Ï¡ÓнðÊô (Ó¢ÎÄ°æ) 2013,32(05),524-531

Feasibility study on heap bioleaching of chalcopyrite

National Engineering Laboratory of Biohydrometallurgy,General Research Institute for Nonferrous Metals

Õª Òª£º

Bioleaching of chalcopyrite often encountered the formation of passivation layer, which inhibited the leaching process and resulted in a low leaching rate.This inhibitory effect can be eliminated by thermophilic bioleaching.The industrial test of BioCOP technology based on thermophiles was successfully completed, which confirmed the feasibility of chalcopyrite bioleaching.However, industrial leaching rate of chalcopyrite heap bioleaching is lower.This paper described the development status and industrial test of chalcopyrite heap bioleaching technology.The reasons for the lower efficiency of chalcopyrite heap bioleaching were analyzed.The strategies for successful chalcopyrite heap bioleaching were proposed.

ÊÕ¸åÈÕÆÚ£º23 January 2013

»ù½ð£ºsupported by the National High Technology Research and Development Program (Nos. 2012AA061501, 2012AA061502);the National Natural Science Foundation of China (No. 50934002);

Feasibility study on heap bioleaching of chalcopyrite

Bo-Wei Chen Jian-Kang

National Engineering Laboratory of Biohydrometallurgy, General Research Institute for Nonferrous Metals

Abstract£º

Bioleaching of chalcopyrite often encountered the formation of passivation layer, which inhibited the leaching process and resulted in a low leaching rate. This inhibitory effect can be eliminated by thermophilic bioleaching. The industrial test of BioCOP technology based on thermophiles was successfully completed, which confirmed the feasibility of chalcopyrite bioleaching. However, industrial leaching rate of chalcopyrite heap bioleaching is lower. This paper described the development status and industrial test of chalcopyrite heap bioleaching technology. The reasons for the lower efficiency of chalcopyrite heap bioleaching were analyzed. The strategies for successful chalcopyrite heap bioleaching were proposed.

Keyword£º

Chalcopyrite; Heap bioleaching; Thermophiles; Pyrite; Feasibility;

Author£º Bo-Wei Chen e-mail: biohydrometallurgy@163.com;

Received£º 23 January 2013

1 Introduction

The world¡¯s copper resources are mainly chalcopyrite which accounts for 70%of the world¡¯s copper reserves[1].In China, the sulfide ores account for 87%of the copper resources, of which 90%are primary copper sulfide ores (mainly chalcopyrite) [2].Currently, the main technology for treatment of chalcopyrite is flotation-pyrometallurgy, but with the decrease of the high-grade copper ore resources, large quantities of low-grade copper ores o\0.4%cannot be economically utilized with existing technologies.

In recent years, biohydrometallurgy technology for heap bioleaching of low-grade secondary copper sulfide ores is industrialized, but heap bioleaching of chalcopyrite is still in the research stage[3].This is mainly because that chalcopyrite has a lattice energy of 17, 500 k J?mol^-1, which is five times higher than that of secondary copper sulfide ores, such as chalcocite, covellite, thus the oxidative dissolution of chalcopyrite requires higher energy consumption[4].Moreover, the use of mesophiles and moderate thermophiles for chalcopyrite bioleaching can form a passivation film on mineral surface, thereby impeding the further dissolution of chalcopyrite[5].The main view points for the passivation film are jarosite layer, sulfur layer, and intermediate sulfur product layer (polysulfide) , but there is no uniform understanding for the main kind of layer in academia circle[6, 7].In order to eliminate the passivation film during chalcopyrite bioleaching, researchers developed such technologies as silver ion catalyzed bioleaching, bioleaching under low potential and extremely thermophiles bioleaching.The first two technologies can improve the leaching efficiency, but owing to cost and engineering difficulties, there is no successful industrial application.From 1995, BHP Billiton started agitation leaching of chalcopyrite concentrate and successfully developed the Bio COP^TMtechnology based on the BIOX^?process.In 2003, an annual output of 20, 000 t of copper cathode plant was built with this technology in Codelco mine, Chile[8].Extremely thermophiles were used to leach chalcopyrite concentrate (Cu 33%, S 35%, As 4.5%) with temperature of 78¨C80¡ãC, and the copper leaching rate reached 95%in a leaching cycle of7¨C10 days.The successful operation of this plant confirmed that the leaching of chalcopyrite with extremely thermophiles (high temperature leaching) was feasible.However, owing to the need of corrosion resistance equipment for agitation leaching and high cost of energy consumption, investment and operation, the plant was once shut down.Heap bioleaching of secondary copper sulfide ores shows large advantages in the investment and operating costs, therefore, researchers work on heap bioleaching of chalcopyrite.

