Heavy mineral sands deposits are made up of various assemblages of titanium-bearing minerals, such as rutile, leucoxene, ilmenite, xenotime and monazite (containing rare earth metals), and the industrial minerals zircon, kyanite, and garnet. Mineral sands mining operations are currently located on Australia’s east coast in northern NSW and southern Queensland, and in the south west of Western Australia. Dry mills and synthetic rutile plants are located in Western Australia at Geraldton, Eneabba, Muchea, Bunbury and Capel.
Titanium is mainly used in white paints and dyes, and in some alloys where properties of high heat resistance and high corrosion resistance are required. Zircon, because of its high melting point (>2,500 degrees Celsius), is used for manufacturing refractory bricks, ceramics and glazes. Monazite is mainly used in visual electronics.
The production of mineral sands generally follows a three-stage process:
- Mining – dredge or dry mining;
- Concentration and separation – using gravitational, magnetic and electrostatic processes; and
- Synthetic rutile production – involving chemical reduction, leaching, aeration and physical separation.
Representative processes are discussed below, and example flow diagrams are provided as a source of reference. Facilities are advised to use their own specificprocess flow diagrams to assist in structuring and addressing the NPI reporting process and requirements.
Some facilities do not produce synthetic rutile, undertaking physical separation processes only. Other facilities may need to divide their mining and synthetic rutile production operations into separate facilities for the purpose of NPI reporting because of their physical separation (see Section 2.0, Step 1). As such, not all sections of this manual will be relevant to each facility.
The Mineral Sands and other mining and mineral processing operations produce various waste material streams which are generically called tailings. Tailings are the sludge, mineral residue and wastewater (apart from final effluent) resulting from ore extraction and processing. Tailings vary widely in properties and constituents depending on the mineral being processed and the extraction process used. In turn the environmental impact and human health effects that tailing have vary widely. Some sectors of the Mineral Sands industry are using terms other than tailings to reflect more accurately the nature of the various process streams in their operations. In future the terminology used by the NPI may be altered, where appropriate, to reflect changes in terminology used and understood by various sectors of the community.
Mining begins with the removal of vegetation (and subsequent burning or use in land rehabilitation) from the mine site, and the removal and stockpiling (or transfer to previously disturbed sites) of topsoil and sub-soil. Orebodies are often interspaced by non-mineralised material (overburden and interburden) which must be removed to gain access to the ore. This is either stockpiled or backfilled into voids from previously mined areas. Revegetation is often part of the operations at mineral sand mining and processing facilities. Replanting of removed vegetation often occurs for embankment stabilisation and as a seed resource for future revegetation.
Mineral sands bearing ores (and the associated soils and overburden) are recovered by either wet dredging or dry mining techniques. The technique used is largely dependent on the location of the orebody in relation to the local groundwater.
Dredge mining involves the recovery of ore from a void filled with groundwater. The dredge utilises a cutting head to disturb the ore, which is then recovered as a slurry, and pumped to a wet separation plant via a floating pipeline. Water is often added to the dredge pond to maintain optimum water levels for operation.
Dry mining uses conventional mining equipment, generally scrapers for overburden removal and loaders for ore recovery, although excavators and haul trucks may also be used. The recovered ore is screened to remove oversized material, slurried, and then passed through a trommel to remove any remaining oversized material.
Recovered ore is passed through a heavy mineral recovery plant to yield heavy mineral concentrate (HMC). The heavy mineral sands are separated from the lighter tailings (waste material) using gravity spirals. Tailings are then either returned directly to the mine void, or placed in drying ponds from which water may be recovered for re-use. Solids are later returned to the mine void, or sealed and rehabilitated. Flocculants are often used on tailings prior to release, to allow for better decant recovery and void fill ratios.
Dry/Wet Separation of HMC
HMC is passed though various separation processes to recover the minerals present. Separation exploits the physical properties of each different mineral, including conductivity (electrostatics), magnetism (magnetic), and density (gravity) to isolate the various minerals. This produces the saleable product for the majority of mineral sands. Only zircon and ilmenite receive further treatment. Zircon processing may include acid washing to improve the cosmetic value of the product. Some ilmenite may be further processed to produce high-grade synthetic rutile as described in 3.4 below.
The non-valuable tailings process streams are generally stockpiled before being returned to the mine void, or used in waste pond construction.
Synthetic Rutile Production
In some cases ilmenite product may be processed further to yield a higher-grade synthetic rutile product. This is achieved using reduction kilns to remove iron from the mineral matrix by reducing it to metallic iron. Coal is used as the fuel source and sulfur or boron compounds as reducing agents; the ilmenite ore is passed through a reduction kiln.
The ilmenite is separated from any char and aggregates that may have formed during reduction, and is passed through an aeration phase. During this phase ammonium chloride is used as a catalyst to oxidise the iron, which can then be separated from the synthetic rutile using cyclones. Prior to drying and shipment the product is passed through a leaching stage using sulfuric acid to remove any remaining impurities (Mulligan 1996).
Wastes produced during this process are dewatered either in tailings storage ponds or using filtration systems and are then sealed in mine voids or remain in the ponds and are rehabilitated.
Most wastewaters are reclaimed and returned to the process although some are treated and released to the environment.
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