Bayer process

The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide). Bauxite, the most important ore of aluminium, contains only 30–60% aluminium oxide (Al2O3), the rest being a mixture of silica, various iron oxides, and titanium dioxide.[1] The aluminium oxide must be purified before it can be refined to aluminium metal.

Process

Bayer-process-en
The Bayer process

Bauxite ore is a mixture of hydrated aluminium oxides and compounds of other elements such as iron. The aluminium compounds in the bauxite may be present as gibbsite 2(Al(OH)3), boehmite (γ-AlO(OH)) or diaspore (α-AlO(OH)); the different forms of the aluminium component and the impurities dictate the extraction conditions. Aluminum oxides and hydroxides are amphoteric, meaning that they are both acidic and basic. The solubility of Al(III) in water is very low but increases substantially at either high or low pH. In the Bayer process, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution (caustic soda) at a temperature of 150 to 200 °C. At these temperatures, the aluminium is dissolved as sodium aluminate (primarily [Al(OH)4]-) in an extraction process. After separation of the residue by filtering, gibbsite is precipitated when the liquid is cooled and then seeded with fine-grained aluminum hydroxide crystals from previous extractions. The precipitation may take several days without addition of seed crystals.[2]

The extraction process converts the aluminium oxide in the ore to soluble sodium aluminate, 2NaAlO2, according to the chemical equation:

Al2O3 + 2 NaOH → 2 NaAlO2 + H2O

This treatment also dissolves silica, but the other components of bauxite do not dissolve. Sometimes lime is added at this stage to precipitate the silica as calcium silicate. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap and with the aid of a flocculant such as starch, to remove the fine particles. The undissolved waste after the aluminium compounds are extracted, bauxite tailings, contains iron oxides, silica, calcia, titania and some unreacted alumina. The original process was that the alkaline solution was cooled and treated by bubbling carbon dioxide through it, a method by which aluminium hydroxide precipitates:

2 NaAlO2 + 3 H2O + CO2 → 2 Al(OH)3 + Na2CO3

But later, this gave way to seeding the supersaturated solution with high-purity aluminium hydroxide (Al(OH)3) crystal, which eliminated the need for cooling the liquid and was more economically feasible:

2 H2O + NaAlO2 → Al(OH)3 + NaOH

Some of the aluminium hydroxide produced is used in the manufacture of water treatment chemicals such as aluminium sulfate, PAC (Polyaluminium chloride) or sodium aluminate; a significant amount is also used as a filler in rubber and plastics as a fire retardant. Some 90% of the gibbsite produced is converted into aluminium oxide, Al2O3, by heating in rotary kilns or fluid flash calciners to a temperature in excess of 1000 °C.

2 Al(OH)3Al2O3 + 3 H2O

The left-over, 'spent' sodium aluminate solution is then recycled. Apart from improving the economy of the process, recycling accumulates gallium and vanadium impurities in the liquors, so that they can be extracted profitably.

Organic impurities that accumulate during the precipitation of gibbsite may cause various problems, for example high levels of undesirable materials in the gibbsite, discoloration of the liquor and of the gibbsite, losses of the caustic material, and increased viscosity and density of the working fluid.

For bauxites having more than 10% silica, the Bayer process becomes uneconomic because of the formation of insoluble sodium aluminium silicate, which reduces yield, so another process must be chosen.

1.9-3.6 tons of bauxite is required to produce 1 ton of aluminum oxide. This is due to a majority of the aluminum in the ore is dissolved in the process.[2] Over 90% (95-96%) of the aluminium oxide produced is used in the Hall–Héroult process to produce aluminium.[3]

Waste

Red mud is the waste product that is produced in the digestion of bauxite with sodium hydroxide. It has high calcium and sodium hydroxide content which makes a complex chemical composition. This makes it very caustic and a possible source of pollution. The amount of red mud produced during the process is considerable: this led to scientists and refiners attempting to find a use for it. It turned out to be useful in ceramic production. Red mud dries into a fine powder that contains iron, aluminum, calcium and sodium. It becomes a health risk when some plants use the waste to produce aluminum oxides.[4]

