Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 1, 2020
Article Number 105
Number of page(s) 8
DOI https://doi.org/10.1051/metal/2019073
Published online 24 January 2020

© EDP Sciences, 2020

1 Introduction

Steel slag is one of the typical solid wastes in iron and steel enterprise, with production around 15% of crude steel production [1]. In 2018, China’s crude steel production is about 928 million tons [2], which is converted into 139.2 million tons of steel slag. However, the effective utilization rate of steel slag is only 22% in China [3,4], far behind the developed countries [5]. With the increasing accumulation of unused steel slag, more than 300 million tons of steel slag have been discarded so far, which would cause environmental problems for China [6].

Outdated treatment approaches are one of the reasons for the low utilization rate of steel slag in China. Pretreatment methods to treat steel slag adopted by China’s iron and steel enterprises mainly includes the self-disintegrated steel slag by steam, layer pouring process, rotary cylinder process, air-granulated process, etc. [7]. Moreover, the slag treated in these ways have the following disadvantages: volume instability, poor grindability and low cementitious activity. The steel slag pretreated by these methods do not meet the requirements of the follow-up product (such as cement, concrete aggregate, etc.) and restrict the secondary utilization of steel slag [8,9]. According to the characteristics of steel slag, a small amount of steel slag is used in internal recycling, road construction, civil engineering and cement production, while most of the slag are final placed in a landfill in China [1012]. The high content of f-CaO in slag is the main reason to restrict its application in civil engineering and road construction [13,14]. This problem could be solved by adding silicon-containing materials that react with f-CaO to improve the volume stability of steel slag [15].

Based on the fact that the output of steel slag in China is high but the utilization rate is low, a new steel slag treatment process known as steel slag gas-quenching method is explored. In this process, compressed air through a Laval nozzle is injected into the liquid slag stream at a high pressure, and the liquid slag stream is broken into small droplets of different sizes, then the small droplets cool rapidly during flight and solidified into beads in the collection room finally. In order to obtain the steel slag beads with higher bead formation rate and lower content of f-CaO, the steel slag was modified and reconstituted by adding another solid waste BF slag containing high content of SiO2 in this paper. Moreover, process parameters were optimized to improve the physicochemical properties of the beads. Natural fine aggregate (such as river sand) is an extremely scare resource in many countries [16] and its consumption had been surpassed by artificial sand in 2010 in China [17]. The beads have the following advantages: uniform particle size, stable performance and excellent stability, which just meet the requirements of fine aggregate used in construction.

In this paper, the solid modified slag was remelted by a small electric furnace for gas quenching into beads, which is an exploration of steel slag treatment. Further more, it will be more beneficial if BOF slag and BF slag can be mixed in the molten state and then gas quenched directly into beads, which is supposed to be an alternative method of steel slag treatment in iron and steel plant.

2 Materials and methods

2.1 Samples

The samples of BOF slag and BF slag were provided from TangSteel, China. The chemical composition and basicity of BOF slag, BF slag and mixed slag are shown in Table 1. Pure steel slag is basic slag with the basicity of 2.88. Basic slag has the characteristics of short slag: when the slag temperature drops to a certain value, the viscosity increases sharply [18]. The formation of gas-quenched beads requires steel slag to have better fluidity [19], so it is necessary to modify the steel slag. The basicity of BF slag is 0.99. The BOF slag modified by BF slag can reduce the basicity of steel slag effectively and change its fluidity, so that it can have better gas-quenching conditions. Moreover, the BF slag has a high content of SiO2, which can effectively dissolve f-CaO and play a role in stabilizing calcium [20,21]. In addition, the combination of the two slags has a significant effect on the solid waste treatment of steel companies.

Table 1

Chemical compositions and basicity of slag (wt. %).

