简介概要

Sulfur distribution in preparation of high titanium ferroalloy by thermite method with different CaO additions

来源期刊：Rare Metals2019年第8期

论文作者：Chu Cheng Zhi-He Dou Ting-An Zhang Jian-Ming Su Hui-Jie Zhang Yan Liu Li-Ping Niu

文章页码：793 - 799

摘要：Ferrotitanium is used as a deoxidizer and alloying agent during steelmaking process,which has a high demand for sulfur control.Sulfur was introduced from raw materials in the process of producing ferrotitanium by thermite method,where CaO was used as fluxing agent.At the same time,CaO has a great desulfurization capability.Effects of CaO addition on the distribution of sulfur in high titanium ferroalloy prepared by thermite method were studied in this work.The equilibrium diagram of Ti-AlFe-S system was calculated by FactSage 6.4 software package with FactPS and FTmisc database.The alloy and slag samples were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,inductively coupled plasma atomic emission spectrometer（ICP-AES）,X-ray fluorescence（XRF）and high-frequency infrared ray carbon sulfur analyzer.The result indicates that the sulfur in the alloy firstly exists in the form of liquid FeS,thereafter TiS（s）and eventually Ti₂ S（s）during cooling.The sulfur is mainly distributed in the alloy,and only a small amount of sulfur remains in the slag.Moreover,it is noted that the sulfur in the alloy does not distribute homogeneously,and it exists in the form of solid solution phase,（Ti,Al,Fe）S.S content in the slag,the sulfur capacity of the slag and the sulfur distribution ratio（L_S）all increase with the increment of CaO addition,while S content in alloys decreases.

稀有金属(英文版) 2019,38(08),793-799

Sulfur distribution in preparation of high titanium ferroalloy by thermite method with different CaO additions

Chu Cheng Zhi-He Dou Ting-An Zhang Jian-Ming Su Hui-Jie Zhang Yan Liu Li-Ping Niu

School of Metallurgy,Northeastern University

Key Laboratory of Ecological Utilization of Multi-Metal Intergrown Ores of Ministry of Education,Northeastern University

作者简介：*Zhi-He Dou e-mail: douzh@smm.neu.edu.cn;

收稿日期：2 September 2017

基金：financially supported by the National Natural Science Foundation of China(Nos.51422403 and51504064);the Fundamental Research Funds for the Central Universities(No.N162505002);the National Basic Research Program of China(No.2013CB632606);

Sulfur distribution in preparation of high titanium ferroalloy by thermite method with different CaO additions

Chu Cheng Zhi-He Dou Ting-An Zhang Jian-Ming Su Hui-Jie Zhang Yan Liu Li-Ping Niu

School of Metallurgy,Northeastern University

Key Laboratory of Ecological Utilization of Multi-Metal Intergrown Ores of Ministry of Education,Northeastern University

Abstract：

Ferrotitanium is used as a deoxidizer and alloying agent during steelmaking process,which has a high demand for sulfur control.Sulfur was introduced from raw materials in the process of producing ferrotitanium by thermite method,where CaO was used as fluxing agent.At the same time,CaO has a great desulfurization capability.Effects of CaO addition on the distribution of sulfur in high titanium ferroalloy prepared by thermite method were studied in this work.The equilibrium diagram of Ti-AlFe-S system was calculated by FactSage 6.4 software package with FactPS and FTmisc database.The alloy and slag samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),inductively coupled plasma atomic emission spectrometer(ICP-AES),X-ray fluorescence(XRF)and high-frequency infrared ray carbon sulfur analyzer.The result indicates that the sulfur in the alloy firstly exists in the form of liquid FeS,thereafter TiS(s)and eventually Ti₂ S(s)during cooling.The sulfur is mainly distributed in the alloy,and only a small amount of sulfur remains in the slag.Moreover,it is noted that the sulfur in the alloy does not distribute homogeneously,and it exists in the form of solid solution phase,(Ti,Al,Fe)S.S content in the slag,the sulfur capacity of the slag and the sulfur distribution ratio(L_S)all increase with the increment of CaO addition,while S content in alloys decreases.

