The mechanisms in the carbothermic production of silicon have been examined in this report. Through the investigation of a pilot scale furnace as well as five small scale induction furnace experiments, a better understanding of this comlex process has been obtained. Especially the silicon producing reactions and mechanisms have been studied.Samples of all raw material as they travelled downwards in the pilot scale furnace was investigated along with samples from the induction furncace. SiO gas formed in the hot zone (2000 °C) travelled upwards and deposited as a condensate mixture of SiO2 and Si. In the pilot scale furnace this condensate went back down with raw material as the furnace was stoked or the raw materials slowly melted. The condensate decomposed as the temperatures increased; silicon accumulated and escaped from the SiO2-matrix in the condensate.A typical experiment conducted in the induction furnace had a specic power consumption of roughly 62 MWh per ton silicon produced, far more than normal industrial power consumption of 11-13 MWh per ton Si produced. Mass balance demonstrated that just over half of the silicon produced were left in the silicon pool in the bottom of the crucible. The other mere half had to end as silicon droplets in the condensate deposited in the upper portion of the crucible.The main silicon producing reaction is: SiO(g) + SiC(s) = 2Si(s,l) + CO(g), but the findings in this thesis have shown that the perspiration of silicon from the condensate is very important. A significant contribution to the total amount of silicon produced can therefore come from the following reaction: 2SiO(g) = SiO2(s,l)+ Si(s,l)Compression tests have been made on the agglomerate caused by the deposits of condensate (SiO2+Si). The compression strength varied from 115 to 396 MPa. Samples exposed to temperatures above 1670 °C had the lowest strenght, while those exposed to lower temperatures were the strongest.

As the silicon content in a silicomanganese alloy increase, silicon carbide becomes the stable carbon phase. Little work is published on the formation of silicon carbide in the SiMn process. This thesis examines the formation of SiC through the reaction between slag, metal and coke. The goal of the thesis has been to determine where and how SiC is formed in the silicomanganese process. Focus has been given to formation through liquid-solid reactions.The investigation was carried out by heating metal and slag together with coke. XRD analysis of SiC formed in the process, as well as wettability testing of slag and metal toward SiC and graphite substrates were also examined. The raw materials consisted of two industrial SiMn alloys, named Metal 1 and 2 in the thesis. These had 19 and 28 wt.% Si, respectively. Two slags, Slag 1 and 2, were used in combination with the metal. SiC was found to form on the coke particle through reaction with both slag and metal. The formation was fastest through the metal phase and the effect of increasing silicon content in the metal was evident. A temperature increase from 1600 C to 1650 C resulted in less SiC formed on the coke particle surface for Metal 1, but gave more SiC at the surface of the coke particle for Metal 2. The decrease in SiC on the coke particle for Metal 1 is likely to be caused by a decreased driving force, as the coexistence point of C and SiC increases with increasing temperature. For Metal 2 the relative distance is smaller, making the effect neglectable. The formation of SiC occurs at the surface of the coke particle. The metal samples indicate that carbon diffuses through the SiC layer to react with silicon in the alloy. Formation through the slag is likely to go through the reduction of SiO2 to Si, before reacting to form SiC. The XRD analysis determined beta-SiC to be the dominant phase formed through both slag and metal. alpha-SiC was found in one of the industrial slag samples, indicating temperatures higher than 1700 C. The wettability testing showed both slag and metal to be non wetting toward the SiC substrate. All angles were stable with increasing temperatures. Both slags were found to be non wetting toward graphite. The wetting angle of Slag 2 was stable with increasing temperature. Slag 1 showed decreasing wetting angle with temperature, before an abrupt change back to an angle larger than the initial angle. The sudden change is likely to be caused by a reaction product, possibly SiC, forming between the substrate and slag.

This master thesis is based on reduction experiments of pelletized Tyssedal ilmenite. The focus is set on the effect of changing parameters such as gas composition, gas flow, pellet size and pre-oxidation condition. The results are interpret regarding values of conversion, degree of metalization, XRF-analysis and microstructure images. Based on these results the aim was to find out how these parameters influenced reduction. Another goal was to reveal what hampered reduction inside both grains and pellets and caused the two stage reduction behaviour. EPMA was used to examine pellets and grains and to look for explanation for the slow reduction. XRD-analysis was decisive for explaining the importance of optimum pre-oxidation for the subsequent reduction. Pre-oxidized and green pellets with a size fraction of 8-10 mm were heated in a thermogravimetric analyzer. Ar was used during heating and cooling. Reduction occurred at 930-940 °C, with either CO, H2 or both for up to 45 minutes. Gas flows used were 4.8, 7 and 9 Nl/min. In total 18 experiments were carried out. Weight before and after reduction was measured, calculations of degree of metalization, XRF- and XRD-analysis was done. Microstructure images, point analysis, line scan and mapping were obtained by EPMA. Highest final values of conversion, c. 0.9, were obtained for pre-oxidized pellets from 2011 reduced with H2. Maximum degree of metalization, 94-99 % were obtained after reduction of pre-oxidized pellets from 2010. Final value of conversion and degree of metalization were 0.23 and c. 24 % higher for pre-oxidized pellets from 2010 than from 2011. Only green pellets from 2011 was harder to reduce with CO. Reducibility was not increased with the pre-oxidation conditions at ETI Tyssedal in January 2011. In spite of that the pre-oxidized pellets from 2010 and 2011 are made in the same manner and from the same raw material they react differently during reduction. This should be kept in mind when comparing results from different sources. Reduction of pre-oxidized pellets occurred in two distinct linear stages separated by a clear bend. A normal behaviour with steadily decreasing oxygen removal rate was observed with green pellets. An increased flow of CO from 7 Nl/min to 9 Nl/min resulted in decreased reduction. Examination by EPMA confirmed the presence of the barrier effect and higher amount of oxygen 0.6-1.0 mm from the surface. This indicates hampered reduction in grains and pellets. This combined with slow migration of CO/CO2 in the pellets can partly explain the unusual reduction behaviour. Pre-oxidized pellets from 2010 contains higher amount of the easily reducible M3O5, and less M2O3 compared to pre-oxidized pellets from 2011. The less reducible M3O4 was also found in the pre-oxidized pellets from 2011, making them harder to reduce.

