Gold and silver separation

The separation of gold and silver is carried out by the nitric acid method. The basic principle is to react silver and other base metals with nitric acid to form soluble nitrates. Gold is not reacted with nitric acid and deposited on the bottom of the reaction vessel to achieve the purpose of gold and silver separation.
For the separation of ordinary gold and silver alloy ingots by nitric acid method, the mass fraction of silver must be greater than 75% to prevent the surface of the particles from being wrapped by a dense gold film and affecting the smooth progress of the reaction. However, for the electrowinning gold mud, because the deposition speed of gold and silver in the reduction process and the adsorption performance on the cathode are different, the formed product is relatively loose, the particles are fine, and the state in which the silver is wrapped by gold is rarely present. Therefore, when the electrowinning gold or gold-bearing steel wool is divided into silver by the nitric acid method, the content of the silver is not critical, and the separation by the direct use of the nitric acid method can also achieve the purpose of refining.
In the gold mud and steel wool produced by the electrowinning, there are higher concentrations of cyanide and hydroxide and a small amount of chloride ions (from sodium hydroxide). Before the silver separation, the gold mud must be washed repeatedly with water. The production of HCN gas against acid addition endangers human health, pollutes the environment, and affects the separation of gold and silver.
The volume of the gold mud after washing in the reaction vessel should not be too large, preferably 20%. In the initial stage, since the reaction between nitric acid and gold mud is very intense, it is required to add acid to follow the principle of a small amount of excavation to prevent metal loss caused by the running tank. Stirring cannot be heated in the initial stage. After the reaction is stable, heating and stirring can be performed to increase the reaction rate. The amount of acid added and the amount of water added are flexibly controlled according to the reaction conditions, and the liquid-solid ratio is generally maintained at 3:1.
For gold-loaded steel wool, it can be pre-dissolved with 10%~15% nitric acid (formed with hot water) and then treated with electrowinning gold mud. The carrier can also be accompanied by the magic hair drying certain amount of saltpeter, borax and silica sand mixed melted, then pour gold and silver beads separated.
The separation of gold and silver by the nitric acid method is carried out in stages. After each stage of the reaction, the solution is separated from the precipitate, and the precipitate is washed with hot water for 2 to 3 times to completely separate the dissolved silver and other base metals from gold, and overcome the passivation phenomenon of the silver surface to facilitate the lowering. The phase reaction proceeded smoothly.
When nitric acid is added, dilute nitric acid should be added at the beginning, and the nitric acid concentration is gradually increased as the reaction progresses. This not only saves the amount of nitric acid, but also allows the reaction to proceed more thoroughly. Generally, in the initial stage, the volume ratio of nitric acid to water is 1: (4~8), and in the later stage of the reaction, the volume ratio of nitric acid to water is 1: (2~4). This is because in the initial stage, the content of silver and other base metals is relatively large, especially gold mud contained magic hair formation, a greater amount of iron-containing, only dilute nitric acid, in order to make silver, iron or the like to obtain a sufficient leaching, And avoid the production of trivalent iron ions, so as to avoid troubles for subsequent operations. It can also be seen from the following reaction formula that the use of dilute nitric acid and concentrated nitric acid will produce different acid consumption, which is due to concentrated nitric acid and dilute nitric acid. It is caused by reduction to different products during the reaction.
Ag+2HNO 3 (concentrated) =====AgNO 3 +NO 2 ↑+H 2 O
3Ag+4HNO 3 (lean)=====3AgNO 3 +NO↑+2H 2 O
Cu+4HNO 3 (concentrated)=====Cu(NO 3 ) 2 +2NO 2 ↑+2H 2 O
3Cu+8HNO 3 (lean)=====3Cu(NO 3 )2+2NO↑+4H 2 O
4Zn+l0HNO 3 (very thin)=====4Zn(NO 3 )2+NH4NO 3 +3H 2 O
In the later stage of the gold and silver separation reaction, the content of silver and other base metals is very small, and the content of gold is as high as about 90%. At this time, only the rich nitric acid is added and the stirring is strengthened to fully dissolve the silver and other base metals. To further improve the quality of gold mud.
Practice has proved that it is necessary to heat the reactants during the silver separation process. Generally, the reaction temperature is controlled at about 90 ° C, the temperature is too low, the reaction rate is slow, the temperature is too high, the operation is difficult to control, and the nitric acid is seriously decomposed.
After the gold-silver separation reaction is completed, the precipitated gold mud is thoroughly washed with hot water, and the effect of washing with boiling water is better. The washed solution is guaranteed to be neutral and no white precipitates appear after the addition of saline. The purity of the gold mud treated by the nitric acid method can generally reach more than 90%, and can be smelted after being filtered and dried.

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