Thermodynamic of Liquid Iron Ore Reduction by Hydrogen Thermal Plasma

Round 1

Reviewer 1 Report

I thank the authors for addressing the questions raised in the last review. The inclusion of some experimental evidence and some attempt to justify the assumption of equilibrium has improved the paper.

I wasn't satisfied with the response to the issue of reversion. Let me explain further: clearly in the plasma, there excited ions at high temperature. At the point where the plasma impinges on the molten material, I would assume that there would be very high temperature gradients and various flow patterns associated with the temperature gradient and sudden physical barrier in front of the barrier. Wouldn't you get quenching of the gases on the edge of the plasma (away from the centre of impingement) and wouldn't you expect this to result in fine oxides forming (like the fume associated with the region near a arc melting/welding process)? I would expect that this sudden quenching of the plasma would result in yield loss and formation of fine iron oxide dusts at the end of the impingement area. Is that incorrect? (is the whole system relatively uniform in temperature to avoid this or ???). I apologise if I have mis-understood this aspect of the process but it is not clear to me and I suspect that other people in the field not familiar with Plasma processes would wonder about the same issue.

I think this point needs some explanation before the paper is published.

Author Response

Subject: Response to the Reviewer No. 1

Dear Reviewer,

Thanks for your kind reply. I tried to response your comments and I hope it would be satisfaction.

Best Regards

Masab Naseri

I wasn't satisfied with the response to the issue of reversion. Let me explain further: clearly in the plasma, there excited ions at high temperature. At the point where the plasma impinges on the molten material, I would assume that there would be very high temperature gradients and various flow patterns associated with the temperature gradient and sudden physical barrier in front of the barrier. Wouldn't you get quenching of the gases on the edge of the plasma (away from the centre of impingement) and wouldn't you expect this to result in fine oxides forming (like the fume associated with the region near a arc melting/welding process)? I would expect that this sudden quenching of the plasma would result in yield loss and formation of fine iron oxide dusts at the end of the impingement area. Is that incorrect? (is the whole system relatively uniform in temperature to avoid this or ???).

There are two different aspects for this issue:

1-     Dust: The dust amount depends on the gas flow rate, velocity of gas and particle size of iron oxide particles. Reh diagram, which illustrates fluid dynamic states for different gas-solid reactor processes, has been applied for calculation of the dust amount. I have calculated the dust amount for different conditions of HPSR process and attached the results in the following table. It is clear that the amount of dust per case depends on the gas velocity, grain size distribution of the iron ore and the flow rate of the off-gas. Therefore, the temperature of the particles is not an influencing parameter on the dust emission.

 Dust amounts in different cases

Case	Off-gas flow [Nl/min]	Batch time [min]	Velocity [m/s]	Particle size [µm]	Dust amount [kg/batch]	Dust stream [g/Nm³]	Dust stream [g/min]
Average	2520	62	0,4	5	5	33	80
Minimum	1461	34	0,23	2	2,5	50	73,5
Maximum	3783	111	0,6	15	6,5	16	58,5

 

2-     Fumes: definitely, plasma arc causes to generate fumes of iron oxide and lime particles due to its high temperature. Fumes and gas particles, after leaving plasma arc, can be condensed on the liquid surface or on the refractory lining or somewhere in off-gas duct. It is a drawback of this process. However, this process is happening in the conventional metallurgical plant such as electric arc furnace and ladle furnace. It is correct that the amount of fumes in HPSR can be more than that of EAF or ladle furnace. This point also was discussed in the paper page 11:

6. Mechanism of the Hematite Reduction Reaction in HPSR

…………………… Murphy et al. [54] simulated the temperatures, velocities, and the vaporization of iron ore in the arc zone for a 150 A tungsten inert gas (TIG)  welding arc. They showed that, with the use of helium, Fe is vaporized and the concentration of Fe can reach 7 mol% due to the high temperature of the weld pool, which is approximately 2773 °C. With the use of argon as a plasma gas, the temperature of the liquid metal at the interface and the Fe concentration are 2273 °C and 0.2%, respectively.

