PAPERS OF GENERAL INTEREST

 

Experimental study on phosphorus distribution ratio and capacity of environment-friendly dephosphorization slag for high-phosphorus hot metal pretreatment

 

 

F. Yang; X.G. Bi; J.D. Zhou

Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan, China

 

 

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SYNOPSIS

In this investigation, the phosphorus distribution ratios in the CaF₂ system and B₂O₃ system dephosphorization slags for high-phosphorus hot metal pretreatment were measured by an indirect method under laboratory conditions. Firstly, the phosphorus distribution ratio between liquid slag and solid iron was measured, and then the phosphorus distribution ratio between liquid slag and hot metal was calculated. Phosphorus capacity was calculated in terms of the composition and optical basicity of the slag. Dephosphorization slag was also studied by scanning electron microscopy, energy dispersive analysis, and X-ray diffraction analysis. The experimental results show that the phosphorus capacity of the B₂O₃ slag system is much greater than that of the CaF₂ system. It is demonstrated thatB₂O₃ can completely replace CaF₂ as fluxing agent for high-phosphorus hot metal pretreatment. With CaF₂ as fluxing agent, the phosphorus capacity increases with increasing CaO content in the slag, but the phosphorus distribution ratio decreases. B₂O₃, when used as fluxing agent, can react with high melting point phases such as 2CaOSiO₂ and 3CaOP₂O₅ in the slag to form new phases with low melting points such as 11CaO-B2O3.4SiO2, 2CaO^O3.SiO2, and Ca_9.93(P_5.84Bo₁₆O₂₄) (B_0.67 O_1.79), thereby acting as a fluxing agent. When the ratio of w(B₂O₃)/w(CaO) is 0.16, the phosphorus distribution ratio reaches its maximum value, that is, the dephosphorization ability of the slag is at its maximum.

Keywords: hot metal pretreatment; dephosphorization; phosphorus distribution ratio; phosphorus capacity

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Introduction

As international iron ore prices continue to rise, the development of China's steel industry is facing a lot of pressure. Against this background, development and utilization of domestic high-phosphorus iron ore resources is particularly important. When high-phosphorus iron ore is used in ironmaking, the phosphorus content in the hot metal can reach 0.35% or more, and the hot metal must be pretreated. For hot metal with 0.08-0.10% dissolved phosphorus, the traditional hot metal dephosphorization pretreatment technologies are relatively mature. However, when using these methods to dephosphorize hot metal containing 0.35% P or more, the amount of dephosphorization slag can reach 100-150 kg per ton of hot metal. Furthermore, the traditional dephosphorization methods require the addition of large amounts of CaF₂ as fluxing agent, and this results in high fluoride levels in the dephosphorization slag. The fluorine in the slag can partially dissolve in water and thereby causes high fluoride concentration in the water and soil. Once the fluoride is absorbed by humans or animals, it can result in toxic effects on the central nervous system and myocardium. In addition, fluoride can accumulate in the environment, and thereby pose a health risk to animals and humans through the food chain (Li, 2007; Liu, Wang, and Dong, 2011, Diao, 2013). The objective of this research is to replace CaF₂ with B₂O₃ as fluxing agent while achieving the expected goals of hot metal pretreatment.

The melting point of B₂O₃ (450°C) is much lower than that of CaF₂ (1419°C). B₂O₃ can interact with FeO, CaO, MgO, and other oxides to form compounds with low melting points such as CaO-B₂O₃ (m.p. 1154°C). Compared with CaF₂, B₂O₃ has a greater advantage in rapid slagging (Hamano and Horibe, 2004). The purpose of this article is to investigate the influence of environment-friendly dephosphorizing slag composition on the dephospho-rization process, and to provide a theoretical basis for the preparation of an environment-friendly dephosphorization agent that has strong dephosphorization ability. The phosphorus distribution ratio and phosphorus capacity of the CaF₂ slag system were also determined.

