In practice the grass is packed tightly into upright stationary digesters and a definite quantity of caustic soda solution added, the amount of chemical used being equal to 15-18 per cent. of the weight of grass packed into the digester. The form of digester almost universally employed is that known as the Sinclair's “vomiting” boiler, which is constructed so that a continuous circulation of the liquid is maintained by means of what are called “vomit” pipes. These are fitted to the sides of the digester in such a manner that the caustic soda solution circulates from the bottom of the digester, up through the “vomit” pipes, and is discharged downwards upon the contents of the boiler through a perforated plate fixed in the upper part of the digester. The requisite quantity of caustic soda solution is placed in the digester, and steam admitted into the bottom of the vessel while the grass is being thrown in. In this way a much larger weight of grass can be boiled at one operation, since the bulk is greatly reduced when the grass has become thoroughly soft and wet.
Fig. 21.—Sinclair's “Vomiting” Esparto Boiler.
When the boiler is loaded the inlet is closed up and steam turned on to the full pressure of about 40 or 50 lbs., this being maintained for a period of about four hours. The non-fibrous constituents of the esparto are gradually dissolved out by the caustic soda, and when the operation is completed the black liquor is run off from the digester into large store tanks, and the esparto grass which remains in the digester is then completely washed until the soda is almost entirely washed out.
Fig. 22.—A Porion Evaporator.
The conditions for boiling and bleaching esparto are varied by the paper-maker as circumstances require. A maximum yield of fibre is obtained when the least possible quantity of caustic soda is used, but a larger percentage of bleaching powder may be necessary to ensure a well bleached pulp. The use of an excess of caustic soda is probably the general practice for several reasons, amongst which may be noted the advisability of guarding against irregularities in the quality of the esparto, and consequent insufficient boiling, as well as the advantage of having some free caustic in the spent liquors to prevent the furring up of the tubes of the evaporating apparatus in the soda recovery department.
The following experiments, given by a contributor to the Paper Trade Review some years ago, are interesting as showing the effect of varying proportions of caustic soda used per unit of grass:—