Processing

  General Description

Lihir processing plant is designed to process 3.6 Mtpa of ore containing 7.2% sulphide sulphur at a gold grade of up to 13 g/t and a plant availability of 86%. The instantaneous design feed rate is 375 t/h of ore containing 27 t of sulphide sulphur. There is a legal obligation under the mining permit to examine the potential for a production expansion. A simplified process flow diagram ^PDF (58 KB) is attached. (Download Acrobat Reader).

The gold plant comprises facilities for crushing, grinding, flotation, pressure oxidation (POX), counter current decantation (CCD) washing, neutralisation, carbon-in-leach (CIL) cyanidation and tailings disposal. Gold recovery facilities include acid washing, carbon stripping, electrowinning, smelting and carbon regeneration. Two cryogenic air separation plants supply 1800tpd of oxygen to the POX circuit.

Run-of-mine stockpiles and primary crushing facilities are located in the Ladolam Creek area, about 500 m east of the ore deposit. All other processing facilities are at Putput Point, approximately 1 km north-east of Ladolam Creek.

Crushing, Stockpiling and Reclaim

The primary crushing circuit incorporates a gyratory crusher to produce the required feed for semi-autogenous grinding (SAG). Run-of-mine ore is delivered to the primary crushing facility by rear-dump haul trucks and front-end loaders. Ore delivery from the mine is on a 24 hour per day schedule. The crushing and conveying facilities are sized for a possible increases in plant throughput.

While ore can be dumped directly to the crusher dump hopper, most of the ore is blended on the ROM stockpile.

Ores that contain too much clay to pass easily through the gyratory crusher will be either blended with rock ore types prior to crushing, or fed to the MMD toothed-roll sizer which is installed in parallel with the gyratory crusher. Stockpiled ore is reclaimed by a front-end loader as required.

A belt conveying system delivers crushed ore to the crushed ore stockpile at Putput, terminating in a radial stacker capable of a 120° movement. The reclaim system includes three apron feeders, with two located under the stockpile and the third positioned to the side of the pile and fed only by front-end loader. Ores needing special treatment, such as those with excessive clay and those with high sulphur, are stockpiled away from the under-pile feeders for controlled reclaim and blending into the SAG mill feed by means of a front-end loader through the external feed hopper.

Primary crusher operation is controlled from a remote DCS console in the crusher control room, with the feed rate controlled through a variable speed drive on the apron feeder.

Ore is reclaimed and conveyed to a SAG mill operating in closed circuit with a trommel screen.

SAG mill discharge, together with the ball mill discharge, is diluted with mill water in the cyclone feed sump and pumped to a cyclone cluster. Cyclone underflow flows by gravity to feed the ball mill. Cyclone overflow is directed to a Delkon linear screen with a 1300 µm cloth aperture for removal of trash or tramp oversize. Trash screen undersize flows by gravity to the grinding thickener feed tank or the Flotation circuit.

  Flotation

The ball mill cyclone overflow can be split to divert up to 120tph at 30% solids through the pilot Flotation circuit. Design flotation time is 14 minutes and concentrate production is 40-50 dtph. Feed can also be obtained from the Pre oxidation storage tanks.

The feed is conditioned in two tanks (6 minutes) and then passed on to the rougher cells in a 2-2-1 arrangement. The concentrate is pumped to the grinding thickener and the tails go to the CIL circuit or to Plant tailings.

The grinding thickener is a high-rate type that requires a relatively low pulp density of about 10% to achieve the most efficient use of flocculent. Approximately 2500 m³/h of raw water is added to the grinding thickener feed tank to reduce the soluble chloride content in the thickener underflow to 50 g/t of ore or less, which also satisfies the dilution requirement. Flocculent addition can be adjusted automatically, based on the clear water pulp interface in the thickener. Thickener underflow at design of 55% solids is pumped to one of three pre-oxidation tanks.

