Electrolytic Extraction of Aluminum from Bauxite and Alumina

In examining the electrolytic extraction of aluminum from bauxite and alumina, we must first briefly explain the element called aluminum and the minerals and ores that contain this very important metallic element. Aluminum is the third most plentiful element in the earth’s crust, being found abundantly as trioxosilicates(IV) in rocks and clays. The main source of aluminum is the mineral bauxite, Al2O3.2H2O, Other important minerals include kaolin, Al2O3 .2SiO2.H2O, cryolite, Na3AIF6, corundum, AI2O3, and mica, K2O.AI2O3.6SiO2.

Extraction of Aluminum from Bauxite 

Although aluminum occurs so abundantly in rocks and clays, no economical process has yet been devised for extracting the metal from them, owing to their high silica content. Instead, aluminum is extracted from bauxite by electrolysis. This process is known as the electrolytic extraction of aluminum. The extraction proceeds in two stages. In the first stage, the crude bauxite is purified to yield pure anhydrous aluminum oxide, which is then electrolyzed in the second stage.

Purification of Bauxite       

The bauxite is first heated with caustic soda solution under pressure to form soluble aluminate (III), NaAl(OH)4.

4Al2O3(s) + 2NaOH(aq) + 3H2O → 2NaAl(OH)4(aq)

Since the impurities in the ore, notably iron(III) oxide and the various trioxosilicates(IV), do not react with or dissolve in the alkali, they can be filtered off as a sludge. The filtrate which contains the aluminate (III) is then seeded with aluminum hydroxide crystals to induce the precipitation of aluminum hydroxide.

NaAl(OH)4(aq) → Al(OH)3(s) + NaOH(aq)

The aluminum hydroxide precipitate is then filtered off, washed, dried and heated strongly to yield pure aluminium oxide or alumina, A12O3, while the sodium hydroxide is concentrated and used again.

2A1(OH)3(s) → Al2O3(s) + 3H2O(1)

In the electrolysis of alumina, the electrolytic cell is an iron container lined with graphite. The graphite lining serves as the cathode while the anodes consist of graphite rods dipping into the electrolyte which is a solution of pure alumina in molten cryolite, Na2A1F6. The electrolyte is maintained at about 9500C by the heating effect of large current. This makes the process very expensive.

During the electrolysis of alumina, oxygen is given off at the anodes. As a result, the anodes are slowly burnt away as carbon(IV) oxide, and must be replaced continuously. This adds to the cost of the whole process. Molten aluminum collects at the bottom of the cell and is tapped off at intervals.

Alumina consists of aluminum and oxygen ions. At the cathode the aluminum ions gain three electrons each as the cathode to become deposited as metallic aluminum.

                   3A1 + 3e → A1

At the anode the oxygen ions donate two electrons each to from atomic oxygen, which then pair off to from gaseous molecules.  

Overall electrolytic reaction can be seen below:-

4A13+(1) + 602-(1) → 4A1(s) + 3O2(g).

Aluminum is the third most plentiful element in the earth’s crust, being found abundantly as trioxosilicates (IV) in rocks and clays. The main source of aluminum is the extracted from bauxite by electrolysis. The extraction proceeds in two stages. In the first stage, the crude bauxite is purified to yield pure anhydrous aluminum oxide, which is then electrolyzed in the second stage.

Electrolysis of Alumina 

In the electrolytic extraction of aluminum, the electrolytic cell is an iron container lined with graphite. The graphite lining serves as the cathode while the anodes consist of graphite rods dipping into the electrolyte which is a solution of pure alumina in molten cryolite, Na3A1F6. The electrolyte is maintained at about 9500C by the heating effect of a large current. This makes the process very expensive.

During the electrolysis of alumina, oxygen is given off at the anodes. As a result, the anodes are slowly burnt away as carbon(IV) oxide, and must be replaced continuously. This adds to the cost of the whole process. Molten aluminum collects at the bottom of the cell and is tapped off at intervals.

Chemistry of the Electrolysis of Alumina

In the electrolytic extraction of aluminum from alumina, we must understand the ionic composition of alumina in the electrolytic reaction process. Alumina consists of two ions. These ions are:- Aluminum and Oxygen ions.

Reaction at the Cathode: The aluminum ions gain three electrons each at the cathode to become deposited as metallic aluminum.

          413++ 3e → A1

Reaction at the Anode: The oxygen ions donate two electrons each to from atomic oxygen, which then pair off to form gaseous molecules.

          O2- → O + 2e

          O + O → O2                     

The Overall Electrolytic Reaction

4A13+(1) +6O2- (1) → 4A1(s) + 3O2(g)

Aluminum ranks above carbon in the reactivity series and so cannot be extracted by reduction using carbon. It is instead extracted through electrolysis.

Pure aluminum oxide has too high a melting point to be electrolysed. It is therefore mixed with molten cryolite, an aluminium compound which has a lower melting point.

