production of high-purity gold from zinc precipitates and
TRANSCRIPT
?.
'
I 8 tr
41 It (
j,..
ill t
^ '-1
5. t^r;
^V\>A.^*'-^ :?
5 '
)
.1' ^^
4 'tr
t i-
I ^rf
**'%
^ -^^ c'^'' 4^o^. ^<=>^
o_ *o,. '».^'' ^0
-^..^^^
-^^0^
^^" ..'•*
.5 ^ A^V
^V"
^<b^
',V1'* ^> V^^"^^ .^ /^^^/U-o -^^ ^<^"^
iO* ,'••
• . s * >
^oV'/.•i;^^-.^, .co^.-i^-.^^o ./\.i;^./^„ c'
.• .1*°-_ -.
..V-^-/ -iv'^^*/ ^;^B-y v^^'.o' V'^V V<'\'
. .^°.-^i-°-^ .-«* '^a;-\ '°/^i> ./.•^;^^-\ /.•^:."°oG^ % *?5r^* A
V^
?j \Kr » /•...
• .^'\ ^3w-./\
- %.<.*'.-^M. X,^*" '*^\'
oV.-..*o, /..-^^/--.^ C<.^'.>0 y.^i^-V ,^°.-^i.°- ^/.•Jv^
^0^ ^oV"
./xV..^" • %/
v .
/ ^''J^^^^ 0°^!^;:% //>;^^\ ^°/^<>- / '^'^
.^-^^^ '^^w*' 0'°"^^-.''^!^° /°"- ^^S:- ^''"^- "'
./%
/ <x^'
O. *9,-,
'^../^^^aK^^^^.^^^^:c^«^oX./ '^^^^^«>^- A^^^Av;^^^^. .^^^^^^^
o""""^ ^
^-l^*.
o VV-0^
w\'
. *.^T^' AA'
n^o^S *
i" oV ^oVa5°<.
^^.
vP'3
>" X;'^"'%o'5 %'^:To^\^'^' -^^"^-•'^°\.. "<
•^^^^^
a"^ . t .», "'^..•^ ^^ *>^;v A
A . "^'"•^^
IC 9002
Bureau of Mines Information Circular/1985c^^V
Production of High-Purity Gold From ZincPrecipitates and Steel Wool Cathodesby 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 ZincPrecipitates and Steel Wool Cathodesby 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 MINESRobert 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
CONTENTSPage
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 10Handling 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 WOOLCATHODES 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 woolcathodes. Precious-metal-bearing zinc precipitates and steel wool cath-odes are unrefined products from conventional cyanidation and heapleaching-cyanidation operations. The zinc precipitates contained 14.40pet Au and 0.35 pet Ag. The precious-metal-bearing steel wool cathodescontained 20.65 pet Au and 4.84 pet Ag. The precipitates and cathodeswere 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 regiasolution with oxalic acid, sulfurous acid, sodium bisulfite, and gaseoussulfur dioxide. The leaching-precipitation experiments recovered 99.9pet of the gold. The gold precipitates ranged in fineness from 997 to999 fine.
The chemical refining method provides a viable technique for thesmaller 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 pricessince 1975 has stimulated extensive ex-ploration programs. Concurrently, theBureau of Mines (_3, 8^)^ has devised im-proved leaching technology for processinggold-silver ores. Many of the preciousmetal resources are too small to warrantexpenditure of large amounts of capitalto recover the contained values. Theoperators of small properties are limitedin processing technology to low-costheap leaching or small-scale agitationcyanidation.
Regardless of the leaching technologyemployed, the dissolved precious metalsare recovered from the pregnant solutioneither 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, theprecious-metal-bearing zinc precipitatesand steel wool cathodes are fire-refinedto obtain dore bullion. The dore bullionis either sold to a refiner or refined on
site to obtain pure gold and silverbullion.
