Molten salt destruction of alkali and alkaline earth metals

A process for destroying alkali metal and alkaline earth metal-containing wastes, such as sodium, by feeding such waste into a molten bath containing a molten salt such as sodium carbonate, or a mixture of salts having a lower melting point, such as a mixture of sodium carbonate and an alkali metal halide, e.g. sodium chloride, or mixtures of alkali metal chlorides, feeding a mixture of carbon dioxide and oxygen into the molten salt bath and reacting the alkali metal or alkaline earth metal such as sodium in the waste with the carbon dioxide and oxygen to form alkali metal carbonate, e.g. sodium carbonate, in the molten salt bath.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the destruction of alkali metal and alkaline 
earth metal waste. It particularly relates to a molten salt process for 
destroying alkali metals such as sodium, potassium, and lithium, and 
mixtures thereof, present in liquid metal reactor coolants. 
2. Description of the Prior Art 
Alkali and alkaline earth metal waste e.g. radioactive sodium waste from 
reactor heat transfer media, are hazardous wastes because of the 
characteristic of reactivity. Before they can be disposed of, this 
characteristic must be destroyed. A large inventory of such metals, 
particularly sodium, potassium, lithium and mixtures of these metals are 
also radioactive. 
A number of processes for the disposal of these metals such as sodium 
exists. These include reaction of sodium with water, reaction of sodium 
with concentrated caustic, reaction with alcohol, burning of sodium in 
oxygen, calcination with silica, and reactions of sodium with ammonia, 
hydrogen, halogens and nitrous oxide. However, these present methods for 
the destruction of the characteristic of reactivity of alkali metals such 
as sodium have been proven generally unsatisfactory and disadvantageous 
for various reasons, such as the production of hydrogen, an explosive gas, 
or oxides, which are highly corrosive. Thus, the provision of a suitable 
method and the design of facilities to process large quantities of alkali 
and alkaline earth metal waste, such as sodium, is very challenging. 
SUMMARY OF THE INVENTION 
According to the present invention, alkali and alkaline earth metals such 
as sodium, are reacted with a mixture of carbon dioxide and oxygen (or 
air) in a molten salt bath, e.g. molten alkali carbonate such as sodium 
carbonate. The alkali carbonate, e.g. sodium carbonate salt, resulting 
from the oxidation reaction is not hazardous and merely adds to the volume 
of salt in the bath. If the feed metal such as sodium is radioactive, most 
of the radioactivity remains in the molten salt bed. When operated as 
either a batch or continuous process, there are no reactive off-gas 
products. The reaction rate and operating temperature are readily 
controllable and there is essentially no release of radioactive materials 
from the salt bath. 
The salt composition of the molten salt bath can be tailored to lower the 
melting point of the salt. For example, if sodium is converted to sodium 
carbonate by reaction with oxygen and carbon dioxide, the melting point of 
the salt will be about 855.degree. C. The reaction can be carried out at 
much lower temperatures, e.g. within the limits of about 200.degree. C. to 
about 900.degree. C., provided that the molten salt bed contains salts 
which result in lower melting mixtures. For example, these salts may 
include or consist of mixtures of alkali metal halides, e.g. chlorides and 
alkaline earth halides, e.g. calcium chlorides, and mixtures thereof. The 
properties of the salt composition may also be tailored by adding sulfate, 
phosphate or nitrate salts. Some make-up, for example, chloride salt, will 
be needed to maintain the low melting point. At lower molten salt bath 
temperatures, the salt vapor pressure and radioactive element carryover 
will be lower, corrosion will be reduced and metal containment vessels, 
such as Inconel 600, can be used. As another alternative, if chloride 
salts are used, chlorine gas or chlorinated hydrocarbons can be sparged 
through the molten salt bed to convert carbonates to chlorides. 
OBJECTS OF THE INVENTION 
It is accordingly one object of the present invention to provide an 
improved method for the destruction of alkali and alkaline earth metal 
hazardous waste. 
