Mineral Dissolution Kinetics and Modeling

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MINERAL DISSOLUTION KINETICS AND MODELING

A thesis submitted

BAHAUDDIN ZAKARIYA UNIVERSTY, MULTAN

Fulfillment of requirements for the degree

DOCTOR OF PHILOSOPHY

CHEMISTRY

NADEEM RAZA

INSTITUTE OF CHEMICAL SCIENCES, BAHAUDDIN ZAKARIYA UNIVERSITY MULTAN, PAKISTAN 2015

DEDICATED TO

My parents, brothers, wife

Children

DECLARATION

I hereby declare that the work described in this thesis has been carried out under the supervision of Prof. Dr. Zafar Iqbal Zafar and Associate Prof. Dr. M. Najam-ul-Haq at Institute of Chemical

Sciences, Bahauddin Zakariya University, Multan, for the degree of Doctor of Philosophy in

Chemistry.

I also hereby declare that the content of this thesis have neither been submitted elsewhere nor are being concurrently submitted for any other degree.

I further declare that the findings presented in this thesis are the result of my own research and the research studies of other investigators have been duly acknowledged.

Nadeem Raza

PhD scholar

APPROVAL CERTIFICATE

The thesis entitled “MINERAL DISSOLUTION KINETICS AND MODELING” submitted to

Bahauddin Zakariya University, Multan, Pakistan by Mr. Nadeem Raza in the partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry is hereby approved.

Supervisors/Internal Examiners:

1. Prof. Dr. Zafar Iqbal Zafar Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, Pakistan

2. Dr. Muhammad Najam-ul-Haq Associate Professor Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, Pakistan

External Examiner:

Prof. Dr. Imtiaz Ahmad Institute of Chemical Sciences, University of Peshawar, Peshawar, Pakistan

Director:

Prof. Dr. Tariq Mahmood Ansari Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, Pakistan

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ACKNOWLEDGEMENTS

All praises be to ALMIGHTY ALLAH, who bestowed man with intelligence, knowledge and sight to observe and ponder. Peace and blessing of ALLAH be upon the Holy Prophet MUHAMMAD (SAW), who exhorted his followers to seek knowledge from cradle to grave.

I feel great pleasure in expressing my sincere gratitude and profound thanks to my research supervisors,

Prof. Dr. Zafar Iqbal Zafar, and Dr. M. Najam-ul-Haq, Institute of Chemical Sciences, Bahauddin Zakariya

University, Multan for their kind guidance and full cooperation throughout the research work. I am also thankful to Dr. R.V. Kumar, Department of Materials Science and Metallurgy, University of Cambridge,

United Kingdom, for his guidance and thought provoking discussions during my six months study visit sponsored by Higher Education Commission (HEC), Pakistan. I am thankful to HEC for providing a golden opportunity to continue research activities in brilliant environment of University of Cambridge. I am also grateful to Prof. Dr. Tariq Mahmood Ansari, Director, Institute of Chemical Sciences, Bahauddin Zakariya

University, Multan, for providing the research facilities.

I feel greatly obliged to Prof. Dr. Muhammad Aslam Malana, Dr. Farzana, Dr. Muhammad Naeem Ashiq and Dr. Mazhar, Dr. Ishfaq, Dr. Zahid Shafique, Institute of Chemical Sciences, Bahauddin Zakariya

University, Multan, for their help and valuable guidance during the research work. I am also thankful to all the other kind faculty members of Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, for their continuous support. I am thankful to the Institute of Chemical Sciences, Bahauddin Zakariya

University, Multan, PCSIR laboratory Lahore, Mineralogy Department Abbottabad for providing me the research facilities, Pak-Arab Fertilizer and NFC Institute of Chemical Engineering and Technological

Centre, Khanewal Road, Multan, for help in sample crushing, grinding and sieving.

I would like to express my gratitude for Dr. Najeeb-U-llah, U.E.T. Peshawar, for his kind counceling. I am thankful to Asif Chemicals, Hattar industrial estate, for their help in collection of magnesite ore samples.

I am highly obliged to my elder brother Mohsin Raza for his moral support and encouragement during the higher education. I feel myself lucky to find sincere friends, like Dr. Sajid, Asad Raza,

Dr. Muhammad Shoaib, Dr. Imran, Dr. Sajjad, Faiz Rasool, Abdul Ghafoor, Dr. Zahoor Ahmed,

Muhammad Akram, Zulfiqar Ali, Hassan Saeed, Nadeem Hussain, Abdul Ghaffar, Tanvir Alam,

Muhammad Nawaz, Muhammad Sheraz, Dr. Muhammad Safdar Sami, Abdullah Sham, Samar

Abass, Muhammad Sajid, Yasir Lodhi, Professor Akram Mirani, Ch. Javaid Akhtar, Ch.

Muhammad Akbar, Shakil Ur Rahman, Muhammad Ali Saleh, Sana Ijaz, Javeria Batool, Javeria

Zahra and Muhammad Mukhtiar, who were eager to listen about my success.

Many thanks to the staff of laboratories of Inorganic, Organic, Physical and Analytical Chemistry,

Shafi Dogar, Ajmal, Riaz, Ishfaq, Yousaf, Ashraf, Akram and Javaid, who arranged all the requirements whenever I needed.

I also acknowledge the prayers and well wishes of my elders, Ghulam Murtaza, Sheikh Tahir

Mahmood and youngers, Kalim Raza, Muhammad Zubair, Sheikh Usman and Waseem Raza who encouraged me all the time during the research work. I would like to extend my thanks to Husnain

Raza, Muhammad Danish, for their help in writing thesis and collecting the magnesite samples. I am grateful to my college Principal and all the colleagues whose moral support encouraged me to complete the research work. I also gratefully acknowledge the pleasant feelings of my family, brothers, sisters and my children, Muhammad Shahmeer Raza, Muhammad Shayan Raza and

Eshal Fatima, who also felt my absence during the research work, as I could not spare time for them. Concluding this work, I feel pleasure that I may spend much time with them in future.

Nadeem Raza

PhD scholar

CONTENT Page No.

DEDICATION i

DECLARATION ii

APPROVAL CERTIFICATE iii

ACKNOWLEDGEMENTS iv

CONTENTS vi

LIST OF TABLES x

LIST OF FIGURES xi

EXPLANATION OF SYMBOLS xiii

ABSTRACT xiv

CHAPTER NO 1 1

1. Introduction 2

1.1 Economic evaluations 6

1.1.1 Overview 6

1.1.2 Global distribution of magnesite 7

1.1.3 Applications of magnesite and its compounds 12

1.2 Concentration techniques 13

1.2.1 Multi gravity separation 13

1.2.2 Heavy media separation 13

1.2.3Jigging 15

1.2.4 Tabling 15

1.2.5 Flotation 16

1.2.6 Hand sorting/picking 17

1.2.7 Magnetic separation 18

1.2.7.1 Dry magnetic separation 19

1.2.7.2 Impact of magnetic field strength 19

1.2.7.3 Influence of drum speed 20

1.2.8 Electrostatic separation 20

1.2.9 Bioleaching 21

1.2.9.1 Effect of temperature 22

1.2.9.2 Effect of shaking 22

1.2.10 Chemical beneficiation 23

1.3 Dissolution 23

1.4 Methods of mining 27

1.5 Reaction kinetics modeling 32

1.6 Instrumental techniques 38

1.6.1 Atomic absorption spectrophotometer 38

1.6.1.1 Working of AAS 38

1.6.1.2 Light source 38

1.6.1.3 Atomization chamber 39

1.6.1.4 Measuring system 40

1.6.1.4.1 Absorption mode 40

1.6.1.4.2 Emission mode 40

1.6.1.5 Chemical interferences 42

1.6.2 Scanning electron microscope 43

1.6.2.1 Principle of SEM 43

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1.6.2.2 Preparation of samples 44

1.6.2.3 Working of SEM 45

1.6.2.4 Energy dispersive X-ray analysis 49

1.7 Chemistry of leaching process 49

CHAPTER 2 53

2 Experimental procedures 54

2.1Collection of magnesite samples 54

2.2 Size reduction and sieving analysis 58

2.3 Preparation of magnesite sample for analysis 61

2.4 Detection measurements and analytical procedures 65

2.5 Reagents and chemicals used 66

2.6 Equipment used 67

2.7 Sample characterization 68

2.7.1 AAS 68

2.7.2 Scanning electron microscope 71

2.7.3 X-ray Diffractometer 75

CHAPTER 3 76

3 Results and Discussion 77

3.1 Influence of reaction time on leaching of magnesite at different temperatures 77

3.2 Impact of reaction temperature on leaching of magnesite with formic acid 83

3.3 Impact of formic acid concentration on dissolution of magnesite 85

3.4 Influence of liquid/solid ratio on dissolution of magnesite 92

3.5 Influence of particle size on conversion of magnesite 100

viii

3.6 Kinetic studies 106

3.6.1 Leaching kinetics of magnesite with formic acid 106

CHAPTER 4 117

4. Recovery of consumed formic acid 118

4.1 Economy of dissolution process 120

Conclusions 123

References 125

Appendix A 142

LIST OF TABLES

Page No.

Table 1.1 Comparison of Kraubath and Veitsch type magnesite 4

Table 1.1.2.1 World magnesite production 8

Table 1.1.2.2 Magnesite production in Pakistan on yearly basis 10

Table 1.1.3.1 Generalized industrial applications of magnesium and its compounds 12

Table 2.2.1 Mesh size, average size, weight fraction and weight percent 59

Table 2.3.1 The magnesite rock samples analysis 63

Table 2.5.1 A list of reagents and chemicals used 66

Table 2.6.1 A list of equipment used in the analysis of magnesite 67

Table 2.7.1 Chemical composition of natural Magnesite 69

Table 2.7.2.1 Elemental analysis of natural magnesite ore 74

Table 3.1.1 Influence of reaction time on leaching of magnesite at different temperature 78

Table 3.3.1 Impact of formic acid concentration on % conversion of magnesite 86

Table 3.4.1 Impact of liquid/solid ratio on % conversion of leaching of magnesite 93

Table 3.5.1 Influence of particle size of magnesite on % conversion of natural magnesite 101

LIST OF FIGURES

Page No.

Fig 1.4.1Open pit mining of magnesite in Pakistan 28

Fig 1.4.2 Mining scheme 29

Fig 1.4.3 Narrow Vein mining of magnesite rock in Pakistan 30

Fig.1.6.1.1 AAS (A-1800) Hitachi used for quantitative analysis of magnesite 41

Fig 1.6.2.1 Sputtering coater (EMITECH- K550) 45

Fig 1.6.2.2 Schematic of a typical scanning electron microscope 47

Fig. 1.6.2.3 SEM JEOL 5800LV used for the analysis of magnesite samples 48

Fig. 2.1.1 Magnesite rock deposit in Abbottabad 55

Fig. 2.1.2 Magnesite rock deposits of Amirabad Sarhad Province 56

Fig 2.1.3 Weathered magnesite rock Abbottabad mines 57

Fig. 2.2.1 Average particle size and weight % of magnesite sample 60

Fig. 2.3.1 Influence of average particle size of magnesite on loss of ignition 64

Fig.2.7.1 Calibration curve for magnesium concentration versus absorbance 70

Fig.2.7.2.1 SEM images of natural magnesite sample on different magnifications 72

Fig. 2.7.2.2 EDX spectrum of the sample 73

Fig. 2.7.3.1 XRD pattern of the natural magnesite sample 75

Fig. 3.1.1 Impact of reaction time on % conversion of natural magnesite at 45 oC 79

Fig.3.1.2 Influence of time of reaction on % conversion of natural magnesite at 55 oC 80

Fig. 3.1.3 Impact of time of reaction on % conversion of magnesite at 65 oC 81

Fig. 3.1.4 Impact of time of reaction on % conversion of magnesite at 75 oC 82

Fig. 3.2.1 Effect of temperature on leaching of magnesite ore 84

Fig. 3.3.1 Effect of 2 % formic acid concentration on % conversion of magnesite 87

Fig. 3.3.2 Impact of 4 % formic acid concentration on % conversion of magnesite 88

Fig. 3.3.3 Effect of 6 % formic acid concentration on % conversion of magnesite 89

Fig. 3.3.4 Influence of 8 % formic acid concentration on % conversion of magnesite 90

Fig. 3.3.5 Influence of 10 % formic acid concentration on % conversion of magnesite 91

Fig.3.4.1 Effect of 6 mL/g liquid/solid ratio on % conversion of magnesite 94

Fig. 3.4.2 Effect of 8 mL/g liquid/solid ratio on % conversion of magnesite 95

Fig. 3.4.3 Impact of 10 mL/g liquid/solid ratio on % conversion of magnesite 96

Fig. 3.4.4 Effect of 12 mL/g liquid/solid ratio on % conversion of magnesite 97

Fig. 3.4.5 Impact of 14 mL/g liquid/solid ratio on % conversion of magnesite 98

Fig. 3.4.6 Impact of 16 mL/g liquid/solid ratio on % conversion of magnesite 99

Fig. 3.5.1 Effect of particle size 500-707 µm on % conversion of magnesite 102

Fig. 3.5.2 Impact of particle size 250-354 µm on % conversion of magnesite 103

Fig. 3.5.3 Influence of particle size 177-210 µm on % conversion of magnesite 104

Fig. 3.5.4 Effect of particle size 125-177 µm on % conversion of magnesite 105

Fig. 3.6.1.1 Time versus 1-(1-x)1/3 at 45 oC 108

Fig. 3.6.1.2 Time versus 1-(1-x)1/3 at 55 oC 109

Fig. 3.6.1.3 Time versus 1-(1-x)1/3 at 65 oC 110

Fig. 3.6.1.4 Time versus 1-(1-x)1/3 at 75 oC 111

Fig. 3.6.1.5 Time versus 1-(1-x)1/3 at 45-75 oC 112

Fig. 3.6.1.6 Arrhenius plot of ln k vs 1/T 114

Fig. 3.6.1.7 Scatter diagram 116

Fig. 4.1.1 Schematic diagram suggested for the recovery of spent acid. 122

xii

Explanation of symbols

-1 Ea activation energy (J mol )

-1 Ao pre-exponential factor (min )

2+ cxp or X exp dissolved fraction of Mg

2+ cal or Xcal predicted fraction of Mg

C acid concentration (Mol/L)

L/S liquid/solid ratio (cm3g-1)

D average diameter of particle (µm) t reaction time (min)

T reaction temperature (K)

SS stirring speed (min-1)

ER relative mean square of errors

N number of experimental data k reaction rate constant (min-1)

Mtpy million tons per year

Pa per annum mt metric ton cps cycles per second

MGS multi gravity separator

EDX energy dispersive X-ray analysis or electron diffraction X- ray

SEM Scanning electron microscope

AAS Atomic absorption spectrophotometer

XRD X ray diffractometer

xiii

ABSTRACT

The dissolution kinetics of natural magnesite is carried out using formic acid as a leaching agent.

The effect of various reaction parameters such as temperature, acidic solution concentration, particle size and liquid to solid ratio is studied regarding the leaching kinetics of natural magnesite.

The results indicate that formic acid can be used to leach the magnesium contained in naturally occurring magnesite rock as it removes the magnesium content of the rock from other impurities.

In order to elucidate the leaching kinetics of magnesite rock with formic acid, the influence of reaction temperatures has been investigated using the known particle fractions of the magnesite samples, formic acid concentration and liquid-solid ratio. The rate of leaching of magnesite increases with an increase in reaction temperature. A kinetic model is suggested to describe

1/3 1 42078/ RT 1 (1 x)  59.41  10 e t . the leaching process of magnesite by the analysis of kinetic data. The rate curves for the leaching of magnesite are assessed to check the validity of shrinking core models for the heterogeneous systems. The kinetic data is examined by graphical and statistical methods. The findings show that the dissolution process of magnesite with formic acid is controlled by the chemical reaction at the liquid-solid interface. The apparent activation energy of leaching process of magnesite with formic acid is found to be 42.08 kJ mol-1 over the reaction temperature range of 318 to 348 K. Moreover, this energy of activation is in close agreement with the similar works of fluid-solid reaction systems. In order to find the applicability of suggested kinetic model, the correlation between experimental conversion values and the values calculated from suggested empirical equation is determined by constructing scatter diagram. The correlation coefficient of this agreement also reveals that the workability of kinetic model is good.

xiv

Chapter

1. INTRODUCTION

A naturally occurring solid substance (element or compound) in the earth crust represented by a chemical formula, having inorganic nature, abiogenic origin and ordered atomic structure is referred as a mineral. The aggregates of minerals or non-minerals without a specific chemical composition are named as rocks and the rocks containing sufficient amounts of economically extractable elements involving metals are called as ores. These ores from various locations may exhibit varied quantities of metallic and non-metallic impurities. The costs of extraction of a desired metallic component from an ore depend on the concentration of the metal and its chemical nature. An efficient extraction of a metal demands an ore with its high percentage.

The most important ores of metals include carbonates, silicates, oxides and sulfides. These deposits of metals deliver limited amounts of metals and are considered as non-renewable.

Today many metals are obtained by recycling of metal scraps.

Minerals are essential ingredients to supply food and feed for mankind and animals and there is no substitute for mineral rocks as raw materials in the production of a number of important mineral products in the world. As the world population continues to increase, so does the demand of such mineral products.

Due to the presence of a number of impurities depending on the type, location and nature, the mineral rocks from these sources cannot be directly used for the production of valuable mineral products unless certain technologies such as crushing, grinding, sieving, dissolution and leaching are not applied. The mineral contents of the indigenous rocks vary from deposit to deposit and area to area having wide diversity in composition.

Generally, it is regarded that Pakistan is endowed with certain extensive geological potential.

As compared to the other well developed countries possessing good mineral endowment,

Pakistan has not so far been able to give promotion to its growth or products and alleviate poverty by increasing and using its natural mineral resources up to its maximum possible levels.

The mineral resources like Rock salt (NaCl), Limestone (CaCO3), Barites (BaSO4), Bauxite

(Al(OH)3), Magnesite (MgCO3), Bentonite (Al2O3 4SiO2 H2O), Celestine (SrSO4), China clay

(Al2O3 2SiO2 2H2O), Chromite (FeCr2O4), Coal (C137H97O9NS), Dolomite(CaMg(CO3)2,

Feldspar(KAlSi3O8), Fluorite (CaF2), Gemstones (Be3Al2SiO6), Gypsum (CaSO4.2H2O), Silica sand (SiO2), Chalcopyrite (CuFeS2) and many more are being produced only with its short and limited supplies. Interest of the government is growing day by day, however, the contribution of the mineral exploitation in Gross Development Product of Pakistan is only 0.4% of the whole.

In comparison to the other countries having same sort of geological endowment, these could produce valuable growth and development in local economics. If such sort of occurrences contribute in the country’s potential up to 25 %, the mineral sector with available capital and good investment has the tendency to contribute in annual resources and foreign exchange within the range of 2-3 % of Gross Development or $1.5-2.0 billions. This will stimulate secondary and tertiary economic activity resulting into promotion of growth, providing sources of earnings and development at community level particularly in far furlong and remote areas of the country.

During recent years mineral sector of Pakistan has gone through substantial transformations.

Vigorous efforts have been made in the right direction, which have started producing fruitful results regarding exploitation of mineral resources. Therefore, most of the indigenous minerals data have shown profound increase in mineralogical production.

Magnesium occurs naturally in many minerals however, brucite, magnesite, carnalite, dolomite and olivine are considered as more important for commercial benefits. Sea water, well and lake brines and bitterns also produce magnesium and its compounds. The raw materials for manufacturing of magnesium and its compounds include magnesite ore. Magnesite exists in two physical forms: Cryptocrystalline or amorphous magnesite and the Macro crystalline

3 magnesite. Kraubath type or microcrystalline also called as bone magnesite [1] is mostly found in contact with or close to serpentinized ultramafic rocks. The deposits of macro crystalline or

Veitsch type magnesite have syngenetic origin and are found in regions of orogenic activities.

