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Technical Principle and Type Analysis of Coal Water Slurry Additives

2019/5/21 Click:375
There are many kinds of CWS additives on the market. According to their different functions, there are dispersants, stabilizers and other auxiliary chemicals, such as defoamers, pH regulators, antienzymes, surface modifiers and accelerators. Among them, dispersants and stabilizers are indispensable. Additives are closely related to the properties of raw coal and water. Reasonable formula of additives must be determined according to the properties of coal for slurry making and the requirements of users for the quality of coal-water slurry products.
I. Coal Water Slurry Dispersant
1. The mechanism of dispersant (1) The hydrophilic dispersant on coal surface is a chemical agent that can promote the uniform dispersion of dispersed phase (coal particles in coal water slurry) in dispersing medium (water in coal water slurry). Coal is a non-polar hydrocarbon and a hydrophobic substance. The wettability of coal can be divided into four grades according to the contact angle of water on its surface. Coal with zero antennae is called strong hydrophilic coal; coal with less than 40 degrees is called weak water-based coal; coal with hydrophobicity is called 40 degrees to 90 degrees; coal with strong hydrophobicity is called coal with more than 90 degrees. The surface of all kinds of coal shows hydrophobicity. In addition, the surface tension of water is large and the surface tension of coal is small. Only by reducing the surface tension of water, increasing the surface tension of coal and reducing the interfacial tension between solid and liquid, can the full wettability be achieved. Even if the surface of coal particles is wet, its huge specific surface area will also promote them to gather together and not disperse evenly. Dispersants used in pulping are amphiphilic surfactants. One end is a non-polar lipophilic group composed of hydrocarbons, the other end is a hydrophilic polar group. The non-polar hydrophobic end is easily bound to the coal surface of hydrocarbons, adsorbed on the surface of coal particles, and the other end of the hydrophilic group is introduced outward into water. The strong hydrophilicity of polarity makes the surface of coal particles change from hydrophobic to hydrophilic, and a layer of hydration film can be formed. Effectively reduce the surface tension of water and increase the surface tension of coal particles, so that the wetting contact angle can be reduced to less than 50 degrees. Hydration membranes can separate coal particles and reduce the resistance between them, so as to reduce the viscosity of coal particles. Experiments show that the dispersant should have good water solubility, but not the better wettability of coal, the better viscosity reduction effect. Wetting agent and penetrating agent can make coal particles extremely hydrophilic (contact angle is equal to zero), but they can not be used as dispersant of coal water slurry. (2) The well-known DLVO theory for enhancing the electrostatic repulsion between coal particles holds that the precondition for stable dispersion of colloidal particles is that the electrostatic repulsion between particles exceeds the van's gravity between particles. Ionic dispersant can not only improve the hydrophilicity of coal surface, but also enhance its electrostatic repulsion and further promote the dispersion of coal particles in water medium. Although people attach great importance to the stabilization of electrostatic repulsion on dispersed suspension of coal particles, some people even think that the main function of dispersant is to change the surface electrical properties of coal particles. They think that when the potential difference between sliding surface and solution reaches - 50 mV, the fluidity and stability of coal water slurry will be expected. However, a lot of studies have shown that increasing the potential difference is beneficial to improve the fluidity and stability of coal water slurry. Improving the fluidity of CWS, on the contrary, is beneficial to improving its stability, but neither of them plays a decisive role. The steric barrier effect of space isolation is more practical. (3) The water in the steric barrier effect hydration film is different from the free water in the system. It is aligned in orientation due to the attraction of surface electric field. When the particles are close to each other, the hydration film is extruded and deformed, and the gravitational force tries to restore the original orientation. This makes the hydration film show certain elasticity, makes the coal particles disperse evenly and the dispersant on the surface of the particles have certain thickness. When the two particles with the adsorption layer overlap each other, the degree of freedom of the molecular movement of the dispersant in the adsorption layer is hindered, and the entropy of the adsorption molecules decreases. Because the entropy of the system always increases spontaneously, the particles tend to separate again to avoid aggregation. When the dispersant is a macromolecule, the adsorbed molecules have long hydrophilic chains and form three-dimensional hydration film on the surface of coal. When the particles are close to each other, the strong repulsive force will be produced, which will lead to the dispersal and suspension of coal particles. The repulsion force is space isolation steric barrier or three-dimensional obstacle. In a word, the characteristics of high-efficiency coal-water slurry dispersant are effective adsorption on coal surface, improving the hydrophilicity of coal, and forming double electric layer and three-dimensional barrier on coal surface. 2. Dispersants commonly used can be divided into ionic and non-ionic types according to dissociation or not. Ionic type can be divided into anionic type, cationic type and amphoteric type according to the property of charge. Amphoteric type refers to the anionic characteristic when the solution is alkaline and the cationic characteristic when it is acidic. The international price ratio of anions, nonions, cations and amphoteric dispersants is 1:2:3:4. Pulping dispersants are usually anionic. The main anionic dispersants are naphthalene sulfonate, lignosulfonate and sulfonated humic acid salt.
Editorial paragraph 2. Coal water slurry stabilizer
The stability of coal water slurry (CWS) refers to the uniformity of CWS during storage and transportation. The destabilization of CWS comes from the precipitation of solid particles. Coal water slurry (CWS) is a kind of coarse suspension, which belongs to dynamic unstable system. The main way to stabilize CWS is to make it thixotropic. That is to say, coal slurry is structured and has high shear stress when it is stationary. When it is applied, once it is acted by external force, its viscosity can be reduced rapidly, and it has good fluidity. When it is stationary, it can restore its original structural state. Rheology calls this fluid thixotropic or time-related fluid. Stable
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