Journal of Environmental Radioactivity 130 (2014) 15e21
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Radon emanation from backfilled mill tailings in underground uranium mine Patitapaban Sahu a, Devi Prasad Mishra a, *, Durga Charan Panigrahi a, Vivekananda Jha b, R. Lokeswara Patnaik b, Narendra Kumar Sethy b a b
Department of Mining Engineering, Indian School of Mines, Dhanbad 826 004, Jharkhand, India Environmental Assessment Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2013 Received in revised form 13 December 2013 Accepted 20 December 2013 Available online 9 January 2014
Coarser mill tailings used as backfill to stabilize the stoped out areas in underground uranium mines is a potential source of radon contamination. This paper presents the quantitative assessment of radon emanation from the backfilled tailings in Jaduguda mine, India using a cylindrical accumulator. Some of the important parameters such as 226Ra activity concentration, bulk density, bulk porosity, moisture content and radon emanation factor of the tailings affecting radon emanation were determined in the laboratory. The study revealed that the radon emanation rate of the tailings varied in the range of 0.12 e7.03 Bq m2 s1 with geometric mean of 1.01 Bq m2 s1 and geometric standard deviation of 3.39. An increase in radon emanation rate was noticed up to a moisture saturation of 0.09 in the tailings, after which the emanation rate gradually started declining with saturation due to low diffusion coefficient of radon in the saturated tailings. Radon emanation factor of the tailings varied in the range of 0.08e0.23 with the mean value of 0.21. The emanation factor of the tailings with moisture saturation level over 0.09 was found to be about three times higher than that of the absolutely dry tailings. The empirical relationship obtained between 222Rn emanation rate and 226Ra activity concentration of the tailings indicated a significant positive linear correlation (r ¼ 0.95, p < 0.001). This relationship may be useful for quick prediction of radon emanation rate from the backfill material of similar nature. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Underground uranium mine Backfilled mill tailings 226 Ra activity concentration Emanation factor 222 Rn emanation rate Moisture content
1. Introduction The uranium content of Indian ores is low. The existing uranium processing technology in India is based on leaching of uraninite ore in sulphuric acid medium in presence of pyrolusite oxidant followed by filtration, ion exchange separation and product recovery in the form of magnesium di-uranate (MgU2O7). As a result of the low uranium content, large quantities of solid and liquid wastes are produced during the production of uranium concentrate, which are mixed together, neutralized at elevated pH (>9.5) for precipitation of radionuclides and heavy metals. The slurried tailings is separated into coarse and fine fractions, the former is sent back to underground uranium mines for backfilling the stoped out areas and later is discharged into a geologically stable structure with proper embankment and barriers called tailings pond (Tripathi et al., 2008). Depending on the ore processing and waste
* Corresponding author. Tel.: þ91 9430191673; fax: þ91 326 2296628/2296563. E-mail address: [email protected] (D.P. Mishra). 0265-931X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.12.017
immobilization technology, these tailings may contain elevated amounts of 226Ra. In mine atmosphere, the concentration of radon gas (222Rn, t1/ 226 Ra, largely 2 ¼ 3.82 d) which is the immediate decay product of depends on activity concentration of 226Ra and 222Rn emanation rate from the tailings used as backfill material (Raghavayya and Khan, 1973). The main source of internal dose for miners and members of public is due to exposure of 222Rn and its short-lived decay products. Prolonged exposure of short-lived radon progeny at elevated level is carcinogenic and may lead to lung carcinoma (Field et al., 2000; Gulson et al., 2005; Al-Zoughool and Krewski, 2009; ICRP, 1993, 1989; UNSCEAR, 2006). In view of the nature of hazard associated with radon progeny, monitoring of activity concentration of 222Rn in uranium mines is required to minimize the internal exposure of miners within the limits stipulated by regulatory agencies (ICRP, 1993; IAEA, 1996). The 222Rn gas emanation from the tailings into mine opening is caused by its diffusion and transport through the matrices. In diffusion, the 222Rn gas moves through the fluid (air and water) filling the pore spaces of the associated matrices, whereas in transport, the fluid carries the radon gas through these pores. 222Rn
16
P. Sahu et al. / Journal of Environmental Radioactivity 130 (2014) 15e21
emanation is primarily governed by the factors such as porosity (Tanner, 1964, 1978; Evans, 1969; Lawrence et al., 2009), distribution of 226Ra and grain size (Garver and Baskaran, 2004; Sakoda et al., 2011; Somlai et al., 2008) and moisture content of the matrix (Bossew, 2003; Adler and Perrier, 2009; Breitner et al., 2010). Apart from this, temperature (Iskandar et al., 2004; Girault and Perrier, 2011) and pressure (Schroeder, 1966; Pohl-Rueling and Pohl, 1969; IAEA, 1981) also have important bearing in deciding the fate of radon emanation. Despite low radium content, emanation potential of radon from the tailings may be high as compared to the bulk ore from which it is originated due to the increased porosity and surface area (Khan and Raghavayya, 1973; Thompkins, 1982). The presence of low moisture content in pore spaces of the tailings enhances radon emanation (Strong and Levins, 