Method for reclaiming metal compounds from a flow battery system

Abstract

The present invention relates to a method for reclaiming metal compounds from a flow battery system and to a flow battery system in which such method for reclaiming metal compounds is implemented. An object of the present invention is to provide a method for reclaiming metal compounds from a flow battery system in which performance instability and environmental issues are prevented or reduced to a minimum.

Description

[0001] Title: Method for reclaiming metal compounds from a flow battery system

[0002] Description:

[0003] The present invention relates to a method for reclaiming metal compounds from a flow battery system and to a flow battery system in which such method for reclaiming metal compounds is implemented.

[0004] Flow battery systems are well known in the art. For example, International application WO 2023 / 121454 in the name of the present applicant relates to a flow battery system, comprising a first tank including a hydrogen reactant, a second tank including a bromine electrolyte, at least one cell including a hydrogen reactant side operably connected to the first tank through an H2 feed and return system and a bromine electrolyte side operably connected to the second tank through a bromine electrolyte feed and return system. Moreover, European patent application EP 3 087 634 relates to a hydrogen-redox flow battery assembly. Other examples of flow battery systems are, inter alia, disclosed in International applications WO 2013 / 086100, WO 2018 / 201070, WO 2021 / 052782, and US 2012 / 299384, US 2015 / 188178, and US 2016 / 0322653.

[0005] Hydrogen-based flow-battery systems supplant the liquid electrolyte in traditional flow batteries with gaseous hydrogen that undergoes the reaction shown in the first equation as shown below. The positive electrode reaction is given in the second equation as shown below with the potential range of the positive redox species dependent on the redox-active species at the positive electrode:

[0006] Negative: E<> = 0.00 V

[0007] Positive: X + ne- nX- E° = 0.77 - 1.74 V

[0008] During cell discharge, H2 gas is supplied to the negative electrode, while X, a redox-active species that could be a halogen, oxygen, solvated ion, etc., is fed at the positive electrode. The H2 is reacted into protons and electrons on the catalyst (typically Pt), and the generated protons transfer through the membrane separator where they subsequently react with the electrons that are forced to go through the external circuit and the redox molecule in the positive electrode. During cell charge operation, the electrochemical reactions proceed in the opposite direction (i.e. species on the positive electrode side are consumed to produce H2 at the negative electrode and X on the positive electrode).

[0009] CN114883596A discloses an all-vanadium liquid flow battery management system, which includes an electrolyte temperature control device for maintaining normal operation of the entire system and a central control device including a positive electrode sediment recovery device. Such device includes a positive electrode electrolyte separation storage tank for storing the positive electrode electrolyte flowing out through the positive electrode electrolyte circulation outlet pipe, a positive electrode sediment sensor for monitoring the amount of positive electrode sediment deposited at the bottom of the positive electrode electrolyte separation storage tank, a positive electrode sediment collection unit for collecting the positive electrode sediment deposited at the bottom of the positive electrode electrolyte separation storage tank, an evaluation unit for measuring the weight of the positive electrode sediment and evaluating the performance of the electrolyte, and a baffle that can be remotely controlled to open and close for blocking the electrolyte to prevent it from falling due to gravity.

[0010] A scientific publication by Doulati Reza et al: "Sustainable Recycling of Electrolytes for Vanadium Redox Flow Batteries", December 31 , 2023 (2023 -12 -31), pages 1-35 discloses the development of a non-ammonium-based precipitation method to provide a unique and sustainable recycling procedure for Vanadium Redox Flow Battery (VRFB) electrolytes, wherein copper impurities significantly decreased battery performance. This study contributed to VRFB electrolyte recycling by presenting an environmentally friendly method yet potentially solving the problem of transportation of VRFB electrolytes and expensive recovery processes in the mining industry.

[0011] A common problem in flow battery systems is metal precipitation. Such precipitation has a negative influence on the performance of the flow battery systems and should therefore be minimized or prevented. As the metals in the electrolyte can precipitate in parts of the flow battery system, these precipitated metals need to be removed.

[0012] An option to clean such flow battery systems is to use external streams for dissolving the metal precipitation. Examples of external steams are acidic streams. However, these external streams have to be withdrawn from the flow battery systems and the streams thus obtained need to be treated before they can be used again. In some situations, these streams have to be discarded which is undesired from an environmental point of view.

[0013] Another option is that precipitation can be avoided by an increase of the temperature of the flow battery system. And by increasing the temperature the metal precipitation can redissolve into the process stream. In order to introduce such method specific equipment has to be incorporated in the flow battery systems, e.g. steam tracing and electrical tracing. This countermeasure is typical to avoid precipitation in dead ends of pipelines, such as high point vents and instrument connections.

