DRAKELANDS MINE, formerly known as HEMERDON MINE or the Hemerdon Ball
or Hemerdon Bal Mine, is a tungsten and tin mine. It is located 11 km
(7 miles) northeast of
* 1 Geology
* 2 History
* 2.1 1867–1959 * 2.2 1960–2006 * 2.3 2007–present
* 3 Processing
* 3.1 Crushing and sizing * 3.2 Gravity separation * 3.3 Dense media separation * 3.4 Concentrate processing
* 4 See also * 5 References * 6 External links
The Hemerdon Ball granite is an outlying cupola intrusion surrounded
by Devonian slates, known regionally as killas . Fractures in the
granite and killas have been penetrated by mineralising fluids
containing metallic ores in the area around the mine. Two types of
vein are discernible with three different orientations.
The mineralisation begins at surface and extends to depths of at least 400 metres (1,300 ft). The vein system is hosted in a dyke like granite body, extending from the Hemerdon Ball towards the Crownhill Down granite. It is flanked by killas, formed by contact metamorphism , which also contains veins although wolframite and cassiterite is found as a lower percentage of the rock bulk. Kaolinisation occurs to depths of up to 50 metres (160 ft) in the granitic body.
The locality is renowned for its high quality scorodite specimens,
which are among the best in Europe.
Mine buildings World War II mill structures
The Hemerdon tungsten-tin deposit was discovered in 1867. In 1916, due to war associated tungsten shortages, an exploration and development program was initiated, which outlined a tin-tungsten stockwork suitable for open-cast extraction. In 1917, Hemerdon Mines Ltd decided to construct a 140,000-tonne per year mill, and shortly afterwards open-cast ore mining operations began. The mine was operated in 1918–1919, during which time it processed 16,000 tonnes of ore. When the British government stopped accepting tungsten ores under the war pricing scheme the mine was forced to suspend mining operations.
Several attempts were made to establish a higher and stable price for
tungsten from the government, including an application supported by
The Ministry of Supply carried out extensive evaluation of tungsten deposits in the UK, and it was concluded by 1942 that Hemerdon offered the most potential for producing tungsten on a large scale. The government took over the mine from Hemerdon Wolfram Ltd. A resource of 2.5 million tonnes of 0.14% tungsten trioxide in addition to tin was outlined, and a new plant was hastily constructed.
The new plant took over operation from the old plant in 1943, and theoretically should have been able to treat over 1 million tonnes per year; however labour shortages and mechanical faults resulted in a much lower production. Ore output from a mixture of underground and open-cast mining methods was documented as over 200,000 tonnes, with a resulting 180 tonnes of tin/tungsten concentrate during the period of government operation. Operations ceased in June 1944 due to access to overseas supplies being restored.
The plant was kept in place after the war, and the government was
rumoured to have planned restarting production during the tungsten
shortages associated with the
However, in the mid-1960s work on the prospect was recommenced by
The leases were transferred to Hemerdon Mining and Smelting Ltd in 1976. They initiated a drilling programme shortly before they entered a joint venture to develop the project with international mining firm AMAX in 1977. An extensive exploration programme costing in excess of $10 million was completed between 1978 and 1980. By the end of 1978, deeper drilling enlarged the resource size to 20 million tonnes of ore. In 1979 this was expanded to 45 million tonnes. At the end of the exploration programme in 1980, over 14,000 metres (46,000 ft) of diamond drilling had been undertaken, outlining a resource of 0.17% tungsten trioxide and 0.025% tin over 49.6 million tonnes.
Bulk sampling of the deposit using an underground drift for ore, and a pilot HMS and Gravity plant for processing, was undertaken in 1980. On average recoveries of around 65% were made, although in excess of 70% was achieved. The final revision of the mining feasibility study concluded in 1982 that a within a global resource of 73 million tonnes of ore, at grades of 0.143% tungsten trioxide and 0.026% tin, there was an in pit reserve of 38 million tonnes, at grades of 0.183% tungsten trioxide and 0.029% tin.
The venture was joined by Billiton Minerals Ltd, providing further
finance and expertise, and forming a consortium that planned to
commence production in 1986. The initial planning application was
made in 1981, but a public enquiry and 'calling in' of the application
by the Secretary of State resulted in an initial refusal of the
application in 1984. This resulted in Billiton Minerals Ltd pulling
out of the consortium. Hemerdon Mining and Smelting Ltd also sold
their 50% stake in the project to AMAX. After making a revised
application, permission was finally obtained in 1986. By then a
collapse in the prices of both tin and tungsten had damaged the
economic feasibility of making an investment in opening the mine. Its
tungsten assets were passed on to a newly formed holding company,
However, during a review of peripheral assets in 1999, it decided that with the depressed prices of tungsten, the Hemerdon prospect was not central to its future. With upkeep costs of in excess of C$150,000 per annum, almost a third of the company's annual costs, attempts were made with the mineral rights holders to reduce fees. The negotiations were unsuccessful and therefore during 2000, two of the three mineral rights were surrendered. To further reduce costs, it disposed the remaining assets of the Hemerdon prospect in 2003.
