HOT BLAST OPERATION

Hot blast operation involves preheating the cupola blast air and was originally conceived as a means of reducing coke consumption. It was first adopted for foundry use in the late 1940s, early 1950s and by heating the blast air to about 500ºC, charge coke reductions of about 30% were experienced compared to cold blast operation. However, hot blast could also be employed to increase metal temperatures and carbon pick-up and this permitted the use of more steel scrap in the charge mixture.

The advantages of hot blast operation may be summarised as follows:

 reduced coke consumption

 increased metal temperature

 higher melting rate

 reduced sulphur pick-up

 lower melting losses of silicon

 increased carbon pick-up.

It is not possible to obtain all of these benefits simultaneously.

Hot blast cupolas have not been universally used in all European countries but have been very popular in Germany. In many countries initial environmental legislation was more stringent for hot blast cupolas than for cold blast. This is not the case now and the cost of the recuperator is relatively easy to justify on operating cost savings.

Most hot blast cupolas are operated on long campaigns, many with externally water cooled unlined shells in the melting zone. Several of this type of furnace have been used for the production of low sulphur ductile base iron using basic slags.

Blast heating has been carried out using both independently fired and recuperative hot blast systems. However, the high fuel costs and generally poor performance of the independent units has resulted in recuperative systems incorporating combustion of the cupola off-take gases being the most common arrangement.

In recent years there has been an interest in the use of higher blast air temperature in excess of 700ºC. Such very high blast temperatures will result in further enhancement of recarburisation of the iron, while the lower blast rates for a given melt rate and the higher bed temperatures should allow the use of smaller charge pieces such as borings to be successfully melted. It has been suggested that superheated hot blast systems will allow lower grade, smaller coke to be employed without the tapping temperature decrease, which would be experienced with conventional cupolas.

Oxygen Enrichment

Although the benefits of oxygen were known for a considerable time, it was only in the 1970s when the costs of bulk oxygen, pig iron and coke were such as to make its employment economical that it came into common use.

Compared with conventional operation, the continuous use of oxygen resulted in:

 higher metal temperatures and carbon pick-up and lower silicon losses at the same coke levels – allowing metallic charge costs to be reduced by pig iron replacement and a reduction in silicon additions

 reduced coke consumption for a given temperature

 improved tapping temperature recovery at a start of melting or following shutdown periods.

The oxygen could be introduced by three different processes – blast enrichment, tuyere injection, or well injection – in order of increasing effectiveness. However, blast enrichment was simpler and the majority of cupolas adopting oxygen technology employed this system.

More recently, the use of supersonic oxygen injection into cupolas via the tuyeres is becoming an accepted technique and a number of cupolas in Europe have installed such facilities. In this process the tuyere lance nozzles are specially designed to provide an outlet velocity in the range 2–2.5 Mach. It is claimed that this approach results in better oxygen and air blast penetration with a consequent improvement in coke bed temperature.

The injection lances are self-cooled by gaseous oxygen and are mounted centrally in each tuyere (generally tuyeres are water cooled) at a distance of between 100 and 300 mm from the exit. It is claimed that the following effects result:

 blast volume is reduced

 blast air/oxygen distribution is more uniform and furnace internal pressure is reduced

 heat losses from cupola are reduced due to more even coke bed combustion

 charge preheating is improved because there is less temperature variation over the cross-section of the cupola

 charge coke additions are reduced

 melting rate variations of -50% to +40% of nominal melting rate are possible

· furnace shell losses are lower as higher temperatures are achieved in the centre of the cupola

 metal tapping temperature is increased

 silicon losses are reduced

 the blast temperature on hot blast cupolas is increased by the higher off-take gas temperature.

It is also claimed that the process is the first oxygen method which demonstrates a reduction in coke consumption at a constant melting rate such that coke savings fully compensate for the cost of oxygen. It is further suggested that it should be possible to use lower grade coke with consequent further cost savings.

Long Campaign Cupolas

In recent years there has been considerably increased interest in operating cupolas for extended periods, both on a daily basis and also from a refractory campaign point of view (repair after weeks rather than after melting day).

