Fuzzy Control Applied to Aluminum Smelting
G. Vanilson, Oliveira Júnior, M. Fbio · InTech eBooks · 2012
Applications 254 20 smelters are responsible for 65% or world production.Most companies work only with Aluminum, but 20% of them work with Aluminum with other metals or mines.Half of Aluminum production is done by companies vertically integrated, from bauxite mining to metal recycling (IAI, 2010).For all these reasons, the Aluminum can be considered a highly important metal, and therefore, its production is a target for many research activities.Researchers all around the world make efforts in making Aluminum production a less costly process, since it spends a lot of energy, and is very complex.In this chapter, we are going to present the whole context, and why and where fuzzy control is important to assist plant operators.The impact and consequences of this work is the use of rules defined by process operators indirectly through the huge database which provides historic information including control decisions made by them.Since this strategy emulates process operator, it can be said that an expert system can provide this personnel more time to concentrate on other activities.Moreover, this technique will be continually improved by revising its rules and evaluation, provided that fuzzy decisions will also have an impact, and this should be analysed and adjusted. Aluminum production processAluminum has been produced through the Hall-Héroult process, named after its inventors.So far, this is the only industrial way to produce this metal.Primary Aluminum is produced in a liquid form, through an electrolytic reduction of alumina (Al 2 O 3 ) in a cryolite bath (Na 3 AlF 6 ).This reaction takes place in electrolytic pots, as shown in Figure 2. Fig. 2. Sketch of an Alumina reduction pot of prebake type (adapted from Kola & Store, 2009). www.intechopen.comFuzzy Control Applied to Aluminum Smelting 255 Inside these pots, also often called cells, Alumina is fed through silo and it is electrically consumed by the carbon anodes (Solheim, 2005), and as shown in the equation ( 1), the anode is also consumed during the electrolytic process.At the bottom part of the cell, there is a thermal isolated steel covering made of refractory material, named cathode block.The liquid Aluminum is formed above the cathode, and under the anode the electrolytic bath is formed.The cathode, in an electrochemical sense, is an interface between liquid Aluminum and the electrolytic bath, according to equation (2).AlF Na e Al NaFThe full reaction inside the reduction pot is shown in equation ( 3).Al O C Al CO 23 2 13 3 24 4 (3)The pure electrolytic bath, i.e. cryolite, has a melting point at 1,011ºC.In order to lower this point, called liquidus temperature, some additives are added into the bath, from which the main are Aluminum Fluoride (AlF 3 ) and Calcium Fluoride (CaF 2 ).The chemical composition of the bath in the reduction pot is 6-13% of AlF 3 , 4-6% of CaF 2 and 2-4% of Al 2 O 3 .With a low liquidus temperature, pot operation is performed with low bath temperature, allowing reducing alumina solubility inside the bath.Therefore a good alumina concentration control system is required.Usually an aluminum reduction pot is operated under temperatures from 940ºC to 970ºC.