Lime Mud Reburning. Properties and Quality of the Lime Produced
It is of utmost importance that the white liquor used in the digester of a sulphate pulp mill is of high quality. The white liquor is prepared in the causticizing plant, through causticizing of the sodium carbonate in the green liquor. The causticizing is made with slaked lime, which, in turn, has been calcined in the lime mud reburning plant and slaked in the green liquor. To enable high quality white liquor to be produced the lime has to be of high quality. A high quality lime is reactive in the slaking and causticizing and the later-formed lime mud has to be easily separated from the white liquor.
This study has shown that the quality of the lime is closely correlated to the specific surface area of the lime. Lime calcined in different types of equipment was slaked and causticized in laboratory scale equipment. The specific filtration resistance of the later-formed lime mud was then determined. Reburned lime mud from a rotary kiln, a pilot scale fluidised bed and from a laboratory oven were all more reactive when the specific surface area of the lime was large. However, the later-formed lime mud was more easily separated from the white liquor if the lime had a small surface area.
Since the size of the specific surface area of the lime is extremely important, the effects of various reburning conditions on the surface area were more closely examined. Experiments in a laboratory oven showed that the specific surface area of the lime is decreased by an increased temperature or increased residence time in the oven. Limes that were not completely calcined had a specific surface area between that of the parent and the calcined material. Furthermore, experiments in an atmosphere of pure carbon dioxide, at non-calcining temperatures, showed considerable sintering of lime mud before calcination. This is important since the characteristics of the parent material is of importance to the final product. Calcium carbonate pro analysi used as a reference, had ten times larger specific surface area. This is likely the effect of less impurities and of the different structure of the parent material.
Lime mud and calcium carbonate were calcined in pure nitrogen in a quartz glass reactor. These experiments showed the largest reported specific surface area of calcined lime mud, 9.2 m2/g, when treated for 300 s at 700 °C. The specific surface area of calcium carbonate p. a.calcined in the quartz glass reactor was about ten times larger than that of the lime mud. The decrease in specific surface area with time was found to follow the German and Munir equation. The temperature dependence was empirically estimated and included in the German-Munir equation.
The .gamma. value, 3, indicates solid phase diffusion as the mechanism of sintering.
The lowest specific surface area was obtained for lime produced in a rotary kiln, 0.2 m2/g. The lime from the pilot scale fluidised bed was more soft-burned and had a specific surface area that was ten times larger, 2 m2/g. This is presumably due to the good mixing in the fluidised bed reactor, which enables high heat and mass transfer in the reactor. Accordingly, the temperature and the concentration of carbon dioxide is kept at a low level. The largest surface area obtained in this study was that of calcium carbonate p. a.calcined in pure nitrogen, 99 m2/g.