Flotation hydrophobic

Skin Flotation. Hydrophobic particles can be removed in the form of a thin, usually one particle thick layer on top of a trough, giving rise to the skin flotation process. 

Figure 5.43. Elementary act of separation by flotation hydrophobic particles, hydrophilic particles. The former adhere to ascending air bubbles.

Flotation reagents are used in the froth flotation process to (/) enhance hydrophobicity, (2) control selectivity, (J) enhance recovery and grade, and (4) affect the velocity (kinetics) of the separation process. These chemicals are classified based on utili2ation collector, frother, auxiUary reagent, or based on reagent chemistry polar, nonpolar, and anionic, cationic, nonionic, and amphoteric. The active groups of the reagent molecules are typically carboxylates, xanthates, sulfates or sulfonates, and ammonium salts. 

Interaction of Solids With Flotation Reagents. For flotation to occur with the aid of reagents, such compounds must adsorb at the sohd—hquid interface unless the soHd to be floated is naturally hydrophobic. In this latter case only depression can be attempted by the use of additional ions or depressants that hinder bubble—particle adhesion. Frothers (typically long-chain alcohols) and/or modifying agents such as hydrocarbon oils can, however, be used to enhance the collection of naturally hydrophobic soflds such as M0S2, talc, or plastics. 

First-order kinetics (ie, n = 1) is frequently assumed and seems adequate to describe the kinetics of most flotation processes. However, highly hydrophobic particles float faster and very fine particles or coarse ones outside the optimal flotation size range (see Fig. 1) take longer to coUect in the froth layer. ExceUent reviews of the subject are available in the Hterature (27). 

In the removal of contaminating ions such as (PO or Fe " a precipitate such as Ca2(P0 2 Fe(OH)2, after oxidizing ferrous ion to ferric, is formed and the soHd is removed. The addition of surfactants is usually not essential (nor desirable) since most waters contain natural surfactants that would render the soflds sufficiently hydrophobic for flotation to occur. Such surfactants derive from the degradation of organic matter, and humic substances abundantly available in nature (30). 

Froth flotation (qv) is a significant use of foam for physical separations. It is used to separate the more precious minerals from the waste rock extracted from mines. This method reHes on the different wetting properties typical for the different extracts. Usually, the waste rock is preferentially wet by water, whereas the more valuable minerals are typically hydrophobic. Thus the mixture of the two powders are immersed in water containing foam promoters. Also added are modifiers which help ensure that the surface of the waste rock is hydrophilic. Upon formation of a foam by bubbling air and by agitation, the waste rock remains in the water while the minerals go to the surface of the bubbles, and are entrapped in the foam. The foam rises, bringing... 

The carbon black (soot) produced in the partial combustion and electrical discharge processes is of rather small particle si2e and contains substantial amounts of higher (mostly aromatic) hydrocarbons which may render it hydrophobic, sticky, and difficult to remove by filtration. Electrostatic units, combined with water scmbbers, moving coke beds, and bag filters, are used for the removal of soot. The recovery is illustrated by the BASF separation and purification system (23). The bulk of the carbon in the reactor effluent is removed by a water scmbber (quencher). Residual carbon clean-up is by electrostatic filtering in the case of methane feedstock, and by coke particles if the feed is naphtha. Carbon in the quench water is concentrated by flotation, then burned. 

Up to 0.4 g/L of the iodine stays in solution and the rest precipitates as crystallized iodine, which is removed by flotation (qv). This operation does not require a flotation agent, owing to the hydrophobic character of the crystallized element. From the flotation cell a heavy pulp, which is water-washed and submitted to a second flotation step, is obtained. The washed pulp is introduced into a heat exchanger where it is heated under pressure up to 120°C to melt the iodine that flows into a first reactor for decantation. From there the melt flows into a second reactor for sulfuric acid drying. The refined iodine is either flaked or prilled, and packed in 50- and 25-kg plastic-lined fiber dmms. 

Collectors ndFrothers. Collectors play a critical role ia flotation (41). These are heteropolar organic molecules characterized by a polar functional group that has a high affinity for the desired mineral, and a hydrocarbon group, usually a simple 2—18 carbon atom hydrocarbon chain, that imparts hydrophobicity to the minerals surface after the molecule has adsorbed. Most collectors are weak acids or bases or their salts, and are either ionic or neutral. The mode of iateraction between the functional group and the mineral surface may iavolve a chemical reaction, for example, chemisorption, or a physical iateraction such as electrostatic attraction. 

The functional group ia collectors for nonsulfide minerals is characterized by the presence of either a N (amines) or an O (carboxyUc acids, sulfonates, etc) as the donor atoms. In addition to these, straight hydrocarbons, such as fuel oil, diesel, kerosene, etc, are also used extensively either as auxiUary or secondary collectors, or as primary collectors for coal and molybdenite flotation. The chain length of the hydrocarbon group is generally short (2—8 C) for the sulfide collectors, and long (10—20 C) for nonsulfide collectors, because sulfides are generally more hydrophobic than most nonsulfide minerals (10). 

