- Chlor-alkali producers evaluate safer alternatives to asbestos - By William Stadig Contributing Editor p m s 26932 ntil recently, 75% of all U.S. U capacity for producing chlor-alkali-more than 40% of the world s capacity-has used asbestos diaphragm-cell technology. Although the Environmental Protection Agency continues to exempt asbestos use in diaphragms from restrictions, producers are considering alternatiyes. n Germany, stringent regulations dill ban asbestos in chlor-alkali production after 1994. Heavy fines were levied recently against some chlor-alkali producers in the United States when EPA inspectors found asbestos fibers in cell renewal areas. Restrictions on the mining of asbestos raise the cost of obtaining adequate amounts of high-quality asbestos and gradually raise the cost of transporting and discarding spent diaphragms. Two alternatives are to use newly developed, non-asbestos diaphragms or to convert to existing ion-exchange membrane-cell technology. Only the former seems economical in the United States. The non-asbestos diaphragm is based on an inorganic polymer composite developed in 1988 as an asbestos substitute. The composite received Du Pont s Plunkett Award for nnovation with TeflonTM, landed on the National Development Association s 1991 Honor Roll and became a 1991 R&D 100 Award winner. Membrane technology is used to replace mercury-cell units (see sidebar). Converting from diaphragm-cell to membrane-cell technology requires far more significant capital investment than switching to non-asbestos diaphragms. Choosing when to switch is a site-specific decision that depends on the cost of the new diaphragms, their performance and service life, electricity consumption and disposal costs for asbestos. The schematic flow through individual diaphragm and membrane cells is shown on p. 45, and block diagrams depicting the entire chlor-alkali process using diaphragm and membrane technology are shown on p. 42. Prior to the early 1970s, conventional diaphragms were made of sheets of pure asbestos fiber, but most have been replaced with polymer-modified asbestos diaphragms (PMA) that encapsulate about 75% by weight asbestos in a net- ::. More than 40% of the world s capacity for producing chlor-alkali has used diaphragm-cell technology. n this photograph, the coated cathode is removed from the deposition bath. work of fibrous polytetrafluoroethylene (PTFE). Although PMA diaphragms present no dusting hazard in service because they are totally wetted and placed in a confined atmosphere, the industry s goal is to eliminate asbestos use. The potential for release of fibers exists whenever asbestos is physically,handled, as in fabrication, installation and disposal of diaphragms. n 1988, the newly developed nonasbestos material mentioned above became available as a replacement for both asbestos and PMA diaphragms. The material is a composite fiber with a TeflonTM (PTFE) backbone and metaloxide particles, preferably zirconium oxide, impacted into the fiber during manufacture. From a functional viewpoint, the material has many of the porosity and flow characteristics of asbestos and can be easily vacuumdeposited on cathodes in a process similar to that of forming asbestos diaphragms. Since PTFE is hydrophobic, the metal oxides provide the hydrophilicity necessary in a diaphragm. Preparation of coated cathodes is on-site (for more information, see the sidebar). Environmental Factor The composite has a branched-chain structure of varying length that may be termed a fibrid. The fibrids, much longer and coarser than asbestos, have diameters several orders of magnitude larger. These dimensions and the greater density attributed to the metal oxides make the material very difficult to suspend in the air. Results of air-quality sampling in depositing and manufacturing facilities have shown the workplaces to comply well within OSHA standards. Tests also indicate that the fibrids high density prevents their inhalation in standard procedures. The EPA has listed CHEMlCAl PROCESSNG.18!3.MARCH 4 1
, -Raw Caustic solution Precipitants Residue Chlorine Gas HydrocMonc Acid Purification By on-exchange Punfied Hydrochloric Acid :, - < hlyle 1 Caustic solution Water Sodium Hydmxir!a Hydrogen 3 Liquefaction Evaporation 1 Chlorine - Y Cooling Hydroeen Chlonne Sodium Hydmxide these particular fibridddiaphragms as an article, meaning they are not subject to further toxicology testing. Because the expected service life of the diaphragms is at least three times as long as that of PMA diaphragms, the volume of waste is not only non-hazardous but also significantly smaller. Economic Factor As long as EPA continues to exempt the use of asbestos diaphragms from restrictions, the conversion to the material from PMA will depend on factors both economic and environmental. These diaphragms originally cost up to 10 times as much as PMA diaphragms, but this cost is partially offset by their service life, which is three times as long. Modifications reducing the diaphragms weight and thickness help further lower 4 2 MARCH.lfifi3.CHEMEAL PROCESSNG
5 their cost. Therefore, the switch in the Unite% States depends mainly on enhanced performance, especially with electrical requirements, which account for approximately 50% of the manufacturing cost. This will come from improved diaphragm permeability and reduced hydrogen production, attributed to iron contamination of the brine feed. Market Development nitially, from two to four test cells were demonstrated at 25 plants in the United States and Europe, using many cell designs, modifiers and current densities. These were quite successful: most cells lasted more than one year, with an average of about two years. One cell has performed for more than five years. Subsequent tests at eight sites, ipcluding Occidental Chemical, Vulcan and BASF in Germany, using large, commercialsize blocks of 25 or more cells, have yielded significant performance data. Tests also provided plants with experience in vacuum-depositing diaphragms on site. Full commercialization (Le., complete plant conversion) began in 1992 at several plants, including those of General Electric, Huls and Vulcan. ped during in the '70s:a Wller 1 o concentrate the cau ly, numerous other adva ane cell technology hav Gary Madison of Consulting Resources Corp. in Lexington, MA, says that the industry is accepting these diaphragms enthusiastically, especially in Europe, because of environmental concerns. nasmuch as the power consumption is a "push" compared to the consumption of conventional PMA diaphragms, the switch is growing. Madison says that the conversion to membrane technology would require significant capital investment upstream in brine purification and downstream in evaporation equipment. Other than the diaphragm or the expensive alternative of converting to membrane technology, no major competitive non-asbestos diaphragm has been offered to the chlor-alkali industry. PolyramixTM diaphragm technology-oxytech Systems, nc., Chardon, OH. Circle 409 TeflonTM-El dupont de Nemours, Wilmington, DE. Circle 410 Nafion membranes-du Pont Co., Wilmington, DE. Circle 411 CHEMCL PROCESSNG.lBB3.MARCH 4 5
and sodium hydroxide (caustic soda), the mmodity chemicals in the group comprising or-alkali industry rank as the eighth and ninth e chemicals used in the United States. Although some production comes from alternative processes, more than 98% is from rine, producing chlorine, caustic soda age is not economical, most chlorine gas production is used\captively. n recent years, the demand for highpurity liquefied chlorine has risen significantly. Of the roughly 10 million tons of caustic soda produced annually in the United States, half is used to manufacture inorganic chemicals and organic intermediates, polymers and surfactants as a catalyst, for ph ControHeat Bolt-On Jackets Beat Tube Tracing Cold Anywhere you need lots of heat, this jacket is ready to go. t s ready for sulfur, pitch, phthalic anhydride or polymers. To 750 F, continuous. With complete heat coverage, flange to flange. There are no cold spots. No freeze ups. No rosebuds. And, its reusable which gives you full payback every time you replace a valve. So give the maintenance crew a break today, write or call us: Controls Southeast, nc., P.O. Box 7500, Charlotte, N.C. 28241. Tel: (704)588-3030, Fax: (704)588-3039. * 4 6 MdRCH.1993.CHEMCAL P R O C E S S N G Circle 129 -