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Preparation, characteristics and mechanisms of the composite sintered bricks produced from shale, sewage sludge, coal gangue powder and iron ore tailings
Construction and Building Materials 232 (2020) 117250
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation, characteristics and mechanisms of the composite sintered bricks produced from shale, sewage sludge, coal gangue powder and iron ore tailings Liqun Luo a, Keyao Li a, Weng Fu c, Cheng Liu a, Siyuan Yang a,b,⇑ a b c
School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, China State Key Laboratory of Mineral Processing, Beijing 102600, China The University of Queensland, School of Chemical Engineering, St Lucia 4072, QLD, Australia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Iron ore tailings and coal gangue were
used to prepare the composite sintered bricks. Sewage sludge and shale could be applied as the binder for the brickmarking. The properties of sintered bricks met the requirement of bearing bricks. The organic matter and water determined the mass and energy changes during sintering. The brick strength was increased by the molten glass phase wrapped crystal particles.
a r t i c l e
i n f o
Article history: Received 29 June 2019 Received in revised form 3 October 2019 Accepted 12 October 2019
Keywords: Iron ore tailings Coal gangue powder Sewage sludge Sintered brick Microscopic characteristics
a b s t r a c t Iron ore tailings and coal gangue powder are the main sources of industrial solid wastes, the dispose of which has become severe and urgent with the increasing demand of environmental harmony. The present study firstly proposed a reasonable way to deal with the large amount of iron ore tailings and coal gangue powder by making them as the sintered bricks with sewage sludge and shale as the binder, considering the advantages in both economic and environmental aspects. The bulk density, sintering shrinkage, water absorption and compressive strength of the composite sintered bricks were tested to determine the optimum preparation conditions, including sludge content, molding pressure, sintering temperature and holding time. The aforementioned characteristics as well as the leaching toxicity of composite sintered bricks produced under the optimum conditions were all met the requirement of Chinese standard of bearing-brick. Thermogravimetry and differential scanning calorimetry (TG-DSC) were applied to reveal the phase transition of components during the sintering process. It was found that most of the mass and energy changes were caused by the phase transition of organic matter and water. The scanning electron microscopy (SEM) found that the section of unburned brick has discrete particles with disordered size and arrangement. During the sintering process, the newly generated crystal-like mineral particles significantly increased, and the molten glass phase also appeared to wrap and cement these tiny crystal-like particles, resulted into the increasing compressive strength of sintered brick. Ó 2019 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (S. Yang). https://doi.org/10.1016/j.conbuildmat.2019.117250 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
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1. Introduction Iron ore is one of the most common resource widely used in industry while the tailings discarded during the beneficiation and smelting process of iron ore become an important hazardous waste worldwide [1,2]. In China and Brazil, more than 10 billion tons of iron ore tailings have been discharged as waste and normally deposit in tailings dam while less than 7% had been recycled as secondary resources [3,4]. Coal gangue powder is another common tailing discarded after the mining and cleaning of raw coal [5]. Currently, the amount of coal gangue powder stored in China reaches 5 billion tons and is growing at a rate of 0.15–0.2 billion tons per year but the rate of reuse is less than 15% [6]. The primary disposal method of the iron ore tailings and coal gangue powder waste is storage, which not only occupy the huge land and pollute the environment but also cause security risks due to the possibility of dam-break [7,8]. The discarded tailings are now recognized as the renewed commercial resources, if compared with other low-grade ores [9]. Iron ore tailings are a fine, dense, stable and crystalline material, which are mainly comprised of iron oxides, silica and alumina, and are normally not present as the hazardous materials. It has been successfully employed in many fields, such as cement-based composites, geopolymer and road infrastructure [10–13]. The primary chemical constituents of coal gangue powder are SiO2, Al2O3 and similar mineral oxides, and its primary mineralogical phases are feldspar, quartz and similar silicates [14]. Coal gangue powder can be used to generate electricity, used to produce cement materials and glass-ceramics and used to backfill mines [15–17]. It can be concluded that both iron ore tailings and coal gangue powder can be used as the materials for brick making. To ensure the strength requirement of brick, binder is the necessary component due to its strong ability of bonding the raw materials together [18]. Among the potential materials currently available, clay is the conventional and widely-used natural binder in brick-marking [19]. However, clay is limited in many cases for its resource location and political policy which cannot satisfy all the demand. For instance, clay has been prohibited in the brick-making industry by Chinese government since the beginning of this century due to the requirement of saving cultivated land [20]. Shale can be used as a substitute for clay in sintered fly ash, due to the similar physical and chemical property to clay [21]. Sewage sludge generated from the process of wastewater treatment can also be considered as the good candidate of binder due to its high organic contents [22,23]. The researchers already applied the sewage sludge in producing the blended cements [24]. Note that the sewage sludge is also an important industrial waste which is normally treated by the landfilling or disposal into the ocean, causing many environmental problems [25]. In the present study, the iron ore tailings and coal gangue powder were used as the main raw materials with shale and sewage sludge as the binder to prepare the composite sintering bricks. The effects of sludge content, molding pressure, sintering temperature and holding time on the performance of the sintering bricks were investigated to obtain the optimum preparing conditions. TG-DSC conducted to explain the transition of mass and energy during the sintering process while SEM were carried out to reveal the microstructure of components. This work will provide an innovative and comprehensive way for the utilization of the common industrial wastes in China, including iron tailings, coal gangue powder and sewage sludge.
2. Experimental 2.1. Materials and their physical properties Iron ore tailings were obtained from the Wolong magnetite mine in Chengde, China. The appearance of which presents as the gray black grain with coarse size distributions (15.64% particle size below 74 lm). The iron ore tailings have the density of 2.95 g/cm3 and the low plasticity (8 ~ 9). Coal gangue powder present as the charcoal grey particles were acquired from Yuanbaoshan coal mine in Chifeng, China. 39.90% particle size of coal gangue powder particles below 74 lm, which have slightly better plasticity (~10) and 2.02 g/cm3 density. Sewage sludge was given by the Chengde Sewage Plant, which is the dark black muddy matter with the acrid smell. It has the water content more than 68.34% and nearly half of the organic materials. As the light red granular powder, shale with small size range (90.48% particle size below 74 lm) and 2.50 g/cm3 density was obtained from Xingrong county of Chengde, China. It has the plasticity larger than 12, thereby the shale used in this study can be attributed to the moderately plastic shale. Note that all the samples were wet-sieved and dry in a vacuum-oven to measure the particle size distribution, the details can be found in the Table S1 in the Supplementary information. In addition, the plasticity index of materials in this study was obtained from a compression plastimeter (Rapid Plastimeter MKV-P14) according to the Chinese Standard QB/T 1322–2010 [26]. The plasticity index is the numerical difference between liquid limit and plastic limit for the materials. The liquid limit is the maximum water content when the material has plasticity while the plastic limit is the minimum water content when the material has plasticity. Lastly, the volume of samples is obtained by the water displacement method which submerging the testing sample in a known volume of water and measuring the change in water level as the volume. 2.2. The composition of raw materials The X-ray diffraction (XRD) and X-ray fluorescence (XRF) were used to obtain the mineral and chemical composition, respectively. The samples were ground in an agate ball mill to a grain size smaller than 150 mm and dried in a vacuum oven at 105 °C for 16 h. The XRF analysis was performed on a Bruker D8 Advance with the scanning angle range from 5° to 70° and step size of 0.02°. XRF analysis was performed on the PANalytical Axios Advanced. Major, minor, and trace chemical elements were measured on fused glass disks, which were prepared with 4.2 g spectro flux (LiBO22H2O/Li2B4O7) and 0.8 g of sample powder. The mixture was automatically molten and fused at 1200 °C. More details can be found in the reference [27]. 2.3. The preparation method of composite sintered brick The preparation procedure of composite sintered brick is shown in Fig. 1. The certain amount of iron ore tailings, coal gangue powder, sewage sludge and shale were mixed at different weight ratios. Afterwards, the raw materials were homogenized by adding the 10% of water in the mortar mixer for 5 min. The wetting materials were then natural aged by sealing at 20 °C for 8 h to allow the fully permeation of water and increase the plasticity. The aging materials were then suppressed in a mold (diameter of 46 mm, length of 30 mm) by a 769YP-30T tablet machine with desired molding pressure for 10 s to obtain the brick body. To remove the water content, the brick body was dried at room temperature (20 ± 5) °C for 48 h and then put it in the oven at 105 °C for 24 h. Afterwards, it was sintered in a SX212-16A high temperature gradient muffle furnace with a heating rate of 5 °C/min to the required temperatures, and then were maintained at the highest temperatures for a desired holding time to ensure the full agglomeration. Finally, the composite sintered bricks were obtained after cooling down to the room temperature. To clarify the preparing conditions of sintered bricks for different experiments in this study, excepted from the details presented in the different experiments, the preparing conditions of specimens in the experiments of this study is summarized and presented in Table S2 in the Supplementary information 2.4. Characterization of sintered bricks The quality of sintered bricks were characterized by some important parameters (bulk density, sintering shrinkage, water absorption and compressive strength) to determine the optimum preparation conditions [28]. The bulk density (BD) meant the density of bricks after sintering, which can be calculated from Eq. (1).
