What is a Filter Press?

History

old filter pressFilter presses are sometimes called "Plate-and-Frame Filters." This refers to the style of filter element that was most prominent in the mid 1800's until the late 1960's.

While this specialized style of the filter press is still manufactured today, the current manufactured units are more accurately referred to as "Recessed Chamber Filter Press” and/or “Diaphragm (Membrane) Filter Press".


Definition

The primary usage of a filter press is as a "fixed volume, batch pressure filter." As a fixed volume filter, it is built to hold a specific quantity of solids.  The term "batch" refers to the fact the cycle must be stopped to discharge the collected “filter cake” prior to restarting a cycle. "Pressure" is created by the feed pump in the separation process.

filter platesThe filter press may also be used as a "polishing" filter to remove minute quantities of solids from an influent stream. In these applications the press is not sized for the quantity of solids holding capacity but for maximum filtration area and hydraulic throughput. When used as a polishing filter, generally a dry filter cake is not developed. Rather, when throughput flow rates drop to an unacceptable level, the cycle is ended.

The last most common usage of the filter press is as a "variable volume" filter through the use of a filter plate known as a diaphragm or membrane plate. This type of plate has a flexible drain-field which when sealed around the edges forms an integral bladder or diaphragm that may be inflated to physically press additional liquid from the filter cake. This process can significantly reduce the typical elapsed time for a press cycle and produce a dryer cake product, or more uniform cake dryness from cycle to cycle.

How does it work?

The filter press is made up of two principle components:
- Skeleton
- Filter pack

This is true for all filter presses of the regardless of design and includes presses of both the sidebar and overhead plate suspension types.

Components

The skeleton has one chief function—that is to hold the filter pack together against the pressures developed internally during the filtration process. The terms used to describe the skeleton may vary slightly from manufacturer to manufacturer, but the sub-components remain essentially the same:
- Stationary head
- Follower head
- Hydraulics 
- Manifold

filter press diagramThese components will be tied together either by the use of sidebars or overhead beams.

The filter pack is where the actual liquid/solid separation process takes place. The pack consists of a series of filter elements that form a series of chambers when held together in the press skeleton. Each chamber wall has a series of raised cylinders or "pips," which are then covered with a porous cloth medium. 

filter plate diagram

These pips form a flow path for the liquid draining from the press. At corners of the drain-field, interconnecting holes join the drain-field to the four corner discharge ports. When the plates are held together in a plate pack, the corner discharge eyes form individual manifolds connecting the drain-fields of the plates with the external piping of the press. The center feed (or less frequently a corner feed) slurry inlet port also forms a manifold that connects with the individual chambers of the plate pack.

Process

In operation, slurry is pumped under pressure into the press chambers through the manifold at the stationary head of the filter press. As each chamber fills with slurry, the liquid passes through the cloth medium, across the drain-field, through the drain ports and exits via gravity out of the corner discharge eyes.

filtration diagram

The prime function of the filter media is to provide a porous support structure for the filter cake as it develops and builds. Initially, some solids may pass through the cloth media causing a slight turbidity in the filtrate, but gradually the larger particles within the slurry begin to bridge the openings in the media reducing the effective opening size. This allows smaller particles to bridge these reduced openings initiating the cake filtration process. Once a layer of solid particles achieves 1 to 2 mm in thickness, this "precoat" layer serves to separate out finer and finer particles as the cake builds in thickness, yielding a filtrate that is very low in turbidity.

The Principle

The pressure behind the slurry (typically 100 psi, but up to 900 psi [7 to 60 bar]) is provided by a feed pump—sometimes a positive displacement or centrifugal pump. With a gravity drain on the filtrate side of the press, a pressure differential between the feed pressure and the gravity discharge is created across the media and the filter cake solids as they build in thickness.

It is the existence of this pressure differential, not just the feed pump pressure, that causes the filtering action to occur. Solids within the slurry will flow to the area of cake development with the lowest pressure differential, resulting in a filter cake that builds uniformly over the drain-field on either side of the chamber walls.

This fill cycle continues until the filter cakes forming on the chamber walls bridge at the center, completely filling the press with solids. It is at this point that the filtration process is complete.

Once this is achieved, the hydraulic closure of the press is retracted, the individual filter elements are separated and the filter cakes are discharged, usually by gravity, to an appropriate receptacle.