CHEMICAL RECOVERY
A. Evaporation.
The black liquor that comes out of the pulping sequence is approximately 10-15% solids. In order to maximize the burning efficiency and get out as much energy as possible from the recovery boiler, the black liquor solids content must be increased to somewhere in the vicinity of 60-80% solids. The most common way of doing this is via multiple-effect evaporators. These evaporators remove the bulk of the water by operating in series while at different pressures. Therefore the vapor from one evaporator body can be the steam supply for the next unit. In this approach the original feed steam performs the final concentration and the vapor becomes the steam for the next less-concentrated evaporator (i.e., countercurrent operation).
B. Recovery Boiler.
The recovery furnace/boiler is the heart of the kraft chemical recovery process. Its major functions include:
The heavy black liquor from the last concentration stage is fed into the furnace. The liquor droplets dry and partially pyrolyze before falling onto a bed of charred products. Incomplete combustion causes the carbon and carbon monoxide present to act as reducing agents which converts the sulfate (SO4-2) and thiosulfate (S2O3-2) into sulfide (S-2). The heating causes melting of the newly formed sodium salts which filter through the bed of char onto the floor of the furnace. The molten material (smelt) then flows into the dissolving tanks forming the green liquor.
C. Recausticizing
The purpose of the recausticizing plant is to convert sodium carbonate (Na2CO3) into sodium hydroxide (NaOH) and remove most of the impurities that are introduced from the furnace and lime kiln. The recovery furnace smelt is dissolved in water to form the green liquor. The green liquor is clarified (filtered) to remove insolubles (dregs) and reacted with lime (CaO) to form the white liquor. The white liquor is then clarified to remove the precipitated lime mud (CaCO3). At this point the white liquor can be submitted to the digester for chip delignification. The lime mud is reburned to form CaO in the lime kiln, and the material can be used again in converting the NaCO3 to NaOH.
D. Recovered Commodities
During recovery operations one can obtain two materials of commerical value, turpentine and tall oil, both of which originate from softwood species. Turpentine is a volatile fraction obtained during digester venting or during black liquor concentration. It is typically sold as a solvent, or to chemical companies which can use it for disinfectant or for starting material for fragrances or specialty chemicals. The US pulp and paper industry produces somewhere in the vicinity of 31 million gallons annually. Tall oil is the fatty acids and resin acids of softwoods (abietic acid for example). This material usually floats to the top of black liquor solutions during processing and is skimmed off and processed elsewhere. It finds use in soaps and paper sizing. About 450,000 tons of this material is recovered annually in the United States, mostly from Southern pines. Black liquor itself is a commodity that can be sold between mills depending on the capacity of each mills recovery boiler. In most cases, mill output is directly tied to the capacity of black liquor recovery operations.