In Khuzdar area of Pakistan most of the deposits of magnesite are of amorphous type while economic deposits of Veitsch type are found in early Cambrian dolomitic limestone of

Abbottabad [2,3].

Table 1.1 Comparison of Kraubath and Veitsch type Magnesite after Jensen and Bateman, 1981

[4]

Kraubath-type Magnesite Veitsch type Magnesite

Compact having conchoidal properties Aggregates of granules

Without cleavage Have rhombohedral cleavage

Porcelain in shape Crystalline in shape

Small sized, veins filling of ultramafic rocks Large size, bedded deposits, lens shaped

Found with opal, chalcedony, chlorite, Associated with Siderite, pyrite, calcite

Serpentinized Harzburgite Host rocks are dolomite and lime stone

In Pakistan, occurs in Khuzdar district Abbottabad

The magnesite [5] is found in five forms: a replacement mineral in carbonate rocks, alternated products in ultramafic rocks (major portion of igneous rocks consists of one or more dark colored ferromagnesian mineral), a sedimentary rock, a vein-filling material and as nodules found in the environment of lacustrine (lake). The deposits of replacement-type magnesite contain rich fluids of magnesium entering limestone via openings to produce both dolomite and magnesite. The deposits of alteration-type are produced by the action of CO2 rich waters on magnesium rich serpentinites (the rock made by the alteration of iron silicate and magnesium minerals). The resulting magnesium carbonate may be found purely. Commonly,

4 the deposits of sediments exist as thin layers of varied magnesium carbonate concentrations.

The magnesite deposits of lacustrine contain nodules of cryptocrystalline magnesium carbonate which came into being in an environment of the lake. The occurrences of both sedimentary magnesium carbonate and vein fillings are rarely mined on industrial levels. The formation of magnesite [5] in such deposits can be explained by the following equations (1.1.-1.5):

Antigorite quartz Talc

Mg 3 Si2O5 (OH )4  2SiO2  Mg 3 Si4O10 (OH )2  H 2O (1.1)

Antigorite Magnesite + Quartz

Mg Si O (OH )  3CO  3MgCO  2SiO  2H O 3 2 5 4 2 3 2 2 (1.2)

Antigorite

2Mg 3 Si2O5 (OH )4  3CO2  Mg 3 Si4O10 (OH )2  3MgCO3  3H 2O (1.3)

Olivine Magnesite + quartz

Mg2Si O4 2CO2  2MgCO3  SiO2  (1.4)

Talc Magnesite + quartz

Mg Si O (OH )  3CO  3MgCO  4SiO  H O 3 4 10 2 2 3 2 2 (1.5)

The only use for magnesium metal at the higher level is in production of alloys of aluminium, which makes approximately 50 % of the total magnesium metal utilization [6]. In order to produce high-strength and corrosion-resistant alloys, magnesium metal is added to aluminium.

About 20 % of magnesium is used in castings and wrought products (machinery, tools and other consumer products such as mag wheels for cars).

Prospects for supply and trade cannot reasonably be addressed without some estimation of demand in any commodity business. It is expected that global magnesium consumption may 5 rise at an overall rate of about 6 % per annum up to 2012 and can result in a market of more than 1.1 million tons per year (Mtpy) [6,7]. The global primary magnesium demand grew 14

% in 2013, to 792,000 mt (metric ton) from 693,000 mt in 2012. It is the only largest use; it is also hoped that die cast alloys for the automotive industry may expose the fastest rate of about

10 % pa, which is greatly underpinned by the production of Chinese vehicles. It is also expected that the usage and consumption of Mg based cast alloys may grow strongly in communications, computers and consumer electronics, applied for the desulphurization of steel, and for injection molded housings.

The major sources of magnesium all over the world include dolomite and magnesite minerals along with recycled magnesium [6]. In Pakistan, both forms of magnesite (macro crystalline and cryptocrystalline) are abundantly found in Khuzdar areas of Baluchistan province and

Abbottabad areas of Khyber Pakhtunkhwa province [2,3]. The estimated deposits of magnesite in Pakistan are around 12 million tones [8] with an average of 45 % MgO.

1.1 Economical evaluations

1.1.1 Overview

Minerals are essential ingredients to supply food and feed for mankind & animals [9] and are considered as one of the principal natural resources necessary for the economical developments of a country. It is not the reason of supernumerary for mineral rocks as the material in its raw form is used in the production of number of important mineral products in the world. The increasing world population necessitates the rising demand of such mineral commodities.

During the mineralogical formations, various kinds of metallic or non-metallic foreign particles may be incorporated and may become the part of minerals. The mineral content of indigenous rocks varies from deposit to deposit and area to area having wide diversity in composition.

Therefore, the mineral rocks from these sources cannot be directly applied to produce valued mineral products prior to crushing, grinding, sieving, dissolution and leaching. In order to fulfill

6 the demands of mineral commodities, Pakistan has to import huge quantities of mineral based chemicals and products from different countries of the world.

The extensive geological potential both for metallic and non-metallic minerals is present in

Pakistan, but the optimal exploitation of these mineral resources has not been achieved on account of various problems. Therefore, it has not yet been able to promote growth and alleviate poverty. From the mineral resources of Pakistan, only limited amounts of industrial minerals are being produced. It is not substantially contributing in the gross national products.

The most important minerals of magnesium include magnesite, dolomite, brucite, carnalite, serpentine and olivine. In order to overcome the demand of magnesium and its compounds, seawater, lake brines and bitterns are being used. But these sources of magnesium require significant amount of energy consumption and also requires development in inexpensive technologies. Magnesite ore is considered as the basic material in its raw form for the production of magnesium along with its compounds.

1.1.2 Global distribution of magnesite

The major sources of magnesium all over the world include dolomite and magnesite minerals along with recycled magnesium. The estimation in case of the world economic reserves of magnesite is about 8.60 billion tons [10]. The reserves of magnesite are widely distributed all over the world but its main reserves are located in China, Russia, Korea, Brazil and Turkey

[11]. Russia was the top most in magnesite production till 1985 with America second and China third. But now China has become the top most magnesite producing country. The most of USA magnesite reserves were consumed before 1990, that's why presently, it is not considered as top magnesite producing country. World magnesite production is given in Table 1.1.2.1.

Table 1.1.2.1 World Magnesite Production [12]

Country 2009 2010 2011 2012 2013 Australia 344,000 275,000 640,000 300,000 450,000 Brazil 409,909 757,063 476,805 480,000 480,000 Canada e, 4 140,000 150,000 150,000 150,000 r 150,000 China 13,000,000 14,000,000 19,000,000 16,000,000 17,000,000 Greece 380,834 396,000 541,813 351,266 r, 3 350,000 Guatemala 17,247 - 311 27,132 r, 3 17196 3 India e 253,000 r 301,000 r 236,000 r 224,000 r 213,000 Iran 130,575 126,702 172,697 r 170,000 r 170,000 Korea, North e 150,000 150,000 254,000 r 178,000 r 250,000 Pakistan 2639 r 5159 r 4908 r 54,44 r, 3 5176 3 Poland 47,000 63,000 75,000 84,000 84,000 Russia e 1,000,000 12,00,000 1200,000 1300,000 1300,000 Slovakia 807,000 r 800,000 800,000 r, e 600,000 700,000 South Africa 47,600 27,700 31,900 31,000 31,000 Spain 163,930 195,893 239,000 r 275,000 r 280,000 Turkey 861,180 2316,763 2588,276 r 23,00,000 2500,000 United States W W W W W Zimbabwe 449 - - - - Total 18,300,000 21,200,000 r 27,300,000 23,300,000 24,800,000

e Estimated. r Revised, Withheld to avoid disclosing company proprietary data.

1World total and estimated data. 2Figures represent crude salable magnesite.3Reported figure.

4Magnesite dolomite and brucite.

Recently, Pakistan is included as a magnesite producing country on the world map of magnesite producers. The extensive magnesite exploitation in Pakistan is not only fulfilling the local market demand but also it has made possible to export magnesite to China. In order to meet the domestic demands for chemical, metallurgical and other industrial applications more serious efforts are required towards exploitation of magnesite. The magnesite production in Pakistan on yearly basis is shown in Table 1.1.2.2.

Table 1.1.2.2 Magnesite Production in Pakistan on Yearly Basis [5,12]

Year Production (Metric tons)

2003 2645

2004 6074

2005 3029

2006 1884

2007 1400

2008 1700

2009 2639

2010 5159

2011 4908

2012 7544

2013 5176

1.1.3 Applications of magnesium and its compounds

There are plethora of possible applications of magnesium and its compounds. Roughly 50 % of total consumption of magnesium metal is in aluminium alloying and results in high-strength, corrosion-resistant alloys. Castings and wrought products which are consisted of machinery and tools being used and other consumer products such as mag wheels for cars accounts for 20

% of magnesium. The efforts are being made to reduce the weight of vehicles to minimize fuel consumption and harmful emissions. From the current market demand of magnesium and its compounds, it can be expected that the usage of magnesium cast alloys in different fields like computers, communications and consumer electronics and its major usage for injection molded housings and for the purpose of desulphurization of steel will grow strongly. Magnesium compounds find varied applications in the production of fluxes, animal feed, pharmaceuticals

[13], high impact resistance portable computer casings and parts [14], cement, paper and chemicals. The widely applied compounds of magnesium are caustic calcined magnesia (used in environmental applications, agricultural supplements and chemical industry) and refractory magnesia (dead burned and fused used for refractory products used by the steel, cement and glass industries). The insulating properties of magnesium carbonate are used in emergency flares, tiles to cover the space shuttles and firebricks because of high melting temperature of magnesium and its light weight nature. A few of the applications of magnesium and its compounds are given in Table 1.1.3.1.

Table 1.1.3.1 Generalized Industrial Applications of Magnesium and its Compounds [12-14]

Magnesium Applications component of aluminium die casting production of Titanium to remove sulfur in steel and iron as aerospace construction metal Automotive industry in engine, frames, paddle shifters, mag wheels As a metal Electronics (cameras, tablet computers, laptops, mobile phones Flash photography, flares, pyrotechnics, firework sparklers Grignard reagent Nodular graphite production in cast iron In Roofing, dry cell battery walls and printing industry, beverage cans, sports equipment, archery bows In ribbon form useful in purification of solvents MgO in refractory bricks, agricultural chemical, construction industries, electrical insulator in fire resistant cables, pharmaceutical preparations

Mg(OH)2 as milk of magnesia used in antacids, cathartic and laxatives

Mg SO4 used in paper industry Magnesium phosphate used in production of fireproof wood Magnesium hexaflorosilicate used in textile as mothproofing Magnesium sulfate, borate and salicylate used as antiseptics As mineral MgSO4 hepta hydrated as soluble fertilizer

MgCl2 , Mg gluconate, malate, orotate, citrate, glycinate as magnesium supplement Magnesium stearate used to prevent sticking of tablets to machines during compression

MgCO3 used in gymnastic apparatus

MgCl2 used for dust control, soli stabilization, wind erosion, in hydrogen storage Magnesium formate in agricultural applications and used in analysis of organic compounds Mg peroxide used as a bleach for dyes and silk

1.2 Concentration techniques

In any naturally occurring significant magnesite deposit, the magnesite mostly does not occur pure enough to be applied directly. A variety of gangue minerals i.e., carbonates, oxides, and silicates may be present in the magnesite ore deposits. The major use of magnesite in the local market is in the production of refractory bricks. The impurities contained in magnesite rocks may reduce the refractoriness [15]. The challenge in magnesite industrial sector is the reduction of impurities so that these rocks may be used for the production of magnesium and its compounds. Different techniques are used to minimize impurities in magnesite raw material and to concentrate it [16,17]. The most adequate method or technique to concentrate magnesite ore is selected on the basis of various parameters. The effective parameters to select an efficient technique to concentrate magnesite ore are associated with ore mineralogy and type of impurities [18-20]. In commercial applications, the concentration or enrichment of minerals from impurities is achieved by taking a benefit of the variation in characteristics of minerals and impurities to be isolated. These sort of characteristics may involve color (optical sorting), density (gravity separation), magnetic or electric (magnetic and electrostatic separation), and physicochemical processes (flotation separation). More efficient techniques applied to concentrate magnesite ore and to reduce the contents of impurities include heavy medium separation, hand sorting, gravity separation, flotation, magnetic separation and chemical beneficiation.

1.2.1 Multi gravity separation

This technique is preferred in the situations where two minerals to be separated from each other possess differences in specific gravity. The success of separation also depends on the ore particle size along with the density of minerals. This technique is adequate to treat the particles having maximum size up to 0.5 mm. The gravity separation is considered as a primary method to concentrate iron, tungsten, tin and coal ores. The gravity separation is controlled by two

13 competing forces, i.e. gravity (dependent on specific gravity) and resistance to movement

(usually water drag).

In this technique, multi gravity separator (MGS) is required to concentrate base metals, scavenging of precious metals from tailings or slimes streams, pre concentrating industrial minerals (Barytes, Chromites) and treatment of alluvial ores [21-23]. A little tapered open- ended drum of MGS revolves in the clockwise direction and is shaken in a sinusoidal form in an axial direction. Generally, the parameters that affect the efficiency of separation of multi gravity separator are the drum speed (100 to 300 rpm), shake amplitude (10 to 20 mm), tilt angle (0 to 90), amount of wash water (0 to 10 liters per minute), feed pulp density (10 to 50

% solids by weight) and shake frequency (4.0 to 5.7cycles per second) [24-26]. The wash water and slurry are introduced onto the inner surface of the drum through perforated rings positioned near the open end of the drum. The high centrifugal forces and shearing effect [27] of the shake cause the dense particles to migrate through the slurry film and to form a semisolid layer opposite to the wall of drum of multi gravity separator [21]. The dense layer discharges into the concentrate launder with the aid of scrapers positioned at the open end of the drum [28], whereas the light particles are taken by the flowing wash water into the tailing launder located at the back end of the drum. The shake amplitude, shake frequency and rotational velocity of the drum are the most important variables of multi gravity separator [21,27,21]. A rise in amplitude and shake frequency increases shearing action on the particles, which cause significant effects on the separation efficiency. The small shake amplitude generally requires high shake frequency and vice versa. The concentrate grade increases by increase in shake amplitude while the recovery is reduced. Fine and/or low-density minerals require a small tilt angle while coarse and/or high-density minerals necessitate a larger tilt angle. Drum speeds of

160 to 300 rpm along with the drum surface of 6.5 to 24 g are generally varied according to the nature of the tested material. The operation of the MGS is effected by the speed of drum in two

14 different ways. The increased drum speed of MGS not only increases the flow rate of mineral slurry towards the rear end of drum but it increases the inertial mass of mineral particles too and reinforces them to attach with the drum wall and make a solid layer [21]. The techniques which apply gravity separation principle include heavy media separation, jigging and tabling.

1.2.2 Heavy media separation

The differences in the specific gravities of gangue and ore make the foundation to isolate gangue particles from the ore. In this technique media slurry having specific gravity is produced by using a mixture of fine media materials (magnesite or ferrosilicon) suspended in water. This will permit low density materials to float and materials with high density to sink. The specific gravity of media slurry usually ranges from 1.45 to 2.5. In heavy media separations, ore particle size usually ranges from 10 mesh to 8 inch and sometimes low to 28 mesh because this affects the separation size.

1.2.3 Jigging

This technique separates particles even with close specific gravities while with large specific gravity variations, its efficiency improves. For varied objectives and considerable efficiencies, many kinds of jigs have been manufactured including the oldest one (HATZ jig). The new jigs form a constant fast pulsion followed by a slower constant suction which facilitates maximum dilation across the bed during pulsion and minimum compaction during suction.

1.2.4 Tabling

This technique is based on the differences in specific gravity where liquid films in laminar flow hold velocity that is not same in all depths of the film. The internal frictions offered by one layer upon the other causes no flow at the bottom but maximum at or very near to the top. In this way lighter particles are washed off and the heavier particles gather and are occasionally removed (panning of gold).

1.2.5 Flotation

In order to concentrate high grained minerals, one of the most widely used extractive methods is considered as flotation. By this method, different minerals can be selectively separated e.g.

Pb-Zn-Cu floatation. It takes an advantage of variations in physiochemical surface characteristics (wettability may be a natural property or altered artificially) of minerals. The froth flotation involves attachment of specific mineral particles to air bubbles which are taken by the water in the froth. A complete separation of mineral particles through flotation comprises of three steps: a) Liberation b) Establishing hydrophobic film on minerals that are not naturally hydrophobic. c) Promote bubble formation.

The minerals can be divided into two categories on the basis of degree of polarity: Polar minerals (sulfides < sulfates < carbonates < phosphates < oxides < hydroxides < silicates) and non-polar minerals (coal, oil, molybdenite, diamond, talc and gold).

In this technique, the hydrophilicity or hydrophobicity of surfaces of mineral particles are varied. By this way, mineral particles which are suspended in water can adhere to the bubbles of air or remain in the pulp while passing through a flotation cell. The froth is formed by the movement of air bubbles to the higher surface of pulp and is skimmed off along with the attached minerals having hydrophobic properties. Tailings which contain the minerals with hydrophilic nature can be isolated from lower part of the cell. In order to achieve high grade of magnesium carbonate concentrate, flotation is the preferred technique used for the separation of fine magnesite particles [29,30]. The dissolution of a mineral has vital role in controlling the surface charge, solution composition and floatability [31]. On the basis of flotation response, magnesite may be further classified into salt type minerals and can be floated with anionic type collector. The impurities as noticed in magnesite samples are floated during reverse flotation

16 with anionic and cationic collectors [32]. The most crucial parameter in anionic flotation is to regulate pH. Each mineral has a specific pH above which it does not float and is termed as the critical pH which depends on the concentration of the collector used. The controlling of pH is achieved by the addition of some alkali. Sodium carbonate is generally used on account of its increase in pH and properties of precipitation [31,33]. The separation of magnesium carbonate from quartz, silicates and different iron compounds is done readily by the process of flotation because the surface properties of these compounds are different. However, simple flotation process cannot be used to separate magnesite from dolomite (CaMg(CO3)2) because they have similar surface properties and same crystal structure. Moreover, both possess sparingly soluble nature in aqueous solutions. In such cases reverse flotation is considered as the preferred option to isolate magnesite [32]. In reverse flotation, Aslani et al., [32] used Armoflot 17, Armoflot

18 (Akzo chemicals, USA) and white oil as foaming agents and collected in the cells of flotation (Denver flotation machine). They described that the wastes attached to the bubbles are taken to the upper layer of the cell. The removal of the formed bubbles on the pulp surface is done till growth of the bubbles come to an end and the color is changed [34,35].

1.2.6 Hand sorting/picking

Totting or hand sorting technique involves the manual removal of materials direct from the hand-sorting line and is helpful in concentration of valuable materials. An efficient hand- sorting requires the removal of fine fraction (usually -40 mm size). Typically, hand sorting is performed on a small pile of mineral kept on the surface of floor for the purpose of sorting flourished by manual methods. On bulk levels totting is achieved by using mechanical shovels, or other plants to rake/shift through the piled material to expose stuffs for the isolation by hand.

In Evoia Island, Greece, hand-sorting of coarse magnesite, after prior removal of the -40 mm fraction was achieved by magnesite processing plant and resulted in the formation of significant stockpiled waste material with a relatively high content in magnesite. These rejected fine ore

17 particles can be successfully treated by hand sorting. In a comparative study of application of concentrating techniques (sorting (in the form of hand-sorting), gravity separation with heavy liquids, and magnetic separation) to recover a commercial magnesite concentrate [36], it was observed that totting and gravity separation can be applied successfully on the tailings of hand- sorting with considerable recovery rate.

