UNIT: MAHAN ALUMINIUM Singrauli, (M.P) P PR RO OJ JE EC CT TR RE EP PO OR RT T PROJECT TITLE: AN OVERVIEW OF THE MANUFACTURING PROCESS OF ALUMINIUM Project Supervisor Sanjay Chaturvedi Project Coordinator Sushil Kumar Submitted by: Richa Tripathi Print to PDF without this message by purchasing novaPDF (http://www.novapdf.com/) CONTENTS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. Aluminium Application Extraction of Aluminium Bauxite Mining Purification of Bauxite Aluminium Reduction Description of Raw Material The Electrolysis Process Environmental Control Other Process of Aluminium Production Aluminium Alloys Casting Process Defects in Casting Health and Safety References Print to PDF without this message by purchasing novaPDF (http://www.novapdf.com/) nepheline. The major impurities include iron oxide. Lightness: Use in aerospace and transport industries. in thermite reactions for welding and for the manufacture of hardener alloys such as ferrotitanium. especially in overhead cables. connecting rods. Also used in pistons. meaning that less energy is required to overcome inertia. silicon dioxide and titanium dioxide. which produces molten iron. thus giving greater rigidity.novapdf. cooking utensils. andalusite. such as kaolin. etc. food industry packaging. labradorite.com/) . This is done by making the article the anode of a direct current electrolysis cell using an electrolyte solution of approximately 15% sulfuric acid. The extraction of aluminium. It is essentially an impure aluminium oxide. It deals with the use of electricity for smelting or refining of metals. should be easy because the ore occurs in oxide form which can be reduced by a suitable reducing agent to give the metal. In addition. The principal aluminium ore is bauxite Al2O3. reduced friction and lower bearing loads. as a deoxidant in steels. leucite.ALUMINIUM APPLICATIONS These properties lead to a variety of specialized uses. many other types of rock contain considerable amounts of alumina.2H2O. to give better balance. Aluminium has great affinity for oxygen hence it can not be reduced by usual reducing agents. This property also makes possible the thermite reaction. etc. and alunite. In these applications a finely powdered form (and hence a high surface area to weight ratio) is used. Corrosion resistance: This is made use of in chemical plant. However. aluminium oxide can not be reduced that easily. Thermite reaction: 2Al + Fe2O3 → Al2O3 + 2Fe + heat EXTRACTION OF ALUMINIUM The extraction of aluminium is called electrometallurgy. Affinity for oxygen: This allows it to be used in explosives. building and marine applications. However this requires a high purity grade (in excess of 99. in practice. The oxide film can be thickened by anodizing. as its lightness enables a greater volume of metal to be used. Aluminium paint is widely used. and the film can be dyed in a wide range of colours. in principle.93%). Print to PDF without this message by purchasing novaPDF (http://www. Electrical conductivity: Used extensively for electrical conductors. pistons. Thermal conductivity: Extensive usage in heat exchangers. India. quartz) and titania. Iron is third at about 5%. Hence aluminium is obtained by the electrolysis of pure alumina. the alumina refineries and the aluminium smelters are not on one site. Bauxite. Bauxite contains smaller amounts of iron oxide. the temperature required for the reduction is very high. Print to PDF without this message by purchasing novaPDF (http://www. Bauxite is a weathered rock containing two forms of hydrated aluminum oxide. It is very common that the bauxite deposits. and Africa. If at all reduced by carbon. Bauxite occurs mainly in the tropics and in some Mediterranean countries. The consequence is extensive power-grids and mass bulk-transportation to make the aluminium. Bauxite contains 40 to 60 mass% of alumina and will no doubt continue to be the most favored aluminum ores for many decades.Aluminium is too high in the electrochemical series (it is a highly reactive element) so it can not be reduced by hydrogen or carbon.novapdf. Usually the electrical power comes from very remote locations. which usually gives it a reddish-brown colour. This does not make the process economic. a monohydrate or a trihydrate.com/) . A summary of production steps from the bauxite mine through casting is given in Fig. The principal ore from which aluminum is extracted is called bauxite after the town of Les Baux in southern France where the ore was originally discovered.Bauxite is the most important aluminous ore for the production of alumina. the main mining locations are in Latin America. Today. Aluminum comprises approximately 8% of the earth’s crust.7%). Australia. making it second only to silicon (27. silicates (clay. (i) Serpek’s process – This process is used when bauxite ore contains appreciable amount of silica (above 7 %) and low amount of Fe2O3 (less than 1 %). The bauxite containing iron oxide as major impurity is called red bauxite and the bauxite containing silica as major impurity is called white bauxite. Aluminium nitride is hydrolyzed with hot water. . If it contains silica. Fe2O3 as the major impurity.BAUXITE MINING