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

Chapter 4
efficiency. When the cell is put back into operation, the iron oxides are reduced to iron and
become points for the evolution of hydrogen leading to increased hydrogen levels in chlorine
[ 216, O'Brien et al. 2005, Section 4.6.6 ]. Techniques to refresh plugged diaphragms and to
remove iron are available (e. g. by washing with hydrochloric acid) [ 216, O'Brien et al. 2005,
Section 4.7.6 ]. , since an accumulation of corrosion products over several years may result in
high levels of hydrogen in chlorine before the end of the useful diaphragm life. Patented
procedures to overcome the plugging of a diaphragm with brine impurities are available (for
example from OxyTech). A plugged diaphragm is frequently the cause of low cell efficiency.
Removing iron from the diaphragm is also important, as iron also can be a source of elevated
hydrogen levels in chlorine. However, the preferred method of operation is to avoid these
problems by using pure and in particular filtered brine, especially if vacuum salt from a caustic
evaporator is used. Techniques to protect the cathode during shutdown include the injection of
reducing agents and a polarisation rectifier [ 31, Euro Chlor 2010 ].
General Electric in Mount Vernon (USA), was fully converted to PMX ® diaphragms at one time
but problems with iron in the brine forced General Electric GE to switch back to PMA
diaphragms. The brine contained high levels of iron and gluconate originating from the
recycling of waste water from a polycarbonate production unit. The impurities plugged the
diaphragms within 9 – 15 months while the gluconate prevented any acid washing. The
asbestos-free diaphragms were therefore not economical [ 221, Florkiewicz 1998 ].
Problems with iron impurities were also reported by the Zachem plant in Bydgosczc (Poland)
during test runs with asbestos-free diaphragms in 1999/2000. The plant recycles salt-containing
waste water from other production units back into the brine system (see Section 4.3.2.1.3).
Zachem reports that the use of asbestos-free diaphragms would therefore require major changes
of the brine treatment system [ 220, Polish Ministry 2011 ].
Economics
Economic benefits of using non-asbestos diaphragms come from [ 31, Euro Chlor 2010 ]:
reduced operating costs due to lower cell voltage;
the reduction of reduced cell renewal labour costs due to the longer lifetimes of the
diaphragms and steel cathodes (fewer shutdowns lead to less corrosion); and the
reduction of the
reduced waste handling and disposal costs due to asbestos-free materials.
Despite the higher purchase costs (up to 20 times the costs of a PMA diaphragm), the interest of
using asbestos-free diaphragms has been proven industrially in several plants [ 31, Euro Chlor
2010 ]. However, this needs to be offset against the substantially higher purchase costs (up to 20
times the cost of a PMA diaphragm) and the costs due to the need for closer control and
monitoring. According to [Florkiewicz, 1997] a 3 year life of the PMX diaphragm is the
minimum required to achieve breakeven.
On-site capital requirements for a conversion remain highly site-specific and the configuration
of the cells (cells with large active area, linked to the chlorine production per m 2 of diaphragm)
may greatly influence the cost of a conversion. The additional equipment required for
WORKING DRAFT IN PROGRESS
conversion may include a diaphragm preparation facility, a reducing agent injection system and
a polarisation rectifier to protect the cathodes against corrosion as well as additional brine
purification equipment [ 31, Euro Chlor 2010 ]. The total cost (everything included) for
converting a diaphragm cell plant with a chlorine capacity of 160 kt/yr an annual capacity of
160 kt Cl2 to PMX ® diaphragms is was reported to be EUR 1.4 – 2 million in 1999 [ 75, COM
2001 ] euros. A new diaphragm preparation plant will be built to supply two sites with PMX
diaphragms in France.
ChlorAlp in Pont de Claix (France) has estimated the costs for the modification of existing
equipment at 0.4-0.8 million euros (costs of raw materials not included).
176 December 2011 TB/EIPPCB/CAK_Draft_1