J. Cent. South Univ. (2016) 23: 1883-1889

DOI: 10.1007/s11771-016-3243-5

Conversion of coal gangue into alumina, tobermorite and TiO₂-rich material

LUO Jun(罗骏), LI Guang-hui(李光辉), JIANG Tao(姜涛), PENG Zhi-wei(彭志伟),

RAO Ming-jun(饶明军), ZHANG Yuan-bo(张元波)

School of Minerals Processing & Bioengineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: A large amount of coal gangue from coal mining and processing is regarded as waste and usually stockpiled directly. In order to recycle the valuable elements from the coal gangue, an integrated process is proposed. The process consists of three steps: 1) concentrating alumina from the coal gangue via activation roasting followed by alkali leaching of SiO₂ which produces alumina concentrate for alumina extraction by the Bayer process; 2) synthesizing tobermorite whiskers from the filtrated alkali liquor containing silicate via a hydrothermal method and reusing excess caustic liquor; and 3) enriching titanium component from the Bayer process residue by sulfuric acid leaching. Alumina concentrate with 69.5% Al₂O₃ and mass ratio of alumina to silica (A/S) of 5.9, pure 1.1 nm tobermorite whisker and TiO₂-rich material containing 33% TiO₂ are produced, respectively, with the optimal parameters. Besides, the actual alumina digestion ratio of alumina concentrate reaches 80.4% at 270°C for 40 min in the Bayer process.

Key words: coal gangue; recycle; alumina; titanium dioxide; tobermorite

1 Introduction

During the past decades, most of coal resources have been exploited merely as fuel, despite the association of many coal-derived minerals with coal measure strata [1-2], which are primarily composed of kaolin, bentonite, quartzite, diatomite, refractory clay, etc. [3-5]. Specifically, coal gangue, as the mixture of ore- bearing rocks, mainly consists of claystone, carbonatite and aluminous rocks. However, coal gangue has always been regarded just as useless waste, and is facing with two possible endings: accumulation on the ground or keeping in the ground. Generally, the direct disposal of coal gangue requires vast areas of land and is costly and environmentally contaminative [6-7].

Actually, coal gangue is an important secondary resource because of its abundance of elements. The most abundant elements are aluminum, silicon, iron, magnesium, sodium, potassium and calcium. Besides, some minor/trace elements such as V, Ga, Sc, Hf, Nb, Sr and rare earths may also be present [8].

Recently, many researchers and engineers are trying to exploit the coal gangue discharged by the coal preparation plants for improving economic value and reducing environmental risks. Coal gangue with calorific value can be used directly as fuel to generate heat, steam and electricity. Low-carbon coal gangue is used as the filling for the mining subsidence, the base and subbase of highways and railways. The coal gangue is also used as building materials and fertilizer [9-10]. Besides, coal gangue contains a large number of aluminum-silicate minerals, which are advantageous to the quality of brick and cement [11-12]. Ammonium sulfate is also produced from coal gangue with high pyrite content by a roasting-soaking-neutralization process. High Al-containing coal gangue can be used to produce aluminum chloride, aluminum sulfate or alumina [13-14]. Obviously, there are limited techniques to comprehensively utilize the coal gangue presently. Lots of valuable constituents have not yet been recycled due to lack of effective measures.

The aim of this work is to verify the feasibility of comprehensive recovery of valuable components from coal gangue. An innovative technological route was proposed for recycling aluminum, silicon, and titanium in stages. Moreover, the main parameters of the process were investigated by using multiple techniques such as X-ray fluorescence (XRF), atomic absorption spectrometry (AAS), X-ray diffraction (XRD) and scanning electron microscope (SEM).

2 Experimental

2.1 Materials

2.1.1 Coal gangue

The coal gangue was taken from Guangxi district, China. Preliminary decarbonization and desulfurization were carried out by flotation of coal and pyrite. The particle size of gangue is 80% passing 74 μm. According to the XRD result, the coal gangue mainly consists of diaspore and clay minerals including illite- montmorillonite, kaolinite, and pyrophyllite. Its chemical composition is listed in Table 1. This gangue is characterized by high contents of alumina (46.2%) and silica (28.3%). Besides, the gangue contains 5% TiO₂ in the form of anatase. Pyrite is the main host mineral of iron and sulfur.

2.1.2 Reagents

Analytical grade chemicals including caustic soda, aluminum hydroxide, sodium chloride, lime and sulfuric acid were used in this work.

