358

MODERN PAPER-MAKING

increases towards the discharge end. It revolves about once per minute.
Ignition of the liquor is caused in the first place by the flame from an auxiliary
furnace with coal, oil, or wood at the start of a run, and with careful control
the liquor will then remain alight throughout the run. In one type of furnace
the liquor is sprayed in similarly to an oil-fired boiler.

Combustion is sustained by the burning of the organic matter brought
forward with the liquor from the digesters.

IJk

[George Scott and Sons

FIG. 156.—CAUSTICISBRS, AND IN REAR ROTARY VACUUM
FILTER

There are several methods of subsequent treatment of the black ash. One
method is to complete the combustion of all remaining carbon on a travelling
chain grate over forced draught. Another consists in removing the ash from
the furnace while combustion is still incomplete and spreading this ash over
a perforated floor under which an air draught is induced. Complete com-
bustion- requires about two weeks under these conditions. This method has
the disadvantage of requiring a long time and a good deal of floor area. The
calcined ash, or 'black ash5, is then soaked with boiling water, which absorbs