Studies on mercury occurrence in inorganic constituents of Polish coking coals
Environmental Science and Pollution Research
Studies on mercury occurrence in inorganic constituents of Polish coking coals
Tadeusz Dziok 0
Andrzej Strugała 0
Adam Włodek 0
0 Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology , Al. A. Mickiewicza 30, 30-059 Krakow , Poland
1 Tadeusz Dziok
During the cokemaking process, a significant amount of mercury occurring in a coal blend is released to the atmosphere. One of the ways of reducing this emission is to reduce mercury content in a coal blend. This could be obtained through the coal washing process. The optimization of this process requires the knowledge of mercury occurrence in coal, especially in its inorganic constituents. A qualitative analysis of mercury occurrence in the inorganic constituents of Polish coking coals was performed using an electron probe microanalyzer (EPMA). For that purpose, selected samples of rejects and middling products derived from the washing process in dense media separators and jig concentrators were examined. The obtained results have confirmed a strong connection between mercury occurrence and the presence of sulfides (pyrite, marcasite, and chalcopyrite) in Polish coking coals. Significant amounts of mercury were also noticed for barite, siderite, and aluminosilicates. The highest value of mercury content, at the level of 0.100%, was obtained for marcasite. For the analyzed coals, the effectiveness of mercury removal in the washing process was determined by the forms of pyrite occurring in coal. The highest values of effectiveness of mercury removal were obtained in the case of coals for which the large framboidal pyrite aggregates with chalcopyrite overgrowths were noticed. It was also found that middling products were characterized by the occurrence of the Hg-rich overgrowths of pyrite on organic matter. To achieve a significant reduction in mercury content in clean coal, it is necessary to develop an effective method of removing this form of pyrite from hard coal.
Coking coal; Inorganic constituents; Mercury; EPMA; Modes of occurrence
Introduction
Mercury is characterized by highly toxic properties
(Li and
Tse 2015)
. According to
Pirrone et al. (2010)
, 30% of mercury
emission to the environment is caused by human activities,
and coal utilization processes constitute one of the main
sources of its emission
(Pacyna et al. 2016)
. This issue is really
important for such countries as Poland and China, because
their energy production sector is based on coal (Burmistrz
Responsible editor: Philippe Garrigues
Faculty of Energy and Fuels, AGH University of Science and
Technology, Al. A. Mickiewicza 30, 30-059 Krakow, Poland
et al. 2016; Wang et al. 2011). The annual mercury emission
in Poland in 2014 was estimated at the level of 9.6 Mg
(KOBiZE 2016)
. The proportion of mercury emissions from
the coal-related sectors was as follows: combustion 93.4%,
pig iron and steel production 4.6%, and coke production
0.3%.
The issue of mercury emission is very important for coke
industry. The annual production of coke worldwide exceeds
700 million Mg
(Mysiak and Jarno 2016)
. In the coal coking
process, a significant amount of mercury occurring in a coal
blend is released to the atmosphere
(ACAP 2005)
. This
emission occurs during the charging and pushing operations as
well as in leaks on the battery
(US EPA 1997b)
. Mercury
emission factors from coking plants range from 0.01 to
0.038 g/Mg of coke produced
(ACAP 2005; Konieczyński
et al. 2012; US EPA 1997a)
. A certain amount of mercury
remains in coke and the rest passes into raw coke gas and then
to coke gas cleaning products. Coke is commonly used in the
processes of pig iron and steel production and, thus,
contributes to mercury emission from this industry sector (Wang et al.
Raw coal
Coal prepara on and
classifica on
<0.5 mm
Cleaning by
flota on
Clean coals
Middling products
Rejects
2016). It should be also mentioned that the use of sorbents
(Lopez-Anton et al. 2016; Yu et al. 2016)
in order to reduce
the mercury emissions from coking plants has a limited
possibility of application.
One of the ways of reducing mercury emission from coking
plants as well as of lowering mercury content in coal coking
products is to reduce mercury content in the coal blend. This
could be obtained in the coal washing process, which is well
known and commercialized
(Rallo et al. 2012)
. The possibility
of removing significant amounts of mercury from coal in this
process was confirmed in the following studies: Dziok et al.
(2015b),
Zajusz-Zubek and Konieczyński (2014)
,
Pan et al.
(2017)
, and
Pyka and Wierzchowski (2016)
. However, the
effectiveness of mercury removal from coal is varied and it
is closely related to the mode of mercury occurrence in coal.
Therefore, the optimization of mercury removal from coal in
the process of coal washing requires the knowledge of
mercury occurrence.
Generally, mercury in coal ca (...truncated)