The operators of small mining proper-ties in many instances cannot afford toconstruct a precious metal refinery andmust sell their zinc precipitates orsteel wool cathodes to a custom refiner.Marketing their product in this mannerdecreases the small operators' profitsbecause of loss of interest on the priceof the metal while it is idle, assaycharges, costs for packing, shipping, andinsurance while in transit, and the re-
finers' fees and profits.
The objective of this investigation wasto develop a hydrometallurgical refiningprocedure to enable the smaller operatorto produce high-purity precious metalproducts from zinc precipitates or steelwool cathodes. The chemistry used inthis procedure is essentially the same as
used for chemically refining jewelry ordental scrap (6-7).
EXPERIMENTAL PROCEDURE
Gold- and silver-bearing zinc precipi-tate used in the experiments was obtainedfrom a large operating gold mine. Anal-ysis of the zinc precipitate was as
follows:Element pet
Ag 0.35Au 14.40CaO 10.2CO2 and others 16.45Cu 9
Hg 6
Pb 2.1
S 4.6Si02 25.7Zn 9.4Other 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 atthe end of this report.
were made by fire assay, atomic absorp-tion, and wet-chemical methods. Spectro-graphic analysis showed that the zincprecipitate contained small amounts of
other metals.
The wet zinc precipitate was air-driedat room temperature for 120 h. The dryprecipitate was sized on a 100-meshscreen. 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 woolcathode was generated from the Bureau'scarbon stripping-electrowinning pilotdemonstration 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. Thesteel 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 forprecious metal values. Solutions wereanalyzed 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 wereH2SO4 , HCl, and HNO3 . After leaching,the pulps were filtered, and the residueswere washed with 100 mL of deionized wa-ter. Residues were dried, weighed, andanalyzed 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 agitatedfor 2 h at 85° C in 100 mL of 6M HNO3
.
After pretreatment, the pulp was filteredand the filtrate was analyzed for silverand base metal content. The residue wasleached three times. The final residuewas washed with 400 mL H2O and was as-sayed for silver,
A 62-g steel wool cathode was digestedat 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 aquaregia leaching to recover the containedgold values.
PRECIPITATION OF SILVER
Silver precipitation experiments wereconducted by adding known quantities of
NaCl to slowly agitated silver-bearingHNO3 pregnant solution at room tempera-ture. When initial AgCl precipitationceased, 1 g of additional NaCl was addedand 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 fromthe acid solution by filtration, washedwith distilled water, and dried.
AQUA REGIA DISSOLUTION OF GOLD
The gold-bearing residues from acid-preleaching zinc precipitates and steelwool cathodes (10 to 15.7 g) were agi-tated at 90° C for 1 h in dilute aquaregia. ^ After leaching, the slurrieswere filtered on glass fiber filter paperand washed with distilled water.
A simulated two-stage aqua regia leach-ing experiment was conducted on an HNO3-leached residue. Ten grams of residuewere agitated at 90° C for 1 h in 175 mLof dilute aqua regia. The pulp was fil-tered, and the filtrate was used to leacha fresh 10-g charge of residue at 90° Cfor 1 h. The slurry was filtered, andthe washed residue was leached with di-lute aqua regia.
PRECIPITATION OF METALLIC GOLD
One-hundred milliliters of gold-bearingdilute aqua regia solution was heatedto 80° C. The stoichiometric amount of
oxalic acid (0,69 g (C00H)2 per gram Au)
required to precipitate the containedgold was added slowly and gently agi-tated. The solution was digested at
temperature until gold precipitationstopped. Additional (C00H)2 was addedto the solution in 1-g increments untilprecipitation 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 toform. The gold products were combined,washed with distilled water, dried, andassayed.
A second method for gold precipitationwas to slowly bubble SO2 gas through a
fritted glass dispenser into 100 ml of
gold-bearing aqua regia solution at roomtemperature for periods ranging from15 min to 1 h. The gold precipitate waswashed with distilled water, dried, andassayed.
^Dilute aqua regia refers to a solutioncontaining 75 mL of 36.5-pct HCl and 50
mL of 70.0-pct HNO3 per liter of water.