1.2.7 Magnetic separation

The process of separation of magnetically susceptible materials from the non-magnetic substances in a mixture using magnetic force produced by magnets is called magnetic separation. The fall of feed particles in a particular size spectrum (0.1 to 1 millimeter) determines the success of magnetic separation processes. Generally, strongly magnetic minerals such as magnetite and franklinite are separated from gangue materials by magnetic separators with low-intensity while high-intensity separators are preferably used to separate iron containing manganese, titanium, tungsten ores, magnesium ores and iron containing silicates. High intensity induced roll magnetic separator can be employed to separate magnetic impurities from the magnesite rocks. An efficient separation necessitates the feed material to be dry, free flowing and free from slimes. The controlling parameters such as magnetic field strength and drum speed significantly affect the efficiency of the separation process. The magnesite rocks usually contain iron, silica and calcium impurities. In the situations when goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) (weakly magnetic minerals) are contained in dolomitic portion while magnesite portion contains hematite and magnetite (strongly magnetic mineral), neither flotation nor gravity separation can be used to separate dolomite from magnesite. In such cases magnetic separation can be applied on account of differences in the magnetic properties between dolomite and magnesite [15]. Most of the magnesite associated with strong magnetic minerals (magnetite and hematite) is separated as strong magnetic fraction while the dolomitic part tainted with goethite and lepidocrocite is separated

18 as weak magnetic fraction. Moreover, using magnetic separation technique magnesite can be cleaned from the non-magnetic minerals such as quartz. A simple magnetic separator consists of a belt which moves on two rollers. One of them is a strong magnet. The magnetic field produced by two pole pieces and a coil facilitates the separation of magnetic materials. As fine ore is dropped on the belt from one end (non-magnetic) and at the other end (magnetic), the magnetically susceptible particles are attracted and fall close to the roller while non-magnetic particles fall away from the roller. On the basis of strength of applied magnetic field, magnetic separator finds different applications: a) Low intensity magnets are applied to concentrate strong magnetic minerals like magnetite. b) High intensity magnets (electromagnet) are used to concentrate weak magnetic minerals. c) Heavy magnets can be applied to remove fine iron tramp from entering the crusher.

1.2.7.1 Dry magnetic separation of carbonate sample

The difference in magnetic susceptibility for minerals under examination makes the foundation for magnetic separation (physical process). Magnetic susceptibility (ξ) [15] can be narrated as:

ξ = ρ χ (1.2.7.1)

χ is the specific magnetic susceptibility and ρ shows the density of material. The significant susceptibilities are as follows:

Strongly magnetic ξ > 10–5 m3/kg

Weekly magnetic ξ = 10–6 to 0.5×10–7 m3/kg

Non-magnetic ξ < 10–8 m3/kg

1.2.7.2 Impact of magnetic field strength

The magnetic attractive force radially inwards, centrifugal force radially outwards and gravitational force downwards govern the useful collection of fine particles in dry higher

19 intensity magnetic separation. The researchers [37] found that the magnetic attracting forces on a particle are product of particle magnetization and magnetic field gradient.

1.2.7.3 Influence of drum speed

The rotation speed of drum determines the centrifugal force which dictates grade and recovery of the component. Moreover, the centrifugal force can control retention time in the magnetic zone, isolation of non-magnetic particles having high specific gravity and detachment of fine gangue particles adhering to the drum. A precise and effective adjustment between centrifugal and magnetic forces is important to achieve equilibrium.

1.2.8 Electrostatic separation

It is an environment friendly technique. It uses electrostatic charges to separate particles of various electrical charges and, as possible, of various sized crushed material particles.

Generally all minerals exhibit varied behaviour towards conductivity and can be separated by this method. The minerals found in heavy sands from beach or stream places are separated by electrostatic separation. This technique of concentrating ore particles can be used with magnetic separators for the isolation of heavy sand mixtures & their components. The roll-type corona electrostatic separator (RTS) [38] is taken as a classical separator however, it has problems regarding the stability of separation process between the capacity of production and the quality of separation, the middling products and their further handling and impurity of non- conductive products due to the aggregation of fine particles. In order to overcome these issues a new two-roll type corona-electrostatic separator (T-RTS) [38] is formed which has promising performance. The working of this technique is based on corona discharge and involves the polarization of mineral particles by the discharge from point electrode. The conducting minerals get positively charged by losing the electrons when high voltage is applied across the two plates placed close together. The material particles possessing different polarities are taken

20 into an electrical field and pursue various motion trajectories and are caught separately. The positively charged mineral particles are repelled from the positively charged roll into the repellent container, while in the case of dielectrics & semiconductors and insulators may retain their polarity for longer time due to electrostatic action. In order to attract or repel differently charged material particles, electrostatic charges are used. When the force of attraction is applied, the conducting particles stick to an oppositely-charged body (metal drum) and get separated from the mixture. In the case of applied repelling force, the trajectory of falling particles will be changed and will fall on different places.

1.2.9 Bioleaching

The recovery of metallic substances from their ores carried out by means of living organisms

(fungi and bacteria) is referred as bioleaching. Being much cleaner than traditional extraction techniques, it is considered as one of several applications within bio hydrometallurgy [39-43].

In bioleaching, a specific strain of bacteria (Acidithiobacillus dissolves gold) eats the metal content of ore. It is preferably used for relatively small amounts of metal ore and when environmental treatment is a demand. The bioleaching can also work as an exploration device for mining small ore bodies.

The microorganisms used in bioleaching are usually ferrous iron and sulfur oxidizing bacteria

(Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans) [44] and many species of fungi such as Aspergillus niger and Penicillium simplicissimum.

Bioleaching avoids precious steps such as roasting and smelting and requires small amounts of metals in the ore. It is economical, environment friendly, cause less landscape damage; however, it is a slow process and can cause biosafety failure.

The bioleaching process requires supply of the proper strain of bacteria, precise conditions to encourage rapid growth and reproduction, shipping to the site and pumping into the ore body.

A recovery well is utilized to pump the solution to surface after the completion of soaking time.

This solution is passed through a purification plant to separate the metals from bacterial mix.

The solution is passed through an enrichment section to reload any of the deficiency of bacteria to continue the cycle. Thiobacillus ferrooxidans play significant role in bioleaching of metals

[45-48]. The importance of heterotrophic microorganisms in release of silica [49,50] is well- established. Mohanty et al., [51] have isolated some silicate bacteria that have ability to release significant quantity of silica from magnesite ores. On an industrial scale the optimum conditions will be required to ensure bacterial adaptation and their silica tolerance level considering the nature of mineral substrates. Following are some parameters which significantly affect the efficiency of bioleaching process:

1.2.9.1 Effect of temperature

Temperature performs an effective role in release of silica by the Bacillus species. Mohanty et al., [51] has described that an increase in temperature from 27 to 37 °C, increases the release of silica from 12 to 42 µg/mL and further rise in temperature from 37 °C inhibits the release of silica. The highest recovery at 37 °C may be due to the metabolically greater synthesis of the silica solubilizing reagent.

1.2.9.2 Effect of shaking

The adequate shaking rate is required to aerate the culture for proper growth because of their aerobic nature. A shaking rate of 50 to 100 rpm has been found [51] promising for the production of the highest solubilization. It was also observed that higher shaking rates than 100 rpm cause reduction in silica release probably due to problems in the synthesis of silica solubilizing reagent and in the intimate contact between the ore and the bacteria. Other factors which can affect the recovery of metals and bioleaching efficiency include nature of media, nitrogen and phosphate sources, yeast extracts and contents of manganese sulfate.

1.2.10 Chemical beneficiation

The concentration of natural magnesite to refractory grade magnesite by mechanical/physical beneficiation methods often neither yield a product of adequate purity nor are economically attractive. In order to obtain sufficiently pure magnesite by physical beneficiation methods we have to sacrifice the higher yields. Presently chemical methods are gaining importance for the purification of magnesite for various commercial grades. The chemical beneficiation methods generally require dissolution of magnesium contents of magnesite, filtration of gangue material, purification of the process liquor and precipitation of magnesium as magnesium carbonate. Moreover, at the precipitation stage of magnesium, different magnesium compounds can be prepared for commercial and industrial applications as in this study

(magnesium formate). The resultant chemically beneficiated magnesite is of good quality and can be further processed to obtain high grade dead burnt magnesia suitable for use in the production of basic refractories.

The carbon dioxide atmospheric leaching process of magnesite involves grinding, calcination and slaking of magnesium oxide to magnesium hydroxide [52]. The slurry of magnesium hydroxide is carbonated with carbon dioxide gas to convert to soluble magnesium bicarbonate at atmospheric pressure. The filtration of slurry is carried out to obtain soluble magnesium bicarbonate solution and the residue containing silica, calcium carbonate, etc. The heating and aeration of magnesium bicarbonate solution produces basic magnesium carbonate which is then sintered to obtain the refractory grade magnesia product.

1.3 Dissolution

Extraction of magnesium from its minerals usually involves three methods: pyrometallurgical, electrolysis and hydrometallurgical. The first two methods require high energy inputs and have limited applications for the production of magnesium and its compounds. The concentration of natural magnesite to refractory grade magnesite by mechanical/physic al beneficiation methods

23 often neither yield a product of adequate purity nor are economically attractive. In order to obtain sufficiently pure magnesite by physical beneficiation methods, we have to sacrifice the higher yields. Presently, chemical methods are gaining importance for the purification of magnesite for various commercial grades.

Therefore, hydrometallurgical routes for the recoveries of metals are preferred. In order to obtain magnesium and its compounds required for commercial and industrial applications from magnesite ore, the hydrometallurgical methods are usually used [53]. A wide range of studies have been reported on leaching and dissolution kinetics of rocks with variety of leaching agents

[54-106]. In this study, it has been described that the leaching kinetics of different metal ores may vary with the change in nature and type of rock deposits.

The study concerning the leaching kinetics of magnesium carbonate in water saturated by chlorine gas [107] as well as with SO2 gas in aqueous medium [108,109] indicated that the leaching rate was governed by surface chemical reaction step. Pokrovsky et al., [110-112] investigated leaching kinetics of calcite, dolomite and magnesium carbonate in various acidic media and found that elevated reaction temperature and high partial CO2 pressure decreased carbonate mineral reactivity in aqueous medium.

Leaching of magnesium carbonate rocks can be achieved by acids (inorganic/organic) or bases and their salts [113]. The researches, in the dissolution process of magnesium carbonate by inorganic acids such as HCl, H2SO4 [114-116] showed that the controlling mechanism for the dissolution of magnesite is a chemical process. Inorganic acids have issues of selectivity, froth formation and scaling [117]. Conversely, organic acids can act as active leaching reagents because most of the leaching reactions are done in mild acidic conditions (pH 3–5). Moreover, organic acids have a low risk of corrosion and can be utilized for carbonaceous rocks. Another advantage of organic acids as leaching agents is their biodegradability, which generally depends on the carbon chain and other attached groups. Fredd and Fogler [118] investigated

24 the leaching kinetics of calcite in aqueous solutions of acetic acid. The analysis of the data showed that, below pH 2.9, the dissolution of calcite was affected by the transport of both reactants and products [118]. However, above pH 3.7, the rate of leaching was governed mainly by the surface reaction step [119].

From leaching investigations of naturally occurring magnesite materials in gluconic acid and lactic acid solutions, it was found that leaching kinetics were driven by chemically controlled mechanisms [120,121]. Demir et al., [122] and Lacin et al., [113] studied the magnesite leaching kinetics by citric and acetic acids respectively, tested for shrinking core models for fluid-solid system and developed a semi-empirical kinetic model. Experimental data, in both of the studies described that the leaching reaction is surface chemical reaction controlled. The energy of activation calculated for leaching reaction of magnesite with citric acid as a leaching agent was found to be 61.35 kJmol-1, while the activation energy of the similar study using aqueous solutions of acetic acid was found to be 78.40 kJmol-1. In these studies the reason for the significant variation in energy of activation using these two organic acids was not addressed

[123]. However, this difference in activation energy may be attributed to the nature of both acids and the type of different chemical entities existing in the leaching solutions.

The above cited literature illustrates that organic acids can be utilized for the leaching kinetic studies of magnesite rocks. On account of their low dissolving power organic acids are reasonably weak as compared to inorganic acids and cannot dissolve impurities as in the case of inorganic acid and also can act as selective leaching agents. Magnesite deposits are widely located in the Ornach Cross, Abottabad, Baran Lak, Nal and Drakalo and are being mined in the vicinity [124] and show marked variations in MgO content. The magnesite rocks of these deposits may contain significant concentrations of calcareous material; silica, manganese and iron compounds. Therefore, these deposits cannot be used directly for various applications unless its magnesium oxide concentrations are improved up to industrially required values. In

25 literature no appropriate strategies are available concerning the dissolution kinetics of indigenous magnesite rock using organic acids. The previously used concentration methods are tedious, laborious and costly. These methods generally require high energy inputs (calcination,

Electrolysis) and obtained products from these strategies do not meet the requirements of indigenous market and require mostly huge investments and personnel. In the present study, it has been intended to use formic acid as an alternative lixiviant to leach the indigenous magnesite deposits and to investigate the dissolution kinetics. In this study, following aims and objectives have been made:

 To view the role of some organic acids in leaching of natural magnesite in contrast to

inorganic acids.

 To use such leaching agents which can be recovered easily

 To access the type of impurities in natural magnesite of Abbottabad and Khuzdar

areas of Pakistan.

 To reduce the purification time of natural magnesite.

 To obtain highly pure end products of impure magnesite rock.

 To obtain useful products for pharmaceutical (magnesium acetate), analytical

(magnesium formate) and daily life applications (pure magnesite)

 To develop the kinetic models to find the optimized reaction conditions, so that these

experimental parameters with their optimized values may be used on commercial

scales.

1.4 Methods of mining

Mining exploitation is achieved by different methods (post pillar, block caving, sublevel caving, cut and fill mining, narrow vein mining) however, two methods i.e. open pit mining and underground mining are mostly used. Open pit method is non-selective, usually used to exploit low grade shallow ore bodies and results into two waste streams: waste rock which contains no economic quantity of minerals which must be removed to gain access to the ore body, and tailings which are the result of a mineral separation process in the concentrator. Open pit mining of magnesite being done in Pakistan is shown in Fig. 1.4.1.

Fig. 1.4.1 Open pit mining of magnesite rock in Pakistan

In nested pits, mining rate may vary from 20,000 to 100,000 tons per day. A series of nested pits are developed, each larger in area than the previous pit in open pit method. In case of steep slopes benches are formed at the bottom of each slope to contain any slope failures. In strong rocks some pits may permit double benching where slopes about 20‐30 meters high are built.

Fig. 1.4.2 shows mining scheme of a magnesite deposit. Underground mining with rate of mining less than 20,000 tons per day is used to exploit high grade deep ore bodies and result in one waste stream tailings generating little waste rock.

Fig. 1.4.2 Mining scheme after Scott Dunbar University of British Columbia [125]

Narrow vein mining (Fig. 1.4.3), a selective method, used for narrow ore bodies, as small as half meter wide, results in waste rock which is left in the hanging wall and the footwall.

Fig.1.4.3 Narrow vein mining of magnesite rock in Pakistan

A post pillar mining method with uncemented backfilling may be preferred in the upper exploitation areas. The rectangular shaped pillars roughly have a width of 4 m and a length of

15 m and the height may vary because of the variation in height of the exploitation areas.

Cut‐and‐fill is a selective mining method, relatively expensive, applied to steeply dipping ore bodies in stable rocks, done only in high grade mineralization and chosen for ore bodies with irregular shape and scattered mineralization.

In cut‐and fill mining, ore is removed in horizontal slices, starting from bottom undercut and advancing upward. The minerals of concern are drilled, blasted and removed from the stope.

After removing a stope, the void is backfilled with slurry of tailings or cement which supports the stope walls and provides a working platform for equipment for the mining of next slice.

Two kinds of cut and fill mining methods are used: one is overhand and the other is underhand.

In overhand method, the ore remains underneath of the working area and the roof is backfilled whereas in underhand cut and fill, the ore overlies the working area and the machines work on backfill.

The magnesite mining is carried out in 3.5 m high slices. At the deepest horizon of exploitation area, the initial slice is made by advancing drifts with a height of 7 m and width 6–7 m. The drilling rigs equipped with a computer, allow the operator to choose the best drilling array for the varied rock conditions and show the holes to be drilled on the screen of the computer. These are also applied to control advance of drifts. These drills improve the process of blasting. The uncemented backfilling matter formed in open pit is used to backfill a layer of 3.5 m. The height of the room to 7 m is achieved by drilling and blasting another 3.5 m high slice of the roof. This progression is continued until the full heights of exploitation areas are extracted.

This method of mining provides high effectiveness up to an overburden of some 700 m and high flexibility for quality control but the support (rock bolts) of the roof may be demolished while taking the next hanging wall.

In the case, when overburden is higher than 700 m, the pillars are redesigned due to higher stresses and the high risk of rock fall resulting in large size pillars which may cause loss of magnesium carbonate. With the help of an operational grade modeling program each bit of the mineral deposit can be estimated in terms of quality. Before the delivery to the sintering plant, the extracted magnesite is passed through an automated quality control section.

In 2006, a new procedure (OXEA) was adopted to make an online analysis of the rock samples to access quality distribution within the mine. In Pakistan, both methods (open pit mining and underground mining) of mining are used to extract the magnesite from mines. After assessing the quality of mine, the open pit method is applied with the help of other concurrent activities such as blasting and drilling.

1.5 Reaction kinetics modeling

The reaction kinetics modeling play vital role in several fields of research including environmental issues, chemical formulations, synthetic schemes, biological reactions, geological weathering, mining and dissolution studies, structural analysis, etc. [126,127]. The development of chemical systems and processes requires understanding of all the affecting parameters, their efficiencies and mechanisms of the processes.

Chemical kinetics is the branch of chemistry which deals with the mechanism of reaction, factors affecting rate of reaction, speeds or rates at which a chemical reaction proceeds during a specified interval of time. The word “kinetic” originates from some movement or change.

Similarly, kinetic energy is the energy caused due to the motion of a material particle.

1.5.1 Rate of reaction

The reaction rate is the change in the concentration of a reactant or a product with change in time. The rate of a chemical reaction is an ever changing entity because during the course of a chemical reaction, the concentration of reactants decreases with change in time. According to

Law of mass action, the rate at which a specie reacts is directly proportional to its active mass.

Since the concentration of a reactant is consumed with the passage of time, hence, the rate of reaction also varies. At the start of a reaction, the rate of reaction of a specie is maximum and decreases with time on account of decrease in its concentration. There are many reasons for studying the rate of a reaction, for example: why chemical reactions have such vastly different rates? On the basis of reaction rates, chemical reactions are classified into three basic types: fast (photosynthesis and nuclear chain reactions), slow (the curing of cement and the conversion of graphite to diamond) and moderate reactions (hydrolysis of ester and synthesis of ammonia). The knowledge of reaction rates has substantial importance in drug design, drug delivery systems, in pollution control, and in food processing. Industrial production of materials normally demands optimized reaction rates to obtain maximum yield. During the course of a reaction, reactants are consumed while products are formed, the progress of a reaction can be visualized by monitoring either the decrease in concentration of the reactants or the increase in concentration of the products. Physical or chemical methods are applied to find out the change in concentration of reactants or products during the course of reaction.

1.5.2 Order of reaction

Order of a reaction shows the sum of exponents of the concentration terms as given in rate equation of a chemical reaction. The rate expression or equation is used to represent rate law.

The order of reaction may be zero, negative, in fraction, and up to 3 in value. The order of reaction may be higher but the determinations become quite difficult. In a zero order reaction

(photosynthesis and decomposition of ammonia in tungsten) rate of reaction remains independent of the concentration of reactants throughout the course of reaction. In first-order reactions (decomposition of uranium and thermal decomposition of hydrogen peroxide), the rate of reaction depends on the first power of the concentration of a single reactant. A second order of reaction (thermal decomposition of ozone and alkaline hydrolysis of ethyl acetate) depends on the concentrations of two reacting species. A reaction in which rate law is governed

33 by the concentrations of three reacting species is said to be third order of reaction (formation of nitrosyl chloride or bromide and reaction between stannous chloride and ferric chloride).

