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.