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Using Nickel as a Catalyst in Ammonium Thiosulfate Leaching

for Gold Extraction

Harunobu Arima1, Toyohisa Fujita2 and Wan-Tai Yen3

1Graduate Student, Queen’s University. Present address: Akita Zinc Co., Ltd., Akita 011-8555, Japan2RACE, (Department of Geosystem Engineering), The University of Tokyo, Tokyo 113-8656, Japan3Department of Mining Engineering, Queen’s University, Kingston, Ontario, K7L 3N6, Canada

The use of copper as a catalyst for gold leaching in ammonium thiosulfate solution might cause the high consumption of thiosulfate. Also,the high copper consumption is resulted in the zinc precipitation process for recovering the gold from the pregnant solution. In this investigation,nickel was used as a catalyst to minimize the reagent consumption. On a 100mass%-75mm of silicate type gold ore containing 16 g/t Au and0.2mass% of Fe and C, the nickel catalyzed ammonium thiosulfate solution could extract 95% of gold with the 1.2 kg/t-ore of ammoniumthiosulfate consumption in 24 hours at the most favorable reagent combination of 0.0001mol/dm3 NiSO4, 0.05mol/dm3 (NH4)2S2O3 and0.5mol/dm3 NH4OH at pH9.5, while the standard cyanidation at 0.02mol/dm3 (1.0 g/dm3) NaCN consumed around 1.5 kg/t-ore NaCN. In theconcentration range of 0:0001�0:005mol/dm3 Ni2þ, the ammonium thiosulfate consumption was 1�5 kg/t-ore, while the ammoniumthiosulfate consumption of copper catalyzed lixiviant was greatly increased from 3 kg/t-ore to 21 kg/t-ore as the increase of Cu2þ concentrationfrom 0.0001mol/dm3 to 0.001mol/dm3. The feasibility of recycling barren solution was confirmed with zinc precipitation at nearly 100% ofgold recovery. Nickel consumption on the cementation process was less than 50%. For extracting gold from the copper bearing sulfide ore, ahigher ammonia and thiosulfate concentrations were required with 0.0001mol/dm3 of Ni2þ. The ammonium thiuosulfate consumption withnickel as catalyst on the copper bearing sulfide ore was about 1�5 kg/t-ore less than that using copper as catalyst.

(Received March 17, 2003; Accepted December 1, 2003)

Keywords: ammonium thiosulfate leaching, cementation, nickel, gold

1. Introduction

The solubility of gold in a diluted solution of potassiumcyanide was demonstrated by Elsner in 1846, as shown in eq.(1):1)

4Auþ 8KCNþ O2 þ 2H2O ! 4KAu(CN)2 þ 4KOH ð1Þ

In 1887, cyanidation was patented by MacArthur and theForrester brothers, and further developed to the practical usefor gold recovery. Since then, cyanidation has been used as astandard gold extraction process. The gold pregnant solutionhas been processed by cementaion, CIP/CIL (carbon in pulp/carbon in leach) or ion exchange for gold recovery.2–4)

However, cyanide has a high toxicity with the timeweighted exposure average limit of 5mg/m3.5) Someincidents have occurred with spilled cyanide and a resultingenvironmental pollution.6,7) Several countries such as theCzech Republic, Turkey and the USA in states, such asColorado, Wisconsin and Montana, have campaigned to bancyanide applications.8–12) Also, when processing refractoryore containing sulfide and carbonaceous minerals, it isdifficult to extract gold by cyanidation without pre-treatment,such as roasting, pressure oxidation and bio-oxidation,etc.13–15)

Due to above environmental, political and technicalconcerns, the development of non-cyanide process has beenfocused on an alternative in the study of gold extraction.

There are various non-cyanide lixiviants being proposedby many investigators for gold extraction: thiourea, bromide,iodine, thiocyanate and thiosulfate.16–24) Among these lix-iviants, thiosulfate has been suggested as the best cyanidesubstitute.25–32)

Thiosulfate (S2O32�) is the least toxic and cheapest

chemical which can stabilize gold in aqueous solution. Up

to 12 g thiosulfate can be taken daily by mouth with no illeffects.5) The current market price for ammonium thiosulfateis only US$0.30/kg.33) In addition to thiosulfate as a majorreagent, both ammonia and cupric ion are required in the goldleaching process. Ammonia is required to stabilize thecatalyst of cupric ion and modify the pH. The time weightedexposure average limit of ammonia is 35mg/dm3.5)

There has been extensive research on thiosulfate leachingfor gold extraction since the 1970s.29–31,34–41) Ammoniumthiosulfate leaching process is also applicable for the heapleaching process.28,31) However, there are problems incommercializing the thiosulfate process in the gold millingindustry, the problems are the most notably high reagentconsumption and varying gold extraction results for thedifferent ores under the same leaching conditions. Further-more, the conventional gold recovery process of CIP (carbonin pulp) on cyanidation can not be applicable in non cyanideprocess.28,41,42) The alternative gold recovery process bycementation using zinc, copper and aluminium powders aswell as ion exchange has not been systematically studied,hence a favourable process is not still established.41,43–51)

In our previous studies on the copper catalyzed ammoniumthiosulfate leaching system, the most favorable reagentcombination to give high gold recovery with small thiosulfateconsumption was extensively researched and evaluated byboth thermodynamic and experimental approaches.52,53) Thebest experimental result showed that 94% of gold extractionand 3 kg/t-ore of ammonium thiosulfate consumption couldbe achieved using 0.3mol/dm3 NH4OH, 0.0001mol/dm3

CuSO4, 0.05mol/dm3 (NH4)2S2O3 at pH 9.5 for a�200mesh (�75 mm) silicate type ore containing 16 g/tAu. Using zinc powder, nearly 100% of gold could berecovered from the pregnant solution. However, thiosulfategold leaching using copper as a catalyst had the following

Materials Transactions, Vol. 45, No. 2 (2004) pp. 516 to 526#2004 The Mining and Materials Processing Institute of Japan

problems:(1) Since the stability of the cuprous thiosulfate complex is

higher than that of the cuprous ammine complexthermodynamically, the cupric ammine complex isreduced to a cuprous thiosulfate complex, causing anoxidation of thiosulfate to tetrathionate through theintermediate product of a cuprous ammine complex asshown in eqs. (2) and (3).

