of molecules or at very low temperatures due to the quantum tunnelling (Van Santen, 2013;
Sholl and Steckel, 2011). The most important input for TST is the explanation of a better understanding of how even very complicated reactions take place. Although Arrhenius
Law (Equation 2.38) is a good approximation to the temperature-dependency, TST provides detailed consideration and significance of frequency factor, ko and activation energy, Ea (Laidler and King, 1983).
2.2.5.3 Activation energy and temperature dependency
Activation energy is the miminum energy required to be attained inorder for a reaction to occur. For the reaction to occur, the reactants must overcome an energy barrier or activation energy, Ea. The energy to overcome their barrier comes from the transfer of the
49
kinetic energy from molecular collisions to internal energy (e.g. Vibrational Energy).The temperature-dependency of reactions is determined by the activation energy and temperature level from Arrhenius Law. The modification of Equation 2.38 provides
Equation 2.51. A plot of lnk vs 1/T (Figure 2.7) gives a straight line of slope = -Ea/R and intercept lnko, with large slope for large E and small slope with a small Ea.
퐸푎 푙푛푘 = 푙푛푘 − … … … … … … … … … … … … … … … … … … … … … … … … (2.51) 표 푅푇
There are several comments associated with Figure 2.7:
i) Reactions with high activation energies are very temperature-sensitive; reactions
with low activation energies are relatively temperature-insensitive;
ii) A given reaction is much more temperature-sensitive at low temperature than at
high temperature and
iii) From the Arrhenius Law the frequency factor ko does not affect the temperature-
sensitivity of a reaction.
High Ea ln k Low Ea
Ea Slope , K R
1
T
Figure 2.7: Temperature-dependency of the reaction rate
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Theoretical predictions rarely match experiment by a factor of two. Therefore for engineering design this kind of information should not be relied on, and experimentally found rates should be used in all cases. Thus, theoretical studies may be used as a supplementary aid to suggest the temperature-sensitivity of a given reaction from a similar type of reaction and the upper limits of reaction rate and design invariably relies on experimentally determined rates.
2.2.6 Experimental methods for studying kinetics of reactions
There are several methods for studying kinetic of reactions which are based on chemical analysis or studying the change of physical property. The nature of the reactants or products will decide the appropriate method to be used.
2.2.6.1 Chemical methods
Chemical methods involve collecting samples from the reaction mixture at intervals, without interrupting the reaction and analysing them. The reactions inside the samples are firstly stopped. This may be tedious but sometimes the only possible way. Stopping a reaction must be rapid so that the time interval measured is accurate. There are several ways to achieve this:
i) Quenching the reaction mixture:i.e rapid cooling to decrease the rate to neglible
levels and
ii) Decreasing the concentration rapidly by dilution or by precipitating a certain
component.
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2.2.6.2 Physical methods
Physical methods are more preffered because are cheaper and most of the time involve only a meter reading. Any physical property which varies during the course of reaction may serve in monitoring its progress such as:
i) When the gas is formed during the reaction, the rate may be followed by
measuring its volume as a function of time;
ii) When one of the products is easily condensed;
iii) When one of the gaseous components is highly soluble e.g SO2 or H2S then the
gases can be passed in an absorbing solution whose concentration can be
continuously monitored;
iv) When one of the reactants is a gas, the rate of its reaction may be followed by
carrying out the reaction in a closed vessel and noting the change in pressure as the
reaction proceeds;
v) The use of radioactive isotopes to study the rates of reactions;
vi) Optical microscopy is usually used to examine samples before and after reaction;
vii) Scanning electron microscope not only shows the shape and size of reacting
particles but also chemical composition of certain areas and
viii) Solid-gas reactions are best studied by noting the change in weight as a function of
time and temperature.
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2.3 Extractive Metallurgy of Pyrite Roasting
Extractive metallurgy is a branch of metallurgy which deals with the extraction of metals from naturally occurring and man-made resources. It is further divided into three classes
(Habashi, 2014) as:
i) Electrometallurgy: the process of obtaining metals through electrolysis. Starting
materials is either molten salt or aqueous solution. The driving force is based on
difference in standard electrode potential. The method is used for electro winning
or electro refining purpose. Aluminium extraction is a good example of this
method which is based on the fuse salt electrolysis. Copper and zinc are other
metals produced by this method;
ii) Hydrometallurgy: the use of aqueous chemistry for the recovery of metals from
ores, concentrates, and recycled or residual materials. The method is superior in
treatment of complex ores and low-grade ores. Cyanide leaching is a good
example of this method in extraction of gold. Other typical ores treated by this
method are uranium and aluminium and
iii) Pyrometallurgy: the oldest sector of extractive metallurgy which involves the
thermal treatment of dry minerals and metallurgical ores and concentrates to bring
about physical and chemical transformations in the materials to enable recovery of
valuable components in the liquid state. Typical ores treated by this method are
those of iron, copper and lead.
2.3.1 Pyrometallurgical processes
There are several pyrometallurgical unit processes such as drying and calcinations, roasting, smelting, converting, refining etc. Each of these unit processes serves a specific purpose from the point of view of separation. They require specialised reactor depending upon the phases (solid/liquid/ gases) involved, mode of contact, temperature,
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environmental measures etc. Calcination and roasting are used as pre-treatment prior to other pyro- and hydro- metallurgical operations. Some common pyrometallurgical processes can be explained as follows:
i) Drying and calcination: thermal treatment of an ore to eliminate water or gases in a
substance (hydrated water or gases). The endothermic calcination of limestone is a
good example (Equation 2.52). Calcination is used as pre-treatment prior to other
pyro- and hydro- metallurgical operations;
...... (2.52) CaCO3(s) CaO (aq) + CO2(g)
ii) Smelting: the process of recovering the metallic elements from an oxide or sulfide
by heating with fusion in the presence of a reducing agent as e.g. carbon. During
the smelting, metal compound (e.g. oxide of metal) is reduced to metallic form,
and the undesirable impurities (gangue) combine with flux to form slag. Reduction
smelting is carried out for oxides. Immiscibility of metal and slag together with
density difference forms the basis for separation. Smelting products are top layer
slag composed of silicates with specific gravity of 3.6. Matte is the next layer
molten sulphides of non-ferrous metals of specific gravity of 5.2; speiss is the third
layer molten arsenides and antimodes with specific gravity of 6; and bullion is the
bottom layer with specific gravity greater than 6 and composed of metal.
Reduction of metal oxide is done by smelting process and blast furnace for iron
making is good example of this and
iii) Roasting: Means heating of material to a point somewhat short of fusing with
access of air to expel volatile matter or effect oxidation (Maycock et al., 2007).
Roasting process has been the traditional method of pretreating refractory gold
ores or concentrates and is one of the most cost-effective processing conditions in
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which any organic carbon present in the ore is destroyed (Runkel and Sturm, 2009;
Marsden and House (2006); La Brooy, et al., 1994; Komnitsas and Pooley, 1989).
2.3.2 Objectives of roasting
The following are the reasons why roasting is very important:
i) Converting the MeS to MeO which is easy to be reduced by carbon which is
abundant. To separate MeS is difficult because Gibb’s free energy for sulphides is
close together; and the use of carbon is ruled out because of formation of CS or
CS2 is much less stable than most sulphides. Not only that, during roasting sulfur
dioxide is being released and can be traped and used for production of sulphuric
acid;
ii) Obtaining the metal soluble substance that can be extracted by hydrometallurgical
method-the solubility of sulfide minerals is very low compared to oxide or sulfates
minerals, thus in order to treat sulfide mineral by leaching, the ore must first be
roasted to increase solubility and
Removing volatile matter- volatile materials made of carbon are easily removed
from the material by just roasting where by most of carbon compounds are
removed as carbon dioxide gas.
Although there are many objectives of roasting but in this study the major objective of the roasting process is to convert the sulphur contained in the ore into calcine product and SO2 containing gases. Practically, during roasting various products appeared which can be illustrated by the equilibrium relation in a system which contains Me-S-O (section 2.3.3).
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2.3.3 Thermodynamics of roasting and Kellog Diagram
According to Gibb’s phase rule, roasting may produce five phases: four-condensed Me,
MeO, MeSO4, MeS and gas. With the aid of Kellog Diagram, a type of chart that can be used to show conditions (pressure, temperature, volume, etc.) at which thermodynamically distinct phases occur and coexist at equilibrium. A Kellog Diagram can tell about equilibrium phases as a function of P, T and composition. There are several rules for developing Me-S-O system (Rosenqvist, 2004):
i) For a given temperature, the composition of gas mixture is defined by the partial
pressure of any of two roaster gases (SO2 or SO3);
ii) For a fixed gas composition, the composition of condensed phases is fixed;
iii) MeS + O2 form MeO + SO2 (pure roasting);
iv) MeS +2 O2 = MeSO4 and
v) MeSO4 =MeO + SO2 + O2
The stable phases are shown in a plot of log PSO2versus log PO2. As shown in Figure 1.4,
for pure metal, if PO2is increased eventually the oxide phase will become stable, PSO2is of no effect, until it is high enough for the sulfide to be stable.
According to Cheng et al. (2013); Runkel and Sturm (2009) and Tveit (2013) the roasting of pyrite is represented by reactions shown by Equations 2.12; 2.53 and 2.54. The predominant of the reactions depend on partial pressure of oxygen and the temperature range. Other potential reactions to occur are (Equation 2.55˗2. 57).
Fe O + 6 SO ...... (2.53) 3FeS2+ 8 O2 3 4 2 ∆ H= -2381 kJ/mol
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FeSO + SO ...... (2.54) FeS2+ 3 O2 4 2 ∆ H= -1054 kJ/mol
FeS+ S ...... (2.55) FeS2
Fe O + 3SO 3FeS+ 5 O2 3 4 2 ...... (2.56)
Fe O + 6S 3FeS2+ 2 O2 3 4 ...... (2.57)
Since the three chemical equations (Equations 2.13; 2.53 and 2.54) are exothermic reactions, a considerable amount of excess heat is developed during the roasting process in the reactor system and is normally recovered in the waste heat boiler system. The oxygen necessary for the roasting reactions is supplied to the reactor system as atmospheric air. The products gained from the roasting process are calcine cinder and
roasting gas. The calcine cinder consists of Fe2O3, Fe3O4, FeSO4 and gangue material,
while the roasting gas has SO2, SO3, O2, N2 and H2O as steam. The design of a roasting plant depends on the type of concentrate or ore. Some of the main process relevant factors are the sulfur content and the particle grain size. Additional important process factors are the reaction temperature and the concentrate throughput (Runkel and Sturm, 2009).
2.3.4 Factors affecting pyrite roasting
Pyrite roasting is a good example of non-catalysed heterogenous reaction. Most of heterogeneous reactions are affected by the following factors:
i) Concentration of reactants: reaction rates usually increase with increasing the reactant
concentration. However, increased reagent concentration may be economical or may
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lead to secondary undesirable reactions. A good example is in leaching plant, where it
is important to know minimum concentration of leaching agent before applying to
avoid wasting of excess reagents. If one of the reactants is a gas, like in pyrite
roasting, the effect of pressure should be known so that a decision could be made
regarding the use of pressure vessels;
ii) Size of the pyrite particles: small size influences roasting kinetics, but very fine
grinding leads to the generation of dusts in the industry and also causes to the use of
excessive energy. A very fine grinding is necessary for slow reactions; iii) Temperature: the rate of most of reactions increase with increasing temperature.
However, some reactions involving solids are more appreciablly affected by
temperature than others. Temperature range better for roasting is between 500 up to
1000° C; iv) Impurity present in the ore: for example in pyrite other elements like silver,
aluminum, silicon can have negative effects in roasting;
v) Air flowrates: atmospheric oxygen should be available for the reaction to take place;
it should exceed or be equal to the theoretical amount required; vi) Porosity: this means the ability of the air to diffuse in the sample through pores, more
porous sample will cause faster diffusion of gases and hence faster reaction. Porosity
will influence the rate of reaction between solids and gases. A solid reacts faster
when it is porous because of the larger surface of contact. However, if the pores have
extremely small diameter, they may hinder large molecules to pass through.
Therefore, the mechanism of movement of the gas molecules in and out of the pores
should be known; vii) Phase mixing: stirring of solids in a liquid or changing gas flow rate in solid-gas
reaction is an important variable, since in some cases it greatly increases the rate
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while in others it plays practically no role. This information is therefore necessary for
reactor design; viii) Interface: this occurs in heterogenous reactions which involve more than one phase.
Section 2.3.5 further explains the effect of interface and rate equations involved;
ix) Catalysis: both homogenous and heterogenous reactions may be catalysed or non-
catalysed. A good example of a homogenous catalysed gas reaction is the oxidation
of SO2 by oxygen in the presence of NO2 as a catalyst which is the bases of Lead
Chamber Process for making sulphuric acid. In catalysed heterogenous reactions, the
surface of a solid, e.g. Fe, Ni, Pt, etc acts as a catalyst. That is why reactions of this
type are usually called contact catalysis. These reactions may take place in the gas
phase, e.g. SO3 formation from SO2 over vanadium catalysis.
Metallurgical reactions are heterogenous non˗catalysed reactions. Only two phases
are actually taking part in any heterogenous process.
2.3.5 Solid interface
Solids play a prominent role in metallurgy, their nature particularly area and geometry of interface will affect the rate of reaction.
2.3.5.1 Area of interface
For heterogenous reactions, the reacting molecules are transferred from one phase to the other; the rate of transfer will depend on the surface area of the interface. It is therefore obvious that in reactions involving solids, fine particles react faster than course because of their large surface area.
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2.3.5.2 Geometry of interface
The shape of a solid undergoing reaction with a liquid or a gas plays an importaornt role in determining the rate of the process. If it is in the form of a plate or disc the surface area will be constant through out the reaction and therefore the rate will be constant. If, however, the solid is in the form of a sphere or pellet, the surface area will be changing continuously as the reaction proceeds and therefore the rate will be changing.
Consider a solid-liquid reaction e.g. metal dissolving in an acid. Assuming that the concentration of acid is kept constant, then rate equation is expressed by Equation 2.58.
When the geometry of the solid is varied, different kinetic equations are obtained.
Consider two scenarios flat surface and sphere as follows:
푑푤 푟 = − = 푘퐴퐶 … … … … … … … … … … … … … … … … … … … … … … … (2.58) 푑푡
Where: w = weight of solid at time t
A=surface area
C=concentration
k=velocity constant
And the (-) indicates a decrease in weight during dissolution. The surface area varies with shape such that:
i) For flat surface, the surface area (A) will be constant during dissolution then
modification of Equation 2.57 forms Equation 2.60 on intergration. The plot of wo-
w against t should give a straight line of slope, kAC from which k can be
calculated.
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w t − ∫ dw = kAC ∫ dt … … … … … … … … … … … … … … … … … … … … … … (2.58) wo 0
푤표 − 푤 = 푘퐴퐶푡 … … … … … … … … … … … … … … … … … … … … … … … … … … (2.60)
ii) For sphere, the surface area (A) will decrease with time. Equation 2.57 applied in
this case however A and w are expressed as function of sphere radius (R).
2.3.6 Formation of solid reaction products
There are numerous metallurgical processes in which a product layer is formed on the reacting solid e.g. solid-gas oxidation of sulphide to form MeO and SO2. The kinetics of these reactions will be governed by the character of this coating whether it is porous or non-porous (Brem and Brouwers, 1990).
Incase the reaction product is a porous layer, there will be no resistance to the reagents reaching the interface and the rate will not be affected. However, for non porous case, the reagent has to diffuse through protective film before it reaches the interface. The kinetics of such reaction will therefore differ markedly.
Porosity (Z) of a solid reaction product is determined by the ratio of molecular volume of the product to that of the reactant as expressed by Equation 2.61.
1 M ⁄ρ 푍 = ( p p) … … … … … … … … … … … … … … . . … … … … … … … … … … … … (2.61) γ MR⁄ρR
Where: Mp=molecular weight of solid product
MR=molecular weight of solid reactant
ρp=density of solid product
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ρR=density of solid reactant
γ=stochiometric factor
If Z is less than 1, the product will occupy less volume than the reactant and as a result, pores will be created within the solid. Otherwise, Z greater than one, the product will occupy more volume than the reactant solid that is being replaced and as a result a dense non-porous layer will be formed.
