CHEMICAL WOOD PULP. The oldest process of freeing cellulose from the incrusting woody mat ter, producing a material for making white pa per, is the soda process, patented by Watt & Bur gess in England in 1854. A soft wood, usually poplar in America, pine in Europe, is barked, chipped across the grain into small pieces. and cooked under steam pressure with a solution of caustic soda. The alkali dissolves everything but the cellulose, and after washing and bleach ing a soft white fibre of good quality is pro duced, of little strength, but very useful to supplement other fibres.
The sulphite process is apparently an Ameri can invention, the first patent being granted in 1867 to 13. C. Tilghmann. He used sulphurous acid to produce pure cellulose, and while this is the base of the modern process, it was not brought to a practical success till bisulphite of magnesium or calciuni was used instead. Mitschelich in Germany brought the process to a commercial basis, and of late years it has developed to very large proportions. Various modifications of the process are used, but in all of them the wood. properly barked. chipped. and dusted, is digested under steam pressure in a solution of the hisulphite, washed, and bleached if necessary. 'Sulphite' fibre has good strength
and color, and much of it is used unbleached.
The sulphate process is not used in America, but produces a very good pulp at a higher cost than sulphite. It is similar to the soda process in theory, but sulphate of soda is used. An objection to it consists in the offensive odor of the by-products.
In the different processes of treating wood various forms of boilers are used, stationary, rotary, cylindrical, and spherical. They have to be lined to resist the action of the chemicals, and lead or cement is commonly used for this purpose. A large factor in the successful con duct of pulp manufacture lies in recovering the chemicals from the spent liquor. In the soda process the liquor is evaporated and finally incinerated in a furnace from which nearly all the caustic soda used is recovered. There are similar economies practiced in the other processes which not only save valuable chemicals, but prevent the pollution of streams.