Wonarah NI 43-101 Technical Report – Preliminary Economic Assessment

This article summarises the two alternative processing routes described in the September 2012 technical report for the Wonarah phosphate project.

Report context

The Wonarah NI 43-101 Technical Report – Preliminary Economic Assessment, dated 28 September 2012, describes the evolution of the Wonarah project from direct shipping ore to on-site downstream processing. Two alternative plant configurations were considered appropriate for the new project strategy: a wet acid process (WAP) route and an improved hard process (IHP) route.

Processing route

Wet Acid Process Route

The WAP route involves a beneficiation plant at Wonarah, a rock slurry pipeline from Wonarah to Tennant Creek, and a chemical plant complex at Tennant Creek comprising a sulphuric acid plant, a phosphoric acid plant, and a DAP/MAP fertiliser plant.

The beneficiation plant design was modified significantly from an earlier Lycopodium design based primarily on operations at Phosphate Hill. The new design represents an initial process concept based on review of test data and information provided by Minemakers, and experience in process development of similar sedimentary phosphate deposits. The mass balance was used as the primary basis for capacity factoring for development of the inside battery limits beneficiation plant capital cost. The report emphasises that the process flowsheet is unproven and was developed primarily to provide a basis for developing comparative estimates of capital and operating costs as part of the PEA.

The beneficiation plant comprises five main processing areas: ore crushing and storage, feed preparation, flotation, water recycle, and reagent tank farm. Run of mine ore is delivered by truck and dumped through a grizzly into a nominal 100 t surge hopper. Ore is removed by vibrating feeder to an open circuit toothed double roll crusher (primary crusher). Primary crusher product is conveyed to a secondary crusher, also configured in open circuit. Secondary crusher product is then conveyed to an ore storage system consisting of a homogenising circular stacker/reclaimer, a slewing stacker, and a rail-mounted bridge type reclaimer.

The feed preparation area contains a rotary drum scrubber, a rod mill configured in open circuit, a ball mill configured in closed circuit with hydraulic sizers, and hydrocyclones for removing fine waste material. Reclaimed ore is conveyed to a drum scrubber designed to disaggregate the ore prior to screening, grinding and desliming. Drum scrubber discharge at about 65% solids reports to a vibrating screen equipped with 1.7 mm openings. Washed screen oversize reports to a rod mill. Rod mill discharge and scrubber screen underflow are combined, repulped and pumped to the hydraulic sizers. Hydraulic sizer underflow (coarse discharge) reports to a ball mill while hydraulic sizer overflow (fines) reports to the cyclone feed tank.

Slimes are removed by hydrocyclones. Hydrocyclone overflow (fines) flows to the waste thickener while cyclone underflow (coarse discharge) reports to the feed thickener. The feed thickener provides surge capacity between feed preparation and flotation areas and provides high solids feed to the reagent conditioners.

The flotation circuit includes conditioning with flotation reagents and three stages of flotation. Preconditioning adjusts the pH of the 65% solids slurry. Conditioning with a collector follows, where the collector is adsorbed on the surfaces of the phosphate particles, rendering them hydrophobic. The slurry exiting the conditioners (vertical stirred tanks) is diluted and fed to the rougher flotation circuit where a phosphate froth product is recovered and cell underflow is rejected as tailings. Rougher concentrate flows to the first cleaner circuit, then the second cleaner circuit. Froth product from the second cleaner is final concentrate, which is pumped to dewatering hydrocyclones. Thickened concentrate is collected in large agitated tanks and pumped by slurry pipeline to the chemical plant.

Tailings from rougher and cleaner circuits are combined with waste cyclone overflow solids and report to the waste thickener. Waste thickener underflow solids are pumped to the tailing impoundment area. Impoundment area water is returned to the process water tank.

Reagents used in the flotation process include: fatty acid soap (phosphate collector used in rougher conditioners), sodium silicate (silica depressant used in rougher flotation), and caustic (sodium hydroxide) used to saponify the fatty acid and adjust conditioning and flotation pH.

For pipeline transport, rock from the beneficiation plant must be ground to about 100% passing 165 microns to 200 microns and 20% to 30% minus 45 microns depending on rock properties. Rock slurry is pumped at between 50% and 60% solids. The report notes that the grind for beneficiation will likely meet the pipeline grind specification, meaning no additional grinding would be needed.