For oxidation of pyrite expressed by Equation 2.62, Z = 0.6 as calculated by Equation
2.63. Because of low value of Z, it would be expected that the rate of the reaction is unaffected by oxide film on the surface and this was verified experimentally (Habashi,
2014).
2Fe O + 8SO ...... (2.62) 4FeS2+ 11 O2 2 3 2
2 Z = (160⁄5.2) = 0.6 … … … … … … … … … … … . . … … … … … … … … … … … … (2.63) 4 120⁄5
2.3.7 Homogenous and heterogenous reaction
Homogenous reactions are of limited importance to metallurgy. Reaction rate is normally influenced by concentration and temperature. In most cases, the rate is described by first and second order reactions. Higher orders are rare, but fractional orders are common. As explained in section 2.2.2, the order of reaction does not necessarily coincide with either its molecularity or correspond to the stochiometric equation.
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However, heterogeneous reactions play an important role in chemical industry, metallurgy, geology and in engineering (Amrei et al., 2014). Non-catalytic fluid-solid reactions (good examples of heterogeneous systems) have been mentioned in many chemical and metallurgical processes such as metal oxides reduction (Ebrahim and
Jamshidi, 2001; Berg and Olsen, 2000; Szekely et al., 2001), roasting of metallic sulphides (Kimura, 1989; Prasad and Pandey, 1989), adsorption of acid gases (Efthimiadis and Sotirchos, 1993), coal gasification (Delikouras and Perlmutter, 1993), activated carbon production (Lozano-Castello et al.,2002; Lu and Do (1992) and catalyst regeneration (Bai et al.,1998; Ramachandran(1975).
In treating heterogeneous reactions, more additional factors that affect chemical kinetics are considered, which is mass transfer and contacting patterns (Silva et al., 2004). This work is not intended to design a chemical reactor, and hence reactor performance equation will not be considered. However it is important to discuss the behavoir of heterogeneous non catalysed reactions such as fluid˗particle reactions.
Fluid-particle reactions, particularly solid-gas reactions are of great importance in chemistry and metallurgy. A fluid contacts a solid, reacts with it and transforms it into product in three ways.
A fluid bBsolid fluid (1) solid (2)
fluid and solid (3)
For non-catalytic reactions of particles with surrounding fluid, two simple idealised models or behavior are considered:
i) The Progressive˗Conversion Model (PCM) and
ii) The Shrinking˗Core Model (SCM).
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(a)
(b)
Figure 2.8: Behavior of reacting solid particles (Source: Levenspiel, 1999).
In PCM the solid particle does not change in size. It is converted continuously and
progressively throughout the particle as shown in Figure 2.8(a). According to Levenspiel
(1999), the practical examples of this kind of model are:
i) Roasting of sulphide ores to yield the metal oxides-conversion of pyrite and
zinc blend to their oxides as depicted by Equations 2.62 and 2.64.
2ZnO+ 2SO ...... (2.64) 2ZnS+ 3O2 2
ii) Reduction of metal oxides into its pure form-it is used in the production of iron
as depicted in Equation 2.65.
Fe3O4 + 4H2 3Fe+4H2O...... (2.65)
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iii) Nitrogenation of calcium carbide to produce cynamide-this is shown by
Equation 2.66.
2CaCN + C(amorphous) ...... (2.66) 2CaC2+ N2 2
iv) Plating of metals
The SCM is very common in our daily life where size of solids is changing during reactions (Figure 2.8 (b)). Good examples are carbonaceous materials (Levenspiel, 1999) such as coal briquettes and wood. Equations 2.67 and 2.68 show some few examples however large numbers of industrial applications, favour this kind of model. Other applications are rusting of iron, dissolution reactions, manufacture of sodium cyanide from amide, manufacture of carbon disulphide etc (Levenspiel, 1999).
CO ...... (2.67) C+ O2 2
2CO+ H ...... (2.68) C+ H2O(g) 2
In addition, Marsden and House (2006) illustrated the mechanism of pyrite roasting as shown in Figure 2.9 and suggested that the phenomenon can be explained by SCM.
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Figure 2.9: Mechanism of Pyrite Roasting (Source: Marsden and House, 2006).
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2.4 Techno-economical viability and environmental feasibility of the
process
Before initiating the development of the process, at various stages in its development, and before attempting the design of a process and plant, process engineers must take economic evaluations. The evaluations determine whether they should undertake a project, abandon it, continue with it (but with further research), or to take it to the pilot plant stage. There are several steps that are normally involved during economic evaluations which are:
i) Preparing a Process Flow Diagram (PFD);
ii) Calculating mass and energy flows;
iii) Sizing major equipment;
iv) Estimating the production cost and
v) Estimating the return on investment.
The major objective is to determine the production cost of a chemical based on engineering point of view. Estimating the product-sales price is beyond the scope of this study. Process engineers are required to make quick, rough, cost estimates to decide between alternative designs and for project evaluation (Towler and Sinnott, 2012).
2.4.1 Project costing
The investor or management of an organisation needs to know the estimates of the production cost and the capital for a proposed process. The total investment cost is divided into the following cost analysis:
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i) Fixed capital cost is total cost of the plant ready for start-up. It is mainly paid for
equipment constructions and extra costs such as installation, piping, buildings,
auxiliary facilities and contingencies. It is a once-only cost: Not recovered at
the end of the project life, other than the scrap value; ii) Working capital cost is the additional investment needed, over above the fixed
capital, to start the plant up and operate it to the point when income is earned.
It can vary from 5% - 30% of the fixed capital. Most chemical plants use an
initial working capital in the range of 10% - 20% of the total capital
investment. It is better estimated from the production costs rather than capital
investment. Most of the working capital is recovered at the end of the plant
life; iii) Variable costs of production are costs that are proportional to the plant output or
operation rate. They can usually be reduced by more efficient design or
operation of the plant. It Includes raw materials, utilities, consumables
(solvents, acids, bases, inert materials, corrosion inhibitors, additives, catalysts
and adsorbents that require continuous or frequent replacement), packaging
and shipping (drums, bags, tankers, freight charges) and iv) Fixed costs of production are costs that are incurred regardless of the plant
operation rate or output. It includes the following :
1. Operating labour;
2. Supervision : ~ 25% of operating labour;
3. Direct salary overhead : usually 40 - 60% of operating labour + supervision;
4. Maintenance;
5. Property taxes and insurance;
6. Rent of land (and/or buildings);
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7. General plant overheads - charges to cover corporate overhead functions such
as:
a) human resources;
b) research and development;
c) information technology;
d) finance, etc.
8. Corporate overhead varies widely depending on the industry sector;
9. Plant overhead is typically taken as 65% of total labour (including
supervision and direct overhead) + maintenance;
10. Allocated environmental charges to cover payments;
11. Licence fees and royalty payments;
12. Capital charges - these include interest payments due on any debt or loans
used to finance the project and
13. Sales and marketing costs in some cases these are considered part of general
plant overhead.
Fixed costs should never be neglected, even in the earliest stages of design, as they can have a significant impact on project economics .Very few big chemical plants carry less than 1 million USD of fixed costs. Apart of that, fixed costs are not easily influenced by better design or operation of the plant.
2.4.2 Economic indicators
The purpose of investing money in chemical plant is to earn money. However, some means of comparing the economic performance of projects is needed. For small projects and simple choices between alternative processing schemes and equipments, the decisions can usually be made by comparing the capital and operating cost. For more sophisticated evaluation techniques and economic criteria are needed when decisions have to be made
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between large, complex projects, particularly when the projects differ widely in scope, time scale and type of product (Sinnot, 1993).
Some commonly used techniques of economic evaluation and criteria used to judge economic performance are Net Present Value (NPV), Internal Rate of Return (IRR), Pay-
Back Period (PBP) and Cash Flow Analysis (Gitman and Zutter, 2012).
NPV (Equation 2.69) is a sophisticated capital budgeting technique; found by subtracting a project’s initial investment from the present value of its cash inflows discounted at a rate equal to the firm’s cost of capital (Vlysidis et al., 2011; Singhabhandhu and Tezuka,
2010).
1 in 1 NPV CAPI P ...... (2.69) i1 in
Where: CAPI = the capital costs;
i = the interest rate;
n =the lifetime of the plant (plant life time) and
P = the annual net profit
In decision making, financial theory suggests if:
i) NPV is greater than zero (NPV>0) i.e. NPV is positive: the investment would add
value to the firm and therefore the project is economically viable and may be
accepted and implemented and
ii) NPV is less than zero (NPV<0) i.e. NPV is negative: the investment would
subtract value from the firm and therefore the project is not economically viable
and should be rejected.
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The positive side of NPV is simple to assess the project and it takes into account the time value of money while its shortfall, it is difficult to calculate.
IRR is the most sophisticated of the above metrics and often is used to analyze large, multi-year investments. IRR equals the percentage rate by which one have to discount the net benefits for the time period until the point that they equal the initial costs. IRR is related closely to net present value. The discount rate one would need to apply to the benefits to obtain a net present value of zero is the rate of return calculated by IRR. The pros and cons are:
i) It takes into account the time value of money;
ii) It is particularly good for measuring uneven annualized returns;
iii) It is complex to calculate and
iv) It offers no magnitude for a project;
Decision criteria:
i) If the IRR is greater than the cost of capital, accept the project and
ii) If the IRR is less than the cost of capital, reject the project.
These criteria guarantee that the firm will earn at least its required return. Such an outcome should increase the market value of the firm and, therefore, the wealth of its owners.
The most accepted and recommended methods for profitability evaluation and the economic comparison of alternatives are NPV and IRR (Chatfield and Dalbor, 2005).
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The PBP for a project is the length of time it takes to get the initial investment back. It is the time from the initial cash outflow to the time when the project’s cash inflows add up to the initial cash outflow. Payback period is important when time and cash flow are an issue. Simple payback period shows how long it will take for an investment to show a profit. It is the time it takes for the project to recoup the funds expended and normally is expressed in years or months. Shorter paybacks are better than longer paybacks. The pros and cons are:
i) The payback method is widely used by large firms to evaluate small projects and
by small firms to evaluate most projects;
ii) Its popularity results from its computational simplicity and intuitive appeal;
iii) By measuring how quickly the firm recovers its initial investment, the payback
period also gives implicit consideration to the timing of cash flows and therefore
to the time value of money and
iv) Because it can be viewed as a measure of risk exposure, many firms use the
payback period as a decision criterion or as a supplement to other decision
techniques.
Decision criteria:
i) The length of the maximum acceptable payback period is determined by
management;
ii) If the payback period is less than the maximum acceptable payback period, accept
the project and
iii) If the payback period is greater than the maximum acceptable payback period,
reject the project.
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2.4.3 Cash Flow Analysis
The flow of cash is the life blood of any commercial organisation. The cash flow diagram
(Figure 2.10) provides financial data for the entire life of the project. The area below horizontal shows financial position is still in debts. The intersection to horizontal is the break-even point, the pay-back period of the project. Beyond the horizontal line, cumulative cash flow is positive. The project is earning a return on the investment.
Towards the end of project life, the rate of cash flow may tend to fall off due to increased operating costs and falling sale volume and price. The net cash flow is a relatively simple and easily understood concept, and forms the basis for the calculation of other, more complex, measures of profitability.
PBP
.
Figure 2.10: Cash Flow Curve
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2.4.4 Model and simulation
For large scale production, the use of model and simulation in performing economic evaluation is paramount. Process simulation is a model-based representation of technical processes and unit operations using computer software. Process simulation is used to interpret process flow sheets and to predict the performance of process. The task is facilitated by the use of simulators which are capable to visualise the process as follows: to simulate process flows; to perform mass and energy balances and to evaluate process economics. There are several commercial process simulators, such as Aspen Plus,
SuperPro Designer, SIMSCI, HYSYS, CHEMCAD and SimMet-(Simulation of
Comminution and Classification Circuits – integrating all tasks within one package).
SuperPro Designer (produced by Intelligen Inc.) was employed. SuperPro Designer® is a
Computing Environment for Designing and Optimising Integrated Specialty: Chemical,
Biochemical, Pharmaceutical, Consumer Product, Food, Agricultural,
Hydrometallurgical, Packaging as well as Water Purification Processes, Wastewater
Treatment Processes and Air Pollution Control Processes.
Process simulation with SuperPro Designer software can predict the behaviour of a process using basic engineering relationships such as mass and energy balances, phase equilibrium and kinetics reactions (Mlay et al., 2014). Due to its flexibility, the software allowed interactively change of specifications such as operating conditions, flowsheet configuration and feed compositions to run new cases and analyse alternatives. SuperPro
Designer is the only commercial process simulator that can handle equally well batch and continuous processes as well as combination of batch and continuous processes (Mlay et al., 2014). The simulator includes mathematical models that perform cost analysis and project evaluation calculations. The project feasibility can be assessed based on the Net
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Present Value (NPV) profitability indicator. It is a standard method for using the time value of money to appraise long-term Projects (Sinnot, 1993; Peters and Timmerhaus,
1981). Other common economic indicators such as IRR, PBP and Cash Flow Analysis can be obtained.
The built in reports in SuperPro Designer are the Stream Report (SR), Economic
Evaluation Report (EER), Itemized Cost Report (ICR), Throughput Analysis Report
(TAR), Environmental Impact Assessment Report (EIR), Cash Flow Analysis Report
(CFR) and Emission Report (ER). Each report has specific information, for instances EIR is used to analyse economic viability while EIR and ER are used to analyse environmental viability.
2.4.5 Sensitivity analysis
The economic analysis of a project can only be based on the best estimates that can be made of the investment required and the cash flows. The actual cash flow achieved in any year will be affected by any changes in raw materials cost and other operating costs and will be very dependent on the sales volume and price. A sensitivity analysis is a way of examining the effects of uncertainties in the forecasts on the viability of a project (Blank and Tarquin, 1998).
The economic evaluation of producing sulphuric acid by using local available pyrite is still missing. This study will provide the required information that will attract the commercial use of pyrite in a sustainable way.
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2.5 Pyrite roasting technology
Pyrite technology is normally alternative to sulphur burning when the pyrite raw material is available (Runkel and Sturm, 2009). The common reactor used to perform roasting reaction is Fluidized Bed Reactor (FBR) (Mutler and Warren, 2009; Runkel and Sturm,
2009).
FBR is a type of reactor device that can be used to carry out variety of multiphase reactions such as gas-solid, liquid-solid and gas-liquid-solid (Rhee, 2006). FBR is considered as very efficient reactor in performing multiphase reaction because is more effective for increasing mixing, heat and mass factors. Those factors are additional to those affecting homogenous reactions and are normally complex one to deal with it
(Fogler, 1986; Levenspiel, 1999). Pyrite roasting reaction involves multiphase reaction particularly gas-solid reaction; this is the reason behind that roasting reactions are performed in FBD. One of the weakness of FBD is that they are not manufactured for laboratory scale that mean are mainly used for large-scale applications and in fact scale up of FBD is complex (Murzin, 2012).
There are two types of FBR used for pyrite roasting namely as Stationery Fluid Bed
Furnace (SFB) and Circulating Fluid Bed Roasting (CFB).
Figure 2.11 shows SFB. This kind of roaster is used for dry pyrite feeding and designed to recover maximum amount of heat for steam production (on cogeration of power). The roasting process is normally isothermal. The temperature of the fluid bed is kept constant by indirect cooling. The excess heat in the fluid bed is removed by immersed cooling elements which form a part heat revocer by producing high-pressure steam for electricity
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production. The largest single line SFB is capable to process 1,130 tons/day of pyrite concentrate (Runkel and Sturm, 2009).
Figure 2.11: Stationery Fluid Bed (SFB) (Source: Runkel and Sturm, 2009)
CFD is depicted in Figure 2.12. This kind of FBR is more controllable and efficient in roasting compared with the former one. The first industrial scale was built by Outotect and commissioned in Australia in 1991. CFB is operated at more than twice of gas velocity in SFB. Due to high gas velocity, the remained-fine solids are recycled to the roaster via a cyclone (Runkel and Sturm, 2009)
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Figure 2.12: Circulating Fluid Bed (CFB) (Source: Runkel and Sturm, 2009)
In general roasting technology is grouped in five processes namely as dead
roasting; partial roasting; sulphating roasting; two-stage roasting sulphate
decomposition and direct reduction. Most of these proceeses are presented in
the Table 2.7.
