Converting The Magnesium Liabilities into Opportunities in Nickel Laterite Hydrometallurgy Process

Nickel saprolite ore typically contains 8–15% magnesium (Mg), making it the second or third most abundant element after silica (SiO₂) and iron (Fe). In nickel hydrometallurgy, more than 90% of Mg is dissolved under acid leaching conditions. Stoichiometrically, 1 g Mg consumes ~4 g H₂SO₄. For example, processing 1 Mt of saprolite ore with 10% Mg requires about 400,000 t H₂SO₄, dissolving roughly 90,000 t Mg into solution.

Heap leaching of low-grade saprolite produces a pregnant leach solution (PLS) with Mg as the predominant impurity. High Mg levels not only drive excessive acid consumption but also create downstream processing challenges and increase effluent salinity, making discharge to surface waters environmentally problematic.

Indoterra Metallurgy Solutions has evaluated the recovery of dissolved magnesium from heap-leach effluent through evaporation and crystallization, producing high-purity magnesium sulfate. The process achieved 67% Mg recovery, yielding Epsomite (MgSO₄·7H₂O) with 93.3% purity, a product widely used in agriculture. The economic assessment indicates a margin of ~US$69 per ton, demonstrating the potential to transform waste streams into valuable by-products while enhancing both environmental sustainability and strengthening the economic viability of low-grade saprolite hydrometallurgy.