## 2 Grinding and Classification Circuit
### Classification and Recycles  
Quality of grinding depends mostly on the classification. Too large grains need to be recycled and grinded again. This task is done in the electromagnetic mill circuit twice: in preliminary and main classifier. Ratio of the air stream that passes the working chamber to the feed inflow should be properly adjusted to let only larger grains fall down into the working chamber while smaller (and lighter) grains are swept upwards to the main classifier (there is no need to grind them). The main air stream keeps the load of the working chamber nearly constant and balances inflow of the larger grains with the outflow of grinded grains that are swept upwards in a similar way as lighter grains of the feed. This time, however, lighter (grinded) grains are not transported through the central pipe of the preliminary classifier, which is partially blocked by inflow of the fresh material, but rather through the outside as it is shown in Figure 3 by red dash-dotted line. Special construction of the preliminary classifier [19] allows for yet another classification: the path of grinded grains slows down their speed, which again allows for the larger grains fall down together with inflow of the fresh material. In this way, the internal recycle of the larger grains is formed. Only grains of smaller size are swept upwards to the main classifier. However, the first and preliminary classification and inner recycle provides only a partial barrier for the grains of the larger size. Thus, precise classification is applied.  
The main classifier performs precise classification. The classifier is of the inertial type, which means that the cut-off grain size depends on the input speed of the grains (in the air stream). In order to adjust cut-off grain size, additional air stream into the main classifier is applied. A quality of classification using the inertial classifier follows from the shape of cumulated mass of input. Figure 5 presents an example of the cumulated mass curve of copper ore grinded by electromagnetic mill with working chamber of 200 mm diameter [20].  
It is clear from Figure 5 that the longer the grinding time, the curve is more flat, which means that there is greater gap between lower and larger grain sizes. For example, the case of \(F\!f=0.077\) Mg/h (orange line) shows that about 50% of grains are less than 0.1 mm of average size, while 40% of grains are greater than 1 mm. Only 10% of grains are between 0.1 and 1 mm. In the case of \(F\!f=0.154\) Mg/h (grey line), 31 % of grains are less than 0.1 mm of average size and 45% of grains are greater than 1 mm. There are 24% of grains between 0.1 and 1 mm. The first case is much simpler to be tackled by the classifier. If the task is to separate grains less, then the 0.1 mm the air stream should be adjusted with a large margin (it is not relevant where the cut-off size is placed between 0.1 and 1 mm). On the other hand, the second case illustrates sensitivity of the classification process on air stream value. It should be noticed that the sensitivity depends on the cut-off size due to the cumulated mass curve is more corrugated between 0.1 and 1 mm of grain size in the second case. This property of the main classifier needs to be taken into account as the objective of the control system.  
The recycle flow created by the main classifier is fed back to the working chamber. Average time of grinding is then extended and the quality of grinding is improved. The material is not stored in the recycle path. Thus, in a steady state inflow of the fresh material to be grinded needs to be equal to the product out-flow from the main classifier. This is one of the most important objectives of the control system.