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Agents that promote the uniform dispersion of material particles in the medium to form a stable suspension. Dispersants are generally divided into two categories: inorganic dispersants and organic dispersants. Commonly used inorganic dispersants include silicates (such as water glass) and alkali metal phosphates (such as sodium tripolyphosphate, sodium hexametaphosphate and sodium pyrophosphate, etc.). Organic dispersants include triethylhexyl phosphate, sodium dodecyl sulfate, methyl amyl alcohol, cellulose derivatives, polyacrylamide, guar gum, fatty acid polyethylene glycol esters and polyethers, as well as organic silicones and silicone oils.
In the mineral processing process, dispersants are a type of regulator used in combination with collectors, foaming agents or selective flocculants. Dispersants enable collectors or flocculants to interact better with different minerals to achieve the purpose of sorting. It is generally used to disperse ore mud to effectively separate the mineral to be selected from the gangue. For example, when selecting kaolin, in order to remove quartz and impurity minerals such as iron and titanium, dispersants such as sodium hexametaphosphate are added to make kaolin highly suspended and stable, and then selective flocculants are added to make one of the minerals (kaolin or impurity minerals) form flocs and settle, while the other is still fully dispersed, so as to achieve sorting and improve the grade and natural whiteness of kaolin concentrate.
In the flotation process of sulfide ore, an appropriate amount of water glass is used as a dispersant to disperse the gangue fine mud, so that the sulfide ore and the reagent can interact better, float out of the slurry, and improve the grade of the concentrate. The amount of dispersant should be moderate. Excessive dispersant will cause the loss of the mineral to be selected, or it will not have a dispersing effect, but an inhibitory effect. In addition to being used in the mineral processing industry, dispersants are also widely used in light industry, food, chemical industry and other industries.
The mechanism of action of dispersants can be summarized as steric hindrance and electrostatic stabilization. Dispersants have a great characteristic in molecular structure, namely "amphiphilicity", hydrophilicity and lipophilicity. The lipophilic group of traditional dispersants is generally a hydrocarbon chain structure, and mainly an alkane chain. The molecular structure of superdispersants includes two parts: anchoring group and solvation chain. Dispersants have two mechanisms of action: ① Liquid non-water-soluble raw materials are difficult to disperse due to excessive surface tension, such as oils and organic solvents with very weak polarity, which form an oil/water interface in water. The interfacial tension is very large, so the oil/water is divided into two layers. Although the oil can be dispersed into oil droplets under strong mechanical stirring, it will quickly separate under the action of interfacial tension after stopping stirring.
Sulfur becomes liquid after melting at 114°C, and for the same reason, it cannot be mixed with water. However, if a surfactant is added to the oil/water system and the oil is dispersed by mechanical force, a monolayer of surfactant will be immediately formed on the surface of the oil droplets, surrounding the oil droplets so that the oil droplets cannot condense, and the generated oil droplets can be stably dispersed in the water phase.
Adding a dispersant to the suspension can reduce the interfacial tension between the solid and the liquid and effectively wet the particles. Taking the aqueous dispersion medium as an example, the lipophilic group of the dispersant is adsorbed on the surface of the solid particles, and the hydrophilic group is solvated by the water medium and extended into the water phase medium, thereby forming a charged protective barrier around the particles, and the double layer surrounds the particles, generating electrostatic repulsion between the particles, making the dispersion stable; for the hydrophilic non-ionic dispersant, the hydration film formed by the long polymer chain can prevent the flocculation of solid particles and make the dispersion stable.
② There is also water/solid interfacial tension between the solid powder particles and the water medium. At the same time, the air film adsorbed on the surface of the solid powder particles also hinders affinity with water, which is conducive to the flocculation of the powder particles. Surfactants can also remove the air film on the surface of powder particles and reduce the water/solid interfacial tension, thus making the powder particles easier to disperse.
The action mechanism of hyperdispersants can be divided into anchoring mechanism and stabilization mechanism. Its anchoring group is tightly adsorbed on the surface of solid particles through interactions such as ionic bonds, hydrogen bonds and van der Waals forces; its solvation chain has good compatibility with the dispersion medium and adopts a relatively extended conformation in the dispersion medium. When the particles adsorbed with hyperdispersants approach each other, the particles bounce off each other due to the spatial barriers of the solvation chain, thereby achieving stable dispersion of the particles in the medium.
Dispersants can be divided into: natural polymer compounds, such as gelatin, pectin, starch, gum arabic, methyl cellulose, carboxymethyl cellulose, etc.; synthetic polymers, such as polyvinyl alcohol, vinyl alcohol copolymer, styrene-maleic anhydride copolymer, vinyl acetate-maleic anhydride copolymer, polyacrylic acid and its salts, polyvinyl pyrrolidone, sulfonated polystyrene, etc.; insoluble salts, such as copper sulfate, calcium sulfate, barium carbonate, magnesium carbonate, calcium phosphate, etc.; inorganic polymers and metal oxides, such as talc, bentonite, kaolin, alumina, diatomaceous earth, limestone, etc.
There are many types of dispersants. According to the different dispersion media, we can divide dispersants into dispersants used in aqueous media and dispersants used in non-aqueous media, and the former includes anionic and non-ionic types. Among dispersants, there is a type of polymer dispersant, called superdispersant, which was first developed to solve the dispersion problem of pigment particles in organic media.
1) Using dye dispersants in the dyeing process can prevent disperse dyes used in cellulose acetate from precipitating during dyeing. It can also be used as a leveling agent when dyeing cellulose fibers with vat dyes. It is also used for dyeing polyester fibers.
2) It is often used as a dispersant for aqueous and granular agents in pesticides. The dispersant used in aqueous agents can be alkylnaphthalene sulfonates, alkylbenzene sulfonates, and ethylene oxide alkylphenyl ethers; the dispersant used in granules can be alkylbenzene sulfonates, polycarbonates, dioctyl sulfosalicylate, etc.
3) In food, dispersants such as polyethylene glycol, microcrystalline cellulose, and polysorbate are also used to prevent fruit juice from condensing and precipitating, and to make pigments and other additives more evenly dispersed in food.
4) Water-based dispersants used in the coating and ink industries, such as polyacrylates, sodium maleate and olefin copolymers, polyethylene oxide or aryl ethers, and non-aqueous dispersants, such as sorbitan fatty acid esters, polyethylene oxide alkylamines, alkyl diamines, etc.
5) In addition to the above, dispersants also have important uses in the fields of cosmetics, civil engineering, and environmental protection. In recent years, the use of polymer compounds as dispersants for mud generated in internal combustion engines and lubricating oils, as well as as dispersants for coal-heavy oil mixed fuels, has received attention. Some dispersants are also used as dispersants for natural rubber and synthetic rubber latex. For example, dispersant Darvon NO.1 (generated from sodium alkylnaphthalene sulfonate), dispersant Dispersiol LR (made from sodium disulfonate), dispersant Tamol 731 (a carboxylated polymer electrolyte sodium salt), etc.