Electrochemical Oxidation Leaching of Gold-Arsenic Concentrates and Other Sulfide Concentrates

In recent years, due to the material composition of complex sulphide concentrates and strengthen environmental protection work, prompting the hydrometallurgical treatment of sulphide concentrates growing up. After extensive research, many new process options have emerged. For gold - leaching pyrite and arsenopyrite concentrate arsenic associated with fine particles, the electrochemical oxidation is a method wherein the.
Arsenic pyrite and pyrite are minerals that are relatively chemically stable and are not suitable for direct leaching with acid solutions and alkaline solutions. They can only decompose in the presence of oxidants (because they can change their potential in the positive direction). The electrochemical conditions for the oxidation of arsenic pyrite and pyrite in acidic and alkaline solutions were investigated. The results demonstrate that the chemical stability of arsenopyrite and pyrite depends to a large extent on the nature of the solvent used. The most advantageous conditions for oxidizing these two minerals can be caused in an alkaline solution. When oxidation is carried out in this solution, the change in electrode potential is minimized and the required power consumption is the lowest. In this case, due to the different electrochemical conditions for the oxidation of arsenopyrite and pyrite, it is possible to preferentially leach the most difficult arsenopyrite when processing the sulfide concentrate.
When electrochemically leaching gold-arsenic concentrate, it is most desirable to use a caustic soda solution. Caustic soda is the cheapest of all alkalis, and the caustic soda solution is highly conductive. The secondary process of transferring gold into the liquid phase does not occur with the caustic soda solution, which is produced when the ammonia solution and the chloride solution are used. The dissolution of gold leads to a redistribution of gold between the liquid phase, the cathode deposit and the leaching residue, and as a result, the process of recovering gold in the next step is more complicated;
The kinetic study of electrochemical leaching of arsenopyrite and pyrite with caustic soda solution showed that the leaching process was carried out in the kinetic range at temperatures above 50 °C. In the range of 50-80 ° C, according to the different oxidation conditions, the calculated activation energy of the mineral oxidation is: 6.1 to 14.4 kcal / gram for arsenic pyrite, for pyrite The words are 8.1~16.3 kcal/mole. At lower temperatures, due to oxidation products formed on the mineral surface. As the density increases, the leaching process takes place in the internal diffusion zone. The oxidation kinetics on the single crystal was studied by artificial polarization method, and the equation is as follows;

Under the same oxidation conditions, when the temperature is 70 ° C and the caustic soda concentration is 3.75 mol / liter, the ratio of arsenic pyrite and pyrite oxidation rate according to the above equation is V _FeAsS /V _Fes2 =0.345 [△φ]. According to the above ratio, it can be concluded that arsenic pyrite has a faster oxidation rate than pyrite. As the oxidation conditions increase, the difference in oxidation rate between the two minerals also increases. For example, when the potential changes by 50 mV, the oxidation rate of arsenopyrite is 1.98 times that of pyrite, and when the potential changes by 100 mV, it is 2.62 times.
Studies on the oxidation of the single mineral fraction of arsenic pyrite and pyrite in an electrolytic cell have shown that for milled materials, the difference in oxidation rate between the two minerals is relatively small, only 1.4 times and 2 times . The difference in the oxidation rate of arsenic pyrite and pyrite in the milled material is due to the fact that the effect of the oxidant and anode surface on the oxidized particles in the cell is very complicated and related to the dispersibility of the material. The impact and so on. [next]
Studies on single mineral fractions have also shown that, in practice, the leaching kinetics of arsenopyrite and pyrite depend not only on certain known laws, but also on the composition of the suspension, and the salt concentration of the solution. In many mineral suspensions, the oxidation rate of arsenopyrite will increase, while the oxidation rate of pyrite will decrease. Increasing the concentration of sodium sulfate in the solution (sodium sulfate is a decomposition product of certain minerals) reduces the rate of leaching of the two minerals. Pyrite, especially the part of the arsenic pyrite, has a high reactivity.
In caustic soda solution, the electrochemical leaching mechanism of arsenic pyrite and pyrite is based on many continuous reaction stages. The electrochemical oxidation is from OH ^- ions are adsorbed at the start of the sulfide surface. According to the influence of the anion of the solvent on the leaching process of arsenic pyrite and pyrite in alkaline and acidic solutions and the electrochemical properties of oxidation, the adsorption complex formed on the surface of the mineral can cause electrons to be vulcanized. It is easier to transfer the oxidant to the oxidant and promote leaching under the more "moderate" oxidizing conditions.
In the initial stage of sulfur oxidation, after sulfur is transferred to the liquid phase, many compounds are formed with oxygen. However, since the electrolytic cell has an oxidizing medium, the majority of the compounds are present for a short period of time. Depending on the oxidation conditions and oxidation time, 80 to 90% of the sulfur is oxidized to sulfate ions. Arsenic is transferred to the solution in the form of trivalent and pentavalent ions. In the initial stage, the ratio between the trivalent and pentavalent ions is approximately equal, and then most of the arsenic is oxidized to pentavalent.
Studies on the ratio between sulfur and iron have shown that sulfur is preferentially leached when leached under milder oxidizing conditions and at low temperatures. As the potential and temperature of the solution increase, the selectivity of sulfur leaching decreases, while the ratio of sulfur to iron reaches the stoichiometric value of the original mineral. Arsenic also has similar behavior. But arsenic is transferred to the solution at a lower rate than sulfur.
At the beginning of oxidation, the solid phase is a hydroxide of ferrous iron. These hydroxides are when they are moved in the positive direction of potential, the adsorption of OH ^- ions and iron ions in the mineral crystal firmly bonded. In the presence of an oxidizing agent, the hydroxide of the ferrous iron in the alkaline medium is oxidized to a trivalent hydroxide and then converted to the more stable compound Fe ₂ O ₃ .
The electrochemical oxidation process can be illustrated by the following reaction equation:

