Adsorption is the predominant form of interaction between phase interfaces during flotation. For example, the foaming agent is mainly adsorbed at the gas-liquid interface, the collector is mainly adsorbed at the solid-liquid interface, the emulsifier is mainly adsorbed at the liquid-liquid interface, and the ions in the slurry can be adsorbed at different interfaces, and the like. The result of the adsorption results in a change in the nature of the phase interface, allowing the flotation process to be adjusted and carried out.
The adsorption-flotation process of the gas-liquid interface uses a foaming agent to form a stable bubble into the air introduced into the slurry. The foaming agent is mostly a surfactant, and is adsorbed in a molecular form and aligned at a gas-liquid interface, the non-polar group faces the gas phase, and the polar group faces the water. Therefore, the adsorption of surface active substances at the gas-liquid interface should be studied during the flotation process. At the gas-liquid interface, the equilibrium concentration c of the surface active material and the surface tension σ and the gas-liquid interface adsorption density
The law of variation can be calculated by Gibbs isothermal adsorption equation Less than zero, then If it is greater than zero, that is, the concentration of the adsorbate in the surface layer is greater than the bulk concentration, it is called positive adsorption, and the adsorbate is called a surface active material. For example, long-chain hydrocarbon carboxylates, sulfates, sulfonates, and amine collectors commonly used in flotation. If the adsorbate increases the surface tension, ie Greater than zero, then Less than zero, when the concentration of the adsorbate in the surface layer is less than the bulk phase concentration, it is called negative adsorption. Such adsorbates are referred to as non-surface active materials such as inorganic acids, bases, salt regulators, etc. used in flotation. For the homologue of the surfactant, the surface activity is increased by about three times for each additional CH2, and the performance of the foaming agent is closely related to its surface activity. This relationship is called the Traube rule.
2 Adsorption of solid-liquid interface---The adsorption of solid-liquid interface in flotation system is quite complicated. In the flotation process, no matter what kind of agent is added, in most cases, adsorption occurs at the solid-liquid interface, which changes the interface properties. Therefore, the study of solid-liquid interface adsorption is mainly to study the adsorption of different drug sites at the solid-liquid interface.
The adsorption of solids is similar to the adsorption of liquids, which occurs only in the surface layer and does not penetrate into the interior, which is a surface phenomenon. If the adsorbed material spreads deep into the solid, it is called absorption. The adsorbed solute is called an adsorbate, and the substance adsorbing the adsorbate is called an adsorbent. Because of the surface energy of the solid surface, there is also a tendency to adsorb a substance to lower the surface energy. Therefore, the adsorbent always adsorbs substances that can reduce its surface energy. The adsorption of the agent in the solution at the solid-liquid interface is subject to the Langmuir adsorption isotherm:
The equilibrium adsorption amount, the saturated adsorption amount, the adsorption mass equilibrium concentration and the constant are respectively. The adsorption results of the monolayers at high and low concentrations can be accurately calculated using the formula [4-6-6]. This formula also applies to the adsorption of gases on solid surfaces. The analysis of the Langmuir adsorption isotherm shows that the constant b value is equal to the adsorption amount of 0.5. The concentration of adsorbate at the time. For multi-layer adsorption of uneven solid surface, the empirical equation can be calculated by Ferrandlich adsorption:
Where k and n are empirical constants, which can be obtained by means of a double logarithmic graph. In aqueous solutions, the adsorption of the collector on the mineral surface often conforms to this formula. In addition, the adsorption of the solid-gas interface can also be adopted.
â‘¢ liquid-liquid interface adsorption eleven froth flotation process is often used in non-polar hydrocarbon oil as collector, in particular when suspended in hydrocarbon oil selected from coal is the main agent. In addition, a large amount of non-polar hydrocarbon oil is also required for oil, full oil or emulsification flotation. However, in the flotation flotation, the oil should be dispersed in water to form an O/W (oil-in-water) emulsion; for the whole oil flotation, etc., the water should be dispersed in the oil to form a W/O/water-in-oil type. Emulsion.
