Dewatering Fine Coal Tailings with Recessed Chamber or Membrane Plate Filter Presses

Dewatering Fine Coal Tailings with Recessed Chamber or Membrane Plate Filter Presses

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P. Godwin2 , C. Jenson1 and T. Park1 1 McLanahan Corporation2 McLanahan Corporation Pty Ltd

ABSTRACT Many coal mines have recently turned to the use of recessed chamber or membrane plate filter presses for dewatering fine coal tailings. This change is being brought about due to increasing environmental constraints and cost savings versus alternative technologies (eg tailings dams, belt presses, centrifuges). This paper gives an overview of how filter presses work and the typical styles and options available. It concludes with several case studies of filter presses dewatering fine coal tailings.

RECESSED CHAMBER AND MEMBRANE PLATE FILTER PRESSES

FILTER PRESSES OF TODAY VERSUS PLATE AND FRAME PRESSES OF THE PAST

Typical Process DescriptionThe underflow from a thickener is pumped to a stirred holding tank, ‘surge tank’, rather than a tailings pond. The tank discharges via a high pressure slurry pump, feeding the filter press, ‘feed pump’. The filter press operates in batch cycles, controlled by a Programmable Logic Controller (PLC). The cycle starts by the press closing and locking. The feed pump then fills the chambers inside the press and begins dewatering the slurry using the fluid pressure generated from the feed pump. The press retains the solids, but passes the water (filtrate) through the filter media and away.

‘Recessed Plate’ – When the desired cake moisture is achieved, based on filtrate flow, the feed pump stops. The press is then unlocked, and the filter plates open discharging dewatered cakes.

‘Membrane Plate’ – When the desired amount of feed solids has been pumped into the recessed membrane plates, the feed pump stops. Water is pumped behind the membranes squeezing the individual filter cakes. Once the desired squeeze time has been reached the membrane water is discharged to depressurize the membranes, the press is then unlocked, and the filter plates open discharging the dewatered cakes.

The filter cakes are typically discharged onto a conveyor or directly onto the ground. Then the cycle repeats. A typical flow diagram is given in Figure 1.

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Figure 1 Typical Flow Sheet for a Filter Press with Core Blow and Dual Feed Inlets

General Composition of Filter PressesFilter presses are composed of filter plates which are held together during dewatering. The size of the press is determined by the number, size and depth of the filter plates. When these plates are closed together they create cavities which can be pumped full of solids for dewatering. The plates are covered with a filter media that traps the solids inside the cavities formed between the plates and allows the water (filtrate) to pass through the cloth and be recycled. Filter plates can be divided into three general styles (see Figure 2):

• plate and frame style;

• recessed chamber style; and

• membrane plate style

Filter presses using the three styles of plates noted above are in wide use today in many dewatering applications in a wide range of industries.

Figure 2 Different Plate Styles


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Plate and Frame Style Filter Plates‘Plate and frame’ style plates are one of the oldest styles available and are frequently confused with newer styles. In order to form a chamber on a ‘plate and frame’ filter press two flat plates where closed against a frame to form each chamber. The total number of frames would result in the number of cakes to be discharged each cycle with each frame being enclosed between two plates. The frame thickness between the flat plates would create the depth of the chamber and resulting cake thickness which would form inside of this frame. With the formation of the filter cake inside the frame there were many issues with removal of the cakes once the flat plates where pulled apart from the frame encasing the filter cake. Operator intervention was often required to help knock the cakes out of the frames of the plate and frame style plates.

Although these issues have been resolved with the newer plate styles which will be discussed below many people still unknowingly call all styles of filter presses ‘plate and frame’ presses even if the filter press uses newer style ‘recessed chamber’ or ‘membrane’ style filter plates. This is probably brought about by a misconception of thinking the frame used in ‘plate and frame’ refers to the frame of the machine when it actually refers to the style of filter plates being used on the filter press.

