Bentonite is a kind of lamellar silicate, the main component is montmorillonite, the cations between the layers are easily exchanged, and it has a large ion exchange capacity. Bentonite is a kind of non-metallic mineral with many uses, and enjoys the name of "universal" clay. According to the type and content of exchangeable cations between montmorillonite layers, bentonite is divided into sodium-based bentonite, calcium-based bentonite, magnesium-based bentonite and aluminum (hydrogen) bentonite. This article mainly introduces the different properties of bentonite, mainly including properties, methods and application areas.
Bentonite is a kind of lamellar silicate, the main component is montmorillonite, the cations between the layers are easily exchanged, and it has a large ion exchange capacity. "Bentonite" is derived from the place name of the clay producing area in Wyoming, USA, and is also called "Bentonite" or "Bentonite". Pure bentonite is rare, most of which contain unequal amounts of impurities, such as quartz, feldspar, mica, zeolite, pyrite, etc. Bentonite is usually white, but also light gray, creamy, light red, flesh red, brick red, maroon, yellow-green, black, mottled, etc., showing grease luster, waxy luster or earthy luster, shell-like or jagged Shaped fracture.
Bentonite is known as "universal clay" because of its good physical and chemical properties. It can be used as a binder, suspending agent, thixotropic agent, stabilizer, purifying and decolorizing agent, filler, feed, catalyst, etc., and is widely used in metallurgy, Petroleum, foundry, food, chemical, environmental protection and other industrial sectors.
The content of montmorillonite is generally above 65%, so its properties determine the properties of bentonite. The main component of montmorillonite is silica tri-alumina, and contains a small amount of magnesium, calcium, potassium, sodium, and iron plasma. Its chemical formula is: (Na,Ca)0.33(Al,Mg,Fe)2[(Si ,Al)4O(OH)2•nH2O. Its structure is a 2:1 crystal structure composed of two silicon-oxygen tetrahedrons and a layer of aluminum-oxygen octahedrons.
Bentonite is mainly composed of dioctahedral montmorillonite-bederite, and also contains illite, kaolinite, chlorite, halloysite and other mineral components. As we all know, the content of montmorillonite can represent the grade of bentonite. The literature shows that the general formula of the unit half-cell structure of montmorillonite is:
It can be seen from the unit cell structure that it presents the characteristics of a 2:1 layered silicate, and the structural unit layer is composed of Si-O4 tetrahedrons sandwiching an AlO4(OH)2 octahedron. The oxygen atom at the top of each tetrahedron points to the center of the structure layer and shares with the octahedron.
Bentonite can be divided into calcium-based, sodium-based, lithium-based and hydrogen-based bentonite according to its interlayer cation species. At present, more than 90% of bentonite discovered in exploration belongs to calcium-based soil, and the content of montmorillonite is between 65 and 90%. The impurities mainly include a small amount of quartz, feldspar, mica, kaolinite, chlorite, calcite and organic matter. Its main mineral chemical components are silicon oxide, aluminum oxide, water, and a small amount of iron trioxide, manganese dioxide, magnesium oxide, calcium oxide, potassium oxide, sodium oxide, titanium dioxide, etc. However, the origin and geology of bentonite will directly affect its mineral composition and chemical properties, and also determine the physical and chemical properties and application scope of bentonite .
According to the type and content of exchangeable cations between the montmorillonite layers, bentonite is divided into sodium-based bentonite, calcium-based bentonite, magnesium-based bentonite and aluminum (hydrogen)-based bentonite. The interlayer domain of bentonite is the distance between its structural layers. Because the interlayer domain has the advantages of interlayer exchange, interlayer polymerization, interlayer pillaring and interlayer adsorption, it is used as an environmental adsorption material. The classification of bentonite is determined by the type and quantity of interlayer cations. If the interlayer is dominated by sodium ions, the bentonite belongs to sodium bentonite; if the interlayer is dominated by calcium ions, the bentonite belongs to calcium bentonite. The type of bentonite determines its interlayer spacing. The interlayer spacing of bentonite is generally 0.96~2.14nm, the interlayer spacing of calcium-based bentonite is about 15nm; the interlayer spacing of sodium-based bentonite is about 1.2nm .
Chemical Composition Of Sodium Bentonite And Lithium Bentonite
In recent years, it has become a research hotspot in various fields to adjust the interlayer spacing of bentonite through pillaring, ion exchange and other methods and change its properties to prepare functional materials.
