<<

~ IRII8836 U.S. Bureau of M' S I mas ~ Bureau of Mines Report of Investigations/1983 PO fane Research Cente f. 315 MOl1tgomer A r Spokane WA .. Y ve. 9207 \-\ LlBRAR/ ~

I,

Removal of and Metals From Mineral Processing Waste

By Joseph E. Schiller

UNITED STATES DEPARTMENT OF THE INTERIOR Report of Investigations 8836

Removal of Cyanide and Metals From Mineral Processing Waste Waters

By Joseph E. Schiller

UNITED STATES DEPARTMENT OF THE INTERIOR William P. Clark, Secretary

BUREAU OF MINES Robert C. Horton, Director Library of Congress Cataloging in Publication Data:

Schiller, Joseph E Removal of eyanide and metals from mineral proeessing waste waters.

(Report of investigations; 8836) Bibliography: p. 8. Supt. of Docs. no.: I 28.23:8836. 1. Sewage-Purificadon-Cyanide removal. 2. Sewage-Purifica­ don- l'emoval. 3. Mineral industries-Waste disposal. I. Title. II. Series: Report of investigations (United States. Bureau of Mines) ; 8836. TN23.U43 [TD758.5.C93] 622s [669] 83·600305 CONTENTS

Abs t r ac t. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • .. • • • • • • • 1 ExperimentalIntroduction. work...... • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • .. • • • • 32 Results and discussion •••••••••••••••••••••••••••••• o...... 4 Cyanide and metals removal...... 4 Theory of operation...... 4 Effect of pH on free-cyanide reaction rate...... 5 Reaction of Fe2+ with cyanide...... 5 Mechanism of metals and removal...... 6 Use of Fe3+ salts for Cu 2+ removal...... 6 Activated carbon and prussian blue...... 7

Canclus ••••••••••••••••••••••••••••••• III • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 7 References. • • ••••• •• •• ...... •• •• •••••• • • •• ••••• ... •• ••• .... •••• ...... • ••• •••• • • • 8

TABLES

1. Analysis of cyanide-containing solutions before treatment •••••••••••••••••• 3 2. Pseudo first-order rate constant for the disappearance of free cyanide (initially at 10 mg/L) in a solution initially containing 6 x 10- 3 M peroxide and 10-2 M ••••••••••••••••••••••••••• 5 3. Effect of pH on residual ~ree cyanide when 0.5 mmol/L Fe 2+ ts added to a lO-mg/L cyanide solution •••••••••••••••••••••••••••••••••••••••••••••••••• 6 4. Residual cyanide after addition of Fe 2+ to a solution of free cyanide at

pH 8.5 ...... (0 • C' • • • • 6 5. Copper remaining in a solution treated with Fe2(S04)3 in the pH range of 5

to 8 ...... $ ••••• 7 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

°C degree Celsius min minute

h hour mmol/L millimole per liter ;J I, M molar (mole per liter) mol mole l' mg/L milligram per liter ppm part per million REMOVAL OF CYANIDE AND METALS FROM MINERAL PROCESSING WASTE WATERS

By Joseph E. Schiller 1

ABSTRACT

The Bureau of Mines is investigating methods to treat waste that contains cyanide. This report describes a new method that removes heavy metals and all forms of cyanide. In the first step, and sodium thiosulfate are added at pH 7 to 9 in a 1:2 molar ratio to convert free and weakly complexed cyanide to nontoxic . Then steryldimethylbenzylammonium chloride is added to precipitate ferro­ cyanide, and finally, fe·rric sulfate is added as a sweep floc and sequestrant for heavy metals. After the suspended solids settle, the water is polished by filtering. The water initially contained 1 to 5 mg/L Fe, 1 to 5 mg/L Cu, and 10 to 30 mg/L total cyanide; the level of each of these constituents was reduced to less than 0.02 mg/L.

lsupervisory physical scientist, Twin cities Research Center, Bureau of Mine;, Min­ neapolis, MN. 2

