Method for encapsulating arsenic-contaminated wastes

BACKGROUND OF THE INVENTION
The present invention relates to the disposal of arsenic-contaminated waste material and more particularly relates to a method for encapsulating arsenic-contaminated waste material whereby it can be used as a permanent landfill material without the risk of the arsenic compounds later being leached into contacting ground waters.
In many known commercial processes, a waste material is generated which contains substantial amounts of arsenic and/or arsenic compounds. For example, synthetic hydrocarbons obtained from coal, oil shale, tar sands, or the like, normally contain substantial quantities of arsenic and/or arsenic compounds which have to be removed before the hydrocabons can be marketed. One process for treating such hydrocarbons is disclosed and fully described in U.S. Pat. No. 4,051,022, issued Sept. 27, 1977, and involves contacting the hydrocarbons with a guard bed material comprising a metal or combination of metals (e.g. iron, cobalt, nickel) and the oxides or preferably sulfides of said metals. The metal or metal compounds can be used by themselves or can be combined with a conventional support (carrier) e.g. silica, as understood in the art. Upon contact under prescribed conditions, the guard bed material removes and retains the arsenic from the feed hydrocarbons.
When the guard bed material becomes spent, a problem arises in properly disposing of same. The spent material cannot be used directly as a landfill material since it has been found that the arsenic and/or arsenic compounds will readily leach from the spent waste material when contacted by ground waters associated by the landfill site. Since this is ecologically unsound, a disposal process is necessitated which will prevent any substantial leaching of the arsenic from the spent material once it has been used as a landfill material.
Known disposal processes for waste materials of the type described above involve encapsulating the waste material into a concrete substrate and using the finished concrete as a landfill material. For example, both U.S. Pat. Nos. 3,980,558 and 4,142,192 disclose processes wherein arsenic contaminated wastes are mixed with cement and various additives to form a concrete landfill material. However, both references teach that Portland cement is the preferred cement for use in the disclosed processes.
Unfortunately, spent guard bed material of the type discussed above contains sulfur or sulfur compounds, in addition to arsenic. This is due to the original composition of the guard bed and/or from sulfur compounds, e.g. H.sub.2 S, which may be collected by the guard bed material from the hydrocarbon feed stock during treatment. Portland cement contains substantial amounts of tricalcium aluminate which react with the sulfur compounds, i.e. sulfates, in the spent guard bed materials to form ettringite which, in turn, expands upon as it forms and this causes the concrete to crack and disintegrate thereby effectively destroying the structural integrity of the concrete. Since ettringite may take a relatively long time (e.g. one year) to form, a concrete mass which appears structurally sound at the time of disposal may effectively disintegrate at the end of a prolonged period thereby allowing the arsenic encapsulated in the concrete to become exposed to leaching upon contact with water flowing through the disintegrated concrete. Further, if the ground water itself, (e.g. water containing gypsum) contains sulfur compounds, it may react with the tricalcium aluminate of the Portland cement with the same results.
Accordingly, where the encapsulated waste material contains sulfur and/or sulfur compounds or is likely to be in contact with waters which contain sulfur or sulfur compounds, Portland cement appears to be undesirable as an encapsulating material especially where the encapsulated material is used as a permanent landfill material which is likely to come into contact with ground waters.

SUMMARY OF THE INVENTION
The present invention provides a method for encapsulating an arsenic-contaminated waste material such as spent guard bed material in concrete whereby the final concrete products maintains its structural integrity even after prolonged periods of exposure to sulfates.
More specifically, the arsenic-contaminated material is mixed with a non-Portland, tricalcium aluminate-free hydraulic cement and water to form a mixture having a wet concrete consistency. Aggregate such as sand may be added to add strength to the mixture. The mixture is allowed to harden to form a concrete product which is highly sulfate and acid (to pH4) resistant. As used throughout, tricalcium aluminate-free hydraulic cement refers to those hydraulic cements which do not contain tricalcium aluminate as a constituent or, if any is present, tricalcium aluminate appears as an impurity in the cement in amounts so small they are considered negligible. An example of a tricalcium aluminate-free cement is "aluminous cement."
