Abstract: This invention relates to a process for multi-stage alkaline pulping of ligno-cellulosic raw materials and recovery of delignification spent liquors, i.e. black liquor from digestion with white liquor and oxy-liquor comprising bleach plant effluent extracted from alkaline oxygen delignification of brown stock.According to the invention, oxy-liquor and intermediate black liquor in the digester are passed through that part of the material column to which the cellulosic raw material, e.g. wood chips, enters. There, hydroxide deriving from two sources, white liquor and oxy-liquor, is exhausted prior to its discharge as terminal black liquor. White liquor is consequently injected to the level of the pre-digested material in the column. This injection is controlled by continuous measurement, e.g. by automated titration analysis, of the hydroxide concentration in the joint volume of oxy-liquor and white liquor in the main cooking liquor circulation system.

Abstract: A method of preventing or reducing the emission of odorous sulphur compounds and/or sulphur combustion products from the black liquor recovery tall oil splitting processes in kraft pulping of pines is described. Sulphur dioxide and/or bisulphite is first reacted with black liquor sulphidic compounds occluded in the soap skimmings simultaneously with the soap acidulation by using a pressure vessel at a temperature of approximately 100.degree.-110.degree. C. Then the sulphite solution with the sulphur compounds thus obtained is separated and further utilized for stabilizing sulphides in various kraft pulping process liquors, preferably the black liquor. Alternatively, the sulphite solution can be passed through a neutral sulphite semichemical (NSSC) pulping arranged for spent liquor cross-recovery with the kraft process. The sulphur dioxide is substituted either for conventionally used acid chlorine dioxide reactor effluent or sulphuric acid. The process may regenerate sulphur dioxide for splitting.