Bauxite occurs close to the surface in seams varying from one meter to nine meters. Hence bauxite mining is an open pit mining process. Al2O3 .n H2O + 3C + N2 → 2 AlN + 3 CO + n H2O SiO2 + 2C → Si + 2 CO Silicon volatilizes at this temperature.Powdered bauxite is mixed with carbon and heated up to 18000C in a current of nitrogen . it is purified by Baeyer’s process or Hall’s process. dried and ignited at about 15000C to get pure alumina. it is purified by Serpek’s process. If bauxite contains iron oxide.com/) . Bauxite transportation is a very dirty process especially during the rainy season and all the water introduced in the bauxite clay will have to be removed in the bauxite preparation of the refining process. Aluminium from bauxite is converted to aluminium nitride while silica is reduced to silicon. Iron and silicon both make aluminium metal brittle and liable for corrosion hence they must be eliminated. It precipitates aluminium hydroxide. SiO2 as the major impurity. formed as small reddish pebbles (pisolites). PURIFICATION OF BAUXITE (THE ALUMINA PLANT) Bauxite contains iron oxide or silica as major impurity. 2 Al(OH)3 → Al2O3 + 3 H2O Print to PDF without this message by purchasing novaPDF (http://www. The bauxite from the mine is crushed and ground and transported to alumina plant.novapdf. AlN + 3 H2O → Al(OH)3 ↓ + NH3 The precipitate of Al(OH)3 is washed. 01–0. is separated from the sodium aluminate solution. Vacuum filters separate the hydroxide precipitate. Print to PDF without this message by purchasing novaPDF (http://www. It is then digested with a hot and strong solution of caustic soda (45 %) in an autoclave under 80 lb. Na3Al(OH)6 + 2H2O → 3NaOH + Al(OH)3. The end product of the alumina plant is alumina. The disposal of red mud can present an environmental problem simply because there is so much of it.03% Fe2O3. white powder.02% SiO2.b) Baeyer’s process – This process is used when bauxite ore contains appreciable amount of Fe2O3 (7 to 10 %) and low amount of silica (less than 1 %).01–0.3H2O → Al2O3 + 9H2O This powder is technical purity alumina. known as red mud. Al2O3 (present in bauxite) + 6NaOH + 3H2O → 2Na3Al(OH)6 Sodium meta aluminate ( soluble ) Some crystalline forms of SiO2 can also dissolve by the reaction SiO2 + 4NaOH → Na4SiO4 + 2H2O These two new species are soluble. when aluminium hydroxide precipitates. and 0. sodium meta aluminate solution is diluted with water.com/) . pressures at 1500C for 2 to 8 hours. a dry white powder that is the feedstock for aluminum smelting. After filtration.novapdf. but Iron and titanium oxide is insoluble in this solution and can be filtered out.3H2O ↓ The precipitation is speeded up by initially adding a small quantity of pure crystalline aluminium hydroxide to act as sites for crystal growth. containing as impurities at most 0. aluminium oxide dissolves in NaOH to form sodium meta aluminate (NaAlO2) while ferric oxide and titanium dioxide remain undissolved. At this stage. (Alternatively CO2 can be passed till the solution becomes acidic). The ore is first calcined and then finely ground.3–0. Calcination in rotary kilns or in fluidized beds at 1100C to 1300C finally converts the hydroxide to a dry. 0. with stirring and cooled to 600C. This waste material. From a few alumina plants. and then pumped to disposal areas. 2Al(OH)3. The filtrate containing caustic soda is concentrated and used again. which is then washed with pure water and then dried. red mud is deposited on the sea bed under strictly controlled conditions. washed to recover the caustic soda.6% NaO2. The impurities Fe2O3 and SiO2 remain on the filter paper. and it is usual to differentiate between two main grades.e. “floury” alumina. ceramics and refectory industries. Na2CO3 to give water soluble sodium meta aluminate.com/) . washed and ignited to obtain pure alumina. causing losses of caustic soda and alumina which increases input material costs. NaAlO2 leaving behind Fe2O3 and SiO2.novapdf. 2 Al(OH)3 → Al2O3 + 3 H2O Pure alumina Alumina: It is mainly extracted from bauxite using Bayer Process. when Al(OH)3 is precipitated. A high silica content of the bauxite is undesirable because insoluble sodium-aluminumsilicate will form. Alumina is also used in abrasive. When designing an alumina plant. 2 NaAlO2 + CO2 + 3 H2O → 2 Al(OH)3 ↓ + Na2CO3 The precipitate is filtered.This process is used for low grade bauxite ores.c) Hall’s process . factors like the type of bauxite ore to be used as feed material and the form of alumina to be produced have to be taken into consideration. bauxite ore is fused with sodium carbonate. which is highly calcined and “sandy” alumina. Energy consumption is another consideration. Al2O3 + Na2CO3 → 2 NaAlO2 + CO2 The fused mass of sodium meta silicate is extracted with water and filtered. which makes it suitable for use in dry scrubber systems for fluoride abatement at aluminum reduction plants. The grade of the alumina (particle size) can be influenced by precipitation and calcining conditions. In alumina plant bauxite is added at Print to PDF without this message by purchasing novaPDF (http://www. i. In this process. The sandy alumina has a large. which calcined to a lesser degree and contains Al2O3 in the hydrated form. The filtrate containing NaAlO2 is warmed and CO2 is passed through it. active surface area. Inexpensive fuel is desirable because the process needs a large amount of thermal energy. The economical operation of the Bayer process requires the rational use of energy for steam generation and calcining. red mud is separated at an intermediate temperature. and alumina is precipitated at the low temperature point in the cycle.novapdf.com/) . ALUMINIUM REDUCTION Flow Sheet of the Process Print to PDF without this message by purchasing novaPDF (http://www.the high temperature point. 