Silver & Gold Cyanide Leaching of Copper Ore

Much work has been done on the effect of copper in cyanide solutions on the leaching of gold. It is generally accepted that copper in cyanide solutions can form complex ions such as Cu(CN)2-, Cu(CN)3=, and Cu(CN)4=-, although Cu(CN)3= is considered the most probable of these. According to leach scientists, the complex having an empirical formula KCu(CN)2, is not a solvent for gold and silver. It was once stated that solutions containing copper “foul” at certain stages and the extracting power falls in efficiency. This occurred when solutions contained 0.3% copper or greater.

The dissolving activity of a solution begins to decrease when its copper content is somewhere between 0.5 and 1.0%. The same authors state that copper in amounts that would be considered economical to carry in the mill solutions has little effect on gold and silver extraction provided the solution contains enough free cyanide. In their work, they determined free cyanide in solutions containing copper by the usual methods of titration.

Effect of Copper on Leaching of Pure Gold

Metallurgists have long investigated the effect of copper in cyanide solutions on the leaching of pure gold and on the extraction of gold from ores. In their tests on pure gold, they prepared cyanide solutions containing copper by dissolving relatively pure cuprous cyanide, CuCN, in sodium cyanide. They prepared various solutions in which the molecular ratio of total sodium cyanide equivalent, as determined by distillation with hydrochloric acid, to copper, varied from 2.6 to 1 to 4.2 to 1. In their work they used 1100 ml. of solution in which a weighed gold disc having a surface area of 10 sq. cm. was suspended by means of a glass thread. Air at the rate of 1 cu. ft. per hour was passed into the solution so as to sweep the surface of the gold. Weighings were made every hour. The rates of leaching thus determined were compared with those obtained in pure solutions of sodium cyanide. In addition to deter-mining the cyanide in the copper-bearing solutions by distillation, the authors titrated the solutions with silver nitrate with and without potassium iodide present. The results showed that the silver nitrate-potassium iodide titration of the copper-bearing cyanide solutions was not a true measure of their dissolving efficiency when compared with pure sodium cyanide solutions. For example, a copper-bearing solution analyzed 0.20% copper and 0.454% total NaCN by distillation. The molecular ratio of total NaCN to copper was 2.91 to 1; it thus corresponded closely to die complex Na2Cu(CN)3.

When titrated with silver nitrate using potassium iodide as an indicator, the titration indicated 0.008 percent free NaCN; when titrated with silver nitrate and with no potassium iodide present, that is, using the first appearance of opalescence as the end-point, the amount of free cyanide indicated was 0.131 percent NaCN. The rate at which this solution dissolved gold was only about 30% of that of a pure sodium cyanide solution having the same titration, i. e. 00008% NaCN using the potassium iodide indicator. Similar effects were observed with a second solution, which was prepared by dissolving chalcocite in sodium cyanide. This solution analyzed 0.20% copper, 0.457% total NaCN by distillation, and titrated 0.017% NaCN by the silver nitrate-potassium iodide titration. This solution dissolved gold at only 22% of the rate at which a copper-free cyanide solution containing 0.017% NaCN did. In the third case, with a solution of cuprous cyanide in sodium cyanide containing 0.94% copper and correspondingly high in total cyanide, the effects were even more pronounced. The effect mentioned earlier in this section was observed with all three solutions, that is, the silver nitrate-potassium iodide titration did not measure fully the sodium cyanide increments made to the solutions when the ratio of total NaCN to copper was increased from 2.6 to 1 to 4.2 to 1. For example, when 0.20% NaCN was added to the solution analysing 1.95% total NaCN and 0.94 percent copper, the silver nitrate-potassium iodide titration indicated an increase in the free cyanide content from 0.003% NaCN to 0.048% NaCN whereas an increase to 0.203% NaCN would have been expected.