Table 1 Chemical composition of the coal gangue (mass fraction, %)

2.2 Methods

2.2.1 Experimental procedure

The experimental flowsheet is presented in Fig. 1. Thermochemical activation (TCA) process followed by alkali leaching was adopted in desilication of the gangue firstly. Then, the alumina was extracted from the alumina concentrate via the Bayer process, while alkali liquor containing silicate was used for synthesis of 1.1 nm tobermorite with simultaneous recycling of excessive caustic alkali liquor. Finally, enrichment of TiO₂ from red mud by sulfuric acid leaching was considered.

2.2.2 Desilication

The coal gangue was granulated into pellets with diameters of 4-8 mm in a disc pelletizer (d 1000 mm) and then dried at 110 °C for 4 h. Activation roasting was carried out in a lab-scale rotary kiln (d 300 mm×1000 mm) and rotating at 2 r/min. At given temperature, the dried pellets were loaded into the kiln and roasted isothermally for a given time period. The cooled pellets were ground to powders with a particle size of 90% passing a 74 μm-size standard sieve for the subsequent alkali leaching.

Alkali leaching was conducted in a DY-8 autoclave. At the beginning of each trail, caustic soda solution and activated minerals were added to the pots in the autoclave. The sealed pots were soaked in the bath and leached at 140°C for 30 min with 140 g/L NaOH liquor,and agitated at 20 r/min. Filtration was performed immediately after leaching, and alumina concentrate and alkali liquor containing silicate were collected, respectively.

Fig. 1 Experimental flowsheet of comprehensive utilization of coal gangue

2.2.3 Alumina extraction

For experiments, the spent liquor with Na₂O_k 250 g/L, Al₂O₃ 118 g/L and the caustic molar ratio (α_k) of 3.5 was mixed [15] and transferred to a rotating stirred autoclave. It was isothermally digested at uniform temperatures for a given time period, and the stirring speed was fixed at 150 r/min. After digestion, red mud was separated by filtration and dried.

2.2.4 Tobermorite whisker preparation

Alkali liquor containing silicate was mixed with the lime milk at the molar ratio of Ca/Si=1.0, and sodium chloride was added into the suspension at the molar ratio of Ca/Cl=5.0 to facilitate the growth of whisker [16]. The mixed solutions were poured into the autoclave and heated at desired temperatures and time periods at the stirring speed of 150 r/min. After the hydrothermal reaction, the suspensions were filtrated and washed with distilled water, and then dried at 70°C.

2.2.5 TiO₂ enrichment

The red mud obtained from the Bayer process was leached with sulfuric acid solution. Specific amounts of red mud and acid were placed in a beaker. The mixture was heated to the desired temperature in the electro- thermostatic water bath and stirred throughout the reaction period. Finally, the suspension was filtered and washed with distilled water, and the filtered cake was dried at 110 °C for 4 h.

2.2.6 Instrumental techniques

The chemical compositions of solid powders were examined using an X-ray fluorescence spectrometer (XRF, PANalytical, Axios mAX, Netherlands), and the contents of elements in liquor were determined using an atomic absorption spectrometry (AAS, HITACHI 180/80, Japan).

The mineral constituents were determined by using an X-ray diffraction (XRD, RIGAKU, D/Max 2500, Japan) under the conditions of radiation: Cu K_α, tube current and voltage of 250 mA, 40 kV, scanning range of 10°-70°, 2θ step size of 0.02° and scanning speed of 8°/min.

The morphology of hydrothermal synthetic was characterized by using a scanning electron microscope (SEM, FEI, Quanta-200, Netherland). SEM images were recorded in backscatter electron modes operating in low vacuum mode at 66.66 Pa and 20 keV.

3 Results and discussion

3.1 Extraction alumina from coal gangue

3.1.1 Desilication via TCA process followed by alkali leaching

Flotation and TCA process have been used for removing silica and improving the A/S ratio of high silicon content of bauxite ores [17-19]. TCA process followed by alkali leaching is a very effective technology for desilication of bauxite ores containing multi- aluminosilicates. During oxidative roasting, the aluminosilicates, such as pyrophyllite and kaolinite, are capable of being thermally activated and transformed into amorphous SiO₂. Subsequently, amorphous SiO₂ is dissolved in the caustic soda liquor by alkali leaching.

