The thiourea soluble gold was confirmed by experiments. In the presence of oxidant, gold was transferred into the thiourea acidic solution in the form of Au(SCN2H4)2+ complex cation. Thiourea gold is an electrochemical corrosion process, and other chemical equations can be expressed by the following formula:
Au + 2SCN 2 H 4 = Au(SCN 2 H 4 ) + 2 +e
Choosing a suitable oxidant is a key issue for thiourea acidic gold. The preferred oxidant is Fe 3+ and dissolved oxygen. Therefore, the chemical reaction formula of thiourea gold can be expressed as:
Au+2SCN 2 H 4 +Fe 3+ = Au(SCN 2 H 4 ) + 2 +Fe 2+
Au +1/4O 2 +H+ +2SCN 2 H 4 = Au(SCN 2 H 4 ) + 2 +1/2H 2 O
The precious liquid obtained by dissolving gold in thiourea can be immersed in gold by iron or aluminum replacement or electrowinning according to the amount of gold contained in the gold, and the gold mud is smelted to obtain the combined gold. The gold mud smelting process is the same as that of cyanide gold mud.
The leaching rate of thiourea dissolved in gold mainly depends on the pH value of the medium, the type and amount of oxidant, the amount of thiourea, the mineral composition and gold particle size, the leaching temperature, the leaching time and the immersion gold process.
Thiourea is unstable in alkaline liquid and easily decomposes into sulfide and cyanamide. However, thiourea is relatively stable in acidic media. Therefore, from the stability of thiourea, the thiourea dilute sulfuric acid solution is generally used as the leaching agent in the thiourea gold extraction, and it should be noted that the thiourea is added after the acid is added first, so as to prevent the thiourea from being hydrolyzed and failed due to the excessive local temperature of the slurry.
The acidity of the medium is related to the concentration of thiourea. The acidity decreases with the increase of thiourea concentration. Under the condition of thiourea at room temperature, the pH value of the medium is less than 1.5, but the acidity should not be too large, otherwise the acid solubility of impurities will increase.
When thiourea dissolves gold, a certain amount of oxidant needs to be added. The preferred oxidants are manganese dioxide, dithioformamidine, high-valent iron salts and dissolved oxygen. When the thiourea acidic solution is dissolved in gold, as long as the concentration of dissolved oxygen in the slurry is maintained, the high-priced iron salt can be regenerated.
Thiourea is an organic complex. It can form complex cations with many metal cations in acidic solution. In addition to mercury , the stability of thiourea complexes of other metals is small, so thiourea acidic liquid gold has high selectivity. . However, the raw material of copper, bismuth oxide will be acid soluble, complexed with thiourea and reduced Thiourea Thiourea effect and increase the amount of raw materials containing higher amounts of acid soluble material (e.g., a divalent iron carbonate , non-ferrous metal oxides, etc.) and reducing components increase the consumption of oxidants and sulfuric acid, and reduce the leaching rate of gold. But less harmful effects of copper, arsenic, antimony, lead and other sulfide minerals thiourea gold dissolution, thus an acidic thiourea solution gold dissolution can be extracted from a complex gold silver metal selectively refractory mineral raw materials.
Gold particle size is one of the factors affecting the gold leaching rate.
The thiourea dissolution rate increases with the leaching temperature, but the heat stability of thiourea is small, the temperature is too high, and it is easy to be hydrolyzed and failed. The temperature of the slurry should not exceed 55 degrees, and thiourea is generally extracted at room temperature.
The leaching rate of gold generally increases with the increase of the amount of thiourea. Since thiourea is mainly based on high-valent iron ions as oxidant, the high-valent iron ion concentration in the solution is much higher than the dissolved oxygen concentration and can be adjusted, so the thiourea dissolves gold. The concentration of thiourea is relatively high, and the amount of sulfur varies depending on the amount of gold contained in the raw material, and its unit consumption (kg/ton) is several kilograms to several tens of kilograms.
The leaching rate of gold generally increases as the leaching time increases.
The leaching rate of gold is related to the immersion gold process. The one-step method (such as carbon slurry method and carbon leaching method) can significantly shorten the immersion time.
The thiourea method is a new non-toxic gold extraction process. This method has been used in China to treat re-election gold concentrates and flotation gold concentrates. However, this process still has a problem of high cost.
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