Keyword：

Desulfurization; Sulfur partition ratio; Optical basicity; High titanium ferroalloy; Thermite method;

Received： 2 September 2017

1 Introduction

Ferrotitanium (FeTi) is one of the most important master alloys containing titanium.It is mainly used as a deoxidizer and alloying agent during the steelmaking,which plays a great role in improving the quality of the high alloy steel in military and aeronautical industries [ 1, 2] .Sulfur is a wellknown harmful impurity in the steelmaking,and it exists in the form of sulfide (Mn,Me)S in steels,which is detrimental to mechanical property and hot-working characteristics of steel [ 3] .The sulfur content in steels is generally controlled under 0.020 wt% [ 4, 5] .Therefore,ferrotitanium,as a deoxidizer or alloying agent,has a high demand for sulfur control.The sulfur content in high ferrotitanium must be controlled under 0.04 wt% [ 6] .

Traditional methods for ferrotitanium preparation include vacuum arc furnace melting and the thermite process [ 7, 8, 9] .For the rcmelting process,titanium scrap and iron are used as raw material and the sulfur content is well controlled.However,this method is not only restricted by the raw material source of titanium scrap,but also results in a high production cost.Rutile or high-titanium slag is normally used as raw material in thermite method.Besides,aluminum is used as a reductant,CaO is used as a slag former,and KClO₃ is used as an exothermal agent.This method has many advantages including wide range of selecting raw material sources,low energy consumption and low production cost.However,O and Al content in the high-grade ferrotitanium is quite high,because of a high number density of A1₂O₃ inclusions and the formation of Ti-Al intermetallic compound [ 10, 11, 12] .More seriously,sulfur is introduced from raw materials,such as titanium concentrate,rutile or high-titanium slag containing sulfide or elemental sulfur,and this part sulfur is difficult to be eliminated [ 13] .The preparation and deoxidizing mechanism of high ferrotitanium based on thermite method have been widely reported.Chumarev et al. [ 14] researched the technological possibility of manufacturing high ferrotitanium form crude ore.The results showed that high ferrotitanium (60 wt%-70 wt%) with oxygen content<5 wt%could not be produced directly by the out-offurnace thermite method.Dou et al. [ 15, 16] investigated the forming mechanism of oxygen in high ferrotitanium.The result indicated that the deoxidation in thermite method is not complete.The existing forms of oxygen in the ferrotitanium are complicated inclusions and titaniumoxygen solid solution.However,those researches focused on the deoxidization mechanism and the removal of inclusions,but researches on the sulfur in high ferrotitanium by thermite method have rarely been reported.

CaO is widely used as a desulfurizer in process metallurgy.Many researchers studied the desulfurizztion and mechanism of Al₂O_3-CaO slag in steel refining process [ 17, 18, 19, 20, 21, 22] .Those researches indicated that CaO has a strong ability of desulfurization.CaO reacts with sulfur generating CaS,and then CaS is absorbed by the refining slag.The optical basicity has a great influence on the desulfurization ability of a slag [ 23, 24] .And,at the same time,CaO,as a fluxing agent in thermite method,reacts with A1₂O₃ generating calcium aluminate with low melting point,which promotes the slag-metal separation.Therefore,the additional amount of CaO may have a great influence on the sulfur in high titanium ferroalloy prepared by thermite method.

Effects of CaO addition on the distribution of sulfur in high titanium ferroalloy prepared by thermite method were studied.The equilibrium diagram of the Ti-Al-Fc-S system was calculated by FactSage 6.4 software package.The alloys and slags were characterized by an X-ray diffraction(XRD),scanning electron microscopy (SEM),an inductively coupled plasma atomic emission spectrometer (ICP-AES),X-ray fluorescence (XRF) and a high-frequency infrared ray carbon sulfur analyzer.The occurrence state of sulfur and effects of optical basicity on the sul fur content in the alloy and the sulfur distribution ratio with different CaO additions were studied.

