Precipitation of rare earths

From the diluted acid solutionf the rare earth oxalates are precipitated by the addition of a small excess of hot concentrated oxalic acid solution. Usually about 2 kg. of oxalic acid is required. The precipitated oxalates are thoroughly washed with hot water, converted to hydrojis oxides by boiling with concentrated sodium hydroxide solution (see synthesis 12D) and the washed hydrous oxides dissolved in the least amount of concentrated (16 N) nitric acid (about 21.). This solution is then diluted to 121. 

Solid sodium sulfate is slowly sifted, with thorough stirring, into the dilute nitrate solution at room tempera- 

JTo ensure complete precipitation, the excess of oxalic acid must be sufficient to form a complex with all of the iron present. 

To the double sulfate precipitate, 2 1. of water and 2 1. of 15 iV ammonia solution are added and the mixture is well stirred until the textxire of the precipitate indicates conversion to hydrous oxides (see synthesis 12D). The hydrous oxide precipitate is washed (by decantation) with large volumes of water in a 5-gal. crock until the washings, removed by siphoning, are only very slightly basic. The precipitate is then dissolved in the least amount of concentrated nitric acid. Cerium is next removed from this solution by the bromate method (see synthesis 14), and the remaining rare earths are converted to double magnesium nitrates (synthesis 15). 

The yttrium group solution is treated with a slight excess of ammonia solution. The resulting hydrous oxides are washed and then converted to bromates (synthesis 17) for fractional crystallization. 

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Anhydrous ammonium oxalate is obtained when the monohydrate is dehydrated at 65°C. The monohydrate is a colorless crystal or white powder, and dissolves in water at 0°C up to 2.17 wt %, and 50°C up to 9.63 wt %. It is slightly soluble in alcohol and insoluble in ether. It is used for textiles, leather tanning, and precipitation of rare-earth elements. 

All the early work on plutonium was done with unweighable amounts on a tracer scale. When it became apparent that large amounts would be needed for the atomic bomb, it was necessary to have a more detailed knowledge of the chemical properties of this element. Intensive bombardment of hundreds of pounds of uranium was therefore begun in the cyclotrons at Berkeley and at Washington University in St. Louis. Sepa-ration of plutonium from neptunium was based on the fact that neptunium is oxidized by bromate while plutonium is not, and that reduced fluorides of the two metals are carried down by precipitation of rare earth fluorides, while the fluorides of the oxidized states of the two elements are not. Therefore a separation results by repeated bromate oxidations and precipitations with rare earth fluorides. 

Full-scale simulations of the formic acid denitration and the precipitation of rare earths (simulating americium) were carried out in plant equipment before processing the actual stream containing the Am. 

Ambient aqueous routes and hydrothermal routes The synthesis of rare earth orthophosphate NCs could be carried out in aqueous solutions by the precipitation of rare earth cations by phosphate anions, due to the low aqueous solubility of RPO4. A typical synthesis begins with soluble rare earth salts in aqueous solution, such as nitrates or chlorides, and a phosphate source like phosphate M H3 P04, triphosphate M5P3O10 (M = Na, K, or NH4), or PI3PO4. The two solutions are mixed at room temperature or elevated temperature to form gel-like precipitation as precursor. Then the mixture of precipitation and mother liquor is... 

The filtrate from (A) (which contains H" ", SO4 , H2P04, Th", La+ , Ce" " , Nd" " , etc.) is diluted to a total volume of 168 1. in a stoneware jar or wooden barrel and stirred for at least 1 hour. Then the pale, blue-gray, heavy, gelatinous precipitate is allowed to settle for 8 to 12 hours. This precipitate consists of thorium phosphate together with some cerium and other rare earth phosphates. To be certain that all the thorium has been removed, a sample is filtered and tested by further dilution. This simple test is sufficient to determine whether the removal of thorium has been complete, since a tremendously greater dilution would be necessary to cause precipitation of rare earth phosphates. The main precipitate is removed by filtration and washed free of rare earth ions. If it is desired to obtain the thorium from this phosphate precipitate, the material should be washed free of sulfuric acid and air-dried. If the phosphate is allowed to dry with sulfuric acid present, it will become hard and glassy in character. It is then almost entirely unreactive toward concentrated acids and bases and yields only to a basic fusion. 

