Magnesium: Resources and Production – Part 2 – Occurrence, extraction, electrolytic process and thermal Reduction

Occurrence in nature

Among the magnesium minerals, the most common are the dolomite (a compound of magnesium and calcium carbonates, MgCO₃·CaCO₃), and magnesite (magnesium carbonate, MgCO₃)[2]. Other are carnallite (MgCl₂·KCl·6H₂O), brucite, Mg(OH)₂, olivine [(Mg,Fe)₂SiO₄], and serpentine [(Mg,Fe)₃Si₂O₅(OH)₄]. Magnesium is obtained by reducing magnesium oxide with silicon, or by the electrolysis of molten magnesium chloride. Some important magnesium minerals are shown in Figure 1.

Annual production of magnesium

Magnesium resources are spread all over the world. The metal could be recovered from seawater at many places along the world's coastlines. However, highly concentrated magnesium reserves are found to be only in three countries: Russia, China and Korea. China being rich in magnesite is the biggest producer of magnesium in the globe leaving others far behind. The United States also has rich magnesium reserves as brines. The Dead Sea and the Great Salt Lake in Utah are abundant magnesium resources famous around the world. The data of the world’s annual primary production of magnesium is given in Table 1 [2,3].

These figures are for primary production from the ores and do not include secondary production from recycled materials. Secondary magnesium is recovered from scrap at magnesium ingot and casting plants and from aluminium alloy scrap at secondary aluminium smelters. Only about 3 % of the total magnesium produced is annually recycled. An estimated quantity of approx. 27,000 tons of secondary magnesium was recovered in 2020. Aluminium-base alloys accounted for about 55 % of the secondary magnesium recovered, and magnesium-based castings, ingot, and other materials accounted for about 45 % [4].

In the early nineties, the production of magnesium in China was only around ca. 5 % (Fig. 2). Today, China is producing well above 80 % of the world's magnesium. Though the country has rich deposits of magnesium ores, magnesite and dolomite. But it was the rapid economic growth of the country, cheap labour, and the advantage of waste heat energy associated with coal gas production to drive the production, which they acquired virtually free by co-locating with coking ovens. The magnesium manufacturing processes consume huge amounts of energy. China’s low-cost magnesium revolution led to the shut-down of magnesium manufacturing plants in many countries, as it became uneconomic and unviable. Consequently, there are a lot of fluctuations in the magnesium annual production data from the various countries.Fig. 2: Global magnesium production (a) thousand metric tons [5], (b) % share [6]

Magnesium production Concentrating and Mining

Magnesium can be produced from magnesium chloride, which can be plentifully acquired from the oceans. Magnesium chloride is extracted using solar energy to vaporize a dynamic stream of preconcentrated seawater flowing along an inclined preferential salt separator. Naturally occurring magnesium-containing brines such as the Great Salt Lake and the Dead Sea, typically contain 1.1, and 3.4 % by weight magnesium, respectively. The magnesium chloride obtained from these sources is concentrated and dried in large ponds by solar evaporation to make it anhydrous, before extraction. The first magnesium extraction plant from seawater was established in 1948 by Dow Chemicals at their Freeport in Texas. It operated until 1998, but, presently, the only remaining saltwater magnesium production plant, the Dead Sea Magnesium Ltd., is a joint venture between Israel Chemicals Ltd. and Volkswagen AG [7].

The magnesium mineral dolomite and magnesite are mined and concentrated by conventional methods. Carnallite is dug either as ore or separated from other minerals that are brought to the surface by solution mining [8].

Extraction and refining

Magnesium is strongly reactive with oxygen and chlorine in both the liquid and gaseous states. This means that extraction of the metal from raw materials is an energy-intensive process requiring well-tuned technologies. Magnesium is commercially produced by two completely different methods:

• electrolysis of magnesium chloride or
• thermal reduction of magnesium oxide generally referred as 'Pidgeon process’

Before the expansion of production in China in the early 21st century, the electrolytic process was the preferred choice in countries where electrical energy is produced relatively cheaply. Chinese plants, however, use a modernized version of thermal reduction or Pidgeon Process. The Pidgeon process was originally developed in Canada by Dr. Lloyd Pidgeon in the 1940s to boost production during World War II. Though the Pidgeon process is both energy and labour-intensive, environmentally unfriendly as coal is used as its main energy source, and is less efficient than electrolysis, China’s low cost of labour and strategically cheap thermal energy enabled it to be economically viable.

