Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

MANGANESE FROM PYROLUSITE. 17

It is upon this strong tendency of aluminium to oxidize that the

processes of aluminothermy are based.

Mixtures of aluminium and oxides react together energetically, whereby

the aluminium is converted into oxide and the metal which originally was in

the form of the oxide is set free. Since one gram of aluminium on com-

bustion yields over 7000 calories, almost as much as carbon, and because

there are no gaseous products of combustion formed which would, carry

away heat from the reaction mixture, the temperature is raised to considerably

above 2000° C.

A mixture of ferric oxide and aluminium, the "thermite" of commerce,

is used for the rapid production of high temperatures in a small space, thus

in welding, riveting, etc., by the Goldschmidt process. The aluminium

oxide separates out from the reaction in the form of a very hard crystalline

substance which can be used as an abrasive; often well-formed needle-like

crystals are to be found in the hollow spaces of the slag. Metals like man-

ganese (No. 2), chromium (No. 3), molybdenum (No. 166) and vanadium,

which were formerly difficult to prepare, or could at best be obtained only in

an unfused state and very impure, can now be prepared without difficulty as

fused masses, free from carbon, by the reduction of their oxides with aluminium.

Even silicon and boron can be obtained by an alumino-thermic process,

if sulphur as well as an excess of aluminium powder is added to the oxides.

The sulphur unites with aluminium and forms a slag of aluminium sulphide,

by which reaction the high temperature required for the reduction of the

oxides is reached; under the slag a fused mass of metal is formed from which,

after dissolving away the excess of aluminium, crystals of silicon, or boron,

are obtained. Magnesium, if used in these reactions instead of aluminium,

produces similar results.

2. Manganese from Pyrolusite.

The reaction between aluminium powder and pyrolusite is so

violent as to be almost explosive; it is better therefore to trans-

form the pyrolusite first into a lower oxide before carrying out the

process. Place 500 g. of finely powdered and sifted pyrolusite

in a Hessian crucible and heat in a charcoal furnace. Mix the

mangano-manganic oxide, Mn 3 O,t, thus obtained (about 420 g.),

with one-third its weight of aluminium powder.

1

Choose a

Hessian crucible, of such a size that it will be about three-fourths

filled by the mixture, and embed it in sand in a large, shallow

dish. At first add only three or four spoonfuls of the mixture

to the crucible; upon the middle of this shake about 2 g. of mag-

(^1) Use the coarser aluminium powder prepared especially for the ther-
mite process, and not the fine powder employed for aluminium paint.