Manganese - Mine To Metal

• Manganese is an essential constituent of majority of steels. Technology has not yet been developed to substitute manganese in steel which combines the relatively low price with outstanding technical benefits. Manganese imparts numerous beneficial mechanical properties viz., hardness, toughness and workability to steel in addition to improving its strength, making it irreplaceable in steel making.
• Manganese input is given in the form of raw ore, manganese based alloys (ferromanganese or silicomanganese) and in rare cases manganese metal. It acts as a de-sulphuriser in iron making and used in the form of Blast Furnace (BF) grade manganese ore (28-30% Mn) whereas the manganese based alloys (+65% Mn) are used primarily in steel making as deoxidizer, de-sulphuriser and as alloying element.
• Manganese ores form the basic raw materials for manganese based alloys. In India, about 90% of manganese ore is converted into manganese alloys partly of which is consumed in domestic steel production while remaining are exported. Manganese requirements for non-metallurgical uses do not represent a quantity large enough to significantly affect the evolution of the overall manganese demand. Therefore, the iron and steel industry is the main consumer of manganese ore in the country.

About 10 kilograms of BF-grade manganese ore (28-30% Mn) is required per ton of iron produced in Blast furnace. Whereas, around 15 kilograms of manganese based alloys (Grade-65-72% Mn) is required per ton of steel production.

• The requirement of manganese alloy varies widely depending upon the process of steel making and the product quality envisaged. The consumption pattern of ferro-manganese (high carbon and refined together) and silicomanganese for the above purpose in Indian steel sector would be in the ratio of 2:3.
• Due to limited availability of high grade (+44% Mn) manganese ore reserves in the country, BF-grade (30-35% Mn) manganese ore have to be upgraded to produce high grade concentrate for ferromanganese making. However, feed to the silicomanganese making necessitates manganese ore assaying around 35% Mn only. 

Manganese Distribution in India & its Geology

• India is bestowed with large resource of manganese ore that occurs in different geological formations throughout the peninsular area. The total State-wise distribution of the resources in the country indicates that, Odisha tops with 44% share followed by Karnataka 22%, Madhya Pradesh 13%, Maharashtra 8%, Andhra Pradesh 4% and Jharkhand & Goa 3% each. Remaining resources occur in Rajasthan, Gujarat and West Bengal.
• Manganese ore in India occurs in diverse geological environment and geographic locations and is formed by a number of geological processes like hydrothermal, sedimentary and surficial.
• Sedimentary deposits are the most abundant types. In this type both metamorphosed and un-metamorphosed sedimentary deposits are in place. The host rocks for un-metamorphosed sedimentary deposits are shale, sandstone, conglomerates, tuff, basalt, ortho-quartzite-clay association, limestone-jasper association and limestone-dolomite association. The variation in mineralogy in un-metamorphosed suit dominates rhodochrosite, pyrolusite, cryptomelane and psilomelane, etc. In metamorphosed sedimentary deposits, it occurs in regionally metamorphosed manganiferous sediments with association of sericite schist, pelitic rock/volcanic rock, quartzite, marble, BHQ, slates, dolomites etc. The variation in mineralogy in metamorphosed suit dominates braunite, bixybite-hollandite-hausmanite-jacobsite.

Most of the exploration was carried out at a cut-off of 20% Mn as against the present threshold of 10% Mn. The exploration agencies lay emphasis on establishing the resource of manganese ores to over 25% Mn grade (BF grade) for its use in iron industry.

Need For Processing

• At present, only processed manganese ores are used in industry. However, the manganese ore processing in the country is restricted to meet the physical and chemical standards. Therefore, the entire winning of ROM manganese ore across the country is carried out by selective mining followed by multi-stage crushing, dry screening and manual sorting of the sized fraction. In this process, around 50% by weight of the valuable manganese is recovered and rests are lost in fines. This practice generates large amount of fines left unused at the mine site.
• In spite of sizeable available manganese ore reserves, inadequate production coupled with incomplete processing of exploited material in the country, it is very interesting to note that the Indian manganese based alloy industry at present is producing sufficient quantity of manganese alloys to meet the domestic demand and export. This is because of the fact that in the present scenario Indian manganese-alloy Industries import substantial quantities of medium and high-grade manganese ore and produce manganese-alloy after blending it with indigenous ores to a limited extent.

Manganese ore characteristics of Indian deposits vary widely. The deposits in Maharashtra, Andhra Pradesh and Madhya Pradesh are mostly siliceous in nature with medium to high phosphorus content while deposits in Odisha, Karnataka and Goa are ferruginous in nature with low phosphorus and high alumina content. Geologically also the pattern of mineralization varies drastically from massive and compact ore bodies in Maharashtra and Madhya Pradesh to lean mineralization occurring as lenses and pockets in Orissa.

• Characterization of manganese ore in Indian context can broadly be classified as siliceous, ferruginous and high phosphorus ore. Siliceous ores contain major gangue of quartz, clay and other silicates minerals. The separation of such minerals can be affected after exploiting physical properties of density or magnetic susceptibility or electrical conductivity at its liberation mesh. The process of gravity concentration (HMS, HMC, Jig, Spiral and Table etc.), Magnetic separation (DHIMS/WHIMS) and Electrostatic Separation (HTS) are generally deployed.
• Ferruginous manganese ore resources predominate in Indian context. In such type of ore main gangue is of iron oxide minerals viz., limonite/goethite, hematite etc. Since, both the manganese and iron minerals have similar physical properties like specific gravity and magnetic susceptibility even after liberation; their separation is in difficult proposition deploying conventional physical beneficiation techniques. Separation of such gangue impurities can only be obtained after conversion of hematite/goethite to magnetite by reduction roasting, which can then be separated by low intensity magnetic separation (1200 gauss) at which the manganese minerals are non-magnetic.
• High phosphorous manganese ores render medium and high-grade manganese ore unsuitable for metallurgical use. The phosphorus is mainly contributed by apatite or at times present in the form of solid solution. The latter impurity cannot be reduced by any mineral beneficiation technique and can be used only after suitable blending with low phosphorus ores. The apatite impurity can be reduced substantially deploying (i) Wet High Intensity Magnetic Separation (WHIMS) rejecting apatite and silicate minerals in non-magnetic fraction; and (ii) flotation of apatite followed by manganese leaving behind silica in the tails.
• Of the three agglomeration techniques available, briquetting is not adoptable because of phase change during metallurgical operations. Pelletisation is ruled out because of quantum and cost involved. Thus, sintering is the most appropriate and acceptable technology for manganese ore fines because of its flexibility and strength of the sinters.

Disclaimer: The views expressed are personal and after interactions with Industry Experts

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