WO2019206755A1

WO2019206755A1 - Method for the biological recovery of metals in electric and electronic waste

Info

Publication number: WO2019206755A1
Application number: PCT/EP2019/059886
Authority: WO
Inventors: Antonio David DORADO CASTAÑO; Xavier GAMISANS NOGUERA; Montserrat SOLÉ SARDANS; Concepción LAO LUQUE; Eva BENZAL MONTES
Original assignee: Universitat Politecnica De Catalunya
Priority date: 2018-04-25
Filing date: 2019-04-17
Publication date: 2019-10-31
Also published as: ES2707353A1; ES2707353B2

Abstract

The present invention relates to a method for the biological recovery of metals in electric and electronic waste, which comprises a) introducing a series of iron-oxidizing microorganisms and a mineral medium into a reactor (10); b) performing a first step of biological oxidation of the iron II ions present in said mineral medium to iron III ions; c) separating the iron-oxidizing microorganisms present in suspension inside the reactor (10), providing a solid phase comprising the iron-oxidizing microorganisms and a liquid phase comprising the iron III ions; d) continuously irrigating the liquid phase into a column (30) configured for housing an electrical material or one or more printed circuit boards from which metals are to be recovered, the iron III ions being reduced to iron II oxidizing the metals, and separating the metals by means of the dissolution thereof; and e) extracting said metals from the solution.

Description

Method for the bioloqicai recovery of metals in electric and electronic waste

Field of the Art

The present invention relates generally to methods for recycling electric and electronic waste. In particular, the invention relates to a method for the biological recovery of metals in electric and electronic waste, for example from printed circuit boards (PCBs) (for example, from mobile telephones, among others) or from any electrical material in general.

Background of the Invention

Printed circuit boards (PCBs) are common components in most electrical and electronic equipment. Electric and electronic waste consisting of discarded computers, televisions, mobile telephones, music equipment, and other electronic equipment have become a significant problem worldwide. Rapid technological advancements cause electrical and electronic products to become obsolete within a short period of time. This, together with the exploding sales of consumer electronics, means that more products are being discarded, even though they still work. One of the main problems with this type of electric and electronic waste is that they are filled with toxic chemicals such as arsenic, lead, mercury, polychrome flame retardants, among others. Moreover, the electric and electronic waste also contains a significant part of valuable metals such as copper, silver, or goid, which may be recovered to be reused.

Chemical methods are generally used for recovering these metals from the mineral matrix or plastic (electronic waste) matrix. Finally, bioleaching has been tested as an alternative to chemical processes, showing significant advantages such as a low cost, high efficiency, and environmental friendliness.

Bioleaching is based on the release of metals from metallic concentrates, such as electric and electronic waste, using microorganism activity.

Some patents or patent applications in this sector are known.

For example document CN105734284A discloses a bioleaching treatment of PCB electronic waste, promoted by means of using biocarbon. According to the method, through oxidation- reduction radicals in the biocarbon, the metal copper in the PCB is bioleached by Fe <2+> microorganisms. The oxidation-reduction capability of the biocarbon and the bioleaching are combined, so that the Fe <2+> is oxidized to obtain Fe <3+>, and the leaching rate of the copper metal is increased. Documents CN 103898550, CN102Q91711 , and CN202519343 are based on using acidophilic bacteria, Acidithiobacillus, for leaching copper. Patent CN 104328280 describes a method and equipment to perform bioleaching with Acidithiobacillus ferrooxidans for extracting copper, gold, and nickel from RGBs, in patent CN104862475, bioleaching with Acidithiobacillus ferrooxidans was improved by means of the proliferation of the bacterium under conditions of low dissolved oxygen level to maximize growth, and then increasing the oxygen concentration when PCBs are added to maximize copper leaching. The application of patent CN 105039704 is based on bioleaching with Thiobaciilus acidophilus (reclassified as Acidiphilium acidophilum) for copper extraction.