1982). Washington and Rose (1990) reported that temperature changes in the porous materials have less effect on radon emanation in dry materials than in moist materials. Clements and Wilkening (1974) reported that the change of pressure in pore spaces of the materials from 1 to 2% changes the radon emanation rate from 20 to 60%. When there is a pressure drop in mine environment, the radon laden air filling the pores moves out into the mine opening carrying the accumulated radon along with it. The radon atoms leaving the tailing matrices, within which they are formed, may directly enter into the interstitial flowing water and transported along with it. Earlier, in Elliot Lake mine (Wheeler, 1982; Archibald and Nantel, 1984) and Saskatchewan mine (Cheng, 1981) of Canada, the radon emanation rates from the tailings materials were estimated based on diffusion theory which does not consider some important mechanisms such as adsorption and advection. Under very dry condition, radon diffusion from the tailings decreases due to the effect of re-adsorption of the recoiled radon atoms on the surfaces of the pores (IAEA, 1981). On the other hand, when these pores are saturated with water, the recoiling ions escaping into the pores encounter a dense absorber and have a greater probability of remaining in the pores (Meslin et al., 2010; Hassan et al., 2011). However, when the interstices of the tailings are filled with water, radon is transported along with flowing water. The concentration of radon in water may approach a much higher level than the pores filled with air (Andrews and Wood, 1972). Due to the aforementioned reasons, there may be uncertainty in the estimated radon emanation rate. Raghavayya and Khan (1973) used a 25 L capacity steel drum from which air samples were drawn after a time lapse varying from 7 to 65 h for measurement of radon emanation rate from backfill material in the mine. However, this technique has also some limitations like occurrence of back diffusion and leakage of air due to longer experiment period giving rise to uncertainty in the results and inconvenience of carrying the large size drum for radon emanation studies at different locations inside the mine. In the present study, we measured the radon emanation rate from in situ backfilled tailings in Jaduguda mine based on accumulation technique using an accumulator of approximately 7 L capacity. The experiments were carried out within a smaller accumulation period of 1 h to minimize the errors due to alterations in the parameters affecting radon emanation, back diffusion of radon and air leakage (Jha et al., 2000; Mayya, 2004). Radon emanation factor of the tailings was determined using both in situ measurement of radon emanation rate and empirical function developed by Rogers and Nielson (1991) for predicting the effective diffusion coefficient. This study intended to investigate the effects of tailings parameters such as moisture content, bulk density, bulk porosity and 226Ra activity concentration on emanation potential of radon. It also aimed at developing an empirical relationship using the primary data of backfilled tailings for quick prediction of radon emanation rate from backfill material of similar nature.
2. Materials and methods 2.1. Study area This study was carried out at Jaduguda uranium mine (Latitude 22 390 and Longitude 86 220 ) located in the Singhbhum Thrust Belt of Jharkhand, India. The geological map of Singhbhum Thrust Belt is shown in Fig. 1 (Sarangi, 2003). Uranium bearing minerals in Jaduguda mine occur in the Precambrian meta-sedimentary rocks. The host rocks are autoclastic conglomerate (brecciated quartzite) and quartz-chlorite-biotite-magnetite schist. The uranium minerals are associated with a wide variety of sulphides of copper, nickel, cobalt, molybdenum, arsenic and bismuth (Krishnamurthy et al., 2004). The mine has two parallel lodes starting from the surface and extending to a depth of 905 m. The strike of the main ore body dipping towards north is in the eastewest direction. Horizontal cut-and-fill method using de-slimed coarser fraction mill tailings as backfill is the principal stoping method adopted in Jaduguda mine. The tailings are pumped into the mine in the form of aqueous slurry for backfilling, which settle and consolidate by its own weight to stabilise the excavations after draining out of water. 2.2. Methodology The radon emanation rate from in situ backfilled tailings was measured at 16 locations of the backfilled stopes using a cylindrical accumulator. Thereafter, the settled core samples of backfill tailings were collected from the locations of radon emanation study by inserting a stainless steel cylinder of 36 mm diameter and 5 cm length. The samples were kept in plastic vials marked with proper identification of the location and date of collection. The desired physical and radioactive properties of the tailings were determined in the laboratory. 2.2.1. Measurement of physical properties of tailings The moisture content (dry weight basis) of backfilled tailings was determined by heating the samples at 105 5 C for 24 h as per standard ASTM D2216. An undisturbed core of backfill material was collected inserting a stainless steel cylinder for determination of bulk density. The mass of sample oven dried at the aforementioned temperature for 24 h was noted. The bulk density was determined by dividing the mass by the cylinder volume. The specific gravity of the tailings was determined using water pycnometer as per standard IS: 2386 (Part III). The bulk porosity (4) of the tailings was estimated using Eq. (1)
Fig. 1. Geological map of Singhbhum Thrust Belt (Sarangi, 2003).
