Patent Number: 056407041
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides methods for immobilizing and solidifying waste materials that contain either heavy metals, radionuclides, or nitrate compounds that result in the waste being classified as either hazardous, radioactive, and/or mixed waste. These waste materials must be properly treated prior to disposal to ensure that the toxic materials are immobilized to an acceptable level. The methods of the present invention provide for the acceptable immobilization and solidification of such wastes by means of admixing a cement-based composition to the wastes to produce a cured, solidified mass that effectively immobilizes the toxic waste species. The waste materials that are contemplated as being processed by the inventive methods set forth herein are those containing waste materials that are classified as either toxic, hazardous, and/or radioactive. Typically such wastes contain heavy metals like metals from Groups VIII, IB, IIB, VIIB, VIB, VB, and IVB, particularly metals such as nickel, chromium, tin, lead, copper, vanadium, selenium, or mercury. These metals are usually complexed in either soluble salt forms or precipitated as relatively insoluble metal hydroxides or oxides. The waste material can also contain radioactive species like uranium or cesium compounds present in similar molecular form to the heavy metals described above. Cesium, an alkali metal, is generally highly soluble and is relatively mobile from a disposal standpoint. The waste material can contain various amounts of the heavy metal or radioactive species. Generally, however, these species will be present individually in an amount less than 20% wt., and more commonly less than 10% wt., but typically more than about 0.1% wt., and more commonly more than about 0.5% wt. of the overall waste material on a dry basis. Typically, the radioactive species, such as uranium or cesium, will be present individually below about 10% wt., but in an amount of at least about 0.05% wt., and more commonly in an amount of at least about 0.5% wt., and in some cases more than about 1% wt. The waste material is typically in the form of an aqueous dispersion, such as a sludge or liquor that contains solubilized nitrate and phosphate compounds. Common industrial examples of such waste materials include sludges and liquors generated by the operation of and decontamination of electrolytes from plating operations. One particular example is the sludge that results from the aluminum or nickel cladding of uranium oxide pellets to prepare nuclear reactor target fuel for weapons production. Typical waste materials can contain from about 5-90% wt. water, and it is generally preferred to reduce the water content to below about 75% wt. prior to treatment of the waste to reduce disposal costs. Typical solids contents range from about 10-95% wt., commonly from about 10-50% wt., and more commonly from about 20-45% wt. The water content can be lowered by filtering the waste by means of conventional techniques such as pressure filtration or evaporation. Such typical waste material is commonly referred to as a sludge, and the heavy metal or radioactive species can either be present in dissolved or precipitated form. The waste material is then treated to immobilize and solidify the desired waste species such as the heavy metals and any radionuclide species. The waste material is admixed with a cement material that acts to provide a solidification matrix upon curing the resultant admixture. Various cement materials are commercially available and are all readily employed in the present invention, including microfine cements. Preferred cement materials are the Portland cement materials that are made by heating a mixture of limestone and clay, containing oxides of calcium, aluminum, iron, and silicon, in a kiln and pulverizing the resultant clinker. The most common constituents used to manufacture Portland cement include: CaO, SiO.sub.2, Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, MgO, SO.sub.3, Na.sub.2 O, K.sub.2 O, CO.sub.2, H.sub.2 O, and CaSiO.sub.5. The calcium oxide reacts with the acidic compounds at high temperatures to yield portland cement clinker that contains the following compounds: Ca.sub.3 O.sub.3, CaSiO.sub.5, CaSiO.sub.4, Ca.sub.3 Al.sub.2 O.sub.5, and Ca.sub.4 Al.sub.2 Fe.sub.2 O.sub.10. The Portland cements are available as the Portland Type I and Type II cements, with the Portland Type I cement being preferred. The amount of cement to be admixed with the waste material is primarily dependent on the water content of the waste material. In the preferred processes of the present invention the water content is from about 40-75% wt., more preferably from about 50-70% wt., of the waste material. In such instances, the amount of cement to be admixed with about 100 parts by weight of waste material is from about 5-50 parts by weight, preferably about 5-35 parts by weight, more preferably from about 10-25 parts by weight. The immobilization of the desired waste species, such as the heavy metal and/or radionuclide species, is advantageously enhanced by the incorporation of a complexant compound into the waste material and cement admixture. The preferred complexant compound is one that contains iron. It is believed that the iron, when present in the aqueous-based waste material mixture, attains the hydrated ferric or ferrous state to form hydrous iron oxides that act as inorganic ion exchangers and that co-precipitate the heavy metal (toxic) and radionuclide (radioactive) constituents. The hydrous iron oxide that is beneficial in the grout admixture of the waste material and the cement can be provided in the form of a hydrous iron oxide, however such forms are more expensive than other iron compounds. Thus, it is preferred to add the iron in the form of a commonly available salt, such as a Group VIIA, VIA, or VA salt, typically such as a sulfate, chloride, or phosphate salt of iron. The iron can take either its ferric or ferrous state in such salts. Iron in the form of FeSO.sub.4 has been found to be particularly suited for incorporation into the present invention. The iron is thought to be particularly beneficial in providing an ionic exchange site to help bind the radionuclides, particularly