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Electric furnace melting is more susceptible to FeO-contaminated steel scrap than is cupola melting. Typical cupola silicon-oxidation loss is 25-30%, compared to EF silicon loss of 10-15%.

Controlling Iron Oxide to Stop Carbon and Silicon Losses

June 30, 2020
Iron foundries must address and eliminate the effects of oxidation by controlling FeO in slag contacting the molten metal.

Carbon and silicon oxidation losses always occur in iron melting. The losses are costly, but they can be prevented easily with newly available technology.

Free-oxygen atoms present in the molten iron cause oxidation: These unwanted atoms are supplied by iron-oxide molecules contacting the molten metal surface. Oxygen atoms cannot be “removed” effectively from molten iron. Nothing can be done to stop the “oxidation” process once they enter molten iron.  

Oxygen atoms seek out and combine with other atoms present in the molten iron, i.e., carbon, silicon, and manganese. They are oxidized according to the Laws of Thermodynamics, which identifies the primary element or elements oxidized according to the existing conditions of the molten bath, based primarily on the molten metal’s temperature.

The secret to stopping oxidation losses is to cut off the supply of free-oxygen atoms. Iron oxide (FeO) contacting the iron surface establishes an equilibrium chemical reaction: FeO = Fe + O. So, stopping the supply of the oxygen atom becomes stopping iron-oxide contact with molten iron. It sounds easy but in fact it’s very difficult, and it has been the basis of Mastermelt’s +20-year project.

Primary steelmakers simply change the slag containing unwanted FeO, a step they call “cleansing the slag”. A clean, lime-based slag, free of iron oxide, replaces the unwanted slag. The new, FeO-free slag creates a barrier to atmospheric contact, preventing new iron-oxide formation.

In iron melting, few special precautions are taken to prevent iron-oxide formation, and up to now ferrous foundries have accepted the consequences of its presence. That is changing with DeOX and DeOX de-oxidation technology.

This is the second in a series of reports addressing the effects of oxygen in iron melting. Read the first: Harmful Effects of Molten Metal Oxidation.

Mastermelt engineers have determined FeO contacting the molten iron surface produces numerous detrimental effects in molten iron. The first step to controlling the problem is to determine the current extent of those oxidation losses, and assign costs to them. Then, a plan can be selected for halting the losses.

The two most common iron melting methods – cupola melting and electric furnace melting – need to be addressed separately, as each has its own sources for iron-oxide contamination during melting.

Cupola Melting — In cupola operations, metallurgical tests soon after the tap-out indicate the metal chemistry before FeO has built up inside the cupola; before that, iron oxide has had a chance to reduce carbon and silicon levels dramatically. It takes about an hour of melting before levels of FeO inside the cupola reach steady state. Once iron-oxide levels stabilize, carbon and silicon loss in the cupola will stabilize somewhat. Hence, tap-out chemistry indicates the conditions “before” oxidation’s effect, and metal chemistry an hour or so following tap-out indicates the condition “after” oxidation has taken hold.

It is important to realize that in cupola melting the descending molten-iron droplet is exposed to atmospheric oxygen in the tuyere raceway, forming FeO at that point. Free-molecular oxygen is available only at tuyere level.

Iron-oxide formation on the molten droplet’s surface at tuyere level cannot be stopped; it forms as soon as the molten iron surface is contacted by oxygen-laden blast air. The iron oxide, formed at tuyere level, systematically spreads throughout the cupola (and described in greater detail here.) Understanding cupola oxidation technology is critical to solving cupola oxidation loss.

1. Silicon oxidation takes place throughout the melt zone and retained slag layer.
2. Carbon oxidation occurs primarily in the tuyere raceway section, where the higher temperatures favor its oxidation. In the lower temperature areas of the cupola, silicon oxidation preferentially occurs.

Overall oxidation losses go hand-in-hand with the iron-oxide level in the cupola. In turn, blast-air humidity levels influence iron-oxide levels significantly. Water contained in blast air accelerates iron-oxide formation many times over. Consequently, high-humidity summer days wreak havoc on cupola melting. Mastermelt’s experiences with the “summertime” problems for ferrous foundries could be the basis for a novel.

Determining cupola oxidation losses. Compare tap-out chemistry with metal chemistry after the first hour or so of melting. Both carbon and silicon levels gradually decline after tap-out to points determined by the melting “conditions of the day.”