These difficulties are overcome when solutions of certain rare earth acetates in aqueous potassium citrate are used as electrolytes. Such solutions may be made alkaline without causing precipitation of rare earth hydrous oxide. 

The precipitation of rare earth elements was done by adding oxalic acid to both leach liquors obtained from digesting the ore sample with 9M sulfuric acid at the acid/ore ratios 5.0/ 1,0 and 10.0/1.0. The oxalate precipitates of rare earth relative to calcium (Pre) were given in Figure (3), p j. = [REI. 

The acidic solution was then subjected to pH adjustment at ambient temperature using ammonium hydroxide for the classical precipitation of rare earths in the range 1.1 - 9.7. The fractions contents were semi-quantitatively analyzed by EDAX-SEM. 

Gypsum and/or anhydrite were separated in the insoluble residue. The formation of either calcium sulfate compounds is independent on leaching factors. The rare earth elements were transferred to the leach liquor with efficiency reached 97.8% by leaching the ore sample with 9M sulfuric acid at 100 C for 2 houi s at the acid/ore ratios 10.0/1.0. The precipitation of rare earth from sulfate leach liquor by oxalic acid is preferred at lower acid/ore ratio. 

The second method, oxalate precipitation of rare earth standards, provides chemically bonded constituents. However, relative reproducibilities of 17 percent were obtained with this type of preparation, only slightly better than with dry blended standards. A source of error in attempting broad range quantification of different intensity levels is the fluctuation in the total ion beam composition at the beam monitor. Any determinations dependent upon relative exposure levels will include this error. 

Figure 6.10.2 The effect of the initial PO/ RE mole ratio on precipitation of rare earth phosphates from NaCI-2CsCI based melts. Phosphate added as Na PO, temperature 550° C, time of experiments = 4h...

Re OPe . The final step in the chemical processing of rare earths depends on the intended use of the product. Rare-earth chlorides, usually electrolytically reduced to the metallic form for use in metallurgy, are obtained by crystallisation of aqueous chloride solutions. Rare-earth fluorides, used for electrolytic or metaHothermic reduction, are obtained by precipitation with hydrofluoric acid. Rare-earth oxides are obtained by firing hydroxides, carbonates or oxalates, first precipitated from the aqueous solution, at 900°C. 

Separation and Recovery of Rare-Earth Elements. Because rare-earth oxalates have low solubihty in acidic solutions, oxaUc acid is used for the separation and recovery of rare-earth elements (65). For the decomposition of rare-earth phosphate ores, such as mona ite and xenotime, a wet process using sulfuric acid has been widely employed. There is also a calcination process using alkaLine-earth compounds as a decomposition aid (66). In either process, rare-earth elements are recovered by the precipitation of oxalates, which are then converted to the corresponding oxides. 

A mixture of dimethyl sulfate with SO is probably dimethyl pyrosulfate [10506-59-9] CH2OSO2OSO2OCH2, and, with chlorobenzene, it yields the 4,4 -dichlorodiphenylsulfone (153). Trivalent rare earths can be separated by a slow release of acid into a solution of rare earth chelated with an ethylenediaminetetraacetic acid agent and iodate anion. As dimethyl sulfate slowly hydrolyzes and pH decreases, each metal is released from the chelate in turn and precipitates as the iodate, resulting in improved separations (154). 

The pH effect in chelation is utilized to Hberate metals from thein chelates that have participated in another stage of a process, so that the metal or chelant or both can be separately recovered. Hydrogen ion at low pH displaces copper, eg, which is recovered from the acid bath by electrolysis while the hydrogen form of the chelant is recycled (43). Precipitation of the displaced metal by anions such as oxalate as the pH is lowered (Fig. 4) is utilized in separations of rare earths. Metals can also be displaced as insoluble salts or hydroxides in high pH domains where the pM that can be maintained by the chelate is less than that allowed by the insoluble species (Fig. 3). 

