Refractories based on sintered and fused periclase-chromite

Abstract

The technology for magnesia-spinel refractories developed in the USSR specifies the production of goods from sintered periclase powder and chromite ore added to the batch in granular form (PKhS) [i]; in the form of finely dispersed mixtures with sintered periclase powder (PShS) [2]; combinations of both versions (PKhV), and also on the basis of granular fused periclase-chromite mixture (PKhVP) [4]; granular fused periclase chromite, chrome-alumina spinel, and finely dispersed fused periclase (PShPKh) [5]; fused (PShPV) and sintered (PShSV) [6,7] granular aluminomagnesia spinel and sintered periclase powder, sintered periclasechromite clinker (KPShS) [8]. Goods from clinker obtained with nonpelletized [9] and pelletized [i0] fine mixtures of caustic magnesite and chromite calcined in rotary kilns 90 m long at 1760-1790~ (at the flame) and cooled in the range tmax-900"C at a rate of 95-100~ are slightly different with respect to the standard (porosity, strength, thermal-shock resistance, refractoriness under load of 0.3 N/mm2), but are suitable distinguished from PShS goods, both in terms of thermal strength, thermal fatigue, porous structure and of wear resistance in the linings of open-hearth roofs [9, i0].At present one of the best high-duty magnesia-spinel refractories is PShPKh, based on fused periclase-chromite, chrome-alumina spinel, periclase, obtained by fusing in the block in arc furnaces. The preparation of fused periclase-chromite is connected with a high specific consumption of batch, electrodes, energy and labor; separating the blocks is laborious and limits the increase in production of PShPKh goods, and greatly affects their cost.In order to study the possibility of replacing fused periclase-chromite in the batch for PShPKh goods by sintered material, we carried out industrial experiments in the TsMI-2 section of the Magnezit Combine.The sintered periclase-chromite was prepared from caustic magnesite and chrome concentrate (Table i) by combined milling in the ratio 80:20 to a residue on a screen with i0,000 apertures per cm 2 of less than 10%, moistening during blending with a water solution of LST having a density of 1.24 g/cm 3 to a moisture content of 2.8-3.6%, pelletizing in the PR-7 press at i00-Ii0 N/mm 2 in the form of flat articles 460 mm and 520 mm long, with a cycle time of 26-30 sec. The pressed green brick had excess pressing cracks, an apparent density of 2.10-2.22 g/cm 3 in a monolithic piece, and possessed adequate strength for mechanized loading onto the kiln cars.The pellets were fired in tunnel kiln No. 6 at TsMI-2 (Magnezit Combine) at a maximum temperature of 1920~ and with 18 pushings of the car in 24 h, with a soak in the temperature zone above 1700~ of 10-11 h. The pellets were cooled in the range 1700-900~ using the standard cycle for the kiln at a rate of 70-72~The fired pellets were milled using the standard TsMP-4 scheme with screening to obtain respectively 73 and 27% fractions 3-I and I-0 mm (control weighing of powders). The open porosity of the pellet fractions 3-1 mm ...