The most important non-metallurgical application of manganese is in the
form of manganese dioxide, which is used as a depolariser in dry-cell
batteries. Dry cell consumption in the world exceeds 20 billion units per year.
The way manganese works 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 oxidise 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 depolariser.
In the alkaline Mn02 zinc cell, which was put on the market
in the 1950's, the cathode (Mn02/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 electrochemically; 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 Mn02 are
Gabon, Brazil, China, Mexico, and India. These natural grade
battery ores are ground into a fine powder before being used directly in
the cathode mixture.