An important feature of plastics is that their properties such as formability, hardness, elasticity, and strength or temperature resistance vary when additives and fillers are admixed and can be adapted to the needs of the respective application, whereby additives are normally being applied in smaller quantities. Filling materials are primarily extenders which replace expensive raw materials with less costly ones for plastics production. However, some of these also improve various final product properties.
The most frequently used filling materials for plastics are calcium carbonate and talc powder. Calcium carbonate improves the mechanical strength properties and the weathering resistance while talc powder improves stiffness and resistance to thermal distortion.
The particle size distribution of a filling material is a decisive factor for the properties of a plastic. When manufacturing the filling materials, a precisely defined particle size distribution is achieved by grinding and subsequent separation, the so-called classification.
This classification step of the particles is a general requirement after grinding in a mill. While it is preferred to integrate the separation process into the mill itself, external air classifiers are commonly being used. Such is the case especially with ball mills.
Even prior to grinding, the use of an air classifier can be advantageous. If the feed material for the grinding process already contains a large quantity of desired final product, it makes sense to separate the fine material fraction in advance. Firstly, this material no longer burdens the mill and secondly overgrinding of such material is being avoided.
The NEUMAN & ESSER air classifier GRC (Guide Ring Classifier) is such an air classifierallowing both types of above-mentioned operations. The initial GRC layout was designed for the classification of roughly 200μm to 20μm (1μm = 0.001mm) particles.
The air classifier divides the so-called feed material into fine material on one side and coarse material on the other side. The size GRC 820 at our Test Center achieves throughputs of several tons per hour for medium fineness. For classification in the higher fineness range, throughput is reduced to several hundred kilograms per hour because the number of particles to classify increases exponentially: if 0.1 mm sized particles are crushed to 0.01 mm, the number of particles increases thousand-fold at the same time.
The feed material is fed into the GRC from the top using a vertical pipe. After falling onto a distribution plate above the classifier wheel the material is thrown to the outer diameter of this plate. There, the material drops into a ring gap between the classifier wheel on the inside and the air guidance ring on the outside.
The inflowing air now carries the particles into the classification zone. Here fine particles are allowed to pass whereas too large particles are being rejected. After passing the classifier wheel the fines leave the classifier housing directly beneath the classifier heading for the downstream filter where the finished product is being discharged from the system. The rejected fraction drops to the bottom of the housing and leaves it via a discharge airlock.
To improve the already existing high performance, we made essential improvements to the GRC air classifier. Ideally a perfect classifier allows only a very specific size of particle and below to pass. Since this is technically not feasible on a production scale the range of particle size above and below the target value defines the quality of such a classifier; the so called “cut efficiency”. The smaller the range, the better the classifier.
Our process solution experts focused on how to intensively separate fine and coarse in the feed material, dispersing as it is called by the experts and on efficiently guiding the material through the GRC classification process.
Only if the solids exist as individual particles in the classifying zone, the classifier can effectively remove the desired fraction from the feed material, thus classifying two precisely defined partial quantities.
The new GRC design features now a distribution cone in the inlet of the feed material above a significantly redesigned dispersing unit. After sliding down the cone and passing the dispersion zone, the material reaches the known ring gap. In contrast to the former design now a virtuously engineered guide ring handles the transportation into the direction of the classification zone.
Extensive and detailed testing with our new classifier revealed an unexpected magnitude of improvements not only in yield, throughput and energy consumption but also in the range of achievable fineness. The new design now allows widening the possible fineness range from 200μm to 2μm instead of 20μm.
Different sizes of the revolutionary new GRC air classifier are available for various throughputs and applications not only in plastics but also for minerals, paints, and coatings, in the chemical sector or for applications in the food industry, wherever they are needed.
