Selective recovery of gold and silver from carbonate eluates

A process is disclosed for the direct and selective recovery of gold and/or silver from carbonate solutions, such as sodium carbonate which is used as an eluant for gold in a carbon-in-pulp process (CIP). The process comprises reduction precipitation of gold and/or silver from a carbonate solution having a pH in the alkaline range by the addition of stabilized alkali metal borohydride, such as sodium borohydride. The resulting gold and/or silver precipitate is generally of high purity and readily separated by filtration. The barren solution is in a condition such that it can be recycled to the upstream process.

FIELD OF THE INVENTION 
This invention relates to a process for the recovery of metallic gold 
and/or silver from gold and/or silver-containing carbonate solution. The 
invention further relates to a process for the recovery of metallic gold 
and silver generally of high purity from pregnant solutions obtained as a 
result of the leaching of gold and silver ore or concentrates by processes 
generally known in the industry. 
DESCRIPTION OF RELATED ART 
Hitherto, there have existed two principal methods of recovering gold from 
gold and silver ores or concentrates. The first method involves 
cyanidation followed by the Merrill-Crowe process wherein gold is 
recovered from solution by cementation with zinc powder which must then be 
refined to obtain gold metal. The process offers high gold recovery, but 
with low purity. 
The second method comprises cyanidation followed by recovery using 
activated carbon followed by electrolysis. Elution of gold and silver from 
loaded carbon is most commonly performed using an alkali cyanide solution. 
Because of the toxicity of cyanide, additional steps are required for its 
handling and subsequent elimination. This significantly increases the 
operating costs of these processes. 
The following Table 1 summarizes various systems used for gold elution and 
the problems encountered in each system. 
TABLE 1 
______________________________________ 
Elution of gold from carbon (3) 
Tempera- Time 
Process Eluant ture (.degree.C.) 
(h) Problems 
______________________________________ 
USBM, 1% NaOH, 98 48 Slow desorption 
Zadra 0.2% NaCN 
Zadra, 1% NaOH, 140 12 Autoclave elu- 
modified 0.2% NaCN tion, under 
pressure 
Smoky 1% NaOH, 88 52 1 day presoak 
Valley 0.1% NaCN 
Anglo presoak in 100 10 Quality of H.sub.2 O 
5% NaCN & is critical 
1% NaCN, 
Deion-H.sub.2 O 
Duval/Battle 
20% Ethanol, 
77 24 Solvent loss & 
Mountain 1% NaOH, fire hazard 
1% NaCN during EW 
Micron NaCN 6 + 2 Solvent loss 
presoak, 
Methanol 
reflux 
Murdoch 1% NaCN/ 4 + 2 
Acetonitrile 
reflux 
______________________________________ 
The current practice for recovering gold and silver from carbonate eluates 
is electrowinning. Although this is generally effective, it has been found 
that electrowinning of solutions of relatively low pH (&lt;12), is associated 
with many problems such as anode corrosion, scale formation and less than 
successful gold and silver recovery. On the other hand, cementation was 
found to be unsuccessful in the recovery of gold and silver from carbonate 
solution. 
More recently, Awadalla et al have developed a method for precious metal 
recovery from acidic aqueous solutions using sodium borohydride (See 
Canadian Patent Application No. 2,016,492 and U.S. Pat. application Ser. 
No. 647,988). However, there is no well established method of recovering 
gold and silver from basic solutions, such as carbonate solutions. 
The reducing power of sodium borohydride has long been exploited for 
industrial applications such as pollution control and the removal and/or 
recovery of various metal cations from solution. Currently, sodium 
borohydride is finding application in the recovery of silver from spent 
photographic liquor (thiosulfate solution), as disclosed in U.S. Pat. No. 
3,082,079, or spent electrolyte and platinum group metals from acidic 
leach liquor. However, there has been no suggestion of a sodium 
borohydride reduction process for the recovery of gold and silver from 
carbonate liquors. Dietz, Jr. et al (Canadian Patent No. 1,090,584) 
teaches a reduction precipitating agent containing aluminium, an alkali 
metal borohydride and a hydrazine compound for recovering precious metal 
values including gold from aqueous alkaline cyanide solutions. This prior 
process suffers from cyanide effluent problems as well as material losses 
due to the necessity for cyanide effluent destruction. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a simple and economic 
method for recovering high purity metallic gold and/or silver directly 
from aqueous carbonate solution. 
