Grain Cleaner

Harvested grain rarely arrives at a storage bin or processing plant in a completely clean condition. Wheat, maize, rice, soybean and other grains normally contain a mixture of dust, straw, husks, broken kernels, stones, soil clods, weed seeds and sometimes metal particles.

A grain cleaning system is used to separate these unwanted materials before the grain enters storage, drying, milling, processing or feed production.

A Grain Cleaner is therefore not simply one machine. Depending on the condition of the incoming grain, a complete cleaning section may include a pre-cleaner, vibrating screen cleaner, aspiration system, destoner, magnetic separator and other equipment.

The key is to match each machine with the type of impurity it is designed to remove.

What Does a Grain Cleaner Do?

The basic purpose of grain cleaning is to separate impurities from usable grain while keeping grain loss as low as possible.

Different impurities require different separation methods.

The cleaning process is based mainly on differences in particle size, weight, density and aerodynamic behavior. Modern grain cleaners often combine several of these principles in one machine or cleaning line.

Main Grain Cleaning Equipment

Steel silo impurity removal equipment

1. Grain Pre-Cleaner

A pre-cleaner is normally installed near the raw grain receiving point.

Its job is not to achieve the final cleaning quality. Instead, it removes the larger and easier-to-separate impurities before the grain reaches the finer cleaning equipment.

Typical materials removed include:

• Straw and stalks
• Leaves
• Large pieces of husk
• Corn cobs
• Large soil clods
• Rope and foreign materials
• Oversized agricultural residues

How Does a Grain Pre-Cleaner Work?

The incoming grain is distributed over a screen or perforated cleaning surface.

As the screen moves or rotates, grain particles and impurities travel through different paths according to their size.

Large materials remain above the screen and are discharged separately, while grain and smaller particles pass through the appropriate openings.

For higher-capacity installations, a pre-cleaner may use several screening sections and an aspiration system to remove light materials at the same time.

Typical Reference Data

The actual capacity should be selected according to the incoming impurity level rather than simply matching the nominal grain flow.

2. Vibrating Grain Cleaner

The vibrating grain cleaner is one of the most widely used machines for general grain cleaning.

It is suitable for wheat, maize, rice, soybean, sorghum, barley and other grains when separation by particle size is required.

A typical machine uses two or more screen decks.

The upper screen removes oversized materials, while the lower screen separates smaller particles from the grain.

Working Principle

Raw grain enters the machine and is distributed evenly across the vibrating screen.

The screen body moves continuously, causing the grain to spread into a relatively thin layer.

During movement:

Oversized impurities → remain above the screen

Qualified grain → follows the required screen path

Small impurities → pass through the screen openings

The screen aperture is selected according to the grain being processed.

For example, a screen suitable for maize should not automatically be assumed to be suitable for wheat or rice. Screen opening, grain size and impurity conditions all affect the final result. Vibrating screen cleaners commonly use interchangeable screen sections for this reason.

Typical Reference Data

3. Air Aspiration System

Not every impurity can be removed effectively by a screen.

Dust, husks, chaff, empty kernels and other lightweight materials may have a similar size to the grain but a much lower weight.

This is where air aspiration becomes important.

An aspiration system separates materials according to their aerodynamic behavior.

How Does Aspiration Work?

Grain passes through an air channel while a fan creates a controlled airflow.

Light particles are carried upward or toward the suction duct, while heavier grain particles continue downward.

The removed material is then transported to a dust collection system, such as a cyclone or dust filter.

The airflow must be controlled carefully.

Too little airflow can leave light impurities in the grain.

Too much airflow can carry good grain away with the waste.

Aspiration is widely used together with screening because screening and air separation solve different impurity problems.

Typical Reference Data

4. Grain Destoner

A destoner is used when the grain contains stones, gravel or other heavy materials that are difficult to remove with ordinary screening.

This is especially important for grain collected directly from fields, where soil and stones can enter the harvested material.

Why Is a Destoner Necessary?

A stone can have a similar physical size to a grain kernel.

Therefore, simply using a screen may not separate it effectively.

The destoner uses the difference in specific gravity and aerodynamic behavior between grain and stones.

Working Principle

Grain enters an inclined vibrating deck.

Air passes through the material layer from below while the deck vibrates.

The lighter grain becomes partially suspended, while heavier stones and dense particles remain closer to the screen surface.

The vibration then moves the two materials in different directions.

As a result:

Clean grain → moves toward the grain outlet

Stones and heavy impurities → move toward the stone outlet

This separation depends heavily on airflow, deck inclination, vibration and feeding uniformity.

Typical Reference Data

A destoner should not be regarded as a replacement for the main screen cleaner. It has a more specific job: removing heavy impurities that are close to grain in size but significantly different in density.

5. Magnetic Separator

A magnetic separator is a relatively simple but valuable protection device in a grain cleaning line.

It is designed to remove ferrous metal particles such as:

• Iron pieces
• Nails
• Wire
• Metal fragments
• Other magnetic contaminants

These materials may enter the grain during harvesting, transportation, storage or handling.

Working Principle

The grain passes through or over a magnetic field.

Ferrous particles are attracted to the magnetic surface and separated from the grain flow.

The cleaned grain continues to the next stage.

Typical Reference Data

A magnetic separator does not replace a screen or destoner. It performs a different protective function.

6. Combined Grain Cleaner

For many commercial grain projects, using several individual machines is not always necessary.

A combined grain cleaner can integrate functions such as:

Screening + aspiration + destoning

into one cleaning unit.

