6.   METALLURGICAL PROCESSING

6.4.1  

Ticor SA

The metallurgical process at Hillendale Mine comprises three distinct sections:

• The Primary Wet Plant (“PWP”) where RoM material is initially treated to produce Heavy Mineral Concentrate (“HMC”) which is then fed to the Mineral Separation Plant (“MSP”);
• The MSP where HMC is separated into ilmenite and non-magnetic minerals, such as zircon, rutile and leucoxene, both being further processed prior to smelting or sale; and
• The smelter where ilmenite concentrate undergoes smelting to produce titanium dioxide slag and low manganese pig iron.

The Hillendale Mine PWP, with a design throughput rate of 1,200tph has been retrofitted with a desliming circuit, in order to improve the spiral recovery efficiency. The PWP flow essentially comprises de-sliming, screening, spiral gravity separation, low intensity wet magnetic separation and slimes disposal. Finally, HMC is de-watered by hydrocyclones ahead of road transport to the MSP and the smelter at the Central Processing Complec (“CPC”) at Empangeni.

The MSP operation comprises several different processes in a very complex, although fairly typical application of appropriate technology. The HMC is initially screened and separated into a magnetic fraction (ilmenite) and a non-magnetic fraction (zircon, rutile and leucoxene) using Wet High Intensity Magnetic Separation (“WHIMS”) techniques. Ilmenite is further upgraded through two parallel process streams in order to remove chromite. The non-magnetic fraction separated in the WHIMS section is further processed by wet gravity and electrostatic/magnetic separation techniques to separate zircon from rutile and leucoxene.

Smelting uses electrical power to convert the ilmenite feed material into titanium dioxide slag and LMPI. The slag, after cooling, undergoes crushing, drying, grinding and screening to produce two size fractions. The recovery factors relating to the feed preparation section, the ilmenite upgrading sections, the roasting plant and the zircon/rutile dry mill as assumed by Ticor SA are considered to be reasonable. In an operation of this nature, the process represents a significant risk. Ticor SA has attempted to minimise such risk by undertaking extensive test work on a variety of ores. Primary concentration, mineral separation and smelting have been tested at pilot scale.

The No. 1 furnace at the CPC in Empangeni was commissioned during 2003. During September 2004 the No. 2 furnace was shut down in order to make improvements to it identified during the commissioning process. These improvements will in the near future also be applied to the No. 1 furnace.

Hillendale Mine and PWP: RoM (around 660 to 750ktpm) is supplied to the PWP at the correct density and tonnage. The RoM is screened to discard oversized material. The undersized material is sent through a desliming circuit. The overflow of the de-sliming circuit reports to the thickeners. The underflow is sent through banks of spirals. The spirals split the slurry into three main streams:

• The HMC (around 50ktpm) is sent through a magnetic separator to remove magnetite. The HMC is stacked on a stockpile at the Hillendale Mine for transport to the MSP.
• The sand tails are pumped to dewatering cyclones in the mining void and deposited as backfill in the mined out areas or dune. The slimes (particle size <45µm) collected from the spirals are sent to the thickeners where flocculent is added.
• The underflow is pumped to the sub-aerial deposition dam where it is dried over a 21-day cycle.

MSP: The HMC is tipped into receiving bunkers at the MSP. From these bunkers the HMC is screened of oversize. The screened HMC is then fed into the WHIMS. Here the ilmenite is separated from the nonmagnetic material. The non-magnetic material (rutile, leucoxene and zircon) contains some silica and the silica is removed from the non-magnetic material in a gravity circuit.

The crude ilmenite from the feed-preparation circuit is refined to a low chrome content ilmenite. This is done by the use of specialised drum magnets. The high chrome material is more magnetic and can therefore be separated easily from the lower magnetic material. The ilmenite is sold as final product and used as smelter feed.

