A filter press is one of the few pieces of process equipment that does not run continuously. It receives a slurry, turns it into solid cakes and clear filtrate, then opens, empties and starts again. Everything in its design and operation revolves around that cycle: its duration sets capacity, its sequence sets cake moisture, and every step has a setting. This article describes the machine, the full cycle and the parameters that matter — for mine tailings and concentrates, though the principle is the same in chemicals, water treatment or food processing.
The parts
The frame. Two side rails (or an overhead beam on large machines) connect a fixed head and a moving head; they support the plate pack and take the closing force.
The plates. Square or rectangular polypropylene frames (sometimes cast iron or steel for high pressures), hung side by side. Each plate carries a recess on both faces; two plates face to face form a chamber. Channels run through the plate: the feed, usually at the centre, and the filtrate outlets in the corners. Membrane plates carry, in addition, a flexible membrane on their faces — the comparison membrane vs chamber details the difference.
The cloths. A filter fabric stretched over each plate face. The cloth is what retains solids — at first; very quickly it is the cake itself that filters, with the cloth serving only as a support. Its weave, material and permeability are chosen for the slurry.
The moving head and the cylinder. A hydraulic cylinder pushes the moving head against the pack to compress the plates together and seal the chambers against the feed pressure. The closing force is the first safeguard of the process: if it is insufficient, the chambers leak.
The feed pump. A pump able to deliver a high flow at low pressure at the start of the cycle, then a low flow at high pressure at the end: diaphragm pumps, piston pumps, or centrifugal pumps in series depending on the case.
The discharge system. A mechanism that shifts the plates one at a time or in groups at opening, often completed by a cloth shaker; below, a chute, a conveyor or a bin receives the cakes.
Cloth washing. A high-pressure nozzle bar that travels along the pack, plate by plate, to unblind the cloths at regular intervals.
The cycle, step by step
1. Closing. The cylinder clamps the pack. Closing pressure must be reached before feeding begins.
2. Filling and filtration. The pump sends slurry into the chambers through the central channel. Filtrate passes through the cloths and leaves through the corner channels; solids build up against the cloths on both sides of each chamber, and the cake thickens toward the centre. Filtrate flow, high at first, declines as the cake resists; feed pressure rises to its setpoint. The step ends either on a fixed time or — better — when filtrate flow drops below a threshold.
3. Squeeze (membrane plates only). The membranes are inflated with air or water and compress the cake: part of the remaining water is expelled and the cake becomes homogeneous. This is the step that gives the driest cakes.
4. Cake washing (if required). Water or a solution is pushed through the cake to displace the interstitial liquid — useful when a dissolved product is being recovered (leaching) or when a contaminant must be removed. On mine tailings the step is often absent.
5. Blowing. Compressed air clears the slurry left in the central channel (core blow) and, on some configurations, passes through the cake (cake blow) to lower its moisture further. Without a core blow, the cake tears at opening and residual slurry runs onto the conveyor.
6. Opening and discharge. The cylinder retracts, the mechanism shifts the plates; the cakes drop by gravity, helped if necessary by the shaker. A well-formed cake releases as one block; a sticking cake is the first sign of a poorly tuned cycle or blinded cloths.
7. Cloth washing. Not every cycle, but at a frequency set by the slurry — every few dozen cycles, for example. This is what maintains cloth permeability and therefore cycle time.
8. Re-closing. And the cycle starts again. A PLC sequences the steps; on large plants several presses run staggered to smooth the feed and the cake output.
The parameters that drive the result
| Parameter | What it changes |
|---|---|
| Feed pressure | Filtration speed and cake moisture; limited by plate and cloth ratings |
| Chamber thickness | Thick chamber = more solids per cycle but slower filtration at the end; thin chamber = short cycles, drier cakes |
| End-of-filtration criterion | Stopping too early leaves a wet cake; too late wastes time for little dryness |
| Squeeze pressure and time (membranes) | Final moisture and cake homogeneity |
| Blowing | Channel cleanliness, moisture, discharge quality |
| Cloth | Filtrate clarity, filtration speed, cake release, washing frequency |
| Slurry conditioning | Flocculant, percent solids, temperature: a thickened, well-flocculated slurry filters far faster — see tailings dewatering |
What sets the capacity
A filter press's capacity is not a slurry flow rate: it is a mass of solids per cycle, multiplied by the number of cycles per day. The mass per cycle comes from the chamber volume (number of plates × chamber volume) and the cake density; the number of cycles comes from the duration of the complete cycle — filtration, squeeze, blowing, discharge, washing. Cutting a few minutes off the cycle on a machine doing forty cycles a day is worth as much as adding plates. That is why tuning the sequence at commissioning, on the actual slurry, weighs as much as the sizing.
Common problems and their likely cause
- Cake sticking to the cloths: blinded cloths, insufficient blowing, poorly flocculated slurry, or the wrong cloth.
- Cloudy filtrate: cloth too permeable or holed, or the cycle started too fast before the cake formed.
- Leaking chambers: insufficient closing force, warped plates, or cloths folded into the seal.
- Cycle getting longer week after week: cloths blinding — increase washing frequency or revisit the cloth.
- Uneven cakes from one chamber to the next: feed not filling every chamber — partly blocked channel, undersized pump, air in the slurry.
Safety
The attention points are well known: the closing zone (pinch between plates — light curtains and emergency stops), hydraulic and slurry pressure (relief valves, pressure switches), spray at opening (side guards), and access to the discharge zone (lockout before intervention). A well-automated filter press requires no presence between the plates in normal operation.
Going further
The choice between chamber and membrane plates, and the sizing — number of plates, size, chamber thickness — are decided on a filtration test with your slurry, not from a catalogue: see membrane vs chamber and the Filter presses & hydrocyclones product page. Tuning the sequence on the delivered machine — pressures, step-end criteria, blowing, washing frequency — is part of our commissioning service. For a specific case, tell us about your slurry and the target moisture.
Frequently asked questions
How long is a filter press cycle? It depends entirely on the slurry, the chamber thickness and the plate type: from a few tens of minutes to several hours. The laboratory filtration test gives the order of magnitude for your material.
Does a filter press replace a thickener? No, it complements it: a press fed with dilute slurry runs long cycles for little cake. Upstream thickening reduces the volume to filter and stabilizes the feed.
What moisture can be reached? It depends on the material — particle size, mineralogy — and the plate type. No general figure holds for every case; the moisture-versus-time curve from the filtration test is what answers.
Hicham Marouazi, ing., PMP
Engineer and project manager (PMP) at Induscoat Solutions. Over 16 years of experience on mining, energy and petrochemical sites in Canada and internationally — selecting, installing and commissioning industrial equipment.