Because the interface area of ​​the liquid-liquid interface and the free energy storage of the interface are extremely high, the system is in a multi-phase thermodynamically unstable state, and strives to develop in the direction of lowering free energy, in order to achieve a stable state, that is, to reduce the liquid-liquid interface area. Therefore, the emulsion formed by physical action can only temporarily increase the interface area, and the emulsion (oil and moisture layer) will soon occur. To maintain the stability of the emulsion, one way is to reduce the oil with a surfactant. The free energy of the water interface can obtain the emulsion with suitable dispersion and high stability. The preparation of such emulsion has practical application value for the hydrocarbon oil flotation agent, so the flotation research liquid-liquid interface Adsorption is mainly to study the distribution of surface active substances at the oil-water interface and the effects on oil and water dispersion properties and forms.
A large amount of surface active substances are used in the flotation process, and in most cases, they are positively adsorbed. Figure 4-6-27 shows the relationship between surface tension and concentration of three types of materials. Curve 1 is the σ-c curve of the inorganic salt surface inert substance. This kind of material is negative adsorption, the surface tension is slightly higher than water; curve 2 is the commonly used surface active material in flotation, mostly the repolarization with short hydrocarbon chain. A molecule such as an alcohol below C9 and a carboxylic acid, a terpene alcohol, an ether alcohol, an oxyalkyl derivative, and a xanthogenate, and the like. Most of these substances are excellent foaming agents and sulfide mineral collectors in flotation, so they are usually called surface active substances; curve 3 is mostly alkyl sulfate or sulfonate and amine with longer carbon chain. Salts, soaps and polyols, characterized by a significant reduction in surface tension at low concentrations, and the lowest point on the curve to mark the critical point of the micelles formed in the solution, while the back surface tension increases with concentration Pick up. Such materials are commonly referred to as surfactants. In the flotation, it is widely used as a collector for oxidized minerals, and also as an emulsifier and dispersant for non-polar oils.
When a surface active substance is added to the water, the hydrophobic groups of these long hydrocarbon chain surfactant molecules are repelled by water, and the hydrophilic groups are attracted by water. Since the hydrocarbon chain is longer, the force of being repelled by water is greater than the attraction of water to the polar group, and thus such a single molecule is unstable in water. In order to stabilize the state, this type of molecule can only adopt two forms of existence: one is directed adsorption at the oil-water interface, leaving the hydrophilic group in water, and the hydrophobic group is inserted into the oil, as shown in Figure 4-6-28. (a) and (b); the second is to form micelles in water to minimize the contact of hydrophobic groups with water, the hydrophobic groups are close together by molecular cohesion; when the concentration of surfactant in water is very low It is possible to form aggregates of two or three molecules as shown in the figure. When the surfactant concentration is increased to a certain extent, the surface tension and conductivity will be abrupt (curve 3 in Figure 4-6-27). The concentration of this turning point is the critical concentration of micelle formation, called CMC. Ten to several hundred spherical, thin-leaf, small rod-shaped micelles, see Figures 4-6-29, which are several nanometers in size.
CMC indicates the concentration at which the surfactant forms micelles and can also be used as a marker for measuring surfactants. The smaller the CMC, the lower the concentration required to form the micelles by the surfactant, and the lower the concentration required to change the interfacial properties. The critical micelle concentration is also a turning point in the significant change in the properties of the surfactant solution. In general, for a homologue, the CMC value decreases as the number of carbon atoms in the hydrocarbon chain increases, and can be expressed by the following empirical formula:
Where n is the number of one CH2 group in the hydrocarbon chain; A and B are constants, determined by molecular type and temperature, see Table 4-6-6. The relationship between CMC and hydrocarbon chain length is shown in Figure 4-6-30.
A small amount of hydrocarbon oil is added to the micelle solution, which immediately penetrates into the hydrocarbon chain of the surfactant molecule to form an interlayer of thin leaf micelles, called a bulky micelle, as shown in Figure 4-6-31 [a) and As shown in [b). The volume is expanded compared to the original micelles, and the size of the micelles can reach 50 nm. If an auxiliary agent is added to the above system, the bulky micelles can be further developed into spherical micelles, that is, microemulsions. Its size is from 30 to 80 nm to 100 nm, as shown in (c) and (d) of Figure 4-6-31.
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