Shichao Yu ,Rui Liao ,Baojun Yang ,Chaojun Fang ,Zhentang Wang ,Yuling Liu ,Baiqiang Wu ,Jun Wang ,,Guanzhou Qiu

1 School of Minerals Processing &Bioengineering,Central South University,Changsha 410083,China

2 Key Lab of Biohydrometallurgy of Ministry of Education,Changsha 410083,China

3 College of Chemistry and Chemical Engineering,Henan Polytechnic University,Jiaozuo 454000,China

4 Wanbao Mining Ltd.,Beijing 100053,China

Keywords:Chalcocite Bioleaching Heap leaching Kinetics Copper sulfides Hydrometallurgy

ABSTRACT There has been a strong interest in technologies suited for mining and processing of low-grade ores because of the rapid depletion of mineral resources in the world.In most cases,the extraction of copper from such raw materials is achieved by applying the leaching procedures.However,its low extraction efficiency and the long extraction period limit its large-scale commercial applications in copper recovery,even though bioleaching has been widely employed commercially for heap and dump bioleaching of secondary copper sulfide ores.Overcoming the technical challenges requires a better understanding of leaching kinetics and on-site microbial activities.Herein,this paper reviews the current status of main commercial biomining operations around the world,identifies factors that affect chalcocite dissolution both in chemical leaching and bioleaching,summarizes the related kinetic research,and concludes with a discussion of two on-site chalcocite heap leaching practices.Further,the challenges and innovations for the future development of chalcocite hydrometallurgy are presented in the end.

1.Introduction

Copper is an important metal used in many aspects of people’s lives,but there is an imbalance between its supply and demand[1].Medd [2,3] investigated the global copper resource data and reported that the estimated Cu mineral reserves and mineral resources increased with an increase in global copper production.However,the ore mineralogy is becoming more complex,and the ore grades for processing decrease gradually.Therefore,there is an urgent need for techniques capable of recovering copper from low-grade primary copper deposits and secondary copper resources(electronic and other copper-containing waste materials,scrap copper) [4,5].For low-grade copper resources,there are many difficulties in copper extraction through traditional pyrometallurgy methods.Hydrometallurgy is an efficient method for extracting the contained valuable minerals in low-grade copper resources [6,7].Various chemical and biological methods have been employed in copper hydrometallurgy,and the chemical processes can be classified into sulfate and chloride processes [8].Main challenge in the sulfate processes of copper sulfides is how to overcome incomplete leaching due to a large amount of elemental sulfur produced on mineral surfaces,which inhibits copper minerals’ wetting by the leaching liquid [9].The chlorides’ severe corrosion on equipment and its drawback in solvent extraction/-electrowinning(SE/EW)limit the application of chloride processes in copper hydrometallurgy industries [8,10].

1.1.Industrial practices for chalcocite hydrometallurgy

Chemical leaching and bioleaching are two main techniques employed in processing marginal copper sulfides.Based on these two techniques,heap leaching as a low-cost copper recovery method has been employed in many copper mines worldwide,especially in Chile,Australia,and America[11–14].The heap leaching of copper oxide ores by SE/EW has been established successfully and it has been applied further to the mixed oxides and chalcocite ores [15–17].The historical and current commercial copper heap leaching plants are listed in Tables 1 and 2.

Table 1 Main historical and current commercial chalcocite heap leaching plants

Table 2 Main historical and current commercial copper ores heap leaching plants

The permeability of heaps mainly consist of copper oxide ores must be considered because a large amount of insoluble substances is easy to be produced in acid leaching,which is different from alkaline leaching [49].Moreover,mines have adopted and implemented different strategies to mitigate the difficulties in leaching processing brought by compositions of primary ores.For instance,heaps’ height in Tuwu Copper Mine was 2–4 m and the percentage of fine ores was controlled to ＜8% to ease compaction of heaps,and sulfuric acid was used in Sarcheshmeh Copper Mine to extract copper from oxide ores in Iran.For acid leaching of the high-clay copper oxide ores from Yangla Copper Mine in China,a number of technological improvements,including antiscalant addition,ore washing,classified crushing and screening,thinlayer conveying and dumping,and mechanical/chemical activation,have been applied to improve heap permeability and copper extraction from low-grade copper oxides [16,50,51].In addition to the permeability problem,problems about leaching kinetics must be considered during copper sulfides’ heap leaching.

