About Kinetics of chemical cyanide process, the cyanide solution is pure gold and pure conducted under laboratory conditions. Under industrial conditions, the cyanide solution also contains a large amount of impurities, and the gold ore also contains a large amount of other minerals that can interact with cyanide. Therefore, it is of great practical significance to study the leaching factors of gold ore under industrial conditions.
1) Cyanide concentration and oxygen concentration The concentration of cyanide and oxygen is the two most important factors determining the dissolution rate of gold. The relationship between the dissolution rate of gold and the cyanide concentration is shown in Fig. 1.
It can be seen from the figure that when the concentration of cyanide is below 0.05%, the solubility of gold rises linearly to the maximum as the concentration of cyanide in the solution increases. Then, as the cyanide concentration increases, it rises slowly until the cyanide concentration increases to 0.15%. After that, the concentration of cyanide is further increased, and the dissolution rate of gold is slightly decreased. When gold ore is treated with a low concentration cyanide solution, the solubility of gold and silver is large, but the solubility of various non-precious metals is small, so that not only the consumption of cyanide is reduced, but also the dissolution of gold is facilitated.
When gold and silver are dissolved, the required concentration of cyanide and oxygen is proportional, which is the conclusion obtained in the above discussion. That is, 1 mol (molecular) oxygen requires 4 mol (molecular) of CN - , and the average ratio of the diffusion coefficients of the two is 1.5. The concentration of O 2 in the cyanide solution which is known to be saturated with air is 8.2 mg/L, or 0.27 x 10 -3 moL (molecule). Then [CN - ] = 4 × 1.5 × 0.27 × 10 -3 = 6 × 0.27 × 10 -3 mol (ion), or 0.01%. In actual production, an aqueous solution containing a mass fraction of 0.02% to 0.06% NaCN is usually used. [next]
Under normal conditions, the solubility of oxygen in cyanide solution is 7.5~8.0mg/L, and it reaches a certain constant value in the thin cyanide solution. Therefore, when the cyanide concentration increases or exceeds a certain limit, the ratio of the concentration of cyanide to the concentration of oxygen is destroyed, so that excessive cyanide is retained and cannot be effectively utilized. Therefore, the basic factor for strengthening the gold dissolution process is to increase the concentration of oxygen in the solution, which can be achieved by osmotic solution or cyanidation under high pressure.
In the production of gold by cyanidation, the concentration of cyanide in the leaching solution varies depending on the conditions of each plant. In general, the maximum dissolution rate of gold at atmospheric pressure is in the range of 0.05% to 0.10% of cyanide; and in some cases in the range of 0.02% to 0.03%. Higher cyanide concentrations can be used when diafiltration cyanidation, concentrate cyanidation, and recycling of lean liquid leaching. Conversely, a lower cyanide concentration should be used under conditions of agitation leaching, total mud cyanidation, and low levels of impurities in the solution.
In a low concentration cyanide solution, the dissolution rate depends on the concentration of cyanide; however, when the cyanide concentration is increased, the dissolution rate is independent of the cyanide concentration and increases with the oxygen supply pressure (Fig. 2). . It is therefore possible to use an oxygen osmotic solution or high pressure aeration to enhance the process of gold dissolution. If cyanidation is carried out under aeration conditions of 709.275 kPa (7 atm), the dissolution rate of gold in different characteristic ores can be increased by 10 times, 20 times, or even 30 times, and the recovery rate of gold can be increased by about 15%.
[next]
2) Stirring Studies on gold ore leaching have shown that the gold dissolution process has diffusion characteristics in most cases. Therefore, all factors that accelerate the diffusion of CN- and O2 should be possible ways to strengthen the cyanidation process.
The diffusion rate increases as the agitation speed increases, so that the dissolution rate can be greatly increased during vigorous stirring. This important conclusion has been widely used in the practice of mixing and leaching of gold mines.
3) Temperature Temperature affects the cyanidation process from two aspects. On the one hand, increasing the temperature will lead to an increase in the diffusion coefficient and thinning of the diffusion layer; on the other hand, it will reduce the solubility of oxygen and reduce the concentration of oxygen in the solution. These two contradictory effects largely offset the effects of temperature, so the surface activation energy is small.
If the temperature is within the allowable variation of the gold dissolution operation, the reactant concentration will increase with increasing temperature and diffusivity, and the reactant concentration will increase by about 20% for every 10 °C increase in temperature. That is to say, increasing the temperature accelerates the chemical reaction rate, that is, for every 10 °C increase in temperature, the decomposition rate increases nearly twice. But increasing the temperature will reduce the solubility of oxygen. When the slurry temperature approaches 100 ° C, the solubility of oxygen is close to zero. In general, the dissolution rate of gold reaches its limit at a temperature of about 85 ° C (Fig. 3). At this time, the temperature increases again, and the dissolution rate of gold decreases as the solubility of oxygen decreases.
[next]
If the temperature of the liquid is too high, it will lead to an increase in the cost of the atmosphere, pollute the environment and reduce the purity of the leachate. Such as: accelerate the evaporation of cyanide solution; increase the side reaction rate, make the base metal in the ore easier to dissolve; promote the hydrolysis of cyanide; increase energy consumption. Therefore, in addition to the warming of the cold area in the winter in order to prevent the slurry from freezing, the airing is generally carried out under normal temperature conditions of not less than 15 to 20 °C.
