Since May 2019 Swissmill, the largest grain miller in Switzerland, is grinding extrudates of oatmeal or wheat bran with a Hosokawa Alpine Zirkoplex classifier mill in an energy-efficient, resource-saving and very flexible way. Especially with abrasive products the Zirkoplex classifier mill - a combination of a Turboplex classifier with a fine impact mill - guarantees customized fineness values and a high throughput.
Whether crusty bread, muesli flakes or biscuits, Swissmill ensures that many people can enjoy high-grade cereal products every day. The company, rich in tradition, with headquarters in Zurich has been in business since 1843. With over 200,000 tonnes of ground grain per year, Swissmill counts today as the biggest grain miller in Switzerland. They are well known for their high standards of quality, taste, technology and innovation.
To remain on this level, the company modernised its production and integrated a new extruder line and a new grinding system. "On the one hand we wanted to achieve a significantly higher throughput and thus generate more capacities. And on the other hand, we wanted to manufacture products with customized fineness values to match customer requirements", says Roland Eggli, Extrusion Manager at Swissmill. In pursuit of a solution, the Hosokawa Alpine 400 ZPS Zirkoplex classifier mill in pressure-compensated design was chosen.
Looking for an efficient grinding system for difficult feed materials
Previously different fractions had been produced via a screening machine after a beater mill. "The screens clogged up in a relatively short period of time, meaning that significantly lower quantities of the desired product were generated", explains Roland Eggli.
The main challenges: the system has to be able to grind abrasive products and has to fit furthermore into the limited space available. Due to its large grinding chamber, the Hosokawa Alpine Zirkoplex classifier mill ZPS is suitable for abrasive products and is well known for its flexibility in the product fineness. The classifier mill enables the production of powders with a precise top cut and steep particle size distribution. Therefore, Swissmill decided to extend the process with a classifier mill. "What convinced us, was ultimately the competent consulting provided by the sales and technology departments and the entirely positive grinding tests in Hosokawa Alpine's application testing centre," says Roland Eggli.
Hosokawa Alpine has its own application testing centre in Augsburg equipped with over 60 machines, which provides the customers with the possibility of having trials carried out, whose aim is to find the right system configuration and determining the process-technological warranty parameters.
Ultrafine grinding of extrudates
In a first step, the feed material is charged pneumatically to the supply bin of the grinding system. The feed material is then transferred using an inclined screw conveyor and dosed to a rotary valve. The rotary valve upstream of the mill fulfills a barrier function.
This results in an explosion decoupling effect before the feed material is charged to the ZPS. Air is supplied to the classifier mill via a side socket underneath the grinding track. This ensures that the product is transported from the grinding track to the classifier and simultaneously has a cooling effect on the processed material.
The integrated classifier produces a product with a precise top cut and a steep particle size distribution without spatter grain. Particles of the desired final size are discharged immediately without putting an unnecessary strain on the grinding unit.
Particles that are too coarse are rejected by the classifying wheel and returned to the grinding elements in the lower part of the mill housing for final comminution. The particles are comminuted until they reach the specified fineness. Only then can they pass through the classifying wheel for collection in the automatic bag filter and being discharged via the rotary valve.
The important thing is that in terms of dust explosion risks, the fine end product is considered an ST1 product. The grinding system is therefore in pressure-relieved design. To this end, a flameless explosion-venting valve was installed. In the event of an explosion, the integrated rupture disc bursts as the result of the pressure increase and the pressure is vented without flames to the outside.
The project planning, automation and commissioning of the 400 ZPS were carried out by a team of process-technology and electro-engineering experts. Meanwhile the system has been installed at the historical site in Sihlquai, Zurich –in spite of a very limited space. As well the existing extruder line has to be integrated into the feed system.
"In this way, the classifier mill can be charged from either of the two extruder lines. This makes us more flexible in production", says Roland Eggli.
System far exceeds expectations
"We have achieved our objectives regarding quality and efficiency. And the system throughput exceeds all our expectations", emphasises Roland Eggli.
Put into figures: since commissioning in May 2019, Swissmill has profited from higher throughputs and saving operating hours and thus considerable personnel resources. "The entire project handling was characterised by an extremely pleasant and solution-oriented atmosphere", says Jörg Krahnen, Operations Director Food Division at Hosokawa Alpine.