In cooperation with Elkem, Maria Førde Møll has written a Ph.D. thesis titled Solidification of Silicon – Macro- and Microstructure as Functions of Thermal History and Composition. Førde Møll’s project has been partially financed by the Norwegian Research Council’s industrial Ph.D Scheme. The scheme funds industry-oriented doctoral research fellowships and was established to facilitate the recruitment of researchers to Norwegian industry. The industrial Ph.D programme aims to support research work that has a close connection to a company’s long term challenges. Casting was and still is such an area in the silicon industry. Førde Møll has through her work strongly demonstrated the effect that such a support scheme might have. The Elkem Silicon Materials division currently has ongoing development projects at its three production sites in Norway that can be traced back to Førde Møll’s work.
Maria Førde Møll defended and passed her thesis in the end of January 2014. We spoke to her about the project’s results, importance and next steps.
What were the results?
Several industrial and laboratory scale experiments were performed in order to determine the factors that had the largest impact on the cast product. This was coupled with a heat transfer model of the casting. The experiments illustrated that several factors determined the silicon grain size. In addition to the effect of cooling rate - other factors were: inclusions present such as SiC, amount of alloying elements, the temperature of the melt and the formation of solidification layers during casting. The layers acted as a barrier to further growth and were important to the overall silicon macro and micro structure. This work supports that the cooling rate after the solidification of primary silicon had an effect on the size and form of the intermetallic phases. This may be utilized industrially to achieve a wanted intermetallic structure. The distribution of the intermetallic phases also depended on solidification layers, available space between the silicon grains and the amount of alloying elements. The main intermetallic phases observed in this work were consistent with former work on the same alloys. The observed intermetallic phases contained 10 to several hundred times more trace elements than the bulk analysis. A weak trend indicated that the abrasion strength of material MG-Si2 increased when the cooling rate increased. In addition an initial test where MG-Si2 was purified by means of magnetic separation showed promising results.
Why does it matter?
It is important for the silicon producer to be aware of the factors that will influence the cast product. From the furnace the melt can have the wanted bulk composition, but the casting procedure may lead to an inhomogeneous product that is not optimal in downstream processes such as hydrometallurgical leaching and production of methyl chloro silanes. For metallurgical grade silicon many of the casting procedures used at most silicon plants today are in principle the same as those developed decades ago and there is a need for improvement. The main reasons for a new and improved casting process are:
- Increasing the post taphole yield and hence reducing the total energy consumption
- Improving the product quality regarding customer value – this may include: defined silicon grain size, distribution of intermetallic phases, trace elements and homogeneity
- Meeting new requirements for working environment
- Reducing the cost for the solidification and silicon forming step
What methods did you use?
The chemical analyses of the samples were performed by XRF (X-Ray Fluorescence) and ICP-OES (Inductively Coupled Plasma- Optical Emission Spectrometry). For microstructural observations the samples were investigated in the SEM (Scanning Electron Microscopy). Analyses of the intermetallic phases were performed by EPMA (Electron Micro Probe Analyzer). A tumbler test was used to determine the abrasion strength and for the magnetic separation a dry (Permroll) and a wet (SLon100) separator was used. The heat transfer model was developed in COMSOL Multiphysics.
What are the next steps?
My PhD study has been supported by the Research Council of Norway as an industrial study and the work has been in close contact with Elkem. Some results are already in the process to be utilized. The heat transfer model with parameters and boundary conditions will be used to optimize the existing casting processes regarding yield and costumer value of the silicon products. The silicon casting process is one of the sources of diffusive emission so new and improved casting methods will be necessary.
The silicon product may seem simple – but the silicon structure, the distribution of alloying and trace elements may be important to the total performance of the product behavior in the following processes on the road to the final product.
The figure compares the result from the heat transfer model and the casted silicon. The model may be used to optimize existing casting processes and in the design of new casting methods.