RAD IANT-H EAT, S PRAY-CALCI NATION PROCESS FOR SOLIDIFICATION OF RADIOACTIVE WASTE RUDOLPH T. ALLEMANN AND BENJAMIN M . JOHNSON Hanford Atomic Products Operation, General Electric Co., Richland, Il'ash. The application of the radiant-heat spray calciner to the continuous calcination of simulated radioactive waste was studied to develop more reliable methods of high-level waste storage. The experimental equipment is described, and the effects of various operating variables on factors such as capacity and completeness of calcination are discussed. The process has several characteristics which make it attractive for waste calcination: Operation is insensitive to changes in waste composition and is readily adaptable to many different types of feed. The simplicity of equipment and operation indicates a remote installation should not b e difficult to run and maintain.
calcination is one of several methods which are being studied at laboratories of the U. S. Atomic Energy Commission for the solidification of radioactive wastes. Development was begun a t the Hanford Laboratories as the result of interest in studies made at the K-25 Plant a t Oak Ridge (2) on the spray calcination of uranyl nitrate to uranium trioxide and extensive work on a similar process for the pyrolysis of sulfite liquors done by the Pulp and Paper Research Institute of Canada ( 3 ) . The practice of storing the current high-level waste volume of approximately 60,000,000 gallons in giant underground tanks is a t present adequately safe and the most economical method. However. the projected volume of this type of waste over the next 40 )-ears, based on a moderately ambitious program of nuclear px+-er production and continued use of solvent extraction for fuel reprocessing, ranges from 0.6 to 2.4 billion gallons. The presence of transuranic elements, radioactive strontium, cesium! and several other long-lived fission products necessitates the storage of these large volumes of waste for an indefinite period (hundreds of years). Permanent storage of wastes by the present method would require multiple tanks, since the soundness of an individual tank cannot be assumed over such a length of time. Hence. the total tank volume required may be several times the initial volume. Conversion of these aqueous wastes to a stable, confined solid will allow for long-term storage of these wastes in reasonable volume. Several methods of calcining radioactive wastes on a continuous or semicontinuous basis have been investigated. As yet no engineering-scale tests have been run on full-level (undiluted) wastes within the AEC. The work reported in this paper was with simulated, nonradioactive, waste solutions. ADIANT-HEAT
the steam, droplets, and resultant solid particles. Spray configuration must be such that the droplets do not hit the walls of the column before drying; otherwise a deposit of calcined solids builds up at the point of contact. Gauvin and Gravel ( 4 ) have described the fluid mechanical aspects of this general type of operation. The solid particles, predominantly - 325 mesh. and off-gases leave the bottom of the column together and are separated by filters. Since a massive solid is desired as a final product. the powder is further heated to its melting point in the final storage container, or alternatively in an interim melt-pouring furnace. The treatment of the off-gases involves condensation and re-evaporation of condensables, treatment of noncondensables to remove volatilized fission
Filter Element: Spring Loaded Against Tapered Fitting at the Bottom
One of 15 Filter Elements
\/
232
I & E C PROCESS D E S I G N A N D DEVELOPMENT
"
ment e
Process Description
In the radiant-heat, spray-calcination process (7), the waste solution or slurry is atomized into the top of a tower with hot walls (about 800' C , ) : where the droplets are evaporated, dried. and calcined as they proceed down the tower. Steam is used (in a two-fluid nozzle) to atomize the liquid feed. No other foreign gas is employed to effect the drying, since the heat is transferred primarily as radiant energy from the hot walls to
For Blow Back
Induction Heated
Scale
&
Figure 1.
Schematic of proposed calciner
Press u r e Gage
Air Steam
0 Off Gas to Samplers
lrod Heaters Banded
Variable Recirculating Speed Pump Pump
Low Voltage Supply 6.4 Volts, 3900 Amps. Sch. 10 Stain less
\
I nconel Section 16 Gage
Ye'
Cooling Section
Air
Scrub Solution Receiver
Time Condensate Receiver Pressure Drop Manometer Figure 2.
