Table of Contents
Cyanidation as applied to ordinary gold and silver ores is a relatively simple process. When cyanicides {cyanide-consuming elements) are encountered in small amounts in the treatment of such ores, the various schemes already discussed, such as use of a lead salt or wasting barren solution, can usually be resorted to and successful operation maintained. When, however, the problem concerns the treatment of an ore that does not respond to these simple expedients, certain modifications of the cyanide process must often be considered.
In the present chapter the various cyanicides and methods of controlling them are discussed, as well as the causes of refractoriness in certain ores, together with recommended treatment procedures. Where these fail, it may be desirable to incorporate cyanide regeneration or roasting into the treatment scheme. Here is a list of methods to help you on how to manage gold leaching when cyanide consumers are present.
Recovering Gold from Iron Sulphides
While the oxidized iron minerals ordinarily have little effect in cyanidation, the sulphides—pyrite, marcasite and pyrrhotite—tend to decompose in cyanide solution.
Pyrite is the most stable and least troublesome of the sulphides. Flotation concentrates high in pyrite content are frequently cyanided without undue consumption of reagents. Marcasite decomposes more readily than pyrite, and for this reason what follows for pyrrhotite is to a lesser extent true for certain occurrences of marcasite also.
Where pyrrhotite is present in an ore, trouble is usually experienced both in regard to cyanide consumption and gold extraction, for the reactions involved tend to reduce both the free cyanide and the oxygen content of the solutions.
Pyrrhotite, which has the general formula FemSm+1, differs in composition from iron pyrite and many other sulphide materials, inasmuch as one sulphur atom appears to be loosely held in chemical composition and is easily capable of forming additive compounds, such as sodium thiocyanate, NaSCN, from cyanide. (On “weathering,” the mineral readily yields elemental sulphur.) The [FeS] remaining is particularly prone to oxidation, forming ferrous and ferric sulphate, which interact with cyanide to form complex cyanides. These reactions show that pyrrhotite not only is a powerful cyanicide but also tends to rob the cyanide solution of much of the oxygen necessary for gold dissolution.
Ores containing pyrrhotite are always difficult to treat satisfactory by cyanide owing to the easily decomposable nature of that mineral. Pyrrhotite, if.kept dry, is stable but in most atmospheres breaks down rapidly, and in contact with water the rate of decomposition is still more accelerated. The action is essentially one of part oxidation, and the products are not dissimilar from those which occur when pyrite or marcasite are weathered. The main difference is that the rate of decomposition of pyrrhotite is markedly greater than that of the other common pyrite rich minerals and larger quantities of ferrous compounds are formed and have to be dealt with than is usually the case with non-pyrrhotitic ores.
To overcome this effect various methods of pre-aerating such pulps before cyanidation with special attention to alkalinity control have been used in operating plants, with a considerable degree of success.
How Thiocyanate Affect Gold Leaching
The reason for the lower cyanide consumption obtained with intense aeration is probably connected with the reactions which take place when alkaline sulphide decomposes in cyanide solution. Alkaline sulphide is one of the initial products of the reaction between pyrrhotite and alkaline cyanide solution. This may be detected during the early stages of contact. The alkaline sulphide, in the presence of oxygen, may decompose in two ways simultaneously. On the one hand, by a series of reactions during which various oxidized sulphur compounds, such as thiosulphate, thionate, sulphite, and sulphate are formed:
2Na2S + 2O2 + H2O = Na2S2O2 + 2NaOH
Na2S2O2 + 2NaOH + 2O2 = 2Na2SO4 + H2O
and on the other hand, to the thiocyanate
2Na2S + 2NaCN + 2H2O + O2 = 2NaCNS + 4NaOH
It is suggested that the relative proportions of alkaline sulphide decomposed by the two series of reactions depend on the intensity of aeration.
Using Lead Salts in Gold Leaching
For retarding the above decomposition of sulphides in alkaline cyanide solution the addition of a lead compound (the nitrate or oxide, for instance) is frequently found to be effective. Two possible reactions are apparently involved:
- Any soluble sulphides formed by the rapid solution of the finer particles of iron sulphide are preferentially precipitated as the highly insoluble lead sulphide, and further consumption of the cyanide avoided.
