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In 2020, fossil energy sources in the EU accounted for 37%, and renewable energy sources – 38%.The main sources of greenhouse gas emissions in metallurgy are blast furnaces, open hearth furnaces, gas heating furnaces and converters. Greenhouse gases are formed at almost all stages of metallurgical redistribution: during fuel combustion, during metal decarburization reactions, during deoxidation of the melt with carbon, during decomposition of flux components, etc.

Sometimes researchers follow the path of improving or optimizing existing technologies and options for processing cast iron or recovering iron [1–3]. It was shown, for example, that the simultaneous use of oxygen converters and arc furnaces for the processing of cast iron can reduce the total emission of carbon dioxide by up to 20%.

Reduction of iron ore with pure hydrogen in a direct reduction mine furnace depends on many factors, in particular, on temperature, type of iron ore (magnetite, hematite, goethite), ratio of  H₂/CO in the mixture of gases, porosity of the mineral (ore), gas consumption, concentration of the diluent of the gaseous atmosphere (He, Ar, N₂), etc. [4–8].

Solid–phase reduction of iron oxides is recommended to be carried out either in countercurrent mine furnaces or in fluidized bed furnaces [9, 10].

As can be seen from the diagram Fe – O, there is a possibility of realizing the iron recovery process from the oxide melt at a temperature above 1870 K. The analysis of literature data shows that most of the research is aimed mainly at the recovery of iron ore in the solid state, and there is not enough research in the field of hydrogen recovery of liquid iron ore.

During plasma welding, the temperature of the arc is more than 4000 °C and dissociation of hydrogen molecules, their ionization and excitation take place in the arc zone [11–13]. It was established that the rate of recovery of iron above the melting temperature of wustite becomes 13.2 times greater than in the solid phase.

It should be noted that magnetite is a mixture of hematite and wustite. Then the reaction of hematite reduction using hydrogen takes place in two stages:

 

Fe₂O_3(liq) + H_2(g) → 2FeO_(liq) + H₂O_(g),                                          (1)

FeO_(liq) + H_2(g) → Fe_(liq) + H₂O_(g),                                                 (2)

 

which agrees with the opinion of the authors of the works [12, 13].

Currently, attention is being paid to the efficiency of hydrogen use in recovery processes and there are studies [13–15] in which, in addition to research on iron recovery, optimization of technological processes is carried out to reduce the consumption of hydrogen, taking into account the rather high cost of its production, storage and transportation.

Recovery of iron during plasma melting is ensured by the intensification of recovery processes both due to the use of active hydrogen (H⁺, H₂⁺, H₃⁺, H), and high plasma temperature. What is the effect of high temperature, apart from the creation of plasma, and inert gas, which is used as a diluent for hydrogen and forms a gaseous reducing atmosphere together with hydrogen? The answer to this question is provided by classical thermodynamics. In the table 1 and in fig. 1 shows the data for calculating the equilibrium temperatures of decomposition reactions of iron oxides.

 

Table 1 – Equilibrium temperatures of iron oxide decomposition reactions

Reaction

Reaction equilibrium temperature
(ΔG = 0), K

Reaction

Reaction equilibrium temperature
(ΔG = 0), K

6Fe₂O₃ = 4Fe₃O₄ + O₂

1580

2Fe₃O₄ = 6FeO + O₂

3148

2Fe₂O₃ = 4FeO + O₂

2250

1/2Fe₃O₄ = 3/2Fe + O₂

3515

2/3Fe₂O₃ = 4/3Fe + O₂

3154

2FeO = 2Fe + O₂

3640

 

 

 

a

b

 

Fig. 1. Gibbs free energy change of iron oxide decomposition reactions:

a – hematite (1 – 6Fe₂O₃ = 4Fe₃O₄ + O₂,

2 – 2Fe₂O₃ = 4FeO + O₂,

3 – 2/3Fe₂O₃ = 4/3Fe+ O₂);

b – magnetite (1 – 2Fe₃O₄ = 6FeO + O₂,

2 –1/2Fe₃O₄ = 3/2Fe + O₂);

c – wustite (2FeO = 2Fe + O₂)

c

 

 

 

From the given data it follows: firstly, that reaching temperatures above 3000 K is possible only with the use of highly concentrated heating sources (for example, plasma), and secondly, that there is no reason to use hydrogen for the reduction of iron from the very beginning of melting, if at the plasma temperature all the given reactions go to the end. As can be seen from the table 2, only small kinetic restrictions can be imposed on the course of reactions, which will lead to the slow progress of some of them at temperatures above 4000 K.

As can be seen from the table 2, any type of iron ore can be thermally decomposed: hematite, magnetite, goethite (hydroxide FeO(OH) composition: Fe₂O₃ – 89.9% and H₂O – 10.1%). However, the reactions of hematite or goethite decomposition after its dehydration have the greatest thermodynamic probability of passage. Reduction reactions can take place simultaneously. As a result, an iron melt will be obtained and reduction of hematite using hydrogen can be considered a one– and not a two–stage process.

 

Table 2 – Gibbs free energy change and equilibrium constant of iron oxide decomposition reactions at 4000 K

Reaction

 

ΔG^{4000 K}, J

Reaction

 

ΔG^{4000 K}, J

6Fe₂O₃ = 4Fe₃O₄ + O₂

17.0610

–1303463

2Fe₃O₄ = 6FeO + O₂

3.5956

–274702

2Fe₂O₃ = 4FeO + O₂

7.1900

–549314

1/2Fe₃O₄ = 3/2Fe+O₂

1.8681

–142725

2/3Fe₂O₃ = 4/3Fe + O₂

3.6132

–276049

FeO = Fe + O₂

1.1761

–89853

 

In order to check the possibility of decomposition of iron oxides, samples of pellets were melted using argon as a plasma–forming gas. In fig. 2 shows the result of X–ray tested of the original pellet. The composition of the pellet is a mixture of 95.0% hematite and 5.0% quartz sand as an accompanying substance.

 

 

 

Fig. 2. Result of X–ray tested the original unfluxed pellet sample

 

 

After plasma–arc melting in an argon atmosphere for 60 seconds, the amount of hematite in the solidified sample decreased to 8.9%. But magnetite (59.8%), wustite (23.2%) and iron and/or calcium silicate (Fe,Ca)₂SiO₄ appear in the amount of 8.1% (Fig. 3). Rather, 2FeO·SiO₄ is formed, since calcium was not detected in the initial sample of the unfluxed pellet.

 

 

 

Fig. 3. Result of X–ray tested the solidified unfluxed pellet sample after plasma–arc melting of the pellet for 60 s in an argon atmosphere

The recalculation of the composition of the samples for the oxygen content showed that its amount in the original pellet is 31.2%, and after plasma–arc melting the oxygen content in the sample decreased by 4.3% to 26.9%, i.e. by 13.8% from the original oxygen content.

So:

1      – the reaction of thermal decomposition of hematite to iron in a liquid state begins at a temperature of 3150 K;

2      – judging by the extrapolation of dependencies 1 and 3 in Fig. 1a at temperatures above 5000 K, the hematite decomposition reaction to iron and not to magnetite will prevail;

3      – melting of hematite by plasma-arc melting in argon atmosphere allows to reduce oxygen content in melt by about 15%;

4      – the thermal decomposition of hematite by hydrogen deoxidation should not be underestimated, since it is one of the steps for producing green iron and saves hydrogen.

 

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