Common-sense improvements to electric smelting at Impala Platinum

Transcription

1 Common-sense impovements to electic smelting t Impl Pltinum by V. Coetzee* J o u n l Synopsis In 1992, ppe ws compiled highlighting common-sense ppoch to electic smelting t Impl Pltinum. Since then, the smelte complex hs undegone consideble chnges. Minel economics, UG2 exploittion, nd chnging envionmentl legisltion hve enfoced numeous pocess upgdes, yet common-sense ppoch hs lwys been dopted. This document detils the mjo chnges in the smelte complex t Minel Pocesses (Minpo) ove the pst 15 yes, nd povides the tionle fo the chnges. Keywods: Pyometllugy, funce, PGM, pltinum, nickel, coppe, electic smelting, six-in-line, Peice-Smith, Impl Pltinum. Compny bckgound Impl Pltinum Holdings Limited (Implts) is in the business of mining, efining, nd mketing pltinum goup metls (PGMs) nd ssocited bse metls. The goup hs thee-ponged opetionl sttegy, nmely: Mine-to-mket opetions, whee Implts hs opetions on the Bushveld Igneous Complex (BIC) in South Afic (on the Westen Limb Impl Pltinum Limited, nd on the Esten Limb Mul Pltinum), nd the Get Dyke in Zimbbwe (Zimplts, which is listed on the Austlin Stock Exchnge) Impl Refining Sevices (IRS), which ws estblished s n entity in 1998 to tke dvntge of the goup s smelting nd efining infstuctue Sttegic llinces nd investments with othe plyes in the industy, vitl to the Implts s gowth sttegy Impl (Rustenbug opetions) Impl Pltinum Limited, the goup s pimy opeting unit, hs opetions bsed some 25 km noth of the town Rustenbug, locted in the Noth West Povince, with efining opetions pefomed in the Est Rnd town of Spings, Guteng. This unit contibutes moe thn 60% of the totl Implts poduction nd ppoximtely thee-qutes of the net pofit. Gowth in Rustenbug hs continued, nd the unit cuently poduces 1.2 million ounces (Moz) of pltinum (1.9 to 2.0 Moz PGMs) nnully, opeting with 13 shft systems nd 5 declines (coveing totl e of 250 km2), 30 un-of-mine mills (17 Mt milled), nd two immesed c electic funces (0.8 Mt smelted t the Minel Pocesses complex). Oe types mined The wold s lgest known deposits of PGMs e concentted in the Bushveld Igneous Complex (BIC), which extends fo moe thn 400 km in the nothen pts of South Afic. The BIC system is divided into n esten nd westen lobe, nd includes two mjo nowsemed eefs exploited by Impl, nmely the Meensky eef nd the UG2 eef. The Meensky eef, which is estimted to contin some tons of PGMs, genelly hs moe sulphides thn the UG2 eef. The UG2 eef, which is moe consistent thoughout the BIC, genelly contins much highe levels of chome thn the Meensky eef, nd significntly less bse metls. The UG2 eef is believed to contin PGM eseves twice s lge s those of the Meensky eef, nd is consequently eceiving moe ttention in pesent opetions, with the depletion of Meensky eseves, which hve been successfully exploited since the lte 1920s. The incesed tonnges of UG2 oe hve been poviding chllenges in smelting o funce pocessing, s esult of the incesed chome content. The chnges in mining nd concentting sttegies hve lso led to need to conside diffeent smelting ppoches, lthough the fundmentls hve not chnged much. * Impl Pltinum, Rustenbug, South Afic. The South Aficn Institute of Mining nd Metllugy, SA ISSN X/ This ppe ws fist published t the SAIMM Confeence, The South Aficn Pyometllugy 2006 Intentionl Confeence, M P p e The Jounl of The South Aficn Institute of Mining nd Metllugy VOLUME 106 NON-REFEREED PAPER MARCH

2 Common-sense impovements to electic smelting t Impl Pltinum Concentting Oe is deliveed fom undegound vi il netwok of 92 km to two concentto plnts: the centl concentto, compising the Meensky nd MF2 sections, nd the UG2 concentto. Meensky oe is cushed to top size of 150 mm by jw cushes, pio to delivey to the silos. The 15 Meensky mills un in pllel s 15 single-stge un-of-mine bll-milling cicuits with single-stge hydocyclone clssifiction. The milling cicuit is followed by single-stge bulk sulphide flottion cicuit. Two flottion bnks e un in pllel, eceiving feed fom 8 nd 7 mills, espectively. With the cuent cicuit design, the Meensky plnt ecoveies e stongly dependent on mss pull. Incesed tonnges, togethe with mechnized mining, hve esulted in stedy decese in hed gde ove the pst decde. In n effot to mintin metl ecovey, plnt mss pulls hve stedily incesed to the cuent level of 5%. This, coupled with the substntilly impoved thoughput of the mills, hs esulted in lge volumes of concentte sent to the smelte. In n effot to educe concentte volumes, second stge of flottion is cuently being tested on the Meensky plnt. UG2 oe is lgely pocessed t the UG2 plnt, with the excess being tken up t the MF2 plnt, long with some Meensky nd ll the opencst mteil. The UG2 plnt consists of two pimy utogenous mills, which both eceive un-of-mine oe. The mill dischge is sceened, nd this effects the oe seption fo which the cicuit is known: the sceens septe the chomite-ich fction fom the silicteich fction. The fine, chomite-ich mteil epots to the undesize, long with bout 90% of the PGMs, nd is theefoe known s the high-gde fction. The