Process for removing impurities from the precipitation system of the bayer process

Claim 1

IN THE BAYER PROCESS WHEREIN ALUMINA IS EXTRACTED FROM ALUMINOUS MATERIAL BY DIGESTING SAID ALUMINOUS MATERIAL IN A CAUSTIC LIQUOR AND SAID ALUMINA IS PRECIPITATED FROM PREGNANT CAUSTIC LIQUOR CONTAINING OXALATE AS A CONTAMINANT, WHICH OXALATE ALSO PRECIPITATES AND CONTAMINATES THE PRECIPITATED ALUMINA HYDRATE, AND THE PRECIPITATED SOLIDS ARE SEPARATED FROM THE MAJOR PORTION OF THE MOTHER LIQUID AFTER WHICH THE CONCENTRATED SOLIDSCONTAINING SLURRY IS CLASSIFIED ACCORDING TO PARTICLE SIZE INTO PRODUCT ALUMINA HYDRATE AND ALUMINA HYDRATE SEED FOR RECYCLE TO THE AFORESAID PRECIPITATION, THE IMPROVEMENT IN PARTICLE SIZE CONTROL AND INCREASED CAPACITY OF PRECIPITATION COMPRISING SEPARATING THE SPENT CAUSTIC LIQUOR FROM THE ALUMINA HYDRATE SEED BY FILTERING AND SUBJECTING THE ALUMINA HYDRATE FILTER CAKE TO COLD WATER WASHING, THE AMOUNT OF COLD WATER USED BEING NOT IN EXCESS OF THAT WHICH IS SUFFICIENT TO REMOVE SUBSTANTIALLY ALL OF THE SPENT CAUSTIC LIQUOR REMAINING IN THE SEED SOLIDS, RECYCLING OF THE SPENT CAUSTIC LIQUOR TO THE DIGESTION PHASE OF THE BAYER PROCESS, SUBJECTING THE COLD-WATER-WASHED-ALUMINA HYDRATE TO A WASHING WITH HOT WATER TO DISSOLVE SUBSTANTIALLY ALL OF THE SODIUM OXALATE CONTAMINANT, SEPARATING THE WASHED SEED FROM THE OXALATE-CONTAINING WASH LIQUOR, AND THEREAFTER RECYCLING THE WASHED SEED TO THE PRECIPITATION REACTION.