Cellroom mercury

The fundamental question is should mercury cellrooms for the production of chlorine and caustic soda be phased out in the near future, or should the industry be allowed - on the basis of its existing voluntary commitment - to move away from this obsolescent technology as it reaches the end of its economic life over the next 20-25 years ... 

There is one further potential source of mercury emissions which, in fact, totally overshadows all discussion of emissions, discharges and losses but which, until recently, was not on the regulatory radar screen. There are about 12 000 tonnes of pure mercury contained in operating cellrooms in Western Europe. What happens to this mercury when the cellrooms close After all, it represents some 1500 years of emissions at present rates from operating cellrooms, and it also represents some 15 years or more of global mercury production at present rates. Clearly the resolution of this issue is of importance not only for the environment but also for the mercury mining industry. 

Why then are conversions carried out at all The answers are of course quite simple. As a mercury cellroom reaches the end of its economic operating life - typically after some 40-50 years of operation - it has either to be replaced or closed. And any state-... 

The results speak for themselves. There is neither an environmental nor an economic case for accelerating mercury cellroom conversions beyond the time-scale of 20-25 years to which the European industry has already expressed its commitment. The only rationale is political - which brings us to the final section of this analysis. 

For mercury cellrooms it expects that the application of BAT will result in emission levels well below 2.0 g/te (of chlorine capacity) and notes that there are examples of plants achieving levels well below 1.0 g/te. Numerical emission limit values were set at 0.01 g/te for new plants (deliberately equivalent to a ban on mercury technology) but will not be decided for existing plant until two years after the Protocol enters into force (say, by 2004). 

All of this work went on in parallel with the development and implementation of best practice in every aspect of the operation, decommission and demolition of mercury cellrooms. Particular attention was paid to the atmospheric emissions, which now dominate, and subsequently to a range of complex issues concerning waste disposal and, not least, the appropriate fate of the 12 000 tonnes of pure mercury contained in operating cellrooms. 

There will be no increase in mercury chlor-alkali production capacity. This is an unequivocal reiteration of a commitment made in 1995. It represents a de facto commitment to phase-out as mercury cellrooms reach the end of their working life. 

Remaining mercury cellrooms will close or convert to non-mercury processes when they reach the end of their economic lives. The exact date will depend on the availability of capital and on macroeconomic factors more under the control of governments than industry. All available independent analyses point to this equating to an end for the mercury process in Western Europe somewhere in the 2020s. 

As previously mentioned, the mercury cellroom at Runcorn is operated as a waste brine process with some of the brine recycled through the cells. This process involves a trade-off between electricity and brine costs since with increased brine flow the cells operate at a higher average concentration, which saves power. 

As every operator of a mercury cellroom knows, corrosion of rubber-lined parts of the cell is a general occurrence. The softer the rubber, the higher the speed of corrosion. 

A typical cdlroom (Fig. 3.5) consists of a large number of cells in electrical series so as to make use of the available 480 V, i.e, about 100 cells. Mercury cells operate at current density in the range 0,8-r4 Acm so that the total cell current will vary between 180 and 315 kA and the cellroom power requirement will be 80-160 M W, It may also be noted that such a cellroom will be somewhat larger than a soccer pitch and be capable of producing about 250000 ton of chlorine each year. 

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