In the United States, the waste is disposed in large impoundments, a sort of reservoir created by a dam. The impoundments are typically lined with clay or synthetic liners. The US does not approve of the use of the waste due to the dangers it risks to the environment. The EPA identified high levels of arsenic and chromium in some red mud samples.[5]

Ajka Alumina Plant Accident

On October 4, 2010, the Ajka alumina plant in Hungary had an incident where the western dam of its red mud reservoir collapsed. The reservoir was filled with 700,000 m3 of a mixture of red mud and water with a pH of 12. The mixture was released into the valley of Torna river and flooded parts of the city of Devecser and the villages of Kolontár and Somlóvásárhely. The incident resulted in 10 deaths, more than a hundred injuries, and contamination in lakes and rivers.[6]

History of the Bayer process

The Bayer process was invented in 1888 by Carl Josef Bayer.[7] Working in Saint Petersburg, Russia to develop a method for supplying alumina to the textile industry (it was used as a mordant in dyeing cotton), Bayer discovered in 1887 that the aluminium hydroxide that precipitated from alkaline solution was crystalline and could be easily filtered and washed, while that precipitated from acid medium by neutralization was gelatinous and difficult to wash.[7] The industrial success of this process caused it to replace the Le Chatelier process which was used to produce alumina from bauxite.[7]

The engineering aspects of the process were improved upon to decrease the cost starting in 1967 in Germany and Czechoslovakia.[7] This was done by increasing the heat recovery and using large autoclaves and precipitation tanks.[7] To more effectively use energy, heat exchangers and flash tanks were used and larger reactors decreased the amount of heat lost.[7] Efficiency was increased by connecting the autoclaves to make operation more efficient.[7]

A few years earlier, Henri Étienne Sainte-Claire Deville in France developed a method for making alumina by heating bauxite in sodium carbonate, Na2CO3, at 1200 °C, leaching the sodium aluminate formed with water, then precipitating aluminium hydroxide by carbon dioxide, CO2, which was then filtered and dried. This process (known as the Deville process) was abandoned in favor of the Bayer process.

The process began to gain importance in metallurgy together with the invention of the Hall–Héroult electrolytic aluminium process, invented just one year earlier in 1886. Together with the cyanidation process invented in 1887, the Bayer process marks the birth of the modern field of hydrometallurgy.

Today, the process produces nearly all the world's alumina supply as an intermediate step in aluminium production.

See also

References

  1. ^ Harris, Chris; McLachlan, R. (Rosalie); Clark, Colin (1998). Micro reform – impacts on firms: aluminium case study. Melbourne: Industry Commission. ISBN 978-0-646-33550-6.
  2. ^ a b Hind, Andrew R.; Bhargava, Suresh K.; Grocott, Stephen C. (January 1999). "The surface chemistry of Bayer process solids: a review". Colloids and Surfaces A: Physicochemical and Engineering Aspects. 146 (1–3): 359–374. doi:10.1016/S0927-7757(98)00798-5.
  3. ^ "The Aluminum Smelting Process". Aluminum Production. aluminumproduction.com. Retrieved 12 April 2018.
  4. ^ Hind, Andrew R.; Bhargava, Suresh K.; Grocott, Stephen C. (1999). "The Surface Chemistry of Bayer Process Solids: A Review". Colloids and Surfaces A: Physicochemical and Engineering Aspects. 146 (1–3): 359–374. doi:10.1016/S0927-7757(98)00798-5.
  5. ^ "TENORM: Bauxite and Alumina Production Wastes". www.epa.gov. United States Environmental Protection Agency. 2015-04-22. Retrieved 12 April 2018.
  6. ^ Ruyters, Stefan; Mertens, Jelle; Vassilieva, Elvira; Dehandschutter, Boris; Poffijin, Andre; Smolders, Erik (2011). "The Red Mud Accident in Ajka (Hungary): Plant Toxicity and Trace Metal Bioavailability in Red Mud Contaminated Soil". Environmental Science & Technology. 45 (4): 1616–1622. doi:10.1021/es104000m. PMID 21204523.
  7. ^ a b c d e f g "Bayer's Process for Alumina Production: A Historical Production" (PDF). scs.illinois.edu. Fathi Habashi, Laval University. Retrieved 6 April 2018.
Alton and Southern Railway

The Alton and Southern Railway (reporting mark ALS) is a switching railroad in the Greater St. Louis area in Illinois. It is a wholly owned subsidiary of the Union Pacific Railroad.

