The reason why cyanide selectively dissolves gold , silver and some of their complexes is due to the aeration of alkali metal (or alkaline earth metal) cyanide. In practice, an aerated solution of sodium cyanide (or calcium cyanide) is usually used, along with a few bases (usually lime). The addition of the base is to inhibit the hydrolysis of cyanide to avoid the loss of vanatilization. If the dissolution reaction is considered to be an electrochemical erosion process, the dissolution of gold can also be considered as the process of dissolving gold into the solution on the anode surface (Fig. 1). When oxygen on the surface of the cathode receives electrons, the reaction between the anode and cathode sections is: Anode area Au+2CN - Cathode area O 2 +2H 2 O+2e - Further reaction H 2 O 2 +2e In electrochemical corrosion systems, the most important factor affecting the polarization of anodes and anodes is concentration polarization, which is determined by Fick's law: In the middle D CN and (CN - ) and (O 2 )- are the concentration of CN - and O 2 in the whole solution, respectively, mol (molecular) ∕ mL; (CN - )i and (O 2 )i- are the concentration of CN - and O 2 at the interface, respectively, mol (molecular) ∕ mL; A 1 and A 2 - are the surface area of ​​the reaction between the cathode and the anode, respectively, cm 2 ; δ-Nernst interface layer thickness, cm. Figure 1 Diagram of the dissolution of gold in cyanide solution In metal interface CN - and through the stagnant layer O 2 as compared to the speed, assuming the chemical reaction at the interface quickly, then, as soon as they reach the metal surface will be consumed immediately, that is to say: (CN - )i=0 (O 2 )i=0 Therefore, equations (1) and (2) can be simplified to: Because the metal dissolution rate is twice the rate of oxygen consumption and is one-half the rate of cyanide consumption, it is: Gold dissolution rate = 2 Or the dissolution rate of gold = When the above reaction formula reaches equilibrium, 2 However, because the total surface area of ​​the metal in contact with the water phase is A = A 1 + A 2 , thus: Gold dissolution rate = It can be seen from this equation that when the cyanide concentration is low, the first term is negligible compared to the second term of the denominator. Therefore, equation (3) can be simplified as: Gold dissolution rate = The value calculated by this formula is in agreement with the experimental results of Fig. 2, that is, when the cyanide concentration is low, the dissolution rate depends only on the cyanide concentration. Fig. 2 Effect of different pressures and different NaCN concentrations on the dissolution rate of silver at 24 °C Similarly, when the cyanide concentration is high, the second term is negligible compared to the first term of the denominator, then equation (3) can be simplified as: Gold dissolution rate = 2 The value calculated by this formula also coincides with the experimental results of Fig. 2, that is, when the cyanide concentration is high, the dissolution rate depends only on the oxygen concentration. when which is At this point, the dissolution rate reaches the limit. The average value of the diffusion coefficient found in Table 1 is: The average ratio of the two diffusion coefficients: That is, the molar average ratio of the ultimate dissolution rate of the two is: 4 Table 1 Diffusion coefficient and average Temperature ∕ °C KCN∕% D CN ∕cm 2 ·s -1 D O2 ∕cm 2 ·s -1 18 - 1.72×10 -5 2.54×10 -5 1.48 25 0.03 2.01×10 -5 3.54×10 -5 1.76 27 0.0175 1.75×10 -5 2.20×10 -5 1.26 average value - 1.83×10 -5 2.76×10 -5 1.50 At this time, the dissolution rate of gold reaches the limit dissolution rate. It is in good agreement with the experimental values ​​shown in Table 2, 4.6 to 6.8. Table 2 Ratio of cyanide and oxygen concentration to limiting dissolution rate metal Temperature ∕ °C P O2 ∕Pa (O 2 )∕mol·L -1 (CN - )∕mol·L -1 gold 25 101325 1.28×10 -3 6.0×10 -3 4.69 25 21278 0.27×10 -3 1.3×10 -3 4.85 25 101325 1.28×10 -3 8.8×10 -3 6.88 silver twenty four 757911 9.55×10 -3 56.0×10 -3 5.85 twenty four 344505 4.35×10 -3 25.0×10 -3 5.75 Japanese-style Furniture with its fresh and natural, simple and unique, forming a unique style of furniture, Japanese-style home environment to create a leisurely, customers in this environment can be happy to eat. QianYu Food Machinery Co., Ltd provides rotary sushi conveyor belts, while providing Japanese-Style Furniture supporting facilities to meet the needs of our customers. Japanese-style Furniture Japanese-Style Furniture,Sushi Furniture Of Simple Appearance,Solid Wood Sushi Furniture,Sushi Bar High Leg Stool Chair Guangzhou QianYu Food Machinery Co., Ltd. , https://www.qianyusushimachinery.com
Au(CN) 2 - +e H 2 O 2 +2OH - 2OH - A 1 [(O 2 )-(O 2 )i] (1) A 2 〔(CN - )-(CN - )i) (2) with - the diffusion rate of CN - and O 2 , respectively, mol (molecular) / s; - the diffusion coefficient of cyanide and dissolved oxygen, respectively, cm 2 /s; A 1 [O 2 ] A 2 [CN - 〕 A 1 [O 2 ] A 2 [CN - 〕 A 1 〔O 2 〕= A 2 [CN - 〕 (3) (CN - )=k 1 (CN - ) [O 2 ]=k 2 [O 2 ] [CN - 〕=4 [O 2 ] =2.76×10 -5 cm 2 ∕s =1.83×10 -5 cm 2 ∕s ≈1.5 =4 ≈6
From a process point of view, it is not only the concentration of dissolved oxygen (ie, the degree of aeration of the solution), but also the concentration of free cyanide, but the ratio of the two concentrations. Therefore, if only the desired aeration is achieved, and sufficient free cyanide in the solution is ignored, a good cyanidation effect will not be obtained, and the maximum cyanide dissolution rate cannot be achieved. Conversely, if an excessive amount of cyanide is added and the oxygen content in the solution is lower than the theoretical value, the excess cyanide is obviously a waste. In order to achieve maximum dissolution rate, the free cyanide and oxygen content in the solution must be simultaneously analyzed and controlled so that the molar ratio of the two is approximately equal to 6.