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Ind. Eng. Chem. Res. 2010, 49, 1792–1798
Leaching Characteristics of Hanford Ferrocyanide Wastes Matthew K. Edwards,* Sandra K. Fiskum, Rick W. Shimskey, and Reid A. Peterson Department of Separations and Radiochemistry, Pacific Northwest National Laboratory, 902 Battelle BouleVard, P.O. Box 999, Richland, Washington 99352
A series of leach tests were performed on actual Hanford Site tank wastes in support of the Hanford Tank Waste Treatment and Immobilization Plant (WTP). The samples were targeted composite slurries of highlevel tank waste materials representing major complex, radioactive, tank waste mixtures at the Hanford Site. Using a filtration/leaching apparatus, sample solids were concentrated, caustic leached, and washed under conditions representative of those planned for the Pretreatment Facility in the WTP. Caustic leaching was performed to assess the mobilization of aluminum (as gibbsite, Al[OH]3, and boehmite AlO[OH]), phosphates [PO43-], chromium [Cr3+], and, to a lesser extent, oxalates [C2O42-]). Ferrocyanide waste released solid phase 137 Cs during caustic leaching; this was antithetical to the other Hanford waste types studied. Previous testing on ferrocyanide tank waste focused on the aging of the ferrocyanide salt complex and its thermal compatibilities with nitrites and nitrates. Few studies, however, examined cesium mobilization in the waste. Careful consideration should be given to the pretreatment of ferrocyanide wastes in light of this new observed behavior, given the fact that previous testing on simulants indicates a vastly different cesium mobility in this waste form. The discourse of this work will address the overall ferrocyanide leaching characteristics as well as the behavior of the 137Cs during leaching. 1. Introduction During defense-weapons production from the 1940s through the 1980s, millions of gallons of radioactive waste were put into underground storage tanks on the Hanford Site, Richland, WA. These wastes are highly complex, multiphased, chemical mixtures, containing mixed activation and fission products. The wastes were formed from discrete spent nuclear fuel processes such as the bismuth phosphate precipitation process (resulting in high bismuth and phosphate concentrations), reduction oxidation (REDOX) processing (high Al concentration), and plutonium-uranium extraction (PUREX) processing (high Al concentration), as well as a plethora of minor production activities. Detailed descriptions of the waste in the tank farms have been previously reported.1,2 One waste form selected specifically for efficacy of leaching and filtration studies is ferrocyanide waste. A history of ferrocyanide waste production in the Hanford tank farms has been previously described.3 In essence, a ferrocyanide salt such as potassium ferrocyanide (K4Fe[CN]6) or sodium ferrocyanide (Na4Fe[CN]6) was added to the metal waste recovery process stream and to underground storage tanks with NiSO4 in an effort to scavenge 137Cs from the aqueous phase as a precipitate (e.g., as Cs2NiFe[CN]6). Reducing the aqueous 137Cs concentration (along with 90Sr as a SrSO4 precipitate) allowed the aqueous phase to be discharged to the cribs (in compliance with thencurrent regulations and policies), thus freeing up more underground tank storage space for new process wastes. Approximately 140 tons of ferrocyanide was added to Hanford tank wastes during the treatment program from 1954 to 1957. This waste type is characterized by a high iron concentration accompanied by relatively high nickel and 137Cs concentrations. In the 1990s, extensive testing was performed on the ferrocyanide wastes. The principal focus was on the thermal properties of ferrocyanide and the possible reactions with oxidizers in the waste. Secondary studies were also done on * To whom correspondence should be addressed. Tel.: (509) 3764595. Fax: (509) 373-9675. E-mail:
[email protected].
