Chapter V.

When an alkaline treatment is adopted, the alkali employed is, at least in this country, caustic soda. In Germany the sulphide of sodium processes have been very largely adopted. The alkali method is, however, being superseded by the more perfect and more economical method of treatment with sulphurous acid or acid sulphites.

_“Broke” Paper._—Under this head may be included all the partially formed paper which is always obtained in greater or less degree when a paper-machine is started, or such portions as are occasionally unavoidably damaged in its passage over the drying cylinders, together with the imperfect or rejected portions. It may also include used or waste paper, a large proportion of which, if not too dirty, is re-made into paper. The cleaner portions, especially if they have not been dried, are frequently returned direct to the beaters, and mixed with other pulp. That which has been actually made into paper requires to be softened by boiling in water and {105} gentle breaking in an engine. It may be necessary to heat it for a short time in a weak solution of caustic soda. This may be done either in a breaker or in special tanks provided for the purpose. Paper which has been printed upon requires a rather more drastic treatment, and it must of course be used for an inferior quality of paper, as it is impossible to get it to as good a colour as the original pulp from which it was made—at least not economically. “Broke” paper may be advantageously disintegrated by means of an edge-runner. It consists of a pair of stones arranged in the same way as an ordinary mortar-mill (Fig. 27).

_Mechanical Wood Pulp._—A very large quantity of pulp is used in the commoner kinds of paper, such as cheap news, &c., which is obtained by disintegrating wood by mechanical means alone, no chemicals being employed. The idea of making paper in this way dates back about 100 years, but owing to the want of suitable machinery it is only lately that a good product has been obtained. The following are, in a general way, the details of the process employed:—

The wood is first cut up into blocks, the size of which is determined by the width of the stones used for grinding; any knots present are cut out with an axe. The stones are made of sandstone, and are covered over three quadrants with an iron casing, the remaining quadrant being exposed. The surfaces of the stones are made rough by the pressure of a steel roll studded with points, and which is pressed against it while revolving. In addition to this, channels about 1⁠/⁠4 in. deep are cut into the stone at distances of 2–3 in. They are made in two sets, crossing each other in the centre of the stone, and serve to carry off the pulp to the sides of the stone, in addition to giving increased grinding-surface. The pressure of the blocks of wood against the stones is steadily maintained by screws worked by suitable gearing; this is necessary in order to obtain a pulp of uniform character. A stream of water is kept constantly playing on the stone; by this means, the pulp as fast as it is formed can be conveniently carried away. It is first passed {106} through a rake, which retains small pieces of wood that have escaped grinding. The stream of pulp then passes through the sorters, the object of which is to keep back such portions of the wood as have not been sufficiently disintegrated. These consist of cylinders about 3 ft. long and 2 ft. in diameter, covered with a coarse wire-cloth. The fibres that are retained by this wire fall into the refiners, which consist of a couple of horizontal cylinders of sandstone, the upper one only of which revolves. Here they are further {107} disintegrated, and are again passed through the wire-cloth; this is repeated until all the fibres have passed through. The pulp, after passing through the first sorter, may be conducted through a series of gradually increasing fineness, and, by this means, be separated into different qualities. Though pulp so prepared cannot compete with chemically-prepared stuff, as the fibres are extremely short, and have comparatively little felting-power, it may be used with advantage as a sort of filling-material.

Various modifications of the foregoing process have from time to time been proposed; among others, that of softening the wood by previous soaking in water, or steaming, seems to be valuable, as by so doing, it is highly probable that a longer fibre could be obtained, the soft wood being more readily torn away by the stones. Some inventors have proposed to replace the sandstone by an artificial stone containing a large quantity of emery.

An improved method of preparing mechanical wood pulp, lately patented by Mr. A. L. Thune, of Christiania, has been communicated to us by Mr. Carl Christensen.

The apparatus employed is shown in Figs. 28, 29, and 30. Fig. 28 illustrates an arrangement of grinding apparatus fixed direct on to a turbine. The stone is fastened on to the shaft S worked by the turbine T. The wood in the form of small blocks is kept in contact with the stone by a number of hydraulic presses P. {108}

A somewhat similar arrangement, but placed horizontally, is shown in front and side elevation in Figs. 29 and 30. The same letters correspond.

