Method of producing white liquor streams of high and low sulphidity

A method of producing two types of white liquor in the recovery of chemicals in a sulphate pulp process, a first type having enhanced sulphidity and a second type having reduced sulphidity comprising the steps of

• a) providing a calcium sulphide rich smelt of inorganic chemicals resulting from simultaneous combustion of black liquor and processing of lime sludge,
• b) treating said calcium sulphide rich smelt with an aqueous solution to yield a mixture wherein the calcium sulphide is in solid form and the rest of the chemicals are in solution,
• c) separating the solid calcium sulphide from the mixture to yield a solution,
• d) processing the solution as green liquor by causticization according to the reaction formula
• Ca(OH).sub.2(S) +[CO.sub.3.sup.2- ].fwdarw.CaCO.sub.3(S) +[2 OH]
• followed by separation of the solid CaCO.sub.3(S), the lime sludge, to yield said first type of white liquor having reduced sulphidity,
• e) mixing the solid calcium sulphide (CaS) obtained in step c) with a required amount of white liquor to react the [OH.sup.- ] of the white liquor with the CaS according to the reaction formula
• CaS.sub.(S) +[H.sub.2 O]+[2 OH.sup.- ].fwdarw.Ca(OH).sub.2(S) +[HS.sup.- ]+[OH.sup.- ]
• f) separating the solid Ca(OH).sub.2 from the mixture obtained in step e) to yield the second type of white liquor having enhanced sulphidity.