emining 10% of the PGMs e ssocited with silictes, nd epot to the sceen ovesize with the cose silicte, o low-gde, fction. The high-gde fction is teted though clssic MF2 cicuit, with the low gde teted in MF1 cicuit, simil to the Meensky oe. The convesion of the UG2 plnt to n open-cicuit pimy milling opetion hs led to moe efficient milling of the high-gde fction, nd hs educed ove-ginding of chomite. The effect of this hs been seen s eduction in chome in concentte, nd n ssocited incese in PGM ecovey. Howeve, incesed thoughput t the UG2 plnt hs ctully esulted in lge quntity of chomite being pocessed t the smelte. Smelting A simplified metllugicl flowsheet of the Minpo smelting complex is shown in Figue 1. Electic smelting is still believed to be the most economicl option to tet the flottion concentte, when comped to hydometllugicl ltentives. This belief stems minly fom the eltively low minel (bse metl) content of the concentte. Flottion concentte is teted togethe with IRS toll concenttes, though thickening cicuit to mximize on wte ecovey, nd to fcilitte blending. The thickened poduct is fed to fou opetionl col-fied Nio-technology spy-dying units, whee n lmost bone-dy poduct is pneumticlly dischged to silo stoge units. Two ectngul six-in-line funces, with combined powe of 73 MW, e used fo sulphide concenttion. The sulphide mtte (which is the PGM cie) is futhe concentted though ion emovl in Peice-Smith convetes. A low funce mtte fll enbles conveting edundncy, s only thee to fou of the six vilble Peice-Smith convetes e in opetion t ny time, llowing fo enough tunound flexibility. Off-gses withdwn fom the electic funces, genelly contining below ppm SO 2, e teted in Sulfcid pocess. The pocess utilizes ctivted Loction of Implts opetos Loction mp of the Impl lese e Mkwio Pltinum Zimbbwe He Get Dyke N Bulwyo Mimos Pltinum Mine Botswn South Afic Impl Pltinum Rustenbug N Bushveld Complex Impl Refineies Johnnesbug Mul Pltinum Cuent shfts Decline pojects New plnned shfts Figue 1 Mps of Impl Pltinum s opetions 156 MARCH 2006 VOLUME 106 NON-REFEREED PAPER The Jounl of The South Aficn Institute of Mining nd Metllugy

3 Common-sense impovements to electic smelting t Impl Pltinum J o u n l P p e Figue 2 Simplified flowsheet of the Impl Pltinum smelte Tble I Typicl convete mtte nlysis Nickel 46.7% Ion 0.5% Coppe 31.0% Sulphu 20.5% Coblt 0.3% PGE & mino impuities < 1% cbon s ctlyst, to poduce wek sulphuic cid solution with concenttions less thn 20% H 2 SO 4. The stonge off-gs stem, emoved fom the Peice-Smith convetes, is teted in conventionl single ctlysis, single bsoption Lugi-designed cid plnt with SO 2 ( ppm) fixted in 94 98% H 2 SO 4 poduct. All sulphuic cid poducts e sold ove the fence to n djcent fetilize poduce. Smelte expnsion Tble II displys the sequencing of mjo cpitl events t the Minpo smelte complex. Little cpitl ws employed duing the 1990s, while exciting gowth with viety of pojects is evident theefte. The mjoity of the pojects hve been imed t eithe incesing poduction, educing gseous nd pticulte emissions, o simply impoving efficiency. The eminde of the document is focused on the common-sense ppoch followed in ll cpitl expnsions: ppoching impovement though incementl dvnces, without dding unnecessy pocess isks. Funce feed peption systems Nio-spy dye design Duing the 1970s, technology chnge fom dum filtes nd tubo-ty dyes to Nio-spy dyes ws systemticlly intoduced. The chnge ws initited becuse of numbe of poblems ssocited with the hndling nd subsequent chging of moist concenttes to the funces. The min poblems included filte-medium blinding, concentte hngups, feede blockges, nd funce blow-bcks esulting fom the genetion of stem in the funces. These wee mjo concens both in tems of poduction nd sfety. The 28 MW spy dye instlltion in 1991 (double the cpcity of the pevious lgest dye) ws done with some design impovements. The hot-gs geneto type chosen ws type L tvelling-gte stoke, which ws the sme s the peceding instlltion. The min diffeence in design ws utomtic hydulic speed contol on the gte. The utomtion llowed fo bette dye thoughput contol, without compomising on col-combustion efficiencies. Chnges mde on the dying chmbe included eplcing the nnul intenl i-cooled ducting with n intenl efctoy-insulted mild steel ducting; the instlltion of thick-wlled mild steel dying chmbe; the chnge to fibeglss of the mteil of constuction of the stck; nd Tble II Chonologicl sequence of mjo cpitl pojects t the Minpo smelte ove the pst 15 yes MW spy dye nd 30 MVA funce 1996 Concentte pneumtics instlled fo funces 2 & Upgded funce off-gs ESPs 2000 Two new 12 x 24 Peice-Smith convetes nd n cid plnt cpcity upgde 2001 New 38 MVA funce nd 1800 ton feed silo 2002 Sulfcid plnt instlltion MW spy dye, 50 t/h high-te thickene nd ton feed silo 2004 New convete mtte gnultion fcility 2006 Toll business off-loding nd smpling instlltion The Jounl of The South Aficn Institute of Mining nd Metllugy VOLUME 106 NON-REFEREED PAPER MARCH