In 2014,Chile and Peru,as two big copper producers in the world,produced 42% and 33% of the worldwide copper through bioleaching,respectively [52,53].Pilot bioleaching plants have been well established worldwide,and most of them have similar optimal operating parameters,including moderate temperature usually below 50 °C,redox potential value controlled between 800 and 900 mV (vs.standard hydrogen electrode),total iron concentration of ＜10 g·L-1,and operating pH value of 1.5–2.0[54–57].However,these bioleaching practices cannot be directly employed in the Zijinshan copper mine of China.Currently,copper hydrometallurgy is not widely used in most copper mines of China,but the successful bioleaching practices in the Zijinshan mine can be a meaningful exploration of the efficient use of low-grade copper ores[25,58].The main copper sulfides with pyrite intimately associated in the Zijinshan mine are digenite,covellite,and enargite.The high content of pyrite(5.8%(mass))in the runs of mines causes the accumulation of acid and iron substances[59],which is a major challenge for copper extraction by bioleaching.The dominant growth of leaching bacteria was first regulated to depress pyrite dissolution,and iron oxidizer growth is inhibited while sulfur oxidizer growth is enhanced [60].However,this selective leaching effect is weakened by the continuous expansion of the scale and the lengthened production period.An effective means by controlling the potential was then established to guarantee the successful practice of selective leaching of different sulfides[61,62].Based on the semiconductor band theory,quantum mechanics have been used to accurately simulate mineral interface electron transfer.Combined with the kinetics of the electrochemical reaction under constant potential,the electrochemical control mechanism and synthetically dynamic model have been established for selective leaching of low-grade complex polymetallic sulfides [24,63].Currently,a characteristic copper bioleaching system in the Zijinshan mine has been established,where copper can be recovered efficiently at a low operating pH range (0.8–1.0),high concentration of total iron ions (＞50 g·L-1),high temperature (up to 60 °C),and with sulfur-oxidizing microorganisms dominant in heaps [58].

1.2.The existing problems in chalcocite hydrometallurgy practice

Watling summarized the primary reasons for the failure of sustained commercial copper production through copper sulfide hydrometallurgy [10].First,the poor copper production owing to the low kinetics of bioleaching limits the commercial-scale pilot application,among which kinetic problems are mostly centered at the widely discussed passivation problems in the bioleaching of copper sulphide [64].Second,there is no universal method for treating different primary copper ores from different mines.Other accompanied impurity valuable elements also affect copper sulfide dissolution,and it is difficult to regulate these parameters [65].Hydrometallurgy is low cost,less energy-intensive,and ecofriendliness and is beneficial to recovering metal from low-grade ores and several other industrial wastes.However,the two problems mentioned above offset the inherent advantages of hydrometallurgy [8].

The slow oxidation of chalcocite depends on the properties of local mineral ores and inhibiting secondary reaction products,some of which are passivating substances,which limits the contact of minerals with oxidants [66].These challenges can be divided into thermodynamic problems related to mineral crystalline structure and kinetic problems related to ion diffusion in the leaching process [10].

2.The Structure of Chalcocite

Chalcocite has a complex structure,although with a simple elemental composition.High and low chalcocites have been reported as the main chalcocite species [67].High-chalcocite and lowchalcocite are related by the transformation at 105°C.In high chalcocite,sulfur atoms exhibit hexagonal close packing [68],and below the transition point (105 °C),superstructure substances with an orthorhombic unit cell,which is known as lowchalcocite,have been reported by Buerger and Wuensch [69].Evans reported low-chalcocite structures and explored the structural relationship with another secondary phase,such as Cu1.96S(Fig.1).They reported bonding interactions between Cu atoms,which affect the stability of the chalcocite species [70,71].

In addition to the chemical bonds,the electronic structure of copper sulfide solids and leaching solution should be considered in analyzing the anodic electrochemistry of chalcocite [72].Chalcocite and covellite are p-type semiconductors with bandgap greater than 1 eV,therefore,minerals are dissolved in the anode because of the injection of holes in the valence band [73].In the chalcocite surface regions that come into contact with the Fe3+/Fe2+leaching liquid,free Cu+in the chalcocite crystal structure is oxidized to Cu2+by Fe3+and released into leaching solution.Then,S2-in the chalcocite crystal structure is oxidized and bonds with Cu+to form CuS,and elemental sulfur is not produced [74].However,there is a requirement to examine some semiconducting effects observed in laboratory results,and additional experimental studies should be conducted to confirm the dissolution mechanism of copper sulfides [75,76].

The peak position of Cu 2p3/2in chalcocite has been reported to be 932.9 eV,and the corresponding oxidation state of Cu is monovalent(Cu+)[77].Covellite is a kind of secondary mineral emerging in the chalcocite oxidation process.Nakai conducted a study by Xray photoelectron spectroscopy(XPS)and reported that Cu in covellite is monovalent [78].Sulfur element of chalcocite is detected by XPS as S2-,with S 2p3/2binding range of 160.2–163.8 eV,and S2-is gradually oxidized into,then covellite is formed byand immigrant Cu+in the chalcocite crystal structure [79].

Fig.1.The crystal structure of chalcocite [70].

3.Chemical Leaching of Chalcocite

3.1.Chemistry of leaching

The extraction rate of Cu from chalcocite using oxidants,such as ferric ions (Fe3+) and acid,is considerably higher than that from chalcopyrite.Chalcocite dissolution can be divided into two stages.The first stage rapidly proceeds with low activation energy (Eqs.(1),(3) and (5)),but the reaction of secondary substances (CuS and Cu1.2S) with oxidants is slow and incomplete (Eqs.(2),(4)and (6)).The reaction rate of the first stage can be controlled by the solid-state diffusion of Cu in the sulfide lattice and mass transport of Fe3+to the mineral surface,and that of the second stage is determined by the rate of charge transfer in the anodic decomposition process.The regeneration of Fe3+in Eq.(7) is the ratedetermining step that can accelerate copper sulfide dissolution,and this process can be efficiently enhanced by iron oxidizers for bioleaching.

