EQP-06 through EQP-07 — Final cement grinding, classification, and storage
Chamber 1 is optimized for impact grinding (cataracting mode). Large 90-60 cm steel balls and aggressive lifter bars (80-120 mm high at 200-300 mm spacing) carry the ball charge above 60% of the mill diameter. The balls then fall in a parabolic trajectory, striking the clinker nodules at the toe of the charge at 3-6 m/s. This high-energy impact fractures clinker from feed size (1-25 mm) down to 500-1,000 micrometers. The lifter bar geometry is critical: without sufficient lifter height, the balls would not achieve the cataracting trajectory and the mill would operate in less efficient cascading mode, losing the impact energy needed for clinker breakage. Ball wear rate: 150-300 g per tonne of cement.
The diaphragm separating Chamber 1 from Chamber 2 is a radially-slotted steel casting with slot widths of 6-12 mm. The slots are tapered from the Chamber 1 side (wider) to the Chamber 2 side (narrower) to prevent material blinding. The diaphragm retains the larger 90-60 cm grinding balls in Chamber 1 while allowing ground material (typically 500-1,000 micrometers) to pass into Chamber 2. It prevents over-grinding by limiting residence time in Chamber 1.
Chamber 2 is lined with classifying liners whose lifter height decreases from the diaphragm to the discharge end. This gradient directs smaller balls (40-15 cm) toward the discharge for fine grinding while retaining larger balls near the diaphragm. The balls roll and tumble down the surface of the charge (cascading mode), producing fine grinding through ball-to-ball attrition and interparticle compression. Material enters at 500-1,000 micrometers and exits at 15-45 micrometers (Blaine fineness of 3,000-5,000 cm²/g). Ball wear rate: 50-100 g per tonne of cement.
Gypsum (3-5% by weight, sourced from Al-Juhfa Quarry) is added at the mill inlet simultaneously with the clinker feed. Gypsum controls the cement's setting time by reacting with C₃A (tricalcium aluminate) in the presence of water to form ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O), which forms a protective barrier around the C₃A grains and delays the hydration reaction. Without gypsum, the cement would set within minutes of mixing with water (flash set). The mill exit temperature is maintained below 110°C to prevent gypsum dehydration to hemihydrate, which would cause false set.
The air separator classifies mill discharge by balancing centrifugal force (Fc = m × ω² × r) from the rotating rotor cage against drag force (Fd = ½ × Cd × ρ × A × v²) from the inward air flow. Particles smaller than the cut point (~10 µm) experience Fd > Fc and pass through the rotor blades as finished product. Particles larger than d50 experience Fc > Fd and are thrown outward to the rejects cone for return to the mill (circulating load 200-400%). Rotor speed is the primary fineness control: doubling rotor speed increases centrifugal force by a factor of four, shifting the d50 to approximately half its previous value. The Tromp curve quantifies separation performance, with ideal sharpness index (d25/d75) above 0.8 and bypass fraction below 15%.
Finished cement is conveyed to four storage silos with a combined capacity of 90,000 MT (15-20 days of production). Each silo is equipped with aerated discharge cones, fluidizing pads, and level monitoring. Dispatch occurs through bulk loading (pneumatic spouts at 100-200 TPH per station with dust collection, serving both truck and rail) and bagged cement (6-spout rotary packers filling 50 kg bags at 2,000-3,000 bags per hour, automatically palletized and shrink-wrapped). Supporting utility systems include compressed air (screw compressors), water supply (cooling tower), steam boiler, and 132 kV substation with backup diesel generators.