Recessed Chamber Filter Plates‘Recessed chamber’ style plates were created to overcome the problems associated with cake release in older plate and frame style plates. The plates in recessed chamber filter presses are recessed on each side, hence the name ‘recessed chamber’ Filter Plates. As a result when two ‘recessed chamber’ filter plates are closed next to each other a cavity is formed between the two plates resulting in the ‘chamber depth’ or ‘cake thickness’. The cake thickness with recessed chamber plates is then equal to the summation of two adjoining plates (eg 20 mm recess on the right side of Plate #1 plus 20 mm recess on left side of plate #2 = 40 mm cake thickness when the two plates are closed against each other). The first and last plates in a plate pack are only recessed on one side so that a partial cake does not form between the end plates and the frame of the filter press itself. As a result a ‘recessed chamber’ filter press with 150 plates would form 149 filter cakes.

Dewatering with a recessed chamber filter press is similar to dewatering with plate and frame style plates. The slurry to be dewatered is pumped into the empty chambers formed between the closed plates and the pressure created from the slurry forces more solids into the press until the chambers are packed full of solids. Various methods such as time, back pressure, feed or filtrate flow rates can determine when the cakes are fully formed. Once the cakes are formed the feed pump is shut off and the filter plates are pulled apart using various opening mechanisms. Once the plates are pulled apart the filter cakes are able to easily fall out from between the plates as they are not encased in a frame as they were with the older style ‘plate and frame’ plates.

A common misconception with ‘plate and frame’ and ‘recessed chamber’ filter plates is that the filter press squeezes the cakes to dewater the slurry. In actuality the plates are held closed, typically with hydraulic ram(s) sufficient to overcome the force created by the slurry, and the dewatering is done using the fluid pressure created from the feed pump. The opening and closing of the filter press is only required to separate the plates and allow the cakes to discharge at the end of each cycle.

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Membrane Filter Plates‘Membrane’ filter plates are a variation on ‘recessed chamber’ filter plates. The membrane filter plates are recessed on each side of the plate like recessed chamber plates but also have a diaphragm or membrane which lines each side of the membrane filter plate. The membrane is used at the end of each cycle to physically squeeze each cake after the cavities formed between the plates have been filled with solids. The cavities formed between the plates are filled with solids by pumping slurry into the plates generally with pressures of no more than 690 kPa. Then the membranes are pressurized using water or air behind the membranes to push the membrane into the cavity and physically squeeze each individual cake. It is generally recommended to use water for squeeze pressures over 860 kPa for safety reasons due to the compressive and explosive nature of compressed air.

The more compressible the solids are that are being dewatered the more benefit can be attained by using membrane style filter plates. An analogy would be that if you squeeze a slurry full of ball bearings in membrane filter plates you may only be able to rearrange the ball bearings a little better resulting in a slightly drier cake. If however the solids being dewatered are compressible like a sponge then a significant difference can be attained. Due to the flexing of the membranes each cycle, membrane plates will eventually fail and need to be replaced due to fatigue from flexing. As a result, the costs/benefits of membrane style plates have to be weighed against the dewatering requirements to meet the needs of the end use of the filter cakes (eg compaction in an impoundment, hauling costs, etc.).

In tests performed by McLanahan, dewatering fine coal tailings, similar capacities for a given machine length have been seen using ‘recessed chamber’ or ‘membrane’ style plates. The percent solids by weight of the filter cakes using ‘recessed chamber’ plates is typically between 70 – 80% solids w/w with the percent solids by weight of the filter cakes from ‘membrane’ style plates typically being a 73 – 83% solids w/w or a few points higher. However in some cases air can be blown through the filter cakes ‘cake blow’ on more permeable filter cakes which in some cases results in similar percent solids using either style of filter plate.

Membrane plates are supplied in two main plate configurations:

• All plates on the filter press being composed of ‘membrane’ plates; and

• Every other plate being a ‘membrane’ plate and the alternate plates being composed of ‘companion plates’ which are essentially the same as ‘recessed chamber’ plates. This configuration is called a ‘mixed pack membrane’ filter press and is the more frequently supplied configuration for these types of applications as it results in less ‘membrane’ platesand consequently lowers upfront capital costs and lower long-term operating costs.

Membrane plates are available in two main styles:

• Welded membranes – The membranes are heat welded to the body of the plate. When the membrane fails from flex fatigue the entire plate has to be replaced; and

• Replaceable membranes – The membranes are replaceable and attach to the plates in variousmethods depending on the filter plate manufacturer. This allows the membranes to be replaced when they fail without replacing the entire plate.