Last [128] was quite successful in explaining an understanding of 1st order reactions regarding gas reactions occurring on the surface of a catalyst. In such reactions, a rise in gaseous pressure results in the rise in rate of reaction to a certain point called optimum value. Beyond this optimum value, further rise in gaseous pressure does not influence the initial rate, and reaction kinetics alters from first to zero order. One explanation to this concept is that when the catalyst is covered with a monolayer of adsorbed molecules and/or in the case of the enzyme catalyzed reaction, the rate approaches to optimum when all the active sites are utilized. The reaction kinetics will change from 1st to zero order when the rate of reaction will become independent of concentration of reactants.

Birk and Gunter [129] used a physical analog technique to illustrate different kind of order of reactions and expected that the water dipping method can be used to obtain simulated data for zero or first or second order processes in any reversible or irreversible and successive or parallel systems.

According to Harsch, [130] there are two basic questions in chemical kinetics: which events at the molecular level are responsible for the concentration changes? and how the amounts of the reactants, transition states, and products in a chemical reaction alter with change in time? The answer to first question can be obtained theoretically by postulating a sequence of elementary reactions provided that these reactions fulfill the stoichiometric and energetic requisites and explain the observed kinetic behaviors. In order to determine the mechanism of an experimentally studied chemical reaction we have to follow following steps:

a) postulating mechanisms for experimentally studied reaction

b) to write the rate laws for all mechanisms and substances

c) to integrate the rate laws in order to get the time laws

d) to compare these laws with the experimental data.

If the plot of experimental concentration versus time curves are in agreement to one of the theoretical time laws, the mechanism under investigation may be considered as a possible one, otherwise it would be wrong. However, the proof of a mechanism is a quite difficult job.

Chemical kinetics may never provide enough information for a positive proof of a unique mechanism.

The second question can be answered experimentally by determining the amounts of reactants, intermediates and/or products at various reaction intervals while keeping the reaction temperature constant throughout the course of the reaction.

It is established that the derived kinetic models provide valuable measures for designing and optimizing chemical reactors, for the development of catalyst and for the investigations based on reaction mechanisms. These models are derived and tested by the applications of simulations [131]. The computer simulations, in the field of chemical reaction engineering, are useful in terms of investigating and boosting a particular reaction process. With the aid of modeling of chemical reactions, the engineers are able to understand the chemistry, design and size of the system. The chemical engineers carry out computer simulations by means of three commonly used models (model of independent reactions, models of successive and competitive reactions) [132]. The model of independent reactions is related with assumption that yield of each component in a chemical reaction is independent of the other while the competing reaction models depend on ultimate analysis of the original reacting material.

A serious problem occurring globally in several mine sites is the release of acid drainage [133] from mining waste disposal areas. Franklin [133] used STEADYQL to predict the steady state composition of drainage produced from reactions between the aqueous and solid phases and found that this model can be applied for coherent representation of the monitored conditions.

In case of leaching reaction studies, three different kinetic models are used to study the influence of kinetic parameters affecting the reaction and to optimize their efficiencies.

As mentioned earlier, reaction kinetics modeling can be applied to hydrometallurgical processes. In the heterogeneous systems, the leaching kinetics may be controlled [53,134,135] by one of the given mechanistic models:

a) Diffusion from fluid films, t  k 1 1 x o     (1.5.1)

  b) Ash to product layer, 2 / 3 t  ko 1 31 x  2(1 x)   (1.5.2)

c) Chemically controlled reaction t  k 1 1 x 1/ 3 o     (1.5.3)

In above kinetic models, t for reaction time and x is the fraction of product formed. Concerning the leaching kinetics of magnesite with formic acid, Raza et al., [53] tested the kinetic data for above three kinetic models and evaluated that the results for dissolution reaction of magnesite with formic acid follows surface chemical reaction. The rate equation for chemically controlled reaction is denoted as:

1 (1 x)1/ 3  kt (1.5.4)

In terms of Arrhenius equation, Eq. 1.5.4 can be stated as:

1 (1 x)1/ 3  k eEa / RT t (1.5.5)

In Eq.1.5.5, Ea is the energy of activation and R is general gas constant. The reaction kinetic model as given in (Eq. 1.5.5) may illustrate only the influence of temperature on leaching process. In order to probe the effect of different experimental parameters such as sample particle size, acidic concentration, rate of stirring and liquid to solid fraction at various reaction temperatures, the cumulative parametric impact [136] on Arrhenius variables is determined.

The rate constant k can be written as:

E / RT a a k k oC e (1.5.6)

a b Ea / RT k k oC (L/ S) e (1.5.7)

a b c Ea / RT k k oC (L / S) D e (1.5.8)

a b c d Ea/RT k koC (L/ S) D S e (1.5.9)

In above equation, D sample particle size, k reaction rate constant, C acid concentration, S stirring rate, L/S liquid/solid ratio, T, reaction temperature, Ea activation energy and R, ko, a, b, c, d, constants.

Following semi-empirical model can be constructed by comparing Eqs. (1.5.5) & (1.5.9):

1 (1)1/3 k Ca(L / S)b DcS d eEa /RT t . o (1.5.10)

With the help of above semi empirical model Eq. (1.5.10), an industrial unit may be designed in such a way to give better results regarding the situations when other experimental conditions

(acid solution concentration, stirring rate, liquid to solid proportion and particle size of sample) also affect the rate constant beside the reaction temperature.

1.6 Instrumental techniques

1.6.1 Atomic absorption spectrophotometer (AAS)

An atomic absorption spectrophotometer (Fig. 1.6.1.1) is utilized to investigate the magnesium content in magnesite rock samples and leached samples (in aqueous phases) obtained during the progress of leaching kinetic reactions. It is a sensitive instrument and can measure concentrations up to ppb of a gram and perform accurate and efficient elemental analysis in almost all kind of fields. The metals present in the mines are estimated by AAS to find whether it is worth mining to extract the desired metallic component.

1.6.1.1 Working of AAS

The characteristic wavelengths of light are absorbed by the atoms of different elements in a sample. During analysis by AAS, the sample is aspirated and atomized. A beam of electromagnetic radiation emitted from hollow cathode lamp (HCL) is passed through the vaporized sample particles. A part of the electromagnetic radiations is absorbed by the analyte.

The quantity of light absorbed is directly related to the number of metal atoms present in the sample. In this manner a signal is fed to the measuring system which determines the amount of analyte by comparing the input and output electromagnetic radiations. From the calibration curves, the result of required analyte in the sample is evaluated. Three components: a light source; atomization chamber to form gaseous atoms; and a mechanism of computing particular component of light absorbed construct the basic parts of a typical atomic absorption spectrophotometer.

1.6.1.2 Light source

The hollow cathode lamp is mostly employed as a light source in AAS. HCL comprises of a cylindrical hollow cathode made of the element whose concentration is to be found and a tungsten anode. HCL is encapsulated in a glass tube which is filled with an inert gas (neon or

38 argon). During the process called sputtering excited atoms are produced which emit characteristic radiation of the metal as they return to ground state and re-deposited on the cathode. Many lamps can be housed in a rotating turret. By the rotation of turret a specific lamp for an element can be quickly selected.

1.6.1.3 Atomization chamber

The atomization of sample is carried out by two mechanisms: direct suction of solution of the sample into flame; and electro thermal atomization. In electro thermal atomization, a drop of sample is placed into graphite tube and heated electrically. In some instruments both atomization systems are available with one set of lamps. When an appropriate lamp is selected, it is directed towards specific atomization system. Some other atomization methods are also utilized for specialized purposes. A few of them are given below;

a) Glow-discharge atomization

It can simultaneously introduce and atomize the sample. Two electrodes existent in the argon atmosphere disrupt the argon gas resulting in positively charged ions which are then accelerated into the cathode surface having sample and cause sputtering. Atomic vapors created by this glow discharge consist of ions, ground state atoms, and excited atoms. The glow of low- intensity is discharged, as the excited atoms come back into their ground state.

b) Hydride atomization

In this method of atomization, hydride generation is done by adding an aqueous solution of acid with the sample to 1 % aqueous solution of sodium borohydride in a glass vessel. Hydride is swept by an inert gas into the atomization chamber and undergoes decomposition and forms an atomized form of the analyte.

c) Cold-vapor atomization

The technique is restricted for finding of mercury content in samples because mercury is the only metallic element that has a sufficiently high vapor pressure at working temperatures. It is an excellent mercury atomization method and its limit of detection is in parts-per-billion.

1.6.1.4 Measuring system

In order to determine the spectral line absorbed by the sample and to eliminate the other ones, a monochromator is used. The selected light is focused towards a detector that is typically a photomultiplier tube which creates electrical signals proportional to the intensity of light.

Two types of AAS instruments are marketed depending on the modes of absorption or emission.

1.6.1.4.1 Absorption mode

The flame of AAS is used to convert the incoming aerosol particles into ground state atomic vapor which absorb specific wavelength coming from primary light source (HCL). The quantity of light radiations absorbed by the metal atoms is determined to find the amount of metal in the aqueous solution.

1.6.1.4.2 Emission mode

In this technique, the flame works in a dual manner; converts the aerosol particles into ground state and activates atomic vapors atoms to an excited electronic state. The exited atoms upon coming back to the ground state emit specific radiations which are read by the instrument. The intensity of emitted radiations is directly related with the content of metal atoms in the solution of sample.

Fig.1.6.1.1 AAS (Model A-1800, Hitachi) used for quantitative analysis of magnesite

1.6.1.5 Chemical interferences

The efficiency of creation of neutral atoms in the flame is greatly restricted by chemical interferences and may cause problems in both absorption and emission modes [137]. Other atoms or molecules (unvaporized solvent droplets, or compounds of the matrix) apart from those of analyte may absorb or scatter some radiations from the light source and may become a reason for erroneous results. In order to correct the background absorption, two light sources are usually applied. One is the hollow cathode lamp appropriate to the analyte while the second one is deuterium lamp which produces broad band radiation with no specific spectral lines. The total absorption (absorption due to analyte atoms plus background) is measured by alternating the measurements of two light sources. The amount of radiation absorbed by the analyte is obtained by subtracting background from total absorption. The atomization is also affected by the presence of other species (matrix interferences) in sample solution. Such species may result in the formation of refractory compounds with test element, usually by an anion in aspirated solution [138] and may lead to decrease in signal strength. During analysis of calcium, presence of phosphate ions may cause the formation of calcium pyrophosphate which does not dissociate in flame and causes low results for calcium. These chemical inferences are minimized by adding an appropriate releasing agent in sample solutions. The releasing agents either compete for interfering substances or displace them from the analyte.

This error may be reduced by adding lanthanum nitrate in calcium solution. Lanthanum nitrate may react with phosphate to give more volatile compound that is dissociated easily and makes calcium absorbance independent of the amount of phosphate. Sometimes addition of high concentrations of ethylene diammine tetra acetic acid in the sample solution may also eliminate the interference of phosphate by forming a chelate with calcium and prevent its reaction with phosphate. Moreover, the addition of an external reagent in sample solution may lead to change in the composition of solution or nature of analyte. In such situations, it is recommended that

42 the releasing agents or modifiers must be added in standard solutions of test element. It has also been observed that the application of high temperature flames can commonly eradicate chemical interferences. For example in estimation of calcium, phosphate interference does not occur in nitrous oxide-acetylene flame [139].

In the cases, when atomization is done by electrothermal method, chemical modifiers are added in sample solutions which react with interfering substances in sample and make more volatile species than the analyte. The volatile species vaporize at relatively low temperature than analyte and are isolated during the low and medium temperature stages of electrothermal atomization.

1.6.2 Scanning electron microscopy

The instrument used for investigation of morphological appearances, apparent topographies of specimens at variable magnification powers and used in qualitative and quantitative determinations is called scanning electron microscope and the technique is referred as scanning electron microscopy (SEM). It is a kind of electron microscope, scans sample surfaces by focusing beam of electrons and produces images. Various signals generated by the interaction of beam of electrons with atoms in the sample are detected.

1.6.2.1 Principle of SEM

SEM works in three modes (secondary electron, back scattered electron and cathode luminescence) of detection but the commonly used one is secondary electron imaging in which secondary electrons contained in the sample are excited by focused electron beam. The secondary electrons are detected resulting in an image containing information of sample topography. All scanning electron microscopes are unanimously equipped with secondary electron imaging detectors, have wide range of magnifications (10 to 500,000 times) and are capable to produce high-resolution images of sample surfaces. In back-scattered electrons

43 imaging (BSEI), focused electron beam is reflected from sample surface due to elastic scattering. BSEI is used in analytical SEM’s and can describe the distribution of elements and their relative amounts in the sample. A typical SEM consists of: an electronic optic column built on the sample chamber, pumping circuit causes high vacuum, and detectors collect all the signals coming from the sample, a video screen to inspect the pictures and an electronic device to control all functions.

1.6.2.2 Preparation of samples

Metallic stubs of appropriate sizes are used to hold electrically conductive fine samples. Carbon and copper tapes can be utilized for powder mounting. The sample powder is sprinkled lightly with spatula and is pressed to seat. The sample holder is turned upside down and taped it to remove loose material. Samples with non-conductive surfaces have tendency to charge during scanning by electron beam and especially in SEI mode cause faults in scanning and other image distortions or drifts. In order to avoid these problems such samples are coated by sputtering coater (Model EMITECH- K550) (Fig. 1.6.2.1) with an electrically conducting material which is deposited on the sample surface.

Fig.1.6.2.1 Sputtering coater (Model EMITECH- K550)

Low acceleration voltage may be utilized to minimize the charge effect if sample cannot be coated with conductive coating. The thin coating of electrically conducting material (gold, platinum, chromium and tungsten) is either achieved by low-vacuum sputtering or by high- vacuum evaporation. The coating thickness may be many nanometer to tens nanometer and depends on if the coating interfere with the morphology of specimen. After coating, the sample is mounted with conductive bridge (carbon/copper tapes, or silver paint) connected from the top surface of the sample to sample holder.

1.6.2.3 Working of SEM

The working of a typical SEM involves focusing of emitted electron beam (energy ranging 0.2 to 40 keV) from electron gun by one or two condenser lenses to a spot of 0.4 to 5 nm diameter.

The electron beam passes through pairs of scanning coils in an electron column where it is deflected in x and y axes and scans the rectangular surface of sample in a raster fashion. During the interaction of electron beam and sample surface, electrons lose energy by repeated random scattering and absorption with in interaction volume and result in reflection of high energy electrons by elastic scattering, the emission of electromagnetic radiation and emission of secondary electrons by inelastic collisions and each of which can be sensed by specialized detectors. The schematic diagram of SEM is shown in Fig. 1.6.2.2.

Fig. 1.6.2.2 Schematic of scanning electron microscope [140]

The dimensions of interaction volume depend on atomic number of specimen, electron's landing energy and sample's density. Various types of electronic amplifiers are in use to amplify the collected signals. Each pixel of computer is coordinated with position of the beam on sample and results in an image. The resolution of SEM is controlled by size of the electron spot, size of interaction volume, volume of sample that interrelates with the beam of electron.

Fig. 1.6.2.3 shows SEM (Model JEOL 5800LV).

Fig. 1.6.2.3 SEM (Model JEOL 5800LV) used for the analysis of magnesite samples

1.6.2.4 Energy dispersive X-ray analysis

Energy-dispersive X-ray diffraction (EDX) differs from conventional X-ray diffraction because it uses polychromatic photons as a source and is commonly operated at a fixed angle with no need for goniometer. It is used with synchrotron radiation which enables it to use for measurement within real engineering materials. In most of cases scanning electron microscope is equipped with an EDX to ensure the qualitative analysis by finding the elemental composition of samples of interest. The EDX acts as an integrated part of a scanning electron microscope and is not used for analysis on its own. It is not only helpful in the detection or identification of desired materials and their contaminants but also it is useful for the estimation of relative concentrations of components comprising the samples. The peak height in EDX spectrum indicates concentration of a component in the sample.

1.7 Chemistry of leaching process

The metallic and non-metallic impurities contained in magnesite rocks can be removed by the process of selective leaching. The type and nature of impurities in magnesite rocks differ from deposit to deposit throughout the world. Moreover, the types and quantities of impurities may vary with in the same deposit. Therefore, each magnesite deposit may require special attention with respect to enrichment or beneficiation techniques and technologies. The nature, strength and concentration of leaching agent are considered as the important factors contributing to selective dissolution of magnesite rocks and to avoid the dissolution of impurities.

A lot of literature is available concerning the leaching studies of various ores with inorganic acids as lixiviants; however, the research related to leaching of ores with organic acids are restricted. It has been described that organic acids have better selectivity towards dissolution reactions of different metals in contrast to inorganic acids and the use of organic acids has increased in recent years because of their properties. With respect to dissolving capability, organic acids are relatively weak but because of high selectivity they can act as alternate

49 lixiviants for extraction of various metals. The use of inorganic acids as extracting agents in scale up studies have limitations in terms of high carbon dioxide pressure and froth formation due to their fast dissolution rates and may lead to some risks. On the other hand organic acids may act as promising extractant for different applications because they are used at intermediate acidic conditions (pH 3-5) [123] and their biodegradability is easy [141]. Moreover, the problems of scaling and corrosion [142] are less in case of organic acids. However, their use at relatively high temperatures is restricted because of their low boiling temperature and the risk of their disintegration at elevated temperatures [113,122]. The levels of impurities in desired materials can be minimized by the utilization of dilute solutions of organic acids such as formic acid, acetic acid, citric acid and succinic acid. The selection of good organic extractant depends on: various reaction conditions applied in the leaching process, nature and properties of organic acid, nature and characteristics of extractable materials. It has been observed that the selectivity of an acid is inversely proportional to its strength because of high polarity of hydroxyl group.

Therefore, strong organic acids are not considered as preferable leaching agents in terms of selectivity. The dilute solutions of these acids can be used as water molecules can drop the effect of polarity of O-H bonds due to higher extent of ionization and can result in selective leaching processes. The criteria for selection of organic acids also depend on experimental parameters required for the completion of leaching processes; the liquid phase should be easily distinguishable for regeneration and reprocessing.

In this study, dilute solution of formic acid is utilized to probe the dissolution kinetics of natural magnesite rock. Dilute formic acid may be recycled to the process by treating magnesium formate with any mineral acid (sulfuric acid) depending upon the required useful products i.e. magnesium sulfate. The chemical reaction between magnesium carbonate and formic acid is written as follows:

MgCO3 + CH2O2  Mg (CHO2)2 + CO2 + H2O (1.7.1)

In the above reaction, carbon dioxide is liberated as a gas and does not participate in making the reaction reversible. The comprehensive mechanism representing essential steps for thorough understanding of this fluid solid reaction system is written as:

(a) Ionization of formic acid:

+ 1- CH2O2  2H + CHO2 (1.7.2)

+ (b) The movement of H ions to exposed surfaces of magnesite rock particle.

CO2+ H2O  H2CO3 (1.7.3)

+ - H2CO3  H + HCO3 (1.7.4)

- + 2- HCO3  H + CO3 (1.7.5)

+ 2- 2H + CO3  CO2 + H2O (1.7.6)

(d) Attack of hydrogen ions on the magnesite rock particles.

+ + 2+ H + MgCO3 H2CO3 + H + Mg (1.7.7)

The hydrogen ions that take part in these reaction steps may come from carbonic acid formed in leach slurry or from formic acid (leaching agent).

