Effect of water vapour on distribution of phosphorus between liquid iron and MgO saturated slag relevant to flash ironmaking technology

Abstract

As part of ongoing effort to develop a novel flash ironmaking process using reduction of iron ore concentrate by natural gas or hydrogen, equilibrium phosphorus distribution between MgO saturated slag and iron was determined in the temperature range of 1550–1650°C under three different gas atmospheres: H₂/H₂O, H₂/H₂O/CO/CO₂ and CO/CO₂. The distribution ratios of phosphorus L _P were 8, 17 and 20 respectively under these atmospheres. In the flash process, over ∼25 wt-%P volatilises during ore concentrate downward flight in the reactor shaft in addition to avoiding the use of coke. Therefore, it is expected that the proposed method would produce iron with less phosphorus content than that of the blast furnace.

Introduction

Given the issues related to high carbon dioxide emissions and energy consumption in the existing ironmaking technologies, a novel flash ironmaking technology was conceived by Sohn^1–3 at the University of Utah. This technology is the first ironmaking process that will convert iron ore concentrate directly to metallic iron in-flight. This process will produce iron directly from the concentrate without requiring pelletisation or sintering and avoid the need for coke. In addition, this process concept takes full advantage of the fine particle size of concentrate with a large surface area, which permits rapid reduction by a gas. In this novel ironmaking process, potential reductants and fuels are H₂, natural gas and coal gas. Therefore, an iron slag bath is expected to be at equilibrium with gas atmospheres of mainly H₂/H₂O or CO/CO₂/H₂/H₂O respectively, corresponding to the use of H₂ or natural gas/coal gas. Since the effects of an H₂O containing atmosphere on slag properties have not been studied before, it is critical for the development of the novel flash process to determine these effects. In H₂–H₂O–CO–CO₂ gas mixtures under ironmaking conditions, H₂O was found to have the most pronounced effect on the slag chemistry.^{4,XPATH ERROR: unknown variable "nextb".} In addition, it was found in this laboratory that water in the gas atmosphere affected the distribution of Mn and S between slag and molten iron.^6,7 For phosphorus, previous research focused on Ar,⁸ Ar–CO mixture,⁹ CO,¹⁰ CO/CO₂ (Ref. 11) or N₂/CO.¹² Therefore, it was necessary to investigate the effect of water vapour content in H₂–H₂O–CO–CO₂ mixtures on the equilibrium distribution of phosphorus between CaO–MgO_sat–SiO₂–Al₂O₃–FeO–MnO (0.2–0.8 wt-%)–P₂O₅ (0.1–0.9 wt-%) slag and molten iron as part of the development of the novel flash ironmaking technology.

Experimental

The experimental apparatus, the materials and other detailed procedures have been described elsewhere.^6,13–15 A brief summary of salient details is presented here.

Dry powders of SiO₂, Al₂O₃ and MgO were premelted at 1600°C for homogeneity. The premelted slag was ground to <40 μm particle size and mixed for 36 h in a tumbler mixer (Bioengineering Inc., Cambridge, MA, USA). Then, the synthetic slag was stored in a desiccator to be used in the experiments. A horizontal electrical resistance furnace heated by MoSi₂ heating elements with an alumina reaction tube (8 cm o.d., 7 cm i.d., 120 cm length) was used in the experimental set-up, which was equipped with a water vapour generator system. Temperature was controlled and monitored inside the tube by two B-type thermocouples (Pt6%Rh/Pt30%Rh). One was connected to a 708P temperature controller (MTI Corporation, Richmond, CA, USA) with an advanced proportional-integral-derivativecontroller (PID) adjustment to control the power supplied to the heating elements in the furnace with an accuracy of ±1°C over an isothermal zone of ∼10 cm. The other thermocouple monitored the experimental temperature near the samples, indicating a constant temperature within 0.2–0.5°C. The gas flowrates were controlled by mass flow controllers (AALBORG, Orangeburg, NY, USA) with an accuracy of ±0.1, ±2, ±1.5 and ±1.5 mL min⁻¹ respectively, for SO₂, H₂, CO and CO₂ gases. Water was injected as a liquid using a MASTERFLEX digital peristaltic pump drive (Cole-Parmer Instrument, Vernon Hills, IL, USA), which provided flowrates from 0.001 to 3400 mL min⁻¹ (using a set of 10 different diameter tubings) with the same brand pump head with an accuracy of ±0.1% of the flowrate.