 

Experimental method

The phosphorus distribution ratio and the phosphorus capacity of B₂O₃ dephospho-rization slag for high-phosphorus hot metal pretreatment were determined under laboratory conditions using a thermodynamic equilibrium furnace. The interior structure of the furnace is shown in Figure 1. The compositions of the CaF₂ and B₂O₃ slag systems are shown in Table I and Table II, respectively. The compositions of the pure iron crucible and iron foil are shown in Tables III and IV, respectively. In the experiments, 99.999% high-purity argon gas was purged to provide an inert furnace atmosphere. Slag samples were thoroughly mixed and placed into pure iron crucibles. The crucibles were then loaded into the furnace, and the furnace was heated to 1400°C. The temperature was kept constant for 1 hour, and then lowered to 1350°C, before being kept constant at 1350°C for 11 hours. Finally, the crucibles were removed from the furnace and immediately quenched with water to avoid oxidation reactions that would drive the reactions of Fe₂O₃-FeO-Fe to equilibrium. The pre-melted slag from the equilibrium experiments was crushed, and placed again in pure iron crucibles, together with pure iron flakes of about 1 g weight and 0.1 mm thickness. The crucibles containing the pre-melted slag and iron flakes were then loaded into the furnace, and the temperature was raised to 1300°C and kept constant for 12 hours under argon gas protection to obtain a full equilibrium between the slag and iron flakes. Finally, the crucibles were removed and quenched with water to avoid oxidation that would drive the reaction of P-P₂O₅ to equilibrium. The samples obtained from the experiments were treated as follows.

(1) The slag that had adhered on the iron flakes was ground and cleaned up with supersonic waves in an acetone solution

(2) The phosphorus dissolved in the iron flakes was measured with the molybdenum blue light absorption method. The microstructure and elemental composition of the slag were analysed with scanning electron microscopy using energy-dispersive X-ray spectroscopy (SEM-EDS). The minerals in the slag were characterized by X-ray diffraction.

 

 

 

 

 

 

 

 

 

 

 

Calculation of the phosphorus capacity and phosphorus distribution ratio

According to the composition of the slag obtained from the equilibrium experiment, phosphorus capacity is calculated by the following formula (Wei, Yang, and Sommerville, 2002).

where is the phosphorus capacity, T the reaction temperature (K), and Λ the optical basicity. The Λ is the weighted average of the optical basicity of all components in slag, and is calculated using the formula:

where A_Bis the optical basicity of oxide, and x_Bis the molar fraction of cations in oxide.

x_Balso denotes the molar fraction of oxygen of oxide in the slag, and is calculated by formula:

where xB is molar fraction of oxide, n_O is the number of oxygen atoms in the oxide molecule. It is specified that one fluorine atom in the fluoride molecule is equal to 1/2 oxygen atoms. xB can be calculated by:

where w_Bis the percentage of oxide and M_Bis the molar mass of oxide.

In this paper, the measured value of the phosphorus distribution ratio is not between liquid slag and carbon-saturated hot metal but between liquid slag and solid iron flake, thus a conversion is needed. Im, Morita, and Sano (1996) calculated the phosphorus distribution ratio between liquid slag and solid pure iron flakes (Tsukihashi, Nakamura, and Orimoto, 1990; Knacke, Kubaschewski, and Heselmann, 1991; Im, Morita, and Sano, 1996; Zhou et al., 2011). The expression of phosphorus distribution ratio is as follows:

where is the phosphorus distribution ratio between liquid slag and a iron, the phosphorus distribution ratio between liquid slag and y iron, %P the mass percentage of P in liquid slag, [% P]^Fe-a the mass percentage of P in a iron, and [% P]^Fe-r is the mass percentage of P in y iron.

After thermodynamic conversion, the distribution of phosphorus between carbon-saturated iron and a iron and y iron is obtained by:

where [% P]^Fe-C is the mass percentage of P in carbon-saturated hot metal.

 

Results and discussion

Phosphorus capacity and phosphorus distribution ratio of CaF₂ slag system

Equilibrium compositions and phosphorus capacity calculated for the CaF₂ slag system are shown in Table V.

The phosphorus contents in the CaF₂ slag system and solid iron flake, and the phosphorus distribution ratio between liquid slag and carbon-saturated hot metal are also shown in Table VI. The relationship between phosphorus distribution ratio and CaO content is depicted in Figure 2.

