PROCESS FOR REGENERATING A BATH FOR CHEMICAL ETCHING OF TITANIUM PARTS

Abstract

Disclosed is a method of regenerating a nitric and hydrofluoric acid bath contained in a machining vessel, the method including, when the etching bath is spent, performing steps of: transferring a portion of the spent etching bath, referred to as the spent solution, from the machining vessel into a reactor; adding NaF and NaNO.sub.3 to the spent solution, to form HF, HNO.sub.3, and Na.sub.2TiF.sub.6; separating the resulting precipitate from the supernatant; transferring the supernatant, which is a regenerated solution, into a tank; measuring the concentrations of HF, of HNO.sub.3, and of dissolved titanium in the tank and in the machining vessel; and determining the volume of regenerated solution that can be added to the spent etching bath to obtain a regenerated bath in which the concentrations of HF, of HNO.sub.3, and of dissolved titanium lie in acceptable concentration ranges, and transferring the regenerated solution into the machining vessel.

Claims

1. A method of regenerating a nitric and hydrofluoric acid bath for chemically etching parts made of titanium or titanium alloy and contained in a machining vessel (100), the method comprising determining whether said etching bath is spent, and if so, in performing the steps consisting in: a) transferring a portion of the spent etching bath, referred to as the spent solution, from the machining vessel (100) into a reactor (1); b) adding a quantity of NaF and a quantity of NaNO.sub.3 to the spent solution, and allowing it to react to form HF, HNO.sub.3, and Na.sub.2TiF.sub.6; c) settling to separate the resulting precipitate from the supernatant; d) transferring the supernatant, which is a regenerated solution, into a tank (2); e) measuring the concentrations of HF, of HNO.sub.3, and of dissolved titanium in the tank (2) and in the machining vessel (100); and f) determining the volume of regenerated solution that can be added to the spent etching bath in order to obtain a regenerated bath in which the concentrations of HF, of HNO.sub.3, and of dissolved titanium lie in respective predefined acceptable concentration ranges, and transferring said volume of regenerated solution into the machining vessel (100).

2. A method according to claim 1, wherein in step b), NaF and NaNO.sub.3 are added in quantities that are proportional to the molar quantities that correspond to stoichiometric reactions of NaF and of NaNO.sub.3 with the dissolved titanium.

3. A method according to claim 1, wherein in step b), NaF and NaNO.sub.3 are added in quantities that are 1% to 8% less in molar terms than the molar quantities corresponding to stoichiometric reactions of NaF and of NaNO.sub.3 with the dissolved titanium.

4. A method according to claim 1, wherein in step b), the mixture is allowed to react under stirring for a period of 2 h to 4 h at a temperature lying in the range 25 C. to 40 C.

5. A method according to claim 1, wherein in step c), the settling is performed at a positive temperature that is less than or equal to 15 C.

6. A method according to claim 5, wherein the settling is performed in the reactor (1), or after the content of said reactor has been transferred into a settling vessel.

7. A method according to claim 1, wherein the resulting regenerated solution is filtered at the inlet or at the outlet of said tank, using a device (20, 21) suitable for retaining chemical species of size greater than 5 m.

8. A method according to claim 1, wherein the regenerated solution is heated to a temperature identical to the temperature of the machining bath prior to being poured into the machining vessel (100).

9. A method according to claim 1, wherein: said range of concentrations that are acceptable for HF in the machining bath extends from 0.5N to 1N; and said range of concentrations that are acceptable for HNO.sub.3 in the machining bath extends from 1.4N to 1.8N.

10. A method according to claim 1, wherein at said range of concentrations that are acceptable for titanium in the machining bath extends from 10 g/L to 40 g/L.

11. A method according to claim 1, wherein said range of concentrations that are acceptable for titanium in the machining bath extends from 18 g/L to 25 g/L.

12. A method according to claim 1, wherein it is determined whether the etching bath is spent by the operations consisting in: measuring the concentration of dissolved titanium in the etching bath; comparing said measured concentration with said predefined range of acceptable concentrations; and if said measured concentration is greater than the maximum value of said range, triggering step a) of transferring a portion of the spent etching bath from the machining vessel to said reactor.

13. A method according to claim 1, wherein the concentration of sodium in the etching bath is measured, and if said sodium concentration is greater than a predetermined limit value, a fraction of the etching bath is removed from the machining vessel.

14. A method according to claim 1, wherein said predetermined limit value for the concentration of sodium in the etching bath is no greater than 7 g/L.

15. A method according to claim 1, wherein the concentration of vanadium dissolved in the etching bath is measured, and if said vanadium concentration is greater than a predetermined limit value, a fraction of the etching bath is removed from the machining vessel.