Filter Press SizesFilter plates generally come in dimensions from 400 x 400 mm to 2000 x 2500 mm with chamber thickness ranging from 15 to 50 mm. For plate and frame and recessed chamber plates the ‘chamber thickness’ is equal to the final ‘Cake Thickness’ as the cakes are not squeezed during


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the dewatering. The plates are most commonly made of polypropylene and manufactured through compression molding. The number of filter plates on a filter press can range from as little as 3 or 4 plates to more than 180 filter plates on a single press. The frames which support the weight of the plates and cakes are generally broken down into two categories denoted as Overhead Beam or Side Beam filter presses which will be discussed in more detail later. The number, size and chamber depth of the plates determines the filter press size.

Cake ThicknessBesides plate size, plate style and number of plates, another key component in dimensioning the filter press is to determine the proper chamber thickness or cake thickness. With chamber thicknesses available from 15 to 50 mm it is important to evaluate the dewatering rates using various thicknesses. For very permeable materials the layer of solids building up on the filter media does not have a great impact on the dewatering rates and thicker chamber thicknesses can be used.

In tailing applications where permeability of the cake forming on the filter media often decreases significantly as the cake builds up, using a chamber that is 10 or 15 mm thicker can often result in cycles that are up to 2 – 3 times longer resulting in much greater fluctuations in cycle times throughout the day. As a result thinner ‘recessed chamber’ cakes (25 mm versus 40 mm) or ‘membrane’ plates (40 mm which will then be compressed) are generally preferable in tailings applications. Tests can be performed to evaluate changes in permeability with varying cake thicknesses and a conservative approach leads to selection of thinner recessed chamber plates or membrane plates to reduce impacts of feed changes on cycle times and dewatering performance.

Filter Press CapacityOnce the total volume of the filter press is determined, Mass Balance calculations are used to determine the weight of solids that will be discharged per cycle and the number of cycles that will be discharged per hour. Capacity of a filter press/hr is then calculated as:

• Volume discharged per cycle = Volume of each chamber × Total number of Chambers;

• Weight of solids discharged per cycle = Volume discharged per cycle × relative density of thefilter cake × Percent solids of the filter cake;

• Cycles per hour = Cycle time in minutes/60 minutes; and

• Capacity per hour = Weight of solids discharged per cycle × Number of cycles per hour.

Cycle TimeThe cycle time is typically broken down into the following components:

• Closing time – Time it takes for the filter press to close and build sufficient pressure to holdthe plates closed during dewatering;

• Fill time – Time it takes to fill the empty volume of the press with slurry;

• Ramp time – Time to build pressure during filtration to the terminal or final pressure;

• Filtration time – Time the pump continues to pump at the terminal pressure;

• Cake Blow time (w/Cake Blow option) – Time air is blown through the filter cakes;

• Core Wash/Core Blow time (w/Core Wash/Core Blow option) – Time for water and air to flushout the feed core of slurry; and

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• Opening time – Time to open the mobile plate and all the filter plates to discharge the cakesand be ready to start another cycle.

Note: It is important to allow for extra time (Safety Factor) to stages of the process that can be variable. For example in a tailings application the filtration time is impacted by the underflow density, feed gradation, clay content and chemical dosing in the thickener. A safety factor can be added to this time to allow for these variations. For more consistent feeds such as dewatering metal concentrates less safety factor is needed.

Filter MediaThe selection of proper filter media is critical for optimal cake release as well as achieving the desired filtrate clarity and wear life. The main criteria used in determining the correct filter cloth for an application are:

• air permeability;

• weave;

• type of fibres used (most commonly Nylon, Polypropylene of varying types) and their electrical affinity or chemical degradation with respect to the material being dewatered;

• thickness or diameter of each fibre;

• are the fibres ‘Mono-filament’ or essentially single strands like a fishing line which are better for cake release or are they ‘Multi-filament’ like a rope composed of twisted strands which canbe better for particle capture;

• density per unit of area of the cloth. Higher densities are more durable to support release ofhigher density filter cakes to prevent ‘Blousing’ or stretching of the filter cloth; and

• coefficient of Friction.

Side Beam Filter Presses‘Side Beam’ style filter presses use beams running along the sides of the filter press to support the weight of the filter plates and filter cake as well as the dynamic loads created by the closing forces of the filter press. Advantages typically noted for this style of filter press are:

• fast opening system;

• easier to implement plate shaking systems;

• allows for faster low pressure wash systems used commonly in mineral concentrateapplications; and

• shorter height for small portable applications.


