Mixed Hydroxide Precipitate (MHP)

Magnesium Oxide for Mixed Hydroxide Precipitate (MHP) Production in HPAL Processing

Mixed hydroxide precipitate (MHP) is a key intermediate in producing battery-grade nickel and cobalt. High-reactivity magnesium oxide (MgO) is increasingly used in HPAL circuits to selectively precipitate these metals.

Nickel is widely used in steel alloys, electroplating, batteries, and magnetic materials due to its excellent mechanical strength, ductility, magnetism, and chemical stability. Nickel has become a crucial energy metal due to its growing use in lithium-ion batteries. The International Energy Agency states “Electric car sales in 2023 were 3.5 million higher than in 2022, a 35% year-on-year increase. This is more than six times higher than in 2018, just 5 years earlier.”

The global nickel market can be divided into two segments. Class 1 nickel has higher purity (>99.8%) and lower iron content. It comes from nickel sulfide ores from Canada, Russia, and China, and is primarily used in battery cathodes. Class 2 nickel is lower purity (<99.8%) with a higher iron content and historically has primarily been used to produce stainless steel. It comes from equatorial laterite deposits in countries such as Indonesia and the Philippines. However, in recent years, laterite deposits are more commonly used to produce material such as MHP for battery cathodes.

According to the Nickel Institute, mixed hydroxide precipitate (MHP) has become increasingly popular over the last ten years. MHP is produced from laterite ores primarily in Indonesia and is an intermediate form of nickel that is precipitated along with cobalt.

Magnesium oxide is commonly used to precipitate the metals from sulfuric acid in the High-Pressure Acid Leach (HPAL) process. The wet ore is dissolved in sulfuric acid at 250°C. The acid solution is neutralized in several stages and washed, producing a solution primarily containing nickel and cobalt. Ground limestone is commonly used in the early stages while magnesium oxide is commonly used in the final stage.

The most common alkalis used for hydroxide precipitation are caustic soda (NaOH), lime (CaO), and magnesium oxide (MgO). Produced by the electrolysis of sodium chloride, caustic soda is typically more expensive than lime and magnesium oxide. The introduction of calcium oxide (or quicklime) can lead to the formation of insoluble calcium sulfate (or gypsum) increasing the volume of the precipitate. A 2021 article in the Journal Minerals Engineering states “MgO is more suitable for the hydrolytic precipitation of nickel and cobalt from the Mg-rich solution because no other ions can be introduced into the solution.”

Not only has mixed hydroxide precipitate and the HPAL process drastically changed the nickel supply chain, but it has disrupted the cobalt industry as well. Historically, the vast majority of cobalt was mined in the Democratic Republic of the Congo and co-produced with copper. However the Cobalt Institute states that in 2024, 12% of global output was from Indonesia. The Cobalt Institute’s Report states that for Indonesia this was an 82% year-over-year growth rate in cobalt production from 2023.

A 2025 article in the Resources Policy journal states “Federal policies in the United States are aimed at increasing domestic mining and refining of critical minerals such as nickel and cobalt.” The designation of a critical minerals list receives bipartisan support. Nickel was added to the critical minerals list in 2022, and cobalt has been considered strategically important for decades.

Despite broad public support for the development of a sustainable battery supply chain, many technical and economic challenges exist. However, magnesium oxide, a key chemical in the processing of nickel and cobalt, is widely available and produced synthetically in the United States.

Martin Marietta manufactures a high purity light burned grade of magnesium oxide in Manistee, Michigan. MagChem^®40 is a fine powder calcined between 700°C and 1000°C to produce optimal reactivity for hydrometallurgical applications as a nickel and cobalt precipitation reagent. A technical white paper regarding magnesium oxide for hydrometallurgy is available on the Magnesia Specialties website. Contact Martin Marietta to discuss MgO specifications, sampling, and supply for nickel and cobalt precipitation circuits.

• E.A. Holley, A. Malone, L. Fahle, N.M. Smith, J. Calderon, R. Eggert, D.E. Spiller, M.D. Bazilian, Nickel and cobalt from U.S. mines and refineries: Assessment of five dimensions of mineral availability, Resources Policy, Volume 108, 2025
• Kui Liu, Yu Wang, Hua Long, Yafei Cheng, Yao Wu, Yezheng Cai, Juantao Jiang, Recovery of cobalt and nickel from magnesium-rich sulfate leach liquor with magnesium oxide precipitation method, Minerals Engineering, Volume 169, 2021