1) Coke : Coke is a by.DESCRIPTION OF RAW MATERIALS Anodes: As explained above. Calcined petroleum coke and pitch as a binder are used for raw materials.novapdf. 2) Pitch : Pitches are used as a binder agent. anode material is consumed in the classical Hall-Héroult reaction. Pitches are a complex mixture of aromatic and heterocyclic organic compounds. other gaseous and liquid products. due to its sustainability as a binder and economics Print to PDF without this message by purchasing novaPDF (http://www.com/) . Coal tar pitches are used most commonly.product of petroleum refining. Most smelters use prebaked carbon anode blocks. Carbon quality is of great interest to the Potroom not only because it is an expensive operating supply but because of the effect poor quality can have on the on the pot operation Anodes are made of carbon containing as few impurities as possible. 415 Kg of carbon is used for each MT of metal production. which carbonize on heat treatment. Function and Properties of pitch a) Good wetting of coke grains & mixing properties b) Gradual release of volatiles during anode baking c) High coking value: A good pitch will yield a good amount of graphitised carbon or coke when baked. Another important property of pitch is its wetting power.com/) . Measured quantities of each grain size are now drawn off to enter the dry mix. how well the liquid pitch wets the coke particles. i. Quantities are taken from a recipe designed to give a dense mix voids between the coarse particles have to be filled with medium size particles and voids between these to be filled by small size particles. etc. this is called cocking value. Wetting power is measured in terms of viscosity. screened again and classified into grain sizes.novapdf. An average recipe for dry mix is given below:- Print to PDF without this message by purchasing novaPDF (http://www. A too high value will produce such a dense "pitch coke" that the escaping pitch vapours will expand the anode during baking. The over size is passed through a impact crusher. Aggregate Calcined petroleum coke is received as a mixture of coke particles ranging from fine dust to pieces 2-3 cm in diameter. d) Strong bonding between coke & carbonized pitch e) Good mechanical properties of carbonized pitch f) Low ash and sulfur g).e. If it is low not enough links will be established between the original coke grains. This coke is screened and sorted into 3-4 different sizes. Low reactivity h) High electrical conductivity Process of Making Anodes 1. The exact quantity needed is related to be dry mix recipe. It is required to bake the green anodes above the 1000ºC because the operating temperature of electrolysis cell is around the 1000ºC. Inside the press blocks are formed either by pressure alone or by vibration and / or pressure. To separate out the specific volatiles by the controlled temperature application is called Baking. Average quantities are 16-20% pitch.com/) . Mixing Pitch is added to the dry mix and every thing is well blended in a mixer at 160-180 degree centigrade.novapdf. c) Low chemical reactivity against the attack of carbon dioxide and ambient air.0 – 1.1 g. Smaller particles have a larger surface area to be coated with pitch hence more pitch is needed for the same weight of coke. too much pitch will give various problems during the next stages of anode making resulting in an inferior finished anode. The quantity of pitch added should just achieve that.cm-3 to 1.65 g. The baked anode is sent to rodding to produce rodded anode from where it is sent to potroom to be used in pots.55 – 1. At this temperature the pitch becomes liquid and will coat the coke particles. 3. The objective of green anode baking is to transform its binder pitch into pitch coke so as to produce baked anodes with the following characteristics: a) Sufficient mechanical strength to withstand the handling forces and thermal shock. This is so that the finished blocks retain their shape. b) High electrical conductivity for lower voltage drop and energy losses. The green blocks are cooled further and put in the temporary storage until they are put into the furnace for baking. Baking In the furnace the green anodes are slowly heated up to the range of 1100 – 1120 0C and slowly cooled down again.2. higher electrical resistively and chemical unbalance. Block Forming The hot green paste coming out of the mixer has to be cooled to about 110-120 degree Celsius before it is fed into the block press. The uniform baking up to high enough temperatures gives low carbon consumption per ton of metal produced. The total heating cycle may last 16 – 28 day. By block forming processes the bulk density of the product mix is usually raised from 1. 