Patent Application
20170247805
The invention is directed to a process and apparatus for metal refining, in particular for refining a mixture of conductive particles, such as mixtures of metallic particles. More particularly, the invention relates to a process and apparatus to separate concentrated metal streams, such as copper cathodes, zinc solutions or noble metal concentrates, from mixtures of heavy non-ferrous particles, containing for example copper, zinc, lead, and noble metals, with the particle sizes less than e.g. 40 mm or less than e.g. 15 mm or less than e.g. 5 mm, preferably with particles sizes smaller than 1 μm or smaller than 100 μm excluded.
Mixtures containing heavy non-ferrous particles are generally produced by shredding or incinerating waste (including waste to energy incineration) and secondary streams, such as municipal refuge and scrapped cars. From the generated mixture magnetic particles and low density materials are removed. The ferromagnetic particles are commonly removed with magnetic belt separators, while the low density materials such as plastics are often removed by some combination of eddy current separators and density separators. Lighter non-ferrous metals can be removed with heavy medium separators (as for instance disclosed in EP2412452), magnetic fluid separators or a variety of other well-known processes such as gravity separation, sensor sorting. In this way, a mixture of heavy non-ferrous particles is produced. Its composition depends strongly on the feed of the shredding or incineration process.
In the art, the mixture of heavy non-ferrous particles is usually sold to pyrometallurgical metal refiners. These refiners separate and upgrade the mixture by different processes which can contain melting, adding virgin metals, oxidizing at elevated temperatures and casting anodes, which are only then electrolytically refined. The value of the mixture of heavy non-ferrous particles is this way therefore generally lower than the sum of the value of the pure metals in the mixture. Also the pyrometallurgical processes have a high energy consumption and require large feed streams to be economical, which results in distant transporting of the mixture to central locations. The present invention does not require casting of anodes, adding virgin etcetera.
Some existing hydrometallurgical methods, as for instance disclosed in U.S. Pat. No. 7,799,184 purify valuable metals from mixtures of conductive particles by electrochemical decomposition of powders. However, this method is not versatile since less noble metals are not separated from the mixture of conductive particles. The presence of less noble metals hampers the purification of the desired metal by build-up of less noble metals in the electrolyte.
In alternative hydrometallurgical methods as e.g. disclosed in WO 2011/074948, WO 98/58090, US 2009/0074639, US 2006/0011014, and US 2009/0183997, valuable metals of interest are purified from mixtures using a combination of leaching and electrowinning. The metal of interest is first dissolved in a leaching step and subsequently recovered from solution by a electrowinning step. It would be desirable to make these processes more energy efficient and find a solution to the liquid waste stream created in these processes.
The present invention seeks to provide a process and an apparatus to purify metals, including but not limited to copper, zinc, lead, gold and silver from mixtures of conductive particles in order to give them more value and to reduce energy consumption compared to currently known processes. Also the invention aims to be more suitable for a smaller feed stream, thereby preventing distant transport of the mixture. Furthermore, production of environment-polluting residues is minimized.
It was found that these objects can be met by a process for recovering a metal of interest from a mixture of conductive particles comprising said metal of interest, which process comprises:
a) feeding said mixture of conductive particles to a separation unit, wherein a less noble metal is separated from the metal of interest by selective leaching, thus producing a concentrated stream comprising the less noble metal and a conductive stream comprising the metal of interest;
b) followed by a step of separating the metal of interest from said conductive stream by electro-refining, thus producing a stream comprising the metal of interest and a stream of concentrated conductive materials.
In the second step (b) of the present invention, the metal of interest can be plated onto cathodes using an electrical current whereby said conductive stream comprising the metal of interest is used as anode. To great advantage, this anode might be placed vertically using for example two perforated plates to hold the conductive stream, through which the electrolyte is pumped. A vertically placed anode allows for a good electrical field in combination with a vertical cathode, thereby producing high quality plated metal with little impurities. Preferably, the recovered metal of interest on the cathode has a purity of more than 95 wt. %, more preferably more than 98 wt. %, most preferably more than 99.5 wt. %.