Another object is the provision of a relatively simple process for the 
destruction of such hazardous waste, e.g. sodium, in a molten salt bath to 
form non-hazardous alkali and alkaline earth metal salts. 
A particular object is the provision of a method for destruction of alkali 
and alkaline earth metals, especially sodium, by reaction with a mixture 
of carbon dioxide and oxygen in a molten carbonate salt bath.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the broadest aspects of the invention, there is provided a process for 
destroying alkali metal and alkaline earth metal hazardous waste and 
converting it into non-hazardous salt which comprises feeding the alkali 
metal or alkaline earth metal-containing hazardous waste into a molten 
salt bath containing a molten salt selected from the group consisting of 
an alkali metal carbonate, such as sodium carbonate, an alkali metal 
halide, such as sodium chloride, an alkaline earth halide such as calcium 
chloride, and mixtures thereof, feeding a mixture of carbon dioxide and 
oxygen into the molten salt bath, the proportions of carbon dioxide and 
oxygen being at least sufficient to react stoichiometrically with the 
alkali metal or alkaline earth metal in the waste, and reacting the alkali 
metal or the alkaline earth metal with the carbon dioxide and oxygen in 
the molten salt bath at a temperature above the melting point of the salt 
in the molten salt bath, and converting the alkali metal or the alkaline 
earth metal into a non-hazardous carbonate salt in the bath. The salts 
formed in the molten carbonate bath are alkali metal or alkaline earth 
metal carbonates such as sodium, potassium, lithium or calcium carbonate. 
Sulfate such as sodium sulfate, phosphate such as sodium phosphate, or 
nitrate such as sodium nitrate, can be added to the molten salt bath to 
tailor specific properties of the salt such as melting point, fission 
product retention, physical form of salt waste, physical form of salt 
collected on the filter, and the like. 
There are two aspects of the invention. The first is the in-situ conversion 
of the alkali or alkaline earth metal, e.g. sodium metal, to sodium 
carbonate in the presence of a molten, e.g. sodium carbonate, salt which 
has a partial pressure of oxygen and a partial pressure of carbon dioxide 
in the salt bath, as by bubbling or sparging the CO.sub.2 and O.sub.2 into 
the molten salt bath. Thus, for the conversion of sodium to sodium 
carbonate by reaction with oxygen and carbon dioxide in molten sodium 
carbonate alone, the reaction takes place at a temperature of about 
800.degree. C. to about 900.degree. C., more particularly about 
855.degree. C., the melting point of sodium carbonate. 
The proportions of CO.sub.2 and O.sub.2 must be sufficient to react 
stoichiometrically with the alkali metal or alkaline earth metal, to form 
the alkali metal or alkaline earth metal carbonate, e.g. sodium carbonate. 
It has been found that if the amount of oxygen is insufficient to react 
with sodium to form sodium carbonate, then some CO by-product can be 
formed. If there is an excess amount of oxygen then only CO.sub.2 will be 
formed in the reaction, with no CO present. It is preferred to operate 
with an excess amount of oxygen and an excess amount of carbon dioxide, 
e.g. of at least 10%, and ranging from an excess of from about 10% to 
about 200%, of the stoichiometric amount required for reaction with the 
alkali metal or alkaline earth metal in the bath. The alkali metal or 
alkaline earth metal reacts with the CO.sub.2 and O.sub.2 gases in the 
molten salt bath so that the gases are being consumed and converted into 
carbonate salts, with the alkali metal or alkaline earth metal. 
Theoretically, complete conversion of the alkali metal or alkaline earth 
metal to carbonates can occur with no off-gases. It is preferred to 
operate with at least a 10% excess of CO.sub.2 and O.sub.2 simply to 
maintain turbulence and mixing in the molten salt bath. In practice, 
preferably not more than a 100% excess of the CO.sub.2 and O.sub.2 is 
employed. 
