British Columbia Mine Reclamation Symposium

Migration of acid substances in Sullivan tailings : a column study Ames, Susan Eveline 1979

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Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 309 MIGRATION OF ACID SUBSTANCES IN SULLIVAN TAILINGS A COLUMN STUDY Paper presented by: S. Ames Dept. of Soil Science University of British Columbia Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 311 MIGRATION OF ACID SUBSTANCES IN SULLIVAN TAILINGS - A COLUMN STUDY INTRODUCTION Reclamation of the tailings pond at the Sullivan Mine, at Kimberley, British Columbia, involves some unique problems.  Although the pond is still active, procedures are being developed for the ultimate reclamation of the pond.  Attempts at establishing vegetation directly on the tailings pond have met with little success because of the high acid production potential and the high salt content of the soil.  This is because the reduced pyritic tailings are weathered in an oxidizing environment which converts sulfides to sulfates. Several procedures for reclamation have been considered.  One procedure involved covering the tailings with overburden or soil to some depth and the establishment of vegetative cover on this material.  The question raised is:  Will the acid produced by the oxidizing tailings slowly rise through the overburden and, with time, contaminate the overburden with acids, soluble salts and heavy metals to make the overburden ultimately ineffective? To test this, a laboratory study was initiated in cooperation with Cominco Ltd. at the Department of Soil Science, University of British Columbia. The objectives of the study were: (1) To test whether an overburden that is applied directly onto the surface of the Sullivan Mine iron tailings would become severely contaminated with time, contamination occurring as a function of the potential upward migration of acids and toxic materials from the tailings, and (2) To test whether a barrier, in this case a layer of gravel, would provide "insulation" to the overburden from the iron tailings, thus inhibiting potential contamination. Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 312 METHODS AND MATERIALS The oxidized iron tailings used in the study were collected from the sur- face of the iron tailings pond at Sullivan Mine.  The tailings were taken to the laboratory, dried, and crushed for uniform packing in plexi-glass columns.  The chemical  composition of the reduced tailings (unoxidized) is given in Table 1.  Some chemical and physical properties for the unoxidized and oxidized tailings are given in Table 2.  Overburden (glacial till) was removed from a ditch north of the iron tailings pond.  The overburden was felt to be representative of the type of material that would be used if overburden were to be placed over the tailings.  Routine chemical and physical analyses were conducted on the tailings material and overburden. The study was designed as a nine-month project which consisted of two treatments.  Nine plexi-glass columns, 190 centimetres in height, with an inside diameter of approximately 15 centimetres were constructed for each treatment.  Treatment 1 consisted of 45 centimetres of overburden placed directly onto the surface of a 30 centimetre deep layer of iron tailings (Figure 1).  Treatment 2 consisted of 45 centimetres of overburden separated from a 30 centimetre deep layer of tailings by a 5 centimetre layer of coarse gravel.  The gravel layer had two functions in this study:  (1)  To provide a hydrologic barrier between the tailings and the over- burden from the tailings.  The upward movement of acids and toxic materials was considered to be a function of capillary rise through a uniform pore continuum.  The gravel would break this continuum and therefore effectively act as a barrier under unsa- turated water flow conditions, and Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 313 TABLE 1 Chemical composition of Unoxidized Iron Tailingsa  Mineral % Chemical Formula Pyrrhotite 85 Fe7 S8 Pyrite 5 FeS2 Sphalerite 1 Zn S  Galena 1 Pb S Quartz 5 SiO2 aFrom Gardiner and Stathers  (3). TABLE 2 Physical and Chemical Properties of Iron Tailingsb Unoxidized Iron      Oxidized Iron Property Tailings Tailings Total S(%) 34 7.3 CEC (Milli equivalents/ 100 grams) 1.1 5.1 pH 6.0 2.2 EC (mmhos/centimetre)     2.6 18.0 Colour (Munsell)         5Y3/1 10YR5/6 bModified from Gardiner and Stathers  (3). Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 314 FIGURE  1 COLUMN   DESIGN TREATMENT   I TREATMENT  2    Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 315 (2)  To provide a means of reclaiming the waste rock dump.  The mining process at Sullivan Mine produces waste rock.  