P. Sahu et al. / Journal of Environmental Radioactivity 130 (2014) 15e21
4 ¼ 1 r=r
(1)
g
where r is the bulk density of material (kg m3) and rg is the specific gravity (kg m3). The fraction of pore space filled with water or moisture saturation (m) was estimated using Eq. (2) (Rogers and Nielson, 1991)
100m ¼
rMw rw 4
(2)
where Mw is the water content in material (dry weight, %) and rw is the density of water (kg m3). The particle size analysis of the tailings oven dried at 105 C for 24 h was carried out with a standard sieve deck. The specific surface area (A, cm2 g1) of tailings was calculated using Eq. (3) (Raghavayya, 1976)
A ¼
n KS X mj=X MKV d j
! (3)
j¼1
where Ks is the surface area shape factor, Kv is the volume shape factor, d is the true solid density (g cm3), n is the total number of groups, mj is the mass of backfill material falling in jth group (g), Xj is the mean particle size in jth group (mm) and M is the total mass of the sample (g). Since the predominant constituent of the tailings is quartz, the volume (Kv) and surface area (Ks) shape factors for quartz were taken as 0.24 and 1.95 respectively (Saha, 1972). 2.2.2. Measurement of radon emanation rate The radon emanation rate from the backfilled tailings was measured at 16 locations of the backfilled stopes using a cylindrical accumulator of 17 cm diameter and 30 cm height (w7 L capacity) as shown in Fig. 2. One end of the accumulator was closed with a circular disk provided with a stopcock for sucking air samples into the scintillation cells through the filter paper to prevent entry of the radon daughters. The open end of the accumulator was placed on the surface of the backfill. The accumulator was buried up to a
17
depth of 5 cm in the backfill for proper sealing and minimizing air leakage. For estimation of radon activity concentration, air samples from the accumulator were collected into the evacuated ZnS (Ag) scintillation cells of 140 ml capacity at the beginning (t ¼ 0) and at the end of experiments. Before sampling, the scintillation cells were vacuumed to a pressure of 1.3 Pa using a vacuum pump. These cells were connected to the photomultiplier assembly after a delay period of about 200 min (the instant of sampling is reckoned as zero time) to ensure equilibrium between the radon and its progeny in the cells. The activity concentration of radon (C) was estimated using Eq. (4) (Raghavayya, 1981; Panigrahi et al., 2005)
C ¼
6:967 105 c EVs els 1 elT
(4)
where c is the net alpha counts during the counting duration “T”, E is the efficiency of the system (74%), Vs is the volume of scintillation cell (m3), l is the decay constant of radon (2.09 106 s1), s is the delay time after end of the sampling (s), T is the counting duration (s) and the constant 6.967 105 represents the radioactive decay of 222Rn in unit time in the scintillation cell (s1). The efficiency of the system was calculated using the formula E ¼ (average counts per minute in the system/21,500) 75, where, 21,500 is the activity (counts per minute) and 75 is the efficiency (%) of the standard scintillation cell. The alpha counts were noted for 10 min at 95% confidence level to estimate the radon activity concentration of each sample. Assuming total radon activity in the accumulator at any time t as q, the rate of change of radon activity in the accumulator is given in Eq. (5)
dq=dt ¼ JA lq
(5)
where J is the radon emanation rate (Bq m2 s1) and A is the area of cross section of the accumulator (m2). Eq. (5) is a first order differential equation. Applying the boundary conditions at t ¼ 0, q ¼ q0; t ¼ t, q ¼ qt and substituting radon concentration, C ¼ q/v (Bq m3), where volume of the accumulator v ¼ A.h (m3), it becomes
J ¼
lh Ct C0 elt 1 elt
(6)
where C0 is the initial radon concentration in accumulator at t ¼ 0 (Bq m3), Ct is the final radon concentration after time t (Bq m3) and h is the height of the accumulator (m). In this study, Eq. (6) was used for calculating radon emanation rate from the backfilled tailings. For longer build-up period (>1.5 h) of radon in the accumulation chamber, back diffusion may occur and the correction factor may be more significant. Back diffusion correction factor is approximately 2 for an accumulation period of 1.5 h in small chamber, which reduces exponentially with decrease in build-up time of radon in the chamber (Mayya, 2004). Back diffusion correction factor is insignificant for short time accumulation in the chamber as maximum radon concentration occurs within few hours (Jha et al., 2000). Based on the aforementioned observations, the build-up period of radon in our experiments was varied from 27 to 55 min to minimize back diffusion correction factor.
