A Text-book of Paper-making

by C. F. Cross; E. J. Bevan

(1) Pecto-celluloses.—From what we have seen of the nature of the pecto-celluloses, it follows that for their resolution a tolerably simple treatment with soda will be sufficient.

The chief members of this group in which the paper-maker is interested, are esparto and straw. The former being but very slightly lignified, its resolution can be effected at a very low pressure; in fact it is even possible by using tolerably strong solutions to do it by boiling in open vessels.

Straw, on the other hand, though it possesses certain features in common with esparto, is more lignified, and in {64} consequence, necessitates a much more energetic action to complete its resolution. It is therefore usual to boil it at somewhat high pressures (40–70 lb.), and with larger quantities of soda than are demanded in the case of esparto. It is, moreover, essential, so to regulate the treatment, that even the most resistent portions of the straw, such, for example, as the knots, shall be completely resolved, otherwise unbleached portions are liable to find their way into the finished paper. By increasing the quantity of soda, it is possible to boil under reduced pressure, and vice versâ; this may be taken as a general principle applicable in all other cases.

We now come to the second class of celluloses, that is to say, the ligno-celluloses. Of these, perhaps the most important are jute, manilla, and wood.

For their complete resolution, it is imperative to employ very strong solutions at very high temperatures and pressures; thus in the case of wood it is necessary to boil at temperatures corresponding to a pressure of 100–120 lb. per square inch.

The use of strong caustic soda solutions at high temperatures is attended with very serious objections, apart from the question of cost. Among these may be mentioned the destructive action of the soda upon the cellulose itself, involving a considerable loss of pulp; the de-hydration, condensation, and oxidation of the non-cellulose portion of the fibre (which, as we have seen (p. [25]) is of the nature of an aldehyde, and therefore peculiarly susceptible of such changes), and the formation of dark coloured products which remain intimately combined with the residual cellulose; the inconvenience of working at very high temperatures and pressures.

With a view to removing the difficulties attending the use of soda, various processes have from time to time been proposed, in which the hydrolytic agent is a solution of sulphurous acid in water, either alone or in combination with a base. These processes are comprised under group B. {65} Several of them have lately acquired great commercial and industrial importance, and a clear understanding of them is therefore essential to the student of paper-making.

Though applicable to lignified fibres generally, they have only been applied up to the present time to wood, and it is in this connection, therefore, that we propose to consider them.

In order to bring out more clearly the fundamental differences underlying the two classes of processes, it will be well to consider, somewhat in detail, these processes, which, although they have now a merely historical interest, will assist the student in thoroughly grasping the rationale of the modern and improved methods.

Water Process.—Wood is to a certain extent resolved by treatment with water at a high temperature, the degree of disintegration being, within certain limits, proportional to the temperature at which it is digested. The volatile products of the resolution are chiefly acids of the acetic series, furfurol and terpenes. The non-volatile soluble products are for the most part acids. This process was studied by Fry, who, in 1866, made it the subject of a patent. He found that under the most favourable conditions of treatment, the yield of brown pulp amounted to 70 per cent. of the wood employed. The objections to the process were sufficiently obvious: the pulp produced was of low quality, and the soluble products acted powerfully on the iron digesters employed. The chemical causes of its failure lie in the prominence of the conditions which determine re-combinations of the products of resolution with one another, viz. oxidation, high temperature, and the presence of acids. It is worthy of note that in one experiment the amount of acids produced from 100 parts wood were equivalent to 3·4 parts of soda (Na₂O). This was by far the most serious defect in the process, and it eventually caused its abandonment in favour of the older method of treatment with caustic alkali.[9]