This is usually done in large cylindrical
revolving furnaces, through, which flames from a fire-grate, or from the
burning of gaseous fuel, pass; the waste heat is utilised for boiling
down "black ash" liquor, obtained by lixiviating the black ash. A
mixture of salt-cake, limestone or chalk (calcium carbonate), and
powdered coal or coal slack is charged into the revolving cylinder;
during the process the mass becomes agglomerated, and the final product
is what is known as a "black-ash ball," consisting chiefly of crude
sodium carbonate and calcium sulphide, but containing smaller quantities
of many other substances. The soda ash or sodium carbonate is obtained
from the black ash by lixiviating with water, and after various
purification processes, the solution is boiled down, as previously
stated, by the waste heat of the black-ash furnace. The alkali is sold
in various forms as soda ash, soda crystals, washing soda, etc.
Caustic soda is manufactured from solution of carbonate of soda by
causticising, that is, treatment with caustic lime or quicklime.
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