Traditional off-line fineness measurement - Blaine
Many plants control cement fineness using an air permeability apparatus to measure the Blaine number, a parameter related to the cement particle size. The Blaine surface area measurement has been used since the 1940s to determine cement quality and despite its longevity the limitations of this off-line technique are widely recognised. It is slow and suffers from poor reproducibility, which can lead to significant errors, including unnecessary or inaccurate changes to the classifier speed. In addition, the fact that it produces only a single result without generating a particle size distribution means that two cements with a different proportion of fines can give the same Blaine number.
A typical regime might involve collecting composite samples, which are then sub-sampled every 4 hours to give 50g cement for measurement. These are placed on paper for weighing and then used in Blaine fineness measurements. While every effort is made to achieve maximum precision, performing this manual off-line measurement is prone to errors, including:
- Loss of sample during transfer of cement from the scales to the permeability cell
- Failure to sufficiently compress the sample
- Using two papers instead of one
- Temperature fluctuation
- Humidity effects
- Losing manometer fluid
What this means in practice
In one USA cement plant (Ash Grove, Utah), shift working meant that Blaine cement measurements could be carried out by any one of twenty people. The possible consequences can be illustrated by taking as an example the production of a cement with a desired Blaine of 3700 cm2/g. The single operator precision as stated in ASTM C-204 is 3.4% for the measurement of two identical samples ie ±126 cm2/g. Ash Grove Cement found that when two different laboratories measure the same sample, the difference can be as much as 6.0% ie. ±222 cm2/g. This was the scenario faced at every shift change, with variation in Blaine cement measurement inherent in each change of personnel. As a result, the separator speed was often being changed unnecessarily. In an attempt to minimize the effect of measurement variation, separator set points were changed only if the sample varied by more than 100 cm2/g. Since the separator speed was held constant between measurements, the actual Blaine value of the cement could drift significantly.
On-line particle size measurement has helped overcome these problems. The product is now monitored continuously every 30 seconds, which leads to several kilograms being measured over the course of 4 hours.