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Sand Processing














Dividends for sand plants that follow the waste stream

Producers are discovering that fines are not a redundant resource in the sand processing application. As Chris Knowles explains, there are major opportunities for savings, revenue and environmental benefits in developing ultra fines from slimes and tailings.

Productivity of a sand wash plant is typically seen as a combination of throughput, quality and yield – any one will affect the other. Ideally, maximising all three is the goal but as we all know, in the end, it’s a compromise. Throughput seems to take precedence in many cases, and fines are the first thing sacrificed. More often than not, fines will contain some lost product and vice versa.

Ultra fines recovery is a small footprint, small investment approach that all sand operators can make, which will often capture back quality fine sand that can be blended into existing products or used to take advantage of niche markets.

Ultra fines plants are often modular and skid-mounted, so they can be plugged into an existing operation when a seam or deposit delivers a higher concentration of fines.

They may also be used as a clean-up system for a screw washer or log washer overflow, as they offer significantly more precision in selecting cut points.

By utilising multiple cyclones in a central manifold it is possible to manage variable feed volume by turning off one or more cyclones based on flow readings.

One of the most important contributions ultra fines recovery makes is in the reduction of tailings going to the dam each year. Tailings are often considered as a single stream, but the equipment to manage the tailings is identified and managed separately.

In fact, tailings are usually quite a number of different products delivered at different rates with different specific gravity, percentage solids and consistency. The tailings system that often includes dewatering, thickening, flocculation and filtration must be considered as one system and the changes in one component can significantly affect the others.

Dry stack tailings

Dry stack tailings are the ideal scenario when assessing the management of fine waste material that comes from the sand plant. No matter what size tailings dam you have, it will become smaller as time goes on as heavy rainfall events will become larger and more frequent. A great example of long-term planning occurred at a quarry in the north-west urban growth corridor of Melbourne.

"The sand processing plant is substantial and provides a resource that is critical to the city’s burgeoning construction industry."

The sand processing plant is substantial and provides a resource that is critical to the city’s burgeoning construction industry. The company’s sustainability overview recognises the importance of delivering the triple benefits of environmental and social responsibility along with worker safety at site.

By applying long-term sustainability principles to the operation, the producer eliminated the use of a tailings dam on-site and significantly reduced costs of dam management while reducing carbon footprint.

Multiple horizontal belt filters were de-commissioned and replaced by a single recessed plate filter. Early phase lab testing indicated a much drier tailings cake could be achieved using this technology.

Dry stacked tailings using a recessed plate press ensures liquefaction does not occur during transport and recovers much higher volume, cleaner water, using less flocculant.

Thickener optimisation

Optimising the thickener by reducing and changing the flocculant dosing methodology and the location reduced cycle times and improved capacity of the filter press.

Thickener feed is derived from both the coarse and fine cyclone overflow. Due to the varying nature of the deposit, the relative mass proportions and the size distribution of the cyclone overflow varied. These ranged from three to five per cent weight per weight (w/w) solids and a P80 of 25μm to 35μm. As the thickener was not custom-designed for this application, the feed system was considerably oversized. The 750mm feed pipe led to a 5m feedwell.

To maintain feed line velocity, a large pump provided external dilution to the feed box. This had a consequence of further diluting the already diluted feed.

Sand processing equipment supplier McLanahan conducted a series of simple jar tests that demonstrated the feed, as delivered from the combined cyclone overflow, settled well. Therefore, over the optimisation period, the overflow recirculation pump was shut off. While reducing the feed line velocity to a lower than ideal rate it had the advantages of:

  • Reducing power consumption.

  • Reducing rate of wear in the feed line.

  • Creating a quiescent feedwell to promote low shear settling of flocculated particles.

  • Reducing the rise rate in the thickener.

In this instance it was recommended the floc dosing position from the feed well to the feed line be changed, and to utilise a lower dose rate, controlled automatically, based on set points. The calculated rise rate on the thickener without the external dilution was 2.0 to 2.5 metres per hour (m/h), indicating the target settling rate could be reduced.

Over the optimisation period the target settling rate was progressively decreased while monitoring overflow clarity. This is an important exercise all operators should consider because it achieves an acceptable overflow clarity with up to 50 per cent less flocculant.

It often happens that sand producers choose a polymer supplier based on service levels and the costs of the flocculant material rather than thickener optimisation or even filter operation. Some polymers used in the thickener process can deliver a better outcome for cake release in a recessed plate filter press.

Costs, benefits

The benefit of installing a recessed plate filter (pictured) is the removal of redundant slime cells.
The benefit of installing a recessed plate filter (pictured) is the removal of redundant slime cells.

The most immediate cost-benefit for installing a recessed plate filter in this plant included the removal of the four redundant slimes cells, given the previous significant costs to dig out, handle and transport the dewatered slimes.The customer estimates a cost saving of about $1 million per year in labour, machinery use, and slimes cell (pond) maintenance.

The removal of the polymer dosing system used to condition the slimes for the belt press saved about $60,000 per year. As the filter press dewaters the cake to much lower moisture than the old belt presses (from about 50 per cent moisture with the belt press to about 26 per cent from the filter press), about twice as much water is being returned as process water, therefore reducing the reliance on the external water allocation.

The large surface area of the slimes cells (ponds) were subject to evaporation and their removal has also achieved additional water savings.

After some investigation into the nature of the filter cake resulting from the filter press, it is now understood to have a commercial value as a raw material in brick manufacture. What was a by-product adds value and offsets the cost of disposal. Transport by road was not previously possible with the dewatered slurry from the belt presses, but with the much drier cake from the filter press, there is no chance of liquefaction during transportation.

The producer’s commitment to its environmental policy sees it:

  • Complying with relevant environmental legislation, regulations, standards and codes.

  • Reducing greenhouse gas emissions and carbon offsets through changing process.

  • Reducing waste, water and energy consumption.

  • Eliminating the risk of dams overtopping or breaking.

  • Eliminating slimes cells.

  • Reducing evaporation. 


REFERENCES AND FURTHER READING

Godwin P, Jenson C. Park T. Dewatering fine coal tailings with recessed chamber or membrane plate filter presses. In Matthewson D, Eschebach D (eds). Proceedings of the 16th Australian Coal Preparation Conference. Australian Coal Preparation Society, Wollongong, 2016; 358-374.

Jenson C. A pressing need. In: World Coal, March 2012; 28-34.

Williams R, O’Brien S. (in prep) Filtered tailings add value in competitive cost industries. McLanahan Corporation, Perth.












ABOUT THE AUTHOR
Chris Knowles

Chris Knowles is McLanahan Corporation’s sales and marketing director for the Australia Pacific region









Wednesday, 20 June, 2018 09:39pm
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