Bioleaching of anilite using pure and mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus caldus

Anilite oxidation was evaluated with two acidophilic thiobacilli that are important in bioleaching processes. The experiments were carried out in shake flasks in the absence and presence of energy sources such as 2 g/L powdered sulphur and 10 g/L Fe²⁺ (as ferrous sulphate) at pH 2.0, 150 rpm, 35 °C. Tests showed that copper extraction in a mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus caldus was higher than in pure cultures with added sulphur in the presence of anilite. The effect of supplemental iron clearly improved Cu leaching by the A. ferrooxidans culture and the mixed culture. The oxidation of anilite by A. caldus was negligible and this bacterium seemed to have no ability to initiate anilite solubilization. On the other hand, an important potential of A. caldus to leaching anilite was indicated. It can decrease pH of the medium and supply a suitable bioleaching environment.     

Dynamic microbial populations in heap leaching of zinc sulphide ore

Polymerase chain reaction (PCR) technology was used to monitor the microbial population within two 6-m-high sulphide ore heaps where zinc was bioleached. Genomic DNA was isolated from microorganisms attached to the ore particles. The microbial population varied in diversity at different heap heights and at different stages of the leaching cycle. Nine bioleaching microorganisms were identified, including iron oxidizers, sulphur oxidizers, whether mesophiles, or moderate thermophiles. No bioleaching extreme thermophiles were detected. Moderate thermophiles were present at average heap temperatures of 40 °C or higher. No moderate thermophiles were present in the top 2 m of the heaps. Mesophiles were present throughout the lifetime of the heaps and at all heap depths. Cultures of Acidithiobacilluscaldus and Sulfobacillusthermosulfidooxidans originally introduced to inoculate the heaps did not proliferate but were instead displaced by other species; no benefits were obtained from heap inoculation. The most predominant species were Acidithiobacillus thiooxidans and Acidithiobacillus albertensis. Leptospirillum sp. was not predominant in the heaps, in contrast to the dominant role this organism plays in tank bioleaching operations.     

Bioleaching of gold and copper from waste mobile phone PCBs by using a cyanogenic bacterium

Chromobacterium violaceum (C. violaceum), a cyanide generating bacterium has been used to leach out gold and copper from the waste mobile phone printed circuit boards (PCBs) containing ∼34.5% Cu and 0.025% Au in YP (yeast extract and polypeptone with glycine) medium. The bioleaching was carried out in an incubator shaker (150 rpm) at 30 °C and 15 g/L pulp density in the pH range 8-11. Dissolution of gold and copper increased from 7.78% (0.225 ppm) to 10.8% (0.46 ppm) and 4.9% (419 ppm) to 11.4% (879 ppm) in 8 days with increase in pH from 8 to 11 and 8 to 10 respectively. Supplementing oxygen with 0.004% (v/v) H₂O₂ increased the copper leaching to 24.6% (1743 ppm) at pH 10 in 8 days whereas improvement in gold leaching was insignificant with the recovery of 11.31% Au at pH 11.0. The waste PCBs can thus be recycled in environmental friendly manner.     

Respirometry studies of bioleaching of low-grade chalcopyrite ore using six acidophilic strains

Respirometry was used to study the growth and activity of six pure cultures of acidophilic bioleaching strains grown on a concentration series of low-grade chalcopyrite ores under various pH conditions. Sulfolobus metallicus, Acidithiobacillus ferrooxidans, Acidianus brierleyi and Leptospirillum ferriphilum were able to grow on a very low-grade ore (equivalent to 0.1% Cu content). However, the two sulphur-oxidising bacteria Acidithiobacillus caldus and Acidithiobacillus thiooxidans grew poorly on the low-grade ore. Growth rates of all strains, except for perhaps S. metallicus at highest ore grades, displayed growth that was limited by substrate availability on this low-grade ore (0.5% Cu content in the ore). The decrease in solution pH from 3.0 to 1.0 enhanced both the cell growth and Cu dissolution.     