Finally, international patent application WO 03006696 discloses a method for extracting zinc from a mineral with sulfur which comprises bioleaching the mineral with acidophilic microorganisms.

Summary of the Invention

Embodiments of the present invention provide a method for the biological recovery of metals in electric and electronic waste which comprises, first of all, introducing a series of aerobic iron-oxidizing microorganisms and a mineral medium formed by different salts in solution into a reactor, and performing, in the mentioned reactor, a first step of biological oxidation of the iron II ions present in said mineral medium to iron III ions. The first step is catalyzed by the metabolic activity of said iron-oxidizing microorganisms and performed within a previously fixed range of temperature, under constant mechanical stirring, controlling the pH of said mineral medium. Particularly, the first step lasts for at least two hours.

Once the first step has been performed, the iron-oxidizing microorganisms present in suspension inside the reactor are separated, out of the reactor, providing a solid phase comprising the iron-oxidant microorganisms and a liquid phase comprising the iron ions III.

Next, the mentioned liquid phase is continuously irrigated into a column with dimensions suitable for housing one or more printed circuit boards of an electrical material or of an electronic waste from which metals are to be recovered, the iron III ions in contact with said liquid phase being reduced to iron II oxidizing the metals, and the metals of interest are separated by means of the dissolution thereof. The mentioned printed circuit board or boards are in contact with the liquid phase inside the column for at least one hour. Finally, the metals of interest are extracted from the solution. It must be indicated that instead of the mentioned printed circuit board (or boards), said column can house any electrical material from which the metals are to be recovered. The solid phase with the iron-oxidizing microorganisms is preferably returned again to the reactor.

To improve contact between the electrical material or electronic waste and iron III, and to assure that all the incoming iron 111 ions react with the metals, said iron III ions can also be re-circulated from a lower part of the column to an upper part thereof,

Particularly, the different steps of the proposed method are performed continuously.

In other embodiments, instead of or in addition to being performed within a previously fixed range of temperature, the first step can also be performed with oxygen and/or redox control.

The mentioned previously fixed range of temperature can be comprised between 25 and 35°C, preferably 30°C, and the constant mechanical stirring is preferably maintained at a speed of 200 rpm.

In one embodiment, a structuring agent which facilitates liquid percolation is additionally used when the mentioned electrical material or the mentioned printed circuit board or boards are contacted with the leaching liquid. This structuring agent can be the actual plastic material of the printed circuit board/boards.

In one embodiment, leaching liquid renewal cycles are performed in the mentioned column and the printed circuit board/boards (or said electrical material) are stirred. The required time is thereby reduced and better biological recoveries obtained.

In one embodiment, when the metals of interest have been extracted from the solution, they are reduced from soluble state to metallic state by means of a cementation process which provides a spontaneous reaction between soluble copper II, extracted from the electrical material or from the printed circuit board or boards, and the metallic iron. In the mentioned spontaneous reaction, the soluble copper II is reduced to copper metal and the metallic iron is oxidized to soluble iron II. In this embodiment, the cementation process is performed in a tank mechanically stirred at a speed comprised in a range of 120 to 140 rpm at room temperature.

Alternatively, in another embodiment the reduction of the extracted metals of interest to metallic state is performed by means of electrolysis or by means of a precipitation technique. In one embodiment, when the metals of interest have been extracted from the solution, the obtained soluble iron II is re-circulated to the first step of biological oxidation to reduce the amount of iron required therein.

In one embodiment, the different salts of the mineral medium include an iron II salt and salts that provide nitrogen, sulfur, phosphorus, magnesium, potassium, and calcium. Preferably, the composition of the mineral medium introduced in the reactor comprises: 30 g/L de FeS0₄ 7 H_zO, 3 g/L de (NH₄)₂S0_4; 0.5 g/L de MgSO„ 7H₂0, 0.5 g/L de K₂HPG₄, 0 10 g/L de KCI, and 0.01 g/L of Ca(N0₃)₂ · 4H₂0. Likewise, the pH of the mineral medium is controlled in a range between 1.7 and 1.8 by means of adding an acid or a base, for example, by means of adding sulfuric acid at a concentration of 10%.