The amount of collector used is necessarily very small because surface coverages of a monomolecular layer or less are required to impart sufficient hydrophobicity to the mineral. The usages typically range from 1—100 g of collector per ton of ore treated for sulfide flotation (typically 0.2—10% value metal content ia the ore) and 100—1000 g/1 for nonsulfide flotation (1—20% value mineral content) (10). 

The most important compound of Mo (TV) is molybdenum disulfide [1317-33-5] M0S2 (21). The layered stmcture of M0S2 is reflected in the flat plate-like hexagonal gray-black crystaUites found in natural and synthetic samples. The stmcture consists of pairs of close-packed layers of sulfur which are echpsed with respect to each other. The close-packed sulfur surfaces are naturally hydrophobic, which faciUtates the extraction of M0S2 ore by flotation. 

Zinc ores are generally floated at the mine (18). In the case of simple zinc sulfide ores, flotation is carried out by treatment with copper sulfate to activate the sphalerite causing it to be wet by the organic collector (eg, xanthate). The now-hydrophobic zinc ore particles attach themselves to the rising bubbles. Oxidized ore particles present must be sulftdized with sodium sulfide to be floated (19). Flotation produces concentrates which are ca 50—60% zinc. In mixed ore, the lead and copper are usually floated after depressing the sphalerite with cyanide or zinc sulfate. The sphalerite is then activated and floated. 

The flotation process usually iuvolves three steps (/) the conditioning of the coal surface iu a slurry with reagents, (2) adhesion of hydrophobic coal particles to gas bubbles, and (J) the separation of the coal-laden bubbles from the slurry. In the conventional flotation process, when the coal particles become attached to air bubbles, the particles ate allowed to rise to the top of the flotation cell and form a stable froth layer (9). A mechanical scraper is used to remove the froth layer and separate the clean coal product from the refuse-laden slurry. 

In 1981, a novel flotation device known as the air-sparged hydrocyclone, shown in Figure 3, was developed (16). In this equipment, a thin film and swid flotation is accompHshed in a centrifugal field, where air sparges through a porous wall. Because of the enhanced hydrodynamic condition, separation of fine hydrophobic particles can be readily accompHshed. Also, retention times can be reduced to a matter of seconds. Thus, this device provides up to 200 times the throughput of conventional flotation cells at similar yields and product quaHties. 

The pH of the pulp to the flotation cells is carefliUy controlled by the addition of lime, which optimizes the action of all reagents and is used to depress pyrite. A frother, such as pine oil or a long-chain alcohol, is added to produce the froth, an important part of the flotation process. The ore minerals, coated with an oily collected layer, are hydrophobic and collect on the air bubbles the desired minerals float while the gangue sinks. Typical collectors are xanthates, dithiophosphates, or xanthate derivatives, whereas typical depressants are calcium or sodium cyanide [143-33-9] NaCN, andlime. 

CMC), reverses the effect that the surfactant has on contact angle at lower concentrations, and at or above the CMC there is no further lowering of surface tension. At the higher concentrations, the surfactant loses some of its beneficial effect on dewateriag, as shown ia Figure 5. The beneficial effects of surfactants on dewateriag are most pronounced ia cakes that have been partially deslimed or ia cakes of partially hydrophobic particles (eg, flotation concentrates) that are adsorbed onto each other. Surfactants at or above CMC have Httle practical effect on extremely fine cakes, where pores are small and the cake has no further opportunity to consoHdate. A number of filter cakes do not respond to surfactant addition at any level. 

Mixed liberated particles can be separated from each other by flotation if there are sufficient differences in their wettability. The flotation process operates by preparing a water suspension of a mixture of relatively fine-sized particles (smaller than 150 micrometers) and by contacting the suspension with a swarm of air bubbles of air in a suitably designed process vessel. Particles that are readily wetted by water (hydrcmhiric) tend to remain in suspension, and those particles not wetted by water (hydrophobic) tend to be attached to air bubbles, levitate (float) to the top of the process vessel, and collect in a froth layer. Thus, differences in the surface chemical properties of the solids are the basis for separation by flotation. 

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 ). 

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. 

Surface properties such as the absorptional ability and the wettability of minerals are again of significant technical importance. On the wettability scale, as for example, minerals are classified as hydrophilic minerals (which are easily wetted by water) and hydrophobic minerals (which are not wetted by water). Hydrophobicity is very helpful in obtaining enrichment of ores by flotation. 

Figure 2.21 (A) Separation of hydrophobic from hydrophilic particles in flotation.

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. 

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