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Kurt Aasly, Norwegian University of Science and Technology, Geology and Mineral Resources Engineering (Norway)
Terje Malvik, Norwegian University of Science and Technology (Norway)
Edin Myrhaug, Elkem AS (Norway)
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Metallurgical quartz is an expression for quartz that is used as source for Si in the silicon and ferrosilicon processes. Typical sources for metallurgical quartz are hydrothermal vein quartz, quartzite and boulders from e.g. fluvial deposits. Quartz from large pegmatite cores are also potential sources but are presently not frequently used. Optimal performance of the silicon furnace requires charging of lump quartz with a typical size of 10 to 150 mm.
The thermo-mechanical properties of quartz are important for avoiding generation of fines in the furnace as a result of the temperature shock as the quartz is charged on the furnace. The initial temperature at the top of the charge is somewhere between 700 and 1300 °C (Grdahl et al., 2000. Environment and furnace processes part III. SINTEF report no STF24F00600).
Previous investigations (Aasly et al., 2007. Advanced methods to characterize thermal properties of quartz. INFACON XI, 381-392) indicated that the phase transformation from β-quartz to cristobalite may be significant for the thermo-mechanical strength of quartz on the silicon furnace.
Shock heating experiments shows that different types of quartz transforms to cristobalite at different rates and that the transformation is initiated at different temperatures. Samples from six types of metallurgical quartz where heated to temperatures from 1250 to 1550 °C for ten minutes. The cristobalite content at 1550 °C varies from 0.4% to 9.8%. Cristobalite was detected in five of the six samples at 1250 °C. Only one sample showed initial formation of cristobalite at higher temperature, 1350 °C. This is also the sample that shows the lowest cristobalite content at 1550 °C. Macroscopic inspection of the products of shock heating shows that the most cristobalite rich sample leaves coarser fragments after heating than the least cristobalite rich. However, the content of fines (-1.658 mm) is higher in the cristobalite rich sample.
Cathodoluminescence (CL) microscopy and spectroscopy investigations show a peculiar feature for two of the heated samples. This is a spotted red CL seen as small red luminescent areas along grain boundaries and micro fractures and is most pronounced in the most cristobalite rich sample. This is probably related to none-bridging oxygen hole centres and are probably caused by transformation of quartz to cristobalite or an intermediate amorphous phase that follows the phase transformation.
The investigations presented have shown that the formation of cristobalite results in more fine material caused by the break down of Si-O bonds in the quartz during transformation. The rate of cristobalite formation may be important for how the quartz behaves in the ferrosilicon or silicon furnace.
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