Decantation, countercurrent

Washing by successive dilution is used when the soHds are separated into a slurry, such as in filter thickeners. The soHds, thickened into a small amount of mother Hquor, are diluted into a wash Hquid and then separated again, diluted, separated, etc until clean of mother Hquor. The consumption of the wash Hquid can be reduced in countercurrent washing systems, sometimes referred to as countercurrent decantation. Cocurrent dilution washing, however, can be built into some dynamic filter-thickeners such as the Escher-Wyss filter. 

The product stream from the kilns is collected in storage bins. Black ash from the bins is fine-ground in a ball mill and fed to a leacher circuit, which is a system of stirred tanks, where it is dissolved in water and the muds are separated by countercurrent decantation. The solution from the decantation is passed through filter presses the muds are washed, centrifuged, and discarded. The filtered product, a saturated solution containing 12—13 wt % strontium sulfide, is sent to an agitation tank where soda ash is added to cause precipitation of strontium carbonate crystals ... 

Carbonate leaching under ambient conditions is extremely slow with poor recoveries. Therefore, the ore is typically leached in an autoclave with air providing most of the needed oxygen. The leach Hquor is separated from the soHd in a countercurrent—decantation system of thickeners, and the uranium is precipitated from the clarified sodium carbonate solution with addition of sodium hydroxide (eq. 9) (23). 

Extraction of Bertrandite. Bertrandite-containing tuff from the Spor Mountain deposits is wet milled to provide a thixotropic, pumpable slurry of below 840 p.m (—20 mesh) particles. This slurry is leached with sulfuric acid at temperatures near the boiling point. The resulting beryUium sulfate [13510-49-1] solution is separated from unreacted soflds by countercurrent decantation thickener operations. The solution contains 0.4—0.7 g/L Be, 4.7 g/L Al, 3—5 g/L Mg, and 1.5 g/L Fe, plus minor impurities including uranium [7440-61-1/, rare earths, zirconium [7440-67-7] titanium [7440-32-6] and zinc [7440-66-6]. Water conservation practices are essential in semiarid Utah, so the wash water introduced in the countercurrent decantation separation of beryUium solutions from soflds is utilized in the wet milling operation. 

Control philosophies applied to continuous countercurrent decantation (CCD) thick eners are similar to those used for thickeners in other applications, but have emphasis on maintaining the CCD circuit in balance. It is important to prevent any one of the thickeners from pumping out too fast, otherwise an upstream unit could be stai ved of wash liquor while at the same too much underflow could be placed in a downstream unit too quickly, disrupting the operation of both units as well as reducing the circuit washing efficiency. Several control configurations have Been attempted, and the more successful schemes... 

By comparing this simple diagram with the conventional continuous countercurrent decantation circuit, shown in Fig. lb (Coulson and Richardson, 1968, 1978 Treybal, 1955, 1968, 1980) for carrying out the same duty, it is easy to deduce the following characteristics for fluidized leaching and washing ... 

Three thickeners in series are run on a countercurrent decantation system. Overflow from the second thickener goes to the agitators, and overflow from the first thickener is to contain 10 per cent sodium sulphide. Sludge from all thickeners contains two parts water to one part barium carbonate by mass. How much sodium sulphide will remain in the dried barium carbonate precipitate ... 

Solids in the feed were mentioned as one of the major causes of subsequent crud formation in the solvent extraction circuit. Good clarification, therefore, is necessary to minimize crud and thereby operating costs. Table 7.4 itemizes some benefits of good clarification [48]. In North America and South Africa, the objective is to obtain approximately 10 ppm suspended solids in the feed to extraction. This is usually obtained by the use of sand filters after a countercurrent decantation (CCD) circuit. At least one plant has reported that crud quickly developed when the sand filters were not in... 

T0883 Westinghouse Savannah River Company, Countercurrent Decanting T0886 Westinghouse Savannah River Company/EnviroMetal Technologies, Inc. 

By comparing this simple diagram with the conventional continuous countercurrent decantation circuit, it is easy to deduce the following characteristics for fluidized leaching and washing ... 

The major use of phosphate is to supply phosphorous, one of the three essential plant foods, nitrogen, phosphorus, and potassium. Phosphate rock extraction from its ore, and its subsequent conversion into fertilizer materials and industrial chemicals, is a relatively mature art. Single superphosphate, a mixture of monocalcium monohydrate and gypsum formed by the reaction of sulfuric acid with phosphate rock, has been used as a fertilizer since the mid-1800s. Phosphoric acid, derived by the treatment of phosphate rock with sulfuric acid so as to produce gypsum in a separable form, was manufactured in many locations by batch and countercurrent decantation methods in the 1920s. 

The selective leaching of the nickel and cobalt from the gangue material is carried out in Pachuca-type leaching vessels with 98% sulfuric acid at temperatures between 450 and 500°F and with equivalent pressures between 400 and 600 psi. A typical leach liquor contains 6 gm/liter Ni, 0.6 gm/liter Co, 0.6 gm/liter Fe, 2.3 gm/liter Al, and 67 gm/liter SO4. Each batch is leached for about 90 min to extract 95% of the metal values. Preheating and heating of the ore slurry is by direct steam absorption in the reaction vessel. The barren residue is separated from the leach liquor by countercurrent decantation to a density of 55-60% solids. The solids are washed with very little dilution and about 99.5% of the soluble value are recovered. 

Ion exchange and extraction with organic solvents have proved effective in separating uranium from the leaching solutions. Combinations of the ion exchange proce.ss with solvent extraction are also known. This is normally preceded by separation of the leaching solution from solids by multistage filtration or countercurrent decantation followed by clarification e.g. over a bed of sand. 

Control philosophies applied to continuous countercurrent decantation (CCD) thickeners are similar to those used for thickeners in... 

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