The Chlor-Alkali Industry

The electrolysis of aqueous sodium chloride, normally brine obtained directly from natural salt deposits, to yield chlorine, sodium hydroxide and hydrogen is the largest of the electrolytic industries. In the USA, the annual production of chlorine is of the order of 10 tons while in the UK it is about 1.7 x 10 tons. 

Both chlorine and sodium hydroxide (normally traded as a 50% aqueous solution) must be considered to be main products and which is in greater demand varies with time their major uses are summarized in Table 3.1. The hydrogen is also used, where possible as a chemical (e.g. for the hydrogenation of fats), otherwise as a fuel for the power station which is inevitably on site at most chlor-alkali facilities because of their large demand for electricity. 

Major uses of chlorine and caustic soda (sodium hydroxide).  

Preparation of chlorinated organic solvents, e.g. methylene chloride, chloroform, carbon tetrachloride, per- and trichloroethylene, 1,1,1-trichloroethane (18) Preparation of propylene oxide (10) 

Synthesis of other organic compounds, e.g. chlorobenzenes, alkyl chlorides (particularly methyl chloride for lead alkyls), herbicides (10) 

The chlor-alkali industry should be particularly interesting to the student of electrochemical technology. Firstly, it is by any standards, a process carried out on a very large scale and at many sites and it is central to the chemical industry as a whole. In addition, three different electrolytic technologies based on mercury, diaphragm and membrane cells are all currently used. Moreover, 

The majority of the chlorine produced is used internally within the chemical industry for the manufacture of polyvinyl chloride, chlorinated hydrocarbons, propylene oxide, etc. (Table 3.1). Hence, it is common to find chlor-alkali plants as part of very large, integrated chemical complexes and the capacity of such plants may be 0.5 x 10 tons Cl2/year. On the other hand, concern about the transport and storage of liquid chlorine has led to a different trend towards smaller plants sited close to the user. This is particularly attractive when there is an almost balanced requirement for both chlorine and sodium hydroxide, e.g. in pulp and paper mills (Table 3.1). A typical plant in this application may have a capacity of 10 tons Cl2/year, On an even smaller scale, the same concerns lead to a need for plants, for example, to provide CI2 to prevent biological growth on 

In order to understand how anti-corrosive rubber linings are used in the caustic soda industry it is useful to have a broad understanding of the design, construction and operation of the process, mainly about the cell house where corrosion is severe. A brief description of design, construction and operation of mercury cells in the caustic soda industry is given next [11]. 

The mercury cells within the cell house consist of three units  

The electrolyser consists of a rectangular trough with a cover on top connected at both ends to the end boxes. The box at the mercury inlet is called the inlet box, and the other is the outlet box. The trough is of mild steel construction with ebonite lined bottom and sides. The covers are of mild steel construction with ebonite lining on the inside surface or only of rubber sheets called flexible cell covers. The covers have holes for fixing anodes. 

The anode gaskets and rings are of soft and ebonite rubber, respectively. The inlet box is of rubber lined mild steel construction having a feed brine (sodium chloride) distributor and mercury seal. 

The inlet box is constructed so that it distributes the mercury uniformly across the entire width of the cell trough. The anodes are fixed to the cover to hang over the cell bottom. The trough is installed on an inclined frame. The mercury and the feed brine flow from the inlet to the outlet end and the seal arrangement in the outlet box allows only the amalgam to flow out, while the depleted sodium chloride solution (brine) is taken out from the overflow nozzles fixed at the end of the trough. 

Initially, the electrolysis of aqueous brine was carried out in amalgam cells [4,10, 11]. Typical conditions employed were an electrolyte feed of 25% NaCl at pH 4, a temperature of 343 K, and a current density in the range of 0.7 -1.4 A cm . The desired anode reaction, the evolution of chlorine, was accompanied by corrosion of the carbon at a rather rapid rate. Sodium amalgam was formed at the mercury cathode, which was then reacted with water to give 50% sodium hydroxide and hydrogen gas in a catalytic reactor known as a denuder. 