In this paper, in order to give some inspirations for industrial heap bioleaching of chalcopyrite, feasibility study is conducted from bacteria, ore characteristics and process conditions based on the literature review and our research results.

2 Technology development status for heap bioleaching of chalcopyrite

Currently, some companies have developed inoculation and temperature control techniques suitable for chalcopyrite heap bioleaching, which are used in pilot plant researches.

2.1 Geocoatüà

Geocoat, developed by Geo Biotics in USA, is a heap leaching technology using flotation concentrates.In this process, flotation concentrate was coated onto crushed and size-sorted support rock and the coated material was stacked for heap leaching[9].This technology combined the high recoveries of tank processes with the low costs of heap-based processes and was used for Agnes Gold Mine, South Africa, with daily capacity of 4, 400 tons ore.In the area of chalcopyrite bioleaching, column leaching test was completed.

2.2 GeoleachTM

Geoleach was invented by Geo Biotics.The original intention of the development of the technology was that a lot of heat would be generated during heap bioleaching of sulfide ores, causing the temperature elevation in the heap[9].However, in practice, heap temperature could rise too high due to improper operation or lack of temperature control.Geoleach was designed to maximize heat conservation through careful control of aeration and irrigation rates.

A demonstration plant based on this technology was established in Quebrada Blanca, Chile, using mesophiles, moderate thermophiles and extremely thermophiles[10].The copper leaching rate increased to 91%, which was70%before technology applied.

2.3 Hot HeapTM

Hot Heap, developed by Geo Biotics, is an operation and control technology to insure the heap temperature for raw ore or flotation concentrate[9].The heap temperature during heap bioleaching is decided by the ambient temperature, spray, evaporation, convection, aeration, and oxidation reactions, which only can be regulated through spray and aeration.And this is just the core of Hot Heap adjustment technology.In the early stages of the heap leaching, owing to lower levels of biological oxidations, heat generated by bacterial sulfide oxidations was very important and heap temperature was regulated by the oxygen demand of bacteria.When the heap temperature reached operating temperature, then heap temperature was maintained by controlling evaporation.In the later stage of heap leaching, biological oxidations slowed down and temperature was controlled again by the oxygen demand of bacteria.

2.4 Bio Proüà

Bio Pro, a pre-inoculation technology for heap leaching, was developed by Newmont Gold Company[11].Cultured bioleaching bacteria was directly sprayed on the surface of heap during traditional heap bioleaching, while cultured bacteria was mixed with ore before stacking heaps in Bio Pro technology, which could insure that the bacteria were uniformly distributed within the heap, shortening the period of stagnation and leaching cycle, thus accelerating the initial biological oxidation process.

Currently, this technology was used in the heap biooxidation of refractory gold ores in Gold Quarry by Newmont, which could be a reference for heap bioleaching of chalcopyrite.

2.5 Smart ColumnTMand Heap Starüà

Smart Column is a column leaching device designed by Mintek, which can simulate temperature changes when actual heap leaching[12].Ideally, a laboratory column should represent a cylinder of ore away from the sides of a large commercial heap, where the ore is well insulated on all sides by material of similar temperature which also generates its own heat, thereby eliminating sideways conduction.At present, the heap leaching process becomes more and more complex, the parameters that need to be included in the design and operation of heap leach plants are growing in number, and the amount of data to be collected and processed, and the number of decisions to be taken daily, are also increasing, so Mintek development Heap Star management and consultation software, which can be a guidance system to insure that the correct method implemented in different stages of the leaching process[12].Under the Smart Column concept, BHP Billiton built a6 m high column leaching device with loading capacity of7 tons ore.In the test, an average ore temperature of 70¡ãC was achieved and the copper leaching rate was 75%in about 280 days[13].

