by Bill Langer

My wife, Pam, reads, critiques, and has veto power over every Carved in Stone article sent to Aggregates Manager. She said the topic of fines is stupefying and hinted that future articles on the subject might end up in the recycling bin.

Because this is my last chance, I have included a list of potential markets for fines. Of course, each use requires fines of specific physical and chemical properties, and not all fines can be marketed for all listed uses.

If you can’t market all your fines, you might want to limit their production. The amount of fines generated during aggregate production is, in part, a function of the mineral composition and texture of the rock. Less energy is generally used to separate individual mineral particles than to break them apart. All other things being equal, blasting and crushing a coarse-grained rock will generate fewer fines than a fine-grained rock.

Similarly, softer minerals break down more easily than harder minerals, and processing rocks with low abrasion resistance produces more fines than rocks with high abrasion resistance. The shape of the fine particles is also influenced by mineralogy. Processing a rock with a high content of flaky or elongated minerals will tend to produce more flaky-shaped fines than processing a rock with equigranular minerals.

Blasting also affects the production of fines. Increasing explosive energy produces smaller feedstock for the primary crusher, resulting in increased productivity and reduced costs for crushing. However, too much fragmentation can generate an excess of fines. Blasting commonly is optimized according to handling and crusher requirements, but could include a marketing strategy for all the products generated, even the fines.

During processing, multiple stages of crushing and screening are commonly used to maintain a relatively low reduction ratio (ratio of particle-size of feed material to particle-size of crusher product), which is more efficient and results in fewer fines being generated at each stage. However, using many stages with low reduction ratios may cumulatively produce more fines than a process using fewer stages with higher reduction ratios.