attenuation potential of clay liner carbon-in-leach tailings storage

8/13/2019 Attenuation Potential of Clay Liner Carbon-In-Leach Tailings Storage

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IntroductionThe environmental fate of free cyanide andmetal-cyanide complexes have been under in-tensive investigation over the last threedecades. Cyanide is naturally occurring andnatural mechanisms exit both geochemicallyand biogeochemically exist to retard or atten-uate its movement in subsurface, and to de-grade its compounds into low-hazard reactionproducts such as nitrogen gas, ammonia, ni-trate, and carbon dioxide. The attenuation ca-pacity of subsurface earth materials can bemeasured, quantied, predicted, and most im-portantly can be enhanced by rather simpleand inexpensive procedures. The fact that nat-ural earth materials have quantiable and pre-dictable ability to remove contaminant traceelements from migrating seepage has beendocumented for more than ten years (Griffin

1976, Griffin 1977).According to Griffin (1976),the overall pollution from contaminationsources could be reduced if liners of naturalearth material were designed for lower hy-

draulic conductivities. According to Ghosh etal. (2006a), various factors effluence thecyanide cycle in the environment and it in-cludes but not limited to pH, temperature,sunlight intensity and OMC of soils.

Study AreaThe study area comprised of the TSF at GFGLwith specic emphasis on he substrata. The fa-cility is located in the Tarkwa Nsuem Munici-pality in the Western region of Ghana. Tarkwais on latitude 515 N and longitude 200 W andlocated within the South Western equatorialclimatic zone. This zone is described as a moisttropical climate characterised by high yearround temperatures and rainfall pattern. Rain-fall peaks is from April to June and October toNovember. Average rainfall over the last 50years at the mine site is 1600 mm per annum.

The average daily temperatures range from24.5 C to 27.8 C with peak readings in themonths from February to April (Arthur et al.2004). The three TSFs (TSF 1, TSF 2 and TSF 3)

Attenuation Potential of Clay Liner Carbon-in-Leach Tailings StorageFacilities A Case Study at Gold Fields Ghana Limited, Tarkwa Mine

(GFGL)M.A.Sackey , G. K.Anornu , Y.Kumah , B. K.Kortatsi , Ko Antwi Yeboah

Department of Civil Engineering, KNUST, KumasiUMat P.O. Box 237, Tarkwa Ghana

CSIR-Water Research Institute, P. O. Box M32, Accra, GhanaNewmont Ghana Limited, Ahafo Mine

Abstract This study presents research ndings on cyanide attenuation potential of the sub-strata material of Tailings Storage Facilities (TSF) at Gold Fields Ghana Limited (GFGL), TarkwaMine. The attenuation properties of the substrata considered and evaluated were hydraulic con-ductivity (K), permeability characteristics and geochemical properties; pH, Organic Matter Con-tent (OMC), Cation Exchange Capacity (CEC), Iron Oxide and Anion Exchange Capacity (AEC).Substrata material samples were taken before and a er an insitu cyanide leaching test at velocations in and around the three TSF; TSF 1, TSF 2 and TSF 3. Total and Weak Dissociable Cyanide(WAD) were analyzed a er leaching test.

Keywords Tailings Storage Facility, hydraulic conductivity, permeability, geochemical, organicmatter content, cation exchange capacity


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were substrata were the prime focus of thestudy.