Obviously the well-established hot blast technology already discussed in an earlier section can fulfil this role successfully, However, largely as a result of work in the USA, a number of cold blast units has been adopted for the same purpose.

The cold blast furnaces include similar features to those found in hot blast plants in that the cupola shells (which may be lined or unlined) are fully water cooled and are fitted with water cooled projecting tuyeres. Tuyere blast velocities are considerably higher than conventional cold blast practice to ensure complete penetration of the coke bed, the generation of very high bed temperatures and a minimisation of heat losses, particularly through an unlined shell.

METALLURGICAL AUTOMOBILE RECYCLING PLANT

Metallurgical Automobile Recycling Plant

The use of cupola plant for recycling considerable quantities of steel scrap containing non-metallic material is not new. However, the use of cupola technology as the heart of an integrated reprocessing system for automobile body shells would result in a low cost bulk metal supply route for large foundry operations where there is a readily available source of suitable feedstock.

Over recent years the increasing pressures to reduce waste and improve reprocessing opportunities has resulted in many automobile manufacturers claiming a higher degree of recyclability for their products. This, together with increased use of valuable recoverable material, is likely to lead to more selective dismantling. Nevertheless, significant quantities of organic materials in the form of paints, lubricants and plastic components and the trim are likely to remain in the resulting car body shell. Zinc coated steel will almost certainly feature in the body construction, while wiring looms and electric motors may be a source of unwanted copper contamination unless removed prior to reprocessing. Car body shells can be prepared by either baling or fragmentation. Fragmentation scrap with appropriate magnetic separation and screening can be rendered clean and at adequately low levels of contamination. However, its generally small size and thin section do not make for an ideal cupola feedstock. Newer, modified cupola techniques to avoid excessive oxidation could improve this situation and lead to greater acceptability.

Baled scrap from known sources has been fairly widely used for cupola charges, although in smaller furnaces the tendency to scaffolding problems has limited its use. The use of hot blast and a tapered shell arrangement will alleviate the scaffolding difficulty and reduce oxidation problems with such feedstock.

Ancillary Equipment

Ü Environmental Control

The design of effective emission control equipment for cupolas is more difficult than for other foundry processes due to the wide range of gas temperatures (100–1200ºC), particle sizes (<1 mm to 10 mm) and flow rates involved. In addition, the presence of sulphur dioxide can cause corrosion problems and any combusted volatile matter may create condensation difficulties with filter media. The high levels of carbon monoxide can be hazardous and unacceptable from a discharge point of view unless burnt.

Most modern emission control plant for cupolas is based on the use of dry bag filter systems. Cold blast operation can result in effluent gas temperatures which vary during the campaign and are generally well in excess of that capable of being handled by economical fabric filters. Gas cooling is therefore required to prevent damage to the filter bags. However, too low a gas temperature may result in condensation of both volatile matter and water vapour leading to adverse effects on plant performance. These include blinding of the filter fabric, corrosion problems in the plant itself, and risk of fire or explosion damage due to flammable deposit ignition.

In order to heat the blast in hot blast cupolas, the off-take gases are burnt in a recuperator but the system actually used may vary depending on the condition of the gases to be handled.

Ü Dust and Waste Injection Systems

Much research and development effort has been expended over the years in attempting to perfect methods for introducing particulate materials in cupolas and these can be categorised as follows:

 exploitation of swarf and borings

 concentration of metallic elements, in particular zinc, for subsequent recovery

 recycling of waste materials such as collector waste, used sands, etc.

Most injection systems have been based on lancing systems incorporation in the tuyeres but it has been found that the cooling effect involved has restricted the opportunities for significant continuous injection because of the need to melt the material collected in the coke bed. It is, therefore, of considerable interest to increase the continuous injection rates and to establish practical feed rates for various waste materials.

Some work undertaken in Germany has been carried out using oxy-fuel burners in the tuyeres of a hot blast cupola. The technology should be transferable to most types of cupolas.

A mixture of natural gas and oxygen is supplied to the burner with dust being injected through the burner pipe from a specially designed transporter unit and ejected through the burner outlet.

Oxy-fuel burners result in very high flame temperatures compared to fuel-air systems, since no nitrogen has to be heated.

