Method for combusting a mixture of spent magnesium sulphite digestion liquor and alkaline oxygen bleach liquor

A method for combining and recovering spent magnesium sulfite digestion liquor and alkaline oxygen bleaching liquor, without forming a smelt of sodium compounds during combustion of the combined spent liquors, comprising:

• (a) digesting pulp in a magnesium sulfite-containing cooking acid liquor;
• (b) washing the digested pulp with a washing liquid so as to remove at least 97% of spent cooking acid liquor from the digested pulp, said spent cooking acid liquor containing magnesium;
• (c) oxygen bleaching said digested pulp in the presence of oxygen and from about 5 to about 40 kilograms of sodium hydroxide per ton of dry unbleached pulp, thereby obtaining a sodium-containing spent liquor;
• (d) using said sodium-containing spent liquor as said washing liquid, thereby obtaining a mixed liquor of said magnesium-containing spent liquor and said sodium-containing liquor wherein said mixed liquor has a ratio of magnesium to sodium compounds measured as a molar ratio of magnesium oxide to sodium hydroxide of at least 1;
• (e) combusting said mixed liquor to produce a mangesium oxide-containing dust;
• (f) removing a sufficient amount of the sodium compounds from said magnesium oxide containing dust to prevent a build-up of sodium in said cooking acid liquor such that a smelt of sodium compounds is not formed during said combusting step;
• and
• (g) using said dust in the preparation of the magnesium sulfite-containing cooking acid liquor.