British Columbia Mine Reclamation Symposium

Remote mine site acid rock drainage treatment Foote, M. W.; Jordan, D. M. 1998

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nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  REMOTE MINE SITE ACID ROCK DRAINAGE TREATMENT  M. W. Foote, Ph.D. D.M. Jordan, Process Engineer MSE Technology Applications, Inc. 200 Technology Way P.O. Box 4078 Butte, MT 59702-4078 ABSTRACT As a part of the Mine Waste Technology Program (MWTP) a pilot-scale technology was tested for the remediation of acid mine drainage at a remote mine site. A remote mine site is one without infrastructure or power sources. The technology chosen for this demonstration was the AQUA-FIX (system) because of its capability to run with minimal operator assistance in a self regulating manner. Weather conditions, variable flow rates, and drainage compositions were considered during the design phase. For this demonstration, acidic drainage treatment consisted of removing toxic, dissolved, metallic and nonmetallic constituents from acidic mine drainage and increasing pH such that the pH of the effluent was near neutral. The MWTP is funded by the U.S. Environmental Protection Agency (EPA) and is jointly administered through an interagency agreement with the U.S. Department of Energy (DOE). Work was conducted through the DOE Federal Energy Technology Center at the Western Environmental Technology Office under the DOE Contract Number DE-AC22-96EW96405. The project was intended to illustrate the viability and feasibility of using the technology to remove numerous contaminant species from an aqueous point source discharge for up to one year. INTRODUCTION  The site was the lower Crystal Mine portal adjacent to Uncle Sam Gulch. The Crystal Mine is located approximately 7 miles north of Basin, Montana, at an elevation of 7,500 feet in Jefferson County, Montana. The site was selected based on a number of criteria including weather conditions; variable flow rates; drainage compositions; characterization data quantity; and location. The Crystal Mine workings consisted of two collapsed horizontal levels and a sublevel between the two major levels. Trench-like workings were apparent along the surface exposing the mineralized zone. The subsurface workings are approximately 5,100 feet in length. Acidic, metal-laden water drains from the lower portal into Uncle Sam Creek.  125  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  Drainage parameter characteristics are: − − −  constant stream temperature of 33-350F; pH of approximately 2-3; and dissolved heavy metals (Tables 1 and 2).  Acid mine drainage from the lower Crystal Mine workings flows at a rate of less than 20 gallons per minute (gpm) for most of the year. Flows increase to as much as 100 gpm during the spring snowmelt (May-June). Acid mine water discharges into Uncle Sam Creek which travels along a 7-mile stretch before discharging into the Cataract Creek. The increase in toxic metal concentration causes aquatic damage to Cataract Creek. Figures 1 through 6 show the Crystal Mine adit , acidic drainage flow, and Uncle Sam Creek.  Table 2. Dissolved metals under peak flow conditions. (mg/L)  126  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  127  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  128  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  TECHNOLOGY DESCRIPTION  A treatment technology train was constructed at the mouth of the lower portal of the Crystal Mine and consisted of six unit operations. The process flow diagram is shown in Figure 7.  Initial Oxidation This unit operation consisted of a length of corrugated polyvinyl chloride (PVC) pipe. The turbulent flow of water through the pipe caused air to be dissolved into the aqueous mine drainage and partially oxidized the dissolved ferrous iron in the solution to ferric iron. The second purpose of this unit was to direct the aqueous discharge to the alkaline reagent addition unit. Two weir boxes were installed within this flow to separate the aqueous discharge into two streams. Stream one flowed under the wheel of the system while stream two flowed over and supplied power to the wheel. These streams were split to control reagent addition. The stream two weir box regulated water flow relative to variances in flow from the mine portal. Figure 8 and 9 respectively show the Crystal Mine portal and the building that housed the system.  Alkaline Reagent Addition This unit operation consisted of a water-driven solid reagent feeder which added an alkaline reagent, pebble quicklime (CaO), into the acidic solution to raise the pH to a value near 10.0 This pH was required to ensure removal of manganese hydroxides as precipitates. Figure 10 shows the lime fed system. The lime storage hopper delivered quicklime to a smaller hopper mounted on the system's feeder. The storage hopper was a 3,000-gallon cone-bottom tank located immediately adjacent to the building. This tank held approximately 11 tons of reagent, which was more than adequate for a year of operation. No conversion of CaO to CaCO3 was observed during the operation. The system used approximately 10% more reagent than the stoichiometry of the reactions which was due to excess lime buildup within the ponds.  