Refractory gold ore is an ore that is difficult to extract gold directly by cyanidation leaching. Refractory gold ore usually needs to be pretreated and then leached for gold extraction. There are 6 common pretreatment methods:
Roasting method is the first method applied to the pretreatment of refractory gold ores. Usually, under high temperature conditions, the sulfur and arsenic in the ore are decomposed into SO2 and As2O3 by air or oxygen rich roasting, so that the carbonaceous material is oxidized and deactivated to obtain loose porous calcine, thus exposing the gold in the ore and creating favorable conditions for the subsequent cyanide leaching of gold. The generated SO2 and As2O3 can be comprehensively utilized through the flue gas recovery process. Roasting method is especially suitable for refractory ores with both sulfide wrapped gold and carbonaceous material "robbing gold".
According to different roasting conditions, it can be divided into oxidation roasting, calcium roasting, flash roasting, vacuum volatilization and arsenic removal roasting, microwave roasting and other methods.
Pressure oxidation, also known as hot pressure oxidation, is a pretreatment method for oxidizing and decomposing arsenide and sulfide in refractory gold ore by adding acid or alkali at a certain temperature (170-225 ℃) and pressure (total pressure: 1-4mpa), so as to expose gold particles and facilitate subsequent cyanide leaching. In addition to the poor treatment effect on raw materials containing high organic carbon, the pressure oxidation method has strong adaptability to all kinds of ores and concentrates and low sensitivity to material composition. The process can adapt to the high and low sulfur and arsenic grades and the amount of harmful interfering impurities such as antimony and lead.
This method is a wet process with fast reaction speed and short preoxidation time. Through pressure oxidation, the products after oxidation of pyrite and arsenopyrite are soluble, the reaction is complete, the recovery rate of gold is high, and the oxidation process does not cause flue gas pollution. The generated waste residue exists in the form of relatively stable arsenate precipitation, with low environmental protection risk. It is an environmentally friendly process.
However, at present, there is no suitable method for comprehensive recovery and utilization of arsenic and sulfur minerals in this process. At the same time, silver is always lost in jarosite during preoxidation, resulting in low recovery rate of silver. In addition, attention should be paid to controlling temperature and oxygen partial pressure to avoid the formation of elemental sulfur, which requires high equipment materials, large investment and high production cost. Therefore, the pressurized oxidation process is more suitable for large-scale or high-grade gold mines, The processing capacity shall be above 1200t / d.
The solution medium used in the pressurized oxidation process is selected according to the properties of the materials. When the gangue minerals of gold deposits are mainly acidic substances (such as quartz and silicate), the acid method is often used for pressure oxidation; When gangue minerals are mainly alkaline substances (such as carbonate containing calcium and magnesium), alkali method is used for pressure oxidation.
In order to significantly reduce the temperature and pressure of pressurized oxidation pretreatment, scientific researchers have developed a nitric acid oxidation catalytic system, so that sulfide minerals can be rapidly oxidized and decomposed under lower temperature and pressure.
Nitric acid is a strong oxidizing acid. The research shows that under the condition of 75-85 ℃, when 150-200g / L nitric acid is used to decompose arsenic containing flotation gold concentrate, the consumption of nitric acid per ton of ore is 100-300kg. If the nitrate in the solution is denitrified at 350 ℃, the consumption of nitric acid can be reduced by 1 / 2-2 / 3.
When oxygen is introduced into nitric acid medium or nitrate is used as catalyst for air oxidation, the required conditions are temperature 100 ℃, pressure 400-800kpa, nitric acid is used as oxygen carrier to oxidize sulfide minerals, nitric acid is reduced to nitric oxide, and then changed to nitric acid after oxygen oxidation, which is equivalent to nitric acid as catalyst to realize catalytic oxidation acid leaching of sulfide minerals. When nitric acid catalytic oxidation is used for the pretreatment of arsenic containing sulfur-containing flotation gold concentrate, its process flow is composed of five unit operations: acid treatment before oxidation, catalytic oxidation acid leaching, solid-liquid separation and washing, solution treatment and gold cyanidation leaching.
The outstanding advantages of nitric acid oxidation pretreatment method are that the leaching speed is fast (1-3H), air is used as oxidant, the leaching agent (nitric acid) is renewable, and the reaction equipment can use ordinary structural materials, such as stainless steel, polyvinyl chloride or glass fiber reinforced plastic.
The basic principle of alkali leaching pretreatment is to inflate the alkaline ore slurry in advance before cyanide leaching to fully oxidize some minerals that affect cyanide leaching, such as iron sulfide, arsenopyrite, stibnite and soluble sulfide, and reduce or eliminate the interference to the subsequent cyanide process. For gold bearing arsenopyrite minerals, alkali leaching pretreatment causes the surface oxidation to form arsenate compounds. NaOH, Koh, Ca (OH) 2 and ammonia are commonly used for alkali leaching pretreatment.
Chlorine oxidation is an effective pretreatment method for carbonaceous refractory gold ores. It oxidizes carbon and organic compounds into carbon monoxide and carbon dioxide through chlorine, releases encapsulated fine gold particles, and eliminates the "gold robbing" effect of carbonaceous materials. The "gold robbing" effect of carbonaceous materials in fine-grained carbonaceous gold deposits is mainly due to the fact that some cryptocrystalline graphite has a chemical structure similar to that of activated carbon for adsorbing gold. Chlorine or hypochlorite can oxidize the "gold robbing" functional groups on carbonaceous materials of activated carbon and humic acid, or replace the sulfur in organic carbon with chlorine, or bind to organic carbon in other ways, thus passivating the adsorption of carbonaceous materials on gold cyanide complexes.
Bacterial oxidation pretreatment of arsenic containing refractory gold ore is to utilize the ability of chemical autotrophic acidophilic microorganisms to oxidize sulfide ore, oxidize and decompose sulfide minerals (such as arsenopyrite, pyrite, realgar, orpiment, pyrite, pyrrhotite, etc.) wrapped with fine gold particles, so that the gold particles are exposed and remain in the oxidized slag to facilitate more effective cyanide or other methods of leaching and extracting gold, It also avoids the disadvantages of other pretreatment processes such as harmful waste gas and high energy consumption.
Bacterial oxidation is a complex process with chemical, electrochemical and kinetic phenomena in the process of physiological growth including bacterial oxidation of Fe2 +, elemental sulfur, etc. and the oxidation and decomposition of sulfide ores. It has both direct and indirect effects of bacteria.