In one embodiment, tracking of the activity of the iron-oxidizing microorganisms in the reactor is furthermore performed, for example by means of:

- preparing a solution with the same mineral medium composition but without the iron salt FeS0₄ 7 H₂Q;

- extracting a specific amount of sample, preferably 2 ml, from the reactor and centrifuging for 10 minutes at 5000 rpm;

- eliminating the surplus and adding another specific amount, preferably 2 ml, of the mineral medium but without the iron salt FeS0₄ 7 H₂0;

- stabilizing the temperature by putting the sample in a thermostatic bath at 30°C;

- adding a specific amount, preferably 2 ml, of the mineral medium with the iron salt FeS0₄ 7 H₂0 and performing homogenization;

- extracting a specific amount, preferably 1 ml, of the mineral medium and introducing it in a container which is again placed in said thermostatic bath under mechanical stirring;

- introducing an oxygen microsensor in said container until contacting the sample or introducing the sample in said container with a sensor, for example an optical sensor, adhered thereto; and

- recording the progression of the oxygen concentration by means of said microsensor or by means of said sensor and determining the biological activity of the sample based on the time progression slope obtained.

Likewise, the activity of the sample can be linked with the concentration of iron-oxidizing microorganisms (i.e., biomass) by means of a prior calibration, for example by means of determining the biological activity of several known concentrations of biomass and correlating them with said activity and concentration parameters. In one embodiment, calibration of the oxygen microsensor in an oxygen-free aqueous medium and under saturation conditions at a constant temperature is also performed.

In one embodiment, the mentioned step of irrigation is performed at room temperature and at a pH less than 1.8.

Brief Description of the Drawings

The foregoing and other features and advantages will be better understood based on the following detailed description of several merely illustrative non-limiting embodiments in reference to the attached drawings, in which:

Fig. 1 is a flow chart illustrating a method for the biological recovery of metals in electric and electronic waste according to the present invention.

Fig 2 schematically illustrates a bioleaching plant for implementing the proposed method. Detailed Description of Several Embodiments

With reference to Fig. 1 , therein it is shown an embodiment of the proposed method for the biological recovery of metals in electric and electronic waste. As seen in the figure, first in step 101 , a series of aerobic iron-oxidizing microorganisms and a mineral medium formed by different salts in solution are introduced into a reactor 10 (see Fig. 2) such as a jacketed reactor, among others. Next, in step 102, a first step of biological oxidation of the iron II ions present in the mentioned mineral medium to iron III ions is performed in the cited reactor 10. This first step is catalyzed by the metabolic activity of said iron-oxidizing microorganisms and performed within a previously fixed range of temperature between 25 and 35°C, preferably 30°C, under constant mechanical stirring at a speed of about 200 rp , controlling the pH of the mineral medium. The first step lasts for at least two hours to enable assuring proper contact between the microorganisms and the iron II solution. Subsequently, in step 103, the iron-oxidizing microorganisms present in suspension inside the reactor 10 are separated out of the reactor, thereby providing a solid phase comprising the iron-oxidizing microorganisms and a liquid phase comprising the iron III ions, and in step 104, the liquid phase is continuously irrigated into a column 30 arranged for housing one or more printed circuit boards of the electrical material or electronic waste, the iron III ions being reduced to iron II oxidizing the metals, and the metals of interest are separated by means of the dissolution thereof. Finally, in step 105, the metals of interest are extracted from the solution. It must be indicated that instead of the mentioned printed circuit board (or boards), said column can house any electrical material from which the metals are to be recovered.

