ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
BOF Slag Glass-ceramics Prepared in Different Atmospheres from Parents Glasses with Various Reduction Degree
Wenbin Dai Yu LiDaqiang CangZhaobo LiuYong Fan
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2014 Volume 54 Issue 12 Pages 2672-2677

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Abstract

As a special material with better mechanical properties than that of traditional glasses and ceramics, glass-ceramics which can be made from the bulk of industrial solid wastes from the perspective of protecting the environment, could be applied into the fields of construction and industry. For BOF slag, its residual iron element contained inside also can be recovered by a reduction process when it was melted to prepare parent-glass. This is an effective way to resolve the problems of how to utilize such metallurgical slags in large scale and recover the remaining metal component. In this study, the parents glass with different reduction degree were made from BOF slag, fly ash and varied amounts of coal powders, and glass-ceramics were obtained by respectively heating the various parents glass in air or in nitrogen. Results have shown that crystallization is occurred in all parents glasses heated in air, but is suppressed in samples heated in N2 except for the parent glass with TFe content of about 4.5%. Since the TFe content of parents glass is above 7.5%, the shape of crystalline exothermic peak is high and sharp, and the main crystal phases are diopside(-ferrian) and augit. A maximum bending strength of glass-ceramics heated in different atmospheres is obtained for samples with 8% coal powders mixed. The N2 atmosphere could promote the optimal bending strength and decrease its heating temperature.

1. Introduction

With the rapid development of economy, the crude steel output in mainland of China was 716 million tons in 2012, accounting for 46.3% of global steel output.1) The annual production of metallurgical slag in China was therefore above 100 million tons, leading to the problems of how to completely and effectively use these secondary resources. BOF slag is a kind of steelmaking wastes, and some of them has been used in the cement industry,2,3,4,5) but most of them are still discarded without further processed. The total utilization ratio of BOF slag is only 10%–20% in China while this ratio is in excess of 95% in some developed countries.6,7) This untreated slag will occupy large tracts of lands and pollute the environment, such as the leaching liquid penetration of some heavy metal ions in slag.8,9) Besides, profits to dispose of BOF slag are also decreased for the loss of the residual non-magnetic iron element contained inside.10,11,12)

The manufacture of glass-ceramics offers a promising route for the commercial use of metallurgical slag. Such types of glass-ceramics can exhibit good fracture strength, chemical resistance and a highly attractive appearance that is suitable for application as wall and floor tiles.13,14,15) The removal of slag from the environment and the possibility of replacing natural stones, such as marble and granite, by glass-ceramic products, are highly appealing from the environmental standpoint and constitute a strongly motivating factor for researches in this field. However, there are lots of challenges for the manufacture of glass-ceramics based on BOF slag because of low concentrations of SiO2 and a variable chemical composition.14) Thus, acidic modifiers like sands, granite, fly ash, tailings, etc., should be added into the raw materials for the needed modified glass melts.15,16)

In the previous studies,17,18,19,20,21) glass-ceramics with satisfied performances were obtained, of which the weigh ratio of CaO to SiO2 (also called basicity) was 0.6. This basicity could lower the amount of acidic modifiers used and the relative energy consumption. In the industrial production of glass-ceramics based on BOF slag, the reduction degree of different parts of parent glasses for the recovery of iron metal will vary at a large scale, which will affect the properties of glass-ceramics heated from these parent glasses. Besides, the heat treatment of slag glass-ceramics was also improved under the environment of nitrogen (N2) in previous. This phenomenon may be caused by the residual iron oxides inside and the related oxidation. Therefore, this paper focus on the slag glass-ceramics with different reduction heated under the different environments. Effects of these conditions on the structure and performance of slag glass-ceramics are also explained hereinafter.

2. Experimental Procedures

Experimental compositions were prepared by the mixtures of BOF slag (–80 mesh, grinded by the cement ball mill with 5 kg capacity), fly ash (–100 mesh), quartz sand (–100 mesh), flux (Na2CO3, analytical reagent) and coal powders (anthracite, –100 mesh, labeled as CP) in an appropriate amount. Three types of parent glasses with different degree of reduction were obtained by adding varied amount of coal powders as the reducing agent, which was proportion to the BOF slag weight in the batch, with the proportion of other raw materials remaining unchanged. Chemical compositions of these raw materials and coal ash after ignition were measured by X-ray fluorescence (XRF-1800, SHIMADZU, Japan), as shown in Table 1, and the coal content added in batches is shown in Table 2. Due to the composition complexity of these materials and the accuracy of the test instrument, the content of all kinds of oxides tested by XRF technique has 2% error range, especially for the oxides with low content, which has been testified previously through some experiments using pure reagents. Therefore, the content variation of these constituents in this error range by XRF is considered as normal or ignored in this study.

