A practical example of recovery improvements in a ...

A PRACTICAL EXAMPLE OF RECOVERY IMPROVEMENTS IN A BACTERIAL OXIDATION PLANT 265

IntroductionThe orebody in Obuasi is considered by many metallurgiststo be complex due to the double refractory nature of thegold carriers. An insight into the ore mineralogy and thedesign criteria of the sulphide treatment plant (STP) isimportant to the current optimization drive.

Gold mineralization in Obuasi occurs in extensive shearsystems and takes the form of quartz veins and massivesulphide orebodies. The gold in the quartz veins isassociated with a distinct suite of base metal sulphides,including galena, chalcopyrite, sphalerite and tetrahedrite.The gold in the quartz reef is generally of microscopic size,but free milling and recoverable by gravity separation. Incertain sections of the mine visible gold occurs.

Refractory gold of the sulphide ores is mainly hosted inarsenopyrite. Other sulphides associated with thearsenopyrite are pyrite, pyrrhotite, chalcocite and sphalerite,but are not known to be gold bearing to any significantdegree. Associated with the ores are carbonaceous matterassociated with the graphitic shear zones and carbonates.Gold is generally submicroscopic, apparently hosted in thecrystal lattice of arsenopyrite, which is lath shaped varyingin size between 50 and 200 μm. Gold deportment of the oreis as follows:

• 12–19% free gold• 52–73% arsenopyrite• 13–26% binaries• 2–4% in gangue.

The BIOX® process utilizes a mixed population ofAcidithiobacillus ferrooxidans, Alt. thiooxidans andLeptospirillum ferrooxidans to break down the sulphidemineral matrix, thereby liberating the occluded gold forsubsequent cyanidation. The typical operating pH range inthe BIOX® process is 1.2 to 1.8 and the optimumtemperature is between 40ºC and 45ºC1.

See Figure 1 for the simplified flowsheet for an overviewof the plant.

The STP was commissioned in 1994 to treat 180 000 tpmof carbonaceous, refractory gold ores from surface andunderground mining operations. The STP recovers gold bygravity concentration and flotation with the regrinding ofthe sulphide concentrate before bioleaching (BIOX®) toliberate gold locked in sulphide minerals. The contact timein the BIOX® reactors is typically 4 days at design tonnageand a minimum sulphide sulphur grade of 6% is required tokeep the bacteria active.

The reaction in the BIOX® is summarized as follows:

STEYN, B. and BENEWOE, V. A practical example of recovery improvements in a bacterial oxidation plant. World Gold Conference 2009, The SouthernAfrican Institute of Mining and Metallurgy, 2009.

A practical example of recovery improvements in abacterial oxidation plant

B. STEYN and V. BENEWOEAngloGold Ashanti—Obuasi Mine

AngloGold Ashanti’s Obuasi Mine, in Ashanti Province, Ghana, operates a plant utilizingbacterial oxidation technology. The mineralization consists predominately of arsenopyrite withgold associated in the sulphides.

The plant was commissioned in 1994, at that time was the largest BIOX® plant in the world,and has been operated by AngloGold Ashanti since 2004. The constraints identified in the plantwere the flotation capacity, BIOX® feed size and disruptions to the BIOX® process.

This paper focuses on the practical steps that were followed in order to increase the recovery ofthe plant. The original plant design was based on a head grade of 10 g/t and an overall recovery of86.5%. In 2007 the plant was operating at an overall recovery of 79.2% at a head grade of 5.6 g/tand this has improved to 80.6% at a head grade of 5.8 g/t during the last quarter of 2008.Additional improvement to 81.5% are expected by the end of April 2009.

Improvements that have been implemented include:• Replacement of the regrind mill• Installation of additional float capacity• Installation of a scavenging circuit• Installation of a complete water treatment plant.

In addition we have focused on the effect of the water circuit on the BIOX® process and thesteps involved in correcting this problem. The water circuit was originally designed as an opencircuit and through environmental considerations the circuit was closed towards the end of 2007.The bacteria in the BIOX® are particularly sensitive to cyanide and cannot tolerate levels above0.05ppm (free cyanide), this required extensive modifications to the water circuit.