In the present invention, first the less noble metals is separated from the mixture (step a), thereby making the process more versatile. As a result, the process may also be used to treat mixtures comprising these less noble metals.
Advantageously, the present invention enables concentration of metals preferably by plating them onto cathodes, separate as fine particles or separate as large particles, such that the separation of metals is more facile and the energy intensive leaching/recovery cycle is not necessary for the metals of interest.
The present invention does not use a two-step process for the separation of metals of interest, i.e. leaching the metal of interest followed by electrowinning. Instead, a concentrated metal stream, comprising the metals of interest, is produced directly from a conductive stream and in the same unit, preferably by electro-refining, thereby decreasing energy consumption. The leaching in the present invention involves the less noble metal is are not electrolytically recovered (i.e. selective leaching).
In the present invention, the degree of nobility of metals is related to the metal of interest in accordance with the galvanic series in the environment. For example, if the metal of interest is copper, zinc would be a less noble metal and gold would be a more noble metal.
For purpose of clarity and concise description, the metal of interest may be one or more metals of interest. Likewise, the less noble metal may be one or more less noble metals. The more noble metal may be more than one more noble metal.
In a preferred embodiment, the process of the present invention comprises an additional step c) wherein a more noble metal is separated as fine particles from said concentrated conductive stream, thus producing said stream of more noble metals and a stream of concentrated conductive particles.
In a particular embodiment of the present invention, said metal of interest comprises copper.
In another particular embodiment of the present invention, said less noble metal comprises zinc.
In yet another particular embodiment of the present invention, said more noble metal comprises gold.
In another preferred embodiment, in an additional step of the present invention, the spent acids or complexing agents are treated with the stream of concentrated conductive particles, so that the acids or complexing agents can be re-used.
The metal of interest is present in a mixture of conductive particles, which is used as a feed for the present invention. Typically, the mixture of conductive particles is a concentrate that originates from ash (e.g. bottom ash) from an incineration process (including waste to energy incineration) or from a shredding process of e.g. scrapped cars. Preferably, the mixture of conductive particles is a non-ferrous mixture of conductive particles since ferrous particles are typically removed from waste before or after the incineration of the waste.
The mixture of conductive particles is thus generally a secondary (or non-primary) source of the metal of interest. Metal ore that is extracted in the traditional mining industry is herein considered as the primary source for the metal of interest. It may be appreciated that the mixture of conductive particles originating from secondary sources differs significantly from ore mixtures from the mining industry.
The mixture of conductive particles in accordance with the present invention commonly comprises a large number of different metals, e.g. up to about 18 different metals. Typically, the mixture of conductive particles comprises three or more metals, such as between five to twenty different metals in amount of more than 0.1 ppm of the total atomic composition.
The mixture typically comprises 20 to 80 wt. %, preferably 30 to 60 wt. % copper (based on the total mass of the dry solids in the mixture of conductive particles).
The mixture of conductive particles further typically comprises 5 to 40 wt. %, preferably 10 to 30 wt. % zinc (based on the total mass of the dry solids in the mixture of conductive particles).
Typically waste streams from e.g. shredders or municipal waste treatment facilities also comprises small amounts of noble metals such as silver and gold. Consequently, the mixture of conductive particles in accordance with the present invention generally comprises gold in an amount of 1 to 300 ppm, preferably in an amount of 10 to 100 ppm (based on the total mass of the dry solids in the mixture of conductive particles). The mixture typically further comprises silver in an amount of 50 to 4000 ppm, preferably in an amount of 1000 to 2500 ppm (based on the total mass of the dry solids in the mixture of conductive particles).
Typically, the mixture of conductive particles comprises at least three, preferably more than ten, most preferably more than twelve different metals, which are preferably selected from the group consisting of silver, gold, platinum, palladium, lead, copper, nickel, zinc, tin, alumina, iron, cadmium, beryllium, chromium, cobalt, manganese, antimony and vanadium.
It may be appreciated that the mixture of conductive particle may also comprise non-conductive and/or non-metallic particles such as silicates originating e.g. from glass waste as long as the conductive properties of the mixture as a whole are not rendered null.