Accordingly, the invention provides a process for recovering high purity 
metallic gold and silver from a basic carbonate solution containing gold 
and/or silver values, which comprises: (a) adjusting the pH value of the 
gold and/or silver-containing solution with acid to a level between 7 and 
11; (b) treating the alkaline solution with stabilized alkali metal 
borohydride in an amount at least stoichiometrically equal to the amount 
of gold and/or silver compounds present in the solution so as to 
precipitate metallic gold and/or silver; and (c) separating the metallic 
precipitate. 
Suitably, stabilized borohydride solutions containing about 0.45% to 5% by 
weight borohydride stabilized in a caustic medium containing 1.7% to 17% 
by weight hydroxide may be used for reduction precipitation according to 
the invention. Both the borohydride and the hydroxide may be in the form 
of sodium compounds (sodium borohydride and sodium hydroxide) or potassium 
compounds (potassium borohydride and potassium hydroxide) or any 
combination of these yielding compositions lying in the stated ranges. 
In a preferred embodiment of invention, reduction precipitation is effected 
by stabilized sodium borohydride in the form of VenMet* solution (12% 
NaBH.sub.4, 40% NaOH). 
FNT *trade-mark

DETAILED DESCRIPTION OF THE INVENTION 
It has been also found that an important factor in the use of stabilized 
sodium borohydride for reduction precipitation is pH control. Through 
extensive experimentation it was found that the reduction efficiency of 
stabilized sodium borohydride peaked at approximately pH 8.0 to 9.0, which 
is the most preferred range for operating the process of the invention. 
Furthermore, it was found that satisfactory reduction efficiency occurred 
when using stabilized sodium borohydride in a pH range of about 7.0 to 
10.0. To avoid the evolution of toxic hydrogen cyanide gas, the pH is 
suitably kept in an alkaline range of 8.0 or more. 
For illustration purposes, two solutions obtained from two different gold 
mills currently using carbonate elution were used for experimentation. 
Solution A was a carbonate solution with an initial pH of 9.43; and 
Solution B was a mixture of carbonate and hydroxide with an initial pH of 
12.88. Other metallic elements contained in Solutions A and B are shown in 
Table 2. 
TABLE 2 
______________________________________ 
The analysis of plant carbonate solutions derived 
from elution of loaded carbon in CIP process 
Concentration, ppm 
Element Solution "A" 
Solution "B" 
______________________________________ 
Au 165 287.8 
Ag 7.6 49.2 
Cu 3.5 7.8 
Ni 36.0 40.1 
Fe 7.4 4.3 
Mg 4.6 ND 
Co 1.1 ND 
Na 7800 9000 
CN-free 28.0 42.0 
total 9.4 209 
______________________________________ 
ND = Not determined 
INITIAL pH OF THE CARBONATE SOLUTION 
Analysis has shown that the initial pH of the carbonate solution is 
determinative of the efficiency of reduction precipitation. At the natural 
pH of carbonate solution (pH=9.4), it was found that the precipitation of 
gold and silver is substantial. No reduction precipitation was observed at 
an initial pH of 11 (FIG. 3). 
The maximum recovery of gold from Solution A was attained at a pH between 
8.0 to 9.0, whereas no reduction of gold was observed in Solution B. It 
was found that the final pH was increased slightly through the addition of 
VenMet solution because VenMet solution contains 40 wt % NaOH. For 
example, on adding 0.1 ml of VenMet solution to ml of Solutions A and B 
and initial pH=8.0, the final pH values obtained after 10 minutes reaction 
time were 9.0 and 8.5, respectively. It was also found that the time 
required to initiate observable precipitation of gold increased with 
increasing initial solution pH, e.g.: at initial pH values of 8, 9 and 10, 
this induction period for Solution A was 1, 2 and 10 minutes, 
respectively. 
