Mineral Processing and Extraction of Rare Earths

Mineral Processing and Extractive Metallurgy of the Rare Earths... 

Study of membrane extraction processes is a matter of primary importance for intensive development of separation and concentration methods of different nature substrates, especially such valuable ones as rare and scattered metals. The imique properties of rare earth metals (REM) allow using them in different realms of modem science and technology when making selective catalysts, magnets (samarium and neodymium), optical systems, luminophors, and ceramic capacitors. REMs are used in metallurgy for production of special cast iron grades, steel, and nonferrous metals alloys. REM additives increase quality of metallurgical products improve their properties, particularly shock resistance, viscosity, and corrosion resistance. Such materials are used primarily in aerospace industry. Extraction of REM from minerals is a complex process. 

Of the principal minerals of Ca listed in Table 1, the most important ores are the various deposits of CaCOj, especially limestones, which occur as immense sedimentary beds over extensive parts of the earth s surface. Extraction of Ca from CaCOj is a simple and relatively inexpensive process. Although the other Ca-bearing minerals are rarely considered as potential Ca sources, they are widely distributed and extensively mined fluorite and apatite for their fluoride and phosphate content, gypsum and anhydrite for their use in construction. 

Uranium minerals may be obtained in solution, in a suitable condition for estimation, by the following process. The ore is dissolved in aqua regia, or, if necessary, fused with alkali bisulphate and extracted mth hot hydrochloric acid. After evaporation to drjmess, the residue is taken up with dilute hydrochloric acid, and the solution saturated with hydrogeir sulphide in order to remove any copper, lead, bismuth, arsenic, antimony, or any other metal yielding an insoluble sulphide. The filtrate is concentrated and treated with ammonium carbonate, which precipitates the carbonates of the alkaline earths, iron, and most of the rare earths. The filtrate is neutralised by hydrochloric acid, evaporated to dryness, and the residue ignited to drive off ammonium salts, and then redissolved in dilute acid. The remaining rare earths, and particularly thorium, are next precipitated by the addition of oxalic acid. The filtrate, which contains the uranium in the uranyl condition, may now be precipitated by any of the methods described above. 

Reiterating, the phosphatic mineral of such phosphorites is essentially francolite, a carbonate fluorapatite of somewhat variable composition (McConnell, 1971 Rooney and Kerr, 1967). Although not proven to be contained within the apatitic phase through isomorphic substitution, some of the continental phosphorites are of considerable interest because of accumulations of uranium, thorium, yttrium, rare earths, scandium, and vanadium therein. These rarer components are thought to be related to diagenetic processes, in which case they were extracted from sea water during the early formative histories of the phosphorites. 

Abstract This chapter is about mineral processing of the rare earths (making the mined ore into a concentrate of the valuable minerals), and extractive metallurgy of the rare earths (how to get the metals out of the concentrate). The mineral processing of three well-known exploited ore deposits is discussed in more detail. 

Extraction. The process of extraction of the rare earths from minerals and their isolation as a group from the other elements with which they are generally associated is not a complicated one but, in most cases, it does demand the use of large-scale apparatus. In general, the silicates such as allanite, cerite, and gadolinite are opened up, or cracked, by treatment of the finely powdered mineral with hot concentrated nitric or hydrochloric acid... 

Billions of years were required for the formation of the earth s crust with its minerals and ores—a process bearing witness to many whims of nature which, to be more exact, reflect the laws of geochemistry. Some elements were less fortunate they did not succeed in forming their own minerals, that is, those in which they would be the principal or, at least, a noticeable component. They exist only as admixtures to all sorts of minerals consisting of other elements. They seem to be widely dispersed in the earth s crust and are called trace elements. Only in the rarest cases do they form their own minerals and if the scientists were lucky to come across them, the new element immediately became the target of chemical analysis. As we shall see later, this was the case of germanium extracted from argyrodite, a uniquely rare mineral. 

---