Overview of Various Strategies for Balancing Saltcake, Chloride and Potassium Levels in an ECF Kraft Mill, 1997 Minimum Effluent Mills Symposium Proceedings

As mill closure increases, so does the accumulation of non-process elements in the kraft liquor cycle. The proposed EPA Cluster Rule requires the implementation of best management practices (BMPs) to contain both pulp and liquor spills. This will require the liquor cycle to be closed more than is normally practiced. For the kraft recovery boiler, mill closure causes increased chloride and potassium concentrations in the as-fired liquor, resulting in a lower melting point ash in the boiler. This can lead to accelerated plugging in the upper superheater and boiler bank sections, resulting in downtime for water washing. Two technologies for removing chloride and potassium from precipitator dust are described in detail. One is an evaporator/crystallizer based process, while the other uses ion exchange. The evaporator/crystallizer process is effective for both chloride and potassium removal (90%), whereas the ion exchange process is more efficient at chloride removal (95%) and less selective in removing potassium (5- 10%). The latter is more effective at saltcake recovery (95%) compared to the former (80%). The best suited technology depends on the requirements of each mill. Computer simulation was used to model an actual ECF kraft pulp mill. It was used to simulate various degrees of recovery cycle closure for a BMPs case, resulting in a need for chloride and potassium removal. The two chloride and potassium removal technologies were evaluated for this case, showing only 40 to 45% of the total precipitator dust required treatment by either process to prove effective.