Chunguang Song ,Hongling Zhang, ,Yuming Dong ,Lili Pei ,Honghui Liu ,Junsheng Jiang ,Hongbin Xu

1 CAS Key Laboratory of Green Process and Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

3 University of Chinese Academy of Sciences,Beijing 100049,China

4 RH Mining Resources Limited,Hong Kong,China

Keywords:Acid-leaching tailings Red mud Waste treatment Sintering Preparation Lightweight aggregate

ABSTRACT Proper treatment of acid-leaching tailings (ALTs) of vanadium-bearing stone coal minerals is of great urgency.One approach is adding it into the raw materials during the preparation of lightweight aggregate(LWA).But clay is always needed.In this paper,another solid waste,red mud,was mixed with ALTs as a source of flux components instead of clay.Evaluation of the physical characteristics,morphological structures,as well as crystal phases during the sintering process were investigated.When their mixtures with a proper ratio were sintered at 1080 °C,a glassy phase with certain viscosity was formed,and the gases generated simultaneously were encapsulated by the melt.Finally,LWA with a one-hour water absorption as low as 1.46%,a bulk density as low as 728.76 kg﹒m-3 and a compressive strength as high as 10.77 MPa was fabricated.

1.Introduction

Acid-leaching tailings (ALTs) of vanadium-bearing stone coal minerals is the solid waste discharged in the industry of vanadium extraction from stone coal.120–150 tons of ALTs are generated per ton of V2O5extracted [1–3].ALTs is acidic and contains harmful elements (e.g.,S).The accumulation of ALTs already caused issues related with land occupation and environmental pollutions.In the process of accumulation,small particles and dust might enter into the air under the action of wind,causing air pollution.The residual waste acid and heavy metals of tailings might contaminate the groundwater,rivers and lakes.The storage of tailings occupies a large amount of land,causing the destruction of vegetation and landform,and affecting the degradation ability and selfregulation function of soil [4].With a SiO2content higher than 60 wt%,ALTs could be added into the raw materials during the preparation of lightweight aggregate (LWA).For example,Shi et al.[5] prepared LWA using stone coal cinder,clay,sodium hexametaphosphate and organic salt.LWA with a bulk density of 776 kg﹒m-3,a water absorption of 7.5%and a cylinder compressive strength of 2.1 MPa was obtained.Zhao et al.[6] prepared LWA from vanadium tailing,potassium feldspar and clay.After been roasted at 1125 °C for 30 minutes,the obtained LWA showed a bulk density of 631 kg﹒m-3,a water absorption of 3.1%and a compression strength of 9.1 MPa.However,some problems were encountered:the sintering temperature is relatively high,clay usually was needed to adjust the composition of raw materials,and the water absorption capacity of the LWA is relatively high.Utilization of clay can be a waste of natural resources and the mining of clay also imposes negative impact on the environment.Lower water absorption capacity is required for one abundant application of LWAs,cement.

Replacing clay by various wastes as LWA raw materials has been attracting numerous studies [7–12].The sintering process has a closely correlation with the contents of flux components(Fe2O3,FeO,CaO,MgO,K2O and Na2O) in raw materials,which is beneficial to the decrease of sintering temperature and the formation of glassy phase [13].That glassy phase could be helpful to smooth the surface roughness,leading to a decrease in the water absorption capacity [14].In consideration of this,another solid waste,red mud,which has a high contents of flux components,was selected to adjust the composition of raw materials.Red mud is an alkaline solid waste discharged in the process of alumina production [15].1–1.5 tons of red mud are generated per ton of alumina produced.It is estimated that about 90 million tons red mud are generated every year in the world [15,16].The red mud produced by different areas and processes is quite different,but their main components are similar,which are SiO2,Al2O3,Fe2O3,CaO,Na2O etc [16].Moreover,all of them are alkaline and have a high contents of flux components.So the adding of red mud might be beneficial to the decrease of sintering temperature,the formation of glassy phase and the increase of the compressive strength[14].When using ALTs and red mud as raw materials to prepare LWA,the composition requirements can be met by adjusting the proportion of ALTs and red mud.Meanwhile,the alkaline components of red mud could neutralize the weak acidity of the ALTs.Gases produced by the decomposition of inorganic substances in ALTs can expand the LWA,so pore-forming agent such as pulverized coal or sodium nitrate can be saved [17,18].