The chemistry change correlates to the oxidation losses occurring. Losses reach a steady state, leveling out when FeO contained in the slag exiting the cupola equals the amount of FeO continually forming inside the cupola. Iron-oxide levels within the cupola control the severity of the losses.

In some cupolas, the oxidation losses are consequential. Silicon loss can exceed 40-50%. In better cupola operations, silicon losses near 20-30%.  Actual carbon loss can exceed 1.20% C in some cupolas but typically it nears 0.70% C. The lowest carbon loss recorded has been 0.40% C.

Silicon loss can be avoided with elevated coke rates combined with treatment of FeO in the retained slag layer. Carbon oxidation loss, however, cannot be eliminated without de-oxidation in the tuyere raceway via tuyere injection of a de-oxidation material.

In all cupola operations, both silicon and carbon oxidation loss can be eliminated easily, without requiring minimal coke rates. DeOX tuyere injection stops FeO formation in the tuyere raceways. Because no iron oxide is vaporized to spread throughout the cupola, coke rates can be adjusted primarily based on the metal’s temperature exiting the cupola. This fact opens the door to numerous cupola design and operating techniques (to be addressed in future entries to this series.)

Oxidation mechanisms — Carbon oxidation in the cupola occurs in the tuyere raceway, the area of extreme high temperature within the cupola. Iron oxide forms when blast air containing the oxygen molecule (O2) contacts the molten iron droplet. Iron oxide instantly forms on the droplets surface: 2Fe + O2 = 2FeO. Temperatures reach near 5,000°F in this area.

This high-temperature zone ends up vaporizing a portion of whatever is present there: molten iron, FeO, and coke ash.  The vaporized gas ascends the cupola, rising to above the upper melt zone where temperatures cool it, initiating condensation of the gas into a liquid. The liquid formed is a combination of liquid iron, liquid iron oxide, and liquid coke ash (SiO2). The liquid coats the descending charge materials.

Steelmakers have documented this vaporization phenomenon extensively in literature on blast furnace operations. Cupola operation differ from blast furnaces: while BF combustion gas products contain primarily carbon monoxide (CO), cupolas are operated with much less CO in the combustion gasses.

The operational difference between blast furnace and cupola combustion gasses determines the fate of the FeO descending from the upper melt zone of a cupola. Iron oxide will remain intact and will not be chemically reduced by CO, if low levels of carbon monoxide exist.

Coke rates in cupola melting determine combustion gas ratios: CO / CO2. Mastermelt engineers suggest a 12% minimum coke rate is required to produce adequate carbon monoxide to effectively chemically reduce FeO levels produced by blast air containing the humidity levels typically recorded in the continental U.S.

Thus, when FeO continues to exist in the upper melt zone, where temperatures are much lower than at the tuyere level, silicon will be preferentially oxidized there. If adequate carbon monoxide is available, iron oxide is chemically reduced in the upper melt zone and silicon oxidation is forestalled. When this occurs, the only silicon oxidation possibly taking place will be silicon lost to iron-oxide chemical reactions in the retained slag layer.

Returning to the tuyere raceway: Iron oxide is formed in the raceway, which instantly reacts with the descending molten metal droplet, and because of the high temperature, oxidizes some of the carbon in the molten metal. A portion of the FeO formed at tuyere level is vaporized with the remaining FeO descending with the droplet to the cupola’s retained slag layer. Slag adhering to descending metal droplets is wiped clean as the droplet travels downward through the slag layer, exiting through the taphole as cleaned metal. Slag accumulates in the retained slag layer, eventually gaining enough mass and forcing all molten metal to exit the cupola while allowing some slag to exit the same taphole.

So the picture develops: Silicon is oxidized at times in the upper melt zone; carbon is oxidized always in the tuyere raceway. Silicon is oxidized sometimes in the retained slag layer.

Once the metal droplets have descended past the high-temperature zone, they descend through the retained slag layer located above the cupola’s taphole. Slag/metal reactions always occur in the retained slag layer. The length of time that droplets are exposed to these slag/metal chemical reactions depends on the depth or volume of slag in the retained slag layer.

Time, temperature, chemistry. Chemical reactions, such as the slag/metal reactions, are greatly influenced by time and temperature. The time of contact between the iron droplet and slag significantly controls the extent of the chemical reactions. High retained-slag amounts inside the cupola extend the contact time between the slag and the metal droplet, which can significantly increase silicon oxidation losses.