Each type of particle influences the overall melting process in a different way, for instance the carbon containing flue ash/cupola dust acts as a supplementary fuel. In these trials the injection rates were restricted by iron quality considerations rather than blockage of the coke bed in front of the tuyeres.

Other advantages of tuyere oxygen injection, such as charge coke reduction, reduced blast air requirements, better carbon pick-up and metal temperatures can be achieved by use of the burners when injection is not being employed. This makes for increased furnace flexibility. Recycling of dry bag filter dusts by cupola injection can result in the concentration of zinc such that it can be sold to a zinc reclaimer.

Ü Waste Heat Recovery

The exhaust gases leaving a cupola are at a high temperature and contain combustible elements which could be exploited as an energy source. Clearly this energy is utilised in hot blast cupolas to good effect but even in these cases less than 50% of the energy available is employed The available waste heat could be used for a variety of other applications by employing suitable heat exchanger systems. Indeed a few examples do exist of hot water and steam production for space heating and process heat. Space heating and hot water production for ablutions and other uses may not be required consistently but one interesting example of a process heat application is for steam generation to power an electricity generating set and compressor. Foundries usually require both electricity and compressed air during their operating hours and therefore there is a continuous outlet for the waste heat from the cupola plant. It is claimed that the use of this system on a 50 tonnes/hr hot blast facility results in an energy utilisation approaching 100%.

Other possible applications of cupola exit gas energy could be for charge material and coke drying and preheating duties to prevent condensation in bag filter outlet chimneys, dust extraction systems and shake-out drums.

The need for high efficiency gas cleaning equipment on modern cupola installations provides the possibility of waste heat recovery, which requires evaluation.

INTRODUCTION ON METAL CASTING

Introduction on Metal Casting

In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow

cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also

known as a casting, which is ejected or broken out of the mold to complete the process. Casting

is most often used for making complex shapes that would be difficult or uneconomical to make

by other methods.

Molding or moulding (see spelling differences) is the process of manufacturing by shaping liquid

or pliable raw material using a rigid frame called a mold or matrix. This itself may have been

made using a pattern or model of the final object.

A mold or mould is a hollowed-out block that is filled with a liquid or pliable material like

plastic, glass, metal, or ceramic raw materials. The liquid hardens or sets inside the mold,

adopting its shape. A mold is the counterpart to a cast. The very common bi-valve molding

process uses two molds, one for each half of the object. Piece-molding uses a number of different

molds , each creating a section of a complicated object. This is generally only used for larger and

more valuable objects.

The manufacturer who makes the molds is called the moldmaker. A release agent is typically

used to make removal of the hardened/set substance from the mold easier. Typical uses for

molded plastics include molded furniture, molded household goods, molded cases, and structural

materials.

Casting processes have been known for thousands of years, and widely used for sculpture,

especially in bronze, jewellery in precious metals, and weapons and tools. Traditional techniques

include lost-wax casting, plaster mold casting and sand casting.

Metal casting is one of the most common casting processes. Metal patterns are more expensive

but are more dimensionally stable and durable. Metallic patterns are used where repetitive

production of castings is required in large quantities.

Casting is a 6000 year old process.The oldest surviving casting is a copper frog from 3200 BC.

The modern casting process is subdivided into two main categories: expendable and nonexpendable

casting. It is further broken down by the mold material, such as sand or metal, and

pouring method, such as gravity, vacuum, or low pressure.

Casting is a manufacturing process by which a liquid material is usually poured into a mold,

which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified

part is also known as a casting, which is ejected or broken out of the mold to complete the

process. Casting materials are usually metals or various cold setting materials that cure after

mixing two or more components together; examples are epoxy, concrete, plaster and clay.

Casting is most often used for making complex shapes that would be otherwise difficult or

uneconomical to make by other methods.

Epoxy is the cured end product of epoxy resins, as well as a colloquial name for the epoxide

functional group. Epoxy resins, also known as polyepoxides are a class of reactive prepolymers

and polymers which contain epoxide groups. Epoxy resins may be reacted (cross-linked) either

with themselves through catalytic homopolymerisation, or with a wide range of co-reactants

including polyfunctional amines, acids (and acid anhydrides), phenols, alcohols, and thiols.

