Thickener underflow pumps

The softened seawater is fed with dry or slaked lime (dolime) to a reactor. After precipitation in the reactor, a flocculating agent is added and the slurry is pumped to a thickener where the precipitate settles. The spent seawater overflows the thickener and is returned to the sea. A portion of the thickener underflow is recirculated to the reactor to seed crystal growth and improve settling and filtering characteristics of the precipitate. The remainder of the thickener underflow is pumped to a countercurrent washing system. In this system the slurry is washed with freshwater to remove the soluble salts. The washed slurry is vacuum-filtered to produce a filter cake that contains about 50% Mg(OH)2. Typical dimensions for equipment used in the seawater process may be found in the Hterature (75). 

A low-pressure-drop liquid cyclone is sometimes used to clarify liquor discharged from the evaporator. The driving force is the pressure drop across the circulating pump. Thickened slurry is returned through a wide-open cyclone underflow connection to the circulating piping before the pump suction. 

Thickener The lime reactor products flow by gravity to a thickener where the calcium solids settle. The thickener overflow, which is a clear liquor, is separated and is piunped to a clarifier to remove any dissolved calcium sulfate. The thickener underflow is pumped to a rotary vacuiun filter to further concentrate the calcium sulfite solids. 

The feed rate is manually set as required for either pH or reactant stoichiometry control. Soda ash is added to the fourth reactor as sodium makeup. Reactor effluent slurry flows by gravity to the thickener centerwell. Clarified liquor overflows from the thickener to the forward feed hold tank from which it is pumped to the tray tower. A horizontal belt filter is used for further dewatering of the thickener underflow solids. 

Another factor was the inability of the thickener underflow pumps (inappropriately sized for the service) to handle slurries containing much more than 15% solids. This required the dilution of the thickener underflow before being pumped to the filter. A much higher demand was then placed on the already limited capabilities of the filter. The slurry had to be dewatered from 15% solids to 55% instead of from 25 to 55% thus, more than doubling the amount of filtrate to be removed from the same amount of insoluble solids. 

Good design practice typically includes dedicated thickener underflow recirculation pumps to deliver adequate volumes of precipitate as seed, and blending of seed slurry and feed liquor ahead of limestone addition to the first neutralization vessel. 

Depending of the case, the slurry from the atmospheric reactors may be pumped straight to the autoclave feed tank. If there is need to adjust the autoclave feed solid content, then the slurry from the reactors may first go to thickener. The thickener underflow is pumped to the pressure leaching feed tanks. Thickener overflow is directed straight to the iron removal circuit. 

From the flash tanks the slurry flows by gravity to the solids and liquid separation phase, first to the leach thickener. Thickener underflow slurry is pumped to the filter feed tank. The filter cake is washed to remove the acidic and PLS solution in the cake. The filtrate is pumped back to the main process. The further treatment of the leach residue depends of the amount of precious metals in the matte. If the precious metals concentration is high in the matte, filter cake containing insoluble precious metal components (such as iron silicates etc.) may be transported to the PGM cake treatment area. If the precious metals do not play important part, the filter cake is transported to the disposal pond. 

Many older systems pond the natural byproduct (and coal ash). Usually, the slurried solids are pumped to the pond at a solids concentration less than 50%. The slurry either is the thickener underflow or comes directly from the scrubber, with the pond acting as the thickener/clarifier. Qarified liquor is usually returned to the scrubber (Jones, 1977). The two major environmental considerations with ponding are the potential water pollution problem associated with the soluble material and the land-degradation potential of non-settling or physically unstable solids. If the disposal site is to be reclaimed when the pond is retired, the retention of water in the byproduct is a serious concern. 

In the case of thickeners, the process of compaction of the flocculated material is important. The floes settle to the bottom and gradually coalesce under the weight of the material on top of them. As the bed of flocculated material compacts, water is released. Usually the bed is slowly stirred with a rotating rake to release trapped water. The concentrated slurry, called the underflow, is pumped out the bottom. Compaction can often be promoted by mixing coarse material with the substrate because it creates channels for the upward flow of water as it falls through the bed of flocculated material. The amount of compaction is critical in terms of calculating the size of the thickener needed for a particular operation. The process of compaction has been extensively reviewed in the Hterature (41,42). 

Rake-Lifting Mechanisms These should be provided when abnormal thickener operation is probable. Abnormal thickener operation or excessive torque may result from insufficient underflow pumping, surges in the solids feed rate, excessive amounts of large particles, sloughing of solids accumulated between the rakes and the bottom of... 

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... 

Overflow Pumps These can be omitted if the thickeners are located at increasing elevations from first to last so that overflows are transferred by gravity or if the mixture of underflow and overflow is to be pumped. Overflow pumps are necessary, however, when maximum flexibility and control are sought. 

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