2FeAsS+10NaOH+7O ₂ =Fe ₂ O ₃ +2Na ₂ SO ₄ +2Na ₃ AsO ₄ +5H ₂ O,

2FeS ₂ +8 NaOH + 7.5 O ₂ = Fe ₂ O ₃ + 4Na ₂ SO ₄ + 4H ₂ O.

At this time, in addition to the sodium sulfate, there were sodium sulfite Na ₂ S0 ₃ , sodium thiosulfate Na ₂ SO ₂ , and other compound Nan (SxOy). In the arsenic pyrite leaching process, sodium arsenite NaAsO _{2 is} also formed simultaneously with sodium arsenate Na ₃ AsO ₄ . In the process, a reaction capable of generating iron sulfate and magnetite also occurred. However, the yield of the products obtained by these reactions generally does not exceed 2 to 5%.
According to the data on the kinetics and mechanism of the electrochemical oxidation process of arsenic pyrite and pyrite, when the caustic soda concentration is 2.5~3.75 mol/L and the solution temperature is not lower than 50~60 °C, this Both minerals have the best leaching speed. In actual concentrates, the leaching speed of the two minerals depends on the structure of the cell, the oxygen required per unit volume of solution, the current density of the anode, the particle size characteristics of the material, and the composition of the suspension.
Electrochemical leaching of three gold-arsenic concentrates with different levels of arsenic pyrite and pyrite was studied. The results of the study show that this method works well (see the process flow diagram). Under the optimal conditions, the oxidation of this concentrate can make the leaching of arsenic pyrite up to 72~87% and the leaching rate of pyrite is 45~53%. Therefore, the gold crystal position in the tailings after cyanidation is substantially the same as the gold grade of the tailings when the concentrate is processed according to the roasting-baking cyanidation process.
After removal of arsenic and sulfur, the electrochemical leachate can be returned to use without degrading its process specifications. When the solution is treated with lime, the arsenic is dissolved in the form of slightly soluble arsenic acid: calcium, and after the solution is concentrated to 150 g/L with caustic soda and cooled to 16 to 17 ° C, the sulfur is in the form of anhydrous sodium sulfate. Take off. Therefore, the research, test and technical-economic evaluation of the electrochemical oxidation leaching process of gold-arsenic concentrates prove that this method should be further expanded and processed in various non-ferrous metals containing poisonous sand impurities. When mineral processing products are used, it is possible to use mineral beneficiation methods to achieve mineral sorting.

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