4.cm-3. The under heating may cause to less strength and hardness. Print to PDF without this message by purchasing novaPDF (http://www. If not enough pitch is added ‘ dry pockets’ will result . which can be reached without too much difficulty in reduction cells. This starts with examining the raw materials. Bath: The bath used in aluminium reduction has a number of unique properties. a double fluoride of sodium and aluminium. •Have a density low enough that it floats on top of aluminium metal in the cell to prevent the aluminium from oxidation. Cryolite: Cryolite is the main constituent of the electrolyte used in alumina electrolysis. •Have a low volatility such that losses are not excessive. Chemically it is Na3AlF6. The petroleum coke used must be sufficiently calcined. which is found as a natural mineral. Another method of checking the degree of calcination is to determine the remaining H2 content of the coke. Further it should give a ringing sound when stuck by a hammer while being suspended. •Ionize and conduct electricity so that current can flow.5. CaF2 +H2SO4 = 2HF (g) +CaSO4 (s) HF is then reacted with sodium oxide Na2O and alumina to produce cryolite.novapdf. The requirement of bath is as follows•Being molten at a high temperature. This is tested by checking the electrical specific resistively. laboratory tests have to be made. •Used as a solvent for alumina. To gain further information on quality of anode. Synthetic Cryolite is made from fluorspar (CaF2). Fluorspar is treated with sulphuric acid to produce hydrofluoric acid HF. It is a white granular powder. which make it the only suitable material for the purposes. A total amount of ash may be 20% of which iron and silicon compounds make up the bulk. Impurities must of course be at a minimum.com/) . Print to PDF without this message by purchasing novaPDF (http://www. It should have a light grey colour and show no deformation or cracks. Anode Quality To a certain extent we can judge the quality of baked anode by its physical appearance. Certain impurities may give it a grey or pink discoloration. The freezing point of cryolite is 1009°C. It is used as a source of aluminum and is much cheaper alternative to sodium fluoride.0 mass% LiF.0 mass% MgF. It is marked as a white powder but it may show shades of colour as a result of impurities. Print to PDF without this message by purchasing novaPDF (http://www. When the cost of producing one tonne of primary aluminium is broken down almost one third is devoted to electrical power. which reacts with aluminium fluoride dissolved in the electrolyte to form dissolved lithium fluoride.novapdf. The DC power required depends on the total number of cells or pots installed & the potline DC current. it reaches steady state concentration of 3-7 mass%. This chemical compound does not occur in nature.com/) . The chemical formula is Na2CO3. CaF2 in the electrolyte varies due to the type of alumina added. Its consumption is 1kgs of MgO per ton of aluminium to maintain a concentration of 2. Eventually. The electrolysis process used to produce aluminium requires large quantities of electrical power. Lithium Fluoride: It is superior to all other additives with respect to the physicochemical properties of the electrolyte. It is made from fluorspar in a similar process to cryolite. Magnesium Fluoride: Magnesium Fluoride (MgF2) is present in the electrolyte of some aluminium smelters.5 – 3. It is added in the form of lithium carbonate (Li2Co3). The actual addition is made in the form of MgO or MgCO3. Great care has therefore to be taken during addition.Aluminum Fluoride: Aluminum fluoride is added as an additive to the cryolite electrolyte used in the electrolysis. large quantities of gas will be produced as carbon dioxide. Power: Aluminium is a power intensive industry. It reacts with aluminum fluoride dissolved in the electrolyte to form calcium fluoride. This concentration is stable because the rate of addition of CaO is balanced by the rate of the loss of calcium into the aluminum produced in the anode gases. Its consumption varies 2-3 Kgs of Li2Co3 per ton of aluminium to maintain a concentration of 1. Sodium Carbonate or Soda Ash: Soda ash is used as an additive to the electrolyte in alumina electrolysis. Its chemical formula is AlF3. Calcium Fluoride: Calcium Fluoride (CaF2) is always present in the electrolyte because it occurs as calcium oxide impurity in the alumina feed.0– 4. When soda ash is added to the electrolyte. sometimes in connection with the use of lithium containing melts. It is a white granular material. the electrolyte is molten cryolite (Na3AlF6) in which 2–8% of alumina (Al2O3) is dissolved. this aluminum is siphoned off by vacuum into crucibles. A crust of frozen bath and alumina Print to PDF without this message by purchasing novaPDF (http://www. lined with SiC to contain the highly corrosive molten fluoride electrolyte. is shaped like a shallow rectangular basin. Liquid aluminum settles on the bottom of the cell since it is denser (specific gravity 2. such as aluminum fluoride (AlF3) and