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Ceramics International 40 (2014) 9709–9714www.elsevier.com/locate/ceramint

Fe–Sialon–Ti(C,N) composites from carbothermal reduction–nitridationof low-priced minerals and their application in taphole clay refractories

Xueyin Liu, Minghao Fang, Yan-gai Liun, Zhongjun Qian, Hao Ding, Zhaohui Huang

School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China

Received 13 October 2013; received in revised form 12 February 2014; accepted 12 February 2014Available online 20 February 2014

Abstract

Fe–Sialon–Ti(C,N) composite materials were first synthesized via carbothermal reduction–nitridation reaction with low-priced mineralmaterials, such as ilmenite, quartz, aluminum dross and coke powders, as starting materials. Effects of temperatures on the phase compositionsand microstructures of products were investigated. The investigation results indicated that the optimal synthesis temperature was 1550 1C. Rise insynthesis temperature was beneficial for the synthesis of Sialon and Ti(C,N). The as-fabricated Fe–Sialon–Ti(C,N) composites were used asadditives to produce taphole clay refractories, and the bending strength and erosion resistance against blast furnace slag of taphole clayrefractories were studied. Taphole clay refractories with 20 wt% Fe–Sialon–Ti(C,N) composite materials still owned good slag resistance.Bending strength of taphole clay refractories increased with the addition of Fe–Sialon–Ti(C,N) composite materials; when 15 wt% Fe–Sialon–Ti(C,N) composite materials were added, bending strength reached its highest value of 13.36 MPa.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sintering; B. Composites; C. Corrosion; E. Refractories

1. Introduction

Taphole clay refractories are functional refractory materialswhich can be used to protect blast furnace wall. The propertiesof taphole clay are important to the safety and high efficiencyof ironmaking blast furnace [1]. Taphole clay refractories aretraditionally prepared with raw materials of corundum, clay,Fe–Si3N4, bauxite chamotte, tar and coke [2]. The high cost ofraw materials (corundum and Fe–Si3N4) restrains the large-scale applications of taphole clay refractories. Lowering thecost of refractories is of great significance to the steel makingplant as the iron and steel industry enters its recession period.Therefore, the new ways to produce high performancematerials with low-cost natural minerals have gained consider-able attention [3–7].

10.1016/j.ceramint.2014.02.05314 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel./fax: þ86 10 82322186.ss: liuyang@cugb.edu.cn (Y.-g. Liu).

Sialon–Ti(C,N) composites have excellent properties, such ashigh melting point, high hardness, good wear resistance, excellentcorrosion resistance and good thermal conductivity [8–12]. Withthese outstanding properties, Sialon–Ti(C,N) composites havebecome promising raw materials for taphole clay refractories. Ironin Fe–Sialon–Ti(C,N) composites usually exists in the ferro-silicon alloy phase, which can promote the sintering property andfracture toughness of taphole clay refractories [13].Ilmenite (FeTiO3) is an oxide mineral of iron and titanium.

Nowadays, ilmenite is mainly used to extract iron andtitanium, while other utilizations are not common [14,15]. Asthe richest mineral resource in the earth crust, quartz is verycheap. Aluminum dross is an industrial waste in aluminumsmelting and can pollute water, land and air [16].In this study, Fe–Sialon–Ti(C,N) composites were synthe-

sized via carbothermal reduction–nitridation method withlow-priced mineral materials as starting materials. The as-fabricated Fe–Sialon–Ti(C,N) composites were used to replacepartial expensive raw materials in taphole clay refractories. The

Fig. 1. XRD figures of samples sintered at different temperatures.

X. Liu et al. / Ceramics International 40 (2014) 9709–97149710

replacement program might serve as a new technical approach forcutting off the cost of taphole clay refractories and efficientutilization of aluminum dross and natural mineral resources.

2. Experimental procedure

2.1. Synthesis of Fe–Sialon–Ti(C,N) composites

Ilmenite, quartz, and aluminum dross were used as rawmaterials and coke acted as a reducing agent. Ilmenite used inthis experiment was obtained from Panzhihua, Sichuan Pro-vince in China. Aluminum dross was obtained from analuminum plant in Hebei Province in China. The chemicalcompositions of the raw materials are listed in Table 1. Theraw materials were ground to a diameter smaller than0.075 mm by a vibration mill. Ilmenite was weighed accordingto 20 wt% of the sample first to synthesize Ti(C,N) and ironand 80 wt% of the sample was composed of aluminum drossand quartz. β-Sialon (Si4Al2O2N6) with a Z value of 2 wasdesigned to be synthesized with Al2O3 and SiO2 from rawmaterials. After chemical calculation, 36.24 wt% aluminumdross, 43.76 wt% quartz, and 25.19 wt% coke powders wereweighed in the sample according to the chemical compositionsof raw materials and using the following equations:

