Transcript
UNIT: MAHAN ALUMINIUM
Sin gr auli, (M.P) (M.P)
P O E T PR JE R OJ EC CT T R R EP PO OR R T
PR O J ECT T TITL E:
AN O OV ER VI EW O OF T TH E MANUFACTUR ING P PR OC ESS O OF AL UMINIUM
Pr o je ject S Super visor
Pr o je ject C Coor dinator
San ja ja y C y C Chatur vedi
Sushil K Kumar
Submitted b b y: y: Richa T Tr ipathi
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CONT ENTS 1. Aluminium Application Appl ication 2. Extraction of Aluminium 3. Bauxite Mining 4. Purification of Bauxite 5. Aluminium Reduction 6. Description of Raw Material 7. The Electrolysis Process 8. Environmental Environmental Control 9. Other Process of Aluminium Production 10. Aluminium Alloys 11. Casting Process 12. Defects in Casting 13. Health and Safety 14. References
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ALUMINIUM APPLICATIONS These properties lead to a variety of specialized uses. Lightness: Use in aerospace and transport industries, as its lightness enables a greater
volume of metal to be used, thus giving greater rigidity. Also used in pistons, connecting rods, etc. to give better balance, reduced friction and lower bearing loads, meaning that less energy is required to overcome inertia. Electrical conductivity: Used extensively for electrical conductors, especially in overhead
cables. However this requires a high purity grade (in excess of 99.93%). Thermal conductivity: Extensive usage in heat exchangers, cooking utensils, pistons, etc. Corrosion resistance: This is made use of in chemical plant, food industry packaging,
building and marine applications. Aluminium paint is widely used. The oxide film can b e thickened by anodizing, and the film can be dyed in a wide range of colours. This is done by making the article the anode of a direct current electrolysis cell using an electrolyte solution of approximately 15% sulfuric acid. Affinity for oxygen: This allows it to be used in explosives, as a deoxidant in steels, in
thermite reactions for welding and for the manufacture of ha rdener alloys such as ferrotitanium. In these applications a finely powdered form (and h ence a high surface area to weight ratio) is used. This property also makes possible the thermite reaction, which produces molten iron. Thermite reaction: 2Al + Fe2O3
Al2O3 + 2Fe + heat
→
EXTRACTION OF ALUMINIUM The extraction of aluminium is called electrometallurgy. It deals with the use of electricity for smelting or refining of metals. The principal aluminium ore is bauxite Al2O3.2H2O. It is essentially an impure aluminium oxide. The major impurities include iron oxide, silicon dioxide and titanium dioxide. In addition, many other types of rock contain considerable amounts of alumina, such as kaolin, nepheline, andalusite, leucite, labradorite, and alunite. The extraction of aluminium, in principle, should be easy because the ore occurs in oxide form which can be reduced by a suitable reducing agent to give the metal. However, in practice, aluminium oxide can not be reduced that easily. Aluminium has great affinity for oxygen hence it can not be reduced by usual reducing agents.
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Aluminium is too high in the electrochemical series (it is a highly reactive element) so it can not be reduced by hydrogen or carbon. If at all reduced by carbon, the temperature required for the reduction is very high. This does not make the process economic. Hence aluminium is obtained by the electrolysis of pure alumina. Bauxite- Bauxite is the most important aluminous ore for the production of alumina. Aluminum comprises approximately 8% of the earth’s crust, making it s econd only to silicon (27.7%). Iron is third at about 5%. 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 occurs mainly in the tropics and in some Mediterranean countries. Today, the main mining locations are in Latin America, Australia, India, and Africa. Bauxite contains 40 to 60 mass% of alumina and will no doubt continue to be the most favored aluminum ores for many decades. Bauxite contains smaller amounts of iron oxide, which usually gives it a reddish-brown colour, silicates (clay, quartz) and titania. Bauxite is a weathered rock containing two forms of hydrated aluminum oxide, a monohydrate or a trihydrate. A summary of production steps from the bauxite mine through casting is given in Fig.
It is very common that the bauxite deposits, the alumina refineries and the aluminium smelters are not on one site. Usually the electrical power comes from very remote locations. The consequence is extensive power-grids and mass bulk-transportation t o make the aluminium.