In order to know which elements are present in the plasma arc, the arc was monitored by an optical spectrometry. There were many peaks from iron particles as well inside the arc. I will submit another paper in January next year, in which I will explain the related issues.

I apologise if I have mis-understood this aspect of the process but it is not clear to me and I suspect that other people in the field not familiar with Plasma processes would wonder about the same issue.

I think this point needs some explanation before the paper is published.

It was explained by:

In the plasma arc, not only iron and iron oxide can be released from the iron ore and liquid bath but also carbon is released from graphite electrode. The amount of iron, iron oxide and carbon vapor depends on the process parameters [35].

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This is an interesting and original piece of work. The background is broad (cleaner steelmaking) but the application is narrow (hydrogen thermal plasma). The paper will mostly interest the readers engaged in a similar research and those who wish to be aware of the breakthrough steelmaking processes studied in universities. The results come from thermodynamic calculations. The title should thus be best rewritten as “Thermodynamic Aspects of the Reduction of Liquid Iron Ore by Hydrogen Thermal Plasma”. 

 

Other general remarks:

The English writing is average and should be improved by a native English speaker or an English editing service.

The references are numerous, but often cited in large groups [x-y]. Those could be somewhat reduced, being more selective.

 

Specific remarks along the text:

p. 1. 1401 kg CO2/t of liquid hot metal [1]. This is a low value, the usual figure being 1800-2000. Please explain this choice or use a more usual figure, e.g. Birat et al., 2009, Rev. Met., doi: 10.1051/metal/2009060. 

p. 1. Refs [2-6]. The paper Ranzani et al., 2013, J. Cleaner Proc., doi: 10.1016/j.jclepro.2012.07.045 should be included in this list. 

p. 1. Replace ‘at the distance of the tip of the electrode and the slag surface’ with ‘from the tip of the electrode to the slag surface’.

p. 2. Fig. 1. A zoom on the important part (electrode, plasma, and bath) should be added and commented. 

p. 2. ‘To analyze the arc, [...] to monitor the arc.’ Please rewrite.

p. 3. Please add sentences for briefly defining CTE and LTE. ‘Because, the particles that diffuse to the LTE location have enough time to equilibrate.’: unclear, please rewrite. ‘Furthermore, the temperature of the ions and electrons are equal in hot plasma [19–21].’ this is true but ill-placed here. 

p. 3. ‘standard Gibs free’: Gibbs. 

p. 3. ‘monoatomic hydrogen is able to reduce metal oxides more feasibly’. H or H+ or both? Also replace feasibly with readily. 

p. 3. Figure 2 shows... 

p. 3. FactSageTM 7.1 (Database: FactPS 2017): Were all the necessary data there, including those related to H+ and H2+ or some data had to be separately determined?

p. 4. ‘are two separate processes’ yes, but this disagrees with ‘simultaneously’ in the title of Fig. 3. 

p. 6. ‘3.53 × 10−16 cm2’: Set it in m2. 

p. 8. ‘To assess the effect of the polarity [...] the results are shown in Figure 6.’ Fig. 6 does not show the effect of the polarity. 

p. 9. ‘which is 13.6 eV,’: Nothing at 13.6 eV is visible in Fig. 7.

p. 10. Please add a conclusion after the discussion at the end of section 5. 

p. 10. Eqs (16-17): these are the reactions from the liquid phase. But some of the species are gaseous in the higher part of the temperature range 1500–3000°C. Please discuss somewhere the relative contributions of the reactions from the liquids and from the gases. 

p. 11. ‘Therefore, the maximum hydrogen utilization degree using molecular hydrogen is 43%.’ The utilization ratio is one thing, another (more important) is the final degree of reduction of the oxides. If my understanding of Fig. 9 is correct, these results show that it is not possible to completely reduce FeO, whatever the temperature between 1500 and 3000°C, with the HPSR device! If this is the case, it means that the technique is eventually not really efficient, and this should be stated. 

p. 11. ‘However, it is expeted to be higher when using hydrogen in a plasma state’: (1) ‘expected’ instead of ‘expeted’; (2) why not having drawn a figure similar to Fig. 9 using H+ or H2+? 

