42 MODERN PAPEK-MAKING
The plant occupies part of three floors, the top floor containing the water
pipes and a weak liquor tank. Provision must also be made for lifting tackle
for emptying barrels, and driving gear for the agitators (not shown in the
illustration). The second floor serves as a support for the mixing tanks, though
an arrangement of girders is all that is necessary. Below the mixers is a tank
(No. 4), which is used as a final settling tank for the last mixing of lime sludge.
The store tank is covered in, and should be of ample size.
The method is to charge a mixer with water and bleaching powder. The
agitator is started and run for 20 to 30 minutes; it is then stopped and the lime
allowed to settle. This takes from 2 to 3 hours, often a good deal longer,
but plenty of time must be allowed for the lime to settle below the level of
the draw-off valves, which are about 12 inches above the bottom. If the solu-
tion is drawn off before the lime has settled properly, the lime will foul the
pipes and store tank and get into the bleaching potcher, which is very un-
desirable.
If the size of the mixing tanks (Nos. i, 2 and 3) is taken at 9 feet deep
by 6 feet diameter, each will hold about 1500 gallons of solution, allowing
12 inches at the bottom for the sludge; the mixer is not filled to the brim on
account of the agitators splashing the liquid over the edge.
Two" barrels, approximately 7 cwt. each, are emptied into No. I with
fresh water. This ought to give a solution of 10° to 12° Tw., which is too
strong for general use. To reduce it to 6° Tw. (working strength), No. 2
mixer is filled with water, and the two are run together to the store tank. If
the solution is under 12° Tw., of course a little less water must be run from
No. 2. A Tw. hydrometer is constantly used to check this strength, and a
measuring stick marked off in loo-gallon sections is necessary for the amount
of liquor or water in the mixers.
No. i mixer will now contain sludge, which must be again stirred up
with fresh water. If the mixer is half filled, the solution should stand about
working strength (6° Tw.). If it is stronger, a little more water will correct
the density as it is let down to the store tank. If weaker, the strength may
be brought up from the charge in No. 2. No. I mixer may now be filled
with water and stirred for the third time, and the whole contents run into
No. 4 tank, with a light flush of water to wash out all the sludge.
This last lot, after settling, will give a weak solution, which may be pumped
to the weak liquor tank, from whence it can be drawn to furnish the mixers
instead of, or in addition to, fresh water. In the meantime, No. 2 mixer is
charged and worked in the same way, and then No. 3.
The mixers will thus be used in turn for making a strong solution, a work-
ing solution and a weak solution for diluting purposes, and, by having three