Where the concrete mixture would require prolonged hardening times, an accelerator chemical, e.g. iron chloride, is added to the concrete mixture to speed its hardening. Further, in some instances, silica flour is used as part of the added aggregate to increase the density of the final concrete product. Likewise, the concrete mixture may be cured or hardened in the presence of steam and/or vibrated during curing to further increase the density of the concrete product. The actual operation and apparent advantages of the present invention will be better understood by referring to the following detailed description.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In accordance with the present invention, an arsenic-contaminated waste material is encapsulated in a concrete mixture formed by mixing the waste material with an aggregate, water and tricalcium aluminate-free hydraulic cement. Where, the curing time is prolonged, an accelerator is added to speed up the hardening time of the concrete mixture. The concrete mixture is allowed to harden into blocks, clinkers, or the like and is then disposed of as permanent landfill material or used for a similar purpose.
In the present invention, the arsenic-contaminated waste material is primarily a solid, granular material having arsenic or arsenic compounds contained therein such as the spent guard bed material used to remove arsenic from synthetic hydrocarbons such as described in U.S. Pat. No. 4,051,022. Also, as used herein, "arsenic" and "arsenic compounds" are used interchangeably and are intended to include arsenic in whatever form, elemental or combined, that may be present in the waste material.
To better understand the improvements provided by and the apparent advantages of the present invention, brief reference is made to the known prior art wherein it is disclosed that arsenic-contaminated wastes may be encapsultated in a concrete mixture prior to disposal. In U.S. Pat. No. 3,980,558, a technique is disclosed for disposing of a liquid, arsenic-contaminated waste by encapsulating it in a concrete mixture formed by mixing the material with a cement and various additives while U.S. Pat. No. 4,142,912 discloses a similar technique for disposing of a solid, arsenic-contaminated waste. It is noted, however, that in both of these references, it is clearly taught that the cement to be used is Portland cement or mixtures containing Portland cement. As will be shown from the following discussion, the present invention requires the use of non-Portland cement, that is, a cement which is free of tricalcium aluminate.
To fully understand the differences between processes for encapsulating arsenic-contaminated wastes which use Portland cement and one which uses a non-Portland cement, the following experimental data and discussion is now set forth. Two experiments were conducted to determine the characteristics of durability and setting a concrete mixture formed by mixing arsenic compounds with Portland cement.
In the first experiment, arsenic trioxide (As.sub.2 0.sub.3), calcium arsenate (Ca.sub.3 (As0.sub.4).sub.2), and arsenic trisulfide (As.sub.2 S.sub.3) were each blended with Portland cement and water to an arsenic concentration equivalent to 2.5 percent As.sub.2 S.sub.3 in each mixture. The water/cement ration was kept at 0.45. Each mix was poured into 2" cubes and allowed to set under water for 24 hours before the forms were taken off. Each cube was rinsed and soaked in 250 ml of water for one month. The leachate from each cube contained less than 1 ppm of arsenic. The cubes were dried and placed in a drawer for storage. One year later the cubes were moved to a new location and the cube containing As.sub.2 S.sub.3 was cracked and had lost structural integrity, thereby exposing a black interior. It was concluded that the cube had lost its strength. This was most likely due to a reaction of As.sub.2 S.sub.3 with some components of the cement. No further work was done with the cubes.
In the second experiment, arsenic disulfide, trisulfide and spent guard bed material were incorporated into a Portland cement concrete at concentrations of 2.5 wt percent bsed on the dry concrete mixture. The setting process of these mixtures was greatly retarded.
From the experiments it was concluded that two independent reactions had taken place. Both indicated that Portland cement appeared to be unsuitable for encapsulating of arsenic containing spent guard bed material.
In order to understand the apparent destruction of Portland cement by arsenic compounds, the mechanism of hardening was then reviewed. A typical Portland cement clinker contains four major minerals as shown in Table 1.