The effects of roasting temperature and time on desilication of coal gangue are studied and the results are shown in Fig. 2. It can be seen that the desilication ratio of activated minerals improves as temperature increases, while the desilication ratio decreases at temperatures above 1150 °C. When the roasting time varies from 10 to 15 min, the desilication ratio improves as time is extended despite that the desilication index declines slowly with the time exceeding 15 min. There are many kinds of aluminosilicate minerals (illite-montmorillonite, kaolinite, and pyrophyllite) in the coal gangue, and the dissolution of silica in these aluminosilicates requires different temperatures for thermal activation [20-22].The roasting temperature of 1100-1150°C meets the demand of aluminosilicates activation in coal gangue. Under the roasting conditions of 1100-1150°C and 15-20 min, the desilication ratio is about 73%, and the recovery ratio of alumina in concentrate is nearly 98%.

Fig. 2 Effect of TCA conditions on desilication of coal gangue:

3.1.2 Extraction of alumina from concentrate by Bayer process

To check the feasibility of extraction of alumina from the concentrate after desilication, the digestion tests for alumina extraction are investigated by the Bayer process. The main composition of the alumina concentrate after roasting at 1150°C for 15 min is shown in Table 2. The alumina content and A/S of alumina concentrate are 69.5% and 5.9, respectively.

Table 2 Main chemical composition of alumina concentrate (mass fraction, %)

The alumina concentrate is digested at 240-270°C for 40 min without lime addition via the Bayer process. The results in Fig. 3 show that digestion temperature has a great impact on alumina digestion. The alumina digestion ratio of concentrate increases with digestion temperature. When the material is digested at 270°C for 40 min, the actual and relative alumina digestion ratios increase to 80.4% and 96.6%, respectively. Meanwhile, the A/S ratio of the red mud decreases to 1.2. It indicates that the alumina concentrate prepared by TCA process followed by alkali leaching has a good digestion in the Bayer process.

Fig. 3 Alumina extraction of concentrate by Bayer process (digestion time of 40 min)

3.2 Preparation of tobermorite whisker from alkali liquor containing silicate

A large amount of silicate exists in alkali liquor containing silicate and should be separated first for recycling the excessive caustic soda to desiliconize. Alkali liquor containing silicate is used for synthesis of 1.1 nm tobermorite (1.1 nm refers to the type of tobermorite whisker, not its size [23]) and recovery of the excessive caustic alkali liquor.

The composition of alkali liquor containing silicate is listed in Table 3. The liquor contains a lot of sodium ions and small amounts of aluminum ions. In hydrothermal synthesis of tobermorite, sodium ions accelerate the formation of crystallized calcium silicate hydrates, while aluminum ions inhibit the bonding of the bridging tetrahedral sheets in the structure of tobermorite, forming pure 1.1 nm tobermorite with a single tetrahedral sheet structure [24-25].

Table 3 Elements concentration of alkali liquor containing silicate (Unit: g/L))

The XRD patterns of the synthetic products are shown in Fig. 4. It can be seen from Fig. 4 that 1.1 nm tobermorite and clinotobermorite are the main crystalline phases at 200 °C. The presence of clinotobermorite is ascribed to the incomplete transformation of clinotobermorite to 1.1 nm tobermorite, while the formation of calcium carbonate is probably attributed to the reaction between remaining calcium hydroxide and CO₂ in air during drying. As the temperature increases, the diffraction peaks of calcium carbonate disappear gradually.

Fig. 4 XRD patterns of synthetic products

Furthermore, as seen in Fig. 4, pure 1.1 nm tobermorite crystals are formed at 240 °C for 6 h. The pectolite crystallizes and replaces the 1.1 nm tobermorite when the time is prolonged to 20 h. It is suggested that high hydrothermal treatment temperature and appropriate time are favorable to the synthesis of 1.1 nm tobermorite whiskers. Furthermore, Fig. 5 shows the SEM photographs of the synthetic products after treatments at 240 °C for 3 h and 6 h, respectively. It can be seen from Fig. 5 that the length and length to diameter (L/D) ratio of tobermorite whiskers improve significantly with increasing synthesis time, and 1.1 nm tobermorite whiskers have the shape of small and thin needles at the synthesis temperature of 240 °C and time of 6 h. They are 5-10 μm in length, 0.2-0.5 μm in diameter, and 20-50 in L/D ratio.