16. A regeneration method according to claim 1, wherein the nitrate and hydrofluoric acid chemical etching bath is a bath for chemically machining or a bath for pickling parts made of titanium or titanium alloy.

17. A method according to claim 1, wherein in step b), NaF and NaNO.sub.3 are added in quantities that are 5% less, in molar terms than the molar quantities corresponding to stoichiometric reactions of NaF and of NaNO.sub.3 with the dissolved titanium.

18. A method according to claim 1, wherein said predetermined limit value for the concentration of sodium in the etching bath is less than 5 g/L.

19. A method according to claim 2, wherein in step b), NaF and NaNO.sub.3 are added in quantities that are 1% to 8% less in molar terms than the molar quantities corresponding to stoichiometric reactions of NaF and of NaNO.sub.3 with the dissolved titanium.

20. A method according to claim 2, wherein in step b), the mixture is allowed to react under stirring for a period of 2 h to 4 h at a temperature lying in the range 25 C. to 40 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The present invention can be better understood, an associated details appear in the light of the following description given with reference to the accompanying figures, in which:

[0062] FIG. 1 is a diagram of equipment of the invention for regenerating chemical etching baths; and

[0063] FIG. 2 shows crystals of the Na.sub.2TiF.sub.6 precipitate as obtained after washing and drying the regeneration sludge, and as seen using a scanning electron microscope.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Example 1: Regeneration Techniques

[0064] A machining vessel one hundred containing an etching bath based on HNO.sub.3 and on HF receives parts made of titanium or titanium alloy. As machining operations progress, the bath picks up titanium, while HNO.sub.3 and HF are consumed. These concentrations are measured at a regular time intervals in order to monitor the state of the bath and to restore its state, should that be necessary. For each compound, acceptable concentrations correspond to a range of concentrations that are predetermined as being those concentrations within which it is desired to conduct the machining of parts. The acceptable ranges given below are given by way of example for a bath for machining parts made of titanium or titanium alloys. [0065] Ti: 10 g/L to 40 g/L (better 18 g/L to 25 g/L); [0066] HF: 10 g/L to 20 g/L; and [0067] HNO.sub.3: 88 g/L to 113 g/L.

[0068] Since the etching reagents HNO.sub.3 and HF are consumed continuously, an adjustment is made each time that is necessary, whenever at least one of them is at the bottom limit of its acceptable range, with this being in spite ofand independently ofany addition constituted by returning regenerated solution.

[0069] The bath is considered as being spent, either when its titanium concentration reaches 40 g/L (or 25 g/L if opting for higher reaction efficiencies and shorter operating cycles), or else when its concentration is going to reach this limit during the next operation of machining parts. A portion of the spent bath, e.g. amounting to 10% thereof, is then transferred to the regeneration circuit.

[0070] In practice, it is advantageous to estimate the rate at which the concentration of titanium increases in the bath, in order to anticipate the moment when it is going to reach the upper limit of the acceptable range. It is also possible to evaluate the frequency at which it is necessary to transfer spent solution to the regeneration circuit, and to proceed with transferring a portion of the bath regularly at said frequency, thereby facilitating automation. When the method is automated, a buffer tank 110 may be interposed between the chemical etching vessel 100 and the regeneration reactor 1, so as to pour the spent solution therein at the appropriate time, and trigger a regeneration operation subsequently, without any need to synchronize those two actions.

[0071] The spent solution is poured into the reactor 1, either directly from the machining vessel 100 or else from the buffer tank 110. The regeneration reagents NaF and NaNO.sub.3 are then introduced into the reactor 1 via feed pipes 30 and 31 provided for this purpose. The quantities added are calculated as a function of the titanium content in the spent solution, on the basis of the molar quantities of reactions (3) and (4). They are determined stoichiometrically for a total reaction minus 1% to 8% so as to avoid the machining bath containing too much NaNO.sub.3 and NaF that has not reacted with titanium in the reactor 1. These compounds could react with the titanium in the machining vessel 100 and precipitate, which it is desired to avoid, even though it appears that precipitating a small quantity of titanium salt does not lead to problems with chemical machining. Table 1 below gives the quantities of reagents that are to be added as a function of the quantities of titanium dissolved in the spent solution (minus 5% relative to stoichiometric reactions).

TABLE-US-00001 TABLE 1 Concentrations of NaNO.sub.3 and of NaF added as a function of the content of titanium in the spent solution [Ti] in (g/L) [NaNO.sub.3] (g/L) [NaF] (g/L) 18.0 60.8 30.1 20.0 67.6 33.4 22.0 74.4 36.7 24.0 81.1 40.1 26.0 87.9 43.4 28.0 94.6 46.8 30.0 101.4 50.1 35.0 118.3 58.45 40.0 135.2 66.8

[0072] The reactor one is maintained at a temperature selected in the range 25 C. to 40 C. for a period of 2 h to 4 h, while stirring. In order to optimize the reaction, it is preferable to select a temperature close to 40 C., and this does not require a large amount of energy to be supplied, since this is the machining temperature. The reaction produces HF, HNO.sub.3, and Na.sub.2TiF.sub.6. The content of the reactor 1 is cooled to a temperature selected in the range of 0 C. to 15 C., preferably to about 5 C., in order to enhance the precipitation of Na.sub.2TiF.sub.6.