2.2 Experimental procedure

In this work, a small electric arc furnace (capacity 100 kg) was used to heat and spray the mixed slag and the mixed slag was added into the furnace at three times. After reaching the preset temperature, the liquid slag was poured out, and then the spraying system was opened. The total mass of the slag mixture was set at 60 kg and the particle size of mixture was less than 50 mm; the power (coke combustion) at the early stage was controlled at 30 KW and the power (mixture melting) at the later stage was gradually increased to 90 KW. Moreover, the melting situation of the mixture was observed every 10 minutes and the power was adjusted appropriately to accelerate the slag melting. Furthermore, a temperature measuring gun and thermocouple were used together to measure temperature accurately. The experimental scheme is shown in Table 2. After the gas-quenching experiment was finished, the collected gas-quenched steel slag beads were used for further measurements. Process diagram of gas quenching steel slag is presented in Figure 1.

Table 2

Experimental scheme.

thumbnail Fig. 1

Process diagram of gas-quenched steel slag.

2.3 Methods

In this work, in order to obtain the gas-quenched steel slag beads with uniform particle size and stable performance, the main parameters such as the addition amount of BF slag and slag tapping temperature were adjusted during the experiment. Moreover, the properties of different kinds of beads were compared.

The square hole screen of different pore sizes (sieve size 0.15, 0.3, 0.6, 1.18, 2.36, 4.75, 9.50 mm) were applied for measuring the particle size distribution and fineness modulus (an indicator of sand particle size) of the gas-quenched steel slag beads (the analysis standard is according to the industrial standard of steel slag fine aggregate for road in China, YB/T 4187-2009); the main mineralogical composition were measured by X-ray diffraction (XRD: 40 kV/100 mA, 2θ = 5° -90 °, step size = 0.02); the microstructure of different kinds of steel slag beads were observed and measured by using Zeiss Axio Scope A1 Microscope; f-CaO was determined by the method of DETA complexometric titration combined with ethylene glycol extraction (f-CaO analysis standard according to the industrial standard of method for determination of content of free calcium oxide in steel slag in China, YB/T 4328-2012).

3 Results and discussion

3.1 Gas-quenching effect

In this experiment, in order to achieve the best gas-quenching effect, two parameters were mainly adjusted: the addition amount of BF slag (the ratio between BF slag and (BOF + BF) slag) and slag temperature (1600 °C, 1650 °C). The gas-quenching effect is mainly reflected by the gas-quenching rate and the bead formation rate of steel slag, as shown in Figures 2 and 3. The gas-quenching rate of the steel slag is calculated from equation (1) and the bead formation rate from equation (2). (1) (2) where, η1 and η2 represent the gas-quenching rate and the bead formation rate of steel slag, respectively; m0 is the total amount of slag in the furnace before gas quenching; mA and mB represent the gas-quenched slag (including beads and flakes) production in the collecting room and the total amount of beads in gas-quenched slag after gas quenching, respectively.

Figure 2 shows that at 1600 °C, when the pure steel slag is directly gas-quenched, the gas-quenching rate is fairly low, about 35%; with the increase of addition amount of BF slag, the gas quenching rate gradually increases. When the addition amount of BF slag reaches 15% or more, the gas-quenching rate increases slowly. When the addition amount of BF slag is 35%, the steel slag gas-quenching rate is 90%. Higher gas-quenching rate is found when the fluidity of the liquid slag is improved with the increase of its degree of superheat. Figure 3 shows that at 1600 °C, when no BF slag is added, the bead formation rate is close to 100%. As the addition amount of BF slag increases, the bead formation rate decreases gradually. When the addition amount of BF slag increases from 25 to 35%, the bead formation rate suddenly drops from 80% to below 50%. When the slag temperature is 1650 °C, the curves of the steel slag gas-quenching rate and bead formation rate are basically consistent with the condition at 1600 °C and it was not found that the performance of beads changed significantly with the increase of temperature through the performance tests. Due to the fact that heating from 1600 to 1650 °C consumes too much energy and takes too long time, the properties of the beads prepared at 1650 °C are not included in this discussion.

thumbnail Fig. 2

Gas-quenching rate of the slag.

thumbnail Fig. 3

Bead formation rate of the slag.

