1. Principle: thiourea forms a cationic complex with gold under the presence of acid and oxidant. The ideal oxidants are manganese dioxide, two sulfur amiprid, high valent iron salts and dissolved oxygen. In industrial production, Fe3+ and dissolved oxygen are usually used as oxidants, so the chemical reaction formula for thiourea dissolution is:
Au + 2 (NH2) 2CS + Fe3+ = Fe2+ + Au[(NH2) 2CS]2+ 4Au + 8 (NH2) 2CS + 4Au * = (()) + +
2. Advantages: 1) Thiourea is nontoxic or low toxic, much less toxic than cyanide, and its lethal dose is 10g/kg body weight. 2) The leaching speed of gold is fast. In general, the leaching rate is very high after less than 4 hours, but the cyanide leaching time is longer. 3) Under acidic conditions, gold can be extracted from materials that are unstable in alkaline solutions and can react directly with cyanide. 4) suitable for handling acidic materials. When a variety of metals are treated and recovered comprehensively by acid leaching (i.e. other valuable metals are recovered by acid leaching first), thiourea method does not need to wash residue with water. A small amount of base metal has little effect on the process. 5) The leaching tailings and wastewater can be treated without special treatment. The process is simple and the dehydration process can be omitted. The acid-resistant non-metallic leaching equipment can be used to save steel.
3, shortcomings: 1), reagent consumption is large, mainly thiourea oxidative decomposition, accounting for about 20% of the total consumption. 2) the technology needs further study and improvement. 3) The elemental sulfur produced in the gold leaching process is highly dispersed (very small) and easily wrapped on the surface of the material, thus passivating the gold surface and delaying or preventing the gold leaching. 4) it is not necessarily suitable for some minerals that consume acid substances, such as carbonate minerals, iron hydroxide, etc. Because the presence of these minerals in acidic systems consumes a lot of acid.
