Section 1: Introduction

Since its discovery in 1774 and general acceptance as an element by 1815, chlorine has been used as a bleaching agent for linen, cotton, and paper. The first patent associated with an industrial use for chlorine (bleaching) is dated 1799.

Chlorine and chemicals related to its production, soda ash (Na2CO3) and caustic soda (NaOH), are among the most important industrial chemicals, with applications that cover the production of soap and detergents, plastics, glass, petrochemicals, fertilizers, and explosives and water/wastewater treatment. The importance of chlorine is now in the synthesis of organic chemicals, where it enters into several of the intermediate reactions.

During the last half of the 19th century, chlorine was produced commercially by the Weldon and Deacon processes, both chemically based processes using catalytic oxidation of hydrochloric acid by air. Because of various inherent inefficiencies in these two processes, and the fact that caustic soda was not produced, they were replaced by more efficient industrial processes, which produced both chlorine and caustic soda.

Since 1853, caustic soda has been produced industrially by the addition of calcium hydroxide (Ca(OH)2) to soda ash in a batchwise chemical process. Although electrolytic production of caustic soda was known in the 18th century, commercialization of this process had to await the development of high capacity, direct current, electrical generating equipment.

In 1892, the process of producing chlorine and caustic soda in an electrolytic cell was developed. This process employed the electrolysis of brine (NaCl) using a moving liquid mercury cathode to produce chlorine at the anode and a sodium-mercury amalgam at the cathode.

This so-called "mercury cell process" constituted the main commercial production process for chlorine and caustic soda from the 1890s to the middle of the 20th century. Although mercury cells are still in operation throughout the world, they are being increasingly replaced with alternative, mercury-free electrolytic processes. These processes, the diaphragm cell and membrane cell processes, produce chlorine and caustic soda, use the same basic raw materials (brine, water, and electricity), and are similar in terms of generating and treating product gases. In this way, they are similar to the mercury cells. Differences exist, however, in the design and operation of and emissions from the various processes, the primary difference being the absence of mercury in effluents and emissions coming from diaphragm and membrane cells.

In the past, chlorine and caustic soda were produced in Canada predominantly via the mercury cell process, which is reflected in the 15 mercury cell plants that operated in Canada from 1935 to the mid-1970s (Table 1). By 1979, however, only five operating mercury cell plants remained. In December 1990, the PPG Canada Inc. plant in Beauharnois, Quebec, converted to a membrane cell.

Reasons for the decline of chlor-alkali1 mercury cell plants in Canada vary, but the primary reason for their replacement with nonmercury-based processes was the growing concern in 1969/1970 over unnaturally high levels of mercury in surface waters, fish, and other aquatic life in various locations throughout the country. Investigations by govemment authorities of those industries using mercury showed that areas around chlor-alkali plants were highly contaminated with mercury. As a result, the mercury cell chlor-alkali industry, being a large point-source contributor, became the focal point for regulatory action, even though mercury is discharged from other industrial sources as well.

Federal regulations for effluents followed in March 1972 (Chlor-Alkali Mercury Liguid Effluent Regulations, promulgated under the Fisheries Act and revised in July 1977). Recognizing that airbome mercury is also a source of contamination, regulations were promulgated in July 1978 for emissions (Chlor-Alkali Mercury National Emissions Standards Regulations, promulgated under the Clean Air Act, revised in February 1990, and incorporated into the Canadian Environmental Protection Act).

In addition to federal regulations, mercury cell plants in Canada are also subject to provincial legislation. In some cases, these are equivalent.

This report summarizes the status of mercury cell chlor-alkali plants and their compliance with federal mercury effluent and emission regulations for the period 1986 to 1989.

A conventional mercury cell consists of two separate but integrated sections, the "electrolyzer" and the "denuder" (or decomposer). The electrolyzer is usually a long (typically 15.2 m long by 1.2 m wide), covered, slightly inclined, rectangular cross-sectioned, flat-bottomed steel trough in which brine (for electrolysis) flows uniformly. The cathode for the process is a thin layer of metallic mercury completely covering and flowing at the bottom of the trough under the brine. Projecting through the electrolyzer cover are the anodes, which are now made of titanium-covered metal and are referred to as dimensionally stable anodes (DSAs). These are horizontal and are hung on insulated rods front the top of the cell. Typical spacing between the dimensionally stable anodes and mercury cathode is on the order of several millimetres. To further assist chlorine gas to leave the cell, the dimensionally stable anodes are in the form of grills.

