Table of Contents
The extraction of aluminum oxide from its ore is the initial step in the commercial production of metallic aluminum. This is most commonly done by the Bayer Process in which a caustic solution is used- to dissolve hydrated alumina from the insoluble materials with which it is associate in bauxite.
Sherwin Plant Alumina Process
At the Sherwin Plant near Corpus Christ!, Texas, over 2,000 tons of alumina are produced daily by a modification of the Bayer Process. The indicated main process stream or mother liquor circulates continuously in a closed loop system. This liquor is essentially a solution of sodium aluminate, equivalent to fifteen per cent caustic soda containing varying amounts of dissolved Al2O3 and Na2CO3. Bauxite is monitored into the main stream on a continuous basis, and flow of the slurry through pressure digesters results in the extraction of available alumina from the ore. Raw caustic is added to the spent liquor stream to make up for soda losses in the process and maintain a constant plant inventory.
The first step in the continuous countercurrent decantation occurs in the clarifier-thickeners as enriched washwater is mixed with digester discharge slurry. The clarified overflow process liquor from these vessels thus contains the enriched washwater from the multi-stage washing operation. The hot thickened underflow sludge from each clarifier-thickener is pumped to the top of a five-tray washing thickener where the second through the sixth repulping and decantation steps take place. The thickened underflows from the several five-deck thickeners are combined, repulped and pumped across a single compartment thickener for the seventh stage of CCD washing and repulped again across a similar vessel for the eighth stage. The underflow from the last thickener is mixed with fresh water and pumped to one of a series of drying beds where decantation of the supernatant liquid effects the ninth washing stage.
CCD Washing of Red Mud
The red mud washing circuit at the Sherwin Plant has progressed from a seven-step system to a nine-step system. The original system consisted of only the clarifier-thickeners, the five-tray washing thickeners and a final washing by decantation from a drying bed. This seven-step system was converted to eight steps by the installation of a single-compartment earthen thickener to wash the combined underflow of the five-tray washing thickeners prior to the drying bed.
Deviations from calculated CCD losses are investigated by comparing CCD equilibrium concentrations for a specific set of operating conditions with chemical analysis of process samples from each stage of the system. Discrepancies from the calculated curves are indicative of one or more of the following irregularities in the CCD wash circuit:
- Deficiency or excess of washwater flowing through the wash system.
- Soda or water entering at some intermediate point in the wash circuit.
- Loss of sludge compaction in the system.
- Channeling or pluggage between decks of the washing tray thickeners.
- Incomplete mixing in the repulping operation between stages.
Efficiency of The Five-Compartment Washing Tray Thickener
The value for the pseudo-pulp consistency in an operating thickener is arrived at from stepwise analyses of supernatant liquor concentrations together with metered inputs and outputs of the vessel. With the 12:1 ratio representing the poorest internal performance of the washing tray thickener, the effect of improvement in the four pseudo-pulp consistences on the overall soda loss from eight and nine stage washing systems.
A potential eighty-five per cent reduction in soda loss is indicated from the poorest to the optimum condition of interdeck pulp consistencies for both the eight and nine stage systems shown. The ordinate as pounds of soda per ton of mud is numerically equivalent to the tonnage of soda lost daily from a 2,000 ton per day plant.