Regular cleaning of filter cloths ensures that quality filter cakes are being produced by each press cycle.
One of the areas where mining companies are focusing on, as part of their thrust to enhance efficiency as cost effectively as possible in mineral processing, is solid-liquid separation in sludge dewatering. In solid liquid separation using filter presses, the ultimate objective is to achieve clear filtration, dry filter cake and shorter filter cycle in the dewatering process. And ound preventative maintenance of filter cloths in the filter presses is one of the avenues through which they can meet this objective. Usually, in a filter press cycle, the state of the filter cloth has an impact on the quality of the filter cakes produced.
High wear rate in mining filter cloth
Preventative maintenance of filter cloth is twice is important in mining than in industrial wastewater treatment plants, as in mining, a filter cloth lasts half the time as it does in industrial wastewater treatment plants. This is one of the observations that Bob Bosgraaf from Evoqua Water Technologies, a US-based filter press supplier, makes when discussing Four factors that impact on filter cloth life span. “In typical industrial wastewater treatment applications, filter cloths typically last 750 to 1,000 cycles (750-1,000 hours of filtration time). However, in the mining industry, filter cloths typically last 4,000 to 5,000 cycles (400-500 hours of filtration time),” he points out.
Bosgraaf single out abrasion, velocity, oxidation and heat wear out filter cloths as common causes of filter cloth wear. In some way each of these can impact on the performance of a filter cloth.
The shape, hardness and structure of the solid particles being filtered can impact cloth life. Irregularly shaped particles with sharp edges can cut the small fibers that make up the fabric of a filter cloth.
Pertaining to mining, abrasion can have a bigger impact in mining, Bosgraaf notes. “Abrasion is a great concern when it comes to filter cloths used in the mining industry since many of the particles being filtered through the dewatering systems are highly abrasive. This is less of a concern in general industrial wastewater treatment since the particles filtered by the dewatering systems are mildly abrasive or not abrasive at all.”
The velocity that filtrate flows through a cloth can impact the filter cloth’s lifespan, particularly at the beginning of the filtration cycle when fine particulate passes through the cloth openings. High velocity and abrasive particulates (as seen in the mining industry) can cause filter cloth failures, especially around feed and filtrate inlet and outlets.
Most filter cloth fibers are made from polypropylene, which can be deteriorated by strong oxidising agents. Oxidising agents include chemicals like chlorine, bromine, fluorine, hypochlorites and peroxides. The speed at which a filter cloth deteriorates depends on the strength of the agent and the length of time it is in contact with the cloth.
UV radiation also impacts the lifespan of filter cloths. Prolonged exposure to sunlight can cause cloth fibers to become stiff and prone to tearing.
The high temperatures encountered in some process applications can lead to premature failure of the filter cloth material because of oxidation. This process of heat aging and deterioration is known as thermal degradation.
- Routine cleaning and washing
These four factors can hinder the performance of a filter press, leading to poor quality filter cake being produced by each press cycle in the dewatering process. Thus, a critical element of preventative maintenance of filter clothes should be routine cleaning and replacement, as and when necessary, based on the specific operating conditions.
How to choose the right filter cloth for your dewatering process
Filter cloths come in a wide variety of materials and styles. Using the correct type of filter cloth is crucial to having a well-running filter press and successful dewatering system. Using a cloth that has a tight weave might restrict the filtration rate. On the flip side, using one that is too porous will cause too many solids to pass through.
When you are choosing a new filter cloth, conducting laboratory testing should be the first step in the process. You can also keep an eye out for and evaluate a cloth based on the following characteristics:
- Material of construction
One of the most widely used filter cloths is Polypropylene. It is extremely sturdy and can withstand most dewatering processes. However, for applications where less corrosive slurry is being dewatered, cloths made of polyester, nylon (and in rare instances, cotton) can also be used.
The weave of a filter cloth’s fabric can have a big impact on many aspects of the dewatering process including cake release and cloth blinding. It is important to choose a weave that will perform. The types of filter cloth weaves include plain weave, satin and twill
- More pliable and stronger than a plain weave
- Typically used for backing (support) cloth and basic filtration applications
Filter cloths can also be made using felted fabric. Most felted cloths are used in abrasive applications like dewatering mineral concentrates.
- Type of fibres
There are 3 primary types of fibres used to create filter cloths: monofilament, multifilament and spun filament. The cloths can be made exclusively of one type of fiber or a mix. One of the most widely used filter cloths uses a monofilament warp (the vertical fibres) and a multifilament weft (the horizontal fibres).
So, why is the type of fibre used in the filter cloth so important? The type of fibre impacts the strength of the cloth and what type of applications it can be used in. The stronger the cloth, the more abrasive slurry it can withstand.
- Thread count
Thread count isn’t just something to consider when selecting bed sheets or towels, but also when selecting filter cloths. The thread count of a cloth directly impacts the particle retention of the cloth. A cloth that has a low thread count will have larger openings, allowing more particles to pass through it; compared to a cloth with a high thread count, which will have smaller openings and allow fewer particles through.
Credits: Evoqua Water Technologies