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BAUXITE MINING Bauxite occurs close to the surface in seams varying from one meter to nine meters, formed as small reddish pebbles (pisolites). Hence bauxite mining is an open pit mining process. The bauxite from the mine is crushed and ground and transported to alumina plant. 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.
PURIFICATION OF BAUXITE (THE ALUMINA PLANT) Bauxite contains iron oxide or silica as major impurity. The bauxite containing iron oxide as major impurity is called red bauxite and the bauxite containing silica as major impurity is called white bauxite. Iron and silicon both make aluminium metal brittle and liable for corrosion hence they must be eliminated. If bauxite contains iron oxide, Fe2O3 as the major impurity, it is purified by Baeyer’s process or Hall’s process. . If it contains silica, SiO2 as the major impurity, it is purified by Serpek’s process. (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 %).Powdered bauxite is mixed 0
with carbon and heated up to 1800 C in a current of nitrogen . Aluminium from bauxite is converted to aluminium nitride while silica is reduced to silicon.
Al2O3 .n H2O + 3C + N2 → 2 AlN + 3 CO + n H2O SiO2 + 2C → Si + 2 CO
Silicon volatilizes at this temperature. Aluminium nitride is hydrolyzed with hot water. It precipitates aluminium hydroxide.
AlN + 3 H2O → Al(OH)3 ↓ + NH3 0
The precipitate of Al(OH)3 is washed, dried and ignited at about 1500 C to get pure alumina.
2 Al(OH)3 → Al2O3 + 3 H2O
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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 %). The ore is first calcined and then finely ground. It is then digested with a hot and strong solution of caustic soda (45 %) in an autoclave under 80 lb. pressures at 1500C for 2 to 8 hours. At this stage, aluminium oxide dissolves in NaOH to form sodium meta aluminate (NaAlO2) while ferric oxide and titanium dioxide remain undissolved. Al2O3 (present in bauxite) + 6NaOH + 3H 2O
→
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, but Iron and titanium oxide is insoluble in this solution and can be filtered out. This waste material, known as red mud, is separated from the sodium aluminate solution, washed to recover the caustic soda, and then pumped to disposal areas. The disposal of red mud can present an environmental problem simply because there is so much of it. From a few alumina plants, red mud is deposited on the sea bed under strictly controlled conditions. After filtration, sodium meta aluminate solution is diluted with water, with stirring and 0
cooled to 60 C, when aluminium hydroxide precipitates. Na3Al(OH)6 + 2H2O
→
3NaOH + Al(OH)3.3H2O ↓
The precipitation is speeded up by initially adding a small quantity of pure crystalline aluminium hydroxide to act as sites for crystal growth. (Alternatively CO2 can be passed till the solution becomes acidic). Vacuum filters separate the hydroxide precipitate, which is then washed with pure water and then dried. The filtrate containing caustic soda is concentrated and used again. Calcination in rotary kilns or in fluidized beds at 1100C to 1300C finally converts the hydroxide to a dry, white powder. 2Al(OH)3.3H2O
Al2O3 + 9H2O
→
This powder is technical purity alumina, containing as impurities at most 0.01–0.02% SiO2, 0.01–0.03% Fe2O3, and 0.3–0.6% NaO2. The end product of the alumina plant is alumina, a dry white powder that is the feedstock for aluminum smelting.
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c) Hall’s process - This process is used for low grade bauxite ores. In this process, bauxite ore is fused with sodium carbonate, Na2CO3 to give water soluble sodium meta aluminate, NaAlO2 leaving behind Fe2O3 and SiO2.
Al2O3 + Na2CO3 → 2 NaAlO2 + CO2
The fused mass of sodium meta silicate is extracted with water and filtered. The impurities Fe2O3 and SiO2 remain on the filter paper. The filtrate containing NaAlO 2 is warmed and CO2 is passed through it, when Al(OH) 3 is precipitated.
