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Quality control (QC) standards for the in vitro antimicrobial susceptibility testing of two fastidious veterinary pathogens, Actinobacillus pleuropneumoniae and Haemophilus somnus, were developed in a multilaboratory study according to procedures established by the National Committee for Clinical Laboratory Standards for broth microdilution and disk diffusion testing. The medium recommended for the broth microdilution testing is cation-adjusted Mueller-Hinton broth supplemented with 2% lysed horse blood, 2% yeast extract, and 2% supplement C. This medium has been designated veterinary fastidious medium. The medium recommended for the disk diffusion testing is chocolate Mueller-Hinton agar. The recommended QC organisms are A. pleuropneumoniae ATCC 27090 and H. somnus ATCC 700025. The QC MICs of ceftiofur, enrofloxacin, florfenicol, gentamicin, penicillin, tetracycline, tilmicosin, and trimethoprim-sulfamethoxazole were determined for each isolate, as were the zone size ranges. Of the results from the participating laboratories, 94.0% of the zone diameter results and 97.0% of the MIC results fell within the suggested QC ranges for all compounds. These QC guidelines should allow greater accuracy in interpreting results when testing these antimicrobial agents against fastidious pathogens.

The effect of SiO2 addition on the solid state reduction of a Greek chromite concentrate, at 1300 and 1400 degrees C, has been studied. Addition of SiO2 in the chromite concentrate-graphite mixture generally promoted reduction at 1400 degrees C. However, there was a critical content of SiO2 after which lower final reduction degrees at higher silica contents were attained. This was due to the diluent effect of SiO2 on the contact between graphite and chromite. Increase in the carbon content of the charge resulted in higher critical silica contents. Two mechanisms of reduction took place: (i) solid state reduction by graphite and (ii) smelting reduction by dissolution of chromite into the molten siliceous slag and subsequent reduction of chromium and iron oxides at the slag/graphite and/or slag/metallic carbide interface. The addition of SiO2 did not have any significant effect at 1300 degrees C due to the non-formation of eutectic siliceous phases for the occurrence of the smelting reduction mechanism. (C) 1997 The Institute of Materials. Ironmaking & Steelmaking

The mechanism of the solid state reduction of a Greek chromite concentrate by graphite in the temperature range 1100-1470 degrees C is studied. Reduction proceeds via six stages: (i) nucleation of metallic phase at the chromite/graphite interface; (ii) outward growth and propagation of metallic nuclei (the chromite grain consists of an unreduced chromite core enveloped by a partially reduced spinel shell); (iii) formation of a metallic rim at the core/shell interface; (iv) metallisation within the chromite core; (v) sintering due to interaction of the silicate gangue with the spinel grains; (vi) disintegration of the spinel grain and penetration with metal. A diffusion mechanism is postulated that involves an outward diffusion of Fe2+ ions produced by reduction of Fe3+ by Cr2+ ions, which have diffused into the interior of the particle, and in the later stages an outward diffusion of Cr2+ and Cr3+ ions. Diffusion of ions which do not take part in the reduction reactions (i.e. SiO42- and Al3+) also occurs. Ironmaking & Steelmaking

The solid state reduction of a chromite concentrate from Northern Greece in the temperature range 1100-1470 degrees C has been studied. The degree of reduction of chromite by graphite was found to increase with increasing temperature, time, and carbon addition. The maximum reduction percentage attained (defined as the mass of carbon monoxide evolved expressed as a percentage of the carbon monoxide that would be evolved if the reduction was complete) was 75%. The maximum metallisation of iron and chromium was 91 and 67%, respectively. The main reduction products were: (i) metallic products, in order of succession of formation, Fe-C alloy, (Cr,Fe)(7)C-3, and (Cr,Fe)(23)C-6 and (ii) non-metallic products, spinels rich in MgO and Al2O3 that contain chromium in the divalent state and forsterite. Melting of chromite, prereduced to 72.5% reduction, at 1550 degrees C for 1.5 h led to 97.5% chromium recovery and 100% iron recovery. Satisfactory silicon and carbon contents of the metallic products were attained. Ironmaking & Steelmaking

The main chemical and physical changes that took place during firing of a Greek chromite concentrate in air are as follows. From 820 to 1170 degrees C, oxidation of FeO to Fe2O3 occurred. Owing to an excess of R(2)O(3) oxides over RO oxides in the chrome spinel unit cell, Fe2O3 combined with excess R(2)O(3) and, on cooling, solid solutions of Cr2O3, Fe2O3, and Al2O3 separated from the spinel lattice. From 1240 to 1370 degrees C, dissociation of Fe2O3 to FeO.xFe(2)O(3) occurred, leading to the resolution of the sesquioxide solid solutions which reentered the chrome spinel lattice. The serpentine matrix was partially transformed to forsterite. Interaction of the chromite phase with the gangue resulted in the crystallisation of bonds joining neighbouring chromite particles, due to liquid and/or solid phase sintering. The changes that occurred during calcining of the chromite in argon atmosphere were restricted to a partial transformation of the gangue to forsterite and to a localised sintering. Ironmaking & Steelmaking