Water with low sulphate Water with high sulphate LoadingGypsum Sulphuric Acid Lime RegenerationFigure 4: Simple schematic of the Sulf-IXTM Process
The process is particularly suited for the removal of sulphate from lime plant effluent but is applicable for the treatment of any process stream, groundwater, or wastewater high in Total Dissolved Solids (TDS) and Ca/Mg hardness. The complete process cycle includes resin loading, regeneration and rinsing. Feed water is first passed through a series of contactors containing cation exchange resin to remove primarily calcium and magnesium by loading the cations onto the resin, and then through contactors containing anion exchange resin to remove sulphate (equations 2 and 3).
Ca2+(l) + 2Rf-H(s) = 2Rf-Ca(s) + 2H+ (l) (2)
2H + (l) + SO42-(l) + Rf(s) = Rf.H2SO4(s) (3)
Where, (s) and (l) represent solid and liquid phases, respectively, and Rf depicts the resin functional groups.
Ion exchange resins have finite capacities to remove ions from feed water hence must periodically undergo regeneration (equations 4 and 5).
Ca2+(l) + 2H+(l) + 2SO4 2-(l) + Rf-Ca(s) + 2H2O =
Rf-2H + Ca2+(l) + SO42-(l) + CaSO4.2H2O(s) (4)
Rf.H2SO4(s) + 2Ca2+(l) + 2OH-(l) + SO4 2-(l) =
Rf + Ca2+(l)+ SO4 2-(l) + CaSO4.2H2O(s) (5)
As can be seen from reaction (4), sulphuric acid is used for cationic resin regeneration. Solid gypsum dihydrate formed during regeneration is separated from the spent regenerant solution in a clarifier and, following the addition of H2SO4, the “refreshed” regenerant solution is 100% recycled to resin regeneration in subsequent resin cycles. Similarly, reaction (5) shows that lime is used for anionic resin regeneration, with gypsum formed and separated in a clarifier. Following the addition of Ca(OH)2, the “refreshed” regenerant solution is 100% recycled to resin regeneration in subsequent resin cycles.
The technology was initially based on the GYP-CIX technology developed in South Africa (Robertson et al, 1993), which also uses sulphuric acid and lime for resin regeneration. The Sulf-IX™ Process, however, overcomes difficulties of the GYP-CIX process associated with limited process flexibility for varying feed chemistry, mechanical entrainment of gypsum in the regeneration stage, and limitations on sulphate removal when magnesium is present in significant concentration in the feed water. These and other process developments have resulted in a significant reduction in the estimated costs of constructing and operating a commercial plant. Estimates carried out on a number of potential applications indicate that the operating cost to reduce sulphate from the concentration of a typical lime plant effluent to less than 500 mg/L will be in the range US$0.60 to US$6.00 per m3 depending on several site specific factors, including the scale of the plant flow and the concentration of magnesium in the feed. Magnesium can also load on the cationic resin but its removal is much lower than calcium due to the high solubility of magnesium sulphate relative to calcium sulphate and can therefore build up in the cation resin regeneration circuit. If this is the case, treatment of a bleed from the cation regeneration circuit is required to remove magnesium from the circuit, and is part of the Sulf- IX™ technology.
Environmental Benefits of Sulf-IX™ ProcessIn addition to being cost competitive, there are environmental benefits to the Sulf-IX™ Process:
• No hard to dispose of brine is produced as a by-product. The only by-product of the process is clean gypsum, which can be sold as a construction material or safely disposed of in a conventional land-fill.
• Reduced electricity consumption. Case study comparisons have indicated that the electricity consumption can be less than half of that for a membrane process to treat the same flow.