Research

The product of this step is referred to as a “carbon-silica product”. The level of mineral reduction sought depends on what the carbon-silica product will be used for. If it will be used to produce silicon nitride or silicon carbide for advanced ceramic applications, the level of mineral reduction required is moderate. On the other hand, if it will be used to produce silicon nitride that will, in turn, be use to produce photovoltaic silicon, the level of mineral reduction sought is very significant. In particular, for use in producing photovoltaic silicon via silicon nitride, it is critical that the levels of phosphorous and boron in the leach product be very low. The optimum fixed carbon: silica mole ratio in the leach product also depends on what the carbon-silica product will be used for. If the product will be used for production of silicon nitride the optimum fixed carbon: silica ratio is around 2:1. On the other hand, if the product will be used for production of silicon carbide the optimum fixed carbon: silica ratio is about 3:1. For all uses it is desirable for the carbon silica product to have a high level of porosity as this enhances the rate of reaction of the carbothermal step.

Si Options has carried out extensive tests to determine the affects of key leach conditions on the mineral purity, fixed carbon: silica mole ratio and porosity of the sulfuric acid leach product. The leach conditions tested are: 1) concentration of rice hulls in the leach mixture; 2) concentration of sulfuric acid in the leach mixture; 3) temperature of the leach mixture, and 4) leach time. Though its investigations, Si Options has discovered that temperature has a significant affect on both the mineral purity and the fixed carbon: silica mole ratio of the leach product (carbon-silica product). With regard to mineral purity, Si Options has discovered that close to 99% of the total minerals can be removed by leaching with sulfuric acid at temperatures at 94 deg C or less. Under these conditions the minerals of sodium (Na), magnesium (Mg), aluminum (Al), potassium (K), calcium (Ca), and manganese (Mn) can be reduced to levels less than 10 ppm in the carbon-silica product. These conditions also greatly reduce the level of phosphorous, but not to the extent that the other minerals are reduced. The level of phosphorous in the carbon silica product produced at 94 deg C or less is about 60 ppm. The level of boron is naturally very low in rice hulls, so the level of this element remains low in the carbon silica-product. With regard to phosphorous, a critical discovery of Si Options is that almost all of the residual phosphorous remaining after leaching at 94 deg C or less is associated with the surface of the silica particles in rice hulls and this phosphorous is “trapped” by a layer of organic material and this layer of organic material can be solubilized by sulfuric acid at a temperature around 120 deg C. Once the layer is solubilized the sulfuric acid can reach the phosphorous bound to the surface of the silica particles and release it. The resulting phosphorous levels in the leached product are less than 4 ppm and close to 1 ppm. Thus, increasing the leach temperature above “boiling” is critical to achieving the phosphorous levels needed for the production of photovoltaic silicon via Si3N4. Furthermore, Si Options has discovered that the minerals leached in each temperature range are released quickly in about 15 minutes. Si Options has found that acid concentration and rice hull concentration also have some affect on the release of minerals. Though its understanding of the affects of temperature, time, rice hull concentration and acid concentration on the mineral content of leach product (carbon-silica product),Si Options can recommend the optimum set of conditions for producing a carbon silica product with a mineral content suitable for the production of the end product of interest, whether it be silicon carbide or silicon nitride for the production of advanced ceramics or silicon via Si3N4 for the production of photovoltaic panels.
Si Options has also discovered that temperature has a major affect on the fixed carbon: silica mole ratio of the leached product (carbon-silica product). At the relatively low leach temperature of25 deg C (room temperature) the fixed carbon: silica ratio of the carbon silica-product is about 3:1. As the leach temperature increases the ratio declines and reaches a minimum of 2:1 at about 120 deg C. As the temperature increases the ratio in creases and reaches 3:1 at about 160 deg C. At higher temperatures and moderate times the ratio reaches a high level of about 5:1. Si Options has found that acid concentration and rice hull concentration also have some affect on the fixed carbon: silica mole ratio. Though its understanding of the affects of temperature, time, rice hull concentration and acid concentration Si Options can recommend the optimum set of conditions for producing a carbon silica product with a fixed carbon: silica mole ratio suitable for the production of the end product of interest, whether it is silicon carbide, silicon nitride or silicon via silicon nitride.
Moreover, through its understanding of the affects of temperature, time, rice hull concentration and acid concentration on the characteristics of the leach product (carbon-silica product)