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.