Wheat Cleaning for Flour Mills
June 01, 1997
by Teresa Acklin
Industry consultant David Sugden discusses cleaning systems and how to assure optimum performance.
Wheat cleaning in any shape or form is a vital part of flour milling, but the process is not always trouble-free. Understanding the stages of cleaning and knowing how to minimize problems can reap dividends. It pays handsomely to pay close attention to wheat cleaning.
The accompanying diagrams illustrate a classical wheat cleaning system reflecting current typical good practices that are designed to optimize results. Diagram I shows the cleaning stages from wheat intake to the feed for first break rolls, while Diagram II depicts the flow and treatment of screenings or dockage obtained from the cleaning stages in Diagram I. All the material in Diagram II is ground by hammer milling and is sieved and weighed before addition to millfeed from the mill proper.
The entire cleaning system is operated by remote control and is monitored by programmable logic controllers, which are not shown. The system also includes significant exhaust equipment and full dust explosion controls, which also are not shown.
The system's main features are weighers at four separate points for accurate inventory logging. Water addition can take place on intake, at tempering (18) and before first break. Blending facilities are facilitated by impact feeders (12), both under the wheat elevator and the temper conditioning bins.
Intake may be from ship and/or rail, although truck intake is pictured here. Capacity is designed for particular local circumstances. As an example, intake capacity normally should be at least three times ideally five times as great as the mill in order to reduce traffic and demurrage problems. The wheat cleaning capacity after intake should be up to 15% greater than the mill for flexibility and maintenance reasons.
Elevator storage capacity depends on specific needs. The example assumes tempering is likely to be between 24 and 36 hours, although the issue of tempering is open to different and valid theories.Cleaning Stages
The diagrams feature the six time-honored wheat cleaning stages. The first five involve separation according to various measures peculiarity, width, air resistance, specific gravity and length and the sixth is scouring.
The first piece of cleaning equipment for so called “preliminary cleaning” at intake, as well as the main wheat cleaning part, is the magnet (6). Its job is to protect machinery and to reduce fire and dust explosion risk. A magnet also always should be placed before a hammermill.
The magnet separates “peculiar” substances from the wheat. Additional equipment for this purpose would be a metal detector for non-ferrous materials. Metal detectors are not yet widely used in mills to clean wheat, but use is expected to increase based on customer demand.
The next cleaning step is width separation (7) and (14). The drum and milling separators are sieving machines, the former a rotating drum whose axis is nearly horizontal to the ground. The latter is either a reciprocating or rotating screen in the horizontal plane.
The function of the drum unit is to separate large debris (wood, brick, straw, cigarette packets, etc.) from grain. The resulting rubble is disposed of separately.
The milling separator will sieve out maize, soybeans and similar material larger than wheat from the top sieve. The throughs of the second and bottom sieves deal with material such as sand, as well as very small broken wheat. All material that is larger and smaller than wheat in width is sent as dockage to the hammermill in Diagram II.
The third stage is aspiration or air resistance as shown at (8), (14), (17) and (20). All can be either open or closed circuit aspirators separating lighter than wheat material, which also is destined for the hammermill.
Closed circuit aspiration has the advantage of saving 90% of filter dust collection area compared with open circuit systems. But some researchers point to the disadvantage of microbiological build-up when closed circuit aspiration is used at every aspiration step. De-stoning machinery tends to require more maintenance for top performance when closed circuit aspiration is employed because return air fine wheat dust particles block the inlet air grid more easily.
The fourth cleaning stage separates by density or specific gravity (15). This separating machine, using strong exhaust and the adjustable angle of the vibrating screen, separates stones, glass, non-ferrous metal and similar substances. It also categorizes wheat, sending low density pieces from the top sieve and sound wheat from the bottom deck.
Air assistance is provided by a fan coupled to the wheat cleaning exhaust equipment at (24). By using this density characteristic, it is possible to siphon off the so-called lighter phase only to the disc separators (16). The light phase will be about 25% to 30% of full feed, so disc machine capacity, and therefore cost, is reduced accordingly.
