44 MODERN PAPER-MAKING
their existing bleach plant to the needs of the new process. This has been
done very successfully, and, in fact, when the bleach mixer is a plain cylindrical
tank, standing on end and fitted with a simple gate or horizontal type of agitator,
it makes what is possibly the best type of chlorination vessel for practical
purposes. It is desirable that the tank should be fairly deep (say at least 12
feet) and that it should have no baffles or other devices which may cause rapid
vertical currents.
The lime in the process should be a high-grade quicklime which slakes
easily. It is necessary that the temperature of the batch of milk of lime, before
chlorination commences, should not be more than about 65° to 70° F., and
thus a lime which will slake with cold or warm instead of hot water is an
advantage. The charge of lime should be slaked in as small a quantity of
water as possible. The thick milk so produced is pumped to the chlorination
tank and can then be cooled, as well as diluted, by filling up the chlorination
tank with cold water. During the chlorination the temperature should not be
allowed to exceed 90° F.
Chlorine is admitted to the chlorination vessel, in the liquid state, through
pipes reaching almost to the bottom of the vessel. A distributing device, such
as a coil perforated at regular intervals and lying close to the floor of the tank,
is a decided advantage.
The progress of the reaction between,the lime and chlorine is checked as
follows:
Remove a sample of the muddy liquor and allow it to settle in a hydro-
meter jar.
Test with a Twaddell hydrometer graduated in fifths or tenths of a
degree.
When the test shows that the batch is nearing completion, samples
should be taken at close intervals and from these tests the actual end point
can easily be predicted.
This method of testing is quite accurate enough for works control purposes,
but the Twaddell to which the batch can safely be taken must be determined
in the first place by more exact (analytical) methods. A typical batch at one
mill was as follows (initial temperature 67° F.):
Temperature Twaddell
After J hour.......... 70° F. 2.0° Tw.*
» * ».......... 78°F. 5.5° „
„ 1} hours ........ 83° F. 9.4° „
„ i hour 40 min. ...... 84° F. 104° „
* These Twaddells refer to warm liquor five minutes affer sampling. They will, of course, be higher if the liquor
cools.