[0014] In some situations it is also possible to extract a stream from the flow battery system, i.e. a so-called ‘bleed stream’, and to subject that stream to a specific treatment, e.g. filtering solid particulates from that stream. The stream thus filtered is qualified as a clean liquid and returned into the main process. This way of working will impact the material composition of the main process since materials are removed and the concentration thereof in the main process is altered, and the performance or durability is impacted.

[0015] The methods discussed above suffer from a number of disadvantages, such as performance instability, environmental issues, and costs aspects.

[0016] An object of the present invention is to provide a method for reclaiming metal compounds from a flow battery system in which the above identified disadvantages are prevented or reduced to a minimum.

[0017] The present invention thus relates to a method for reclaiming metal compounds from a flow battery system, said method comprising the following: a step of withdrawing an electrolyte stream from the flow battery system, said electrolyte stream comprising dissolved metal compounds and acid, a step of precipitation of the dissolved metal compounds present in the electrolyte stream withdrawn from the flow battery system, a step of transporting the electrolyte stream depleted in dissolved metal compounds to the flow battery system, wherein the step of precipitation is carried out by cooling down the electrolyte stream thus withdrawn from the flow battery system to a temperature at which precipitation of metal compounds occurs.

[0018] On basis of such a method the present object is achieved. The present inventors found that the electrolyte stream from a flow battery system can be used as a medium for solving the technical problem of the metal precipitation in a flow battery system. The construction of a closed loop of the electrolyte and gas stream in which loop several treatments are carried out on the electrolyte and gas stream is an essential technical feature of the present invention. In fact, the electrolyte itself is used as a rinsing liquid for the removal of metal compounds from the inside of the flow battery system, i.e. the stacks. According to the present method the electrolyte is stripped of the metal compounds resulting in a clean acid. The clean acid thus obtained is now able to dissolve any remaining metal compounds in the stacks and the rest of the system as a rinsing liquid and to return such metal compounds back to the electrolyte storage tank. The dissolved metal compounds present in the electrolyte stream are precipitated and the clean acid can be returned to the flow battery system and ready to be used again for the removal of metal compounds from the inside of the flow battery system.

[0019] According to the invention the step of precipitation is carried out by cooling down the electrolyte stream thus withdrawn from the flow battery system to a temperature at which precipitation of metal compounds occurs.

[0020] In an example the metal compounds depleted electrolyte stream is heated before the metal compounds depleted electrolyte stream is transported to the flow battery system.

[0021] In an example the electrolyte stream withdrawn from the flow battery system is stored in an electrolyte storage tank, wherein a part of the electrolyte stream is transported from the electrolyte storage tank to a first tank, in which first tank the step of precipitation of the metal compounds present in the electrolyte stream takes place.

[0022] In an example the metal compounds precipitated in the first tank are returned to the electrolyte storage tank.

[0023] In an example the metal compounds depleted electrolyte stream from the first tank is transported to a second tank, in which second tank a step of heating the metal compounds depleted electrolyte stream takes place.

[0024] In an example the electrolyte stream thus heated is transported from the second tank to the flow battery system.

[0025] The present invention also relates to a flow battery system comprising a closed loop of an electrolyte stream between a flow battery system, an electrolyte storage tank and a first tank, wherein an electrolyte stream comprising dissolved metal compounds and acid is withdrawn from the flow battery system and transported to the electrolyte storage tank, wherein a part of the electrolyte stream is transported from the electrolyte storage tank to the first tank, in which first tank precipitation of the metal compounds takes place, and the electrolyte stream thus depleted from the metal compounds is transported to the flow battery system.

[0026] The metal compound as discussed above are for example solid particles that remain behind, for example if the environment is not acidic enough or if a crack occurs in a membrane, causing electrolyte to move to the hydrogen side and then the moisture dries up, leaving the iron behind as solid particles.

[0027] In an example the acid is sulfuric acid. Other types of acid are hydrochloric acid, methanesulfonic acid and phosphoric acid.

[0028] In an example the metal compounds thus precipitated in the first tank are returned to the electrolyte storage tank.

[0029] In an example the electrolyte stream thus depleted from the metal compounds is heated up before being transported to the flow battery system.

[0030] As discussed above, according to the present invention the electrolyte itself is used in the method for reclaiming metal compounds from a flow battery system. As a first step, a side stream of the process is treated to precipitate the metals from it by cooling down that stream. In a next step a portion of the cleaned liquid is heated up. Such stream may be used as a flushing medium to dissolve the precipitated metals in the flow battery system and bring them back to the electrolyte storage without the need of external streams. The side stream with the remaining portion of the liquid and precipitated metals may be pumped back to be mixed with the main storage. Since the overall composition of the process is not changed, there is sufficient capability in the process to dissolve all metals and no further subsystems or treatments are required. From the above it is clear that the composition of the process is not changed since no materials are removed from or added to the flow battery system. This aspect improves the overall system performance and durability, and the costs for operating the system are significantly reduced. In an example the steps of cooling down and heating up can also be carried out in a single tank or in multiple tanks after the precipitated metals are removed after cooling.