4 Common-sense impovements to electic smelting t Impl Pltinum chnges to the i-dispese mteil nd design. The emining peiod of the lte 1990s sw futhe incementl impovements to the dying cicuits. As pt of n expnsion pln, the lgest dye to dte ( 32 MW spy dye) ws constucted in A decision ws mde to step wy fom the gte-type stoke, nd evet to fluid-bed hot-gs geneto, which ws used in the fist spy-dye instlltion. The chnge in HGG type ws imed t ising efficiencies though ensuing complete col combustion. A numbe of modifictions ws mde to the olde fluid-bed type design. The fundmentl diffeence ws the emegency stck being positioned diectly bove the HGG (foming pt of the HGG) s opposed to septe emegency stck. This eliminted the need fo two citicl shut-off dmpes to be positioned in the exteme het zone (900ºC), nd hs subsequently educed the enegy losses esulting fom indequte seling ound dmpes. It lso educed the likelihood of dmpe filue nd mintennce esulting fom the dmpes wping, nd hs consequently impoved dye vilbilities. The positioning of the emegency stck lso llowed fo impoved tempetue contol to the dying chmbe, without compomising on fluid bed i flow tes (thus significntly educing the pobbility of bed sinteing), by intoducing cool i though the emegency stck. In ddition to the chnge in emegency stck positioning, the use of only fou themocouples (fom moe thn twenty) monitoing the entie bed sufce e llowed fo ese of opetion nd monitoing. The te t which the dye could be bought on-line ws lso impoved with optimized bune design nd positioning (the new instlltion elies on fou bunes positioned bove the bed, s opposed to two bunes positioned in the plenum e on the olde instlltion). The substitution of i-dispeses with ven contct eliminted poblems expeienced on the i-dispeses s esult of wping nd coosion. A wte tp ws lso instlled, nd positioned below the dying chmbe, llowing fo pid cooling of the dying chmbe duing dvese tempetue conditions befoe dmge could be inflicted on the chmbe nd costly tomizes. The design chnges with the lst two mjo dye instlltions hve been vey successful. Futhe to enjoying the pocess impovements, effots wee mde to ise opeting efficiencies nd educe opeting costs. The efficiency of spy dye is lgely dependent on the feed density of the sluy to the dye, nd mximizing the feed density lowes unit col consumption. It ws fo this eson tht emphsis ws plced on thickene nd sluy cicuits to educe col consumption, ultimtely impoving opeting costs nd educing gseous emissions. Sluy concentte hndling Flottion concenttes, togethe with the mjoity of the toll concenttes, e thickened in conventionl nd high-te thickenes pio to dying. This pocess tkes plce in fou lge concete thickenes (30 to 35 metes in dimete). The 32 MW spy dye nd 35 mete high-te thickene intoduced chnge to thickene nd concentte hndling mngement. With the technologicl dvncements on feedwell design, much highe undeflow sluy densities, t lowe thickene stock holding, hve been chieved. This bought bout sevee pumping chllenges. Peistltic pumps wee initilly instlled with the ecent dye, but these poved to be mintennce intensive nd costly to opete becuse of hose filues, pulstion poblems, nd high glyceine consumption. The peistltic pumps wee lso incpble of poviding the equied sluy flow tes t elevted densities. (The pump ppliction is difficult, s sttic hed of ound 35 m, t low flow tes of ppoximtely 30 m3/h e equied. This equies most centifugl pumps to opete f outside the best efficiency point (BEP), nd it ws fo this eson tht peistltic pumps wee initilly instlled.) As esult of the bove-mentioned poblems, in-depth heologicl investigtions nd sluy chcteiztion studies wee undetken, in conjunction with numbe of pump tils. The investigtions hve eveled necessity to bing pocess consultnts on bod with pump selection nd hydulic design fo non-newtonin system designs, the thn elying on pump supplies fo specifictions. The findings fom the studies e cuently been implemented, nd will delive futhe pocess optimiztion. Funce feed Influence of feed chcteistics on funce opetion The min chnge to funce feed chcteistics in the pst 15 yes hs been s esult of the incesed mount of UG2 oe mined, s well s the effect of toll concenttes on the Figue 3 Illusttion of the bsic chnges mde to the new fluid-bed hot-gs geneto 158 MARCH 2006 VOLUME 106 NON-REFEREED PAPER The Jounl of The South Aficn Institute of Mining nd Metllugy

5 Common-sense impovements to electic smelting t Impl Pltinum C2O3 pecentge in funce feed C2O3 pecentge in funce feed J o u n l P p e Figue 4 Chnge in funce feed composition, with specific efeence to bse metl content, togethe with chome concenttions feed mix. The te t which UG2 deposits e being exploited is fst outpcing tht of the Meensky deposits. This hs ccodingly led to substntil incese in the chomium content, s displyed gphiclly in Figue 4. Negtive effects of chome Chomium hs limited solubility in the slg, nd incesed feed concenttions (bove the solubility limit) led to the fomtion of cystlline chomite spinels. The spinels hve high melting point, nd e dense, subsequently settling on the funce heth, esulting in educed funce volume. Intemedite density ccetions typiclly fom mushy lye between the slg nd the mtte, esulting