1. What is a ribbon blender

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 20 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).

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 aler the producer that the mixer operation is not 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
  (500-2000 l) is around 50 rpm with a Froude
  number < 1
• 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

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 theParticle 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.

4. Ribbon blender design – Detailed specifications

Paddle and plow designs

Some manufacturers have adapted their ribbon blender design by
changing the agitator through the use of paddles or plows. Those
designs should not be confused with double
shaft paddle mixers or shear
mixers / ploughshare mixers. Going to a single shaft
paddle design may be advantageous for very poorly flowing materials,
fragile materials or if the batch size may be as low as 15-20% of
the nominal batch size. When using plows, it is possible to have a
better action close to the wall of the mixer, high centrifugal
forces should however not be used and reserved to shear mixers /
ploughshare which have a cylindrical shell, versus a U shaped trough
design for ribbon blenders.

Mixer access

The access to the inside of a ribbon mixer for cleaning or
maintenance is not very easy, considering that the ribbon is
taking a lot of space in the mixer, and has a complex shape.

The most common access features for ribbon blenders is to have
hatches on top of the mixer, with sometimes the possibility to open
fully the cover of the mixer (tilting or sliding cover). Some
manufacturers propose to have an extraction system for the ribbon
which allows to take out the whole ribbon, it is possible but adds
to cost and mechanical complexity.

The opening of the mixer must be detected so that the mixer
cannot be started, for the safety of operators accessing the
inside of the mixer. Accesses must be guarded by locks
blocking the opening when the mixer runs and detecting when it has
been opened.

Some care must be taken for hygienic applications, indeed the
ribbon must be fully welded to avoid any loose part that could get
in the product. The difficulty to access for cleaning means that
risks of cross contamination must be well weighed and managed by
the plant operator.

Discharge valves

3 types of discharge valves can be found on the market, depending
on the suppliers and the need of the customer.

• Simple flap valves : discharge valves have a
  rectangular shape. It is the most common design.
• Hygienic round discharge valve : the valve has a round
  shape and, once closed, mimimizes the gaps where product can
  settle and avoid being mixed, contrary to the 1st type of valve
  that is presenting higher gaps. Those valves have a limited size
  that can increase the discharge time and thus reduce the mixer
  capacity.
• Bomb doors : the bottom of the of the mixer can
  entirely be opened by large flaps. The key advantage is that the
  mixing time is very short (less than 30 s, and that the quantity
  of product remaining in the mixer is very low. However, one must
  be careful to the tightness of the doors once closed, as well as
  to the access for cleaning below the doors.

Depending on the technology chosen, the mixer will discharge is a
hopper that will be entirely connected to the mixer bomb doors, flap
valves or that be connected through a short pipe to the mixer (round
valve)

Instrumentation

The following instrumentation can be found on ribbon mixers :

• Speed sensor : allows to confirm rotation and speed of
  the shafts
• Temperature sensors : positionned on the bearings,
  allow to detect abnormal heating due to broken bearings
• Flowmeter : positionned on the compressed air supply to
  bearing seal flush. Bearing seal flush constitute an important
  function to avoid ingress of product to the bearings, which would
  damage them or make the powder burn, which would constitute an
  ignition source causing dust explosion.
• Vale position sensor : allows to detect that the
  discharge valve is closed
• Safety Locks : placed on each access door, ensure the
  safety of the machine by preventing operator to access the mixer
  while it runs, or start the mixer if an access point is opened.

ATEX – dust explosion risks in ribbon blenders

In order to process powders, ribbon blenders, which are mixing with
a mixing tool tip speed > 1 m/s, must present the following
characteristics in order to prevent dust explosion risks:

• Clearance in between the tip of paddles and the mixer body must
  be large enough to avoid any contact ribbon / housing
• The bearing seals must be pressurized
• During loading and discharge, the mixer speed must be such that
  the tip speed of the paddles is < 1 m/s

The ribbon tip speed can be calculated thanks to the following
formula :

With :
– R is the radius of the mixing tool (center of shaft to tip of
paddle) in m
– n is the mixing speed in rpm

Equation 1
: Tip speed

5. Solids mixer sizing

The mixer should be the bottleneck of the installation of mixing,
which means that it should not be slowed down by the process section
upstream or downstream. The capacity of the installation should be a
given and a batch size should be chosen in consequence, considering
as well an estimated number of batches / h

Batch size (kg) = Capacity (kg/h) / Number batches per hour (/h)

The mixing process being actually volumetric, it is necessary to
know the untapped (loose) density of the mixture to size properly the mixer.

Batch size (l) = Batch size (kg) / Loose density mix (kg/l)

On top of this, it is critical to consider that the system should
never be filled at 100% of its capacity, in order to allow space for
particles movement.

Total mixer size (l) = Batch size (l) / 0.7

Mixers have maximum filling coefficient in between 0.65 to 0.8
usually.

6. Common problems with ribbon blenders

General troubleshooting guide for ribbon mixers

Ribbon blenders are usually reliable equipment, however a certain
number of issues may require some corrections :

Table 1 : common problems with
ribbon blenders

7. Ribbon blender buying guide – How to select a ribbon blender

7.1 Buying a new ribbon blender

When sourcing a new ribbon mixer for your factory, the following
questions need to be asked in order to buy the right specifications
:

• What is the expected throughput of the line ? What is the product density to be mixed ? What
  is the expected mixing
  time and cycle time ? This will give the size of the ribbon
  blender to buy. Don’t forget that the ribbon blender should not be
  filled at more than 80% of its total volume
• Is it an application requiring cleaning ? if yes, consider
  access doors on top of the blender to clean, consider safety locks
  on the access
• Is the application handling an abrasive product ? if yes,
  discuss with the ribbon blender supplier the alloy in which the
  blender can be supplied
• How fast is the blender to be discharged ? How ? This will give
  you the inputs necessary for the discharge valve
• Is it in ATEX
  area ? If yes, the blender must be certified, the clearance paddle
  / housing guaranteed, the seals of bearing must be pressurized and
  possibly their temperature monitored

7.2 Second Hand ribbon blender

Many used ribbon blenders can be found on the market. When looking
for a 2nd hand mixer, you should go through the following checks
:

• Was the ribbon blender used for a similar application to your
  needs ?
• Look for damages on the ribbon, scratches on the inside of the
  shell, measure the gap in between the ribbon and the housing,
  there should not be metal metal contact
• Run the mixer, listen to the bearings, if possible measure
  vibrations
• If necessary, can the mixer be cleaned ?
• Can the cover be modified to accomodate your needs for the
  fittings
• Not all the ribbon are equivalent, if possible run a test with
  product to validate
  homogeneity and product degradation during mixing
• Is the discharge valve functional
• Test all instrumentation that may be coming with the mixer
• Is the mixer ATEX compliant for the area you defined, if not can
  it be retrofitted