2[Cu(NH3)4]2þ þ 2S2O3

2�

! 2[Cu(NH3)2]þ þ S4O6

2� þ 4NH3 ð2Þ[Cu(NH3)4]

þ þ 2S2O32�

! [Cu(S2O3)2]3� þ 4NH3 ð3Þ

(2) Although either the increase of ammonia concentrationor the decrease of thiosulfate concentration can makethe cupric ion stable as a cupric ammine complex, thecupric ion is the important electron acceptor to oxidizegold. And, a substantial amount of cupric ion is requiredin the ammonium thiosulfate leaching of gold extrac-tion. Also, gold extraction kinetics and thiosulfateconsumption are nearly influenced by the level ofcopper concentration.52)

(3) In the study of gold cementation by zinc, copper andaluminium powders, zinc powder was found to be thebest metal powder to recover gold at lower ammonia(0.3mol/dm3) and thiosulfate (0.05mol/dm3) concen-trations.52) But almost all of the copper in the solutionwas co-precipitated along with the gold since thestability of the copper ions is lower than that of zincions thermodynamically.52,53) Thus, the addition ofcupric ions is required for recycling the barren solutionto the leaching circuit, increasing the total reagentconsumption.

In order to reduce the reagent consumption in thethiosulfate leaching of gold, nickel ion was used instead ofcopper ions in the present investigation. Effects of oxygen,ammonia, copper and thiosulfate on both gold extraction andthiosulfate consumption were investigated using the same orestudied on the conventional copper catalyzed system.52) Thegold in the pregnant solution was cemented by zinc powder.The optimal reagent composition found in this investigationwas evaluated thermodynamically. Gold leaching tests for

various sulfide minerals were also conducted and evaluatedfor their feasibility.

2. Thermodynamic Discussion on Gold Leaching andRecovery

Figure 1 shows the Eh-pH diagram of the Ni-NH4OH-S2O3

2� system for the reagent combination of 0.5mol/dm3

NH4OH, 0.5mol/dm3 S2O32� and 0.005mol/dm3 Ni2þ. The

concentrations of reagent except Ni2þ are within the rangeapplied on the previous experimental project of the coppercatalyzed gold leaching.52) Free energies of formation used inthis calculation are listed in Table 1.54–64) Nernst Equationsapplied for this Eh-pH diagram is shown in Table 2. Theresult of Fig. 1 suggests that the electrode potential in thecouple of {Ni3O4/[Ni(NH3)6]

2þ} may work for gold oxida-tion in ammonium thiosulfate solution, while that in thecouple of {[Cu(NH3)4]

2þ/[Cu(NH3)2]þ} works as an oxidant

for gold.53)

In the gold leaching by nickel as a catalyst, nickelous oxide

-1.0

-0.5

0.0

0.5

1.0

1.5

4 6 8 10 12 14

pH

Eh,

V

Ni

[Ni(S2O3)2]2-

NiO

NiO[Ni(NH3)6]

2+

Ni3O4

Ni2O3

NiO2

↑N2

↑N4↑N3

↑N4

↓N6

↓N8↓N5

↓N9

↓N5

↓N7↓N10

↓E2

↓E1

Fig. 1 Eh-pH diagram for the Ni-NH3-S2O32�-H2O system at the

condition of 0.5mol/dm3 NH4OH, 0.005mol/dm3 Ni2þ and 0.5mol/

dm3 S2O32�. Nernst Equations (N1)�(N11) are shown in Table 2.

Table 1 Free energy of formation for various species54–61)

species �G (kJ/mol) species �G (kJ/mol) species �G (kJ/mol)

H2O �237:18 NiO �215:93 Ni(S2O3)22� �1098:02

OH� �157:29 Ni3O4 �711:91 Au(S2O3)23� �1030:15

NH3 �26:57 Ni2O3 �469:94 Cu(S2O3)23� �1065:01

S2O32� �522:50 NiO2 �215:94 Cu(S2O3)3

5� �1595:73

Ni2þ �48:24 Cu2O �146:00

Auþ 163.20 CuO �129:70 Ni(NH3)62þ �256:66

Au3þ 434.18 Au(NH3)2þ �41:4

Cuþ 50.00 HNiO2� �349:22 Au(NH3)4

3þ 65.43

Cu2þ 65.52 NiO42� — Cu(NH3)2

þ �63:51

AuO3� �24:27 Cu(NH3)4

2þ �112:65

Ni(OH)2 �447:3 CuO22� �182:00

Au(OH) 134.86

Au(OH)3 �289:15

Using Nickel as a Catalyst in Ammonium Thiosulfate Leaching for Gold Extraction 517

Table 2 Nernst equations applied in Eh-pH diagram at 298K on Metal-Ammonia-Thiosulfate-Water System (The presence of other polythionate species was not considered.)

