Corrosion Mechanism of A Density-Reduced Steel Ladle Lining Containing Porous Spinel-Calcium Aluminate Aggregates

Abstract

Refractory monolithics for steel ladle linings are typically products with low porosities and high bulk densities. They achieve high temperature, penetration, and corrosion resistance. Despite the high density of these products, which is due to the low porosity of the aggregates, their matrices still exhibit a high amount of pores. Since calcium magnesium aluminate (CMA) has already proven its resistance to penetration and corrosion as a binder in the matrix, this paper investigated if alumina spinel refractories containing microporous calcium magnesium aluminate aggregates can withstand conditions that occur in a steel ladle wall. The objective was to reduce the castable density with the advantage of a lower material requirement for a ladle lining and reduced heat and energy losses. This was achieved by replacing dense alumina aggregates by up to 38% of porous CMA aggregates (grains with 30 vol% porosity), which resulted in a bulk density reduction from 3.1 g/cm3 for the dense alumina castable to 2.8 g/cm3 for the 38% CMA aggregates containing castable. However, the despite the higher porosity, penetration, and corrosion resistance and thermomechanical properties were not impacted negatively for a model alumina spinel castable. A postmortem investigation was conducted on a newly developed dry-gunning mix that was installed in a steel ladle wall on top of a slag penetrated castable and that achieved a service life of 31 heats versus only 18 heats for the reference mix that contained dense alumina and spinel aggregates. This new repair mix contained the newly designed porous CMA aggregates, which in this case partly replaced the dense alumina and spinel aggregates. These porous aggregates consisted of magnesium aluminate and calcium aluminate micro-crystals. The postmortem study revealed two important phenomena that can explain the improved performance: at the hot face in contact with steel and slag, a thin densified zone was observed that blocked the slag penetration into the porous matrix and the porous aggregates. Iron oxides were almost completely blocked from penetration, and only some manganese oxide was observed in the penetrated zone together with some silica and lime from the slag. Clusters of calcium aluminate (CA6) and magnesium aluminate (MA) spinel build the refractory back-bone on the hot side of the material and gussets filled with mostly glassy calcium aluminum silicates close to the hot face and gehlenite further inside the penetrated zone. Alumina grains had a reaction rim consisting of CA2 or CA6 and a very intimate connection to the surrounding matrix unlike the CMA-free mix that showed micro cracks around the alumina grains. At the colder side, the gunning mix with CMA aggregates showed a very good connection to the substrate, supported by a hercynite formation in the gunning mix resulting from a cross-reaction with remains of iron oxide on the CMA containing repair mix. Furthermore, macroscopic observations of a CMA aggregate containing alumina magnesia castable in the metal zone of a steel ladle revealed that macro cracks developed only very slowly, which resulted in a superior service life.

Optimization of Properties for Alumina-Spinel Refractory Castables by CMA (CaO-MgO-Al2O3) Aggregates

“…Hence, the penetration index was one of the important factors to evaluate the slag resistance of castables. It was indicated that the complex phase composition of CMA aggregates was beneficial to form calcium aluminum silicate glass phase, which can prevent slag from deep penetration in refractories [ 14 , 15 , 16 ]. This mechanism was mainly concerned the chemical composition of CMA aggregates.…”

“…The slag resistance of magnesia-carbon (MgO-C) bricks for steel ladle linings was significantly improved with CMA aggregates addition as a consequence of an in situ formation of a protective layer on the surface of bricks [ 14 , 15 ]. The corrosion mechanism of alumina-spinel castables with CMA aggregates addition has been demonstrated by Wöhrmeyer et al The results indicated that a thin densified zone formed and blocked the slag penetration into the porous matrix and the porous aggregates [ 16 ]. However, the comprehensive performance of castables was critical for the applications.…”

Enhanced Properties of Tailored Alumina–Magnesia-Based Dry Ramming Mixes by Calcium Magnesium Aluminate (CMA)

“…Recently, novel calcium magnesium aluminate (CMA) materials containing spinel and calcium aluminate have been developed (MagArmour, Imerys, France). Wöhrmeyer et al reported that the incorporation of CMA in alumina-spinel castables could effectively inhibit the penetration of slag into the matrix and improve the energy efficiency of steel ladles [ 18 ]. Tang et al investigated the influence of CMA aggregates on the properties of alumina-spinel refractory castables.…”

Design, fabrication and properties of lightweight wear lining refractories: A review

“…Later, a series of lightweight porous aggregates (alumina, mullite, spinel, and cordierite) was prepared via an in-situ pore-forming technique by Li et al [14] and Yan et al [15][16][17][18][19]. In addition, lightweight porous aggregates of calcium aluminates and spinel have been reported [20,21].…”

“…Compared to common dense refractory materials, the lightweight refractories fabricated with such lightweight aggregates exhibited lower bulk density and thermal conductivity [19,21].…”