2 NaAlO2 + CO2 + 3 H2O → 2 Al(OH)3 ↓ + Na2CO3
The precipitate is filtered, washed and ignited to obtain pure alumina. 2 Al(OH)3
→ Al2O3 + 3 H2O Pure alumina
Alumina: It is mainly extracted from bauxite using Bayer Process. Alumina is also used in abrasive, ceramics and refectory industries. The grade of the alumina (particle size) can be influenced by precipitation and calcining conditions, and it is usual to differentiate between two main grades, i.e. “floury” alumina, which is highly calcined and “sandy” alumina, which calcined to a lesser degree and contains Al2O3 in the hydrated form. The sandy alumina has a large, active surface area, which makes it suitable for use in dry scrubber systems for fluoride abatement at aluminum reduction plants. When designing an alumina plant, 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. A high silica content of the bauxite is undesirable because insoluble sodium-aluminumsilicate will form, causing losses of caustic soda and alumina which increases input material costs. Energy consumption is another consideration. In alumina plant bauxite is added at
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the high temperature point, red mud is separated at an intermediate temperature, and alumina is precipitated at the low temperature point in the cycle. The economical operation of the Bayer process requires the rational use of energy for steam generation and calcining. Inexpensive fuel is desirable because the process needs a large amount of thermal energy. ALUMINIUM REDUCTION Flow Sheet of the Process
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OF R AW MAT ER IAL S D ESCR IPTION O
Anodes: As explained above, anode material is consumed in the classical Hall-Héroult reaction. Most smelters use prebaked carbon anode blocks. 415 Kg of carbon is used for each MT of metal production. 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. Calcined petroleum coke and pitch as a binder are used for raw materials. 1) Coke : Coke is a by- product of petroleum refining, other gaseous and liquid products. 2) Pitch : Pitches are used as a binder agent. Pitches are a complex mixture of aromatic and heterocyclic organic compounds, which carbonize on heat treatment. Coal tar pitches are used most commonly, due to its sustainability as a binder and economics
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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, this is called cocking value. If it is low not enough links will be established between the original coke grains. A too high value will produce such a dense "pitch coke" that the escaping pitch vapours will expand the anode during baking. Another important property of pitch is its wetting power, i.e. how well the liquid pitch wets the coke particles. Wetting power is measured in terms of viscosity. d) Strong bonding between coke & carbonized pitch e) Good mechanical properties of carbonized pitch f) Low ash and sulfur g). Low reactivity h) High electrical conductivity Process of Making Anodes 1. Aggregate
Calcined petroleum coke is received as a mixture of coke particles ranging from fi ne dust to pieces 2-3 cm in diameter. This coke is screened and sorted into 3-4 different sizes. The over size is passed through a impact crusher, screened again and classified into grain sizes. Measured quantities of each grain size are now drawn off to enter the dry mix. 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. An average recipe for dry mix is given below:-
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2. Mixing
Pitch is added to the dry mix and every thing is well blended in a mixer at 160-180 degree centigrade. At this temperature the pitch becomes liquid and will coat the coke particles. The quantity of pitch added should just achieve that. If not enough pitch is added ‘ dry pockets’ will result , too much pitch will give various problems during the next stages of anode making resulting in an inferior finished anode. The exact quantity needed is related to be dry mix recipe. Smaller particles have a larger surface area to be coated with pitch hence more pitch is needed for the same weight of coke. Average quantities are 16-20% pitch. 3. 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. This is so that the finished blocks retain their shape. By block forming processes the bulk density of the product mix is usually raised from 1.0 – 1.1 g.cm-3 to 1.55 – 1.65 g.cm-3. Inside the press blocks are formed either by pressure alone or by vibration and / or pressure. The green blocks are cooled further and put in the temporary storage until they are put into the furnace for baking. 4. Baking
In the furnace the green anodes are slowly heated up to the range of 1100 – 1120 0C and slowly cooled down again. The total heating cycle may last 16 – 28 day. 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. b) High electrical conductivity for lower voltage drop and energy losses. c) Low chemical reactivity against the attack of carbon dioxide and ambient air. To separate out the specific volatiles by the controlled temperature application is called Baking. It is required to bake the green anodes above the 1000ºC because the operating temperature of electrolysis cell is around the 1000ºC. The under heating may cause to less strength and hardness, higher electrical resistively and chemical unbalance. The uniform baking up to high enough temperatures gives low carbon c onsumption per ton of metal produced. The baked anode is sent to rodding to produce rodded anode from where it is sent to potroom to be used in pots.
