Filter media is a critical component of any filtration process and selecting the right type can be highly complex. Choosing the optimal filter media depends on the application, operating environment and the required performance of the filter media.
For instance, a mining application may require a tough filter cloth, which remains open for a long time but a chemical application, on the other hand needs the filter cloth to be tight, with low air permeability in order to provide clear filtrate and prevent loss of product.
Juha Höijer and Robert Grimm from Outotec spell out the various considerations involved in selecting the most appropriate filter media for industrial applications.
Filter cloth selection
Step 1: Select the material
Filter cloth material is available in two main options, polypropylene and polyester with other synthetic materials such as peek used for specialised applications.
Polypropylene is the most cost efficient material and can still result in excellent performance, provided the optimum fibre, weave and finish are selected. Peek is highly temperature resistant, the most expensive option and most suited to vacuum belt filters in special chemical industry processes.
Step 2: Select the fibre
Fibre options include monofilament, multifilament, film and spun yarn. Outotec MARO S60, for example, is a monofilament double layer cloth that combines excellent throughput and cake release, still producing clear filtrate. MARO S60 is mainly used in vacuum belt filters in hydrometallurgical applications.
- Monofilament fibres are available as a single, long continuous strand of synthetic fibre extruded in fairly coarse diameter and are good for cake or surface filtration
- Multifilament fibres comprise of several long monofilament fibres, twisted together to form one yarn, which is widely used in tower press cloths, especially as warp yarns
- Film yarns are strips of plastic with short length cuts twisted to form a yarn
- Spun or staple yarns are made from filaments cut into short lengths and twisted or spun together, and are used when tight cloth is needed
Step 3: Choosing the right weave pattern
- Plain weave is the simplest form of thread crossing, easy to clean with high elasticity
- Twill weave offers good cake release
- Satin weave offers high flow rate, very smooth filter cake side to ensure excellent cake release
- Double layer weave aids cake filtration, offers high strength, and are more expensive
Step 4: Cloth finish
Filter cloth selection also depends upon the type of finish on the cloth including washing, pre-shrinking, pre-stretching, calendaring and anti-static among others.
All Outotec OEM cloths are heat treated to prevent stretching. Further treatments are available for more demanding applications. Calendaring can ensure lower permeability and better cake release while heat treatment with stretching before the cloth is calendared results in a non-stretching cloth.
Outotec’s Aino T30, for example, is heat treated to ensure a particularly robust performance in mining applications. Calendaring the same cloth will make it tighter and suitable for applications such as pigment filtration.
Testwork is key
An experienced supplier should organise appropriate test filtration on the slurry, as without proper slurry preparation or getting the slurry from the right place in the process, the resulting filter cloth will, at best not deliver the optimum process results and at worst, is rendered unusable.
In slurry preparation, the slurry must be truly representative of the actual ‘real life’ process because factors such as slurry density play a major role along with pH, other chemistry and actual slurry temperature. During testwork the sample must also be fresh, thereby avoiding issues such as biological oxidisation, agglomeration and any further crystallisation.
OEM providers such as Outotec provide process optimisation services including optimising the cycle time for sites that need to increase capacity or reduce to lower cake moisture. Simple service checks such as reviewing feed-, pressing- and drying pressures will help extend the lifecycle of pressing diaphragms.
It is also advisable to get the filter cloth and filtration solutions from the same provider to ensure that the cloth is designed specifically for the filter technology, delivering truly optimised performance. One also benefits from a single supply chain for spare parts, flexible ordering of filter cloths, short delivery times and agreements with availability guarantees.
Additionally, filtration issues can be resolved more quickly if the cloth and filters are from the same supplier, thereby minimising downtime costs.
Variations in the application/process and the filter condition can significantly affect the life of the filter cloth as well as the economics of the operation and quality of the product. Proper cloth installation and machine maintenance are essential for optimum performance in all industries.
In mining, in the right conditions many thousands of cycles can be expected but upstream process variations, poor cloth washing or filter condition can limit the life to hundreds of cycles or a few days of operation.
It is common in chemical and pharmaceutical applications for cycle times to be tens of times longer than in mining and cloth life is often counted in months but equally if cloth wash nozzles are worn or blocked and the cloth does not get cleaned properly, the cloth may blind and require to be changed after a relatively short time.
It is important to ensure that the filter uses clean water with sufficient pressure (min 6 bar) for cloth washing and that the washing nozzles are open. It is also mandatory to use proper cloth tracking to prevent the cloth edges from fraying.
Economic and environmental benefits of the right filter cloth
There are significant economical and environmental benefits to be derived from efficient solid-liquid separation technology.
The right filters and filtration solutions are designed to achieve significant savings in energy and/or water consumption in most of processes. Efficient filtration also leads to better quality waste water, thus reducing the environmental impact.
Despite the multiple options in filter cloths, choosing the optimum filter media is much more of a science than a black art, requiring an exact and scientific approach that involves initial testwork, correct slurry preparation and working with an experienced, professional supplier.
Once these are in place, decisions such as material, fibre, weave and finish for the filter media can be easily and correctly made.
Juha Höijer is currently Sales Manager for Filter Cloths in Outotec (Filters) Service Product Center, Finland. Robert Grimm is currently Filtration Spare Parts Manager - Outotec South East Asia Pacific.