Since the advent of time, cupola furnaces have melted the vast majority of cast iron worldwide and in the U.S. While the number of operating cupolas in the U.S. has decreased from about 200 to 42 cupolas since the turn of the century, advances in cupola technologies, increases in cupola efficiency and the metallurgical and cost benefits of cupolas have ensured they remain as a significant, and leading technology for iron production. The 42 cupola foundries in the U.S. produce approximately 53% of all domestic cast iron. Nearly 62% (3.75 million tons per year) of all gray iron production and nearly 38% (2.3 million tons per year) of all ductile iron production is produced by cupola melting.

Projections from AFS Metalcasting Forecast & Trends show that total annual shipments for iron castings is 6.74 million tons per year.  By applying typical yield rates for the various SIC codes (ductile pipe, municipal, agricultural, automotive, etc.), estimates show that total tons melted for all iron types totals about 11.45 million tons per year. By applying knowledge-based estimates of the cupola industry, it is estimated that of the total iron melted, about 6.07 million tons per year are melted by 23 companies using cupolas.

There are SIC sectors, such as ductile iron pipe and high production automotive/industrial/municipal  foundries, where cupolas are the mainstay. In other SIC sectors, demand for a wide variety of metallurgical grades have made the cupola less attractive. Cupolas have, however, remained highly competitive, due to their flexibility, giving them a competitive advantage over electric melting methods.  

While cupolas are a big part of the iron melting picture in the U.S., the landscape of the cupola has changed. Stricter environmental regulations in the last three decades, combined with the changing metallurgical demands in foundries, have led many to believe that cupolas are going away. While it is true that there are fewer cupolas, it can be said with certainty they are not going away anytime soon. Cupolas continue to have some advantages, compared to electric melting, but there are six primary myths about cupolas that should be dispelled.

Myth 1: Cupolas are going away.
Many old, inefficient, obsolete cupolas have mostly gone away. While some cupolas are challenged by operation at low production rates, the remaining cupolas mostly have increased thermal efficiencies and many have incredibly high efficiency rates. Some technical advances can be made with minimal capital expenditure. The post combustion heat recovery systems are better, and cupola shops are pursuing methods to reclaim waste heat and increase hot blast temperatures (up to 1,150F) to decrease coke rates while many have found ways to beneficially reuse slag. Alternative carbon and fuel sources have been established and chemistry transition practices have been modernized. The bottom line is cupolas now are more efficient with higher productivity per unit. Many cupola shops continue to invest in technical improvements in furnace efficiency and heat recovery systems as part of their long term strategy for sustainability.

Myth 2: Cupolas are economically infeasible.
Continuous production rates and flexibility in scrap input gives cupolas an economic advantage. Production rates are steady and predictable. A wide range of scrap input can be handled, such as dirt, high zinc, shredder contaminants, bundles, iron briquettes, high oxide content and self-reducing briquettes. In stream desulfurization for ductile or compacted graphite base iron allows for the melting of low-cost high sulfur scrap. Cupolas can accommodate up to seven iron chemistry changes per day, supplemented with in-spout alloy additions. Cupolas are well known to have lower labor cost per unit of iron, especially for high production cupolas, say above 20-25 tons per hour.

Myth 3: It is impossible to obtain permits for cupolas.
While regionally designated environmental areas can make permitting difficult for any new foundry, new cupolas can be permitted and meet the strictest requirements. New gas combustion and dust collection technologies can meet MACT particulate matter and hazardous air pollutant and volatile organic compound standards. Gas concentrations like SOx and NOx are generally not of great concern, even in some EPA designated ozone non-attainment areas. State of the art pollution controls for cupolas have been in place in Europe for two decades, where regulations are more strict than in the U.S., proving that properly designed air pollution control systems make cupolas viable. In fact, properly designed control systems provide ultra low maintenance costs, with extended filter life (bag changes) on the order of 8-10 years.

Myth 4: Cupolas are energy intensive and inefficient.
Cupolas are inefficient only if they are following “old” methods and designs.  The high efficiency cupola plants of today produce iron that is 15% less energy intensive than iron produced from coal-produced electricity (medium frequency).  When methods for waste heat reclamation are installed (cupola flue gases), cupolas are 40-60% more energy effective than electric melting.  As electricity prices and natural gas prices increase in the future, the benefits of cupola melting, using coke as a the heat source, will become more attractive. We should remember that only about 85% of the coke charged into a cupola is burned for heat, and that about 15% of the coke is dissolved into the cast iron as metallurgical alloy, a very effective means of carburizing iron, especially for high steel charge mixes.
Technology advances extend beyond the furnace, to further to reduce the energy intensity of the entire cupola plant. Approximately 20 U.S. states have now defined industrial process waste heat as a renewable resource, making it, in some cases, not only recognized as a resource for harvest, but also making the capital investment eligible for funding and/or tax credits. The best, most efficient cupola shops have already installed, or are actively pursuing the installation of waste heat capture systems that can harvest and re-use the heat for a variety of processes.  These include:

Plant and office makeup air heating.
Cupola blast air dehumidification.
Core oven heating.
Electric power generation.
Domestic water heating.
Other process heating.

Investment in these technologies will significantly reduce the electric and/or natural gas consumption of the foundry and reduce the overall energy intensity of the final product, iron castings.

Myth 5: Coke is too expensive.
It is true that coke is somewhat costly, but prices have significantly dropped and stabilized in the last five to eight years. Electricity prices, meanwhile, have increased and been somewhat unpredictable. It appears that high quality European coke and formed coke suppliers have been placing pressure on domestic coke pricing.  

Myth 6: Coke is not going to be available in the future.
According to David C. Ailor, president, American Coke and Coal Chemicals Institute (ACCCI), the present foundry coke demand is only 38.2% of the four domestic merchant member coke-producers’ capacity. Like cupolas, domestic producers are fewer, but they are strategically suited for the long-term supply of the foundry industry. The merchant coke production capacity is 1.78 million tons, while the current domestic demand is about 0.8 million tons. The four merchant members producing foundry coke have taken several steps to continue as sustainable partners for U.S. metalcasters by:

Tailoring operations to meet the coke requirements/needs.
Organizing to engage regulatory agencies as the need arises.
Using many different coal blends.
Tailoring transportation/delivery infrastructure to meet customer needs.
Maintaining batteries to ensure long-term, reliable battery operation, performance and environmental compliance.

What does the future hold?
Cupolas continue to produce the majority of iron in the U.S., especially gray iron. The benefits of modern cupola melting systems include:

The ability for in-stream desulfurization for ductile and compacted graphite iron.
Flexibility in alloying (charge or in-stream).
Improvements in heat reclamation and heat recovery.
Low labor cost.
Low scrap cost.
Flexibility in scrap material selection.

It is clear that “old” cupola technology will not work in the foreseeable future, especially as the U.S. sees continued competition globally. Many U.S. foundries still use methods and equipment that has been surpassed by competitors in Europe, Asia andSouth America. Without the necessary investment, U.S. cupola foundries could fall behind.  But it is also clear, that those  foundries who continually advance their cupola technologies and improve energy efficiency, will realize a strong, competitive and sustainable means of cupola melting, well into the future.   

This article is adapted from AFS Melting Methods & Material  Division panel (18-122) with presentations by David Kasun and David Ailor, at the 2018 AFS Metalcasting Congress.

Click here to see this story as it appears in the August 2018 issue of Modern Casting