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5. Anode Quality
To a certain extent we can judge the quality of baked anode by its physical appearance. It should have a light grey colour and show no deformation or cracks. Further it should give a ringing sound when stuck by a hammer while being suspended. To gain further information on quality of anode, laboratory tests have to be made. This starts with examining the raw materials. The petroleum coke used must be sufficiently calcined. This is tested by checking the electrical specific resistively. Another method of checking the degree of calcination is to determine the remaining H2 content of the coke. Impurities must of course be at a minimum. A total amount of ash may be 20% of which iron and silicon compounds make up the bulk. Cryolite: Cryolite is the main constituent of the electrolyte used in alumina electrolysis. Chemically it is Na 3AlF6, a double fluoride of sodium and aluminium. It is a white granular powder. Certain impurities may give it a grey or pink discoloration. The freezing point of cryolite is 1009°C. Synthetic Cryolite is made from fluorspar (CaF 2), which is found as a natural mineral. Fluorspar is treated with sulphuric acid to produce hydrofluoric acid HF. CaF2 +H2SO4 = 2HF (g) +CaSO4 (s) HF is then reacted with sodium oxide Na 2O and alumina to produce cryolite.
Bath: The bath used in aluminium reduction has a number of unique properties, which make it the only suitable material for the purposes. The requirement of bath is as follows•Being molten at a high temperature, which can be reached without too much difficulty in reduction cells. •Have a density low enough that it floats on top of aluminium metal in the cell to prevent the aluminium from oxidation. •Used as a solvent for alumina. •Ionize and conduct electricity so that current can flow. •Have a low volatility such that losses are not excessive.
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Aluminum Fluoride: Aluminum fluoride is added as an additive to the cryolite electrolyte used in the electrolysis. Its chemical formula is AlF 3. This chemical compound does not occur in nature. It is made from fluorspar in a similar process to cryolite. It is marked as a white powder but it may show shades of colour as a result of impurities.
Calcium Fluoride: Calcium Fluoride (CaF 2) is always present in the electrolyte because it occurs as calcium oxide impurity in the alumina feed. It reacts with aluminum fluoride dissolved in the electrolyte to form calcium fluoride. Eventually, it reaches steady state concentration of 3-7 mass%. CaF2 in the electrolyte varies due to the type of alumina added. 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. Sodium Carbonate or Soda Ash: Soda ash is used as an additive to the electrolyte in alumina electrolysis. It is used as a source of aluminum and is much cheaper alternative to sodium fluoride. The chemical formula is Na2CO3. It is a white granular material. When soda ash is added to the electrolyte, large quantities of gas will be produced as carbon dioxide. Great care has therefore to be taken during addition. Lithium Fluoride: It is superior to all other additives with respect to the physicochemical properties of the electrolyte. It is added in the form of lithium carbonate (Li2Co3), which reacts with aluminium fluoride dissolved in the electrolyte to form dissolved lithium fluoride. Its consumption varies 2-3 Kgs of Li2Co3 per ton of aluminium to maintain a concentration of 1.5 – 3.0 mass% LiF. Magnesium Fluoride: Magnesium Fluoride (MgF2) is present in the electrolyte of some aluminium smelters, sometimes in connection with the use of lithium containing melts. The actual addition is made in the form of MgO or MgCO3. Its consumption is 1kgs of MgO per ton of aluminium to maintain a concentration of 2.0– 4.0 mass% MgF. Power: Aluminium is a power intensive industry. The electrolysis process used to produce aluminium requires large quantities of electrical power. When the cost of producing one tonne of primary aluminium is broken down almost one third is devoted to electrical power. The DC power required depends on the total number of cells or pots installed & the potline DC current.
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TH E EL ECTR OL YSIS PR OC ESS In the Hall-Héroult process, the electrolyte is molten cryolite (Na3AlF6) in which 2–8% of alumina (Al2O3) is dissolved. To lower the melting point, industrial c ryolite-alumina mixtures also contain various amounts of other salts, such as aluminum fluoride (AlF3) and calcium fluoride (CaF2); sometimes lithium carbonate (Li2CO3) is present and, less frequently, magnesium fluoride (MgF2) is introduced. These additions also improve current efficiency and reduce evaporation losses.
