Filter Presses

The main factors impacting Filter Press dewatering efficiency include:

• Feed slurry percent solids
• Slurry particle size distribution
• Chamber thickness
• Cake percent solids
• Solid material permeability

Pump options for Filter Presses include: 

• A lower pressure centrifugal feed pump 
• A high-pressure centrifugal feed pump (two impellers on one shaft) 
• Two centrifugal pumps in series, low-pressure pump and booster pump 
• A single positive displacement pump 
• A low-pressure centrifugal pump in parallel with a high-pressure positive displacement

Filter Presses and Belt Presses are different in several ways. With Belt Presses, slurry is dewatered between two moving synthetic cloths or belts. Water initially is released by gravity then by squeezing the slurry/belts between rollers. The filtrate water is collected below the press unit in a pit. The dewatered cake material is scraped off the belts as they separate at the discharge end of the press. A dewatering polymer chemical is required to flocculate the slurry particles prior to the slurry entering the press. The chemical also aids in allowing the water to release from the solid material.

A plate Filter Press is composed of a frame holding a series of recessed plates lined with filter cloths. The plates are held tightly together, creating a seal around their perimeters using hydraulic pressure. A high-pressure slurry dewatering pump forces slurry into the chamber spaces between the plates for dewatering. The slurry solids are captured between the plates, while the clean filtrate water passes through the filter cloth mesh and exits through ports in the filter plates. When the chamber spaces are full of dewatered slurry solids, the slurry feed pump is automatically stopped. The hydraulic pressure holding the plates together is released so the plates can be separated individually, allowing the cake to fall by gravity out of the press. Typically, no chemicals are required to aid dewatering on a plate filter press.

Recessed plates are solid plates made of plastic or metal or a combination of metal and plastic. The plates have recessed surfaces, except for the perimeter edge surface that allows the plates to seal against each other when they are clamped together using a hydraulic ram. With the plates held tightly together, all dewatering takes place as a result of the slurry feed pump press. 

Membrane plates are similar to recessed plates, having recessed surfaces except for their perimeter edges. The membrane plates are constructed so compressed air or water can be injected inside the plate surface, inflating the plate surfaces. These plates are also held tightly together with a hydraulic ram while the slurry is pumped into the press and dewatered. Once the chambers are filled with dewatered slurry, the feed pump is stopped. With plates still clamped together, the membrane plate surfaces are inflated to compress/squeeze the dewatered cake material. If the material can be compressed, additional water is removed from the solids. The plate surfaces are deflated by relieving the air or water pressure, then the plates are unclamped to allow the plates to be separated and the cakes are discharged by gravity.

Recessed plate Filter Presses require routine filter cloth replacement and slurry feed pump rebuilds. The cloth's life is typically in thousands of press cycles. Pump rebuilds are required due to internal pump wear caused by the slurry material. The rebuild frequency can vary greatly depending on pump materials of construction, slurry material abrasiveness and hours of operation, but in general would occur once to twice per year.

The savings most frequently gained using a Filter Press are: 

• Reduction in waste volume and weight, due to the creation of a drier, dewatered material, also resulting in reduced transportation, disposal and footprint costs. 
• Reduction or elimination of chemicals used for dewatering vs. alternate technologies. 
• Reduction in labor through automation vs. alternate technologies. 
• Higher-value end product due to lower inherent moisture, or in cases where additional drying is necessary through non-mechanical means such as rotary driers, the BTU costs can be reduced significantly through the reduction in product moisture prior to the drier.

Filter Presses are often used to create a closed loop process. This eliminates the need for a settling pond or tailings pond, while also recycling process water back for reuse. By eliminating the need for tailings ponds or settling ponds, valuable land reserves can be protected or freed up for mining rather than being covered with tailings.

In many cases, the use of a closed loop process can also reduce regulatory permits while also putting forth an environmentally positive message for investment companies.

Many companies are now waiting years for permits for new settling ponds without assurance that this permit will be approved. Moving to a closed-loop process can eliminate the need for some of these long permitting processes.

Water availability and cost are becoming more of an issue as large population areas continue to grow. These areas also require quarried and mined materials, but lack of water can be restrictive for new plant considerations. Plant feasibility is greatly improved using a closed loop approach to wet processing with a filter press due to the maximum recovery of recyclable water, minimizing the need for new water sourcing.

The most commonly considered options when purchasing a Filter Press are:

• Chamber thickness - The thickness of the chamber has large impacts on the dewatering performance and should be selected through testing or experience in similar materials.
• Recessed chamber plates or membrane plates.
• Feed pressure – 125 to 225psi is the typical operating pressure for most high-pressure Filter Presses with modern filter plates.
• Surge tank or holding tank – Surge tanks are often used to allow Filter Presses, which operate in a batch process, to be used in a continuous process. Surge tanks of different sizes can be considered to ensure a smooth and controlled link from your continuous process to the batch dewatering process.
• Feed pump – The feed pump acts as the heart of a filter press operation. Filter Presses can be fed with centrifugal pumps, two-stage centrifugal pumps, positive displacement pumps, diaphragm pumps, screw pumps or a combination of these pumps. The selection of the right pumping system is critical to performance and operating costs.
• Core blow – This prevents slurry from being discharged with the filter cake and also reduces wear on the filter cloths and can improve downstream processes.
• Open or closed filtrate discharge - Open filtrate discharge allows for easy identification of damaged filter cloths. This can greatly reduce the wear and tear of the filter plates.
• Filter cloth selection and style of attachment of the filter cloths to the filter plates.
• Automatic wash system – This is used to wash the filter cloths to reduce operator requirements and improve filtration performance when blinding of the filter cloths is an issue.

The capacity of a Filter Press is determined primarily by the following items:

• Type and preparation of feed material
• Volume of the chambers of each Filter Press
• Specific gravity of the solids being dewatered
• Percent moisture of the filter cake
• Number of filter plates
• Number of cycles per hour the Filter Press can discharge

The number of cycles per hour is dependent on the:

• Fill time
• Dewatering time
• Cake/Wash/Blow when used
• Core/Wash/Blow when used
• Open and closing time of the Filter Press
• Pump capacity (flow and pressure capabilities of the feed system)
• Feed characteristics of the material - size gradation, permeability, chemicals used