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"^ '" •^^ IC 9002 Bureau of Mines Information Circular/1985 c ^^V Production of High-Purity Gold From Zinc Precipitates and Steel Wool Cathodes by Hydrometallurgical Refining By G. E. McClelland, M. D. Wroblewski, and J. A. Elsele UNITED STATES DEPARTMENT OF THE INTERIOR 75^ AMINES 75TH AV5J^ I r^-for f^iOL-K c^ c \ i^culcLT ( U A » red Stoies . S^nreau o-f Mi'^tes ) Information Circular 9002 Production of High-Purity Gold From Zinc Precipitates and Steel Wool Cathodes by Hydrometallurgical Refining By G. E. McClelland, M. D. Wrobiewski, and J. A. Eisele UNITED STATES DEPARTMENT OF THE INTERIOR William P. Clark, Secretary BUREAU OF MINES Robert C. Norton, Director /r\^ (\^ ]}}< .kO^ fj & Op- Library of Congress Cataloging in Publication Data: McClelland, G. E Production of high-purity gold from zinc precipitates and steel wool cathodes by hydrometallurgical refining. (Information circular / United States Department of the Interior, Bu- reau of Mines ; 9002) Bibliography: p. 11. Supt. of Docs, no.: I 28.27:9002. 1. Gold— Metallurgy. 2. Hydrometallurgy. 3. Precipitation (Chem- istry). 4. Zinc. 5. Cathodes. 6. Steel. I. Wroblewski, M. D. (Matt D.). II. Eisele, J, A. (Judith A.). III. Title. IV. Series: Information cir- cular (United States. Bureau of Mines) ; 9002. TN295.U4 [TN769] 622s [6(59'. 22] 84-600263 s CONTENTS Page Abstract 1 Introduction 2 Experimental procedure. 2 Acid pretreatment 3 Precipitation of sliver 3 Aqua regla dissolution of gold 3 Precipitation of metallic gold 3 Refining of precipitated gold. 4 Cyanldatlon of acld-leached residues for recovery of residual precious metals 4 Results and discussion 4 Acid pretreatment 4 Precipitation of silver 4 Aqua regla dissolution of gold 5 Precipitation of metallic gold 5 Refining of precipitated gold 7 Cyanldatlon of acid leached residues for recovery of residual precious metals 7 Slurry solids for acid leaching procedures 8 Recommended procedure for zinc precipitates 8 Recommended procedure for steel wool cathodes 10 Handling and disposal of acid solutions and acid wastes 10 Summary and conclusions 11 References 11 ILLUSTRATIONS 1. Flowsheet for chemical refining zinc precipitates. 8 2. Flowsheet for chemical refining steel wool cathodes 9 TABLES 1. HNO3 pretreatment of zinc precipitates.. 5 2. Precipitation of silver from HNO3 leaching solutions 5 3. Results of aqua regla leaching of HNO3 residues,. 6 4. Precipitation of gold by reduction with (C00H)2 6 5. Precipitation of gold by reduction with SO2 7 6. Results of flre-ref Inlng precipitated gold 7 7. Results of cyanide leaching of aqua regia-leached residue 8 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT °c degree ce Lsius mg/L milligram per liter g gram min minute h hour ml, milliliter kg kilogram oz/ton ounce per ton L liter pet weight percent M molar PRODUCTION OF HIGH-PURITY GOLD FROM ZINC PRECIPITATES AND STEEL WOOL CATHODES BY HYDROMETALLURGICAL REFINING By G. E. McClelland/ M. D. Wroblewski,^ and J. A. Eisele^ ABSTRACT The Bureau of Mines Investigated chemical methods for producing high- purity gold from precious-metal-bearing zinc precipitates and steel wool cathodes. Precious-metal-bearing zinc precipitates and steel wool cath- odes are unrefined products from conventional cyanidation and heap leaching-cyanidation operations. The zinc precipitates contained 14.40 pet Au and 0.35 pet Ag. The precious-metal-bearing steel wool cathodes contained 20.65 pet Au and 4.84 pet Ag. The precipitates and cathodes were treated with dilute acid to solubilize the silver and/or base met- als. The gold-bearing residue was leached in dilute aqua reqia to solu- bilize the gold. High-purity gold was precipitated from the aqua regia solution with oxalic acid, sulfurous acid, sodium bisulfite, and gaseous sulfur dioxide. The leaching-precipitation experiments recovered 99.9 pet of the gold. The gold precipitates