in mtte entinment. Incesed C in the funce mtte lso leds to chomite/mgnetite fomtion in the convetes. The high melting point spinels obviously lowe the fluidity (incesed viscosity) of the slg nd this leds to tpping poblems. Highe chome concenttions lso esult in decesed electicl conductivity (highe esistivity), leding to funce electicl contol poblems. Funce electodes typiclly lift out of the bth to compenste, esulting in indequte immesion (foming flt points on the electodes) nd bush cing. Hndling highe chome Chome-hndling chllenges led to numeous industy-wide studies, nd bought bout vious debtes bout spects of funce design nd the use of high poductivity funces opeting with high powe densities. Impl hs lwys dopted wht it believed ws simplistic ppoch to hndling chome though incementl dvnces nd ceful mngement, undestnding nd contol of chome levels. A numbe of simple chnges wee mde to meliote the effects of highe chome, without necessitting dstic technologicl chnges. The fist mjo chnge implemented ws opening the convete slg cicuit. All convete slg ws ediected to the concentto slg plnt, nd no longe etuned to the funces. The slg plnt tets ll cuent isings fom the convetes nd funces. Convete slg typiclly contins ound 2.3% C 2 O 3, nd eintoduction into the funces ntully ised the ovell chome concenttion, nd limited flexibility of input options. The decoupling between the convetes nd funces, howeve, lso esulted in lowe funce slg ion concenttion, which in tun esulted in highe esistivity (convete slg contins on vege ppoximtely 62% ion oxide). This offset the effects of highe C 2 O 3, which lowes the esistivity. The second mjo chnge ws stopping flux dditions to the funces. Lime ddition ws pedominntly equied to ise the slg bsicity, nd educe chemicl ttck on the efctoies. It ws lso needed to lowe the slg viscosity nd the liquidus tempetue. With the instlltion of coppecooled funces, since the ely Nineties, plte cooles wee used to ensue tht feeze lining kept the slg out of diect contct with the efctoies. This negted the need fo lime fluxing. The lowe slg bsicity futhe ssisted with the chome chllenges expeienced, in tht lowe CO levels incesed the solubility of chomite in the slg. The development nd use of pyometllugicl simultion pckges getly ided the undestnding nd contol of feed souces such s UG2/Meensky nd toll concenttes. Simultion hs become n impotnt ctivity t Minpo, to ssist in the mngement of toll concenttes. Accute clcultion of spinel fomtion, theml conductivities, tempetue pofiling, nd likely mtte/slg compositions, e edily vilble fo ny feed mix. The bove-mentioned inititives hve helped Impl to incese C 2 O 3 solubility fom 0.9% to ppoximtely 1.8%. This is enough to cope with foeseeble feed blends. To move beyond this point would, howeve, equie futhe esech nd development. Funce feed system Dy funce feed ws pogessively intoduced in the lte Eighties. Although the spy dyes wee cpble of poducing bone-dy poduct, pddle mixes wee still used to contol the feed moistue content to 5%. This ws necessy to eliminte the high dust losses esulting fom the use of belt conveyos. The Jounl of The South Aficn Institute of Mining nd Metllugy VOLUME 106 NON-REFEREED PAPER JANUARY

6 Common-sense impovements to electic smelting t Impl Pltinum Pneumtic conveynce upgdes wee cied out on ll dyes, nd silo stoge units (ppoximtely tons cpcity) wee intoduced to povide buffe between the funces nd dyes. The dy-feeding upgdes esulted in modifictions to the funce chging systems, with dmtic eduction in the numbe of chge pipes needed pe funce. The chnge ultimtely led to bette contol on blcktop levels, dsticlly educing the likelihood of funce blow-bcks, nd bolished the need fo mnul bbling. The dy concentte lso povided some smelting te impovements. Pneumtic conveynce futhemoe llowed fo chnge fom limestone to bunt lime, duing the ely Nineties. The inheent hzdous popeties of bunt lime peviously limited its use, but the intoduction of n enclosed chging system mde this possible, nd led to n ppoximte 5% incese in the smelting te. The emovl of ll flux to the funces theefte, not only ssisted with the chome chllenges, but lso povided dditionl benefits. A futhe incese of bout 5% ws obseved in the smelting te, nd mssive woking cost eduction ws elized. The fist pneumtic conveynce system instlled (on the 35 MVA funce) ws len-phse high velocity system, but hd high we tes on the system components. Choking poblems wee egully expeienced, especilly when the system ws stted with ny poduct in the tnsfe line. Since 1996, ll pneumtic systems instlled hve been densephse, low velocity systems lmost ppoximting plug-flow. These hve poved to hve low we tes, nd cn be stted with poduct in the tnsfe line. Electic funces Funce sizing nd powe input The bsic geomety chnges with funce instlltions to dte e povided in Figue 5. The optimum tio of intenl width to electode dimete nd funce sufce e ws estblished ely in both the coppe-cooled nd non-coppecooled funces. Afte ll the dimensionl chnges, sevee sidewll eosion issues wee solved, nely doubling the efctoy life in the ffected es. With funce geometies