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Article

Thermal Treatment of Mercury Mine Wastes Using a Rotary Solar Kiln

1
Department of Fluid Mechanics, Polytechnic University of Catalonia (UPC), Colón, 7, 08222 Terrassa, Spain
2
Solar Platform of Almería (PSA), P.O. Box 22, Tabernas, E-04200 Almería, Spain
*
Author to whom correspondence should be addressed.
Minerals 2014, 4(1), 37-51; https://doi.org/10.3390/min4010037
Submission received: 20 November 2013 / Revised: 24 January 2014 / Accepted: 24 January 2014 / Published: 28 January 2014
(This article belongs to the Special Issue Mine Waste Characterization, Management and Remediation)

Abstract

:
Thermal desorption, by a rotary kiln of mercury contaminated soil and mine wastes, has been used in order to volatilize mercury from the contaminated medium. Solar thermal desorption is an innovative treatment that uses solar energy to increase the volatility of contaminants, which are removed from a solid matrix by a controlled air flow system. Samples of soils and mine wastes used in the experiments were collected in the abandoned Valle del Azogue mine (SE, Spain), where a complex ore, composed mainly of cinnabar, arsenic minerals (realgar and orpiment) and stibnite, was mined. The results showed that thermal treatment at temperatures >400 °C successfully lowered the Hg content (2070–116 ppm) to <15 mg kg−1. The lowest values of mercury in treated samples were obtained at a higher temperature and exposition time. The samples that showed a high removal efficiency (>99%) were associated with the presence of significant contents of cinnabar and an equivalent diameter above 0.8 mm.

1. Introduction

Thermal treatment has been used to treat mercury-contaminated soil and waste by typical desorption units, which operate at temperatures ranging from 200 to 700 °C [1,2]. In these systems, wastes are placed in the thermal desorber (direct-fired rotary kilns, indirectly heated screw, auger systems, etc.), after being heated, to volatilize the mercury. In these thermal devices, the off-gas generated is passed through a filtration system, where Hg0, finally, is collected [3,4]. Several mercury desorption experiments have demonstrated the feasibility of mercury removal at temperatures between 127 and 600 °C [5,6,7,8]. Experimental remediation of mercury-polluted soils by low-temperature thermal desorption has also shown mercury removal of over 99% in sand [8] and the volatilization of at least 99% of mercuric sulfide from polluted soil [5].
Besides, thermal treatment of mercury contaminated sediments showed that the percentage of mercury removal raises with the temperature during the treatment of solid matrices [9,10]. Furthermore, the treatment of mercury wastes from the chloralkali industry showed that treatment for 1 h at 800 °C allowed for a removal efficiency above 99.7% [11]. The results of the thermal desorption of mercury from different contaminated wastes showed that thermal decontamination at temperatures >400 °C successfully decreased the Hg content [12,13,14].
Previous experiments of solar thermal desorption using a fluidized bed kiln showed that when soils and mine wastes were heated to 400–500 °C, the mercury removal was significant [15]. The main objective of this research was to evaluate the potential of solar thermal desorption (STD) for the removal of mercury from mining contaminated soils and wastes. A rotary kiln system was designed and used to evaluate the effectiveness of STD and to verify its efficiency, comparing the results with previous experiments using other technologies.

