Hydrophobic minerals, flotation

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. 

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. 

Pyrrhotite is a relatively slow floating mineral, especially monoclinic pyrrhotite, which is usually present in these ore types. The floatability of pyrrhotite also decreases when using certain hydrophobic mineral depressants, such as guars and dextrins. The flotation of pyrrhotite may improve with small additions of copper sulphate (CuS04). 

Kartio, I. J., Basilio, C. I., Yoon, R. H., 1996. An XPS study of sphalerite activation by copper. In R. Woods, F. Doyle, P. E. Richardson (eds.), Electrochemistry in Mineral and Metal Processing IV. The Electro-Chemical Society, 25 - 34 Kelebek, S., 1987. Wetting behaviow, polar characteristics and flotation of inherently hydrophobic minerals. Trans. MM, Sec. C, 96 103 - 107... 

The flotation of minerals is based on different attachment forces of hydrophobized and hydrophilic mineral particles to a gas bubble. Hydrophobized mineral particles adher to gas bubbles and are carried to the surface of the mineral dispersion where they form a froth layer. A mineral is hydrophobized by the adsorption of a suitable surfactant on the surface of the mineral component to be flotated. The hydrophobicity of a mineral particle depends on the degree of occupation of its surface by surfactant molecules and their polar-apolar orientation in the adsorption layer. In a number of papers the relationship was analyzed between the adsorption density of the surfactant at the mineral-water interface and the flotability. However, most interpretations of adsorption and flotation measurements concern surfactant concentrations under their CMC. 

Chelating agents that can form insoluble, hydrophobic chelates on the surface of minerals are potential collectors for the selective flotation of minerals.3 4 As early as 1927, Vivian5 reported the use of cupferron, a well-known analytical reagent, as a collector for the flotation of cassiterite (Sn02). Since then, there have been a number of reports on the use of chelating agents in flotation. 

Flotation of Naturally Hydrophobic Minerals. Flotation response of naturally hydrophobic minerals correlates very well with elec-trokinetic measurements. Figure 3 shows that the flotation of coal correlates well with zeta potential of demineralized coal (5.). The flotation rate is maximum where the zeta potential is zero and it decreases with increase in the magnitude of the zeta potential. Similar observations were made earlier by Chander and Fuerstenau (6 ) for the flotation of molybdenite. The decrease in flotation rate with increase in zeta potential is because of the electrical double layer repulsion between the charged particle and the air bubble. 

Scalping flotation is where a first flotation step is practised in order to remove a minor, hydrophobic mineral. This mineral could be one that is valuable, such as MoS2 in a Cu-Mo ore [91], or one that is unwanted, such as talc. 

Mineral flotation is a method for selective separation of mineral components out of polymineral dispersions of ground ores in water (ca. 5-35 vol.% of the solid) by using dispersed gas (usually air) bubbles. The method consists in the different adhesion of hydrophobized and hydrophilic mineral particles to an air bubble. Hydrophobized mineral particles adhere to the air bubble and are carried out as a specifically lighter aggregate to the surface of the mineral dispersion where they form a foam (froth) layer. This foam, called concentrate, is mechanically removed (Fig. 1A). A mineral is hydrophobized by adsorption of a suitable surface-active compound (surfactant, collector) on the surface of the mineral component to be flotated. All other nonhydrophobized particles remain dispersed in the mixture (Fig. IB). 

The rest of the chapter has been devoted to special topics and in materials science there are many possibilities. Those selected include the mechanism of the flotation of minerals in which the addition of a certain organic to the solution causes a specific mineral to become hydrophobic so that it is exposed to air bubbles, the bubbles stick to it and buoy the mineral up to the surface, leaving unwanted minerals on the bottom of the tank. It turns out that the mechanism of this phenomenon involves a mixed-potential concept in which the anodic oxidation of the organic collector, often a xanthate, allows it to form a hydrophobic film upon a semiconducting sulfide or oxide, but only if there is a partner reaction of oxygen reduction. This continues until there is almost full coverage with the dixanthate, and the surface is thereby made water-repelling. 

The above descriptions show the monomeric structures of starch, dextrin, cellulose, and guar gum. In reality, these polysaccharides can be extracted from different sources and the chain length and configuration, molecular weights, and the contents of impurities may vary considerably. Generally, starches have been used mainly as flocculants or flotation depressants for iron oxide minerals and phosphate minerals while the associated silica is floated. Dextrin has been mainly tested as depressants for inherently hydrophobic minerals such as talc, molybdenite, and coal [96]. Applications of polysaccharides in other mineral systems, both in the laboratory and in commercial processes, have also been frequently reported. As can be seen, the polysaccharides have been used or tested as selective depressants in practically all types of mineral systems, ranging from oxides, sulfides, salt-type, and inherently hydrophobic minerals. 

