Trihydrate crystal modifier for the bayer process

An improved Bayer process for the production of aluminum

• wherein bauxite ore is pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures, forming an aqueous sodium aluminate liquor containing caustic-insoluble constituents,
• wherein said caustic-insoluble constituents are then substantially separated from the aqueous phase of said aqueous sodium aluminate liquor to form an alumina trihydrate precipitation feed, said alumina trihydrate precipitation feed containing no more than about 50 mg of caustic-insoluble constituents per liter, and
• wherein said alumina trihydrate precipitation feed is then cooled and optionally also seeded with crystals of alumina trihydrate whereby a solid alumina trihydrate product is precipitated out of said precipitation feed and subjected to settlement, whereupon a supernatant is formed, and said aluminum trihydrate precipitate is collected as product,
• characterized by adding to said precipitation feed after said caustic-insoluble constituents are substantially separated from said precipitation feed so said precipitation feed contains no more than about 50 mg of said caustic-insoluble constituents per liter, and prior to the completion of said alumina trihydrate precipitation, a water-soluble anionic hydroxamated polymer in an amount effective to improve the crystal formation of said alumina trihydrate precipitate by increasing the yield of coarser alumina trihydrate particles formed.

A method of producing alumina trihydrate crystals from a Bayer process stream, having an upwardly shifted particle size distribution,

• wherein an aqueous sodium aluminate liquor containing caustic-insoluble suspended solids is formed upon digestion of bauxite ore,
• wherein the Bayer process stream is clarified to substantially remove the suspended solids;
• and
• wherein alumina trihydrate is precipitated in the Bayer process stream after the caustic-insoluble solids have been substantially removed, the method comprising:
• (a) substantially removing said insoluble suspended solids by at least one separation method to form an alumina trihydrate precipitation feed having no more than about 50 mg of said insoluble suspended solids per liter of said alumina trihydrate precipitation feed;
• and
• (b) after said removal of said suspended solids is completed and when said alumina trihydrate precipitation feed has no more than about 50 mg of said insoluble suspended solids per liter, adding an anionic water-soluble hydroxamated polymer to the alumina trihydrate precipitation feed in an amount effective to upwardly shift the particle size distribution of the alumina trihydrate crystals precipitated from said alumina trihydrate precipitation feed so that said alumina trihydrate crystals have a reduced percentage of alumina trihydrate fines.