Thickening decanter

In a thickening operation, where all that is wanted is partial removal of some liquid, both of the two main flow options are possible, but the type of control with each could be different. One control option is to run the bowl with a pond level close to the level of solids discharge (conventionally called neutral ) and to control the differential at a precise low level. A second option is to run the pond much shallower and operate the differential much higher and less 

However, if necessary, the cake could be allowed to over thicken, by setting the decanter to dewater, without any need for fine control, with some feed bypass mixed in with the cake at discharge, to reduce dryness to the desired level. This enhances overall capacity and can reduce overall flocculant consumption when used, as the by-pass requires no flocculant. This option is best used with those slurries that are more readily dewatered. 

The B range of decanters manufactured by KHD (now Bird Humboldt) are unique in having one. or possibly two, nozzles in the bowl wall, allowing the discharge of the bulk of the thickened product. The remainder of the product is scrolled up the beach, which is much steeper than normal (as much as 45°). Discharge from the nozzle can be made intermittent by providing a zero pitch to the conveyor for less than one turn at the nozzle location, thus covering the nozzle for a fraction of each conveyor turn. 

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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. 

Most carbide acetylene processes are wet processes from which hydrated lime, Ca(OH)2, is a by-product. The hydrated lime slurry is allowed to settle in a pond or tank after which the supernatant lime-water can be decanted and reused in the generator. Federal, state, and local legislation restrict the methods of storage and disposal of carbide lime hydrate and it has become increasingly important to find consumers for the by-product. The thickened hydrated lime is marketed for industrial wastewater treatment, neutrali2ation of spent pickling acids, as a soil conditioner in road constmction, and in the production of sand-lime bricks. 

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. 

The leached solids must be separated from the extract by settling and decantation or by external filters, centrifuges, or thickeners, all of which are treated elsewhere in Sec. 18. The difficulty of solids-extract separation and the fact that a batch stirred tank provides only a single equilibrium stage are its major disadvantages. 

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... 

Temperature rise, centrifugal pump, 207-209 Terminal particle velocity, 228, 230 Particles, different densities, 238 Single spheres, 274 Solids in air, 237 Solids in water, 237 Test pressure, piping, 18 Thickeners and settleix/decanters, Decanter, 242... 

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 ... 

After the final purification, the suspension of nitrocellulose in water is led into cylindrical decantation tanks, 6-8 m high and 1.5-2.5 m dia. equipped with stirrers. The contents of the tanks are allowed to stand for a time, and then the water is decanted either by means of a funnel (Fig. 163) or through a valve in the side of the tank. The stirrer is then set in motion and the thickened pulp of nitrocellulose in water is delivered to a centrifuge. 

Sedimentation. This is a general term for an operation wherein suspended sulids are removed from a liquid by gravitational settling. The two major forms of sedimentation equipment are (I) thickeners, and (2) clarifiers. The term decanting also is sometimes used In designate sedimentation. 

Ivory- nut meal [Plate V (at end of chapter), Fig. 38] is detected by the following microscopical test The flour is treated with 3% soda solution and left for half an hour, after which the liquid is decanted off and the residue washed with water and examined under the microscope. The ivory-nut meal is readily recognised by its characteristic structure, since the cells of the endosperm of this seed exhibit an enormous thickening of the walls, so that the aperture of the cell is distinctly reduced. These thickenings are interrupted here and there by channels connecting the cells.8... 

Let it be supposed that a test sample has been prepared showing a proportion of water to solids of 14.04 to 1 and that after 17 hr. the pulp has settled to 1.13 parts of water to 1 of solids and only to 1.12 to 1 after 24 hours, then evidently the economic point of thickening is 1.13 to 1. Settlement tests show an average rate of 1.78 in. per hour, hence there can be decanted 1.78 cu. ft. times 62.3 or 111 lb. of water per hour per square foot of tank surface. Since the feed contains 14.04 water to 1 of solids and the discharge 1.13 water to 1 of solids the overflow must contain the difference between the two ratios or 12.91 parts of water in the overflow to 1 part solids settled and is 111 lb. per hour. The solids consequently represent 8.59 lb. solids settled per hour or 206 lb. of solids in 24 hr. For a ton there would then be required 9.7 sq. ft. for 24 hr. which is the capacity of the tank with the dilution given. 

Continuous Counter-current Decantation.—This system is being quite generally used in the chemical world to replace the older intermittent agitation and decantation washing systems as well as more modern filter installations. It has as its basis the application of the Dorr thickener to remove suspended solids contin-... 

The power consumption of such a machine is less than horsepower for mechanisms, in tanks up to 40 ft. diameter. The labor required by six thickeners is one-half of one man s time per shift and on larger installations will be proportionately less per thickener. The size tank required is the theoretical area required to settle out the solids continuously which is considerably less area than required by an intermittent system due to the fact that the time-consuming period of decanting and removing the solids is done away with by continuous operation. 

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