5.1 Filter Press Types
There are three filter press configurations: recessed chamber, membrane, and plate and frame. The recessed chamber press has a series of plates with a cavity on adjoining plates to form an empty chamber. The membrane press has either every other plate with an inflatable bladder on each side of the plate or every plate with a bladder on every adjoining side. The first pattern, alternating companion and membrane plates, is called a “mixed-pack membrane plate stack”. The other pattern is called a “full membrane plate stack”. The mixed pack is more common because it is less expensive and normally our testing uses a mixed pack configuration. The plate and frame consists of alternating flat or “flush” plates with hollow frames. This is the oldest style of filter plate and is commercially rare. Some of test plate configurations use a plate and frame configuration alone or in combination with a membrane plate stack or recessed chamber plate stack to obtain the desired cake thickness. The data obtained can be used to properly size a recessed chamber or membrane press using the conventional plate configurations.
Depending on the process requirements and goals, the recessed chamber and/or membrane configurations may be tested. The determination of which configuration will be used should be done before shipping the filter press. The two primary process requirements are whether cake washing is required; the membrane press is much better suited for cake washing applications or if the driest possible cake is desired; then a membrane press is again indicated.
The recessed chamber configuration is the simplest and is less expensive than the membrane filter press. For many waste slurries this is the preferred press.
Membrane tests can be done on the HPL300, HPL470, and HPL500 filter presses. If accurate cake washing data is required, the HPL470 and HPL500 are the used.
If the application is for a true plate and frame configuration, recessed chamber plates can be substituted for the test.
5.2 Process Goals
Typical process goals are to improve process efficiency or to separate the waste into liquid and solid streams for easier and less expensive disposal.
Improving process efficiency is not always the same as optimizing the filter press performance but is achieving the cake and filtrate characteristics the needed for other downstream processes. Since each process will have different requirements, each process must be considered individually for the most likely filter press type that will be suitable. Depending on the process goals either a recessed chamber or membrane press (or both) may need to be tested.
Separating waste slurries into a liquid and solid streams results in more cost effective disposal of both. Here, the disposal requirements will dictate the optimum filter press to be tested.
Before testing the filter press, please consult your R&B salesman to determine the types of tests that should done for your specific application.
5.3 Filter Cloths
There are numerous filter cloths available for use on a filter press and the proper selection is important for successful filter press operation. Each cloth has specific characteristics such as weave, porosity, thread types, and surface finish to achieve a balance between solids capture, cake release, blinding resistance, and cloth life. However, one must note that chemical conditioning is more important than the filter cloth. Poorly conditioned slurries will not filter well regardless of the filter cloth used.
The testing of filter cloths will only be able to determine if solids capture and cake release are acceptable. Long term blinding resistance and cloth life will not be determined by filter press testing because these problems occur after numerous cycles have been run on the cloth (many more than is commonly done during a testing program).
The filter cloth must be considered before the start of any test. If the slurry is to be chemically conditioned R&B’s preference is to use as a cloth that has excellent cake release and blinding resistance with reasonable solids capture. Otherwise, the cloth selection should be done to maintain adequate filtrate clarity, cake release, and resistance to blinding (the pores of the cloth becoming so plugged that cloth becomes almost impervious to liquid flow).
Smaller particle sizes generally require tighter filter cloths for adequate clarity. The determination of “adequate” clarity will vary according to the process goals. Chemical conditioning tends make large particles that normally do not require a tight cloth for reasonable clarity.
Filter cloth tightness is determined by the type of yarn used, thread count, and cloth permeability. Many people mistakenly believe that the permeability data is directly related to the particle size that the cloth will capture. It is actually more complex, because two cloths with the same permeability but very different yarns and thread counts will actually have different capture characteristics. For specific cloth, a lower permeability will lead to capturing of smaller solids. Two similar cloths with the same permeability but different thread counts will typically have different solids capture, with the higher thread count usually have the better solids capture. Different yarns will have very different ability to trap particles within themselves and the fabric.
Common cloth materials are polypropylene, nylon, and polyester. Other materials are occasionally used for very specific processes.
Polypropylene has good strength and very good general chemical resistance. Polypropylene will have a reduced life when exposed to organic solvents, they cause the polypropylene to swell and lose some mechanical strength. It is usually the least costly fabric.
Nylon is very strong but has poor resistance to mineral acids and tends to have higher minimum porosity than equivalent polypropylene cloths. Nylon does not “calendar” to a tight finish because of its relatively high melting point. Thus, nylon fabrics can not be as tight as the equivalent polypropylene fabrics. Nylon tends to have the longest life because of its mechanical strength.
Polyester is normally used when polypropylene and nylon are unacceptable usually do to chemical attack.