The Kraft Recovery Cycle

Direction of black liquor flow Incraasinfl solids content 

The black liquor needs to be concentrated to at least 65% solids before it can be burnt in the recovery furnace. Traditionally the viscous liquor was concentrated further by direct contact with the hot flue gases from the recovery furnace for example in a cascade rotary evaporator where the liquor is picked up from a vat and 

Black liquor is not an ideal fuel, partly because of its moisture and partly because of the very high ash content, i.e. the inorganic elements. It has been difficult to achieve steam temperatures above 480°C, compared to 540°C with coal. The ability to generate electricity can be achieved only with higher combustion temperatures by burning at a higher solids content (80%). Newer steels and designs can limit corrosion from small amounts of chlorides and potassium. New recovery furnaces can have rated capacities of over 2500 tonnes of dry solids per day. 

Losses of sodium and sulphur, e.g. in pulp wash and in flue emissions, used to be made up by the addition of sodium sulphate and sodium carbonate, hence the term the sulphate process. The sodium sulphate undergoes reduction to sodium sulphide in the recovery furnace. With systems moving to total closure of chemical and water cycles, the presence of small amounts of sulphur in the wood itself and in the magnesium sulphate coming to the evaporators from the oxygen delignification plant is such that the sodium intake exceeds sulphur losses. In that case the addition of Na2S04 would result on excessive sulphidity. Hence only NaOH may be required. 

The ideal of having only Na2S and Na2C03 in the smelt is not true there are nonprocess chemicals, Na2S04, Na2S203 and contaminant salts, in the green liquor. The calculated causticity and sulphidity have to adjust for the presence of these salts. 

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The additional sulfur for polysulfide pulping can upset the sodium—sulfur balance in the kraft recovery cycle and increase sulfur emission problems. In the MOXY (Mead Corp.) process, polysulfide is formed from kraft white Hquor by catalytic oxidation of sodium sulfide in the white Hquor using air. 

Figure 13.14. The kraft recovery cycle it is a closed loop.

Modijications to the Recope Cycle. The recovery system is a principal capital cost in a kraft mill. Consequently, any recovery process that is less expensive to build can improve pulping economics. There have been numerous attempts to improve the kraft recovery process. Two examples are the direct alkaline recovery scheme (DARS) and the autocausticizing scheme using sodium borates (37). Both schemes eliminate the lime loop of the conventional kraft mill. As of 1996, neither is commercially used. 

Rapp HJ and Pfromm PH. Electrodialysis for chloride removal from the chemical recovery cycle of a kraft pulp mill. J. Membr. Sci. 1998 146 249-261. 

Foster, R., et al., Optimization of the Chemical Recovery Cycle of the Kraft Pulping Process, Laboratory for Applied Industrial Control, Report 54. Purdue University, West Lafayette, IN, 1973. 

M. Paleologou, R.M. Berry, R. Thompson, and J.T. Wearing, Electromembrane process for the treatment of Kraft mill electrostatic precipitator catch, US Pat. 5,567, 293 H.-J. Rapp and P.H. Pfromm, Electrodialysis for chloride removal from the chemical recovery cycle of a Kraft pulp mill, J. Membr. Sci., 1998, 146, 247-261 P.H. Pfromm, S.-P. Tsai and M.P. Henry, Electrodialysis for bleach effluent recycling in Kraft pulp reduction Simultaneous control of chloride and other no-process elements, Can. J. Chem. Eng., 1999, 77, 1231-1238. 

The paper pulping industry is reportedly not, in total, a very large emitter of sulfur oxides, although individual plants may present local problems. Kraft mills emit more malodorous reduced sulfur compounds, whereas sulfite mills are more important as emitters of sulfur dioxide. The pulping processes (particularly sulfite) are most interesting because the chemical recovery cycles use basic chemistry that could well be applied to recovery of sulfur dioxide and sulfur from the flue and process waste gases of other types of sources. 

Figure 7-38 illustrates the recovery and conversion of the kraft cooking chemicals. In the case of sulfur-free pulping (soda process, soda-oxygen process, and anthraquinone-alkali pulping) only sodium carbonate is recovered and chemical losses are compensated by adding sodium carbonate. The hydroxide-carbonate and lime cycles are the same as for the kraft process. 

In spite of these efforts to optimize the combustion of black Hquor together with many improvements of the energy efficiency of pulp and paper mills, there have also been activities to improve significantly the efficiency of the energy (i.e., the evident production of electric power) and chemical recovery cycle in kraft pulping by one... 

Despite the electronic communications revolution, paper production continues to grow worldwide at a rate of 2% to 3% annually. Much of the incremental increase is based on pine because no other long-fibered softwood offers such high yield rates per hectare coupled with a short growth cycle. New kraft pine pulpmills and most existing mills will continue to practice tall oil recovery because non-removal of tall oil soap from kraft black liquor adversely affects evaporator and recovery boiler capacity (14). Increased tall oil production has been accompanied by new fractionator capacity in Europe and incremental expansion in the United States. Since the paper industry is the largest user of rosin size, it might be expected that the increased paper production would absorb the increased availability of tall oil rosin. 

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