With reference now to Fig. 2, it is shown an embodiment of the bioleaching plant used for carrying out the present invention. As can be seen in the drawing, according to this embodiment the plant consists of 4 steps, a first step where the biological oxidation of the iron II is performed, a second sedimentation step, a third waste leaching, and a fourth cementation step. The different reactions involved in each phase are: Biological Activity ,

1^st step: 4 Fe²⁺ + 4 H⁺ + 0₂ - » 4 Fe³⁺ + 2 H₂0 (Eq. 1) 3^rd step: Cu + 2 Fe³⁺ ® Cu²⁺ + 2 Fe²⁺ (Eq, 2) 4^th step: Fe° + Cu^z+ ® Fe²⁺ + Cu° (Eq. 3)

The objective of the first step is to oxidize the iron II ion to an iron III ion catalyzed by the metabolic activity of the iron-oxidizing microorganisms which obtain energy from iron oxidation. During this step which takes place in the mentioned reactor 10, preferably at a temperature of 30°C, under constant mechanical stirring, the pH is controlled between 1.7 and 1.8 by means of adding an acid or a base. In one embodiment, said control is performed by means of adding of 10% sulfuric acid. Stirring used in this system is of the mechanical type and is maintained at a speed of about 200 rpm. The material of the stirrer is Teflon since this material does not undergo chemical etching nor does it react with any of the compounds found in the reactor 10. Given that the iron-oxidizing microorganisms used are aerobic, the reactor 10 is aired with air from the main supply with a flow rate of about 30 L/h The proper operation of the method is assured with continuous pH monitoring (optionally or additionally, an oxygen and/or redox control can also be performed) which allows knowing the level of oxidation achieved at any given time (about 600 mV in condition).

Preferably, the reactor 10 is continuously fed with a mineral medium with the following composition: 30 g/L of FeS0₄ · 7 H₂0, 3 g/L of (NH₄)2S0₄, 0,5 g/L of gS0₄ · 7H_zO, 0.5 g/L of K₂HP0₄, 0.10 g/L of KCl, and 0.01 g/L of Ca(N0₃)₂ 4 H₂0. It must be indicated that other mineral media or different compositions may also be used.

To prepare the mineral medium (about 1 liter), all the salts (except the iron salt) are dissolved in 700 ml of deionized water and the pH of this solution is adjusted to 1.75 with 10% sulfuric acid, for example. The iron salt is then dissolved in 300 ml of deionized water and the pH of this solution is adjusted to 1.75, also with 10% sulfuric acid. Finally, the two solutions are pooled and the pH result is verified, and where necessary, re-adjusted again to a pH of 1.75 with 10% sulfuric acid.

The tracking of the activity of the iron-oxidizing microorganisms in the reactor 10 can be performed by means of microrespirometric (oxygen consumption) techniques, for example by preparing a solution with the same mineral medium composition but without the iron salt FeS0₄ 7 H₂0; extracting a specific amount of sample, preferably 2 ml (this is non-limiting as other amounts may be extracted) from the reactor 10, and centrifuging for about 10 minutes at 5000 rpm; eliminating the surplus and adding another specific amount, preferably also 2 ml, of the mineral medium but without the iron salt FeS0₄ 7 H₂0; stabilizing the temperature by putting the sample in a thermostatic bath at about 30°C; adding another specific amount, preferably 2 ml, of the mineral medium with the iron salt FeS0₄ 7 H₂0 and performing homogenization; extracting a specific amount, preferably 1 ml, of the mineral medium and introducing it in a container which is again placed in the thermostatic bath under mechanical stirring; introducing an oxygen microsensor into the container until contacting the sample, or alternatively, directly introducing the sample into the container with a sensor, for example an optical sensor, adhered thereto; and recording the progression of the oxygen concentration by means of the microsensor or the sensor and determining the biological activity of the sample based on the time progression slope obtained.

The preceding process allows detecting the activity of the iron-oxidizing microorganisms with very small volumes of sample of less than 2 ml_ of sample.