Guo Guangsi et al. In the article on the influence of sodium bentonite on the properties of bauxite coatings, bauxite and talc are used as refractory aggregates, sodium bentonite and CMC are composite suspending agents, composed of sodium hexametaphosphate and water glass Binder, take water as carrier, add proper amount of defoamer and surfactant. The amount of CMC added is 0.5%, water glass is 5%, and sodium hexametaphosphate is 6% (the amount of each component added is the mass fraction of the refractory aggregate). Change the added amount of sodium bentonite, test the density, pH value, viscosity, suspension, leveling, coating, dripping and strength of the coating, and study the effect of sodium bentonite on the performance of the coating. The test found that the coating has the best comprehensive performance when the sodium bentonite is added at 5%.
Physicochemical properties of calcium bentonite
XRD image (left) and SEM image (right) of calcium bentonite
On the left is the XRD pattern of calcium bentonite. Analysis shows that its main components are montmorillonite (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2•xH2O and saponite Ca0.2(Al,Mg)2Si4O10( OH) 2.4H2O and SiO2.
The picture on the right is the SEM image of calcium-based bentonite. The analysis shows that the calcium-based bentonite has a sheet-like structure with random distribution, and the particle size distribution is very uneven and has no specific shape.
Jiang Lei In the research on resistance reduction and anti-wear of the new calcium bentonite grounding composite material, the commonly used metal-based and carbon-based conductive particles and the common corrosion inhibitors on the market were preselected, and then the calcium-based bentonite was determined by the orthogonal experiment. The optimal mass fraction of each component of the grounding composite material, and the main index performance of the calcium-based bentonite grounding composite material were tested, and the simulated embedding experiment and water retention experiment were carried out. The research results showed that:
Calcium-based bentonite is doped with different mass fractions of Zn, Fe, Al, graphite, carbon black, and carbon fiber. The results show that as the mass fraction of conductive particles increases, the resistivity gradually decreases and then tends to a stable value; Particle density, particle size, dispersion and shaft diameter ratio have a great influence on the resistivity of composite materials;
Among calcium-based bentonites, sodium nitrite, sodium molybdate, sodium silicate, sodium chromate, sodium borate, sodium polyphosphate and sodium phosphate have the same effect on Q235 (carbon structural steel with a yield point (σs) of 235MPa). Good anti-corrosion performance; comprehensive consideration of anti-corrosion performance, environmental protection and economic effects, etc., finally determine sodium molybdate and sodium polyphosphate as suitable corrosion inhibitors in calcium-based bentonite; through orthogonal experimental research, determine the grounding of calcium-based bentonite The best process plan for various mass fractions of composite materials is: carbon fiber 2.5%, graphite 9%, sodium polyphosphate 1.5%, sodium molybdate 1.5%, conductive cement 40%, calcium bentonite 45.5%; corrosion rate to Q235 Lower, the size is 0.027mm/a, the corrosion products are FeO(OH), Fe2(CO3)(OH), Ca2.8, Fe8.7Al1.2 Si0.8(OH), Fe2O3, Fe3O4; water retention experiment shows : Water content has little effect on the water retention of composite materials, while temperature has a greater impact on water retention. When the temperature is 30℃, the water retention of composite materials is the best; its main indicators and testing indicators meet the requirements of the power industry for resistance reducing agents. Claim.
Hu Yang studied the effect of different types of bentonite raw materials, heating temperature, different types and concentrations of modifiers on the magnesization modification of bentonite in the article on Bentonite modification and its effect on the permeability of cement-based materials. The results show that when calcium-based bentonite is used for modification, the higher the temperature, the greater the degree of magnesization of calcium-based bentonite; when sodium-based bentonite is used for modification test, the conversion to magnesium-based bentonite can be realized at room temperature.
Generally speaking, when the molar concentration of Mg2+ in the modifier is 0.5 mol/L, it can meet the needs of magnesization of bentonite, and the use of magnesium sulfate as the modifier can generate magnesium-based bentonite to a greater extent. The bentonite before and after modification is analyzed. The calcium-based bentonite and the magnesium-based bentonite obtained after modification have a large interlayer ionic attraction, which can absorb more water molecules, has a denser micro-layered structure and weak water absorption capacity, while sodium The gravitational force of ionic charges between the base bentonite layers is small, the number of adsorbed water molecules is small, the micro-layered structure is looser and the water absorption capacity is strong.
