Computer thermodynamic modeling of the producing silicon alloys from the Kentau Combined Heat and Power Plant ash

. The article presents the results of studies on the production of silicon alloys from the Kentau Combined Heat and Power Plant ash by the computer thermodynamic modeling method using the HSC-6.0-Chemistry software package. The effect of temperature (from 500 0 C to 2000 0 C) and the amount of carbon (from 30% to 40% of the ash mass) on the equilibrium distribution degree of elements between the condensed and gas phases, the grade of resulting ferroalloys. In the system under consideration, the formation of FeSi25 ferrosilicon occurs in the temperature range of 1500-1530 0 C, and FeSi45 ferrosilicon-at the temperature of 1610-1700 0 C; FS50 ferrosilicon is formed in the temperature range of 1700-1900 0 C in the presence of 30% of carbon. The formation of FeSi45A10 ferrosilicoaluminium is possible at 2000 0 C and 30% of carbon, in this case the concentrations of the metals in the alloy are 46.51% of Si, 8.57% of Al; 42.27% of Fe. FeSi45A15 ferrosilicoaluminium is formed in the system at 40% of carbon and 2000 0 C; the elements’ conc entrations in the alloy are 45.62% of Si, 14.76% of Al, 37.29% of Fe. Processing of 1 tonne of the ash makes it possible to produce 459 kg of FeSi45A10 ferrosilicon


Introduction
Traditional technological production processes are associated with the formation of industrial waste, which is placed on the territory in volumes exceeding the natural ability of the environment for self-purification and self-production.The main polluters of the natural environment are enterprises of non-ferrous metallurgy, heat power engineering, mechanical engineering and metalworking [1][2][3][4][5][6].
The problem of neutralization and disposal of industrial waste is one of the most pressing in the city of Kentau, where the most large-tonnage wastes are JSC "Achpolimetal" polymetallic ores' concentration tailings and Combined Heat and Power Plant (CHPP) ash and slag waste, which account for 135 and 405 million tonnes, respectively [7] .
To date, scientific and technical developments are known for the use of ash as the main component or an additive in the production of building bricks, binders, thermal insulation materials, glass, glass-crystalline materials, agloporite, etc. [8][9][10][11][12].At the same time, the ash utilization degree in Kazakhstan reaches only 2% of the annual output.
The main components in thermal power plant ashes are SiO2 and Al2O3.Therefore, it is of interest to analyze the literature data on the production of commercial products using raw materials containing predominantly SiO2 and Al2O3.
A technology for producing a ferroalloy and a slag containing residual aluminium oxide (a raw material for the production of alumina) from the ash formed at thermal power plants has been developed by scientists of KazNTU (Nurkeyev S.S.).The experiments were carried out in a Tamman furnace at 1650-1800 0 C. Conducted studies on the recovery of the Ekibastuz coals' slag have shown the possibility of obtaining conditioned ferrosilicon and iron-free slag, suitable for producing high-quality grades of coagulant and alumina [18].
Researchers of Zh.Abishev Chemical Metallurgical Institute (Baisanov S., Tolymbekov M. et al.) proposed a method for producing ferrosilicoaluminium in an ore-thermal furnace using a carbonaceous rock containing 15-35 wt.% of carbon as a silicon-aluminiumcontaining material.During the smelting process, the quality of carbon in the charge, loaded into the furnace, was maintained as the stoichiometrically required amount within the range of 3-12% by adding coke or quartzite to the carbonaceous rock as necessary.The invention makes it possible to increase the technical and economic indicators of ferrosilicoaluminium production by stabilizing the production process of silicon-aluminium alloys containing from 5 to 35% of aluminium and improving their quality due to the elimination of the carbide formation process and decrease of friability of the alloys [19,20].
Russian scientists (Zhuchkov V.I., Marshuk L.A., etc.) has proposed to use a charge to produce ferrosilicoaluminium, which contains aluminium-containing slag of the secondary aluminium production as a silica-alumina-containing material and a reducing agent, and lime as a slag-forming agent, with the following ratio of components, wt.%: aluminium slag -47.0-55.0,lime -22.5-26.5, scrap steel -22.5-26.5.The given invention makes it possible to develop a new charge composition allowing us to process cheap non-traditional materials to obtain an alloy and slag with a given composition [21].
In Russia (Gusarov N.), ferrosilicoaluminium was obtained [22] by melting an aluminosilicate charge in an electric furnace; the aluminosilicate charge was preliminarily heated to reduce Al2O3 and SiO2 to a total formation of 10% of aluminium and silicon carbides and oxycarbides.The disadvantage of this method is the absence of carbon in the aluminosilicate charge; it impairs the gas permeability of the charge due to the absence of capillaries in the form of carbon veins.The result of this is the formation of "fistulas" (the breakthrough of high-pressure hot gases through the charge) and the intensification of the process of formation of gaseous suboxides and aluminium and their intensive removal through the "fistulas".These processes lead to disruption of the furnace operation stability and a decrease in the technical and economic indicators of the process.In addition, the amount of the reducing agent (coke) proposed in the method is greater than the amount stoichiometrically required for the preliminary formation of aluminium and silicon carbides and oxycarbides.An excess amount of coke leads to the almost complete reduction of SiO2, Al2O3 and formation of silicon and aluminium carbides in the metal.The presence of these carbides in the metal leads to their accumulation in the furnace, disruption of the process, and increase in the alloy friability, which deteriorates its quality.
The presented material shows that one of the directions for processing of raw materials containing silicon and aluminium oxides is the production of ferroalloys.
The purpose of the work is to thermodynamically predict the possibility of producing a silicon-containing ferroalloy from the Kentau CHPP ash.