Half cell reaction Nernst equation

B1 Au + 2S2O32� = Au(S2O3)2

3� + e� B1 E ¼ 0:15þ 0:059 log½Au(S2O3)23�� 0:118 log½S2O3

2��B2 Au + 2NH3 = Au(NH3)2

þ + e� B2 E ¼ 0:12þ 0:059 log½Au(NH3)2þ� � 0:118 log½NH3�

B3 Au(NH3)2þ + 2NH3 = Au(NH3)4

3þ + 2e� B3 E ¼ 0:89þ 0:059 logf[Au(NH3)43þ]/[Au(NH3)2

þ]g � 0:059 log½NH3�B4 Au(S2O3)2

3� + 4NH3 = Au(NH3)43þ + 2S2O3

2� + 2e� B4 E ¼ 0:81þ 0:059 logf[Au(NH3)43þ]/[Au(S2O3)2

3�]þ 0:059 log½S2O32�� 0:118 log½NH3�

B5 Au(NH3)2þ + 3H2O = AuO3

3� + 2NH3 + 6Hþ + 2e� B5 E ¼ 3:50þ 0:030 logf[AuO33�]/[Au(NH3)

2þ]g þ 0:059 logðNH3Þ � 0:177pH

B6 Au(S2O3)23� + 3H2O = AuO3

3� + 2S2O32� + 6Hþ + 2e� B6 E ¼ 3:48þ 0:030 logf[AuO3

3�]/[Au(S2O3)23�]g þ 0:059 logðS2O3

2�Þ � 0:177pH

B7 Au(NH3)2þ + 3H2O = Au(OH)3 + 2NH3 + 3Hþ + 2e� B7 E ¼ 2:29þ 0:059 log½NH3� � 0:089pH� 0:030 log½Au(NH3)2

þ�B8 Au(S2O3)2

3� + 3H2O = Au(OH)3 + 2S2O32� + 3Hþ + 2e� B8 E ¼ 2:27þ 0:059 log½S2O3

2�� 0:089pH� 0:030 log½Au(S2O3)23��

B9 Au + H2O = Au(OH) + 2Hþ + e� B9 E ¼ 3:03� 0:059pH

N1 Ni = Ni2þ + 2e� N1 E ¼ �0:25þ 0:030 log½Ni2þ�N2 Ni + 2S2O3

2� = Ni(S2O3)22� + 2e� N2 E ¼ �0:27þ 0:030 log½Ni(S2O3)2

2�� 0:059 log½S2O32��

N3 Ni + 6NH3 = Ni(NH3)62þ + 2e� N3 E ¼ �0:50þ 0:030 log½Ni(NH3)6

2þ� � 0:177 log½NH3�N4 Ni + H2O = NiO + 2Hþ + 2e� N4 E ¼ 0:13� 0:059pH

N5 3NiO + H2O = Ni3O4 + 2Hþ + 2e� N5 E ¼ 0:897� 0:059pH

N6 2Ni3O4 + H2O = 3Ni2O3 + 2Hþ + 2e� N6 E ¼ 1:31� 0:059pH

N7 Ni2O3 + H2O = 2NiO2 + 2Hþ + 2e� N7 E ¼ 1:43� 0:059pH

N8 3Ni(NH3)62þ + 4H2O = Ni3O4 + 18NH3 + 8Hþ + 2e� N8 E ¼ 2:74� 0:089 log½Ni(NH3)6

2þ� � 0:236pHþ 0:531 log½NH3�N9 3Ni(S2O3)2

2� + 4H2O = Ni3O4 + 6S2O32� + 8Hþ + 2e� N9 E ¼ 2:05þ 0:177 log½S2O3

2�� 0:236pH� 0:089 log½Ni(S2O3)22��

N10 2Ni(S2O3)22� + 3H2O = Ni2O3 + 4S2O3

2� + 6Hþ + 2e� N10 E ¼ 1:80þ 0:118 log½S2O32�� 0:177pH� 0:059 log½Ni(S2O3)2

2��N11 Ni(S2O3)2

2� + 2H2O = NiO2 + 2S2O32� + 4Hþ + 2e� N11 E ¼ 1:61þ 0:059 log½S2O3

2�� 0:118pH� 0:030 log½Ni(S2O3)22��

E1 2Hþ = H2 + 2e� E1 E ¼ �0:059pH

E2 2H2O + O2 = 4OH� + 4e� E2 E ¼ 1:23� 0:059pH

F1 NH4þ + H2O = NH4OH + Hþ F1 ½NH3�� ¼ ½NH3�T��=ð1þ 109.25-pHÞ; �½NH3� ¼ ½NH4OH�, ��½NH3�T ¼ ½NH4

þ� þ ½NH3�

Equations B2, B3, B4, B5, B7, N3 and N8 are calculated with eq. F1.

518

H.Arim

a,T.Fujita

andW.-T

.Yen

(Ni3O4), formed from nickel ammine complex, act as anoxidant for gold. The oxidation reaction of nickel amminecomplex is shown as the combination of anodic reaction (4)and cathodic reaction (5):

3Ni(NH3)62þ þ 8OH�

! Ni3O4 þ 18NH3 þ 4H2Oþ 2e� ð4Þ

1/2O2 þ H2Oþ 2e� ! 2OH� ð5Þ

The nickelous oxide (Ni3O4) in eq. (4) is reduced back to anickel ammine complex with the oxidation reaction of goldas shown in the combination of cathodic reaction (6) andanodic reaction (7):

Ni3O4 þ 18NH3 þ 4H2Oþ 2e�

! 3Ni(NH3)62þ þ 8OH� ð6Þ

2Auþ 4NH3 ! 2[Au(NH3)2]þ þ 2e� ð7Þ

The overall reaction from (4) to (7) is expressed as thefollowing reaction (8):

Catalyzed by [Ni(NH3)6]2þ

#4Auþ 8NH3 þ O2 þ 2H2O

! 4[Au(NH3)2]þ þ 4OH� ð8Þ

Since the aurous ammine complex is not stable, the aurousammine complex in eq. (8) is immediately converted to astable aurous thiosulfate complex according to reaction (9),as discussed in the previous study on copper catalyzedsystem.52)

[Au(NH3)2]þ þ 2S2O3

2� ! [Au(S2O3)2]3� þ 2NH3 ð9Þ

The sum of reactions (8) and (9) gives reaction (10), which isthe overall gold oxidation reaction in a nickel catalyzedsystem.

4Auþ 8S2O32� þ O2 þ 2H2O

! 4[Au(S2O3)2]3� þ 4OH� ð10Þ

Figure 2 shows the Eh-pH diagram of the Au-NH4OH-S2O3

2� system for the reagent combination of 0.5mol/dm3

NH4OH, 0.5mol/dm3 S2O32� and 5� 10�5 mol/dm3 Au.