2. Materials and Methods

2.1. Characterization of Soils and Mine Wastes

Samples of soils and mine wastes were collected in the Valle del Azogue mine (SE, Spain). The Valle del Azogue mine is located in the Betic Ranges and was exploited from approximately 1873 to 1890. The ore is composed of stibnite, cinnabar, arsenic minerals (realgar and orpiment), sphalerite, siderite, chalcopyrite, pyrite, quartz, calcite and barite [16]. The sampling area, comprising the North of Sierra Almagrera, is located 90 km NE of the city of Almería (SE, Spain) in a semi-arid and intensively cultivated region. This abandoned mining area together with the Iberian Pyrite Belt and the Cartagena mining district is the oldest metallurgical and mining area in the Iberian Peninsula [16].
Approximately 1.5-kg samples of mine wastes and soil (Figure 1) were manually extracted and crushed to 10 mesh in a jaw crusher, quartered and pulverized in an agate mortar, re-homogenized and repacked in plastic bags. Soil samples were taken from a depth of approximately 0–0.25 m and were sent to Actlabs (Ontario, Canada) with other mine samples. Au, Ag, As, Ba, Br, Ca, Ce, Co, Cr, Cs, Eu, Fe, Hf, Hg, Ir, La, Lu, Na, Ni, Nd, Rb, Sb, Sc, Se, Sm, Sn, Sr, Ta, Th, Tb, U, W, Y and Yb were quantitatively analyzed by instrumental neutron activation analysis (INAA), and Mo, Cu, Pb, Zn, Ag, Ni, Mn, Sr, Cd, Bi, V, Ca, P, Mg, Tl, Al, K, Y and Be were analyzed by inductively coupled plasma emission spectroscopy (ICP-OES). The thermally treated samples were analyzed in the same way. The accuracy of the analytical data may be evaluated around 10%, because of the heterogeneity of the solid samples.
Figure 1. (a) General view of Valle del Azogue area. (b) Detail of the calcines and old metallurgical plant (location of MA4 and MA5 samples). (c) Detail of weathered superficial mine wastes (location of MA1 and MA2 samples).
Figure 1. (a) General view of Valle del Azogue area. (b) Detail of the calcines and old metallurgical plant (location of MA4 and MA5 samples). (c) Detail of weathered superficial mine wastes (location of MA1 and MA2 samples).
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Hg phases were determined by solid-phase Hg-thermodesorption (SPTD), based on the specific thermal desorption or decomposition of Hg compounds from solids at different temperatures [16,17]. Mercury thermo-desorption curves were determined by means of an in-house apparatus, consisting of an electronically controlled heating unit and an Hg detection unit. Measurements were carried out at a heating rate of 0.5 °C/s and a nitrogen-gas flow of 300 mL/min. The lowest level of detection under the given conditions is in the range of 40–50 ng if all Hg is released within a single peak [17]. The results are depicted as Hg-thermodesorption curves (Hg-TDC), which show the release of mercury versus temperature.
Mine waste samples were studied using transmitted and reflected light microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with an attached energy dispersive X-ray spectroscopy system (EDS) at the Electronic Microscopy Laboratory of the Barcelona Autonomous University.
Furthermore, some physical characteristics of the soil were evaluated: pore size distribution, porosity, field capacity and bulk density (Table 1). Particle size distribution was determined by sieve analysis using aperture ranges between 4000 and <350 μm.
Table 1. Metal concentration in soils and mine wastes. Values in parts per million, except Fe (%), Na (%), S (%), Ca (%), K (%) and Mg (%). MW, mining wastes; CS, contaminated soil; CAL, calcine; IV, intervention values for soil remediation of the Dutch regulations [18].
Table 1. Metal concentration in soils and mine wastes. Values in parts per million, except Fe (%), Na (%), S (%), Ca (%), K (%) and Mg (%). MW, mining wastes; CS, contaminated soil; CAL, calcine; IV, intervention values for soil remediation of the Dutch regulations [18].
SampleMA1MA3MA5MA6MA2MA4MA7IV
TypeMWCSCALMWMWCALCAL---
As (ppm)59814313204625501,55047755
Ba (ppm)59,8007,35020,40015,80078,90093,00042,800625
Co (ppm)9<1258<1<4<4240
Cr (ppm)<2064<604483<43<9380
Fe (%)3.193.442.722.492.273.062.34---
Hg (ppm)2,070116<259358651301,24010
Na (%)0.831.032.231.180.640.691.24---
Sb (ppm)2,880357>10,0002,0003,290>10,0002,85015
Se (ppm)<18<6<37<9<10<31<30.7 *
Ta (ppm)<2.5<0.5<6.0<2.5<0.5<3.3<0.51 *
Th (ppm)<2.011.6<7.07.26.5<3.84.8---
U (ppm)<6.0<0.9<15.0<3.0<2.9<8.9<1.9---
W (ppm)<5<1<13<4<1<6<1---
Ag (ppm)16.51.158.715.725.734.515.315 **
Cu (ppm)36273238314228190
Cd (ppm)3.8151.410.66.60.812
Mo (ppm)4<1<112<1<1200
Pb (ppm)2131341,8205121,2101,190536530
Ni (ppm)25552034202318210
Zn (ppm)1,2004245031,3303,1902,230854720
S (%)2.620.472.811.492.052.020.28---
Be (ppm)23132221.1 *
Ca (%)1.72.497.332.280.273.090.12---
K (%)1.861.981.142.071.871.451.49---
Mg (%)0.461.280.940.690.231.160.54---
Mn (ppm)83823641085317131---
V (ppm)5191256357305342 *
Notes: * target values for sediments; ** indicative values of serious contamination for sediments.