Aluminium

Aluminium or aluminum is a chemical element with symbol Al and atomic number 13. It is a silvery-white, soft, nonmagnetic and ductile metal in the boron group. By mass, aluminium makes up about 8% of the Earth's crust; it is the third most abundant element after oxygen and silicon and the most abundant metal in the crust, though it is less common in the mantle below. The chief ore of aluminium is bauxite. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals.Aluminium is remarkable for its low density and its ability to resist corrosion through the phenomenon of passivation. Aluminium and its alloys are vital to the aerospace industry and important in transportation and building industries, such as building facades and window frames. The oxides and sulfates are the most useful compounds of aluminium.Despite its prevalence in the environment, no known form of life uses aluminium salts metabolically, but aluminium is well tolerated by plants and animals. Because of these salts' abundance, the potential for a biological role for them is of continuing interest, and studies continue.

Aluminium hydroxide

Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric in nature, i.e., it has both basic and acidic properties. Closely related are aluminium oxide hydroxide, AlO(OH), and aluminium oxide or alumina (Al2O3), the latter of which is also amphoteric. These compounds together are the major components of the aluminium ore bauxite.

Aluminium recycling

Aluminium recycling is the process by which scrap aluminium can be reused in products after its initial production. The process involves simply re-melting the metal, which is far less expensive and energy-intensive than creating new aluminium through the electrolysis of aluminium oxide (Al2O3), which must first be mined from bauxite ore and then refined using the Bayer process. Recycling scrap aluminium requires only 5% of the energy used to make new aluminium from the raw ore. For this reason, approximately 36% of all aluminium produced in the United States comes from old recycled scrap. Used beverage containers are the largest component of processed aluminum scrap, and most of it is manufactured back into aluminium cans.

Aluminium smelting

Aluminium smelting is the process of extracting aluminium from its oxide, alumina, generally by the Hall-Héroult process. Alumina is extracted from the ore bauxite by means of the Bayer process at an alumina refinery.

This is an electrolytic process, so an aluminium smelter uses prodigious amounts of electricity; they tend to be located very close to large power stations, often hydro-electric ones, and near ports since almost all of them use imported alumina. A large amount of carbon is also used in this process, resulting in significant amounts of greenhouse gas emissions.

Bauxite

Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)), mixed with the two iron oxides goethite and haematite, the aluminium clay mineral kaolinite and small amounts of anatase (TiO2) and ilmenite (FeTiO3 or FeO.TiO2).In 1821 the French geologist Pierre Berthier discovered bauxite near the village of Les Baux in Provence, southern France.

Bauxite tailings

Bauxite tailings, also known as red mud, red sludge, bauxite residue, or alumina refinery residues (ARR), is a highly alkaline waste product composed mainly of iron oxide that is generated in the industrial production of alumina (aluminium oxide, the principal raw material used in the manufacture of aluminium metal and also widely used in the manufacture of ceramics, abrasives and refractories). Annually, about 77 million tons of the red special waste are produced, causing a serious disposal problem in the mining industry. The scale of production makes the waste product an important one, and issues with its storage are reviewed and every opportunity is explored to find uses for it.

Over 95% of the alumina produced globally is through the Bayer process; for every tonne of alumina produced, approximately 1 to 1.5 tonnes of bauxite tailings/residue are also produced. Annual production of alumina in 2015 was approximately 115 million tonnes resulting in the generation of about 150 million tonnes of bauxite tailings/residue.