[0073] Alternatively, the content of the reactor 1 is initially transferred into a settling tank (not shown), which is likewise cooled. The solubility of the Na.sub.2TiF.sub.6 salt can thus be reduced by nearly one half. Under the conditions described, precipitation and settling eliminate about 95% by weight of Ti in the form of Na.sub.2TiF.sub.6(H.sub.2O). Dissolved Na.sub.2TiF.sub.6 may thus remain in the solution, at a quantity of about 5%. The precipitate that is formed and that contains essentially solvated Na.sub.2TiF.sub.6 is eliminated as sludge from the bottom of the reactor 1.

[0074] Certain titanium alloy metals present in the spent solution are also eliminated, at least in part, by precipitating and settling, since they form complexes with the regeneration reagents. This applies in particular to aluminum, which precipitates the form of Na.sub.3AlF.sub.6 and is eliminated to more than 99%, of tin (75% elimination), of zirconium (61% elimination), and of iron (18% elimination).

[0075] The major fraction of water in the sludges is then removed in a filter 11, and the sludges are then stored prior to being eliminated. Techniques for elimination, including drying, recycling, or other treatments, do not come within the ambit of the present invention. Nevertheless, it can be understood that the volumes of waste for treatment are significantly less than the volumes of waste from conventional chemical etching.

[0076] The supernatant is a regenerated solution, that contains essentially HF and HNO.sub.3, together with some titanium and Na.sup.+ that have not reacted, and finally a proportion of Na.sub.2TiF.sub.6 that remains partially soluble. This solution is transferred into the tank 2 before being returned to the machining vessel 100.

[0077] In order to eliminate any crystals that might form by precipitation of the Na.sub.2TiF.sub.6 still present in the solution, it is possible to arrange respective filters 20, 21 at the inlet and/or the outlet of the tank 2. By way of example, it is possible to use a membrane filter, selected to retain particles of size greater than 10 m, and preferably greater than 5 m. As shown by the image given in FIG. 2, the crystals of Na.sub.2TiF.sub.6 are of a size enabling these filter means to be effective.

[0078] Prior to being poured into the machining vessel 100, the regenerated solution is heated to a temperature identical to that of the etching bath, i.e. about 25 C. to 45 C. The etching bath thus returns more quickly to its operating conditions, such that the machining process need not be interrupted for more than one hour. It can thus operate in quasi-continuous manner.

[0079] The volume of regenerated solution that is returned to the machining vessel 100 is determined on the basis of measuring the concentrations of the various species present in the regenerated solution and in the etching bath. After adding said volume of regenerated solution, new measurements are taken and adjustments are made, either by adding water, or by adding HNO.sub.3 and/or HF.

[0080] In general manner, concentrations are measured regularly at various points in the equipment, thereby serving to monitor and to control the various steps of the process, automatically, continuously, and in reliable manner.

[0081] Simultaneously, it is verified that there is no excessive accumulation of sodium in the machining vessel 100, so as to avoid any risk of undesirable secondary reactions. If sodium exceeds a previously set limit value, e.g. 7 g/L or preferably 5 g/L, a fraction of the spent bath is offloaded into and offloading vessel 120.

[0082] Also, certain alloyed metals are not eliminated during regeneration. This applies in particular to vanadium, which becomes concentrated little by little in the etching bath. Its concentration is monitored, and if it reaches an excessive level, e.g. set in the range of 5 g/L to 10 g/L, a fraction of the spent bath is offloaded to the offloading vessel 120.

[0083] It is possible to adopt another approach for avoiding undesirable compounds accumulating in the etching bath, which approach consists in regularly eliminating a fraction of the bath at predefined time intervals or at particular times when certain operations are performed. For example, it is possible to eliminate a fraction of the etching bath prior to each occasion when spent solution is drawn off to be regenerated. The fraction of the bath that is to be eliminated may be greater or smaller, e.g. 5% or 10%, depending on the nature of the parts being machined and on the nature of the treatment. The concentrations, in particular of vanadium and of sodium, are thus maintained at levels that are moderate. Such concentrations are thus measured more for monitoring purposes than for triggering emptying.

