Bulk material processors in the fine chemicals industry are increasingly resorting to contract mixing services that combine extensive know-how with otherwise unachievable flexibility. This applies equally to the form of the delivery containers and to the filling containers. However, the service provider can only guarantee the required high flexibility if it has the right equipment at its disposal.
Helmut Kaiser, Karl Horinger
When designing mixing and filling systems for contract mixers, where flexibility, rapid recipe changes and easy cleaning are essential, meticulous upfront planning is a top priority. Providers of contract mixing services have to switch their production process almost daily to adapt to new customer trends and requirements. Two basic kinds of conversion process are carried out by contract mixers in the fine chemicals industry: on the one hand simple conversion, where one container is transferred to another, and on the other hand conversion combined with refining – by far the more complex alternative. With the latter method, the raw materials have to be refined in accordance with a specification and then filled into delivery containers.
Flexibility in container delivery
A distinction is normally made between three kinds of delivery: sacks, Big Bags or – for large quantities – tankers. The contract mixer provides the necessary equipment for picking up and feeding the product into the processing system. Pick-up hopper stations made by Azo, in which the sacks are emptied into a closed pneumatic conveying system, can be used for this purpose, for example. Little or no dust is created in the process. Filter and aspiration systems protect both the operator and the product. As large quantities are sometimes supplied in sacks, it makes sense to use fully automatic Azo sack emptying systems in the product discharge area. These systems can open sacks on pallets and empty them with no residue. To ensure that no impurities get into the automatic emptying machines, cyclone screeners are employed for controlled screening. This is an efficient way to prevent scraps of paper or foil and agglomerated product from reaching the next processing stage.
Dumping systems are also available from Azo for raw materials delivered in Big Bags. Big Bags delivered on pallets are transported to a Big Bag dumping station, where they are lifted and attached to the docking system. The double ring design of the docking system prevents dust from escaping when the Big Bags are docked onto the closed conveyor system. The bags can be either completely emptied or assigned as buffer containers.
External silos that can be filled from tankers are useful for large quantities of clay minerals, such as silicic acid, bentonite, kaolin, silica or calcite. These silos are equipped with suitable filters, level indicators and a discharge system such as aeration bottoms. To ensure that the throughput and fill level are recorded precisely, they also have sensors that allow the quantity delivered to be checked at any time and the stock quantity monitored.
If one container has to be transferred to another, the bulk material is conveyed from the pick-up points to packing silos via piping systems with automatic two-way pipe diverters. These silos act as buffer tanks with very precise dosing mechanisms installed downstream. The Big Bags are then pre-weighed and filled here. Alternatively, the same filling systems can also fill the Big Bags to an exact weight.
Mixing and refining processes
Refining in combination with mixing is obviously a much more complex process for a contract mixer. The raw materials are given defined properties by adding liquids or other additives in the mixing systems. Suction weighing systems fill the mobile containers from the mixers. Their size depends on the throughput. This also means that the weighing systems have to be exactly adjusted to the required weight range, so that there are small suction weighing systems for smaller quantities offering a weighing precision of 50 to 100 g. Large systems weigh with 100 to 200 g precision.
If necessary, special additives can be manually added to the mixer by the operator using a terminal that doses any quantity right down to a single gram. Pick-up hopper stations equipped with appropriate discharge and dosing mechanisms are also used. They fill Azodos dosing systems, which then dose into mobile Componenter scales with very strict tolerances. Azodos is designed as a negative weighing system that records and documents precisely the quantity removed. The linear Compo-nenter allows adjacent mixers to be operated if the process so demands. The fully automatic, dust-free docking system merits particular attention here. As some of the mixing processes involve hot mix systems, special explosion protection measures must be taken. This is particularly the case with the filter and exhaust systems used to vent the mixers. Many recipes also require liquids to be weighed and metered precisely prior to mixing. They are supplied in drums and pumped into liquid weighing scales in a heated and insulated room, then passed to the mixing process.
The mix requested by the customer is prepared in a special mixing process according to the specification and the stipulated properties. The extremely homogeneous mixes can then be transported via a pneumatic conveyor system, which handles the prod-uct very carefully and can be adapted to the product in question. Packing silos remote from the mixing process are safely filled and the product is transferred to the filling process.
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