In the hydrometallurgical processes of non-ferrous metals such as gold, silver, and copper, acids (e.g., sulfuric acid, hydrochloric acid) or alkalis (e.g., sodium hydroxide, ammonia) are commonly used as leaching agents or regulators. Since many reactions are extremely sensitive to pH, controlling the acidity and alkalinity of the solution becomes crucial. Magnesium oxide (MgO), as a slow-reacting, low-solubility alkaline material, plays an important role as a “buffer” in these processes.
I. Buffering Mechanism of Magnesium Oxide
Magnesium oxide reacts with water to form magnesium hydroxide, which is sparingly soluble but strongly alkaline. When the solution becomes acidic, the hydroxide ions (OH⁻) released neutralize H⁺, thereby increasing the pH.
Sustained Alkali Release: Compared to strong alkalis such as NaOH, magnesium oxide releases alkalinity slowly, resulting in more stable pH regulation and avoiding drastic fluctuations.
II. Application Examples
1. pH Adjustment in Copper Ore Leaching:
During the sulfuric acid leaching of copper ore, the addition of magnesium oxide can control the pH between 1.5 and 2.0, which is conducive to the selective leaching of copper while inhibiting the dissolution of impurities such as iron (Fe), aluminum (Al), and silicon (Si).
2. Alkalinity Control in Gold Ore Cyanidation:
In the cyanidation process of arsenic-bearing gold ores, magnesium oxide can be used to maintain the solution pH > 10, preventing the escape of hydrogen cyanide (HCN) and simultaneously passivating the activity of arsenic, reducing its interference with gold encapsulation.
III. Advantages of Magnesium Oxide Buffering
- Good sustained release: Slowly dissolves, making it difficult to cause drastic pH changes.
- Operational safety: Milder than NaOH, less likely to corrode equipment or harm personnel.
- Strong precipitation control: Helps form stable hydroxide precipitates.
- Targeted precipitation of impurities in different pH ranges: Such as iron, aluminum, and arsenic.
IV. Issues to Consider in Application
- Low dissolution rate: It is necessary to control the particle size or increase the temperature to improve the reaction rate.
- Excessive amount easily generates precipitates that encapsulate metals: Inhibiting metal ion migration or leaching.
- Impurity content needs to be controlled: Impurities in industrial-grade magnesium oxide (such as SiO₂, CaO) may affect the metallurgical effect.
V. Future Research Directions
- Development of high-activity magnesium oxide: Improving reaction activity through calcination processes or the addition of additives.
- Composite buffer systems: Combining MgO with other neutralizing agents to optimize pH regulation.
- Intelligent dosing systems: Achieving precise buffer control through online monitoring and intelligent control systems.