TABLE I__________________________________________________________________________FUNCTION OF MINERALS IN PORTLAND CEMENTTYPICALCOMPOSITION__________________________________________________________________________10% C.sub.3 A CAUSES SET BUT NEEDS RETARDATION (BY GYPSUM)52% C.sub.3 S RESPONSIBLE FOR EARLY STRENGTH (AT 7-8 DAYS)21% C.sub.2 S AND C.sub.3 S RESPONSIBLE FOR FINAL STRENGTH (.about. 1 YEAR) 9% C.sub.4 AF, Fe.sub.2 O.sub.3 , Al.sub.2 O.sub.3 LOWER CLINKER TEMPERATURES, GIVE COLORMgO, alkali__________________________________________________________________________ WHEREIN:- C = CaO A = Al.sub.2 O.sub.3 S = SiO.sub.2 F = Fe.sub.2 O.sub.3
Each of the minerals has a specific function and must be present at a certain concentration to assure proper setting and hardening. Setting is the initial stiffening which occurs within a few hours. Hardening is the development of strength, which is a slower process. The reactions that cause setting and hardening are collectively referred to as hydration reactions. The mechanism of hydration is shown in Table 2. Any interference with the hydration leads to a prolonged setting time or even to complete inhibition.
TABLE 2______________________________________HYDRATION MECHANISM OF PORTLAND CEMENTEQUA-TION______________________________________1 C.sub. 3 A + 6H.sub.2 O .fwdarw. C.sub.3 A . 6H.sub.2 O CRYSTAL2 C.sub.3 S + xH.sub.2 O .fwdarw. C.sub.2 S . xH.sub.2 O AMORPH + Ca(OH).sub.2 CRYST.3 C.sub.2 S + xH.sub.2 O .fwdarw. C.sub.2 S . xH.sub.2 O AMORPH + Ca(OH).sub.2 CRYST.4 C.sub.4 AF + xH.sub.2 O .fwdarw. C.sub.3 A . 6H.sub.2 O CRYST. + CaO . Fe.sub.2 O.sub.3 (x-6)H.sub.2 OETTRINGITE FORMATION IN PORTLAND CEMENT5 C.sub.3 A + 3(CaSO.sub.4 . 2H.sub.2 O) C.sub.3 A . 3CaSO.sub.4 . 31H.sub.2 O + 25 H.sub.2 O ETTRINGITE (CEMENT BACILLUS)______________________________________ WHEREIN:- C = CaO A = Al.sub.2 O.sub.3 S = SiO.sub.2 F = Fe.sub.2 O.sub.3
Further, if during or after hardening, new hydration products with greater volume requirements are formed, disruption of the exiting concrete structure occurs. The classical example is the formation of ettringite. It is shown, Table 2, equation 5, that tricalcium aluminate (C.sub.3 A) can react with calcium sulfate (CaSO.sub.4.2H.sub.2 O) to give ettringite. Ettringite causes complete disruption of the concrete because of its expansion. It is a slow reaction and takes place when Portland cement is attacked by sulfates over prolonged periods. From the above, it is concluded that ettringite formation was the likely cause of the disintegration of the Portland cement blocks. Any sulfides present in or in contact with Portland cement can, over a long period of time, undergo oxidation to sulfates. The constant presence of calcium hydroxides leads then to the formation of calcium sulfate which will react with the tricalcium aluminate to form ettringite.
The retardation effect observed in the second experiment can be explained as follows. Most cement slurries contain small amounts of dissolved alumina. If at any time ions of arsenic, boron or phosphate enter the solution, the dissolved alumina is precipitated and forms a film around cement grains, thus retarding or completely inhibiting their reaction. Different types of retarders are known, those that effect the solubility of alumina temporarily and those that form insoluble salts that hold up setting and hardening indefinitely. Examples of the latter type are Na.sub.3 PO.sub.4, Na.sub.2 B.sub.4 O.sub.7, Nahd 3AsO.sub.4, and Ca (CH.sub.3 COO).sub.2. To counteract the effect of retarders, accelerators can be used as will be more fully discussed below. Accelerators react preferentially with the retarder, thus allowing hydration to proceed.
