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MANUFACTURE of SODIUM DITHIONITE from SODIUM- MERCURY AMALGAM and AQUEOUS SOLUTION of SULFUR DIOXIDE by RAMAN NAYAR B. Tech

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MANUFACTURE of SODIUM DITHIONITE from SODIUM- MERCURY AMALGAM and AQUEOUS SOLUTION of SULFUR DIOXIDE by RAMAN NAYAR B. Tech |5"IV) MANUFACTURE OF SODIUM DITHIONITE FROM SODIUM- MERCURY AMALGAM AND AQUEOUS SOLUTION OF SULFUR DIOXIDE by RAMAN NAYAR B. Tech. (Hon.), I.I.T., Kharagpur, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of CHEMICAL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA November, 1972 In presenting this thesis in partial fulfullment of the requirements for an advanced degree at the University of British Columbia/ I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that publication, in part or in whole, or the copying of this thesis for financial gain shall not be allowed without my written permission. RAMAN NAYAR Department of Chemical Engineering The University of British Columbia Vancouver 8, Canada Date K^~ot Xo' \^ 7 3 ii ABSTRACT A relatively dilute (approximately 1 to 2%) water solution of sodium dithionite was produced from sodium-mercury amalgam and aqueous solution of sulfur dioxide in a simple "once through" reactor [proposed process]. The reactor could be run in conjunction with the Castner-Kellner type cell. The manufactured solution could then be used directly for the brightening of groundwood pulp. The bench scale experiments were carried out in a continuous-flow-stirred-tank reactor where the aqueous and amalgam phases formed an interface. The effects of important process variables on the steady-state concentration of sodium dithionite in the reactor and yields of sodium dithionite on sulfur dioxide in the aqueous feed and on sodium consumed in a single pass were determined. The above-mentioned yields are important in assessing the economic feasibility of the pro• posed process. The steady-state yield of sodium dithionite on sodium in the amalgam entering the reactor and'conversion of sodium to different products in the reactor were also determined. The present investigation showed that the process variables can be controlled to give approximately 2.3% sodium dithionite solution with steady-state Na2S20^ yields of about 21% on sulfur dioxide in the aqueous feed and about 67% on sodium consumed. The yields obtained depend on the levels of process variables such as: 1. the concentration of sodium in the amalgam enter• ing the reactor, 2. the concentration of total sulfur dioxide in the aqueous feed solution, 3. the agitation in the aqueous phase, 4. the agitation in the amalgam phase, 5. the residence time in the aqueous phase, 6. the residence time in the amalgam phase, 7. the interfacial-area/aqueous-volume ratio, 8. the temperature of the aqueous phase, and 9. the pH of the aqueous phase. This experimental study indicates that it may be economically feasible for a pulp mill to change .from zinc dithionite produced in situ to sodium dithionite produced in situ by the proposed; process. Further, the proposed process compared to the manufacture of zinc dithionite in situ avoids the discharge of zinc ions which act as biocidal agents when discharged into the effluent receiving waters. The models suggested by Ketelaar (44) and Gerritsen (30) were found inadequate to explain the processes occurring iv in the reacting system sodium-mercury amalgam and aqueous sulfur dioxide. A qualitative model has been suggested on the basis of the experimental work and the information available in the literature. This work also sheds some light on the type of reactor which would be suitable for the proposed process. V ACKNOWLEDGEMENT The author wishes to express his thanks to the faculty and staff of the Chemical Engineering Department, The University of British Columbia. Special thanks are extended to Dr. F.E. Murray, who suggested the project and under whose guidance this work was undertaken. The author is indebted to the Chemical Engineering Workshop personnel for their assistance in assembling the experimental equipment. The author wishes to thank Mr. E. Rudischer, in particular, for his assistance and cooper• ation . Financial support for this research was most gratefully received from the National Research Council of Canada and from the British Columbia Research Council. The author is also indebted to his wife Jane for her invaluable help throughout this work. vi TABLE OF CONTENTS CHAPTER PAGE I. INTRODUCTION 1 II. REVIEW OF PERTINENT PRIOR WORK 7 A. Manufacturing Processes for Sodium Dithionite 7 1. Zinc dust: sodium carbonate process ... 7 2. Electrolytic or cathodic reduction process 9 3. Sodium formate process 9 4. Sodium borohydride process 10 5. Sodium amalgam process 11 (a) Advantages of the sodium amalgam process 11 (b) Types of the sodium amalgam process ... 12 (i) Sodium amalgam: S02-organic solvent process 12 (ii) Sodium amalgam: gaseous SO2 process and sodium amalgam: liquid SO2 process 13 (iii) Sodium amalgam: S02-NaHS03/Na2S03 buffer process 13 B. Recommended Conditions for Improving the Yield of Sodium Dithionite in the Sodium Amalgam: S02-NaHS03/Na2S03 Buffer Process . 