Formation Mechanism of In Situ Intergranular CaZrO 3 Phases in Sintered Magnesia Refractories
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I.
INTRODUCTION
MAGNESIA-BASED refractories have been widely applied in high-temperature industries such as steelmaking and cement production owing to their high melting point and excellent corrosion resistance to iron-rich and basic slags.[1–3] The fabrication of magnesia aggregates also plays a crucial role in the preparation of magnesia-based refractories. Since magnesia aggregates have several shortcomings, such as low sinterability,[4] high thermal conductivity,[5] poor penetration resistance, and poor thermal shock resistance,[6] various additives such as oxides and some halides are frequently used to improve their performance.[7–9] Meanwhile, CaZrO3 is widely utilized in magnesia-based refractories as a second phase for improving their mechanical strength and resistance against basic phases and alkali attack.[10,11] The CaZrO3 phase has high refractoriness (~ 2340 C), compatible with MgO, since these materials cannot react with each other, and
YONGSHUN ZOU, HUAZHI GU, AO HUANG, and LVPING FU are with the The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, No. 947 Heping Avenue, Qingshan District, Wuhan 430081, Hubei, P.R. China. Contact e-mail: [email protected]. Manuscript submitted April 2, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
do not form any liquid phases at temperatures lower than 2060 C.[11,12] Generally, the CaZrO3 phase is introduced into magnesia-based refractories via two methods: (1) synthetic CaZrO3 powders or aggregates are directly mixed with magnesia aggregates and sintered at high temperature,[13,14] and (2) zirconia (ZrO2) or zircon (ZrSiO4) is mixed with dolomite (MgCa(CO3)2) or MgO-CaO aggregates and sintered at high temperature.[12,15,16] Nonetheless, for the first method, since CaZrO3 is quite rare in nature and needs to be pre-synthesized, this method is limited by the high cost of purchasing synthetic CaZrO3.[11] Furthermore, for the second method, the introduction of CaO or SiO2 may increase the formation of low-melting phases in magnesia aggregates, resulting in decreased performance. Meanwhile, it is known that natural magnesia aggregates may contain impurities such as CaO, SiO2, and trace amounts of Fe2O3 (and/or FeO) or Al2O3, most of which are located at magnesia grain boundaries.[3,17] The interactions among these impurities at high temperature can lead to the formation of low-melting phases such as monticellite (CMS) and merwinite (C3MS2). Therefore, it is worth investigating if it is possible to synthesize a CaZrO3 phase while also altering the intergranular-phase compositions by relying on the interactions between added ZrO2 and CaO impurities at magnesia grain boundaries.
It should be mentioned that ZrO2 is also one of the most commonly used additives in the fabrication of magnesia aggregates.[18–21] Many previous works have proposed that the addition of ZrO2 not only improves the sintering of magnesia aggregates by intensifying secondary liquid phase formation and promoting diffusion through vacancy creation,[1
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