The pin mixer is a horizontal, medium-shear industrial mixer used in a wide variety of settings. It is most well known for its ability to densify and de-dust; from chemical processing to energy production, it is the equipment of choice when pelletizing or agglomerating ultra-fine materials.
A diverse machine, the pin mixer can be used as a stand-alone agglomeration device, or as part of a larger agglomeration process, where it preconditions material for downstream processing.
Most notably, the pin mixer is a key piece of equipment in the following applications:
With such significant flexibility, pin mixers are most effective when they are designed around the characteristics of the material to be processed. Here is a look at how this diverse industrial mixer is engineered to produce an optimal processing solution.
Questions to Ask Prior to Pin Mixer Design
While pin mixers are ideal in many settings, they are not the proper choice for every application. A few questions should precede the sizing and design process to ensure the best mixing solution has been chosen.
Is the pin mixer’s action appropriate for the application?
Pin mixers utilize a high-intensity spinning action that quickly densifies material. Materials that require more gentle handling are often better processed in a pugmill mixer. Pug mills use a kneading and folding over motion that still provides a homogeneous mixture, but is less aggressive on the material.
Will the material’s consistency be a problem?
Pin mixers perform best when working with finely divided solids such as dusts and fines, particularly when those solids will be mixed with a liquid binder.
Materials containing large lumps may require a pre-grinding step. Similarly, materials that have the potential to be accompanied by tramp material are not advisable to process in a pin mixer; as a result of the high-speed spinning action, tramp material entering the mixer has the potential to cause significant damage.
A sticky material can also sometimes be problematic in feeding the mixer. Testing is recommended to assess and account for this.
Will testing be needed?
Depending on the material, the process goals, and what data is already known, testing may or may not be necessary.
Processes that are well established typically do not require testing. However, if the material is known to vary across sources, or the process is somewhat novel, testing in a facility such as the FEECO Innovation Center is recommended to work out process variables and gather the data necessary for scale-up.
Design Considerations
The process of sizing and designing a pin mixer that meets its intended application requires a careful balance of several factors.
Not unlike the pugmill mixer, sizing centers around the retention time and capacity that the unit will need to meet, along with the desired agglomerate characteristics.
Retention Time
Retention time, or residence time, is the amount of processing time required for the material to discharge at the proper specification. The rotational speed of the shaft can be used to control retention time, along with feed rate.
The optimal retention time for a material is based on the goal of processing; is the mixer being used to produce agglomerates within a specific size range? Will it need to accommodate multiple feed components? Is a homogeneous mixture of liquid and solid feed the intent?
A retention time in a pin mixer anywhere from 10 to 20 seconds is common. If the retention time is not known, this can be determined through testing.
Capacity
Available in many sizes, pin mixers can be designed to process material in a wide range of scales – typically anywhere from 20 lb/hr to 70 TPH, depending on the application.
Customizations
The standard pin mixer is comprised of a single rotor shaft fitted with pins and encased in a cylindrical housing. A stainless steel liner on the housing improves operation by providing a smooth surface that promotes material release. This liner also helps prevent corrosion of the carbon steel housing, and is easily replaced when worn.
Pin mixers can be customized in a number of ways to address the unique challenges of the material to be processed. Common customizations are summarized here.
The Spray System
When a liquid binder will be incorporated, a spray system will be required. Liquid is added either via a simple open coupling, or it is sprayed in using specially designed bulkheads.
Multiple ports are utilized during the introduction of liquid binder, and various locations are provided, since this must be adjusted during process startup and provides flexibility.
Drive Assembly
The following drive assemblies are available. Drive selection is most often based on buyer preference.
- Direct drive
- Belt and Sheave
In cases where the allotted footprint of the unit is limited, an in-line drive can also be used to save on space.
Pins
The pins, or rods mounted onto the mixer shaft, are generally made of stainless steel or carbon steel. However, when the material to be processed is especially abrasive or corrosive, pins can be provided with a coating to minimize wear, of which there are many options. Due to the intense spinning action that occurs between the material and the pins, abrasion-resistant coatings are extremely important when working with an abrasive material.
Pins can also be provided in a variety of arrangements, depending on the desired particle size and shape, as well as how the mixer is being used (as a stand-alone device or as a preconditioning step). Pin arrangement options include:
- Full sweep
- Partial sweep
- Single helix
- Double helix
Raising Cover Assembly
A raising cover assembly facilitates easier machine cleaning and pin adjustment or replacement. Screw jacks are used to raise and lower the cover sections. A bottom that drops down for easier cleaning is also available.
Miscellaneous Customizations
Materials of Construction
Various materials of construction are available depending on the degree of corrosion and/or abrasion protection necessary.
Mechanical Components
Many of the pin mixer’s mechanical and control components can also be customized.
An optional variable frequency drive, or VFD, allows the motor to sense and respond to changes in the mixer. This can be used to prevent motor trips due to excessive amp draw, and also allows the rotor speed to be adjusted to aid in fine-tuning the process.
Zero-speed switches can also be helpful in this type of situation to alarm the operator when the mixer drops below a specified set point. Often both a VFD and zero-speed switches are recommended.
Automation
Pin mixers can also be integrated into a process control system, so process data can be collected, monitored, and even trended through a user interface for the operator. This might include data points such as:
- Current (amps)
- Solid Feed rate
- Liquid Feed Rate
- Horsepower
- Torque
- And more…
Testing in the Design Process
Testing is often an important step in the development of a commercial-scale pin mixer. As mentioned, testing in a facility such as the FEECO Innovation Center provides a way to gather and analyze various data to make informed decisions during the design process.
“One of the nice things about the Innovation Center, is that we can test material on a continuous basis as part of a larger process,” says Chris Kozicki, FEECO Agglomeration Process Sales Engineer. “A lot of our customers appreciate that we can incorporate drying, screening, and a recycle circuit if needed. It really helps them to get their concept worked out on a high level.”
Conclusion
Pin mixers are a flexible mixer ideal for agglomerating and de-dusting ultra-fine materials. As they are best designed around the specific demands of the project at hand, a number of considerations should be taken into account in order to produce the most ideal processing solution possible. While this can be based solely on known data points, testing is often necessary to define process variables.
FEECO has been providing custom pin mixers since 1951. With experience around hundreds of materials, we can test, design, and manufacture a pin mixer suited to your exact needs. For more information on our pin mixers, contact us today!