Sedimentation, gravity

Note - In designing a system based on the settling velocity of nonspherical particles, the linear size in the Reynolds number definition is taken to be the equivalent diameter of a sphere, d, which is equal to a sphere diameter having the same volume as the particle. 

Sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening, i.e.. 

Typical diameters are up to 150 ft. and machines have operating torques to 750,000 fir-Lbs. The Bridge - or Beam or Truss - spans the diameter of the tank and supports the drive and rake mechanisms. The underflow is removed from the discharge cone at the bottom center. 

The diameters on these machines are over 400 ft., with operating torques to 2,400,000ft.-Lbs. The stationary center pier supports the drive and rake mechanisms. The truss extends from the center pier to the tank periphery supports walkway, power lines and feed lasunder. 

Diameters are typically 140 ft. or more with operating torques of 600,000ft.-Lbs. This is a high throughput machine designed for large loadings. 

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Gravity sedimentation Gravity separation Gravure Gravure inks... 

In particle-size measurement, gravity sedimentation at low soHds concentrations (<0.5% by vol) is used to determine particle-size distributions of equivalent Stokes diameters ia the range from 2 to 80 pm. Particle size is deduced from the height and time of fall usiag Stokes law, whereas the corresponding fractions are measured gravimetrically, by light, or by x-rays. Some commercial instmments measure particles coarser than 80 pm by sedimentation when Stokes law cannot be appHed. 

Tim LarOS/ M.S./ Mineral Processing, Senior Proce.ss Consultant, El MCO Proce.ss Equipment Co. Member, Society of Mining, Metallurgy, and Exploration (SME of AIME). (Gravity Sedimentation Operations Filtration)... 

GMrge Priday/ B S / Ch E / EIMCO Process Equipment Co. Member, Amenean In.sti-tute of Chemieal Engineering (AJChE), Instrument Soeiety of Amenea (ISA). (Gravity Sedimentation Operation.s)... 

Both particle size and the liquid viscosity affect the rate of particle settling. The rate of settling due to gravitational force decreases with decreasing particle size and increasing viscosity. The process mechanisms are sensitive to the relative rates of filtration and gravity sedimentation. 

Filters generally achieve a lower final moisture content than obtained by gravity sedimentation and are often fed from thickeners, as indicated in the schematic particulate process shown in Figure 9.2. In this chapter the principles of slurry filtration will be described and certain simplified filter design equations derived. For more complex derivations the reader is referred to specialist texts e.g. Coulson and Richardson (1991), Wakeman (1990a) and Purchas (1981). 

The capacity of filtration centrifuges is very dependent on the solids concentration in the feed. For example, at 10 per cent feed slurry concentration 9 kg of liquid will be centrifuged for every 1 kg of solids separated whereas with a 50 per cent solids concentration the quantity will be less than 1 kg. For dilute slurries it is well worth considering using some form of pre-concentration such as gravity sedimentation or a hydrocyclone. 

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