Pickling of refractory metals

Abstract

Refractory metal pickling bath of HF/H 2 O 2 aqueous solution with recovery of the etched metal as a salt thereof and separation of dissolved impurities taken from the refractory metal, with avoidance of drawbacks of state of the art HF/HNO 3 pickling solutions.

What is claimed is:  
     
       1. Process for surface finishing of tantalum parts in the course of mill processing or product fabrication to remove lubricant residue and surface contaminants comprising pickling the metal in an aqueous pickling solution for 5 to 100 minutes at 10 to 50° C., the solution comprising 100 to 400 g/l HF, 10 to 50 g/l H 2 O 2 , the bath being entirely free of nitrous oxide precursors; 
       treating the bath by addition of an alkali metal fluoride to form an alkali metal tantalum fluoride double salt that crystallizes out of solution in a form usable as a tantalum precursor and recovering the double salt while substantially avoiding salt deposition on the pickled tantalum/niobium surface; and  
       operating the process substantially continuously with continuous or periodic: (a) removal of the double salt, (b) addition of the fluoride, and (c) replenishment of pickling solution components.  
     
     
       2. The process of  claim 1  wherein the process is also established in a way to limit dissolution of Ta. 
     
     
       3. The process of  claim 1  wherein the additive is potassium fluoride and the crystallized double salt is K 2 TaF 7 , the solution is chilled during crystallization to a depressed temperature compared to the normal pickling temperature to insure complete removal of the salt from the pickling solution. 
     
     
       4. The process of either of claims  1  or  3  wherein the pickling bath is agtitated. 
     
     
       5. The process of either of claims  1  or  3  wherein the pickling is conducted in a first tank and pickling fluid is then removed to a second tank for conversion of the refractory metal ions in solution to a precipitable compound by alkali metal fluoride addition and the precipitation occurs primarily in the second tank with liquid effluent from the second tank being recycled to the first tank with replenishment of pickling bath components from external sources as necessary. 
     
     
       6. The process of  claim 5  wherein some of the alkali metal fluoride is also added to the first tank to accelerate pickling and reduce the rate of stainless steel material handling fixture dissolution in the pickling solution. 
     
     
       7. The process of either of claims  1  or  3  wherein the pickling and crystallization baths are controlled to limit H 2 O 2  decomposition.