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Workshops >> Tailings Closure Workshop
Agenda Tailings Impoundment Closure Workshop Sponsored By
Location:
Note: All information gathered and presented will be posted on the web for broader access to the national and international community at www.unr.edu/mines/mlc. Contact Information:
Tuesday, June 8, 20048:30 – 9:00 Welcome and Review of Tailings Impoundment Closure Workshop 9:00 – 10:00 1 ) Modeling Discharge of Interstitial Water From Tailings Following Deposition – Part 1 2 ) Barrick Goldstrike’s Approach to Tailings Closure 10:00 – 10:30 Refreshment Break 10:30 – 12:00 4) Performance of the Goldstrike Tailings Facilities 12:00 – 1:30 Lunch to be provided. 1:30 – 3:00 5) The Affect of Tailings Characteristics on Mine Reclamation and Closure 6) Closure of the Big Springs Tailings Facility, Elko County, Nevada 3:30 – 5:00 7) Field Performance Monitoring of Mine Tailings Cover Systems 5:00 Reception Wednesday, June 9, 2004 8:30 – 10:00 9) Modeling Discharge of Interstitial Water From Tailings Following Deposition – Part 2. 10) Material Characterization for Tailings Impoundments Closure Analysis. 10:30 – 12:00 11) Tailings Dam Closure: Designing Inside Out and Backwards. Open Forum for Short Presentations and Discussions. 12:00 Closure. Modeling Discharge of Interstitial Water from Tailings Following Deposition Abstract: This paper deals with the evaluation of entrainment water from tailings impoundments for engineering design and environmental planning. An integrated review of all physical processes experienced by tailings following deposition, closure and reclamation periods is presented. The review focuses on the description of the phenomena, and their modeling on a hypothetical constitutive surface. A methodology for quantitative assessment of tailings discharge throughout the processes is proposed by putting together a stream of modeling tools available in the current practice. Important remarks are made regarding initial conditions for unsaturated flow analysis in the sense of recognizing non-uniformity in the void ratio profile inherited from previous consolidation stage. It is also called attention to modeling behavior of silt-like materials. The approach was applied to a gold tailings project in a semi-arid climate in northern United States. Barrick Goldstrike’s Approach to Tailings Closure Ron Espell, Barrick Goldstrike Tel: (775) 778-8191 respell@bgmi.com , Joe Giraudo, P.E. Barrick Gold Strike, Carlin, NV, JGiraudo@bgmi.com, and Mike Henderson, P.E. Vector Colorado, LLC henderson@vectoreng.com Tel: (303) 522-1191 Abstract: Barrick’s Goldstrike operation is one of the largest gold mines in the world, having produced over 20 million ounces of gold since 1986 when Barrick purchased the property. Tailings from this operation have been hydraulically deposited in two tailings impoundments: the AA Block Tailings Impoundment which was operated from 1987 to 1994 and the North Block Tailings Impoundment which operated from 1994 to date. The two tailings facilities will store up to about 300 million tons of fine grained tailings, when they reach full capacity. Barrick’s corporate goals related closure of these tailings facilities are:
This paper discusses Barrick’s management approach to attain these overall goals related to closure. Closure Options for the Goldstrike Tailings Facilities Mike Henderson, P.E., Vector Colorado, LLC, henderson@vectoreng.com Tel (303) 522-1191 Pete Kowalewski, P.E., Vector Nevada, LLC, Tel (303) 522-1191 kowalewski@vectoreng.com Ron Espell, Barrick Goldstrike, (775) 778-8191 respell@bgmi.com and Joe Giraudo, P.E., Barrick Goldstrike, JGiraudo@bgmi.com Abstract: Barrick’s Goldstrike operation is one of the largest gold mines in the world, having produced over 20 million ounces of gold since 1986 when Barrick purchased the property. Tailings from this operation have been hydraulically deposited in two tailings: the AA Block Tailings Dam which was operated from 1987 to 1994 and the North Block Tailings Impoundment which was operated from 1994 to date. The two tailings facilities will store up to about 350 million tons of fine grained tailings, when they reach full capacity.
