Mechanical process engineering, a sub-discipline of process engineering, deals with the conversion processes of substances based on mechanical action. Mixing is one of the four main process groups:
- Comminution or atomisation of liquids → the shift of the particle size range towards smaller sizes
- Agglomerating → shifting the particle size range towards larger sizes
- Mixing → combining at least two material systems of different composition into one material system. Here we distinguish between dispersions, suspensions and emulsions.
- Separation, sieving → the splitting of a disperse system into two disperse systems of different composition with the aid of the physical and chemical properties of the material systems
Historically, the storage, conveying and dosing of solid systems and liquid goods is also usually categorised as mechanical process engineering, even if this does not generally involve any changes to the system.
All these process steps can be very energy-intensive, although normally no phase boundaries are overcome. For example, crushing processes are said to account for four per cent of global electricity demand (of which ‘only’ one per cent is used for cement production).
During mixing, energy is introduced into the mix via the ‘mixing tool’. This tool is located inside the mixing pan - in process engineering, a machine whose mixing components are predominantly present as a solid phase. Stirrers, on the other hand, refer to machines whose main mixing components are mainly present as a liquid system.
But regardless of whether stirring or mixing - the primary objective of both processes is to achieve an even distribution of the components to be mixed. This can be quantified via the so-called mixing quality.
In a mixing process, the ingredients - at least two separate mixing components - are therefore repositioned by relative movements in such a way that a new arrangement pattern / scheme is created.
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Examples of arrangement schemes → Mixed states
A: Complete separation of the individual components
B: Ideal mixture (cannot be achieved in reality)
C-F: Real mixing states
F: Homogeneous mixing (target)
In dynamic mixers (rotating tools), this application of force through relative movements is generated with the aid of mixing tools inside the mixing pan. This involves circulating, shearing, pushing and throwing the mix in a radial and axial direction.
Distributive mixing - the uniform distribution of all particles in the moulding compound - requires lower throwing, centrifugal and shearing forces than dispersive mixing - the breaking up of the components to be mixed.
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Distributive mixing |
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Dispersive Mixing |
Other examples of dispersive mixing depending on the consistencies are suspension (main phase liquid/additional phase solid), emulsification (liquid/liquid) and gassing (liquid/gaseous).
The centrifugal, throwing and shear forces acting on the mix require a correspondingly high energy input to achieve the desired end product, depending on the mix components and the consistency of the mix. The mixing tool is therefore a decisive interface that combines high mix quality and the shortest possible mixing time. Focussing on this, the specific energy requirement essentially depends on the consistency of the mix (dry, crumbly, pasty/plastic, liquid), the speed at which the mixing blades move in the mix and the design and mode of operation of the mixing tool.
Examples: A dry mix (e.g. cement, dry mortar) or a slip / slurry (e.g. porcelain) therefore require a lower specific energy input than, for example, a plastic mass (e.g. clay). On the other hand, the energy requirement of a mixing tool that works downwards and compacts is higher than that of a mixing tool that works upwards and loosens.
The need to optimise the energy requirement in the mix results in specific requirements for the design of a mixing tool. For example, a mixing blade must be arranged at an angle to the vertical on the tool holder in order to impart an upward or downward movement impulse to the mix. In addition to the speed, the mixing blade cross-sections, the number of mixing blades and the swept area of the mixing blade diameter also have an effect on the mixing result.
The mixing process can take place in a continuous1) or discontinuous2) mixer . A distinction is also made between active and passive or static and dynamic mixers. Mixers whose mixing tools are mounted on shafts can also be categorised according to the number of shafts (single-shaft mixer, multi-shaft mixer).
In active mixers, the energy required for the relative displacement of particles of the starting materials is not obtained from the starting materials themselves. Examples are ultrasonic waves, vibrations caused by rising bubbles and pulsating inflow. In passive mixers, the required energy is extracted from the incoming raw materials.
KRAUSKOPF relies on dynamic, vertically mounted compulsory mixers with a rotating pan to achieve the best possible mixing quality. Other criteria for good mixing systems include a high throughput rate and repeat accuracy, no screen line displacement and long and short maintenance intervals. The design of a mixer is defined by the geometry of the mixing pan (e.g. drum, cylinder, cone, cube, tetrahedron, etc.) and the capacity (test scale e.g. 2 litres, pilot plant scale e.g. 20 litres, production scale e.g. 1,000-3,000 litres).
1) In continuous operation, the filling level of the mix is approximately constant. The components to be mixed are fed in continuously and the mixture is discharged continuously. One example is mixing in a conveyor pipe.
2) In discontinuous operation, the cycles are repeated: filling of the components to be mixed; mixing; discharging of the mixture. An example is a batch vessel.
Related topics:
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| Development project MIM | Mixing tool system TuneMixx | Industries |