optimized ove the yes, the min chllenge emined the incese in enegy intensity in ode to mintin constnt tempetue pofiles thoughout the bth, nd to minimize spinel fomtion. This led to consideble industy innovtion when high-intensity cooling elements enteed the mket, nd debte ged on ectngul vesus ound funces. Impl still believes in well-designed funce with poven cooling elements nd optimized geomety. It hs lso been elized tht contol of funce inputs nd mintining constnt powe nd funce utiliztion is essentil to hndling highe chome contents. This hs led to inititives to minimize electode bekges nd ensue constnt opetion, which will be descibed in gete detil in fothcoming sections. Appliction of highe phse voltge, without ising cuent levels, llowed fo incesed powe input nd highe funce powe densities, without ffecting sidewll efctoy life. The chnge in both the phse voltge nd powe density is illustted in Figue 6. The figue illusttes step chnge in instlled powe density nd the phse voltge with the newe coppe plte funce instlltions. The most ecent instlltions, opeting ound 180 kw/m2, hve poved to be well within sfe opetion fo the selected cooling element type. Incesed powe density hs, howeve, hd n dvese effect on efctoy life. Instlltions in the ely Sixties sw funce lives of ound 5 yes, nd this ws incesed to 10 yes with the chnge to wide funces. Minpo funces opeting tody with plte cooles e equiing ptil nd full ebuilds evey 5 6 yes (s ws the cse in the 1960s). This hs led to investigtions into efctoy selection to impove efctoy lives nd will be discussed in the elevnt section. Funce bck-up cooling wte system To mximize funce efctoy life, nd ensue optiml funce opetion, the inteuption of wte supply to the Funce Instlltion Desciption ye Length (L) Width (W) Electode dimete (D) Electode spcing (E) Mtte/electode dist. (M) Slg/electode dis. (S) (Old) (New) L MATTE M W D D S SLAG Figue 5 Chnges in bsic funce geomety 160 JANUARY 2006 VOLUME 106 NON-REFEREED PAPER The Jounl of The South Aficn Institute of Mining nd Metllugy

7 Common-sense impovements to electic smelting t Impl Pltinum Heth Powe Density (kw/m2) Phse Voltge Figue 6 Chnges in instlled heth powe density nd phse voltge J o u n l P p e cooles must be limited to pevent cceleted eosion in the slg zone. With the newest 38 MVA funce, bck-up diesel geneto ws instlled to ensue wte to the funces duing peiods of powe nd pump filue. The bck-up system hs been put to the test ove the yes, nd hs poved to be essentil to limiting wll degdtion. The success of the instlltion is, howeve, highly dependent on the vilbility of the geneto. The infequent need fo its use necessittes mintennce nd opetionl checks on egul bsis, to ensue its vilbility when equied. Electode mngement To limit spinel fomtion, nd mximize funce thoughput, focus ws plced on mking full utiliztion of funce powe. Opetion fo polonged peiods t educed powe levels hs to be minimized to pevent build-ups in the colde funce es. Electode bekges led to educed powe levels, nd esult in colde es foming ound the ffected phse e. Impl mkes use of Södebeg electodes. The following design chnges wee implemented on the ltest funce instlltion, nd significnt benefits in tems of electode bekges hve been obseved. Contct pd wte contol system Impovements wee mde to minimize electode bekges by focusing on poducing sound, dense electodes t ll times. In ddition to incesed ttention to electode mngement by mintining dequte pste levels, minimizing pste contminnts nd ensuing good qulity welding on the electodes csings, design chnges wee implemented to futhe educe the isk of electode bekges. An impovement in poducing dense electodes ws chieved by ising the tempetue of the cooling wte supplied to the bus tubes nd contct pds. The fist coppecooled funce instlltion mde use of cold wte to the bus tube nd contct pd cicuit. This esulted in tempetues below the pste softening point t the electode pd e. In tun this esults in gps/pockets foming in the bked pste, which ffects the integity of the electode. With the ltest funce instlltion, the inlet wte to the bus tubes nd contct pds ws mintined t ppoximtely 50ºC. This ws chieved by utilizing vlve, contolled on tempetue loop, nd tnk to mix the etun wte fom the contct pds (ppoximtely 60ºC), with wte supplied fom the cooling cicuit (45ºC). The chnge ultimtely ensued moe constnt tempetue pofile, nd impoved electode integity. Use of Rogowski coils The funce tnsfome is electiclly connected to the electode column vi 40 ka wte-cooled bus nd flexible wte-cooled cbles. The distibution of electicl contct is djusted with coppe shoting bs t cuent distibution blocks mounted t the end of the wte-cooled bus nd on the electode column. Unifom cuent distibution to ll eight contct pds is essentil, to pevent the fomtion of holes in the electode csing. This my esult in pitting t the electode tip, enhncing the pobbility of electode bekges. The olde funce systems elied on mnul mesuements (monthly) to ensue equl cuent distibution. Rogowski coils wee instlled on the newest funce to impove this. The coils e connected to the down tubes, nd llow fo on-line mesuement nd pid engineeing esponse, ensuing pope cuent distibution nd minimized csing bun-though. Puge i system In ddition to cuent distibution contol, mintining clen contct between the pds nd the electode is essentil. Olde systems equied switching off once pe dy to blow the electode pds (this ws obviously not efficient becuse of the powe loss). An utomted clening system ws instlled, s consequence. The puge i system mkes use of compessed i, nd is instlled on ing-min ound the electode contct pd es. The use of nozzles nd solenoids llows fo clening of the contct pds, ensuing good contct t ll times without ffecting poduction tes. The Jounl of The South Aficn Institute of Mining nd Metllugy VOLUME 106 NON-REFEREED PAPER JANUARY