Dixon classified dissolution into three types based on the Butler–Volmer equation [80,81].The first stage of chalcocite leaching is Type III leaching,and the second stage is a mixture of Type I leaching and Type II leaching.Reactions of Type III leaching are rapid and mainly controlled by the mass transfer of oxidants towards mineral surfaces.However,for Type I leaching,the exchange current densities of anodic and cathodic cells are similar;subsequently,the rate of leaching is proportional to the square root of the ferric ion concentration.In Type II leaching,the leaching rate is proportional to the square root of the ratio of Fe3+/Fe2+[82,83].

3.2.Sulfuric acid systems

In 1930,Sullivan [84] examined Cu extraction from chalcocite in sulfuric acid systems,and two primary leaching stages of chalcocite were determined.Chalcocite dissolution in the first stage is the transformation from Cu2S to CuS,and different copper sulfide intermediate products were detected in this stage,including Cu31S16,Cu7S4,and Cu8S5.The reactions in the second stage were CuS oxidation,and oxygen was the main acting oxidant of the sulfuric acid leaching system.Kinetic studies about acid leaching of chalcocite are investigated by Grizo [85].

3.3.Sulfuric-acid–ferric-sulfate systems

Based on sulfuric acid systems,ferric ions,as efficient oxidants,have been applied to chalcocite chemical leaching,and the kinetics have been extensively investigated [24,86,87].Both oxygen and ferric ions are available oxidants in sulfuric-acid–ferric-sulfate systems,and the main dissolution stages,as demonstrated in Section 3.1,are suitable.The first stage has first-order dependence on Fe3+concentration;therefore,it is faster than the second stage.The second stage is more sensitive to temperature and has halforder dependence on the ratio of ferric to ferrous concentrations[88,89].Kinetic modeling has shown that chalcocite dissolution in the first stage is controlled by the diffusion of oxidants from the solution to the mineral surfaces,whereas,in the second stage,it is controlled by the rates of both chemical reactions and diffusion.To summarize,widespread shrinking core models are used to describe chalcocite dissolution in sulfuric-acid–ferric-sulfate systems [24,87,89,90].

3.4.Hybrid–sulfate–chloride systems

Reaction kinetics in the hydrometallurgy of copper sulfide ores are accelerated with chloride substances in the leaching solutions.In a study focusing on the chalcopyrite concentrate electrochemical oxidation in a mixed chloride–sulfate electrolyte,the formation of sulfur products with a more porous structure was reported,and the formation of a metal-deficient sulfide was hindered in the initial leaching period [91,92].

Generally,chalcocite dissolution in a chloride leaching system proceeds in two steps.The first step is slow(the phase transformation from Cu2S to CuS),which is the same for both sulfate and chloride systems [93].However,copper complexis formed in chloride systems (Eq.(8)),during which Cu in chalcocite is extracted without undergoing the oxidation process from Cu+to Cu2+in sulfate systems,and the reaction rate of Eq.(8) was reported to be seventy times faster than Eq.(3)[94].However,contrary views on the role of chlorides in chalcocite leaching have been reported[95].Chen reported that the complex effect of chlorides with Cu ions is of secondary importance,but the promoted formation of amorphous sulfur products enhanced chalcocite dissolution kinetics [96].

The reaction rate in the first stage of chalcocite dissolution(Eq.(8))is directly proportional to the variations of chloride concentration and initial surface area,and it is controlled by chloride ion diffusion through the solution boundary layers to the chalcocite surface.The rate in the second stage of chalcocite dissolution(Eq.(9))is independent of the variations of hydrogen ion concentration,chloride concentration,and initial surface area,whereas it is controlled by the electron transfer step in the anodic dissolution reaction [94].The second stage of chalcocite dissolution can also be divided into two substages.The dissolution rate in the first substage is limited by surface chemical reactions;however,in the second substage,it is limited by both the surface chemical reaction and pore diffusion process[97].The apparent activation energy in this second leaching stage is 69.0 kJ·mol-1,which shows that the reaction in the second stage is under chemical reaction or mixed control[96].

In addition to the copper complexes formed in chloride systems,Fe complexes are considered in analyzing the reaction mechanism[98].Ferric chlorides are efficient oxidants;however,the reactions related to iron oxidants are much less reversible on chalcopyrite surfaces than pyrite.This is because secondary metal-deficient polysulfides on the surface slow the electron transfer and diffusion of oxidants.A mixture of Fe3+/Cu2+chlorides is an effective oxidant for chalcopyrite leaching,which favors the constant occurrence of two electrochemical reactions,including fast electron transfer from corroded chalcopyrite to Cu2+and the oxidation of Cu+by Fe3+[99].This chloride mixture can be also employed for chalcocite leaching [100].

Torres [101] used brine waste and manganese nodules from seawater desalination plants to effectively improve the leaching kinetics of chalcocite.Compared with seawater,waste brine showed a higher chloride concentration,and the contained low concentration of MnO2could improve the dissolution kinetics of chalcocite in a short time,which is important for continuous leaching operations.Under a high concentration of chloride,there was no requirement for a high concentration of H2SO4.