With all three solutions the authors increased the ratio of total NaCN to copper, by adding solid NaCN, until close to the maximum rate of gold dissolution was obtained. The rate of gold leaching was plotted against the molecular relationship of total sodium cyanide to copper. The curves showed that with all three solutions, there was a sharp increase in the rate of gold leaching in the zone corresponding closely to a molecular relationship of 3 mols. of total sodium cyanide to 1 mol. of copper, that is, corresponding to the complex Na2Cu(CN)3. The rate of gold dissolution at the end of this stage was approximately 60% of the maximum obtained with sodium cyanide. (Maximum rate with pure cyanide solutions was within the range 0.01 to 0.05% sodium cyanide). After this stage, there was a slow increase in the rate of gold leaching until the maximum was reached when the molar ratio of total sodium cyanide to copper was about 4 to 1; this would correspond to the complex Na3Cu(CN)4. Thus, ultimately, the sodium cuprocyanide solutions, became equal in dissolving effect on gold to a pure sodium cyanide solution containing 0.01% NaCN or more. (In order to determine at this stage the exact equivalent of cuprocyanide solutions in terms of sodium cyanide, a more accurate method than the rate of gold dissolution would have to be employed.)

In explanation of these findings, the hypothesis is suggested that the leaching of gold in cuprocyanide solutions is due to the presence of relatively small amounts of CN ions resulting from the dissociation of these complexes. As the ratio of total cyanide to copper decreases in passing from Na3Cu(CN)4 to NaCu(CN)2, the extent of this dissociation decreases and also the dissolving effect. In the silver nitrate titration of cuprocyanides, the silver nitrate reacts with the dissociated CN ions to form the relatively stable sodium silver cyanide. During this reaction, progressively decreasing amounts of CN ions are liberated from the cuprocyanides until equilibrium is established between the cuprocyanides and silver nitrate whereupon a precipitate of silver iodide is formed. The chain of reactions indicating the dissociation of sodium cuprocyanides may be represented thus:

Na3Cu(CN)4 <–> 3 Na+  + Cu(CN)4=-  <–> -CN + Cu(CN)3= <–> CN- + Cu(CN)2-

In other words the silver nitrate titration measures the total reservoir of CN ions in a sodium cuprocyanide solution which are ultimately available for use in cyanidation, and not the instantaneous free cyanide strength of that solution. These CN ions are liberated as a small stream, the flow of which becomes slower as they react with silver nitrate.

Cyanidation of a Copper-Bearing Gold Ore

Gold industry metallurgist investigated the effect of copper in cyanide solutions on the extraction of gold from an ore assaying 2.2 oz./ton gold, 0.82% copper, no non-sulphide copper, 1.08% lead, and 2.62% zinc. The principal minerals in this ore were chalcopyrite, pyrite, sphalerite, galena, chalcocite, covellite, gold, bomite, hematite, quartz, talc, chlorite, sericite and carbonate. Chalcopyrite, the most abundant copper mineral, was commonly coated by a narrow rim of chalcocite or covellite, or both. The gold was relatively coarse; particles up to 50 microns in size were present.
Cyanidation of this ore in practice was not particularly successful; even after 15 to 23 days of treatment residues averaged 0.35 oz./ton gold. A partial analysis of the mill solution is shown here below.

The above results show that the free cyanide content of this solution, as indicated by the usual method of titration, was 0.076% NaCN; this would be considered adequate for maximum gold extraction. The molar relationship of total NaCN to Cu was 3.47 to 1. This would indicate from the authors’ previous work on pure gold, a mixture of the complexes Na2Cu(CN)3 and Na3Cu(CN)4. Using this mill solution, a series of cyanidation tests were ran on the copper-bearing gold ore. The ore was ground to 66% minus 325 mesh; tests were run at 25% solids in glass bottles which were agitated on rolls for 48 hours. The amount of cyanide added in each test was varied. At the end of the tests the pregnant solutions were analyzed and the ratios of total cyanide to copper determined. The effects of varying the ratio of total cyanide to copper in the solution on gold extraction, and the solution analyses are shown here below.

