First, the effect of cyanide and oxygen concentration on the dissolution rate of gold
When gold and silver are dissolved, the required concentration of cyanide and oxygen is proportional. According to the reaction formula (1), 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 cyanide solution which is known to be saturated with air contains [O 2 ] = 8.2 mg of ∕L, or 0.27 × 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 0.02% to 0.06% NaCN is usually used.
4Au+8NaCN+O 2 +2H 2 O 
4NaAu(CN) 2 +4NaOH (1)
The adjustment of the cyanide concentration in the solution is achieved by controlling the amount of cyanide input. The oxygen concentration is achieved by inflating the solution by means of an aeration machine. Under normal conditions, the aeration of the aerated machine can make the solubility of oxygen in the solution reach 7.5 ~ 8mg , L, only a certain constant value can be reached in the thin cyanide solution. Practices in most plants have shown that under atmospheric aeration conditions, the maximum dissolution rate of gold is in the range of 0.05% to 0.1% cyanide concentration; in individual cases, it is in the range of 0.02% to 0.03%. A higher cyanide concentration is used only when diafiltration cyanidation is carried out, or if ore containing more cyanide-depleted impurities is consumed, and de-gold-depleted liquid containing cyanate is recycled for recycling.
Tests have shown that when the cyanide concentration is less than 0.05%, the solubility of oxygen in the solution is large, and the diffusion rate of oxygen and cyanide in the dilute solution is faster, and the dissolution rate of gold increases with the concentration of cyanide. The line rises to the maximum. Later, as the cyanide concentration increases, the dissolution rate of gold rises slowly. When the cyanide concentration exceeds 0.15%, although the cyanide concentration is increased, the dissolution rate of gold does not increase, but decreases slightly (Fig. 1). This may be due to oxygen and CN - ratio imbalance. And the pH of the solution is increased to cause hydrolysis of cyanide ions:
CN - +H 2 O HCN+OH -
Fig.1 Effect of different cyanide concentrations on the dissolution rate of gold and silver
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). . To this end, the process of gold dissolution can be enhanced with an oxygen osmotic solution or high pressure aeration. 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%.
Fig. 2 Effect of different pressures and different NaCN concentrations on the dissolution rate of silver at 24 °C
Second, the effect of impurities on the dissolution rate
Adding certain elements to the cyanide solution accelerates the dissolution of gold. Some researchers proved that, under certain conditions, adding a small amount of lead, thallium, bismuth and mercury, to improve the dissolution rate of gold. At least, the small amount of lead present can act as a synergist for the dissolution of gold (Figure 3). However, a large amount of lead, particularly at a high pH, ​​forms a Pb(CN) 2 film on the surface of the gold particles to suppress dissolution of gold.
Figure 3 Effect of lead ion concentration on the dissolution rate of gold in 0.1% NaCN solution
The presence of sulfur ions forms an insoluble film of gold sulfide on the surface of the gold particles, making gold difficult to dissolve. Alternatively, cyanide is consumed by cyanide to form a thiocyanate which does not dissolve the gold. Even if the sulfide content in the solution is very low (5 × 10 -4 %), the dissolution rate of gold is significantly reduced (Fig. 4).
Figure 4 Effect of Na 2 S concentration on the dissolution rate of gold and silver in 0.25% KCN solution
When cyanidation flotation concentrates, xanthate and black medicine brought into the cyanide solution by the concentrate also reduce the dissolution rate of gold. When the concentration of xanthate in the cyanide solution of a gold-selecting plant in China increased from 33 mg ∕L to 110 mg/L, the cyanidation leaching rate of gold decreased from 74.2% to 55.6%. This is mainly because the surface of the gold particles is covered by a gold film of xanthogen. In order to improve the recovery rate of gold, the flotation concentrate or tailings must be removed before cyanidation.
The deionization of concentrates is usually carried out after washing and concentration of concentrates after flotation to achieve the purpose of drug removal. The grinding grain size of a mine is 65%~0.074mm (200 mesh). After flotation, it is better to remove the xanthate and 2 # oil. After the flotation concentrate is detoxified by the cyclone, it is ground to overflow. The fineness is 98% to 100% 0.074mm (-200 days) and concentrated, and the flotation agent can be removed by 96%. Finally, the concentrate is sent to cyanide and gold, and the annual average leaching rate of gold is 90.57%.
The carbon present in the ore and the hydroxide formed by silicon, aluminum , iron, etc. all have an adsorption effect, which is disadvantageous to the cyanidation operation.
Third, the effect of pH on the dissolution rate of gold
A certain amount of base is usually added during the cyanidation operation to prevent hydrolysis loss of cyanide. However, when the amount of alkali is too high and the pH is too high, the dissolution rate of gold is significantly lowered. This is because at high pH, ​​the reaction kinetics of oxygen is detrimental to the dissolution of gold. In addition, in the presence of calcium ions, when the pH is increased, the dissolution rate of gold is significantly reduced by the formation of a calcium peroxide film on the metal surface (Fig. 5).