ranged in fineness from 997 to 999 fine. The chemical refining method provides a viable technique for the smaller operator to produce high-purity gold without using pyrometallur- gical refining methods. ^Metallurgist, Reno Research Center, Bureau of Mines, Reno, NV (now with Hienen- Lindstrom Associates, Sparks, NV) . ^Physical science technician, Reno Research Center, Bureau of Mines, Reno, NV. ■^Supervisory chemical engineer, Reno Research Center, Bureau of Mines, Reno, NV. INTRODUCTION The increase in precious metals prices since 1975 has stimulated extensive ex- ploration programs. Concurrently, the Bureau of Mines (_3, 8^)^ has devised im- proved leaching technology for processing gold-silver ores. Many of the precious metal resources are too small to warrant expenditure of large amounts of capital to recover the contained values. The operators of small properties are limited in processing technology to low-cost heap leaching or small-scale agitation cyanidation. Regardless of the leaching technology employed, the dissolved precious metals are recovered from the pregnant solution either by precipitation on zinc dust (J^, _5, 9-10) or by carbon adsorption- desorption-electrowinning 0-2, A~5» 11~ 12). In larger scale operations, the precious-metal-bearing zinc precipitates and steel wool cathodes are fire-refined to obtain dore bullion. The dore bullion is either sold to a refiner or refined on site to obtain pure gold and silver bullion. The operators of small mining proper- ties in many instances cannot afford to construct a precious metal refinery and must sell their zinc precipitates or steel wool cathodes to a custom refiner. Marketing their product in this manner decreases the small operators' profits because of loss of interest on the price of the metal while it is idle, assay charges, costs for packing, shipping, and insurance while in transit, and the re- finers' fees and profits. The objective of this investigation was to develop a hydrometallurgical refining procedure to enable the smaller operator to produce high-purity precious metal products from zinc precipitates or steel wool cathodes. The chemistry used in this procedure is essentially the same as used for chemically refining jewelry or dental scrap (6-7). EXPERIMENTAL PROCEDURE Gold- and silver-bearing zinc precipi- tate used in the experiments was obtained from a large operating gold mine. Anal- ysis of the zinc precipitate was as follows: Element pet Ag 0.35 Au 14.40 CaO 10.2 CO2 and others 16.45 Cu 9 Hg 6 Pb 2.1 S 4.6 Si02 25.7 Zn 9.4 Other metal oxides.. 15.3 Analyses for specific elements and com- pounds contained in the zinc precipitate ^Underlined numbers in parentheses re- fer to items in the list of references at the end of this report. were made by fire assay, atomic absorp- tion, and wet-chemical methods. Spectro- graphic analysis showed that the zinc precipitate contained small amounts of other metals. The wet zinc precipitate was air-dried at room temperature for 120 h. The dry precipitate was sized on a 100-mesh screen. The oversize was ground to pass 100 mesh and mixed with the minus 100- mesh material in a glass bottle by roll- ing and tumbling. A precious metal-bearing steel wool cathode was generated from the Bureau's carbon stripping-electrowinning pilot demonstration unit (PDU) in Reno, NV. The cathode was oven-dried and weighed. Pieces were cut from different regions of the cathode and hand-blended to make a head sample. The sample was assayed by conventional fire assay methods. The steel wool cathode assayed 20.65 pet Au, 4.84 pet Ag, and 0.14 pet Cu. Solid products (residues, Au sponge, AgCl precipitate, etc.) from all experi- ments were analyzed by fire assay for precious metal values. Solutions were analyzed by atomic absorption spectropho- tometry for metal values. ACID PRETREATMENT Acid pretreatment experiments were con- ducted on 25-g charges of zinc precipi- tate which