MethodologyField Leaching Test Leaching test was carried out at ve locationsin the TSF basin. At each station, an auger drillwas used to drill a hole to a depth of 1.5 m andtted with a 2 m length Polyvinyl Chloride(PVC) pipe (101.6 mm diameter) which hasbeen perforated. Four monitoring holes, at adistance of 3 m from the leaching point holewere also similarly constructed as the leachingpoint. A depth of 1.5 m was chosen to conformto the thickness of the clay in the TSF basin.Tailings slurry with 50 mg/L WAD concentra-

tion was introduced into the leaching pointsat the respective leaching sites, up to the 1.5 mlevel. Concentration of slurry conformed toGFGL mine limits for WAD in tailings dis-charged which averaged 48 mg/L from eldsampling programme at various spigotingpoints.The leachates in the monitoring pointsat the respective leaching points were sampledand analyzed for Total and WAD cyanide a era month. Cyanide leachate in the monitoringholes surrounding the leaching points weresampled monthly and analyzed for WAD andTotal cyanide. This was repeated over sixmonths period. Samples of the substrata weretaken and analyzed to verify the impact of theleaching on the attenuation potential of theTSF clay liner.

Soil Sampling

A total of ten (10) clay soil samples were col-lected from the ve leaching points in July2010 prior to the leaching test. The same num-ber of samples was taken in June 2011 to ana-

lyze for geochemical parameters a er theleaching test. These disturbed clay samples of varied texture were taken from depths rangingfrom 1 to 1.5 m. The samples were coned andquartered on a clean polyethylene samplesheet and representative samples taken andtransferred into a new, clean polyethylenesample bag. Samples earmarked for dry sieveanalysis weighed 7 kg each, whiles that ear-marked for hydrometer tests and geochemicaltests weighed 1.5 kg each. The attenuationproperties of the clay samples considered arethe hydraulic conductivity, permeability char-acteristics and geochemical properties.

Results and Discussion

Geochemical ParametersGeochemical investigations carried out on claysamples collected from ve identied sitesshowed the results of the geochemical param-eters as presented in Table 1.

Hydraulic Conductivity (K), Permeability (k i )and Porosity (n) of Soil Samples

The Alyamani and Sen (1993) empiricalformula and relevant parameters derived fromgrain size analyses were used to determine thehydraulic conductivity of the clay samples col-lected from the ve respective stations. Theeld calculated hydraulic conductivity valueswere in the range of 6.2 10 to 1.41 10 m/dwith a mean value of 8.3 10 m/d which ispresented in Table 2. The maximum value of Kwas obtained from clay sample from Station 3,whiles the minimum value of K was obtained

from clay sample from Station 5. Hydraulicconductivity values of silt, clay and mixturesof sand, silt and clay were in the orders of 10and 10 m/d (Brassington 2007). Based on this

Table 1 Geochemical param-

eters of sampled clays

Sample ID pHC.E.C

(meq/100 g)A.E.C

(meq/L )OMC

(% )Fe O

(ppm)Stat ion 1 4.5 2.01 1278.25 0.36 49.57Station 2 4.5 2.89 1057.50 0.53 63.14Station 3 4.8 2.34 881.25 0.78 66.43Station 4 4.3 1.92 1057.50 0.43 22.86

Station 5 4.5 2.37 881.25 0.98 29.00


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to the leaching test.WAD cyanide results fromthe leaching test falls within limits stipulated,for TSFs with planned measures for cyanidedegradation, where concentrations range from 1-10 mg WAD-CN/L (NICNAS, 2010). More-over, the Total and WAD cyanide results fallswithin stipulated limits of EPA-Ghana, whichstates a Total and WAD cyanide limit of 1 mg/L

and 0.6 mg/L respectively and hence, poses nothreat to groundwater resource. However, theaverage cyanide detected with time at the vestations remained fairly constant as time in-creased.

Effect of hydraulic conductivity and permeability on cyanide attenuationTotal cyanide concentration from the leachateat Station 3 recorded the highest value due tothe high hydraulic conductivity of the sub-strata. Clay material from Station 5 had theleast hydraulic conductivity and attenuated50 mg/L concentration of NaCN to 0.010 mg/Lof Total cyanide and WAD cyanide respectively.Based on the geochemical results from theleaching stations, it was observed that clay ma-terial from Station 5 had unfavorable geo-chemical parameters which gave it a weak

cyanide attenuation property. Such propertiesare; least AEC (881.25 meq/L), highCEC(2.37 meq/100 g) and low Iron-oxide con-tent (29 ppm). However, it exhibited acidic (av-