Al2O3(s)þ4SiO2(s)þ3N2(g)þ9C(s)-Si4Al2O2N6(s)þ9CO(g) (1)

2FeTiO3(s)þ6C(s)þN2(g)-2Fe(l)þ2TiN(s)þ6CO(g) (2)

Then starting powders were mixed mechanically for 6 h byball milling. The mixture powders were pressed into columnarsamples with the size of Ф¼20 mm� 10 mm under thepressure of 25 MPa. After drying at 110 1C for 12 h, thesamples were sintered at 1400 1C, 1450 1C, 1500 1C and1550 1C in a flowing nitrogen atmosphere (the purity of99.999%) for 3 h. The phase composition was determined byX-ray diffraction (XRD; D8 Advance diffractometer, Ger-many) using Cu Kα1 radiation (λ¼¼1.5406 )̊ with a step of0.021 (2θ) and a scanning rate of 41 min�1. The microstructurewas observed by a scanning electron microscope (SEM, JEM-6460LV, Japan).

2.2. Preparation of taphole clay refractories containingFe–Sialon–Ti(C,N) composites

The original taphole clay refractories were collected from asteel plant in China. Different contents of the as-synthesizedFe–Sialon–Ti(C,N) composites were added into original tap-hole clay refractories. The contents of Fe–Sialon–Ti(C,N)

Table 1Chemical compositions of raw materials (wt%).

TiO2 FeO MgO Al2O3

Ilmenite 51.50 38.72 4.04 1.99Aluminum dross 1.60 – 7.10 69.60Quartz – – – 0.63

composites added to taphole clay refractories were 0 wt%,5 wt%, 10 wt%, 15 wt% and 20 wt%.The samples were shaped with the size of 150 mm� 25

mm� 25 mm under the pressure of 100 MPa for strength tests.Crucible samples for slag resistance test were shaped with thesize of Ф¼50 mm� 50 mm and an inner hole (Ф¼20mm� 25 mm). All samples were heated at 1400 1C in thereducing atmosphere. Then the bulk density, apparent porosity,and bending strength of samples were tested.Blast furnace slag (10 g) was added in crucible samples.

Slag resistance test was carried out at 1500 1C in the reducingatmosphere for 3 h. After the slag resistance test, cruciblesamples were cut along the central axes to examine slagresistance.

3. Results and discussion

3.1. Effects of synthesis temperature on phase behaviors ofFe–Sialon–Ti(C,N) composites

The effects of different heat treatment temperatures on thephase compositions of the products are shown in Fig. 1. Themain phases of samples sintered at 1400 1C and 1450 1C wereSi1.8Al0.2O1.2N1.8 (O–Sialon), Ti(C,N), ferro-silicon alloy, anda small quantity of corundum. The appearance of O–Sialonsuggested that carbothermal reduction–nitridation reaction ofaluminum dross and quartz occurred at 1400 1C and 1450 1Cand that partial SiO2 was converted into silicon oxynitride.Then silicon oxynitride reacted with Al2O3 to form O–Sialonthrough the solution reaction. The main phases of samples

SiO2 CaO MnO Fe2O3 K2O

1.23 0.30 0.71 – –

5.90 2.80 – 4.00 2.6097.80 0.08 – 0.13 0.05

X. Liu et al. / Ceramics International 40 (2014) 9709–9714 9711

sintered at 1500 1C and 1550 1C were composed of β-Sialon(Si4Al2O2N6) and Ti(C,N). 15R (Si4Al2O2N6) appeared in thesamples sintered at 1550 1C. With the increase of sinteringtemperature, O–Sialon was transformed into β-Sialon and 15R,indicating that the higher temperature was conducive for theformation of β-Sialon. There was no peak of the corundum phasein the samples sintered at 1500 1C and 1550 1C, indicating that allAl2O3 had been solid-solubilized into β-Sialon phase.

From the above analysis, the optimal synthesis temperatureof Ti(C,N) and β-Sialon was 1550 1C.

3.2. Effects of synthesis temperature on the microstructure ofFe–Sialon–Ti(C,N) composites

The SEM photographs of the samples sintered at differentsynthesis temperatures (1400 1C, 1450 1C, 1500 1C and 1550 1C)are shown in Fig. 2. It can be seen from Fig. 2(a) that there are ahandful of fibroid materials, numerous pores and tiny particlesevenly distributed on the smooth fracture surface of samplessintered at 1400 1C. When synthesis temperature was 1450 1C,bigger holes and some agglomerate combinations appeared in thesamples, resulting in rough fracture surface and a lot of tinyparticles on the surface (Fig. 2(b)). A lot of short-columnar Sialonexisted in the samples sintered at 1500 1C (Fig. 2(c)). Sialoncrystals grew into long-columnar shape when the temperature rosefurther to 1550 1C (Fig. 2(d)). This phenomenon can be explainedas follows: carbothermal reduction–nitridation reaction at 1400 1Cproceeded slowly and the development and growth of Sialon wasrestrained. Meanwhile, less liquid phase was formed from ilmeniteand ferrosilicon alloy melting. When samples were sintered at

Fig. 2. SEM photographs of fracture morphology of samples sintered at different temand d: sintered at 1550 1C).

