Patent Application
20170175223
The invention relates to a bioleaching method and facility that allow the extraction of metals and the reuse of these metal resources thus extracted.
Since the demand for metal compounds is constantly increasing, the reuse of resources formerly deemed as not very accessible now poses a growing economic interest. These resources can be old deposits of mining waste containing residual quantities of metals or ores having lower concentrations of metal and/or of a more complex nature, comprising various metals and/or comprising a high level of impurities. Moreover, these metals are often in the form of sulphide compounds, the treatment of which is technically complex and requires heavy investments.
One of the methods most frequently used to treat such ores is pyrometallurgy. After concentration of the sulphides via a physico-chemical treatment of the ore, this method involves a thermal treatment that allows the sulphides to be “burned” via an oxidation reaction and generates a calcine rich in iron and a solid product with a high concentration of metals of value (matte). Such methods involve the emission of toxic gases, the treatment of which can be very disadvantageous, and are only slightly effective for the treatment of ores comprising a high level of carbonate.
With respect to pyrometallurgy, hydrometallurgical methods in general require less investment and are particularly suited to the treatment of metal resources having a complex composition and/or having a low concentration of the metal of interest. Among the hydrometallurgical methods, a solution that is generally very satisfactory from the environmental and economic points of view, called biohydrometallurgy, involves extracting the metals by using microorganisms. In particular, this solution allows both mining waste and sulphide ores having low concentrations to be treated.
The process of degradation of the sulphide ores by microorganisms forms the basis of the bioleaching method used in biohydrometallurgy. These microorganisms draw the energy necessary for the functioning of their metabolisms from the reactions of oxidation, in a highly acidic medium, of iron and sulphur, major components of sulphide ores that contain significant quantities of metals of high economic value (copper, nickel, cobalt, zinc, gold, molybdenum, silver . . . ). Bioleaching also allows the extremophilic metabolic capabilities of certain microbes such as Sulfolobus metallicus, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans to be used to extract these metals of interest.
Heap bioleaching is a particular technique involving the rough crushing of the ore to be treated and then the storage thereof in heaps on impermeable pads. These heaps can reach a height of more than 100 m. Then, a solution containing microorganisms and a suitable nutritive medium is scattered at the top of the heap. While the solution percolates through the heap, the solution is progressively enriched with metal. After percolation through the heap, this enriched solution is recovered at the base of the heap. Heap bioleaching has slow or even very slow kinematics (sometimes years of continuous treatment) and yields that are variable (between 30 and 90%) and thus sometimes low, and is difficult to implement for the treatment of polymetallic ores and/or ores containing carbon. The slow kinematics of this method are in particular linked to the difficulty of applying and maintaining the optimal conditions (temperatures, concentration and dispersion of the nutritive medium, concentration and dispersion of the oxygen) homogeneously inside the heap.
In order to achieve high yields and more attractive operation times, bioleaching can be carried out in a reactor that is mechanically stirred and thermally regulated. After grinding of the ore to be treated and concentration of the sulphide phase via physico-chemical means, the ore is mixed in reactors with an aqueous phase containing microorganisms and a nutritive medium, and thus a suspension is obtained. The reactors, manufactured from non-oxidisable materials, comprise means for stirring, injection of air and heat exchange, respectively allowing the ore to be maintained in suspension and the gas to be dispersed in the suspension, oxygen to be supplied for the reactions and for the microorganisms, and the temperature of the suspension to be controlled in order to maintain the fastest possible bioleaching kinematics.
These reactors are therefore necessarily limited in size and only allow the treatment of suspensions having limited concentrations of ore (approximately 15 to 20% solid mass with respect to the total mass of the suspension). Despite these advantages, the investment in equipment and the operational cost in terms of energy required for the implementation of such facilities are only justified for the treatment of metal ores having high value, such as gold-containing ores.
The present invention thus relates to a method for bioleaching a metalliferous ore, the investment costs and the environmental constraints of which are limited, as well as to a bioleaching facility suitable for implementing said method.
