Radon exhalation study of manganese clay residue and usability inbrick production
Kovács, T., Shahrokhi, A., Sas, Z., Vigh, T., & Somlai, J. (2016). Radon exhalation study of manganese clayresidue and usability in brick production. Journal of Environmental Radioactivity.
Published in:Journal of Environmental Radioactivity
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Download date:12. Feb. 2021
1
Radon exhalation study of manganese clay residue and
usability in brick production
Abstract
The reuse of by-products and residue streams is an important topic due to environmental
and financial aspects. Manganese clay is a residue of manganese ore processing and is
generated in huge amounts. This residue may contain some radionuclides with elevated
concentrations. In this study, the radon emanation features and the massic exhalation rate
of the heat-treated manganese clay were determined with regard to brick production. From
the manganese mud depository, 20 samples were collected and after homogenization radon
exhalation characteristics were determined as a function of firing temperatures from 100
to 750 ºC. The major naturally occurring radionuclides 40K, 226Ra and 232Th concentrations
were 607 ± 34, 52 ± 6 and 40 ± 5 Bq kg-1, respectively, comparable with normal clay
samples. Similar to our previous studies a strong correlation was found between the internal
structure and the radon emanation. The radon emanation coefficient decreased by ~ 96 %
from 0.23 at 100 ºC to 0.01 at 750 ºC. The massic radon exhalation rate of samples fired at
750 ºC reduced by 3 % compared to samples fired at 100 ºC. In light of the results, reusing
of manganese clay as a brick additive is possible without any constraints.
Keywords
Manganese clay, Heat-treatment, Radiation hazard, Radon, Emanation, Exhalation,
Building material
2
Introduction
The importance of the utilization of by-products and residue streams has grown over recent
decades due to the concern about sustainability of the human environment; of course, the
use of residues sometimes provides better financial solutions. The building industry seems
to be an appropriate solution as certain residues and waste materials have long been used
in the construction industry (Adam, 1994). The use of certain mining by-products such as
waste rock also has a past (Hooke, 2000). The use of other wastes, i.e. coal slag and fly-
ash has become frequent after the development of cement production (Blezard, 2004).
However, the use of these by-products has to be restricted in several cases as, on the one
hand, waste impairs the structural properties of the end-product (Ducman et al., 2007), on
the other hand, different contaminants dissolved and entered the environment causing harm
to the environment or to human health (Uhde et al., 2007).
The quantity and quality of additives usable in building materials has long been set out by
strict regulations. The radioactivity of materials usable in building materials is regulated
explicitly by the standards of only relatively few countries. However, some have been
established for many years: Hungary the regulation setting out the radioactivity limit of
building materials has been in effect since 1960 (no. 26/1960 Directive of Hungarian
Ministry of Construction).
Over recent years more and more studies have been published on the harmfulness of
NORM materials, and there has been no unified recommendation (regulation) on the
restriction of their use until recently, yet the newest EU-BSS (Council directive
2013/59/EURATOM, European Basic Safety Standards) emphasizes the restrictions
related to these materials. The reference level applying to indoor external exposure to
gamma radiation emitted by building materials, in addition to outdoor external exposure,
shall be 1 mSv per year.
In this study the potential usage of the residue of manganese ore mining in the building
industry, i.e. manganese clay, was investigated from a radiological point of view.
Manganese clay is the residue of manganese mining, it is not classified as a by-product as
it is listed as a secondary raw material (Farkas et al., 2004; Vigh, 2005). The underground
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manganese ore mine in Úrkút, Hungary is one of the biggest European manganese mines
(Polgari et al., 2000).
Over recent decades 2.8 million tons of manganese clay has been deposited on the land
surrounding the mine covering a territory of 20 hectares (Szabo, 2006; Vigh, 2005). The
recent study of Vigh et al., 2013 investigated the radioactivity of a manganese clay. That
study showed that the activity concentrations of the primordial radioisotopes (238U, 226Ra, 40K) of different black shales (manganese clay) do not exceed average soil activity
concentrations. In the other study Kavasi et al., 2012, and Sas et al. 2015a investigated its
utilization as a building material, however, this study only considered the gamma dose and
did not include radon exhalation measurements. However, as several of our previous
surveys have proved the measurement of gamma dose or even that of 226Ra alone is not
suitable for the estimation of building material radiation dose as the majority of radiation
dose is provided by radon and its progenies (Somlai et al., 1997; 2006). The amount of
radon emitted greatly depends on the inner structure of the used building material (Somlai
et al., 2008, Sas et al., 2012, 2015b). Therefore, measuring radionuclide concentration is
insufficient for an accurate dose estimation and it is necessary to take into account the
processing used in the building industry to derive useful radon exhalation data.
