Carbon extracting process - desorption of gold-loaded charcoal

The cyanide slurry plus activated carbon adsorbs gold- loaded gold-bearing charcoal, which is desorbed by gold and silver after washing and removing wood chips and other impurities. A panker developed in South Africa (Fig. 1) is particularly effective in removing wood chips from wood chips containing 13.8% of wood chips. This type of panker should be widely used to treat wood-loaded gold charcoal precipitated from Sweco sieves.

Figure 1 Sawdust Washer

1-Sweco sieve; 2-220V mixer; 3-distribution chute;

4- observation hole; 5--automatic valve; 6-manual valve; 7-loaded gold charcoal tank

An 28-mesh screen of EPAC for wood chips and sundries developed by the Mintek plant in South Africa is of the same type as the immersion screen of Figure 2. It is also installed on the side of the tank for static operation to treat the slurry entering the adsorption tank. Due to the presence of a bubble curtain on the screen surface, the wood chips, fibers and coarse particles filtered on the 28 mesh screen surface can be kept loose and not adhered to the screen surface. Its structure and operation process are shown in Figure 3.

Figure 2 immersion sieve and its arrangement

Figure 3 EPAC in addition to wood chips

As mentioned above, the adsorption of gold (silver) by activated carbon is an exothermic reaction. The higher the temperature of the slurry, the smaller the capacity of carbon adsorption to gold. This property becomes the high temperature desorption (elution) of gold on gold-loaded charcoal. The basis of the process. According to А. И. Grabowski and other tests AY-50 loaded gold charcoal under dynamic conditions, using 0.1% NaCN and 0.4% NaOH as desorbent, at 50, 70 and 93 ° C, different time gold The desorption curve of silver and silver is shown in Fig. 4. At the same time, they also carried out the desorption test of КАД and CKT-loaded gold charcoal. The results of all tests (including static conditions) showed that the desorption of gold and silver showed a significant peak at 93 °C, the peak at 70 °C was small, and there was no peak at 50 °C. And the peak of silver appeared early, and the desorption rate reached 50% at the beginning of 2~3h. The peak of gold appeared late, and the highest concentration of gold in the desorbent appeared 4 to 6 hours after the start, 9 to 12 hours.

The desorption rate is only 50%. This provides conditions for the initial separation of gold and silver. The desorption operation is divided into two sections. The desorbed liquid with low silver high gold is taken from the previous one, and the desorbed liquid with low gold and high silver is produced in the latter stage. These tests also demonstrate that the desorption of gold-loaded carbon must be carried out at higher temperatures, which is consistent with the widely used 80-93 °C. And the use of NaOH-containing desorbent can also remove silicon deposited on charcoal to prevent charcoal poisoning.

Figure 4 Desorption curve of gold and silver in AY-50

-·-Au, -Ag, a, 1-93 ° C; b, 2-70 ° C; c, 3-50 ° C

Some researchers in the former Soviet Union believe that the adsorption of gold on gold-loaded charcoal is the formation of a stable neutral covalent compound, Au(CN)2, which precipitates on charcoal. Therefore, it has a highly water-insoluble property, so it can be desorbed only by using a high-concentration agent capable of forming a strong anion with gold (such as CN- or SCN-, etc.), or pre-concentrated with a higher concentration. After the CN-agent is treated, it is washed with hot water, and the gold adsorbed on the gold-loaded carbon can be effectively desorbed. South Africa has also studied a method in which gold-loaded charcoal is first washed with dilute HCl (preferably HNO3), which reduces the desorption time of charcoal to 8 h. If, before the desorption washed with 3% HCl or a 5% HNO3, but also to remove calcium magnesium carbonate is deposited on the carbon, these salts can effectively eliminate the poisoning caused by the first use of carbon.

The desorption method of gold-loaded carbon is widely used in production today:

First, Zade desorption method.

The law was invented by JB Zadra in 1952. The gold-loaded carbon was desorbed in a hot (80-93 ° C) solution of 0.1% to 0.2% NaCN and 1% NaOH at atmospheric pressure for 24 to 48 hours, so that 98.4% of the gold was desorbed.

Second, the alcohol desorption method.

The law was developed by the Renault Research Center of the US Bureau of Mines. The gold-loaded charcoal is desorbed in a solution of 20% alcohol, 1% NaOH and 0.1% NaCN heat (80-85 ° C) at normal pressure for 5-6 hours, so that 99% of the gold can be desorbed from the charcoal. However, alcohol is flammable and explosive and toxic. It should be used with care.

Third, the high pressure desorption method.

In this method, 99% gold is desorbed by using 1% NaOH and 0.1% NaCN at a pressure of 350-400 kPa and 160 ° C for 2-6 hours. However, this method requires the use of high-pressure equipment, and the desorbent must be cooled before decompression to avoid splashing and causing loss.

Fourth, deionized water elution method.

South Africa Anglo American Company pretreated with a carbon volume of 10% NaOH (or 0.5% NaCN and 1% NaOH) in a hot (90 ~ 110 ° C) solution for 2 ~ 6h, then washed with 5 ~ 7 volume of deionized hot water 5 ~ 7h, the entire operating cycle is 9 to 20h.

The gold-loaded charcoal of the Yangjishan gold mine was pre-washed with 0.6 volume 3% HNO3 for 16 h and then decanted with tap water. Since the alkali concentration of paleocalcium in the tap water is high, the upper portion of the carbon [NaAu(CN)2] is converted into Ca[Au(CN)2]2, which lowers the desorption rate of gold. The addition of 0.1% NaOH in tap water was stirred and clarified, and the supernatant was eluted. The desorption rate of gold was increased by 4.6%. The pretreatment of gold-loaded charcoal was pretreated at 98 °C for 3 h. The amount of gold in the upper part of the desorption column was dried due to the evaporation amount, which affected the desorption rate of gold, and then the pretreatment time was changed to 30 min. After the reform, the desorption rate of gold reached 98%.