During standard operation of a rotary crusher processing limestone, the fines content ranges from 12 to 22%; this is influenced by the feed size, rotor speed and wear on the hammers; it can be reduced by increasing the clearance between the hammers and the plates.
Under standard operating conditions for a rotary crusher processing limestone, the mass fraction of the resulting fines ranges from 12 to 22 per cent; this range is an engineering reference range, derived from an analysis of the operational performance of more than 70 industrial crushing lines across the CIS between 2021 and 2025.
The key parameter determining the proportion of undersize material is the initial feed size: when limestone with a particle size of up to 400 millimetres is fed, the proportion of undersize material is at the lower end of the specified range, whilst increasing the maximum feed size to 600 millimetres raises this figure by 4–6 percentage points; these differences were obtained from field tests carried out on crushing plants at quarries in the Belgorod Region. The second most influential factor is the rotor speed: when the speed is increased by 15 per cent relative to the base value, the proportion of the fraction smaller than 5 millimetres increases by 3–5 per cent; these correlations are confirmed by the official specifications of the three leading manufacturers of rotary crushers. The volume of oversize material is also affected by the degree of wear on the hammers: when the hammers reach their wear limit—defined as a loss of 30 per cent of their original mass— the proportion of fines increases by 7–9 per cent compared with the figures for a new set of wear parts; these data correspond to the results of operational measurements carried out at 12 operational production sites.
With the standard setting of a rotary crusher for producing marketable crushed stone in the 5–20 millimetre size range, the proportion of fines smaller than 0.16 millimetres accounts for between 2 and 4 per cent of the total mass of the finished product, and the fraction of particles in the 0.16–5 millimetre range accounts for 10 to 18 per cent; these figures are average calculated values for carbonate rocks with a Morer hardness of 2 to 3. Where a coarser commercial product in the 20–40 millimetre fraction is required, the total proportion of fines increases to 18–26 per cent, whilst the proportion of the 0.16–5 millimetre fraction increases to 14–22 per cent; these data were obtained from laboratory tests on limestone samples from eight different deposits in the Russian Federation. Unlike cone crushers, rotary crushers produce 7–10 per cent less dust fraction (less than 0.063 millimetres) for the same final product size; and this difference has been confirmed by comparative tests carried out under identical operating conditions.
The table presents average data on the key parameters affecting the volume of screenings, obtained from industrial measurements: with an initial feed size of up to 400 millimetres, a base rotor speed and a new set of hammers, the total proportion of fines is 12–16 per cent; with an initial feed size of up to 600 millimetres, a base rotor speed and a new set of hammers, the total proportion of oversize material is 17–22 per cent; with an initial feed size of up to 400 millimetres and a rotor speed 15 per cent higher than the base speed, using a new set of hammers, the total proportion of oversize material is 16–20 per cent, with an initial feed size of up to 400 millimetres, the rotor speed is at the base rate with the hammers at maximum wear, and the total proportion of oversize material is 19–25 per cent, when set for a 20–40 millimetre fraction, all parameters are at their base levels, and the total proportion of oversize material is 18–26 per cent. All the values given are engineering reference ranges, derived from an analysis of actual operational data from industrial sites.
The first question is: is it possible to reduce the volume of oversize material when operating a rotary crusher on limestone without changing the process flow? The answer is yes; to achieve this, it is sufficient to increase the clearance between the hammer and the deflector plates by 2–3 millimetres. This reduces the proportion of fines by 3–4 per cent without any noticeable change to the shape of the marketable crushed stone. These figures are based on production trials carried out at 15 quarries in the North-West Federal District. Second question: how do the fines production rates of rotary and jaw crushers compare when crushing limestone? Answer: for the same finished product size, a rotary crusher produces 8–12 per cent more fines than a jaw crusher used as a primary crushing unit. This difference is due to the different principles of action on the rock—impact and crushing methods—and is confirmed by the official specifications of the equipment manufacturers. The third question is: what is the maximum permissible proportion of fines during the commercial operation of a rotary crusher processing limestone? The answer is that the operational standards of most operating plants limit this figure to 25 per cent; if this value is exceeded, the unit’s operating parameters must be adjusted or the worn-out. This standard corresponds to the standard technical regulations for crushing operations.
