Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/661453
AGGREGATES MANAGER / April 2016 23 the press to discharge the cakes and clos- ing the press to start another cycle. The simplicity and limited movement of filter presses is what lends them to high levels of automation and reliability. The ability to select cycle times, cake thickness, feed pressures, and plate styles are what allow filter presses to achieve much drier cakes than many competing technologies. Filter press operation is a batch pro- cess, and when asked about the capacity of the press, we must answer that the capacity is strictly a function of the cycle time. Each press comes with a finite volume to form cakes. Shorter cycle times (the time it takes to go from start to start) are promoted by: • Optimizing cake thickness – Thinner cakes form quicker than thicker ones. • Higher permeability of solids – The easier it is for water to pass through the cake, the quicker the filtration stage. • Optimum cake moistures – If 18 percent moisture is achievable in 40 minutes, is 20 percent after perhaps 30 minutes acceptable? • Higher feed solids concentrations – The less water to push through the cake, the more rapid cake formation. • Faster opening and closing – Dis- charging cakes and resetting the press for the next fill stage can play a big role in press capacity on a tons-per- hour basis. Once again, testing of your material is always a good place to start when looking at recessed plate filter presses. Most manufacturers have facilities for in- house testing to determine filtration times and moistures as a function of feed solids concentration and cake width. Types of filter presses Filter presses are frequently categorized into two main styles each with their own advantages and disadvantages. Overhead beam – Denoting a press in which the filter plates hang from overhead beams. Key features include the following: • No parts on the sides of the press, pro- viding unobstructed access to change filter cloths on the press without plate removal; • Much wider plate openings, often exceeding 3 feet between the plates, creating more movement to aid cake release, while also allowing easier ac- cess for cloth washing and inspection; and • Dynamic loads are typically isolated from the beams of the press using ten- sion shafts in various configurations. Side beam – Denoting a press in which the filter plates are supported by beams running along the side of the press. Key features include the following: • Easier to implement shaking systems for applications requiring shaking systems; • Lower upfront cost; and • Less access to plates for cloth wash- ing and inspection. Automatic wash systems should be considered more heavily with side beam style presses due to limited plate access. Maintenance As with any capital equipment, a success- ful maintenance program begins well be- fore the first change of oil. No matter how large a maintenance budget is or how highly skilled the maintenance crew is, it is too difficult to overcome the challenges of an improperly sized filter press or a poorly planned facility. In batch processes like that of the filter press operation, the process can be small batches with a high number of cycles or large batches with fewer cycles to achieve the same output capacity. The later lends itself to lower maintenance cost, but it may have higher capital cost. Under sizing of filter presses will result in high inlet velocities that cause exces- sive wear on the feed pipes, filter cloths, and, eventually, filter plates. Inlet veloci- ties should be considered when selecting filter cloth styles and cloth materials with their relative replacement cost associated with each. Under sizing of the filter press also causes excessive opening and closing speeds resulting in high wear and tear on those corresponding components. It is also imperative that slurries be tested to properly select a filter press for optimal performance and low maintenance cost. Just as important as a properly sized filter press, the facility in which it is installed greatly affects the maintainability Slurry is pumped into the cavity between sealed plates. The filter cloth keeps solids inside the plates, while filtrate passes through. Once the chamber is full of solids, the plates separate and allow the dewatered cakes to fall below.