A.P. Luz

Artigo revisão: uso da molhabilidade na investigação do comportamento de corrosão de materiais refratários

Due to the high corrosion presented by refractory materials used in the steelmaking processes and the need to minimize and understand the interactions between molten slag and metals with these ceramics, the wettability technique seems to be appropriate. Nevertheless, there are few works applying this approach to refractory materials. Therefore, the use of this technique could be the first step to try to reduce the chemical corrosion that these materials are subjected. The theoretical concepts, experimental techniques and the factors that can influence the wetting behavior are discussed in the present review article. To understand the slag / refractory corrosion process, it is first necessary to understand the mechanisms of the wettability when these materials first come into contact. Once this process is fundamentally equated, then one can proceed to understand the sequential steps of the corrosion such as in the development of the high performance materials.

Sintering Effect of Al and a Boron Source in High-Alumina Nano-Bonded Refractory Castables

Petrochemical refractory end-users face difficulties in finding commercial products with optimized thermo-mechanical properties for the low operational temperatures (<900°C) in fluid catalytic cracking units. Blending colloidal binders with sintering additives in castable compositions is an interesting alternative that may overcome the limited densification of conventional cement-bonded systems. This work investigated the effects associated with use of Al and/or boron sources added to high-alumina colloidal silica-bonded castables sintered at temperatures up to 1000°C. In situ measurements of hot elastic modulus and hot modulus of rupture, and XRD and SEM analyses were performed to explain the reaction mechanisms and phase evolution associated with Al and boron-based compound performance at high temperatures. The test results indicate that added boron induced formation of Al4B2O9 and generated liquid phase in the microstructure of the castables. Aluminium powder was oxidized (giving rise to alumina), which resulted in increased castable stiffness. This latter transformation was only observed in colloidal silica-containing compositions. The best thermo-mechanical performance in the 600–815°C range was obtained for castables containing both Al and boron.

Slag melting temperature and contact angle on high carbon containing refractory substrates

In this work the sessile drop technique was used in order to evaluate the melting temperature of synthetic and industrial slags, and their contact angle as a function of time and temperature on substrates of an Al2O3-SiO2-SiC-C castable containing 10 wt.% of carbon or its matrix. The best experimental conditions for the evaluation of the wetting behavior of refractory castables were attained when 25 x 25 x 10 mm3 matrix substrates and cylindrical slag pellets of 3 mm diameter and 5 mm high were used. This substrate - slag set resulted in contact angle values with good reliability. Small differences between the contact angle behavior on ground and polished matrix surfaces for synthetic slags were also detected. Additionally, industrial slags containing various oxide components, showed high values of contact angles and lower liquid spreading when compared with synthetic slags. This difference can be associated to the interaction liquid-solid at the material interface. Besides that, the wetting experiments were presented as a good alternative for measuring the melting temperatures of slags, as their values are difficult to determine due to the high oxide variety in the industrial slag compositions and the interaction between the slag and the substrate.

Melting Temperature and Wetting Angle of AlN/Dy2O3 and AlN/Yb2O3 Mixtures on SiC Substrates

This work aims to evaluate the melting temperature and wetting behavior of AlN/Re2O3 (Re = Dy, Yb) mixtures when in contact with SiC substrates at high temperatures, in order to define whether these compounds can be further used to induce a more effective liquid phase sintering of SiC-based products. The prepared samples were placed on SiC plates and thermally treated up to 1900 °C under argon. The melting point and spreading evolution of different compositions of AlN/Re2O3 on SiC were determined by analyzing images captured as a function of the heating temperature. The contact angle and melting point were measured using the ImageJ software and according to DIN 51730, respectively. Based on the obtained wetting curves, all evaluated conditions resulted in the decrease of the contact angle values with temperature. The mixtures presenting improved wetting (θ ~ 1° and 3°) on SiC plates were the ones above the selected eutectic point.

Características e mecanismos de desgaste dos refratários MgO-C usados na linha de escória de panelas de aço

Steel ladles are special vessels used for liquid steel transportation from the primary refining step up to its solidification at the casting shop. However, due to the increasing market demand for clean steel, new procedures have been implemented in the refining process resulting in more aggressive environments to the ladle refractory materials. In general, the upper region of the vessel, where the contact between slag and refractory takes place (known as slag line), is subjected to intense wearing and corrosion. This fact has significant implications, leading to a reduction steel ladles availability at the steelshop, early refractory lining replacement repairs, production costs increase and reduction in safety. In this context, this work presents a review of the main factors involved in the degradation process of MgO-C refractory, which is the most widely used material for slag lines in steelmaking. Additionally, some alternatives to reduce or inhibit such wearing are also discussed.

In situ α-alumina hydrophobization dynamics at acid pH: effectiveness limitations of short-chain amphiphilic molecules

The present work highlights that α-alumina hydrophobicity (attained by the in situ modification of its surface properties with water soluble organic monoacids) cannot be increased by using molecules with low acidity (pKa) values due to the alumina zeta potential profile at pHs below approximately 4. In order to certify this statement, the effects of short-chain fatty acids and fluorinated ones on the alumina contact angle (θe) with water were analyzed. It is also pointed out that the θe values derived from the experiments carried out using fluorinated acids are the highest possible when monoacid surfactants are added in α -alumina suspensions in acidic pH. This result indicates that there is another constraint to the in situ hydrophobization procedure, which was not previously elucidated or fully settled in literature.

Avaliação do molhamento da matriz de um concreto refratário (Al2O3-SiC-SiO2-C) por escórias sintéticas contendo distintos teores de MgO

The wetting behavior of four synthetic slags was investigated by the sessile drop technique, when in contact with the matrix of a high carbon containing refractory castable. The wetting evolution was monitored at 1450, 1550 and 1650 oC during 1800 s. Besides that, thermodynamic simulations were carried out using the FactSage® software aiming to attain a better understanding of the chemical reactions at the materials interface. It was observed that the chemical composition of the liquid directly affected its spreading and the higher the MgO content in the slag composition, the higher contact angles values were attained. However, the formation of bubbles on the liquid surface and the slags infiltration in the refractory pores at high temperatures and long time intervals, were observed. These factors affected and made the collected wetting results less conclusive, pointing out that there is still the need of some improvements in the wettability technique in order to evaluate complex materials, such as refractory castables.