British Columbia Mine Reclamation Symposium

Environmental & reclamation measures : Equity Silver Mines Ltd. Patterson, Robert J. 1987

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
59367-1987 - Patterson - Environmental and Reclamation Measures.pdf [ 984.9kB ]
Metadata
JSON: 59367-1.0042120.json
JSON-LD: 59367-1.0042120-ld.json
RDF/XML (Pretty): 59367-1.0042120-rdf.xml
RDF/JSON: 59367-1.0042120-rdf.json
Turtle: 59367-1.0042120-turtle.txt
N-Triples: 59367-1.0042120-rdf-ntriples.txt
Original Record: 59367-1.0042120-source.json
Full Text
59367-1.0042120-fulltext.txt
Citation
59367-1.0042120.ris

Full Text

th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -117-  ENVIRONMENTAL & RECLAMATION MEASURES EQUITY SILVER MINES LTD. ROBERT J. PATTERSON Environmental Co-Ordinator Equity Silver Mines Limited Houston, British Columbia January, 1987  ABSTRACT Acid Mine Drainage (A.M.D.) formed from oxidation of disposed pyritic wastes presents a potential pollution hazard for Equity Silver Mines Limited and the mining industry in general. Failure to collect and treat these acidic effluents can eventually affect the quality of receiving streams and endanger aquatic life forms. Treatment and discharge of A.M.D. to the receiving environment must comply with stringent anti-pollution laws. Wastes mined at Equity Silver Mines Ltd. are predominantly acid generating and must be handled according to a materials management plan to minimize impact on the environment. Effluent collection and treatment facilities have been constructed to address the short term environmental concerns with reclamation and research programs being developed to address and monitor abatement measures for control over the long term. The purpose of this paper w i l l be to offer a brief overview of these measures. Key topics for discussion w i l l be: kinetics of Acid Mine Drainage, acid generating potential, water treatment and sludge handling, special wastes, waste dump construction, waste rock amendments, bacteriocide research, simulations, and reclamation measures. Programs designed to evaluate impact on the receiving environment, although they form an extensive part of the program, will not be discussed in this paper.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -118-  INTRODUCTION Equity Silver Nines Limited is located in the Central Interior of British Columbia, within the Omineca Mining Division,  35  approximately  kilometers 575  southeast  kilometers  by  air  of  Houston  north-northeast  and of  Vancouver (Figure 1).  FIGURE 1:  LOCATION OF EQUITY SILVER MINES LTD  The deposit consists of three economic mineralized zones: The Southern Tail, Main and Waterline Zones. Mining of the Southern Tail commenced in April, 1980 and was completed in the first quarter of 1984. Main Zone production was initiated during the last quarter of 1983 and  the  Waterline  Zone  remains  undeveloped  to-date.  Remaining reserves are in the order of five to six years contingent on favourable metal prices.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -119The three deposits (Figure 2) occur within a window of interbedded volcanic and minor sedimentary rocks. The older volcanic sequence is bounded on the west by a small granite-like stock and on the east by a gabbro-monzonite complex. A l l are overlain by a series of younger lava flows. Economic  mineralization  (Cu-Ag-Au)  occurs  as  tetrahedrite and chalcopyrite. Pyrite is the predominant sulphide mineral throughout the waste and mineralized zones and occurs in disseminated, massive and crystalline forms. Oxidation of these sulphides results in the formation of Acid  Mine  Drainage  (A.M.D.).  mechanisms  are  complex,  identified  as  moisture,  the  key  Although parameters  oxygen,  a  source  of  reaction have  been  sulphide  minerals and the presence of certain strains of iron and sulphur oxidizing bacteria. (Kleinman & Erickson, 1983). Elimination of any one of these parameters w i l l suppress the acid producing reactions. Equity  Silver  Mines  Ltd.  has  encapsulate wastes with glacial eliminate oxidation  control and  factors  subsequent  measures along with  the  and  till  is  in an  governing  production  continuing  of  water treatment  the  to  effort  to  rate  of  A.M.D.  These  processes and  research programs will be outlined in the paper.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -120-  FIGURE 2:  EQUITY SILVER MINES LTD. SITE PLAN ILLUSTRATING KEY GEOGRAPHIC FEATURES  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -121KINETICS OF ACID GENERATION The degree to which acid generation occurs varies with the nature and concentration of available sulphide (pyrite) material. Pyrite mineralization at Equity Silver ranges from disseminations to the massive form. Where these massive forms occur along fracture planes, liberation and exposure  due  to  blasting  is  thought  to  be  maximized, likewise acid generation processes.  