Fig. 5 SEM photographs of synthetic products:

3.3 Enrichment of TiO₂ from alumina red mud

The main chemical composition of red mud, derived from digestion of alumina concentrate via the Bayer process at 270°C for 40 min is presented in Table 4. It shows that the titanium dioxide content is 12.3%.

Table 4 Main chemical composition of red mud (mass fraction, %)

Figure 6 shows the effect of H₂SO₄ concentration on leaching of red mud when leaching is carried out at 30 ^oC for 10 min, liquid-solid ratio of 10:1, and stirring speed of 200 r/min. It can be observed from Fig. 6 that the dissolution ratios of Al₂O₃ and SiO₂ reach about 85% and 92%, respectively, when the H₂SO₄ concentration exceeds 10%. This is because the sodiumaluminosilicates (Na₂O·Al₂O₃·nSiO₂·mH₂O), which mainly consist of host sodium, aluminum and silicon in red mud, can be leached with diluted sulfuric acid under atmospheric conditions without any pretreatment [26-27]. Furthermore, the TiO₂ content increases with concentration of diluted sulfuric acid liquor. The content increases from 25% to 33% with the H₂SO₄ concentration from 5% to 30% (mass fraction). At the same time, the recovery of TiO₂ decreases from 96.8% to 91.9%.

Fig. 6 Effect of H₂SO₄ concentration on leaching of red mud

Figure 7 shows the effect of leaching temperature on leaching of red mud. The conditions are leaching with 20% H₂SO₄ liquor for 10 min, liquid-solid ratio of 10:1, and stirring speed of 200 r/min. Increasing temperature is beneficial to the leaching of red mud and to the dissolution of Fe₂O₃, Al₂O₃ and SiO₂. The TiO₂ content in the slag increases correspondingly. It can be seen that the TiO₂ content goes up to 35.4% when the temperature is 90 °C. However, the recovery of TiO₂ decreases from 94.4% at 30 °C to 74.5% at 90 °C. Thus high leaching temperature is unfavorable for the enrichment of TiO₂.

From the above results, the appropriate H₂SO₄ concentration is 10%-20%, and the leaching temperature of 30 °C is recommended. As to leaching of Al₂O₃ and SiO₂, the dissociation ratios of Al₂O₃ and SiO₂ reach 85.4% and 96.5%, respectively, with 20% H₂SO₄ at 30 °C for 10 min. The Fe₂O₃ content varies from 31.3% to 41.6% in TiO₂-rich material, indicating difficult dissolution of Fe₂O₃ from red mud at low temperatures. Hence, the intensifying pyrite flotation processing plays an important role in concentrating TiO₂. The obtained TiO₂- rich material is worthy of further investigation to produce high titania-slag and marketable pig iron by a carbonthermal reduction or melting process.

Fig. 7 Effect of leaching temperature on leaching of red mud

4 Conclusions

1) Desilication of coal gangue is performed by TCA process followed by alkali leaching. An alumina concentrate with 69.5% Al₂O₃ and A/S of 5.9 is prepared from a gangue containing 46.2% Al₂O₃ and A/S of 1.6 under the conditions: roasting at 1150 °C for 15 min, and alkali leaching at 140 °C for 30 min with 140 g/L NaOH liquor. Subsequently, an actual alumina digestion ratio of 80.4% with the relative alumina digestion ratio of 96.6% is achieved after digestion by the Bayer process at 270°C for 40 min.

2) Tobermorite whisker is synthesized by hydrothermal processing from the alkali liquor containing silicate derived from the desilication stage. Pure 1.1 nm tobermorite whiskers (5-10 μm in length, 0.2-0.5 μm in diameter, 20-50 L/D ratio) are obtained at 240°C for 6 h.

3) TiO₂ is enriched by sulfuric acid leaching of alumina red mud. The TiO₂ content is nearly 33% in the product of TiO₂-rich material and its recovery is approximately 95% when leached at 30 °C for 10 min with 10%-20% sulfuric acid liquor.

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(Edited by FANG Jing-hua)

Foundation item: Projects(51234008, 51174230) supported by the National Natural Science Foundation of China; Project(NCET-11-0515) supported by the Program for New Century Excellent Talents in University, China; Project supported by Co-Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources, China

Received date: 2015-06-16; Accepted date: 2015-11-19

Corresponding author: LI Guang-hui, Professor, PhD; Tel/Fax: +86-731-88830542; E-mail: liguangh@csu.edu.cn