Computer models give accurate iron melting method economics.

New spreadsheet models include all cupola, induction and arc furnace melting variables to provide reliable operating cost data.

Major operational decisions in a foundry are difficult enough to make without the problem of insufficient data. For iron foundrymen looking to evaluate the cost- and productivity effectiveness of their current melting operation, scanty information has been a particular hindrance.

To overcome this impediment, several attempts have been made to develop computer-based analysis tools for comparing the operation of coke-fired cupolas and electric induction furnaces for melting iron. Most computer models have been either too complicated for easy use or so simplified that they ignore critical parameters, such as the impact of oxygen enrichment or changing the charge composition.

One computer model, however, has been developed to assist in analyzing the costs of melting using various technologies. in fact, three models now exist: cupolas, induction furnaces and arc furnaces. The models are simple to use, yet have the flexibility to consider a variety of input variables to more accurately predict the costs of each melting technology. Data for the models came from existing information published by the Center for Materials Production and others. Since the models are spreadsheet-based, they can be easily modified to fit the conditions of an individual foundry operation.

The Cupola Model

The primary input variables in the cupola model consist of melt-rate requirements, tap temperature, energy costs, charge material quantities and costs, metal-to-coke ratio, operating labor requirements, labor costs, maintenance labor requirements, maintenance intervals and material costs, and environmental costs. Other cupola-specific variables include desulfurization costs, oxygen enrichment percentage and cost per standard cubic foot, hotblast options, and credits for the sale of coke fines.

Energy costs per ton of melted iron are calculated based upon the quantities and costs of coke, gas for hotblast, electricity for auxiliaries and oxygen. Changing these parameters shows the impact of changing the hotblast temperature and oxygen enrichment. Foundry officials can then make decisions to optimize the hotblast temperature and oxygen enrichment percentage as the cost for gas, oxygen and coke change. Energy costs also include the cost of operating the cupola auxiliaries such as blast blowers, cooling water pumps, baghouse fans and stack-gas afterburners.

The number of operators required for the cupola, the cost per labor hour and the stated melt-rate in tons per hour are factored in to provide operating labor cost per ton of melted iron. Environmental costs are based upon typical slag and dust accumulation factors of 0.07 and 0.2 tons of slag or dust, per ton of metal melted, respectively. This is then multiplied by the cost of disposal for slag and dust.

Maintenance costs include both routine maintenance and an estimate of the labor required for major maintenance. Refractory costs are included in an item called "start-up costs." This parameter may require estimating outside the model as it can contain several pieces of data, including labor, refractory, bottom sand or other costs associated directly with starting a cupola melt campaign.

Charge costs depend upon the ratio of the various charge materials and the cost of those materials. Each major material is itemized (pig iron, scrap steel, cast iron, foundry returns and ferrosilicon). Other charge materials could be added if considered as major cost items. If a variety of iron chemistries are produced, the model can determine a weighted average charge to represent an average cost of production.

The results of the model are classified into six different cost components: energy, labor, maintenance, start-up costs, charge materials and environmental costs. A summary page shows these results as a cost per ton of melted iron and as an annual operating cost. Examples of each spreadsheet model may be found in Tables 1 through 9. The examples show all the input parameters, intermediate calculation results and the summary of costs for the case study that completes this article.

Table 1. Cupola Operating Cost Model For Thin-Wallet,
Water-Cooled Cupolas -- Input Data

Operating Data
Tons Per Hr Required:       28     tph
          Hr Per Day:       10     hr/day
         Days Per Wk:        5     days/wk
      Weeks Per Year:       50     wks/yr
Annual Tons Produced:   70.000     tons/year
  Required Tap Temp.:     2750     F
               Blast:   12.500     SCFM
      Hot Blast Temp:        0     F
   Oxygen Enrichment:      1.5     %
    Metal:Coke Ratio:        7  :  1
         Cupola Size:       90     in

Raw Material Data
           Coke Cost:   $200.00  / ton
            Gas Cost:     $1.80  / MMBTU
         Demand Cost:     $6.88  / kW
         Energy Cost:     $0.025 / kWh
         Oxygen Cost:     $0.00  / 100 SCF
     Cast Iron Scrap:   $140.00  / ton
            Pig Iron:   $214.85  / ton
   Gray Iron Returns:   $110.00  / ton
         Steel Scrap:   $157.00  / ton
        Ferrosilicon:  $1040.00  / ton
     Desulfurization:     $1.95  / ton of product

Labor

              People:         6  / shit
               Wages:    $15.00  / hr

Maintenance
             Routine:        80    man-hr/wk
               Major:       300    man-hr/wk
Environmental Data
       Slag Disposal:     $0.00  / ton
       Dust Disposal:    $10.00  / ton

