Recovery of precious metals from difficult ores

Precious metals such as gold and silver are recovered from difficult-to-treat ores, particularly those containing manganese, by lixiviating using an ammonium thiosulfate leach solution containing copper, sufficient ammonia to maintain a pH of at least 7.5, and at least 0.05% sulfite ion.

FIELD OF INVENTION 
The present invention relates to the recovery of minerals from ores and, 
more particularly, to the extraction of precious metals by lixiviation, 
particularly from ores which are otherwise difficult to handle. 
BACKGROUND OF THE INVENTION 
Lixiviation is a technique used to extract a soluble component from a solid 
mixture by washing or percolation, i.e. leaching. World-wide present 
practice for extracting precious metals by lixiviation is carried out 
using cyanide solutions, mainly sodium cyanide. Because cyanides are so 
highly toxic, and because they cause substantial environmental problems, 
the use of cyanides is now falling into disfavor. Moreover, cyanides are 
costly materials which makes their use economically disadvantageous. 
Moreover, the use of cyanide solutions is at best difficult and at worst 
impossible with respect to some ores, especially those containing copper 
and/or manganese, since these materials easily contaminate the cyanide; 
and such materials are frequently present to the extent that poor 
recoveries of the precious metals are obtained using cyanide solutions. 
Indeed, with respect to the last problem mentioned immediately above, there 
are many difficult-to-treat ores in existence which contain manganese and 
significant quantities of silver and/or gold, and from which it would be 
desirable to extract these precious metals, and if a suitable and 
sufficiently inexpensive technique existed for such recovery. However, 
present techniques are simply not adequate and these ores remain an 
untapped mineral resource. 
Copper-bearing sulphur-containing ores, such as chalcopyrite, often contain 
small quantities of gold and silver which, desirably, should be recovered. 
Although the problem of recovering such precious metals, as well as the 
copper, has received considerable attention in the past, much of the work 
carried out in this connection, insofar as commerical processing is 
concerned, has involved the recovery of precious metals using 
pyrometallurgical processes for the recovery of the copper. 
One attempt to solve the above identified problems is disclosed in the 
Genik-Sass-Berecowsky et al U.S. Pat. No. 4,070,182. This patent proposes 
the use of ammonium thiosulfate as a secondary leach for the recovery of 
silver and gold, in conjunction with a hydrometallurgical process for the 
recovery of copper from the copper-bearing sulphidic ore. FIG. 3 of this 
patent shows a flow diagram for the extraction of precious metals from 
chalcopyrite concentrate before the main leaching step for extraction of 
copper. However, this patent appears to provide no instruction as to how 
to maintain the thiosulphate radical stable, and does not even appear to 
recognize the problem of thiosulphate instability. This patent also does 
not clearly teach the necessity of maintaining an alkaline pH in the 
thiosulfate leach liquor when starting with a raw ore, although the need 
for an alkaline pH is mentioned in conjunction with thiosulfate extraction 
following a copper recovery leach. Furthermore, this patent provides no 
guidance with respect to the extraction of precious metals from difficult 
ores containing manganese. 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the instant invention to overcome 
deficiencies in the prior art, such as indicated above. 
It is another object to provide for the improved extraction of precious 
metals from ores by lixiviation. 
It is a further object to provide an improved process of extracting 
precious metals, such as gold and silver, by lixiviation, using an 
ammonium thiosulfate leach liquor. 
It is yet another object of the instant invention to provide for the 
extraction of precious metals from difficult-to-treat ores, and 
particularly such ores containing copper and/or manganese, and most 
particularly such ores containing manganese. 
It is yet a further object to provide a method for recovery of precious 
metals from an ore containing same, which method comprises lixiviating the 
precious metals, using an ammonium thiosulfate leach liquor at an alkaline 
pH and containing copper and sulfite ions. 
These and other objects and the nature and advantages of the instant 
invention will be more apparent from the following detailed description of 
various embodiments, such detailed description being offered 
illustratively and not limitatively. 