1450 1C, more liquid phase appeared under the effect of surfacetension and tiny particles of Sialon crystals were formed from thecarbothermal reduction–nitridation reaction. Then Sialon crystalsgrew up at 1500 1C. At 1550 1C, gas–liquid–solid phase (VLS)reaction occurred in the samples and promoted the growth ofSialon crystals.Fig. 3 shows SEM photos and EDS spectra of ferrosilicon

alloy and Ti(C,N) in Fe–Sialon–Ti(C,N) composite materials.In Fig. 3(a), among the columnar Sialon crystals, there aremany spherical particles generated. According to EDS spectrashown in Fig. 3(c), the spherical particles are ferrosilicon alloyphase. In Fig. 3(b), other agglomerated particles of about 1 μmare also found in Fe–Sialon–Ti(C,N) composites. In Fig. 3(d),the elements of the particles are Ti and N. Although no C isdetected, Ti(C,N) phase exists in composites according to theXRD analysis of Fig. 1 and TiN is easy to be solid-solubilizedwith TiC. Therefore, the agglomerated small particles were Ti(C,N) phase that consisted of TiN, TiC or Ti(C,N).

3.3. Synthesis mechanism analysis

The synthesis mechanism of Fe–Sialon–Ti(C,N) via car-bothermal reduction–nitridation reaction with illmenite, alumi-num dross and quartz was explored. It is assumed that duringthe process of carbothermal reduction–nitridation reaction,illmenite is reduced by the coke powders. The reactions givenare as follows:

FeTiO3(s)þC(s)-Fe(l)þTiO2(s)þCO(g) (3)

SiO2(s)þ2C(s)-Si(s)þ2CO(g) (4)

peratures (a: sintered at 1400 1C, b: sintered at 1450 1C, c: sintered at 1500 1C,

Fig. 3. SEM photographs and EDS spectra of ferrosilicon alloy and Ti(C,N) in Fe–Sialon–Ti(C,N) composite materials (a: distribution and morphology of ferrosiliconalloy, b: distribution and morphology of Ti(C,N), c: EDS spectrum of spherical particle marked “1” in (a), and d: EDS spectrum of particle marked “2” in (b)).

Fig. 4. Bulk density and apparent porosity of samples with different contentsof Fe–Sialon–Ti(C,N) composites.

X. Liu et al. / Ceramics International 40 (2014) 9709–97149712

Fe(l)þSi(l)-FeSi(l) (5)

5Fe(l)þ3Si(l)-Fe5Si3(l) (6)

Then the resultant TiO2 would be reduced gradually: TiO2-Ti3O5-Ti2O3-TiN (or TiC) [17]. According to thermody-namic calculation results, when the reaction temperature wasabove 1350 1C, the final carbothermal reduction–nitridationproducts of TiO2 were TiN or TiC. Ti (C,N) in our experimentwas caused by a solid solution from TiC and TiN. SiO2 in rawmaterials would be reduced by C as shown in Eq. (4) and theproduct would be Si. Then a solid solution of Si and Fe wasobtained (generated in Eq. (3)) to form ferrosilicon alloy (Eqs.(5) and (6)). When synthesis temperature was below 1500 1C,Si1.8Al0.2O1.2N1.8 was synthesized with Al2O3 and SiO2 accord-ing to Eq. (7); while temperature was above 1500 1C, β-Sialon(Si4Al2O2N6) and 15R (Si4Al2O2N6) were formed (Eqs. (8) and(9)). This can be confirmed by the XRD patterns (Fig. 1).

1.8SiO2(s)þ0.1Al2O3(s)þ2.7C(s)þ0.9N2(g)-Si1.8Al0.2O1.2N1.8(s)þ2.7CO(g) (7)

4SiO2(s)þAl2O3(s)þ9C(s)þ3N2(g)-Si4Al2O2N6(s)

þ9CO(g) (8)

SiO2(s)þ2Al2O3(s)þ6C(s)þ2N2(g)-SiAl4O2N4(s)þ6CO(g) (9)

3.4. Effects of Fe–Sialon–Ti(C,N) composites on physicalproperties of taphole clay refractories

Taphole clay refractories were prepared with the originaltaphole clay refractories and different contents of the as-fabricated Fe–Sialon–Ti(C,N) composites.