Start-Up Cost
      Cost Per Start: $5,000.00
      Parts Per Week:       4.5

Auxiliary Costs
           Blast Fan:       300    hp
       Bag House Fan:       250    hp
 Cooling Water Pumps:       100    hp
       After-Burners:  10 MMBTU  / hr

Charge Data
        Total Weight:      8000 lb
     Cast Iron Scrap:      3400 lb
            Pig Iron:       870 lb
   Gray Iron Returns:      1800 lb
         Steel Scrap:      1900 lb
        Ferrosilicon:        30 lb

Table 2. Cupola Operating Cost Model -- Calculations

Estimated Melt Energy by Cost Per Ton of Metal

      Amount of Coke:     286 lb /  ton
        Cost of Coke:     $28.60 /  ton
    Hot Blast Energy: 0.00 MMBTU /  ton
      Hot Blast Cost:      $0.00 /  ton
   After-burner Cost:      $0.64 /  ton
              Oxygen:    402 SCF /  ton
         Oxygen Cost:      $0.00 /  ton
  Electricity Demand:        557    kW
     Electric Energy:     20 kWh /  ton
       Electric Cost:      51.15 /  ton
               Total:     $30.40 /  ton

Charge Cost Per Ton of Metal

     Cast Iron Scrap:     $59.50 / ton
            Pig Iron:     $23.36 / ton
   Gray Iron Returns:     $24.75 / ton
         Steel Scrap:     $37.29 / ton
        Ferrosilicon:      $3.90 / ton
     Desulfurization:      $1.95 / ton
               Total:    $150.75 / ton

Maintenance Cost Per Ton of Metal

               Labor:      $0.92 / ton
           Materials:      $3.53 / ton
               Total:      $4.45 / ton
Labor Cost Per Ton of Metal

               Wages:      $3.21 / ton
               Total:      $3.21 / ton

Environmental Cost Per Ton of Metal

       Slag Disposal:      $0.00 / ton
       Dust Disposal:      $0.20 / ton
     Water Treatment:      $0.00 / ton
               Total:      $0.20 / ton

Start-Up Costs
      Cost Per Start:   $5000.00
         Annual Cost: $1,125,000
        Cost Per Ton:     $16.07 / ton
               Total:     $16.07 / ton

Table 3. Cupola Operating Cost Model--Summary

Total Cupola Cost Per Ton of Metal
              Energy:     $30.40 / ton
              Charge:    $150.75 / ton
            Start-up:     $16.07 / ton
               Labor:      $3.21 / ton
       Environmentai:      $0.20 / ton
               Total:    $205.08 / ton

Total Cupola Cost Per Year
              Energy:   $2,127,742
              Charge:  $10,552,500
            Start-Up:   $1,125,000
         Maintenance:     $311,500
               Labor:     $224,700
       Environmental:      $14,000
               Total:  $14,355,442 / yr

Table 4. Arc Furnace Operating Cost Model--Input Data

Operating Data
Tons Per Hour Required:         8  tph
         Hours Per Day:         3  hr/day (average)
         Days Per Week:         5  days/wk
        Weeks Per Year:        51  wk/yr
        Annual Tonnage:      6120  tons/yr
     Required Tap Temp:      2750  F
       No. of Furnaces:         2  furnaces
                  Size:         6  tons/furnace
          Power Supply:      2500  kVA/furnace

Charge Data
          Total Weight:    10,000 lb
               Returns:      6000 lb
             Cast Iron:       730 lb
                 Steel:      3000 lb
              Graphite:       130 lb
          Ferrosilicon:       140 lb

Raw Material Data
           Demand Cost:     $3.04 / kW
           Energy Cost:  0.023493 / kWh
              Gas Cost:     $1.80 / MMBTU
              Pig Iron:   $214.85 / ton
      Low Copper Steel:   $220.00 / ton
              Graphite:    $55.00 / ton
          Ferrosilicon:  $1040.00 / ton
               Returns:   $110.00 / ton
Labor Data
                People:         4 / shift
                 Wages:     13.20 / hr

Maintenance
               Routine:        40 man-hr/wk
        Relining Labor:        23 man-hr/reline
       Tons Per Lining:      1100
       Electrode Costs:      1.80 / lb
       Electrode Cons.:      9 lb / ton
       Environmental Data
         Slag Disposal:     $0.00 / ton
         Dust Disposal:    $10.00 / ton

Table 5. Arc Furnace Operating Cost Model--Calculations

Estimated Melt Energy Cost Per Ton of Metal
                Demand:    4000   kW
                   kWh:     547   kWh/ton
           Energy Cost:  $36.69 / ton
                 Total:  $36.69 / ton

