Industrial and Metallurgical
Manganese and Steelmaking
Steel is basically an alloy of iron and carbon, consisting of an iron phase and iron carbides. Crude steel produced from iron contains an undesirable amount of oxygen and some sulphur. Manganese plays a key role because of two important properties: its ability to combine with sulphur and its powerful deoxidation capacity. When there is insufficient manganese the sulphur combines with iron to form a low melting point sulphide, which melts at hot rolling temperatures, causing a surface cracking phenomenon known as “hot shortness”. Desulphurisation processes reduce the need for manganese in this respect. Some 30% of the manganese used today is still used for its properties as a sulphide former and deoxidant.
The other 70% of the manganese is used purely as an alloying element. These alloying uses depend on the desired properties of the steel being made. Steel, as has been noted, contains iron and carbon. At room temperature, iron crystallises into a body-centred cubic structure named alpha iron (ferrite). At a high temperature (above 910 degrees C), the structure is transformed into a face-centred cubic form, which is called a gamma iron (austenite). When the steel is cooled down slowly, the carbon, soluble in austenite, precipitates as an iron carbide called cementite, the austenite transforms to ferrite and they precipitate together in a characteristic lamellar structure known as pearlite.
Manganese plays an important role as it lowers the temperature at which austenite transforms into ferrite, thus avoiding cementite precipitation at ferrite grain boundaries, and by refining the resulting pearlitic structures. The strength and toughness of steel depend, first of all, on the grain size and the volume fraction of pearlite contained. Alloying elements, including manganese, also contribute some solution-hardening of the ferrite, but this effect is limited compared to that of carbon, nitrogen, phosphorus and even silicon. When the cooling process is accelerated by quenching, austenite transforms into structures with high strength such as bainite and martensite.
Manganese improves the response of steel to quenching by its effect on the transformation temperature. Manganese is also a weak carbide former. Both properties are advantageous in heat-treated steels specified by mechanical engineers. Another important property of manganese is its ability to stabilize the austenite in steel, as does nickel. Since manganese is not as powerful as nickel in its ability to stabilize austenite, more manganese is required to achieve the same effect. However, manganese has the advantage of being much less expensive. The effect of manganese in forming austenite can be reinforced by combining it with nitrogen, which is also an austenite-forming element. Manganese also increases hardenability rate, used to significant advantage, depending on the steel type and the end product, to improve mechanical properties.
Manganese Content in Steel Today
The bulk of steel production results in multi-purpose low carbon steels containing from 0.15% to 0.8% manganese. A large proportion consists of low carbon steel sheets with less than 0.3% manganese, some with even less than 0.2% for extra deep drawing qualities. High strength steels with a yield strength over 500MPa, and representing 3 to 4% of the tonnage of steel produced, contain over 1% manganese. A large number of these are high strength low alloy steels (HSLA). They are low carbon, controlled-rolled steels containing higher manganese levels (1.0% to 1.8%), taking advantage of its beneficial effect on the austenitic transformation temperature to obtain a very fine ferrite structure. Micro-alloying additions help to refine the structure or to strengthen the steel through carbide or nitride precipitates which are evenly distributed in the ferrite matrix. These steels are widely used for oil/gas pipelines, shipbuilding and in transportation equipment in order to reduce weight.
Engineering steels comprise either HSLA or heat-treated grades; either pure chromium-manganese grades, or with nickel, chromium, molybdenum, vanadium additives and often 0.6 to 0.8% manganese. A few grades containing 1.0% to 1.5% manganese (as well as chromium or boron) are popular with the automobile industry.
Stainless steels which represent less than 2% of total world steel production make use of chromium and nickel. They also contain about 1% manganese. There are also manganese-stainless steels, where nickel is replaced partly or entirely by manganese, giving a manganese content of 4 to 16%. These are not yet produced in large quantities, but they could develop in the future depending on the evolution of the nickel price compared with manganese and on the marketing effort devoted to them. Most commonly known as Series 200 stainless, this high Mn content stainless steel uses Manganese metal. China is the largest producer of Mn metal, producing over 470,000 mt, 90% of the world’s production, in 2004. Series 200 was originally created in India, and is still made there as well.