FIGURE 3:  TYPICAL OXIDATION REACTION FOR SULPHIDE (PYRITE) MINERALS  Metal and acid forming ions liberated during the initial  stages  of  oxidation,  combine  with  alkaline  materials inherent in the host rock and settle out of solution as soluble metal precipitates (Reactions 1 and 2 Figure 3). With continued oxidation (Reaction 3) and the lowering of solution ph, metallic precipitates are taken back into solution. This step occurs when available  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -122alkalinity in the waste is depleted. Liberated metal ions, particulariIy iron in the ferric state (Fe+3), act as an oxidizer which further react with sulphide materials. As the process accelerates and the ph of the solution drops below 2.5, the rate of activity of the ferric iron (Fe+3) is governed by the combined effect of bacterial oxidation of ferrous iron (Fe+2), the reduction of ferric iron (Fe+3) by pyrite, (Reaction 4) and the associated formation of ferric sulphate and hydroxal complexes. If the reactions are allowed to proceed uncontrolled the cycle w i l l continue until the sulphide supply is depleted.  ACID GENERATING POTENTIAL In assessing the reactivity of an ore body, waste materials  are  tested  for  acid  producing  potential  and  buffering (alkalinity) capacity. Results of testing at Equity are quoted as a ratio, Acid Generating Potential : Acid Consumption Capability. For instance, an index ratio of 4.5, merely means testing has indicated that the waste material has the potential to produce 4.5 times more acid than it is capable of neutralizing. Where these ratios are low,  kinetic  substantiate  tests static  in  columns  acid-base  should  be  accounting.  used  to  Release  of  neutralizing capacity is greatly affected by factors such as  particle  size  and  hydroxide  coating.  Erroneous  conclusions based on static test results alone are not uncommon.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -123The acid producing potential of pyritic waste material within the Nain Zone Pit varies between a low of 0.15 to a high of 65, with the majority of values falling within the 10 to 20 range (Sperling, 1984). Zones of acid generating potential are illustrated in Figure 4.  FIGURE 4:  COMPOSITE CROSS SECTION OF MAIN ZONE ORE BODY WITH ACID PRODUCING POTENTIAL ZONING  Non-acid producing wastes occuring on the east wall of the Main Zone Pit are selectively mined for construction purposes and used at sites where A.M.D. must be eliminated, i.e. tailing dams. Use of non-acid producing materials for these projects precludes the possibility of reducing acid  producing  potential  through  acid-base  accounting  by  strategic placement of wastes. All wastes classified as acid producers must be stockpiled on a common dump and reclaimed to reduce oxidation rates.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -124-  WATER TREATMENT AND SLUDGE HANDLING Approximately 800,000 cubic metres of A.M.D. must be collected  annually  and  processed  through  a  lime  neutralization treatment plant. Treated water from the plant complies with stringent quality objectives and is discharged to the receiving environment at dilution ratios governed by metal content and stream flow rates.  Table 1 - A.M.D. Treatment. Statistics  Estimated costs for pumping and treating A.M.D. at present acidity levels is in the order of $0.85 per cubic  metre.  Typical  A.M.D.  treatment  statistics  are  illustrated in Table 1. The sediment (sludge) produced from  the  treatment  process  consists  mainly  of  metal  hydroxides. Yearly accumulation of this sludge represents not only a short term storage problem but a long term reclamation concern.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -125-  FIGURE 5 - RESULTS OF SPECIAL WASTE TESTING USING VARIOUS RATIOS OF A.M.D. SLUDGE AND TAILING A.M.D. sludge, through testing, has been classified a Special Waste according to the proposed Special Waste Act.  