BD ¼
WAS VAS
ð1Þ
where WAS stands for the weight of bricks after sintering, VAS is the volume of brick after sintering. The sintering shrinkage (SS) was another important parameter to evaluate the deformation possibility of sintered bricks, which was determined by Eq. (2).
SS ¼
VBS VAS VBS
ð2Þ
3
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Fig. 1. Preparation procedure of composite sintered brick by iron ore tailing, coal gangue powder, sewage sludge and shale.
WASW WBSW WA ¼ WBSW
ð3Þ
where WASW represents the weight of materials after soaking in water for 24 h, WBSW stands for the weight of materials before soaking in water. The compressive strength (CS) of sintered bricks was measured via a YAW-300D automatic pressure testing machine according to the Chinese Standard QB/T 2542–2012, which was obtained from Eq. (4).
CS ¼
FB FA
ð4Þ
where FB is the force to break materials while FA is the force area of materials. The testing sample was placed in the center of the compression plate and loaded perpendicular to the compression surface. The loading speed was set as 2.5 kN/s until the fracture of the sample happened, and the maximum load FB is recorded. Note that the characterizations of sintered bricks were tested twice to obtained the average value and reported with error bars in this study. Error bars represent one standard deviation of uncertainty obtained from two independent runs. 2.5. Leaching toxicity The concentrations of hazardous heavy metal ions in the raw materials, composite sintered bricks and its leachate were analyzed by the inductively coupled plasma optical emission spectrometry carried on an Optima 4300DV instrument (ICP-OES, Perkin-Elmer, America). The leachate was obtained by the leaching toxicity-sulfuric acid and nitric acid method (HJ/T299-2007) which is the standard solid waste-extraction procedure for evaluating the toxicity of metals and their harm to the environment by simulated acid rain [29]. The results were compared with the Chinese standard 5085.3-2007 to ensure the safety of presented sintered bricks in its usage.
3. Results and discussion 3.1. The analysis of raw materials The chemical assays of these samples were analyzed by the XRF and shown in Table 1. In addition, the XRD was conducted to obtain the components of the raw materials and shown in Fig. 2. As shown in Table 1 and Fig. 2, the components of the raw materials are complicated with types of different minerals, which are mainly the silicate minerals including quartz, illite, diopside, gypsum and orthoclase. Hence, it will be beneficial to the sintering and solidification of the bricks in this study. 3.2. Effects of the sludge content on the characteristics of composite sintered bricks The weight ratios of four raw materials were considered in the first place. Note that the preliminary determined the percentage of coal gangue powder and shale should be fixed at 30% and 10% respectively to obtain the suitable product, thereby only the
Iron ore tailings
2.6. Thermogravimetry and differential scanning calorimetry (TG-DSC) The mass and energy transitions of mixing raw materials (the optimum mass ratio of iron ore tailings, coal gangue powder, shale and sewage sludge equals to 54:30:10:6) during the sintering process were tested by the TG-DSC, using a DIL402C heat analyzer. The specimen were heated from room temperature (25 °C) to 1300 °C at a rate of 10 °C/min in air atmosphere.
2
2
5 2
2
5 2 1 15 15 1
5 1
5 11 1
5 1
6 2 1
1
Sewage sludge
2 7 Coal gangue 2 9 4
21
2.7. Scanning electron microscopy (SEM)
1 Quartz 4 Orthoclase7 Chlorite 8 Calcite 5 Albite 2 Illite 3 Diopside 6 Feldspath 9 Gypsum
1
2 1 2 9 1 8
Shale
To analyze the microstructure of the composite sintered bricks, a scanning electron microscope (JEOL 5610LV, JEOL Ltd., Tokyo, Japan) under an accelerating voltage of 20 kV was applied to take the images of the section of bricks before and after sintering. Prior to the SEM experiments, the sections of bricks with required size were obtained by a cutting machine and then platinum coated in a sputter coater for preparing samples.
1
1
Intensity (Counts)
where VBS is the volume of materials before sintering which equals to the volume of mold. The water absorption (WA) strongly affect the strength and durability of bricks, and it yielded:
10
1 4
20
6 22 2 11 1
2
7 6 4
2
1 9 84
32 2 1
4
30
4
4 3 2
4 3 41
40 2θ /(°)
4 3
5 4
50
60
70
Fig. 2. XRD pattern of raw materials. (a- iron ore tailings, b-sewage sludge, c-coal gangue powder, d-shale).
Table 1 Composition analysis of raw materials (unit: %). Samples
SiO2
Fe2O3
Al2O3
CaO
MgO
P2O5
K2O
Na2O
S
Loss on Ignition
Iron ore tailings Coal gangue powder Sewage sludge Shale
51.59 8.42 41.47 59.69
8.22 2.26 3.53 5.01
12.75 3.12 15.95 15.64
11.91 2.98 1.23 5.14
6.07 1.77 1.79 2.11
1.86 3.45 0.13 2.66
1.32 0.62 1.58 2.19
2.51 0.34 0.58 0.91
1.23 0.34 0.85 0.15
0.83 68.37 31.88 5.24
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contents of iron ore tailings and sludge were investigated in this study. The effects of sludge content on the characteristics of composite sintering bricks under 20 MPa molding pressure, 1100 °C sintering temperature and 3 h holding time were shown in Fig. 3. As shown in Fig. 3(a), the bulk density (BD) was largely dropped from 1.74 to 1.63 with the addition of 3% sludge while it slightly decreased with the increasing sludge content from 3% to 12%. The variation of BD with different sludge contents indicated that the organic components in the sludge had been transition into the gas and then volatilized through a chemical reaction during the sintering process, leading to a decreasing trend of BD. A declining trend of sintering shrinkage (SS) from 9.7% to 7.9% with the increasing sludge content from 0 to 12% was also presented in Fig. 3(a). The generated gas by sludge during the sintering process filled in the gap after the evaporation of water, causing the gradually decrease of SS. Note that the increase of sludge content brought more water into raw materials, which also led to the decrease of BD and SS. In addition, the increasing content of fine sludge particle may promote the improvement the BD of sintered brick but it was offset by other factors considering the small amount of sludge content in the raw materials (12%). Fig. 3(b) revealed the effects of sludge content on the water adsorption (WA) of sintering bricks. The WA can reflect the ability of sintered brick to resist the harsh environment. According to the national standard of China (GB5105-2003), the WA of the qualified sintering bricks should be less than 18%. As shown in Fig. 3(b), the WA of sintering bricks without sludge as the component is 14.2% but it increased with the addition of sludge. When the sludge
content increased to 9%, the WA also increased to the value over 18% which did not meet the national standard. Another main parameter of sintering bricks, the compressive strength (CS), was also highly related to the sludge content which is shown in Fig. 3 (b) as well. With the sludge content increased from 0 to 6%, the CS decreased from 18.2 to 14.24 MPa which could satisfy the Chinese national standard of bearing brick (MU10). When the sludge content continuously increased, the CS dropped to 8.4 MPa which could only meet the Chinese national standard of non-bearing brick (MU5). 3.3. Effects of the molding pressure on the characteristics of composite sintered bricks The molding pressure had the strong effects on the shape and strength of brick body. Fig. 4 presented the effects of molding pressure on the characteristics of composite sintered bricks with 6% sludge content of raw materials, 1100 °C sintering temperature and 3 h holding time. As shown in Fig. 4(a), the BD of sintered bricks linearly enhanced from 1.635 to 1.641 g/cm3 the with the increasing molding pressure from 10 to 30 MPa. However, the SS dropped with increasing molding pressure, especially in the range from 15 to 20 MPa where SS decreased from 9.08% to 8.12%. With the increase of molding pressure, the particles in the raw materials were squeezed by the external force to free the gas inside the gaps of particles, hence the BD increases linearly with a small amplitude. During the sintering process, the removal of residual gas compacted the particles further, leading to the reduction of SS.
Fig. 3. Effects of sludge content on the characteristics of composite sintered bricks: a) bulk density and sintering shrinkage; b) water absorption and compressive strength.
Fig. 4. Effects of molding pressure on the characteristics of composite sintered bricks: a) bulk density and sintering shrinkage; b) water absorption and compressive strength.
