## 1 Introduction  
Shredding of the raw material is a part of many technological processes, mostly concerning mineral materials, but chemical, food, and pharmaceutical industries are typical examples as well [1]. Usually grinding process is run by a mill where fed material is crushed with grinding media being put into motion. Construction of the mill, its size, and output depend on particular demands following from the application. In the most classical drum mills, grinding media of ball, cylebps, or rod type, rotate in a cylindrical chamber. Conventional grinding is carried out by material removal by the grinding media [2]. Disadvantage of classical mills is a high energy consumption that leads to low efficiency due to the high loss of energy supplied to put the grinding media and the mill shell into motion [3,4]. Another problem concerns the control of the grain shape. The installation of dry grinding in the electromagnetic mill offers ability to control grain size, flow of the grinding media, exact classification and recycle of oversized grains. Parameterization of the installation allows for influencing grain shape, moisture, temperature, area, and energy surface properties. An interesting comparison of sorption ability of mechanically and thermally activated sodium bicarbonate is presented in [5]. Three types of mills: fluid bed opposed jet mill, fine impact mill,and electromagnetic mill were compared. The electromagnetic mill, as the alternative to standard mills is presented in [6] where high-energy milling was used to obtain tetragonal form of PbO. Generally, the electromagnetic mill is used to achieve grinding efficiency increased even by 50%. Heat recovery system is used for drying and heating the material that increases milling capacity. All of the mentioned characteristics of the electromagnetic mill have not been developed yet enough deeply, and, in turn, not reported. Lack of the researches and huge potential of the electromagnetic mills is a serious motivation to undertake the challenges. The paper presents the electromagnetic mill installation that is a unique solution. In the literature only, one other solution was presented in [7], namely an electromagnetic mill having inductor with hidden poles. The solution presented in this paper uses apparent poles and the physical structure of the mill is different. The most important novelty is vertically opposed pneumatic transport of grinding material which causes instability of the setup in an open loop. Thus, to meet the multi objective requirements, a set of control schemes has been developed in every level of control: direct regulation, supervisory control, and optimization so the system performs stably and robustly. Important challenge was involved by the states of the setup, which cannot be measured directly. The paper presents developed indirect measurement methods and data processing that is applied in the system. All of the advantages of the electromagnetic mill can be achieved by a specially designed control system. It is shown in the paper that the installation is a complex multi-objective plant. To put all the objectives in an order, multi-rate control is proposed. Structure of the control system is then layered according to the time horizon of validity of control laws that are related to a certain set of objectives. Control theory calls such a structure hierarchical control. Hierarchically structured control system constitutes the only arrangement to manage multi-objective technological plant. Section 2 of the paper is devoted to the description of grinding and classification circuit, which uses the electromagnetic mill. Three subsections focus on the main elements of the installation: the electromagnetic mill, pneumatic transporting system, and classifiers.