What is the role of coal water slurry dispersants that are often added during the preparation of coal water slurry?

1. Mechanism of action of dispersants 1) Increasing the hydrophilicity of the coal surface. A dispersant is a chemical agent that facilitates the uniform dispersion of the dispersed phase (coal particles in coal water slurry) within the dispersion medium (water in coal water slurry). Coal is primarily composed of non-polar hydrocarbons and is a hydrophobic substance. The wettability of coal can be divided into four categories based on the contact angle of water on its surface. Coal with a contact angle of zero is referred to as highly hydrophilic coal ； Coal with a value of less than 40° is considered weakly hydrophilic ； Those ranging from 40° to 90° are referred to as hydrophobic coal ； Those with an angle of over 90° are referred to as highly hydrophobic coals. The surfaces of various types of coal are hydrophobic. Furthermore, water has high surface tension while coal has low surface tension; only by reducing the surface tension of water, increasing the surface tension of coal, and decreasing the interfacial tension between the solid and the liquid can thorough wetting be achieved ； Even when the surface of coal particles is wet, their large specific surface area causes them to aggregate together, preventing them from dispersing evenly. The dispersants used in pulp production are all amphiphilic surfactants, with a non-polar lipophilic group composed of hydrocarbons at one end, and a polar hydrophilic group at the other end. The non-polar hydrophobic end readily binds to the surface of coal, which is made of hydrocarbons, adhering to the surface of the coal particles while exposing the hydrophilic group at the other end into water. The strong hydrophilicity of the polar groups transforms the surface of coal particles from hydrophobic to hydrophilic, allowing a hydration film to form. It effectively reduces the surface tension of water and increases the surface tension of the coal particles, reducing the wetting contact angle to below 50 degrees. The water film is used to separate the coal particles, reducing the resistance between them and thereby lowering the viscosity. Tests show that dispersants should have good water solubility, but it is not the case that the better their wetting ability for coal is, the better their viscosity-reducing effect will be. Wetting agents and penetrants can make coal particles highly hydrophilic (with a contact angle of zero), but they cannot be used as dispersants for water-coal slurry. (2) Enhancing the electrostatic repulsion between coal particles. The well-known DLVO theory states that a prerequisite for the stable dispersion of colloidal particles is that the electrostatic repulsion between them exceeds the van der Waals attraction between them. In addition to improving the hydrophilicity of the coal surface, ionic dispersants can also enhance its electrostatic repulsion, further promoting the dispersion of coal particles in an aqueous medium. Although there is considerable emphasis on the stabilizing effect of electrostatic repulsion on the dispersion and suspension of coal particles, and some even believe that the main function of dispersants is to alter the surface electrical properties of these particles, arguing that when the potential difference between the sliding surface and the interior of the solution, namely the zeta potential, reaches -50 mV, the water-coal slurry gains desirable fluidity and stability; numerous studies have shown that increasing this potential difference improves the fluidity of the water-coal slurry, while reducing it enhances its stability, yet neither factor plays a decisive role. The steric effect of spatial separation is more practically significant. (3) Spatial isolation steric effect: The water in the hydration layer is different from the \"free water\" in the system; it arranges itself in an ordered manner due to the attraction of the surface electric field. When the particles come close to each other, the hydration film is compressed and deformed; gravity then tries to restore its original orientation. This gives the hydration film a certain degree of elasticity, allowing the coal particles to be evenly dispersed. The dispersant on the particle surfaces also has a certain thickness. When two particles with adsorption layers overlap, the freedom of movement of the dispersant molecules in those adsorption layers is restricted, resulting in a decrease in the entropy of the adsorbed molecules. Since the entropy of a system always tends to increase spontaneously, the particles have a tendency to separate again, thereby preventing them from aggregating. When the dispersant is a macromolecule, the adsorbed molecules possess long hydrophilic chains that form a three-dimensional hydration film on the coal surface; when the particles come close to each other, a strong repulsive force is generated, resulting in the dispersion and suspension of the coal particles. This repulsive force is known as spatial steric hindrance or steric barrier. In summary, efficient water-coal slurry dispersants are characterized by their ability to adsorb effectively on the coal surface, enhancing the hydrophilicity of the coal, and forming a double layer as well as geometric barriers on its surface. 