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Melting Iron Without Forming Slag

Sept. 6, 2021
Oxidation is a natural effect, but eliminating the process of slag formation is simple, and the results can be significant for material quality and foundry productivity.

Regardless of the iron melting technique — EF or cupola — slag will form during the process. Slag consists of oxides that form during the melting cycle. The driving force for oxide formation is the contact of molten iron with the atmosphere, which forms iron oxide, starting the oxidation process.

Iron exposed to the atmosphere starts the chemical reaction that forms iron oxide, and it cannot be stopped unless the contact between molten iron and air is prevented. Steel industry technologists go to great lengths to shroud molten steel, attempting to stop the liquid metal’s contact with air. Their efforts are less than completely successful.

In iron melting and molten iron handling processes there are no attempts to limit air contact, which means that iron oxide is present throughout ironmaking; it becomes an integral part of the melting, holding, and casting process.

Iron oxide contacting molten iron supplies free-oxygen atoms to the iron bath. When iron oxide is present in surface slag, free-oxygen atoms will be present in the molten iron bath. The equilibrium chemical equation – FeO = Fe + O – is the supply mechanism for free-oxygen atoms.

Currently, all the iron that is melted contains some volume of free oxygen unless it has been specifically deoxidized. Free-oxygen atoms produce a dynamic, ever-changing condition within the molten metal mass. Oxides are continuously formed. Solid oxides, such as SiO2 and MNO, agglomerate and continuously rise to the bath’s surface. Gaseous oxide, carbon monoxide, can remain in supersaturated solution, the super saturation caused by ferro-static pressure preventing the precipitation and agglomeration of the oxide.

Once deoxidation occurs, the oxygen atom is removed from the supersaturated carbon-monoxide molecule, freeing the carbon atom to re-enter the molten iron mass. Occasional, one-time increases of 0.40-0.50% C have been noted in the bath’s chemistry, caused solely by the deoxidation process.

A new possibility – Deoxidized molten iron is a fascinating new material. The turmoil resulting from the presence of free-oxygen atoms is gone. The molten iron ”lays flat”, awaiting defect-free casting.

The melting process in coreless furnaces causes significant stirring of the molten iron, continuously exposing fresh iron surfaces to atmospheric contact. In effect, the stirring action in a specific coreless furnace correlates to the oxidation loss occurring in that furnace. In loop-heated furnaces, such as vertical channel furnaces, minimal exposure of the molten iron to the atmosphere occurs, making deoxidation much easier to accomplish.

Once melting has ceased very little stirring action occurs, which greatly reduces the formation of iron oxide on the surface of the molten iron bath. Because of this, reoxidation of the iron bath is reduced. In steel melting, reoxidation is of grave concern; but, in iron melting, holding, and pouring, reoxidation can be disregarded in most instances.

When iron oxide forms in EF melting, a portion of the iron oxide residue embeds in the furnace lining. This residue reduces the working volume of the furnace and requiring a special “melt out” process to remove it. Generally, this process is a high temperature run-up and hold that causes embedded iron-oxide slag to melt out.

It should be noted that many foundries add special dangerous fluxes to aid in melting out the built-up slag. Because DeOX Metal Treatment eliminates iron oxide, no build-ups occur and no flux is needed or wanted.

The sidewall buildup is a worse-case in cast iron melting, but it occurs to some extent in all iron melting. DeOX Metal Treatment molten-iron deoxidation eliminates all sidewall furnace build-ups, which greatly enhances the melting process.

Inductor heated furnaces suffer loop plugging, which hinders furnace service life. DeOX Metal Treatment stops all inductor loop plugging, and in some applications it has reduced existing buildups. Furnace inductor life can be extended by 2X-4X with DeOX application.

Furnace pour spouts and inlets plug on to most furnaces containing molten iron. Plugging is caused by iron-oxide slags. Iron oxide also causes slag “crustiness” and “stickiness.” Eliminating iron oxide with DeOX also resolves the crustiness and stickiness, changing slag into a free-flowing, insulating material and turns a detriment into a beneficial asset.

