EQP-05 — Vertical Roller Mill for raw meal grinding
Five parallel feed bins, each with a weight-controlled belt feeder and an individual wet feeder for moisture adjustment, supply material to the raw mill. The metal detector at the mill inlet removes any tramp metal before it can damage the grinding elements.
The grinding table, driven by a bevel-planetary gear reducer, rotates the material bed under the four stationary rollers. Each roller is pressed against the table by dual hydraulic cylinders (eight total) connected to nitrogen-charged accumulators pressurized to 120-160 bar. The accumulators serve dual functions: they provide the grinding force and act as hydraulic springs, allowing the roller to lift vertically when uncrushable material (tramp iron, oversized rock) passes under the roller, preventing mechanical damage. The rollers are not driven — they rotate by friction contact with the material bed on the rotating table. The roller tires have a cylindrical or conical profile that optimizes pressure distribution across the grinding track. Grinding bed thickness (30-80 mm) is maintained by balancing feed rate against grinding capacity, monitored via mill differential pressure and vibration sensors.
Hot gas from the kiln preheater enters the mill through the ring nozzle, an annular slot surrounding the grinding table. The gas velocity at the nozzle throat reaches 80-120 m/s, creating a high-velocity air stream that lifts ground material from the table into the classifier zone above. The gas serves three functions: (1) conveying ground material upward; (2) drying the raw material from 8-15% moisture to below 0.5% product moisture; (3) providing the classifying air flow for the dynamic separator. The gas velocity decreases as it moves upward through the mill's increasing cross-sectional area. According to Stokes' law, particles coarser than approximately 200 µm have terminal velocities exceeding the upward gas velocity in the expanded zone and fall back to the grinding table for re-grinding (internal circulating load of 200-400%).
Particles reaching the classifier enter the rotating cage (the dynamic classifier rotor). The cage speed (40-120 RPM) creates a centrifugal force field that rejects particles above the cut size (d50 at approximately 350 µm) back to the grinding table. Acceptable fines pass through the cage, exit the mill with the gas stream at 80-100°C, and are collected in a baghouse or cyclone. The classifier speed is the primary control for product fineness: increasing speed rejects more material (finer product), while decreasing speed allows coarser product (higher throughput). The recirculation loop returns coarse rejects from the classifier to the grinding table for further size reduction, achieving the internal circulating load of 200-400%.