the stability of the ferrocyanide complex. Aging and simulant studies revealed contradictory evidence. Aging studies displayed that sodium nickel ferrocyanide reacted with free hydroxide to form nickel hydroxide and ammonia under tank conditions and, when driven to completion, increased the cesium content in solution.4 Ferrocyanide simulant studies, however, concluded that cesium nickel ferrocyanide was a stable compound in up to 4 M sodium hydroxide.5,6 No studies were done, however, on the stability of the cesium nickel ferrocyanide in aged real waste and under tank conditions. Hanford tank wastes are destined to be removed from the underground storage tanks and vitrified at the Hanford Tank Waste Treatment and Immobilization Plant (WTP) as part of a U.S. Department of Energy (DOE) agreement with the U.S. Environmental Protection Agency (EPA) and the state of Washington. Before vitrification, the waste slurry is to be processed at the Pretreatment Facility (PTF) to remove glasslimiting elements such as aluminum, chromium, and phosphorus from the high-level waste (HLW) to increase the solubility of the radioactive waste in the glass matrix. The first step in treating HLW sludge (Figure 1) at the PTF will involve cross-flow ultrafiltration to concentrate the sludge phase by removing much of the supernatant. After ultrafiltration, the concentrated sludge will be caustic leached to mobilize aluminum (gibbsite and boehmite), chromium, and phosphate from the solids phase, thus removing the bulk of the waste volume from the HLW stream. The leached slurry is again filtered to remove most of the leach solution and then washed and filtered in iterative steps to further remove residual leach material from the slurry. The leached and washed concentrated sludge solids are later vitrified as HLW, while the aqueous phase undergoes further treatment before vitrification as low-level waste. Comprehensive solids phase and supernatant characterizations were conducted on the initial material and the leached and washed product to assess the leach behavior. The ferrocyanide waste was then subjected to prototypic pretreatment operations (caustic leaching and washing) to assess the mobilization of phosphate-, chromium-, and aluminum-bearing phases from the
10.1021/ie901034m 2010 American Chemical Society Published on Web 12/21/2009
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
Figure 1. Schematic representation of the key processes to be performed in the PTF. (Note: This is for illustrative purposes only; it is not meant to be a comprehensive view of the functions performed within the WTP.)
solids phase to the aqueous phase. Post pretreatment of the wastes identified elemental, radioisotope, and crystal information of solid phases. 2. Experimental Material The actual tank waste leach testing program was limited to eight groups that would account for ∼75 wt % of the materials of interest in the expected feeds to the WTP with respect to leaching (tank waste Best Basis Inventory):7,8 group 1: Bi phosphate sludge9 group 2: Bi phosphate saltcake9 group 3: PUREX cladding waste sludge10 group 4: REDOX cladding waste sludge10 group 5: REDOX sludge11 group 6: S-saltcake11 group 7: TBP sludge12 group 8: ferrocyanide sludge13 Items of specific interest in the group 8 ferrocyanide sludge were Al, Cr, and phosphate, which cause limitations in the quantity of HLW glass that is produced, and iron, which has issues related to the filtration of iron hydroxide. Tank waste sludge samples with known ferrocyanide process history were used to construct a representative ferrocyanide waste (group 8) composite. The TWINS database was queried in 2007 to identify the tanks containing >70% ferrocyanide waste type identified as the following: • ferrocyanide sludge from in-farm scavenging of 1C (bismuth phosphate first cycle decontamination waste) supernatants in TY-Farm (1955-1958) • ferrocyanide sludge from in-plant scavenged supernatant (1954-1955) • ferrocyanide sludge produced by in-tank or in-farm scavenging (no date provided) [The TWINS database is a U.S. Department of Energy (DOE) owned resource. It is a Web-based interface providing access to information about a wide variety of Hanford tank waste information. It is available at URL http://twins/twins3/twins.htm.] Ferrocyanide wastes were selected from core samples retrieved from Hanford Tanks BY-104, BY-105, BY-108, and BY-110. These samples had been stored in the sample archive for ∼12-15 years. The selected sample materials were com-
