The purpose of flour milling is to provide quality flour that is fit for purpose and free from any foreign matter or tainted in any way.
To make this happen, the miller must prepare the wheat for milling by ensuring that all foreign matter is removed prior to the milling process. Typically raw wheat can be contaminated by the following on arrival at the mill:
Diseased or shrivelled grain, straw, chaff, other cereal grains, seeds of all descriptions,string, paper, wood, stones, sand, dust, nuts, bolts, wire, glass. The larger contaminants and some dust are usually removed at intake to prevent potential damage to equipment, so the task of final clean is done in an area of the flour mill called the screenroom. The design and layout of screenrooms and machinery used in the modern flour mill can vary greatly from one country or mill to another, but one thing remains constant - they all employ the following five principles of separation to remove these impurities:
- Separation by size
- Separation by specific gravity
- Separation by air resistance
- Separation by natural peculiarity
- Separation by shape
Separation by size
This is by far the easiest of the cleaning principles and is essentially sieving. By employing revolving, oscillating or reciprocating sieves, contaminants that are larger or smaller than the grain can be separated. A generic term for this type of machine would be a "Milling Separator" or "Drum Separator".
Separation by specific gravity
Where contaminants may be the same size as a grain of wheat (eg. a stone), separation by size is not possible. By means of a vibrating, inclined porous deck with an adjustable air flow being drawn through the deck, the denser material can be retained on the deck and will travel up the deck and fall off the back, whereas the lighter grain will float above the deck and will travel downwards and fall off the front of the deck. Various machines have been developed over the years which can make multiple separations and can therefore separate heavy grains as well as light grains from the original contaminant. This is particularly useful to split grains for separate treatments specific to their size or nature further down the process.
Typically these machines would have such names as Gravity tables, Dry stoners, Concentrators or Combinators.
Separation by air resistance
As well as the naturally lighter material such as dust, sand and chaff that comes in with the wheat, more dust and chaff is created by the mechanical transport of the wheat around the process.
By means of selective aspiration, a strong current of air is drawn through a falling curtain of wheat. This lifts the lighter contaminants out of the wheat due to its velocity. This dust laden air is then allowed to decelerate in a special expansion chamber which causes it to drop the impurities it had collected from the wheat.
Typically this machine would be called an Aspirator. Aspirators are normally installed immediately after any impact machine, as these machines generate dust, loosen dried mud from the crease of the grain as well as loosen bran layers.
Separation by Shape
Where the size and the density of a contaminant is similar to a grain of wheat, it may be possible to separate by shape. Most seeds, for example, are round and will "roll" better than a grain of wheat. There are two methods for removal of these impurities.
The first is the spiral seed separator which is built very much like a fairground spiral slide. When product is fed into the top of this inclined spiral, the round seeds will roll faster than the grain and by means of centrifugal force, will be forced to the outside edge of the slide. By having varaible flaps at the exit of the spiral it is possible to split the feed running on the inside of the spiral from that on the outside. The second method is the disc or cylinder separator which uses indented pockets in a rotating disk or a cylinder to collect or reject grain or impurities according to their shape. By selecting specific size pockets or dimples, a very fine selection or rejection of size can be controlled. Typically the selected shape is collected by the pockets as they rotate through the feed and are thrown into a collection conveyor for disposal or further treatment.
Separation by natural peculiarity
Normally this refers to metal contamination, but in recent times has also come to include colour.
There are many types of magnets used to do this from rotating drum magnets to cascade magnets, to bar magnets and even electro-magnets. The function of all of these is the same. The removal of metal, apart from the obviuos contaminant aspect, is the danger this poses to the process equipment in terms of damage or wear, and more specifically the potential to generate a spark leading to a dust explosion. With colour sorting it is now possible to identify not only different colours but also different shades of colours. By means of electronic "eyes" and pneumatic air jets it is possible to remove single grains from a stream of product that may be running through the screenroom at several tonnes an hour.