2+ 1- (e) Reaction between Mg and CHO2

2+ 1- Mg + CHO2  Mg (CHO2)2 (1.7.8)

The formation of magnesium formate depends on experimental factors, such as reaction temperature, concentration of leaching acid, reaction time, grain size and nature of natural magnesite ore used in the dissolution process. The general reaction [123] representing the leaching process can be simplified as:

MCO3(s) + H2Y (aq)  MY(s/aq) + CO2 + H2O (1.7.9)

2+, 1- o Where M = Mg Y = HCO2 . The ionization constant of formic acid, pKa = 3.75 at 20 C and

-6 o the solubility product constant for MgCO3 is 6.8 × 10 at 25 C [143,144]. Ashraf et al., [123] 51 described that the magnitudes of constants (ionization constant and solubility product constant) are temperature dependent and efficiency of leaching reaction is restricted by the kinetics and

CO2 (pK1 = 6.35, pK2 = 10.33 for H2CO3) [123]. The values of these constants will be contingent to reaction temperature and reaction kinetics may govern the success of leaching reaction. Formic acid as an alternative lixiviant for chemical beneficiation of natural magnesite was preferred over other organic acids because of its higher leaching ability and efficiency as compared to other previously used organic acids in the similar studies. It would not only require lesser time for beneficiation but also reduce the energy inputs. Formic acid is found naturally in the venom of ants and also in atmospheric conditions because of forest emissions.

Synthetically it can be prepared from oxidation of methanol, as intermediate in acetic acid synthesis and biodegradation of biomass. Its cost per metric ton is 500-600 US dollar comparable to that of acetic acid. It is preferentially used in leather and textile processing and is considered as green organic solvent because of its compatibility with the environment.

Methanoic acid and its salts are easily biodegradable. When formic acid is subjected to biodegradation, it produces just one molecule of CO2 half as compared to acetic acid. However, it is more toxic as compared to other organic acids; that’s why, its handling requires more precautionary measures.

Carbon dioxide formed in the course of chemical reaction of magnesite rock and organic acids is removed and may be collected for different applications if leaching reaction is carried out on industrial scale. Normally, the direction of equilibrium for reaction (Eq. 1.7.9) remains in the right and makes it an irreversible reaction and may not cause complications in chemical reaction. In order to get maximum recovery of the desired component (in this study magnesium from magnesite) from a rock, the experimental parameters (reaction temperature, acidic concentration, reaction time, solid particle size, stirring rate and liquid/solid ratio) of leaching process must be optimized keeping in attention the economic and environmental aspects.

Chapter

2. EXPERIMENTAL PROCEDURES

2.1 Collection of magnesite samples from Khuzdar and Abbottabad areas

The deposits of magnesite, located in western fold belt of Pakistan consist of sedimentary rocks ranging in age from Jurassic to Miocene [5] sandwiched with Bela Ophiolite. The Bela

Ophiolite, linked regionally with Alpine-Himalayan belt, consists of an extended belt starting from Khuzdar and ending at the coast of Godani Balochistan, Pakistan. It is one of the important mineral belts of Pakistan containing deposits of magnesite, chromium, asbestos, soapstone, talc, manganese, nickel and copper [145]. The significant deposits of magnesium carbonate are present in Baran lak, Drakalo, Nal, Ornach cross, Khushal, Ustam butt and Pahar khan bidrang localities and are being used locally [124,146]. Bashir et al., [5] divided the ophiolitic belt into three zones (northern, central and southern based on the occurrence of magnesite deposits and the nature of host rock).

The central zone running from Baran lak in the south to Gangu in the north exhibits major deposits of magnesite having magnesium oxide content ranging from 31.75 to 46.95 % [5].

Magnesite is distributed throughout the ophiolite belt in the form of veins and commonly originates from the modification of magnesium rich rocks after coming in contact with carbonate rich solutions. These magnesite rocks are generally cryptocrystalline and have silica as opal or chert. In the present study, sampling of natural magnesite was carried out from two different provinces of Pakistan. Magnesite samples in the form of big lumpy stones, ranging in weight from 8-10 kg, were collected from Baran lak deposits close to Regional Corporation for

Highway (RCD) situated in Khuzdar district, Balochistan, Pakistan. Some samples of magnesite were also collected from Amirabad area of Abbottabad (Khyber Pakhtunkha). A few of the magnesite deposits occurring in Pakistan are shown in Figs. 2.1.1-2.1.3.

Fig. 2.1.1 Magnesite rock deposit in Abbottabad

Fig. 2.1.2 Magnesite rock deposits of Amirabad, Khyber Pakhtunkhwa Province

Fig 2.1.3 Weathered magnesite rock in Abbottabad mines

2.2 Size reduction and sieving analysis of magnesite samples

Magnesite ore samples collected from Baran Lak area were in the form of lumps of varied grain size and weights. The size reduction of magnesite samples was achieved by crushing with jaw crusher. In order to get fine particle fractions, a ball mill was used for further grinding. Different size fractions (500–707, 250–354, 177–210 and 125–177 μm) were obtained from screening of

2 kg ground sample using ASTM standard sieves. The samples of above mentioned fractions were separated and weighed. The percentage of each size fraction was determined and the results are shown in Table 2.2.1 and Fig. 2.2.1.

Table 2.2.1 Mesh size, Average size, Weight Fraction and Weight Percent of Magnesite

Samples

Average Weight Weight Mesh # Size (µm) size (µm) Fraction (g) percent

-25+35 500-707 603.52 588.63 29.43

-45+60 250-354 302.03 624.82 31.24

-70+80 177-210 213.53 547.84 27.39

-80+120 125-177 151.05 95.93 4.79

-120+140 105-125 115.03 47.82 2.39

-140+200 74-105 89.52 95.15 4.75

Fig. 2.2.1 Average particle size and weight % of magnesite sample

Fig. 2.2.1 illustrates that the weight percent of each fraction varies with the particle size. A marked increase in weight percent can be observed during the average particle sizes (150-210

µm). This rising trend continues slowly till the average particle size of 302 µm and after that increased particle size causes reduction in weight percent.

2.3 Preparation of magnesite sample for analysis

In order to dry the ground samples of magnesite, samples were placed in an electric oven at

100 °C for 24 h. These samples were brought to ambient temperature and stored in cleaned air tight glass jars. Further analysis (Qualitative and quantitative) was carried by using these stored samples. To find loss on ignition (LOI), each sample fraction was placed in prewashed, cleaned and heated fire-clay crucibles at 110 oC in an electric furnace. These magnesite sample fractions were examined for the loss on ignition (LOI) [123] and carbon dioxide amount as given in

Table 2.3.1.

The loss on ignition is defined as the reduction in weight of sample after heating from 550 to

950 °C. LOI also shows the quantity of CO2 contained in sample in the form of carbonates.

The fraction of magnesite (Xexp) leached during the dissolution process is calculated by using following equation 2.3.1:

Amount of magnesium in the leached solution x  (2.3.1) exp Total amount of magnesium in ore sample

The instrumental techniques such as AAS, SEM, EDX XRD and other conventional methods

[147,148] were applied for the qualitative and quantitative analysis of magnesite rock fractions.

The analysis of magnesite sample is given in Table 2.3.1. The effect of average particle size of magnesite on loss of ignition is shown in Fig. 2.3.1 which illustrates that the rate of loss of carbon dioxide increases gradually with increase in average particle size from 100-350 µm.

But this rate of loss of carbon dioxide increases rapidly with increase in average particle size

61 from 350–600 µm probably because of larger particles of magnesite exhibit higher concentrations of carbon dioxide content.

The formic acid (methanoic acid), EDTA (ethylene diammine tetra acetic acid) and EBT

(Eriochrome black T) used during analysis were of reagent grade. The stock solutions were prepared and their further dilutions were made by using deionized water. Eriochrome black T and disodium salt of EDTA are used in volumetric quantitative determinations of magnesium in leached solutions. In complexometric titrations of magnesium [149], EBT acts as an indicator while EDTA is used as a complexing agent for Mg.

Table 2.3.1 Magnesite Rock Samples Analysis

Sample size (mesh #) Size (µm) Average size (µm) LOI (%)

-25+35 500-707 603.5 53.50

-45+60 250-354 302.0 52.61

-70+80 177-210 213.5 52.08

-80+120 125-177 151.0 51.73

-120+140 105-125 115.0 51.37

Fig. 2.3.1 Influence of average particle size of magnesite on loss of ignition

2.4 Detection measurements and analytical procedure

Atomic absorption spectrophotometer (Model Hitachi-1800) was used for the determination of

Mg in natural magnesite and leached solution samples. Scanning electron microscope (Model

JEOL JED-2300) was used to observe the particle morphology of raw magnesite. Leaching studies of magnesite samples were investigated in a reactor made up of pyrex glass with 500 mL capacity. A hot plate (Model IKA C-MAGHS-7) equipped with a temperature sensor (ETS-

D5) was used to stir, heat and control the temperature (±0.5 K) of the reactor content. In each experiment, fixed volume of 8 % formic acid having L/S ratio of 14:1 mL/g was gradually introduced to the reactor with 5 g of sample. These entities were agitated with stirring rate of

350 rpm at known times and temperatures. After completion of the reaction, the hot solution was filtered to remove gangue minerals from magnesium formate. The filtrate solution was analyzed to find the percentage of conversion of magnesite [149].

2.5 Reagents and chemicals used

Table 2.5.1 List of Reagents and Chemicals Used

Chemical Name Formula Mol. Wt. % age Purity Company

Formic acid CH2O2 46.03g/mol > 96 % Sigma Aldrich

Eriochrome Black T C20H12N3NaO7S 461.38 Indicator grade Sigma

Succinic acid C4H6O4 118g/mol 99.9% Sigma

Acetic acid C2H4O2 60.05 g/mol 99.8 % Sigma Aldrich

Sulphuric acid H2SO4 98.08g/mol 97 % RDH

EDTA C10H14N2Na2O8. 2H2O 372.24 g/mol 99 % Sigma Aldrich

Nitric acid HNO3 63.01g/mol 65 % Riedal-ed Hasen

Ammonium NH4OH 35g/mol 35 % MERK hydroxide

Calcium nitrate tetra Ca(NO3)2. 4H2O 236.15 g/mol 99 % Sigma Aldrich hydrate

Magnesium chloride MgCl2 95.21 g/mol 99 % Sigma

Sodium hydroxide NaOH 40g/mol 97 % Fluka

Phenolphthalein C20H4O4 318.32 g PH=8.3-10 Fluka

2.6 Equipment used

Table 2.6.1 List of Equipment Used in Analysis of Magnesite

Equipment Name Made Model No. Company

Hot Plate Germany (C-MAGHS-7) IKA

Oven United states ESC 2 OMRON

Furnace United Kingdom BCF13/5-2408CP ELITE Thermal System

Water Bath Germany WNB 14 Memmert

AAS Singapore 1800 Hitachi

SEM Japan JED-2300 JEOL Limited

Weighing Balance USA PM 480 Mettler-Toledo

Sputtering Coater Italy Emitech K 550 SDI

Deionized water plant UK Select HP Purite Ltd.

2.7 Sample characterization

2.7.1 Atomic absorption spectrometer

A commonly used procedure to determine the quantity of element of interest is calibration curve. This method is used in the cases when interferences are either absent or are completely in control. In this study, calibration curve method was applied for qualitative and quantitative determinations of magnesite rock samples and leached solutions. The stock solution (100 ppm) of magnesium was prepared by dissolving magnesium metal (99.9 %) in 5 M hydrochloric acid solution. A series of standard solutions of magnesium in the range of 0.1 to 0.6 ppm were prepared in deionized water. In order to calibrate atomic absorption spectrophotometer, the absorbance of these standard solutions was measured and plotted against the concentrations of known solutions.

Fig. 2.7.1 shows the calibration curve for magnesium which is linear over the range (0.1-0.6 ppm) of the standard solutions used. The concentration of magnesium in standards, rock samples of magnesite and leached samples was determined at a wavelength of 285.2 nm in absorption mode using a hollow cathode lamp specific for magnesium.

The maximum sensitivity can be obtained by adjusting the burner head relative to the light path of the instrument. Different operating conditions (lamp current 4 mA, silt band width 0.7 nm, flow rates of acetylene 2 and air at 17 l/min) were adjusted to get the accurate and precise results. The chemical composition of natural magnesite sample as determined by atomic absorption spectrometer is shown in Table 2.7.1 which indicates that the magnesium is present in relatively higher concentrations than the other elements.

Table 2.7.1 Chemical Composition of Natural Magnesite after Raza et al., [53]

Component [Mass %]

MgO 43.47

CaO 0.38

Fe2O3 1.11

SiO2 1.54

Loss on ignition [at 950 oC for 24 h] 53.5

0.40

0.35

0.30

0.25

0.20

Absorbance 0.15

0.10

0.05

0.00 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Mg Concentration (ppm)

Fig.2.7.1 Calibration curve for magnesium concentration versus absorbance.

2.7.2 Scanning electron microscope

In order to evaluate the morphological characteristics of magnesite sample, scanning electron microscope (Model JEOL JED-2300 and JSM- 5800 LV) has been applied. The magnesite in pure form is usually white in appearance but becomes colored due to inclusion of impurities such as iron. The SEM images of raw magnesite at different magnification powers represent the apparent morphological properties of magnesite and are shown in Fig. 2.7.2.1. Magnesite particles in the form of grains and sheets are evident and their sizes range from few micrometers to 500 nm. The material appears to be non-granular with surface roughness, which may be due to the evolution of volatiles during weathering and formulation of the ore.

Fig. 2.7.2.1 SEM images of the natural magnesite sample at different magnifications after Raza et al., [53,71]

001

2020 µmµm

The EDX spectrum of intact ore of magnesite (Fig. 2.7.2.2) depicts the elemental composition.

The elemental composition showing mass and atomic % of elements in the indigenous magnesite is given in Table 2.7.2.1. From the EDX signature, it can be inferred that material under study is magnesium rich. The profile also exhibits the presence of carbon and oxygen indicating the presence of carbonates.

Fig. 2.7.2.2 EDX spectrum of the sample after Raza et al., [53]

Table 2.7.2.1 Elemental Analysis of Natural Magnesite Orea after Raza et al., [53]

Element [keV] [Mass %] [Atomic %]

C 0.27 14.59 20.44

O 0.52 57.56 60.51

Mg 1.25 26.06 18.28

Si 1.74 0.718 0.43

Ca 3.69 0.26 0.11

Fe 6.39 0.78 0.23

a Results obtained from EDX (Model JEOL, JED-2300)

2.7.3 X-ray diffractometer

In order to find the mineralogical composition of magnesite rock sample, an XRD (Philips X pert pro 3040/60) using Cu Kα radiation in step mode between 0o and 70o is employed. The identification of peaks in XRD spectrum is carried out by comparing with the ICSD XRD patterns. The mineralogical analysis (Fig. 2.7.3.1) shows that the magnesite ore sample is mainly composed of magnesium carbonate. The XRD pattern also reveals the presence of silica and calcium carbonate as impurities. Some mineral phases present in minor or trace amounts cannot be detected because XRD diagnostic reflections are not significantly above the background.

Fig. 2.7.3.1 XRD pattern of natural magnesite sample after Raza et al., [71]

Chapter

RESULTS & DISCUSSION

3. RESULTS AND DISCUSSION

3.1 Influence of reaction time on leaching kinetics of magnesite at different temperatures

The dissolution kinetics of natural magnesite ore is restricted by experimental parameters such as temperature, acidic solution concentration, size of the particle, stirring rate of reaction solution, liquid to solid ratio and time for completion of reaction. In order to find the impact of reaction time on leaching of natural magnesite at different reaction temperatures, a large number of experiments are performed and the results are shown in Table 3.1.1 and Figs. 3.1.1-

3.1.4. From the experimental results, it is evident that the rate of dissolution of natural magnesite increases with the increase in time of reaction depending on reaction temperature.

Rendering the experimental observations, the optimum value of reaction time to achieve maximum recovery of magnesium is found to be between 50 to 60 minutes at 65 oC using the acid concentration of 8 % and a liquid/solid ratio of 14:1 mL/g.

The experimental observations illustrate that with the rise in temperature of reaction, the degree of conversion of magnesite increases with corresponding decrease in reaction time resulting in higher reaction rates along with its higher solubility at elevated temperatures. However, the leaching reactions at elevated temperatures may lead to contamination of CO2 gas stream with water and formic acid vapors (vaporized during the reaction relatively at higher temperature).

Table 3.1.1 Influence of Reaction Time on Leaching of Natural Magnesite at Different

Temperatures

Time (min) 45 oC 55 oC 65 oC 75 oC

10 10.11 21.92 27.22 39.91

20 15.23 32.84 47.24 67.42

30 22.12 49.82 67.92 84.93

40 32.21 58.64 75.84 94.9

50 42.13 67.15 82.41 96.48

60 51.80 73.21 89.42 97.04

Fig. 3.1.1 Impact of reaction time on % conversion of natural magnesite at 45 oC

Fig.3.1.2 Influence of time of reaction on % conversion of natural magnesite at 55 oC

Fig. 3.1.3 Impact of time of reaction on % conversion of magnesite at 65 oC

Fig. 3.1.4 Impact of time of reaction on % conversion of magnesite at 75 oC

3.2 Impact of reaction temperature on leaching of magnesite with formic acid

Number of experiments have been performed to elucidate the influence of reaction temperature on rate of leaching of magnesite while keeping other experimental conditions constant (177–

210 μm particle size, 8 % formic acid, liquid/solid ratio of 14:1 mL/g and stirring speed of 350 rpm). The reaction temperature was varied from 45 to 75 °C. The experimental results are shown in Table 3.1.1 and Fig 3.2.1. The results illustrate that an increase in reaction temperature causes a rise in rate of conversion of the magnesite. The elevation of reaction temperature from 55 to 75 °C in 30 min causes an increase in % recovery of magnesite from

49.8 to 84.9 %. This indicates that an increase in reaction temperature elevates rate of chemical reaction. Typical rate curves in Fig 3.2.1 represent the temperature increase, which reduces reaction time needed to achieve maximum conversion of magnesite. About 95 % dissolution of magnesite is achieved at 75 °C in 40 min of leaching time. High reaction temperature usually affords high solvent evaporation, corrosion and some handling problems. Other experiments were made at 65 °C to determine effect of formic acid concentration, L/S ratio in the medium and particle size of magnesite on the dissolution kinetics of magnesite.

Fig. 3.2.1 Effect of temperature on leaching of magnesite ore (formic acid concentration, 8

%; particle size, 177–210 μm; stirring speed, 350 rpm; liquid/solid ratio, 14:1 mL/g)

3.3 Impact of formic acid concentration on dissolution of magnesite

The formic acid concentration can play a significant role in leaching of magnesite. The rise in acidic concentration is expected to raise the % conversion of magnesite, but the high strength of formic acid can affect the economy of dissolution process. In order to observe the influence of concentration of formic acid on leaching of magnesite, the concentration of formic acid was varied from 2 to 10 % at a reaction temperature of 65 °C, stirring rate 350 rpm and particle size fraction 177–210 μm. The results are shown in Table 3.3.1 & Figs. 3.3.1-3.3.5, which show that an increase in acid concentration accelerates the rate of leaching of magnesite. However, concentrations higher than 8 % do not have an appreciable effect as expressed in Figs. 3.3.1-

3.3.5. This situation may be described according to the fact that relatively higher concentration of leaching agent may attack the gangue minerals present in the ore. Raza et al. [53] described that an increase in the concentration of leaching agent in reaction vessel may increase the product layer formation and produce a solid film layer surrounding the particles and this may reduce the rate of leaching process. Therefore, formic acid concentration (8 %) is considered as an optimum in the present case of leaching of magnesite.