L. Luo et al. / Construction and Building Materials 232 (2020) 117250
Fig. 4(b) showed the WA of sintered bricks decreased from 18.6% with the increasing molding pressure, especially in the range from 15 to 25 MPa where the WA largely reduced from 17.6% to 15.5%. Thus, the WA of sintered brick using the molding pressure larger than 15 MPa will meet the requirement of Chinese national standard. The CS of sintered bricks significantly increased from 9.8 to 14.24 MPa with the increasing molding pressure from 10 to 20 MPa, which met the standard of MU10. When the molding pressure continuously increased, the CS only present slightly improving trend. In summary, the 20 MPa of molding pressure was the most appropriate condition for the brick-making. The increase of molding pressure compressed the raw materials and then reduced the gaps between them, leading to the improvement of CS. 3.4. Effects of the sintering temperature on the characteristics of composite sintered bricks It is all known that the sintering temperature determines the properties of sintered bricks. Fig. 5 showed the effects of sintering temperature on the characteristics of composite sintered bricks with 6% sludge content of raw materials, 20 molding pressure and 3 h holding time. It can be seen from Fig. 5(a) that the BD of sintered bricks gradually enhanced from 1.617 to 1.651 g/cm3 the with the increasing sintering temperature from 950 to 1150 °C. SS also presented an increasing trend with the improvement of sintering temperature. As the temperature rises, the crystal-like and liquid phases can be formed inside the brick body to compact the particles, leading to the increase of BD and SS.
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Fig. 5(b) revealed the WA of sintered bricks dropped from 18.07% to 15.72% while the CS enhanced from 7.64 to 14.24 MPa with the increasing sintering temperature from 950 to 1100 °C. When the temperature increased to 1150 °C, both WA and CS presented the decrease trend. Note that the WA of sintered bricks only qualified when the sintering temperature over 1100 °C, considering the requirement of Chinese national standard (<18%). On the other hand, the peak value of CS (14.24 MPa) was obtained at 1100 °C sintering temperature, which might be attributed to the formation of calcium sulphate from the sulphate and calcium in the raw materials when temperature larger than 1050 °C, causing the so-call ‘‘black cores” phenomenon [30]. In addition, a high temperature might result into the excess liquid glass phase in the brick body which can also decrease the compressive strength of bricks [31]. Thus, an appropriate sintering temperature was necessary to obtain the maximum CS of the samples in this study. 3.5. Effects of the holding time on the characteristics of composite sintered bricks After the sintering temperature reached desired value, a suitable holding time was needed to ensure the transition of mixed raw materials into tight structure. Fig. 6 revealed the influences of holding time on the characteristics of composite sintered bricks with 6% sludge content of raw materials, 20 MPa molding pressure and 1100 °C sintering temperature. As shown in Fig. 6(a), the BD decreased with the increasing holding time while the SS increased linearly as the improvement of holding time.
Fig. 5. Effects of sintering temperature on the characteristics of composite sintered bricks: a) bulk density and sintering shrinkage; b) water absorption and compressive strength.
Fig. 6. Effects of holding time on the characteristics of composite sintered bricks: a) bulk density and sintering shrinkage; b) water absorption and compressive strength.
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Fig. 6(b) revealed the increase of holding time is beneficial to the parameter of WA which decreased with the improvement of holding time. When the holding time was larger than 2.5 h, the WA already met the requirement (<18%). In addition, the CS of sintered bricks was also shown in Fig. 6(b), in which the CS firstly increased with the increasing holding time until it reached 3 h where the CS became stable with the continuous enhancement of holding time. The continuous increase of holding time allowed fully reactions between the raw materials, causing the tight and sturdy structure of sintered bricks, resulted into the increase of CS [31]. Interestingly, when the holding time was larger than 3.5 h, the CS began to decrease which might be attributed to the unknown reaction among the raw materials. 3.6. The leaching toxicity of composite sintered bricks Except from the physical requirement of sintered bricks, the leaching toxicity is also necessary to be measured [32]. Table 2 showed the main types of heavy metal ions in the raw materials. The leaching efficiency (LE) was defined as the following equation.
LE ¼
CL CO
ð5Þ
where the CO stands for the concentration of the heavy metal in the original sample, CL represents the concentration of the heavy meatal in the leachate of the samples. A higher value of LE means that the heavy metal of sample is more easily to be leached out in the acid environment. As is shown, the rank of heavy elements’ content in the raw materials was Cu > Zn > Cr > Pb. However, the rank presents different in the leachate of raw materials which was Zn > Cu > Pb > Cr. Thus, the Zn in the raw materials was easier to be leached out with 38.94% leaching efficiency, followed by the Pb with 20.83% leaching efficiency. The Cu and Cr shows stable characteristics with 5.29% and 2.72% respectively. These hazardous heavy metal elements were also tested in the sintered bricks and its leachate. The results were shown in Table 2, in which we can see that the rank of elements’ content in the sintered bricks is the same as in the raw materials but all the elements’ content in sintered bricks are larger. It could be attributed to the increase of BD during the sintering process. In addition, as can be seen from Tables 2, all heavy metal contents in the leachates were far less than the toxicity threshold prescribed in GB5085.32007 [33]. However, the elements’ contents in the leachate of sintered bricks were smaller than that of in the leachate of raw materials, meaning the most of the heavy metal elements contained in the material were immobilized during the sintering process [34]. The heavy metals could form phases that can exhibit sufficient stability due to irreversible phase transformation by interacting with silicate minerals in high temperature [35,36]. Thus, the order of leaching efficiency of sintered bricks was Pb > Cu > Zn > Cr. 3.7. Transition of the mixing raw materials during sintering To better understand the quality and energy changes in the sintering process, the TG-DSC analysis of sintering raw materials in
Fig. 7. Thermal analysis on the mixing raw materials during sintering.
the optimum ratio (iron tailings: coal gangue powder: shale: sludge = 54:30:10:6) was conducted and shown in Fig. 7. As can be seen from it, the thermogravimetry (TG) curve changed gently, indicating that there is no obvious reaction during the sintering process, which can avoid the obvious cracks but might lead to the open of porosity in sintered bricks due to the large amount of gases discharged in a short time. According to the differential scanning calorimetry (DSC) curve, an endothermic peak was appeared at 59.3 °C, which could be attributed to the heat absorption by volatilization of free water during the sintering process. When the sintering temperature continuously increased, an exothermic peak was generated at 390.6 °C, which may be caused by the combustion of organic matter [37]. In addition, a strong endothermic peak and a weak endothermic peak were appeared at 450 °C and 750 °C respectively, which may be resulted from the removal of crystallization water from minerals such as kaolinite in the raw materials [38]. However, no obvious weight loss phenomenon occurred at the same range of the TG curve, suggesting the solid phase reaction may just began. At 1230.2 °C, the DSC curve showed an endothermic valley, which was caused by the melting of mineral particles at high temperatures [39]. As for the derivative thermogravimetry (DTG) curve, a valley was firstly generated at 56.7 °C, which mainly due to the loss of weight caused by the volatilization of residual free water in the raw materials with a loss rate of about 2.80%. The second obvious valley at 383.1 °C was mainly caused by the combustion of organic matter in the raw materials, after which the DTG curve does not change significantly. Overall, there were only few peaks and valleys appeared in the TGDSC analysis, thereby the thermal effect and solid-phase reaction were not obvious during the sintering process in this study. SEM was used to observe the microstructure of the sections of composite bricks before and after fired at 1000 °C, 1050 °C and 1100 °C, the results of which are shown in Fig. 8. It can be seen from Fig. 8(a) that the sections of unburned bricks were mostly the discrete particles with different sizes and disordered arrangements. Fig. 8(b) presented the small silicate layers and crystallike minerals particles were initially generated in the section of
Table 2 Concentrations of hazardous heavy metal elements in the raw materials and sintered bricks and their leachate. Samples
Hazardous heavy metal elements
Cu
Pb
Zn
Cr
Raw Materials
Raw materials (mg/L) Leachate of raw materials (mg/L) Leaching efficiency (%) Composite sintered bricks (mg/L) Leachate of sintered bricks (mg/L) Toxicity thresholds (mg/L) Leaching efficiency (%)
1.466 0.0775 5.29 1.646 0.0489 100 2.97
0.131 0.0273 20.83 0.146 0.0053 5 3.63
1.485 0.5783 38.94 1.604 0.0077 100 0.48
0.268 0.0073 2.72 0.314 0.0011 15 0.35
Sintered Bricks
L. Luo et al. / Construction and Building Materials 232 (2020) 117250
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Fig. 8. Microstructures of the sections of sintered bricks at different temperatures. (a-raw materials; b-1000 °C; c-1050 °C; d-1100 °C).