2. Common dispersants Dispersants can be divided into two main categories: ionic and non-ionic, based on whether they dissociate or not. Ionic types can be further divided into anionic, cationic, and amphoteric categories based on the properties of their charge. Amphiphilic means that it exhibits anionic properties when the solution is alkaline, and cationic properties when it is acidic. The international price ratio of anionic, non-ionic, cationic, and amphoteric dispersants is 1:2:3:4. Anionic types are commonly chosen as pulping dispersants. Anionic dispersants mainly include naphthalenesulfonates, lignosulfonates, sulfonated rosin sulfonates, polycarboxylate-based substances, etc. There are many articles available for you to download and read. Furthermore, water has high surface tension while coal has low surface tension; only by reducing the surface tension of water, increasing the surface tension of coal, and decreasing the interfacial tension between the solid and the liquid can thorough wetting be achieved ； Even when the surface of coal particles is wet, their large specific surface area causes them to aggregate together, preventing them from dispersing evenly. The dispersants used in pulp production are all amphiphilic surfactants, with a non-polar lipophilic group composed of hydrocarbons at one end, and a polar hydrophilic group at the other end. The non-polar hydrophobic end readily binds to the surface of coal, which is made of hydrocarbons, adhering to the surface of the coal particles while exposing the hydrophilic group at the other end into water. The strong hydrophilicity of the polar groups transforms the surface of coal particles from hydrophobic to hydrophilic, allowing a hydration film to form. It effectively reduces the surface tension of water and increases the surface tension of the coal particles, reducing the wetting contact angle to below 50 degrees. The water film is used to separate the coal particles, reducing the resistance between them and thereby lowering the viscosity. Tests show that dispersants should have good water solubility, but it is not the case that the better their wetting ability for coal is, the better their viscosity-reducing effect will be. Wetting agents and penetrants can make coal particles highly hydrophilic (with a contact angle of zero), but they cannot be used as dispersants for water-coal slurry. (2) Enhancing the electrostatic repulsion between coal particles. The well-known DLVO theory states that a prerequisite for the stable dispersion of colloidal particles is that the electrostatic repulsion between them exceeds the van der Waals attraction between them. In addition to improving the hydrophilicity of the coal surface, ionic dispersants can also enhance its electrostatic repulsion, further promoting the dispersion of coal particles in an aqueous medium. Although there is considerable emphasis on the stabilizing effect of electrostatic repulsion on the dispersion and suspension of coal particles, and some even believe that the main function of dispersants is to alter the surface electrical properties of these particles, arguing that when the potential difference between the sliding surface and the interior of the solution, namely the zeta potential, reaches -50 mV, the water-coal slurry gains desirable fluidity and stability; numerous studies have shown that increasing this potential difference improves the fluidity of the water-coal slurry, while reducing it enhances its stability, yet neither factor plays a decisive role. The steric effect of spatial separation is more practically significant. (3) Spatial isolation steric effect: The water in the hydration layer is different from the \"free water\" in the system; it arranges itself in an ordered manner due to the attraction of the surface electric field. When the particles come close to each other, the hydration film is compressed and deformed; gravity then tries to restore its original orientation. This gives the hydration film a certain degree of elasticity, allowing the coal particles to be evenly dispersed. The dispersant on the particle surfaces also has a certain thickness. When two particles with adsorption layers overlap, the freedom of movement of the dispersant molecules in those adsorption layers is restricted, resulting in a decrease in the entropy of the adsorbed molecules. Since the entropy of a system always tends to increase spontaneously, the particles have a tendency to separate again, thereby preventing them from aggregating. When the dispersant is a macromolecule, the adsorbed molecules possess long hydrophilic chains that form a three-dimensional hydration film on the coal surface; when the particles come close to each other, a strong repulsive force is generated, resulting in the dispersion and suspension of the coal particles. This repulsive force is known as spatial steric hindrance or steric barrier.

Is it an additive? Only gray water contains a dispersant. . . . . . . . . . . . . . . . . .