Foam layer analysis

Two-phase foam

The two-phase foam is a distribution form of the gas phase in the liquid phase, which is characterized in that the gas phase volume is much larger than the liquid phase volume, so the liquid phase is a thin layered gas. The bubble dispersion in the liquid phase rises to a stable accumulation of the liquid surface to form a foam, and the foam without solid phase participation is called a two-phase foam. A foam containing a large amount of ore particles or other third phase material formed by flotation is called a three-phase foam. Two-phase foams are relatively simple and are often used in the evaluation and study of foaming agents and foams.

Figure 4-6-40 shows the structure of a two-phase foam. In the case where the adjacent bubble sizes are not much different, the angle between the water layers is often 120. The three-bubble structural unit is separated by a trigeminal water layer. This trigeminal water layer structure is also known as the Plano boundary. The Plano boundary acts on the dewatering of the foam. Figure 4-6-41 shows the structure of a three-phase foam layer. In the lower part of the foam layer, the small bubbles are mainly spherical, the separated water layer is thick and contains more water; the upper layer of the foam layer is gradually thinner, and the bubble gradually becomes larger, mainly composed of large bubbles of polyhedron, and the water layer is thin. Little water. The three-phase foam is similar to the two-phase foam except that there are a large number of ore particles in the separated water layer.

Like other binary dispersions (such as emulsions, aerosols), foam is an unstable system. The foam layer is always in the surface of the old bubbles continue to burst and the lower part of the dynamic balance of fresh air bubbles. The main reasons for the bursting of bubbles in the foam layer are as follows:

Figure 4-6-42 Plantation Triangle

1 Dehydration - The water layer separating the bubbles in the foam layer is gradually thinned by the action of gravity, evaporation and surface tension. Gravity causes the water in the upper part of the foam layer to flow downward and eventually enter the liquid phase. The water layer on the surface of the foam layer is further thinned by the evaporation of moisture. The effect of surface tension is shown in Figure 4-6-42. Concave at the junction of the Plano boundary (marked with x), according to the surface pressure formula (The pressure difference between the liquid phase and the gas phase interface is The radius of curvature of the liquid surface is R), the ΔP at the concave surface is negative, and the ΔP at the flat surface y is zero, so the pressure of the water layer at x is less than y, so that the water at y is collected at x , the water layer is thinner at y.

3 rupture of the separated water layer - the dehydration of the foam layer causes the water layer to be thinned to a limit thickness (1.0~1.5) X10-8m, and the water film at this time is called Black Spot or Black film. The gas-liquid surface is very close, and the foaming agent molecules and water molecules in the interface layer are attracted to each other by hydrogen bonding and molecular force, and the electrostatic repulsion of the interface electric double layer makes the black film extremely stable, such as no external force. It persists for a certain period of time, and once it is affected by external forces and chemicals, it breaks down instantly.

It can be seen that the bursting of the two-phase foam layer can be divided into two sections: thinning of the water layer and rupture of the black film. The foam life is also determined by the progress of these two stages, and the rupture rate of the black film is often decisive.

2. Three-phase foam----mineralized foam layer

As mentioned above, the foam containing the ore particles or the third phase is a three-phase foam. The three-phase foam has many similarities with the two-phase foam. For example, the bubbles in the foam layer are changed from large to small from top to bottom, and the separated water layer is thinned from top to bottom, and large bubbles in the upper portion of the foam layer are significantly deformed. A common mineralized foam layer is shown in Figure 4-6-41.

The ideal three-phase foam in the flotation process is composed of bubbles with sufficient mineralization and moderate size. It is not sticky and has good fluidity. Except for the bubble tip, other bubble surfaces are mineralized. Sometimes mineralized foam formed by hydrophobic flocs and many small bubbles can be observed during the flotation process. This foam contains a large amount of hydrophobic ore particles and a small amount of water, which has high stability and is not easily broken. In the process of sweeping, "bubble" is often encountered. This kind of foam has a poor mineralization degree, contains a large amount of water, and the bubbles are large and fragile. Sometimes, water spray mist appears on the foam layer. This phenomenon is mostly caused by excessive foaming agent, which makes the bubbles become brittle and the water is increased. This kind of fragile foam is often poorly mineralized, indicating the use of flotation reagents. Improper, the flotation process is out of tune.