Another way of stating this: If high iron-oxide levels in retained slag cannot be prevented, silicon oxidation losses will be reduced if the retained slag level is minimized within the cupola. Minimal retained-slag levels will minimize the time molten iron is exposed to the FeO present in the retained slag. Mastermelt’s operational plastic cupola model demonstrates retained slag’s relationship to operational variables (also to be addressed in future entries to this series.)

Minimizing the slag/metal contact time effect is demonstrated several ways. Consider: When the metal dam height is reduced, silicon levels increase; or, when the taphole “blows”, indicating no retained slag in the cupola, silicon levels also increase. Less slag or no slag reduces the slag/metal reactions occurring in the cupola, reducing loss and causing the silicon level to rise.

The slag/metal chemical reactions occurring in the retained-slag layer are determined by the chemical composition of the slag layer. Iron oxide resides in the slag layer, and this causes a new round of chemical attack on the key elements in the molten iron, namely carbon, silicon, and manganese.

Mastermelt engineers have controlled the iron-oxide level in the retained-slag layer and in doing so have controlled silicon-oxidation loss. Cupolas have been operated with no silicon loss only to have the silicon loss occur unfortunately as the humidity in the blast air increases.

Mastermelt tuyere injection in a southern U.S. cupola reduced silicon oxidation to zero for a portion of a day until humidity rose, posting a 74°F dewpoint. Then, the silicon-oxidation rate in the cupola rose to 45%. The following day, operating under the same conditions, but increasing the coke rate from 10 to 12%, silicon oxidation loss was eliminated throughout the day. The 12% coke rate produced enough carbon monoxide to chemically reduce the iron oxide causing silicon oxidation in the upper melt zone. In this cupola, Mastermelt tuyere injection neutralized the iron oxide in the retained slag layer, but needed the assist of CO to neutralize the FeO in the upper melt zone.

The solution to carbon and silicon oxidation in cupola melting is to chemically reduce iron oxide, which is produced continuously throughout melting. But, iron oxide is always produced in tuyere raceways, and cannot be prevented. To stop carbon- and silicon-oxidation losses:  DeOX must be injected with blast air so that as FeO forms, it is chemically reduced to inert by-products.

Electric Furnace Melting — EF melting has always incurred some silicon and carbon loss. The carbon loss is typically attributed to “poor graphite.” Most EF experts declare the silicon loss to be inevitable and unavoidable. Mastermelt engineers disagree.

Electric furnace melting is less susceptible to iron-oxide formation during the melting process than is cupola melting, which results in less silicon and carbon oxidation loss. Electric furnace melting is more susceptible to FeO-contaminated steel scrap than is cupola melting. Overall, oxidation losses in EF melting are less than cupola melting. Typical cupola silicon-oxidation loss is 25-30%, compared to EF silicon loss of 10-15%.

Mastermelt engineers have proven nearly all carbon and silicon-recovery issues in EF melting occur due to FeO present in the furnace slag. It is the same technology as described above for cupola melting: FeO in the cover slag adds oxygen atoms to the bath, which causes the oxidation reactions. Molten metal temperature determines whether carbon or silicon is oxidized.

When iron oxide in cover slag is eliminated in EF melting, no oxidation losses are encountered. Simple addition of DeOX to a coreless furnace charge will produce full recovery of both carbon and silicon. In one example, carbon recoveries nearing 75% with standard melting practice increased to full 100% recovery, and silicon recovery of nearly 85% also increased to full 100% recovery with DeOX treatment.  It is a new technology, and  simple to evaluate.

In typical EF melting, some FeO is produced by exposing molten iron to atmospheric contact during the melting and superheating stirring process. Typical slag levels in coreless melting are near the 15 to 25% FeO range. Typical free-oxygen levels of EF melted iron exceed typical cupola free-oxygen levels. But, with small amounts (0.25% - 0.40%) of DeOX added with the furnace charge, the iron-oxide level in the resulting EF slag is typically less than 0.5% FeO, which is similar to cupola slag’s FeO content during full carbon- and silicon-recovery periods.

The new de-oxidation technology means iron foundries’ carbon and silicon losses can be eliminated by controlling iron oxide in slag contacting the molten iron.

Ron Beyerstedt is the president of Mastermelt LLC. Contact him at [email protected]

This is the second in a series of reports addressing the effects of oxygen in iron melting. Read the first installment: Harmful Effects of Molten Metal Oxidation.