Since most arsenic-contaminated wastes, especially spent guard bed material, contains sulfur or sulfur compounds or since the encapsulated arsenic-concrete mixture, if used as landfill, is likely to be contacted by sulfur-containing waters, it was concluded that Portland cement because of its high content of tricalcium aluminate was unacceptable as an encapsulating material.
Now turning to the present invention again, the arsenic-contaminated material is mixed with water, aggregate, and a non-Portland hydraulic cement which is free of tricalcium aluminate to form a concrete mixture which, when hardened, may be disposed of as a permanent landfill material without risk of the arsenic subsequently leaching into groundwaters even after prolonged periods. As used throughout, tricalcium aluminate-free hydraulic cement refers to those hydraulic cements which do not contain tricalcium aluminate as a constituent or, if any is present, it appears as an impurity in the cement in amounts so small as to be negligible. The preferred tricalcium aluminate-free hydraulic cement used in the present invention is one commonly known as "aluminous cement" which is made by heating a mixture of limestone and buxite to fusion and has monocalcium aluminate as its chief component. Other examples of cements which are substantially tricalcium aluminate-free hydraulic cements are those commonly known as "slag cements" and pozzalana cements. For a more complete description of tricalcium aluminate-free hydraulic cements, see THE CHEMISTRY OF CEMENTS, Taylor, Academic Press, 1964, Vol. 1, pp. 9-10 which is incorporated herein by reference.
To further understand the difference between Portland cement and tricalcium aluminate-free hydraulic cements such as aluminous cement, a comparison of the chemical composition of Portland cement and a typical commercially available aluminous cement (Fondu Calcium Aluminate Cement, sold by Lone Star Lafarge, Inc., Norfolk, Va.) is shown in TABLE 3 while the average chemical analysis of the two are shown in TABLE 4 below.
TABLE 3______________________________________MINERAL CONSTITUENTS OF PORTLAND ANDALUMINOUS CEMENTPORTLAND CEMENT ALUMINOUS CEMENT______________________________________C.sub.3 A 3CaO . Al.sub.2 O.sub.3 --C.sub.3 S 3CaO . SiO.sub.2 CA CaO . Al.sub.2 O.sub.3C.sub.2 S 2CaO . SiO.sub.2 C.sub.11 A.sub.7 11CaO . 7Al.sub.2 O.sub.3C.sub.4 AF 4CaO . Al.sub.2 O.sub.3 . Fe.sub.2 O.sub.3 C.sub.4 AF 4CaO . Al.sub.2 O.sub.3 . Fe.sub.2 O.sub.3 -- C.sub.2 AS 2CaO . Al.sub.2 O.sub.3 . SiO.sub.2______________________________________
TABLE 4______________________________________AVERAGE ANALYSIS OF PORTLANDAND FONDU CEMENTPORTLAND CEMENT ALUMINOUS CEMENT% (FONDU) %______________________________________CaO 61-67 24-40SiO.sub.2 18.6-23.3 2-6.5Al.sub.2 O.sub.3 3.9-7.5 70-50Fe.sub.2 O.sub.3 1.5-6.2 .about.1MgO 0.6-5.2 0.6-1.0SO.sub.3 0.8-2.3 0.2-0.3______________________________________
More specifically, in carrying out the present invention, an arsenic-contaminated waste materials, e.g. spent guard bed material, is mixed with tricalcium aluminate-free hydraulic cement, an aggregate (e.g. Ottawa sand), and water to form a concrete slurry. As understood by those skilled in the art, the actual ratio between components will follow recognized American Society for Testing and Materials (ASTM) Standards used by the trade in forming high strength concretes and will vary depending on the actual mixtures involved. In actual practice, various mixtures will be made and strength tests will be carried out to arrive at the most desirable mixture for a particular waste material. Typical mixture ranges are as follows: 1 to 50 wt. percent of arsenic-contaminated material; 20 to 30 wt. percent tricalcium aluminate-free hydraulic cement; and 10 to 30 wt. percent aggregate, with the water/cement ratio being 0.37 to 0.60. Of course, the arsenic/contaminating material when in particulate form acts as an aggregate itself, and the final ratios will take this factor into consideration.