were agitated for 2 h at 80° C in 200 mL of 6M acid. Acids used were H2SO4 , HCl, and HNO3 . After leaching, the pulps were filtered, and the residues were washed with 100 mL of deionized wa- ter. Residues were dried, weighed, and analyzed for residual metal values. A multistage HNO3 pretreatment experi- ment was conducted on a 25-g charge of zinc precipitate to improve silver recov- ery. The zinc precipitate was agitated for 2 h at 85° C in 100 mL of 6M HNO3 . After pretreatment, the pulp was filtered and the filtrate was analyzed for silver and base metal content. The residue was leached three times. The final residue was washed with 400 mL H2O and was as- sayed for silver, A 62-g steel wool cathode was digested at 90° C for 1 h in 1 L of 5M HCl to dis- solve excess iron. The solids were sepa- rated from the acid solution by filtra- tion. The solids were washed with dis- tilled water, dried, and saved for aqua regia leaching to recover the contained gold values. PRECIPITATION OF SILVER Silver precipitation experiments were conducted by adding known quantities of NaCl to slowly agitated silver-bearing HNO3 pregnant solution at room tempera- ture. When initial AgCl precipitation ceased, 1 g of additional NaCl was added and the solution was gently agitated. If further AgCl precipitation was observed, additional NaCl was added in 1-g incre- ments until precipitation was complete. The AgCl precipitate was separated from the acid solution by filtration, washed with distilled water, and dried. AQUA REGIA DISSOLUTION OF GOLD The gold-bearing residues from acid- preleaching zinc precipitates and steel wool cathodes (10 to 15.7 g) were agi- tated at 90° C for 1 h in dilute aqua regia. ^ After leaching, the slurries were filtered on glass fiber filter paper and washed with distilled water. A simulated two-stage aqua regia leach- ing experiment was conducted on an HNO3- leached residue. Ten grams of residue were agitated at 90° C for 1 h in 175 mL of dilute aqua regia. The pulp was fil- tered, and the filtrate was used to leach a fresh 10-g charge of residue at 90° C for 1 h. The slurry was filtered, and the washed residue was leached with di- lute aqua regia. PRECIPITATION OF METALLIC GOLD One-hundred milliliters of gold-bearing dilute aqua regia solution was heated to 80° C. The stoichiometric amount of oxalic acid (0,69 g (C00H)2 per gram Au) required to precipitate the contained gold was added slowly and gently agi- tated. The solution was digested at temperature until gold precipitation stopped. Additional (C00H)2 was added to the solution in 1-g increments until precipitation stopped and the bright yel- low color of the solution disappeared. The acid solution was decanted and al- lowed to stand overnight at room tempera- ture for additional gold precipitate to form. The gold products were combined, washed with distilled water, dried, and assayed. A second method for gold precipitation was to slowly bubble SO2 gas through a fritted glass dispenser into 100 ml of gold-bearing aqua regia solution at room temperature for periods ranging from 15 min to 1 h. The gold precipitate was washed with distilled water, dried, and assayed. ^Dilute aqua regia refers to a solution containing 75 mL of 36.5-pct HCl and 50 mL of 70.0-pct HNO3 per liter of water. The third method evaluated was sodium bisulfite or sulfurous acid precipitation of gold from 100 mL of gold-bearing aqua regia solution. The sodium bisulfite (1.8 g NaHS03 per gram Au) or sulfurous acid (2.5 g H2SO3 per gram Au) was added to the solution in one increment. The mixture was gently stirred for 15 min at 80° C. Precipitation was complete when no additional precipitation was observed and when the yellow color of the solution disappeared. The barren solution was an- alyzed for residual gold. The precip- itate was filtered, washed, dried, and assayed, REFINING OF PRECIPITATED GOLD Separate charges of gold precipitated by (C00H)2 or by SO2 gas were mixed with nitre (KNO3), silica (Si02), and borax (Na2B4 07) and heated in clay crucibles to 1,000° C for 1 h. The molten charges were poured into cast iron molds and cooled. The gold beads were fire-assayed for fineness, and the slags were fire- assayed for residual gold content. In a separate experiment, gold precipi- tated with SO2 was upgraded in purity by digesting in 5M HNO3 at 90° C for 4 h. The gold sponge was washed with distilled water, dried, and assayed. CYANIDATION OF ACID-LEACHED RESIDUES FOR RECOVERY OF RESIDUAL PRECIOUS METALS Gold- and silver-bearing siliceous residue from aqua regia leaching of zinc precipitates (~7.5 g) was agitated at room temperature for 24 h in 100 mL H2O containing 0.3 to 1.0 g NaCN and 0.2 g CaO. The cyanide leached residues were filtered, washed, and dried. RESULTS AND DISCUSSION The chemical technique for refining zinc precipitates differs from the pro- cess for steel wool cathodes. For refin- ing zinc precipitates, the silver and base metals must be separated from the gold and silica. Since the steel wool cathodes contained no silica, separation of the gold from the silver by acid pre- leaching was not necessary. The residue generated from the aqua regia leaching of the HCl pretreated steel wool cathodes was a nearly pure AgCl product which assayed 74 pet Ag. ACID PRETREATMENT Preliminary tests showed that HNO3 was the only effective acid for dissolving the silver from the zinc precipitates. The results of HNO3 leaching experiments are shown in table 1. The data show that leaching with 6M HNO3 for 6 h dissolves 76 pet of the silver and most of the base metals and that multiple leaches are not justified. HCl pretreatment of the steel wool cathodes generated an acid filtrate which contained 99 pet of the Fe, 28 pet of the Cu, 0.016 pet of the Au, and 1.7 pet of the Ag. The leached residue was 80.5 pet Au, 19.0 pet Ag, 0.4 pet Cu, and <0.1 pet Fe. PRECIPITATION OF SILVER Silver can be precipitated as AgCl chloride from the HNO3 leaching solutions with NaCl according to the following equ- ation: NaCl + AgN03 ^ AgCl(s) + NaN03 . (1) The amount of silver contained in the HNO3 leaching solutions must be known to ensure the addition of the proper amount of NaCl. The results of NaCl precipita- tion of silver from HNO3 leaching solu- tions with stoichiometric and excess NaCl are shown in table 2. The silver was precipitated, but a large excess of NaCl was needed. The dry AgCl precipitate is suitable for fire refining, or is marketable if it contains between 70 and 75 pet Ag. TABLE 1. - HNO3 pretreatment of zinc precipitates Acid leaching HNO3, M Extraction, pet number Ag Cu Fe Pb Zn SINGLE LEACHING WITH 200 mL OF SOLUTION 1.5 3.0 6.0 6.0 6.0 50.2 45.5 64.8 69.1 69.8 97.8 >99 >99 >99 >99 92.0 >99 >99 >99 >99 72.4 81.9 85.7 86.3 85.9 97.9 >99 >99 >99 >99 MULTIPLE LEACHING WITH 100 mL OF SOLUTION 1 6.0 6.0 6.0 17.0 61.1 .1 86.9 12.1 .9 48.5 45.6 5.8 48.2 27.9 20.0 89.4 2 10.2 3 .3 Total NAp 78.2 >99 >99 96.1 >99 SINGLE LEACHING WITH 200 mL OF SOLUTION, 6 h 1 6.0 75.9 >99 1 >99 96.0 >99 NAp Not applicable. NOTE. — Leaching taled 6 h. time = 2 h, multiple leaching to- TABLE 2. - Precipitation of silver from HNO3 leaching solutions Pet of Silver, mg/L Silver stoichiometric NaCl In pregnant solution In barren solution precipitated, pet 100 120 120 180 120 120 110 95 56 20 2.1 8.3 500 20.8 1,000 68.9 1,500 83.3 3,000 98.3 AQUA REGIA DISSOLUTION OF GOLD Dilute aqua regia leaching experiments were conducted to determine if gold could be extracted from residues from the HNO3 pretreatment of zinc precipitates and HCl pretreatment of steel wool cathodes. The amounts of HCl and HNO3 used were 150 to 300 pet in excess of the stoichio- metric amounts required by the following equation: Au + 3HNO3 + 4HC1 ->■ HAUCI4 + 3NO2 + 3H2O. (2) The experimental results are shown in ta- ble 3 and indicate that agitated leaching in hot dilute