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Figure 3 McLanahan Side Beam Filter Press

Overhead Beam Filter Presses‘Overhead beam’ style filter presses use beams located above the plates to support the weight of the plates. Advantages typically noted for this style of filter press are:

• Suspension of the plates from overhead beams allows the dynamic loads/forces created fromthe hydraulic ram(s) holding the filter plates closed to be isolated or reduced from loading the beams which support the weight of the filter plates. These forces can be as high as 6670 kN of force on a 2000 mm x 2000 mm press operating at a feed pressure of 1550 kPa.

• Easier access for cleaning and change out of the filter cloths as there are no beams in the wayon the side of the filter press

• Wider opening between the filter plates (up to approximately 900 mm) resulting in:

º Less abrasion as cakes are released;

º More movement to help cake release;

º Plates are too far apart for the filter cakes to be attached to the cloths on both platesresulting in half the surface tension and improved cake release;

º Fast plate opening to keep cycle times down; and

• Simple and fast opening with limited moving parts.

• More suitable for high-pressure cloth washing systems due to increased space between the plates. High-pressure wash can be done with an operator or automated system.

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Figure 4 McLanahan Overhead Beam Filter Press

FILTER PRESS OPTIONS

Filtrate Discharge OptionsThe filtrate can typically be discharged from the filter plates in two commonly used methods:

• Internal – The filtrate passes through a manifold created by the holes located in the corners ofthe filter plates. This is a lower cost option and is desirable when the filtrate contains chemicals that should not be exposed to the air; and

• Open Filtrate Discharge – The filtrate exits each plate through individual spigots found on each plate. This option is preferable to allow easy identification of damaged filter cloths for the operator. Quick identification of damaged filter cloths prevents unnecessary abrasion tothe filter plates.

Figure 5 Open Filtrate Discharge


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Dual Feed InletsDual feed inlets reduce the inlet velocity in half. This option helps reduce wear on filter cloths and also ensure a more even distribution of the feed pressure for dewatering in large filter presses. Having dual feed inlets also allows for easy addition of ‘Core Wash/Core Blow’ options which require dual feed inlets.

Core Wash/Core BlowWhen it is desirable to eliminate the discharge of the feed core which is not dewatered during normal filtration ‘Core Wash’ and/or ‘Core Blow’ options are used.

• Core Wash – ‘Core wash’ uses a pressurized water line to wash out the feed core prior to opening of the filter press. This prevents buildup of slurry on the plates which can reducecloth washing and blowouts (leaks between the filter plates); and

• Core Blow – ‘Core blow’ uses pressurised air to evacuate the slurry from the feed core. This prevents buildup of slurry on the plates which can reduce cloth washing and blowouts. If a ‘core wash’ is used prior to the ‘core blow’ then the pressurized air only needs to remove the water from the feed core. This combination of using a ‘core wash’ followed by a ‘core blow’ isthe most efficient option.

CAKE BLOWOn materials that can be dewatered quickly due to their high permeability it is possible to blow air through the filter cakes at the end of the cycle in order to remove additional amounts of moisture from the filter cake. In some coal tailings applications with limited amounts of clay this can be as much as 5 additional points (eg 80% vs 75% solids w/w.)

Automatic Filter Cloth WashMost tailings applications only require filter cloth washing once every week to two weeks. When desired to reduce operator requirements it is possible to automate this function.

Drip-Tray or Bomb Bay DoorsWhen an automated cloth wash is used it is typically necessary to use a drip-tray or bomb bay doors underneath the filter press to capture the water coming from the cloth wash and prevent it from going onto the filter cakes.

FILTER PRESS ANCILLARIES

Feed PumpThe feed pump is a slurry pump or series of slurry pumps designed to generate the necessary flows and pressures required during the cycle. Depending on the flows and pressures required typically centrifugal, double diaphragm or piston diaphragm pumps are used.

Surge TankThe surge tank receives the thickened slurry and buffers the flow between the thickener (continuous process) and the filter press(es) (batch process). Proper selection of the surge tank ensures a smooth transition from the continuous process coming from the wash plant to the batch process of the filter press. Surge tank(s) are typically sized to store 1.5 filter press cycles or more of slurry. The filter press is typically automated to wait until there is enough slurry in the surge tank for a complete cycle before starting a cycle.