The oxygen microsensor is calibrated in an oxygen-free aqueous medium (nitrogen shift) and under saturation conditions (oxygenating by means of a diffuser until achieving a stable signal) at constant temperature.

The objective of the second step is to separate the iron-oxidizing microorganisms found in suspension inside the reactor 10 This step is performed by assuring a solution settling time of at least two hours for iron-oxidizing microorganism sedimentation. The solid phase in the lower part of the sedimentation tank 20 with the iron-oxidizing microorganisms is preferably returned again to the reactor 10. The supernatant with iron 111 solution is driven to the following step at the same speed as the speed at which the mineral medium was introduced into the reactor 10. A part of the sediment is periodically purged from the system to maintain process continuity.

The objective of the third step is to oxidize the metals in metallic state contained in the printed circuit boards of the electric and electronic apparatus to be recovered. The chemical element responsible for the oxidation of the metals is iron III which undergoes reduction into iron II as it chemically etches the metal, and allows extracting the metals of interest from the solid-liquid system by means of the dissolution thereof.

The third step is carried out in a column 30 inside which there is placed one or more printed circuit boards. During the operation of the plant, the column 30 is constantly irrigated with the iron III solution such that the boards contact the solution and the chemical reaction which allows extracting the metals of interest takes place. To improve the contact between the waste and iron III and to assure that all the iron III reacts with the metals, preferably part of the leachate obtained in the lower part of the column 30 is re-circulated again to the upper part of the column 30. A contact time inside the column 30 of at least one hour is required to assure an efficient leaching of the metals contained in the printed circuit board/boards.

To facilitate leachate percolation, a structuring agent can additionally be used when performing the mentioned contact.

Likewise, liquid renewal cycles can be performed in the mentioned column 30, stirring the boards. The required time is thereby reduced and better biological recoveries obtained.

The process takes place at room temperature and at a pH below 1.8. It is important to keep the pH below this value to prevent iron III precipitation which would reduce the effectiveness of the process, and therefore the effectiveness of the metal extraction. To carry out this pH adjustment, pH control in the liquid re-circulated to the column 30 can be performed.

The objective of the fourth step is to reduce the metal extracted from the printed circuit board/boards from soluble state to metallic state to enable re-using it as a raw material. In this embodiment, the process used for obtaining the metal in metallic state is cementation. This process consists of the spontaneous reaction between soluble copper II and metallic iron, in which the copper is reduced to a metallic copper and the iron is oxidized to a soluble iron II. An electrorefining can additionally be performed to achieve a higher purity.

In this case, cementation takes place in a stirred tank 40 under mechanical stirring at a speed between 120 and 140 rpm and at room temperature. The contact time between the two metals to assure complete reaction is at least one hour. Once this time has elapsed, metallic copper in the form of powder is obtained by means of a filtration 41 whereas iron II and other soluble elements that may have leached out in the preceding step remain in the liquid. Alternatively to the mentioned filtration 41 , another separation mechanism, for example, a decantation mechanism, can be used. The iron II solution resulting from this fourth step can be re-circulated to the first step to reduce the need for iron of the biological process.

The scope of the present invention is defined in the attached claims.

Claims

1. A method for the biological recovery of metals in electric and electronic waste, comprising: a) introducing a series of aerobic iron-oxidizing microorganisms and a mineral medium formed by different salts in solution into a reactor (10); b) performing, in said reactor (10), a first step of biological oxidation of an iron II ions present in said mineral medium to iron III ions, wherein said first step is catalyzed by the metabolic activity of said iron-oxidizing microorganisms and performed within a previously fixed range of temperature, under constant mechanical stirring, controlling the pH of said mineral medium, and wherein said first step lasts for at least two hours; c) separating the iron-oxidizing microorganisms present in suspension inside the reactor (10) out of said reactor (10), a solid phase comprising the iron-oxidizing microorganisms and a liquid phase comprising the iron III ions being provided; d) continuously irrigating the liquid phase into a column (30) configured for housing an electrical material or one or more printed circuit boards of an electrical material or electronic waste from which metals are to be recovered, the iron 111 ions being reduced to iron II oxidizing the metals, and separating the metals of interest by means of the dissolution thereof, wherein the electrical material or the printed circuit board or boards are in contact with the liquid phase inside the column (30) for at least one hour; and e) extracting said metals of interest from the solution.

