There are three commonly used desorption methods for gold- loaded carbon: Zhadra desorption method, high temperature and high pressure desorption method, and alkaline ethanol solution desorption method. In addition, in recent years, the desorption method proposed by the Anglo-American company laboratory in South Africa has also appeared.

First, Zade desorption method

Although it has long been known that activated carbon is a good adsorbent for gold cyanide, the early method of recovering gold from gold-loaded charcoal is to burn off the gold-loaded charcoal and then smelt the gold from the ash. The activated carbon can only be used once and the cost is high. After World War II, the United States due to the excess of activated carbon, low prices, but also of interest to people recovering gold and silver from the cyanide solution with activated carbon. Since then, the US Bureau of Mines has been working on the study of gold-carbon desorption methods. In 1950, a method for desorbing gold from gold-loaded carbon with an alkaline sodium sulphide solution was published, which had little effect on the silver on the gold-loaded charcoal. In 1952 Zade published a method for desorbing gold-loaded carbon with a hot sodium hydroxide-sodium cyanide mixed solution, namely Zhadra desorption. The desorption method is to use a 1% sodium hydroxide solution and a 0.1% sodium cyanide mixed solution to heat to 90-95 ° C, and pass through the gold-bearing carbon layer at a certain speed, and the gold and silver on the gold-loaded charcoal are desorbed and enter. In solution. The result of the laboratory is desorption for 4-6 hours, and the desorption rate of gold and silver is over 90%. However, in actual production, it usually takes 50 to 70 hours to achieve a high desorption rate. Despite the long desorption time, the Zade process makes it possible to reuse activated carbon, which greatly reduces the operating cost of gold mine heap leaching operations.

Second, high temperature and high pressure desorption method

The high temperature and high pressure desorption method is to install the gold-loaded carbon in a pressure vessel, and mix the solution with 0.4% to 1% NaOH and 0.1% NaCN (may not), at a temperature of 130 to 160 ° C, at 3.6 to 5.9 kg. Under the pressure of cm 2 , pass the gold carbon layer. Within 2 to 6 hours, the gold and silver desorption rates of gold-loaded carbon exceeded 90%. Compared with the Zade process, the pressure desorption method has a much shorter desorption time, and the desorption process also consumes less reagents than the Zade process. The equipment and process of the pressure desorption method are shown in Figure 1. The desorbed rich liquid was cooled to 90 ° C and electrolyzed into the electrolytic cell.

Figure 1 Schematic diagram of the pressure desorption equipment

Third, alkaline ethanol solution desorption method

The alkaline ethanol solution desorption method is to add 20% ethanol (or propanol) in a mixed solution of 1% NaOH and 0.1% to 0.2% NaCN, and desorb at 64 ° C under normal pressure desorption within 6-8 hours. The volume of the desorbent and the washing liquid is only 7 to 10 times the bed volume. Practice has shown that desorption with an alkaline ethanol solution can reduce the number of carbon regenerations. However, ethanol is flammable and explosive, and strict safety measures should be considered. Increasing the recovery of alcohols is critical to reducing the cost of the alcohol solution desorption process.

The alkaline ethanol solution desorption method successfully studied by the nuclear industry uses 40% to 50% ethanol, the temperature is 78-80 ° C, NaOH 1% to 2%, NaCN 0.5% to 1.0%, and desorption at atmospheric pressure. Soak the gold-loaded charcoal with the above solution for 1 to 2 hours, and then soak it once with 30% to 35% of the ethanol solution. The solutions of these two operations were successively placed in a distillation still for distillation. The distilled steam passes through the desorbed carbon layer, and the ethanol vapor is cooled by the condenser into an alcohol solution and flows into the alcohol storage tank. The distilled alcohol recovered by the pump is pumped to the desorption dome and flows through the carbon layer from top to bottom. Repeated distillation 3 to 4 times to complete a desorption cycle, generally 8 ~ 10h. The desorption rate is greater than 98% and the recovery of ethanol is greater than 90%. The concentration of the rich liquid gold obtained by this method can reach 5-8 g ∕L (the gold grade of the char). Therefore, the current efficiency during electrolysis is also higher than other methods. Since the alcohol vapor repeatedly passes through the carbon layer during desorption, it is beneficial to restore the activity of the carbon, and the desorbed carbon can be reused without high-temperature regeneration.

In recent years, Caps-Kahid United has developed a mobile adsorption desorption device suitable for small and medium-sized heap leaching operations, as shown in Figure 2. The main equipment consists of six carbon adsorption/rinsing towers, power supply and various storage tanks. It is packed in two containers for easy on-site movement and installation. Its greatest feature is that both adsorption and desorption are carried out in the same equipment, without the need to transfer charcoal. The left side of the figure is a schematic diagram of the four-column series adsorption, and the right side is a schematic diagram of the two towers desorption.

Figure 2 Six towers process and equipment

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