Particle Size Analysis of Cement in Real Time by On-line Laser System

Cement energy costs
More than 1% of the world’s energy is used to grind cement to a certain fineness such that it is an appropriate size for its application. Mills are used to grind the cement down to a suitable size, but they are generally inefficient. Grinding systems in cement production make up approximately 85 to 90% of total plant electrical energy consumption but less than 5% of this energy is usefully used to grind the cement.

With global yearly cement production running at approximately 4,100 million metric tons/year for each of the last 7 years, till this year, there are plenty of savings to be made. Cement energy costs can be reduced by good housekeeping, conversion to more energy-efficient processes and capital-intensive investments in upgrades to existing plant.
However improved monitoring and control systems and equipment energy management are always going to provide the quickest returns on investment. The large power demands of finish milling mean that improving the efficiency of grinding and separation of the ground clinker can yield significant energy efficiency improvements and cost savings. How is this best achieved? The answer is on-analysis.

Real-time measurement of cement
We know laboratory-based laser particle sizing systems improve the quality of the cement but how can they be adapted on-line to produce the most optimized cement fineness?
The particle size is determined by a ruggedized version of the laser diffraction system used in cement labs but this system is much more stable, resistant to the intense vibrations occurring in a cement plant environment. The Bettersize BT-Online1 particle size analyzer will continually measure this representative quantity of sample in real time. Up to 10 kg of cement/hour is sub-sampled by the flute/eductor from the sampling point. As cement is an abrasive product, the sample flow path is ceramic lined up to and after the measuring cell. The measurement is similar to the laboratory operation in that the particles pass through the measuring cell, light is scattered and collected by the receiver lens and focused onto a detector where it is scanned, recorded and digitized at high speed for continuous real-time analysis (Figure 1).

Figure 1. Front and back view of BT-Online1 with a sampling flute shown by the arrow.

Comparison of on-line to laboratory results
How do the on-line results compare to the Bettersize laboratory system and other historical techniques? Bettersize has done tests comparing results measured on both their laboratory laser diffraction system and the on-line system.
Due to the different optical systems and algorithms used by the on-line and laboratory instruments, there is a slight difference in the measurement results. It should be noted that good consistency of results can be obtained by both different types of instruments, so a good correlation between results can be achieved but the absolute values are not always exactly equal.
It can be seen from Figures 2 and 3 that the typical particle size values and contents of cements of different strength grades measured by the on-line and laboratory systems are quite consistent. As the compressive strength of cement increases, the average particle size (D50) obtained by both instruments gradually decreases, and the content of 3-32μm % (the most important contribution to the 28-day strength of cement) increases slowly. Therefore, the measurement results produce a good correlation.

Figure 2. Mean particle size (D50) of different cements obtained by laboratory and on-line systems

Figure 3. Content between 3-32 μm of different cements obtained by laboratory and on-line systems

In addition, tests have been made comparing the on-line system with wet sieve analysis. It is widely known that one of laser diffraction’s advantages is to measure all orientations of the particle so all 3 dimensions are taken into account. Sieves however only report the second largest dimension and hence undersize irregularly shaped particles through ignoring the longest particle dimension. In situations like this we have to accept there will be an undersized shift in the sieve results, they are not wrong but they can be a little different depending on their aspect ratio of the particles.

Figure 4. The wet sieve results and the on-line process results of above 45um cement content distribution

However, the important thing is to check that the sieve results track the on-line process results and as can be seen in Figure 4, when tests were made, comparing the on-line results with those measured by sieves, they tracked with great success. If there was an increase in the percentage of particles above 45 microns on the Bettersize on-line, then there was also a proportional increase on the sieves and vice versa.

Conclusions
For cement industry, it is essential to install a ruggedized Bettersizer on-line laser system, fully automated and based on the same principles but with no need for human input. It can measure the cement fineness providing key parameters such as the % < 3 µm, % > 3µm and < 32 µm, % > 45 µm in real time. It can compare these current values to the optimal values saved in the computer placed in the cement control room. Measurement, control, automation and optimization of the grinding circuit lowers production costs, ensures increased consistent high-quality product is produced all the time with the resulting increased profitability of your cement plant.

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