- A surface reaction takes place on the larger sulphide particles whereby a film of the same insoluble lead sulphide is formed, and this protective coating inhibits further reaction.
How Ferrocyanides influences Gold Leaching
Where the pH of a pulp containing pyrrhotitic mineral is allowed to drop and oxidation is permitted to proceed without sufficient protective alkalinity, the FeS is converted to FeSO4 which reacts with the free cyanide present to form ferrocyanide:
FeSO4 + 2NaCN = Fe(CN)2 + Na2SO4
Fe(CN)2 + 4NaCN = Na4Fe(CN)6
An important paper describes the successful working out of a method for cyaniding a pyrite concentrate produced by floating the tailing from the copper-flotation circuit. The sulphide content of this feed was 99 per cent, of which 10 to 20 per cent was pyrrhotite and 0.3 to 0.7 per cent chalcopyrite. There was no free gold. The exposed gold was attached to pyrite grains or in fine veinlets, some particles less than 1 micron in diameter. There was available for dissolution about 0.05 oz. per ton of gold.
In the course of several years of laboratory and plant testing the following conclusions were reached as to the best operating conditions:
- The economical limit of fine grinding was 90% minus 325 mesh.
- A well-aerated pulp saturated with calcium sulphate, using a minimum alkalinity of 1.0 lb. per ton of calcium equivalent.
- Cyanidation for a period of 6 to 8 hr. at 50 per cent solids with a cyanide strength of not more than 0.35 lb. free NaCN per ton of solution.
The problem had two aspects: a study of conditions in the flotation circuit ahead of cyanidation that would provide maximum elimination of pyrrhotite and chalcopyrite and of conditions within the cyanide circuit itself that would overcome the effect of copper and sulphide sulphur. The chemical reactions involved have already been discussed.
Effect of pre-aeration on gold leach recovery
In the paper “Treatment of Gold Ore containing Pyrrhotite at Sub-Nagel, Limited” the authors describe investigations carried out to determine the cause of high residues at the aforesaid property and the measures taken to correct the difficulty. The following treatment was evolved:
Pre-aeration in three 33- by 48-ft. Pachuca tanks in series, followed by cyanide treatment in eight 50 ft. diameter by 12 ft. side by 4 ft. cone mechanical air-lift agitators arranged in two rows of four tanks in series. The period of pre-aeration is approximately 12 hr., and cyanide treatment 42 hr.; in the winter months, however, it is usually possible to reduce the time of treatment by taking one to two cyanide-treatment agitators out of the circuit. With the exception of the cyanide present in the solution used for transfer and dilution, the first addition of cyanide is made after pre-aeration, when the strength is brought up to approximately 0.02 per cent NaCN.
It has been found essential to keep the alkalinity of the pulp low during pre-aeration and cyanide treatment, and to this end alkalinity in the thickeners is maintained at about 0.002 per cent CaO in the summer months and 0.004 to 0.005 per cent in the winter, depending upon conditions of settlement in the thickeners. After the addition of precipitated and by-passed unprecipitated solution to bring the specific gravity of the pulp to 1.40 (45 per cent solids), the alkalinity leaving the pre-aerators is about 0.002 per cent CaO, equivalent, with normal buffer action of dissolved zinc, etc., to pH 9.6. No further lime is added to the pre-aeration and treatment circuit until the last agitators, when the alkalinity is brought up to about 0.01 per cent CaO for the sake of filtration and precipitation. Alkalinity in the thickeners and treatment circuits is determined by the ordinary titration method, but the main control is obtained by frequent checking with a glass electrode pH meter. A constant-reading pH meter in the pre-aeration agitators would be preferable, but there appear to be technical difficulties in designing a reliable meter to work in agitated cyanide pulp. Milk of lime and dissolved cyanide are fed by independent pumps to the various points in the circuit where additions are required, the quantities of solid lime and cyanide to the mixers being controlled by electric vibrating feeders.