According to NEN (Netherlands Standardization Institute) 5699:2001 EN standard entitled
Radioactivity measurement - Determination method of the rate of the radon exhalation of
dense building materials the exhalation rate (radon activity that diffuses per unit of time
from a material into the air surrounding the material, in Bq s-1) can be divided either the
area of the exhaling surfaces or by the mass, the areic (radon flux Bq m-2 s-1) and massic
radon exhalation rates (Bq kg-1 s-1). It is important to clarify that the areic exhalation can
be a characteristic parameter of investigated materials only if the sample thickness is
greater than the diffusion length of the radon belongs to investigated matrix. In case of
massic exhalation the sample thickness has to be very small against the diffusion length of
radon to estimate the radon exhalation without significant loss as a result of its decay inside
the matrix. This assumption can be used for comparison (Kovler et al., 2005) if the sample
thickness of porous material is less than 5 cm (López-Coto et al. 2009). Under this
4
circumstance all the radon has a chance to exhale and the massic radon exhalation rate can
be determined (Sas et al., 2015b).
From the aspect of building industry usage this parameter provides tangible information.
From the aspect of brick manufacturers, the maximum amount of by product that can be
added is important, and this may be driven by the amount of radon emitted. It is not easy
to comply with both requirements, the amount of radon emitted is simulated using complex
models in most cases (Risica et al., 2001). However, for modelling it is rather important to
identify the radon potential of materials. As already mentioned above radon emission is
considerably influenced by the inner structure of the building material (Sas et al., 2012;
2015b) or materials (Jobbagy et al., 2009) and for red mud it has been shown that specific
surface area and porosity greatly influence radon emission. However, both specific surface
area and porosity can be affected by thermal treatment of the material (Vigh, 2005; Sas et
al., 2015b).
Due to other major components of manganese clay (Seil et al., 1928) it is potentially useful
in brick productions. Thermal treatment (firing) is the basic treatment method of brick-
making and this has implications on the amount of radon emitted.
In this study the radon emanation characteristics of manganese clay were measured at
different temperatures identifying the optimum firing temperature in order to minimize
radon exhalation and provide useful data for later modelling and also for construction
companies, and information for authorities on the maximum amounts of additives.
Materials and methods
Sampling and sample preparation
The manganese mining is based on a sedimentary manganese deposit containing a very
fine grained, laminated ore of Jurassic age and the main ore minerals of rhodochrosite and
kutnahorite. Samples were taken from a waste depository site of Úrkút manganese mine
located near the village of Úrkút in the Balaton Upland region of Hungary (Figure 1).
5
Fig. 1- Location of the Úrkút manganese mine
In total 20 grab clay samples (about 10 kilograms) were taken from the difference part of
the depository site, the depository site is marked for environmental monitoring, after
removing the 70 cm thick upper layer (from 0 to 40 cm deep) and then each samples were
dried at room temperature then crushed and homogenized. For gamma spectrometry,
samples were crushed to under 0.63 mm in grain size and then dried in an oven at 105 °C
for 24 h to remove any moisture and to achieve mass constancy. The prepared samples
were weighed, sealed in aluminium Marinelli vessels of 600 cm3 in volume and stored for
at least 27 days in order to reach the radioactive equilibrium between 226Ra and its decay
products prior to counting using gamma spectrometry (Somlai et al., 2008).
Kovler et al., 2005 was found that the massic exhalation cannot be considered as
characteristic values of the tested materials, but can be used rather for comparison because
the massic exhalation rate should depend on some more factors, such as degree of
compaction and geometry (thickness of the layer, mainly) of the powder/granular sample.
If the sample thickness is much lower against diffusion length all emanated radon has a
chance to exhale from the matrix. In that case the massic exhalation can be a characteristic
value. This phenomena was examined by Sas et al., 2012 and it was found that the required
sample thickness should be thinner than 1 cm in the case of humid clayish materials to
avoid the diffusion inhibition effect of the sample thickness on radon exhalation. To
6
evaluate the effects of heat-treatment on morphological attributes, radon emanation and the
exhalation rate, about 1 kilogram of mixed homogenized manganese clay was moulded
into small spheres of 1-2 mm in diameter to ensure the required condition to measure the
massic exhalation rate. In case of massic exhalation the sample thickness has to be very
small against the diffusion length of radon to ensure the radon exhalation without
significant loss as a result of its decay inside the matrix (Sas et al., 2015b, López-Coto et
al. 2009). Then prepared samples were fired each of 100, 250, 350, 450, 550, 650 and 750
ºC for 24 hours (150 gram of prepared spherical samples for each temperature) (Sas et al.,
2012). Owing to occurred loss of mass uniformly 140 grams of each fired sample was
weighed and placed into glass emanation tubes, which were sealed and stored for 30 days
to achieve an equilibrium radon concentration (Jobbagy et al., 2009).
