Carbon slurry process operation and equipment (3)

The desorption method of gold- loaded carbon is widely used in production today:
1) Zade desorption method This method was invented by JB Zade in 1952. It is also called atmospheric desorption method. It is 1% sodium cyanide and 1% hydroxide at 85 °C. The sodium solution desorbs gold from the gold-loaded charcoal. The method is carried out under normal pressure. According to the chemical composition and operation method of the solution, the gold-loaded carbon is in a hot (80 ° C ~ 93 ° C) solution with a mass fraction of 0.4% to 0.2% NaCN and 1% NaOH at atmospheric pressure. Under the conditions of desorption for 24~48h, 98.4% of the gold can be desorbed. The method is simple, with low capital and production costs, and is suitable for small-scale production.
2) Alcohol desorption method The alcohol desorption method was developed by the Renault Research Center of the US Bureau of Mines. The method is to desorb gold from gold-loaded charcoal at a temperature of 80-85 ° C and atmospheric pressure using a mass fraction of 0.1% NaCN, 1% NaOH, and 20% by volume of alcohol as a desorbent. Adding alcohol to the desorbent can significantly shorten the desorption time to 5-6 hours, which can desorb 99% of the gold from the charcoal. The advantage of this method is that it can reduce the scale of the desorption section and shorten the desorption time. The main disadvantages are that the alcohol is flammable, extremely unsafe, and the volatile loss of alcohol causes the production cost to increase. When designing such a desorption device, it is necessary to take safety fire prevention measures, and at the same time, it is necessary to install a device that effectively recovers alcohol.
3) High pressure desorption method High pressure desorption method is carried out by using a mass fraction of 0.1% NaCN and 1% NaOH solution at 160 ° C and 350 ~ 400 kPa, desorption for 2 ~ 6h, which can desorb 99% of the gold. Alternatively, the gold-loaded charcoal is pretreated with a mass fraction of 5% NaOH and 1% NaCN solution for 0.5 to 0.1 h, and then 5 hot waters of gold-loaded carbon volume (flow rate of 3 loaded gold charcoal per hour) The charcoal was desorbed at an operating temperature of 110 ° C, an operating pressure of 0.5 × 10 ⁵ to 1.0 × 10 ⁴ Pa, and a total desorption time (including pickling) of 9 h.
The advantages of using high pressure desorption are low reagent consumption and short desorption time. However, this method requires high pressure and high temperature, so the equipment is relatively expensive, and in order to avoid rapid evaporation, the liquid discharged before decompression must be cooled to prevent the solution from splashing and causing loss.
4) Deionized water elution method South Africa Anglo American Company pretreated with a hot solution (90~110 °C) containing 10% NaOH (or 0.5% NaCN and 1% NaOH) for 2~6h, then 5~7 The volume of deionized hot water is washed for 5~7h, and the whole working cycle is 9~20h.
The gold-loaded charcoal of Yangjishan Gold Mine was pre-washed with 0.6 volume of 3% HNO ₃ for 16 h, and then eluted with tap water. Due to the high alkali concentration of calcium in tap water, the upper part of the carbon [NaAu(CN) ₂ ] It is further converted to Ca[Au(CN) ₂ ] ₂ to lower the desorption rate of gold. In the tap water, the mass fraction was added to 0.1%. After NaOH was stirred and clarified, the supernatant was eluted, and the desorption rate of gold was increased by 4.6%. The pretreatment of gold-loaded carbon was pretreated at 93 °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%. [next]
4. Electron accumulation
The desorption solution is a pure gold and silver cyanide solution, and the volumetric concentration of gold is 300-600 g/m ³ . If such a dilute solution is subjected to conventional plate cathode electrolysis, the current efficiency is extremely low. Electrolysis was carried out by expanding the surface area of ​​the cathode, and satisfactory results were obtained.
Rectangular electrolyzers made of plastic are widely used in China. The anode is a drilled stainless steel plate and the cathode is stainless steel (in a nylon mesh or plastic basket). Cathode deposition of gold, silver and evolved hydrogen during electrolysis:
Au(CN) ₂ ^- +e ^- =====Au+2CN ^-
Ag(CN) ₂ ^- +e ^- =====Ag+2CN ^-
2H ₂ O+2e ^- =====H ₂ +2OH ^-
Oxidation products of oxygen and cyanide ions from the anode:
4OH ^- -4e ^- =====2H ₂ O+O ₂
CN ^- +2OH ^- -2e ^- =====CNO ^- +H ₂ O
2CNO ^- +4OH ^- -6e ^- =====2CO ₂ +N ₂ +2H ₂ O
Steel wool has a maximum immersion weight of 20 times its own weight and should normally be removed before reaching this value. Gold adheres to the steel wool, and the obtained gold powder is treated with hydrochloric acid and then smelted into gold ingots.
The electrolyte (desorbed liquid) passes through several electrolytic cells equipped with several pairs of anodes and cathodes, the current density is 8~15A/m ² , the cell pressure is 2.5~3.5V, and the deposition rate of gold is over 99%. After electrolysis, NaCN and NaOH are added to make a desorption solution or return to leaching.
The adsorption of gold by activated carbon is carried out in a multi-component complex solution containing a large amount of impurities such as copper , zinc , nickel , iron , sulfur and the like. Through the selective adsorption of carbon, most of the impurities remain in the tailings slurry and are separated from the gold. The noble liquid obtained by desorption of the gold-loaded carbon sieved from the slurry has a high concentration of gold and silver cyanide complex ions. However, the impurity ions are greatly reduced, and it provides a relatively ideal solution for the reduction of gold and silver from the noble solution by electrowinning or precipitation.
There are four types of industrial electrolyzers that recover gold from desorbed liquid. They are the Zadra electrolyzer developed by the US Bureau of Mines (Fig. 11d) and the parallel electrode electrolyzer (Fig. 11a), the electrolysis cell developed by the South African Anglo-American and Rand Company (AARL) (Fig. 11c) and the National Institute of Metallurgy, South Africa. (NIM) developed graphite crumb cathode electrolyzer (Fig. 11b). [next]

