GB866715A - Process for separating and purifying elements forming insoluble or difficultly soluble oxalates - Google Patents

Abstract

A process for the separation and purification of difficulty separable elements capable of forming insoluble or difficulty soluble oxalates especially the rare earth elements, comprises forming as a liquid system a solution of a water-soluble complex of the elements with a di- or poly-basic aliphatic carboxylic acid, an additional water-soluble complex former being present in the case of elements which form insoluble salts with the said aliphatic acid, which additional complex former may be present in all cases, and subjecting said soluble complex mixture to a fractional precipitation in the presence of oxalate ions, which may either be provided by the addition of precipitants yielding oxalate ions, or may already be in the solution, to precipitate the elements as oxalates, by altering one or more parameters (as defined below) of the liquid system up to but not beyond the point when crystal nuclei of a compound or compounds of said element or elements are first formed as determinable by the Tyndall effect and then maintaining said one or more parameters unchanged until separation of the so-induced precipitate or crystallisation is complete, the liquid system being maintained substantially homogeneous with respect to said one or more parameters during the process. The word "parameter" is defined as a variable a change in which will induce a precipitation or crystallisation of a compound of the desired element. Specified di- or poly-basic acids are oxalic, citric, and tartaric acids, and chlorinated or sulphonated di- or poly-basic aliphatic carboxylic acids. It is preferred to carry out the step-wise precipitation very slowly so that the changes taking place in the reaction medium conditions, e.g. reactant concentrations, pH value and temperature, are such as to give the best conditions for separation at each stage. Precipitation in each step begins with nuclei formation which produces a slight turbidity (Tyndall effect) and as soon as this appears the addition of the precipitant is discontinued until precipitation of the particular fraction is complete. The additional complex former is suitably a compound of an additional metal, e.g. of aluminium, beryllium, scandium, yttrium, boron, gallium, indium, thallium, germanium, zirconium, tin, vanadium, niobium, tantalum, arsenic, antimony, chromium, molybdenum, tungsten, uranium or iron. Water soluble complexes may suitably be obtained in this way by stirring or digesting freshly precipitated oxalates with aluminium salts, e.g. ammonium alum, or p aluminiumnitrate, sulphate or -chloride. The step-wise decomposition of the oxalate complexes is preferably effected by successive dilution with water, addition of ammonium oxalate, addition of oxalic acid and finally of dilute mineral acids. Soluble complexes of the rare earth elements may also be formed by using citric acid and ammonia and step-wise decomposition may then be performed by the successive addition of ammonium oxalate, of oxalic acid and finally mineral acids with pH control at constant temperature in each case. Other organic or inorganic acid groups, e.g. acetate, nitrate, sulphate, chloride, fluoride or phosphate may be included and organic solvents, several types of which are specified, may also be present. In another embodiment the water-soluble complexes may be formed by reaction of soluble compounds of the elements, especially salts, with a metalcarboxylic acid complex of the type disclosed in Specification 866,718, e.g. with an aluminium oxalic acid complex of the formula H6[Al2(C2O4)6]H2O. Examples are given for the separation of various rare earths and for the separation of the individual metal values from a mixture of cerium, thorium and uranium oxides. The process is also suitable for the separation of gallium, indium, thallium, niobium, tantalum, hafnium, the platinum metals, manganese, rhenium, cobalt, nickel, calcium, barium, strontium, zinc and cadmium. Specification 866,716 also is referred to.