EconomicsGo ldOre

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ECONOMICS OF REFRACTORY GOLD ORE PROCESSES

John E. Litz and R. Wayne Carter

119th TMS Annual MeetingFebruary 18-22, 1990

Hazen Research, Inc.4601 Indiana StreetGolden, Colorado 80403

AbstractRefractory gold ore pretreatment processes continue to receive much attention. Thispaper compares the relative economics of treating ores by oxygen pressure oxidation,roasting and bacterial oxidation with or without preconcentration by froth flotation.Capital and operating cost estimates for each method of pretreatment were prepared andthese estimated costs and ore sulfide grades then are used to compare the processeconomics.The study indicates that the most significant item affecting the economics is theamenability of the ore to flotation. The cost benefits of flotation are $10-13 per tonneore processed (2%-4% Sa, 10:1 ratio of concentration). In most cases bacterial oxidationhas the lowest overall cost, especially if less than complete oxidation of the sulfides willgive acceptable recovery, however bacterial oxidation has yet to be practiced on a largescale. In all situations pressure oxidation shows the highest cost. However, the costdifferences between the treatments are small, less than $7 per tonne.


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INTRODUCTIONRefractory gold ores present a number of problems to the gold producer. Much hasbeen written about the suitability of particular pretreatment processes for specificgold ores. We at Hazen Research have had the opportunity to participate in thedevelopment and application of a number of gold ore processes and to review others'results from applying processes to gold ores. This experience gives us thebackground necessary to compare the available processes on their technical merit.However, technical merit does not necessarily translate to the bottom line. It is thebottom line which is most important to the gold producer.

DISCUSSION

Potential ProcessesA number of processes have been applied to the treatment of refractory ores andconcentrates. These include:

Alkaline air oxidationChlorinationChlorination and alkaline air oxidation tend to be the lowest capital costpretreatment methods, and in cases of ores with very low sulfide content, they alsocan have very low operating costs. These processes, however, do not have wideapplication as only a few ore types show good response to them. These are notconsidered in this paper since their applicability is limited.

Chloride catalyzed, oxygen pressure oxidation (Cashman)Nitric acid catalyzed oxidation (Nitrox, Redox)The chloride and nitric catalyzed processes have excellent technical merit, but asyet have not been thoroughly tested at either pilot or commercial scale. Therefore,they are not considered in this paper.

Bacterial oxidationOxygen pressure oxidationRoasting

This paper is restricted to comparing the economics of biological oxidation, oxygenpressure oxidation and roasting.


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3Basis of CostingThe basic process evaluatedis shown in Figure 1. Thesteps essentially includecrushing and grinding,followed by cyanidation andCIP recovery of the gold.The effect of concentrating bymeans of flotation also wasinvestigated in each process.Process flowsheets weredeveloped for each of thethree pretreatment processes.These flowsheets are shownin Figures 2, 3 and 4. Aseries of cases then wasdeveloped using orescontaining 2 and 4% sulfidesulfur. The cases consideredtotal ore cyanidation,cyanidation of flotationconcentrates, and cyanidationof either total ore or flotationconcentrates afteroxidative pretreatment. Forthe cases which includedflotation, 90% recovery of thesulfide into 10% of the weightwas assumed.

BASIC PROCESS FLOWSHEET

r---

L _

FIGURE 1

The biological oxidation process uses an initial 20% solids content. The venting aircontains sufficient heat that no additional heating or cooling is required to maintainthe desired 35C operating temperature, if 80% oxidation of the sulfide and 5%utilization of the oxygen in the air are assumed. Others have shown that 50 to80% sulfide oxidation is suitable for good liberation of the gold and that oxygenutilization greater than 5% is difficult to achieve.The oxygen pressure oxidation of total ore uses a 40% solids feed and uses heatexchangers on both the flashed slurry and vapor to preheat the autoclave feed priorto oxidation at 200C for 90 minutes. Oxygen utilization during leaching isassumed at 80%. The flashed slurry is neutralized and cooled to 40C prior tocyanidation. In the case of the


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FIGURE 2

~ FLOCCULANT,~

TH ICKEHE" r NEUTRAL PULPTO CYANIDATI

NEUTRALIZATION

HYDRATED LIME

BIO-OXIDATION

ACTERIAL OXIDATION FLOWSHEETFEEDSLURRYFEED PREPARATION

THICKENER

TOGRIND

BLOWER

COOLINGWATER


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PRESSURE OXIDATION FLOWSHEET

TO ATMOSPHERE

FEED ~LURRY ... ITO GRIND~

THICKENR I j~ WATERIhlFEED PREPARATION

OXYGEN

HEATEXCHANGER

r- HYDRATED LIME11-11NEUTRALIZATION

HEATEXCHANGER

~ FLOCCULANT

J _ I' I 'I _lr-OVERFLOW TRECYCLETHICKENER

NEUTRAL PULPTO CYANIDATION

COOLING TOWER

FIGURE 3


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BLEED

HYDRA TEDLIME

FIGURE 4

NEUTRAL PULPTO CYANIDATION

TO ATMOSPHERE

COOLI NG TOWERNEUTRALIZATION

DUST SCRUBBER

CYCLONE

FEED PREPARATION

FLUID BED ROASTING FLOWSHEET

FEED ~LURRY ., ,iTO ----}l ~ ~RIND ~THICKENER

FUEL


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7concentrate oxidation, preheating is not required and the oxygen leach temperatureis controlled by reducing the feed density to 17% solids with the 2% sulfide ore and


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8Table 1

Estimated Capital Costs2000 tonnes per day, 2% sulfide orelotation concentrate(OOO'sdollars)Oxygen