The aim of the thesis is to produce high grade silicon from off-grade materials like sculls and process slags from Elkem Thamshavn. The methods investigated are dense medium separation, optical separation, flotation and metallurgical separation by remelting. Dense medium separation trials are conducted using magnetite suspended in water and aim to find the suspension density where only one phase will float. Optical separation experiments are done to identify light intensity thresholds between refractories, silicon and slag. The viability of flotation without surface activators and at neutral pH are investigated by measuring the zeta potential of each phase. Remelting experiments are done, building on previous work by the author, with the aim of investigating whether adding CaO or MgO to the slag will increase the settling efficiency. No results were obtained in the DMS experiments, because the viscosity of the suspension increased to infinity before the density of either slag or silicon was reached. The flotation experiments showed that flotation is not viable at neutral pH without surface activators, as the zeta potential of slag and silicon is nearly identical. The optical separation experiments were a success. The product fraction had an average silicon content of 74 wt%, compared to 52 wt% in the original off-grade material, while the waste fraction contained 7.5 wt% silicon. The separation efficiency was best for coarser grains, which is the expected result based on theory. The remelting experiments concluded that adding CaO or MgO to the slag will increase the settling efficiency. This was established both by chemical analysis and by surface area analysis of slag samples from each experiment. MgO seemed to contribute more to the settling efficiency than CaO, but because of the large variance between samples, this is not conclusive. The quality of the produced silicon was unaffected or improved by adding CaO, but additions of MgO increased the magnesium content slightly.

Tap hole refractories constitute critical parts of the refractory lining in submerged arc furnaces. For several hours every day, molten slag and metal flow through the tap hole calling for thorough selection of refractory materials able to withstand the intense thermal, corrosive and erosive conditions present in this area. Carbon-based refractories have shown excellent thermal properties and high strength, as well as low wettability towards process materials, and are therefore utilized in silicomanganese production furnaces both as side lining, in the hearth, and in the tap hole area. The aim of this investigation was to determine the compatibility of five refractory materials utilized in the tap hole area of an industrial silicomanganese furnace with two industrial silicomanganese slags: •Investigate the suitability of the selected refractory materials for confining the process materials during industrial production of silicomanganese alloys. •Identify critical refractory wear mechanisms upon slag-refractory interaction at industrial tapping temperatures.Compatibilities were investigated through 12 static crucible tests and two static plate tests in a vertical tube furnace redesigned during this investigation. Slag-refractory interaction was studied after two and four hours holding time at 1367°C ± 1.8°C, 1464°C ± 2.1°C and 1600°C - 0.6°C /+ 0.2°C. Holding temperatures were verified through the wire-bridge method at the melting points of gold and palladium. Visual inspection, as well as optical microscopy and SEM, were utilized to examine the samples after heat treatment.During compatibility experiments, dissolution of refractory matrix due to solubility of oxide refractory binder phases in silicomanganese slags was observed, as well as disintegration of refractory particles due to gas formation at slag-refractory interface, or expansion as a result of phase transformations in refractory material. Direct reduction of manganese oxide from slags and iron oxide present in refractories by carbon and silicon carbide was also observed. Establishment of partial slag-metal equilibriums between iron oxide and silicon metal originally present in slag was observed, as well as formation of silicon carbide at the slag-refractory interfaces. The latter may serve to protect the refractory from wear caused by slags.Based on observations of extensive interaction between silicomanganese slag sample I and ramming paste at 1600°C, the ramming paste investigated cannot be recommended for usage during tap block repair in an industrial silicomanganese furnace. Incipient electrode paste disintegration by slags and silicon carbide tap block – slag interaction were observed after compatibility tests at 1464°C, calling for further investigations of these refractory materials. Tap hole clay and carbon tap block showed minimal signs of interaction with process materials at 1464°C. Refractory porosity seemed to have a larger effect on refractory wear than refractory ash content. Contrary to industrial observations, silicomanganese slag sample I was more corrosive towards the ramming paste and electrode paste investigated than silicomanganese slag sample II.