I
R. G. WERKEMA and G. P. LANG Mallinckrodt Chemical Works, St. Charles, Mo.
Uranium Recovery from Magnesium Fluoride Slag Carbonate Leaching Carbonate leaching recovers essentially all uranium in slag and may become commercially useful free leach liquor to prepare a uraniumbearing cake‘suitable as a refinery feed material. Preliminary Grinding. Carbonate leaching, unlike acid leaching, dissolves little except uranium; therefore, the slag must be finely ground. I t was concluded that a preliminary grind to achieve at least 80% of -325 mesh material was necessary on roasted slag (7). Also, uranium must be converted to a more soluble form-air roasting converts it to uranium oxide (black oxide) which dissolves more rapidly in the carbonate solutions. Roasting. When pure uranium is roasted in air, the roast temperature below 1000° C. has little effect on oxide dissolution rate in carbonate solutions. This is not true of slag. There is an optimum roasting temperature of 425 O f 50’ C. above which the amount of uranium that cannot be leached markedly increases. A slag roasted at this temperature is gray-being a mixture of white magnesium fluoride and. black
either an acid or carbonate leach. The carbonate leach method was chosen for investigation because corrosion problems are less, capital equipment costs are lower, less filtration capacity is needed, and operations such as drumming, handling, and storage are less expensive.
M m r m s m M FLUORIDE slag is a byproduct from uranium metal manufacture by a thermite-type reaction in which uranium tetrafluoride is reduced with magnesium in a refractory-lined steel vessel. While the reactants are still hot, the denser uranium settles to the bottom of the vessel, leaving magnesium fluoride slag at the top. Separation however, is never complete and a certain amount of uranium freezes as pellets in the slag. After the uranium ingot has cooled and been broken out, the slag and liner material is crushed and ground, and a selected portion re-used as lidear material. The excess slag, which contains from 1 to 5% of uranium, may be processed for its uranium content. Therefore, work was undertaken to reduce the uranium content of this slag to less than 0.05% on a dry basis, and to produce a uranium concentrate suitable as a refinery feed. Of the methods (4)proposed for accomplishing this, all but one involves
The Process The carbonate process selected involves the following basic steps: grinding the slag to expose uranium included in slag crystals; air roasting the slag to oxidize the uranium to UaOs; leaching the slag with a carbonate-bicarbonate solution containing an oxidant to convert the UsOs to the soluble uranyl tricarbonate complex; further grinding the slag in part of the leaching circuit to complete the quantitative exposure of uranium to the leach solution; washing the leached slag cake to remove residual soluble uranium; and caustic precipitation of the uranium from slagI.
I
I
I
I
STANDARD CONDITIONS AT START OF LEACH 0 . 4 g EXCESS NaHCOS
SLAG
8
350 % / I .
T I M E = Ihr. TEMP. * 1 9 S o F . U CONC. OF SLAG * 6 . 8 %
I I 0.0 I
STANDARD CONDITIONS AT START OF LEACH
THEORETICAL AMOUNT NECESSARY TO OXIDIZE CONTAINED U s 0 8
T O T A L SOLUBLE
0.0lH SLAQ 40 9.11. TEMP. * 160.F.
1
I
I 0.1
0.2
C o p * I.OH
KMn04
a3
I
I
I
I
0.4
0.5
0.e
0.7
RATIO OF KMnO,
20
T O U, g./g.
