The nature of thiourea
Thiourea, also known as sulfurized urea, is a white and shiny diamond-shaped six-sided crystal, which is bitter, slightly toxic and non-corrosive. The molecular formula is SC(NH 2 ) 2 , the relative molecular mass is 76.12, the density is 1.405 g/cm 3 , and the melting point is 180-182 ° C. It is easily soluble in water, with a solubility of 9% to 10% in water at 20 ° C and 14% at 25 ° C, which can meet any requirement for the concentration of thiourea leaching gold . The aqueous solution of thiourea is neutral and has no corrosive effect.
Thiourea is unstable in alkaline solution, easily decomposed into sulfur ions and cyanamide, and cyanamide can be hydrolyzed to urea:
SC(NH 2 ) 2 +2NaOH 
Na 2 S+CNNH 2 +2H 2 O
CNNH 2 +H 2 O CO(NH 2 ) 2
In an alkaline medium, the S 2 - formed by the decomposition of thiourea can also form a sulfide precipitate with various metal cations such as Au + , Ag + and Cu 2 + in the solution.
Thiourea has a reducing property in an acidic (pH 1 to 6) solution, but its formulation is stored for a long period of time, and itself can be oxidized to form a variety of products, so it should be used now. For example, in an acidic medium at room temperature, thiourea can be self-oxidized to dithiocarbamidine or dithiourea if it is left for too long:
Or 2SC(NH 2 ) 2 (SCN 2 H 3 ) 2 +2H + +2e (1)
2SC(NH 2 ) 2 (SCN 2 H 4 ) 2 2 + +2e (2)
The standard potential of the (SCN 2 H 3 ) 2 ∕SC(NH 2 ) 2 pair is 0.42V, which is 0.38 than the standard potential of the Au(SCN 2 H 4 ) 2 + ∕Au pair at 25 °C. V is high, so the dithiocarbamate formed by oxidation actually becomes an active oxidant in the process of gold dissolution at an appropriate pH, and reduces itself to thiourea. In an acidic medium at room temperature, thiourea is also oxidized to form products with higher oxidation states such as S 0 and HSO 4 - , SO 4 2 -, etc., but their reaction rate is very slow. NP Finkelstein also confirmed that even in solutions with a pH < 4, thiourea undergoes a small amount of acid decomposition to form H 2 S:
SC(NH 2 ) 2 H 2 S+CNNH 2
The stability of thiourea depends mainly on the pH of the medium, the concentration of thiourea and the temperature. At a suitable temperature, when the concentration of sulphur is constant, thiourea tends to be more stable as the pH of the medium decreases. Conversely, when the pH of the medium is constant, the thiourea becomes easier as the concentration of thiourea increases. Oxidized. In order to maintain the stability of thiourea during the gold dissolution process, a low pH thiourea solution should be used for the gold extraction operation. It is also only necessary to lower the pH of the solution to properly increase the concentration of thiourea in the solution.
Although the increase of temperature can accelerate the initial speed of thiourea dissolved gold, it will seriously affect the stability of thiourea, so that the speed of gold dissolution will decrease with time, or even invalid. In most literatures, the temperature of the selected thiourea solution is not higher than 25 ° C, although it is not necessarily the best choice, but the test proves that with the temperature of the medium (whether acidic, neutral or alkaline) Increasing the oxidation rate of thiourea will increase. When the thiourea solution is heated, hydrolysis occurs to produce ammonia, carbon dioxide and liquid H 2 S:
SC(NH 2 ) 2 +2H 2 O 2NH 3 +CO 2 +H 2 S
H 2 S can be further broken down into S 0 . When the thiourea solution is boiled, the thiourea is rapidly hydrolyzed to generate or fail with S 2 - , S 0 , HSO 4 - and SO 4 2 - and the like.
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