On the Dissolution of Native Gold in Media Formed During the Oxidation of Certain Sulfides

A. P. Gribanov

(Presented by the scientific seminar of the Department of Mineralogy, Petrography, and Geology of Mineral Deposits)

Thanks to the efforts of a large number of scientists of the present and the end of the last century, the view held by geologists concerning the chemical inertness of gold in hypergene processes has been overcome. The forms of its migration may be varied. As one such form, many researchers consider the transport of the noble metal by thiosulfate complexes.

The possibility of the dissolution of gold by thiosulfates is regarded as proven, owing to well-known works [4, 10]. It has also been established experimentally that appreciable amounts of thiosulfates are formed, and that substantial dissolution of gold takes place, during the oxidation of sulfides, carried out in an aqueous medium with oxygen or air blown through it at room temperature and a total pressure of 1 atm. [7 et al.]. The conditions of the experiments mentioned were considered to correspond, to a certain degree, to the water–air environment of the oxidation zone. One can agree with this assumption only in part, since an intensive supply of free oxygen to descending waters can occur only in the upper part of the oxidation zone [9 et al.]. The delivery of free oxygen to the lower part of the oxidation zone, however, is considerably impeded. In this connection it must be acknowledged that the possibility of the formation of appreciable amounts of thiosulfates and, consequently, of intensive dissolution of gold under those physicochemical conditions \([pH, Eh]\) that may arise there in the course of oxidation of heavy-metal sulfides remains experimentally uninvestigated. This prompted the author to carry out experiments whose purpose was to investigate the possible scales of thiosulfate accumulation and gold dissolution during the oxidation of natural sulfides under conditions of normal temperature and

pressure without forced supply of free oxygen. In carrying out the experiments, it was planned to determine, in the liquid phases, the contents of gold, silver, sulfate, sulfite, and thiosulfate ions, polythionates, and also the sum of sulfide and hydrosulfide ions. Determination of other components in the solutions and elucidation of changes occurring in the composition of the solid phases in the experiments under consideration were not planned.

In carrying out the experiments, distilled water with a slightly alkaline (close to neutral) reaction and oxidizing properties was used, as were natural sulfides (pyrite, galena, and chalcopyrite) and calcite characterized by certain impurity elements (Table 1), and native gold. Since the latter usually contains one or another amount of silver, we considered it possible to use in the experiments küstelite of 260 fineness.

Monomineralic fractions of sulfides and calcite were first ground to a particle size of less than 100 microns in diameter. Küstelite particles separated from crushed ore quartz had sizes from 0.5 to 1 mm; they were introduced into the solid phases in an amount of 0.1 g per liter of distilled water.

Table 1

Results of spectral analysis of the minerals used in carrying out the experiments

As is known [3], the aggregate state of gold particles does not affect its dissolution by sulfur compounds with the formation of complex ions. This made it possible to dispense with the use of finely dispersed gold, which, upon periodic shaking of the contents of the vessels, could form colloidal solutions and thereby lead to distortion of the experimental results.

The experiments were set up for prolonged exposure with periodic shaking. The gas phase in each flask was connected with the external environment by means of a pair of glass tubes with an internal diameter of 4 mm, bent at a right angle and passed through rubber stoppers. The solid phase was located at the bottom of the flasks.

Before the beginning of the experiments, electrometric measurements of the pH and Eh of the distilled water were made. The same measurements were carried out in the solutions after completion of the experiments. The measurement results are given in Table 2.

To carry out the necessary analyses, the solutions obtained in the experiments were separated from the solid phases by means of a siphon and filtered through a double “blue ribbon” paper filter. In the resulting filtrates, quantitative determination was made of various sulfur-containing anions \([SO_4^{2-},\ SO_3^{2-},\ HS^{-}\ \text{and}\ S^{2-},\ S_2O_3^{2-}\ \text{and polythionates}]\) according to a known procedure [2], and polarographic determination of gold and silver was performed by a method developed at the Department of Physical and Colloid Chemistry of the Tomsk Polytechnic Institute [5, 6]. The quantitative determination of anions was carried out by the author at the Department of Geology of Mineral Deposits, and the polarographic analyses were performed by analytical chemist M. S. Chemeris in the Problem Geological Laboratory of the Tomsk Polytechnic Institute.

A total of 3 series of experiments were conducted. In the first of these, solid phases consisting of gold with pyrite (experiments 3a, b, c) and galena (7a, b, c) were used. The sulfides were taken in an amount of 1 g per liter of distilled water. The duration of the experiments in this series, as in the others, was 20, 40, and 260 days.

The second series of experiments differed from the preceding one in that calcite was also introduced into the composition of the solid phases, in an amount of 0.3 g per 1 g of pyrite (experiments 4a, b, c) or galena (experiments 12a, b, c).

In the third series of experiments, unlike the second, the solid phases also contained chalcopyrite in an amount of 0.1 g per 1 g of pyrite (experiments 5a, b, c) or galena (experiments 13a, b, c). The results of analyses of the liquid phases are contained in Table 2.

Table 2

Results of experiments on the dissolution of native gold in media arising during the oxidation of certain sulfides

Note: \(S^{2-}\), \(HS^{-}\), and polythionates were absent in all solutions.

The first series of experiments made it possible to establish that the oxidation of pyrite (experiments 3a, b, c) was accompanied by a considerable increase in acidity and by substantial fluctuations in the oxidation-reduction...

of the oxidation–reduction potential of the solutions. This phenomenon indicates the absence of stable equilibrium between the solid and liquid phases. The absence of such equilibrium also occurred in other series of experiments. In the composition of the liquid phases, small amounts of sulfate and sulfite ions were found, and in experiment 3a the presence of thiosulfate ions was established. Dissolution of gold was observed only in experiment 3a, with a duration of 260 days. Silver was recorded in the weakly alkaline solution obtained in experiment 3c (duration 20 days), and in a smaller amount in the acidic liquid phase obtained in experiment 3a (duration 260 days).