The method according to the invention is particularly suited to the treatment of metalliferous ores having low value and/or a complex composition, but is also advantageous for the treatment of metalliferous ores that are rich, have high value and/or have a simple composition.
In the description of the invention, metalliferous ore designates: an ore extracted from a mine, mining waste, or a concentrate resulting from the mineralurgical treatment of an ore and/or of mining waste.
Such a metalliferous ore can comprise one or more metals to be released from the mineral matrix via bioleaching. Thus, the references hereinafter to a “metal”, in the singular, present in the metalliferous ore or released via bioleaching can refer to both a single metal or to (a combination of) several metals.
In the present context, suspension is understood as: any liquid continuous phase comprising a solid phase dispersed in the liquid phase.
A first object of the invention is a method for bioleaching, for example via “lagooning”, a metalliferous ore, said method comprising the following steps:
a) a ground metalliferous ore, a medium comprising a bioleaching microbial consortium, and a nutritive medium for the microorganisms of the microbial consortium are added into a basin,
b) a suspension is obtained in the basin via at least one stirring system for placing and/or maintaining the metalliferous ore in suspension in a liquid phase,
c) a gas containing oxygen and optionally carbon dioxide that can be metabolised by the microorganisms is injected into the suspension,
d) the metalliferous ore is bioleached by the microbial consortium in such a way as to obtain released metal,
e) a liquid product containing the released metal and consisting of (a) a liquor and (b) a solid residue is recovered.
According to the invention, the temperature of the suspension is controlled by regulating the flow rates and the composition of the gas, as well as optionally the concentration of solids in the suspension. More particularly, the temperature of the suspension is controlled in such a way as to be maintained in a predetermined range suitable for bioleaching.
The metal released via bioleaching of the metalliferous ore that made said metal inaccessible for direct treatment may be (partially or totally) in dissolved form and thus present in the liquor of the product recovered at the end of the bioleaching. The released metal may also be (partially or totally) in solid form (that is to say, not dissolved) and thus present in the solid residue of the recovered product.
The temperature of the suspension is considered suitable for bioleaching when said temperature allows sufficient activity of the microorganisms of the consortium and thus efficient bioleaching kinematics.
In practice, the thermal regulation is not only related to the microbial activity, but also to the geometry of the bioleaching basins, and in particular to the volume to surface ratio, as well as to the ambient conditions and the variability of said conditions.
Thus, according to the invention, the temperature of the suspension can be controlled without using an external thermal regulation system such as heat exchangers or other heating and/or cooling elements.
By allowing the temperature of the suspension to be maintained in an optimal range for bioleaching without the need for heating or cooling elements, the present invention not only allows energy to be saved, but also allows optimal bioleaching kinematics to be obtained in basins of a large size or even open air basins.
The metalliferous ore that is used as a substrate in the method according to the invention can, for example, come from mining waste or be an ore with a low concentration of metals to be recovered.
The invention is particularly useful for a sulphide ore and/or an ore with a complex composition, for example a polymetallic ore or an ore containing carbon (for example carbonate). One advantage of the invention is to allow, under acceptable economic conditions, the treatment of a metalliferous ore for which the concentration of metal of interest is relatively low. The types of sulphide ores used can comprise, for example, pyrite, copper sulphides, galena or sphalerite. For example, Kupferschiefer copper-containing black shales are ores having a complex composition that could be used as substrates. The metalliferous ore to be treated can also contain a high proportion of carbon, for example 5% carbonate.
The ore used is ground. The corresponding particles can have a particle size (corresponding to D90) from 10 μm to 300 μm, preferably from 10 μm to 200 μm, and typically of approximately 50 μm; D90 indicates that 90%, by weight, of the particles considered have a size less than D90, the remaining 10% by weight having a size of at least D90.
The microbial consortium used in the method according to the invention preferably comprises autotrophic and acidophilic microorganisms. In a useful manner, the microbial consortium is mesophilic and/or moderately thermophilic.
A mesophilic consortium is a consortium that grows at temperatures from 20° C. to 40° C. A moderately thermophilic consortium is a consortium that grows at temperatures from 40 to 60° C.