Table 1. Chemical compositions of raw materials.
wt.%BOF slagFly ashQuartz sandcoal ash*
CaO36.064.790.55.76
SiO215.0354.9894.6547.68
MgO7.050.710.270.83
Al2O31.2129.282.1634.19
Fe2O320.965.250.574.96
MnO2.990.050.09
KNaO0.411.81.932.11
MFe12.28<0.1
*  Coal ash was obtained after ignition at 1000°C in air for 3 h.

Table 2. The proportion of coal powders in batches and chemical compositions of parent glasses.
Now.(CP)/w.(BOF slag)chemical composition wt.%
SiO2CaOAl2O3MgONa2OFe2O3#
G00%31.4621.8813.183.973.3217.56
G44%33.3723.6615.904.273.4712.76
G88%35.9725.2517.066.023.656.17
#  Total iron content (TFe) was given in Fe2O3 form by XRF technique, with about 2% error.

The proximate analysis of coal powders is shown in Table 3, measured according to the national standard GB/T 212-2008. These raw materials were mechanically mixed evenly after crushing (jaw crusher, with 5–30 mm crushed gap), ball milling (ND8-4L, NDTZ, China, 360° rotating and oscillating with stainless steel balls moving in the steel tanks), sieving (80–100 mesh of standard sieves) and weighing, and melted in alumina crucibles at 1550°C for 1 hour under the environment of nitrogen. Then, the glass melts were quenched in cold water to gain the original state of the melts at high temperature. After separating the reduced iron metal particles by magnetic process, the rest of solid glasses were ball milled till the particle size below 0.074 mm (–200 mesh). Chemical compositions of these parent glasses and the experiment procedure are shown in Table 2 and Fig. 1(a), respectively.

Table 3. Proximate analysis of coal powders.
Mad / %Ad / %Vd / %FCd / %
0.5610.1811.9477.89
Fig. 1.

Protocols of the experiment process (a) and the test analysis (b). (Online version in color.)

Because the contents of chemical constituents of samples were varied in the above error range after heat treatment (seeing Tables 2 and 4), the chemical composition (in oxide form) is considered unchanged. The discussion in the following is focused on the samples with different reduction degrees and sintering atmospheres.

Table 4. Chemical composition of heated samples in different atmosphere.
Samplesintering atmosphereheating temperatureSiO2CaOAl2O3MgONa2OFe2O3
G0N2800°C30.5121.9313.693.652.8518.24
G4N2900°C32.5223.915.983.872.813.5
G8N2900°C35.6325.8317.44.283.267.21
G0air1000°C30.8221.713.013.792.8318.27
G4air1000°C32.8623.8315.913.852.8513.64
G8air1000°C35.4826.0417.24.133.067.27

The glass powders were suppressed into rectangle shape with the size of 50×6×(4~7) mm3 at the pressure of 9 tons (294 MPa) in a steel mold. All these green samples were heated under the environment of air or N2 at temperatures among 700–1000°C for 1 hour holding time, and then cooled to the room temperature, where the heating rate was 7°C·min–1 and the cooling rate was 5°C·min–1.

The bending strength (labeled as BS) of these sintered samples was determined by a three-point bending test with 30 mm outer span and the speed of 0.5 mm·min–1 down (FPZ100, Germany, the results are the average of measurements made on, at least, 5 bars). The crystallization process was investigated by the differential thermal analysis (DTA, HCT-3, Hengjiu, China) with a heating rate of 10°C·min–1 in air or N2 atmosphere, taking an empty alumina crucible as reference. The crystalline phases formed were determined by X-ray powder diffraction (XRD, M21X apparatus and Cu radiation, MAC Seience Co. Ltd, Japan). The microstructure of heated samples etched in 5% HF solution for 1 minute was observed by the scanning electron microscope (SEM, EV018, Carl-Zeiss, Germany). Protocol of test analysis can be seen in Fig. 1(b).

The iron content (TFe and Fe2+) was tested by the chemical method using hydrochloric acid (HCl) solution to dissolve samples (–200 mesh) and potassium bichromate (K2Cr2O7 solution) titration to determine, details referring to the national standard GB/T 223.7-2002 and GB/T 1549-2008. The content of Fe3+ was calculated from the equation Fe3+ = TFe – Fe2+, because iron metal (MFe) was less than 0.1 wt.% in parent glass powders after magnetic separation and sieving, which is ignored and not given in the following for simplifying analysis. The weight ratio of Fe3+/Fe2+ was thus calculated by the equation (TFe – Fe2+)/Fe2+.

