WO1994008661A1

WO1994008661A1 - Product and method for treating asbestos

Info

Publication number: WO1994008661A1
Application number: PCT/US1993/009682
Authority: WO
Inventors: Alan J. Kenyon
Original assignee: Cross-Link Technology
Priority date: 1992-10-08
Filing date: 1993-10-06
Publication date: 1994-04-28
Also published as: AU5356494A

Abstract

A method for eliminating the hazardous properties of asbestos waste is provided. The waste is treated with alkali and heat to crosslink the asbestos fibers and form a nontoxic, amorphous, silica aggregate. An apparatus for carrying out the method is also provided.

Description

PRODUCT AND METHOD FOR TREATING ASBESTOS

The present invention relates generally to the safe disposal of asbestos waste, that is, asbestos and asbestos-containing waste materials. In particular, the invention provides a method and apparatus for eliminating the hazardous properties of asbestos waste by altering the morphology of asbestos fibers.

BACKGROUND OF THE INVENTION

Disposal of waste material is one of the major problems facing modern society. The problem is particularly acute with regard to asbestos waste, not only because asbestos is toxic to man and animals but also because this material has been in wide use for many years and the resulting volume of asbestos waste being generated is substantial.

It is a common practice to landfill asbestos waste even though parties who generate such waste remain responsible for its deposition in landfills. Thus, to reduce the liabilities attendant on this method of disposal, the abatement industry must operate in compliance with the expensive and labor intensive disposal procedures set forth in the Comprehensive Environmental Response Compensation and Liability Act.

As an alternative to the direct disposal of asbestos waste in landfills, a high temperature vitrification process has been developed. While such a process converts asbestos to a form which is apparently harmless, this procedure requires large amounts of energy and is economically not practical. Other asbestos treatments are known, but do not produce materials that are suitable for waste disposal. For example, Charleton, U.S. Patent No. 1,256,296 discloses the treatment of "teased" asbestos fibers with magnesia or dolomitic lime at an elevated temperatures and superatmospheric pressure, to produce fibers which retain their original appearance but have a greater tendency to mat. This product has applications similar to those of ordinary asbestos, and is described as a superior packing or insulation material. Thus, Charleton is concerned with asbestos use, not asbestos removal. Charleton preserves the fibrous nature of asbestos, he does not destroy it.

Jackson, U.S. Patent No. 1,254,230 discloses that naturally-occurring chrysolite and other silicates can be decomposed into acid-soluble alkaline and magnesium silicate materials by treatment with a large excess of caustic alkali at temperatures of about 350 °C to about 600 °C and higher, for a period of about 2 hours. According to Jackson, these temperatures require the use of caustic alkali, not carbonates, and a large excess of alkali of more than 2:1, preferably 4:1 is essential. The silicates produced by this process are extracted with water, filtered, and dissolved in acid to produce a useful soluble magnesium salt, such as magnesium sulfate. This process and product is far removed from the treatment of asbestos to form a non-fibrous material for waste removal purposes.

U.S. Patent No. 3,94,184 of Harada et al. discloses a method for converting chrysotile asbestos to a composition rich in magnesium hydroxide, by repeated treatments in a pressure chamber with a 20 to 60% solution of potassium hydroxide at 150 to 200 ^*>C for 20 hours per treatment. The treated asbestos is useful as a potassium hydroxide resistant mat material for a fuel cell matrix in a hydrogen-oxygen fuel cell. In particular, Harada teaches that the morphology of the asbestos reaction can be maintained fibrous, so that the desirable flexibility and capillarity for a fuel cell matrix mat is maintained. The final material is composed of about 80 to 90 percent magnesium hydroxide fibers by weight, and the remainder consists of untreated asbestos fibers. The magnesium hydroxide fibers, which are crystalline, retain the morphology of the original asbestos fibers. Harada also teaches that more severe reaction conditions result in a less fibrous and desirable product.

