GB2393969A

GB2393969A - Metal depletion in nitrate electrolytes by electrodialysis; nitrate recovery

Info

Publication number: GB2393969A
Application number: GB0323454A
Authority: GB
Inventors: Guenter Wiltschko; Juergen Hackenberg
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2002-10-10
Filing date: 2003-10-07
Publication date: 2004-04-14
Also published as: GB0323454D0; DE10247318A1

Abstract

A process for metal depletion of aqueous nitrate electolyte systems occurring during electrochemical machining (ECM) and containing impurities in the form of heavy metal cations comprises removing the electrolyte from the aqueous nitrate electrolyte system by electrodialysis. The heavy metals may be Ni, Co, Cr or Mo. A preferred system for carrying out the process comprises two circulation systems separated from one another by cation exchange membranes (13) and an ion exchange membranes (14). The nitrate, and nitrite, ions migrate out of the dilute chambers 15 into the concentrate chambers 16 whereas for example nickel and chromium ions remain in the dilute chambers because they cannot pass through the appropriate membrane. As electrodialysis continues the nitrate concentration increases in 16 and reduces in 15. In a second circulation system 19 the content of concentrate chambers 16 is pumped around continuously. A third circulation system 20, designated the "electrode flushing" system exists completely independent of the other two circulation systems.

Description

Process for metal depletion in nitrate electrolytes The invention relates

to a process for metal depletion of 5 aqueous nitrate electrolyte systems occurring during electrochemical machining (the ECM process), in accordance with the precharacterising clause of the main claim.

Background to the invention

The term ECM is understood to mean the defined removal of metals (or metal alloys) by electrochemical means, conventionally using electrolyte solutions containing chloride or nitrate.

The ECM process is used to shape metal workpieces and treat the surface thereof, with the physical principles of electrolysis forming the basis for machining. Here, the workpiece to be machined is usually connected up as the anode 20 and the tool as the cathode. Applying a direct current brings about chemical dissociation of the workplace at points which have been precisely defined beforehand. The volume removed is in this case influenced by the level of the working current and the period of its operation but also by controlled 25 flushing with electrolyte and appropriate construction of the tool. The ECM process is used for example for debarring, polishing and etching.

The advantages of the ECM process include a controlled 30 machining which can always be repeated with reproducible precision and the absence of any mechanical or thermal load on the workpiece.

When the ECM process is used, iron contained in the system is precipitated quantitatively as iron hydroxide and so can be removed by filtering. However, depending on the type of 5 alloy, heavy metal cations such as Ni2+, Co2+, Mo2+ and so on remain in solution and result in disruptive depositions of metal on the tool electrode (cathode). These may have an adverse effect on both the geometry and the conductivity of the precision tool. In production conditions, the 10 electrolytes are therefore disposed of or renewed in part at regular intervals.

Conventional electrochemical treatment processes such as electroflotation can only be used with ECM electrolytes which 15 behave inertly during the treatment procedure (such as chloride). In the case of electrolytes which themselves form a redox system (such as nitrate), this method is unsuitable.

Thus, when electrolytes containing nitrate are processed, for example, nitrite and ammonia are formed.

Electrodialysis (a membrane process) is a process enabling anions and cations to migrate in an electrical field. Here, a

basic feature is that no electrochemical reactions take place. The classic area of application for electrodialysis is 25 the desalination of seawater to obtain drinking water. In this case, the "disruptive" ions, that is to say the sodium cations and chloride anions, are removed from the aqueous system. 30 DE 39 03 024 Al describes a process for desalinating mixtures containing metal salts, of water, and highly active water-

soluble organic solvents, in which the mixtures are subjected

to electrodialysis using commercially available ion exchange membranes. Preferably, in this case flushing with an electrolyte solution free of sulphate ions is carried out.

5 DE 43 10 366 C1 describes a process for regenerating aqueous coating baths which operate without an external current, in which metal ions and a reducing agent (hypophosphite) are used. In this process, a combination of an ion exchange process and the electrode reactions of electrolysis is 10 proposed.

Advantages of the invention The process according to the invention has the advantage over 15 the prior art that, in contrast to chemical/physical

processes, no additional chemistry or addition of salts is required. Advantageous further developments of the invention emerge 20 from the measures mentioned in the subclaims.

Thus, it is advantageous if the circulation systems used are pumped around continuously.

25 Additional advantages are produced from the possibility of using a third circulation system for flushing the electrodes.

