Introduction

The electron-beam method of refinement is used in the vacuum metallurgy since the last decades of the last century. Due to the development of the electron-beam technology and the corresponding efficient equipment aimed at the electron-beam melting (EBM) and refinement, this method turned into one of the main methods used for the production of charge materials in industrially developed countries. The results of studies in this important branch of industry are presented in numerous works [1,2,3,4,5,6,7,8,9].

The methods of EBM and refinement were created as the sole possible procedures that can be used for the processing of refractory and chemically active metals, such as Mo, W, Ti, Nb, Ta, Zr, Hf, V, Pt, and their alloys. At present, they are extensively used in the production of various high-purity metals and alloys in a vacuum and also for the repeated use of metallic scrap, etc.; see, e.g., [5, 10,11,12,13,14,15]. For the purposes of EBM, the electron beam is used as a heat source. This procedure is usually carried out under the conditions of high vacuum. In this case, the processes of refinement are accelerated. Under the action of electron beams, the metal is not overheated [10]. This guarantees high level of purification of the metal from gases and metallic or nonmetallic admixtures, as well as the preservation of a given chemical composition, structure, and properties of finished metallic ingots. The application of a water-cooled copper crucible for melting, refinement, and solidification excludes the possibility of additional contamination caused by the contact with the surfaced of ceramic crucible used in the ordinary metallurgy.

As an additional advantage of EBM, we can mention the absence of special requirements to charge (source) materials. The indicated characteristics, in combination with the possibility of long-term holding of melted metal in the overheated state make the EBM procedure convenient for applications in various metallurgical processes: (1 ) in the final stage of production of high-purity metals (Cu, Ti, Mo, etc.) from the concentrates obtained after thermal reduction; (2 ) for the reprocessing of wastes containing valuable metals (Ta, Hf, Zr, etc.), which were oxidized or contaminated by admixtures in the previous stages of production or in the process of operation [12, 16].

The EBM method gives no wastes and is environmentally safe. It enables one to produce metals and alloys from the natural ores and anthropogenic wastes in the case of deficiency of natural raw materials. The global stocks of various valuable metals are continuously exhausted. At the same time, significant amounts of wastes containing these metals are accumulated.

The process of production of each metal is unique. It is especially important to be able to selectively remove admixtures from the raw (source) materials. Therefore, it is necessary to develop special technological conditions for different source materials.

The aim of the present work is to determine the parameters of EBM promoting the removal of admixtures in the process of refinement of various chemically active and nonferrous metals and in the course of reprocessing of the scrap of Cu, Hf, and Zr and to perform the analysis of the results of application of the EBM method.

Results and Discussion

Specific features of the process of electron-beam melting. In Fig. 1, we present a schematic diagram of the EBM (electron beam drip melting) method. The process of melting is carried out in a vacuum chamber. The formed electron beam is directed by one (or several) electron-optical system into the reaction zones: to the remelted rod 1 and to the surface of liquid bath 3 of the ingot. The electrons collide with the refined metal and heat it. The drops of melted metal 2 fall into the water-cooled copper crucible with moving bottom, where they are solidified. The surface of melted metal in the crucible is also heated by the electrons. The processes of refinement mainly occur on the melted-metal/vacuum interface in the front part of the source (raw) material. The feeding rod creates drops and the liquid bath in the crucible (Fig. 1).

Fig. 1.
figure 1

Schematic diagram of the electron-beam drip melting (EOS — electron-optical system): 1 ) horizontal feeding rod of the source metal; 2 ) generated drops; 3 ) bath of melted metal in a water-cooled crucible.

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The main parameters of the process controlled by the operator are as follows: startup power, focusing current of the beam, melting rate and/or casting rate (or the rates of feeding and drawing), as well as the cross section and density of the supplied metal. By varying the power of the beam and casting rate, it is possible to control the geometry of the bath of liquid metal, the distribution of temperature in the zone of interaction of the beam with the metal, and the duration of the thermal influence affecting the processes of refinement and the quality of the metal obtained as a result of EBM. The choice of the corresponding parameters of the technological processes guarantees the possibility of creation of the thermodynamic, hydrodynamic, and kinetic conditions required for the removal of various admixtures.

In the present work, all experiments were carried out with the use of electron beams with a power of 60 kW (ELIT 60 installation) with a single electron gun, a horizontal feeder, a water-cooled copper crucible, and a withdrawal system (pulling mechanism) (Fig. 1).

Application of EBM as the final stage of an ordinary metallurgical processes of production of pure metals. The indicated possibility of application of the EBM method makes it possible to exclude numerous intermediate technological operations some of which are hazardous for the environment and dangerous for health. In many cases, it is possible to include EBM in the earlier stages of the ordinary metallurgical process. In this case, metals with high contents of admixtures can be used as source (raw) materials. This trick enables one to decrease both the time losses and the costs required for the production of defect-free billets of pure metals with the required shapes and sizes.

It is worth noting that there are no special requirements to the type and parameters of the source material for the application of EBM. Thus, as the source material in the process of EBM of copper, one can take copper billets taken after different stages of the ordinary electrolytic (anodic, cathodic, etc.) process of production.

