Environ. Sci. Technol. 1994, 28, 882-889
DXRD Studies of Sodium Nickel Ferrocyanide Reactions with Equimolar Nitrat e/Nit rit e Salts Joseph N. Doddst and Wllllam J. Thomson’ Department of Chemical Englneering, Washington State University, Pullman, Washington 99 164-27 10
Dynamic X-ray diffraction (DXRD) has been used to identify and quantify the solid-state reactions that take place between sodium nickel ferrocyanide, NazNiFe(CN)6, and equimolar concentrations of sodium nitrate/nitrite, reactions of interest to the continued environmental safety of certain waste storage tanks at the Hanford site in eastern Washington. The results are supportive of previous work that indicated that endothermic dehydration and melting of the nitrates take place prior to the occurrence of exothermic reactions, which begin about 300 “C. The DXRD results show that a major reaction set at these temperatures is the occurrence of a series reaction which produces sodium cyanate, NaCNO, as an intermediate in a mildly exothermic first step. In the presence of gaseous oxygen, NaCNO subsequently reacts exothermally and at a faster rate to form metal oxides. Measurements of the rate of this reaction are used to estimate the heat release, and comparisons of this with heat-transfer rates from a hypothetical “hot spot” show that, even in a worse case scenario, the heat-transfer rates are approximately eight times higher than the rate of energy release from the exothermic reactions. Introduction Beginning in the 19509, ferrocyanide ions, [Fe(CN)@, were used to scavenge radioisotopes of cesium (137Cs)from waste streams resulting from the recovery of uranium at the Hanford Reservation in eastern Washington. After nickel sulfate was added to the waste stream to cause precipitation of cesium nickel ferrocyanide, the slurry was pumped to large (750 000-gal capacity) single-shelled carbon steel storage tanks following neutralization with sodium hydroxide. More than 900 t of ferrocyanidecontaining compounds were added to the single-shelled tanks (I). Large quantities of sodium nitrate, which resulted from the use of nitric acid during uranium recovery operations, were an integral constituent of the waste and were partially decomposed into sodium nitrite and oxygen. Thirty years of storage in a highly basic and highly radioactive environment nava prompted concerns as to the continuedsafe storage of these materials. This concern is due to the possibility of an explosive reaction which can occur between the ferrocyanides and the accompanying nitrates and nitrites under appropriate conditions (2). In order to address these concerns, a number of studies have been conducted in an effort to identify the reaction(s) which occur, to determine the necessary reaction conditions, and to quantify the magnitude of the energy released during reaction(s) (3-9). While much insight has been obtained during these studies, the identification of the specific reactions that occur and the quantification of the prevailing reaction rates has yet to be determined. t Present address: UNOCAL, Fred L. Hartley Research Center, Brea, CA 92621. 882
Environ. Sci, Technoi., Vol. 28, No. 5, 1994
Since thermally explosive reactions only occur when the reaction rates exceeded the heat transfer (cooling) rates, the present study was undertaken in an effort to obtain the necessary reaction and rate data. Specifically,dynamic X-Ray diffraction (DXRD)(10, 111, which can follow a reaction between solids and show the appearance or disappearance of reacting species, was used to determine the reactions that occur between sodium nickel ferrocyanide and equimolar sodium nitrate and sodium nitrite mixtures. The measured rate of reaction for key reaction(s) can then be compared to the rate of heat dissipation in the tank to assess the possibility of runaway reaction(9).