The electrolysis cell, or “pot,” shown schematically in Fig. 2.4, is shaped like a shallow rectangular basin. It consists of a steel shell with a lining of fireclay brick for heat insulation, which is, in turn, lined with SiC to contain the highly corrosive molten fluoride electrolyte. Steel bars carry the electric current through the insulating bricks into the carbon cathode floor of the cell. Carbon anode blocks are suspended on steel rods, and dip into the electrolyte. As the electric current flows through the electrolyte, it breaks down the dissolved alumina into its component elements as metallic aluminum and oxygen gas. The oxygen reacts with the carbon anodes, forming bubbles of CO and CO2 gas. Liquid aluminum settles on the bottom of the cell since it is denser (specific gravity 2.3 at 960°C) than the electrolyte (specific gravity 2.1). Periodically, this aluminum is siphoned off by vacuum into crucibles. To replace the alumina consumed in the reaction, more alumina must be added. Today, computer-controlled devices called point feeders automatically inject the alumina powder through the top surface crust of solidified electrolyte. Pots may each have two or more point feeders, depending on their size. A crust of frozen bath and alumina
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covers the molten bath. Thermal insulation is designed to provide sufficient heat loss to maintain a protective ledge of frozen electrolyte on the walls of cells, but not on the bottom under the anodes. 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. Breakdown of Voltage Requirement At 4–4.5 volts per cell, the operating voltage is considerably higher than the theoretical decomposition voltage of aluminum oxide. The difference is due to various voltage losses, which are unavoidable under industrial conditions. The resulting excess power generates heat, which maintains electrolyte temperature. More heat comes from the slow bur ning of the carbon anodes. The cell is controlled mainly by regulating the anode/cathode distance and the direct current, which can be up to 300,000 A in modern cells. In modern smelters, process-control computers connected to remote sensors ensure optimal operation, this being one of the main reasons for today’s high energy efficiency. The individual cells are connected in series, bringing the supply voltage to over 1000 V, which is the optimum operating voltage of thyristor power supplies. Thus, a modern potline consists typically of 264 cells in series, supplied at 1150 V. Aluminum busbars carry the current from one cell to the next. The anode-to-molten aluminium spacing, anodecathode distance, ranges from 4.5-5 cm. Chemical reaction The charge consists of cryolite ( 85 % ), CaF2 ( 5 % ), AlF3 ( 5 % ) and Al2O3( 5 % ) .The electrolysis is carried out at temperature of 9500C and with a voltage of 5.5 volts in a graphite lined steel tank which acts as a cathode. The anodes are made of graphite. The Al2O3 is added from feeder at the top. 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. The simplified mechanism of electrode reactions is given below. Na3AlF6 At anode
2 Al2O3 + 12 F
→
At cathode
3 NaF + AlF3 ;
→
-
4 Al3+ + 12 e-
→
4 AlF3
4Al3+ + 12 F-
→
-
4 AlF3 + 3 O2 + 12 e ;
→
4C + 3O2
4 Al
→
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2 CO2 + 2 CO
→
ENVIR ONM ENT CONTR OL Substances emitted are gaseous and particulate fluorides, alumina and carbon dust, and gaseous sulfur dioxide, carbon monoxide and carbon dioxide. Of these, the fluorides are of the most environmental concern, potentially causing damage to humans selected plant and animal life. 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. These fumes are collected by hoods over modern cells. HF is removed by adsorption on the alumina surface: Al2O3 + HF(g)
→
Al2O3.HF(ads)
Particulates are physically trapped by the alumina at the same time. The alumina used in the dry scrubber is transported to the reduction cell, and used to form aluminium, and the fluoride. Under unfavorable conditions, the pots may produce small quantities of the fluorocarbon compounds CF4 and C2F6, which are known to take part in the “greenhouse effect” of the upper atmosphere. However, this emission happens only during the so-called “anode effect,” 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, the cell voltage climbs from the normal 4.5 volts to over 40 volts. In modern electrolysis pots, which are fitted with pneumatic alumina transport and feeding systems, the alumina concentration can be held at a n almost constant level. With the aid of modern, microprocessor-controlled potroom control, the frequency of anode effects, and hence the emission of fluorocarbons, can be much reduced. Formerly, about one per day and per cell, anode effects can now be reduced to one every two months by automated alumina feeding. FOR PR ODUCING AL UMINUM OTH ER PR OC ESS ES F
Other processes for extracting aluminum have been developed. Here, it is worth mentioning two electrolysis processes (using aluminum chloride or, alternatively, aluminum sulfide electrolytes) and two metallothermic ones (the Toth process, in w hich manganese reduces aluminum chloride, and the carbothermic reduction process, in which carbon reduces alumina). Except for the aluminum chloride electrolysis, technical or