In summary,intensive refining and electrolysis are used to obtain high-purity cathode copper and these procedures require high-energy input.In particular,a large amount of liquid is to be processed in the solid–liquid separation process,which further limits the application of the process in chalcocite chloride leaching.Although the leaching kinetics is higher than that in sulfate systems,chlorides also increase the separation difficulty in EW at the same time.Through the comparison of various acid leaching of chalcocite at atmosphere pressure(Table 3),it can be concluded that sulfuric-acid–ferric-sulfate leaching systems can overcome the separation difficulty in EW and are suited for bioleaching systems using iron-oxidizing microorganisms.Therefore,they have more prospects in practical application.

Table 3 Summary and comparison on various acid leaching processes of chalcocite at atmosphere pressure

3.5.Electrochemical study on chalcocite oxidation

Studies on the electrochemical behaviors of chalcocite in different leaching mediums are required to verify the proposed dissolution mechanisms.Most electrochemical studies on chalcocite dissolution impose an external electrical press on the mineral anodes and detect the different electrical signals,through which the corresponding reaction paths under different conditions can be determined.

Arce[105]examined the electrochemical behavior of chalcocite in H2SO4systems via cyclic voltammetry.Chalcocite was gradually oxidized to release Cu2+in the leaching liquid.Moreover,intermediate products,including Cu1.92S,Cu1.60S,and CuS,were detected during the process,which was consistent with the leaching results.Liao[106] used electrochemical methods,including cyclic voltammetry,steady-state polarization,Tafel method,and X-ray photoelectric spectroscopy,to examine chalcocite oxidation and the electrochemical behaviors of the process in bacterial and sterile systems.The above mentioned results confirmed the two-step dissolution mechanism of chalcocite discussed in Section 3.3.The first oxidation reaction is the continuous oxidation of chalcocite to produce CuxS (1 ≤x ＜2),which are intermediate products of Cudeficient substances,and this stage can occur at a low potential.The second step is the oxidation of the intermediate product,CuS,which occurs at a high potential.Moreover,the reaction rate is low,and thus it is generally considered as the limiting step of chalcocite oxidation.To explore the reason for the slow kinetics in the second stage,Elsherief [107] recorded the anodic polarization curves of chalcocite in an H2SO4system,and reported that Cu2-xS produced in the first stage of chalcocite oxidation was more difficult to dissolve,thus resulting in the slow kinetics in the second step of chalcocite dissolution.Bolorunduro [82] detected the electrochemical steady-state polarization parameters of chalcocite dissolution in a sterile leaching system and reported that the passivation of chalcocite oxidation is attributed to the formation of insoluble CuS and S.Moreover,the higher oxidation reaction rate in the first stage is attributed to the active Cu atoms in the chalcocite crystal lattice;furthermore,the inert nature of Cu atoms in the intermediate CuS product results in a low dissolution rate in the second stage.

4.Bioleaching of Chalcocite

Sulfide bioleaching has been investigated in four aspects,including bioleaching microorganism communities,processoptions,bioleaching mechanism,and kinetics [108,109].Microorganisms,processes,and mechanisms involved in chalcocite bioleaching are discussed herein,and relative bioleaching kinetics will be discussed in Section 5.2.

The enhanced Cu extraction in bioleaching is primarily attributed to microorganisms naturally coexisting with copper sulfide.Most of them are acidophilic bacteria,which can live at pH below three,and they can oxidize ferrous ions or inorganic sulfur compounds.It has been reported that the main roles of the leaching microorganisms include acidolysis (the formation of organic or inorganic acids (protons)),complexolysis (the excretion of complexing agents),and redoxolysis (oxidation and reduction reactions) [110].Firstly,the formation of H2SO4by leaching microorganisms not only provides a suitable living condition for themselves but also accelerates chalcocite dissolution (Eqs.(3)and (4)),and the participation of Cu2+and Fe2+ions is inhibited.Secondly,leaching microorganisms can regulate leaching kinetics by affecting the Fe and S cycle in chalcocite dissolution.As discussed in Section 3.1,ferric ions are essential to the reactions in the first stage (Eq.(1));therefore,the accelerated circulation of Fe2+/Fe3+in chalcocite bioleaching is helpful for rapid copper extraction.For the second stage of chalcocite dissolution (Re (1)),which is controlled by a mix of oxidant diffusion and chemical reactions,the elimination of sulfur intermediates from mineral surfaces is more important than the elevated ferric ion concentration (Eq.(10)).Iron oxidizers,such as Acidithiobacillus ferrooxidans(A.ferrooxidans),and sulfur oxidizers,such as Acidithiobacillus caldus (A.caldus),which are commonly used for copper sulfide bioleaching,have been extensively investigated [111,112].

