Sodium hydroxide dissolving bauxite

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 processing of bauxite aluminium ore (hydrated alumina oxides) exemplifies these processes. In the Bayer process hot concentrated sodium hydroxide dissolves alumina which is subsequently separated and crystallized as aluminium hydroxide (hydrometallurgy). This is calcined to anhydrous alumina (pyrometallurgy) before being reduced in the Hall process (electrometallurgy) wherein alumina serves as the electrode from which elemental aluminium is deposited on the cathode. 

Aluminum is the most abundant metallic element in the Earth s crust and, after oxygen and silicon, the third most abundant element (see Fig. 14.1). However, the aluminum content in most minerals is low, and the commercial source of aluminum, bauxite, is a hydrated, impure oxide, Al203-xH20, where x can range from 1 to 3. Bauxite ore, which is red from the iron oxides that it contains (Fig. 14.23), is processed to obtain alumina, A1203, in the Bayer process. In this process, the ore is first treated with aqueous sodium hydroxide, which dissolves the amphoteric alumina as the aluminate ion, Al(OH)4 (aq). Carbon dioxide is then bubbled through the solution to remove OH ions as HCO and to convert some of the aluminate ions into aluminum hydroxide, which precipitates. The aluminum hydroxide is removed and dehydrated to the oxide by heating to 1200°C. 

Some metals that are chemically combined with oxygen (metal oxides) also dissolve in sodium hydroxide. For example, aluminum ore (known as bauxite) is treated with sodium hydroxide to isolate pure aluminum oxide, from which pure aluminum is obtained. Sand (silicon dioxide) will also dissolve in sodium hydroxide to form a chemical known as sodium silicate or water glass. 

BAYER PROCESS. Process for making alumina from bauxite. The main use of alumina is in the production of metallic aluminum. Bauxite is mixed with hot concentrated sodium hydroxide, which dissolves the alumina and silica. The silica is precipitated, and the dissolved alumina is separated from the solids, diluted, cooled, and then crystallized as aluminum hydroxide. The aluminum hydroxide is calcined to anhydrous alumina, which is then shipped to reduction plants. 

Alumina, A1203, is obtain from the cleaned bauxite ore in the Bayer process. The ore is first treated with aqueous sodium hydroxide, which dissolves the amphoteric alumina as the aluminate ion, Al(OH)4 (aq) and the acidic silica, Si02, as the metasilicate ion, SiO,2. The insoluble iron and titanium oxides precipitate in a red mud, which is filtered off ... 

Aluminum hydroxide is produced from bauxite. The ore is dissolved in a solution of sodium hydroxide, and aluminum hydroxide is precipitated from the sodium aluminate solution by neutralization (as with carbon dioxide) or by autoprecipitation (Bayer process) (HSDB 1995 Sax and Lewis 1987). 

The Hail-Herouli process for obtaining aluminum metal is one of the most important metallurgical processes of the present day. The ore, bauxite (AI2O3), is dissolved in sodium hydroxide solution (separating it from the nonamphoteric iron oxides) the hydroxide is reprecipitated with carbon dioxide and the hydroxide is heated, fused with cryolite, NasjAlFe, and electrolyzed with carbon electrodes. The molten metal, formed at the cathode, is drawn off from the bottom of the cell. 

Bauxite is not pure aluminum oxide (called alumina) but also contains the oxides of iron, silicon, and titanium, and various silicate materials. The pure hydrated alumina (A1203 wH20) is obtained by treating the crude bauxite with aqueous sodium hydroxide. Being amphoteric, alumina dissolves in the basic solution ... 

Bauxite is a mixture of aluminum minerals [AlHO., Al(OH)g], Which contains some iron oxide. It is purified by treatment with sodium hydroxide solution, which dissolves hydrated aluminum oxide, as the aluminate ion, Al(OH)4, but does not dissolve iron oxide ... 

Aluminum production is a two-step process. First, aluminum oxide is separated from bauxite by the Bayer process. In this process, bauxite is mixed with sodium hydroxide (NaOH), which dissolves the aluminum oxide. The other compounds in bauxite are left behind. 

In the Bayer process, ground bauxite is digested with aqueous sodium hydroxide (200 to 350 g/L sodium hydroxide) at temperatures of 140 to 250°C in autoclaves or in continuous tube reactors, in which the hydrated aluminum hydroxide is dissolved as sodium aluminate. The iron-containing very finely divided so-called red mud is then separated in thickening and filter units and washed. Ca. 1 to 2 t of red mud is produced per t aluminum oxide. 

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... 

Knowledge of the aluminium species present under alkaline conditions is of critical importance to industrial chemical operations such as the Bayer process in which the aluminium present in bauxite ores is dissolved in concentrated sodium hydroxide solution. Al NMR has been used to investigate the A1 coordination states in the resulting sodium aluminates after freeze drying (Bradley and Hanna 1994). At an OH Al ratio > 4.4, the predominant species is Na[Al(OH)4] (Q ) with a Al resonance at 86.6 ppm. A broader resonance at 71.3 ppm is probably composed of a variety of other poly-oxoanionic species such as [Al(OH2)(OAl)2] (Q ), [Al20(0H)6] (Q ) and... 

Aluminum is the third most abundant element in the earth s crust (after oxygen and silicon), accounting for 8.2% of the total mass. It occurs most commonly in association with silicon in the aluminosilicates of feldspars and micas and in clays, the products of weathering of these rocks. The most important ore for aluminum production is bauxite, a hydrated aluminum oxide that contains 50% to 60% AI2O3 1% to 20% FeiOs 1% to 10% silica minor concentrations of titanium, zirconium, vanadium, and other transition-metal oxides and the balance (20% to 30%) water. Bauxite is purified via the Bayer process, which takes advantage of the fact that the amphoteric oxide alumina is soluble in strong bases but iron(III) oxide is not. Crude bauxite is dissolved in sodium hydroxide... 