The consortium advantageously comprises microorganisms of the species Leptospirillum ferriphilum, Acidithiobacillus caldus and/or Sulfobacillus benefaciens, which can be found in the DSMZ strain collection (Deutsche Sammlung von Mikroorganismen and Zellkulturen). The consortium can be, for example, the microbial consortium BRGM-KCC, which is described in the article Morin, D., d'Hugues, P. (2007). “Bioleaching of a cobalt-containing pyrite in stirred reactors: a case study from laboratory scale to industrial application” in: Rawlings, D. E., Johnson, D. B. (Eds), Biomining, Chapter 2, Springer-Verlag, Berlin, pp. 35-55.
The specific composition of the consortium can vary according to the metalliferous ore to be leached. A nutritive medium adapted to the consortium is advantageously inserted into the medium to allow the development of the microorganisms and thus promote the bioleaching. This medium can be advantageously derived from a “9K” medium described by Silverman and Lundgren in “Studies on the chemoautotrophic iron bacterium Ferrobacillus ferrooxidans. I. An improved medium and harvesting procedure for securing high cell yields.”, Silverman, M. P. and Lundgren, D. G., J. Bacteriol., 77: 642-647. (1959) and adapted to the specific consortium. Thus, a medium having the composition (NH4)2SO4, 3.70 g·L−1; H3PO4, 0.80 g·L−1; MgSO4.7H2O, 0.52 g·L−1; KOH, 0.48 g·L−1 is particularly suited to the growth of a microbial consortium as previously described when the mineral substrate is cobalt-containing pyrite. The specific composition of the nutritive medium can vary according to the species present in the microbial consortium and according to the ore to be treated.
pH
The suspension is generally an aqueous suspension.
In the basin, it is preferred that the suspension be maintained at a pH greater than 0.8. Preferably, the pH of the medium is maintained in a range from 0.8 to 2.5; advantageously, the pH is maintained between 1 and 1.5.
To lower or raise the pH to the preferred values, for example sulphuric acid or calcite, calcium carbonate, quick lime or slaked lime, respectively, can be added to the suspension.
The pH range can vary according to the species present in the microbial consortium and according to the composition of the ore to be treated.
The invention has the particular advantage of allowing not only the treatment of suspensions having a low concentration of solid particles, but also of suspensions having a high concentration of solid particles of metalliferous ore.
Indeed, in the conventional methods for bioleaching in a stirred reactor, the use of air to supply the oxygen requires the injection of a very large volume of gas, the dispersion of which requires high power in the dispersion system, which correspondingly reduces the power available to place the solid particles in suspension via the stirrer. The use of a gas with a higher oxygen content allows more oxygen to be supplied with a lower total gas flow rate while also promoting the dissolution of the oxygen in the liquid. Moreover, the possibility of using a plurality of gas dispersion systems allows the flow rate of gas injected at each system to be lowered accordingly.
The power available for stirring the medium, in particular in order to place and maintain the solid particles in homogeneous suspension, is therefore increased because of the reduction of the power required for the dispersion of the gas.
If the stirring system consists of stirrers, the fact that the geometry of a lagoon is such that the ratio of the diameter of the mobile element of the stirring system to the diameter of the lagoon is in general lower than for a stirred reactor as used in the known bioleaching methods is added to this. The lower this ratio, the easier it is to create and maintain the suspension. In other words, the stirring speed required to return the particles to or maintain the particles in suspension is lower in large spaces than in confined spaces. Thus, when the method according to the invention is implemented in one or more basins of a large size, the power necessary for placing the solid particles in suspension is reduced.
According to the invention, the solid concentration by weight in the suspension is advantageously from 15 to 40%, preferably from 22 to 38%, and more preferably from 25 to 35%, for example approximately 30%, with respect to the total weight of the suspension.
The ground metalliferous ore can be dispersed in a liquid before being inserted into the basin.
According to the invention, the stirring system can comprise a circulator of the suspension or a stirrer, or even a combination of the two. Preferably, the stirring system comprises at least one stirrer. The stirring system advantageously comprises at least one and preferably a plurality of floating stirrers.