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TESTING FOR SIZING AND SELECTION PURPOSES

Lab TestsCommon tests performed on Coal Tailings for selection of a filter press for Coal Tailings applications include:

• Filtration tests. Tests are performed at varying feed solids, cake thickness, and pressures todetermine filtration times and percent solids achievable;

• size distribution;

• % solids; and

• Relative density (RD). It is important to accurately determine two components of the RD for accurate sizing:

º RD of the slurry – the feed density has a large impact on the cycle times; and

º RD of the solids – to accurately determine the weight of solids discharged from the filterpress each cycle at a given cake moisture the relative density of the solids is required.

Pilot TestingFor sites with existing Coal Prep Plants it is possible to bring demonstration equipment on site to perform on site filtration tests.

CASE STUDY 1Dewatering Fine Coal Tailings in Bishop, WV with two McLanahan ‘Recessed Chamber’ filter presses

BACKGROUNDMcLanahan was approached by Taggart in 2011 (now DRA Global) about a project where Justice Corporation needed to dewater their fine coal tailings at a greenfield site in Bishop, WV without the use of settling ponds. Forge (ie Taggart) supplied slurry samples and design criteria to McLanahan for machine selection to meet these requirements. The McLanahan recessed chamber filter presses were purchased by DRA Global and commissioned in February of 2013 and have been operating since that time without the use of settling ponds.

Design CriteriaMaterial: Thickener Underflow (Coal) Solids Flow Rate: 68 dry t/h

RD of Solids: 1.6

Thickener Underflow Density: 30% Solids by weight

Objective: Dewater thickener underflow into a ~75% solids w/w cake

Lab TestingLab tests were performed to evaluate filtration times and cake moistures achievable on the thickener underflow sample submitted from Forge Group.


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Figure 6 Dewatered Filter Cakes from 5 and 10 Minute Filtration Tests on Submitted Samples

Equipment Selected2 x filter presses 2 x 2 m with 155 – 25 mm recessed chamber plates

2 x feed pumps 130 kW 10 x 8 double stage feed pumps

Discharge volume/press: 13.1 m3/cycle

Dry solids discharged/cycle/press: 13.6 tonnes

To achieve the 68 t/h required each press needed to achieve 2.5 cycles per hour or 24 minute cycles. 2 Presses x 2.5 Cycles/Hour/Press x 13.6 tonnes/Cycle = 68 t/h

Figure 7 McLanahan Overhead Beam Filter Presses at Justice Coal Prep Plant in Bishop, WV

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InstallationInstallation was done by DRA Global with assembly supervision of the filter presses from McLanahan Field Service personnel. DRA Global provided the turnkey installation of the entire coal prep plant.

CommissioningCommissioning was done with personnel from McLanahan Corporation and DRA Global from February 22 to February 28, 2013.

PerformanceThe filter cakes have consistently exceeded the 75% solids w/w required and have been tested to be typically around 79 – 80% w/w solids with cycle times within the expected range based on the initial lab testing.

Complete cycles have been able to run as short as 15 minutes per cycle resulting in up to 108 dry t/h with both presses running. The installation of the McLanahan filter presses has allowed Justice Corporation to meet their goals of running without any settling ponds while keeping up with the tailings generated from the Coal Prep Plant.

Filter Cloths – To date approximately 6 of the 310 filter cloths have been replaced far exceeding the estimated wear life of 3000 – 6000 hours.

Figure 8 Dewatered Filter Cake at 79% Solids w/w and Recycled Water (Filtrate) from the McLanahan Presses at Justice Corporation in Bishop, WV

CASE STUDY 2Dewatering of Coal Tailings at a coal mine located in Indiana USA, using 2 x McLanahan Recessed Chamber filter presses and 2 x McLanahan Membrane Plate filter presses

BACKGROUNDThe existing tailings dam(s) were approaching capacity and the decision was made to convert wet tailings to dry cake for disposal with coarse rejects. The equipment selection would need to produce a dry enough cake to meet compaction requirements and thereby eliminate the need for future tailings dams.