Schematic of experimental reactor
products, and final fi1:ration through a n AEC filter befoie dispersal. ,4 portion of the condensate may be recycled to the calciner to retain within the system those solids which passed through the filter. The off-gas treatment is similar to that proposed for other types of calcination processes for high-level radioactive wastes. Since the capital cost of the calciner itself \Till be minor relative to site costs, the actual configuration of the unit may be dictated by factors other than optimum design of the reactor. An axample of a calciner capable of processing 50 gallons per hour of waste solution is shown in Figure 1. Experimental Equipment
T h e calcination reactor used in the work reported here (Figure 2) was a n 8-imh-diameter column, 10 feet long, constructed in three sections: a 4'/~-foot top section and a 2-foot bottom section of Schedule 10 stainless steel pipe, with a 3'/2foot center section of 16-.gageInconel sheet Heat was supplied by ten 2000-watt Calrod heaters bonded to the side of the upper section of the column, and by low voltage current passed through the entire column wall. Seventy-five per cent of the energy from this latter source was localized in the center section, which was operated a t a temperature up to 875' C. T h e low-voltage current was obtained from a welding transformer equipped to supply secondary voltage in 16 steps up to 6.4 volts a t 3900 amperes. T h e maximum allowable sheath temperature of the Calrod units limited the power to the top section of the column to about 13 kw. U p to 4 gallons per hour of simulated waste solution was fed to the top of the unit 1:hrough a pneumatic atomizing nozzle which protruded through a water-cooled insert a t the center of the top plate. Wall temperatures, measured a t six points along the column, and gas temperatures, measured a t two points within the
reactor. served to indicate the quality of atomization and to detect off-standard operation within the reactor. The solid particles and vapor were cooled somewhat toward the bottom of the column and passed into the filter section. During many of the runs, the filter elements were porous ceramic rated a t 5-micron retention and having a total surface area of 4.2 sq. feet. Each filter was cleaned periodically cvith a short pulse of high pressure air introduced through a tube located concentric to, and just downstream of, a Venturi section in the filter outlet. Solids were collected in a receiver which was sealed to the bottom of the filter section. The receiver could be heated to the sintering or melting point of the solids to form a coagulated product. Off-gases passed through a downdraft condenser and were then scrubbed with caustic to remove remaining acidic gases before being exhausted. Samples were taken of condensate and noncondensable gases. Gases were analyzed either by a n absorption train or a mass spectrograph; particulate content was determined by passing the gas through a cellulose nitrate membrane filter and analyzing the filter for one of the insoluble constituents of the solid, generally iron. Operating Procedure
Although the unit was intended for preliminary studies and was equipped \vith a minimum of control instrumentation. operation was very simple After the reactor had been preheated, which required about 45 minutes, superheated steam and then water were introduced through the atomizing nozzle. During the initial period, pressure and flow rates were stabilized by manual adjustment of feed rates and gas flow to the vacuum system of the building. T h e water to the atomizing nozzle was then replaced by simulated waste solution, and appropriate changes were made to permit the collection of the condensate and scrub solution and the analysis of the off-gas. Time required to reach steadyVOL. 2
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state operation was generally less than 5 minutes after introduction of the feed. Thick slurries as well as solutions were processed conveniently, so long as solids were finely divided and could be maintained in suspension within the feed tank. Poor atomization or excessive throughput could be detected by rapid changes in temperature indicated by the probe located near the center of the reactor 50 inches beneath the nozzle. Shutdown procedure involved reverting to water to flush the feed lines and reactor system before terminating operation.
Table 1.