8 Common-sense impovements to electic smelting t Impl Pltinum Figue 7 Electicl cuent distibution nd the effect of doppe cble lengths Doppe cble design A futhe simple impovement ws mde fom olde designs, to ensue equlized cuent distibution t the contct pds. Olde designs used doppe cbles distibuting fom single nodl point to the eight contct pds. This esulted in hlf the electode eceiving lmost 80% of the cuent, while the othe pds connected to the longest doppe cbles eceived the cuent blnce, schemticlly shown in Figue 7. This led to the fomtion of holes in the electode csings, nd ultimtely esulted in mny electode bekges. The system chnge eliminted the unequl cuent lod by splitting the feed point, nd utilizing doppe cbles of equl length. The effects wee immeditely noted with the chnge, nd electode bekges wee futhe educed. Funce efctoy selection The effect of bsic funce geomety on funce efctoy life hs been ddessed. Despite the pesence of feeze-linings, the intoduction of coppe cooling nd highe heth powe densities led to eduction in funce efctoy life. The lives obtined in funces tody e simil to wht ws obseved in the Sixties (5 6 yes) befoe funce geometies wee optimized to educe side-wll eosion. Highe chome levels pevent ny futhe incese in geomety s this will incese the likelihood of funce build-ups. At Minel Pocesses, high-gde mgnesi efctoies hve tditionlly been utilized in the woking lining of the funces. Although the Minpo slg is n cidic slg, the bsic mgnesi efctoies wee selected due to thei bility to withstnd high slg tempetues, nd due to the good theml conductivity of this bick type. As discussed, lime nd limestone wee lso tditionlly dded to the funces, theeby incesing the bsicity of the slg, nd educing the slg coosion of the bsic efctoies. The mgnesi efctoies e, howeve, pone to hydtion nd ect with SO 2 gses t fvouing tempetues. Hydtion is likely to occu when mgnesium oxide is exposed to moistue t tempetues below 350ºC (the ection is cceleted t tempetues between 80ºC nd 180ºC). Mgnesi poducts will lso ect with sulphu nd sulphu compounds t tempetues below 1100ºC. In the yes befoe coppe cooling (side-wll cold-fce tempetues in excess of 300ºC), efctoy hydtion ws unhed of t Minpo, nd sulphuous degdtion of efctoies hd negligible effect. Coppe cooling, howeve, bought the efctoy linings into the tempetue zone whee these effects cn hve detimentl impct. Typicl coppe plte coole hot-fce tempetues t Impl e of the ode of 180ºC, while coppe coole cold-fce tempetues cn be s low s 45ºC. At pesent, the effects of hydtion nd sulphuous ttck on the efctoies e evident in es djcent to the cooling elements in both opeting funces t Impl. Sulphuous ttck of the coppe plte cooles is lso evident. This is, howeve, not mjo concen, s thee is no isk of wte enteing the funce s esult of this type of ttck, since ll cooles t Minpo e shllow-cooled. The need to conside ltentive efctoies fo coppecooled funces ws elized, nd viety of combintions hve been tested on the hot nd cold fces in sevel funce pnels. Two genel types of efctoies tested (luminchome nd mgnesi-chome) poved to be vey successful. The funce pnel tests wee lso confimed in lbotoyscle slg induction funces. The lumin-chome bicks esist dissolution in slg to highe tempetues thn ll the othe efctoy options, e immune to hydtion, nd esist sulphuous tmosphees when used bove 500ºC. These bicks e, howeve, moe expensive thn the mgnesi bicks, nd hve lowe theml conductivity. As such, the inteuption of wte supply to the cooles must be limited to pevent cceleted eosion in the slg zone. Mgnesi-chome bicks hve lwys been good cndidtes in non-feous cid slg pocesses, nd povide specific dvntges ove high mgnesi ltentives. Once gin, bette esistnce to slg dissolution is evident, nd the bicks e consideed to be esistnt to SO 2 ttck bove 1200ºC. The mgnesi content still mkes the bick pone to hydtion, limiting its use on the cold wll nd es djcent to coppe cooles. New funce instlltions t Impl will see move wy fom conventionl mgnesi efctoies, nd combintions of lumin-chome nd mgnesi-chome will be utilized on the hot nd cold fces. Tp-hole epis Pio to 1996, un-outs wee expeienced on egul bsis duing 700 mm dilling epis of mtte tp-blocks. This led to pocedul eview, with pticul ttention plced on the dill peption phse of the epi. The following chnges wee implemented to id sfe mintennce pctice. The cooling wte to the tp-hole block nd elevnt coppe coole is now fully opened, while the funce liquid level is dopped until slgging occus, nd the mtte eches n cceptble level. Immeditely fte chieving the equied mtte level, the hole is plugged with cly stoppe. Appoximtely 6 tons of evets (cold dope) is then dded diectly behind ech tp hole, nd the fist electode phse is isolted while the othe two phses e educed to 1 MW ech. 162 JANUARY 2006 VOLUME 106 NON-REFEREED PAPER The Jounl of The South Aficn Institute of Mining nd Metllugy