This could be used as a source of coarse material for the hydrologic barrier. In order for upward migration to occur, water must move up through the columns by capillary rise.  Thus, each column is equipped with a water table located at a height halfway up in the tailings material.  Four sampling portals are specifically located to include sampling areas below and above the water table in the tailings material, at levels A and B, respectively; and, a sampling area in the overburden fairly proximate to the tailings overburden boundary at level C.  A fourth portal is located near the top of the columns in the overburden, at level D (Figure 1). Solutions were extracted each month from each sampling portal for chemical analyses. The upward movement of acids and toxic materials was promoted by accelerat- ing evaporation at the surface of the columns.  This accelerated evaporation was imposed by placing the columns in a wooden growth chamber constructed in the laboratory.  The wooden structure is approximately 1.8 metres x 2.1 metres and is roughly 2.5 metres in height.  It was equipped with a heater, insulation and a thermostat.  The temperature was maintained at approximately 32 C.  Arid conditions were mandatory for the success of this project.  In an environment of room temperature and pressure the rate of reaction would be minimal.  The experimental approach of evaporation processes as opposed to leaching processes is justified by considering the climatic zone  in which the mine site is located.  Kimberley has been described as in the 'dry belt' of British Columbia.  It receives approximately 378 millimetres of precipi- tation annually of which roughly 229 millimetres are allocated to rainfall. The climatic data for Kimberley is given in Table 3.  With summer tempera- tures reaching a maximum of 32 C and a relatively long (181 day) growing season, evaporation is considered to be the dominant process as opposed to leaching. Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 316 TABLE 3  Climatic Data for the Kimberley Areac   Precipitation (mm)  Mean annual 377.6 Mean annual rainfall 228.8 Maximum rainfall 51.8 (June) Mean annual snowfall 154.5 Maximum snowfall 42.4 (December) Temperatures (°C)  Maximum 32   (August) Minimum -21.1 (March)  Growing Season (days)      181 Frost free 95 cFrom Canada Dept. of Transport (1) and (2).    Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 317  Chemical Analyses Standard pH measurements were carried out on the solutions to monitor changes in acidity.  Electrical conductivity is important as it indica- tes soluble salt content.  Iron, aluminum, zinc, and copper were ana- lysed by atomic absorption spectrophotometry as these elements were in relatively high concentrations in the iron tailings and were considered toxic to vegetation when in large amounts.  Data is presented for the solutions extracted from the overburden at levels C and D.  The experi- ment has been in operation for six months. RESULTS AND DICUSSION Preliminary analyses indicated that the gravel barrier was effective in preventing the upward movement of salts, acids, and other toxic materials into the overburden from the tailings material.  The overburden material placed in direct contact with the tailings became contaminated by these substances.  The extent of the chemical changes occurring in Treatment 2 in comparison to Treatment 1 can be effectively discussed if each type of ana- lysis is considered separately. Analysis of pH As noted previously the most intense reactions were predicted to occur at level C in the columns.  The barrier in Treatment 1 was effective in preventing acid movement (Figure 2).  Changes in pH did not occur in this treatment during the experimental period.  However changes in pH occurred in Treatment 2.  The pH dropped from an initial basic reaction to a very acid condition at level C.  The absence of a barrier resulted in a pore continuum between the tailings and overburden providing a direct path into the overburden.  Indications that acids moved up to the D level in the columns became apparent as measurements decreased 1 pH unit during the sampling period.  The reaction was much more extreme at level C, as movement to the sampling portal at level C took place over a distance of 7 centimetres (from the tailings-overburden Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 318  Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 319 boundary).  The optional pH range for vegetation success is roughly pH 6.5-7.0.  The levels of acidity persisting at level C in Treatment 2 would pose serious problems to the growth and maintenance of vegetation. Variation in pH measurements in the overburden at time "0", between the two treatments, was the result of rapid migration of some acid substan- ces immediately after the columns had been packed. Soluble Salts Electrical conductivity measurements reflecting soluble salts are plotted in Figure 3.  