Other surface modification of asbestine minerals have also attracted attention in prior studies. These modifications consist of: coating the surface with phosphate, polyphosphate, magnesium carbonate or oxides of polyvalent metals. For example, Flowers, U.S. Patent No. 4,328,197 discloses a metal-micelle polymer coating for asbestos which is said to reduce the toxicity of asbestos materials. Some procedures have consisted of coating fibers with organic detergent to gain fiber dispersion. These methods, though interesting, are relatively costly and complex, and have not achieved meaningful commercial recognition. Accordingly, it is an object of the present invention to provide a low energy, economical method wherein the hazardous properties of asbestos are eliminated.

It is a further object of the invention to provide a method wherein asbestos waste is rendered non-toxic and may be disposed of in landfills with a lower classification of waste (e.g. as designated in certain areas as a Type II waste) .

It is a further object of the invention to provide an apparatus for carrying out such a method.

SUMMARY OF THE INVENTION

The present invention meets these and other objectives by providing a method for eliminating the hazardous properties of asbestos waste by altering the morphology of asbestos fibers. The method includes the steps of reacting asbestos waste with alkali at a concentration of about 10M and a temperature of about 300°C. This produces a melding of fibers to form amorphous silica aggregates. The invention further provides an apparatus for carrying out this process. The apparatus includes a reactor in which asbestos waste is treated with alkali at relatively low temperatures and means for loading and recirculating alkali used in the process.

BRIEF DESCRIPTION OF FIGURES

Figure 1 depicts the effect of fiber density by comparing pelletized and non-pelletized reactions with NaOH

(1-20M) at 200°C, for 30 minutes. The control represents values where no NaOH was present in the reaction mixture.

The values are expressed as mean Fiber Counts Per

Photographic Field (Federal Register Vol. 52, No. 210, October 30, 1987; Counting Guidelines Used in Determining

Asbestos Structures) .

Figure 2 shows the effect of various temperatures on the chemical alteration of chrysotile. This graph demonstrates that temperatures above 200°C and molar concentrations of NaOH above 1M were effective in the molecular alteration of fiber structures. The plateau of values at 50° to 100°C indicates that there may be fibers varying in resistance to the conditions used. This was also true for crocidolite and amosite asbestos at temperatures of 50° to 100°C.

Figure 3 depicts the effect of temperature and molar concentration of NaOH on crocidolite.

Figure 4 depicts the effect of temperature and molar concentration of NaOH on amosite. Figure 5 shows that asbestos does not leach-out or reform once it has been chemically altered by NaOH and mild heat (200°C) for 30 minutes. Figures 1 to 4 show a precipitous drop in fiber counts even at 1M NaOH. Figure 5 shows that, even at 68 days, there was no conversion back to the fibrous structures .

Figure 6 shows transmission electron microscope (TEM) images of untreated chrysotile asbestos fibers. Figure 7 shows amorphous granules resulting from treatment of chrysotile fibers with 10M NaOH at 300°C for 30 minutes. No detectable fibers are present.

Figure 8 shows the Selected Area Electron Diffraction (SAED) pattern in which a crystal image is used to define the presence of asbestos (e.g. in a sample such as that used in Fig. 6) .

Figure 9 shows the lack of any SAED crystalline structure in the NaOH treated sample (from Fig. 7) .

Figure 10 compares three different alkalis, NaOH, KOH, and NH₃0H which are used under standard conditions of 300°C at 30 minutes. The molar concentration of each was 1, 5 and 10M. It was apparent that NaOH is superior to KOH and NH₃0H under these conditions. Ammonium hydroxide was without detectable effect. Figure 11 shows resulting asbestos aggregation on refractory (firebrick) surfaces. The presence of other masonry materials found in abatement work were shown not to interfere with the chemical effect of NaOH and mild heat on asbestos fiber aggregation. Figure 12 shows the rate of aggregation of chrysotile fibers at 300°C with 10M NaOH.

Figure 13 shows the rate of aggregation of asbestos fibers taken from steam pipe insulation.

Figure 14 depicts the amount of NaOH required to aggregate 80 grams of damp (amended water) steam pipe insulation.

Figure 15 illustrates the effect of materials used in abatement work on the rate of aggregation. Figure 16 is a bar graph showing that chrysotile,. crocidolite and amosite are aggregated extensively at 10M NaOH at 300°C.

Figure 17 shows in schematic representation a reactor for chemically cross-linking asbestos fibers.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described with reference to several examples. It will be recognized, however, that the invention is not limited to any particular examples or embodiments and may be practiced broadly by persons of ordinary skill in the pertinent art or arts.