Most runs were for 1 to 3 hours, though several extended runs as long as 23 hours were made. Results and Discussion
T h e concept of waste solidification involving the radiant-heat spray calciner which developed of these studies the calciner Principally as a dryer and the subsequent container as the calcination vessel. Originally the spray
Compositions of Simulated Wastes Calcined in the Radiant-Heat Spray Calciner
Composition Purex I. Acid killed-high sodium-low sulfate, vol. = 25 gal./ton of U Na 2.50 M NO? 2.86M Fe 0.50 M SO; 1.25 M A1 0.25.44 Ni 0,015 M Cr 0.025 M H 0.50M Purex 11. Acid killed-low sodium-high sulfate, vol. = 25 gal./ton of U Na 1.00 M NO3 0.52 M Fe 0.60 M SO4 2.60M AI 0.20 M Ni 0 . 0 1 Ad Cr 0.02 M H 2.22M Purex 111. High-acid-neutralized \Y/caustic, vol. = 50 gal./ton of U Supernate Na 7.65.44 NO3 7.15 M SO1 0.50 M Solids Fe( OH), 0 . 2 mole/l. AI(OH)3 0 . 1 mole/l. Ni(0H)z 0 006 mole/l. Cr(0H)a 0 010 mole/l. Purex IV. High acid plus coating waste, vol. = 246 gal./ton of U Supernate Na 2.7 M NO3 0.842 M NO2 0.835 M SO4 0.203 M A102 0.55 M Si04 0.017 A4 Solids Fe(0H)a 0.061 mole/l. Al(OH)a 0,677 mole/l. Cr(0H)a 0.002 mole/l. Purex V. Coating waste, vol. = 209 gal./ ton of U Xa 3.43 M A102 1.44 M OH 0.57 M NO3 0.31 M TBP-25. Vol. = 0.11 - 0.22 gal./g. U-235 .41 1.6M No3 5.5 M NHa 0.05 M SO4 0.02 M 0.01 M 0.002 M H 0.5 M Darex. Vol. = 0.5 gal./g. U-235 Fe 0.68 M No3 6.1 M Cr 0.16 M A1 0.123 M Ni 0,075 M H 2.94 M Zirflex, Neutralized NH4 1 . 9 M ZrFG-* 0.06 M F 0.6 M NO3 0.08 M a Added to the final powder.
Additives
Powder
Density, G./Cc. M e l t or sinter
Volume Reduction Factor, Vol. Feed Vol. M e l t or Sinter
No additive W / p hosphate HaPo4 = 1.62 M \%'/phosphate and sugar = 250 g./l.
0.4 0.8-0.9
2 . 4 (S) 2.8 (M)
12 12.5
0.9-1.1
2.9 (M)
13
No additive W/sugar = 150 g./l. W/DhosDhate HaPo4 = 1.16 M \%'/phosphateand sugar = 250 g./l.
0.7-0.8
2 . 3 (M-2 phase) 2 . 3 (M-2 phase) 2.8 ( M ) 2 . 9 (M)
12 12 12.5 13
0.7-0.8
0.8-0,9 0.9-1.1 0.8
...
No additives W/sugar = 200 g./1.
0.85
(Cannot be calcined) 2.3
No additive
0.65 0.64
W/calcium, Ca(OH)2
W/sugar = 150 g./l. W/sugar and
=
=
1. 0 M
75 g./l."
2 . 3 (Mj'
5.5
8.5
No additive W/sugar = 150 g./l. W/sugar and H ~ B O = I 75 g./I."
0.57 0.20
No additive W/borate, NasB404 = 1.22 M
0.18 0.70
No additive
0.81
...
7
Neutralized w/NHdOH
1.30
...
11
No additive
0.67
...
... 2.3 (M)
...
2.3 (M)
10
37
F$
234
I & E C PROCESS DESIGN A N D DEVELOPMENT
3
85
Radiant-Heat Spray Calciner w i t h Simulated High Sulfate Purex Waste Feed Rate 120 mllniin 2
A
Neutralized Waste
c 0 m
8
2
Em
.-
9 " E
V
L
1.0
10 L
0
Figure 4.
50
100 150 Sugar Added to Feed (gramslliter)
200
250
Sugar is effective in reducing unstable sulfates
I
I
I
300
400
500
Figure ciner
5.
