Sediment, centrifuging

Prethickening of filter feeds can be done with a variety of equipment such as gravity thickeners, hydrocyclones, or sedimenting centrifuges. Even cake filters can be designed to limit or completely eliminate cake formation and therefore act as thickening filters and be used in this thickening duty. 

Equation 8 provides the basis of comparison for the performance of various bottle centrifuges having the same material, and also, under certain circumstances, of other types of sedimentation centrifuges, if geometric dissimilarities are also considered. 

Centrifugal Sedimentation Equipment. Commercial sedimentation centrifuges are characterized priacipaHy by how soHds are discharged, and the general dryness of these soHds. There are batch and automatic batch soHd bowl machines which coUect the soHds at the bowl wall. SoHds are removed very dry. Almost any soHd is coUectable, even those that are very soft and compressible. 

Effect of Fluid Viscosity and Inertia The dynamic effect of viscosity on a rotating liquid slurry as found in a sedimenting centrifuge is confined in veiy thin fluid layers, known as Ekman layers. These layers are adjacent to rotating surfaces which are perpendicular to the axis of rotation, such as bowl heads, flanges, and conveyor blades, etc. The thickness of the Ekman layer 6 is of the order... 

Imperforate Bowl Tests The amount of supernant hquid from spin tubes is usually too small to warrant accurate gravimetric analysis. A fixed amount of shiny is introduced at a controlled rate into a rotating imperforate bowl to simulate a continuous sedimentation centrifuge. The liquid is collected as it overflows the ring weir. The test is stopped when the solids in the bowl build up to a thickness which affects centrate quality. The solid concentration of the centrate is determined similarly to that of the spin tube. 

TABLE 18-12 Specifications and Performance Characteristics of Typical Sedimenting Centrifuges... 

Purchase Price Typical purchase prices, including drive motors, of tubular and disk sedimenting centrifuges are given in Table 18-16. The price will vary upward with the use of more exotic materials of construc tion, the need for explosion-proof elec trical gear, the type of enclosure required for vapor containment, and the degree of portability, and this holds for all types of centrifuges. 

TABLE 18-16 Typical Purchase Prices/ Including Drive Motors/ of Tubular and Disk Sedimenting CentrifugeS/ 1996... 

We have an emulsion of oil in water that we need to separate. The oil droplets have a mean diameter of lO " m, and the specific gravity Of the oil is 0.91. Applying a sedimentation centrifuge to effect the separation at a spedd of 5,000 rpm, and assuming that the distance of a droplet to the axis of rotation is 0.1 m, determine the droplet s radial settling velocity. 

Determine the settling velocity of a particle (d = 4 x 10" m and Pp = 900 kg/m ) through water in a sedimentation centrifuge operating at 4,000 rpm. The particle velocity is a function of distance from the axis of rotation, as shown by the following data ... 

Figure 4.20 Thin layer sedimenting centrifuge Transport equations...

Slow filtering slurries are handled in pressure filters or sedimenting centrifuges. 

Separation of phases is sometimes a problem, especially in the case of mixtures of alkaline aqueous solutions and organic liquids. Addition of materials that decrease the surface tension, filtration of the emulsion to remove interfacial contamination, and the use of sedimentation centrifuges should be considered if the separation time is too long. 

The main factors to be considered are summarised in Table 10.4. As a general rule, sedimentation centrifuges are used when it is required to produce a clarified liquid, and filtration centrifuges to produce a pure, dry, solid. 

There are four main types of sedimentation centrifuge ... 

This approach to describing centrifuge performance has become known as the sigma theory . It provides a means for comparing the performance of sedimentation centrifuges and for scaling up from laboratory and pilot scale tests see Ambler (1952) and Trowbridge (1962). 

A selection guide for sedimentation centrifuges by Lavanchy et at. (1964), which includes other types of solid-liquid separators, is shown in Figure 10.19, adapted to SI units. 

Most of the pollutants may be effectively removed by precipitation of metal hydroxides or carbonates using a reaction with lime, sodium hydroxide, or sodium carbonate. For some, improved removals are provided by the use of sodium sulfide or ferrous sulfide to precipitate the pollutants as sulfide compounds with very low solubilities. After soluble metals are precipitated as insoluble floes, one of the water-solid separators (such as dissolved air flotation, sedimentation, centrifugation, membrane filtration, and so on) can be used for floes removal.911 The effectiveness of pollutant removal by several different precipitation methods is summarized in Tables 5.15-5.17. 

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