To understand what the aluminum casting process is we must first look at the properties of aluminum. Aluminum is the most abundant metallic element in the earth's crust, of which it forms nearly 8%. It always occurs as a compound, some of its minerals being bauxite, cryolite, corundum, alunite, diaspore, turquoise, spinel, and such silicates as kaolin, feldspar, and mica. Bauxite a ferruginous aluminum hydroxide, is the basic raw material from which the metal aluminum is produced.
Impurities are removed from the bauxite by chemical processing to make alumina (aluminum oxide). The bauxite is crushed, mixed in a caustic soda (sodium hydroxide) solution, and then heated by steam under pressure to dissolve the alumina. Settling and filtration remove impurities. After cooling, the solution is agitated, and crystals of hydrated alumina are formed. These crystals are washed and then dried in long rotating kilns. The final product, alumina or aluminum oxide is a very hard crystalline substance having the appearance of refined sugar. Four pounds of bauxite yields approximately two pounds of alumina.
Electrolyzing a solution of alumina in molten fluorides makes aluminum; this electrolysis takes place in reduction pots or cells at a reduction plant. Aluminum reduction plants produce pure, high quality primary aluminum. The reduction process removes the oxygen from the alumina, which consists of almost equal parts of aluminum and oxygen and leaves pure aluminum. Two kg of alumina yields one kg of aluminum.
Characteristics of Aluminum
Aluminum is lightweight, has excellent strength, high thermal and electrical conductivity, high reflectivity, good corrosion resistance, excellent workability, and attractive appearance. It can be given almost any finish. It is nonmagnetic, nontoxic, and non- sparking.
Aluminum weighs 32.523887897 kg/dm3 (kilogram / cubic decimeter), approximately 1/3 the density of steel, copper, and brass. Some of the stronger aluminum alloys exceed the strength of mild steel. The melting point of aluminum is 758 degrees Celcius.
The high thermal conductivity of aluminum is a marked advantage in any application where it is desirable to conduct or dissipate heat quickly and uniformly. On a weight basis, aluminum is the most efficient heat conductor of the common metals.
The metal casting industry is one of the most energy-intensive manufacturing sectors with the melting process accounting for over half (55%) of its energy consumption. Although its high energy expenses have been a significant concern for metal casters, the industry continues to use melting technologies with poor energy efficiency.
The seemingly simple melting operation – heating metals to turn them into liquids for pouring – is actually a very complex process, involving a series of steps that incur both material and energy losses.
These losses are attributable to several factors: undesired conduction, radiation and convection, stack loss (flue gases), and metal loss. The extent of the losses depends on the furnace design, the fuel used, and the method of imparting heat to the metals.
The low thermal efficiency of current furnaces calls for high-priority actions to improve melting technologies.
The melting of any industrial metal used in manufacturing involves the following steps:
- Preparing the Metal and Loading – removing dirt and moisture and sometimes, preheating the charge material, such as scrap metal or ingot; and introducing solid charge into the furnace system
- Melting the Metal – supplying energy from combustion of fuels, electricity or other sources to raise the metal temperature above its melting point to a pouring temperature
- Refining and Treating Molten Metals – introducing elements or materials to purify, adjust molten bath composition to provide a specific alloy chemistry and/or affect nucleation and growth during solidification
- Holding Molten Metal – maintaining the molten metal in molten state until it is ready for tapping
- Tapping Molten Metal – transferring the molten metal from the furnace to transport ladle
- Transporting Molten Metal – moving the molten metal to the point of use and keeping the metal in molten state until it is completely poured
Material and energy losses during these process steps represent inefficiencies that waste energy and increase the costs of melting operations. Modifying the design and/or operation of any step in the melting process may affect the subsequent steps.
The stack furnaces are receiving more attention in recent years due to their higher energy efficiency than of the reverberatory furnaces. It can be considered as a modified reverberatory where its efficiency is improved by better sealing of the furnace and the use of the flue gases to preheat the charge materials. The charge materials slide down the shaft and reach the melting zone where they are melted by the burners, and the molten metal flows down to the holding area. The hot exhaust gases from the melting zone flow through the shaft to preheat the incoming charge, improving the energy efficiency of the stack furnace by 40 to 70%.
The main disadvantage of the stack furnace was that due to the charging mechanism, the height of the stack furnace was too high. with our ECO furnace we have solved this problem and brought this technology into dimensions that fits every workshop and fit the direct feeding of one or two diecasting machines without any transport of liquid metal!