Carl Josef Bayer

Carl Josef Bayer (also Karl Bayer, March 4, 1847 – October 4, 1904) was an Austrian chemist who invented the Bayer process of extracting alumina from bauxite, essential to this day to the economical production of aluminium.

Bayer had been working in Saint Petersburg to develop a method to provide alumina to the textile industry that used it as a fixing agent in the dyeing of cotton. In 1887, he discovered that aluminium hydroxide precipitated from an alkaline solution which is crystalline and can be filtered and washed more easily than that precipitated from an acid medium by neutralization. In 1888, Bayer developed and patented his four-stage process of extracting alumina from bauxite ore.

In the mid-19th-century, aluminium was so precious that a bar of the metal was exhibited alongside the French Crown Jewels at the Exposition Universelle in Paris 1855. Along with the Hall-Héroult process, Bayer's solution caused the price of aluminum to drop about 80% in 1890 from what it had been in 1854.

Deville process

The Deville process was the first industrial process used to produce alumina from bauxite.

The Frenchman Henri Sainte-Claire Deville invented the process in 1859. It is sometimes called the Deville-Pechiney process.

It is based on the extraction of alumina with sodium carbonate.

The first stage is the calcination of the bauxite at 1200 °C with sodium carbonate and coke. The alumina is converted in sodium aluminate. Iron oxide remains unchanged and silica forms a polysilicate.

In the second stage sodium hydroxide solution is added, which dissolves the sodium aluminate, leaving the impurities as a solid residue. The amount of sodium hydroxide solution needed depends upon the amount of silica present in the raw material. The solution is filtered off; carbon dioxide is bubbled through the solution, causing aluminium hydroxide to precipitate, leaving a solution of sodium carbonate. The latter can be recovered and reused in the first stage.

The aluminium hydroxide is calcined to produce alumina.

The process was used in France at Salindres until 1923 and in Germany and Great Britain until the outbreak of the Second World War.It has now been replaced by the Bayer process.

Hall–Héroult process

The Hall–Héroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide (alumina) (obtained most often from bauxite, aluminium's chief ore, through the Bayer process) in molten cryolite, and electrolysing the molten salt bath, typically in a purpose-built cell. The Hall–Héroult process applied at industrial scale happens at 940–980°C and produces 99.5–99.8% pure aluminium. Recycled aluminum requires no electrolysis, thus it does not end up in this process.

Hydroxide

Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. A hydroxide attached to a strongly electropositive center may itself ionize, liberating a hydrogen cation (H+), making the parent compound an acid.

The corresponding electrically neutral compound HO• is the hydroxyl radical. The corresponding covalently-bound group –OH of atoms is the hydroxy group.

Hydroxide ion and hydroxy group are nucleophiles and can act as a catalysts in organic chemistry.

Many inorganic substances which bear the word "hydroxide" in their names are not ionic compounds of the hydroxide ion, but covalent compounds which contain hydroxy groups.

Industrial processes

Industrial processes are procedures involving chemical, physical, electrical or mechanical steps to aid in the manufacturing of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry.

List of countries by aluminium oxide production

Aluminium oxide is an amphoteric oxide of aluminium with the chemical formula Al2O3. It is also commonly referred to as alumina or aloxite in the mining, ceramic and materials science communities. It is produced by the Bayer process from bauxite. Its most significant use is in the production of aluminium metal, although it is also used as an abrasive due to its hardness and as a refractory material due to its high melting point.

Orbite Technologies

Orbite Technologies Inc. is a Canadian cleantech company based in Montreal, Canada. It specialized in extracting processes for mining industry, especially alumina extraction. Its main asset is the Orbite process which can be used as a cheaper and pollution-free replacement of the Bayer process as well as a way to treat red mud. In April 2017, the company declared bankruptcy, due to ongoing delays and issues relating to its construction of a new plant in Cap-Chat, Quebec. As of January 2018, the bankruptcy process was still ongoing.

Raschig–Hooker process

The Raschig–Hooker process is a chemical process for the production of phenol, named after German chemist Friedrich Raschig. The Raschig-Hooker process replaced the classical, Dow and Bayer process for phenol production which used non-recyclable sodium hydroxide to form sodium chloride to neutralize chlorine ions. The Raschig-Hooker process allows for the chlorine ion to be regenerated as hydrogen chloride. The hydrogen chloride can then be recycled in the first step of the reaction.