Effect of SDS on planctonic Acidithiobacillus thiooxidans and bioleaching of sand samples

The inhibition of bioleaching by sodium dodecyl sulphate (SDS), previously used in large scale percolators in Romania (Schippers et al., 2001), was shown for pure cultures of sulphur-oxidizing Acidithiobacillus thiooxidans DSM 622 or German sand samples.A decrease of 25-75% in planctonic cell number from an initial 10¹⁰A. thiooxidans cells, counted with a Thoma-chamber 30 min after exposure to SDS concentrations from 0.5 to 10 g/L, suggested a cell lysis. Additionally a release of nucleic acids was found.To apply these results in a more complex habitat, columns filled with aquifer material from an East German lignite mining area containing 1% pyrite were treated. Columns were washed once with 2 g/L SDS and afterwards with rainwater. Most-probable-number determinations of flow-through revealed no growth of iron- and sulphur-oxidizing microorganisms within 26 weeks, while up to 10⁶ cells per millilitre were determined in the control. Elution of sulphate dropped to 25%.     

Oxidation of isochemical FeS2 (marcasite–pyrite) by Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans

A research-grade mineral sample that contained marcasite and pyrite (FeS₂) was subjected to the oxidation by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. Oxidation of FeS₂ by A. ferrooxidans produced acid, and the redox potential increased with sulfide dissolution and the oxidation of Fe²⁺. Jarosite was detected in solids from spent cultures. Preferential oxidation of either mineral was not consistently observed across all treatments. Neither iron sulfide was oxidized by A. thiooxidans.     

Bioleaching of djurleite using Acidithiobacillus ferrooxidans

The bioleaching of djurleite using Acidithiobacillus ferrooxidans (LD-1) was investigated in this paper. Experiments were carried out in shake flasks at pH 2.0, 160 r/min and 30 °C. The leaching residues were analyzed using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The total copper extraction of djurleite under optimal condition reached 95.12%. The XRD analysis indicated the residues mainly consisted of ammoniojarosites and S₈. It was observed by the SEM image that the djurleite was heavily etched. The XPS results confirmed the intermediate product formed during djurleite leaching was CuS. The result indicates the reaction pathway is: Cu₃₁S₁₆ → CuS → tCu²⁺ and S⁰.     

Reductive dissolution of ferric iron minerals: A new approach for bio-processing nickel laterites

Nickel laterites represent the major ore reserves of this base metal present in the lithosphere. However, processing these ores by conventional technologies involves considerable energy or reagent expenditure and consequently is less cost-effective than extracting nickel from sulfide ores. Biological options, using metal-complexing organic acids and mineral acids generated by fungi and bacteria, have been investigated but generally found to be ineffective in terms of extraction dynamics or yields. We have examined the possibility of using bacteria that can bring about the reductive dissolution of ferric iron minerals and thereby facilitate the extraction of nickel from a lateritic ore at relatively low (<30-45 °C) temperatures.Four species of iron-reducing acidophilic bacteria were screened for their abilities to solubilise nickel from a limonitic laterite ore in which the major iron mineral present was goethite. One of these (Acidithiobacillus ferrooxidans) was selected for further study only the basis of it being able to use a cost-effective energy source (elemental sulfur) to mediate the dissolution of goethite at mildly acidic conditions (pH < 2). Cultures were set up in 2 L bioreactors, maintained at pH 1.8 (±0.1) and 30 °C, and initially aerated (to promote growth of the bacteria on sulfur) and then switched to anaerobic conditions when nickel laterite ore (crushed to <6 mm, with a nickel grade of 0.5%) was added. Over 70% of the nickel present in the ore was solubilised within 14 days, and solubilised metals remained in solution due to the low pH of the leachate. In contrast, only 10% of the nickel was solubilised (by non-reductive acid dissolution) when the cultures were continuously aerated. The results suggest that biological processing of limonitic nickel laterites is technically feasible and, more generically, that reductive dissolution can be used to bioprocess ferric oxide mineral ores.     

