Real-Time Blaine Cement Fineness Measurement

'Laser diffraction' Blaine versus 'pressure drop' Blaine

The laser diffraction particle size measurement technique employed by the process instrument varies greatly from the packed-bed pressure-drop method that the typical Blaine number measurement device utilizes. However, a recent measurement sensitivity analysis has verified that the optical instrument was able to resolve the 7% variations in Blaine numbers of the samples tested.

In addition, a good correlation exists between the "laser-diffraction" Blaine cement measurement and "pressure-drop" Blaine measurement. To develop this correlation, various samples of a particular type of cement that represented the high-acceptable, low-acceptable, and average-acceptable Blaine characteristics (i.e. the control range of interest) were measured.

Blaine apparatus measurements were then performed on these samples by the product manufacturer, and compared with the analysis performed using the Insitec optical technique. The results indicate that the Blaine number, or specific surface area (SSA), yielded by the instrument can be used as a surrogate for a pressure-drop Blaine analysis. The optical instrument determines the specific surface area by effectively integrating the light scattering from individual particles. Note that there is not an exact numerical comparison between the two techniques, and that no error-bar uncertainties have been assigned to the measured Blaine numbers.
Allen (Allen, Terence, "Particle Size Measurement," Vols. I & II, 5th Edition, Chapman and Hall, 1997) has analyzed the reliability of the Blaine concept and states: "The assumptions made in deriving the Carman-Kozeny equation (on which the Blaine number is based) are so sweeping that it cannot be argued that the determined parameter is a surface," and "The determined surface areas are usually lower than those obtained by other measuring techniques." This last comment is obviously consistent with the current optical results, which give higher values than the Blaine number. Allen points out a range of other limitations for permeametry techniques. Thus we conclude that the precision and accuracy of the Blaine number is limited.

From an operational point of view, the Blaine test takes time, which delays optimal mill/classifier adjustment. Over-grinding increases energy consumption, while under-grinding reduces product quality. The ideal goal is to meet quality requirements and maximize production rates (use of capital equipment) with minimum energy consumption. A current optical instrument user has found a three-fold reduction in product variation from 8% to 10% down to 3%. Although some customers still require Blaine tests, these measurements are more a confirmation of the optical instrument results.

These initial successful comparisons of Blaine and SSA measurements encouraged more comprehensive test comparisons. The figure below shows the results of a two-week test series comparing independent Blaine results and the optical SSA. The SSA values were normalized to the Blaine number results by a constant determined in a previous test series. On this graph, the Blaine number error bars are indicated for one standard deviation (about 1.5% as estimated by ASTM for an individual technician). For multiple technicians, it is known that different techniques may be used and the standard deviation would be of the order of 2.5%.

Note that the two different measurement techniques agree quite well, with a few data points at the beginning and end of the test series showing deviations exceeding 1.5%. An error bar has not been applied to the optical instrument measurements, but it can be assumed that it is of the same order, namely 1% to 1.5%. The error bars of the two sets of measurements would overlap in all cases, indicating that the two techniques are statistically in agreement. That is, there is no statistical discrimination for choosing one method as being more accurate or precise than the other.