Initially the coarse ore is mechanically reduced to a finely divided form and stirred with the requisite concentration of aqueous base prior to pressure leaching. The alumina from trihydrate bauxite (A1203 -3H20 natural bayerite or gibbsite, AllOHlj) is relatively easy to dissolve using 15-20% aqueous sodium hydroxide, under pressure, at temperatures of 120-140°C (Eq. 12.2). 

Aluminum hydroxides are separated from bauxite by the Bayer process, in which these hydroxides are dissolved in sodium hydroxide to separate them from the other unwanted constituents of the bauxite. The dissolution reactions are carried out at about 285°C and 200 atm. pressure, and are ... 

The extraction of alumina from bauxite (the Bayer process) leaches the ore with sodium hydroxide at high temperatures and pressures to dissolve the amphoteric aluminum oxide ... 

Commercial alumina is produced chiefly from bauxite, the total world production being millions of tons per year. The bauxite ore contains, besides oxides of aluminium, silica and ferric oxide, which are removed by the Bayer method as follows. First the ore is ground fine, and it is then treated with sodium hydroxide solution in an iron autoclave under a pressure of 4 atm and at 160°—170°C. The alumina dissolves, forming sodium aluminate ... 

Most alumina used for chromatography is prepared from the impure ore bauxite AljOg- [JcHjO + Fe203- The bauxite is dissolved in hot sodium hydroxide and filtered to remove the insoluble iron oxides the alumina in the ore forms the soluble amphoteric hydroxide A OH) ". The hydroxide is precipitated by CO2, which reduces the pH, as Al(OH)g. When heated, the Al(OH)g loses water to form pure alumina AljOg. 

Electrowinning of Aluminum. Aluminum, the most abundant metal in the Earth s crust, did not become readily available commercially until the development of the Hall-Heroult process. This process involves electrolysis of dry aluminum oxide (alumina) dissolved in cryolite (sodium aluminum hexafluoride). Additional calcium fluoride is used to lower the melting point of the cryolite. The process runs at about 960 degrees Celsius and uses carbon electrodes. The alumina for the Hall-Heroult process is obtained from an ore called bauxite, an impure aluminum oxide with varying amounts of compounds such as iron oxide and silica. The preparation of pure alumina follows the Bayer process The alumina is extracted from the bauxite as a solution in sodium hydroxide (caustic soda), reprecipitated by acidification, filtered, and dried. The electrolysis cell has a carbon coating at the bottom that forms a cathode. 

Aluminum is the most commercially important nonferrous metal. Its chemistry and uses will be discussed in Section 27-7. Aluminum is obtained from bauxite, or hydrated aluminum oxide, AI2O3 XH2O. Aluminum ions can be reduced to A1 by electrolysis only in the absence of H2O. First the crushed bauxite is purified by dissolving it in a concentrated solution of sodium hydroxide, NaOH, to form soluble Na[Al(OH)4].Then Al(OH)3 XH2O is precipitated from the filtered solution by blowing in carbon dioxide to neutralize the... 

Aluminum oxide, also known as alumina, is the main component of bauxite, the principal ore of aluminum. The bauxite ore is made up of impure AI2O3, Fe203, and Si02. Typically, the content of AI2O3 in bauxite is about 40wt%. In the Bayer process, AI2O3 is dissolved in sodium hydroxide, then precipitates as Al(OH)3. 

Bauxite ore contains not only aluminum oxide (AI2O3), but oxides of iron, silicon, and titanium. Therefore, the aluminum oxide (called alumina) must first be separated from the other compounds in the ore. The pure hydrated alumina (Al203 nH20) is obtained by treating bauxite with sodium hydroxide, which dissolves the alumina but does not dissolve the other compounds in the ore. The alumina solution is then separated from the remaining solid compounds and reprecipitated to obtain pure alumina. The purified alumina is dissolved in molten cryolite, Na3AlFg, at 970°C in an electrochemical cell, and the aluminum ions are reduced to aluminum metal. The liquid aluminum is denser than the molten cryolite and alumina therefore, the molten aluminum metal settles to the bottom of the cell and is drained off periodically. 

The ore is first treated with sodium hydroxide under pressure. The aluminium largely dissolves as the aluminate, the iron oxide is insoluble and the silica also remains in the form of a sodium aluminium silicate, which leads to a loss of aluminium. Hence, the best bauxites are those low in silica. After filtration, the hydrated aluminium oxide is repredpitated by seeding and the sodium hydroxide solution may be reused. The alumina is washed and then healed at 1200 C to remove water The final step in the production of aluminium metal has to be electrolytic since the reduction of alumina with carbon is only possible at very high temperatures and the reverse reaction occurs on cooling. Moreover, because of the chemistry of aluminium, the electrolysis medium cannot be water in fact, almost all commercial production of aluminium during the last 90 years has used an electrolysis in molten cryolite, NajAlF. ... 

In the process to produce alumina (Fig. 1), bauxite is crushed and wet ground to 100-mesh, dissolved under pressure and heated in digesters with concentrated spent caustic soda solution from a previous cycle and sufficient lime and soda ash. Sodium aluminate is formed, and the dissolved silica is precipitated as sodium aluminum silicate. The undissolved residue (red mud) is separated from the alumina solution by filtration and washing and sent to recovery. Thickeners and Kelly or drum filters are used. The filtered solution of sodium aluminate is hydrolyzed to precipitate aluminum hydroxide by cooling. The precipitate is filtered from the liquor, washed, and heated to 980°C in a rotary kiln to calcine the aluminum hydroxide. 

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