2.2. Rotary Kiln

In this study, an experimental rotary kiln was used in the solar devices of the Solar Platform of Almería (PSA-CIEMAT (Centro de Investigaciones Energéticas, Mediombientales y Tecnológicas)) in Spain. This solar system (Figure 2 and Figure 3) essentially consists of a continuous solar-tracking flat heliostat, a parabolic concentrator mirror (collector), an attenuator (shutter) and an experimental kiln located in the concentrator focus center [19].
The heliostat reflects horizontal and parallel solar rays on the parabolic collector, which again reflects and concentrates them in its center, located at the rotary kiln-window (Figure 3). When the shutter is 100% opened and with a direct solar irradiance of 100 W/m2, the focus is characterized by an irradiance peak of 3051 kW/m2, a total power of 70 kW and a focal diameter of 26 cm.
The experimental rotary kiln consists of a rotating cylindrical device, where heat is transferred through a quartz window focused to a parabolic collector (Figure 3). The contaminants are volatilized and transported to the emission control system by a controlled air-flow system. The process gas is filtered through an activated carbon unit, where mercury and other contaminants are adsorbed.
Figure 2. Experimental solar system comprising a continuous solar-tracking flat heliostat, a parabolic concentrator mirror (collector), an attenuator (shutter) and the experimental kiln located in the concentrator focus center.
Figure 2. Experimental solar system comprising a continuous solar-tracking flat heliostat, a parabolic concentrator mirror (collector), an attenuator (shutter) and the experimental kiln located in the concentrator focus center.
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Figure 3. Detail of the rotary kiln and the quartz window.
Figure 3. Detail of the rotary kiln and the quartz window.
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3. Results and Discussion

