Calcite floculate control of
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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 :-
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.
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