Mechanical sampling systems are essentially small-scale material handling and processing plants. These plants often require scaled-down production equipment to function effectively. Many of the low-tonnage crushers available today were born from experience with larger-scale crushers and crushing applications driven by the need to incorporate low-tonnage crushing in sampling systems.
Outside of sampling systems, these smaller crushing machines lend themselves to another 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.
Multiple stage mechanical sampling systems often include sample crushing. Some multiple stage systems may include several sample crushers. The need to incorporate crushing into mechanical sampling systems is primarily based on the concept that smaller particle-size materials can be reduced into samples of smaller total mass.
A sample that is small enough to be handled in a laboratory is usually comprised of particle sizes that are less than 3/8" (10 mm) and are often comprised of particle sizes that are 4 mesh (~0.0.1875" or 4.76 mm) or less. Consider that particle sizes sampled from many process flows are 2" (50 mm) or larger, and the need for crushing within sampling systems begins to come into better focus.
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. It is this niche where sampling system-oriented crushers reside.
While there are some good rules of thumb for selecting an appropriate sample crusher or crushers for an application, there is still a bit of an art to the selection process. It's a balancing act where all considerations related to the material being crushed and the sampling application come together to form a best possible solution.
Often, we must choose the best possible solution because the ideal solution does not exist.
How To Choose the Right Sample Crusher
Choosing the type of sample crusher that suits your application largely follows the same selection process as larger production machines.
Type of Material
The type of material to be crushed is the most important piece of information.
Mechanical sampling systems can be used to process nearly any dry bulk material, and any application involving larger particle sized material and sampling is likely to require a crusher. The type of material and its associated physical properties will often limit options for sample crushing or exclude some types of crushers entirely.
From there, we begin to look at specific characteristics of the material in question (e.g. moisture content, abrasiveness, stickiness), and then some other application-specific items like particle size and size distribution of the material feed. We also look at particle size requirements (sometimes size distribution requirements) of the crushed product.
Once a crusher type is selected, the correct size machine is determined based on feed rate requirements and crusher drive components (motor size, type of drive, etc.).
Some other considerations include:
- feed size versus crushed product size
- crushed product size versus crusher capacity
As mentioned, the crushed particle size in sampling applications tends to be on the lower end of a crusher’s output size range. When crushers are being used to make crushed product at the lower end of their range, crushing capacities are reduced (i.e. the smaller the crushed product, the lower the available feed rate).
Crushers also have a limit on the crushed output size that is possible based on a particular feed size. This is called crushing ratio. The crushing ratio varies primarily by crusher type, but other items (e.g. material characteristics) can influence crushing ratio as well.
Types of Low-Tonnage Sampling Crushers
There are many variations to the design of sampling system crushers, such as features designed into a specific machine or design alterations necessary to meet a particular application, but in general terms, the types of crushers commonly used in sampling systems can be broken down into four main categories:
- Roll Crushers
- Jaw Crushers
- Cone Crushers
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.
How Sampling Hammermills Work
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.
An example of Hammermill crushing action in a downward running mill with fixed (replaceable) crushing screens.
Types of Material Sampling Hammermills Process
This type of crusher is best suited to crushing relatively soft to moderately hard, friable materials.
The maximum particle size for the feed material is generally based on the feed opening of the Hammermill, but most sampling system-oriented crushers will accept a 3” (75 mm) feed. Some larger or smaller sample crushers may accept a larger or smaller maximum feed size, respectively.
Reduction Ratio of Sampling Hammermill Crushers
Hammermills are capable of high crushing ratios. These ratios could be as high as 16:1 to 20:1 and are usually governed by the type of material being crushed (higher ratios with softer or more friable materials) and the condition of the material (lower ratios for more moist or sticky materials).
Hammermills are known to produce a higher amount of fine particles in the crushed product. This is advantageous if a finer crushed product is desired, since Hammermills can typically produce a finer crushed product than other crusher options. This can also be a disadvantage if a coarser particle size distribution is desired or if fines generation in general is not desirable.