Internal structure related measurements
The relative densities were measured using Density Kit and the reference liquid (White
Spirit 150/200) with a density of 0.775 g cm-3 at 15 °C. A graduated glass body of defined
reference liquid volume (V1) was weighed in air and marked as M1; the samples were
dipped into the holder and placed on an ultrasound wave shaker for 30 minutes and then
the new volume and weight were recorded as V2 and M2. Then their density was calculated
by dividing ∆M by the reference displacement volume (Speight, 2015).
The porosity features and specific surface area were calculated by changing the absorbed
component of samples in a nitrogen gas tube under specific conditions (Jobbagy et al.,
2009). Total pore volumes from 1 nanometre to 15 micrometres were determined by
combining results from ASAP 2000 gas absorption (Micromeritics, U.S.A) and mercury
penetration (Micromeritics, U.S.A) methods.
Gamma spectrometry
A high resolution gamma ray spectrometer, using an ORTEC GMX40-76 HPGe
semiconductor detector with efficiency of 40 %, and an energy resolution of 1.95 keV at
7
1 332.5 keV was used to determine the concentrations of 40K, 226Ra and 232Th present in
the samples. The activity concentration of 40K was determined by the 1461 keV gamma
peaks, and for 232Th by the 911 keV gamma peaks of 228Ac, and the 2614 keV gamma
peaks of 208Tl. The 226Ra concentrations were determined by measuring the activities of
its decay product 214Pb using 352 keV gamma peaks (IAEA, 2007; Somlai et al., 2008).
Radon emanation and exhalation
For calibration and determining the efficiency of a Lucas cell, a certified Genitron EV
03209 calibration chamber, a RN2000A solid 226Ra source with an activity of 104 ± 0.4
kBq (Pylon Electronics Inc., Canada) and an AlphaGUARD PQ2000Pro radon monitor
(Saphymo Gmbh, Germany) as a reference were used.
The grab sampling method using Lucas scintillation cells (1 dm3 Lucas covered by
ZnS(Ag), MÉV Ltd., Hungary) and an EMI photomultiplier were used to determine the
radon concentrations.
The sample container (sealed glass tube) was broken inside a special breaker and radon-
free N2 gas transferred the evaluated radon gas to the Lucas cell. After transferring the
Lucas cells were stored for 3 hours to reach secular equilibrium between 222Rn and its
decay products. Each cell was measured for 3 × 2000 s. The calculated MDA was
determined by the Currie method (Currie, 1968). Radon concentration activity can be
calculated using Eq. (2) (Kovacs et al., 2003):
ARn-222 = N / (V×E×S×F×t×3) (2)
where ARn-222 is the activity of the 222Rn concentration (Bq m-3), N is the net count, V is the
volume of the scintillation cell in litres, E is the counting efficiency of the cell (cpm/dpm),
S is the decay correction factor for the time interval between sampling and measuring, F is
the transfer efficiency (0.95), t is the measurement time and 3 is the number of alpha
emitters under equilibrium conditions which are reached after 3 hours.
The radon exhalation rate in terms of mass was calculated using Eq. (3) (Sas et al., 2015b):
8
tMass et
tmVCE λ
λ−−⋅
⋅⋅⋅
=1
(1)
where:
• C = accumulated radon concentration [Bq m-3]
• EMass = massic exhalation rate [mBqkg-1 h-1]
• t = accumulation time [h]
• V = volume of the accumulation kit [m-3]
• m = mass of the sample [kg]
• λ = decay constant of radon [h-1]
The emanation coefficient of each investigated sample was calculated from obtained
massic exhalation results. (Kovler et al., 2005).
Results and discussion
The relative detection efficiencies and minimum detectable activities of gamma
spectrometry for 40K, 226Ra and 232Th were calculated as 1.2%, 2.4% and 1.4% and the
MDAs as 46, 1.3 and 2.3 Bq kg-1, respectively.
The concentration of 40K, 226Ra and 232Th in the manganese clay were determined in Bq
kg-1 as 607±34, 52±6 and 40±5, respectively. The concentrations of 40K and 226Ra with the
exception of 232Th were higher than the world average mean radionuclide concentration of
soils reported in UNSCEAR 2008 Annex B (226Ra: 32 Bq kg-1, 232Th: 45 Bq kg-1, 40K: 412 Bq kg-1) and RP112 (226Ra: 40 Bq kg-1, 232Th: 40 Bq kg-1, 40K: 400 Bq kg-1).