These co-reactants are often referred to as hardeners or curatives, and the cross-linking reaction

is commonly referred to as curing. Reaction of polyepoxides with themselves or with

polyfunctional hardeners forms a thermosetting polymer, often with strong mechanical properties

as well as high temperature and chemical resistance. Epoxy has a wide range of applications,

including metal coatings, use in electronics / electrical components, high tension electrical

insulators, fiber-reinforced plastic materials, and structural adhesives. Epoxy resin is employed

to bind gutta percha in some root canal procedures.

Epoxy resins are low molecular weight pre-polymers or higher molecular weight polymers which

normally contain at least two epoxide groups. The epoxide group is also sometimes referred to as

a glycidyl or oxirane group.

A wide range of epoxy resins are produced industrially. The raw materials for epoxy resin

production are today largely petroleum derived, although some plant derived sources are now

becoming commercially available (e.g. plant derived glycerol used to make epichlorohydrin).

Epoxy resins are polymeric or semi-polymeric materials, and as such rarely exist as pure

substances, since variable chain length results from the polymerisation reaction used to produce

them. High purity grades can be produced for certain applications, e.g. using a distillation

purification process. One downside of high purity liquid grades is their tendency to form

crystalline solids due to their highly regular structure, which require melting to enable

processing.

An important criterion for epoxy resins is the epoxide content. This is commonly expressed as

the epoxide number, which is the number of epoxide equivalents in 1 kg of resin (Eq./kg), or as

the equivalent weight, which is the weight in grams of resin containing 1 mole equivalent of

epoxide (g/mol). One measure may be simply converted to another:

Equivalent weight (g/mol) = 1000 / epoxide number (Eq./kg)

Plaster, Concrete, Or Plastic Resin

Plaster and other chemical setting materials such as concrete and plastic resin may be cast using

single-use waste molds as noted above, multiple-use 'piece' molds, or molds made of small rigid

pieces or of flexible material such as latex rubber (which is in turn supported by an exterior

mold). When casting plaster or concrete, the finished product is, unlike marble, unattractive,

lacking in transparency, and so it is usually painted, often in ways that give the appearance of

metal or stone. Alternatively, the first layers cast may contain colored sand so as to give an

appearance of stone. By casting concrete, rather than plaster, it is possible to create sculptures,

fountains, or seating for outdoor use. A simulation of high-quality marble may be made using

certain chemically-set plastic resins (for example epoxy or polyester) with powdered stone added

for coloration, often with multiple colors worked in. The latter is a common means of making

attractive washstands, washstand tops and shower stalls, with the skilled working of multiple

colors resulting in simulated staining patterns as is often found in natural marble or travertine.

Resin Casting

Resin casting is a method of plastic casting where a mold is filled with a liquid synthetic resin,

which then hardens. It is primarily used for small-scale production like industrial prototypes and

dentistry. It can be done by amateur hobbyists with little initial investment, and is used in the

production of collectible toys, models and figures, as well as small-scale jewelry production.

The synthetic resin for such processes is a monomer for making a plastic thermosetting polymer.

During the setting process, the liquid monomer polymerizes into the polymer, thereby hardening

into a solid.

INOCULANTS

Inoculants

The most commonly used inoculants are high silicon materials (ferro silicons and calcium silicide) and high purity highly crystallised graphite.

Silicon based inoculants require the presence of small amounts of minor elements such as aluminium, barium, calcium, cerium or strontium to obtain maximum effect.

More has been written about the effects of inoculation than practically any other metallurgical problem.

Inoculation gives the following advantages and disadvantages:

 reduces tendency to chill and promotes graphite formation

 reduces formation of fine graphite and associated ferrite

 promotes uniform structures

 increases strength

 enables high strength, low carbon equivalent irons to be cast free from chill

 increases tendency to unsoundness.

Size range

 Silicon inoculants – granules 2–12 mm

 Graphite inoculants – powder or fine particles

Control

 Specify requirements and ensure that each delivery conforms to specification, including grading.