A portion of the grinding thickener overflow is recycled to the grinding circuit for mill water use, including SAG mill feed dilution, trommel sluice water and recycled feed dilution. Grinding thickener overflow is also used as dilution water in the neutralisation circuit. The remaining thickener overflow is discharged by gravity to the No. 2 CCD thickener, positioned just below the grinding thickener, for use in washing autoclave discharge slurry.

Grinding thickener underflow at 55% solids can be pumped into any one of the three pre-oxidation tanks. Some acidic slurry from the autoclave flash tank can be recycled back to the pre-oxidation tanks to remove contained carbon dioxide in the ore in the form of carbonates, and to control the overall sulphide content by recycling solids that have already been oxidised. In addition, the tanks also act as surge volume between the grinding and POX facilities. Discharge from the pre-oxidation tanks is piped to the POX facilities and delivered to the autoclave circuit by one of three POX booster pumps.

The three autoclaves flash and quench vessels, and scrubbers operate in parallel. Each autoclave has its own variable-speed, positive displacement, high pressure feed pump. The autoclave is divided into six compartments by partial titanium walls: the enlarged first compartment has three agitators, while the others each have a single agitator.

Gaseous oxygen is sparged below the bottom impeller of each agitator. High pressure quench water is also injected as required into each compartment to control the operating temperature to 210°C (maximum) throughout the length of the autoclave. Oxygen is supplied from the oxygen plant at 98% purity. Autoclave pressure is controlled 2400 to 2700 kPa(g) by controlled venting of free gases to the quench vessel.

Slurry discharges from the last compartment through a dip pipe, and thence through a modulating valve to a fixed ceramic choke located inside the flash vessel. High pressure water is added to the slurry between the autoclave and flash tank and is used, in conjunction with the modulating valve, to control flow rate. The super-heated slurry is cooled by flashing steam as the pressure drops in passing through the choke. Steam exits from the flash tank and together with the vent gas passes through a quench vessel where a majority of the steam is condensed in a sea water spray. Non-condensable gases from the quench vessel pass through a venturi scrubber for the collection of any residual slurry droplets or mist prior to atmospheric discharge. Slurry from the flash tank flows by gravity to the first CCD thickener, as do sea water streams from quench vessel and scrubber.

Two CCD thickeners wash acid and soluble salts from the slurry prior to neutralisation. A combination of fresh water from the grinding thickener and sea water is used for the counter-current washing of the solids. Sea water use is limited to optimise the lime consumption during neutralisation.

This facility includes nine tanks and associated screens arranged for cascade gravity flow; a pre-mix tank, a neutralisation tank, a leach tank followed by a Delkor trash screen, and six CIL tanks with inter-stage Kemix carbon screens, with discharge from the last tank flowing through a Delkor carbon safety screen. Any individual tank or screen can be by-passed for maintenance, with a minimal effect on production.

Underflow from the second CCD thickener feeds the small, intensely agitated pre-mix tank, in which milk-of-lime is added to neutralise the residual acid, precipitate the solubilised metals, and raise the pH to 10, which is the alkalinity required for subsequent cyanidation. Final pH adjustment is made in the neutralisation tank by adding milk-of-lime. Grinding thickener overflow is used to dilute the neutralised slurry to the density and viscosity required in CIL.

The neutralisation tank overflows to the leach tank, where the bulk of the sodium cyanide solution required for leaching is added. These two tanks, as well as the first CIL tank, are sparged with low pressure air to ensure sufficient oxygen for the dissolution of contained gold. A Delkor linear screen with 800 µm aperture cloth between the leach tank and the first CIL tank removes any oversized material or trash that would otherwise contaminate the carbon if it were allowed to pass to CIL.

Leach discharge flows through the trash screens to the six CIL tanks. Granular 6 x 12 mesh carbon and 1.5mm diameter extruded carbon at a concentration of 10 to 15 kg/m³ of slurry, is maintained in the pulp to absorb the soluble gold cyanide complex. The carbon is retained in each tank by two vertical Kemix cylindrical retention screens. Slurry flows from tank to tank, and carbon is advanced counter current by back pumping slurry from tank to tank up stream.