Figure 1. Amount of uranium leached remained constant when more than 0.2 gram of potassium permanganate was added per gram of uranium
? e‘
40
60
60
too
OF COZ INITIALLY PRESENT AS CARBONATE
Figure 2. Slag leaching improves if bicarbonate rather than carbonate is used VOL. SO, NO. 12
DECEMBER 1958
1781
oxide. Slag roasted below this temperature is not completely oxidized and still contains uranium metal pellets; in fact, the optimum temperature is barely high enough for this oxidation to take place at a reasonable rate. Slag roasted above this temperature is lemonyellow to orange. Hydrogen reduction of this yellow slag at 750" to 800' C. can restore its original gray color and leachable properties. An explanation for these reactions was deduced from x-ray diffraction studies of synthetic magnesium uranates prepared by roasting various ratios of magnesium oxide and uranium trioxide in air at temperatures between 500' and 1000" C. Of the two uranates prepared in this way, magnesium diuranate is the dominant form at lower ignition temperatures (500' to 700" C.) or at higher temperatures when the uranium trioxide-magnesium oxide ratios are greater than 10. At 1000" C., magnesium uranate is the stable phase. These same magnesium uranates have been found in the overroasted yellow slag and are largely responsible for the unleachable characteristics of this material. Leaching and Oxidation. The ability of carbonate solutions to leach sexivalent uranium from solids is due to the extremely stable uranyl tricarbonate complex u o ~ ( c o 3 ) 3 - ~ .The estimated equilibrium constant for the formation of this complex from U02+2 and COS-2 is 2 X lo1*. A second species, UOZ(CO3)2 (HzO)z-2, also exists but its equilibrium constant of 4 X 1014 is considerably lower; so that in solutions containing an excess of carbonate ion only the tricarbonate complex can exist. The pH of the solution determines the stability of these complexes. The hydrolysis reactions of the uranyl ion in connection with these carbonate complexes have been described by Sutton (5) and are summarized as follows: The U O Z +ion ~ is stable below about
c
2 v)
d
ai 'r
p: D
h
W
U J v)
Figure 3. Leaching improves with increase in bicarbonate concentration ,
I HR.
0 W
LEACH
E o
s
-I
5
STANDARD CONDITIONS START OF L E A C H
AT
K M n 0 4 = O.Ol& SLAQ = 40 g . / l , T E M R * l 6 0 ' F.
02
0.4
0.8
0.8
1.0
INITIAL MOLARITY OF NaHCO,
pH 2. As the pH increases, the species U20b+2,U30g+2, U3OgOH+, and U308(OH)z are formed. Above pH 9, a number of mixed uranates precipitate; the yellow and orange tetrauranates are the most stable. If carbonate or bicarbonate ions, the relative concentrations being controlled by the pH, are added to this system, the
very strong carbonate complexes are formed in the slightly alkaline pH region. At low pH's these are unstable and form carbonic acid and the hydrolyzed species of the UO2+2 ion; at high pH values they form the very insoluble polyuranates. Successful leaching, therefore, depends on an excess of carbonate and addition of enough bicarbonate ion
i 0.3 W E X C E S S
NaHCOa
0 . 8 Y EXCESS I
-0.02 IO0
I20
140
180
LEACH TEMPERATURE
Figure 4.
1782
180
200
%
Higher temperatures increase leach efficiency
INDUSTRIAL AND ENGINEERING CHEMISTRY
0
0.02
0.04
0.06
0.08
0,lO
CONCENTRATION OF EXCESS CAUSTIC, IbJgol.
Figure 5. An excess of sodium hydroxide improves uranium recovery
NUCLEAR T E C H N O L O G Y
1.
Figure 6.
The pilot plant could operate continuously at a slag feed rate of about 300 pounds per hour
to destroy any hydroxyl ions formed during the reaction. One other consideration is involved-uranium in the quadrivalent oxidation state will form a soluble carbonate which has been written as U ( C O ~ ) S -(4). ~ This species is unstable in alkaline solutions and forms an extremely insoluble uranous hydroxide, U(OH)4, having a K,, of 10-45. Therefore, quadrivalent uranium compounds must first be oxidized to bring them to a leachable form. Because roasting converts the uranium in slag largely to “black oxide,” UaOs, one third of the uranium still is in the quadrivalent state and only about 40% of the uranium in roasted slag can be dissolved without additional oxidation. This oxidation step, which may be represented by U808(s) f ‘/z 0 2 (aq.) + 3UO&) (2), is probably the rate-controlling step in the leaching operation. A number of oxidizing agents have been tested. Hydrogen peroxide is effective, but is unstable and must be continually added. Ozone has the highest standard oxidation potential of the oxidants tested, but its solubility in aqueous solution is too low to be effective. Hypochlorite added either as its sodium salt or as chlorine gas is a very effective oxidant but is unstable in steel equipment and was, therefore, unsuitable because of its corrosive properties. Potassium permanganate has none of these objections and was the oxidant finally used. The stoichiometric amount necessary
