Cloud Hill Joins The Jet Set

New jet washer system transforms waste fines into added-value product

Today virtually all quarry operators are seeking to reduce their waste stockpiles and increase their added-value product sales. As a result, the market for washing/treatment plants designed to remove unwanted fines and produce clean sands has burgeoned in recent years. However, at Cloud Hill Quarry in the East Midlands, operators Ennstone Johnston have forgone the conventional wash plant approach to this problem in favour of a highly innovative jet washing system that eliminates the need for traditional scrubber barrels, log-washers and silt ponds.

Using this novel jet wash system in conjunction with a modern, small-footprint water-treatment plant, all supplied by Ealing-based PM Systems (PMS), Ennstone Johnston have found a highly efficient and cost-effective way to recover valuable, high-quality concrete sand from their ever-growing stockpile of semi-waste limestone fines. According to PMS, the system generates considerably more attrition than conventional scrubbing techniques but with less component wear and considerably reduced energy costs.

At the heart of the new system is PM Systems’ 1850 Track Jet Screen unit, which aroused considerable interest when it was first unveiled to the public at Hillhead 2005. The unit now installed at Cloud Hill is only the second machine of its kind in existence, the first having been supplied to Ireland, where it is successfully processing a –50mm all-in sand and gravel material at feed rates of up to 300 tonnes/h.

The 1850 Track Jet Screen is based on a standard Fintec 570 tracked three-way split screener featuring an inclined 18ft x 5ft double-deck screen box, but with subsequent modifications carried out by PMS to create the machine’s jet washing capabilities. These include: the installation of a secondary feed box with high-pressure water jet nozzle and associated feed pipe and impact box; the fitting of multiple rinsing bars on the screen box; the provision of pipework and valves for high and low pressure water supplies; together with arrangements piping or fluming the washed product from the machine to an adjacent sand plant.

Raw material is fed into the 1850 Track Jet Screen by wheel loader via the machine’s standard feed hopper and tipping grid arrangement. From here a belt feeder discharges the material directly into the secondary feed box, where the first stage of the washing process takes place. At this point water is piped into the box from a number of overhead inlets to agitate and fluidize the feed material and thus help prevent bridging.

Meanwhile, at the base of the feed box, a continuous high-pressure jet of water is emitted through a specially designed high-chrome-steel nozzle, the force of which literally blasts the feed material through a 6m long x 8in diameter inclined feed pipe to a point at the top of the screen box. The huge force of this water jet is created by a 132kW inverter-driven pump, capable of delivering between 8 and 16 bar pressure, combined with a range of interchangeable nozzles featuring a variety of aperture diameters to suit different feed material characteristics. At no point in the process does any abrasive material pass through the pump itself or the nozzle.

The feed pipe, which is mounted to the frame that would have supported the main feed conveyor to the screen box on a standard Fintec 570 unit, terminates at an impact box, which helps to generate further cavitation and turbulence within the slurry prior to discharge on to the grading screens. But it is within the feed pipe itself, where the hydrodynamic forces are at their greatest, that the bulk of feed material attrition takes place. To further enhance this effect, PMS are currently working with Imperial College London on ways to generate even more cavitation and turbulence within the system, to increase material disintegration and dispersal still further.

At Cloud Hill Quarry, where the jet wash machine’s primary function is the washing of limestone fines, with only occasional processing of 20–25mm scalpings, the 1850 Track Jet Screen’s twin-deck screen box is equipped with 20mm aperture wire mesh on the top deck and 5mm apertures on the bottom deck. Both decks are equipped with a series of spray bars to assist the passage of the underflow. The machine, which is designed to operate at feed rates up to 250 tonnes/h (300 tonnes/h peak), currently produces around 150 tonnes/h of clean –5mm sand, although according to PMS the unit is capable of delivering 170 tonnes/h at peak output. With the inverter-driven pump operating at 10–11 bar pressure, the plant utilizes 400m3 of water per hour (1,500 gal/min), the majority of which is recirculated, via the downstream water treatment plant, for reuse.

Any +20mm or –20mm +5mm materials rejected by the screen decks are discharged to ground stockpiles via the machine’s standard side conveyors, while the –5mm underflow is discharged, via a specially adapted flume arrangement, to a 150 tonnes/h sand plant comprising an 8/6 Warman horizontal slurry pump, a 650mm diameter hydrocyclone and a dewatering screen. The dewatered concrete sand, which has a final moisture content of 12–15%, is currently discharged to a ground stockpile but in future will be fed to a dedicated 5,000-tonne capacity concrete stock bay situated adjacent to the plant.

The silt-laden process water discharged from the sand plant is dealt with by a fully automated, PLC-controlled PMS T12 water-treatment system. This incorporates a 12m diameter flat-bottomed thickener equipped with a torque-sensitive rotating rake that can be raised and lowered according to changes in the settled solids level within the tank. Settlement rates are monitored every 60s and flocculant levels adjusted accordingly via a fully automated flocculant control, mixing and dosing system. A PCM pump is used to draw off the thickened sludge from the base of the thickener for delivery to a fully automated PMS 1515 side-bar filter press. This 65-chamber machine with 1.5m x 1.5m plates produces 13 tonnes of dry cake (20–25% moisture) per press cycle, equivalent to 18 tonnes per hour.

Meanwhile, the clean water overflow from the thickener tank is discharged directly into an 8m clean-water tank for subsequent recirculation to the to the 1850 Track Jet Screen plant. The clean water requires only minimal top up (15–35 gal/min) to make up for moisture losses in the finished sand.

Commenting on the new jet washing installation, Ennstone Johnston’s operations director, Phil James, said the solution supplied by PM Systems was providing a highly effective synthetic replacement for conventional concrete sand, turning the quarry’s semi-waste product in to an added-value product.

‘Our sister company, Ennstone Building Products, are delighted with the product,’ he said. ‘They are currently using 250–300 tonnes a day for concrete pipe production, and we expect more synthetic concrete sand to be used internally as Ennstone Johnston’s new ready-mixed concrete plants come on stream. We are also receiving considerable interest in the product from external customers.’

The versatility of the new plant has also opened up a number of other new possibilities for Ennstone Johnston, such as the potential to produce washed chippings and clean recycled materials. Moreover, the company intends to recycle foul water from a number of new wheel wash installations through the water-treatment plant in order to minimize routine maintenance of the wheel washers. This was planned for from the outset with the necessary underground pipes already installed and the treatment plant deliberately oversized to accommodate this additional requirement.

Moreover, PMS say plans are already afoot to develop a fully mobile version of the jet wash system, including mobile sand plant and mobile filter press units, which will allow plant hirers to offer contract washing facilities and equipment on a price-per-tonne basis as well as providing benefits for small-scale or shorter-lived extraction operations where operators are seeking to avoid lengthy planning applications and civils costs.

Acknowledgement

The editor wishes thank Ennstone Johnston for permission to visit Cloud Hill Quarry and, in particular, Damian Shields and Graham Lamond of PM Systems, and Phil James, operations director with Ennstone Johnston, for their help in preparing this report.