## 4 Results and Discussion  
### Experimental Research of the Pneumatic Classification Process for Granular Materials  
The experimental stand was based on a pneumatic classification device for granular materials with the changed shape of the operating volume. The proposed geometry allowed increasing the separation quality and reducing hydraulic resistance for the consequent increase in the specific volume rate and efficiency of the pneumatic classification process. Experimental research of granular materials was carried out in the laboratory model of the pneumatic classifier. The related experimental stand and its technological scheme are presented in Figure 3.  
The initial mixture of a bulk material entered the loading tank of the apparatus. After that, the mixture moved into the middle part of the pneumatic classifier (PC). Air flow entered the lower part of the apparatus. The flow rate was regulated by a calibrated collector with the regulating valve (RV). A fine fraction was removed by the air flow to a cyclone (C). As a result, fine particles were deposited in the tank T2, and the air flow was removed from the apparatus. Coarse particles fell down into the lower part of the apparatus and they were removed into the tank T1 through the switchgear. A suction air flow was created using the gas blower (GB). The separation occurred directly in the apparatus body with the two following operating areas:  
Figure 2: Dependence between the fine fraction concentration and vertical coordinate in time. 1. Lower part--separation area with rotating and mixing material. In this part, fine and coarse fractions were removed from a rotating layer. 2. Upper part--area in which the small fraction was dispersed and removed from the apparatus. The fine fraction removed from the apparatus passed through the cyclone and deposited into the tank. The coarse one fell down and moved into another tank. It should be noted that, in previous work [24], the factors influencing the hydrodynamics of the device were substantiated. In particular, the shape of the body is rhombic with a diffuser angle of about 8-9\({}^{\circ}\), and a diffuser angle of 10-11\({}^{\circ}\). Moreover, since continuous product loading has a negative impact on the separation quality, periodic loading should be used. As a result, using periodic loading allows ensuring the appearance of rolling swirls in the separation section since these swirls prevent removal of the gas-dispersed mixture from a suspended layer. The research methodology of studying the efficiency of the pneumatic classification process was as follows:  
1. As a model material, a binary mixture of quartz sand was selected. The river sand was dried in a muffle furnace. 2. The use of a set of sieves (0.63, 0.4, 0.32, 0.20, and 0.16 mm) allowed obtaining the different charges of sand. 3. After the charge was weighed by an electronic scale \"Moment-6000\" with an accuracy of 0.1 g, the material was loaded into the hopper for the consequent raw material feeding. 4. The gas blower was switched on, and the air flow was controlled by the clamp pressurizing the air flow. The pressure in the apparatus body was measured by the \"U-shaped\" liquid pressure gauges. Air velocity was measured using the Pitot-Prandtl tubes inserted at different cross-sections of the  
Figure 3: Experimental stand (**a**) and its technological scheme (**b**) for studying the process of pneumatic classification: 1—air flow; 2—outlet mixture; 3—coarse fraction; 4—fine fraction; 5—air flow with a fine fraction; C—cyclone; PC—pneumatic classifier; GB—gas blower for creation of a suction flow; T1—tank for collecting a coarse fraction; T2—tank for collecting a fine fraction; RV—regulating valve. body. The tubes were connected to the micromanometer with a sloping tube \"MMN-240(5)-0.1\", which measured the pressure difference at the cross-sections of the apparatus body. The pressure values were recalculated into the velocity. The material charges were periodically loaded into the device.