One of the nickel industry problems in our country is a sharp decrease in the extraction and processing of oxidized nickel ores (ONO). In 2010 ores of four deposits were developed and the nickel share from the ONO in Russian production reached 10% but in recent years mining operations have been carried out on only one deposit, and production does not exceed 5% . The current situation is mainly due to the lack of profitable technologies for the extraction and processing of oxidized nickel ores. At the same time, in recent years, the production of nickel–containing alloys and steels has significantly increased in the world. In Russia, electrolytic nickel ( 99% Ni) is mainly used for their smelting, the high price of which does not contribute to the development of the production of nickel–containing steels . At the same time, in our country, there is a shortage of relatively cheap nickel and complex ferroalloys containing 10–30% Ni [3,4], which can be produced from domestic ONO [5,6]. At present, a number of methods have been developed , which make it possible to efficiently process poor oxidized nickel ores with the production of new types of ferroalloys [8,9], however, such alloys are still in low demand in steelmaking. To use new ferroalloys in the processing of steel, it is necessary to know their physicochemical characteristics. One of the most important characteristics of the alloy intended for steel alloying is the time of its melting and dissolution in the iron–carbon melt , on which depend both the absorption and the distribution of the leading elements of the ferroalloy in the metal volume [11,12]. The mechanism of alloys melting is a fundamental factor determining the time of its melting in an iron–carbon melt . Extensive research was conducted in this area by both domestic [11,14,15] and foreign [12, 16–21] metallurgists.
In this work, to study the melting process of complex nickel–containing alloys in an iron–carbon melt, we used a mathematical model for calculating the melting time developed by scientists of UrFU and IMET UB RAS. The model includes hydrodynamic and thermal parts. The hydrodynamic part of the model determines the speed of ferroalloys movement in relation to the melt of the processed metal, considering the forces acting on the alloys at each time point. The thermal part of the model is based on the classification of ferroalloys depending on the ratio of their liquidus temperature (T ), bath temperature (T ) and crystallization of the iron–carbon melt (T ) into low–melting (T T ); refractory (T T T ) and ultra–refractory (T T ). The course of the melting process is mainly determined by the temperatures ratio of T , T , T .
For the study, there were selected samples of complex nickel–containing ferroalloys with a composition corresponding to the alloys that can be smelted from domestic oxidized nickel ores [22,23]. The chemical composition of the samples and their thermophysical properties are presented in the table.
– time of the first melting period; – time of the second melting period; – time of the third melting period; - total melting time
The simulation results are presented in Figures 1–3 . In the calculations, it was assumed that T = 1530 C, the initial temperature of the ferroalloy T = 25 C, the temperature of the iron–carbon melt bath is 1600 C; crystallization heat L = 1,79 10 J/m , thermal conductivity coefficient of steel in the liquid state λ = 30 W/m K . In real conditions, the initial temperature of a ferroalloy piece is always lower than the temperature of steel crystallization, and therefore a crust of solid steel 25 is formed on its surface. The following notation for the melting time is taken: are the duration of individual periods, is the total melting time. The thermal part of the model includes the differential Fourier thermal conductivity equation for each of the phases (crust, solid core, liquid ferroalloy layer), the number of which depends on the process period and the type of ferroalloy, and the boundary conditions determined by convective heat transfer and melting process.
All the alloys under consideration belong to the group of low–melting ferroalloys (see table), in which T T . The process of their melting in an iron–carbon melt takes place in three periods. During the first period (Figure 1, In the second period (Figure 1,
In the third period (Figure 1, ), the solid piece of ferrosilicon nickel is melted, directly contacting with liquid steel. Theoretically, in periods 1, 2 there is no direct contact of the ferroalloy with liquid steel. Therefore, the interaction between the ferroalloy and the liquid metal (deoxidation reactions, mass transfer, etc.) is absent. The process of ferroalloy melting accelerates its movement in the iron–carbon melt.
The influence of the initial piece size is unambiguous: with increasing diameter, the total melting time increases (Figure 2). With an increase in the ferroalloy size up to 50 times (from 1 to 50 mm), the total melting time increases up to 450–1800 times.
An increase in the Si content from 5 to 18% in the considered alloys leads to a decrease in the melting time (Figure 3), a further increase in the Si concentration to 31%, on the contrary, is accompanied by an increase in the melting time of the alloys, and a subsequent increase in Si to 53% leads to a smooth decrease of . This is related to the liquidus temperatures values of the corresponding alloys (Table) and the possible formation in the region for silicon concentration of 17–23% for low–melting Fe Si and Fe Si phases, characterized by a low liquidus temperature of 1261 C [26,27]. It should be noted that the effect of the silicon concentration in the alloy on the total melting time increases significantly with increasing of ferroalloys fractional composition.
Thus, the work studied the melting kinetics of complex nickel–, silicon–containing ferroalloys in a steel melt. It is shown that an increase in the fraction of all the alloys under consideration from 1 to 50 mm leads to a stable increase in the melting time, and an increase in the silicon concentration from 5 to 18% reduces , a further increase in the Si content to 31%, on the contrary, is accompanied by an increase in of alloys, a following increase of Si content to 53% leads to a gradual reduction of . In general, complex nickel–, silicon–containing ferroalloys are characterized by a much faster course of the melting process in steel compared to ferronickel.