Research methods
The studies were carried out by computer thermodynamic modeling using the HSC-6.0software package [23][24][25], based on the fundamental principle of minimizing the Gibbs energy.The method makes it possible to determine the influence of temperature, pressure, and the ratio of the initial components on the equilibrium distribution of elements in the system between the initial and final components.The HSC-6.0 software package presents primary information about the process under study in the form of a quantitative (kg) distribution of elements among substances.In the work, we used the calculation algorithm developed at M. Auezov SKSU [26] to determine the equilibrium distribution degree of elements among interaction products in %.
The object of the study was ash formed at the Kentau CHPP, which runs on the Ekibastuz deposit coal containing up to 63% of mineral components [27].The chemical composition of the ash formed at burning the Ekibastuz coal is given in Table 1.During the research, the amount of iron was constant and amounted to 15% of the ash mass.The parameters studied were the effect of temperature (in the range from 500 0 C to 2000 0 C) and the amount of carbon (from 30% to 40% of the ash mass) on the elements' equilibrium distribution degree and the composition of the resulting alloy.

Results and discussion
Figure 1 shows the primary information on the quantitative (kg) distribution of siliconcontaining substances at 30% and 40% of carbon.It can be seen that the main siliconcontaining substances in the systems are CaSiO3, MgSiO3, Na2SiO3, FeSi, Si, SiO(g), FeSi2, TiSi.The formation of FeSi begins at 1200 0 C, Si and TiSiat 1400 0 C, FeSi2 and SiO(g)at 1500 0 C. Significant formation of FeSi and Si occurs at temperatures above 1800 0 C. Undesirable noticeable formation of gaseous SiO, which leads to loss of silicon, occurs at temperatures above 1700 0 C.Moreover, with an increase in the amount of carbon in the charge, the loss of silicon in the form of SiO increases.An increase in carbon in the charge has virtually no effect on the amount of FeSi formed, while the amount of silicon formed increases, for example at 1900 0 C, from 10.6 to 12.1 kg (i.e. by 14.1%).The temperature influence on the equilibrium distribution degrees (%) of silicon, iron and aluminium in the system containing 40% of carbon is presented in Figure 2.With increasing the temperature, the transition degrees of silicon into elemental silicon and FeSi increase and reach 50% and 30.22%, respectively, at 1900 0 C. When the temperature grows the transition degree of iron into iron silicide in the form of FeSi increases and reaches a maximum (90.21%) at 1700 0 C, then it decreases to 89.1% at 2000 0 C. Iron to a small extent goes into FeSi2; its maximum equilibrium distribution degree is 1.304% (1700 0 C).
At 1700 0 C, 0.87% of elemental aluminium is formed.Then, with an increase in temperature to 2000 0 C, the amount of aluminium formed increases to 70.19%.
Figure 3 shows the effect of temperature on the total transition degree of silicon and aluminium into ferroalloy at a carbon content of 30, 36, and 40%.It can be seen that the transition of silicon from the ash to the alloy in the temperature range of 1600-2000 0 C increases with increasing carbon in the charge.The maximum total transition degree of silicon into the alloy (80.17%) occurs at 1900 0 C and 40% of carbon (Fig. 3A).The transition of aluminium into the ferroalloy increases with increasing carbon in the charge and increasing temperature from 1700 0 C to 2000 0 C (Figure 3B).
Table 2 and Figure 4 show the effect of temperature on the metals' concentrations in the alloy.From Table 2 and Figure 4A it is clear that the concentration of silicon in the alloy, depending on temperature and the amount of carbon, is 22.76% at 1500 0 C and 30% of carbon and 50.37% at 1900 0 C and 40% of carbon; up to 1900 0 C the silicon concentration in the alloy increases with increasing carbon in the charge from 30% to 40%.The concentration of aluminium in the alloy also increases with increasing temperature and amount of carbon (Figure 4 B).The maximum concentration of aluminium in the alloy at 2000 0 C is 8.57% (30% of carbon) and 14.76% (40% carbon).In the system under consideration, at From the information shown in Figure 4A it follows that FS25 ferrosilicon in the system under consideration is formed at 1500-1530 0 C, and FS45 gradeat 1610-1700 0 C. FS50 ferrosilicon is formed at 1700-1900 0 C and 30% of carbon [28].
The research has shown that, according to [29], in the system under study, at 40% of carbon and a temperature of 2000 0 C, ferrosilicoaluminium is formed: -FS45A15 grade containing 45.62% of Si, 14.76% of Al, and 37.29% of Fe; -FS45A10 grade (at 30% of carbon) containing 46.51% of Si, 8.57% of Al, and 42.27% of Fe.
Using the HSC-6.0software package, we calculated the material balance for the production of ferroalloy from the Kentau CHPP ash.Thus, under conditions of 30% of carbon and 15% of iron, 459 kg of FS45A10 ferrosilicoaluminium are formed from 1 tonne of the ash.

