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Wet Screening: Linear or Rotaspiral Screens?

PSS supplies both the Rotaspiral and Linear Screens with their own unique features:

Linear Screens:

The Linear screen presents a woven Polyester (plastic) cloth with apertures perpendicular to the slurry flow.

The screen cloth travels around a set of pulleys, a drive pulley and a cloth tracking pulley. The Linear screen uses a feed box with HDPE distribution plates to distribute the inflow of slurry over the screening area. Once the slurry passes through the cloth the slurry is guided out of the underflow chute to the side outlet of the screen.

Oversize remains on the screen cloth and is discharged/washed off at the end of the screen.

Traditionally these cloths are polyester. Polyester is hydrophilic with PH limitations 3 – 11 (it needs to operate between these perameters) Above PH 11 the polyester becomes brittle.

The polyester cloths are normally joined into an endless mesh using clipper seams.

Use of the correct lacing hooks is important whereby the individual hooks are offset, and a locking wire of 2.0 mm can easily slot into a 2.5 – 3.5 mm loop. Specialized glue which is strong and flexible prolongs the seam life.

The main features of PSS Linear Screens are:

• Use of a Pneumatic take up (prevents cloth creasing and allows for correct tensioning)
• Using the Buistex Light Weight Rollers, at ± 70 % lighter than conventional rubberlined mild steel rollers. Turn much easier, are 100 % true (extruded) and equal in strength to rubberlined mild rollers. This renders maintenance much easier, provides longer cloth and roller life.
• Elimination of negative cloth bending, simpler design, PSS does retrofit PSS pneumatic design to standard design screens.

Linear Screen Size (size of feed box area)

Rotaspiral Screens:

The Rotaspiral screen is a rotary drum screen with a build in spiral to remove oversize fast.

Rotaspirals use steel mesh with a high open area. The mesh is neutral and allows a higher throughput through the mesh than Polyester Linear Screen cloths.

Steel mesh cut points are more accurate. PSS devised a very simple mesh installation method for fast replacement on very abrasive slurries mesh life in excess of 6 months is normal.

The drum is suspended on two shafts with wheels on top, one of which is driven by a gearbox and motor.

Rotaspirals only have 4 bearings.

Feed is introduced inside the drum using a HDPE feed pipe with distribution holes.

The mesh is cleaned on the top downwards using a spray bar, and the wash water collected and guided to the last spiral.

The Rotaspiral can be used for sizing applications and screen down to 75 – 100 microns.

For sizing applications the meshes are slotted in a ratio of 1 : 4 to prevent “pegging” or “blinding”

ROTASPIRAL VS LINEAR SCREEN

The Rotaspiral has the following advantages:

1. Less Weight
2. Small Footprint
3. No moving cloth
4. Less maintenance
5. Lower capital cost
6. No tracking system
7. No air requirements
8. Less power (± 30 %)
9. Much Longer mesh life
10. Steel vs Polyester (Hydrophilic)
11. No rollers to support mesh media
12. Steel is UN affected by PH (Polyester limit is 11.5)
13. Better washing of woodchips (Because they tumble in the last spiral)
14. Can accept surges – woodchips cannot by pass to the product flow/underflow

Efficiency:

Linear Screens are efficient because the woodchips fall flat and do not move hence cannot pass through the mesh.

Rotaspirals turn and a woodchip is deposited/tumbles a few times, hence in theory not as efficient as Linear Screens.

However a woodchip on a Linear Screen can splash to the side, they can pass over the side of the cloth to the underflow. This cannot happen on a Rotaspiral Screen.

Particles larger than the Rotaspiral aperture cannot by pass to the underflow.

Arguably:

Rotaspirals are more efficient than Linear Screens on carbon scavenging duties.

Arguably:

Linear Screens are more efficient than Rotaspiral Screens on a Woodchip/Splinter Screen duty.

Maintenance:

The Rotaspiral is cheaper and easier to maintain and requires much less attention.

Preferences:

Gold Mines prefer Linear Screen for woodchip removal – Gold Mines should use Rotaspiral Screens for Carbon Scavenging.

