The main factors affecting the replacement of gold from precious liquids by zinc are:
1. Concentration of sodium cyanide and alkali   When zinc is replaced by gold, the concentration of cyanide and alkali in the noble liquid is required. In general, this concentration is determined by the leaching operation.
The concentration of cyanide and alkali is too high, which accelerates the dissolution rate of zinc. When the alkali concentration is too high, zinc can be dissolved under anaerobic conditions, resulting in an increase in zinc consumption. However, the dissolution of zinc causes the zinc to continuously expose the fresh surface and facilitate the replacement of gold. Generally, the concentration of cyanide ion normally controlled between 0.0 and 3% to 0.06%, the alkali concentration be controlled at about 0.01%.
2. Oxidized concentration   Dissolved oxygen in the solution is detrimental to the displacement. In the presence of oxygen, gold dissolves and affects the displacement. In addition, the presence of oxygen accelerates the dissolution rate of zinc, increases zinc consumption, and produces Zn(OH) 2 and Zn(CN) 2 in large amounts to affect displacement. The amount of dissolved oxygen in the solution is generally required to be less than 0.5 mg / liter in production.
3. The amount of zinc   Zinc is used as a precipitant, and its amount of use plays a decisive role in the gold substitution effect.
Due to the different nature of the precious liquid produced by different ores, the amount of zinc used is also different. The appropriate amount in production should be determined by experiment.
The main factors affecting the amount of zinc are: the concentration of cyanide and alkali in the solution, the amount of metal to be replaced, the deoxidation, the time and temperature of the replacement, and the nature of the zinc.
The amount of zinc used in the replacement of zinc powder is relatively low, generally 15 to 50 g / m3 solution, and the amount of zinc replaced by zinc wire is 200 to 400 g / m3 solution. The quality of zinc should be good, especially zinc powder should contain more than 98% of metal zinc and 95% of fineness of -325 .
4. Lead salt   Lead is an indispensable element in the zinc replacement process because lead accelerates the replacement of gold. In the displacement reaction, hydrogen ions in the solution are precipitated by obtaining electrons from the lead electrode to generate hydrogen gas. The precipitated hydrogen reacts with oxygen in the solution to form water, thereby consuming dissolved oxygen. Lead ions can also eliminate impurities in the solution, such as lead ions can be removed by reacting with sulfur ions in the solution to form lead sulfide precipitates.
Lead nitrate and lead acetate are commonly used in production. Excessive use of lead will be detrimental to gold replacement. Excess lead will cover the surface of the zinc and slow down the displacement. When lead enters the gold mud, the gold mud will increase and the gold grade will decrease. In production, the amount of lead in the gold mud cyanidation method is 5 to 10 g / m3, and the solution in the cyanidation of the concentrate is 50 to 100 g / m3 .
5. Temperature   The reaction rate of zinc replacement gold is related to temperature, and the rate of displacement reaction depends on the rate of diffusion of gold cyanide ions onto the zinc surface. The temperature increases, the diffusion speed increases, and the reaction speed increases. And the temperature is lower than
6. Impurities in the solution   Copper , mercury , nickel and soluble sulfides in solution are all harmful impurities in zinc replacement gold.
The copper complex reacts with zinc, and the copper is replaced by zinc to consume zinc, and copper forms a thin film on the surface of zinc to hinder the replacement of gold. Its reaction formula is:
2NaCu(CN) 3 +Zn=2Cu ↓ +Na 2 Zn(CN) 4 +2NaCN
Mercury reacts with zinc in the following reactions:
Na 2 Hg(CN) 4 +Zn=Hg ↓ +Na 2 Zn(CN) 4
The generated mercury forms an alloy with zinc to make the zinc brittle and affect the effect of displacement.
Soluble sulfides act with zinc and lead to form zinc sulfide and lead sulfide on the surface of zinc and lead, reducing the conversion of zinc to gold.
The suspension in the precious liquid (mainly fine-grained slime) will contaminate the surface of the zinc during the replacement and reduce the rate of gold exchange. When using zinc powder replacement, a large amount of slime entering the displacement filter will clog the filter cloth, making replacement impossible. At the same time, the suspended solids will enter the gold mud and affect its quality. In production, the suspended solids content in the precious liquid should be controlled
The oil in the precious liquid forms an oil film on the surface of the zinc and affects the replacement of gold.
In industrial production, zinc wire or zinc powder is generally used to replace the precipitated gold from the gold-containing cyanide solution, which is called zinc wire method or zinc powder method.
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