The kinetics of dissolution of gold and silver in cyanide solution are thought to be mainly controlled by diffusion. However, there are different opinions on the reaction mechanism of gold and silver. The dissolution theory of gold and silver proposed in various cyanide solutions is mainly as follows: I. Elsner (1846) The theory of oxygen. The theory holds that oxygen is essential when gold is dissolved in a cyanide solution: 4Au+8NaCN+O 2 +2H 2 O The dissolution of silver can be expressed in a similar reaction formula: 4Ag+8NaCN+O 2 +2H 2 O 2. The theory of hydrogen peroxide in Bodländer (1896). He believes that the dissolution of gold in cyanide solution is carried out in two steps. Its total reaction formula is in accordance with the above-mentioned Elsner equation. 2Au+4NaCN+O 2 +2H 2 O H 2 O 2 +2Au+4NaCN Hydrogen peroxide formed in the formula is an intermediate product. He confirmed his opinion by detecting the presence of H 2 O 2 in the solution. 3. The hydrogen theory of Jenin (Janin, 1888, 1892). Jenin does not recognize that oxygen is an essential part of dissolving gold, and is convinced that hydrogen is inevitably released during the reaction: 2Au+4NaCN+2H 2 O Barsky measured the free energy of the formation of the cyanide and silver cyanide ions. By calculating the free energy of the reaction, the thermodynamic equilibrium constants of the above reactions are: Equation (1), K = 10 66 ; Equation (2), K = 10 16 ; Equation (3), K = 10 49.8 ; Equation (4), K = 10 - 99 . According to △G 0 =-RTlnK Then equation (1), △ Equation (2), △ Equation (3), △ Equation (4), △ Therefore, Elsner and Bodland proposed that the reactions of [formula (1) to (3)] can be carried out, and the reaction of [formula (4)] proposed by Jenin cannot be carried out. Fourth, Christy (1896) cyanism. According to the theory, for the dissolution of gold, the cyanide gas must be aerobic, and the cyanide formed is an activator that promotes the reaction of gold: 2Au+2NaCN+(CN) 2 But Skey and Parker (1898) presented clear evidence that the cyanide-containing aqueous solution did not dissolve gold or silver. 5. The corrosion theory of Thompson (1934). He believes that the dissolution of gold in cyanide solution is similar to metal corrosion, in which oxygen dissolved in the solution is reduced to hydrogen peroxide and hydroxyl ions. And pointed out that the reaction formula proposed by Bodland should be further divided into the following steps: O 2 +2H 2 O+2e - H 2 O 2 +2e - Au Au + +CN - AuCN+CN - These reactions have been confirmed by experiments. Sixth, the kinetics of cyanide leaching. Habashi (1966, 1967) pointed out that the kinetics of leaching gold from cyanide solution is essentially an electrochemical dissolution process, which generally follows the reaction formula (2): 2Au+4NaCN+O 2 +2H 2 O A similar reaction formula can be written for the dissolution of silver. This conclusion is based on the following facts: (1) 1 mol (molecular) of oxygen is consumed per 2 mol of metal dissolved; (2) 2 mol (molecular) cyanide is consumed per 1 mol of metal dissolved; (3) Hydrogen peroxide is formed when gold or silver is dissolved, and 1 mol (molecular) H 2 O 2 is produced per 2 mol of metal dissolved; (d) Experiments have shown that the dissolution of gold and silver in NaCN+H 2 O 2 is a slow process in the absence of oxygen (see table below). because: 2Au+4NaCN+H 2 O 2 The reaction is rare. In fact, if a large amount of hydrogen peroxide is present in the solution, the cyanide is oxidized to a cyanide ion which does not dissolve in gold due to the following reaction: CN - +H 2 O 2 It inhibits the dissolution of gold and silver. Table gold, silver dissolution rate Dissolution mass ∕mg Time required ∕min Remarks Cyanide + O 2 Cyanide + H 2 O 2 Gold 10 5~10 30～90 1943 Silver 5 15 180 1951 Despite the differences in the theory that gold is dissolved in cyanide solutions, H 2 O 2 experiments, such as Swollen (Zurilla, 1969), which collect and measure diffusion from the gold surface (no longer participate in the reaction), show: 85% The gold is dissolved according to Bodland's Hydrogen Peroxide Theory (Formula 2), and only 15% of the gold is dissolved according to Elsner's Oxygen Theory (Formula 1). That is, the reduction of O 2 does not directly produce OH - but always involves the formation of the intermediate H 2 O 2 . The resulting H 2 O 2 can promote the formation of Au ((CN) 2 - : O 2 +2H 2 O+2e 2Au+4CN - +H 2 O 2 This reaction is achieved by O 2 dissolved by supplying air to the solution. This conclusion can increase the cyanide dissolution rate of gold by adding a small amount of H 2 O 2 to the solution. If a large amount of H 2 O 2 is added, the surface of the gold particles is passivated and the dissolution rate is lowered to obtain a strong proof. From a thermodynamic point of view, the driving force for the precipitation of gold and silver from the CN - solution can be quantitatively expressed by the reduction potential. which is Au + (Ag + )-e - Concrete Crusher,Demolation Shear,Excavator Shear,Hydraulic Shear, Concrete Pulverizer Jiangsu Origin Machinery Co., Ltd , https://www.originmachinery.com
4NaAu(CN) 2 +4NaOH (1) 4NaAg(CN) 2 +4NaOH 2NaAu(CN) 2 +2NaOH+H 2 O 2 (2) 2NaAu(CN) 2 +2NaOH (3) 2NaAu(CN) 2 +2NaOH+H 2 (4) <0; <0; <0; >0. O 2 +2NaCN+H 2 O (CN) 2 +2 NaOH 2NaAu(CN) 2 H 2 O 2 +2OH - 2OH - Au + +e - AuCN Au(CN) 2 - 2NaAu(CN) 2 +NaOH+H 2 O 2 2NaAu(CN) 2 +2NaOH CNO - +H 2 O H 2 O 2 +2OH - 2Au(CN) 2 - +2OH - (5)
Theory of dissolution of gold and silver in cyanide solution
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