Agglomeration, or particle size enlargement, is central to many processes. Among the many common applications that benefit from agglomeration are pharmaceutical tablet-making operations; waste dust concentration reductions; filter cake processing; iron ore pellet production and other mineral processing applications; and fertilizer pellet and other basic chemical manufacture.
The pin mixer can be a major piece of equipment, serving as a preconditioner to high-value agglomeration processes. Pin mixers are used for:
Seed formation for pan, drum and other agglomerators.
Premixing of feeds and binders for agglomerators.
Dedusting and improved powder flowability.
Predensification and compaction.
The transport of fines into molds and dies of tableting, pelleting and briquette machines is much more difficult than is the transport of larger particles. The pin mixer makes larger particles and enhances the filling operation of such machines by increasing material flowability into the mold.
The larger particle size also allows the powder to fill more evenly in the mold. The resulting higher porosity permits air to escape upon compression. Trapped air inside a pellet and/or briquette, made from finer material, could cause gas releases that destroy the pellet or briquette.
Pellets pick up dust as they are formed and as they roll, resulting in dedusting of the material. Agglomerates have to form from seeds, which typically are made in pin mixers and become the feed to pan or drum agglomerators. The pin mixer serves as a pre-agglomeration unit in these cases.
In many processes that handle coal fines, fly ash, slag and other waste streams, the pin mixer is the first processing step. Overall, many processes are improved substantially when a pin mixer is incorporated into the processing circuit. Processes handling fly ash and carbon black, for example, can benefit greatly from a pin mixer. Applications for pin mixers continue to grow as they are added into established processes.
Pin mixer operation
A pin mixer, also called a turbulator in agglomeration circuits, consists of a number of pins placed on a rotating shaft inside a solid cylinder or shell. See Fig. 1.
Figure 1. Pin Mixer
The figure shows the basic design of a typical pin mixer.
The pins extend almost to the wall of the shell, but a clearance remains between the pins and the wall. The shaft rotates at high speeds, e.g., typically between 500 revolutions per minute (rpm) to 1,000 rpm. The hold-up (the amount of material held in the vessel) in pin mixers is very small, and the material's residence time is very short, on the order of three seconds to five seconds. The pin mixer quickly agglomerates powder into small spheres.
In many applications, a liquid sprayed within the vessel acts as a binder. Typically the liquid is water and might contain an additive to bring about the desired effect. The liquid spray typically is applied directly to the agitated bed. When it comes into contact with the particles, the liquid coalesces them into a single mass.
Pin mixer design
The pin mixer's pins can be configured in numerous arrangements, including straight, random or spiral. The spiral arrangement is preferred because it aids the passage of material through the mixer. The random pin arrangement has not been shown to be as effective and, therefore, is not used extensively.
The important design variables for a pin mixer include:
The tip speed of the pins (pND).
The diameter or cross-sectional area of the pins (pd2)/4.
The clearance of the pins from the wall, C.
N is the rotational speed of the mixer.
D is the length of the pins.
d is the cross-sectional diameter of the pins.
C is the clearance of the pins.
See Fig. 2.
Figure 2. Pin Passing through Material
The pin's tip speed, clearance and diameter are important mixer design considerations.
Optimization of a pin mixer's design is difficult, given the variety of solids processed by the mixer. Optimization tends to be process-specific, therefore, and testing is highly recommended before design. Unfortunately, any optimization efforts before action operations might just be guesses.
In general, designs should be as versatile as possible. Pin number, orientation, length and clearance are fixed by the equipment's design. The only variables that can be optimized in the plant are hold-up, feeding rate, rotational speed and possibly equipment tilt. Information-sharing and visits to other facilities using pin mixers are recommended.
Although the flow inside a pin mixer has not been determined through technical studies, it is assumed that the pin mixer tilt and the pins' rotation move material through the mixer. The material is exposed to the pins' high-speed rotation and thrown to the wall. Consequently, the mixer forms a ring around the wall.
The material essentially is centrifuged to the wall by the pins. The pins impact the material and usually push or pull it through the mixer. This pumping action is enhanced if the pins are orientated in a forward spiral configuration. If rotated backwards, the pins impede material flow unless the shell's tilt causes the material to move forward through the mixer by gravity.
The pins can be pictured as a flat surface moving through the material (Fig. 2). If the material is fluid-like, the pins shear it under and around them. Some particles in the clearance and in front of the pin are caught and broken up by the high shear rates in these zones.
The shearing areas are the pins' tip and the submerged areas. The shear rate is proportional to the pins' rotational speed, N.