Three bond modes of basic refractories used for Ruhrstahl Heraeus degassing process

Abstract

Magnesia-spinel brick and unburnt periclase-spinel-Al brick are being employed as a substitution of traditional magnesia-chrome brick in the chromium-free campaign of lining materials in Ruhrstahl Heraeus (RH) degasser. These three materials are investigated, in terms of physical properties, corrosion resistance and flexibility by wedge splitting test. Tracking their physical alterations and chemical reactions through burning or heating, three bond modes are discovered. Magnesia-chrome brick is subject to a series of phase transformation with rising temperature to yield a liquid envelop around chromite-ore particles, to further form porous rim while liquid is gradually absorbed by surrounding magnesia and eventually to precipitate secondary chromite spinel lied between magnesia particles by thoroughly dissociating chrome ore. The precipitated chromite spinel functions as the featured bond that enhances hot strength and corrosion resistance to slag, and additionally liquid coexistence improves the flexibility. The direct bond mode of magnesia particles in magnesia-spinel brick endures slag penetration by immanent character of MgO. Spinel incorporation in magnesia effectively improves thermal shock resistance. Due to minor negative value of permanent linear change after reheating, further sintering (densifying) in using at high temperature would bring a risk of loosening and open joints of magnesia-spinel lining. While used in RH degasser, unburnt periclase-spinel-Al bricks undergo a miraculous process of metallic Al melting, gaseous AlN and AlON formation, MgAlON whiskers germination combined with gaseous Mg reduced, and micron-size whisker network bond domination in their matrix. Such a whisker-network bond renders the material a successful eco-friendly alternative to magnesia-chrome refractory.

Whisker Bond: From Findings to Concept in Refractories

“…Nowadays, as the standard material used in the snorkels and lower vessels of RH degassers, periclase-spinel-Al bricks demonstrate excellent performance. Post-mortem analyses have been performed on the residual bricks dismantled from a snorkel with a service life of around 100 heats and a lower vessel with a service life of approximately 230 heats [7][8][9].…”

“…Such a bond supported by interwoven-whisker networks makes the material a successful ecofriendly alternative to magnesia-chrome refractory in RH degassers. Three bond modes were made to compare various properties of magnesia-chrome bricks with reprecipitated-chromite bond, magnesia-spinel bricks with direct bond, and periclase-spinel-Al bricks with whisker bond [9]. The re-precipitated chromite bond of magnesia-chrome bricks resisted slag infiltration and corrosion.…”

“…The mineral phases and whiskers with temperature gradients were identified from hot face to cold face along a used residual brick [8]. According to the phase evolution during the burning, the properties of basic bricks with three bonding modes were further studied and compared, demonstrating enormous advantages of unburnt periclase-spinel-Al brick over magnesia-chrome brick and magnesia-spinel brick [9].…”