Alumina

Aluminum production is an enormous industrial undertaking involving large chemical plants and huge mining operations. Australia has four long term bauxite mines at Weipa, Gove and Huntly-Willowdale and six alumina refineries at Boddington, Yarwun, QAL, Kwinana and Worsley.

The aluminum smelting process uses electrolysis to convert bauxite into alumina and then to metal aluminum. During electrolysis electricity runs through an electrolytic cell (it’s like a battery) which is an iron container lined with graphite and filled with a solution of pure alumina in molten cryolite Na3AlF6. The anode electrodes are carbon rods which dip into the electrolyte. The cathode electrodes are shaped like mirrors reflecting the anode rods to create a complete electrical circuit.

During the electrolysis of alumina in the anode cell aluminum oxide decomposes to form metallic aluminum and oxygen gas. The ions in the alumina react with the oxygen from the anode to form carbon dioxide. This process requires a lot of energy which comes from coal powered electrical power stations.

The alumina is then processed to produce activated alumina which is the main raw material for all other forms of aluminum production such as alloys, sheet and foil. Activated alumina also has many other uses in areas such as rubber, plastic and paint. The aluminum from bauxite can be recycled over and over again requiring only about 5% of the original energy required to extract it from bauxite using electrolysis.

Bauxite

Bauxite is a sedimentary rock that contains hydrated aluminum oxides, predominantly gibbsite (Al2O3*H2O) and diaspore and bohmite (both basic aluminum oxide, AlO(OH)), as well as iron oxides (hematite and goethite) and silica. The presence of these minerals gives bauxite its reddish hue. The bauxite mining industry is highly integrated with alumina refineries, and the majority of Australia’s bauxite mines supply alumina refineries located close to the mines.

Bauxites are excavated from open cut and underground mines. The bauxite is extracted using machines such as bulldozers, loaders and scoops. It is then loaded into trucks, rail cars or conveyors and transported to the alumina refinery. Before bauxite is reacted with caustic soda in the Bayer process it must be crushed, washed and dried. The resulting ‘red mud’ waste is then stored in residue lakes.

The bauxite is dissolved in caustic soda at high temperatures in tanks called precipitators. The hot solution is then seeded with small amounts of aluminum hydroxide seeds to induce the precipitation of solid aluminum hydroxide. This is then filtered, washed and heated to yield pure aluminum hydroxide – known as alumina – which is stored in tall tanks called precipitators. This is a key step in the smelting process. XRD plays an important role in this by directly measuring the mineralogy of the alumina. By knowing which minerals are present and what their proportions are alumina can be refined further reducing the production costs of aluminum metal.

Electrolysis of Bauxite

Aluminum is extracted by a process called electrolysis. The bauxite is refined into pure aluminum oxide, which is then electrolysed to produce molten aluminum metal. The resulting product is then used to make a range of industrial products. These include aircraft components (the low density of aluminum makes them ‘lighter’ than equivalent steel parts), greenhouse and window frames, car bodies, electricity pylons (aluminum is a good conductor of electricity but the pylons need to be strengthened with steel to prevent damage by lightning strikes) and many more.

In the electrolytic extraction of aluminum, we must first realise that aluminum has a very high melting point, so cannot be electrolysed directly from the ore. It must first be reduced to aluminum oxide using the Bayer Process, which is then dissolved in a solution of molten cryolite – another aluminum compound – to reduce energy costs. The addition of cryolite lowers the temperature needed for the cell, which saves energy by allowing ions to move more easily at a less extreme temperature.

The aluminum oxide is placed in an electrolytic cell and connected to a power source. A direct current, supplied by the power source, causes the ions in the electrolyte to be attracted to one of the electrodes. The negative electrode, made of graphite, is known as the cathode and the positive electrode is called the anode. The molten aluminum metal then forms on the anode.

Reactions of Bauxite

Aluminum is the second most malleable metal and is ductile, with good corrosion resistance. It is also non-toxic and non-magnetic. It is easily shaped and machined, and can be cast into thin sheets. It can be alloyed with copper, magnesium and silicon to make stronger and more useful metals. It is used in aircraft components (because of its low density), greenhouses and window frames, car parts (because it is strong), overhead power lines (because it can withstand a lot of strain), and other uses. It resists corrosion due to the very strong thin layer of aluminum oxide that covers its surface.

Bauxite, the ore from which aluminum is extracted, contains a mixture of minerals including silica. The process of purifying bauxite to produce pure aluminum involves a series of reactions that are described below.

The first reaction is digestion of the bauxite in a steam heated unit called a digester with a solution of caustic soda. This reaction produces a solution of sodium aluminate or green liquor and a precipitate of sodium aluminium silicate or red mud.

The next step in the electrolytic extraction of aluminum is electrolysis. The aluminate is dissolved in molten cryolite to create an electrolyte mixture. Alumina has a very high melting point of over 2000C, but when mixed with cryolite it melts at a lower temperature near 900C. A graphite rod is used as the anode and a carbon coated steel tank serves as the cathode. A direct current of 100 A passes through the electrolyte causing the oxygen from the alumina to combine with the carbon of the anode forming gaseous aluminum dioxide.