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posited with deionized (DI) water and mixed and subdivided for analytical, physical, and process testing. The sample composite was characterized before filtering/ leaching. The radioanalytical results and chemical compositions for the initial slurry as well as those of the separated supernatant liquid and solids are provided in Table 1. Details of analytical processing were previously reported.13 The supernatant was composed primarily of sodium salts (nitrate, carbonate, nitrite, hydroxide, phosphate, sulfate, and oxalate). As expected, the iron and nickel concentrations were relatively high. The 137Cs was fractionated largely in the solids phase (75%), which was characteristic of the Cs scavenging effect of the nickel ferrocyanide precipitation reaction.3 The aqueous phase freehydroxide concentration was 0.3 M. The X-ray diffraction (XRD) mount of the water-washed ferrocyanide solids was prepared with rutile as the internal standard. The raw XRD pattern included a broad hump from about 10 to 35° 2θ, indicating that the solids probably contained some amount of amorphous materials. The background-subtracted XRD pattern with stick figure phase identification is shown in Figure 2. Phases present at a lower peak area/height are shown in decreasing intensity down the display. The phase identification is based on the best fit per the published data and may not necessarily be exact, based on chemistry. [Published data include known chemistries (Fiskum et al.12,13), and phase identification was done with the JADE search match routines (version 8.0.10, 12/15/06, Materials Data Inc.) with comparison with the International Centre for Diffraction Data (ICDD) database PDF-2 release 2007, version 2.0704, which includes the Inorganic Crystal Structure Database (ICSD) maintained by Fachinformationszentrum (FIZ), Karlsruhe, Germany).] Several features were identified to specifically evaluate during typical pretreatment operations: • Al (present at 8.8 wt % in the solids) is represented by gibbsite and hydroxycancrinite. According to gibbsite waste studies, gibbsite leaching is complete and rapid at relatively low free-hydroxide concentrations and temperature.10,14 • P (present at 3.8 wt % in the solids) is in part represented by calcium phosphate, which does not leach in caustic solutions.15 Phosphorus may also be a component of the large amorphous fraction of the material. • Cr (present at 0.22 wt % in the solids) was not present in a high enough concentration to challenge waste loading into glass. However, leach behavior was collected to increase the understanding of tank waste. • The fission product 137Cs was largely fractionated to the solids phase (∼75%), unlike most other tank waste types. • Ferrocyanide compounds are not identified via XRD despite the fact that cesium is supposed to be incorporated into ferrocyanide compounds. • The high uranium and iron mass components were of significant interest in relation to filtration behavior, but are not addressed in this paper. 3. Experimental Methods The ferrocyanide waste leaching and filtration studies were conducted with the cross-flow filtration/leaching test (cell unit filter [CUF]) apparatus diagramed in Figure 3. All sample handling and filtration/leaching activities were conducted in hot cells to shield operators from high-level radiation. The filter feed flowed through the inside of the filter element axially, while the feed permeate passed through the tube walls radially. The filters purchased for this testing were supplied from
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Figure 2. Background-subtracted XRD pattern with possible stick-figure peak identification of the ferrocyanide solids based on the best fit per the published data. Table 1. Characterization of Preleached Ferrocyanide Waste slurry
liquid fraction
solids fraction
mass
1.98 kg
1.74 kg
0.23 kg
wt % of slurry
100 wt %
ICP-OES analyte
g
Al Bi Cr Fe Mn Na Ni P S Si Sr U
2.3 × 10 1.4 6.3 × 10-1 2.5 × 101 3.1 × 10-1 1.4 × 102 8.5 1.3 × 101 4.6 4.0 9.7 2.8 × 101
radionuclides 60
Co Cs 154 Eu 241 Am gross alpha gross beta 90 Sr 239+240 Pu 238 Pu 137
1
µCi 2.5 1.2 × 105 3.3 × 101 4.4 × 101 2.0 × 102 7.3 × 105 3.0 × 105 1.3 × 102 3.1
88.2 wt %
11.8 wt %
g
µg/mL
2.2 0.0 1.2 × 10-1 3.6 × 10-2 1.2 × 10-4 1.2 × 102 8.2 × 10-2 3.7 3.4 2.6 × 10-2 1.6 × 10-4 1.7 × 10-2
1.5 × 10