In addition, the article also studied the effects of calcium-based bentonite, sodium-based bentonite, and modified magnesium-based bentonite on the working performance, strength performance, and anti-permeability performance of the mortar, and combined SEM/EDS and MIP test analysis to explore the modification The mechanism of the front and back bentonites improving the waterproof and anti-permeability properties of mortar
Magnesium sulfate and magnesium nitrate are respectively dissolved in ultrapure water to prepare modifier solutions of different concentrations for later use. Then mix the Ca-based bentonite, Na-based bentonite and the configured modifier solution in a ratio of 1:20, and then place them on a magnetic stirrer, and set different stirring temperatures at a stirring speed of 1000r/min for 6h . Finally, the heated and stirred bentonite slurry is filtered, dried at 105° C., pulverized and finely ground to obtain modified Mg-bentonite.
Sheng Kai used the microwave method to prepare aluminum-based pillared bentonite and its adsorption performance. Using calcium-based bentonite from a certain place in Liaoning as a raw material, it was modified by sodiumization as a carrier, and the aluminum-based pillared bentonite was prepared by a microwave method. , Iron-aluminum pillared bentonite, zirconium-aluminum pillared bentonite, and then use a variety of test methods to characterize the pillared bentonite, and explore the best conditions for its adsorption of simulated dye wastewater.
Shao Hong et al. used aluminum chloride hexahydrate and sodium lauryl sulfate to composite modified bentonite to prepare modified bentonite with higher adsorption performance. When the dosage is 3g, the stirring time is 20min, and the stirring speed is 200r/min. , When the ammonia nitrogen wastewater concentration is 300mg/L, the ammonia nitrogen removal rate reaches 88.41%. Meng Hailing used high-efficiency microwave radiation to prepare ferric sulfate-modified sodium bentonite and aluminum sulfate-modified sodium bentonite with ferric sulfate and aluminum sulfate as modifiers. The pores, interlayer spacing and specific surface area of the two modified clays Both are significantly increased, and the adsorption performance is significantly improved. The removal rate of phosphorus in water by the two modified bentonites is above 98% .
Bentonite waterproof blanket is a kind of composite waterproof material with tiny voids made of bentonite particles evenly spread between two geotextiles according to certain quality requirements. The civil fabrics mainly play a protective role. The bentonite waterproof blanket mainly uses the principle of bentonite swelling in contact with water. The sodium bentonite used in the waterproof blanket can swell at least dozens of times when exposed to water. Needle-punched fibers lock the bentonite between the two layers of geotextiles to form a high viscosity and hydraulic power. A uniform and dense colloidal system with a small permeability coefficient can effectively waterproof.
Bentonite waterproof blanket was first used in landfills, underground garages, artificial lakes and many other industrial and civil construction projects in the late 1980s to prevent seepage. Practice has proved that bentonite waterproof blanket has been widely used. Now the bentonite waterproof blanket has been recognized all over the world as an impermeable material for engineering .
There are currently three main types of bentonite waterproof blankets. The raw materials used are sodium-based or calcium-based bentonite, but the production method is different.
Bentonite waterproof blanket is used for waterproof and seepage prevention in various fields. Mainly solve the problems of waterproof and seepage prevention of various projects. Used in landfills, the bentonite waterproof blanket is used for seepage prevention and reinforcement; used in water conservancy projects, the bentonite waterproof blanket can not only improve the seepage prevention effect of irrigation canals, but also strengthen the strength of dams and river banks, and reduce soil and water Loss, to solve the hidden danger of piping in the levee; the application of subway engineering mainly uses the waterproof and anti-seepage of the bentonite waterproof blanket to prevent the underground buildings from being infiltrated by water, causing economic losses, and improving the service life of the underground engineering; it is used in the metal tailing pond, It is mainly to prevent metal ions from being stored for a long time and penetrating into surrounding rocks and water, causing unnecessary environmental pollution and endangering human health; in mines, artificial lakes, tunnels, swimming pools, etc., it can improve the reliability and durability of waterproofing.
Bentonite-cement slurry anti-seepage wall is a slurry made of ordinary Portland cement, bentonite, and water in a certain proportion and poured into a pre-made groove to form a kind of anti-permeability. Seepage wall. Beginning in the 1950s, European countries used cement-bentonite vertical impermeable wall technology to prevent the spread of contaminated groundwater, and then extended the technology to the landfill of reservoirs, dams, lakes, and even heavy metal or high-level radioactive waste. During the treatment, however, the chemical components and pollutants in the groundwater environment will interact with the cement-bentonite solids, which will have a great impact on the permeability and strength of the wall.
In addition, bentonite can also be made into bentonite mortar and bentonite water-stop strips to be used for waterproofing of construction joints or joints of prefabricated components.