calcium fluoride (CaF2). Today. Periodically.novapdf. which is.3 at 960°C) than the electrolyte (specific gravity 2. To lower the melting point. less frequently. industrial cryolite-alumina mixtures also contain various amounts of other salts. Carbon anode blocks are suspended on steel rods. Steel bars carry the electric current through the insulating bricks into the carbon cathode floor of the cell. in turn. 2. The oxygen reacts with the carbon anodes. and dip into the electrolyte.1).com/) . or “pot. it breaks down the dissolved alumina into its component elements as metallic aluminum and oxygen gas. As the electric current flows through the electrolyte. To replace the alumina consumed in the reaction. forming bubbles of CO and CO2 gas.4. sometimes lithium carbonate (Li2CO3) is present and. It consists of a steel shell with a lining of fireclay brick for heat insulation.THE ELECTROLYSIS PROCESS In the Hall-Héroult process. The electrolysis cell. computer-controlled devices called point feeders automatically inject the alumina powder through the top surface crust of solidified electrolyte. depending on their size.” shown schematically in Fig. magnesium fluoride (MgF2) is introduced. These additions also improve current efficiency and reduce evaporation losses. more alumina must be added. Pots may each have two or more point feeders. In modern smelters.5-5 cm.. The ledge serves to stabilize the temperature of the bath by freezing to a greater thickness when heat generation is low and thinning when heat generation increases.5 volts in a graphite lined steel tank which acts as a cathode. Thus.5 volts per cell.→ 4 AlF3 + 3 O2 + 12 e. At cathode → 4 Al3+ + 12 e. which are unavoidable under industrial conditions. The difference is due to various voltage losses. The cell is controlled mainly by regulating the anode/cathode distance and the direct current. which can be up to 300.→ 4 Al 4C + 3O2 → 2 CO2 + 2 CO Print to PDF without this message by purchasing novaPDF (http://www. More heat comes from the slow burning of the carbon anodes. a modern potline consists typically of 264 cells in series. The individual cells are connected in series. which maintains electrolyte temperature. bringing the supply voltage to over 1000 V. Some coke is thrown on the surface of charge to control the oxidation of the metal. The electrode reactions are complicated and their exact nature is not known.000 A in modern cells. CaF2 ( 5 % ). The anode-to-molten aluminium spacing. The Al2O3 is added from feeder at the top. the operating voltage is considerably higher than the theoretical decomposition voltage of aluminum oxide. anodecathode distance. this being one of the main reasons for today’s high energy efficiency. which is the optimum operating voltage of thyristor power supplies.novapdf. The resulting excess power generates heat. ranges from 4. The simplified mechanism of electrode reactions is given below. process-control computers connected to remote sensors ensure optimal operation. Aluminum busbars carry the current from one cell to the next. The anodes are made of graphite. Thermal insulation is designed to provide sufficient heat loss to maintain a protective ledge of frozen electrolyte on the walls of cells. Na3AlF6 → 3 NaF + AlF3 . supplied at 1150 V.covers the molten bath. Chemical reaction The charge consists of cryolite ( 85 % ). AlF3 ( 5 % ) and Al2O3( 5 % ) .The electrolysis is carried out at temperature of 9500C and with a voltage of 5. 4 AlF3 → 4Al3+ + 12 FAt anode → 2 Al2O3 + 12 F.com/) . Breakdown of Voltage Requirement At 4–4. but not on the bottom under the anodes. Of these. which are fitted with pneumatic alumina transport and feeding systems. With the aid of modern. Formerly. alumina and carbon dust. the cell voltage climbs from the normal 4. the fluorides are of the most environmental concern.com/) . Under unfavorable conditions. aluminum sulfide electrolytes) and two metallothermic ones (the Toth process. and used to form aluminium. can be much reduced. microprocessor-controlled potroom control. The fluoride fumes are emitted more or less constantly and are roughly 50% particulate (condensed cryolite droplets and condensed sodium tetrafluoroaluminate vapour) and 50% gaseous HF. Other processes for extracting aluminum have been developed. The alumina used in the dry scrubber is transported to the reduction cell. technical or OTHER PROCESSES FOR PRODUCING ALUMINUM Print to PDF without this message by purchasing novaPDF (http://www. and the carbothermic reduction process. HF is removed by adsorption on the alumina surface: Al2O3 + HF(g) → Al2O3. These fumes are collected by hoods over modern cells. Here. However. In modern electrolysis pots. and the fluoride. it is worth mentioning two electrolysis processes (using aluminum chloride or.HF(ads) Particulates are physically trapped by the alumina at the same time. in which manganese reduces aluminum chloride.ENVIRONMENT CONTROL Substances emitted are gaseous and particulate fluorides. anode effects can now be reduced to one every two months by automated alumina feeding.” which occurs when the alumina concentration drops below a critical threshold When the alumina concentrations too low and cryolite is reduced instead. 