Chemical Composition & Characteristics of Asbestos

The term asbestos is generally applied to a group of naturally occurring fibrous silicate minerals that are commercially important due to their fibrous characteristics. There are three principal types of asbestos which have appeared in world-wide commerce. These are chrysotile from the serpentine group and crocidolite and amosite from the amphibole group. Of these, chrysotile accounts for 95% of the world's asbestos production. Chrysotile is a hydrated magnesium silicate with the general formula Mg₃Si₂0₃(0H-)₄. Structurally, chrysotile consists of sheets rolled up and formed from two layers. The first layer is a continuous matrix of silica (Si0₂) tetrahedra. This layer is bound through oxygen atoms held in common with a second layer ot Mg(0H)₂ octahedral. The shells or walls of asbestos fibers are composed of a number of individual sheets contorted into scrolls with magnesium hydroxide layers on the outside.

It has now been discovered that the use of mild alkali and relatively low temperatures will render asbestos harmless, without resorting to complex coating procedures or high-energy disposal means. Moreover, it has been discovered that this treatment changes the fiber morphology to a non-crystalline amorphous structure.

All forms of asbestos break down into simpler components when heated to temperatures of 1,000°C or higher. The reactivity of asbestos to some acids and alkalis has been studied, and all varieties of asbestos will resist prolonged attack by strong alkali (e.g. 5M NaOH) to temperatures at least as high as 100°C. Surprisingly,it has now been discovered that asbestos fibers treated with a combination of relatively moderate alkali and temperature conditions will undergo substantial cross-linking or aggregation. The morphology of these fibers is permanently altered by this treatment and an insoluble, amorphous, silicate aggregate results.

Treatment for Waste Disposal

This method of cross-linking asbestos fibers was tested with asbestos (chrysotile, crocidolite, and amosite) contained in asbestos waste. This included materials arising from abatement work, such as Tyvek suits, cotton towels, disposable gloves, polybags, fiberglass, polyethylene sheets, wood, concrete, refractory from boilers, and metal bands. The data presented below shows that an alkaline reaction conducted at relatively low temperatures (100°C - 300°C) is able to render asbestos into amorphous silicates even when the asbestos forms only a part of such waste materials.

All laboratory analyses were conducted by licensed personnel in a rigid plastic glove box with exhaust through a HEPA filter at 600 liters per minute and then expelled through the roof via an 8 inch diameter duct (fiber counts in the exhaust were less than 0.0045 f/cc) .

According to the present invention, asbestos waste was treated with alkali at temperatures ranging from 50°C to about 400°C with the optimal temperature being about 300°C with respect to economy. Preferably, the alkali is a univalent base such as NaOH or KOH, but mixtures may also be used. It will be understood, however, that the invention is no way limited to this regard and that a broad range of both organic and inorganic bases may be employed. The alkali was present in concentrations ranging from about 1M to 20M, most preferably 10M for economy reasons. The reaction is allowed to proceed for periods of up to 60 minutes with the preferred duration being about 20 to about 30 minutes depending on the concentrations of alkali used.

Analysis of Treated Waste

As seen in Figure 1, the amount of asbestos, whether concentrated in a pellet or as a loose suspension, did not affect the rate of reaction. A marked reduction in fiber count is observed as the result of these treatments. The rate of aggregation as a function of temperature and concentration of NaOH was determined for chrysotile (Figure 2) , crocidolite (Figure 3) , and amosite (Figure 4) . It is apparent that there is a stoichiometric relationship between temperatures and concentrations of NaOH. The data shown in Figure 5 indicates that once the aggregates are formed they are not dissolved or returned to original form.

Figures 6 through 9 were generated by transmission electron microscopy (TEM) and together illustrate the dramatically altered morphology of asbestos fibers after treatment according to the invention. The highly developed fibrous morphology of chrysotile asbestos illustrated in Figure 6 is typical of all forms of asbestos prior to treatment with alkali and heat. Transmission electron microscopy was also used to generate the data shown in Figure 7, which shows that the asbestos morphology has clearly been altered after treatment at 300°C with 10M NaOH. The fibers have cross-linked to form an amorphous silica aggregate which is granular and can no longer be characterized as fibrous or as asbestos.