Compared to Fig. 1, the electrode potentials of aurousammine complex at pH9.5 is located in higher regions thanthose of nickel ammine complex. Thus, it is considered thatthe stable formula of Auþ is [Au(S2O3)2]

3� rather than[Au(NH3)2]

þ in the nickel catalyzed ammonium thiosulfatesolution. As well, the higher stability of nickel ion againstaurous ion suggests minimizing the loss of the catalyst ofnickel ion in the gold cementation by zinc. It means that thereagent cost is reduced to recycle the barren solution to thegold leaching stage, while the copper catalyzed system co-precipitated the entire catalyst of cupric ion along with gold.The chemistry of gold cementation by zinc powder isexpressed as eq. (11).

2Znþ 2[Au(S2O3)2]3� þ 4NH3 þ 2H2O

! [Zn(S2O3)2]2� þ [Zn(NH3)4]

2þ

þ 2S2O32� þ 2OH� þ H2 " þ2Au #

ð11Þ

Also, some nickel precipitation by zinc powder could occur

as shown in eq. (12):

2Znþ [Ni(NH3)6]2þ þ 2S2O3

2� þ 2H2O

! [Zn(NH3)4]2þ þ [Zn(S2O3)2]

2�

þ 2NH3 þ 2OH� þ H2 " þNi #ð12Þ

Figure 3 summarizes the schematic electrochemicalmechanism of ammonium thiosulfate leaching of gold, whichcorresponds to eqs. (4) to (10).

3. Experimental

3.1 Gold leaching3.1.1 Ore sample

Gold ore sample was supplied from Hishikari Mine,Kagoshima Japan by Sumitomo Metal Mining, Co., Ltd. Thetotal amount of ore sample was 150 kg, and the ore size wasinch 15�20 cm (0�8 inch). Those ore was crushed to�1:70mm (�10mesh) by a jaw crusher (Denver H5902CB),a gyratory crusher (Allis-Chalmers) and a crushing roll

-0.5

0.0

0.5

1.0

1.5

4 6 8 10 12 14

pH

Eh,

V

Au

[Au(S2O3)2]3- [Au(NH3)2]

+

[Au(NH3)4]3+

↑B1

↑B4

↑B3

↑B2

↑E2

↑E1

Fig. 2 Eh-pH diagram for the Au-NH3-S2O32�-H2O system at the

condition of 0.5mol/dm3 NH4OH, 0.5mol/dm3 S2O32� and

5� 10�5 mol/dm3 Au Nernst Equations (B1)�(B4) are shown in Table 2.

e-

Gold surface Ammonium Thiosulfate Solution

2NH3, Au(S2O3)23-

Au(NH3)2+

2S2O32-

3Ni(NH3)62+, 8OH-

Ni3O4 (hydr), 18NH3, H2O

1/2O2, H2O

Au

Fig. 3 Schematic gold dissolution mechanisms in ammonium thiosulfated

solution on gold surface using Ni.


(Sturtevant Laboratory Roll). The crushed ore was dividedinto 2 kg each using a rotation vibratory feeder. Finally, the2 kg crushed ores were passed through a riffle 3 times, andthen 0.3 kg samples were packed into paper bags for leachingsamples.

Table 3 shows the results of chemical analysis of the ore.The ore matrix was silicate. The gold grade was 16.26 g/t bythe fire assay method.65,66) From a 0.5 g of pulverized sample(100mass%-75 mm), 0.18mass% of iron was detected byaqua regia leaching. Copper was not detected. Sulfur andcarbon contents determined by a sulfur and carbon combus-tion analyzer (Leco1 SC-444DR) were 0.00mass% and0.19mass%, respectively.3.1.2 Leaching procedure

For thiosulfate leaching, ammonium thiosulfate, ammo-nium hydroxide, copper sulfate and nickel sulfate were used.For cyanidation, sodium cyanide and calcium hydroxidewere used. All chemicals were reagent grade supplied fromAldrich Chemical, Alfa Aesar and EM science.

A 0.3 kg-crushed sample was ground by a rod mill (DenverSala) with 666 cm3 of water at certain grind time, and theground sample was filtered by a filter press. The filtered cakewas cut into quarter, and then the half of sample, composedof two opposite quadrants, was used for leaching. The 150 gground ore was put into a 2 dm3 bottle, and the calculatedamounts of ammonia, ammonium thiosulfate and coppersulphate or nickel sulfate were fed into a bottle, and then awater was added to make a 33 solids% of slurry. The bottlewas rolled on a bottle roller for 24 hours at an ambienttemperature. Rotation speed of the bottle was about 60 rpmmeasured by a VMR digital taco meter.

After the leaching test, the gold pregnant solution wasfiltered using a vacuum pump with a buchner funnel andcollected for gold assay. The filtered cake (residue) waswashed with a 100 cm3 of water 4 times to wash out all goldpregnant solution trapped in the residue. The total volumes ofpregnant and washed solutions were measured by a cylinder.The gold concentrations were measured by an atomicadsorption spectrophotometer. The gold leached residuewas dried in an oven, and assayed for gold by the fire assaymethod.65) The percentage of gold extraction was calculatedfrom the total amount of gold in the leached residue and thepregnant solution as following:

Gold Extraction [%]

¼ð½Au�preg.sol. þ ½Au�washed.sol.Þ

ð½Au�preg.sol. þ ½Au�washed.sol. þ ½Au�residueÞ� 100

ð12Þ

where½Au]preg.sol.: total amount of gold in pregnant solution [mg]½Au]washed.sol.: total amount of gold in washed solution [mg]½Au]residue: total amount of gold in leached residue [mg]

3.1.3 Thiosulfate analysisThe consumption of thiosulfate ion in a gold pregnant

solution was assayed by the iodimetric titration method afterthe leaching.67) Procedure of the iodimetric titration was asfollows:(1) After a 10 cm3 of thiosulfate solution was taken into a

50 cm3 beaker, pH of the solution is measured. If the pHis higher than 8, the solution pH was neutralized to bearound pH 7�7:5 by 10 vol% H2SO4 solution (The totaladded volume of 10 vol% H2SO4 solution should benoted). This neutralized thiosulfate solution was trans-ferred into a 25 cm3 burette for the iodimetric titration.