aqua regia dissolved metal- lic gold from the acid-leached residues. A two-stage leach produced a pregnant AUCI3 solution low in residual HNO3, which improves the precipitation of gold from the aqua regia leaching solution. PRECIPITATION OF METALLIC GOLD (C00H)2 was added to the pregnant di- lute aqua regia solution to precipitate gold as metallic crystals, according to equation 3: 3(COOH)2 + 2HAUCI4 ->■ 2Au + 6CO2 + 8HC1. (3) The experimental results for the chemi- cal reduction of gold with (C00H)2 are shown in table 4. TABLE 3. - Results of aqua regia leaching of HN03-leached residues 36-pct HCl/70-pct HNO3, mL^ Pretreated residue weight, g Volume of dilute aqua regia used, mL Au recovered , g Au extracted, pet Residual 2 HNO3, M Single leaching: 10/7 , 15/10 , 2-stage leaching: Stage 1, 15/10 , Stage 2, NAp , Leaching of 2d-stage resi- due, 15/10 , Residue from HCl leach of steel wool cathodes, 50/30. 5l2 ^12 310 Mo NAp 15.7 175 175 175 175 175 500 3.51 3.74 3.19 6.63 .14 12.77 91.6 99.9 99.9 97.9 100 (total) 99.75 >0.5 >.5 .48 <.l .88 >.5 NAp Not applicable. ^ Pairs of figures in column entries are the respective amounts of 36-pct 70-pct HNO3; e.g., 10/7 = 10 mL HCl and 7 mL HNO3. ^Residual HNO3 determined by chemical analysis. ■^Residue from HNO3 leaching of zinc precipitate. TABLE 4. - Precipitation of gold by reduction with (C00H)2 HCl and Test 1 Test 2 Test 3^ Test 42 Test 5^ Test 6^ (C00H)2 added g.. Gold precipitated g. . Gold precipitated pet. . Fineness Gold in barren mg/L. . Urea added g.. NaOH added g . . Retention time h. . Residual HNO3 .M. . Final pH 15 1.48 96.54 999 200 24 0.5 <0 20 5.20 98.95 995 300 24 0.5 <0 5 1.05 99.91 999 5 2 24 <0.1 <0 15 3.22 99.43 997 110 24 <0.1 <0 10 3.14 99.99 998 3 14 2 <0.1 1.7 11.6 5.50 99.95 998 4 8 3 2 <0.1 1.0 'Pregnant solution generated precipitates. 2 Pregnant solution generated by zinc precipitates. by dilute aqua regia leaching of pretreated zinc 2-stage dilute aqua regia leaching of pretreated Pregnant solution generated by dilute aqua regia leaching of pretreated steel wool cathode. NOTE. — All tests made at 70° to 80° C. The data show that reduction with (C00H)2 precipitated high-purity gold (998 to 999 fine) from dilute aqua regia leaching solutions. Five-tenths molar residual HNO3 in the leach solution re- sulted in slightly less precipitation of gold. Gold precipitation was higher from aqua regia leaching solutions from the second stage of the two-stage leaching experiment, which contained less than O.IM HNO3. Residual HNO3 decreased by adding urea equation 4, could also be according to 6HNO3 + 5CO(NH2)2 ->■ 8N2 + 5C02 + I3H2O, (4) increasing the percent of gold precip- itated. Increasing the pH with NaOH TABLE 5. - Precipitation of gold by reduction with SO2 Test 1 Test 2 Test 3 Test 4 Test 5 SO2 source Quantity used g . . Gold precipitated g . . Gold precipitated pet,. Fineness Gold in barren mg/L. . Reaction time min. . Residual HNO3 M» • ^Weight of pure substance, NOTE. — Pregnant solutions were generated by the aqua regia leach- ing of Zn precipitates. Tests using SO2 gas were carried out at room temperature (25° C). Tests using NaHS03 and H2SO3 were heated to 80° C. Gas Excess 2.39 99.63 988 59 15 0.5 Gas Excess 1.55 99.62 983 42 15 <0.1 Gas Excess 2.23 99.61 985 67 60 <0.1 NaHS03 ^2.9 1.61 99.44 979 71 15 0.5 H2SO3 I3 1.15 99.72 970 33 15 0.5 decreased the time required to precipi- tate the gold. The crystalline gold pro- duced by (GOGH) 2 reduction is bright and lustrous. SO2 , NaHS03 , or H2SO3 was added to pre- cipitate gold as metallic sponge, accord- ing to equation 5, 2AUCI3 + 3S02 + 6H2O -»- 2Au + 6HC1 + 3H2SO4. (5) The experimental results for the reduc- tion of gold with S02-producing compounds are shown in table 5. The gold precipi- tate was not as pure as gold precipitated with (C00H)2 and did not have the lus- trous appearance. REFINING OF PRECIPITATED GOLD If the precipitated gold