The pharmaceutical production of tablets is practically unthinkable without the use of a sieve or a mill. These are needed whenever active and inactive materials are handled and tablets or other products are being manufactured.

Sieving machines and mills are used for milling solids to reduce their size. This fulfils two purposes: Firstly, it enlarges the particle surface which increases the speed of dissolution. Secondly, it evens out differences between particle sizes to ensure that mixing can be homogeneous.

Due to their versatility, milling and sieving machines play a crucial role in pharmaceutical production. They can be integrated into complex production lines, for example for filling/emptying or transfer operations at process machines, or for filling and decanting containers.

Decisive factors for the use and functionality of mills and sieves include their flexibility and the fact that the equipment requires very little room. In particular, in pharmaceutical clean rooms where there is often not much space, the small footprint of these systems is a great benefit. In addition, certain sieving systems and mills are equipped with built-in lifting columns so that they can be integrated into almost any conceivable process scenario.

Another feature that increases the efficiency of the processes is the mobility of the machines. For use or for cleaning, they can be moved or even disassembled quickly and easily.

However, mills and sieves are subject to constant development and improvements. In addition to some current trends, this article presents an example of how the use of new hammer mills achieved an increase in production efficiency.

Conical sieving machine

Conical sieving machines are used for the uniform size reduction of pharmaceutical ingredients. In addition, they can be used for mixing, sieving, and dispersing. They are available in a wide variety of different sizes – from laboratory equipment to high-capacity systems.

Compared to other milling technologies available on the market, the sieve provides numerous benefits for production: It runs very quietly, produces a narrow distribution of particle sizes, and offers great design flexibility and higher capacity. This means that for the processing of low-density materials, a sieving system of this type can achieve a throughput that is 50 per cent higher than that of conventional mills.

The patented Turbo Sieve BTS made by LB Bohle Maschinen + Verfahren GmbH (Ennigerloh) is used for the calibration of particle sizes and thus guarantees an optimal particle size distribution. The use of various conical sieve inserts (hole sieves and friction sieves) supports the processing of both dry products and wet products at speeds of 150 to 1500 rpm (revolutions per minute). This makes it possible to achieve product capacities of two to three tonnes in one hour.

Operators work with an easy-to-use 7-inch touch panel that provides access to user management and recipe management as well as PDF batch print management and includes a USB interface. The explosion-proof version of the Turbo Sieve can also be used for applications where compliance with the ATEX directives is required.

The hygienic design of the sieve makes manual cleaning quick and convenient. The sieve container is easy to remove using the combination of bayonet mount and clamp ring. This makes it very easy to clean the individual components. As an option, the sieve housing can be equipped with cleaning nozzles for simple WIP cleaning where material residue is quickly removed from inside the sieve container. Cleaning nozzles on the sides of the sieve insert and below the angled drive are included in the scope of supply. They receive the cleaning fluid from a central water connection point.

Hammer mills

Hammer mills (also called turbo mills) are suitable for research and development as well as for batch production and continuous production. They are used for the size reduction of powders, agglomerates, and granulates. Hammer mills are used in particular if the application demands a precise particle reduction for hard-to-mill active ingredients and other substances. In addition, hammer mills are used to re-mill fragmented tablets.

Tablets that have already been produced undergo inspection and may fail to meet the customer’s requirements for various reasons. Such criteria include for example defects in their appearance, or not having the correct weight or hardness. If the tablet does not fulfil the specifications, the manufacturer can use the hammer mill to pulverise the tablets again and re-use the materials. The re-milling of the tablets and the recycling of the materials in production ultimately reduces the rejects rate and increases productivity.

Hammer mills such as the BTM series (Bohle Turbo Mill) by LB Bohle can reach speeds between 600 rpm and 6000 rpm and produce up to 1500 kg/h. In order to achieve this, the BTM is equipped with an automatic dispenser unit (rotary feeder). This allows the operator to feed the material into the milling chamber evenly and without overloading. In addition, such automatic feeder devices can control the powder flow in the milling chamber. Further benefits: Less heat is generated, and the process can be repeated using the same parameters.

The internal rotor has two sides with different functions that support applications with wet or dry products. The knife side mills sticky products. The hammer side is used for crushing hard, crystalline products.

In the hammer mill, the particle size is defined by the sieve size selected for the mill. The BTM provides different sieve inserts that can be used to reduce the material size to as little as 0.1 mm.

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