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

2.3 The diaphragm cell technique process
Chapter 2
{The paragraphs under this section have been rearranged to follow the same logic as the
mercury cell technique. Section 2.3.1 description of electrolysis, products and qualities, extent
of brine depletion, operating conditions, advantages/disadvantages; Section 2.3.2: description
of cells with cathodes, anodes and diaphragms, monopolar/bipolar cells.}
2.3.1 General description
The diaphragm cell technique process was developed in the 1880s in the US and was the first
commercial technique process used to produce chlorine and caustic soda from brine. In North
America, diaphragm cells are still the primary technique, accounting for roughly 70 % of all US
production. The technique process differs from the mercury cell technique process in that all
reactions take place within one cell and the cell effluent contains both dissolved salt and caustic
soda. A diaphragm is employed to separate the chlorine liberated at the anode, and the hydrogen
and caustic soda produced directly at the cathode (Figure 2.2). Without the diaphragm to isolate
them, the hydrogen and chlorine would spontaneously ignite and the caustic soda and chlorine
would react to form sodium hypochlorite (NaClO), with a further reaction to produce producing
sodium chlorate (NaClO3) [ 17, Dutch Ministry 1998 ] [Kirk-Othm,er 1991].
The diaphragm is usually made of asbestos and separates the feed brine (anolyte) from the
caustic-containing catholyte. Purified brine enters the anode compartment and percolates
through the diaphragm into the cathode chamber. The percolation rate is controlled by
maintaining a higher liquid level in the anode compartment to establish a positive and carefully
controlled hydrostatic head. The percolation rate is determined as a compromise to maintain a
balance between a low rate that would produce a desirably high concentration of caustic soda in
the catholyte (which provides the cell effluent) and a high rate to limit back-migration of
hydroxyl ions from catholyte to anolyte, which decreases cathode current efficiency [ 17, Dutch
Ministry 1998 ] [Kirk-Othm,er 1991].
All diaphragm cells produce cell liquor that contains ca. 11% 10 – 12 wt-% caustic soda and
18% 15 – 17 wt-% sodium chloride. Generally, this solution is evaporated to 50 wt-% NaOH by
weight at which point all of the salt, except a residual of approximately 1.0 wt-% 1.0-1.5% by
weight, precipitates out. The salt generated is very pure and is typically used to make more brine
[ 10, Kirk-Othmer 2002 ]. This high quality sodium chloride is sometimes used as a raw
material for a mercury or membrane cell technique an amalgam or membrane process. A flow
diagram of a possible integrated plant site is shown in Figure 2.5 on page 17.
Brine
Purification
H 2
Cl 2
Mercury or
membrane
cells
WORKING DRAFT IN PROGRESS
Saturation
H 2
Reclaimed salt
TB/EIPPCB/CAK_Draft_1 December 2011 27
Cl 2
Diaphragm
cells
Concentration
(only for 50 wt-% NaOH
membrane
technique)
Depleted brine
Concentration
50 wt-% NaOH
1 wt-% NaCl
Figure 2.5: Flow diagram of the integration of the membrane or mercury and the diaphragm
cell techniques
{This figure was updated.}