Bacterialxidationressureoasting 80% Oxid. 20% Oxid.5252525252 38080808080 2,244,244,244,244,244 62222222222 21313131313 22222222222 22222222222 4,855,855,855,855,855 6,890,914,702,093 2,472,472,472,472,472 566666 --.M-.M-.M-.M-.M 2,612,612,612,612,612 19494949494 3,387,387,387,387,387 655555 17676767676 59191919191 64040404040 33232323232 10303030303 5.488.488.488.488.488 12,9559,8459,8696,6576,048 1,321,024,027,699,637 ----B11Q 14,8902,8102,8379,1458,445


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Estimated Operating Costs2000 TPD - 2% Sulfide Ore(Dollars/tonne)Oxygen

Bacterialxidationressureoasting: 80% Oxid. 20% Oxid..18.18.18.18.18 0.33.33.33.33.33 1.80.80.80.80.80 1.27.27.27.27.27 0.05.05.05.05.05 0.10.10.10.10.10 0.01.01.01.01.01 4.74.74.74.74.74 0.54.54.54.54 7.79.70.95.75 0.07.07.04.04 0.08.15.08.08 0.12.85.37.09 0.85.37.25.38.38 8.97.86.33.85 1.27.27.27.27.27 0.02.02.02.02.02 0.01.01.01.01.01 1.30.30.30.30.30 0.08.08.08.08.08 1.21.21.21.21.21 1.29.29.29.29.29 7.336.304.195.660.18


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The data generatedfrom the capital andoperating costestimates were used togenerate a number ofgraphs which comparethe costs for thevarious treatmentprocesses at differingtonnages. Figure 5compares the capitalcosts for a plant totreat a 2% sulfidesulfur bearing are.The capital costs arevery similar at the 500te/d rate and showabout a 10% spread at4000 te/d.

COMPARISON OF CAPITAL COSTS2% SULFIDE

"o Io LL LL IA 0A N5 5

1000 1500 2000 2500 3000 3500 4000SIZE. ~fl'ERDAY

FI GUR E 5

COMPARISON OF OPERATING COSTS2% SULFIDE

The operating costs fortreating the same areare graphed inFigure 6. Because ofhigh reagentrequirements, costs forpressure leachingremain high regardlessof the tonnage. Sincebiological oxidationrequires the leastreagent, the costsdecrease the most withincreasing tonnage.

$35$3025

p 0 Eo A $20TA 0 NNE $15$10

1000 1500 2000 2500 3000 3500 4000

~.'.;.;.;'; ..liltF IGU RE 8


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11The bottom lineincludes the recoveryof capital, which oftencan be the mostsignificant per tonnecost. Figure 7includes a four-yearrecovery of capitalwith the operatingcosts in comparing theprocesses. When therecovery of capital isincluded, the roastingcosts are a few dollarsper tonne less thanpressure leaching.Biological oxidationhas the lowest cost atall tonnages, but byonly $3-4 dollars pertonne.The inclusion offlotation to the processreduces the capitalcosts significantly.Figure 8 compares thecapital costs for plantswhich include flotationas well as oxidativepretreatment byoxygen pressure,bacterial oxidation androasting. Flotationreduces the totalcapital costs about50%.

COMPARISON OF OPERATING PLUSCAPITAL RECOVERY COSTSFOR 2% SULFIDE

po Eo RLLARS

1000 1500 2000 2500 3000 3500 4000SUE. 'fCNrEB PER DAY

FIGURE 7

COMPARISON OF CAPITAL COSTS2% SULFIDE WITH FLOTATION

...o ,o LL LL ,A 0R NS S

$1

1000 1500 2000 2500 3000 3500 4000SIZE. TCNEII PER DAY


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COMPARISON OF OPERATING COSTS2% SULFIDE WITH FLOTATION

12Concentrating the oreby flotationsignificantly reducesthe operating costs.Figure 9 shows that athigh tonnages the totaloperating costs withflotation are only $14-18/tonne versus $16-22/tonne with wholeore. po EORLL TA 0R NS N

E

1000 1500 2000 2500 3000 3500 4000SIZE. TCNM:S PEA DAY

FIGURE

Figure 10 comparesoperating plus capitalrecovery costs forroasting 2% sulfidesulfur total ore andconcentrates derivedtherefrom.

COMPARISON OF OPERATING PLUSCAPITAL RECOVERY COSTS

FOR 2% SULFIDE WITH FLOTATION

Figure 10 also showsthat when flotation isused, the overall costsfor the three types ofpretreatment convergeas the tonnageincreases to 4000 te/d.

$40$35

$30$25

T

~ $20e $15$10

1000 1500 2000 2500 3000 3500 4000F IGURE 10


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The effect of flotationon the costs isillustrated in Figure11 which comparesoperating plus capitalrecovery for ClP,roasting and biologicaloxidation.This graph shows howflotation dramaticallyreduces the overallcosts to only $20/tonneat 4000 te/d versus$27-32/tonne withwhole ore.

The sulfide sulfurcontent of the ore hassignificant impact onthe operating costspius recovery ofcapital. As shown inFigure 12, the increasein costs when thesulfide doubles from 2to 4% ranges fromonly $2 per tonne forroasting to $5-10 pertonne for bacterialoxidation.

EFFECT OF FLOTATION ON OPERATING PLUSCAPITAL RECOVERY COSTS USING 810-OXIDATION AND ROASTING ON 2% SULFIDE

?) E" RLL TA 0" N5 NE

1000 1500 2000 2500 3000 3500 4000F1Gl. II 11

EFFECT OF SULFUR CONTENT ONOPERATING PLUS CAPITALRECOVERY COSTS

po Eo RLL TA 0R N5 N

E

1000 1500 2000 2500 3000 3500 4000FIGURE 12


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