Electrorefining for direct decarburization of molten iron

Application of electromotive force between molten iron-carbon and slag is shown to decarburize iron. Electrorefining decarburizes by direct interfacial electrochemical reaction, resulting in low solubilized oxygen in iron, even at low carbon concentration.

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Industrial plants that transform raw materials into steel are referred to as steel mills or steelworks. There are two main types of steel mills, integrated mills that produce steel using mostly iron ore and mini-mills that produce steel using mostly scrap. Recycling steel scrap with electric arc furnaces in mini-mills produces drastically less carbon emissions and pollutant discharge. Hence, increased recycling of steel scrap is needed for sustainable development and for closing material loops to enable circular economies. However, the quality of recycled steel is often subpar compared with virgin steel, and there is an urgent need for new refining technologies for recycling steel. In our article, we investigated a new electrorefining technology for this application where we electrolytically decarburize molten iron in slag.

Many existing decarburization procedures are known to proceed indirectly. Typically, oxygen gas is blown through lances and tuyeres to decarburize iron melts at high temperature. Here, much of the oxygen gas dissolves in molten iron according to

(1)       ½ O2(g) = [O]

where the square brackets indicate oxygen is dissolved in iron. Solubilized oxygen then reacts with carbon dissolved in molten iron and forms carbon monoxide gas that evolves from melt,

(2)       [C] + [O] = CO(g)

Hence, decarburization proceeds indirectly. That is, the molten iron bath is first oxidized before decarburization occurs. This leads to high residual oxygen that must be removed by subsequent deoxidization processes (typically aluminum is used as a deoxidizer). The latter adds cost and can also result in oxide inclusions in the steel microstructure. It would be quite lucrative if iron could be decarburized without excess oxidation of the metal bath.

In our work, we investigated electrorefining of molten iron in slag as a possible new route for decarburization. We first demonstrated that electrochemical decarburization of iron in slag is possible by corelating visual gas evolution and carbon monoxide concentration in the off-gas with anodic polarization of iron in slag. We then performed in-depth electrochemical studies on different iron-carbon alloys. Our electrochemical analyses show that decarburization takes place by direct interfacial electrochemical reaction,

(3)       (O2−) + [C] = CO(g) + 2e

and not by indirect reaction,

(4)       (O2−) = [O] + 2e 

(2)       [C] + [O] = CO(g) 

Proof of this comes from certain electrochemical measurements and theory, the heart of which are shown in Figure 1.

Figure 1. Electrochemical measurements of molten iron-carbon alloys in slag. (a) Anodic polarization curves of different iron-carbon alloys. Points are experimental measurements and lines are fitted Tafel equations. (b) Dependence of the Tafel exchange current on carbon activity.

First, we observe a Tafel region in the anodic polarization of alloys. This means a semi-logarithmic plot of overpotential versus the logarithm of current gives a straight line according to Tafel’s equation. This equation applies to reactions slowed by the actual charge transfer or chemical reaction rate and not mass transfer to or from the interface. So, such a response proves that the rate-determining step is an interfacial reaction under charge-transfer control or chemical reaction control. Again, this means the reaction takes place at the metal/slag interface with no influence of product or reactant diffusion (whether in metal or slag) on the reaction rate. This is explained schematically in Figure 2. If the rate-determining step were to involve diffusive processes, there would be no Tafel behaviour and limiting currents would be observed.

Figure 2. Schematic depiction of the metal/slag interface for different rate-limiting processes. Concentration profiles in iron and slag are shown along with characteristic polarization curves.

So, we know that an interfacial reaction is occurring with no influence of diffusion. Now we examine how the characteristics of this reaction change for different iron-carbon alloys. We discovered that Tafel slopes remained constant for different alloys (Figure 1a), which indicates the reaction mechanism remains the same across the carbon range studied. However, we do see that carbon has a strong effect on increasing exchange current of the Tafel process. The plot in Figure 2b shows the relation between carbon activity and Tafel exchange current. In our opinion, this is our most exciting finding because it proves that decarburization takes place by direct interfacial electrochemical reaction of oxide in slag with carbon in iron, according to Reaction (3).

If decarburization were to proceed through Reaction (4) and Reaction (2), carbon concentration would have no effect on the electrochemical reaction rate of the interfacial rate-determining step because carbon is not involved in the interfacial reaction (i.e., Reaction (4)). Of course, carbon activity may exert influence on the reaction rate of Reaction (2), but since this reaction takes place away from the interface it cannot represent what is being measured by Tafel plots! In fact, even carbon’s influence on the activity of dissolved oxygen, [O], plays no role in the electrochemical kinetics of Reaction (4) because the activity of dissolved oxygen does not appear in kinetic equations if Reaction (4) were to proceed under charge-transfer control. By analogy, this is similar to how partial pressure of carbon monoxide gas (pCO) never appears in any kinetic equations for any rate-determining step of Reaction (3).

Our measurements therefore prove that electrochemical decarburization occurs by direct electrochemical interfacial reaction of carbon with oxide in slag. We believe this to be the key finding and major significance of our work. As readers from the steel industry are no doubt aware, decarburization by Reaction (2) occurs extensively in conventional decarburization processes. Our work highlights the possibility of a new decarburization route for steelmaking. It offers potential benefits of lower oxidation of the metal bath, less use of reagents, recovery of metals from slag, use of less corrosive slags (less FeO) to reduce refractory consumption, easy process control, and more. Of course, much more scaled-up testing is required to understand how these benefits can be realized in industry.

In summary, using electricity to refine molten steel is an interesting possibility for producing more sophisticated steel grades. Electrorefining is particularly well suited for mini-mills that recycle steel with electric arc furnaces given the existing electrical infrastructure and know how. This may allow higher value steels to be produced from lower value recycled steel scrap, thereby offering a key piece of the puzzle for sustainable development and closing material loops in the circular steel economy.

William Judge

PhD, University of Toronto