Cupola vs electric a battle for melting efficiency.

For the first half of the 20th century when a foundry melted iron, it was melted in a cupola. However, with the development of electric melting in the 1960s, the landscape for melting iron changed. Iron foundries were presented with a simpler process, reduced environmental concerns and lower energy costs.

In today's foundry world, however, higher melt rates, upcoming Maximum Achievable Control Technology (MACT) environmental standards and increased energy costs have reconfigured the landscape yet again. The result for foundries is a decision between cupola or electric melting that isn't clear.

A Comparative Analysis

When a foundry is choosing between state-of-the-art cupola or medium frequency coreless induction melting for its operation, the discussion centers on key factors such as energy costs, environmental regulations, charge materials, labor and production levels. Underlying these factors is one cost unit--dollars per ton of molten iron. To perform a proper analysis of what system is best for an operation, all factors must be quantified in dollars per ton of molten iron and then totaled to determine the cost-effective melt solution.

This article summarizes a cost analysis of a 40 ton/hr cupola and a 40 ton/hr medium-frequency coreless induction melt system. The systems being analyzed are for greenfield installation and 4000 hr/yr of operation. For this analysis, the melt systems are for a midwest U.S. foundry that requires base iron for ductile iron and will melt 16 hr/day, five days a week.

This analysis reviews the following factors for both melt systems and assigns them a cost in dollars per ton of iron:

* charge material/treatment cost;

* operational cost;

* labor cost;

* environmental cost;

* investment and breakeven analysis.

These factors then can be totaled to determine the melt system that is a more cost-effective option for the specific production situation.

This article only is a summary of the full comparative analysis performed. For the full report, visit the For More Information section at www.moderncasting.com.

Melt System Basics

The two systems being compared are:

Cupola Melt Center--The cupola is designed with recuperative hot blast, refractory lining, front slagging, water-cooling and below charge takeoff. The gas cooling and cleaning system includes a recuperator/gas cooler, baghouse and exhauster. Also incorporated in the system is a combustion chamber with pilot burner system for carbon monoxide combustion. The charging system for the cupola melter includes a vertical hoist unit and storage and weighing for metallic and non-metallic charge.

Electric Furnace Melt Center-This system consists of three medium frequency coreless induction furnaces. Each is equipped with exhaust hoods connected to a baghouse for gas cleaning. The batching and weighing system consists of hoppers, a crane, conveyors and a scale. A charging car designed for low emission charging will be used on each furnace.

Charge Materials

A major difference between the compared melt processes is the ability to use differing quality charge materials. Oxidation and reduction reactions take place within and above the melt zone during cupola melting, which allows for the usage of highly oxidized and low quality scrap material. Electric melt furnaces are more sensitive to low quality charge materials and contaminants, resulting in premium scrap costs. A reductive atmosphere is not present and therefore iron oxide will not be reduced. This increases iron loss through the slag.

Another charge material difference between the melt processes is the cost of alloys and nonmetallic additions. An electric operation uses a high-grade silicon carbide to adjust iron chemistry. In addition, pure carbon in the form of graphite is used for carburization. These additions are costs not required for the cupola.

For comparative reasons, the cost for returns in this analysis was set at $150.00 for both cupola and electric melt operations. A typical charge composition for a cupola is shown in Table 1. This charge mix leads to the necessary charge weights for coke and silicon carbide (SiC) in order to reach the typical ductile iron carbon and silicon spout chemistry. Table 1 also shows a typical metallic charge composition for a medium frequency furnace.

Based on these cost differences, charge materials for the medium frequency melt system are $19.98/ton of iron melted higher than for the cupola.

Energy & Utilities

Figure 1 is a comparison of operation costs, The electric melt shop will use considerably more power than the cupola melt shop. The cupola uses coke as its primary energy source, while the cost for oxygen, water and compressed air is not included in the calculation as the cost of these items is negligible compared to the primary energy input. Also, this analysis assumes the use of state-of-the-art cupola systems, which includes the use of high efficiency melt, charging and gas cleaning equipment.

In the final analysis, total energy cost per ton of iron melted is $23.42 for the cupola and $29.58 for the medium frequency electric furnace.

Labor

A cupola operation with state-of-the-art equipment has the following labor requirements: two cupola operators, one charge crane operator, one supervisor, two maintenance personnel and one laboratory technician. This results in seven workers per shift and a $3.98 labor cost/ton of iron melted.

A medium frequency melt system's labor requirements are: two furnace operators, two charger and crane operators, one supervisor, two maintenance personnel and one laboratory technician. Eight workers per shift are required for melting within the outlined operation, which results in a $4.52 labor cost/ton of iron.

Refractory

The cupola is designed for a two-week melt campaign. After the two weeks, the anticipated repairs would be the breast and tap hole, spout, well and melt zone. In addition, major replacements and repairs for the spout, well, melt zone and brick lining above the melt zone must be considered.

For the three medium frequency furnaces, the refractory and labor items to consider are major furnace relining, general/small furnace repair and pouring spout repair.

Figure 2 portrays the systems' differences in refractory cost and labor. The largest differences are in general refractory and small repairs because of the increased level of materials and man-hours for a three-furnace system. The medium frequency system does have the advantage of not using a holding furnace, which saves on refractory cost. However, as an overall evaluation, the cupola's higher throughput per campaign provides the largest advantage. In terms of refractory cost, the cupola has a $1.16/ton of iron melted advantage.