Background Sodium (or potassium) ferrocyanide, known also as sodium (or potassium) hexacyanoferrate(II), and nickel sulfate were used in the 1950s to scavenge radioisotopes of cesium from nitrate-containing liquid waste streams produced during the recovery of uranium. The primary purpose was to reduce the volume of liquid waste, which required long-term storage. Alkali nickel ferrocyanides precipitated out of solution, scavenging the radiocesium. The resulting solids were then stored in large underground single-shelled tanks (SST). In the 1970s the SSTs were found to be slowly leaking, and as a result, pumpable liquid from these single-shell tanks was removed, leaving behind a wet solid residue (I). Prior to the pumping, the heat generated from the radioactive decay of the 137Cswas dissipated through the evaporation of water. This and the concerns relative to the reactivity of the remaining wet sludge and salt cake, components of which were known to have thermallyinduced explosive potential (2), required further study. That is, since the radiolytic decomposition product of sodium nitrate is sodium nitrite, it is likely that the ferrocyanide coexists in the tank along with sodium nitrate and nitrite. Despite the fact that the measured temperatures in these tanks continue to drop (3 OC/year) and the highest temperature currently recorded is 56.7 “C (I2), there has been a good deal of speculation as to the possibility of “hot spots” forming in the tanks. This, in turn, has stimulated numerous technical investigations and safety analyses. As part of this technical effort, synthetic sodium nickel ferrocyanide solids were produced by Battelle Pacific Northwest Laboratory (PNL) and the Westinghouse Hanford Company (WHC) in order to study the behavior of the precipitated ferrocyanide in a nonradioactive environment. These synthetic solutions were produced by following the flowsheets (8,13) of the chemicals added during the uranium extraction process, and in addition to ferrocyanide, the synthetic waste also contained sodium nitrate, sodium nitrite, and sodium sulfate. Battelle PNL has determined that synthetic sodium nickel ferrocyanide has an estimated chemical formula NazNiFe(CN)g3HzO 0013-936X/94/0928-0882$04.50/0
0 1994 American Chemical Society
based on elemental analysis using inductively coupled argon plasma atomic emission spectroscopy (ICP/ AES), scanning thermogravimetry (STG), and total cyanide analysis (9). Early studies used differential scanning calorimetry (DSC), (I,5 , 6 , 8 ) ,scanning thermogravimetry (STG) (6, 8, 9), differential thermal analysis (DTA) (3, 7), time to explosion (TTX), (6,7),pyrolysis/mass spectroscopy (MS) (8-IO), and thermodynamics (4, 5, 8 ) to determine the thermochemical reactivity of ferrocyanide nitratehitrite reactions. These studies indicated that the reaction between sodium nickel ferrocyanide and sodium nitrate and sodium nitrite is complex and multistep. DSC, which measures enthalpy changes, detected only endothermic reactions prior to 230 "C. Exothermicity began at 230 "C, was followed by a rapid exothermic reaction at 300 "C, and ended with a final endothermic reaction at 410 "C (I, 6 , 8 ) . STG, which measures weight changes that occur during a chemical reaction, showed weight losses that accompanied the exothermic and endothermic reactions (6,8). DTA, which provides a measure of the initiation of a reaction and its onset temperatures, did corroborate the results of the DSC measurements (3,7),and in addition, DSC, DTA, and STG experiments all indicated that there are differences in the mechanisms between the nitrate reaction and the nitrite reaction with ferrocyanide. However, since these three methods can only measure the net reaction effects and are not capable of distinguishing between simultaneous events, they did not provide any data as to the reaction sequences or products which form. TTX experiments, on the other hand, measured the time in seconds until the occurrence of an explosion (indicated by either a flash of light or loud noise) and employed a 50-100-mg sample containing 20-30 mg of ferrocyanide which was placed in a metal block at temperatures ranging from 280 to 400 "C. The TTX results (6, 7) varied with the sample composition and handling procedures but the variations did corroborate the overall complexity of the reaction. The TTX data (6) also indicated that the reaction mechanism changes at about 350 "C, but no explanation was available. A "pyroprobe" equipped with a mass spectrometer (MS) was also used to measure the mass of gaseous chemical species that are produced at different temperatures during a chemical reaction (8).As the temperature was raised at 5 "C/min, a "washed" flowsheet ferrocyanide sample (8) produced H2O up to 250 "C, N2 and/or CO between 200 and 500 "C, N2O and/ or C02 between 200 and 500 "C, NO between 150 and 300 OC, and HCN between 200 and 250 OC. Finally, thermodynamic calculations (6, 8, 9) predicted that the most energetic reactions occur when the nitrogen present in the cyanide is releasd as N2, and these are shown in eqs 1 and 2.
-
Na,NiFe(CN),
+ 6NaN0,
Na,NiFe(CN),
+ 10NaN0,
4Na2C0, + NiO + FeO + 2C02 + 6N2 (1)
-
6Na2C0, + NiO + FeO + 8N2 (2)
jv
Wells (14) reported on the crystal structure of complex ferrocyanides and showed that several of them were simple cubic. He also addressed the insertion of waters of hydration into the structure and reported that the water of hydration can, in some cases, displace cyanides out of
X-RAY SOURCE
DETECTOR
GAS
I
I
I r - I
TEMPERATURE CONTROLLER
I
Flgure 1. DXRD schematic diagram of reaction chamber.
the cubic lattice. The structure of sodium nickel ferrocyanide was later corroborated by Hallen at PNL (8) on the basis of X-ray pattern matching to similar ferrocyanide structures.