During electrolysis, dissociation of brine produces sodium and chlorine ions (Na+, Cl-). At the mercury cathode, Na+ combines with mercury to form a Na-Hg amalgam, which leaves the cell and flows to the denuder, an enclosed rectangular cross-sectioned steel duct mounted below or alongside the electrolyzer. Here, demineralized water is added to the Na-Hg amalgam to free the mercury (which is recycled to the electrolyzer) and produce sodium hydroxide (NaOH) and hydrogen (H2). There is no electrical input to the denuder.

At the dimensionally stable anodes, chlorine gas is produced, which leaves the cell. The gas is then cooled (for water removal), dried (with sulphuric acid), and liquified.

Chemical reactions in each section of the cell can be represented as follows:

Electrolyzer:
NaCl + Hg
Na-Hg + ½ Cl 2 (g)
Denuder:
Na-Hg + H 2 O
NaOH + ½ H 2 (g)
Net reaction:

2NaCl + 2H 2 O
2NaOH + Cl 2 (g) + H 2 (g)

Representative photographs of an operating mercury cell room, a mercury cell, and dimensionally stable anodes from ICI Ltd., Cornwall, are shown in Figures 1, 2, and 3 respectively. A schematic diagram of a mercury cell is presented in Figure 4.

Mercury Cell Room, ICI Ltd., Cornwall, Ontario
Mercury Cell Undergoing Maintenance, ICI Ltd., Cornwall, Ontario
Dimensionally Stable Anodes Undergoing Maintenance at ICI Ltd., Cornwall, Ontario (note connecting rods and grill-type DSAs)
Schematic Diagram of Mercury Cell


Because of increasing environmental concerns over mercury in effluents, emissions, and solid wastes from chlor-alkali plants, alternate processes now operate commercially in Canada. These processes, the diaphragm cell and membrane cell processes, are electrolytically based, use the same basic raw materials (brine, water, and electricity), produce chlorine and caustic soda, and are similar in terms of generating and treating product gases. However, neither process uses mercury.

In the diaphragm cell process, the parallel anodes and cathodes are separated by a porous layer of asbestos. This layer or diaphragm, which covers the steel wire mesh cathode, allows ions to pass through by electrical migration, but reduces diffusion of products that could result in the formation of unnecessary compounds (chlorates) and reduce current efficiency.

Purified, saturated brine is electrolyzed to produce chlorine, caustic soda, and hydrogen. Chlorine is liberated at the dimensionally stable anodes and removed in the chlorine outlet. At the cathode, hydrogen gas and hydroxide ions are formed. A 10-15% NaOH solution is generated in the cathode compartment. Diaphragm cells permit the construction of compact cells of lowered resistance because the electrodes can be placed close together. Because of their resistance, these cells require a higher voltage and a higher hydrostatic pressure on the brine feed.

The membrane cell process, developed during the early 1970s as a result of progress made in membrane technology, uses a cation-exchange membrane as a selective partition between an anode and a cathode compartment. The perfluorocarboxylic acid membrane allows Na+ to migrate to the cathode, prevents OH- from migrating te, the anode, and ensures that Cl- is retained in the anode area, thereby ensuring a higher purity NaOH in the cathode compartment.

In operation, saturated salt solution is passed through a brine pretreatment system followed by an ion-exchange resin bed to remove multivalent cations. The treated salt solution is then directed to the anode compartment. Sodium ions migrate through the cation-exchange membrane to the cathode side and with hydroxide ions form sodium hydroxide. Pure water is injected into the catholyte stream to maintain the concentration of sodium hydroxide obtained in the catholyte. The concentration of NaOH produced by this process (about 20%) requires evaporation te, achieve a concentration of about 50%. Chlorine gas generated at the anode is used directly or is liquified and stored.

Both the diaphragm cell and membrane cell processes operate with distinct environmental advantages over the mercury cell process, the most important advantage being the absence of mercury in the process operation and in process effluents, emissions, solid wastes, and products. However, the fact that mercury cell plants still operate in Canada (although in 1989 there were only five plants) is due to the higher concentration of caustic soda produced compared with that produced using the diaphragm and membrane cell processes and the cost of replacement and site decommissioning. The comparative advantages and disadvantages of all there processes are presented in Table 2.

Table 3 presents chlorine and caustic soda production in Canada by the mercury, diaphragm, and membrane cell processes for 1986 and 1989. The decreasing share of the caustic soda - chlorine market once enjoyed by the mercury cell process is presented in Table 4.


1 Chlor-alkali derives from "chlorine" and an alternate expression for caustic soda, "alkali." Hence, chlor-alkali.

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