For economic and environmental reasons, chemical recovery processes are used in chemical and semi-chemical pulp mills to reclaim spent cooking chemicals from the pulping process. At kraft and soda pulp mills, spent cooking liquor, referred to as “weak black liquor,” from the brown stock washers is routed to the chemical recovery area. The chemical recovery process involves concentrating weak black liquor, combusting organic compounds, reducing inorganic compounds, and reconstituting the cooking liquor.
In black liquor concentration, the weak black liquor from pulping processes – containing wood lignins, organic materials, oxidized inorganic compounds (Na2SO4, Na2CO3), and white liquor and having a solids concentration of about 12 to 15% – passes through a series of evaporators to increase solids content. Depending on the type of evaporators used in later stages (direct or indirected), an intermediate oxidation step may also be included. The solids content of the black liquor following the final evaporator/ concentrator typically averages 65 to 68% (US EPA, 2010).
Combustion of organic compounds takes place in a recovery furnace. The black liquor has a has an energy content of approximately 14 to 16 MJ/kg-dry solids. Consequently, steam for mill processes and/or electricity can be produced from this combustion. Recovery boilers typically have a thermal efficiency of around 65% and this efficiency can be increased with increased solids content in black liquor (around 2% increase in steam production per 5% increase in solids contents). During combusion, the inorganic process chemicals are reduced to a molten smelt, which is removed from the bottom of the boiler and is further refined in subsequent steps (Kramer et al., 2009. p.18).