The concrete roads constructed around the Second World War mills up
to the open-cast area at the top of the hill were used by the Plymouth
Motor Club and
Sustained tungsten metal price rises resulted in a five-fold increase in the price of ammonium paratungstate (an intermediate product of tungsten), from around US$60 per STU in 2003, to in excess of US$240 per STU from 2006. This has resulted in increased tungsten mining exploration and development activities globally since 2005.
In June 2007, ASX -listed specialty metal exploration and development company, Wolf Minerals , suspended trading of shares pending the acquisition of mineral leases. On 5 December 2007, trading recommenced following the public announcement of acquiring the mineral leases for the Hemerdon Mine project. The mineral leases were made for a period of 40 years, with the Hemerdon Mineral Trust and the Olver Trust. An agreement with Imerys to purchase remaining mineral rights and freehold land was also made. Following agreements with local landowners to acquire surface rights, Wolf Minerals renamed the project the Drakelands Mine to "recognise the local community".
SRK Consulting were commissioned to produce a JORC -compliant resource using previous drilling data. This was released in March 2008. Subsequently, it has been updated twice by SRK Consulting to incorporate new drilling data and revised geological modelling. The resource of over 300,000 tonnes of tungsten metal makes Drakelands the fourth largest tungsten deposit in the world. In 2009, funding for a DFS (definitive feasibility study) was achieved with the support Resource Capital Funds and Traxys, completed in May 2011. Mining operations commenced in 2014, with first ore into the plant June 2015 and first concentrate production scheduled for September 2015. The project has planning permission dating back to 1986, which is valid until 2021. If production levels are achieved as anticipated, the mine would be the largest tungsten concentrate producer in the world. A planning application is in process to extend the pit slightly further to the southwest to further increase reserves.
Hemerdon Ball JORC Revised resource estimate completed by SRK Consulting in June 2010 RESOURCE CATEGORY ORE TONNAGE (MT) SN GRADE (%) WO3 GRADE (%) CONTAINED SN (TONNES) CONTAINED W (TONNES)
Measured 48.53 0.02 0.19 9700 72800
Indicated 22.39 0.02 0.18 4500 40300
Inferred 147.61 0.02 0.18 29500 206700
Total 218.53 0.02 0.18 43700 318800
The Drakelands processing plant relies on a number of different processes to recover tin and tungsten and discard gangue minerals such as arsenopyrite and haematite. Broadly speaking, the process involves crushing and sizing, followed by gravity separation on fine material and dense media separation (DMS) on coarse material. The concentrates from these processes are then milled, followed by flotation and roasting, finishing with magnetic separation and further gravity separation to produce the final tungsten and tin concentrates respectively.
The processing plant was built by GR Engineering Services from Perth and consists of a primary/secondary crusher building near the mine and stockpile, feeding the main processing plant via conveyor, and a tertiary crusher building. Design recoveries of tin and tungsten are in the range of 58–66% depending on feed type (soft granite near-surface, hard granite towards depth), with grades of over 60% tungstate and tin as final products.
CRUSHING AND SIZING
Two Sandvik hybrid rolls crushers perform the primary and secondary crushing duties at gaps of approximately 60 and 40 mm respectively. These crushers were preferred over jaw crushers as they should cope better with the high clay content of the ore in the early years of operation. The secondary crusher product is conveyed into a Sepro scrubber where the material is washed to remove fines sticking to the coarser material. The majority of material from the scrubber reports to a double-deck screen, sizing at 9 and 4 mm. Oversize material from the scrubber and this screen (over 9 mm) is conveyed to two Sandvik cone crushers with a closed size setting of 12–15 mm, before returning onto the scrubber screen. Material between 9 and 4 mm in size reports to the DMS circuit. The undersize material from the scrubber screen (less than 4 mm) is pumped onto a second screen where it is sized at 0.5 mm. The oversize for this screen makes up further DMS feed, and the undersize from this screen (less than 0.5 mm) reports to a large holding tank that stores feed for the gravity circuit.
Wolframite and cassiterite are heavy minerals, making them very suitable for recovery by gravity separation. The gravity separation process at the Drakelands processing plant starts two steps of desliming using Multotec deslime cyclones, designed to cut at 63 and 45 micron respectively. The underflow from these spirals goes to three banks of eleven 3-start MG6.3 Mineral Technologies spirals (99 spirals in total), producing a rougher concentrate that reports to the cleaner spirals, a middlings product that goes to a bank of 33 middlings spirals and tailings that go to the 25 m diameter thickener. The middlings spirals (also MG6.3) tailings go to the thickener, and the concentrate is sent to the cleaner spirals. The cleaner spirals (four MG6.3) tailings are recycled to the rougher spirals and the concentrate is sent to Holmans tables for further refining.
After dewatering using Multotec cyclones, two Holmans Wilfley shaking tables are used to produce a rougher table concentrate. This concentrate is sized at 90 μm using a Derrick screen and dewatered using cyclones followed by two further steps of cleaning/recleaning (also on Holmans Wilfley shaking tables) to produce the final coarse and fine gravity concentrate. The tailings and middling from the rougher table reports back to the rougher spiral feed, whilst cleaner table middlings/tailings are sent back to the rougher tables and re-cleaner table middlings and tailings are sent back to the cleaner tables.
DENSE MEDIA SEPARATION
The −9 +0.5 mm fraction produced by the crushing/washing/sizing circuit is stored in a feed bin with approximately 4-5h capacity. A prep screen washes any remaining