The samples were made up of 1.0 g slag and 1.0 g iron powder, which were mixed well to reduce the time to reach equilibrium in magnesia crucibles (1.8 cm o.d., 4 cm height, 0.25 cm wall thickness) supplied by Ozark Technical Ceramics, Inc. (Webb City, MO, USA). Dense sintered magnesia crucibles were selected after careful consideration of the properties of different crucible materials, such as mechanical strength, high temperature stability and resistance to thermal shock and on the basis of the trials conducted with different materials. Thus, the slags tested became saturated with MgO. The furnace was heated to the target temperature under a flow of N₂, and a four-sample alumina holder was placed in the even temperature zone of the furnace. Then, N₂ was switched to the experimental gas mixture. From preliminary experiments,^14,15 it was confirmed that equilibrium was attained within 10 h. To assure three-phase (gas–slag–metal) equilibria, 15 h was chosen in this work. After equilibrium, the system was purged with ultrahigh purity N₂ for 5 min. The holder was then quickly pulled out of the furnace, and the samples were quenched in an ice bath or cold water. The crucible itself together with the sample was crushed, and iron was separated from the slag. Then, the slag was finely ground.

The composition of the slag and iron were analysed by inductively coupled plasma optical emission spectrometry. Before analysis, the samples were digested in closed Savillex microwavable vessels. Reproducibility of the experiments was confirmed by the consistency of the results of repeated experiments under the same conditions, as well as the reproducibility of the analysis method.^14,15 The experimental accuracy was within ±10%.

Results

The chemical analysis and the experimental conditions are presented in Table 1. The equilibrium gas compositions are listed in Tables 2–^{Table 3} 4. The effect of temperature, slag composition and gas type on the distribution of manganese will be discussed. In the cases of CO_x containing atmospheres, the dissolved carbon in the molten iron is expected to be in the range of 0.1%.¹⁶ The effect of C content on the distribution of phosphorus was therefore neglected in the following discussion.

Table 1.

Chemical analysis of slags and experimental conditions of present work

Sample	Gas	T/°C	pO₂	CaO/SiO₂	FeO/wt-%	MgO/wt-%	CaO/wt-%	Al₂O₃/wt-%	SiO₂/wt%	L _P
1	CO/CO₂/H₂/H₂O	1550	1.9×10⁻⁹	0.8	21.7	24.6	20.3	13.0	25.5	39.5
2		1550	1.6×10⁻¹⁰	0.8	10.1	24.4	26.2	16.3	34.8	8.9
3		1550	1.6×10⁻¹⁰	1	9.8	17.9	30.7	8.4	31.8	15.0
4		1550	1.6×10⁻¹⁰	1.2	10.8	16.8	33.5	18.8	29.8	25.6
5		1600	3.6×10⁻¹⁰	0.8	13.0	23.4	26.0	15.7	32.3	6.9
6		1600	3.6×10⁻¹⁰	1	12.7	19.2	30.0	17.0	30.9	7.7
7		1600	3.6×10⁻¹⁰	1.2	12.4	16.6	32.8	17.2	28.6	13.7
8		1630	6.4×10⁻¹⁰	1.2	11.2	19.2	30.1	17.0	30.5	5.4
9	CO/CO₂	1550	1.6×10⁻¹⁰	0.8	35.5	10.2	24.5	13.9	22.8	14.0
10		1550	1.6×10⁻¹⁰	1.2	16.9	26.2	26.3	15.0	23.6	25.4
11		1600	3.6×10⁻¹⁰	0.8	14.7	22.1	25.7	15.2	31.9	4.4
12		1600	3.6×10⁻¹⁰	1	16.9	16.9	28.5	15.2	29.4	17.7
13		1600	3.6×10⁻¹⁰	1.2	15.3	14.7	30.7	16.6	26.7	27.4
14		1630	6.4×10⁻¹⁰	0.8	13.7	15.7	24.7	23.4	32.9	15.9
15		1630	6.4×10⁻¹⁰	1	14.6	14.3	28.0	17.0	36.4	15. 5
16		1630	6.4×10⁻¹⁰	1.2	26.4	14.8	25.2	14.3	27.4	15.6
17	H₂/H₂O	1550	1.6×10⁻¹⁰	1	9.9	20.7	34.9	5.1	27.8	8.6
18		1550	1.6×10⁻¹⁰	1.2	9.6	19.7	31.2	17.1	32.1	7.9
19		1600	3.6×10⁻¹⁰	1.2	10.5	16.1	33.7	17.7	29.5	9.8
19R		1600	3.6×10⁻¹⁰	1.2	9.5	16.9	33.8	18.1	29.6	7.7

