Methods of Reducing Consumption of Sulphuric Acid in Regeneration of Strong Cation Exchangers in Water Desalination

Abstract

If calcium is present on a strong cation exchanger limited concentrations of sulphuric acid regenerant can be used to avoid CaSO₄ precipitation which would plug fixed bed contactors. This adversely affects water recovery and economy. At low concentrations calcium selectivity is enhanced and regeneration is less efficient requiring a greater excess of sulphuric acid than if HCl or HNO₃ were used offsetting the cost advantage of the cheaper H₂SO₄.

A new ion exchange flowsheet has been developed which overcomes these limitations and puts CaSO₄ precipitation to use to gain a number of major advantages. The loaded cation containing a mixture of cations is first treated with a CaCl₂ solution (the waste from the lime regenerated weak anion resin step in a desalination process) to convert it almost entirely to the Ca⁺⁺ form. In the subsequent sulphuric acid regeneration step precipitation of CaSO₄ is controlled (by promoting nucleation through recirculating seed crystals) to produce a soft sludge and prevent formation of hard scale. The fine precipitate is disengaged in a settler and the H₂SO₄, solution recycled to the regeneration unit. This results in using less than 10% excess acid giving major acid and neutralization agent (limestone) cost savings. A cost breakdown is given. The only waste from the sulphuric acid regeneration is solid CaSO₄, with small amounts of other cations and unused H₂SO₄. Water recovery is increased in this process.