Treatment 1 proved to be a successful means of preventing the movement of salts.  Measurements of soluble salts fall below those levels considered deleterious to vegetation-set at 4 millimhos per centimetre or lower.  Unsuitably high concentrations of salts moved into the overburden at level C in Treatment 2.  The short distance of 7 centimetres from the tailings boundary resulted in "rapid" contamination of the overburden at this level.  Such high salt levels persisted in the overburden, illustrating the potential source of salts from the tailings. The salts in Treatment 2 migrated in a linear manner due to the greater distance (roughly 40 centimetres) before reaching the sampling portal at level D.  High salt contents appeared to be "long-term".  Future analysis will verify this. Elemental Analysis Elemental analysis was carried out in the solutions extracted from the overburden at level C, which indicated that the acids and salts moving into the overburden in Treatment 2 were accompanied by concentrations of iron, aluminum, zinc, and copper (Figures 4 and 5).  Therefore, ele- ments did not migrate into the overburden in Treatment 1.  The barrier resulted in effectively insulating the overburden from the tailings Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation  320 Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 321 FIGURE 4 ELEMENTAL   CONCENTRATION    vs   TIME  Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 322  material.  However, Treatment 2 was not effective as a reclamation pro- cedure, as the overburden was contaminated.  Relatively high con- centrations of iron migrated upwards (Figure 4).  This was attributed to the formation of soluble iron salts in the tailings when oxidation of the iron pyrite occurred.  Data from two columns, arbitrarily num- bered 13 and 15, indicated differential movement.  This may give an indication of the kinds of variation that may occur in a field situation.  The concentrations of iron, aluminum and zinc, rise and fall through the sampling period.  This was attributed to precipitation and dissolution of precipitates under extreme acid conditions.  Total analysis carried out on the overburden will verify this, or provide an alternate explanation for this pattern.  As noted, this data is plotted from solutions extracted from the columns. Aluminum and zinc concentrations were also relatively high.  These high concentrations are predictable if a source is available, as these metals are soluble under acid conditions.  On completion of the experi- ment, total analysis on the overburden will indicate whether the con- centrations of these elements will be at a level toxic to vegetation. Copper concentrations did not follow the same peak-depression pattern as the other three metals, iron, aluminum, and zinc.  Explanation for this change in pattern may be attributed to the comparatively low con- centrations of this element in the tailings.  Higher concentrations may be required for precipitation. CONCLUSIONS The results of this study indicated that the overburden in Treatment 2 was contaminated by the migration of acids, soluble salts, iron, aluminum, zinc and copper.  The barrier in Treatment 1 was effective because the pH remained high and the electrical conductivity remained low.  Detectable iron, aluminum, zinc, and copper did not migrate up into the overburden. This data indicates the progress of the experiment to date.  As was for- merly noted, this data was plotted from analysis of solutions extracted from the overburden material.  It is predicted that total analysis on the Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 323 FIGURE  5 ELEMENTAL   CONCENTRATION   vs  TIME  Proceedings of the 3rd Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation 324 overburden will confirm that the reclamation of Sullivan Mine iron tailings may be successful if a barrier is placed between the tailings material and the overburden.  It is also predicted that overburden if placed directly onto the tailings pond may become contaminated in time. ACKNOWLEDGMENT The author wishes to thank Cominco Ltd. for providing a Grant-in-Aid for this research. BIBLIOGRAPHY 1. Canada Department of Transport, Meteorological Branch.  1968. Climatic Normals, Precipitation, Vol. 11. 2. Canada Department of Transport, Meteorological Branch.  1968. Climatic Normals, Temperature, Vol. 1. 3. Gardiner, R.T. and J.E. Stathers.  1975.  Mined Land Reclamation Research at Cominco Ltd., Sullivan Operations, Kimberley, B.C. Progress Report.

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