Ore Processing Plant
After entering the ore processing plant the metal ore is crushed using two hybrid roll crushers, which were preferred over jaw crashers as they are believed to cope better with ore of high clay content. After the crushing, the ore undergoes a “pre-concentration” process, the aim of which is to increase its metal concentration. 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 coarse particles and fine particles. Oversized material (larger than coarse screens) is diverted to a cone crusher before being returned to the scrubber and the screens. The fine material is collected in the processing plant’s 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 metal 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 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 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 metal ore is then 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 the metal ore and various impurities such iron oxides, 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. The reduced ore from the kiln is cooled and fed onto a low intensity magnetic separator which is designed to remove the now highly magnetic iron oxides, which are sent to the tailings thickener. The resulting non-magnetic product is sized on a dry screen before free-flowing to a multi-stage high gradient disc electromagnetic separator, with the goal of separating the metal from non-magnetic minerals such as cassiterite and silicate.