According to the composition of LWA [19,20],raw materials with an appropriate chemical composition might be obtained by adjusting the ratio of ALTs and red mud without the addition of clay.However,the transformation of micro morphology and crystal phases during the sintering process needs to be studied further for the successful fabrication of lower water absorption LWA.Therefore,the main goal of the present study is to utilize the relatively high aluminum content of the red mud to improve the compression strength for LWA,and study the effect of flux components in red mud on the sintering process and the water absorption capacity of LWA.Affecting factors on the physical characteristics,morphological structures,as well as crystal phases of the as-prepared LWA samples were investigated systematically.Finally,low water absorption LWA with desired characteristics was fabricated.Utilization of ALTs and red mud as raw materials for low water absorption LWA can be an effective way for the dispose of solid wastes in the industry of vanadium extraction from stone coal and aluminum extraction from bauxite.It can also be an approach for the fabrication of low water absorption LWA without the addition of clay.

2.Materials and Methods

2.1.Materials

The chemical compositions of ALTs and red mud are shown in Table 1.The ALTs used in the experiment was provided by Dunhuang Sunshine Zhaojin Mining Co.,Ltd (Gansu,China),with an average particle size of 31.49 μm and a maximum particle size of 239.88 μm.The red mud used in the experiment was provided by Kaiman Aluminum Co.,Ltd(Henan,China),with an average particle size of 10.82 μm and a maximum particle size of 181.97 μm.All materials were dried to constant mass at 105 °C using a constant temperature air-drying oven DHP-9012 (Shanghai,China),ground by a planetary ball mill BHY-PM4 (Beijing,China) and passed through a No.100 mesh before using.

Table 1 Chemical composition of main raw materials (wt%)

2.2.Methods

2.2.1.Preparation of LWA

ALTs and red mud were completely mixed according to certain mass ratios of 90:10,80:20,70:30,60:40 and 50:50 by a planetary ball mill BHY-PM4 (Beijing,China).Raw pellets with diameters of 5–6 mm were prepared using the mixture.Then they were dried at 105 °C in an oven DHP-9012 (Shanghai,China) until invariable mass was achieved.The sintering of those pellets were conducted in an electrically heated furnace SX-G08163(Tianjin,China),using procedures as follows:starting temperature～20 °C,ramp rate～8 °C﹒min-1,the 1st dwell temperature～500 °C and the 1st dwell time～10 min,the 2nd dwell temperature as preset (i.e.1000,1025,1050,1060,1075,1080 and 1085 °C) and the 2nd dwell time～30 min.Finally,the sintered samples were naturally cooled to room temperature.

2.2.2.Characterization

Simultaneous thermogravimetric analysis (TGA) and differential scanning calorimeter(DSC)analysis of the samples were examined by a SDT Q600 (USA),with a heating rate of 8 °C﹒min-1from room temperature (RT) to 1200 °C under air flow.

Simultaneous thermogravimetric/Fourier transform infrared spectrometry (TG-FTIR) analysis were examined by a Nietzsche STA449C(Germany,for TG analysis)and a Bruker Equinox55(Germany,for FTIR analysis of the volatiles which were transferred into its gas cell simultaneous),using a heating rate of 8 °C﹒min-1from RT to 1200°C under air flow.The transferring line and gas cell were kept at 220 °C to avoid condensation.The scan resolution was 2 cm-1in the wavenumber region of 400–4000 cm-1.