Fig. 2. Experimental setup for measurement of radon emanation rate from backfilled tailings.
2.2.3. Measurement of 226Ra activity concentration The emanometry technique was used in this study for measurement of 226Ra activity concentration of the backfilled tailings (Raghavayya, 1990). Dried tailings sample of 2.0 g was repeatedly
18
P. Sahu et al. / Journal of Environmental Radioactivity 130 (2014) 15e21
leached in 8 N HNO3, filtered and the leached aliquot was made up to 100 ml. Thereafter, 50 ml of the aliquot was transferred to the radon bubbler and diluted to w100 ml. Before starting the experiment, the dissolved radon initially present in the solution was removed using a vacuum pump. The solution in the bubbler was allowed to stand for a desired period of 20 days to ensure secular equilibrium between 226Ra and 222Rn. The accumulated radon in the bubbler was transferred to an evacuated scintillation cell using a swedgelok quick connector device. The scintillation cell was left for >200 min to ensure secular equilibrium between radon and its short-lived progeny. The alpha activity was counted for a period of 10 min at 95% confidence level. However, a longer counting duration can also be used if the expected level of 226Ra is appreciably low. The activity of 226Ra (Bq kg1) in the sample was determined using Eq. (7) 226
Ra ¼
6:967 105 c Vts 1000 Vsb w Eels 1 elf 1 elT
(7)
where c is the number of net counts, E is the efficiency (%), Vts is the total volume of solution prepared from the sample (ml), Vsb is the volume of the solution in bubbler (ml), f is the build-up period in bubbler (s) and w is the weight of sample (g). The minimum detectable 226Ra activity in the solution taken in the bubbler depends on the factors like duration of radon build-up, efficiency and background count rate of the scintillation cell and counting duration. The background of the cell was 0.5 cpm (counts per minute) and average efficiency of the system worked out to be 85% using a standard scintillation cell as explained in Section 2.2.2. Allowing the maximum build-up period, minimum detectable activity worked out to be 6.8 mBq (Jha et al., 2010). 2.2.4. Calculation of radon emanation factor Radon emanation factor (f) of in situ tailings was estimated using Eq. (8) (IAEA, 1992)
f ¼ J=226 RarðlD Þ1=2 e
(8)
where J is the estimated radon emanation rate (Bq m2 s1) and De is the effective diffusion coefficient (m2 s1). The effective radon diffusion coefficient (De) in the material was determined from an empirical relationship given by Rogers and Nielson (1991)
T 0:75 De ¼ D0 4 exp 6m4 6m144 273
(9)
where D0 is the diffusion coefficient of radon in air at ambient temperature and pressure (1.1 105 m2 s1),4 is the bulk porosity of the tailings, m is the fraction of pore space filled with water and T is the absolute temperature (K). 3. Results and discussion 3.1. Physical properties of backfilled tailings The physical properties of the backfilled tailings are summarised in Table 1. The particle size analysis revealed that the backfilled tailings were of predominant size >75 mm with average specific surface area of 460.42 cm2 g1. The average bulk density, specific gravity and bulk porosity of the tailings were found to be 1790 kg m3, 2700 kg m3 and 33.7% respectively. The moisture content of the tailings varied in the range of 0e10.06%. The high bulk porosity and larger surface area of the tailings may result in greater emanation of radon into mine atmosphere.
