Routine to estimate composition of concentrated metal–nitric–hydrofluoric acid pickle liquors

Hydrometallurgical processes need feasible methods for controlling the composition of concentrated metal and acid solutions that provide real-time measurements of the main process parameters. A large contrast between controlling methods appears when accuracy and speed are evaluated. This paper proposes a new titration method for determining the composition of concentrated solutions of nitric, hydrofluoric acids containing iron, chromium and nickel, originating from stainless steel pickling. Methods for controlling this process depend usually on probes in direct contact with the pickling bath which are used to calculate, in an approximate manner, bath composition. The proposed analysis consists of determining the pickling bath composition by fitting simulated titration curves to experimental plots, using various metals, acid and anion concentrations as parameters. Sodium and potassium hydroxide were used to titrate 12 synthetic pickling liquors. Results show that sodium hydroxide cannot be used as titration agent as sodium–fluoride ion interaction displaces equilibrium and makes singular points of titration curves to overlap. Titration with potassium hydroxide provides good correlation indexes and low errors in concentration prediction and is recommended for composition measurement. Mean relative error for metal concentrations is 0.4% for iron and nickel and 1.4% for chromium; while the mean relative error for anions is 4% for fluoride and 1% for nitrate.