The mixing of refractory castable and water to make refractory castable slurry is an extremely important process in its construction. Mixing is done at constant speed in specialized mixing equipment. The mixing force generated during the mixing process, the time required for uniform mixing, and the temperature of the refractory castable slurry increase.
The refractory castable mixture contains a certain amount of fine powder and micropowder, and the powder generally has a natural agglomeration tendency. The forces of particle adhesion and agglomeration are van der Waals force and capillary force in the presence of water, and the roles of the two in powders with different compositions are more complicated.
In the initial stage of adding water, the particles are covered by the so-called liquid film of the adsorption layer, and at the same time, connecting "liquid bridges" appear between the particles. The overlapping of the adsorption layers produces adsorption forces that lead to particle agglomeration. The adsorption force increases as the contact surface of the particles expands, thereby increasing the strength of the agglomerates.
When water is further added or the water in the particles is improved to separate them, the number of particles covered by water can be increased, and the torque can be increased at the same time.
When the water content reaches a critical (turnover) level, "liquid bridges" form between the particles. The shear resistance of the system increases sharply (at this time there is capillary attraction). In general, shear resistance increases with increasing particle surface area (ie, decreasing powder particle size) (due to excess "liquid bridges" forming)
When the water in the refractory castable is just enough to fill the pores between the particles (pores) and cover the surface of the particles to a critical value (the turning point), the capillary force is the strongest. Further addition of water results in a dramatic reduction in the number of "liquid bridges" and a consequent reduction in the shear resistance of the mud (slurry).
In the initial stage of mixing with water, refractory castables often form water-containing agglomerates (some agglomerates are also wrapped with a large amount of free water, that is, non-adsorbed water), which will seriously affect the flow properties of the refractory castable slurry. Therefore, only when these agglomerates are broken up to form smaller mobile monomers (particles or clusters) can the refractory slurries have rheological properties.
The mixing process of refractory castables needs to go through the following three processes:
(1) Break the agglomerates of the dry powder and homogenize the powder particles.
(2) Add water to the powder to transform it into a fluid state due to the powder state. This process requires sufficient mixing energy, which is usually called the turning point (transition point) of the refractory castable.
(3) After adding all the water required, mix the material to a suitable stable and homogeneous state. Sometimes, high shear rates can be applied in the final stage to compensate for the effects of insufficient mixing of the two processes.
(4) The agglomerates formed by the fine particles during mixing become stronger due to the enhancement of the van der Waals effect and the capillary force generated by the reduction of the particle size. Therefore, these forces must be overcome during mixing in order to break up the agglomerates and homogenize the corresponding refractory castable slurry.
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