As shown in Fig. 11, the parallel electrode electrolytic cell is an electrolytic cell for performing multi-stage electric current, and the other three kinds of cells are single-stage electrolytic electrolytic cells. Although Au and Ag in the desorbed solution can be extracted by the first-order electrowinning, in practice, due to the complicated structure of the electrolytic cell, failure of any one electrode will affect the extraction rate of Au and Ag, and it is not convenient. Continuous monitoring of the discharge of the drain outlet to detect the faulty electrode in time. Therefore, multi-stage cyclic electrowinning operations are used.
When using a parallel electrode multi-stage electrolytic cell, the number of cathodes charged in the tank is determined according to the effective volume in the cathode frame (ie, the packing density of steel wool or graphite chips), and not the anode and the anode are filled in each of the strips. When the current is accumulated for one cycle, the first stage cathode has been taken out after Au and Ag are saturated, the second stage becomes the first stage of the next cycle, and so on. If several single-stage electrolyzers are used, when the first tank is saturated with Au and Ag, the second tank becomes the first tank of the next cycle. In order to achieve cycle work, and the cathode material is loaded with the most gold.
The structure and specifications of the EWC-96 parallel electrode electrolyzer and cathode frame designed by HBS for Xiqiao Carbon Paste Plant are shown in Figures 12 and 13. Its technical performance is as follows:

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Since the designed electrolyte temperature is 90 ° C, the electrolytic cell and the cathode frame are both made of polypropylene material, and the cathode frame is uniformly drilled on both sides of the frame plate so that the electrolyte flows uniformly. Each cathode was powered by steel wool inserted into the cathode frame by four Ф8 mm × 500 mm stainless steel rods welded to the conductive frame. The anode is made of 316 stainless steel plate with a thickness of 1.5~2mm. The surface of the plate is evenly distributed with Ф5mm drill holes. The anode and the anode are embedded in the strips with flexible materials on both sides of the groove wall. The bottom of the groove is also covered with flexible material. The electrolyte is prevented from passing through the sides and bottom of the plates, forcing it to flow through the bore of the plates. The bottom of the tank is inclined toward the end of the cleaning nozzle to periodically remove the sediment.
One of the patents invented by British people for the adsorption, desorption and electrolytic gold extraction from carbon-containing cyanide leachate using carbon fiber, which is a carbon fiber produced by British Activated Carbon Fabric Co., Ltd., which is a flexible fabric. The preparation method (patent) of the fabric is that the artificial fiber is first immersed in a chemical agent, taken out, carbonized by dry distillation at 350 ° C, and then activated at 900 ° C, so that the fiber is 100% activated carbon. If the fabric is made into a ring-shaped electrodeless belt, the gold adsorption, desorption and electrolytic gold extraction process can be continuous. Fig. 14 is a schematic view showing the connection of equipment for adsorption, desorption and electrolytic gold extraction of activated carbon. [next]

In the figure, the drive wheel drives the endless carbon belt for slow motion. The electric conductor wheels are connected to the positive power supply. The remaining wheels are plastic guide wheels. The gold-containing solution from the cyanide leaching is injected into the carbon ribbon on the adsorption tank, and after the gold is adsorbed from the tank, the impurities are washed through the washing tank to remove the impurities and desorbed into the desorption tank A. The desorbed gold is also deposited in a tank or a stainless steel cathode plate in a tank by closed-circuit electrolysis in a tank A. The decarburized strip is washed by the washing tank B and activated by heating in an activation furnace (250 ° C), and then again into the adsorption tank C. Adsorption of gold to form a continuous operation.
The activated carbon fiber fabric adsorbs gold, the adsorption rate is higher than that of granular activated carbon, and the speed is much faster. The adsorption equilibrium can be achieved within 20~25min, while the granular charcoal needs 150min.

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