The cavity size and shape can change by tuning the furnace parameters. Zherdev et al. [61] carried out five excavations in furnaces rotated at different revolution speeds, and compared the profiles to a static furnace (Figure 39a). A slow rotation speed (Figure 39b) does not alter relevantly the cavity size. A shorter revolution time (Figure 39c) gives a smaller cavity close to the electrode, as well as a distortion and loss in symmetry. Higher revolution speeds (Figure 39d) displace the high temperature zones into the furnace bath. The highest rotation speed (Figure 39e) gave the smallest cavity, but the difference between 70-hours and 50-hours revolution time was not large.
The movement of the furnace increases the temperature in the space around the electrode path. The electrical current distribution is altered if the furnace rotates too fast, but rotation is necessary to avoid clogging and enhance the mixing of the charge. Rotating over a critical speed will give an asymmetric, shorter, wider cavity.
This practice is not recommended, as it may affect the electrode position.
Figure 39: Shape of the gas spaces around the electrodes in a 16.5 MVA furnace used for melting FeSi75. a) Furnace not rotated; b-e) Furnace rotated at different speeds. The numbers close to the electrode are times for a single revolution, in hours.
Another study from Zherdev et al. [62] showed that the shape of the cavity changes by tuning the alloy composition (Figure 40). Table 4 gathers the measurements made by Zherdev et al. in three different FeSi furnaces, producing FeSi45, FeSi75 and FeSi90 respectively. Each of them has a power of 9 MW. If the alloy produced is richer in silicon, the cavity and the distance between electrode and crater walls will increase.
An alloy richer in silicon gives a smaller charge layer thickness. Such a process dissipates high amounts of heat, and higher amounts of SiO(g) can reach the offgas system. When this occurs, the process is not economically profitable, as massive amounts of Si are lost through the offgas.
63 Figure 40: Shapes of cavities and cross sections of furnaces producing different ferrosilicon alloys [62].
Table 4: Cavity size and thickness of charge layer around electrode in three different pilot scale FeSi furnaces [62]
Furnace alloy Max. distance between electrode and crater walls
(m)
Max. height of cavity (m) Thickness of charge layer in contact with the
electrode (m)
FeSi 45 0.20 1.20 0.50
FeSi 75 0.60 1.40 0.32
FeSi 90 1.20 1.70 0.06
Figure 41a provides a schematic view of the furnace excavation results by Schei and Sandberg [4] and Schei [1], [6]. The sketch in Figure 41b looks correct qualitatively, and it has been accepted for long in the silicon industry community as a good examples of a typical silicon furnace cross section. However, there is a major flaw in the dimension of the arc.
If we could scale this picture to real size, the arc would be about 50 cm long. The arc size influences the size of the cavity. On the other hand, Sævarsdóttir [63] modelled the arc size in silicon furnaces to be maximum 15 cm long. Besides, one should not forget that the cavity size is depending on the history of each furnace, as shown in the previous chapter.
Muller et al. [58], [64] excavated two pilot scale furnaces at 150 kW power, producing FeSi67 and FeSi75. When stoking a pilot scale furnace with 150 kW power, they found a top cavity as well as a secondary bottom cavity (Figure 42). Where condensates form, and quartz is not melted yet, secondary cavities would form. Stoking a furnace has the purpose to break the roofs of these cavities. If secondary cavities are not stoked, the materials below will flow down, but new materials would accumulate at the top. This was also the case in the experiments by Myrhaug [65] and Tangstad et al. [7]. These cavities are generated by the materials consumed below the stuck charge, which leaves empty space below.
A double cavity was found also at laboratory scale experiments. In the works by Tangstad and Ksiazek [53] and Myrhaug [65], one cavity is located up in the furnace, and the other in the inner zone. At the end of the experiment the two cavities have collapsed on each other at some point during the process. The authors explain that these cavities are formed by the consumption of raw materials below the condensation temperature. the It can be acknowledged that the geometry of the cavity formation is strongly dependent on the experimental setup and furnace operation condition.
Figure 41: a): Ferrosilicon furnace excavation after smelting. Legend: 1=Ferrosilicon; 2= Coarse SiC crystals with voids filled with FeSi; 3=Cavity; 4=Green layer made of melted quartz, green lumps of (SiC+SiO2+C), ferrosilicon drops covered by brown condensate; 5=Brown condensate (more abundant close to the electrode), unreacted charge; 6=Graphite electrode. [1] b): Proposed inner structure of a submerged arc furnace [1].
Figure 42: Excavation of pilot scale furnace, revised after Müller et al. [64]
Finally, it is worth also to mention the cavity formation mechanism proposed in literature, thus completing the knowledge of gas generation and condensation. Otani et al. [3] hypothesized the cavity formation for a charge which is continuously descending in the hearth. The mechanism is sketched in Figure 43.
The mechanism of formation consists of three steps. First, the raw materials are accumulated in the furnace, at the bottom of the electrode. Secondly, the raw materials located at the highest temperatures will turn into Si(l), SiO(g) and CO(g) (Reaction -1,-2, 4). Then, the gas will flow upwards, until it meets a substrate cool enough to trigger the condensation reactions. The condensate will bind the charge and build up the cavity roof, while the charge at the hot zone will transform into SiO(g) and CO(g). The cavity will grow up to a certain height, and raw materials will be suspended by the cavity roof, unless stoked.
(b) (b)
(a) (b)
65 Figure 43: Mechanism of formation of the cavity in industrial furnaces, according to Otani et al. [3]
Vangskåsen [59] used a laboratory scale furnace with a temperature gradient, to estimate the temperature of formation of a cavity in a static heated charge. If a SiO2-SiC charge is heated, it will react to SiO(g) and CO(g).
Vangskåsen demonstrated that cavity roof is positioned always at the same temperature, even by changing the thermal history and the charge composition [15].
Figure 44 shows sections of laboratory scale crucibles after 10, 30, 40 and 60 minutes at 2000°C. The dimension of the cavity increases drastically between 10 and 30 minutes as the bottom raw materials are producing gases, as well as the amount of condensate in the top charge layer increases. Some charge materials are still at the bottom of the crucible after 30 and 40 minutes. These can either be unreacted charge, or lumps fallen from the cavity roof.
After these experiments, Vangskåsen suggested that the cavity grows downwards. A sketch of Vangskåsen’s cavity formation mechanism is proposed in Figure 45. Above a critical temperature, gas formation will dominate, whereas the charge would remain unreacted and fixed with condensates at the colder zone. The point where the cavity roof is located is now fixed in the system. The charge will start reacting to SiO(g) and CO(g), and the cavity will grow downwards from the critical temperature.
Otani and Vangskåsen propose similar mechanisms. Both state that a certain isotherm will determine the position of the cavity roof. The difference between Vangskåsen and Otani is that Otani thinks that the isotherm will move upwards, and Vangskåsen does not.
Figure 44: Time evolution of cavity in laboratory scale experiments [59].
Figure 45: Cavity formation mechanism proposed by Vangskåsen [15].