Water absorption,bulk density and apparent density tests were employed to characterize the quality of the sintered pellets.Water absorption,bulk density and apparent density were determined according to GB/T 17431.2-2010.A sintered LWA pellet with a diameter of 5–8 mm was placed vertically on the platform of an automatic material testing machine WDW-100E (Jinan,China)and was pressed at a crosshead speed of 5 mm﹒min-1until it was crushed.Comprehensive strength was the average value of eight tests per sample,and it was given by S=2.8Pc/(πX2),where X was the diameter of the pellets and Pcwas the rupture load[21,22].The bloating index (BI) was given by BI=(Vi– Vf)/Vi× 100,where Viwas the estimated volume based on the mean diameters of the raw pellets and Vfwas the estimated volume based on the mean diameters of the sintered pellets [23].

The chemical composition of materials were determined by Xray fluorescence spectrometry (XRF) using an AXIOS system(PANalytical,Netherlands).The microstructures of the materials were examined by a scanning electron microscope (SEM,JSM-7800F,15 kV).After been grinded to fine powder,the crystal phases of raw materials and sintered pellets were determined by X-ray diffraction (XRD).XRD analysis was performed on an X-ray powder diffractometer(Empyrean,Netherlands)with Cu Kα radiation(λ=0.15406 nm),operating at 40 kV and 40 mA,with the scan range from 5° to 90°.

Fig.1.Effect of red mud content on the physical characteristics of LWA.

Fig.2.Three-component diagram of LWA.The dash circle indicates the suitable area for fabricating expandable LWA[26].The blue dots indicate the composites of ALTs,red mud and mixtures (with the red mud contents from 10% to 50%,as indicated in the brackets).

3.Results and Discussion

3.1.Determination of the proper content of red mud content

Bulk density,apparent density and one-hour water absorption of LWAs sintered at 1075 °C using different contents of red mud were represented in Fig.1.The bulk and apparent densities increase gradually for red mud content from 10%to 30%,and then,decrease gradually when the red mud content is above 30%.The change of water absorption with red mud content increasing is opposite to those of densities.A maximum bulk density(888.32 kg﹒m-3),a maximum apparent density (1920.64 kg﹒m-3),and a minimum water absorption (5.82%) were obtained when the red mud content is 30%.

Fig.3.DSC-TGA curves of ALTs/red mud=70/30.

Water absorption rate can reflect the degree of sintering[24,25].The lower the water absorption rate,the higher the degree of sintering[14].Clearly,LWA fabricated with a red mud content of 30% has the highest sintering degree.

Three-component diagram consisting of SiO2,Al2O3,and flux components (Fe2O3,FeO,CaO,MgO,K2O and Na2O) was shown in Fig.2[26].To be suitable for fabricating expandable LWA,chemical compositions of candidate materials need to fall inside a limited area (dash circle in Fig.2) [26–29].When the red mud content is 30%,the chemical composition of raw pellets is in the range of expandable LWA,which is more conducive to sintering reaction,and LWA with appropriate bulk density and low water absorption can be obtained.Therefore,30% was selected as the optimal red mud content.

3.2.Thermal behavior analyses

DSC-TGA curves of ALTs and red mud mixture with ALTs/red mud=70/30 were shown in Fig.3.A total weight loss about 19.67% was observed when the temperature increases from RT to 1200°C.The weight loss(～3.81%)when the temperature increases from RT to approximately 550 °C might attribute to the evaporation of the absorbed water and the structural water in ALTs and red mud.This is in consistent with the endothermic peak in the DSC curve.A weight loss approximately 11.26% was seen in the TGA curve from 550 °C to 850 °C,which could be attributed to the decomposition of carbonous materials and combustion of carbon.A corresponding exothermic peak can be observed in the DSC curve.Further weight loss and thermic change could also be observed from 910 °C to 1200 °C,which might correspond to the decomposition of calcium sulfate and the crystallization of albite,anorthite,etc.Gases might be generated during the decomposition of carbonous materials and calcium sulfate,and the combustion of carbon,which is beneficial for the fabrication of LWA.

