Operating a filter press
Mechanical and process engineering
The filter press is a discontinuously working apparatus. During a batch a number of process steps are necessary for the solids to be separated from the liquid.
Both an ideal machine as well as sophisticated process engineering are necessary for the operation of the system and that the results meet the requirements.
Technical equipment of a filter press.
The heart of the filtration system is the filter press itself but also the entire periphery is of the utmost importance. The possibilities for the operation of such a system are defined here.
The layout of this system shows the required assemblies and components:
- A filter press with hydraulics and transport means for the opening of the chambers.
- Polypropylene chamber-filter elements covered with filter cloths.
- Valves for sludge, wash water and pressurized filtrate air.
- Feed pump with controls including pressure and flow measurements.
- High pressure cleaning system for the cleansing of the filter cloths.
- Filter cloth acidification plant for chemical cleaning of filter cloths and filter elements.
Type of Filter Presses
Sometimes there are only slight differences between the various brands. Each machine- builder can design and build a filter press.
The main distinguishing characteristics of filter presses are in the operation, the result of the separation and the reliability of the systems.
For the operator, the following aspects are important:
- Rugged, robust and reliable engineering. Good filter press frames are simple in design.
- Use of proven components.
- Easy maintenance, prompt service by the manufacturer and cost-effective operation.
- User-friendly, fully automatic control of the filter press.
Deviations may be found in practice again and again. Some filter press manufacturers save on plate transport, others build expensive, movable end plates so that tolerances of the machine can be compensated.
Chamber filter plates:
- For over ten years, chamber filter plate made from polypropylene have been proven to work well. They are light and inexpensive.
- Chamber filters made of cast steel are used when the material to be filtered has a high temperature. However, these plates are very heavy and very expensive.
Filter cloths are absolutely critical to the separation effect and also for easy throw of the filter cake from the filter chambers.
There are many suppliers in this industry. The differences in the fabric qualities, and especially in the careful assembly of the filter cloths are very important.
Various details are particularly important in the choice of the filter cloth:
- As a commercial material polypropylene is generally used.
- The fibers of the fabric consist of mono-and multifilament yarns.
- The surface of the fabric must be thermally smoothed and thermally fixed.
- The optimal fit of the finished filter cloth must be guaranteed.
Controlling the feed pump
Generally, mud pumps are used to feed the filter presses.
- Piston diaphragm pumps
- Ball diaphragm pumps
- Rotary piston pumps
It is important to control and regulate these pumps.
Important for the operation of the feed pump:
- The feed pump should be operated with a frequency converter.
- As controlled variables for the filling pump the filtration pressure, the flow, the volume flow and the absolute input per load have to be included.
- The control of the feed pump must be able to set-up individual parameters in certain areas. In particular these are:
- The maximum-permissible filtering pressure
- The start-up characteristics of the charge-pump
- The flow of filtrate curve
- The shut-down condition of the charge-pump
- The minimum and maximum input per batch
- To ensure that this operation is possible, the measurement and control technology (german: MSR-technology) of the filter press requires pressure sensors, a flowmeter and an optimal controller.
High pressure washer to clean the filter cloths
Filter cloths should be cleaned depending on how much they are soiled. Modern filter presses are equipped with a fully automatic high pressure washer.
This system cleans the filter cloths from each filter plate automatically with clean water at pressures up to about 100 bar.
Important criteria for the high-pressure washing system are:
- The filter cloths must be cleaned, depending on the sludge composition, the self-cleaning behavior of the filter cloths and the number of daily batches. The cleaning intervals can therefore be between one cleaning per week, ranging up to a cleaning per cycle.
- Regarding the cleaning time and the cleaning effect, one should choose a high-pressure washer with 100 bar operating pressure. Washing systems with lower pressure are not as effective.
- Clean water is usually used for the cleaning.
Acidification system to clean the filter cloths and filter elements
Calcium deposits in the filter fabric and on the filter elements can of course not be removed with a high pressure washer. For this purpose the chambers of the filter press must be flooded with diluted hydrochloric acid.
In the past, several companies did without such feature. Either the cloths were removed and occasionally acidulated or a chemical cleaning was not even performed. In consequence to that, the practice showed a set of problems.
The advantages of a chemical cleaning are:
- Calcification in and behind the filter fabric can only be removed with a chemical cleaning.
- Calcification on the surface of the filter elements and especially in the filtrate drainage holes are accessible only by the chemical cleaning.
- With good cleaning systems the diluted acids are continuously recirculated. This method is more effective and cheaper than any other.
- Several manufacturers offer cleaning systems that work both manual and automatic. The automated systems definitely offer more consistent results than manually controlled cleaning systems.
