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Bulletin UASVM Agriculture 69(2)/2012Print ISSN 1843-5246; Electronic ISSN 1843-5386

Environmental Issues within the Chlor-Alkali Manufacturing Industry –Mercury Cell Process

Tania MIHAIESCU1), Radu MIHAIESCU2), Antonia ODAGIU1)

1) Faculty of Agriculture, Department of Environment and Plant Protection, University of AgriculturalSciences and Veterinary Medicine, 3- 5 Manastur Street, Cluj-Napoca, Romania;

tmihaiescu@yahoo.com, aodagiu@gmail.com2) Faculty of Environmental Science and Engineering, Babes-Bolyai University, 33 Fantanele Street,

Cluj-Napoca, Romania; RaduMihaiescu@yahoo.com

Abstract. At present is estimated that around 12000 tones of mercury are contained in mercury cellsused for chlorine production in the EU. Mercury contamination risk, remains active for long periodsafter the removing of the pollutant source, and is recognized as extremely dangerous for humans andenvironment due to the toxicity of various mercury compounds that can accumulate in the trophicchains. Almost all facilities using mercury –cell process reports unaccounted for mercury losses. Theannual mercury balance for a site is never zero. This is because mercury accumulates in plantequipment and structures during the life of the plant. The majority of mercury losses occur, in thevarious process wastes. Considerable emissions of mercury can also occur with run-off water. The soilat many sites is contaminated with mercury due to deposition of atmospheric diffuse emissions and/orhistorical disposal of mercury contaminated wastes. An even greater problem is represented by thedecommissioning of obsolete facilities when large quantities of mercury can escape in theenvironment despite all precaution measures.

Keywords: mercury, natural sources, anthropogenic sources, chlor-alkali industry, mercury cellprocess, environment contamination, impact

INTRODUCTION

Mercury is among the most extensively studied of all the environmental pollutantsbecause of its high toxicity, its environmental ubiquity and persistence, bioaccumulation inthe food chain and the developmental effects observed at relatively low levels of exposure..Humans and wildlife throughout the world are exposed to mercury, often at levels that raiseconcern for health and environmental effects (AMAP, 2011; Mergler et al., 2007). Mercury ismost toxic in its alkylated forms (methyl/ethyl mercury) which are soluble in water andvolatile in air. Methyl mercury (MeHg) is a neurotoxin and endocrine disruptor a potentneurotoxin that is readily accumulated by aquatic biota and is strongly biomagnified along thefood chain (Clarkson, 1997). Bioconcentration factors in the order of 104–107 have beenreported (Stein et al., 1996), and more than 90% of mercury in fish is generally present in theMeHg form (Becker and Bigham, 1995). The main route of exposure for humans to MeHg isconsumption of contaminated fish (Clarkson et al., 2003). High levels of Hg in thebloodstream may harm the developing central nervous systems of unborn babies and youngchildren, ultimately making the child less able to think and to learn (Obiri et al., 2010).Incidents related to mercury contamination such as Minamata, are well studied and highlightthe severity of effects that are attributed to mercury pollution (Ekino et al., 2007).

Still mercury enters the environment in various ways, mercury emissions beingdocumented from several generic sources, and from concentrated sources originating from theactivities that use large quantities of this element. Mercury is therefore a global concern.

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Taking into account the mercury related hazards, especially in the areas of intensive use,various strategies intended to minimize the use and therefore the risk of mercury pollutionwere designed at international level.

In the Earth’s biogeochemical system mercury exists in several forms: elemental(metallic) mercury, inorganic mercury compounds and organic mercury compounds. Each hasdistinct properties that affect its distribution, uptake and toxicity, but all forms can beconverted to toxic organic compounds. Environmental contamination by mercury (Hg) is amajor concern because of the impact of human activities on its cycling, toxicity andbioconcentration potential (Lindberg, 1998). Since the advent of humans, and particularlysince the industrial revolution of the late 18th and 19th centuries, anthropogenic sources havebecome a significant contributor to the environmental distribution of mercury and itscompounds (WHO, 2003).

According to UNEP (2002) the releases of mercury to the biosphere can be grouped infour categories: natural sources; primary anthropogenic sources; secondary anthropogenicsources; re-mobilisation of historic anthropogenic mercury releases previously deposited insoils, sediments, water bodies, landfills and waste/tailings piles. Natural sources of mercuryinclude volcanic eruptions, weathering of rocks, and emissions from forest, lakes and oceans(Dommergue et al., 2002). An inventory of the global anthropogenic emissions of mercury for2005 was prepared in a joint UNEP/AMAP project in 2008. The global emissions ofanthropogenic mercury to air for 2005 were estimated to be 1921 metric tones (Table 1). Themajor contributions are from fossil-fuel fired power plants, artisanal small scale gold mining,non-ferrous metals manufacturing, cement production, waste disposal and chlor-alkaliindustry) (AMAP/UNEP, 2008; Pacyna et al. , 2010; Pirrone et al., 2010).

