Manganese itself has a value
for steel-making; or, rather, for the making of spiegeleisen and
ferro-manganese, which are used in the Bessemer and Siemens processes.
For this purpose the percentage of the metal (Mn) is required.
Consequently the minerals of manganese may be considered in two
aspects--(1) as a source of oxygen; and (2) as a source of manganese.
These will require separate consideration.
The black oxide is mainly used in the preparation of chlorine,
liberation of which it brings about when treated with hot hydrochloric
acid, or with a mixture of common salt and sulphuric acid. The quantity
of chlorine which is obtained depends upon the proportion of dioxide
present;[78] and in assaying may either be measured by its equivalent of
iodine liberated, or by the oxidising effect on an acid solution of
ferrous sulphate. When the ore also carries substances which have a
reducing effect (such as ferrous compounds), such assays will give, not
the total dioxide (MnO_{2}), but less, by the amount required to oxidise
these impurities; and this is exactly what is required in valuing such
an ore for commercial purposes. Manganese compounds are characterised by
the readiness with which they may be converted into highly-oxidised
bodies. Solution of manganese in hydrochloric acid, rendered alkaline
with ammonia, yields a clear solution,[79] which rapidly takes up oxygen
from the air, forming a brown precipitate of the oxide (Mn_{2}O_{3}).
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