Theoretical Analysis

The ultimate goal of the project is to create a boron-containing aluminum-silicon ferroalloy for the steelmaking industry. The task at hand was to determine its optimal phase and chemical compositions. The common practice is to produce a complex alloy of ferrosilicoaluminum (FSA) from high-ash coal, due to its containing all the necessary components for alloy production: silicon oxide (SiO2), aluminum oxide (Al2O3), and carbon (C) acting as a reducing agent. Kazakhstan has large reserves of such coals.

In this study, the production of boron-containing ferrosilicoaluminum is planned. The market offers Kazakh borate ore from the Inder deposit, boron anhydride (B2O3), and high-quality Turkish borate ore (colemanite). It was decided to produce the new alloy from coals from the Kuu-Chek deposit and the Borly deposit with ash content of 47% and 55%, respectively.

The success of experimental research depends on establishing the correct temperature level of the process. At low temperatures, some elements may not be reduced, while at excessively high temperatures, they may transition into the gaseous phase. To find the optimal temperature level, a complete thermodynamic modeling of the studied process was performed in the temperature range of 300–3000 K using the “Terra” software complex. The calculation results for the production of the FS45A10B1 alloy are shown in Figs. 1 and 2. It can be seen that metal formation does not occur until 800 K. In the temperature range of 300–800 K, the interaction of free oxides in the charge leads to the appearance (Fig. 1) of dicalcium borate Ca2B2O5 (curve 6), calcium monoborate CaB2O4 (curve 3), magnesium borate MgB4O7 (4), and hercynite FeAl2O4 (curve 5). Beyond 800 K, the interaction of free iron (from the chips) and carbon from the coal leads to the appearance of iron carbide Fe3C (curve 3 in Fig. 2) according to reaction

Fig. 1.
figure 1

Slag-phase composition dynamics versus temperature: 1 — SiO2; 2 — Al2O3; 3 — CaB2O4; 4 — MgB4O7; 5 — FeAl2O4; 6 — Ca2B2O5; 7 — FeSiO3; 8 — B2O3, 9 — CaAl2O4.

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Fig. 2.
figure 2

Metal phase composition dynamics versus temperature: 1 — C; 2 — Fe; 3 — Fe3C; 4 — FeSi; 5 — Si; 6 — AlB12; 7 — CaSi2.

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$$3\mathrm{Fe}+\mathrm{C}={\mathrm{Fe}}_{3}\mathrm{C}.$$
(1)

In the temperature range of 1700–1800 K, there is a sharp decrease in the amount of Fe3C, SiO2, and carbon (C), and the appearance of FeSi. The possible cause of this is the reaction

$${\text{Fe}}_{3}{\text{C}}+5{\text{C}}+3{\text{SiO}}_{2}=3{\text{FeSi}}+6{\text{CO}}\left({\text{g}}\right)$$
(2)

the negative change in Gibbs energy of which starts at 1773 K and amounts to ∆G1773K = –17.025 kJ/mol (Fig. 3).

Fig. 3.
figure 3

Thermodynamic characteristics of FeSi formation.

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The two-calcium borate Ca2B2O5 (see curve 6, Fig. 1) and alumina (see curve 2, Fig. 1) are preserved in the charge until high temperatures. At a temperature slightly above 2000 K, there is a sharp and simultaneous decrease in their quantity in the charge, accompanied by the appearance of aluminum boride AlB12 in the metal (see curve 6, Fig. 2), and calcium monoaluminate CaAl2O4 in the slag (see curve 9, Fig. 1).

It can be assumed that this is a consequence of the reaction occurring:

$$12{\text{Ca}}_{2}{\text{B}}_{2}{\text{O}}5+25{\text{Al}}_{2}{\text{O}}_{3}+39{\text{C}}=2{\text{AlB}}_{12}+24{\text{CaO}}\cdot {\text{Al}}_{2}{\text{O}}_{3}+39{\text{CO}}\left({\text{g}}\right).$$
(3)

The negative value of the change in Gibbs energy is observed at 2073 K and amounts to ∆G1773K = –215.328 kJ/mol (Fig. 4).

Fig. 4.
figure 4

Thermodynamic characteristics of AlB12 formation.

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The obtained data allow us to conclude that in order to obtain metal of the desired composition, the temperature in the melting furnace should be maintained at a level of at least 2073 K (1800°C). Exceeding this temperature is undesirable due to the transition of the components of the charge into the gas phase (Fig. 5).

