*G. Lane1, B. Foggiatto1, R. Braun1,M. P. Bueno1, P. Staples2 and S. Rivard2
1 Ausenco Services, 144 Montague Road, Brisbane, Australia QLD 4101
(*Corresponding author: firstname.lastname@example.org)
2 Ausenco Engineering Canada, 555 Boulevard René-Lévesque Ouest, Montréal, Canada QC H2Z
This paper demonstrates the selection of grinding circuit technology for a number of recent projects including discussion of the key drivers and the role of energy efficiency. Decreasing head grades combined with a strong and sustained increase in operating costs has forced miners to examine increased project size, increased equipment capacity and efficiency improvements in order to reduce their unit operating costs and improve overall project economics. Comminution energy consumption, safety and capital cost considerations play a critical role in the success of projects. This paper presents some key guidelines for a successful and cost effective comminution circuit design and includes a case study on three projects, outlining the issues and the drivers which led to the ultimate circuit selection and the critical role that energy efficiency played.
Comminution, SAG Mill, Cost Effective Design, Energy Efficiency
Comminution circuits for minerals processing are selected based on their suitability and efficiency in reducing run-of-mine ore to the desired feed size for the downstream process. The input material (ore) properties and downstream process requirements are the key determinants and there are standard approaches for each circumstance.
The minerals processing industry has been a fast adaptor of efficiency improvements based on increasing capacity of equipment (e.g. trucks, crushers, mills, pumps) but less successful in adapting radically new technologies that disrupt incumbent flowsheets or processes.
The installed power of grinding mills has increased linearly over the past 25 years (Meka & Lane, 2010) and pumps and classification equipment have kept pace. The autogenous grinding (AG) and semi- autogenous grinding (SAG) mills displaced stage crushing as a dominant method of coarse rock breakage in many applications causing crusher manufactures to increase the capacity of cone crushers to meet the competitive demand.
Examples of new comminution technology in the 20th and 21st centuries are the development and adoption of cyclone classification in the 1940s (Newtech, 2016), AG milling in the late 1950s, vertical grinding mills in the 1980s, high pressure grinding rolls (HPGRs) also in the 1980s and ultrafine milling technology in the 1990s (Meka & Lane, 2010).
These technologies each took decades to become generally accepted to be able to reliably improve process efficiency and the geographical pattern of acceptance has corresponded with the commercial realities driven by the technical requirement.An example is the slow adoption of SAG milling in Brazil where a combination of technical and cultural issues have led to lingering reticence to accept SAG milling to the same degree as the remainder of the Americas, Africa and Australia (Fountain, Libanio & Lane, 2011). However, the dominant evolutionary improvement in the last 30 years is in automation, process measurement, reliability monitoring and process control.Whilst the potential of this area is yet to be fully exploited, we are measuring and collecting data that allows better forward planning and prediction.
The role of efficiency in the above is comprised of two parts that come together in one measure; operating cost. The two parts are material inputs (the cost of liners, grinding media and energy) and people inputs (the cost of labour, including the key consideration of safe work practices). Machine capacity increases have led to simpler circuits (fewer parallel process lines) that are easier and safer to maintain. Automation and process control have reduced the degree of manual intervention in circuit operations and lead to lower labour requirements and less physical labour.
Technology improvements, such as AG/SAG milling, have reduced the complexity of the process and resulted in fewer items of equipment to maintain. AG milling, where applicable, has led to reductions in operating cost of, for example $2/t, due to the elimination of steel grinding media from the process.
AG and SAG milling, in their early development stages, claimed improvements in energy efficiency and improved product quality (narrower size distribution). There are indications that in some applications, in particular single stage AG milling, these circuits can be very energy efficient in a pilot plant setting (Bueno & Lane, 2012).However, in the main and particularly for competent ore, SAG milling is less energy efficient than multi- stage crushing and particularly HPGR circuits (Bailey, Lane, Morrell, & Staples, 2009).The use of HPGR in the comminution of competent ores provides a process with materially higher energy efficiency even though some of this benefit is lost due to the increased power required for screen classification and associated materials handling and maintenance.
The dominant requirement of good plant design is personal safety, followed by capital intensity and efficiency.Efficiency improvement is achieved by a mix of reliability, simplicity, maintenance cost and operating materials and labour cost. There is recognition that energy efficiency in comminution is a desirable outcome from community and environmental perspective. The CEEC (Coalition for Energy Efficient Comminution) initiative demonstrates corporate support from mining houses, designers/engineers, equipment vendors and consumables suppliers for these outcomes.
Most process selection processes include consideration of energy efficiency as one of the criteria for selection of a flowsheet. However, energy efficiency remains a modest determinant of flowsheet selection unless energy costs are extreme and principally associated with power production from diesel generation (was greater $0.30/kWh in some locations compared with less than $0.10/kWh for the majority of operating plants on grid power).
This paper discusses three case studies. The first is the Constancia Project where energy efficiency was not a major consideration in equipment selection. The other two are the Boddington Project and Cadia East Project where energy efficiency played a key role in the selection of HPGR technology. These case studies are based on data in the public domain.
