Silicon for the direct process to methylchlorosilanes

Abstract

The available literature in the field of the direct process to methylchlorosilanes has been reviewed and commented. An apparatus and operation procedure have been developed to study the effect of structure on the direct process to methylchlorosilanes. The reproducibility was found to be better than ±5% on both reactivity and selectivity. However, a very careful control is required to be able to obtain such a reproducibility. Reproducible stirring and vibration (movement in the bed) conditions were found to be the most critical factors. There is a significant difference between the Low Aluminium (LA, 0.12 wt% Al) and the High Aluminium (HA, 0.6 wt% Al) samples. HA is approximately 30% higher in reactivity and 15% poorer in selectivity (Tri/Di) compared to LA. The effect of higher Al on reactivity is reduced by serious agglomeration problems. These agglomeration problems occur not only at start-up, but during the whole run. Al is expected to be necessary in the start up period, but as the silicon is activated, lower levels can be accepted. Heavy deposition of AICI3 in the tubing was observed for the HA samples and is expected partly to be the reason for the poor selectivity. The two samples with low Al had very different gram structures. Surprisingly, the slow cooled sample had the finest gram structure, which was believed to be caused by a change in the crystallisation path from columnar growth to equiaxed growth due to the very slow cooling conditions. Compared to the quickly cooled sample (LAR) the slow cooled sample (LAS) had 8% improved reactivity and 1 1 % improved selectivity. Metallographic examination of the LAS sample showed the gram structure similar to a rapid cooled silicon, but the impurity distribution typical for an annealed silicon. Between the two high Al samples only minor differences were observed. No significant difference was observed in reactivity between the rapid cooled (HAR) and the slow cooled (HAS) samples. But it should be noted that HAS showed improved selectivity at higher silicon conversion compared to HAR. Clogging and agglomeration was a problem for both samples, but especially for sample HAS, probably due to larger intermetallic phases. This examination shows that a finer grained material has improved performance, but the effect of structure is more important for qualities with low Al content. The effect of structure is much less important than the effect of chemical composition, and it does not make sense to start optimisation of structure if the chemical composition and metallic impuritiy content are out of control. The gram boundaries were found to react much faster than the rest of the silicon, being responsible for the improved reactivity in a fine grained material. Areas with large amounts of defects are expected to behave similarly since they, as gram boundaries, have a high dislocation density. It is shown that it is possible to produce a small grained silicon quality with slow cooling. The mechanism for this process is not fully understood. Very slow cooling, that probably is required, is difficult to realise in industrial production. Annealing of silicon was found to change the shape of the intermetallic phases and the defect structure. The long thin bands of impurities normally observed in silicon were changed to more spherical inclusions. The defect density was increased by annealing, probably due to the stress stored in the solidified ingot, but the amount of cracks are lower in the annealed sample. Thick castings showed a structure similar to the annealed samples. Annealing is expected to be beneficial for the performance of the silicon, botn due to the increased defect density increasing reactivity, and the round intermetallic phases and change in intermetallic phase composition improving selectivity. CaSi2 was found to be detrimental to performance in the direct process, mainly because it surrounds the other intermetallic phases and thereby reduces the availability of Al. The start-up period of such a quality is especially critical because it is mainly then the Al is required. High Ca is also associated with a higher amount of slag in the silicon due to the lower viscosity and higher density of the slag phase. Oxygen compounds in the methyl chloride were found to be very detrimental to the reactivity and selectivity. If these compounds are present during activation of the silicon a poor distribution of the catalyst on the silicon surface is observed. The catalyst seems to agglomerate rather than deposit on the silicon particles. As in other studies, higher temperature was found to increase the reactivity. But in addition it was found that the increase in reactivity depends on whether the methyl chloride was purified or not. With purified methyl chloride the reactivity increased by 40% when the temperature was increased from 300 to 320°C. With unpurified methyl chloride an increase of 130% was observed for the same temperature rise. This result indicates that the operating temperature should be adjusted depending on the amount of oxygen present in the methyl chloride. A pure methyl chloride should allow a lower reaction temperature than a less pure methyl chloride. It was also found that lower temperature improves the selectivity, indicating that very pure methyl chloride and low reaction temperature should give the best results in the direct process. Oxygen in silicon was found to be less important. The main influence was the lower amount of active Al and Ca due to the presence of these impurities in the slag phase instead of in intermetallic phases. However, the results obtained here are from a batch reactor and it is likely that other results will be observed in a continuous reactor due to build up of inert particles in the reactor. A ternary chloride melt of CuCI, ZnCtø and SnCtø was found likely to be present and the synergy effect of Sn and Zn is probably due to the reduction of the melting point. Other chloride combinations that lower the melting point and do not form stable bonds with silicon are likely to be potential promoter systems. Elements giving complexation effects with CuCI are also potential promoters. The surface oxygen was not examined in this study. The intermetailic phases that were found to be without beneficial influence on performance when mixed into the contact mass, were found to behave different if they are present in the silicon. The intermetallic phases will distort the silicon lattice and act as nucleation points for the catalyst deposition. The small cracks created by the intermetallic phase are very active areas. The round intermetallic phases formed upon annealing seem to generate more cracks than the thin bands. Again the long term influence of the impurities has not been examined, but a study is in progress to determine the influence of the main impurities. A chain mechanism involving a surface temperature higher than the bulk gas temperature was considered to explain the sudden start up of the direct process. Probably due to the heat generated in the chlorination of the impurities like Al.