 Store materials under cover in an area free from damp or in a waterproof container.

 Finely divided graphite absorbs moisture.

 Silicon inoculants react with moisture which reduces their efficiency and makes them dangerous in use.

 Segregate each consignment in case of variations or problems.

Using Inoculants

 Weigh each inoculant addition.

 Add it at the last possible moment before casting.

 Ensure uniform distribution throughout the metal.

 Add the inoculant to clean metal free from slag or dross.

 Ensure good mixing. Add inoculant to the metal stream or to a partly filled ladle.

 When using transfer ladles, add inoculant to the casting ladles as above.

 Do not delay pour. Inoculants are subject to fade, in most cases over a very short period of time. 

 Use late stream or mould inoculation where possible.

 Where possible use inoculant in the form of a wire as a late addition to the metal stream.

Alloy Additions

 Electric furnaces, cupolas or cupola ladles.

 The specification of the desired cast iron will determine the production route.

 In general, highly alloyed irons – Ni hard, Ni resist, high chromium and high silicon irons – are made in electric furnaces.

 Low alloy grey or ductile irons can be made in either electric furnaces or cupolas.

 Alloy additions used to provide the alloying elements and their recovery in cupola and electric furnace practice are given in Exhibit O.

Control

 All alloy additions should be accurately weighed.

 Ladle additions should not exceed a total of 1% unless the alloy addition is of a low melting point or is absorbed very easily. Metal temperatures should be as high as possible when addition is made.

 Avoid the use of alloy scraps which can give rise to obnoxious fumes or gases

Avoid the use of relatively low cost alloy scraps which contain elements deleterious to the alloy cast iron being manufactured.

 Specify requirements, type, chemical composition, size range and check each consignment.

 Segregate each consignment.

 With high alloy cast irons it is necessary to check chemical compositions of the molten metal by spectrograph and adjust before tapping.

 A summary guide to maintaining quality in foundries is given in Exhibit P. This illustrates some of the major problems which can occur if strict quality control is not carried out.

 Remember that “quality assurance” is not just two words, it is a way of life which encompasses the whole of the business. Quality is only as good as the weakest link in the chain.

 Weighbridge

Large and medium-sized foundries should have their own weighbridges to weigh accurately all incoming and outgoing materials, whereas small foundries may have to share a facility with a nearby large company or use a public weighbridge close to the plant.

The weighbridge should have a deck size suitable for road vehicles, with electronic load cells accurate to ±5 kg and have provision for checking axle loads. The computer print-out should be accessible to the production control computer system. The types of materials that the weighbridge will be used for include:

 incoming (all metallic, coke, sands, coal dust, bonding agents, ferro-alloys, etc)

 outgoing (finished castings, waste sand, slag, etc)

Foundries must keep close control on the receipt and availability of raw materials within the plant. These controls help to reduce overall costs and identify stock excesses.

 Stockyard

Foundry stockyards should be concreted areas with adequate drainage and the facility to clean out the individual raw material bays, which will be typically for:

 steel scrap

 cast iron scrap

 pig iron

 foundry returns

 ferro alloys

 coke

 limestone

Purchased scrap, pig iron and foundry returns should be stored under cover, organised in individual bays by usage frequency, density, specification, or alloy content.

Stockholding will vary according to the size of the foundry and the space available, but a minimum of three days’ supply of all raw materials should be held on site. If space is available, those items subject to price fluctuation, eg steel scrap, should be purchased and stored when prices are low. Each delivery should be from an approved supplier to avoid the need for the checking of each load. However, the quality of materials supplied must be kept under constant review to ensure quality levels do not drop.

Charge make-up can be by a crane fitted with an electro magnet or grab or, in very small foundries, by hand. However, all materials must be weighed and an automatic print-out of the weight of each constituent material should be retained manually or, preferably, on computer for traceability purposes.

For cupola melting, coke and limestone should be stored in separate hoppers and the desired weight automatically dispensed into the charging skip. The coke and limestone should be charged into the cupola separately from the metallic to minimise the crushing and fracturing of the coke and so maintain its optimum size. The metallic skips should be charged alternately with the coke/limestone skips.