Tab. 1Global emissions of total mercury from major anthropogenic sources in 2005

(Adapted from AMAP/UNEP, 2008; Pacyna et al., 2010; Pirrone et al., 2010)

AMAP/UNEP (2008); Pacyna et al.,(2010)

Pirrone et al. (2010)

Anthropogenic sources tonnes contribution, % tonnes contribution,%

Stationary combustion 880 46 810 35Non-ferrous metal production 132 7 310 13Gold production 111 6 -Cement production 189 10 236 10Waste incineration and other waste 116 6 187 8Pig iron and steel production 61 (54.8) 3 43 2Artisanal and small-scale goldproduction 323 17 400 17

Chlor-alkali industry 47 2 163 7Mercury production - - 50 2Other - - 65 3Overall inventory* 1921 100 2320 100

*Includes other sectors not listed in rows above**Differences of the estimated global anthropogenic emissions of mercury can partly be

explained by use of different emission factors.

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CHLOR-ALKALI INDUSTRY – MERCURY CELL PROCESS – AS A SOURCE OFMERCURY EMISSIONS

Pollution from mercury released from the chlor-alkali plants has been a cause ofenvironmental concern since 1950s. The chlor-alkali industry is the third major mercury userworldwide. The chlor-alkali (also called "chlorine-caustic") industry is one of the largestelectrochemical technologies in the world. Chlorine and sodium hydroxide are among the topten chemicals produced in the world and are involved in the manufacturing of a wide varietyof products used in day-to-day life. These include pharmaceuticals, detergents, deodorants,disinfectants, herbicides, pesticides and plastics.

Main types of processes used worldwide to manufacture chlorine and caustic sodafrom brine are (Table 2): Mercury cell; Diaphragm cell; and Membrane cell. Each processdiffers in keeping chlorine produced at the anode separate from the caustic soda and hydrogenproduced, directly or indirectly, at the cathode.

Table 2Comparison of chlor-alkali processes

(Adapted from www.eurochlor.org and EC, 2011)

Mercury Cell Diaphragm Cell Membrane CellAdvantages

-produces high-quality caustic soda andchlorine gas with nearly no oxygen.

Advantages-more energy efficient process(2,300-3,100 (kWh/t chlorine as theadjusted total energy use);-no mercury or asbestos emissions.

Disadvantages-less efficient process (3,100-4,400kWh/t chlorine as the adjusted totalenergy use);-requires a pure brine solution withlittle/no metal contaminants to avoid therisk of explosion through hydrogengeneration in the cell-produces Hg emissions

Disadvantages-less efficient process (2,700-3,300 kWh/t chlorine as theadjusted total energy use andmore steam than membranecell;-some still use asbestos incells with the potential forrelease into the air

Disadvantages-requires steam to concentrate thecaustic to the commercial grade(50%)-requires complete overhaul of olderprocesses and associated capitalinvestment.

The mercury cell process is the oldest electrochemical processes used for obtainingchlorine. Castner and Kellner developed it in 1892. In the mercury cell process are involvedthe electrolysis cell (produces chlorine gas) and the amalgam decomposer (produces hydrogengas and caustic solution). In the electrolytic cells occur the following electrochemical andchemical reactions:

2Cl- ==> Cl2 + 2e- (anodic reaction) [1]

2Na+ + 2Hg + 2e- ==> 2Na (in Hg) (cathodic reaction) [2]

2Cl- + 2Na+ + 2Hg ==> Cl2 + 2Na (in Hg) (overall cell reaction) [3]

2Na (in Hg) + 2H2O ==> H2 +2NaOH + Hg (decomposer reaction) [4]

The cathode is a flowing layer of elemental mercury running over a steel bedplate (cellbottom). Te brine flows on top of a continuously fed mercury stream (which acts as thecathode in this process). An electric current is applied, causing a reaction that produces

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chlorine gas at the anodes suspended in the top of the cell and a mercury-sodium amalgam atthe cathode. The chlorine is collected at the top of the cell while the amalgam proceeds to thedecomposer. In the decomposer, the mercury amalgam comes in contact with deionized waterwhere it reacts and regenerates into elemental mercury and produces caustic solution andhydrogen. Caustic solution and hydrogen are transferred to other processes for purification,and the mercury is recycled back into the cell. A schematic diagram of the process is shown inFigure 1.