The beneficial effect of pre-aeration and treatment in solution of low alkalinity is considered to be twofold; there is possibly complete oxidation of a relatively small portion of finely ground pyrrhotite which has already come nearly to that state from exposure to the atmosphere, particularly during grinding, and the remainder is inhibited by a coating of the products of oxidation formed during the period of pre¬aeration. The hypothesis of inhibition appears the more tenable of the two, since analyses of residues have shown that there is little or no reduction in the pyrrhotite content of the ore during cyanide treatment. In either case, however, the intensive pre-aeration ensures that an excess of oxygen is supplied, so that sufficient remains for efficient cyanide extraction.
The residues were reduced from an average value of 0.473 dwt. per ton for a 6 months’ period during 1939 and 1940 before the improved scheme was used to 0.278 dwt. per ton for a similar period in 1941 and 1942 after installation of same. Cyanide consumption also fell from 0.93 lb. NaCN per ton to 0.78 lb. per ton over the same interval.
How the method of pre-lime treatment affects gold leaching
The Mines et Usine de Salsigne in the south of France is cyaniding 100 tons a day of a complex gold ore containing besides arsenopyrite, pyrite, and pyrrhotite, sulphides of bismuth, lead, zinc, and nickel as well as small amounts of chalcopyrite.
A method of pre-lime treatment was worked out by M. IT. Carron, professor of metallurgy at Delft University, and J. D. Grothe of the Dorr Company. This consists of grinding in water, adding a predetermined amount of lime such that after 24 hr, agitation in Pachuca tanks the solution has dropped to an alkalinity of about 0.01 per cent CaO, filtering, washing, and following this pretreatment step with conventional cyanide treatment. This scheme of treatment made it possible to reduce the cyanide consumption to less than one-half the figure obtained by direct cyanidation.
Grothe ascribes the beneficial effects of this pre-lime treatment to three factors:
- the removal of readily soluble sulphur in solution,
- the oxidation and slow precipitation of ferrous iron compounds, which is probably responsible for
- the rendering of the remaining sulphides passive to attack by cyanide.
It is noted, for instance, that the iron content of the solutions increases rapidly as soon as the alkalinities get very low. Thus the conditions which favor a rapid precipitation of flocculant Fe(OH)3 do not prevail; on the contrary a slow precipitation can be expected as the pH drops to the range of 8 to 9, and it is presumed that this results in the formation of a colloidal film of iron hydrate which would adhere tenaciously to the sulphide surfaces and account for the passivity effect. We have all observed the formation of such colloidal films on the walls of glass containers.
Direct Leaching in Grinding Circuit
Among the minerals containing iron in the ferrous state are pyrrhotite, chlorite, cummingtonite (an iron-magnesium amphibole), and a dolomitie carbonate. Some of these carry as much as 30 per cent iron. Arsenopyrite and pyrite are also constituents. The pyrrhotite oxidizes rapidly, perhaps it would be more precise to say steadily, as the action continues seemingly throughout the treatment cycle. In oxidizing it acts as a cyanicide, producing thiocyanate, and withdraws oxygen from the working solutions. Cyanidation should be applied promptly after comminution, and oxygen must be supplied to maintain dissolution of the gold.
The gold occurs both in the free state and associated with the sulphides, so that amalgamation is used, followed by cyanidation after grinding to finer than 80 mesh to release the values. The sulphide minerals yield their gold readily, but special attention has to be paid to the chemical and mechanical preparation of the pulp for cyanidation.
Up to the present time crushing in water, rather than in cyanide solution, has been practiced, followed by addition of lime and subsequent aeration prior to cyanidation. Tests indicate that aeration must be carried to the extent of low alkalinity; presence of excess lime or addition of more lime after aeration interfere with gold dissolution. Direct cyanidation by grinding in solution is unsatisfactory, with poor recovery and high cyanide consumption, unless addition of lead compounds such as litharge is practiced. With such additions results are more favorable. Much less thiocyanate is formed, and cyanide consumption is markedly reduced, while with identical lime feed the alkalinity of the working solutions is increased. This leads to the belief that some sort of film must be produced on the pyrrhotite which acts as an oxidation inhibitor.
The addition of very small quantities of mercury compounds also appears to be of some benefit in increasing the rate of dissolution of the gold.