Table 1 Physical properties of the backfilled tailings. Parameters
Average values
Bulk density, kg m3 Specific gravity, kg m3 Bulk porosity, % Particle size analysis results D77, mm D8, mm D15, mm Specific surface area, cm2 g1
1790 2700 33.7 75.0 46.0
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Radon emanation from backfilled mill tailings in underground uranium mine Patitapaban Sahu a, Devi Prasad Mishra a, *, Durga Charan Panigrahi a, Vivekananda Jha b, R. Lokeswara Patnaik b, Narendra Kumar Sethy b a b
Department of Mining Engineering, Indian School of Mines, Dhanbad 826 004, Jharkhand, India Environmental Assessment Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2013 Received in revised form 13 December 2013 Accepted 20 December 2013 Available online 9 January 2014
Coarser mill tailings used as backfill to stabilize the stoped out areas in underground uranium mines is a potential source of radon contamination. This paper presents the quantitative assessment of radon emanation from the backfilled tailings in Jaduguda mine, India using a cylindrical accumulator. Some of the important parameters such as 226Ra activity concentration, bulk density, bulk porosity, moisture content and radon emanation factor of the tailings affecting radon emanation were determined in the laboratory. The study revealed that the radon emanation rate of the tailings varied in the range of 0.12 e7.03 Bq m2 s1 with geometric mean of 1.01 Bq m2 s1 and geometric standard deviation of 3.39. An increase in radon emanation rate was noticed up to a moisture saturation of 0.09 in the tailings, after which the emanation rate gradually started declining with saturation due to low diffusion coefficient of radon in the saturated tailings. Radon emanation factor of the tailings varied in the range of 0.08e0.23 with the mean value of 0.21. The emanation factor of the tailings with moisture saturation level over 0.09 was found to be about three times higher than that of the absolutely dry tailings. The empirical relationship obtained between 222Rn emanation rate and 226Ra activity concentration of the tailings indicated a significant positive linear correlation (r ¼ 0.95, p < 0.001). This relationship may be useful for quick prediction of radon emanation rate from the backfill material of similar nature. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Underground uranium mine Backfilled mill tailings 226 Ra activity concentration Emanation factor 222 Rn emanation rate Moisture content
1. Introduction The uranium content of Indian ores is low. The existing uranium processing technology in India is based on leaching of uraninite ore in sulphuric acid medium in presence of pyrolusite oxidant followed by filtration, ion exchange separation and product recovery in the form of magnesium di-uranate (MgU2O7). As a result of the low uranium content, large quantities of solid and liquid wastes are produced during the production of uranium concentrate, which are mixed together, neutralized at elevated pH (>9.5) for precipitation of radionuclides and heavy metals. The slurried tailings is separated into coarse and fine fractions, the former is sent back to underground uranium mines for backfilling the stoped out areas and later is discharged into a geologically stable structure with proper embankment and barriers called tailings pond (Tripathi et al., 2008). Depending on the ore processing and waste
* Corresponding author. Tel.: þ91 9430191673; fax: þ91 326 2296628/2296563. E-mail address: [email protected] (D.P. Mishra). 0265-931X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.12.017
immobilization technology, these tailings may contain elevated amounts of 226Ra. In mine atmosphere, the concentration of radon gas (222Rn, t1/ 226 Ra, largely 2 ¼ 3.82 d) which is the immediate decay product of depends on activity concentration of 226Ra and 222Rn emanation rate from the tailings used as backfill material (Raghavayya and Khan, 1973). The main source of internal dose for miners and members of public is due to exposure of 222Rn and its short-lived decay products. Prolonged exposure of short-lived radon progeny at elevated level is carcinogenic and may lead to lung carcinoma (Field et al., 2000; Gulson et al., 2005; Al-Zoughool and Krewski, 2009; ICRP, 1993, 1989; UNSCEAR, 2006). In view of the nature of hazard associated with radon progeny, monitoring of activity concentration of 222Rn in uranium mines is required to minimize the internal exposure of miners within the limits stipulated by regulatory agencies (ICRP, 1993; IAEA, 1996). The 222Rn gas emanation from the tailings into mine opening is caused by its diffusion and transport through the matrices. In diffusion, the 222Rn gas moves through the fluid (air and water) filling the pore spaces of the associated matrices, whereas in transport, the fluid carries the radon gas through these pores. 222Rn
16
P. Sahu et al. / Journal of Environmental Radioactivity 130 (2014) 15e21