The sulphuric acid plant uses the double-contact double-absorption (DCDA) type to comply with environmental regulations. Prilled sulphur is received by railcars, melted using steam, filtered and stored in molten sulphur storage tanks. Molten sulphur is pumped to the sulphuric acid plants. Steam generated from the exothermic reaction is sent to a turbine-generator.

The phosphoric acid plant uses the dihydrate (DH) process. Phosphate rock is reacted with 98% sulphuric acid to produce nominally 28% P2O5 phosphoric acid and gypsum. Reactor slurry is fed to a filter where product acid is separated from gypsum solids. Filter product acid at 28% P2O5 is pumped to clarification and storage. Recycled process water is mixed with gypsum filter cake and pumped to the gypsum stack. Fluoride vapours are vented and sent through a fume scrubber.

For DAP/MAP production, concentrated 52% P2O5 acid from storage and ammonia are partially reacted in a preneutraliser and the resultant ammonium phosphate slurry is pumped into the granulator. Fertiliser granulation occurs in a rotary granulator drum where slurry is sprayed onto a bed of recycled dry material. Additional ammonia is added through a sparger pipe to fully ammoniate the slurry and complete the reaction to produce either DAP or MAP depending on how much ammonia is added. Damp granules leave the granulator and fall into the dryer.

Gypsum slurry from the reaction area is pumped to the top of the gypsum stack. The initial gypsum stack is designed for a five-year storage capacity and will be approximately 60 m high. The stack is isolated from the ground by an HDPE liner.

The turbine generator converts steam produced by the sulphuric acid plant to electric power for internal use and sale. If a larger market can be found for surplus electricity, a heat recovery system could increase electricity available for export from 32 MW to 48 MW.

Improved Hard Process Route

The IHP route involves a beneficiation plant at Wonarah, an IHP phosphoric acid plant at Wonarah, and storage for raw materials and products. The IHP beneficiation plant is much simpler than the WAP plant. The design is based primarily on screening and washing tests conducted by Minemakers in June 2011 using -12.7 mm crushed feed metallurgical composite 1. The report emphasises that the process flowsheet is unproven and was used primarily to provide a basis for developing comparative estimates of capital and operating costs as part of the PEA.

The IHP beneficiation plant comprises three main processing areas: ore crushing and storage, feed preparation, and water recycle. Compared to the WAP beneficiation plant, rod and ball mill grinding are not required. Flotation is also not required, and therefore dewatering of flotation feed, reagent conditioning, rougher, cleaner, and scavenger flotation circuits, concentrate thickening, and the reagent tank farm are not required.

ROM ore is delivered by truck and dumped through a grizzly into a nominal 100 t surge hopper. Ore is removed by vibrating feeder to an open circuit toothed double roll crusher (primary crusher). Primary crusher product is conveyed to the secondary crusher configured in closed circuit with a vibrating screen. Secondary crusher product is conveyed to a vibrating screen that makes a separation at about 10 mm. Screen oversize is returned to the primary crusher discharge while screen fines (nominally -1.5 cm) are conveyed to the product storage pile. Crushed ore is placed on the storage pile using an elevated tripper conveyor mounted over a chevron shaped storage pile.

The feed preparation area contains a rotary drum scrubber, hydrocyclones for removing fine waste material, and a belt filter for dewatering the product. Reclaimed ore is conveyed to a drum scrubber designed to disaggregate the ore prior to desliming. Drum scrubber discharge at about 65% solids reports to the cyclone feed pump box where it is repulped and pumped to the waste cyclones. Waste cyclone underflow reports to the belt filter where it is further dewatered and then conveyed to the IHP plant.

Waste cyclone overflow (fines) is directed to a waste thickener. Waste thickener underflow solids are pumped to the waste impoundment area. Impoundment area water is returned to the process water tank.

To produce 1 Mtpa of P2O5, the IHP phosphoric acid plant configuration requires five IHP trains at 200 ktpa of P2O5 each. The process is divided into raw material handling, dry side operation, and wet side operation.

In raw material handling, phosphate rock (with a lower grade than required for WAP), petroleum coke, silica sand and bentonite are brought into the plant and dumped in large storage piles. Rock is brought by conveyor from the nearby beneficiation plant. Coke, sand and bentonite are brought in by truck. These raw materials are fed from storage piles at accurately controlled flows and combined in a conveyor system to feed the five trains of the IHP dry side operation. Other raw materials include: binder (a liquid additive used in the dry side mixer operation), lime (an alkaline solid mixed into a slurry and used in the wet side flue gas desulphurisation operation), and fuel oil (used for auxiliary firing at the dry side dryers and kiln).