COMMINUTION CIRCUIT SELECTION
The determination of grinding efficiency is discussed in papers by Siddall (1999), Lane and Siddall (2002), Morrell (2008), Lane, Foggiatto, Bueno and McLean (2013) and numerous other practitioners.Grinding efficiency plays an important role in selecting the most appropriate circuit and associated equipment, particularly in determining the transfer sizes between stages in AG/SAG and ball milling circuits. However, energy efficiency per se is then rolled up into the operating and capital cost outcomes for an overall quantitative financial analysis. Qualitative measures (preferences and perceptions) play an important role within the flowsheet selection and design processes and are also important in parallel assessment of the financial analysis.
Project size and business drivers also impact on selection criteria. Large project operating costs are dominated by consumables and maintenance costs, with labour costs being a lesser input. The reverse is true of smaller projects. Major mining houses are better able to fund large capital intensive, low operating cost projects, whereas junior resource companies are generally capital constrained and more strongly influenced by the need for cost-effective design (Lane & Dickie, 2009 and Lane, Dakin & Elwin, 2011).
Projects treating competent ores, such as Boddington selected a multi-stage crushing circuit incorporating HPGR on the basis of energy efficiency, even though the capital cost was higher than for a more conventional SAG mill based circuit (Parker, Rowe, Lane & Morrell, 2001). The Cadia East Project used HPGR to improve the energy efficiency of an existing SAG mill based circuit when moving from the treatment of open pit ore to a much more competent and potentially coarser, block cave ore source (Engelhardt, Robertson, Lane, Powell & Griffin, 2011; Engelhardt et al., 2015).
Projects treating less competent ores, such as Constancia (Lane, Dakin, Stephenson, Johnston & Granados, 2015) are able to achieve reasonable energy efficiency with primary crusher and SAG mill-based circuits.Staples, Lane, Braun, Foggiatto and Bueno (2015) pointed to the use of secondary crushing to improve SAG mill throughput and energy efficiency when SAG mill circuits are constrained by ore competency, as an intermediate option between the HPGR-based flowsheet and the primary crusher to SAG mill-based flowsheet. In Ausenco’s general experience, secondary crushing prior to SAG milling leads to improved energy efficiency when processing competent ores, supporting Putland, Siddal and Gunstone’s (2004) assessment of Mt Rawdon’s SAG and ball mill circuit and the observations, shown in Figure 1, of Lane and Siddal (2002).
Figure 1 – Relationship between %HPGR in feed and SABC grinding circuit efficiency (from Lane & Siddal, 2002)
Many papers have been written on comminution circuit selection. Appendix 1 contains a selection of references. To exemplify the key considerations three case studies are provided in summary form from the Constancia (from Lane et al., 2015), Cadia East (from Engelhardt et al., 2011, 2015) and Boddington (from Hart, Parker, Rees, Manesh & Mcgaffin, 2011; Veillette & Parker, 2005) projects.
A summary of ore characteristics is provided in Table 1. The energy efficiency factor was determined using the method described by Lane et al. (2013) and is based on the specific energy requirement of a multistage crushing, rod and ball mill circuit as the benchmark energy efficient process.
CONSTANCIA PROJECT CASE STUDY
The flowsheet for the Constancia project was based on a conventional SABC circuit design (Lane et al., 2015). As the ore to be treated in the initial years of production was less competent than the design ore case, pebble crusher installation was deferred. A twin line geared mill system consisting of a total of four 16 MW mills, illustrated in Figures 2 and 3, was selected on the following basis:
Figure 2 – Constancia dual line grinding circuit of four 16 MW mills (from Lane et al., 2015)
Figure 3 – Constancia grinding circuit viewed from the primary crusher (from Lane et al., 2015)
The major design parameters and equipment specifications are summarized in Table 2.
As shown in Table 1, the calculated energy consumption for the comminution circuit was about 130% of the theoretical energy calculated using Ausenco’s Ausgrind methodology (Lane et al., 2013) based on laboratory test work. However, the high degree of fracture in the core and the lower ore competency in the initial years of operation were expected to reduce this to between 115% and 120% for the initial period of operation.
CADIA EAST PROJECT CASE STUDY
The expansion of the original Cadia Hill circuit was required to begin treating ore from the new Cadia East underground mine (Engelhardt et al., 2011, 2015). Cadia East ore is extracted using panel caving which is an underground mining method commonly used to allow high mining rates to be achieved at a relatively low cost for hard rock mining. However, this method can result in coarser size distributions than other mining methods and was therefore expected to produce a coarser and fines deficient feed for the new comminution circuit. In addition, Cadia East ore is more competent than Cadia open pit ore, as shown in survey results presented by Foggiatto, Hilden and Powell (2015).
The expansion was accomplished by installing screening and HPGR crushing equipment ahead of the existing SAG mill, converting existing pebble crushers into secondary crushers and adding additional ball milling capacity to the original circuit (as well as additional downstream equipment). This is the only circuit of its type and the flowsheet is shown in Figure 4. A panoramic view of the crushing-HPGR circuit is illustrated in Figure 5.
Figure 4 – Cadia low-grade plant comminution circuit with survey sampling points (from Engelhardt et al, 2015)
Figure 4 – Cadia low-grade plant comminution circuit with survey sampling points (from Engelhardt et al, 2015)