Experimental Section All measurements were made using the DXRD apparatus shown in Figure 1. Specific details of the DXRD equipment can be found elsewhere (IO, II), but the equipment consist of a Siemens D-500 diffractometer, a position sensitive detector, and an Anton-Paar hot stage (a schematic of the latter is shown in Figure 1). Powder samples were placed on an Inconel heating strip for both non-isothermal (2 "C/min to 20 "Urnin) and isothermal ( f l K) conditions. Heating strip temperatures were measured and controlled by a type S Chromel-Alumel thermocouple attached to the base of the heating strip. The entire enclosed hot stage, the "DXRD chamber", allows for gases such as helium or compressed air to be passed through the apparatus (at 250 mL/min). Attempts were also made to qualitatively monitor the gases exiting the DXRD chamber using nondispersive infrared (IR) detectors for CO and C02 and a chemiluminescence NO/ NO, detector. However, due to the small sample sizes (0.1 g), CO or C02 in the product gases were below the detection limits (200 ppm). The maximum NO concentration observed during these reactions was on the order of 50 ppm and may also include nitrous oxide as well, since the production of ozone by the detector can oxidize N2O to NO. Nitric oxide and/or nitrous oxide production was observed for a 1:l:lmolar ratio of ferr0cyanide:nitrate: nitrite between 180 and 350 "C in helium and between 180 and 500 "C in air, which indicated that differences existed between the air and helium environments. However, the gases produced in a helium environment and their corresponding temperature range did corroborate the results obtained using the MS/pyroprobe at PNL (8). The sample of sodium nickel ferrocyanide, which was used in this work, was obtained from PNL and was designatedas FeCN36. It was necessary to remove sodium sulfate from this sample due to considerable peak overlapping, and consequently, it was ion exchanged with nitrate to replace the sulfate and had a final chemical formula of NazNiFe(CN)v3HzO (8). Subsequent XRD scans of FeCN36 confirmed the absence of sodium sulfate peaks, but also indicated that there were no excess nitrates. Envlron. Sci. Technol., Vol. 28, No. 5, 1994
883
It is possible that these were removed during the washing of the ion-exchanged ferrocyanide. An equimolar nitratelnitrite mixture was produced by weighing the sodium nitrate and sodium nitrite, grinding them into a fine powder using a mortar and pestle, melting them on a hot plate to obtain uniform concentrations, and then regrinding the cooled mixture. The mean particle diameter of FECN36 was measured as 0.4 f 0.05 pm using a Quantachrome Microscan Particle Size Analyzer and confirmed with SEM micrographs. All the ferrocyanideequimolar nitrate/nitrite mixtures were prepared by sample weighing and admixing, followed by grinding with a mortar and pestle. The samples employed in this work consisted of equimolar mixtures of sodium nitrate and nitrite with FECN36:NaN03:NaN02 ratios equal to l:l:l, 1:4:4, and 1:88. Selected experiments with the latter were conducted since it most closely corresponds to estimates of what is the most likely concentrations that exist in the tanks at this point in time ( I ) . The 1:4:4 mixture was studied because it is the ratio required to completely oxidize the ferrocyanide according to Burger and Scheele (6) and is shown as eq 3. However, the eutectic melting of the Na,NiFe(CN),
Table 1. Observed Cyanide XRD Peaks
major peaks species
d-spacing
2b
NasNiFe(CN)e3HzO
4.8659 3.5392 2.5149 2.2448 1.7768 4.6493 3.8909 3.464 2.5034 2.3792 2.2181 5.8343 2.7811 1.9649 2.9161
21.201 29.301 41.701 47.001 60.502 22.201 26.601 29.951 41.901 44.201 47.601 17.651 37.551 54.202 35.752
NazNiFe(CN)e
NahFe(CN)a NaCNO
equimolar nitrate/nitrite salts used at higher molar ratios (1:88) resulted in significant sample melting, which made DXRD observations somewhat difficult. Consequently, the 1:l:l mixture was used so that the ferrocyanide XRD peaks could be easily followed along with the nitratehitrite peaks. Chemical tests (15) for the detection of ferrocyanide ions [Fe(CN)#- in the product powders were used to corroborate the XRD data. The test consisted of placing a small sample (0.05 g) of powder into a 5-mL test tube and adding distilled water. The sample was centrifuged to separate undissolved solids, and the supernatant was placed into another 5-mL test tube. A few drops of Fe3+ ions from a 10% ferric chloride solution was added to the supernatant to promote the formation of Prussian blue, a dark blue precipitate of Fer(Fe(CN)&. A detection limit of 20 ppm [Fe(CN)sl" has been reported by Charlot (15). Fourier transform infrared (FTIR) analyses were also selectively performed to confirm the presence or absence of ferrocyanide, sodium cyanate, nitrate, and nitrite in the reacted sample powders.
