Thiourea

Thiourea, also known as urea sulfide, molecular formula for SCN2H4, is an organic compound, white with glossy rhombic hexahedron, soluble in water, aqueous solution is neutral. Thiourea gold extraction mainly has thiourea leaching pyrite gold concentrate, thiourea leaching chalcopyrite concentrate containing gold, thiourea leaching antimony concentrate containing gold, thiourea leaching arsenic antimony sulphur flotation gold concentrate, etc.; Thiourea gold extraction is to make the gold in the gold concentrate dissolved in the acidic thiourea solution containing oxidizing agent and be extracted. It has the advantages of fast speed of dissolving gold, small toxicity, easy to handle and renewable solution after removing gold.

Thiourea can dissolve gold for the test confirmed, in the presence of oxidizing agent, gold in the form of Au (SCN2H4) 2 + complex cation into the thiourea acidic solution. Thiourea dissolves gold is electrochemical corrosion process, other chemical equations can be expressed as follows:

Au + 2SCN2H4 = Au(SCN2H4)+2 +e

Selection of suitable oxidizing agent is the key issue of thiourea acid dissolution of gold, the more suitable oxidizing agent for Fe3+ and dissolved oxygen, so the chemical reaction equation of thiourea dissolution of gold can be expressed as:

Au+2SCN2H4 +Fe3+ = Au(SCN2H4)+2 +Fe2+

Au +1/4O2 +H+ +2SCN2H4 = Au(SCN2H4)+2 +1/2H2O

The precious liquid obtained from thiourea gold dissolution, according to its gold content, can be used iron, aluminum replacement or electrolytic deposition method of gold deposition, gold mud smelting to get composite gold. Gold mud melting process is the same as cyanide gold mud.

Thiourea gold leaching rate depends mainly on the medium pH value, oxidizer type and dosage, thiourea dosage, mineral composition and gold particle size, leaching temperature, leaching time and leaching process and other factors.

Thiourea is unstable in alkaline liquid, easy to decompose into sulfide and aminocyanide, but more stable in acidic medium. Therefore, from the stability of thiourea considerations, gold extraction generally use thiourea dilute sulfuric acid solution as leaching agent, and should pay attention to the first add acid after adding thiourea, so as to avoid the local temperature of the slurry is too high and make thiourea hydrolysis failure.

Medium acidity and thiourea concentration, acidity in the concentration of thiourea with the increase and decrease in the amount of thiourea at room temperature under the conditions of medium pH less than 1.5 is appropriate, but the acidity should not be too large, otherwise it will increase the amount of impurities of the acid dissolved.

Thiourea dissolved gold need to increase a certain amount of oxidizing agent, the more ideal oxidizing agent for manganese dioxide, dithiocarbamidine, high-value iron salts and dissolved oxygen. As long as the concentration of dissolved oxygen in the slurry is maintained when dissolving gold in thiourea acidic solution, the high-value iron salt can be regenerated.

Thiourea is an organic complex, in the acidic liquid and many metal cations can form complex cations, in addition to mercury, other metal thiourea complex cation stability is small, so the thiourea acidic liquid dissolution of gold has a high selectivity. But the raw material of copper, bismuth oxides will be acid soluble, and thiourea complex and reduce the effect of thiourea gold leaching and increase the dosage of thiourea, raw materials containing a larger amount of acid-soluble substances (such as divalent iron, carbonate, non-ferrous metal oxides, etc.) and reducing components will increase the consumption of oxidant and sulfuric acid, and reduce the leaching rate of gold. However, copper, arsenic, antimony, lead and other sulfide minerals on the thiourea dissolution of gold has less harmful effects, so the thiourea acid solution dissolution of gold can be selectively extracted from the complex difficult to select gold mineral raw materials of gold and silver.

Gold grain size is one of the factors affecting the gold leaching rate.

Thiourea dissolved gold rate with the leaching temperature increases and improve, but the thermal stability of thiourea is small, the temperature is too high and easy to hydrolyze and failure, the slurry temperature should not be more than 55 degrees Celsius, generally at room temperature for thiourea gold extraction.

The leaching rate of gold is generally increased with the increase of thiourea dosage, because thiourea gold mainly rely on high-valent iron ions as an oxidizing agent, the concentration of high-valent iron ions in the solution is much higher than the concentration of dissolved oxygen and can be adjusted, so thiourea dissolved in gold thiourea concentration is higher, the sulfur dosage with the raw material content of gold varies, and its unit consumption (kg/ton) for a few thousand to dozens of kilograms.

The leaching rate of gold generally increases with the increase of leaching time. Gold leaching rate and leaching process, the use of one-step method (such as charcoal slurry method, charcoal leaching method) gold extraction process can significantly reduce the leaching time.

Summary:

In the process of gold extraction, thiourea serves to dissolve gold from the ore in which it is present for further purification and processing. Specifically, thiourea reacts with gold to form a complex that is stable in an acidic solution, thus allowing the gold to dissolve. In order to make this process can be carried out smoothly, it is necessary to add oxidizing agent, in the role of oxidizing agent, thiourea and gold reaction is more rapid and complete, so as to improve the dissolution efficiency. It should be noted that although thiourea can dissolve gold, this process requires strict conditions and operational control. For example: temperature, acidity, concentration of thiourea and oxidizer will affect the dissolution effect!

Author：Shanghai Shijian Chemical