Mercury-Cell Plants

FIGU RE 22.5 General wastewater treatment process flow diagram at a mercury cell plant for the production of chlor-alkali. 

Orica Australia Pty Ltd, formerly ICI Australia, is the largest manufacturer of chlorine and caustic soda in Australia. In December 1998, the Orica Board sanctioned a project to replace two mercury cell plants with new membrane plants. The plants are due to be commissioned sequentially at the end of 2000/mid-2001 at a total capital cost of US 100 million. 

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. 

Orica is the largest producer of chlorine in Australia and currently operates three chlor-alkali plants on the east coast. Two of these plants (in Melbourne and Sydney) are mercury cell plants dating back over 50 years while the third plant is a small, modern membrane cell plant in the central Queensland town of Gladstone. The mercury cell plants have both reached the end of their useful economic lives. 

Reinvest significant sustenance capital to bring the mercury cell plants up to the world s best practice. 

The existing mercury cell plant in Melbourne (ICI Mark 1 cells) is 14 000 tonnes per annum capacity whereas the market demand in Victoria, South Australia and Tasmania is 30 000 tonnes per annum. Owing to the age, condition and location of this plant (adjacent to residential areas and 5 km from the centre of Melbourne), it was decided to close this site and construct a new Greenfield plant in Laverton North, approximately 10 km away. The new plant includes chlor-alkali manufacture, sodium hypochlorite, hydrochloric acid, liquid chlorine storage and packing and chlor-paraffin manufacture. 

An industrial grade, skid-mounted unit (see Fig. 11.2) has been operating successfully at Occidental Chemical s Delaware City mercury cell plant since September 1997. Operating results have met both the expectations of the client and Kvaerner. This chapter discusses the operating experience at OxyChem and the resulting optimised commercial product, the Kvaerner Chemetics Sulphate Removal System or SRS . The acceptance in the market-place has been excellent four systems have been ordered and three are in the process of manufacture, with operations commencing later in 2000. Approximately 50 firm price proposals have been issued in the past two years. Several of these evaluations are in the final stages and will lead to the sale of units in the near future. 

Alercury has a high vapor pressure at the normal cell operating conditions hence it is always found in the reaction products. Although the mercury is almost completely recovered and returned to the process, environmental problems associated with mercury, combined with the less efficient eneigy utilization compared to the modem membrane cell process, has effectively stopped the building of new mercury cell plants. Furthermore, in the 1990s, membrane cells will most likely replace most of the present mercury cells. For details related to mercury cells, see references 8 and 16 and general references. 

A comparison with the investment costs for the other two varieties does not make sense at the moment, because since 1991 only a few diaphragm cell plants and no mercury cell plants were built [3, p. 123],... 

The typical layout of a mercury cell plant for producing Cl2 and NaOH is presented in Fig. 6. The brine purification procedure is... 

With the growth of synthetic fibers, such as rayon, a tremendous demand for very pure (salt free) caustic soda developed. A significant number of mercury-cell plants were built in the years around World War II. 

After DuPont introduced Nafion membranes. Diamond Shamrock intensified its research efforts in membrane-cell technology. Initial research tests with the membranes began in 1970 using laboratory cells. In 1972, a commercial-size electrolyzer and pilot plant were placed in operation in Painesville, Ohio. Four years later, a 20-ton per day demonstration plant was built in Muscle Shoals, Alabama. Hiis unit was integrated with an existing mercury-cell plant. 

On the other hand, in bipolar cells, only the terminal cells are connected by intercell conductors, and there are typically many unit cells electrically in series between the terminal cells (Fig. 5.2). The two basic types of bipolar cells are the flat plate cell and the finger type cell. A group of bipolar cells that have a common piping system for the fluids, via manifolds, is referred to as an electrolyzer or sometimes a series or a stack. Within a single bipolar electrolyzer, there are sometimes more than one set of terminal cells. Bipolar electrolyzers can be connected via an external bus within a DC circuit in series or in parallel, but usually not both. Furthermore, in the case of mercury-cell plant conversions to membrane cells, the electrolyzers are connected electrically in parallel as shown in Fig. 5.3. 

When membrane cells operate in tandem with mercury or diaphragni cells, there is an opportunity to purge sulfate by sending at least part of the depleted brine that otherwise would be recycled to the other cells. This approach is less effective with mercury cells. The water balance is extremely tight in a mercury-cell plant, and any import of water must be balanced by an export. Since export would spread the potential for mercury pollution, we reject it as an operable solution. Only merciuy-cell plants with some capacity for addition of water to the brine loop, as for example a plant with a brine evaporator, could be integrated in this way. 