Discharge from the last CIL tank passes through the final Delkor carbon safety screen to recover any carbon that might otherwise be lost. These screens are slightly finer than the tank screens in order to recover abraded near-size carbon. Screen undersize flows to the cyanide detoxification circuit.

All gold recovery and carbon facilities are designed for a maximum of 20 t/d of carbon loaded at 5500 g/t of gold. This represents approximately 50% excess capacity over the daily average of the highest predicated yearly gold ore grade (8.3 g/t Au), and is incorporated into the flowsheet in order to handle occasional periods when high grade ore will be encountered.

Carbon is advanced to gold recovery as a slurry by the No. 1 CIL tank carbon advance pump. The slurry is removed from the carbon on a vibrating screen, with screen underflow returned to the No 1 CIL tank discharge launder by gravity. Water sprays on the screen wash most of the adhering ore slurry from the loaded carbon, which is then discharged to one of the two acid wash columns.

The gold recovery facility includes several unit operations to remove the gold from the carbon and produce gold bullion as a final product. Loaded carbon is washed with a 3% hydrochloric acid solution, primarily to remove calcium scale which precipitates during the leach and CIL process. Acid solution is circulated upward through the wash column from the acid wash recycle tank, and overflow from the column is returned to the same tank. This wash cycle is maintained for 1 hour and is followed by draining the residual acid and rinsing the carbon with filtered fresh water for 3 hours to remove residual acid. Rinse water and acid bleed stream are pumped through a plate and frame filter press for recovery of carbon fines. Filtered solution reports to the tailings detoxification circuit.

Desorption (removal of the gold cyanide complex from the carbon) takes place in a single continuous Anglo American Research Laboratories (AARL) process utilising a pre-soak vessel feeding a separate elution column. This system consists in soaking the loaded carbon in a hot caustic cyanide solution in the first vessel followed by stripping with clean, demineralised water in the elution column. The design of the desorption system allows for its operation as either a continuous process or as a conventional batch AARL elution system (if any operating problems are encountered with the continuous carbon transport system).

The carbon is transferred from the acid wash column in 6 t batches by pressurising the column with plant air. It is de-watered on a screen prior to discharge to the upper vessel where the soaking takes place. A pre-soak solution, comprising 7.5 g/L of caustic and 16 g/L of sodium cyanide, circulates through the chamber continuously at 90°C, with the temperature maintained by a steam heated exchanger in the circulating loop. The carbon moves by gravity through a double valve transfer chamber at the conical bottom of the pre-soak vessel and into the elution column.

Water for desorption is heated to between 100 and 115°C by steam in an in-line heat exchanger. The water is injected near the bottom of the column and overflows from the top of the vessel to the pregnant solution tank. Pressure in the column is controlled to prevent flashing of the hot water. The pregnant solution can be routed through a recuperative heat exchange if necessary to reduce its temperature to below boiling point. Pregnant solution is collected in a tank, from where it is fed continuously to electrowinning.

Carbon is discharged from the bottom of the vessel through a regulating valve. Cold water is added to the discharge pipe to cool the carbon prior to its discharge to the stripped carbon tank. Stripped carbon is transferred by recessed impeller pump to a single horizontal carbon regeneration kiln for reactivation. Carbon is first dewatered on a stationary dewatering screen, with supplemental dewatering occurring in the kiln feed bin.

The kiln is heated indirectly by No. 2 fuel oil. Residual moisture in the carbon provides sufficient steam to make the internal atmosphere relatively inert, thus preventing oxidation of the carbon as it is heated to about 600 to 650°C. The heating process volatilises any adsorbed hydrocarbons and reactivates the carbon.

The hot reactivated carbon discharges into an agitated sump flooded with water, where the carbon is quenched. Make-up virgin carbon is added, as required, to a separate agitated tank for pre-attrition before use. Recessed impeller pumps deliver the regenerated carbon to a dewatering screen at No. 6 CIL tank. The water reports to a carbon water tank for reuse in transporting carbon, with a bleed stream to tailings to prevent a build up of carbon fines within the circuit.

