Patent GB-705025-A (GB)
Title: Process of preparing fibre-pulp from pith-containing plant materials and products produced thereby
Inventor: MCELHINNEY THOMAS ROBERT
IPC: D21C3/02
Filed: 1951-09-10
Abstract: A cellulose fibre pulp is obtained from pithcontaining plant material by digesting the material in a hot alkaline liquor containing caustic soda of a concentration which is never greater than 0.2 per cent. based on the weight of digestion liquor, but which is at all times sufficient to solubilize the ligneous content of the material without degrading the cellulose content. Alternatively, an aqueous liquor containing calcium hydroxide may be used instead of caustic soda solution, and in that case the concentration is constantly maintained at such a value that the liquor has a pH of 7.5-9, corresponding to a state below saturation, measured at room temperature and pressure, and the digestion is effected at 239-338 DEG  F.  When caustic soda is used, the temperature of the digestion may be within the range from that temperature at which the liquor boils at atmospheric pressure up to 338 DEG  F. and the digestion may be continued until 4-25 per cent. of caustic soda is consumed, based on the weight of the pith-containing fibrous material.  The treatment may be carried out in two stages, in the first of which the concentration of caustic soda is maintained at or below 0.1 per cent. and in the second of which a stronger solution, up to 0.2 per cent., is used. If desired, a treatment with calcium hydroxide liquor may be followed by a treatment with caustic soda solution.  If desired, the raw material may be submitted to a preliminary treatment with sulphuric acid solution to hydrolyse and destroy the pentosans. The required concentration of caustic soda or lime is maintained by adding additional quantities of reagent during the treatment. According to the intensity and duration of the treatment, pulp suitable for making boards or coarse or fine paper may be obtained.  The raw material may be bagasse, maize stalks, reeds or grasses.  In an example, bagasse is placed in a digester and heated up with a circulating current of water, rendered alkaline with sodium carbonate, until the temperature reaches 240 DEG F. and 10 lbs. per sq. in. gauge pressure.  Strong caustic soda solution is then added continuously at such a rate that the concentration is at all times not greater than 0.1 per cent. by weight of the cooking liquor.  The treatment is continued for about four hours until 7.5 per cent. of caustic soda has been consumed, based on the weight of raw material treated.  The pulp obtained is used in making corrugated board or coarse paper.  In another example, bagasse is placed in a rotary digester with water adjusted to pH 9.0 by addition of lime, and the contents are heated with live steam until the temperature reaches 295 DEG F. at 50 lbs. per sq. in. gauge pressure.  At intervals the pH value of samples of the liquor is determined and sufficient lime water is added to the liquor in the digester to restore the pH to about 8.5.  The process is continued for three hours or until a total of 4.5 per cent. of calcium hydroxide has been consumed.  The pulp obtained may be used for making wall board or pressed board.