Platinum Mines prefer Rotaspiral Screens because they can cope with high flows and very abrasive (Chromium) slurries and the efficiency are acceptable and better than Linear screens on these duties

1. Berzillius Stolberg – GMBH  Rotaspirals: 2 of RS 1.2 – 1.8  Duty: Classification @ 250 micron Oversize 50 % lead slag 2 – 6 tph each (10 – 30 m³/h each)
2. Montana Winery – New Zealand Rotaspirals: 2 of RS 0.8 – 1.0 Duty: Classification @ 100 micron Oversize 1 – 2 % Organics 15 – 20 m³/h each
3. Anglo Platinum - SA Rotaspirals: 1 of RS 1.2 – 1.8 Duty: Tramp screening @ 2 mm Oversize 1 – 2 % Tramp & Woodchips 300 m³/h @ 1.3 Merensky reef
4. Lonmin Platinum - SA Rotaspirals: 1 of RS 1.2 – 1.8 Duty: Tramp screening @ 2 mm Oversize 1 – 2 % Tramp & Woodchips 300 m³/h @ 1.3 slurry SG
5. MMS - Russia Rotaspirals: 4 of RS 1.6 – 3.0 Duty: Classification @ 400 micron Oversize 70 % Fine coal 30 tph @ 1.2 slurry SG
6. MAED – Talas GM - Ukraine Rotaspirals: 2 of RS 1.6 – 3.0 Duty: Tramp screening @ 600 micron Oversize 1 – 2 % Tramp & Woodchips  800 m³/h (150 tph)
7. Siyanda Chrome - SA Rotaspirals: 1 of RS 1.6 – 3.0 Duty: Classification @ 100 micron  Oversize 50 % Chromium 80 m³/h (15 tph)
8. Aquarius Platinum – SA Rotaspirals: 1 of RS 0.8 – 3.0 Duty: Classification @ 100 micron Oversize –1 % Deswik overflow 20 m³/h (20 tph)
9. Bindura Nickel – Zimbabwe Rotaspirals: 1 of RS 1.6 – 3.0 Duty: Trash Screening @ 1000 micron Oversize –2 % Flotation Circuit feed 1100 m³/h
10. Chrome-Tech Holdings – Lonmin SA Rotaspirals: 1 of RS 1.2 – 1.8 Duty: Trash Screening @ 2 mm Oversize 1 – 2 % Feed 400 m³/h
11. Outotec (Filters) Oy – Finland Outotec has standardized on PSS Rotaspiral sand Linear Screen for Larox – feed screening to Remove trash (January 2013)
12. Apogee Silvell Mine – Bolivia Rotaspirals: 1 x RS 0.8 – 1.0 Duty: Trash Screening at 1 mm Oversize 1 %  Feed 30 m³/hr
13. AngloGold Ashanti – Savuka Mine Rotaspirals: 1 x RS 0.8 – 1.0 Duty: Mill Return Water Screening @ 500 micron Oversize ± 1 - 2 % Feed 250 m³/hr

The First Rotaspiral Screens

Experience & References:  Berzillius GMBH

The Rotaspiral Fine Screen is a new development in fine screening technology, which may be reminiscent of trommel screens of previous times. There are, however, significant differences in design and application. During November 2001, Particle Separation Systems (PSS) delivered two Rotaspirals to Berzillius Stolberg GmbH of Germany to separate fines from granulated slag in the lead smelter.  Feed rate to the parallel Rotaspirals (1,2m ø model RS12-18 with 1.1 kW drive) is between 15 and 20 t/h each.  The QSL furnace at Stolberg produces granulated slag with a particle size of less than 2 mm with about 10% minus 200 micron. The fines have an economic metal content and this portion can be profitably recycled to the smelter. It was thought that vibrating screens would not perform well in this application. Alternatively, hydro-separators were not able to provide a clear cut, recovering too many coarse particles with the fines because of gravity and shape effects.  This would have resulted in recycle of nearly 30% of the slag instead of the target of less than 10%.

PSS have recently started marketing Rotaspirals, which were developed in South Africa initially for platinum mineral processing. The design of the Rotaspiral allows cut points down to 100 micron. The unique woven stainless steel mesh specification is designed to maintain clean and precise apertures.  Recovery of fines is +95% and wear rates are negligible because of the low velocities between mesh and particles.  An axial spiral inside the screen provides fast transport of the oversize, and high velocity sprays at multiple points guarantee a sharp separation.  Additional benefits of this technology include the low energy demand and negligible noise levels.

R BUISMAN