Table 3.3.1 Impact of Formic Acid Concentration on % Conversion of Magnesite

Acid

Time (min) Concentration 2 % 4 % 6 % 8 % 10 %

10 5.04 10.56 17.98 29.54 35.34

20 9.11 21.79 36.28 49.92 54.49

30 12.72 32.49 47.06 63.93 68.17

40 15.57 41.67 59.19 73.8 77.95

50 19.21 54.87 73.48 81.44 86.87

60 24.89 64.33 82.36 90.21 93.74

Fig. 3.3.1 Effect of 2 % formic acid concentration on % conversion of magnetite

Fig. 3.3.2 Impact of 4 % formic acid concentration on % conversion of magnesite

Fig. 3.3.3 Effect of 6 % formic acid concentration on % conversion of magnesite

Fig. 3.3.4 Influence of 8 % formic acid concentration on % conversion of magnesite

Fig. 3.3.5 Influence of 10 % formic acid concentration on % conversion of magnetite

3.4 Influence of liquid/solid ratio on dissolution of magnesite

The influence of liquid/solid ratio on leaching kinetics of natural magnesite is elucidated by varying the L/S ratio (vol./wt. basis) from 6:1 to 16:1 mL/g, keeping other experimental parameters constant. Results have been expressed in Table 3.4.1 and Figs.3.4.1-3.4.6. The rate curves in Figs.3.4.1-3.4.6 show that an increase in L/S ratio causes an improvement in dissolution of magnesite. However, its effect on the leaching process is not as dominant as that of the temperature and formic acid concentration. Higher liquid/solid ratio results in an increase of volume of leaching agent and a modification of ratio between Mg and formic ions.

Moreover, the increased volume of reaction mixture may increase filtration time and its handling process. The results also show that liquid/solid ratio greater than 14:1 mL/g causes less significant role. Therefore, the liquid/solid ratio of 14:1 mL/g is found to be promising subjective to applied reaction conditions in the leaching reaction.

Table 3.4.1 Impact of Liquid/solid Ratio on % Conversion of Magnesite

Time L/S Ratio (mL/g) 8 10 12 14 16 (min) 6

10 15.32 21.21 27.89 34.98 39.55 43.38

20 25.93 31.53 39.84 46.08 53.24 57.82

30 32.27 39.12 50.94 60.01 67.14 69.77

40 40.87 47.81 58.71 68.89 74.8 77.95

50 47.65 54.84 66.84 76.48 82.97 85.78

60 53.69 64.52 73.64 81.36 87.53 92.91

% Conversion 20

0 0 10 20 30 40 50 60 Time (min)

Fig. 3.4.1 Effect of 6 mL/g liquid/solid ratio on % conversion of magnetite

% Conversion

0 0 10 20 30 40 50 60 Time (min)

Fig. 3.4.2 Effect of 8 mL/g liquid/solid ratio on % conversion of magnesite

% Conversion 30

0 0 10 20 30 40 50 60 Time (min)

Fig. 3.4.3 Impact of 10 mL/g liquid/solid ratio on % conversion of magnesite

% Conversion

0 0 10 20 30 40 50 60 Time (min)

Fig. 3.4.4 Effect of 12 mL/g liquid/solid ratio on % conversion of magnesite

100

% Conversion

0 0 10 20 30 40 50 60 Time (min)

Fig. 3.4.5 Impact of14 mL/g liquid/solid ratio on % conversion of magnesite

100

% Conversion 40

0 0 10 20 30 40 50 60 Time (min)

Fig. 3.4.6 Impact of 16 mL/g liquid/solid ratio on % conversion of magnesite

3.5 Influence of particle size on conversion of magnesite

Various experiments were carried out to probe the particle size impact. The leaching kinetics of magnesite were performed using 4 sample sizes (500–707, 250–354, 177–210 and 125–

177μm) at 65 °C, 8 % formic acid solution, stirring speed of 350 rpm and a liquid/solid proportion of 14:1 mL/g. The results are described in Table 3.5.1 and Figs. 3.5.1-3.5.4, which show that the increase in particle size of magnesite samples has an inverse impact on the dissolution process. The findings show that efficiency of leaching process of magnesite with formic acid increases with reduction in the particle size of magnesite samples. This situation may be related to the fact that with decrease in particle size of magnesite, the surface area of particles for reaction may become more available for the leaching process. But the extent of increase in efficiency of leaching process will require further reduction of particles by grinding, thus necessitating the need of greater amount of energy required for the purpose of grinding and size reduction which may result in an extra or higher value of cost to the leaching process.

100

Table 3.5.1 Influence of Particle Size of Magnesite on % Conversion of Natural Magnesite

Particle size (µm) Time (min) 250-354 177-210 125-177 500-707

10 15.06 24.46 38.14 41.78

20 28.05 39.11 51.84 56.85

30 39.16 57.61 65.93 70.83

40 52.75 66.49 73.82 77.78

50 60.87 75.08 81.97 85.99

60 68.98 80.85 88.93 91.46

101

% Conversion

0 0 10 20 30 40 50 60 Time (min)

Fig. 3.5.1 Effect of particle size 500-707 µm on % conversion of magnesite

102

% Conversion

0 0 10 20 30 40 50 60 Time (min)

Fig. 3.5.2 Impact of particle size 250-354 µm on % conversion of magnesite

103

Fig. 3.5.3 Influence of particle size 177-210 µm on % conversion of magnesite

104

Fig. 3.5.4 Effect of particle size 125-177 µm on % conversion of magnesite

105

3.6 Kinetic studies

3.6.1 Leaching kinetics of magnesite with formic Acid

Hydrometallurgical processes have a great interest in industrial applications and usually involve solid liquid reaction systems. The kinetic data makes the foundation for successful and efficient performance of chemical reactors for these processes. The kinetic parameters and rate controlling steps [119] for the dissolution of natural magnesite in formic acid solutions can be determined by the analysis of kinetic data. In solid–liquid reactions, the leaching kinetics is governed [113,134] by one of the following mechanisms:

d) Diffusion from fluid films

e) Ash to product layer

f) Chemically controlled reaction

The outcome was examined from shrinking core model to evaluate the rate determining step and reaction conditions affecting leaching kinetics of magnesite. On this ground, a general reaction of solid with fluid can be expressed as:

A( fluid)  bB(Solid)  product (3.6.1.1)

Only two controlling mechanisms (diffusion from fluid films or chemically controlled reaction) may be considered during the reaction if no ash or product layer is produced. If the conversion fraction of natural magnesite be x at any interval t then the integral rate equations for fluid- solid systems can be denoted as:

For the film diffusion controlling mechanism,

t  ko 11 x  (3.6.1.2)

For the chemically controlled reaction,

1/ 3 t  ko 11 x  (3.6.1.3)

For the ash layer diffusion controlled,

106

  2 / 3 (3.6.1.4) t  ko 1 31 x  2(1 x)  

Statistical and graphical methods were applied to test the soundness of experimental data. It was inferred that the experimental results for dissolution reaction of magnesite follow a surface chemical reaction. The integral rate for surface chemical reaction can be expressed as:

1 (1 x)1/ 3  kt (3.6.1.5)

From the analysis of kinetic data, it is found that the Eq. (3.6.1.5) gives the best straight lines in contrast with other kinetic models. Using a known particle size of magnesite rock and formic acid concentration with specific liquid/solid proportion, the results at various reaction

1/ 3 temperatures are plotted between1 (1 x) vs. t as shown in Figs. 3.6.1-3.6.1.5.

107

Fig. 3.6.1.1 Time versus 1-(1-x)1/3 at 45 oC

108

Fig. 3.6.1.2 Time versus 1-(1-x) 1/3 at 55 oC

109

Fig. 3.6.1.3 Time versus 1-(1-x) 1/3 at 65 oC

110

Fig. 3.6.1.4 Time versus 1-(1-x) 1/3 at 75 oC

111

Fig. 3.6.1.5 Time versus 1-(1-x) 1/3 at 45-75 oC

112

From the slopes of straight lines [123] in Figs 3.6.1.1-3.6.1.4, the values of apparent constants are calculated. Using the Arrhenius law,

 E / RT k k e a (3.6.1.6)

Eq. 4.6.1.5 can be expressed as:

1/ 3 Ea / RT 1 (1 x)  ko e t (3.6.1.7)

In Eq. 3.6.1.7, Ea is the activation energy and R is general gas constant. The values of energy

1 of activation and ko are obtained by plotting ln k vs. as shown in Fig. 3.6.1.6. T

113

Fig. 3.6.1.6 Arrhenius plot of ln k vs 1/T

114

By putting the values of ko and energy of activation, Eq. 3.6.1.7 can be represented as:

1 (1 x)1/3 59.41  101e42078/ RT t (3.6.1.8)

The value of activation energy (42.08 kJmol-1) depicts that the dissolution process of natural magnesite in formic acid solution is chemically controlled reaction and this value is in accordance with the results described in published research studies [113,122,123].

In order to find the validation of kinetic model (Eq. 3.6.1.8), experimental and calculated conversion values for magnesite were plotted and the results are shown in Fig.3.6.1.7. From the scatter diagram (Fig. 3.6.1.7) it can be seen that the correlation of experimentally determined conversion values and calculated conversion values is good and the value of correlation coefficient (0.9862) of this agreement also reveals that the workability of kinetic model is good.

115

1.0

0.8

0.6

Xcal 0.4

0.2

0.0 0.0 0.2 0.4 0.6 0.8 1.0 X exp

Fig. 3.6.1.7 Scatter diagram showing relationship between experimental and calculated

conversion values of magnesite

116

Chapter

RECOVERY OF CONSUMED FORMIC

ACID AND ECONOMY OF THE

DISSOLUTION PROCESS

117

4. RECOVERY OF CONSUMED FORMIC ACID

The economic value of dissolution reaction depends on various factors such as the price of lixiviant and raw material, the expenditures in terms of energy consumption and the cost of recovery of leaching agent. The cost of overall leaching process can be reduced by the means of recovery of spent leaching agent (formic acid). Formic acid can be regained from magnesium formate solution by a number of ways as reported in literature [2]. No loss of formic acid can be expected during the dissolution process rendering to condition that the spent leaching agent must be recycled and may be used as a fresh solution. The fate of leaching agent recovery depends on the formation of insoluble salts and isolation of regenerated lixiviant from the mother liquor using inexpensive separation techniques. A few of the methods, applied for the recovery of spent formic acid, are discussed below:

(a) Carbonation of magnesium formate solution

Formic acid may be regenerated from magnesium formate solution by passing carbon dioxide gas at suitable conditions. The carbon dioxide, liberated during the dissolution reaction of magnesite ore with formic acid, can be used for this purpose. The chemical reaction can be shown as:

MgCO3 + 2CH2O2  2Mg (CHO2)2 + CO2 (4.1)

Mg(CHO2)2 + CO2  2CH2O2 + MgCO3 (4.2)

The resultant product (magnesium carbonate) finds various applications in different fields.

(b) Using ion exchange resins

The cation exchange resins can be applied to recover formic acid from the magnesium formate solution. The chemical reaction occurring during the treatment of cationic bed to recover formic acid from magnesium formate solution can be written as:

Mg(CHO2)2 + 2HR  2CH2O2 + MgR2 (4.3)

118

In order to regenerate the cationic bed, hydrochloric acid solution may be used as shown in equation (4.4):

MgR2 + 2HCl  MgCl2+ 2HR (4.4)

Formic acid may be regenerated by treating magnesium formate solution with the phosphoric acid as given in equation:

Mg(CHO2)2 + 2H3PO4  2CH2O2 + Mg3(PO4)2 (4.5)

Magnesium phosphate produced in this reaction can be used for various applications.

(d) Using alkaline solutions

The recovery of formic acid may be achieved by treating the magnesium formate solution with any strong alkali such as sodium hydroxide.

Mg(CHO2)2 + 2NaOH  2NaCHO2 + Mg(OH)2 (4.6)

NaCHO2 + HCl  CH2O2 + NaCl (4.7)

Concerning the future horizons of this study, the regeneration needs further work and details regarding process and designing parameters.

119

4.1 Economy of the dissolution process

Various factors such as availability of the magnesite rock, price of leaching agent and the price of its recovery from the spent liquor, total energy consumption, demand and application of the resultant product contribute towards the dissolution process. In the situations, when the leaching agent consumed in the dissolution process is easily recoverable, encourages the usability of the process for commercial applications. In a closed circuit plant loss of leaching agent may become low. The recovered formic acid and its washing solutions may be recycled to the leaching process which will result in a decrease in the amount of fresh formic acid. In this way a small amount of fresh formic acid will be required to make-up the leaching agent concentration in the dissolution process. The schematic (Fig. 4.1.1) of the recovery of formic acid shows the magnesite dissolution process with formic acid and recovery of formic acid.

Formic acid can be recovered by treating magnesium formate (leaching reaction product) with phosphoric acid. Magnesium phosphate is filtered from the formic acid solution.

The filtrate solution, obtained after filtration of mother liquor, contain magnesium formate salt which may be evaporated to yield crystalline magnesium formate and the product formed

(magnesium formate) can be used in animal feed formulations and in general chemical applications [150]. It can also be used for determining pesticides in fruits and vegetables and for the simultaneous determination of tropane alkaloids and glycol alkaloids in grains and seeds. Another alternative is to sell the different useful products (magnesium sulfate, magnesium hydroxide, magnesium metal) by treating magnesium formate with suitable reagents.

The refractory materials, required in various industries especially steel industry are usually of high quality magnesia. Magnesium oxide is generally obtained from thermal decomposition of naturally occurring magnesite. The industrial production of high quality magnesia (calcined, dead burned and electro fused) from magnesite requires high temperature inputs ranging 1000-

120

3000 °C depending upon the kind of magnesia required, thus demanding high energy consumption and extended time intervals. Alternatively, the magnesium oxide can be obtained by the thermal decomposition of magnesium formate (resultant product of magnesite with formic acid) around 400 °C. The production of magnesium oxide by this route may require relatively less energy consumption and may reduce the calcination time.

Moreover, the by-product CO2 as produced during the dissolution reaction can also be marketed to various industries in order to meet the cost of commercial formic acid and its recovery. It may be anticipated that the selling of carbon dioxide to other industries may pay back some cost regarding operating system of the leaching process of natural magnesite.

121

Fig. 4.1.1 Schematic diagram suggested for the recovery of spent acid.

122

CONCLUSIONS  Concentration of natural magnesite is achieved by physical, thermal and chemical

methods having their own limitations. Usually, thermal and electrolytic methods,

employed for the production of magnesium oxide and magnesium, consecutively

require high energy inputs, that’s why there is a need to find out an efficient and

economical method for the beneficiation of natural magnesite. To avoid high energy

consumptions in a process, hydrometallurgical methods are preferred. In

hydrometallurgical methods inorganic/organic acids or bases and their salts are used.

Because of high selectivity and less corrosion effects, organic acids are preferably used

in contrast to inorganic acids. The calcination analysis of magnesite samples show that

the loss on ignition depends on particle size of magnesite. Organic acids can be utilized

as attractive solvents for the selected leaching and beneficiation studies for various

rocks as these acids have advantages over the mineral acids. Previous studies carried

out for the leaching of magnesite with organic acids have shown that the activation

energy of dissolution process is higher than that found in this research. The answer to

this reason may be due to the fact that formic acid has higher dissociation constant than

that of routinely used organic acids (acetic acid, gluconic acid) and can leach

magnesium content more effectively from magnesite rock. The results obtained from

the present study indicate that formic acid can be employed as a solvent for leaching of

magnesite as it increases the extent of beneficiation required for industrial applications.

A formic acid concentration of 8 % with liquid solid ratio of 14:1 mL/g was found to

be promising for the leaching of magnesite.

 Kinetic data as examined and analysed on the grounds of reaction kinetic models

illustrate that the dissolution process of natural magnesite in formic acid is controlled

123

by surface chemical process. The energy of activation of leaching reaction is 42.08

kJmol-1, which is in agreement with chemically controlled reactions.

 The applicability of suggested kinetic model, i.e.,

1/3 1 42078/ RT 1 (1 x)  59.41  10 e t . for leaching of magnesite with formic acid

is evaluated from the correlation between calculated conversion values and the

experimental ones. The scatter diagram illustrates that the correlation of experimentally

determined and calculated conversion values is good and the value of correlation

coefficient of this agreement also reveals that workability of the kinetic model is good.

 The economy of leaching process is mainly dictated by the cost of leaching agent and

the price of its recovery. The regeneration of organic leaching agents is critical for

further applications and recycling of dissolution process. Formic acid may be recovered

from its soluble salt magnesium formate in aqueous solutions by number of routes.

 In the dissolution reaction of magnesite in formic acid solutions, the reaction product

formed (magnesium formate) can be used in various animal feed formulations as well

as in general chemical applications. Alternatively, magnesium formate can be

employed for the industrial production of magnesia on account of less energy

consumption as compared to trivial route of decomposition of magnesite at elevated

temperatures for prolonged durations.

It is suggested to extend research study in the future, regarding conditions of regeneration process, the process design parameters and evaluation of characteristics of magnesia obtained by the thermal decomposition of magnesium formate.

124

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125

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Appendix A

Published Work

[1] Raza, N., Zafar, Z.I., Najam-Ul-Haq, M., 2014. Utilization of formic acid solutions

in leaching reaction kinetics of natural magnesite ores. Hydrometallurgy 149, 183-

188.

[2] Raza, N., Zafar, Z.I., Najam-Ul-Haq, M., Kumar, R.V., 2015. Leaching of natural

magnesite ore in succinic acid solutions. International Journal of Mineral

Processing 139, 25-30.

142

Hydrometallurgy 149 (2014) 183–188

Contents lists available at ScienceDirect

Hydrometallurgy

journal homepage: www.elsevier.com/locate/hydromet

Utilization of formic acid solutions in leaching reaction kinetics of natural magnesite ores

Nadeem Raza ⁎, Zafar Iqbal Zafar, Muhammad Najam-ul-Haq

Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, 60800, Pakistan article info abstract

Article history: In the present research work, dissolution kinetics of natural magnesite is carried out using formic acid as a Received 12 November 2013 leaching agent. The effect of various reaction parameters such as temperature, acidic solution concentration, par- Received in revised form 22 June 2014 ticle size and liquid to solid ratio was studied regarding the leaching kinetics of natural magnesite. The ﬁndings Accepted 16 August 2014 show that the dissolution process is controlled by the chemical reaction (intrinsic) at the liquid-solid interface; Available online 24 August 2014 −ðÞ− 1=3¼ : 1 −42078=RT : Keywords: 1 1 x 59 41 10 e t Magnesite Formic acid The apparent activation energy of the leaching process of magnesite with the formic acid was found to be − Dissolution kinetics 42.08 kJ mol 1 over the reaction temperature range of 318 to 348 K. Regression analysis © 2014 Elsevier B.V. All rights reserved. Kinetic model

1. Introduction 1996). Conversely, organic acids can act as active leaching reagents because most of the leaching reactions are done in mild acidic conditions Magnesium, the 6th most common element, does not occur freely in (pH 3–5). Moreover, organic acids have a low risk of corrosion and nature because of its high reactivity. It is abundant in magnesite, peri- can be utilized for carbonaceous rocks. Another advantage of organic clase, asbestos, meerschaum, serpentine, talc and epsomite (Deangelis acids as leaching agents is their biodegradability, which generally de- et al., 2007). Magnesium is important in many aspects of life and its pends on the carbon chain and other attached groups. uses involve synthesis of Grignard reagent, alloy formation, sulfur From leaching investigations of naturally occurring magnesite mate- removal in iron and steel production, refractory materials, fertilizers, rials in acetic acid, gluconic acid, citric acid and lactic acid solutions, it medicinal products, and fireproofing (Bukovisky, 1997; Jones et al., was found that leaching kinetics were driven by chemically controlled 2000). mechanisms (Bayrak et al., 2010; Lacin and Bakan, 2006; Lacin et al., A wide range of studies have been reported on leaching and dissolu- 2005). tion kinetics of rocks with a variety of leaching agents (Demirkiran, Magnesite ore deposits are found abundantly in the Khuzdar areas of 2008, 2009; Demirkýran and Künkül, 2007; Demirkiran and Kunkul, Balochistan (Pakistan). The Khuzdar area consists of a number of 2008; Dogan and Yartasi, 2009; Kovacheva et al., 2001; Kuslu and Colak, Kraubath type magnesite deposits associated with alpine type bela 2010; Mergene and Demirhan, 2009; Sandstrom and Samuelsson, 2010; ophiolite dating from the cretaceous period (Bashir et al., 2009). These Zhang and Nichol, 2010). In these research studies, it has been described are mostly serpentinized and harzburgite confined mostly in the that the leaching kinetics of different metal ores may vary with the lowermost segment of bela ophiolite. These deposits have not been change in nature and type of rock deposits. Leaching of magnesite considered extensively for leaching kinetic investigations up to this rocks can be achieved by acids (inorganic/organic) or bases and their time. Moreover, no literature has been reported concerning formic salts (Lacin et al., 2005). The studies on the dissolution process of mag- acid as a leaching agent for the study of leaching kinetics of indigenous nesite in inorganic acids such as HCl, H2SO4 (Abali et al., 2006; Chou magnesite. et al., 1989) showed that the controlling mechanism for the Therefore, the current research work was intended to investigate the dissolution of magnesite is a chemical process. Inorganic acids have leaching reaction kinetics of the indigenous magnesite ore by formic issues of selectivity, froth formation and scaling (Housmanns et al., acid. The product formed (magnesium formate) can be used in animal feed formulations and in general chemical applications (Shi et al., 2005). It can also be used for determining pesticides in fruits and vege- ⁎ Corresponding author. Tel.: +92 300 6340861. tables and for the simultaneous determination of tropane alkaloids and E-mail address: [email protected] (N. Raza). glycol alkaloids in grains and seeds.