The use of floating stirrers significantly increases the flexibility of the method according to the invention and is in particular useful for basins that are non-circular and/or have a high surface area and/or are open-air. Indeed, the position of a floating stirrer in the basin and the number of floating stirrers in a basin can be easily modified.
The speed of rotation of the stirrers is chosen from a range from 40 to 500 rpm, preferably 200 to 350 rpm. On the laboratory scale, however, the speed of rotation can reach 1500 rpm.
A suspension circulator can, for example, have the following form: the bioleaching suspension is sucked into a pipe outside of the basin via a pump suitable for a liquid of this type. The gas is injected (simple injection or the use of a venturi or of a porous element, for example) into this pipe and mixed with the suspension (for example via a static mixer). Downstream of the injection of gas, a sufficient line length of the pipe ensures good transfer efficiency. The suspension thus “enriched” with gas is then re-injected into the basin, ideally at the bottom of the basin, in several locations in order to ensure homogeneous transfer of the gas throughout the basin. The pump can advantageously be a vortex pump, known to not be “traumatic” to the microorganisms.
According to the invention, a gas containing oxygen is injected into the suspension in order to supply the oxygen necessary for the development of the microbial consortium and for the microbial lysis reaction. If the metalliferous ore does not contain enough carbon that can be metabolised by the microbial consortium in order to ensure the growth of said consortium, CO2 is advantageously also injected into the suspension, preferably in the form of a gaseous mixture with the oxygen. In the latter case, the gas containing oxygen thus also contains CO2.
In general, the ambient temperature around the basin in which the method is carried out is lower than the temperature suitable for the activity of the microbial consortium and the development thereof. In the absence of a heat exchange system and particular constraints on the gas pipe (thermal insulation after compression, short length of the pipes, etc.), the gas injected is in general close to the ambient temperature. The injection of a significant flow of gas can therefore lead, despite the exothermic nature of the reaction, to the cooling of the suspension to temperatures that do not allow the microbial activity to be maintained at an adequate level.
According to the invention, this is prevented by adjusting the flow rate of gas injected in order to maintain the temperature of the suspension in a predetermined temperature range suitable for bioleaching. The supply of oxygen necessary for the reaction is then ensured by adjusting the composition of the gas, in particular the concentration of oxygen in the gas.
In particular, if it is not possible to maintain both a flow rate of gas that allows the temperature to be maintained in the predetermined range and the supply of oxygen necessary for the reaction, it is also possible to act on the quantity of metalliferous ore injected into the basin, for example to dilute the suspension by adding liquid or, on the contrary, to increase the concentration of metalliferous ore by adding metalliferous ore to the suspension.
According to yet another particularly advantageous aspect of the invention, the stirring system comprises at least one device for ejection/dispersion of a gas, in particular a gas containing oxygen and/or carbon dioxide. Such a suspension circulator that integrates a gas injector has already been described above. Likewise, when the stirring system comprises a stirrer, and in particular a floating stirrer, said stirrer is advantageously provided with a gas injector.
A gaseous mixture suitable for the method according to the invention can contain, for example, 1% carbon dioxide, 49% nitrogen and 50% oxygen by volume.
Advantageously, a gas obtained by mixing an oxygenated gas and a dilution gas is injected into the suspension. The oxygenated gas has an O2 concentration greater than the O2 concentration in the air, typically an O2 concentration of 50 to 100% vol, preferably of at least 75% vol, and more preferably of at least 85%. The dilution gas advantageously comprises between 0 and 21% O2 by volume. The dilution gas can be a gas that is inert with respect to bioleaching reactions, such as nitrogen, and does not contain any oxygen. However, it is also possible to use a gas having a relatively low (and in any case lower than the oxygen concentration of the oxygenated gas) concentration as a dilution gas, namely air for example. The gas injected into the suspension optionally contains carbon that can be metabolised, preferably in the form of CO2.
Although the invention is suitable for being used on a small scale, or even on the laboratory scale, the invention is, as indicated above, particularly useful for the treatment of metalliferous ores on a large scale.