2. The method according to claim 1 , wherein said step e) comprises reducing said extracted metals of interest from soluble state to metallic state through a cementation process which provides a spontaneous reaction between soluble copper II, extracted from the electrical material or from the printed circuit board or boards, and the metallic iron, wherein in said spontaneous reaction the soluble copper II is reduced to copper metal and the metallic iron is oxidized to soluble iron II.

3. The method according to claim 1 or 2, further comprising re-circulating the soluble iron II obtained in step e) to said first step of biological oxidation to reduce the amount of iron required in said first step.

4. The method according to any one of the preceding claims, wherein said steps a) to e) are performed continuously.

5. The method according to claim 1 , wherein the different salts of the mineral medium include an iron II salt and salts that provide nitrogen, sulfur, phosphorus, magnesium, potassium, and calcium.

6. The method according to claim 5, wherein said mineral medium comprises the following composition: 30 g/L of FeS0₄ · 7 H₂0, 3 g/L of (NH₄)2S0₄, 0.5 g/L of MgS0₄ 7H₂0, 0.5 g/L of K₂HP0₄, 0.10 g/L of KCI, and 0.01 g/L of Ca(N0₃)₂ 4 H₂0.

7. The method according to any one of the preceding claims, wherein the pH of said mineral medium of step b) is controlled in a range comprised between 1.7 and 1 ,8 by adding an acid or a base.

8. The method according to any one of the preceding claims, further comprising tracking the activity of said iron-oxidizing microorganisms in the reactor (10) in said step b).

9. The method according to claim 8, wherein said tracking is performed by the following steps: preparing a solution with the same mineral medium composition but without the iron salt FeS0₄ 7 H_zO; extracting a specific amount of sample, preferably 2 ml, from the reactor (10) and centrifuging it for 10 minutes at 5000 rpm; eliminating the surplus and adding another specific amount, preferably 2 ml, of the mineral medium but without the iron salt FeS0₄ 7 H₂0; stabilizing the temperature by putting the sample in a thermostatic bath at 30°C; adding another specific amount, preferably 2 ml, of the mineral medium with the iron salt FeS0₄ · 7 H_zO and performing homogenization; extracting a specific amount, preferably 1 ml, of the mineral medium and introducing it in a container which is again placed in said thermostatic bath under mechanical stirring; introducing an oxygen microsensor in said container until contacting the sample or introducing the sample in said container with a sensor adhered thereto; and recording the progression of the oxygen concentration via said microsensor or said sensor and determining the biological activity of the sample based on the time progression slope obtained.

10. The method according to claim 9, further comprising; linking the activity of the sample with the concentration of iron-oxidizing microorganisms by means of a prior calibration; and/or calibrating the oxygen microsensor in an oxygen-free aqueous medium and under saturation conditions at a constant temperature.

11. The method according to claim 1 , wherein step d) further comprises re-circulating iron III from a lower part of said column (30) to an upper part thereof.

12. The method according to claim 1 or 10, wherein step d) is performed at room temperature and at a pH less than 1.8.

13. The method according to claim 2, wherein the cementation process is performed in a tank (40) mechanically stirred at a speed comprised in a range of 120 to 140 rpm at room temperature.

14. The method according to claim 1 or 13, wherein step e) lasts for at least one hour.

15. The method according to claim 1 , wherein said previously fixed range of temperature is comprised between 25 and 35°C, preferably 30°C, and the constant mechanical stirring is maintained at a speed of 200 rpm.