Tuesday, 25 November 2008

The Refinery Flow of palladium and other PGMs


The flow of material through the refining process follows a branching series of circuits in which the palladium and other PGMs follow one path, and various other materials are separated and follow other paths. Most steps take advantage of conditions that dissolve one metal, but not another. The numbered steps shown are not all sequential, but are illustrated in a simplified flow in the sidebar. This clickable flow chart will access a virtual tour through the process with a flow chart of each step.

The three stage leach and metal recovery process selectively extracts nickel, iron, copper and others from a Platinum Group Metal (PGM) bearing granulated matte from the smelter. These activities result in a higher grade PGM filter cake being produced.

1. Grinding

The smelter matte is received in open top bulk bins. The matte is finely ground in a tower mill batch grinding process. The ground matte, now in slurry form, is stored in tanks to provide surge capacity before reporting to the continuous Nickel Atmospheric Leach Circuit.

First the smelter matte is ground into a slurry in the Tower Mill.

2. Nickel Atmospheric Leach Circuit

The function of NAL is primarily for leaching nickel and iron from the ground matte slurry. The ground matte slurry, along with acid and process water, is fed into the first of five continuously stirred reactors in series. The slurry from the atmospheric leach is transferred to a thickener. NAL thickener underflow, which contains the PGM solids, is pumped to the Copper Dissolve Autoclave Circuit. The overflow liquid from the thickener is filtered and pumped to the Iron Removal Circuit.

Overflow from the thickener is fed to the iron removal circuit (shown next). The PGM containing underflow is pumped to the Copper Dissolve Autoclave(step 5).

3. Iron Removal Circuit

The function of the Iron Removal Circuit is to precipitate iron from solution. Oxygen is sparged into the IR reactors for iron oxidation and ammonium hydroxide is added for pH adjustment to precipitate the iron. The iron slurry is then filtered through a filter press and the iron residue (goethite/jarosite) is returned to the smelter. The nickel-rich filtrate from the iron residue filter feeds the Nickel Crystallizer Circuit.

The NAL overflow is sparged with oxygen in the IR reactors to precipitate the iron oxides formed with pH control provided by ammonium hydroxide.

4. Nickel Crystallizer Circuit

The function of this circuit is to evaporate water and produce a dry nickel sulfate crystal product. The water evaporated from heating the feed liquor is condensed and recycled as process water throughout the Refinery. As water evaporates, the liquor reaches nickel saturation and crystals are formed. The resulting nickel crystal slurry passes through a cyclone to separate the crystal solids from the liquid. The solids then proceed through a centrifuge for further liquid separation. The crystal solids are then fed to a fluid bed dryer for further drying after the centrifuge. The final nickel product (nickel sulfate hexahydrate) is packaged for shipment.

The IR overflow is treated by evaporating water, crystalizing nickel sulfate which then can be separared from the liquid.

5. Copper Dissolve Autoclave Circuit

The function of this circuit is to leach copper from the NAL discharge solids. Feed slurry from NAL thickener underflow, along with acid and process water, is continuously fed to the copper dissolve autoclave. With oxygen pressure and elevated temperature, copper is leached from the slurry solids into solution and the loaded slurry is discharged to a thickener. The copper dissolve thickener underflow slurry feeds the Batch Polish Autoclave Circuit. The copper-rich thickener overflow solution flows through a series of filtration steps before reporting to the Selenium/Tellurium Removal Circuit.

Copper is dissolved from the NAL discharge solids which contain PGMs. PGM containing slurry is fed to the Polish Autolcave (7) while the copper solution is treated next.

6a. Selenium/Tellurium Removal Circuit

The function of STR is to precipitate selenium and tellurium from solution, at an elevated temperature, using sulfurous acid. The resultant solid is filtered, packaged in drums, and shipped as a concentrate. The copper pregnant filtrate is transferred to the Copper Electrowinning Circuit.

Selenium/Tellurium is precipitated and removed then copper is captured through electrolytic deposit.

6b. Copper Electrowinning Circuit

The function of the electrowinning circuit is to remove dissolved copper from solution by electrolytic deposition onto stainless steel sheets. The filtrate from STR provides the copper in solution that is deposited as copper metal. The copper deposit, commonly referred to as copper cathode, is harvested weekly and stacked on pallets for shipment. Discharge solution from EW is high in acid and is recycled to the three leach circuits.

Selenium/Tellurium is precipitated and removed then copper is captured through electrolytic deposit.

7. Batch Polish Autoclave Circuit

The function of the Batch Polish Autoclave Circuit is to upgrade the final PGM concentrate by leaching the small amounts of residual copper, nickel, and iron remaining in the solids. The copper dissolve leach thickener underflow is mixed with acid and process water in a polish autoclave feed tank. The polish batch is then transferred into a polishing leach autoclave, steam heated, and pressurized with oxygen. The batch leaching allows for precise operating conditions which, dissolve nearly all the remaining copper, nickel and iron, maximize filter cake PGM concentration and minimize precious metals losses. The batch polish leach discharges to the PGM Filtration Circuit.

Small amounts of residual copper, nickel and iron remaining in the matte are leached out.

8. PGM Filtration Circuit

The function of this circuit is to separate the PGM concentrate from the batch polish discharge slurry and to prepare the final PGM filter cake for shipment. The batch polish slurry is pumped through a series of filters to separate the Refinery final product, PGM filter cake. The wet cake reports to a delumper and on to electric ovens where the free moisture is evaporated away from the filter cake. The dried filter cake is degglomerated in a pin mill, blended, packaged and shipped off site for further refining. The copper-rich filtrate from this circuit reports to the Selenium/Tellurium Removal Circuit then onto copper electrowinning.

The PGM material is now filtered from the now dissolved residual metals.

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