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Table 2.7: Roasting Process Technologies
Process Type Aim and Process Feed Materials End Products Application Conditions
Dead roasting SFB/ Total sulphur elimination, ZnS, FeS2, CuS, MeO, Sulphuric Zinc, pyrite, gold, (exothermic) CFB product is pure oxide MeS, Au acid, Steam copper, nickel
Temp: 550–950° C Oxygen: 3–5 vol%
Partial roasting SFB Removal of As; sulphur is As elimination in “As-free” Cu- Copper roaster (exothermic) removed partially from Cu- concentrate concentrate, 34% to 20% Sulphuric acid
Temp: 550–700° C Oxygen: < 1 vol%
Two-stage SFB Removal of As (partial As containing “As-free” Au- Gold roaster roasting roasting), Removal of S and Au concentrate concentrate, (partial C (dead roasting) Sulphuric acid, roasting + dead Steam roasting) Temp: same as partial/dead roasting, Oxygen: same as partial/dead roasting
Sulphating roasting SFB Transformation of metal MeS-like MeSO4, mainly Copper/cobalt (exothermic ) sulphides into metal Cu/Co-sulphide CuSO4 /CoSO4 roaster sulfates
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Temp: 650–750° C Oxygen: ~ 6 vol%
Decomposition CFB/ Removal of SO4 from metal, MeSO4 MeO, Sulphuric Pigment (endothermic) SFB Removal of chloride from (e.g. FeSO4 , acid, Hydrochloric industry, metal CaSO4 ) acid nickel, alumina Temp: 600–1000° C Oxygen: 1–2 vol% MeClx (e.g. NiCl2 , AlCl ) 3
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2.5.1 Dead roasting
Dead roasting is defined as removal of all sulphur, arsenic, or other volatile
components from an ore. It is a common used process applied to zinc
sulphide, pyrite and gold-bearing sulphide mineral. Dead roasting is the most
commonly used roasting process and is mainly applied for zinc and pyrite
roasters. When using a dead-roasting process for concentrates, the sulphides
are completely transformed into oxides. The gas temperature in this type of
plant is typically between 800 and 950° C. The hot gas generated in the
process is used for heat recovery and is suitable for high-pressure steam
production in a waste-heat boiler (Runkel and Sturm, 2009; Jones, 2002).
Roasting of sulphide concentrates has been practised for centuries. Oxidative
roasting of pyrite (FeS2) is a standard way of producing sulphuric acid. In
many base metals plants, roasting is used on an industrial scale, for example
for the production of zinc, copper, and nickel, and even tin, molybdenwn, and
antimony. In many cases, the roasting operations take place in conjunction
with one or more leaching or smelting operations. Roasting is used in order to
capture some of the sulphur present in the concentrates produced from
sulphidic ores. The general approach used in sulphide roasting is to oxidize
some (or all) of the sulphur, and treat the resulting SO2, most commonly
producing sulphuric acid in an acid plant. Other options for recovery of
sulphur include the production of elemental sulphur, or liquid SO2 (Jones,
2002).
Modern roasting processes, since 1960, usually use fluidized-bed reactors,
which are energy-efficient, and have a high productivity because of their
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favourable kinetic reaction conditions. The SO2 content in the off-gas is
typically 8 to 15% by volume (Jones, 2002).
2.5.2 Partial roasting
Partial roasting process is used as a pretreatment step prior to the flash
smelting process for removing impurities such as arsenic and antimony from
feed material of copper. In this process the sulphur is only partially roasted,
with the quantity depending on the concentrate analysis and the desired calcine
grade (Ondrey, 2012; Bartlett et al., 1985).
Partial roasting may be used to prepare sulphide concentrates for subsequent
pyrometallurgical or hydrometallurgical operations. For pyrometallurgical
processing, the usual purpose of roasting is to decrease the sulphur content to
an optimum level for smelting to a matte. Partial roasting is accomplished by
controlling the access of air to the concentrate; a predetermined amount of
sulphur is removed, and only part of the iron sulphide is oxidized, leaving the
copper sulphide, for example, relatively unchanged. For hydrometallurgical
extraction, roasting forms compounds that can be leached out (Jones, 2002).
2.5.3 Two-stage roasting
Two-stage roasting plants are mainly used for treating double-refractory gold
concentrates. A two-stage roasting process combines partial roasting in the
first stage with dead roasting in the second stage. The first stage is used as a
de-arsenifying process step in an oxygen-deficient roasting atmosphere, while
the second stage operates in an oxidizing atmosphere to fully transform the
metals into their oxide form (Iglesias and Carranza, 1994; Robins and
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Jayaweera, 1992).
2.5.4 Sulphating roasting
Sulphating roasting is generally used for processing copper and nickel
concentrates in combination with electrowinning. The copper sulphide is
transformed into water-soluble copper sulphate and the iron sulphide into
insoluble iron oxide. Standard sulphating roasters run with a slurry feed and do
not require a waste-heat boiler system in the off-gas stream (Ozer et al., 2017;
Sithole et al., 2017; Güntner and Hammerschmidt, 2012). Fluid bed roasters are
typically used to react wet concentrate with air in an autothermal manner.
Developments in fluidized bed techniques have made it possible to firmly
control process conditions such as temperature, material distribution and
reaction gas atmosphere (Güntner and Hammerschmidt, 2012). Under these
controlled conditions, cobalt and copper occur primarily as sulphates which are
subsequently leached, while iron compounds form oxides that are insoluble
under mild leaching conditions. The recovery of cobalt and copper from the
pregnant leach liquor is typically achieved by solvent extraction followed by
electrowinning. The sulphation roasting off gas is generally used for the
production of sulphuric acid. This technology is well established for
processing cobalt and copper concentrates and has been in operation
throughout the world for many decades. The sulphation roasting operations
are generally uncomplicated and profitable at low capacities (Güntner and
Hammerschmidt, 2012).
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2.5.5 Sulphating decomposition
Sulphating decomposition is useful for removal of sulphate or chloride from metal (Beerkens, 2003). Alumina is the most useful product which is obtained from thermal decomposition of aluminium sulphate hydrate with a chemical formula Al2(SO4)3.18H2O. For many decades alumina has been the standard raw material for water treatment industry. It has large molecular size and weight in combination with its low cost, makes it an excellent flocculants for the treatment of both drinking water and industrial waste water. In addition to water treatment, alumina finds use in a diversity of other areas including construction products, oil and fat processing, and paper manufacturing (Cigli and Cetisli, 2009).
2.5.6 Direct reduction
Direct reduction is for spongy iron production. The products gained from the roasting process are calcine and roasting gas. The calcine consists of Fe2O3,
Fe3O4, FeSO4 and gangue material, while the roasting gas has SO2, SO3, O2, N2 and H2O as steam. Depending on the chlorine content in the roaster feed, HCl may also exist in the roasting gas in corresponding concentration (Long et al.,
2016; Runkel and Sturm, 2009).
Direct reduction is the process of reducing iron oxides to metallic iron at temperatures below the melting point of iron. The product of such solid-state processes is called direct reduced iron. The reducing gas is a mixture of gases, primarily hydrogen (H2) and carbon monoxide (CO). The process temperature is typically at 800 to 1200° C (Grobler and Minnitt, 1999). Direct-reduced iron is also called sponge iron. It is produced from the direct reduction of iron ore (in the form of lumps, pellets or fine powders) to iron by a reducing gas or
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elemental carbon produced from natural gas or coal. Reduced iron derives its name from the chemical change that iron ore undergoes when it is heated in a furnace at high temperatures in the presence of hydrocarbon-rich gases, carbon monoxide or elemental carbon (Grobler and Minnitt, 1999).
The sponge iron has a highly porous structure when viewed under the microscope and hence this product is called sponge iron (Chatterjee, 2010; Sun and Lu, 1999). It is used as prime source in steel making process using the electric furnace or Induction furnace route. The quality of sponge iron is primarily ascertained by the percentage of metallization (removal of oxygen), which is the ratio of metallic iron to the total iron present in the product
(Chatterjee, 2010; Fruehan et al., 2000)
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CHAPTER THREE
MATERIALS AND METHODS
3.1 Study Sites
The pyrite samples were collected from Merelani (Simanjiro district). Potential areas with pyrite in terms of quality and quantity in Tanzania are Samena and Merelani. Merelani was selected as case study of this work because of accessibility and quality. Merelani’s pyrite is a sulphide waste rock (overburden) which its presence is even serious AMD pollutant. It is easily accessible because it is just a waste rock. Pyrite from Samena might be of much better quality and quantity however accessability is a real challenge, sometimes one needs to excavate up to 200-400 m deep to get it. So the best option, with time frame was to choose pyrite from Merelani as case study. Two pyrite ores of about three kilograms each were collected from two different sites of Block D in Merelani
(Figure 3.1).
The Merelani graphite-tanzanite deposit is situated in northern part of Tanzania, about 70 kilometres from both Arusha and Moshi Municipalities. Merelani gem-mining is located in the Merelani Hills, on the slope of the Lelatema Antiform (Figure 3.1). The Laletema
Antiform belongs to the fault-bounded granulite-gneiss complexes of the Pan-African
Mozambique Belt in Tanzania (Muhongo and Lenoir, 1994). Merelani gem-mining area is rich in minerals, where tanzanite was first found in 1968. Tanzanite is the blue gemological trade name of vanadium bearing with a chemical formula of
Ca2Al3Si3O11(OH). Merelani is the only known deposit of gem-quality tanzanite in the world (Malisa, 2003).The occurrence of gem-quality tanzanite and grossular has been found to occur mainly in boudinaged pegmatitic veins and hydrothermal fracture fillings.
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These fracture fillings occur in brecciated and hydrothermally altered graphite-bearing gneiss in a mineral association containing glass-clear quartz, diopside, zoisite, graphite and calcite (Harris et al, 2015).
Figure 3.1: The Geological Map of Merelani (Source: Hiji et al., 2018; Harris et. al, 2015)
The tanzanite mining area at Merelani is about 8 km by 2 km and is divided into 4 blocks as Blocks A, B, C and D. Block C is owned by Tanzania One Company and the remaining are under small-scale artisanal miners. Harris et. al. (2015) reported the presence of pyrite in several holes as shown in Table 3.1.
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Table 3.1: Represantive Data of Merelani-Tanzanite Deposit Samples Block Sample Name Location Mineral
D WTD5 01 Shaft B Drive 5 Quartz,tanzanite, tremolite, graphite
D WTD5 02 Shaft B Drive 5 Quartz,tremolite, graphite,pyrite
D WTD5 03 Shaft B Drive 5 Quartz,tremolite, graphite
D WTD5 04 Shaft B Drive 5 Quartz,tremolite, graphite,pyrite
D WSA Santana Shaft A Tsavorite, quartz, pyrite
3.2 Methodology
This study involved three parts: analytical tests, laboratory experiments and desktop work.
The specific objectives no. 2 and 3 were achieved by laboratory experiments, while the first and fourth specific objectives involved analytical tests and desktop work respectively.
There were two types of experiments for:
i) Investigating the effect of temperature, particle size and concentration on
conversion of pyrite ore to sulphur dioxide and
ii) Developing the appropriate protocol of converting pyrite ore to sulphuric acid.
3.3 Chemical and Mineralogical Characterisation of Pyrite Ore
3.3.1 Sample preparation
This involved primary and secondary size reduction process. The collected samples of pyrite ore of about 1 kg weight was reduced by hammer into diameter of about 5 cm and then by a jaw crusher into 2 mm diameter. About 500 gm of 2 mm diameter was subjected
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to grinding machine for pulverisation to a small size below -75 micron (200 mesh) which was used in further tests and roasting experiments.
3.3.2 Chemical characterisation of pyrite ore
X-Ray Fluorescence (XRF) and Atomic Absorption Spectroscopy (AAS) tests were used to analyse elemental composition. Portable XRF machine was used to analyse all elemental composition of the sample while AAS examined particularly the composition of
Fe. Elements quantified by XRF are shown in Appendix B.
3.3.2.1 XRF test
Portable XRF machine from Geological Survey of Tanzania (GST) was used to analyse the chemical composition of samples by determining the major, minor and trace elements.
Figure 3.2 shows a further elaboration between major, minor, trace and ultratrace. A representative sample of around 500 gm of 75 microns was prepared. 25 mg of the sample was placed into XRF cup. The XRF run conditions were: Media (Air for portable XRF);
Power (28 kV, 75 mA); Filter (Aluminium is recommended for sulphur); Time (60 s);
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Figure 3.2: Classification of Constituent Types by Analyte Level (Source: Skoog, 2013)
3.3.3 Mineralogical characterisation of pyrite ore
X-Ray Diffraction (XRD) was used to analyse the mineral composition of the sample.
The results obtained in XRD results are qualitative. To quantify it, modified ASTM D-
2492 was used which utilises techniques in XRF and AAS to come into conclusion.
3.3.3.1 XRD test
X-Ray Diffraction with Cr-Kα target from Wazo Hill Cement Company was used to determine the presence of minerals speciess in a solid-powdered sample, as well as identify phases. The XRD run conditions were: Power (40 kV, 150 mA); Scanning rate (2
ᵒ/min in the 2θ range of 2.6-7 ᵒ).
3.3.3.2 Quantitative analysis of pyritic sulphur
Section 2.1.6 explained in detail the theory behind which involves Equation 2.5. In addition, Appendix A shows procedures for dissolution of finely ground rock samples of high concentration of the metals such as Cu, Zn, and Ni etc. AAS tests analyses were
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done at GST laboratory. The AAS run conditions were: Lamp current (5 mA); Lamp wavelength (248.3 nm); Fuel (acetylene); Support (air); and Flame stoichiometry (248.3 nm).
3.3.3.3 Analysis of inorganic sulphur
A sample of 0.25 gm was placed into a tephlon beaker, and then 5 mL of 70% HCl was added. The beaker was heated on hot plate to evaporate the solvent. It was then removed from the hot plate and allowed to cool for 5 minutes. 5mL HCl were mixed with the residue and evaporated again.The beaker was again removed from the hot plate and allowed to cool for 5 minutes, followed by addition of 5 mL of HCl and 40 mL of distilled water. Precipitates were allowed to settle. The solution was filtered into the volumetric flask. The filter paper was washed well with distilled water. The volumetric flask was filled to the mark. The concentration of the element was determined by AAS.
3.3.3.4 Analysis of pyritic sulphur
A sample of 0.25 gm of sample was placed into tephlon beaker, followed by addition of
10 mL of 70% HNO3. The beaker was covered with a watch glass and heated on the hot plate until the brown fumes were no longer produced. The watch glass was rinsed with distilled water. The solution was evaporated to dryness to avoid too much baking. The beaker was removed from the hot plate and allowed to cool for 5 minutes. 5 mL HCl were mixed with the residue and evaporated again.The beaker was again removed from the hot plate and allowed to cool for 5 minutes. 5 mL of HCl and 40 mL of distilled water were added. Precipitates were allowed to settle. The solution was filtered into the volumetric flask. The filter paper was washed well with distilled water. The volumetric flask was filled to the mark. The concentration of the element was determined by AAS.
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Pyrite sulphur is determined by taking amount of iron extracted by dilute hydrochloric acid (section 2.1.6) and subtracting it from the iron extracted by nitric acid, the difference being iron present as pyrite iron (FeS2), which was then computed by Equation 2.6.
3.4 Chemical kinetics of conversion of pyrite ore to sulphur dioxide
The kinetics of roasting of pyrite is based on equations 2.12, 2.18 and 2.32 where pyrite is reacting with oxygen gas to form sulphur dioxide and calcine cinder (mainly as Fe2O3).
The acquisition of chemical kinetic data which utilise the technique of concentration-time measurements in a batch reactor was collected and recorded as t (min) and w (mg).