p. 12. Fig. 9: improve the scale division, e.g. 1500–3000.

pp. 12-13 and the role of water dissociation. This discussion is interesting, but two points are not dealt with. (1) What happens between 2268 and 2850°C? This is not discussed. It seems that reduction takes place (FeO decreases) whereas O2 is present, and O2 is said to oxidize. (2) the water dissociation gives O2 and H2. O2 can oxidize iron and H2 can reduce FeO. Which prevails? Could you explain why only considering the oxidation? Please try to improve the discussion.

p. 14. Conclusion: if my remark above about the low final degree of reduction is right, please state it again in the conclusion. 

p. 14. Ref. 10: Delete ‘(None)’. Ref. 11: add the year.

Author Response

Subject: Response to Reviewer No. 1

Dear Reviewer,

Thanks for your helpful comments. I tried to response your comments and revise my paper based on your comments and I hope it would be satisfaction.

Best Regards

Masab Naseri

 

The title should thus be best rewritten as “Thermodynamic Aspects of the Reduction of Liquid Iron Ore by Hydrogen Thermal Plasma”. 

The title has been changed to:

‘Thermodynamic of Liquid Iron Ore Reduction by Hydrogen Thermal Plasma’

The English writing is average and should be improved by a native English speaker or an English editing service.

This paper has been revised by Elsevier English editing service for the first time and then by MDPI English editing service. I have attached the English edited version for your consideration.

The references are numerous, but often cited in large groups [x-y]. Those could be somewhat reduced, being more selective.

Two of those have been removed:

Ushio, M. Arc discharge and electrode phenomena. Pure and Applied Chemistry 1988, 60, doi:10.1351/pac198860050809.

Hiebler, H.; Plaul, J.F. Hydrogen plasma- smelting reduction- an option for steel making in the future. METABK 2004, 43, 155–162.

Specific remarks along the text:

p. 1. 1401 kg CO2/t of liquid hot metal [1]. This is a low value, the usual figure being 1800-2000. Please explain this choice or use a more usual figure, e.g. Birat et al., 2009, Rev. Met., doi: 10.1051/metal/2009060 . 

Regarding direct and indirect emission it is more. I have revised the sentences to consider the total CO2 emissions.

p. 1. Refs [2-6]. The paper Ranzani et al., 2013, J. Cleaner Proc., doi: 10.1016/j.jclepro.2012.07.045  should be included in this list. 

Added

p. 1. Replace ‘at the distance of the tip of the electrode and the slag surface’ with ‘from the tip of the electrode to the slag surface’.

Done

p. 2. Fig. 1. A zoom on the important part (electrode, plasma, and bath) should be added and commented. 

Done

p. 2. ‘To analyze the arc, [...] to monitor the arc.’ Please rewrite.

Revised

p. 3. Please add sentences for briefly defining CTE and LTE. ‘Because, the particles that diffuse to the LTE location have enough time to equilibrate.’: unclear, please rewrite. ‘Furthermore, the temperature of the ions and electrons are equal in hot plasma [19–21].’ this is true but ill-placed here. 

Completely revised

p. 3. ‘standard Gibs free’: Gibbs. 

Revised

p. 3. ‘monoatomic hydrogen is able to reduce metal oxides more feasibly’. H or H+ or both? Also replace feasibly with readily. 

Revised

p. 3. Figure 2 shows... 

Revised

p. 3. FactSageTM 7.1 (Database: FactPS 2017): Were all the necessary data there, including those related to H+ and H2+ or some data had to be separately determined?

All data are included in the mentioned database

p. 4. ‘are two separate processes’ yes, but this disagrees with ‘simultaneously’ in the title of Fig. 3. 

Revised

p. 6. ‘3.53 × 10−16 cm2’: Set it in m2. 

Revised

p. 8. ‘To assess the effect of the polarity [...] the results are shown in Figure 6.’ Fig. 6 does not show the effect of the polarity. 

It shows Gibbs free energy changes, and based on Gibbs free energy changes, the effect of polarity can be assessed.

