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What is a Dewatering Filter?

 Most dewatering filter presses are sometimes called "plate-and-frame filters," but this term accurately describes only a certain style of filter plates in wide use during the mid- to late-1800s. Although true plate-and-frame filters are still employed today, the most commonly used system of plates in modern dewatering operations are more accurately called "recessed chamber” and/or “diaphragm (membrane) plates."

Filter presses are "fixed volume” or “batch pressure” systems. A fixed volume filter is one that holds a specific maximum quantity of filtered solids. The word "batch" refers to the fact the filtering cycle must be periodically stopped to remove the “filter cake” of collected solids. “Pressing” is actually a misnomer since the filter only presses together enough keep the plate stack closed to prevent leakage. The pressure in the system actually comes from the pump that feeds the filter with the material to be dewatered.

The filter press can also be employed as a "polishing" filter intended to remove very small concentrations of solids from an influent stream. When used for polishing, the press size is chosen to provide optimal filtration area and hydraulic throughput. In this kind of operation, a filter cake usually does not accumulate. Instead, the decrease in the throughput flow rate is monitored and the filtering cycle is ended when the rate sinks to an unacceptably low level.

The third of the most common uses for a filter press is as a "variable volume" filter. This requires the use of a type of filter plate, known as a diaphragm or membrane plate, that has a flexible drain-field. When it’s sealed around the edges, the membrane plate forms an integral diaphragm or bladder that can be inflated, which presses additional liquid from the filter cake. This significantly reduces the time required for a press cycle and also results in a drier filter cake; or, the process can be manipulated to produce more uniform cake dryness from cycle to cycle.

Components

A filter press has two main parts, the skeleton and the filter pack. All filter presses, whether of the sidebar or overhead plate suspension types, and regardless of any other design differences, include these two main components.

The skeleton holds the filter pack together against the internal pressure created during the filtration process. Different manufacturers may use different terminology to describe the parts of the skeleton, but in all makes the sub-components are essentially the same and consist of:

These components will connected by either sidebars or overhead beams.

The actual separation of solids from liquid takes place in the filter pack, which consists of a series of filter elements that form a series of chambers when held together by the skeleton. The walls of each chamber have a set of raised cylinders, called "pips," which are covered with a porous cloth medium.

The pips form a flow path for the liquid that drains from the filter press. At the corners of each drain-field, a series of interconnecting holes connect the drain-field to four discharge ports at the corners. When the filter plates are joined together in a plate pack, the corner discharge eyes form individual manifolds that connect the drain-fields of the plates to filtration system pipes located outside of the press. The center feed slurry inlet port (corner feed inlet ports also exist, but are less common) also forms a manifold connecting to the various chambers within the plate pack.

How the Dewatering Process Works

During the dewatering process, slurry is pumped into the filter press through the manifold located on the stationary head. As the slurry fills each chamber of the press, the liquid component passes through the cloth medium and across the drain-field, then through the drain ports before exiting the chamber through the corner discharge eyes under the force of gravity.

The main function of the filtering media is to provide a supportive framework for development of the filter cake. Some solid particles may pass through the cloth media at first, which can result in a slightly turbid filtrate initially. During the cycle, the larger particles in the slurry begin to gradually bridge the openings in the filter media, reducing the size of the openings. Smaller particles then bridge these reduced openings, allowing the filter cake to form. When an accumulation of solid particles 1 to 2 mm thick builds up, forming what is called a "precoat" layer, finer and finer particles are separated from the incoming slurry, resulting in a filtrate that’s very low in turbidity.

A feed pump, which is sometimes a positive displacement or centrifugal pump, provides the pressure that forces the slurry into the filter press. A typical operating pressure is 100 psi, but pressures can run up to 900 psi (7 to 60 bar). A pressure differential arises across the media and the growing filter cake between the slurry feed and the gravity discharge. This pressure differential, in concert with the feed pump pressure, is what causes the filtering action to take place. Slurry solids naturally flow to the areas of the developing filter cake where the pressure differential is lowest, so that the filter cake accumulates uniformly over the drain-field on both walls of each filter chamber.

The cycle continues until the filter cakes forming on either wall of each filter chamber meet and the press is completely filled with solids. The hydraulic closure of the press is then retracted and the filter elements are separated to discharge the filter cakes, usually just by gravity, to a collection receptacle.

There are many applications for filter presses in both the industrial and municipal sectors. From wineries to refineries, the rule is that any slurry containing solid particles can be dewatered. Filtronics is a preferred provider of high-quality Grupo TEFSA filter presses. TEFSA has optimal solutions for specific dewatering applications. The product line includes overhead filter presses, sidebar filter presses, high yield filter presses and belt filter presses. We offer a complete after-sale service program to keep your press running efficiently.

All TEFSA presses have the following characteristics:

To take the first step, simply complete our General Mineral Analysis (GMA) form, which will tell us your water quality and site profile. From that, we will be able to custom design a solution to meet your needs, prepare a budget proposal and arrange a pilot test.

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