The Most Prolific Process in The World?
Soap. Swimming pools. Plastic. Paper. Pretzels. Fertilizers.
All are united by the fact that they rely on the Chlor-Alkali industry. This remarkable process takes brine – essentially seawater – and splits it into a host of different chemical products. These products and their derivatives are so key in this age that an understanding of this process, as well as ways that it can be improved, and the catastrophes that have resulted in history because of mistakes that have been made with its misuse, is vital.
The story starts with salt – in this case sodium chloride. We need not remind ourselves that it’s a common occurrence: it is speculated that around 2.9% of the world’s oceans are comprised of this compound. There are around 1000000000000000000 tons of seawater in the oceans (1018/ 1 Exaton ) – this means there is a huge amount of salt, and a huge amount of raw material to utilise. Brine is seawater with about 5% salt by weight.
Suppose you were to build a very tall wall around the UK – all 30,000 kilometers of its coastline. Now imagine that you were to start filling this volume with table salt. Continuing in this fashion, you would reach the earth’s atmosphere before you ran out of salt taken from the oceans.
Early Chlor-Alkali processes used mercury and asbestos to go from reactants to products. The basic principle is that a redox reaction occurs, and the salty water is electrolysed. The Castner-Kellner process, for example, uses a cathode of flowing mercury which forms an amalgam with the sodium from the brine. When the mercury comes into contact with water, the sodium is brought out and sodium hydroxide is created. However, this technique is environmentally catastrophic. For example, the use of this technique by the Dryden Chemical Company in Ontario in the 60s and 70s lead to the dumping of over 9,000 Kg of elemental mercury into the local water systems, leading to a disease called “Ontario Minamata disease” – so-called because of a similar outbreak of convulsions, loss of speech abilities and eventually coma seen in a similar Japanese case a decade earlier.
Diaphragm cells, which use a layer of asbestos fibres to ensure that the right chemicals are produced at each electrode, obviously have the associated downfall of asbestos, now a confirmed carcinogen, being swept away in the products or waste. So both of these historical techniques, although effective, are slightly foolish options.
Nowadays, most of the 80-billion-dollar chlor-alkali industry uses what is called a membrane cell. This uses a complex copolymer membrane derived from Teflon which separates the two “half-cells” of the reaction and allows some things to pass through it but not others. This revolutionary material, which like most polymers was developed by DuPont, is now found in 60% of Chlor-alkali cells.
So what about Ruthenium? This rare ‘noble’ metal is only mined in quantities of about thirty tonnes per year. But it is rare also in its ability to resist reaction: for example, it only tarnishes when heated to extreme temperatures and is very resistant to the attack of acids and bases. Even Titanium is quite pathetic when used for prolonged periods in harsh conditions, and so the electrodes of membrane cells use mixed metal oxide electrodes. The core of these is still made of titanium in sheet or mesh form, but this is then coated with Iridium Dioxide and Ruthenium Dioxide, resulting in an electrode which has a significantly longer lifetime. The use of these noble metal coatings also results in purer products, because they prevent a phenomenon called overpotential.
Considering that electrodes in this process started as graphite, which decomposes and must be replaced every few months, modern-day electrodes really have come a long way, lasting between six and eight years. And it is the use of these noble metal oxides which, crucially, is responsible for this.
This story has once again been a reminder of the pivotal roles that metals – no matter how rare – play in modern day society. 90% of European drinking water, for example, is purified with chlorine. Hydrogen from the chlor-alkali process is used in the Haber process to manufacture fertilisers. Even pretzels as we know them, with their brown hyaline shells and chewy centres, depend upon the chlor-alkali process since they are dipped into a bath of lye (Sodium Hydroxide) before baking.