(2) The thiosulfate solution prepared in procedure 1 wasfiltered against 5 cm3 of I2 solution, containing a2�3 cm3 of 0.5 g/dm3 starch solution as indicator, ina 50 cm3 beaker for iodimetiric titration. The end pointis reached, when the colour of I2 solution changed fromdeep brown to colour less. The total volume ofthiosulfate solution required for titration was appliedto evaluate the thiosulfate concentration.

To analyze a 0:2�0:5mol/dm3 thiosulfate solution, a0.05mol/dm3 iodine solution was used. In the range of a0:01�0:2mol/dm3 thiosulfate solution, a 0.02mol/dm3

iodine solution was used. The iodine solution was preparedby dissolving 0.05mol/dm3 I2 with 40 g of KI in 1 dm

3 flask.The starch solution was made by dissolving a 1.0 g of solublestarch in a 500 cm3 water by heating. The accuracy of theiodimietric titration was confirmed using standard ammoni-um thiosulfate solutions.

From the data obtained from the iodimetric titrationpresented above, thiosulfate concentration was evaluated tothe following equation:

MT ¼2�MI � L1

L2 �V1

V2

ð14Þ

Where, MT is thiosulfate concentration [mol/dm3]; MI isiodine concentration [mol/dm3]; L1 is initial iodine volume(In this procedure, 5 cm3 at constant); L2 is thiosulfatevolume used to titrate iodine solution [cm3]; V1 is initialthiosulfate solution volume (In this procedure, 10 cm3 atconstant); V2 is thiosulfate solution volume [cm3] after pHadjustment.

Based on the thiosulfate concentration obtained from eq.(14), the total ammonium thiosulfate consumption duringleaching was evaluated to the following:

Ammonium Thiosulfate Consumption [kg/t-ore]

¼148� Vlixviant � ½S2O3

2��consumed

Wsample

ð15Þ

where, 148 is the molecular weight of ammonium thiosulfateat 1mol [g/mol]; Vlixiviant is total volume of ammoniumthiosulfate lixviant utilized in leaching [dm3];[S2O3

2�]consumed. is total amount of thiosulfate consumed[mol/dm3];Wsample is ore sample weight utilized for leaching[kg].

Table 3 Assay results of Hishikari ore.

Au (g/t) Fe (mass%) Cu (mass%) S (mass%) C (mass%)

16.26 0.18 0.00 0.00 0.19

Ore Matrix: Silicate Type

520 H. Arima, T. Fujita and W.-T. Yen

3.2 Gold cementation from ammonium thiosulfate solu-tion

Zinc powders (supplied from Alfa Aesar) were used forgold cementation. The average diameter of the powder was45 mm and purity was more than 98%. A known amount ofthe metal powder was added to a 50 cm3 pregnant solution ofthe leaching experiments described in the section 3.1, and thesuspension was gently agitated by a magnetic stirrer for 30minutes at an ambient temperature under atmosphere.

After the gold cementation, the pulp was filtered imme-diately with a micro filter. The filtrate was assayed for gold,nickel, thiosulfate and ammonia. Gold, nickel and thiosulfateconcentrations were evaluated in the same manner describedin section 3.1. The ammonia concentration was measured byNessler’s method (JIS 0102). Nessler’s method is a semi-quantitative method, and the standard deviation is around�0:5mol/dm3 NH4

þ/NH3.To investigate the feasibility of recycling the ammonium

thiosulfate solution, the concentration of ammonia, nickeland thiosulfate before and after the gold cementation processwere evaluated.

4. Results and Discussion

4.1 Preliminary gold leaching testFigure 4 shows the gold extraction result from a silicate

type ore by using the reagent combination of 0.005mol/dm3

NiSO4, 0.5mol/dm3 (NH4)2S2O3 and 0.5mol/dm3 NH4OHat pH 9.5. The gold extraction at 24 hours was 95% and theammonium thiosulfate consumption was about 5 kg/t-ore,while 21 kg/t-ore of ammonium thiosulfate was consumedwhen 0.001mol/dm3 CuSO4 was used as a catalyst.52) Theammonium thiosulfate consumption with copper catalyst wasabout 4 times higher than that with a nickel catalyst. It wasobvious that the nickel ion significantly decreased thethiosulfate consumption.

4.2 Effect of ammonia, nickel and thiosulfate on goldextraction and ammonium thiosulfate consumption

The effects of ammonia, nickel and thiosulfate concen-trations on gold extraction and thiosulfate consumption wereinvestigated at the standard reagents composition of 0.5mol/dm3 NH4OH, 0.005mol/dm3 NiSO4 and 0.5mol/dm3

(NH4)2S2O3. The reagent concentrations was varied in therange of 0:1�0:5mol/dm3 NH4OH, 0:0001 � 0:005mol/dm3 NiSO4 and 0:01�0:5mol/dm3 (NH4)2S2O3.

Figure 5 shows the effect of ammonia on gold extractionand ammonium thiosulfate consumption with 0.005mol/dm3

NiSO4 and 0.5mol/dm3 (NH4)2S2O3. The gold extractiondecreased from 95% to 60% as the ammonia concentrationdecreased from 0.5 to 0.1mol/dm3, while the ammoniumthiosulfate consumption increased from 5 to 9 kg/t-ore. Thisresult concludes that the increase of the nickel thiosulfatestability at the lower ammonia concentrations reduced thecatalytic effectiveness for gold oxidation. Thus, the bestconcentration for gold extraction is 0.5mol/dm3 NH4OH.

