Mineral sands mining is a comparatively simple process compared with underground mining. The stages involved are
The front end loader is pouring mined sand through a screen to remove coarse pebbles, tree roots and other debris. The sand is then sent to the processing plant to separate the valuable heavy minerals from unwanted sand.
The first part of mineral sands processing involves separating the high density, valuable minerals (ilmenite, leucoxene, zircon) from the lighter unwanted sand (mostly quartz and feldspar). The mined ore is passed through a series of wet cyclones. These are spiral gutters and the ore is washed down them with water. The heavy minerals tend to concentrate on the inside of the spiral where the speed of the water is lowest. The lighter quartz and feldspar sand grains tend to wash to the outside of the spirals. At the bottom of the cyclone the heavy and light minerals are washed into different tanks. There are two final products; one is the valuable heavy minerals (the heavy mineral concentrate, or HMC), the other is quartz rich tailings material.
Bank of cyclones used to separate the HMC from sand.
The HMC consists mostly of ilmenite, leucoxene, zircon and minor rutile. This is transferred to a stockpile to await further processing at another site.
The grey stockpile behind this settling tank is the heavy mineral concentrate.
The quartz rich sand is called tailings, and is placed on a tailings stockpile to be used later in rehabilitating the mined area.
Stockpile of tailings sand, from which the heavy minerals have been removed. This sand will be used to backfill mined out areas prior to rehabilitation.
The second stage of processing involves drying the HMC in a kiln, and then separating it into its constituent minerals. The separating techniques are quite simple, making use of the differing physical properties of the heavy minerals, including magnetic and electrical conductivity properties, and density. The main mineral produced at Capel is ilmenite. Ilmenite is magnetic and is therefore separated from the HMC using a magnetic separator.
Magnetic separator.
Other minerals are separated using further magnetic, gravity or electrostatic separation.
The ilmenite generally contains 58% titanium oxide (chemical formula = TiO2, the same as rutile) and 42% iron oxide (chemical formula FeO). Some of the ilmenite mined at Capel is converted in a value-adding process into synthetic rutile in the Synthetic Rutile Plant. The synthetic rutile can then be used in the production of pigment for paints, plastics, cosmetics etc.
The synthetic rutile plant at Capel.
The conversion is a two-step process known as the Becher Process, which was developed in Australia;
1. The ilmenite is mixed with coal from Collie, and heated to a temperature greater than 1200°C in a 68m long rotating kiln. This converts the iron oxide part of the ilmenite into metallic iron, and leaves the titanium oxide part as synthetic rutile. However the metallic iron is still mixed in with the rutile, and must now be separated from it, to produce pure synthetic rutile.
The rotating kiln in which ilmenite is roasted at 1200°C with coal.
2. The material in the kiln is cooled, and the synthetic rutile and metallic iron are separated from the coal. They are mixed with chemicals that convert the metallic iron to rust, but do not affect the synthetic rutile. The rust and rutile are then separated using wet cyclones. The last traces of rusty iron are removed from the synthetic rutile using acid leaching. The final product contains more than 93% pure titanium dioxide.
