Universal ChemicalsManufactures of
|
|||||||||
|
|||||||
It is well recognized that a low pH in the finished paper is detrimental to the
strength and permanence of the paper. This is one objection to alum as a
precipitant, since the use of alum alone with rosin size makes it difficult to
obtain good sizing without lowering the pH to such an extent that the strength
of the paper is affected.
In order to overcome the undesirable effects of too low a pH, a small amopunt
of alkali such as sodium hydroxide can be used in combination with alum. This
permits sizing to be obtained under pH conditions where the permanence of the
paper is not so greatly affected, the corrosion of the mill equipment is
lessened, the wire life is extnded, and there is less embrittlement and loss of
folding endurance in the paper.
Another method is to use a basic aluminium sulfate (sodium aluminate) together
with alum for precipitating rosin sizr at the desired pH. Sodium aluminate
(NaAl02) (Na2Al204) is an alkaline
source of alumina and differs from alum in that it contains about three times
as much alumina, and this alumina is in the negative radical. An excess of
sodium alydroxide is present over that necessary to neutralize all the laumina
becasue pure sodium aluminate is unstable in solution. The best commercial
products contain about 5% excess sodium hydroxide.
Sodium aluminate used in conjection with alum offers independent control of pH
and of the amount of alumina added to the system with a minimum of sulphate
addition. For an equivalent amoun of alumina, sodium aluminate plus alum adds
only one quarter the amount of sulphate ion as is added by alum and sodium
hydroxide, as shown by the following equations :-
Al2(SO4)3
+ 3Na2O Al2O3 -> Al2O3
+ 3Na2SO4
By using sodium aluminate in combination with alum, it is possible to obtain satisfactory
sizing over a wide range of pH. Schur and Levy found that sizing begins to
decrease rapidly at pH values higher than 4.3 when alum alone is used, but when
sodium aluminate and alum are sued together, good sizing can be obtained at pH
values up to 6.5. Also using combinations of sodium alumiante and alum. Libby
and Dohne were able to obtain good sizing even with low free rosin size, at a
pH between 6.0 and 8.5, depending upon the amount of white water recircualted.
The results of Wilson shown in figure XIII-6, show that in the pesense of
increasing sodium aluminate, optimum sizing is obtained at higher and higher pH
values. In every case, 1% alum was added and the amount of sodium aluminate was
varied from 0 to 2%.
In addition to mixing sizing possible at a higher pH, sodium alumiante is
claimed to have benefits in controlling foam (by coagulating finely dispered
colloidal material), improving the closeness or formation of the sheet, and
increasing the retentionof filters. To obtain these results, it is best to add
the aluminate fairly close to the machine, e.g at the mixing box or at the
headbox, after the alum.