Foam is an important part of flotation, and minerals float with the foam.
First, the formation and nature of foam (a) the reasons for formation
The foam is a crude dispersion system in which the gas is a dispersed phase, and the solid phase or the liquid phase is a dispersion medium. This section mainly discusses a foam in which the dispersion medium is an aqueous solution. Since the density of the gas and the liquid are far apart, the bubbles formed in the liquid quickly rise to the liquid surface, forming a bubble aggregate surrounded by a liquid film of a certain thickness, which is a foam, but it is difficult to form a foam in pure water. Even if it forms, it will immediately break down and disappear. The liquid that produces the foam must have more than two components. In order to make the foam easy to form and stable, it is necessary to add additives in pure water, usually called a foaming agent, mostly a surfactant, which acts on the surface of the liquid film outside the bubble to form a firmer film. , delay the liquid discharge speed in the film to increase the foam life.
When the foam is generated, the energy of the system increases correspondingly with the increase of the surface area of ​​the liquid. When the foam is destroyed, the energy of the system also decreases accordingly. Therefore, from the energy point of view, low surface tension is advantageous for foam formation (that is, foams of the same total surface area can be produced with less work), but the foam is not guaranteed to have good stability. Low surface tension helps the foam to stabilize only when the surface film has a certain strength and can form a polyhedral foam. Because according to the Laplace formula, the pressure difference between the plateau Plateau and the planar film is proportional to the surface tension; when the surface tension is low, the pressure difference is small, so the liquid discharge speed is slow, and the liquid film becomes thinner and slower, which is favorable for stability.
Many phenomena indicate that there is no definite corresponding relationship between liquid surface tension and foam stability. For example, the flotation agent, terpene alcohol and pine oil, can significantly reduce the surface tension of the solution, and the corresponding foaming ability is large; while the flotation oil reduces the surface tension of the solution more than the two foaming agents, but the foaming ability is Small, on the contrary, cresol does not significantly reduce the surface tension of the solution, but also has good foaming. In addition, some protein aqueous solutions have higher surface tension than aqueous surfactant solutions, but have higher foam stability.
Marangoni believes that when the foam liquid film is impacted by external force, local thinning occurs, the surface area increases, the surface adsorption molecular density decreases, and the surface tension increases. Therefore, the surfactant molecules try to migrate to the thinned portion, so that the surface adsorption density increases and the surface tension drops to the original level. At the same time, when the surface molecules migrate, it will drive the adjacent thin layer of liquid to move together, so that the thinned liquid film strength is restored, which shows that the foam has good stability, which is the "repair" effect of surface tension. Therefore, the surfactant is adsorbed on the surface liquid film, and has the ability to resist the expansion or contraction of the surface of the liquid film even if the interface film has film elasticity. Pure liquids have no surface elasticity, and their surface tension does not change with area, and thus cannot form a stable foam.
For the Marangoni effect, two different processes should be considered. One is the process of migrating surfactant molecules from the low surface tension region to the high surface tension region (as described above); the other is the process of adsorbing molecules from the solution onto the surface. As a result of this process, the surface tension of the impacted liquid film can be restored to the original value, and the density of the surface adsorbed molecules is restored. However, if the latter process is carried out faster (fast adsorption speed), the surfactant molecules required in the liquid film expansion portion will be largely provided by the solution to complement, rather than by surface migration. Thus, although the surface tension and the adsorbed molecular density at the impact are recoverable, the thinned liquid film is not re-thickened (the solution is not brought by the migration molecules). Such a liquid film is obviously inferior in strength and thus in foam stability.
(two) the nature of foam
A foam is a collection of a plurality of individual bubbles, generally a pentagonal dodecahedron structure, which contains large and small bubbles, and a rib interface is formed between the bubble and the bubble. The minimum foam size is a few micrometers. The maximum can reach several tens of millimeters. Depending on the film thickness, the density of the foam is similar to that of the continuous phase itself, and some are close to the gas density inside the foam.
The foam is elastic and rigid. The destruction of the foam is mainly caused by the thinning of the liquid film to the difficulty of recovery and the diffusion of gas in the bubble. When the liquid film is thinned to 5-10 nm, the foam is destroyed.
