Computer models give accurate iron melting method economics.
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.
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|Title Annotation:||Computers in the Foundry|
|Author:||Cooley, Edwin M.|
|Date:||Sep 1, 1996|
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