Molecular changes associated with altered morphology have shown that treated asbestos will no longer polarize light, has an altered X-ray spectra of Mg, Fe and Si, and has no crystalline structure when examined by electron diffraction. Tests with crocidolite and amosite asbestos produced similar results.

The data presented in these figures clearly indicates a change from a fibrous to non-fibrous morphology.

X-ray analysis further revealed a shift in atomic ratio of

(Mg + Fe) Si, as shown in Table 1 for treatment with NaOH or

Hcl.

TABLE 1 X-RAY ANALYSIS SPECTRA

Treatment with NaOH as above caused a shift in Mg and Fe ions for all asbestos tested.

As stated above, a broad range of both organic and inorganic bases may be employed in the present invention. Figure 10 illustrates a comparison of three different alkalis, NaOH, KOH,a and NH₃0H, all of which were used an 300° for 30 minutes. The molar concentrations of each was 1, 5, and 10M. It is apparent from Figure 10, that while KOH is entirely suitable for the present invention, NaOH achieved more extensive aggregation of the fibers under these particular reaction conditions. Ammonium hydroxide produced only a minor degree of cross-linking, and it is believed that the ammonium ions may have volatilized off during the early phase of heating. Accordingly, volatile bases such as NH₃0H are probably not as suitable for the present invention. Hydrochloric acid did not produce any of the molecular changes seen with NaOH (change in fiber morphology) .

The effect of additional materials mixed with asbestos on the present invention is further illustrated in Figure 11. The sample used in Figure 11 was taken from an asbestos-containing refractory surface. The presence of masonry materials contained in the sample was shown not to interfere with the chemical effect of alkali (e.g. NaOH) and heat on fiber aggregation. It has been found that the cross-linking reaction proceeds more efficiently if the bulk size of the asbestos waste is reduced prior to treatment. Reducing the bulk size of the waste by, for example, grinding, crushing or shredding increases the surface area of the waste in contact with the alkali at any given point in the reaction.

Figure 12 shows the rate of aggregation of chrysotile fibers at 300°C with 10M NaOH. Under these conditions, the asbestos fibers are eliminated in three stages: about 10 percent or less of the fibers react during the first 10 minutes or so of treatment, about 60 percent of the fibers are rapidly eliminated during the second 10 minutes, and the remaining 30 percent or so were more slowly eliminated in about the last ten minutes. No residual fibers were present after 30 minutes.

Asbestos fibers taken from steam pipe insulation can be aggregated much more rapidly under similar conditions. As shown in Figure 13, all of the fibers were aggregated by 20 minutes when heated at 300°C, and all but about 5 percent or less were eliminated in the first ten minutes. The lower starting level of 64% (compared to Figure 12) was due to samples containing other particulate material. In general, the presence of other particulate material is indicated by a lower starting point or control value, but this does not alter the rate of aggregation.

Asbestos fibers may be wet or damp,particularly when taken from steam pipe insulation. Figure 14 depicts the amount of NaOH required to aggregate 80 grams of damp

(amended water) steam pipe insulation. As shown, equal weights of damp asbestos and NaOH eliminated all residual fibers.

Other materials may be present when asbestos material is removed from a contaminated site, and the abatement process itself may add materials to the asbestos waste. The affect of various materials used in abatement work on the rate of asbestos aggregation according to the invention was evaluated. These materials had no affect on aggregation or on the new method of aggregating asbestos fibers when equal weights of NAOH were used. The tested materials were Tyvek, poly bags, sweeping compound, and HEPA filters. The results are shown in Figure 15.

The preferred working conditions, according to the invention, are 10M NaOH at 300°C. These conditions produce rapid, efficient, cost-effective aggregation of potentially harmful asbestos fibers into harmless melded aggregates. As shown in Figure 16, these conditions are suitable for chrysotile, crocidolite and amosite asbestos.

