Recovery of gold from arsenopyrite and calcined arsenopyrite with sodium sulfide as an adjuvant

Abstract: It has been found that the addition of sodium sulfide allows the leachate to redox about -350 mV (compared to a standard calomel electrode) to promote cyanide leaching of gold in arsenic yellow iron or calcined arsenopyrite with lower potential. The solubility of gold decreases, which is related to the formation of a gold-sulfide passivation layer on the gold surface. It is believed that sodium sulfide can promote the dissolution of gold, because sodium sulfide can prevent the formation of cyanide-depleted iron oxide on the ore surface. The oxides present can be converted to sulfides.

The cyanide concentration has a significant effect on the solubility of gold in the calcined arsenopyrite. At a fixed cyanide/ore ratio, reducing the cyanide concentration reduces the amount of CN ^- adsorbed on the ore surface and effectively increases the solubility of gold. This can be explained by the assumption that the solubility of gold is limited by the diffusion of cyanide to the Au ⁰ lattice.

Previous studies have reported the suitability of gold-bearing iron ore for the oxidizer-assisted cyanide leaching process. It has been determined through research that the oxidation gold can be dissolved to the highest when the oxidation-reduction potential is controlled in the range of -50 to 0 mV, without hindering the simultaneous pyrite oxidation. This finding is related to the oxidation of iron on the pyrite electrode at pH 9.2 with an oxidation-reduction potential of about 50 mV. It has been shown that the oxidized iron surface depletes cyanide and forms a passivated surface film.

Under alkaline pulp conditions, arsenopyrite is more susceptible to oxidation than pyrite. Betty and Pauline found that at a pH of 10.6 and the presence of Cl ^- , the calcined arsenopyrite electrode was oxidized at a redox potential of about -150 mV to form high-valent iron ions. Recent studies indicate that Sanchez and Xi Siji, arsenopyrite oxidation is not significantly lower than at pH 11.9, but as CN ^- the complexing agent and the like, has been demonstrated arrival occurs pH 8.0 oxidized. They found that as the CN ^- concentration increased and the pH decreased, the iron-oxidized cathode disappeared at an oxidation-reduction potential of about -150 mV, while the arsenic pyrite-oxidized cathode wave was more pronounced. This phenomenon was observed by the insoluble iron hydroxide is CN ^- interpreted dissolved (Scheme 1). However, a green-blue surface film (such as a characteristic of iron ferrocyanide such as Fe ₄ [Fe(CN) ₆ ] ₃ ) is found on the surface of the arsenopyrite.

Fe(OH) ₃ +6CN ^- +3H ⁺ +e ^- →[Fe(CN) ₆ ] ^{4 -} +3H ₂ O (1)

The content of this paper is the possibility of carbonic acid cyanidation leaching of arsenopyrite concentrate with Na ₂ S as an adjuvant. The purpose of the reducing agent in the cyanidation leaching process is to prevent the formation of high-valent iron ions which consume cyanide, thereby preventing the formation of a ferric ferrocyanide passivation layer on the surface of the ore and effectively increasing the diffusion rate of CN ^- to Au ⁰ .

Arsenic pyrite is sometimes treated with high temperature calcination to release gold that is immobilized in the mineral lattice. The solubility of gold is always increased by calcination, but this may mask the fact that even iron oxide which hinders gold cyanide is formed. In this case, the addition of Na ₂ S converts the oxide to a sulfide to increase the solubility of the gold. Therefore, the role of Na ₂ S in the cyanidation leaching process of calcined arsenopyrite was also examined in this study.

First, the test

The first sample is a flotation concentrate containing arsenic pyrite provided by New Zealand's McRae, with a gold grade of 20.5 g/t. According to the X-ray diffraction reflectance analysis of the sample, the artificial ore sample prepared from Spanish pure pyrite, Mexican pure arsenic pyrite and silica is estimated to contain 12% to 15% pyrite and 10% to 12% arsenic pyrite. The gangue mineral consists of various alternative silicates, quartz and gypsum .

Sample No. 2 was obtained from roasting arsenic yellow iron from the Samantha mine in western Australia, with a gold grade of 105 g/t. The peak of sulfide was not detected by X-ray diffraction studies. The gangue mineral is the same as the No. 1 sample except for iron oxide.

