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Metal Profile: Manganese

What is manganese?

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Metal Profile: Manganese

Electrolytic manganese flakes.

Image © Strategic Metal Investments Ltd.

Manganese is a key component in the production of steel. Although classified as a minor metal, the quantity of manganese produced worldwide each year falls behind only iron, aluminum, copper and zinc.

Properties:

  • Atomic Symbol: Mn
  • Atomic Number: 25
  • Element Category: Transition Metal
  • Density: 7.21 g/cm³
  • Melting Point: 2274.8°F (1246°C)
  • Boiling Point: 3741.8° F (2061 °C)
  • Mohs Hardness: 6

Characteristics:
Manganese is an extremely brittle and hard, silvery-grey metal. The twelfth most abundant element in the earth's crust, manganese increases strength, hardness and wear resistance when alloyed in steel.

It is manganese’s ability to readily combine with sulphur and oxygen, which make it critical in the production of steel. Manganese's proclivity to oxidize helps to remove oxygen impurities, while also improving the workability of steel at high temperatures by combining with sulphur to form a high melting sulphide.

History:
The use of manganese compounds stretches back more than 17,000 years. Ancient cave paintings, including those in Lascaux France, derive their color from manganese dioxide. Manganese metal, however, was not isolated until 1774 by Johan Gottlieb Gahn, three years after his colleague Carl Wilhelm Scheele, had identified it as a unique element.

Perhaps the biggest development for manganese came nearly 100 years later when, in 1860, Sir Henry Bessamer, taking the advice of Robert Forester Mushet, added manganese to his steel production process in order to remove sulphur and oxygen. This increased the malleability of the finished product, allowing it to be rolled and forged at high temperatures.

In 1882, Sir Robert Hadfield alloyed manganese with carbon steel, producing the first ever steel alloy, which is now known as Hadfield steel.

Production:
Manganese is primarily produced from the mineral pyrolusite (MnO2), which, on average, contains more than 50% manganese. For use in the steel industry, manganese is processed into the metal alloys silicomanganese and ferromanganese. According to the International Manganese Institute, 11.7 million metric tons of manganese alloys were manufactured in 2009. Of this, silicomanganese accounted for 7.4 million metric tons and ferromanganese accounted for 4.3 million metric tons.

Ferromanganese, which contains 74-82 % manganese, is produced and classified as high carbon (>1.5% carbon), medium carbon (1.0-1.5% carbon) or low carbon (<1% carbon). All three are formed through the smelting of manganese dioxide, iron oxide and coal (coke) in a blast or, more often, an electric arc furnace. The intense heat provided by the furnace leads to a carbothermal reduction of the three ingredients, resulting in ferromanganese.

Silicomanganese, which contains 65-68% silicon, 14-21% manganese and about 2% carbon is extracted from the slag created during high carbon ferromanganese production or directly from manganese ore. By smelting manganese ore with coke and quartz at very high temperatures, the oxygen is removed while quartz converts to silicon, leaving silicomanganese.

Electrolytic manganese, with purities between 93-98%, is manufactured by leaching manganese ore with sulphuric acid. Ammonia and hydrogen sulphide are then used to precipitate unwanted impurities, including iron, aluminum, arsenic, zinc, lead, cobalt and molybdenum. The purified solution is then fed into an electrolytic cell and through an electro-winning process creates a thin layer of manganese metal on the cathode.

China is both the largest producer of manganese ore, accounting for about 22% of the manganese mined in 2009, and the largest producer of refined manganese materials (i.e. ferromanganese, silicomanganese and electrolytic manganese). In 2009, China produced 6.6 million metric tons of manganese alloys, approximately 57% of total global production, which included 64% of global ferromanganese production and greater than 95% of global electrolytic manganese production.

Applications:
About 90% of all manganese consumed annually is used in steel production. Of this, about 30% is used as a desulpherizer and deoxidizer, while the remaining 70% is used as an alloying agent.

Steel:
Standard steels have a manganese content of between 0.15-0.8%, which is introduced as ferromanganese or silicomanganese to deoxidize carbon steels. The silicon in the silicomanganese reacts with oxygen to silicate, which readily separates from steel.

According to the World Steel Association, global crude steel production in 2011 was in excess of 1.5 billion metric tons. Steel is used in countless applications from building, ship and bridge construction to automobiles, lampposts and shopping carts.

High strength low alloy (HSLA) steels contain 1.0-1.8% manganese and are used in oil and gas pipelines and shipbuilding, while engineering steels that alloy manganese along with other metals, including chromium, nickel, vanadium and molybdenum, are often used in the auto industry. High strength steels account for about 3.4% of all steel production

Stainless steels account for about 2% of global steel production and require roughly 1% manganese. Stainless steels are used to manufacture everything from oil and gas pipelines to kitchenware, surgical instruments and food storage equipment. High manganese content stainless steels (e.g. 200 series stainless steel) replace some nickel content with manganese, increasing the manganese content to 4-9%. These often also contain a high chromium content and are used in freight containers, lighting columns, hose clips and washing machine drums.

Once manganese content reaches 4%, steel becomes more brittle, however, this again decreases as the manganese content continues to increase. Steels containing 8-15% manganese can have tensile strengths of up to 863 MPa.

For this reason, the aforementioned Hadfield steel has a manganese content greater than 13%. Its high tensile strength makes it tough and durable; ideal for use in rock crushing equipment and railway components.

Non-magnetic steels, used in specialty applications such as the retainer rings for turbo alternators and collars on oilrigs and cryogenic steels, contain 10-12% manganese.

Other Alloys:
After steel, alloys with aluminum are the largest applications for manganese. Aluminum with a manganese content of about 1.5% has an increased resistance against galvanic corrosion. Aluminum alloys 3004 and 3104, which have manganese contents of 0.8-1.5%, respectively, are the alloys used most for beverage cans. Aluminum-manganese alloys and aluminum-magnesium-manganese alloys are also used in kitchenware, roofing and auto parts.

Small additions of manganese to molten copper (0.1-0.3%) deoxidise while improving castability and mechanical strength of the final product. Slightly larger quantities are also used to improve strength and workability.

Alloys combining 20% or more manganese with copper, have a high thermal expansion coefficient, a useful attribute in temperature control devices. They are also and non-magnetic, making them suitable for small watch parts.

Various electronic applications that require non-magnetic components use small quantities of manganese with alloys of zinc, gold, silver and bismuth.

Sources:
The International Manganese Institute. www.manganese.org
The World Steel Association.http://www.worldsteel.org
Newton, Joseph. An Introduction to Metallurgy. Second Edition. New York, John Wiley & Sons, Inc.

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