Patent ID: 6946057

Claim:
An alkaline process for the production of a pulp from lignocellulosic material and the recovery of pulping chemicals used in said process comprising the steps of: a) providing a feed stream of finely divided lignocellulosic material, b) contacting lignocellulosic material in a digester with an alkaline aqueous buffer solution comprising at least one of a sodium or potassium compound and a boron compound, during a period of time and at a temperature sufficient to obtain a stream of substantially delignified lignocellulosic material, c) further treating said substantially delignified lignocellulosic material to obtain a pulp product, d) extracting spent liquor comprising dissolved lignin components and spent chemical substances from step b), e) partly or fully oxidizing spent liquor originating from step d) in a recovery boiler or gas generator providing one gaseous stream comprising carbon dioxide and one solid or liquid ash stream comprising at least one of a sodium or potassium compound and a boron compound, wherein i) a boron compound in the alkaline buffer solution in step b) is a metaborate or tetrahydroxy metaborate ion, B(OH) 4 , originating from the dissolution of alkali borates in an aqueous liquid, said metaborate and metaborate ion being present in an amount providing a sodium plus potassium to boron ((Na+K)/B) molar ratio in the alkaline buffer solution in the range from about 1:1 to about 10:1, ii) the solid or liquid ash stream comprising sodium or potassium compounds and boron compounds provided in step e) is dissolved in an aqueous solution to provide an alkaline buffer solution comprising metaborate and carbonate ions, whereof at least a portion is transferred to step b) or c) without prior subjection to treatment with lime or calcium compounds for the generation of hydroxide ions, iii) the solid or liquid ash stream in e) comprises alkali metaborate and alkali carbonate, which substances, or corresponding ions after dissolution of the solid or liquid ash stream in an aqueous solution, are present in a combined concentration which is higher than the combined concentration of other dissolved compounds originating from dissolution of said solid or liquid ash stream in the aqueous solution.