If you’ve been perusing the internet in search of a crushing machine for your low-tonnage application, you may have noticed it is relatively easy to locate the type of machine you are looking for; however, this type of machine is often much larger than necessary, or it is smaller and lighter duty than required for a process application.
Before we delve too much further into types of crushers for low tonnage applications, we should probably add some definition to the phrase “low-tonnage crushing”. For our purposes, “low-tonnage crushing” will mean the flow rate of material to be processed is less than 20 tph.
Machines geared toward typical production needs and laboratory environments are common — but what if you require a smaller crusher with the same function and durability of larger production machines? Do you try to make due with a smaller, lighter-duty machine that isn’t quite what you are looking for? Can you afford to buy the larger, more expensive machine that is everything you are looking for (and then some) but is likely to break your budget?
Crushing machines that strike a balance between these two extremes is a bit of a niche market (and therefore not an area in which many manufacturers will invest a great deal of time and resources) since there really isn’t the potential high sales volumes of standardised “cookie cutter” type machines.
This niche in low-tonnage crushing applications dovetails nicely with another niche market — mechanical sampling systems. Mechanical sampling systems are essentially small-scale material handling and process plants, and these plants require scaled-down production equipment to function effectively.
This is, in fact, how McLanahan came to develop many of the low-tonnage crushers available today — a familiarity with larger-scale crushers and crushing applications coupled with the need to incorporate low-tonnage crushing in sampling systems.
Many multiple-stage sampling systems incorporate some sort of crushing — the particular type of crusher being dependent upon the type of material being sampled.
Outside of sampling systems, these machines lend themselves to another very similar market — pilot plants. Again, the common theme is the need for production-quality equipment of durable design in smaller-scale material handling and processing systems that require lower-tonnage crushing.
Types of low-tonnage crushers
Many variations are possible for sampling system and pilot plant crushers — features designed into a machine or design alterations necessary to meet a particular application, but the McLanahan offering for these types of crushers can be broken down into three main categories.
The term “hammermill” is a relatively generic term that can apply to a variety of designs. In general terms, however, the same basic operating principles apply. Material to be crushed is fed into the top of the crushing chamber, where it is impacted by rotating hammers. This impact fractures the larger particles and allows them to be drawn further into the crushing chamber, along with whatever finer particles were already contained in the material feed. As the material to be crushed is drawn into the lower portion of the crushing chamber, it is forced against crushing screens, where it is further reduced in particle size by attrition.
Example of Hammermill crushing action.
The photo above shows an example of hammermill crushing action in a downward running mill with fixed (replaceable) crushing screens. Crushed product size in this example is primarily controlled by the size of the openings in the crushing screen, with fine-tuning of the crushed product being accomplished by adjusting the rotational speed (and therefore hammer tip speed) of the crusher rotor.
Hammermills tend to have high crushing ratios (attainable ratios vary by material, physical properties, etc.), and typically produce larger amounts of fines.
Similar to Hammermills, the term “roll crusher” can be applied to a number of different designs and differing quantities of rolls within the crusher. Material to be crushed is fed into the top of the machine, where it is either drawn downward between crushing rolls or between a crushing roll (or rolls) and a breaker plate (combs).
Inward crushing Double Roll Crusher.
In either instance, crushing occurs as a result of compression of material between rolls (or rolls and breaker plate) and shearing action from the roll teeth or roll teeth and breaker plate. Sized material then exits the bottom of the machine.
Roll Crushers generally have crushing ratios of 4:1 to 6:1, and are known to produce a more cubical product with minimum fines.
Jaw Crushers have several sub categories, but the most common configuration is typically the overhead eccentric-style design. In this machine, material to be crushed is fed into the top of the machine between a moving jaw die and a stationary jaw die. The material is fractured by compression and then moves further downward into the crushing chamber, where it is repeatedly compressed until it reaches the bottom of the crushing chamber (and the appropriate size) and is discharged from the bottom of the machine.
Jaw Crushers tend to produce smaller amounts of fines and generally work better with harder, but friable, materials.
Single toggle, overhead eccentric jaw crushing action.
Selecting a low-tonnage crusher
Choosing the type of crusher that suits your application largely follows the same selection process as larger production machines.
The type of material to be crushed is the most important piece of information. From there, assuming that we will follow our premise of lower tonnage, we begin to look at specific characteristics of the material in question for a given application (e.g. moisture content, abrasiveness, stickiness). We then look at some other items, such as particle size and size distribution of the material feed and particle size requirements (sometimes size distribution requirements) of the crushed product. From there, the correct size machine is selected and the drive requirements (motor size, type of drive, etc.) are developed.
If you find yourself in an “in between” situation where you need a durable production-quality crusher but have a low-tonnage pilot plant or sampling plant application that doesn’t warrant the use of a typical full-size crusher, a low-tonnage crusher may be just what you need.