By Ross Milverton
The chlor-alkali process, as complicated as it sounds, is simply the separation of the chlorine and sodium atoms found within brine. When solid salt is dissolved, the sodium and chlorine atoms become ions, which are free to move around in the solution.
The sodium and chlorine ions are charged positively and negatively, respectively. Two electrodes can be placed into the solution, one positive and one negative. The chlorine atoms, being negatively charged, travel to the positive electrode. Here each chlorine ion loses an electron to the positive electrode, the anode, and reacts to become a neutrally charged chlorine molecule.
Around 65 million tonnes of chlorine are produced each year, for which the largest use is in the chemical industry. Polyvinyl chloride is the third most synthesised plastic in the world. 57% by weight of PVC is made up by chlorine, and roughly the same amount of chlorine produced is used to make solvents. When I think of chlorine and its use, my naïve first thought is sanitation of swimming pools. Even taking the sanitation of all water used, only 5% of chlorine produced is used for this purpose.
The chlor-alkali process has been implemented for over 100 years, and with these uses, it is clear to see that chlorine production will always be wanted. One of the key ways to improve efficiency, is improving the electrodes. The electrodes need to be electrical conductors, to allow current to flow for the process, and corrosion resistant, in order to reduce the downtime in a continuous process.
A catalyst can be used in order to decrease the energy required for the reaction. It contains a mixture of 30% ruthenium oxide and 70% titanium oxide, provides good conductance and gives the electrode a lifespan of 10-12 years when added. If too much ruthenium oxide is used, the conductance is too low, resulting in a low yield of chlorine. If too much is used, then a side reaction forming oxygen occurs at the anode also, reducing the purity of the chlorine gas produced.
Roughly 4.5 tonnes of ruthenium are used per year worldwide to create these electrodes, which is 14% of the total production. Other platinum group metals share the essential properties needed to make the electrodes efficient, such as Platinum and Iridium. Perhaps other materials could replace ruthenium and cause a huge shift in the market, but given its fantastic suitability to the job, I am not holding my breath.
By Ross Milverton