Peroxymonosulphuric acid or Caro’s acid is an oxidant employed for destroying free and weakly acid dissociable cyanide (WAD) in gold tailings or barren solutions. Due to its inherent instability, Caro’s acid is prepared and used on site immediately after preparation. Basically, this reagent is employed in some gold operations due to its oxidant ability to reach the typical targets of cyanide in effluents to prevent waterfowl mortalities, the very fast reaction and its low capital and operating cost. Caro’s acid is prepared by mixing hydrogen peroxide and sulphuric acid, H2O2 + H2SO4 = H2SO5 + H2O.
The product of this reaction is an equilibrium mixture of H2SO5, hydrogen peroxide, water and sulphuric acid. Considering a 2.5/1 mole ration of H2SO4/H2O2 and 93-95% and 66-72% concentrations of sulphuric acid and hydrogen peroxide, the composition in equilibrium is 23-26% H2SO5, 55-58% H2SO4, 3.2-3.8% H2O2 and 14-16% H2O. Caro’s acid is prepared cold or hot and most gold operations transfer hydrogen peroxide and sulphuric acid from a process loop controlled skid unit to a funnel-static mixer. And adiabatic temperature rise to near 56-58 oC in the static mixer when a 2.5/1mole ratio of sulphuric acid and hydrogen peroxide are added. The heat catalyzes the formation of Caro’s acid so rapidly that the reagent can be used immediately after preparation. There is a cold preparation process where the reagents are and the products are cooled. The Caro’s acid is stored for an intermediate time in a container located on the generator skid before being mixed with tailings slurry or barren solution.
Part of hydrogen peroxide is converted to Caro’s acid and this fraction is influenced by the ratio of H2SO4/H2O2 and the concentrations of both reagents. It is possible a reduction of the fraction of hydrogen peroxide that reacted if the final solution stands for any length of time. For this reason it is important to use the solution almost immediately. It has been noted that higher the H2SO4/H2O2 mole ratio, more hydrogen peroxide is converted to Caro’s acid. Nevertheless, for economical aspects, the amount of hydrogen peroxide must be carefully controlled. In this way, the economical ratio between sulphuric acid and hydrogen peroxide is 2.5-5/1 that represents approximately 66-79% of hydrogen peroxide converted to Caro’s acid. It is possible to employ higher ratios, but the production cost will increase. Also, if more sulphuric acid is added, the final pH will be lower than necessary and the only way to control this problem is to add lime and the preparation cost will be higher.