3. Results and Discussion

3.1. Reduction of Parent Glasses

The reduction degree of parent glasses can be reflected through the content of the total iron (TFe) and iron ions like Fe2+ and Fe3+, as shown in Fig. 2. With more coal powders added into the glass melts for reduction, the contents of TFe and Fe2+ are decreased approximately in a linear shape. While for the Fe3+ ions, its content is reduced to a lower level. The reason for this is that the Fe2+ ion has a stronger stability than Fe3+ ion at high temperature in silicate melts,22) and the rapid cooling rate like water quenching could keep them at the same state in solid glasses.

Fig. 2.

Content of the total iron and iron ions of parent glasses.

3.2. Thermal Characterization

In order to investigating the influences of different atmospheres and reduction degrees on the crystallization of parent glasses, the simultaneous thermal analysis, DTA, was carried out in the 20°C–1000°C range. In the DTA curves shown in Fig. 3, the initial crystallization temperatures (Ts) of these parent glasses are nearly the same, but the crystalline peak temperature (Tc) and the shapes of these peaks are varied. While the TFe is below 7.5 wt% (G4), the Tc of glasses is increased, and the crystalline peak is blunt and distributed in a larger temperature scale, compared with the other glasses exhibiting sharp crystalline peaks. This difference illustrates that the crystallization will be promoted in a lower temperature and a shorter time while more iron oxides in glasses. This may be probably for the effect of iron oxides on the [SiO4] network breaking.

Fig. 3.

DTA curves of parent glasses heated in different atmospheres.

In addition, the Tc of samples heated in N2 (samp-N) is slightly higher than that of samples heated in air (samp-A). Two exothermic peaks in DTA curves of samp-N are observed, while only one peak for each samp-A. The trend of the difference value of TcTc) in samp-N is shown in Fig. 4. As can be seen, the value of ΔTc is decreased sharply in a quadratic polynomial trend with the TFe rise. These phenomena above could be explained that the Fe2+ oxidation in the surface of glass particles of samp-A possibly promotes the nucleation and crystallization of parent glasses. The crystallization of samp-A is induced in a lower heating temperature and in a quick rate. Therefore, two peaks in DTA curves of samp-A are not shown in Fig. 3. While for samp-N, the oxidation is absent and the devitrification is suppressed. The effect of iron oxides on the silicate network breaking lowers the glass melting temperature. The crystallization is thus not so enough that the exothermic peak at higher temperature is occurred in the DTA curves of samp-N.

Fig. 4.

The ΔTc change of samp-N with the content of TFe.

3.3. Bending Strength Characteristics

In Fig. 5, the bending strength (BS) values of these sintered samples could be divided into two groups, one of the samp-A (dashed lines) and the other of the samp-N (solid lines). The BS value of samp-A increases gradually with the sintering temperature rise, probably due to the air inside discharged mostly and the glass crystallization preventing shrinkage. Then, the BS value remains steady among 800°C–900°C range. As approaching to the melting temperature of the material, the densification is promoted and the BS value is increased again.

Fig. 5.

Bending strength of samples heated in different atmospheres. (Online version in color.)

For the samp-N, the maximum BS values of G0–G8 are obtained at temperature of 800°C for G0 and 900°C for both G4 and G8. Those temperatures are lower than that of the sampl-A, with the related BS values all above 80 MPa. This indicates that the N2 atmosphere could promote the densification of parent glasses at a lower sintering temperature, and reduce energy consumption compared with traditional manufacturing processes of glass-ceramics.

The maximum BS value of samp-A and samp-N are both obtained from the glass G8 which has the lowest iron content. This could be explained according to the changes of the weight ratio of Fe3+/Fe2+, and the mole ratio of non-bridging oxygen atoms to number of tetrahedrally-coordinated atoms (NBO/T), which reflects the degree of depolymerisation of silicates,23) as shown in Fig. 6. With the NBO/T value increasing along with the amount of TFe in parent glasses, the depolymerization of silicates is reinforced and the crystallization is restrained. The mechanical property of materials is hardly improved for the sample G0 and G4 consequently. But as the heating temperature exceeds Tc in the related DTA curves, the sintering and crystalline processes as well as the mechanical property are promoted. While for the samp-N heated at 1000°C, the BS value decreases sharply probably due to the bubbles generated inside and the corresponding volume expansion of samples. This may be caused by the positive effect of Fe2+ ions much more than that of Fe3+ irons on lowering the melting temperature of the samples.

Fig. 6.

The values of NBO/T and Fe3+/Fe2+ of parent glasses.

3.4. Crystallization Characteristics

The crystalline phases of these samples with the maximum BS values in Fig. 5 are shown in Fig. 7 by the XRD technique. The diopside(-ferrian) and the augit are the main crystalline phases.

Fig. 7.

XRD spectra of the samples heated in different atmospheres.