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economic reasons have prevented these processes from developing beyond the laboratory or pilot scale. For a few years from 1976, Alcoa operated an aluminum chloride electrolysis plant with a capacity of 15,000 tonnes per year. However, this was later shut down, the reason given being the excessive cost of producing anhydrous aluminum chloride feedstock by chlorinating alumina. Aluminum chloride feedstock is dissolved in an electrolyte consisting mainly of sodium chloride (NaCl) and potassium chloride (KCl) or lithium chloride (LiCl). Electrolysis releases aluminum metal and chlorine gas. The latter is recycled by chlorinating alumina. The main advantage of the chloride process over the Hall-Héroult process is a saving of 30% in electric power consumption. In addition, the process avoids fluoride emissions and makes more optimum use of anode coke, since it can work with multipolar graphite electrodes. At 700°C, the temperature is much lower than that in the Hall-Héroult cells, so that it needs less heat energy. In spite of these advantages, its chances of success remain uncertain because, as mentioned above, the economics of producing aluminum chloride feedstock remain unclear. Primary Aluminum Smelters produce primary aluminum (as opposed to secondary, or recycled, aluminum) with a purity of 99.7–99.9%. The main impurities are iron and silicon, together with smaller amounts of zinc, magnesium, manganese, and titanium. Typical analyses also show traces of copper, chromium, gallium, sodium, lithium, calcium, vanadium, and boron. Passing chlorine gas through the molten aluminum can remove traces of sodium, lithium, calcium, and, if necessary, magnesium. Filtering can remove suspended particles, such as oxides and carbides. Hydrogen, the only gas soluble to any extent in aluminum, can be removed by degassing with chlorine, nitrogen, or, better still, argon. Aluminum for electrical use must not exceed fairly low maximum levels of titanium, vanadium, manganese, and chromium, because these elements greatly reduce conductivity. Conductor-grade aluminum is generally produced by selecting the purer metal available from the best cells. If the level of these elements is still too high, adding b oron can precipitate them as insoluble borides, which have little effect on conductivity. International standards distinguish two types of unalloyed aluminum: “pure aluminum” of 99.0–99.9% and “high-purity” aluminum of at least 99.97%, which is p roduced by further refinement.
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High-purity Aluminum Metal produced by Hall – Herouit’s process is almost 99.9 percent aluminium and it contains small amounts of iron, silicon from the bath and some alumina and carbon. High-purity aluminum of at least 99.97% aluminum content is n ecessary for certain special purposes (e.g. reflectors or electrolytic capacitors); So Hoope’s electrolytic refining process is used to refine the metal . Purity grades of aluminium
Hoope’s process: In this process, fused salt electrolyte is used. The cell uses three liquid layers of different densities. (i) The bottom anode layer consists of impure aluminium. (ii) The middle layer consists of cryolite , alumina and barium fluoride acting as electrolyte. (iii) The top cathode layer is of pure metal. This aluminium layer is connected with graphite electrode to the mains. The cell is made of iron box. It is lined from inside with carbon. The cell is shown in Fig. On passing electric current, aluminium from the middle layer passes into the top layer and equivalent amount of aluminium passes from the bottom layer to
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the middle layer. From time to time , pure aluminium is removed from the top and aluminium of lower purity is added to the bottom layer. Thus, there is transfer of aluminium from the base to the top while impurities are left behind. Pure aluminium is tapped from the top. The refined aluminium has purity of 99.99 %.Higher purities of up to 99.9999% (“six nines” aluminum) can be obtained using one or two additional zonerefining operations. Zone-refining traps impurities in a molten zone that moves gradually from one end to the other of a specially prepared ingot. Lesser purities in the range 99.97–99.98% are today produced in limited quantities by fractional crystallization. Alloying Here other elements are deliberately added to improve the properties in some way. Many alloys have been developed, the aim being to improve strength while retaining the desirable properties of aluminium, most notably its lightness and corrosion resistance. In general though, while the addition of an alloying element increases the strength, it reduces the resistance to corrosion, making a compromise of properties necessary. A possible exception to this is magnesium alloys, which have improved corrosion resistance in marine environments. Aluminium-copper alloys have very poor r esistance to corrosion, and sheets are often produced in sandwich form with thin layers of pure corrosion resistant aluminium on the outside.
