Casting in metal industry

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General description

The process can bedivided into the following major activities:

  • melting and metal treatment: the melting shop
  • preparation of moulds: the moulding shop
  • casting of the molten metal into the mould, cooling for solidification and removing the

casting from the mould: the casting shop

  • finishing of the raw casting: the finishing shop.


Foundry process.PNG

Figure 1: The foundry process


Starting from foundry scrap (selected scrap according to a certain chemical composition) or ingots, the foundry produces finished castings. Usually these are components which will require further treatment or assembly to yield a final product. On the moulding side, a basic distinction is made between permanent and lost moulds. Foundries casting in permanent moulds, buy these metal moulds (dies) externally, but typically operate an in-house mould repair and maintenance shop. Foundries casting in lost moulds, often buy wooden, metal or plastic patterns (for their mould design) and operate an in-house pattern maintenance and repair shop. Moulds, cores and lost models are generally produced as part of the foundry process.

Traditionally in the foundry sector the main distinction made is between ferrous and non-ferrous foundries. This is mainly because the applied processes in both sectors differ. Non-ferrous foundries often apply die-casting techniques. These allow a better surface finish, which is important for many of the aluminium and brass applications. Due to the high cooling rate, castings with a high mechanical strength are produced. However, this technique does not allow the production of massive or large pieces, which require sand casting techniques. Sand casting techniques are applied in non-ferrous foundries for those products that are not produced in large series. The non-ferrous metals (and their alloys) discussed in this document are:

  • aluminium
  • magnesium
  • copper
  • zinc
  • lead.

Ferrous foundries generally apply the lost mould techniques. Due to their greater stiffness and strength, ferrous alloys are used in different applications to non-ferrous alloys. The size of the products that can be produced is almost unlimited. Ferrous metals have a higher melting point and therefore require different melting techniques. The ferrous metals and alloys discussed in this document are the various types of cast iron (which may be classified according to their properties or by the graphite type) and cast steel.

Superalloys with a high content of alloying elements, such as nickel, will also be discussed. Foundries utilise mechanisation and automation depending on the need for reproductivity and on the series sizes. The most flexible installation is typically the jobbing foundry. This produces a variety of products in small numbers (<100). In general, this type of foundry applies manual moulding techniques with resin-bonded sand moulds. The melting furnace works batch wise to allow an easy change of alloy. This implies the use of induction or rotary furnaces. For medium-sized series (<1000 parts), mechanised moulding and casting lines are used. Lost mould foundries utilise mould making machines. This implies the use of green sand, which allows fast mould making. The size of the mould making machine limits the maximum size of the castings. Casting can be performed manually or by using a pouring machine. Auxiliary side processes, such as sand preparation, are operated in a semi-automated way with remote control. Both continuous furnaces (cupola, shaft) and batch furnaces are used. For non-ferrous alloys, die-casting techniques are applied.

Large series of small castings are made in flaskless green sand moulding. For specific applications, die-casting also can be used in ferrous foundries if the final casting quality requires it, although in reality the technique finds only limited implementation. The main difference for medium-sized series is the further automation of the finishing, the quality control and the mould assembly. For die-casting in non-ferrous alloy facilities, further automation is often applied, this is especially the case in pressure die-casting shops.

Specific casting techniques, such as full mould casting, centrifugal casting and continuous casting are applied where the product type requires it.


Mass stream overview for the foundry process

Mass stream foundry.PNG


Best available technologies (BATs)

Ferrous metal melting

Furnace selection

Steel is melted in both electric arc furnaces (EAF) and induction furnaces (IF). The choice between furnace types is based on technical criteria (e.g. capacity, steel grade). Due to its refining ability, the EAF allows the melting of lower grade scrap. This is an advantage in terms of the recycling of metals, but requires an appropriate flue-gas capture and cleaning system, as will be presented below.

For cast iron melting: cupola, electric arc, induction and rotary furnaces are applicable. The selection will be based on technical and economic criteria. For the operation of any selected furnaces type, elements of BAT are given below.


Cupola furnace melting of cast iron

For the operation of cupola furnaces, BAT is all of the following, to:

  • use divided blast operation (2 rows of tuyères) for cold blast cupolas
  • use oxygen enrichment of the blast air, in a continuous or intermittent way, with oxygen levels between 22 and 25 % (i.e. 1 % - 4 % enrichment)
  • minimise the blast-off periods for hot blast cupolas by applying continuous blowing or long campaign operation. Depending on the requirements of the moulding and casting line, duplex operations must be considered
  • apply good melting practice measures for the furnace operation
  • use coke with known properties and of a controlled quality
  • clean furnace off-gas by subsequent collection, cooling and dedusting using a combination of the techniques. BAT for dedusting is to use a bag filter or wet scrubber.
  • apply post combustion in the cupola shaft of CBC, if the off-gases can burn autothermally and then to recover the heat for internal use. For HBC, use a separate combustion chamber, and recover the heat for blast air preheating and other internal uses
  • evaluate the possibility of waste heat utilisation from holding furnaces in duplex configuration and to implement heat recovery if applicable
  • prevent and minimise dioxins and furan emissions to a level below 0.1ngTEQ/Nm³, using a combination of measures. In some cases this may result in a preference for wet scrubbing. Industry has expressed doubts on the implementation of secondary measures that have only been proven in other sectors and in particular questions the applicability for smaller foundries
  • use a wet scrubber system when melting with basic slag (basicity up to 2). Residues produced by cupola melting include dust, slags and coke breeze.