Pregnant solution containing about 400 g/t of gold is pumped to electrowinning, which consists of two banks of two cells each in parallel. In electrowinning, the soluble gold is plated on to stainless steel wool cathodes. Barren solution, at about 5 to 10 g/t of gold, is pumped to leach for utilisation of the contained cyanide and recovery of any remaining gold values.

Loaded cathodes are removed periodically and placed on a rack in one of two cathode wash tanks where the gold, in the form of a sludge, together with wash-down from the cells themselves, flows to a conical bottom filter feed tank from where it is pumped to a plate and frame filter press. A portion of the filtrate is re-used as cell wash-down water and the rest reports to the barren solution tank. The gold sludge is recovered as a wet cake and placed in SS trays.

Oxide ore contains some mercury and a mercury retort is included in the gold room equipment as a worker hygiene precaution. The trays of gold sludge are placed in the retort overnight for the removal of mercury and moisture prior to smelting. Dried slimes are mixed with flux consisting of borax, sodium nitrate, feldspar and soda ash, and placed in an induction furnace. The charge is melted, the slag poured off, and the metal poured into bars. Assay samples from each bar are taken with vacuum tubes before solidification. The bars are cleaned, weighed, stamped and stored in a safe inside the vault until shipment. The product is gold bullion containing mostly gold and some silver, with minor impurities.

Slag is crushed and screened inside the gold room to recover gold prills, with the clean slag recycled to the grinding circuit. Assuming that 100 g of slag are produced for each kilogram of gold, approximately 250 kg of slag would be produced each month.

  Cyanide Detoxification & Tailings Disposal

Tailings from the carbon safety screen underflow feeds the cyanide detoxification circuit by gravity. This circuit includes two intensely agitated tanks operated in series, in which a portion of the iron bearing overflow solution from the No. 1 CCD thickener is mixed with the CIL tailings slurry. The iron reacts with a majority of the cyanide contained in CIL tailings forming non-toxic iron cyanide complexes. An average detoxification efficiency of about 90% has been demonstrated. Both detoxification tanks are covered and are equipped with a common scrubber.

Tailings from the detoxification circuit are diluted with the cooling water return streams from the oxygen and power plants. Diluted tailings are then combined with the remainder of the No. 1 CCD thickener overflow (which contains the bulk of the acid formed in the autoclave) for submarine disposal.

A de-aeration tank is located offshore in about 3 m of water at low tide. It has sufficient cross-sectional area to enable entrapped air and soluble gases to escape from the slurry. It is equipped with two underwater one-way relief valves connected to two sea water intake lines running to a location offshore where they will be always fully submerged at low tide. The level in the de-aeration tank is self regulating, with water drawn from the intake valves as required to prevent draw down of the tank and air entrainment in the tailings line.

The tailings line (nominally 1050 mm in diameter) extends from the de-aeration tank to a depth of 122 m, well below the range of ocean mixed layer depths measured in the area. At discharge, the solids settle along the steep slope of the sea bed and flow into the ocean depths. The solutions dissipate in the ocean waters at depth and pose no identifiable risk to aquatic life in the upper levels of the ocean.

  Process Control

A distributed process control system is used to provide control, data acquisition, process computation, alarm initiation and operator interface to all process plant areas, and will interface with vendor supplied control systems in the power, lime and oxygen plants.

The main plant control room is located adjacent to the grinding and pre-oxidation areas. Operator work stations in that control room interface with and provide control for the following process areas:

• grinding

• thickening and washing

• pre-oxidation

• oxidation

• sea water pumping

• fuel storage and pumping

In addition, this control room monitors the activities of remote operator work stations for crushing, CIL and gold recovery (including neutralisation and carbon handling) and the oxygen plant.

Although the remote operator work stations will normally control these areas, control can if necessary be switched to the main plant control room. The oxygen plant control system, while supplied by Linde as a stand-alone system, is also a Foxboro system as in the process plant. The Praxair oxygen plant uses an Allen Bradley PLC supervised by a Windows NT SCADA package.