By  mixing  A.M.D.  sludge  with  mill  tailing  discharge at ratios as low as 1:4, extracted metals from the composite meet guidelines laid out in the act. Figure 5  illustrates  that  tailing  complies  with  Special  Waste  guidelines, whereas A.M.D. sludge far exceeds the limits set forth. Mixtures of A.M.D. sludge to tailing between the  ratios  of  1:10  and  1:6  show  little  change  from baseline extractions within the tailing. Deviation from this trend becomes evident at ratios of 1:4 or less. Permitting has been completed to accommodate mixing the two products at a 1 : 10 ratio. Disposal of the material will be handled in this fashion during the period when  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -126the mine is in operation. Alternate options for sludge handling after abandonment include disposal in large ponds or beneath water within the Main zone pit. The mechanics of pumping sludge has met with several operating difficulties. Pumping was initially carried out in a pond containing a surface layer of water. This resulted in a vortex around the pump with sludge being supported by water thus preventing lateral movement of the material. A waste rock divider dyke within the two cell arrangement was subsequently sealed with glacial t i l l to prevent short-circuiting of water between the active pond and pond being excavated. Elimination of the water sources improved sludge pumping and at 5 percent solids sludge migrated over the full length of the pond supplying feed to the pump.  WASTE DUMP CONSTRUCTION Disposal  of  all  acid  generating  waste  material  is  restricted to a common dump site (Figure 2). Prior to identifying  A.M.D.  occurrences,  pyritic  wastes  were  liberally used for fills throughout much of the property. Sites difficult to access for purpose of A.M.D. collection have  required,  generating throughout  where  material. the  contamination.  possible,  Similar  mine  life  excavation  clean-up to  will  be  consolidate  of  acid  scheduled areas  of  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -127-  FIGURE 6:  TYPICAL DUMP PROFILE AMD IMPERVIOUS CLAY SEALS  Initially the waste dump was constructed by disposal along  a  common  horizon.  This  construction  technique  produced high dump faces with static loads in excess of shear till,  strengths consequently  of  the  resulted  underlying  in  several  glacial  minor  localized  failures. Geotechnical investigation (Klohn Leonoff, 1984) indicated that dump faces would have to be reduced from the natural angle of repose (37 degrees) to an overall slope of 20 degrees to achieve a safe weight distribution. Dump faces  were  upwards,  reconstructed  eventually  merging  from with  the  lower  limits  the  existing  dump  head. In achieving the design slope angle, the dump had to be reconstructed in individual lifts of 10 meter  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -128-  thickness, each overall  stepped  in  to  conform  to  the 20 degree  slope (Figure 6).  Reconstruction of the dump coincided with the period when A.M. D. occurrences became most prevalent. The new design  accommodated  a  continuous  reclamation  program  whereby completed berms and side slopes could be topdressed with a one meter layer of glacial t i l l and seeded to establish a protective vegetative cover. Air exchange within dump structures accelerates during the  winter  months up  as gases produced from  process  set  convection  maximum  atmospheric  currents  pressure  air  exchange,  within  an  intermediate  during  variation,  process as in a fireplace chimney.  the  periods  much  the  of  same  To further reduce this  layer of till  the dump during the course  oxidation  was  of construction  placed (Figure  6). Glacial t i l l is removed from active mining areas and from  stockpiles  inventoried  during  the  pre-production  stage. Disposal of t i l l for reclamation purposes during the mining cycle precludes the need to stockpile material and in turn decreases final abandonment costs. Glacial till hauled to the dump represents a small incremental cost over and above normal operating expenditure. To accommodate waste disposal demands, the dump will be extended easterly (Figure 2) to backfill the Southern Tail pit. By placing a portion of the waste below water within the pit, oxidation processes w i l l be eliminated.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -129Wastes (especially the fine fraction) placed below the water table provide a supply of available alkalinity. Provided the acid generating processes within the area are minimized,  residual  alkalinity  should  maintain  water  quality within permit specification. A one meter layer of inert material (Gabbro) was placed at the appropriate horizon to accommodate fluctuation in the water table. This will safeguard alternate wetting and drying of pyritic wastes that would otherwise contribute to A.M.D. generation. A spillway has been constructed at the south end of the pit to ensure that a constant water level is maintained. Those materials placed above the water table will be encapsulated with a clay layer to decrease oxygen diffusion. Results of existing reclamation efforts have yet to be fully assessed, although where these measures have been instituted at localized sites there has been a downward trend  in  Monitoring  acidity  and  programs  dissolved  are  metal  presently  concentrations.  