4NaAlF6 + 3C → 4Al + 3CF4 + 12NaF During the anode effect. in which carbon reduces alumina). this emission happens only during the so-called “anode effect. the pots may produce small quantities of the fluorocarbon compounds CF4 and C2F6.novapdf. Except for the aluminum chloride electrolysis. which are known to take part in the “greenhouse effect” of the upper atmosphere. the alumina concentration can be held at an almost constant level. potentially causing damage to humans selected plant and animal life. and hence the emission of fluorocarbons.5 volts to over 40 volts. alternatively. carbon monoxide and carbon dioxide. and gaseous sulfur dioxide. the frequency of anode effects. about one per day and per cell. gallium. so that it needs less heat energy. The main advantage of the chloride process over the Hall-Héroult process is a saving of 30% in electric power consumption. However. The latter is recycled by chlorinating alumina. and boron.0–99. In spite of these advantages. Electrolysis releases aluminum metal and chlorine gas. vanadium. Primary Aluminum Smelters produce primary aluminum (as opposed to secondary. nitrogen.com/) . the only gas soluble to any extent in aluminum. since it can work with multipolar graphite electrodes.97%. such as oxides and carbides. The main impurities are iron and silicon. Typical analyses also show traces of copper. If the level of these elements is still too high. aluminum) with a purity of 99. Filtering can remove suspended particles. and chromium. because these elements greatly reduce conductivity. can be removed by degassing with chlorine. Conductor-grade aluminum is generally produced by selecting the purer metal available from the best cells. Print to PDF without this message by purchasing novaPDF (http://www. and. Passing chlorine gas through the molten aluminum can remove traces of sodium. argon. manganese. calcium. adding boron can precipitate them as insoluble borides. its chances of success remain uncertain because. magnesium. if necessary. Alcoa operated an aluminum chloride electrolysis plant with a capacity of 15. or recycled. the temperature is much lower than that in the Hall-Héroult cells. lithium. lithium. together with smaller amounts of zinc. the process avoids fluoride emissions and makes more optimum use of anode coke. magnesium. International standards distinguish two types of unalloyed aluminum: “pure aluminum” of 99. as mentioned above. the reason given being the excessive cost of producing anhydrous aluminum chloride feedstock by chlorinating alumina. which is produced by further refinement.9%.000 tonnes per year. At 700°C.novapdf. chromium. Aluminum for electrical use must not exceed fairly low maximum levels of titanium. or. better still.7–99. calcium. Hydrogen. the economics of producing aluminum chloride feedstock remain unclear. In addition. For a few years from 1976. sodium. and titanium. manganese. Aluminum chloride feedstock is dissolved in an electrolyte consisting mainly of sodium chloride (NaCl) and potassium chloride (KCl) or lithium chloride (LiCl).9% and “high-purity” aluminum of at least 99. this was later shut down.economic reasons have prevented these processes from developing beyond the laboratory or pilot scale. which have little effect on conductivity. vanadium. (i) The bottom anode layer consists of impure aluminium. This aluminium layer is connected with graphite electrode to the mains. So Hoope’s electrolytic refining process is used to refine the metal. High-purity aluminum of at least 99. alumina and barium fluoride acting as electrolyte. reflectors or electrolytic capacitors). aluminium from the middle layer passes into the top layer and equivalent amount of aluminium passes from the bottom layer to Print to PDF without this message by purchasing novaPDF (http://www.High-purity Aluminum Metal produced by Hall – Herouit’s process is almost 99.97% aluminum content is necessary for certain special purposes (e. silicon from the bath and some alumina and carbon. (iii) The top cathode layer is of pure metal. The cell uses three liquid layers of different densities.9 percent aluminium and it contains small amounts of iron. On passing electric current.com/) .g. fused salt electrolyte is used. The cell is made of iron box. It is lined from inside with carbon.novapdf. Purity grades of aluminium Hoope’s process: In this process. The cell is shown in Fig. (ii) The middle layer consists of cryolite . and sheets are often produced in sandwich form with thin layers of pure corrosion resistant aluminium on the outside. it reduces the resistance to corrosion. which have improved corrosion resistance in marine environments. Lesser purities in the range 99. pure aluminium is removed from the top and aluminium of lower purity is added to the bottom layer.98% are today produced in limited quantities by fractional crystallization. From time to time . there is transfer of aluminium from the base to the top while impurities are left behind.9999% (“six nines” aluminum) can be obtained using one or two additional zonerefining operations. Many alloys have been developed. Aluminium-copper alloys have very poor resistance to corrosion. In