Sintering in the Kraft Pulp Mill - Studies in the Rotary Kiln and Recovery Boiler

Abstract: The conversion of black liquor to white liquor is carried out in the chemical recovery system, which is made up of several units; in two of them, i.e. the recovery boiler and the rotary kiln, sintering plays an important role.

To study what influences sintering of recovery boiler dust, several dusts of different compositions were heat treated and their sintering behaviour examined. Sintering was measured as the reduction of the specific surface area. It was shown that, besides temperature and sintering time, the first melting temperature of the dust strongly influences the degree of sintering. The lower the first melting temperature, the lower the specific surface area. The reduction of the specific surface area with time was found to agree with the German Munir equation.

It is important, for several reasons, to know what influences sintering of reburned lime mud in the rotary kiln. Different kinds of experiments were run. First, a quartz glass tube reactor was used to reburn lime mud of different compositions in different sintering atmospheres. It was found that the degree of sintering depends on the compound the lime mud was doped with. Generally speaking, a higher concentration of the foreign compound resulted in a higher degree of sintering. The results suggest that the melting point of the doped solid influences the sintering process as well, with more sintering for low melting point solids. It was also shown that carbon dioxide in the sintering atmosphere strongly effects sintering. Even small amounts enhanced the sintering behaviour significantly and, at partial pressures of carbon dioxide corresponding to equilibrium, the reduction of the specific surface area was very rapid. The German Munir equation was used as a base to develop models that describe the reduction of the specific surface area as functions of time, temperature, partial pressure of carbon dioxide and concentration of the different doping compounds. Pellets formed either of lime mud or reburned lime mud were heat treated in a muffle furnace and their strength was measured. It was shown that not only the temperature and time but also the compact pressure markedly influence the strength of the pellets. Also, when forming pellets out of already calcined lime mud, the pellets became much stronger. Finally, lime mud was reburned in a pilot scale rotary kiln. The concentration of carbon dioxide in the sintering atmosphere was varied, and it could thereby be seen how the specific surface area depends on the residual carbonate. Two different lime muds were used, and they showed different behaviours. The first reached a minimum in specific surface area at about 10 weight-% of residual carbonate, whilst for the second, the specific surface area decreased with increasing residual carbonate over the whole range investigated. Rings and balls were formed, and the strength of the balls increased with increasing residual carbonate.