AIR CLASSIFICATION OF FINE AGGREGATES
Doctoral thesis, 2014
Aggregates are defined as particles of rock which in combination are used as a civil engineering material. It is used in bounded form in asphalt and concrete. Historically natural sand has been used as fine aggregates for concrete production in Sweden. Due to environmental reasons the Swedish government is limiting the extraction of natural sand resources. Manufactured sand is therefore being investigated in Sweden as a replacement product.
Properties of the fine aggregates such as shape and particle size distribution are affecting the qualities of both the uncured and finished concrete. It is possible to produce manufactured sand with a good shape; however the crushing process results in a relatively large amount of fines (particles smaller than 63 µm). Research shows that the amount of fines in manufactured sand affects concrete properties such as, workability, water demand, drying shrinkage, compressive and flexural strength. To achieve a good concrete the manufactured sand should have somewhere between 5-10 mass percent of fines depending on rock type. It is common that manufactured sand has around 15-25 mass percent of fines after the crushing stage. In this thesis, dry air classification has been investigated to reduce the amount of fines in manufactured sand.
There exists a number of air classifiers produced for different industrial use. This work has investigated four different air classifiers using different design approaches. Two air classifiers using an internal aerodynamic cycle and two with an open aerodynamic cycle.
Both of the air classifiers with internal aerodynamic cycle investigated in the performed research are based around a centrifugal-crossflow separation zone. The classification process are for both of these air classifiers controlled by two fans, the circulating fan and the separator fan. The circulating fan creates the internal circulating air flow. The air flow created by the separator fan changes the cut size. Experiments performed on the air classifiers showed that it was possible to reduce the amount of fines from 15 mass percent to 5 mass percent.
The investigated air classifiers with an open aerodynamic cycle were a two-stage air classifier and a mobile air classifier. The first stage of the two-stage air classifier used the same design as the mobile air classifier and is designed around a gravitational-crossflow separation zone modified to allow for recirculation of particles towards the separation zone. The second stage is based around a vertical standing centrifugal-counterflow zone modified to allow for recirculation of material. Both air classifiers can produce a product with the desired amount of fines. The performed empirical measurements and CFD simulations showed that the recirculation is important for the classification process, reducing the influence of particle to particle interaction.
In conclusion, air classification has been technically proven to be able to reduce the amount of fines in the finished product. The classification result will depend on the chosen technology.