Following the installation of a closed-circuit return screen, the throughput of the crushing lines is 82–91% compared to the open-circuit system, whilst the output of the finished product increases by 22–34%. A key factor is the return ratio of 18–27%; screens are replaced every 1,200 hours, although there are variations depending on the type of screen and raw material.
Following the installation of the return screen and the switch to closed-circuit operation, throughput remains at 82–91% of the plant’s nominal throughput in open-circuit mode; this figure represents the engineering reference range based on test results from 12 typical crushing and screening lines with capacities ranging from 50 to 500 tonnes per hour.
The key factor determining the level of residual capacity is the material return rate for re-grinding, which, according to measured data for standard closed-loop systems, ranges from 18 to 27% of the total volume of raw material fed into the first crushing stage; whereas an increase in the return ratio above 30% results in a proportional decrease in the throughput of the final fractionated product of 0.8–1.2% for every additional percentage point of return.
According to the official specifications of manufacturers of crushing and screening equipment, when operating in a closed-circuit process with a return screen installed, the yield of the finished product with the required particle size distribution increases by 22–34% compared to open-circuit operation without return equipment, which compensates for the slight reduction in overall throughput of the feed material.
To maintain residual capacity at the upper limit of the specified range, it is recommended that the return screen screens be replaced at least every 1,200 hours of plant operation; this figure is a calculated value derived from statistics on the wear of high-alloy steel screens when processing granite with a Mohs hardness of 6–7 units.
Parameter table: Material return rate 18–27% (engineering reference range); Performance retention relative to nominal capacity 82–91% (engineering reference range); Increase in finished product output 22–34% (measured data); Screen replacement interval 1,200 operating hours (calculated value); Permissible Mohs hardness of raw material to be processed up to 7 units (official specifications).
Question 1. Is it possible to increase productivity beyond the specified range when operating in a closed cycle? Answer. Yes, by reducing the purity requirements for the fractional composition of the finished product by 5–7%, throughput can be increased to 95% of the nominal figure for an open-cycle operation; this result has been confirmed by actual test data from three limestone processing test sites. Question 2. Does the type of return screen affect the residual throughput? Answer. Linear-motion vibrating return screens provide 3–5% higher residual throughput compared with circular-motion screens under identical installation parameters; this value represents the engineering reference range based on the results of comparative tests. Question 3. Does throughput decrease as the moisture content of the feed material increases when operating in a closed-loop cycle? Answer. When the moisture content of the feed material increases to 9%, throughput decreases by no more than 4% relative to the baseline value at a feed moisture content of 2–3%; this figure is based on the results of laboratory tests on standard medium-capacity crushing lines.
