APPLICATION OF DOLOMITES AS THERMAL INSULATION OF HIGH PRESSURE CONTAINERS OF SIX-PUNPING PRESSES
Abstract
The aim of the work was to determine the thermal stability of dolomite powders and its compacted mixtures with water glass (sodium silicate) used as a binder for isostatic pressing of parts. The main research methods were differential scanning calorimetry, thermogravimetric analysis and infrared spectroscopy, which made it possible to study the transformations of dolomite materials upon heating and to determine the nature of the observed thermal effects.
As a result of research, the temperature boundaries of phase transformations were determined; using the methods of scanning differential calorimetry and thermogravimetry, transformations in natural dolomite powders with grain size of 1000/200 and 160/20 microns were studied at atmospheric pressure and temperatures up to 1200 оC; the temperature limits of phase transformations were determined – the dissociation of CaMg(CO3)2 into calcite, magnesite and carbon dioxide at 310 – 750 оС with further decomposition of CaCO3 into CaO and СО2 in the temperature range 600 – 935 оС. It was found that the transformation of the fine fraction proceeds at a temperature lower by 82 оC and requires 104 J/g less heat.
Studies carried out using infrared spectroscopy made it possible to determine the characteristic peaks of heat absorption in the infrared range of optical waves, from which it was concluded that a material with an optimal content of sodium silicate as a binder increases the level of absorption of thermal radiation, as a result of which the thermophysical characteristics are improved. Assessment of thermal properties for such materials was performed for the first time and is original; it allows us to conclude that the thermal insulation of the material is improved by 2,5 times compared to the original composition.
Studies of transformations during the annealing of dolomite-water glass mixtures at temperatures of 160 - 300 oC made it possible to optimize the composition of the material for its use as thermal insulation and the manufacture of parts of high-pressure cells used for containers of six-punch press installations when growing diamond single crystals by the temperature gradient method. This made it possible to ensure the reliability and stability of heating the growth cells in cycles lasting 200 – 300 hours.
The research carried out makes it possible to significantly expand the class of heat-insulating materials for use in large-volume compression structures of six-punch presses with a plunger diameter of up to 1000 mm and more.
