Direct Alloying of Silicon in Liquid Steel by Molten Slag Electrolysis

Abstract

The present study aimed at completing the direct alloying operations of Si in liquid steel by the way of electrochemical method. In the traditional steelmaking process, the ferrosilicon is prepared by preliminary smelting the oxide ores, and then added into the liquid steel for alloying. In this study, the direct alloying operations will be completed by electrolyzing CaO–SiO₂–Al₂O₃ molten slags, in which process the alloy elements are produced in the slag-steel interface and then diffuse into the liquid steel when direct current pass through the two electrodes which are placed in liquid steel (cathode) and slag (anode), respectively. It is a new attempt to applying the electrochemistry on the steelmaking process. Experimental results showed that after electrolysis for 2 hours, as applying larger current and using slag with a higher content of SiO₂, there will be a higher content of Si generated in the liquid steel. It was found that during the electrolysis process, Al content in the steel also slightly increases, while there was no obvious change of Ca content. Meanwhile, reduction of Al₂O₃ and CaO is not the main factor to decrease current efficiency (calculated by the ratio of generated Si in liquid steel to the theoretical silicon by Faraday law).

1. Introduction

The addition of alloy elements can significantly change the performance of the steel. For instance, adding a certain amount of silicon in steel can improve the strength, hardness and elasticity of steel. In the process of smelting constructional steel (0.40%–1.75% Si), tool steel (0.30%–1.80% Si) and spring steel (0.40%–2.8% Si), it must add a certain amount of ferrosilicon alloy into the liquid steel.^1,2,3,4) Silicon also possesses other excellent properties, such as large specific resistance, low thermal conductivity and strong permeability. A certain amount of Si can improve the magnetic conductivity of steel and reduce the hysteresis loss and eddy current loss of the steel. So, during smelting the transformer silicon steel (2.81%–4.80% Si), ferrosilicon alloy is also used to for silicon alloying.^5,6,7)

At present, the ferrosilicon alloy is first produced in arc furnace, and added into the liquid steel for alloying. The production of ferrosilicon is a high energy consumption process. If the ferroalloy production and the alloying of liquid steel can be coupled together, and the generated Si go directly into liquid steel to complete alloying process, it will not only shorten the production cycle, but also save energy consumption and reduce the environment load. In this study, the electrochemical method will be used to direct alloying silicon into the liquid steel.

2. Experimental Procedures

In this study, the compositions of each sample compiled in Table 1 were chosen. In Table 1, slag A has a low basicity, while slag B has a high basicity. Slag samples were prepared using reagent grade SiO₂, Al₂O₃ and CaCO₃ powder, all of which were calcined at 1000°C for 10 h in a muffle furnace to decompose any carbonate and hydroxide before use. Then the prepared CaO and other reagents were precisely weighted according to the compositions shown in Table 1, and mixed in an agate crucible thoroughly. In the following sections, 40 g slag samples were used in each electrolysis experiment. Liquid steel was obtained by smelting high purity iron powder, the composition of which is shown in Table 2.

Table 1. The compositions of each sample (mole percent).

	CaO	SiO2	Al2O3
Slag A	40	40	20
Slag B	57	15	28

Table 2. The composition of iron powder (mass percent).

Element	Fe	C	S	Cr	Si	Mn	Ni
Content	99.99	0.003	0.002	0.001	0.002	0.001	0.001

In the experiment, the furnace with heating elements of MoSi₂ was employed. The inner diameter of alumina working tube was 50 mm. A self-made Mo (50 mol%)-ZrO₂ cermet connected by a Mo wire (0.8 mm in diameter) was used as the cathode and placed in the liquid steel, while a platinum piece connected by a platinum wire (0.3 mm in diameter) was used as the anode and placed in the molten slag. The cathode and anode were passed down by two support tubes which are fixed in furnace tube plug, so that the cathode and anode were separated for 12 mm. The cathode and anode could be moved up and down along the two support tubes.