is 2 moles of potassium permanganate per 9 moles of uranium in black oxide, or about 0.19 gram of potassium permanganate per gram of uranium. This ratio was closely approached in actual beaker scale leaching of slag, where the amount of uranium leached remained constant when more than 0.2 gram of potassium permanganate per gram of uranium was added (Figure 1). In the presence of the proper oxidizing agent, pure black oxide is readily and completely dissolved by a single leach in carbonate solution. Variables such as carbonate concentration, pH, and solids content are all of secondary importance. Yet in order to leach uranium completely from slag, all these variables must be closely controlled and a second leach is usually necessary. Slag leaching improves if bicarbonate rather than carbonate is used (Figure 2) and leaching also improves if a higher concentration of bicarbonate ion is used (Figure 3). Only a slight increase in leach efficiency is found with a decrease in the solids concentration of the leach slurry between 80 and 400 grams of slag per liter of leach solution. Higher temperatures cause a marked increase in leach efficiency (Figure 4). All these effects are found only after the bulk of the uranium has been dissolved and the optimum conditions determined by this work are necessary to remove only the final portion of the exposed uranium. Several reasons for this have been proposed. Some metallic uranium still persists through the grind-
ing and roasting steps and is only slowly leached. Some magnesium uranates are formed ecen at the optimum roasting temperature or during the thermite reaction and these leach very slowly. A small portion of the uranium is still trapped in the magnesium fluoride matrix and is not accessible to the leach solution; further grinding is necessary during the leaching step, and was accomplished by performing the leaching in a closed circuit grinding operation. Precipitation. The soluble uranium obtained from the leaching operation can be almost quantitatively recovered by precipitation with hydroxyl ions, which destroys the tricarbonate complex to form the polyuranates. The equilibria involved are not well understood; however, the degree of precipitation is dependent on the concentrations of the uranyl tricarbonate, carbonate, and hydroxyl ions and on the cations present. I t was necessary to obtain a n excess of about 10 grams of sodium hydroxide per liter over that necessary to convert all bicarbonate to carbonate to obtain residual liquors containing less than 0.01 gram of uranium per liter (Figure 5 ) . Pilot Plant Equipment. The equipment used in the grinding, roasting, and screening involved in preparing the slag for feed to the wet grinding and leaching steps of this process consisted of an 18 X 30 inch Denver ball mill, a Sturdevant roll mill with adjustable 5-inch rolls and a 20 X 3G inch, two-screen Gump sifter, and an American Gas Furnace Co. Model 2GA rotary gas carburizing VOL. 50,
NO. 12
DECEMBER 1958
1783
Wet Grinding and Classification System Standard Flows These are the optimum flows and compositions of streams (Basis, 1 lb. of 73% -326-mesh roasted slag feed) Fraction of Solids Content, Solids, - 325 Stream Flow, Gal. Lb./Gal. Mesh, % 1 lb. 73.0 Slag feed 0.78 4.1 77.8 Ball-mill feed 0.75 4.1 86.2 Ball-mill discharge 0.66 1.5 99.0 Hydroclassifier overflow 0.74 3.0 80.3 Hydroclassifier underflow 0.37 Dilution liquor 0.30 Cone wash Table 1.
...
... ...
furnace. The remainder of the processing was carried out as shown in the flowsheet (Figure 6). Wet grinding and classification sections of the leach step were carried out in a 16 X 32 inch Denver ball mill and a Denver hydroclassifier 36 inches in diameter. Leached slag filtration and washing experiments were conducted on a n Oliver rotary drum precoat filter 3 feet in diameter by 1 foot wide. The pilot plant had sufficient tankage to permit semicontinuous operation a t a slag feed rate of about 300 pounds per hour to duplicate this flowsheet through the first-stage leaching operation. The majority of the equipment was mild steel. There was no evidence of any corrosion that could be attributed to the leach liquor. Wet Grinding Circuit Operation. The closed circuit grinding operation was investigated to determine equipment capacities and satisfactory operating ranges for slurry solids concentration in the various streams. The pilot plant hydroclassifier was intended to provide about the same particle size “cut” as a 325-mesh screen, In most runs, however, trace amounts of plus 325-
... ...
mesh material were found in the hydroclassifier overflow steam, amounting to less than 1% of the overflow solids. I n the runs where more than 1% of the overflow solids were plus 325 mesh, one or more of the following conditions were responsible for the excessive oversize entrainment: The concentration of solids in the overflow was 1.8 pounds per gallon or greater; the underflow solids concentration was greater than 4.0 pounds per gallon; or the ratio of underflow volume to cone wash was 1.1 or less. Overflow liquor rates up to 300 gallons per hour were successfully processed when the preceding conditions were not observed. The estimated maximum allowable slag rate for the pilot
Table 111.