Fig. 1. The mercury cell process (Source www.eurochlor.org)

Mercury is not “used up’’ in the mercury cell process. It is used only to conduct anelectric current. Replenishment is only necessary when mercury leaks into the plant or thesurrounding environment or when it leaves the plant in the form of waste or residue. Ideally,facilities would report all loss of mercury through products, air, water, soil and waste thatleaves the plant.

In practice, however, facilities routinely report buying and adding much morereplenishment mercury to the process than they report as releases. Every year the chlor-alkaliindustry reports unaccounted for mercury losses (Johnson, 2004). Since mercury is a potentneurological toxin, these unaccounted for mercury losses from the chlor-alkali industry are ofconcern, as they could be a source of exposure for humans, wildlife and the environment.Even if in theory mercury circuits and electrolysis cells are sealed and capsulated, mercuryloses still appear in the form of emissions to air, water and mercury contenting wastes andfinal products. The main mercury emission sources at a mercury cell chlor-alkali plant arepresented in Table 3.

In present, approximately 100 facilities globally, mostly been built before the 1970’s,still use this process and have been identified as one of the major sources of Hg releases to theenvironment. Now, they are gradually being phased out in Europe, the U.S. and India anddecommissioned or replaced by cleaner technologies (EC, 2011; UNEP, 2011). Voluntarycommitments to ensure phase-out of mercury technology have already been put in place byindustry in India (by end 2012) and in the European Union, Switzerland and Norway (by 2020at the latest). In the United States, the four remaining mercury cell facilities are discussing apotential 31 December 2018 deadline for closure or conversion, depending upon pendingcongressional legislation and the economic viability of converting the plants. In India, all

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remaining mercury-cell chlorine production is scheduled to cease by 2012, according to anagreed timetable drawn up by the Government and industry bodies. Voluntary commitmentsto ensure phase-out of mercury technology have already been put in place by industry in India(by end 2012) and in the European Union, Switzerland and Norway (by 2020 at the latest). Asa result of all these measures the number of mercury cell chlor-alkali plants remaining is 55,located in 24 countries, with an aggregate chlorine production capacity of about 1.7 milliontonnes per year compared to approximately 2.6 million tonnes actually decommissionedduring the period 2002–2011.

Table 3Emission factors in environment components and the main emission sources of mercury sources at a

mercury cell chlor plant(Data from EC, 2011; Euro Chlor, 2011)

Mercury emission factorsg/tonne of annual chlorine

capacity*

Com

pone

nt

Sources of mercury release

Min Max Median

Cell-room ventilation 0.132 1.298 0.587

Air

Process exhaust-purged air from cell end boxes-vents from wash water collection tanks-exhaust from any vacuum system used to collect spilled mercury-hydrogen burnt or sold as fuel-hydrogen emitted to the atmosphere-vents from caustic soda pumping tanks-vents from caustic soda filters-exhaust and vents from distillation units for Hg contaminated solidwastes-vents from storage of metallic Hg and waste contaminated with Hg-vents from workshps where contaminate equipment is handled

0.00030 0.180 0.0065

Wat

er

-the process waste water: brine purge, (back)washing water from brinepurification, condensate and wash liquor from treatment of chlorineand hydrogen, condensate from caustic concentration units, brineleakage, ion-exchange eluate from process water treatment-the wash water from the cell cleaning operations: inlet and outletboxes-the rinsing water from the electrolysis hall: cleaning of the floors,tanks, pipes and dismantled apparatus-the rinsing water from maintenance areas outside the electrolysis hall,if they are cleaned with water-with run-off water

> DL 1.65 0.02

Prod

ucts

Mercury in produced chlorine, hydrogen and caustic (may therefore beemitted to the environment during subsequent uses).Hg contained in caustic is usually the largest contributor to emissionsvia products

0.01 0.43 0.05

Was

tes

-solids from brine purification-solids from caustic filtration-solids from waste water treatment-solids from sludge (sewer, traps, channels)-activated carbon from treatment of gaseous streams-graphite from decomposer packing-residues from retorts-wastes from maintenance and renewal.

0 98 5.0

* Data referred to plants in OSPAR countries in 2009 (Euro Chlor, 2011, EC, 2011

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Still, it is generally agreed that the mercury-cell process is no longer an economicallyor environmentally preferred technology for new chlor-alkali facilities or capacity additions,and that existing facilities should be phased out (UNEP, 2011). Many plant operators havephased out this technology and converted to the more energy-efficient and mercury-freemembrane process, to have efficient production and ensure environmental and healthprotection. Innovation in mercury technology processes has focused particularly on reductionof electricity consumption, improvements around cell maintenance efficiency and thereduction of fugitive mercury emissions. Others companies have plans to do so, and stillothers have not announced any such plans. In many cases, governments have worked withindustry representatives and/or provided financial incentives to facilitate the phase-out ofmercury technology. Recently governments and international agencies have createdpartnerships with industry to encourage broader industry improvements with regard to themanagement and releases of mercury.

