The organic complexing agent is thiourea, can form complexes with many metal ions.
It is a white glossy diamond-shaped six-sided crystal, which is bitter, slightly toxic and non-corrosive. It has a density of 1.405g/cm 3 and a melting point of 180-182 ° C. It decomposes at a higher temperature and is easily soluble in water. The solubility in water is 9%-10% (142g/L) at 20 °C, and the heat of dissolution is 22.57kJ/ d, its aqueous solution is neutral.
In the past 40 years, due to the rapid development of physical technology, the chemical structure of thiourea molecules has been further identified as the following resonance:
That is, it is a pair of free electrons possessed by N and s atoms in the molecule, adsorbed on the surface of the gold particles, and the redox potential of gold is greatly reduced, so that the gold is easily oxidized and dissolved into the human solution.
Thiourea is unstable in alkaline solution and easily decomposes into sulfide and cyanamide:
SC(NH 2 ) 2 +2NaOH====Na 2 S+CNNH 2 +2H 2 O
The cyanamide produced by decomposition can be converted into urea:
CNNH 2 +H 2 O=====CO(NH 2 ) 2
This results in the consumption of thiourea. In an alkaline medium, 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 reducing properties in acidic (pH 1-6) solutions and can be oxidized to produce a variety of products. It is relatively easy to oxidize to dithiocarbamidine (SCN 2 H 3 ) 2 at room temperature:
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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 thiourea 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 more easily as the concentration of thiourea increases. Oxidation. 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. Only by lowering the pH of the solution can the thiourea concentration in the solution be appropriately increased. The main thermodynamic data of 298K thiourea are shown in Table 1.
Table 1 Â Main thermodynamic data of thiourea | ||||
Molecular formula | status | △Hf Ө /kJ | S Ө / ( J·K -1 ) | △G f Ө /kJ |
CS ( NH 2 ) 2 | Crystal | -92.4 | 302.8 | -36.66 |
CS ( NH 2 ) 2 | Aqueous solution | -59.8 | 383.8 | -38.2 |
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 degree of gold dissolution will continue to 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 thiourea is heated to 60 ° C in an acidic or alkaline solution, hydrolysis occurs to produce ammonia, carbon dioxide and liquid H 2 S:
SC(NH 2 ) 2 +2H 2 O====CO 2 +2NH 3 +H 2 S
H 2 S can be further broken down into S. When the thiourea solution is boiled, the thiourea fails due to rapid hydrolysis to form S 2- , S, HSO 4 - and SO 4 2- .
An important characteristic of thiourea is the formation of stable complex cations with transition metal ions in aqueous solution. The reaction formula is as follows:
Me n+ +x(TU)====[Me(TU) x ] n+
Of which: Tu-thiourea; n-valence; x-coordination number.
The cumulative production constants β x of several metal thiourea complexes are listed in Table 2. [next]
Table 2 Â Cumulative generation constant of metal thiourea complex | |||
Complex ion | Igβ x | Complex ion | Igβ x |
Au ( TU ) 2 + | 21.98 21.50 | Zn ( TU ) 2 2+ | 1.77 |
Ag ( TU ) 3 + | 13.1 | FeSO 4 ( TU ) + | 6.44 |
Cu ( TU ) 4 2+ | 15.4 | Hg ( TU ) 4 2+ | 26.3 |
Cd ( TU ) 4 2+ | 3.55 | Hg ( TU ) 2 2+ | 21.9 |
Pb ( TU ) 4 2+ | 2.04 | Bi ( TU ) 4 2+ | 11.94 |
*Measure standard potential calculation Au+2(TU)=====Au(TU) 2 + +e - | |||
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