general though. Pure aluminium is tapped from the top.novapdf.com/) . The refined aluminium has purity of 99.the middle layer. Print to PDF without this message by purchasing novaPDF (http://www. most notably its lightness and corrosion resistance. A possible exception to this is magnesium alloys. Thus.97–99. Alloying Here other elements are deliberately added to improve the properties in some way. the aim being to improve strength while retaining the desirable properties of aluminium.99 %. while the addition of an alloying element increases the strength.Higher purities of up to 99. making a compromise of properties necessary. Zone-refining traps impurities in a molten zone that moves gradually from one end to the other of a specially prepared ingot. cold working operations and heat treatment are clearly shown in Table. CASTING PROCESS Print to PDF without this message by purchasing novaPDF (http://www. Cold working operation s (e. Heat treatment. or a master alloy or hardener (e. iron.g. wire drawing) 2. silicon. magnesium and copper. rolling.g. manganese as a 10% Mn -90% Al hardener. magnesium. usually by adding the element direct.novapdf.com/) . copper and magnesium alloys.The strength of aluminium alloys may be further increased by: 1. e.g. especially on silicon and zinc. Alloying is carried out by the addition of suitable quantities of the alloying element to molten aluminium. This is done in a special holding furnace. using a process called "homogenization". While comparing with steels it must be noted that for equivalent sizes the aluminium alloy components will have a weight only about one-third that of the steel.) The effect of alloying. Print to PDF without this message by purchasing novaPDF (http://www. It is used for producing many components of home appliances (e.novapdf. common examples are tin. and it allows the cast part to be withdrawn from the mold more easily.g rice cookers.com/) . usually cast iron. motors. Die casting Die casting is a very commonly used type of permanent mold casting process. copper. or refractory alloys. and require significant lead time to fabricate. • In a hot chamber process (used for Zinc alloys. Die casting molds are expensive. There are two common types of die casting: hot. molten metal refills gooseneck cylinder. toys and hand-tools. steel. steel. pulling molten metal back through nozzle and gooseneck. plunger returns. magnesium) the pressure chamber connected to the die cavity is filled permanently in the molten metal. and lead. Also. fridges). As plunger uncovers inlet hole. magnesium. bronze. metal is held under pressure until it solidifies. The surface of the mold is coated with clay or other hard refractory material – this improves the life of the mold. aluminum. e. and therefore yields high throughput rates. Before molding. stoves. zinc. etc. casting stays in ejector die. (iii) die opens and cores. (iv) ejector pins push casting out of ejector die.Permanent mold casting Here. washing and drying machines. Surface finish and tolerance of die cast parts is so good that there is almost no post-processing required. the surface is covered with a spray of graphite or silica. This has two purposes – it improves the flow of the liquid metal. which acts as a lubricant.and coldchamber die casting. The process can be automated. are retracted. if any. The basic cycle of operation is as follows: (i) die is closed and gooseneck cylinder is filled with molten metal. The hot chamber process is used for metals that (a) have low melting points and (b) do not alloy with the die material. it produces very good tolerance and surface finish.g. the molds are made of metal. (ii) plunger pushes molten metal through gooseneck passage and nozzle and into the die cavity. fans. they are commonly called dies. It is commonly used for producing ingots of low melting point metals. the molten metal is poured into the cold chamber in each cycle. The operating cycle is (i) Die is closed and molten metal is ladled into the cold chamber cylinder. if there are cores. Print to PDF without this message by purchasing novaPDF (http://www. This process is particularly useful for high melting point metals such as Aluminum. they are retracted away. the metal is held under high pressure until it solidifies.• In a cold chamber process. and Copper (and its alloys).novapdf. (iii) die opens and plunger follows to push the solidified slug from the cylinder. (iv) ejector pins push casting off ejector die and plunger returns to original position. (ii) plunger pushes molten metal into die cavity.com/) . In the case of aluminium.Porosity is detrimental to the ductility of a casting and its surface finish. Dissolved gases may be removed from the molten metal by flushing or pouring with an inert gas or by melting and pouring the metal in vacuum.novapdf. Causes: High pouring temperature. Pin holes porosity: As molten metal solidifies it looses temperature which decreases solubility of gases and thereby expelling dissolved gases. we are particularly concerned about hydrogen. abrupt bends and other turbulances. ladle and during flow in mould. To produce a sound casting proper design consideration should also be kept in mind. Print