Patent ID: 7314604

Claim:
A method of selectively precipitating arsenic from a solution containing copper, ferric iron and ferrous iron whilst minimising copper losses which includes the steps of: (a) introducing an acidic solution containing arsenic(V), copper, ferric iron and ferrous iron in succession into each of a series of continuously stirred tank reactors; (b) adjusting the pH of the solution in each of said tank reactors and adding air to the solution to oxidise a portion of the ferrous iron to ferric iron and heating the solution to an elevated temperature to increase the rate of ferric arsenate precipitation and to minimise copper co-precipitation; (c) recycling a portion of selectively precipitated ferric arsenate compounds exiting a final tank in the series to a first tank in the series; (d) seeding the solution with ferric arsenate compounds to provide seeds for enhanced crystalline formation; (e) maintaining the pH of the solution in a second tank in the series at a pH of at least 1.5 and selectively precipitating ferric arsenate compounds from the seeded aqueous solution with a first calcium-containing neutralising agent; and (f) maintaining the pH of the solution in a third tank in the series at a pH which is higher than the pH in the second tank and which is at least 1.9 and selectively precipitating ferric arsenate compounds from the solution with a second calcium-containing neutralising agent.