3 Industrial test status of chalcopyrite heap bioleaching technology

3.1 Escondida Copper Mine, Chile

Escondida Copper Mine, located in 170 km southeast of Antofagasta, Chile, with an altitude of 3, 100 m, is the largest copper mine in the world.The main copper sulfide minerals of Escondida copper mine are chalcopyrite, chalcocite, covellite, and digenite.The mineral deposit has a total of 2.88 billion tons ore with copper grade below 1.5%for bioleaching, of which 2.56 billion tons ore are sulfide ore with copper grade of 0.3%¨C0.7%, others are oxide and sulfide mixed ore.The heap leaching field in Escondida copper mine is the largest in the world with a length of4.9 km, width of 2 km, height of 126 m in seven layers, each of 18 m.The plant has a copper leaching rate of 50% (sulfide ore leaching rate of 30%¨C35%) in 250 days, with annual output of 180, 000 t copper cathodes[3].

3.2 Sarcheshmeh copper mine, Iran

Sarcheshmeh copper mine, considered to be the second largest copper deposit worldwide, is located in the Kerman province of Iran.The region¡¯s altitude averages about2, 600 m.The mineral deposit has 1.2 billion tons ore with average grade of 0.70%copper and 0.03%molybdenum.In 2005, Sarcheshmeh cooperated with Mintek and built a test heap of 20, 000 t of ore with height of 6 m and particle size\25 mm.Fifty three percent of the copper is present in the form of chalcopyrite.Copper leaching rate reached60%in 200¨C300 days with mesophiles and extremely thermophiles and the maximum heap temperature was55¡ãC[14].

3.3 Mt Sholl nickel¨Ccopper mine, Australia

The Mt Sholl ore body is located in Pilbar region of Western Australia.Sulfide minerals account for about15%of the total mineral, which are mainly pyrrhotite, chalcopyrite and pentlandite.The ore contains 0.92%Cu, 0.67%Ni, 4.10%S and 11.10%Fe.A pilot heap of5, 000 tons ores with height of 5 m and particle size of7.5 mm was built using technology by Pacific Ore.The average heap temperature is about 50¡ãC.The leaching rate of more than 50%copper and 90%nickel were reached in400 days[15].

3.4 Dexing copper mine, China

Dexing copper mine, located in Jiangxi Province, China, is one of the world¡¯s largest porphyry copper mines.In 1997, a dump heap leaching plant was built in Dexing copper mine.The plant has an annual processing capacity of 18million tons ore with copper grade of 0.09%.The annual output of copper cathodes is 1, 500 t and copper leaching rate is only 9%.In 2008, a segmentation leaching technology using mesophiles, moderately thermophiles and extremely thermophiles for waste rock of Dexing copper mine was conducted by General Research Institute for Nonferrous Metals and heap leaching pilot test of10, 000 tons ore with 0.12%Cu and 2.32%S was completed.The maximum heap temperature reached 55¡ãC and annual copper leaching rate increased to more than 20%[16].

3.5 Dahongshan copper mine, China

A 2 t ore heap bioleaching test for Dahongshan high iron low-grade copper mine was conducted by Kunming Metallurgical Research Institute.The ore has an average of0.75%Cu, 0.79%S, 28.99%Fe and 65.33%of copper is chalcopyrite.After 120 days leaching, copper leaching rate was 14.2%[17].

From the above mentioned five cases on chalcopyrite heap bioleaching, it can be summarized that copper leaching rate is\35%at production plant and\60%at pilot plant.Heap temperature is basically 55¡ãC and sometimes can reach more than 70¡ãC.Generally speaking, the heap leaching rate of chalcopyrite is lower than that of secondary copper sulfide ores.

4 Feasibility study on heap bioleaching of chalcopyrite

At present, a lot of research work was done on chalcopyrite bioleaching, but industrial applications of chalcopyrite heap bioleaching has not yet got breakthrough.The most fundamental reason for this is that the inner heap environment fails to achieve the optimum growth conditions for extremely thermophiles, of which temperature is the fundamental condition.This means that in order to obtain a higher leaching rate, chalcopyrite heap bioleaching firstly has to be a high temperature heap leaching.Can chalcopyrite heap bioleaching or high temperature heap leaching be achieved?Two cases of high temperature heap bioleaching can be referenced.