Table 2.

Calculated gas partial pressures at experimental temperatures and P _T = 0.85 atm for CO/CO₂/H₂O/H₂ gas mixtures (1 atm = 101.3 kPa; Rxx, sample designation)

	Temperature/°C	Partial pressure/atm	Sample designation	Partial pressure/atm	Sample designation
pO₂/atm	1550	1.9×10⁻⁰⁹	R10–R12	1.6×10⁻¹⁰	R19–R21
	1600	4.8×10⁻⁰⁹	R13–R15	3.6×10⁻¹⁰	R22–R24
	1630	8.0×10⁻⁰⁹	R16–R18	6.1×10⁻¹⁰	R25–R27
pCO/atm	1550	2.4×10⁻⁰¹	R10–R12	2.7×10⁻⁰¹	R19–R21
	1600	2.4×10⁻⁰¹	R13–R15	2.7×10⁻⁰¹	R22–R24
	1630	2.4×10⁻⁰¹	R16–R18	2.7×10⁻⁰¹	R25–R27
pCO₂/atm	1550	4.4×10⁻⁰²	R10–R12	1.4×10⁻⁰²	R19–R21
	1600	4.2×10⁻⁰²	R13–R15	1.4×10⁻⁰²	R22–R24
	1630	4.2×10⁻⁰²	R16–R18	1.4×10⁻⁰²	R25–R27
pH₂O/atm	1550	2.4×10⁻⁰¹	R10–R12	1.0×10⁻⁰¹	R19–R21
	1600	2.4×10⁻⁰¹	R13–R15	1.0×10⁻⁰¹	R22–R24
	1630	2.4×10⁻⁰¹	R16–R18	1.0×10⁻⁰¹	R25–R27
pH₂/atm	1550	3.3×10⁻⁰¹	R10–R12	4.7×10⁻⁰¹	R19–R21
	1600	3.3×10⁻⁰¹	R13–R15	4.7×10⁻⁰¹	R22–R24
	1630	3.2×10⁻⁰¹	R16–R18	4.7×10⁻⁰¹	R25–R27
Flowrates/mL min⁻¹	H₂	90	R10–R18	128	R19–R27
	H₂O	0.054		0.022
	CO	69		77
	CO₂	13		5

Atmospheric pressure at Salt Lake City.

†

Flowrates are calculated at 25°C and 0.85 atm (atmospheric pressure at Salt Lake City).

Table 3.

Calculated gas partial pressures at experimental temperatures and P _T = 0.85 atm for CO/CO₂ gas mixtures (Rxx, sample designation)

