A method of which includes the steps of introducing an acidic solution
containing (V), copper, ferric iron and ferrous iron into a first tank of
a series of continuously stirred tank reactors and, in the first tank,
adding air to the solution; heating the solution to an elevated
temperature; recycling a portion of selectively precipitated ferric
arsenate compounds to the said first tank; and seeding the solution with
ferric arsenate compounds.