Any overview of high Mn steels must include Hadfield steel, named after its 19th century UK inventor and the first alloy steel ever invented. This steel contains 13% or more manganese. It has unique properties which make it indispensable for applications in which great toughness and wear resistance are required. Among these can be cited gyratory crushers, jaw-crusher plates, railway points and crossover components, teeth for earth-moving equipment, etc. High manganese (10-12%) non-magnetic steels are used for such products as retainer rings for turbo alternators and collars on oil rigs. Grades with a similar chemistry are used as cryogenic steels. A high manganese stainless "memory" steel has been developed.
Other Metallurgical Uses of Manganese
Although ranking far behind steel, the second most important metal in which manganese plays an important alloying role is aluminum. Some 23 million tons of aluminum are produced annually. Small amounts of manganese are found in many of these Al alloys, enhancing corrosion resistance. The explanation for this beneficial effect is simple. Intermetallic compounds, formed with iron and silicon, have an electrolytic potential which is far less negative than that of aluminum. This means that the aluminum surrounding such particles will be attacked under corrosive conditions, with disintegration further spreading as a series of deep pits are formed which spread the process to other particles. Manganese replaces iron-silicon compounds with manganese-iron-silicon compounds which have an electrolytic potential very close to that of aluminum. As a result there is no potential difference and therefore no corrosion.
Manganese is used as an alloying element up to its solubility limit of about 1.5%. Aluminum-manganese alloys and aluminum-manganese-magnesium alloys, which have been sold under different trade names, have found applications in such diversified areas as kitchenware, roofing, car radiators and transportation. By far the most important use of aluminum-manganese alloys is for beverage cans, of which some 100 billion units are produced each year. The market for aluminum-manganese cans has grown steadily, thanks to the fact that such cans can be recycled.
Aluminum alloys containing up to 9% Mn have promising properties, but they cannot as yet be economically produced. Technologies to produce these commercial “amorphous” metals through very fast cooling are of potential interest but the processes used are still very expensive and can only be applied to high value materials used in the aerospace industry.
Manganese is probably the most versatile element which can be added to copper alloys. Small additions of manganese (0.1 to 0.3%) are used to deoxidise the alloy and improve its castability and mechanical strength. Manganese has a high solid solubility in copper and in binary systems with copper and aluminum, zinc or nickel as the binary constituent. Many commercial copper alloys contain around 1 to 2% manganese to improve strength and hot workability. In order to reduce costs, manganese can replace part of the nickel in nickel-silver alloys.
Far higher levels of manganese content are found in some alloys for specific applications. Although most have levels of 10 to 20% there are some alloys with >50% manganese content. These are produced in small quantities for such specific properties as damping capacity or high-thermal expansion coefficient. One such alloy, 72% Mn, 18% Cu, 10% Ni, is used for bimetallic strips in temperature control devices fitted to cars and other vehicles. Another alloy, sold under the commercial name “Manifor” is a non-magnetic high-strength alloy (60% Cu, 20% Mn, 20% Ni), used to manufacture small parts for the watchmaking industry. However, copper alloys represent only two million tons per year and hence provide a limited market for manganese.
Manganese is also an alloying element addition to other metals. An alpha-beta titanium-base alloy contains 8% manganese and was used for the Gemini re-entry control module in the 1960’s.
Manganese is used in zinc alloys, but only at contents of 0.1 to 0.2%. It can also be used in magnesium alloys with the same content range, with the one exception of an alloy with 1.5% Mn.
Manganese can also be added to gold, silver, bismuth etc., to give alloys which are used for very specific applications, generally related to the electronic industry. The quantities involved are very small.
Non Metallurgical Uses
The most important non-metallurgical application of manganese is in the form of manganese dioxide, which is used as a depolarizer in dry-cell batteries. Dry cell consumption in the world exceeds 20 billion units per year. The function of manganese in batteries is simple. In the battery cell, the anode and cathode are essentially humid. During discharge, hydrogen generated at one of the electrodes coats the latter with a gas film, preventing any further wetting, hence cutting off electrical generation. The role of the manganese dioxide is to oxidize the hydrogen and form water. The rate at which this occurs depends on the reactivity of the dioxide.