Bioleaching of phosphorus from rock phosphate containing pyrites by Acidithiobacillus ferrooxidans

Pyrites can be oxidized by the bacterium Acidithiobacillus ferrooxidans (At. f.), producing H₂SO₄ and FeSO₄. Rock phosphate is dissolved by H₂SO₄, forming soluble phosphorus. Fe²⁺ in FeSO₄ is oxidized to Fe³⁺, producing energy to sustain the growth of At. f. The effects of four factors (rock phosphate dosage, pyrite dosage, culture temperature and time) on the fraction of phosphorous leached were investigated. It is suggested that the optimal conditions are as follows: rock phosphate dosage 1 g/L, pyrite dosage 30 g/L, culture temperature 30 °C, culture time 84 h. The fraction of phosphorous leached is up to 11.8%.     

Electrochemical dissolution process of chalcopyrite in the presence of mesophilic microorganisms

The dissolution process of chalcopyrite in the presence of mesophilic microorganisms was investigated by electrochemical measurements. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis, accompanied by leaching experiments. Results proved that the presence of Acidithiobacillus ferrooxidans enhanced the initial reduction of chalcopyrite to intermediate species (Cu₂S), thus promoting the dissolution of chalcopyrite in the initial stage of bioleaching. However, chalcopyrite tended to be directly oxidized to polysulfide (S_n²⁻) and CuS in the later stage of bioleaching when redox potential was higher than 0.5 V (vs. Ag/AgCl), the formed polysulfide and jarosite can be responsible for the passivation of chalcopyrite in the later stage of bioleaching by A. ferrooxidans. On the contrary, Chalcopyrite was mainly directly oxidized to polysulfide and CuS in the presence of sterile 9K medium or Acidithiobacillus thiooxidans, and the initial reduction reaction was still the rate-limiting step, thus resulting in low copper extraction rate.     

Effect of pulp density on planktonic and attached community dynamics during bioleaching of chalcopyrite by a moderately thermophilic microbial culture under uncontrolled conditions

An enriched and adapted moderately thermophilic culture showed good performance during bioleaching of chalcopyrite under uncontrolled conditions. The copper extractions were up to 85.0%, 77.3% and 56.3% at pulp densities of 10%, 15% and 20% (w/v) within 22 days, respectively. The real-time quantitative PCR was employed to monitor planktonic and attached community dynamics during bioleaching of chalcopyrite by the moderately thermophilic microbial culture. Only three species, including Acidithiobacillus caldus, Sulfobacillus acidophilus and Ferroplasma thermophilum, were detected during the whole bioleaching process. The results show that pulp density had significant effects on planktonic and attached microbial community dynamics. The succession of attached cells was different from community dynamics of their planktonic counterparts. F. thermophilum and A. caldus preferred to attach to mineral surface, especially at pulp densities of 15% and 20%.     

The effect of temperature and culture history on the attachment of Metallosphaera hakonensis to mineral sulfides with application to heap bioleaching

Temperatures in excess of 60 °C are required for efficient bioleaching of chalcopyrite. Within heaps, colonisation of the mineral with thermophilic archaea is important in reaching and maintaining these high temperatures. The effect of temperature and culture history on the attachment of Metallosphaera hakonensis, an extreme thermophilic acidophile identified as a key player in heap bioleaching, to sulfide concentrates and low-grade ore was investigated in shake flasks and packed beds. Attachment studies were conducted at 25 °C, 45 °C and 65 °C. The results show a clear relationship between increasing temperature and attachment efficiency for both suspended and packed bed systems. Attachment at 25 °C was low. Increasing the temperature to 45 °C improved attachment efficiency by between 50% and 100% while a further increase to 65 °C improved attachment by an additional 20-50%. Cells cultured on elemental sulfur as energy source prior to contacting showed 1.3 times greater affinity for the mineral concentrate than those cultured on sulphide mineral concentrates or ferrous sulphate. In contrast to previous studies using mesophilic organisms the selective attachment of Metallosphaera to sulfide minerals, relative to gangue, was less pronounced. Attachment efficiency was lower in the packed bed system which more closely mimicked flow through a heap. The cell surface properties surface charge and hydrophobicity as well as metabolic activity were investigated to provide insight into the observed phenomena. The data suggest that retention of thermophiles within the heap could be enhanced by a secondary inoculation following elevation of the temperature above 40 °C by the mesophilic pioneer species.     

