To prevent the build-up of magnesium and other elements in the circulated zinc solution (the inventory) a fraction of the spent acid is sent to a process called "the discard plant". Here the spent acid is neutralized in two small tanks with NaOH and NaSO4 and ZnOH is formed. The solution goes to a belt filter where ZnOH removed, then the solution is cooled in a crystallizer and NaSO4 removed with a belt filter. The NaSO4 is reused in the jarosite process. The effluent is discharged to the effluent water treatment plant. The pH in the second tank is controlled automatically by a controller and a valve which adds sodium hydroxide. The pH in the third tank is measured.