The metal of interest may be present in their metallic or oxidized form, as long as it is still conductive and may be (further) oxidized. The present inventors realized that typically the metals of interest, in particular copper, are frequently present in metallic form and that in order to select the metals and to concentrate them, it is very important to select the right acid or complexing agents or the right combination of acids or complexing agents, both with the right pH.
Furthermore, it is highly advantageous to select the right voltage for the electrochemical recovery.
In the first step (a) of the present invention, the less noble metal is preferably separated from a conductive supply stream (herein also referred to as the mixture of conductive particles) in the presence of a lixiviant. The lixiviant typically contains one of more acids or complexing agents, e.g. sulfuric acid, hydrochloric acid, nitric acid, ammonia, cyanide, hydroxide or the like. Said less noble metal is chemically converted into a salt that is dissolved in aqueous media, such as water. This process may be referred to as selective leaching. The metal of interest that is to be recovered is not converted into an in aqueous media soluble salt. Step a) thus involves selective leaching of the less noble metal to separate this from at least one metal of interest. This results in high less noble metal concentrations. For example in zinc concentrations of 20-200 g/L in a sulfate solution with a pH range of 0-1. Preferably this step is operated at elevated temperatures, more specifically at temperatures around 50° C. Typically the following reaction occurs, wherein zinc is used as an example:
Zn(s)+2H+(aq)→H2(g)+Zn2+(aq) (1)
After the less noble metal is converted into in aqueous media soluble metal salts and removed as a liquid stream, preferably to a considerable extent, e.g. 75% (based on weight) or more, the metals of interest is separated from said conductive stream (step b of the present invention), preferably to a considerable extent, e.g. 70% to 90% (based on weight). This is preferably done with an electro-refining step. The conditions in the electro-refining step need to be chosen such that a selection and purification of the metal of interest is achieved.
Electro-refining is a method to purify a mix of different metals. This step is typically carried out using an electrolytic cell, preferably wherein one or more pairs of negative cathodes and positive anodes are present. More preferably, the number of anodes is one more than the number of cathodes, such that the metal of interest may be plated on both sides of each cathode (
The used electrolyte is preferably 20-30 wt % sulfuric acid with a concentration of ions of the metal of interest (e.g. Cu2+ and/or ions of other metals of interest) between 20 and 45 g/L and/or with a pH range of −0.5 and 0.5. Electro-refining is preferably performed at elevated temperatures, more preferably at temperatures around 50° C.
By illustration, the following two half-reactions may take place, wherein copper is used as an example:
at the anode: Cu(s)→Cu2++2e−,
and at the cathode: Cu2++2e−→Cu(s).
The concentration of ions of the metal of interest in the electrolyte needs to be kept constant or substantially constant. However, these ions are expended as long as there are less noble metals present in the anode resulting in spent electrolyte. For instance zinc versus copper, the leaching of zinc follows the reaction: Zn(s)+Cu2+→Zn2++Cu(s), which depletes the electrolyte.
By illustration, the following two half-reactions may take place for zinc and copper:
at the anode: Zn(s)→Zn2++2e−,
and at the cathode: Cu2++2e−→Cu(s).
For this reason the spent electrolyte is additionally reprocessed in a step of the process of the present invention . In this step the metal of interest is dissolved in the spent electrolyte, in order to compensate for the depletion of ions from the electrolyte (this step may also be referred to as regeneration of the electrolyte). This is preferably done by adding an electron acceptor, e.g. an oxygen containing gas, such as air, to a bed of particles containing a dissolvable metal of interest.
After the electro-refining step b), the stream of concentrated conductive materials typically still comprise part of the metal of interest (i.e. the part that is not yet recovered in step b), also referred to as notyet recovered metal of interest), typically in an amount of 10-30% of the amount of the metal of interest in the mixture of conductive particles. Preferably, this not yet recovered metal of interest is used for the regeneration of the electrolyte. For example, a packed bed of particles containing dissolvable not yet recovered metal of interest and an electron acceptor are contacted with the spent electrolyte in which the metal of interest dissolves. By illustration, the following two half-reactions may take place by addition of electron acceptor in order to compensate for the depletion of the electrolyte. For example by using an oxygen containing gas as electron acceptor and copper as the metal of interest:
anodic: Cu(s)→Cu2++2e−,
cathodic: O2+2H++4e−→2OH−.