The electrolytic process

The process involves two stages: the production of a feedstock containing magnesium chloride from sea water or brine, and the dissociation of this compound into magnesium metal and chlorine gas in electrolytic cells.

Production of feedstock of magnesium chloride from sea water or brine:

Where sea-water is the raw material. The dolomite is crushed, roasted, and converted to mixed oxides by heating to a high temperature. The sea water (or brine) is mixed with this dolomite in large tanks. Magnesium hydroxide precipitates, while calcium hydroxide remains in the solution. Magnesium hydroxide is filtered off and on heating readily forms the magnesium oxide. To produce molten magnesium chloride, the oxide is heated, mixed with coke, and reacted with a stream of chlorine at a high temperature in an electric furnace (Fig. 3). Magnesium chloride is then electrolyzed, releasing magnesium, which floats to the surface. The chemical reactions of the process can be represented as follows:

2MgO(s) + C(s) + 2Cl₂(g) → 2MgCl₂(s) + CO₂(g)

2Cl₂(g) + C(s) + H₂O(g) → 2HCl(g) + CO(g)                  <1>

MgO(s) + 2HCl(g) → MgCl₂(s) + H₂O(g)Fig. 3: High temperature electric furnace illustrating the production of MgCl2 from MgO [3]

In industrial processes, cell feeds consist of molten salts containing anhydrous or partially dehydrated magnesium chloride [9]. Partly dehydrated magnesium chloride is obtained by the Dow process, in which seawater is mixed in a flocculator with lightly burned reactive dolomite. An insoluble magnesium hydroxide precipitates, and settle down at the bottom of the tank, whence it is pumped as a slurry, filtered, converted to magnesium chloride by reaction with hydrochloric acid, and dried in a series of evaporation steps to 25 % water content. Final dehydration takes place during smelting. Anhydrous magnesium chloride is produced by two principal methods: dehydration of magnesium chloride brines (Norsk hydro process) or chlorination of magnesium oxide (I.G. Farben process).

Norsk Hydro process: This process developed by Norwegian aluminum and renewable energy companies in the late sixties is used where magnesium chloride-rich brines are the source of magnesium [10-12]. In this process the brines are first purified by removing impurities by precipitation and filtering. The purified brine containing about 8.5 % magnesium is concentrated by evaporation in several stages and converted to particulates. The last stage of dehydration is carried out in prilling towers in the presence of hydrogen chloride gas to avoid hydrolysis of the magnesium chloride:

Mg(OH)Cl(s) + HCl(g) → MgCl2(s) + H₂O(g)                     <2>

I.G. Farben process: In I.G. Farben chlorination method, lightly burned dolomite is mixed with seawater in a flocculator, where magnesium hydroxide is precipitated out, filtered, and calcined to magnesium oxide [9]. This is mixed with charcoal, formed into globules with the addition of magnesium chloride solution, and dried. The globules are charged into a chlorinator, a brick-lined shaft furnace, and heated at 1,000-1,200°C. Chlorine gas introduced ­through portholes in the furnace reacts with the magnesium oxide to produce molten magnesium chloride, which is tapped at intervals and sent to the electrolytic cells. This process was developed by I.G. Farben Industries of Germany in 1930 and is currently used in China, Russia and the United States.

