Cellroom

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. 

It is tempting for environmentalists to ignore economics and finance. After all, they say, the environment is more important than mere money. However, in the real world - where the rest of us live - we too want to improve and protect the environment, but this costs money and we recognise that without infinite resources we have to prioritise where we spend our money. We also recognise that if business does not make profits there will not be any money to spend So I make no apology for addressing the economics of cellroom conversion. 

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-... 

Quantification of the macroeconomic effects of cellroom closure is extremely difficult. But it is significant that this difficulty arises precisely because the chlor-alkali industry forms the base for such a large proportion of so many diverse sectors of manufacturing industry - and thus of the tax base for national governmental budgets. 

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. 

The Diaphragm Electrolysis Plant Delfzijl has a cellroom containing 184 OxyTech MDC-29 cells and an annual liquefaction capacity of 130 000 tons of chlorine. 

Most, if not all, of the world s diaphragm plants are written off since their average age is 15 years. This clearly has an impact on the thinking of cellroom owners when they consider the future of their assets (see Fig. 15.1). 

For the membrane cellroom of the same capacity there are two choices of technology type either monopolar or bipolar electrolysers. In the case of monopolar membrane electrolysers (Fig. 15.9), such as the ICI FM1500, one membrane electrolyser can replace one diaphragm cell. Since the membrane electrolyser has smaller dimensions there is an overall space saving. The monopolar membrane electrolysers may use the same pipework galleries and overhead crane from the... 

Owing to limitations on the water balance of the plant, a 40-50% conversion can be undertaken before a salt evaporator needs to be installed to remove excessive water. The feed brine for the membrane cellroom is taken from brine made from fresh imported salt and the weak brine is returned, combined with the diaphragm brine, restrengthened with recovered salt from the evaporators and fed to the diaphragm cells. The concentration of brine to the diaphragm cells could be weaker than normal, at around 270-280 g l-1 rather than 300-310 g l-1 to assist with the water balance. 

If a completely new cellroom is being considered and space is available close to the... 

It has been shown in this chapter that financial benefits result from the conversion of a diaphragm cellroom to membrane technology. Tonnage may be increased for a similar power consumption or power usage maybe decreased for the same amount of tonnage when a conversion takes place. 

Fig. 16.7 Relative costs for a 50 000 metric tonnes per year NaOH plant as a function of the current density. Only the costs of the cellroom and rectifier were taken into account.

Before considering the validation process in detail it is important to realise that the model considers only one cell and yet the particular cellroom under study at Runcorn has 106. This means that the model must predict average values of parameters for the whole cellroom rather than, for example, each individual k-factor. 

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. 

Unlike in the other two electrolysis processes, the brine is not recirculated and the temperature in the system can be chosen according to optimum conditions and therefore comparatively little titanium is used in a diaphragm cellroom. However, there are some clear candidates. An example is the cell blanket where Permascand has a newly patented design comprising bellows welded to the anode collar. The chlorine header and also the cell top are other components that could be manufactured from... 

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. 

Current is fed into the electrolyzer by means of anodic and cathodic end elements. The anodic compartment of each cell is joined to an independent brine feed tank by means of flanged connections. Clilorine gas leaves each cell from the top, passing through the brine feed tank and then to the cellroom collection system. Hydrogen leaves from the top of the cathodic compartment of each cell the cell liquor leaves the cathodic compartment from the bottom through an adjustable level connection. 

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