In the solutions formed during the oxidation of galena, lower contents of sulfite ions were found than during the oxidation of pyrite. The formation of thiosulfate ions and the dissolution of gold were recorded in experiments 7b (duration 40 days) and 7a (duration 260 days). Silver was detected only in the weakly alkaline solution obtained in experiment 7c with a duration of 20 days.

The solutions obtained in the second series of experiments were characterized by a stable alkaline reaction and low values of the oxidation–reduction potential. During the oxidation of pyrite (experiments 4a, b, c) in the presence of calcite, a small amount of sulfate and sulfite ions accumulated in the solutions, whereas during the oxidation of galena mixed with calcite (experiments 12a, b, c) only sulfate ions accumulated. Gold and silver were absent from these solutions.

The solutions obtained in the third series of experiments, as in the second series, were characterized by a stable alkaline reaction, fluctuations in pH, and low values of the oxidation–reduction potential. During the oxidation of pyrite mixed with chalcopyrite and calcite (experiments 5a, b, c), somewhat larger amounts of sulfate and sulfite ions were recorded in the liquid phases than in the experiments of the second series. During the oxidation of galena mixed with chalcopyrite and calcite (experiments 13a, b, c), the content of sulfur compounds in the solutions was also somewhat higher than in the case of oxidation of galena mixed with calcite. Gold was absent from the liquid phases obtained in the third series of experiments. Silver was found in the liquid phase obtained in experiment 5c (duration 20 days).

Polythionates, sulfide ions, and hydrosulfide ions were absent from all solutions. None of the solutions showed opalescence or other signs indicating the presence of colloids. During the oxidation of solid phases containing pyrite, with or without chalcopyrite, the formation of reddish-brown coatings—probably hydrated ferric oxides—was visually observed during the first four days. This process was most intense during the first day, especially in the oxidation of pyrite without an admixture of calcite (experiments 3a, b, c). The liquid phase obtained after the end of experiment 3a contained a significant amount of ferric iron, which, upon oxidation with hydrogen peroxide, caused the formation of a sol of ferric oxide.

Comparison of the results of the experiments considered here with the experiments mentioned above [7], in which the solutions were forcibly supplied with free oxygen, shows that in the liquid phases obtained by the author, oxygen-containing sulfur compounds, as well as gold, accumulate on a considerably smaller scale. At the same time, intermediate oxygen-containing sulfur compounds (sulfite and thiosulfate ions) are either entirely absent from the solutions (experiments 12a, b, c and 13a, b, c), or are observed in small amounts. The total content of these components in the liquid phases, as follows from Table 2, is in direct proportional dependence on the magnitude of the oxidation–reduction potential.

Polythionates, sulfide and hydrosulfide ions do not accumulate in solutions under these conditions, although in the experiments already mentioned [7] they were sometimes recorded directly or indirectly. Consequently, the kinetics of oxidation of heavy-metal sulfides in the experiments conducted by the author differs from that which occurs when solutions are intensively supplied with oxygen.

Since gold was recorded only in those liquid phases that contain thiosulfate ions (experiments 3a, 7a, 7b), it may be assumed that it was dissolved there specifically by thiosulfates. This assumption is supported by the fact that gold cannot be dissolved by sulfates at the grain size of kostelite particles used in the experiments [8]. Chloride ions, even if they were present in the solutions, likewise could not dissolve gold at the pH and Eh values recorded in Table 2 [1].

As the experimental results show, the dissolution of gold is in direct proportional dependence on the amount of thiosulfate ions in the liquid phases. In addition, the same dependence on the pH value is observed (experiments 7a and 7b), which agrees well with known data [7].

If the sulfur content in natural sulfides is taken as equal to the theoretical value, then by a simple calculation we find that, upon oxidation of one ton of gold-bearing pyrite under the conditions corresponding to experiment 3a, approximately 0.67 g of gold will dissolve; upon oxidation of one ton of galena under the conditions of experiment 7b—15 mg, and under the conditions of experiment 7a—2.8 mg of gold.

Silver, as is evident from Table 2, was found only in liquid phases containing intermediate oxygen compounds of sulfur. At the same time, its dissolution shows no dependence on the amount of thiosulfate ions in the solutions. Nevertheless, a certain tendency is observed for it to dissolve in weakly alkaline solutions during the initial stages of sulfide oxidation at the lowest concentration of sulfate ions.

There are as yet insufficient data for judging what caused the dissolution of silver in the acidic liquid phase (experiment 3a).

The results of the experiments performed make it possible to draw the following conclusions:

1. The oxidation of heavy-metal sulfides in an initially weakly alkaline medium, with insufficient entry of free oxygen into the solutions, is accompanied by a slight accumulation of intermediate oxygen compounds of sulfur, in particular thiosulfates. This accounts for the low solubility of gold, which nevertheless will be quite sufficient to lead to a substantial redistribution of the noble metal in oxidizing pyritic ores characterized by gold contents of up to 1–2 g/t.

2. The presence of carbonates, which increase the alkalinity of solutions, prevents the formation of thiosulfates and other intermediate oxygen compounds of sulfur and, consequently, the dissolution of gold, whereas with an intensive supply of oxygen to the solutions the directly opposite phenomenon was observed [7].

REFERENCES

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