I
I
600 700 Temperature, OC,
I
I
800
900
Temperature profile during operation of col-
Spray calciner temperature profile f e e d r a t e 50 mi. per minute
nearly complete calcination of the powder during its short residence time in the reactor. If sufficient sodium sulfate is present in the solid product, the material will melt in the vicinity of 850' to 950' C. T h e addition of sodium sulfate to the feed solution is one obvious method of promoting melt formation. Other studies on the formation of melts and glasses from Purex high-level wastes indicated that adding phosphoric acid to the feed liquor (sufficient to form orthophosphates of all cations) resulted in a low melting solid (800' to 900' C.). Borates and silicates were also considered as fluxing agents. Borates were used primarily with aluminum-bearing wastes, while silicates were considered only briefly because of their high melting point and high viscosity as melts. The addirion of these fluxing agents generally increased the bulk density of the powder if the temperature of the reactor was sufficiently high to result in partial melting of the droplets and formation of more dense and spherical particles. The final solid material, produced by melting or sintering the powder, ranged in density of from 2.3 to 3.0 grams per cc. Volume reduction factors from the original waste solutions to the coagulated solid ranged from 9 to 12. The addition of sugar to the feed solution proved to be of value in a number of situations. Principally, it was used as a reducing agent to decompose sodium nitrate and unstable ferric and aluminum sulfates. Neutralized wastes, which generally contain a high concentration of sodium nitrate, cannot be calcined successfully without adding sugar to decompose the sodium nitrate. This constituent is an essentially stable liquid a t the temperatures used in the spray calciner. I n this form it coated the column walls and could not be processed unless converted to a higher melting compound. The reaction between the sugar and the nitrate to form sodium carbonate is very rapid a t the operating temperature of the reactor. High-sulfate \saste-i.e., solutions which form ferric and aluminum sulfates upon initial drying, such as Purex 11, Table I-were not completely calcined during the short residence time in the column: which was from 5 to 13 seconds, 236
I&EC PROCESS DESIGN AND DEVELOPMENT
depending on the feed rate. Incompletely calcined powders decomposed further to oxides of iron and aluminum when heated for an extended period a t 900' C. The addition of more than 150 grams per liter of sugar, which served as a reducing agent, was effective in accelerating the decomposition of these compounds, as shown in Figure 4. The decomposition of the sugar also provided a carbonaceous residue which ignited to augment the particle temperature and improved the powder characteristics of phosphate-treated waste, presumably by heating the particles to the melting point. Under these conditions the gas and solids temperature exceeded the wall temperature in the center section of the column because of the ignition of the sugar residue (Figure 5). Wall Deposits. With some types of simulated waste solutions, powder was returned to the top of the column by turbulent backmixing in the nozzle zone and adhered to the wall on contact. Generally, this buildup was easily removed by vibration or a sharp tap on the column by means of a pneumatic hammer. Plugging of the column by this deposit did not appear to be a problem. However, solutions that form low-melting, temperature-stable solids? such as sodium nitrate, coat the column walls and cannot be processed unless converted to a higher melting solid. The fraction of the total powder that was held,upon the wall for a portion of its total residence time in the reactor was determined by placing in the bottom of the column a system of baffles which caught the agglomerated wall deposit Lvhile permitting the other powder to pass through. The fraction of wall deposit generally tended to increase with higher processing rates and lower atomizing steam rates; under some conditions it was as high as 507,. One method of reducing backmixing and powder buildup was studied briefly and is incorporated in the design of a proposed spray calcination reactor (Figure 1). The proposed design uses a scheme called the "channel-recycle" principle. A heated interior wall is hung from the top of the column and equipped with ports a t the top to provide for recirculation of gases from the annular region. I t allows the gas needed
100
= -
30
1
-
I
~
” 0
8
x
-
-.-
Few Kale OL rniimin Simulated Purex Hiah-Sulfate Waste
Acid - No Sugar
xx--
-
a.
I
Acid
0
- 50 g / P Sugar
u l (3
z 10 - Neutralized - 200 g/R Sugar 1 -
n \
0)
-
/ Residual Unstable Sulfate
L
W
W c
0 L
d
l
0 1
Figure 6.
2 3 Feed Rate, Gal./Hr.
