Silicon for the direct process to methylchlorosilanes

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 optimisa