INTRODUCTION

~~e Bureau of Mines is conducting re­ are small and located in remote areas. search to improve methods for treating Therefore, a treatment process is needed mining and mineral processing waste that is reliable, is capable of being waters. Water is needed in virtually automated, and does not use difficult- every mineral operation, and new technol­ to-handle chemicals. ogy is needed to purify the aqueous wastes so that the resultant clean water This report describes a new approach can be recycled or discharged safely. for removing heavy metals and strongly Process waters may contain a variety of complexed from water and con­ contaminants, including suspended par­ verting free and weakly complexed cyanide ticles, undesirable dissolved metals or to nontoxic thiocyanate and . The nonmetal ions, and chemicals used in min­ method utilizes three steps that accom­ ing and beneficiation processes. One plish the following: (1) conversion of current need is for better methods to free and weakly complexed cyanides to handle mineral processing wastes that nontoxic forms, (2) precipitation of contain cyanide. strongly complexed cyanides and metals, and (3) sedimentation and filtration to Cyanide is used widely in leaching pre­ remove solids. In the first step, hydro­ cious metal ores, and it is sometimes gen peroxide reacts with the thiosulfate employed in flotation of sulfide miner­ to form an intermediate (1)2 that, in als. For over 50 years, cyanide leaching turn, reacts with most of the free and has been the method of choice for dis­ weakly complexed cyanide to form solving finely disseminated and thiocyanate. silver from their ores. In typical com­ mercial operations, a 0.1- to 0.3-pct The method described in this report was caustic cyanide solution is percolated evaluated in laboratory-scale experi­ through crushed or ground ore to dissolve ments. It was developed mainly for the precious metals. The gold and silver treating waste waters from precious metal are won from the "pregnant" leach solu­ ore leaching, but it could be used for tion, usually by cementation with zinc other wastes containing similar metals dust. This leaves a waste solution that and forms of cyanide. Depending on site­ contains some of the original free cya­ specific agreements, Federal and State nide along with complexed cyanides, base regulations generally require that the metals (such as copper and iron), and total cyanide content of water discharged thiocyanate. These ions were formed by to the environment be less than 0.02 mg/L reaction of cyanide with various gangue (2). Since mining and mineral process­ minerals in the ore. The dilute solu- ing operations seldom release water tions, diverse forms of cyanide and met­ into municipal treatment systems, treat­ als, and high make effective ment methods used by these operations treatment of precious metal leaching must enable their discharged water to wastes a definite necessity and a signif- meet these requirements. icant challenge. Technical or economic problems make None of the present cyanide treatment existing treatment methods undesirable to methods are very effective with precious the mining industry. Either the cost is metal leach solution. Therefore, the considered excessive, or the cyanide con­ Bureau is conducting studies to devise a tent is not reduced to sufficiently low method for handling the constituents com­ levels. Common industrial processes to monly found in mining wastes. Due to the present high price for gold and silver, 2underlined numbers in parentheses re­ many new cyanide leaching operations are fer to items in the list of references at being started. A large fraction of these the end of this report. 3

treat cyanide wastes include oxidation, chemical industry prior to secondary ion exchange, complexation, volatiliza­ treatment. This method is unsuitable as tion with or without cyanide recovery, the sole treatment, because the cyanide and other lesser used methods (3-4). remains soluble and appears in the total Although mining companies h'ave evaluated cyanide analysis. By acidifying a waste these methods on a small scale, none have to liberate gaseous , gained universal acceptance. Most com­ most of the free cyanide in a waste can monly, mines in North America treat be removed. This form of cyanide can be cyanide wastes by oxidation or by dis­ recovered by absorption in a base or charging waste water after dilution and vented if quantities are small. detention in a tailings pond. The most widely used oxidation system is alkaline Two very new methods specifically chlorination, where reactions are quite designed for mining wastes use sulfur rapid and most metals are removed. How­ dioxide or sulfite salts (5-6). In the ever, ferrocyanide is not destroyed in International Nickel CO. (INCO)3 process, this method, and chlorine consumption can 2 to 5 pct 802 in air is bubbled through be very high, since thiocyanate and other a waste containing at least 50 mg/L dis­ sulfur species react with chlorine. solved Cu. Cyanide levels are reduced from several hundred milligrams per liter Alternatively, and hydrogen per­ to 1 mg/L in 10 to 30 min. This method oxide have been investigated as oxidants. would best be applied to very concen­ Both are usually more costly than chlo­ trated wastes. In the second method, rine, and they do not destroy ferro­ sodium sulfite and Fe2+ are reacted with cyanides. Anion exchangers are effective the waste water at pH 5. Ferrocyanide is for multiple-charged complex cyanides, converted to insoluble prussian blue, but they are not sufficient for free Fe 4 [Fe(CN)6 J3' and the free and weakly cyanide and also tend to become fouled in complexed cyanide is converted to prod­ actual use. Complexation of free cyanide ucts (unidentified) that do not appear in with iron is often employed in the the cyanide analysis procedure.