 

 

 

It can be clearly seen from Table V that when the CaO content in the slag is the highest, the phosphorus capacity of the CaF₂ slag system reaches the maximum value. That is, at a CaO content in slag of 49.72%, the phosphorus capacity is 11.66x10²². At a CaO content in slag of about 44%, the change in phosphorus capacity is small. As can be seen from Table VI and Figure 2, increases with decreasing CaO content. For example, as the CaO content decreases from 49.72% to 42.06%, the increases from 126.84 to 189.98. is influenced synthetically by slag basicity, fluxing agent content (effect of slag melting), and FeO content, among other factors. of the slag in test 1-0-4 is 189.98, the maximum value. The basicity of 1-0-4 slag is 3.76. However, of 1-0-1 slag reaches the minimum value, with a basicity of 17.20. When the basicity is too high, the dephosphorizing slag contains solid CaO particles deposited from the bath. The higher the basicity, the more CaO particles are deposited. Figures 3 and 4 show elemental analyses and a micrograph of CaO solid particles in melted slags from tests 1-0-2 and 1-0-3, respectively. It can be clearly seen that the quantity and size of CaO particles in melted slag with 44.52% CaO are much greater than in slag with 43.88% CaO. This explains why bears an inverse relationship to CaO content. More CaO particles increase the viscosity of slag, which in turn decreases . In this condition, it is required to decrease the basicity of the slag, increase the amount of fluxing agent, or increase the FeO content of the slag.

 

 

 

 

Phosphorus capacity and phosphorus distribution ratio of B₂0₃ slag system

Equilibrium compositions and phosphorus capacity calculated for the B₂O₃ slag system are shown in Table VII.

Phosphorus distribution ratios obtained for the B₂O₃ slag system between liquid slag and carbon-saturated hot metal are shown in Table VIII. The relationship between phosphorus distribution ratio and w(B₂O₃)/w(CaO) is depicted in Figure 5.

 

 

 

 

As can be seen from Table VII, the phosphorus capacity of all B₂O₃ slag system is generally higher. The highest value is as high as 57.54x10²³. It can be seen from Table VIII and Figure 5 that at a w(B₂O₃) to w(CaO) ratio of 0.16, the phosphorus distribution ratio reaches the highest value of 156.12. As the B₂O₃ content continues to increase, the phosphorus distribution ratio begins to decrease. As such, this kind of slag system (test number 3-0-5) has the highest dephosphorization ability.

When the quantity of B₂O₃ is appropriate, during the dephosphorization reaction, B₂O₃ reacts with 2CaO-SiO₂ to form 11CaO-B₂O₃-4SiO₂ and 2CaO-B₂O₃-SiO₂. Moreover, B₂O₃ continues to react with 3CaO-P₂O₅, which is formed by reaction between CaO and P₂O₅, to form Ca₉₉₃(P₅₈₄ B₀₁₆O₂₄) (B₀₆₇O₁₇₉), which has a low melting point. Thus B₂O₃ acts as a fluxing agent and is beneficial for dephosphorization reactions. In this case, the phosphorus distribution ratio and capacity are the highest. The XRD pattern from test 3-0-5 is shown in Figure 6.

 

 

Conclusions

With CaF₂ as fluxing agent, the phosphorus capacity of dephosphorization slag for high-phosphorus hot metal is between 2.331x10²² and 11.66x10²². Phosphorus capacity increases with increasing slag basicity. However, too high a CaO content adversely affects the phosphorus distribution ratio because the melting of the dephosphorization slag becomes difficult, and this reduces the fluidity of the slag, hindering diffusion and decreasing the dephosphorization rate.

The phosphorus capacity of the B₂O₃ slag system is between 1.41x10²³ and 57.54x10²³, which is one order of magnitude higher than CaF₂ system. It has been demonstrated that B₂O₃ can completely replace CaF₂ as fluxing agent for high-phosphorus hot metal pretreatment.

The phosphorus distribution ratio of the B₂O₃ system is 111.44-156.12. At a w(B₂O₃) to w(CaO) ratio of 0.16, the phosphorus distribution ratio reaches its highest value. The dephosphorization ability of this slag system is therefore the strongest.

 

References

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Paper received May 2015
Revised paper received Nov. 2015.