Manganese is one of the most heavily used metals in the steel industry after iron and aluminum, and almost every heat of steel depends on it. Its greatest role is deoxidation and desulfurization in steelmaking: residual oxygen and sulfur in pig iron make steel brittle, and adding manganese combines them into slag that floats off; at the same time, manganese alloys with iron to raise strength, hardness and wear resistance, so rail steel, crusher liners, excavator buckets and wear-resistant castings all contain high-manganese steel—which work-hardens rapidly on its surface under severe impact, growing harder the more it is abraded. In batteries, manganese is the cathode-active material of alkaline zinc–manganese dry cells and lithium-manganese primary batteries, and is an important metal in lithium-manganate and ternary power batteries; manganese-based materials are also being widely used in sodium-ion battery cathodes. Among non-ferrous metals, adding a small amount of manganese to aluminum improves the corrosion resistance and strength of aluminum alloys, with the 3000-series Al-Mn (anti-rust) aluminum the most common example. Manganese is also used in welding fluxes, trace-element plant fertilizers and food additives. Manganese is indispensable in steel because it is the cheapest and most efficient deoxidizer and desulfurizer, and because it imparts outstanding wear resistance to steel through phase transformation.
Manganese dioxide (MnO2) is the main constituent of pyrolusite and the core active material in the cathodes of dry cells and lithium-manganese batteries; in the glass industry it acts as a decolorizer by oxidizing green divalent iron into lightly colored higher-valent iron, and in environmental protection it is used as a filter medium for iron and manganese removal in drinking water and as a catalyst for flue-gas denitrification; in the laboratory it is heated with potassium chlorate to produce oxygen. Potassium permanganate (KMnO4) is a deep-purple strong oxidizer; dilute solutions are common external antiseptics and water-treatment agents in households and hospitals, it serves as an oxidant in organic synthesis, and it is used as the primary standard in redox titrations in analytical chemistry. Manganese(III) oxide (Mn2O3) is a brownish-black oxide used mainly as an intermediate for preparing other manganese compounds, and as a catalyst in soft magnetic materials, electronic ceramics and some organic syntheses; it is also studied in lithium-battery precursor materials. With the rise of new-energy manganese-based cathode materials such as lithium iron manganese phosphate, demand for manganese dioxide and manganese sesquioxide as precursor raw materials is rising rapidly.
