Cesium from Pollucite by Aluminum Reduction

Aluminum metal was found to liberate cesium almost quantitatively from pollucite ore when reacted under conditions of operation determined by experiments described in the paper. In the treatment, ore ground to pass a 150 mesh sieve is mixed with atomized aluminum and the mixture heated under an inert atmosphere in a retort. Pollucite ore from one source yielded its cesium when treated at 950°C for 4 h while pollucite ore from another source required a temperature up to 1050°C to yield essentially all of its cesium. The cesium metal obtained from pollucite by aluminum reduction is about 90% pure; it contains small amounts of sodium, potassium, rubidium, lithium and other elements. Recovery of pure cesium from this J>roduct could be by fractional distillation or by further well established chemical treatments. Metallic cesium was of interest only for laboratory studies until about 40 years ago when it was first used as a getter in radio tubes. Until relatively recent developments, however, its uses in photoelectric cells, infra-red viewing systems an.d a few other devices have constituted most of the requiremen,ts for the metal. The future possibilities of cesium metal in the development of power for certain applications point to a marked increase in the demand for that metal. However, present production is still on a relatively small scale and processes covering a wide range of chemical treatments are being employed to obtain cesium from its ore. The more important processes include acid leaching and direct metal replacement. The study reported here deals with cesium recovery from pollucite ore concentrates by treatment with aluminum metal. Although small amounts of cesium occur in many minerals, pollucite is the only known mineral of high cesium content. A flotation method has been developed for concentrating pollucite from associated minerals1 and the resultant ore concentrates usually contain 20 wt % or more cesium. Pollucite is considered by some to be represented by the chemical formula 2Cs20 · 2Al20 3 • 9Si02 · H20 while others represent pure pollucite as Cs20 · Ab03 • 4Si02. The natural mineral, however, will contain considerable amounts of sodium, potassium and lesser amounts of rubidium replacing some of the cesium. The formula for the naturally occurring mineral containing sodium has been represented by (Cs20h-x · (Na20)x · AbOs · 4Si02 ·2.xH202 where .x apparently ranges between 0.1 to 0.3. Although aluminum has been used to reduce free cesium oxide, 3 a literature 1 Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission. Contribution No. 1885. • Phelps Dodge Researoh Labor8il:Ol:y, Morenci, Arizona. 81 1 Gault et al.: Cesium from Pollucite by Aluminum Reduction Published by UNI ScholarWorks, 1966 82 IOWA ACADEMY OF SCIENCE [Vol. 73 semch did not reveal the use of aluminum in liberation of cesium from compounds such as pollucitc. The acid leaching process is most commonly used for obtaining cesium from pollucite; a mineral acid, usually HCl or H2Sn1, is employed. Cesium is subsequently precipitated from the leach solution as a double salt, generally as cesium antimony chloride 3CsCl · 2SbC13 or as cesium alum Cs2S04 • A12(S04 ) 3 • 24H20. Trnaitment of either double salt can give a purified single cesium salt. For metal preparation, the cesium salt is ordinarily prepared as or converted to cesium chloride. Active metals are then employed to release cesium from its purified compounds. In a typical cesium metal preparation, purified cesium chloride obtained from the decomposition of the antimony double s1aJt is reduced with calcium by heating in vacuum to yield the metal. Experimental work on direct metal replacement employing a calcium reduction of pollucite in vacuum was reported by Hackspill and Thomas4 in 1950. According to their report, one part by weight of pulverized pollucite ore, dehydrated at 900°C, is mixed with three parts of metalic calcium chips. As this mixture is heated in a vacuum, a reaction apparently starts near 750°C. With operating temperatures of 900°C, cesium yields of 85% are reported. Although this method appears interesting, the l~ge excess of calcium used to release the cesium poses an economic factor for other than laboratory operations. A sodium metal process for the direct extraction of cesium from pollucite has been briefly desciibed.5 According to this description, the ground ore was mixed with sodium and heated to liberate the cesium. The mixture was then cooled and a liquid alkali metal alloy containing cesium was filtered from the ore residue. It was claimed that this treatment removed about 90% of the cesium from the pollucite. A recent patent for the direct extraction of cesium from pollucite covers the use of lithium, sodium or potassium. 