Role and contribution of pure and mixed cultures of mesophiles in bioleaching of a pyritic chalcopyrite concentrate

This study compares the capacity of pure and mixed cultures of mesophilic bacteria for bioleaching of a low grade, pyritic chalcopyrite concentrate. In pure culture form, Acidithiobacillus ferrooxidans was found to have a higher bioleaching capacity than Leptospirillum ferrooxidans and Acidithiobacillus thiooxidans with the capability of the latter to bioleach copper being very limited. Mixed cultures, MixA (At. ferrooxidans, L. ferrooxidans and At. thiooxidans) and MixB (L. ferrooxidans and At. thiooxidans) were shown to perform better than the pure cultures with the highest extraction of copper (62.1% Cu) being achieved by MixA. Copper bioleaching performances of the cultures were observed to agree with their respective growth pattern. The results also indicated that the increase in the pulp density (1–5% wt/vol) adversely affected bioleaching process regardless of the pure and mixed cultures used having led to the decrease in the extent of final copper extraction i.e. 50.3% Cu recovery at 1% wt/vol for At. ferrooxidans compared with 38.6% Cu at 5% wt/vol. This study underlines the importance of mixed cultures and, iron and sulphur-oxidising activity of a bacterial culture to efficiently oxidise chalcopyrite.     

Microbial solubilization of phosphorus from phosphate rock by iron-oxidizing Acidithiobacillus sp. B2

Procedures for solubilization of phosphorus from insoluble phosphates by acidophilic autotrophs and heterotrophs are considered to be research subjects of great importance in sustainable soil management, and could become, in the future, an alternative to current expensive methods of industrial fertilizer production. Furthermore, this biological production of fertilizer can be considered to be more environmentally friendly than current production methods.The object of this study was to investigate the possibility of phosphorus leaching from phosphate rock apatite using sulphuric acid generated from pyrite by iron-oxidizing Acidithiobacillus sp. B2, and to test the possible application of the mixture of these two minerals to produce a natural phosphate fertilizer.Iron-oxidizing Acidithiobacillus sp. B2 was isolated from copper sulphide mine wastewater (Lake Robule) in Bor, Serbia. The bacterium was identified by 16SrDNA oligonucleotide sequence and characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS).The leaching experiment was carried out in laboratory conditions at 28 °C for a period of four weeks.The results showed that up to 34.5% of the inorganic phosphorus was leached from the test mixture when in suspension with iron-oxidizing Acidithiobacillus sp. B2, while at the same time, the degree of leaching from a control suspension (without iron-oxidizing Acidithiobacillus sp. B2) was only up to 3.8%.Due to its low cost and environmental acceptance, treatment of soil with phosphate minerals in combination with pyrite and acidophilic iron- and sulphur-oxidizing bacteria could become an attractive and alternative way to improve the quality of alkaline soils.     

Leaching of a zinc ore and concentrate using the Geocoat™ technology

In this study, the Geocoat™ technology was used for the extraction of zinc from a mineral concentrate obtained from the Kooshk mine (Yazd, Iran) by a culture dominated by the mesophilic bacterium Acidithiobacillusferrooxidans in a packed column bioreactor. A low grade sphalerite ore was used as support for the concentrate coating. During the 100 days of leaching pH, Fe³⁺, Fe^total, microbial population density and zinc extraction were measured. The final zinc extraction from concentrate and low grade support was 97% and 78%, respectively, and it was found that leaching from the support does not proceed significantly before leaching from the coating is completed.     