being  developed  or  modified as assessment tools. Temperature probes installed in the southern flank of the dump, have been used over the past 3 years to monitor internal dump temperatures. Observations to date reveal that the reaction produced through oxidation has elevated internal temperatures to 55 degrees celsius. A recent decline of several degrees has been observed and is thought to be a product of reclamation measures. A program of  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -130thermal  imaging  completed Vancouver,  by  of  pits  Environmental  B.C.  The  and  dumps  has  Protection  practice  of  been  Service  photo  heat  recently based  in  tracing  (Thermography) is presently under investigation as a tool to assess the effect of till covers on pyritic wastes and a detection method for pin-pointing problematic zones yet to be reclaimed.  FIGURE 7:  Test  plots  SCHEMATIC OF RECLAMATION TEST PLOTS  (Figure  7)  representing  scaled  down  versions of the waste dump have been developed to evaluate present reclamation measures. Glacial t i l l covers have been added to wastes at thicknesses comparable to present use on the waste dump. Uncovered control units will be used to assess the effect of present t i l l thicknesses.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -131A qualitative water monitoring program has been in place since 1983 to evaluate the impact of reclamation efforts. This program has recently been expanded to provide hydraulic  balances  balances,  along  of  with  runoff metal  within  domains.  analyses,  will  Hydraulic  be  used  to  calculate contaminant flows out of the waste dump and delineate problematic zones.  WASTE ROCK AMENDMENTS The anionic surfactant (Sodium Lauryl Sulphate) has been  tested  to  evaluate  bacterial  suppression  methods  within waste fills. Although the initial lab testwork looked encouraging, results of in-field testing proved to be  less  product  than  favourable.  should  not  be  Nevertheless,  overlooked  use  pending  of  this  favourable  conditions. To be effective, a sufficient dosage of the soap product must be available to completely infiltrate the spoil. Application rates at Equity Silver were based on treating  the  top  meter  of  waste  as  outlined  by  past  experience at coal operations. Recent findings at Equity Silver indicate that this oxidation zone, in coarse rock, can be in excess of 10 meters. Because of this oxidation depth, spoil piles could not be effectively saturated with treatment solutions. Recent tests carried out by the U.S. Bureau  of  Mines  on  Equity  waste  material  support  this  assessment (Watzlaf, 1986). As a further complication the surfactant used biodegraded rapidly under the acidic  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -132-  conditions thus would require reapplication more frequently than the two to three months period experienced in the coalfields.  Results  of  lab  simulations  using  sodium  lauryl  sulphate (S.L.S.) are illustrated in Figure 8. Where S.L.S. was used in a test column containing pyritic waste, oxidation rates and dissolved metal content were suppressed in relationship to an uncontrolled test. Copper was used as a trend indicator, although other metals, sulphates or acidity  values  could  have  been  similarity  used.  After  surfactant addition was discontinued, copper values in the test column approached the trend set in the control column, thus  indicating  re-population  of  acceleration of oxidation processes.  bacterial  colonies  and  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -133Precipitates  formed  from  the  treatment  of  A.M.D.  consist of met hydroxides and residual lime. Injection of these precipitates into waste rock voids was initially considered an option for sealing and adding alkalinity to pyritic wastes. Tests were carried out to evaluate the impact of this amendment on oxidized pyritic waste rock.  FIGURE 9:  A.M.D. SLUDGE INJECTION INTO OXIDIZED PYRITIC WASTE  Injection  of  the  product  revealed  a  significant  improvement on water quality. However, over an elapsed period acid generation processes within the waste exceeded the  buffering  capacity  of  the  sludge  resulting  in  a  deterioration of effluent quality. Sludge injected into the waste was redissolved, further adding to the metal loading  in  the  effluent  (Figure  9).  Under  less  severe  conditions alkaline sludges may be a suitable amendment for  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -134control of A.M.D. Operational aspects of a scaled up program were not investigated nor the geotechnical effect on dump structures.  