Different types of mixers

As a follow-up to the article on the mechanisms and methods of mixing, this article discusses:

  • The different types of mixers for powders and their specific advantages and disadvantages

The experts at Dinnissen Process Technology are available to answer all your questions:

Get in touch with Juul Jenneskens 077 467 3555

Discover the different types of mixers and their advantages and disadvantages

Tumble mixers consist of a tank that rotates on one or more shafts. In some cases baffles have been placed in the mixing chamber. The flow of the particles varies with the rotational speed of the mixer. The powder particles will be moved by gravity. At low speeds, powder particles will slide across the bottom and there is no mixing. At high speeds, centrifugal forces will arise and the product will be pressed against the wall, so that there is no mixing involved. Between the phase in which powder particles slide over the bottom and the phase in which centrifugal forces predominate, two other phases can be distinguished in which the actual mixing takes place.

The first stage of mixing occurs when the critical tilt angle is exceeded by the rotation of the mixing chamber. If powder particles are poured onto a horizontal surface, a conical pile will form. The internal angle between the horizontal angle and the conical stack is known as the critical angle of inclination. This angle of repose is dependent on various powder properties such as bulk density, surface structure, shape and mutual cohesion of the particles. If this angle of repose is exceeded, the powder falls down in parts. Mixing occurs in this phase by shearing in the highest material layers.

The second mixing phase occurs when the speed increases further. Powder particles fall gradually in this phase. This changes the mixing principle from shear to diffusion. Tumble mixers are usually used in batch processes, but there are possibilities to use them as continuous mixers.

By applying different shapes such as V shapes, double cones, baffles and movements over multiple axes, more chaotic flows can be created in the mixing chamber.

 

The following types of tumbler mixers are available in the market:

  • IBC or container mixers
  • Drum or drum mixers
  • V blenders
  • Double cone blenders

The specific advantages and disadvantages of tumble mixers are:

advantages

  • Can be filled and emptied vertically,
  • Brings little or no shear into the product,
  • Have a low energy consumption in relation to the volume to be mixed,
  • Can be cleaned well when wet,
  • No contamination.

disadvantages

  • Have long mixing times,
  • Difficult to dry clean,
  • Many provisions have to be made to integrate them into a process flow,
  • Homogeneous mixtures with small additions (<1 to 2%) is critical.

The specific advantages and disadvantages of drum mixers are:

advantages

  • Vertical filling and emptying,
  • Short mixing times,
  • Little shear,
  • Large variation in fill level possible,
  • No heat input into product.

disadvantages

  • Internal inspection or dry cleaning is not optimal,
  • Difficult to install hygienically and dust-proof in a process line,
  • Shielding of the mixer necessary due to rotating parts,
  • Homogeneous mixtures with small additions (<1 to 2%) is critical.

Chaotic flows can be created in the mixing chamber by applying different shapes

Convection mixers have a wide variety of performances. The similarity they all have is that they consist of a mixing chamber with a rotating shaft or shafts, possibly with internal baffles. The shaft or shafts that rotate in the mixing chamber are equipped with one or more paddles that rotate with the shaft in the mixing chamber and thus in the product to be mixed. Mixing takes place due to the rotating shaft with paddles that ensures convection and shear in the product. Just like tumble mixers, convection mixers can be used in both batch and continuous processes.

Horizontal convection mixers are widely used in a wide variety of markets such as pharmaceuticals, food, pet food, feed and chemicals. The mixers are used in particular for powders, granules and granulates with a relatively low cohesion. A spiral ribbon, screw or paddles are attached to the rotating horizontal shaft or shafts that set the product to be mixed in motion. This creates mixing by convection. This means that dynamic areas with powder particles are moved through the mixer and mixed with it. This does not mean that diffusion and shearing does not take place in these mixers. If the powder particles are moved or fluidized as with a Dinnissen pegasus dual paddle mixer, they will hit other particles and each particle will take up another place in the mixture as a result of gravity. In addition, the rotating shaft will move some powder particles more than other particles, causing shearing in the mixture. The extent to which the different mixing principles occur will differ per type of mixer.

Three types of horizontal convection mixers are available:

  • Dual shaft paddle mixers
  • Horizontal ribbon blenders
  • Drum mixers

The specific pros and cons of dual shaft paddle mixers are:

advantages

  • Vertical filling and emptying,
  • Very short mixing times,
  • Low installation height,
  • One-sided bearings possible for retractable axles for easy and good cleaning,
  • Versions available with fully openable hinged bottom for complete emptying of the mixer,
  • Versions available with fully opening mixing chamber for good and easy cleaning,
  • Fast emptying of the mixer by means of a hinged bottom,
  • Many different sizes available,
  • Little shear,
  • Very small components can be added (<0.1%).

disadvantages

  • Poor emptying of the last remaining powder if hinged bottoms can or cannot be opened completely,
  • Gaskets below product level.

The specific advantages and disadvantages of horizontal ribbon blenders are:

advantages

  • Vertical filling and emptying,
  • Low installation height,
  • Many different sizes available,
  • Inexpensive versions available in the market.

disadvantages

  • Poor emptying of the last remnants of powder,
  • Difficult to clean,
  • Gaskets below product level,
  • Long mixing times
  • Moderate homogeneity for cohesive powders,
  • Homogeneous mixtures with small additions (<1 to 2%) is critical.

Vertical convection mixers can be divided into 3 classes:

  • Shear mixers
  • Planetary mixers
  • Vertical ribbon mixers

The shear mixers are generally batch mixers with a low capacity. The vertical shear mixers are mainly used for mixtures with a relatively high cohesion. A rotor knife or paddles are attached to the rotating vertical shaft to set the product to be mixed in motion. This creates mixing by convection. This means that dynamic areas with powder particles are moved through the mixer and mixed with it. This is optionally supplemented with a rotating knife in the side wall of the mixer. Because not all powder particles are in motion in this type of mixer, dynamic areas of powder are created in the powder mixture that move past each other at a different speed. Shearing takes place at the contact surface of these areas. Individual powder particles will transfer from one area to another independently of each other. The difference in speed causes a surface tension between the different dynamic areas on the interfaces. As a result of this surface tension, the dynamic areas of powder can disintegrate into pieces. This creates shear in the mixer.

Planetary mixers have a conical shape. In most cases the shaft at the bottom of the mixer is fixed to the bottom by means of a bearing. The shaft or shafts with screw rotates on its own axis. These shaft or shafts are attached to a planetary shaft that ensures that the screw rotates in the cone. The screw will transport the powder towards the top of the mixer creating convection mixing. Because not all powder particles are in motion in this type of mixer, dynamic areas of powder are created in the powder mixture that move past each other at different speeds. Shearing takes place at the contact surface of these areas. Individual powder particles will transfer from one area to another independently of each other. The difference in speed causes a surface tension between the different dynamic areas on the interfaces. As a result of this surface tension, the dynamic areas of powder can disintegrate into pieces.

Juul Jenneskens

Name: Juul Jenneskens
Advisor

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Vertical ribbon mixers are available in single and double shaft versions. With the double types, the ribbons of the axes overlap. The product to be mixed is set in motion by rotating the shafts. This creates mixing by convection. This means that dynamic areas with powder particles are moved through the mixer and mixed with it. Due to the principle of this mixer, there will be large speed differences in the product to be mixed. In the vicinity of the bottom and the center of the shaft the speed is almost 0. With double shaft ribbon mixers, transfer will take place between the product that has been set in motion by one shaft with product that has been set in motion with the other shaft. Little shearing takes place in this type of mixer. Liquids can be applied with extra facilities such as nozzles in combination with a blade motor.