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Energy Efficient Comminution Circuits – a Modified Grinding Strategy and the Selection of a Target Product Size

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Energy Efficient Comminution Circuits – a Modified Grinding Strategy and the Selection of a Target Product Size Energy Efficient Comminution Circuits – A modified grinding strategy and the selection of a target product size Z. Pokrajcic University of Queensland, JKMRC, QLD Introduction increasing need for more efficient grinding This paper describes two techniques that can be systems and innovative comminution technologies used in the design of comminution circuits for that not only increase the efficiency of the improved efficiency and a reduction in total comminution process but also act to minimise the energy consumption. They are: consumption of steel media and liners. This together with a new approach to choosing a target • A modified grinding strategy where efficient product size can significantly increase the energy grinding equipment is used and techniques efficiency of a comminution circuit. employed to decrease grinding media consumption Methodology / Experimental Technique • A more reasoned and thorough approach to The two parts of this investigation were the selection of target product size for a undertaken according to the following: comminution circuit involving a better A modified grinding strategy understanding and interpretation mineral liberation data. All of the modifications made to the configuration of an existing comminution circuit were Background undertaken using the modelling and simulation There has been much discussion about the energy package JKSimMet. It is a valuable tool in intensive nature of comminution processes. It has predicting the performance of a comminution been quoted several times that comminution circuit, including the equipment power draw, processes account for 3%-4% of the global product size distributions, classification electrical energy consumption. However, the efficiency, etc. comminution process remains inherently The aim of the eco-efficient modifications was to: inefficient. It is understood that 85% of the energy • Reduce the direct energy usage. That is, used comminution is dissipated as heat, 12% is decrease the grinding power used in the size attributed to mechanical losses and only 1% of the reduction process. total energy input is used in size reduction of feed material (Alvarado et al. 1998). • Reduce the indirect energy usage. That is, Another aspect of comminution which contributes decrease the consumption of comminution significantly to the energy intensity of the process consumables such as steel media and mill is the consumption of media and mill liners. The liners. quantity of energy necessary for the fabrication of The strategy used to achieve these aims involved the media and/or liners from extraction, to better utilisation of the comminution energy. This transport, to manufacture and assembly is can be achieved by purposely targeting the considered part of the total energy utilisation of comminution energy at specific problematic size the comminution process. This is also quite an fractions such as the critical size material in the energy intensive process and opportunities to primary mill and the very fine fraction in the minimise steel consumption in comminution secondary mill. Through the use of more efficient circuits should be realised. comminution devices such as HPGR (high The third factor which contributes significantly to pressure grinding rolls) a product size distribution the energy intensity of comminution circuit is the that is narrower in size range can be generated. chosen target product size, or grind size. As the This size distribution contains less material in the target product size decreases the energy required coarse fraction to aid overall mineral liberation to achieve the product size increases significantly. and less material in the fine fraction to minimise As the particles become smaller their strength and the effects of slimes. therefore their resistance to breakage increases. The strategy also exploited opportunities to For small particles as internal flaws are depleted, minimise the load in grinding mills and in some the observed strength approaches the high cases employ autogenous grinding techniques to intrinsic strength of the solid. decrease indirect energy consumption. This is the prospect facing all comminution design and operating engineers. Hence, there is an Selection of a target product size • Existing mill sizes were reduced This concept was illustrated using a sample from When the performance of this new circuit, of rod mill discharge from Mount Isa. Rod mill denoted ABC-HPGR-PM, was compared to the discharge can be equated to SAG mill discharge in original SABC circuit, the following (based on terms of product size and product size distribution JKSimMet and mass balancing