Chloride and potassium are harmful non-process elements in the kraft chemical recovery cycle. In high concentrations they can cause severe corrosion and fouling of the recovery boiler heat exchangers. Traditionally their levels in circulation have been controlled by dumping a portion of ESP ash. However, environmental concerns and cost of make-up chemicals are increasing the interest towards more affordable and environmentally friendly removal methods. This paper presents the principles of two common chloride and potassium removal methods: ash leaching and evaporative recrystallization. Experiences and the suitability of these two methods in different situations are reviewed based on operational data collected from recent installations. In the ash leaching process chloride and potassium are leached out of ESP ash with hot water. In ash crystallization the ash is completely dissolved in water and the mixture is evaporated until sodium sulfate crystallizes from the solution. In both processes valuable sodium is returned to the recovery cycle by mixing the solids with heavy black liquor while chloride and potassium are removed by purging the liquid reject. When comparing the two processes, leaching is simple and the more straightforward option while crystallization requires a bit more sophisticated process design. The composition of ESP ash has a major impact on process performance but generally the crystallization method yields better removal and recovery efficiencies. Selecting the most suitable removal method for a mill is always case specific since it depends on raw materials, make-up chemicals and degree of closure of the mills' chemical cycles. As a rule of thumb, leaching is most suitable for smaller removal needs and retrofits while crystallization can handle larger capacities, making it a good option for new and larger mill projects.
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