Compared with calcium montmorillonite, sodium montmorillonite has better water dispersibility, suspension, expansion, thickening and lubricity, so it has a wider range of uses.
However, the vast majority of bentonite ore produced naturally in China is calcium-based bentonite, and the commonly used method in the industry is to increase the expansion of bentonite by adding sodium modification agent under certain conditions.
1, montmorillonite sodium modification process and equipment.
There are many processing methods for sodium modification of montmorillonite, which can be generally divided into dry, semi-dry and wet modification.
Due to the difficulty of ion migration under dry conditions, the modification effect is poor, so the use of more wet and semi-dry methods, including stirring, ball milling, homogenization, extrusion sodium, etc., by increasing the shear force, reduce the dispersion particle size, extend the aging time and other ways to speed up the ion exchange process, improve the reaction efficiency and reaction degree, so as to achieve the purpose of promoting the modification effect.
Laboratory modification using stirring sodium is the most common, in the pulping at the same time to add montmorillonite and modifier, after soaking pulping dehydration, drying, crushing of sodium bentonite, often combined with wet purification. This method requires low equipment, the process is easy to achieve, but the stirring equipment line speed is low, the shear force is small, although the way of water bath heating to a certain extent to speed up the ion exchange process, but the dispersion of montmorillonite particle size is relatively coarse, so the modification effect is limited, sodium is not sufficient.
The ball milling modification is to drive the ball milling media such as aluminum balls and zirconium balls to move together through the high-speed rotation of the ball milling tank. Due to the action of inertia and centrifugal force, they fall to a certain height and produce strong impact on the grinding materials. At the same time, the medium ball rotates and revolves in the ball milling tank, and the extrusion impact force, shear force and friction force are generated between the medium balls and the ball milling tank, under the combined action of these forces, the montmorillonite particles become fine and the completion of the sodium reaction is promoted. However, affected by the speed of the ball mill tank, the size of the medium ball, the grinding limit, etc., the particle size of the montmorillonite modified by ball milling is finer than the sodium sample, but coarser than the colloid mill modified sample.
Colloidal mill and ball mill modification are both ultrafine montmorillonite slurry and sodium modification. When the colloid mill is modified, the rotor and the stator move at a relatively high speed. When the material passes through the gap between the two, it is subjected to strong friction, shear force, spiral impact force, high-frequency vibration and other forces, so that the material is effectively mixed, Dispersion, ultrafine, etc. The general line speed is 8-12 m/s, and the processing particle size is finer than the particle size modified by ball milling.
Due to the high cost of wet processing and drying, semi-dry sodium is used more in actual production. Generally, after adding a certain amount of sodium solution to the raw ore, mix well, pile up for 7-10 days, and often turn and stir, sodium conversion efficiency is low and the cycle is long. At the same time, the application of a certain shear force, or the use of microwave radiation process, can improve the efficiency of ion exchange, shorten the sodium reaction time.
2. Common problems in sodium modification of montmorillonite
Common sodium methods generally have the following problems:
(1) sodium for a long time, the reaction is not complete. In actual production, calcium-based montmorillonite ore is generally sodized when the water content is more than 10%. Montmorillonite contains a large amount of adsorbed water and interlayer water, which to a certain extent prevents sodium ions from entering the interior of montmorillonite particles. After the surface of the particles contacts with the modifier sodized, a barrier layer is formed. Generally, the barrier layer is broken by increasing extrusion or strong shearing, and the sodium ions are forced into the montmorillonite layer by external force to promote the ion exchange reaction.
(2) The divalent iron in the structure is not oxidized to trivalent iron. Al3 is in the octahedral lattice of montmorillonite, when Fe3 replaces Al3, it is electrically neutral; when Fe2 replaces Al3, it is negatively charged, and under the action of Coulomb force, the crystal layer is more closely combined, the expandable crystal layer is less, and the expansion is small. When the montmorillonite crystal internal Fe2 oxidation to Fe3, more overlapping crystal lamellae will be separated, the proportion of expandable crystal layer increases, improve the expansion.
On the one hand, under the condition of natural weathering, the divalent iron in montmorillonite is slowly oxidized into trivalent iron. On the other hand, sodium ions gradually enter the montmorillonite layer by prolonging the time to promote the slow progress of sodium reaction, but it is often placed for several months or even more than half a year, with a long cycle.