3.1. Geochemistry of Soils and Mine Wastes: Physical Characteristics

The mean concentrations of mercury and other metals in the soil and mine waste samples are shown in Table 1. The total mercury concentration in soils and wastes from the Valle del Azogue area varies between <25 and 2070 mg/kg. In the waste samples, Pb contents vary between 134 and 1820 mg/kg. Furthermore, high concentrations of As, Ba, Sb and Zn were detected, above the intervention values for soil remediation of international regulations (Table 1). Due to the mining and metallurgical activities, plants have disappeared or have been severely affected by the very high mercury and metal content in most part of this area [20].
The main physical parameters of soils used in the experiments showed a mean bulk density of 1450 kg/m3, a mean porosity of 0.40 and a mean field capacity of 0.08 (Table 2). The particle sizes showed that the abandoned mine wastes and contaminated soils are largely comprised of sandy material fraction (Figure 4). The coarser sample 1 is associated with mining wastes and, possibly, to overburdened ore deposit. Remaining samples have a particle size that may be suitable for thermal treatment.
Table 2. The main physical characteristics of soils used in the column experiments.
Table 2. The main physical characteristics of soils used in the column experiments.
SampledeερFC (%)
MA13.4---13308
MA21.80.3414308
MA31.00.45131010
MA40.8---141010
MA50.5---14707
MA61.9---14008
MA71.30.4114908
Notes: de, equivalent diameter (mm) obtained by sieve analysis; ε, porosity obtained by water displacement in a test tube; ρ, bulk density (kg/m3); FC, field capacity.
Figure 4. (a) Grain size distribution of samples MA1 to MA3. (b) Grain size distribution of samples MA4 to MA7.
Figure 4. (a) Grain size distribution of samples MA1 to MA3. (b) Grain size distribution of samples MA4 to MA7.
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Hg-thermodesorption curves (Hg-TDC) belonging to mining wastes, soils and calcine samples showed the predominant release of Hg in two temperature ranges: 200–250 °C and 300–330 °C (Figure 5). The first temperature range was assigned to a release of Hg from the soil matrix components based on the Hg-TDCs of standard materials [17]. Thus, we assume that most of the Hg present in the calcine material is bound to mineral components mainly by iron oxides, which were formed when the cinnabar-bearing ore was being roasted. It has been suggested in earlier studies that Hg0 formed during thermal breakdown of cinnabar is re-condensed during the cooling of the material and adsorbed to iron oxide surfaces [21]. In addition to matrix-bound Hg, some calcine samples contain traces of cinnabar. This could be explained by an incomplete breakdown of cinnabar ore during the roasting process.
Figure 5. Hg-thermodesorption curves (Hg-TDC) of samples: (a) MA1 (mining wastes); (b) MA2 (mining wastes); (c) MA3 (contaminated soil); (d) MA4 (calcine); (e) MA5 (calcine); (f) MA6 (mining wastes); and (g) MA7 (calcine).
Figure 5. Hg-thermodesorption curves (Hg-TDC) of samples: (a) MA1 (mining wastes); (b) MA2 (mining wastes); (c) MA3 (contaminated soil); (d) MA4 (calcine); (e) MA5 (calcine); (f) MA6 (mining wastes); and (g) MA7 (calcine).
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The second temperature range was assigned to Hg release from cinnabar, which was the predominant Hg mineral in contaminated soils and mining wastes (host rock and low-grade stockpiles). Cinnabar and a possible Hg sulfate were also detected in several samples [16]. No free metallic Hg, which is typically released at temperatures below 100 °C, was found in any of the samples studied.
Mercury phase characterization by soil sample X-ray showed the presence of cinnabar (HgS), corderoite (Hg3S2Cl2), laffittite (AgHgAsS3), metacinnabar ((Hg)S), shakhovite (Hg4SbO5(OH)3), schuetteite (Hg3(SO4)O2) and tiemannite (HgSe) (Table 3). The proportionally Hg predominant phase is cinnabar, which is concordant with the SPTD analyses.
The detailed SEM and EDS systems study of mine waste samples showed the presence of primary and secondary cinnabar associated with barite, pyrite and botryoidal pyrite. Furthermore, SEM observations showed several small particles containing both Hg and Cl and that may be associated with calomel (Hg2Cl2). Moreover, some particles containing both Hg and Br were observed and may be associated with kuzminite (Hg2(Br,Cl)2) [15,22]. Besides, in the gangue, main minerals are quartz, barite and silicates.
Table 3. Identified minerals in the Valle del Azogue soil and mine wastes and dominant Hg minerals from Hg-thermodesorption curves. Modified from Navarro et al. [22].
Table 3. Identified minerals in the Valle del Azogue soil and mine wastes and dominant Hg minerals from Hg-thermodesorption curves. Modified from Navarro et al. [22].
MineralsFormula
Primary minerals (soils and mine wastes)
Quartz *SiO2
Barite *Ba(SO4)
Cinnabar *HgS
Dolomite *CaMg(CO3)2
Calcite *Ca(CO3)
Huntite *Mg3Ca(CO3)4
Stibnite *Sb2S3
Realgar *AsS
OrpimentAs2S3
ChalcopyriteCuFeS2
Arsenian pyrite *Fe(S1-x As x)2
SphaleriteZnS
OrthoclaseK(Al,Fe)Si2O8
GoldAu
Illite *Al4(Si4O10)(OH)8
Secondary minerals (soils and mine wastes)
Hg0Hg
MetacinnabarHgS
GoethiteFeOOH
JarositeKFe3(SO4)2(OH)6
HematiteFe2O3
InyoiteCaB3O3(OH)5·4H2O
FerrihydriteFe(OH)3
KaoliniteKAl2Si3AlO10(OH)2·3H2O
GypsumCa(SO4)·2H2O
Secondary minerals (soils and mine wastes)
SchuetteiteHg3(SO4)O2
TiemanniteHgSe
CorderoiteHg3S2Cl2
ShakhoviteHg4SbO5(OH)3
CalomelHg2Cl2
KuzminiteHg2(Br,Cl)2
SampleDominant Hg-mineral phase
MA1 (mining wastes)Hg matrix, cinnabar
MA2 (mining wastes)Hg matrix
MA3 (contaminated soil)Hg matrix, cinnabar
MA4 (calcine)Hg matrix
MA5 (calcine)Hg matrix
MA6 (mining wastes)Hg matrix, cinnabar
MA7 (calcine)Hg matrix
Note: * high–medium abundant minerals.