100 67.8 97.0 48.9 48.2 100 45.3 62.4 44.3 53.8 53.2 65.3 100 66.7 100
Cobalt radiation (A = 1.790). Intensity
-
+ 4NaN0, + 4NaN0, NiO + FeO + 5Na,C03 + CO, + 7N, + 1/20, (3)
relative intensity
5
6462
-1
Flgure 2. Sample 1:l:l (FECN36:NaN03:NaN02),2 "C/min, helium. Key: 1, Na2NIFe(CH)s.3H20;2, NaNO3/NaNO2;3, NaCN03; 4, Na4Fe(CN)*; 5, Ni; 6, N13N. Intensity
7764
4
Results First of all, Table 1 lists the XRD lines that were measured during the course of this study and are associated with the cyanide-containing species, which are not commonly found in the literature. Both the hydrated and dehydrated phases of sodium nickel ferrocyanide are included in Table 1along with sodium ferrocyanide. The latter was found only at temperatures greater than 350 "C in helium experiments using the 1:l:l mixture or sample FECN36 in the absence of nitratednitrites. The line for sodium cyanate is also shown, and this was observed in all experiments containing any nitratebitrite at temperatures above 290 "C. Figures 2 and 3 show typical DXRD nonisothermal results in helium with 1:l:l mixture and in air with a 1:4:4 mixture, respectively. Based on these results, the reaction 884
Envlron. Scl. Technol., Vol. 28, No. 5, 1994
Figure 3. Sample 1:4:4 (FECN36:NaN03:NaN02),2 'C/min, air. Key: 1, Na2NiFe(CN)~.3H20; 2, NaN03/NaN02;7, NIO 8, F82O3; 9, Na2C03.
sequence is hypothesized to proceed in three separate steps designated as events A, B, and C (see Figures 2 and 3): (A) dehydration of the ferrocyanide at 200 f 20 "C; (B)
Table 2. Products from Reactions above 350 OC
species
helium air FECN:NOJNOZ-C 1:00 1:l:l 1:44 1:00 1:l:l
1:44
FeaOgPb X X X x x X X Ni NisN X X NiOa X x x x x NazCOa' X x x X X NaCNO X X X NQF~(CN)~ X X Na202 X X a In helium, the iron oxide may be Feg04, a clear distinction was not possible. b Only observed ae minor phases in helium. 2Z13 50
Flgure 4. Sample FECN30, 2 "Clmin, helium.
2 - Na,NiFe(CN),
i
,,
Key
to Figure 5
45 8Y
TEIlP
C1
5 % Steam Added
Flgure 5. Sample FECN36, 2 "Wmin, helium.
melting of the equimolar nitrate/nitrite at 235 f 5 "C; (C) appearance of degradation products at 300 f 10 "C. The first major structural change (event A) occurs at 200 f 20 "C and is attributed to the dehydration of the ferrocyanide (eq 4). Since dehydration would be reversible, Na,NiFe(CN),.3H,O
Na,NiFe(CN),
+ 3H2O
(4)
this was corroborated by experiments where the synthetic FECN36 in helium was heated and cooled with and without the addition of water vapor, and the results are shown in Figures 4 and 5. The results in Figure 4 are for an experiment where FECN36 was heated in dry helium from 50 to 222 "C and then cooled back down to 45 "C. As can be seen, this results in a crystallinity change at 200 "C, which remains unchanged even after cooling back to room temperature. On the other hand, when 5 ?4 steam is added after the crystallinity change at 200 "C,the original XRD reflections are recovered upon cooling back to room temperature (Figure 5). Given these results, there is little doubt that the crystallinity change a t 200 "C is due to the dehydration of the ferrocyanide. Although sodium nitrate melts at 308 "C and sodium nitrite melts at 270 "C, mixtures of nitrate and nitrite salts exhibit eutectic melting at temperatures well below the normal melting point of either salt (6). Following the dehydration of the ferrocyanide, Figures 2 and 3 show that the equimolar nitrate/nitrite melts at 235 f 5 "C
(event B) as evidenced by the disappearance of the combined NaN03/NaN02peaks at this temperature ("2" in Figures 2 and 3). Since the melt is noncrystalline, the nitrate/nitrite peaks are not detected in the scans above 240 "C. At about 300 OC, the ferrocyanide reacts to form sodium cyanate (NaCNO), and this is the primary degradation product associated with event C. While the sodium cyanate persists throughout the experiments in a helium environment and is clearly evident in Figure 2, it reacts in the presence of oxygen at about 320 "C to form Na2C03, NzO, and Cog, and thus it is difficult to see in Figure 3. As the temperature is further increased, the degradation products are more numerous, and these are listed in Table 2 as a function of reactant ratios and gas environment. From Table 2, it is readily apparent that reduced species such as Ni and NiN3 are only produced in helium and then only at low concentrations of nitratehitrite. Although Table 2 shows iron oxide, nickel oxide, and sodium carbonate forming in a 1:l:l mixture in helium, these products formed in low quantities, and resulting low XRD peak heights are not obvious in Figure 2. Because different products did form using the different reactant ratios, complete degradation of the ferrocyanide is dependent on the total quantity of equimolar nitratehitrite present in the mixture. The formation of oxide products such as nickel or iron oxide was far more substantial in an air environment than in the helium atmosphere. This is probably due to additional reactions with oxygen in the air rather than the oxygen formed during some of the degradation reactions (see below). Finally, it should be pointed out that when the sodium nickel ferrocyanide was exposed to high temperatures in helium in the absence of nitratehitrate, NaFe(CN)6was formed along with Ni and NiN3. There were also a number of unidentified XRD peaks that appeared during these experiments, and it is likely that they are other ferrocyanide species. Based on the DXRD observations and the wet chemistry analyses, eqs 5-11 (shown in Scheme 1)are the reactions which are hypothesized to occur in both helium and air immediately following the eutectic melting. However, we should be careful to point out that the reaction network here is probably quite complex. These reactions are hypothesized solely on the basis of the DXRD measurements and are supported by limited wet chemistry analyses. The NO given off in reaction 8 between 270 and 320 "C is consistent with previous pyroprobe measurements (8)and small (Le., too small for accurate integration), but detectable traces of NazO were observed by DXRD at Envlron. Scl. Technol., Vol. 28, No. 5, 1994