Many possible supply sources for oxygen atoms are possible. In fact, a technical conference organized by The University of Wisconsin identified more than 125 possible sources. Technical investigation by Mastermelt,LLC, revealed only two free-oxygen atom sources were pertinent:
1) FeO = Fe + O
2) SiO2 = Si + 2O

Ultimately, all other possible sources of a free-oxygen atom supply to the iron bath were of little influence. Primary sources – The main factor in slag formation is No.1 but bringing that reaction under controlled puts an end to the iron bath’s free-oxygen atom supply.

The second reaction results in limited amounts of free-oxygen atoms to the molten bath, but not an excess supply. The volume of free-oxygen atoms supplied by the No.2 reaction is limited to the amount of silicon present in the molten iron. In simple terms, silicon content within the iron supplies a finite amount of free oxygen to the bath, and this amount is basically inert regarding secondary reactions that the free-oxygen atoms could initiate.

Example: The inert level of free oxygen in steel is 1 PPM, whereas in cast iron the inert free-oxygen level irises to 2-3 PPM. In steel, silicon levels near 0.35% Si; and in iron, silicon levels near 2.00% Si. The higher silicon level present in cast iron contributes more free-oxygen atoms to the bath, raising the inert oxygen level of the bath. Silicon influences the inert-oxygen level but it does not supply excess oxygen atoms to the bath, as is the case for iron oxide.

Iron oxide is unique. Widely varying concentrations of iron oxide can form in an iron bath’s cover slag, making it the variable supplier of free oxygen that must be controlled.

Reaction No.1, which is the iron oxide-based supplier of free-oxygen atoms, depends on the supply of iron oxide contacting the molten iron bath, with the amount of iron oxide present varying widely.

In simple terms, the higher the amount of iron oxide present in slag contacting the iron bath, the greater will be the amount of free-oxygen atoms supplied to the bath

Oxygen atoms are high-energy nano-sized atomic particles that seek reaction with other elements in the iron bath. The oxygen atoms react quickly within the iron bath. They exist as free atoms for a maximum of two minutes, ending up as an oxide molecule, combining with elements within the bath, primarily carbon, silicon, or manganese.

Because of the limited time that free-oxygen atoms can exist within the molten iron bath, these atoms must be continually supplied if oxidation is to be ongoing.

Mastermelt’s DeOX Metal Treatment process homes in on the supply side of free-oxygen atoms, effectively cutting off the supply of oxygen by neutralizing the iron-oxide molecule in the bath’s cover slag, chemically reducing it. When the supply of free-oxygen atoms is cut-off, the iron bath’s level of free-oxygen atoms declines quickly to the inert level of oxygen. At the inert level, oxygen-induced reactions are stopped, and no-longer affect iron’s melting and casting processes.

Slag formation is the result of oxidation; by-products of oxidation formed during the melting cycle. When the melting cycle undergoes deoxidation, no slag forms. This is true for all EF melt processes and cupola operations as well.

In cupola operation some slag results from melted coke ash (SiO2), but 80% of cupola slag is eliminated with deoxidation of the melting process.

Beyond belief – Melting without slag formation contradicts the common understanding. Few iron melting technologists can visualize iron melting without slag formation, but that remarkable condition occurs with DeOX Metal Treatment. All oxidation is stopped resulting in no slag formation.

Iron oxide contamination of iron bath cover slag causes many unwanted operating issues. However, cover slag is needed to prevent iron exposure to air. DeOX Metal Treatment removes the iron oxide component in slag, converting the crusty-slimy slag into a free-flowing cover barrier that enhances the melting, holding, and pouring processes, stopping the reoxidation of the iron bath

Deoxidation of the iron melting process is the key, and at this time the DeOX Metal Treating process is the only technology available to accomplish it.

Iron has been melted without deoxidation since the beginning of the Iron Age, in 500 BC. It can continue to be melted today without deoxidation, but many casting quality and manufacturing advantages result from deoxidation and the savings are remarkable.

Eliminating slag formation in iron melting and casting is simple -- add small amounts of DeOX to the melting process. Deoxidation will occur quickly.

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