Abstract: The residual acid mixture containing metals which has been produced in the steel pickling process is regenerated to become new HF/HNO.sub.3 pickling acid by applying H.sub.2 SO.sub.4 addition and evaporation. The metal sulphate salt produced in evaporation is converted into scarcely soluble iron jarosite Na[Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 ] and chrome and nickel hydroxide, the last mentioned metal hydroxide being obtained as a separately utilizable precipitate.The metal sulphate salt is separated from 60% H.sub.2 SO.sub.4 solution, wherein the main part of the salt is also crystallized. For promoting the crystallization, part of the salt is crystallized in a side line where 80% H.sub.2 SO.sub.4 concentration is used. The acid slurry produced is carried back to the crystallizing stage in the main line.

Abstract: An immersion evaporator is disclosed having, connected to its evaporation vessel, a pipe for feeding a solution to be concentrated by evaporation into the evaporation vessel, a riser pipe for withdrawing the concentrated solution and any crystals from the evaporation vessel, a pipe for withdrawing flue gases and vapors from the evaporation vessel, and an immersion pipe extending vertically inside the evaporation vessel, the upper end of the immersion pipe being connected, by means of an angle joint piece to the outlet of the combustion chamber, there being at the opposite end of the combustion chamber, a burner for liquid or gaseous fuel, the burner having a cylindrical combustion gas turbulence chamber for an oxygen-bearing combustion gas, at one end of which there is a substantially tangential combustion gas inlet conduit and a fuel dispersion pipe extending coaxially into the turbulence chamber, the opposite end of the turbulence chamber being convergent.