Reagent Consumption Analysis The process used these amounts of reagents Lb./Lb. U Leached NaHCOs consumption Theoretical“ required to dissolve U308 1.49 Decomposedbto NazCOs
+ coz
1.34
Consumed by other reactions Table II.
Efficiency of the Leach Pilot Plant Was High U Content Leach of Slag Effi(Dry Basis), ciency, Leaching % % Roasted slag feed 6.3 1st stage product (wecoat filter cake) 0.068 98.92 2nd stage product (beaker leach) 0.047 99.25 I-hr. leach 0.039 99.35 5-hr. leach
Bled off with product
Precipitation Liquor bled off for precipitation, g. U/l. 18.6 Liquor after precipitation, g. U/1. 0.036 Precipitation efficiency 99.8
NaOH consumption TheoreticalCrequired for neutralization and precipitation Excess Ratio of product liquor volume to input water volume a 3UsOs 38HCOs-
U
F
46.5 0.14
a Includes both washed and unwashed filter cakes.
1 784
2.95
3.15 6.13
KMnOc consumption
...
Precipitation Cake Cornxna, % 46 Hz0
0.12 __
Theoretical“ required to dissolve Us08 Consumed by other reactions
0.21 -
Bled off with product
0.36 0.21 __
0.15
Lb./Lb. U Pptd.
+
1.4
0.9 __ 0.86 gal./gal.
+ 2Mn04+ 19Hz0 +
+
guO~(c03)3-~ 2C03-’ SCOZ 2Mn02. 2HCOs- 6 4H20 COz. HCOaOH- +COO-* H 2 0 . 2UOz(COJ)s-4 2 X a + 6OH+- NazU?O7 6COa-2 3Hz0.
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
+ +
+
+ + +
+
Conclusions The experimental work on this carbonate leach process demonstrated that under the proper grinding, roasting, and leaching conditions, the uranium content of magnesium fluoride slag can be reduced to less than 0.05% on a dry basis and that a uranium concentrate can be obtained which is sufficiently low in fluoride to be suitable as refinery feed material.
literature Cited
0.57
+
plant wet grinding system was about 310 pounds per hour. Table I lists the optimum flows and compositions of the various streams according to data obtained in these operations. Slag Filtration. A series of runs was made to obtain filtration rate data on a precoat filter. Filtrations were carried out under 15-inch mercury vacuum with a precoat cut of from 0.001 to 0.002 inch per revolution. Drum submergence was 30%. The filtration rates obtained varied from 79 gallons per hour per square foot at 1.5 pounds of dry solids per gallon and 1 r.p.m. to 252 gallons per hour per square foot at 4.0 pounds per gallon and 4 r.p.m., these rates being computed on the submerged filter area. Diuranate Precipitation. The optimum conditions for diuranate precipitation were found to be a digestion temperature of 200” F.; a final caustic concentration of about 10 grams per liter excess; and a digestion time of about 15 minutes after the final caustic concentration had been obtained. All bleed-off liquors were polished prior to diuranate precipitation. The average fluoride content of the unwashed diuranate cake was 0.3% on a uranium basis. Washing of the diuranate cake resulted in decreasing the fluoride concentration to less than the specified maximum concentration of 0.1%. Over-all results obtained from the pilot plant runs are summarized in Tables I1 and 111.
(I) Berry, N. E., ed., “Compendium of
Mallinckrodt Reports on Slag Processing,” MCW-1387 (February 1956). ( 2 ) Forward, F. A., Halpern, J., Peters, Z., Can. Mining Met. Bull. 46, 643 (October 1953). (3) McClaine, L. A., Bullwinkel, E. P., Huggins, J. C., Proc. Intern. Conf. Peaceful Uses of Atomic Energy 8, 525 (1956). (4) “Processing of Uranium-Magnesium Fluoride Slap.” Technical Information Meetinr. TfD-7528 iPt. 1 and 21, (Dec. 4’1956). (5) Sutton, J., J . Chem. SOC. 1949, pp. 5275-86. RECEIVED for review April 7, 1958 ACCEPTED October 20, 1958 Division of Industrial and Engineering Chemistry, Symposium on Preparation and Recycle of Feed Materials for Nuclear Fuel, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
