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Abstract

Mercury is a very toxic element. Due to its volubility it is present everywhere in the environment at ambient temperature. High temperature processes increased levels of mercury on the global scale. Global and European action plans were established to reduce mercury emissions and remove mercury from the environment. Cement plants contribute significantly to the global mercury emissions, but data about emissions from cement plants are limited and even less is known about processes of mercury cycling inside the production process due to a counerflow of materials and gasses and recycling of mercury with filter dust back into the high-temperature process.
The main aim of the study was the evaluation of mercury emissions from the cement plant and study of mercury cycling within the process of cement clinker production. Measurements and sampling procedures were performed according to the preprepared plan, which included initial and three successive measurements periods. The plan enabled to observe individual states in different operating conditions. Measurements of mercury in all important materials and gas streams were performed in three sampling periods on about 300 solid samples and about 80 samples taken from gas streams. At the same time mercury species concentrations were measured in gaseous streams as total Hg(t), particulate Hg(p), elemental Hg0(g) and reactive Hg2+(g) in ionic form at three characteristic parts of the process.
Additionally, the spatial distribution and accumulation of mercury in the surrounding of the cement plant were evaluated with biomonitoring methods using epiphytic lichens at seven of the most representative sites around the cement plant and one distant reference site. Active biomonitoring was performed with transplanted lichens Pseudevernia furfuracea (L.) Zopf for periods of 3, 6 and 12 months and passive biomonitoring with natural in-situ lichens. Biomonitoring methods were combined with measurements of elemental mercury in the outdoor air, multielement analysis of lichens and some measurements from environmental and process monitoring of the cement plant
Mercury mass flows and mass balances of the whole, and in certain parts of the process were evaluated. It was confirmed that the process comprises many mercury cycles which are strongly dependent on the operating conditions and technological specifics affecting also chemical forms of mercury in the process. In the annual mercury input of about 27 kg, raw materials accounted for nearly 60 % and fuels for about 40 % (i.e. petroleum coke 31 %, waste tyres 10 % and waste oil 0.4 %). Two main outputs from the system are emission of exhaust gas and cement clinker. The annual emission of mercury represented 40 – 70 % of the inputs, while cement clinker only contained about 10 %. The points in the process with the highest mercury mass flows and the lowest powder solid material flows were identified. Such points represent an opportunity to remove a significant amount of mercury from the process at low removed quantity of processed material. Mercury was mainly emitted in gaseous form with 92 % (direct mode) or 89 % (combined mode) as Hg(g) of total mercury concentration Hg(t) on average. Shares of individual mercury species in the last sampling period were 65.7 % Hg2+(g), 34.0 % Hg0(g) and 0.3 % Hg(p) on average. Ratios between individual mercury forms were found to be related to operating modes. In the gaseous part of mercury Hg(g) the average percentage of reactive mercury Hg2+(g) was 83 % in the direct mode and 51 % in the combined mode of operation in the last sampling period. The quantities of Hg(t), Hg(g) and Hg2+(g) emitted were higher when operating with the raw mills off (direct mode).
Concentrations of elemental mercury in air were relatively low (on average below 10 ng/m3). In-situ lichens showed Hg concentrations comparable to lichens taken from the background area for transplantation, indicating that the local pollution is not severe. Transplanted lichens showed an increase of mercury in all three periods of exposure (3, 6 and 12 months), especially at one site located near the cement plant. Mercury uptake measured in vital transplanted lichens was in a good correlation with the working hours (i.e. emitted Hg quantity) of the kiln.
The main novelty of the study is the obtained knowledge about mercury species, cycling and mass flows inside the process, which could contribute to techniques for mercury control and reduction in cement plants. Mass balances could be used for emission prediction in the case of changed inputs in the process. The contribution of the study is also the result of biomonitoring methods, which showed an important correlation between accumulation of mercury in transplanted lichens and mercury emissions from the plant. Biomonitoring with proper validation and standardisation could be a perspective and also cost effective method for the study of spatial distribution and deposition near point sources of mercury. According to the new regulation in the filed of air quality and directions of UNEP partnership programme this area will become even more important in the future.