EXTRACTION OF Al – Si MASTER ALLOY AND ALUMINA FROM COAL FLY ASH

Coal fly ash from coal power plants is a potential raw material for the production of alumina. An objective aluminothermic reduction method for the preparation of Al-Si master alloy and alumina from coal fly ash was investigated. The kinetic analysis using non-isothermal differential scanning calorimetry indicated that the reduction of Al6Si2O13, Fe2O3, and TiO2 by aluminum in coal fly ash occurs at 1618 K, 1681 K, and 1754 K, respectively. Moreover, the influence of reaction temperature on product composition was studied. The phases and morphologies of the products obtained by the aluminothermic reduction of coal fly ash at 1373-1773 K were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy, respectively. The results from X-ray diffraction show that no oxide reduction has taken place at 1373 K and 1473K, the compositions of the product obtained by aluminothermic reduction of fly ash at 1573K1673 K are Al2O3, mullite, Al and Si, while the compositions of the product at 1773 K are Al2O3, Al, and Si. In addition, the chemical compositions of Al-Si alloy obtained at 1773 K are 86.81 wt% Al and 13.19 wt% Si.


Introduction
With the rapid development of the energy and power industry, the coal power plants of China are spewing out huge amounts of fly ash every year.The irregular accumulation and inappropriate disposal of coal fly ash will not only take up quantities of farmland but also highly pollute the environment.At present, the application of coal fly ash in the manufacturing of construction and road-building materials are the most widespread use of coal fly ash in China; however, the economic benefits of coal fly ash remain under-exploited.
On the other hand, with the expansion of alumina production and the excessive consumption of bauxite resources, especially in China, bauxite resources cannot meet the increasing aluminum industry demand.The coal fly ash produced by coal power plants in the northern Shanxi and Shaanxi provinces as well as in southern Inner Mongolia in China contain 40-45 % alumina; in contrast, bauxite, a naturally occurring alumina ore, contains about 30-60 % alumina [1,2].Therefore, it is imperative to extract alumina from coal fly ash to use this waste material and develop a new aluminum source.To date, a large number of laboratory studies have been performed on the extraction of alumina from coal fly ash and complicated oxides similar to coal fly ash; these methods can be classified into acidic methods (including sulfuric acid, hydrochloric and nitric acid), alkali sintering methods, acid-alkali methods, and electrochemical reduction methods [3][4][5][6][7][8].
Alkali sintering method has been used for alumina extraction from coal fly ash in Chinese chemical plant in recent years.However, the energy consumption and production cost would be markedly reduced if alumina can be acquired from coal fly ash via a short process.In recent years, numerous experiments have established that metals and alloys such as Si [9][10][11], Mg [12], Al-Ti-Fe alloy [13,14], and V-Ti-Fe alloy [15,16] can be obtained from their metal oxides via aluminothermic reduction.However, few studies have focused on the extraction of metals from the mixed oxides of coal fly ash via aluminothermic reduction.In the present paper, an alternative method for the extraction of alumina by the aluminothermic reduction of coal fly ash is proposed (Fig. 1); after mechanical separation from aluminum-based alloys, the alumina-rich residue containing a few amount of CaO and K 2 O can be utilized for aluminum production using the Hall-Héroult process.In the present study, the thermodynamic feasibility and kinetic mechanism of the aluminothermic reduction of coal fly ash were studied, and the influence of reaction temperature on product composition was investigated.

Experiment 2.1 Materials
The raw coal fly ash material used in this investigation was obtained from the Datang International Recycling Resource Development Company Limited (Inner Mongolia, China).The chemical composition of the coal fly ash was analyzed by X-ray fluorescence (XRF; ZSX 100e; Rigaku Corporation, Tokyo, Japan), and the major oxides of the coal fly ash are Al 2 O 3 (52.75wt%), SiO 2 (38.05 wt%), CaO (3.88 wt%), TiO 2 (1.97 wt%), Fe 2 O 3 (1.86 wt%), and K 2 O (0.51 wt%).The mineralogy of the coal fly ash material was analyzed by X-ray diffraction (XRD; PW3040/60; PANalytical, Almelo, The Netherlands) operated with Cu-Kα radiation generated at a pip voltage of 40 kV and a current of 100 mA.As shown in Fig. 2, the main phases of the coal fly ash are mullite (Al 6 Si 2 O 13 ) and alumina.
As coal fly ash is compounds of Al 2 O 3 , SiO 2 , TiO 2 , and Fe 2 O 3 , the process of coal fly ash reduction by aluminum is complicated.Therefore, it is necessary to investigate the aluminum reduction of single oxide (SiO 2 , TiO 2 , and Fe 2 O 3 ).Silica (SiO 2 , 99.9% pure, size 100 mesh), iron oxide (Fe 2 O 3 , 99.9% pure, size 100 mesh), titanium dioxide (TiO 2 , 99.9% pure, size 100 mesh), and aluminum powder as reducing agent (grade-C, 99.9% pure, size 100 mesh) were purchased from Sinopharm Chemical Reagent Company Limited (Shanghai, China).The above oxides and coal fly ash were heated at 673 K for 24 h to drive off moisture and other volatile impurities.The dehydrated oxides were then used as materials for aluminothermic reduction.Aluminum block (99.9% pure) was obtained from the Fushun aluminum smelter of China.