3.2. Thermal Desorption Experiments

Rotary kiln experiments were conducted under global radiation of approximately 800 W/m2 with an exposure time of between 120 and 300 min. Table 4 shows the initial and final mercury concentration in the treated samples and the mercury removal efficiency of each experiment. Efficiency was calculated as the percentage of removed mercury with respect to the untreated sample.
Table 4. Results of mercury removal by solar thermal desorption with the rotary kiln. US, untreated sample; TS, treated sample; RE, removal efficiency; T, mean temperature reached.
Table 4. Results of mercury removal by solar thermal desorption with the rotary kiln. US, untreated sample; TS, treated sample; RE, removal efficiency; T, mean temperature reached.
SampleHg (US) (ppm)Hg (TS) (ppm)RE (%)T (°C)
MA12070<1099.5550
MA3116<199.1750
MA5<25<25----790
MA6935<599.4700
MA2865<599.4700–800
MA4130<1588.4700–800
MA71240<599.6700–800
The results showed that the lowest values of mercury in treated samples were consistently obtained at higher exposition times and desorption temperatures of the main Hg minerals (Table 5). However, in sample MA1, a low temperature exposition allowed for a greater mercury removal (99.5%). Besides, the lead removal in the thermal experiments was also significant, since the removal efficiency reached values of 48.5%–89.0% in all samples, with the exception of the MA1 and MA2 samples (Table 6). At lower temperatures, Pb was not removed, suggesting the presence of Pb as a trace metal in other minerals other than galena, such as barite and pyrite, as the volatilization temperature of galena is really low (503 to 654 °C). High amounts of Pb in pyrite from the study area have been reported, reaching concentrations up to 0.29% in weight.
Table 5. Desorption temperatures of different mercury minerals.
Table 5. Desorption temperatures of different mercury minerals.
PhaseDesorption temperature (°C)
Hg0<100
Hg2 Cl2170
HgCl2<250, 220
HgO420–550
HgSO4450–500
HgS (cinnabar)310–330
Hg in pyrite>450
Hg in Sphalerite600
Hg matrix200–300
Table 6. Results of lead removal by solar thermal desorption with the rotary kiln. US, untreated sample; TS, treated sample; RE, removal efficiency; T, mean temperature reached.
Table 6. Results of lead removal by solar thermal desorption with the rotary kiln. US, untreated sample; TS, treated sample; RE, removal efficiency; T, mean temperature reached.
SamplePb (US) (ppm)Pb (TS) (ppm)RE (%)T (°C)
MA1213245---550
MA31346948.5750
MA5182079556.3790
MA651226049.2700
MA2121074538.4700–800
MA4119013089.0700–800
MA75366887.3700–800
Figure 6 reveals the decrease of mercury with temperature in sample MA7, coinciding with a removal above 99% when the thermal treatment reached 400 °C, above the decomposition temperature of cinnabar (310–330 °C). Mercury removal became highly efficient (more than 90% removal) when the temperature was higher than 400 °C. Similar results were reported by other studies [6,11,15].
Figure 6. Evolution of mercury removal during the treatment of sample MA7.
Figure 6. Evolution of mercury removal during the treatment of sample MA7.
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This temperature may be the lowest limit treatment in order to remove mercury from solid wastes associated with cinnabar and matrix-bound Hg. However, if contaminated samples comprise mercury associated with pyrite, sphalerite and HgSO4, the treatment temperature should be over 600 °C, above the decomposition temperature of these mineral phases [15,23,24]. Moreover, the experimental results showed that the rotary kiln is more efficient than a fluidized-bed reactor in order to remove mercury from contaminated soils and wastes. The results reported by Navarro et al. [15] showed a lower removal efficiency (4.5%–76.0%), using similar samples and a solar fluidized reactor.
Furthermore, experimental results showed that metalloids, like Sb, Se and As, remain in treated samples at elevated concentrations. Thus, thermal treatment has been shown to cause, possibly, a stronger binding between the metal/metalloid with the soil matrix, which may cause increasing difficulty in subsequent remediation. Therefore, solar thermal treatment is inefficient in the metalloid removal.