Response Surface Methodology (RSM) which is dedicated on finding the factor settings that optimise the response was employed. The factors under investigation were temperature (T), particle size (PS) and concentration and the response is reaction order, α.
For each design, the concentration of pyrite was uniform. Table 3.2 shows the details of factors and their levels. With aid of Minitab Statistical Software (MSS), Central
Composite Design (CCD) table was used with internal input data information shown in
Table 3.3. The experimental design table is shown in Table 3.4. For each data in Table
3.4, 10 experiments were conducted determine corresponding reaction order, α, after data treatment. Atleast 100 experiments were conducted. The obtained value of α was filled in
Table 3.4.
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Table 3.2: Parameters for Optimisation (Source:Runkel and Sturm, 2009) Factor Remarks
T Low value is 500° C and high value is 1000° C
PS Very Fine Particles (VFP<75 µm); Fine Particles ((FP<150 µm); Coarse Particles (CP<425 µm);
Table 3.3: CCD Internal Input Information Property Value
Factors 2
Base runs 13
Base blocks 1
Replicates 2
Total runs 26
Total blocks 1
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Table 3.4: Experimental Design Temperature Particle StdOrder RunOrder Blocks (T) Size (PS) α
3 1 1 573 CP
10 2 1 750 CP
9 3 1 750 CP
6 4 1 1000 CP
7 5 1 750 VFP
4 6 1 927 CP
2 7 1 927 CP
11 8 1 750 CP
1 9 1 573 FP
13 10 1 750 CP
12 11 1 750 CP
5 12 1 500 CP
8 13 1 750 CP
3.4.1 Acquisation of kinetic data
Experiments were done batchwise according to Table 3.4. One gram of powdered-pyrite ore sample was placed into the sample holder. The furnace (CS30000G) analyser from
GST was used as a roaster (Appendix I). It is a separate component of CS analyser. It consists of a column which is divided into three zones; the first zone which contains mainly pure oxygen from the gas cylinder, the second zone is the heating zone where ceramic crucible is placed under isothermal condition and final zone contains roasting gases that are within ordinary room temperature. After setting of appropriate temperature, the ceramic-crucible sample holder was placed into heating zone of the furnace for
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specified duration of time. Then the sample holder was removed from the furnace, after cooling, the sample was analysed by XRF, and the weight of sulphur, W was recorded.
The exercise was repeated until the full batch of data was completed before moving to the second batch.
3.4.2 Analysis of kinetic data
Matlab tool was developed to determine lnk value and reaction order, α, by analysing data from Table 3.4 on each batch. The tool needs only three information, the sample ID number, the upload of excel file for t data and its corresponding w data.
The tool computed or formulated the following:
i) x_data= ln[W];
ii) f(i)=[W(i+1)-W(i)]/(2*delta);
iii) y_data=ln(-f) ;
iv) plot(x,y);
v) Reaction order as slope and rate constant as intercept;
vi) R2 and
vii) Confirming reaction order.
The output from the tool was the uploaded data, analysis of intergral and differential methods and their corresponding plots, R2 and reaction order for each batch. Activation energy determination for coarse particle was done in separate graph, because CP was performed in different temperatures.
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3.4.3 Evaluation of RSM and optimisation
The value of pseudo-rection order, α obtained in each batch in section 3.4.2 was filled in
Table 3.4 as the response and analysed by RSM using linear regression and Analysis of
Variance (ANOVA). An estimated regression coefficient table and ANOVA table were results of evaluation. When the p-value selection of linear regression was less than 0.05, then the selection was supported and the regression coeffient values could be fitted to linear polynomial equation. This is an indication that the null hypothesis is false. Equation
3.1 represents linear polynomial equation for two variables (StatSoft, 2013). In a case where the selection model was rejected, then re-run was needed and selection of quadratic regression was done. In such a condition, regression coefficient values would be fitted to quadratic polynomial equation (Equation 3.2). When the selected model fit well the experimental data, the optimum condition could be determined.
y b1 x1 b2 * x2 b12 * x1 *x 2 ...... (.3.1)
Where: y stands for the predicted response;
b1, b2 stands for the regression coefficients (linear coefficients);
b12 stands for the interaction coefficients;
x1, x2 are the operating variables.
y b1 x1 b2 * x2 b3 * x3 b12 * x1 *x2 b13 * x1 * x3 b23 * x2 * x3...... (3.2)
Where: y stands for the dependent variable values (the predicted response);
b1, b2, b3 stands for the regression coefficients (linear coefficients);
b12, b13, b23 stands for the interaction coefficients;
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x1, x2, x3 are the operating variables.
3.5 Conversion of pyrite ore to sulphuric acid
The proposed protocol is according to Williams (2003) who utilised the basics of Lead
Chamber Process (Jones, 1950). Equations involved in this protocol are no. 2.12-2.15.
Figure 3.3 shows an experimental setup where a large flat-bottomed flask with air inlet
was connected to three smaller flat-bottomed flasks. Into the first flask 5 mL of H2O was added, while 10 gm of pyrite-ore powder was placed in the fourth flask, and into the third flask 5 gm of Cu turnings and 10 mL of dilute HNO3, 5 mL water were added. Ring stand was hanged on, and slowly heated the flasks containing H2O and FeS2. Notice the fumes that passed into the second flask which involves combination of gaseous products from all other flasks was done.
Figure 3.3: Experimental Setup for Production of Sulphuric Acid
Some modification had been done to the method presented by Williams (2003). The set up for production is shown in the Figure 3.4. The roaster in this case is plug flow reactor
(PFR) instead of round bottom flask. PFR are very useful in performing gas reactions
(Levenspiel, 1999; Fogler, 1986).
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Figure 3.4: Production of Sulphuric Acid
3.5.1 Quantitative analysis of produced sulphuric acid
Quantitative analysis of produced sulphuric acid involved three major steps and analysis of results.
3.5.1.1 Preparation of NaOH standard titration solution
Preparation of 0.5 M NaOH was done by dissolving 5 gm NaOH into 250 mL water in a volumetric flask. Other molarites can be used but ideally in the range of 0.4 M to 1.0 M.
Normally, a simple way to prepare 0.X M NaOH in 250 mL water simply measure X gm
NaOH.
3.5.1.2 Preparation of conc. H2SO4 solution
Transferred 5 mL of concentrated H2SO4 using a volumetric pipette into a 100 mL volumetric flask and gently add water to the mark to make a 1:20 dilution (5:100).
3.5.1.3 Titration
20 mL of the dil H2SO4 was transferred into four 100 mL flasks. Few drops of phenolphthalein indicator were added. The NaOH standard solution was poured in a
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burette. The acid solution with the NaOH was titrated to achieve neutralisation that is when the indicator colour turned pink. The volume of NaOH used was measured by the burette for the three flasks and average the results out with aid of titrimetric tool (section
3.5.1.4).
3.5.1.4 Titrimetric tool
Another Matlab tool was developed to analyse titrimetric experiments based on Equations
2.1, 2.2, 2.3 and 2.4. The input parameters were: mass of acid (3 readings) and its corresponding volume; four titration readings as shown in Table 3.5; molarity of sodium hydroxide solution (CNaOH); dilution factor; and volume of sulphuric acid solution (Vacid) and relative molar mass (RMM) of sulphuric acid.
The tool computed according to internal programmed instructions and displays the following:
i) All uploaded information;
ii) Density of acid in g/mL;
iii) Complete filled titration data (Table 3.5);
iv) Molarity of sulphuric acid (Cacid);
v) % Cacid;
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Table 3.5: Titration Data Volume of sulphuric acid Burrete readings Molarity of sodium hydroxide solution solution pipetted out: 25 mL used : 0.5 M
Dilution factor: 20 Titration Start Final Volume of sodium hydroxide solution delivered (final – start) mL mL mL 1 2 3 4 Mean titration value :
3.5.2 Qualitative analysis of produced sulphuric acid
Two qualitative analysis tests were conducted to confirm presence of acid character and sulphate ion.
3.5.2.1 Acidic character
Litmas paper was used to check acidic character. One mL of the sulphuric acid was poured into blue litmas paper, the colour turned to red indicates presence of acidic solution.
3.5.2.2 Sulphate ion test
One mL of sulphuric acid was mixed with little amount of barium nitrate solution.
Formation of white precipitate (cloudness) of barium sulphate according to Equation 2.5 is a confirmation test of presence of solube sulphate and sulphuric acid.
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3.6 Establishment of techno-economical viability of the process
This was achieved by aid of process simulator SuperPro Designer. SuperPro Designer is superior in performing reaction kinetics as well as economic evaluations
(Shanklin et al., 1999). The input information are: (1) properties of components and mixtures and their corresponding economic data; (2) feed stream data; (3) mass transfer data; (4) equipment cost data; (5) data for economic parameters such as project life and discount rates; and (6) data for other technical parameters, including set-up time, processing time, temperatures, flow rates, among others.
Figure 3.5 shows a proposed block diagram of producing sulphuric based on Lead
Chamber Process. There are five units operations/processes namely; grinder, pulveriser, pyrite roaster, H2SO4 reactor, and NO2 reactor.
The economical and environmental data were obtained after running the model. The sensitivity analysis was performed to investigate relationship between production scale and unit cost; plant capacity and payback period; plant capacity and capital investment;
pyrite price and sulphuric acid unit production cost.
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H2O
H2SO4 Acid Pyrite Roasting Pulverising Chamber Grinding Process Operation Operation Process n
NO2 NO
NO2 HNO3 Chamber
Process O2
Cu
Figure3.5: Proposed Block Diagram of Producing Sulphuric Acid from Crude Pyrite Ore from Merelani
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CHAPTER FOUR
RESULTS AND DISCUSSION
4.1 Chemical analysis
The results for chemical analysis are presented in Table 4.1 and 4.2 which indicate that in both samples (Sample A and Sample B), S and Fe are predominant. In addition, by considering the 1% error in XRF analysis, the geochemistry of both samples based on major elements (Figure 3.2) appears to be identical although there is existence of two additional elements, P and Zr in Sample B. This indicates that the mineral composition from one sample to another on the same block is not the same (Table 3.1) consequently the chemical constituent will have different pattern.
Table 4.1: Chemical Composition of Sample A
Element Amounts LE 35.87% S 28.72% Fe 27.14% Al 5.72% Si 1.02% Ca 0.55% Ni 1092 ppm K 993 ppm As 153 ppm Ag 28 ppm Zn 22 ppm
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Table 4.2: Chemical Composition of Sample B
Element Amounts LE 36.02% S 29.16% Fe 27.09% Al 6.4 %
Si 6597 ppm P 4070 ppm K 1432 ppm
Ni 667 ppm Ca 225 ppm As 157 ppm
Zr 101 ppm Ag 29 ppm Zn 20 ppm
The portable XRF is less capable to detect or quantify lighter elements (LE) particularly lanthanide series (Brouwer, 2006). However, the LE in this case were Br, La, Yb and Re which are within elements detected by using MINPAL4 XRF which utilises the helium˗filled spectrometer. MINPAL4 XRF was not used in this work in general due to the lack of standard reference sample.
Most of pyrites used worldwide are flotation one with w/w % sulphur (40-50) and iron
(37-45) (Table 2.1). In comparison with the chemical results in Tables 4.1 and 4.2 which shows percentage of sulphur and iron are 29 and 27 respectively and the ore is crude one,
Merelani’s pyrite is potential and attractive.
Both Tables 4.1 and 4.2 show that, the major elements are S, Fe, Al and Si. This indicates that there is a potential of having pyrite (FeS2) and quartz (SiO2). In addition, the geochemistry of Merelani shows the presence of dolomite (CaMg(CO3)2) and tremolite
(Ca2(Mg,Fe)5Si8O22(OH)2). Without iron in the formula of tremolite will have the typical
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creamy white colour. With just a small amount of iron tremolite will be green. Increasing iron content will raise the specific gravity, refraction index and darken the colour (Amrei et al., 2014; Malisa, 2003). The analysed samples did not contain any of green colours.
Tables 4.1 and 4.2 also show that, the pyrite ore from Merelani does not contain toxic and hazardous substances such as arsenic, selenium and antimony in a concentration that is serious hazard to environment; that mean it does not need special or additional treatment.
4.2 Mineralogical analysis
The XRD pattern is a fingerprint that enables to figure out what is in the sample. It is a plot of intensity of X-ray scattered measured in counts at different angles by the sample.
Experimental XRD data are normally compared to reference patterns to determine what phases are present. Each phase produces a unique diffraction pattern. The position and intensity of the reference should match the experimental data. A small amount of mismatch in peak position and intensity is acceptable experimental error (Speakman,
2009).
The International Centre for Diffraction Data (ICDD) developed a database of powder diffraction patterns, the Powder Diffraction File (PDF) which includes the d-spacings
(related to angle of diffraction) and relative intensities of observable diffraction peaks. In
2016, ICDD and the Joint Committee on Powder Diffraction Standards (JCPDS) released
848,000 PDF of unique material data sets. Each data set contains diffraction, crystallographic and bibliographic data, as well as experimental, instrument and sampling conditions, and selected physical properties in a common standardised format. The PDF are called ICDD or JCPDS cards. Any experimentally determined patterns can be compared with ICDD or JCPDS cards to identify the phases and the quality. The ICDD
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database softwares are capable to match practical experimental patterns and JCPDS cards and provide both qualitative and quantitative phase analysis of the sample.
The XRD patterns of pyrite from Merelani (Figure 4.1), showed peak intensity at 2θ=28.5,
33.1, 37.1, 41, 47.5 and 56.5. The peak angles and ratio of intensity corresponds with work published by Cheng et. al. (2013) as shown in Figure 4.2 confirming the presence of pyrite. Comparison with JCPDS 42-1340 (Figure 4.3) suggests a high degree of pyrite.
Such kind of pyrite has a cubic structure with lattice constant a = 5.419 and crystalline shape as shown in Figure 4.4.
In addition, Figure 4.1 shows another peak angle 2θ=9.5. This is another phase, different from pyrite. The XRF results show other elements apart of Fe and S containing up to 40% w/w. The major ones are Al, Si and Ca. Based on XRF results (Tables, 4.1 and 4.2) and reported geochemistry data of Merelani (Harris et al, 2015), the result suggest the presence of other mineral such as quartz (SiO2), dolomite (CaMg(CO3)2) and tremolite
(Ca2(Mg,Fe)5Si8O22(OH)2).
106
Figure 4.1: XRD Pattern of the Crude Pyrite from Merelani
107
Figure 4.2: XRD Pattern of the Coal-derived Pyrite (Source: Cheng et. al., 2013)
108
Figure 4.3: Image result for JCPDS card for pyrite (Source: http://pubs.rsc.org)
109
Figure 4.4: Pyrite Crystalline Shape (Source: Cheng et. al., 2013)
4.3 Pyritic sulphur analysis
Although section 4.2 shows mineralogical analysis, forms of sulphur present in the ore is crucial since the depletion of sulphur during pyrite roasting was used to study kinetics. The results and analysis of pyritic sulphur are shown in Tables 4.3, 4.4 and 4.5. Pyrite sulphur
(29.29) obtained by conversion is slightly higher than total sulphur (28.72) obtained in
XRF (Table 4.5). By considering 1% marginal error in XRF reading, the results are the same. This shows evidence that, all sulphur in the pyrite ore exists as pyrite (FeS2). In combination with Tables 4.1 and 4.2 and Figure 4.1; about 60% w/w of the ore is pyrite
(FeS2). This kind of sulphide-rich mineral deposits is the potential AMD pollutant. Special remediation options are needed to save the environment.