Conclusion
Based on the results obtained at the thermodynamic modeling of the producing siliconcontaining alloys from the Kentau CHPP ash, the following conclusions can be drawn: -the main silicon-containing substances in the systems under study are CaSiO3, MgSiO3, Na2SiO3, FeSi, Si, SiO(g), FeSi2, TiSi; -the effect of temperature (from 500 0 C to 2000 0 C) and the amount of carbon (from 30% to 40% of the ash mass) on the equilibrium distribution degrees of the elements between the condensed and gas phases, the grade of the resulting ferroalloys and the silicon and aluminium content in them was determined; -the transition of aluminium into the ferroalloy begins at 1700 0 C; with an increase in the temperature from 1700 0 C to 2000 0 C, the transition degree of aluminium into the ferroalloy increases from 0.87% to 70.19%;

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-FeSi begins to form at 1200 0 C; Si and TiSiat 1400 0 C; FeSi2 and SiO(g)at 1500 0 C. Significant formation of FeSi occurs at temperatures above 1800 0 C, siliconat temperatures above 1800 0 C; -undesirable noticeable formation of gaseous SiO, which leads to the loss of silicon, occurs at temperatures above 1700 0 C.Moreover, with an increase in the amount of carbon in the charge, the loss of silicon in the form of SiO increases; -an increase in the amount of carbon in the charge increases the transition degrees of the metals into the alloy: Siup to 80.17%, Alup to 70.19%; -in the systems under consideration, FS25 grade ferrosilicon is formed at 1500-1530 0 C, FS45 grade ferrosilicon is formed at 1610-1700 0 C; FS50 grade ferrosilicon is formed in the system at 30% of carbon and 1700-1900 0 C; -at 2000 0 C and 30% of carbon, a ferroalloy is formed, which is FS45A10 ferrosilicoaluminium; the metals' concentrations in the alloy are 46.51% of Si, 8.57% of Al, and 42.27% of Fe; -in a case of 2000 0 C and 40% of carbon it is possible to obtain FS45A15 ferrosilicoaluminium; the metals' concentrations in the alloy are 45.62% of Si, 14.76% of Al, and 37.29% of Fe; -459 kg of FS45A10 ferrosilicon can be produced from 1 tonne of the ash.

Fig. 1 .
Fig. 1.Effect of temperature and amount of carbon (A -30%, B -40%) on the equilibrium quantitative distribution of silicon-containing substances in the ashcarboniron system.

Fig. 2 .
Fig.2.Effect of temperature on the equilibrium distribution degree of substances containing silicon (A), iron (B) and aluminium (C) in the ash -nC -Fe system at 40% of C.

Fig. 3 .
Fig. 3.The effect of temperature and amount of carbon on the transition degree of silicon (A) and aluminium (B) from the ash to the alloy

4 .
0 C, due to the development of the silicon and aluminium reduction processes and their transition into the alloy, the concentration of iron in the alloy decreases with increasing the temperature.1 -FS45 ferrosilicon, 2 -FS25 ferrosilicon Fig.Effect of temperature and amount of carbon on the concentration of silicon (A), aluminium (B), and iron (C) in the alloy, %.

Table 1 .
Chemical composition of the Kentau CHPP ash.

Table 2 .
Effect of temperature on the silicon, aluminium and iron concentrations in the alloy (%) at 30% and 40% of carbon.