EXPERIMENTAL WORK

The overall treatment process developed thiosulfate and 2 to 3 mol peroxide are during this research consists of three used for each mole of free and weakly steps: (1) destruction of free and complexecl CN- present. Water that con­ weakly complexed cyanide with liberation tains 10 mg/L or less of free cyanide of metal ions, (2) precipitation of fer­ requires for treatment 30 mg/L hydrogen rocyanide and metal ions, and (3) set­ peroxide and 150 mg/L sodium thiosulfate. tling and filtration to remove solids. The first step in cyanide conversion is 3Reference to specific trade names or reaction with peroxide and thiosulfate. manufacturers does not imply endorsement The optimum pH is about 8, and 3 to 4 mol by the Bureau of Mines.

TABLE 1. - Analysis of cyanide-containing solutions before treatment, milligrams per liter

Sample origin Cyanide Cu Fe SCN""" Free Total Prepared KeN ••••••••••••••••••••••• 9.4 9.4 0.0 0.0 0.0 Prepared KCN, Cu80 4 , and K4Fe(CN)6"······················· • 21.4 24.2 5.3 1.5 .0 Precious metal leaching wastes ••••• 5.6 11.4 1.6 1.0 120 Molybdenum flotation waste ••••••••• .40 .45 .10 1.5 1-2 4

After a reaction time of 2 to 4 h, 5 mol determined colorimetrically as the ferric oleyl- or steryldimethylbenzylammonium complex, and metals were measured using chloride is added for each mole of ferro­ atomic absorption . The reac­ cyanide to be removed, and then 100 mg/L tion products of free cyanide and the ferric sulfate is added while the pH is combination of peroxide and thiosulfate maintained between 8 and 9. Finally, 1 were identified by reacting a 20 mg/L CN­ 3 mg/L anionic polyacrylamide flocculant solution with 1 x 10- ~ H202 and S203~. (Separin AP-30, Dow Chemical Co.) is After the solution had stood for 4 h, the ~ added to reduce settling time and in­ cyanide was determined with a specific r, crease floc size; then the mixture is ion electrode (Ag/S=), and an aliquot was filtered. withdrawn for colorimetric thiocyanate analysis using Fe3+. The remainder of Four different waters were treated the reaction mixture was acidified to according to this procedure. The first convert any cyanate formed to was simply a dilute solution of carbonate. The presence of cyanate was cyanide. The second was a prepared solu­ confirmed by collecting the liberated CO 2 tion containing potassium cyanide, cupric in barium hydroxide solution. Chemical sulfate, and . The equations for cyanate identification remaining two were waste waters from pre­ follows: cious metal leaching and molybdenite flo­ tation, respectively. Typical analyses CNO- + 2~ + H20 + NH4+ + CO 2 (gas); are given in table 1. CO 2 + Ba(OH)2 Cyanide analyses were done using the standard distillation procedure with + H20 + BaC0 3 (white precipitate). MgCl 2 as a catalyst (l). Thiocyanate was RESULTS AND DISCUSSION

CYANIDE AND METALS REMOVAL cyanide. In the first step, where hydro­ gen peroxide and sodium thiosulfate are After treatment of each solution listed added, free and weakly complexed cyanide in table 1, free and total cyanide con­ is converted primarily to thiocyanate and centrations were below 0.02 mg/L, which to lesser amounts of cyanate. Peroxide is the analytical limit of detection for and thiosulfate react together to give a cyanide. Copper and iron levels were mixture of tetrathionate and sulfate (1), less than 0.02 mg/L and 0.05 mg/L, re­ but when cyanide is present, an interme­ spectively, which is their limit of diate in the peroxide thiosulfate reac­ determination using available methods. tion converts cyanide to thiocyanate according to the reactions THEORY OF OPERATION

