What is a Filter Press?
In some batch filtration processes, highly permeable suspensions dewater fast compared to the rest of the process. This work explores the impact of fast-filtering compressible materials on the throughput of fixed-chamber filter presses. The dewatering properties for a compressible yet highly permeable minerals processing slurry are used as inputs to a standard filter press model to explore the effects of operating and design parameters. For fast-filtering materials, maximum throughput is achieved with wide cavities and minimal handling time, while membrane resistance can be significant. Pressure affects the maximum achievable concentration, as given by the strength of the material. Overall, this work demonstrates the combined use of material characterisation and device modelling for filter press optimisation.
This article answers three common inquiries. What is a filter press? How does a filter press work? What is a filter press used for?
We’ll also give you some advice on sizing your equipment (including your feed pump). Our Sales and Service Team is looking forward to answering any other questions you might have.
WHAT IS A FILTER PRESS?
A pressure leaf filter is a batch operation, fixed volume machine that separates liquids and solids using pressure filtration. A slurry is pumped into the filter press and dewatered under pressure. It is used for water and wastewater treatment in a variety of different applications ranging from industrial to municipal.
M.W. Watermark manufacturers filter presses ranging from .06-600 cubic feet.
Slurry is pumped into the filter press. The solids are distributed evenly during the feed (fill) cycle.
Solids begin to build on the filter cloth. Most of the solid/liquid separation is done by the filter cake building on the cloths. At first some fines may pass through the cloth (1), but eventually the solids begin to form a layer on the filter cloth (2) much like a pre-coat. That layer traps the fine particles and forms a filter cake (3).
As the vertical pressure leaf filter builds pressure, the solids build within the chambers until they are completely full of filter cake. When the chambers are full, the fill cycle is complete. The filtrate (liquid) exits the filter pack (plates) through the corner ports into the manifold; when the correct valves in the manifold are open, the filtrate exits the press through one single point, the filtrate outlet.
HOW LONG DOES A FILTER PRESS CYCLE TAKE?
The Total Cycle time is the Fill Cycle time plus a constant. For presses of 125 cubic feet and under this constant is usually around 45 minutes. This is the time required to close/open the press, perform the Air Blow Down and discharge the filter cake. If the particular application requires operations such as Core Blow or Cake Wash, for example, this constant is longer.
HOW LONG DOES A FILL CYCLE TAKE?
The Fill Cycle is dependent on many parameters. The most important parameter is the nature of material to be dewatered. A sand slurry releases its water readily and dewaters quickly. On the other hand, an Aluminum Hydroxide waste slurry from beverage can manufacture does not readily release its water and dewaters slowly.
The next most important parameter is the concentration of the solids by weight in the slurry. The Fill Cycle for a 5% solids slurry is about twice as long as a 10% solids slurry (with all other parameters being equal). This is because the press has to process half of the water to fill with solids.
Other parameters include the thickness of the filter cake, the maximum feed pressure which the slurry is fed to the press, and the filter cloth selection. These parameters are typically fixed during the proposal process.
CAN YOU GIVE SOME EXAMPLES OF FILL CYCLES?
With 32mm (1.25”) cake chamber thickness, 100 psi max feed to the press and a 3-5 SCFM filter cloth, a 5% sand slurry would be expected to dewater in 20-30 minutes and a 10% sand slurry in 10-15 minutes.
Conversely, the 5% Aluminum Hydroxide slurry may take 4-6 hours to dewater, while the 10% slurry would dewater in 2-3 hours.
We have an in-house laboratory where we can test a sample of your slurry to determine the Fill Cycle time as well as the other outputs from pressure filtration testing. Give us a call.
WILL A SMALL PRESS FILL FASTER THAN A LARGER PRESS?
The Fill Cycle times for a 1 cubic foot, 10 cubic foot and 100 cubic foot press are approximately the same. Press volume is the ability to remove solids. Associated with this volume is the square feet of surface area in the press.
Square footage is the ability to process fluid. As volume is added, square feet of surface area is proportionally added and the cubic foot to square foot ratio remains (roughly) constant. Therefore, the Fill Cycle Time is basically the same.
A 1 cubic foot press at 32mm cake thickness has 22 square feet of surface area for a 0.045 cf/sf ratio. A 10 cubic foot press at 32mm cake has 211 square feet of surface area for a 0.047 cf/sf ratio.
WHAT TYPE OF PUMP SHOULD I USE TO FEED MY PRESS?
For press capacities of 125 cubic feet or less, the double Air Operated Diaphragm pump (AOD) is uniquely suited for vibrating filter operations. The filter press is not a constant flow device. As the solids build up within the press, the resistance to flow increases and the flow rate through the press decreases. At a given air pressure supply, the time between pump strokes for an AOD pump, then, constantly increases with no harm done to the pump.
On presses larger than 125 cubic feet in capacity, AOD pumps become impractical because three or more large (3”) pumps would be required.