Fig. 4 shows the bulk density and apparent porosity ofsamples with different contents of Fe–Sialon–Ti(C,N) compo-sites. In Fig. 4, as the content of Fe–Sialon–Ti(C,N) compo-sites increases, the bulk density of samples first increases andthen decreases. The sample without Fe–Sialon–Ti(C,N) com-posites had the lowest bulk density, 2.02 g/cm3. And thehighest bulk density (2.21 g/cm3) was when the content of Fe–Sialon–Ti(C,N) composites was 15 wt%. Meanwhile, theapparent porosity of samples first declined and then rose withincreasing content of Fe–Sialon–Ti(C,N) composites. Theapparent porosity of the sample without Fe–Sialon–Ti(C,N)composites was 21.93%. When the content of Fe–Sialon–Ti(C,N) composites accounted for 15 wt%, the apparent porosityreached its lowest value of 16.44%.According to the analysis results (Fig. 4), the addition of

Fe–Sialon–Ti(C,N) composites in taphole clay refractories wasbeneficial for increasing the bulk density and decreasing theapparent porosity.

X. Liu et al. / Ceramics International 40 (2014) 9709–9714 9713

The bending strength of the samples with different contentsof Fe–Sialon–Ti(C,N) composites is shown in Fig. 5. Thebending strength of samples first increased and then decreasedwith increasing content of Fe–Sialon–Ti(C,N) composites(Fig. 5). The bending strength of the sample without Fe–Sialon–Ti(C,N) composites reached its minimum value,11.90 MPa; when the content of Fe–Sialon–Ti(C,N) compo-sites was 15 wt%, bending strength reached its maximumvalue, 13.36 MPa. This phenomenon can be explained asfollows: adding certain content of Fe–Sialon–Ti(C,N) compo-sites can improve physical properties of the samples becauseiron alloy exists in the liquid phase at a high temperature.However, too many Fe–Sialon–Ti(C,N) composites wouldprovide too much liquid phase, which would destroy thesample structure. Therefore, the bending strength decreased

Fig. 5. The bending strength of samples with different contents of Fe–Sialon–Ti(C,N) composites.

Fig. 6. Profile morphology of samples with different contents (a: 0 wt%, b: 5 wcorrosion test.

when 20 wt% Fe–Sialon–Ti(C,N) composites were added intosamples.In conclusion, adding certain content of Fe–Sialon–Ti(C,N)

composites can improve the bending strength of taphole clayrefractories.

3.5. Effects of Fe–Sialon–Ti(C,N) composites on slagresistance properties of taphole clay refractories

Fig. 6 shows the profile morphology of samples with differentcontents of Fe–Sialon–Ti(C,N) composites after slag corrosiontests. It can be seen from Fig. 6 that after corrosion with slag at1500 1C, the samples with different contents of Fe–Sialon–Ti(C,N)composites have no obvious corrosion signs and that the hole ofcrucible maintained the original topography. This result indicatedthat adding Fe–Sialon–Ti(C,N) composites would retain the slagresistance of taphole clay refractories. Fe–Sialon–Ti(C,N) compo-sites had good slag resistance properties.Fe–Sialon–Ti(C,N) synthesized with low-priced mineral

materials can be used in taphole clay refractories to replacepartial expensive raw materials in the original taphole clayrefractories. The taphole clay refractories with Fe–Sialon–Ti(C,N) not only maintained good slag resistance but alsoimproved bending strength to a certain degree.

4. Conclusions

Fe–Sialon–Ti(C,N) composite materials were synthesizedsuccessfully with ilmenite, quartz, aluminum dross and cokepowders as raw materials. The optimal synthesis temperatureof β-Sialon and Ti(C,N) was 1550 1C. Adding certain contentof Fe–Sialon–Ti(C,N) composites in taphole clay refractoriescan improve the bulk density, decrease the apparent porosityand enhance the bending strength of samples. The optimal

t%, c: 10 wt%, d: 15 wt%, and e: 20 wt%) of Fe–Sialon–Ti(C,N) after slag

X. Liu et al. / Ceramics International 40 (2014) 9709–97149714

added content was 15 wt%. It can be confirmed that Fe–Sialon–Ti(C,N) composites synthesized with low-pricedmineral materials can replace partial expensive raw materialsin the original taphole clay refractories, maintain slag resis-tance of the taphole clay refractories and improve bendingstrength and slag resistance to a certain degree.

Acknowledgment

This work was supported by the National Natural ScienceFoundation of China (Grant no. 51032007), the NationalScience Technology Support Projects for the 12th Five-yearPlan (Grant no. 2011BAB03B08), the Program for NewCentury Excellent Talents in University of Ministry of Educa-tion of China (Grant no. NCET-12-0951), the FundamentalResearch Funds for the Central Universities (Grant nos.2012067) and the New Star Technology Plan of Beijing (Grantno. 2007A080).

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