Experimental process
Differential scanning calorimetry (DSC) was performed on an STA 409 CD simultaneous thermal analyzer (Netzsch, Germany) with the sample and reference crucibles made of α-Al 2 O 3 .The temperature was calibrated using copper [17].The reductant Al powder was mixed thoroughly with SiO 2 , Fe 2 O 3 , TiO 2 , and coal fly ash by ball milling.Considering that Al is oxidized instantly, the powders were mixed inside a glove box filled with inert argon (99.999 % pure).To accelerate the process of aluminothermic reduction, excess reductant Al powder was used; the ratio of actual Al dosage to theoretical Al dosage was three.Reactant mixture samples with masses between 3 mg and 10 mg were placed into crucibles with hermetic lids and then placed into the DSC cell using an auto sampler.The reference sample was an empty hermetically sealed α-Al 2 O 3 crucible.Heating was performed from room temperature to 1773 K at a heating rate of 10 K/min under argon flow (30 mL/min, argon purity of 99.999 %).Cooling was conducted at 20 K/min.All records were corrected by the subtraction of a baseline determined from a run without pans under the same conditions.
The reactant mixture composed of coal fly ash and reductant Al powder were mixed thoroughly and then poured into a hermetically sealed α-Al 2 O 3 crucible.The reactant mixture was then rammed mildly to improve the solid-to-solid contact.The α-Al 2 O 3 crucible was then placed inside a stainless-steel crucible with continuous argon circulation during the experiment.As seen in Fig. 3, the aluminothermic reduction process was performed in a MoSi 2 resistance furnace for 30 min at temperatures of 1373 K, 1473 K, 1573 K, 1673 K, and 1773 K.After natural cooling, the products were characterized by XRD, scanning electron microscopy (SEM; SSX-550; Shimadzu corp., Kyoto, Japan) and energy-dispersive spectrometer (EDS, Shimadzu corp.) to identify the phases and microscopic morphology.
The standard Gibbs free energy changes of the above equations were calculated by FactSage 6.4 and shown in Fig. 4. From thermodynamic point of view, it is thermodynamically desirable for a chemical reaction with a large negative change in standard Gibbs free energy to proceed autogenously.The standard Gibbs free energy changes associated with equations ( 2)-( 8) are negative, indicating that from the thermodynamic point of view, these reactions occur spontaneously at temperatures ranging from 1373 K to 1773 K; in contrast, the reaction between CaO and Al is not thermodynamically favorable.In addition, Fig. 4 suggests that the reduction of TiO 2 to Ti is more difficult than the reduction of Fe 2 O 3 to Fe because the standard Gibbs free energy changes associated with equations ( 4) are more negative.Moreover, it was reported that Si and Ti could be prepared from their oxides via aluminothermic reduction in molten salts such as NaCl-KCl-AlCl 3 [9] and NaF-AlF 3 [11,18].According to the negative standard Gibbs free energy changes associated with equations ( 5) and ( 8), the reduced silicon and titanium can dissolve in molten aluminum and form alloys such as Al 3 Ti and Al 3 Fe.Thus, Al can be used as a reducing agent to extract metals (Si, Fe, and Ti) and enrich Al 2 O 3 from coal fly ash using suitable reactant combinations and temperatures.
In addition, Ellingham diagram of metal oxides involved in the system of coal fly ash can be seen in