The conclusion arrived at from these tests was that provided the ratio by weight of sodium cyanide to copper in the solution is over 4 to 1, which is equivalent to a molar ratio of about 5 to 1, close to the maximum extraction of gold can be obtained.

Additional tests were run in which the copper content of the mill solution was increased to over 1% by the addition of cuprous cyanide. In these tests sufficient sodium cyanide was also added to maintain the weight ratio of sodium cyanide to copper at about 4 to 1. High extractions of gold were obtained provided that this ratio was maintained, thus indicating that solutions containing over 1% copper are not necessarily “foul” with respect to their capacity to dissolve gold.

Looking at some operating data obtained from a mill treating a copper-bearing gold ore containing about 25% sulphides of which a high percentage was pyrrhotite. Chalcopyrite and chalcocite were present, the copper content of the ore being about 0.3%. When this mill was started, the recovery of gold was about 95%. During the ensuing two months there was a steady drop in extraction to about 78%. Bleeding solution failed to improve the results for any appreciable length of time. The cyanide consumption during this period was about 7 lbs. of NaCN per ton of ore. An analysis of the pregnant solution during the period of poor gold extraction is shown below.

The ratio of cyanide to copper of 3.14 to 1 indicated a possible deficiency of free cyanide for rapid leaching of gold, although the usual method of titration indicated 0.041% NaCN. The additions of cyanide to the mill solutions were, therefore, increased so that the usual method of titration indicated 0.07 to 0.09% NaCN. This would insure a sodium cyanide to copper weight ratio of at least 4 to 1. Average mill results for two weeks under these conditions were compared with those of the previous two weeks when solutions were carried at the lower strength. With the higher cyanide strength the tailings dropped from 0.067 to 0.034 oz. gold per ton, cyanide consumption increased from 7.9 to 9.05 lbs. per ton of ore.
As pointed out previously in the dissolution of gold sheet, the maximum rate of leaching was reached when the molar ratio of total sodium cyanide to copper was about 4 to 1 (3.08 to 1 on a weight basis); for the extraction of gold from an ore, the molar ratio giving best results was 5 to 1 or higher (4 to 1 on a weight basis). This possibly indicates that the procedure followed for the dissolution of gold sheet, i.e., with intense aeration, was more efficient than the rolling bottle technique employed in testing ores. In the latter case the possible inefficiency of agitation and aeration was counteracted by the higher ratio of total cyanide to copper. On the other hand, during the cyanidation of an ore, copper cyanogen complexes other than those described might be formed depending on the various minerals present. In these complexes the ratio of cyanide to copper might be higher than any of those tested for the leaching of gold sheet.

NaCN Research concludes that in the cyanidation of copper-bearing gold ores, copper combines with more cyanide than was hitherto suspected. Part of this combined cyanide reports as free cyanide by the usual methods of analysis, but this indicated free cyanide fails to dissolve gold as readily as true free cyanide does. Therefore, under such conditions, the operator has an erroneous impression of the efficiency of his solutions and he frequently condemns such solutions as “foul” when they merely need the correct addition of cyanide to bring them back to normal. In the absence of a reliable method for the determination of true free cyanide in complex mill solutions, the dissolving efficiency of any solution under question should be determined by actual tests on the particular ore treated. This may be done by running laboratory cyanidation tests on the mill feed or on the mill tailing, using the cyanide solution under question. In one test, the free cyanide strength as indicated by the usual method of titration should be maintained at the usual mill strength. In other tests, the cyanide strength of the suspected solution should be increased and held at various concentrations. At the same time, similar cyanidation tests should be run using fresh cyanide solutions for purposes of comparison. If the results show that the efficiency of the solution under investigation increases when held at a higher indicated cyanide strength, then this solution can be analyzed for copper, and for total cyanide by distillation. The critical ratio of total cyanide to copper can thus be determined. If economically practicable, the mill solution can be increased in cyanide strength accordingly. On the other hand, if the solution, after being increased in cyanide strength, is still an inefficient solvent for gold, then it is probably advisable to discard solution.