When crushing limestone to 0–40 mm using various crushers, the yield of the 0–5 mm fraction ranges from 32 to 45%, depending on the gap width, moisture content, strength of the raw material and impurities.
In the standard process for crushing limestone into 0–40 mm fractions using a jaw crusher with an adjustable 5 mm outlet gap, the proportion of finished material in the 0–5 mm size range is 38–42 per cent; this figure represents the engineering reference range.
The main parameter determining the yield of the target fraction is the size of the outlet gap in the crushing equipment; when the gap is set at exactly 5 mm, the proportion of material smaller than 5 mm increases by 7–9 per cent compared with a gap set at 7 mm; this difference is based on actual data obtained from production lines with a capacity of 150 tonnes per hour. The result is also influenced by the moisture content of the feed limestone: at a moisture content of 2–3 per cent, the yield of the 0–5 mm remains within the baseline range, whilst if the moisture content exceeds 6 per cent, the proportion of the target material decreases by 4–6 per cent. This relationship has been confirmed by test results from quarries in the Central Federal District of the Russian Federation.
According to the official specifications of crushing equipment manufacturers, when processing limestone with a compressive strength of 80–120 MPa, the output of the 0–5 mm fraction is 32–36 per cent; this figure is specified in the equipment’s official specifications. When using a rotary crusher with two working rotors, the corresponding figure reaches 41–45 per cent, which is due to the higher degree of material comminution achieved through impact action; this result was obtained during production trials at plants processing non-metallic construction materials.
| Crusher type | Output slot clearance (mm) | Percentage of the 0–5 mm fraction | Data category |
|---|---|---|---|
| Medium-crushing jaw crusher | 5 | 38–42 | Scope of the Engineering Reference |
| Cone crusher for fine crushing | 5 | 32–36 | Official specifications |
| Twin-rotor rotary crusher | 5 | 41–45 | Actual test data |
| Medium-crushing jaw crusher | 7 | 29–33 | Calculated value |
The first question is: what will be the yield of the 0–5 mm fraction when processing limestone with a compressive strength exceeding 150 MPa? The answer is that, as the material’s compressive strength increases to 150–180 MPa, the proportion of the target fraction decreases by 5–7 per cent relative to the baseline values for standard limestone; this figure is within the engineering reference range. The second question is: is it possible to increase the proportion of 0–5 mm material without replacing the crushing equipment? The answer is that to achieve this, it is sufficient to reduce the line’s throughput by 12–15 per cent relative to the nominal value; this will allow for an increase in the residence time of the material in the crushing chamber and will increase the yield of the target fraction by 4–6 per cent. This result is based on the findings of production tests. The third question is: how does the presence of clay impurities in the feedstock affect the yield of the fraction? The answer is that, where the content of clay inclusions is up to 10 per cent of the total mass of the 0–40 mm feedstock, the proportion of material in the 0–5 mm decreases by 3–5 per cent; if the impurity content exceeds 15 per cent, blockages may form in the crushing chamber, which further distorts the particle size distribution of the finished product. This relationship is confirmed by the official recommendations of crushing plant manufacturers.
