Sulphate Pulp.—The solvent used in making sulphate pulp is a mixture of caustic soda, sulphide of soda and sulphate of soda. Sprucewood, largely, is used and the pulp produced is exceedingly strong. Unbleached sulphate pulp is used, notably, in the making of Kraft wrapping-paper. The soda is recovered from the spent liquors.
Sulphite Pulp.—Sulphite pulp is produced by the use of bisulphite of lime; this, being acid, necessitates a special brick lining in the digesters, which are of the vertical type. Sprucewood is the best raw material and yields a strong, fairly long fiber, capable of being bleached to a good white color.
Mitscherlich Pulp.—A special method for making sulphite pulp was invented in Germany by Professor Mitscherlich. It varies from the ordinary process in that the cook is continued over four times as long under lower steam pressure, and yields a fiber of greater strength.
The steps subsequent to cooking chemical pulps of all kinds are similar. After emptying the digesters, the soft, discolored mass of fibers is washed and bleached. The yield of cellulose fiber is close to fifty per cent of the air-dry weight of the wood. The shives and undigested particles are removed by screening, and the pulp is either run out like ground wood on wet machines, or made up into rolls, or sheets, on a paper-machine. The soda pulp is shipped in rolls and the sulphite in sheets, as this is the most favorable form in which to handle them at the paper-mill. If the pulp is to be used on the premises, it is made up into laps on the wet machine and is not artificially dried. The so-called “air dry” pulp contains about 10% moisture, and pulp containing not over this amount of moisture is billed at its actual weight.
CYLINDER MACHINE FOR DRYING PULP, THE BROWN CO.
The web of pulp is shown as it passes from the cylinder mold over the couch roll toward the driers.
Esparto and Straw.—Esparto pulp is made by the soda process from a grass obtained in the circum-Mediterranean countries, and is used most extensively in England and somewhat on the Continent, but freights have been prohibitive for American manufacturers.
Straw pulp is similarly made, and while occasionally used on medium grades of writing-papers, its principal use in this country is for strawboard and cheap wrappings. It is expensive to reduce to a clean, bleached pulp on account of its knots, and the large quantities of silicious matter it contains.
Waste Papers.—The next largest source of paper-making fibers to wood is the waste paper, such as old books, magazines, newspapers, binders’ waste, paper shavings and miscellaneous waste. This stock is collected by regular packers, sorted, and sold by grade to the mills.