2 Experimental

High-titanium slag containing 86.03 wt%TiO₂ with particle size of less than 3 mm (Panzhihua Iron&Steel Group Co.in China),and Fe₂O₃ powder with a purity of 9 8.89%and a particle size between 0.1 and 0.3 mm (Sinopharm Chemical Reagent Co.,Ltd in China) were used as raw material.The chemical composition of raw materials is shown in Table 1.Aluminum powder with a purity of99.5%and particle diameter between 0.1 and 0.3 mm(Jinzhou Metal Co.,Ltd.,China) was used as reductant.KClO₃ with a purity of 99.8%and particle diameter between 0.1 and 0.3 mm,CaO with a purity of 99.5%and particle diameter between 0.1 and 0.3 mm and magnesium powder with a purity of 99.5%and particle diameter betwween 0.05 and 0.2 mm were used.

For preparing raw materials before the synthesis,hightitanium slag,Fe₂O₃,KClO₃ and CaO were dried in an electrothermal constant-temperature dry box at 473 K for24 h.Thereafter,the powders were weighed and put in a vacuum tank.Al powder and steel balls were subsequently aadded into the vacuum tank,and then the tank was isolated by a lid and was vacuumed to 1×10^-5 Pa,following which they were mixed by a can mixer for 60 min.Finally,they were preheated in a vacuum drying oven at 383 K for1 h to supply extra heat for the reaction system.The ingredients (total amount is about 5-7 kg) were placed into a home-made conical graphite reactor enclosed by magnesia lining with a volume of 10 L.2-3 g Mg powder was used as an easy ignition agent placed on the top of the ingredients.The Mg powders were ignited to induce selfpropagation high-temperature synthesis (SHS) reaction and to lead to a high-temperature melt,which flowed into the separator of the alloy and slag at the bottom of the conical graphite reactor.After tens of seconds,the melt was cast into a graphite crucible enclosed by magnesia lining,and then was cooled to room temperature by air-cooling.Samples of the alloy and slag after SHS experiment were collected.

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Table 1 Chemical composition of raw materials (wt%)

Fig.1 Diagram of Ti-Al-Fe-S system (A1 12.0 wt%,S 0.5 wt%)

The mass ratio of high-titanium slag of Al/Fe₂O₃/KClO₃/CaO is 1:0.61:0.23:0.35:0.126-0.226.The stoichiometric ratio of A1 powder used as a reductant for hightitanium slag is 100%.The ratios of CaO in the ingredients can be expressed as R_(C/A)(the molar ratio of CaO to Al₂O₃,where Al₂O₃ is a combustion product of SHS reaction in theoretical stoichiometry).R_(C/A) in the experiments is 0.20,0.25,0.30 and 0.35,respectively.The adiabatic temperature of the system is calculated as2150-2200 K.

The equilibrium diagram of Ti-Al-Fe-S system was calculated by FactSage 6.4 software package with FactPS and FTmisc database.The phases of the alloys and slags were identified by XRD (Model D8 Bruker,Germany) with a Cu Kα1 source at 40 kV and 40 mA.A high-frequency infrared ray carbon and sulfur analyzer (G4 ICARUS.Bruker Ltd,Germany) was used to measure sulfur content.Inductively coupled plasma atomic emission spectrometry(ICP-AES,Model ICP-Prodigy,Optima 4300 DV,Lehman,USA) was used to analyze the chemical compositions of alloy samples.XRF was used to analyze the composition of the slags.Scanning electron microscope coupled with energy-dispersive X-ray spectroscopy (SEM-EDS,SU-8100,Hitachi,Japan) was used to observe the microstructure of the alloy and slag samples.Preparing process of alloy samples is as follows:the sample was firstly manual polished using SiO₂ paste.Thereafter,the polished sample was etched for 10-20 s.Finally,the etched sample was cleaned by ultrasonic wave and was dried by a hair dryer.The etching solution is 1-3 ml HF,2-6 ml HNO₃ and 91-97 ml distilled water.