Since the reaction takes place under acidic conditions, it may be appreciated that the two-half reactions may also be represented by:
anodic: Cu(s)→Cu2++2e−,
cathodic: O2+4H++4e−→2H2O
To great advantage the dissolution of the metal of interest in the spent electrolyte and thus the regeneration of the electrolyte is accelerated by using ammonia as catalyst. To great advantage this dissolution is accelerated by elevating the temperature to at least 50° C.
In the concentrated conductive stream, typically high value metals remain present that are more noble than the metal of interest. The concentrated conductive stream that is obtainable by the present invention is very characteristic in its chemical composition, in particular in case the mixture of conductive particles originates from secondary sources (vide supra).
The process in accordance with the present invention typically results in a concentrated conductive stream that comprises silver and/or gold. In a preferred embodiment, the concentrated conductive stream comprises at least three, preferably more than ten, most preferably more than twelve different metals, preferably selected from the group consisting of silver, gold, platinum, palladium, lead, nickel, tin, alumina, iron, cadmium, beryllium, chromium, cobalt, manganese, antimony and vanadium. More preferably, the concentrated conductive stream comprises at least gold and/or silver.
The concentrated conductive stream preferably comprises less than 20 wt. %, more preferably less than 10 wt. % copper and/or less than 10 wt. %, more preferably less than 5 wt. % zinc (based on the total mass of the dry solids in concentrated conductive stream).
Preferably, the concentrated conductive stream comprises gold in an amount of more than 150 ppm, preferably more than 500 ppm. Typically, the concentrated conductive stream comprises gold in an amount of up to 5000 ppm, more preferably in an amount of up to 10000 ppm (based on the total mass of the dry solids in the concentrated conductive stream).
Preferably, the concentrated conductive stream comprises silver in an amount of more than 4000, preferably more than 10000 ppm. Typically, the concentrated conductive stream comprises silver in an amount of up to 25000 ppm, more preferably in an amount of up to 100000 ppm (based on the total mass of the dry solids in the concentrated conductive stream).
Exact concentrations and even the presence of the metals may vary based on the chemical composition of the feed stock (i.e. mixture of conductive particles). Metals that are typically present in the concentrated conductive stream are provided in table 1 (concentrations based on the total mass of the dry solids in the concentrated conductive stream). For sake of conciseness and clarity, all metals are given in a single overview. However, it may be appreciated that, for the reasons provided above, the composition of the concentrated conductive stream may be any combination of any of the given metals in any of the given concentrations.
The concentration of in particular nickel, iron, tin, aluminum, cadmium, beryllium, cobalt, manganese, antimony and/or vanadium are generally lower than the concentration of other metals and may vary significantly on the type and composition of the source material used for the present invention. Concentrations are therefore indicated as either small or trace (i.e. less than small) amounts.
From the concentrated conductive stream, which remains after the electro-refining step, additionally a more noble metal may be separated (i.e. step c) by for example filtration. The present inventors realized that such a more noble metal of interest, e.g. gold or silver, is usually present as a coating of the conductive particles. When the conductive particles are partially dissolved in prior leaching and/or electro-refining steps, small particles of this more noble metal will remain. These small particles are then concentrated by using a filter. This is preferably done during the electro-refining step, by circulating the electrolyte through the electrolytic cell. The electrolyte will transport the small particles out of the electrolytic cell. A concentrated noble metal stream can be obtained by filtering the electrolyte. For instance, gold may typically be obtained in a concentration of 2000 to 8000 ppm (based on the total mass of the concentrated noble metal stream).
