Titanium Ores and the Commercial Mining of Titanium
Titanium is the 9th most common elements on the planet. It is found in both oxide and silicate minerals. The earth’s crust is 90% silicate with titanium present in
concentrations of between 0 and 1%. In such low concentrations it is not economical to extract. Titanium’s oxides meanwhile have concentrations between 15 and >95%. Those with above 25% are the best sources for production. Of titanium’s oxides rutile (Ti02) and ilmenite (FeTiO3) have the highest concentrations and are the world’s primary commercial titanium oxides.
Titanium can be mined from intrusive crystalline rocks, weathered rock and unconsolidated sediment. Half of all Titanium mined comes from unconsolidated sediments known as shoreline placer deposits. Placers are alluvial deposits formed by rivers as they reach the sea. Suspended sediments have different densities known as specific gravities. A river will deposit different sediments as its speed fluctuates, forming separate layers of sediment. Titanium’s ores, ilmenite and rutile are both found in placers worldwide.
How are Rutile and Ilmenite Formed?
Rutile has traditionally been the primary input in the manufacture of Titanium metal. Its name comes from the Latin rutilus meaning red. Its deep red colour is caused by iron impurities in its lattice. Rutile is formed under high pressures and temperatures as an accessory mineral in metamorphic rocks like eclogite. It is not economical to mine rutile from primary rock so it is recovered from weathered deposits in mineral sands.
Sierra Leone is the world’s largest exporter of rutile with among the world’s largest natural deposits of rutile. Important sites include Gbangbama, Rotifunk, Sembehun and Kambia. Titanium ores are Sierra Leone’s second largest export after diamonds and play an important part in the country’s recovery in the wake of its civil war.
The ore ilmenite is much more abundant than rutile. It is formed in magma chambers in intrusive rocks like nortite, anothosite and gabbro. Ilmenite solidifies at a much lower temperature than other minerals. This causes it to sink to the bottom of the chamber as it cools. This process known as “magmatic segregation” takes hundreds of years and causes distinct layers of minerals to be formed. Unlike rutile which is an accessory mineral these layers of ilmenite are considered primary deposits.
Ilmenite can be mined from both layered intrusive deposits and heavy mineral deposits. It is often found alongside rutile in heavy mineral deposits. Ilmenite is used to make titanium dioxide
pigment or it can be processed into feedstock that can be used in the manufacture of titanium. This has become more common as viable rutile deposits become increasingly scarce. South Africa and Australia are among the world’s largest producer of Ilmenite, each extracting over a million metric tonnes per year.
The Mining of titanium from intrusive rock is restricted to ilmenite and its weathered derivative leucoxene. The largest opencast ilmenite mine is Tellnes in Norway’s municipality of Sokndal.
Mining of Titanium Placer Deposits
Placer deposit mining is either done as a wet dredge or dry mining operation. The height of the water table where the deposit is found dictates which method is required.
Wet Dredge Mining of Titanium
In wet dredge mining, an artificial pond is created by digging below the water table. Some operations fill mining ponds with water pumps. A suction bucket wheel attached to a floating dredge is used to remove heavy mineral sediment from the ground. Sediment is then concentrated by passing it through a set of inclined cylindrical trommel screens, as these rotate the material that is too small for processing falls through the screens.
The particles that make it this far are then sorted by a spiral concentrator, a chute sorts particles suspended in water based on their size and density. They are flushed down the chute with water. The high-density particles stay closest to the inside of the spiral chutes cross-section with the lower density particles on the outside edge. The sorted sediments are then collected in separate containers the high-density ores separated from the much lower density silicates which make up 98 of the excavated sediment.
Dry Mining of Ilmenite and Rutile
Dry mining meanwhile is carried out with conventional earth moving plant including excavators, scrapers, loaders and bulldozers. Like wet dredging, the sediments from dry mining also need to be concentrated this is the same as the above process but is done without water in the spiral concentrator.
After the minerals have been concentrated they are put through the feed preparation plant where they are cleaned with attrition scrubbers and subjected to additional gravity concentration before undergoing froth flotation which can remove sulphides or other local unwanted sediment.
The last step is the dry mill, where a combination of magnetic and electrostatic separation is used to improve the quality of the ore. Titanium’s ores ilmenite and rutile are conductive because of their iron content and can be easily separated from zircon and unwanted silicates. After the dry mill, the ore is ready for export.