Effect of Partial Borate Autocausticizing on the Causticizing Rate and Equilibrium, 2002 Fall Technical Conference

Borate autocausticizing can replace - total autocausticizing, or supplement the kraft lime cycle - partial autocausticizing. Partial autocausticizing has recently been commercialized in one kraft mill and is undergoing trials in several additional mills. In the on-going partial borate autocausticizing trails, a common but unexpected observation is that the causticizing efficiency increases. The objectives of this study are to determine the effect of borate on the causticizing rate, equilibrium, and rate controlling parameters with the aim of defining the conditions that can increase causticizing efficiency. The causticizing experiments were conducted using analytical grade sodium carbonate, sodium borate, sodium sulfide and commercial burnt lime. The major findings are that when the degree of completion of the causticizing reaction is rate limited (which is true in commercial operations), partial borate autocausticizing can increase causticizing efficiency due to the higher initial hydroxide content. Temperature was found to be a critical variable for achieving high causticizing efficiencies after partial borate autocausticizing.