British Columbia Mine Reclamation Symposium

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

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rd  Proceedings of the 3 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  rd  Proceedings of the 3 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. raised is:  The question  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.  rd  Proceedings of the 3 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 surface 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  given in Table 1.  composition of the reduced tailings (unoxidized) is  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. (1)  The gravel layer had two functions in this study:  To provide a hydrologic barrier between the tailings and the overburden 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 unsaturated water flow conditions, and  rd  Proceedings of the 3 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  %  Pyrrhotite  Chemical Formula  85  Fe7 S8  Pyrite  5  FeS2  Sphalerite  1  Zn S  Galena  1  Pb S  Quartz  5  SiO2  a  From Gardiner and Stathers  (3).  TABLE 2 Physical and Chemical Properties of Iron Tailingsb Property  Unoxidized Iron Tailings  Total S(%)  Oxidized Iron Tailings  34  7.3  1.1  5.1  6.0  2.2  CEC (Milli equivalents/ 100 grams) pH EC (mmhos/centimetre) Colour (Munsell) b  2.6 5Y3/1  Modified from Gardiner and Stathers  18.0 10YR5/6 (3).  rd  Proceedings of the 3 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  rd  Proceedings of the 3 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. mining process at Sullivan Mine produces waste rock.  The 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 accelerating 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. insulation and a thermostat. approximately 32 C. project.  It was equipped with a heater,  The temperature was maintained at  Arid conditions were mandatory for the success of this  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 site is located. British Columbia.  in which the mine  Kimberley has been described as in the 'dry belt' of 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.  rd  Proceedings of the 3 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  c  95  From Canada Dept. of Transport (1) and (2).  rd  Proceedings of the 3 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 analysis 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. occurred in Treatment 2.  However changes in pH  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  rd  Proceedings of the 3 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation  318  rd  Proceedings of the 3 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation  319  boundary). 6.5-7.0.  The optional pH range for vegetation success is roughly pH  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 substances 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  rd  Proceedings of the 3 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation  320  rd  Proceedings of the 3 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation  321  FIGURE 4  ELEMENTAL CONCENTRATION  vs TIME  rd  Proceedings of the 3 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 concentrations 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 concentrations 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  rd  Proceedings of the 3 Annual British Columbia Mine Reclamation Symposium in Vernon, BC, 1979. The Technical and Research Committee on Reclamation  323  FIGURE 5  ELEMENTAL CONCENTRATION vs TIME  rd  Proceedings of the 3 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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