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Table 4.3: AAS Results for the Total Iron in the Pyrite Ore
Run Sample name Amount of Fe in Actual amount of iron pyrite ore (%) in a sample (%) Blank 0.84 0 1 Pyrite Ore 25.25 24.41 2 Pyrite Ore 26.50 25.66 3 Pyrite Ore 27.76 26.92
Table 4.4: AAS Results for Iron in the Sulphate Sulphur
Run Sample name Concentration of Fe in Actual amount of iron pyrite ore (%) in a sample (%) Blank 0.29 0 1 Pyrite Ore 0.44 0.15 2 Pyrite Ore 0.43 0.14 3 Pyrite Ore 0.44 0.15
Table 4.5: Pyritic Sulphur Analysis
Analysis % Reference Total Iron 27.14 XRF; Table 4.1 Total Sulphur 28.72 XRF; Table 4.1 Total Iron for Sulphate Sulphur and 25.66 AAS; Table 4.3 Pyrite Sulphur Total Iron for Sulphate Sulphur 0.15 AAS; Table 4.4 Total Iron for Pyrite Sulphur 25.51 25.66-0.15 Total Pyrite Sulphur 29.29 Equation 2.6
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4.4 Chemical kinetic results
The objective of studying chemical kinetics is to elucidate the chemical reaction order and rate constant. The rate constants were used to deduce the activation energy (Ea). However, there were several settings made before analysing kinetic parameters as follows:
1) The concentration data are normally calculated on molar basis however in this
work, the relative mass and the sulphur concentration were calculated on mass
basis in order to be compatible with the experimental data and the selected
quantification of a rate equation based on unit mass of solid in fluid-solid reaction
(Equations 2.18 and 2.32 );
2) Since the reactions are highly spontaneous, only the forward rate reaction was
considered;
3) The experiments were performed in excess of oxygen surrounding the samples. As
a result, the reaction order obtained would be pseudo with respect to solid reactant
and
4) All the sulphur was in the form of FeS2.
Figure 4.5 shows the plot of sulphur content versus time data from Table 4.6. Each graph shows the loss of weight of sulphur, and it is comparable with theory. During roasting, pyrite was converted to hematite and SO2 (Equation 2.12). Since pure oxygen was used in the roaster, the reactions were very fast and last within ten minutes. In the first few minutes, the slope is very steep meaning the iniatial rates were very high and a large amount of sulphur was removed. The roasting mechanism was controlled by the arrangement of pyrite particles and free flow of air. After some time, when significant
MeO were formed (this is analogue with formation of ash during combustion of ash), the roasting mechanism changed and was controlled by diffusion, and the slope is less steep.
112
Beyond that, FeSO4 was predominant and the concentration of sulphur increased. The effect of increasing the sulphur concentrations are clearly illustrated by Kellog Diagram
(Figure 1.4). The dependence of temperature and particle size is clearly observed where the first graph is 1000 CP followed by 927 FP only 927 CP deviate from this trend. Figure
4.6 shows mechanistic suggestion for roasting of pyrite into SO2.
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Table 4.6: Variation of Sulphur Content with Time and Particle Size (For details Appendix C)
SULPHUR CONTENT, % t, min 500, CP 927,CP 927, FP 573, FP 573, CP 750, VFP 750,CP 1000, CP 24.64 24.64 25.1 25.1 0 25.1 25.1 25.1 25.1 24.64 21.85 9.4 6.28 2 22.7 17.49 20.72 18 9.26 5.67 16.87 0.46 0.38 4 18.27 4.68 9.31 6.57 4.16 0.37 0.69 0.38 0.34 6 0.72 2.12 0.91 2.76 0.6 0.29 0.43 0.39 0.38 8 0.33 0.27 0.46 0.59 0.23 0.32 Run2 10 Run1
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30
25
500 CP 20 573 CP 750 CP 15 927 CP 1000 CP 10 Sulphur Sulphur content,wt% 573 FP 750 VFP 5 927 FP
0 0 1 2 3 4 5 6 7 8 9 Time, min
Figure 4.5: Sulphur Content versus Time for Various Temperature
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Figure 4.6: Mechanistic Suggestion for the SO2 Released by Thermal Decomposition of Pyrite (Source: Cheng et al., 2013)
The data in Table 4.6 were further analysed by Two Point Method (Table 2.6) with aid of developed computer kinetic data tool (Appendix D). The results from the computer tool are presented in Table 4.7 which involves uploaded data and analysed data. All eight experiments developed by RSM were analysed and the graphs from integral and differential methods are shown in Figures 4.7a and 4.7b. Integral Method is normally used for confirmation of reaction order, and in this case all data were fitted into appropriate model which were assessed by the value of R2. The value of R2 obtained varies between
0.86 and 0.97 (Table 4.7). The results are good, although the higher value approaching one, the better it is. This slightly deviations are due to experimental errors.
116
Table 4.7: Results from Kinetic Tool
.... RESULTS FOR KINETIC TOOL DEVELOPED BY HIJI, M (UDOM 2018)..... the uploaded data for sample IDs 500CP; 573FP; 573CP; 750VFP; 750CP
t w1 w2 w3 w4 w5
2 22.7 17.49 20.72 18 9.26
4 18.27 4.68 9.31 6.57 4.16
6 0.72 2.12 0.91 2.76 0.6
8 0.33 0.27 0.46 0.59 0.23 the uploaded data for sample IDs 927CP; 927FP; 1000CP
t w6 w7 w8
0 25.1 25.1 24.64
2 9.4 6.28 5.67
4 0.46 0.38 0.37
6 0.38 0.34 0.29
...... INTEGRAL METHOD CONFIRMATION......
Sample ID Estimated Order Confirmed Order R-Square(integral)
500 CP 0.91004 1 0.88951
573 FP 0.94798 1 0.96733
573 CP 1.0847 1 0.94923
750 VFP 0.89154 1 0.98333
750 CP 0.99769 1 0.97363
927 CP 1.3773 1 0.9008
927 FP 1.5139 2 0.85859
1000 CP 1.332 1 0.92288
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Figure 4.7a: Differential and Integral Method for Determination of Chemical Reaction Order
118
Figure 4.7b: Differential and Integral Method for Determination of Chemical Reaction Order
119
The order of the reaction obtained by Differential Method was filled in Table 4.8 as a response. After running the model, the regression Table 4.9 was obtained on which regression equation was developed.
Table 4.8: Experimental Design Table
StdOrder RunOrder Type Blocks T PS Order 7 1 -1 1 750 VFP 0.89 13 2 0 1 750 CP 1 11 3 0 1 750 CP 1 5 4 -1 1 500 CP 0.91 8 5 -1 1 750 CP 1 6 6 -1 1 1000 CP 1.3 9 7 0 1 750 CP 1 10 8 0 1 750 CP 1 1 9 1 1 573 FP 0.95 4 10 1 1 927 CP 1.4 2 11 1 1 927 FP 1.5 12 12 0 1 750 CP 1 3 13 1 1 573 CP 1.1
Table 4.9: Response Surface Regression: Order Versus T, PS
Term Coef SE Coef T P Constant 0.295601 0.229320 1.289 0.226 T 0.000990 0.000280 3.530 0.005 PS 0.000171 0.000331 0.518 0.616 S = 0.140382 PRESS = 0.401283
R-Sq = 56.00% R-Sq(pred) = 10.41% R-Sq(adj) = 47.20%
The analysis was done using uncoded units
120
Table 4.10: Analysis of Variance for Order
Source DF Seq SS Adj SS Adj MS F P Regression 2 0.250822 0.250822 0.125411 6.36 0.016 Linear 2 0.250822 0.250822 0.125411 6.36 0.016 Temp 1 0.245540 0.245540 0.245540 12.46 0.005 PS 1 0.005282 0.005282 0.005282 0.27 0.616 Residual Error 10 0.197070 0.197070 0.019707 Lack-of-Fit 6 0.197070 0.197070 0.032845 * * Pure Error 4 0.000000 0.000000 0.000000 Total 12 0.447892
The selected linear model fits well the data with p-value less than 0.05. The lack of test is insignificant and the model is appropriate (Table 4.10). By using Equation 3.1 and regression coefficient results, the effects of two variables (T and PS) on reaction order is according to Equation 4.1.
Order 0.2956010.00099T 0.000171PS...... (4.1)
The magnitude and sign for each coefficient affects the reaction order. The p-value less than 0.05 on temperature, shows this variable has significant effect towards response.
This is supported by Table 4.7 where only change in temperature that change the reaction order to second order. Díaz et al. (2016) also observed that the particle size did not affect significantly the conversion rate of pyrite roasting.
Table 4.11 shows recommended optimum condition, which in combination with Table
4.7 suggests that the pyrite roasting is pseudo first order with respect to the solid. The results are compared with reported work of pyrite roasting in high-arsenic copper concentrate which showed zero order with respect to the solid and first order with respect to the gas (Devia et al., 2012). The results and methods used are not the same.
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The setting conditions made in this work, is what is actually happening during roasting or any combustion reaction involving gas-solid system.
Table 4.11: Recommended Optimum Conditions
Factor Optimum Value Low Level High Level
T 750 573 927
PS 250 100 400
Since five experiments that were run with the same particle size and different temperature conditions (Table 4.6) were available, the intrinsic rate constants were studied versus temperature for elucidation of activation energy, Ea. Table 4.12 shows values of ln K versus T. The activation energy was calculated from the slope of Arrhenius equation (Equation 2.51) from the plot of ln K versus 1/T as shown in Figure 4.8. Two data from Table 4.12 were not fitted to the model constructed in Figure 4.8. The lnK for
T equal to 750 and 1000° C deviated from the model. The former seemed to be an experimental error and the latter, seemed to be too high temperature that is not supported by roasting process. It seemed useful to study more data at 600 and 700° C as shown in
Table 4.12 to improve precision of results.
Table 4.12: Calculated Rate Constant Values
T (° C) 500 573 600 700 750 927 1000 T(K) 573 846 873 973 1023 1200 1273 lnK 1.2847 1.306 1.47 1.675 1.102 2.0388 1.749
122
2.5
2
1.5
] ]
K ln [ ln 1 y = -0.1749x + 3.4722 R² = 0.9591
0.5
0 0 2 4 6 8 10 12 14
-1 4 1/T[K x 10 ]
Figure 4.8: Rate Constants Versus Temperature
The average Ea of 15 kJ/mol was calculated from slope of Figure 4.8 (slope=-Ea/R) as R equal to 8.314 J/molK. This low activation energy value, indicate the following:
1) Pyrite roasting reactions are highly exothermic where only a small amount of
energy is required in initiating a chemical reaction;
2) Since it has been well established that SCM is the best and simplest
representation for the majority of non-catalytic gas-solid systems including
pyrite roasting (Marsden and House 2006; Saha et al., 2005). The typical
SCM involves three steps namely as: gas filming resistance controlling;
diffusion through a layer control; and the chemical reaction controlling. The
first step was not accountable because the reaction was carried out in plentiful
supply of oxygen. Since the Ea value was small, the roasting reaction was not
controlled by the chemical reaction. Hence, it was suggested that the roasting
reaction was controlled by diffusion of oxygen through the ash layer to the
reacting surface.
123
The value of Ea obtained was comparable with Marin et al. (2005) who studied the roasting kinetics of molybdenite (MoS2) and obtained the value of 22 kJ/mol. Another publication, Aracena et al. (2016) reported Ea value of 60·5 kJ/mol for pyrite oxidation under oxygen–nitrogen atmosphere and concluded that roasting reaction was controlled by the chemical reaction. The roasting atmosphere of this work is different from latter reporter, how ever it shows that the mechanism of pyrite roasting depend on roasting atmosphere.
4.5 Production of sulphuric acid
4.5.1 Quantity of produced sulphuric acid
By using the setup according to Figure 3.4 which is modification protocol from Williams
(2003), 20 mL of dil H2SO4 was obtained. Titration procedures were performed and results were analysed and displayed by prior developed tool for performing analytical analysis - Titration tool (Appendix E). The results from Titration tool are shown by Table
4.13.
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Table 4.13: Results from Titration Tool
...... THE UPLOADED DATA...... W1 is : 6.719 g and V1 is: 5.00 mL W2 is : 7.786 g and V2 is: 6.00 mL W3 is : 9.121 g and V3 is: 7.00 mL dilution factor used is 20.000
****************TITRATION ANALYSIS********************
Volume of sulphuric acid Burrete readings Molarity of sodium hydroxide solution solution pipetted out: 25 mL used : 0.5 M
Dilution factor: 20 Titration Start Final Volume of sodium hydroxide solution mL mL delivered (final – start) mL 1 0.00 21.00 21.00 2 0.00 22.00 22.00 3 0.00 21.50 21.50 Mean titration value : 21.50
******* THE FINAL QUANTITATIVE RESULTS *************** Density of acid in (gm/mL) is 1.31 molarity of acid is 5.38 Percentage concentration of acid is 40.06
The strength of sulphuric acid obtained in this study was about 40% w/w. It can be used in battery acid which needs only 30% grade and also can be further diluted for laboratory use (Table 2.2). According to Williams (2003), the protocol can be used in small to large scale production. The roaster used in this case is PFR.
125
4.5.2 Quality of produced sulphuric acid
Presence of soluble sulphate and sulphuric acid were confirmed. However, it seemed useful also to check the contamination of nitrogenous compounds in the products since they were among the raw materials.
4.5.2.1 Confirmatory test by using litmus paper
One mL of the sulphuric acid produced was transfered into the test tube, and then the litmus paper was wetted by distilled water then dropped into the solution, the color of the indicator turned to red showing acidic character.
4.5.2.2 Confirmatory test by using barium chloride solution
Sulphate ion presence was tested. One mL of sulphuric acid produced was mixed with little amount of barium chloride solution. The result obtained was white precipitate of
2- barium sulphate. This shows the presence of SO4 ion according to Equation 4.2.
BaCl2 + H2SO4 → BaSO4solid + HClaq……………………………………………………………..(4.2)
4.5.2.3 Confirmatory test by using barium nitrate solution
This is characteristic test of sulphuric acid and soluble sulphates where little amount of
Ba(NO3)2 was added into one mL of sulphuric acid produced. The white precipitation
(cloudiness) of barium sulphate formed according to Equation 2.5 confirmed the presence of sulphuric acid.
4.5.2.4 Confirmatory test of NO2
Little amount of acidified KMnO4 was poured into one mL of sulphuric acid produced,
The purple color of KMnO4 was not decolorised showing the absence of NO2.
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4.5.2.5 Confirmatory test of NO3
Few drops of FeSO4 solution was added into the little amount of sulphuric acid produced.
The brown ring was not seen which confirmed the absence of NO3 (Figure 4.9).
Figure 4.9: Presence and Absence of Brown Ring Test
4.6 Techno-economic analysis
Techno-economic analysis of producing sulphuric acid was done with aid of SuperPro
Designer simulator. The simulator provides all necessary steps required for performing techno-economic analysis such as development of PFD, performing material and energy balances, sizing of unit processes and operation and estimations of production cost and return of investment.
4.6.1 Process description
The process of producing sulphuric acid was developed based on Williams’ protocol, which utilised the basics of Lead Chamber Process. The Williams’ protocol can be used in small and large scale production of sulphuric acid (Sidana, 2016; Williams, 2003).The
PFD is depicted in Figure 4.10.
The process involved six major steps namely: size reduction; pyrite roasting; gas cleaning; formation of NO; conversion of NO to NO2; and formation of sulphuric acid.
127
Note that NO and NO2 are generated in the same reactor, it is firstly configured to produce NO and then NO2, the process is batch wise. The information about major equipment specification is attached in Appendix G.
4.6.1.1 Size reduction
This involves primary and secondary size reduction process. The pyrite ores of about 5 cm diameter are subjected to grinding machine for pulverisation to a coarse particle of about 1 mm in diameter.
4.6.1.2 Pyrite roasting
Pyrite roasting is the process of heating a pyrite into a temperature below its melting point in the presence of air. The pyrite is converted into solid oxides and gaseous sulphur dioxide at a temperature of 600° C -1000° C . Roasting in a fluid-bed furnace remains the core technology. Fluid-bed roasters are easy to operate and provide an excellent control of the calcine quality (Runkel and Sturm, 2009). Several reactions (Equations
2.12, 2.40 and 2.53) are involved in the roaster, however, hematite (Fe2O3) and SO2 are the most required products. Since all three reactions are strictly exothermic, a considerable amount of heat is generated during the roasting process in the fluidised bed reactor and which can be recovered by generating steam which is a potential source of electricity. The predominant reaction depends on a partial pressure of oxygen and the temperature range. The products gained from roasting process are calcine and roasting gas. The calcine consists of Fe2O3, Fe3O4, and FeSO4 and gangue material while the roasting gas has SO2, SO3, O2, N2 and H2O as steam. Table 4.14 shows currently installed fluidised bed roasters worldwide.