DETAILED DESCRIPTION OF EMBODIMENTS 
In accordance with the instant invention, it has been found that the 
problems extant in the prior art, including those indicated above, are 
largely overcome by lixiviation in ammonium thiosulfate solutions 
containing copper and at least a trace of sulfite ions. With the use of 
such a leach liquor good recoveries are achieved in less time compared 
with the prior art use of cyanide, and without the possibility of 
contamination of streams and surroundings. Moreover, the process 
constitutes an improvement of the thiosulfate leaching of U.S. Pat. No. 
4,070,182 by providing better control of the stability of the thiosulfate 
radical. 
After the lixiviation has been completed, recovery of the precious metals 
from the leach liquor can be carried out in the same ways as are 
conventionally used for recovering such metals from cyanide solutions, 
namely by the use of metallic zinc, iron or copper; by electrolysis; or by 
the addition of soluble sulfides to recover a sulfide precipitate. The 
stripped ammonium thiosulfate solution is thereby rejuvenated and can be 
recycled for reuse in the instant process. 
The present process is especially advantageous for the recovery of precious 
metals from difficult-to-treat ores, namely those which are contaminated 
by copper and/or manganese, less frequently arsenic, antimony, selenium, 
tellurium and possibly other base metals as well. Copper and manganese, 
and particularly manganese, are especially poisonous to cyanide solutions, 
and because thiosulfate is much less expensive, stronger leach solutions 
may be used, which will overcome the disadvantages of such poisons as 
manganese and lead. A particular problem exists with manganese ores, as 
many such ores presently exist which, under previous technology, are 
simply unusable. The instant invention overcomes the disadvantages of high 
manganese content, and good recoveries are obtained merely at the expense 
of the use of more reagent and, at times, maintaining a higher quantity of 
sulfite in the leach solution than would otherwise be needed in accordance 
with the instant invention. 
In any lixiviation process, the strength of the leach solution is an 
important consideration. However, with cyanide lixiviation the high 
toxicity and the high cost of the chemical prohibit consideration of using 
more than about 1-2% solutions, thereby requiring long retention times and 
resultant large solution tanks. These problems are eliminated by the 
instant invention; thus, in the present invention ammonium thiosulfate, 
which is a relatively low cost and non-toxic material, can be used in much 
stronger solutions than is permissible with cyanide, namely as high as 
60%. Solutions in the range of 12-25% are particularly satisfactory, it 
being understood that the higher the solution strength the less the time 
needed for completing the leaching. In some ores, as little as 2% ammonium 
thiosulfate gives adequate results. 
An important aspect of the present invention is the inclusion of copper in 
the leach solution or lixiviation liquor. This, of course, presents no 
problem if the ore itself contains copper, such as the ores treated in 
accordance with U.S. Pat. No. 4,070,182. Some copper must be present for 
good recovery, and if the ore itself contains copper, this will most 
generally suffice. If not, a copper salt or copper containing ore should 
be added to supplement and maintain the concentration required for best 
results. In general, and consistent with U.S. Pat. No. 4,070,182, it has 
been found that a copper concentration of 1-4 g/l is desirable, although 
this will vary somewhat from ore to ore. 
Another important requirement is to maintain the pH of the leach solution 
in the alkaline range, preferably at least 7.5 and most preferably at 
least 8. Ammonium hydroxide (ammonia titratable with dilute standard acid) 
is the preferred means for maintaining the desired pH. Available ammonia 
not only accelerates the rate of solution of the precious metal in the 
leach liquor, but also helps to stabilize the ammonium thiosulfate. 
The presence of sulfite ions in the leach solution is an essential aspect 
of the invention. The sulfite ion is necessary to inhibit the 
decomposition of the thiosulfate which, if permitted to occur, would 
result in precipitation of silver sulfide with resultant loss of recovery. 
While the quantity of sulfite present need not be great, as noted below, 
it is essential that the sulfite be present throughout the lixiviation 
process. Quantities as little as trace amounts of sulfite will assure 
stability of the solution, but in view of the continuously changing 
conditions which inherently occur in the lixiviation process, it is 
desirable that the sulfite ion be present in a quantity of at least 0.05%. 
