After the plates are closed and sealed, the feed pump fills the chamber formed between the recessed and membrane plates. Once the required amount of solids is introduced into the chamber, high-pressure water can be pumped behind the membrane to physically squeeze the cake. The addition of membranes adds some complexity to the operation when compared with recessed plates, but some materials allow for reduction in cycle times and drier cakes, which can justify the increased capital cost and maintenance.

Other considerations to keep in mind with membrane plates:

• Membranes have a much shorter life than recessed filter plates due to the flexing required of the membrane every cycle under high-pressures. Life of the membrane will depend on the number of cycles performed and materials used.

• Some membrane plates are available with replaceable membranes. Others require replacement of the entire filter plate when the membrane needs to be replaced.

• For pressures higher than 100 to 125 psi, it is generally recommended to pressurize the membranes with water, rather than air, for safety. Recommendations of the plate manufacturer should be followed.

• Some membrane plates require a minimum amount of solids to be introduced into the cavity before the membranes can be pressurized. Premature pressurization of these membrane plates can result in overextension and potential failure of these membrane plates. More expensive membrane plates are known as “empty chamber” membrane plates and can be pressurized without failure with any volume of solids.

Recessed chamber plates vs. membrane plates and cake thickness

For both recessed chamber and membrane plates, lab and/or pilot testing should be performed to determine cycle times and achievable cake moistures. Recessed chamber plates offer simplicity and reduced maintenance. Membrane plates may be able to achieve drier cakes or shorten cycle times. Depending on the clay content, recessed plates operating at 225 psi typically produce cakes of 70- to 85-percent solids on aggregate tailings. Membrane plates may increase the percent solids and reduce cycle times while treating the same slurry. The addition of membrane plates may be considered if higher solids content will be required to meet compaction requirements.

The selection of the proper cake thickness is also critical to the proper functioning of the press. Cake thicknesses typically range from 15 mm to 50 mm. As the filter cake becomes thicker, the permeability of the cake decreases. When large amounts of clay are present, even small increases in the cake thickness can result in large increases in the cycle time (i.e. increasing cake thickness from 25 mm to 35 mm can double or triple the cycle times on less permeable slurries). Testing should be performed to determine the proper cake thickness, and thinner cakes, such as 25 mm, are more commonly used when clays are a concern.

Overhead beam vs. side beam presses

Filter presses are frequently categorized into two main styles, each with their own advantages and disadvantages:

• Overhead beam — Denoting a press in which the filter plates hang from overhead beams.

- No parts on the sides of the press, allowing unobstructed access to change filter cloths on the press without plate removal.

- Much wider plate opening, often exceeding 3 feet between the plates. This creates more movement to aid cake release, while also allowing easier access for cloth washing and inspection.

- Dynamic loads are typically isolated from the beams of the press using tension shafts in various configurations.

• Side beam — Denoting a press in which the filter plates are supported by beams running along the side of the press.

- Easier to implement shaking systems for applications requiring shaking systems.

- Lower upfront cost.

- Less access to plates for cloth washing and inspection. Automatic wash systems should be considered more heavily with side beam style presses due to limited plate access.