Each of these potential disposal options are in the early days of evaluation, particularly the last two which are in the category of research and development, and must be demonstrated to meet Barrick’s strict standards related to environmental management. However, each has the potential to improve the reliability and reduce the risk associated with final closure of the tailings facilities. Performance of the Goldstrike Tailings Facilities Mike Henderson, P.E., Vector Colorado, LLC, henderson@vectoreng.com Tel: (303) 522-1191; Pete Kowalewski, P.E., Vector Nevada, LLC, Tel (303) 522-1191, kowalewski@vectoreng.com; Ron Espell, Barrick Goldstrike, Tel : (775) 778-8191 respell@bgmi.com, Joe Giraudo, P.E. Barrick Goldstrike JGiraudo@bgmi.com and Johnny Zhan, PhD, Barrick Management Corporation, Tel: (801) 741-467 jzhan@barrick.com Abstract: Barrick’s Goldstrike operation is one of the largest gold mines in the world, having produced over 20 million ounces of gold since 1986 when Barrick purchased the property. Tailings from this operation have been hydraulically deposited in two tailings impoundments: the AA Block Tailings Impoundment which was operated from 1987 to 1994 and the North Block Tailings Impoundment which was operated from 1994 to date. The two tailings facilities will store up to about 350 million tons of fine grained tailings, when they reach full capacity. Barrick has undertaken several studies over the life of the facilities to understand their geotechnical, hydrogeologic and geochemical behavior and properties. This paper presents the technical characteristics of these tailings, summarizing the data from these studies which cover a period from 1987 to date. The tailings are considered representative of Nevada gold tailings which were subjected to a range of cyanide recovery processes including CIL, CIP, autoclave and roaster circuits. The Affect of Tailings Characteristics on Mine Reclamation and Closure M. Milczarek, M. Yao, GeoSystems Analysis, Inc, Tucson, AZ, Tel (520) 628-9330 mike@gsanalysis.com and T. Thompson, Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85721 Abstract: Mine tailing properties are so different from other types of mine waste that the reclamation design, criteria for reclamation success and post-closure monitoring require a very different approach from the standard methods used for waste rock and heap leach material. Research has shown that although tailings can be directly revegetated under some conditions, tailings alone are typically a poor growth media due to lack of structure, poor moisture retention characteristics, and lack of fertility and microbiota. Nonetheless, when cover material is used, unless the tailings are highly acidic, plant roots will propogate into the tailings and reduce deep percolation, effectively acting as a evapotranspirative cover. Evidence also suggests that limited oxygen ingress occurs into the finer grained tailings and long-term acid generation may be limited to the near surface. These same high moisture retention characteristics can mean that tailings drainage will occur for centuries, depending on the size and construction methods of the impoundment. All of these factors require careful consideration during closure and reclamation design. Tailings can generally be classified into three textural types corresponding to location within the impoundment: coarse- to fine-grained sands at the dams, sandy silts in perimeter mixed zones, and silt (slimes) in the decant area. The latter two types comprise the majority of tailings. Coarse- to fine-grained sands have poor moisture retention and limited plant available water holding capacity. Silts and sandy silts have high moisture retention but limited permeability, which can reduce infiltration and impede root penetration. Although a number of studies have shown that the addition of organic matter to tailings (such as biosolids and green waste) improves textural properties and fertility, the long-term effects of amendments may be limited. Moreover, for sandy tailings at the impoundment slope areas, limited water availability and high erosion rates make revegetation virtually impossible. The addition of nominal (one foot or less) amounts of growth media significantly affects the revegetation of non-acid tailings. Primary root growth occurs in the growth media, but rooting extends at depth into the tailings in order to extract moisture during dry periods. Indeed, high moisture retention in the sandy silts and slimes can result in vigorous growth of deep-rooted trees and shrubs after reclamation. Even with thick covers where the roots do not extend into the tailings, the moisture retained in tailings can be wicked via evapotranspirative demand during dry periods. The high moisture retention characteristics of most tailings also results in extremely long drainage periods. Model simulations of tailings drainage indicate that the bulk of the tailings solution drains from the dam and mixed zones within a relatively short period of time, but that extremely low rates of drainage occurs from the slimes area for a sustained period. Tailings consolidation due to overburden pressure will increase the rate of drainage, however, many impoundments larger than hundreds of acres may take decades for the majority of drainage to occur and centuries, if not thousands of years, to completely drain. The implications of tailings characteristics for reclamation design are: 1) Non-acid tailings can be revegetated with nominal cover and/or amendments; 2) Revegetated tailings/cover systems can serve as highly efficient evapotranspirative covers; 3) Acid tailings can be covered, however, wicking of tailings solution into the cover may occur; 4) Long-term tailings drainage can take decades or more. Closure of the Big Springs Tailings Facility, Elko County, Nevada Jonathan Gorman, Anglo Gold (Nevada) Corp., Elko, NV JGorman@AngloGoldNA.com Abstract: The Big Springs tailings facility, located in northeastern Nevada, is owned by Anglo Gold (Nevada) Corp. and was operated by its predecessors from 1988 through 1994. The 50-acre facility incorporates a low-permeability soil liner with a full drainage blanket and zoned earth fill embankments. The facility was raised several times using a combination of upstream and downstream construction and was operated employing sub-aerial deposition techniques. The design, construction, and operation of the sub-aerial