predictions) was in SABC circuit (SAG mill with recycle crushing observed. and a single stage ball mill). • The ABC-HPGR-PM circuit produced a both The sample underwent staged grinding, using a a finer and steeper product size distribution laboratory scale rod mill and ball mill to produce which promoted better flotation performance, samples of product sizes ranging from approximately a 2% increase gold recovery P80=0.015m to P80=1.500mm. • Direct energy comparisons show that the Samples from each of the product sizes were ABC-HPGR-PM circuit had an operating analysed using MLA techniques to assess the work index of 15.9kWh/t compared to extent of liberation of individual mineral 24.3kWh/t for the base case SABC circuit. components. This is an energy saving greater than 8kWh/t In order to maximise the potential target grind size for the whole circuit. Significant to say the and therefore minimise required energy input for least. the circuit, the following concepts were applied: • Comparison of indirect energy consumption • minerals were grouped to form larger entities for the two circuits is showed that again the and therefore deemed liberated at coarser ABC-HPGR-PM out performed the standard sizes SABC circuit with a 79% saving in indirect • particles were deemed recoverable in a energy based on media and liner wear alone. flotation process, as opposed to liberated, The SABC circuit consumed 3535kW while when the particle contains greater than 15% the ABC-HPGR-PM circuit consumed mineral of interest 753kW. These figures are based on 6kWh/kg Together the above two points act not only to required to manufacture the grinding increase the target product size but it also enable consumables. the separation and rejection of material (typically Selection of a target product size liberated gangue) from the circuit. The rejection Analysis of the MLA data for the various product of gangue from the feed stream can limit the sizes to include liberated and recoverable amount of over-grinding but more importantly it particles, showed that: will reduce the mass of material proceeding to the • 70-80% of galena and sphalerite is liberated at next stage of size reduction and/or separation. P =76m This allows for more efficient separation and 80 significantly lower energy requirements for • 70-80% of galena and sphalerite is downstream size reduction. recoverable at P80=150m Key Results / Findings • When galena and sphalerite are combined and analysed as one mineral, 85% of this mineral A modified grinding strategy is recoverable at P80=150m Eco-efficient modifications were made to an existing SABC according to the following: • >95% of the sulphides (combined galena, sphalerite, pyrite and pyrrhotite) are • A pre-crush stage using a HPGR was included recoverable at P =1607 m to minimise the generation of critical size 80 material in the primary mill. The aim was to • 45% of non-sulphide gangue is liberated at reduce a portion of the feed material below P80=1607m critical size and generate more fines to These points can be used in the design a decrease the load in the primary mill. Not all comminution circuit where product size is finer of the feed material was pre-crushed to allow than 1.5mm. for sufficient abrasion breakage in the primary mill. The aim is to exploit the recoverability of valuable minerals and the rejection of liberated gangue in a • The SAG mill was converted to an AG mill separation step at the coarsest product size. and the ball mill was converted to a pebble mill to decrease liner and media consumption 7KH VHSDUDWLRQ VWHS FRXOG EH D ÀRWDWLRQ VWDJH RU D Conclusions and Future Direction UHODWLYHGHQVLW\VHSDUDWLRQVWDJH2UHSURSHUWLHVZLOO 2SSRUWXQLWLHV H[LVW WR LPSURYH FRPPLQXWLRQ GHWHUPLQHWKHPRVWVXLWDEOHPHWKRG HI¿FLHQF\DQGGHFUHDVHFRPPLQXWLRQFLUFXLWHQHUJ\ 7KH FKRVHQ FLUFXLW PD\ UHVHPEOH WKDW VKRZQ LQ FRQVXPSWLRQ E\ PRGLI\LQJ WKH VWDQGDUG 6$%& )LJXUH circuit to include: D+3*5SUHFUXVKVWDJH DXWRJHQRXVJULQGLQJ 7KH VWUDWHJ\ LV WR WDUJHW FRPPLQXWLRQ HQHUJ\ DW problematic fractions such as critical size material E\ GHFUHDVLQJ UHFLUFXODWLQJ ORDGV DQG HPSOR\LQJ DXWRJHQRXVJULQGLQJWHFKQLTXHV ,WLVDOVRSRVVLEOHWKURXJKDEHWWHUXQGHUVWDQGLQJRI mineral liberation characteristics of coarse stream 3 aPP LQ D FRPPLQXWLRQ FLUFXLW WR H[SORLW RSSRUWXQLWLHV WR UHMHFW OLEHUDWHG JDQJXH IURP WKH stream and therefore the comminution circuit thereby UHGXFLQJ WKH VL]H DQG JULQGLQJ HQHUJ\ UHTXLUHPHQW IRU GRZQVWUHDP SURFHVVHV 7KHVH HOHPHQWV DOORZ Figure 1: Design of separation circuit post primary SURYLVLRQIRUDFRDUVHUJULQGSURGXFWVL]HEDVHGRQ grinding. the liberation and recoverability characteristics of PLQHUDOV 7KHFLUFXLWIHDWXUHVDFRDUVHSDUWLFOHÀRWDWLRQVWHSWR VHSDUDWHWKHOLEHUDWHGQRQVXOSKLGHJDQJXHWRWDLOIURP 7KH FRPELQDWLRQ RI WKH PRGL¿HG JULQGLQJ VWUDWHJ\ WKHUHFRYHUDEOHVXOSKLGHV7KHVXOSKLGHVWKHQSURFHHG WRJHWKHUZLWKDPRUHUHDVRQHGDSSURDFKWRSURGXFW WRDIXUWKHUVWDJHRIJULQGLQJEHIRUHEHLQJVHSDUDWHG
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