Diversion of Water Main to Allow Construction of the Water Treatment Plant
Following the water treatment trials that took place between November 2016 and March 2017, and the subsequent awarding of a water discharge permit by the Environment Agency in October 2017, Strongbow engaged Siltbuster Process Solutions Ltd to undertake flow sheet design and optimisation work on a full-scale plant capable of treating 25,000m3/day of mine water.
The pH of the mine water is neutral (pH of 6.5). The process for removing dissolved metals from solution involves aeration (addition of hydrogen peroxide), followed by addition of lime to increase the pH to precipitate metals from the water, addition of a flocculent to bind metals for recovery in a settling tank, and then addition of CO2 to reduce the pH before discharge of clean, treated water that meets strict Environment Agency water quality standards. The main metals recovered in the process are iron and manganese, plus lesser amounts of copper and arsenic.
The optimisation works successfully focused on reducing lime consumption, reducing estimated operating costs, through increasing aeration in the Stage 1 mixing tanks. This action had the additional benefit of reducing the thickening area required to settle the sludge containing precipitated metals from the mine water during the treatment process. Nomenca PLC was engaged to work with Siltbuster to complete the detailed design and engineering of the optimised plant.
To expedite the construction of the plant, Strongbow took the decision to begin groundworks during the dry summer months. However, before excavation could commence a mains water pipe that crossed the construction site had to be diverted and historical mine workings were capped.
The water main was diverted and re-laid around the perimeter of the site by Balfour Beatty on behalf of South West Water. This work was completed in June 2018 enabling excavation of the main water treatment plant site to commence. The excavation works were undertaken by Henderson Mine Research Ltd, a local company that specialises in historical mine research and remediation.
Excavation, Shaft Capping and Foundation Preparation for Mine Water Treatment Plant
During the excavation, all historical shafts, adits and stopes that were identified during the site investigation were recorded and subsequently capped with reinforced concrete. Cornwall Archaeology Unit undertook a watching brief during the work to ensure that all features of historical interest were recorded.
The capping works were completed in August 2018. The ground was then prepared for the main plant construction. All drainage and electrical ducts were installed before placing down compacted engineered fill material. This brought the ground level up to the formation level of the reinforced concrete slab that will form the foundations of the plant. All groundworks are complete, ready for concrete to be poured and plant construction to commence.
Rising Mains and Shaft Lifting Frame for Installation of the Mine Dewatering Pumps
In order to enable the mine to be dewatered in an efficient and effective manner, Strongbow engaged International Mining Engineering Consultants Ltd (“IMEC”) to assist in devising a dewatering scheme. The scheme utilises a phased approach to dewatering using high-head multi-stage submersible pumps to pump mine water from NCK shaft directly to the mine water treatment plant.
Phase 1 will dewater from surface to the 195 fathom level (350 metres below surface); Phase 2 will dewater from 195fm level to the 400fm level (730 metres below surface); and Phase 3 will dewater from 400fm level to 470fm level (the bottom of the mine, 870 metres below surface).
Two pumps will be installed in the shaft, each capable of delivering 525m3/hr of water to the treatment plant.
The pipe column, shaft deck and temporary lifting frame has been designed by IMEC and fabricated in Navan, Ireland, at a specialist fabrication shop that undertakes work for the Irish mining industry. The fabrication work is complete and the components are awaiting shipment to the mine site.
Factory Acceptance Test Variable Speed Drives for Mine Dewatering Pumps
Two submersible dewatering pumps and electric motors are due for delivery to the mine in mid-November 2018.
The pump motors are designed to be controlled by a variable speed drive so that the speed of the pump can be varied from zero to 3,000 rpm. This speed variation is necessary to maintain a constant 1,050m3/hr of water delivery to the treatment plant with the varying head of water that will exist in the mine as the water level is lowered. Additionally, the speed control enables the flow rate of water through the whole treatment plant to be controlled, ensuring Strongbow can meet the conditions of its environmental permit and limit the discharge of treated mine water into the Red River during flood conditions. The variable speed drives, manufactured by Schneider Electric, have been delivered to site.
Clearance of blockage in New Cook's Kitchen Shaft
After the mine closed in 1998 a new tunnel was mined from the Tuckingmill Decline to intersect NCK shaft 50 metres below surface. The development of the tunnel resulted in a partial blockage of the NCK shaft. The blockage has been removed and the shaft is now clear for the installation of the pumps.
Strongbow has also undertaken preparation works in Middle Engine Shaft and the Dolcoath Deep adit. This work will eventually allow treated mine water from the water treatment plant to be discharged into the adit system via Middle Engine Shaft, as per the conditions of Strongbow’s water discharge consent.
Underwater Camera Survey in New Cook's Kitchen Shaft
Finally, an underwater camera survey was undertaken in New Cook’s Kitchen shaft to ensure that the shaft compartments were open and in suitable condition to allow the dewatering pumps to be installed. As the pumps and pipe column are planned to be centralised within their respective shaft compartments by brackets supported from the existing steel guides within the shaft, a full survey of the guides was also undertaken. The survey successfully demonstrated that the shaft was open to shaft bottom, 780m below surface.