The 50 g iron powders and 40 g slag samples were packed orderly into an alumina crucible and then placed at the constant temperature zone of furnace, where the temperature variations was measured by a type B (Pt-6 pct Rh/Pt-30 pct Rh) thermocouple. During the heating process, the tips of the electrodes were located at about 2 cm above the slag surface. After the target temperature 1873 K was reached and held for 30 min, the Mo–ZrO₂ cermet and platinum were lowered slowly into liquid steel and molten slag, respectively. The systematic diagram of experiment apparatus is shown in Fig. 1. The two electrodes were connected to a DC power supply and different constant currents were applied. H₂–Ar (3%H₂ + 97%Ar) mixed gas was used as the protecting gas during the whole experimental process. The electrolytic time for all experiments is 2 hours. After the experiment, the electrodes were lifted.

Fig. 1.

The systematic diagram of experiment apparatus. 1- Mo wire 2- alumina protective tube 3- alumina protective crucible 4- alumina experimental crucible 5- Mo-ZrO2 cermet electrode 6- Pt wire 7- alumina protective crucible 8- platinum piece electrode 9- molten slag 10- liqiud steel.

3. Results and Discussion

The platinum piece is a inert anode, so the anodic reaction can be expressed as follows:   

$${\left(\text{O}\right)}^{\text{2}-}-\text{2e}=\text{1/2}\mathrm{\hspace{0.17em} }{\text{O}}_{\text{2}}$$(1)

  

$${\text{SiO}}_4^{4-}+4\text{e}=\text{Si}+{\text{4O}}^{2-}$$(2)

The Mo–ZrO₂ cermet together with liquid steel was used as cathode. The standard decomposition voltage of CaO, SiO₂ and Al₂O₃ follows the sequence: SiO₂<Al₂O₃<CaO, consequently when applying direct electric current, the silicon ion has the priority to gain electrons in the slag-steel interface as shown in Eq. (2) to generate silicon which is further diffuse into the liquid steel. Si content in steel in the case of different electrolysis currents at 1873 K is shown in Fig. 2. For comparison, the Si content in the blank sample without applying direct electric field is less 0.01%. It can be seen from Fig. 2 that when using CaO(57 mol%)-SiO₂(15 mol%)-Al₂O₃(28 mol%) slag, Si content in liquid steel is 1.30%, 1.76% and 2.08% as current is 2.5 A, 3.5 A and 4.5 A, respectively; while, when using CaO(40 mol%)-SiO₂(40 mol%)-Al₂O₃ (20 mol%) slag, Si content is 1.69%, 1.95% and 2.20% as current is 2.5 A, 3.5 A and 4.5 A, respectively. It can be known from the results that the effect of direct alloying of silicon is obvious. Meanwhile, as increasing current, the Si content also increases, since the increase of electric field strength leads to the promotion of kinetic conditions. Compared with CaO(57 mol%)-SiO₂(15 mol%)-Al₂O₃(28% mol%) slag, the Si content is slightly higher when using CaO(40 mol%)-SiO₂(40 mol%)-Al₂O₃(20 mol%) slag. The reason for this may be that the concentration of silicon ion in the latter slag is higher than the former one.

Fig. 2.

Si content in steel for different electrolysis currents at 1873 K.

Figure 3 shows the change of alloying rate with electrolysis current for both two slags. From Fig. 3, it can be seen that the alloying rate increases as increasing the electrolysis current for both slags, in addition, the alloying rate is higher when using CaO(40 mol%)-SiO₂(40 mol%)-Al₂O₃(20 mol%) slag. Higher concentration of SiO₂ , as well as large current which provides more electrons and larger driving force, can both improve alloying rate.

Fig. 3.

The alloying rate in the cases of different electrolysis current for both slags.

Current efficiency is an important factor for electrolytic experiment. The current efficiency was calculated by the ratio of generated Si in liquid steel to the theoretical silicon by Faraday law. Figure 4 shows the current efficiency for different electrolysis current at 1873 K. It can be seen that the current efficiency varies from 47% to 67%. As increasing electrolysis current, the current efficiency always decreases. Apart from the experiment error, the occurrences of side reactions are the main factor to affect the current efficiency. In the presence of Si ion concentration polarization, many side reactions may happen, such as,   

$${\text{Al}}^{3+}+\text{3e}=\text{Al}$$(3)

  

$${\text{Ca}}^{\text{2}+}+\text{2e}=\text{Ca}$$(4)

Fig. 4.

