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How to increase mill efficiency

30 September 202210 min reading

The mill must have a planned inspection and prioritized preventive maintenance program in place and all the relevant consumable spares available at all times. Planned maintenance when applied correctly will prevent breakdowns and lost time. Lost time is an unrecoverable cost to the company and loss of revenue.

Ronald D. Sebastian
Operations Management Advisor
Milling Consultant



Infestation prevention prior to storage of grain

Infestation results in losses of raw materials due to the effect of the insects in the grain in storage. Insects can consume up to 5% of the total grain in storage. The inclusion of a phostoxin pellet or tablet dispenser in the grain flow just prior to storing in the silos, can combat infestation and prolong the storage time for the grain.  The movement of the insects in the stored grain can also result in the generation of friction, which can then result in spontaneous combustion in the silo.   

Pre-cleaning of the grain will improve storage capacity and prevent choking of dosing equipment, spouting during discharge. Removal of as much dust as possible enhances the grain life span especially if the natural moisture content exceeds the normal requirements

The process of monitoring, measuring and recording flour extraction is necessary on a regular basis (daily, weekly, monthly or quarterly, at the least) to monitor efficiency and to take corrective action where needed. With extraction values, flour millers can ensure optimum financial performance commensurate with the product quality necessary to satisfy all customers. Extraction is also one indication, among several, of product consistency.

Extraction is defined as, the targeted acceptable quantity of high-quality flour, desired by customers, to be obtained from wheat. There are three methods of calculating extraction, viz.

 On raw wheat brought to the mill against total of all products obtained.

 On cleaned dry wheat conditioned then milled, against total products obtained.

 On the total quantity of wheat, passed through the b1 weigher, against total products obtained.

For proper monitoring, the miller has to ensure that the measuring and dosing equipment is properly maintained and accurately functioning. Equipment used includes mechanical or electronic batch weighers, gravimetric, constant weight, loss in weight (transflowtrons) or load cells mounted under bins, mechanical scales, flow balancers, variflow measurers (in some cases).

In the screens room, the miller or screen room operator must ensure that all or as much of the impurities are removed from the grain. This is to ensure that, the wheat, in the conditioning process, will not be affected by the “impurities”, absorbing the moisture that is intended for the grain, to obtain optimum milling condition. 


Failure to remove these, can and will result in, out of specification products. Products out of speck result in returns and rework. Returns costs are non-recoverable (a dead loss to the company). Millers often overlook the screen room and allow the operators to make adjustments that are not effective.

 Regular inspection and cleaning of magnets before high-speed equipment and the 1st break rolls must be done and the quantity of ferrous metal weighed and recorded, fragments of metal in the wheat flow can causes irreversible damage to machinery and cause sparks that can then result in dust explosions and fires.

BI FLOW RATE AND DISTRIBUTION

Ensuring even distribution of the feed, across the entire roller length is critical to the mills efficiency. The best method of ensuring this is by weighing the grind of all like rolls then making the necessary adjustment. Unbalanced like rolls has an impact on the efficiency of the plansifters and mixing of stocks to the purifiers. Measuring the moisture content and HLM of the feed to the rolls will provide the miller with the potential extraction possible from the grain. Target moisture content must be obtained and maintained through the run.

Inspection of the roll fluting at regular intervals for wear is necessary. This is to ensure that the required releases are achieved every time without increasing the energy demand. Worn roll surfaces require more energy to achieve the desired results and, cause flaking of the endosperm particles against the larger bran particles. This segment of the feed then flows to the second break where the effort in removing the endosperm often results in the generation of bran powder and a subsequent increase in the ash content of the flour.


Regular inspection will alert the miller of the need for roll changes. Replacing rolls with the correct flutes and roll disposition is critical. Do not use rolls with different fluting because you don’t have stock.

Note must be taken that smooth rolls require taper to avoid over expansion of the ends of the rolls causing the stock passing through the middle of the roll length with ineffective grinding of the semolina, especially in the head reduction passages. Shiny smooth rolls do not have the abrasive surface needed to crush the endosperm to a fine powder.

Inspection of the separations of the sifter allocation and comparison provides a good guideline for the miller, relating to the settings of the purifiers, for thorough efficient separation of the various particle sizes of the semolina, to be ground into saleable high-quality flour.