is not of high enough purity, refining by smelting or leaching with hot HNO3 may be necessary. Gold precipitated by SO2 was upgraded by digestion in hot HNO3 from 985 to 998 fine. The experimental results obtained from melting (C00H)2- or S02-precipitated gold with nitre, borax, and silica are shown in table 6. Gold produced by chemical refining can be purified, if necessary, to 999.9 fine by fluxing with nitre, borax, and silica at 1,000° C, TABLE 6. - Results of fire-refining precipitated gold' Test 1 Test 2 Test 3 Weight, g: Nitre (KNO3).... Borax (Na2B4 07). Silica (Si02)... Au bead. ........ 2.9 3.8 2.9 0.99 979 997 3.4 3.2 2.4 0.99 990 999.9 3.4 3.2 2.4 0.99 Fineness: Before. ......... 995 After 999.9 ' 1-g gold samples, CYANIDATION OF ACID LEACHED RESIDUES FOR RECOVERY OF RESIDUAL PRECIOUS METALS The siliceous residues from leaching zinc precipitates contained residual pre- cious metals. The results of cyanidation to recover the precious metal values are shown in table 7, Approximately 96 pet of the gold and silver in the residues was recovered by leaching in alkaline cyanide solutions. The precious-metal- bearing residues could be recycled to the plant's cyanide leaching circuit. TABLE 7. - Results of cyanide leaching of aqua regia-leached residue NaCN g. Gold, oz/ton: Feed Tailing Gold extracted, ., ,pct. Silver, oz/ton: Feed Tailing Silver extracted. .pet. Time h. Test 1 1.0 2.90 0.13 95.52 171.2 1.9 98.89 24 Test 2 1.0 14.00 0.51 96.36 118.1 1.7 98.56 24 Test 3 0.2 33.00 1.00 96.97 196.8 8.6 95.63 1 SLURRY SOLIDS FOR ACID LEACHING PROCEDURES The slurry solids recommended for all the acid-leaching procedures are very low and depend on the amount of gold and/or silver contained in the specific feed. If the pulp solids are too high, the solubility limits for gold, silver, and other metals in the specific acid- leaching liquor may be attained, and pre- mature precipitation of precious metals and other metals may occur. The recom- mended pulp solids should be used unless it is determined by metallurgical testing that the slurry solids can be changed. RECOMMENDED PROCEDURE FOR ZINC PRECIPITATES The recommended flowsheets based on the results of this study are shown in fig- ures 1 and 2. Acid pre treatment of zinc precipitates is required to dissolve the silver and base metals and leaves a gold- bearing siliceous residue. One kilogram Zinc precipitates H2O Residue -^ recycle to cyanide leaching Urea- Oxalic acid- NaOH- HNO: H2O HgO Residue Aqua regia leaching , 90° C. I h Filtration Filtrate Precipitation of nnetallic gold, 80° C Acid pretreatment, 85° C, 6 h n Filtration NaCI Filtrate H2O AgCI product H2O Gold sponge Barren acid to waste 1 Silver precipitation U Filtration Filtrate to waste Gold product FIGURE 1. - Flowsheet for chemical refining zinc precipitates. steel wool cathode HCI HgO H2O - Residue I Aqua regia leaching, 90' C, I h H2O O Filtration Filtrate Urea Oxalic acid NaOH Precipitation of metallic gold, 80* C Acid pretreatment, 85" C, 6 h n Filtration -Filtrate to waste ^ AgCI residue product H2O Gold sponge •Barren acid to waste FIGURE 2. - Flowsheet for chemical refining steel wool cathodes. Gold product of zinc precipitate is slowly added to 10 L of 6M HNO3. The slurry is heated to 85° C, agitated, digested at temperature for 6 h, and filtered hot. The residue is washed with water. The wash water and acid filtrate are combined and saved for silver recovery. The silver contained in the acid fil- trate and wash water is precipitated as AgCl by the addition of NaCl. A tenfold stoichiometric excess of NaCl is added to the HNO3 leaching solution at room tem- perature. (The stoichiometric amount of NaCl is 0.54 g NaCl per gram of silver in solution.) The solution is agitated gently for 5 min and filtered. One gram of NaCl is added to the filtrate. If additional AgCl precipitation is ob- served, the above procedure is repeated, NaCl is added in the above