The third method evaluated was sodiumbisulfite or sulfurous acid precipitationof gold from 100 mL of gold-bearing aquaregia solution. The sodium bisulfite(1.8 g NaHS03 per gram Au) or sulfurousacid (2.5 g H2SO3 per gram Au) was addedto the solution in one increment. Themixture was gently stirred for 15 min at80° C. Precipitation was complete whenno additional precipitation was observedand when the yellow color of the solutiondisappeared. The barren solution was an-alyzed for residual gold. The precip-itate was filtered, washed, dried, andassayed,
REFINING OF PRECIPITATED GOLD
Separate charges of gold precipitatedby (C00H)2 or by SO2 gas were mixed withnitre (KNO3), silica (Si02), and borax(Na2B4 07) and heated in clay crucibles to1,000° C for 1 h. The molten charges
were poured into cast iron molds andcooled. The gold beads were fire-assayedfor fineness, and the slags were fire-assayed for residual gold content.
In a separate experiment, gold precipi-tated with SO2 was upgraded in purity bydigesting in 5M HNO3 at 90° C for 4 h.
The gold sponge was washed with distilledwater, dried, and assayed.
CYANIDATION OF ACID-LEACHED RESIDUESFOR RECOVERY OF RESIDUAL
PRECIOUS METALS
Gold- and silver-bearing siliceousresidue from aqua regia leaching of zincprecipitates (~7.5 g) was agitated at
room temperature for 24 h in 100 mL H2Ocontaining 0.3 to 1.0 g NaCN and 0.2 gCaO. The cyanide leached residues werefiltered, washed, and dried.
RESULTS AND DISCUSSION
The chemical technique for refiningzinc precipitates differs from the pro-cess for steel wool cathodes. For refin-ing zinc precipitates, the silver andbase metals must be separated from thegold and silica. Since the steel woolcathodes contained no silica, separationof the gold from the silver by acid pre-leaching was not necessary. The residuegenerated from the aqua regia leaching of
the HCl pretreated steel wool cathodeswas a nearly pure AgCl product whichassayed 74 pet Ag.
ACID PRETREATMENT
Preliminary tests showed that HNO3 wasthe only effective acid for dissolvingthe silver from the zinc precipitates.The results of HNO3 leaching experimentsare shown in table 1. The data show thatleaching with 6M HNO3 for 6 h dissolves76 pet of the silver and most of the basemetals and that multiple leaches are notjustified.
HCl pretreatment of the steel woolcathodes 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 petAu, 19.0 pet Ag, 0.4 pet Cu, and <0.1 petFe.
PRECIPITATION OF SILVER
Silver can be precipitated as AgClchloride from the HNO3 leaching solutionswith NaCl according to the following equ-ation:
NaCl + AgN03 ^ AgCl(s) + NaN03 . (1)
The amount of silver contained in theHNO3 leaching solutions must be known to
ensure the addition of the proper amountof NaCl. The results of NaCl precipita-tion of silver from HNO3 leaching solu-tions with stoichiometric and excess NaClare shown in table 2. The silver was
precipitated, but a large excess of NaClwas needed.
The dry AgCl precipitate is suitablefor 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, petnumber Ag Cu Fe Pb Zn
SINGLE LEACHING WITH 200 mL OF SOLUTION1.5
3.06.0
6.0
6.0
50.245.564.8
69.1
69.8
97.8>99>99
>99>99
92.0>99
>99
>99>99
72.481.985.786.385.9
97.9>99>99
>99
>99
MULTIPLE LEACHING WITH 100 mL OF SOLUTION1 6.0
6.06.0
17.061.1
.1
86.912.1
.9
48.545.65.8
48.227.920.0
89.42 10.2
3 .3
Total NAp 78.2 >99 >99 96.1 >99
SINGLE LEACHING WITH 200 mL OF SOLUTION, 6 h1 6.0 75.9 >99
1>99 96.0 >99
NAp Not applicable.