Figure 6 shows the effect of nickel concentration on goldextraction and ammonium thiosulfate consumption with0.5mol/dm3 NH4OH and 0.5mol/dm3 (NH4)2S2O3. Thenickel concentration was varied in the range of0:000�0:005mol/dm3. The result shows that both goldextraction and ammonium thiosulfate consumption wererespectively constant at 95% and 5 kg/t-ore in the range of0:0001�0:005mol/dm3. At the nickel concentration of0mol/dm3, the gold dissolution was not observed. Theevidence suggests that nickel may act as a catalyst for goldextraction with small thiosulfate consumption. Also, theresult, in which the gold oxidation was not initiated withoutthe presence of a catalyst, confirms the reports from otherresearchers.37,39) The main cause of thiosulfate consumptionin the nickel catalyzed system could be attributed to thenatural degradation of thiosulfate in the gold leachingprocess. The minimum nickel concentration can be selected

0

20

40

60

80

100

0 0.1 0.2 0.3 0.4 0.5 0.6

NH4OH, M/mol . dm-3

Au

Ext

ract

ion

(%)

0

3

6

9

12

15(N

H4)

2S2O

3 C

onsu

mpt

ion

(kg/

t-or

e)

Fig. 5 Effect of ammonia concentration on gold extraction and ammonium

thiosulfate consumption with 0.005mol/dm3 NiSO4 and 0.5mol/dm3

(NH4)2S2O3 on a 100mass%-75mm ground ore. Pulp density: 33mass%,

Ambient temperature, Atmospheric condition, Retention Time: 24 hours,

Rotation speed: 60 rpm, pH 9.5. : Au extraction, : (NH4)2S2O3

consumption.

0

20

40

60

80

100

0 5 10 15 20 25 30 35

Retention Time, t /hour

Au

Ext

ract

ion

(%)

Fig. 4 Gold extraction kinetics at the condition of 0.5mol/dm3 NH4OH,

0.005mol/dm3 NiSO4 and 0.5mol/dm3 (NH4)2S2O3 at pH 9.5. Pulp

density: 33mass%, Ambient temperature, Atmospheric condition, Rota-

tion speed: 60 rpm.


as 0.0001mol/dm3.Figure 7 shows the effect of thiosulfate concentration on

gold extraction and reagent consumption in the range of0:01�0:5mol/dm3 (NH4)2S2O3. Nickel and ammonia con-centrations were maintained at 0.005mol/dm3 NiSO4 and0.5mol/dm3 NH4OH. The results show that the maximumgold extraction of 95% with the lowest ammonium thio-sulfate consumption of 2 kg/t-ore was obtained at 0.05mol/dm3 (NH4)2S2O3. The increase of thiosulfate concentrationfrom 0.05 to 0.1mol/dm3 increased the ammonium thio-sulfate consumption from 2 to 5 kg/t-ore. In the range of0:1�0:5mol/dm3 (NH4)2S2O3, the ammonium thiosulfate

consumption remained at 5 kg/t-ore. The 0.05mol/dm3

(NH4)2S2O3 is the minimum concentration for achievinghigher Au extraction under this reagent composition.

From the above results, the optimum reagent combinationcan be proposed as 0.0001mol/dm3 NiSO4, 0.05mol/dm3

(NH4)2S2O3 and 0.5mol/dm3 NH4OH at pH 9.5. Figure 8shows the result of gold extraction with this reagentcomposition, which resulted in 95% recovery in 24 hours.The ammonium thiosulfate consumption was 1.2 kg/t-ore,while the reagent consumption of copper catalyzed lixiviantwas 3�5 kg/t-ore under 0.0001mol/dm3 CuSO4, 0.05mol/dm3 (NH4)2S2O3 and 0.5mol/dm3 NH4OH at pH 9.5 and thesodium cyanide consumption was about 1.8 kg/t-ore with the0.02mol/dm3 (1.0 g/dm3) NaCN at pH 10.5. This exper-imental result demonstrates the advantage of nickel catalyzedammonium thiosulfate leaching.52,53)

For further confirmation, ammonia and thiosulfate con-centrations were varied with 0.0001mol/dm3 NiSO4.Figure 9 shows that the decrease in the ammonia concen-tration from 0.5 to 0.1mol/dm3 causes a decrease in the goldextraction from 95 to 75% and an increase in the ammoniumthiosulfate consumption from 1.2 to 3.5 kg/t-ore. The resultsupports that 0.5mol/dm3 NH4OH is an optimum concen-tration with the combination of 0.0001mol/dm3 NiSO4 and0.05mol/dm3 (NH4)2S2O3.

Figure 10 shows that the decrease of thiosulfate concen-tration from 0.05mol/dm3 to 0.01mol/dm3 decreased thegold extraction from 95% to 30%. The result also supportsthat 0.05mol/dm3 (NH4)2S2O3 is an optimum concentrationwith the combination of 0.0001mol/dm3 NiSO4 and0.5mol/dm3 NH4OH. Thus, the results of Figs. 9 and 10confirmed that an optimum reagent combination is0.0001mol/dm3 NiSO4, 0.05mol/dm3 (NH4)2S2O3 and0.5mol/dm3 NH4OH, which resulted in the 95% of goldextraction with 1.2 kg/t-ore of (NH4)2S2O3 consumption,experimentally. This optimum reagent combination was usedto construct the Eh-pH diagram as shown in Fig. 11. At the

0

20

40

60

80

100

0 5 10 15 20 25 30 35

Retention Time, t /hour

Au

Ext

ract

ion

(%)

Fig. 8 Gold extraction kinetics of the nickel catalyzed ammonium

thiosulfate leaching at the condition of 0.5mol/dm3 NH4OH,

0.0001mol/dm3 NiSO4 and 0.05mol/dm3 (NH4)2S2O3 at pH 9.5. Pulp

density: 33mass%, Ambient temperature, Atmospheric condition, Rota-

tion speed: 60 rpm.