Machinery that combines these functions to save space is available from suppliers. All-in-one machines put together the milling separator, stoner and specific gravity principles with the addition of closed circuit aspiration. Most certainly, for high-standard wheat cleaning, it is necessary to include all these steps.
The fifth cleaning stage is length separation by the disc (or cylinder) separators (16). Discs with designed pockets are immersed almost to the central axle. In the case of oats and barley, which are longer than wheat, wheat falls in to the pocketed discs and discharges to the center of the machine. Oats and barley are then discharged to dockage at one end. For separation of seed smaller than wheat, the reverse is true, with a separately configured pocketed disc separator.
These machines are fully adjustable on the run. Length separation is effected by the disc running through the grain mass. Material that sticks out of the designed pocket, that is too long, will not be retained and is separated.
So-called trieur separators perform a similar job. These are large cylindrical pocketed drums, into which wheat is centrally fed from one end. Material longer than the pocket design discharges at the opposite end, whereas the smaller material sits in pockets until it falls from the top of its cycle by gravity into an adjustable angle trough conveyor.
The final cleaning phase is scouring (17 and 20). Scouring involves a horizontal machine containing a central rotating shaft with beaters surrounded by a small wire mesh or round hole static screen. Wheat is fed centrally at one end and scoured as it travels to the opposite end. Fine bran, crease dirt and small broken wheat go through the screen with whole sound kernels fed to an aspirator to lift off light material.Problems and Solutions
The first step in avoiding problems is to assure that all cleaning machines are in sound working order.
Magnets (6) should be cleaned daily, an obvious point but sometimes overlooked. All machines should be checked physically daily for performance and adjusted if necessary.
The quickest method is to look at the material feeding the hammermill (26) and then trace faults such as whole sound kernels back to the responsible machine and adjust. Also, a look at cleaned wheat feeding the first break rolls will show unwanted impurities; then trace back and adjust. Impurities, including some dangerous ones, can be handled by the sort of diagram shown here provided the extent of impurities is within reasonable bounds.
Color sorting is relatively new and now is being used in a few plants. It has two locations in the flow, the first on full feed and the second in place of or alongside disc separators (16).
The colors that can be identified are much better than a year or so ago. But the capital cost puts off many, even though the return on investment, especially in big plants, is excellent.
Blending by dilution is a very common and effective answer to trouble. Sometimes it is necessary to deliberately “overset” screen room machines, although this must be done temporarily and judiciously.
Some customers put flour products and whole wheat for human consumption under the microscope, looking for perfection. So another help to the miller is to run cleaned wheat around one or even two more times. It is incredible how much impurity exists even on the third cleaning and even from a well-adjusted set of machines in a comprehensive flow.
The sequence of the main wheat cleaning machinery in the diagrams reflects a distinct logic. While it is possible to alter the order in which each occurs in the flow, some drawback usually surfaces.
For example, the scourer and aspirator (17) would not perform as well if moved from the last position before damping. Stones not removed would be likely to damage the screen of the scourer all too easily.
The stoner and density separator, if placed before the milling separator and aspirator (14), would need to deal with a wider range of sizes and specific gravities (sand, maize and soybeans), thus diminishing performance. Further, to gain maximum clear separation of oats, barley and seeds, the milling separator and stoner need to come beforehand.
What are the implications of well-set or not so well-set screen room machinery? A typical percentage of screenings could be 2%, more or less. A 0.2% increase to 2.2% of sound wheat in dockage represents a significant financial penalty. But worse still would be a lost customer because of an impurity complaint that could have been avoided.
David Sugden, independent consultant to the grain industries, may be reached at The Coach House, Killigrews, Margaretting, Ingateston, Essex CM4 0EZ, U.K. Tel: 44-1245-352048. Fax: 44-1245-251162.