Even though the involvement of graphite in the chlor-alkali industry is now history, this application remains important to this chapter, because it remains the largest and most reliable source of information about the mechanism of graphite corrosion and ways to limit the rate of corrosion [1 -3]. The graphite lost from the anode was found partly as sludge in the base of the cells, but CO and CO2 were also found to contaminate the chlorine off-gas. Evidently, graphite consumption results from anodic oxidation 

An important sector of heavy inorganic chemical manufacturing is the production of chlorine and sodium hydroxide — the chlor-alkali industry. The manufacture of these chemicals has a long history. Today they are produced simultaneously by the electrolysis of sodium chloride solutions, but this was not always the case. The two chemicals were originally manufactured by different routes. In the 19th century chlorine was made by the oxidation of hydrogen chloride (itself made by reaction of salt with sulfuric acid) using the Deacon process. Sodium hydroxide was prepared by the reaction of calcium hydroxide with sodium carbonate — the lime-soda process. 

There is, however, a drawback with membrane technology, due to the hostile environment in which the cells operate. The highly alkaline environment means that the expensive membranes must be replaced every 2-3 years. 

The number 2 means the fire brigade can use water, fog or spray equipment on this chemical. The X is in the contain section, indicates the spillage must be contained, and not allowed to enter drains or watercourses. If it was in the dilute section, the substance could be flushed away. The X also indicates that full body protective clothing must be warn if tackling a spillage if there were a V next to the letter (not the case here) then the chemical is deemed to be highly reactive and potentially explosive. Finally, the letter E advises evacuation of the area should be considered. 

Since the production of chlorine and sodium hydroxide is inextricably linked, a surge in demand for one creates problems with sales of the other. For example, increased demand for chlorine for vinyl chloride production meant that there was an abundance of sodium hydroxide. As a result it replaced sodium carbonate in some of its applications. 

Over one-third of chlorine produced is used to make PVC via ethylene dichloride (1,2-dichloroethane). 

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For the chlor-alkali industry, an emergency preparedness and response plan is mandatory for potential uncontrolled chlorine and other releases. Carbon tetrachloride is sometimes used to scrub nitrogen trichloride (formed in the process) and to maintain its levels below 4% to avoid fire and explosion. Substitutes for carbon tetrachloride may have to be used, as the use of carbon tetrachloride may be banned in the near future due to its carcinogenicity. 

This trade-off may not even occur in some cases. Membranes used in the PEMFC have been developed for the chlor-alkali industry and have 40,000-hour durability (shutdowns are prohibitively expensive in stationary applications), require only 5,000-hour durability (corresponding Co 100,000 miles) for automotive applications. Hence, it maybe possible to develop less expensive membranes that still meet automotive requirements. 

Hine, F. Chemistry and Chemical Engineering in the Chlor-Alkali Industry 18... 

Thanks to its high stability and permselectivity, Nation has been used as a Na+ conductor in membrane electrolysis of brine in the chlor-alkali industry. This application was introduced in the early 1980s and is by far the most important use of ionomer membranes. 

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28... 

OSPAR (2007) Mercury losses from the chlor-alkali industry (1982-2005). ISBN 978-1-905859-56-6... 

At one time, sodium hypochlorite was manufactured electrochemically on a substantial scale. Now it is regarded as a by-product of the chlor-alkali industry [10]. On the other hand, there are many situations where low volumes of hypochlorite may be required or the requirement is irregular. Aqueous solutions of hypochlorite are much safer than chlorine gas but contain < 15wt% of active chlorine. Hence, storage and transportation costs are relatively high. Often the most convenient and cost-effective solution is to eleetrolytically generate OC1- in situ [10]. 

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. 

This chapter examines the business of the chlor-alkali industry. Without a need for chlorine there is no chlor-alkali business. This industry is now in its second century and having passed the millennium bug without any serious interruption, the future can now be examined. In this chapter there is an emphasis on chlorine rather than alkali as the alkali industry is thousands of years old and has been documented extensively since the time of the ancient Egyptians. 