NOTE.—Leachingtaled 6 h.
time = 2 h, multiple leaching to-
TABLE 2. - Precipitation of silver from HNO3leaching solutions
Pet of Silver, mg/L Silverstoichiometric
NaClIn pregnantsolution
In barrensolution
precipitated,pet
100 120
120180
120
120
110
9556
20
2.1
8.3500 20.8
1,000 68.9
1,500 83.33,000 98.3
AQUA REGIA DISSOLUTION OF GOLD
Dilute aqua regia leaching experimentswere conducted to determine if gold couldbe extracted from residues from the HNO3pretreatment of zinc precipitates and HClpretreatment of steel wool cathodes.The amounts of HCl and HNO3 used were150 to 300 pet in excess of the stoichio-metric amounts required by the followingequation:
Au + 3HNO3 + 4HC1
-> HAUCI4 + 3NO2 + 3H2O. (2)
The experimental results are shown in ta-ble 3 and indicate that agitated leachingin hot dilute aqua regia dissolved metal-lic gold from the acid-leached residues.
A two-stage leach produced a pregnantAUCI3 solution low in residual HNO3,which improves the precipitation of goldfrom the aqua regia leaching solution.
PRECIPITATION OF METALLIC GOLD
(C00H)2 was added to the pregnant di-lute aqua regia solution to precipitategold 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 areshown in table 4.
TABLE 3. - Results of aqua regia leaching of HN03-leached residues
36-pct HCl/70-pct HNO3, mL^
Pretreatedresidue
weight, g
Volume of
diluteaqua regiaused, mL
Aurecovered
,
g
Auextracted,
petResidual
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
91.699.9
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.
.
FinenessGold in barren mg/L.
.
Urea added g..NaOH added g .
.
Retention time h.
.
Residual HNO3 .M.
.
Final pH
15
1.4896.54
999200
24
0.5<0
20
5.2098.95
995300
24
0.5<0
5
1.0599.91
9995
2
24<0.1
<0
15
3.2299.43
997110
24<0.1
<0
10
3.1499.99
998
3
14
2
<0.1
1.7
11.6
5.5099.95
9984
8
3
2
<0.1
1.0
'Pregnant solution generatedprecipitates.
2 Pregnant solution generated byzinc 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 woolcathode.
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 regialeaching solutions. Five-tenths molarresidual HNO3 in the leach solution re-sulted in slightly less precipitation of
gold. Gold precipitation was higher fromaqua regia leaching solutions from thesecond stage of the two-stage leachingexperiment, which contained less than
O.IM HNO3. Residual HNO3decreased by adding ureaequation 4,
could also beaccording 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 sourceQuantity used g .
.
Gold precipitated g .
.
Gold precipitated pet,.FinenessGold 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 heatedto 80° C.
GasExcess
2.3999.63
98859
15
0.5
GasExcess
1.5599.62
9834215
<0.1
GasExcess
2.2399.61
98567
60<0.1
NaHS03^2.9
1.61
99.44979
71
15
0.5
H2SO3I3
1.1599.72
9703315
0.5
decreased the time required to precipi-tate the gold. The crystalline gold pro-duced by (GOGH) 2 reduction is bright andlustrous.
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 precipitatedwith (C00H)2 and did not have the lus-trous appearance.
REFINING OF PRECIPITATED GOLD
If the precipitated gold is not of highenough purity, refining by smelting orleaching with hot HNO3 may be necessary.Gold precipitated by SO2 was upgraded bydigestion in hot HNO3 from 985 to 998fine. The experimental results obtainedfrom melting (C00H)2- or S02-precipitatedgold with nitre, borax, and silica areshown in table 6. Gold produced bychemical refining can be purified, if
necessary, to 999.9 fine by fluxing withnitre, borax, and silica at 1,000° C,
TABLE 6. - Results of fire-refiningprecipitated 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
979997
3.43.2
2.4
0.99
990999.9
3.43.2
2.4
0.99Fineness:
Before. ......... 995After 999.9
' 1-g gold samples,
CYANIDATION OF ACID LEACHED RESIDUESFOR RECOVERY OF RESIDUAL
PRECIOUS METALS
The siliceous residues from leachingzinc precipitates contained residual pre-cious metals. The results of cyanidationto recover the precious metal values areshown in table 7, Approximately 96 petof the gold and silver in the residueswas recovered by leaching in alkalinecyanide solutions. The precious-metal-bearing residues could be recycled to theplant's cyanide leaching circuit.