The main steps in this process are the production of chlorobenzene from benzene, hydrochloric acid and oxygen, and the subsequent hydrolysis of chlorobenzene to phenol. The first step uses either a copper or iron chloride catalyst and exposes the materials to air at 250℃. In the second step, the resulting chlorobenzene is introduced to steam at 450℃ over a silicon catalyst that hydrolyses the chlorobenzene, giving phenol and hydrogen chloride that can then be recycled back to the first step. Due to the two step nature, the Raschig-Hooker process can be used to produce either chlorobenzene or phenol.

The ability to recycle the hydrogen chloride made the Raschig-Hooker process preferable to the Dow and Bayer process. The reaction, however, takes place at very high temperatures in a very acidic environment with hydrogen chloride vapor and therefore the industrial setting must use highly corrosion resistant equipment for the reaction. While the Raschig-Hooker process does recycle the hydrogen chloride it produces, its use of catalysts that need to be replaced. The harsh chemical environment, use of catalysts, and large energy consumption has made it a target for green chemistry alternatives.

The Raschig-Hooker process suffers from selectivity issues in both steps. In the first step, the reaction is only run to 10% to 15% conversion to prevent the second addition of a chlorine atom to the desired chlorobenzene. Despite this, the overall selectivity of the reaction is 70% to 85%. The second step shares the low conversion rate and high selectivity of the first step. The small amount conversion per reaction offsets the monetary benefit of recycling the hydrogen chloride due to the large initial cost of the reaction. Therefore, the Raschig-Hooker process needed to be run at high concentrations in large reactors to be industrially economical.

Sodium aluminate

Sodium aluminate is an inorganic chemical that is used as an effective source of aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as NaAlO2, NaAl(OH)4 (hydrated), Na2O·Al2O3, or Na2Al2O4. Commercial sodium aluminate is available as a solution or a solid.

Other related compounds, sometimes called sodium aluminate, prepared by reaction of Na2O and Al2O3 are Na5AlO4 which contains discrete AlO45− anions, Na7Al3O8 and Na17Al5O16 which contain complex polymeric anions, and NaAl11O17, once mistakenly believed to be β-alumina, a phase of aluminium oxide.

Sodium borohydride

Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a reducing agent that finds application in chemistry, both in the laboratory and on a technical scale. It has been tested as pretreatment for pulping of wood, but is too costly to be commercialized. The compound is soluble in alcohols, certain ethers, and even water, although it slowly hydrolyzesThe compound was discovered in the 1940s by H. I. Schlesinger, who led a team seeking volatile uranium compounds. Results of this wartime research were declassified and published in 1953.

Sodium hydroxide

Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations Na+ and hydroxide anions OH−.

Sodium hydroxide is a highly caustic base and alkali that decomposes proteins at ordinary ambient temperatures and may cause severe chemical burns. It is highly soluble in water, and readily absorbs moisture and carbon dioxide from the air. It forms a series of hydrates NaOH·nH2O. The monohydrate NaOH·H2O crystallizes from water solutions between 12.3 and 61.8 °C. The commercially available "sodium hydroxide" is often this monohydrate, and published data may refer to it instead of the anhydrous compound. As one of the simplest hydroxides, it is frequently utilized alongside neutral water and acidic hydrochloric acid to demonstrate the pH scale to chemistry students.Sodium hydroxide is used in many industries: in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents, and as a drain cleaner. Worldwide production in 2004 was approximately 60 million tonnes, while demand was 51 million tonnes.

Yarwun, Queensland

Yarwun is a town and locality in the Gladstone Region, Queensland, Australia. In the 2011 census, Yarwun had a population of 239 people.

Mineral processing
(by physical means)
Pyrometallurgy
(by heat)
Hydrometallurgy
(by aqueous solution)
Electrometallurgy
(by electricity)
Co-products

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