|The mixing of solids in powder or granular form is a strictly physical unit operation, which has the aim of homogenising the composition of a mixture consisting of two or more solids.
Mixing is brought about by three different mechanisms due to:
- Diffusive phenomena;
- Convective phenomena;
- Cutting forces.
The equipment used for this unitary operation, indicated by the term mixers, can be classified into moving casing mixers and fixed casing mixers.
A moving casing mixer, or rotating drum, consists of a casing of varying shape (cylindrical, conical, double cone, etc.), sometimes fitted with internal baffles, which, by rotating around an axis, bring about the mixing mainly by means of diffusive phenomena.
Mixing by diffusion is ideal for particulate solid mixtures in which the constituents have the same physical characteristics (especially particle size and density).
For materials, which tend to show high degree of cohesion or form agglomerates, as well as materials subjected to segregation, the equipment adopted must be capable of high cutting and convection actions, like a fixed casing mixer.
In a fixed casing mixer, mixing is brought about by using internal moving tools.
The mixing quality is mainly affected by:
- the average speed of the mass,
- the characteristic dimension of mixing chamber,
- the length and shape of tools,
- density, dynamic viscosity, granulometry and adhesiveness of the ingredients.
Mixing of solids generally never gives a perfectly uniform distribution of the components; the aim is to create a global uniformity of the mixture (random mixing).
A rotary blade mixer consists of a fixed cylinder inside which a rotating horizontal shaft is present, which is fitted with ploughshare or arc-shaped blades; the rotation of the blades allows the material to be lifted from the drum walls and moved vigorously. Often, the mixer is provided with devices capable of breaking the lumps present.
A ribbon mixer consists of a horizontal trough, which houses a ribbon agitator, but usually, two concentric ribbon agitators contra pitches.
Mixing takes place mainly by convection, based on a gentle radial movement; axial movement is limited and accomplished by some shear.
A double-shaft mixer consists of a mixing chamber inside which are placed two shafts rotating in opposite directions at different speeds, on which alternating blades are fitted, so as to create a fluid bed of material above the area of intersection of the two shafts, where the mixing occurs mainly due to convection and diffusion. In this case, the material is treated gently, is conferred with limited amount of energy and subjected to limited abrasion.
Depending on the loading and unloading conditions, the mixers are classified as continuous and discontinuous (or batch). In the batch mixer the particles move randomly throughout (or almost throughout) the mixing chamber, and tend to form a homogenous mixture as time goes by; on the other hand, in the case of a continuous mixer the particles are subjected to random motion in a radial direction and axial motion that is limited to the maximum. In batch mixers the mixing time is determined by the rotation time of the tools, while in continuous mixers it varies according to the resistance to flow that the product encounters while passing through the mixer; so theoretically, the stay time is the same for all the particles.
The choice of a continuous or batch mixing system is determined by a large number of factors,
1) The type of system in which the mixer is to be inserted (continuous or batch);
2) The properties of the components, especially the size-distribution and the shape of the particles, their specific weight, the flow characteristics, moisture content, the tendency to cohese or agglomerate, their adhesiveness to the walls, possible degradation if subjected to cutting forces, the tendency to segregation phenomena;
3) The flexibility required by the plant and evaluation of possible cross-contamination, which require cleaning with every change in the mixer operation;
4) The space available.
If the mixer is provided with a jacket for heat exchange, the drying, heating and cooling operations can be carried out simultaneously with the mixing process. A service fluid (water vapour, water or diathermic oil) is made to circulate through the jacket which, in the presence of a suitable temperature gradient between solid product and service fluid, guarantees heat exchange through the mixer walls.
The humidification of solids can be brought about in different ways, depending on the type of operation to be carried out on the solid product and the nature of liquid to be added; it can be done by:
a) Injecting liquid into the solid material inlet section of the mixer;
b) Injecting liquid in the mixing chamber;
c) Injecting liquid at the mixture outlet outlet section of the mixer.
If, however, it is necessary to add small quantities of liquid into the product to be distributed uniformly throughout the mass, the liquid may be injected together with air; this will improve nebulization of the ingredient and fluidisation of the mass.
The granulation operation, i.e. obtaining mixtures of granules having dimensions greater than the dimension of the particles of the individual components, is brought about by adding liquid in the mixing chamber, from such a distance with respect to the solid material inlet section, where the mixing process is already active.
Special care must be taken while selecting the material used for constructing the mixers; the operating conditions, especially for continuous fixed casing mixers operating at very high speeds, can result in excessive wear of the moving mechanical devices.