[0031] The sole Figure shows a flow battery system according to the present invention.

[0032] The present invention will be illustrated by way of an example. In the sole figure a flow battery system 9 is schematically shown. An electrolyte stream 11 comprising dissolved metal compounds and acid is withdrawn from flow battery system 9 and sent back to electrolyte storage tank 1. A similar stream 12 can also be sent to electrolyte storage tank 1 through the gas piping of the system. From electrolyte storage tank 1 a stream 2 is transported to a first tank 3. In first tank 3 stream 2 is cooled down thereby inducing precipitation of the dissolved metal compounds. First tank 3 is provided with cooling means 4. The precipitation comprising metal compounds is transported via line 10 to electrolyte storage tank 1. The metal particles are transported from first tank 3 to electrolyte storage tank 1 to rebalance the total metal content of electrolyte storage tank 1. The metal compounds are dissolved by the higher temperature in electrolyte storage tank 1.

[0033] A metal compounds depleted electrolyte stream 5 is transported to a second tank 6 provided with heating means 7. In second tank 6 metal compounds depleted electrolyte stream 5 is heated to the temperature prevailing in flow battery system 9. Second tank 6 is provided with a heat exchanger 7. The thus heated stream 8 is transported to flow battery system 9 thereby creating a closed loop. Stream 8 is now ready for take-up of any metal compounds present in flow battery system 9 and the resulting stream 11 or 12 comprising the dissolved metal compounds and acid can be retreated as discussed above. Electrolyte storage tank 1 and flow battery system 9 can be operated at a temperature between 30 and 70 °C. First tank 3 can be operated at a temperature between 1 and 10 °C. Second tank 6 can be operated at a temperature between 40 and 70 °C.

[0034] In a case wherein metal particles, e.g. iron particles, pass through a membrane of the flow battery system 9 and end up in the hydrogen section and accumulate there, the heated acid from tank 6 will be used to dissolve the iron in the hydrogen line and it flows back to the electrolyte tank 1 via gas / liquid cross-over line 12 where the acid and iron are dissolved again in the electrolyte itself. The gas / liquid cross-over line 12 is a fixed line in the system that carries both the hydrogen gas and the liquid that crosses from the electrolyte to the hydrogen via the membrane back to electrolyte tank 6 to be reused there in the electrolyte and the hydrogen itself.

[0035] Although the process shown in the sole Figure consists of a first tank 3 and a second tank 6, it is also possible to carry out the method with multiple or only one single tank in which both cooling and heating of the electrolyte takes place.

Claims

CLAIMS1. A method for reclaiming metal compounds from a flow battery system, said method comprising the following: a step of withdrawing an electrolyte stream from the flow battery system, said electrolyte stream comprising dissolved metal compounds and acid, a step of precipitation of the dissolved metal compounds present in the electrolyte stream withdrawn from the flow battery system, a step of transporting the electrolyte stream depleted in dissolved metal compounds to the flow battery system, wherein the step of precipitation is carried out by cooling down the electrolyte stream thus withdrawn from the flow battery system to a temperature at which precipitation of metal compounds occurs.

2. A method according to claim 1 , wherein the electrolyte stream depleted in dissolved metal compounds is heated before the electrolyte stream depleted in dissolved metal compounds is transported to the flow battery system.

3. A method according to any one or more of claims 1-2, wherein the electrolyte stream withdrawn from the flow battery system is stored in an electrolyte storage tank, wherein a part of the electrolyte stream withdrawn from the flow battery system is transported from the electrolyte storage tank to a first tank, in which first tank the step of precipitation of the metal compounds present in the electrolyte stream takes place.

4. A method according to claim 3, wherein the metal compounds precipitated in the first tank are returned to the electrolyte storage tank.

5. A method according to any one or more of claims 3-4, wherein the electrolyte stream depleted in dissolved metal compounds from the first tank is transported to a second tank, in which second tank a step of heating the electrolyte stream depleted in dissolved metal compounds takes place.

6. A method according to claim 5, wherein the electrolyte stream thus heated is transported from the second tank to the flow battery system.

7. A flow battery system comprising a closed loop of an electrolyte stream between a flow battery system, an electrolyte storage tank and a first tank, wherein an electrolyte stream comprising dissolved metal compounds and acid is withdrawn from the flow battery system and transported to the electrolyte storage tank, wherein a partof the electrolyte stream is transported from the electrolyte storage tank to the first tank, in which first tank precipitation of the metal compounds takes place, and the electrolyte stream thus depleted in metal compounds is transported to the flow battery system.

8. A system according to claim 7, wherein metal compounds thus precipitated in the first tank are returned to the electrolyte storage tank.

9. A system according to any one or more of claims 7-8, wherein the electrolyte stream thus depleted from the metal compounds is heated up before being transported to the flow battery system.

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