How Pulses Are Cleaned for Export

A pulse cleaning line is a sequence of separations, and each stage exploits a different physical difference between the product and what has to come out of it. Size is handled by screens. Weight is handled by air. Density is handled by destoners and gravity tables. Colour is handled by cameras. No single machine produces an export specification — the specification is the cumulative result of the sequence, run at settings chosen against the contract.

The order matters as much as the equipment. Coarse contamination is removed first because it protects and unloads everything downstream: a gravity table fed with straw and mud performs badly, and an optical sorter fed with dust wastes camera time on material a $500 screen could have removed.

Stage 1 — Intake and sampling

Before anything is cleaned, the incoming crop is sampled and assessed: moisture, visible foreign matter, damaged and immature grain, admixture, and live insect activity. The purpose is to decide what this delivery can be used for. Material outside the tolerance for the intended contract is either diverted to a lower specification or refused. Each accepted delivery is given a lot identity that follows it through the line.

Stage 2 — Conditioning, where required

Crop bought early in the harvest, or after unseasonal rain, may arrive above the moisture target. It is dried before it enters the line — on tarpaulins or drying floors, or mechanically where equipment is available. This step is applied only where it is needed, but where it is needed it cannot be skipped, because moisture is the one parameter that cannot be corrected after loading.

Stage 3 — Pre-cleaning

The first mechanical pass removes bulk field contamination: pod fragments, stalk, leaf, string, dust, mud balls and large stones. It is unglamorous and it does more for the final result than any single downstream machine, because every subsequent separation works better on a stream that is already free of coarse trash.

Stage 4 — Aspiration

A controlled air stream lifts everything lighter than sound grain out of the flow: dust, husk, chaff, shrivelled and immature grain, and insect-hollowed seed that has lost enough weight to be carried. Aspiration is usually integrated into the pre-cleaner and repeated after screening. Setting the air is a balance — too little leaves light defects in, too much carries sound grain out with them.

Stage 5 — Screening and sizing

Stacked vibrating decks split the stream by size. The top deck scalps anything larger than the product; the lower decks drop out undersize material, broken pieces, sand and small weed seed. For commodities sold on size — mung beans, Kabuli chickpeas, all beans — this is also where the crop is graded into the band being contracted.

Stage 6 — Destoning

A destoner separates material of the same size as the grain but higher density, using a fluidised inclined deck. This is what removes small stones, soil pellets and dense fragments that pass straight through screening because they are dimensionally identical to the pulse. Stone content is one of the first things a mill or a canner checks, so this is a routine stage rather than an optional one.

Stage 7 — Gravity separation

A gravity separator refines the density split, dividing sound, well-filled grain from material of similar size but lower specific weight: insect-hollowed seed, partly filled grain, weathered and heat-damaged kernels. On pigeon peas and cowpeas this stage does most of the practical work of reducing insect-damage percentages, because bruchid-attacked seed is dimensionally normal and only density reveals it.

Stage 8 — Magnetic separation and metal detection

Magnets positioned before the bagging point capture ferrous fragments picked up in threshing, transport and handling. In-line metal detection at the packing head is available at some facilities and not others; where a buyer requires it as a contract condition it should be confirmed for the nominated processing point rather than assumed.

Stage 9 — Optical colour sorting

Where the specification calls for it, product passes camera arrays in a thin stream and grains outside the accepted colour range are ejected by compressed air. This removes discoloured, stained, mould-marked, heat-damaged and off-variety seed, along with remaining foreign seed distinguishable by colour. It is the only stage that addresses colour, and it is specification dependent rather than universal.

Stage 10 — Hand picking, where specified

For the highest visual grades, product is run over picking belts and residual defects are removed manually. It is slow, it adds cost, and it is quoted specifically. It is used where the visual standard exceeds what machine sorting alone achieves — some Kabuli chickpea grades and certain white and speckled bean contracts.

Stage 11 — Final check, weighing and bagging

A composite sample from the finished lot is checked against the contract parameters, and retained samples are kept. Product is then weighed to the contracted net weight, bagged, stitched and marked with the agreed description, weight, crop year, origin and lot reference.

The difference between mechanical cleaning and optical sorting, in commercial terms. Machine cleaned vs Sortex →