The plant improvement project is ongoing and the current scope of work should be complete bythe end of April 2009. Further research and development work is being planned, which includes a‘white rot bacteria’ that can consume the preg-robbing carbon and oxidize the sulphide in a singlepass.

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WORLD GOLD CONFERENCE 2009266

2FeAsS + 702 + H2SO4 + 2H2O → 2H3AsO4 + Fe2(SO4)3

Important secondary reactions include the precipitation offerric arsenate, according to the following reaction:

2H3AsO4 + Fe2(SO4)3 → 2FeAsO4 + 3H2SO4

From the graph, in Figure 2, it can be seen that therecovery curve follows the grade profile except for 2007and 2008 when the recovery dropped despite the graderemaining reasonably stable. This trend has changed in2009 as the recovery improves despite the grade dropping,this being the result of the various recovery improvementinitiatives taking effect.

Sustaining an active bacterial culture is critical for thesuccessful of a bioleaching plant. It is therefore important toprevent toxic elements from entering the process or to buildup in concentrations that inhibit the mineral oxidizingbacteria. The risk at Obuasi is confined to:

• Cyanide• Thiocyanate• Grease and oil• Water treatment reagents• Certain flotation reagents and nutrients

This danger makes it critical that any change in reagent,or even change of supplier is tested for BIOX®

compatibility.

Problems identifiedVarious problem areas were identified as contributing tothis adverse trend seen in 2007 and 2008. The mostsignificant were:

• Frequent power interruptions• Low availability of the regrind mill• Flotation plant under pressure• Mechanical breakdowns• Cyanide contamination of process water.

The power interruptions were mostly associated withproblems on the national grid. The BIOX® process isextremely sensitive to power outages and it takesapproximately 3 hours to get the activity of the bacteriaback to normal, after a 1 hour power outage.

The regrind mill was in a poor condition and runningtimes were as low as 20%. The original design specificationcalled for a feed to BIOX® grading of 90% passing 45micron. Actual feed size had dropped to 76% passing 45micron.The coarse feed together with the powerinterruptions had caused a significant build-up of solids inthe BIOX® reactors. The graph, in Figure 3, below clearlyillustrates the relationship between the regrind millavailability and the build-up of inventory in the BIOX®

reactors.2007 and the first half of 2008, were associated with

significant changes in plant inventory and associatedaccounting difficulties as a result of the poor regrind millperformance and frequent power interruptions.

The flotation capacity has long been identified as thebottleneck in the plant. This limited the milled tonnage toabout 6 500 tpd and the lack of an automated reagent feedsystem further restricted the performance of the float.

Figure 1. Flowsheet of the STP plant

Figure 2. Recovery and theoretical grade (R+r) trends

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Mechanical breakdowns on the SAG and ball millsduring 2007 and the first half of 2008 resulted in difficultyobtaining steady-state conditions and plant efficiencysuffered.

In September 2007 the mine began the process of closingthe water circuit and treating the return water to destroycyanide. This presented significant problems as temporarytreatment facilities were quickly established, using lime andhydrogen peroxide, while the permanent water treatmentplant was being constructed. In times of heavy rain limeand hypochlorite were used and this water released to theenvironment. In June/July 2008 the BIOX® was severelypoisoned by cyanide and it took about 2 weeks for thebacteria to recover.

Remedial actionTo fix a plant that has been under pressure takes capital,time and a lot of patience and a dedicated team focused onachieving set targets.

The target was to get the overall recovery up to 81.5% bythe end of April 2009 and eventually to 83% by addressingthe issues identified.