Effect of thiocyanate on BIOX® organisms: Inhibition and adaptation

Stringent environmental legislation and the desire to become zero discharge have motivated mining operations to treat and recycle process water. Cyanidation tailings effluent contains elevated concentrations of cyanide and thiocyanate (SCN⁻), precluding recycling to the BIOX® process without prior treatment to reduce SCN⁻ to below 1 mg/l. The current study investigated the effect of SCN⁻ on individual microbial species. Iron oxidation by Leptospirillum ferriphilum was not affected by SCN⁻ concentrations below 0.5 mg/l, with concentration dependent inhibition observed between 0.75 and 1.25 mg/l and complete inhibition of iron oxidation above 1.25 mg/l. Sulphur oxidation by Acidithiobacillus caldus showed a similar trend, with limited inhibition below 1.25 mg/l and almost complete inhibition above 1.25 mg/l. Repeated sub-culturing at low concentrations induced adaptation, with adapted cultures currently growing at SCN⁻ concentrations of 7 mg/l. The phenomenon of inhibition at low concentration, with subsequent adaptation was repeated in stirred tank reactors, leaching a pyrite/arsenopyrite concentrate.     

Density-based separation in a vibrated Reflux Classifier with an air-sand dense-medium: Tracer studies with simultaneous underflow and overflow removal

The continuous separation of tracer particles in the air-sand dense-medium Reflux Classifier was investigated. The Reflux Classifier consisted of a 1 m long vertical fluidized bed section with a 2 m long channel inclined at 70° to the horizontal mounted above, both with a 20 × 100 mm cross section. Silica sand of 220 μm average diameter (−355 + 125 μm) was used as the dense medium. The Reflux Classifier produced good density separations for tracer particles ranging in size from 6.35 down to 1.0 mm. The density cut-point could be varied from 1418 to 2130 kg/m³ by varying the underflow rate and the Ep was within the range of 0.06-0.46 × 10³ kg/m³ depending on particle size and gas rate. At certain gas flowrate and underflow conditions the density cut point ranged between 1534 and 1619 kg/m³ across six particle sizes, suppressing the effects of particle size on the density cut point. As air rates increased from 4.03 to 5.64 × 10⁻⁴ m³/s the density cut-point increased, as did the Ep. The results were compared with separations in a vertical fluidized bed of the same total length. Ep values in the vertical fluidized bed ranged between 0.07 and 1.49 × 10³ kg/m³ over the same experimental conditions as the inclined bed and the density cut point showed more variability with the conditions. The addition of an incline above the fluidized bed provides a more stable system allowing for greater separation efficiency and minimizing the effect of changing conditions. Increasing the flow of sand medium to the underflow decreased the density cut point while raising the gas rate increased the density cut point. Raising the gas rate also increased the variability of the system which resulted in a lower separation efficiency.     

The influence of the density of a gas-solid fluidized bed on the dry dense medium separation of lump iron ore