The filtration membranes are sensitive to fouling as well as to free chlorine. This situation is least troublesome in a membrane-cell plant, where the problem components already have been removed from the brine. In mercury-cell plant applications, an installation in the brine recycle loop should include some means of dechlorination. The usual choice is treatment with activated carbon, which is covered in Section 7.5.9.3B. The membranes are in spiral-wound modules placed in cylindrical housings and assembled as on the skid shown in Fig. 7.81. Figure 7.82 shows the construction of a modular element. The low-sulfate permeate flows through the membranes into spacer channels... 

A. Vacuum Dechlorination. Dechlorination with the aid ofa vacuum is probably the most frequently used method. It had been the standard process in mercury-cell plants for many years and was naturally adapted to membrane cells. Any of the standard methods of producing vacuum (Section 12.6.1) will be satisfactory. Steam jets are quite acceptable as vacuum sources, since their effluent can simply join the cell gas flowing to the chlorine coolers. 

An option to consider is operation without pressure control. The deeper vacuum that results will release more chlorine from solution. In a membrane-cell plant, this will reduce the cost of the secondary dechlorination, which is discussed below, but the approach may not be workable in a mercury-cell plant, where higher free chlorine concentrations are desirable. More water will evaporate along with the incremental chlorine. The load on the vacuum condenser will therefore increase, but not by as much as when the addition of steam is used for pressure control. At the same time, the dechlorinated brine will become cooler. This can only help the pumping operation. The cooler brine can also... 

P.M. Mayo, Conversion of Mercury-Cell Plants to Membrane-Cell Technology, 36th Chlorine Institute Plant Operations Seminar, Washington, DC (1993). 

In mercury-cell plants, the ventilating air itself can be a source of mercury emission. The air used to ventilate the end boxes that seal the feed and overflow at each cell is an especially rich source of mercury. Established practice now is to limit these flows and collect and treat any air that escapes the end-box system. This change alone has done much to reduce average air emissions. 

Istas [20] described an aluminum busbar system in a mercury-cell plant. The design-basis current density for the buswork was about 900kAm operation was close to 800kAm. The buswork joints were clamped. Joints between aluminum and copper or steel were through 19 ftm of nickel plate. Exposed aluminum surfaces were covered... 

IB. Hydrogen. Low-pressurehydrogeniseasily handled, but designers and operators should be aware that there is always liquid in a hydrogen header. At its most iimocuous, this is simply water that condenses as the gas cools. Entrained liquor from the cells can make the liquid in the pipe conductive. In a mercury-cell plant, there is the added complication of liquid mercury condensing in the lines. 

Small towers sometimes are reinforced PVC, and there is no need for a lining. A potential problem with PVC or other nonconductive material is the accumulation of a static charge. The explosion of a set of three PVC dryers in a mercury-cell plant was attributed to static discharge in the presence of an explosive gas [27]. The root cause of the incident was the failure of the power supply to the mercury pumps, which allowed mercury to drain from the cells and uncover the steel bottoms. This led to the formation of large quantities of hydrogen. The investigators concluded that towers should be constructed of acid-proof conductive material that can be held at ground potential. 

Combining mercury and diaphragm cells is straightforward (Fig. 9.72). Some of the evaporator salt is used to resaturate the diaphragm-cell brine after chemical treatment. The rest is available to resaturate mercury-cell depleted brine. A given mercury-cell plant will have a certain need for salt. This fixes the minimum size of the diaphragm-cell plant required as a salt producer. Only in an ideal world would the two types of cell remain forever in perfect balance, and the plant design must accommodate any imbalances. 

This chapter was contributed by Thomas A. Weedon, Jr. In twenty-six years with the Central Engineering Department of Diamond Shamrock, he worked on many new and existing diaphragm- and mercury-cell plants. Subsequently, with his own company, Instrumentation Technology, Inc., he has worked closely with ELTECH Systems Corp. on the development of control strategies for their membrane-cell technology. 

The occasion for decommissioning a mercury-cell plant may be the abandonment of production or a decision to convert to membrane-cell technology. In either case, decisions also are required on the reuse of buildings, material, and equipment. In a total shutdown of a facility, site remediation wiU also be important. 

Euro Chlor [22] has published methods for decontamination of the various components of a mercury-cell plant. The discussion here is based on that publication. Table 13.9, taken from Appendix 3 of the reference document, is a brief summary. Discussion of the methods referred to in the last column follows the table. 

TABLE 13.9. Mercury-Cell Plant Decontamination Techniques... 

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