A cupola charge is sized from three ratios: iron-to-coke (the melting ratio, often 6:1 to 10:1), limestone-to-coke (the flux ratio, generally 10% to 45% by weight), and the fixed-carbon fraction of the coke itself (commonly 85%) [S5][S7].
The 8:1 example commonly given in textbooks works out to 0.125 kg coke per kg iron, with 0.375 kg/hr per cm² of hearth area as the specific melting rate for an 80 cm diameter cupola, illustrating how the three numbers feed straight into a tonnage target [S7].
Coke Rate and the Iron-to-Coke Melting Ratio
Cupola efficiency is reported as the tonnage of iron melted per tonne of coke burned, so the iron-to-coke ratio is the primary lever an operator can pull in an hour-to-hour window [S3]. Cold-blast cupolas commonly run at 6:1 to 8:1, and hot-blast, recuperated units with blast preheat around 400 to 600°C can push the ratio to 10:1 or better because a fraction of the CO leaving the stack is re-burned in the recuperator [S2].
For a baseline 8:1 charge, 1000 kg of metal needs 125 kg of coke, and at an 85% fixed-carbon coke the stoichiometric minimum air is roughly 4.6 kg O2 per kg C, which sets the blower capacity required for a target melting rate [S7]. A 35-tph production cupola illustrates the upper end of the operating envelope, where tuyere-zone temperatures near 5,000°F drive silicon and carbon pickup chemistry and force tight control of the iron-oxide formation reaction [S5].
Limestone-to-Coke Ratio and Slag Chemistry
Limestone addition is set as a weight percentage of the coke charge, with operating shops reporting 10% to 45% limestone and the 10% figure flagged as a baseline that should reproduce across similar cupolas [S5]. The limestone calcines inside the stack (CaCO3 to CaO + CO2), and the resulting CaO combines with SiO2, FeO, and Al2O3 carried down from the melt zone to form a fluid slag that drains with the iron.
Charge order matters: the textbook sequence is coke (bed plus incremental), limestone and other fluxing agents on top of the coke, and then the iron-bearing materials (pig iron, returns, steel scrap) loaded last so the descending metal passes through the preheated coke column [S4]. Slag basicity targets, often expressed as CaO/SiO2 in the 1.2 to 1.6 range on acid-lined North American cupolas, are adjusted by moving the limestone percentage up or down within that 10% to 45% band [S3].
Comparison: Iron-to-Coke and Limestone-to-Coke Across Cupola Types
Operating point selection is a trade-off between melting rate, spout temperature, and scrap tolerance, so the same ratios do not transfer between designs [S2].
Cold-blast cupola, melting rate 2 to 10 t/h, iron-to-coke 6:1 to 8:1, limestone 30% to 45% of coke, blast temperature ambient, spout iron around 1,450 to 1,500°C, and only limited steel scrap in the charge. Hot-blast cupola, melting rate 10 to 35 t/h, iron-to-coke 8:1 to 10:1, limestone 20% to 30% of coke, blast preheat 400 to 600°C, and spout iron 1,500 to 1,550°C with greater scrap tolerance. Gas-assisted or oxygen-enriched cupola, melting rate comparable to hot-blast, partial coke substitution by natural gas or oxygen injection, and limestone 10% to 20% because higher tuyere temperatures lower the slag volume needed to cover the metal [S2][S5].
Oxidation Loss: The Hidden Tax on the Charge
Cupola iron melting and superheating combined is roughly 60% of the coke energy; the rest leaves as stack gas and slag sensible heat, so any oxygen that slips into the metal stream is a direct efficiency loss [S1]. Worst-case iron-oxidation losses in cupola melting approach 65% of metallic iron charged, against 20% to 30% in electric furnace melting, which is why the same nominal iron-to-coke ratio gives different metal yields on different days [S5].
Blast humidity is one of the few inputs the operator can change quickly to dampen the tuyere raceway oxidation, and injection of deoxidizing agents such as silicon-bearing or carbon-bearing powders into the raceway is a documented method of cutting the FeO feedback into the slag [S5]. This is the variable that most often makes a charge that worked at 8:1 last week underperform at 8:1 this week, and it is the reason the cupola furnace operating record tracks backpressure, blast temperature, and slag FeO at the same time as the weigh-hopper numbers.
Step-Through: A Worked 8:1 Charge Example
For an 80 cm inner diameter cupola, melting ratio 8:1, specific melting rate 0.375 kg/hr/cm², and coke at 85% fixed carbon, the hourly iron yield at the spout is 0.375 × π/4 × 80² ≈ 1,885 kg/hr, with coke consumption of 1,885 / 8 ≈ 236 kg/hr [S7]. Limestone at 30% of coke is 71 kg/hr, and blast air at stoichiometric ratio for 85% carbon is 236 × 0.85 × 4.6 ≈ 923 kg O2/hr, or roughly 3,800 Nm³/hr of air at 21% O2 [S7].
That same charge, applied as a layer cake, becomes 236 kg coke, 71 kg limestone, and 1,885 kg metal per hour, repeated with a 30 to 60 minute cycle delay between top charging and spout response [S3]. Practical schedules group the metal into multiple buckets per "drop" so that the iron-to-coke ratio and the limestone-to-coke ratio both stay inside their working windows even when the iron mix swings between high-pig-iron and high-scrap heats [S6].
Limits of the Ratio Method
The ratio approach assumes the coke is consistent, the blast is dry, and the lining is acid, so a swap to a basic refractory or a wet blast moves the optimum limestone percentage outside the published band [S3]. North American acid-lined cupolas dissolve a basic slag faster than the limestone can be re-adjusted, and operators on those furnaces keep CaO/SiO2 in the slag in the 1.2 to 1.6 range rather than chasing the higher basicities used on basic-lined European practice [S3].
Charge calculation is a steady-state, top-of-furnace calculation, and the 30 to 60 minute transport delay means an operator changing the iron-to-coke ratio today sees the iron chemistry at the spout roughly an hour later, which is why most foundries run ratio changes in small steps rather than single large jumps [S3][S6]. Cast iron grade targets, especially the carbon and silicon ranges that define gray versus ductile base iron, are ultimately the constraint that the charge ratio has to satisfy, and the crucible furnace comparison only holds in the narrow band where both furnace types can hold temperature long enough to settle the melt.
Sourcing and Standard References
The charge-calculation method traces back to Bureau of Mines work on a gas-assisted cupola, where the 60% thermal efficiency figure for melting and superheating was first quantified, and to foundry-lexicon data on cold-blast versus hot-blast operation that anchors the 400 to 600°C blast preheat window [S1][S2]. Industry-operating articles provide the 10% to 45% limestone band and the tuyere-zone 5,000°F figure for a 35-tph cupola that sets the upper limit on the iron-to-coke ratio in production service [S5]. The textbook 8:1 example with melting rate 0.375 kg/hr/cm² and 85% carbon coke remains a standard student calculation for an 80 cm diameter cupola [S7].
Trackable signals for the next six months: (1) any shift in published limestone-to-coke percentages as oxygen enrichment and natural-gas-assisted cupola trials continue, and (2) any revision to the 65% worst-case iron-oxidation loss figure as tuyere-injection deoxidation methods accumulate more operating hours.
Background reading: EPDM Modulus and Stiffness: Engineering Inputs for Gasket Design.