Many years ago, the chemical structure of thiourea molecules was assumed to be That is, it is a pair of free electrons possessed by N + and S atoms in the molecule, and adsorbed on the surface of the gold particles to greatly reduce the redox potential of gold, so that gold is easily oxidized and dissolved into the solution. The dissolution of gold in acidic thiourea solution is generally considered to be an electrochemical corrosion process, and oxygen must be involved in the process. That is, the thiourea molecule reduces Au 3 + to Au + , and the oxidant further oxidizes Au + with thiourea to form stable Au(SCN 2 H 4 ) 2 + into the solution. The oxidant is reduced. When H 2 SO 4 is used as the pH adjuster and Fe 3 + is used as the oxidant, the reaction is: 2Au+4SCN 2 H 4 +Fe 2 (SO 4 ) 3 2Ag+6SCN 2 H 4 +Fe 2 (SO 4 ) 3 At this time, the stability constants of gold and silver complexes were formed: Au (SCN 2 H 4 ) 2 + was 10 22 to 22.5 , and Ag (SCN 2 H 4 ) 3 + was 10 13.1 to 13.9 . It is assumed that the gold particles are a microbattery in the acidic thiourea solution containing an oxidizing agent, and its surface has a cathode region and an anode region. When the cathode region (positive electrode) is oxidant Fe 3 + . The electron obtained is reduced to Fe 2 + ; and the Au + of the anode region (negative electrode) is oxidized to form Au(SCN 2 H 4 ) 2 + due to the loss of electrons (see Fig. 1). Figure 1 Diagram of gold dissolution in thiourea solution That is, in the cathode region: Fe 3 + +e In the anode region: Au Au + +2SCN 2 H 4 Total reaction formula: Au+Fe 3 + +2SCN 2 H 4 A similar reaction occurs in silver: Ag+Fe 3 + +3SCN 2 H 4 Since Fe 3 + is oxidized to rely on the solution of dissolved oxygen blast Fe 2 + so as to reproduce, it is in fact still depends on the role of the oxidizing agent dissolved air bubbled into the solution oxygen, and therefore (3) in turn Written as: Au+2SCN 2 H 4 +H + + The standard oxidation-reduction potential of the Au + ∕Au pair is known to be 1.73V. When gold is leached with thiourea, the potential of gold is lowered by the formation of complex cations. In the (2) reaction, the standard oxidation-reduction potential of the Au(SCN 2 H 4 ) 2 + /Au pair was (+0.38 ± 0.1) V at 25 °C. Although the potential value is greatly reduced, it is much higher than the Au(CN) 2 - /Au electric potential -0.64V when the gold is leached by the cyanidation method, and the gold thiourea complex ion is a cation, and the gold cyanide complex ion is Anion, so the complexing ability of thiourea to gold is weaker than that of cyanide. If the "two-step method" of adsorption (or displacement) after leaching is used, the recovery rate of thiourea gold extraction is often lower than that of the cyanidation method. However, in order to overcome the above difficulties, the dissolution rate of gold by thiourea is about 10 times faster than that of cyanidation (6×10 -9 mol/(cm 2 ·s)), and the initial dissolution rate of thiourea to gold is extremely fast. To increase the recovery of thiourea gold, it is necessary to lower the potential of the Au(SCN 2 H 4 ) 2 + ∕Au pair. According to the Nernst formula, the potential of the Au(SCN 2 H 4 ) 2 + /Au pair can be expressed by the following formula: = =0.38+0.0591lg It can be seen from the formula that in the acidic thiourea solution, the potential of the Au(SCN 2 H 4 ) 2 + ∕Au pair is only balanced with the concentration of Au(SCN 2 H 4 ) 2 + ions and SC(NH 2 ) 2 molecules. Concentration related. That is, the application of acid thiourea liquid gold extraction should make the dissolved gold complex ions in the solution in a non-equilibrium system to reduce the electrode potential of the negative electrode and improve the recovery rate of gold. The methods that can be used for this purpose are: (1) increasing the concentration of thiourea in the leachate; (2) using a "one-step method" of adsorption (or displacement) while leaching, and most of the dissolved gold in the slurry at any time ( Or a part) proposes to keep the concentration of gold in the infusion at a lower level and increase the free thiourea content in the solution. But some researchers believe that the latter is not very important. Oxidation of thiourea is the main cause of excessive thiourea consumption. In