Laboratory studies on treatment of used niobium electro polishing solution and neutralization tank design

Niobium superconducting radio-frequency cavities (SRF) have to undergo several steps of processing for achieving its intended performance. Electropolishing is one of the techniques of choice to obtain optimally smooth surfaces for high field superconducting RF cavity applications. Used electropolishing solution consists of mainly sulphuric acid, hydrofluoric acid and fluorides of niobium in complex forms. Treatment of niobium electropolishing solution is essential to neutralize the acidity of used solution and also for the removal of sulphates, fluorides and niobium ions present in this solution before disposal. Three different methods (i) treatment with calcium oxide (5% w/v solution) (ii) treatment with sodium hydroxide (10% w/v) up to pH 4 followed by pH adjustment using calcium oxide solution (5% w/v) and (iii) treatment with calcium oxide (5% w/v) up to pH 4 followed by pH adjustment using sodium hydroxide (10% w/v) were adopted for testing their removal efficiency. Ion chromatography was used for finding out the concentrations of fluoride and sulphate before and after treatment. Treatment with calcium oxide alone was found to remove fluoride and sulphate very efficiently. A dilution factor of 20 was optimized for treating 50 litres of used solution in a single batch for which a reactor vessel was also designed. (author)