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Metallurgical and Materials Transactions B, 2015, Volume 46, Issue 1, pp 62-73. Jeff Kline1,2, Merete Tangstad1, Gabriella Tranell1
- Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7491, Trondheim, Norway and
- Cliffs Natural Resources, Northshore Mining, Cleveland, USA.
Abstract
Raman spectroscopy as an instrumental technique for the determination of silicate structure is widely accepted. This method was utilized for analysis of structural modifications associated with the addition of network modifying species. Silicate melts are described by the extent of oxygen bridging. Therefore, understanding the effect different oxides have on modifying the silicate structure will provide valuable information regarding the removal of boron from silicon in slag refining processes. Samples in the range CaO/SiO2 = 0.56 to 1.2 were evaluated with and without varying concentrations of B2O3. As expected, increasing the CaO content resulted in an increase in the Q 0and Q 1 species and a decrease in the Q 3 species indicating a depolymerization of the silicate network. The addition of B2O3 to the 36 wt pct CaO-64 wt pct SiO2 system resulted in a decrease in ring-typed structures associated with the vibrational mode near 600 cm−1, an increase in the Q 3species and a decrease in the Q 2 species. Adding B2O3 to the 54.5 wt pct CaO-45.5 wt pct SiO2system resulted in decrease in the Q 0 and Q 2 species and an increase in the Q 3 species. Thus, both systems indicate the introduction of B2O3 to the more polymerized structural units in the silicate network. The increase in the peak near 630 cm−1 signifies some formation of ring-type metaborate groups or ring-type danburite groups. A correlation between the experimentally determined Qn distribution and optical basicity is proposed. Viscosity and optical basicity are correlated for the CaO-SiO2 system as well as viscosity and Qnexp
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Ssignificance statement
This publication provides an evaluation of the structural modifications associated with the addition of calcium oxide and boron oxide to the silicate network. The development of relationships used to characterize the structure of silica and the current understanding of alterations to the silicate network posed by calcium oxide and boron oxide is reviewed. The incentive in determining silicate structures and the modifications associated with the addition of calcium oxide stems from the connection to physical properties of industrially relevant compositions.
The lack of an extensive knowledge base on the thermodynamics, kinetics, diffusivity, density, viscosity, and other physicochemical properties of silicate slag systems provides an opportunity for fundamental research as these properties are crucial to the design of high temperature reactors. Improved understanding of these properties will provide insight into new means for optimizing purity and reaction time. However, the high temperature processes involved in the production of silicon complicates accurate determinations of these fundamental properties. For this reason, accurate and reliable data is limited.
Understanding the thermodynamic and kinetic properties of silicate liquids is crucial for modeling high temperature processes as well as optimizing commercial slag compositions. Most metallurgical processes involve heterogeneous chemical reactions. For instance, metal refining and purification processes often depend on the transfer of matter between immiscible phases. Typically, these phases are the molten metal (or metalloid, in the case of silicon) and slag.
In a refining process, the extent of polymerization directly affects the transport of silicon and impurity species between the metal and slag. Consequently, silicate melt structures are frequently characterized by the extent of polymerization in the system, which is determined by the acidic or basic properties of the melt. A prevalence of acidic components, such as silica, results in extensive polymerization, while basic melts exhibit a lower degree of polymerization. The degree of polymerization is related to the predominant form(s) of oxygen bonding in the melt. Information about the extent of bridging oxygen atoms in the melt assists with the understanding of the structure of the melt.