manner until no precipitation is observed. In some cases a 30-fold excess of NaCl may be re- quired. The AgCl product is washed with distilled water and dried at low tempera- ture (<100° C). Gold is recovered from the HNO3- preleached siliceous residue by leaching with aqua regia. The HN03-leached resi- due is divided into two equal charges. One charge is slurried with dilute aqua regia. For every 100 g of charge, 2 L of dilute aqua regia containing 150 mL of 36.5-pct HCI and 100 mL of 70-pct HNO3 are used. The solution is heated to 90° C and digested with continuous agita- tion for 1 h. The slurry is filtered hot on glass fiber filter paper. The acid filtrate is removed, and the residue is washed with distilled water until no yel- low color can be seen in the filtrate. The washes are combined with the acid filtrate. The first aqua regia-leached 10 residue is saved for recycle to cyanide leaching. The acid filtrate is slurried with the second charge of HN03-leached residue and treated as above. The second aqua regia leached residue is saved for further dilute aqua regia leaching. Metallic gold is precipitated from the aqua regia leaching solution by (C00H)2. The pregnant AUCI3 solution (dilute aqua regia leaching solution) is heated to 80° C and treated as follows: 1. Slowly add urea prills in 1-g increments until gas evolution stops. Add the stoichiometric amount of (C00H)2 required to precipitate the known or estimated amount of gold contained in the pregnant solution (0.69 g (C00H)2 per gram of gold in solution) . Precipita- tion should start within 10 min. If precipitation does not start, or if it stops, add enough 30-pct NaOH solution to bring the pH to 1,0, Digest the solution at 80° C until precipitation stops. 2. Add 10 pet of the previously calcu- lated amount of (C00H)2. If precipita- tion is not observed, or if it stops, add enough NaOH to bring the pH up to 1.0, Digest the solution until precipitation stops. Repeat until the yellow color of the solution disappears. Combine the gold precipitate into a single sponge by using a spatula to free entrained gases and to scrape the sides and bottom of the vessel. When all of the gold has been collected, decant the barren solution and analyze for residual gold, 3, Wash the gold sponge with distilled water and dry at 100° C, The dry gold sponge should assay between 997 and 999 fine. RECOMMENDED PROCEDURE FOR STEEL WOOL CATHODES HCl pretreatment of the cathodes is re- quired to dissolve the steel wool. One kilogram of steel wool cathode is slowly added to 10 L of 5M HCl, which is heated to 90° C while stirring. The slurry is digested at temperature for 1 h, filtered hot, and washed with water. The HCl-pretreated cathode residue is leached with dilute aqua regia to recover the gold and silver. The HCl leach resi- due is divided into two equal charges. One charge is slurried with dilute aqua regia. For every 100 g of charge, 2 L of dilute aqua regia containing 150 mL of 36.5-pct HCl and 100 mL of 70-pct HNO3 are used. The pulp is heated to 90° C, digested, and agitated for 1 h. The hot slurry is filtered on glass fiber filter paper, and the acid filtrate is removed. Displacement washing of the residue with distilled water is continued until no yellow color is visible in the filtrate. The washes are combined with the acid filtrate. The residue from the first aqua regia leach is an AgCl product which should contain 70 to 74 pet silver. The second charge of HCl-leached residue is slurried with the combined filtrate and treated as above. The second aqua regia- leached residue is saved for further di- lute aqua regia leaching. The procedure for precipitating me- tallic gold from the resultant solution with (C00H)2 is the same as for zinc precipitates. HANDLING AND DISPOSAL OF ACID SOLUTIONS AND ACID WASTES The acid solutions used are corrosive and should be handled with extreme care. All leaching must be done in vessels that