In the samp-A, the characteristic peaks of the crystalline phases are nearly the same with each other. But for the samp-N, the crystallization similar to the samp-A is only obtained in the sample G8. Crystallization of G0 and G4 of the samp-N are not well developed probably due to the lack of the heating time or crystalline time in heat treatment. Besides, it can be concluded that the air atmosphere could promote the crystallization in samp-A with different reduction. The N2 atmosphere only promotes the crystallization while the TFe is below 3.96% in parent glasses. The crystals arising from the oxidation in samples will increase the melting temperature of parent glasses. For the sample G8 heated in N2, the effects of iron oxide on lowering melting temperature and the NBO/T value on depolymerization of silicates are reduced. The crystallization of it shows a similarity with the samp-A consequently.

3.5. Microstructural Characterization

With the scanning electron microscope (SEM) technique, the microstructure and the relative mean grain size of these samples are shown in Fig. 8 and Table 5, respectively.

Fig. 8.

SEM images of heated samples. (a) G0, air, 1000°C; (b) G4, air, 1000°C; (c) G8, air, 1000°C; (d) G0, N2, 800°C; (e) G4, N2, 900°C; (f) G8, N2, 900°C.

Table 5. The mean diameter size of crystals.
Grain sizeG0-airG4-airG8-airG0-N2G4-N2G8-N2
d/μm4.170.834.192.141.561.50

For the samp-A, crystals of G0 and G8 are formed like long rod with the same grain size 4.17–4.19 μm. While for G4, cubic shape crystals are formed and distributed uniformly in the matrix, of which the mean grain size is only 0.83 μm. The main XRD peaks of G4 are slightly higher than that of G0 and G8 in Fig. 7. The differences described above may be caused by the crystal phase fraction (labeled as CPF) and the development of these crystals. For the smaller difference of the BS values between G8 and G0, which nearly have the same microstructure and grain size, this may be resulted from the density difference caused by heat treatment as well as the effects of iron ions content and the chemical composition change on the glass crystallization and related crystal properties.

For the samp-N, crystals with different shapes are obtained. G0 crystals are formed like sphere balls while G4 crystals are like cones. For the sample G8, the cubic shape crystals are formed, and this shape is nearly similar to but two times in the grain size than that of G4 heated in air. Besides, the main XRD peaks of G4 in air and G8 in N2 are slightly higher than that of the other samples in Fig. 7. This could be attributed to the high CPF and good developed but low size crystals, which is possibly due to the lack of heating time.

From another perspective, although the crystallization is not enough for G0 and G4 sintered in N2, the maximum BS values of them are nearly the same as that of the sample G0 and G4 sintered at 1000°C in air. Therefore, the N2 atmosphere could introduce the glass-ceramics to achieve satisfying values of mechanical property at a lower heating temperature. Besides, the one step method of heat treatment used in this study is met for the requirement of mechanical strength of glass-ceramics compared with the traditional method, which consists of the sintering and crystalline processes. This method applied above could lower the energy consumption in the industrial production. But the approach for improving the glass crystallization in N2 should be investigated in the next studies.

4. Conclusions

(1) As amounts of coal powders from 0 to 8% mixed into the raw materials, the amount of TFe in batches of parents glasses, which was mainly existed as Fe2+, were reduced from 10.6% to 3.96%, and the calculated values of NBO/T reflecting the degree of de-polymerisation of silicates were also reduced. Since the TFe is below 7.5% (G4), the Tc of glasses is increased, and the crystalline peak is blunt and distributed in a larger temperature scale, compared with the other glasses with sharp crystalline peaks. The decrease of Fe2+ content and NBO/T in the parent glasses would contribute to the change of their crystalline peak.

(2) After heated in air or N2 atmosphere with the same heating temperature, performance of glass ceramics prepared from the parent glasses shows an obvious difference. In order to obtain the optimal bending strength of the samples, heating in N2 could decrease the optimal sintering temperature compared with heating in air, and both samples have almost the same optimal bending strength values. Therefore, energy consumption could be decreased when the parent glasses were heated in N2.

With the sintering method of one-step process, the bending strength of slag glass-ceramics is high enough for their application, and the heating time of this process is so shorter than that of the traditional two-step process that the heating energy consumption could be further reduced.

(3) The crystallization is occurred in all samples heated in air, but is suppressed in samples heated in N2 except for the sample G8. The diopside(-ferrian) and the augit are the main crystal phases in these vitrified samples, and they are formed like long rods, cubic shape, balls or cones. Overall, the grain size of the samples heated in air is larger than that of the samples heated in N2, probably for the crystallization process promoted by the oxidation of glasses heated in air.

Acknowledgment

This study was financially supported by the State Key Program of National Natural Science of China (Grant No. 51034008, 51274042).

References
 
© 2014 by The Iron and Steel Institute of Japan
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