Mercury cell technology

Dr M Harris ICI Halo chemicals, PO Box 13, The Heath, Runcorn, Cheshire, WA7 4QP, UK. Phase-out Issues for Mercury Cell Technology in the Chlor-Alkali Industry. 

Phase-out Issues for Mercury Cell Technology in the Chlor-Alkali Industry... 

The Sydney plant is a combination of older mercury cell technology (updated ICI Mark 1 cells) and modern derivative plants (hydrochloric acid, sodium hypochlorite and ferric chloride) combined with liquid chlorine packing. The mercury cells had originally been in excess of 80 000 tonnes per annum capacity and cells had been progressively taken off-line following closure of chlorinated solvents (CTC/PCE) and EDC/VCM manufacture over a period of 10 years. This had left an asset significantly oversized compared with the present market demands. 

The unit operations in a commercial chlor-alkali plant can be generally classified as follows (1) brine purification, (2) electrolytic cells, (3) H2 and Cl2 collection, and (4) caustic concentration and salt removal. In this section, the general process flowsheets for diaphragm, membrane, and mercury cell technologies are discussed with emphasis on the need for brine purification and the manner in which it is carried out. 

Production of CI2 and NaOH by electrolysis of NaCl is a huge industry with annual production capacity in excess of 50 million tons of NaOH per year. Membrane cells are the state-of-the-art technology, but mercury and diaphragm cells are still used because the capital cost for their replacement is substantial. The mercury cell technology is more than a century old and stiU accounts for nearly half of the world s production capacity. Chlorine evolves from a DSA (dimensionally stable anode) situated above a pool of mercury with NaCl brine in between. Mercury reacts with sodium to form sodium amalgam, which is removed and hydrolyzed in a separate reactor. 

In 2002, approximately 67% of the chlorine in the United Sates was produced using the diaphragm-cell technology, 20% by the membrane-cell technology, and 10% by the mercury-cell technology. Five companies—Dow Chemical, Occidental Chemical Corporation, PPG Industries, Olin Corporation, and Formosa Plastics Corporation— manufacture 79% of the chlorine. Twenty-eight percent of the total US chlorine production was by Dow Chemical, whereas twenty-two percent of the production was by Occidental Chemical. PPG, Olin, and Formosa Plastics produce 12%, 9%, and 7% of the chlorine production in the United States, respectively. 

Diaphragm-cell technology contributes 62% of the total chlorine produced in Canada, while membrane- and mercury-cell technologies produce 36% and 2% chlorine, respectively. Dow Chemical Canada, Nexen Chemicals, PCI Chemicals Canada, and PPG Canada are the major producers of chlorine. 

Presently, there are five mercury-cell technologies [70,71] practiced in the world, and their operating characteristics are summarized in Table 5.2. The only technology supplier that is active in optimizing the cell design and the cell operations to achieve negligible mercury emissions is DeNora [72]. 

Brine Dechlorination. Operation of the primary dechlorinator should be established next. It is not easily possible to simulate actual dechlorination. However, where vacuum dechlorination is used, it is important to establish that the vacuum and condenser systems operate properly. In the case of a conversion from mercury-cell technology, this may already be established. 

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