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Magnesium melt is produced in steel crucibles. The main application is found in pressure die-casting foundries. The furnaces can be heated by induction resistance or by fuels. In recycling operations with annual throughputs of > approx. 3,000 t/p.a. and scrap dimensions of approx. 500x200x50, the inductively-heated steel crucible furnace manufactured by ABB is used successfully. These furnaces are available with capacities of approx.800 kg with 400 kW and 1,500 kg with 560 kW. Ceramically-lined magnesium furnaces have not proven popular for recycling purposes due to the high level of dross. Ceramically-lined magnesium furnaces of 7 and 10 t capacity are operated in the USA, although these are operated with 1/3 sump and charged pigs. The pigs come from Russian and Chinese production, and contain impurities which are then cleaned out in these furnaces. The furnaces have to remain in operation continuously, since the tamping compound must not be allowed to cool off.

Machine casting refers to the production of castings for the engineering industry.

Loss performance is the overall term for the difference in performance between the performance provided via the cable from the transformer to the furnace and the performance actually induced in the melt. The main types of loss performance are listed below: Switch system losses, condenser losses, performance and cable losses, coil losses, thermal furnace losses and cover losses.

These overall losses can total up to 40% of the nominal system output. Depending on the method of operation of a furnace system, these losses can be reduced to approx. 32%.

The inner surface of the induction furnace coil and the ceramic upper construction (upper concrete ring) must form a smooth surface, without any deposits, which tapers gradually to form a cone at the top. If a crucible removal device is used, the conicity should be 0.8%. When working with an outer lining with approx. 40 mm overlap at the bottom, the coil plastering should not be applied conically. In this case, the conicity of the outer lining is sufficient. The coil plastering and the outer lining are applied with a water content of 5 – 8%. This moisture leads to a long drying time, and thus also to extended starting times. In order to remove this moisture from the coil plastering and the outer lining quickly, these can be dried out with a gas burner and the tamping form. The coil plastering can be dried out at a maximum of 150 °C and the outer lining at a maximum of 350 °C in the lower area.

The drying time in both cases should be at least 24 hours, if the outer lining has been cast immediately after application of the coil plastering, the drying time should be at least 36 hours. The temperature increase should be between 30 to 50 K/h.

If no burner is available, the drying can also be carried out inductively for medium-frequency furnaces.
If neither gas nor inductive heating are to be used, the coil plastering and the outer lining should be allowed to dry out normally in air for approx. 12 hours, and then dried out for a further 24 hours with a 3 kW or 6 kW fan heater placed on the bottom of the furnace, depending on the furnace size. Some customers use resistance-heated “baskets”, which are supplied with a line through the furnace bottom. In the case of electrical heating, the heating should be carried out with the furnace cover closed.

Before applying the tamping compound, the inner surface of the coil is “papered” with 0.5 mm of Cogemikanit or similar material up to the edge of the furnace. Micanites with added flow materials are expensive, and provide hardly any heating benefits.

The furnace bottom with the bottom electrodes is now prepared for the application of the bottom compound. The tamping compound is filled in to such a level that the bottom electrodes can be tamped over to a depth of approx. 10 mm. This is usually carried out after manual ventilation using a bottom agitator, which is powered electrically or pneumatically. Agitation continues for 5 – 10 minutes depending on the furnace size. For bottom thicknesses of over 350 mm the work should be carried out in 2 layers, paying special attention to the bottom electrodes. After the bottom agitator has been removed, the bottom height from the furnace edge must be checked and recorded. Now an outer ring is well keyed around the furnace bottom up to the cone of the tamping form (at least 15 mm), in order to provide a good connection to the crucible wall. The tamping form is installed, centred and fixed in place at the crucible edge using wooden wedges. The crucible wall is filled up to the edge of the furnace in layers of 300 to 400 mm. After every layer, “Neptune zinc” ventilation is carried out to ensure better final compaction. Compaction must now be carried out very carefully in the area of the bottom cone. For the crucible wall, the self-rotating wall agitator is used. This must be installed in the lowest possible position using a crane, and agitation applied in this area for approx. 5 minutes. Agitation then continues for 2, 3 or 4 minutes for furnaces with up to 800 mm diameter and 100 mm height difference, up to 1,200 mm diameter and 125 mm height difference and over 1,200 mm diameter and 150 mm height difference respectively. Since the crucible wall will be subjected to high mechanical stresses in the upper area, the greatest care must be exercised here. The upper 200 mm are compacted by hand after removal of the wooden wedges. Die tamping compound should finish approx. 50 – 70 mm below the furnace edge. Tamping compound should be added continually during agitation to avoid the formation of layers between the compacted compound and the added compound. A thin patch layer is then applied to the ring of the crucible wall to prevent the tamping compound trickling out at the 1st tapping. The compound is not scraped out of the casting spout. In the case of disposable and permanent templates, the casting spout should also be filled with compound in order to achieve a higher filling level during sintering. In the case of a disposable template, the compound is removed from the casting spout following completion of the sintering process, and the area dried with a welding burner. After approx. 10 minutes, this part of the tamping form becomes so soft that a “breakthrough” can be made to the casting spout using a crowbar. The iron now flows into the casting spout and the furnace can be tipped for the 1st time.

