Screening is a basic operation in process engineering.
Almost all bulk goods are screened during their production, i.e. sorted by particle size.
Often, the process of screening is a crucial procedural step for the quality of the final product.
Especially for fine and medium sized particles tumbling screening machines have become the preferred type of screening machine.
Due to their manifold possibilities of adjustment, lots of different bulk goods can be screened at the same time efficiently and to a high grade.
Important characteristic numbers of screening processes are the maximum possible input mass flow and the grade of separation.
The grade of separation is evaluated in terms of the percentage of miss-sorted gangue particles.
Usually the percentage of gangue particles is evaluated separately for all decks of the machine and divided into undersized and oversized gangue particles.
The choice of an appropriate screening machine and optimal process and adjustment parameters is a task of multi criteria optimization, in which the mentioned characteristic numbers behave contrariwise.
Usually the user defines limit values for the characteristic numbers.
In the range of these limits the screening process has to be optimized.
For industrial application this optimization is performed experimentally in pilot plant scale.
The simulation of the screening process is based on the Discrete Element Method.
The equations of motion are integrated numerically for every particle inside of the machine.
Interactions between particles are described by suitable force laws in terms of a penalty approach.
Both the tumbling screening machine and the screening mesh are described using surface elements.
A main part of the simulation approach is the realistic modelling of the screening mesh with sinusoidal woven wires.
In order to speed up simulations while keeping this sophisticated model of the wire mesh a hierarchical approach is introduced.
The mesh is reproduced of single nodes of wires using replicated geometry.
Speeding up the simulation is realized as the geometrical data of the moving triangles describing the wires is only transformed if a particle interacts with a cell containing one node of wires.
Thus, expensive computations to determine the positions of every triangle can frequently be avoided by this hierarchical description yielding considerably reduced computation time.
The simulation results show a good qualitative and quantitative agreement with experimental results.
In order to reduce experimental effort or to improve the design of screening machines reference values for the behaviour of bulk goods during screening and for suitable machine adjustments can be obtained from simulations.