The primary dry circuit is an electrostatic circuit where conductive and non-conductive materials are separated from each other rutile (conductive) and leucoxene (conductive) is separated from zircon (nonconductive)). This separation is effected by the use of high-tension roll separators and electrostatic plate separator machines. The zircon from the primary dry circuit is iron stained and is sent to the hot acid leach circuit for leaching. The rutile and leucoxene is sent to the rutile dry circuit for further separation between the rutile and leucoxene. The requirement of world markets is that the iron content in the zircon must be less than 0.06%. The zircon that enters the hot acid leach circuit contains 0.18% iron. The reason for this high percentage iron is due to a thin layer of iron oxide around the zircon particles. This layer is removed by leaching the zircon with sulphuric acid. The acid solution is neutralised using gypsum. The water is re-used in the circuit.

The leached material from the hot acid leach circuit is fed to the wet zircon circuit. Here the trash minerals still contained in the wet zircon circuit feed, namely kyanite, hornblendes and silica are removed from the zircon concentrate. The separation is done with a gravity circuit as the tails in this circuit have a lower particle density than zircon. The concentrate from the wet zircon circuit is fed to the final zircon beneficiation stage the dry zircon circuit.

In the dry zircon circuit the final refining of the zircon is done. Here a small amount of rutile and leucoxene is removed and the monazite in the zircon is separated from the zircon with induced roller magnetic separator. The zircon concentrate from this circuit is the final zircon concentrate. The product is fed to three zircon product bins. From here the zircon is either taken to the bulk terminal or to a bagging plant with road tankers.

In the dry rutile circuit, the feed from the primary dry circuit, that contains leucoxene and rutile, is split into two products. The rutile and leucoxene are separated from one another using an induced roll magnet separator. Both these products are stored in product bins that are unloaded with road tankers that take the product to the bulk terminal.

Ticor Smelter: The smelter comprises two 50 MVA DC arc furnaces. The ilmenite from the MSP is fed into the furnace together with anthracite (reductant). The mixed feed is fed via a hollow electrode. The feed can be fed as a cold material or as a hot material. Hot material is achieved by feeding the ilmenite through a preheater that heats the material up to a range of between 800°C and 900°C. This allows the furnace to operate at a lower energy consumption rate than when feeding cold material. The ilmenite and anthracite is fed in the correct ratio in order to obtain the correct chemistry and thermal balance within the furnace.

The main chemical reaction that takes place is:

FeO.TiO₂ + C = Fe (metal) + TiO₂ (slag) + CO (gas).

The iron and slag are tapped from the furnace from different tap holes. The carbon monoxide gas is burnt off in a flame above the furnace building. The iron is processed further at the metal processing plant. At the metal processing plant the tapped iron is treated with calcium carbide to reduce the sulphur content of the iron. Ferro-silicon and carbon are added to achieve the final customer specification for the iron. The iron is finally cast into small ingots (pigs) and stored in bunkers. From the bunkers, the pigs (Low Manganese Pig Iron) are transported by road to the bulk terminal for export. All scrap that is generated from the metal processing plant is collected and sold locally.

The titanium dioxide slag that is tapped from the furnace is allowed to cool and then processed further at the slag processing plant. At the slag processing plant, the slag is crushed, dried, screened and then classified according to coarse and fine size fractions. The coarse material becomes the final product of chloride grade slag and the fine material becomes the final product of sulphate grade slag. Both final products are exported.

6.4.2  

Tiwest JV

Tiwest JV mines and processing plants: The Tiwest JV mines and processing plants operate in an integrated manner such that ilmenite originating from the mines is processed through to the finished product of titanium dioxide pigment.

HMC is produced at the primary concentrating plants (“PCP”) at Cooljarloo then transported to Chandala. Mineral products are produced in the MSP at Chandala. Ilmenite from the MSP is taken directly into the adjacent synthetic rutile plant (“SRP”). Synthetic rutile either is transported to the pigment plant at Kwinana or exported. Waste products from the processing plants at Chandala and Kwinana are returned to Cooljarloo for final disposal.