CUPOLA FURNACES

Cupola Furnaces

a) Introduction

Since the first cupola patent was taken out in England in the late eighteenth century, the furnace has remained the predominant melting unit in iron foundries on a tonnage basis. Over the two intervening centuries, the basic operations of coke fired cupola have remained relatively unchanged, although the understanding of the process involved has improved considerably.

The conventional cupola has been perceived as a relatively low cost plant which, particularly for smaller foundries, is capable of achieving the appropriate quantity of iron, especially grey iron. Thus the status of the furnace was virtually unchallenged in iron foundries until the late 1950s, early 1960s.

At that time, electric furnaces, particularly coreless induction types, began to make inroads into the market. This market penetration has accelerated in recent years with the development of more efficient and powerful solid state powered equipment. The introduction of electric furnaces was promoted by environmental concerns, by the ability to produce more easily a range of alloys, by increasing customer expectations regarding quality, and improvements of both electric furnace equipment and associated refractory systems. Many of the small to medium sized electric furnace plants installed in recent years have been justified on this basis, even though capital costs may be higher than for a simple cupola plant.

The introduction of environmental legislation requiring rigorous control of emissions to the external atmosphere has been perceived to mitigate against cupola melting in favour of electric melting, the latter being considered by most foundrymen to be cleaner. However, this is not necessarily true, since deterioration of scrap quality, the need to control workplace environment and the continued scrutiny and tightening of outside emission limits apply whatever the melting unit. Various recent waste regulations mean that the dumping of waste from furnace emission control systems will become more difficult and expensive, and the use of the cupola to reprocess such material will become a more important benefit in the future.

The largest melting cost item is charge metallic and the ability to process lower quality scrap through a cupola, due to its refining action and greater tolerance of included non-metallic material, is of considerable benefit, particularly for tonnage applications. This is why large tonnage outputs of a restricted range of unalloyed cast irons are almost invariably produced from cupolas. Such plants may include hot blast facilities and will for the most part be fitted with fume cleaning plant of sufficient efficiency to satisfy the appropriate environmental requirements. These cupola installations will employ electric holding furnaces to allow for optimum operation of the prime cupola melter.

Most iron foundries have had to consider either upgrading their emission control equipment or changing their melting practice to ensure compliance with the relevant requirements. The choice in reality has been between fitting dry bag filtration systems or changing to induction furnace operation. These options are expensive, but installing suitable emission control equipment on an existing cupola plant has no economic justification other than keeping the company out of Courts and allowing them to continue in business.

The majority of cupola plants are based on conventional cold blast units, which may or may not be fitted with improvements such as automatic blast control, oxygen enrichment, and divided blast equipment. In addition, the higher output furnaces, especially where long campaign operation is involved, have for many years been largely of the hot blast type. However, relatively recently, the impending legislation on emissions and to a lesser extent waste, the increased interest in long campaign operation and the ever present need to reduce costs – both capital and operating – have all conspired to ensure that cupola technology has not become stagnant. The following sections attempt to review the current status of the cupola furnace.

b) Conventional Cold Blast Cupola Operation

The conventional cold blast cupola is a vertical shaft furnace operating on the counter-current principle. Cold charge materials are fed into the top of the unit and are preheated by the products of combustion of coke as they descend and melt before being tapped out near the base (see Exhibit Q).

An incandescent coke bed is established in the lower part of combustion air (blast air) introduced via a series of tuyeres arranged in this area to ensure that appropriate combustion conditions are generated. Blast pressure, velocity and disposition are important and require adequate control if the correct bed conditions and temperature distribution are to be maintained.

The cupola furnace is not a “dead melting” furnace in that the melting process is accompanied by a number of compositional changes – carbon and sulphur pick-up, and oxidation losses of silicon and manganese.

Pick-up of carbon depends on a number of factors including:

 initial carbon content of the charge mixture

 the tapped silicon and phosphorus levels

 the amount and quality of the coke

 coke bed temperature

 cupola well depth

 method of tapping

 metal tapping temperature.

Clearly some of these factors are inter-related.

Sulphur pick-up in turn is governed by a number of factors, notably the sulphur content of the coke, slag basicity and charge make-up and composition.

