silicon for the chemical and solar industry XIII

Silicon for the Chemical and Solar Industry XIII

Kristiansand, Norway, June 13 – 16, 2016

Growth of impurity phases below the melting point of silicon

and consequences on fluidized bed reactor efficiency

A. Broggi, H. Tveit

Norwegian University of Science and Technology, Faculty of Natural Sciences and

Technology, Trondheim (Norway)

Abstract

One of the challenges with silicones production is the accumulation of impurities

from metallurgical grade silicon (MG

Si) in the fluidized bed reactor (FBR). Previous

studies have demonstrated that the amount of intermetallic impurities is dependent on

the thermal history of silicon. Two different thermal histories were chosen to

investigate compositional changes. XRF chemical analysis in different areas of the

cast confirm the effects of oxidation refining and segregation. The software SiStruc

calculated the expected contents of phases based only on thermodynamics. EPMA and

BSE

SEM image analysis estimated the phase composition of an untreated and on the

treated samples. A comparison between expectations from SiStruc

and EPMA

BSE

combined analysis is also presented. It can be stated that there could be reactions

between intermetallic phases. The effects of the annealing and refining on the

composition are compared to the expected change on reactivity and selectivity in the

silicones production.

1. Introduction

Silicones production is the most valuable application of silicon in chemical industry.

Composition, concentration and shape of the intermetallics in MG

Si are key aspects

in silicones production. The composition of the intermetallics is affected by the purity

of the raw material, as well as by the refining and casting processes [1]. The

composition of both the raw material and the furnace electrode sets the quantities of

Fe and Ti, whereas Ca and Al contents can be corrected through ladle refining [2].

The casting technique influences the intermetallics' shape and size. The shape of the

intermetallics can be characterized by controlling the thermal history of the

solidification of silicon [3]. The aim is the best size and shape distribution, as well as

low accumulation of particles in the fluidized bed reactor. When this occurs, the

reactor is damaged and the reaction is more difficult to control [4].

Changes in shape and composition of the intermetallic compounds occur during

solidification. The driving forces of these transformations are mainly diffusion and

surface energy reduction [5-7]. Studies have been performed concerning the

characterization of the intermetallics in MG-Si, by controlling the thermal history.