Disintegration of Lime Mud Particles during the Causticizing Operation
It is of essential importance that the white liquor used in processing wood chips to produce pulp in the sulphate pulp process is of high and uniform quality. The white liquor is, together with lime mud, produced in the causticizing plant where sodium carbonate is causticized with lime, slaked in green liquor. The operations in this plant are influenced greatly by the physical properties of the solid particles involved which influences the reactivity of the lime and the separation properties of the lime mud. Therefore, in order to produce white liquor of high quality at a minimum of energy demand, it is important to understand how the quality of the lime and lime mud particles are influenced by the process conditions and the design of the equipment.
This work is focused on how different conditions in the slaking and causticizing reactor influence the size of the lime mud particles formed. The influence of lime quality, reaction temperature and mechanical forces caused by the mixing operation have been investigated. It is probable that, in the future, slaking and causticizing will be performed in pressurized vessels; it was therefore decided to use closed vessels which may be pressurized in the causticizing experiments.
The mixing systems were characterized before the slaking and causticizing experiments were performed since no papers could be found in the literature dealing with the mixing conditions in vessels suitable for pressurization. The first study focused on the power demand of different equipment configurations: the results showed that the major influence on the Power number was the type of mixing impeller. Also, the clearance between the impeller and the vessel bottom was of some importance. In order to minimize the power demand at the off-bottom suspension point, a small impeller located close to the bottom was found to be an appropriate choice. In the other characterization study, the flow conditions in the impeller region of the vessel generated from a high-flow impeller, a pitched-blade turbine and a Rushton turbine were investigated using LDV-technique. It was found that the flow rate at constant impeller speed increased in the order: high-flow impeller / Rushton turbine / pitched-blade turbine. The corresponding order for the turbulence fluctuation is: high-flow impeller / pitched-blade turbine / Rushton turbine.
Causticizing tests were performed in steel reactors. During the tests measurements of the size distribution were made. The experimental data was used to investigate models describing the disintegration of the lime mud agglomerates.
This study has shown that the decrease in particle size during slaking and causticizing is due to two different mechanisms. The reactions involved influence disintegration during the first ten minutes in the reactor and, in this period, the quality of the reburned lime mud is of utmost importance. The quality of the reburned lime mud in this work was characterized by measuring the B.E.T. specific surface area; it was found that the inverse of the average agglomerate diameter is a function of the inverse of the specific surface area.
The breakage of the agglomerates for longer residence times is due to mechanical forces caused by the mixing operation. The agglomerate size was influenced by the mechanical energy input from the impeller as well as its design, and it was found that the bulk flow, i.e. kinetic energy in macro-scale, is of major importance. When modelling disintegration in this second period in the reactor, grinding theory was useful. The average agglomerate diameter was found to be a function of the totalenergy input to the suspension from the impeller. The result of this work suggests that, for residence times longer than ten minutes, the disintegration of the lime mud agglomerates is caused by collisions between the agglomerate and either the impeller or the walls of the vessel.