EXPERIMENTS IN DEAD-BURNING DOLOMITE AND MAGNESITE1

Abstract

Stability of calcined dolomite; effect of fineness of raw material, nature of flux and temperature of burning.—Dolomite is difficult to dead-burn because the absorption of moisture from the air produces a gradual slacking due, probably, to the hydration of lime and various lime compounds, and this tends to cause disintegration. With all the fluxes tried, 100 mesh raw material gave greater resistance to slacking than 8 mesh size. This resistance decreased, however, with increasing temperature of calcining, by an amount which seems to depend on the silica and alumina content of the flux, being greatest for basic open hearth slag and kaolin, less for flue dust, and iron ore, and least for roll scale. The stability also varied with the proportions of flux used, being greater, as a rule, for smaller proportions. Shrinkage, porosity and vitrification of calcined dolomite; effect of flux and burning temperature.—The shrinkage and porosity curves for mixtures containing flue dust, iron ore or roll scale are very similar. The higher the proportion of flux, the lower the temperature required to get maximum shrinkage and minimum porosity. Kaolin and basic open hearth slag behaved peculiarly in that vitrification depended more on the burning temperature than on the proportion of flux used. Experiments in making dolomite brick.—It was found that ageing of the wet calcined material was necessary to prevent cracking of the bricks upon drying. With 5 per cent MgCl2 as binder, comparatively strong brick were obtained. High shrinkage on firing caused excessive cracking, but the bricks were extremely dense and did not disintegrate until after four to six months. Unfired tar-bonded brick were made with sufficient strength to stand shipping and with a storage life of about four months. Fired tar-bonded brick showed less shrinkage and cracking than similar brick made with water. If dipped in tar to protect them from moisture, such brick will last five or six months before disintegration. It is possible, then to make dolomite brick with a storage life long enough to allow shipping and placing in furnaces. Stability, shrinkage and porosity of calcined magnesite; effect of flux and temperature of burning.—Unlike dolomite, magnesite showed less slacking effect the higher the temperature of burning. Roll scale gave greater stability than iron ore, probably because of its lower silica content. The product was comparatively porous and with low shrinkage, even with high proportions of roll scale.