There Are Numerous Options to Consider When Integrating Particle Size Reduction at Your Aggregate Facility.
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The feed enters a multi-row cage mill’s innermost cage. The left and right cages rotate in opposite directions.
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The single-row cage mill is simple to operate and widely used in limestone, sand and gravel operations.
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The design of the flared style base prevents material buildup allowing the mill to processes wet, tacky, or sticky materials without clogging.

Different materials require different breaking methods. Horizontal shaft impactors and vertical shaft impactors produce different results than a cage mill or hammer mill.

With a general understanding of your operation, application and results of cage mills, you can achieve desired particle sizes.

We get emails every day from producers wanting advice and options for crushing limestone with “cage grinders.” Here are answers to common questions we get. Keep in mind for fine grinds, the challenges are even higher.

Impact Crushing Methods

Okay then, a little review: Impact crushing produces size reduction by adding energy to the feed; the material collides with wear parts providing the reduction.

Testing different impact crushers such as cage mills, horizontal shaft impactors, vertical shaft impactors, and others identify the best crusher for an application.

Here, we’re focused on cage mills. Especially the applications that are asking for “one-step crushing to produce the final product without a closed-circuit system.”

When you understand cage mill components and functionality, you can achieve the most efficient and effective results. Multi-row mills consist of two cages and, in most cases, an even number of rows: two, four, or six.

A multi-row cage mill has multiple stages of impact reduction. The material enters the center of the innermost cage, where the material bounces around, getting struck until it is small enough to exit that row. Particle impact velocity increases the farther from the center the material travels.

Many calls we get, producers expect that a classifier or screener will be needed to meet product specifications after material passes once through the cage mill.

That’s not always the case. In most applications for fine limestone grinding, one pass will meet specifications. That eliminates the need for another piece of equipment – something that might have slipped your attention.

Particle Size Distribution Control

Now for a little technical talk. The cage counter-rotation allows the succeeding rows, moving from the innermost outward, to act on the particles that have not reduced to the desired size. Crushed particles of proper size pass through the subsequent rows of the cages without being further reduced. This characteristic of cage mills minimizes the amount of oversize and undersize particles found in the final product’s particle size distribution.

Configurations for Different Applications

Single-row cage mills are a legacy line predating horizontal shaft impactors in North America. These robust, compact, simple-to-operate impact crushers widely used in limestone, sand, and gravel for many years. Accepting large feed sizes, producing cubical shaped particles, and processing high tonnages (up to 800 tph) are hallmarks of the single-row cage mills.

Two-row and four-row cage mills output finer gradations than a single-row cage mill because they impart more energy into the material.

The same multi-row cage mill configuration can produce a variety of different sized final products. For example, high-calcium limestone is processed through a cage mill to produce:

  • Animal feed supplements.
  • Chicken grits.
  • Barn lime.
  • Ballfield striping.
  • Aglime (in one pass, no screening required).
  • Mineral filler materials for plastics and roof shingles.
  • MSHA rock dust.
  • Flue gas desulfurization.
  • Feed for hydrators (hydrated lime).

Six-row cage mills are a unique solution for fine grinding and work well for high calcium limestone or calcium carbonate. Understanding the material hardness and chemistry is a crucial application qualifier. The six-row is the final step before getting into higher cost, specialized fine or ultra-fine grinding equipment such as vertical roller mills, ball mills or jet mills.

Aglime Production

And we get calls about processing aglime – a finely crushed limestone used as a soil conditioner. Aglime increases the pH of acidic soil provides a source of calcium and magnesium for plants. These benefits increase the uptake of nitrogen, phosphorus, and potassium and enhance water penetration for lower crop production costs.

The primary methods for the production of aglime are impact-crushing with a cage mill or hammer mill, and the use of screenings of otherwise waste materials. Cage mills produce a Particle Size Distribution (PSD) that passes most state specs without screening. Many agricultural lime suppliers are “looking at better crushers for reducing the limestone to powder.”

Keep this in mind: The specific PSD used in the farmer’s field determines how rapidly the soil absorbs the aglime. The smaller the aglime particle, the higher the percentage absorbed in the ground within the first year of application.

One hundred to 50 mesh particles are wholly absorbed within the first year of implementation, considered 100% effective. Aglime between the 10 to 50 mesh size is 50% effective over three years. (Material particles more substantial than 10 mesh are too large to dissolve within a three- or four-year period.)

Mill Maintenance

Of course, we also field calls about machine maintenance. Here’s what you need to know: Cage configuration controls the feed’s flow through the machine. The design concentrates the wear on the pins, which are made of hard alloys, to maximize the life span before replacement.

To increase the life of the cage sleeves, change the cage direction frequently, allowing the sleeves to wear on both sides. Indexing or rotating the sleeves 90 degrees also increases the wear surface allowing for higher utilization of wear part material.

Testing Helps Select the Correct Mill for an Application

And then there are the quarries that call us wanting to purchase the “right tool from the start.” That’s the kind of call we love to answer.

Testing a variety of impactors and mill feed gradations offers insight as to which mill works best in either open or closed-circuit systems.

Testing material at different speeds ensures meeting the right particle size-reduction requirements while keeping up with production demands.

It would be best if you remembered the best results come from testing done at the specific moisture content and bulk density, as found in the field.

Since the material can change during transportation, field analysis of the material is critical to determine if any changes occurred that may affect any test results.

Abrasion tests determine wear-surface consumption and replacement schedules.

Summary

Yes, the cage mill is a different type of impact crusher. Still, once you understand how it operates and its advantages, you will be more likely to consider testing it on your feed.

If you have an old cage mill that’s not performing up to speed, reviewing the initial testing and results can help get you back on track.

Eric Marcotte is director of units and systems sales of Stedman Machine Co., based in Aurora, Ind.