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INTRODUCTION

BLAST furnace (BF) slag is one of the largest by-products in the extraction and refining of metals with an estimated generation rate of 320 Mt annually.[1] Due to its similar composition with Portland cement, one of the most desired uses of the slag is as cement feedstock. For this application, the molten slag must be cooled rapidly to form an amorphous, glass-like structure as otherwise, crystallized material leads to swelling of concrete upon weathering. It is therefore crucial to gain sufficient knowledge on the crystallization behavior of BF slag with the purpose of controlling the cooling conditions or manipulating the slag composition so that the desired amorphous structure is achieved. There have been several studies on crystallization kinetics of mold fluxes[1–7] due to their important role in controlling the heat transfer and lubrication in the steel continuous casting mold. However, similar investigations for metallurgical slags are scarce.[8–11] In a recent study[11] the transformation kinetics of a slag with composition close to BF slags has been studied but to this date, a comprehensive study on the effect of composition on

SHAGHAYEGH ESFAHANI is with Acuren Group Inc., Mississauga, Canada. Contact e-mail: [email protected] MANSOOR BARATI is with the Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, M5S 3E4, Canada. Manuscript submitted September 30, 2015. Article published online January 29, 2018. 590—VOLUME 49B, APRIL 2018

the rate and extent of crystallization is lacking. In the present study, the crystallization behavior of CaOSiO2-Al2O3-MgO (CSAM) synthetic slags with varying basicity was studied, aiming to establish quantitative relations between the kinetics of amorphous to crystalline transformation and slag chemistry.

II.

EXPERIMENTAL APPARATUS AND METHOD

The kinetic study was carried out using the single hot thermocouple technique (SHTT) which was first invented by Ordway[12] and was later adopted and advanced by other researchers.[3,13,14] In this technique, the thermocouple acts as the heating element at the same time that it measures the temperature of the sample in direct contact with the thermocouple. In addition to high accuracy of temperature readings, the technique allows rapid heating and cooling of the material under study, as well as direct observation using a camera mounted on top of the heating chamber. In situ visualization of the material undergoing crystallization provides data on the transformed fraction as a function of time, essential for quantifying the kinetics. Detailed information about the setup of the hot thermocouple technique can be found in Reference 15. Four CSAM slag samples with varying basicity (C/ S = CaO/SiO2) are prepared as shown in (Table I).[16] For each slag composition, 10 g of powder was mixed and melted in a graphite crucible (D = 4 cm, h = 4 cm) by heating the material in an argon atmosphere and to approximately 50 K (50 C) above the

METALLURGICAL AND MATERIALS TRANSA