By : Novozymes and Buhler

Milling is an ancient art. It is a technology, the marriage of food science with the art of the miller. 

Principles of modern milling are much the same as they were many years ago using rotary grinding stones, except now wheat is ground between pairs of cast-iron rolls before the stocks are sieved and then reground. 

Wheat milling consists of several processing stages. These stages are wheat intake, pre-cleaning, cleaning, milling and packing. Each stage is described in detail below :

1. Intake of Wheat

Wheat arrives at the mill either by boat, truck or rail. If it arrives by boat, it is transported directly to the mill. In rest of the cases, it is downloaded in a pit from where it is transferred to the mill.

Unload of wheat from ship

Supply by road is mainly carried out by trucks. For this purpose, vehicles that can be loaded with up to 40 tons of wheat are used. 

Generally, we distinguish between the following types of vehicles for bulk transport : self-emptying trucks with hoppers; self tipping vehicles and vehicles that need a local tipping device.

For the transport of different grains by rail, special bulk cars are used. Depending on the length of the rail wagons, there are up to 5 individual outlet hoppers. The loading capacity of such carriage can exceed 50 tons. 

These silo or bulk wagons are completely self-emptying. And their unloading capacity depends on the intake capacity of the wheat reception. A small intake pit and a chain conveyor along the rails are generally sufficient.

Wheat from seagoing vessels is unloaded either pneumatically or mechanically. These ships have a limited “docking time”, during which the product must be unloaded. If this time is exceeded, the party receiving the grain has to pay quite a high penalty. 

For this reason, ship unloading installations work with high hourly capacity and usually operate around the clock. Pneumatic unloading units can reach capacities of up to 600t/h, while mechanical units can unload up to 1,200 t/h, or even more. 

The pneumatic ship unloading units have the advantage that they work dust free, and are also suitable for cleaning up the residues. However, the power consumption per ton of grain handled is considerably higher than that of the mechanical system. 

There are different types of mechanical wheat unloading systems. The best known are the chain conveyors. Bucket elevators and double belt conveyors are also used.

Mechanical unloading requires a lot of manual work. Ahead of the intake scale, it is necessary to install a buffer bin to prevent the product from building up during interruptions of the weighing process.

2. Pre-cleaning Wheat

Depending on the harvesting method and country origin, the grain contains a larger or smaller amount of impurities such as : straw, stones, wood, metal pieces, paper, strings, mud balls, sand, chaff, dust and other. 

If not removed, these impurities can damage the machinery, block bin outlets, and adversely affect the quality of the grain. 

Typically the pre-cleaning stage of wheat involves passing through a drum sieve, a separator/ aspirator and a magnet. Subsequently, the grain is scaled and transferred to the storage bins. The drum sieve removes only the coarse impurities from the grain. 

The product is led to the inside of the rotating sieve drum via an inlet channel, from where it directly falls into the outlet hopper after passing the sieve cylinder. Coarse impurities are retained and moved to the outlet by the rotating motion of the sieve cylinder. 

The sieve is kept clean by a strong brush that acts upon the sieve drum from the outside. The sieve drum consists of two halves with different sizes of perforations. 

The holes are square shaped and range for the coarse section between 30-60 mm and for the fine one between 10-30 mm. Ferrous particles taken into the system can damage machines or even cause a dust explosion by sparking. 

Magnets are therefore, installed as close as possible to the start of the processing line. This could be prior to the following options : pre-cleaning section, cleaning section, grinding section, flour silo, and flour packing.

Hard-metal magnets that where mostly used in the past have been replaced by an iron-cobalt-nickel-aluminium alloy today. 

This special metal alloy ensures the magnetic effect remains nearly indefinitely and the magnets are less susceptible to vibrations. Irrespective of which type of magnet is installed, the following should be observed under all conditions :

The product flow should be evenly distributed over the entire surface of the magnet

The grain should flow with a low speed over the separating area

The magnet must be installed in such an accessible positon

Magnets are usually provided with a retarding gate with counterweight, which functions as speed breaker and distributor at the same time.

Separators are used both in the pre-cleaning and cleaning sections. The difference between separators in the pre-cleaning and cleaning section is usually limited to the selection of screens and inclination of the sieve box. 

Grain separators work according to the principle of separation by size and with the help of perforated sieves. In most cases, an aspiration channel is attached for the separation of dust and light impurities. 

The rotating movement causes an excellent separating effect and allows for high throughput rates. In a more recent design (“Classifier”), the sieve box is vibrated with the help of vibrommotors. With the vibrating movement, a very good separating efficiency is obtained, particularly for fine impurities.

The wheat flows via a baffle plate onto the wheat sieve. A special feeding system is not required because the rotating movement causes the product to automatically distribute over the entire sieve width. 

All impurities that are bigger than the wheat over tail the top sieve and leave the machine on one side via the cross channel. The so called “curtain”, attached to the top sieve, prevents the sound wheat kernels from tailing over together with the coarse impurities.

The troughs of the top sieve, wheat and fine impurities, drop onto the sand sieve, which is placed underneath. 

The wheat now overtails the sand sieve and either leaves the machine directly or reaches the aspiration channel, if attached. Sand and other fine impurities passing through the sand sieve and afterwards are discharged via the second cross channel.

The sieve box is connected to the central aspiration system via a connection pipe, to avoid dust emission. For the separation of light impurities, an aspiration channel must be added. 

To obtain an optimal performance of this channel, a constant feed, evenly distributed across the entire width has to be secured.


Cross section of a separator (Picture is courtesy of Buhler AG, Uzwil, Switzerland)

The aspiration channel, often also called air classifier, is used wherever light particles have to be separated from a stream of the product. For example in grain cleaning, it is usually used with the separator and after a scourer.

Performance of the aspiration channel depends largely on the way the material is fed into the machine. Only with a constant flow of stock, evenly distributed along the entire width of the channel, can good results be expected.

The aspiration channel is fitted with a vibratory feeding system. The vibration is caused by a vibro-motor. In a more recent design, instead of the vibro-motor, a motor with a V-belt and an accentric shaft cause an oscillation of the feeding shoe and thus provides for a uniform feeding of the products. 

The stream of product  emerges horizontally from the feed hopper and air is drawn through it along the entire width of the channel. Light particles such as husks, dust and shriveled grain are lifted into the aspiration channel. 

This is the place where the final separation takes place. In the aspiration channel, the lifted particles either drop back to join the product again or finally carried with the airflow. This separating process can be observed via the transparent rear wall.

Cross Section of aspirator (Picture is a courtesy of Buhler AG, Uzwil,Switzerland)


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