Gold extraction by nitric acid pre-oxidation method - Redox method gold leaching process

The Redox method (reduction oxidation method) is an alias for the Arseno method under high temperature operation. In 1988, the Redox method was also another process using nitric acid oxidation. Minproc engineering company near ManitobaSnowLake Canada stockpiling, grade of 12g / t around tailings containing arsenic recovery process to do the research. Such arsenic pyrite tailings (45%), pyrrhotite (12%) and gangue. Most of the gold exists in the form of inclusions or solid solution in the arsenic pyrite grains. The chemical composition is: gold 10.7g/t, iron 24.6%, arsenic 19.7%, total sulfur 15.0% (including sulfide sulfur 12.5%). , elemental sulfur 0.85%), calcium 1.93%, acid insolubles 29.7%. In order to recover gold by conventional cyanidation, it is required to pretreat the ore. It is further considered that a stable arsenic product can be produced to avoid environmental pollution problems. And proposed to use the Redox pretreatment method. The Redox process is also a process that uses HN0 ₃ to oxidize hard-to-dip sulphide ore.
1) The chemistry of the Redox process
The Redox process is the pretreatment of difficult-to-dip sulphide ore with nitric acid as an oxidant prior to cyanidation. SnowLake tailings contain FeAsS and FeS, which are oxidized at elevated temperatures according to the following general reaction formula:

3FeAsS+14HN0 ₃ +3H ⁺ ==== 3Fe ³⁺ +3S0 ₄ ^2- +3H ₃ AsO ₄ +14N0+4H ₂ 0

FeS+3HN0 ₃ +H ⁺ ==== Fe ³⁺ +SO ₄ ^2- +2H ₂ 0+3N0

The NO produced by the HNO ₃ reduction reaction reacts with the oxygen supplied to the process to form NO ₂ o. When there is a high partial pressure of NO and O _{2 in the} reactor, the formation of NO ₂ is extremely rapid. Generated NO ₂ in the solution and is absorbed into solution, and then generates _{HN0 3 (4N0 + 30 2 +} 2H 2 0 ==== 4HN0 3) for the oxidation of nitric acid in this case plays a role of a catalyst. During the oxidation process, iron is precipitated from the solution mainly in the form of iron arsenate (FeAsO ₄ ). At high temperatures, the iron can be goethite, jarosite precipitation and hematite form:
Fe ³⁺ +H ₃ As0 ₄ ==== FeAsO ₄ +3H ⁺

Calcium is added as a solution with nitric acid or as limestone to remove sulfate:

2H ⁺ +SO ₄ ^2- +CaC0 ₃ ==== CaSO ₄ +H ₂ 0+CO ₂

Arsenic pyrite and pyrrhotite have been shown to be oxidized to sulfates. However, under certain conditions, oxidation can also produce elemental sulfur. In order to minimize the formation of elemental sulfur (which can wrap gold particles and increase the consumption of cyanide), the process conditions should be selected.
2) Intermediate test In the intermediate test, in order to ensure complete oxidation of sulfide and avoid the generation of elemental sulfur, high temperature operation of 190~210 °C is selected. During the sedimentation operation, scale was found on the reactor wall, and the modified reactor was added with surfactant sodium lignin sulfonate (addition amount 2 kg/t) to effectively prevent scaling. When solid-liquid separation and recovery of nitric acid, a thickener is needed, followed by a storage tank. The storage tank is kept at 80-90 ° C for 9 h, which helps the hydration of gypsum and promotes the growth of crystallization, while paying attention to arsenic. The iron ratio is greater than 4:1 to precipitate iron arsenate.
After intermediate tests, the selected Redox process parameters are shown in Table 1. The parameters of solid-liquid separation and recovery of HN0 ₃ are shown in Table 2. These data are all from the best conditions for a 25 kg/h intermediate plant. [next]
Table 1 Redox process parameters

Studies have shown that Redox residue must be exposed to gypsum and anhydrite- coated gold after 15 minutes of fine grinding. The cyanide leaching is carried out for 24 hours, and the gold leaching rate is above 91%. It should be noted that in the absence of calcium in the reactor, the gold leaching rate can be higher than 96%. Each ton of ore consumes 1.25 kg of sodium cyanide and consumes 9.7 kg of lime.
Intermediate tests have successfully demonstrated that the Redox process can be used to pretreat arsenopyrite to dissociate gold recovered by cyanidation. When treated by the Redox process, arsenic precipitates from a feed containing Fe and As 1 to 1.7 mol in a stable residue. In the Redox process, the residence time is less than 8 min and the total loss of nitric acid in the gas and liquid phases is less than 2%.
According to the pilot study, a factory feasibility study with a throughput of 125,000 tons/a has been completed. In July 1994, a 12t gold concentrate gold extraction plant was built in Auezv, Kazakhstan.

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