Each step in the treatment process is intended to handle a certain form of

H202 + 82°3= + [H202·S203~]*

[H202·8203~]* + CN- + 8C~ + H20 + 804=. (free or weakly complexed) 5

Direct oxidation of cyanide by peroxide various pH values in the range of 7 to gives cyanate. After reaction of free 10. At various times during each reac­ and weakly complexed cyanide, generally tion, aliquots were withdrawn and ana­ the only cyanide species remaining is lyzed for cyanide. Table 2 shows the ferrocyanide. This is precipitated with initial reaction rate for disappearance octyl- or steryldimethylbenzylammonium of cyanide as a function of pH. Although chloride. Since ferrocyanide carries a the reaction proceeds throughout the -4 charge, the large organic singly range of 7 to 10, the optimum pH is near charged cation would give a ferrocyanide 8. salt of molecular weight about 2,000. The nonpolar character of the organic TABLE 2. - Pseudo first-order rate con­ groups and their high molecular weights stant for the disappearance of free are sufficient to render the quaternary cyanide (initially at 10 mg/L) in a ammonium ferrocyanide insoluble. In pre­ solution initially containing 6 x 10-3 liminary experiments, an amorphous white M hydrogen peroxide and 10-2 M sodium precipitate formed when the quaternary 'thiosulf ate ammonium compound was added to solutions of ferrocyanide. Pseudo first-order rate constant, 1 The rate of conversion of free and 10-2 min- 1 weakly complexed cyanide bo thiocyanate is determined by the rate of the 7 ...... 5.5 peroxide-thiosulfate reaction. Adding 8 •••••••••••••••••••• 8.7 excess peroxide and thiosulfate will 8.5 ...... 6.4 shorten the time needed for the first 9 ...... 5.7 process step, but higher costs and sec­ 10 •••••••• Q ...... 1.5 ondary problems with residual peroxide or 1The rate of cyanide reaction depends thiosulfate may not justify the reduced on the concentrations of hydrogen per.ox­ reaction time. Lower temperatures may ide, thiosulfate, and cyanide. However, extend the time needed for thiocyanate by using a large fixed excess of hydrogen formation. In common with most other peroxide and thiosulfate, their concen­ chemical reactions, decreasing the tem­ trations can be lumped into a pseudo rate perature by 10° C approximately halves constant, and the concentration of cya­ the rate of the peroxide-thiosulfate nide is essentially the only variable in reactions. Thus, very cold water (0° to the rate expression. 5° C) may need 12 to 16 h reaction time, but water warmer than 25° C would need REACTION OF Fe 2 + WITH CYANIDE less than 4 h. Fe 2 + reacts with cyanide to form ferro­ EFFECT OF pH ON FREE-CYANIDE cyanide according to the reaction: REACTION RATE

The pH during reaction of peroxide and thiosulfate with cyanide must be con­ However, this reaction alone cannot re­ trolled at about 8 to 9 to obtain the duce free cyanide levels below about 2 fastest reaction rate possible. The ppm. Table 3 shows the effect of pH on effect of pH on the rate of disappearance the reaction, and table 4 indicates the of free cyanide during the peroxide­ lower limit of free cyanide obtainable thiosulfate reaction was determined by with Fe 2 +. To obtain these data, O.OlM reacting a solution initially comprising ferrous sulfate solution was added to a 3.8 x 10-4 M cyanide, 6 x 10-3 M hydrogen potassium cyanide solution while the pH peroxide, and 10-2 M sodium thIosulfate. was m,aintained at the values indicated in Separate reactions-were performed at tables 3 and 4. Trends in both tables 6