Controlling the filter press
Today only PLC controls are used in filter presses.
For the operator, the following criteria should serve as guidelines:
- Parameters such as pressure, flow and temperature must be measured and integrated into the processing technology.
- The operator must be able to simply change all variable parameters. Intelligent control systems are therefore required.
- Today, LCD screens are very inexpensive. This technology can easily be integrated to display the current processes.
- It is essential to document all the relevant process parameters as well as all individual changes. In many cases, one can‘t look for errors or for irregularities, as there is no documentation from the operation of the system.
Der verfahrenstechnische Ablauf einer Charge kann sehr genau definiert werden. Natürlich sind für eine ganze Reihe von Prozessparameter variable Bedingungen möglich. Mittels Setup der Anlagensteuerung und auch intelligenter MSR-Technik können die variablen Parameter manuell oder automatisch angepasst werden.
The procedural sequence of a batch can be very precisely defined. Naturally variable conditions are possible for a range of process parameters. With the setup of the system control and intelligent control technology, the variable parameters are adjusted manually or automatically.
In the following sections, the influences are presented and defined widely on the basis of the operation of a batch.
1. Closing the filter press
2. Filtering sludge
3. Drying the filter cake
4. Opening the filter press and the discharge of the filter cake.
Step 1: Closing the filter press
To close the filter chambers, the filter plates are closed with a defined clamping force. All filter press manufacturers use a hydraulic cylinder with a hydraulic unit.
Important aspects are:
- The filter plates need to hang correctly guided in the filter press. It is to be made sure that no objects are trapped between the filter plates. The sealing surfaces of the filter plates must remain clean.
- At an operating pressure of approximately 10 bar, a hydraulic closing force of 1585 kN is necessary.
- The clamping force must be kept within narrow boundaries. Therefore the closing pressure of the hydraulic filter press is monitored. Decreases the hydraulic pressure, the hydraulic pump must built up pressure. Increases the hydraulic pressure past the maximum limit, it will be reduced by a pressure relief valve.
In practice various problems are known:
- At high closing pressures, the filter plates are deformed and a total destruction on the filter plates can occur.
- The filter cloths are overloaded at the sealing edge. The life of the filter cloths is considerably reduced.
- The filter press and the filter elements in the filter press must be geometrically arranged correctly. Worn and misaligned filter plates lead to permanent damage.
Step 2: Filtration of sludge
When the sludge storage tank contains a certain minimum volume the filter press can then be fed with sludge. At the beginning, when the press chambers are empty, the pump delivers a high flow – towards the end, when the chambers are nearly full, the filtering pressure rises and the flow rate of the pump is reduced.
Important notes for practical operation:
- The pump should be started with a so-called soft start( = specific method for starting and run-up of centrifugal pumps in filter presses).
- The power of the pump must be adapted to the filterability of the sludge, therefore for continuously variable pump control a frequency converter is absolutely beneficial.
- It is very important that the chambers of the filter press are filled evenly.
- Too little chamber filling, partially reduces the life of the filter elements considerably.
- It is extremely important that the operator knows the particular sludge composition and the properties of the sludge. Only on this basis the optimal parameters can be set-up in the control for the best possible filtration.
- For the end of the filtration, certain conditions must meet. The feed pressure, the filtration performance and the minimum input values must achieve the defined values.
- It is very important that the control of the pumps, allow very simply necessary changes, in the set-up of the pumps. The operator can then react very quickly to the flexible operating conditions to then allow the optimal operation.
Step 3 : Drying the filter cake
At the end of the pressing process, in most cases, the filter cake is dried with compressed air. In this process, capillary water is blown from the filter cake by compressed air. The process of drying is thus very short.
In practice, several criteria are important:
- In order to receive an even drying result, the filter cakes should here be as uniform as possible.
- The air pressure and the drying time must be carefully matched to the filter cake.
- The control of the system must ensure that the setup for the optimal parameters are easy to implement. These include the parameters for air pressure and drying time.
Step 4 : Opening the filter press and the discharge of the filter cake
When all the steps are completed, it is essential that the filter press is put in a ” pressure-free state.” Only then, can the filter press be opened and the filter chambers emptied.
Usually, the filter cake falls automatically from the filter chambers. However, the operator should monitor this.
The following aspects are important for practical operation:
- The filter cloths must always be in clean condition, so that the filter cakes fall off easily. Where necessary, the filter cloths need to be washed with the high pressure cleaning system and from time to time a chemical cleaning of the filter cloths must be carried out.
- The operator must monitor the unloading of the chambers.
- The operator should monitor the filling of the chambers and possibly correct the setup parameters.
- In the laboratory, the solids content of the filter cakes should be determined once or twice per shift.