Homepage » Pharma »

A fluid way to a perfect particle size

More safety and flexibility through wet grinding
A fluid way to a perfect particle size

Dry grinding of organic substances requires elaborate exhaust and filter fans that are meant to avert the danger of dust explosions. Wet grinding processes offer an efficient alternative and are able to achieve even very small particle sizes. Heavy-duty dispersing machines such as the Dispax DR 2000 and DRS 2000 Reactors boast various advantages: closed systems, high shearing forces, minimal maintenance requirements and, last but not least, a high level of safety.

Christof Maier

Whether dealing with a solution or a suspended solid – the synthesis of medical substances is always followed by processing and grinding. If the medication has a defined dosage form, optimal mixing of two liquids – or of one liquid with a solid – is a top priority. The medicinal substance must be evenly distributed within the recipe, independently of the grinding process. Closed systems are clearly more advantageous than open systems, even in this phase. Dosing and product feeding can be easily and precisely controlled with inline systems such as the IKA Dispax Reactors. Due to the risk of toxicity, it is not only imperative to use filter and exhaust fans during dry grinding; there are also strict safety regulations to comply with, such as the Atex directive. Atex is intended to protect people whose work involves a danger of explosion. Costly safety measures are no longer necessary for wet grinding in closed inline devices because the grinding material is fed to the device via pipes or pumping systems. This continuous process enables the applicable mixture volume to be precisely controlled.
Since the same particle size is not needed for every substance, the grinding performance must be adaptable to the desired size. Some substances have very specific material properties that require individual treatment. Dry mills are only partially suitable for vastly fluctuating demand.
The entire mill head often has to be switched off, if a complex mixing result is required. Wet mills can easily adapt to changing processes and demand. The speed range of the rotor-stator grinder can be extended with a frequency inverter. Smaller particle sizes can also be achieved by increasing the number of passes. In the event that even these small sizes are not sufficient, simply switching off the tools helps to achieve the desired mixing efficiency.
High energy input
A high energy input is necessary for the successful dispersion of liquids and solids. High-power rotor-stator systems are particularly suitable for this challenging task. Rotation is another key feature of the Dispax systems for achieving small particle sizes. Here, the matter of the mixtures is accelerated with a high level of kinetic energy input in a radial flow channel between the rotor and the stator. The shearing action produced by this rotation can be compared to stretching. The stator’s exhaust ports generate additional pressure differences and whirling. The high crushing dynamics enable particle sizes of just a few micrometres.
The material properties of the products to be dispersed determine the achievable particle size. The hardness and structure of the solid and the viscosity of the liquid play a major role here, along with the solid content and the temperature. Matter accelera-tion, expressed as part of the circumferential speed, is one important parameter. The mechanical and geometric design of the rotor-stator system – which includes the number of tools and toothed rings as well as their specific geometry – also has an effect on the mixing results. The geometry is particularly crucial when it comes to grinding solids, as it makes a significant contribution to optimal results. It is not pos-sible simply to set all these variables arbi-trarily. Mechanical barriers such as resistance, bearing clearance, critical speeds and the generation of turbulence limit the performance of the rotor-stator system. As a result, the particle sizes cannot be reduced infinitely. Increasing the number of passes is a popular way to optimise the dispersion result.
Wet grinding processes facilitate a high output and the option of processing very large source materials without prior size reduction – something that comes in very handy during production.
Multiple applications
Amongst other things, wet grinding can be used for particle size control. Techniques such as precipitation are used to clean the substances. Integrating wet grinding into this precipitation process allows the particle size distribution to be influenced directly. Wet grinding prevents the formation of oversized particles and the substances remain part of the process.
Wet grinding can also take place immediately before drying, for example prior to a spray drying process. The upstream process produces fine powders directly in the drying tower, as individual particles dry quicker than agglomerates. Thanks to the defined, narrow particle size distribution, the dryer’s working point can be made more precise. This can dramatically improve the efficiency of existing equipment.
A third example concerns the use of wet grinding as a reaction accelerator. Many chemical reactions occur at an interface. In the event that this interface lies between two liquids, the Dispax machines have a significant influence on the reaction speed and intensity. This is especially important when it comes to precipitation reactions. A mass transport and exchange take place at the interface due to the intensive mixing in the mixing chamber. The positive effect of rotor-stator machines on phase transfer limited reactions has been widely described and confirmed in the industry.
Even if liquids with very different levels of viscosity need to be mixed, wet mills represent a clear improvement owing to the rotor-stator principle. This improvement is reflected in an enormously reduced mixing time, for example. If a chemical reaction consists of two educts of varying viscosity levels in one phase, these can frequently be the cause of a low reaction speed. Wet mills help reduce the differences in viscosity and thus increase the reaction rate. Even with high system loads, rotor-stator systems achieve better results than other process technologies.
Emissions management
Most highly active substances are toxic in high concentrations. This is why the release of dust must be prevented by means of a suitable containment concept during dry grinding, for example. Every dry grinding process produces fine dust. Time and cost-intensive filtering and cleaning are neces-sary to protect people and the atmosphere from contamination as a result of toxic emissions. However, not only the environment but also the mixing devices themselves must be thoroughly cleaned so that no residues remain, or cross-contaminations may occur with substances from downstream grinding processes. The pharmaceutical industry therefore makes very tough demands when it comes to hygiene. The machines used to make pharmaceutical products must be CIP and SIP capable. The Dispax Reactor system was developed with hygienic pharmaceutical design criteria in mind and is based on EHEDG and FDA guidelines. It is distinguished by its extremely compact dimensions. Due to the design, the surfaces to be cleaned are significantly smaller than with traditional wet grinding units. IKA’s dispersing machines conform to a broad spectrum of specifications in the pharmaceutical industry, such as surface certificates or raw material certificates for all product-contacting component parts. Since the inline dispersing machines function as closed systems, active emissions management is not essential for wet grinding. In addition, potentially toxic substances are bound in a liquid during the grinding process, so that they are prevented from being released.
The production of pharmaceutically active substances is very expensive. The compact design of IKA’s dispersing units has two great economic advantages: firstly, it minimises material losses during processing and secondly, it is extremely convenient as far as cleaning and maintenance are concerned. The Dispax Reactors can be sterilised with steam, for instance. The wet grinding process itself provides good operating efficiency, as the high-performance dispersers often achieve the most common particle sizes in a single pass.
All Whitepaper

All whitepapers of our industry pages

Current Whitepaper

New filtration technology for highly corrosive media


Industrie.de Infoservice
Vielen Dank für Ihre Bestellung!
Sie erhalten in Kürze eine Bestätigung per E-Mail.
Von Ihnen ausgesucht:
Weitere Informationen gewünscht?
Einfach neue Dokumente auswählen
und zuletzt Adresse eingeben.
Wie funktioniert der Industrie.de Infoservice?
Zur Hilfeseite »
Ihre Adresse:














Die Konradin Verlag Robert Kohlhammer GmbH erhebt, verarbeitet und nutzt die Daten, die der Nutzer bei der Registrierung zum Industrie.de Infoservice freiwillig zur Verfügung stellt, zum Zwecke der Erfüllung dieses Nutzungsverhältnisses. Der Nutzer erhält damit Zugang zu den Dokumenten des Industrie.de Infoservice.
AGB
datenschutz-online@konradin.de