The use of a rotary boiler is open to certain objections. In the first place, it is doubtful if they are as economical of soda as the vomiting boilers, especially the more recent patterns; and, secondly, the rotation of the boiler so disintegrates the pulp, that a certain proportion of the cellulose, especially those fibres which are short, is liable to be lost during the subsequent treatment. It seems probable that the best results would be obtained if the rotation of the boiler were reduced to a minimum, in fact, sufficient only to produce thorough circulation of the liquor.
The proportion of soda necessary to boil straw thoroughly is, for reasons already stated, greater than is the case with esparto. The amount varies from 10 to 20 per cent. on the raw material. {100}
The different processes to which straw is subjected vary greatly. The following may be taken as a general indication of the methods employed. The straw is usually cut into short pieces of about 1–2 in. in length by means of an ordinary chaff-cutter. The cut straw is carried by means of a blast of air along a wooden tube or shaft into a chamber, the sides of which are made of coarse iron gauze. This chamber is itself inclosed in another chamber, in which the dust and dirt accompanying the straw collect. The clean straw is then placed in sacks and conveyed to the boiler-house.
Owing to the bulky nature of straw, it is difficult to get a large charge into a boiler at one operation; it is therefore usual, after having filled it as completely as possible, to run in a portion of the lye required, and to turn on the steam for a few minutes. This has the effect of so far softening the straw as to make it lie closely at the bottom of the boiler and to allow of a further quantity being put in. This having been accomplished, the remainder of the lye required is run in, together with the requisite quantity of water, and the steam turned on. The pressure may vary from 10 to 50, or even 80 lbs. per sq. in., and the time from 4–8 hours.
When the boiler has sufficiently cooled, the charge is run out by a cock in the bottom. Owing to the rotary action of the boiler the straw is in the state of fine pulp, having been almost completely disintegrated; so fine, in fact, that it flows readily through a 3-in. pipe. It is run into large tanks with perforated tile bottoms, where the excess of liquor is allowed to drain away and the pulp washed by the addition of water. It is then dug out and taken to be bleached. Before this is done, however, it may be necessary to give it a further washing. This may be done in the potcher itself. Instead of washing the straw in tanks, it may be washed in an ordinary washing engine, such as has been already described. Owing to the finely divided state of the pulp, however, this method, unless the meshes of the wire-cloth covering the drum washer are very fine, entails {101} a considerable loss of fibre. It is more suitable in cases in which the straw has been boiled in stationary boilers, and in which therefore it is less disintegrated. When stationary boilers are employed, it is not necessary to cut the straw very fine; in fact, it is sometimes put into the boiler whole. The cutting has this advantage, that it loosens the adhering dirt. In cases where the action of the soda has not been carried far, or when the straw has been put into the boiler whole, the pulp will not be disintegrated to the same extent, and it would not flow through a narrow pipe. It is necessary therefore to discharge the boiler through the doors used for filling.
A novel form of washer especially adapted for straw, whereby the washing is effected with the minimum quantity of water, has been used on the Continent to a considerable extent, and to some extent in this country. The pulp is caused to pass along a series of revolving cones covered with wire-cloth, through which the liquor escapes. As it reaches the end of each cone, the pulp is emptied into a small tank containing water from the cones further on in the series. It is carried forward by means of hollow bent arms connected with the inside of the next cone, which then discharges it at the other end, to be again carried forward.
As already stated, the presse-pâte system is largely adopted for straw pulp; it has been described under Esparto. It may sometimes be employed with advantage before bleaching, though it is generally used after.
A method of treating straw is sometimes adopted which produces a pulp of higher quality than is obtainable by the ordinary method. The washed pulp is mixed in a chest provided with stirrers, with a large quantity of water, and is then pumped into another chest placed at a higher level, from which it flows between a pair of hard granite millstones, the surfaces of which are cut into radial grooves. The stones are fixed horizontally, and are made to revolve at a very rapid rate. During the passage of the pulp through the stones, the knots, weeds, and other hard portions of the {102} straw which may have resisted the action of the alkali are reduced to a fine state of division, and are thus more readily acted upon in the subsequent operation of bleaching. The degree of fineness to which the pulp is reduced can be determined by regulating its flow and the distance between the stones. Pulp produced in this way is of a very even character, and is comparatively free from unbleached particles.
Within the last few years a process of treating straw has been introduced which, on account of the high quality of the pulp produced, has attracted considerable attention from paper-makers. It consists in exposing pulp obtained in the usual way by the action of caustic soda to the action of chlorine gas. The method was patented in England on March 3rd, 1880 (No. 938), by Friedrich Carl Glaser. The amount of soda in the preliminary boiling is reduced much below that necessary for perfect pulping, the result being that the cellulose is much less liable to be destroyed, and thus a greater yield is obtained. The pulp is thoroughly washed and partially freed from moisture in a centrifugal machine until only about 70 per cent. remains. The partially dried pulp is then exposed in leaden or stone chambers for some hours in an atmosphere of chlorine, produced by the action of hydrochloric acid upon manganese dioxide. By the action of the chlorine, those portions of the straw which have been but imperfectly acted upon by the caustic soda in the boiling process are completely disintegrated, or, rather, are rendered more susceptible to the action of the bleaching liquor employed subsequently. The consequence is that when bleaching liquor is added, the whole of the straw is rapidly reduced to the state of pure cellulose, and the finished product is remarkably free from anything like unbleached portions. Owing to the fact that the action of the caustic is minimised, a considerable proportion of the more easily degraded cellulose survives, and the yield of pulp is thereby increased. The process is, however, necessarily expensive. The action of the chlorine will be readily understood by a reference to p. [19]. {103}
Owing to the fact that a considerable proportion of the pulp obtained from straw consists of cellular tissue (see Fig. 13, p. [58]), which, although cellulose, is devoid of fibrous structure, paper made from straw only is found to be much weaker than that made from fibres such as esparto. On the other hand, being a cheap material, and one capable of yielding a very white pulp with a moderate quantity of bleach, it is held in considerable repute by many paper-makers, especially as a material for mixing with esparto. It is sometimes mixed with cotton and linen rags, even in the finest qualities of paper.