The test procedure was introduced in previous reports. The leaching solution oxidation-reduction potential (Eh) was adjusted to the desired value with a solution of 5% (w/v) Na ₂ S·9H ₂ O, and then a dilute solution having a concentration of 0.5% was added by a peristaltic pump to make the cyanidation process Eh remains within the scope of this study. After extracting gold from the No. 1 sample by the carbon slurry method, the slurry was passed through a 125 μm sieve several times to remove charcoal from the tailings. The accurately weighed dry tailings sample was then calcined at 650 ° C for 4 hours in air and then dissolved in aqua regia for 5-6 hours until dry. The dry residue was mixed with 2M HCl and the solution was filtered into a vial. Gold was enriched in a layer of methyl isopropyl ketone (MIBK) containing 1% trioctylmethylammonium chloride (quaternary ammonium chloride) and on a Varian Spectr AA-300 spectrometer with external control software. Atomic absorption spectrometry was performed.

Second, the results and discussion

(1) Arsenic pyrite flotation concentrate (No. 1 sample)

1, cyanide consumption

The first sample is cyanide. The redox potential of the leaching solution is usually -100 to -50 mV. Since arsenopyrite is oxidized at a potential of about -150 mV in the presence of cyanide and a pH of 10.0, the formation of cyanide-depleted Fe(OH) ₃ in the first alkaline slurries is considered to be advantageous. . The leaching of No. 1 sample at pH 10.0 and the consumption of free NaCN in the absence of ore are shown in curve 2 and curve 1 of Figure 1, respectively, indicating that the consumption of free NaCN is almost independent of ore properties (Figure 1, About 50% of the initial solution concentration of 1000 ppm of NaCN is adsorbed on the surface of the ore sample). This situation is consistent with the study with pyrite as the electrode, indicating that by the chelation of CN ^- chelation, free CN ^- impermeable insoluble iron ferrocyanide is formed, thereby causing active iron oxide on the surface of the ore. The lattice produces deactivation.

Figure 1 Consumption of cyanide solution when the next sample is cyanide leached at pH 10.0 (curve 1) and when no ore is added (curve 2)

The solubility of Fe and Cu in the cyanidation leaching of sample No. 1 at a pH of 10.0 indicates that 20.5 ppm of Fe and 49.3 ppm of Cu have dissolved after 24 hours at a CN ^- initial concentration of 530 ppm. Assumes the form _{^{[Fe (CN 5)] 4}} - and _{^{[Cu (CN) 3] 2}} -, and the 57PPm 61PPm of CN ^- are consumed by Fe and Cu. Due to the added CN ^- about 50% adsorbed on the sample surface, so that the total amount is consumed by Fe and Cu apparently due CN ^- adsorption results.

2. Recovery of gold by carbon slurry method using sodium sulfide as an auxiliary agent

Some reducing anions including S ₂ O ₃ ^{2 -} , HSO ₃ ^- , SO ₃ 2 ^- and S ^{2 -} were added to the alkaline cyanide slurry of sample No. 1 in order to reduce the potential of the slurry liquid. It has been shown that the redox system studied is relatively insensitive to sulfhydryl groups, and only S ^{2 -} can significantly reduce Eh. A series of tests were carried out in which Na ₂ S was added to shift Eh to a negative direction, and the test results are shown in Table 1. Since it is affected by the 2, 3 reaction process leading to H ⁺ consumption, it is necessary to periodically add 3M HCl to maintain the pH at 10.0. The reaction process is:

S ^{2 -} +H ⁺ →HS ^- (2)

HS ^- +H ⁺ →H ₂ S (3)

Table 1 Results of carbonic acid cyanidation leaching of No. 1 sample with sodium sulfide as an adjuvant

Conditions: pH 10.0; NaCN 2.5 kg / t; activated carbon 30 g / l; leaching time 24 hours.

* Calculated as % of original ore grade.

+ initial concentration %.

≠Eh was initially adjusted to -350mV but was not maintained.

When Na ₂ S was added to reduce the potential of the leachate to -350 mV, the cyanide leaching process of the first gold sample was promoted. The highest recovery rate of gold was 92% (Table 1), but the gold recovery rate was significantly decreased when the potential was lower than -350 mV. When Na ₂ S was not added and Eh was higher than -100 mV, the gold recovery was 76% under normal conditions. Table 1 also shows the results obtained when a portion of Na ₂ S was added to adjust Eh to -350 mV and the immersion time was extended. The addition of this Na ₂ S resulted in a gold recovery of 84%. This means that the Eh value must be maintained in order to get the best gold recovery.