3.2 Physical property

The form of the gas-quenched steel slag is different when the modified slag was gas quenched into beads at 1600 °C, as shown in Figure 4. Grading analysis of different types of gas-quenched steel slag was carried out, and the particle size distribution and physical properties of the beads are shown in Tables 3 and 4. It can be seen that when the addition amount of BF slag is low, as shown in Figures 4(a) and (b), the particle size is relatively uniform, and the particles are gray beads. When the addition amount of BF slag is 15%, a small number of light green flakes appear among the beads, as shown in Figure 4(c). When the addition amount of BF slag continues to increase, the number of light green flakes increases greatly, and even becomes a major part, as shown in Figures 4(d) and (e). In summary, with the increase of addition amount of BF slag, the color of the gas-quenched steel slag gradually changes from gray to light green; the flakes in gas-quenched slag increase gradually, which is the main reason for the decrease of bead formation rate. According to the literature [22], the metallurgical slag can even be blown into slag fibers at the right basicity.

It can be seen in Table 3 that the five kinds of gas-quenched steel slag are of continuous sizes, and they fall into the category of fine aggregate with good continuous gradation according to the industry standard of steel slag sand. The fineness modulus of the five types of gas-quenched steel slag are 2.66, 2.78, 2.89, 2.92, 3.01 respectively, and they belong to the category of medium sand according to the specifications of sand. Reasonable particle size distribution can reduce the voidage to the minimum and improve the comprehensive performance of concrete. However, as can be seen from Figures 4(d) and (e), when the addition amount of BF slag is 25 or 35%, the gas-quenched slag contains too much flaky glass, which is not suitable for fine aggregate. Therefore, considering the industry standard of steel slag sand in China as well as the gas-quenching rate, physical and chemical properties, the gas-quenched steel slag (BF slag addition: 5%, 15%) can be directly used as fine aggregate without secondary processing. In other words, the direct utilization of steel slag beads reduces the cost in the secondary treatment on solid steel slag such as crushing, screening, etc.; it can be seen in Table 4 that the main physical properties (bulk density, percentage of voids, and index of crushing) of the three kinds of gas-quenched steel slag beads all meet the industry standard, while the apparent density is slightly lower than the minimum limit of 2900 kg/m3 required by the industry standard. Particularly, the percentage of voids has a great influence on the compactness and strength of concrete and mortar [23]. The continuous particle-size distribution of steel slag beads result in lower percentage of voids (26.3, 27.5, 27.8%, in Tab. 4).

thumbnail Fig. 4

The different kinds of gas-quenched steel slag beads: (a) pure steel slag; (b) modified slag (95%BOF slag + 5%BF slag, 1600 °C); (c) modified slag (85%BOF slag + 15%BF slag, 1600 °C); (d) modified slag (75%BOF slag + 25%BF slag, 1600 °C); (e) modified slag (65%BOF slag + 15%BF slag, 1600 °C).

Table 3

Particle size distribution (residue on sieve) of gas-quenched steel slag beads (%).

Table 4

Physical properties of gas-quenched steel slag beads.

3.3 Mineralogical composition and microscopic characteristics of gas-quenched slag

The modified slags with different BF slag ratio before and after gas quenching have great differences in mineralogical composition and micro-structure. The comparisons of the five kinds of modified slag XRD patterns before and after gas quenching are shown in Figure 5.

Figure 5(I) shows that the mineralogical composition of the five kinds of modified slag has little difference before gas quenching. All slags contain akermanite (Ca2MgSi2O7 ) and gehlenite (Ca2Al2SiO7), while dicalcium silicate (Ca2SiO4) are just presented in BOF slag and calcium ferrite (Ca2Fe2O5) are presented in the modified slag that BF slag is added. However, compared with before gas quenching, mineralogical composition of slag changed greatly after gas quenching. Also, the BF addition amount has a great influence on mineralogical composition of the slag after gas quenching as shown in Figure 5(II). With the increase of addition amount of BF slag, the mineral composition changes greatly and the peak shape becomes more and more diffuse. When no BF slag is added, the bead mainly contains Ca2Al2SiO7 (C2AS), dicalcium silicate (Ca2SiO4), magnetite (Fe3O4) and limonite (Fe2O3). However, C2AS, Fe3O4 and Fe2O3 disappear after BF slag is added, and Ca2SiO4 (C2S) and MgO are found. When the addition amount of BF slag reaches 35%, the diffraction peaks are too diffuse and almost gas-queched slag bead phase are amorphous state.