brick when the sintering temperature reached 1000 °C. Note that the gaps between materials were still large which means the sintered bricks were not consolidated and developed, leading to the lower CS and higher WA rate. When the sintering temperature further increased to 1050 °C, a large amount of liquid glass phase was found in the inner part of the sample [40], which was shown in Fig. 8(c). The molten glass phase wrapped and cemented the fine crystal-like mineral particles, filled the gaps and holes between materials and made them close to each other, leading to the increase of BD and CS [41]. Fig. 8 (d) showed the sintered bricks under the optimum sintering temperature 1100 °C, the degree of melting and consolidation of the sintered bricks were very high with the flat sections, leading to the dense structure. The sections presented the small and evenly distributed pores as well as the cementing of liquid phase, which increased the BD and CS while reduced the porosity and WA rate of the sintered bricks. 4. Conclusions As the increasing requirement of environmental harmony, the cyclic utilization of solid waste become more and more important. In this study, the composite sintered bricks were successfully prepared by using the iron ore tailings and coal gangue powder as the main materials, the shale and sewage sludge as the binder. The product was studied by changing the preparation conditions and investigating the related characteristics. We found that the qualified composite sintered bricks with 1.638 g/cm3 of bulk density (BD), 8.3% of sintering shrinkage (SS), 17.47% of water absorption
(WA) and 14.24 MPa of compressive strength (CS) could be obtained by the optimal conditions: 54:30:10:6 mass ratio of iron ore tailings: coal gangue powder: shale: sewage sludge, 20 MPa of molding pressure, 1100 °C of sintering temperature and 3 h of holding time. Most of the hazardous heavy metal elements in the sintered bricks were immobilized during the sintering process with the leaching efficiency order of Pb > Cu > Zn > Cr, which met the requirement of Chinese national standard. The transition of the mixing raw materials during the sintering process was also investigated. We found that the most of the mass and energy changes were caused by the phase transition of organic matter and water but the transformation of partial minerals in high temperature could also lead to the changes. The microstructure of unburned bricks showed that most of them were mineral particles with disordered size and arrangement. With the increase of sintering temperature, the newly formed crystal-like mineral particles increased obviously, and the molten glass phase was also appeared to wrap and cement the fine crystal-like mineral particles, thereby the surface became more uniform and compact with the reduction of the porosity. Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the
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review of, the manuscript entitled ‘‘Preparation, characteristics and mechanisms of the composite sintered bricks produced from shale, sewage sludge, coal gangue and iron ore tailings”. Acknowledgements This work was funded by the National Natural Science Foundation of China (51904214, 51874219) and the Open Foundation of State Key Laboratory of Mineral Processing (BGRIMM-KJSKL2019-02). The authors are also thankful for the reviewers due to their constructive suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.117250. References [1] S. Yang, L. Wang, Measurement of froth zone and collection zone recoveries with various starch depressants in anionic flotation of hematite and quartz, Miner. Eng. 138 (2019) 31–42. [2] W.C. Fontes, J.M.F. de Carvalho, L.C. Andrade, A.M. Segadães, R.A. Peixoto, Assessment of the use potential of iron ore tailings in the manufacture of ceramic tiles: From tailings-dams to ‘‘brown porcelain”, Constr. Build. Mater. 206 (2019) 111–121. [3] S. Zhao, J. Fan, W. Sun, Utilization of iron ore tailings as fine aggregate in ultrahigh performance concrete, Constr. Build. Mater. 50 (2014) 540–548. [4] C. Li, H. Sun, J. Bai, L. Li, Innovative methodology for comprehensive utilization of iron ore tailings: Part 1. The recovery of iron from iron ore tailings using magnetic separation after magnetizing roasting, J. Hazard. Mater. 174 (1–3) (2010) 71–77. [5] Y. Taha, M. Benzaazoua, R. Hakkou, M. Mansori, Coal mine wastes recycling for coal recovery and eco-friendly bricks production, Miner. Eng. 107 (2017) 123– 138. [6] C.L. Wang, W. Ni, S.Q. Zhang, S. Wang, G.S. Gai, W.K. Wang, Preparation and properties of autoclaved aerated concrete using coal gangue and iron ore tailings, Constr. Build. Mater. 104 (2016) 109–115. [7] S. Zhang, X. Xue, X. Liu, P. Duan, H. Yang, T. Jiang, D. Wang, R. Liu, Current situation and comprehensive utilization of iron ore tailing resources, J. Min. Sci. 42 (4) (2006) 403–408. [8] W. Wang, C. Wang, S. Wang, W. Ni, S. Zhang, G. Gai, Preparation and properties of autoclaved aerated concrete using coal gangue and iron ore tailings, Constr. Build. Mater. 104 (2016) 109–115. [9] C. Li, H. Sun, Z. Yi, L. Li, Innovative methodology for comprehensive utilization of iron ore tailings: Part 2: The residues after iron recovery from iron ore tailings to prepare cementitious material, J. Hazard. Mater. 174 (1–3) (2010) 78–83. [10] L.A.D.C. Bastos, G.C. Silva, J.C. Mendes, R.A.F. Peixoto, Using iron ore tailings from tailing dams as road material, J. Mater. Civ. Eng. 28 (10) (2016). 04016102. [11] P. Duan, C. Yan, W. Zhou, D.J. Ren, Fresh properties, compressive strength and microstructure of fly ash geopolymer paste blended with iron ore tailing under thermal cycle, Constr. Build. Mater. 118 (2016) 76–88. [12] J.L.B. Galvão, H.D. Andrade, G.J. Brigolini, R.A.F. Peixoto, J.C. Mendes, Reuse of iron ore tailings from tailings dams as pigment for sustainable paints, J. Cleaner Prod. 200 (2018) 412–422. [13] L. Weishi, L. Guoyuan, X. Ya, H. Qifei, The properties and formation mechanisms of eco-friendly brick building materials fabricated from lowsilicon iron ore tailings, J. Cleaner Prod. 204 (2018) 685–692. [14] D. Li, X. Song, C. Gong, Z. Pan, Research on cementitious behavior and mechanism of pozzolanic cement with coal gangue, Cem. Concr. Res. 36 (9) (2006) 1752–1759. [15] X.X. Miao, J.X. Zhang, Analysis of strata behavior in the process of coal mining by gangue backfilling, J. Mining Safety Eng. 4 (2007). [16] N. Zhang, H. Sun, X. Liu, J. Zhang, Early-age characteristics of red mud–coal gangue cementitious material, J. Hazard. Mater. 167 (1–3) (2009) 927–932. [17] M. Yang, Z. Guo, Y. Deng, X. Xing, K. Qiu, J. Long, J. Li, Preparation of CaO– Al2O3–SiO2 glass ceramics from coal gangue, Int. J. Miner. Process. 102 (2012) 112–115.
[18] F. Gouny, F. Fouchal, P. Maillard, S. Rossignol, Study of the effect of siliceous species in the formation of a geopolymer binder: understanding the reaction mechanisms among the binder, wood, and earth brick, Ind. Eng. Chem. Res. 53 (9) (2014) 3559–3569. [19] L. Muñoz Velasco, P. Muñoz Velasco, M.P. Morales Ortíz, M.A. Mendívil Giró, Fired clay bricks manufactured by adding wastes as sustainable construction material – A review, Construct. Build. Mater. 63 (2014) 97–107. [20] X. Lingling, G. Wei, W. Tao, Y. Nanru, Study on fired bricks with replacing clay by fly ash in high volume ratio, Constr. Build. Mater. 19 (3) (2005) 243–247. [21] S. Mu, B.G. Ma, G. De Schutter, X.G. Li, Y.C. Wang, S.W. Jian, Effect of shale addition on properties of sintered coal fly ash, Constr. Build. Mater. 25 (2) (2011) 617–622. [22] Z. Liu, Q. Chen, X. Xie, G. Xue, F. Du, Q. Ning, L. Huang, Utilization of the sludge derived from dyestuff-making wastewater coagulation for unfired bricks, Constr. Build. Mater. 25 (4) (2011) 1699–1706. [23] K.-Y. Chiang, P.-H. Chou, C.-R. Hua, K.-L. Chien, C. Cheeseman, Lightweight bricks manufactured from water treatment sludge and rice husks, J. Hazard. Mater. 171 (1) (2009) 76–82. [24] Z. Pavlík, J. Forˇt, M. Záleská, M. Pavlíková, A. Trník, I. Medved, M. Keppert, P.G. ˇ erny´, Energy-efficient thermal treatment of sewage sludge for Koutsoukos, R. C its application in blended cements, J. Cleaner Prod. 112 (2016) 409–419. [25] A.M. Mahon, B. O’Connell, M.G. Healy, I. O’Connor, R. Officer, R. Nash, L. Morrison, Microplastics in sewage sludge: effects of treatment, Environ. Sci. Technol. 51 (2) (2017) 810–818. [26] C. Xu, C. Xu, F. Han, F. Zhang, W. Wei, Z. Zhong, W. Xing, Fabrication of high performance macroporous tubular silicon carbide gas filters by extrusion method, Ceram. Int. 44 (15) (2018) 17792–17799. [27] J. Schönenberger, T. Momose, B. Wagner, W.H. Leong, V.R. Tarnawski, Canadian field soils I. Mineral Composition by XRD/XRF Measurements, Int. J. Thermophys. 33 (2) (2012) 342–362. [28] H. He, Q. Yue, Y. Su, Y. Gao, B. Gao, J. Wang, H. Yu, Preparation and mechanism of the sintered bricks produced from Yellow River silt and red mud, J. Hazard. Mater. 203 (2012) 53–61. [29] J. Li, Z. Chen, J. Shen, B. Wang, L. Fan, The enhancement effect of pre-reduction using zero-valent iron on the solidification of chromite ore processing residue by blast furnace slag and calcium hydroxide, Chemosphere 134 (2015) 159– 165. [30] L. Gredmaier, C.J. Banks, R.B. Pearce, Calcium and sulphur distribution in fired clay brick in the presence of a black reduction core using micro X-ray fluorescence mapping, Constr. Build. Mater. 