Environ. Sci. Technol. 2000, 34, 2293-2298
A Novel Approach for the In-Situ Chemical Elimination of Chrysotile from Asbestos-Containing Fireproofing Materials JACOB BLOCK,† LEONIDAS PETRAKIS,‡ L E O N A R D E . D O L H E R T , * ,† D A V I D F . M Y E R S , § L . L O U I S H E G E D U S , §,| RONALD P. WEBSTER,‡ AND LAWRENCE E. KUKACKA‡ W. R. Grace & Co., 7500 Grace Drive, Columbia, Maryland 21044, Brookhaven National Laboratory, Upton, New York 11973, and W. R. Grace & Co., 62 Whittemore Avenue, Cambridge, Massachusetts 02140
We report here the development of a method for the chemical digestion of chrysotile asbestos in asbestoscontaining fireproofing to levels lower than the regulatory threshold. The resulting fireproofing, no longer defined as asbestos-containing, can remain in place with properties intact. In the process, chrysotile fibers are digested without generating excessive gaseous byproducts, and the foam-based delivery system essentially eliminates release of airborne fibers. New X-ray diffraction methods quantified chrysotile levels with far greater precision than standard optical microscopic methods. Full-scale field testing confirmed the laboratory phase of the project. Fire testing of the treated fireproofing showed that the treated material functions as well as the original fireproofing.
Introduction Asbestos (Greek for inextinguishable) is a commercial term that is applied to six naturally occurring hydrated silicates that crystallize as fibers (1). The unique properties of asbestos, especially its resistance to high temperatures, are well-known since antiquity (2-4). During World War II, asbestos was classified as a “strategic material” and was used to insulate boiler and piping systems of ships and was also sprayed on inside walls. After the war, asbestos-containing products were sprayed on structural steel for fire protection in high-rise and other buildings. The U.S. Environmental Protection Agency (EPA) (5, 6) has reported that at the height of asbestos use some 3000 products were manufactured. Worldwide consumption of asbestos for all uses in 1976 was 4.8 million tons (7). There are two groups of asbestos minerals: serpentines, with chrysotile (Mg6[(OH)4Si2O5]2) being the most common and accounting for almost 90% of the worldwide asbestos production, and amphiboles, which include amosite, actinolite, anthopholite, crocidolite, and tremolite (2, 3, 8). These minerals share the ability to form high aspect ratio (length/ diameter) fibers. * Corresponding author e-mail:
[email protected]; phone: (410)531-4188; fax: (410)531-4068. † W. R. Grace & Co., Columbia. ‡ Brookhaven National Laboratory. § W. R. Grace & Co., Cambridge. | Present address: Elf Atochem North America, Inc., 900 First Avenue, King of Prussia, PA 19406. 10.1021/es990432d CCC: $19.00 Published on Web 04/27/2000
2000 American Chemical Society
U.S. regulatory agencies define “asbestos fibers” as serpentines or amphiboles having aspect ratios greater than or equal to 3 and lengths greater than 5 µm; “asbestoscontaining material” (ACM) is any material “containing more than 1% asbestos” (9, 10). The U.S. Government has regulated asbestos because inhalation of large quantities of asbestos fibers over extended periods has been implicated in increasing the risk of lung disease (11-13). Indeed for many years the focus of regulatory agencies was that of worker exposure. However, in the 1980s there was an increasing concern about exposure of occupants in schools as well as commercial and residential structures. In 1973, the EPA banned the spray application of asbestoscontaining fireproofing (National Emissions Standard for Hazardous Air Pollutants, NESHAP, Asbestos Standards) (9). In 1986, the U.S. Congress enacted the Asbestos Hazard Emergency Response Act (AHERA), which required the EPA to regulate ACMs in school buildings. Compliance with AHERA requirements was estimated to cost school districts $3 billion. The legislation also required the EPA to estimate the amount of ACM in other public and commercial buildings (5, 14). In 1988, the EPA reported to Congress that there were 750 000 public and commercial buildings in the United States that contained ACMs and that the abatement effort could cost $50 billion. Other estimates have put the potential cost for asbestos removal in the United States as high as $100150 billion (15, 16). In the United States, an asbestos abatement industry with about $3 billion in annual revenue has arisen. Current practices for asbestos abatement present several options to building owners. The first, at lowest cost, is “management in place”, involving monitoring the ACM and establishing work practices that minimize exposure of building occupants to asbestos. A second option is encapsulation of the ACM. The third, and most costly option, is complete asbestos abatement. Current removal practice, as specified by the Occupational Safety and Health Administration (OSHA), requires erection of a negative-pressure enclosure to completely seal the work area, wetting the ACM, scraping the ACM from the substrate, and disposing the ACM as a regulated waste. The negativepressure enclosure is then cleaned and disposed of as a regulated asbestos waste. Finally, new nonasbestos insulation is applied to replace the removed material. Worker protective equipment is designed to maintain airborne asbestos fiber exposures below the OSHA permissible exposure limit (PEL) of 0.1 fiber/mL of air on an 8-h time-weighted average basis. Much of the asbestos-containing fireproofing in the United States is a composite of gypsum (CaSO4‚2H2O), vermiculite (a hydrated magnesium-aluminum-iron silicate), and chrysotile asbestos, typically 63, 25, and 12 wt %, respectively. This is the composition used in all our experiments. Chrysotile asbestos is considered relatively nonreactive chemically, although a number of approaches for chemically eliminating chrysotile have been reported (17-22). These include use of sulfuric acid (17) or other strong acids to dissolve the magnesium oxide (8), a combination of a strong acid and fluoride ions (HF) (18-21), and complete fluorination of the chrysotile (22). The effectiveness of fluorosulfonic acid to decompose chrysotile was recently demonstrated (23). In the attack of chrysotile by strong acid (8), temperatures exceeding 50 °C are used over long periods of time to completely digest the asbestos. This is necessitated by chrysotile’s crystal structure (which consists of alternating layers of magnesia and silica) and by the incomplete removal of magnesium from the structure. Mass transport of reactants VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and reaction products is likely to be severely limited within the narrow channels formed between the acid-resistant silica layers when the magnesia is removed by an acid, leading to incomplete removal. As an additional consequence of the poor reactivity of silica with acids, the removal of the Mgrich layers can leave behind a silica-rich structure that appears fibrous under the microscope. A combination of HF and strong acids can completely dissolve chrysotile by removing both MgO and SiO2 (23, 24). However, the use of large quantities of HF is hazardous and is therefore not practical. The compositions of Mirick et al. (18-21) combine an organic acid and a fluoride salt, thereby avoiding the direct use of HF in the formulation. Since the fluoride salts proposed by Mirick et al. all dissociate very rapidly under acidic conditions, our experience suggests that they produce significant amounts of HF in both the liquid and gas phases. Recent patents to Block (25-29), Block et al. (30), and Hartman (31) describe a process using inorganic acids and “fluoride generators”. It is this process that is described in this paper.