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The strength of aluminium alloys may be further increased by:
1. Cold working operation s (e.g. rolling, wire drawing) 2. Heat treatment, especially on silicon and zinc, magnesium and copper, copper and magnesium alloys, using a process called "homogenization". 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, usually by adding the element direct, e.g. magnesium, iron, silicon, or a master alloy or hardener (e.g. manganese as a 10% Mn -90% Al hardener.) The effect of alloying, cold working operations and heat treatment are clearly shown in Table. 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.
PR OC ESS CASTING P
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Permanent mold casting Here, the molds are made of metal, usually cast iron, steel, or refractory alloys. The surface of the mold is coated with clay or other hard refractory material – this improves the life of the mold. Before molding, the surface is covered with a spray of graphite or silica, which acts as a lubricant. This has two purposes – it improves the flow of the liquid metal, and it allows the cast part to be withdrawn from the mold more easily. The process can be automated, and therefore yields high throughput rates. Also, it produces very good tolerance and surface finish. It is commonly used for producing ingots of low melting point metals, e.g. copper, bronze, aluminum, magnesium, etc. Die casting Die casting is a very commonly used type of permanent mold casting process. It is used for producing many components of home appliances (e.g rice cookers, stoves, fans, washing and drying machines, fridges), motors, toys and hand-tools. Surface finish and tolerance of die cast parts is so good that there is almost no post-processing required. Die casting molds are expensive, and require significant lead time to fabricate; they are commonly called dies. There are two common types of die casting: hot- and coldchamber die casting. • In a hot chamber process (used for Zinc alloys, magnesium) the pressure chamber connected to the die cavity is filled permanently in the molten metal. The basic cycle of operation is as follows: (i) die is closed and gooseneck cylinder is filled with molten metal; (ii) plunger pushes molten metal through gooseneck passage and nozzle and into the die cavity; metal is held under pressure until it solidifies; (iii) die opens and cores, if any, are retracted; casting stays in ejector die; plunger returns, pulling molten metal back through nozzle and gooseneck; (iv) ejector pins push casting out of ejector die. As plunger uncovers inlet hole, molten metal refills gooseneck cylinder. The hot chamber process is used for metals that (a) have low melting points and (b) do not alloy with the die material, steel; common examples are tin, zinc, and lead.
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• In a cold chamber process, 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; (ii) plunger pushes molten metal into die cavity; the metal is held under high pressure until it solidifies; (iii) die opens and plunger follows to push the solidified slug from the cylinder, if there are cores, they are retracted away; (iv) ejector pins push casting off ejector die and plunger returns to original position. This process is particularly useful for high melting point metals such as Aluminum, and Copper (and its alloys).
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Porosity is detrimental to the ductility of a casting and its surface finish. Porosity caused by shrinkage can be reduced or eliminated by adequate liquid metal feeding and by the use of external and internal chills. To produce a sound casting proper design consideration should also be kept in mind. Sharp corners should be avoided.