According to a second aspect of the invention, the reaction can be carried 
out at a lower temperature and in liquid sodium carbonate, by employing a 
mixture of salts that has a lower melting point, such as a mixture of 
alkali metal or alkaline earth metal halides, e.g. chlorides such as 
sodium chloride, or by employing a eutectic alkali carbonate mixture, 
consisting for example of 50% Na.sub.2 CO.sub.3 and 50% K.sub.2 CO.sub.3, 
by weight, melting at 710.degree. C. Use of such mixtures will reduce the 
temperature in the molten salt bath, e.g. to a range of say 
600.degree.-800.degree. C. Also, a lower melting point mixture of two or 
more salts such as NaCl, KCl or CaCl.sub.2, can be employed, alone, or in 
combination with alkali carbonate, e.g. sodium carbonate. 
One can initially start the process employing molten pure alkali carbonate 
salt or pure chloride salts. Carbonates such as sodium carbonate are 
formed in the molten bath and increasing the amount thereof in the bath as 
the process continues, unless a source of chlorine such as chlorine gas or 
chlorinated hydrocarbons is simultaneously added to the molten salt bath 
to convert the sodium carbonate to chlorides. The molten salt bath can be 
100% alkali metal carbonate, e.g. sodium carbonate, or 100% chlorides, or 
any mixture thereof, and the molten salt bath temperature will vary 
depending on the composition thereof. 
To maintain a reduced temperature of about 750.degree. C. in the molten 
salt bath, a mixture of carbonate and chloride approximately corresponding 
to the eutectic composition of about 50% sodium carbonate and about 50% 
sodium chloride, by weight, can be employed, and such composition is 
maintained near the eutectic composition by draining salt as it is 
produced in the molten salt bath. However, the composition of the molten 
salt bath can vary from the eutectic sodium carbonate-sodium chloride 
composition e.g. by about 25%, so long as the desired reduced molten salt 
bath temperature is maintained. The chloride content can also be 
maintained constant by controlled addition of chlorine gas or a 
chlorinated hydrocarbon. 
As to the composition of the molten salt bath, when employing alkali metal 
halides or alkaline earth metal halides in combination with sodium 
carbonate, it has been found that there is no preference to having calcium 
chloride mixed with sodium or potassium chloride. Usually, an alkaline 
earth halide, such as calcium chloride, would not be employed unless one 
is attempting to dispose of calcium metal, barium metal or magnesium 
metal. However, if it is desired, for example, to dispose of calcium metal 
waste, then it is preferable to employ calcium chloride in the molten salt 
bath. Likewise, if the object is to dispose of sodium or potassium waste, 
the corresponding chloride would be employed in the molten bath, 
preferably in combination with alkali carbonates such as sodium carbonate. 
As previously noted, temperature of operation of the molten salt bath can 
range from about 200.degree. to about 900.degree. C., usually from about 
600.degree. to about 900.degree. C. If it is desired to operate for 
example at 600.degree. C., the molten salt composition must have a melting 
point less than 600.degree. C. If it is desired to operate at 900.degree. 
C., then various molten salt compositions can be employed because of the 
high melting point. The operating temperature will dictate which salt 
composition must be employed in the molten salt bath. 
Referring to the drawing, schematically illustrating a system for carrying 
out a typical process for the disposal of alkali metal waste, such as 
sodium waste, by oxidation of the sodium to sodium carbonate in molten 
sodium carbonate, molten sodium waste is fed from an alkali metal feed 
tank 10 via line 12 through a tube 14 into the molten salt bath, e.g. 
molten sodium carbonate 16 in a reactor tank 18 positioned in a suitable 
furnace 19. Carbon dioxide is simultaneously fed from a CO.sub.2 supply 
tank 20 via line 22 into tube 14 and into the molten sodium carbonate bath 
16, and oxygen is fed from an oxygen supply tank 24, via line 26 through 
an inlet tube 28 into the molten salt bath 16. Cooling air is fed at 30 
into an annulus 32 in furnace 19 around the molten salt reactor 18, to 
maintain the desired temperature of the molten salt bath 16, the heated 
cooling air exiting at 34. 
As the oxidation reaction proceeds in the molten salt bath 16 between the 
sodium feed and the CO.sub.2 and O.sub.2 introduced into the salt bath to 
convert the sodium to sodium carbonate, carbonate salt melt is drawn off 
from the molten salt bed via outlet line 36, for salt disposal at 38. 
