In order to achieve maximum micronization with a minimum of power consumption, a highly efficient prototype micronizer has been designed. The apparatus is based on the principle of countercurrent jet milling in closed circuit with a pneumatic classifier capable, among other things, of controlling the top size of the final product. Systematic experimental investigations allowed to analyse the dynamics of breakage due to impact and attrition and to optimize the fluid dynamics circuit, the design and the resulting prototypes, the adjustable operating parameters, the size class characteristics of the feed and the performance achievable. The results obtained for a variety of industrial ores (talc, kaolin, baryte) are presented, with particular reference to the micronization of a high grade baryte concentrate with an elevated degree of whiteness.