to PDF without this message by purchasing novaPDF (http://www. Porosity caused by shrinkage can be reduced or eliminated by adequate liquid metal feeding and by the use of external and internal chills. If moisture is present in the atmosphere then the water will decompose in contact with the aluminium as follows: 3 H2O + 2 Al = 3 H2 + Al2O3 It is likely that the liquid aluminium will gain hydrogen in this environment.com/) . Remedies: less turbulences. (a) Poor design (sharp corner) (b) rounded edges Gas holes: The gases absorbed by molten metal in furnace. Sharp corners should be avoided. as it is soluble in aluminium in all proportions. proper pouring temperature and proper gating. The hydrogen picked up by molten metal from unburnt fuel or dissociation of moisture inside mould may escape solidifying metal leaving behind very small diameter and long pin holes showing path of escape. if not allowed to escape gets trapped inside the casting. part of which is entrapped in casting.novapdf.Blow holes: These are spherical. Remedy: Proper drying of mould. cube will solidify first. TS depends on size and shape of casting by relationship known Chvorinov's Rule Solidification time = C (volume/surface area) 2 Where. metal characteristics and temperature. Due to heat of molten metal. Solidification Time Total solidification time TS is the time required for casting to solidify after pouring. the moisture is converted into steam. C is a constant that depends on mold material and thickness. Print to PDF without this message by purchasing novaPDF (http://www. cubical and cylindrical shapes with the same volume.com/) . flattened or elongated cavities present inside the casting or on the surface. For a given piece of metal of spherical. These defects are moisture left in mould or core. the control of their pressure. since it rapidly converts to steam. etc. The presence of fuels for furnace. as well as splashing of the molten metal during the transfer or pouring. commonly known as sows. 2. Improper handling of fluxes. Bulk aluminum intended for re-melting is often cast in the form of large shapes. weighing 700 to 2000 pounds. operations must make every effort to avoid charging sows that contain moisture. into molten aluminum Print to PDF without this message by purchasing novaPDF (http://www. either entrapped or surface. The exposures in aluminium smelting include noise. 7. alumina. Molten aluminum contacting any part of the human body can cause serious burns. Sows are also subject to surface moisture and other contaminants. tools. 8.com/) . thus requiring proper ventilation and safety equipment for the workers. If extensive. which are hygroscopic. which can become reservoirs for collecting large amounts of water.novapdf. creating severe danger of explosion 5. Molten Aluminum is typically handled at 1300-1450 degrees Fahrenheit to avoid premature solidification. fluorides and sulphur dioxide. and other locations. safety is an important consideration. Fumes from molten metals and lubricants. thus absorbing moisture and creating a danger 6.HEALTH AND SAFETY As in all other manufacturing operations. molds. and other equipment for wear. coal tar pitch. 9. The introduction of water into molten aluminum can result in an explosion ranging from a small to very violent event causing extensive equipment damage and endangering human life. the proper operation of valves. Therefore. The presence of water and moisture in crucibles. Inspection of crucibles. 4. Dust from sand and other compounds used in casting. The sow-casting process generally results in unavoidable internal shrinkage cavities. these burns can be fatal. Where there is possibility of splash or other direct exposure. cracks. personnel working with molten aluminum wear eye and face protection and protective clothing. particularly because of the following factors: 1. etc. 3. R. ISBN 3-87017-169-3. Metals Handbook American Society for Metals. 9.G. ALUSELECT . Chichester. 1984. Aluminium-Taschenbuch Edited by Aluminium-Zentrale. Ohio. 2 diskettes. Mateds: The Materials Technology Education System.LITERATURE/REFERENCES 1. SkanAluminium. Print to PDF without this message by purchasing novaPDF (http://www.novapdf. KTH 1990 Sandström. D. 1983/1988.: Introduction to Materials Selection. England 1987 12.An Introduction to the Metallurgy of Aluminum Fabrication 7. 1990 3. Ellis Harwood Series in Metals Materials.. The Properties of Aluminum and its Alloys. XXVI/1094 pages. Aluminium-Verlag Düsseldorf.The European Engineering Properity Database for Wrought Aluminium and Aluminium Alloys. Aluminum Federation Ltd. ISBN 3-87017-138-3 8. 342 tables.Hatch (Editor): Aluminium .EAA. ISBN 0-87179-176-6 4. King: Aluminum and its Alloys. 699 figures. KTH 1990 6. 2. Ellis Harwood.Edition. 1982. KTH 1992 5. Altenpohl: Aluminum Viewed From Within . 11. Registration Record of International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys. Aluminum Association. Aluminum Association Inc.Properties and Physical Metallurgy American Society for Metals. Metals Park. F. 2nd edition. 14. Birmingham 1993 10. E.com/) . Washington DC. Print to PDF without this message by purchasing novaPDF (http://www.novapdf.com/) .