Selection of Comminution Circuits for Improved Efficiency

This paper examines the design of comminution circuits and the issues that impact on comminution circuit efficiency. Circuit efficiency is discussed, principally from the perspective of power efficiency. However, operating efficiency, maintenance efficiency, capital expenditure and return on investment are often the key efficiency factors in project development outweighing power efficiency considerations. Comminution circuits have progressed from circuits comprising simple crushing machines, for example stamp batteries, through multi-stage crushing and staged grinding, to single stage crushing and semi-autogenous grinding (SAG) and ball mill (SAB) circuits. This progression has, not necessarily, been accompanied by an improvement in the efficiency of power utilisation in achieving the target grind size, but has seen an improvement in the overall efficiency of mineral processing as measured by the overall cost of grinding to the required size for effective downstream beneficiation of the value metal or mineral. In the future, projects that add value by improving power efficiency and reducing CO2-equivalent output may earn Emission Reduction Units (ERU's). The value of an ERU is hard to predict and the achievable reduction will be a function of comminution technology and site specific factors such as the method of power production. WHAT IS GRINDING EFFICIENCY? Technically, grinding circuit efficiency refers to the amount of energy used to grind from a certain feed size to a product size as measured against a benchmark, such as that determined by Bond formulae adapted by Rowland (Rowland 1982). From a business perspective, efficiency may be measured according to the cost per unit of the grinding circuit product. That cost is comprised of the following components: • cost of capital • labour; • maintenance materials; • power, and • consumables. The relative importance of each of these components will vary depending upon the ore characteristics, plant location, circuit selection and plant capacity. Impact of Circuit Capacity In general, optimising fixed costs, such as labour costs, is most leveraging for low tonnage plants whilst optimising variable costs, such as energy consumption and media costs is most leveraging for high tonnage plants. Consider two cases, a modest flotation-based operation at 1 Mt/a and a large tonnage operation of 20 Mt/a. Table 1 summarises the operating costs for these plants. Obviously the location and type of plant impacts significantly on operating costs. For instance, the Cadia Hill plant operating costs are approximately $3.60 /t due to the coarse grind size, simple plant and low power cost. Conversely, a 1 Mt base metals flotation plant in Tasmania had operating costs of $16/t due to the fine grind size and complex flotation circuit. The distribution of costs between the cost centres for the two plants is very different. For a modest plant, nearly 50% of costs are labour related, 17% of costs are for comminution power. For the large plant, less than 10% of costs are labour related and 28% of costs are for comminution power. Table 1 SUMMARY OF PLANT OPERATING COSTS 1 Mt/a Plant 20 Mt/a Plant Area Cost $/t Cost $/t Personnel number (including maintenance) cost with on-costs 60 $6 M/a 6.00 120 $ 12 M/a 0.60 Power power cost, $/kWh comminution circuit, kWh/t other plant, kWh/t $0.12/kWh 18 kWh/t 12 kWh/t 2.16 1.44 $0.08/kWh 18 kWh/t 12 kWh/t 1.64 0.96 Maintenance Materials comminution circuit other plant $0.70M /a $0.55M /a 0.55 0.55 $9 M/a $4 M/a 0.45 0.20 Consumables grinding media reagents and general consumables $1M /a $1M /a 1.00 1.00 $20 M/a $20 M/a 1.00 1.00 TOTAL 12.70 5.85 A 1 Mt/a plant may have a capital cost of between $25 000 /t/h and $50 000 /t/h compared with between $10 000 /t/h and $20 000 /t/h for a 20 Mt/a plant, dependent on complexity. Approximately $9M and $120M is expended on the comminution circuit (crushing, stockpiling and grinding) representing 35% and 50% of plant capital cost for the 1 Mt/a and 20 Mt/a plants, respectively. Comminution costs include power, grinding media and maintenance materials. These costs represent 30% of costs for the 1 Mt/a plant and over 50% of costs for the 20 Mt/a plant. Thus, the “efficiency” of a modest plant is improved by simplifying the comminution circuit and reducing labour requirements. Improving the grinding efficiency of the comminution circuit by 10% will improve plant operating costs by less than 3%. The “efficiency” of a large plant is not greatly impacted by the labour costs (within sensible limits). A 10% improvement in comminution circuit grinding efficiency leads to a 5% improvement in plant costs. Impact of Circuit Type Comminution circuits in mineral processing plants is Australasia fall into the following categories: • single stage crush and single stage grind with steel media, eg Cosmos Nickel Mine (Fleay et al 2000). • single stage crush and single stage autogenous grind, e.g. Kambalda Nickel Mines (Esvelt, 1997), Olympic Dam, post 1995. • single stage crush, SAB, e.g. Macraes Gold Mine. • single stage crush, SABC, e.g. Cadia Hill Copper/Gold Mine (Dunne et al 2001, Hart et al 2001). • single stage crush, ABC, e.g. Ridgeway Mine (gold/copper), Olympic Dam, 1986-1995. • single stage crush, rock and pebble mill as operated at Mt Edon Gold Mine and Forrestania Nickel Mines (Rantanen et al, 1996). • three stage crush, rod and ball mill, e.g. Renison Bell Tin Mine. • three stage crush, two stage ball mill, e.g. Marvel Loch Gold Mine. • two stage crush, two stage ball mill, e.g. Bronzewing Gold Mine (Lane et al, 1997). • three stage crush, ball mill e.g. (Three Mile Hill). • two stage crush, ball mill, e.g. Pajingo. • a crushing circuit including high pressure grinding rolls (HPGR) at Argyle Diamond Mine. Recent feasibility studies for the Boddington Expansion Project have considered the use of HPGR for hard rock multi-stage crushing and ball mill circuits at high tonnage rates (Parker et al, 2001). Table 2 provides a simple comparison of the various comminution circuits. The primary mill may be operated in open circuit or closed by a classifier, such as a screen or a hydrocyclone. Closing the primary mill may result in an improvement in operating efficiency for some operations, or may facilitate a reduction in the primary mill product size. 1 Plant relocated to Black Swan Table 2 SUMMARY OF COMMINUTION CIRCUITS Steel Media Circuits Semi-autogenous Circuits Autogenous Circuits Crushing two stage crush three stage crush crushing circuit including high pressure grinding rolls three stage crush, rod and ball mill, three stage crush two stage crush single stage crush single stage crush single or two stage stage crush single stage crush single stage crush single stage crush Primary Milling rod mill ball mill ball mill SAG mill SAG mill and pebble crush AG mill and pebble crush rock mill (with pebble crush) Secondary Milling ball mill ball mill ball mill ball mill ball mill ball mill single stage SAG mill ball mill ball mill ball mill single stage autogenous grind pebble mill Examples in Australia Pajingo, Karonie Harbour Lights Proposed for Boddington Renison,