Use of Borate Autocausticizing to Supplement Lime Kiln and Causticizing Capacities, 2004 Paper Summit, Spring Technical & International Environmental Conference, (including Papermakers; Process Control, Electrical & Information; Product & Product Quality)

In borate autocausticizing, sodium hydroxide is produced directly on dissolving trisodium borate in the green liquor, which forms sodium hydroxide and sodium metaborate. Sodium metaborate then follows the liquor cycle and is reconverted to trisodium borate in the kraft recovery boiler. Although borate autocausticizing can replace part or all of the causticizing/calcining cycle, this project has focused on partial autocausticizing. Partial autocausticizing is especially applicable to mills, which have limited causticizing/calcining capacity. Work conducted at Western Michigan has provided fundamental knowledge and understanding in the following areas: the furnace decarbonization reactions, the effect of borate on the causticizing reactions, the effect of the borates on black liquor properties and the effect of residual borate in the white liquor on pulping. The decarbonizing furnace reactions were found to start at about 600 oC and are highly dependent on temperature and the removal of the carbon dioxide. The addition of borate has only minor effects on the black liquor’s boiling point rise and viscosity, and may increase causticizing efficiency. One of the most significant findings was that borate has the potential to increase softwood yield. A number of mill trials have been conducted in North America. These trials generally support the results of this research.