uranium, and co-precipitating them into insoluble species for better fixation within the grout crystalline structure formed from the cement. The iron species is thought to beneficially reduce the valence of the mobile uranium species to render such species into an immobile state. The amount of iron to be added to the admixture can be defined as the amount of elemental iron admixed in relation to the amount of cement to be added, and generally the weight ratio of elemental iron to cement is from about 1:3 to about 1:50, preferably from about 1:5 to about 1:35, and more preferably from about 1:10 to about 1:20. Additional complexants can also be used in the processes of the present invention in addition to the iron compound. Such compounds as sodium sulfide and organic chitosan can be used in amounts of weight ratios of those materials to cement of from about 1:1 to about 1:10, preferably from about 1:2 to about 1:5. If the waste material contains such compounds as cesium or other daughter or activation products, then such complexants as zeolites or organic ion exchange media can be additionally added with the iron compound, generally in amounts of from about 1-50% wt. of the iron compound. One process of the present invention is depicted in FIG. 1. The waste material is stored in a waste storage tank 10. The waste material is then transported via line 13 by means of pump 12 into the waste mixing tank 14. The waste material is blended into a homogeneous state by means of agitator 16. This homogenized waste material is then transferred via line 17 by means of pump 18 into the waste blending tank 20. The complexant material is then admixed with the waste material. The complexant is transferred from complexant tank 24 via line 25 into the waste blending tank 20. The complexant and waste material are then blended by means of agitator 22. This complexant/waste blend is then transferred via line 27 by means of pump 26 to a high intensive mixer 34. Cement, from cement tank 30, is transferred via line 31 to the intensive mixer 34. The intensive mixer 34 is designed to provide for the efficient blending of the waste material, complexant, and cement materials to form a grout admixture. Such mixers are commercially available in various design capacities. Following the blending of the waste material, complexant, and cement materials in the intensive mixer 34, the resultant grout admixture material is transferred via line 37 by means of pump 36 to filter 40. The filter 40 can be any of a various array of possible designs, such as a vacuum type filter or a high pressure filter press. The purpose of the filter 40 is to drain off excess aqueous fluids from the filter 40 that are transferred via line 46 by means of pump 48 to a separate storage or waste removal facility. The resultant filter cake is then conveyed via line 42 to a suitable containment vessel 44. The containment vessel 44 can be of any commercially available design and generally will be dictated by the nature of the solidified waste. The waste material, now immobilized and solidified in the grout material and complexant, is allowed to cure for a sufficient period of time in the containment vessel 44, typically such curing times being from about 8-72 hours. In a preferred embodiment the grout composition containing the waste material, complexant, and cement is washed with an aqueous solution while being treated at the filter 40 to remove easily mobilized nitrate compounds. The aqueous solution can contain surfactants to improve the efficiency of this washing step. EXAMPLE 1 In this example, a waste sludge formed from a nickel plating of uranium operation is treated according to the present invention to effectively immobilize and solidify the waste material species. The chemical analysis of the waste is set forth in Table 1 wherein the elemental results are shown by either inductively coupled plasma spectrophotometry (ICP), atomic absorption (AA), ion chromatography (IC), or ion selective electrode (ISE). The values are in weight percent on a dry basis. TABLE 1 ______________________________________ Sludge Chemical Analysis ELEMENT DRY WT. % ______________________________________ Al(ICP) 11.145 Ca(ICP) 0.325 Fe(ICP) 0.495 Mg(ICP) 0.154 Mn(ICP) 0.539 Na(ICP) 8.83 Li(ICP) 0.002 Ni(ICP) 0.595 Si(ICP) 9.613 Cr(ICP) 0.015 B(ICP) 0.06 U (D-G) 5.246 Sr(ICP) 0.004 Zr(ICP) 0.032 Ti(ICP) 0.04 K(AA) 0.122 P(ICP) 1.548 Ba(ICP) 0.008 Pb(ICP) 0.161 Mo(ICP) 0.011 Zn(ICP) 0.96 Cu(ICP) 0.018 Cl(ISE) 0.113 SO.sub.4 (IC) 0.5 NO.sub.3 (IC) 2.989 % SOLIDS (105.degree. C.) 35.37 % SOLIDS (90.degree. C.) 34.43 ______________________________________ The testing was conducted by admixing the waste material with cement and FeSO.sub.4 as a complexant in the amounts shown in Table 2. The test compositions were prepared by admixing the stated amount of waste material with the stated amount of complexant and cement. The cement A was Portland Type I cement and cement B was Portland Type II cement. About 7-20 ml of NaOH solution was added to the mixtures to bring the mixtures to a pH 11. The mixtures were then filtered and washed with the stated amount of rinse water and then filtered again. The resultant filter cake was then allowed to cure for about 24 hours. This filter cake was then tested in accordance with standard EPA test methods as set forth in EPA standard test methods (3d Edition, 1992) "Test Methods for Evaluating Solid Wastes", Physical/Chemical Methods, SW-846, Standard Method #1311, Toxicity Characterization Leaching Process (TCLP). The results for the nickel, uranium, and nitrates are set forth in Table 3. TABLE 2 ______________________________________ Compositions of Examples Component Ex. 1 Ex. 2 Ex. 3 Ex. 4 ______________________________________ Waste Material (g) 608 608 608 608 FeSO.sub.4 (g) 10 30 20 10 Cement A (g) 46 138 92 Cement B (g) 46 Rinse Water (ml) 475 475 475 475 ______________________________________ TABLE 3 ______________________________________ TCLP Results Tested Species Ex. 1 Ex. 2 Ex. 3 Ex. 4 ______________________________________ Nickel 0.017 0.014 0.017 0.03 Uranium ND* ND* ND* ND* Nitrates 17.4 29.1 24.7 16.3 ______________________________________ *denotes that values were below detectable limits of the instrumentation.