Novel screening unit provides alternative to conventional shale shaker

In October 1997, the Pansep screen was introduced in the minerals-processing industry to treat slurries containing fine mineral fractions in the 37-2,000 um range and to dewater waste liquid streams containing sludge and solids (Fig. 1).

Over 50 units are in commercial operation for such companies as Anglo American Corp. (patent holder); Cleveland Potash Industries Ltd., Ireland; and Jindex Ltd.

In addition, the unit has been extensively used on coal-washing plants in South Africa, Australia, and Canada.

An alternative

• Cyclones
• Vibrating screens
• Linear screens
• Sweco screens
• Trommel screens
• Derrick screens
• Spiral concentrators
• Rake classifiers
• Sieve bends
• Centrifuges.

Existing technology

• Poor cuttings size recovery. Undersize cut point recoveries greater than 98.5% w/w (weight by weight) on the D50 partition number-the desired point where 50% w/w of feed solids reports to the undersize and oversize, respectively-are difficult to achieve.
• Lack of a self-cleaning mesh.
• Vibrating features that tend to cause component failures.
• Blinding of the mesh surface requiring repeated cleansing with water hoses.
• Screening promotion restricted exclusively to vibrating motion.
• High losses of undersize material in relation to oversize material.
• No means to optimize recoveries or screening efficiencies.
• Unit bypassing resulting in oversize buildup in the downstream settling tank.

Unit description

The primary operating components include:

• Drive-Shaft-mounted gearbox that moves the inter-linked pans.
• Drive sprocket-Moves the inter-linked pans.
• Rear tail guide-Guides the inter-linked pans.
• Linkage shafts-Connect the pans together.
• Screen pans-Double-decker pans used to accept slurry feed on both sides (top and bottom runs).
• Coarse-screen mesh-Screens out big lumps and trash while protecting the fine-screen mesh.
• Fine-screen mesh-Rectangular or slotted steel mesh that screens the feed slurry to the required size. Pneumatic tensioner on mesh panels eliminates mesh creasing and sagging.
• Top and bottom feed boxes-Feeds top and bottom pans with slurry.
• Underpans-Collects undersize fines.
• Oversize chutes-Collects oversize solids from the top and bottom runs.
• Spray bars-Promote and increase undersize screening efficiency.
• Preclassifier plate-Laser-cut feed plate used to increase undersize-screening efficiency.

Top and bottom screens

The oversize cut is retained on the mesh and then discharged from the top of the screen into the top pass oversize discharge chute as the inter-linked pans rotate around the drive sprocket.

Slurry is fed to the bottom feed box, distributing the mixture onto the bottom moving deck of pans. The screen area that was previously on the bottom side of the pans in the top side screen pass, now becomes the screen area in the bottom side screen pass.

The undersize cut of slurry drains through the mesh on the pans and is collected in the bottom pass under the pans. The oversize cut is retained on the mesh and is then discharged from the top of the screen into the bottom pass oversize discharge chute after the inter-linked pans have rotated around the rear tail guide.

Spray water, solids classification

Spray water is used to maximize undersize solids recovery, minimize misplaced undersize solids in the oversize discharge, fluidize the solids on the mesh, and clean the mesh prior to receiving fresh slurry feed on each pass.

Solids-feed preclassification plates may be provided to preclassify undersize solids as the slurry is fed onto the mesh using a feed solids concentration and pulp specific gravity greater than 65% w/w and 1.9, respectively.

Fig. 5 [89,748 bytes] shows the preclassification plates for each feed distributor. The prelaydown of undersize solids ensures that the undersize solids are placed at a minimum vertical distance from the mesh surface, thereby improving and increasing the recovery of undersize material.

Mesh selection, drainage

Current weaving technology allows for custom-designed meshes to be selected with wire diameters as low as 0.02 mm for a 20-um aperture of minimum width. The cost of steel mesh primarily increases with an increase in the number of weaves required per unit area. Thus, for small apertures, the cost of the steel mesh increases.

Drainage

Zone 1. The section where the mesh receives the feed and contains the primary drainage and solids separation components.

Zone 2. Solids located on the meshes that are washed and fluidized with sprays from the top side and bottom side of the mesh for each feed pass.

Zone 3. Final drainage of the wash water and removal of free surface water prior to discharge of the oversize solids.

Where drainage rates are low, the mesh's horizontal travel length is increased for the application (the width of pans is not changed on units).

In order to calculate the mesh open area, Equation 1 may be used (see Equation box) [148,335 bytes]:

Performance measurements

As it is virtually impossible to measure the flow rates of the feed (undersize and oversize streams in real time operations), the partition numbers for the various size fractions must be determined from sample data gathered in a steady state for the three streams using an analytical equation.