Theoretically, in the typical diffusion control process, the decomposition activation energy of gold and silver should be between 8.37~20.93kJ (2~5 kcal)/mol.
4) Size and shape of gold particles The size and shape of gold particles are one of the most important factors determining the rate of cyanide. Since the specific surface of the coarse gold is smaller than the specific surface of the fine gold, the dissolution rate of the coarse gold is small, and the time of complete dissolution is long, so that it is not economical to use cyanidation. In addition, during grinding, the gold particles have a strong toughness, which makes it impossible to achieve the desired fineness like other ores.
Assuming that the dissolution rate of gold is 3 mg/(cm 2 ·h), it takes 14 hours for the spherical gold particles having a diameter of 44 μm (325 mesh) to completely dissolve, and 48 hours for the spherical gold particles having a diameter of 149 μm (100 mesh). Therefore, coarse gold must be extracted first before cyanidation to increase the recovery rate of gold and shorten the cyanide operation time as much as possible.
During the cyanidation process, the gold particles are usually divided into three sizes according to the characteristics of the cyanidation operation: coarse gold of more than 74 μm (200 mesh) and fine particles of 37-74 μm (200-400 mesh). Gold, less than 37 μm (400 mesh) is fine gold. For ease of operation, gold particles larger than 495 μm (32 mesh) are sometimes referred to as extra coarse gold.
For coarse and extra coarse gold, it dissolves slowly in the cyanidation operation and takes a long time to completely dissolve. For this part of the gold particles, it is often uneconomical to use extended cyanide time, because the gold in most gold ores is mainly composed of fine particles and particles. The method of recovering coarse and extra coarse gold in the recovered ore used in many cyanidation mines at home and abroad is often carried out by amalgamation or re-selection before cyanidation, so as to avoid the loss of undissolved coarse gold in cyanide. In the tailings.
In addition, in closed-circuit grinding systems, coarse gold is easily enriched in the circulating material and embedded in the mill liner and medium, so if possible, cyanide can be added to the mill to effectively accelerate Leaching of coarse gold. [next]
For fine-grained gold with a particle size range of 37-74 μm, it is generally possible to obtain monomer separation or exposure from the surface of associated minerals after grinding. The cyanidation treatment can achieve good results. In industrial production, the exposure of gold particles is related to the fineness of grinding. The finer the grinding particle size and the more complete the gold grain exposure, the faster the leaching speed. The reasonable grinding fineness of cyanide ore should be determined through experiments, based on the actual leaching effect of gold and the cost of grinding, consumption of chemicals and washing conditions of cyanide. Generally speaking, gold ore particles are uniform, and ore with less fine particles is suitable for coarse grinding, and the particle size of the whole mud cyanide ore is often coarser than that of flotation concentrate. China's concentrate cyanide plant, grinding fineness mostly requires -325 mesh accounted for 80% to 95%, while the grinding mud fineness of the whole mud cyanide plant is mostly controlled at -325 mesh accounted for 60% to 80%.
Particulate gold with a particle size of less than 37 μm is difficult to separate or expose from the encapsulated mineral during grinding, and therefore is not suitable for recovery by cyanidation. If gold is encased in a useful mineral (such as a sulfide ore), the gold can be enriched in the concentrate by flotation, recovered by pyrometallurgy along with other elements, or cyanidated after the concentrate is calcined. Recycling. Some gold-bearing oxidized ores, although the gold particles are very fine, the ore is porous, and in the case of coarse grinding, a better cyanide leaching effect can be obtained. Therefore, it can be considered that the size of the gold particles in the ore is often one of the important factors determining whether or not to use the cyanidation method.
The shape of the gold particles has a great influence on the gold leaching process. In ore, the shape of the gold particles is rounded, flaky, veined or dendritic, inner cavities and other irregular shapes. The round gold has a smaller specific surface, and the leaching speed is slower. At the same time, as the leaching action continues, the surface area of ​​the round gold particles is continuously reduced, thereby causing the leaching speed of gold to gradually decrease. Other shapes of gold have a larger specific surface than round gold, and the leaching speed is generally faster. In the form of flake gold, the surface area does not decrease with the leaching time, so the amount of gold leaching is nearly uniform during the leaching process; after the gold particles with inner pores are leached for a period of time, the surface area of ​​the inner pores increases, and the dissolution rate of gold increases. The sooner you come.
Automatic Mask Production Line
Longyi Machinery Equipment Co. Ltd independently develops High-speed Mask Machine and provides the most affordable price.The high-speed mask produce equipment which can reaches from 400 to 800 pieces per minute yields three-layer filter mask. The mask used the core melt blown cloth by Lonyi Company self-develops , and its features are designed to widen the ear belt which is one time shaped and doesn't make the ears uncomfortable during useing for a long time. Lonyi Company has provided product sources for many cooperative manufacturers at present through passed inspection by provincial testing department.
Sincerely welcome friends from all walks of life to visit the company , exchange , study , and find common development.
Full Automatic Mask Making Production Line Market,automatic face mask production line,exportable mask production line,high quality automatic mask production line,best price automatic mask making production line
Dengzhou Longyi Machinery Co., Ltd , https://www.nylongyi.com