erage pH is 4.5) values, highOMC (0.98 %), 0.010 mg/L Total and WADcyanide concentration respectively. Clay mate-rial from Station 2 was able to attenuate the 50mg/L NaCN to 0.020 mg/L of Total cyanide and0.011 mg/L of WAD cyanide due to the fact thatit was acidic (pH-4.5), had a high AEC (1057.50meq/L) and high Fe-oxide content (63.14 ppm).The sample however, had the highest CEC (2.89

meq/100g) and low OMC (0.53 %), thereby ren-dering it ineffective in cyanide attenuation.De-spite the fact that there was 0.01 mg/L of WADcyanide in leachate samples of Station 3, Totalcyanide values were the highest (0.030 mg/L)compared to leachate from Station 2(0.020 mg/L) and Station 5 (0.010 mg/L). Al-though its clay material had the highest Fe-oxide content (66.43 ppm) and high OMC(0.78 %), the differences in cyanide attenuationwas due to the fact that it had the least AEC(881.25 meq/L), high CEC (2.34 meq/100g) andhad the highest pH (4.8).

Clay material from Station 4 attenuatedboth Total and WAD cyanide to 0.01 mg/L in-spite of its low OMC (0.43 %) property and low-est Fe-oxide content (22.86 ppm). This could beattributed to the fact that clay material fromStation 4 was highly acidic (4.3), had lowest CEC

(1.92 meq/100g) and high AEC of 1057.50 meq/L. Similarly, clay material fromStation 1also attenuated both Total and WADcyanide to 0.01 mg/L, though having the low-

Fig. 1 Hydraulic Conductiv-ity Results from the respec-

tive stations


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est OMC (0.36 %) and a relatively lower CEC(2.01 meq/100g). The cyanide attenuation inStation 1 was due to the fact that Station 1 wasacidic (4.5), had the highest AEC (1278.25meq/L)and Fe-oxide content of 49.57 ppm. An assess-

ment of the hydraulic conductivity, permeabil-ity and geochemical results of clay materialfrom the ve leaching sites with respect to theTotal and WAD cyanide concentrations fromtheir leachate, revealed that low hydraulic con-ductivity and a combination of low acidic pH,relatively high AEC and low CEC were themajor attenuation properties that inuencedcyanide attenuation (Fig. 1). This agrees withstudies conducted by Dzombak et al. (2006a)

Signicant variations were recorded in allthe geochemical parameters in samples takena er the leaching test and these are the pH,AEC, CEC, FeO and OMC. pH decreased signif-icantly for all the ve leaching sites with valuesranging from 3.0 to 3.4 and a corresponding in-crease in AEC ranging from a minimum valueof 1650 ppm to a maximum value of 2116.67 ppm. According to nding by Dzombak

et al. (2006); Ghosh et al. (2006a) the substratamaterial a er the leaching test increased effec-tiveness of the attenuation potential of theclay material but considering the higher CECand OMC. OMC for Station 5 reduced from0.98 % to 0.79 % rendering it less effective inattenuating cyanide and other minerals.FeOlevels for Stations 1(14.5 %), 2 (60.9%) and3 (49.5 %) decreased a er the test whiles thatof Station 4 (27.3%) and 5 (8.0 %) were elevated.The pH reduced a er the test at all the ve sta-tions ranging from 3.0 to 3.4 comparing to pre-leaching pH range of 4.3 to 4.8. Stations 1 and3 reduced by 33.3 % with Stations 2, 4 and 5 re-ducing by 24.4 %, 23.3 and 27.8 % respectively.

