Il processo di estrazione della bauxite e dell'allumina

Dr Donoghue and Mr Frisch are full-time employees of Alcoa of Australia.

Bauxite is the primary raw material for producing aluminium and can be found in large deposits worldwide. Bauxite is crushed and cooked in caustic soda solution before being seeded with seeds of aluminium hydroxide called gibbsite to induce its precipitation into red mud waste, leaving an environmentally damaging red slime residue which has had serious repercussions in many countries.

Bayer Process

Bauxite ore, mined for the production of aluminum, contains hydroxides and oxyhydroxides of aluminum2 (gibbsite, bayerite and nordstrandite), oxides of iron2 (ferrisite and goethite) and titanium dioxide2 (rutile and diaspore). The Bayer Process chemically converts this bauxite ore into alumina – the first step toward producing pure aluminum metal through Hall-Heroult electrolytic process.

Digestion and clarification steps in the Bayer Process are both vitally important. Any insoluble residue produced during digestion is known as red mud, and must be cleaned up for environmental reasons. Furthermore, an aluminate solution left after clarification must be purified – using NaCl-free caustic solution, ion exchange purification or high temperature sintering may all help control impurity levels in these two streams.

Clarification of an aluminate solution is accomplished via gravity settling and rinsing with water in thickeners (also referred to as raking thickeners), creating a concentrated sodium aluminate solution suitable for precipitation into alumina trihydrate. Coarse particles are extracted using crude cyclones while finer solids are retained by filtering; then the solution is chilled further to increase supersaturation before being pumped to tall silolike precipitators; this produces precipitated alumina hydrate which must then be washed, filtered and dried before being decomposed into commercially pure alumina2.

Under the Hall-Heroult electrolytic process, metallic aluminium with small amounts of ferrite impurities (minor impurities) is separated from molten salt (a mixture of sodium chloride and calcium chloride) by passing an electric current through an aluminate solution containing carbon from an anode anode electrode, where carbon reacts with other impurities forming white-crystalline compound called Aluminate which is transferred into large holding furnaces where impurities are removed while alloying elements added in to produce pure aluminium.

Maintaining impurity levels within the Bayer process blow down stream at below predetermined levels is a significant challenge, while producing effluent that can be discharged directly to natural receiving bodies without additional treatment. To accomplish this feat, factors like managing changes to bauxite mineralogy, optimising production rates, controlling competing reactions, minimizing losses from red mud deposits must all come together in harmony for it to succeed.

Fusione

Bauxite, an aluminum oxide rock, can be mined and refined to produce aluminium. Mining and refining processes produce significant waste material containing toxic elements like TENORM (Trace Elements of Natural Origin). Alumina then goes onto an aluminium smelter for electrolysis conversion into metal; to minimize waste generated during this process it would be beneficial if methods could be found to reuse as much alumina as possible.

Alumina is widely utilized as a raw material in industrial products like glass, porcelain and metallic paints for cars. Alumina also boasts many other applications due to its low electric conductivity, resistance to chemical attack, extreme strength and hardness (9 on Mohs’ hardness scale).

As part of the mining and refining processes for alumina, it is necessary to clear land of vegetation and trees that could obstruct operations. Bauxite is broken apart using blasting, drilling, ripping and very large bulldozers before being loaded onto trucks, railroad cars or conveyor belts for transport to refineries located nearby.

At the start of the alumina refining process, alumina is broken up and immersed in a caustic soda solution to form sodium aluminate supersaturated solution. It is then transferred into a slurry reactor where it is heated at high temperatures in order to remove chemically combined water through a process known as calcination, producing solidified alumina hydrate that is then stored in brick-lined pots before carbon anodes are introduced and an electric current passed through them in order to electrolyze alumina into pure aluminium which then transferred back into pots before finally transferred back into cast houses where billets with various shapes are formed by hand-in.

After the billets have been cut into ingots, they are then used to roll long rods called rebar, which can then be cut to various lengths and used accordingly. Finally, these long rods are melted to produce ingots for further processing.

Casting

Aluminum is a central element in today’s modern world, used everywhere from buildings and cars to packaging and sporting equipment. But aluminum doesn’t occur naturally as pure metal; rather it undergoes an intricate conversion process from its origins as bauxite ore to become the lightweight metal we all know and enjoy today.

First, bauxite is mined from the earth using techniques including drilling, blasting and ripping with large bulldozers. Crushing and washing then take place before transport to an alumina refinery nearby that uses Bayer refining process to produce alumina.

Hot caustic soda solution is used to dissolve the aluminium-bearing minerals found in bauxite (gibbsite, bohmite and diaspore) to form supersaturated sodium aluminate solution known as pregnant liquor, then sent off to be calcinated in an aluminium oxide (Al2O3) furnace for calcination before being washed, dried and heated again to drive off any remaining water of crystallisation to produce high purity alumina which then forms dry white sandy material known as Alumina Hydrate.

Alumina is used in various products including refractory materials, chemical products and raw materials such as gypsum and silica; some is even recycled back into the refining process itself.

Refining of alumina generates two to three tons of wastes for every ton produced; these materials, known as red mud, must be handled carefully to minimise environmental impacts.

Once refined, alumina is cast into ingots for use in various applications. They may be rolled into sheets, foil and bars; drawn into wire which is stranded into cable for electrical transmission lines; moulded into complex shapes or extruded profiles such as window frames; they may even be recycled back into ingots again – making aluminium an infinitely recyclable metal! Furthermore, different alloys exist that offer specific properties to meet different customer requirements.

Recycling

Aluminum is one of the most widespread metals used in numerous products ranging from soda cans to car components, yet does not naturally occur as its pure form. Instead, it must be extracted from bauxite rocks containing various forms of aluminium oxides and refined into alumina before finally becoming aluminum in this three-step process which takes both energy and water resources.

As part of its mining process, Alumina Mining necessitates first removing overburden–layers of rock and clay covering bauxite–using blasting or drilling methods. Once loosed, this material can be loaded onto trucks for transport to an alumina processing plant for processing.

At a bauxite processing plant, the ore is crushed to reduce its size before being submerged in hot caustic soda to dissolve aluminium-bearing minerals like gibbsite and bohmite before being separated from silica to become sodium aluminate supersaturated solution known as pregnant liquor.

Some of the pregnant liquor is processed to create aluminium hydroxide, more commonly referred to as slaked lime, while the rest of it is sent directly to an alumina refinery where its components will undergo chemical reactions that will turn it into aluminium metal.

Alumina is an indispensable industrial material, used for many diverse applications in industries from plastics manufacturing to abrasives. Alumina also plays a key role in many types of fire retardants.

While alumina mining and refining may be highly recyclable, some environmental concerns remain associated with its mining and refining. These include noise pollution, ergonomic issues, trauma risks and caustic soda splashes to skin or eyes. Furthermore, both bauxite dust and alumina powder have been associated with respiratory complications including irritation or inflammation of lungs.

Some workers have reported health concerns such as cancer, kidney disease and liver damage attributed to exposures to bauxite dust and alumina dust as well as various contaminants; however, studies conducted have determined that current best-practices in mining and refining do not appear to increase risks of these problems.