Rob O'Connell

Asieve or screener is an essential part of every pharmaceutical production process, particularly as product quality and integrity are so important. The use of a sieve gets rid of oversized contamination to ensure that ingredients and finished products are quality assured during production and before use or despatch. 			

However, the design of sieving equipment has undergone radical changes in recent years to satisfy the new demands of pharmaceutical manufacturing companies. These demands include improving productivity, product quality and most importantly, the health and safety of the operators. The latest sieve generation has greatly improved operator safety by effectively containing the powders being processed and, therefore, has adhered to occupational exposure limits (OELs).  

In basic terms, a sieve consists of a housing containing a removable wire mesh of a defined aperture size. This assembly is vibrated by an electric motor so that small particles can pass through the mesh apertures and any particles or contamination that are too big remain on the top. Most units used in the pharmaceutical industry tend to be circular and of a high-quality good manufacturing practice (GMP) design by ensuring accurate separation (Figure 1). Stainless steel mesh with a high tolerance on the apertures is also specified to give excellent product quality. 

 Figure 1 Most units used in the pharmaceutical industry tend to be circular and of a high-quality GMP design.

There are two main types of sieving — safety screening and grading. This article will concentrate on sieves used for safety screening, but will also provide a brief explanation of grading.  			

Safety screening, sometimes known as control sieving or security/check screening of powders, is done to ensure the correct product quality of powders. Basically, the sieve removes any oversized contamination from the powder, which could be something that has accidentally found its way into the process line (e.g. some packaging, a piece of personal protective equipment or extraneous particles that may be inherent in the material). Removal of this contamination improves the quality of the powder and final product and, therefore, safeguards the pharmaceutical company's reputation. 

Grading or sizing of powders or granules is done to separate different ranges of particle sizes. For example, primary and intermediates must be sieved to remove oversize and undersize particles to ensure a correct particle size distribution ready for granulation and subsequent tablet pressing. 			

Most pharmaceutical processes are hazard analysis and critical control point (HACCP) controlled. This means that a process analysis is done to determine the location of potential hazard areas. Critical control points are identified and some form of prevention is put in place. Sieving equipment will help to eliminate risk at any of the points where contamination could enter the process.  			

These critical control points are found in many different areas of the production process, including the following: 					

Where raw ingredients are debagged (because of the potential for parts of the bag to be accidentally introduced into the process).

Where mixing or blending takes place (because this is a potential area for contamination to enter the powder).

Sieves used for check screening are designed to be extremely simple to operate and maintain, with the emphasis on making them easy to strip down and clean effectively. Their compact design means that they can be placed in small or restricted height areas of the production process — possibly where a sieve was not originally deemed necessary, but is now essential. 							

The sieve mesh itself is removable so that its aperture size can be changed according to the powder being processed. Modern units use mesh that is securely bonded with adhesive to a frame, which gives a much higher tension in the mesh than older styles, which secured the frame with a clip or screws. Having a consistent and high tension level gives better throughputs and reduces blinding or blocking of the sieve apertures. Another recent development is the use of FDA-approved adhesive to bond the sieve mesh to the frame. 			

All other contact parts of the sieve are manufactured from stainless steel and can be polished to very low Ra (surface roughness) values to ensure good flow properties and easy cleaning. These components are simple to remove and wash in an autoclave or other cleaning vessel, therefore, any chance of cross contamination between different material batches is removed. 

Most powders can be screened quickly and accurately by a standard sieve, however, some pharmaceutical powders may be sticky or have irregular-shaped particles, which can cause mesh-blinding problems (Figure 2). The method of ultrasonically exciting the stainless steel mesh wires of a powder screening machine by high-frequency, low-amplitude vibration to prevent apertures blocking has been used for over 25 years.  

Figure 2 Some pharmaceutical powders may be sticky or have irregular shaped particles, which can cause mesh-blinding problems. Photo courtesy of G. Bopp & Co. Ltd, UK			

The ultrasonic frequency is applied to the sieve mesh via an acoustically developed transducer (Figure 3). This breaks down the surface tension, effectively making the stainless steel wires friction free and preventing particles, both slightly greater and smaller than the mesh, from blinding or blocking it. Screen blinding or blocking is a common problem in the sieving of difficult powders on screens of 500?μm and below. It occurs when either one or a combination of particles sits on or in an aperture of the mesh and stay there, or particles adhere to the mesh wires preventing other particles from passing through these openings. This is particularly common with sticky powders or materials that contain a large number of particles of a size similar to that of the apertures of the mesh. When blocking occurs, the useful screening area is reduced and, therefore, capacity drops.

Figure 3 To prevent mesh-blinding an ultrasonic frequency is applied to the sieve mesh via an acoustically developed transducer. 			

