A new process for sulfate removal from industrial waters

A New Process For Sulfate Removal From Industrial Waters
June 20, 2001
Many industrial wastewaters, particularly those associated with mining and mineral processing, contain high concentrations of sulfate. These concentrations typically exceed the secondary drinking water standard of 250 mg/L and may be subject to discharge limits between 250 and 2000 mg/L.
The Cost Effective Sulfate Removal(CESR) process was developed to address the shortcomings of other technologies used for sulfate removal. The advantages of the CESR process are:
* Low concentrations of sulfate in treated water,
* Additional removal of metals and other parameters,
* No liquid waste, and
* A minimal volume of hazardous solid waste.
Process Description
The most common method for removing highconcentrations of sulfate from water is through addition of hydrated lime (Ca(OH)2), which precipitates calcium sulfate:
Na2SO4 + Ca(OH) 2 => CaSO4 + 2 NaOH
Calcium sulfate, which hydrates to become the common mineral gypsum, has a solubility of approximately 2000 mg/L as sulfate. Sulfate reduction below 2000 mg/L has been possible in the past only through expensive technologies such asreverse osmosis (RO) or ion exchange (IX). Large volumes of liquid waste are generated with RO and IX, which typically create additional treatment and disposal costs. The CESR process can reduce the sulfate concentration in most wastewaters to less than 100 mg/L through use of a proprietary powdered reagent.
Addition of the CESR reagent to lime-treated water precipitates sulfate as a nearly insolublecalcium-alumina-sulfate compound known as ettringite. Ettringite formation can also provide a polishing effect, allowing precipitation of difficult-to-remove metals such as chromium, arsenic, selenium and cadmium, often below their respective analytical detection limits. Boron, fluoride and up to 30 percent of the chloride and nitrate in water have also been removed. Metals and other constituentswhich the ettringite removes are typically not leachable, allowing disposal as a nonhazardous waste. The CESR process uses a sequential design to separate any metal hydroxide sludges from the other precipitates.
The CESR process is an extension of wastewater treatment with lime in that it can meet more stringent requirements for sulfate removal. Lime is inexpensive, readily available andproduces stable products which can be reused or disposed in landfills. Unlike treatment methods such as sodium aluminate addition, all of the chemicals added during the CESR process can be precipitated. Water treated by the CESR process typically meets or exceeds recommended drinking water standards for sulfate, metals and other parameters. The process produces a net reduction in total dissolved solids(TDS).
The ability to adjust the CESR process to achieve desired sulfate concentrations allows the process to be economically used by a wide variety of industries. Over 20 treatment plants in Europe now use the process, at flow rates up to 350 gpm. The CESR process essentially consists of four steps:
1. Initial precipitation of sulfate as gypsum
2. Precipitation of metals as hydroxides in agypsum matrix
3. Additional sulfate removal via ettringite precipitation
4. pH reduction using recarbonation.
Figure 1 shows the simplest version of the process where only final sulfate removal is required. Only Steps 3 and 4 of the CESR process are shown. The CESR process can be added to an existing wastewater treatment plant, and designed as a fully automated continuous process or a manualbatch system. Separate mixing and settling tanks may be used, or equipment such as a lime-softening clarifier could be used to combine mixing and settling.

Step 1 -- Initial Sulfate Precipitation
For wastewater with a high metals content and a sulfate concentration greater than 8000 mg/L, hydrated lime is used initially to precipitate most of the sulfate as gypsum. This precipitation occurs at...