4.1 Heap bioleaching of secondary copper sulfide ores, Zijinshan, China

Zijinshan copper mine is located in Shanghang county, Fujian province, China.On December 2005, an annual production of 10, 000 t of high purity copper cathode of heap bioleaching plant was established at the mine.The ore has an average of 0.39%Cu, 2.6%S and metal sulfide is mainly present as pyrite, which accounts for 5.80%of the overall minerals[18].The ore particle size for heap leaching is-30 mm.At the beginning of leaching, mesophiles were inoculated.The heap temperature reached60¡ãC through microbial oxidation and heat release, but extremely thermophiles were not found by molecular biological methods.

4.2 Heap bioleaching of nickel¨Ccobalt¨Ccopper sulfide ores, Talvivaara, Finland

Talvivaara mine, located in east of Finland, is one of the biggest metal mine in the world.The ore has a deposit of one billion tons of ore with 0.23%Ni, 0.13%Cu, 0.02%Co, 0.50%Zn and 8.40%S.Sulfide minerals account for 22%of the overall minerals, of which pyrrhotite is more than50%.The ore particle size is 80%passing 8 mm.When leaching, no inoculation is needed, but the heap temperature varied from 30 to 90¡ãC regardless of the cold weather.The output in 2011 was 16, 087 t Ni and 31, 815 t Zn[19].

In the two cases, the initial leaching temperature is atmospheric temperature, but the heap temperature can reach 60 and 90¡ãC separately.It can be seen that, based on the nature of the ore with higher levels of sulfur, the higher heap temperature can be achieved.On the other hand, the two cases confirmed that high temperature heap bioleaching can be achieved.

From the previous analysis, it can be seen that bioleaching of chalcopyrite using extremely thermophiles and high temperature heap leaching were feasible.However, the existing chalcopyrite heap bioleaching plant had a low leaching rate.The three main reasons are:heap temperature fails to reach the optimum temperature of the extremely thermophiles;the inoculated extremely thermophiles fail to play a role or the inoculation strategies have problems;insufficient supply of physical and chemical conditions in the heap.The three reasons can be attributed to two factors:microbiological factor and mineralogical factor.

4.3 Microbiological factor

Owing to its unique nature, extremely thermophiles were mostly isolated from hot springs or deep sea vent and did not exist in heap bioleaching environments because of its lower temperature.Therefore, heap bioleaching of chalcopyrite needs to be inoculated, but if the temperature, physical and chemical conditions are improper, inoculated extremely thermophiles would disappear from the leaching environment.

Table 1 shows the changes of microbial communities during column bioleaching of chalcopyrite at ambient temperature.At the initial stage, equivalent of eight kinds of microorganisms were inoculated.It can be seen that although extremely thermophile Acidianus was inoculated in early leaching stage, L.ferriphilum and A.caldus became dominant at later stages[16].In another paper, equivalent of11 kinds of microorganisms (4 kinds of extremely thermophiles, 4 kinds of moderately thermophiles and 3 kinds of mesophiles) were inoculated during chalcopyrite bioleaching at different temperatures.At the end of leaching, moderately thermophiles were dominant at 35 and 45¡ãC and the composition of microorganisms was similar.Moderately thermophiles and extremely thermophiles were dominant at 55¡ãC, whereas extremely thermophiles only exist at 65¡ãC[20].

Table 2 shows the optimum and range of growth for p H and temperature of extremely thermophiles[21].It can be seen that the difference among the range of growth temperature for extremely thermophiles is large and the minimum temperature is between 50 and 55¡ãC and the optimum temperature is over 65¡ãC and closer to the maximum temperature.Therefore, in order to achieve high temperature heap leaching, the heap temperature should be above 50¡ãCwhile the temperature should be controlled above 65¡ãC to make the inoculated extremely thermophiles effect.