4
Effect of feed rate on extent of calcination
for entrainment in the spray jet to be provided from a region lower down in the column where the solid particles have become throughly dry. Preliminary studies have indicated that the recycle principle is very effective in eliminating local backmixing and stabilizing the configuration of the spray jet. Gas-Solids Separation. Efficient separation of solid powder from the off-gas is essential in the application of a calciner to waste fixation. Although not studied extensively, the behavior of various solids separation devices in the spray calciner system was evaluated. Cyclone separators were of marginal value. Most particles were too small and light to be effectively removed and often more than 20% would pass on through to the condenser. Porous-metal filters were generally the most effective, but the Type-31 6 stainless steel elements initially used were corroded by the off-gases from some feed solutions. A more corrosion-resistant a!loy lvould have to be used for long-term service. Solids de-entrainment factor through the elements (50-micron pore size), determined by concentration of sodium in feed/concentration of sodium in concentrate, was about 1.5 X lo4. The pulse blowback cleaning system kept the pressure drop a t 1.8 to 2.4 inches of mercury for gas flow rates of 13 cu. ft.,’hr.,sq. ft. during runs of 18 to 20 hours. Tubular ceramic filter elements of 5-micron pore size were less effective because of difficulty in obtaining good blowback cleaning. The combination of doivn-draft condenser and scrubber contributed a decontamination factor of about 104. T h e entire system could be operated ivith a de-entrainment factor of about 2 X 108. This !vas determined by filtering all off-gases through an 0.8-micron membrane filter and analyzing the paper colorjmetr ically for iron. A calcination system processing full-level radioactive waste lvould have to achieve a decontamination factor somewhat in excess of that (about 1O’O) before the off-gas could safely be vented to the atmosphere. Capacity. It is difficult to predict, except from experience, the capacity of a radlant-heat spray calciner, although considerable similarity exists between this type of process and spray drying. Studies by Hoffman (5) have indicated that heat is ultimately supplied as radiant energy from the reactor walls,
0
20
I I I I I 40 60 80 1CO 120 Oxygen Added-Per Cent Stoichiometric
1 140
Figure 7. Oxygen is needed to initiate reduction of SUIfate b y sugar
but the primary mechanism for heat transfer to the evaporating droplets is by convection from the hot steam within the column because of the high velocity of the droplets and rhe resultant large convective heat-transfer coefficient. Once the particles are dried, hoivever: heating is probably by radiant heating from the walls. The first indication that the capacity of the reactor has been exceeded is that incompletely dried particles hit the walls of the reactor and build up a crust. Studies of the floiv regime existing in the reactor, involving measurement of the eddy diffusivity, shoived it to be highly turbulent. As further evidence, the temperature of the gas and particles (they are very nearly the same temperature) are approximately uniform radially across the column. The only exception is in the region a short distance below the nozzle as shown in Figure 5, a typical longitudinal temperature profile of the calciner during operation. Consequently, scale-up of the reactor is not expected to be hampered seriously by attenuation of radiant energy through the cloud of solid particles. Thus, for reactors u p to perhaps 3 feet in diameter the temperature should not vary significantly across the radius. Possibly the capacity of the spray calciner may be predicted by utilizing the method of Hottel and Cohen (6) as adapted by Hoffman (5) to determine the net rate of heat input into the system. However, the method involves a number of questionable assumptions and its application is very time-consuming. Its use has not been studied in connection with this work. Four gallons per hour was the maximum feed rate processed in the experimental unit, but the system was limited by the capacity of the off-gas facilities and not the reactor. Operating Variables. The presence of ferric and aluminum sulfate in the product of a high-sulfate Purex waste provided a convenient method for observing the effects of several operating variables on the extent of calcination. Throughput. Increasing the feed rate up to the maximum of 4 gallons per hour slightly decreased the extent of calcination (increase in weight loss on subsequent heating of the powder) with typical simulated waste solutions (Purex 11, Table I) as shown in Figure 6. Neutralized waste, with sugar added, behaved in similar fashion. However, acid waste with sugar
VOL.
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JULY 1963
237
1.4
1.3 1.2
ci
1.1
1
I
Acidic Waste
eutralized Waste
--
-U ... cn
*.-
1.0 --
VI
a! c
a
f
a!
0.9 .-
a!
9
0.8
-
’
Acidic Waste
W/l5@/l.Sugar
0.6 0.5
0
1
2
3
4
Feed Rate - Gal. Ihr.
