Hall-Heroult Process. • Electrochemical Process to Reduce Alumina to Aluminum . – Alumina is dissolved in a molten fluoride solvent called cryolite. 2 Al. 2. O. Aluminium is produced according with the Hall-Heroult process. During this complex electrolyte two-phase electrolysis, it is difficult to model the current. Hall-Héroult process: metallurgy: Electrolytic smelting: In the Hall-Héroult smelting process, a nearly pure aluminum oxide compound called alumina is dissolved.
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It involves dissolving aluminium oxide alumina obtained most often from bauxitealuminium ‘s chief ore, through the Bayer process in molten cryoliteand electrolysing the molten salt bath, typically in a purpose-built cell. Recycled aluminum requires no electrolysis, thus it does not end up in this process.
Elemental aluminium cannot be produced by the electrolysis of an aqueous aluminium salt because hydronium ions readily oxidize hsroult aluminium. In reality much more CO 2 is formed at the anode than CO:. Besides having a relatively low melting point, cryolite is used as an electrolyte because among other things it also dissolves alumina well, conducts electricity, dissociates electrolytically at higher voltage than alumina and has a lighter density than aluminum at the temperatures required by the electrolysis.
Aluminium fluoride AlF 3 is usually added to the electrolyte. The density of liquid aluminum is 2. The density of the electrolyte should be less than 2. In addition to AlF 3other additives like lithium fluoride may be added to alter different properties melting point, density, conductivity etc. This causes liquid aluminium metal to be deposited at the cathode while the oxygen from the alumina combines with carbon from the anode to hwll mostly carbon dioxide.
Cells in factories are operated 24 hours a day so that the molten material in them will not solidify. Temperature within the cell is maintained via electrical prkcess. Oxidation of the carbon anode increases the electrical efficiency at a cost procews consuming heroultt carbon electrodes and producing carbon dioxide. Herult aluminium sinks to the bottom of the electrolytic cell, where it is periodically collected.
The liquid aluminium is removed from the cell via a siphon every 1 to 3 days in order to avoid having to use extremely high temperature valves and pumps. Alumina is added to the cells as the aluminum is removed.
Collected aluminium from different cells in a factory is finally melted together to ensure uniform product and made into e. The electrolytic mixture is sprinkled with coke to prevent the anode’s oxidation by the oxygen evolved. The cell produces gases at the anode.
Hall-Héroult process | industrial process |
The exhaust is primarily CO 2 produced from the anode consumption and hydrogen fluoride HF from the cryolite and flux AlF 3. In modern facilities fluorides are almost completely recycled to the cells and therefore used again in the electrolysis. Escaped HF can be neutralized to its sodium salt, sodium fluoride. Particulates are captured using electrostatic or bag filters.
The CO 2 is usually vented into the atmosphere.
Agitation of the molten material heoult the cell increases its production rate at the expense of an increase in cryolite impurities in the product.
Properly designed cells can leverage magnetohydrodynamic forces induced by the electrolysing current to agitate the electrolyte. In non-agitating static pool procesx the impurities either rise to the top of the metallic aluminium, or else sink to the bottom, leaving high-purity aluminium in the middle area. Electrodes in cells are mostly coke which has been purified at high temperatures.
Pitch resin or tar is used as a binder. The materials most often used in anodes, coke and pitch resin, are mainly residues from petroleum industry and need to be of high enough purity so no impurities end up into the molten aluminum or the electrolyte.
The anode is in a frame and as the bottom of the anode turns mainly into CO 2 during the electrolysis the anode loses mass and being amorphous it slowly sinks hsll its frame.
More material to the top of the anode is continuously added in the form of briquettes made from coke and pitch. The lost heat from the smelting operation is used to bake the briquettes into the carbon form required for reaction with alumina. Prebake technology is named after its anodes, which are baked in very large gas-fired ovens at high temperature before being lowered by various heavy industrial lifting systems into the electrolytic solution.
There usually are 24 prebaked anodes in two rows in one cell. Each anode is lowered vertically and individually by a computer as the bottom surfaces of the anodes are eaten away during the electrolysis.
Prebake technology also has much lower risk of the anode effect see below happening, but cells using it are more expensive to build and labor-intensive to use as each prebaked anode in a cell needs to be removed and replaced once it has been used. Alumina is added to the electrolyte from between the anodes in prebake technology using cells. The remains of prebaked anodes are used to make more new prebaked anodes. Prebaked anodes either are made in the same factory where electrolysis happens or are brought there from elsewhere.
The inside of the cell’s bath is lined with cathode made from coke and pitch. Cathodes also degrade during electrolysis, but much more slowly than anodes do, and thus they need neither be as high in purity nor be maintained as often. Cathodes are typically replaced every 2—6 years. This requires the whole cell to be shut down. Anode effect is a situation where too many gas bubbles form to the bottom of the anode and join together forming a layer.
This increases the resistance of the cell when smaller areas of the electrolyte touch the anode. These areas of the electrolyte and anode heat up when the density of the electric current of the cell focuses to go through only them. This heats up the gas layer and causes it to expand thus further reducing the surface area where electrolyte and anode are in contact with each other.
Anode effect decreases the energy-efficiency and the aluminium production of the cell. It also induces the formation of tetrafluoromethane CF 4 in significant quantities, increased formation of CO and to lesser extent also causes the formation of hexafluoroethane C 2 F 6.
Aluminium is the most abundant metallic element in the Earth’s crust, but it is rarely found in its elemental state. It occurs in many minerals but its primary commercial source is bauxitea mixture of hydrated aluminium oxides and compounds of other elements such as iron.
The heeroult was complicated and consumed materials that were in themselves expensive at that time. This meant the cost to produce the small amount of aluminium made in the early 19th century was very high, higher than for gold or platinum.
Bars of aluminium were exhibited alongside the French crown jewels at the Exposition Universelle ofand Emperor Napoleon III of France was said to have reserved his few sets of aluminium dinner plates and eating utensils for his most honored guests.
Some authors claim Hall was assisted by his sister Julia Brainerd Hall ;  however, the extent to which she was involved has been disputed. It later became the Alcoa corporation. This in turn helped make it possible for pioneers like Hugo Junkers to utilize aluminium and aluminium-magnesium alloys to make items like metal airplanes by the thousands, or Howard Lund to make aluminium fishing boats.
The Hall-Héroult Process Basics – The Aluminum Smelting Process