Abstract
	Decreasing consumption of magnesite refractory materials as well as increasing demands on their quality in modern steelmaking stimulate looking for "non-traditional" applications of natural magnesites both in industries and agriculture. An information about the relationship between the properties of burned magnesite products and their reactivity is generally necessary for their successful use. Leaching of magnesium plays the key-role practically in all cases of practical interest: it is the most important stage of chemical processes for producing high-grade synthetic magnesia and other pure magnesium compounds, it takes place during the action period of fertilizers and/or feedstuffs with added magnesium, and the laboratory methods for testing the reactivity of burned magnesium products are based on leaching, as well.
	The macrokinetics of leaching are a result of the surface area, morphology and real structure of used solid as well as the interactions between chemical reactions and transport phenomena. The kinetic study of leaching represents therefore an important step of its serious theoretical and/or engineering analysis. It is a connection between laboratory testing and full-plant operation and provides an information about the mechanism of the process as well as the data for the scale-up methods.
	Two model reaction systems were studied: (1) calcined magnesite - ammonium chloride - water, which is an example of leaching of a porous solid by solution of a hydrolysing salt, and (2) dead-burned magnesite - hydrochloric acid - water, which represents leaching of a non-porous solid by solution of an acid. Simple mathematical expressions (models) were used to describe and follow the behaviour both burned magnesite products during leaching. These models can be used to study the mechanism of leaching and/or to build-up more complex simulation models for chemical reactor (or other kind of "chemical reaction system") design.