The undersize and oversize partition numbers are derived in Equations 2-5.

Solving Equations 2-4 to eliminate flow rate terms, Equation 6 can be applied to the oversize partition number.

The partition curves are plotted on a semi logarithmic graph with partition number on the y-axis vs. the particle size on the x-axis (Fig. 6 [52,187 bytes]). The partition curves are a useful tool to establish the sharpness of the separation cut for the equipment type being considered.

The gradient from the D80 inflection point A1, to the D20 inflection point at B1, must be maximized in order to obtain maximum screening efficiency and to minimize misplaced material in the oversize and the undersize ranges.

The partition curve gradients and the particle size bandwidths from particle size spans A1A2 and B1B2 are the key parameters to be checked when the performance of the equipment is being determined or when various equipment types are being considered for a particular process.

Partition number basis

In the case of cyclone sizing, the D50 partition number particle size is required. In addition, with the Pansep unit, the D50 partition particle size is also required because the sizing of the unit is primarily based on the quantity of oversize material above the D50 cut point. This must be retained on the mesh at less or equal to a maximum thickness specification.

For industrial applications, the conversion of a D80 specification to a D50 specification is often calculated with the Lynch Rao equation as shown in Equation 7.

For example, a D80 undersize partition number at a particle size cut of 74 um is required to operate a milling circuit. The equivalent D50 undersize particle size cut point needed to meet the required D80 cut-point specification is calculated as follows:

PN_U = 80
PN_O = 20 (Equation 5)
D₂ = 74 um
D₁ = 111 um (Equation 7)
The Lynch Rao equation partition numbers should be used as the base reference to compare the actual partition numbers achieved for a specific equipment type.

Screening efficiency

The undersize screening efficiency is calculated in the same manner as the oversize, however for the solids, it becomes less than the D50 cut point size.

Screen partition results

Tests were completed to establish if the first pass release of gold could be maximized and the lock up of gold in the mill could be minimized by taking advantage of the high separation efficiencies using the Pansep screen to remove solids at a lower particle size cut point.

The tests were completed with 140-um x 400-um aperture-slotted steel mesh with 112-um x 160-um gauge wire (39.7% open area). A cut-point specification of 150 um at the D50 partition number was the preliminary requirement.

The analysis data for the tests are detailed in Table 1 [33,342 bytes] while the partition numbers are calculated from Equations 4, 5, and 6 (Table 2) [63,728 bytes].

The partition number results are shown in Fig. 7 where they are compared with cyclones for the same D50 partition number and feed conditions.

Pansep screen vs. cyclone

Screening efficiencies for the Pansep screen are shown in Table 3 [42,019 bytes] with selected reference cut points of 130, 140, 150, and 212 um. The screening efficiencies are very high for undersize solids.

For this processing application, a 350-um aperture length would be selected in order to reduce the 163-um D50 partition number size to 150 um.

Because of the rectangular dimensions of the mesh, slipage of 1.5-2.0 oversized solids in with the undersized are typical values achieved with other Pansep screen installations. The primary objective for these tests has been satisfied whereby undersize recovery has been maximized, in turn minimizing over-grinding in the mill circuit as the undersized solids exit the circuit sooner.

Pansep screen sizing

The oversize layer thickness is a key parameter because the thinner the layer, the easier it is for small particles to pass through the mesh and the more effective becomes the topside and underside sprays in fluidizing the solid particles.

Solids washing promotes undersize screening efficiency by minimizing the opportunity for smaller particles to accumulate on the top of the layer while reducing the possibility of agglomerating or adhering to oversize solids.

Screen sizing incorporates the variable speed capability of the unit. Current units are operated to a maximum horizontal pan speed of 36 m/min. When drainage time is a design-limiting factor, the unit length must be increased.

Several models are available with different lengths but the same width. Units may be installed in parallel or series. If a two-feed deck unit is used, dual independent feed supplies may be processed, subject to the mixing of the product stream requirements.

Pansep screen sizing for the output from four mills is shown in Table 4 [177,030 bytes]. The sizing program used for the Pansep screen is available in a spreadsheet program for use by engineers and companies.

Fluid screening

The process configuration required for screening drilling fluids is detailed in Fig. 8 [54,080 bytes]. Drilling fluids are fed to the top screen pass after the steel mesh has been washed with wash water. Wash water sprays are mounted above and below the top pass screening level to promote undersize screening efficiency.

A solids lifter or riffle allows dry oversize solids (maximum 3% w/w free moisture) to be obtained. Wrappers or knockers located at the oversize solids discharge end minimize solids holdup on the underside of the mesh of the return pass. An air knife can also be installed for solids removal.