25 (12) (2011) 4477–4486. [31] J.G. Song, F. Wang, X.B. Bai, D.M. Du, Y.Y. Ju, M.H. Xu, G.C. Ji, Effect of the sintering technology on the properties of fired brick from quartz sands, J. Ceramic Process. Res. 12 (4) (2011) 357–360. [32] A. Ukwatta, A. Mohajerani, Leachate analysis of green and fired-clay bricks incorporated with biosolids, Waste Manage. 66 (2017) 134–144. [33] F.-H. Wang, F. Zhang, Y.-J. Chen, J. Gao, B. Zhao, A comparative study on the heavy metal solidification/stabilization performance of four chemical solidifying agents in municipal solid waste incineration fly ash, J. Hazard. Mater. 300 (2015) 451–458. [34] Z. Haiying, Z. Youcai, Q. Jingyu, Utilization of municipal solid waste incineration (MSWI) fly ash in ceramic brick: Product characterization and environmental toxicity, Waste Manage. 31 (2) (2011) 331–341. [35] L. Zhang, Z. Zhang, A. Li, X. Wang, Stabilization/solidification of municipal solid waste incineration fly ash via co-sintering with waste-derived vitrified amorphous slag, Waste Manage. 56 (2016) 238–245. [36] G. Wang, X. Ning, X. Lu, X. Lai, H. Cai, Y. Liu, T. Zhang, Effect of sintering temperature on mineral composition and heavy metals mobility in tailings bricks, Waste Manage. 93 (2019) 112–121. [37] X. Li, Y. Lv, B. Ma, S. Jian, H. Tan, Influence of sintering temperature on the characteristics of shale brick containing oil well-derived drilling waste, Environ. Sci. Pollut. Res. 18 (9) (2011) 1617–1622. [38] K. Traoré, F. Gridi-Bennadji, P. Blanchart, Significance of kinetic theories on the recrystallization of kaolinite, Thermochim. Acta 451 (1) (2006) 99–104. [39] Z.B. Ma, J. Bai, Z.Q. Bai, L.X. Kong, Z.X. Guo, J.C. Yan, W. Li, Mineral transformation in char and its effect on coal char gasification reactivity at high temperatures, Part 2: char gasification, Energy Fuels 28 (3) (2014) 1846– 1853. [40] A. Schwarz-Tatarin, S. Freyburg, Influence of scrap glass powders and the maturing process in the unfired state on the body properties of a kaolinitic clay after firing, J. Eur. Ceram. Soc. 30 (7) (2010) 1619–1627. [41] M. Romero, A. Andrés, R. Alonso, J. Viguri, J.M. Rincón, Sintering behaviour of ceramic bodies from contaminated marine sediments, Ceram. Int. 34 (8) (2008) 1917–1924.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation, characteristics and mechanisms of the composite sintered bricks produced from shale, sewage sludge, coal gangue powder and iron ore tailings Liqun Luo a, Keyao Li a, Weng Fu c, Cheng Liu a, Siyuan Yang a,b,⇑ a b c
School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, China State Key Laboratory of Mineral Processing, Beijing 102600, China The University of Queensland, School of Chemical Engineering, St Lucia 4072, QLD, Australia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Iron ore tailings and coal gangue were
used to prepare the composite sintered bricks. Sewage sludge and shale could be applied as the binder for the brickmarking. The properties of sintered bricks met the requirement of bearing bricks. The organic matter and water determined the mass and energy changes during sintering. The brick strength was increased by the molten glass phase wrapped crystal particles.
a r t i c l e
i n f o
Article history: Received 29 June 2019 Received in revised form 3 October 2019 Accepted 12 October 2019
Keywords: Iron ore tailings Coal gangue powder Sewage sludge Sintered brick Microscopic characteristics
a b s t r a c t Iron ore tailings and coal gangue powder are the main sources of industrial solid wastes, the dispose of which has become severe and urgent with the increasing demand of environmental harmony. The present study firstly proposed a reasonable way to deal with the large amount of iron ore tailings and coal gangue powder by making them as the sintered bricks with sewage sludge and shale as the binder, considering the advantages in both economic and environmental aspects. The bulk density, sintering shrinkage, water absorption and compressive strength of the composite sintered bricks were tested to determine the optimum preparation conditions, including sludge content, molding pressure, sintering temperature and holding time. The aforementioned characteristics as well as the leaching toxicity of composite sintered bricks produced under the optimum conditions were all met the requirement of Chinese standard of bearing-brick. Thermogravimetry and differential scanning calorimetry (TG-DSC) were applied to reveal the phase transition of components during the sintering process. It was found that most of the mass and energy changes were caused by the phase transition of organic matter and water. The scanning electron microscopy (SEM) found that the section of unburned brick has discrete particles with disordered size and arrangement. During the sintering process, the newly generated crystal-like mineral particles significantly increased, and the molten glass phase also appeared to wrap and cement these tiny crystal-like particles, resulted into the increasing compressive strength of sintered brick. Ó 2019 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. E-mail address: [email protected] (S. Yang). https://doi.org/10.1016/j.conbuildmat.2019.117250 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
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1. Introduction Iron ore is one of the most common resource widely used in industry while the tailings discarded during the beneficiation and smelting process of iron ore become an important hazardous waste worldwide [1,2]. In China and Brazil, more than 10 billion tons of iron ore tailings have been discharged as waste and normally deposit in tailings dam while less than 7% had been recycled as secondary resources [3,4]. Coal gangue powder is another common tailing discarded after the mining and cleaning of raw coal [5]. Currently, the amount of coal gangue powder stored in China reaches 5 billion tons and is growing at a rate of 0.15–0.2 billion tons per year but the rate of reuse is less than 15% [6]. The primary disposal method of the iron ore tailings and coal gangue powder waste is storage, which not only occupy the huge land and pollute the environment but also cause security risks due to the possibility of dam-break [7,8]. The discarded tailings are now recognized as the renewed commercial resources, if compared with other low-grade ores [9]. Iron ore tailings are a fine, dense, stable and crystalline material, which are mainly comprised of iron oxides, silica and alumina, and are normally not present as the hazardous materials. It has been successfully employed in many fields, such as cement-based composites, geopolymer and road infrastructure [10–13]. The primary chemical constituents of coal gangue powder are SiO2, Al2O3 and similar mineral oxides, and its primary mineralogical phases are feldspar, quartz and similar silicates [14]. Coal gangue powder can be used to generate electricity, used to produce cement materials and glass-ceramics and used to backfill mines [15–17]. It can be concluded that both iron ore tailings and coal gangue powder can be used as the materials for brick making. To ensure the strength requirement of brick, binder is the necessary component due to its strong ability of bonding the raw materials together [18]. Among the potential materials currently available, clay is the conventional and widely-used natural binder in brick-marking [19]. However, clay is limited in many cases for its resource location and political policy which cannot satisfy all the demand. For instance, clay has been prohibited in the brick-making industry by Chinese government since the beginning of this century due to the requirement of saving cultivated land [20]. Shale can be used as a substitute for clay in sintered fly ash, due to the similar physical and chemical property to clay [21]. Sewage sludge generated from the process of wastewater treatment can also be considered as the good candidate of binder due to its high organic contents [22,23]. The researchers already applied the sewage sludge in producing the blended cements [24]. Note that the sewage sludge is also an important industrial waste which is normally treated by the landfilling or disposal into the ocean, causing many environmental problems [25]. In the present study, the iron ore tailings and coal gangue powder were used as the main raw materials with shale and sewage sludge as the binder to prepare the composite sintering bricks. The effects of sludge content, molding pressure, sintering temperature and holding time on the performance of the sintering bricks were investigated to obtain the optimum preparing conditions. TG-DSC conducted to explain the transition of mass and energy during the sintering process while SEM were carried out to reveal the microstructure of components. This work will provide an innovative and comprehensive way for the utilization of the common industrial wastes in China, including iron tailings, coal gangue powder and sewage sludge.