TABLE 3 ..... Effect of pH on residual free MECHANISM OF METALS AND FERROCYANIDE cyanide when 0.5 mmol/L Fe 2+ is added REMOVAL to a 10 ....mg/L cyanide solution When waters were treated that contained Free cyanide dissolved copper and ferrocyanide, a pink pH remaining, mg/L precipitate formed during the peroxide­ thiosulfate reaction. This precipitate 7.75...... 5.6 appeared to be CU2(Fe(CN)6), produced by 8.00...... 3.5 the reaction 8.25...... 3.6 8.50...... 2.8 8.75...... 3.3 9.00...... 4.4 It would form after the free cyanide and weakly complexed cyanide were destroyed. TABLE 4 ..... Residual cyanide after addi .... At that point in the reaction, uncom­ tion of Fe 2 + to a solution of free plexed Cu 2+ had been liberated. The pink cyanide at pH 8.5 precipitate dissolved in excess cyanide at above pH 9 but was stable in acid. A Free CN-, similar solid was observed after mixing Fe 2+ added, mmol/L mg/L Cu 2+ and Fe(CN)6-4 solutions. This pre­ cipitate indicates an important mechanism o...... 10.0 by which copper, iron, and strongly com­ 0.164...... 4.4 plexed cyanide are removed. It may be 0.328...... 3.8 the major pathway in solutions where the 0.488 ..••...... ••• '...... 3.0 Cu:Fe is near 2:1. 0.656...... 2.9 0.816...... 2.8 Ferric sulfate and a quaternary ammo­ nium salt are generally required to lower result from the competition between hy­ copper and ferrocyanide to below 0.1 droxide and cyanide for Fe2+. The opti­ mg/L. Quaternary ammonium salts, such as mum pH for the Fe 2+-cyanide reaction is oleyl- or steryldimethylbenzylammonium near 8.5. This pH represents a balance chloride, make insoluble compounds with between the of Fe(OH)2 and the the tetravalent ferro cyanide by the HCN .... CN- equilibrium. At pH values below reaction 8.5, where Fe 2+ is more soluble, a great­ er proportion of cyanide exists as HCN, so its lone pair of electrons is not free to complex with metals. Above pH 8.5, When solutions of these salts are added the amount of CN- is greater, but the to ferrocyanide solutions, a white, Fe 2+ encounters more hydroxide and pre­ cloudy mixture results. Ferric sulfate cipitates before it has a chance to com­ functions as a coagulant for the quater­ plex. The same reasoning applies when nary ammonium ferrocyanide salt and as a greater amounts of Fe 2 + are added at a sequestrant for copper arid other heavy fixed pH. As shown in table 4, addition metal ions. Addition of an anionic poly­ of more Fe 2 +, even at the optimum pH of acrylamide gives a solid that settles and 8.5, does not substantially reduce the can be filtered more readily. free cyanide level below 2 to 3 mg/L. The excess Fe 2+ that is added simply USE OF Fe 3+ SALTS FOR Cu 2+ REMOVAL forms Fe (OH)2' since the CN- level has been lowered to the point where it can no When ferric salts are added to water, longer effectively compete with OH-. the pH is lowered, and hydrated Fe(OH)3 7