The basin or basins used in the method according to the invention are thus advantageously of dimensions suitable for the industrial treatment of the ores, such as lagoons. These dimensions are dependent, for example, on the flow rate of the supply of suspension/slurry and on the residence time necessary for the leaching of the ore. For example, such basins have a depth that can reach 6 m and have a total surface area of up to 1500 m2.
The residence time necessary for the leaching of the metalliferous ore varies according to the conditions of reactions and the starting materials used. This residence time is generally approximately 4 to 8 days, for example 6 days.
Via the present invention, it is possible to reach an adequate bioleaching yield for a reduced residence time with basins, without the need to use conventional heating and/or cooling systems such as heat exchangers.
According to the invention, the bioleaching of the metalliferous ore can in particular be carried out in a single basin or in a plurality of basins in series.
The invention also relates to a lagooning facility comprising a bioleaching basin, preferably open-air, said basin comprising a liquid phase, typically an aqueous phase, a ground metalliferous ore, a bioleaching microbial consortium, a nutritive medium for the microorganisms of the microbial consortium. The facility further comprises a stirring system for placing and/or maintaining the metalliferous ore in suspension in the liquid phase. The stirring system of the facility comprises a plurality of floating stirrers. The facility also comprises at least one injector for the injection of a gas into the suspension of metalliferous ore.
According to the invention, said at least one injector of gas is connected to a source of oxygenated gas and a source of a dilution gas. The oxygenated gas has an O2 concentration greater than the O2 concentration of air. The oxygenated gas typically has an O2 concentration of 50 to 100% vol, preferably of at least 75% vol, and more preferably of at least 85%. The source of oxygenated gas can thus be a unit for separating the gases in air, a pipeline of oxygenated gas (for example of industrial oxygen), or a tank of liquefied oxygenated gas.
The dilution gas advantageously comprises between 0 and 21% O2 by volume.
The dilution gas can be a gas that is inert with respect to bioleaching reactions, such as nitrogen, and does not contain any oxygen.
However, it is also possible to use a gas having a relatively low (and in any case lower than the oxygen concentration of the oxygenated gas) concentration as a dilution gas, namely such as air.
In the first case, the source of dilution gas can be a facility that produces the inert gas, namely such as a unit for separating the gases in air that produces not only oxygen that can be used as the oxygenated gas, but also nitrogen. The source of dilution gas can also be a tank of the dilution gas, liquefied if necessary.
In the second case, the source of dilution gas is advantageously an air compressor.
The facility also comprises a regulator of oxygenated gas and a regulator of dilution gas. The regulator of oxygenated gas regulates the flow rate of oxygenated gas to the at least one injector. The regulator of dilution gas regulates the flow rate of dilution gas to said at least one injector.
The (at least) one injector of the facility is optionally also connected to a source of carbon gas that can be metabolised. The carbon gas that can be metabolised typically contains from 50 to 100% CO2 by volume, preferably at least 75% vol, and more preferably at least 85% vol. Such a source of carbon gas that can be metabolised is, for example, a tank of liquefied CO2. In this case, the system advantageously also comprises a regulator for regulating the flow rate of carbon gas that can be metabolised to the at least one injector.
The gas regulators are, in a useful manner, regulator valves. When the source of dilution gas is an air compressor, the dilution gas regulator can form an integral portion of said compressor, in particular in the case of a compressor having an adjustable flow rate.
The facility according to the invention thus allows both the flow rate of O2 injected into the basin and optionally the flow rate of CO2 injected into the basin to be regulated according to the needs of the bioleaching reactions, and allows the overall flow rate of gas injected into the basin to be regulated separately. According to an advantageous aspect of the invention, this allows the use of a facility without a system for conventional regulation of the temperature of the suspension in the basin, in particular such as a heat exchanger.
