Prediction of metals distribution in mill processes, Part 3 of 3: NPE management in kraft chemical recovery

Most transition metals are precipitated in green liquor as dregs. Completely removing dregs from green liquor will control transition metals in chemical recovery systems. A regular, small purge of lime mud controls phosphorus in lime mud at acceptable levels. The Samoa Pacific mill installed an X-filter and a pre-coat dregs filter to improve dregs removal. Mill measurements show significant improvements in lime mud quality, lime availability, and lower nonprocess dement content in the white liquor. It took more than a year for the new system to purge the NPE accumulation in the recovery system. Using equilibrium calculation tools, mill and laboratory measurements are interpreted and incorporated into metals equilibrium models. Combining the equilibrium models with a particular mill process configuration, process simulation successfully predicts metals content in kraft mill recovery systems. The metals distribution and concentrations are very process dependent. (Part 1 of this study was published in TAPPI JOURNAL Vol. 3 No. 1, and Part 2 was published in TAPPI JOURNAL Vol. 3 No. 2.) Application: This study will help mills manage non-process element (NPE) on a mill-wide basis. In particular, better dregs removal achieves significantly drier lime mud to the kiln.