Journal of Metals 45 (12):51-53 (1993) The Biological Leaching of an Auriferous Pyrite Ore Marja Riekkola-Vanhanen, Seppo Heimala, Carita Sivelä, Felipe Viguera, Irma Varjola, Seppo I. Niernelä, and Olli H. Tuovinen INTRODUCTION . The biological leachingof gold-containing refractory sulfide ores using acidophilic iron- and sulftrr-oxidizing bacteria (T/rroån cillis t'errooxiinns,T.thiooxitlans,f*pbspirithrnt t'errooxidatts) is usually amenable to concenträtes that contain pyrite (Fe$), aisenical pyrite, and arsenopyrite (FeAsS) as the main sulfide minerals.u The biological leach- ing process involves oxidative dissolution of the sulfide mineral matriceslhat would otherwise interfere with cyanide extraction. Pyrite and arsenopyrite do not directly react.with cyanide, but may physically maskthe contact of cyanide solution witir colloidal-size gold disseminåted in the matrix. Some reaction products (e.g., elemental sulfur) on mineral surfaces from pyrite and arsenopyrite oxidation incräase cyanide consumption and thereby. reduce the gold recovery. Economically, the bioiogical process-compares favorably with conventional methods of roastin§ for gold reiov- ery-l{ Therelationship between the amount of the Fe$- and FeAsS-iontent oxidizecl and thegold recovered bycyanidation is characteristic täeach ore.5.6 Usually, complete dissolution of the sulfideminerals is not required for high-efficiency gold cyanidaiion. The purpose of the work described here was to eval-uate the bi6lägicalieaching of a Sold-containing pyriteoresample. Thematerial was tested atdiffereit pulpdenslTies uslng shake flask and reactor leaching techniques. The results demonstrate that different leaching techniques may yield-contrasting estimates for gotd recovery. MATERIALS AND METHODS The ore tested contained pyrite as the predominant sulfide phase. Sphalerite (ZnS) ,"d llullgplrite(CuFesJ were-p^resentai minorconstituents. i'hesamplewas grountl \o 19).!Vo -75 gm and 88_,27, -32 pm size fraction. The composition (by weigirt) was 57.27os,44.7%Fe,7.277o2n,0.99%pb,0.64vocu,and 0.237o'As.Theore'conta'ined,on average,5.3 ppm Au and_40 ppm Ag. The gold recovery in previous tests was t+riby direct cyanidation and 4Zy. bi roasling ariä cyanidati6n. Two mixed cultures of aciäophilic-Fesr-oxidizing bacteria were used as test or- ganisms- The cultures were initially enriched froä two different mine sites anc-l designated as LASandSB/p-ll. The LAscultureand thepyriteorewere from thesame mine. TheSIl,i P-l I culture was originally enriched with a mixed sulfide black schist ore and subsequt'ntly adapted to grrxv wiih tlre pyrite ore sample as the substrate. Both mixed cultr.rre's contained iron-oxidizingfliibncittus ferrooxidans as a predominant lomponent. l'he cultures 1e_r9 grgyn in a mineral salls solution 13.0 mM (NH.)2SO., 2.? TM K2HL'oy and 1.6 mM Mgsoo.TH,ol that contained either ferrous sutraii(tzil mM) or-pyri(e ore (50-150 g/1, pulp-aeniity S-llyo) as a substrate. fne.lna.te fiask experim_ents_ (i80'rpm) wäre carried out at 29"C. The pulp densities varied betr.r'een 57o and 15v, (wt./voL). The stirred tank reactor experiments were carried out at 30oC- using-pulp densities betweenl0?o and lsTo.The working volunre was I I, with aeration at I l,/min. and stirring at 5fi) rpm. solids fronr tlre leaching experiments we.eiecore.eå by centrifugation, fouowed by wa9\ing with clistilled HrO änd ajld-ry-ing, F9r gol{ cyanidation] samptes (l g dry weight) of solids were sdspencled in fo rit of distilled water. Tlie suslrensioriwas adiusted.to pH 11.G.-11.5 with Ca(OH)r, followed by addition of 250 mg NaCN. The, ryanidation rvas carried out for l8 h wiih magnetic jtirring. The gold diåolved in the cyanide solrrtion was analyzed by graphite örnace-atomlc abso"rption spectrometry (GF-AAS). other dissolvea metäb-(Få, Cu, zn, As) were analyied byhame-aA§. soluble iron i. Ieach residues was analyzed by AAS after dissotuiion in ö.ozs N Hrso. for 30 min. at 20oC. RESULTS AND DISCUSSIOH Both the LAS and sB/P-II cultures reached a density of approximatelv 10e bacteria per milliliter Lrased on microscopic counts in shake fläst ldatning expe'riments. The microscopic counts areunderestimates because the fraction associited with pyriteand other.mineral particles could not be accounted for with this enumeration tähnique. Both test cultures were consortia of acidophilic bacteria, and no atte.npi*u, made to enumerate the nrajor groups such as iron-oxidizers by techniques baled on cultural t"coY"Y. Rot'l-shaped iron-oxidizing bacteria (7. ferroåxittans) were the main constittr- ents in both crrltures. 1993 Decembcr o JOM The oxidation of an auriferous pyite ore sample was aaluated in biological leaching experiments for subsequent gold recooery oia cyanidation. In batch cultures, organ- isms derfued from the mine site oxidized pyrite and ferrous iron at pHualues as low as pH 0 .6. The recoztery of gold was aaiable in chnlcc fl n <k pynprim cn lc I a c t i w,] in-L hi ^-- ' " -t"-'--'t' nctor lcaching, gold recooery toas propor- t_i9ry| t_o the extent ot' iron dissohdion by bioleaching.The leaching of arsenic t'rom tie snmpleuas also directly proportional to iron dissolution. > 800 E E 7OO c 3 600 o a x 5OO o E # roo o.5 r.o | .5 2.O 2-5 J.O pH Figure 1. The relationship between the pH and redox potential in sterile and inoculated pyrite suspensions. The data have been pooled lrom shake llask and stirred tank re- actor experiments, representing measure- rflents taken at difierent iime intervais. Ail cultures were adjusted to initial pH 1.8-2.5 at the beginning of lhe experiment. o 10 20. 30 40 50 Rotc (m9 Felt h) Figure 2. The relationship between the total rate and maximum calculated rate of pyrite dissolution measured as the release ol iron in solulion. The data are pooled lrom shake flask and bioreactor leaching experiments. t l60 ,lI I 40 E rzo 3 loo o _80 t .§ 60 §,0 20 0 5l § i. t.- "1.r. .. I r,.t.rj. .