When, according to the present invention, the temperature of the suspension is controlled in such a way that the temperature of the suspension is maintained in the predetermined range, this control being carried out by regulating the flow rates and the composition of the gas injected, as well as optionally the concentration of solids in the suspension, and the dilution gas contains no or very little O2, the oxygen concentration of the gas injected is determined by the ratio on one hand of the flow rate of the oxygenated gas and, on the other to the flow rate of the dilution gas or to the sum of the dilution gas flow rate and the flow rate of the carbon gas that can be metabolised. When the dilution gas and/or the gas containing the CO2 has a non-negligible concentration of O2, the supply of O2 by the dilution gas and/or the gas containing the CO2 is taken into account during the adjustment of the composition and in particular of the O2 concentration of the gas injected into the suspension.
The regulation of the composition and of the flow rate of the gas injected can be manual or automatic, continuous or interrupted (by intervals).
Thus, the facility according to the invention can comprise a control unit for the control of the regulator of the oxygenated gas and of the regulator of the dilution gas and optionally also of the regulator of the carbon gas that can be metabolised, in order to regulate the flow rate of said gases and thus also the overall flow rate and the composition, and in particular the O2 and CO2 concentration, of the gas provided to the injector of gas and injected into the suspension.
The facility advantageously comprises at least one system for measuring temperature, for measuring the temperature of the suspension in the basin. In this case, the control unit is advantageously connected to said system for measuring temperature in such a way as to allow the regulation of the overall gaseous flow rate and the oxygen concentration of the injected gas by the control unit according to the temperature measured.
The basin can be a basin that does not comprise heating or cooling elements. The facility according to the invention can comprise a single basin or a plurality of basins, for example a plurality of bioleaching basins in series.
The microbial consortium is typically an autotrophic consortium. Said consortium is preferably mesophilic to moderately thermophilic.
The elements of this facility can advantageously be as described in reference with the method of the invention, for example with respect to the stirring system. Likewise, the elements included in the description of the facility also apply to the method according to the invention.
During the bioleaching, the metal present in the metalliferous ore is progressively released. The released metal is typically present in dissolved form. In this case, the liquid phase of the medium is progressively charged with dissolved released metal. As described above, the released metal can also be partially or totally present in solid form. The suspension obtained at the end of the bioleaching can be subjected to liquid/solid separation (for example via decantation and/or filtration) and thus be separated into a liquid phase and a solid residue. The liquid phase thus obtained is also called liquor. When at least a fraction of the released metal is present in dissolved form, the dissolved released metal is present in the liquid phase that can be refined via known methods in order to allow the recovery of the dissolved metals of value. When at least a fraction of the released metal of value is present in solid form, this fraction can also be recovered via known methods. The solid residue can, for example, be recovered in order to undergo a new bioleaching step in other conditions, for example in order to allow another type of metal that can be reused (precious metals) to be recovered. Said residue can also be stored as waste or used for other purposes.
The present invention also relates to a method for regulating temperature for a bioleaching suspension comprising a metalliferous ore, a bioleaching microbial consortium and a nutritive medium for the microorganisms of the consortium. According to this method, the temperature of the suspension is maintained in a predetermined range by regulating the flow rate and the composition of a gas containing oxygen and optionally carbon dioxide that is injected into said suspension, as well as optionally by regulating the concentration of solids in the suspension.
The invention also relates to the use of a ground metalliferous ore, a bioleaching microbial consortium, and in particular such a microbial consortium that is autotrophic and mesophilic to moderately thermophilic, a nutritive medium of the microorganisms of the consortium, and a stirring system, for creating a facility or in a method for bioleaching via lagooning.
Finally, the products resulting from the method and/or the facility as previously described are also part of the invention, in particular the suspension, the liquid product comprising the metal released via bioleaching, the liquor containing the metal dissolved via bioleaching, the solid residue, which can comprise non-dissolved metal released from the metalliferous ore via bioleaching, as well as the metals recovered from the suspension/from the liquor, such as copper, zinc, molybdenum, antimony, nickel, gold, silver and cobalt.
The invention will be better understood by reading the examples and the drawings that follow, which are not in any way limiting and in which:
A pilot facility was created on the laboratory scale in conditions that can be easily extrapolated to the industrial scale.
The ore treated is cobalt-containing mining waste from a European mine, containing approximately 60% (by weight) pyrite (iron disulphide). This ore has a cobalt concentration of approximately 800 ppm, as well as gold at 1 ppm and copper at 1900 ppm.
