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Expansibility of Bentonite
Published:
2022-10-25
Bentonite expands when it meets water. The main reason for this natural phenomenon is that the spacing between the crystal layers of bentonite minerals increases, and water molecules enter the crystal layer of the minerals. In addition, the cation exchange effect of bentonite minerals also causes the expansion of bentonite. The expansibility is greatly related to the properties of bentonite and the content of montmorillonite. The expansibility of sodium bentonite is significantly stronger than that of calcium bentonite. In addition, the expansibility of bentonite with higher purity and high content of montmorillonite is stronger. Therefore, in practical applications, if we mainly want to utilize the expansibility of bentonite minerals, we should first select sodium bentonite minerals when considering the types of bentonite minerals, and then consider sodium bentonite with high montmorillonite content. In mechanical casting and iron ore pellet work, there is a high requirement for expansibility. A large amount of calcareous bentonite cannot meet the use requirements, so it is necessary to modify the calcareous bentonite before use.
The degree of dispersion of sodium bentonite is higher than that of calcium bentonite, and the water absorption and expansion ratio of sodium bentonite are high. The reasons for the different results of water absorption and expansion on sodium bentonite and calcium bentonite are:
1) Cations can bind bentonite particles together, restricting the dispersion of bentonite particles. The charge density of multivalent ions is higher than that of monovalent ions, which generates strong electrostatic attraction between particles, making the bentonite particles have a strong ability to connect. Therefore, the dispersion ability of calcium bentonite is weaker than that of sodium bentonite.
2) The negative charge generated by lattice replacement of montmorillonite needs to adsorb ions with opposite electrical properties to balance the electrical properties of the solution. These ions with opposite electrical properties exist in solution as hydrated ions, and negatively charged montmorillonite particles adsorb hydrated cations to form a double electric layer. The thickness of the double electric layer is inversely proportional to the power of the counter ion valence, i.e., the cation valence is high, the hydration film is thin, and the expansion factor is low; However, the cationic valence efficiency is low, the hydration film is thick, and the expansion factor is high.
3) The thickness of sodium bentonite crystal layer adsorbing water is three layers, and the thickness of calcium bentonite crystal layer adsorbing water is four layers. Under the action of polar water molecules, due to the small electrostatic attraction, large crystal layer spacing can be generated between the crystal layers of sodium bentonite. However, due to the large electrostatic attraction between the crystal layers of calcium bentonite, polar water molecules are not easy to enter between the crystal layers. Therefore, the distance generated between the crystal layers of calcium bentonite is significantly smaller than that of sodium bentonite, which is manifested in that calcium bentonite is more difficult to disperse in water and has a lower expansion factor than sodium bentonite. In fact, the expansion of montmorillonite is controlled by its chemical composition, and montmorillonite with more sodium ions can continue to expand until it becomes a gel state. There is a limit to the expansion of montmorillonite with high calcium ions from the dry state to the aqueous state. After understanding the deep-seated reasons that affect the expansibility of bentonite, we can artificially and effectively control the expansibility of bentonite minerals to achieve the best use effect.
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Keywords: foundry bentonite, montmorillonite, cat's litter, bentonite raw ore, papermaking soil, impervious particles, metallurgical pellets