9 Common-sense impovements to electic smelting t Impl Pltinum The chnges to the peption pocedue hve educed the un-out fequency duing ny tp-hole epis to zeo. This hs educed the sfety isks considebly, nd hs enhnced poductivity by educing the time equied to dill to the equied depth fo the epi of tp-block. Peice-Smith convete chnges Opetionl chnges To cope with high chome peks in the funce feed, electodes e pushed down to ise bth tempetues nd ensue fluidity. The effects of highe funce mtte tempetues duing these times e seen in Peice-Smith convete opetion. The ections occuing duing the blowing of funce mtte in the convetes e exothemic, esulting in ise in bth tempetue. Unless contolled, though ddition of evets (cold dope), the high bth tempetues would esult in much incesed efctoy we. Funce mtte tempetues hve been seen to ise beyond 1355ºC t highe chome levels, comped to ppoximtely 1300ºC t noml levels. This esulted in eviewing blowing pocedues, nd hevy elince on evet ddition to limit efctoy we s well s bse-metl losses. Much emphsis ws subsequently plced on optimizing convete lives, nd significnt impovements wee obseved when utilizing lge quntities of cold dope immeditely fte mtte is poued into the convetes. This hs become the nom duing peiods when highe funce mtte tempetues e expeienced. The use nd mintennce of pyometes fo tempetue contol lso becme incesingly moe impotnt. Chnges in convete design Impl s unpecedented gowth necessitted the instlltion of two new convetes double the volume, nd double the blowing cpcity of the existing convetes. The two new 12 x 24 Peice-Smith instlltions, opeting with i flow tes of Nm3/h (though 32 tuyees with 50 mm dimete), wee to supplement the existing fou smlle convetes ech opeting t Nm3/h. The min eson fo exctly doubling the blowing te, convete cpcity nd mtte ldle cpcity ws tht the bsic blowing ecipe could be mintined, thus equiing miniml opeto tining nd poduction inteuption. Both the old nd the newe instlltions opete with Kennecott punching systems, nd e lined with mgnesichome efctoies. A numbe of chnges wee mde on the new instlltions to simplify opetion nd ensue mximum utiliztion. These chnges e biefly discussed below: A weighing system ws intoduced on the flux nd evet btching bins, to enble known quntities of flux nd evets to be dded. In ddition, feede fom the bin ws instlled with vible speed dive to enble contol of the flux ddition te. The min benefits tht the system chnges llowed fo wee: Enhnced contol of flux nd evet ddition, enbling moe consistent slg compositions Bette contol on bth tempetues, llowing fo finetuning with the te ddition. This hs become incesingly moe impotnt with cmpigns to impove convete lives, nd hndling the tempetue spin-off effects fom highe chome hndling. Impoved isle coodintion. The demnd on cne ctivities fo evet ddition being eliminted Impoved poduction tes, since convetes e not equied to tun out of stck fo evet ddition. Dive systems The new convetes wee instlled with electic dive motos fo noml opetion, nd i motos fo emegency conditions (e.g. duing powe filues). The i motos e essentil to contol the convete position duing powe outges o electic moto filue. The olde smll convete instlltions hd only i motos. The biggest design chnge ws the use of Bogi-flex dive consisting of spide, gebox, couplings, bkes, nd clutches. The Bogi-flex dive povided two min benefits when comped to the tditionl positioning t the bottom of the ing ge. The pimy benefit is tht the dive positioning on the side of the convete eliminted the isk of dmge duing bun-though o in the sitution of hot molten slg o mtte unning long the shell onto the dives. In ddition to this, the Bogi-flex dives wee ble to ccommodte the convete shell expnsion due to the exteme opeting tempetue nges, nd lso bending of the shell ove the long-tem. Adibtic dy-wll evpotive cooles Anothe mjo chnge fom the olde conveting systems ws the intoduction of dy-wll evpotive cooles fo ech of the new convete instlltions. The cooles wee intoduced to limit the finl gs tempetue deliveed to the cid plnt scubbe, thus limiting the need fo mjo chnges to the scubbe plnt nd ll ssocited uxiliies. The system is completely utomted, with contol diect fom the PLC SCADA, nd hs poven to be touble-fee system equiing miniml mintennce. The min es of we equiing monitoing e on the wte lnces. J o u n l P p e Automted flux nd evet ddition systems A new flux nd evet (cold dope) system ws incopoted with the upgde. The system consists of two utomted skips nd two btching bins fo hndling the evets nd the flux septely. Envionmentl chllenges Sulfcid instlltion The need to tet funce off-gses in septe pocess fom the convete off-gses, led to n in-depth eview of ll The Jounl of The South Aficn Institute of Mining nd Metllugy VOLUME 106 NON-REFEREED PAPER JANUARY