Most bioleaching plants are operated under conditions that are suitable for bioleaching microorganism growth;however,it is not beneficial to chalcocite dissolution [13,25,55].Increased temperature significantly enhances chalcocite and covellite dissolution,but most heaps in bioleaching plants remain cool because native thermophilic microorganisms are rare in high-temperature heaps.The Zijinshan copper bioleaching mine in China with high pyrite content is an exception.First,pyrite oxidation produces much heat in heaps[54].Second,the microorganism activity of iron oxidizers is reduced with the increase of temperature;accordingly,the content of direct oxidant Fe3+is reduced in a way,and only sulfur oxidizers are left to operate the bioleaching process [113].Finally,most acidophilic microorganisms used in mineral processing are inhibited because the leaching operation is kept at pH of ＜1.0[114].However,the Zijinshan mine still operates in good condition with high temperature,high ferric iron concentration,low pH value,and low microbial activity [58–60,111].Thus,researchers paid attention to the species of indigenous microbial cultures.Zou [115] conducted column bioleaching experiments and reported that the dissolution of low-grade copper ores (mainly contained digenite,covellite,chalcocite,pyrite,and alunite) at 60°C and pH 1.0 is chemically driven by Fe3+in absence of microorganisms,whereas the dissolution at 30 °C is enhanced by the high redox potential (Eh) owing to indigenous microbial cultures.Because of the small amount of acid-consuming gangue in the primary ores of the Zijinshan mine,aeration and inoculation are not necessary to enhance microbial activities in heaps.The absence of recirculation of leaching liquid causes Fe3+to participate as potassium jarosite at pH 1.7,which is just the mechanism of iron removal and control at Zijinshan copper heaps [58].

Bioleaching is the chemical leaching catalyzed by leaching microorganisms,and microorganisms participate in bioleaching in two ways:1) accelerated oxidation of ferrous to ferric ions(Eq.(7));2) accelerated oxidation of sulfur to sulfate,producing additional acid for bioleaching (Eq.(10)).In early studies,Sakaguchi [116] reported that microorganisms can directly oxidize Cu2S and CuS,thus producing sulfate rather than sulfur.However,most researchers disagreed with this opinion and state that secondary elemental sulfur is more important than sulfates in chalcocite bioleaching.The secondary reaction products,also called “passivation over-layers” have attracted considerable attention in chalcopyrite hydrometallurgy,and the passivation mechanisms are not clarified yet,“passivation over-layers” is reported to be the complex copper-rich layer comprising sulfides,polysulfides,or elemental.However,passivation in chalcocite bioleaching has been rarely studied,the complex copper-rich layer cannot be directly detected on surfaces of chalcocite dissolution,and only covellite-like copper sulfide and elemental sulfur can be detected.Cheng [117] examined digenite (Cu1.75S) and covellite bioleaching by A.ferrooxidans and A.caldus and reported that supplemental sulfur could improve copper extraction from Cu1.75S,but could not improve copper extraction from CuS,and large amounts of jarosite and elemental sulfur(S0)were clarified in chalcocite leaching residues.Cheng[118]reported that the produced S0is porous,and Niu[90]reported that raised temperature results in a more porous sulfur layer,which remarkably influences the ion diffusion rate in the second chalcocite dissolution stage.Therefore,the morphology of secondary sulfur in chalcocite bioleaching should be considered in the kinetics of chalcocite dissolution to clarify the reasons for slow kinetics during the second stage.

5.Kinetics

5.1.Kinetics in chemical leaching of chalcocite

The dissolution kinetics of chalcocite is discussed in two aspects.One aspect focuses on the first-stage dissolution of chalcocite,and the other analyzes covellite dissolution kinetics in the second stage.

Fe3+plays a different enhancing role in the two stages of chalcocite dissolution.In the first stage,the rate of chalcocite dissolution linearly depends on the Fe3+concentration,but when the Fe3+concentration exceeds 0.1 mol·L-1at 25 °C,the dependence is reduced[89].Bolorunduro[82]reported that the first-stage rate is first-order dependent on the Fe3+concentration when the Fe3+concentration is below 0.058 mol·L-1,and half-order dependent on the Fe3+concentration with a higher Fe3+concentration.The connection between chalcocite dissolution and all other leaching kinetic factors has been examined by kinetic models fitting,and the first stage reaction rate is demonstrated to be primarily controlled by diffusion [83,87,89,96,119,120],which mainly refers to the diffusion process of Fe3+to the chalcocite surfaces (Table 4).Accordingly,a low activation energy (4–25 kJ·mol-1) in this stage has been reported [122].

The first stage of chalcocite leaching is controlled by oxidant diffusion from the solution to the mineral surface,whereas there is a different rate-limiting step in the second stage of chalcocite leaching,and temperature is an important kinetic factor in the second stage [84],and the related kinetic researches are listed in Table 5.For the slow kinetics in the second stage,some studies have reported that chemical reactions control the reaction rate owing to the high activation energy (55–105 kJ·mol-1) in the second stage of chalcocite dissolution [25,124].However,other researchers have reported that the elemental sulfur layer produced in this stage hinders the diffusion of Cu+and oxidants,thereby blocking reactions [85,125],many studies have reported that the rate of the second stage varies with temperature,and the sulfur morphology is affected by the temperature [90].However,activation energy that related directly to temperature is not the only factor that determines the reaction rate in chemical reactions;and some other kinetic factors,including particle size and the redox potential of the systems must be considered.

