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Εκλεκτική λειοτρίβηση λατερίτη Καστοριάς με σφαιρόμυλο

Marini Angeliki-Evangelia

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Summary

During this project, grinding tests were performed in a ball mill, at different times and ball sizes. The initial material, which was Kastoria’s laterite, was smashed and sifted into individual fractions, as a result the formation of three granulometric fractions (-3.35+1.70 mm, -1.18+0.600 mm and -0.300+0.150 mm), which constituted the feeds of the ball mill. A grinding test was performed for each one of the feeds, in four different times (t = 0.25 min, t = 0.5 min, t = 1 min and t = 2 min) and three different ball sizes (d = 0, d = 6.5 mm and d = mm) in each time. Each test corresponds to a different energy consumption, which depends on the power of the ball mill, the mass of the material and the grinding time.The output of each grinding test was divided into three individual fractions, coarse, intermediate and fine, which was the fraction size -0.075 mm. By the end of the tests, X-ray fluorescence spectroscopy (XRF) and X-ray diffraction analysis were performed. The main oxides which were found are NiO, Fe2O3, SiO2 and MgO and the main mineral phases of the sample were nepuite ((Ni,Mg)3(Si2O5)(OH)4), quartz SiO2 and goethite FeO(OH). Results showed that the highest NiO content (%) is mainly present in the finer product (-0.075 mm). It was observed that as the grinding time increases, NiO content (%) remains nearly constant, while its distribution (%) is significantly increased. Most of the results showed that self-grinding (d=0) leads to a higher increase in NiO content in the grinding output, with a really low distribution (<1%). The highest distribution (56.94%) was observed in the fine product (-0.075 mm) of -0.300+0.150 mm feed, in the grinding with balls of diameter d = 6.5 mm, grinding duration t=2 min with 1.92% NiO content, (an increase of 18.6% compared to the initial feed). Satisfactory NiO content and distribution showed in the -0.075 mm product of -1.18+0.600 mm feed, in the test with d=6.5 mm balls and grinding duration t=2 min, in which the distribution is 43.59% and the NiO content 1.85% (an increase of 17.1%). Moreover, in the fine product (-0.075 mm) of -0.300+0.150 mm feed, for d = 12.7 mm balls and t = 2 min grinding duration, there is a distribution of 37.36% and a NiO content of 1.81%.From the study of size reduction ratio(〖d1〗_80/〖d2〗_80 ), it emerged that in short grinding times, and therefore in low consumption energies, the size reduction ratio of the different feed sizes does not show major changes. In self-grinding tests (d=0), the size reduction ratio remains nearly constant, regardless the feed size and energy consumption. In tests using d=6.5 mm balls and with a energy consumption of ε=1.61 kWh/t (t=2 min), the size reduction ratio is higher in the finest feeds (-0.300+0.150 mm, 〖d1〗_80/〖d2〗_80 = 1.87) (-1.18+0.600 mm, 〖d1〗_80/〖d2〗_80 =1.66) and lower in the coarser fraction (-3.35+1.70 mm, 〖d1〗_80/〖d2〗_80 = 1.44). On the other hand, in tests using larger balls (d=12.7 mm) and an energy of ε = 1.61 kWh/t (t =2 min), the size reduction ratio is higher in the intermediate fraction (-1.18+0.600 mm, 〖d1〗_80/〖d2〗_80 = 2.48 ) and lower in the coarser (-3.35+1.70 mm, 〖d1〗_80/〖d2〗_80 = 1.68) and in the fine fraction (-0.300 + 0.150 mm, 〖d1〗_80/〖d2〗_80 = 1.56). Results show that the smaller the size of the feed, the smaller the balls required, in order to grind the material more efficiently and vice versa. If larger diameter balls were used than d=12.7 mm, it would be observed that the largest feed (-3.35 + 1.70 mm) would be more efficiently ground.Finally, the enrichment ratio (ER) was calculated, which is defined as the ratio of the content of an element or oxide (in our case, NiO) in the product to its initial feed content. The NiO enrichment ratio was calculated for the fine fraction -0.075 mm at grinding time t=2 min, for each feed (-3.35+1.70 mm, -1.17+0.600 mm, -0.300+0.150 mm). From the enrichment ratio diagram it is observed that although the coarser feed fraction (-3.35+1.70 mm) has the highest enrichment ratio (for d=0, ER = 1.47), its distribution is very small (1.60%) , while the two finest fractions (-1.18+0.600 mm, -0.300+0.150 mm) for d=6.5 mm balls, have a higher distribution (42.64% and 20.01%, respectively) and a significant enrichment ratio (ER= 1.17 and ER = 1.19).

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