When using a permanent template, this must be provided with a supporting film of micanite or with a separating agent prior to installation. There are conical complete templates available, as well as folding templates.
In the case of the conical complete template, this is heated up to approx. 450 °C after agitation at a rate of approx. 150 K/h, and then cooled down after 1 hour to 200 °C with compressed air or cooling fans. To remove it, the template must be pulled directly upward in a single movement. The folding permanent template is not heated, and is removed by folding in one of the 120° segments. Following removal of the templates, an “impact protection” layer of recycled material is applied to the furnace bottom. The height of this layer should be 200 – 300 mm, depending on the furnace size. The crucible must now be filled up to the casting spout as quickly as possible. No power should be switched to the furnace during this time. Once the maximum filling level has been reached, power is switched to the furnace, providing a temperature increase of approx. 100 K/h, i.e. about 40 kW/t, or 200 kW for a 5-t furnace. At this output, the melt is brought up to sintering temperature and maintained at this level for approx. 2 hours. The 1st tapping is then carried out, and a total of 3 charges should be processed.

Lining is also the designation for the melting crucible or wear crucible without the concrete rings or fixed ceramic components. Nowadays dry tamping compounds are generally used. Damp compounds are sometimes still used in aluminium and zinc furnaces. There are 3 different types of material: acidic, basic and neutral compounds. Mixtures of the 3 above types of material are also used for certain applications.

Acidic compounds contain approx. 98.5% SiO2
Neutral compounds contain approx. 84.5% Al2O3 and up to 13% MgO
Basic magnetic compounds contain approx. 88.0% MgO, up to 10% Al2O3 and approx. 2% SiO2. The application temperatures of acidic compounds are normally in the area of 1,600 °C and can reach up to a maximum of 1,700 °C for short periods.

For neutral compounds, the normal application temperatures are in the area of 1,650 °C and can reach up to a maximum of 1,750 °C for short periods.
Basic magnetic compounds have a normal application temperature of 1,650 °C and a maximum of 1,800 °C for short periods.

In a 1-t crucible furnace with 1,000 kW for example, the maximum temperature can be reached within 2 minutes, after which the furnace must be emptied immediately and re-charged with scrap. The furnace is now brought up to temperature for approx. 3 minutes at high output, so that the crucible is not cooled down too much from the extremely high temperature required. With this procedure, charge numbers of from 40 to 70 can be achieved, depending on the method of operation, compound and charge materials.

Dry compounds are usually supplied in 25 kg sacks or disposable containers weighing up to 1,600 kg. The compounds are supplied ready for use with sintering agents, and do not need to be mixed. Acidic compounds containing boric anhydride contain no crystalline water, so that these can be used with temperature increases of up to 150 K/h. High-alumina and magnesitic compounds are specially mixed and supplied by manufacturers for individual applications.

See also

• Fireproof – the material property a lining must meet
• Furnace lining – general term for a furnace’s refractory lining

Intermediate layers are used as winding insulation between electrical conductors. The HGW material is cut to the external coil diameter. For small furnaces 1 – 100 mm, for furnaces from 1 – 30 t |= 150mm, for furnaces over 30 t |= 200/250 mm.

The intermediate layers are glued in offset by 1/3 of their length, in order to an open design of the coil for the expulsion of humidity. Ground intermediate layers are of very accurate dimensions and have optimum adhesion properties. Underground material is in general 15/100 mm over size, thus changing the coil length, and the adhesion properties are very poor.

Iron piles or yokes are used for the return and direction of the magnetic flux outside the furnace coil. The iron piles are physically hold the furnace coil. The construction of large crucible induction furnaces would be impossible without these iron piles. Iron piles are used with operating frequencies of up to approx. 2,000 Hz. The loss performance is between 0.35 W/kg and 1.5 W/kg of iron. The iron piles are used with a minimum projection over the coil length/height of 100 mm or the distance between the melt and the coil diameter. The iron piles are very important in the design, and the dimensions depend on the magnetic load and the mechanical requirements with regard to stability. In the case of high electrical outputs and medium frequency, the iron piles are water-cooled. Iron piles of crucible furnaces are made of ungrained plate.

Insulation in induction furnace construction refers either to electrical or heat insulation. The insulation materials are 100% asbestos-free. For electrical applications, epoxy glass hard fibres (Diverrit E), micanite and ceramic-organic pressed materials are used, while thermal applications make use of ceramic fibre materials and pressed materials such as Isoplan, Nevalit and similar materials. Silica-glass materials are used for both applications.

Inoculation is the introduction of nucleating agents, usually based on ferrosilicon, into an iron melt. It increases the number of crystallization nuclei and thereby controls graphite formation during solidification. The chemical composition of the melt changes only slightly; this is what distinguishes inoculation from alloying. The inoculants are usually introduced into the casting stream in powder form during casting. If a very large quantity of inoculant is needed because of special requirements, the use of wire injection devices is recommended. Inoculation prior to the filling of the iron into a heated casting unit can lead to increased formation of deposits in the channel inductor or the inductor neck.

References

1. Herbert H. Netzel: Induktionsofenhandbuch. 3rd edition, Eigenverlag IES, Witten 2003, p. 80.

Ingot moulds are permanent moulds for the production of die-cast products. Ingot moulds are usually water-cooled, and are made of graphite or metallic materials.