The magnetic band separator is provided for the enrichment of strongly magnetic ores, as, for example, magnetite ore. The process of magnetic flocculation occurs under the influence of magnetic field. The forming particle aggregates (floes) contain non-magnetic particles in their structure which deteriorates the separation results. In the band separator the material is subjected to several remagnetizations on its separation path during which non-magnetic particles are being liberated from the floe volumes. The separation results depend on the characteristics of the separator magnetic system and magnetic properties of the raw material. Starting from the equations ofmagnetic field the author calculated the distribution of magnetic field and force in the band separator. On this basis he also determined the optimum pole pitch of the magnetic system which depends on particle sizes of the enriched raw material. Despite the magnetic force, also mechanical forces act upon particles. The balance of forces acting upon the particle enabled the value of separation magnetic susceptibility lo be calculated according to which the raw material is divided into magnetic and non-magnetic particles. Taking into account magnetic interactions between magnetite inclusions in the particle, the dependence of particle magnetic susceptibility on the volume content of magnetite was determined and, next, theoretical indexes of magnetite ore enrichment ability were calculated.

In magnetic separators the phenomenon of magnetic flocculation is an inseparable feature of enrichment of strongly magnetic ores. Non-magnetic particles are bound in the floe internal structure by means of magnetic, surface and mechanical forces which leads to the deterioration of enrichment results. The intensity of flocculation depends on magnetic field intensity, content of the magnetic component in the feed and ore feed rate. The above mentioned factors affect the enrichment results. The paper presents the separation analysis in the band magnetic separator with respect to the magnetic field distribution in the separator working space as well as internal and external mechanical forces, acting on the particle. The author determined the effect of the magnetic compound content in the feed and the amount of washing water on the recovery of this component in the concentrate as well as the effect of the magnetic component content in the feed and the magnetic force density on the residue of the non-magnetic component in the concentrate. The analysis was performed according to the physical model of magnetic separation, presented in the paper. The theoretical dependences, derived from this model, are in good agreement with the results of empirical research, found in the literature.

This paper describes comminution processes using the theories of limiting states, elasticity, and plasticity to explain some effects observed in the process of crushing brittle materials. It further describes the phenomena occurring during crushing in high-pressure roll presses and analyzes the effects of selected factors upon crushing results. The evaluation of the usefulness of various hypotheses for interpretation of the crushing process in the high-pressure grinding roll was carried out by means of experimental investigations. A series of laboratory crushing tests were also conducted in which limestone samples were pressed in a hydraulic piston-die press. Comminution conditions in this press are similar to those observed in the working chamber of HPGR presses. The limestone aggregate, placed in a steel cylinder, was exposed to pressure exerted by the stamp of the press. Samples had various particle size distributions, and experiments were conducted for two values of pressing force. Operating pressure was the main parameter influencing the obtained comminution effects, but the particle size distribution also has an impact on the process effects. A comparison of the results of the investigations indicated that there exists a significant potential for adjusting the operational parameters of high-pressure grinding rolls. Internal stresses are a derivate of crushing actions such as compression, impact, bending, and shearing. The result of crushing in a particular crusher depends on the strength properties of particles reacting to a specific type of crushing actions. In every crusher there are many crushing actions out of which one is dominating due to the crusher type. Impact is a dominating factor in impact or hummer crushers. Various actions of crusher elements on the crushed material are beneficiary. For example, the shape of the jaw surface in jaw crushers, cone surface in cone crushers, or roll surface in roll presses are important.

Theoretical tensile strength of brittle materials is 102-104 times larger than real strength. The particle structure, apart from the force of atomic bonds in an ideal crystal, affects real strength. A single-phase particle, spatially continuous and homogenous from the point of view of its chemical composition, is assumed to be the basis. A real particle is formed by means of introducing pre-existing microcracks and other geometrical and physical internal defects. The size distribution and number ofthese defects, affecting the particle strength, form the mechanical structure ofa particle. Since both the number and size ofpre-existing microcracks are randomvariables, respectively the particle tensile strength iś°"a random variable, described by Weibull's distribution. This paper has analysed the effect of particle structure on its tensile strength from the point of view ofthe weakest link theory and the statistical theory of fracture. In both cases for the distribution is obtained whose parameters are connected with the distribution ofmicrocracks lengths (formulas 5 and 15). The next part of the paper shows the results ofempirical tensile strength tests oflimestone and porphyry particles. The authors set distribution functions of tensile strength (formulas 20-22 and figures 3-5) and calculatedWeibull's moduli of the tested samples and the average particle strength. The average particle tensile strength is connected with the particle size by one of the formulas (25), depending on the fact whether particle fracture resulted from stimulating the volume, surface or edge microcracks. In case of limestone (one-component material) the particle fracture is caused by edge microcracks (formula 27a) while for porphyry (multi-component material) by surface microcracks (formula 27b). The dependence of crushing ratio on particle strength is described by an increasing power function (formulas 29-32). The exponent in this dependence is correlated with Weibull's modulus (formula 33). The presented results concern two raw materials. Further investigations will decide whether the obtained results are of general character, concerning all materials.