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

This chapter examines an issue that is one of the key determinants of the future of the West European chlor-alkali industry. It examines the environmental, economic, financial and political aspects of a debate whose effects are likely to spill over into North America and Asia - and to affect the development of the chlor-alkali industry world-wide. 

For over a hundred years the chlor-alkali industry has used the mercury cell as one of the three main technologies for the production of chlorine and caustic soda. For historical reasons, this process came to dominate the European industry - while in the United States the asbestos diaphragm cell took the premier position. Over the last two decades developments in membrane cells have brought these to the forefront, and membrane cells of one kind or another now represent the technology of choice worldwide. 

We can t do much about the volcanoes, but we do need to address the anthropogenic emissions. To do so, we need to know where these come from. Historically, there is no doubt that the chlor-alkali industry was one of the biggest anthropogenic sources. However, the industry has reduced its emissions by an order of magnitude and it no longer represents a major source. Annual emissions of the West European... 

Quantification of the macroeconomic effects of cellroom closure is extremely difficult. But it is significant that this difficulty arises precisely because the chlor-alkali industry forms the base for such a large proportion of so many diverse sectors of manufacturing industry - and thus of the tax base for national governmental budgets. 

The practical implementation of the Directive takes place by means of the development of a series of BAT Reference Documents (BREFs) which serve as guidance documents either for individual industrial sectors (such as the chlor-alkali industry) or, in the case of the so-called lateral BREFs , for cross-cutting activities. 

High current density has become a reality in the chlor-alkali industry. More than half of the plants starting up between 1997 and the end of 2000 are designed for operation at 4-6 kA m-2. Furthermore, many plants designed for operation up to 4 kA m-2 are successfully operating at higher current densities to meet increased demands, or to... 

This laboratory serves the chlor-alkali industry in many ways. These services include ... 

Electric energy is the predominant cost in the manufacture of chlorine and is the driver for most of the technical progress in the chlor-alkali industry. The busiest areas of development over the past 20 or 30 years have been related to reductions in energy consumption. Approximately 60% of the papers presented in this book deal with improvements in chlor-alkali cell internals, namely the anolyte/catholyte separator (primarily membranes) and the electrodes. 

Australia has a population of only 18 million people, spread geographically over an area similar to that of the mainland USA. The chlor-alkali industry in Australia is small by world standards and has a product mix unique in being aligned neither with chlorinated solvents nor the vinyl chain. The chlorine/caustic soda balance is heavily skewed towards caustic use in the alumina industry however, none of the locally produced caustic soda reaches this market. This has resulted in an industry limited by outlets for the chlorine half of the ECU and based on a relatively large number of small plants scattered across the country to serve localised chlorine market needs. Australia remains a large importer of caustic soda to fill the gap between local manufacture and demand, primarily for the alumina industry. 

This chapter gives an overview of the chlor-alkali industry in Australia and examines the background to the decision to replace the mercury cell plants. It then describes the new plants, their technical and safety features and the process used to arrive at the selection of the technology supplier. 

The chlor-alkali industry in Australia is dispersed, with chlorine production capacity of 127 000 tonnes per annum from a total of nine plants spread across the country. 

Among the various desulphation systems available in the chlor-alkali industry, the RNDS offers excellent advantages in terms of its low initial and running costs and environmental issues. In addition, it is a simple system and the process design results... 

This chapter has introduced the RNDS application in the removal of impurities from brine destined for chlor-alkali electrolysis. On top of this, however, RNDS has potential use in other markets, including water treatment. Chlorine Engineers will continue its innovative work to meet the various requests coming from the chlor-alkali industry. 

World-scale producers use spreadsheet analysis to evaluate the economics of different options over the lifetime of the plant (often 20 years is assumed), taking account of operating, maintenance and capital costs. The chlor-alkali industry also expects the current density (CD) to increase in a manner that is dependent on membrane development. Other important factors expressed by producers about membrane technology choice included component lifetimes and reliability. 

In the chlor-alkali industry titanium brings its properties to application as a material in activated metal anodes. In fact this is the major use of titanium in the chlor-alkali industry. 

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