There is invariably some oxidation loss of silicon during cupola melting, normally between 10% and 20% of the charged level of element. The actual figure will vary depending on the melting conditions, with high tapping temperatures favouring a reduction in losses and the presence of steel scrap and high silicon briquette additions having the opposite effect. Manganese is subject to oxidation losses of the order of 20–25% of the charged level, although again melting conditions will have an influence on the practical values achieved.

The cupola is refractory lined and as the melting campaign proceeds, the refractory is mechanically and chemically eroded until the remaining thickness is insufficient to allow for continued safe operation. This erosion situation limits the melting campaign and usually requires the provision of two furnaces in an installation. One will be melting while he second unit is being repaired. Eroded lining material, together with coke ash, dirt from the charge materials and appropriate fluxes (usually limestone) results in the formation of an acid slag which is tapped either with or separate to the metal.

From the above, it can be observed that there are a number of areas of possible improvements that could be explored when considering the limitations of the conventional cold blast cupola. These include:

 increasing coke bed and metal tapping temperatures

 improving carbon pick-up and reducing sulphur pick-up

 reducing silicon manganese oxidation losses

 increasing campaign length

 increasing melting rates.

In addition, the increasingly stringent emission and waste regulations have resulted in the development of re-use systems to minimise their effects on the foundry’s operations and costs.

These factors have resulted in considerable research and development of a wide variety of cupola technology in an effort to improve the performance of the furnace.

c) Divided Blast Cupola

The divided blast cupola, a development of the balanced blast cupola of earlier years, was developed in the early 1970s and became very popular as a means of improving the performance of many cold blast cupola plants installed years before and struggling to meet increasing quality standards. It also became popular in new installations at that time.

In the balanced blast system, however, the cupola is provided with two rows of tuyeres set 900 mm apart and the air blast is controlled in both areas. A distribution of 50/50 between the two rows of tuyeres appears to give the best results independent of the amount of charge coke or blast rate. Control can be obtained by using separate fans for each row of tuyeres or by installing proportioning control equipment in the ducts from a single fan (see Exhibit R).

The system enables a higher metal tapping temperature and higher carbon pick-up to be obtained for a given coke charge or a reduction in charge coke and an increase in melting rate whilst maintaining a given tapping temperature. It is necessary to recognise that all of these advantages cannot be obtained at the same time.

Exhibit S shows the relationship between coke consumption, metal temperature and melt rate. The divided blast cupola may be further enhanced by the use of oxygen applied to the lower row of tuyeres.

SUBSIDIARY RAW MATERIALS FOR MELTING

Subsidiary Raw Materials  for melting:

These materials, which must be controlled, include:

 coke

 carburisers

 fluxes

 inoculants

 alloying additions

Coke

Coke is the primary energy source in cupola operations and coke quality is the key to carbon and sulphur pick-up. Carbon pick-up is of particular importance when high steel charges are used.

Chemical and physical properties should have the following characteristics: moisture	–	4% max
ash	–	9% max
volatile matter	–	1.0% max
sulphur	–	0.9% max
50 mm shatter index	–	90% min
mean size	–	110 mm min
undersize	–	not more than 4% of pieces less than 50 mm

Serious coke breakages can occur through inefficient handling at all stages from the coke oven, to and in the foundry.

Each coke delivery should be checked visually for size to ensure that there has been no serious degradation.

Examination of the coke structure may give a clue to its behaviour in the cupola. A good close/dense grain structure normally denotes a good operating coke. Coke strength is related to its final size and weak friable coke will result in excessive breakage and fines.

Serious coke breakages can occur at the foundry, resulting in a high percentage of small coke below 50 mm in size.

Small cokes can reduce tapping temperatures and carbon pick-up and may require an increased coke percentage to maintain good cupola operation. Increasing coke percentage will reduce melting rates unless air volumes are increased.

Care should be taken if coke is stocked for any length of time as this will result in degradation, and possibly a reduction in carbon pick-up and tapping temperatures.

Control

Inspect each delivery for size and structure. Record the name of the supplier, haulier and time and delivery of each consignment.

