Magnesium oxide (MgO) plays a crucial role in the processing of laterite nickel ore, going beyond a simple neutralizing or precipitating agent to become a key process control tool. It enables precise pH adjustment, improves impurity control, enhances nickel and cobalt recovery efficiency, and boosts product quality, making it indispensable in green metallurgy and the new energy material industry chain.
I. Background: Characteristics and Processing Challenges of Laterite Nickel Ore
Laterite nickel ore is a significant component of global nickel resources. Its characteristics include:
- Low nickel and cobalt content, typically 1–2% Ni and <0.2% Co
- High content of impurities such as iron, aluminum, silicon, and magnesium
- Strong reducing properties, easily reacting with acids to form complex ionic systems
- Categorized into magnesian and ferruginous types
The main processing methods for laterite nickel ore include:
- High-Pressure Acid Leaching (HPAL)
- Atmospheric Acid Leaching (AL)
- Reduction Roasting-Magnetic Separation
- Ammonia Leaching (less commonly used)
In the acid leaching process, the introduction and use of magnesium oxide play a crucial role in controlling the reaction system, selective metal precipitation, and impurity removal.
II. Main Functions of Magnesium Oxide
- pH Adjuster and Neutralizer:Magnesium oxide is a basic oxide that reacts quickly with acids to generate Mg²⁺ and OH⁻, neutralizing the acidic solution and increasing the system’s pH, which helps selectively precipitate Ni and Co and avoid metal loss.
- Precipitant – Nickel and Cobalt Precipitation:In acid leaching solutions, magnesium oxide provides a stable source of OH⁻, causing Ni²⁺ and Co²⁺ to form hydroxide precipitates, facilitating separation and extraction:
(Equations omitted for brevity, but conceptually: Ni²⁺ + 2OH⁻ -> Ni(OH)₂↓ and Co²⁺ + 2OH⁻ -> Co(OH)₂↓) - Inhibiting Impurity Co-precipitation:Using magnesium oxide to control the pH within a suitable range (e.g., 9–10) can inhibit the precipitation of impurity ions such as iron, aluminum, and manganese, improving product purity.
- Reducing Carbon Footprint and Calcium Ion Contamination:Compared to traditional lime (CaO) or lime milk (Ca(OH)₂), magnesium oxide does not introduce Ca²⁺ impurities, avoiding interference with subsequent precursor synthesis. It also has lower carbon emissions, making it environmentally friendly.
III. Specific Roles of Magnesium Oxide in Different Processes
- Treatment of Residual Liquid after High-Pressure Acid Leaching (HPAL):The HPAL process generates acidic leachate that needs to be neutralized before Ni/Co recovery. Magnesium oxide is used as a neutralizer in this stage, offering advantages such as slow release, pH control, and reduced pollution.
- Selective Precipitation of Ni/Co:Appropriately controlling the amount of magnesium oxide to maintain the system pH between 9 and 10 can effectively selectively precipitate Ni/Co, while Fe³⁺ and Al³⁺ remain dissolved, making them easier to remove.
- Synergistic Effect with Magnesian Laterite Ore:Some laterite ores contain Mg (e.g., magnesite). During the acid leaching process, Mg²⁺ will be dissolved. The addition of magnesium oxide can further increase the Mg²⁺ concentration, regulate the system’s buffering capacity, and facilitate Ni/Co precipitation.
IV. Application Advantages and Precautions
Advantages:
- Fewer impurities are introduced, improving the quality of subsequent materials (especially suitable for ternary precursor production)
- More stable pH control and more controllable precipitation conditions
- Does not form calcium-magnesium hard precipitates or affect ion exchange
- Reduces sludge volume, facilitating environmental governance
Precautions:
- Low-activity magnesium oxide does not react sufficiently, leading to metal loss
- Unreasonable particle size distribution can affect the precipitation rate and particle morphology
- Excessively high Mg²⁺ concentrations may affect some subsequent processes (e.g., crystallization, ion exchange)
V. Future Development Directions and Research Suggestions
- Develop dedicated magnesium oxide materials with high activity and low impurities to adapt to different laterite ore processing conditions
- Use in combination with other precipitants (e.g., MgO + Na₂CO₃) to improve selectivity and efficiency
- Construct a dynamic pH regulation system for magnesium oxide to achieve precise pH control and selective precipitation in continuous flow
- Study the enrichment behavior of Mg²⁺ in the recycling system and optimize wastewater treatment processes.