Figure 5 Blocking effect of calcium ion on gold dissolution rate
Numerous studies have shown that the optimum pH for gold cyanide leaching is 9.4. The optimum pH range for actual production operations can be between 9.4 and 10. If conditions permit, the cyanide leaching operation will take the lower limit, and the zinc replacement operation will take the upper limit. The latter pH value will increase the reaction advantage of zinc and water and reduce the consumption of zinc.
The corresponding pH values ​​for different potassium cyanide concentrations are listed in the table below. The dissolution rates of gold and silver at different pH values ​​(ie different KOH concentrations) are shown in Fig. 6. It can be seen from the figure that the dissolution rate of the KOH concentration is 0.1 mol ∕L or more.
Table The corresponding pH values ​​of various concentrations of KCN solution
KCN∕% | pH |
0.01 | 10.16 |
0.02 | 10.31 |
0.05 | 10.40 |
0.10 | 10.51 |
0.15 | 10.66 |
0.20 | 10.81 |
Figure 6 Effect of pH value of solution on dissolution rate of gold and silver
Fourth, the effect of temperature on the dissolution rate of gold
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 the trouble is that increasing the temperature will affect the solubility of oxygen. When the pulp temperature approaches 100 ° C, the solubility of oxygen has dropped to near zero. In general, the maximum dissolution rate of gold reaches its limit at a temperature of about 85 ° C (Fig. 7). If the temperature is increased again, the dissolution rate of gold is lowered by the decrease in the solubility of oxygen. In order to increase the temperature of the slurry, a large amount of fuel is consumed, which increases the cost of the cyanide operation. In particular, as the temperature of the slurry increases, the rate of cyanide dissolving the base metal increases, and the hydrolysis of the alkali metal cyanide and the metal cyanide on the alkali is accelerated, resulting in an increase in the consumption of cyanide. These adverse effects are not compensated by increasing the temperature of the slurry to increase the dissolution rate of gold and shorten the cyanide time. Therefore, in addition to heating in the cold region in the winter in order to prevent the slurry from freezing, the cyanidation is generally carried out under normal temperature conditions of not less than 15 to 20 °C.
Fig. 7 Effect of temperature on the dissolution rate of gold in 0.25% KCN solution
In the typical diffusion control process, the decomposition activation energy of gold and silver ranges from 8.37 to 20.93 kJ (2 to 5 kcal) / mol (molecular).
Fifth, the effect of gold particle size on the dissolution rate of gold
The size of the gold particles is a major factor in determining the rate of gold dissolution. Assuming that the dissolution rate of gold is 3 mg cm (cm 2 ·h), it takes 14 hours for the spherical gold particles having a diameter of 44 μm (325 mesh) to be completely dissolved, and 48 hours for the spherical gold particles having a diameter of 149 μm (100 mesh). To this end, coarse gold must first be removed prior to cyanidation to increase gold recovery and minimize cyanide operation time.
In the cyanidation process, the gold particles are generally classified into three sizes according to the characteristics of the cyanidation operation: larger than 74 μm (200 mesh) is coarse gold, and 37 to 74 μm (200 to 400 mesh) is fine gold. Less than 37 μm (400 mesh) is fine particle gold. For the convenience of operation, gold particles larger than 495 μm (32 days) are sometimes referred to as extra coarse gold.
Coarse and extra coarse gold are very slow to dissolve in cyanidation and take a long time to dissolve completely. For such gold particles, it is often uneconomical to use extended cyanide time because the gold in most gold ores is mainly 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 mixing or re-selecting the mercury before cyanidation, so as to avoid the loss of undissolved coarse gold in the tail. In the mine.
Fine gold is well soluble during normal cyanidation operations. This is because most of the dissociation is monomeric gold at the corresponding grinding particle size.
The fine gold particles are often dissociated from the monomer during the grinding operation, and most of them are still in the package of other minerals or gangue. Particulate gold in sulfide minerals is often subjected to oxidative roasting prior to cyanidation. The fine-grained gold coated with quartz gangue is difficult to leach during cyanidation. The recovery of such particulate gold by cyanidation usually requires the ore to be finer to increase the degree of dissociation of the gold particles. This will increase the grinding cost and bring difficulties to the solid-liquid separation of the cyanide slurry, increasing the loss of cyanide and dissolved gold. For some particulate gold ore, it is often impossible to use cyanidation because the ore grinding size is not fine enough.
Therefore, it can be considered that the size of gold in the ore is often one of the important factors determining whether cyanidation can be used.
6. Effect of slime content and slurry concentration on gold dissolution rate
Sludge content and pulp concentration will directly affect the dissolution rate of gold. The concentration of the slime and ore in the slurry is large, which will affect the contact between the gold particles and the solution and the diffusion rate of the effective components in the solution, and the dissolution rate of gold is lowered. Under normal circumstances, the concentration of granular ore in cyanide pulp should be no more than 30% to 33%. When the slurry contains more slime, the concentration of solid materials in the cyanide slurry should be less than 22% to 25%.
The hazard of the slime is mainly to increase the viscosity of the slurry. Whether it is the primary slime brought by the ore, or the secondary slime produced by the grinding, they all enter the slurry with highly dispersed fine particle size, and the gel which is extremely difficult to precipitate is suspended for a long time, and is reduced. The dissolution rate of gold, and the washing and filtration of the slurry is difficult, so that the dissolved gold is lost in the tailings slurry.
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