The grinding device most applicable to the production of aggregate is the rod mill. It may be employed where abrasive rock must be comminuted to sand sized particles. The reduction ratio is typically between 5 : 1 and 20:1 with a maximum particle size in the feed of about 20 mm.
The rod mill comprises a cylindrical shell, lined with replaceable liner plates, which rotates upon a horizontal axis. The mill contains a mass of hardened steel rods up to 100 mm in diameter as media which occupies approximately 40% of the volume. The mill is supported at each end by hollow, cylindrical bearings, the trunnions, and driven by a peripheral ring gear. The feed material enters the mill through one or both of the trunnions and is discharged through ports in the shell, either end-peripheral discharge or centreperipheral discharge if fed through both trunnions. The mill is operated wet in the vast majority of mineral processing plants i.e. a solid to water ratio of about 2:1 on a weight basis. Within the aggregate industry, however, the material may be dry or moist.
The rotation of the mill causes the rod charge to tumble and cascade producing comminution by a combination of compression and impact. The action of the rods is favourable to the production of 'fine' aggregate since the larger particles holding the rods apart are crushed preferentially and a minimum quantity of very finely sized particles is generated by abrasion and attrition. Nevertheless, the rod mill suffers serious disadvantages when compared to fine crushers; notably lower efficiency of energy utilization and the costly replacement of worn rods.
Other grinding mills have limited application to the production of aggregate from typically strong, abrasive rocks. Finely sized aggregate and dust are generated unavoidably by all forms of comminution device and can be recovered or separated from coarser aggregates by specialized screens, classifiers or cyclones and fabric filters included in dust collection systems. Pulverizers of the edge-runner mill type are extensively employed to produce powders from soft minerals for major end-uses such as agricultural limestone and dolomite, chemicals, fillers for rubber, plastics, paper and paint, fire retardant in coal mines, pulverized coal and raw meal feed to cement kilns. Although none of these uses can be categorized as aggregates, the equipment will be included within the processing plant and similar mineral powders may be incorporated in masonry cements and asphalt. The edge-runner mill comprises alloy steel-tyred rollers bearing upon a rotating horizontal table or fixed cylindrical anvil. The load upon the rollers is created either by the mass of the rollers or springs or centrifugal force. The mill operates dry utilizing air, often preheated to dry moist rock, to transport the ground product to the air classification circuit. Finely ground rock is recovered by cyclones and bag filters whilst oversize particles are recirculated to the mill for further reduction.