Drakelands Mine
Some History
The origins of the Drakelands Mine, or the Hemerdon Mine as it was formerly known, date back to the 18th and 19th century when the area was worked for tin. In the 19th century 2 mines were operating on the south-western slopes of Hemerdon Ball, namely Mary Hutchings and the Hermerdon Consoles. Recorded outputs for the Mary Hutchings Mine were 426 tons of tin and 230 tons of arsenic for the period between 1866 and 1880, while the Hemerdon Console recorded 26 tons of tin in 1855.
In 1897 Tungsten/Wolframite deposits were discovered on top of Hemerdon Ball. In 1916 with an increased demand for tungsten due to the start of World War I, a mineral exploration program was initiated, which outlined a tin-tungsten deposit suitable for open-cast extraction. In 1917, Hemerdon Mines Ltd decided to construct a 400-tonne per day mill, and shortly afterwards open-cast ore mining operations began. The mine operated for three years between 1918 and 1920. However in 1920 Hemerdon Mines Ltd had to suspend operations and sell off some of the milling equipment since the British government stopped accepting tungsten ores under the war pricing scheme.
Increases in the tungsten prices in 1934 led to renewed prospecting of the deposit, along with some metallurgical testwork. In 1937-38 the Hemerdon deposit was prospected by the Hemerdon Syndicate and the British (Non-Ferrous) Mining Coproration who sank three 20m shafts for sampling purposes. The average samples retrieved contained about 3kg of wolfram/tungsten and black tin per ton, of which milling tests showed a possible recovery of 55 per cent. At the then prices of wolfram this was not a very attractive economic proposition, but because of the outbreak of Worl War II, the Hemerdon Wolfram Ltd. was tasked with exploiting the mine. In 1939 they erected a mill capable of treating 3000 tons of mineral ore per day. Due to a large need of tungsten during WWII the government took over the mine in 1942 and hastily erected a new mill with a larger capacity. The new plant took over operation from the old plant in 1943 but ceased operation only a year later as access to cheaper markets were being restored in 1944. It seems that after 1944 production of tungsten at the Hemerdon Mine ceased completely until the early 1960s.
In the mid-1960s work at the Hermerdon mine was recommenced by British Tungsten Ltd, owned by Canadian entrepreneur W.A.Richardson. In 1976, the Hemerdon Mine was acquired by Hemerdon Mining and Smelting Ltd. and in 1977 they brought in the international mining firm AMAX Inc. as a joint venture partner. Between 1977 and 1980 AMAX undertook an extensive exploration programme and constructed a pilot plant for processing of the mineral ore. The final version of the study released in 1982 concluded that the total reserve of mineral ore at the Hemerdon site was 73 million tons of ore at grades of 0.143% tungsten trioxide (1.43kg/ton) and 0.026% tin (260g/ton). Hemerdon Mining and Smelting Ltd. was then joined by Billiton Minerals Ltd. and the consortium planned to commence production in 1986. However, their planning application, made in 1981, was refused in 1984 with the result that Billiton Minerals pulled out of the consortium and Hemerdon Mining and Smelting Ltd. sold their 50% stake in the mine to AMAX. After making a revised application, permission for the mining operation was finally obtained in 1986. However, by then a collapse in the prices of both tin and tungsten, rendered the opening of the mine economically unviable and the assets of the Hemerdon Mine were passed on to a newly formed holding company, Canada Tungsten Ltd, in 1986. Canada Tungsten Ltd. then passed into the ownership of Aur Resources, which, in 1997, was purchased by the North American Tungsten Plc. During the early 2000s the North American tungsten Plc, surrendered the mineral rights and disposed of the assets at the Hemerdon Mine, due to the sustained low prices of tungsten on the international markets.
In 2007 the specialty metal exploration and development company, Wolf Minerals purchased the the mineral leases for the Hemerdon Mine project. 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.
The Process
After entering the processing plant the tungsten ore is crushed using two Sandvil hybrid roll crushers, which were preferred over jaw crashers as they are believed to cope better with the high clay content of the ore. After the crushing, the ore undergoes a “pre-concentration” process, the aim of which is to increase its concentration of wolframite in the ore. At the start of this process, the crushed ore is conveyed to scrubbers where it is washed to remove the fines (finely crushed or powdered material) from the coarser material. After scrubbing the ore moves through a variety of screens, the aim of which is to divide the material into 2 different size bines, namely coarse particles sized between 0.5 mm to 9 mm and fine particles smaller than 0.5mm. Oversized material (i.e. larger than 9mm) is diverted to a cone crusher before being returned to the scrubber and the screens. The fine material is collected in a large holding tank while the coarse material is stored in a feed bin with approximately 4-5 hours capacity.
