Effect of calcium oxide on the hydration activity of the powders of electrical-engineering periclase

Abstract

The powders of electrical-engineering periclase obtained from natural brucite mainly meet the specifications of the second and the third classes according to GOST 13236-83. The reason for the poor quality of the powders is their high calcium oxide content and this owes to its presence in large quantities in the original brucite (exceeds 2.0~ in the calcined substance). This fact made it necessary to carry out additional and more detailed studies on the properties of the periclase powders having a high calcium oxide content.Moisture (water) absorption is one of the most important parameters determining the service characteristics of the periclase powders in TEN [i, 2]. Calcium oxide has a considerable hydration ability [3] and, consequently, it can make periclase powders highly prone to moisture absorption.It was established [4] that the change in the weight content of calcium oxide in a periclase powder from 0.75 up to 2.20~ does not have a significant effect on its moisture absorption. We note that in these experimental powders, calcium oxide was present in the form of silicate compounds.However, in periclase powders, calcium oxide can be present in free state (besides calcium silicates).This paper deals with a study of the aspects concerning the hydration activity of a periclase powder, pure magnesium and calcium oxides, and the calcium-containing compounds present in periclase in the form of additional (impurity) phases. Imported periclase powders were also studied for the purpose of comparison.The granulometric composition of the experimental periclase powders that were obtained from brucite met the specifications concerning the 'fine grade' according to GOST 13236-83. Pure magnesium and calcium oxides and silicate compounds (monticellite CaO'MgO'SiO 2, mervinite 3CaO-MgO'2SiO2, and dicalcium silicate 2CaO'SiOe) had a grain (particle) size less than 0.063 mm.The powders were calcined at 1000~ and were held in the medium of air having a humidity of 97-98~ at 20-25~The duration of holding varied from i up to 50 days. Hydration activity of the powders was evaluated on the basis of the change in their weight during calcination at 1000~In view of the fact that the processes of moisture adsorption and subsequent hydration occur mainly at the surface of the grains, it is necessary to take the specific surface of the powders into account. The data presented in Table I show that the powders of calcium and magnesium oxides and the impurity-phase are characterized by uncomparable (widely differing) values of the specific surface. Therefore, in order to evaluate their hydration activity, we used the parameter of weight change during calcination (recalculated) per unit surface area (Table 2).It was established that the hydration activity (ability to hydrate) of calcium oxide exceeds that of magnesium oxide by approximately 8 times. Monticellite, mervinite, and dicalcium silicate hydrate very strongly as compared to pure calcium oxide.Thus, calcium oxide present in the periclase powders in free state must have a signific...

A microstructural study of surface hydration on a magnesia refractory

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Experimental Study of Magnesium (Mg) Production by an Integrated Calcination and Silicothermic Reduction Short Process

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