Messi Biology explains that the role of magnesium oxide is primarily manifested in hydrometallurgy (such as gold cyanidation, thiosulfate method, chlorination, etc.). Magnesium oxide is not a primary reagent directly involved in the gold extraction reaction but plays a crucial role as a pH regulator, neutralizing agent, or precipitating agent. The detailed mechanisms are analyzed below:
1. pH Regulator and Neutralizing Agent
- Gold extraction often employs the cyanide leaching method, and the cyanidation system is highly sensitive to pH, which is typically maintained between 10.5 and 11.5.
- Magnesium oxide (MgO) is a weakly alkaline oxide that slowly dissolves to form magnesium hydroxide (Mg(OH)₂). This is used to maintain the alkalinity of the system, preventing the volatilization of hydrogen cyanide (HCN) gas and ensuring the efficiency of gold leaching and operational safety.
- Compared to traditional lime (CaO), magnesium oxide has a slow-release effect, providing a more gentle pH adjustment, which is suitable for fine control.
2. Reducing the Introduction of Calcium Ions to Avoid Scaling and Precipitation Issues
- Lime is widely used in the gold extraction process, but it introduces a large amount of Ca²⁺, which can form scale (e.g., CaCO₃, CaSO₄) with carbonate or sulfate ions, leading to pipeline blockages or affecting filtration performance.
- Replacing a portion of lime with magnesium oxide can reduce the concentration of Ca²⁺, mitigating problems with scaling and precipitation and improving process stability.
3. Precipitating Impurity Metal Ions to Enhance Selective Gold Leaching
- During the cyanide leaching process, impurity metals such as copper, zinc, and iron can also enter the solution, competing for cyanide ions and affecting gold recovery.
- Under alkaline conditions, magnesium oxide can promote the precipitation of certain impurity metals (such as Fe³⁺, Al³⁺, Cu²⁺) in the form of hydroxides, which helps to increase the selective dissolution rate of gold.
4. Application in the Thiosulfate Method
- Magnesium oxide can be used as a neutralizing agent in the thiosulfate gold extraction method to adjust the alkalinity of the leach or desorption solution.
- It is compatible with the stability of thiosulfate and does not easily cause its decomposition, making it suitable for complex gold ores and “refractory ores” containing elements like arsenic and carbon.
5. Environmental Protection and Tailings Treatment
- The tailings liquid after gold extraction still contains residual cyanide, metal ions, and other pollutants that require neutralization and treatment.
- Magnesium oxide can be used to neutralize acidic substances in the tailings and remove metal impurities through precipitation. It serves as an environmental treatment agent for tailings, particularly suitable for wastewater containing chromium, copper, and iron.