It is only at this stage that the “pre-concentration” process begins. Increasing the wolframite concentration in the fine and coarse ore is achieved using two different methods namely gravity separation and dense media separation (DMS). Gravity separation, is basically a method for separating particle mixtures of the same size but with difference in specific weight. Let us look at the former process, i.e. gravity separation, first.
From the holding tank, the finely crushed ore is fed into a couple of desliming hydrocyclones. Slime basically consist of particles approximately 200 times smaller than the finest mesh-size of the screen suspended in a liquid. Because of their small size this particles tend to remain suspended indefinitely. Hydrocyclones (see schematic) create a separation between coarse/high specific gravity particles and fine/low specific gravity particles based on their geometry and the centrifugal motion of the flow inside them acting on the particles accordingly. When slurry is fed under pressure tangentially into the pipe shaped body of a cyclone, the centrifugal force tends to throw the heavier particles towards the outside in preference to the lighter ones. The outer particles then move down the cone under pressure and are forced out of the underflow spigot, while the lighter particles and the liquid (i.e. the slime) on the inside of the vortex rise up into the vortex finder and discharge as an overflow.
The primary flow is essentially the slurry flowing down the spiral trough under the force of gravity while the secondary flow pattern is radial across the trough. This radial secondary is crucial for the separation of particles. The underflow from the desliming hydrocyclones is then fed into banks of gravity separator spirals. A gravity separator spiral is basically an inclined chute with a complex cross section wrapped around a central column. The principle is that a combination of gravitational and centrifugal forces acting upon particles of differing specific gravities cause fine heavier particles and coarse light particles to segregate. A spiral unit is composed of a profiled channel swept helically around a central post creating a spiraling trough. Slurry containing the underflow from the desliming cyclones enters the spirals on the top and on its way to the bottom of the spiral, the denser particles generally reports inward while the less dense particle normally flow towards the outer part of the trough.
The mechanism of separation involves primary and secondary flow patterns. As the slurry film flows down the trough, the dense particles settles faster and are carried inward by the lower layer of the secondary flow, while the less dense particles stay on top of the dense materials and are then carried outward by the top layer of the secondary flow. After dewatering using hydrocyclones, the output from the gravity separator spirals the concentrate ore from the spirals is send to Holman’s Wilfley shaking tables for further refining. Similarly to the previous devices, shaking tables allow for the separation of particles based on their specific gravities and sizes. In this case the separation is achieved by an inclined table which oscillates backwards and forwards essentially at right angles to the slope, in conjunction with riffles which hold back the particles which are closest to the deck. This motion and configuration causes the fine high specific gravity particles to migrate closest to the deck and be carried along by the riffles to discharge upper-most from the table, while the low specific gravity coarser particles are able to jump of the riffles, thus discharging over the lowest edge of the table.
The “pre-concentrating” process of the coarser ore via dense media separation is in some sense a much simpler process as it mostly relies on the use of hydrocyclones. The main difference between this process and the desliming/dewatering process is that the medium within the cyclones is much denser than water as otherwise all the coarse feed would escape through the underflow and now separation would be achieved. The required concentration of the wolframite ore is achieved by sending the coarse feed through a primary and a secondary DMS circuit which both consist of hydrocyclones.
Once the slurry has gone through the gravity separation and the dense media separation processes, it consists mainly of wolframite, cassiterite, iron oxides and some silicates and arsenic materials. This slurry is now fed into the “concentrate processing” section where most of the impurities are removed. In a first step arsenopyrite is removed by pumping the slurry into a conditioning tank where several chemicals are added to enable the separation of the arsenic compound via sulphide flotation. Once the arsenopyrite has been removed the slurry is thoroughly dried before feeding into a reduction kiln. This kiln uses diesel as a reductant to generate carbon monoxide, which reacts with haematite and other iron oxides in the feed at approximately 700 °C, to create magnetite or maghemite whilst leaving other minerals largely unaffected. This process changes paramagnetic haematite into ferromagnetic maghemite/magnetite. Wolframite, like hematite, is paramagnetic and without this reduction step separation of haematite and wolframite would be impossible using magnetic separators.
The reduced ore from the kiln is cooled and fed onto a low intensity magnetic separator (LIMS) which is designed to remove the now highly magnetic iron oxides, which are sent to the tailings thickener. The non-magnetic product from the LIMS is sized at 150 μm on a dry Derrick screen before free-flowing to a multi-stage high gradient disc electromagnetic separator (VOG HIMS), with the goal of separating tungsten from non-magnetic minerals such as cassiterite and silicate. These HIMS produce six streams of varying quality tungsten concentrate grading up to over 60% tungstate.
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