Selection of mixers must take into account any tendency towards segregation.
This may be evaluated form a "heap test", in which a well-mixed material is poured
through a funnel to form a heap. If the composition of samples taken from the outside
varies significantly from compositions of samples from the centre of the heap,
the material is likely to segregate during mixing or later processing.
Mixers can be classed into two groups with respect to segregation:
Segregating mixers have mainly diffusive mechanisms, encouraging
the movement of individual particles, making segregation more significant.
Non-impeller type mixers tend to be of this type.
Less segregating mixers have mainly convective mixing mechanisms.
These are typically impeller types in which blades, screws, screws, ploughs, etc.,
sweep groups of particles through the mixing zone.
Tumbler mixers
- operate by tumbling the solids inside a revolving vessel
- may be fitted with baffles, etc., to assist mixing, or with internal rotating devices to break up agglomerates
- operate at speeds up to about 100 rpm (about half the critical speed -- at which the centrifugal force on the particles exceeds the pull of gravity)
- working capacity is about 50 to 60% of volume
- best suited to gentle blending of particles with similar physical characteristics, segregation can be a problem
- equilibrium is generally reached in about 10 to 15 minutes
Horizontal trough mixers
- consist of semi-cylindrical horizontal vessels in which one or more rotating devices (such as screw conveyors or a ribbon mixer) are located
- in a typical ribbon mixer one ribbon moves the material slowly in one direction, while the other moves it quickly in the opposite direction, so there is a net movement of material, and the system can be used as a continuous mixer
- particle damage can occur due to the small clearance between the ribbon and the vessel wall, and the mixer has a high power requirement.
- Segregation is less of a problem
The Hayes & Stolz Industrial Mfg. Co., Inc. uses a ribbon blenders in its
Continuous Blender (see
http://www.hayes-stolz.com/continumxr.htm) and in its
Counterpoise Mixer (see
http://www.hayes-stolz.com/cntrpoimxr.htm).
Vertical screw mixers See Brennan Fig 5.14 for diagrams
- a rotating screw located in a cylindrical or cone shaped vessel
- the screw may be fixed centrally or may rotate around the vessel near the wall
- quick and efficient, and good for mixing a small quantity into a larger one.
- good for materials prone to segregation
Fluidised bed mixers
- effective for materials that will fluidise and with similar settling characteristics
- added jets of air that produce "spouting" are said to be effective in decreasing the time required to achieve good mixing
- very rapid -- 1 to 2 minutes compared with around 15 minutes for a tumbler
Other types
- examples include heavy paddle mixers, pan mixers, and Z-arm blenders
- injecting additives into pneumatic lines carrying solid particles can be effective
- continuous mixers -- for example Littleford Day Inc., see
http://www.littleford.com/km.html
Other mixing tasks
Mixers are also available for specialized tasks such as
Some web sites of interest:
Littleford Day
-- http://www.littleford.com
Eastern-Cleveland Mixers
-- http://www.emimixers.com
The Hayes & Stolz Industrial Mfg. Co. --
http://www.hayes-stolz.com/
Food Processing Equipment Company
-- http://www.fpec.com