885
---
Scheme 1. Hypothesized Reaction Sequence
270-320 "C 2NaNOz NazO + 2NOt t 1/202? NaNO3 NaNOz + 1/202t 290-310 "C BNazNiFe(CN)e + 12NaN03 18NaCNO+ 3Ni + Fes04 + 12NOt + Ozt 300-350 "C 2NaCNO + 5/202 NazCO3 + 2N07 + COnt Ni + 1/202 NiO Ni + NaN03 NiO + NaN03 2FesOr + 1/202 3FezOs
-
-+
0.01
0
, 5
(5) (6) (7) (8) (9) (10) (11)
I
1
I
10
15
20
Time (minutes)
Figure 8. Dehydration rate constants, first-order fit, sample 1:4:4 (FECN38:NaN03:NaN02),air.
these temperatures. In all experiments with nitrates/ nitrites, the sodium nickel ferrocyanide reacted to form NaCNO as shown in eq 7, which is then further oxidized via eq 8. The major difference in the two gaseous environments is that, in helium, the oxygen reactant is supplied autogenously. Thus reactions 8,9, and 11occur very slowly since the generated oxygen can easily escape from the thin samples employed in the experiments. In air, on the other hand, oxygen is readily supplied from the surrounding gas, and species such as Ni and FeaOl are rapidly oxidized so that they are undetected by DXRD. Finally, it should be noted that reaction 8 proceeds very slowly in helium, fueled only by the liberation of 0 2 from reactions 5-7. It should also be noted that previous observations (17) indicate that this reaction is accelerated in the presence of small quantities of iron and nickel oxide. In discussing these reactions, it must be emphasized that several of these reaction steps could be bypassed if heating rates were excessive. No attempts to identify alternate reaction pathways at high heating rates were made in this study since temperature changes in ferrocyanide-containing wastes would not be expected to be rapid, even in the presence of a hot spot. Safety issues relative to the explosivity of exothermic reactions are a consequence of the comparative rates of chemical reaction and heat transfer (cooling). Consequently, once the various reaction events had been identified, attempts were made to quantitatively model the pertinent reaction rates. With respect to the dehydration of FECN36 in air, Figure 6 shows the results of an attempt to model the dehydration as a first-order reaction. Only the low conversion data were able to be 888
Environ. Scl. Technol., Vol. 28, No. 5. 1994
used for this fit as a consequence of the overlapping of the hydrated and dehydrated XRD peaks at higher conversions. In addition to the first-order fit to the low conversion data, Figure 6 also lists the rate constants between 443 and 473 K. A subsequent Arrhenius analysis of the rate constants resulted in an activation energy of 8.8 & 2.3 kcal/mol. The key reactions in this system correspond to event C. From the behavior observed in the DXRD experiments, it was apparent that reactions 7 and 8 occur by way of a series reaction system. I t is theoretically possible to model these two reactions in an air environment (where the oxygen pressure is known and constant) and to then determine the reaction rate constants by fitting the data to the model results. However, there was only a very narrow temperature range where both the NazNiFe(CN)s and the NaCNO X-ray peaks could both be detected. That is, at temperatures above 300 "C, reaction 7 was essentially complete by the time isothermal conditions were achieved. Consequently, the approach which was taken was to first verify that the reactions were indeed in series and to then concentrate on the reaction rates of reaction 8. In order to achieve the first objective, it was assumed that reaction 7 was first order with respect to both nitrate and ferrocyanide and that reaction 8 was first order with respect to sodium cyanate. The differential equations describing these two reactions are then dCCN -=
dt
-~~CCNCNO,
-dCcNo-k dt 1CCN CNO,-kZCCN$O,
(13)
or, since Poz is constant
where k'2 = k2P0,. Using the stoichiometry of reactions 7 and 8 and, for the 1:l:l mixture, these equations were then solved using Simusolv (I&?), a software program capable of solving sets of differential equations by a variety of methods. To show that reactions 7 and 8 were in series, the values of k1 and k'2 were manipulated until the simulation matched the time at which the sodium cyanate concentration reached a maximum. An initial estimate of the rate constant, k l , was obtained from the initial slope (t 0) of the decrease in the ferrocyanide disappearance; Le., at a t i q e where the rate of reaction 8 would be much lower than that of reaction 7. Similarly, an initial estimate of k'2 was taken from the latter stages of the disappearance of NaCNO, and then the values of kl and k'z were adjusted in the Simusolv program until a good match in time was obtained. As can be seen from Figure 7A,B, an excellent match in time is obtained for both the disappearance of the sodium nickel ferrocyanide and the intermediate species,sodium cyanate, verifying the series nature of these reactions. It should be kept in mind, however, that the experimental data shown in these figures are not absolute in nature, and thus no particular significance can be given to the ability of the model to match XRD intensity data. Because of these limitations, it was not possible to obtain quantification of the two rate constants from the match of the Simusolv model. However, by concentrating on the
-
0.34
0.33
~~
1
f R
Table 3. Rate Constants for Reaction 8
1200 l4O0
I
0.32
-
1000
--
800
.5
-- 600
d .--h e
h
i w
0.31
4
0.3
*
c3
--
0.29
400
o.28!
