Maximizing Cupola Performance

Iron melting operations face the same challenge with electric furnaces and cupola furnaces — molten metal oxidation. Oxidation is caused by molten iron’s contact with the atmosphere. Last year in these pages we addressed in detail the factors that contributed to oxidation in EF melting, and the technology to counter it. Cupola melting faces a greater challenge: a much higher volume of iron oxide produced during the melting cycle.

In cupola melting, blast air contacts the descending molten metal droplets, instantly forming iron oxide coatings on the droplets’ surfaces. Molten metal’s exposure to atmospheric contact is much greater in cupola melting; it cannot be avoided, which makes cupola oxidation losses magnitudes more severe as compared to EF melting.

Over the years, proponents have touted EF melting for better iron chemistry control. Lesser oxidation losses in EF melting accounted for this claim.

Cupola melting was favored for higher production volumes, but metal quality suffered at times. The metal quality issue revolves around iron oxide that forms during melting — but the process by which that happens is unknown to most cupola operations.

The cupola melting cycle needs to be examined to pinpoint exactly how the oxidation losses occur and how they can be overcome.

Cupola Process Detailed

Blast air enters the cupola through tuyeres extending into the melting zone. The oxygen-laden air contacts incandescent coke, causing combustion and liberating heat. In the process, the oxygen-laden  air contacts the descending molten metal droplets. Iron oxide formation on the droplets’ surfaces cannot be avoided. If the blast air contains oxygen molecules, which are needed for combustion, iron oxide always will be formed.

Oxygen-laden blast air is only available in a shallow-limited zone at tuyere level.  In the remainder of the cupola, no oxygen molecules exist to form iron oxide. In this area, called tuyere raceways, temperatures reach 5,000° F, which causes partial vaporization of whatever is present. Molten iron, iron oxide, and SiO2 coke ash all are vaporized, ascend the cupola as a gas, and condense back to a liquid above the melt zone. This process, which is identical to what occurs in blast furnaces, distributes liquid iron-oxide throughout the upper cupola. All the cupola charge ingredients are coated with iron oxide. Thus, all cupola furnaces become contaminated with iron oxide during normal operation.

In blast furnaces, coke combustion is controlled to favor large amounts of carbon monoxide production, which chemically reduces iron oxide. Iron oxide is the primary melting ingredient in blast furnaces.

In cupolas, iron oxide is not intended to be part of the metallic charge but occurs only as a contaminant. The fact that iron oxide is distributed throughout the upper cupola during normal melting must be addressed.

Iron oxide has been identified as the root cause of EF melting problems. It is also true in cupola melting. Iron-oxide formation cannot be stopped in cupola melting, which means some method must be formulated to neutralize its presence.

Iron oxide production in the tuyere raceways and ultimately its contamination of the entire cupola cannot be stopped. But, the iron oxide contamination in the upper cupola can be countered by regulating the coke rate.  Coke rates exceeding 12% produce enough carbon monoxide to chemically reduce the vapor-deposited iron oxide in the upper melt zone.  Coke rates less than the 12% allow some iron oxide to remain, which leads to silicon oxidation during the initial iron melting stages occurring in the upper melt zone.

Key to success. To be successful, cupola operators must realize that iron oxide is always produced during normal melting cycles and iron oxide contamination of the upper melt zone always occurs. Silicon-oxidation losses can exceed fifty percent, in the worst-case scenarios.

In addition to silicon loss in the upper melt zone, significant carbon oxidation occurs in the tuyere raceway area, the high-temperature zone of the cupola. This carbon loss can exceed one percent carbon present in the molten iron, which is very significant in view of overall carbon levels near 3.50% C. Over one-third of carbon contained in the molten iron can be lost to oxidation.

Important operational point. Carbon loss at the tuyere raceway level has not been recognized in most cupola operations. Molten iron chemistries at tap-out reflect metal chemistries before iron oxide contamination has spread throughout the cupola. Carbon and silicon levels present at this point represent near-true chemistry without oxidation losses.  Oxidation losses mount rapidly as melting continues, with losses reaching steady-state 1.0-1.5 hours after melting starts.

Attention Cupola Operators

Compare tap-out chemistry to the cupola run chemistry after one hour. See for yourself the level of oxidation losses that are costing your foundry bottom-line losses far exceeding anything you have previously realized. Coke rates can approach 6%-7% with de-oxidation of the cupola melt process.  Normal cupola coke rates are 10%-14%.

DeOX tuyere injection instantly reduces iron oxide formed in the tuyere raceways. DeOX cannot stop iron oxide formation, but it instantly reduces the oxide volume after it forms. Iron oxide is reduced to inert by-products, which no longer supply oxygen atoms to the molten iron. It is the only material that accomplishes this. Cupolas need to be de-oxidized and DeOX metal treatment is the only technique to accomplish that.

Important new technology. Carbon oxidation losses, always occurring in cupola tuyere raceways, are stopped by de-oxidation, as is the vaporization of iron oxide and its spread throughout the cupola.  

Cupola melting produces straight-line chemistry results when iron oxide is neutralized. Iron oxide is, by far, the greatest detrimental influence in cupola operation, and in the past its presence has gone unchecked due to the inability to counteract it. DeOX metal treatment removes that limitation, and now cupola melting can be free of iron oxide contamination.

Cupola coke rates. Today, coke rates are adjusted upward to increase carbon levels in the melted iron. Generally, the carbon increases are minimal when compared to the carbon losses occurring in tuyere raceways. 

Carbon losses in the raceways exceed one-percent, and coke rate increases to elevate carbon levels by 0.10%-0.15%C are commonly sought.  The carbon oxidation losses occurring in the raceways end up being huge sources of carbon level increases.

Cupolas effectively melt iron at 6%-7.5% coke rates, which continues now in U.S. metalcasting operations.  But, soon as the melting constraint of requiring carbon level increases from coke is applied, coke rates are elevated upward. Eliminating carbon oxidation loss favors reduced coke rates of 6%-7%.

Controlling oxidation. Heat is produced in cupola melting through the combustion of coke, which is an oxidation process. The successful cupola operation allows the coke combustion to continue unhindered but draws the line at iron oxidation. Iron oxide formation must be countered; it cannot be stopped.

Molten iron contacting oxygen-laden blast air produces iron oxide on the molten iron’s surface. DeOX de-oxidation removes that oxide coating without hindering the coke combustion process.

Free-oxygen atom technology. Iron oxide, when present in surface slag, adds free oxygen atoms to the iron bath. These oxygen atoms spread throughout the bath, quickly forming “oxides” once the enter the iron bath. Cutting off the oxygen atom supply – ridding the surface-slag contacting the iron bath of iron oxide  – becomes the key to de-oxidation.

Key Technology. Iron oxide is the only significant source of free-oxygen atoms in molten iron. Silicon, which oxidizes to SiO2, adds some free oxygen atoms, but iron oxide is by far more influential. As detailed in the previous series examining EF melting practices, iron oxide control is paramount to successful cupola melting. Cupola de-oxidation is easily accomplished by chemically reducing iron oxide as it forms in the cupola raceways. DeOX metal treatment is the only technology that accomplished that goal.