Effect of Al 2 O 3 Concentration on Density and Structure of (CaO-SiO 2 )-xAl 2 O 3 Slag
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INTRODUCTION
CAO-SIO2-AL2O3
is one of the important silicate melts in the iron- and steel-making processes. Thermo-physical properties of the slag such as density, viscosity, surface tension, and thermal conductivity are crucially important to optimize the high temperature processes. Notably, these properties are highly dependent on the silicate structure.[1] Therefore, advanced knowledge of structural information is essential to develop a thorough comprehension of the slag system. In the CaO-SiO2-Al2O3 slag system, CaO is a basic oxide that acts as a modifier in silicate melts, i.e., it depolymerizes the silicate structure and provides non-bridging oxygen (NBO). The structural units of silicate melts are monomer (SiO44), dimer (Si2O76), chain (SiO32), sheet (Si2O52), and fully polymerized structures (SiO2).[2] These structural units are denoted by Q0, Q1, Q2, Q3, and Q4, respectively (Qn notation indicates n bridging oxygens per tetrahedron). Mysen et al. suggested that these structural units are equilibrated according to the quantity of non-bridging oxygen atoms per cation (NBO/T).[3,4] Q1 ¼ Q0 þ Q2 ðfor NBO=T ¼ 2:14:0Þ
½1
Q2 ¼ Q0 þ Q3 ðfor NBO=T ¼ 1:02:1Þ
½2
RAMARAGHAVULU RAJAVARAM, HYELIM KIM, and JOONHO LEE are with the Department of Materials Science and Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul, 02841, Republic of Korea. Contact e-mail: [email protected] CHI-HOON LEE, WON-SEUNG CHO, and CHI-HWAN LEE are with the Department of Materials Science and Engineering, Inha University, Inharo 100, Nam-gu, Incheon, 22212, Republic of Korea. Manuscript submitted September 29, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
Q3 ¼ Q2 þ Q4 ðfor NBO=T ¼ 0:11:0Þ
½3
In this study, NBO/T is estimated according to Eq. [4].[5,6] NBO 2XCaO 4XAl2 O3 ¼ T XSiO2 þ 2XAl2 O3
½4
On the other hand, Al2O3 is an amphoteric oxide whose equilibrium forms are suggested by Eqs. [5] through [7].[2] Al2 O3 ¼ 2Al3þ þ 3O2
½5
Al2 O3 þ 5O2 ¼ 2AlO5 4
½6
Al3þ þ 4O2 ¼ AlO5 4 :
½7 3+
ions that In a silicate system, alumina forms Al balance the free oxygen ions and also combine with free oxygen ions to form AlO45 ions. Accordingly, the relation between Al3+ and AlO45 ions will depend on the slag basicity (or the activity of oxygen ions); Al3+ is more stable in the acidic region, whereas AlO45 in the basic region.[2] If the metal cation Ca2+ balances the local charge, Al3+ may fit into the tetrahedral sites of the silicate structure. Accordingly, Si(Al) coupling occurs upon substitution of Si by Al in the tetrahedral structure of the silicate melt.[3,7–9] If there are insufficient metal cations, Al3+ may dissociate from the tetrahedral sites and act as a network modifier. This structural modification may yield changes in the thermo-physical properties of silicate melts. For example, Grundy et al. reported the relationship between viscosity and structure of the CaO-SiO2-Al2O3 system at
the Al2O3 content was below 15 wt pct, while Al3+ was high when Al2O3 content was above 15 wt pct. Although dens
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