Studies have shown that sulfur is deposited on the surface of sulfur-containing minerals that are in contact with the S ^{2 -} and oxygen bubble streams. However, the volumetric measurements of Woods and his colleagues showed that the deposition of sulfur on the surface of gold and sulfide ore was not large in the range of redox potentials studied, but showed that when the potential was about -350 mV and the pH was 9.2, A gold-sulfide layer is formed on the surface of Au ⁰ . Therefore, the first cyanide leaching process is carried out at a concentration of S ^{2 -} sufficient to maintain Eh below -350 mV, which may hinder the recovery of gold due to the formation of a passivation layer on the gold surface. The measurement data of free NaCN (Table 1) is in good agreement with the possibility of reducing the gold recovery at low potential due to the reaction between S ^{2 -} and CN ^- .

One explanation for the promoting effect of S ^{2 -} on gold cyanidation is that this action prevents the oxidation of Fe ^{2 +} and forms Fe ^{3 +} which depletes cyanide. The amount of CN adsorption calculated for all Eh conditions listed in Table 1 was 52.5% of the initial concentration of added NaCN (1000 ppm). Since Eh has no effect on the adsorbed CN, it can be inferred that S ^{2 -} prevents the formation of high-priced iron, thereby reducing the amount of CN adsorption of the high-valent iron complex, which confirms the results of Barkley and his colleagues. This result indicates that pyrite is not oxidized when treated with Na ₂ S in an oxidizing liquid. The oxidation of Fe ^{2 +} is prevented, the formation of insoluble iron cyanide which deactivates the mineral surface is suppressed, and the diffusion rate of cyanide to Au ⁰ is lowered. However, the authors have shown that the recovery of gold is always increased when the redox potential is significantly lower than -150 mV; and when the potential is higher than -150 mV, the arsenopyrite is oxidized to iron oxide. Na ₂ S has been used as an activator for base metal oxides and partial oxidation ores, the main function of which is to convert oxides into sulfides. The presence of iron oxide in the first sample may also contribute to an increase in gold recovery due to inhibition of iron cyanide and an increase in the diffusion rate of cyanide to Au ⁰ .

Under normal conditions, oxygenation in the alkaline cyanide slurry of sample No. 1 did not affect the recovery of gold. The auxiliary effect of adding oxidant to the arsenic pyrite concentrate treatment process is considered to be unhelpful. Because the inherent oxidant foot has kept Eh above the value required to achieve the highest gold recovery. The intrinsic oxidant includes, in addition to the solid phase oxidant on the ore surface, an oxidant dissolved from the ore. The latter appears to be particularly important because the redox potential is below -350 mV and S ^{2 -} does not prevent electron transfer from the Au ⁰ surface before the formation of gold sulfide. It is known that in alkaline liquids, hydrated metal oxides such as Fe(OH) ₃ and Co(OH) ₃ are precipitated by the following redox reaction:

M(OH) _x +e ^- →M(OH) _{x - 1} +OH ^- (4)

(2) Roasted arsenic pyrite (sample No. 2)

1, the impact of cyanide concentration

The leaching potential in the cyanidation leaching of the calcined arsenopyrite is usually in the range of 0 to 50 mV, which is consistent with its oxidizing properties. It has been found that the liquid to solid ratio in the leaching has a significant effect on the solubility of gold in the calcined ore. When the liquid volume of cyanide leaching is increased, that is, the cyanide concentration is lowered, the solubility of gold is significantly increased (Table 2). The above problems can be reasonably explained in consideration of the presence of a colloidal suspension of a metal oxide in a liquid (shown by the following formula).

(5)

In an alkaline liquid, CN ^- replaces OH ^- from the metal lattice (M ^{n +} ). Usually the adsorption isotherm is based on the hypothesis that the adsorbed molecules are in equilibrium with the ion molecules:

M+CN ^- ≒MCN _ads (6)

Therefore ^, the amount of CN ^- adsorption (CN _ads ) is proportional to the CN ^- concentration and the number of lattice vacancies on the surface of the calcined ore (M). If the crystal lattice is assumed to be unsaturated and quickly reaches equilibrium, the liquid cyanide concentration (CN _ads ) depends on the initial CN concentration (CNi). A decrease in CNi causes a decrease in CN _ads , which increases the rate at which CN-displaces the surface of the ore for a fixed cyanide/ore ratio.