0.27
1 0 0
10
20
30
40
50
Time (min) 2800
0.03
2600
0.025 2400
4
2200
0.02
a C
h
u"
.-.-
s
Zoo0
8 0.015
s
.-.-e" 8 Y,
2.
1800 'p
.-
g!
0
5?
5 e'
1600
0.01
1400 0.005 1200
1000
0
0
10
20
30
40
50
Time (min)
Flgure 7. (A, top) Comparison of sodium nickel ferrocyanide experimentaldata with Simusolv results (1:1:1 mixture, FECN36NaN03: NaN02, at 300 "C). (B, bottom) Comparison of sodium cyanate experimental data with Simusolv results. (1:1: 1 mixture, FECN36: NaN03:NaN02,at 300 "C).
latter stages of NaCNO disappearance (where reaction 7 was complete), it was possible to obtain estimates of kz for a number of temperatures, and these are listed in Table 3. While there were relatively few unobscured data points at each temperature (due to XRD peak overlapping), an Arrhenius analysis of these rate constants yielded an
T (K)
kz (kPami@-' x 104
T (K)
kz (kPamin)-1 x 104
573 583
2.9 3.6
593 613
10,5 13.3
activation energy of 134 f 8 KJ/mol, consistent with the high-temperature sensitivity usually associated with fast oxidation reactions.
Discussion The results of this DXRD study of the reaction between sodium nickel ferrocyanide and equimolar nitratehitrite salts compare well with DSC, STG, and MS results (8,9) from previous work. Observed endothermic reactions which occur up to 250 O C correspond well to the dehydration of ferrocyanide and the melting of nitratehitrite, which are both observed with DXRD. Gas analysis of separate experiments using FTIR spectroscopy (16) determined that water vapor was released at up to 220 "C, which corresponded to the temperature determined by DXRD as the point of crystal structure changes from the loss of water of hydration. Observed exothermic reactions which occur at temperatures greater than 300 "C correspond to the oxidation of sodium cyanate to sodium carbonate and to the formation of iron and nickel oxide, which are also observed with DXRD. A temperature of 300 "C was determined as the initiation of the exothermic part of the reaction sequence, reactions 8-11, and is consistent with DXRD data which show product formation beginning at 300 "C. Again separate FTIR traces (16) also detected carbon dioxide and nitrous oxide released over this same temperature range, and this is consistent with the decomposition of sodium cyanate observed in the DXRD results. The results obtained for the 1:l:l and 1:4:4 sample mixtures were very similar, differing only in the fact that the 1:4:4 produced larger quantities of the same oxidized products, namely, sodium carbonate, iron oxide (FezOd, and nickel oxide. That is, the results from both the 1:l:l and 1:44 sample mixtures showed dehydration of ferrocyanide between 180 and 220 "C and had very similar dehydration rate constants and activation energies, and both formed nickel oxide, iron oxide, and sodium cyanate in both helium and air. Complete oxidation of the cyanide ions did occur in the 1:4:4 mixture in both helium and air, but not in the 1:l:l mixture in helium which formed partial oxidation products such as sodium cyanate, unreacted products such as sodium ferrocyanide, and reduced products such as elementalnickel. Due to the great excess of equimolar nitratehitrite in the 1:8:8 mixture, the ferrocyanide peaks were almost undetectable and resulted in no useful DXRD data concerning the dehydration rate. The effect of the nitratehitrite melting was enhanced to the point of yielding a liquid solution on the DXRD hot stage (Figure 1). In this case, typical oxidation products did form but were produced as a result of solidifying out of the liquid nitrate/nitrite solution. The primary difference between experiments conducted in helium and those in air was that the helium experiments showed incomplete oxidation of the ferrocyanide when using a less than stoichiometric quantity of equimolar nitratehitrite (i.e., less than 4). Although oxygen is Environ. Sci. Technol., Vol. 28, No. 5, 1994
887
released by reactions 5 and 6 in both helium and air, the oxygen produced in the helium environment would tend to rapidly diffuse out of the sample because of the small sample sizes employed. As a result, the incomplete oxidation of the ferrocyanide in helium formed sodium ferrocyanide (Na4Fe(CN)G),sodium cyanate, nickel, and nickel nitride with small amounts of nickel and iron oxide, whereas the products from an air environment were predominantly nickel oxide, iron oxide, and sodium carbonate. It should be emphasized that the sodium cyanate forms during reactions in both helium and air, but the lack of 0 2 in the helium atmosphere results in very little additional reaction (reaction 8)of the sodium cyanate. In