(a) Poor design (sharp corner)
(b) rounded edges
Gas holes: The gases absorbed by molten metal in furnace, ladle and during flow in mould, if not allowed to escape gets trapped inside the casting. Causes: High pouring temperature, abrupt bends and other turbulances. Remedies: less turbulences, proper pouring temperature and proper gating. Pin holes porosity: As molten metal solidifies it looses temperature which decreases solubility of gases and thereby expelling dissolved gases. 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. 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. In the case of aluminium, we are particularly concerned about hydrogen, as it is soluble in aluminium in all proportions. If moisture is present in the atmosphere then the water will decompose in contact with the aluminium as follows: 3 H2O + 2 Al = 3 H 2 + Al2O3 It is likely that the liquid aluminium will gain hydrogen in this environment.
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Blow holes: These are spherical, flattened or elongated cavities present inside the casting or on the surface. These defects are moisture left in mould or core. Due to heat of molten metal, the moisture is converted into steam, part of which is entrapped in casting. Remedy: Proper drying of mould. Solidification Time Total solidification time TS is the time required for casting to solidify after pouring. TS depends on size and shape of casting by relationship known Chvorinov's Rule Solidification time = C (volume/surface area) 2 Where, C is a constant that depends on mold material and thickness, metal characteristics and temperature. For a given piece of metal of spherical, cubical and cylindrical shapes with the same volume, cube will solidify first.
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AND SAF ET H EA EAL TH A ETY
As in all other manufacturing operations, safety is an important consideration, particularly because of the following factors: 1. Dust from sand and other compounds used in casting, thus requiring proper ventilation and safety equipment for the workers. 2. Fumes from molten metals and lubricants, as well as splashing of the molten metal during the transfer or pouring. 3. The presence of fuels for furnace, the control of their pressure, the proper operation of valves, etc. 4. The presence of water and moisture in crucibles, molds, and other locations, since it rapidly converts to steam, creating severe danger of explosion 5. Improper handling of fluxes, which are hygroscopic, thus absorbing moisture and creating a danger 6. Inspection of crucibles, tools, and other equipment for wear, cracks, etc. 7. The exposures in aluminium smelting include noise, alumina, coal tar pitch, fluorides and sulphur dioxide. 8. Molten Aluminum is typically handled at 1300-1450 degrees Fahrenheit to avoid premature solidification. Molten aluminum contacting any part of the human body can cause serious burns. If extensive, these burns can be fatal. Where there is possibility of splash or other direct exposure, personnel working with molten aluminum wear eye and face protection and protective clothing. 9. Bulk aluminum intended for re-melting is often cast in the form of large shapes, weighing 700 to 2000 pounds, commonly known as sows. The sow-casting process generally results in unavoidable internal shrinkage cavities, which can become reservoirs for collecting large amounts of water. Sows are also subject to surface moisture and other contaminants. 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. Therefore, operations must make every effort to avoid charging sows that contain moisture, either entrapped or surface, into molten aluminum
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L IT ER ATUR E/R EF ER ENC ES 1. Aluminium-Taschenbuch Edited by Aluminium-Zentrale. XXVI/1094 pages, 699 figures, 342 tables, 14.Edition, 1983/1988, ISBN 3-87017-169-3. 2. Metals Handbook American Society for Metals, 2nd edition, 1990 3. E.Hatch (Editor): Aluminium - Properties and Physical Metallurgy American Society for Metals, Metals Park, Ohio, 1984, ISBN 0-87179-176-6 4. ALUSELECT - The European Engineering Properity Database for Wrought Aluminium and Aluminium Alloys, 2 diskettes.EAA, KTH 1992 5. Mateds: The Materials Technology Education System, SkanAluminium, KTH 1990 Sandström, R.: Introduction to Materials Selection, KTH 1990 6. D.G. Altenpohl: Aluminum Viewed From Within - An Introduction to the Metallurgy of Aluminum Fabrication 7. Aluminium-Verlag Düsseldorf, 1982, ISBN 3-87017-138-3 8. The Properties of Aluminum and its Alloys. 9. Aluminum Federation Ltd., Birmingham 1993 10. F. King: Aluminum and its Alloys. 11. Ellis Harwood Series in Metals Materials. Ellis Harwood, Chichester, England 1987 12. Aluminum Association Inc. Registration Record of International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys. Aluminum Association, Washington DC,
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