Kinetic analysis
The DSC curves of the Al-coal fly ash system, Al-Fe 2 O 3 system, Al-SiO 2 system, and Al-TiO 2 system at heating rate 10 K/min are shown in Fig. 6.The endothermic signal at approximately 933 K (660 o C) is obvious attributed to the melting of aluminum.The exothermic peaks at 1628 K (Fig. 6, peak F), 1493 K (Fig. 6, peak D), and 1583 K (Fig. 6, peak E) are believed to be associated with the aluminothermic reduction of Fe 2 O 3 , SiO 2 , and TiO 2 , respectively.
More interestingly, three exothermic signals were observed in the DSC curve of the Al-coal fly ash system (Fig. 6a).To identify the reactions corresponding to the exothermic peaks of Fig. 6a, the peak area was evaluated with the reaction enthalpy.A comparison of the standard reaction enthalpies (Fig. 7) shows that the standard reaction enthalpy of the aluminothermic reduction of TiO 2 is slightly smaller than that of Al 6 Si 2 O 13 , while the standard reaction enthalpy of the aluminothermic reduction of Fe 2 O 3 is almost three times larger than that of Al 6 Si 2 O 13 .Moreover, through comparison of the standard Gibbs free energy changes of equation ( 2) with ( 5) and ( 8), the reduction of TiO 2 to Al 3 Ti is more difficult than the reduction of Fe 2 O 3 to Al 3 Fe, while the reduction of Fe 2 O 3 to Al 3 Fe is more difficult than the reduction of Al 6 Si 2 O 13 to Si.Therefore, the reactions responsible for the exothermic signals peak A (1618 K), peak B (1681 K), and peak C (1754 K) can be expressed as Eq. ( 2), Eq. ( 5), and Eq. ( 8), respectively.Compared to the Al-Fe 2 O 3 , Al-SiO 2 , and Al-TiO 2 systems, the initial temperatures of the corresponding reactions in the Al -coal fly ash system are higher, and the areas of the exothermic peaks are smaller.These phenomena are primarily attributed to the fact that the stability of the SiO 2 component in Al 6 Si 2 O 13 is higher than that in SiO 2 .Therefore, a higher temperature is required for the chemical reactions involved in the aluminothermic reduction of coal fly ash than for those involved between Al and single oxides.Compared to the reduction of pure Fe 2 O 3 (s) and TiO 2 (s), the order of exothermic peak temperatures of Fe 2 O 3 (s) and TiO 2 (s) in coal fly ash is changed, which is primarily attributed to the fact that the reduction of TiO 2 to Al 3 Ti is more difficult than the reduction of Fe 2 O 3 to Al 3 Fe.In addition, the initial reaction temperature of the reduction of pure Fe 2 O 3 (s) is lower than that of pure TiO 2 (s).
The kinetic analysis of the DSC curve is based on a combination of a formal kinetic equation and the empiric Arrhenius expression [19], and it is universally described as (9) where α = ΔH(t)/ΔH is the fraction of conversion at time t, ΔH(t) is the partial heat of reaction at time t, ΔH is the total heat of reaction, A is the frequency factor, n is the order of reaction, E is the activation energy of the reaction, and R is the gas constant.By inserting the constant heating rate, β = dT/dt, into equation ( 9), equation ( 9) can be expressed as (10) Using the integral method developed by Freeman and Carroll [20], the reaction order n and the activation energy E can be determined from the equation  1.
The obtained kinetic parameters are E = 89.38 kJ/mol and n = 2.4 for equation ( 2), E = 73.77kJ/mol and n = 1.4 for equation (5), and E = 380.16kJ/mol and n = 0.9 for equation (8).These apparent activation energies were huge, indicating that aluminothermic reactions between coal fly ash and aluminum were mainly controlled by chemical reaction.Moreover, the reaction enthalpy of the reaction associated with Peak B (Fig. 6) is about 2.26 times larger than that of the reaction associated with Peak C (Fig. 6); this finding is in agreement with the results shown in Fig. 7 along with the associated discussion.

Influence of Reaction Temperature on Product Composition
The phases of initial mixture of Al and coal fly ash before heat treat, and the products obtained by the aluminothermic reduction of coal fly ash were identified by XRD (Fig. 11).The XRD patterns of the powder blends isothermally heated at 1373 K and 1473 K for 30 min are nearly identical to that of the as-blended sample.This implies that in this temperature range, the phase structure was largely retained, and no oxide reduction took place (Figs.11a  and b).On the other hand, silicon was detected for the first time in the XRD pattern of the powder sample isothermally heated at 1573 K (Fig. 11c), indicating that an exchange of oxygen atoms between Si and Al occurred (i.e.mullite was reduced by Al at this temperature).According to Al-Si binary diagram [21], the maximum solubility of Si in Al occurs at the eutectic temperature (850.15K) and is 12.6 wt%.When the percent of silicon in Al-Si alloy ranges between 0.1 wt% and 12.6 wt%, Al-Si alloy can be produced at temperature of 1573 K-1773 K.The formation of Al-Si alloy can be expressed as Eq.(12).
Al (l) + Si (l) = Al-Si (l) (12) The peak of Al 2 O 3 at 1773 K (Fig. 11e) is stronger than that at 1673 K (Fig. 11d), while the peak of Al 6 Si 2 O 13 at 1673 K (Fig. 11d) is weaker than that at 1573 K (Fig. 11c).Moreover, no Al 6 Si 2 O 13 peaks were observed in the XRD pattern of the powder sample  isothermally heated at 1773 K, which suggests that Al 6 Si 2 O 13 is completely reduced at this temperature (Fig. 11e).Therefore, as the heating temperature increases, the coal fly ash and Al powder react more thoroughly.In addition, no peaks were observed for Fe or Ti or their compounds, possibly due to their low contents in the alloys.
SEM and EDS were used to examine the general distribution of elements in the Al and coal fly ash powders blends that were isothermally heat treated at 1373-1773 K for 30 min (Fig. 12 and Table 2).The EDS results of points No. 1 (Fig. 12a) and No. 2 (Fig. 12b) taken from the white areas indicate that no individual metals or alloys were produced in this temperature range.These results agree well with the XRD results.The small gray particles (points Nos. 5 and 6) are Al 2 O 3 , and the excess is aluminum.The composition at point No. 4 (Fig. 12d) is 91.91 wt% Al and 8.09 wt% Si, while that at point No. 7 (Fig. 12e) is 86.81 wt% Al and 13.19 wt% Si.The content of Si increases with increasing temperature.Therefore, it is reasonable to induce that the higher the temperature is, the more completely the aluminothermic reduction reaction proceeds.

