SE435302B - SET FOR TREATMENT OF PREPARES FROM DELIGNIFICATION OF LIGNOCELLULOSAMATER WITH CHLORIDE-CREATING BLEACH - Google Patents

Abstract

Lignocellulosic fibrous materials are delignified in two or more stages. An introductory stage of digestion discharges brown stock and a black liquor which contains i.a. sulphur compounds wherein sulphur has an oxidation number that equals either -II (e.g. Na2S) or IV (e.g. Na2SO3). Optionally, the brown stock is further delignified by means of oxygen, whereby oxystock and oxyliquor are obtained. Stock from digestion, i.e. brown stock or oxystock, is subjected to delignifying oxidative bleaching using chlorine or chlorine compounds, the reaction products of which subsequently form chlorides in the bleaching effluent. The non-chloride liquors, i.e. black liquor and oxyliquor, possibly also a low chloride fraction of bleaching effluents, are joined to form a lye which is recovered and combusted and includes the major part of the sulphur compounds used for digestion. The lye combustion furnace releases compounds comprising sulphur of oxidation number -II and IV. The chloride forming effluents, optionally including a minor lye fraction, are combined to form a brine which is subjected to another combustion which principally produces chlorides and compounds of sulphur of oxidation number IV(e.g. SO2) and VI (e.g. SO3 and Na2SO4). This makes it possible to eliminate sulphur compounds, e.g. sulfidic compounds with oxidation number -II, from the brine recovery operations including separation of chloride, while the digestion chemical recovery sysstem only handles the lye. By this means the digestion as well as the entire pulp mill system is relieved from chlorides, however, at least two combustion furnaces and two separate evaporation lines, one for lye and one for brine, are required. The combined water evaporation from lye and brine can be kept on the same level as for a conventional single-line black liquor operation because brine substitutes wash water.