The thin watery "slip" or slurry flows into large settling tanks ("backs") where the solids in suspension are deposited; the water is drawn off, leaving behind an intimate mixture of chalk and clay in the form of a wet paste.
According to present practice the raw materials are mixed in a wash mill with so much water that the resulting slurry contains 40 to 50% of water.
The slurry, which is wet enough to flow, is ground between millstones so as to complete the process of comminution begun in the wash mill.
The drying of the slurry is generally effected by the waste heat of the kilns, so that while one charge is burning another is drying ready for the next loading of the kilns.
The kilns commonly employed are "chamber kilns," circular structures not unlike an ordinary running lime kiln, but having the top closed and connected at the side with a wide flue in which the slurry is exposed to the hot products of combustion from the kiln.
The slurry, in drying on the floor of the flue, forms a fairly tough cake which cracks spontaneously in the process of drying into rough blocks suitable for loading into the kiln.
A layer of dried slurry is loaded on this, then a layer of coke, then a layer of slurry, and so on until the kiln is filled with coke and slurry evenly distributed.
Fresh slurry is run on to the drying floors, and the kiln is started.
The Chimney Section Grate Drying space for Slurry Plan FIG.
An ordinary kiln, which will contain about 50 tons of slurry and 12 tons of coke, will take two days to get fairly alight, and will be another two or three days in burning out.
Besides the chamber kilns which have been described, there are the old-fashioned bottle kilns, which are similar, to the chamber kilns, but are bottle-shaped and open at the top; they do not dry the slurry for their next charge.
At the upper end the raw material is fed in either as a dry powder or as a slurry; at the lower end is a powerful burner.