It may be interesting at this point to say something about the substances which are removed from the esparto by the caustic soda. On referring to p. [57] it will be seen that the original grass contains nearly half its weight of extractive matter, the removal of which has to be effected. Only a small proportion of this is extractive matter in the ordinary sense of the term, i.e. that can be extracted by the usual solvents; the remainder is intimately combined with the cellulose. The action of the caustic soda is to resolve these bodies, the cellulose remaining behind, and the other constituents being dissolved as resinous bodies by the soda. A certain proportion of the mineral constituents, notably the silica, dissolves in the lye, the latter forming silicate of soda.
On neutralizing the liquor with an acid, the bulk of the dissolved constituents is thrown down as a dark brown resinous mass. If this crude product be purified, it is found to consist of a definite body having the formula C21H24O8. By the action of chlorine on this resin a bright yellow chlorine compound is formed, resembling the compound from jute described on p. [18]. If the resin be treated with concentrated nitric acid, a bright yellow body is formed, which forms definite compounds with bases, and which has the property of dyeing animal fibres a bright orange colour. In addition to this body, a yellowish-white wax can be isolated. It sometimes happens that this wax is but imperfectly dissolved in the caustic lye; if the waste liquor be allowed {98} to stand, the wax is occasionally found to collect in small quantities on the surface.
Various attempts have been made from time to time to remove from the waste liquors the soluble matter derived from the esparto by the addition of lime. This throws it down as a very voluminous precipitate, exceeding difficult of filtration. The filtrate contains only caustic soda. The difficulties of removal of the precipitate, however, are almost insurmountable. The usual method of disposing of the liquor is to evaporate it to dryness and ignite it, as will be subsequently described under the head of “Soda Recovery,” Chapter XII.
Straw.
The kinds of straw in general use are wheat, oat, rye, and barley; the two first constituting the bulk of the raw material, at least in this country. Some idea of the composition of straw may be gathered from the analyses given on p. [59].
It will be seen that the amount of cellulose is quite as high as in esparto, but for the reasons above given and from the fact that a large proportion of the cellulose consists of cellular tissue (see Figs. 12 and 13), which is easily attacked by soda and readily passes through the meshes of the {99} drum-washers and the wire-cloth of the presse-pâte and paper machine, the yield obtained in practice falls considerably below that from esparto.
In addition to the numbers already quoted on p. [59], the following results of analyses made from time to time by the authors, may be interesting. The percentages are calculated on the absolutely dry material.
| Cellulose per cent. | |
|---|---|
| Oat Straw | 52·0 |
| Oat Straw | 53·5 |
| Wheat Straw | 49·6 |
| Rye Straw (Foreign) | 53·0 |
| Oat Straw (Foreign) | 46·5 |
| Wheat Straw (Foreign) | 50·2 |
The yield of pulp is greatly influenced by the conditions under which it is obtained; high pressures and temperatures exerting considerable influence on the result. This is probably the case with straw more than any other fibre, on account of the physical and chemical nature of the cellulose. It is doubtful whether much more than 35 per cent. is actually obtained in practice.
Straw is usually boiled in cylindrical rotary boilers. Some paper-makers, however, prefer to use one or other of the different forms of vomiting boilers already described.