Fig. 5.
figure 5

Gas phase composition dynamics versus temperature: 1 — CO; 2 — SiO; 3 — Fe; 4 — P; 5 — Si; 6 — Al; 7 — B.

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However, in industrial furnaces, local overheating of the metal and slag (under electric arcs) cannot be avoided [1], and therefore it is necessary to work with a closed and high chimney to capture the fumes, and to use aspirated dust as an additive in the production of pellets and agglomerates, as it contains valuable components [2,3,4].

In the exhaust gases, CO dominates as a product of carbon reduction of oxides, but there are also losses of silicon, iron, aluminum, and boron in the form of silicon monoxide and gaseous elements.

Materials and Procedure

Based on the obtained thermodynamic models, large-scale laboratory tests were conducted on the smelting of boron-containing aluminum-silicon ferroalloy. The practice of smelting aluminum-silicon alloys is limited [5,6,7,8]. It has been established that the deficiency of the reducer in the charge should be 2–3%. Borly coal contains 17–19% volatiles, which hinder the access of oxygen in the air to the hearth during electric smelting, thereby mainly impeding the combustion of carbon [9,10,11].

For the production of the complex alloy FSAB, a ore-thermal electric furnace with a current-conducting bottom of 250 kVA power was used. The electric furnace is powered by the transformer TSE 250–380/50. The arc discharge temperature of 2500–4500°C is provided by a graphite electrode with a diameter of 200 mm. The furnace is lined with firebrick. The furnace bath is elliptical with axes of 75–90 cm, elongated towards the tapping side. The distance from the electrode to the tapping block is 25–29 cm, and to the back wall of the furnace is 41–45 cm. The depth of the bath is 85–90 cm. The furnace bottom is made of ramming mass that has been coked for 8 hours under current on a cushion of blast furnace coke with periodic furnace shutdown. The furnace transformer has 10 voltage levels, V: 20; 24.5; 28.1; 33; 40; 49; 56.2; 66 and 80.

The furnace’s electrical parameters were selected to conduct the technological process with high technical and economic indicators — 40 V and a load of 400 A on the high side [12,13,14]. In order to carry out the tests, the preparation of the charge materials was performed in accordance with the technological requirements. The experimental batch of charge materials underwent technical operations for sampling for chemical analysis through the use of triple-quenching and mixing.

Chemical and technical compositions of the selected charge materials:

– High-ash coal from the Borly deposit, fractions 10–60 mm, with a technical composition: C — 55.04%; V — 19.03%; W — 0.6%; S — 25.66% and mineral components of ash: SiO2 — 57.8%; Fe — 4.35%; Al2O3 — 29.07%; CaO — 1.4%; MgO — 3.56%;

– High-ash coal from the Kuu-Chek deposit, fractions 10–60 mm, with a technical composition: C — 35.12%; V — 20.8%; W — 0.7%; S — 44.08% and mineral components of ash: SiO2 — 64.4%; Fe — 2.65%; Al2O3 — 29.0%; CaO — 1.5%; MgO — 1.9%;

– Colemanite, with the following chemical composition: B2O3 — 40.2%; Fe2O3 — 0.09%; Al2O3 — 0.4%; P2O5 — 0.02%; CaO — 27.6%; SiO2 — 5.5%; hydrated moisture — 23.19%;

– Quartzite, with the following chemical composition: SiO2 — 97.76%; Fe2O3 — 0.66%; Al2O3 — 0.77%; P2O5 — 0.03%; CaO — 0.78%;

– Shavings, with the following chemical composition: Fe — 98.98%; Si — 0.31; P — 0.03%; C — 0.24%; Mn — 0.41.

The electric furnace was preheated for 3 hours on a coke cushion, which served as an electrical conductor and maintained the temperature. After the preheating period, the electric furnace was thoroughly cleaned of any remaining coke cushion residue. The electrical parameters during the preheating period were as follows: secondary voltage of 40 V, current strength of 200–210 A on the high side. The smelting process was carried out continuously, with the charge being loaded in small portions as the hearth settled, and periodic release of metal into the cast iron molds. Each metal release was weighed, and samples were taken for chemical analysis.

The main objective of the smelting process was to achieve complete reduction of all major elements from the oxides of high-ash coal, using a continuous, stable, and easily adjustable slag-free process.