Table 2 Effect of leaching solution ratio on gold solubility in No.2 sample

Conditions: NaCN/ore = 2.5 kg/t; leaching time 24 hours.

*% by original ore grade.

The reduction of CN ₁ did not affect the consumption of cyanide liquid by sample No. 2, as shown in curves 3 and 4 of Figure 2.

Figure 2 Consumption of cyanide liquid at pH 10.0

1- not adding ore; 2-adding Na ₂ S to leaching sample No. ₂ at -350 mV;

When 3,4-not added Na ₂ S, II samples of different initial concentrations of leached with NaCN

2. Recovery of gold with sodium sulfide as an adjuvant

The effect of S ^{2 -} , HSO ₃ ^- , S ₂ O ₃ ^{2 -} and SO ₃ ^{2 -} on the gold cyanide leaching of No. 2 sample was studied under the condition of liquid-solid ratio of 33: ¹ . Although SO ₃ ^{2 -} can increase the solubility of gold somewhat, only S ^{2 -} can significantly shift Eh in the negative direction, which significantly increases the solubility of gold during cyanidation. As observed for sample No. 1, the gold solubility reached a maximum at -350 mV, i.e., the gold solubility decreased as the potential of the leachate decreased (Table 3). The presence of an oxidizing atmosphere on the surface of the calcined arsenopyrite indicates again that when the potential of the leachate is low, a precipitate of a solid phase redox reaction (reaction 4) is generated during the oxidation of Au ⁰ .

Table 3 Results of cyanidation leaching of No. 2 sample with sodium sulfide as an adjuvant

Conditions: pH 10.0; NaCN 2.5 kg/t; liquid-solid ratio 33:1; leaching time 24 hours.

*: Calculated as % of raw ore grade.

+ initial concentration %.

≠: Eh range without Na2S.

It can be seen from Fig. 2 that the consumption of free NaCN (curve 2) is lower than the positive value of the positive potential (curve 3, 4) when the S ^{2 -} is -350 mV, and the consumption is lower than when the ore is not added. The amount has increased. However, although the solubility of gold increased continuously at a potential of -350 mV (Table 3), free NaCN increased only at a potential of -50 mV. The NaCN consumption calculated at -350 mV was 1.4 x 10 ^-3 min ^-1 (Fig. 3 curve 3). This indicates that 2.4 × 10 ^-3 min ^-1 (curves 1, 4, 5) calculated when the positive value of S ^{2 -} Eh is not larger is significantly lower than that of 1.1 × 10 calculated without adding ore. ^-3 min ^-1 (curve 2) slightly increased. Based on these results and the previously obtained data for the increase in gold solubility at oxidation-reduction potential values ​​greater than -50 mV, it is believed that at least part of the reason is the decrease in CN ^- oxidation rate. of. Increasing the concentration of MCNads by the equilibrium of Equation 6 increases the diffusion rate of CN to Au ⁰ . The increase in gold solubility at potential values ​​less than -50 mV can only be explained by the vulcanization of iron oxide. As discussed for sample No. 1, the diffusion of cyanide to Au ^{0 was} also accelerated by retarding the reaction of CNads with iron oxide.

Figure 3 Relationship between In[NaCN] and leaching time at pH 10.0

1, 4, 5 - carry out the entry and exit of the second sample at different initial concentrations of NaCN;

2- without leaching under ore conditions; 3-leaching at -350 mV and addition of Na ₂ S

Third, the conclusion

When Na ₂ S is used as an adjuvant and leaching at an oxidation-reduction potential of -350 mV, the solubility of gold in the ore studied can be maximized. The oxidation reduction potential value for the liquid leads to a particularly high cyanide consumption and higher degree of oxidation of the pulp concerned, S ^{2 -} improving effect of the gold cyanide show aspects have limitations.

It is speculated that in the treatment of highly oxidized ores such as calcined sulphide ore, a large amount of consuming iron cyanide oxide limits the diffusion of the gold cyanidation process. under these circumstances. The solubility of gold may be increased by lowering the cyanide concentration at a fixed cyanide/ore ratio.

Hand Hydraulic Stacker