Modification of Black Liquor Properties to Enhance the Chemical Recovery Efficiency

The black liquors generated from different raw materials have different ratios of organic to inorganic constituents, varying concentrations of process and non-process elements, widely ranging viscosities, and thermal properties. These situations can result in inefficient operations of chemical recovery systems, and over a long period can lead to terrible situations with the build-up of many problems in almost all unit operations. Modification of black liquor properties by subjecting it to controlled chemical, mechanical, and thermal treatments is one of the solutions to make black liquors suitable for smooth operation in chemical recovery unit operations. The agro residue and bamboo based black liquors are generally difficult to handle. Bagasse, wheat straw, rice straw and bamboo black liquors have some unusual characteristics of high viscosity and scaling during evaporation in multiple effect evaporators. As a result, they pose problems in subsequent recovery operations like combustion in recovery boiler, clarification of white liquor and reburning of lime sludge in lime kiln. Firing of low solids black liquor due to highly viscous semi-concentrated black liquor, significantly affects the thermal efficiency of recovery boilers and results in uneconomical operation. Silica present in straw and bamboo black liquors causes problems in smooth evaporation in multiple effect evaporators (MEEs), due to poor heat transfer and equipment down time. Liquor heat treatment and desilication of black liquor are discussed in this chapter to address these problems by modifying black liquor properties after subjecting liquors to chemical and thermal treatments. The impact of modification of black liquor properties is evident on operation of various other chemical recovery processes.