In the case were the ores being treated are refractory ores, in particular 
ores containing significant quantities of manganese, up to three or four 
percent, e.g. 1-4%, of sulfite ions are desirable to maintain stability of 
the ammonium thiosulfate. 
Sulfite ions can be provided in a number of ways. The most direct is by 
simply adding ammonium sulfite or ammonium bisulfite to the leaching 
solution; other sulfite salts may also be used. In some cases it is 
desirable to maintain sulfite concentration by adding sulfur dioxide to 
the ammoniacal leach solution, but if this method is chosen, precaution 
must be taken to assure that the solution does not become acid and that 
the pH is preferably maintained at 8 or above, it being understood that 
sulphur dioxide is an acid source. 
The importance of maintaining at least a trace of sulfite anion 
(SO.sub.3.sup.= ) in the leaching liquor during lixiviation is important 
because without the presence of sulfite, the thiosulfate radical becomes 
unstable resulting in the production of sulfide and the precipitation of 
silver as represented by the following equation: 
EQU (1) CaO+(Ag).sub.2 S.sub.2 O.sub.3 .fwdarw.Ag.sub.2 S+CaSO.sub.4 
This equation is representative of the irreversible reactions which take 
place not only in the presence of calcium oxide, but also with the oxides 
of iron, aluminum, manganese and copper; and such a reaction may even take 
place with ammonium hydroxide in the absence of the sulfite anion. The 
sulfite ion prevents the formation of any free divalent sulphur necessary 
for the formation and precipitation of silver, and automatic entrainment 
and loss of gold. In treating raw ores containing oxides of the metals 
listed above which serve to poison the extraction process, particularly 
manganese, the conditions are quite variable depending on the ore and thus 
it is essential to prevent decomposition of the thiosulfate following the 
general mechanism of formula (1) above. Maintaining at least a trace of 
sulfite ion, preferable at least 0.05% and most preferably 0.1-2% sulfite 
ion, has been found to stabilize the thiosulfate and thereby preventing 
precipitation of already dissolved precious metal. 
An equilibrium reaction occurs in the thiosulfate leach liquor as 
represented by the following equation (2): 
EQU (2) 6H.sup.+ +4SO.sub.3.sup.= +2S.sup.= .revreaction.3S.sub.2 O.sub.3.sup.= 
+3H.sub.2 O 
It is clear that without the sulfite ion being present, the equilibrium 
would move to the left, thereby producing divalent sulfide sulphur 
(S.sup.=) which precipitates metal sulfides thereby losing them from the 
leaching solution. Equilibrium reaction (2) thereby readily illustrates 
the need for continued presence of some sulfite to drive the reaction (2) 
to the right thereby preventing the decomposition of the thiosulfate with 
loss of not only reagents but loss of values from the leaching solution. 
Manganese containing precious metal ores have an unusually high requirement 
for sulfite ion, because of the oxydizing capability of various manganic 
compounds, especially prevalent among which is manganese oxide 
(MnO.sub.2). This high requirement for sulfite is demonstrated by equation 
(3) below: 
EQU (3) MnO.sub.2 +2(NH.sub.4).sub.2 SO.sub.3 +2H.sub.2 O.fwdarw.MnS.sub.2 
O.sub.6 +4NH.sub.4 OH 
The reaction demonstrated by equation (3) is beneficial with many ores, 
because the metals are in a complex combination with manganese, and such 
reaction serves to free the desirable metals from the manganese so that 
such desirable metals can then be lixiviated. However, the undesirable 
aspect of this reaction is that it consumes sulfite anion and it is 
therefore important that when acting on manganese containing ores in 
accordance with the present invention, special precautions be taken to 
assure the continued presence of sulfite thereby preventing equation (2) 
from going to the left with the resultant loss of these desired precious 
metals from the leaching solution. 
The lixiviation is preferably carried out at a temperature of 
40.degree.-60.degree. C., preferably 50.degree.-60.degree. C. Temperatures 
much greater than 60.degree. C. make it difficult to retain the ammonium 
hydroxide content needed for best results. Temperatures below 40.degree. 
C. adversely affect the speed of the process, i.e. the time it takes for 
the desired precious metals to become solubilized is undesirably extended. 