Chemical Digestion of Chrysotile in Gypsum-Vermiculite Fireproofing Chemistry. In developing the subject process, the two main objectives were to (i) safely and economically destroy chrysotile in gypsum-vermiculite fireproofing, with the reaction product being nonfibrous; and (ii) have the capability of leaving the treated fireproofing in place so that it would continue to function as fireproofing. These objectives presented two technical challenges. First, chemistry would need to be developed that would be effective in digesting chrysotile fibers without generating large quantities of HF or other substances of concern. Second, the compounds used to destroy the chrysotile would need to be selected such that good thermal and mechanical properties would be maintained in the treated fireproofing, so the latter can remain fully functional. The system developed has two components: phosphoric acid and a fluoride salt that hydrolyzes very slowly under acidic conditions. The acid was selected to provide optimum removal of the magnesium component of the chrysotile and to generate reaction products that actually improve the mechanical properties of the fireproofing. In this way, elimination of asbestos from the matrix does not affect fireproofing properties. It was also critical that the acid be selective for chrysotile relative to other components of the fireproofing. The fluoride compound was selected so as to generate fluoride slowly at a rate sufficient for the reaction with chrysotile silica to proceed, but not so rapidly that significant free HF is generated. The fluorides found to be most effective were the soluble fluorosilicate or fluoroborate salts. Experiments were conducted on gypsum-based, sprayapplied, fireproofing, a common commercial product that typically contains 60-65% gypsum as a binder and 23-27% vermiculite as a lightweight filler. The remainder, nominally 12% by mass, is chrysotile asbestos. The phosphoric acid was found to successfully remove magnesium without attacking the gypsum and vermiculite in the fireproofing. The chemical reaction between the phosphoric acid and the chrysotile generates magnesium phosphate and other phosphate salts as reaction products; these serve to reinforce the gypsum binder, thus distinguishing phosphoric acid from the other acids tried. The result is greatly improved mechanical properties in the fireproofing. Phosphoric acid only attacks the magnesium-rich layers in the chrysotile structure, leaving the silica-rich layers largely untouched. In choosing the fluoride source needed to 2294
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FIGURE 1. Scanning electron microscope images of untreated ACM fireproofing containing 12% chrysotile asbestos. overcome this limitation, it was found that small quantities of salts were effective in chemically eliminating chrysotile with extremely low HF emissions. Figures 1 and 2, which are scanning electron microscopy (SEM) images of the fireproofing (cubes) before and after treatment, show that the reaction products are no longer fibrous due to the complete reaction caused by joint action by the acid and the fluoride generator. X-ray diffraction analysis (XRD) confirmed the absence of chrysotile asbestos (see Figure 3). The formulation works according to the following proposed mechanism. Phosphoric acid removes the magnesium from chrysotile to produce magnesium hydrogen phosphate, silica, and water:
3MgO‚2SiO2‚2H2O + 3H3PO4 ) 3MgHPO4 + 2SiO2 + 5H2O However, the reaction does not go to completion. The fluorosilicate or fluoroborate component slowly hydrolyzes to produce fluoride ions
SiF62- + 2H2O ) 6F- + SiO2 + 4H+ or
BF4- + 3H2O ) 4F- + BO33- + 6H+ The fluoride ions can then react with the silica of the MgOdepleted chrysotile to form soluble hexafluorosilicate anions:
SiO2 + 6F- + 4H+ ) SiF62- + 2H2O The hexafluorosilicate anions will also hydrolyze to produce more fluoride and amorphous silica:
SiF62- + H2O ) F- + SiO2(amorphous)
TABLE 1. Product Composition following Application of Digestion Agenta component
wt %
CaSO4‚2H2O, Ca(H2PO4)2‚H2O, CaHPO4‚2H2O MgHPO4‚‚7H2O vermiculite CaF2, AlPO4, Fe2O3 amorphous SiO2 chrysotile
55-65 15-25 10-20