The current efficiency for different electrolysis current at 1873 K.

These side reactions can decrease the current efficiency. Meanwhile, the larger the current, the easier the concentration polarization of Si ion will be. In this cane, more side reactions will be taken place, which leads to a lower current efficiency. From Fig. 4, it can also be seen that CaO(40 mol%)-SiO₂(40 mol%)-Al₂O₃(20 mol%) slag with a higher silica concentration also has a larger current efficiency. Based on the thermodynamic calculation by Factsage 6.2, SiO₂ activity are 9.11×10⁻² and 1.22×10⁻³ for CaO(40 mol%)-SiO₂(40 mol%)-Al₂O₃(20 mol%) and CaO(57 mol%)-SiO₂(15 mol%)-Al₂O₃(28% mol%) slags, respectively. Therefore, a higher activity of silica can decrease the concentration polarization of Si ion near the cathode, which is beneficial for the enhancement of current efficiency.

The acid-soluble Al content in steel for different electrolysis currents at 1873 K is shown in Fig. 5. From Fig. 5, Al content is 8 ppm for blank sample. In the case of using CaO(57 mol%)-SiO₂(15 mol%)-Al₂O₃(28% mol%) slag, Al content is 10 ppm, 17 ppm and 15 ppm when current is 2.5 A, 3.5 A and 4.5 A, respectively. In the case of using CaO(40 mol%)-SiO₂(40 mol%)-Al₂O₃(20 mol%) slag, Al content is 29 ppm, 28 ppm and 31 ppm when current is 2.5 A, 3.5 A and 4.5 A, respectively. It can be known from above results that Al content slightly increases in the direct alloying process of Si. Figure 6 shows the Ca content in liquid steel for different electrolysis currents at 1873 K. From Fig. 6, it can be seen that Ca content varies from 4 ppm to 8 ppm under the present experimental conditions. In other words, the electric field doesn’t obviously improve Ca content in the steel. However, taking CaO(40 mol%)-SiO₂(40 mol%)-Al₂O₃(20 mol%) slag for example, when current is 2.5 A, the increments of Al and Ca content are 21 ppm and 2 ppm, relative to the blank samples without exerting electric field. Compared to the current efficiency of 67% when the increment of silicon content is 1.69% in the same experimental conditions, reduction of Al₂O₃ and CaO in slag should only account for a very small proportion for deterioration of current efficiency. In other words, reduction of Al₂O₃ and CaO is not the main factor to decrease current efficiency. The detailed reason is unclear.

Fig. 5.

The Al content in steel for different electrolysis currents at 1873 K.

Fig. 6.

The Ca content in steel for different electrolysis currents at 1873 K.

4. Conclusions

The present study investigated the direct alloying operation of Si into the liquid steel by electrolyzing CaO–SiO₂–Al₂O₃ molten slag. Experimental results show that as increasing electrolysis current and concentration of SiO₂ in slag, both Si content in the steel and alloying rate of Si increase. During the electrolysis, a little amount of Al can be generated and lead to the increase of Al content in liquid steel, but there is no obvious change of Ca content.

Acknowledgements

Thanks are given to the financial supports from National Natural Science Foundation of China (51304018).

References

• 1)   X. Y.  Li and  K. N.  Tandon: Wear, 225 (1999), 640.
• 2)   M. D.  Meyer,  D.  Vanderschueren and  B. C.  De Cooman: ISIJ Int., 39 (1999), 813.
• 3)   H. E.  Townsend: Corrosion, 57 (2001), 497.
• 4)   F.  Nahif,  D.  Music,  S.  Mraz,  H.  Bolvardi,  L.  Conrads and  J. M.  Schneider: Surf. Coat. Technol., 235 (2013), 250.
• 5)   C.  Suryanarayana: Prog. Mater. Sci., 46 (2001), 1.
• 6)   K. C.  Hwang,  S.  Lee and  H. C.  Lee: Mater. Sci. Eng., 254 (1998), 282.
• 7)   G. A.  Fontalvo and  C.  Mitterer: Wear, 258 (2005), 1491.