Task 2: Sulfate Removal from Mine Impacted Waters

Task Sponsor:  Freeport McMoRan

Background

Mine sites use large amounts of water as part of mineral processing where, this water is reused during the life of the mine. Eventually, the mines lower or stop operations, eliminating the option to reuse this impacted water consequently requiring treatment of the remaining water prior to discharge off site. The selection of a water treatment process is site-specific and has to meet the required discharge regulations for the state. One of the most common constituents on mining impacted water is sulfate.  The secondary drinking water standard for sulfate is 250 mg/L.  Above this level, the water has a salty taste.  The EPA does not enforce the secondary standards because these are primarily aesthetic standards as opposed to human health standards. More and more mines are being regulated to the secondary standards particularly if the water can impact drinking water supplies.

Sulfate in mining impacted water usually comes from pyrite, or other sulfur-based minerals found in the ore bodies. During mineral processing of these ores, the sulfur-based minerals can oxidize or dissolve to release sulfate into the mineral processing waters. While this can result in acid rock drainage (ARD), often there is sufficient alkalinity in the ore, primarily from calcite, to neutralize any acid generated as shown in the following chemical equation:

2FeS2 (pyrite) + 7.5O2 + 4CaCO3  + 11H2O è 2Fe(OH)3 + 4CaSO4.2H2O + 4CO2    (Eq.1)

Even if acid is generated, the most economical treatment for ARD is neutralization with calcium oxide (lime) as shown below.

H2SO4 + CaO + H2O è CaSO4.2H2O (Eq.2)

Both of these processes (Eq.1 & Eq.2) result in waters that are saturated with respect to gypsum.  Unfortunately, gypsum is slightly soluble in water and the resulting water can contain 1400-1800 mg/L of sulfate.There are many options available to treat the sulfate left after neutralization of ARD such as membrane filtration, chemical precipitation, ion exchange, biological processes and much more. More stringent discharge regulations in the future may promote more research to remove sulfate from water, using cost effective and sustainable technologies in comparison with the current options.

Problem Statement

Your team will research, evaluate and design a water treatment technology to remove sulfate from mine water impacted with calcium sulfate at circumneutral pH. The water treatment approach or technology should reduce the amount of sulfate from about 1,500 mg/L to below 250 mg/L in the treated water.

Design Considerations

Your proposed design should provide specific details and outcomes as follows:

  • Remove sulfate from water from about 1500 mg/L to below 250 mg/L
  • Recover at least 60% of the water in the clean water stream
  • Membrane filtration, chemical precipitation, ion exchange, biological or other technologies can be used.
  • A combination of technologies can be used
  • Cost of treatment is an important factor due to the large potential volumes of this water and should be addressed. Estimated costs for the full-scale design should include capital and operating cost for a 2000 gpm water treatment plant. Operating costs should be expressed in yearly operating cost and in $ per 1000 gallons treated.
  • Address residual management such as brine or solids
  • Address expected water quality and residual dissolved constituents in the treated water
  • Address process difficulties, such as solids precipitation scaling or other issues

Bench-Scale Demonstration

Demonstrate your design at bench scale using a synthetic gypsum saturated water. Your bench-scale working prototype should be able to treat at least 20 mL/min continuously. Synthetic gypsum saturated water will be provided to your team and will be prepared as follows.

  1. Mix 3 grams of solid gypsum per liter of deionized water in a 55-gallon plastic barrel.
  2. Stir for at least 3 hours with a mixer of sufficient power to suspend the gypsum
  3. Allow the excess gypsum to settle out
  4. Decant the clear water

Success will be measured using a Hach turbidity sulfate measurement and a conductivity probe.

Written Report Requirements

The written report should demonstrate your team’s insight into the full scope of the issue and include all aspects of the problem and your proposed solution. The report will be evaluated for quality of writing, organization, clarity, reason, and coherence. Standards for publications in technical journals apply. In addition to the listed requirements, your report must address in detail the items highlighted in the Problem Statement, Design Considerations, and Evaluation Criteria.

Evaluation Criteria

Each team is advised to read the Participation Guide for a comprehensive understanding of the contest evaluation criteria. Upon registration, WERC will provide you with a copy of the Public Involvement Plan and Participation Guide.

Additionally, your proposed solution will be evaluated on the following:

  • Technical fundamentals, performance, safety and other issues stated in the problem statement
  • Potential for real-life implementation
  • Thoroughness and quality of the economic analysis
  • Originality, innovativeness, functionality, ease of use, maintainability, reliability, and affordability of the proposed technology
  • How well the bench-scale represents your full scale design concept
  • The quality of your treated water – the bench-scale processed water will be evaluated for treated water volume, separation efficiency, and time to process

Optional tour to the Freeport-McMoRan Water Treatment Test Facility

A tour can be provided during the fall of 2017 (date to be determined) for interested students to visit the Water Treatment Test Facility at the Freeport McMoRan Sierrita Mine, which focuses on developing water treatment technologies for mining impacted water. The Sierrita mine is located 20 miles southwest of Tucson, Arizona. For more information about the Sierrita mine visit http://www.fcx.com/operations/USA_Arizona_Sierrita.htm. Students who are interested in the tour will have to have to cover their own transportation cost.