Abstract
	A new chemical plant was recently built up in Hačava-Hnúša, Slovakia for producing high-grade synthetic calcined and/or sintered magnesium oxide [1-6]. Since magnesite flue-dusts from dust separators of rotary and/or shaft kilns for burning Slovak natural magnesites are used as the raw-materials for this technological process, the feed represents a mixture of particles of original raw magnesite, calcined magnesite (with a high content of reactive MgO) and dead-burned magnesite. The reactivity of calcined magnesite is much higher, therefore the dissolution of dead-burned magnesite in hydrochloric acid according to Eq. (1) can, under certain conditions, control the overall output of  the leaching stage.
	Chemical dissolution of periclase, which is the main component of dead-burned magnesite, has been intensively studied and it was observed by various authors that the overall rate can be controlled by diffusion of liquid reactants/products or by the surface reaction, in dependence on the reaction conditions [8-14]. However, the experiments were carried out using diluted acids (pH>1) which limits the use of published data for an egineering analysis. The aim of the present work was to study the dissolution of dead-burned magnesite in hydrochloric acid to ascertain the effect of process parameters viz. temperature, concentration of HCl and chemical composition of the solid.
	Two samples of dead-burned magnesite with different chemical composition (samples MSK/95 and MKJ/113 in Table 1) were obtained by dry-screening of two different rotary-kiln products. Leaching behaviour of both samples was tested in an isothermal well-mixed glass batch reactor (Fig.1) under reaction conditions which were as follows: temperature from 50°C to 75°C and concentration of HCl from 1 M to 5 M. 
	The shrinking particle model [15-17] was chosen to describe the dissolution of magnesium during leaching (Fig.2) and to analyse the kinetic data. It was concluded that the dissolution is controlled by the chemical reaction of MgO with H+ ions at the liquid-solid interface. Assuming the reaction rate expression in the form of Eq. (2) resulted in similar (and negative) values of the reaction order, n (Table 2), though better correlation between model and actual dependence of the reaction rate on activity of H+, aH+, was obtained for the sample MSK/95 - Fig.3. The apparent activation energy was found to be practically independent on the composition of the solid - the values 63.0 kJ.mol-1 and 60.9 kJ.mol-1 were obtained for the leaching of samples MSK/95 and MSJ/113, respectively (Arrheniusī plots are shown in Fig.4). Figs.5a,b illustrate a good correlation between model prediction and experiments, observed for both samples under the conditions of experiments in the present work.