Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol Structure and its effect on viscosity of fluorine-free mold flux: Substituting CaF 2 with B 2 O 3 and Na 2 O Tae-min Yeo a , Jung-Wook Cho a, ⁎ , Marco Alloni b , Simone Casagrande b , Riccardo Carli b a Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang, 37673, Republic of Korea b Research & Development Department, Prosimet S.p.A., 10 24040 Filago BG Italy ARTICLE INFO Keywords: B 2 O 3 Na 2 O CaF 2 Viscosity Mold flux Sodium Raman spectra Solid-state NMR spectra ABSTRACT Fluorine in mold fluxes causes environmental pollution by dissolution into cooling water or evaporation, which considered seriously hazardous to human health and casting facilities. In order to develop a successful fluorine- free mold flux system, viscosity of borosilicate-based fluxes were investigated in consideration of structural evolution of borates when replacing CaF 2 with B 2 O 3 and Na 2 O. Measured viscosity decreased at the beginning of replacing CaF 2 (15.74 to 10.77 wt%) with B 2 O 3 (0 to 4.49 wt%) and then increased drastically further sub- stituting CaF 2 (10.77 to 4.71 wt%) with B 2 O 3 (4.49 to 9.02 wt%). An actual determinant of viscosity is not the degree of B 2 O 3 replacement, but rather the portion of tetrahedral borate structures (3-D) which necessarily need support of cations (sodium). Moreover, due to 19 F NMR, it is suspected that fluorine tends to interact selectively with sodium, and consequently interferes with formation of tetrahedral borate structures. 1. Introduction Fluorine in mold fluxes is used as an additive to (i) control melting temperature [1], (ii) form principal crystalline phase (Cuspidine), (iii) reduce viscosity by depolymerization of silicate network [2]. Despite its criticality, fluorine has been limited due to some environmental and industrial issues: (i) evaporation and formation of HF [3], (ii) erosion/ corrosion of SEN and caster frame [4,5], (iii) cost for pretreatment before discharging waste water [1], etc. In order to reduce these detrimental effects, B 2 O 3 and Na 2 O have been regarded as the alternative to CaF 2 in conventional mold flux systems. It is well known by previous studies that the role of B 2 O 3 and Na 2 O is similar to that of CaF 2 as a flux to reduce melting temperature [6]. However, the contribution of B 2 O 3 in terms of viscosity is quite controversial. Mills et al. [7] found that viscosity of mold fluxes de- creases when replacing CaF 2 with B 2 O 3 . On the other hand, Benavidez et al. [8] reported that by replacing fluorine with boron oxide, a great increase of viscosity was produced. To elucidate uneven viscosity behavior, importance of chemical structure identification has come to the fore [9]. Shin et al. [10] proved that variation of viscosity depends on the form of borate; [BO 3 ]-trian- gular or [BO 4 ]-tetrahedral structure. Measured viscosity showed trend that increases at first with increasing B 2 O 3 contents, however, it turned into decreasing tendency with further B 2 O 3 additions. At the early stage, most of borates existed in the form of [BO 4 ]-tetrahedral struc- ture, which could combine successfully with silicate anions, thus visc- osity increases. When further amounts of B 2 O 3 are added, [BO 3 ]-tri- angular structures are predominantly formed due to limited alkali oxide. Increasing viscosity with [BO 3 ]-triangular structures is extremely hard because most of [BO 3 ] units are generally presented in the form of boroxol rings themselves so far from combining with silicate networks [1]. Moreover when excessive amounts of B 2 O 3 are added, partial re- placement between [BO 3 ]-triangular and silicate unit is doubted which may lead to decrease viscosity [2]. By degree of polymerization analysis it is verified that [BO 3 ]-triangular structures could simplify and even attack silicate network. In order to develop a successful fluorine-free mold flux, a series of new slag systems is designed by application of B 2 O 3 and Na 2 O in sub- stitution for CaF 2 . Until now, there are few experimental works in- vestigating the effect of B 2 O 3 structure on viscosity substituting CaF 2 with B 2 O 3 . In this research, the effect of polymeric structure on visc- osity has been investigated using NMR and Raman Spectroscopy with an emphasis on stability of charge compensation between B and Na at elevated temperature. https://doi.org/10.1016/j.jnoncrysol.2019.119756 Received 22 July 2019; Received in revised form 2 October 2019; Accepted 26 October 2019 ⁎ Corresponding author. E-mail address: jungwook@postech.ac.kr (J.-W. Cho). Journal of Non-Crystalline Solids xxx (xxxx) xxxx 0022-3093/ © 2019 Elsevier B.V. All rights reserved. Please cite this article as: Tae-min Yeo, et al., Journal of Non-Crystalline Solids, https://doi.org/10.1016/j.jnoncrysol.2019.119756