Since it has been determined that the presence of arsenic compounds (e.g. As.sub.2 S.sub.3, arsenic trisulfide) will inhibit aluminous cements, it is preferred to add an accelerator to quicken the setting time of the wet concrete mixture. The normal accelerators for aluminous cements such as CaCl.sub.2, Portland cement, and Cal-seal respectively, were found to have no apparent effect in the presence of As.sub.2 S.sub.3. However, FeCl.sub.3 and AlCl.sub.3, upon experimentation, had a good effect on the inhibiting effect of As.sub.2 S.sub.3. A series of other compounds such as sodium carbonate, sodium silicate, ferric ammonium oxalate, and sodium acetate were tested for their accelerating power but none of them showed any potential. The results of tests involving the above discussed accelerators are shown in TABLES 5 and 6 below.
TABLE 5__________________________________________________________________________ACCELERATION STUDIES OF FONDU CEMENT CONTAINING As.sub.2 S.sub.3 359-1 359-2 359-3 359-4 359-5 359-6__________________________________________________________________________FONDU CEM. g. 6.13 6.13 6.13 6.13 6.13 6.13OTTAWA SAND g. 16.87 16.87 16.87 16.87 16.87 16.87As.sub.2 S.sub.3 g. 0.70 0.70 0.70 0.70 0.70 0.70WATER ml. 3.0 3.0 3.0 3.0 3.0 3.0ACCELERATOR CaCl.sub.2 FeCl.sub.3 PORT.CMT. CAL-SEAL AlCl.sub.3 -- (4.9%) (5%) (5%) (5%)* (5%) 0.300 g 0.308 0.308 0.308 0.308WATER/CEMENTRATIO 0.49 0.49 0.49 0.49 0.49 0.49SET TIME, INITIAL >17 hours 2 hours not at all >17 hours 2 hours not at allFINAL only 24 not at all only 24 not at all partially partially__________________________________________________________________________ *CaSO.sub.4 . 1/2H.sub.2 O
TABLE 6__________________________________________________________________________ACCELERATION STUDIES OF FONDU CEMENT CONTAINING As.sub.2 S.sub.3 363-1 363-2 363-3 363-4 363-5 363-6__________________________________________________________________________FONDU CEM. g. 6.13 6.13 6.13 6.13 6.13 6.13OTTAWA SAND g. 16.87 16.87 16.87 16.87 16.87 16.87WATER ml. 3.0 3.0 3.0 3.0 3.0 3.0As.sub.2 S.sub.3 g. 0.7 0.7 0.7 0.7 none 0.7ACCELERATOR Na.sub.2 CO.sub.3 Na-SILICATE Fe. AM. OX.** Na-ACETATE none NaOH (3%) (3%) (3%) (3%) (3%) 0.184 g 0.184 g 0.184 g 0.184 g 0.184 gWATER/CEMENTRATIO 0.49 0.49 0.49 0.49 0.49 0.49SET TIME, INITIAL 24 hours 2 hours 24 hours 96 hours 2 hours 96 hoursFINAL never never never never 24 never__________________________________________________________________________ **Fe(NH.sub.4).sub.3 (C.sub.2 O.sub.4).sub.3 . 3H.sub.2 O
Further, since arsenic-contaminated material such as spent guard bed material contains not only arsenic compounds (e.g. FeAs; FeAs.sub.2 ; FeSA.sub.s) but may also contain some iron sulfides, the effects of the latter on the concrete mixture was also considered.
A spent guard bed material containing approximately 20 to 25 percent arsenic and a sample of ferrous sulfide were, therefore, tested with the results being shown in Table 7. Both acted as inhibitors especially on the initial setting time.
These results further indicate the need for a suitable accelerator. Based on the results shown in TABLE 5, it was determined that either AlCl.sub.3 or FeCl.sub.3 will effectively control the setting time of an aluminous cement-concrete. FeCl.sub.3 is preferred because of its low price and its availability.