Brief description of the drawings

30 Example embodiments of the invention are illustrated in the drawings and explained in more detail in the description

below. In the drawings:

Fig. 1 shows diagrammatically how the ions migrate through the membrane circulation systems) and 5 Fig. 2 shows diagrammatically how the process according to the invention is incorporated into the overall ECM process; Example embodiments 10 The present invention is particularly suited to the treatment of ECM electrolytes which contain nitrate and are contaminated with heavy metal cations. It is not possible to separate the cations by conventional electrochemical methods (such as a membrane cell) because these electrolytes have 15 very high concentrations of nitrate. The cations of the actual ECM electrolyte, such as Na+ or K+, are monovalent and so they preferentially migrate during membrane processes, whereas the heavy metal cations (low concentration, polyvalent, highly charged) hardly diffuse through the 20 polymer membranes at all.

Electrodialysis provides a procedure by means of which ionic constituents can be removed from aqueous mixtures in a controlled manner. In electrodialysis an item of apparatus 25 which has a positive electrode (anode) of large surface area and a negative electrode (cathode) respectively is used. The space between the electrodes is divided up, by a plurality of alternately arranged cation exchange and anion exchange membranes, into a plurality of narrow chambers separated from 30 one another by the membranes. The membranes taken as a whole, the frame associated therewith, the sealing elements and the incoming and outgoing lines are also designated a stack.

For the process according to the invention, ion exchange membranes comprising organic polymers having side chains with ionic activity may be used. Cation exchange membranes 5 contain, for example, sulpho or carboxyl groups in the polymer matrix, while anion exchange membranes may have for example tertiary or quaternary amino groups as substituents of the polymeric basic material.

10 Chambers having an anion exchange membrane on the cathode side and a cation exchange membrane on the anode side form the so-called concentrate chambers. Chambers having the anion exchange membrane on the anode side and the cation exchange membrane on the cathode side form the so-called diluate 15 chambers. The diluate chambers are first of all filled with the solution to be regenerated (ECM electrolyte). An electrode flushing solution, which generally comprises an aqueous salt solution, is applied to the chambers in which the electrodes are located. Under the effect of the voltage 20 applied across the electrodes, the ions migrate out of the diluate chamber into the concentrate chamber, through that membrane which is permeable to them. It is not possible for them to migrate on through the next membrane, the one which is impermeable to the corresponding type of ion, and the ion 25 remains in the concentrate chamber. The liquids in the diluate, concentrate and electrode chambers are pumped around in separate circulation systems, for example with the aid of centrifugal pumps or the like.

30 Figure 1 shows diagrammatically an example of the structure of an electrodialysis device 10 having the electrodes (a cathode 11 and an anode 12), and by way of example two cation

exchange membranes 13 and two anion exchange membranes 14.

Located between the membranes are the diluate chambers 15 and the concentrate chambers 16. Arrows extending through a membrane indicate that the ion labelled with the arrow passes 5 through the membrane, and bent arrows show that the corresponding ion cannot pass through the membrane.

At the beginning of electrodialysis, the diluate chambers 15 are filled with the ECM electrolyte 17 which has been 10 contaminated by heavy metal ions. Once an electrical field is

applied, the ions migrate out of the diluate chambers 15 into the concentrate chambers 16, respectively through the membrane which is permeable to them. The liquid in the diluate chambers 15 is pumped around continuously by way of 15 the circulation system 18.

At the beginning of the process according to the invention, the circulation system 18 therefore contains the contaminated ECM electrolyte from production, which may for example 20 contain > 250 g/l of sodium nitrate and 0.2 to 1.0 g/l of nickel cations. In other words, there is a very large amount of nitrate by comparison with the heavy metals. The nickel cations disrupt the process and would moreover necessitate a waste water treatment. Because the conductivity of the 25 electrolyte is a function of the nitrate concentration, it provides a measurement value which indicates symbolically the nitrate content of the solution. A high level of conductivity therefore represents a high nitrate content, in this case. In the present example, the conductivity of the electrolyte in 30 the circulation system 18 is approximately 120 mS/cm.

