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

Source: [ 146, Arkema 2009 ]
Figure 4.2: Deposition of an asbestos-free diaphragm on a cathode
Chapter 4
PPG's Tephram ® diaphragms are composed of three components: basecoat, topcoat, and dopant.
The basecoat, containing PTFE fibres, PTFE microfibres, a perfluorinated ion-exchange resin
and other constituents, is vacuum deposited on the cathode screen. The PTFE fibres form the
base mat and the microfibres provide a suitable porosity. The ion-exchange resin contributes to
the wettability of the PTFE mat. The topcoat, with inorganic particles (metal oxides such as
zirconium oxide or titanium dioxide), is applied on the basecoat by vacuum deposition to
achieve adequate permeability and uniformity. The third component, which contains soluble and
insoluble materials such as magnesium hydroxide and magnesium aluminosilicates, is added to
the anolyte during start-up and also intermittently during operation, to form a gel layer in the
micropores of the diaphragm mat to improve the performance of the diaphragm. The final
diaphragm is typically about 2.5 mm thick. Tephram ® diaphragms have been used in Diamond
MDC-type, Columbia N-type and Glanor cells [ 216, O'Brien et al. 2005, Section 4.7.6 ], [ 218,
Ahmed and Foller 2003 ]
The PPG’s Tephram diaphragms are composed oa base diaphragm and a topcoat. The base
diaphragm is made of PTFE fluoropolymer, PTFE microfibrils and a perfluorinated ion
exchange resin. The topcoat, composed of inorganic particulate materials (metal oxides such as
zirconium oxide or titanium dioxide) is incorporated into, and becomes an integral part of, the
base diaphragm. The purpose of the topcoat is to adjust permeability and uniformity. The
Tephram diaphragms can in some cases directly replace other diaphragms in chlor-alkali cells.
[Dilmore-DuBois, 1995]
The differences between the patents lie in the way the hydrophobe PTFE fibres are treated and
deposited in order to form a permeable and hydrophilic diaphragm and in the mineral fillers
used.
WORKING DRAFT IN PROGRESS
Operation and applicability of non-asbestos diaphragms
Non-asbestos diaphragms can be applied at new and existing diaphragm chlor-alkali plants.
However, some operators still have doubts concerning the economics and safety of the asbestosfree
diaphragms.
The Dow diaphragm cell is optimised for low current density (~ 0.5 kA/m 2 ) and requires a large
active area (~ 100 m 2 for one single bipolar element) [Kirk-Othmer, 1991]. The flow rate is
lower compared to other industrial cells and the company reports that there is a danger of
chlorine and hydrogen getting mixed, because of the specificity of their cells, which could lead
TB/EIPPCB/CAK_Draft_1 December 2011 173