ECONOMICS OF REFRACTORY GOLD ORE PROCESSES John E. Litz and R. Wayne Carter 119th TMS Annual Meeting February 18-22, 1990 Hazen Research, Inc. 4601 Indiana Street Golden, Colorado 80403 Abstract Refractory gold ore pretreatment processes continue to receive much attention. This paper 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 and these estimated costs and ore sulfide grades then are used to compare the process economics. The study indicates that the most significant item affecting the economics is the amenability of the ore to flotation. The cost benefits of flotation are $10-13 per tonne ore processed (2%-4% Sa, 10:1 ratio of concentration). In most cases bacterial oxidation has the lowest overall cost, especially if less than complete oxidation of the sulfides will give acceptable recovery, however bacterial oxidation has yet to be practiced on a large scale. In all situations pressure oxidation shows the highest cost. However, the cost differences between the treatments are small, less than $7 per tonne.
Transcript
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 Street
Golden, Colorado 80403
Abstract
Refractory 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.
Evelyn
Rectangle
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INTRODUCTION
Refractory 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 Processes
A number of processes have been applied to the treatment of refractory ores andconcentrates. These include:
Alkaline air oxidationChlorination
Chlorination 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.
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 oxidation
Roasting
This paper is restricted to comparing the economics of biological oxidation, oxygenpressure oxidation and roasting.
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Basis of Costing
The 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 35°C 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 200°C for 90 minutes. Oxygen utilization during leaching isassumed at 80%. The flashed slurry is neutralized and cooled to 40°C prior tocyanidation. In the case of the
FIGURE 2
~ FLOCCULANT
,~
TH ICKEHE" r NEUTRAL PULPTO CYANIDATION
NEUTRALIZATION
HYDRATED LIME
BIO-OXIDATION
±BACTERIAL OXIDATION FLOWSHEET
FEED
SLURRY
FEED PREPARATION
THICKENER
TO
GRIND
BLOWER
COOLING
WATER
PRESSURE OXIDATION FLOWSHEET
TO ATMOSPHERE
FEED ~SLURRY ... ITO GRIND~
THICKEN£R I j~ WATER
IhlFEED PREPARATION
OXYGEN
HEAT
EXCHANGER
r-HYDRATED LIME
11-11NEUTRALIZATION
HEATEXCHANGER
~ FLOCCULANT
J _ I' I 'I _lr-OVERFLOW TO~ RECYCLETHICKENER
NEUTRAL PULP
TO CYANIDATION
COOLING TOWER
FIGURE 3
BLEED
HYDRA TED
LIME
FIGURE 4
NEUTRAL PULP
TO CYANIDATION
TO ATMOSPHERE
COOLI NG TOWER
NEUTRALIZATION
DUST SCRUBBER
CYCLONE
FEED PREPARATION
FLUID BED ROASTING FLOWSHEET
FEED ~SLURRY ., ,iTO ----}l ~ ~GRIND ~
THICKENER
FUEL
7
concentrate 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<10% solids with the 4% sulfide ore.
The roasting cases utilize low cost, high sulfur fuel oil when necessary to reach the650°C operating temperature. The air volume used in all cases is sufficient to leave4-5% oxygen in the roaster off-gas. When total ore is used, dry grinding to 10-meshis assumed prior to roasting. A fluidized cooler on the bed overflow is used topreheat the combustion and oxidation air in all cases except the concentrate from4% sulfide ore. The quenched slurry is neutralized and cooled to 40°C prior tocyanidation.
Preparation of Costs
Mass and energy balances using the METSIM Process Simulation software wereprepared for each oxidation process and each sulfur level with and without flotation.The mass and energy balances then served as the basis for preparing a series ofcapital and operating cost estimates. Assumptions used in preparing the estimateswere that the plant would be sited in central Nevada, power would be available atthe property line, and fuel oil would be the source of thermal energy. No costs fortailings disposal are included as such costs are too site-specific for inclusion in ageneral study.
The costs were developed using the United States Bureau of Mines CostingHandbook for the crushing, grinding, flotation, cyanidation and CIP portions of theflowsheets. The pretreatment portions were costed by pricing the major equipmentitems and factoring the installed costs. Since at this time there is no "standard"commercial apparatus for the biological oxidation, we made an estimate for theequipment which we felt would accomplish the oxidation. We have assumed lowpower type agitation in vertical tanks having 48-hours retention for the oxidation.
Table 1 shows the capital cost estimates determined for treating 2000 tonnes of 2%sulfide sulfur total ore per day by all three pretreatment methods after flotation toproduce 200 tonnes of concentrate.
Operating costs then were estimated for each size and type process. Operatingcosts for the crushing, grinding, flotation, leaching and CIP circuits weredetermined using the Bureau of Mines Handbook; whereas the operating costs forthe treatment steps were estimated separately for each size and type of process.The operating costs assume that the ore is not acid consuming and that lime will berequired to neutralize all acid or sulfur dioxide produced during the preoxidation.Table 2 lists the operating costs for the cases listed in Table 1.
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Table 1Estimated Capital Costs
2000 tonnes per day, 2% sulfide oreFlotation concentrate(OOO'sdollars)
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
FIGURE 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
$30$25
p 0Eo AL $20L T
A 0A N5N
E $15
$10
1000 1500 2000 2500 3000 3500 4000
~.'.;.;.;'; ..'
liltFIGURE 8
11
The 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 COSTS
FOR 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
COMPARISON OF OPERATING COSTS2% SULFIDE WITH FLOTATION
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Concentrating the oreby flotationsignificantly reducesthe operating costs.Figure 9 shows that athigh tonnages the totaloperating costs withflotation are only $1418/tonne versus $1622/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
$25T
~ $20e
$15
$10
1000 1500 2000 2500 3000 3500 4000
FIGURE 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