In the dry side operation, the combined mixture of phosphate rock, petroleum coke, silica sand and bentonite is fed to the feed dryer to remove moisture. Solids leaving the dryer are screened to remove large lumps and material passing through the screen is finely ground in ball mills. Exhaust air from the dryer passes through a feed dryer cyclone and feed dryer scrubber to recover entrained solids particles.

The mixed phosphate material from the ball mills is fed to the mixer by the mixer feed conveyor. Solids on the conveyor pass under a laser-induced breakdown spectroscopy unit that continuously analyses the material on the belt for Ca, Mg, and Si to confirm correct feed formulation. If additional silica is required, it is automatically brought in directly to the mixer feed conveyor from the sand bin.

In the mixer, ground material from the ball mills is vigorously mixed with water and special binding reagent. Wet solids leaving the mixer fall into the balling drum where they are mixed with additional water so solids are formed into 10 mm to 13 mm balls. Material leaving the balling drum passes over the roller screen where fines and oversize are separated and conveyed back to the balling drum. Product sized balls from the roller screen are fed to the grate dryer.

In the grate dryer, balled solids are spread evenly over a grating and dried by warm air rising up through the grating. Fines that fall to the floor of the grate dryer are conveyed back to the balling drum. The dryer heat source is normally warm air from the aggregate cooler or fuel oil burners.

From the grate dryer, dried balls are conveyed to the variable speed, refractory lined kiln, where solids are roasted and P4O10 gas is liberated from the phosphate rock. The kiln is equipped with radial ports around the shell to distribute air over the kiln bed to oxidise phosphorus and carbon monoxide and create heat in a controlled manner. Air is blown through the kiln counter-currently to the flow of material. Heat is provided by combustion of petroleum coke and supplemented as required by fuel oil. Aggregate solids from the kiln are air cooled in a rotary cooler and conveyed to storage piles. Warm air from the cooler is either used as a source of heat in the feed and grate dryers or exhausted to atmosphere.

In the wet side operation, P4O10 gas from the kiln is absorbed in water in the hydrator to form superphosphoric acid containing 68% to 70% P2O5. Gases leaving the hydrator are scrubbed in the venturi scrubber, cyclone scrubber and mist eliminator to recover product not absorbed in the hydrator. Vent gases are further scrubbed with lime slurry in the flue gas desulphurisation tower for final cleaning before exhausting to atmosphere. SPA draining from the hydrator is collected in the strong acid surge tank, filtered to remove suspended solids, and pumped to storage. The flue gas desulphurisation system absorbs SO2 and converts it to calcium sulphate which is purged to a settling pond.

For DAP/MAP considerations, SPA product from the IHP plant can be sold for use as raw materials for liquid fertilisers or to produce DAP/MAP. IHP acid is very pure with an almost zero MER (% Al2O3 + % Fe2O3 + % MgO)/% P2O5. IHP acid cannot be used directly to manufacture commercial grade DAP since impurities are needed to promote granulation. To make solid fertiliser, IHP acid could be blended with phosphoric acid having a high level of impurities (high MER acid or HMA). This would require a relatively small wet process phosphoric acid plant or a source of sludge acid or raffinate from a purified acid plant. An acceptable MER for DAP would be about 0.04 and for MAP about 0.06.

Key reported parameters

Technical qualifications

The report states that the process flowsheet for the WAP beneficiation plant is unproven and was developed primarily to provide a basis for developing comparative estimates of capital and operating costs as part of the PEA. The IHP beneficiation plant flowsheet is similarly described as unproven and used primarily to provide a basis for developing comparative estimates.

The report notes that the beneficiation plant design for the WAP route was modified significantly from an earlier Lycopodium design that was based primarily on operations at Phosphate Hill. The new design represents an initial process concept based on review of test data and information provided by Minemakers, and experience in process development of similar sedimentary phosphate deposits.

The IHP beneficiation plant design is based primarily on screening and washing tests conducted by Minemakers in June 2011 using -12.7 mm crushed feed metallurgical composite 1.

Storage capacities are noted as requiring refinement in conjunction with logistics studies, as storage requirements depend on tonnages consumed and produced as well as the reliability of transport. Storage quantities at Darwin Port are not included in the reported storage figures.

Source: Minemakers Australia Pty Ltd, Wonarah NI 43-101 Technical Report – Preliminary Economic Assessment, 28 September 2012, Sections 17.1–17.6.3.