Mill design and construction

by Emily Buckley
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By Gavin Owens

Millers want a process plant that can deliver the desired quantity of product, within specification and at the lowest capital and running cost. The latter two requirements can be mutually exclusive, and thus, a compromise has to be reached.

The engineer responsible for taking a mill project from concept to completion must take the mill owner’s requirements into account. The engineer must, at the same time ensure that the technology adopted is sound, reliable and efficient. Efficiency encompasses the use of resources and materials during construction as well as operation.


A mill project starts on paper where basic requirements are laid out. This is easiest where mill upgrades or replacements are involved because product and volume data are known. New projects involve a lot of unknowns and significant product and market research needs to be carried out. Ultimately there is a lot of educated guesswork involved.

The choices made at this point have an enormous influence on the form that a mill will take, because there is no such thing as a universal mill that can produce all products. Therefore choices must be made as to the products that it will not be possible to mill as well as to those it will. The choices made here can result in the inclusion or exclusion of whole classes of equipment, such as pin mills.

Mill capacity is a customer specified parameter, but ultimately is decided by the size of the market that the miller wishes to service. From the engineer’s perspective, the capacity delivered is a simple multiple of the smallest practical unit that can achieve the product requirements specified. There is some scope for efficiencies of scale, but the magnitude of the savings can be surprisingly small.

The availability of raw material can play an enormous role in the final configuration of a mill design, because this factor will determine whether storage
is needed at all or if large amounts of storage are needed to cater for consignments from ships or trains. There may be some extra facilities required on the milling site to deal with raw materials being received. For example the grain may need to be dried or treated for insect infestation on reception. Special cleaning regimes may even need to be considered if particular characteristics about the raw material supply are known. An example is the extra de-stoning equipment that is required in China.

Handling equipment for grain reception must be chosen to be capable of handling the largest envisaged consignment of grain in the shortest possible time. This means that there is an enormous amount of redundant capacity installed in receiving points. This is absolutely necessary to minimize the risk of penalties from transporters, particularly shipping companies. The other point that must be taken into account is reliability. This is crucial because of the intermittent nature of the equipment’s use. The equipment must also be as simple as it is reliable because specialized expertise may not be available to remedy any problems that may manifest.

The storage of raw materials is always a difficult one to discuss in global terms, because storage requirements and conditions vary so much across the globe. For example, bins are the most popular form of storage, because automation is simple, but flat stores are sometimes used because they can serve more than one purpose. Consider also the option of steel bins versus concrete silos. Condensation can be a major problem in steel silos in desert climates where day and night temperatures vary dramatically and so are sometimes considered unsuitable for use in these applications.

The product list for a mill project will determine finished product storage configurations. The number of products, the type of products and the volumes involved will all need to be discussed. The method of delivery to the customer will also be a major consideration as provisions for delivery to packaging will be very different from those for bulk deliveries. The logistics of getting the material into and out of the site will also affect the placement of silos, etc. relative to the mill and local infrastructure.


The equipment required for a particular project will largely be determined by customer preferences. If the mill owner opts for a conventional build, the equipment listing will be virtually identical between manufacturers. This can be useful in obtaining competitive quotes for a particular project. However each supplier of turnkey plants has specific, individual technologies that offer process efficiencies or process adaptability. However no one manufacturer has the full repertoire of machine types available to them and so it is difficult to build a an entire mill containing the best technology available. The end result is thus a compromise between what is available and what it is possible to secure with a performance guarantee from a particular manufacturer.

The list of processing machines largely determines equipment layout, but there is scope for creativity. For example, it is possible to opt for a low profile layout by using pneumatic conveying in a more extensive manner than would be normal. The use of pin mills, de-branning equipment and double roller mills can also result in compact layouts, thus minimizing the space required to house the process.

The building layout is principally determined by the equipment configuration but is also influenced by external factors such as the size of the available site, existing buildings, planning regulations and even capital available. Concrete buildings integrating raw material and product storage are popular. They have numerous advantages including space efficiency and good thermal and hygiene characteristics. However, they have the major disadvantage of high capital cost. Steel frame buildings are popular in feed milling and there are examples of flour mills housed in steel frames. Cost, speed of erection and flexibility of design are the hallmark advantages of this type of construction.


The operator, as opposed to the owner, will have specific wishes and opinions regarding the plant, and these must be taken seriously. The operator will probably have experience with the raw materials that the future project will use and also with the requirements of customers in terms of product characteristics. In essence, experience should not be ignored. This can give valuable insight into factors that are not obvious to the owner or designer.

An operator can have significant insight into how a plant should be controlled. This should be taken into consideration because it is an area fraught with potential pitfalls. While complete automation of any plant is possible, this is not always a sensible or practical option. Automation is notoriously expensive and in many environments there is a negative payback involved. While automated plants are as flexible as non-automated plants in the global sense, it can be difficult to customize these plants for minor products. This is highlighted by examples of milling groups where highly automated plants are used to produce mainstream products while custom products are produced by non-automated plants. Thus automation must be carefully considered and adopted with due consideration for its usefulness.

Other aspects of technological advancement have more tangible benefits. These have served to improve efficiency and make construction more cost effective. Examples are high capacity roller mills and large section plansifters. There is always the risk, however, of adopting technology that is unproven or which has no real benefit — technology for technology’s sake. While the automation and machine evolution discussed above are obviously areas where technology advancement has had an impact, they are also the areas where obsolescence occurs the quickest. It is therefore advisable not to invest too heavily in these areas, particularly where payback criteria are borderline.


Ultimately mill design and construction projects must have a payback period. This may be an extended timeframe where strategic goals are being pursued, but reasonable timeframes are expected in developed markets and open economies. New mills are more efficient and have greater potential than old mills. They can be better places to work in and can produce a product that is technologically better and ‘safer.’ However, in the absence of sound management and technological expertise, the result can be disastrous. In other words, it is possible for new projects to fail.


For more information, contact Dr. Owens.