The mineralogical composition and microscopic characteristics of the samples are shown in Table 5 and Figure 6, respectively.

It can be seen from Table 5 and Figure 6(a) that when no BF slag is added, the sample has uniform overall structure, simple mineralogical composition and generally no clear crystal shape. It is characterized by the interphase uniform distribution of the irregular magnetite, limonite and melilite. In this sample, the main metal phases are magnetite and limonite, followed by metallic iron; the non-metallic phase is mainly melilite and contains a small amount of glass phase.

It can be seen from Table 5 and Figure 6(b), (c), and (d) that when the addition amount of BF slag is 5%, the glass phase of the sample is distributed over a large area, as much as 90%, and the melilite disappears almost exclusively, only existing in a small amount. Moreover, a trace amount of periclase and dicalcium silicate are present, and various minerals are unevenly distributed in the glass phase. When the addition amount of BF slag is gradually increased to 25%, the volume fractions of dicalcium silicate and periclase in the sample gradually increase, and the glass phase gradually decreases.

It can be seen from the Table 5 and Figure 6(e) that when the addition amount of BF slag is continuously increased to 35%, the mineral content changes greatly compared with the previous three samples. The content of glass phase does not decrease but increases to 93% or more. There is also a slight increase in the content of metallic iron, but minerals such as periclase and dicalcium silicate are greatly reduced, only existing in a trace amount.

thumbnail Fig. 5

The XRD patterns comparison among different modified slags before and after gas quenching: (I) before gas quenching, (II) after gas quenching; (a) pure steel slag, (b) modified slag (95%BOF slag + 5%BF slag, 1600 °C), (c) modified slag (85%BOF slag + 15%BF slag, 1600 °C), (d) modified slag (75%BOF slag + 25%BF slag, 1600 °C), (e) modified slag (65%BOF slag + 15%BF slag, 1600 °C).

Table 5

Mineral structure and volume fraction of different kinds of gas-quenched steel slag beads.

thumbnail Fig. 6

Microstructure of different kinds of slag after gas quenching (a) pure steel slag, (b) modified slag (95%BOF slag + 5%BF slag, 1600 °C), (c) modified slag (85%BOF slag + 15%BF slag, 1600 °C), (d) modified slag (75%BOF slag + 25%BF slag, 1600 °C), (e) modified slag (65%BOF slag + 15%BF slag, 1600 °C).

3.4 f-CaO in gas quenched slag

Free CaO (f-CaO) of slag before and after gas quenching was determined by the method of DETA complexometric titration. The elimination rate of f-CaO is calculated as equation (3). The analysis result of f-CaO is shown in Figure 7. (3) where, ωA is the content of f-CaO in the modified slag after gas quenching, %; ωB is the content of f-CaO in the modified slag before gas quenching, %; η is the elimination rate of f-CaO in the slag after gas quenching.

Figure 7 shows that f-CaO of steel slag and modified slag are higher than 3% before gas quenching at high temperature, which is higher than the national standard of China. The slag instability caused by high f-CaO content seriously restricts the secondary utilization of steel slag. The modified slag is gas quenched into beads at 1600 °C and the beads after cooling were analyzed. Figure 7 shows that the f-CaO content in the samples after gas quenching is lower than that before gas quenching. The more the addition amount of BF slag is, the higher the elimination rate of f-CaO will be. When no BF slag is added, the f-CaO value is 2.43% and the elimination rate is 20.75%. According to Table 5 and Figure 6, the possible reason for decrease of f-CaO is that the f-CaO reacts with SiO2 and Al2O3 in steel slag under the condition of high temperature to form gehlenite (Ca2Al2SiO7).

