Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Reexamination Certificate
2001-04-02
2003-07-29
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
C423S122000
Reexamination Certificate
active
06599489
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is in the technical field of alumina trihydrate crystal formation, that is, the nucleation and growth of alumina trihydrate crystals, in the Bayer process for the recovery of alumina from bauxite ore.
BACKGROUND OF THE INVENTION
In the typical Bayer process for the production of alumina, bauxite ore is pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate product is precipitated out of the solution and collected as product.
The Bayer process is constantly evolving and the specific techniques employed in industry for the various steps of the process not only vary from plant to plant, but also are often held as trade secrets. As a more detailed, but not comprehensive, example of a Bayer process, the pulverized bauxite ore may be fed to a slurry mixer where a water slurry is prepared. The slurry makeup water is typically spent liquor (described below) and added caustic. This bauxite ore slurry is then diluted and passed through a digester or a series of digesters where about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. The digested slurry is then cooled, for instance to about 230° F., employing a series of flash tanks wherein heat and condensate are recovered. The aluminate liquor leaving the flashing operation often contains from about 1 to about 20 weight percent solids, which solids consist of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solid particles may be removed from the aluminate liquor with a “sand trap” cyclone or other means. The finer solid particles may be separated from the liquor first by settling and then by filtration, if necessary. For instance, the slurry of aluminate liquor and finer solids may be first fed to the center well of a mud settler, or primary settler, and as the mud settles (which settling can be enhanced by the presence of a flocculant) clarified sodium aluminate solution, referred to as “green” or “pregnant” liquor, overflows a weir at the top. This overflow from the mud settling tank is passed to the subsequent process steps. If the aluminate liquor overflowing the settler contains an unacceptable concentration of suspended solids (at times from about 50 to about 500 mg of suspended solids per liter), it would then generally be further clarified by filtration to give a filtrate with no more than about 10 mg suspended solids per liter of liquor. The treatment of the liquor collected after the primary settlement to remove any residual suspended solids before alumina trihydrate is recovered is referred to as a secondary clarification stage. In another section of such a Bayer circuit, the settled solids of the primary settler (“red mud”) may be withdrawn from the bottom of the settler and passed through a countercurrent washing circuit for recovery of sodium aluminate and soda.
The Bayer process can vary from plant to plant as to both minor modifications and/or as to major process techniques. For instance, not all Bayer process plants use primary settlers or even mud washers. Some plants send the digester blow-off slurry directly to red mud filters, and the filtrate liquor is the green liquor of a typical Bayer process.
The present process generally is independent of the method of red mud separation. The characteristic qualities of the clarified liquors will remain within a reasonable range, and not so different as to become unresponsive to the present process.
The clarified sodium aluminate liquor is usually seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, Al(OH)
3
. The alumina trihydrate particles or crystals are then separated from the concentrated caustic liquor, and the remaining liquid phase, the spent liquor, is returned to the initial digestion step and employed as a digestant after reconstitution with caustic.
The clarified sodium aluminate liquor (which may be the overflow from primary settler or the supernatant), also referred to as “green liquor”, is a hot caustic liquor, generally containing the highest values of dissolved sodium aluminate. Sodium aluminate-containing liquor is kept at elevated temperatures during the beneficiation steps so as to retain its high values of dissolved sodium aluminate. It is charged to a suitable precipitation tank, or series of precipitation tanks, and almost always seeded with recirculated fine particle alumina trihydrate crystals. In the precipitation tank(s) it is cooled under agitation to induce the precipitation of alumina from solution as alumina trihydrate. The fine particle alumina trihydrate crystal seeds act as crystal nucleation sites for this precipitation process.
Alumina trihydrate crystal formation (the nucleation and growth of alumina trihydrate crystals), and the precipitation and collection thereof, are critical steps in the economic recovery of aluminum values by the Bayer process. Bayer process operators strive to optimize their crystal formation and precipitation methods so as to produce the greatest possible product yield from the Bayer process while producing crystals of a given particle size distribution. A relatively large particle size is beneficial to subsequent processing steps required to recover aluminum metal. Undersized alumina trihydrate crystals, or fines, generally are not used in the production of aluminum metal, but instead are recycled for use as fine particle alumina trihydrate crystal seed. If too much of the overall product yield is formed as fines, the production rate of alumina trihydrate crystals usable for aluminum metal production is diminished, the seed/product production balance is skewed, and the fraction of the overall product yield that is of sufficient particle size to be used for aluminum metal production routinely will still suffer from a less than optimum particle size distribution for the electrolytic production of aluminum metal.
After formation, the alumina trihydrate particles or crystals are separated from the concentrated caustic liquor, and the remaining liquid phase (the spent liquor) is returned to the initial digestion step and employed as a digestant after reconstitution with caustic. This separation or recovery of alumina trihydrate as product in the Bayer process, or for use as precipitation seed, is generally achieved by settling and/or filtration. Coarse particles settle easily, but fine particles settle slowly and to some extent are lost product or, if recovered by filtration, blind the filters. Thus another reason it is desirable to limit the formation of alumina trihydrate fines is to improve the subsequent step(s) directed to the separation of alumina trihydrate crystals from the spent liquor.
U.S. Pat. No. 4,608,237, August 1986, Roe et al., uses an acrylic acid containing polymer, added to the pregnant liquor of a Bayer process immediately prior to crystallization of alumina trihydrate, to reduce the percent alumina trihydrate crystals smaller than 200 microns in diameter. U.S. Pat. No. 4,737,352, April 1988, Owen et al., uses a surfactant/oil blend, added to the pregnant liquor of a Bayer process immediately prior to crystallization of alumina trihydrate, to increase the percent alumina trihydrate crystals coarser than −325 mesh.
It is an object of the present invention to provide an upward shift in the particle size distribution of alumina trihydrate crystals formed in the Bayer process. It is an object of the present invention to reduce the percent of alumina trihydrate crystal fines formed in the Bayer process. It is an object of the present invention to provide an upward shift in the particle size distribution of alumina trihydrate crystals formed in the Bayer process substantially without any decrease in
Mahoney Robert P.
Schnieders, Jr. William B.
Bos Steven
Breininger Thomas M.
Brumm Margaret M.
Ondeo Nalco Company
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