The comparison of different alumina exctraction technologies
During the process of coal fly reduction by aluminum, the alumina in the raw fly ash does not react with aluminum, while alumina-rich oxides can be produced by aluminothermic reduction of other oxides in fly ash (as shown in Eqs. ( 2)-( 8)).After  As for the energy consumption of the process of coal fly reduction by aluminum, the electric power consumption of the MoSi 2 resistance furnace (1773K) that heated aluminum and coal fly ash from room temperature (298K) to experimental temperature (1773K) should be considered.In order to simplify the calculation, mullite (Al 6 Si 2 O 13 ) was used to substitute complex coal fly.According to Kirchhoff's Law, the enthalpy change ΔH can be expressed as Eq. ( 13) Where a, b, and c are thermodynamic temperature coefficient, as shown in Table 3; T is the final temperature of heating.
Taking all parameters into Eq.( 13), the calculated ΔH of heating aluminum and coal fly ash from 298 K to 1773 K are 56.36 kJ/mol and 747.06 kJ/mol, respectively.In order to produce 1t alumina (refer to Eq. ( 2)), the required aluminum and mullite are 603.55mol and 226.33 mol, respectively.In addition, aluminum was used as raw materials.The electrolytic reaction of aluminum production can be expressed as Eq. ( 14), and the enthalpy change ΔH equal to 549.42 kJ/mol (1223 K, endothermic).Therefore, the total energy consumption is 5.35×10 5 kJ/t Al 2 O 3 .According to this analysis, the theoretical energy consumption during the aluminothermic process is lower than that in the lime-soda sinter process and sinter-acid leach combination process, as shown in Table 4.

Figure 1 .
Figure 1.Flow chart of metal and oxygen extraction from mixed oxides by aluminothermic reduction and molten salt electrolysis

Figure 2 . 3 .
Figure 2. XRD pattern of coal fly ash material used in present study

Fig. 5 .
The position of the line for a given reaction on the Ellingham diagram shows the stability of the oxide as a function of temperature.Reactions closer to

Figure 3 .Figure 4 .Figure 5 .
Figure 3. Schematic diagram of the MoSi 2 resistance furnace used for aluminothermic reduction of fly ash

ZFigure 8 .Figure 9 .Figure 10 .
Figure 8. Freeman-Carroll curve of Al and coal fly ash system at exothermic peak A

Table 1 .
Kinetic parameters calculated by Freeman-Carroll method

Table 2 .
EDS mapping analysis of samples obtained by aluminothermic reduction of coal fly ash

Table 3 .
2 , Fe 2 O 3 , and TiO 2 at 1373-1773 K are negative, which predicts that the reduction of coal fly ash by aluminum is thermodynamically feasible.The reduction of Al 6 Si 2 O 13 , Fe 2 O 3 , and TiO 2 by aluminum in coal fly ash occurs at 1618 K, 1681 K, and 1754 K.The components of the product obtained by the aluminothermic reduction of fly ash at 1573 K are Al 2 O 3 , Al 6 Si 2 O 13 , Al, and Si, while the components of the product obtained at 1773 K are Al 2 O 3 , Al, and Si.The composition of the alloy obtained at 1673 K is 91.91 wt% Al and 8.09 wt% Si, while the composition obtained at 1773 K is 86.81 wt% Al and 13.19 wt% Si.With increasing heating temperature, the coal fly ash and Al powder react more thoroughly.The calculation datasheet of ΔH of aluminum and mullite

Table 4 .
The comparison of different alumina recovery technologies