air ejection (air ejection)

Air ejection is the actuation mechanism used in nearly every modern optical sorting system, including NIR, XRF, XRT, and visible-light camera-based sorters. It consists of a row of high-speed solenoid air valves arranged along the conveyor belt edge, each connected to a compressed-air manifold and a precisely aimed nozzle that fires a short burst of air at a target fragment as that fragment crosses the line of sight at the belt discharge.

Mechanism in detail: The sensor identifies a target fragment somewhere along the scan line. The control system calculates the fragment's belt position, velocity, and predicted position at the discharge edge — typically 200-400 mm downstream of the scan line. After a precisely timed delay (5-30 milliseconds depending on belt speed), the appropriate air-valve solenoid opens for 5-20 milliseconds, releasing a compressed-air pulse at 3-7 bar pressure through a nozzle aimed at the target's predicted trajectory. The air pulse deflects the fragment sideways or downward off the natural belt-discharge parabola, dropping it into a separate collection chute rather than the main reject stream.

Engineering parameters: A typical industrial air-ejection bar has solenoid valves spaced 6-12 mm apart along the belt width — a 2-metre-wide sorter therefore has 170-330 individual valves, each capable of firing 200-400 times per second. The valves are typically rated for 100-500 million actuations before replacement. Compressed-air consumption is one of the highest operating-cost drivers in any optical sorter — a 2-metre sorter operating at 80% loading consumes 3,000-6,000 normal cubic metres per hour of compressed air at 6 bar, translating to 20-40 kW of compressor electrical load.

Trade-offs and optimisation: Smaller fragments need shorter air pulses but produce less reliable deflection because air drag dominates over momentum; larger fragments need longer pulses, which increases air consumption and limits the achievable sorting rate. Operators tune valve-open duration and nozzle pressure to the specific target fraction. Failure modes include compressed-air leaks (which degrade purity by causing valves to under-fire), nozzle blockage by sticky or dust-laden fragments, and miscalibration of the scan-to-eject timing window (which causes the sorter to miss targets entirely or eject adjacent fragments). Compressed-air system reliability — typically supplied by an oil-free screw compressor with dryer and filtration — is therefore a critical maintenance focus in any optical-sorting plant.