TABLE 7. - Results of cyanide leaching of
aqua regia-leached residue
NaCN g.Gold, oz/ton:
FeedTailing
Gold extracted, ., ,pct.
Silver, oz/ton:FeedTailing
Silver extracted. .pet.Time h.
Test1
1.0
2.900.13
95.52
171.21.9
98.8924
Test2
1.0
14.000.51
96.36
118.11.7
98.5624
Test
3
0.2
33.001.00
96.97
196.88.6
95.631
SLURRY SOLIDS FOR ACIDLEACHING PROCEDURES
The slurry solids recommended for allthe acid-leaching procedures are very lowand depend on the amount of gold and/orsilver contained in the specific feed.
If the pulp solids are too high, thesolubility limits for gold, silver,and other metals in the specific acid-leaching liquor may be attained, and pre-mature precipitation of precious metalsand other metals may occur. The recom-mended pulp solids should be used unlessit is determined by metallurgical testingthat the slurry solids can be changed.
RECOMMENDED PROCEDURE FOR ZINC PRECIPITATES
The recommended flowsheets based on theresults of this study are shown in fig-ures 1 and 2. Acid pretreatment of zinc
precipitates is required to dissolve thesilver and base metals and leaves a gold-bearing siliceous residue. One kilogram
Zinc precipitates
H2O
Residue -^recycle to
cyanideleaching
Urea-Oxalic acid-
NaOH-
HNO:
H2O
HgO
Residue
Aqua regialeaching
,
90° C. I h
Filtration
Filtrate
Precipitation ofnnetallic gold,
80° C
Acidpretreatment,85° C, 6 h
nFiltration
NaCI
Filtrate
H2O
AgCIproduct
H2O
Gold sponge
Barrenacid towaste
1Silver
precipitation
UFiltration
Filtrate
to waste
Goldproduct
FIGURE 1. - Flowsheet for chemical refining zinc precipitates.
steel wool cathode
HCI
HgO
H2O -
Residue
IAqua regia
leaching,
90' C, I h
H2O OFiltration
Filtrate
UreaOxalic acid
NaOH
Precipitation ofmetallic gold,
80* C
Acidpretreatment,85" C, 6 h
nFiltration -Filtrate
to waste
^ AgCI residue product
H2O
Gold sponge
•Barrenacid towaste
FIGURE 2. - Flowsheet for chemical refining steel wool cathodes.
Goldproduct
of zinc precipitate is slowly added to10 L of 6M HNO3. The slurry is heated to85° C, agitated, digested at temperaturefor 6 h, and filtered hot. The residueis washed with water. The wash water andacid filtrate are combined and saved forsilver recovery.
The silver contained in the acid fil-trate and wash water is precipitated asAgCl by the addition of NaCl. A tenfoldstoichiometric excess of NaCl is added tothe HNO3 leaching solution at room tem-perature. (The stoichiometric amount of
NaCl is 0.54 g NaCl per gram of silver insolution.) The solution is agitatedgently for 5 min and filtered. One gramof 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 untilno precipitation is observed. In some
cases a 30-fold excess of NaCl may be re-quired. The AgCl product is washed withdistilled water and dried at low tempera-ture (<100° C).
Gold is recovered from the HNO3-preleached siliceous residue by leachingwith aqua regia. The HN03-leached resi-due is divided into two equal charges.One charge is slurried with dilute aquaregia. 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 HNO3are used. The solution is heated to90° C and digested with continuous agita-tion for 1 h. The slurry is filtered hoton glass fiber filter paper. The acidfiltrate 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 acidfiltrate. The first aqua regia-leached
10
residue is saved for recycle to cyanideleaching. The acid filtrate is slurriedwith the second charge of HN03-leachedresidue and treated as above. The secondaqua regia leached residue is saved forfurther dilute aqua regia leaching.
