There are several methods employed for estimating the amount of cellulose in plants. The process giving a maximum yield is known as the chlorination method, the details of which are as follows:—About ten grammes of the air-dried fibre is dried at 100° C. in a water oven for the determination of moisture. A second ten grammes of the air-dried fibre is boiled for thirty minutes with a weak solution of pure caustic soda (ten grammes of caustic soda in 1,000 cubic centimetres of water), small quantities of distilled water being added at frequent intervals to replace water lost by evaporation. The residue is then poured on to a piece of small wire gauze, washed thoroughly, and squeezed out. The moist mass of fibre is loosened and teased out, placed in a beaker, and submitted to the action of chlorine gas for an hour. The bright yellow mass is then washed with water and immersed in a solution of sodium sulphite (twenty grammes of sodium sulphite in 1,000 cc. of water). The mixture is slowly heated, and finally boiled for eight to ten minutes, with the addition of 10 cc. of caustic soda solution. The residue is washed, immersed in dilute sodium hypochlorite solution for ten minutes, again washed, first with water containing a little sulphurous acid and then with pure distilled water. It is finally dried and weighed.
The second process for estimating cellulose is based upon the use of bromine and ammonia. About ten grammes of the air-dried fibre is placed in a well-stoppered wide-mouthed bottle with sufficient bromine water to cover it. As the reaction proceeds the red solution gradually decolourises, and further small additions of bromine are necessary. The mass is then washed, and boiled in a flask connected to a condenser with a strong solution of ammonia for about three to four hours. The fibrous residue is washed, again treated with bromine water in the cold, and subsequently boiled with ammonia. The alternative treatment with bromine and ammonia is repeated until a white fibrous mass is obtained.
In practice the paper-maker is confined to two or three methods for the isolation of the fibres, viz., alkaline processes, which require the digestion of the material with caustic soda, lime, lime and carbonate of soda, chiefly applied to the boiling of rags, esparto, and similar pecto-celluloses; acid processes, in which the material is digested with sulphurous acid and sulphites. The latter methods are at present almost exclusively used for the preparation of chemical wood pulp.
Yields of Cellulose in the Paper Mill.—The object of the paper-maker is to obtain a maximum yield of cellulose residue at a minimum of cost. Usually the amount of actual bleached paper pulp obtained in the mill is less than the percentage obtained by careful quantitative analysis, for reasons easily understood.
In the first place, the raw material is digested for a stated period with a carefully measured quantity of caustic soda, for example, at a certain temperature. Now the conditions of boiling may be varied by altering one or more of these factors, the period of boiling, the strength of solution, or the steam pressure, and the paper-maker must exercise his judgment in fixing the exact relation between the varying factors so as to produce the best results.
In the second place, the mechanical devices for washing the boiled pulp and for bleaching cause slight losses of fibre, which cannot be altogether avoided when operations are conducted on a large scale. Frequently, also, a greater yield of boiled material may involve a larger quantity of bleaching powder, so that it is evident the adjustment of practical conditions requires considerable technical skill and experience.
The percentage of cellulose in the vegetable plants employed more or less in the manufacture of paper is given in the following table:—
Table Showing Percentage of Cellulose in Fibrous Plants.