Abstract: The sulphur content and/or the sulphur compound composition of a melt in a recovery boiler of a pulp mill is adjusted by bringing a gas into contact with the melt. The gas may be a gaseous sulphur compound or a reducing gas. Bringing the gas into contact with the melt increases the sodium sulphide content of the melt while decreasing the sodium carbonate content, so that the sulphidity of the melt increases. The sulphur-containing gas may be produced by pressure heat treatment of black liquor, so that the black liquor burned in the recovery boiler has a lower sulphur content and thereby reduces the boiler sulphur emissions, and boiler corrosion problems.

Abstract: In the bleaching of paper pulp without chlorine, such as using an OTZE or OZP bleach sequence, filtrate from subsequent stages is used as a wash liquid in preceding wash stages. The filtrate is treated with a fiber filter, and then passes through a magnesium filter, which has magnesium chips with a maximum dimension of about 3 mm or less. The magnesium removes heavy metal ions, and if any of the magnesium dissolves in the filtrate it provides a beneficial affect on the pulp. The pH of the filtrate is typically maintained in the 2-6 range to facilitate dissolution of Cu, Mn and Fe while the magnesium stays out of solution. Two filters may be provided in parallel so that if one becomes contaminated the other may be quickly put into service, and any explosive gases from the magnesium filter are vented.

Abstract: Sodium sulphate and/or sodium carbonate containing material from a cellulose pulp mill, such as fly ash from a recovery boiler, or a by-product from a chlorine dioxide production plant, is brought into contact with a a gas containing hydrocarbons and/or hydrogen sulphide which chemically reacts with the material to produce sodium sulphide. Preferably the gas contains one or both of ethene and hydrogen sulphide. The sodium sulphide produced is used to effect sulphate cooking of cellulose pulp. The gas may be brought into contact with the melt of a soda recovery boiler to increase its sulphidity, either prior to, after, or simultaneously with bringing the gas into contact with the material. The sources of the fly ash and melt may be a soda recovery boiler combusting black liquor under oxidizing conditions.