The Process of Mining Bauxite
Bauxite is a mineral that contains varying amounts of combined water and several impurities of which ferric oxide and silica are usually predominant. It is found in a belt around the equator and is generally extracted by open-cast mining. Before mining can commence the land needs to be cleared of timber and vegetation.
- Milling: Several meters of rock and clay are removed by blasting, drilling, and bulldozers. The bauxite is then transported to the crushing and washing area, reducing the particle size and increasing the available surface area for the digestion stage. The material that is discharged from the mill is called slurry
- Digestion: The slurry is pumped to a digester where the chemical reaction to dissolve the alumina takes place. A hot caustic soda solution is used to dissolve the aluminum-bearing minerals in the bauxite to form a sodium aluminate supersaturated solution. The slurry is pumped into a series of flash tanks to reduce the pressure and heat before it is transferred into settling tanks.
- Clarification/Settling: The first step is to separate the solids from the pregnant liquor, this is done by filter cake. Chemical additives are added to assist the sedimentation process. The bauxite residue sinks to the bottom of the settling tanks, then transferred to the washing tanks, where it undergoes a series of washing to recover the caustic soda. Further separation of the pregnant liquor from the bauxite residue is performed utilizing a series of security filters. The purpose of the security filters is to ensure that the final product is not contaminated with impurities present in the residue. Then it is cooled and pumped to the precipitators.
- Precipitation: Bauxite is recovered by crystallization from the pregnant liquor, which is supersaturated in sodium aluminate. Seed crystals are added to start the precipitators of alumina particles as the liquor cools. The alumina crystals begin to grow around the seeds, then settle to the bottom of the tank where they are removed and transferred to thickening tanks.
- Classification: The crystals formed in precipitation are classified into size ranges. This is normally done using cyclones or gravity classification tanks. The coarse size crystals are destined for calcination after being separated from spent liquor utilizing vacuum filtration, where the solids are washed with hot water.
- Calcination: Calcination is a heating process to remove the chemically combined water from alumina hydrate. The hydrate is filtered and washed to rinse away impurities and remove moisture. The calcining kiln is brick-lined inside and gas-fired to a temperature of 1,100°C. It slowly rotates and is mounted on a tilted foundation which allows the alumina to move slowly through it to cooling equipment. Alumina is formed into a white powder, this is the final product of the Bayer Process, ready for shipment to aluminum smelters.
Smelting – Extracting and refining aluminum
Alumina is dissolved in reduction pots; these are deep rectangular steel shells lined with carbon or graphite. The steel shells are filled with a molten electrolyte consisting mostly of a compound of sodium, aluminum, and fluorine, called cryolite. Carbon anodes are immersed into the electrolyte carrying electrical current which then flows into the molten cryolite containing dissolved alumina. The chemical bond between aluminum and oxygen in the alumina is broken, the aluminum is deposited in the bottom of the cell, while the oxygen reacts with the carbon producing carbon dioxide bubbles. Once passed through, the electrical current flows into the molten aluminum deposit and is then collected by the bottom of the pot, called cathode.
The molten aluminum is siphoned from the cells into large crucibles. From there the aluminum is poured directly into molds to produce foundry ingot. It then can be transferred to furnaces for further refining or for alloying.
Liquid steel is poured into the ladle and the ladle is then taken to Continuous Casting Machine. Continuous casting is the process whereby molten metal is solidified into a "semi-finished" billet, bloom, or slab. Billets are processed by hot rolling them through plain or grooved cylindrical rotating rolls to produce plates, sheets, rods, structural sections, and tubes etc. Billets are then packaged and ready for distribution.
Step 1. Billet Pre-heating
Billets are first cut to the desired length based on the product specifications. Then billets are moved to a tunnel heater and must be heated to approximately 800-925° F. The exact temperature is based on what the make-up of the aluminum is.
Step 2. Extrusion
After a billet reaches the desired temperature, it is transferred to the loader where a thin film of smut or lubricant is added to the billet and to the ram, to prevent sticking together. Then the billet is transferred to the cradle.
A set of dies that match the customer’s specifications are placed in line. The ram then applies pressure to the dummy block which, in turn, pushes the billet until it is inside the container. Under pressure the billet is crushed against the die, becoming shorter and wider until it has full contact with the container walls. The billet is forced through these dies using a ram and anywhere from 1600 to 6000 tons of pressure. While the aluminum is pushed through the die, liquid nitrogen flows around some sections of the die to cool it. This increases the life of the die and creates an inert atmosphere which keeps oxides from forming on the shape being extruded.
As an extrusion exits the press, the temperature is taken with a True Temperature Technology (3T) instrument mounted on the press platen. The main purpose of knowing the temperature is to maintain maximum press speeds. The target exit temperature for an extrusion is dependent upon the alloy.
Step 3. Cooling
Not all of the billet can be used. The remainder (butt) contains oxides from the billet skin. The butt is sheared off and discarded while another billet is loaded and welded to a previously loaded billet and the extrusion process continues.
Step 4. Stretching and Cutting
After the aluminum has cooled and moved along the cooling table, it is then moved to the stretcher. Stretching straightens the extrusions, increases hardness and improves the aluminum strength.
After extrusions have been stretched they are transferred to a saw table and cut to specific lengths.
Step 5: Aging
The extrusions are loaded on a transportation device and moved into age ovens. Heat-treating hardens the metal by speeding the aging process in a controlled temperature environment for a set amount of time.