Slurries settling characteristics

Slurry-settling characteristics Slurry-filtering characteristics... 

Flow of slurries in rotary drums depends on, among other factors, slurry settling characteristics. For the case of highly settling slurries flowing in a rotary drum with open-end and no lifters, these conclusions were obtained ... 

Figure 5.4 Coding the slurry settling characteristics (adapted from Purchas and Wakeman, 1986).

FIG. 18-135 Coding the settling characteristics of a slurry. (Purchas, Solid/Liqnid Separation Equipment Scale-Up, Uplands Press, Croydon, England, 1977, p. 11, hy permission.)... 

Alumina- and silica-abrasive slurries display different settling characteristics for the blend as compared only to abrasive component. In most alumina-based slurries, normal blends (created by diluting abrasive with DI water and/or chemical additive component) settled more quickly as compared only to the abrasive component, whereas in silica-based slurries the settling behavior was... 

The transportation of sludges and slurries in pipelines is advantageous, but poses more problems arising from high viscosity, nonhomogenity of the fluid system and the tendency of suspended materials to segregate and settle. The tendency to settle varies with the particular flow condition. Particle density, shape and size as well as size distribution, concentration and composition influence the settling characteristics. 

Solids concentrations can vary from a few percent to well over 50% in a typical stirred tank. Solids concentration, particle shape, and the viscosity of the suspending phase are the main factors affecting the rheology and settling characteristics of the slurry. Cubic- and spherical-shaped solids tend to form Newtonian slurries, while needle-, oblong-, and plate-shaped solids form thixotropic slurries. Such slurries exhibit yield stresses even at quite low solids concentrations. This can lead to the development of caverns, as shown in Section 9.4. Proper design can usually overcome these stagnation problems. 

The second step consists of running a simple test to identify the settling characteristic of the slurry. A sample is allowed to settle in a one liter... 

The results from the test procedure are coded as shown in Figure 5.5, using a third set of characteristic letters. For example, a slurry which forms a cake at the rate 0.5 cm min is coded K. Combining this with the settling characteristics gives a total preliminary description of the separation characteristics of the slurry (e.g. BEG, K). If the proposed duty is simply to thicken a slurry then it is not necessary to carry out a filtration test. However, for a total separation of the solid from the liquid (as obtained in a filter, for example) both settling and filtration tests need to be performed. 

The duty specification, jar sedimentation and filtration tests enable the slurry settling and filtering characteristics to be broadly classified, and a selection problem to be specified through a series of letter codings. In order to select and rank equipment from this information it is necessary to provide charts and/or tables which relate equipment performance to the letter codings. Comparisons between the user defined specifications and the tables/charts enable the selection process. 

Also, it is essential to take more account of the properties of the solids and liquids to be handled during the separation these are listed in Tables 5.10 and 5.11. The properties of the slurry are also important, but any particular property will normally be dominated by the corresponding property of one of the phases to be separated. Many properties of the slurry (e.g. solids concentration in the slurry, filtrate fluxes, settling and filtration rates, slurry flow characteristics, particle size distribution and their state of aggregation in the slurry and pH value) will have been ascertained in a well designed experimental programme. It is also important to perform some kind of sensitivity analysis to assess the Ukelihood and extent of variations in flow rate and solids content of the feed slurry, the particle size distribution in the feed and... 

CaCOj, suspended in a 10% solution of sodium hydroxide, NaOH, 0.125 kg suspended solid/kg solution. This is settled, the clear sodium hydroxide solution withdrawn and replaced by an equal weight of water, and the mixture thoroughly agitated. After repetition of this procedure (a total of two freshwater washes), what fraction of the ori nal NaOH in the slurry remains unrecovered and therefore lost in the sludge The settling characteristics of the sluny, determined under conditions representing the practice to be followed in the process [Armstrong and Kammermeyer, ItuL Eng. Ckem., 34, 1228 (1942)], show adsorption of the solute on the solid. 

Limited uranium concentralion. In solution reactors, uranium concentration is limited b solubility or corrosion effects, and in slurries, by the effective iscosity and settling characteristics. In H20-moderated reactors, in particular, a high uranium or thorium concentration is necessary for a high convcr.sion ratio. Concentrations up to 1000 g/liter, however, may be con.sidcred for solutions and up to 4000 g/liter for fluidized beds. 

Brief studies made with thorium hydroxide indicated [33] that it is probably not a good source material for the production of slurry oxide. As precipitated from nitrate solution, the hydroxide formed a bulky precipitate which was hard to filter and wash, was amorphous to x-rays, and contained con.siderable nitrate impurity. Drying at 300 to 500°C yielded a crystalline oxide product which was difficult to slurry. Autoclaving a slurry of the hydroxide (without previous drying) at 250°C gave a bulky slurry (settled volume 300 to 500 g Th/liter) exhibiting a characteristic TI1O2 x-ray diffraction pattern. 

High-temperature sedimentation characteristics. Slurry settling rates at temperatures in excess of 100 C have been obtained in quartz tube 8 mm in diameter [54]. These data, obtained with a slurry of thorium oxide prepared by a 650°C calcination of thorium formate [55], indicated that the slurry was already in the compaction zone of settling above 500 g Th/kg H2O at 200 to 300°C. At a concentration of 1000 g Th/kg H2O, no settling occurred at temperatures above 100°C. The small diameter of the tube probably affected the concentration at which the slurry went into compaction. 

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