2. Experimental procedures that the material under study is magnesium rich. The profile also exhibits the presence of carbon and oxygen indicating the presence of 2.1. Sample protocol and analyses carbonates. The magnesite in pure form is usually white in appearance but becomes colored due to the inclusion of impurities. The SEM Samples of magnesite involved in the current study were collected image of the raw magnesite ore representing the apparent morpholog- from the Khuzdar area in the province of Balochistan (Pakistan). ical properties of the magnesite is given in Fig. 2. The material appears to Samples were ground in a ball mill followed by a mortar grinder. Differ- be non-granular with surface roughness, which may be due to the ent size fractions (500–707, 250–354, 177–210 and 125–177 μm) were evolution of volatiles during the weathering and formulation of the ore. obtained from screening of ground samples by ASTM sieves, and then dried in an electric oven at 100 °C for 24 h. These samples were brought 3.2. Chemical reactions taking place in the glass reactor to room temperature and stored. Atomic absorption spectrometry, scanning electron microscopy and other conventional methods Chemical reactions occurring in the reaction vessel containing (Furmann, 1963) were applied for the analysis of magnesite rock frac- magnesite and formic acid solution is represented as:(a) Formic acid tions. The formic acid (methanoic acid), EDTA (ethylene diamine tetra ionization acetic acid) and EBT (eriochrome black T) used during analysis were ↔ þ þ ‐1 ð Þ of reagent grade. The preparation of stock solutions and their further CH2O2 H CHO2 1 dilutions were made using deionized water. Eriochrome black T and EDTA were used in volumetric quantitative determinations of magne- (b) Diffusion of H+ ions(c) Attack of H+ ions on the magnesite particles sium in leached solutions. In complexometric titrations of magnesium þ þ ↔ þ 2þ ð Þ (Gulensoy, 1974), EBT acts as an indicator while EDTA is used as a 2H MgCO3 H2CO3 Mg 2 complexing agent for Mg. (d) Reaction between Mg2+ and formate ions 2.2. Detection measurements and analytical procedure 2þ þ −1↔ ðÞ ð Þ Mg 2CHO2 Mg CHO2 2 3 Atomic absorption spectrophotometer (Hitachi-1800) was used for the determination of Mg in natural magnesite samples. Scanning elec- The ionization constant of formic acid is pKa = 3.75 at 20 °C and the −6 tron microscope (JEOL JED-2300) was used to observe the particle mor- solubility product constant for MgCO3 is 6.8 × 10 at 25 °C (Harned phology of raw magnesite. Leaching studies of magnesite samples were and Embree, 1934; Visscher et al., 2012). Ashraf et al. (2005) described investigated in a reactor made up of glass with 500 mL capacity. A hot that the magnitudes of the constants (ionization constant and solubility fi plate (IKA C-MAGHS-7) equipped with a temperature sensor (ETS-D5) product constant) are generally temperature dependent and ef ciency was used to stir, heat and control the temperature (±0.5 K) of the of the leaching reaction is restricted by the kinetics. reactor contents. In each experiment, a fixed volume of 8% formic acid having an L/S ratio of 14:1 mL/g was gradually introduced to the reactor 3.3. Effect of time and reaction temperature with 5 g of sample. These entities were agitated with stirring rate of 350 rpm at known times and temperatures. After completion of the The reaction temperature was varied from 45 °C to 75 °C to eluci- fl reaction, the hot solution was filtered to remove gangue minerals date the in uence of temperature on the rate of dissolution of magne- – from magnesium formate. The filtrate solution was analyzed to find site while keeping the other experimental conditions constant (177 μ the percentage of conversion of magnesite (Gulensoy, 1974). 210 m particle size, 8% formic acid, liquid/solid ratio of 14:1 mL/g and stirring speed of 350 rpm). The results are shown in Fig. 3, which illustrates that an increase in reaction temperature causes a rise in the 3. Results and discussion rate of conversion of the magnesite. The elevation of reaction temperature from 55 °C to 75 °C in 30 min causes an increase in % recovery of 3.1. Sample characterization magnesite from 49.8% to 84.9%. This situation indicates that an increase in the reaction temperature increases the rate of chemical reaction. Typ- To find loss on ignition, magnesite rock samples were heated to ical rate curves in Fig. 3 represent the temperature increase, which re- 950 °C for 24 h in a furnace. The carbonates of different elements duces the reaction time needed to achieve the maximum conversion. were converted into their oxides with the liberation of CO . The chemi- 2 About 95% dissolution of magnesite is achieved at 75 °C in 40 min of cal composition of the calcined magnesite sample (Table 1)wasdeter- leaching time. A number of experiments were carried out to determine mined by atomic absorption spectrometer. Table 1 indicates that the effect of formic acid concentration, L/S ratio in the medium and the magnesium is present in relatively higher concentrations than the particle size of magnesite on the dissolution kinetics of magnesite at other elements. The EDX spectrum of the intact ore of magnesite 65 °C. (Fig. 1) depicts the elemental composition. The elemental composition showing the mass and atomic % of different elements in the indigenous 3.4. Influence of acid concentration and L/S ratio magnesite is given in Table 2. From the EDX signature, it can be inferred In order to find the influence of concentration of formic acid on the leaching of magnesite, the concentration of formic acid was varied from 2% to 10% at a temperature of 65 °C, stirring rate 350 rpm and par- Table 1 ticle size fraction 177–210 μm. The results are shown in Fig. 4, which a Chemical composition of natural magnesite ore . shows that an increase in acid concentration accelerates the rate of Component (Mass.%) leaching of magnesite. However, concentrations higher than 8% do not have an appreciable effect as expressed in Fig. 4. This situation may be MgO 43.47 CaO 0.38 attributed to the fact that a relatively higher concentration of the

Fe2O3 1.11 leaching agent may attack the gangue minerals present in the ore. SiO2 1.54 Ozmetin et al. (1996) described that an increase in the concentration Loss on ignition (at 950 °C for 24 h) 53.5 of leaching agent in the reaction vessel may increase the product layer a Results obtained from analysis of magnesite ore by AAS (Hitachi-1800). formation and produce a solid ﬁlm layer surrounding the particles and N. Raza et al. / Hydrometallurgy 149 (2014) 183–188 185

Fig. 1. The EDX pattern of natural magnesite.

this may reduce the rate of the leaching process. The influence of the liq- 4. Kinetic analysis uid/solid ratio was elucidated by varying the L/S ratio from 6:1 to 16:1 mL/g, keeping the other experimental parameters constant. Exper- Hydrometallurgical processes usually involve solid–liquid reaction imental results have been expressed in Fig. 5. The rate curves in Fig. 5 systems. In solid–liquid reactions, the leaching kinetics is governed show that an increase in the L/S ratio causes an improvement in the dis- (Lacin et al., 2005; Levenspiel, 1972) by one of the following mechanisms: solution of magnesite. However, its effect on the leaching process is not a) Diffusion from fluid films as dominant as that of the temperature and formic acid concentration. b) Ash to product layer Higher liquid/solid ratio results in an increase of volume of leaching c) Chemically controlled reactions agent and a modification of the ratio between Mg and formic ions. Moreover, the increased volume of the reaction mixture may increase The outcome was examined from the shrinking core model to eval- filtration time and its handling process. uate the rate-determining step and reaction conditions affecting leaching kinetics of magnesite. On these grounds, a general reaction of solid with fluid can be expressed as: 3.5. Influenceofparticlesizeofmagnesite þ → ð Þ AðÞfluid bBðÞSolid product 4 Various experiments were performed to probe the particle size impact. The leaching kinetics of magnesite were performed using 4 sample sizes (500–707, 250–354, 177–210 and 125–177 μm) at 65 °C, 8% Only two controlling mechanisms (diffusion from fluid films or formic acid solution, stirring speed of 350 rpm and a liquid/solid propor- chemically controlled reaction) may be considered during the reaction tion of 14:1 mL/g. The results are summarized in Fig. 6, which shows if no ash/product layer is produced. If the conversion fraction of natural that the increase in particle size of magnesite samples has an inverse magnesite is x at any interval t, then the integral rate equations for impact on the dissolution process. This situation may be related to the fluid–solid systems can be denoted as: fact that with the decrease in particle size of the magnesite, the surface For the film diffusion controlling mechanism, area of the particles for reaction may become more available for the leaching process. ¼ ½−ðÞ− ðÞ t ko 1 1 x 5

Table 2 Elemental analysis of natural magnesite orea.

Element (keV) (Mass %) (Atomic %)

C 0.277 14.591 20.446 O 0.525 57.567 60.501 Mg 1.253 26.086 18.2767 Si 1.739 0.7186 0.4314 Ca 3.690 0.268 0.1127 Fe 6.398 0.7728 0.2321

a Results obtained from EDX (JEOL, JED-2300). Fig. 2. SEM image of natural magnesite. 186 N. Raza et al. / Hydrometallurgy 149 (2014) 183–188

Fig. 5. Effect of liquid/solid ratio on leaching of magnesite ore (formic acid concentration, – μ Fig. 3. Effect of temperature on leaching of magnesite ore (formic acid concentration, 8%; 8%; particle size, 177 210 m; stirring speed, 350 rpm; temperature, 65 °C). particle size, 177–210 μm; stirring speed, 350 rpm; liquid/solid ratio, 14:1 mL/g). as shown in Fig. 7. Considering Arrhenius equation, Eq. (8) can be expressed as:

For the chemically controlled reaction, −ðÞ− 1=3 ¼ −Ea=RT ð Þ 1 1 x koe t 9 hi ¼ −ðÞ− 1=3 ð Þ t ko 1 1 x 6 In Eq. (9), Ea is the activation energy and R is the general gas constant. The values of energy of activation and ko are obtained by plotting For the ash layer diffusion controlled, :1 lnkvs T as shown in Fig. 8. By putting the values of ko and energy of ac- hi tivation, Eq. (9) can be represented as: ¼ − ðÞ− 2=3þ ðÞ− ð Þ t ko 1 31 x 21 x 7 = − = 1−ðÞ1−x 1 3¼59:41 101e 42078 RTt ð10Þ Statistical and graphical methods were applied to test the soundness of the experimental data. It was inferred that the experimental results − for the dissolution reaction of magnesite follow a surface chemical reac- The value of activation energy (42.08 kJ mol 1) depicts that the dis- tion. The integral rate for the surface chemical reaction can be expressed solution process of natural magnesite in formic acid solution is a chem- as: ically controlled reaction and this value is in accordance with the results described in the published research studies (Ashraf et al., 2005; Demir = 1−ðÞ1−x 1 3¼kt ð8Þ et al., 2003; Lacin et al., 2005). In order to ﬁnd the validation of the kinetic model (Eq. (10)), exper- Using the conversion values for various reaction temperatures, the imental conversion values and calculated conversion values for magne- apparent rate constant k, is determined by plotting 1 − (1 − x)1/3vs. t site were plotted and the results are shown in Fig. 9. From the scatter diagram (Fig. 9), it can be seen that the correlation of the experimentally determined conversion values and calculated conversion values is good

Fig. 4. Effect of formic acid concentration on leaching ore (particle size, 177–210 μm; stir- Fig. 6. Effect of particle size on leaching of magnesite ore (formic acid concentration, 8%; ring speed, 350 rpm; liquid/solid ratio, 14:1 mL/g; temperature, 65 °C). stirring speed, 350 rpm; liquid/solid ratio, 14:1 mL/g; temperature, 65 °C). N. Raza et al. / Hydrometallurgy 149 (2014) 183–188 187

− − 1/3 Fig. 7. Plot of 1 (1 x) and 1/t. Fig. 9. Agreement between experimental conversion values and calculated conversion values of magnesite. and the value of correlation coefﬁcient (0.9862) of this agreement also Explanation of symbols reveals that the workability of the kinetic model is good.

−1 5. Conclusions Ea activation energy (J mol ) t reaction time (min) T reaction temperature (K) • Organic acids can be used as attractive solvents for selective leaching EDX energy-dispersive X-ray analysis and beneﬁciation studies for various rocks as these acids have some SEM scanning electron microscope advantages over the mineral acids. AAS atomic absorption spectrophotometer • The results indicate that formic acid can be employed as a solvent for leaching of magnesite. A formic acid concentration of 8% with a liquid/ solid ratio of 14:1 mL/g was found to be promising for the leaching of magnesite. Acknowledgement • The kinetic data analyzed on the basis of different reaction kinetic models illustrates that the dissolution process of natural magnesite The authors are grateful to the anonymous reviewers for their con- in formic acid is controlled by surface chemical reaction. The energy structive comments and improvement to the manuscript. The authors of activation of the leaching reaction is 42.08 kJ mol−1. also thank the Institute of Chemical Sciences, B.Z.U., Multan for provid- • In the dissolution reaction of magnesite in formic acid solution, the ing the facilities. reaction product formed (magnesium formate) can be used in various animal feed formulations as well as in various more general chemical References applications. Abali, Y., Yavuz, M., Copur, M., 2006. Determination of the optimum conditions for

dissolution of magnesite with H2SO4 solutions. Ind. J. Chem. Tech. 13, 391–397. Ashraf, M., Zafar, I.Z., Ansari, T.M., 2005. Selective leaching kinetics and upgrading of low- grade calcareous phosphate rock in succinic acid. Hydrometallurgy 80, 286–292. Bayrak, B., Lacin, O., Sarac, H., 2010. Kinetic study on the leaching of calcined magnesite in gluconic acid solutions. J. Ind. Eng. Chem. 16, 479–484. Bashir, E., Naseem, S., Sheikh, S.A., Kaleem, M., 2009. Mineralogy of kraubath-type magnesite deposits of the Khuzdar area, Balochistan, Pakistan. J. Earth Sci. Appl. Res. 30, 169–180. Bukovisky, V., 1997. Yellowing of newspaper after deacidiﬁcation with methyl magnesium carbonate. Int. J. Preserv. Libr. Arch. Mater. 18, 25–38. Chou, L., Garrels, R.M., Wollast, R., 1989. Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals. Chem. Geol. 78, 269–282. Deangelis, M.T., Labotka, T.C., Cole, D.R., Fayek, M., Anovitz, L.M., 2007. Experimental investigation of the breakdown of dolomite in rock cores at 100 MPa, 650–750 °C. Am. Mineral. 92, 510–517. Demir, F., Donmez, B., Colak, S., 2003. Leaching kinetics of magnesite in citric acid solutions. J. Chem. Eng. Jpn 36, 683–688. Demirkýran, N., Künkül, A., 2007. Dissolution kinetics of ulexite in perchloric acid solutions. Int. J. Miner. Process. 83, 76–80. Demirkiran, N., Kunkul, A., 2008. Dissolution kinetics of ulexite in acetic acid solutions. Chem. Eng. Res. Des. 86, 1011–1016. Demirkiran, N., 2008. A study on dissolution of ulexite in ammonium acetate solutions. Chem. Eng. J. 141, 180–186. Demirkiran, N., 2009. Dissolution kinetics of ulexite in ammonium nitrate solutions. Hy- drometallurgy 95, 198–202. Dogan, H.T., Yartasi, A., 2009. Kinetic investigation of reaction between ulexite ore and phosphoric acid. Hydrometallurgy 96, 294–299. Furmann, N.H., 1963. Standard Methods of Chemical Analysis, sixth ed. D. Van Nastrand Fig. 8. Plot of lnk and 1/T. Company, New Jersey, USA. 188 N. Raza et al. / Hydrometallurgy 149 (2014) 183–188

Gulensoy, H., 1974. Kompleksometrik titrasyonlar ve kompleksometrinin temelleri. Fatih Mergene, A., Demirhan, H.M., 2009. Dissolution kinetics of probertite in boric acid Yayinevi, Istanbul, Turkey. solutions. Int. J. Miner. Process. 90, 16–20. Harned, H.S., Embree, N.D., 1934. The ionization constant of formic acid from 0 to 60°. J. Ozmetin, C., Kocakerim, M.M., Yapici, S., Yartasi, A., 1996. A semi empirical kinetic model

Am. Chem. Soc. 56, 1042–1044. for dissolution of colemanite in aqueous CH3COOH solutions. J. Ind. Eng. Chem. Res. Housmanns, S., Laufenberg, G., Kunz, B., 1996. Rejection of acetic acid and its improve- 35, 2355–2359. ment by combination with organic acids in dilute solutions using reverse osmosis. Sandstrom, A., Samuelsson, C., 2010. Dissolution kinetics of tetrahedrite mineral in Desalination 104, 95–98. alkaline sulphide media. Hydrometallurgy 103, 167–172. Jones, P.T., Blanpain, B., Wollants, P., Ding, R., Hallemans, B., 2000. Degradation mecha- Shi, D., Yu, X., Roth, L., Morizono, Y., Hathout, N., Allewell, M., Tuchman, M., 2005. Expres- nisms of magnesia-chromite refractories in vacuum oxygen decarburization ladles sion, puriﬁcation, crystallization and preliminary X-ray crystallographic studies of a during production of stainless steel. Iron Making and Steel Making. 27, pp. 228–237. novel acetylcitrulline deacetylase from Xanthomonas campestris. Acta Crystallogr. Kovacheva, P., Arishtirova, K., Vassilev, S., 2001. MgO/NaX zeolite as basic catalyst for F61, 676–679. oxidative methylation of toluene with methane. Appl. Catal. 210, 391–395. Visscher, A.D., Vanderdeelen, J., Nigsberger, E.K., Churagulov, B.R., Ichikuni, M., Tsurumi, Kuslu, S., Colak, S., 2010. Leaching kinetics of ulexite in borax pentahydrate solutions M., 2012. IUPAC-NIST solubility data series. 95. Alkaline earth carbonates in aqueous saturated with carbon dioxide. J. Ind. Eng. Chem. 16, 673–678. systems. Part 1. Introduction, Be and Mg. J. Phys. Chem. Ref. Data 41 (1). Lacin, O., Donmez, B., Demir, F., 2005. Dissolution kinetics of natural magnesite in acetic Zhang, S., Nichol, J.M., 2010. Kinetics of the dissolution of ilmenite in sulfuric acid acid solutions. Int. J. Miner. Process. 75, 91–99. solutions under reducing conditions. Hydrometallurgy 103, 196–204. Lacin, O., Bakan, F., 2006. Dissolution kinetics of natural magnesite in lactic acid solutions. Int. J. Miner. Process. 80, 27–34. Levenspiel, O., 1972. Chemical Reaction Engineering, second ed. John Wiley & Sons, New York. International Journal of Mineral Processing 139 (2015) 25–30

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International Journal of Mineral Processing

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Leaching of natural magnesite ore in succinic acid solutions

