Dalradian's 2016 study for Curraghinalt in County Tyrone designs a 1,500 t/d plant that floats about 18% of the feed, leaches the reground concentrate in ten CIL tanks and filters the flotation tailings for dry stacking.
The Curraghinalt gold project lies in County Tyrone, Northern Ireland, about 127 km west of Belfast by road, 15 km northeast of Omagh and 7 km east of Gortin. Dalradian Resources Inc. acquired the licences in 2009, and by 2016 it held six licences covering more than 120,000 hectares. The feasibility study by JDS Energy & Mining and SRK, effective December 12, 2016, covers a development project. It would mine up to 16 vein systems from a ramp-access underground mine over an 11 year production period. The report gives no construction start date. Ore would feed a 1,500 t/d flotation and cyanide leach plant built into a cutting in the hillside, with filtered tailings going to a dry stack facility or underground as paste.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Plant throughput | 1,500 | t/d | Nominal |
| Grinding rate | 68 | t/h | Fresh feed |
| Plant availability | 92 | % | Milling and leaching |
| Design gold grade | 7.94 | g/t | Average of metallurgical composites |
| Gold recovery | 94.3 | % | Life of mine, from test work |
| Silver recovery | 57.6 | % | Silver not in the economics |
| Crusher product size | 110 | mm P80 | Single stage jaw crusher |
| Bond ball mill work index | 13.7 | kWh/t | At 212 µm |
| SAG mill | 6.1 m x 2.44 m | Not applicable | 1.3 MW motor |
| Primary grind | 240 | µm P80 | SAG mill with cyclones |
| Flotation mass pull | 18 | % | Rougher concentrate |
| Regrind size | 50 | µm P80 | 244 kW mill, ceramic media |
| CIL tanks | 10 | Not applicable | 48 hours residence |
| Cyanide addition | About 1.23 | kg/t | Per tonne of plant feed |
| Detox discharge | Less than 5 | mg/L CN(WAD) | SO2/air with zinc sulphate |
| Tailings filter cake moisture | 15 | % | Dry stack |
Overview
Test work supported a flowsheet in which only the flotation concentrate is leached. Ore is crushed in one stage and ground in a SAG mill to a coarse 240 µm. Rougher flotation takes about 18% of the mass as concentrate, and the rest of the feed reports to tailings without cyanide. The concentrate is thickened, reground to 50 µm and leached in a carbon-in-leach (CIL) circuit with oxygen. Loaded carbon is acid washed, stripped and regenerated, and electrowinning and smelting produce doré. CIL tailings pass cyanide destruction and join some of the flotation tailings as paste backfill. Most flotation tailings are thickened and filtered for the dry stack facility.
Key Process Stages
Crushing and stockpile
Thirty tonne haul trucks bring ore from underground to the run-of-mine pad or tip it straight through a static grizzly into a dump pocket, where a rock breaker deals with oversize. An apron feeder draws about 189 t/h into a 1,200 mm by 870 mm, 160 kW jaw crusher. The crusher can run around the clock but can finish the day's tonnage in 12 hours to limit noise. A dome covers the crushed ore stockpile, about 35 m across and 14.7 m high, with 1,500 t live capacity, enough for 24 hours of plant feed. Two belt feeders in a corrugated pipe tunnel each deliver up to 68 t/h.
Grinding
The SAG mill runs at 70% solids in closed circuit with a cyclone cluster. A trommel on the discharge sends oversize trash to a bin. Cyclone underflow returns to the mill, and overflow at 240 µm flows by gravity to the flotation conditioning tank.
Flotation and regrind
Rougher cells in series, with a 200 mm step between cells, give 18 minutes of residence time and can recover up to about 18% of the feed as concentrate. Rougher tailings go to final tailings. The concentrate passes a vibrating trash screen to a 9 m thickener dosed with anionic polyacrylamide flocculant, which produces a 50% solids underflow. A 244 kW regrind mill with ceramic media takes it to 50 µm.
Carbon in leach
The reground concentrate feeds the first of ten CIL tanks, each 6 m in diameter and 6 m high, which stand outside next to the main building. Each tank has an agitator, a carbon transfer pump and an interstage screen. Oxygen is sparged at the tank base, and lime in the first two tanks holds pH at 10.0. Carbon averages about 25 g/L, with 50 g/L in the first tank, and moves counter-current to the slurry. About 2 t/d of loaded carbon leaves the first tank. A stationary safety screen catches stray carbon from the last tank before the slurry goes to a 9 m CIL tailings thickener.
Acid wash, elution and regeneration
The 2 t daily batch of loaded carbon is washed in 3% hydrochloric acid to remove calcium, magnesium, sodium salts, silica and iron, then rinsed. A recessed impeller pump moves it to a 2 t strip column. Solution with about 1% sodium hydroxide and 0.1% sodium cyanide circulates at 140 °C and 450 kPa, with an electric boiler as the heater and a heat recovery exchanger. Stripped carbon is screened to remove fines and regenerated in an electric horizontal kiln that treats 2 t a day, the full carbon inventory turnover. Fresh carbon replaces attrition losses.
Refining, detox and tailings
Electrowinning plates gold sludge onto steel cathodes. The sludge is washed off, filtered, dried and smelted with fluxes in an induction furnace, and the doré is held in a vault. Two 72 m3 tanks treat the CIL tailings thickener underflow by the SO2/air process for three hours, using sodium metabisulphite, lime at pH 8.5 to 9.0 and zinc sulphate as catalyst, to below 5 mg/L CN(WAD). The thickener overflow is detoxified separately for 1.5 hours before going to the water treatment plant. Detoxed slurry and part of the flotation tailings, 31 t/h of solids in total, go to the paste plant at 75% density. The remaining flotation tailings go to a 12 m thickener at 55% solids and an eight hour stock tank. Two filters, one on standby, treat about 46 t/h to 15% moisture for trucking to the dry stack facility.
Additional Interesting Data and Summary
Process water comes from the concentrate and tailings thickener overflows. Fresh water use in the plant is estimated at 15 to 20 m3/h for reagent make-up, gland water and boiler cooling. Reclaim water from catchment impoundments or the water treatment plant totals about 100 m3/h. Separate compressors serve the crusher, the filters and the plant's instrument and service air.
The plant would employ 56 people, including expatriate specialists in the early years. Dalradian drilled 347 holes totalling 102,691 m on Curraghinalt from 2010 and ran underground bulk sampling and test stoping in 2015 and 2016. The work raised the overall resource sevenfold. The site design aims to keep the plant low in the landscape, set into the hillside behind existing trees and clad in dark green.
Key Processes
- Single stage jaw crushing to a covered stockpile
- SAG milling in closed circuit with cyclones to 240 µm
- Rougher flotation to a concentrate of about 18% of the feed
- Concentrate thickening and ceramic media regrinding to 50 µm
- Oxygen-assisted cyanide leaching in ten CIL tanks
- Acid washing, pressure elution and electric kiln regeneration of carbon
- Electrowinning and induction furnace smelting to doré
- SO2/air cyanide destruction with paste backfill
- Filtration of flotation tailings for dry stacking
Source: NI 43-101 Feasibility Study Technical Report on the Curraghinalt Gold Project, Northern Ireland, December 12, 2016.
Project website: Curraghinalt Gold Project