Separate PCPs operate in the two mining areas at Cooljarloo. In the Cooljarloo South mine two dredges mine in parallel to feed the PCP at 2,200tph, which floats in the dredge pond. In the Cooljarloo North mine scrapers dump ore into a slurrying hopper to feed the land-based PCP at 650tph. This PCP can be relocated when the mine advances beyond an economic pumping distance. The PCPs have similar flowsheets.

Gravel, rock and clay lumps are removed from the slurried ore by a rotating trommel. The resulting sand slurry is deslimed by hydrocyclones before treatment in a conventional spiral circuit. HMC containing approximately 96% heavy minerals is stockpiled by hydrocyclone. Sand tailing is pumped back to the mined out area to reform the land surface. Slime tailing from the North mine, derived from higher slime containing ore, is thickened before pumping to shallow solar drying areas. Slime tailing from the South mine settles in the dredge pond from where it can also be pumped to solar drying areas when required. The combined output of concentrate from the two PCPs is up to 800,000tpa. The two concentrates are blended before trucking to Chandala.

The maximum treatment rate of HMC at the MSP is 90tph. The total consumption is 740ktpa leaving the possibility of stockpile accumulation at the mines. The flowsheet required to separate mineral products in the MSP is complex. The HMC is first attritioned to remove coatings from the mineral grains and screened to remove coarse oversize grains. The resulting concentrate is dried and heated before electrostatic separation to separate the conducting minerals ilmenite, rutile and leucoxene from the non-conducting mineral zircon and most of the trash minerals including quartz, monazite and kyanite. Ilmenite, rutile and leucoxene are separated into products by magnetic separators. The non-conducting minerals are classified into two size ranges and magnetically cleaned before reslurrying then concentrating on wet tables, spirals and Kelsey jigs.

The concentrates are dried then zircon is produced after many stages of electrostatic and magnetic separation. A small output of staurolite, derived from the coarse non-conductor magnetics, has recently been added to the products.

The Becher Process employed in the Chandala SRP is an elegant combination of pyro and hydrometallurgy which removes most of the iron oxide portion of ilmenite and about 50% of the manganese oxide impurity resulting in synthetic rutile containing up to 93% TiO₂.

Ilmenite and Collie sub-bituminous coal are mixed in a rotary kiln. The coal is combusted to provide carbon monoxide for reduction and heat up to 1,100ºC. Iron oxide is reduced to metallic iron within the ilmenite grains. Sulphur is also introduced to the kiln to sulphidise manganese oxide within the grains. The reduced ilmenite is cooled and separated from coal char, which is recycled to the kiln.

Excess char is cleaned for sale as activated carbon. The reduced ilmenite is then slurried in aeration tanks with a weak solution of the oxidising catalyst ammonium chloride. Each batch is sparged with air while being agitated until the metallic iron has been reoxidised. During this process iron migrates from the reduced ilmenite to form separate particles of iron oxide less than 5 microns in diameter. Iron oxide is separated from synthetic rutile by hydrocyclones. The synthetic rutile is then agitated with dilute sulphuric acid to remove manganese sulphide and residual iron.

Treatment and disposal of waste products is a major part of the process, which occurs in the waste management plant. Iron oxide slurry is thickened and filtered. Acidic effluent is neutralised and the precipitates thickened and filtered. Sulphurous liquor from the kiln waste gas scrubber is treated with a double alkali process and the precipitates thickened and filtered. All liquors are recycled to the SRP. All filter cakes are returned to the mine for disposal.

The capacity of the SRP has been gradually increased to 220ktpa. An increase to 240ktpa is currently underway and is forecast to produce at that rate in 2006. Ilmenite consumption will rise to a maximum of 400ktpa, which still allows excess ilmenite from the MSP to be stockpiled at Chandala. This stockpiled ilmenite of high quality has the potential to maximise kiln capacity in the future if blended with lower quality ilmenite from orebodies developed to replace Cooljarloo North.

Kwinana Pigment Plant: The plant is situated in Kwinana, Western Australia where the process employed is the Chloride Process. The technology is provided by the Tronox Inc of Oklahoma. The initial capacity of the plant was 54.6ktpa when the plant was commissioned in 1991. The plant has since been expanded, largely by de-bottlenecking, to the point where the capacity is now 108,000tpa.

