System for removing sodium chloride contaminants from a magnesium base pulping process

Claim 1

IN A CHEMICAL RECOVERY SYSTEM WHEREIN CELLULOSIC MATERIALS CONTAINING SODIUM CHLORIDE ARE PULPED IN A MAGNESIUM BASE COOKING LIQUID, AND THE RESIDUAL LIQUOR FROM THE PULPING PROCESS CONTAINS THE SODIUM CHLORIDE FROM THE CELLULOSIC MATERIALS, THE RESIDUAL LIQUOR IS CONCENTRATED BY EVAPORATION AND BURNED TO PRODUCE HOT FLUE GASES CONTAINING SULPHUR DIOXIDE AND HYDROGEN CHLORIDE AND ENTRAINED SOLIDS INCLUDING SODIUM SULPHATE AND MAGNESIUM OXIDE, THE STEP OF REMOVING ENTRAINED SOLIDS FROM THE GASES, TREATING THE SOLIDS TO FORM A SLURRY OF MAGNESIUM OXIDE AND TO DISCARD THE SODIUM SULPHATE, PASSING THE FLUE GASES THROUGH A SULPHUR DIOXIDE ABSORBING ZONE IN CONTACT WITH THE MAGNESIUM OXIDE CONTAINING SLURRY TO FORM MAGNESIUM SULPHUR COMPOUNDS, THE IMPROVED STEP OF WASHING THE FLUE GASES BEFORE THEY ARE PASSED TO THE SULPHUR DIOXIDE ABSORBING ZONE TO SELECTIVELY REMOVE THE HYDROGEN CHLORIDE GASES BY ABSORPTION IN A MAGNESIUM CONTAINING LIQUID, THE MAGNESIUM CONTAINING LIQUID BEING FORMED BY SOME OF THE ENTRAINED MAGNESIUM OXIDE SOLIDS PASSING WITH THE FLUE GASES TO THE HYDROGEN CHLORIDE WASHING STEP AND COMBINING WITH THE WASHING LIQUID TO FORM MAGNESIUM CHLORIDE AND SOME OF THE GASEOUS SULPHUR OXIDES COMBINES TO FORM A SOLUTION OF MAGNESIUM BISULPHITE IN THE LIQUID, RECIRCULATING A MAJOR PORTION OF THE LIQUID TO THE WASHING STEP TO ABSORB HYDROGEN CHLORIDE, DISCARDING A PORTION OF THE LIQUID, ADDING MAKE UP WATER TO CONTROL THE CONCENTRATION OF MAGNESIUM CHLORIDE AND MAGNESIUM BISULPHITE IN THE RECIRCULATED LIQUID, REMOVING SOME OF THE LIQUID FROM THE WASHING STEP TO BE REACTED WITH MAGNESIUM HYDROXIDE TO PRECIPITATE MAGNESIUM SULPHITE, THE REACTED LIQUID IS SEPARATED FROM THE PRECIPITATE WITH THE LIQUID RECYCLED TO THE WASHING STEP, AND THE PRECIPITATE SOLID MAGNESIUM SULPHITE SUBSEQUENTLY UTILIZED IN THE SULPHUR DIOXIDE ABSORBING STEP IN FORMING COOKING LIQUOR.