The reduction of iron oxides from liquid slags is the fundamental stage of many newly developed, future-oriented technologies of obtaining liquid iron (pig iron), which are referred to as reduction iron smelting processes, whose main distinguishing feature is the use of non-coking coals. The reduction processes in those technologies are heterogeneous processes in which the following phases take part: a solid phase (carbon reducer), a liquid phase (molten iron oxides in the salg), and a gaseous phase (carbon monoxide and carbon dioxide). This is the reason for the complexity of the reduction process mechanism, which makes the kinetic description of these reaction difficult. In the present study, a methodology of examining the reduction was adopted using the so called graphite rotating disk. The investigations carried out within the present study were aimed to determine the kinetic area for the reduction of iron oxides in the temperature range of I 350-l 420°C in the CaO-Fe0-SiO2 slags and with an FeO content from 20 to 60% by weight, with the use of a graphite reducer, as well as to evaluate the kinetic parameters of the process. The applied methodology of the rotating disk tests has enabled a conclusion to be drawn of the suitability of this method for identifying the kinetic area of reduction in most of the reduction variants used. It has been found that for slags with a basicity from 0.3 1 to 1.53 and in the viscosity range from 4.23 to 0.31 dPa·s the reduction rate is determined by the diffusion of FeO to the slag-reducer interface. Parameters of the kinetic diffusion area have been calculated, such as: FeO diffusion coefficient, the limiting diffusion layer thickness and mass transfer coefficient. The analysis of the calculated values of FeO diffusion coefficients made based on the available literature data indicates that these should be regarded as specific for the diffusion coefficients of oxygen ions in the liquid slag. The determined mass transfer coefficients have enabled the reduction rate to be calculated for particular variants of reduction. A good agreement with the experimental results has been obtained at low disc rotations.

This article considers the problem of the rise in temperature of the windings of an induction motor during start-up. Excessive growth of thermal stresses in the structure of a cage winding increases the probability of damage to the winding of the rotor. For the purpose of analysis of the problem, simplified mathematical relationships are given, enabling the comparison of quantities of energy released in a rotor winding during start-up by different methods. Also, laboratory tests were carried out on a specially adapted cage induction motor enabling measurement of the temperature of a rotor winding during its operation. Because there was no possibility of investigating motors in medium- and high-power drive systems, the authors decided to carry out tests on a low-power motor. The study concerned the start-up of a drive system with a 4 kW cage induction motor. Changes in the winding temperature were recorded for three cases: direct online start-up, soft starting, and the use of a variable-frequency drive (VFD). Conclusions were drawn based on the results obtained. In high-power motors, the observed phenomena occur with greater intensity, because of the use of deep bar and double cage rotors. For this reason, indication is made of the particular need for research into the energy aspects of different start-up methods for medium- and high-power cage induction motors in conditions of prolonged start-up.