Dockage and screenings in flour mills

by Teresa Acklin
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Industry consultant David Sugden describes treatment and disposal systems commonly used to assure flour quality and protect profit margins.

   Wheat cleaning systems, the handling of dockage and screenings and expected results are subjects that often are neglected, at a serious cost. The incorrect setting or poor maintenance of any type of wheat cleaning machinery can result in a loss of money, flour quality or both.

   Dockage and screenings have different connotations in varying parts of the world. As used in this article, dockage is defined as any material separated from wheat at intake or preliminary cleaning. This operation is carried out at a relatively high speed or capacity compared with later cleaning stages. Screenings, on the other hand, refer to matter taken away from wheat during the main wheat cleaning processes.

   While many variations are possible, the accompanying flow diagrams represent some of the main cleaning machinery typically in use today. The diagrams characterize the types of dockage and screenings found and their destinations.

   Diagram l shows dockage separation at wheat intake. The common machinery sequence is a magnet, a dockage or rubble separator and an aspirator. Conveying, weighing and exhaust equipment is not shown.

   It is vital to begin cleaning flows with a magnet. In large installations, the magnet preferably would be a motorized self-cleaning electromagnet. The magnet protects the ultimate consumer and the subsequent machinery, and it mitigates dust explosions by minimizing the possibility of sparks.

   Material separated by magnets, known as tramp metal, is a waste product collected in some suitable receptacle and ultimately thrown away. But tramp metal should be examined daily because magnetic metallic waste often contains surprises for the operative and maintenance engineer. A little detective work can identify potential failures upstream, such as bucket elevator bolts that are broken or damaged chain conveyor parts.

   The rubble separator sorts out any large crude debris, which is also waste and therefore is disposed of as garbage. The principle of separation is by size, accomplished by a rotary sieve.

   Again, a daily inspection of this debris can prove advantageous. A problem sometimes develops at this point if there is an excessive quantity of straw, a nuisance that can easily plug the machine. More commonly, all manner of bits and pieces turn up — wood, paper, cigarette packets or evidence of the previous cargo carried by the truck or vessel currently discharging.

   The third machine is the aspirator, which collects dust and chaff or husk. This debris may be treated in several ways. It can be directed to a system as outlined in Diagram 4 for ultimate blending into millfeed, it can be sold separately, or in some cases, it may be burned in furnaces to raise steam or for central heating.

   The so-called preliminary cleaned wheat is then destined for elevator storage.

      Screenings Separation.

   Diagram 2 is an illustration of the main wheat cleaning machines, their positions in the flow and the character of the separated elements. Again, only the relevant machinery is shown.

   Another magnet starts the sequence, although at a much lower capacity that is similar to or up to 15% greater than the mill capacity proper. The ensuing tramp metal is disposed of as before and for the same reasons.

   A reciprocating milling sieve separator takes over to differentiate and eject material larger or wider than wheat, e.g. maize and soybeans. It also sieves out sand, grit, dirt and some seeds smaller than wheat. These screenings and all the remaining machine-separated elements that are not mainstream wheat, tramp metal or stones are directed to the system in Diagram 4 for screenings grinding.

   An aspirator carries out a more comprehensive operation than in Diagram 1, namely separating light wheat as well as dust and chaff, all by means of air resistance differentiation. The next system component uses specific gravity separation and sorts out stones for waste disposal. Seed separators of the disc or trieur cylinder type then divide oats and barley, as well as seeds smaller than wheat, by length.

   Another magnet is placed before a scourer, which rubs or scours wheat to eliminate surface dirt and small, immature and light unfilled kernels. The final aspirator, either closed or open circuit, deals with material loosened by the scourer, yet light enough to be lifted away. Wheat is then ready for the conditioning or tempering process and bins. A similar flow is seen in Diagram 3, which illustrates separation at the feed to first break stage, with a final magnet preceding the scourer and aspirator.

   At this time it is useful to stress the importance of inspecting the performance not only of magnets, but also of all the cleaning machinery. This advice applies to all equipment that might be installed, including metal detectors, color sorters and gravity tables.

   Wear and tear, burst screens, leaks, worn sieve cleaners, blocked air channels and other difficulties all produce malfunction and poor performance in some form. For example, in a 300 tonne per day mill, it is incredibly easy to lose 0.5% of sound wheat to screenings. This means that 1.5 tonnes per day — or a massive 470 tonnes per year — are lost, at a price equal to the difference between wheat cost and millfeed revenue.

   Accordingly, it is clear that each machine needs a daily inspection. The trick is, of course, to look for sound wheat directed into screenings grinding or conversely, for screenings directed into the first break roll. The latter situation will lead to dirty flours, high ash, low extraction and perhaps one or two other unpleasant consequences. Investigation will show which machine is the culprit.

      Screenings Treatment.

   Diagram 4 shows a common method of treatment of screenings. Importantly, the system begins with an in-line production weigher, which measures and records the proportion of screenings taken out of wheat.

   Depending on the source, total screenings are likely to be between 1% and 2% by weight, a fairly broad range. It should be possible to establish fairly quickly, after a day or two, a probable range for a collection of wheat.

   Then, by setting an upper and lower alarm limit on the screenings weigher, a tell-tale signal will emerge for abnormal readings. Although the weigher alarm is all very well and useful, it still will not beat proper daily inspection of all cleaning equipment.

   From the weigher, screenings proceed to the sifter, with a 20W (1 millimeter) or finer cloth. The tails go to the coarse unground screenings bin and eventually to the fine ground screenings bin.

   The coarse screenings bin discharge vari-feeder is set to ensure a steady feed according to the full load amperes drawn by the hammermill. Note the magnet preceding the hammermill.

   The hammermill will probably have a 1.6- to 2-mm screen, depending on the capacity and final particle size required. The beauty of a hammermill is that the direction of the motor can be reversed to even wear. The flow continues to a filter receiver and fan, then via a seal and screw to the fine ground screenings bin.

   From there, a vari-feeder discharges proportionately into millfeed, to a pellet mill or even to be sold separately for bulk, in bags or tote bins, with end product depending on the local market. Some mills do not grind screenings at all and mix them with millfeed for sale. Others pellet everything without grinding screenings.

   A look at some other risks is useful in determining disposal. The purpose of wheat cleaning, imperfect though it is, is clearly to clean wheat. It follows that difficulties caused by mite, weevil, mycotoxins, ergot, bunt or stinking smut, rodent hair and many other contaminants unfortunately will end up in screenings. Depending on the nature of the problem, the option to dispose of screenings as a separate sale is prudent.

   A point of interest is a system I have seen practiced in only one plant, in which the miller kept a large elevator bin for broken wheat separated from the seed cylinders. Every three months, he short tempered and milled it, binning the flour as low grade. The extraction was about 65%!

   Proportionately, something like 40% of screenings comes from the milling sieve separation, the scourer and their aspirators. Around 20% comes from small seed separators, with the balance spread among the remaining separations.

   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.