Charge Cost Per Ton of Metal
               Returns:  $66.00 / ton
             Cast Iron:  $15.68 / ton
                 Steel:  $66.00 / ton
              Graphite:   $7.15 / ton
          Ferrosilicon:  $14.56 / ton
       Desulfurization:   $0.00 / ton
                 Total: $169.39 / ton

Maintenance Costs Per Ton of Metal
        No. of Relines:       6 / yr
          Reline Labor:   $0.30 / ton
      Lining Materials:   $1.54 / ton
         Routine Labor:   $4.40 / ton
       Electrode Costs:  $16.20 / ton
                 Total:  $22.44 / ton

Labor Cost Per Ton of Metal
                 Wages:   $6.60 / ton
                 Total:   $6.60 / ton

Environmental Cost Per Ton of Metal
         Slag Disposal:   $0.23 / ton
         Dust Disposal:   $0.23 / ton
                 Total:   $0.23 / ton

Table 6. Arc Furnace Operating Cost
Model--Summary

Total Cost Per Ton of Metal
                Energy:         36.69 / ton
                Charge:       $169.39 / ton
           Maintenance:        $22.44 / ton
                 Labor:         $6.60 / ton
         Environmental:         $0.23 / ton
                 Total:       $235.35 / ton

Total Cost Per Year
                Energy:      $224,543 / yr
                Charge:    $1,036,667 / yr
           Maintenance:      $137,333 / yr
                 Labor:       $40,392 / yr
         Environmental:        $1,408 / yr
                 Total:    $1,440,343 / yr

Table 7. Medium Frequency, Batch Melting Induction Furnace
Operating Cost Model--Input Data

Operating Data
Tons Per Hour Required:       20   tph
         Hours Per Day:       14   hr/day
         Days Per Week:        5   days/wk
        Weeks Per Year:       50   wk/yr
        Annual Tonnage:   70,000   tons/yr
     Required Tap Temp:     2750   F
       No. of Furnaces:        3   Furnaces
                  Size:        8   tons/furnace
          Power Supply:     5000   kW/furnace

Raw Material Data
               Demand:     $3.04 / kW
               Energy:    $0.025 / kWh
             Gas Cost:     $1.80 / MMBTU
             Pig Iron:   $214.85 / ton
          Steel Scrap:   $157.00 / ton
             Graphite:   $550.00 / ton
              Returns:   $110.00 / ton
         Ferrosilicon: $1,040.00 / ton

Labor Data
               People:         5 / shift
                Wages:    $15.00 / hr

Maintenance

              Routine:        35 man-hr/wk
       Relining Labor:        50 man-hr/reline
      Refractory Cost:    $3.250 / reline
      Tons Per Lining:      2000

Environmental Data
        Slag Disposal:     $0.00 / ton
        Dust Disposal:    $10.00 / ton

Auxiliary Costs
        Bag House Fan:       $75   hp
  Cooling Water Pumps:       100   hp

Charge Data
         Total Weight:      8000 lb
             Pig Iron:       560 lb
          Steel Scrap:      5300 lb
             Graphite:       330 lb
              Returns:      1780 lb
         Ferrosilicon:        30 lb

Table 8. Induction Furnace Cost Model--Calculations

Estimated Melt Energy Cost Per Ton of Metal
                 Demand:    12887   kW
                    kWh:      506   kWh/ton
      Total Energy Cost:   $19.36 / ton

Charge Preheating Potential Savings
           Preheat Temp:     1000   F
      Added Energy Req.:     0.60   MMBTU/ton
        Cost of Preheat:    $1.08 / ton
Est. Electrical Savings:       15   %
                               76   kWh/ton
                            $1.90 / ton
            Net Savings:    $0.82 / ton

Labor Costs Per Ton of Metal
                  Wages:   $3.75 / ton
                  Total:   $3.75 / ton

Charge Cost Per Ton of Metal
               Pig Iron:  $15.04 / ton
              Steelcrap: $104.01 / ton
               Graphite:  $22.69 / ton
                Returns:  $24.48 / ton
           Ferrosilicon:   $3.90 / ton
                  Total: $170.12 / ton

Maintenance Cost Per Ton of Metal
        No. of Relines:       35 / yr
          Reline Labor:    $0.38 / ton
             Materials:    $1.63 / ton
         Routine Labor:    $0.38 / ton
       Other Materials:    $0.76 / ton
                 Total:    $3.15 / ton

Environmental Costs Per Ton of Metal
         Slag Disposal:    $0.00 / ton
         Dust Disposal:    $0.05 / ton
                 Total:    $0.05 / ton

Table 9. Induction Furnace Operating Cost Model--Summary

Total Induction Cost Per Ton of Metal
                Energy:       $19.36 / ton
                Charge:      $170.12 / ton
           Maintenance:       $3.15 / ton
                 Labor:       $3.75 / ton
         Environmental:       $0.05 / ton
                 Total:     $196.43 / ton