Excess CO.sub.2 and O.sub.2 is discharged through exit 40 at the top of 
the reactor 18 and passes via line 42, together with any occluded sodium 
carbonate, to a filter 44, which separates such sodium carbonate and 
discharges it via line 46 for disposal at 38. Excess CO.sub.2 and O.sub.2 
from filter 44 is passed via line 48 to a pump 50 for recycling via line 
52. 
The following are examples of practice of the invention process: 
EXAMPLE 1 
In one example of molten salt oxidation of sodium waste, on a 
stoichiometric basis, 46 lbs of sodium is reacted with 44 lb of CO.sub.2 
and 16 lb of oxygen (or 69 lb of air) to yield 106 lb of Na.sub.2 
CO.sub.3. Heat is released at the rate of 5400 BTU/lb of sodium. The 
sodium is continuously fed into a reactor, previously filled partially 
with sodium carbonate, maintained at a temperature of 875.degree. to 
1000.degree. C. The carbon dioxide and oxygen (or air) are introduced 
separately into the reactor vessel. The above amounts of gases are the 
minimum amount of gas to produce Na.sub.2 CO.sub.3 product. An excess 
amount of carbon dioxide and oxygen are added to maintain mixing in the 
reactor. At least 10% excess gas is needed to maintain turbulent mixing. 
The product salt is withdrawn from the reactor on a batchwise basis or 
continuously. Excess heat is removed by natural cooling of the reactor or 
by forced convection on the outside of the vessel. The gas leaving the 
salt bed is filtered in a fabric filter and recycled into the salt 
reactor. Periodically, the off-gas is released into the atmosphere to 
remove tramp gas, such as nitrogen or argon. As an option, the gas leaving 
the reactor is released on a continuous basis, and no gas recycle is used. 
EXAMPLE 2 
As an example of molten salt oxidation of radioactive sodium waste, the 
sodium contains radioactive contamination in the form of 1 mCi Cs-137, 1 
mCi Co-60, and 1 mCi H-3 (tritium). The radioactive sodium is continuously 
fed into a reactor, previously filled partially with sodium carbonate, 
maintained at a temperature of 875.degree. to 1000.degree. C. A small 
amount of sodium sulfate, less than 5 wt %, is maintained in the reactor 
vessel to aid in reducing volatility of the radioactive components. Carbon 
dioxide and oxygen (or air) are introduced separately into the reactor 
vessel. An excess amount of carbon dioxide and oxygen are added to 
maintain mixing in the reactor. At least 10% excess gas is needed to 
maintain turbulent mixing. 
The product salt is withdrawn from the reactor on a batchwise basis or 
continuously. Excess heat is removed by natural cooling of the reactor or 
by forced convection on the outside of the vessel. The gas leaving the 
salt bed is filtered in a fabric filter and recycled into the salt 
reactor. Periodically, the off-gas is released into the atmosphere to 
remove tramp gas, such as nitrogen or argon. As an option, the gas leaving 
the reactor is released on a continuous basis, and no gas recycle is used. 
The radioactive cesium and cobalt are retained in the salt bath. The 
radioactive hydrogen (tritium) leaves the vessel as tritiated water, HTO, 
and is collected in a condenser or other typical water vapor adsorption 
device. 
EXAMPLE 3 
The following is an example of molten salt oxidation of sodium-potassium 
(NaK) waste in a salt bed containing about 40% Na.sub.2 CO.sub.3 and 60% 
K.sub.2 CO.sub.3 by weight. On a stoichiometric basis, 33.88 lb of NaK (22 
wt % Na-78 wt % K) is reacted with 23.39 lb of CO.sub.2 and 8.48 lb of 
oxygen (or 36.57 lb of air) to yield 25.3 lb of Na.sub.2 CO.sub.3 and 
40.45 lb of K.sub.2 CO.sub.3. Heat is released due to the oxidation of the 
NaK. 