TABLE 7__________________________________________________________________________SETTING-TIME STUDIES OF FONDU CEMENT CONTAINING FeSAND GUARD BED MATERIAL (IRON ARSENIDE) 357-1 357-2 358-1 358-2__________________________________________________________________________FONDU CEMENT g. 6.13 6.13 6.13 6.13OTTAWA SAND g. 15.61 15.61 16.87 16.87WATER ml. 3.0 3.0 3.0 3.0ADDITIVE As-GUARD -- FeS -- BED 1.26 g 1.0 gWATER/CEMENTRATIO 0.49 0.49 0.49 0.49SET TIME, INITIAL <43 hours 2-3 hours <17 hours 2-3 hoursFINAL 48 hours 24 hours 24 hours 24 hours__________________________________________________________________________
TABLE 8__________________________________________________________________________SETTING-TIME ACCELERATION WITH FeCl.sub.3 OF FONDU CEMENTCONTAINING GUARD BED MATERIAL (IRON ARSENIDE) 360-1 360-2 360-3 360-4 363-5__________________________________________________________________________OTTAWA SAND g. 14.57 14.57 14.57 14.57 16.87FONDU CEMENT g. 6.13 6.13 6.13 6.13 6.13WATER ml. 3.00 3.00 3.00 3.00 3.00As-GUARD BED g. 2.30 2.30 2.30 2.30 --ACCELERATOR none FeCl.sub.3 (1%) FeCl.sub.3 (2%) FeCl.sub.3 (4%) -- (0.062 g) (0.124 g) (0.248 g)WATER/CEMENTRATIO 0.49 0.49 0.49 0.49 0.49SET TIME, INITIAL <96 hours 24 hours 2 hours 2 hours 2 hoursFINAL .about.120 hours 48 hours 24 hours 24 hours 24 hours__________________________________________________________________________
TABLE 8 above shows the effect of 1, 2, and 4 percent of FeCl.sub.3, calculated with respect to cement, on the setting time of a concrete containing 10 percent spent guard bed material. The spent guard bed material had a loading of approximately 28 percent of arsenic (samples 360-2, 360-3, and 360-4). Sample 360-1 also contained spent guard bed material but no accelerator. It is severely inhibited and reaches a reasonable strength only after five days. In comparison, sample 365-5 contains neither spent guard bed material nor accelerator and has a setting time of one day. The amount of FeCl.sub.3 needed ranges between 1 and 2 percent.
To establish strength and durability characteristics of the arsenic-contaminated material encapsulated in a concrete formed from tricalcium aluminate-free hydraulic cement, leaching studies were carried out on concrete samples identified as numbers 360-1 through 360-4, and the results are summarized on TABLE 9. Each sample is cylindrical (33 mm diam.times.12 mm h) and weighed approximately 25 g. At an age of 7 days each sample was placed in a plastic box containing 400 ml of deionized water. In intervals of 1, 5, 12, 30, 41, 58, and 111 days, the water was analyzed for changes in pH and arsenic values. New water was then added.
TABLE 9__________________________________________________________________________LEACH-WATER ANALYSIS OF FONDU CONCRETE SAMPLES(as in parts per million, ppm) LEACHING PERIOD (DAYS)SAMPLE NO. FeCl.sub.3 1 5 12 20 30 41 58 111__________________________________________________________________________ % ppm ppm ppm ppm ppm ppm ppm ppm360-1 none 0.06 <0.02 0.02 0.04 0.02 0.098 0.08 0.028360-1 pH 9.7 9.4 8.9 8.1 8.2360-2 1 0.02 <0.02 0.11 0.11 0.31 0.227 0.18 0.018360-2 pH 9.3 9.1 8.8 8.1 8.2360-3 2 <0.02 <0.02 0.23 0.16 0.50 0.205 0.11 0.016360-3 pH 9.0 9.0 8.7 7.8 8.2360-4 4 <0.02 <0.02 0.20 0.18 0.33 0.246 0.19 0.022360-4 pH 8.9 8.9 8.6 7.8 8.1__________________________________________________________________________
The initial arsenic concentration in the leach water increased during the first 30 days and consistently dropped off after that time as shown by the 41, 58, and 111 day values.
The lowest arsenic concentration was found in the ferric chloride free sample (360-1). A total of 0.366 ppm of arsenic was leached from this sample over a period of 111 days.
