Updated magnetic separation techniques to improve grinding circuit efficiency.PDF

Technical note Updated magnetic separation techniques to improve grinding circuit effi ciency Daniel Norrgran a,*, Timothy Shuttlewortha, Greg Rasmussenb a Eriez Magnetics, 2200 Asbury Road, Erie, PA 16506, USA b Kemess Mines Limited, British Columbia, Canada Received 1 April 2004; accepted 28 May 2004 Abstract Grinding ball fragments discharging from SAG mills and ball mills cause extreme wear to downstream processing equipment. These ball fragments, circulating in a milling circuit, will cause excessive wear to sumps, pumps, hydrocyclones and interconnecting piping. Grinding ball fragments contribute very little to the grinding process while consuming power. Magnetic separation applied to milling circuits to remove the recirculating grinding media has evolved over several years. Mag- netic separation techniques have been successfully applied to ball mill circuits as well as SAG mill circuits. The Trunnion Magnet separator was developed and improved over the years to remove grinding ball fragments directly from the mill discharge. The removal of grinding media from the ball mill discharge has resulted in several benefi ts in the milling circuit. Reported benefi ts taken from various installations are Extended the pump life and the hydrocyclone life of approximately 250. An increase in mill throughput of a nominal 5. A nominal 8 reduction in mill power consumption. More effi cient grinding resulting in a nominal 10 reduction in the mill work index. Magnetic separation has also been recently applied to the SAG mill discharge. Suspended electromagnetic and rotating drum type magnetic separators have been designed to ‘‘pick-up’’ the grinding media in the coarse oversize recirculating product. In addi- tion, rotating drum type magnetic separators have been developed to remove grinding media in the fi ne undersize product that that reports to the sump. 2004 Elsevier Ltd. All rights reserved. Keywords Comminution; Magnetic separation 1. Magnetic separation techniques Magnetic separation systems to remove grinding ball fragments from the mill discharge have been developed andsuccessfully appliedin themilling circuit. Thesemag- netic separation systems were fi rst applied at the Escon- dida copper concentrator in Chile in 1996 Norrgran and Mankosa, 1999. The fi rst magnetic separator ap- plied to the milling circuit consisted of an arc of perma- nent magnets mounted at the discharge end of the trommelscreen tomagneticallycollecttheoversizegrind- ing media. This separator was termed Trommel Magnet. 0892-6875/ - see front matter 2004 Elsevier Ltd. All rights reserved. doi10.1016/j.mineng.2004.05.020 * Corresponding author. Tel. 1 814 835 6000; fax 1 814 833 3348. E-mail address dnorrgran D. Norrgran. This article is also available online at Minerals Engineering 17 2004 1287–1291 SincetheinceptionoftheTrommelMagnet, magneticseparationtechniqueshaveevolvedand have become more eff ective and effi cient. Magnetic separation has now been applied to various milling operations worldwide. Descriptions of the applications follow. 2. SAG mill pebble crusher The magnetic collection of grinding media from mill- ing circuits fi rst began with the introduction of pebble crushers. Recirculating SAG mill trommel oversize to a pebble crusher required the eff ective removal of grind- ing ball fragments. This was necessary to remove the oversize grinding ball fragments to prevent blockage and damage to the crushers. Fig. 1 illustrates how a grinding ball will traverse a cone leaving damage in its trail. The usual outcome is that crusher cone will fail along this trail of damage. The usual to remove these grinding balls is with suspended electromagnets over the belt feeding the pebble crusher. This application required an electro- magnet designed to generate a high magnetic fi eld to eff ectively collect grinding balls. The more recent and more eff ective however is to use a magnet posi- tioned over the deck screens or at the discharge of the SAG trommel screen. 