The morphological attributes of the clay are related to the firing temperature with the
exception of pore volume. The effects of firing on the samples in terms of different
temperatures show in Table (1).
Table 1- The modification of morphological attributes as a function of firing
temperature
9
Temperature (ºC)
Density (kg m-3)
100 4500 250 4400 350 4200 450 3800 550 4000 650 3600 750 3200
Increasing the firing temperature resulted in gradual decreases of specific surface area and
density decreased except at 550 ºC (Fig. 2.).
Fig. 2 Morphological modifications as a function of temperature
The efficiency and MDA of scintillation cells were calculated to be between 60 and 70 %
and 32 to 41 Bq m-3, respectively. A significant difference among firing temperatures was
observed for 222Rn exhalation and emanation rates and temperature (Table 2).
Temperature (ºC) 100 250 350 450 550 650 750 Massic exhalation (Bq kg-1 h-1) 76 67 74 30 51 46 3
Emanation factor 0.25 0.22 0.24 0.11 0.17 0.16 0.01
10
Table 2 – Radon concentration, exhalation rates and emanation factors for
manganese clay in terms of firing temperatures
In Fig. 3, the relationship between cumulative pore volume and exhalation rate and the
emanation factor are shown.
Fig. 3 The relationship between pore volume and radon concentration, exhalation
rate and the emanation factor
Fig. 4 shows Cumulative pore volume distribution of fired manganese clay. The obtained
results clearly proved that in the case of high temperature range the pore size distribution
significantly shifted towards bigger pore diameter compared with at low temperatures. As
a result, it can be stated that low emanation and exhalation rates at high temperatures can
be caused by the modified porosity features. Furthermore, it can be concluded that by
firing, the radon emanation and exhalation features can be significantly reduced, which can
ensure safer building material production from manganese clay in terms of a radiological
point of view.
11
Fig. 4 Cumulative pore volume distribution of fired manganese clay
__baIn the light of the obtained results it can be stated that similar behaviour was found
between presented results of manganese clay, previously studied red mud (Sas et al.,
2015b) and clay (Sas et al., 2012). In the case of red mud and clay study the exhalation
characteristic of heat-treated samples was obtained with accumulation chamber technique,
whilst in the case of currently presented study the radon exhalation results were obtained
with sealed glass tube technique. Despite of different methods the radon emanation and
exhalation decrement was observed in all cases around the high temperature range. In the
case of cumulative pore volume distribution of red mud also similarity was found. The
porosity changes of clay material were not investigated. The cumulative pore volume of
red mud and manganese clay has decreasing tendency in the function of increasing firing
temperature in both cases. Generally, the applied heat in the case of brick production is
around 800 °C, which means technological modification would not be required if these
material will be used for brick production.
Conclusions
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This study characterized manganese clay in terms of major naturally occurring radioactive
materials, internal morphological modifications, and radon emanation and exhalation rates
as a function of firing temperature. The concentration of 226Ra and 40K presented in clay
were higher than the global average values, while the 232Th concentration was lower than
the global average. Results show a link between firing temperatures and changes in
morphological nature, and subsequently the radon emanation and exhalation rates. The
density, specific surface area and total pore volumes decreased as temperature increased.
The radon exhalation rate reduced by 97% from 75.7 to 2.4 mBq kg-1h-1. To change
properties of manganese mine slag waste as a by-product and reduce the radon exhalation
rate to eliminate health hazards for usage in the building and ceramic industries, firing at
750 °C is recommended. All in all, it can be stated that the high temperature treatment has
beneficial effect on internal structure of all type of investigated clayish materials, which is
favourable from building material production point. However, the application of by-
products (manganese clay and red mud) on its own can arise concerns from mechanical
point of view. On the basis of presented results, the possibility of the application of
manganese clay and also red mud as additive material is considerable, which justifies
further experiments of their clay-based mixtures focusing on mechanical and radiological
properties.
Acknowledgements
The author(s) would like to acknowledge the contribution of the COST Action TU1301.
www.norm4building.org.
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List of Figures
Fig. 1- Location of the Úrkút manganese mine
Fig. 2- Morphological modifications as a function of temperature
Fig. 3- The relationship between pore volume and radon concentration, exhalation rate
and the emanation factor
Fig. 4- Cumulative pore volume distribution of fired manganese clay
List of Tables
Table 1- The modification of morphological attributes as a function of firing temperature
Table 2 - Radon concentration, exhalation rates and emanation factors for manganese
clay in terms of firing temperatures