In Table 1, we present the results of EBM of copper with various levels of initial purity: A ) commercial purity (99.202% Cu) with high contents of oxygen and metallic inclusions, B ) anodic purity (99.510% Cu), and C ) commercial copper after reduction with carbon (99.816% Cu). The source material (feeder) had the form of a rod 60 mm in diameter and 350 mm in length. We used the following parameters of the EBM process: the beam power P was equal to 6.9 or 17.3 kW and the growth rates vC were equal to 1 and 2 mm/min. The measured surface temperature of the liquid metal in the crucible T = 1500 – 1700 K. We also determined the total efficiency of removal of the inclusions η and the concentration of admixtures Ci (ppm) (1 ppm = 10 – 4 wt.%).

Table 1. Results of the electron-beam refinement of copper
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For all inclusions, we attained a high efficiency of removal of the admixtures from copper on the following level: η = 93.65 – 97.96% (Table 1). After EBM, the contents of oxygen and all monitored metallic admixtures (Ci ) strongly decreased. In the process of refinement, many admixtures play the role of accumulators, which take oxygen away from the copper oxides and form stable oxides. These stable oxides are removed from the liquid metal in the process of distillation. Thus, the contents of oxygen and metallic admixtures in refined copper decrease and its purity becomes higher than for the electrolytic copper (Table 1).

The accumulated results demonstrate that if the initial material is preliminarily saturated with carbon up to 0.03%, then the content of oxygen in copper after EBM becomes 117 times lower (version 1C, Table 1). For versions 1A, 2A, and 1C, the concentration of oxygen becomes about 45, 90, and 50 times lower, respectively (Table 1).

The refined metals were produced in the form of ingots 50, 60, 80, or 100 mm in diameter and up to 25 cm in length (these sizes are determined by the applied equipment).

We studied the microstructure of ingots with the help of an IM-3MET optical microscope with recording images in an AxioCamERc 5s digital camera. In Fig. 2, we show the microstructure of refined copper typical of high-purity metals. It is easy to see that the grain boundaries are free of inclusions.

Fig. 2.
figure 2

Microstructure of the copper ingot in a cross section (made in the vertical direction) after EBM carried out for P = 17.3 kW and vC= 2 mm/min.

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EBM for the repeated usage of metallic wastes enriched with gases. Some countries (including Bulgaria) are not traditional producers of various metals active with respect to oxygen at high temperatures (such as titanium, hafnium, zirconium, etc.) due to the absence of ores. In view of the unique combination of physicochemical and chemical properties, these metals are of significant interest for the production of components of electronic circuits, vacuum and special equipment, artificial joints, medicinal implants, etc. At the same time, significant amounts of high-quality scrap of these metals are formed as a result of their production and operation. This scrap is oxidized and, hence, its repeated usage is possible only after the removal of oxygen (or at least as a result of significant lowering of its content). Thus, the EBM method proves to be suitable for the refinement of wastes of this kind.

Oxygen is present in solid wastes in the form of oxides, which pass into the bath of liquid metal in the course of heating with electron beams. A part of oxides is dissolved or dissociated in the volume of the liquid bath, whereas the other part comes to the surface of the ingot (melt/vacuum interface) and dissociates (due to the low partial pressure of oxygen in the atmosphere). The degree of dissociation of oxides and the efficiency of the process of refinement are quite high due to the high temperature on the surface of ingot and its contact with a vacuum.

In Table 2, we present the data on the refinement of hafnium raw materials for various technological conditions of the EBM process. The experiments were carried out with the use of chips of oxidized hafnium with a low content of admixtures. The chips were preliminarily cleaned from oil and pressed into disks 60 mm in diameter and 10 mm in height. The EBM process was carried out for the beam powers P = 12, 15, and 17 kW. The duration of refinement τ was either 2 min or 4 min. The maximum degree of refinement (37.41%) was attained for τ = 2 min and P = 12 kW. In this case, the purity of hafnium was as high as 99.96 %. The process of removal of Si, Mg, Bi, and Sn is complicated by the thermodynamic restrictions specific for the analyzed modes of refinement.

Table 2. Results of Electron-Beam Melting and Refinement of Hafnium
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The degree of refinement does not increase as the power of the electron beam increases. The results of our investigations demonstrate that the long-term overheating of the metal does not promote refinement due to the evaporation of hafnium. The losses of the metal for evaporation in the process of overheating increase from 1.01 to 3.64%. The best efficiency of refinement of hafnium chips is observed for a beam power of 12 kW.

The experiments on the refinement of zirconium wastes by the EBM method also revealed a noticeable removal of metallic admixtures (Table 3). In this case, the concentration of oxygen became about 7 times lower.

Table 3. Results of the electron-beam refinement of zirconium
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The obtained results show that, after the procedure of treatment with electron beams, the contents of oxygen in hafnium and zirconium become 4 and 7 times lower, respectively. It is also possible to remove the volatile admixtures (aluminum, copper, etc.). Their concentration becomes up to 30 times lower.

Conclusions

Electron-beam melting is an efficient method for the production of high-purity metals and reprocessing of metallic wastes. This method is environmentally safe and enables one to save the energy. On the basis of the performed studies, we developed several recommendations concerning the use of electron-beam refinement of copper, hafnium, zirconium and reprocessing of their wastes.