Metallic gold is precipitated from theaqua regia leaching solution by (C00H)2.The pregnant AUCI3 solution (dilute aquaregia leaching solution) is heated to80° C and treated as follows:
1. Slowly add urea prills in 1-g
increments until gas evolution stops.Add the stoichiometric amount of (C00H)2required to precipitate the known orestimated amount of gold contained inthe pregnant solution (0.69 g (C00H)2 pergram 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 tobring the pH to 1,0, Digest the solutionat 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, addenough NaOH to bring the pH up to 1.0,Digest the solution until precipitationstops. Repeat until the yellow color ofthe solution disappears. Combine thegold precipitate into a single sponge byusing a spatula to free entrained gasesand to scrape the sides and bottom of thevessel. When all of the gold has beencollected, decant the barren solution andanalyze for residual gold,
3, Wash the gold sponge with distilledwater and dry at 100° C, The dry goldsponge should assay between 997 and 999fine.
RECOMMENDED PROCEDURE FOR STEEL WOOL CATHODES
HCl pretreatment of the cathodes is re-quired to dissolve the steel wool. Onekilogram of steel wool cathode is slowlyadded to 10 L of 5M HCl, which is heatedto 90° C while stirring. The slurry is
digested at temperature for 1 h, filteredhot, and washed with water.
The HCl-pretreated cathode residue is
leached with dilute aqua regia to recoverthe gold and silver. The HCl leach resi-due is divided into two equal charges.One charge is slurried with dilute aquaregia. 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 HNO3are used. The pulp is heated to 90° C,
digested, and agitated for 1 h. The hotslurry is filtered on glass fiber filter
paper, and the acid filtrate is removed.Displacement washing of the residue withdistilled water is continued until no
yellow color is visible in the filtrate.The washes are combined with the acidfiltrate. The residue from the firstaqua regia leach is an AgCl product whichshould contain 70 to 74 pet silver. Thesecond charge of HCl-leached residue is
slurried with the combined filtrate andtreated 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 solutionwith (C00H)2 is the same as for zincprecipitates.
HANDLING AND DISPOSAL OF ACID SOLUTIONS AND ACID WASTES
The acid solutions used are corrosiveand should be handled with extreme care.All leaching must be done in vessels thatwill not be corroded. The solutionsshould not contact the skin or eyes.Protective clothing should be worn to
prevent accidental contact. Should theacid solution contact a person's skin oreyes, flush with cold water for a minimumof 15 min and consult a physician.
The fumes from the acid solutions andchemical reactions with the solutions canbe very hazardous. All operations shouldbe 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 chemicalrespirator and full protective clothingat all times.
11
The acid waste solutions should be neu-tralized with alkaline solutions beforedisposal. Violent reactions may occurwhen mixing acid solutions with alkalinesolutions. Extreme care should be taken.The solutions should be slowly mixed to
prevent an uncontrollable reaction. ThepH of the neutralized acid waste shouldbe between 6 and 7. The neutralizedsolutions should be disposed of accord-ing to local, State, and/or Federalregulations.
SUMMARY AND CONCLUSIONS
The experimental results showed thatzinc precipitates and steel wool cathodescan be chemically refined to produce a
pure metallic gold and an AgCl precipi-tate. Chemical refining of zinc precipi-tates and steel wool cathodes recovered99.9 pet of the contained gold. HNO3preleaching separated silver and basemetals 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 separatedbase metal from the gold and silver con-tained in steel wool cathodes. Silver
was recovered as an AgCl precipitatewhich contained some gold.
Aqua regia leaching dissolved gold inthe residues from HNO3 and HCl preleach-ing. (C00H)2 and SO2 precipitated metal-lic gold from the aqua regia leachingsolutions. The gold produced by (C00H)2reduction was 998 to 999 fine. Gold pro-duced by SO2 reduction was 970 to 988fine, 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 inSouth Africa. Chamber of Mines of SouthAfrica, 1972, pp. 203-255.
