An in-line mixing apparatus is especially useful for iodine extraction from brine. Three fluids (brine, an oxidant such as sodium hypochlorite, and an acid such as HCl) are mixed in the apparatus. The apparatus includes an inner tube and an outer tube. Openings are present in the sidewall of the inner tube to connect the interior volume of the inner tube with the annular volume between the inner tube and the outer tube. Fluid is passed into the inner tube and flows through the openings into the annular volume, causing immediate and vigorous mixing and chemical reaction to obtain elemental iodine.