128
COMMINUTION PROCESS ROASTING AND CLASSIFICATION NO2 PRODUCTION
P y rit e o re C o p p e r S - 1 0 2
S - 1 0 6 H N O 3
A ir in le t P-3 / Gas Cyclone Pyrite roaster S - 1 0 3 P-5 / NO2 reactor G rin d e r S - 1 0 4 S - 1 0 1
Pyrite Cinder D ust particles
PRODUCTION OF H2SO4
NO
P-4 / Acid reactor W a t e r
H 2 S O 4
Figure 4.10: Proposed PFD for Production of Sulphuric Acid from Pyrite Ore
129
Table 4.14: Installed Fluidised Bed Roaster (Source: Jones, 2006)
Year Location Capacity 1983 Finland 420 t/d 2009 Mali 590 t/d 1996 Wengfu, China 600 t/d 2004 Turkey 630 t/d 1984 Spain 725 t/d 2007 Tongling, China 1130 t/d
4.6.1.3 Gas cleaning
This is the important stage in which the SO2 is cleaned for the aim of removing impurities such as dust particles. The roasting gases which mainly contain SO2, N2, O2 and dust or fine particles of pyrite are passed through dust chamber followed by electrical precipitator or centrifugal separator in order to remove dust or fine particles of ore. Gas cleaning is very serious in Contact Process or when having ore with toxics. The impurities must be removed otherwise catalyst loses its efficiency (catalyst poisoning).
4.6.1.4 Formation of NO
Production of NO takes place in the reactor according to Equation 2.13. This reaction appears only once during plant start-up because NO is regenerated from acid chamber. It will not be economical to continue to produce NO by this method while there is a possibility of recycling it.
4.6.1.5 Conversion of NO to NO2
The NO gas produced is passed through the air, thus the gas will be oxidised into NO2
(Equation 2.14).This stage is very important because the NO2 produced will act as the oxygen carrying catalyst for the conversion of SO2 to SO3 in an acid chamber. The main advantage is that, once NO2 is produced, it donates oxygen to become NO and NO takes
130
oxygen from the air to form NO2 (Equation 2.15).
4.6.1.6 Formation of sulphuric acid
This is the last step in which sulphuric acid is produced. In this reactor, steam is charged in to react with SO2 and NO2. Firstly, steam and SO2 will react to form sulphorous acid
(H2SO3), then NO2 will oxidize sulphorous acid to sulphuric acid along with evolution of
NO gas. The acid is then collected and stored into tanks (Equation 2.15).
4.6.2 Assumptions and limitations on process simulation and plant design
The following assumptions and limitations were made on the process of simulation:
1) According to the protocol of producing sulphuric acid by Lead Chamber Process,
nitric acid and copper are used only once during start up. It is difficult to
implement it into the model. The price equivalent to one batch of nitric acid and
copper was distributed over total amount of it used annually. As a result economic
calculation was not affected;
2) The reactor used by reacting nitric acid and copper, was also used for converting
NO to NO2;
3) The model did not consider the treatment of waste streams;
4) The plant operates 24 hours;
5) The price of all raw materials are based on actual price in Tanzania and
6) The prices of all equipments are based on online searching price.
4.6.3 Simulation results
The direct cost, indirect cost, direct fixed capital, working capital, total capital investment and operating cost were estimated by standard engineering cost estimation methods in evaluating process economics. The estimation of cost was done with aid of built in reports
131
in SuperPro Designer which utilise the standard instruction from Sinnot, 1993 and Peters and Timmerhaus, 1981. Table 4.15 shows the major finding from process simulation. The capital investment is the sum of fixed capital and working capital. The distribution of fixed capital is further shown by Table 4.16 and determination of capital investment is shown in Table 4.17.
Figure 4.11 shows the ingredients of operating cost. The major source is labour of about
41.5%. This is contributed by the model itself because it is set as batch process. It is likely to be reduced during practical work. The utility also is very low, which is unlikely due to operation of the roaster. However, high temperature roasted gas, can be used to produce steam which can generate electricity. Therefore, the increased cost of utility can be off set with potential additional of electricity as auxiliary revenue (cogeration).
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Table 4.15: Major Finding from Process Simulator
Category Value Units
TOTAL CAPITAL INVESTMENT 4,368,000 USD
CAPITAL INV. CHARGED TO THIS PROJECT 4,368,000 USD
OPERATING COST 4,383,000 USD/year
PRODUCTION RATE 1,198,761 kg/year of H2SO4
UNIT PRODUCTION COST 3.656 USD/kg of H2SO4
TOTAL REVENUES 5,994,000 USD/year
GROSS MARGIN 26.88 %
RETURN ON INVESTMENT 30.54 %
PAYBACK TIME 3.27 years
IRR AFTER TAXES 18.05% %
NPV (at 7.0 % interest) 3,503,000 USD
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Table 4.16: Fixed Capital Estimate Summary
Category EPC factor Cost [USD] A. TOTAL PLANT DIRECT COST (TPDC) (physical cost) 1. Equipment Purchase Cost (EPC) 557,000 2. Installation 0.60 497,000 3. Process Piping 0.35 195,000 4. Instrumentation 0.40 223,000 5. Insulation 0.03 17,000 6. Electricals 0.10 56,000 7. Buildings 0.45 251,000 8. Yard Improvement 0.15 84,000 9. Auxiliary Facilities 0.40 223,000 SUM 1 2.50 TPDC = EPC(1+ SUM 1) TPDC = 2,101,000
B. TOTAL PLANT INDIRECT COST (TPIC)
10. Engineering 0.25 525,000 11. Construction 0.35 735,000 SUM 2 0.60 TPIC = 1,261,000 TPC = TPDC(1+ SUM 2) C. TOTAL PLANT COST TPC = 3,362,000 (TPDC+TPIC)
12. Contractor's fee 0.05 168,000 13. Contingency 0.10 336,000 SUM 3 0.15 (12+13) = 504,000 DFC = TPC(1+ SUM 3) D. DIRECT FIXED CAPITAL (DFC) TPC+12+13 386,6000
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Table 4.17: Profitability Analysis
Category Cost [USD]
A. DIRECT FIXED CAPITAL 3,866,000
B. WORKING CAPITAL 308,000
C. STARTUP COST 193,000
D. UP-FRONT RandD 0
E. UP-FRONT ROYALTIES 0
F. TOTAL INVESTMENT (A+B+C+D+E) 4,368,000
G. INVESTMENT CHARGED TO THIS PROJECT 4,368,000
H. REVENUE STREAM FLOWRATES
kg/year of total flow (in H2SO4) 1,198,761
I. PRODUCTION (UNIT) COST
USD/kg of H2SO4 3.656
J. SELLING/PROCESSING PRICE
USD/kg of total flow (in H2SO4) 5
K. REVENUES (USD/year)
H2SO4 5,994 ,000
L. ANNUAL OPERATING COST 4,383 ,000
M. GROSS PROFIT (K-L) 1,611,000
N. TAXES (40 %) 644,000
O. NET PROFIT (M-N + Depreciation ) 1,334,000
135
0.29
6.23
Raw Materials 16.46 35.52 Labor-Dependent Equipment-Dependent Laboratory/QC/QA Utilities 41.5
Figure 4.11: Operating Cost
The overall process data are shown in Table 4.18. The residence time for one batch is 5.39 h, with raw crude pyrite of 200 kg and a product of 310.89 kg. After almost two hours, new batch start. For one day there were 13 batches and total of 4318 batches annually.
The plant operates in 330 days per year. Gantt chart (Figure 4.12) elaborates one batch cycle showing actual time and action on each equipment while Figure 4.13 shows actual equipment utilisation period and schedule.
Table 4.19 shows equipment capacity utilisation. In overall, Pyrite roaster has the highest combined utilization (68.38%) and therefore it is the limiting step of the entire flowsheet.
In other words, if one tries to increase batch throughput (material processed per batch) without installing extra equipment, Pyrite roaster will become the first throughput bottleneck. The bottleneck effect can be seen clearly in Figure 4.13 where there is no any gap from one batch to another in throughput analysis of Pyrite roaster.
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Table 4.18: Overall Process Data
Item Value Units
Annual Operating Time 7919.89 h
Annual Throughput 1198761.02 kg MP
Batch Throughput 277.62 kg MP
Plant Batch Time 5.39 h
Number of Batches Per Year 4318
MP = Main Product = Total Flow in H2SO4
Table 4.19: Equipment Capacity Utilisation
Capacity Equipment Combined Equipment Utilization [%] Uptime [%] Utilization [%]
NO2 reactor 25.96 110 28.55
Grinder 66.67 54.55 36.36
Pyrite roaster 68.38 100 68.38
Gas Cyclone 0 54.55 0
137
Time [h] 0 1 2 3 4 5 6
Total occ time Grinder Roaster Transfer in Charge React Vent Transfer out Cyclone Acid reactor Transfer in_1 Transfer in_2 Charge React
Vent NO2 reactor Charge_1 Charge_2 React Vent
Figure 4.12: Gantt Chart
138
Legend Batch # Color Code # 1 # 2 # 3 # 4 # 5 # 6 # 7 # 8 # 9 # 10 # 11 # 12 # 13
Grinder
Pyrite roaster
Equipment Gas Cyclone
Acid reactor
0
24 Time in h
Figure 4.13: Equipment Utilisation Chart
139
Table 4.20 provides information on raw material requirements for the entire flowsheet.
The quantities are displayed in kg/year, kg/batch, and kg/kg MP (main product). Around
277 kg of H2SO4 is produced per batch. Note the large amounts of N2 and O2 that are required per batch. Table 4.21 shows that the large amount of N2 and O2 will not affect overall cost analysis. Presence of of N2 and O2 is mostly by default in the simulaltor software. Almost major cost of raw material is from pyrite ore (Table 4.21). It means the cheaper availability of pyrite will bring significant reduction in operating cost.
Table 4.20: Overall Materials Balances
Raw Material kg/Year kg/Batch kg/kg MP
Nitrogen 611329.75 141.577 0.51
Oxygen 1547670.25 358.423 1.291
Water 863600 200 0.72
Nitric Acid 1122680 260 0.937
Pyrite Ore 863600 200 0.72
Copper 280670 65 0.234
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Table 4.21: Raw Materials Cost
Raw Unit Cost Annual Amount Cost
Material [USD] [kg] [USD/yr] %
Nitrogen 0 611329.75 0 0
Oxygen 0 1547670.25 0 0
Water 0.001 863600 432 0.03
Nitric Acid 0.001 1122680 1347 0.09
Pyrite Ore 1.8 863600 1554480 99.86
Copper 0.002 280670 449 0.03
Table 4.22 shows information on component balance and stream report for the entire flowsheet. The overall process is under atmospheric pressure except for steam inlet
(vapour) to sulphuric acid reactor. The component flow in each unit is shown clearly in both kg/batch and m3/batch. The additional information concerning component flowrates is shown the Appendix H.
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Table 4.22: Component Balance and Stream Report
STREAM NAME Air inlet Dust Particles H2SO4 Water HNO3 SOURCE INPUT P-3 P-4 INPUT INPUT
DESTINATION Pyrite Roaster OUTPUT OUTPUT Acid Reactor NO2 Reactor STREAM PROPERTIES TEMP ° C 25 25 25 110 25 PRES bar 1 1 1 1.5 1 DENSITY g/l 1.3 92.1 1000 0.8 1502.7 COMPONENT FLOWRATES (kg/Batch) Dust Particles 0 20 0 0 0 Nitric Acid 0 0 0 0 260 Nitrogen 141.5771 0.1808 0 0 0 Oxygen 358.4229 0.0548 0 0 0 Sulfuric Acid 0 0 95.0902 0 0 Water 0 0 182.5293 200 0 TOTAL (kg/batch) 500 20.2356 277.6195 200 260 TOTAL (m3/batch) 397.7228 0.2198 0.2776 235.7017 0.173
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4.6.4 Viability of the Project
Some commonly used techniques of economic evaluation and criteria used to judge economic performance are Net Present Value (NPV), Internal Rate of Return (IRR), Pay-
Back Period (PBP) and Cash Flow Analysis.
NPV is a sophisticated capital budgeting technique; found by subtracting a project’s initial investment from the present value of its cash inflows discounted at a rate equal to the firm’s cost of capital.
The NPV for this project is 3,503,000 USD which is greater than zero (NPV>0) i.e. NPV is positive: the investment would add value to the firm and therefore the project is economically viable and may be accepted and implemented. The bigger value of NPV also shows the magnitude of the project. The NPV value is not too high which is in alignment of method used ‘modified Lead Chamber Pocess’. The method is appropriate for small to medium scale of production of sulphuric acid.
IRR is the most sophisticated of the above metrics and often is used to analyse large, multi-year investments. The IRR for this project is 18.05%. The value guarantees that the firm will earn at least its required return. Such kind of outcome increases the market value of the firm and, therefore, the wealth of its owners. The IRR value is higher than discount rate in several banks in Tanzania which is around 16% reducing balance. That mean: banks are more likely to lend money if the IRR is higher than the bank’s interest rate.
The most accepted and recommended methods for profitability evaluation and the economic comparison of alternatives are NPV and IRR (Sinnott, 1993)
143
The PBP for this project is about three years. This shows that the investment can be quickly recovered and PBP is widely used to evaluate small projects. The risk exposure for investing sulphuric acid is low.
The flow of cash which is the life blood of any commercial organisation is depicted in
Figure 4.14. PBP is clearly shown and the performance of investment is shown for about
15 years and it is good and recommended for implementation. Appendix F shows additional information used in constructing cash flow curve.
2000 1500 1000 500 PBP 0 0 2 4 6 8 10 12 14 16 -500 -1000
-1500 Net Cash Flow [thousandUSD] -2000 -2500 Time [yrs]
Figure 4.14: Cash Flow Curve
4.6.5 Sensitivity Analysis
Figure 4.15 and Figure 4.16 shows the sensitivity analysis of IRR and unit production cost versus plant production scale of 1, 1.1, 1.2, 1.3 and 1.4 respectively. It is only feasible to plant production scale of 1.4, beyond that it needs redesign according to simulation results. This is aligned with the method used which is only feasible in small- scale production. The increase in annual plant capacity increases the IRR, which is good
144
sign (Figure 4.15) and decreases the unit production from 3.66 to 2.99 USD (Figure 4.16) although is not much significant beyond production scale of 1.2.
39.61 IRR IRR [%] 36.8 33.52 21.95 18.05
1 1.1 1.2 1.3 1.4 Plant production scale
Figure 4.15 IRR as Function of Plant Production Scale
3.7
3.6
3.5
3.4
3.3
3.2 Unit Unit productioncost
3.1
3
2.9 1 1.1 1.2 1.3 1.4 1.5 Production scale
Figure 4.16 Relationship Between Unit Production Cost and Plant Production Scale
145
4.7 Environmental aspects of sulphuric acid production
Production of sulphuric acid from pyrite is not an environmentally benign activity. There are potential environmental pollution from handling of raw materials, processing and product handling and by-products. The impact to environment can be severe when the feed materials contain hazardous substances such as arsenic, lead, cadmium and selenium.
4.7.1 Handling of raw materials
Pyrite should be covered during storage and transport to avoid dust. Depending on the climate, three problems can be aroused for uncovered pyrite:
i) Dust problems can be expected under dry conditions. A dust atmosphere,
especially inside the buildings can cause a fire or an explosion and the
favourable conditions;
ii) Water in contact with pyrite becomes acidic (AMD formation) under wet
conditions and
iii) With too high moisture content, the pyrite will give clogging problems in the
internal transport system at the plant.
4.7.2 Processing
Almost all steps in the process of producing sulphuric acid emit gases of varying content and volume. However, most of the gases can be collected and treated in the acid plant.
Weak gases and fugitive emission must be treated by alternative method.
In the pyrite roaster, uncontrolled pyrite roasting process can emit large quantities of particulate matters, trace elements and sulphur oxides which have adverse effects on human health. Suphur dioxide, the sulphates and sulphuric acid aerosols form in the atmosphere, can cause lung irritants and aggravate asthma (Mousavi, 2012).
