I
DAVID J. CROUSE, Jr., and KEITH B. BROWN O a k Ridge National Laboratory, O a k Ridge, Tenn.
The Amex Process for Extracting Thorium O r e s with
Solvent extraction processes offer b better purity b high recoveries b good separation o f by-product uranium a n d rare earths
Alkyl Amines i
made with the organic as the continuous phase (water-in-oil dispersion), this precipitate separates readily. Extending the contact time from 2 to 10 minutes increases stripping and precipitation.
Alkyl amines are versatile extractants for economical recovery of relatively high grade thorium products from ore process sulfate liquors. Relative extraction power for thorium and for other metal values is strongly dependent on the amine type and alkyl structure. Thus, appropriate choice of amine permits efficient recovery and separation in multicycle extraction operations.
Recovery from Monazite
THORIUM,
which is a potentially useful fertile material for breeder reactors, has been obtained chiefly from monazite. A further major source was recently discovered in the uranium-thorium ores of the Canadian Blind River district. T h e amine extraction (Amex) processes developed a t the Oak Ridge National Laboratory for uranium recovery (5, 6, 9-72) have been extended to achieve efficient low-cost recovery from both sources. Through choice of amine type and alkyl structure, these processes provide good control over extraction power and metal selectivity (Table I). As with uranium, a number of reagents are effective for stripping the extracted thorium from amines (73- 75). Thorium is present in the extract as a thorium-amine-sulfate complex, with excess amine sulfate and/or bisulfate. This complex contains three to four amine sulfates per thorium, depending on amine type (77). When the extract is treated with C1 or NO3, thorium is displaced from the organic phase. When the extract is treated with neutral or acidic nitrate or chloride solutions, thorium is displaced with the nitrate or chloride anion. Nitrate is the more effective stripping anion. Following nitrate stripping the amine is regenerated to the free amine form for recycle by contact with a base such as ammonium hydroxide. This recovers the relatively expensive nitrate for recycle and maintains extraction efficiency by preventing contamination with nitrate. Regeneration after chloride stripping is optional. Hydroxide solutions precipitate thorium directly from the solvent. However, the slimy thorium precipitate is difficult to separate from the solvent. Carbonate solutions strip thorium as a soluble carbonate complex, provided there is sufficient excess carbonate. With insufficient carbonate excess, part or essentially all of the stripped thorium precipitates, but if phase contact is
Battelle Memorial Institute and Ames Laboratory previously developed methods for recovering thorium, uranium, and rare earths from monazite sands. T h e Battelle process ( 4 )uses caustic, and the Ames process (2, 3, 22, 24) uses sulfuric acid to open u p the sands. T h e elements are partially separated by precipitation and the thorium-rich precipitate is redissolved and purified by tributylphosphate (TBP) extraction (76). T h e Amex process (7, 73, 75) provides essentially complete recovery and separation of the elements, and the relatively pure thorium product is more amenable to TBP extraction. By adjustment of operational procedures, it may be possible to produce nuclear grade thorium oxide as a direct result of the amine extraction treatment, thus obviating need for T B P purification. Digestion of Monazite Sand. T h e liquors used in these studies were prepared by digesting Indian monazite sand in 93% sulfuric acid, followed by water dilution to solubilize the metal
1 sulfates ( 2 ) . The digest slurries were filtered to provide clear liquors analyzing 5 to 7 grams of thorium, 0.2 gram of uranium, and 35 to 45 grams of rare earth oxides per liter (Table 11). Extraction of Thorium. Owing to the high acidity and relatively high concentrations of sulfate and phosphate, coefficients for extraction of thorium from monazite liquors are appreciably lower than those shown in Table I; the relative extraction power of amine classes is similar (Table 111). T h e strong affinity (E: > 500) of the primary amines for thorium is again apparent. In comparison, the extraction coefficient for di(tridecy1)amine is low (-5) although adequate for process use.
Table I.