After BF slag is added, the content of f-CaO in beads decreases greatly. When adding 35% BF slag, the content of f-CaO reaches the lowest value of 0.60% and the elimination rate is 84.96%. According to Table 5 and Figure 6, the mineralogical composition of gas-quenched steel slag beads changed greatly with the increasing BF slag ratio. On one hand, the f-CaO in the slag is captured by SiO2 in BF slag with the product of Ca2SiO4 [20], on the other hand, f-CaO reacts with other phases to form vitreous after gas quenching at high temperature result in a large amount of the possible reason for decrease of f-CaO. These are mainly responsible for eliminating f-CaO.

thumbnail Fig. 7

Analysis results of f-CaO in the slag.

4 Conclusions

In this study, conclusions are drawn as follows:

  • with the increase of addition amount of BF slag, the gas quenching rate of steel slag increases and the bead formation rate decreases above 1600 °C. The physical properties of beads were analyzed, and the gas-quenched slag beads with 5 and 15% BF slag were more in line with medium sand standard. The bead could be used as fine aggregate;

  • the mineral composition of the gas-quenched steel slag beads changed greatly with the increase of addition amount of BF slag after the modified slag was gas quenched at high temperature. When no BF slag was added, the main mineral phases were magnetite, limonite and melilite in beads. However, a large amount of glass phase and some Ca2SiO4 and MgO were found after BF slag was added;

  • the content of f-CaO in beads was more than 3% before the slag was gas quenched, which was higher than national standard of China. After the slag was gas quenched at high temperature, the content of f-CaO decreases down to 0.6%. The elimination rate of f-CaO increases with the increase of addition amount of BF slag.

Acknowledgments

This work was financially supported by the Natural Science Foundation of Hebei Province (No. E2017209201), the key research and development program of Hebei Province (19273806D) and the Applied Basic Research Project of Tangshan City of Hebei Province (No. 18130229a).

References

  1. J.T. Gao, S.Q. Li, Y.T. Zhang, J. Iron Steel Res. Int. 18, 32 (2011) [CrossRef] [Google Scholar]
  2. China National Bureau of Statistics, China statistical yearbook, China Statistics Press, Beijing, 2019 [Google Scholar]
  3. P.Y. Mahieux, J.E. Aubert, G. Escadeillas, Constr. Build. Mater. 23, 742 (2008) [Google Scholar]
  4. H. Yi, G.P. Xu, H.G. Cheng, Procedia Env. Sci. 16, 791 (2012) [CrossRef] [Google Scholar]
  5. M.F. Engströ, D. Adolfsson, Q. Yang, Steel Res. Int. 81, 362 (2010) [CrossRef] [Google Scholar]
  6. J.L. Guo, Y.P. Bao, M. Wang, Waste Manage. 78, 318 (2018) [CrossRef] [Google Scholar]
  7. H.M. Yu, Q. Wang, Steel slag treatment and resource utilization, Metallurgical Industry Press, Beijing, China, 2015 [Google Scholar]
  8. J.N. Murphy, T.R. Meadowcroft, P.V. Barr, Can. Metal. Quart. 36, 315 (1997) [CrossRef] [Google Scholar]
  9. Y.C. Ding, T.W. Cheng, P.C. Liu, W.H. Lee, Constr. Build. Mater. 146, 644 (2017) [Google Scholar]
  10. C. Kamboleab, P. Greena, W.K. Kupolatia, Constr. Build. Mater. 148, 618 (2017) [Google Scholar]
  11. J.N. Murphy, T.R. Meadowcroft, P.V. Barr, Can. Metall. Quart. 36, 315 (1997) [CrossRef] [Google Scholar]
  12. H. Qasrawi, F. Shalabi, I. Asi, Constr. Build. Mater. 23, 1118 (2009) [Google Scholar]
  13. L.Z. Yildirim, M. Prezzi, Adv Civil Eng. 2011, 1 (2011) [CrossRef] [Google Scholar]
  14. J. Vaverka, K. Sakurai, ISIJ Int. 54, 1334 (2014) [CrossRef] [Google Scholar]
  15. Q. Wang, P.Y. Yan, J. Hazard. Mater. 186, 1070 (2011) [Google Scholar]
  16. E. Anastasiou, K. Georgiadis Filikas, M. Stefanidou, Constr. Build. Mater. 50, 154 (2014) [Google Scholar]
  17. S.L. Hu, A brief analysis of aggregate consumption of sand, number of mines and processing technology in China in 2019. 2019. http://www.huaon.com/story/432336 [Google Scholar]
  18. Z. Liu, L. Pandelaers, P.T. Jones, Proceedings of 10th International Conference on Molten Slags, Fluxes and Salts, Washington, USA, 2016, pp. 439–446 [CrossRef] [Google Scholar]
  19. Y.X. Jiang, Iron & Steel. 5, 89 (2011) [Google Scholar]
  20. X. Yin, C.M. Zhang, G.C. Wang, Ironmak. Steelmak. 1 (2018). DOI: 10.1080/03019233.2017.1419656 [Google Scholar]
  21. X. Yin, C.M. Zhang, G.C. Wang, Metall. Res. Technol. 115, 414 (2018) [CrossRef] [EDP Sciences] [Google Scholar]
  22. Q.Q. Ren, Y.Z. Zhang, Y. Long, J. Iron Steel Res. Int. 24, 601 (2017) [CrossRef] [Google Scholar]
  23. M.C. Li, X.G. Zhou, Z.H. Su, J. Yantai U (Nat. Sci. Eng. Edition) 30, 335 (2017) [Google Scholar]

