Bayer process residue

CBPC matrix composites can incorporate a high volume of industrial waste streams such as fly ash, mineral waste such as iron taUings and Bayer process residue from the aluminum industry (red mud), machining swarfs from the automobile industry, and forest product waste such as saw dust and wood chips. Table 14.1 lists some of these waste streams and potential products or applications. 

Starch is a polysaccharide found in many plant species. Com and potatoes are two common sources of industrial starch. The composition of starch varies somewhat in terms of the amount of branching of the polymer chains (11). Its principal use as a flocculant is in the Bayer process for extracting aluminum from bauxite ore. The digestion of bauxite in sodium hydroxide solution produces a suspension of finely divided iron minerals and siUcates, called red mud, in a highly alkaline Hquor. Starch is used to settle the red mud so that relatively pure alumina can be produced from the clarified Hquor. It has been largely replaced by acryHc acid and acrylamide-based (11,12) polymers, although a number of plants stiH add some starch in addition to synthetic polymers to reduce the level of residual suspended soHds in the Hquor. Starch [9005-25-8] can be modified with various reagents to produce semisynthetic polymers. The principal one of these is cationic starch, which is used as a retention aid in paper production as a component of a dual system (13,14) or a microparticle system (15). 

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

Residue Disposal. The major environmental problem in the Bayer process is disposal of bauxite residue which is effected by marine disposal, lagooning, use of underdrain lakes, or semidry disposal. Marine disposal in oceans or rivers, diluting the alkaline residue by large quantities of water, is environmentally unacceptable. Lagooning behind retaining dikes built around clay-sealed ground is commonly used, but there have been isolated leaks into aquifers. This has motivated installation of underdrains between the residue and clay-sealed, plastic-lined, lake bottom. This design removes the hydraulic head from the lake bottom and improves consoHdation of the residue. 

One major environmental problem is associated with the Bayer process for alumina production from bauxite. The residue (called red mud ) which is obtained in the process contains unextracted oxides (e.g., alumina, ferric oxide, titanium dioxide, silicon dioxide, calcium oxide) and various insoluble materials. This solid waste is washed and discharged into impoundment ponds or into the marine environment. 

The high stability of the aluminate ion allows the production of concentrated solutions of aluminum with the virtual exclusion of the main metallic impurity, viz. iron as an oxide residue. The resultant impure aluminate solution is clarified and its temperature reduced when the reverse of the above reaction occurs with the formation of A1203,3H20 by a slow crystallization procedure. The high-purity alumina trihydrate product is calcined and then reduced electrochemically in a molten fluoride bath by the well-known Hall-Heroult process. The major problems in the Bayer process have their origin in the coordination chemistry of aluminum in alkaline solutions. The... 

M.E. Afonso de Magalhaes and M. Tubino, Recovering Gallium from Residual Bayer Process Liquor, JOM 43, 37-39, June (1991). 

There are two bulk-waste products from which REEs can be recycled. They are red mud , a bauxite residue generated during the Bayer process for alumina production and phosphogypsum, a residue that is formed during phosphorus production, and contains REE originating from the original phosphorus ore mineral (apatite). 

In the aluminium industry the most widely used method for the extraction of alumina (AI2O3) from bauxite is the Bayer process, in which aluminium and silica are leached from bauxite under pressure with hot caustic soda. The residue is red mud , a silt-like mixture of iron oxides, titanium dioxide, silica, caustic and many impurities. Roughly one ton of insoluble residue of red mud is produced for each ton of alumina. The liquors, following appropriate treatment to recover alumina, are recycled. In view of its high solubility in nature, it seems that most of the uranium must also be leached and possibly build up in the leach liquors. Small mills are now recovering this Uranium, but analyses of red muds show uranium contents that vary from 12 to 40ppm uranium, which suggest that it is not all leached. 

The Bayer process is almost universally employed for the purification of bauxite. In this process, which was develq>ed by Austrian Karl Josq>h Bayer in 1892, the crushed and ground bauxite is digested with caustic soda solution, at elevated ten erature and under pressure, and the alumina is dissolved out as a solution of sodium aluminate. The residue, known as red mud, contains the oxides of iron, silicon, and titanium ard is separated by settling and filtration. Aluminum hydrate is separated from the solution of sodium alu-minate by seeding and precipitation and is converted to the oxide, AI2O3, by calcination. 

White Hydroxide. Tlie soda sinter process appHed to bauxite or bauxite residue produces a hydroxide that is completely free from organic coloring matter and is very wliite. A value of more than 95% is obtained on the GE brightness scale relative to Ti02 as followed in the paper (qv) industry. Tliis compares to about 70% on the same scale for the nomial Bayer product. Tlie wliite hydroxide is preferred in the paper, toothpaste, and artificial marble industries. 

In 1976, Meluch and Campbell at General Motors obtained a patent10 for a steam hydrolysis depolymerization of PUR. High-pressure steam hydrolyzes flexible PUR foams rapidly at temperatures of 232-316°C to form diamines and polyols. The diamines are distilled and extracted from the steam and the polyols are obtained from the hydrolysis residue. In 1977, Bayer AG obtained a patent for a continuous PUR hydrolysis process using a specially designed extruder.11... 

Vandenhove H (2002) European Sites Contaminated by Residues from the Ore-extracting and Processing Industries. In Burkart W, Sohrabi M and BAYER A, eds. High Levels of Natural Radiation and Radon Areas Radiation Dose and Health Effects, pp. 307-315. International Congress Series No. 1225, Elsevier. 

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