15 C. Mercury Contamination of Sodium Dithionite Produced by the Sodium Amalgam: SOj-NaHSO,/ Na„S0o Buffer Process ...... 17 vii CHAPTER PAGE D. Lignin Preserving Bleaching of Ground- wood Pulp by Sodium Dithionite 19 1. Definition of the terms "brightening" and "bleaching" . 19 2. Characteristics of the groundwood bleach• ing process . 20 3. Effects of groundwood brightening . 21 4. Conditions for groundwood brightening by sodium dithionite 21 E. Sodium-mercury Amalgam 24 1. Molecular structure of sodium-mercury amalgam .......... 24 2. Surface tension of sodium-mercury amalgam 25 3. Sensitivity to oxidation of sodium- mercury amalgam ...... 25 4. Density of sodium-mercury amalgam ... 26 F. Sulfur Dioxide Solution in Water ..... 26 1. Principal equilibria ......... 26 2. Diffusion of sulfur dioxide in water ................. 32 G. Important Reactions in the Proposed Process ..... 33 1. The Sodium dithionite formation reaction . ..... 35 2. The water reaction 39 3. The sodium dithionite decomposition reactions ....... 43 (a) Homogeneous decomposition of sodium dithionite 43 (b) Heterogeneous decomposition of sodium dithionite 4 6 viii CHAPTER PAGE 4. The sodium dithionite oxidation reaction ..... 48 III. THEORETICAL MODELS 50 IV. EXPERIMENTAL 54 A. Experimental Materials .... 54 1. Sodium-mercury amalgam ..... 54 2. Aqueous sulfur dioxide solution 54 B. Experimental Apparatus . 55 1. Reactor 55 2. pH measurement of the aqueous phase ... 64 3. Temperature measurement of different streams 65 4. Insulation of the equipment ....... 67 5. Electrical wiring diagram ........ 67 C. Calibration Curves ..... 67 D. An Experimental Run ........ 69 E. Analytical Procedures and Errors ...... 71 1. Sodium-mercury amalgam .... 71 (a) Analysis of sodium-mercury amalgam 71 (b) Accuracy and precision of the analytical procedure 72 2. Aqueous sulfur dioxide solution ..... 75 (a) Analysis of aqueous sulfur dioxide solution 75 (b) Accuracy and precision of the analytical procedure 76 3. Aqueous sodium dithionite solution .... 77 ix CHAPTER PAGE (a) Analysis of sodium dithionite in the product stream 77 (b) Accuracy and precision of the analytical procedures 79 V. EXPERIMENTAL RESULTS 8 2 A. Batch Experiments 82 B. Introduction to Experiments in the CFSTR .......... 87 C. Definitions of Some Important Quantities which are used for the Interpretation of Data ... 90 D. Reproducibility of Experimental Runs in the CFSTR 94 E. Data from CFSTR Experiments ......... 101 1. Concentration of sodium in fresh amalgam 101 2. Concentration of "total" sulfur dioxide in the aqueous feed solution 121 3. Agitation in the aqueous phase 133 4. Flow rate of aqueous sulfur dioxide solution, i.e. residence time in the aqueous phase ...... 144 5. Interfacial-area/aqueous-volume ratio . 153 6. Temperature of the aqueous phase 164 7. pH of the aqueous phase 168 8. Flow rate of fresh amalgam, i.e. residence time in the amalgam phase . 168 VI. DISCUSSION I70 A. Model for the Reacting System in the Proposed Process 170 X CHAPTER PAGE 1. Development of the model 170 2. The model 185 B. Conditions for Improving the Yields of Sodium Dithionite in the Proposed Process 188 C. Economic Feasibility of the Proposed Process ....... 194 D. Reactor for the Proposed Process ...... 198 VII. CONCLUSIONS 199 VIII. RECOMMENDATIONS FOR FURTHER WORK 203 IX. NOMENCLATURE 205 BIBLIOGRAPHY ........ 208 APPENDIX A. EQUIPMENT SPECIFICATION A-l 1. pH measurement A-l 2. Digital temperature recording ........ A-l 3. Calibration curves ....... A-2 B. STATISTICAL EVALUATION OF ACCURACY AND PRECISION B-l 1. Error of a measurement process ....... B-l 2. Evaluation of accuracy B-3 3. Evaluation of precision (or imprecision). B-4 4. Propagation of random error B-8 C. SODIUM-MERCURY AMALGAM C-l 1. Purity of the chemicals in preparing amalgam C-l 2. Problems encountered in preparation of amalgam ..... C-2 xi APPENDIX PAGE 3. Calculation of sodium content in an amalgam sample C-3 4. Estimation of the precision of the analytical procedure C-4 D. AQUEOUS SOLUTION OF SULFUR DIOXIDE D-l 1. Purity of the chemicals in preparing aqueous sulfur dioxide solution D-l 2. Calculation of the total sulfur dioxide con• centration in an aqueous solution sample . D-2 3. Estimation of the precision of the analytical procedure ..... D-3 E. SODIUM DITHIONITE IN THE PRODUCT STREAM ..... E-l 1. Details of the analytical procedures and sample calculations E-l (a) The iodine-formaldehyde method E-l (b) The Rubine-R method E-8 2. Estimation of the precision of the Rubine-R E-16 method F. DATA PROCESSING F-l 1. Mathematical expressions for calculating C Y Y C0NNA X and Na S S204' S02, Na' ' Na / °2< rate of sodium consumption, and sample calculations 2. The 95 per cent confidence limits of the steady-state C8 Y , CONNA, X^, Na/S02 and rate of sodium consumption _ for an experimental run 3.
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