Sifter separations are more often than not, not carried out by the mill operative/ miller, resulting in him/her running around the mill when the laboratory personal call them with production errors in flour quality. The subsequent result of the lack of sifter inspection is the stock to the purifiers, contaminated with bran adhering to the endosperm and the miller spending valuable time trying to adjust the machine instead of identifying the root cause of the problem. (Poor sifter separations or unnecessary excessive roll grinding due to worn rolls.)

Again, balancing the feed to like rolls is an important aspect of the head reduction process so the flour produced here is of uniform quality off the plansifters allocated. Incorrect roll adjustment often has the miller running around the mill trying to correct the flour quality instead of ensuring correct required machine setting. Inspection and adjustment of smooth roll scrapers are essential to the correct operation of the rolls. Too often one will find rolls ringing because the scrapers are worn out, or not properly adjusted to be “just touching” the roll surface.

When carrying out plan sifter maintenance, a miller must make sure that the sieves used are at the correct tension and that the sieve cleaning elements are functioning properly. When replacing torn or burst covers, the correct aperture covers are used and the sieves repacked in their exact location within the cabinet ( miller’s often get the cleaning team to carry out this function, which is carried out incorrectly). This results in the mill stopping almost immediately after restarting, due to incorrect sieves or sieve boxes being miss-placed in the cabinet. 


Incorrect feed distribution to the rolls results in the feed rolls stopping and starting. The effect of this is, the plansifters have erratic flow resulting in bare dressing and high ash flour produced. It is therefore pertinent that the feed to all the rolls in the mill to be constantly fed with full even flow of stock across the entire length of the grinding rolls.

MIXING AND BLENDING

Ensuring that the correct quantity of any additives, enzymes and other necessary ingredients for mixing and blending is drawn by the miller in charge and the quantity remaining after completion of the specific batch, is reconciled with the quantity of product scheduled for mixing.

 Detachers, Impact, pin mills and drum detachers, are installed immediately after reduction rolls to break up any flakes generated by the action of the rolls, allowing for the ease of separation in the following sifters. Regular recommended inspection and adjustment or repair is necessary as the rotating pins and the beaters of the drum detachers do become work. Regular lubrication of bearings of the drum detachers is necessary too.

The introduction of sterilator into the final flour stream helps prevent or reduce infestation of the final product and has to be inspected daily for potential leaks from the seals of the rotating shaft.

FLOUR STREAM INSPECTION AND ANALYSIS

The question here is - how often do millers carry out ash curve analysis in their mills. Millers often don’t even look at flour streams regularly, they only inspect this when there is a complaint from QA. Inspecting at least twice per shift is preferable to ensure constant flour quality.

Scales are an important aspect of the entire milling process. All weighers should be checked for accuracy at least once a month during planned maintenance shutdowns. During this time, the miller must also check the condition of the fluted and smooth rolls and plan for the necessary roll replacements. Poorly surfaced rolls contribute to loss of extraction and out of spec production.

All scales monitoring the raw material into the mill, the screenings taken off in the screens room and pre-cleaning, all flour produced, bran and offal’s produced on the mill, must be accurate when compared with wheat brought in and product released for dispatch. The road and rail weighbridges also need inspection and calibration annually.

As stated already, the accuracy of stock reconciliation daily, weekly, and monthly, allows errors to be corrected easily, rather than once annually or even quarterly.

Production reconciliation has to be completed by the head miller/ mill manager on a daily basis and all anomalies corrected on the same day. The methods of calculation have already been discussed. Another method of checking the accuracy of weighers is to cross-check all packing activities with mills and mixing and blending activities, to confirm that what was produced, was actually packed.

Modern-day mills have online reporting which one can view at any time. The introduction of online quality monitoring devices (NIR online) that help the miller monitor product quality at all times.

Ensuring that all of the above work is carried out while you are on shift, whether you are the head miller or the mill manager or even the plant manager, will certainly ensure that your plant is efficient and productive. All downtime in the mills is to be utilized by carrying out any necessary running maintenance that has been identified.

In some facilities, reconciliation per consignment of grain received at the mill is done. In this case, only small consignments purchased, whereas, in other facilities, larger quantities of grain purchased, making it difficult for this to reconcile. We, therefore, prefer to reconcile all flour and by-products produced against the quantity of raw wheat received at the mill screens room.

Together with all of the above, the mill must have a planned inspection and prioritized preventive maintenance program in place and all the relevant consumable spares available at all times. Planned maintenance when applied correctly will prevent breakdowns and lost time. Lost time is an unrecoverable cost to the company and loss of revenue.

Only the team responsible for the efficient and productive operation of the facility can do this. Diligence, responsibility and commitment by all is truly necessary

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