BAT for residue management is all of the following, to:

  • minimise slag forming using one or more of the in-process measures
  • pretreat the slags in order to allow external re-use
  • collect and recycle coke breeze.


Electric arc furnace melting of steel and cast iron

For the operation of electric arc furnaces, BAT is all of the following, to:

  • apply reliable and efficient process controls to shorten the melting and treatment time
  • use the foamy slag practice
  • capture furnace off-gas using one of the techniques
  • cool the furnace off-gas and dedust using a bag filter. Residues produced by EAF melting include dust and slags.

BAT for residue management is to:

  • recycle filter dust into the EAF furnace.


Induction furnace melting of cast iron and steel

For the operation of induction furnaces, BAT is all of the following, to:

  • melt clean scrap, avoiding rusty and dirty inputs and adhering sand
  • use good practice measures for the charging and operation
  • use medium frequency power, and when installing a new furnace, to change any mains frequency furnace to medium frequency
  • evaluate the possibility of waste heat recuperation and to implement a heat recovery system if applicable
  • use a hood, lip extraction or cover extraction on each induction furnace to capture the furnace off-gas and to maximise off-gas collection during the full working cycle
  • use dry flue-gas cleaning)
  • keep dust emissions below 0.2 kg/tonne molten iron.


Rotary furnace melting of cast iron

For the operation of rotary furnaces, BAT is all of the following, to:

  • implement measures to optimise furnace yield
  • use an oxyburner
  • collect the off-gas close to the furnace exit, apply post combustion, cool it using a heatexchanger and then to apply dry dedusting
  • prevent and minimise dioxins and furan emissions to a level below 0.1ngTEQ/Nm³, using a combination of measures. In some cases this may result in a preference for wet scrubbing. Industry has expressed doubts on the implementation of secondary measures that have only been proven in other sectors and in particular questions the applicability for smaller foundries.


Ferrous metal treatment

If an AOD converter is used for steel refining, BAT is to

  • extract and collect the exhaust gas using a roof canopy. For the production of nodular iron, nodularisation is performed. BAT for nodularisation is to:
  • select a nodularisation technique with no off-gas production or to capture the produced MgO smoke, using a lid or cover equipped with extraction equipment or by using a fixed or movable hood, and to
  • dedust the exhaust gas using a bag filter and to make the dust available for recycling.



Non-ferrous metal melting

For non-ferrous metals, this document (only) considers the melting of ingots and internal scrap, since this is standard practice in non-ferrous foundries.

For aluminium melting, multiple furnace types apply. The selection of the furnace type is based on technical criteria (e.g. regime, capacity, type of casting line). One foundry can use several furnace types. Operational practice and logical reasoning shows that centralised melting in higher capacity furnaces has a more favourable energy efficiency than small crucible melting for large-scale plants. No data are available however to select it as BAT.

For the melting of copper, lead and zinc and their alloys, induction or crucible furnaces are used. For copper alloys, hearth type furnaces are used as well. The selection depends on technical criteria.

For magnesium melting, only crucible furnaces are used. A cover gas is used to prevent oxidation.

Induction furnace melting of aluminium, copper, lead and zinc

For the operation of induction furnaces, BAT is all of the following, to:

  • use good practice measures for the charging and operation
  • use medium frequency power, and when installing a new furnace, to change any mains frequency furnace to medium frequency
  • evaluate the possibility of waste heat recuperation and to implement a heat recovery system if applicable
  • minimise emissions in accordance to the associated emission levels stated below, and if needed to collect the furnace off-gas maximising off-gas collection during the full working cycle, and to apply dry dedusting.


Rotary furnace melting of aluminium

For the operation of rotary furnaces, BAT is to:

  • implement measures to optimise the furnace yield
  • collect the off-gas close to the furnace exit and to evacuate it through a stack, taking into account the BAT associated emission levels given below.


Hearth type furnace melting of aluminium and copper

For the operation of hearth type furnaces, BAT is all of the following, to:

  • collect the furnace off-gas and to evacuate it through a stack, taking into account the BAT associated emission levels given below
  • capture fugitive and visible emissions
Shaft furnace melting of aluminium

For the operation of shaft furnaces, BAT is all of the following, to:

  • allow efficient exhaust collection upon tilting of the furnace, and to evacuate the exhaust gas through a stack, taking into account the BAT associated emission levels given below. Radiant roof furnace holding of aluminium. For the operation of radiant roof furnaces, BAT is all of the following, to:
  • follow the BAT elements for fugitive emissions


Crucible melting and holding of aluminium, copper, lead and zinc

For the operation of crucible furnaces, BAT is to:

  • follow the BAT elements for fugitive emissions and apply hooding.


Degassing and cleaning of aluminium

BAT for the degassing and cleaning of aluminium is to:

  • use a mobile or fixed impeller unit with Ar/Cl2 or N2/Cl2 gas.


Melting of magnesium

For the melting of magnesium, BAT is to:

  • use SO2 as a cover gas or to replace SF6 by SO2 as a cover gas. This applies for installations with an annual output of 500 tonnes and more
  • for smaller plants, use SO2 as a cover gas or take measures to minimise SF6 consumption and emissions. In the case where SF6 is used, the BAT associated consumption level is <0.9 kg/tonne casting for sand casting and <1.5 kg/tonne casting for pressure die-casting.


Note: At the time of reaching this conclusion, other alternatives to SF6 were just being brought into use. SO2 can be of concern on health and safety grounds and can be corrosive to equipment



Source: European Commission, Reference Document on Best Available Techniques in the Smitheries and Foundries Industry, May 2005, p.13-14, 97


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