6 One experiment using two parts metallic sodium by weight to three parts of polh~cite has been described. About 88% of the cesium was exh·acted when this ore-sodium charge was heated for 31 h over temperatures ranging between 400 and 700°C under inert gas pressures of 75 to 250 mm. Any process for cesium metal by direct reduction generally yields metal contaminated with the other alkalies and usually requires subsequent purification. Distillation has been employed in purifying the metal. The boiling points 0£ the alkali metals are in reverse order of their atomic weights; therefore, rubidium (b.p. 694°C) presents the greatest problem in distilling cesium (b.p. 670°C) from the other alkali metals. However, a method of fractional distillation employing a stainless steel column6 has 2 Proceedings of the Iowa Academy of Science, Vol. 73 [1966], No. 1, Art. 15 http://scholarworks.uni.edu/pias/vol73/iss1/15 1966] CESIUM FROM POLLUCITE 8.'3 produced pound quantities of metallic cesium containing less than .02 wt % other alkalies. A preliminary investigation of various means for treating ore to recover cesium led to the present study which has been focused essentially on the release of cesium from pollucite ore by a reative metal. The relea1se of the cesium as metal from the ore combined with the fractional distillation could give an over-all process for pure metallic cesium. If the cesium is released from the ore in a condition that cannot effectively lend itself to fractional distillation, then the recoveiy of pt~rified cesium metal would :in.valve chemical processing of the released: product. These chemical treatments would involve solution foHowed by such operations as precipitation, liquid-liquid extraction or ion exchange to give a purified cesium compound. The purified cesium metal would then be obtained by treatment of this compound with an appropriate reactive metal. RegaQ"dless of whether the cesium released from the ore is to be purified either by distillation of the metal or by chemical processing, the evaluation of the over-all process depends primarily on the effectiveness with which the cesium is released from the ore. The major part of this investigation on cesium removal from pollucite WiliS performed with an ore from Southern Rhodesia which was obtained from the American Potash and Chemical Corporation. A domestic pollucite from Tin Mountain Mine near Custer, South Dakota was employed in only a few tests. Unless otherwise specified, the ore from Southern Rhodesia was employed in the experiments. Ores from both sources have been analyzed by a number of laboratories and the average values of the constituents are recorded in Table I. These ores were received in the form of small lumps; for the experiments, they were ground in finer sizes in a laboratory mill. Table 1. Analyses of Pollucite Ores Constituent Cs20 Rb20 K20 Na20 Li.O MgO Cao Fe20• Al20a Si02 L.O.I. Southern Rhodesia Ore 24.1 wt% 0.9 1.5 2.3 0.5 0.0 0.1 0.1 18.3 47.7 2.1 EXPLORATORY South Dakota Ore 28.5 wt% 0.1 0.7 2.7 0.2 0.3 0.7 0.2 17.6 45.8 3.2 Exploratory experiments on the removal of Cs from pollucite ore included treatments with anhydrous and aqueous hydrogen 3 Gault et al.: Cesium from Pollucite by Aluminum Reduction Published by UNI ScholarWorks, 1966 84 IOWA ACADEMY OF SCIENCE [Vol. 73 fluoride, treatments with sulfur chloride, and direct reductions with carbon and metal reducing agents. The hydrogen floride treatments decomposed the ore but the fluoride products. could not be readily processed. The sulfm chloride treatment was successful in bringing cesium to a recoverable form. 7 The carbon reduction in vacuum or inert atmosphere appeared to require rather high temperatures to release appreciable amounts of cesium. Some metallic reducing agents, however, were found to liberate cesium from the ore at temperatures below 1000°C. The metals used in these tests were selected on the basis of thermodynamic, economic and other practical considerations. Since free-energy data were not available on the cesium containing mineral, data8 on pure cesium oxide were employed in the calculations to give indications of the thermodynamic potentials of the reducing metals relative to one another. The equations employed in these calculations are represented by Cs20 + xM ~ MxO + 2Cs where "M" is the reducing metal considered for reaction with the pollucite. The results of the calculations of the free-energy change for the reaction of each of the reducing metals per mole of Cs20 at various temperatures are shown by the graphs in Fig. 1.