Crushed product size can be adjusted by the size of the openings in the crushing screen or deck, with fine-tuning of the crushed product accomplished by adjusting the rotational speed (and therefore hammer tip speed) of the crusher rotor.
Minimum top size for crushed product is generally 4 mesh (4.76mm) to 8 mesh (2.36mm), and this minimum value often varies with material type and condition.
Sampling Hammermill Sizes
Sample Hammermill crushers are available in a range of sizes that cover feed rates typical of sampling system flows. Given a crushed product size of 4 mesh, this type of crusher can commonly be found with capacities of up to approximately 20 stph (18.1 mtph).
While these machines are rather versatile, it is generally best to avoid materials that are very hard, very abrasive, or are very moist or sticky, especially if smaller crushed particle sizes are desired.
Similar to Hammermills, the term “roll crusher” can be applied to a number of different designs and differing quantity of rolls within the crusher.
How Sampling Roll Crushers Work
Material to be crushed is feed 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). In either instance, crushing occurs as a result of compression of material between rolls (or rolls and breaker plate) and shearing action between the roll teeth or between roll teeth and a breaker plate. Sized material then exits the bottom of the machine.
Inward-crushing Double Roll Crusher.
Types of Material Sampling Roll Crushers Process
This type of crusher is best suited to crushing relatively soft to moderately hard material. There are also some more robust designs that can accept some harder materials.
Maximum particle size for the feed material is generally based on the diameter of the crushing rolls. Larger rolls have a larger nip angle that allows the rolls to properly grab material and pull it through the machine.
Sampling-system-sized machines will generally accept 3” (75 mm) material, though larger Double Roll Crushers are available for applications that require crushing from a larger feed size. However, these larger crushers tend to be more process-type crushers than sampling-system-type crushers.
Reduction Ratio of Sampling Roll Crushers
Roll Crushers generally have crushing ratios of 4:1 to 6:1 and are known to produce a more cubical product with minimum fines. The compromise with the more cubical product is a larger minimum crushed product size.
A typical Double Roll Crusher will generate a minimum crushed product size of approximately ¼” (6mm), assuming the feed size allows for a crushing ratio appropriate for the machine in question.
It is worth noting that there are specially designed Roll Crushers available that are specified to produce smaller crushed product sizes, potentially as small as 4 mesh (4.76 mm).
The takeaway is that it is more difficult to generate crushed product of this size. Achieving this size is dependent on the material being crushed, and producing very small product with a Roll Crusher is typically only accomplished at very low feed rates.
Crushed product size is controlled by setting the distance between crushing rolls or between crushing rolls and combs, as well as altering the tooth pattern on the rolls. For the smallest crushed particle sizes, rolls may not have individual teeth. For fine crushing with roll crushers, beaded or smooth face roll may be incorporated.
Roll Crusher rolls after processing a salt sample.
Sampling Roll Crusher Sizes
Sampling Rolls Crushers are generally available for capacities up to 15-20 stph (13.6-18.1 mtph).
Roll Crushers tend to function better than Hammermills with respect to moist and sticky materials (roll scrapers can also be included in the design), but if the material to be crushed is very moist or sticky, issues can still occur. It is also best to avoid using Roll Crushers for very hard materials.
Jaw Crushers have several sub categories, but the most common configuration is typically the overhead-eccentric-style design.
How Sampling Jaw Crushers Work
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.
Single toggle, overhead eccentric Jaw Crusher.
Type of Material Sampling Jaw Crushers Process
Jaw Crushers can process a wider range of materials than is typically considered for this type of machine. That said, Jaw Crushers are usually used with hard to very hard, but friable materials, and tend to produce smaller amounts of fine particles than other types of crushers.
The maximum feed particle size is governed by the gape, or depth of the crusher chamber opening at the top of the crushing dies. A common rule of thumb is that the maximum particle size is ~80% of the gape value.
How a Jaw Crusher works.