2. Experimental 2.1. Materials and their physical properties Iron ore tailings were obtained from the Wolong magnetite mine in Chengde, China. The appearance of which presents as the gray black grain with coarse size distributions (15.64% particle size below 74 lm). The iron ore tailings have the density of 2.95 g/cm3 and the low plasticity (8 ~ 9). Coal gangue powder present as the charcoal grey particles were acquired from Yuanbaoshan coal mine in Chifeng, China. 39.90% particle size of coal gangue powder particles below 74 lm, which have slightly better plasticity (~10) and 2.02 g/cm3 density. Sewage sludge was given by the Chengde Sewage Plant, which is the dark black muddy matter with the acrid smell. It has the water content more than 68.34% and nearly half of the organic materials. As the light red granular powder, shale with small size range (90.48% particle size below 74 lm) and 2.50 g/cm3 density was obtained from Xingrong county of Chengde, China. It has the plasticity larger than 12, thereby the shale used in this study can be attributed to the moderately plastic shale. Note that all the samples were wet-sieved and dry in a vacuum-oven to measure the particle size distribution, the details can be found in the Table S1 in the Supplementary information. In addition, the plasticity index of materials in this study was obtained from a compression plastimeter (Rapid Plastimeter MKV-P14) according to the Chinese Standard QB/T 1322–2010 [26]. The plasticity index is the numerical difference between liquid limit and plastic limit for the materials. The liquid limit is the maximum water content when the material has plasticity while the plastic limit is the minimum water content when the material has plasticity. Lastly, the volume of samples is obtained by the water displacement method which submerging the testing sample in a known volume of water and measuring the change in water level as the volume. 2.2. The composition of raw materials The X-ray diffraction (XRD) and X-ray fluorescence (XRF) were used to obtain the mineral and chemical composition, respectively. The samples were ground in an agate ball mill to a grain size smaller than 150 mm and dried in a vacuum oven at 105 °C for 16 h. The XRF analysis was performed on a Bruker D8 Advance with the scanning angle range from 5° to 70° and step size of 0.02°. XRF analysis was performed on the PANalytical Axios Advanced. Major, minor, and trace chemical elements were measured on fused glass disks, which were prepared with 4.2 g spectro flux (LiBO22H2O/Li2B4O7) and 0.8 g of sample powder. The mixture was automatically molten and fused at 1200 °C. More details can be found in the reference [27]. 2.3. The preparation method of composite sintered brick The preparation procedure of composite sintered brick is shown in Fig. 1. The certain amount of iron ore tailings, coal gangue powder, sewage sludge and shale were mixed at different weight ratios. Afterwards, the raw materials were homogenized by adding the 10% of water in the mortar mixer for 5 min. The wetting materials were then natural aged by sealing at 20 °C for 8 h to allow the fully permeation of water and increase the plasticity. The aging materials were then suppressed in a mold (diameter of 46 mm, length of 30 mm) by a 769YP-30T tablet machine with desired molding pressure for 10 s to obtain the brick body. To remove the water content, the brick body was dried at room temperature (20 ± 5) °C for 48 h and then put it in the oven at 105 °C for 24 h. Afterwards, it was sintered in a SX212-16A high temperature gradient muffle furnace with a heating rate of 5 °C/min to the required temperatures, and then were maintained at the highest temperatures for a desired holding time to ensure the full agglomeration. Finally, the composite sintered bricks were obtained after cooling down to the room temperature. To clarify the preparing conditions of sintered bricks for different experiments in this study, excepted from the details presented in the different experiments, the preparing conditions of specimens in the experiments of this study is summarized and presented in Table S2 in the Supplementary information 2.4. Characterization of sintered bricks The quality of sintered bricks were characterized by some important parameters (bulk density, sintering shrinkage, water absorption and compressive strength) to determine the optimum preparation conditions [28]. The bulk density (BD) meant the density of bricks after sintering, which can be calculated from Eq. (1).
BD ¼
WAS VAS
ð1Þ
where WAS stands for the weight of bricks after sintering, VAS is the volume of brick after sintering. The sintering shrinkage (SS) was another important parameter to evaluate the deformation possibility of sintered bricks, which was determined by Eq. (2).
SS ¼
VBS VAS VBS
ð2Þ
3
L. Luo et al. / Construction and Building Materials 232 (2020) 117250
Fig. 1. Preparation procedure of composite sintered brick by iron ore tailing, coal gangue powder, sewage sludge and shale.
WASW WBSW WA ¼ WBSW
ð3Þ
where WASW represents the weight of materials after soaking in water for 24 h, WBSW stands for the weight of materials before soaking in water. The compressive strength (CS) of sintered bricks was measured via a YAW-300D automatic pressure testing machine according to the Chinese Standard QB/T 2542–2012, which was obtained from Eq. (4).
CS ¼
FB FA
ð4Þ
where FB is the force to break materials while FA is the force area of materials. The testing sample was placed in the center of the compression plate and loaded perpendicular to the compression surface. The loading speed was set as 2.5 kN/s until the fracture of the sample happened, and the maximum load FB is recorded. Note that the characterizations of sintered bricks were tested twice to obtained the average value and reported with error bars in this study. Error bars represent one standard deviation of uncertainty obtained from two independent runs. 2.5. Leaching toxicity The concentrations of hazardous heavy metal ions in the raw materials, composite sintered bricks and its leachate were analyzed by the inductively coupled plasma optical emission spectrometry carried on an Optima 4300DV instrument (ICP-OES, Perkin-Elmer, America). The leachate was obtained by the leaching toxicity-sulfuric acid and nitric acid method (HJ/T299-2007) which is the standard solid waste-extraction procedure for evaluating the toxicity of metals and their harm to the environment by simulated acid rain [29]. The results were compared with the Chinese standard 5085.3-2007 to ensure the safety of presented sintered bricks in its usage.
3. Results and discussion 3.1. The analysis of raw materials The chemical assays of these samples were analyzed by the XRF and shown in Table 1. In addition, the XRD was conducted to obtain the components of the raw materials and shown in Fig. 2. As shown in Table 1 and Fig. 2, the components of the raw materials are complicated with types of different minerals, which are mainly the silicate minerals including quartz, illite, diopside, gypsum and orthoclase. Hence, it will be beneficial to the sintering and solidification of the bricks in this study. 3.2. Effects of the sludge content on the characteristics of composite sintered bricks The weight ratios of four raw materials were considered in the first place. Note that the preliminary determined the percentage of coal gangue powder and shale should be fixed at 30% and 10% respectively to obtain the suitable product, thereby only the
Iron ore tailings
2.6. Thermogravimetry and differential scanning calorimetry (TG-DSC) The mass and energy transitions of mixing raw materials (the optimum mass ratio of iron ore tailings, coal gangue powder, shale and sewage sludge equals to 54:30:10:6) during the sintering process were tested by the TG-DSC, using a DIL402C heat analyzer. The specimen were heated from room temperature (25 °C) to 1300 °C at a rate of 10 °C/min in air atmosphere.
2
2
5 2
2
5 2 1 15 15 1
5 1
5 11 1
5 1
6 2 1
1
Sewage sludge
2 7 Coal gangue 2 9 4
21
2.7. Scanning electron microscopy (SEM)
1 Quartz 4 Orthoclase7 Chlorite 8 Calcite 5 Albite 2 Illite 3 Diopside 6 Feldspath 9 Gypsum
1
2 1 2 9 1 8
Shale
To analyze the microstructure of the composite sintered bricks, a scanning electron microscope (JEOL 5610LV, JEOL Ltd., Tokyo, Japan) under an accelerating voltage of 20 kV was applied to take the images of the section of bricks before and after sintering. Prior to the SEM experiments, the sections of bricks with required size were obtained by a cutting machine and then platinum coated in a sputter coater for preparing samples.
1
1
Intensity (Counts)
where VBS is the volume of materials before sintering which equals to the volume of mold. The water absorption (WA) strongly affect the strength and durability of bricks, and it yielded:
10
1 4
20
6 22 2 11 1
2
7 6 4
2
1 9 84
32 2 1
4
30
4
4 3 2
4 3 41
40 2θ /(°)
4 3
5 4
50
60
70
Fig. 2. XRD pattern of raw materials. (a- iron ore tailings, b-sewage sludge, c-coal gangue powder, d-shale).