A gas-solid fluidized bed was used for dry dense medium separation of lump iron ore particles based on their floating and sinking in the fluidized bed. The density of the bed was adjusted to different values using mixtures of zircon sand and iron powder as the fluidized media. Float-sink experiments using 30 mm diameter density adjusted spheres in the range of 2100-4500 kg/m³ in density increments of 100 kg/m³ were carried out to determine the partition curves, the density of the bed and the probable error (Ep). It was found that the density could be adjusted in the range of 2500-4200 kg/m³, when the bulk volume fraction of iron powder and the fluidizing air velocity were varied. The Ep values were less than or equal to 0.05, if suitable fluidizing air velocities were chosen. The density of the bed determined using the spheres floating-sinking corresponds to that measured using the height of the fluidized bed. The float-sink performance of lump iron ore particles in the size range of +25-31.5 mm agrees well with the spheres’ float-sink performance. The partition curves, separation density and the Ep values were determined for the lump iron ore particles. The Ep value for the ore particle separation was around 0.03. The theoretical Fe-grade recovery (washability) curve for the ore was determined for separation densities between 2500 and 4200 kg/m³ from the density distribution and Fe content of the lump iron ore particles. The actual Fe-grade and recovery were calculated from the partition curves of the ore particle separation and compared to the theoretical maximum obtainable Fe-grade and recovery.     

Synchrotron X-ray photoelectron spectroscopic study of the chalcopyrite leached by moderate thermophiles and mesophiles

SXPS (Synchrotron X-ray Photoelectron Spectroscopy) and NEXAFS (Near Edge X-ray Absorption Fine Structures) have been applied to study the surface chemical species of chalcopyrite leached by a moderate thermophilic consortia, Leptospirillum ferrooxidans and a mesophilic mixed culture of L. ferrooxidans, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. A sulfur-rich layer dominated by S_n²⁻ developed with time, which was found to control the rate of bioleaching. Fe L_2,3-edge NEXAFS and Fe 2p spectra indicate the formation of jarosite during bioleaching. Thermophiles significantly enhanced the leaching efficiency, in which 1.34 g/L copper was dissolved in 25 days, while less than 0.3 g/L copper was released in 30 °C bioleaching. This was mostly caused by the increased abiotic reaction rate. The solution copper concentration in presence of L. ferrooxidans was higher than that with mesophilic mixed culture, which suggests the synergistic effect of mixed microorganisms did not play a comparably important role as temperature under the conditions used in this study. Explicit evidence of elemental sulfur was only found in samples leached by L. ferrooxidans by Raman spectroscopy. However, the formation of elemental sulfur does not significantly hinder the leach rate.     

Microwave reduction of copper(II) oxide and malachite concentrate

The use of microwave radiation as an energy source in mineral processing and extractive metallurgy has demonstrated both the instantaneous generation of heat by microwaves in a number of compounds and minerals, and the achievement of high temperatures for an efficient time period enable the heating and reduction of metallic oxides and ores. In the present study, the carbothermic reduction of copper oxide (CuO) and one malachite [Cu₂CO₃(OH)₂] concentrate were investigated. To explore feasibility, the dielectric constants [real (ε′) and imaginary (ε″) permittivities] of both materials were measured at the frequencies of 2.45 GHz and 912 MHz, in the temperature range from 25 to 800 °C using the cavity perturbation method. The high ε″ values (between 1.9 and 36.3) observed in the case of CuO suggest strong microwave absorption, while the malachite concentrate values (between 0.1 and 0.4) indicate limited microwave absorption. Experiments showed the microwave heating rate of CuO was considerably higher than that of the malachite concentrate.The carbothermic reduction of CuO oxide was examined, and the effect of power supply, carbon source, carbon content and granularity of the reducing agent on the reduction rate was studied in detail. Using an 800 W power supply, and with addition of lignite as a reducing agent (with carbon content two times stoichiometric), almost complete reduction of 10 g of CuO was achieved in 4 min.The carbothermic reduction of one malachite concentrate was attempted with the same experimental procedure, but the reaction was not possible using only lignite as the reducing agent, since the poor microwave absorption of malachite concentrate-lignite mixture produced a maximum temperature of 200 °C. This difficulty was overcome by the addition of 5% by weight of graphite powder to the mixture. The rapid heating of the malachite concentrate-lignite-graphite mixture (800 °C after 2 min) resulted in sequential malachite calcination and CuO reduction reactions. After 8 min at a power supply of 800 W, the reduction degree of CuO produced by the calcination of malachite was about 90%.