Paddy pre-cleaning and de-stoning: the stage that protects everything downstream

Pre-cleaning sits at the front of a rice milling line, before husking, and it is the stage buyers cut when a budget gets tight.

That decision costs more than it saves. A stone that reaches the whitener can gouge an emery roll and take the line down for a day. Straw wrapped around a husker roll reduces husking efficiency until someone stops the machine and clears it. Soil and dust carried through to the bran stage contaminate a by-product that would otherwise sell.

None of these show up as a pre-cleaning problem. They show up as a husker problem, a whitener problem, a bran quality problem. The cause sits at the beginning of the line.

What arrives with the paddy

Paddy delivered from the field is not clean, and how dirty it is depends on how it was harvested and handled.

Mechanically harvested and threshed paddy from an irrigated scheme arrives relatively clean. Hand-harvested paddy that has been threshed on a road surface or dried on bare ground arrives with a great deal more in it.

The usual contents:

Stones and grit. The most damaging. Field stones, road gravel from threshing surfaces, and grit picked up during sun drying on bare ground. Stones are similar in size to paddy grains, which is exactly why they travel with the grain and why removing them takes a dedicated machine rather than a screen.

Straw, chaff and leaf material. Light, bulky, and prone to wrapping around rotating parts. Reduces flow consistency through the line.

Soil and dust. Contaminates bran, adds ash content to husk sold as fuel, and creates the dust load that an aspiration system has to handle.

Weed seeds and other grain. Passes through into the finished product if not removed, and shows up against the foreign matter limit on an export specification.

Metal fragments. Nails, wire, bolts, fragments from threshing equipment. Less common but capable of serious damage.

Mud balls. Similar in size and density to paddy, which makes them one of the harder contaminants to separate.

For markets where paddy is largely rain-fed and hand-handled, which covers most of West Africa, East Africa and large parts of South Asia, the impurity load is at the high end. Specification needs to reflect that.

Pre-cleaner and destoner are two different machines

These are spoken of interchangeably, and they do different jobs.

A pre-cleaner separates by size and by weight using vibrating or oscillating screens combined with an air aspiration system. Large material is caught on an upper screen. Fine material falls through a lower screen. Light material is lifted away by airflow. Paddy, being the target size and weight, continues through.

A destoner separates by specific gravity, not by size. Stones and paddy grains overlap in size, so a screen cannot tell them apart. A destoner uses an inclined vibrating deck with air blown up through it. The airflow fluidises the bed of grain so lighter paddy floats on top and heavier stones sink to the deck surface. Deck vibration then carries stones up the incline in one direction while paddy flows down in the other.

They complement each other. A pre-cleaner cannot remove stones. A destoner is not designed to handle bulk straw, and feeding it dirty paddy reduces its separation accuracy.

A properly configured line runs pre-cleaner first, destoner second.

How a vibrating pre-cleaner works

Paddy is fed onto a screen deck that oscillates or vibrates on an incline.

The top screen has perforations larger than a paddy grain. Straw, string, large stones and clods stay on top and discharge to a reject outlet. Paddy passes through.

The bottom screen has perforations smaller than a paddy grain. Sand, dust, small weed seeds and broken material fall through and discharge separately. Paddy travels across and continues to the outlet.

An aspiration channel at the discharge draws air across the falling grain stream. Light chaff, empty husks and dust are carried into a cyclone or dust collection system. This is where most of the dust load from pre-cleaning is captured, and it connects directly to the mill's overall aspiration design.

Screen sizes are matched to the grain. A screen set for long grain Indica will not clean short grain Japonica correctly, which matters for mills processing more than one variety.

How a destoner works

The separation is based on density difference. Paddy has a bulk density around 550 to 600 kg per cubic metre. Stone is roughly two and a half times denser.

Grain is fed onto an inclined deck with a perforated or mesh surface. A fan blows air upward through the deck. At the correct airflow, the grain bed fluidises, behaving somewhat like a liquid, and stratifies. Lighter paddy rises to the top of the bed. Denser stones sink and make contact with the deck surface.

The deck vibrates in a direction that drives material in contact with it upslope. Stones, sitting on the deck, are carried up and out at the high end. Paddy, floating above the deck surface, flows down and out at the low end.

Two adjustments control it: airflow and deck angle. Too little air and the bed does not fluidise, so stones never sink and separation fails. Too much air and paddy is lifted along with the stones, sending good grain to the stone outlet. Deck angle sets how aggressively material moves and how long it stays on the deck.

Both need setting for the paddy actually being processed, and both drift as conditions change. Paddy at a different moisture content behaves differently on the same settings.

What happens without it

Whitener damage. Emery rolls are abrasive but brittle. A stone entering a whitening chamber can chip or crack the roll surface. Replacement means downtime plus the part cost, and a damaged roll produces uneven whitening until it is changed.

Husker roll damage. Rubber rolls are consumables with a predictable service life. Stones and grit shorten that life by cutting and gouging the rubber surface, which also makes pressure inconsistent across the roll face and raises breakage. See the rubber roll husker guide.

Contaminated bran. Bran mixed with soil and dust loses value in both feed and oil extraction markets. Given that bran is often the highest-value by-product per kilogram, this is a direct revenue loss. See the rice milling by-products guide.