STOICHIOMETRY 
Sodium borohydride is known to be a strong reductant as eight electrons, 
produced per mole of NaBH.sub.4, reduce eight moles of monovalent ion such 
as Au+ or Ag+: 
EQU 8M.sup.+ +BH.sub.4 +2H.sub.2 O.fwdarw.8M.degree.+BO.sub.2 +4H.sub.2(1) 
wherein M=Au or Ag. 
In practice, however, more than the stoichiometric amount of NaBH.sub.4 is 
required to achieve complete reduction. Gold and silver ions are present 
on the activated carbon as cyanide complexes, and are expected to be in 
the carbonate eluate also as cyanide complexes. The reaction can, 
therefore, be represented as follows: 
EQU 2M (CN).sub.2 +BH.sub.4 +2OH.fwdarw.2M.degree.+4CN.sup.- +BO.sub.2 
+3H.sub.2(2) 
The amount of NaBH.sub.4 required in equation (2) per mole of metal reduced 
is four times that required in equation (1). FIG. 2 represents the effect 
of the amount of NaBH.sub.4 on the recovery of gold from Solutions A and 
B. 
At a molar ratio of NaBH.sub.4 /Au equal to 4 and 2.5, about 90% gold 
recovery was obtained for Solutions A and B (initial pH 8.4) respectively. 
The complete recovery of gold was achieved at a molar ratio of 7.5 
(Solution A) and 5.0 (Solution B). The excess amount of NaBH.sub.4 is 
required due to the complexed nature of the gold and silver in solution, 
as discussed earlier, the presence of other metal ions such as Cu, Fe and 
Ni which also consume some NaBH.sub.4 and finally to the decomposition 
behaviour (hydrolysis) of NaBH.sub.4 in aqueous solutions. 
KINETICS 
It has been found that the reduction from alkaline solution starts after an 
initial induction period, the duration of which depends on the initial 
solution pH as discussed earlier. FIG. 4 represents the variation of gold 
recovery with time for Solution A at initial pH values of 8 and 9. The 
reduction time could be halved if the initial pH decreased from 9 to 8. As 
indicated in FIG. 4, at pH=8.0, the reduction of gold from Solution A 
could reach its maximum value in less than 30 minutes while, at a pH of 
9.0, one hour was required to achieve the maximum reduction. 
In Solution B, the time required to complete the reduction was found to be 
much less than in Solution A at the same initial pH with the same amount 
of NaBH.sub.4. The maximum recovery of gold and silver was achieved in 
about 10 minutes (FIG. 5) Significant amounts of silver, present with gold 
in Solution B (Table 2), could be reduced prior to gold reduction (Table 
3). The reduced silver could enhance the precipitation of gold through an 
additional cementation mechanism. 
TABLE 3 
______________________________________ 
Electrode potentials for cyanide complexes 
Complex E*, mV 
______________________________________ 
[Ag(CN).sub.2 ].sup.- + e = Ag + 2CN.sup.- 
-300 
[Fe(CN).sub.6 ].sup.3- + e = Fe(CN).sub.6.sup.4- 
-360 
[Cu(CN).sub.2 ].sup.- + e = Cu + 2CN.sup.- 
-430 
[Ag(CN).sub.3 ].sup.2- + e = Ag + 3CN.sup.- 
-510 
[Au(CN).sub.2 ].sup.- + e = Au + 2CN.sup.- 
-600 
[Ni(CN).sub.4 ].sup.2- + e = [Ni(CN).sub.3 ].sup.2- + CN.sup.- 
-820 
[Fe(CN).sub.6 ].sup.4- + 2e = Fe + 6CN.sup.- 
-1500 
______________________________________ 
TEMPERATURES 
It has been found that raising the temperature had a positive effect on 
gold recovery. 
TABLE 4 
______________________________________ 
The effect of temperature on the recovery 
of gold from carbonte solution 
V = 100 mL, SBH/Au = 
3.8 for soln. A 
2.5 for soln. B 
pH.sub.init = 9.0, Time = 30 min 
Recovery, % 
Temp. .degree.C. 
Solution A 
Solution B 
______________________________________ 
25 48.6 46.8 
30 81.8 70.3 
40 89.7 76.6 
50 75.9 82.4 
______________________________________ 
For example, the recovery of gold increased from 46% to 82% upon increasing 
the temperature from 25.degree. C. to 50.degree. C. for Solution B. In 
Solution A the recovery increased from about 48% at 25.degree. C. to about 
90% at 40.degree. C. At 50.degree. C., the recovery of gold in Solution A 
was decreased (Table 4) due to the promotion of gold residue redissolution 
by the hot cyanide solution. 
