Dispersion of Floating Solids

The Froude number, Npr, is a predominant correlating parameter in these systems, where liquid surface behavior is so important. Joosten et al. (1977) have developed a correlation that has been used successfully to design a commercial mixing system for suspending floating solids in a 50 m vessel. The correlation is 

Joosten et al. (1977) recommend a down-pumping 45° pitched blade impeller in a vessel with a single baffle whose width is one-fifth the impeller diameter submerged to a depth of one-third the impeller diameter to produce a noncentral vortex. Henmajani et al. (1988) recommend a down-pumping 45° pitched blade 

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Suspension systems of sticky slurry and paste-like liquid explosives with solid particles, based on the dispersion of suspended solid particles, should belong to suspension or coarse multiphase systems in colloid chemistry. In these suspension systems, the main issue is its dynamic instability, because the density of the dispersed particles and the density of the dispersion medium are different (generally, the density particle is greater than that of the medium), settlement or floating can occur with the role of gravitational field to separate the system, resulting in unevenness in composition and density of liquid explosive. Stability is the ability to overcome the so-called sink-and-float separation of two-phase components, therefore, within a certain period of use, the composition and density of explosive and other physical parameters remained unchanged and its properties are stable and reliable. 

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. 

The physical properties of the solid state seen in crystals and powders of both drugs and pharmaceutical excipients are of interest because they can affect both the production of dosage forms and the performance of the finished product. Powders, as Pilpel reminded us, can float like a gas or flow like a liquid but when compressed can support a weight. Fine powders dispersed as suspensions in liquids are used in injections and aerosol formulations. Both liquid and dry powder aerosols are available and are discussed in Chapter 9 some properties of compacted solids are dealt with in Chapter 6. In this chapter we deal with the form and particle size of crystalline and amorphous drugs and the effect these characteristics have on drug behaviour, especially on drug dissolution and bioavailability. 

The solid material normally observed in fluid environmental samples may be inorganic in nature (e.g. floating sludge) but also bacteria are often present. Because many compounds preferentially adsorb on solid materials or are accumulated in bacteria to different extent, simple filtration will alter the overall composition of the sample solution. In environmental samples the presence of humic and fulvic acids deserve special attention. They not only can form complexes with many metal ions but they can also affect surface tension and even viscosity and so have influence on the performance of the analyzing system such as the dispersion of the sample plug in a flow-through analyzer. 

It is possible to remove small particles using dispersed or dissolved gas flotation devices. These units are primarily used for removing suspended hydrocarbons from water. Gas is normally dispersed into the water or released from a solution in the water, forming bubbles approximately 30-120 pm in diameter. The bubbles form on the surfaces of the suspended particles, creating particles whose average density is less than that of water. These rise to the surface and are mechanically skimmed. In the feed stream, chemicals called "float aids" are normally added to the flotation unit to aid in coagulation of solids and attachment of gas bubbles to the solids. The optimum concentration and chemical formulation of float aids are normally determined from batch tests in small-scale plastic flotation models on-site. Because of the difficulty of predicting particle removal efficiency with this method, it is not normally used to remove solids from water in production facilities. 

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. 

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