Sulphite pulping process

Claim 1

A CYCLIC METHOD OF PREPARING A CHEMICAL PULP FROM A CELLULOSIC RAW MATERIAL AND RECOVERING HEAT AND CHEMICALS FROM THE PULP RESIDUAL LIQUOR COMPRISING IMPREGNATING THE RAW MATERIAL WITH A MAGNESIUM BISULPHITE SOLUTION HAVING A PH OF 2.5 TO 5.0 AS MEASURED AT ROOM TEMPERATURE, DIGESTING THE IMPREGNATED MATERIAL AT A TEMPERATURE OF 160-190*C. FOR A PREDETERMINED TIME AND AT A PREDETERMINED SUPERATMOSPHERIC PRESSURE, MAINTAINING THE PH OF THE SOLUTION AS MEASURED AT ROOM TEMPERATURE AT A VALUE BETWEEN 3.0 AND 4.0 AT SAID COOKING TEMPERATURE DURING THE PERIOD WHEN THE MAJOR SULPHONATION OF THE LIGNIN AND DISSOLUTION FROM THE CELLULOSE TAKES PLACE, SEPARATING THE RESIDUAL LIQUOR FROM THE PULP, CONCENTRATING THE LIQUOR TO A CONCENTRATION IN THE RANGE OF 45-70% SOLIDS, BURNING THE CONCENTRATED LIQUOR WITH THE RECOVERY OF HEAT, MAGNESIUM OXIDE AND SULPHUR DIOXIDE, RECOMBINING THE MAGNESIUM OXIDE AND SULPHUR DIOXIDE TO FORM AN AQUEOUS SOLUTION OF MAGNESIUM BISULPHITE ESSENTIALLY FREE OF EXCESS SULPHUROUS ACID, AND USING SAID REFORMED LIQUOR IN THE DIGESTING STEP.