• The frequent power interruptions experienced in 2007have reduced significantly and the company has beenactively engaging with the Volta River Authority(national electricity supplier)

• The regrind mill has been replaced and upgraded froma 300 kW to a 750 kW unit. The feed size to theBIOX® improved to 87% passing 45 microns.Optimisation of the cyclone configuration is currentlybeing undertaken and alternative grinding media isbeing sought to replace the 40 mm steel balls. Thegraph in Figure 4, clearly shows the improvementachieved after the new regrind mill was commissionedin July 2008.The improvement in the fineness of grind has had abeneficial impact on the performance of the BIOX®

section as can be seen in Figure 5.The drop in oxidation efficiency in January 2009 is aresult of lower sulphur grade feed coming into theplant.

• A third tank cell was installed in June 2008 and floatrecovery improved from 86.5% to 87.3%. In additionthe need for a scavenger circuit was identified andcurrently a gravity concentration circuit is beinginstalled on the float tails stream. This plant consists offour 42 inch Knelson concentrators and is due forcommissioning early in May 2009. Laboratory testwork has indicated that scavenger gravity concentrationhas the potential to improve overall recovery by up to1.0%. Figure 6 indicates the improvements made since theinstallation of the third tank cell.

Figure 4. Biox® feed size

Figure 3. Regrind mill availability vs inventory increase



• The SAG and ball mills experienced various problemson the drive trains caused by vibration frommisalignment. This caused problems with pinionbearings, couplings and gearboxes. The trunnionbearings on the SAG mill failed and have since beenreplaced. The use of off mine experts to assist withalignment and adhering strictly to a plannedmaintenance schedule has begun to show benefits. Themill availability has improved from 83.2% to 90.1%. Inaddition, the maintenance effort has improved thecondition of critical units in the plant and spillage isunder control.

• A water treatment plant to remove cyanide and arsenicfrom process water has been commissioned.

Pseudominas bacteria in rotating biological contactors(RBC’S) are used to break down free cyanide from 20ppm to 1 ppm in a single pass. The removal of arsenicand other heavy metals is achieved in an ACTIFLOW®

plant which uses ferrous sulphate and potassiumpermanganate to precipitate the heavy metals that areremoved by a system of flocculation and settling. Thefinal solution to the water treatment will be theseparation of the float tailings and the CIL tails as allthe cyanide will be contained in a small stream and canbe handled easily through the existing water treatmentplant. See the Figure 7 for a simplified flowsheet of the watertreatment plant.

Figure 5. BIOX® oxidation performance

Figure 7. Simplified water treatment flow sheet

Figure 6. Flotation recovery trend



Further workThe most recent diagnostic leaches of the CIL tails are asfollows:

Direct cyanidation 13.4 %Preg robbed 13.1Gold in sulphides 40.3Carbonaceous matter 4.1Gold in quartz (other) 29.1These results show that further work is needed to improve

the leaching conditions and currently cyanide concen-trations and oxygen addition is in the process of beingoptimized. The gold in sulphides is a function ofincomplete oxidation and further work on the regrind milland optimizing conditions in the Biox® is beingundertaken.

The application of alternative bacteria (white rot) stillneeds further research, but the potential to consume preg-robbing carbon and to oxidize sulphide in an alkalinemedium has the potential for far-reaching benefits both interms of cost and environmental impact.

The commissioning and optimizing of the scavengergravity concentration will provide an additional safeguardagainst inefficiencies in the float plant.

ConclusionThis paper was intended to demonstrate that a process ofgoing back to basics and optimizing each unit process willpay dividends. The cause and effect relationships within aplant need to be thoroughly understood and used to doproper troubleshooting.

AcknowledgementsThe assistance of the management of the STP plant isacknowledged and in particular the help given by KwesiAmebley and Charles Eshun needs special mention.

References1. RAWLINGS, D.E., Johnson (eds), D.B., Biomining.

Springer

Bill SteynChief Metallurgist, AngloGold Ashanti—Obuasi Mine

Have been involved in the gold mining industry since 1977 working for the Anglo American

Corporation, and later AngloGold and more rececently for AngloGold Ashanti. Experience has

been obtained at various South African mines starting as a student and progressing to Metallurgical

Manager/Chief Metallurgist at Obuasi Mine in Ghana. 4 years were spent at Namdeb mine in

Namibia


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