Lepidolite processing

Rubidium (78 ppm, similar to Ni, Cu, Zn) and caesium (2.6 ppm, similar to Br, Hf, U) are much less abundant than Na and K and have only recently become available in quantity. No purely Rb-containing mineral is known and much of the commercially available material is obtained as a byproduct of lepidolite processing for Li. Caesium occurs as the hydrated aluminosilicate pollucite, Cs4ALiSi9026.H20, but the world s only commercial source is at Bemic Lake,... 

Rubidium does not exist in its elemental metallic form in nature. However, in compound forms it is the 22nd most abundant element on Earth and, widespread over most land areas in mineral forms, is found in 310 ppm. Seawater contains only about 0.2 ppm of rubidium, which is a similar concentration to lithium. Rubidium is found in complex minerals and until recently was thought to be a rare metal. Rubidium is usually found combined with other Earth metals in several ores. The lepidolite (an ore of potassium-lithium-aluminum, with traces of rubidium) is treated with hydrochloric acid (HCl) at a high temperature, resulting in lithium chloride that is removed, leaving a residue containing about 25% rubidium. Another process uses thermochemical reductions of lithium and cesium ores that contain small amounts of rubidium chloride and then separate the metals by fractional distillation. 

Numerous processes have been proposed for extracting potash from felspar, leucite, alunite, and other minerals rich in this substance, but the cost is so great that very few proposals yet made ofier promise of successful competition with the Stassfurt deposits. This is even the case with alunite, where mere calcination to 1000° drives off water and sulphuric acid, leaving water-soluble potassium sulphate, and alumina. Humphry Davy in his paper On Some Chemical Agencies of Electricity (1807), indicated in Cap. Ill, found that when water was electrolyzed in cavities contained in celestine, fluorspar, zeolite, lepidolite, basalt, vitreous lava, agate, or glass, the bases separated from the acid and accumulated about the cathode. It is therefore probable that if water with finely divided potash minerals in suspension were electrolyzed, the alkali would be separated in a convenient simple way. 

The muin source of cesium is camallite KCI MgCL 6H1O which contains a small percentage of cesium compounds. See also Camallite. Cesium also occurs in pollucile (cesium aluminosilicaie, 35% Cs 0) and lepidolite (lithium aluminosilicaie). See also Lepidolite and Pollucile. In early processes, cesium metal was obtained by the reduction of cesium salts, such as Ihe hydroxide or chloride. In current practice, the metal is produced by electrolyzing the cyanide. The latter compound usually is fused cesium barium cyanide mixture. 

Lithium carbonate, Li2C03.—The carbonate is prepared by boiling a solution of a lithium salt with ammonium, sodium, or potassium carbonate, its slight solubility inducing crystallization and facilitating purification. A process for its manufacture from lepidolite has also been devised.2... 

Like the other alkali metals, cesium is a soft, silvery metal, but it appears golden if it has been exposed to small amounts of oxygen. It is not found in its metallic state in nature it is obtained as a byproduct of lithium processing of the mineral lepidolite. Its most significant ore is pollucite, and the world s largest pollucite deposit is found in Bernic Lake, Manitoba, Canada. Cesium s average crustal abundance is about 3 parts per million. Cesium is the most electropositive stable element and will ignite if exposed to air. Cesium burns blue in the flame test. 

Lithium ores of major economic importance are spodumene, lepidolite, Trona concentrates, and amblygonite. Spodumene is the most abundant source, occurring in a complex matrix named pegmatite, which is inert to chemical treatment at room temperature. The industrially important processes of recovery of lithium from silicate minerals involve either high temperature ion substitution reactions or volatilization, and yield the sulfates, carbonates, hydroxides, or chlorides. These salts are readily interconvertible. Metallic lithium is made by electrolysis of lithium chloride. 

Lepidolite is a lithium potassium mica it also belongs to the class of aluminum silicates. Various empirical formulas have been proposed for it, probably because its actual composition varies somewhat. In fact, none of these minerals are pure crystals of definite composition. Of the lepidolite deposits which have been discovered in the United States so far, none warrant mining today. Lepidolite is processed in the United States, but the ore is imported from southern Africa. 

Rubidium is not too rare in the earth crust, being more abundant than lead. As stone melts crystallized when the earth s crust was formed, rubidium followed potassium in all minerals, as the ionic radii of these two elements are very similar. Consequently there are no typical rubidium minerals. This has been discussed in Chapter 4, Geochemistry. LepidoHte, a hthium-rich mica, is an exception. In that mineral, rubidium can substitute for lithium to such a great extent that as much as 2.5% may be present. One such source is the pegmatite at Bemic Lake, Manitoba in Canada. From mines there, a rubidium-containing lepidolite fraction is obtained and separated as a by-product However, this is not a very profitable business. The demand for rubidium in the whole world is only about 2 tonnes per year and this quantity is obtained in a few hours at Bemic Lake [13.1]. This lepidolite is worked in chemical plants in the US. From the mixed alkali carbonates, rubidium is isolated as sulfate or chloride by advanced separation processes. 

Figure 1.83 Flow sheet for the Bikita lepidolite hand-sorting process, Zimbabwe (Symons, 1961).

China s Yichun Li—Ta—Nb mine in 1998 accounted for 90% of the country s recoverable hthium reserves, and its lepidolite was easily obtained as concentrates from the open pit mine s tantalum and niobium processing. However, initially there was only a small amount of lithium carbonate processed from this ore due to the high cost of the lime sintering process (see the American Potash and Chemical Co. Section, above). To reduce these costs a pressurized ammonium chloride leach process has been suggested by Xu et al. (1998). In this process lepidolite concentrates (Table 1.18) would be initially partially defluorinated by being heated to 850°C for 20 min, and then ground to a —74 pm ( 200 mesh) size. The roasted concentrates would be cooled and made into a 25% aqueous slurry, with 3.5 mol of ammonium chloride being present per mole of total alkaline solids in the concentrate. The slurry would be heated under pressure at 240°C for 90 min, and then cooled, filtered and washed. The process was estimated to leach about 95% of the lithium, but the filtrate would also contain most of the other alkali metals in the ore. The filtrate would consequently be evaporated to crystallize the sodium, potassium. 

The production of lithium carbonate prior to 1966 came primarily from the processing of lithium minerals (since the 1960s primarily spodumene and petalite), but by 1998 this source became phased out except in China (using lepidolite) and... 

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