Magnesium oxide (MgO), a common basic oxide, plays various physicochemical roles in the nickel extraction process. It can be used as an additive, mineral modifier, or reaction regulator in both pyrometallurgical (such as roasting and reduction smelting) and hydrometallurgical (such as acid leaching) processes. The specific mechanisms of action are as follows:
1. Improving Slag Fluidity and Enhancing Metal-Slag Separation (Pyrometallurgy)
In pyrometallurgical processes (especially smelting and reduction roasting), the viscosity and melting point of the slag directly affect the separation efficiency of metals (nickel, iron) from the slag.
Adding an appropriate amount of magnesium oxide can react with silicate minerals (such as olivine and serpentine) to form low-melting-point magnesium silicate compounds (such as forsterite), reducing slag viscosity and improving the enrichment and precipitation of nickel.
This is conducive to the formation of a clear two-phase interface, increasing the recovery rate of nickel-iron alloys.
2. Promoting the Solidification of Harmful Elements and Reducing Nickel Loss
Nickel ores often contain impurities such as iron, chromium, and aluminum, which, if unstable, can form difficult-to-handle intermediate products during smelting.
Magnesium oxide can react with these impurities to form stable composite minerals, such as spinels (MgAl₂O₄, MgCr₂O₄), reducing their solubility in the metal phase and thus increasing the purity of nickel products.
3. Promoting the Transformation of Beneficial Mineral Phases and Releasing Nickel Elements
In high-magnesium nickel ores, nickel often exists in solid solution within the structure of olivine, serpentine, etc., making direct reduction difficult.
Magnesium oxide at high temperatures helps to modify these mineral structures, promoting the migration and precipitation of nickel from the crystal lattice.
During the reduction roasting process, magnesium oxide may facilitate the precipitation of nickel from the solid solution into an independent phase by adjusting the atmosphere or forming new crystal defects.
4. Regulating the Acidity, Alkalinity, and Thermal Stability of the Reaction System
In some combined processes (such as roast-leach), the basicity of magnesium oxide can neutralize some acidic components, regulate the pH of the system, and stabilize the reaction environment.
Magnesium oxide has a high melting point and thermal stability and is not easily decomposed at high temperatures, which can also provide structural support.
5. Possible Adverse Effects and Precautions
Excessive addition of magnesium oxide can significantly increase the viscosity of the slag, making smelting difficult and potentially encapsulating nickel particles, leading to a decrease in recovery rate.
Magnesium oxide is difficult to dissolve in some hydrometallurgical systems, which can increase the cost of residue treatment.
The effects of magnesium oxide also vary in different types of nickel ores (laterite nickel ore, nickel silicate ore, nickel sulfide ore), and specific analysis based on mineralogical characteristics is required.