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

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

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Chapter 2 Criterion Mercury Diaphragm Membrane Caustic concentration Chlorine quality Brine feedstock Variable electric load performance 50% Contains low levels of oxygen (< 0.1%) and hydrogen Some purification required but depends on purity of salt or brine used Good variable electricity load performance, down to 30 % of full load possible for some cell rooms, which is very important in some European countries 12%, requires concentration to 50% for some applications Oxygen content between 1.5-2.5% Some purification required but depends on purity of salt or brine used Tolerates only slight variation in electricity load and brine flows in order to maintain diaphragm performance 33%, requires concentration to 50% for some applications Oxygen content between 0.5% and 2%, depending on whether an acidified electrolyte is used Very high purity brine is required as impurities affect membrane performance Variable electricity load performance less than for mercury (40 – 60 % depending on design load), affects product quality, and efficiency at lower loads Source: [ 1, Ullmann's 2006 ], [ 3, Euro Chlor 2011 ], [ 10, Kirk-Othmer 2002 ], [ 28, EIPPCB 2011 ], [ 58, Euro Chlor 2010 ]after [Kirk-Othmer, 1991], [Lindley, 1997], [Ullmann’s, 1996] and other sources WORKING DRAFT IN PROGRESS 22 December 2011 TB/EIPPCB/CAK_Draft_1

2.2 The mercury cell technique process 2.2.1 General description Chapter 2 The mercury cell technique process has been in use in Europe since 1892 and accounted in 1999 for 58 % of total production in western Europe. As shown in Figure 2.3, the mercury cell technique includes an electrolysis cell and a horizontal or vertical decomposer process involves two 'cells'. In the electrolysis cell primary electrolyser (or brine cell), purified and saturated brine containing approximately 25 wt-% sodium chloride flows through an elongated trough that is slightly inclined from the horizontal. In the bottom of this trough a shallow film of mercury (Hg) flows along the brine cell co-currently along with the brine. Closely spaced above the cathode, an anode assembly is suspended [ 17, Dutch Ministry 1998 ]. NB: A) Electrolysis cell: a) Mercury inlet box; b) Anodes; c) End box; d) Wash box; B) Horizontal decomposer: e) Hydrogen gas cooler; f) Graphite blades; g) Mercury pump; C) Vertical decomposer: e) Hydrogen gas cooler; g) Mercury pump; h) Mercury distributor; i) Packing pressing springs; CW = cooling water. Source: [ 1, Ullmann's 2006 ] {Figure was replaced because original source of figure could not be identified.} Figure 2.3: Schematic view of a mercury electrolysis cell with horizontal and vertical decomposer Flow diagram of mercury cell technology Electric current flowing through the cell decomposes the brine passing through the narrow space between the electrodes, liberating chlorine gas (Cl2) at the anode and metallic sodium (Na) at the cathode. The chlorine gas is accumulated above the anode assembly and is discharged to the purification process. As it is liberated at the surface of the mercury cathode, the sodium immediately forms an amalgam (NaHgx) [Kirk-Othmer, 1991]. The concentration of the amalgam is maintained at 0.2 – 0.4 wt-% Na so that the amalgam flows freely, 0.3 % is the reference figure in [Gest 93/186, 1993]. Na concentrations of > 0.5 wt-% can cause increased hydrogen evolution in the cells [ 1, Ullmann's 2006 ]. The liquid amalgam flows from the electrolytic cell to a separate reactor, called the decomposer or denuder, where it reacts with water in the presence of a graphite catalyst to form sodium hydroxide and hydrogen gas. The sodium-free mercury is fed back into the electrolyser cell and is reused. WORKING DRAFT IN PROGRESS The reaction in the electrolyser is: 2 Na + + 2Cl - + 2 Hg V 2 Na-Hg + Cl2(g) The reaction in the decomposer is: 2 Na-Hg + 2 H2 O V 2 Na + + 2 OH - + H2 (g) + 2 Hg {Reactions were moved to Section 2.1.} The depleted brine anolyte leaving the cell is saturated with chlorine and must be partially dechlorinated before being returned to the dissolvers. TB/EIPPCB/CAK_Draft_1 December 2011 23

Chapter 2<br />

Criterion Mercury Diaphragm Membrane<br />

Caustic<br />

concentration<br />

<strong>Chlor</strong>ine<br />

quality<br />

Brine feedstock<br />

Variable electric<br />

load<br />

per<strong>for</strong>mance<br />

50%<br />

Contains low levels <strong>of</strong><br />

oxygen (< 0.1%) and<br />

hydrogen<br />

Some purification<br />

required but depends on<br />

purity <strong>of</strong> salt or brine<br />

used<br />

Good variable electricity<br />

load per<strong>for</strong>mance, down<br />

to 30 % <strong>of</strong> full load<br />

possible <strong>for</strong> some cell<br />

rooms, which is very<br />

important in some<br />

European countries<br />

12%, requires<br />

concentration to 50% <strong>for</strong><br />

some applications<br />

Oxygen content between<br />

1.5-2.5%<br />

Some purification required<br />

but depends on purity <strong>of</strong><br />

salt or brine used<br />

Tolerates only slight<br />

variation in electricity load<br />

and brine flows in order to<br />

maintain diaphragm<br />

per<strong>for</strong>mance<br />

33%, requires<br />

concentration to 50% <strong>for</strong><br />

some applications<br />

Oxygen content between<br />

0.5% and 2%,<br />

depending on whe<strong>the</strong>r<br />

an acidified electrolyte<br />

is used<br />

Very high purity brine is<br />

required as impurities<br />

affect membrane<br />

per<strong>for</strong>mance<br />

Variable electricity load<br />

per<strong>for</strong>mance less than<br />

<strong>for</strong> mercury (40 – 60 %<br />

depending on design<br />

load), affects product<br />

quality, and efficiency at<br />

lower loads<br />

Source: [ 1, Ullmann's 2006 ], [ 3, Euro <strong>Chlor</strong> 2011 ], [ 10, Kirk-Othmer 2002 ], [ 28, EIPPCB 2011 ], [ 58, Euro<br />

<strong>Chlor</strong> 2010 ]after [Kirk-Othmer, 1991], [Lindley, 1997], [Ullmann’s, 1996] and o<strong>the</strong>r sources<br />

WORKING DRAFT IN PROGRESS<br />

22 December 2011 TB/EIPPCB/CAK_Draft_1

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