0

20

40

60

80

100

0.000 0.002 0.004 0.006

NiSO4-6H2O, M/mol . dm-3

Au

Ext

ract

ion

(%)

0

3

6

9

12

15

(NH

4)2S

2O3

Con

sum

ptio

n (k

g/t-

ore)

minimum Ni2+ conc.

0.0001mol/dm3

Fig. 6 Effect of nickel concentration on gold extraction and ammonium

thiosulfate consumption with 0.5mol/dm3 NH4OH and 0.5mol/dm3




consumption.

0

20

40

60

80

100

0.0 0.1 0.2 0.3 0.4 0.5 0.6

(NH4)2S2O3, M/mol . dm-3

Au

Ext

ract

ion

(%)

0

3

6

9

12

15

(NH

4)2S

2O3

Con

sum

ptio

n (k

g/t-

ore)

Fig. 7 Effect of thiosulfate concentration on gold extraction and ammo-

nium thiosulfate consumption with 0.005mol/dm3 NiSO4 and 0.5mol/

dm3 NH4OH on a 100mass%-75mm ground ore. Pulp density: 33mass%,



consumption.


pH of 9.5, which was the gold leaching condition in Fig. 8,the stable region of the nickel ammine complex was widerthan that of Fig. 1. This is due to the decrease of nickel andthiosulfate concentrations from 0.005 to 0.0001mol/dm3 andfrom 0.5 to 0.05mol/dm3, respectively. The equilibriumelectrode potential for gold to form an aurous amminecomplex was higher than that of nickel as compared to inFigs. 2 and 11. The result supports that metallic gold wasoxidized to thiosulfate complex ([Au(S2O3)2]

3�), while thenickel ammine complex was stable enough to act as a catalystin eqs. (4)�(10).

4.3 Gold cementation and solution recyclingFigure 12 shows the result of gold recovery from a nickel

catalyzed thiosulfate pregnant solution, which was obtainedunder an optimum leach condition of 0.0001mol/dm3

NiSO4, 0.05mol/dm3 (NH4)2S2O3 and 0.5mol/dm3

NH4OH, by zinc powder cementation. The 97% of the goldwas recovered in 10 minutes with a Zn/Auþ mass ratio of 30.The final nickel concentration did not change significantlyand only 0:00001 � 0:00005mol/dm3 of Ni2þ was lost,while the entire copper ion was co-precipitated along withgold in the zinc precipitation from a copper catalyzedammonium thiosulfate solution.2) The stability of nickel inthe ammonium thiosulfate solution during zinc precipitationsuggests that the nickel catalyzed ammonium thiosulfateleaching has an advantage against the copper catalyzedammonium thiosulfate leaching process for minimizing the

0

20

40

60

80

100

0 0.1 0.2 0.3 0.4 0.5 0.6

NH4OH, M/mol . dm -3

Au

Ext

ract

ion

(%)

0

2

4

6

(NH

4)2S

2O3

Con

sum

ptio

n (k

g/t-

ore)

Fig. 9 Effect of ammonia concentration on gold extraction and ammonium

thiosulfate consumption with 0.0001mol/dm3 NiSO4 and 0.05mol/dm3




consumption.

0

20

40

60

80

100

0.00 0.01 0.02 0.03 0.04 0.05 0.06

(NH4)2S2O3, M/mol . dm-3

Au

Ext

ract

ion

(%)

0

2

4

6

(NH

4)2S

2O3

Con

sum

ptio

n (k

g/t-

ore)

Fig. 10 Effect of thiosulfate concentration on gold extraction and

ammonium thiosulfate consumption with 0.0001mol/dm3 NiSO4 and

0.5mol/dm3 NH4OH on a 100mass%-75mm ground ore. Pulp density:

33mass%, Ambient temperature, Atmospheric condition, Retention Time:

24 hours, Rotation speed: 60 rpm, pH 9.5. : Au extraction, :

(NH4)2S2O3 consumption.

-1.0

-0.5

0.0

0.5

1.0

1.5

4 6 8 10 12 14

pH

Eh,

V

Ni

[Ni(S2O3)2]2-

NiO[Ni(NH3)6]

2+

Ni3O4

Ni2O3

NiO2

↑N2

↑N4↑N3

↓N6

↓N8

↓N9

↓N5

↓N7↓N10↓E2

↓E1

↓N11

Fig. 11 Eh-pH diagram for the Ni-NH3-S2O32�-H2O system at the

condition of 0.5mol/dm3 NH4OH, 0.0001mol/dm3 Ni2þ and 0.05mol/

dm3 S2O32�. Nernst Equation (N1)�(N11) are shown in Table 2.

0

20

40

60

80

100

0 10 20 30 40

Time, t /min

Au

Rec

over

y (%

)

Fig. 12 Gold cementation by zinc powders at Metal/Auþ mass ratio of 30

without de-aeration from the pregnant solution contained 0.0001mol/dm3

Ni2þ, 0.05mol/dm3 S2O32�, 0.5mol/dm3 NH4OH and 5� 10�5 mol/

dm3 Auþ.


cost of reagent loss.The feasibility of recycling the nickel catalyzed thiosulfate

barren solution to the leaching stage followed by zincprecipitation was also evaluated. When the barren solutionwas recycled to the leaching stage, the solution compositionwas readjusted based on the analytical results of nickel andthiosulfate consumption. After four stages of recycling test,around 95% of gold was successively extracted with only1�3 kg/t-ore of ammonium thiosulfate consumption. About30�50% of nickel ion was replenished in each stage.Ammonia concentration was kept at the same value duringthe gold cementation process. This confirms that recyclingthe barren solution followed by zinc precipitation would notaffect the gold extraction and the thiosulfate consumption.

