Figure 88: Sample location in Elkem Fiskaa plant (Kristiansand, Norway)
Two condensates samples were extracted from Furnace 11 at REC Solar in Fiskaa, Kristiansand (Norway), where the furnace is used for the production of solar grade silicon. The central inactive zone is located at about 1 meter from the top of the furnace. During operation of the furnace, the operators managed to push out the top of the inactive accumulated material, by using the stoking car. Thus, sample A and B were extracted from the position represented in Figure 88.
Samples A and B are shown in Figure 89. Sample A consists mostly of quartz and carbon particles, tied to each other by different materials. A brown, solid layer is present in large portions of the sample. Some areas have a grey color. Green and white layers cover and bind the charge. The white layers are easier to brake than the brown ones. Some charge pieces are also transparent. Sample A was broken with a hammer, and smaller samples were chosen according to their different colors, as marked in Figure 89. Ten samples were collected from sample A.
Sample B is a solid, black compound with shining grey droplets. This was a clear hint of the presence of SiC and silicon. SiC formation goes out of the scope of this thesis, therefore the sample was analyzed by Sethulakshmy Jayakumari (NTNU – Department of Materials Science and Engineering, Trondheim, Norway) [22].
Table 10 resumes the results of the visual inspection. Samples coming from this furnace will be identified with a prefix “REC” before the sample number. Samples were divided into three groups. Samples from Group 1 contains mostly the green and white compounds. Group 2 gathers samples with high amounts of the transparent compound, which is thought to be molten quartz. Group 3 consists of 5 lumps with high concentration of brown and grey materials. The “Expectation” columns collects the expectations made by Wegge [83] on the content of the samples.
105 Figure 89: Fragments of industrial samples from Elkem Kristiansand. Examples of compounds of different colors are highlighted by the circles in the figure.
Table 10: Overview of samples extracted from sample A, REC Solar.
Sample number Visual inspection Expectation [83]
REC 1.1 Green and white particles Molten quartz and SiC
REC 1.2 Green and white particles Molten quartz and SiC
REC 1.3 Black and green particles Carbon, SiC, quartz
REC 2.1 Transparent and black particles Quartz, carbon, (SiC, quartz in minor amounts)
REC 2.2 Transparent material Quartz
REC 2.3 Black particles and transparent
material Carbon, SiC, quartz.
REC 3.1 Brown and grey compounds
Brown condensate, porous and molten silica, quartz, silicon carbide
REC 3.2 Grey and white particles, small amounts of brown REC 3.3 Brown and grey compounds, mixed
with white particles REC 3.4
Salten
The samples analyzed from furnace n°2 in Elkem Salten (Norway) come from the excavation performed by Ksiazek in 2018 [81] (Figure 90). Samples coming from this excavation will be identified with a prefix “S” before the sample number. Samples S.22 and S.23 were located between port 1 and electrode 2. They have fallen from the furnace top, as the top of the charge was being removed while the samples were being extracted. Sample S.22 is white and pale green. Sample S.23 is orange and white and disintegrates easily into crumbs when handled.
Sample S.36 was found behind the refractory wall of the furnace, between port 4 and 5. The sample is made of a grey-brown compound which embeds green and white particles. Table 11 contains the sample details at visual inspection.
Figure 90: Estimated position of samples S.22, S.23 and S.36. Revised from Ksiazek [84].
Table 11: Overview of samples analyzed from Salten furnace n°2.
Sample number Visual inspection
S.22 White-green solid lump
S.23 Orange porous compound
S.36 Grey-brown compound binding green
and white particles
Wacker 1
Samples extracted from Wacker furnace n°1 come mostly from the electrode zone and upper charge areas. The furnace was excavated by Ksiazek et al. [2], and the samples come from the same study. Samples coming from this excavation will be identified with a prefix “W1” before the sample number.
Figure 91 shows an overview of the samples analyzed, and their position in the furnace. Sample W1-31 was extracted close to the electrode area, at a height of 1.7 m above the taphole. A part of the sample is green and contains mostly SiC and charge. The sample surface has also a soft white and blue coating, easy to remove, which is expected to be white condensate. Sample W1-34 consists of charge bound by green condensate. It is located slightly above sample W1-31, but on the other side of the electrode. Sample W1-43 was taken from the center of the furnace, on the border between loose charge and reacted charge. It is located near sample W1-34, but closer to the furnace core. Samples W1-38, W1-39 and W1-40 were extracted at heights between 1.5-2 m from the taphole level, at the furnace walls. Sample W1-43 comes from the low T central zone.
107 Figure 91: Overview of samples analyzed from Wacker 1 excavation. Revised after Ksiazek et al. [2]
Table 12: Overview of samples analyzed from Wacker 1.
Sample number Visual inspection W1-31 White-blue wool coating W1-34 Brown/greyish condensate
W1-38 Porous white compound
W1-39 Porous white and green compound
W1-40 Porous grey compound
W1-43 White layer on charge lump
Wacker 4
The furnace was excavated earlier by Ksiazek et al. [2] and some samples were also analyzed by Bø [55]. The samples chosen from the excavation of Wacker 4 were taken in proximity of an electrode (Figure 92). Samples coming from this excavation will be identified with a prefix “W4” before the sample number. Samples W4-29 and W4-30 are white, and they come from the area where the charge particles start sticking to each other.
Sample W4-34 comes from the loose charge area. Both Bø and Ksiazek identify it as a condensate at visual inspection. Sample W4-42 comes from the furnace bottom. The sample was identified as a slag, but some portions of it could contain Si-SiO2 condensate .
Table 13: Overview of samples analyzed from Wacker 4.
Sample number Visual inspection
W4-29 White
W4-30 White
W4-34 White/grey lump
W4-42 Black and white lump
Figure 92: Overview of Wacker 4 furnace and sample positions, revised after [55], [57].