Cite this article as: Hui Wang, Wei Zhang, Chao Liu, Hongwei Xing, Chen Guo, Yuzhu Zhang, Preparation and performance analysis of gas-quenched steel slag beads, Metall. Res. Technol. 117, 105 (2020)

All Tables

Table 1

Chemical compositions and basicity of slag (wt. %).

Table 2

Experimental scheme.

Table 3

Particle size distribution (residue on sieve) of gas-quenched steel slag beads (%).

Table 4

Physical properties of gas-quenched steel slag beads.

Table 5

Mineral structure and volume fraction of different kinds of gas-quenched steel slag beads.

All Figures

thumbnail Fig. 1

Process diagram of gas-quenched steel slag.

In the text
thumbnail Fig. 2

Gas-quenching rate of the slag.

In the text
thumbnail Fig. 3

Bead formation rate of the slag.

In the text
thumbnail Fig. 4

The different kinds of gas-quenched steel slag beads: (a) pure steel slag; (b) modified slag (95%BOF slag + 5%BF slag, 1600 °C); (c) modified slag (85%BOF slag + 15%BF slag, 1600 °C); (d) modified slag (75%BOF slag + 25%BF slag, 1600 °C); (e) modified slag (65%BOF slag + 15%BF slag, 1600 °C).

In the text
thumbnail Fig. 5

The XRD patterns comparison among different modified slags before and after gas quenching: (I) before gas quenching, (II) after gas quenching; (a) pure steel slag, (b) modified slag (95%BOF slag + 5%BF slag, 1600 °C), (c) modified slag (85%BOF slag + 15%BF slag, 1600 °C), (d) modified slag (75%BOF slag + 25%BF slag, 1600 °C), (e) modified slag (65%BOF slag + 15%BF slag, 1600 °C).

In the text
thumbnail Fig. 6

Microstructure of different kinds of slag after gas quenching (a) pure steel slag, (b) modified slag (95%BOF slag + 5%BF slag, 1600 °C), (c) modified slag (85%BOF slag + 15%BF slag, 1600 °C), (d) modified slag (75%BOF slag + 25%BF slag, 1600 °C), (e) modified slag (65%BOF slag + 15%BF slag, 1600 °C).

In the text
thumbnail Fig. 7

Analysis results of f-CaO in the slag.

In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.