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Gentle, fast and economical

Mixers and vacuum dryers for all bulk materials
Gentle, fast and economical

In most conventional mixing systems the tools are required to plough through the entire product volume. This leads to excessive stress on the products. When the equipment is used for drying, this effect is enhanced still further. Economical mixers and vacuum dryers, in which only part of the product participates in the process, are produced by a Swiss manu-facturer.

The mixers and vacuum dryers are made of various grades of stainless steel, according to the required resistance to chemicals and abrasion. The production models are manufactured in standard sizes with net capacities ranging from 100 to 10,000 l. A scale-up to larger sizes is possible. Laboratory units holding 25 and 50 l are also available.

The mixing vessel is designed as an upright, downward-tapered cone equipped in the case of the dryers with a double shell. In the centre of the vessel is a removable tube, with the mixing cone attached to its upper end. The products are picked up at the lowest point of the vessel by a vertical screw conveyor and moved through the central tube to the mixing cone (Fig. 2). The cone functions as the actual mixing centre, where the tools that are fixed to the driving shaft mix the product components both axially and radially.
The continuous upward flow from the screw conveyor pushes the product over the edge of the mixing cone. It gradually moves downward along the outer wall of the vessel, to be picked up again at the bottom and returned to the mixing process. The design of the screw conveyor is such that, irrespective of the size of the vessel, the entire product volume is turned over once every 1.5 to 2 minutes.
The product is heated up in the dryers as it slides down the wall. The drying energy is supplied by a standard heat-transfer medium, such as water, steam or oil. The mixing cone and the central tube can optionally be fitted with a double shell. This nearly doubles the heat-transfer area and substantially reduces the drying time. The process room is designed for a pressure range of 1 mbar to 2 bar, and the double shell for a pressure of 3 bar. The optional halfpipe coil jacket for steam heating is made for pressures up to 10 bar.
Easy cleaning and inspection
The torospherical lid is fixed to the vessel with a dismounting flange, which allows the lid to be lifted with all its attachments. The larger units are supplied with the lid welded to the vessel and are equipped with a manhole with a quick-closing lid. A variable electric drive unit and the drive axle are fixed to the lid. Screw-conveyor and mixing tools are attached to the drive axle with a standard-size DIN flange. The lid further contains ports for charging, for the air exit or an exhaust filter and an inspection window. On the dryers, the flanges for the heat-transfer medium and a temperature reading port are attached to the conical shell of the vessel. Thanks to their simple construction, the mixers and dryers are easy to clean and maintain. There are no dead corners. If space permits, the lid with its attachments can be lifted or the mixing vessel can be lowered for cleaning. In the larger models the central tube and the mixing cone can be furnished with hinges, so that the conveyor screw becomes accessible for cleaning. A lower bearing is not necessary, because the double-threaded screw conveyor is self-centring. Thus, apart from the static seals of the ports and the discharging valve, only one rotary shaft seal is required at the entry of the drive axle into the process room. The mixers are normally supplied with a lip seal, whereas the vacuum dryers are furnished with a floating ring seal or special sealing systems, according to customers’ requirements. The product flow for loading and discharging is completely vertical. Loading can be effected manually, by gravity or by mechanical or pneumatic charging systems. The absence of a lower bearing is a major advantage for the discharging process. The conveyor screw extends to the bottom of the conical vessel and moves the product until it leaves this vessel, thus facilitating complete emptying. Discharging is normally controlled by a pneumatic sliding valve. Various kinds of dosing equipment can be attached directly to the valve.
The drive unit’s energy consumption depends on the material to be mixed and is only about half that of conventional cone mixers. Unlike cone mixers with a rotating screw, which mills the products between it and the outer wall, these mixers handle the products very carefully. This is not only reflected by the reduced attrition: the products also do not stick to the outer wall, and the vessel is emptied cleanly. Gentle product handling is also advantageous for mixing abrasive materials. It considerably reduces the usual wear on the mixing tools and the vessel. The only parts subject to wear are the valve and the drive shaft seals.
Compliance with GMP regulations
The simplicity of the mixer’s construction also facilitates compliance with GMP regulations. No lubricant from bearings in the process room is able to contaminate the products. In the optional GMP package, the interior attachments, e.g. for the mixing cone, are either fixed with smooth angles or, if they need to be removable, made with extra-wide joints to facilitate cleaning. The mixing tools and the conveyor screw are welded and ground to produce joints with a wide radius, so that they are also easy to clean. A CIP cleaning system is available as a further option. The mixers are very versatile. At low rotational speeds, and with appropriately designed mixing tools, the products are handled extremely gently. Even sensitive granulates show very little attrition. At higher speeds however, and with a more aggressive tool design, the mixer easily crushes lumps and aggregates. Spraying nozzles can be positioned on the lid at the fringe of the mixing cone. They allow liquids to be injected for dust binding or coating.
The mixing time depends on the products and the mixing ratios and is normally 10 to 15 minutes.
Mixing trials
A mixing trial with finely ground calcium carbonate can be cited as an example (Fig. 3). 1% magnesium oxide was added in one dose. The particle size of both products was mainly below 100 mm. Samples of 10 ml were taken once every minute over a total period of 15 minutes, while the mixer was running. The magnesium content of the samples was determined by atomic absorption spectrometry and compared to a precisely weighed reference sample. The evaluation shows that the precalculated value, and thus a homogenous mixture, was obtained after 11 minutes. The values remained within the 3% error margin for the method. The results of mixing trials with other powdery, crystalline and granular substances are all within the same time range of 10 to 15 minutes.
It is not possible to specify a generally valid drying time for vacuum dryers. This time depends on the properties of the solvent and its bond to the products. Just a few examples for this vacuum dryer: it took about 2 h at 80 °C to reduce the content of ethyl acetate in a sample of shredded PVC from 13% to 0.8%. Within the same time, but at a temperature of 70 °C, the monomer content in a polymer substrate was reduced from 0.1% to 0.01%. Alternatively, the water content in an organic sample was reduced from 14% to 4% at 65 °C. On the other hand, a drying time of 12 h was required to completely remove 2 to 3% water from an acrylic polymer, even though product temperature was allowed to reach 90 °C. These drying times can be reduced even further with a dryer equipped with a heated mixing cone and central tube. These mixers and dryers are suitable for all bulk materials which are pourable, from fine powders to particles in the two-digit millimetre range, and from bulk densities of around 100 g/l to several kilos per litre in the case of metal powders. The mixers are ideal for abrasion-sensitive products as well as for highly abrasive components. The dryers are suited to applications requiring high vacuum and long drying times, e.g. for removing residual solvents. The limitations of these units are reached when it comes to handling wet, sticky or pasty materials that are not picked up by the central conveyor screw.
Mitroba
Fax: ++41/61/425-95 99
Further information cpp 241
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