The ground and sorted pulp is made into thick sheets by means of the board machine shown in Fig. 31. The pulp mixed with water passes down the shoot D into the vat B in which the cylinder K revolves. This cylinder is covered with wire-cloth, and as it revolves it takes with it a certain quantity of pulp in the form of a continuous sheet. This sheet is taken on to the endless travelling felt F by the small couch roll E. When it reaches the rolls C it is wound round the upper one, from which it is removed when a sufficient thickness is obtained. Obtained in this form the pulp is readily transported.

The woods commonly employed are white pine and aspen. The latter yields a pulp of a better colour, but of inferior strength than the former.

Paper containing mechanical wood pulp is very liable to become discoloured by the action of air and light, the ligno-celluloses being much more readily acted upon than the celluloses isolated from them. Such fibre is, moreover, devoid of much felting power; it has, in fact, little to recommend it but its comparative cheapness. It is nevertheless used in large quantities, some cheap papers being made entirely from it. {109}

TABLE OF STRENGTH OF CAUSTIC SODA SOLUTIONS (15° C. = 59° F.) (TÜNNERMAN).

----------+-------------+-----------+----------------------- | | | Equivalent Percentage Specific | Degrees | Per cent. | of 60 per cent. Gravity. |Twaddle.[10] | Na_{2}O. | Caustic Soda. ----------+-------------+-----------+----------------------- 1·0040 | 0·80 | 0·302 | 0·503 1·0081 | 1·62 | 0·601 | 1·001 1·0163 | 3·26 | 1·209 | 2·015 1·0246 | 4·92 | 1·813 | 3·021 1·0330 | 6·60 | 2·418 | 4·030 1·0414 | 8·28 | 3·022 | 5·037 1·0500 | 10·00 | 3·626 | 6·043 1·0587 | 11·74 | 4·231 | 7·051 1·0675 | 13·50 | 4·835 | 8·059 1·0764 | 15·28 | 5·440 | 9·067 1·0855 | 17·10 | 6·044 | 10·073 1·0948 | 18·96 | 6·648 | 11·080 1·1042 | 20·84 | 7·253 | 12·090 1·1137 | 22·74 | 7·857 | 13·095 1·1233 | 24·66 | 8·462 | 14·103 1·1330 | 26·60 | 9·066 | 15·110 1·1428 | 28·56 | 9·670 | 16·117 1·1528 | 30·56 | 10·275 | 17·125 1·1630 | 32·60 | 10·879 | 18·131 1·1734 | 34·68 | 11·484 | 19·140 1·1841 | 36·82 | 12·088 | 20·147 1·1948 | 38·96 | 12·692 | 21·153 1·2058 | 41·16 | 13·297 | 22·161 1·2178 | 43·56 | 13·901 | 23·170 1·2280 | 45·60 | 14·506 | 24·177 1·2392 | 47·84 | 15·110 | 25·170 ----------+-------------+-----------+-----------------------

NOTE.—It must be borne in mind that the above numbers refer only to solutions of pure caustic soda. With liquors containing sodium chloride, sulphate, &c., the specific gravity will give an erroneous view of the amount of alkali present, as these salts of course raise the gravity. For example, a liquor prepared by causticising a solution of recovered soda has a specific gravity of 1·05 (10° Tw.). According to the above table this corresponds to 6·043 per cent. of 60 per cent. caustic. Tested by means of standard acid it showed 4·520 per cent. Too much reliance should therefore not be placed upon determinations of specific gravity, but in important cases the actual amount of alkali should be determined by titration with standard acid.

[Footnote 10: To convert degrees Twaddle into specific gravity, multiply by 5, add 1000 and divide by 1000: thus 7° Tw.,

7 × 5 = 35 1000 ──── 1000) 1035 (1·035 sp. gr. 1000 ──── 3500 3000 ──── 5000 5000 ━━━━]

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