is precipitated according to the 4.5. The capacity of the carbon for cop­ reaction per was quite low, since 100 mg/L C would adsorb only 1 to 2 mg/L at pH 4.5. Work on this possibility was suspended, since most process waste waters are naturally Ferric salts are often employed as coagu­ buffered above pH 7 and large amounts of lants and flocculants for suspended sol­ chemicals would be required to lower the ids, but the hydrous Fe(OH)3 can also aid pH (for carbon adsorption) and to raise in removing dissolved metals. In the it up again. process described in this paper, copper removal is improved by adding ferric sul­ Removing ferrocyanide as pruss ian blue fate; table 5 shows data for iron addi­ was also investigated early in the re­ tion during copper precipitation from a search, but chemical requirements for pH CuS04 solution. From data in table 5, it adjustment were large. For ferrous or appears that 10 to 20 mg/L Fe 3+, when ferric ferrocyanide to be a stable pre­ precipitated, can collect up to about 0.5 cipitate, the pH had to be kept below mg/L Cu 2+. about 5. If the pH were higher, then ferrous or ferric ion formed the hydrox­ TABLE 5. - Copper remaining in a solution ide, and ferrocyanide was put into solu­ treated with Fe2(S04)3 in the pH range tion according to the reactions of 5 to 8, milligrams per liter Fe 4 (Fe(CN)6)3 + 12HzO pH Fe 5 + added 5 mg/L 10 mg/L 20 mg/L + 4Fe(OH)3 + 3Fe(CN)6-4 + 12H+ 5 •••••••••••••• 5.4 4.6 3.1 6 •••••••••••••• 1.2 .83 1.2 and Fe 2(Fe(CN)6) + 4H zO 7 •••••••••••••• .85 <.10 .17 B• ••••••••••••• • 44 <.10 <.10 + 2Fe(OH)z + Fe(CN)6-4 + 4H+ • 1 Original CuI~ concentration was 6.4 mg/L. The reactions could readily be o~served. When base was added to a deep blue acidic ACTIVATED CARBON AND PRUSSIAN BLUE suspension of prussian blue, no change was observed until the pH reached about During earlier phases of this research, 5. The color of the suspension then activated carbon was investigated for changed slowly to green, then brown, as removal of copper. Although carbon is more base was added to bring the pH up to employed for gold and about 6. After the mixture at pH 6 was adsorptibn at pH 9 to 11, copper was filtered, analyses of the filtrate showed adsorbed on carbon most efficiently at pH that ferrocyanide had dissolved.

CONCLUSIONS

In this laboratory study, the treatment solution with hydrogen peroxide and thio­ method described was effective in reduc­ sulfate, (2) precipitating and flocculat­ ing the cyanide concentration of prepared ing with quaternary ammonium salt, ferric solutions and industrial processing waste sulfate, and a polymer flocculant, and waters to less than 0.02 mg/L. The pro­ (3) filtering the solution. Laboratory cedure uses chemicals that are ,relatively results indicate that a major pathway for nontoxic and easy to handle. In addi­ removal of ferrocyanide from solutions tion, since the peroxide-thiosulfate sys­ that contain copper is precipitation of tem does not react appreciably with thio­ Cu z(Fe(CN6) and that quaternary ammonium cyanate, chemical usage is minimized. compounds are effective precipitants for ferrocyanide ions. The method includes three treatment steps: (1) reacting the waste cyanide 8

REFERENCES

1. Schiller, J. E. Reaction of Cy­ 5. Devuyst, E. A., V. A. Ettel, and anide, Peroxide, and Thiosulfate at G. J. Borbely. New Method for Cyanide pH 7 to 9. Submitted to Journal of the Destruction in Gold Mill Effluents and American Chemical Society; available Tailing Slurries. Pres. at 14th Ann. upon request from author, BuMines, Operators Conf. of the Canadian Mineral Minneapolis, MN. Processors Division of the Canadian In­ stitute of Mining, Ottawa, Ontario, Jan. 2. Federal Register. Environmen- 19-21, 1982, 14 pp.; available from the tal Protection Agency. Ore Mining Canadian Institute of Mining, Ottawa, and Dressing Point Source Category: Ontario, Canada. Effluent Limitation Guidelines and New Source Performance Standards, 40 CPR 6. Neville, R. G. A Method for Re­ Part 440. V. 47, No. 114, June 14, 1982, moval of Free and Complex Cyanides From pp. 25682-25718. Water. U.S. Pat. 4,312,760, Jan. 26, 1982. 3. Scott, J. S., and J. C. Ingles. Removal of Cyanide From Gold Mill Efflu­ 7. Rand, M. C., A. E. Greenberg, and ents. Can. Min. J., v. 101, 1980, M. J. Taras, (eds.). Standard Methods pp. 57-61. for the Examination of Water and Waste­ water. American Public Health Associa­ 4. Zabban, W., and R. Helwick. Cya­ tion, Washington, DC, 14th ed., 1976, nide Waste Treatment Technology--The Old, pp. 361":'379. The New, and The Practical. ,Plating and Surface Finishing, v. 67, No.8, 1980, pp. 56-59.

"U.S. GPO: 1985-505-019/5095 INT •• BU.OF MINES,PGH.,PA. 27255