Recovery of naoh and other values from spent liquors and bleach plant

Abstract

Oxidized weak black liquor or effluents from alkaline bleaching stages, particularly oxygen delignification and/or ozone and/or hydrogen peroxide bleaching effluents are treated in process stages which include an electrolytic cell to recover NaOH, lignin, O.sub.2, H.sub.2 and to achieve other important benefits, the most important of these are (i) to unload the evaporator-recovery furnace and lime-kiln causticizing plant, and (ii) to enable a mill to implement low- or no-chlorine bleaching techniques without overloading their recovery furnace and/or lime kiln or requiring a new, larger recovery system, the electrolytic treatment can be carried out with one or a combination of the above types of effluents; it is also possible to increase the conductivity if required and thus the efficiency of the electrolytic cells and the yield/production of NaOH by the introduction of Na.sub.2 SO.sub.4 (make-up or recycle from recovery furnace) and/or NaCl into the feed to the process; the process involves pre-acidification of aqueous alkaline liquid containing lignin to initiate precipitation of lignin, feeding the partly acidified liquid to the anolyte compartment of an electrolysis cell in which the anolyte and catholyte compartments are separated by a cation permselective membrane, and in which the catholyte compartment is provided with a source of hydroxide cations by carrying out electrolysis to effect migration of sodium ions into the catholyte compartment, and simultaneously to acidify the liquid in the anolyte compartment to substantially complete lignin precipitation; sodium hydroxide is recovered from the catholyte compartment and lignin is recovered from the anolyte compartment.