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Abstract

Cement production
is one of the main contributors to mercury emissions in to the
atmosphere. Many national and international legal i nstruments aim to control and reduce
Hg emissions. Hg enters the cement production process as an impurity in fuels and raw
materials, with the latter being the main source of H g. The behavior of Hg in the production
process is influenced by various factors , such as: counter flow of materials and flue gases,
different temperature gradients, recycling of the filter dust to the high temperature process,
operating regimes of raw mills , Consequently , Hg can exist in elemental, oxidized and
particulate form.
This thesis
is centered on methodological approach to study Hg transformation in the
cement production process. In relation to this , the thesis consists of both quantitative and
qualitative approach to the issue. Having an overview of Hg content in input materials for
cement production is useful information in monitoring Hg emissions . Methods that employ
direct thermal decomposition coupled to atomic absorption spectrometer (TPD AAS) have
shown to be suitable for quantification of Hg in various matrices, e specially soils, sediments,
gypsum, etc. A cknowledging the advantages that such techniques provide; the first part of
this study was dedicated to validating the method for determination of total Hg in solid
samples of raw materials used for cement producti on. In this regard, two calibration
approaches were employed: calibration using matrix matched certified reference materials
( CRM), and the so called spike calibration ” where standard calibrating solution was added
to the solid matrix that was free from n atural Hg . While both calibration approaches were
found to be appropriate for the calibration of the instrument, the uncertainty of the mass
fraction and the need for several CRM s are major drawbacks of the matrix matched CRM
calibration. The second approa ch was found to be suitable and had less associated
uncertainty.
The second part of the thesis
is dedicated to studying Hg temperature fractionation in
solid samples. For this purpose, a quadrupole mass spectrometer (QMS) was utilized based
on several adva ntages that it provides: better sensitivity, the possibility to observe the
release of other ions during decomposition process, lack of carrier gas, etc. From the results
of fractionation analysis, it was shown that Hg in primary raw materials, such as lim estone,
is found to be strongly bound to the matrix and exhibited only one fraction. With the
addition of other materials needed for cement production, both the H g content and the
desorption profile change . Once the raw meal enter s the preheater section wh ere the
temperatures are from ~290 °C in the first cyclone to ~900 °C in the fifth cyclone, Hg from
the solid phase evaporates and is transformed in to the elemental form. Clinker as a final
product of pyro process has a very low Hg content and its thermo d esorption profile does
not show any identifiable fraction. Dust from the fabric filter consistently showed the same
desorption curve, with the peak maxima at the temperature range of 150 3 50 C, indicating
the presence of chlorine containing compounds.
The
implemented approach provides better insight into Hg mobility in the cement
production and can therefore be of great support in the development of emission control
technologies.