In order to confirm the generation of gases during the sintering process,simultaneous TG-FTIR was employed [30,31].The absorbance corresponding to CO (2173 cm-1),CO2(2359 cm-1) and SO2(1373 cm-1) were monitored as temperature increases(Fig.4).Clearly,CO and CO2were emerged between 554–795 °C and 542–825 °C,respectively.The result is in consistent with the weight loss between 550 °C and 850 °C,as shown in Fig.3.SO2was observed in the temperature range of 903–1090°C,which corresponds to the weight loss after 900 °C (Fig.3).

Fig.4.Infrared absorbance of main gases products of ALTs/red mud=70/30 at different temperatures.(a) CO,(b) CO2 and (c) SO2.

Fig.5.Effect of sintering temperature on the physical characteristics of LWA.

The FTIR results further confirmed that there were CO,CO2and SO2generated during the sintering process.The generation of gases is of great significance for the fabrication of LWA.Since the TGA and FTIR results showed that there are considerable weight loss and relatively abundant gases generated at a sintering temperature above 900°C,several sintering temperatures including 1000,1025,1050,1060,1075,1080 and 1085 °C were selected in order to obtain LWA.

3.3.Physical characteristics of LWAs obtained under different sintering temperatures and holding time

Several LWA samples were prepared by sintering ALTs and red mud mixture with ALTs/red mud=70/30 at different temperatures.Their physical characteristics were shown in Fig.5.It can be seen from Fig.5 that the bulk and apparent densities gradually increase for sintering temperatures from 1000°C to 1060°C,and then,gradually decrease when the sintering temperature is above 1060 °C.The water absorption capacity rapidly decreases as the sintering temperature increases from 1000°C to 1080°C.The water absorption capacity (1.46%)of the pellets sintered at 1080 °C were lower compared with previous reports [5,6,32].With further increasing of the sintering temperature,the LWA began to deform and adhesion occurred between different pellets and pellets with the container,and the shape of the pellets cannot be maintained.

Fig.6.Effect of sintering temperature on the compressive strength of LWA.

Fig.7.Effect of sintering temperature on the bloating index of LWA.

Compressive strength is a key index to evaluate the mechanical properties of LWA,which is the maximum compressing force a sinter could withstand before it breaks [33].Compressive strength of different LWA samples was listed in Fig.6.Similar with the bulk and apparent densities(Fig.5),the compressive strength gradually increases for sintering temperatures from 1000 °C to 1060 °C,and then,gradually decreases when the sintering temperature is above 1060 °C.When the temperature is between 1060 °C and 1080 °C,LWA with a compressive strength higher than 7.5 MPa,a bulk density less than 900 kg﹒m-3,and a water absorption capacity less than 10% could be obtained,which can be used as LWA in civil engineering [34,35].

With the increase of sintering temperature from 1000 °C to 1060 °C,the bulk density,apparent density and compressive strength of the LWA were improved.This might be attributed to the sintering reaction between the particles [32].BI of the pellets sintered at different sintering temperatures was shown in Fig.7,with negative values indicating bloating of the sintered LWA.It can be seen from Fig.7 that the volume of pellets gradually shrank for sintering temperatures from 1000°C to 1060°C,and then,gradually bloated when the sintering temperature is above 1060°C.The volume shrinkage from 1000 °C to 1060 °C was caused by the sintering reaction.After that,the decomposition of inorganic substances caused the volume bloating.The change of strength and density of the sintered LWA was closely related with the sintering and bloating process.LWA sintered at 1060 °C has the maximum BI,indicating it has the highest sintering degree,corresponding to the maximum bulk and apparent densities(Fig.5).With further increase of the sintering temperature,a viscous glassy phase might be formed accompanying the melting of inorganic oxide,which could encapsulate the gases produced by the decomposition of inorganic substances(Fig.4)[33,36,37].Thus several interior pores might form and the volume of pellets gradually increases,which results in the decrease of densities.The amount of glassy phase might increase as temperature increases.That glassy phase could be helpful to smooth the surface roughness,leading to a decrease in the water absorption capacity [14].More detailed will be analyzed in the next part according to the morphological structure.

