Although the consumption of salt-cake for the manufacture of alkali is now much less than formerly, since the Leblanc alkali process has been greatly restricted, yet it is largely made and will continue to be made for the use of glassmakers, who use it for the ordinary description of glass in the place of soda-ash.
The charge of salt-cake (generally 3 cwt.), limestone and coal is roughly mixed and put upon the back-bed; when the front bed has become empty it is drawn forward and exposed to the full heat of the fire, with frequent stirring.
Nor must it be overlooked that salt-cake must be made as long primary duty of preventing nuisance, but quite as much to showing manufacturers how to make the most of the acid formerly wasted in one shape or another.
- This is commercially known as salt-cake, and is made by decomposing common salt with sulphuric acid of about 80%, the reaction being 2NaC1+H 2 SO 4 =Na 2 SO 4 +2HC1.
Since the work with ordinary hand-wrought salt-cake furnaces is disagreeable and costly, many attempts have been made to construct mechanical salt-cake furnaces.
The process is carried out either in hand-wrought furnaces,or mechanical furnaces, both called " decomposing " or " salt-cake furnaces."
This rich matte is then mixed with coke and salt-cake and melted down in an open hearth furnace.
The mixture is fed in continuously to the central pan (e), whence it overflows into the compartments (c'), (c 2), (c 3) successively until it reaches the circumference, where it is discharged continously by o and into the collecting-box (q), being now converted into salt-cake.
This unavoidable gaseous bye-product of the manufacture of salt-cake was, during the first part of the 19th century, simply sent into the air.
The mixture of hydrochloric acid and air is taken directly from the " decomposing-pan " of an ordinary salt-cake furnace, is first cooled down in pipes sufficiently to condense most of the moisture present 1 ?i; \'\` (together with about 8% of the hydrochloric acid), and then passed through a cast-iron superheater and from this into the " decomposer."