The electrolysis of fused magnesium chloride:

The electrolytic cells (Fig. 4) are brick-lined vessels equipped with multiple steel cathodes and graphite anodes. These are mounted vertically through the cell hood and partially submerged in a molten salt electrolyte. The cell operates at 680 to 750 °C. Power consumption is 12 to 18 kW per hour per kilogram of magnesium produced. The electrolyte consists of alkaline chlorides mixed with 6 to 18 % of anhydrous magnesium chloride produced above. Electrolyte is continuously fed into cells, which are hot enough to melt it. On electrolysis, magnesium and chlorine are produced [13]:

(+) Anode:2Cl- + → Cl2 + 2e-                                         <3>

(–) Cathode:Mg2+ + 2e- → Mg

Chlorine and other gases are generated at the graphite anodes, and molten magnesium metal floats to the top of the salt bath, where it is collected and cast into ingots. The chlorine gas is recycled to the chlorination furnace.Fig. 4: Electrolysis of magnesium chloride [3]

Thermal reduction process

The thermal reduction process is an expensive method of production of magnesium. In this method dolomite ore is crushed and heated in a kiln to produce a mixture of magnesium oxide and lime (calcium oxides); a process known as calcining:

MgCO₃.CaCO₃(s) → MgO.CaO(s) + 2CO₂(g)                     <4>

The next step is reduction of the magnesium oxide by silicon. The reducing agent used is ferrosilicon (an alloy of iron and silicon) which is made by heating sand with coke and scrap iron, and typically contains about 80 % silicon.

The oxides are mixed with crushed ferrosilicon, and made into briquettes for loading into the reactor. Alumina may also be added to reduce the melting point of the slag. The reaction is carried out at 1250 – 1500 °C under very low pressure, close to vacuum. Under these conditions the magnesium is produced as a vapour which is condensed by cooling to about 825 °C in steel-lined condensers, and then removed and cast into ingots [1]:

2MgO(s) + Si/Fe(s) ↔ SiO₂(s) + 2Mg(g) + Fe(s)

2CaO(s) + SiO₂(s) → Ca₂SiO₄, or (2CaO·SiO₂) (l)                 <5>

The basic forward reaction is endothermic, it favours the equilibrium in favour of magnesium oxide. The heat requirements to initiate and sustain the reaction are quite high. To make it feasible at lower reaction temperatures, industrial processes operate under vacuum, and by removing the magnesium vapour as it is produced, the reaction goes to completion. The silica combines with calcium oxide to form the dicalcium silicate slag. The process produces magnesium with up to 99.99 % purity, slightly higher than from the electrolytic processes. There are three different methods to supplying heat, and separating slag.

Pidgeon process: In this original process, ground and calcined dolomite is mixed with finely ground ferrosilicon, briquetted, and charged into cylindrical nickel-chromium-steel retorts. A number of retorts are installed horizontally in an oil- or gas-fired furnace, with an attached condenser system extending out of the furnace. The furnace is heated to a temperature of 1200 °C under a reduced pressure of 13 pascals. After a reaction cycle of about 11 hours, magnesium crystals (called crowns) formed at condensers are removed, slag is evacuated as a solid, and the retort is recharged. The process requires manually filling and emptying of the vacuum tubes at each cycle, and uses about 11 tons of raw materials for every one ton of magnesium produced.

Bolzano process: Here dolomite-ferrosilicon briquettes are stacked on a special charge support system through which internal electric heating is conducted to the charge. A complete reaction takes 20 to 24 hours at 1200 °C below 400 pascals.

Magnetherm process: The Magnetherm process developed by Pechiney-Ugine-Kuhlman during 1963 in France. The process is currently used in France, U.S., and Japan. The dicalcium silicate slag produced in above process has a melting point of about 2000 °C and is therefore present as a solid. In Magnetherm process alumina or bauxite is added to the charge which acts as a catalyst and keeps the calcium silicate slag molten at a reduced temperature of 1550 – 1600 °C, so that it can be tapped in a liquid state [14]. The process has the advantage of directly heating the liquid slag by electric current through a water-cooled copper electrode. The reduction occurs at 1600 °C and 400 – 650 pascals pressure. Vaporized magnesium is condensed in a separate system attached to the reactor, and molten slag and ferrosilicon are tapped at intervals.

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