As shown in Fig. 1, nitrite and nitrate ions migrate out of the diluate chambers 15 into the concentrate chambers 16, whereas nickel and chromium ions, for example, remain in the diluate chambers 15, because they cannot pass through the 5 appropriate membrane. As the reaction continues, the concentration of nitrite and nitrate ions in the concentrate chambers 16 therefore increases, whereas their concentration in the diluate chambers 15 is continuously reduced. The sodium or potassium ions also present in the diluate chamber 10 pass through the cation exchange membranes 13 but because of their small size they can also pass through the anion exchange membranes 14, and so they also migrate into the concentrate chambers.

15 In a second circulation system 19, the content of concentrate chambers 16 is also pumped around continuously. At the beginning of the process, the circulation system 19 only contains deionised (fully demineralized) water with a conductivity of approximately 0.1 mS/cm, but as the duration 20 of electrodialysis increases it is enriched with nitrite and nitrate ions, which may be tracked through the increase in the conductivity value.

Pumping around in separate circulation systems 18, 19 is 25 continued until almost all the content of nitrate ions from the contaminated ECM electrolyte which was initially supplied is in the concentrate chambers 16, which as indicated above can be monitored by way of the conductivity. Once the process according to the invention has been performed, the 30 circulation system 18 therefore contains only nickel and chromium cations with a conductivity of approximately 1.6 mS/cm, whereas the circulation system 19 has almost the

entire quantity of purified ECM electrolyte, which is free of heavy metal ions and has a conductivity of, for example, approximately 115 mS/cm. Thus, the sodium and nitrate ions have migrated out of the circulation system 18 into the 5 circulation system 19.

The purified electrolyte is removed from the system once a predetermined conductivity value is reached and may then be re-introduced into the overall ECM process.

The third circulation system 20, also designated the "electrode flushing" system, is completely independent of the other two circulation systems. It merely serves to maintain a certain conductivity in the overall system. The current flow 15 which is associated with the transport of ions gives rise to electrolytic reactions at the electrodes. As a result of the independent third circulation system, with a suitable electrode flushing solution, undesirable reactions can be avoided and only water is electrolysed, that is to say oxygen 20 and hydrogen are produced in this circulation system 20, with the result that the conductivity of the overall system increases. To avoid ions arriving at the anode or cathode and being 25 discharged there, barrier membranes 21 are provided in the region of the electrodes.

Preferably, the process according to the invention operates with stacks of approximately 25 to 100 membranes. The 30 temperature is below 50 C, preferably room temperature. The voltage applied is preferably approximately 2 V/membrane.

Operation is preferably potentiostatic, in other words the

l voltage is set to a fixed value and the current is regulated accordingly. The current flow is approximately 11 A on average. 5 Fig. 2 shows diagrammatically how the process according to the invention is incorporated into the overall ECM process.

The ECM electrolyte 22 is first of all subjected to a microfiltration 23 before undergoing the electrodialysis process 24 according to the invention. After that, the 10 purified electrolyte 25 is put back into the process circulation system.

The process according to the invention is distinguished in particular by the fact that the nitrate ECM electrolyte can 15 be recovered by means of electrodialysis, and the metal cations present as impurities are separated off as a concentrate. Thus, in contrast to the electrodialysis processes of the prior art, the process does not remove the disruptive ions

from the system, but rather the purified electrolyte is recovered, with the metal cations remaining behind.

In this way, electrolytes which contain a very high 25 concentration of nitrate ions (> 250 Gil) can be regenerated simply and successfully.

Claims

Process for metal depletion in nitrate electrolytes 5 Claims

1. A process for metal depletion of aqueous nitrate electrolyte systems occurring during electrochemical machining (the ECM process) and containing impurities in the 10 form of heavy metal cations, characterised in that the electrolyte is removed from the aqueous nitrate electrolyte system by electrodialysis.

2. A process according to Claim 1, characterised in that 15 the heavy metals are Ni, Co, Cr. Mo and the like.

3. A process according to Claim 1 or 2, characterised in that the nitrate content is > 250 Gil.

20

4. A process according to one of Claims 1 to 3, characterised in that two circulation systems separated from one another by cation and anion exchange membranes are used.

5. A process according to Claim 4, characterised in that a 25 third circulation system for flushing the electrodes is used.

6. A process according to Claim 4 or 5, characterised in that the circulation systems are pumped around continuously.

30

7. A process according to one of Claims 4 to 6, characterised in that the,cation exchange and anion exchange membranes are arranged alternately.

8. A process according to one of the preceding claims, characterized in that barrier membranes are arranged in the region of the electrodes for electrodialysis.

9. A process substantially as herein described with reference to the accompanying drawings.