Table 4 The kinetics of chalcocite leaching of the first stage

Table 5 The kinetics of chalcocite leaching of the second stage

The redox potential(Eh)of leaching systems with Fe2(SO4)3and H2SO4can be predicted and calculated using the Nernst equation(Eq.(11)).The Fe3+/Fe2+ratio affects the redox potential[126].During the initial period of the first stage,ferric ions are rapidly consumed,thus resulting in a sudden drop of Eh.In the second leaching stage of chalcocite,the reaction rate does not vary with Eh because of the slow reaction of Fe3+with CuS [127].There is a requirement to explore the kinetic impact of Eh on chalcocite chemical leaching,and more attention should be paid to the Eh maintaining.

pH values within an acid range have little effect on chalcocite leaching [96],but the formation of Fe3+hydrolysis and jarosite products is related to the leaching system acidity (Eq.(12))[128].Jarosite layers formed on a mineral surface are dense,thus preventing the transportation of electrons and reactants through the liquid–solid interfaces.Therefore,the acidity must be appropriately controlled to accelerate chalcocite dissolution.

The size of chalcocite particles influences only the first leaching stage,whereas the leaching rate in the second stage is independent of particle size.To evaluate the effect of particle size on the kinetics of the two stages,there are two additional points to be considered.First,the topological effect and particle-size distribution effect should be together discussed to ensure copper recovery kinetics[129].Second,newly generated cracks and pores on chalcocite particles increase as the leaching time increases,which will affect the initial particle-size effect and result in the contradiction with the kinetics prediction of the two leaching stages [82,125].

5.2.Kinetics in bioleaching of chalcocite

Chalcocite bioleaching is a complex process synergistically controlled by chemical,biological,and physical factors.Different kinetic models for plant chalcocite bioleaching have been established to predict and explain the mechanism.However,there is no general kinetic model that can be applied in most chalcocite bioleaching plant practices.

For interactions among bioleaching microorganisms,Leahy[130] developed a hydrodynamic model to simulate the bioleaching process and studied the interaction between mesophilic andmoderate thermophilic bacteria (MT) during the process.The proposed model predicted how bacterial species change in the two kinds of leaching fronts in heaps.Bottom-up and top-down leaching fronts appear,and it was found that the bottom-up leaching fronts proceeded faster with only mesophilic bacteria,whereas the top-down leaching front was slower.This was attributed to the high-temperature gradient caused by MT in the heap,which easily triggers the appearance of leaching fronts.These results demonstrated that adding MT extended the optimal leaching period before overheating occurred;therefore,there was a higher copper extraction in the initial stage.To sum up,changes in the environmental temperature and the inoculation methods of bioleaching microorganisms can regulate the bioleaching process.

Ogbonna[131]developed some mathematical methods to simulate the heap leaching process of chalcocite and explored the relationship between the particle size and kinetic processes,such as the diffusion process in bulk heap leaching.An agglomerate scale mathematical model was developed,which considered the particle-size distribution and factors in the large-volume heap leaching,and therefore a more accurate effective prediction for chalcocite bioleaching was realized.

The impact of Eh on chalcocite bioleaching differs from that in chemical leaching.Petersen[122]examined the bioleaching kinetics of chalcocite and reported that solution potential had a greater impact on the biological leaching of chalcocite.The first step of the leaching reaction was controlled by diffusion,during which a large amount of Fe3+was generated on the mineral surfaces and quickly consumed.The Eh value increased with the dissolution rate,as the Eh was at a high potential,the leaching rate in the second step was reduced and less affected by the potential.Therefore,it is recommended to increase the system potential in the first stage to speed up the overall Cu extraction.Li[132]also found that the early stage of chalcocite bioleaching is controlled by the Fe3+concentration,and the system potential increases quickly when the solution Fe3+concentration is high.However,in the later stage of bioleaching,a portion of Fe3+in the solution was transformed to jarosite by precipitation,which attached to the chalcocite surface and then hindered the dissolution process.

The discussion on jarosite during chalcocite bioleaching is relatively less than that in chalcopyrite because Fe is not originally contained in chalcocite crystal lattice.However,the formation process of jarosite and its interation with acidophilic microorganisms in chalcocite bioleaching is still worthy studying in detail,which is beneficial to explaining the hindered chalcocite dissolution in the late stage.

The composition of the passivation layer in chalcopyrite bioleaching has been discussed for many years,and jarosite has been adopted as the main passivation substance by many researchers[133–136].However,the morphology of the formed intermediate jarosite must be considered when referring to its passivation effect on chalcopyrite dissolution [66,137].Jarosite with a good crystal shape is detected to be coating copper mineral surfaces and blocking the contact with oxidants [112],however,in other cases,intermediate jarosite is dispersed and the morphology is loose and porous and thus will not hinder further dissolution of copper sulfides[66].This morphology differences depend on different experimental procedures in a sense,jarosite inclined to crystallize on mineral surfaces during bioleaching when the mineral was ground to the fine powder with a diameter less than 75 μm[138],while less jarosite or dispersed jarosite are reported in the experimental systems with large size minerals for the experiments[139].Although jarosite mineral has a relatively good crystal structure,it is not a type of stable secondary mineral phase[19],and the stability of jarosite can be easily affected by certain external factors including pH,ORP,natural organic matter (NOM),and acidophilic microorganisms in bioleaching[140,141].Acidophilic microorganisms can significantly accelerate Fe2+oxidation,thereby promoting the rapid crystallization of jarosite,and the crystallization of jarosite synthesized by acidophilic microorganisms is usually better than that by chemical methods [79,142,143].Therefore,a further investigation is to be required to tell if the intermediate jarosite on chalcocite bioleaching is the reason for passivation.