As noted above, after recovery of the dissolved precious metals, such as by 
precipitation from the leaching liquor, the ammonium thiosulfate 
containing liquor is desirably recycled for reuse. However, there are 
likely to be certain losses of chemicals, including thiosulfate, both due 
to side reactions and to mechanical losses. In such a case additional 
ammonium thiosulfate to make up for the losses is manufactured in situ by 
the reaction between extra sulfite, i.e. an amount of sulfite above and 
beyond that otherwise needed, and soluble sulfide. Thus, at the conclusion 
of the recovery stage, ammonium thiosulfate may be internally manufactured 
in the liquor by the addition of extra sulfite, either as ammonium 
sulfite, or sulphur dioxide and ammonia, and the addition to the filtered 
liquor of soluble sulfide, preferably as ammonium polysulfide or ammonium 
sulfide. Addition of the soluble sulfide will first precipitate metals 
from solution, and the remaining soluble sulfide will then react with 
sulfite present to produce the desired thiosulfate. This technique can be 
used to restore only the thiosulfate lost during the prior lixiviation 
and/or recovery stage, or it can be used to bring the solution to the 
desired strength. the mechanism of the reaction is according to equation 
(2), above.

The following examples will illustrate the manner in which the invention 
can be practiced. It is to be understood that the specific conditions set 
forth in the examples are not to be considered limiting to the invention. 
EXAMPLE 1 
A manganese containing ore from the State of Sonora, Mexico, in the 
Guereguito region, was obtained, the ore having the following assay: Gold 
0.014 oz per ton; Silver, 12.1 oz. per ton; Manganese, 2.1%. This is a 
very difficult-to-treat ore, the owner having attempted for many years 
without success to have the ore commercially treated to recover the gold 
and silver. 
The ore was split into seven equal parts of 500 grams each. The first 500 
gram portion of the lot was ground fifteen minutes in a solution 
containing 200 grams of ammonium thiosulfate, 12 grams of ammonium 
sulfite, fifty grams of ammonium hydroxide, and sufficient water to bring 
the solution to 1/2 liter. After grinding in this solution, the slurry was 
transferred to a 2,000 ml beaker along with additional water, added while 
washing the mill, to bring the total to 1,200 cc. This was placed on a 
hotplate with stirring and the temperature raised and maintained while 
stirring to between 50.degree. and 60.degree. C. After one and one half 
hours of this heating and stirring, sufficient copper sulfate was added to 
make 4 grams per liter of copper in solution. Stirring and heating 
continued for an additional six hours, adding every hour an additional 
amount of ammonium hydroxide to maintain the volume at a total of 1,200 
cc. At the end of this period the slurry was filtered and the solids 
washed twice with 250 cc of water, after which the solids were dried and 
sent for assay. 
The solution from the above test was analyzed for free ammonia, ammonium 
thiosulfate and sulfite ion, as well as for precious metals content. 
Sufficient ammonium sulfide was added to this solution to precipitate the 
silver content only, according to the following equation: 
EQU Ag.sub.2 S.sub.2 O.sub.3 +(NH.sub.4).sub.2 S.fwdarw.Ag.sub.2 
S+(NH.sub.4).sub.2 S.sub.2 O.sub.3 
If there is any lead in the solution which has been leached from the ore, 
it too would be precipitated along with the silver and therefore 
additional sulfide to precipitate the lead must be added in order to 
precipitate the silver. Some copper also precipitates and some silver will 
remain in the solution. However, if 80% of the silver is precipitated and 
the residual solution containing a little silver but most of the copper is 
recycled to the process, reagent costs are kept low. 
After removing the silver precipitate, the leaching solution was recycled 
to the second 500 gram lot of ore, and the above procedure repeated adding 
only sufficient ammonium thiosulfate, ammonium sulfite and copper to 
maintain optimum strength of the solution. Anhydrous ammonia was used in 
all cycles subsequent to the first, instead of ammonium hydroxide, so as 
to be able to use more water and better wash the values from the leached 
solids. 
