The process uses a series of sequential, dedicated sorting machines staged right after raw pulse receiving, before tempering and dehulling. Each unit targets a specific type of contaminant by size, density, weight, magnetism, color, and aerodynamic difference. The full workflow eliminates light organic trash, heavy mineral stones, metal fragments, weed seeds, moldy defective grains and dust.

Step 1: Primary Vibrating Grading Sieve + Air Aspiration (Size & Light Impurity Removal)

This is the first barrier for bulk raw pulses.

1. Multi-layer vibrating screen with stacked mesh decks
   • Top coarse mesh: Remove oversized contaminants – crop stalks, broken pods, clods of soil, large sticks, big lumps of mud.
   • Middle mesh: Retain whole intact pulses as main process feed.
   • Bottom fine mesh: Screen out undersized impurities – fine dust, broken pulse grit, tiny weed seeds, sand fines.
2. Integrated air aspirator (negative airflow hood)
   Controlled upward air current lifts lightweight low-density waste out of the pulse stream: chaff, husk dust, straw fragments, dried leaf litter, lightweight weed seed husks. Light waste is captured by a baghouse dust collector.

Outcome: Separate all oversized/undersized trash and light fibrous impurities; only uniform whole pulses move forward.

Step 2: Gravity Destoner (Stone & Heavy Mineral Separation – Critical for Milling Protection)

Targets high-density heavy contaminants: gravel, pebbles, rock fragments, glass shards, dense soil clumps.

Working principle

• A perforated fluidized bed deck vibrates horizontally, while controlled upward airflow fluidizes the pulse layer.
• Pulses have lower specific gravity (~1.2–1.3) and float upward on the air cushion, moving forward to the discharge outlet.
• Stones/minerals have much higher density (>2.6) and sink to the deck surface. Vibration pushes heavy stones sideways to a separate waste chute for continuous removal.
• Recirculation loop: Small pulse kernels mixed with stones are sifted and returned to the main feed to avoid yield loss.

Key purpose: 98–99% of damaging stones are removed here to prevent abrasion, pin mill rotor damage, and spark explosion risks.

Step 3: Magnetic Separation (Ferrous Metal Removal)

Installed at transfer chutes after the destoner, two common configurations:

1. Overband cross-belt magnet: Suspended above the conveyor belt; pulls iron nails, wire fragments, harvesting machinery metal shavings, bolts, metal scrap out of the pulse flow. Collected metal automatically drops into a waste bin.
2. Pipeline magnetic trap (magnetic grate): Rod magnets installed inside vertical pneumatic conveying pipes to catch fine metal particles missed by the overband magnet.

Metal contaminants are a major hazard: high-speed impact in pin mills creates sparks that can trigger organic dust explosions and permanently crack ceramic pin coatings.

Step 4: Optical Color Sorter (Defective Seeds, Foreign Grain & Discolored Impurities)

Targets biological and visual impurities that density/size sorting cannot separate:

• Mold-stained, rotten, insect-damaged, discolored pulses
• Foreign grain cross-contamination (wheat, corn, barley mixed with peas/faba beans)
• Dark seed fragments, black foreign weed seeds

Working mechanism

• High-speed CCD cameras scan every single seed as it falls in a thin monolayer stream.
• AI software compares each kernel’s color, brightness, surface texture against a pre-set standard template.
• Compressed air micro-jets instantly eject defective/foreign seeds into a waste hopper when mismatches are detected.
• Two-pass sorting available: first pass removes major defects, second pass reprocesses ejected waste to recover good pulses and minimize yield loss.

This step eliminates tannin-rich moldy material that would create bitter, dark low-value protein concentrate.

Step 5: Optional Secondary Fine Aspiration & Brush Polisher (Surface Dust Residual Cleanup)

For premium food-grade protein lines:

1. Rotary brush polisher: Soft rotating brushes scrub surface dust, residual dirt and loose pod fragments off pulse seed coats.
2. Secondary air aspiration: Suction removes brushed-off fine dust before tempering.

Step 6: Post-Cleaning Buffer Bin (Homogenization)

Fully cleaned pulses are held in a buffer silo with slow agitation to homogenize particle size and moisture, eliminating batch-to-batch variation before tempering and dehulling.

Classification of All Removed Impurities & Corresponding Equipment

Why Complete Stone/Impurity Removal Matters for Pulse Dry Fractionation

1. Protect high-value core equipment: Stones and metal scratch dehull rollers, wear ceramic pin mill pins, bend dynamic classifier wheels, and cause costly unplanned downtime.
2. Eliminate explosion ignition sources: Hard contaminants striking rotating metal parts generate sparks in combustible organic dust atmospheres.
3. Boost final protein purity: Fibrous chaff, weed seeds and damaged grain add non-protein fiber/carbohydrates, diluting protein concentrate purity.
4. Stabilize consistent processing: Uniform clean pulses temper evenly, dehull completely, and produce a predictable bimodal particle size after pin milling for optimal air classification separation.
5. Meet food safety regulations: Removes mycotoxin-contaminated moldy seeds, foreign matter, heavy stone abrasives, and metal foreign bodies required for human food certification.

Full Sequential Pre-Cleaning Flow

Raw pulse intake hopper → Vibrating grading sieve + primary air aspirator → Gravity destoner → Magnetic separator → Optical color sorter → Optional brush polisher + fine aspiration → Clean pulse buffer silo → Tempering → Dehulling → Pin mill → Air classification protein enrichment