Alumina Processing

There are several processes available for the manufacture of cryoHte. The choice is mainly dictated by the cost and quaUty of the available sources of soda, alumina, and fluoriae. Starting materials iaclude sodium aluminate from Bayer s alumina process hydrogen fluoride from kiln gases or aqueous hydrofluoric acid sodium fluoride ammonium bifluoride, fluorosiUcic acid, fluoroboric acid, sodium fluosiUcate, and aluminum fluorosiUcate aluminum oxide, aluminum sulfate, aluminum chloride, alumina hydrate and sodium hydroxide, sodium carbonate, sodium chloride, and sodium aluminate. 

Specialty Aluminas. Process control (qv) teclmiques permit production of calcined specialty aluminas ha nng controlled median particle sizes differentiated by about 0.5 ]lm. Tliis broad selection enables closer shrinkage control of high tech ceramic parts. Production of pure 99.99% -AI2O2 powder from alkoxide precursors (see Alkoxides, metal), apparently in spherical form, offers the potential of satisfying the most advanced appUcations for calcined aluminas requiring tolerances of 0.1% shrinkage. 

At the alumina plant, the bauxite ore is further crushed to the correct particle size for efficient extraction of the alumina through digestion by hot sodium hydroxide liquor. After removal of "red mud" (the insoluble part of the bauxite) and fine solids from the process liquor, aluminum trihydrate crystals are precipitated and calcined in rotary kilns or fluidized bed calciners to produce alumina (AljOj). Some alumina processes include a liquor purification step. 

Processes 7 to 9 in Table III have the common feature of producing sulphur as the end product. They are therefore overall reduction processes. The Cat-Redox and CO/SO2 systems are both direct conversions to elemental sulphur. The latter has a particular advantage of being able to accept effluent gas streams at temperatures of 750 F or higher thus avoiding the need for any cooling of combustion flue gases before clean-up. The Alkalized Alumina Process, however, involves concentration and catalytic... 

In some cases, as for example in catalytic cracking on silica-alumina, processes similar to those involved in sintering and recrystallisation can lead to a change in the texture of the catalyst. Surface areas are diminished and the pore-size distribution is changed. 

Dorre, E. and Hubner, H., Alumina Processing, Properties and Applications, Springer-Verlag, Berlin, Heidelberg, New York, 1984. 

In the late 1970s it became apparent that the existing Ziegler-alumina capacity would not be sufficient to serve the growing needs of the markets, in particular of the catalytic market. This led to the development of the so-called On-Purpose Alumina Process by CONDEA (figure 4), a process which basically copies the chemistry of the Ziegler-process related to the formation of boehmite alumina, but which has not linkage to alcohol production[5]. 

Fig.4. Flow Diagram of the CONDEA On-Purpose Alumina Process...

Ibe use of slaked lime to regenerate caustic soda by causticising sodium carbonate is a feature of a number of other processes (e.g. the Bayer alumina process — section 31.15 and the processing of carbolic oil [31.24]). 

Figure 1 Schematic for a simple activated alumina process for arsenic removal from groundwater without pretreatment.

Activated alumina processes are sensitive to pH, and anions are best adsorbed below pH 8.2, a typical zero point of charge (ZPC), below which the alumina surface has a net positive charge and excess protons are available to fuel Eq. (1). Above the ZPC, alumina is predominantly a cation exchanger, but its use for cation exchange is relatively rare in water treatment. 

I. Activated Alumina Process with Regenerant Reuse... 

Alunite [Kz Al6(QH)i2 (504)4 is a potential source of potassium sulfate, alumina, and sulfur dioxide byproduct. The economics of production hinge on the value of the alumina. Processing involves ore comminution, roasting, and leaching to recover potassium sulfate solution. Filtered solids can be processed in a Bayer-type process plant for alumina recovery [111]. 

E. Dorre and H. Hiibner. Alumina, Processing, Properties, and Applications. 

Hellmann, J. K. (1986) Alumina processing and properties characterization workshop, SAND-86-1224, Albuquerque, NM Sandia National Laboratory. 

One of the routes to dense, fine-microstructured, polycrystaUine abrasive minerals considered by 3M at that time (-1974) was sol-gel technology. Prior to work on abrasives, 3M s Dr. Harold Sowman had been successful utilizing sol-gel approaches for the synthesis of nuclear fuel pellets and production of ceramic fibers. He, along with Dr. Melvin Leitheiser, initiated an effort to develop new aluminous abrasive minerals based on sol-gel technology. They focused their effort on the conversion of colloidal boehmite, y-AlOOH, into dense, polycrystalline alumina. Since that time, boehmite has become, not only the precursor of choice for sol-gel abrasive grains, but the precursor of choice for virtually every sol-gel alumina process. 

Alkalized Alumina Processes Bureau of Mines Proems... 

The Alkalized Alumina process was developed by the U.S. Bureau of Mines and carried through the pilot-scale testing phase (Bienstock et al., 1964 1967). It has not been applied commercially. The process uses dawsonite [NaAl(C03)(OH)2]/sodium aluminate [NaA102] as the sorbent. Ihe material is activated at 1,200°F to form a high surface area, high porosity, dry solid, which removes sulfur dioxide from flue gas at temperatures between 300° and 650°F. This process is no longer being pursued due primarily to an excessive sorbent attrition rate. 

Selection of an alumina precursor for detailed study was guided by a combination of fundamental value and local interest. Basic aluminum sulfate is an intermediate product in the acid alumina process( °) developed in the Mineral Chemistry Division of these Laboratories, and the thermal decomposition of this material has been studied by Davey While... 

Perhaps the most promising regenerative dry process for the removal of sulfur dioxide from stack gases is the alkalized alumina process, which has undergone extensive development by the U.S. Bureau of Mines and others. Here the absorption step takes place at 300-350°C, which is a relatively low temperature compared with the lime absorption processes discussed previously. The absorption unit, perhaps a fluidized bed, would be located between the steam generation section of the boiler and the stack. The alumina provides a support rather akin to a catalyst support for the absorbent proper, which is Na20, at a level of about 20%. The absorption reaction is... 

The major difficulty with the alkalized alumina process appears to be the attrition of the solid as it is recycled from the absorber to the regenerator. A recent report from the Bureau of Mines [83] describes the evaluation of some solid oxides, including alkalized aluminas, for this process. All of the oxides were found to show adequate capacity and reactivity but were subject to excessive attrition, as evaluated by the investigators. 

In a recent study of zirconia-toughened alumina processed by two different routes a considerable difference in fracture toughness behavior was observed according to whether a mechanical or a wet [Pg.273]

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