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A Cavex 500CVX hydrocyclone and Enduron VD1538 dewatering screen in action at a site in South Africa.
A Cavex 500CVX hydrocyclone and Enduron VD1538 dewatering screen in action at a site in South Africa.

The importance of efficient design in fines recovery

Fines recovery is vital to maximising recovery (and dollars) from the available resource, and minimising waste disposal volume and cutting costs. But how much thought goes into designing an efficient fines recovery process? Chris Lowe discusses in detail.

What are “fines”? That depends on what you are processing and what you want to do with it.

In the wet processing of silica sands, fines are typically defined as minus 250 microns (μm) plus 75μm. Ultrafines are typically minus 75μm plus 20μm, while slimes are minus 20μm and clays are about -2μm. People may disagree about the absolute values, but these figures are a good starting point.

Most people want to recover the fines. They may also want to recover a part of the ultrafines, but they generally don’t want the slimes or clays. That means they desire a recovery size somewhere between 75μm and 20μm. For those who know their equipment, that’s an application for a cyclone, not a screen or a classifier or a washer.

Equipment selection

Cyclones are available in a very wide range of sizes, from 10mm or less to 1000mm or more. Most manufacturers cover a good part of that size range, if not all of it. For example, Weir Minerals supplies slurry cyclones with inside diameters ranging from 40mm to 800mm. But which size should be used for fines recovery?

The answer is simple – smaller cyclones cut finer. Fines recovery should always use multiple small cyclones rather than a few larger cyclones.

A dewatering screen’s optimum bed depth is 100mm to 150mm.
A dewatering screen’s optimum bed depth is 100mm to 150mm.

By all means, run the feed pressure high to get more flow through the cyclones, because higher pressures also provides a finer cut size. The feed density should also be low, if possible, below 20 and 25 per cent solids by weight. This is usually not a problem, because fines recovery is generally the last stage in the process, so all the water used to wash the coarser sand is generally available at the fines recovery. In fact, the problem can actually be the opposite – there is too much water – but this can be addressed.

Finally, use long narrow cyclones rather than shorter, broader cyclones. The longer body forces the sand to spin around more inside the cyclone, so it has more chance of being captured into the cyclone underflow.

Fishtail cyclones are very good for coarse sand washing (eg -5mm), but do not recover fines very well because their cut size is coarser than a non-fishtail cyclone, due to the extended length overflow pipe syphoning fines out of the cyclone.

A fishtail cyclone is also typically limited to feed pressures below 100 kilopascals (kPa) so the fishtail operates properly, and this restricts the amount of flow it can handle, and makes the cut size coarser.

Even with a fishtail fitted, fine sands do not dewater as well as coarser sands, so fitting a fishtail to try to get a drier underflow really does not succeed when the sand feed is finely sized.

Other equipment

Cyclones cannot operate on their own – they need a feed supply and a pump to deliver it.

Figure 1. The sump, pump and cyclone is the simplest process for sand fines recovery.
Figure 1. The sump, pump and cyclone is the simplest process for sand fines recovery.

The simplest sand washing flowsheet has three parts – a sump, a pump and the cyclone – and this flowsheet can be used for both coarse sand washing and for fines recovery (see Figure 1). So what equipment should be used for optimum fines recovery?

Again, the answer is simple – big is better for feed regulating sumps. These rely on gravity to settle the fines down into the base of the sump, so the more time that can be provided (in a larger sump), the better the fines recovery will be.

The sump selection process is very simple – we only need to know the total flow going into the sump and the total overflow from the sump. These two determine the “recovery” size of the sump – the size at which half of the particles go to overflow and half go into the underflow. Typical sump “recovery” sizes range from 53μm to 106μm, but coarser and finer sizes can be obtained by selecting smaller or larger sumps, as required.

Sump sizes

Weir’s standard sizes are 1.8m2 (measured inside the overflow launder) up to 6m2, in steps of about 0.6m to 0.7m. Custom sizes can be provided, and people often build their own non-standard sizes, but these standard sizes are generally suitable for most applications.

A linear arrangement of six 150mm Linatex cyclones.
A linear arrangement of six 150mm Linatex cyclones.

Many people build their own sumps, and they work well. Unfortunately, it’s quite easy to get the details wrong and then wonder why the whole process does not work as it should.

Although a sump looks like a simple item of equipment, it is really an integral part of the whole fines recovery process. For example, leaving out the impact plate at the bottom of the feedwell can cause slimes to plunge right down into the pump, where they go up into the cyclone, making it more difficult to remove them.

Returning all the cyclone overflow back into the feedwell can also cause slimes to overload – part of the cyclone overflow should be diverted into the sump overflow so it’s not recycled back into the cyclone.

Finally, mild steel is suitable for building the sumps, and paint will provide reasonable protection. For long-term performance, the rubber lining of the sump interior will keep your investment working for many years with minimal attention.