facility resulted in a well drained, stable, and "full" facility with no surface water inventory at the time of reclamation in the summer of 2000. The tailings surface was regraded in-situ to create the design slopes prior to growth medium placement. The slopes were selected to increase the south and west aspect of the facility and to promote runoff. Potential growth medium cover designs were evaluated using the HELP model. A final cover thickness of 36-inches, with no barrier layer, was selected to optimize evapotranspiration and to minimize infiltration. Water retained in the tailings facility after regrading and cover placement gravity drained to the HDPE-lined reclaim pond where it was land applied to the mill site alluvium in 2000 and 2002. The remaining drainable water retained in the tailings and the year-to-year infiltration volume due to precipitation will be managed in the short-term possibly by another land application program and in the long-term by an evapotranspiration basin connected to a drainfield. Field Performance Monitoring of Mine Tailings Cover Systems M. O'Kane, P.Eng. Tel (306) 955-0702 mokane@okc-sk.com and B. Ayres, P.Eng, O’Kane Consultants, Inc, Saskatoon, SK Tel (306) 241-2263 bayres@OKC-SK.com Abstract: Field performance monitoring of tailings cover systems is typically an essential element of a tailings closure plan. In general, monitoring of tailings seepage to groundwater and surface water systems is required. However, the timeframe over which the impact on tailings seepage quality and quantity is realized following construction of a cover system can be tens of years, if not longer, as a result of drain down and other associated hydrologic and geochemical conditions. Cannon Mine Tailings Dam Brief Operational Review and Description of the Closure Process and Post-Closure Release of the Facility Rick Frechette, RTW, Elko, Spring Creek, NV Tel (775) 778-6624 rf@rtweng.com Lin Callow, Conoco-Phillips, Calgary Abstract: The Cannon Mine was operated between the mid-1980’s and mid-1990’s, as an underground gold and silver mine on the outskirts of Wenatchee in central Washington. The mine was closed in 1994 and reclaimed over the period ending in 1996. The tailings facility was built as a valley fill structure with a 300-ft tall water-retaining dam within a mile of the town and 4 miles from the Columbia River. Reclamation involved a pre-planned and constructed raise of the dam for flood-routing freeboard and installation of an open-channel spillway. During reclamation, a soil cover was placed over the tailings and the surface area reseeded. A small open-water surface was retained at the rear of the impoundment. This presentation will provide an overview of the design and operation of the dam and a description of the closure design and its implementation. A brief synopsis will also be provided of the post-closure chronology covering negotiation with the regulators and stakeholders on permit and bonding release. Material Characterization for Tailings Impoundments Closure Analysis Dobroslav Znidarcic, University of Colorado, Boulder, znidarci@spot.colorado.edu Tel (303) 492-7577 Abstract: Elaborate numerical models have been developed over the last quarter of century for the analysis of tailings impoundments settlements, pore pressures and water balance. The models include consolidation analysis, desiccation and seepage in both saturated and unsaturated regimes. Today, the models are routinely used for predicting impoundments behavior during operations and after closure. While the numerical algorithms do not present major challenge today, the accuracy and reliability of the analysis heavily depends on our ability to correctly characterize the tailings material and determine appropriate parameters as input into numerical models. As soft tailings exhibit highly nonlinear properties, often changing parameters over several orders of magnitude, the determination of these parameters is not a trivial task. The presentation demonstrates the developed and well established technologies for determining the nonlinear tailings properties for consolidation, desiccation and unsaturated flow. The associated testing and test analysis procedures have been in use in routine applications for over a decade now, and it is safe to say that they have been validated extensively with field observations on tailings impoundments. The procedures are equally applicable to the impoundments with sub aqueous and sub aerial depositions. The application of the testing technologies to the analysis of tailings impoundments for closure will be demonstrated on the example of a phosphatic clay disposal area in central Florida. The results of an eighteen months field monitoring program of clay desiccation will be presented. The case study represents probably the extreme test of the validity of the presented methodology, as the hydraulic conductivity of phosphatic clays varies over five or more orders of magnitude. Finally, a simple field monitoring program during impoundment operation will be discussed in order to demonstrate how an inexpensive continuous observation program can greatly simplify, improve accuracy and possibly reduce cost of the closure activities. Tailings Dam Closure: Designing Inside Out and Backwards Bryan Ulrich, Principal, Knight Piésold and Co. Elko, Nevada Tel (775) 738-2265 bulrich@knightpiesold.com Abstract: The current trend in tailings facility design is to consider the closure of the facility during the initial design process. In a sense, you are designing the facility backwards, in that reclamation issues and end land use considerations are often driving factors that determine the initial layout of the facility, the construction staging as well as the operational parameters. It is these and other factors that, over the life of the facility, result in how the facility exists at the time of closure. When the actual time for closure approaches, the final closure strategy can be refined from that developed during the initial design process. If there exists a potential for the tailings to produce acidic effluent or if metals may mobilize from the tailings mass it is important to design the cover system (the outside) based on the geochemical behavior of the tailings material (the inside). In this sense, the closure is designed from the
inside out. |
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University
of Nevada, Reno |