This type of equipment can be useful when installation space is limited or when the project requires several cleaning functions in a compact arrangement.

However, a combined machine should still be selected according to the actual raw grain condition.

If the incoming grain contains a particularly high percentage of stones, very large agricultural residues or unusual foreign materials, separate cleaning stages may provide better control.

How a Complete Grain Cleaning Line Works

A typical grain cleaning section can be arranged as follows:

Raw Grain Receiving

↓

Pre-Cleaning

↓

Screening

↓

Aspiration

↓

Destoning

↓

Magnetic Separation

↓

Clean Grain

↓

Drying / Storage / Milling / Processing

The exact sequence can change according to the project.

For example, large foreign materials should normally be removed early so they do not overload downstream equipment.

Dust and light impurities can then be removed through aspiration.

Stones require gravity-based separation.

Metal contamination is handled separately through magnetic separation.

This is why a grain cleaning system should be designed around the impurity profile of the raw grain, rather than simply choosing the machine with the highest advertised capacity.

Grain Cleaning Equipment Selection Guide

Important Factors When Choosing a Grain Cleaner

Grain Type

Wheat, maize, rice, soybean and other grains have different kernel sizes, shapes, densities and flow characteristics.

A screen and airflow setting that works well for maize may not provide the same result with wheat or rice.

Initial Grain Condition

The amount and type of impurities in the incoming grain are equally important.

A relatively clean commercial grain may only need basic screening and aspiration.

Grain coming directly from harvesting may require pre-cleaning, destoning and stronger dust removal.

Required Capacity

The cleaner should normally be matched to the actual processing flow.

If a grain storage project handles 20 t/h, selecting a cleaner with only 10 t/h effective capacity can create a bottleneck before the grain reaches the dryer or silo.

At the same time, greatly oversizing the machine can increase equipment cost and operating requirements without providing a practical benefit.

Moisture Content

Wet or sticky grain can behave differently from dry, free-flowing grain.

High moisture may reduce screening efficiency, cause material to stick to screens and make stable separation more difficult.

Therefore, the grain condition should be considered when selecting the equipment and screen configuration.

Cleaning Target

Not every project needs the same cleaning standard.

A grain receiving station may focus mainly on removing coarse foreign materials.

A commercial grain storage facility may require more complete cleaning before drying and storage.

A flour mill, rice mill or seed processing plant may require much finer cleaning and additional separation stages.

Grain Cleaner Parameters Should Be Selected as a System

One common mistake is to select a grain cleaner only by its advertised capacity.

For example:

“20 t/h grain cleaner” does not mean every 20 t/h of grain will receive the same cleaning result.

The actual performance depends on:

• Grain type
• Initial impurity percentage
• Impurity composition
• Grain moisture
• Screen opening
• Feeding uniformity
• Airflow
• Vibration conditions
• Required final cleanliness
• Number of cleaning stages

For this reason, the capacity shown on equipment specifications should be treated as a reference value, and the final equipment configuration should be confirmed after understanding the raw material and project requirements.

Common Operating Precautions

Keep Feeding Uniform

Uneven feeding can create a thick material layer in some areas and a thin layer in others. This directly affects screening and air separation.

Select the Correct Screen

The screen aperture should correspond to the grain size and the impurities that need to be removed.

Do Not Use Excessive Airflow

Strong airflow is not automatically better. Excessive suction can remove good grain together with chaff and dust.

Check the Destoner Regularly

The destoner needs appropriate airflow, inclination and vibration. Incorrect adjustment can either leave stones in the grain or increase grain losses in the stone outlet.

Clean Screens Regularly

Blocked or damaged screens reduce effective screening area and can cause unstable cleaning performance.

Control Dust

Grain cleaning generates dust, particularly during receiving, screening and aspiration. A properly designed dust collection system helps maintain a cleaner working environment and reduces dust accumulation around the equipment.

Protect Downstream Equipment

The cleaning section should be viewed as part of the complete grain handling system.

Removing stones and metal before elevators, dryers, milling machines and other processing equipment can reduce the risk of mechanical damage and unnecessary downtime.

Grain Cleaner for Storage and Grain Processing Projects

For a grain silo project, the cleaning system is normally installed before the grain enters the storage process.

A practical arrangement may include:

Truck Receiving → Pre-Cleaner → Grain Cleaner → Destoner → Magnetic Separator → Grain Dryer → Bucket Elevator → Grain Silo

Not every project needs every machine.

For relatively clean grain, a simple screening and aspiration system may be sufficient.

For field-harvested grain containing stones, straw and considerable dust, a more complete cleaning section is normally more appropriate.

The purpose is not to add as many machines as possible. The purpose is to make sure each separation stage handles the impurity it is actually designed to remove.

Grain Cleaner Selection from COBAN

COBAN can configure grain cleaning equipment according to the actual grain handling project rather than supplying a standard machine without considering the upstream and downstream equipment.

For a new grain storage project, the cleaner can be considered together with:

• Grain receiving system
• Pre-cleaning equipment
• Destoner
• Dust collection
• Bucket elevator
• Belt conveyor
• Grain dryer
• Grain silo
• Temperature monitoring
• Silo aeration system
• Grain unloading system

For an existing grain storage facility, the cleaning section can also be upgraded separately.

The most useful information for equipment selection is the grain type, required capacity, incoming grain condition, moisture content, major impurities and required cleaning level.

Once these conditions are known, the appropriate cleaning equipment and capacity can be determined more accurately.