Table 1 Composition analysis of raw materials (unit: %). Samples
SiO2
Fe2O3
Al2O3
CaO
MgO
P2O5
K2O
Na2O
S
Loss on Ignition
Iron ore tailings Coal gangue powder Sewage sludge Shale
51.59 8.42 41.47 59.69
8.22 2.26 3.53 5.01
12.75 3.12 15.95 15.64
11.91 2.98 1.23 5.14
6.07 1.77 1.79 2.11
1.86 3.45 0.13 2.66
1.32 0.62 1.58 2.19
2.51 0.34 0.58 0.91
1.23 0.34 0.85 0.15
0.83 68.37 31.88 5.24
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contents of iron ore tailings and sludge were investigated in this study. The effects of sludge content on the characteristics of composite sintering bricks under 20 MPa molding pressure, 1100 °C sintering temperature and 3 h holding time were shown in Fig. 3. As shown in Fig. 3(a), the bulk density (BD) was largely dropped from 1.74 to 1.63 with the addition of 3% sludge while it slightly decreased with the increasing sludge content from 3% to 12%. The variation of BD with different sludge contents indicated that the organic components in the sludge had been transition into the gas and then volatilized through a chemical reaction during the sintering process, leading to a decreasing trend of BD. A declining trend of sintering shrinkage (SS) from 9.7% to 7.9% with the increasing sludge content from 0 to 12% was also presented in Fig. 3(a). The generated gas by sludge during the sintering process filled in the gap after the evaporation of water, causing the gradually decrease of SS. Note that the increase of sludge content brought more water into raw materials, which also led to the decrease of BD and SS. In addition, the increasing content of fine sludge particle may promote the improvement the BD of sintered brick but it was offset by other factors considering the small amount of sludge content in the raw materials (12%). Fig. 3(b) revealed the effects of sludge content on the water adsorption (WA) of sintering bricks. The WA can reflect the ability of sintered brick to resist the harsh environment. According to the national standard of China (GB5105-2003), the WA of the qualified sintering bricks should be less than 18%. As shown in Fig. 3(b), the WA of sintering bricks without sludge as the component is 14.2% but it increased with the addition of sludge. When the sludge
content increased to 9%, the WA also increased to the value over 18% which did not meet the national standard. Another main parameter of sintering bricks, the compressive strength (CS), was also highly related to the sludge content which is shown in Fig. 3 (b) as well. With the sludge content increased from 0 to 6%, the CS decreased from 18.2 to 14.24 MPa which could satisfy the Chinese national standard of bearing brick (MU10). When the sludge content continuously increased, the CS dropped to 8.4 MPa which could only meet the Chinese national standard of non-bearing brick (MU5). 3.3. Effects of the molding pressure on the characteristics of composite sintered bricks The molding pressure had the strong effects on the shape and strength of brick body. Fig. 4 presented the effects of molding pressure on the characteristics of composite sintered bricks with 6% sludge content of raw materials, 1100 °C sintering temperature and 3 h holding time. As shown in Fig. 4(a), the BD of sintered bricks linearly enhanced from 1.635 to 1.641 g/cm3 the with the increasing molding pressure from 10 to 30 MPa. However, the SS dropped with increasing molding pressure, especially in the range from 15 to 20 MPa where SS decreased from 9.08% to 8.12%. With the increase of molding pressure, the particles in the raw materials were squeezed by the external force to free the gas inside the gaps of particles, hence the BD increases linearly with a small amplitude. During the sintering process, the removal of residual gas compacted the particles further, leading to the reduction of SS.
Fig. 3. Effects of sludge content on the characteristics of composite sintered bricks: a) bulk density and sintering shrinkage; b) water absorption and compressive strength.
Fig. 4. Effects of molding pressure on the characteristics of composite sintered bricks: a) bulk density and sintering shrinkage; b) water absorption and compressive strength.
L. Luo et al. / Construction and Building Materials 232 (2020) 117250
Fig. 4(b) showed the WA of sintered bricks decreased from 18.6% with the increasing molding pressure, especially in the range from 15 to 25 MPa where the WA largely reduced from 17.6% to 15.5%. Thus, the WA of sintered brick using the molding pressure larger than 15 MPa will meet the requirement of Chinese national standard. The CS of sintered bricks significantly increased from 9.8 to 14.24 MPa with the increasing molding pressure from 10 to 20 MPa, which met the standard of MU10. When the molding pressure continuously increased, the CS only present slightly improving trend. In summary, the 20 MPa of molding pressure was the most appropriate condition for the brick-making. The increase of molding pressure compressed the raw materials and then reduced the gaps between them, leading to the improvement of CS. 3.4. Effects of the sintering temperature on the characteristics of composite sintered bricks It is all known that the sintering temperature determines the properties of sintered bricks. Fig. 5 showed the effects of sintering temperature on the characteristics of composite sintered bricks with 6% sludge content of raw materials, 20 molding pressure and 3 h holding time. It can be seen from Fig. 5(a) that the BD of sintered bricks gradually enhanced from 1.617 to 1.651 g/cm3 the with the increasing sintering temperature from 950 to 1150 °C. SS also presented an increasing trend with the improvement of sintering temperature. As the temperature rises, the crystal-like and liquid phases can be formed inside the brick body to compact the particles, leading to the increase of BD and SS.
5
Fig. 5(b) revealed the WA of sintered bricks dropped from 18.07% to 15.72% while the CS enhanced from 7.64 to 14.24 MPa with the increasing sintering temperature from 950 to 1100 °C. When the temperature increased to 1150 °C, both WA and CS presented the decrease trend. Note that the WA of sintered bricks only qualified when the sintering temperature over 1100 °C, considering the requirement of Chinese national standard (<18%). On the other hand, the peak value of CS (14.24 MPa) was obtained at 1100 °C sintering temperature, which might be attributed to the formation of calcium sulphate from the sulphate and calcium in the raw materials when temperature larger than 1050 °C, causing the so-call ‘‘black cores” phenomenon [30]. In addition, a high temperature might result into the excess liquid glass phase in the brick body which can also decrease the compressive strength of bricks [31]. Thus, an appropriate sintering temperature was necessary to obtain the maximum CS of the samples in this study. 3.5. Effects of the holding time on the characteristics of composite sintered bricks After the sintering temperature reached desired value, a suitable holding time was needed to ensure the transition of mixed raw materials into tight structure. Fig. 6 revealed the influences of holding time on the characteristics of composite sintered bricks with 6% sludge content of raw materials, 20 MPa molding pressure and 1100 °C sintering temperature. As shown in Fig. 6(a), the BD decreased with the increasing holding time while the SS increased linearly as the improvement of holding time.
Fig. 5. Effects of sintering temperature on the characteristics of composite sintered bricks: a) bulk density and sintering shrinkage; b) water absorption and compressive strength.
Fig. 6. Effects of holding time on the characteristics of composite sintered bricks: a) bulk density and sintering shrinkage; b) water absorption and compressive strength.
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Fig. 6(b) revealed the increase of holding time is beneficial to the parameter of WA which decreased with the improvement of holding time. When the holding time was larger than 2.5 h, the WA already met the requirement (<18%). In addition, the CS of sintered bricks was also shown in Fig. 6(b), in which the CS firstly increased with the increasing holding time until it reached 3 h where the CS became stable with the continuous enhancement of holding time. The continuous increase of holding time allowed fully reactions between the raw materials, causing the tight and sturdy structure of sintered bricks, resulted into the increase of CS [31]. Interestingly, when the holding time was larger than 3.5 h, the CS began to decrease which might be attributed to the unknown reaction among the raw materials. 3.6. The leaching toxicity of composite sintered bricks Except from the physical requirement of sintered bricks, the leaching toxicity is also necessary to be measured [32]. Table 2 showed the main types of heavy metal ions in the raw materials. The leaching efficiency (LE) was defined as the following equation.
LE ¼
CL CO
ð5Þ
where the CO stands for the concentration of the heavy metal in the original sample, CL represents the concentration of the heavy meatal in the leachate of the samples. A higher value of LE means that the heavy metal of sample is more easily to be leached out in the acid environment. As is shown, the rank of heavy elements’ content in the raw materials was Cu > Zn > Cr > Pb. However, the rank presents different in the leachate of raw materials which was Zn > Cu > Pb > Cr. Thus, the Zn in the raw materials was easier to be leached out with 38.94% leaching efficiency, followed by the Pb with 20.83% leaching efficiency. The Cu and Cr shows stable characteristics with 5.29% and 2.72% respectively. These hazardous heavy metal elements were also tested in the sintered bricks and its leachate. The results were shown in Table 2, in which we can see that the rank of elements’ content in the sintered bricks is the same as in the raw materials but all the elements’ content in sintered bricks are larger. It could be attributed to the increase of BD during the sintering process. In addition, as can be seen from Tables 2, all heavy metal contents in the leachates were far less than the toxicity threshold prescribed in GB5085.32007 [33]. However, the elements’ contents in the leachate of sintered bricks were smaller than that of in the leachate of raw materials, meaning the most of the heavy metal elements contained in the material were immobilized during the sintering process [34]. The heavy metals could form phases that can exhibit sufficient stability due to irreversible phase transformation by interacting with silicate minerals in high temperature [35,36]. Thus, the order of leaching efficiency of sintered bricks was Pb > Cu > Zn > Cr. 3.7. Transition of the mixing raw materials during sintering To better understand the quality and energy changes in the sintering process, the TG-DSC analysis of sintering raw materials in
Fig. 7. Thermal analysis on the mixing raw materials during sintering.
