PART I PROJECT DEVELOPMENT
2. COMMINUTION PROCESS OPTIONS 1. Overview
Flowsheet selection for free-milling ores can be relatively straightforward, with the key issues revolving around comminution circuit selection, the use of heap leaching, treatment of high-silver ores and flotation options for free- milling sulfides.
Complex ores include those associated with base-metal mineralization, par- ticularly copper, that can consume cyanide and create issues in CIP and el- ution. The presence of preg-robbing carbon will demand flowsheet inclusions to achieve acceptable recovery without gross losses of gold to CIL tailings.
While refractory characteristics can be seen in a variety of ore types, in- cluding auriferous base metals and rocks with a high carbon content, the major focus in refractory gold processing has been on gold-bearing iron sulfides, such as pyrite, arsenopyrite, pyrrhotite, telluride and the stibnite family. It is the intention of this chapter to concentrate on the pre-treatment processes available for the latter ore types.
In summary, gold flowsheet options will be examined under the headings of:
Comminution processes;
Free-milling ore processes;
Complex ore processes;
Refractory ore processes.
2. COMMINUTION PROCESS OPTIONS
illustrates an overall decision-making diagram established byLane and Lunt (1997) in which the extent of testwork is dependent upon the scale of the project, hence capital cost, and the targeted schedule. For smaller-scale projects with throughputs of less than about 4–5 Mt/a, the costs of piloting can be offset against the provision of flowsheet safeguards and some addi- tional capacity margin. Certain comminution options such as fully auto- genous milling and high-pressure grinding rolls (HPGR) usually require piloting regardless of throughput.
Preliminary Assessment of Ore Characteristics Based on Available Data
- initial bench-scale testwork evaluation
Evaluation of Project Size and Estimated Comminution Circuit Costs
- project status
- preliminary assessment of project economics - sensitivity to capital and operating costs
Assessment of Test Campaign - bench-scale testwork - pilot-plant operation - testwork costs and schedule
Assessment of Ore Body Variability - sample availability
- sampling costs - geology and lithology
Recommendation on Appropriate Sampling and Testwork Programme
Finalisation of Test Programme - selection of laboratory/consultants - supervision of testwork
Analysis of Test Data and Preliminary Design - evaluation of variability test data, pilot-plant data (if executed), and benchmarking
- use of industry standard and in-house design tools
Assessment of Project Risk and Required Design Contingencies
Final Engineering Design Overall Project Requirements - project type
- development schedule
- discussion of project risk (commercial and technical)
Fig. 2. Comminution circuit decision-making diagram.
Process flowsheet selection 75
Fleay et al. (2002) have provided some excellent guidelines for assessing comminution flowsheet options as illustrated inTable 1and these and other selection considerations are discussed in turn.
2.2. Ore characteristics
Ore competency and hardness is the prime determinant of the circuit con- figuration. Low competency may permit the use of lower capital cost crush- ing and milling equipment such as MMD sizers, single-stage primary mills and SAG mills. High competency will dictate the examination of SABC circuits and, for ores that exhibit extreme resistance to breakage, staged crushing (two or three stages) followed by either ball or SAG milling may have to be considered. At higher throughputs, such circuits can consider the use of HPGRs in place of SAG milling.
Circuits handling moderately competent ores can take advantage of operating cost savings by employing fully autogenous grinding (AG) mills.
2.3. Throughput
As a general guideline, low-capacity projects are driven by the need to minimize capital costs and hence circuit simplicity and minimization of the number of unit operations is a prime goal. Flowsheet considerations include the use of single-stage milling and for soft ores such installations often run in a regime that is intermediate between SAG and ball milling. A further con- sideration is the elimination of intermediate stockpiling between the primary crusher and the mill. While this may indicate an operating cost increase, it has the potential to again reduce capital. Table 2 illustrates a survey con- ducted across a number of smaller-scale gold projects that illustrates the achievement of remarkably high plant availabilities.
Table 1
Matrix of competency and capacity
Competency Griding Circuit Throughput
o0.5 Mt/a 0.5–2 Mt/a 2–6 Mt/a 46 Mt/a
Low Single stage
SAG
Single stage SAG
Single stage SAG
SAB Moderate Single stage
SAG or AG
Single stage AG, SAB and ABC
Single stage AG, SAB and ABC
ABC
High Two stage
crush/ball mill
Stage crush/ball mill and SABC
SABC SABC and stage
crush/HPGR/
ball mill D. Lunt and T. Weeks
76
With the increase in size of projects and the seemingly inexorable increase in the ultimate capacity of crushers and mills and their associated compo- nents, it is worth determining the capacity of a large single-train crusher-SAG mill installation to determine whether this fits with the resource exploitation capability.
2.4. Downstream process requirements
Downstream processes may be sensitive to ore characteristics that are in- fluenced by the comminution circuit such as size distribution and surface chemistry. This is particularly true of sulfide flotation associated with many gold projects where recovery of sulfide, hence precious metal, can be influ- enced by both parameters. The relative advantages of SAG and AG milling may need to be assessed in such situations.
2.5. Operating cost
Higher capacity projects are particularly sensitive to power, maintenance and steel costs. The designer will need to examine the cost offsets of AG in terms of steel savings against the lower inherent power efficiency.
3. FREE-MILLING ORE PROCESS OPTIONS