Gibbs free energy changes are shown in the figure. The polarity causes to change the Gibbs free energy changes. If the positive polarity is applied, it means that ionized hydrogen can not reach on the reaction surface. Therefore, FeO is reduced by H, which the Gibbs free energy is more positive.

p. 9. ‘which is 13.6 eV,’: Nothing at 13.6 eV is visible in Fig. 7.

Revised

p. 10. Please add a conclusion after the discussion at the end of section 5. 

Added

p. 10. Eqs (16-17): these are the reactions from the liquid phase. But some of the species are gaseous in the higher part of the temperature range 1500–3000°C. Please discuss somewhere the relative contributions of the reactions from the liquids and from the gases. 

The transition temperature of FeO from liquid to gas is 3414 °C at 1 atm. I focused mainly on the reduction reactions which occur on the liquid surface.

p. 11. ‘Therefore, the maximum hydrogen utilization degree using molecular hydrogen is 43%.’ The utilization ratio is one thing, another (more important) is the final degree of reduction of the oxides. If my understanding of Fig. 9 is correct, these results show that it is not possible to completely reduce FeO, whatever the temperature between 1500 and 3000°C, with the HPSR device! If this is the case, it means that the technique is eventually not really efficient, and this should be stated. 

There are two different aspects, thermodynamic and kinetics. In HPSR, thermodynamically, it is possible to reach 100% reduction degree. Nevertheless, it needs more hydrogen to be injected to the plasma reactor. I have calculated the required amount and explained it in the paper. However, when the FeO concentration in the slag is decreased, the reduction rate is also decreased. This process is similar to other conventional steelmaking processes.

p. 11. ‘However, it is expeted to be higher when using hydrogen in a plasma state’: (1) ‘expected’ instead of ‘expeted’; (2) why not having drawn a figure similar to Fig. 9 using H+ or H2+? 

Expected: revised

Due to the limitation of the software or thermodynamic knowledge, it is not possible to have this diagram for the hydrogen activated particles.

Fig 9 in at equilibrium. The temperature is the reaction surface temperature,i.e. slag surface. Ionized hydrogen at this range of temperature at equilibrium is not stable. Therefore, there is not any software to calculate this kind of diagrams for the hydrogen activated particles.  

p. 12. Fig. 9: improve the scale division, e.g. 1500–3000.

At the temperatures below 1537°C, Fe is in the solid state. But I want to discuss only the liquid state. The reason for selecting this range has been discussed in the paper page 11:

6. Mechanism of the Hematite Reduction Reaction in HPSR

To study the reduction of hematite using hydrogen at high temperatures, the equilibrium of Fe2O3 and H2 has been assessed by FactSageTM 7.1. For the assessment of the equilibrium, the range of the equilibrium temperature should first be determined. ………………….. Therefore, for the calculations of equilibrium, the temperature range between 1550 and 3000 °C was considered. The lower part of the range (i.e., 1550 °C) was considered in order to be above the melting temperature of pure iron, which is 1537 °C.

 

pp. 12-13 and the role of water dissociation. This discussion is interesting, but two points are not dealt with. (1) What happens between 2268 and 2850°C? This is not discussed. It seems that reduction takes place (FeO decreases) whereas O2 is present, and O2 is said to oxidize. (2) the water dissociation gives O2 and H2. O2 can oxidize iron and H2 can reduce FeO. Which prevails? Could you explain why only considering the oxidation? Please try to improve the discussion.

(1)  revised

(2) Hydrogen is produced by the water dissociation process and released in a reactor where is pure hydrogen. Therefore, the small amount of hydrogen can not considerably affect the reduction rate or utilization rate. But there is a small amount of hydrogen in the reactor which can be reacted 100%. I want to refer you to the following sentence in my paper (page 12):  

Kamiya et al. [26] studied the reduction of molten iron oxide using H2-Ar plasma. They reported that the hydrogen utilization degree can be 60% at low concentration of hydrogen in the gas mixture.

p. 14. Conclusion: if my remark above about the low final degree of reduction is right, please state it again in the conclusion. 

It was explained in the text.

p. 14. Ref. 10: Delete ‘(None)’. Ref. 11: add the year.

Removed.

 

Author Response File: Author Response.pdf