2. Hall, K. B, Homestake 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, andR. E. Lindstrom. Processing Gold OresUsing 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 HomestakeMining Co., Lead, SD.
6. Hoke, C. M. Refining PreciousMetal Wastes. Metallurgical Publ.Co., New York, 1940; 355 pp.; re-prints available from Univ. Microfilms
International, 300 North Zeeb Road, AnnArbor, MI 48106.
7. Lowen, R. The Worshipful Companyof Goldsmiths (London), Tech. Rept. 44/1,1980, 22 pp.
8. McClelland, G. E., and J. A.
Eisele. Improvements in Heap Leaching ToRecover Silver and Gold From Low-GradeResources. BuMines RI 8612, 1982, 26 pp.
9. Merrill-Crowe. Merrill-Crowe Pre-cipitation Process—Details of Operation.Undated, 9 pp.; available upon requestf rom M. D. Wroblewski, BuMines, Reno, NV.
10. Pickett, D. E. Milling Practice inCanada. Can. Inst. Min. and Metall.
,
spec. V. 16, 1978, pp. 45-78.
11. Potter, G. M. Design Factors forHeap Leaching Operations. Min. Eng,
,
Mar. 1981, pp 277-281.
12. Zadra, J. B. , A. L. Engel, andH. J. Heinen. Process for RecoveringGold and Silver From Activated Carbonby Leaching and Electrolysis. BuMinesRI 4843, 1952, 32 pp.
*U.S.CPO: 1985-505-019/5092 INT.-BU.OF MINES, PGH., PA. 27847
1
D DD2 Q. O^ Q- 3
-1- en oin
3 O X)Q O <J
? O I
oVI n- ^ 33 9 Qrrto —n (t 3
» -r,
?-:5 ;;;•
2--^
3;- °
-O3^ in
(» O 3O -< a(n <? C<J n ;«
2
3 i? i:
So3) >
m z
enCD
Oo c
m ™ Cf mo
< m Z> Z mzc w
2Cm zz HH mo oTl COH HI >m Hmz CO
m
O3)
>zmOc>
oT3TJODD
-<
m2T3t-o<m3D
H 357 8§ '
cCO
IsIs
- m""2 2 >t -• 2i O Oo> "n -tl
-H mI mm w
m O
O
^*"% 'WW.' J" "i
o V '^oV^
e„o^-/ "o^*^-^%o'^ ^^,''*^\/ %^^-'/ '*'^^'*3^\/ %**r.
.*'%
'bv"
7^ ^0•^ '#•,
'.'-n^-o^ f
<j5°^
• V**•' - ^<>. c-^
i»/ /\^^CJ^' -^<m^^\ ^^Mc$i oV'^^^'- «>bV^ ^^^^^^^- '-^-^r^^'bV"
^^-n^.
'^o^
, *o.o^*y'' ^'^'^^'^O' "V^^'V?-^ '^^^^^'^^'•"^O
•: \.^^•'
^oV° '-^0
K-?^
o " a
Ml
."^-^^
> ---\/ "-^*^'^%o'^ V^^'y %*^-^%°' '^.>-*^\/ %*•.-.
u^'S.-
.<5 vP- <b^>.
'-K^-^ o^^'^^^ia-" 'J*.^-^' -^mm^^^ ^^A
s^o^ 'bv 'vf-0^
^ ^,-^77.' A
cv *
N !
^•^°<<v
^'^ .s**^. <>o »
'* .^^
"^ -^^^^
vS<^'
HECKMANBINDERY INC.
,<^^ JUL 85*§# N. MANCHESTER,N^^ (NDIANAAfiafiZ.
(•,3 'J.,-, « in /IWWn •'. * At
-JiL ai^iiiHMi
,^V t ^
LIBRARY OF CONGRESS
002 955 869 3
'..^ife, -t
J*).
^^i:-t If
!•