Results and Discussion

The charge was calculated taking into account the complete reduction of ore oxides and coal ash. Thus, high-ash coals from the Borly and Kuu-Chek deposits contain almost half of the oxides that make up the ash [17, 18]. The charge was adjusted by changing the weight of the ore, as changing the weight of the coal would simultaneously change the amount of oxides in the charge.

The working charge was used during smelting with a reductant deficiency, with the calculated stoichiometry, as well as with an excess of the reductant.

When operating with a reductant deficiency of up to 10%, active electrode consumption began, and after the metal was tapped, the slag would solidify on the hearth, which was removed with metal rods. The gas permeability of the furnace lining deteriorated. Normal furnace operation was achieved by adding reductant under the electrode.

When operating with an excess of reductant of up to 10%, carbide deposition occurred under the electrode, which led to electrode expulsion, loss of temperature in the reaction zone, and hindered the tapping of the alloy from the furnace. To stabilize the smelting process when carbides formed in the furnace, quartz was added under the electrode.

Through experimentation, the optimal composition of the charge was found to be 2% reductant deficiency. With this stoichiometry, no slag accumulated in the furnace and no carbides formed. The load on the transformer on the high side remained within 400 A. To actively extract the metal before tapping, the load was increased to 450 A. Periodically, when there was no active self-flow of the charge, the furnace lining around the electrode was repaired.

The duration of the smelting process was 30 hours, not including the time for preheating the furnace lining. The conducted tests showed the absence of active self-flow of the charge, which results in the formation of crucibles around the electrodes covered with the charge from above. The diameter of the crucible was 330– 390 mm. The size of the crucibles was judged by the width of the charge collapse zone during metal pouring. Such operation of the furnace bath has its positive and negative aspects.

The positive aspect is that in the absence of intense self-flow, there is overheating of the reaction zone, ensuring more complete progress of the reduction reactions. On the other hand, overheating of the crucibles increases heat losses and unevenness of the current load. The chemical composition of the obtained boron-containing ferrosilicoaluminum is presented in Table 1.

Table 1. Chemical Composition of Ferro-Silicon Aluminum Obtained by a Single-Stage Slag-Free Method
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The general view of the single-phase ore-thermal furnace is shown in Fig. 6.

Fig. 6.
figure 6

General view of a single-phase ore-thermal furnace — (a) front view, (b) side view: 1 — ore-thermal furnace casing; 2 — upper electrode; 3 — tap-hole block; 4 — cascade molds; 5 — exhaust hood; 6 — water-cooled copper plates; 7 — elements of high-current system; 8 — water cooling elements; 9 — lower electrode; 10 — transformer.

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During the experimental trials, the following amounts were consumed: high-ash coal — 935 kg; colemanite — 39.68 kg; quartzite — 156.08 kg; chips — 68.75 kg. 356.93 kg of complex boron-containing ferrosilicoaluminum was obtained, and the electricity consumption per ton of alloy was 2700 kWh. The electricity consumption was significantly low due to the activity of the silicon-aluminum complex in the alloy composition, as well as the reduction in process temperature thanks to boron.

Fifteen batches were produced, seven batches of FS45A10B1 alloy and eight batches of FS50A20B1 alloy. The insufficient recovery of aluminum and silicon in the first two batches can be attributed to the furnace not reaching the necessary temperature regime. After the seventh batch, a flushing batch was conducted with a change in the charge for the FS50A20B1 alloy.

The aforementioned results of technological tests indicate the possibility of obtaining complex boron-containing ferrosilicoaluminum through a one-stage slag-free method.

Conclusions

Full thermodynamic modeling has been performed, which has shown that boron-containing metal is formed as a result of the formation of the AlB12 phase, and the process must be carried out at a temperature of at least 1800°C to achieve the final composition of the ferroalloy.

Experimental-industrial tests of the technology have been conducted under production conditions using a 250 KVA transformer in an ore-thermal furnace. The possibility of practical production of boron-containing aluminum-silicon ferroalloy has been proven, utilizing high-ash coals from the Kuu-Chek, Borly deposits.

Under experimental-industrial conditions, an alloy with the following mass % composition has been obtained: 28.5% Fe, 49.89% Si, 20.5% Al, 1.03% B, suitable for steel alloying.

This study was conducted as part of the grant funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (grant No. AP09259368).