Fig.8.Effect of holding time on the physical characteristics of LWA.

Several LWA samples were prepared by sintering ALTs and red mud mixture with ALTs/red mud=70/30 under 1080 °C with different holding time.Their physical characteristics were shown in Fig.8.It can be seen from Fig.8 that the bulk and apparent densities gradually increase for holding time from 0 minutes to 20 minutes,and then,gradually decrease when the holding time is above 20 minutes.The water absorption capacity rapidly decreases as the holding time increases from 0 minutes to 30 minutes.With further increasing of the holding time,the water absorption capacity of the LWAs keep almost unchanged.

According to GB/T 17431.1-2010,the water absorption of LWAs of different density grades is between 10.0% and 30.0%,and the water absorption should not be greater than that of the corresponding density grade.When the sintering temperature is 1080 °C,the density grades of LWA samples obtained at different holding times are all between 600 and 1200,and the water absorption should not exceed 10.0%,so the holding time should not be less than 20 minutes.In order to prepare LWA with a lower water absorption and density,the holding time was selected as 30 minutes.

3.4.Morphological structure evaluation

To investigate the effects of sintering temperatures on the morphological structure of LWAs,SEM analyses were conducted on the external surface (Fig.9) and cross section (Fig.10).The surface of raw pellets is very rough (Fig.9a),illustrating the point contact between particles.The surface of LWA specimen sintered at 1000 °C is less rough but extensive intergranular pores between particles can be seen clearly,resulting in a fragile LWA body.Consequently,LWA sintered at 1000 °C yields a compressive strength as less as 3.84 MPa (Fig.6),a water absorption as high as 29.89%and low densities (Fig.5).

Sintering between the raw material particles has occurred at 1000 °C,and glassy phase due to melting of flux such as CaO and Na2O,which were supplied by red mud,was formed (Figs.9b and 10b).The intensity of sintering and the amount of glassy phase gradually increase as temperature increases (Figs.9b–d and 10b–d).So,the volume of the LWA shrinks and the amount of interstitial pores decreases [34].These could be the main reasons for the increase of densities and compressive strength,and the decrease of the water absorption when the sintering temperature is below 1060 °C (Figs.5 and 6).

As illustrated by the TGA and FTIR results (Fig.4),gases would generate during the sintering process.When the sintering temperature is below 1060 °C,the amount and viscosity of the glassy phase might be too low to effectively encapsulate the released gases,resulting in the shrinking and volume reducing of the sintered pellets.Consequently,the bulk and apparent densities increase as temperature increases (Fig.5).With the further increase of the sintering temperature,the amount and viscosity of the glassy phase would increase to high enough to encapsulate the gases generate,and inner pores would form inside the LWA[15,38,39].The pellet will continue expanding with more pores formed as long as the viscous resistance force is lower than the expanding force of the bloating gases.Then it will stop expanding when these two forces are in balance[14].The expansion causes a rapid decrease in densities (Fig.5) and compressive strength(Fig.6)at temperature higher than 1060°C.The glassy phase could effectively prevent water from entering the LWA,so the water absorption rate decreases gradually [40,41].For pellets fabricated at 1075°C and 1085°C,many interconnected pores with diameters around 5–40 μm (Fig.10e) and 50–200 μm (Fig.10f) were observed within the LWA.That might because the viscous resistance force is much lower than the expanding force of the bloating gases at these temperatures.Therefore,the densities and compressive strength decrease abruptly(Figs.5 and 6).Clay is usually used as raw material or excipient to adjust the content of inorganic oxide and control the viscosity of the glassy phase [14].It can be seen that when ALTs and red mud with a ratio of 70/30 were used as raw materials instead of clay,glassy phase of suitable viscosity could also be form at a suitable sintering temperature.Besides,gases were released from the gas generating material,which could be encapsulated by the glassy phase of suitable viscosity and amount.Finally,LWA with low densities,low water absorption,and high compressive strength was obtained.