A metallurgist points out that the principal factors in cyanide consumption and the extraction of gold from the Homestake ore include cyanide concentration, oxygen content of the pulp, hydrogen-ion concentration (pH), and temperature.
While the actual oxygen content of the pulp is difficult to determine, since filtration affects the result, ordinary aeration seems to provide the necessary oxygen demand. It was found, however, that the usual titration of alkalinity was unsatisfactory and pH determination was necessary for this purpose. By controlling cyanide strength, pH, and temperature, the results are systematic and readily reproduced and can be interpreted scientifically.
The control of the temperature of cyanide solution is the subject of U.S. Patent 2,220,212, Clark, Herz, and Adams. It is shown that the reaction between cyanide and the sulphides present in an ore is accelerated by increase in temperature. While the rate of dissolution of gold is also increased, this effect is more than offset by the oxidation of the sulphur compounds and utilization of available oxygen. It is proposed to cool the working cyanide solution to 35 to 50°F. for optimum results consistent with the cost of plant and operation involved.
A trick to reduce cyanide consumption
“Control of Alkalinity of Cyanide Pulps”, contains a very interesting account of a difficult cyanicide problem which was solved by the use of buffer salts and careful pH control. It was found that at alkaline levels above pH 10, the cyanide consumption was reduced but the gold extraction was poor. The formation of thiocyanates indicated an attack of alkali on the pyrrhotite. Below pH 7.0 the destruction of cyanide was very great, with the formation of ferrocyanides and HCN gas.
The best gold extractions and lowest cyanide consumption were reached when no ferrocyanide was found in the solution after 6 hr. In the presence of the optimum lead concentration, a starting pH of about 9.6 using about 2 lb. lime per ton, in the presence of suitable buffers, gave the best results. Soluble lead and mercury salts alone were first tried as buffers, but later it was found that zinc salts returning with the barren solution from the extractor boxes (60 per cent return) could replace about one- third the lead demand. Maximum aeration was found essential to high gold recoveries, but this, of course, increased lime consumption.
The conclusion was reached that in Morro Velho ores the presence of free hydrate was not essential but properly buffered solutions were.
Starting, for instance, with a low alkalinity (trace to phenolphthalein) the solution too quickly fell below pH 7.0. Soda ash, borax, trisodium phosphate, and sodium and calcium acetates were all tried, the phosphate giving perhaps the best result. Lightly buffered, an alkalinity of pH 9.6 at the start of the agitation period would fall to pH 7.6 to 8 in the course of treatment. In unbuffered solutions, the hydrates, subject to rapid alteration, are quite unstable.
Gold Leaching with Copper Minerals present
Copper is probably the most active and one of the most troublesome of the cyanicides. The results of a series of tests to determine the solubility of copper minerals in cyanide solution. They show that asurite, chalcocite, cuprite, malachite, and finely divided metallic copper are readily and completely soluble under the ordinary conditions of cyanidation. Bornite is largely soluble under the same conditions, over 90 per cent of it being dissolved in warm cyanide solution in 24 hr. Enargite and tetrahedrite are soluble enough to cause excessive cyanide loss and fouling of solutions with arsenic and antimony. While chalcopyrite is not normally very soluble, the experience at Noranda showed it to be an active cyanicide when finely ground.
In recent years it has come to be realized, however, that the copper cyanide complexes do have a considerable solvent action on gold. The paper “Copper Cyanogen Complexes in Cyanidation” describes investigations carried out by the Ore Dressing Laboratory of the American Cyanamid Company into the loss of dissolving power of cyanide solutions carrying copper.
It is generally accepted that copper dissolved in cyanide solution exists in the form of complex ions such as Cu(CN)2, Cu(CN)3 , and Cu(CN)4 . It is thought that the first of these is the most common form, although at least one authority considers that Cu2(CNS)2 copper thiocyanate is the complex most frequently encountered in working solution.
While most operators agree that the dissolving power of copper-bearing solutions is not seriously interfered with if the free cyanide strength is kept high enough, the conclusions reached in the present investigation, which was carried out on copper-bearing ores using cyclic tests with mill solutions and later confirmed in actual practice, are:
- The ordinary silver nitrate—KI method of titration is not a measure of the effective free cyanide present.
