Technical Profile: the Discmill

by Emily Wilson
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The Discmill is steadily gaining acceptance in cereal milling applications around the world. The latest model of this machine was introduced to the U.S. market by United Milling Systems, at the Association of Operative Millers’ 2002 conference last May in St. Paul, Minnesota.

The Discmill is particularly suited to applications in which a combination of a high work-rate and controllability is required. In addition, the Discmill can be used where the feed material has not been graded and redressed to remove flour, a situation in which a roller mill could not be contemplated.

The patented Discmill is designed for a range of size-reduction applications. Its main feature is very high productivity, up to 3,000 kilograms per hour, in relation to its size and energy consumption.

The finished product granulation can be controlled by the choice of corrugations on the grinding elements; selection of the peripheral speed of the rotating disc; and the ability to adjust the gap between the discs while in operation.

The Discmill model MHA600S is equipped with a single row of grinding elements, whereas the model MHA600D has two rows for 80% more grinding surface.

Discmill operation

A regular and constant flow of product enters the inlet (A, see illustration), located at the top of the Discmill, and travels through the center of the stationary disc (B) into the milling chamber (C).

The rotating disc (D), mounted on the center shaft (E), is equipped with vanes that accelerate and evenly distribute the product over the grinding surface of the two discs.

The distance between the discs can be accurately adjusted during operation by means of a hand wheel, allowing the desired finished product granulation to be achieved. No integral screen is employed, and in many applications, subsequent sifting is also unnecessary.

The speed of the rotating disc is selected to suit the application, as is the corrugation pattern on the exchangeable grinding elements, which are made from a wolfram carbide alloy that has exceptional wear resistance.

The ground product is discharged via the outlet (F).

It is recommended that the milling chamber be kept under slight negative pressure, either from a suction pneumatic conveying line connected to the outlet or from an exhaust system.

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Applications

In rye mills, the Discmill is being employed in place of roller mills with great success. The simplicity of the Discmill, its ease of operation and its compact size has attracted the attention of rye millers in Germany and Scandinavia. At one German rye mill, Discmills completely replace roller mills as the principal machine for grinding.

The following flow diagram shows how Discmills are employed in a mill with 80 tonnes per day capacity, providing a simple but fully effective system that meets demanding quality requirements.

WHEAT MILLING

In the wheat milling industry, the Discmill has for many years been used to increase the mill capacity by supplementing the first break passage. This approach continues to be a popular solution where there is limited space and when minimal disruption to the existing mill is important.

Each Discmill typically adds 48 tonnes per day to the mill capacity. In some cases it has been unnecessary to add extra sifting capacity, while in others, the so-called "bobtail unit" incorporates dedicated sifting and pre-crushing roller mills, allowing the production of separate specialty flours while also supplementing the capacity of the host mill.

This flow diagram shows a typical application in which the mill capacity has been increased by 96 tonnes per day.

In several mills the Discmill has been successfully employed to replace roller mills on certain passages in the reduction system.

In one Swedish mill, the capacity was significantly increased when a Discmill replaced the first reduction roller mill (1 Midds), and that roller mill was reallocated to supplement the fine reduction system.

The Discmill will achieve a flour release of 55-60% when operating at a capacity of 5,000 pounds per hour. This performance effectively replaces a full 10 x 40 roller mill and its attendant flake disrupters in a typical flourmill. After considering the cost, simplicity of operation and low maintenance, installing a Discmill has proved a significant breakthrough for many facilities.

In the case of the Swedish mill, as is the case in all mills so equipped, the flour quality was unaltered, although the extraction rate increased slightly. The Discmill does not degrade loose bran in the feed product any more than a roller mill, and therefore ash content is comparable. It is only when the feed product contains a lot of bran and endosperm agglomerates that there is danger of bran degradation and ash content increase.

This diagram shows how Discmills can be used in place of roller mills to simplify the reduction system.

The Discmill does not produce the same high levels of starch damage as a roller mill. However, by relieving the load on the reduction system, the Discmill can assist subsequent roller mills to achieve higher levels of starch damage. The end result can be an unchanged, or even increased, level of starch damage in the final flour.

Where a low level of starch damage is needed, such as in mills producing flour for starch production, the Discmill is a viable choice as a substitute for roller mills. One of Europe’s leading starch flour producers is adopting this very
approach.

Speciality flours, such as whole-wheat and brown flour, can be produced very efficiently on the Discmill. At a capacity of more than 3,000 pounds per hour, a Discmill type MHA600D will produce a whole-wheat flour with granulation of less than 5% passing over a No.18US mesh screen and with 85% passing through a No.40US mesh screen. By using a recycle or two-stage system, finer granulations can be obtained.

In the U.K., one of the millers using this system reports that finished product granulation never varies by more than 2% from the required customer specification.

CORN MILLING

In corn processing, the Discmill reduces whole corn to a full-fat meal product, either for direct consumption in some central and southern African countries, or to make composite flour for the Egyptian market.

In southern Africa, the Discmill is being used to grind degermed corn into sophisticated low fat products.

This final diagram (top right) shows a typical corn milling system.

Rollermill vs. Discmill

Rollermills should be used on those applications for which they are best suited: they have the ability to precisely scrape endosperm from bran and to selectively detach endosperm and bran agglomerates. Also, rollermills achieve very precise and narrow ranges of particle size.

For many of the straightforward grinding functions in the milling process, the level of sophisticated control and complex engineering of the rollermill is simply not needed.

The MHA600D Discmill occupies one half of the floor space of a 10 x 40 roller mill. In addition, it weighs about one third as much as a roller mill.

Its single motor is housed neatly and safely in its base, requiring no additional drive system. A single hand-controlled wheel controls the grinding gap and the discs holding the grinding elements can be replaced in a matter of minutes. Element life is from 2 to 4 years, depending on the application.

The processed product can be collected by a pneumatic lift from the same floor as the Discmill, so that it is also practical to locate the Discmill on the ground floor of the mill.

 

Flow diagram: Rye Mill

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Flow diagram: Wheat Mill

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Flow diagram: Wheat Mill

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Flow diagram: Corn Mill

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John Schofield is Managing Director of United Milling Systems.

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