[0084] The presently described method of regenerating the bath is performed in discontinuous manner, by drawing of the spent solution at the desired time and by triggering a regeneration cycle when a certain volume needs to be treated, followed by storing the regenerated volumes of solution in the tank 2. It is also possible to have recourse to equipment that makes use of two reactors 100 operating in alternation. While one spent solution is being regenerated, another is being reintroduced into the production bath. Whatever the system adopted, chemical machining is interrupted only for a short length of time, of the order of one hour. This discontinuous regeneration system is simple to install and requires few resources.

[0085] It is also possible to make use of a continuous regeneration device. Such a system makes it possible to avoid interrupting production during regeneration, but at a cost that is higher and while being more complicated to put into place.

Example 2: Efficiencies

[0086] Tests of regeneration by the method of the invention have been carried out on a bath containing 25.2 g/L of dissolved titanium. The contents of HNO.sub.3 and of HF in the bath were measured as 1.55 moles per liter (mol/L) and 0.81 mol/L respectively, i.e. levels close to the minimum acceptable values in a preferred implementation of the invention (1.4 mol/L and 0.5 mol/L respectively).

[0087] The tests were carried out using three protocols that differ by varying the regeneration reagents: a) NaNO.sub.3 only, b) NaF only, and c) NaNO.sub.3 and NaF, both reagents being added at molar concentrations which are equals. The regeneration reagents were added in stoichiometric quantities minus 5%.

[0088] A portion of the spent bath was taken off and placed in a container at 35 C. under stirring for a period of 2 h, in the presence of one or more reagents, added in stoichiometric quantities. Thereafter the reaction mixture was cooled to 5 C. and allowed to settle. The resulting precipitate was separated from the supernatant. The quantity of titanium in the supernatant (i.e. a regenerated solution) was measured and its decrease (reduction) compared with the initial quantity was calculated. The results are given in Table 2.

TABLE-US-00002 TABLE 2 Reduction of titanium after regeneration a) NaNO.sub.3 b) NaF c) NaNO.sub.3 + NaF Quantity of 170.4 g/L 84.0 g/L 85.2 g/L of NaNO.sub.3 reagent (s) (i.e. 1 mol/L) and 42. g/L of NaF (i.e. 1 mol/L) Final Ti 4.5 g/L 3.8 g/L 3.3 g/L concentration Reduction 82% 85% 87%

[0089] In test c), the contents found in the regenerated solution were 1.9 mol/L of HNO.sub.3 and 1.16 mol/L of HF, which are higher than the maximum acceptable limit values in said preferred implementation (respectively 1.8 mol/L and 1 mol/L), but that is not an impediment since the regenerated solution is subsequently diluted on being returned into the machining vessel. The regenerated solution thus adds a significant quantity of regenerated etching reagents to the bath.

[0090] The above-described results are very encouraging, even though they have not been optimized. They could be improved by adjusting production condition parameters.

Example 3: Influence of Sodium

[0091] Tests were carried out to determine the influence of sodium on parts subjected to chemical etching. The question put was to verify that the sodium-containing reagents used for regeneration do not degrade the quality of the machined parts. Specifically, as mentioned above, reactions (3) and (4) are not total, such that there always remains a certain quantity, even if minimal, of the reagents NaF and NaNO.sub.3 in solution. These compounds are of small size and cannot be eliminated by filtering. They thus end up in the etching bath.

[0092] A test piece of TA6V titanium-vanadium alloy having dimensions of 5 cm5 cm and a thickness of 3 mm, as standardized for hydrogen embrittlement tests and identical to the test pieces used for monitoring chemical machining treatments in mass production, was used under the same conditions as for conventional monitoring of production conditions: removal of material by machining in order to achieve a thickness of 0.6 mm. The test piece was immersed in a chemical machining bath containing nitric acid and sodium fluoride at concentrations of 1.6N (i.e. 100.8 g/L) of HNO.sub.3 and of 0.8N (i.e. 22.5 g/L) of NaF. Thus, in that bath, hydrofluoric acid HF was replaced with NaF, at a concentration that was so high as to be unreachable in the context of the regeneration method of the invention. The etching bath did not contain any dissolved titanium, so that sodium could not react therewith and remained in free form in the bath. The test was thus carried out under conditions that were extremely unfavorable.

[0093] The test piece as machined in that way was subjected to hydrogen analysis, to an intergranular etching test, and to a pitting test. The conclusions of the laboratory were as follows: there was no intergranular etching, no pitting, and no hydrogen embrittlement. For each of the tests, the test piece complied with the requirements of the technical standards defined by the aviation industry for chemical machining of titanium alloys.

[0094] Those tests thus show that sodium, as is present in the presently-described technology, does not embrittle parts subjected to chemical etching. The solubility of NaF in the regenerated solution and returning to the chemical machining bath therefore does not degrade the health of parts that are machined chemically.