Hemp Hurds as Paper-Making Material | Page 7

3 per cent. It
became apparent that a finer screen would probably serve as well and
effect a saving of small but good hurds.
Cooking.--Cooking is the technical term for the operation by which
fibrous raw materials are reduced to a residue of cellulose pulp by
means of chemical treatment. In these tests about 300 pounds of hurds
were charged into the rotary with the addition of a caustic-soda solution,
such as is regularly employed in pulp mills and which tested an average

of 109.5 grams of caustic soda per liter, or 0.916 pound per gallon, and
averaged 85 per cent causticity. Sufficient caustic solution was added to
furnish 25 or 30 per cent of actual caustic soda, calculated on the
bone-dry weight of hurds in the charge. After closing the rotary head, it
was started rotating at the rate of one-half revolution per minute, and in
about five minutes steam at 120 pounds per square inch was admitted at
such a rate that the charge was heated in one hour to 170° C., which is
the theoretical equivalent of 100 pounds of steam pressure per square
inch. It was found, however, that when the temperature reached 170° C.
the pressure was usually 115 or 120 pounds instead of 100 pounds, due
to air and gases inclosed in the rotary. At this point the rotary was
stopped and steam and air relieved until the pressure dropped to 100
pounds, or a solid steam pressure. The temperature was maintained at
this point for the number of hours required to reduce the hurds, which
was found to be about five, after which the rotary was stopped and
steam relieved until the pressure was reduced to zero, when the head
was removed and the stock was emptied into a tank underneath,
measuring 5-1/2 by 6 by 2 feet deep, where it was drained and washed.
Samples of waste soda solution or "black liquor," which were taken
from some of the "cooks" for analysis, were drawn while the stock was
being thus emptied into the drainer.
Determination of yield.--For determining the yield of cellulose fiber the
stock in the drain tank was washed with water until free from waste
soda solution, when, by means of a vacuum pump communicating with
the space between the bottom and the false perforated bottom, the water
was sucked from the stock, leaving the fiber with a very uniform
moisture content throughout its entire mass and in a condition suitable
for removing, sampling, and weighing for a yield determination. Tests
have shown that it is possible to sample and calculate the yield of
bone-dry fiber within 0.05 per cent of the actual amount.
It has been found that stocks from different materials vary greatly in
their ability to mat in the drain tank, thereby enabling a good vacuum to
be obtained, some stocks permitting a 25-inch vacuum to be obtained,
while others will not permit more than 5 inches. For this reason the
moisture content of the stock will vary from 65 to 85 per cent.

Washing and bleaching.--Washing and bleaching were performed for
the purpose of bleaching the brown-colored cooked stock to a white
product, since it was regarded as highly probable that the fiber would
be suitable for book-paper manufacture. The colored stock was charged
into a 400-pound beating and washing engine of regular construction
and washed about one hour, the cylinder washer being covered with
60-mesh wire cloth in order to remove fine loose dirt and chemical
residues. The washer was then raised, the stock heated by steam to
about 40° C., and a solution of commercial bleaching powder was
added in the quantity judged to be necessary, after which the stock was
pumped to a large wooden tank, to remain and bleach over night. If the
stock was bleached sufficiently white it was drained and washed from
bleach residues, and if not more bleach was added until a good color
was obtained. The bleaching powder used was estimated to contain 35
per cent of available chlorin, as this is the commercial practice, and the
amount required was calculated to the bone-dry weight of the
unbleached stock. More bleach is required for undercooked stock than
for stock which is properly cooked or overcooked; therefore, the
percentage of bleach required is an indication of the quality of the
cooked stock. Since bleaching is usually more expensive than cooking,
it is desirable to cook to such a degree that the consumption of bleach
will be held within certain limits, depending on the raw materials used
and the quality of paper to be produced. In these tests it was desirable
so to cook the hurds that the consumption of bleach would not be over
about 10 per cent of the fiber.
Furnishing.--Furnishing is the operation of charging the beating engine
with