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 3 to the spent solution, to form HF, HNO 3 , and Na 2 TiF 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 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 3 , and of dissolved titanium lie in acceptable concentration ranges, and transferring the regenerated solution into the machining vessel.

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 3  to the spent solution, and allowing it to react to form HF, HNO 3 , and Na 2 TiF 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 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 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 3  are added in quantities that are proportional to the molar quantities that correspond to stoichiometric reactions of NaF and of NaNO 3  with the dissolved titanium. 
     
     
         3 . A method according to  claim 1 , wherein in step b), NaF and NaNO 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 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 μm. 
     
     
         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 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 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 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 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 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.