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After testing and working with multiple vendors the customer decided that using either recessed or membrane plate filter presses were the best option in order to meet their compaction requirements. Pilot testing was performed on site by McLanahan to generate enough filter cake to ensure the moisture levels reached would perform adequately to meet their compaction requirements for their reclamation plan.

Design CriteriaMaterial: Thickener Underflow (Coal) Solids Flow Rate; 195 t/h

RD of Solids: 1.7

RD of Slurry: 1.2 to 1.35

Ash Content: 24.5 – 39.4%

Sulfur Content: 2.5 – 5.3%

Thickener Underflow Density: 44% solids by weight

Objective: Dewater thickener underflow into a ~75% solids w/w cake

Lab TestingLab tests were performed to evaluate filtration times and cake moistures achievable on the thickener underflow sample submitted Lab testing indicated:

Pressure 15 Bar with 4 min filtration times would achieve an average of 74% solids w/w.

By adding 3 min of cake blow, cake solids were calculated to increase to an average of 76.5% solids w/w.

Equipment Selected2 x 180 Plate 2 x 2 m x 25 mm Recessed Chamber

2 x 151 Plate 2 x 2 m x 40 mm Membrane Plate

Note that all filter 4 filter presses, shared a common size filter press frame, and all fed from a common single agitated surge tank.

Because the membrane plate presses were able to achieve slightly lower moisture contents in testing (2 – 3 % points drier) it was decided by the customer to install 2 recessed chamber presses which have lower operating costs and 2 membrane plate presses which achieved slightly lower moisture cakes. This would allow the customer to blend the cakes as needed to achieve the compaction requirements.

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Figure 9 McLanahan Overhead Beam Filter Presses installed in Indiana

InstallationGraber Construction did the installation of the equipment with supervision from McLanahan Field Service.

CommissioningCommissioning was done by McLanahan with assistance of Graber and onsite personnel.

PerformanceThe 2 x 180 Plate 2 m x 2 m x 25 mm recessed chamber filter presses operate with total cycle times of 20 – 24 min, at 15 Bar pressure, with the cycle including the 3 min of cake blow. The resulting filter cake has an average cake solids of 77 – 78% w/w.

The 2 x 151 Plate 2 m x 2 m x 40 mm membrane plate filter presses operate with a feeding pressure of 8 Bar and squeeze pressure of 15 Bar with a total cycle times 20 – 24 min including 3 min of cake blow. The resulting filter cake has an average cake solids of 78 – 79% solids w/w.


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Table 1 Comparison of Filter Presses installed in Coal Tailings Applications

Year Location Customer Qty Size and Model Plate Type Feed %

Solids w/wCycle Time

Cake Moisture TPH

2009 IN/USA 1st Site in Indiana 1

1.5 m x 1.5 m x 100 x

35 mmRecessed 25 – 35%

15 – 20 min

22 – 25% 22+

2011 WV/USA Taggart/Bishop 2

2 m x 2 m x 155 x 25

mmRecessed 30%

15 – 22 min

20% 72+

2012 Russia Severstal Resources 2

2 m x 2 m x 170 x 25

mmRecessed 25 – 40% 20

min 17 – 20% 90+

2013 IN/USA 2nd Site in Indiana

22 m x 2 m x 180 x 25

mmRecessed 44%

20 – 24 min

22%

195+

22 m x 2 m x 151 x 40

mmMembrane 44%

23 – 27 min

21%

CONCLUSIONRecessed chamber and membrane plate overhead beam filter presses are currently in use in the US and elsewhere in the world to effectively and economically dewater coal tailings.

Filter presses should be a considered option wherever the site requirements call for lower moisture, higher recovery of water, or dry stacking as an alternative to traditional tailings dams.

ACKNOWLEDGEMENTSThanks are extended to Justice Corporation and DRA Global (formerly Taggart) who were involved in the selection and operation of the McLanahan filter presses in the Case Study.

REFERENCESJenson, C., 2014, “Dewatering Coal Tailings”, 2014 Coal Prep Conference Proceedings.Jenson, C., 2012, “A Pressing Need”, World Coal, V21, N12, pp28–34.Jenson, C., 2014, “Dewatering Fines”, Coal Age, March, pp38–40.

Dewatering Fine Coal Tailings with Recessed Chamber or Membrane Plate Filter Presses

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