Table 1 Changes of microbial communities during column bioleaching of chalcopyrite at ambient temperature  ÏÂÔØԭͼ

Table 1 Changes of microbial communities during column bioleaching of chalcopyrite at ambient temperature

Table 2 Optimum and range of growth for p H and temperature of extremely thermophiles  ÏÂÔØԭͼ

Table 2 Optimum and range of growth for p H and temperature of extremely thermophiles

Table 3 Physiological properties of extremely thermophiles  ÏÂÔØԭͼ

MS-metal sulfides other than pyrite, A-autotroph, F-facultative autotroph and/or mixotroph, na-data not available

Table 3 Physiological properties of extremely thermophiles

Table 3 shows the physiological properties of extremely thermophiles[21].It can be seen that most of the extremely thermophiles are heterotroph.Yeast extract is usually added when isolation and cultivation of extremely thermophiles, which is facilitated for extremely thermophiles, but inhibitory for mesophiles[22].In addition, extremely thermophiles have the ability of oxidation of Fe²⁺, sulfur and metal sulfides, which is beneficial for the growth in leaching environments.

Optimum temperatures for the growth of extremely thermophiles did not always mean higher copper extraction yields, which was correlated with p H, ORP, concentration of Fe²⁺and Fe³⁺of the system and the conditions for the highest copper extraction yields of different thermophiles was different[23].With Sulfolobus metallicus, maximum copper extraction was obtained at 70 (p H 2.0 and 2.5) or80¡ãC (p H 1.5) .With Metallosphaera sedula, the maximum yield was obtained at 75 (p H 1.5 and 2.5) or 80¡ãC (p H2.0) .In contrast, with Acidianus brierleyi at all tested initial p H values (1.5, 2.0, and 2.5) , the maximum yield was obtained at 75¡ãC.The conditions for the maximum copper extraction yields of the three extremely thermophiles were different from their optimum temperature and p H.Therefore, the realization of high temperature heap bioleaching of chalcopyrite was relevant with temperature, p H, ORP, Fe³⁺, Fe²⁺, CO₂, O₂and the inoculated extremely thermophiles, while temperature is the most critical and fundamental factor, which determines the presence or absence of the extremely thermophiles and also determines whether it is the high temperature heap leaching or not.

4.4 Mineralogical factor

The heat in heap bioleaching process mainly comes from the oxidation of sulfide minerals by bacteria.Therefore, certain conditions should be satisfied for a heap temperature over 65¡ãC.

Table 4 shows the oxidation and heat release of different sulfide minerals, almost including all the main metal sulfide minerals of copper sulfide ores[24].If the detailed mineralogy of a certain chalcopyrite ore is available, the released heat by completed oxidation of sulfide minerals per kilogram of ore can be calculated based on the content of each mineral.

For example, a test heap of 5, 000 t ores with bulk density of 1.7 t.m^-3and moisture content of 5%, the ambient temperature is 20¡ãC and the heat required to heat the air in the heap is ignored[25].If the ore contains 1%Cu Fe S₂and 3%Fe S₂, which means that the grade of Cu and S is 0.35 and 1.95%, the heat released of completed oxidation of 1 kg ores is 473 k J and completed oxidation of 5, 000 t ore can release 2.37 9 10⁹k J.However, the heating of the whole heap to 65¡ãC need energy of2.72 9 10⁸k J, which including:

Table 4 Oxidation and heat release of different sulfide minerals  ÏÂÔØԭͼ

Table 4 Oxidation and heat release of different sulfide minerals

It can be seen that the heat generated by the completed oxidation of sulfide ore is 8.7 times of that needed to heat the heap temperature to 65¡ãC, but this is an ideal situation, without considering the heat loss caused by the spray and evaporation during heap leaching process, also without considering the actual oxidation rate of sulfide minerals, such as sulfur oxidation rate of 20%[12].The heap temperature can be basically heated to 65¡ãC through the oxidation of sulfide ore, but if encountering special environment conditions, it will no longer be suitable.Petersen[26]proposed pyrite content of5%, while Brierley and coworkers[11]advised sulfur content in pyrite of 1.8%, equal to pyrite content of 3.375%, and a heap temperature of 60¨C70¡ãC can be reached.Regarding of this, our recommendation is that for calculation of sulfur content, local environmental conditions, heap leaching process conditions, the morphology of the presence of sulfur and theoretical sulfur oxidation rate under the designed copper leaching rate should be taken into account, but a sulfur content of 2%is at least.