In terms of the cementation using copper or aluminumpowders, it was confirmed that a dissolved copper ion causedthe thiosulfate consumption, and that aluminum was insolu-ble at low ammonia (0.3mol/dm3) and thiosulfate (0.05mol/dm3). The stability of copper and aluminum in an ammoniumthiosulfate solution has been discussed in our previousstudy.52,53) Thus, those processes were not feasible.

4.4 Extraction of gold ore containing sulfide mineralsIn this section, the optimum reagent combination of nickel

catalyzed ammonium thiosulfate leaching was investigatedon four gold ores containing iron and copper sulfide minerals.The 24 hour standard cyanidation process at 1.0 g/dm3

(0.02mol/dm3) NaCN was also conducted to compare withthe thiosulfate leaching. Table 4 shows the assay result offour gold ore samples A, B, C and D. Three of the ores (A, Band C) contained 1:5�2:5mass% S, 0:01�0:30mass% Cuand 4�6mass% Fe. Ore D contained 50 g/t Au, 30mass% S,28mass% Fe and 0.1mass% Cu. In the thiosulfate leachingtest, the thiosulfate concentration was varied in the range of0:05�0:4mol/dm3. The ammonia concentration was variedin the range of 0:5�4mol/dm3. The nickel concentration was

maintained at 0.0001mol/dm3.Table 5 shows that the optimum reagent combinations for

the ores were 0:1�0:3mol/dm3 (NH4)2S2O3 and 3�4mol/dm3 NH4OH with 0.0001mol/dm3 NiSO4. Overall, a higherammonia concentration was required for the ores containingcopper bearing sulfide minerals as compared to the silicateore because the dissolved copper content from the sulfidemineral caused the thiosulfate consumption and retarded thegold extraction at the lower ammonia concentration. The bluecolor leach solution was the obvious evidence of copperdissolution. The gold extractions in thiosulfate leaching werelarger than the results of cynidation at 1.0 g/dm3 (0.02mol/dm3) NaCN, but the reagent consumptions in thiosulfateleaching were obviously higher than that for cyanidation. Forthe copper bearing ore of sample A and the massive sulfideore of sample D, 20 and 24 kg/t-ore of ammonium thiosulfateconsumption occurred respectively, while 3�4 kg/t-oreNaCN was consumed with cyanidation. When copper wasused as a catalyst, thiosulfate consumption was 25 kg/t-oreand 30 kg/t-ore respectively for samples A and D. Forsamples B and C, the thiosulfate consumption was 8 and9 kg/t-ore respectively with nickel as a catalyst, while 13 and10 kg/t-ore (NH4)2S2O3 was consumed respectively withcopper as a catalyst. The gold extraction by thiosulfate witheither copper or nickel as catalyst was relatively the same.However, the gold extraction by cyanidation was 8�21%lower than that by thiosulfate leaching.

5. Conclusion

Nickel catalyzed ammonium thiosulfate leaching test wascarried out to investigate the optimum reagent combinationon the variable types of ore at 100mass%-200mesh (75 mm).The optimum reagent combination was found as 0.0001mol/dm3 NiSO4, 0.5mol/dm3 NH4OH and 0.05mol/dm3

(NH4)2S2O3 for silicate type ore studied. Gold extractionwas 95% with a 1.2 kg/t-ore of ammonium thiosulfateconsumption. At an optimum reagent composition, zincpowder was the best media for gold cementation at nearly100% of gold recovery. About 30�50% of nickel was co-precipitated along with gold by the cementation of zincpower. In the barren solution recycling test followed bycementation, the 4 stages of the solution recycling with nickeladdition could give 95% gold extraction with 1�3 kg/t-ore ofammonium thiosulfate consumption successively. For thethiosulfate leaching of gold ores contained sulfide minerals,

Table 5 Comparison of the gold extraction and the reagent consumption between the ammonium thiosulfate leaching and the standard

cynidation at 1.0 g/dm3 (0.02mol/dm3) NaCN for 24 hours gold leaching test.

Optimum reagent combination Au recovery (%) Reagent consumption (kg/t-ore)

(NH4)2S2O3 NH4OH NiSO4/CuSO4 ThiosulfateCyanidation (NH4)2S2O3 (NH4)2S2O3

Sampleat

NaCN(mol/dm3) (mol/dm3) (mol/dm3) Leaching 1.0 g/dm3 Ni Cu

NaCNcatalyzed catalyzed

A 0.2 3 0.0001 98 90 20 25 3.0

B 0.2 3 0.0001 96 88 8 13 2.0

C 0.1 3 0.0001 95 81 9 10 3.4

D 0.3 4 0.0001 77 56 25 30 4.0

Table 4 Assay results of gold ores contained iron and copper sulfide

minerals.

SampleAu Cu Fe S C

(g/t) (mass%) (mass%) (mass%) (mass%)

A 25 0.3 5.2 2.5 0.8

B 7 0.01 6.4 1.7 0.3

C 3 0.1 4.3 1.8 0.5

D 50 0.1 28.2 30 0.5


the requirement of higher ammonia concentration and theresult of higher thiosulfate consumption were unavoidable.The ammonium thisulfate consumption in nickel catalyzedlixiviant was 1�5 kg/t-ore less than that in copper catalyzedlixiviant. The gold extraction was higher than that of thestandard cyanidation at 1.0 g/dm3 (0.02mol/dm3) NaCN.But the thiosulfate consumption on sulfide bearing gold orewas 3�7 times higher than the cyanide consumption.

Acknowledgements

Mr. Toru Onozuka and Mr. Taro Aichi, Dowa Mining Co.,Ltd., Japan are sincerely indebted for their management todefend the invention found in this study, which is ‘‘usingnickel as a catalyst in ammonium thiosulfate leachingprocess’’ as a patent. Also, the authors would like toacknowledge that Sumitomo Metal Mining Co., Ltd. pro-vided the gold ore samples and the financial support thatcame from Canada Centre for Mineral and Energy Technol-ogy (CANMET), Natural Sciences and Engineering ResearchCouncil of Canada (NSERC) and Queen’s School ofGraduate Studies.

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