A quantity of 713 kg of ore was added to a quantity of 1318 kg of nutritive medium and 226 kg of inoculum in a 2 m3 tank in order to obtain a pulp. This tank is thermally insulated in such a way that the results obtained can be easily extrapolated to a lagoon industrial use.
Indeed, the surface-to-volume ratio of such a tank is much higher than for a lagoon, the thermal losses via the edges in such a tank are therefore much greater in proportion to the volume of the suspension, and the insulation of the edges of the tank thus allows the thermal conditions reigning in the volume of a lagoon to be approached.
This pulp was inoculated with a microbial consortium from the BRGM-KCC culture, the main organisms of which are affiliated with the genera Leptospirillum, Acidithiobacillus and Sulfobacillus. This culture was transplanted several times in “batch” mode while progressively increasing the volume of liquid from 2 mL to 200 L.
The nutritive medium used is a medium called “9 Km”. This is a “9K” medium modified and optimised to allow microbial growth on cobalt-containing pyrites. The composition of said medium is the following: (NH4)2SO4, 3.70 g·L−1; H3PO4, 0.80 g·L−1; MgSO4.7H2O, 0.52 g·L−1; KOH, 0.48 g·L−1.
A floating stirrer provided by the company MILTON ROY Mixing under the brand name TURBOXAL® is installed on the surface of the pulp. The stirring speed is 1300 rpm. The pH at the beginning of the reaction is adjusted to 1.8 by the addition of concentrated sulphuric acid. During the reaction, the pH was controlled by the addition of calcite in such a way that the pH was never lower than 0.8.
A single floating stirrer was used in the pilot facility on the laboratory scale. In the case of facilities according to the invention on the industrial scale, the basin comprises a plurality of such floating stirrers.
The Eh value reached in the solution (close to 900 mV) indicates that the totality of the iron in solution is in the form of ferric iron (FeIII), which demonstrates a good microbial activity of oxidation, confirmed by the increase in the microbe concentration.
Because of this strong microbial activity, high levels of extraction of the metals are obtained. The level of cobalt in solution after 6 testing days is 86%, which indicates that 86% of the pyrite contained in the pulp was leached (see
A pilot bioleaching facility imitating lagooning via a series of basins in cascade was created on the laboratory scale under conditions that can be easily extrapolated to an industrial case of lagoons in series.
The same sulphide ore, the same inoculum and the same stirring conditions as described in example 1 were used in a facility comprising a primary basin of 50 L (R1) and two secondary basins of 20 L (R2 and R3). The basins are fed in cascade. In R1, the gaseous mixture injected consists of 50% O2 and 1% CO2, and the flow rate is set to 316 NL/h. In R2 and R3, the flow rate and concentration of O2 were reduced since the demand for oxygen is lower than in the secondary basins (74 NL/h and 40% O2).
These conditions (stirring speed, concentration of oxygen in the gas) can vary to a certain degree according to the composition of the material (sulphide and carbonate concentration, nature of the mineral species . . . ). The adaptation of these conditions is part of routine techniques.
An embodiment of the method according to the invention with three lagoons in series (in cascade) is shown in
A finely ground sulphide ore 1 is placed in a pulp at the desired solid concentration (from 15 to 40% (by weight) and for example 30% (by weight)), in a nutritive medium 2 suitable for the development of the microorganisms used for the bioleaching. The pH of the pulp is adjusted by the addition of concentrated sulphuric acid 3 in order to reach a value of approximately 1.8 (and typically from 0.8 to 1.8). The pulp is then injected into the basins 10, 20, 30 previously inoculated with an autotrophic, mesophilic to moderately thermophilic microbial consortium that combines Leptospirillum ferriphilum, Acidithiobacillus caldus and Sulfobacillus benefaciens microorganisms (for example the microbial consortium from the culture BRGM-KCC). The three types of microorganisms necessary for the bioleaching (Leptospirillum ferriphilum, Acidithiobacillus caldus and Sulfobacillus benefaciens) are available from the DSMZ strain collection.
