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.
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.
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The incoming solution enters into the Filter Press via a diaphragm pump that is plumbed to the center feed pipe. When the plates are closed, the sludge accumulates in the cavity between the plates. The filtered water exits through the filter cloth and works its way through the stack alternating between the left upper and lower corners and right upper and lower corners, due to the porting differences on the on button and three button plates.
Before the operator can begin filling the filter press, it should be confirmed that the plates are in the 1-3-1-3 button alignment. (This alignment is critical in the air blow-down process.) The unit then needs to be completely closed and all valves should be in the appropriate position. This is accomplished by pushing all them in place by extending the hydraulic cylinder and attaining the recommended PSI. ( for automatic and for manual)
As the filter press fills, it is recommended to begin the feed at 25 PSI and to gradually ramp up the pressure at steady intervals (25, 50, 75) to the maximum pressure of 100 PSI. Upon initial filling of the filter press, the pump will thrust at quick intervals. Conversely, as the press fills, the diaphragm pump will slow down significantly. The easiest way to tell that the press is full is, to listen to the diaphragm pump once it hits 100 PSI. It will dramatically reduce to 30 seconds to 1-minute intervals between thrusts or it may deadhead (stop) altogether. This is your indicator that the cycle is complete. Cycle time can vary from as little as 30 minutes to over 4 hours depending upon what is being filtered. When the filter press is full, all of the chambers will be filled with sludge.
Once the press is full, it is time to operate the Air Blowdown. When the three ball valves on the outlet manifold are closed, and the inlet center feed ball valve is closed, the air that enters the filter press via the small ball valve on the upper left-hand corner of the manifold is forced through the filter press.
Because of the porting denoted by the one button or the three button plate, the air enters into the sludge chamber via the upper left-hand corner of the three button plates and exits via the bottom right-hand corner of the one button plates. This process will push excess water out through the outlet manifold. This process allows for a better cake release and a drier filter cake. Think of this a wringing out the sponge. This is why it is imperative to maintain the 1-3-1-3 alignment of the plates.
Once the Sludge Blowndown Process is complete, the filter press is ready to be opened so that the plates can be cleaned. To open the automatic filter press, reverse the air valve on the automatic hydraulic pump to allow the cylinder to slowly pull open the steel pusher plate. On the smaller manual units, release the ram pressure by simply opening the release valve and the pusher plate will retract via an internal return spring.
Now that the plates are released, pull apart the plates one by one. Most of the sludge will fall into the sludge dumpster on its own. A sludge spatula is provided on new Filter Presses to aide in the sludge removal. Sometimes it is necessary to scrape the residual sludge from the cloths. However, care should be taken to ensure that the cloths are intact and the o-ring and O-ring sealing surfaces are in position and free from obstructions.
Once all plates are clean, the filter press is ready to be closed hydraulically. The three outlet manifold ball valves should be opened, the center feed pipe should be opened and the pump is ready to be switched to the on position. Once the diaphragm pump is reset to 25 PSI, the unit is ready to resume the next fill cycle.
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