3. SAG mill screen deck The SAG Mill discharge stream is frequently classifi ed with a screen deck. The screen deck scalps the oversize material, which in turn is routed to the pebble crusher and back to the SAG mill feed. A new approach has been developed to magnetically collect ferrous grinding media from the screen oversize. A rotating magnetic drum mounted over the discharge lip of the screen eff ectivelycollectsferrousgrindingmedia.These drums can often utilize rare earth permanent magnets thus eliminating the need for a power demanding electromagnet. The separator utilizes an electromagnet or permanent magnet to generates a deep magnetic fi eld. The drum rotates around the stationary magnetic element. Ferrous material passing under the drum is attracted to the magnetic element and is held to the surface of the drum. The ferrous material is rotated over the top of the drum and discharged into a separate bin. This is a more eff ective approach than using a suspended electromagnet over the conveyor belt that feeds the peb- ble crusher. The rotating drum off ers the following benefi ts Ore that would otherwise be speeding along on a con- veyor belt is spread out and moving much slower on the screen deck. The ore is presented to the drum essentially as a monolayer. Iron objects are not hindered by a ‘‘dead burden’’ as on a conveyor belt. The magnetic drum can be placed very close 0.2– 0.3m to the discharging ore. The material is agitated while subjected to separation releasing physically entrapped ore. Long bars or rods can be safely removed before they spear a belt. 4. Trunnion magnet The Trunnion Magnet is an enhancement of the initial Trommel Magnet. The enhancement involved both mag- neticaswellasnon-magneticrefi nements.Theseimprove- ments were necessary to magnetically collect grinding ball fragments directly from the mill discharge stream without the use or requirement of a trommel screen. The magnet barrel or blind trommel is bolted directly to the discharge fl ange of the mill and is coupled with an arc of permanent magnets to collect grinding media. The Trunnion Magnet includes four basic compo- nents as shown in Fig. 2. The system consists of the magnet barrel, magnet sector, support structure and the discharge hopper. The magnet barrel is a short extension that bolts directly to the discharge end of the trommel screen. The function of the blind trommel is to provide a fl ange to transport the trommel oversize material through the magnetic fi eld. The magnet sector is comprised of permanent magnets and has an approx- imate 200 arc. This magnetic arc is mounted on a steel support pedestal and is positioned around the mag- net barrel. The discharge hopper and support struc- ture is positioned just inside the magnet barrel to collect the grinding ball fragments. The hopper collectsFig. 1. Cone crusher mantle damaged by a grinding ball. 1288D. Norrgran et al. / Minerals Engineering 17 2004 1287–1291 the grinding ball fragments as they rotate past the end of the magnetic sector at the top of the magnet barrel. In the design of the Trunnion Magnet, consideration was given to the magnetic capture of grinding balls/frag- ments directly opposed by the drag force of the mill dis- charge slurry. Several techniques are used in the magnet design to compensate for the drag force. First, high- energy rare earth magnets have been used in conjunction with the barium ferrite magnets in a hybrid magnetic cir- cuit to increase the magnetic force. This provides a mag- netic force of over three times that required to lift the grinding ball. Second, the magnetic arc can be extended along the length of the magnet barrel to increase the retention time that the mill discharge stream is exposed to the magnetic fi eld. And third, magnetically induced lifters inside the magnet barrel can be used to assist in the collection of ball chips. 5. Trunnion magnetball mill installationoperation/ results A typical installation will take 16–20h. This is on the basis that the necessary preparatory work has been com- pleted. The most involved task is setting the support structure that will hold the magnet arc. This has to be accurately placed relative to the mill discharge. The weight of the support structure with the magnet arc is typically 7000–9000 pounds and must span two beams. All other components are then installed relative to the magnet support structure. The benefi ts derived from the Trunnion Magnet sys- tem installed on a ball mill have been published earlier Shuttleworth et al., 2001; Powell and Smit, 2001. These studies provided a comparison between ball mills oper- ating with and without the Trunnion Magnet at the Los Pelambres copper concentrator in Chile. Another study pered detailed the deleterious eff ects of grind- ing ball fragments in the mill circuit. A study has recently been pered detailing the operating characteristics of a ball mill before and after the installation of the Trunnion Magnet system. This study was conducted at the Northgate Explorations Ke- mess Mine in British Columbia. The Kemess deposit is a gold–copper porphyry. The ore is processed using con- ventional crushing, grinding and fl otation techniques to produce gold–copper concentrates. There are two