Total Induction Cost Per Year
                Energy:  $1,355,200 / yr
                Charge: $11,908,400 / yr
           Maintenance:    $220,500 / yr
                 Labor:    $262,500 / yr
         Environmental:       $3500 / yr
                 Total: $13,750,100 / yr

Additional Savings Identified
     Charge Preheating:       $0.82 / ton
                 Other:       $0.00 / ton
                 Total:       $0.82 / ton

Cost With Charge Preheating
                            $195.61 / ton
                        $13,692,700 / yr

The Induction Model

The variables for induction melting include all the basic data from the cupola model, such as melt rate, tap temperature, energy costs and charge material costs. Unique data includes the number of furnaces, furnace size (tons), power supply kW rating, charge preheating options and relining data (number of heats per lining, cost of refractory, etc.).

The desired melt rate, the tap temperature and furnace data are used to calculate energy consumption, while using the furnace kW ratings and melt energy requirements and the electric costs per kW and kWh will produce energy costs. The cost of auxiliaries such as cooling water pumps and baghouse fans also factor into that number. A sideline calculation can show the costs or benefits of charge preheating based upon the optional charge preheating temperature selected on the input page.

Labor, charge material, and environmental costs are calculated similarly to the cupola model.

The Arc-Furnace Model

This model uses similar input data to that for the induction model, with the addition of electrode consumption and cost parameters. Calculation of energy, charge, labor, maintenance and environmental costs is carried out substantially the same as for induction melting.

A Case Study

A gray and ductile iron foundry with a melt-rate requirement of about 28 tons per hr and a total production volume of 72,000 tons per year operates a cupola and several channel holding furnaces. The cupola is run for relatively short campaigns of 8-10 hr with 4-5 campaigns per week. Iron is tapped out of it and transferred to one of several channel induction holding furnaces.

The shop also has two 3000-kW arc furnaces that have not been used for several years. Because some of the ductile iron requires low copper content, the foundry is using expensive low-copper scrap for ductile charge material into the cupola for all ductile iron and then adding copper to the iron for products that require higher copper content. This is adding unnecessary expense to the high-copper ductile iron products.

Several other problem areas have been identified, including difficulty in scheduling production of ductile iron from the cupola and the numerous transfers of molten metal between furnaces and vessels. As part of an overall process and productivity improvement study, the models were used to provide cost data to the decision making process.

The first task was to model the existing cupola operation to understand the various cost factors and identify possible ways to improve the cupola operation. The second task was to see if the arc furnaces could be used cost effectively to deliver some or all of the ductile iron requirements, particularly the low-copper ductile iron. The third task involved analysis of induction melting. The results of the three computer models were then used as input to the overall decision making process to optimize the foundry operation for maximum profitability.

Using different input parameters, the cupola analysis showed two things: oxygen enrichment should probably be lowered to 0.5% from the existing 2-2.5%, and that reducing the number of cupola starts per week and/or lengthening the melt campaigns would provide substantial savings per ton of metal melted.

The cost of operating the arc furnaces for ductile iron is shown in Table 6. The basic cost of melting in the arc furnaces is substantially higher than for cupola melting (Table 3) and is heavily influenced by the cost of electrodes.

For the induction furnace model, it was determined that melting would occur during the utility's off-peak periods in order to minimize electricity costs. The resulting costs compare favorably with cupola melting. The induction melting costs are shown in Table 9.

In the final analysis there are several significant cost and productivity gains to be obtained by conversion to coreless batch induction melting. These include the following:

* savings in charge material by using expensive low-copper scrap only for low-copper ductile iron;

* no need to add copper back for the higher-copper alloys;

* elimination of one or more of the channel induction holding furnaces amounting to roughly $150,000 per year in energy and maintenance cost;

* reducing the amount of iron produced by the cupola can eliminate at least two starts per week saving about $500,000 per year;

* the basic cost of melting with induction is lower than melting with the cupola;

* reducing the number of vessel-to-vessel transfers of molten metal (which may in turn reduce the number of oxide inclusions) will certainly reduce labor requirements.

These factors have led this foundry to decide to install coreless batch induction melting for all ductile iron needs. In the interim, the foundry is considering the use of the idle arc furnaces to melt only the low-copper ductile iron. Preliminary figures indicate this would be cost effective.

Foundry operations are complex, involving processes that are in many ways interrelated. This firm's experience demonstrates that use of a computer model for comparing the economics of cupola melting versus induction melting can facilitate the screening of various melting system modifications and provide valuable input to the decision making process. The spreadsheet models allow fast and easy determination of the impact from various process modifications. This means the foundryman can quickly determine the value of pursuing a process or technology change without spending an inordinate amount of time analyzing options that are not viable.