3 Results and discussion

3.1 Phase diagram

Figure 1 shows the equilibrium diagram of Ti-Al-Fe-S system,where 12.0 wt%Al and 0.5 wt%S are included.It shows that when titanium content ranges from 58 wt%to60 wt%,the diagram is located in the region between imaginary Line 1 and Line 2.The phase precipitation sequence from 2073 K to room temperature is described as follows:When the temperature of the system is above Line A,S in the melt exists in the form of liquid FeS.When the temperature drops to the value of Line A,TiS (s) begins to precipitate.The liquid FeS dissolves completely,and all S exists in the form of TiS (s) when the temperature decreases to Line B.Ti₂S (s) starts to generate when the temperature decreases to Line C.When the temperature is a value of Line E,TiS (s) dissolves completely and S exists in the form of Ti₂S (s) completely.At the same time,TiAl(s) starts to precipitate.FeTi (s) begins to appear when the temperature arrives to Line F.When the temperature ranges from eutectic Line G to Line H,where the phases in the alloy are Ti (s),TiAl (s),Ti₂S (s) and FeTi (s).Fe₂Ti(s) starts to precipitate when the temperature is a value of Line H.As a summary,the phase in this system is composed of Ti (s),TiAl (s),Ti₂S (s) and Fe₂Ti (s) at room temperature,and the sulfur exists in the form of Ti₂S (s).

Table 2 shows the chemical composition of the high titanium ferroalloy prepared by thermite method with different R_(C/A).It indicates that Ti content is about 60 wt%,A1 content is about 12 wt%,and S content is about0.5 wt%in those alloys.According to the diagram of TiAl-Fe-S system (Fig.1),S should exist in the form of Ti₂S(s) in those alloys.

3.2 S distribution

Figure 2a shows XRD patterns of slags with different R_(C/A).The diffraction peaks can be indexed to CaAl₄O₇and CaAl₁₂O_19.With R_(c/A) increasing,the diffraction peak intensity of CaAl₂O₄ increases,but CaAl₁₂O₁₉ decreases.Figure 2b shows XRD patterns of high titanium ferroalloys prepared by thermite method with differentR_(C/A).The diffraction peaks can be indexed to FeTi and Ti-Al intermetallic compound,which offers a reasonable agreement with the phase analysis results in Fig.1.

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Table 2 Chemical composition of high titanium ferroalloys prepared by thermite method with different R_(C/A)(wt%)

Figure 3 shows the morphology of typical microstructure and the distribution of elements in the slag with a R_(C/A) value of 0.2.Figure 3a indicates that the micro structure of the slag is mainly composed of“smooth plate”,“laminated plate”phases and spherical alloy particles.EDS analysis result (Fig.4) shows that the atom ratio of Ca,Al and O in the“smooth plate”(Point 1) is close to 1:4:7,and that of“laminated plate”phase (Point2) is close to 1:12:19.According to XRD patterns of slags(Fig.2a),the“smooth plate”is identified as CaAl₄O₇ and the“laminated plate,”is perceived as CaAl₁₂O_19.In addition,S distribution (Fig.3c) shows that S is mainly distributed in the alloy,and only a small amount of sulfur exists in the slag.This can be explained by the following reasons.According to the CaO-Al₂O₃ phase diagram [ 25] and Fig.1,the melting temperatures of CaAl₁₂O₁₉ and CaAl₄O₇ slags are more than 2000 K,which is higher than those of both FeS (melting point,1468 K) and alloy matrix(melting point,≤1273 K,as shown in Fig.1).After the self-propagating reaction,the temperature of the system was fast cooled down by air-cooling to room temperature.Subsequently,the viscosity of the slag increases and its liquidity sharply decreases.However,FeS (1) and liquid alloy still had a good liquidity and they mixed with each other and were separated from the slag for the action of gravity.However,FeS had only a little time to react with CaO in the slag,leading to the result that most of the sulfur in the system is transferred into the liquid alloy.In addition,FeS is solidified with rapid cooling rate and could not mix uniformly with liquid alloy,which brings about the uneven distribution of sulfur in the alloy.