Foam stability is the most important property of foam. In addition, the foaming ability of the surfactant is also an important property related to the foam, so the measurement of the general foam properties is mainly to study the stability and foaming property. Foaming force refers to the difficulty of foam formation and the amount of foam generated, while foam stability refers to the length of the "life" of the foam. By its very nature, foam is a thermodynamically unstable system that cannot be stable.
There are many methods for measuring foam stability. There are two main types of foaming methods: airflow method and agitation method. Among them, the RossMiles method has been adopted as an industry standard method in many countries, which can measure foam easily and accurately. Foaming and stability. Generally, the foam height (mm) after 5 minutes of the test liquid flow is used as a measure of foam stability, but it is also often expressed by the time required for the initial foam height and the foam to break half (ie, the foam height is half of the initial height). Foaming and foam stability.
Second, the factors affecting the stability of the bubble
The process of foam destruction is mainly a process in which the liquid film separating the gas is thinned from thick to cracked. Therefore, the stability of the foam mainly depends on the speed of liquid discharge and the strength of the liquid film. [next]
The main factors affecting the stability of the foam, that is, the factors affecting the thickness of the liquid film and the strength of the surface film, are more complicated.
(1) Liquid film and its gas permeability
Generally, the size of the foam is always uneven. As a result of the curved surface pressure, the gas pressure in the small bubble is larger than that in the large bubble, and the gas diffuses from the high pressure vesicle through the liquid film to the low pressure large bubble, resulting in The vesicles become smaller (until disappearing), the large bubbles become larger, and eventually the foam is destroyed. Throughout the process, the liquid film is dependent on the ability of the gas to pass through the liquid film. This is called the gas permeability of the liquid film. The diffusion of this gas through the liquid film clearly manifests itself in a single bubble floating on the liquid surface: the bubble gradually becomes smaller with time, so that it eventually disappears. Generally, the bubble radius and time change rate on the liquid surface are used as a standard for measuring the gas permeability of the liquid film, as shown in the following formula.

Where r ₀ ——the bubble radius at t=0, cm;
R——the radius at time t;
p - atmospheric pressure;
γ - the surface tension of the solution;
k - gas permeability constant.
By plotting r ² versus t, a straight line is obtained with a slope of From this, the constant k can be found. Studies have shown that those with low gas permeability, high surface viscosity, and good foam stability, and vice versa.
The gas permeability of the liquid film is related to the tightness of the adsorption molecules on the surface, and the closer the arrangement is, the more difficult the gas is to pass, and the more stable the liquid film. After adding a small amount of dodecanol to sodium lauryl sulfate, the mixed adsorption film formed on the surface contains a large amount of dodecanol molecules, which has a shielding effect on the ionic charge of the surfactant, and therefore, the intermolecular attraction is strengthened. The molecules are arranged more closely and the gas permeability is reduced.
(2) Surface viscosity and solution viscosity
Surface viscosity refers to the viscosity of a monolayer on the surface of a liquid, which is produced by the adsorption of surfactant molecules on the surface to form a monolayer. Table 1 shows the relationship between surface tension, surface viscosity and foam life of several common aqueous surfactant solutions. These data show that for systems with higher surface viscosities, the life of the formed foam is longer, and there is no definite corresponding relationship between the surface tension of the solution and the stability of the foam.
Therefore, the foaming agent molecules are adsorbed on the surface of the solution to increase the surface viscosity, and the film strength is increased to form a stable foam. The strength of the surface film is related to the attraction between the adsorbing molecules, and the film strength is also large.
Generally, the protein molecules are large and the interaction between molecules is strong, so the stability of the foam formed by the aqueous solution is relatively high. Generally, the surfactant having a large number of branches in the hydrophobic group has a poorer intermolecular action than the linear one, so that the surface viscosity of the solution is small and the foam stability is also poor. Some ionic surfactants adsorb repulsion between molecules. At this time, adding some neutral polar substances can improve foam stability because the neutral molecules are inserted into the surface adsorption layer, which reduces the repulsion between the isotropic ions. Conducive to increase the strength and surface viscosity of the film, see Table 2.