SIMULATIONS Disposal and abatement concepts are tested in columns containing the rock types in question. These test vessels, constructed from five foot lengths of P.V.C. pipe, are equipped with either porous or impervious bases depending on the water course regime being evaluated. Solutions are circulated through each column by purging air through plastic tubing connected to the effluent receptacle and top of column .  FIGURE 10:  COLUMN TESTS  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -135-  Acid  generation  subjecting  wastes  to  processes a  can  constant  be  accelerated  circulating  load  by of  effluent (Figure 10). Essentially this technique enhances leaching  by  cumulating  acidity  and  bacterial  concentrations and works well for testing products such as surfactants as an abatement measure. Similar columns, with sealed bases, have been used to evaluate oxidation processes below a fixed water table. Water is held within the lower section of the column by sealing off the base and providing a drain midway to decant off water introduced through the top section (Figure 10). The decant serves as a sample point for monitoring water quality in the upper horizon above the water table with substrate water quality being monitored from a valved under drain. Results from these tests indicate minimal oxidation of pyritic waste while in the submerged state. RECLAMATION Reclaimed areas are revegetated to stabilize glacial till covers and reduce precipitation infiltration through evapotranspiration  losses.  Various  grass  and  legume  species are evaluated using test plots for selection of suitable mixtures. Assessment of these plots is carried out annually and documented for future reference. Seeding programs  employing  hand  and  aerial  methods  have  been  reasonably effective in establishing vegetation on well graded and unconsolidated soil types and to a lesser degree on the dense consolidated glacial t i l l , especially on side  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -136slopes. Hydroseedling methods were subsequently tested and found to provide better results. As results of the test work were encouraging, Equity Silver has since purchased a hydroseeder to supplement traditional seeding techniques. Although towards  reclamation  reducing  efforts  oxidation  are  within  primarily  wastes  geared  dumps,  the  aesthetic and habitat benefits are also deemed important. An increase in deer population has been observed over the past  few  years  and  is  thought  to  be  a  product  of  reclamation efforts. CONCLUSION Control measures at Equity Silver Nines Limited have been effective in minimizing the environmental impact of A.M.D.  Reclamation  practices  appear  to  have  reduced  oxidation rates at confined sites, although considerably more monitoring will be required to fully evaluate control measures on the larger disposal site. Alternate methods tested  for  suppressing  initially  appeared  curtailed  due  operationally  to and  acid  encouraging questionable that  of  generation however long  have  since  been  benefit,  both  security.  With  term  environmental  processes  existing reserves in the neighbourhood of 5 years, plans are  now  being  abandonment.  This  developed will  to  require  prepare a  host  for of  mine  studies  associated with sludge disposal, tailing pond abandonment and further research into A.M.D. mitigation methods for the waste dump.  th  Proceedings of the 11 Annual British Columbia Mine Reclamation Symposium in Campbell River, BC, 1987. The Technical and Research Committee on Reclamation  -137-  ACKNOULEDGEMENT  As spokesperson for Equity Silver Mines Limited, the author would like to thank the executive of the Reclamation Symposium for the invitation to present this paper, and to the management and operating crews at Equity Silver for their undivided support in dealing with environmental concerns. References Kleinmann, Robert L.P. & Erickson, Patricia M. (1983) Control of acid drainage from coal refuse using anionic surfactants, U.S. Bureau of Mines Report of Investigations RI 8847. Klohn Leonoff Consulting Engineers, 1984. Waste dump stability review. Parameters & Report to Equity Silver Mines Ltd. Sperling, T. (1984) A study of acid generating rock distribution in the main zone (unpublished) Equity Silver in-house report. Watzlaf, G.R. (1986) Control of acid mine drainage from mine wastes using bacterial inhibitors. National meeting of the American Society for Surface Mining & Reclamation March 17-20, 1986.  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.59367.1-0042120/manifest

Comment

Related Items