Minimising the energy consumed during inductive melting of metal alloys is an ongoing concern for plant and equipment manufacturers and operators. Surprisingly enough, little thought is given to the potential provided by the enthalpy of the melt in question. It is demonstrated that the composition of the input materials has a considerable influence on the heat content of alloy melts and therefore on their energy consumption. Enthalpies for different input materials can be derived from tabular thermochemical data and these can be used in inductive melting. This particularly applies to the production of cast iron melts made from scrap steel and various silicon carriers and also to brass melts made from copper and zinc as feed components compared to input materials made of brass.
The heat content (i.e. the specific enthalpy of the melt) is assumed when calculating the energy consumed during inductive melting of metals. For example, it is 385kWh/t for melting one ton of cast iron to 1 480°C. The end energy requirement is then determined by the efficiency of the system and any technical process action taken until the melt is tapped. Plant constructors and operators constantly work to improve furnace efficiency and the process engineering in order to minimise the energy consumed. The potential here can be recognised from the relatively broad spectrum of consumption values in iron foundries, which range from 570kWh/t to over 700kWh/t.
At first glance, the specific enthalpy value may not seem to be amenable to influence. In what follows, it is shown that an appropriate selection of input materials drawing on thermochemical data can also be used to optimise this baseline value for the energy required to melt metal alloys. In this context, the feeding and melting processes deployed in modern, convertor-fed induction furnaces with weight-controlled loading of the charging vehicle and the tapping of the entire melt charge are important factors in the targeted adjustment of the composition and sequence of the iron materials and additives to be melted.
Enthalpy of metal alloys as a heat balance
The enthalpy of alloy melts is made up of the specific heats of each of the materials in solid and liquid states, their heats of transformation and of melting and also of exothermic or endothermic dissolving reactions. There have been tabular compilations listing thermochemical data since the 1970s, firstly for the pure materials, and then also for alloys, which have enabled a quantitative description of technically relevant processes. This applies just as much to stoichiometric reactions as to heat balances. The energy required to melt alloys can thus also be calculated from the tabular enthalpy values, in that a heat balance is created in order to produce the alloy as a product made of different components.
The heat content of cast iron melts greatly depends on the composition of the input materials. One of the main influencing factors is the energy required to dissolve silicon and carbon in the iron melt. Silicon releases energy, whilst energy is required for carbon.
The potential presented by enthalpy optimisation can also be exploited when melting aluminium and copper alloys. Specific enthalpies of some significant alloys, referred to the respective melt temperature and the associated overheating enthalpies, which were melted from recycled material on the one hand and from the single components on the other.
In the attempts to minimise energy consumption during inductive melting of metal alloys, account also needs to be taken of the potential offered by the specific enthalpy of the alloy melts. Tabular thermochemical data demonstrate that the composition of the input materials exercises a considerable influence on the energy requirement. The main factors here are primarily endothermic and exothermic processes of transformation and dissolution in the alloying components, as is shown by the enthalpy values determined for cast iron melts. Siliconising with FeSi instead of with SiC produces an energy benefit of up to 25kWh/t here. It is also recommended to make use of the available thermochemical data in melting practice for aluminium and copper alloys. For example, the energy requirement when melting brass from the components Cu and Zn is up to 40kWh/t lower than if brass materials are fed in.
ABP supplies coreless and channel induction furnaces for ferrous and non-ferrous foundries. ABP is known for its industry leading productivity, reliability, safety and efficiency. ABP offer furnaces with capacities from 5 to 120 tons. With a modular architecture ABP can configure a system to meet your requirements quickly and efficiently. ABP’s Prodapt advanced melt processor calculates the energy requirements according to the furnace content and automatically controls the energy supply for melting and holding operations. ABP offers furnaces with Twin-Power – two furnaces, connected to one that distributes converter power freely between both furnaces. While it holds molten iron in one furnace it simultaneously melts in the other.
This is an extract from a technology report compiled by Dr.-Ing. Erwin Dötsch of ABP Induction Systems GmbH. For further details contact ABP Induction Furnaces on TEL: 011 623 1814/17 or cell number 072 158 1117 or email firstname.lastname@example.org. You can also visit www.abpinduction.com