Eliminate over-grinding

The finish milling circuit consumes significant amounts of energy: throughput is high (generally in the range 20–150 t/h) and the target particle size is relatively fine. Typically, finished cement is produced using ceramic ball mills, although roller mills are becoming increasingly popular. With either mill it is common for a dynamic separator to take a fine product cut, coarse material being recycled to the mill for further grinding.

Flowsheet showing cement finish mill process with location of Insitec

The operational target is to grind the cement just enough to meet the specification. Producing finer cement requires a higher recycle rate, lowering throughput. Most importantly, the relationship between fineness and the energy consumed in grinding is more likely to be exponential rather than linear so energy consumption rises rapidly with decreasing particle size.

Off-line control with manual Blaine measurement

With manual Blaine analysis off-line measurements are generally taken every 2-4 hours. Reproducibility tends to be low due to manual sampling and operator-to-operator variability. Because confidence in the analytical information is low and measurement is relatively infrequent the process is operated some way from the true specification. The cement is milled more than is necessary since too fine a product is more acceptable, in terms of performance, than one that is too coarse. This approach prevents the product drifting out-of-specification between analyses but incurs a penalty in the form of excessive energy consumption. 

Switching to laser diffraction particle size measurement

Laser diffraction particle size analysis gives much richer information than Blaine, enhancing the operator’s view of the process. Furthermore, the repeatability of off-line laser diffraction analysis is normally better than that for Blaine measurement.

On-line laser diffraction sizing, with automatic continuous sampling and measurement, eliminates any possibility of operator-to-operator variability in sampling, as well as measurement, further enhancing reproducibility. Continuous measurement also increases the statistical significance of the data as several kilograms of sample are measured per hour, instead of a few grams every 2-4h.

All of these benefits help to dramatically increase confidence in the particle size data, encouraging the operating team to push closer to the real specification, milling to a coarser particle size that still meets the product specification.  

Diagram showing how over-grinding is reduced using on-line particle size analysis

Automating control

Automatic process control via a simple PID control loop to the classifier, or use of a MIMO controller, gives even tighter control, simultaneously streamlining operation. With this in place, the operator simply "dials-in" the required product specification; the controller moves the plant to its optimal setting.

With tight control and drift eliminated there is no reason to avoid running right up to the specification limit, achieving the goal of reducing particle size just enough to meet performance targets.  Although this will coarsen the product, the specification will be met consistently. Product quality will be enhanced and over-grinding largely eliminated, leading to higher throughputs and lower energy demand.

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