4. Conclusions

Soils and mine waste samples from the abandoned Hg mine of Valle del Azogue (Almería province, Spain) were thermally treated in a rotary solar kiln to test the efficiency of the method on the mercury removal.
The SPTD determinations showed two different temperature ranges in which mercury was released from the samples: 200–250 °C and 300–330 °C. Previous interpretations showed that the first Hg release peak indicates the Hg release from the solid matrix, whereas the second peak at higher temperatures indicates the presence of cinnabar. The first release peak is typically associated with calcine samples, and the second peak release is dominant in mining wastes and soils.
Experimental results showed a removal efficiency above 99% when the thermal treatment reaches 400 °C, above the decomposition temperature of cinnabar (310–330 °C), and similar results were reported by other studies. Mercury removal was highly efficient (more than 90% removal) in samples that showed a significant content of cinnabar and an equivalent diameter above 0.8 mm. Moreover, the experimental results showed that the rotary kiln is more efficient than a fluidized-bed reactor, in order to remove mercury from contaminated soils and wastes.

Acknowledgments

This work was funded by The Spanish Ministry of Science and Technology (project REN2003-09247-C04-03 and ENE2006-13267-C05-01/ALT) in collaboration with the PSA-CIEMAT (Centro de Investigaciones Energéticas, Mediombientales y Tecnológicas), and the 2003–2004 Technical and Scientific Infrastructure Program (FEDER CIEM-E008). The authors wish to thank Dr. Ursula Kelm and Mónica Uribe for XRD analyses and interpretation. The technical support by the PSA Furnace personnel, especially Jose Galindo, is also greatly appreciated.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

Navarro, A.; Cañadas, I.; Rodríguez, J. Thermal Treatment of Mercury Mine Wastes Using a Rotary Solar Kiln. Minerals 2014, 4, 37-51. https://doi.org/10.3390/min4010037

AMA Style

Navarro A, Cañadas I, Rodríguez J. Thermal Treatment of Mercury Mine Wastes Using a Rotary Solar Kiln. Minerals. 2014; 4(1):37-51. https://doi.org/10.3390/min4010037

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Navarro, Andrés, Inmaculada Cañadas, and José Rodríguez. 2014. "Thermal Treatment of Mercury Mine Wastes Using a Rotary Solar Kiln" Minerals 4, no. 1: 37-51. https://doi.org/10.3390/min4010037

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