Tungsten mining in Devon
Think of tungsten and you may picture China – the world’s largest producer. But there are exciting developments much closer to home, just a few miles from one of Britain’s most popular tourist towns. Michael Forrest talks to Humphrey Hale of Wolf Minerals about the company’s work.
In 2011, the European Commission produced a report highlighting the vulnerability of supply of a number of metals and minerals. Defined as critical mineral raw materials, the list of 14 includes tungsten. Although there is no shortage in the geological record, the tungsten mining industry has become concentrated in China, which produces more than 80% of the world’s mined metal. This position has been exacerbated by rising demand within the country at the same time as a nationwide moratorium on the development of small tungsten mines. China’s dominance has caused international governments and mining companies to look elsewhere to develop tungsten mines.
Tungsten as a carbide accounts for about 54% of consumption in hard metals. Another 27% is used in the steels/alloy sector, with mill products and other uses accounting for the remainder. The principal traded tungsten commodity is ammonium paratungstate, a white powder used in the production of a wide number of components. Market growth has averaged around 3% per annum, but projections show an increase to an estimated 6–7% in the next few years.
Making a change
The mining industry has responded to this change, backed by governments that wish to avert supply problems that can stem from a dominant supplying nation. Tungsten occurs commercially as the minerals scheelite and wolframite, both oxide minerals and principally found adjacent to granitic intrusions. Such is the case in southwest England, where Variscan-aged plate movements thrust against the older Caledonian-aged continent. The result in this region is the folding and metamorphism of Devonian- and Carboniferous-aged sediments, and the intrusion of granitic rocks.
To the southwest of Dartmoor, in Devon, an intrusion linked to the main Dartmoor granite forms a dyke-like structure of Hemerdon granite. The surrounding rock is known as killas, a Devonian-aged fine-grained sediment metamorphosed to slate. Around 200 million years later, during the Tertiary Period, the intensive tropical weathering rotted the granite to a depth of around 20 metres. Tin-tungsten mineralisation is hosted within the granite and in the surrounding country rock as sheeted greisens and vein stockworks, while the greisens result from the alteration of the rock by the mineralising fluids, and the contact between the granite and the country rock is steeply dipping.
In the 1970s, the mineralisation at Hemerdon interested mining company AMAX enough for it to carry out some 25,000 metres of drilling within the granite and killas and conduct pilot-scale metallurgical test work on a large 10,000-tonne bulk sample. But no more came of this work. Some 30 years later, Wolf Mining Ltd, led by Executive Managing Director Humphrey Hale, recognised the potential of Hemerdon. At the start of 2007, the company was exploring several properties for tin and tungsten in New South Wales.
Hale explains, ‘Although adequate, these potential targets did not offer the prospects the board and share-holders required’. Later the same year, the company undertook an extensive review of global tin and tungsten resources and in December 2007 signed an option and lease agreement for the mineral rights and the right to mine at Hemerdon. Hale continues, ‘While not of the highest grade, the south Devon resource had a number of advantages. Firstly a lot of investigative work had been completed by AMAX, including drilling and testing. There is support from the local authority for the project, with planning permission renewed in January 2011, and its location just a few miles from the deep water port of Plymouth gives easy access to markets, particularly in Europe.’
However, although these features are desirable, it is the scale of the resources that put Hemerdon at the top of Hale’s list. The resources in the inferred, indicated and measured categories amount to 401.4Mt at an overall grade of 0.13% tungsten trioxide and 0.02% tin oxide. ‘This makes it the fourth largest deposit in the world,’ states Hale.
Wolf has upgraded these resources by new drilling designed to confirm the extensive work completed by AMAX, and by retesting the AMAX core. Initially 12 cores were split and analysed, resulting in an initial inferred result. Subsequently, a further six holes were drilled to validate the block model, resulting in lifting a significant proportion into the indicated and measured categories. These results fall within the pit outline, for which AMAX gained planning permission in 1986, and are essentially confined to the weathered and fresh granite lithologies that form a small portion of the mapped mineralisation. It has given proven and probable (with an indication of economic recovery) of 5.5Mt in the weathered rock and 21.3Mt in hard granite, for an overall grade of 0.18% tungsten oxide and 0.03% tin oxide. These much lower tonnage values reflect the outline of the permitted pit and discount for the time being the 256Mt identified in the killas country rock, the metallurgical recovery from which has yet to be tested.
The mine is located next to the china clay open pits operated by Sibelco and Imerys, eight kilometers from Plymouth. The red outline in the map (right) shows the permitted open pit, while the blue area gives the overall planning permit. All are outside the china clay workings. The current mine plan overdevelops a pit of 880x450m to a depth of 200m, with a programme of mining waste material undertaken prior to ore production for the construction of the tailings dam wall. Overall, the mining operations will have a low strip ratio of 1.5:1, with conservative pit wall angles of 35–40°. Mining will be undertaken by contractors using simple drill and blast technology, while the upper weathered granite can be dug by simple excavator and truck fleets. It may be possible to steepen the pit walls to 50° depending on the results of geotechnical working during mining. If so, an additional three years’ worth of ore may be recovered from within the pit outline. Mining costs as estimated in the definitive feasibility report received in 2010 are £4.90 per tonne.
The ore is processed by simple gravity circuits, with the addition of flotation to remove some sulphide minerals. Three different-sized concentrates using spirals, tables and dense media separation will be fed to a roaster, followed by low and high magnetic separation to produce tin (non-magnetic) and tungsten concentrates that will be sold to downstream processors. The plant will have a capacity of three million tonnes per year of mine ore. In terms of recovery, Wolf’s definitive feasibility study estimates that 66% from the hard rock and 58% from the weathered rock will be possible.
Hale explains, ‘Although the grades at Hemerdon are moderate, most of the developing tungsten mines are underground, with concomitant higher costs and similar recovery rates’. The strategic nature of tungsten and the dominance of Chinese production has troubled traders and users for some time, and since the development of these plans Hemerdon has received considerable interest from European and North American processors. Traxys Processors, one of the largest traders in the tungsten and tin markets, has taken a 9.8% holding of issued shares in the company.
The overall capital expenditure of the project is £104 million, while the operating cost is estimated at US$122 per metric tonne unit (10 kilogrammes). This equates to approximately half the estimated Chinese average cost, and well below the current price of US$400–450 per metric tonne unit. Mining is planned for the third quarter of 2013, with operating capacity of three million tonnes per year planned for 2014. The British mining industry will have to wait a little longer before it begins to compete with its Asian counterpart.
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