Figure 5 shows typical micros true ture and elemental distribution in the titanium ferroalloy.Figure 5a indicates that the alloy is mainly composed of matrix,solid solution containing S and Al₂O₃ inclusion.The elemental distribution (Fig.5c-f) shows that Ti,A1 and Fe are uniformly distributed in the matrix.However,S is intensively distributed in the solid solution phase.EDS analysis (Fig.5b)indicates that the solid solution phase is composed of Ti,Al,Fe and S.The atomic ratio of Ti,Al,Fe and S is12.41:2.76:1.69:1.00,which offers a reasonable agreement with the phase analysis results (Ti,TiAl,Ti₂S and FeTi) in Fig.1.In addition,S content in this phase is 4.04 wt%,which is higher than that of the alloy (Table 2).

Fig.2 XRD patterns of slags and alloys prepared by thermite method with different R_(C/A):a slag and b alloys

Fig.3 a Typical SEM image and element distribution (b Ti,c S,d Ca,e O and f Al distribution) of slag with R_(C/A) of 0.2

Fig.4 EDS analysis of a Point 1 and b Point 2 in Fig.3a

Fig.5 a Typical SEM image,b EDS analysis of Point 1 and elemental distribution (c Ti,d Al,e Fe and f S) of alloys

3.3 S partition ratio

The desulfurization reactions using CaO as a desulfurizer in metallurgical industry can be written as follows:

The basicity is an important indicator of the chemical characteristics of a slag,which has a great effect on its desulfurization ability.According to the theoretical optical basicity of each oxide listed in Table 3 [ 26] ,the following expression is used to calculate the optical basicity (A) of a slag [ 24, 25] :

where i is the number of oxygen atoms or fluorine atoms in the oxide or fluoride,respectively; and are the mole fractions of oxide and fluoride,respectively.

When the optical basicity of a slag is lower than 0.8,its sulfide capacity (C_s) is usually defined by the following equation [ 27, 28] :

According to the chemical composition analysis(Table 4),the optical basicity and the sulfide capacity of slag samples at 2100 K are calculated and listed in Table 4.

Table 4 shows that the optical basicity and sulfur capacity gradually increase with R_(C/a) increasing,which indicates that the desulfurization capacity is gradually improved with CaO addition increasing.Zhao et al. [ 29] and Patsiogiannis et al. [ 30] investigated the desulfurization effect of Al₂O_3-CaO-based slag.The result indicated that,when the optical basicity of a slag is less than 0.8,the increase in R_(C/A) would result in the increase in sulfur capacity,which agrees well with this result.

The sulfur distribution ratio (L_s) is usually defined by the following equation:

where w(S) is the mass fraction of sulfur in the slag;w([S])is the mass fraction of sulfur in the alloy.

According to the chemical compositions of the slags and alloys,Eq.(4) was used to calculate L_s and the result is shown in Fig.6.It shows that S content in the slag and L_sincreases with R_(C/A) increasing,while S content in the alloy decreases.This can be explained as follows:firstly,the increase in R_(C/A) would lead to the increase in sulfur capacity of the slag,which is conducive to the removal of S in the alloy.In addition,when R_(C/A) is greater than 0.3,namely when the optical basicity of the slag is close to 0.8,R(C/A) slightly increases with the increase in R_(C/A),indicating the effect of R_(C/A)on desulfurization is weak.

Fig.6 Effects of R_(C/A) on sulfur partition ratio and sulfur content in alloys and slags

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Table 3 Optical basicity (Λ_i) of oxides

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Table 4 Composition,optical basicity and sulfur capacity of slags prepared by thermite method with different R_(C/A)

4 Conclusion

S in the alloy shown in the equilibrium diagram of the Ti-AlFe-S system firstly exists in the form of liquid FeS,and thereafter TiS (s) and eventually Ti₂S (s) in the lower temperature range during cooling.S is mainly distributed in the alloy prepared by thermite method,and it has uneven distribution in the form of solid solution containing Ti,Al,Fe and S.Only a small amount of S remains in the slag.S content in the slag,the sulfur capacity of slags and S partition ratio (L_s)increase with R_(C/A) increasing,while S content in alloys decreases.In addition,whenR(C/A) is greater than 0.3,the effect of R_(C/A) on desulfurization is weak.

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Nos.51422403 and51504064),the Fundamental Research Funds for the Central Universities (No.N162505002) and the National Basic Research Program of China (No.2013CB632606).