It has also been found that a moderate increase in temperature has a 
positive effect on the kinetics of gold recovery from Solution A. As shown 
in FIG. 6, at 40.degree. C., the maximum gold recovery was attained after 
15 minutes reaction time, while at 25.degree. C., the reaction took one 
hour to reach its maximum recovery starting from the same initial pH and 
only half the NaBH.sub.4 dosage. When the reactants and products were left 
in contact for a longer period of time, a significant decrease in gold 
recovery was noted (FIG. 6). At 40.degree. C. gold recovery was found to 
be stable up to 30 minutes contact (82%) but after one hour the recovery 
of gold also decreased (72%) as the precipitate started to redissolve. 
Thus, in general it is preferred to operate the process at a temperature in 
the range of 25.degree. C. to 40.degree. C. for a period of 15 to 30 
minutes. 
CHARACTERIZATION 
Scanning electron microscopy (SEM) and quantitative energy dispersive 
analysis (EDXA) were used to 10 examine the precipitate derived from 
Solution A. It has been found that the precipitate was heterogeneous with 
gold content from 92 to 97 wt %, silver from 3 to 7 wt %, copper from 0.5 
to 1.4 wt % and nickel from 0 to 0.6 wt %. Traces of discrete particles of 
MgFeSi and CaFeSi were also indicated. 
The morphology of the powder derived from Solution B was found to be 
identical to that derived from Solution A. The chemical constitution was 
also similar except for a difference in gold to silver ratio. It was found 
that gold varied from 73 to 98 wt %, silver from 4 to 21 wt %, copper from 
0 to 3 wt % and nickel from 0 to 7 wt %. The quantity and type of 
contaminants observed in this sample were also identical to those observed 
in Solution A. Hence, the overall purity of gold and silver and silver 
precipitate was found to be very high. 
SELECTIVITY 
It has been found that very small amounts of impurities were precipitated 
with gold and silver as indicated by the chemical analysis and 
quantitative dispersive analysis of the solid products (Table 3). 
The measured potentials were found to decrease upon adding NaBH.sub.4 
(Table 6) and reached a minimum value (-770 mV) after 30 minutes reaction 
time. From Table 6, it is apparent that the sequence of reduction is in 
the following order: 
Ag(CN).sub.2 &gt;Cu(CN).sub.2 &gt;Ag(CN).sup.2.sub.3 &gt;Au(CN).sub.2 
&gt;Ni(CN).sup.2.sub.4 &gt;Fe(CN).sup.4.sub.6. 
Therefore, silver and gold in cyanide form can easily be reduced while the 
cyanide complexes of base metals, except for the monovalent copper 
complex, cannot. In addition, the higher cyanide complexes of base metals 
can be reduced to the lower complexes and not to the metallic form when 
NaBH.sub.4 is employed as the reductant (Table 6). Accordingly, selective 
reduction of silver and gold (over base metals) can be achieved and the 
resulting product is highly pure (Table 5). 
TABLE 5 
______________________________________ 
Recovery of gold and silver from solutions A and B 
by SBH added as VenMet solution 
V = 50 mL, SBH/Au = 
7.6 for Soln. A 
5.0 for Soln. B 
pH.sub.init = 8.0, Time = 10 min at room temperature 
Recovery, % Product purity, %* 
Element Solution A 
Solution B A B 
______________________________________ 
Au 94.9 94.8 94.7 85.1 
Ag 97.7 92.0 4.5 14.1 
Cu 38.5 8.5 0.8 0.2 
Ni 0 4.4 0 0.5 
Fe 0 4.2 0 0.1 
______________________________________ 
*Calculated by difference 
TABLE 6 
______________________________________ 
The change of pH and potentials with time during the 
reduction of gold and silver in carbonate solutions by SBH 
V = 500 mL, SBH/Au = 
7.6 for soln. A 
5.0 for soln. B 
pH.sub.init = 9.0 at room temperature 
Solution A Solution B 
Time, min pH mV pH mV 
______________________________________ 
0 9.00 109 9.00 172 
1 9.33 -205 9.06 -335 
2 9.32 -368 8.93 -540 
5 9.32 -698 -- -- 
6 -- -- 8.53 -741 
10 9.33 -756 8.03 -785 
15 9.33 -764 7.75 -784 
30 9.34 -770 9.27 -790 
60 9.34 -770 9.34 -775 
______________________________________ 
ECONOMICS 
The following Table 7 is a preliminary economic analysis of sodium 
borohydride processing in comparison with electrowinning. 