emanation is primarily governed by the factors such as porosity (Tanner, 1964, 1978; Evans, 1969; Lawrence et al., 2009), distribution of 226Ra and grain size (Garver and Baskaran, 2004; Sakoda et al., 2011; Somlai et al., 2008) and moisture content of the matrix (Bossew, 2003; Adler and Perrier, 2009; Breitner et al., 2010). Apart from this, temperature (Iskandar et al., 2004; Girault and Perrier, 2011) and pressure (Schroeder, 1966; Pohl-Rueling and Pohl, 1969; IAEA, 1981) also have important bearing in deciding the fate of radon emanation. Despite low radium content, emanation potential of radon from the tailings may be high as compared to the bulk ore from which it is originated due to the increased porosity and surface area (Khan and Raghavayya, 1973; Thompkins, 1982). The presence of low moisture content in pore spaces of the tailings enhances radon emanation (Strong and Levins, 1982). Washington and Rose (1990) reported that temperature changes in the porous materials have less effect on radon emanation in dry materials than in moist materials. Clements and Wilkening (1974) reported that the change of pressure in pore spaces of the materials from 1 to 2% changes the radon emanation rate from 20 to 60%. When there is a pressure drop in mine environment, the radon laden air filling the pores moves out into the mine opening carrying the accumulated radon along with it. The radon atoms leaving the tailing matrices, within which they are formed, may directly enter into the interstitial flowing water and transported along with it. Earlier, in Elliot Lake mine (Wheeler, 1982; Archibald and Nantel, 1984) and Saskatchewan mine (Cheng, 1981) of Canada, the radon emanation rates from the tailings materials were estimated based on diffusion theory which does not consider some important mechanisms such as adsorption and advection. Under very dry condition, radon diffusion from the tailings decreases due to the effect of re-adsorption of the recoiled radon atoms on the surfaces of the pores (IAEA, 1981). On the other hand, when these pores are saturated with water, the recoiling ions escaping into the pores encounter a dense absorber and have a greater probability of remaining in the pores (Meslin et al., 2010; Hassan et al., 2011). However, when the interstices of the tailings are filled with water, radon is transported along with flowing water. The concentration of radon in water may approach a much higher level than the pores filled with air (Andrews and Wood, 1972). Due to the aforementioned reasons, there may be uncertainty in the estimated radon emanation rate. Raghavayya and Khan (1973) used a 25 L capacity steel drum from which air samples were drawn after a time lapse varying from 7 to 65 h for measurement of radon emanation rate from backfill material in the mine. However, this technique has also some limitations like occurrence of back diffusion and leakage of air due to longer experiment period giving rise to uncertainty in the results and inconvenience of carrying the large size drum for radon emanation studies at different locations inside the mine. In the present study, we measured the radon emanation rate from in situ backfilled tailings in Jaduguda mine based on accumulation technique using an accumulator of approximately 7 L capacity. The experiments were carried out within a smaller accumulation period of 1 h to minimize the errors due to alterations in the parameters affecting radon emanation, back diffusion of radon and air leakage (Jha et al., 2000; Mayya, 2004). Radon emanation factor of the tailings was determined using both in situ measurement of radon emanation rate and empirical function developed by Rogers and Nielson (1991) for predicting the effective diffusion coefficient. This study intended to investigate the effects of tailings parameters such as moisture content, bulk density, bulk porosity and 226Ra activity concentration on emanation potential of radon. It also aimed at developing an empirical relationship using the primary data of backfilled tailings for quick prediction of radon emanation rate from backfill material of similar nature.
2. Materials and methods 2.1. Study area This study was carried out at Jaduguda uranium mine (Latitude 22 390 and Longitude 86 220 ) located in the Singhbhum Thrust Belt of Jharkhand, India. The geological map of Singhbhum Thrust Belt is shown in Fig. 1 (Sarangi, 2003). Uranium bearing minerals in Jaduguda mine occur in the Precambrian meta-sedimentary rocks. The host rocks are autoclastic conglomerate (brecciated quartzite) and quartz-chlorite-biotite-magnetite schist. The uranium minerals are associated with a wide variety of sulphides of copper, nickel, cobalt, molybdenum, arsenic and bismuth (Krishnamurthy et al., 2004). The mine has two parallel lodes starting from the surface and extending to a depth of 905 m. The strike of the main ore body dipping towards north is in the eastewest direction. Horizontal cut-and-fill method using de-slimed coarser fraction mill tailings as backfill is the principal stoping method adopted in Jaduguda mine. The tailings are pumped into the mine in the form of aqueous slurry for backfilling, which settle and consolidate by its own weight to stabilise the excavations after draining out of water. 2.2. Methodology The radon emanation rate from in situ backfilled tailings was measured at 16 locations of the backfilled stopes using a cylindrical accumulator. Thereafter, the settled core samples of backfill tailings were collected from the locations of radon emanation study by inserting a stainless steel cylinder of 36 mm diameter and 5 cm length. The samples were kept in plastic vials marked with proper identification of the location and date of collection. The desired physical and radioactive properties of the tailings were determined in the laboratory. 2.2.1. Measurement of physical properties of tailings The moisture content (dry weight basis) of backfilled tailings was determined by heating the samples at 105 5 C for 24 h as per standard ASTM D2216. An undisturbed core of backfill material was collected inserting a stainless steel cylinder for determination of bulk density. The mass of sample oven dried at the aforementioned temperature for 24 h was noted. The bulk density was determined by dividing the mass by the cylinder volume. The specific gravity of the tailings was determined using water pycnometer as per standard IS: 2386 (Part III). The bulk porosity (4) of the tailings was estimated using Eq. (1)
Fig. 1. Geological map of Singhbhum Thrust Belt (Sarangi, 2003).