The three lagoons are provided with floating stirrers 11, 21, 31 that carry out the mixing of the suspension and the injection and the transfer of the oxygen and of the carbon dioxide necessary for the functioning of the microorganisms and for the oxidation of the sulphides. Such stirrers are available on the market. Thus, the stirrers sold by the company MILTON ROY Mixing under the brand name TURBOXAL® and described in the patent application No. EP-A-2714256 can be used to carry out the method according to the invention.
The process of bioleaching takes place in these lagoons 10, 2030.
The use of lagoons in series allows the liquor to be concentrated. The operation thereof is the following:
At the output 32 of lagoons, a pulp consisting of a liquor rich in released, dissolved metals 33 and a solid residue 34 containing the non-leachable mineral phases is obtained, the totality of the metal released by leaching being present in dissolved form. After a step of solid/liquid separation (via decantation or filtration), the liquor is sent for refining in order to recover the metals, while the solid residue can be either recovered in order to undergo a new leaching step in other conditions (for example in order to recover the precious metals) or stored as waste.
The depth of the lagoons can vary from 2 to 10 m and the total volume of said lagoons depends on the flow rate at which pulp is fed and the residence time necessary for the leaching of the sulphides contained in the material (approximately 4 to 8 days and for example 6 days). The number of lagoons can vary, for example from 2 to 10.
The stirring speed depends mainly on the concentration of the pulp and the density of said pulp, said speed typically varies in a range from 200 to 350 rpm.
The gas injected into the pulp via the floating stirrers 11, 21, 31 comprises, by volume, approximately 1% vol CO2 (typically from 1 to 3% vol) coming from the CO2 tank 5, a variable concentration of nitrogen of less than 78% vol nitrogen, the nitrogen coming from the nitrogen tank coming from the liquefied nitrogen tank 6, and a variable concentration of oxygen of more than 21% vol, the oxygen coming from the liquefied oxygen tank 4. The gas can thus contain, for example, 49% vol nitrogen and 50% vol oxygen. The oxygen must be injected in a sufficient quantity in order to ensure the dissolution of a quantity of oxygen sufficient to allow the dissolution of the sulphides (e.g.: to dissolve 1 kg of pyrite (FeS2), 1 kg of O2 must be provided).
The oxygen can be injected in concentrated or non-concentrated form. The composition of the gas injected and the flow rate of said gas are also adjusted for each lagoon 10, 20, 30 via flow rate regulators (not shown) in order to compensate mainly for the heat generated by the reaction of oxidation of the sulphides (exothermic reaction), but also for the influence of the environment on the temperature of the pulp in the lagoons, and maintain the system at the temperature required for the functioning of the microbial consortium (between 35° C. and 48° C.)
The basin 51 is provided with a suspension circulator.
A portion of the suspension is extracted from the basin 51 by a perforated aspiration tube 52 via a pump 53.
Said portion is expelled into a recirculation circuit 56.
The circuit 56 is provided with a system 54 for the injection of a gas into a liquid phase, such as a venturi injector or a porous injector. A regulated flow of a gaseous mixture having a controlled concentration of oxygen and optionally also of CO2, from a device 55 for supplying gas, is mixed with the suspension in the recirculation circuit 56 via the gas injector 54. The gaseous mixture is carried by the suspension in the recirculation circuit and injected into the basin 51 with this suspension at reinjection points 57 distributed around the circumference of the basin. Such a suspension recirculator with integrated injection of gas can be combined with other stirring systems such as (floating) stirrers.
In order to adapt the method to the variations in ore, to the microbial activity, to the basins and to the environmental conditions that can affect the method, the following should be done:
For example, when the temperature of the system is not high enough, the O2/N2 ratio increases and the flow rate decreases. On the contrary, when the system needs to be cooled, the O2/N2 ratio decreases and the flow rate increases.
The invention is not limited to the embodiments presented, and other embodiments will be obvious to a person skilled in the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14305865.9 | Jun 2014 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/062592 | 6/5/2015 | WO | 00 |