146
For instances, the estimates of the magnitude of health risks and the influence of SO2 and secondary pollutants from all emission sources cause up to 50,000 premature deaths per year in the United States of America (USA) and Canada (USCOTA, 1984).
The solid byproduct known as pyrite cinder is generated during roasting process is considered as environmentally hazardous. The cinder is an ash residue which is very rich in hematite (Fe2O3) and magnetite (Fe3O4). Although it is hazardous, however, it has a known commercial value. It can be used as excipient in cement formulation or carbothermic reduced to sponge iron (Zhu et al., 2013; Oliveira et al., 2012).
For Lead Chamber Process, traces of NOx gases such as NO and NO2 are common. The
NOx gases are air pollutant and have deleterious health effects by causing asthma-like symptoms (Weinmayr, et al. (2010); Schwartz, (2004); Ao et al., (2003)).
Fugitive emission from furnaces and converters can cause health problems in the work place and situation is more serious in the presence of elevated level of toxic pollutants such as lead and arsenic. Employees are exposed to the highest concentrations of toxic elements because they work in enclosed areas.
4.7.3 Product handling
There is no air pollution problem connected with the storage, handling and shipping of the product because of the very low vapour pressure at normal temperature conditions.
However, concentrated sulphuric acid is extremely corrosive and can cause serious burns when not handled properly. This chemical is unique because it does not only cause chemical burns, but also secondary thermal burn as a result of dehydration. This
147
dangerous chemical is capable of corroding skin, paper, metals, and even stones in some cases. If sulphuric acid makes direct contact with the eyes, it can cause permanent blindness. If ingested, it may cause internal burns, irreversible organ damage, and possibly death (Riddick, 2014).
Exposure to sulphuric acid aerosols at high concentrations leads to severe eye and respiratory tract irritation and tissue damage. Consistent exposure to sulphuric acid aerosols, even at low concentrations, can cause a person’s teeth to erode (Sihto et al.,
2006; Fiedler et al., 2005; Sathiakumar et al., 1997).
4.7.4 Prerequisite environmental mitigation for production of sulphuric acid
The environmental effect for production of sulphuric acid can be modest due to technological and other pollution control because of mitigation features such as:
1) Closely monitoring of the process such as the roasting temperature to ensure
complete combustion;
2) Monitoring the emission of particulate matters, SO2, NO, NO2. Equipment for
monitoring gases are available and suitable for sulphuric acid plants and should be
installed on all plants;
3) Proper use of pyrite cinder and
4) Monitoring of the mist emission in the stack. At present there is no appropriate
equipment for monitoring however can be measured by controlled condensation
method. The sampling procedure and analysis are described in the standard
method EPA 8a and the German standard method VDI 2462 (Mertens et al.,
2015) and
5) When handling pure sulphuric acid in a laboratory or industrial setting, or when
using products that contain concentrated sulphuric acid, it’s important to prioritise
148
safety precautions. The following protective equipment should be worn when
using sulphuric acid:
Respirator;
Long rubber gloves;
Boots;
Industrial apron;
Chemical safety goggles;
Face shield.
It is also a good idea to have access to an eye-flush station if using sulphuric acid at your workplace. Another important consideration when handling this dangerous chemical is that it can react violently if it comes into contact with water.
149
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion
The extensive study of the chemical and roasting kinetics of pyrite ore from Merelani shows the following remarks:
1) The chemical analysis shows that S and Fe are predominant and the ore does not
contain toxic and hazardous substances such as arsenic, selenium and antimony in
a concentration that is serious hazard to ecosystem and human health;
2) All sulphur in the pyrite ore exists as pyrite and about 60% of ore w/w is pyrite
(FeS2) of cubic structure and high degree purity which might be serious AMD
pollutant. Special environmental mitigation is required;
3) The roasting reaction shows pseudo first order kinetic with respect to solid in
fluid-solid reaction with optimum roasting temperature of 750° C and the Ea
value of 15 kJ/mol was obtained which suggests that the roasting reaction is
controlled by diffusion of oxygen through the ash˗like layer to the reacting
surface.
4) The quality and quantity of pyrite in the ore and its kinetic informations show that
pyrite from Merelani can be potential for producing sulphuric acid.
In addition, the pyrite ore was converted into value added chemical, sulphuric acid which can not only provide the proper mitigation to the environment but also provide social- economic income to local people. The modified Lead Chamber Process with maximum acid grade capacity of 80% was used to convert the ore into sulphuric acid. The grade obtained was 40% and is sufficient for battery acid use.
150
From the techno-economic analysis of converting pyrite ore into value added chemical, sulphuric acid, it can be concluded that the process technology is economically viable and can be implemented in small-scale level. The high positive value of NPV (3,503,000 USD) and acceptable IRR range in the local region of 18.05% show that the project is attractive.
In addition, the process of producing sulphuric acid was still viable for plant production scale of 1, 1.1, 1.2, 1.3 and 1.4.
The key input to the knowledge of this study is that the kinetics of pyrite roasting reaction follows pseudo first order with respect to solid in fluid-solid reaction. This information is useful for scaling-up the process, design of roasters and performing economic evaluation.
5.2 Recommendations
The literature survey revealed that most attractive pyrite source in Tanzania, in terms of quality and quantity is from Samena, Geita region. It could be useful and interesting to be part of this work. However due to financial implication and time frame, it was not possible to be engaged. Due to the findings and conclusions made in this work, it is recommended to elucidate the chemical kinetics parameters of pyrite roasting from
Samena. So the current work will be platform of future further work in pyrite value addition projects.
151
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172
APPENDICES
Appendix A: Dissolution of Rock-ore Samples for AAS Analysis
This method is applicable to finely ground rock samples for the determination of fairly high concentrations of the metals mentioned. The quantity of sample to be weighed depends on the dilution factor.
ELEMENTS DETERMINED BY HA 01/ AAS Element Dilution factor = 1000 Dilution factor=100ppm Detection limit range = 0.01% (Detection limit range =1- 10 ppm) Dil. volume Sample weight (g) Dil. volume (g) (mL) Cd 0.25 250 1.0 1.0 Co 0.25 250 1.0 1.0 Cu 0.25 250 1.0 1.0 Fe 0.25 250 1.0 1.0 Ni 0.25 250 1.0 1.0 Pb 0.25 250 1.0 1.0 Zn 0.25 250 1.0 1.0 Mn 0.25 250 1.0 1.0
Procedures for dissolution of the samples:
1. Weigh the appropriate amount of sample into a 150 mL beaker.
2. Add about 10 mL of HNO3.
3. Cover with a watch glass and heat on a sand bath or a hot plate until brown fumes
are no longer produced.
4. Rinse the watch glasses into the beaker with distilled water with the aid of a wash
bottle and remove them.
5. Evaporate the solution to dryness, but avoid too much baking.
6. Take the beakers off the hot plate and allow to cool.
7. Add 5 mL HCl, mix with the residue and evaporate again.
8. Cool again.
9. Add 5 mL HCl, and about 40 mL of water.
10. Allow the precipitate to settle.
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11. Filter into the volumetric flask. Wash the filter paper well with distilled water.
12. Fill the volumetric flask to the mark.
13. Determine the concentration of the elements.
Suggested standard solutions: Elements Standard 1 Standard 2 Standard 3 Fe, Ni, Co, Pb 5 ppm 10 ppm 20 ppm Zn, Cd 1 2.5 5 Mn 5 10 20 Reference: the method is adopted from a procedure used at the Geological Survey of Finland
174
Appendix B: Elements Quantified by XRF (Source: Wilson, 2018)
175
Appendix C: Sulphur Content (%)
500 CP Run 2 PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S
4/1/2017 2 68638.81 5302.85 7909.43 634.87 4174.93 921.11 227624.6 4/1/2017 4 82376.05 5433.59 10181.16 678.72 5373.36 885.62 182733.6 4/1/2017 6 152677 6088.34 16571.25 815.64 11839.44 649.16 7202.97 4/1/2017 8 147075.6 6129.68 16901.27 830.53 12565.4 656.68 3345.91
500 CP Run 2 PART B S +/- Cl Cl +/ - K K +/ - Ca Ca +/- Fe Fe +/ -
1702.72 ND 890.67 40.93 752.11 49.48 298245.7 2064.49 1442.99 ND 865.37 40.06 1772.99 50.22 343804.9 2425.12 200 993.06 67.91 384.25 32.02 3273.78 47.51 619094.8 4743.25 146.41 1181.67 69.5 374.83 32.7 2830.7 43.82 629743.6 4826.19
500 CP Run 2 PART C Ni Ni +/- Cu Cu +/- Zn Zn +/ - As As +/- Unit 888.8 16.28 273.77 62.84 25.81 6.5 179.63 4.62 % 931.96 17.62 482.26 66.86 22.6 7.01 164.59 5.69 % 1302.63 31.04 433.57 122.46 ND ND % 1272.47 31.01 ND ND 50.79 15.91 %
176
500 CP RUN 1 PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S 3/17/2017 2 72454.12 4836.82 8362.78 600.22 4895.2 895.29 246386.8 3/17/2017 4 78664.41 5143.76 8459.54 633.28 7185.94 927.9 218470 3/17/2017 6 94798.31 5318.36 9786.99 660.67 7447.87 878.75 168663.4 3/17/2017 8 168635.3 6016.87 18693.14 849.83 13478.55 681.71 6878.57
500 CP RUN 1 PART B S +/- Cl Cl +/ - K K +/ - Ca Ca +/- Fe Fe +/ - 1690.2 ND 1117.35 40.11 705.94 50.17 273707.4 1751.67 1594.61 ND 894.38 39.81 815.16 48.45 339441.8 2262.15 1314.72 ND 787.81 38.04 1822.04 48.63 405720.8 2758.68 197.15 1228.49 71.62 377.96 32.42 3538.23 50.06 643217.7 4864.55
500 CP RUN 1 PART C Ni Ni +/- Cu Cu +/- Zn Zn +/ - As As +/- Unit 768.1 13.96 276.43 57.94 34.87 6.05 140.43 3.92 % 966.6 17.47 448.42 67.54 39.04 7.23 154.07 5.39 % 1062.33 19.76 283.61 78.64 35.69 8.2 106 7.24 % 1472.06 33.38 ND ND ND %
177
573 CP PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S 3/17/2017 2 81035.99 5117.67 10522.48 663.44 5947.77 898.81 207151.1 3/17/2017 4 133650.1 5831.54 17268.49 828.91 12144.75 857.17 93137.2 3/17/2017 6 171619.1 6254.97 18801.26 882.97 15096.64 742.09 9071.54 3/17/2017 8 186717 6528.54 22094.98 977.34 16964.05 797.37 4586.98
573 CP PART B S +/- Cl Cl +/ - K K +/ - Ca Ca +/- Fe Fe +/ - 1523.32 ND 904.75 39.33 925.78 47.35 338588.1 2255.18 886.69 ND 721.56 38.15 2902.18 50.85 533421.4 3867.89 232.45 1458.76 79.3 574.67 36.34 3531.9 51.95 681074.1 5286.03 179.25 1542.24 81.71 617.84 39.1 4469.74 61.89 731849.7 5955.42
573 CP PART C Ni Ni +/- Cu Cu +/- Zn Zn +/ - As As +/- Unit 912.95 16.87 243.64 66.32 29.17 6.93 155.18 5.42 % 1347.23 25.98 471.94 102.75 62.86 11.04 ND % 1517.21 35.31 ND ND ND % 1556.6 37.65 517.39 143.36 ND 90.49 19 %
178
750 CP PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S 3/13/2017 2 11324.37 1812.14 10653.77 546.56 4172.06 301.63 92571.03 3/13/2017 4 9592.66 1435.08 9202.74 426.05 4622.68 225.32 41629.73 3/13/2017 6 10068.1 1347.7 9481.68 384.46 4932.35 182.65 6002.95 3/13/2017 8 11168.16 1373.35 9794.02 382.54 5158.78 179.48 2346.27
750 CP PART B PART B S +/- Cl Cl +/- K K +/ - Ca Ca +/- Fe Fe +/ - 868.2 128.63 48.62 422.54 202.14 2163.65 349.49 666998.6 6932.15 < LOD: 485.54 94.87 39.34 404.72 1401.08 268.62 691729.8 7448.67 < LOD: 151.39 141.99 37.14 366.05 1568.36 285.57 700473.5 7459.11 < LOD: 99.91 140.84 36.83 280.94 1335.85 274 710786.1 7765
750 CP PART C Ni Ni +/- Cu Cu +/- Zn Zn +/ - As As +/- Unit < LOD: 2841.1 164.93 850.53 86.5 118.18 38.73 48.04 % < LOD: 2772.02 173.97 436.26 74.38 102.3 39.43 48.36 % < LOD: 2796.83 176.5 390.46 73.21 61.33 37.34 51.23 % < LOD: 2703.28 179.47 633.57 86.38 87.29 40.2 60.72 %
179
573 FP PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S 3/14/2017 2 6681.391 2255.443 9028.504 749.751 2816.069 475.667 174978.8 3/14/2017 4 8372.051 1490.487 9230.975 453.952 4870.626 245.485 46794.95 3/14/2017 6 7942.953 1306.408 8433.986 383.843 5175.933 203.798 21207.07 3/14/2017 8 7636.398 1303.216 8367.054 365.564 5423.16 186.473 2681.978
573 FP PART B S +/- Cl Cl +/ - K K +/ - Ca Ca +/- Fe Fe +/ - 1864.918 0 69.669 0 330.85 2639.572 370.555 503747.2 4471.782 532.903 91.364 42.027 329.35 192.478 1789.152 314.185 690811 7310.013 306.171 127.173 37.949 0 287.486 1814.769 301.822 699473.9 7496.197 107.602 157.077 37.894 0 233.664 1441.077 290.774 723282.2 7844.871
573 FP PART C Ni Ni +/- Cu Cu +/- Zn Zn +/- As As +/- Unit 2468.625 122.52 647.33 60.544 80.485 27.22 0 21.402 % 2947.994 175.286 599.342 80.656 76.59 37.887 0 31.732 % 2950.383 180.656 524.01 79.468 90.5 39.672 0 32.05 % 2810.413 181.301 506.51 80.9 103.136 41.38 0 34.247 %
180
927 FP PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S 3/17/2017 0 67336.12 4812.01 9097.28 614.74 4868.7 902.01 250963 4/1/2017 2 131863.8 5851.85 15909.98 793.83 10769.21 764.74 62789.14 4/1/2017 4 143636.9 6172.96 17831.51 856.7 11921.29 657.74 3844.92 4/1/2017 6 146165.3 6049.13 17961.94 840.55 12106.48 645.07 3432.09 4/1/2017 8 165712.1 6267.27 17961.68 864.26 15143.08 722.99 3775.7
927 FP PART B S +/- Cl Cl +/ - K K +/ - Ca Ca +/- Fe Fe +/ - 1708.71 ND 1090.66 40 706.57 51.05 278631.4 1772.33 673.78 ND 687.81 36.11 3221.08 50.24 511759.3 3815.86 156.48 1268.48 71.85 537.64 35 3777.93 52.57 617122.2 4770.38 146.21 1158.91 68.18 580.18 34.23 3467.72 49.02 601981 4610.12 156.31 1097.27 69.97 499.79 34.82 3671.15 52.15 631725.3 4979.58
927 FP PART C Ni Ni +/- Cu Cu +/- Zn Zn +/ - As As +/- Unit 817.6 14.52 338.25 57.8 40.46 6.19 142.14 4.01 % 1173.36 24.77 344.63 95.16 ND 53.18 11.14 % 1304.64 30.73 ND ND ND % 1299.48 30.37 ND ND ND % 1269.62 31.12 ND ND ND %
181
1000 CP PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S 3/17/2017 0 72454.12 4836.82 8362.78 600.22 4895.2 895.29 246386.8 4/1/2017 2 131387.2 6082.28 14164.41 790.23 9617.6 759.24 56712.77 4/1/2017 4 151475.3 6387.84 15845.66 845.33 13174.7 701.92 3714.58 4/1/2017 6 145141.9 6233.71 12774.54 764.06 12197.6 665.11 2981.79 4/1/2017 8 144593.2 5983.12 14821.36 777.92 12254.34 644.55 3281.93
1000 CP PART B S +/- Cl Cl +/ - K K +/ - Ca Ca +/- Fe Fe +/ - 1690.2 ND 1117.35 40.11 705.94 50.17 273707.4 1751.67 649.19 ND 487.35 36.75 2479.93 45.53 545054.7 4158.73 160.56 1176.84 74.33 324.76 35.24 2976.77 47.29 638297.5 5080.56 144.38 1119.79 71.6 337.35 34.55 2832.32 45.07 593745.7 4705.61 144.27 1116.33 67.96 330.35 32.12 2983.1 44.86 604371 4581.72
1000 CP PART C Ni Ni +/- Cu Cu +/- Zn Zn +/ - As As +/- Unit 768.1 13.96 276.43 57.94 34.87 6.05 140.43 3.92 % 1188.88 25.79 ND ND ND % 1002.5 28.91 ND ND ND % 1104.41 28.56 ND ND ND % 1007.44 27.27 ND ND ND %
182
750 VFP PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S 3/8/2017 2 115502.3 5516.73 15602.42 781.58 8159.35 930.04 180007 3/8/2017 4 153341.1 5922.07 21442.01 904.3 13266.13 829.45 65672.28 3/8/2017 6 170216.7 6179.05 24494.97 978.42 14076.73 780.26 27593.24 3/8/2017 8 177802.1 6178.89 23435.98 957.89 15149.83 739.38 5936.46
750 VFP PART B S +/- Cl Cl +/ - K K +/ - Ca Ca +/- Fe Fe +/ - 1436.76 ND 1343.09 46.07 3570.21 63.76 374587.5 2648.55 707.26 516.95 99.14 1087.72 41.76 5101.16 67.1 531429.2 3995.86 418.7 1146.31 86.16 1065.66 41.98 5657.89 71.03 613175.8 4765.89 190.84 1651.03 79.28 954.76 40 5754.87 70.42 633238.7 4946.98
750 VFP PART C Ni Ni +/- Cu Cu +/- Zn Zn +/ - As As +/- Unit 1052.65 19.3 860.73 73.97 45.1 8.07 152.64 6.3 % 1409.34 27.91 864.16 100.93 54.79 11.19 39.94 11.65 % 1569.13 32.97 923.16 124.01 ND 45.89 14.69 % 1523.01 33.54 908.54 131.47 ND ND %
183
927 CP PART A Time Date (min) Al Al +/- Si Si +/- P P +/- S 3/17/2017 0 67336.12 4812.01 9097.28 614.74 4868.7 902.01 250963 3/17/2017 2 127277.6 5926.28 16623.9 835.35 12208.32 875.93 93994.94 3/17/2017 4 184730.7 6417.4 19356.88 908.58 14305.63 728.85 4551.37 3/17/2017 6 175459.6 6143.09 17716.42 844.13 12724.2 668.36 3781.31 3/17/2017 8 174681.2 6118.4 18844.73 864.38 14651.08 703.04 3916.77
927 CP PART B S +/- Cl Cl +/ - K K +/ - Ca Ca +/- Fe Fe +/ - 1708.71 ND 1090.66 40 706.57 51.05 278631.4 1772.33 904.44 ND 690.2 39.26 2821.68 51.23 544079.2 3982.2 175.15 1714.93 82.27 428.1 36.22 3429.67 51.97 720548.3 5756.83 155.48 1338.48 72.72 367.17 33.25 3407.49 49.61 663062.7 5110.42 157.52 1470.96 74.29 399.54 33.29 3337.56 48.88 672068.2 5139.37
927 CP PART C Ni Ni +/- Cu Cu +/- Zn Zn +/ - As As +/- Unit 817.6 14.52 338.25 57.8 40.46 6.19 142.14 4.01 % 1387.65 27.08 640.8 106.05 46.83 11.18 ND % 1628.05 37.98 689.31 145.05 ND 82.7 18.71 % 1305.97 32.57 ND ND ND % 1502.3 34.81 ND ND ND %
184
Appendix D: Kinetic Tool clear clc disp(' .STARTING FOR KINETIC TOOL DEVELOPED BY HIJI, M (UDOM 2018)….