Steam and air blowing features can also be provided for removal of adhered low pour point oil and to reduce the moisture content of oversize solids. The undersize stream discharges into a launder box, which is provided with an overflow partition plate to segregate the liquid portion from the mesh-cleaning section. This stream may have trace amounts of oversize solids.

Liquid from the pump box is pumped to a hydrocyclone, removing any trace of oversize solids originating from the topside pass mesh-cleaning operation. Oversize solids that are removed are fed to the top of the screen.

The hydrocyclone produces recycle wash water for the spray bars. Available spray nozzle orifice sizes are 0.4 to 4 mm in diameter, accounting for the varying water quality residual undersize solids.

Recessed spray nozzles are lifted above the internal bottom of each spray bar. Each spray bar is equipped with an internal removable coarse strainer and a blow-down valve positioned at the end of the respective spray manifold, serving to remove any solids that might settle on the bottom of the spray bar after an extended time period.

Makeup water is added to compensate for downhole drilling fluid losses and solids makeup in the mud tank. As configured, the major advantages of using the screen include:

• Premesh cleaning prior to receiving the drill mud feed
• Optimizing retention time using a variable-speed drive
• Solids washing on the mesh resulting in increased screening efficiency.

Construction and maintenance

Steel mesh is recommended over polyester mesh for the following reasons:

• Steel mesh has a 30% higher drainage rate.
• Steel mesh uses thinner wires to obtain equal strength and wear life; however, there is an increase in open area.
• Steel mesh is unaffected by lime (Ph of +10). Polyester life would be reduced to 2-3 weeks when the feed is highly alkaline.
• Polyamide stretches.
• Polypropylene is not resistant to abrasion.
• When steel mesh wears, the open area increases, whereas with polyester mesh, the open area decreases.
• Steel mesh apertures are more accurate.
• Plastic meshes, with the exception of polypropylene, are hydrophilic and reduce drainage rates.
• Steel mesh is neutral and drains faster than polyester mesh.

Mesh panels on a number of units have not been replaced for more than 9 months of continuous operation. Life expectancy of the steel mesh is improved by the removal of wear otherwise caused by vibration devices along with the elimination of any potential damage caused by operators trying to clean the mesh fixed within a frame.

The provision for a pneumatic tensioner tube on each mesh panel also improves life expectancy as the taught ness of the mesh can be set so there is no sagging or creasing of the mesh when loaded up with solids.

Mesh panels are removable within minutes if a replacement is required. Mesh life is also improved because the solids load is distributed over a larger screening area. An increased mesh life span can also be obtained by increasing the mesh wire gauge.

Downstream applications

Cyclones have been tried by a number of companies with varying degrees of success. However, because coke particles have a low density, the performance of cyclones is generally poor. Excessive wear on high-pressure pumps and lance nozzles caused by residual coke fines is a major problem.

A mini Pansep screen with mesh-cleaning sprays can be considered for ultra-fine dewatering of the recycle water. Minimum apertures as low as 20 um can be considered. The recovered coke fines can be routed to a hopper, rail car, or stockpile. A grizzly screen, roller screen, and mini Pansep screen configured in series would eliminate the requirement for a coke pit.

Produced-water systems

A mini Pansep screen with mesh-cleaning sprays only would be considered for ultra-fine dewatering of the produced water prior to well reinjection. This will also reduce filtration loads on the downstream filters, if present.

The major requirement for produced-water processing is that the water must be degassed before the screen unit. This may be performed upstream of the unit by adding a standpipe fitted with a vapor vent and liquid seal leg.

Acknowledgment

• R. Buisman, Particle Separation Systems, Greenhills, South Africa. Africa manufacturing facility for the Pansep screen.
• W. Altmann, Particle Separation Systems, Bischofszell, Switzerland. Europe manufacturing facility for the Pansep screen.
• Anglo American Ltd. and Anglo American Research Laboratories, South Africa.

Bibliography

1. Besendorfer, C., "Exert the Force of Hydrocyclones," Chemical Engineering, September 1996.
2. Kelton, G.P., Torres, D.L., and Rawlins, H., "Use hydrocyclones to Improve Delayed Coker Operations," Hydrocarbon Processing, March 1998.
3. Beatrix Mining Ltd., Milling Test Report, November 1998.

The Author

Courtney Dehn is a process manager for Lindsay Process Plant. He holds a BS in chemical engineering from Bath University, U.K. He has 18 years' experience as a process-equipment design engineer in the refining, chemical, petrochemical, and mineral processing industries.

Copyright 1999 Oil & Gas Journal. All Rights Reserved.