P. Sahu et al. / Journal of Environmental Radioactivity 130 (2014) 15e21
4 ¼ 1 r=r
(1)
g
where r is the bulk density of material (kg m3) and rg is the specific gravity (kg m3). The fraction of pore space filled with water or moisture saturation (m) was estimated using Eq. (2) (Rogers and Nielson, 1991)
100m ¼
rMw rw 4
(2)
where Mw is the water content in material (dry weight, %) and rw is the density of water (kg m3). The particle size analysis of the tailings oven dried at 105 C for 24 h was carried out with a standard sieve deck. The specific surface area (A, cm2 g1) of tailings was calculated using Eq. (3) (Raghavayya, 1976)
A ¼
n KS X mj=X MKV d j
! (3)
j¼1
where Ks is the surface area shape factor, Kv is the volume shape factor, d is the true solid density (g cm3), n is the total number of groups, mj is the mass of backfill material falling in jth group (g), Xj is the mean particle size in jth group (mm) and M is the total mass of the sample (g). Since the predominant constituent of the tailings is quartz, the volume (Kv) and surface area (Ks) shape factors for quartz were taken as 0.24 and 1.95 respectively (Saha, 1972). 2.2.2. Measurement of radon emanation rate The radon emanation rate from the backfilled tailings was measured at 16 locations of the backfilled stopes using a cylindrical accumulator of 17 cm diameter and 30 cm height (w7 L capacity) as shown in Fig. 2. One end of the accumulator was closed with a circular disk provided with a stopcock for sucking air samples into the scintillation cells through the filter paper to prevent entry of the radon daughters. The open end of the accumulator was placed on the surface of the backfill. The accumulator was buried up to a
17
depth of 5 cm in the backfill for proper sealing and minimizing air leakage. For estimation of radon activity concentration, air samples from the accumulator were collected into the evacuated ZnS (Ag) scintillation cells of 140 ml capacity at the beginning (t ¼ 0) and at the end of experiments. Before sampling, the scintillation cells were vacuumed to a pressure of 1.3 Pa using a vacuum pump. These cells were connected to the photomultiplier assembly after a delay period of about 200 min (the instant of sampling is reckoned as zero time) to ensure equilibrium between the radon and its progeny in the cells. The activity concentration of radon (C) was estimated using Eq. (4) (Raghavayya, 1981; Panigrahi et al., 2005)
C ¼
6:967 105 c EVs els 1 elT
(4)
where c is the net alpha counts during the counting duration “T”, E is the efficiency of the system (74%), Vs is the volume of scintillation cell (m3), l is the decay constant of radon (2.09 106 s1), s is the delay time after end of the sampling (s), T is the counting duration (s) and the constant 6.967 105 represents the radioactive decay of 222Rn in unit time in the scintillation cell (s1). The efficiency of the system was calculated using the formula E ¼ (average counts per minute in the system/21,500) 75, where, 21,500 is the activity (counts per minute) and 75 is the efficiency (%) of the standard scintillation cell. The alpha counts were noted for 10 min at 95% confidence level to estimate the radon activity concentration of each sample. Assuming total radon activity in the accumulator at any time t as q, the rate of change of radon activity in the accumulator is given in Eq. (5)
dq=dt ¼ JA lq
(5)
where J is the radon emanation rate (Bq m2 s1) and A is the area of cross section of the accumulator (m2). Eq. (5) is a first order differential equation. Applying the boundary conditions at t ¼ 0, q ¼ q0; t ¼ t, q ¼ qt and substituting radon concentration, C ¼ q/v (Bq m3), where volume of the accumulator v ¼ A.h (m3), it becomes
J ¼
lh Ct C0 elt 1 elt
(6)
where C0 is the initial radon concentration in accumulator at t ¼ 0 (Bq m3), Ct is the final radon concentration after time t (Bq m3) and h is the height of the accumulator (m). In this study, Eq. (6) was used for calculating radon emanation rate from the backfilled tailings. For longer build-up period (>1.5 h) of radon in the accumulation chamber, back diffusion may occur and the correction factor may be more significant. Back diffusion correction factor is approximately 2 for an accumulation period of 1.5 h in small chamber, which reduces exponentially with decrease in build-up time of radon in the chamber (Mayya, 2004). Back diffusion correction factor is insignificant for short time accumulation in the chamber as maximum radon concentration occurs within few hours (Jha et al., 2000). Based on the aforementioned observations, the build-up period of radon in our experiments was varied from 27 to 55 min to minimize back diffusion correction factor.
