The Application of Thiourea in Gold Extraction from Gold Ore

Thiourea, also known as thiourea, has the molecular formula SCN₂H₄ and is an organic compound. It appears as white, lustrous rhombohedral hexagonal crystals and is highly soluble in water, with its aqueous solution being neutral in pH. Thiourea-based gold extraction primarily involves thiourea leaching of pyrite gold concentrate, thiourea leaching of gold-bearing chalcopyrite concentrate, thiourea leaching of gold-bearing antimony concentrate, and thiourea leaching of gold-bearing antimony-arsenic-sulfur flotation concentrate. Thiourea gold extraction involves dissolving gold from gold concentrate into an acidic thiourea solution containing an oxidising agent to extract the gold. This method has advantages such as fast gold dissolution, low toxicity, easy treatment of the solution after gold removal, and regenerability.

The ability of thiourea to dissolve gold has been experimentally confirmed. In the presence of oxidants, gold is transferred into the thiourea acidic solution in the form of the Au(SCN₂H₄)²⁺ complex cation. Thiourea gold dissolution is an electrochemical corrosion process, and other chemical equations can be expressed as follows:

Au + 2SCN₂H₄ = Au(SCN₂H₄)²⁺ + 2e

Selecting an appropriate oxidising agent is a key issue in thiourea acid gold dissolution. The most suitable oxidising agents are Fe³⁺ and dissolved oxygen. Therefore, the chemical reaction equation for thiourea gold dissolution can be expressed as:  

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

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

Thiourea gold extraction

The precious liquid obtained from thiourea gold dissolution can be processed using iron or aluminium replacement or electrowinning methods to precipitate gold, with the gold slurry then smelted to produce alloyed gold. The gold slurry smelting process is identical to that for cyanide gold slurry.

The leaching efficiency during thiourea gold extraction primarily depends on factors such as the pH of the medium, the type and amount of oxidising agent, the amount of thiourea, the mineral composition and gold particle size, leaching temperature, leaching time, and gold extraction process.

Thiourea is unstable in alkaline solutions and easily decomposes into sulfides and amino cyanide, but it is relatively stable in acidic media. Therefore, considering the stability of thiourea, a dilute sulfuric acid solution of thiourea is generally used as the leaching agent during gold extraction. It is important to add acid first and then thiourea to avoid local overheating of the slurry, which could cause thiourea hydrolysis and render it ineffective.

The acidity of the medium is related to the concentration of thiourea; as the thiourea concentration increases, the acidity decreases. Under normal temperature conditions and thiourea dosage, the pH of the medium should be below 1.5, but the acidity should not be too high, as this would increase the acid solubility of impurities.

When using thiourea for gold leaching, a certain amount of oxidising agent must be added. Ideal oxidising agents include manganese dioxide, dimethyl sulphide, high-valent iron salts, and dissolved oxygen. During thiourea acid leaching, maintaining the concentration of dissolved oxygen in the slurry allows high-valent iron salts to be regenerated.

Thiourea is an organic complex that can form complex cations with many metal cations in acidic solutions. Except for mercury, the stability of thiourea complex cations for other metals is low, thereby conferring high selectivity to thiourea acidic solutions for gold leaching. However, copper and bismuth oxides in the raw material are acid-soluble and can complex with thiourea, thereby reducing the gold leaching efficiency of thiourea and increasing thiourea consumption. When the raw material contains a high amount of acid-soluble components (such as ferrous iron, carbonates, and non-ferrous metal oxides) and reducing components, it increases the consumption of oxidising agents and sulphuric acid and reduces the gold leaching rate. However, sulfide minerals such as copper, arsenic, antimony, and lead have a relatively minor adverse effect on thiourea leaching, so thiourea acidic liquid leaching can selectively extract gold and silver from complex, difficult-to-process gold ore raw materials.

The size of gold particles is one of the factors affecting gold leaching rates.

The leaching rate of thiourea increases with rising leaching temperature, but thiourea has low thermal stability, and excessive temperature can cause hydrolysis and render it ineffective. The slurry temperature should not exceed 55°C, and thiourea gold extraction is typically conducted at room temperature.

Gold leaching efficiency generally increases with higher thiourea dosage. Since thiourea gold extraction primarily relies on high-valent iron ions as oxidants, the concentration of high-valent iron ions in the solution is significantly higher than dissolved oxygen concentration and can be adjusted. Therefore, thiourea gold extraction requires higher thiourea concentrations, with thiourea dosage varying depending on raw material gold content, typically ranging from several kilograms to dozens of kilograms per tonne.

The leaching rate of gold generally increases with the extension of leaching time. The leaching rate of gold is related to the gold extraction process. Using a one-step method (such as carbon slurry method or carbon leaching method) for gold extraction can significantly shorten the leaching time.  

Summary:

In the process of extracting gold, thiourea acts to dissolve gold from the ore in which it is present, enabling further purification and processing. Specifically, thiourea reacts with gold to form a complex that remains stable in acidic solutions, thereby facilitating the dissolution of gold. To ensure this process proceeds smoothly, oxidising agents must be added. Under the influence of oxidising agents, the reaction between thiourea and gold occurs more rapidly and completely, thereby enhancing dissolution efficiency. It is important to note that while thiourea can dissolve gold, this process requires strict control of conditions and operations. For example, factors such as temperature, acidity, and the concentrations of thiourea and oxidising agents all influence the effectiveness of dissolution.