- The total cyanide, as determined by the distillation method (see Appendix B), rather than the free cyanide present is a more correct measure of the effective dissolving power of the solution.
- A ratio of total cyanide to total copper of at least 4:1 must be maintained if serious loss of dissolving power is to be avoided.
As pointed out by the authors, this is a practical method of operation only in so far as maintaining such a ratio is economic. If the cost in cyanide consumption becomes excessive, then the solution must either be discarded or regenerated.
At Noranda a lower ratio, agreeing more nearly with the cyanide-to-copper ratio of 2.3:1, in the complex Na2Cu(CN)3 was found to be effective. In his discussion of this discrepancy, Hedley points out that, since the solvent action on gold is evidently due to the slight concentration of CN resulting from the dissociation of the sodium cuprocyanide, the presence of any other compounds in the ore which disturbed that equilibrium would have a marked effect on the total reserve of CN ions available for gold extraction.
Leaching gold in Calcine
The principal cyanides in the calcine are copper, ferrous iron, and smaller amounts of Mo, Co, Ni, Mn, etc. Most of the cyanicides can be removed by water washing, but this step cannot be used owing to the loss of about $0.70 in gold resulting from the use of salt in the roast.
The cyanide consumption during continuous treatment was about 12 lb. per ton of calcine; after batch treatment was introduced, it dropped to about 9 lb. per ton, and now by the use of “super-aeration,” which apparently converts part of the copper content to a form less soluble in cyanide solution, the consumption has been reduced to around 6 lb. per ton.
Leaching Gold with Zinc Minerals in Ore
Referring again to T.P. 494, U.S.B. of M., the following summary of experimental data on zinc in cyanidation is of interest:
- The zinc minerals smithsonite, hydrozincite, zincite, and calamine are soluble enough under the usual conditions of cyanidation to cause rapid accumulation of zinc in the solution unless special precautions are taken to remove the dissolved zinc. Willemite, sphalerite, and franklinite are dissolved more slowly. Commercial zinc dust is also readily soluble; therefore an excess over that necessary to precipitate the metals from cyanide solutions should be avoided.
- When zinc dust or zinc minerals are dissolved in cyanide solution, 1.5 to 4.0 lb. of sodium cyanide is used for each pound of the zinc dissolved.
- The various zinc cyanide compounds formed in the mill solution are only weak solvents for the precious metals even if the usual titration with silver nitrate shows excess free cyanide; i.e., if considerable zinc is present in the solution, the titration for free cyanide is misleading as to the efficacy of the solution as a solvent for silver and gold.
- The addition of excess lime or caustic soda improves the extraction with solutions containing zinc by liberating free cyanide from the double salt.
- If most of the cyanide present in a solution is combined with zinc, more free cyanide must be added to this solution than to a fresh solution for the two solutions to have equal activity as solvents.
- The deleterious effect of zinc in a solution may be entirely overcome by using solutions excessively strong in free cyanide. The continued addition of the required amount of free cyanide would soon result in an excessive amount of cyanide being tied up in the mill solution. Furthermore, strong cyanide solutions consume more zinc during the precipitation of precious metals, thereby increasing zinc consumption and causing additional fouling of the solution.
It is therefore advisable when the mill solution becomes foul with zinc either to remove the zinc and regenerate the cyanide or discard the solution.
The solution of zinc probably takes place according to the following equation:
4NaCN + ZnO + H2O = Na2Zn(CN)4 + 2NaOH
According to Hamilton, if sufficient free alkali is present, the double salt is decomposed, yielding alkali cyanide and an insoluble alkaline zincate.
Na2Zn(CN)4 + 2Ca(OH)2 = 2NaCN + Ca(CN)2 + CaZnO2 + 2H2O
A second reaction, by which the double cyanide is broken up with the generation of free cyanide, is caused by soluble sulphides formed during contact of the solution with the ore:
Na2S + Na2Zn(CN)4 = ZnS + 4NaCN
These reactions and elimination of soluble zinc with mechanical losses of solution in filter cakes, etc., normally prevent zinc from building up to serious proportions in the circuit.
