Pugmill Mixers

A pugmill mixer works by imparting a kneading-and-folding action on solids as a spray system distributes a liquid binder or additive over the tumbling material bed. The tumbling action combines the solid and liquid feeds to produce a homogeneous mixture.

Yes: In addition to mixing liquid and solid feedstocks into a homogeneous mixture and accommodating chemical reactions, the pugmill mixer is also capable of producing granules. 

When granulation is desired, production is controlled both by the overall sizing of the unit, as well as the adjustment of process parameters based on the characteristics of the material being processed.

The typical consistency of material exiting the pugmill mixer is rough agglomerates mixed with conditioned/de-dusted fines. However, processing parameters such as binder/liquid spray rate, retention time, and more, can all be used to control the consistency of the material exiting the unit, promoting more or less agglomeration.

The pugmill mixer uses a high-torque, medium-sheer kneading and folding action. The counter-rotating twin shafts cause material to be “lifted” up between the two shafts and then sent down the sides of the u-shaped trough. In addition to the mixing that occurs as a result of this action, mixing also occurs between paddles as they pass by each other in the center of the mixer. 

A “pulling” motion occurs along the trough walls as the shafts rotate that causes material to advance toward the discharge end of the mixer. 

A pugmill mixers is a continuous mixer. Some manufacturers may offer them at a batch-processing capacity, but these small-scale mixers are better suited for R&D activities and not commercial-scale production. Continuous units are capable of processing up to 250 TPH.

The main difference between a pugmill and a pin mixer is the motion employed. In contrast to the intense centrifugal motion that occurs in a pin mixer, the kneading and folding action used by the pugmill mixer is still considered medium sheer, but is less intense than the pin mixer. For this reason, granules produced in a pugmill mixer are less dense than those produced in a pin mixer. 

Combined with a higher torque, the pugmill mixer’s action is ideal for processing more malleable or sticky materials such as sludge, as well as materials that are highly abrasive, or that consist of large clumps. 

In addition to their different mode of action, pugmill mixers are also capable of processing a much higher capacity than pin mixers. They also have a longer retention time, making them better for accommodating chemical reactions. 

Pugmill mixers also allow for a bit more flexibility in processing, in that the paddles can be adjusted to modify the mixing pattern.

Paddles can be adjusted by swapping paddles in pairs from one shaft to the other, removing one paddle from the right shaft, and replacing it with the same paddle from the left shaft and vice versa. This allows paddles that were previously pushing material toward one end of the mixer to push material toward the opposite end, thereby increasing retention time.

By removing one or more pairs of paddles pushing toward the discharge, retention time can be increased without significantly increasing mixer brake horsepower. Paddles are affixed to the shafts via bolts, making removal and replacement a simple task. 

In adjusting paddle arrangement, care must be taken to ensure that the paddles are positioned so that the lined side of the paddle (if applicable) is the side that pushes against the material.

Adjusting paddle arrangement is helpful when a change in operating conditions or feedstock characteristics occurs, allowing operators to modify accordingly.

Achieving the right outcome when adjusting paddles is often a process of trial and error. In addition to the overall arrangement, paddle proximity can be adjusted to bring paddles closer together or push them farther apart. The most common paddle adjustment involves removing or reversing some of the paddles. 

A typical retention time for a pugmill mixer is around 60 seconds, with the exact retention time depending on the material’s unique characteristics, as well as the process goals – whether or not granulation is desirable, end product consistency, etc. 

The typical wear items for a pugmill mixer are the paddles, as these experience the most contact with the material. Other components that might require replacement throughout the mixer’s life due to wear include:

• Seals
• Pinions
• Shafts
• Sprockets and drive chain

The benefits of a double-shaft pugmill mixer over a single-shaft mixer include more thorough mixing, resulting in a more homogeneous mixture. The intimate contact created between particles that results from the counter-rotating shafts also promotes a more complete reaction where applicable.

While some manufacturers offer single-shaft pugmill mixers, all FEECO pugmill mixers are of  the dual-shaft design. 

Determining whether a pugmill mixer is the best fit for a given application is often best established through testing in a facility such as the FEECO Innovation Center, though in many cases, the manufacturer’s experience may inform the decision.

While pugmill mixers provide an excellent processing solution for many materials, they are not a fit for all applications. They are generally used in settings requiring heavy-duty mixing. The specific characteristics of the feedstock, as well as the desired end product parameters and process goals ultimately determine the best solution.