Reduction Ratio of Sampling Jaw Crushers
Jaw Crushers typically have a crushing ratio of 6:1, though there are a number of high efficiency Jaw Crushers available that can produce smaller crushed product sizes with higher crushing ratios (possibly as high as 8:1 or 10:1), albeit at lower feed rates. These high efficiency models are more focused toward laboratory applications, but there some designs available on the market that are suitable for continuous use in sampling system applications.
With Jaw Crushers that are specifically designed for small crushed particle sizes, output sizes of approximately 1/8” (3 mm) can usually be achieved. Crushers with the capability to crush to this small of an output size, however, are usually limited with regard to the feed particle size they can accept. Also, generally speaking, as a Jaw Crusher size increases to accept larger feed particle sizes, the minimum crushed particle size will also increase.
Regardless of the exact design of the Jaw Crusher in question, the crushed product size is determined by the closed-side setting of the jaws and the throw of the eccentric mechanism.
The closed-side setting is the gap at the bottom of the crushing chamber where the jaw dies are closest together. The closer the moveable jaw die can be positioned with respect to the stationary jaw die, the smaller the crushed product size will be. How close the moving jaw can be located with respect to the stationary jaw depends on the model and size of the crusher, but most sampling-system-sized jaw crushers can be adjusted to allow only a very small gap in this area.
The throw of the moveable jaw is the amount of movement between the closest point the moveable jaw die will be from the stationary jaw and the furthest point the moveable jaw will be from the stationary jaw during the course of one cycle. This throw value tends to increase as the size of the crusher increases (minimum crushed particle size tends to increase with machine size). On Jaw Crushers with small crushed output size capabilities, the throw tends to be rather small, contributing to the ability for that type of machine to produce smaller output sizes.
Sampling Jaw Crushers of varying capacities.
Sampling Jaw Crusher Sizes
Feed rates are dependent on the size of the crushed product. In sampling applications, these machines may have feed rates as low as several hundred pounds/kilograms per hour, or as high as 5-10 stph (4.5-9 mtph).
Cone Crushers operate by compressing uncrushed material between an eccentrically rotating (gyrating) cone called a mantle and a stationary liner called a concave.
How Sampling Cone Crushers Work
Uncrushed material is fed into the top of the machine and is distributed radially about the top of the mantle. As the mantle gyrates through its rotational motion, material continues to be distributed around the mantle, where it is fractured against the concave.
As material is broken into smaller particles, the particles work further into the crusher, where continued fracturing occurs until material of the intended crushed particle size is discharged from the bottom of the machine.
Type of Materials Sampling Cone Crushers Process
Cone Crushers can process a variety of soft to hard materials and are generally able to achieve higher crushing ratios than Jaw Crushers, potentially as high as 15:1. A more consistent crushed product size with minimal production of fines is also typically associated with this type of crusher.
The maximum feed particle size for Cone Crushers is governed by the opening between the mantle and the concave at the top of the crushing chamber. This opening must be large enough for a particular particle size that the particle can be compressed between the mantle and the concave to allow the compression necessary to achieve the proper crushing action.
Cone Crushers in sampling applications are capable of producing relatively fine crushed product sizes, as small as approximately 1/8” (3 mm). Generally, these smaller crushed product sizes are produced in smaller-sized crusher models with lower feed rate capacity. The final crushed product is dependent on the opening between the mantle and the concave at the bottom of the crushing chamber.
Sizes of Sampling Cone Crushers
While there are a range of model sizes available for various feed rates, sampling-system-sized cone crushers are usually used from feed rates up to approximately 10 stph (8.1mtph), dependent on the target crushed product size.
While the Cone Crusher is a rather efficient and versatile machine, it does not process sticky materials particularly well, and it tends to be more expensive than other potential crushing options for similar materials.
Sample crushing is an important component in many sampling systems. While some sampling specific considerations are needed to properly integrate a crushing solution into a sampling solution, many of the basic concepts and equipment related to typical process-oriented crusher selection in general also apply to the niche of sampling-system-specific crushing.