10 Common-sense impovements to electic smelting t Impl Pltinum vilble technologies. It ws essentil to decouple the two off-gs stems (funces nd convetes) to limit emissions in the cse of eithe pocess being offline. The tetment oute fo low stength SO 2 gs (<1% v/v), poved to be difficult execise, nd, lthough numbe of ltentives exist, mny e undesible options becuse of the by-poducts fomed. Keeping to common-sense ppoch in pocess selection, it ws decided to look t othe industies (outside of pltinum, nd mining in genel) to find the best-suited technology. Of ll the options consideed, the most sustinble nd ttctive pocess ws tht used in pint pigment industies of Euope, nmely the Sulfcid pocess. The technology mkes use of ctivted cbon nd spy systems in numbe of fibeglss ecto sets, llowing fo emovl efficiencies in excess of 80%. The min esons fo selection wee: Vey simple pocess, equiing miniml numbe of opetos nd mintennce stff Poven technology, extensively used in the pint pigment industy woldwide Modul plnt, cn be expnded to impove thoughput nd efficiency A sleble sulphuic cid poduct cn be sold ove the fence to n existing fetilize poduce Low cpitl expenditue, eected fo ppoximtely R65m Low opeting expenditue, only consumbles e the ctlyst, which equies eplcement evey 5 6 yes. Miniml pump instlltions, nd vey little ovell moving pts in the pocess. The Sulfcid pocess elies on exothemic convesion of SO 2 on n ctivted cbon ctlyst, nd sfe opetion is dependent on concenttion limits of sulphu dioxide nd sulphu tioxide fed to the plnt. The genetion of hot spots, though high loclized sulphu concenttions in the ctlyst bed, does pose fie hzd. This hs demnded tht the funces opete with enough blck top to educe feebod tempetues nd limit SO 3 fomtion to the Sulfcid plnt. The funce off-gs is futhemoe diluted with n mbient i intke, to limit the fomtion of possible hot-spots, nd emove ny pocess isk. The plnt hs been in opetion fo nely 5 yes, nd hs poven to be n excellent technology choice due to the vious dvntges it offes ove ltentive technologies. Emission eduction sttegies With sttegies imed t educing gseous nd pticulte emissions, focus hs been plced on pimy btement pocesses cuently instlled, in ode to deive mximum benefit fom these pocesses, befoe poceeding with costly secondy nd tetiy pocessing. This hs been, nd continues to be, n ongoing pocess t Impl, with phsed ppoch to minimize emissions in line with cuent legisltion to limit the effects on the envionment, employees, nd the suounding communities. In ddition to mjo cpcity upgde on the cid plnt, numbe of smlle cpitl inititives hve been instigted to educe fugitive gs emission by impoving the gs cptue mechnisms t souce. Ducting designs hve been evisited on both the convetes nd funces to ensue efficient gs tnspot, limit dust fll-out, nd mintin tempetue pofiles to void gs condenstion. Optimizing these pmetes is essentil to mintin good suction, limit downtime, nd ultimtely educe fugitive gs elese. A ingmin ducting ws instlled on the convete off-gs, to dd edundncy to the cicuit, enbling mintennce without jeopdizing poduction o emission contol. A numbe of smlle chnges wee mde in both the Acid nd Sulfcid plnts, imed t impoving thoughputs nd pocess efficiencies. Excellent esults hve been chieved with miniml cpitl investment. The esult of the inititives implemented, hs been lge eduction in the SO 2 nd pticulte emission te ove the pst few yes. This hs been ttibuted to the impoved cptue of gses t souce, nd vilbility of the btement pocesses, notwithstnding the lge incese in thoughput nd sulphu input. Conclusions This document ws compiled in n ttempt to biefly highlight nd discuss the mjo chnges mde to the Minel Pocesses smelte cicuit ove the pst 15 yes, in ode to cope with pocessing nd economic chllenges pesented. The 1992 document1 with simil title to this one (by Geoge Wtson nd Bin Hvey) stessed the impotnce of: Mtching the pocess to the poduct teted Mximizing use of vilble equipment Avoiding technologicl chnge unless essentil Appoching impovement though incementl lowisk modifictions Developing nd utilizing intenl expetise Teting the poblems t souce Keeping pocesses nd equipment s uncomplicted s possible. This ws Impl s ppoch in the Sixties, nd hs emined so until now. It is still the belief tht commonsense ppoch, bsed on expeience fo specific cicumstnces, is winning fomul, nd it is believed to be woking fo Impl. Refeences 1. WATSON, G.B. nd HARVEY, B.G. A common sense ppoch to pocess impovements to electic smelting of nickel-coppe concenttes t Impl Pltinum, Non-feous Pyometllugy: Tce Metls, Funce Pctices nd Enegy Efficiency, Poceedings of the Intentionl Symposium, R. Begmn et l. (eds.), The Metllugicl Society of the Cndin Institute of Mining, Metllugy nd Petoleum, JANUARY 2006 VOLUME 106 NON-REFEREED PAPER The Jounl of The South Aficn Institute of Mining nd Metllugy