3.5.Crystal phases evaluation

Fig.9.Surface structure for LWA at different sintering temperatures.(a)raw pellets,(b)1000°C,(c)1025°C,(d)1050°C,(e)1075°C,and(f)1085°C.Glassy phase was circled by dash lines.

Fig.10.Interior structure for LWA at different sintering temperatures.(a) raw pellets,(b) 1000 °C,(c) 1025 °C,(d) 1050 °C,(e) 1075 °C,and (f) 1085 °C.Glassy phase was circled by dash lines.Pores were marked by arrows.

Fig.11.XRD patterns of raw materials((a)ALTs,(b)Red mud)and LWA sintered at different temperatures ((c) 1000 °C,(d) 1025 °C,(e) 1050 °C,(f) 1075 °C,and (g)1085 °C).

Fig.11 shows the XRD patterns of different samples.The major crystal phases of ALTs are quartz(SiO2) and muscovite((K,Na)(Al,Mg,Fe)2(Si3.1Al0.9)O10(OH)2).The major crystal phases of red mud are cancrinite (Na6Ca2Al6Si6O24(CO3)2﹒2H2O),calcite (CaCO3) and hematite (Fe2O3).The major crystal phases in the sintered specimens are quartz (SiO2),albite (NaAlSi3O8) and anorthite(CaAl2Si2O8).

When the sintering temperature is 1000 °C,new crystal phases such as albite(NaAlSi3O8)and anorthite(CaAl2Si2O8)were formed,along with the disappearance of some crystal phases including muscovite,cancrinite and calcite.Decomposition of these mineral produces gases and oxides,resulting in mass loss at the temperature from 550 °C to 850 °C,as supported by the TGA result(Fig.3).

As the sintering temperature increases,the peak intensity of quartz decreases,which corresponds to the form of the glassy phase (Figs.9 and 10).Intensities of peaks corresponding to albite and anorthite tend to increase as temperature increases.That’s because more Na+and Ca2+,which was supplied by red mud,would combined withso as to maintain electrical neutrality[35,42,43].The main mineral crystal phases of LWA preparing from ALTs and red mud are quartz and anorthite,which are similar to these of LWA preparing from ALTs with clay[5,6].No sulphur containing crystalline diffraction patterns were observed.

4.Conclusions

Low water absorption LWA was fabricated using ALTs as main raw material and red mud as a source of flux components instead of clay.The effect of flux components in red mud on the sintering process and the water absorption capacity were studied.Mixture with ALTs/red mud=70/30 was found to be an optimal ratio.At temperature higher than 1000 °C,sintering occurred between the raw material particles,and a glassy phase due to melting of flux such as CaO and Na2O,which were supplied by red mud,was formed.When the sintering temperature is below 1060 °C,the amount and viscosity of the glassy phase might be too low to effectively encapsulate the released gases,resulting in the shrinking and volume reducing of the sintered pellets.At temperature higher than 1060 °C,the amount and viscosity of the glassy phase would increase to high enough to encapsulate the gases generate,and inner pores would form inside the LWA.When the sintering temperatures is 1080 °C,LWA with a one-hour water absorption as low as 1.46%,a bulk density as low as 728.76 kg﹒m-3and a compressive strength as high as 10.77 MPa was fabricated.Utilization of ALTs and red mud as raw materials can be an approach for fabrication of LWA without the addition of clay.The properties of LWA can meet the requirements of lightweight aggregates (GB/T 17431.1-2010),and the leaching toxicity of heavy metals in LWA was far below the standard values in GB/T 5085.3-2007.So the LWA can be used in construction field.This approach is also of great significance for the dispose of solid wastes in the industry of vanadium extraction from stone coal and aluminum extraction from bauxite.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial supports from Dunhuang Sunshine Zhaojin Mining Co.,Ltd.and RH Mining Resources Ltd.are gratefully acknowledged.