Analiza mechaniki procesu rozdrabniania surowców mineralnych w kruszarkach i wysokociśnieniowych prasach walcowych

• Published in: Gospodarka surowcami mineralnymi Vol. 28; no. 3; pp. 139 - 153
• Main Authors: Bromowicz, Jan, Figarska-Warchoł, Beata
• Format: Journal Article
• Language: English
• Published: Warsaw Versita 01.10.2012; Polish Academy of Sciences
• Subjects: Aggregates; Analysis; Brittleness; comminution; High-pressure grinding rolls; Materials research; Mining; Prasy walcowe; Production methods; rock particles pressing; rozdrabnianie surowców; Studies; ściskanie ziaren skalnych; United States > US
• ISSN: 0860-0953; 2299-2324
• DOI: 10.2478/v10269-012-0022-2

W artykule przeanalizowano opisy procesu rozdrabniania przy użyciu różnych teorii. Scharakteryzowano pokrótce teorię stanów granicznych, teorię sprężystości oraz teorię plastyczności dla wyjaśnienia niektórych efektów obserwowanych w procesie rozdrabniania materiałów kruchych. Bardziej szczegółowo opisano zja­wiska występujące przy rozdrabnianiu w wysokociśnieniowych prasach walcowych, przeanalizowano wpływ wybranych czynników na wyniki rozdrabniania. Ocenę przydatności różnych hipotez do interpretacji procesu rozdrabniania w wysokociśnieniowych prasach walcowych przeprowadzono na podstawie badań ekspery­mentalnych. Przedstawiono wyniki prób ściskania wapienia w warunkach podobnych do istniejących przy rozdrabnia­niu w prasach walcowych. Kruszywo wapienne umieszczone w stalowym cylindrze poddawano ściskaniu poprzez stempel prasy hydraulicznej. Ściskano próbki o różnym uziarnieniu kruszywa i przy różnej wielkości ciśnienia. Oprócz wpływu wielkości ciśnienia prasowania stwierdzono wpływ uziarnienia nadawy na efekt rozdrabniania. Porównanie otrzymanych wyników wskazuje na duże możliwości regulacji parametrów pracy pras walcowych. 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 themain 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 aremany 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. The character and size of deformations depend on the type and magnitude of applied forces (stresses); therefore, the rocks are characterized by a different strength depending on the load applied. Difficulties in investigations of the strength of brittle materials against direct crushing meant that only approximate methods could be practically used. The transverse compression of thin plates (rollers) with the load displaced on a small surface is generally known as the Brazilian test. It consists of axial compression of a disk of diameter d and thickness h. Tearing stresses appear in the center of the disk, leading to the sample’s destruction. The comparison of the disk’s strength and the strength of irregular particles compressed between two points proved them to be equal. A large scatter of results was observed inmeasurements of the tearing strength ofmineral particles. This scatter is caused by the random character of the phenomena occurring during tearing, such as particle shape, arrangement, load conditions, and the internal structure of a given particle. Consequently, the strength of the rock material is characterized by a certain distribution and the crushing process possesses a statistical character. Increasing a rock’s humidity results in a decrease in the rock’s strength because water is a surface active compound which decreases the free surface energy. It is very important in the crushing processes of rock materials which are usually carried out under conditions of natural humidity. High pressure roller presses constitute a contemporary type of roller crushers which are increasingly applied in the crushing processes of minerals. Due to their low energy consumption, they are becoming more popular in ore processing, cement industry applications, and clinker milling. In crushers and roller presses, crushing occurs between two horizontal rolls rotating towards each other. The rollers may have smooth, grooved, or toothed surfaces. There are different forces acting upon the crushed material between rollers. When both rollers rotate with the same speed, crushing occurs as a result of squeezing (with smooth or grooved rollers) or splitting (with toothed rollers). The material is transported in the so-called choking system, consisting of mounting the self-dumping tank over the press. The material fills the entire space over the device as well as the working chamber of the press. The material is tightly packed and air voids between particles are significantly reduced in such a way that smaller particles are placed between larger ones. The crushing process results in the origination of microcracks and microfissures, which enhance further crushing mainly at the third and fourth degree of crushing in ball mills. The obtained energy consumption, measured by Bond’s index, is reduced by about 20-30% in the case of softer limestone, and by 15-20% for harder materials. The closed circulation of material is a characteristic feature of roller presses. The multiplication factor of circulation is most often 2 or more. The processing of monodispersive material contributes to obtaining higher rates of comminution (by about 30%) than in crushing of polydispersive materials, but the press efficiency at crushing the monodispersive material decreases significantly. It is caused by a deterioration of the consolidation and stabilization of the crushed layer. The density rate can be improved by introducing a portion of fine material into the monodispersive material. Fine particles filling the voids between coarse particles are characterized by higher strength. Experimental tests were carried out to evaluate the above hypotheses of crushing in high-pressure roller presses. The crushing tests on a rock material (limestone) were performed by means of compressing the layer of particles in the cylinder and pressing with the hydraulic press punch. Compression tests were applied to compact (Devon) limestone of different particle size distributions. The tested material was placed in the cylinder and loaded with the press punch. The thickness of the compressed material corresponded to the size of an outlet port on an average roller press tomake the compression conditions close to those in thematerial layer between the rollers. Analyzing the results of the compression of limestone samples in the hydraulic press, it is possible to observe an influence of applied pressure upon the effect of crushing (Fig. 7). The comminution rates of respective samples are given in Table 1. Apart from the pressure effect, there is also an influence from the feed particle composition. For coarse and mono-fraction material, the highest rates of particle size distribution were achieved, i.e. coarse particles of the feed were subject to crushing in a higher degree. The materials of the same upper boundary of particle size distribution, differing by the content of fine particle class, had an almost identical particle composition after compressing. This is advantageous since overmilling of the already crushed material does not occur. It can be stated while analyzing the obtained results, that the roller press has great potential for regulating the particle size distribution. It is possible to regulate the product particle size by means of regulating the gap between rollers, by regulating the exerted pressure, and by delivering the feed of the given particle size distribution, i.e. of the determined cavity rate.