The system works on the power by demand (PBD) principle, which solves the problem of uneven loading. Constant feedback from the separator screen to the PBD controls monitors the throughput of material in the system. When there is a heavy loading on the sieve mesh, PBD increases power, maintaining the amplitude of the ultrasonics to pass materials through quickly and efficiently without blinding. 

There are several knock-on benefits of eliminating blinding via an ultrasonic deblinding device: 					

sieving capacities improve, which increases productivity

manual cleaning is less frequent, because the mesh is kept free from blockage for longer, which reduces the chance of damaging the mesh

powders can be sieved using meshes with smaller apertures, which enables even finer-quality products to be produced than previously possible or even to screen powders that could not be sieved.

Recent legislation has had a large effect on the design of sieving equipment. 			

On 1 March 1996, the European Community adopted a Directive on equipment and protective systems intended for use in potentially explosive atmospheres (94/9/EC). 'Atmosphères Explosibles' is more commonly referred to as the ATEX Directive. 

Its primary function is to eliminate the possibility of explosions in all industries (including food, metal powders, powder paint, pharmaceutical powders and chemicals) involving powders, dusts and vapours. Specifically, it applies to electrical and mechanical equipment intended for use in potentially explosive atmospheres.			

From July 2003 all new equipment purchased for installation and use in a potentially explosive atmosphere must comply with ATEX requirements. 

Design changes to sieving units are mainly centred on making sure that the unit is free of any potential sources of ignition. Therefore, it is essential to properly earth all components and, in the design of the equipment, remove all other possibilities of a spark or excessive heat generation. 

However, when an electrical component is in continuous contact with powder and dust during sieving, there is a further hazard of an explosion. The ultrasonic probe of the deblinding system must be made safe when it is placed inside the sieve (an area often categorized as Zone 20 and Zone 0). Some manufacturers of this equipment have addressed this by totally enclosing the transducer and cable to eliminate the possibility of any explosion. The equipment has to go through rigorous testing procedures and approved by certified bodies. Only then can it be deemed to meet essential health and safety requirements.  

This, in turn, allows difficult-to-sieve powders to be screened effectively and safely, and gives the user complete peace of mind. 

Employers have been using OELs for many years to safeguard their employees' health. They are used to determine the adequacy of the control measures and indicate if a problem ever occurs. This has forced manufacturers of process equipment to design machines that contain dust and fumes much more effectively —for sieving equipment this is especially important because the very action of a sieve vibrating causes dust generation. 			

Traditionally sieves have used either over-centre toggle clamps or circular band clamps to secure the component parts together. These are not ideal mechanisms for ensuring dust-tight operation as they rely on operators to tighten them correctly to ensure an adequate seal.

Figure 4 The latest generation of sieve addresses the issue of improved containment and is the biggest advance in pharmaceutical sieving technology for 10 years. 

The latest generation of sieve addresses this clamping issue (Figure 4). It utilizes a validatable pneumatic clamping system giving large improvements in product containment, and operator health and safety. The unit also complies with the new ATEX legislation.  

The GMP design of the sieve is based on clean lines, which makes sanitation easier and performance greater. Clean down times are reduced because the sieve is simple to disassemble in seconds without the need for tools. Crevice-free and smooth surfaces make the product contact parts very easy to clean. They are also fully washable. 

The unit is clamped together with a revolutionary airlock system. This pneumatic lock provides an even and high clamping force across all sealing faces and, therefore, guards against powder leakage more effectively than traditional band clamps or over-centre toggle clamps (Figure 5).  

Figure 5 This pneumatic lock gives an even and high clamping force across all sealing faces and, therefore, guards against powder leakage more effectively than traditional band clamps or over-centre toggle clamps.

The component parts are placed into the base and located by twisting and locking the lid without the need for tools. The air clamp is then inflated to secure and seal the unit during operation. Also, to assist with FDA process approval, this pneumatic clamping system can be validated because it provides a repeatable and measurable seal.

This new generation of sieve can be fitted neatly into any part of a production process, providing considerable screening capacity with little headroom. Its innovative design means that the screener is less than half the height of a traditional sieving machine. It is ideal for check screening incoming ingredients through to the finished powders.

Sieves or screeners continue play a large part in the safe production of pharmaceutical products. However, it is important that companies using this equipment choose it carefully so that it complies with the new ATEX legislation, and safeguards the health and safety of their operators. 				

 is the marketing manager at Russell Finex Ltd, UK.

Articles

Asieve or screener is an essential part of every pharmaceutical production process, particularly as product quality and integrity are so important. The use of a sieve gets rid of oversized contamination to ensure that ingredients and finished products are quality assured during production and before use or despatch.