Retain a sample from each delivery for chemical analysis. If laboratory facilities are available, check ash, sulphur and volatile matter. If laboratory facilities are not available and coke quality is suspect, arrange for an outside analysis.

If the load contains excessive small coke and fines, do not use unless absolutely necessary. Under these circumstances retain a proportion of the load for supplier’s examination.

Do not store coke except on an emergency basis. Use the stored coke at regular intervals and replace with fresh supplies.

Carburisers

Carburisers are mainly used in electric furnaces to carburise high steel charges to the appropriate carbon level.

They are also used in external treatment processes such as porous plug ladles, and shaking ladles, again to raise carbons to required levels.

Carburising materials available in the UK are mainly based on the synthetic graphites, petroleum cokes and to a lesser degree metallurgical cokes.

Approximate analyses are given in Exhibit N.

Graphites

These materials are used for carburising base irons for ductile irons, malleable irons and also grey irons when it is desirable to restrict nitrogen pick-up, or when sulphur content is in excess of 0.05% and a high degree of nucleation is required.

Non graphites (petroleum or metallurgical cokes)

These materials are used for ductile and malleable iron production. In the case of ductile irons, petroleum and metallurgical cokes are only used when the metal is to be desulphurised.

They are used extensively in the production of grey irons for the following reasons:

 To obtain an increase in tensile strength due to increased nitrogen content

 Care must be taken that the increased nitrogen content does not give rise to nitrogen fissure defects in the finished casting.

 To avoid over-nucleation which could give rise to shrinkage defects.

 To increase sulphur contents of some irons to above 0.05% making them more responsive to inoculation.

Control

 Select the correct carburising material and grade for the application. When purchasing specify these two criteria.

 Grading is of prime importance and can affect solution rate, carburising potential and carbon losses. If the material is too fine, the carbon will float away on thermal air currents. If too coarse, low temperatures and low turbulence will result in slower carbon solution and low recovery.

 Check every consignment for specific type and grading, and store the material under cover.

Segregate carburising agents if more than one type is used.

 Check carbon recoveries on a statistical basis to ensure quality continuity.

Fluxes

 Fluxes are added to the cupola to give a fluid slag with the impurities in the charge, sand, rust, ash, etc, plus refractory lining attacked and removed during the melting process.

 Standard fluxes are limestone and dolomite but certain other fluxes such as fluorspar may be used in special circumstances.

Limestone

 Calcium carbonate which reverts to CaO (lime) and CO2 in the cupola.

 The CaO (lime) combines with the impurities to give a fluid slag.

Dolomite

 This is the double carbonate of calcium and magnesium which calcines to CaO and MgO to form a fluid slag with the impurities. The melting point of the double carbonate is somewhat lower than CaO and gives more flexibility with “sticky slags” of high acidity or basicity.

 Both limestone and dolomite should contain a minimum of 96% CaCO3 and MgCO3.

 Size range should be 2.5 cm to 7.5 cm.

Amount

 Normal cupola practice requires the flux to be 3–4% of the metallic charge.

 This should provide a fluid slag with a basicity, in this case an acidity of 0.6–0.8, using the standard formula CaO + MgO/SiO2.

 It is extremely important to obtain a fluid slag in acid cupola operations.

 Too little flux will give a highly acid viscous slag which in extreme cases can cause bridging.

 Too much flux will tend to give a basic slag which will quickly neutralise itself by attacking the acid refractory lining resulting in a large slag volume and misuse of energy in the form of coke.

 As there are very few basic lined cupolas in operation in Europe, or the rest of the World, there is little or no point in discussing the operating parameters for basic slags.

Fluorspar (calcium fluoride)

 Fluorspar may be used in small amounts as a slag fluidiser or to reduce sulphur pick-up.

It is a very vicious slagging material and will severely attack acid linings.

Control

Limestone and Dolomite

 Specify high purity limestone and dolomite.

 Specify and check the size range.

 Weigh or check by volume the amount per charge.

 Remember too little limestone can mean cupola problems.

 Too much is a waste of coke and refractories.

Fluorspar

Briquettes are recommended for acid lined cupola operation. Specify size of briquettes and ensure the correct number per charge is made.