Foreign matter in finished rice. Export grade specifications set tight limits on foreign matter, and a stone found in a retail bag damages a brand in a way that is hard to recover from. See rice grades explained.

Higher dust load. Every gram of soil entering the mill has to be dealt with somewhere. Most of it ends up airborne, which loads the aspiration system and worsens the working environment.

What the damage actually costs

The case for a destoner is usually made in the abstract. It is more persuasive with numbers.

Take a 20 TPD mill running 280 days a year. A single stone entering the whitening chamber can chip an emery roll. Say that happens three times in a year, which is conservative for uncleaned rain-fed paddy.

The lost margin alone runs to roughly $1,200 a year before counting the rolls themselves. Add the rubber husker rolls wearing faster on gritty paddy, and the bran that fetches a lower price because it carries soil, and the annual cost of not de-stoning comfortably exceeds the cost of the machine within the first year or two.

That calculation is why mills processing rain-fed paddy treat the destoner as core equipment rather than an optional extra. Figures are illustrative — run them with your own roll costs and local margin.

Two Starlight case studies document this stage in practice: a 56-drum destoner at a Nigerian rice cooperative, and a Nigerian impurities and pre-cleaning case covering what arrives with locally sourced paddy.

Specifying for your paddy

Match capacity to the line with margin. Pre-cleaning capacity should exceed the husker's throughput. A pre-cleaner running at its limit passes material it should have rejected, because screens need room for the grain bed to stratify.

Specify for your dirtiest paddy, not your average. Impurity load varies through the season. Paddy arriving during a wet harvest carries more mud than paddy from a dry one. Sizing for average conditions means the machine underperforms exactly when it is needed most.

Confirm screen sizes for your variety. State the grain type when ordering. Mills processing both long grain and short grain need either interchangeable screens or an accepted compromise.

Check the aspiration connection. The pre-cleaner's air system needs somewhere to send the dust it collects. Whether that is an integrated cyclone or a connection to a central system, it should be settled at specification rather than improvised on site.

Do not skip the destoner in rain-fed markets. For paddy dried on bare ground or threshed on roads, a destoner is not optional equipment. It is the machine that prevents the most expensive failures in the line.

What you need, by paddy source

The middle two rows describe most paddy across West Africa, East Africa and large parts of South Asia.

For full line configuration: what machines are needed in a rice mill plant and the rice milling process guide.

Common problems

Stones still reaching the husker. Check destoner airflow first. Insufficient air means no fluidisation and no separation. Then check deck angle and feed rate.

Good paddy in the stone reject. Airflow too high, or deck angle too steep. The machine is being over-aggressive and losing product.

Screens blinding. Grain or debris lodged in the perforations reduces open area and cuts capacity. Most pre-cleaners include ball trays or brushes to keep screens clear, and these need checking.

Separation degrades through the season. Usually paddy moisture change rather than a machine fault. Wetter paddy has different flow characteristics and needs the settings revisited.

Excessive dust in the mill building. Aspiration is undersized, ducting is leaking, or the cyclone is not functioning. Worth addressing as a safety matter, not only a cleanliness one.

For broader diagnosis: common rice milling problems.

Contact Starlight for pre-cleaning and de-stoning equipment

Starlight supplies pre-cleaners and destoners sized for cooperative through commercial scale, with screen and airflow specification matched to local paddy conditions.

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• Pre-Cleaning & De-Stoning
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Frequently asked questions

What is the difference between a pre-cleaner and a destoner? A pre-cleaner separates material by size and weight using vibrating screens and air aspiration, removing straw, dust, sand and oversized material. A destoner separates by specific gravity using a fluidised vibrating deck, removing stones that are similar in size to paddy grains and therefore cannot be screened out. Most commercial lines need both.

Why can a screen not remove stones from paddy? Because many stones are close to the same size as a paddy grain. A screen separates by size, so a stone that fits through the same perforation as paddy will travel with the paddy. Separating them requires exploiting the density difference instead, which is what a destoner does.

How does a destoner actually separate stones? Air is blown upward through an inclined vibrating deck, fluidising the grain bed. Lighter paddy floats to the top while denser stones sink and contact the deck. The deck's vibration carries material touching it upslope, so stones discharge at the high end while paddy flows down to the low end.

What happens if paddy is not properly cleaned before milling? Stones damage husker rolls and whitener rolls, causing downtime and replacement cost. Straw wraps around rotating parts and reduces efficiency. Soil and dust contaminate bran and reduce its sale value. Foreign matter appears in the finished rice and can fail export grade specifications.

Do small rice mills need a destoner? If the paddy is rain-fed, hand-harvested, or dried on bare ground, yes. Those handling methods introduce stones reliably. A destoner costs considerably less than repeated whitener roll damage. Mills processing clean, mechanically harvested paddy from irrigated schemes have more flexibility.

How is a pre-cleaner sized for a rice mill? Capacity should exceed the throughput of the husking stage so the machine is never running at its ceiling. Screens must be matched to grain dimensions. Specification should be based on the dirtiest paddy expected during the season rather than the annual average.

Starlight Machinery is a rice processing machinery manufacturer based in Danyang, Jiangsu, China, supplying combined rice mills, production lines and individual processing machines to commercial operators, cooperatives and distributors worldwide.