Abstract
	"Chloride" processes, based on leaching of burned magnesite by acidic chloride solutions, have been intensively studied during past two decades. The so called Chloride-carbonate process has been recently developed to upgrade Slovak magnesites with higher contents of iron, lime and/or silica using the leaching of calcined magnesite with ammonium chloride according to Eq. (1). 
	Two samples of calcined magnesite with high contents (93 to 95 %) of "chemically soluble" MgO (samples MKK(800/20)95 and MKK(800/30)95 in Table 1) were prepared by calcination of the ground bulk raw magnesite (Table 1) and dry-screening. The samples differed in calcination times and exhibited slightly higher contents of MgO and lower contents of impurities (namely CaO and SiO2) if compared with an average composition of calcined bulk raw magnesite - Fig.1.
	The leaching process has been tested in a pH-stat to ascertain the effect of calcination time on the reactivity of the solid. The main purpose of the approach adopted was to keep the concentrations of H3O+, NH4Cl a NH3 constant during individual measurements. Leaching behaviour of both samples was tested under reaction conditions which were as follows: temperature from 30°C to 70°C, concentration of NH4Cl from 0 M to 2.5 M and pH from 3 to 6. The effect of calcination time on the rate of reaction (1) is shown in Fig.2. In this study, the rate of reaction (1) was characterized using the half reaction time t0.5 - a period of time which is necessary under certain reaction conditions to dissolve one half of the amount of "chemically soluble" magnesium originally present in the calcined magnesite. Under defined reaction conditions, t0.5 depends only on the reactivity of calcined magnesite - the higher the reactivity the lower the value of t0.5. In Fig.3, the values of t0.5 are compared for both samples - it can be concluded that the reactivity of sample MKK (800/20) 95 was higher than that of MKK (800/30) 95 under the conditions of experiments in the present work. The results obtained in this work together with those published earlier by other authors (ref. [8]) indicate that calcination time may be the parameter having the strongest effect on the reactivity of calcined magnesite.
	The kinetic data indicated that the overall rate of leaching is controlled by pore diffusion and surface chemical reaction under the conditions studied in the present work. Presence of diffusional limitations was confirmed by a method based on the random pore model - Fig.5.