A similar comparison of the FeCl.sub.3 accelerated samples shows 1.00 ppm (1% FeCl.sub.3), 1.26 ppm (2% FeCl.sub.3), and 1.21 ppm (4% FeCl.sub.3) of arsenic.
It is apparent from these results that the addition of FeCl.sub.3 enhances arsenic leaching, especially during the first 30 days. From 30 days on the amount of arsenic decreases again and reaches values even lower than that of the blank (FeCl.sub.3 -free) sample. This is particularly true after 111 days.
The effect of FeCl.sub.3 on the acidity of the leach water was clearly noticed during the first 58 days. The acidity increases, as seen in TABLE 9, going from left to right and down within the columns. After 111 days, the leach water was slightly more basic than on the 58th day. It is conceivable that during the time span of 53 days (111 minus 58 days), more base was leached from the concrete, thus neutralizing the acid which derived from FeCl.sub.3. When freshly added water was checked on the 113th day, after only two days exposure to the concrete samples, the pH value of the blank (360-1) was unchanged. However, all of the ferric chloride-containing samples were more acidic (sample 360-2, pH 7.9; 360-3, pH 7.8; 360-4, pH 7.9). These results indicate that there is still residual FeCl.sub.3 present which contributes to the acidity.
Although it may be thought that the acidity or the porosity or both are the determining factors for the amount of arsenic leach out, the samples taken on the 111th day do not support that. In fact, the arsenic levels are higher in the blank (TABLE 8, 360-1) than in the FeCl.sub.3 containing samples. There is, apparently, a leachable limit while the remainder of the arsenic is either inaccessible to the leach water or now have very little solubility. In our experiments the leachants contained from 0.02 ppm to a maximum of 0.5 ppm of arsenic.
In a further modification of the present invention, the final concrete mass formed by mixing an arsenic-contaminating material with aggregate, water and a tricalcium aluminate-free hydraulic cement is further treated by curing the concrete mixture in the presence of steam and/or vibrating the mixture as it hardens. This substantially increases the density of the final concrete product thereby increasing its durability and decreasing its porosity and permeability. Still further, the final concrete product may be made even denser by substituting silica "flour" (i.e. finely ground sand) for up to 25% of the aggregate to be used in the mixture. Even further still, where the final concrete product may be exposed to extremely severe conditions, the solid arsenic-containing material may first be coated with an organic resin before it is mixed with the cement.
To summarize, the present invention provides a method for encapsulating an arsenic-contaminated material in a concrete mixture formed by mixing the material with water, an aggregate, e.g. sand plus silica flour, and a cement which is free of tricalcium aluminate, e.g. aluminous cement. If the hardening time for the mixture is longer than desired or if compounds are present which substantially inhibit the mixture, an accelerator, e.g. FeCl.sub.3, is added to the mixture. The mixture is poured into forms and is allowed to harden, preferably applying steam and vibrating the mixture while it hardens. The final product is removed and disposed of as a permanent landfill material or put to a similar use. Since the final product is highly resistant to both sulfate attack and to acids (up to pH4), it will not disintegrate after prolonged periods of exposure and accordingly any substantial leaching of arsenic even after such prolonged periods of time is prevented.

Number	Name	Date
254530	Budlong	Mar 1982
954692	Phillips	Apr 1910
1875897	Stalhane et al.	Sep 1932
1988821	Werner et al.	Jan 1935
2028420	Stalhane	Jan 1936
3947283	Uchikawa et al.	Mar 1976
3980558	Thompson	Sep 1976
4051022	Myers et al.	Sep 1977
4124405	Quienot	Nov 1978
4132558	Uchikawa et al.	Jan 1979
4142912	Young	Mar 1979

Method for encapsulating arsenic-contaminated wastes

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Taylor, The Chemistry of Concretes, Academic Press, New York, N.Y.; 1964, vol. 1, pp. 9-24, vol. 2, pp. 24-27.
Lea and Desch, The Chemistry of Cement and Concrete, Ed., Arnold (Pubs.) Ltd., London, 1956, pp. 454-459.