1、The role of iron-oxidizing bacteria in stimulation or inhibition of chalcopyrite bioleachingAbstract：A series of bacterial and chemical leaching experiments were conducted to clarify contradictory reports in the literature regarding the role of bacteria in the bioleaching of chalcopyriteTests contai
2、ning a high bacterial concentration showed inhibited leaching,even lower than non-inoculated controls.However，when bacterial cells were washed before inoculation, it was apparent that it was not the bacterial cells but rather the chemical species introduced with them that influenced the leaching rat
3、eIn addition， the results of comparative tests with 0.1M ferrous sulphate or ferric sulphate showed that copper Was leached from the ore 2.7 times faster in leach solutions containing ferrous ion，suggesting that ferric ions inhibit chalcopyrite dissolutionThe results indicated that the chalcopyrite
4、dissolution rate is strongly dependent on the reduction potential E in solution， and that this parameter is far more influential than the number or activity of bacterial cellsThese Results imply that the role of bacteria may only be stimulatory when the prevailing electrochemical conditions are also
5、 favourableKeywords：Bioleaching Chalcopyrite Inoculation1Introduction Chalcopyrite(CuFeS2)is the most important copper bearing mineral in the world 1Unlike many other ores，chalcopyrite is known to be particularly recalcitrant to hydrometallurgical processesResearchers have been striving for decades
6、to understand the reasons for the slow dissolution of chalcopyrite in both chemical and biological leaching eactionsThe main problem hindering commercial application of biohydrometallurgical processing of chalcopyrite is the slow dissolution rateThe bioleaching rates of other copper sulphides such a
7、s covellite (CuS) and chalcocite (Cu2S)are relatively high in the presence of iron oxidising bacteria，since these minerals react favourably with ferric ion,the principal oxidant2 However，the solubilization rate of chalcopyrite in an oxidising medium is characteristically slow The parabolic kinetics
8、of chalcopyrite dissolution are thought to result from the formation of a tenaciouspassivating layerthat causes a dramaic decrease in the rates of copper release by preventing the diffusion of ions to and from the mineral surface36 The passivating layer and the conditions under which it is formed ha
9、ve been investigated, yet the nature of the layer has not been elucidated with certaintyIt has been reported consistently,and observed in the present study，that increasing concentrations of the conventional oxidising agent，ferric ion，do not result in a corresponding increase in the leaching rates of
10、 chalcopyrite 4,7-9In fact，it has been reported that the addition of ferric sulphate depresses chalcopyrite leaching rates 10Interestingly，in recent studies，it has been observed that ferrous ions enhanced chalcopyrite leaching rates by an order of magnitude compared with ferric ion 8,10However, from
11、 a theoretical point of view,an oxidizing agent is needed because the non-oxidative dissolution of chalcopyrite can not be sustained on thermodynamic groundsIn continuing efforts to accelerate the dissolution of chalcopyrite，attention has been focused on the use of bacteria to catalyse the process 8
12、,11-13However，the role of bacteria in the bioleaching of chalcopyrite has been a subject of much controversy，with different authors concluding that their presence either enhances leaching 12，has no effect9or is detrimental to leaching8,10For other mineral sulphide systems,such as pyrite，bacteria are
13、 thought to contribute to bioleaching processes by both direct and indirect mechanismsIn the indirect mechanism,bacteria produce highly oxidising conditions through their ability to oxidise ferrous ion to ferric ion (Eq.(1)normally an effective oxidising agent13The direct mechanism of bacterial atta
14、ck involves their ability to metabolise the sulphide portion of the mineral，possibly by oxidising the sulphide but more likely elemental sulphur particles (Eq.(2). Thus,in the case of other sulphide minerals, the role of the bacteria is clearThey accelerate both the chemical oxidation of the mineral
15、 by ferric ion and the oxidation of sulphides or elemental sulphur to sulphateHowever, since it has been observed on several occasions that ferric ion may have a detrimental effect on the dissolution of chalcopyrite 8,10，it could be asked what is the role of the bacteria in the dissolution of chalco
16、pyrite？ The research presented here is intended to provide a better understanding of the biological and chemical mechanisms of chalcopyrite bioleachingThe effects of increasing the bacterial cell concentration and the individual chemical effects of ferrous and ferricion in chalcopyrite leaching solutions were investigated2Experimental procedures2.1MicroorganismsA mixed culture of mesophilic acidophilic bacteria, enri