Pumps, cyclones

The pump simply needs to deliver enough head and flow to operate the cyclones correctly. Again, bigger is better – a slightly oversize pump is preferred to a pump that is too small.

Smaller cyclones provide a finer cut size.
Smaller cyclones provide a finer cut size.

Cyclone selection is quite straightforward. The client must provide the feed flow and the feed sizing, and the required cut size must be stated.

As noted previously, the cyclone size, the feed percentage of solids and the feed pressure determine the cut size. Multiple cyclones in parallel are selected to handle the required slurry flow – a single cyclone would only be used in a small, low flow application. The solids recoveries to underflow and overflow are calculated (if the feed sizing is provided), and the results reviewed by the client to confirm that’s what’s wanted.

The cyclone feed pressure is then used to confirm the pump selection, pump speed and motor power.

The recovery process

Putting all three pieces of equipment – sump, pump and cyclone – together completes the fines recovery process.

One complicating factor is the feed pressure, which may be as high as 250kPa, so all ancillary items – pipes, hoses, valves – must be rated to handle this. And, as noted, some of the cyclone overflow should be returned to the feed sump to keep it topped up and overflowing, so this requires extra piping and a valve to direct the flow as needed.

Dewatering screens

These screens are suitable for both coarse (-5mm +150μm) and fine sands (-300μm +38μm). On coarse sands the capacity can be as high as 350 tonnes per hour (tph) per screen, but on a fine sand the feed rate to the same screen would be down to only 30 per cent, or 105 tph.

Dewatering screens require a dense feed, eg cyclone underflow, but can handle the slightly dilute feeds from fines recovery cyclone underflows (without fishtails). The typical discharge moisture for a fine sand would be 14 per cent to 16 per cent by weight. Note too that sands finer than 300μm can be treated on dewatering screens, but the throughputs are reduced sharply as the sand becomes finer, and the discharge moistures rise rapidly to 20 per cent or more.

An Enduron sand wash plant, incorporating sump, pump, cyclone and dewatering screen
An Enduron sand wash plant, incorporating sump, pump, cyclone and dewatering screen

However, the sand is still drip-free and conveyable, requiring only one or two days’ drainage in the stockpile before being delivered.

The only complicating factor is the amount of fines going through the screen with the water. With typical fine sands this is about 10 per cent of the feed, but with finer feeds this can rise to 20 per cent or more. This sand should not be wasted, but should be returned to the cyclone feed sump, either by pumping or gravity.

Dewatering screens are available from many manufacturers, including Weir, in sizes from 0.6m wide to 3m wide, or more. Selection is simple: there are published tables listing the solids density (low for coal, medium for sand, high for minerals), the sand sizing (coarse, medium or fine) and the rated capacity of the screen.

Again, bigger is better, because a slightly oversize screen is preferred to one that is too small. But don’t go too large – the optimum bed depth is 100mm to 150mm for fine sands, as the mass of the sand on the deck helps to squeeze the water out of the sand every time the screen moves up.

Compact plants

The photo in the bottom right hand corner shows a typical compact sand wash plant. Weir Minerals supplies four standard sizes, based on coarse sand feed rates of 50 tph, 100 tph, 150 tph and 200 tph.

However, these plants can also be adapted for fines recovery by selecting the screen size to suit the fine sand tonnage, and specifying multiple smaller cyclones rather than one or two large cyclones.

An external pump feeds the cyclones, and the tank under the screen collects the underflow, to be pumped back to the primary feed sump for recovery.

These plants offer a less expensive solution than standalone equipment, in a compact footprint, that can be relocated as needed to match the main wash plant requirements.

They can be added to an existing sand plant, or used to recover fines dredged from ponds or dams, for blending or separate sale.

Equipment selection

Here are a few simple rules for selecting the right equipment:

Sumps – make sure the design follows the rules, ie:

  • The feedwell and impact plate are critical and should be in place and correctly sized for the duty.

  • The pump take-off box should not be too small – this will restrict the throughput and make sand washing more difficult.

  • There should be a wash water addition point on the take-off box – this helps to displace slimes from the slurry before they can enter the pump.

  • Allow for the return of some of the cyclone overflow back into the sump, to keep it topped up and overflowing.

  • The steel should be rubber-lined to protect the investment.

  • Bigger is better in sumps.

Cyclones – choose the right size, ie:

  • Smaller is better. For higher flows, use multiples in parallel in clusters.

  • For feed pressure, higher is better.

  • For feed percentage solids, lower is better.

  • Use long narrow cones rather than shorter cones.

  • Preferably do not use fishtail cyclones.

Dewatering screens – provide a drier product than cyclones, ie:

  • Choose the correct size, neither too large nor too small.

  • Recycle the underflow fines back to the cyclone feed sump.

 












ABOUT THE AUTHOR
Chris Lowe

Chris Lowe is a process engineer for Weir Minerals Australia.









Wednesday, 20 June, 2018 09:36pm
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