Nadeem Raza a,b,⁎, Zafar Iqbal Zafar a, Najam-ul-Haq a,R.V.Kumarb a Institute of Chemical Sciences, Bahauddin Zakariya University, Multan, 60800, Pakistan b Department of Material Science & Metallurgy, University of Cambridge, United Kingdom article info abstract

Article history: The study of succinic acid as a leaching agent has been undertaken to probe the leaching of naturally occurring Received 31 December 2013 magnesite samples. The inﬂuence of various reaction conditions (acidic strength, particle size of the magnesite, Received in revised form 20 February 2015 liquid to solid proportion, reaction temperature and stirring rate) was investigated. The results indicated that Accepted 21 April 2015 the extraction of magnesium depends on acidic strength, reaction temperature, ore particle size, stirring rate Available online 23 April 2015 and liquid solid ratio. The application of graphical and statistical approaches for the analysis of kinetic data Keywords: revealed that the rate of extraction of magnesium from natural magnesite is determined by the chemical reaction −1 Leaching kinetics step. The calculated energy of activation for the dissolution of magnesite is 45.197 kJ mol in the reaction Magnesite ore temperature span of 313 K to 343 K. Succinic acid © 2015 Elsevier B.V. All rights reserved. Statistical analysis

1. Introduction leaching solutions in terms of poor selectivity, high rates of corrosion of the reaction vessels, significant environmental problems, enhanced Magnesite is considered one of the most suitable ores for obtaining froth formation and difficulty in achieving pH control of the reaction magnesium and its various compounds. After aluminum and iron, mag- medium (Raza et al., 2014). nesium is considered the third most commonly used structural metal. Organic acids can act as active leaching reagents in situations where Magnesium and its compounds are widely applied in alloy formation, mild acidic conditions are essential. In the leaching of ores where com- desulfurization of molten iron, fertilizers, pyrotechnics, aerospace, paratively less acidic solution strengths are preferable, organic acidic so- flares, synthesis of Grignard reagent, refractory materials, food, and lutions may be considered more selective than inorganic acid solutions. many other areas (Jones et al., 2000). Moreover organic acids as leaching agents cause less corrosion and froth Magnesite rocks from different locations show differences in acidu- amassing. It is also possible to obtain organic acids from renewable bio- lation processes on account of variation in composition of magnesite de- logical sources. posits. Impurities contained in magnesite rocks in general are silica, Veeken and Hamelers (1999) studied the feasibility of citric acid as a calcium, iron, etc. and may become the causes of undesirable impacts leaching agent to extract heavy metals from sewage sludge. Zafar and on the usage of compounds of magnesium. Ashraf (2007) undertook dissolution studies of calcareous phosphate Many studies concerning leaching of rocks with a variety of leaching rock samples using an organic leaching agent (lactic acid) and evaluated solutions have been carried out (Awe et al., 2010; Demirkıran, 2009; that the dissolution of calcareous rock was governed by chemically con- Dogan and Yartasi, 2009; Mergen and Demirhan, 2009; Zhang and trolled reaction steps. In another research work Ashraf et al. (2005) uti- Nicol, 2010). Leaching of magnesite ore is generally done on industrial lized solutions of succinic acid for the dissolution of phosphate rock scale using hydrometallurgical reactors (Kennedy, 1996). samples. They evaluated that the rate determining step for the dissolu- Leaching of magnesite rocks can be achieved by organic/inorganic tion of phosphate rock is a chemical reaction step at specific process bases or acids or/and their salts. Dissolution of magnesite rock in inor- conditions. ganic acids such as HCl and H2SO4 was found to be chemically controlled Various research studies considering usage of different organic acids (Abali et al., 2006; Kurtbas et al., 1992). Pokrovsky et al. (2009) studied such as ascorbic acid, citric acid and gluconic acid for the dissolution of leaching of dolomite, calcite, and magnesite in various acids and magnesite ores are available (Bayrak et al., 2010; Demir et al., 2003; established that higher values of temperature and partial pressure of Raza et al., 2014). In these studies it was inferred that the rate control-

CO2 cause reduction in reactivity of carbonate mineral in aqueous me- ling step in the leaching kinetics of magnesite samples with organic diums. There are certain restrictions for the use of inorganic acids as acids is surface chemical reaction. Lacin et al. (2005) and Bakan et al. (2006) investigated that the leaching of naturally occurring magnesite in solutions of acetic acid and lactic acid is chemically controlled. No lit- ⁎ Corresponding author. erature has been found related to the application of succinic acid solu- E-mail address: [email protected] (N. Raza). tions for the leaching and kinetic studies of natural magnesite samples.

In Pakistan the natural magnesite ore deposits are widely found in solutions (García-Casal and Layrisse, 2001; Nishi et al., 2011), therefore the Khuzdar area of Balochistan. Bashir et al. (2009) has reported that succinate salts of calcium and iron do not interfere in the analysis of the Khuzdar area consists of a number of Kraubath type magnesite magnesium. deposits associated with alpine type Bela ophiolite of Cretaceous age. These deposits are mostly serpentinized, harzburgite conﬁned mostly 3. Chemical reactions during leaching of magnesite ore in the lowermost segment of bela ophiolite. The leaching reaction kinetics for these deposits has not yet been systematically studied or re- The dissolution of the magnesite with succinic acid ported. Therefore, the present work has been intended to probe the dis- (HOOC(CH2)2COOH) can be displayed as taking place in the follow- solution and kinetic studies of the indigenous magnesite samples in ing steps: Pakistan at different experimental conditions using succinic acid. (a). Ionization of succinic acid HOOC(CH ) COOH. Succinic acid is increasingly available industrially from biological 2 2 sources whereby glucose is subjected to fermentation (Song and Lee, þ − C H O →2H þ C H O 2 : ð1Þ 2006). 4 6 4ðÞaq ðÞion 4 4 4 ðÞion

2. Methods and materials (b). Flow of hydrogen ions to the uncovered surface of rock. (c). Attack of H+ ions on sample particles: 2.1. Sample preparation þ þ → þ 2þ ð Þ 2HðÞion MgCO3ðÞS H2CO3ðÞaq MgðÞion 2 The collection of natural magnesite samples was carried out from Balochistan (Pakistan). The grinding of magnesite samples was The hydrogen ions in the above reactions may be generated from achieved by using ball mill and mortar grinder. The ground samples the carbonic acid and succinic acid. – – 2+ −2 were sieved to obtain particle size fractions (500 707, 250 354, (d). Chemical reaction between Mg and C4H4O4 177–210 and 125–177 μm) using Tyler mesh screens. The purpose of 2þ þ −2 → : ð Þ sieving was to obtain small sized fractions with larger surface area. MgðÞion C4H4O4 ðÞion MgC4H4O4ðÞaq 3 The drying of sieved samples was done in an oven at 100 °C. These samples were brought to ambient temperature and were kept in well dried plastic jars. Instrumental techniques such as EDX (energy dispersive X- ray), AAS (atomic absorption spectrometer) and XRD (X-ray diffractom- The following general equation can be used to represent the overall eter) were utilized to analyze the magnesite sample fractions in addi- leaching process: tion to the other traditional methods such as standard gravimetric and þ → þ þ : ð Þ volumetric (Furman, 1962). Succinic acid (1,4-butanedioic acid), EDTA H2YðÞaq MCO3ðÞS CO2ðÞg MYðÞaq H2O 4 (Ethylene diamine tetra acetic acid) and EBT (Erio Chrome Black T) used in this research work were of reagent grade. EDTA and EBT In the above reaction Y stands for the succinate ion. Solubility prod- −6 were used in volumetric titrations for the quantitative determination uct constant for MgCO3 is 6.8 × 10 at 25 °C and dissociation constants of magnesium in leached solutions. All the stock solutions and their fur- for succinic acid are pK1 = 4.21, pK2 = 5.64. The chemical equilibrium ther dilutions were made in deionized water. for the above reaction (4) is expected to remain in the right direction and thought to be an irreversible reaction because one of the reaction

2.2. Detection measurement product (CO2) is taken out from the medium.

An atomic absorption spectrophotometer (Hitachi-1800) was used 4. Results and discussion for the estimation of Mg in rock and leached solutions. In order to image the ore morphology of magnesite samples, a scanning electron 4.1. Magnesite morphology microscope (Hitachi S-3000H) was utilized. The composition of the magnesite sample was determined using X-ray diffractometer and en- The analytical results obtained from the analysis of calcined magne- ergy dispersive X-ray. site ore using atomic absorption spectrometer are given in Table 1, which indicates the presence of magnesium oxide as a major compo- 2.3. Experimental procedure nent of calcined magnesite rock samples. In order to find the mineralogical analysis of magnesite rock sample, an XRD (Philips X pert pro 3040/ In order to study the leaching of magnesite, samples with altered 60) using Cu Kα radiation in step mode between 10° and 80° was size fractions (500–707, 250–354, 177–210 and 125–177 μm) were uti- employed. The identification of peaks in XRD spectrum was carried lized in a spherical glass batch reactor having a capacity of 500 mL, out by comparing with the ICSD XRD patterns. The mineralogical analy- armed with a magnetic stirrer for effective mixing, a thermostat, timer sis (Fig. 1) showed that the magnesite ore sample is mainly composed of for controlling leaching time and a cooler for terminating the reaction. magnesium carbonate. The XRD pattern also reveals the presence of sil- The experiments were undertaken with succinic acid of various ica and calcium carbonate as impurities. The image of the raw magnesite strengths at a variety of liquid/solid (L/S) proportions. In each experi- sample obtained from the scanning electron microscope is presented in ment a specified volume of the leaching agent was inserted gradually Fig. 2, which illustrates the morphological characteristics of magnesite to the glass reactor having 5 g of magnesite sample of a specified particle size. The reaction variables studied were reaction temperature, liquid to solid proportion, succinic acid concentration, stirring rate, size of ore Table 1 particles and time of leaching reaction. After the completion of chemical Chemical analysis of natural magnesite calcined ore. reaction, the reaction vessel contents were strained to remove insoluble Component [wt.%] fi impurities. The volumetric analysis of the ltrate solution for magne- MgO 46.5 sium contents was undertaken to find percentage of conversion CaO 1.12

(Furman, 1962). Calcium and iron in the natural magnesite ore also Fe2O3 0.35 react with succinic acid and form their respective succinate salts. The SiO2 0.83 Loss on ignition [at 950 °C] 51.2 solubility of calcium and ferrous succinate is very low in aqueous N. Raza et al. / International Journal of Mineral Processing 139 (2015) 25–30 27

Fig. 1. XRD pattern of magnesite ore. sample. Several morphological features corresponding to the siliceous Fig. 3. Effect of temperature on leaching of magnesite (7% succinic acid, 177–210 μmpar- ticle size, liquid/solid ratio 25:1 mL/g, stirring speed 350 rpm). and argillaceous material can be seen from the SEM micrograph. The material seems to be non-granular with the presence of surface roughness. The surface roughness may be due to the evolution of volatiles. conversion of magnesite ore from 47.7% to 88% in 40 min of reaction. Some curls and curvatures are also evident. However, acid concentration higher than 7% has no appreciable effect on the dissolution kinetics of magnesite. It has been reported that 4.2. Impact of reaction temperature on dissolution process when the acid strength is higher than the required one, the hydrogen ions present in the reaction medium may reduce with reduction in Using different experimental conditions (177–210 μm particle size, amount of water (Marinovic and Despic, 1997). From the leaching of 7% (g/100 mL) succinic acid solution along with 25:1 mL/g liquid/solid colemanite ore using acetic acid solutions, Ozmetin et al. (1996) in- proportion and 350 rpm stirring speed), influence of reaction tempera- ferred that a higher concentration of leaching agent in the reaction me- ture (40 °C to 70 °C) on the leaching of magnesite samples was evaluat- dium increased the appearance of a product with the development of a ed. The rate curves are presented in Fig. 3, which shows that an sparingly solid film layer and resulted in lessening of leaching reaction elevation in reaction temperature enhances the rate of dissolution of rate. From Fig. 4, it can be observed that acidic solution concentrations Mg from the magnesite with a corresponding reduction in time of reac- of 7% and 8% have almost parallel reaction efficiencies that's why tion. A rise in temperature from 50 °C to 70 °C increases the % recovery acid concentration of 7% was considered for further experimental of magnesium from 73.3 to 90.9% for reaction duration of 40 min. About observations. 99.9% of magnesium contained in the magnesite sample was recovered In order to find the impact of liquid/solid proportion on the extrac- at a 40 min reaction time and reaction temperature of 70 °C. In order to tion of magnesium, a series of experiments was done by altering the liq- find the effect of other reaction parameters 60 °C reaction temperature uid/solid proportion from 10:1 to 27.5:1 mL/g. The experimental results was used. are presented in Fig. 5, which depicts that the dissolution of magnesite increases from 78% to 86% with an increase in liquid to solid proportion 4.3. Influence of acidic strength and liquid to solid proportion on leaching from 20:1 to 25:1 mL/g. Fig. 5 also shows that there is a slight increase in rate the conversion of magnesite ore after 25:1 mL/g L/S ratio; however its

The impact of succinic acid solution strength and liquid to solid proportion on leaching rate of magnesite was determined. The experimental results are presented in Figs. 4 & 5 respectively. From Fig. 4,itcanbe observed that an increase in acid concentration from 4% to 7% increases

Fig. 4. Effect of succinic acid concentration on leaching (177–210 μm particle size, liquid/ Fig. 2. SEM image of natural magnesite ore. solid ratio 25:1 mL/g, temperature 60 °C, stirring speed 350 rpm). 28 N. Raza et al. / International Journal of Mineral Processing 139 (2015) 25–30

causes intensification of surface area for reaction and results in an increase in dissolution reaction rate. Further lessening of particle size of magnesite samples will increase the consumption of energy required for grinding and resulting in an increase in cost. The influence of stirring rate on leaching kinetics of magnesite ore was inspected by varying rates of stirring from 100 rpm to 400 rpm at 60 °C temperature, 7% succinic acid concentration with 25:1 mL/g liquid to solid proportion and particle size of 125–177 μm of the magnesite ore. The results are presented in Fig. 7, which shows that the influence of stirring rate on the leaching of magnesite ore is not much significant on account of the fact that the dissolution of magnesite sample in succinic acid is neither product nor ash layer controlled. The magnesium contents of four different size fractions (500–707, 250–354, 177–210 and 125–177 μm) are shown in Fig. 8,which indicates that the magnesium content is an exponential function of the particle size. This clearly indicates that the reduction in sample particle size elates the surface area exposed for reaction.

4.5. Succinic acid as leaching agent Fig. 5. Effect of liquid solid ratio on leaching of magnesite (7% succinic acid, 177–210 μm particle size, temperature 60 °C, stirring speed 350 rpm). The cost of a leaching process also relies on the cost of chemicals used in the process. If the expensive chemicals used in the process are regenerated by any suitable method then the cost of process is reduced rate of increase decreases from liquid/solid ratio of 25:1 to 27.5:1 mL/g. and the overall process may become more economical. Succinic acid Therefore 7% acid concentration may be taken as enough to avoid the used in the present research work as a leaching agent may be regener- starvation of leaching agent along with the L/S ratio of 25:1 mL/g. On ated by treating magnesium succinate with any inorganic acid thus the other hand, very high liquid/solid ratios may cause difficulties in making the leaching of magnesite with succinic acid economical. The handling and may increase the time of evaporation to collect the solid selection of inorganic acid would depend on the nature of the end prod- product. ucts and their commercial and/or industrial applications. In this approach selectivity of succinic acid can be favorably used without 4.4. Influence of sample particle size and stirring rate incurring higher costs. In another approach, the solid magnesium succinate can be recovered by evaporation and then combusted in air in Many experiments were undertaken to investigate the impact of order to obtain pure MgO directly similar to processes developed for sample particle size and stirring rate on dissolution of magnesite making PbO from lead citrate crystals (Kumar et al., 2013). It is also pos- samples. Four particle sizes (500–707, 250–354, 177–210 and sible to divert some of the resultant magnesium succinate as a value- 125–177 μm) at 60 °C temperature, 7% succinic acid concentration, added pharmaceutical product for medical applications in order to 25:1 mL/g liquid to solid ratio and 350 rpm stirring rate were used to ex- treat magnesium deficiency. plore the leaching of magnesite samples. Analytical results are shown in Fig. 6. As estimated the rate of dissolution process swells with the reduction of sample particle sizes. From Fig. 6, it can be perceived that the re- 5. Kinetic analysis covery of magnesium increases from 70.8% to 88% by decreasing the particle size from 500–707 to 125–177 μm at 40 min reaction time. The hydrometallurgical processes usually take into account fluid– This situation is due to the fact that reduction in particle size of samples solid heterogeneous systems. According to Levenspiel (1972), the rate

Fig. 6. Effect of particle size on leaching of magnesite (7% succinic acid, liquid/solid ratio Fig. 7. Effect of stirring speed on leaching of magnesite (7% succinic acid, 177–210 μm 25:1 mL/g, temperature 60 °C, stirring speed 350 rpm). particle size, liquid/solid ratio 25:1 mL/g, temperature 60 °C). N. Raza et al. / International Journal of Mineral Processing 139 (2015) 25–30 29

Fig. 8. Magnesium contents in 4 different size fractions. Fig. 9. Plot of 1 − (1 − α)1/3 and reaction time. of reaction in fluid solid systems is normally governed by one of the followings: Using the Arrhenius equation, Eq. (9) can be written as: a) Dispersion through ash or product layer. = − = 1−ðÞ1−α 1 3 ¼ k e Ea RTt ð10Þ b) Diffusion through fluid films. o c) Chemical reactions occurring at the surface of unreacted constituents. where R, k0 are constants, Ea is the activation energy for the reaction temperature T, and the reaction time t, for the dissolution of magnesite. fi A chemical reaction occurring in a fluid and solid system may be de- In order to nd the values of k0 and Ea, a graph was plotted between lnk noted as: and 1/T as shown in Fig. 10. Using the values of k0 and energy of activation, Eq. (10),canbe þ → : ð Þ written as, AðÞfluid bBðÞsolid products 5

= − = 1−ðÞ1−α 1 3 ¼ 2:688 103e 45197 RTt: ð11Þ In the absence of ash or product layer, the chemical reaction or fluid film diffusion may be considered during the reaction Let α be the frac- The magnitude of energy of activation calculated for magnesite tion of conversion of magnesite at any particular reaction time t. The leaching shows that the reaction of magnesite with succinic acid is sur- consolidated equations for these reactions can be written as: face chemically controlled. Moreover this value is in agreement with the If a reaction is controlled by film diffusion, value of activation energy found in a similar study (Abdel-Aal, 2000). To test the workability and reproducibility of the suggested model, ¼ ½−ðÞ−α : ð Þ t k 1 1 6 the experimental data were compared with the calculated ones. The comparison of the two sets of data has been shown in Fig. 11. The scatter

For a surface chemical controlled step, diagram (Fig. 11) represents that the arrangement between αexp and α ﬁ hi cal is good. The value of correlation coef cient (0.9976) indicates that t ¼ k 1−ðÞ1−α : ð7Þ the relationship between the two variables is good.

If the reaction is ash layer diffusion controlled, hi = t ¼ k 1−31ðÞ−α 2 3þ21ðÞ−α : ð8Þ

Fluid/solid reaction models were used for the analysis of data to evaluate the slowest step and the kinetic parameters for the leaching of magnesite. The reliability and validity of the data was examined by statistical as well as by graphical approaches. From the results of the dissolution of magnesite, it was perceived that the rate determining step of the dissolution reaction is surface chemically controlled. The rate expression for leaching reaction of magnesite was found to follow the rate equation given below:

= 1−ðÞ1−α 1 3¼kt ð9Þ k (the apparent rate constant) can be determined by plotting 1 − (1 − α)1/3 and t. The values of reaction rate constant were calculated from Fig. 9. Fig. 10. Plot of lnk and 1/T. 30 N. Raza et al. / International Journal of Mineral Processing 139 (2015) 25–30

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