	Temperature/°C	Partial pressure (atm)	Sample designation	Partial pressure (atm)	Sample designation
pO₂	1550	3.49×10⁻⁰⁸	R37–R39	1.37×10⁻¹⁰	R46–R48
	1600	9.31×10⁻⁰⁸	R40–R42	3.67×10⁻¹⁰	R49–R51
	1630	1.63×10⁻⁰⁷	R43–R45	6.44×10⁻¹⁰	R52–R54
pCO	1550	4.78×10⁻⁰¹	R37–R39	8.10×10⁻⁰¹	R46–R48
	1600	4.78×10⁻⁰¹	R40–R42	8.10×10⁻⁰¹	R49–R51
	1630	4.78×10⁻⁰¹	R43–R45	8.10×10⁻⁰¹	R52–R54
pCO₂	1550	3.72×10⁻⁰¹	R37–R39	3.96×10⁻⁰²	R46–R48
	1600	3.72×10⁻⁰¹	R40–R42	3.96×10⁻⁰²	R49–R51
	1630	3.72×10⁻⁰¹	R43–R45	3.96×10⁻⁰²	R52–R54
Flowrates/mL min⁻¹	CO	103	R37–R45	66	R46–R54
Flowrates/mL min⁻¹	CO₂	30	R37–R45	7	R46–R54

Atmospheric pressure at Salt Lake City.

†

Flowrates are calculated at 25°C and 0.85 atm (atmospheric pressure at Salt Lake City).

Table 4.

Calculated gas partial pressures at experimental temperatures and P _T = 0.85 atm for CO/CO₂ gas mixtures (Rxx, sample designation)

	Temperature/°C	Partial pressure/atm	Sample designation	Partial pressure/atm	Sample designation
pO₂	1550	3.49×10⁻⁰⁸	R64–R66	1.39×10⁻¹⁰	R73–R75
	1600	8.46×10⁻⁰⁸	R67–R69	3.35×10⁻¹⁰	R76–R78
pH₂O	1550	6.43×10⁻⁰¹	R64–R66	1.39×10⁻⁰¹	R73–R75
	1600	6.43×10⁻⁰¹	R67–R69	1.39×10⁻⁰¹	R76–R78
pH₂	1550	2.06×10⁻⁰¹	R64–R66	7.10×10⁻⁰¹	R73-R75
	1600	2.06×10⁻⁰¹	R67–R69	7.10×10⁻⁰¹	R76–R78
Flowrates/mL min⁻¹	H₂	68	R64–R72	97	R73–R81
Flowrates/mL min⁻¹	H₂O	0.04	R64–R72	0.017	R73–R81

Atmospheric pressure at Salt Lake City.

†

Flowrates are calculated at 25°C and 0.85 atm (atmospheric pressure at Salt Lake City).

The phosphorus distribution ratio L _P between slag and metal is defined as 1 where (%P) is the wt-% of phosphorus dissolved in slag and [%P] is the wt% of phosphorus dissolved in metal.

It can be seen from Figs. 1–^{Figure 2} 3 that L _P decreases when CO₂ is replaced by H₂O. In other words, the CO/CO₂ slag ranks first with the highest L _P, CO/CO₂/H₂/H₂O comes next and the H₂/H₂O has the lowest L _P. The redox reaction that dominates the equilibrium of P between molten iron and slag phase can be written as 2 where [ ] and ( ) represent the concentration of species in metal and slag respectively. The above equation suggests that the more the free oxygen in the slag, the more the phosphorus uptake in slag. Based on X-ray photon spectroscopy analysis, the order of the slags based on the relative abundance of free oxygen O²⁻, the oxygen that is not bonded to silicon atom in the slag, is CO/CO₂ (44%)>CO/CO₂/H₂/H₂O (41%)>H₂/H₂O (37%), which is in accordance with the order based on L _P.^4,5,17 It can be seen that at constant pO₂, temperature and basicity, L _P decreases with increasing pH₂O, as Figs. 4 and 5 show. The effect of water on the slag chemistry was further investigated in this laboratory to understand the interactions at the molecular level.^4,5,17,18 The authors also found that water affected the distribution of P, S and Mn between slag and molten iron.^6,17,19 Moreover water was found to affect refractory lining and FeO content in the slag.^20–22 Water vapour was found to be advantageous in terms of lowering FeO uptake in the slag in the novel flash ironmaking process. In addition, it was determined that water vapour in the atmosphere of the new process would keep lining wear low based on the low MgO solubility in slag under its expected operating conditions.