Apparatus for Processing Asbestos Waste

An apparatus according to the invention is shown in Figure 17. The apparatus includes a stack (1), an after burner (2), a scrubber (3), a reaction chamber (4), an alkaline charge (5) , and a two-stage ram (6) . In operation, asbestos containing waste is introduced to a hopper (7) and mixed with an aqueous solution of alkali. The contents of hopper (7) are discharged by ram (6) into the reaction chamber (4) after compaction. The ram or piston (6) is then withdrawn to accept the next charge of waste. The asbestos containing material is aggregated within chamber (4) by heat from an external source (not shown) in the presence of alkali. The vapor phase generated by this process is condensed in scrubber (3) and the liquid phase is recycled to hopper (7) through water return (8) . Other gases are expelled through stack (1) and are eliminated by after burner (2) . The alkaline solution is recycled to hopper (7) by alkaline return (9) . The treated waste is removed via discharge chamber (10) , and may be washed to remove and recirculate additional alkali via alkaline return (9) . In practice, the solid and gas materials discharged from the apparatus (i,e at stack (1) and chamber (10)) are monitored by TEM to establish and ensure contaminate-free operation.

The asbestos containing material treated in this apparatus becomes aggregated and amorphous; it no longer has the properties of asbestos waste. This detoxified material can safely be brought to landfills or used for other purposes.

Toxicity Studies

To establish that asbestos treated with alkali (e.g. NaOH) and mild heat has been stripped of its toxic properties, three animal studies were conducted. Two of these studies consisted of intradermal implants (200 S/D rats) of filters containing asbestos treated with NaOH and appropriate controls or intradermal injections (10ml) of treated asbestos and controls. The third study consisted of intraperitoneal injection of treated asbestos in CD-I mice (25-30g, SPF) . In all cases analyses were made by preparing histologic sections (H+E, 400x) . Table 2 shows that rats inoculated with treated asbestos were free of typical asbestos-induced lesions.

TABLE 2

Mice inoculated with treated asbestos or untreated control asbestos were euthanized at intervals of up to 48 hours after injection. These animals were scored for the presence or absence of histocytes in the splenic follicles or in the thymus-dependent areas surrounding the central arterioles. Untreated asbestos produced large accumulations of histocytes (germinal centers) in both the marginal and thymus-dependent areas, while the treated asbestos produced no distinct response. The treated asbestos inocula produced no increase in follicle or germinal centers. This procedure is described in Kenyon, A.J. Comparison of early splenic changes associated with virus replication in murine monocytes, Am. J. Vet. Res. 45:1054 (1984).

Claims

CLAIMS :

1. A method for treating asbestos waste comprising reacting asbestos waste at temperatures below about 400 °C with an alkali solution having a molar concentration effective to convert asbestos fibers into nontoxic amorphous silica aggregates.

2. The method of claim 1 wherein the temperature is in the range of about 100°C to about 400°C.

3. The method of claim 1 wherein the temperature is about 300 -C.

4. The method of claim 1 wherein the alkali is a univalent inorganic base.

5. The method of claim 1 wherein the alkali is selected from the group consisting of NaOH, KOH and mixtures thereof.

6. The method of claim 1 wherein the alkali is provided as an aqueous solution having a concentration ranging from about 1M to about 20M.

7. The method of claim 1 wherein the concentration of the alkali in the solution is about 10M.

8. The method of claim 1 wherein the step of reacting is preceded by the step of reducing the particle size of the asbestos waste to increase its surface area.

9. The method of claim 1, wherein the reaction is conducted for a period of time ranging from about ten minutes to about 1 hour.

10. A method for treating asbestos waste comprising reacting the waste at about 100 to 400 °C with an alkali solution having at least one of NaOH and KOH in a molar concentration of about 1 to 20M, for a period of time ranging from about ten minutes to about 1 hour.

11. Nontoxic amorphous silica aggregates comprising the reaction product of asbestos fibers and an alkali.

12. Nontoxic amorphous silica aggregates comprising the reaction product of asbestos fibers and an aqueous alkali solution, at temperature of about 100 to about 400 "C.

13. The aggregates of claim 11, wherein the alkali is at least one of NaOH and KOH in a concentration of about 1M to 20M.

14. The aggregates of claim 12, wherein the alkali is at least one of NaOH and KOH in a concentration of about 1M to 20M.

15. The aggregates of claim 14, wherein the reaction product is obtained after a period of time ranging from about 10 minutes to about 1 hour.