The Leclanché cell, incorporating this process, was first developed in 1868. Today a zinc can is used as the anode, and the cathode is a rod made of a mixture of acetylene carbon black surrounded by manganese dioxide. A paste of ammonium and zinc chloride is used for the electrolyte. The manganese dioxide, as has been noted, acts as the depolarizer. In the alkaline MnO2 zinc cell, which was put on the market in the 1950’s, the cathode (MnO2/C) is pressed against the inside wall of a steel container, and the anode is made from zinc powder. Potassium hydroxide serves as the electrolyte. This type of cell has a very low resistance and impedance, giving under certain conditions greater service life than the standard cell. Another cell used for specific purposes is the magnesium chloride-manganese dioxide cell developed for military applications.
Naturally occurring manganese dioxides (NMD) can be used in standard cells. Improved manganese dioxide grades required in high performance cells are obtained synthetically. The products are named after the processes used. EMD, or electrochemical manganese dioxide, is made through electrolysis ; CMD, or chemical manganese dioxide, is produced by a purely chemical process. Combined production of both synthetic types is approximately 200 thousand tons per year, and is growing rapidly.
The market for natural manganese dioxide is about 180-200 thousand tons per year, but very few ores have the properties required for the manufacturing of dry cells. The major producing countries of natural MnO2 are Gabon, Ghana, Brazil, China, Mexico, and India. These “natural grade battery” ores are ground into a fine powder before being used directly in the cathode mixture.
Potassium permanganate is one of the best known manganese products. It is a powerful oxidizing agent with bactericidal and algicidal properties, which enable it to be used in purifying drinking water and treating waste water. It is also used for odour control, including deodorization of discharges from paint factories, fish-processing plants, etc. Permanganate has a variety of other applications as an oxidant.
An important application for manganese is “Maneb” (manganese-ethylene bisdithiocarbamate), an organo-chemical compound sold in the form of a yellow powder. It is sold under different trade names as an agricultural fungicide and is widely used for controlling crop and cereal diseases, downy mildew in vines, scab in fruit trees, banana and peanut diseases among others. Some 200 000 tons of Maneb is estimated as a world-wide consumption figure. An organic manganese compound known as MMT (methylcyclo-pentadienyl manganese tricarbonyl) is used on a small scale as an octane booster or anti-knock agent in gasoline. MMT can dramatically improve oil combustion, reducing boiler clogging and soot levels. This application is important from an environmental point of view as it allows lead to be replaced, but it has not yet been fully developed. There are numerous other applications of manganese oxide and salts. Manganese dioxide is used as a catalyst in the production of artificial flavours like vanilla. It is also used as an oxidizing agent in treating uranium ore to produce the oxide-concentrate known as “yellow cake”. Other applications include the colouring of bricks and tiles, driers and as a pigment for paints, etc. Manganese sulphate is widely used as an end product in fertilizers and animal feed, and as an intermediate product in the chemical industry. Manganese phosphatation is used to produce surface films which, when sealed with oil or wax, can protect steels for internal or mild outdoor use. Manganese phosphating improves wear resistance, prevents welding of metals under load, increases lubrication efficiency by oil absorption and assures rapid and safe running-in of moving parts.
Another important material is manganese ferrite, a soft ferrite widely used in electronics. Large amounts are consumed in the manufacture of television circuit boards. Manganese for this purpose can come from ore, oxides, carbonates and even metallic manganese. Manganese is used in the process of making electrolytic zinc. In MnO2 form it is used to purify the leach solution by oxidizing the iron; as MnSO4 it can be added to the electrolyte to reduce corrosion of the lead anode. By forming a light coating on the cathode, it also facilitates stripping of the zinc deposit.
A novel application under development is that of manganese organo-metallics : they provide an intermediate step in the synthesis of fine organic chemicals of high added value.
The next generation of hybrid electric automobiles and fuel cells will be using lithium manganate as a positive electrode active material for large sized lithium-ion batteries. These batteries are already used today in commercial vehicles and are proving to have excellent performance.
Another upcoming application (still in its research stage) is the use of manganese dioxide to reproduce the photosynthetic process for absorbing large quantities of carbon dioxide emissions.