Final Oxidation This unit operation also consisted of corrugated PVC pipe for turbulent flow. Turbulent flow through this section dissolved oxygen from the air into the aqueous mine drainage. The oxygen oxidized the ferrous iron in the solution to ferric iron. The ferric iron hydrolyzed in the alkaline solution to produce a ferric hydroxide precipitate. Ferric hydroxide adsorbed several metals removing those metals from solution.  Initial Solid-Liquid Separation This unit operation was designed to allow solid metallic hydroxide materials formed in the mine drainage, because of the alkaline reagent addition and oxidation of that fluid, to settle from solution. These solids formed a metallic hydroxide sludge that was periodically removed and disposed of in the sealed subsurface mine workings. This stage consisted of two ponds lined with 40-mil high-density polyethylene (HOPE). The settling ponds operated efficiently. Total 129  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  suspended pond overflow solids did not exceed 10 ppm during more than 90% of the systems operation. pH Adjustment The process consisted of corrugated PVC pipe causing a turbulent water flow through the apparatus causing the air to be entrained in the water. This process stage function was to adsorb carbon dioxide from the air. The entrained gas reacted to form carbonic acid thus lowering pH to near neutral. Final Solid-Liquid Separation  This unit operation consisted of two functions: −  To adsorb carbon dioxide from air to serve the same purpose as the pH adjustment phase and;  −  To allow for settling of any solid materials formed during this or the pH adjustment phase.  This stage consisted of a single pond lined with 40-mil HOPE. The pond served to hold a sufficient quantity of water to allow late-forming solids to precipitate and a sufficient amount of carbon dioxide to react with the water, Figure 11 shows these ponds.  Figure 7. Process Flow diagiram.  130  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  131  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  Results and Discussion The initial field demonstration was scheduled to last for one year; however, due to problems during the first year, the system was modified and operated for a second year. Field operation ceased in June of 1996. Problems associated with the treatment train in the first year emanated from reagent plugging the throat of the lime fed unit. This was caused by a combination of two factors, the first a function of unit design. The system was designed with an auger that was shorter than the tube encasing the auger by 6 inches. This caused the reagent to be deposited within the tube choking the end of the auger on a constant basis. The second factor that enhanced clogging was a breeze that swept up the pipeline that carried the water from the building to the first set of settling ponds. The pipeline faced downhill to the south and functioned as a chimney channeling the breeze into the building and the throat of the unit. The throat was less than 2 feet from the opening of the pipeline, moisture accumulated and enhanced reagent caking. Thus, reagent was not allowed to enter the acidic stream and effluent pH decreased rapidly. During the second year of operation, the process performed as designed 70% of the time. Lack of knowledge by the operational team accounted for 37 of the 91 failed days. These failures occurred in the first two months of the second year during a crew change. In March and again in May of the second year, lower portal collapses resulted in the process interruptions.  Data Interpretation Copper, Iron and Zinc removal efficiency rates over the two-year test period are shown graphically in Figure 12, 13 and 14.  132  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  133  nd  Proceedings of the 22 Annual British Columbia Mine Reclamation Symposium in Penticton, BC, 1998. The Technical and Research Committee on Reclamation  CONCLUSION  The technology treatment train incorporating the AQUA-FIX device was a feasible method of removing heavy metals from acidic drainage; however, the system has limitation and may be a usable alternative while a more efficient, permanent, source control-type of remediation is being developed. Trained operators should visit the site at least once every 2 weeks. Physical changes that are minor in nature should be made to the original design of the system to ensure a smoother operation as a whole. These include a longer auger and a wind screen. With the use of trained operational personnel, the system, as modified, would likely serve the purpose of protecting surface waters from the effects of acidic, metal-laden drainage until the source of the drainage can be controlled.  ACKNOWLEDGMENT  This demonstration was performed for the Mine Waste Technology Program (MWTP) at the Crystal Mine, which is located 7 miles north of Basin, Montana. The MWTP is funded by the U.S. Environmental Protection Agency (EPA) and is jointly administered by the EPA and the U.S. Department of Energy (DOE) through an lnteragency Agreement. Work was conducted through the DOE Federal Energy Technology Center at the Western Environmental Technology Office under the DOE Contract Number DE-AC22-96EW96405.  134  

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