For this series, the solution analysis was brought back with each cycle to 
the approximate analysis as follows: 
______________________________________ 
Ammonium thiosulfate 18% 
Ammonium sulfite 3% 
Ammonium hydroxide 2% 
Copper about 4 grams per liter 
______________________________________ 
This process was continued as above for the seven cycles with an average of 
93.2% recovery for the silver and 86.7% of the gold. Consumption was 
approximately eight pounds of ammonium thiosulfate and three pounds of 
ammonium sulfite per ton of ore. Copper loss was about a pound per ton. 
It is therefore seen that excellent results were achieved demonstrating the 
successful and economical recovery of gold and silver from this 
difficult-to-treat manganese containing ore. 
EXAMPLE 2 
Six different "difficult" ores as identified in the tables below were split 
into duplicate 500 gram samples, and two series of lixiviations were 
carried out, each series with one of the duplicates from each of the six 
samples. The A series of samples were treated in accordance with the 
present invention with 180 grams of ammonium thiosulfate, plus 9 grams of 
ammonium sulfite, and 4 grams of copper (as copper sulfate) and made up 
with water to one liter of slurry. The B series were treated with 
precisely the same leaching liquor, except that no sulfite was used. The 
samples were all placed on a heated-agitator and 4 grams of copper were 
added to each slurry while injecting ammonia to bring the pH to 9.0 and 
the temperature to 50.degree. C. Every two hours throughout the eight hour 
leaching, a sample was taken from the solution and in the case of the A 
series sufficient ammonium sulfite was added to maintain the sulfite 
analysis at about 1%. Ammonia was also added to maintain the pH above 7.5. 
The results are given in the table below in which the identification of the 
ore is given in the left hand column with the gold and silver assays being 
presented in ounces per ton, and the other important ingredients are given 
in percent; gold and silver recoveries are reported in the two series. 
It will be obvious to those skilled in the art that various changes may be 
made without departing from the scope of the invention and the invention 
is not to be considered limited to what is described in the specification. 
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NAME AND ASSAY 
Solution Solids (Tails) 
Solution Solids (Tails) 
Au and Aq Oz/ton 
% thio- Percent 
% thio- Assay oz/t 
Percent 
Others as percent 
sulfate 
% Sulfite 
Assay oz/t 
Recovery 
sulfate 
% sulfite 
oz/t Recovery 
__________________________________________________________________________ 
(1) Belmont Ore 
18.4 
1.3 0.002 Au 
None Au 
17.3 
Nil 0.002 Au 
None Au 
Au 0.002 1.80 Ag 
82.2% Ag 5.78 Ag 
42.8% 
Ag 10.10 
Mn 22.4% 
(2) PROSPECTO - Mexico 18.6 
Nil 0.050 Au 
36.7% Au 
Au 0.079 8.70 Ag 
27.5% Ag 
Ag 12.0 19.7 
0.9 0.004 Au 
95.0% Au 
Mn % 2.1 1.98 Ag 
83.5% Ag 
Cu % 1.1% 
(3) Noranda 2062 17.5 
Nil 0.006 Au 
25% 
Au 0.008 4.40 Ag 
63.3% 
Ag 12.0 17.7 
1.4 0.004 Au 
50.0% 
Mn % 18.0 0.56 Ag 
95.3% 
Cu % 2.0 
(4) Guanacevi Mexico 18.2 
Nil 0.014 Au 
30% 
Au oz/t 0.020 18.8 
0.8 0.040 Au 
80.0% 2.80 Ag 
47.2% 
Ag. oz/t 5.3 0.20 Ag 
96.2% 
Mn % 7.3 
(5) Cruz de Mayo Mexico 16.4 
Nil 0.012 Au 
Zero 
Au oz/t 0.012 19.3 
0.70 0.006 Au 
50.0% 6.22 Ag 
5.8% 
Ag. oz/t 12.9 2.02 Ag 
84.3% 
Mn % 0.90 
(6) Duval Corp.-Battle 17.2 
Nil 0.004 Au 
95.5% 
Mountain 0.05 Ag 
90.0% 
Au oz/t 0.088 18.0 
1.0 0.004 Au 
95.5% 
Cu. % 0.3% 0.02 Ag 
96.0% 
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