The carbon dioxide and oxygen (or air) are introduced separately into the 
reactor vessel. The above amounts of gases are the minimum amount of gas 
to produce Na.sub.2 CO.sub.3 and K.sub.2 CO.sub.3 products. An excess 
amount of carbon dioxide and oxygen are added to maintain mixing in the 
reactor. At least 10% excess gas is needed to maintain turbulent mixing. 
The product salt is withdrawn from the reactor on a batchwise basis or 
continuously. Excess heat is removed by natural cooling of the reactor or 
by forced convection on the outside of the vessel. The gas leaving the 
salt bed is filtered in a fabric filter and recycled into the salt 
reactor. Periodically, the off-gas is released into the atmosphere to 
remove tramp gas, such as nitrogen or argon. As an option, the gas leaving 
the reactor is released on a continuous basis, and no gas recycle is used. 
EXAMPLE 4 
The following is an example of molten salt oxidation of sodium waste in a 
salt bed containing carbonates and chlorides. On a stoichiometric basis, 
46 lb of sodium is reacted with 44 lb of CO.sub.2 and 16 lb of oxygen (or 
69 lb of air) to yield 106 lb. of Na.sub.2 CO.sub.3. Heat is released at 
the rate of 5400 BTU/lb of sodium. 
The sodium waste is continuously fed into a reactor, previously filled 
partially with sodium carbonate and sodium chloride, maintained at a 
temperature of 650.degree. to 1000.degree. C. The reactor is filled with a 
mixture of sodium carbonate and sodium chloride, such that the melting 
point of the salt is below that of pure sodium carbonate (855.degree. C.) 
or that of pure sodium chloride (804.degree. C.). This is approximately in 
the range of 1 mol % Na.sub.2 CO.sub.3 to 80 mol % Na.sub.2 CO.sub.3. 
The carbon dioxide and oxygen (or air) are introduced separately into the 
reactor vessel. The above amounts of gases are the minimum amount of gas 
to produce Na.sub.2 CO.sub.3 product. An excess amount of carbon dioxide 
and oxygen are added to maintain mixing in the reactor. At least 10% 
excess gas is needed to maintain turbulent mixing. The product salt is 
withdrawn from the reactor on a batchwise basis or continuously. Excess 
heat is removed by natural cooling of the reactor or by forced convection 
on the outside of the vessel. The gas leaving the salt bed is filtered in 
a fabric filter and recycled into the salt reactor. Periodically, the 
off-gas is released into the atmosphere to remove tramp gas, such as 
nitrogen or argon. As an option, the gas leaving the reactor is released 
on a continuous basis, and no gas recycle is used. 
From the foregoing, it is seen that the invention provides an improved 
process for molten salt destruction of alkali metal and alkaline earth 
metal waste, particularly from reactor coolants, by reaction with a 
mixture of carbon dioxide and oxygen in a molten carbonate bath. The 
process control is straightforward with no potential for explosion due to 
hydrogen gas production of off-normal operation in which water can work 
its way back from off-gas processes. The waste salt formed in the process 
is non-hazardous and can be disposed of directly with no further 
processing. Since no aqueous processing in required, no primary waste from 
off-gas treatment is generated. The gas system of the invention can 
operate as a closed system, with a small feed and bleed stream to remove 
non-condensibles. Vessel off-gas processing includes only filtration of a 
small fraction of the sodium reaction product generated. The overall cost 
including construction, operation and eventual decommissioning and 
decontamination, is lower than other processes because of the high 
throughput for the compact size of the equipment, low energy usage and no 
need for an extensive off-gas processing system. The compactness of the 
molten salt unit with its few operating components results in low 
maintenance requirements and lower operator exposure to radioactive 
contaminants. The salt product, e.g. anhydrous sodium carbonate, does not 
require neutralization under U.S. Environmental Protection Agency 
regulations. 
It is to be understood that what has been described is merely illustrative 
of the principles of the invention and that numerous arrangements in 
accordance with this invention may be devised by one skilled in the art 
without departing from the spirit and scope thereof.