If, however, zinc is used to precipitate solutions high in copper (see cyanide regeneration), the accumulation of zinc becomes so great as to cause rapid fouling of solution.
How Lead Minerals Affect Gold Leaching & Recovery
Galena (PbS), Anglesite (PbSO4)³
The two lead minerals behave very similarly toward cyanide, as the reactive effect of galena is largely due to the ease with which it oxidizes to sulphate. It is important to use low concentrations of alkali or lime; otherwise excessive amounts of alkaline plumbite will be formed, which will interact with the cyanide present to form very basic insoluble lead cyanide. For example:
4NaOH + PbSO4 = Na2PbO2 + Na2SO4 + 2H2O
and
3Na2PbO2 + 2NaCN + 4H2O = Pb(CN)2,2PbO + 8NaOH
With low concentrations of alkaline plumbite, a less basic lead cyanide is formed. The less basic lead cyanide hydrolyzes with liberation of HCN, which reforms alkaline cyanide with the excess alkali present. Thus:
2Na2PbO2 + 2NaCN + 3H2O = Pb(CN)2, PbO + 6NaOH
and
Pb(CN)2PbO + 4H2O = 2PbO, 3H2O + 2HCN
It has been observed that, if free gold is roasted in the presence of sulfur and lead salts, or if pyritic gold ores containing even a small percentage of lead minerals are roasted to free the gold from the pyrite, lead compounds coat the gold particles enough to make them almost insoluble in the usual mill cyanide solution containing lime for protective alkalinity. Also, the lead compounds remaining in the calcine are slightly soluble in the mill solutions, and such dissolved lead may retard the dissolution of the gold by the removal of oxygen or prevent contact between the gold and the solution by precipitation in some form on the gold surface. Direct cyanide treatment without any protective alkalinity will dissolve this coated gold in time, but the cyanide loss in the treatment of such refractory ores may be prohibitive.
It is shown that high gold recovery can be obtained from such calcines by preliminary treatment with acid brine to remove a large percentage of the lead, followed by cyanidation for the recovery of the gold.
There is the possibility that by close control of caustic used for protective alkalinity in the cyanide solution, so as to keep the pH of the solution under 11, it may be practical to leach such calcine by direct cyanidation without excessive cyanide loss.
Satisfactory results have been obtained from the application of these methods on a laboratory scale to two important commercial ores that have been refractory to all other known leaching methods.
Optimize Gold Leaching & Recovery with Chromium Minerals
Lead Chromite (PbCrO4)5
The lead chromate mineral crocoisite is destructive to cyanide in the presence of high concentrations of caustic alkalis such as lime, by reason of the highly oxidizing salts formed, leading to the production of cyanates, formates, etc., the reaction proceeding thus:
2PbCrO4 + Ca(OH)2 = PbCrO4, PbO + CaCrO4 + H2O
Further reaction may be partially expressed thus,
CaCrO4 + NaCN + 2H2O = NaCNO + Ca(OH)2 + Cr(OH)2
It is therefore indicated that free alkalinity in the cyanide solution should be at a minimum.
In a paper “Chromium in Cyanide Solutions,” it is described that an investigation into the cause of serious precipitation trouble encountered on one of the Transvaal gold-mining properties. The results of this work were summarized as follows:
- The metallurgical difficulties experienced were due to the presence of chromium in the cyanide solutions in the form of chromate.
- The yellow precipitate (which was obtained by the addition of lead nitrate to the plant solutions) was lead chromate, carbonate, and hydrate. It did not contain gold, but its presence in the solutions prevented zinc-dust precipitation of gold from solution.
- The source of the chromium was the particular feldspathic and pyroxine ore.
- All of the chromium must be precipitated by the addition of a solution of a lead salt and the resulting precipitate removed prior to de-aeration when effective precipitation with zinc is obtainable.
- Addition of a soluble lead salt to the cyanide solution gives rise to the formation of hydrocyanic acid, which is readily “fixed” on contact with free alkali.
Cyanidation and concentration of gold and silver ores
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