The electrolytic process
Industrial magnesium was first produced on a large scale in 1886 using a continuous process known as the electrolytic process. This typically involves three main feeds — Farben (fully anhydrous magnesium chloride), Dow (partially dehydrated magnesium chloride) and VAMI (dehydrated carnallite). Magnesium is produced through the molten salt electrolysis of magnesium chloride, the chloride is then extracted and sold, and the magnesium is cast directly or purified by vacuum distillation where required.
The electrolytic process offers a high-purity magnesium product with moderate labour requirement and process intensity. However, it can be a complex and costly method due to the required electrolyte preparation and comes with a high-capital cost.
The Pidgeon process
The Pidgeon process of producing silicothermic magnesium was invented in Canada in the 1940s, it’s operated as a batch process as follows:
- Dolomite is calcined to dolime
- Ferrosilicon is prepared by carbothermic reduction
- The dolime and ferrosilicon are briquetted and loaded into a stainless-steel retort
- The material is heated to 1000 degrees celsius under a vacuum
- Pure magnesium condenses in a crown formation at 20kg per batch, while a large amount of slag is also formed
- The reactor is then stripped and recharged.
This process results in a high-purity magnesium product with a low capital cost, however it requires a high level of labour and energy use, and comes with significant carbon dioxide and slag by-product.
The carbothermal process
Fritz Johann Hansgirg (1891–1949) was an Austrian electrochemist and metallurgist who in 1928 invented the carbothermic magnesium reduction process. In May 1940, Hansgirg arrived in the United States where he joined with the American industrialist Henry J. Kaiser. Due to the pending war aircraft engineering demand increased in the light magnesium alloys. Kaiser paid $750,000 to Winter for the patent in the carbothermic process and received government credits of about $22 million to build the defense plant of Permanente Metals Corporation (PMC) in California. By the start of World War II, the first unit at Kaiser's plant was producing about 5 short tons (4.5 long tons) of magnesium per day. Nine days after 7 December 1941, the attack on Pearl Harbour plunged the United States into World War II, Hansbirg was arrested by the FBI on a presidential Warrant accused of being potentially dangerous to the public peace and safety of the United States. Post the War plants born from carbothermic technology in Korea, Austria, and the US were closed or dismantled.
The carbothermal process is chemically straightforward — at high temperatures, carbon reduces metal oxide to pure metal and carbon monoxide. This requires a low level of energy input with a direct and efficient reaction, it’s applied at an industrial scale for iron and silicon production. While it requires high production intensity, it offers inexpensive reactants and a potentially continuous process with low environmental impact.