Waste Disposal

The wastes accumulating from both systems are slag and dust. The difference lies in the quantity of waste.

Cupolas generate 5% of their melt rate as slag and 20-30 lb of dust and fly ash per ton of iron melted. Within this analysis, this leads to waste of 8000 tons of slag/yr and 2400 tons of dust and fly ash. Cupola slag from this analysis will have a commercial value for beneficial reuse. Therefore, disposal costs for cupola slag do not apply.

Fly ash from the cupola is estimated at 10 lb/ton melted and does not need further treatment. Baghouse dust is estimated at 20 lb/ton melted and for many operations must he treated for leachable heavy metals. Treatment costs are estimated at $0.80/ton melted. It then can be disposed with the fly ash. Disposal costs are estimated at $45.00/ton of material, which results in $0.68/ton melted.

Medium frequency furnace wastes are less in volume. Slag mass is 1% of the melt or 1600 tons/year. This operation does not produce slag for commercial reuse. Therefore, a disposal cost of $0.45/ton melted is assumed for a unit cost of $45.00/ton of waste. In addition, in medium frequency melting, the dust collected (1 lb/ton melted) in a baghouse requires treatment for leachable heavy metals. The estimated treatment costs are $0.04/ton melted. Disposal of the dust is estimated at $0.02/ton melted.

Maintenance

Maintenance costs for both operations are calculated assuming costs for equipment, spare parts, in-house maintenance and outside contractors and based on reports by foundries. Costs for the cupola operation are estimated at $6.40/ton of melted iron and $4.40/ton melted iron for the medium frequency furnace operation.

Buildings and Other

This item includes building operational costs such as electricity and natural gas, office operation, transportation and safety and is based on reports by foundries. This parameter is equal for both operations and is estimated at $10.00/ton melted iron.

Capital Investment

In this analysis, the capital investments for these two systems are based on past projects. Based on this experience, medium frequency melt systems are installed at a 25% reduced cost when compared to cupola systems. In this analysis, the estimated cost of the medium frequency melt system is $13.5 million, while the estimated cost for the cupola system is $18 million.

For the electric melting system, the major cost factors are the furnaces, charging system, buildings with power supply and environmental controls. For the cupola, the capital investment can be most influenced by the cost for gas combustion, cooling and cleaning, the charging system and its facility.

Breakeven Analysis

The profitability of any industrial plant increases and decreases directly with production rates. Figure 3 is a graph comparing the breakeven model for the two systems.

The foundation for the analysis are the estimated investment costs, the liquid metal costs and a 5% profit on the molten metal. For this breakeven analysis, both foundries' investments are financed by a loan with payments calculated at an 8.5% interest rate.

This breakeven analysis illustrates that the cupola melt system will cross from a loss to a profit at an annual production level of 67,000 tons. The medium frequency furnace will require 165,000 tons annually to achieve the same level. A significant disadvantage to the medium frequency melt system in this analysis is the low profit margin on the iron sales price.

These results will shift in a real" foundry as firms can lower their profit margin, increase market share or change other factors. In addition, a profit is achieved not only through liquid metal costs, but through other operation efficiencies and deficiencies.

Which to Choose?

Table 2 illustrates the total cost difference between a cupola and medium frequency melt system at $25.02/ton of iron melted.

Since it gained prominence in the 1960s, electric melting has been an attractive option for iron foundries, but in today's world, the differences aren't as clear. The key is that when the selection of a system is being made, all the proper data and factors must be considered to ensure the most efficient melt operation.

This article was adapted from a presentation (02-142) at the 2002 AFS Casting Congress.

For a free copy of this article circle No. 343 on the Reader Action Card.

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Table 1

Cupola vs. Electric Melt Charge Material Comparison

Charge and Alloy Additions for Cupola

                            $/ton   lb/charge  $/ton of Iron Melted

Plate and Structural Steel   125       800              50

Ductile Returns              150      1000              75

Boring Briquettes           104.40     200            10.44

SiC                         309.60     20              3.10

FeSi 75                     679.50     10              3.40

Coke for Carburization       180       20              1.80

Limestone                    19.80     100             0.99

Total                                2150 lb         $144.73

Charge and Alloy Additions
for Electric Melt

                            $/ton   lb/charge  $/ton of Iron Melted

Steel                        145       820            59.45

Ductile Returns              150      1000              75

Pig Iron                     192       180            17.28

SIC 90%                      447       17              3.80

Graphite                     612       30              9.18

Total                                2047 lb         $164.71
Table 2

Melt System Cost Comparison (dollars ($)/ton of iron melted)

                            Medium Frequency
                           Coreless Induction      Cupola

Metallic Charge                  151.73            135.44

Additive Materials               12.98              9.29

Melt Energy and Utilities        29.58             23.42

Labor                             4.52              3.98

Refractory/Labor                  3.07              1.76

Waste Disposal                    0.51              1.48

Maintenance                       4.40              6.40

Buildings and Other                10                10

Total                        $216.79/ton of    $191.77/ton of
                              iron melted       iron melted

For More Information

View the full report and analysis (AFS Transaction paper 02-142) online at www.moderncasting.com

Cupola Handbook, 6th Edition, AFS, Des Plaines, IL (1999).

MODERN CASTING / July 2002

About the Author

Michaela Boehm is a process/environmental engineer for kuttner-Modern and has been working with engineering and evaluating cupola melt systems for four years.