Gravity separations

Oil-Water Separation Gravity separation Dissolved air floatation 

Chemical coagulation — flocculation sedimentation Coalescers Membrane processes Biological processes Carbon adsorption Inorganics Organics 

In addition, disposal options are presented with a discussion on the reinjection of untreated groundwater during free-phase LNAPL recovery. 

Removal of NAPL can present problems, particularly if emulsion is involved. Emulsion is an intimate mixture of two liquids not miscible with each other, as oil and water. Water-in-oil emulsions have water as an internal phase and oil as the external phase, whereas oil-in-water emulsions reverse the order. Oil-water separation is required prior to downstream treatment processes. Several specific oil removal technologies are presented below. 

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Flotation. Flotation is a gravity separation process which exploits differences in the surface properties of particles. Gas bubbles are generated in a liquid and become attached to solid particles or immiscible liquid droplets, causing the particles or droplets to rise to the surface. This is used to separate mixtures of solid-solid particles and liquid-liquid mixtures of finely divided immiscible droplets. It is an important technique in mineral processing, where it is used to separate different types of ore. 

Another type of gravity separator used for small amounts of oily water, the oil interceptor, is widely used both offshore and onshore. These devices work by encouraging oil particles to coalesce on the surface of plates. Once bigger oil droplets are formed they tend to float to the surface of the water faster and can be skimmed off. A corrugated plate interceptor (CPI) is shown below and demonstrates the principle involved. However there are many varieties available. Plate interceptors can typically reduce oil content to 50-150 ppm. 

Gravity sedimentation Gravity separation Gravure Gravure inks... 

Centrifugal separators are used in many modem processes to rapidly separate the hydrocarbon and used acid phases. Rapid separation greatly reduces the amounts of nitrated materials in the plant at any given time. After an explosion in a TNT plant (16), decanters (or gravity separators) were replaced with centrifugal separators. In addition, rapid separation allows the hydrocarbon phase to be quickly processed for removal of the dissolved nitric acid, NO, etc. These dissolved materials lead to undesired side reactions. The organic phase generally contains some unreacted hydrocarbons in addition to the nitrated product. 

The simplest analytical procedure is to oxidize a sample in air below the fusion point of the ash. The loss on ignition is reported as graphitic carbon. Refinements are deterrninations of the presence of amorphous carbon by gravity separation with ethylene bromide, or preferably by x-ray diffraction, and carbonates by loss of weight on treating with nitric acid. Corrections for amorphous carbon and carbonates are appHed to the ignition data, but loss of volatile materials and oxidation may introduce errors. 

Entrainment Due to Gas Bubbling/Jetting through a Liquid Entrainment generally hmits the capacity of distiUation trays and is commonly a concern in vaporizers and evaporators. Fortunately, it is readily controllable bv simple inertial entrainment capture devices such as wire mesh pads in gravity separators. 

The sharpness of separation of the mineral from the gangue is dependent on (1) the stability of the suspension, which is influenced by the size of the medium (2) the specific gravity of the medium (3) the cleanliness of the medium (4) the cone angle (5) the size and ratios of the internal openings in the cyclone (inlet, apex, and vortex) and (6) the pressure at which the pulp is introduced into the cyclone. A 20° cone angle is the most common. Cyclone diameter will be determined by the separation to be made as well as by the capacity required. The 0.5- and 0.6-m (20- and 24-in) cyclones are most common in coal plants, whereas multiple cones of 0.25- or 0.3-m (10- or 12-in) diameter are used in higher-gravity separations. 

Waste gases (main stack Particulates, CO, SO2, Gravity separators to... 

Use a close coupled ell for drawoff from gravity separators to eliminate backup of hydraulic head. 

They are much more practical for solids or dusts, although even for these situations the flow quantities must be small if the sizes are not to become excessive. With unusually heav) and/or large particles the gravity separator can be used to advantage. 

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