(BAT) Reference Document for the Production of Chlor-alkali ...

Chapter 4
The drawback of using ferrocyanide is that the complex is very stable, so the iron is not oxidised
and precipitated as hydroxide during the primary brine purification. Ferrocyanide is therefore
transferred to the electrolysis cell where it reacts with chlorine to form Fe(III) and cyanide. The
latter is oxidised to cyanate which may be further oxidised to ammonia [ 140, Giatti 2011 ].
Ammonia is an unwanted impurity in the brine because it may lead to the formation of
explosive nitrogen trichloride (see Section 2.6.11.4). Iron is also unwanted in the membrane cell
technique because it deposits on the electrodes or on/in the membranes, leading to shorter
lifetimes of electrodes and membranes, to reduced product qualities and to higher energy
consumption (see Table 2.4).
Ferrocyanide can be decomposed without the formation of nitrogen trichloride by adding
chlorine to resaturated brine at temperatures higher than 80 °C and pH > 7, followed by primary
purification to remove the liberated iron. However, this technique is rarely used due to
additional investment and operating costs [ 187, Debelle and Millet 2011 ].
Iron(III) meso-tartrate was introduced in 2004 as an alternative anti-caking agent. This complex
is thermodynamically less stable so that iron is precipitated as hydroxide during primary brine
purification. The residual tartrate is transferred to the electrolysis cells and fully oxidised to
hydrochloric acid and carbon dioxide. No unwanted substances are formed [ 140, Giatti 2011 ].
Achieved environmental benefits
The achieved environmental benefits of this technique include the following:
reduction of energy consumption;
reduced formation of explosive nitrogen trichloride which reduces the risk of accidental
emissions.
Environmental performance and operational data
Test runs with ferrocyanide-containing brine at the AkzoNobel plant in Rotterdam (The
Netherlands) revealed a cell voltage increase of approximately 50 mV during the first month.
No such increase was found with brine containing iron(III) meso-tartrate [ 140, Giatti 2011 ].
{Please TWG provide more information.}
Initially the anti-caking behavior of the salt was not as good as with the classic ferrocyanide but
the recipe was then further improved [ 142, AkzoNobel 2010 ].
Cross-media effects
Some raw materials and energy are consumed for the production of iron(III) meso-tartrate. This
anti-caking agent is biodegradable and decomposes quickly in soil or water [ 141, AkzoNobel
2011 ]. No halogenated organic compounds are formed in the brine circuit [ 140, Giatti 2011 ].
Technical considerations relevant to applicability
Generally, there are no technical restrictions to the applicability of this technique.
Economics
Slightly higher amounts of iron(III) meso-tartrate are required compared to ferrocyanide. Cost
savings due to a reduction of power consumption can reach up to 5 %. Further savings result
from longer lifetimes of electrodes and membranes and less downtime due to maintenance as
well as increased product quality [ 141, AkzoNobel 2011 ].
{Please TWG provide more information.}
WORKING DRAFT IN PROGRESS
In 2011, iron(III) meso-tartrate was only available in conjunction with the purchase of vacuum
salt from a particular supplier.
Driving force for implementation
The driving forces for implementation of this technique include [ 141, AkzoNobel 2011 ]:
reduction of costs related to energy consumption;
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