parallel grinding circuits, each operating at a nominal rate of 25,000tons per day. Each grinding circuit con- sists of one semi-autogenous grinding mill and one ball mill in combination. Operating characteristics were measured before and after the installation of the Trunnion Magnet system on a ball mill. The results are summarized as follows Total mill feed TPHthe total feed was essentially unchanged averaging approximately 1300 TPH. Total mill power kWthe mill power dropped 8 from an average of 7600–7000kW. Mill work index kW-h/Tthe mill work index dropped 10 from an average of 5.5–5.0kW-h/T. In addition, the amount of ferrous grinding media magnetically collected from the mill discharge was mon- itored. 46tons of ferrous was collected in the fi rst two days. A total of 80tons of ferrous was collected in the fi rst week. 6. Summary Fig. 3 depicts a typical SAG mill/ball mill grinding circuit. The SAG mill discharge is classifi ed with the Fig. 2. Conceptual illustration of the Trunnion Magnet. The magnet barrel is attached to the discharge end of the ball mill. The magnetic arc collects grinding media discharging the mill, rotates the material out of the discharge stream, and drops it into a hopper. D. Norrgran et al. / Minerals Engineering 17 2004 1287–12911289 oversize reporting to a pebble crusher. The pebble crusher product returns to the SAG mill. The SAG mill discharge undersize reports to the hydrocyclone sump. The hydrocyclone is in closed loop with the ball mills. The ball mill recirculating load is classifi ed with the hydrocyclone overfl ow reporting to fl ota- tion and the hydrocyclone underfl ow returning to theballmill.Variousmagneticseparationtech- niques are available to remove the grinding media from the mill discharge streams. Common magnetic separa- tion applications are numbered in the fi gure and detailed below. The SAG mill discharge is typically classifi ed in one of the two ways. The two s are detailed as follows 1 The SAG mill discharge is classifi ed on screen decks. In this application a suspended electromagnet or drum type magnet suspended over the screen deck collects the oversize grinding media as it transverses the screen deck. 2 The SAG mill discharge is classifi ed on a trom- mel screen. In this application the SAG mill discharge is treated with a Trunnion Magnet or a drum type magnet to collect the grinding media as it discharges the mill. The Trunnion Magnet isreplacesthetrommelscreenandremoves grinding ball fragments directly from the mill discharge. In addition, there are a few other applications for magnetic separation. 3 The SAG mill oversize reporting to the pebble crusher is conveyed under a suspended electromag- net.Themagnetcollectsgrindingmediaand removes it prior to the crusher. 4 A Trunnion Magnet is mounted at the ball mill dis- charge. The Trunnion Magnet is replaces the trom- mel screen and removes grinding ball fragments directly from the mill discharge. In each case, the grinding media is collected and dis- charged into a separate hopper. 7. Conclusions Reported benefi ts taken from various installations in- clude extended the pump life and hydrocyclone life, in- crease in mill throughput, reduction in mill power consumption, and more effi cient grinding. It should be noted that the analyses presented in this report should be taken as indicators. It is diffi cult at best to provide a precise comparative analysis in a mineral concentrator. Specifi c characteristics of diff erent mills as well as irregularities in the operation have to be accounted. References Norrgran, D.A., Mankosa, M.J., 1999. Magnetic collection of grinding ball fragments from SAG and ball mill circuits. In 4 1 2 3 COARSE PRODUCT FINE MILLED CYCLONE FINE COARSE NEW FEED CRUSHER PEBBLE CLASSIFICATIONSAG MILLBALL MILL Fig. 3. Typical SAG mill/ball mill grinding circuit detailing magnetic separation points. 1290D. Norrgran et al. / Minerals Engineering 17 2004 1287–1291 ProceedingsoftheConferenceonCOPPER99Fourth International Conference, vol. II. Phoenix, AZ., TMS. pp. 177– 189. Shuttleworth, T., Stipicic, C., Milton, D., 2001. Magnetic separation techniques for SAG grinding circuits. In Proceedings of the Conference on International Autogenous and Semiautogenous Grinding Technology, vol. III. University of British Columbia, Vancouver, BC. pp. 393–407. Powell, M., Smit, I., 2001. Startling eff ect of ball scats removal on SAG mill perance. In Proceedings of the Conference on Interna- tional Autogenous and Semiautogenous Grinding Technology, vol. IV. University of British Columbia, Vancouver, BC. pp. 124–137. D. Norrgran et al. / Minerals Engineering 17 2004 1287–12911291