Bubble attachment

Frothers. These are also surface-active agents added to the flotation pulp primarily to stabilize the air bubbles for effective particle-bubble attachment, cariyover of particle-laden bubbles to the froth. 

FIG. 19-65 Schematic representation of air hubhle-water-solid particle system (a) before, (h) after particle-bubble attachment, and (c) equilibrium force balance. 

Flotation is a physical process involving relative interaction of three phases solid, water, and air. An understanding of the wettability of the solid surface, physical surface, and chemical phenomena by which the flotation reagents act and the mechanical factors that determine particle-bubble attachment and removal of particle-laden bubbles, is helpful in designing and operating flotation systems successfully. 

Particle-Bubble Attachment. In the above, principles leading to creation of desired hydrophobicity/hydrophihcity of the particles has been discussed. The next step is to create conditions for particle-bubble contact, attachment, and their removal, which is simply described as a combination of three stochastic events with which are associated the probability of particle-bubble colhsion, probabihty of attachment, and probability of retention of attachment. The first term is controlled by the hydrodynamic conditions prevaihng in the flotation unit. The second is determined by the surface forces. The third is dependent on the s irvival of the laden bubble by liq ud t irbulence and impacts by the other suspended particles. A detailed description of the hydrodynamic and other physical aspects of flotation is found in the monograph by Schulze (19 ). 

The flotation process was developed in the mining and coal processing industries as a way of separating suspended solids from a medium such as water. As noted above, the flotation process has found uses in other fields, such as wastewater treatment. The process introduces fine air bubbles into the mixture, so that the air bubbles attach to the particles, and lift them to the surface. 

In recapping, DAF is the process of removing suspended solids, oils and other contaminants via the use of bubble flotation. Air is dissolved into the water, then mixed with the wastestream and released from solution while in intimate contact with the contaminants. Air bubbles form, saturated with air, mix with the wastewater influent and are injected into the DAF separation chamber. The dissolved air then comes out of solution, producing literally millions of microscopic bubbles. These bubbles attach themselves to the particulate matter and float then to the surface where they are mechanically skimmed and removed from the tank. Most systems are versatile enough to remove not only finely divided suspended solids, but fats, oils and grease (FOG). Typical wastes handled include various suspended... 

Xanthate compounds are widely used as collectors in flotation. Their function is to render the mineral surface hydrophobic and thus facilitate bubble attachment. The adsorption of xanthates onto sulfide minerals occurs via an electrochemical mechanism involving the reduction of oxygen and the anodic adsorption of xanthate. 

Bubbles with some of them attached with hydrophobic mineral particles shown in the rising mode. Hydrophilic mineral particles with minority presence of hydrophobic mineral particles (those lost chance for contact with bubbles) and bubbles attached with hydrophobic mineral particles (those got mechanically driven along with hydrophilic particles) shown in the descending mode. (B) Froth flotation air bubbles carry nonwetted particles upwards, while wetted mineral particles drown. 

Selective handover of matter from the pulp to the froth by particle-bubble attachment. 

Estimation of microlayer evaporation The model, incorporating the evaporation from a microlayer surf ace underneath a bubble attached to the heater surf ace, was used by Hendricks and Sharp (1964). With water as the fluid, at somewhat subcooled conditions, the heat transfer rates were as high as 500,000 Btu/hr ft2, or... 

Accelerated particle-bubble attachment through coalescence of tiny bubbles frosted on particle surfaces with larger bubbles, instead of the direct particle-bubble contact... 

Nucleation rates are sensitive to the presence of foreign solid particles, because these objects may act as catalysts. If a nucleus is created on a solid particle, it will remain attached during part of the subsequent growth process. The growth equations for bubbles attached to solids have not been worked out mathematically, but it is rather obvious that interfacial tensions will be important as long as the bubbles are small. 

These bubbles attach themselves to oil droplets and suspended solids as thev rise in the flotation cell. 

Such problems arise in steady-state diffusion, such as the equilibrium radius of a vapor bubble attached to a heated surface in contact with liquid. This was considered by Bick (B5), who used spherical Bessel function expansions rather than integral methods. 

Frothers are surfactants that contribute stability to the froth (foam) so that it will last long enough for solely mechanically-entrained particles to fall back into the pulp phase before the froth is skimmed. Frothers are frequently nonionic surfactants, such as cresol. Frothers may also aid the particle-bubble attachment process. Typical frothers include a variety of alcohols including a-terpineol (pine oil), cresol, polypropylene glycols, short-chain alcohols, and methylisobutylcarbinol (MIBC). 

As discussed in Section 1.2.2 the bubble shapes in fairly dry foams and froths (4 gas > 0.83, approximately) are not spheres or distorted spheres, but polyhedrons. In practice there will be distributions of both gas-cell sizes and shapes. In addition to the gas bubbles, froth contains the floated particles, pulp liquor, and a fraction of (hydrophilic) particles that did not float due to bubble attachment, but which were mechanically entrained in the froth. The pulp liquor and these latter particles all have to be allowed to drain back out of the froth. The rate of this drainage will be greatest at the froth-pulp interface (i.e., the bottom of the froth layer) and slowest near the top of the froth layer. Froth drainage equations are discussed elsewhere [53]. The froth needs to be a stable enough foam that some time can be allowed for these drainage processes, and also so that the upper layer(s) of the froth can be swept out of the flotation cell. On the other hand, the froth should not be too stable as a foam so that it will break easily after collection. In addition to the role of the frother, froth stability is also promoted by increasing liquid viscosity. 

The large Canadian oil-sands surface-mining and water-based flotation processing operations involve a number of kinds of emulsions and foams in a variety of process steps. Whereas mineral ore flotation relies on wettability alteration and bubble attachment, oil and bitumen flotation rely more on attachment and filming to create an oleic foam. 

Figure 12.3 Illustration of adsorption of calcium-surfactant ion pairs to form collectors for ink particle to air bubble attachment and subsequent flotation.

Aubertein and Emeury [29] made a detailed examination of the causes of foaming. Oxidation of non-sulphonated resorcinol is the original cause of gas evolution during nitration. When the product is in the form of very fine crystals the gas bubbles attach themselves to the crystals and raise the product to die surface in the form of a foam. This does not occur when the crystals of die product are large. Therefore, die method of nitration should be such as to favour formation of large ctystals of styphnic acid. To achieve this, Aubertein and Emeuty advise ... 

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