Thermodynamics of Boron Removal from Silicon Using CaO-MgO-Al 2 O 3 -SiO 2 Slags
- PDF / 656,343 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 5 Downloads / 233 Views
THE thermodynamic properties of boron in slag and silicon have been in focus for some time due to the possibility of removing boron from silicon by slag refining. Slag refining is one of the few metallurgical refining methods that can be used to remove boron from silicon. This is a part of a method for refining silicon directly to solar grade using metallurgical techniques. Lower cost, less energy use, and a reduced carbon footprint are the potential benefits of using a purely metallurgical refining process instead of a chemical purification method. The conventional technique for making solar grade silicon is a chemical vapor deposition method using chlorosilane or silane. Knowing the thermodynamic properties of boron in slags is important for understanding the refining process. Slag refining for boron removal is an oxidation process where boron dissolved in silicon is oxidized at the slag-silicon interface and absorbed into the slag through the reaction 3 3 B ðlÞ þ SiO2 ðlÞ ¼ Si ðlÞ þ BO1:5 ðlÞ 4 4
½1
LARS KLEMET JAKOBSSON is with Elkem Technology, Fiskaaveien 100, 4675 Kristiansand, Norway. Contact email: [email protected] MERETE TANGSTAD is with the Department of Materials Science and Engineering, NTNU Norwegian University of Science and Technology, Høgskoleringen 1, 7491 Trondheim, Norway. Manuscript submitted June 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The stability of boron oxide is similar to that of silicon dioxide.[1] In addition, there is no strong interaction between boron oxide and silicon dioxide in slags,[2] or between boron and silicon in metal.[3] Altogether, this means that boron does not have a strong affinity for either slag or silicon, as will be seen from the experimental data discussed later in this article. The ability of a slag for extracting boron from silicon can be expressed by the distribution coefficient of boron, LB , which is given by wBin slag xBO1:5 in slag LB ¼ ¼ kw!x ½2 wBin Si xBin Si where w is the weight fraction, x is the mole fraction, and kw!x is the conversion factor going from weight fraction to mole fraction. The concentration of boron in the system is at parts per million level in this work. Since the interaction between boron and silicon is relatively weak in both metallic and oxide form, the assumption in this work is that boron behaves as a Henrian solute in both slag and silicon. This may not be true regarding interaction of boron with other components in the slag, but Henrian behavior of boron was still assumed because of the limited thermodynamic data available to make any other assumption. If this assumption is not valid, the activity coefficient of boron oxide in slag is a regular activity coefficient rather than a Henrian activity coefficient. The measurements are, however, done at low boron contents. Therefore, a possible deviation from Henrian solute behavior is expected to be relatively small.
Hence, the equilibrium for oxidation of boron is given by 3=4
3=4
K1 ¼
aSi aBO1:5 3=4
aB aSiO2
¼
aSi xBO1:5 coBO1:5 3=4
xB coB aSiO2
½
Data Loading...
