February 23, 2018
Over the past several years, the popularity of the filter press has increased dramatically, making it one of the more popular methods to process and dewater tailings. Other outside factors are also pushing producers to invest in a filter press. Due to changes in regulation and permitting processes, it has become increasingly hard to obtain permits for settling ponds. Having enough space at a site can also be an issue. Settling ponds may not fit expansion plans because they prevent access to and use of saleable product. Additionally, equipment such as belt presses often produce a moister cake using chemicals, while filter presses can achieve the needed drier product and require no chemicals.
How does a filter press work?
At its most basic level, the filtration cycle of a filter press consists of covering recessed plates with filter media, applying enough force to create a seal between the filter plates perimeter surfaces, and then using a feed pump to supply the pressure necessary to pump the slurry into the cavities formed between the sealed plates. The filter cloth captures the slurry solids between the filter plates while allowing the filtrate water to pass through the cloth mesh and exit through ports in the filter plates. When the chamber becomes completely full, the feed pump stops and the pressure that seals the plates is released. The plates are separated allowing the dry cakes to emerge from the chamber and fall to the ground below.
Most people think that the filter plates squeeze together to expel water, but the press never moves during the time of dewatering. The only movement by the press is when they are opened and closed to expel the dry cake or to begin a new process. Plates simply connect to form a seal; the feed pump supplies the necessary pressure to dewater the fine solids.
Filter presses were designed for simple and limited movement. This design leads 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.
Optimizing a filter press
Like other machines in your operation, the filter press should be set up to maximize your output. You should consider these variables when designing your filter press:
• Your optimal cake thickness – Thinner cakes dewater faster than thicker ones.
• Permeability of solids – The higher the permeability of the material, the easier for the water to pass through, leading to a quicker cycle.
• Cake moisture – Know your goal for moisture content and the amount of time needed to achieve it.
• Solids concentration in your feed – Less water and higher solids in your feed means faster dewatering.
• Opening and closing speed – Finding a happy medium between dewatering cakes and resetting the press can play a big role in the tons per hour capacity of your press.
• Test your material – Several characteristics can impact dewatering including:
– Clay content,
– Particle size distribution (very coarse, very fine),
– Particle shape, and
– Particle chemistry, pH, and temperature.
Types of filter presses
There are two main types of filter presses: overhead beam and side beam. Each type has its own advantages and disadvantages depending on the user’s needs.
Overhead Beam (OH): Filter plates hang from overhead beams.
– Filter cloth change is easier because the overhead beam doesn’t obstruct access to plates while they are in place on the press.
– Operator has easier access for cloth washing and inspection, due to wider plate openings.
– Dynamic loads are typically isolated from the beams of the press using tension shafts in various configurations.
Side Beam (SB): Filter plates hang from side beams.
– High speed cake discharge systems are available.
– Plate shaking systems aid in cake release.
– They are suitable for portable applications.
– Automated rain style or one-by-one high pressure cloth washing systems are available.
Properly size your filter press
Poor planning in sizing and design can cause unwanted wear problems. If under sizing occurs, it can result in high inlet velocities that cause excessive wear on the feed pumps, feed piping, filter cloths and, eventually, filter plates. Sites should select filter cloth styles and material based on inlet velocity to ensure they have selected the best material that will last the longest. Also, take into consideration costs to replace worn or torn filter cloths. Another problem undersized filter presses face is excessive opening and closing speed resulting in high wear and tear on mechanical components. Along with sizing, testing your slurry is imperative to select the filter press best for optimal performance and low maintenance cost.
Placing a filter press
Where is the filter press going to be placed? Like proper sizing, the location of the filter press is also an important factor. When deciding on the site location of a filter press, several factors should be taken into consideration, including the following:
– Thickener/clarifier location,
– Slurry feed storage tank location,
– Storage space below the press for dewatered cake material or for a conveying system,
– Gravity flow of filtrate water away from the filter press,
– Clearances required for replacing components,
– Ability to deliver components/material to the filter press, and
– Lift capabilities designed to meet the specific tasks.
Walkways, railings, harness tie-offs, and access platforms are important design considerations incorporated to optimize safe operation and maintenance activities. Designers who are working with typical polypropylene filter plates and standard cloth media should consider roof and side enclosures to prevent harmful damage caused by prolonged exposure to the sun’s UV radiation. Ultimately, regardless of the filter press type and size, close cooperation with the end user, facility engineer, and OEM will achieve a properly designed facility that facilitates good, safe maintenance practices.
Like any piece of equipment in your operation, filter presses require maintenance. When maintenance and inspection checks are performed regularly, a filter press can work at its best. Maintenance and inspection tasks to the filter cloth, plate, hangers, power unit, and opening mechanism are all important. Consider these items when inspecting your filter press.
• Should be checked for tears and rips at least once per shift;
• Should be checked for cleanliness at least once per week and washed as needed, ensuring material does not build up between filter plate and filter cloth; and
• An automatic wash system can be installed on applications that require frequent washing.
• Should be checked weekly or if filter cloths tear or rip for wear;
• Can be repaired if damage is in a small shallow area; and
• Can be replaced through the bottom, top, or side depending on the type and configuration of the filter press.
Filter plate hangers and wheels:
• Should be checked weekly for operating alignment and wear; and
• Bolts should be checked periodically and tightened as needed.
Filter plate opening mechanism/chain:
• Chain drive and tail shaft bearings should be lubricated per the recommended schedule; and
• Chain should be checked weekly for proper tension.
Hydraulic power unit:
• Change filters at least every 500 hours of operation or after every oil change;
• Perform oil testing every six months and change as recommended by hydraulic oil supplier; and
• Check calibration of pressure indicators and relief valves annually.
• Lubricate bearings and change oil per the OEM-recommended lube schedule.
• Check gland water condition, level, flow, and pressure every shift;
• Check lubrication of bearings weekly; and
• Perform condition monitoring of pump weekly.
Producers who take into consideration the right sizing and location, as well as perform the proper maintenance are most likely to enjoy optimal results from their filter press
Along with regular maintenance tasks, winterization of the filter press is something to consider. Producers operating in cold climates need to be aware of the damage that ice can cause. Although many filter presses are under roof, freezing can still occur, causing damage to equipment. To avoid damage to your filter press during the cold winter months, review these key points:
• Slurry feed pumps should be opened, cleaned, and drained.
• Gland system for feed pumps should be opened, cleaned, and drained.
• All piping should be opened, cleaned, and drained.
• All valves should be opened, cleaned, and drained.
• Filter cloths and plates should be washed leaving no mud between cloth and filter plates.
• Filter cloths and plates should be dry as possible prior to closing mobile plate.
• Mobile plates should be closed against filter plate stack, but not fully pressurized.
• Threads and machined surfaces, such as cylinder rods, should be protected against rust and corrosion.
• Cloth washing systems need to be drained, cleaned, and blown dry.
• Bay doors should be cleaned and placed in the closed and locked position.
• If it could become exposed to the elements, the filter plate chain opening mechanism should be oiled and covered.
• Hydraulic power units should topped off with oil; oil should be replaced during spring start-up.
As with the rest of your plant, guarding against the impact of cold temperatures will help to ensure your filter press is ready for spring startup.
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