order to address the question of continued safety regarding the storate of ferrocyanides and nitratednitrites in the Hanford tanks, the rate of heat transfer inside the tanks should be compared to the rate of heat generated from the exothermic reactions. First of all, the standard heat of reaction for the first step in the series of reaction sequence, reaction 7, is mildly exothermic and liberates 93.6 kJ/mol of NaCNO formed. The second step, reaction 8,liberates 267.2 kJ/mol of NaCNO reacting; almost three times the heat. While these two reactions occur at roughly the same time, reaction 7 occurs more slowly at the initiation temperature of about 280 "C. While reaction 8 is three times as energetic, it requires oxygen as a reactant. If oxygen is supplied externally from contact with air, then this reaction is quite fast. However, if the surrounding environment is anoxic, then it can only proceed as fast as reaction 7 since that would be its source of oxygen. Of course, this assumes that these two are the only reactions taking place. Strictly speaking, the most thorough rate analysis would result from a model wherein the rates of both heat transfer and chemical reactions are accounted for in a simultaneous fashion. However, the reaction complexities are such that the rates of all of the coexistent reactions are not available. Nevertheless,in a "worse case scenario", we can concentrate solely on reaction 8 and assume that a hot spot does exist and has reached a temperature of 300 "C; Le., at a temperature where reaction 8 begins to take place and where there is an externally supplied oxygen source that is not rate limiting. From the kinetic analysis of reaction 8, a reaction rate for the disappearance of sodium cyanate under isothermal conditions would be given by
Recent core samples of tank 241-(2-112 (19) indicate that the "packing density" of the dry salt cake would be about 0.77 g/mL. Using this value together with the NaCNO concentration at its maximum value (scaled to the tank stoichiometry of 1:8:8), eq 15 and the heat of reaction for reaction 8 yields an estimated heat release of 0.1 KJ/s. If we now assume that a reasonably sized hot spot is at 300 "C and is on the order of 1 f t 3 (0.028m3)and we model it as a cylinder (1.3 f t high and 1.0 f t in diameter), the rate of heat generation within the volume can be compared to the rate of heat transfer from the cylinder. Assuming further that the temperature at a point 5 radii from the center of the cylinder is at the currently detected temperature (57 "C (12))and that the thermal conductivity of the wet salt cake is 0.87 J/m s-1 K-l(20), then the heattransfer rate from the cylinder is calculated to be 0.8 KJ/s or a factor of eight higher. Thus, even in the unlikely 888
Environ. Scl. Technol., Vol. 28, No. 5, 1994
event of a hot spot suddenly existing at 300 "C, the heattransfer rates are still more than sufficient to remove the generated heat.
Conclusions The results showed that a reaction between sodium nickel ferrocyanide and equimolar nitrate/nitrite begins by dehydration of the ferrocyanide at 180 "C, followed by melting of the nitrate/nitrite at 230 " C, and finally resulting in an exothermic reaction at 300 "C. Since the temperatures in the tanks are well below the temperatures required for the exothermic reaction to occur, the danger of an explosive reaction occurring is very low. The results here have shown that a major factor in the reactions at 300 "C is the occurrence of a series reaction that produces NaCNO as an intermediate in a first step, which is mildly exothermic. In an anoxic environment, the NaCNO is relatively inactive but, in the presence of gaseous oxygen, it further reacts, rapidly and exothermically, to form metal oxides. Furthermore, it has been shown that the rate of heat release from the oxidation of the NaCNO, even in the presence of excess oxygen, is still a factor of 8 lower than the estimated heat-transfer rate from a localized hot spot. This means that excessive heat will not buildup in the tank causing the reaction to become uncontrollable. Given the fact that the Hanford waste tanks which are thought to contain ferrocyanides have been thermally stable for almost 20 years, the occurrence of a hot spot would be slow in developing. The reaction rates measured in this study show that a relatively slow intermediate reaction, which is mildly exothermic, precedes the high-energy exothermic reactions. This and all of the other available data lead to the conclusion that the danger of explosion from ferrocyanide-containing wastes and nitrates/nitrites in the Hanford tanks is very low.
