A cupola iron foundry should treat approximately 8:1 to 13.5:1 as the practical reference band for metal-to-coke ratio, with the operating target tied to furnace capacity, coke properties, blast rate, and required metal temperature.
The ratio expresses kilograms of molten metal produced per kilogram of coke: 8:1 means 8 kg of iron per kg of coke, while 13.5:1 means 13.5 kg, and published operating examples span this band.
Define the Ratio and Its Practical Range
Metal-to-coke ratio is the mass of metal melted divided by the mass of coke consumed. The inverse, coke-to-metal ratio, is equally important for costing and fuel accounting: an 8:1 metal-to-coke ratio requires 0.125 kg of coke per kg of metal, whereas 13.5:1 requires about 0.074 kg. [S4]
One published cupola design example specifies a metal-to-coke ratio of 8.1 for a melting rate of 10 t/h in a cupola with a 1.22 m internal diameter [S2]. A separate intelligent-control study reports 13.5% coke by weight in the charge, equivalent to a 13.5:1 metal-to-coke ratio, with a target cast-iron composition of 3.2–3.5% carbon and 2.1–2.4% silicon [S1].
The 8–13.5:1 range should therefore be used as an initial design and benchmark band, not a universal set point. Efficiency is often expressed as tonnes of iron melted per tonne of coke, which is the same comparison on a 1,000:1 basis [S6].
Select the Target by Furnace and Fuel Conditions
Use the lower part of the range when the design requires a high melting rate, strong oxidation control, or a specific tapping temperature, because more coke provides more heat and carbon control than a high-ratio target. The 10 t/h design example at 8.1:1 shows how the ratio is linked to furnace diameter and output rather than selected independently [S2].
Use a higher ratio only when coke quality, burden preparation, air supply, and furnace control can sustain the required heat release and metal chemistry. Experimental work on an 18 in. cupola specifically evaluated blast rate, metal-coke ratio, and coke size as variables affecting melting rate, tapped-metal temperature, and stack-gas composition [S4].
Coke size, blast rate, and metal-to-coke ratio must be considered together. A charge calculation that changes one of these variables without adjusting the others can miss both the energy target and the permitted carbon pickup. Cupola design and operation guidance also treats blowing rate as part of the same design calculation [S2].
Compare High and Low Ratio Targets
At 8:1, 10 kg of metal consumes 1.25 kg of coke; at 10:1, it consumes 1.00 kg; at 12:1, it consumes 0.83 kg; and at 13.5:1, it consumes 0.74 kg. The 100 kg-metal comparison therefore changes coke demand from 12.5 kg to 7.4 kg across the reference band. [S2]
Against a 10 t/h melting rate, an 8.1:1 ratio corresponds to 1.23 t of coke per hour, while 13.5:1 corresponds to 0.74 t/h. These are mass-flow comparisons only, not a guarantee of thermal efficiency, because actual coke consumption also depends on oxidation, burnout, moisture, impurities, and heat losses.
For a foundry, the decision criterion is not simply the highest possible ratio. Compare the options on required coke consumption, obtainable tapping temperature, carbon and silicon control, melting rate, and sensitivity to coke size. The 8.1:1 design case demonstrates that output and furnace diameter constrain the ratio, while the 13.5:1 control case ties the ratio to target iron chemistry [S1][S2].
Fit the Ratio to the Production Objective
A high-ratio target is appropriate where fuel cost and oxidation losses dominate, provided the furnace still reaches the required tap temperature and carbon level. The published 13.5% coke-by-weight target was linked to a specific cast-iron chemistry of 3.2–3.5% carbon and 2.1–2.4% silicon, so it was not a generic efficiency claim [S1].
A lower-ratio target is safer as a starting point for capacity studies, high-output operation, or foundries requiring generous heat for refractory and metal losses. The 10 t/h example links 8.1:1 to a 1.22 m diameter cupola, and its stated blowing requirement shows that blast capacity must be designed with the fuel ratio [S2].
Small furnaces are not automatically high-ratio installations. Published cupola sizes range from about 0.5 m to 4.0 m internal diameter, expressed in foundry practice as roughly 1.5 to 13 ft [S3]. A 0.5 t/h research design also demonstrates that small cupolas remain actively engineered around fuel, air, charge, and output rather than copied from large-furnace ratios [S8].
Control the Variables That Can Break the Target
Poor control of the average coke and metal temperatures is a documented contributor to poor cupola performance [S7]. This matters because a calculated ratio cannot compensate for unstable combustion, uneven burden descent, or insufficient heat transfer through the shaft.
Heat-balance methods can calculate metal, coke, and gas temperatures at different furnace levels using an 18 in. experimental cupola and heat-transfer calculations [S4]. Full-scale measurements using the properties of coke and raw material formed the basis for calibration of a separate mathematical model [S5]. These methods support process adjustment by identifying where heat transfer or combustion is departing from the target.
Common operating variables include blast rate, coke size, metal-coke ratio, raw-material properties, and the height at which metal is tapped. A cupola furnace normally burns coke or charcoal in a vertical shaft, receives air through tuyeres, and melts charged pig iron, scrap, and returns [S3][S9].
Apply the Ratio as a Controlled Process Variable
Start from a mass-flow specification: tonnes of metal per hour divided by tonnes of coke per hour. Verify the value in the plant using weighed coke additions and measured metal output, then adjust one major variable at a time while monitoring tap temperature, carbon content, silicon content, melting rate, and stack-gas composition [S1][S4].
Do not treat a manual ratio sheet as sufficient control. The holding furnace can buffer temperature and chemistry variation after melting, but it does not correct a mismatch between fuel input and required cupola heat. The correct target remains the lowest coke rate that reliably meets metal output, tapping temperature, composition, and campaign-duration requirements.
For traceable plant records, capture furnace diameter, nominal melting rate, coke size range, blast rate, coke consumption, tapped-metal mass, metal-to-coke ratio, tap temperature, and metal chemistry. Revalidate the target after changes in coke supply, charge mix, oxygen enrichment, or casting schedule.
The next trackable signal is consistency at the measured target: if coke consumption rises while tapped temperature or carbon chemistry misses specification, the furnace is operating above its effective ratio, and the blast, coke size, burden, or heat-loss balance should be reviewed.