6.Selective Bioleaching of Chalcocite

Both the presence of pyrite and sulfur-oxidizing bacteria can enhance the leaching process of chalcocite.The external pyrite added into chalcocite bioleaching systems introduces an additional iron metabolism.Related redox transformation of iron in chalcocite systems can not only combine with the sulfur metabolism process systematically but also enhance the formation of EPS(Extracellular Polymeric Substances),which can mediate the “contact mechanism” process,and the copper extraction in the bioleaching process is finally increased by 44.8% [144].However,there are still disadvantages to the presence of iron metabolism in chalcocite dissolution.On the one hand,the weathering of pyrite under natural conditions results in the easy diffusion of heavy metal elements in the AMD (Acid Mine Drainage) environment and a discharge of acid mine wastewater [145].On the other hand,chalcocite surfaces are passivated by jarosite emerging in the leaching process,thereby hindering further chalcocite dissolution.For low-grade secondary copper ore with a high pyrite content,the potential of the leaching process should be controlled ＜600 mV (vs.Ag/AgCl)to achieve selective leaching of chalcocite,thus inhibiting the dissolution of pyrite.

Studies demonstrated that controlling the community composition of leaching bacteria and regulating the oxidation rate of Fe2+can affect the leaching of pyrite [146-148].The redox potential of the leaching solution highly affects the dissolution of pyrite;therefore,adjusting the oxygen content of the bioleaching process is a simple and effective method to control Eh.Chalcocite dissolution is primarily controlled by the diffusion step;hence,it is necessary to limit the growth of iron-oxidizing bacteria and promote the growth of sulfur-oxidizing bacteria to prevent the accumulation of elemental sulfur on mineral surfaces.Wu et al.[61] reported that when the oxidation–reduction potential is ≤760 mV(vs.standard hydrogen electrode),the selective leaching of chalcocite in the presence of pyrite can be achieved by controlling the amount of oxygen in the leaching system.Selective dissolution mechanism is raised based on the related first-principles analysis:The energy level of the leaching solution is higher than ideal pyrite but lower than the chalcocite,which leads to the accumulation of electrons on the surface of the pyrite and the formation of holes on the top of the chalcocite valence zone,thereby inhibiting the dissolution of the pyrite and promoting chalcocite dissolution [63,149].However,as the Fe2+ions are rapidly oxidized into Fe3+ions by ironoxidizers and then the dissolution of pyrite increases.Pyrite dissolution further provides more Fe2+for the growth of iron-oxidizers,and this will change the composition of microorganism communities in the bioleaching system.To summarize,chalcocite can be selectively oxidized,and the increase in solution potential will weaken this selective leaching behavior.

7.Heap Leaching Application in Myanmar

The Monywa copper mine in Myanmar is a large-scale and lowgrade secondary copper sulfide mine.The total amount of resourceful ores discovered is 2.1 billion tons,of which the amount of copper resource is 7.35 million tons,and the average copper grade is 0.35%.There are four mines,including Sabetaung (mine S),Sabetaung South(mine S South),Kyisintaung(mine K),and Letpadaung(mine L).Mine L is the largest among the four mining sections,accounting for～75% of the total resources,and most of the copper ores are chalcocite.The mining area is located in a region of tropical monsoon climate with relatively high temperatures throughout the year,which is suitable for bioleaching.Moreover,the annual rainfall in the region is only 720 mm,and the annual evaporation is 1800 mm.The climatic conditions are beneficial to not only leaching but also the treatment of acidic water.

Studies on copper extraction from S&K mines (Sabetaung and Kyisintaung) in Myanmar have been applied to the heap leaching of local pilots.On the one hand,those studies aim to strengthen recovery of the secondary copper from the historical stacks.On the other hand,there is a need to promote copper recovery from the current leaching stacks.Ore types affect the control of key engineering parameters (Table 6),and the optimized operating conditions for different types of ores in Myanmar have been investigated separately to achieve high copper efficiency.The program of S&K has been in operation for more than 10 years by Ivanhoe Mines Ltd.,and a large part of the copper ore piled under the surface unit is difficult to recycle directly with the underuse technology,this part of copper resource in S&K mines is called “stocked copper resource”.It is difficult to recover the stocked copper resource because the multi-layer heap leaching method has been used to extract copper.Then,the layer under the surface unit is not only the source of leached copper but also absorbs the copper leached from the upper layers,which greatly reduces Cu recovery during heap leaching practices.In order to strengthen the recovery of the stocked copper resource,relevant researchers first determined the correlation between the factors affecting copper leaching efficiency and ore types (Table 7).In brief,low pyrite content and poor permeability are characteristics of copper ores with a high proportion of clay (high-clay) and that with a medium proportion of clay (medium-clay),while the low-clay copper ore has a high pyrite content,good aeration and fast oxidation rate of pyrite.Therefore,for the medium-clay and high-clay copper ores,the permeability problem and the microbial oxidizing activity should be promoted to enhance pyrite oxidation,ensuring the balance of acid and iron ions in heaps.However,for the low-clay copper ores,the oxidation of pyrite needs to be appropriately inhibited to prevent the risk of excessive accumulation of acid and iron ions.