Globally, the number of mercury-cell facilities is on the decline, consistent with theend of the economic life of these facilities. The number of mercury cell chlor-alkali plants inoperation shows a worldwide decrease from 91 to 53 over the period 2002-2011 (-42%) andthe global mercury cell chlorine capacity is estimated to have declined by about 42%, thereremains approximately 5.3 million metric tons per year of mercury cell chlorine productioncapacity worldwide, compared to about 9.1 million metric tons per year in 2002 (Figure 2).

50

60

70

80

90

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Number of plants

5,000

6,000

7,000

8,000

9,000

Capacity of plants(1000 t/y)

Hg plants Capacity

Fig. 2. Trend of number of plants and capacity of mercury electrolysis units in USA/Canada/Mexico,Europe, Russia, India and Brazil/Argentina/Uruguay

(Source: WCC, 2011 and UNEP, 2011)

Global mercury emissions have been further substantially reduced in the period 2002-2011. They went down from 24.6 tonnes/year to about 6.9 tonnes/year, or 72% decrease overthe ten years of reporting by WCC (2011). The emissions expressed in g mercury/tonneannual chlorine capacity show the same trend (Figure 3).

Although is in many places being replaced by alternative techniques, it is still themost commonly used in Europe. However, emissions from existing Hg-cell plants remain

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significant in less developed countries where stringent environmental controls are lacking(EC, 2011; UNEP, 2002,).

Even though the mercury cell chlor-alkali technique has been replaced by alternativetechniques in many locations or the chlor-alkali plants were decommissioned, historicalaccumulation of Hg could still be a secondary contamination source to the environment(Olivero-Verbel et al., 2008; Ullrich et al., 2007a, 2007b).

0.00

1.00

2.00

3.00

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

kg Hg/y

0

10,000

20,000

30,000

g Hg/t Cl2annual cap.

Specific emissions Total emissions

Fig. 3. Trend of the total mercury emissions (air + water + products) for the mercury cell chlor-alkaliplants (USA/Canada, Europe, India and Brazil/Argentina, Uruguay and Russia)

(Source: WCC, 2011 and UNEP, 2011)

The soil and groundwater contamination is due to both the deposition of atmosphericcontaminant and the historical disposal of graphite sludges and other wastes on and aroundthe sites. The mercury may leach from the soil and end up in the run-off water and thegroundwater. Mercury is mostly present in the elemental form, but dissolved inorganic andorganically-bound mercury are also present (Euro Chlor, 2009). The soil beneath theproduction units can be contaminated with mercury up to several meters depth, especially theareas beneath the cell room and the retortig unit where mercury concentration can be as highas some g/kg of dry soil. Mercury concentration in the topsoil could vary from some µg/kg upto some hundreds of mg/kg within the first kilometers downwind from cell room.

Most of the metallic mercury (directly or indirectly) released to air from the plant issubjected to atmospheric long-range transport (Biester et. al, 2002). Some research articlereported concentrations of mercury in soils due to deposition that reach background levels at arelative short distance, a few km, from the plant (Maserti and Ferrara, 1991) while in othercase the distance was reported to be as high as 100 km (Lodenius and Tulisalo, 1984). Whensoluble mercury is bound to organic matter, the contamination reached down toapproximatively 20 – 50 cm in the soil and in sandy soils, contamination was restricted to theupper 5 cm. At some sites, discharge of mercury-containg wastewater led to contamination ofriver sediments (Garcia et al, 2009; 2010).

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CONCLUSION

Among the different technologies of producing sodium hydroxide and chloride, oneof the older but still employed is the mercury cell process. This technology despite allenvironmental protection measures still can emit substantial quantities of mercury inenvironment. The decision to phase out all facilities employing this type of technology until2020 is therefore justified. In the mean time, existing facilities should preoccupy in order tominimize the emissions of mercury in air, water, products and waste products. While inoperation all facilities should comply at least to the requirements of the Best AvailableTechniques (BAT) Reference Document issued for the production of chlor alkali. Taking intoaccount that mercury is a dangerous pollutant which can accumulate in the trophic chainstudies and monitoring regarding the local pollution should be conducted at all sites affectedby the presence of mercury cell chlor alkali plants in operation or after decommission. Aspecial attention should be accorded to historical polluted sites.

ACKNOWLEDGMENTS

This paper was realised with the support of the University of Agricultural Sciences andVeterinary Medicine Cluj-Napoca (Romania), grant No. 1215/19/2012.

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