The difference between sodium bentonite and calcium bentonite
Bentonite is a clay rock, also known as montmorillonite clay rock. Normal bentonite contains a small amount of illite, kaolinite, halloysite, chlorite, zeolite, quartz, feldspar, calcite, etc.; generally white, light yellow Due to the change of iron content, it is light gray, light green, pink, brown, brick red, gray black, etc.; waxy, earthy or grease luster; bentonite is loose as soil, some dense and hard chemical components It is silica, alumina and water, and also contains iron, magnesium, calcium, sodium, potassium and other elements. The content of Na 2 O and CaO has a great influence on the physical and chemical properties and process technology of bentonite. According to the type, content and interlayer charge of montmorillonite exchangeable cations, bentonite can be divided into sodium bentonite (alkaline soil), calcium bentonite (alkaline soil), natural bleaching earth (acid soil or acid clay). Among them, calcium bentonite and calcium bentonite and calcium magnesium base have strong hygroscopicity and swelling property, and can adsorb 8~15 times of their own volume of water, and the volume expansion can be several times to 30 times; in aqueous medium It can be dispersed into gelatinous and suspended form. This kind of mass solution has certain viscosity, thixotropic and lubricity; it has strong cation exchange capacity; it has certain adsorption capacity for various gases, liquids and organic substances. The maximum adsorption capacity can be up to 5 times its own weight; it has plasticity and cohesiveness with the admixture of water, mud or fine sand;
Adsorption is a natural phenomenon in which all solid materials are present. We collect some molecules on the surface of bentonite, which is called the adsorption of bentonite. This adsorption has been widely used in industry. For example, drilling mud often uses the adsorption characteristics of bentonite minerals to adjust the mud parameters for different purposes. For example, adding fluid loss additives is to adsorb the bentonite particles and water molecules through one end of the polymer polymer adsorbed on the surface of the bentonite particles and the other end dissolved in water. There is an indirect connection between them. A bridging effect is formed, which reduces the free water in the mud, changes the performance parameters of the mud, and achieves the purpose of reducing the filtration rate.
Bentonite adsorption can be divided into three types: physical adsorption, chemical adsorption and ion exchange adsorption.
l) physical adsorption. Physical adsorption is caused by the intermolecular attraction between the adsorbent and the adsorbate, which is what we often say by van der Waals forces. Physical adsorption is a reversible adsorption process, and the adsorption rate and desorption rate are dynamically balanced under certain conditions. The main reason for the physical adsorption is the surface energy of the surface molecules of bentonite. Since bentonite is highly dispersed in water, physical adsorption is very obvious.
The factors affecting the adsorption of bentonite minerals are:
1) The effect of bentonite type. The adsorption capacity of sodium bentonite is obviously stronger than that of other types of bentonite minerals such as calcium.
2) The effect of the particle size of the bentonite particles. According to the theory of solid adsorption, the adsorption capacity of the pulverized bentonite mineral is obviously improved, and the finer the pulverized mineral, the stronger the adsorption.
3) The influence of the solution medium. According to the electric double layer theory, bentonite mineral crystals are negatively charged, and ion exchange occurs during formation of the electric double layer. If the ion concentration in the solution is too high, the double layer of the bentonite particles is compressed, the dispersion and diffusion of the bentonite are inhibited, and even the bentonite is coagulated and coalesced.
Bentonite expands when exposed to water. The main cause of this natural phenomenon is that the spacing of the mineral layer of bentonite is increased, water molecules enter the mineral layer, and the cause of swelling of bentonite is the cation exchange of bentonite minerals. The swelling property has a great relationship with the properties of bentonite and the content of montmorillonite. The swelling property of sodium bentonite is obviously stronger than that of calcium bentonite. In addition, the purity is higher, and the bentonite with high montmorillonite content has stronger expansibility. Therefore, in practical applications, if we mainly want to use the swelling property of bentonite minerals, then we should first choose sodium bentonite mine when considering the types of bentonite minerals, and secondly consider the sodium bentonite with high montmorillonite content. In the work of mechanical casting and iron ore pellets, the requirements for expansion are high. A large amount of calcareous bentonite does not meet the requirements for use, so it is necessary to modify the calcium bentonite before use.
The dispersion of sodium bentonite is higher than that of calcareous bentonite. The water absorption of sodium bentonite is high. The reasons for the different swelling results of swelling expansion of large sodium bentonite and calcium swelling are:
1) The cation can bond the bentonite particles together, which restricts the dispersion of the bentonite particles. The multivalent ions have a higher charge density than the monovalent ions, and the strong electrostatic attraction between the particles is generated, so that the ability of the bentonite particles to bond is strong, so the dispersing ability of the calcium bentonite is weaker than that of the sodium bentonite.
2) The negative charge generated by the montmorillonite lattice replacement is to adsorb the oppositely charged ions to balance the electrical properties of the solution. The oppositely-positive ions are present in the solution in the form of hydrated ions, and the negatively charged montmorillonite particles adsorb the hydrated cations to form an electric double layer having an inversely proportional thickness to the counter-ion valence. That is, the cation price is high, the hydration film is thin, and the expansion ratio is low; while the cation is low in cost, the hydrated film is thick, and the expansion ratio is high.
3) The thickness of the adsorbed water of the sodium bentonite layer is three layers, and the thickness of the adsorbed water of the calcium bentonite layer is four layers. Under the action of polar water molecules, due to the small electrostatic attraction, a large interlaminar spacing can be produced between the sodium bentonite crystal layers, while the calcium bentonite has a large electric attraction between the crystal layers, and the polar water molecules It is not easy to enter between the crystal layers. Therefore, the distance between the calcium bentonite crystal layers is significantly smaller than that of the sodium bentonite. It is indicated that the calcium bentonite is more difficult to disperse in the water than the sodium bentonite, and the expansion ratio is low. In essence, the swelling property of montmorillonite is controlled by its chemical composition, and the smectite containing more sodium ions can be continuously expanded until it becomes a gel state. The smectite containing more calcium ions can only be expanded from the dry state to the hydrated state. After understanding the deep-seated factors affecting the swelling of swelling, we can artificially and effectively control the swelling properties of bentonite minerals to achieve the best results.