TABLE 7 
______________________________________ 
Gold recovery from carbon eluate solution by 
electrowinning and VenMet precipitation 
Process capital cost estimate, October, 1990 Canadian dollars 
Item EW VenMet 
______________________________________ 
Delivered equipment 
264,000 53,000 
Installation 69,000 14,000 
Equipment foundations 
29,000 6,000 
Support structures 
26,000 5,000 
Piping 66,000 13,000 
Electrical 40,000 8,000 
Instrumentation 21,000 4,000 
Total direct 515,000 103,000 
Engineering 41,000 8,000 
Management 36,000 7,000 
Construction expenses 
52,000 10,000 
Total indirect 129,000 25,000 
Total direct plus indirect 
644,000 128,000 
Contingency 97,000 32,000 
Fixed capital cost 
741,000 160,000 
Plant operation 
Operating labour 85,200 25,000 
Maintenance labour 
13,100 1,400 
Spare parts 8,000 600 
Operating supplies 
4,200 400 
Total operation 110,500 27,000 
Utilities 
Electric power 1,100 200 
Fuel oil 400 
Total utility 1,500 200 
Reagents 
Steel wool 1,000 
Flux 2,700 
VenMet solution 49,700 
Sulphuric acid 93% 13,700 
Calcium hydroxide 12,200 
Total reagent 3,700 75,600 
Total direct cost 2,700 103,200 
Capital charge 
15 years @ 13% interest 
114,700 24,800 
Total annual cost 230,400 128,000 
Cost per gram of gold .cent. 
15.4 8.6 
______________________________________ 
Notes: 
EW costs cover electrowinning and steel wool cathode smelting system to 
recover gold in dore bars. 
VenMet costs cover acid pH reduction, VenMet reagent precipitation of 
gold, recovery by filtration and readjustment of eluate pH with calcium 
hydroxide. 
The NaBH.sub.4 based process is economically attractive because the capital 
investment required is significantly lower than for electrowinning. 
Operating costs (excluding consumables) are also lower, principally 
because less labour is required for the NaBH.sub.4 process which is 
considerably simpler than electrowinning and is conducted under ambient 
conditions. However, the savings in non-consumables are almost cancelled 
out by the increased reagent costs incurred by the NaBH.sub.4 process. 
Therefore, the economics realized through the use of NaBH.sub.4 are gained 
from the great reduction in capital costs. An added benefit of using 
NaBH.sub.4 is the relatively high grade of the product. The speed of the 
reduction reaction also means that "in-process" gold and silver 
inventories are reduced. 
A key advantage of the invention is that recovery of high purity gold 
and/or silver by reduction precipitation with a stabilized alkali metal 
borohydride can be employed in the final steps of already established and 
commercially viable gold and silver recovery processes as will be 
demonstrated below. 
In the embodiment shown in FIG. 9, gold ore or concentrate is leached with 
a cyanide solution in accordance with known methods. Gold values in the 
pulp are then adsorbed on activated carbon in accordance with established 
carbon-in-pulp technology. Carbon is then separated by sieve filtration 
and the remaining ore residue and liquid (pulp) is further treated or 
expelled as waste. The separated carbon is eluted with sodium carbonate 
and the resultant eluate is then separated and adjusted with sulphuric 
acid to the optimum pH for gold precipitation (pH=8-10). The resultant 
eluate is then treated with stabilized sodium borohydride to induce 
reduction precipitation. The precipitated gold and/or silver powder is 
recovered by filtration. Optionally, the barren solution can be recycled 
to the upstream (stripping) elution stage after pH adjustment with NaOH or 
Na.sub.2 CO.sub.3.