OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 7 5 adjustable layer between the rolls. The grinding parameters were: rotational speed – 50 Hz, pressure – 28 kg/cm3, roll gap – 2 mm. The VSI Barmac 5100SE vertical shaft impact (VSI) crusher operates on a stone-on-stone principle, with a rotor speed of 3,000 rpm and a throughput of 4 tons per hour. The crushing chamber’s lining pockets are filled with compacted silicon carbide, which significantly reduces metal-on-metal abrasion and promotes the formation of more isometric grains. During typical operation, grinding generates a substantial amount of fine silicon carbide particles (dust). Therefore, a dust extraction system is integrated into the crushing chamber to ensure compliance with the abrasive grain quality requirements of GOST R 52381. Input material fractions were obtained by sieving powders using a Ro-Tap machine. Five fractions were selected for analysis, with the nominal cell sizes of the upper and lower control sieves presented in Table 1. The average nominal cell size (Wmi) of the upper (Wui) and lower (Wli) sieves was used as the primary parameter characterizing the grain size of each fraction, calculated as: Wmi = (Wui + Wli)/2. In accordance with GOST R 52381, the ratio Wui/Wli should fall within the range of 1.18–1.21. Ta b l e 1 Grinding powder fractions (GOST R 52381) Fraction designation Nominal size of sieve cells Upper sieve Wu, μm Lower sieve Wl, μm Average value Wmi = (Wu+Wl)/2, μm 1 2,360 2,000 2,180 2 1,700 1,400 1,550 3 1,000 850 925 4 600 500 550 5 300 250 275 Grain profile images were captured using an Altami CM0870–T optical stereoscopic microscope. Image processing and geometric parameter calculations were performed using dedicated software [24]. A total of 800 grains were measured within each fraction. The following geometric parameters were determined and analyzed: grain length (l), grain width (b), and shape factor (l/b ratio). Research Results and Discussion Based on the empirical grain size distribution patterns (Figs. 1–3), four distribution models were evaluated to determine the theoretical distribution: normal, lognormal, gamma, and the law of maximum value. The normal distribution is typically used to model distributions exhibiting symmetrical right and left branches on graphs [25]. a b Fig. 1. Experimental distributions of grain width b for fractions after roller-press (a) and rotary (b) grinding methods
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