Recovery of gold (silver) in precious liquid
In the operation of desorbing the gold- loaded resin by using a thiourea-sulfuric acid solution, a thiourea noble liquid having a high gold ( silver ) content is obtained. The gold is recovered from the thiourea noble liquid. In the past, the displacement precipitation method or the chemical precipitation method was used. At present, the electrolysis method has become the basic method for the gold extraction plant to obtain the finished gold.
Replacement precipitation method
It replaces precipitates with precious metals such as zinc , lead and aluminum to recover precious metals such as gold and silver. For example, replacing with zinc:
2Au(ThiO) 2 + +Zn=====Zn(ThiO) 2 2+ +2ThiO+2Au
The main disadvantage of this method is that the metal precipitant is consumed in a large amount and accumulated in a large amount in the thiourea solution, so that the desorption rate of gold and silver is reduced when the thiourea de-gold solution is returned, and it is necessary to periodically update with new thiourea. , resulting in increased consumption of thiourea.
2. Chemical precipitation method
It is used to precipitate gold by lye precipitation. This process is carried out under heating (50 to 60 ° C). At this time, the gold in the thiourea complex is converted into a hydroxide to precipitate a precipitate. The precipitate was filtered and then burned. The precipitate after burning contained 35% 50% precious metal. The use of a series of steps to treat the precipitate increases the precious metal content, but these process steps are not always advantageous under factory conditions. Although the method is simple and can ensure the complete precipitation of gold and silver, the precious metal content of the precipitated product is not high, and the thiourea is partially decomposed in the alkaline medium, the consumption of acid and thiourea is increased, and due to the accumulation of sodium sulfate in the solution, The desorption capacity of the thiourea solution used for return is lowered.
3. Electrolytic deposition
Electrodeposition is a widely used and effective method for recovering gold from thiourea noble liquid. The advantage is that a noble metal with high grade can be obtained without a lengthy gold mud enrichment process; the consumption of reagents (especially thiourea) is greatly reduced, and the contamination of the circulating thiourea solution by impurities is avoided, thereby improving the index of the resin regeneration process. It is based on gold in the form of thiourea complex cation Au[SC(NH 2 ) 2 ] 2 + in thiourea noble liquid, which is reduced on the cathode during electrolysis and gold is deposited on the surface of the cathode for recovery:
Au[SC(NH 2 ) 2 ] 2 + +e-=====Au+2SC(NH 2 ) 2
During electrodeposition, thiourea is oxidatively decomposed into elemental sulfur at the anode, causing the solution to turbid and contaminate the cathode deposits, and consuming a large amount of thiourea. More importantly, the thiourea anodization product has a high rate of precipitation of gold on the cathode. Adverse effects, in order to eliminate the above harmful phenomena, electrowinning is carried out in a special electrolytic cell. The cathode is composed of carbon fiber or flake graphite ; The cathode chamber and the anode chamber are separated by a cation exchange membrane, the catholyte is a noble liquid (desorbed liquid); and the anolyte is a 2% H2SO4 solution. [next]
The concentration of gold and silver in the electrolytic cell is not high. Although it is electrolyzed on the large surface cathode, in addition to the precipitation of gold and silver, there is also H + co-discharge:
2H + +2e - =====H 2
Therefore, the current efficiency is not more than 10%-15% in gold.
At the anode, water molecules oxidize and precipitate gaseous oxygen:
2H 2 O=====4H + +O 2 +4e -
The generated H + ions pass through the cation membrane and enter the catholyte (see figure below). The thiourea in the negative liquid, due to the barrier of the diaphragm, cannot enter the anode region and thus is not oxidized at the anode. SO 4 2- is also retained in the catholyte (because the cation membrane is impermeable to the anion). Thus, the catholyte undergoes gold depletion and accumulation of thiourea and sulfuric acid, and for each deposition of 1 mol of gold, 2 mol of thiourea and 0.5 mol of sulfuric acid are accumulated, and the anolyte composition does not change. Therefore, this film prevents the thiourea molecules from passing, and thus it is impossible to reach the anode surface, but the anion SO 4 2- can enter the cathode region through the separator. Therefore, the electrolytic cell equipped with the ion exchange membrane ensures the normal progress of the electrolysis process.
Electrolysis is carried out in a series of electrolytic cells. After depositing gold at the cathode, the lean liquid is returned for the preparation of the sulfur vein desorption solution. Under normal operation, the current density is 20~60A/m 2 . In order to increase the deposition rate of gold during electrolysis, it is important to increase the surface area of ​​the cathode. Nowadays, Russia uses a high-efficiency electrolytic cell with an ion exchange membrane, a porous graphite as a cathode, and a mesh as an anode, models 3Y-1 and 3Y-1M. Under optimal conditions, a cathode made of graphite material, 1 kg of graphite matrix can deposit up to 5Okg of gold, while the matrix itself accounts for only 2% of the total weight of the precipitate.
Production practices have shown that increasing the current density, solution flow rate and temperature can increase the deposition rate of gold and silver by a factor of three to five. If the temperature of the electrolyte is raised from 20 ° C to 50 ° C alone, the gold deposition rate is increased by about 1.9 times.
The electrolytic deposition process of gold consists of several process steps: filtration of noble liquid, electrolysis, washing of cathodic deposits, drying and unloading, sediment roasting and smelting. Before the electrolysis, the precious liquid must be filtered to remove suspended solids, wood chips and finely divided resin in the solution to avoid clogging the porous graphite cathode, resulting in deterioration of electrolysis efficiency and sediment quality. When the content in the precious liquid falls below the specified value, the electrolysis operation can be stopped.
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