5 Strategies for heap bioleaching of chalcopyrite

From the previous discussion, it can be seen that chalcopyrite heap bioleaching is entirely feasible in theory and some practical work had been done.But there are still many difficulties.To achieve chalcopyrite heap bioleaching, it is recommended to research in the following areas:

(1) Temperature:temperature is the first element to insure the growth and role of extremely thermophiles.When sulfur content of the ore is low and heap temperature cannot be heated to more than 65¡ãC under complete oxidization of the ore, then it is necessary to provide the heat source by addition of a sulfur source, such as S, Fe S₂, Fe_1-xS.If sulfur source is not available, solar installations can be used to heat the heap.Heap temperature can be adjusted by controlling the aeration rate and spray rate except the external factors.

(2) Bacteria:breeding of efficient bioleaching bacteria is the eternal task of biohydrometallurgy.For chalcopyrite heap bioleaching, not only efficient extremely thermophiles are needed, but also efficient mesophiles and moderately thermophiles are needed in order to enhance the oxidation of sulfide ores and shorten the time of heating the heap temperature to 65¡ãC.

(3) Energy source:energy substances like O₂, CO₂and yeast extract are needed for the growth of extremely thermophiles.Large amount of oxygen will be consumed during the oxidation of sulfide minerals, for example, the complete oxidation of 1 kg Cu Fe S₂and1 kg Fe S₂requires 0.74 and 1 kg O₂separately.It is unable to satisfy the mineral oxidation only by the air existence in the heap.In addition, as most extremely thermophiles are facultative autotroph, good conditions can be provided for the growth of extremely thermophiles by supply extra CO₂and yeast extract.

(4) Inoculation:currently, large-scale cultivated bacteria are directly sprayed to the surface of heap.The disadvantage of this approach is that the heterogeneity of inoculation with some area is not sprayed.The Bio Pro technology solved this problem, but the existence problem is still that whether the inoculated bacteria can adapt to temperature changes in the heap or not.Therefore, bacteria can be inoculated partly by monitoring changes of heap temperature to find out the temperature gradient, which means the bacteria of desired temperature were inoculated according to the heap temperature.

(5) Heat preservation:usually, raffinate is used to spray during heap bioleaching, which has passed multiple processes before spraying into the heap and the temperature is reduced.Therefore, raffinate can be heated before entering the heap in order to reduce heat loss.

(6) Heap stack:heap bioleaching can process thousands of tons of ore annually, so it is difficult to insure uniformity of internal conditions for such a large project.Owing to partial blockage within the heap, permeability would decrease, O₂and CO₂transmission rate would reduce, local anaerobic zone might be developed and nutrients and liquids may not be able to reach in some places.These would affect the normal operation of heap leaching, reducing leaching efficiency, extending leaching cycle.Therefore, more reasonable stacking approach should be adopted in order to uniform the internal heap conditions.Research on internal temperature, transmission of gases and liquids within heap should be conducted to provide theoretical guarantees in order to stabilize the heap conditions more reasonably.

6 Outlook

It has been more than 30 years from the finding of A.ferrooxidans in oxidation of sulfide ores in 1947 to the application of mesophiles in heap bioleaching of secondary sulfide ores in 1980[27].At present, 20%of the world¡¯s copper is produced by this technology, which is suitable for large quantities of low-grade ores.It also went through30 years from the finding of extremely thermophile Sulfolobus in 1972 by Brock T D to the application of Bio COP in 2003[28].Although chalcopyrite heap bioleaching is difficult, chalcopyrite heap bioleaching is feasible and can be successful and important breakthrough is also achieved.The successful development and application of chalcopyrite heap bioleaching technology will make a lot of lowgrade chalcopyrite ore be economically exploited and also make the copper production proportion by this technology increase.The technology will advance secondary copper sulfide heap bioleaching process and also be a promotion for biohydrometallurgy of nickel, cobalt, zinc, and other sulfide minerals.

²Î¿¼ÎÄÏ×

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