will not be corroded. The solutions should not contact the skin or eyes. Protective clothing should be worn to prevent accidental contact. Should the acid solution contact a person's skin or eyes, flush with cold water for a minimum of 15 min and consult a physician. The fumes from the acid solutions and chemical reactions with the solutions can be very hazardous. All operations should be conducted in a fume hood or in a well- ventilated area. If a fume hood is not available, ensure good outside ventila- tion and wear an appropriate chemical respirator and full protective clothing at all times. 11 The acid waste solutions should be neu- tralized with alkaline solutions before disposal. Violent reactions may occur when mixing acid solutions with alkaline solutions. Extreme care should be taken. The solutions should be slowly mixed to prevent an uncontrollable reaction. The pH of the neutralized acid waste should be between 6 and 7. The neutralized solutions should be disposed of accord- ing to local, State, and/or Federal regulations. SUMMARY AND CONCLUSIONS The experimental results showed that zinc precipitates and steel wool cathodes can be chemically refined to produce a pure metallic gold and an AgCl precipi- tate. Chemical refining of zinc precipi- tates and steel wool cathodes recovered 99.9 pet of the contained gold. HNO3 preleaching separated silver and base metals from the gold in zinc precipi- tates. Silver in the HNO3 leaching solu- tion was recovered as AgCl by precipita- tion with NaCl. HCl leaching separated base metal from the gold and silver con- tained in steel wool cathodes. Silver was recovered as an AgCl precipitate which contained some gold. Aqua regia leaching dissolved gold in the residues from HNO3 and HCl preleach- ing. (C00H)2 and SO2 precipitated metal- lic gold from the aqua regia leaching solutions. The gold produced by (C00H)2 reduction was 998 to 999 fine. Gold pro- duced by SO2 reduction was 970 to 988 fine, but was upgraded to 998 fine by di- gestion in HNO3 . The gold can be puri- fied to 999.9 fine by fire refining. REFERENCES 1. Adams on, R. J. Gold Metallurgy in South Africa. Chamber of Mines of South Africa, 1972, pp. 203-255. 2. Hall, K. B, Homes take Carbon- In-Pulp Process. Pres. at Am, Min, Cong. Mtg., Las Vegas, NV, Oct. 7-10, 1974, 16 pp.; available upon request from Home- stake Mining Co., Lead, SD. 3. Heinen, H. J., G. E. McClelland, and R. E. Lindstrom. Enchancing Perco- lation Rates in Heap Leaching of Gold- Silver Ores. BuMines RI 8388, 1979, 20 pp. 4. Heinen, H. J. , D. G. Peterson, and R. E. Lindstrom. Processing Gold Ores Using Heap Leach-Carbon Adsorption Meth- ods. BuMines IC 8770, 1978, 21 pp. 5. Hinds, H. L. , and F. Mosely. Re- fining Gold at Homes take. Undated, 8 pp; available upon request from Homestake Mining Co., Lead, SD. 6. Hoke, C. M. Refining Precious Metal Wastes. Metallurgical Publ. Co., New York, 1940; 355 pp.; re- prints available from Univ. Microfilms International, 300 North Zeeb Road, Ann Arbor, MI 48106. 7. Lowen, R. The Worshipful Company of Goldsmiths (London), Tech. Rept. 44/1, 1980, 22 pp. 8. McClelland, G. E., and J. A. Eisele. Improvements in Heap Leaching To Recover Silver and Gold From Low-Grade Resources. BuMines RI 8612, 1982, 26 pp. 9. Merrill-Crowe. Merrill-Crowe Pre- cipitation Process — Details of Operation. Undated, 9 pp.; available upon request f rom M. D. Wroblewski, BuMines, Reno, NV. 10. Pickett, D. E. Milling Practice in Canada. Can. Inst. Min. and Metall. , spec. V. 16, 1978, pp. 45-78. 11. Potter, G. M. Design Factors for Heap Leaching Operations. Min. Eng, , Mar. 1981, pp 277-281. 12. Zadra, J. B. , A. L. Engel, and H. J. Heinen. Process for Recovering Gold and Silver From Activated Carbon by Leaching and Electrolysis. BuMines RI 4843, 1952, 32 pp. *U.S.CPO: 1985-505-019/5092 INT.-BU.OF MINES, PGH., PA. 27847 1 D DD 2 Q. 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