an acidic solution, thiourea is first oxidized to form dithiomethane, which has an electrical potential of 0.42V. However, in the literature, it has also been reported that thiourea is oxidized to form dithiourea polymerized ions. The reaction between the two: As in the formula: 2SC(NH 2 ) 2 2SCN 2 H 4 As in equation (7): 2SC(NH 2 ) 2 2SCN 2 H 4 Formula (6) has H + participation, and the influence of thiourea and dithiocarbamate on the potential is affected by the change of pH of the medium. Formula (8) produces dithiourea whose electrical potential is not affected by the pH of the medium. Figure 2 is a plot of potential-pH of the Au(Ag)-SCN 2 H 4 -H 2 O system at 25 °C. It is calculated by the reaction of the reaction formula (6) that SCN 2 H 4 ∕(SCN 2 H 3 ) 2 is electrically drawn to the potential = 0.4791-0.0591 pH to draw a 4-line. It can be seen from the figure that gold dissolves to form Au(SCN 2 H 4 ) 2 + line 1 at a potential of 0.3739 V, and silver dissolves to form Au (SCN 2 H 4 ) 2 + line 2 at 0.1142 V. Therefore, from the thermodynamics, the Ag line potential is lower than the Au line, and thiourea dissolves silver more easily than gold. And both the 1 line and the 2 line intersect the 4 line, the pH corresponding to the intersection of the gold dissolution line 1 is 1.78, and the pH corresponding to the intersection of the silver dissolution line 2 is 6.17. That is to say: the pH of gold dissolution is not more than 1.78, and the pH of silver dissolution is not more than 6.17. Only when the pH value of the thiourea solution is lower than the pH corresponding to the intersection point, the oxidant can oxidize gold and silver and complex with thiourea to form a complex ion into the solution. If the pH of the solution is greater than the pH corresponding to the intersection, the oxidation reaction of thiourea will be enhanced, and dithiocarbamate will be formed and decomposed into S 2 - , H 2 S, CNNH 2, etc., and the dissolved Au (SCN) will be dissolved. The 2 H 4 ) 2 + and Ag(SCN 2 H 4 ) 3 + complex ions are reduced to a metal precipitate. The higher the pH value of the solution, the stronger the tendency of thiourea to oxidize, and the more the reduction of dissolved gold and silver complex ions. The dithiocarbamium formed at this time does not oxidize gold or silver. Figure 2 Au(Ag)-SCN 2 H 4 -H 2 O system potential-pH diagram conditions at 25 ° C; SCN 2 H 4 =(SCN 2 H 3 ) 2 =10 -2 mol; Au(SCN 2 H 4 ) 2 + =Ag(SCN 2 H 4 ) 3 + =10 -4 mol; Oxygen pressure = hydrogen pressure = 101.32kPa However, Zhu Xi said: When the ionic strength is constant, the potential of a fixed ratio of thiourea and dithiomethan mixed solution is not affected by acidity change, and their oxidation reaction formula (8), the slope of potential-pH line Should be zero, the 4 line should extend horizontally from a low pH to pH 4.3 instead of the above-mentioned slope -0.0591 pH. Therefore, the above-mentioned oxidative dissolution of gold at pH below 1.78, otherwise the argument that only thiourea can be oxidized is not established. Since the relationship between gold leaching speed and acidity is approximately zero-order kinetics, the reaction rate decreases at pH>2. However, the amplitude is very small, SJ Rodi et al. believe that; since the influence of acidity in the actual leaching process is very complicated, the reaction formula (8) is only the total reaction formula of thiourea oxidation, and does not reflect their reaction mechanism. Some steps of the reaction process have H + participation as in the formula (6), or their oxidation rate is related to the pH value, which is not inconsistent with the total reaction of the formula (8).
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. In the past forty years, due to the rapid development of physical technology, the chemical structure of thiourea molecules has been further identified as the following resonance: [Au(SCN 2 H 4 ) 2 + ]SO 4 +2FeSO 4 [Ag(SCN 2 H 4 ) 3 + ]SO 4 +2FeSO 4 Fe 2 + (1) Au + +e Au(SCN 2 H 4 ) 2 + (2) Au(SCN 2 H 4 ) 2 + +Fe 2 + (3) Ag(SCN 2 H 4 ) 3 + +Fe 2 + O 2 Au(SCN 2 H 4 ) 2 + H 2 O (4) -0.118lg (SCN 2 H 3 ) 2 +2H + +2e (5) (SCN 2 H 3 ) 2 +2H + +2e (6) (SCN 2 H 4 ) 2 2 + +2e (7) (SCN 2 H 4 ) 2 2 + +2e (8) 
Dissolution of gold and silver in thiourea solution