Fast drying of high-alumina MgO-bonded refractory castables

Abstract

Refractory producers face many challenges in terms of producing MgO-containing castables due to the high likelihood of magnesia to hydrate in contact with water, resulting in Mg(OH)2 generation. The expansive feature of this transformation affects the performance of such refractories, as (i) if this hydrated phase is not accommodated in the formed microstructure, ceramic linings with cracks and low green mechanical strength will be obtained; and (ii) if crack-free pieces are prepared, they should present low porosity and reduced permeability, which require special attention when heating these materials. In both cases, spalling/explosion may be favored during the drying step of MgO-containing compositions. Hence, this work investigated the ability of various additives in the optimization of the drying behavior of Al2O3-MgO castables. Vibratable compositions were tested after incorporating polymeric fibers (PF), an organic salt (OAS), SiO2-based additive (SM) or permeability enhancing active compound (MP) into the dry-mixtures. Various experimental measurements were performed to infer the role of the drying agents to prevent the samples' explosion and whether they would also influence other properties of the castables. As observed, OAS and MP helped to inhibit the MgO-bonded samples' explosion even under severe heating conditions (2-20°C/min) and increased their green mechanical strength and slag infiltration resistance when compared to the additive-free composition. On the other hand, the addition of polymeric fibers (PF) or silica-based compound (SM) to the formulations was not able to prevent the castables' explosion when using a high heating rate and other side effects (samples' cracking during drying at 110°C, high linear expansion and increased slag penetration during corrosion tests) could also be observed when testing these materials. Thus, the selection of suitable drying 2 agents is a key issue, as they may allow the development of MgO-bonded castables with enhanced properties and lower spalling risk during their first thermal treatment.

Aluminum lactate role in improving hydration and drying behavior of MgO-bonded refractory castables

“…Reports in the literature suggest that the soluble salts could be replaced by reactive oxides as cation sources [17][18][19]. In these cases, hydration/hydrolysis reactions would provide the desired ions, excludi process [17-Authors' hydrotalcite and magnes alumina or c simply by c suspension, w steps [20] A n-A n-A n- According to the XRD results (Fig. 2), AL addition to the MgO-suspensions inhibited brucite formation.…”

“…Authors' previous work has reported on the formation of hydrotalcite in some systems comprised by a blend of alumina and magnesia reactive sources (respectively, hydratable alumina or calcium aluminate cement and caustic magnesia), simply by combining these raw materials in an aqueous suspension, with no need of additional reactants or purification steps [20]. This paper presents and discusses this process route in order to produce hydrotalcite.…”

Evaluation of the combined addition of aluminum lactate and calcium carbonate to alumina-based castables

“…After the cooling step, the elastic modulus of 3.2CC and 3.2CC_0.5L (120 and 115 GPa, respectively) were higher than the reference composition 6CAC (~110 GPa). However, there are two differences between refractories 3.2CC and 3.2CC_0.5L that are relevant to point out: i) the presence of the aluminum lactate gel after the drying step explains the higher initial elastic modulus of the latter (70 versus 40 GPa); and ii) its decomposition at ~375 °C [14,25] increased the porosity content in the microstructure, causing a stiffness drop of the samples in the 300-400 °C range. Furthermore, Fig.…”

“…Although it was observed a drop in the HMOR values measured between 1200-1450 °C, the results obtained for 3.2CC and 3.2CC_0.5L were still similar to the ones of the reference 6CAC. Moreover, the aluminum lactate gel provided a better thermomechanical performance at 300 °C for 3.2CC_0.5L when compared to 3.2CC (5 versus 3 MPa), whereas its decomposition at ~375 °C [14,25] gave rise to pores in the resulting microstructure (Fig. 5b), which affected its hot modulus of rupture at higher temperatures (600-1450 °C).…”

“…Considering these aspects, a literature survey for additives that could induce a binding effect for alumina refractory castables containing calcium carbonate has been made. It was found out that the addition of aluminum lactate to high-alumina magnesia-bonded castables helped to inhibit the samples' explosion during drying and increased their green mechanical strength [13,14]. Moreover, some studies [15,16] reported a sol-gel aluminum lactate route to produce alumina glass, investigating the pH's effect over the hydrolysis and gelation of this organic salt.…”