In a preferred embodiment of the present invention, part of the spent electrolyte from step b) is used in step a) as lixiviant. This way buildup of ions of the less noble metal(s), e.g. zinc, in the electrolyte is prevented. In this case the reaction in step b) initially changes, whereby ions of the metal of interest are cemented onto the particles, and are thus not lost as aqueous media soluble salt, because they can be extracted again in step b) or d). Typically the following reaction occurs, wherein copper and zinc are used as an example:
Zn(s)+Cu2+(aq)→Cu(s)+Zn2+(aq) (2)
Additionally, the inventors realized that in the remaining concentrated conductive stream, high concentrations of metals, such as noble metals and lead, are present that are not removed during the steps a) and b) of the present process. This remaining solid stream is more concentrated in these metals after steps a and b of the present process, thereby making the mixture more suitable for further refining. This refining is commonly done by pyrometallurgical processes. However, some initial refining can be achieved by a novel step. Accordingly, in a preferred embodiment of the present invention steps a) and b) are followed by a further refining step, thereby separating at least one more noble metal of interest from said concentrated conductive stream, thus producing a stream of concentrated more noble metals and a second stream of concentrated conductive materials. It may be appreciated that in case said step of further refining is applied, step c) may be performed on the second stream of concentrated conductive materials.
In
In a typical example of the electro-refining unit (
In a particular embodiment, multiple anodes and cathodes are used in an alternating setup. Preferably the solids which remain after the electrolytic treatment in electro-refining unit 2 (also referred to herein as the stream of concentrated conductive material), are used in the electrolyte treatment step (16). The spent electrolyte (11) is fed to a filtration unit (3), where a more noble metal of interest, e.g. gold, is filtrated as particles from the spent electrolyte (11). Then the filtrated spent electrolyte (13) is fed to an electrolyte treatment unit (4). An example of a method to treat the spent electrolyte is to blow an oxygen-containing gas, e.g. air, (14) through the spent electrolyte (13) while it is in contact with particles that contain dissolvable metal(s) of interest (i.e. not yet recovered metal of interest), e.g. particles in the stream of concentrated conductive materials which remain after the electro-refining step (16). New (fresh) acid is added in this unit (15) in order to compensate for the acid which leaves the system with the pregnant lixiviant (7). If the remaining particles resulting from the electro-refining unit (16) (stream of concentrated conductive materials comprising non-recovered metal of interest) are used as the source for the ions to regenerated the electrolyte in the electrolyte treatment unit (4), then the remaining solids resulting from this unit (4) contain a high concentration of other conductive particles, i.e. the more noble metal e.g. lead and silver (17).
In a further aspect of the present invention, the inventor found that a particular apparatus may be used that is suitable for carrying out the present invention.
The apparatus comprises a dissolution unit, which comprises an opening for feeding a conductive supply stream, in particular a heavy non-ferrous mixture of particles produced by shredding or incinerating waste and secondary streams; a refining unit, which comprises a duct for receiving the effluent of said dissolution unit; the apparatus preferably further comprises an electrolyte filtration unit, which comprises a duct for receiving the effluent of said refining unit.
Preferably, the apparatus further comprises an electrolyte treatment unit that comprises a duct for receiving the spent electrolyte. Typically said refining unit is an electro-refining unit which comprises perforated plates (typically based on non-conductive materials such as PVC or coated stainless steel) that form a compartment comprising the conductive stream as anode. Adjacent the compartment—and in case of multiple compartments, in between two compartments—a cathode is present. The electro-refining unit preferably comprises one or more vertical anodes and one or more cathodes preferably spaced in such a way that the conductive stream can be prevented from entering the space between each compartment when feeding with a tool and a short circuit is prevented.
In a preferred embodiment, the units are the size of a pallet box, for example, the floor dimensions are near to those of an EURO pallet or an GMA pallet, with a height between 600 and 1200 mm. Also the flooring of the unit is such that it can be lifted and rotated upside down by a forklift, or other jacking device. This way the remaining solids can be easily unloaded from the unit.
In a preferred embodiment, the perforated plates which support the anodes are spaced in such a way, that, when the unit is filled from the top, a sieve or lattice can be used to guide the mixture of conductive particles between the perforated plates at the anode side and not between the perforated plates at the cathode side.
For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013407 | Sep 2014 | NL | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NL2015/050612 | 9/3/2015 | WO | 00 |