Froth flotation separation of minerals requires the mineral of interest to have a hydrophobic surface while the other minerals present in the pulp (a mixture of 25% m/m powdered ore with water) are hydrophilic [361, 362]. The particles of the hydrophobic mineral can be captured by the gas bubbles and raised to the pulp surface where they are collected to the froth. Flotability of the mineral depends not only on the amount of adsorbed hydrophobizing reagent (the collector), which in most cases does not exceed a monolayer, but also on the form in which it is adsorbed [361, 363]. 

Flotation equipment consists of a series of agitated tanks through which the mineral pulp flows and into which air is dispersed as a stream of fine bubbles. A froth is formed on the surface, containing the hydrophobic mineral particles, and is skimmed off into a trough where it collapses and flows into a collection tank. 

Ore dressing Chemicals, often containing sulfur, are added to the crushed ore dispersed in by flotation water. The chemicals are adsorbed on the surface of the valuable mineral grains and make them hydrophobic. When air is blown into the suspension the air bubbles stick to the hydrophobic mineral-grain surfaces and they float to the surface. The gangue grains stay on the bottom of the container. 

It is weU known that the selective adsorption of surfactants at the solid-water interface imparts hydrophobicity to the surface of the solid. The relative hydrophobicity of the solid surface is responsible for various macroscopic properties observed experimentally. For example, in mineral separation, the hydrophobicity of the solid surface leads to selective bubble-particle attachment, which accounts for the selective flotation of minerals in large scale industrial plants. The relative measure of mineral surface hydrophobicity is usually quantified in terms of contact angle measurements and flotation experiments (Fuerstenau 1957, 1970, 2000 Fuerstenau and Herrera-Urbina 1989 Fuerstenau and Pradip 2005 Pradip 1988). Molecular-modeling tools can be successfully employed to compute the interaction energies and contact angle on both virgin and surfactant-covered mineral surfaces. The relative flotation efficacy of different surfactants can thus be related to their molecular structure and properties. 

In this chapter, the use of MD simulations in examining the solution chemistry and interfacial phenomena of selected flotation systems common to mineral processing are examined, including soluble salt minerals, phyllosilicate minerals, oxide minerals, and natural hydrophobic minerals. These initial MD simulation results are discussed in terms of their significance in the understanding of flotation technology for the separation and recovery of mineral resources. 

Naturally hydrophobic minerals such as graphite and talc are common gangue minerals found in sulfide ores and are difficult to separate due to their tendency to float together with valuable sulfide minerals. Despite the relatively successful use of the polysaccharide group of chemicals (e.g., dextrin, guar gum, and carboxymethyl cellulose) as flotation depressants for these naturally hydrophobic minerals, the nature of the adsorption processes remains in debate. Consequently, the adsorption of amphipathic solutes at naturally hydrophobic minerals such as coal and graphite, talc, and sulfur is of interest to many researchers, and a substantial amount of research has been discussed. 

In this chapter the surface chemistry of selected nonsulflde flotation systems, including soluble alkali halide salts, phyllosilicates, quartz, and some naturally hydrophobic minerals, were studied using MD simulation. Issues such as water structure and dynamics, solution chemistry, interfacial water structure, and adsorption states for surfactants and macromolecules were examined. It is clear that MD simulation has been validated as a very useful tool to study the surface chemistry of certain flotation systems. As a complement to experimental studies, MD simulation analysis provides further information and understanding at the atomic level to issues such as water structure, particle dynamics, solution viscosities, mineral surface wetting characteristics, surface charge, and adsorption states. A wide application of MD simulation in the study of mineral surface chemistry is expected to have a significant impact on further advances in flotation technology. 

In a laboratory experiment (Adamson and Gast, 1997), one may use the following recipe. To a 1% sodium bicarbonate solution, one can add a few grams of sand. Then if one adds some acetic acid (or vinegar), the bubbles of CO2 produced cling to the sand particles and thus make these float on the surface. It must be mentioned that in wastewater treatments, the flotation method is one of the most important procedures. When rocks in crushed state are dispersed in water with suitable surfactants (also called collectors in industry) to give stable bubbles on aeration, hydrophobic minerals will float to the surface due to the attachment of bubbles, while the hydrophilic mineral particles will settle to the bottom. The preferential adsorption of the collector molecules on a mineral makes it hydrophobic. Xanthates (alkyl-O-CSj) have been used for flotation of lead and copper. 

Since then, there have been many publications concerning the effect of particles on the formation and stability of foams, froths, and emulsions. Here we are mainly concerned with the destabilizing effect of particles on foams and froths. Practical examples inclnde the effect of hydrophobed mineral particles and collector precipitates on the stability of mineral flotation froths [77, 143], Another is the nse of wax [144] to control the foam of detergent formulations for automatic washing machines. 

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... 

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). 

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