Process and installation for producing alumina trihydrate by alkaline digestion of bauxite ore

An installation for producing alumina trihydrate by digestion of bauxite ore, the installation comprising:

• digestion means for digesting the bauxite ore with a sodium aluminate liquor to obtain a slurry,
• separation means for treating the slurry to separate an enriched sodium aluminate liquor from insoluble residues to produce a clarified liquor, and
• precipitation means connected to the separation means for treating the clarified liquor to precipitate alumina trihydrate,
• the separation means comprising pre-treatment means for adding flocculants to the slurry and for mixing the slurry with the flocculants and obtain a flocculated slurry, the separation means further comprising a gravity settler vessel for settling the flocculated slurry and for producing the clarified liquor, the installation being characterized in that the installation further comprises:
• measurement means for determining a measured value representative of a concentration of solid particles in the clarified liquor,
• comparison means functionally connected to the measurement means for comparing the measured value with a predetermined threshold of the concentration of the solid particles in the clarified liquor, and
• redirecting means functionally connected to the comparison means, for directing the clarified liquor to a feeding line of the precipitation means directly connected to the gravity settler vessel for feeding the clarified liquor directly to the precipitation means when the measured value is less than the predetermined threshold of the concentration of the solid particles in the clarified liquor, and for redirecting the clarified liquor to the pre-treatment means when the measured value is more than the predetermined threshold of the concentration of the solid particles in the clarified liquor.

A process for producing alumina trihydrate by digestion of bauxite ore comprising:

• (a) digesting the bauxite ore with a sodium aluminate liquor to obtain a slurry including an enriched sodium aluminate liquor comprising dissolved alumina and insoluble residues formed of particles of undissolved bauxite ore;
• (b) treating the slurry to separate the enriched sodium aluminate liquor from the insoluble residues to produce a clarified liquor;
• and
• (c) treating the clarified liquor to precipitate alumina trihydrate,
• wherein step (b) comprises:
• b1) pretreating the slurry by adding a flocculant to the slurry and mixing the flocculant and the slurry to obtain a flocculated slurry,
• b2) settling the flocculated slurry in a gravity settler vessel to produce the clarified liquor and a thickened slurry of insoluble residues,
• b3) determining a measured value representative of a concentration of solid particles in the clarified liquor,
• b4) comparing the measured value with a predetermined threshold of the concentration of the solid particles in the clarified liquor,
• b5) feeding the clarified liquor directly to step (c) when the measured value is less than the predetermined threshold of the concentration of the solid particles in the clarified liquor, and
• b6) redirecting the clarified liquor to step b1) when the measured value is more than the predetermined threshold of the concentration of the solid particles in the clarified liquor.