TECH NOLOGY
LAB EXTRACTOR. D. J. Crouse (left) and W. D. Arnold of Oak Ridge National Laboratory adjust their laboratory scale solvent-extraction unit to separate cesium from ore liquors, using substituted phenols as the extractant. The extract is later stripped of cesium by acidulating to produce various cesium salts
Two Processes Benefit Cesium Technology One process upgrades pollucite ores; the other produces high-purity cesium salts from pollucite liquors
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C&EN
DEC.
23,
1963
Cesium and its salts, now costly materials, may notjbe quite so costly in the future as the result of two independent developments in cesium technology. The upgrading of domestic cesium ore is also a benefit. One of the developments—by the Bureau of Mines—concerns the concentration of the cesium mineral pollucite from domestic pegmatite ores. The bureau has patented the process (U.S. 3,107,215) and is offering it for license on a royalty-free basis. The other development comes from Oak Ridge National Laboratory and is an adapted process to get cesium salts in large volume from pollucite liquors. Those concerned with cesium production are looking forward to what is expected to be a relatively large market developing in the 1970's. One of the most easily ionized metals, cesium may be used in tonnage quantities for ion propulsion rocket engines and for
power generation systems such as plasma thermionic converters and magnetohydrodynamic generators. Some suggested new uses now being developed are in welding rods and in many kinds of electronic devices. Currently the market for cesium metal is small—perhaps a few hundred pounds per year. Sales in 1964 are expected to increase slightly. The metal is expensive, listing at from $100 to $375 per pound, depending on quantity and purity. Cesium and its compounds were, until somewhat recently, produced as by-product or coproduct at plants recovering lithium and other alkali metals from the mineral lepidolite. However, they are currently being made from pollucite. According to the Bureau of Mines, large amounts of both imported and domestic pollucite are already on hand in anticipation of new uses.
Adapted. The ORNL process is a solvent extraction and acidification method adapted by laboratory scien tists from a technique used to recover cesium isotopes from fission products. It makes use of substituted phenols in hydrocarbon diluent as extractant. W. D. Arnold of ORNL detailed the process at the Southwest Regional Meeting of the American Chemical Society in Houston, Tex. Three sub-. stituted phenols out of about 60 tested have proved practical to extract cesium from ore liquors. Of these three, 4-sec-butyl-2- ( α-methylbenzyl ) phenol (BAMBP), available in de velopment quantities from Dow Chemical, performs best in an aliphatic solvent such as kerosine. 4-Chloro-2benzyl phenol, available from Mon santo Chemical as Santophen-1, per forms best in chlorinated hydrocarbon diluents. Control of pH is critical to maximum selectivity and, in turn, to efficiency of extraction, Mr. Arnold says. Ex traction power of BAMBP for cesium increases with increasing pH and is adequate for effective recovery at about 13. Of the other alkali earth metals, rubidium is extracted by BAMBP about V20 as well as is cesium. Potassium is third at about V300 the separation of cesium. In most of the work at ORNL, the cesium came from liquors produced by fusing pollucite ore (the pollucite is a hydrated cesium aluminum silicate obtained from Bikita in Southern Rhodesia) with a mixture of two parts sodium chloride and one part sodium carbonate per part of ore, and leaching with water. Cesium recovery from the ore in this step is 97 to 98% at fusion temperatures between 640° and 850° C , Mr. Arnold says. A small amount of caustic is then added to raise the p H to the optimum for ex traction. The liquor contains about 9 grams of cesium per liter, the result of using 25 lb. of water per pound of ore for leaching. Leaching water can be greatly reduced without significantly lowering recovery of cesium, Mr. Arnold says. BAMBP in an aliphatic solvent will recover 99% of cesium contained in the liquor to make an extract containing about 22 grams per liter. Cesium is removed from the liquor by countercurrent contact with the solvent, usually \M BAMBP in kerosine. To increase the separation of cesium from other alkali metals and