Table 6 Minerals composition of Kyisintaung (Mine K),and Letpadaung (Mine L) by Mineral Liberation Analyser (MLA)

Table 7 Optimized leaching factors for different types of ores

The following preliminary solutions have been proposed in S&K program,which is further optimized and adjusted in combination with the practical leaching progress:

(1) The permeability of copper ores is improved with a thorough turning process,and a high-concentration acid solution is used to leach the adsorbed copper.

(2) Huge ores are picked out and broken by hydraulic hammer to a size below 400 mm.All these ores are crushed by a mobile crushing machine and placed on the edge of the heaps for spray leaching.

(3) A kind of specific solution with a high concentration of acid and a low concentration of copper ions is used for spraying to promote the diffusion of leached copper in the heaps.

(4) Spray intensity is increased to promote the diffusion of the solution in the heaps.

Heap-leaching operations in another pilot (Letpadaung) of Myanmar differ from those in S&K mines.A large amount of sulfuric acid is needed to initiate mine L.Sulfuric acid cannot be purchased in the short term because its price in Myanmar is as high as$450·ton-1,and too much sulfuric acid cannot be stored in pilots.A study has proposed the use of water to realize the startup of heaps,and technological development in the research through theoretical analysis,laboratory tests.In the end,industrial tests are utilized properly to realize cathode copper production by heap leaching in mine L.

Studies have focused on revealing the oxidation mechanism,kinetics,and key parameters of pyrite oxidation in neutral,slightly acidic,and acidic environments,as well as the impact of other acidconsuming factors on pyrite oxidation.The water-spraying technology uses clean water to oxidize pyrite by O2under neutral and slightly acidic conditions and by leaching microorganisms and ferric ions under acidic conditions.It can be divided into three steps(Fig.2):(1)“Period 1” is the ore acidification stage.The ore is sprayed with clean water and O2is the main oxidizer.A part of the pyrite is oxidized to produce acid and iron and the pH of “Period 1” is contained under the value of three.(2)“Period 2” is the oxidation stage mainly by microbial organisms,in which enriched microbial organisms are adopted.(3) After the second stage,the pH of the leaching system is reduced (pH～2),and the microbial oxidation growth promotes the acid and iron production from pyrite,which will promote the oxidation rate of chalcocite and pyrite.To conclude,the chemical and biological oxidation mechanism of pyrite under neutral and slightly acidic conditions are theoretically revealed firstly,and then a method for judging the leaching feasibility of water-spraying technology is proposed in this program,and the combination of acidification and leaching of copper ores are realized successfully in the end.

Fig.2.Prototype of water-spraying technology.

8.Summary

Most studies on chalcocite biohydrometallurgy are aimed to prompt copper extraction from chalcocite,emphasizing how to enhance the oxidation kinetics of intermediate covellite-like substances (CuS) in the second stage of chalcocite dissolution.About the research of this field,we suggest two aspects should be considered in the following research.First,now that the low kinetics in the second stage is mainly attributed to the oxidants diffusion controlled by elemental sulfur,the attachment between multiple kinetic parameters and elemental sulfur morphology should be considered during the research in the future.Second,Sulfuric-aci d–ferric-sulfate and chloride-leaching systems are the main leaching media for chalcocite heap-leaching practices,and the sulfuricacid–ferric-sulfate system has been studied extensively.However,hybrid sulfate–chloride leaching of chalcocite can reduce the diffusion-hindering effect of elemental sulfur layers in chalcocite bioleaching effectively,in which condition both Fe3+/Fe2+and Cu2+/Cu+can easily cooperate with iron oxidizers to improve copper recovery efficiency.Nowadays,studies on the reaction mechanism of mixed-ligand complex species in hybrid sulfate–chloride leaching are insufficient,and the interaction between mixedligand complex species and leaching microorganisms needs to be explored.

Although several studies on the kinetics of chalcocite column bioleaching have been reported,there is insufficient knowledge of commercial heap leaching.This is because the control of parameters,such as temperature,aeration,and microbial communities,is more complex,therefore,the long period of heap leaching cannot be accurately controlled.To address all kinds of problems emerging from practical bioleaching,the fundamental dissolution mechanism of chalcocite first stage leaching should be clarified first.It is difficult to clarify the chalcocite dissolution kinetics in the first stage due to the fast kinetics of chalcocite oxidation and a natural high-purity chalcocite sample is hard to obtain.The CuxS component in natural chalcocite minerals must be considered when chalcocite dissolution kinetics are fitted with mathematic models.Additionally,the fundamental density functional theory is needed to clarify the oxidation mechanism of purified chalcocite to confirm the mechanisms of the two stages of chalcocite dissolution.

Besides focusing on the fundamental research of chalcocite bioleaching,the subsequent AMD pollution caused by sulfides bioleaching practice will also be paid attention to,which aims to protect the nearby living environment and extract value metal at most.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (U1932129,51774332,51934009 and 52004086),Natural Science Foundation of Hunan Province (No.2018JJ1041),Fundamental Research Funds for the Central Universities of Central South University (Nos.2021zzts0301 and 2021zzts0299),and authors of this manuscript.We truly appreciate the help provided by Shanghai,Hefei and Beijing Synchrotron Radiation Facilities.