Manganese occurs in nature in the form of minerals. More than 300 minerals are said to contain some manganese, but only a small number have high manganese content. The manganese mineralogy is complex because manganese occurs in divalent, trivalent, and tetravalent states. The most common manganese minerals are oxides, carbonates, and, appearing less frequently, silicates and sulfides. Manganese minerals of significant abundance and economic importance are listed in Table 1:
Braunite and braunite II are common complex silicate minerals usually occurring in association with bixbyite, hausmannite, and pyrolusite in deposits such as the Postmasburg and Kalahari manganese deposits of South Africa where braunite is the principal manganese mineral. Rhodochrosite is a common carbonate mineral in various ores.
There are only a limited number of workable deposits of manganese ores. The most important land-based manganese ore deposits are located in the Republic of South Africa, Australia, Gabon, Brazil, China, India, and in the Commonwealth Independent State (CIS) countries of Ukraine, Kazakhstan, and Georgia. The manganese ores are characterized by their content of manganese, iron, and various impurities such as P and SiO2. The main types of ore are as follows:
The metallurgical ores are mainly used for direct production of high carbon ferromanganese and silicomanganese alloys, whereas the last two categories are used especially in blast furnaces for adjusting the manganese content of produced pig iron. There might be a substantial variation of ore composition even within the
common deposit. For example, Georgian Tchiatura-oxide type ores have 15% to 43% Mn, 13% to 58% SiO2, 2% to 4.5% Fe2O3, and 0.7% to 5.5% Al2O3 with the average manganese content ~27%.
Metallurgical grade ores are produced from open pit and underground operations by conventional mining techniques. Ores are crushed and screened, and washed if necessary. Heavy media separation can be used for ores with a high content of silica and alumina gangue. The average manganese recovery in this operation is usually between 60% and 75%.
Metallurgical grade manganese ores contain typically 40% to 50% manganese. Another important parameter is the manganese-to-iron ratio (Mn/Fe), which is required to be >7.5 by weight for the production of standard ferromanganese alloy with 78% Mn. Extra iron might always be introduced as steel chips and recycled scrap. There are also limitations on alumina and silica contents, as excessive slag formation in the furnace increases the electric energy consumption. Ores and concentrates with more than 10% SiO2 are suitable for use in SiMn production. In some deposits, high phosphorus content is also a concern, and it must be removed before smelting because most of the phosphorus remains in the finished product. South African manganese ores are characterized by low phosphorus content, but, for example, Nikopol ore deposits (Ukraine) have 0.15% to 0.30%P in oxide-type ores and up to 0.6%P in carbonate-type ores. Sulfur is not a problem, neither for metallurgical nor for environmental reasons, as sulfur forms manganese sulfide, which is dissolved and usually removed with the slag.
Most of the mines have sinter plants, where fines are agglomerated. Sintered material is well suited for use in ferromanganese furnaces because it is mechanically strong and thermally stable, allowing the gas to disperse evenly throughout the preheating and pre-reduction zone. The sintering also results in saving energy if the ore is of the carbonate type. If, on the other hand, oxide type ores are sintered, most of the beneficial heat from the exothermic pre-reduction that usually takes place inside the furnace is lost and the energy consumption will increase. However, the use of fluxed sinter, by the addition of dolomite or MgO-contained materials, has been introduced and shown to improve both sinter plant and smelting operations.
A blend of manganese ores is, in most cases, used when manganese ferroalloys are produced either in electric submerged arc furnaces or in blast furnaces. The choice of ores depends on chemical and physical properties as well as on economic factors. Table 2 shows the average analyses of some important metallurgical ores. It is evident that there are important differences in the chemical composition of the various ores. For example, some ores have an unfavorable Mn/Fe ratio but rather low phosphorus content, so a proper ore dressing process must be applied on a case-by-case basis.
After mining, ore is crushed and screened into various particle size fractions ranging from fines (<6 mm) to lump ore (<75 mm). The proportion of fines is often as high as 30% to 70% of the total. Screened ores are upgraded by various methods to produce concentrates. The most common physical separation methods are washing, high-intensity magnetic separation, separation by gravity concentration, and separation by flotation.
Table 2. Typical Compositions of Some Manganese Ores (major components)