Process for oxygen pulping of lignocellulosic material and recorvery of pulping chemicals

A substantially sulfur-free process for the production of a chemical pulp from lignocellulosic material and the recovery of chemicals used in said process comprising the steps of:

• a) providing a feed stream of comminuted lignocellulosic material,
• b) subjecting said feed stream of comminuted lignocellulosic material to a pretreatment,
• c) reacting the pretreated lignocellulosic material from step b) with oxygen or oxygen-containing gas, in the presence of an alkaline buffer solution comprising at least one sodium or potassium compound in order to obtain a stream of at least partially delignified lignocellulosic material,
• d) further treating said at least partially delignified material from step c) to obtain a chemical pulp product,
• e) extracting spent liquor comprising dissolved lignin components and spent chemical substances from step b) or both steps b) and c),
• f) recovering chemical substances from the spent liquor obtained in step e) and preparing fresh alkaline buffer solution to be charged to step c) or both steps c) and b),
• wherein in step b) said comminuted lignocellulosic material is subjected to a mild prehydrolysis in an aqueous solution and thereafter precooked in the presence of an alkaline buffer solution, and
• in step b) an aromatic organic compound is added to promote selective delignification, and
• in step f) the recovery of chemical substances from the spent liquor obtained in step e) comprises,
• f1) treating at least part of said spent liquor to form a concentrated stream of cellulose spent liquor,
• f2) reacting said concentrated cellulose spent liquor stream with an oxygen containing gas at elevated temperature in a gas generator to form a hot gas comprising carbon dioxide and molten droplets of sodium or potassium compounds or an aerosol of sodium or potassium compounds,
• f3) dissolving said sodium or potassium compounds in water to form an alkaline buffer solution and
• f4) recycling and charging at least a portion of said alkaline buffer solution to step c) or both steps c) and b).