Hydrophobic minerals, testing

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. 

Finally, selective separation and dewatering of one suspended substance in a slurry containing different minerals or precipitates is possible by selectively adsorbing a magnetic material (usually hydrophobic) onto a soHd that is also naturally or chemically conditioned to a hydrophobic state. This process (Murex) was used on both sulfide ores and some oxides (145). More recently, hydrocarbon-based ferrofluids were tested and shown to selectively adsorb on coal from slurries of coal and mineral matter, allowing magnetic recovery (147). Copper and zinc sulfides were similarly recoverable as a dewatered product from waste-rock slurries (148). 

The common gangue material quartz (silica) is naturally hydrophilic and can be easily separated in this way from hydrophobic materials such as talc, molybdenite, metal sulphides and some types of coal. Minerals which are hydrophilic can usually be made hydrophobic by adding surfactant (referred to as an activator ) to the solution which selectively adsorbs on the required grains. For example, cationic surfactants (e.g. CTAB) will adsorb onto most negatively charged surfaces whereas anionic surfactants (e.g. SDS) will not. Optimum flotation conditions are usually obtained by experiment using a model test cell called a Hallimond tube . In addition to activator compounds, frothers which are also surfactants are added to stabilize the foam produced at the top of the flotation chamber. Mixtures of non-ionic and ionic surfactant molecules make the best frothers. As examples of the remarkable efficiency of the process, only 45 g of collector and 35 g of frother are required to float 1 ton of quartz and only 30 g of collector will separate 3 tons of sulphide ore. 

One technology to be tested at the CCTF will be a microbubble flotation process developed by Bechtel National, Inc., of San Francisco. Microbubble flotation is a further development of conventional froth flotation of coal. Froth flotation is a physicochemical process that uses the difference in the surface properties of coal and its associated mineral impurities to effect a separation. An aqueous coal slurry is fed into an aerated tank, where the hydrophobic coal particles become attached to, and are buoyed to the surface by, finely dispersed air bubbles and are collected as a clean-coal-froth product. The mineral matter, being hydrophilic, is wetted by water and remains in suspension to be carried off as refuse. 

After eight years of exposure, an extensive examination was started in 1994 to evaluate the effectivity and durability of the water repellent treatments [4]. Macroscopic tests as water uptake measurements were carried out to determine the remaining effect of the protective organosilicon layer. Due to the fact that surface information from hydrophobic treatment of mineral surfaces is supplied by surface sensitive measuring techniques, TOF-SIMS and additional DRJFT-studies on treated and exposed material were performed. 

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