the optimum ratio (iron tailings: coal gangue powder: shale: sludge = 54:30:10:6) was conducted and shown in Fig. 7. As can be seen from it, the thermogravimetry (TG) curve changed gently, indicating that there is no obvious reaction during the sintering process, which can avoid the obvious cracks but might lead to the open of porosity in sintered bricks due to the large amount of gases discharged in a short time. According to the differential scanning calorimetry (DSC) curve, an endothermic peak was appeared at 59.3 °C, which could be attributed to the heat absorption by volatilization of free water during the sintering process. When the sintering temperature continuously increased, an exothermic peak was generated at 390.6 °C, which may be caused by the combustion of organic matter [37]. In addition, a strong endothermic peak and a weak endothermic peak were appeared at 450 °C and 750 °C respectively, which may be resulted from the removal of crystallization water from minerals such as kaolinite in the raw materials [38]. However, no obvious weight loss phenomenon occurred at the same range of the TG curve, suggesting the solid phase reaction may just began. At 1230.2 °C, the DSC curve showed an endothermic valley, which was caused by the melting of mineral particles at high temperatures [39]. As for the derivative thermogravimetry (DTG) curve, a valley was firstly generated at 56.7 °C, which mainly due to the loss of weight caused by the volatilization of residual free water in the raw materials with a loss rate of about 2.80%. The second obvious valley at 383.1 °C was mainly caused by the combustion of organic matter in the raw materials, after which the DTG curve does not change significantly. Overall, there were only few peaks and valleys appeared in the TGDSC analysis, thereby the thermal effect and solid-phase reaction were not obvious during the sintering process in this study. SEM was used to observe the microstructure of the sections of composite bricks before and after fired at 1000 °C, 1050 °C and 1100 °C, the results of which are shown in Fig. 8. It can be seen from Fig. 8(a) that the sections of unburned bricks were mostly the discrete particles with different sizes and disordered arrangements. Fig. 8(b) presented the small silicate layers and crystallike minerals particles were initially generated in the section of
Table 2 Concentrations of hazardous heavy metal elements in the raw materials and sintered bricks and their leachate. Samples
Hazardous heavy metal elements
Cu
Pb
Zn
Cr
Raw Materials
Raw materials (mg/L) Leachate of raw materials (mg/L) Leaching efficiency (%) Composite sintered bricks (mg/L) Leachate of sintered bricks (mg/L) Toxicity thresholds (mg/L) Leaching efficiency (%)
1.466 0.0775 5.29 1.646 0.0489 100 2.97
0.131 0.0273 20.83 0.146 0.0053 5 3.63
1.485 0.5783 38.94 1.604 0.0077 100 0.48
0.268 0.0073 2.72 0.314 0.0011 15 0.35
Sintered Bricks
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Fig. 8. Microstructures of the sections of sintered bricks at different temperatures. (a-raw materials; b-1000 °C; c-1050 °C; d-1100 °C).
brick when the sintering temperature reached 1000 °C. Note that the gaps between materials were still large which means the sintered bricks were not consolidated and developed, leading to the lower CS and higher WA rate. When the sintering temperature further increased to 1050 °C, a large amount of liquid glass phase was found in the inner part of the sample [40], which was shown in Fig. 8(c). The molten glass phase wrapped and cemented the fine crystal-like mineral particles, filled the gaps and holes between materials and made them close to each other, leading to the increase of BD and CS [41]. Fig. 8 (d) showed the sintered bricks under the optimum sintering temperature 1100 °C, the degree of melting and consolidation of the sintered bricks were very high with the flat sections, leading to the dense structure. The sections presented the small and evenly distributed pores as well as the cementing of liquid phase, which increased the BD and CS while reduced the porosity and WA rate of the sintered bricks. 4. Conclusions As the increasing requirement of environmental harmony, the cyclic utilization of solid waste become more and more important. In this study, the composite sintered bricks were successfully prepared by using the iron ore tailings and coal gangue powder as the main materials, the shale and sewage sludge as the binder. The product was studied by changing the preparation conditions and investigating the related characteristics. We found that the qualified composite sintered bricks with 1.638 g/cm3 of bulk density (BD), 8.3% of sintering shrinkage (SS), 17.47% of water absorption
(WA) and 14.24 MPa of compressive strength (CS) could be obtained by the optimal conditions: 54:30:10:6 mass ratio of iron ore tailings: coal gangue powder: shale: sewage sludge, 20 MPa of molding pressure, 1100 °C of sintering temperature and 3 h of holding time. Most of the hazardous heavy metal elements in the sintered bricks were immobilized during the sintering process with the leaching efficiency order of Pb > Cu > Zn > Cr, which met the requirement of Chinese national standard. The transition of the mixing raw materials during the sintering process was also investigated. We found that the most of the mass and energy changes were caused by the phase transition of organic matter and water but the transformation of partial minerals in high temperature could also lead to the changes. The microstructure of unburned bricks showed that most of them were mineral particles with disordered size and arrangement. With the increase of sintering temperature, the newly formed crystal-like mineral particles increased obviously, and the molten glass phase was also appeared to wrap and cement the fine crystal-like mineral particles, thereby the surface became more uniform and compact with the reduction of the porosity. Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the
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review of, the manuscript entitled ‘‘Preparation, characteristics and mechanisms of the composite sintered bricks produced from shale, sewage sludge, coal gangue and iron ore tailings”. Acknowledgements This work was funded by the National Natural Science Foundation of China (51904214, 51874219) and the Open Foundation of State Key Laboratory of Mineral Processing (BGRIMM-KJSKL2019-02). The authors are also thankful for the reviewers due to their constructive suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.117250. References [1] S. Yang, L. Wang, Measurement of froth zone and collection zone recoveries with various starch depressants in anionic flotation of hematite and quartz, Miner. Eng. 138 (2019) 31–42. [2] W.C. Fontes, J.M.F. de Carvalho, L.C. Andrade, A.M. Segadães, R.A. Peixoto, Assessment of the use potential of iron ore tailings in the manufacture of ceramic tiles: From tailings-dams to ‘‘brown porcelain”, Constr. Build. Mater. 206 (2019) 111–121. [3] S. Zhao, J. Fan, W. Sun, Utilization of iron ore tailings as fine aggregate in ultrahigh performance concrete, Constr. Build. Mater. 50 (2014) 540–548. [4] C. Li, H. Sun, J. Bai, L. Li, Innovative methodology for comprehensive utilization of iron ore tailings: Part 1. The recovery of iron from iron ore tailings using magnetic separation after magnetizing roasting, J. Hazard. Mater. 174 (1–3) (2010) 71–77. [5] Y. Taha, M. Benzaazoua, R. Hakkou, M. Mansori, Coal mine wastes recycling for coal recovery and eco-friendly bricks production, Miner. Eng. 107 (2017) 123– 138. [6] C.L. Wang, W. Ni, S.Q. Zhang, S. Wang, G.S. Gai, W.K. Wang, Preparation and properties of autoclaved aerated concrete using coal gangue and iron ore tailings, Constr. Build. Mater. 104 (2016) 109–115. [7] S. Zhang, X. Xue, X. Liu, P. Duan, H. Yang, T. Jiang, D. Wang, R. Liu, Current situation and comprehensive utilization of iron ore tailing resources, J. Min. Sci. 42 (4) (2006) 403–408. [8] W. Wang, C. Wang, S. Wang, W. Ni, S. Zhang, G. Gai, Preparation and properties of autoclaved aerated concrete using coal gangue and iron ore tailings, Constr. Build. Mater. 104 (2016) 109–115. [9] C. Li, H. Sun, Z. Yi, L. Li, Innovative methodology for comprehensive utilization of iron ore tailings: Part 2: The residues after iron recovery from iron ore tailings to prepare cementitious material, J. Hazard. Mater. 174 (1–3) (2010) 78–83. [10] L.A.D.C. Bastos, G.C. Silva, J.C. Mendes, R.A.F. Peixoto, Using iron ore tailings from tailing dams as road material, J. Mater. Civ. Eng. 28 (10) (2016). 04016102. [11] P. Duan, C. Yan, W. Zhou, D.J. Ren, Fresh properties, compressive strength and microstructure of fly ash geopolymer paste blended with iron ore tailing under thermal cycle, Constr. Build. Mater. 118 (2016) 76–88. [12] J.L.B. Galvão, H.D. Andrade, G.J. Brigolini, R.A.F. Peixoto, J.C. Mendes, Reuse of iron ore tailings from tailings dams as pigment for sustainable paints, J. Cleaner Prod. 200 (2018) 412–422. [13] L. Weishi, L. Guoyuan, X. Ya, H. Qifei, The properties and formation mechanisms of eco-friendly brick building materials fabricated from lowsilicon iron ore tailings, J. Cleaner Prod. 204 (2018) 685–692. [14] D. Li, X. Song, C. Gong, Z. Pan, Research on cementitious behavior and mechanism of pozzolanic cement with coal gangue, Cem. Concr. Res. 36 (9) (2006) 1752–1759. [15] X.X. Miao, J.X. Zhang, Analysis of strata behavior in the process of coal mining by gangue backfilling, J. Mining Safety Eng. 4 (2007). [16] N. Zhang, H. Sun, X. Liu, J. Zhang, Early-age characteristics of red mud–coal gangue cementitious material, J. Hazard. Mater. 167 (1–3) (2009) 927–932. [17] M. Yang, Z. Guo, Y. Deng, X. Xing, K. Qiu, J. Long, J. Li, Preparation of CaO– Al2O3–SiO2 glass ceramics from coal gangue, Int. J. Miner. Process. 102 (2012) 112–115.
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