other impurities (both extracted and entrained), the extract is scrubbed countercurrently with water. The spent scrub solution containing some cesium is combined with the feed liquor and returned to the extraction system. Countercurrent contact of the loaded solvent with dilute acid strips the cesium to give a concentrated cesium salt solution. The stripped solvent is recycled to the extractor. Cesium salts are recovered by evap orating the solution, possibly coupling this with a crystallization. Since weak acids very effectively strip cesium from the solvent, many different salts can be prepared, Mr. Arnold points out. Even cesium hy droxide can be made by contact with water at high temperatures, although the solution is dilute, thus making re covery relatively expensive. High Purity. The product solution contains cesium salts with very few other salts as impurities. In a typical laboratory-scale, continuous run ex traction, the combined alkali metal im purities are 0.02% based on cesium in the product. This run used four extraction and three water scrub stages. Flow ratios of feed liquor/ organic extractant/scrub water are 1.9/1/0.3. Stripping the scrubbed extract in two stages with dilute acid gives more than 98% recovery of cesium. Product purity can be increased by using more scrub stages, Mr. Arnold says. It can be improved by evapora tion of the strip liquor and partial crystallization of cesium salts from the liquor; the first 60% of the cesium salt crystallized contains less than 0.005% total alkali metal impurities. Final purity also can be increased by recrystallization of the cesium salts similar to conventional purification of cesium salts following adsorption on ion exchange resins. Reagent costs total about 40 cents per pound of cesium recovered from pollucite ore from Bikita. Bikita ore contains about 2 3 % cesium and is considered a high-grade ore. Of the 40-cent cost, Mr. Arnold estimates that 40% is for loss of BAMBP which he assumes will cost about $5.00 per pound. If the process were scaled up, cost of the extractant would be lower and cost of extractant lost would be correspondingly reduced. Even if reagent costs were not reduced, the process is expected to make highpurity cesium salts at a cost well be
low the present prices—$35 per pound for cesium chloride, for example. Reverse Flotation. In normal flota tion processing, the desired material is air-frothed to the surface of a flota tion vessel and removed for further processing. This situation is reversed in the Bureau of Mines process. Con centrated pollucite ore is treated so that it becomes water-avid and the undesired materials are air-frothed to the surface for removal. Crude pollucite ore is first ground to 48 to 100 mesh, and aqueous pulp solution containing about 3 3 % solids is prepared. The addition of commer cial sulfuric acid brings the p H of the solution to about 1.4 to 2.7. Hydro fluoric acid is then added to act as a depressant for the cesium mineral. Tap water at normal temperatures and pressures is used in the process. Contaminating materials, principally feldspar and micaceous earths, must be made to rise to the surface under agitation and aeration. For this pur pose, the Bureau of Mines process calls for the addition of a collecting agent. Several amines were tried and found satisfactory. In most experi mental runs, however, Armac CD, a product of Armour Chemical, was used. A 5% solution of the amine was fed directly to the pulp. During aeration—usually about 5 cu. ft. of air per minute per liter of water—the unwanted materials sepa rate from the pollucite. Hydrofluoric acid makes the pollucite water-avid but frees the contaminants. The amine collector carries the contami nants to the surface where they can be removed in the froth. The concen trated pollucite pulp is then filtered and dried. The Bureau of Mines process is the work of Karl C. Dean, project coordinator at the Bureau's Salt Lake City, Utah, metallurgy research cen ter. Experiments cited in the patent showed significant efficiencies in two out of three batches of ore. For ex ample, an 8% pollucite ore from Maine was concentrated to about 22% cesium, and a 22% Bikita ore was concentrated to about 32%. Accord ing to the Bureau, pure pollucite con tains only about 37% cesium. Other ores tested included those taken from deposits in South Dakota and Bernic Lake, Man. Based on the results of these ex periments, the Bureau feels that the process is adaptable to a more efficient continuous system. DEC. 23, 1963 C & E N
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