Dispersion floating solids

Little has been done on dispersing floating solids. This is an area in which much work is still needed, especially with fine, air-entrapping and non-wetting solids. It appears that the process is very energy intensive. 

This problem has still rarely been studied. This process is generally more energy-intensive than solids suspension and is complicated by the fact that the topic essentially covers two different aspects. Firstly, solids float when they are large and of density less than that of the fluid. Secondly, and perhaps more commonly, fine solids float because they form cohesive clumps filled with air. In this case, dispersion includes breaking up of these clumps as well as ingestion from the top surface. Both these processes are made more difficult if the soiids cannot be wetted or only wetted with difficulty. After dispersion, such solids as individual particles may be denser than the liquid, giving rise to a... 

When used to separate solid-solid mixtures, the material is ground to a particle size small enough to liberate particles of the chemical species to be recovered. The mixture of solid particles is then dispersed in the flotation medium, which is usually water. Gas bubbles become attached to the solid particles, thereby allowing them to float to the surface of the liquid. The solid partices are collected from the surface by an overflow weir or mechanical scraper. The separation of the solid particles depends on the different species having different surface properties such that one species is preferentially attached to the bubbles. A number of chemicals are added to the flotation medium to meet the various requirements of the flotation process ... 

For suspension of free-settling particles, circulation of pseudoplastic slurries, and heat transfer or mixing of miscible liqiiids to obtain uniformity, a speed of 3.50 or 420 r/min should be stipulated. For dispersion of dry particles in hquids or for rapid initial mixing of hquid reactants in a vessel, an 11.50- or 1750- r/min propeller should be used at a distance Df/4 above the vessel bottom. A second propeller can be added to the shaft at a depth below the hquid surface if the submergence of floating hquids or particiilate solids is other wise inadequate. Such propeller mixers are readily available up to 2.2 kW (3 hp) for off-center sloped-shaft mounting. 

It is important to state the difference between particles and particulate films at the onset of this section. Particles are separate nanometer- to micron-sized colloids dispersed in solution. Physically interconnected colloidal metal particles constitute a particulate film which may be supported by a monolayer floating on an aqueous subphase or be deposited on a solid substrate. 

When liquid samples such as serum, plasma, milk, or honey are not to be extracted using direct liquid-liquid partitions with organic solvents but through use of solid-phase extraction or matrix solid-phase dispersion techniques, dilution with water (323, 324), phosphate buffer saline (325), or phosphoric acid (326, 327) is often the only sample preparation procedure applied. Milk analysis sometimes requires further pretreatment for fat removal (328). Centrifugation at about 7000g at 4-10 C for 20 min is the usually applied procedure for making the fat floating on top of milk readily eliminated. 

Electrolytic formation of finely dispersed bubbles (usually H2 and O2, from water electrolysis) can remove suspended particles from a liquid by floating them to the top. This solid matter can then be skimmed off from the top and subsequently treated. Much like electrodialysis, electroflotation is a process that concentrates the waste, which eventually needs to be treated and/or reused. It was estimated in 1980 that 20 electroflotation plants existed in the UK for treating industrial effluent [139]. Little to no information is available on the use of electroflotation for the decontamination of waste effluents in the USA. 

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