Caustic liquor processing filtration

Separation of the solids, primarily CaCOs, from the caustic solution. Primary separation is in a thickener that produces NaOH solution in the overflow and a suspension of solids in the caustic liquor in the underflow. The solids go to a second set of thickeners for washing with hot water and recycle filtrate (from step 3). The overflow, a dilute caustic solution, is used to prepare the process feed solution. 

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. 

In the process to produce alumina (Fig. 1), bauxite is crushed and wet ground to 100-mesh, dissolved under pressure and heated in digesters with concentrated spent caustic soda solution from a previous cycle and sufficient lime and soda ash. Sodium aluminate is formed, and the dissolved silica is precipitated as sodium aluminum silicate. The undissolved residue (red mud) is separated from the alumina solution by filtration and washing and sent to recovery. Thickeners and Kelly or drum filters are used. The filtered solution of sodium aluminate is hydrolyzed to precipitate aluminum hydroxide by cooling. The precipitate is filtered from the liquor, washed, and heated to 980°C in a rotary kiln to calcine the aluminum hydroxide. 

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. 

The wash liquor may contain about 20% caustic. Depending on the process duty, the solids will range from 2-4% Hg and less than 1% filter aid. At ambient to warm temperature, the resulting filtrate will have a turbidity less than 5 ppm. 

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