What is a Ribbon Blender and How Does it Work?

Ribbon Mixers are a specific type of powder blenders that consist of 2 parts mounted on a central shaft . The mixing medium often consists of inner and outer helical ribbons that are designed to move material both inwards and outwards. The external helical ribbons pull product from the sides of the mixer into the middle and the internal ribbons push the product back to the sides. The back-and-forth folding motion of the materials to be blended creates a convective mixing pattern.

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The ribbons are designed to have precise gap tolerances between the walls of the mixer. In some instances, they can even be designed with soft material wipers to help keep product from building up on the sides. The drive mechanism often consists of a belt drive for the powder transmission between the motor and gear box. This arrangement would use a system of flexible belts and pulleys to produce the desired speed and torque required for the specific mixing application. However, more manufacturers are starting to use direct drives via gear motors that utilize a motors low inertia rotor to specifically match the gearbox unit. The advantage of this arrangement means less maintenance over the life of the industrial mixer.

Make sure to understand the required volume capacity of the mixer you are in need of. Knowing the bulk densities of the ingredients to be mixed is crucial to understanding the required capacity of your mixer, and more importantly the needed motor power required to mix your batches properly. Most standard design mixers will work just fine with a product bulk density of 35 lbs./ft³. However, once you start mixing denser batches the need for heavy duty motors and gear ratio changes will also be required.

1. What is a ribbon blender : definition

Ribbon blenders are made of shell having a U shape in which is rotating an agitator. The agitator is designed as a double helical ribbon which allows to create convective movement of the materials in 2 directions achieving mixing of powders and bulk solids.

Ribbon mixers are very widespread in process industries for bulk solids dry Mixing. Although other Mixers designs exist which are over-performing ribbon blenders in some areas (mixing speed, hygienic design...), ribbon blenders are still a very simple and robust solution to mix dry materials offering decent mixing performances which are sufficient for many applications. It is not rare to have 40-50 years old industrial ribbon blenders in factories, still perfectly operational. Many manufacturers propose industrial ribbon blenders, from few hundreds liters to several cubic meters.

What is a ribbon blender used for ? Ribbon blenders are actually used in many industries and applications : pharmaceuticals, food and bakery, cosmetics, plastics, spices, mixes for drinks, cements grouts and mortars, coffee and tea, tobacco...

This webpage is focusing in the detail design of ribbon mixers in order to offer selection and buying guidelines to operators of mixers.

2. Ribbon blender working principle

What is the mixing mechanism of a ribbon blender ?

Ribbon blenders are convective mixers. The mixture movement is forced by the rotation of the ribbon which is circulating the product in 2 directions : the ribbon is actually made in 2 parts, 1 external ribbon circulates the product in 1 direction while another ribbon located inside the 1st one moves the product in the other direction (it can be possible to have even more complex profiles to attempt optimizing the mixing efficiency and thus the mixing time). By mixing enough time, those 2 axial movement, coupled with some radial movement (blades "cutting" the material when rotating), will allow to reach the required degree of homogeneity. The twist of the helix must be studied by the manufacturer so that the direction in which the outer ribbons is pushing is towards the outlet valve of the mixer, if it is not the case, good discharging rate of the mixer cannot be achieved.

Contrary to double shaft paddle mixers the powder is not fluidized in a ribbon mixer. The optimal mixing speed is advised by the mixer manufacturer, some trials can however be done at higher or lower speed (provided the drive is able to sustain different speed) in order to verify the influence on the mixing quality and the product properties (in general, lower speed will require a longer mixing time but the product may be less damaged).

Additional reading:
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Figure 1 : Ribbon blender drawing

3. Mixing operating parameters

How to reach good homogeneity with a ribbon blender ?

For ribbon mixers, the mixing time is typically 3-5 min. Ribbon blenders have the reputation not to be supposed have a short mixing time, which brings some operators to mix 10-15 min. If the mix has no particularity (injection of liquid...) such long mixing time should alert the producer that the mixer operation may not be optimal.

The mixer performance, i.e. time to reach a desired homogeneity, is a function of the following operating parameters :

• Mixing batch size : 70-80% of mixer total volume. Visually, the top of the blades must be slightly above the level of product, and some head space to the top cover must always be available. It is a very common problem to find ribbon blenders overloaded which is strongly decreasing their performance. To be noted that it is not advised to under-fill a ribbon blender, since the level of powder must reach inner ribbon in order to be moved and thus mixed.
• Mixing speed : one should follow the supplier's recommendation, typical mixing speed for medium size blenders (500- l) is around 50 rpm (note that Froude number < 1 for ribbon blenders)
• Small and Minor ingredients to be introduced in the mixer after the main ingredients (or in sandwich), preferably in the central area of the mixer

Figure 2 : Ribbon mixer recommended and max mixing volume

Recent Discrete Element Modelling (DEM) [Basinskas] of ribbon blenders have confirmed that the key operating parameters influencing on the degree of mixing and thus the mixing time are :

• The mixing speed : higher mixing speed means shorter mixing times
• The filling level : a ribbon blender should not be under filled, typically if the level of powder is below the shaft the impact on the mixing quality will be very high
• The filling sequence : according to the simulation the axial mixing of a ribbon blender is not so good (which is surprising as the ribbon are supposed to bring some axial thrust), which means that it is better to fill the mixer by layers rather than having one side of the blender with some components and the other side of the blender with the rest of the components.

The ribbon movement has quite some impact on the product being mixed. The thrust imposed to the product combined to mixing times that are not very short, usually cause some breakage on the solids processed. Breakage means that the Particle Size Distribution (PSD) will be changed, with creations of some smaller particles due to the breakage of bigger ones. The impact can be minimized by validating the exact time required for mixing (Homogeneity) and thus not mix longer without need, and by optimizing the mixing speed to mix as low as possible. However, breakage is expected to be higher than in twin shaft paddle mixers for example.

Ribbon blenders are operating well with free flowing powders, due to their mode of operation (thrust by a rotating agitator), ribbon blenders may be less effective with cohesive powders and may even block if the the mixture is very cohesive.

The power input required for a ribbon blender is quite low, in the range of 3-5 kW/m3.

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