The feedstock for the plant is synthetic rutile (“SR”) which is obtained from the Tiwest JV synthetic rutile production plant located near Muchea about 70km north of Perth. The feedstock has a TiO₂ grade of around 93%. The TiO₂ part of the synthetic rutile comprises mainly iron oxide as well as other metal oxides. The impurity – TiO₂ metal oxides – must be separated from the TiO₂ because, in general, these TiO₂ metals impart colour to pigment other than the desired white colour. The separation of the TiCl₄ from the other metal chlorides is achieved as described below.

Firstly almost all the SR is converted to a metal chloride vapour. Titanium tetrachloride (“TiCl₄”) is one of the most volatile of the metal chlorides and has the convenient property that TiCl₄ is a liquid at ambient temperature and pressure. The SR is reacted with chlorine and coke to produce a mixture of metal chloride vapour (mainly TiCl₄), carbon monoxide, carbon dioxide and a small amount of hydrochloric acid vapour. This reaction is carried out in a fluid bed reactor referred to as a chlorinator. On leaving the chlorinator the exit gas is cooled. As the exit gas cools most of the metal chlorides other than TiCl₄ freeze out as solids.These solids are separated in a waste solids cyclone and are quenched with water to form an acidic metal chloride solution which is then transferred to the Effluent Treatment Plant. The gas passing through the waste solids cyclone now contains TiCl₄ vapour, carbon monoxide, carbon dioxide and a small amount of hydrochloric acid vapour. The gas is further cooled to condense out the TiCl₄ as a liquid. This TiCl₄ liquid is then treated and distilled to produce a pure form of TiCl₄. This purified TiCl₄ is stored prior to being used as the feedstock for the Oxidation Plant. The non-condensable gases report to the Waste Gas Scrubbing train. Waste gas is scrubbed with water to remove the hydrochloric acid vapour and forms a saleable 28% Hydrochloric Acid product.

TiCl₄ can be burned with oxygen in a special burner (called an Oxidizer) to form rutile (one of the crystal forms of TiO₂) crystal particles in a size range where they exhibit maximum light scattering properties. Light scattering translates to hiding power, which is the chief commercial property of TiO₂ pigment. In order to stabilise the rutile crystal it is doped with a small amount of aluminium, which is achieved by dissolving a small amount of aluminium chloride in TiCl₄ prior to oxidation. Gaseous oxygen is obtained from the L’Air Liquide plant in Kwinana by pipeline.

During the oxidation process chlorine is liberated from the TiCl₄. This chlorine is separated from the TiO₂ product and is recycled to the Chlorination Plant. The TiO₂ product from the Oxidation Plant is referred to as Raw Pigment. Raw Pigment particles do not mix into paint resins very well. To facilitate the incorporation of Raw Pigment into resins an alumina coating is applied to the Raw Pigment particles. Some grades of pigment will also be coated with silica and zirconia to enhance the durability of the paint. These coating steps are carried out in the Pigment Finishing Area. The main reagents consumed in the process to carry out these coatings include Sodium Aluminate, Sodium Silicate, Zirconium Oxychloride, Caustic Soda and Sulphuric Acid.

Raw Pigment slurry is first subjected to Sand Milling to breakup agglomerates of pigment. The product from the Sand Milling section is then cycloned and screened to remove remaining oversize particles. The screened Raw Pigment slurry is then fed to the Treatment Tanks where the coatings are applied. The product slurry from the treatment section is washed on rotary vacuum filters to remove remaining soluble chemical from the treatment process. The washed pigment slurry is then fed to gas-fired Dryers which remove water from the pigment. The pigment is then micronised. Micronisation is a process whereby pigment agglomerates are broken into particles of only a few microns in size. This is accomplished using high pressure steam jets in a device known as a microniser. The micronised pigment is then cooled and bagged into 25kg paper bags or bulker bags. The major part of the pigment produced goes into paper bags. The paper bags are shipped on pallets which are shrink-wrapped for protection against moisture.