GB813072A - Sulphite pulping process - Google Patents

Abstract

<PICT:0813072/IV (a)/1> Cellulosic raw material is pulped with a hot magnesium bisulphite solution at a digestion temperature of at least 160 DEG C., the solution being essentially free of "excess" sulphurous acid and being maintained at a pH of 3.0-4.0 during most of the digestion period. By "excess" sulphurous acid is meant the excess of "free" over "combined" acid. Preferred conditions comprise a temperature of 160-190 DEG C., an initial solution pH of 3.5-5.0, and a pressure such that the sulphurous acid liberated is retained in the digester to accelerate pulping, particularly a pressure slightly greater than the vapour pressure of water at the digester temperature. The pulp may be separated from the residual digesting liquor and the liquor concentrated and burned to recover heat, magnesium oxide and sulphur dioxide. The magnesium oxide may be slurried with water and the resulting magnesia treated with the sulphur-dioxide-containing combustion gases to form a magnesium bisulphite solution fit for further use without being "fortified" with additional sulphur dioxide. The entire process including the recovery of reagent may be effected in the apparatus shown. In stage (1) chips in digester 10 are steamed to displace air. (2) Magnesium bisulphite solution from stage (6), pre-heated in accumulator tank 11, is pumped to 10 until 5-6 atmospheres pressure is attained. (3) Surplus liquor is drained from 10 until the desired liquor to wood ratio is reached and returned to 11. (4) Digester 10 is raised to required temperature and held there for the required time, maintaining a pressure rather above the water vapour pressure at the working temperature. (5) Digester pressure is reduced, steam passing if required to 11, which also absorbs sulphur dioxide in the relief gases from stages (4) and (5). In stage (6) digester contents are discharged to blow tank 12, freeing digester for stage (1), and steam released is condensed in condenser 8, which discharges to hot water accumulator 45. (7) Pulp in 12 is suspended in a portion of the residual liquor recycled from rotary washer 13 to which the thus diluted stock is delivered, where it is washed with dilute liquor from rotary washer 14, pulp passing to stage (8) and remaining residual liquor to stage (9). (8) Partially washed pulp from 13 passes to 14 and is washed with hot water from 45, and washed pulp is then screened, cleaned and/or bleached and converted to sheeted pulp or to paper. (9) Hot residual liquor from (7) is sprayed into evaporator 18 where it meets hot flue gases from magnesia separating cyclones 25 of a chemical recovery furnace unit 24, stage (12), thus cooling the gas and partially evaporating the liquor. In stage (10) the liquor is brought to pH 6-7 at mixer 19 with make-up or recovered magnesium oxide and passes from tank 20 to multiple-effect evaporator 21, where it is concentrated to 45-70 per cent solids. (11) Evaporated liquor is then fired in recovery furnace 24, thus recovering heat and yielding steam, magnesium oxide and sulphur dioxide. (12) Magnesium oxide passes through heat exchangers and to cyclones 25 and in stage (13) is slurried in water at 26, collected in tank 30 and filtered at wash filter 32 to remove soluble alkali salts and calcium sulphate, the filter cake being reslurried and slaked at tank 33. (16) The gas from 25 passes through evaporator 18, where it is saturated with water as in stage (9), and passes to primary absorption towers 28, 29, where it meets a slurry containing washed recovered magnesium hydroxide obtained from slaking tank 33. The sulphur dioxide from the gas is absorbed, so forming magnesium bisulphite and a little magnesium monosulphite. (15) The resultant magnesium bisulphite liquor is filtered at 40 to remove silicates &c. and run from storage tank 44 to the accumulator 11 for return to cycle at stage (2). (16) Effluent gas from stage (14), which has been stripped of magnesium oxide and sulphur dioxide, is cooled by a counter-current of water in condensing tower 42, the hot water produced being available at tank 43 for use in bleaching the pulp and for other purposes. Losses of magnesium oxide from the system are made up by supplying magnesium sulphate to liquor at stage (11) or by adding oxide to hydroxide to slurry passing to stage (14), while sulphur dioxide can be made up at stage (14), or can be added to filtered liquor in accumulator 11. The process may be adapted to continuous digestion and may be applied to mixed stands of wood, including "refractory" species such as pine and Douglas fir. U.S.A. Specifications 2,285,876 and 2,385,955 are referred to.