') disp('Results are also saved as Kinetic1.txt') fileID = fopen('Kinetic1.txt','w');
P=input('Enter the number of pairs of data: '); for i=1:P
fprintf(' For Sample ID number : %d\n',i)
ID{i}=input('Enter Sample ID name : ','s');
filename{i}=input('enter excel file for t data: ','s');
filenameC{i}=input('enter excel file for C data: ','s'); end clc disp(' .... RESULTS FOR KINETIC TOOL DEVELOPED BY HIJI, M (UDOM 2018).....
') fprintf(fileID,'%20s\r\n','.... RESULTS FOR KINETIC TOOL DEVELOPED BY HIJI, M
(UDOM 2018)..... '); disp(' ')
185
fprintf(fileID,' '); for i=1:P
[t]=importdata(filename{i});
[C]=importdata(filenameC{i});
delta=t(2)-t(1);
n=length(t);
str{i}=['the uploaded data for sample ID ',ID{i}];
disp(str{i})
fprintf(fileID,'the uploaded data for sample ID : %4s\r\n',ID{i});
disp(' ')
fprintf(fileID,' ');
disp(' t w')
fprintf(fileID,'%4s %6s\r\n','t','W');
for j=1:n
str{j}=[' ',num2str(t(j)),' ',num2str(C(j))];
disp(str{j})
fprintf(fileID,'%6.4f %8.4f\r\n',t(j),C(j));
disp(' ')
fprintf(fileID,' ');
186
end for j=1:1:n-1
f(j)=[C(j+1)-C(j)]/(2*delta);
y(j)=log(-f(j));
x(j)=log(C(j)); end p=polyfit(x,y,1); order=p(1,1); rate_constant=p(1,2);
disp(' ') yfit = polyval(p,x); subplot(2,P,i) plot(x,y,'+',x,yfit,':') xlabel('ln[W]') ylabel('ln (-f)') title('Differential method') legend(ID{i}) shg;
187
disp(' ')
%Using polyval saves you from typing the fit equation yourself, which in this case looks like: yfit = p(1) * x + p(2);
yresid = y - yfit; %Compute the residual values as a vector of signed numbers:
SSresid = sum(yresid.^2); %Square the residuals and total them to obtain the residual sum of squares:
SStotal = (length(y)-1) * var(y);%Compute the total sum of squares of y by multiplying the variance of y by the number of observations minus 1:
rsqd = 1 - SSresid/SStotal;%Compute R2 using the formula given in the introduction of this topic:
N=p(1,1);
n=round(N);
if n==1;
y=log(C);
[t;C;y];
%m=(slope)
K=polyfit(t,y,1);
slope=K(1,1);
yfit=polyval(K,t);
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yresid = y - yfit;
SSresid = sum(yresid.^2);
SStotal = (length(y)-1) * var(y);
rsqi = 1 - SSresid/SStotal;
subplot(2,P,i+P)
plot(t,y,'+',t,yfit,':');
xlabel('Time, t')
ylabel('log[C]')
title('Integral method')
legend(ID{i})
else
y=1./C;
[t;C;y];
%m=(slope,)
K=polyfit(t,y,1);
slope=K(1,1);
yfit=polyval(K,t);
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yresid = y - yfit;
SSresid = sum(yresid.^2);
SStotal = (length(y)-1) * var(y);
rsqi = 1 - SSresid/SStotal;
subplot(2,P,i+P)
plot(t,y,'+',t,yfit,':');
xlabel('Time,t')
ylabel('1/[C]')
ylabel('log[C]')
title('Integral method')
legend(ID{i})
end
clear x;
clear y;
K=abs(slope);
str3{i}=[' ',ID{i},' ',num2str(N),' ',num2str(n),' 'num2str(rsqi)]; end disp(' ...... INTEGRAL METHOD CONFIRMATION...... ') fprintf(fileID,'%20s\r\n','.... INTEGRAL METHOD CONFIRMATION..... ');
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disp('Sample ID Estimated Order Confirmed Order R-Square(integral)') fprintf(fileID,'%20s\r\n','SampleID Estimated Order Confirmed Order R-
Square(integral)'); for i=1:P
str3{i};
disp(str3{i})
fprintf(fileID,'%6s %8.4f %10.4f %12.4f\r\n',ID{i},N,n,rsqi);
disp(' ')
fprintf(fileID,' '); end fclose(fileID);
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Appendix E: Titration Tool clc clear disp(' .... STARTING FOR TITRATION TOOL DEVELOPED BY HIJI, M (UDOM
2018)..... ') disp('It is a Matlab program for quantitatively analysis for production of sulphuric acid'); disp('Results are also saved as Titration.txt') fileID = fopen('Titration.txt','w');
% 1.Density of acid in(g/mL) disp('Determine Density of acid in(g/mL)')
V1= input('Enter volume one of your choice: ');
V2= input('Enter volume two of your choice: ');
V3= input('Enter volume three of your choice: ');
M1=input('Enter weight in grams for volume one of your product: ');
M2=input('Enter weight in grams for volume two of your product: ');
M3=input ('enter weight in gram for volume three of your product: '); d=((M1/V1)+(M2/V2)+ (M3/V3));
D=d/3; clc
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% 2.Molarity of acid disp('Determine Molarity of acid')
Da=input('Enter dilution factor :');
Mb=0.5;
Va=20;
%Vb=0; % Inial value for i=1:3
fprintf('For titration round %d :',i)
Vi(i)=input('Enter initial burrete value :');
Vf(i)=input('Enter final burrete value :');
Vd(i)=Vf(i)-Vi(i); end
Vb=(Vd(1)+Vd(2)+Vd(3))/3;
Ma=(Mb*Vb*Da)/(2*Va);
P.acid=(98*Ma*100)/(D*1000); clc disp('...... THE UPLOADED DATA...... ')
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fprintf('W1 is : %4.3f g and V1 is: %4.2f mL\n',M1,V1) fprintf('W2 is : %4.3f g and V2 is: %4.2f mL\n',M2,V2) fprintf('W3 is : %4.3f g and V3 is: %4.2f mL\n',M3,V3) fprintf('dilution factor used is %4.3f\n ',Da)
fprintf(fileID,'%20s\r\n','.....THE UPLOADED DATA...... '); fprintf(fileID,'W1 is : %4.3f\r g and V1 in mL is: %4.2f\r\n',M1,V1); fprintf(fileID,'W2 is : %4.3f\r g and V2 in mL is: %4.2f\r\n',M2,V2); fprintf(fileID,'W3 is : %4.3f\r g and V3 in mL is: %4.2f\r\n',M3,V3); fprintf(fileID,'dilution factor used is %4.3f\r\n ',Da); disp('****************TITRATION RESULTS********************') disp('------') disp('TITRATION : 1 2 3') disp('------') fprintf('FINAL VOL : %5.2f %6.2f %5.2f\n',Vf(1),Vf(2),Vf(3)) fprintf('INITIAL VOL: %5.2f %6.2f %5.2f\n',Vi(1),Vi(2),Vi(3)) fprintf('VOLUME USED: %5.2f %6.2f %5.2f\n',Vd(1),Vd(2),Vd(3)) fprintf('The average titration volume (Vb) is : %4.2f mL\n',Vb) disp('------')
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fprintf(fileID,'%20s\r\n','****************TITRATION RESULTS***********'); fprintf(fileID,'%20s\r\n','------'); fprintf(fileID,'%20s\r\n','TITRATION : 1 2 3'); fprintf(fileID,'%20s\r\n','------'); fprintf(fileID,'FINAL VOL : %5.2f\r %6.2f\r %5.2f\r\n',Vf(1),Vf(2),Vf(3)); fprintf(fileID,'INITIAL VOL: %5.2f\r %6.2f\r %5.2f\r\n',Vi(1),Vi(2),Vi(3)); fprintf(fileID,'VOLUME USED: %5.2f\r %6.2f\r %5.2f\r\n',Vd(1),Vd(2),Vd(3)); fprintf(fileID,'The average titration volume (Vb in mL ) is : %4.2f\r\n',Vb); fprintf(fileID,'%20s\r\n','------');
disp('******* THE FINAL QUANTITATIVE RESULTS ***************') fprintf('Density of acid in (g/mL) is %4.2f\n ',D) fprintf('molarity of acid is %4.2f\n ',Ma) fprintf('Percentage concentration of acid is %4.2f\n ',P.acid) fprintf(fileID,'%25s\r\n','**** THE FINAL QUANTITATIVE RESULTS *****'); fprintf(fileID,'Density of acid in (g/mL) is %4.2f\r\n ',D); fprintf(fileID,'molarity of acid is %4.2f\r\n ',Ma); fprintf(fileID,'Percentage concentration of acid is %4.2f\r\n ',P.acid); fclose(fileID);
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Appendix F: Cash Flow Analysis (Thousand USD)
YR CAPITAL DEBT SALES OPERAT. GROSS LOAN DEPREC. TAXABLE TAXES NET NET INVESTM FINANCE COST PROFIT PAYMENT INCOME PROFIT CASH FLOW 1 -1160 0 0 0 0 0 0 0 0 0 -1160 2 -1547 0 0 0 0 0 0 0 0 0 -1547 3 -1468 0 2997 3455 -458 0 367 0 0 -458 -1926 4 0 0 5994 4370 1624 0 367 1257 503 1121 1121 5 0 0 5994 4370 1624 0 367 1257 503 1121 1121 6 0 0 5994 4370 1624 0 367 1257 503 1121 1121 7 0 0 5994 4370 1624 0 367 1257 503 1121 1121 8 0 0 5994 4370 1624 0 367 1257 503 1121 1121 9 0 0 5994 4370 1624 0 367 1257 503 1121 1121 10 0 0 5994 4370 1624 0 367 1257 503 1121 1121 11 0 0 5994 4370 1624 0 367 1257 503 1121 1121 12 0 0 5994 4370 1624 0 367 1257 503 1121 1121 13 0 0 5994 4370 1624 0 0 1624 650 974 974 14 0 0 5994 4370 1624 0 0 1624 650 974 974 15 501 0 5994 4370 1624 0 0 1624 650 974 1476 IRR BEFORE TAXES 26.797 INTEREST 7.00% 9.00% 11.00% IRR AFTER TAXES 18.047 NPV 3503 2564 1795 Depreciation Method: Straight-Line DFC Salvage Factor: 0.05
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Appendix G: Major Equipment Specification and Fob Cost (2018 prices)
Quantity/ Equipment Description Unit Cost Cost Stand-by [USD] [USD] 1/0 Pyrite roaster Stirred Jacket Vessel 400000 400,000 Volume = 324.98 L Diameter = 0.55 m
1/0 Gas Cyclone Cyclone 3000 3,000 Body Diameter = 0.11 m Overall Height = 0.43 m Rated Throughput = 47.64 m3/h
1/0 Acid reactor Stirred Jacket Vessel 30000 30,000 Volume = 293.03 L Diameter = 0.53 m
1/0 NO2 reactor Stirred Jacket Vessel 8000 8,000 Volume = 771.80 L Diameter = 0.73 m
1/0 Grinder Grinder 5000 5,000 Throughput = 300.00 kg/h
Cost of Unlisted Equipment 111,000 TOTAL EQUIPMENT PURCHASE COST 557,000
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Appendix H: Component Flowrates (kg/ Batch)
COMPONENT INITIAL INPUT OUTPUT FINAL Copper 0 65 0.325 0 copper nitrate 0 0 190.9103 0 dp 0 0 20 0 gm 0 20 0 0 Hematite 0 0 77.5096 0 LE 0 60 60 0 Nitric Acid 0 260 3.4172 0 Nitrogen 1.2575 141.5771 141.8947 0.94 Nitrogen Dioxid 0 0 49.0563 0 Nitrogen Monoxi 0 0 29.0952 0 Oxygen 0.3818 358.4229 273.267 0.2773 pyrite 0 120 3.6 0 Sulfur Dioxide 0 0 62.034 0 Sulfuric Acid 0 0 95.0902 0 Water 0 200 219.2131 0 TOTAL 1.6393 1225 1225.4125 1.2173
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Appendix I: CS30000G Furnace and Pyrite Sample in Crucibles
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