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

Chapter 4
same deposition procedures (under vacuum) as those used for making asbestos
diaphragms;
suitable permeability (prevent reaction of chlorine with OH - , isolate chlorine from
hydrogen for safety reasons);
same cell liquor strength and caustic quality as asbestos diaphragms;
chemical stability (inert to oxidation by chlorine, hypochlorite and chlorate; inert under
acidic and strong alkaline conditions and/or to acids and strong base conditions);
mechanical strength; mechanically strong
provide provision of a high current efficiency under a wide range of current
density/brine flow conditions.
Research on asbestos-free diaphragms has been is also carried out with the objectives of [ 31,
Euro Chlor 2010 ]:
achieving an extended service life (longer than asbestos diaphragms);
reducing electricity consumption; obtaining a power consumption advantage
meeting safety and environmental needs.
There are today two patents that are applied on a commercial scale: OxyTech’s Polyramix
(PMX) diaphragm and PPG’s Tephram diaphragm.
In addition, ChlorAlp’s patented microporous Asbestos Free Diaphragm is being installed in the
company’s industrial cell room at Pont de Claix, France. The plant will be fully converted by
2001.
In 2011, two asbestos-free diaphragms were applied on an industrial scale, of which only one
was used in EU-27 countries. The differences between the diaphragms lie in the way the
hydrophobic PTFE fibres are treated and deposited in order to form a permeable and hydrophilic
diaphragm, as well as the mineral fillers used [ 31, Euro Chlor 2010 ].
Composition of non-asbestos diaphragms
OxyTech’s De Nora Tech's Polyramix ® /PMX ® diaphragms are composed of (1) fibres with a
PTFE polymer backbone with zirconia (ZrO2) particles embedded in and on the PTFE and (2)
free zirconia particles. This mixture is deposited from a slurry bath and then fused in an oven.
Together with the replacement of the existing diaphragm cell cathodes (at a minimum, the inner
assembly portion of the cathode), the PMX ® diaphragms can directly replace other diaphragms
in Hooker H-type or Diamond MDC-type cells [ 212, Florkiewicz 1997 ], [ 213, Florkiewicz
and Curlin 1992 ]. [Florkiewicz, 1997], [Florkiewicz-Curlin, 1991]
For the preparation of the diaphragms, a nylon mesh is put on the steel cathode which is then
immersed in the suspension containing the fibres. During some hours, vacuum is sucked
through the cathode and the fibres deposit on the nylon mesh (Figure 4.2). Subsequently, the
cathode box is placed in an oven overnight at a temperature of ~ 350 °C at which the PTFE
fibres melt on the surface to stick together. The thickness of the diaphragms is ~ 0.5 cm. After
cooling down, the anode is carefully placed over the cathode without damaging the diaphragm.
WORKING DRAFT IN PROGRESS
With older anode designs, the distance between anode and diaphragms amounted to a few
millimetres. Newer anode designs rely on the expandable anode which results in a final gap of
approximately 1 mm thereby reducing the electrical resistance of the cell and the consumption
of electricity. Finally, the diaphragm needs to be wetted out for approximately one day prior to
being put into operation. At the end of their lifetime, diaphragms are mechanically removed
from the steel cathodes under dry conditions [ 118, Solvay 2011 ].
172 December 2011 TB/EIPPCB/CAK_Draft_1