Integrated sand washing plant addresses feed contamination

A problem frequently encountered in Australian hard rock quarries is contamination of feedstock through inclusion of clays and silts within the deposit. Often this occurs as overburden, inseparable from the usable rock at source, for which the only solution in the production of high quality sands and aggregates is to reject the contaminated materials without any processing through the crushing and screening plant.

In some cases attempts are made to recover some of this material using additional plant and equipment, such as specialised high frequency screens, but this can prove costly, with complex equipment and high maintenance, and the amounts of saleable material recovered may not justify the investment.{{image2-A:R-w:320}}

The Brolton Group, a leading Australian multi-disciplined engineering solution provider, is introducing innovative solutions to this problem, using technologies developed by two European partners: Whitwick Engineering (Coalville) (WEC) and Advanced Mineral Processing SL (AMP).

These solutions embrace the concept of washing feedstock before final processing, something hitherto restricted to “wet” sand and gravel processing within the quarrying industry. Using techniques suited to the specific nature of the quarried material, waste can be dramatically reduced, while adopting the latest technologies enables the removal of unwanted material without the need for large volumes of water.

Cliffe Hill Quarry in Leicestershire, England, is operated by Hanson UK. The quarry produces 4.5 million tonnes of granite aggregate annually. A major concern was the high waste content in the plant in-feed, and WEC was given responsibility to design and install a suitable process to recover the maximum amount of usable material and restrict waste to less than five per cent, while minimising power and water consumption, within a very restricted footprint.

Like all the major UK granite quarries and most Australian hard rock operations, Cliffe Hill Quarry used to scalp off “unsaleable” material at the primary section, with the variable quality of product making it unsuitable for use other than as a low grade and low value scalps/fill material. {{image3-A:L-w:320}}

Typically, the production of 40mm to 50mm scalps was about 25 per cent of the in-feed, before WEC installed the washing plant.

WEC installed a heavy-duty washing drum that is designed to separate the usable and waste elements. This is followed by a rinsing screen that recovers the clean +5mm coarse aggregate into the main plant circuit, with the wash water and -5mm solids going to a compact sand plant to produce a coarse grit/sand.

The 63µm silt and water are then treated in a plate filter press. The pressed cake is used in restoration and the clean water recovered for reuse. The main items of the plant are a washing drum, rinsing screen, 100 tonnes per hour (tph) compact sand plant, flocculent tank and dosing system, filter press and clean water tank.

The solids capacity is about 200 tph, being determined by Hanson’s production requirements. The feed size is 50mm but the system could easily cope with 150mm, and the throughput would be higher with increased feed material size.

The results suggest the washing drum has been one of the most cost-effective projects Cliffe Hill Quarry has invested in, increasing profitability dramatically and virtually eliminating waste, reducing it from 25 per cent to significantly below five per cent.

WEC installed the plant nine years ago, with downtime and maintenance cost of the rubber-lined barrel virtually zero since the start of operation. As nearly all the water is recovered for recirculation, only a small amount of make-up water is required.

Water is collected from the adjacent catchment area, which drains from the sand and around the facility into a sump for reuse, such that the top-up water is of the order of 2.5 per cent, ie about 25,000m³ per year for one million tonnes throughput for this section.

The system incorporates a modest water storage lagoon and although Cliffe Hill Quarry is not an area of high rainfall, natural top-up is such that imported water does not need to be considered.{{image4-A:R-w:320}}

Cloud Hill Quarry at Breedon Hill in Leicestershire, is operated by Breedon Aggregates, the largest independent aggregates producer in the UK. It is recognised as a Site of Special Scientific Interest within a conservation area of biological and geological interest comprising semi-natural ancient woodland with diverse ground flora, including several rare species.

Operational constraints are, therefore, stringent.

Within the deposit at this quarry there are random bands of clays and mud, which created a significant problem in production of aggregates, as they are irregular and mainly undetectable before blasting. The quantity of waste was high and WEC was contracted to provide a solution to minimise the waste content.

The nature of the contaminant, with fine cohesive materials, including silty sands and dirty scalps, meant using a scrubber barrel or log washers was not viable, due to the lack of coarse material necessary for attrition scrubbing. The solution introduced by WEC was cavitational scrubbing, using an in-house specialised process. This proved to be an ideal solution in these circumstances.

The process works by mixing the feed material and wash water together, inducing cavitation within the wash water, with a combination of high velocity and agitation. The microscopic bubbles then implode violently in the pressure recovery region and aggressively separate the dirty material adhering to the surface of the sand and fine aggregate, removing it into suspension in the water.

The clean product is recovered through a combination of rinsing screen and hydrocyclone plant. This process is ideally suited to material below 40mm in size. At Cloud Hill, the addition of this system was successful in reducing wastage to about one per cent.

At Calcasa Quarry, near Madrid in Spain, the deposit of high quality limestone also suffered from intermittent intrusions of sticky clay and silt. Without a dedicated process, the operating company had discarded all suspect and unusable material to the point that nearly 500,000 tonnes of waste material had accumulated.

At this point, the AMP technical team was asked to deliver a solution, with the target of cutting the amount of waste by at least 75 per cent.

AMP provided a complete self-contained plant, linked to the main plant via a transfer conveyor, so material was washed only when necessary. An independent feed hopper was also employed, enabling material from the waste stockpile to be reintroduced for processing into saleable products. {{image5-A:L-w:320}}

All the contaminated feed passes into a heavy-duty Hydroflux washing drum lined with wear-resistant rubber. The feed material is nominally 0-20mm, although feed sizes up to 250mm may be processed.

From the drum, the washed material goes onto an inclined rinsing screen, separating the coarse fraction into 5-12mm and
12-20mm washed aggregates, with the 0-5mm being treated on a Hydroset compact hydrocyclone/dewatering screen unit, to remove excess minus 63µ superfines and produce a high quality concrete sand.

The fines are carried, in solution, to a thickener/clarifier tank, incorporating a Conves automatic flocculent dispensing system, and then, via a surge tank, to a fully automatic plate filter press. The inclusion of the tank enables a lower capacity plate press to be used, on a 24-hour cycle, whereas the main plant runs between 10 and 12 hours daily.

The resultant silt cake from the press contains low moisture content, so even in the hot and dry Spanish summer conditions make-up water requirements are low. The target of 75 per cent reduction in waste was easily beaten, and Calcasa is now also reducing the large stockpile of scalps through reintroduction into the washing circuit.

AMP also has extensive experience in the elimination of lignite and other contaminants in fine sands using TAK hydroclassifiers. The application of this technology is based on separation by “floating” of organic particles due to their lower density than the sand, allowing the production of a wide range of sands to meet all required specifications and qualities.

The research and development team has extensive experience in the treatment of samples of ores and sands in the laboratory, using in-house pilot plants or field trials to characterise and investigate mineral raw materials. These trials lead to the accurate determination of process parameters, which allow the extrapolation of data from industrial-scale engineering to specify the most suitable operational flow circuit.

Ben Lynch, Brolton’s managing director, believes there are opportunities for Australian quarries to benefit from these techniques.

“By using these methods, proven and field tested by WEC and AMP, we have been able to eliminate much of the learning curve associated with introducing new procedures,” he said.

“While any such process will, by necessity, need to be custom-designed and built to suit specific operations, we can now offer the expertise and competence to achieve the required results, free of risk or experimentation.”

Source: Brolton Group

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