Digestion of bauxite

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). 

AKOHIj and AI2O3 production is industrially almost exclusively by the Bayer process digestion of bauxite in sodium hydroxide... 

The industrial manufacture of aluminum hydroxide and aluminum oxide currently proceeds almost exclusively by the Bayer process i.e. by wet digestion of bauxite. The sinter- and melt-digestion processes with sodium carbonate and/or lime only have minor industrial importance. 

Sodium aluminate (theoretical formula NaA102) has a certain industrial importance not only as an intermediate in the digestion of bauxite in the Bayer process (see Section 3.2.4.2). USA production of sodium aluminate in 1993 was estimated to be 85 10- t/a. Sodium aluminate is produced by dissolving hydrated aluminum oxide in 50% sodium hydroxide. It is utilized in water purification, in the paper industry, for the post-treatment of Ti02-pigments, for the manufacture of aluminum-containing... 

Continuous digestion of bauxite with sodium hydroxide in autoclaves or tubular reactors at 140 to 250°C under pressure using the Bayer process... 

An unraked version of the deep cone thickener was developed for the thickening of red mud (insoluble residue from caustic digestion of bauxite... 

Uses Defoamer in acid digestion of bauxite ore in alumina production... 

A.lkaline Solutions. The most important example of alkaline leach is the digestion of hydrated alumina from bauxite by a sodium hydroxide solution at 160-170°C, ie, the Bayer process (see Aluminumand aluminum alloys). 

Commercial grades of sodium aluminate are obtained by digestion of aluminum tnhydroxide in aqueous caustic at atmospheric pressure and near the boiling temperature (7). Digestion of the aluminum hydroxy oxide in aqueous sodium hycboxide [1310-73-2] requbes pressures of up to 1.38 MPa (13.6 atm) and temperatures of about 200°C. Dry sodium aluminate is obtained by evaporation of water. Several processes for the production of sodium aluminate are known that do not reqube the addition of water. In one process, bauxite reacts with molten sodium hycboxide at approximately 400°C (8) in... 

Iron(III) oxide or alumina is refined from bauxite. Approximately 175 million tons of bauxite are mined annually worldwide, with virtually all of this processed into alumina. Alumina is a white crystalline substance that resembles salt. Approximately 90% of all alumina is used for making aluminum, with the remainder used for abrasives and ceramics. Alumina is produced from bauxite using the Bayer process patented in 1887 by Austrian Karl Josef Bayer (1847-1904). The Bayer process begins by grinding the bauxite and mixing it with sodium hydroxide in a digester. The sodium hydroxide dissolves aluminum oxide components to produce aluminum hydroxide compounds. For gibbsite, the reaction is Al(OH)3 + NaOH —> Al(OH)4 + Na+. Insoluble impurities such as silicates, titanium oxides, and iron oxides are removed from the solution while sodium hydroxide is recovered and recycled. Reaction conditions are then... 

The other major raw material for zeolite production is the sodium aluminate solution. The production route to this important ingredient starts from bauxite minerals, for example, gibbsite [15,20,99,100,119,126,142,150-154]. The ore is extracted and purified via the Bayer process. The process encompasses the digestion of sodium aluminate by means of caustic soda and precipitation of purified aluminum trihydroxide [15,18,20,99,100,119,126,142,150-159]. 

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. 

In 1990, appioximately 66,000 metric tons of alumina trihydiate [12252-70-9] AI2O2 3H20, the most widely used flame retardant, was used to inhibit the flammabihty of plastics processed at low temperatures. Alumina trihydrate is manufactured from either bauxite ore or recovered aluminum by either the Bayer or sinter processes (25). In the Bayer process, the bauxite ore is digested in a caustic solution, then filtered to remove siUcate, titanate, and iron impurities. The alumina trihydrate is recovered from the filtered solution by precipitation. In the sinter process the aluminum is leached from the ore using a solution of soda and lime from which pure alumina trihydrate is recovered (see Aluminum compounds). 

Other important reactions that occur in digestion are desiHcation, causticization of Hquor, and precipitation of impurities. The reactive siHca in bauxite, for example that in kaolin, reacts with caustic to form soluble sodium siHcate [1344-09-8], Na2Si02... 

The optimum conditions for roasting the clay and the optimum strength (30—60%) of the sulfuric acid used depend on the particular raw material. Finely ground bauxite or roasted clay is digested with sulfuric acid near the boiling point of the solution (100—120°C). The clay or bauxite-to-acid ratio is adjusted to produce either acidic or basic alum as desired and soHds are removed by sedimentation. If necessary, the solution can be treated to remove iron. However, few, if any, of the many methods claimed to be useful for iron removal have been used industrially (29). Instead, most alum producers prefer to use raw materials that are naturally low in iron and potassium. 

Alumina (AfOf. Alumina is produced by calcining either bauxite or aluminum hydroxide in rotary Idlns at temperatures from 1250 to 1600 K. In obtaining the highest-purity alumina, the bauxite is digested with alkah to remove impurities the resultant aluminum hydroxide [AlgfOH) ], of approximately 200-mesh size, is then calcined in rotaiy Idlns at 1350 K. 

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. 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

Bayer A process for making pure alumina hydrate from bauxite, used principally as a raw material for the manufacture of aluminum metal. The ore is digested with hot sodium hydroxide solution, yielding a solution of sodium aluminate. Insoluble impurities are separated off in the form of red mud and the solution is then nucleated with alumina hydrate from a previous batch, causing alumina trihydrate to precipitate ... 

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