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

Chapter 2
2.9 Production of caustic potash
Potassium chloride is used as raw material for the production of potassium hydroxide. It occurs
naturally mostly as sylvinite and can be purified by crystallisation from solution, flotation or
electrostatic separation [ 43, Spolchemie 2010 ]. Potassium chloride plants in the EU-27 use
both rock salt and vacuum salt [ 57, EIPPCB 2011 ].
The electrolysis of potassium chloride brine to produce caustic potash differs in some aspects
from the much more common electrolysis of sodium chloride brine. For example, the bromide
content of raw KCl salt is ~ 0.2 % compared to only ~ 0.002 % in NaCl. The resulting higher
concentration of bromine in the produced chlorine gas causes difficulties such as higher
corrosion rates as well as the necessity to use more sophisticated equipment and more energy
for chlorine purification (see Sections 2.6.6 and 2.6.11.5) [ 42, Euro Chlor 2010 ].
Another difference is that the mercury cell technique based on KCl is much more sensitive to
trace quantities of metals such as vanadium and molybdenum in the brine as well as to sodium
in the amalgam, both types of catalysts potentially causing increased hydrogen evolutions in the
cell (see Section 2.2.1). Therefore, a higher brine purity is required which can be achieved by
setting stricter raw material specifications and/or a two-step filtration, along with more frequent
cell openings for maintenance and cleaning. NaCl present in the brine is converted to NaOH
during electrolysis and amalgam decomposition and is transferred to KOH [ 1, Ullmann's
2006 ], [ 42, Euro Chlor 2010 ].
In membrane cell plants, the KOH units use the same membrane as the NaOH units, which was
not the case until recently when membranes with different electrochemical characteristics were
used. The concentration of chlorides and chlorates in KOH from membrane cells is higher
(typically ~ 20 mg KCl/kg of 50 wt-% KOH) compared to KOH from mercury cells
(< 3 mg KCl/kg of 50 wt-% KOH). The migration of chloride through the membrane is driven
by the concentration gradient against the electric potential, and is independent of the current
density. However, the caustic production rate increases with increasing current densities.
Because of this, the concentration of chloride in the caustic decreases with increasing current
densities. At a current density of 6 kA/m 2 , the chloride concentration in caustic soda and caustic
potash is similar, while at current densities of 1.5 kA/m 2 , the chloride concentration in caustic
soda is considerably higher than in caustic potash [ 42, Euro Chlor 2010 ].
Plants producing both NaOH and KOH keep the brine circuit completely separated even if the
electrolytic cells are in the same cell room. The switching of cells or of groups of cells from one
production to the other is possible but is usually avoided because it requires a time-consuming
cleaning process and the caustic solution does not meet the normal quality specifications for a
couple of hours after restart [ 42, Euro Chlor 2010 ].
WORKING DRAFT IN PROGRESS
62 December 2011 TB/EIPPCB/CAK_Draft_1