Caustic liquor processing centrifugation

Recovery of solid salt from the caustic liquor is primarily by centrifugation, normally in two stages. Section 9.3.2.S has already covered this operation. Processing of the recovered salt depends on the measures taken for sulfate removal. Postponing discussion of the sulfate, we consider first the handling of the salt for reuse. 

Two cocrystallization processes employ dibasic crystals as intermediates. The PPG process (199—202) is discussed under commercial processes. The PPC process (203) forms dibasic crystals from lime and recovered filtrates. The dibasic crystals are separated from thek mother liquor by decantation, slurried in caustic solution and chlorinated to produce a cocrystalline slurry of Ca(OCl)2 and NaCl. The slurry is sent to a flotation cell where the larger salt crystals settle out and the smaller hypochlorite crystals float to the top with the aid of ak and flotation agent. The hypochlorite slurry is centrifuged the cake going to a dryer and the centrate to the flotation cell. The salt-rich bottoms from the flotation cell are centrifuged and washed with dibasic mother Hquor. The centrates are recycled to the precipitation step. 

The precoat usually is salt itself. Some of the slurry that ordinarily would go to the secondary centrifuges bypasses that step and goes directly to the filter that requires precoating. It is also possible to use filter feed as the precoat, but the finer salt does not work as well. When the precoat becomes blinded or the chambers filled with salt, backwashing is necessary. Fresh water, cell liquor, or unsaturated brine can be used here, and the salt, precoat as well as process load, dissolves and returns to the process. Water is the most efficient backwash fluid, but the use of brine or cell liquor prevents dilution of the process. The caustic filtrate becomes the final product and is ready for storage or shipping. 

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