Fig. 2. Experimental setup for measurement of radon emanation rate from backfilled tailings.
2.2.3. Measurement of 226Ra activity concentration The emanometry technique was used in this study for measurement of 226Ra activity concentration of the backfilled tailings (Raghavayya, 1990). Dried tailings sample of 2.0 g was repeatedly
18
P. Sahu et al. / Journal of Environmental Radioactivity 130 (2014) 15e21
leached in 8 N HNO3, filtered and the leached aliquot was made up to 100 ml. Thereafter, 50 ml of the aliquot was transferred to the radon bubbler and diluted to w100 ml. Before starting the experiment, the dissolved radon initially present in the solution was removed using a vacuum pump. The solution in the bubbler was allowed to stand for a desired period of 20 days to ensure secular equilibrium between 226Ra and 222Rn. The accumulated radon in the bubbler was transferred to an evacuated scintillation cell using a swedgelok quick connector device. The scintillation cell was left for >200 min to ensure secular equilibrium between radon and its short-lived progeny. The alpha activity was counted for a period of 10 min at 95% confidence level. However, a longer counting duration can also be used if the expected level of 226Ra is appreciably low. The activity of 226Ra (Bq kg1) in the sample was determined using Eq. (7) 226
Ra ¼
6:967 105 c Vts 1000 Vsb w Eels 1 elf 1 elT
(7)
where c is the number of net counts, E is the efficiency (%), Vts is the total volume of solution prepared from the sample (ml), Vsb is the volume of the solution in bubbler (ml), f is the build-up period in bubbler (s) and w is the weight of sample (g). The minimum detectable 226Ra activity in the solution taken in the bubbler depends on the factors like duration of radon build-up, efficiency and background count rate of the scintillation cell and counting duration. The background of the cell was 0.5 cpm (counts per minute) and average efficiency of the system worked out to be 85% using a standard scintillation cell as explained in Section 2.2.2. Allowing the maximum build-up period, minimum detectable activity worked out to be 6.8 mBq (Jha et al., 2010). 2.2.4. Calculation of radon emanation factor Radon emanation factor (f) of in situ tailings was estimated using Eq. (8) (IAEA, 1992)
f ¼ J=226 RarðlD Þ1=2 e
(8)
where J is the estimated radon emanation rate (Bq m2 s1) and De is the effective diffusion coefficient (m2 s1). The effective radon diffusion coefficient (De) in the material was determined from an empirical relationship given by Rogers and Nielson (1991)
T 0:75 De ¼ D0 4 exp 6m4 6m144 273
(9)
where D0 is the diffusion coefficient of radon in air at ambient temperature and pressure (1.1 105 m2 s1),4 is the bulk porosity of the tailings, m is the fraction of pore space filled with water and T is the absolute temperature (K). 3. Results and discussion 3.1. Physical properties of backfilled tailings The physical properties of the backfilled tailings are summarised in Table 1. The particle size analysis revealed that the backfilled tailings were of predominant size >75 mm with average specific surface area of 460.42 cm2 g1. The average bulk density, specific gravity and bulk porosity of the tailings were found to be 1790 kg m3, 2700 kg m3 and 33.7% respectively. The moisture content of the tailings varied in the range of 0e10.06%. The high bulk porosity and larger surface area of the tailings may result in greater emanation of radon into mine atmosphere.
Table 1 Physical properties of the backfilled tailings. Parameters
Average values
Bulk density, kg m3 Specific gravity, kg m3 Bulk porosity, % Particle size analysis results D77, mm D8, mm D15, mm Specific surface area, cm2 g1
1790 2700 33.7 75.0 46.0