The solid particles in the foam layer strongly influence the stability of the foam. For a fully mineralized foam layer, the ore particles are densely arranged at the gas-liquid interface, which is equivalent to “armoring” the foam. When the bubbles are merged, additional energy is required to cause the adhered ore particles to fall off, making mergers difficult to occur; Because of the adhesion of the ore particles at the gas-liquid interface, the separation of the water layer produces a capillary force that keeps the thickness of the water layer constant. When the contact angle of the ore particles is between zero. ~ 90. At the same time, as the contact angle increases, the adhesion of the ore particles on the bubbles becomes stronger, and the stability of the foam layer also increases accordingly. However, if the contact angle of the ore particles is greater than 90. The hydrophobic ore particles become the medium for communicating the air on both sides of the water layer, causing the bubbles to merge.

The shape and size of the ore particles also affect the stability of the foam layer. The ore particles are too thick and the stabilizing effect is reduced. Galena ore particles such as adding 0.1mm isoamyl alcohol solution may cause a foam layer increases the life of 17s hours, 0.3mm square and the foam layer is an aluminum ore can increase the life of 17s 60s. Hydrophobic colloids of less than 0.1 mm destroy the stability of the foam. The shape of the ore particles is also important, and generally flat ore particles produce a relatively stable foam.

The formation and destruction of the flotation foam layer is a dynamic balancing process. The bubbles float up with a certain thickness of the slurry layer, gradually become dense and interfere with each other when reaching the foam layer, and continue to rise at a certain speed; at the same time, due to the dehydration of the foam layer, a part of the leeches entering the foam layer is not sticky or sticky. The loose ore particles are dropped and returned to the slurry.

The dehydration of the foam layer, the mutual elimination of the bubbles, the reduction of the gas-liquid interface, the pulsation of the bubble wall, and the like, so that the ore particles adhering to the gas-liquid interface compete for the gas-liquid interface according to the adhesion firmness. The firmness of mineral grain adhesion depends mainly on its hydrophobicity and shape. The less hydrophobic mineral particles are not firmly adhered, first falling off, and returning to the liquid phase with the downward flowing water. Thus, the mineral composition or grade of the ore in the foam layer varies with the height of the foam layer, the grade of the concentrate increases with the height of the foam layer, and the gangue mineral mainly accumulates in the lower part of the foam layer, as shown in Fig. 4-6-43. The phenomenon of enrichment of this target mineral in the foam layer is called secondary enrichment. The secondary enrichment is beneficial to improve the concentrate grade. In order to make full use of this effect, attention should be paid to the method of foaming and the speed of foaming, and at the same time, the necessary conditions for the falling of the gangue ore particles in the foam layer should be created. Such as adjusting the pharmaceutical system, reducing the viscosity of the foam, expanding the difference in floatability between the target mineral and the gangue mineral, and even spraying the water properly on the foam layer. But pay attention to the amount of water spray, too big or too small is not good.

According to the analysis of the foam layer structure, some researchers at home and abroad have designed a foam sorting machine in recent years. The principle is that the surface of the sorting tank is caused by a thick layer of foam which has adsorbed the foaming agent and the collector by passing an inflator and previously adding an appropriate amount of foaming agent and collector to the tank. In addition, unlike the general flotation method, the flotation feed is added to the surface of the foam layer, and the foam layer has different selection effects on various minerals due to the different hydrophobicity of various minerals. Minerals with good hydrophobicity easily adhere to the foam and enter the concentrate; minerals with poor hydrophobicity penetrate the foam layer due to the gravity of the mineral and enter the slurry, which is eliminated with the tailings. The sorting principle of the foam sorter is that the foam layer has different effects from different hydrophobic minerals.

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