Acknowledgments The authors would like to acknowledge the financial support of Battelle PNL and express their appreciation for the valuable assistance given by R. T. Hallen and G . Schielfebein.
Literature Cited (1) Burger, L. L.; Reynolds, D. A,; Schulz, W. W.; Strachan, D. M. A Summary of Available Information on Ferrocyanide Tank Wastes; Pacific Northwest Laboratory: Richland, WA, 1991; PNL-7822. (2) Hepworth, J. L.; McClanahan, E. D.; Moore, R. L. Cesium Packaging Studies-Conversion of Zinc Ferrocyanide to a Cesium Chloride Product; Hanford Atomic Products Operation Co.: Richland, WA, 1957; HW-48832. (3) Scheele, R. D.; Burger, L. L.; Tingey, J. M.; Bryan, S. A.; Borsheim, G. L.; Simpson, B. C.; Cash, R. J.; Cady, H. H. Ferrocyanide Containing Waste Tanks: Ferrocyanide Chemistry and Reactivity. In Proceedings of Environmental Restoration 91; University of Arizona: Tucson, Sep 1991. (4) Burger, L. L. Complexant Stability Investigations. Task 1. Ferrocyanide Solids; Pacific Northwest Laboratory: Richland, WA, 1984; PNL-5441. (5) Burger, L. L.; Scheele, R. D. 1988, Interim Report on Cyanide Safety Studies; Pacific Northwest Laboratory: Richland, WA, 1988; PNL-7175. (6) Burger, L. L.; Scheele,R. D. The Reactivity of CesiumNickel Ferrocyanide Towards Nitrate and Nitrite Salts; Pacific Northwest Laboratory: Richland, WA, 1990; PNL-7175. (7) Scheele, R. D.; Cady, H. H. Preliminary Safe-Handling Experiments on a Mixture of Cesium Nickel Ferrocyanide
and Equimolar Sodium NitrateJNitrite;Pacific Northwest Laboratory: Richland, WA, 1989; PNL-7928. (8) Hallen, R. T.; Burger, L. L.; Hockey, R, L.; Lilga, M. A.; Scheele, R. D.; Tingey, J. M. Ferrocyanide Safety Project Project F Y 1991 Annual Report; Pacific Northwest Laboratory: Richland, WA, 1991; PNL-8165. (9) Scheele, R.D.; Burger, L. L.; Tingey, J. M.; Hallen, R. T.; Lilga, M. A. Chemical Reactivity of Potential Ferrocyanide Precipitates in Hanford Tanks with Nitrates and Nitrites; WM'92 ConferenceProceedings, Waste Management Symposia Inc.: Tucson, A 2 1992. (10) Thomson, W. J. Dynamic X-ray Diffraction: A Technique for Following Solid-StateReactions. Ceramic Transaction, Vol. 5, Advanced Characterization Techniques for Ceramics;Young, W. S., McVay, G. L., Pike, G. E., Eds.; American Ceramic Society: Westerville, OH, 1989; pp 131-140. (11) Anderson, D. E.; Thomson, W. J. Ind. Eng. Chem. Res. 1987, 26, 1628. (12) Hanlon, B. M. Tank Farm Surveillance and Waste Status Report for December 1990; Westinghouse Hanford Richland, WA, Apr 1991; WHC-EP-0182-33. (13) Sloat, R. S. T B P Plant Nickel Ferrocyanide Scavenging Flowsheet; General Electric Co.: Richland, WA, 1954;HW30399.
(14) Wells, A. F, Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1984. (15) Charlot, G. Qualitatiue Inorganic Analysis; John Wiley and Sons: New York, 1954. (16) Sheifelbein, G. Battelle PNL, Richland, WA, personal communication, Sep 1992. (17) Kirk, R. E., Othmer, D. F. Eds. Encyclopedia of Chemical Technology; Interscience Publishers, Inc.: New York, 1949; VOl. 4. (18) Simusolv, A New Method for Numerical Parameter Estimation; Dow Chemical: 1986. (19) Westinghouse Hanford Co. Report WHC-EP-0640, WHC: Richland, WA, 1991. (20) McLaren, J. M. Single-Shell Tank 104-BY ThermalHydraulic Analysis; Westinghouse Hanford Co. Report WHC-SD-WM-ER-083;WHC: Richland, WA, Apr 1991.
Received for review August 6, 1993. Revised manuscript received January 10, 1994. Accepted January 31, 1994.'
Abstract published in Advance ACS Abstracts, March 15,1994.
Envlron. Sci. Technoi., Vol. 28, No. 5, 1994 889