Figure 1.

Effect of different gas atmospheres on L _P at various pO₂ and temperature values for slags with CaO/SiO₂ of 0.8 and pH₂O of 0 and 0.1 atm in CO/CO₂ and H₂/H₂O/CO/CO₂ respectively

Figure 2.

Effect of different gas atmospheres on L _P at various pO₂ and temperature values for slags with CaO/SiO₂ of 1.0 and pH₂O of 0, 0.1 and 1.0 atm in CO/CO₂, H₂/H₂O/CO/CO₂ and H₂/H₂O/CO respectively

Figure 3.

Effect of different gas atmospheres on L _P at various pO₂ and temperature values for slags with CaO/SiO₂ of 1.2 and pH₂O of 0, 0.1 and 1.0 atm in CO/CO₂, H₂/H₂O/CO/CO₂ and H₂/H₂O/CO respectively

Figure 4.

Effect of pH₂O on L _P for slag compositions with CaO/SiO₂ of 1.0 at various temperatures and pO₂

Figure 5.

Effect of pH₂O on L _P for slag compositions with CaO/SiO₂ of 1.2 at various temperatures and pO₂

In addition to the low phosphorus input from raw materials, a significant amount of phosphorus volatilises during the downward flight of the particles in the proposed flash reactor. To obtain an approximate extent of volatilisation, a representative sample was analysed for phosphorus before and after reduction in a simulated run for the proposed process. It was found that over 25 wt-% of the phosphorus in the ore was lost during particle flight.

The wt-%P in iron is related to L _P by the following relationship obtained from materials balance 3 where [wt-%P] is the phosphorus weight percentage in produced iron, t is the total phosphorus input per ton of iron and a is the slag/metal mass ratio.

In the proposed process, L _P = 9 and 17 (H₂/H₂O and CO/CO₂/H₂/H₂O respectively) versus 18 in under CO/CO₂ and t = 4.5 kg ton⁻¹ iron (considering that 25 wt-% phosphorus in the ore will volatilise in the furnace shaft). Applying the same slag/metal ratio of 0.3 as in BF, the P content in the iron produced by the novel process would be ∼0.12 and 0.07 wt-% under H₂/H₂O (H₂ conditions) and CO/CO₂/H₂/H₂O (natural/coal gas, NG/CG, conditions), as shown in Fig. 6. In contrast, the P input to the blast furnace is 10.3 kg ton⁻¹ iron, based on the same P content in the ore plus a typical coke/iron ratio of 0.4 and P content in coke. Thus, the P content in iron produced under CO/CO₂ would be ∼0.16%. The typical hot metal produced by the BF has ∼0.2 wt-%P.

Figure 6.

Effect of the type of reductant gas on L _P; labels show values of L _P and [wt-%P]

Conclusions

For the slag compositions investigated, MgO saturated CaO–FeO–Al₂O₃–SiO₂–MnO (0.2–0.8 wt-%)–P₂O₅ (0.1–0.9 wt-%), in the temperature range of 1550–1600°C, L _P was found to decrease with H₂O content in the gas. Under H₂/H₂O (L _P = 8) and NG/CG (L _P = 17), L _P was 60 and 15% less than under CO/CO₂ (L _P = 20) respectively. Based on material balance calculations, the iron produced under H₂ ([wt-%P] = 0.12) and NG/CG ([wt-%P] = 0.07) would respectively have significantly less S and P than iron produced under CO/CO₂ ([wt-%S] = 0.3; [wt-%P] = 0.16). Therefore, the novel flash ironmaking technology is expected to produce iron that requires much less dephosphorisation compared with pig iron produced by blast furnace.

Acknowledgements

The authors acknowledge the financial support from American Iron and Steel Institute (AISI) through a Research Service Agreement with the University of Utah under AISI’s CO₂ Breakthrough Program. This material also contains results of work supported by the US Department of Energy under Award no. DE-EE0005751. This report was prepared as an account of work sponsored by an agency of the US Government. Neither the US Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favouring by the US Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the US Government or any agency thereof.

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