Degree of Extraction and Metal Selectivity Depends on Amine Type and Alkyl Structure 1M SO4, pH 1 , -1 gram metal ion/liter; phase ratio 1 / 1 , 0.1M amine in kerosine
Metal Ion Extr. Coefficient, = [Mid [I11 L c (
Amine Type Branched primary Secondary with alkyl branching distant from the nitrogen Secondary with alkyl branching on the first C Tertiary with no branching or branching no closer than the third C
Example Amines Primene JMn and 1-(3ethylpentyl)-4-ethyloctylamine Di(trideeyl)amine* Amberlite LA-1" and bis(1-isobutyl-3,sdimethylhexy1)amine Alamine 336d+ and triisooctylamineeJ
U(T'1)
Th
5-30
>20,000
80 80-120
140
Ce(II1)
>500
10-20
99.8y0thorium recovery could be expected in a single ideal extraction stage while loading to 90% of this value. Extraction of Uranium. Coefficients range from 25 to 50 for extraction of uranium from thorium-barren monazite liquors with 0.05M solution of N-benzyl branched alkyl secondary amines, the most powerful known (77, 72, 78) amine extractants for uranium (Table IV). Coefficients with tertiary amines are much lower (-5), although of usable magnitude; the primary amines are not useful uranium extractants owing to excessive extraction of rare earths. Uranium extraction isotherms for 0.05M tri-n-octylamine solutions indicate the maximum solvent loading obtainable a t p H 0.1 is -0.6 gram of uranium per liter, about 25y0of the loading normally obtained in treating western uranium ore leach liquors. Adjustment of p H to 0.4 prior to extraction appreciably increases the isotherm slope (permitting recovery in fewer extraction stages) but does not greatly improve solvent loading.
1462
Continuous Tests. Continuous countercurrent recovery of thorium and uranium from monazite liquors was demonstrated in small mixer-settler test arrays a t liquor flow rates of about 3 liters per hour. In preliminary runs di(tridecy1)amine was the thorium extractant as it is more selective than primary amines toward rare earths. However, this compound extracts excessive amounts of phosphate which are relatively difficult to remove from the solvent by sulfuric acid scrubbing. Consequently, all subsequent thorium recovery runs were made with a primary amine (Primene JM) which extracts only a minor amount of phosphate with the thorium. Depending on amine choice, either uranium or thorium can be recovered in the primary extraction cycle. Latest results favor the latter approach, although both have been demonstrated successfully. I n using the relatively unselective primary amines, the solvent must be saturated with thorium to assure decontamination from rare earths during the extraction step (73, 75). This also improves the efficiency of the scrub
Table 111. For Thorium Primary Amines Are the Strongest Extractants Monazite liquor containing 6.9 grams of thorium per liter; 0.1M aminea in kerosine; organic/ aqueous phose ratio, 3/1 T h Extr. Amine Coeff.. E: Primaries Primene JMb 1-(3 -Ethylpentyl)-4ethyloctylC Secondaries Di(tridecy1) Amberlite LA-I Tertiary Tri-n-octylb
> 500 >500 4.6 0.15
99.5y0 uranium extraction in eight stages. T h e extract was stripped with sodium carbonate, although other stripping methods (72) would be applicable. Precipitation of uranium by addition of sulfuric acid to destroy carbonate and ammonia to precipitate uranium yielded a product (dried a t 120" C.) analyzing 77% U308, 8% thorium oxide, 97% with -3 pounds of sodium chloride per pound of rare earth oxides. A typical product (dried a t 120' C.) analyzes 40% rare earth oxides, 7.37, sodium, 48% sulfate, 1.1% phosphate, and 11% LO1 at 700' C. T h e rare earths can also be recovered effectively and separated from phosphate by extraction with a primary amine. Extraction isotherms (Figure 4) for 0.2M Primene Jhl indicate that >98% recovery should be obtainable in three ideal stages a t p H 0.4 and in four a t p H 0.2, while loading the solvent to 6 grams of rare earth oxides per liter. The phosphate content of the extract a t this loading is approximately 1% based on the rare earth oxide. Further separation is obtainable by scrubbing the extract with dilute sulfuric acid. Contact with nitrate or chloride solutions, 3 to 4M sulfuric acid, or carbonate solutions readily strips the rare earths (7, 73). With the latter, they are precipitated directly from the solvent, and organic continuous mixing of the phases must be maintained to avoid emulsions. Typical products (dried a t 120' C.) from carbonate stripping contain 71 to 7S% rare earth oxides, 30 to 36% carbonate, and