ConclusionThe following conclusions were deduced fromthe studies carried out;

Estimated mean porosity values of thesubstrata material of the TSF basin establishesit as clayey with further classication of siltyclay (TSF 1 and 2), laterite on slopes in TSF 3 and

silty sand at low lying areas of TSF 3.Mean calculated permeability and hy-

draulic conductivities of samples conforms toacceptable regulatory materials for the con-struction of tailings substrata. Station 5 exhib-

ited the least attenuating geotechnical capabil-ities.Conrmed geochemical parameters of

the substrata effective in attenuating cyanideto tolerable and acceptable limits especially forstations 1, 2 and 3, with Station 5 being less ef-fective.

The substrata material a er the leachingtest increased effectiveness of the attenuationpotential of the clay material but consideringthe higher CEC and OMC. OMC for Station 5 re-duced from 0.98 % to 0.79 % rendering it lesseffective in attenuating cyanide and otherminerals.

pH for all the test stations became moreacidic a er the leaching test. Adsorption on-clay particles and iron hydroxides can reducemetal and WAD cyanide concentrations. Socan chemical reactions and ion replace -

ment/exchange as the uid inltrates throughthe soil prole. Soils containing a clay compo-nent are especially effective in attenuatingmetals.

ReferencesAlyamani MS, Sen Z (1993) Determination of Hydraulic

Conductivity from Grain-Size Distribution. Curves.Groundwater.Vol. 31, No. 4. 551555.

Barnie S (2010) Hydrogeological and HydrochemicalFramework of Groundwater for irrigation in theAtankwidi sub-basin of the White Volta Basin. MSc.Thesis (unpublished), Kwame Nkrumah Universityof Science and Technology, Kumasi, Ghana. Brass-ington, R. (2007). Field Hydrogeology.3 rdEdition.JohnWiley and Sons Ltd. United Kingdom.

Brassington R (2007) Field Hydrogeology.3rdEdition.John Wiley and Sons Ltd. United King-dom.

Dzombak DA, Ghosh RS, Young TC (2006) PhysicalChemical Properties and Reactivity of Cyanide inWater and Soil. In: Cyanide in Water and Soil. Chem-istry, Risk, and Management. Dzombak, D.A., Ghosh,


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R.S. and Wong-Chong, G.M., (eds.), CRC Press, BocaRaton, Florida, USA, (2006), p. 61.

Ghosh RS, Ebbs SD, Bushey JT, Neuhauser EF, Wong-Chong GM (2006a) Cyanide Cycle in Nature. In:Cyanide in Water and Soil. Chemistry, Risk, and

Management. Dzombak, D.A., Ghosh, R.S. andWong-Chong, G.M., (eds.), CRC Press, Boca Raton,Florida, USA, (2006), p. 225.

Ghosh RS, Meeussen JCL, Dzombak DA, Nakles DV(2006c) Fate and Transport of AnthropogenicCyanide in Soil and Groundwater. In: Cyanide inWater and Soil. Chemistry, Risk, and Management.Dzombak, D.A., Ghosh, R.S. and Wong-Chong, G.M.,(eds.), CRC Press, Boca Raton, Florida,USA, (2006), p.197.

Griffin RA (1976) Attenuation of Pollutants in Munici-pal Land ll Leachate by Clay Minerals: Part 1-Col-umn Leaching and Field Veri cation, Ill. St.Geol.Surv. Envir. Geol. Notes No. 76, Urbana, Nov.1976.

Griffin RA (1977) Attenuation of Pollutants in Municipal

Land ll Leachate by Clay Minerals: Part 2-Heavy-Metal Absorption. Ill. St. Geol. Surv. Envir. Geol.Notes No. 79, Urbana, Apr. 1977.

Kresic N (2007). Hydrogeology and Groundwater Mod-eling. Second Edition, CRC Press, Taylor & FrancisGroup, Boca Raton, USA, (2007), p. 807.

NICNAS (2010), Sodium Cyanide, February 2010. Prior-ity Existing Chemical Assessment Report No. 31.Sydney, Australia. Internet accessed on April 3 rd 2010at www.nicnas.gov.au

attenuation potential of clay liner carbon-in-leach tailings storage

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