Slurry particle velocity

Separation involving sedimentation is dependent upon settling velocity, which requires a difference in density between solid particles and the suspending liquid. Gravitational sedimentation operations are divided into clarification and thickening. Clarification involves dilute suspensions and frequently has the objective of liquid recovery. Thickening refers to solid recovery by forming more concentrated slurries. Particle size, liquid and particle densities, and liquid viscosity are important factors in sedimentation processes. 

Figure 5. Three-dimensionsal simulation for 800 /jm particles with a viscosity input model in the IIT slurry bubble column with a 5 cm width (a) instantaneous particle velocity vector plots and solid volume fractions (color bar) at 25 s, and (b) time-averaged particle velocity at the center (c) near the wall from 15 to 36 5. (Fi=2.02cm/s, VG=3,37cm/s)...

There are data to indicate that for particles 70 mesh d = 210 microns) or larger the effect of agitation is pronounced. As the particle size decreases this effect disappears. If there is little mechanical agitation of the slurry, the velocity will be related to the settling of the particles. Calderbank and Jones have equated the settling velocity due to gravitational force to the relative velocity in Friedlander s theory. The resultant correlation for is... 

The ANL capacitive flowmeter was installed at the vertical line of the SLTF (see Fig. 6.19), thus, particle settling problems would not occur. The only parameter that affected the velocity measurement was the velocity profile. Figure 6.24 shows that particle velocities were measured over a range of solids concentrations. For coal concentrations of 3.5-33 wt.%, the measured velocities were independent of coal concentration and 7% accurate when compared with timed-diversion measurements. For higher coal concentrations, the measured velocities deviate from the diversion measurements velocity measurements were = 30-80% higher for 43 and 49 wt.% slurries, but 40%... 

Velocity and concentration profiles are two important parameters often needed by the operator of slurry handling equipment. Several experimental techniques and mathematical models have been developed to predict these profiles. The aim of this chapter is to give the reader an overall picture of various experimental techniques and models used to measure and predict particle velocity and concentration distributions in slurry pipelines. I begin with a brief discussion of flow behavior in horizontal slurry pipelines, followed by a revision of the important correlations used to predict the critical deposit velocity. In the second part, I discuss various methods for measuring solids concentration in slurry pipelines. In the third part, I summarize methods for measuring bulk and local particle velocity. Finally, I review models for predicting solids concentration profiles in horizontal slurry pipelines. 

Beck and co-workers (86, 87) placed the sensors on the pipe wall. As a result, their measurements give particle velocity averaged over the pipe cross-section. Mounting the sensors on the pipe wall is useful for vertical slurry flow where the velocity profile is uniform. This technique, however, is not so useful for the flow of settling slurries in horizontal pipelines. In this case, the solids are not uniformly distributed over the pipe cross-section, and, as a result, particle velocity is a strong function of position in the pipe. 

To measure local particle velocity in slurry pipelines, Brown et al. (88) modified Beck et al. s (86, 87) conductivity method. They developed a new conductivity probe where four electrodes are mounted on an L-shaped probe. The probe has two field electrodes and two sets of sensor electrodes separated by a known distance (Figure 24). The probe is capable of measuring particle velocity in vertical and horizontal slurry... 

For slurry flow in a horizontal pipe, solids concentration and particle velocity profiles are not uniform, especially for highly settling slurries. These profiles are functions of particle settling velocity, bulk velocity, pipe diameter, and shape (89). A brief review of various models to predict vertical solids concentration in a horizontal pipe is given later. 

The results shown in Figures 25 and 31 indicate that for a given Cp the mean solids concentration in the drum with lifters is lower than that for the drum without lifters. This trend is due to higher axial particle velocities encountered in drums with lifters. For the drum with lifters, the variation of the slurry mean solids concentration with the drum speed was much less pronounced than that observed for the drum without lifters. Also, the effect of Cp on the hold-up solids concentration was more pronounced for the drum with lifters. 

In wet grinding carried out in tube mills the axial progress of the material is governed mainly by the flow velocity of the slurry, its water/solids content and the fineness of the raw slurry particles. If the water content is appropriately adjusted, classification of the material according to particle size occurs in the mill, which is advantageous because particles already sufficiently reduced in size will then not unnecessarily be further subjected to grinding action. 

In a typical slurry pipeline design situation, the flowrates and solids concentrations are fixed by process material balances and equipment performance specifications. In these circumstances, a primary goal in design is selection of the optimum pipe diameter. For slurries in turbulent flow, the optimum transport condition almost invariably occurs when all the particles are suspended but moving at the lowest possible mean velocity. By operating the pipeline at the slurry deposition velocity, the frictional energy losses and wear are minimized and the whole of the pipe cross-section is available for flow. 

The ordinate is the ratio of the particle velocity to the bulk velocity of the flow. The single particle velocity ratios are close to the values measured in the slurries. The deposition velocity in all three experiments was probably near 2.5 m/s which is consistent with the results shown in Figure 2. 

Slurry Line velocity has to be greater than solid settling veiodiy to prevent solid particles sectiiog down in line. 

V is slurry line velocity, in cm/sec c is slurry solid concentratiorL, In volume % fL is friction factor of line flow without solid particles, dimensionless C is drag coefficient, dimensionless dx is line inside diameter, in cm g is gravitational constant, in 980 cm/sec2 s — ps/pL, density ratio of solid particle to liquid, dimensionless. 

Fig. 5. The effect of ultrasonic irradiation on the surface morphology and particle size ofNi powder. Initial particle diameters (a) before ultrasound were i 160 fim-, (b) after ultrasound, fim. High velocity interparticle coUisions caused by ultrasonic irradiation of slurries are responsible for the smoothing...

Pressure filters can treat feeds with concentrations up to and in excess of 10% sohds by weight and having large proportions of difficult-to-handle fine particles. Typically, slurries in which the sohd particles contain 10% greater than 10 ]lni may require pressure filtration, but increasing the proportion greater than 10 ]lni may make vacuum filtration possible. The range of typical filtration velocities in pressure filters is from 0.025 to 5 m/h and dry sohds rates from 25 to 250 kg nY/h. The use of pressure filters may also in some cases, such as in filtration of coal flotation concentrates, eliminate the need for flocculation. 

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