by Robert L. Zick
The use of mine pool water can provide significant advantages for new or expanded steam electric power plants that need to turn to non-traditional, alternative sources of water. New water treatment technology can now produce water that meets these strict specifications at significantly less chemical cost and sludge production than conventional treatment approaches.
Although mining practices often vary greatly according to the material produced and the value of the deposit, one common denominator is that mining of materials containing sulfide minerals creates acid mine drainage (AMD). AMD is one of the mining industry’s major environmental challenges.
A number of drivers influence the process selection for treating mine drainage waters. These include environmental regulations and the public’s growing demand for increased environmental stewardship. A growing number of local and regional initiatives also demand that bioaccumulative chemicals of concern, both inorganic and organic, be treated to ever-lower levels.
Consideration of AMD discharge is a major factor that influences treatment option decisions, including the purpose for which the water is to be treated and the method selected for disposal of the concentrated waste. In some cases, new water treatment technologies can efficiently allow for the marketing of mine pools as valuable resources.
New or expanded steam electric power plants frequently need to turn to non-traditional alternate sources of water for cooling. For example, six power plants in northeastern Pennsylvania currently use mine pool water as cooling tower makeup water (U.S. Department of Energy, 2006). Besides cooling tower makeup, there is a growing interest in providing the additional treatment required for AMD water to be used as boiler feed at steam electric power plants.
According to the U.S. Geological Survey, the steam electric power industry withdrew about 136 billion gallons per day of fresh water in 2000 (USGS 2005). With many areas of the United States facing fresh water shortages, and with the increasingly stringent target levels for discharge of treated mine drainage water, the beneficial reuse of this water for cooling and other purposes at steam electric power plants is becoming increasingly attractive to many mine operators and power producers.
Requires Additional Treatment
Although the use of mine drainage water as makeup water for power plants with closed-cycle cooling technology can often provide a number of advantages, including lower costs and a sustainable water source of sufficient capacity, this beneficial use option requires additional treatment to meet the water quality requirements. The problems experienced with mine waters often include:
• High salinity, frequently in the range of 2 g/l to 10 g/l.
• Dissolved and precipitated metals. These can be iron, manganese, aluminum, copper, zinc, cadmium, nickel, selenium and boron, to name a few.
• High levels of sulfate concentration, due to the oxidation of sulfide minerals contained in the ore.
• Water 100% saturated in calcium sulfate, or in some cases much higher.
Determining the optimized process for the particular circumstances requires analytical data on the source water as well as both bench scale testing and pilot testing to ensure the processes selected are reliable and easy to operate.
A power plant being constructed in Pennsylvania, for example, recently evaluated the use of AMD water for the purpose of providing cooling water and boiler feed water. The water supply under consideration came from a number of inactive mines in the area that require treatment of AMD to prevent pollution. The salinity of the water varies between 7,000 and 13,000 mg/L TDS. Dual treatment trains are currently in place to treat the water using a High Density Sludge (HDS) process.
Based on the following criteria, two approaches were evaluated to produce water for use by the power plant:
• The water treatment plant had to have an extremely high flow available to supply up to 6,600 gpm of treated water 24 hours per day, 365 days per year.
• The assumption was made that the concentrate from a reverse osmosis (RO) membrane process could be returned to another portion of the mine and should not have an adverse impact on the future quality of water to be contained in the mine.
Evaluating Treatment Options
One of the options under evaluation was a conventional approach. The treated water from the existing HDS plants would be further treated with a lime soda softening process followed by pH adjustment and filtration to provide water to an RO system using both brackish membranes and seawater membranes to achieve a predicted high recovery (80%-90% of the treated softened water).
This conventional approach required the removal of the potentially fouling and scaling constituents in the water by treating the existing HDS systems effluent through softening and filtration to reduce the calcium sulfate saturation in the RO concentrate to a controllable level.
The second approach utilized AMDRO technology, developed by N.A. Water Systems, a business unit of Veolia Water Solutions & Technologies (VWS). This process incorporates acid addition (either sulfuric or hydrochloric), ACTIFLO clarification, and multimedia filtration followed by cartridge filtration and reverse osmosis (See Figure 1).
The pretreatment processes ahead of the RO are designed to reduce the particulates and retain the scale-forming contaminants such as metals and calcium salts in solution, preventing scaling of the membranes upon concentration. The RO process is operated in double-pass mode with the first pass operated under acidic conditions, which effectively controls scaling due to metals and calcium salts, and the second pass at neutral pH conditions for further removal of dissolved inorganic compounds.
Due to the low pH of the process (2.5 S.U.), the AMDRO technology maintains iron and manganese in solution and uses a proprietary chemistry to remove suspended solids in the ACTIFLO clarification process. Under the acidic conditions, the iron and manganese stay dissolved and have no fouling effect on the RO membranes. In addition, at pH 2.5, a portion of the sulfate ions in the water are present as bisulfate, thereby reducing the tendency for calcium sulfate to precipitate.
A pilot unit was run to demonstrate the AMDRO process system and showed that the technology significantly reduced the chemical demands in the process while achieving the product water quality with the reliability required. Although the technology operated at 60%-65% water recovery, compared to 78% recovery achieved by the conventional approach, the operating costs were greatly reduced. The AMDRO technology also eliminated the impact of adding sodium to the mine water in the softening process. The reject from the conventional treatment process contained approximately 7.4% sodium sulfate.
The calculated chemical cost for the AMDRO technology was significantly less than that from the conventional process, (See Table 1) although capital costs were very similar. Less equipment was required with the AMDRO process, but a higher grade material of construction was needed to enable the system to operate at the low pH of 2.5.
The pilot study showed iron and other metals could be maintained in solution and effectively rejected by the RO membranes, reducing the concentrations to the low levels required for the power plant feed water by using the second pass RO bank (See Table 2).
The determination of costs associated with the use of mine pool water for power plant cooling depends on many factors, including the water influent treatment requirements of the power plant’s cooling and/or boiler system, mine pool water quality, and location of the mine pool resource. Although costs will vary with the complexity of a project, the beneficial use of mine pool water by power generators shows great potential for producing significant cost savings.
The pilot study demonstrated that AMDRO technology could be effectively used to produce the quality of water required for the power plant and at significantly lower chemical costs and lower sludge production than under the conventional approach. This technology would also benefit the quality of the mine water over the long term, as compared to the conventional techniques, by introducing less sodium in the mine pool in the recycled concentrate.
About the Author
Zick is director, mining market, N.A. Water Systems, a business unit of Veolia Water Solutions & Technologies, Inc. He can be reached at 412-809-6688; E-mail: firstname.lastname@example.org.