Silica Flour


Silica flour is used in the production of fiberglass.  Along with other components it is melted in a furnace that operates at high temperatures, roughly 1500C. The molten glass is extruded through bushings that have hundreds of tiny holes. Each hole produces a filament of drawn glass that is only about 10 microns in diameter. Manufacturers of fiberglass put strict specifications on the particle size of the flour that they use because particle size has a direct impact on their manufacturing process. Normally they specify the required fineness as the minimum amount of material that must pass a #325 US standard test sieve. For example, one specification is for “85% grade”, which means that when a sample of the product is analyzed at least 85% of it is finer than the #325 test sieve. Some manufacturers require a finer grind, like a 96% grade. The procedures used to make these quality measurements is time consuming and often yields inconsistent results due to variability in material sampling and the analysis technique of the technician performing the test.

To make silica flour, producers feed granular silica sand to a ball mill where it is ground to a powder that has a fairly wide particle size distribution. The mill discharge is fed to an air classifier that extracts the fine portion of the PSD and sends it to the product silos while the coarse rejects are fed back to the ball mill for further grinding. Producing a consistently high quality product requires that the operators closely monitor the performance of the ball mill and the air classifier. An improper setting on either can lead to the production of off-spec product.  

By definition, the finer the product the more grinding energy required to produce it. Thus for a given ball mill, making a finer product means a lower production rate. So the objective of the mill operator is to maximize production while maintaining product quality. It can be a delicate balance. When using traditional manual sampling and laboratory sieve analyses, operators had to rely on data that was literally hours old by the time they received it. This severely limited their ability to fine tune the system and maximize production because if they pushed the limits too far they ended up with a product that did not meet the fineness requirement. And they wouldn’t know it until hours later, after many tons of off-spec flour were produced.

But now with Xoptix, operators see accurate PSD measurements of the fineness being produced in the milling and classification circuit. And they see the data in real time. This enables operators to fine tune the classifier’s cut point and push the feed to the mill to the optimum rate, without fear of going out of spec. Below is a plot of an Xoptix trend graph showing the quality of silica flour being produced, in this case a 96% product...