Figure 8. Effect of feed rate on powder density was calcined more completely a t the higher feed rate. This apparently anomalous behavior is explained by noting that these runs were made a t a fixed rate of oxygen bled to the top of the column and consequently a stoichiometric decrease in oxygen (based on complete oxidation of the sugar) with increasing feed rate. Figure 7 indicates that some oxygen is desirable to facilitate the reduction of the sulfates by the sugar. but if present in excess of the optimum it decreases the effectiveness of the sugar. I n all runs included in Figure 6, the stoichiometric quantity was in excess of optimum; hence the reduction of sulfates increased with feed rate. This behavior is not evident in the case of neutralized feed and sugar, because the sodium nitrate provided a source of oxygen a t a constant stoichiometric ratio. The density of the powder product also increased with feed rate for most feed compositions, as shown in Figure 8. This was the result of the lower temperature in the nozzle zone a t the higher feed rate. Consequently, drying of the particles was slower, shattering was less pronounced, and the tendency to consolidate in the hottest regions of the reactor was more marked, Temperature control of the upper section of the column was from a point 30 inches below the nozzle; hence, with a heavier evaporative load the top operated a t a lower temperature. An exception to the trend of increasing density with feed rate existed, if the residual quantity of unstable sulfates decreased with increasing feed rate-e.g., acid waste and 200 grams of sugar per liter. These sulfate constituents
238
contributed to the consolidation of the particle in the hightemperature region of the reactor and their removal resulted in a porous particle of lokver density. Atomizing Steam Rate. Within the normal operating range of the two-fluid nozzle, steam pressure did not affect the extent of calcination in any consistent manner. However, the density of the powder increased slightly and the quantity of powder which was temporarily held up on the walls decreased at the higher steam flows. A definite lower limit for the steam-to-feed ratio did exist, nevertheless, below which the system would not operate efficiently. The onset of this unstable flow ratio: a t which the quantity of steam is inadequate to atomize the liquid effectively, depends o n the design of the nozzle and the liquid properties, such as viscosity, density, and surface tension. The lower limit of the ratio of Iveight of steam to weight of liquid was about 0.5. Temperature. Because of the very short residence time for the calcination reactors to take place, the reactor walls were kept a t the highest temperature thought to be safe, from the standpoint of corrosion of the materials of construction. Figure 5 shows the temperature profile during a typical run. No studies were made of the effect of lower reactor temperatures upon the extent of calcination.
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Conclusions
Spray calcination provides a solution to the heat transfer problem of rapidly drying a concentrated salt solution without introducing the large volume of gas that is inherent in conventional spray drying. Its application to high-level waste has been studied only on a laboratory scale and because of the high degree of reliability needed in remote installations involving radioactive materials, additional development work is needed before its capabilities can be fully assessed. For example, a smaller unit approximately 10 inches in diameter and 6 feet tall is being constructed within a high-level cell for operation with both dilute and full-level radioactive wastes. In addition, a large pilot plant unit is being assembled for operation with simulated waste solutions to investigate the engineering aspects of the process, including the reliability ol various components and the scale-up parameters. Literature Cited (1) Allemann, R. T., Johnson, B. M., “Radiant Heat Spray
Calcination Process for the Solid Fixation of Radioactive Waste,” Hanford Atomic Products Operation, Richland, Wash., HW-65806 (February 1961). (2) Allen, A. L., Bernhardt, H. A., Bernstein, S., Harrison, R. H., Powell, E. W., “Spray Decomposition of Uranyl Nitrate Solutions to Uranium Trioxide,” Union Carbide Carp., K-25 Plant, Oak Ridge, Tenn., K-444 (July 1949). ( 3 ) Gauvin, W. H., Tappi 40, No. 11, 866-72 (November 1957). (4) Gauvin, W.H., Gravel, J. J . O., Trans. Inst. Chem. Engrs., in press. (5) Hoffman, T. W., Ph.D. thesis, McGill University, .4pril 1959. (6) Hottel, H. C.: Cohen, E. S., A.Z.Ch.E. J . 4, 3 (1958). RECEIVED for review September 10, 1962 ACCEPTEDMarch 8, 1963