Due to the great similarity of the chemical properties of the rare earth elements, their separation represented, especially in the past, one of the most difficult problems in metallic chemistry. Two principal types of process are available for the extraction of rare earth elements (i) solid-liquid systems using fractional precipitation, crystallization or ion exchange (ii) liquid-liquid systems using solvent extraction. The rare earth metals are produced by metallothermic reduction (high purity metals are obtained) and by molten electrolysis. 

Sholkovitz ER, Elderfield H, Szymczak R, Casey K (1999) Island weathering riverine sources of rare earth elements to the western Pacific Ocean. Marine Chem 68 39-57 Skulan JL, Beard BL, Johnson CM (2002) Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(III) and hematite. Geochim Cosmochim Acta 66 2995-3015 Sumner DY (1997) Carbonate precipitation and oxygen stratification in Late Archean seawater as deduced from facies and stratigraphy of the Gamohaan and Frisco Formations, Transvaal Supergroup, South Africa. Am J Sci 297 455-487... 

Finely-ground monazite is treated with a 45% NaOH solution and heated at 138°C to open the ore. This converts thorium, uranium, and the rare earths to their water-insoluble oxides. The insoluble residues are filtered, dissolved in 37% HCl, and heated at 80°C. The oxides are converted into their soluble chlorides. The pH of the solution is adjusted to 5.8 with NaOH. Thorium and uranium are precipitated along with small quantities of rare earths. The precipitate is washed and dissolved in concentrated nitric acid. Thorium and uranium are separated from the rare earths by solvent extraction using an aqueous solution of tributyl phosphate. The two metals are separated from the organic phase by fractional crystallization or reduction. 

In one acid digestion process, monazite sand is heated with 93% sulfuric acid at 210°C. The solution is diluted with water and filtered. Filtrate containing thorium and rare earths is treated with ammonia and pH is adjusted to 1.0. Thorium is precipitated as sulfate and phosphate along with a small fraction of rare earths. The precipitate is washed and dissolved in nitric acid. The solution is treated with sodium oxalate. Thorium and rare earths are precipitated from this nitric acid solution as oxalates. The oxalates are filtered, washed, and calcined to form oxides. The oxides are redissolved in nitric acid and the acid solution is extracted with aqueous tributyl phosphate. Thorium and cerium (IV) separate into the organic phase from which cerium (IV) is reduced to metalhc cerium and removed by filtration. Thorium then is recovered from solution. 

Yttrium oxide is produced as an intermediate in recovery of yttrium from xenotime and monazite (See Yttrium, Recovery). The oxide is produced after separation of rare earth sulfates obtained from digesting the mineral with sulfuric acid on a cation exchange bed, precipitating yttrium fraction as oxalate, and igniting the oxalate at 750°C. 

On the other hand, the use of rare earth metals for the fixing of os gen and sulfur in light metals for production of conductive copper and conductive aluminum has remained insignificant. Hcwever, the use of rare earth elements as magnesium hardeners remains important, tfere the rare earth metals serve precipitation of intermetallic compounds of high thermal stability. 

Th, thorium, was discovered in 1829 by Jons Jakob Berzelius, who isolated a new oxide from a recently discovered mineral which Jens Esmark had sent to him. He called the oxide thoria and the mineral thorite (ThSi04) after the Scandinavian god Thor. Berzelius subsequently made the metal by the reduction of ThF4 with Na. Th now is extracted from monazite, a phosphate of rare earths and Th. The mineral is heated in concentrated NaOH to give hydrous oxides, which are filtered out. HCl is then added to dissolve the solids and when the pH is adjusted to 3.5, Th02 precipitates and the rare earths remain in solution. The Th02 is solubilized and purified by solvent extraction. 

The long story of the methods for the separation of the individual rare earths may broadly be divided into two main parts a) classical methods b) modern methods. Old-fashioned classical techniques like fractional crystallization, fractional precipitation and fractional thermal decomposition were not only used by the early workers in the past, but still remain as very important methods for economical production of rare earths on commercial scales. Modem methods like solvent (liquid-liquid) extraction, ion exchange or chromatographic (paper, thin layer and gas) techniques have both advantages and limitations. 

Extraction of the rare earths with acetylacetone has been investigated [418, 419] and is found to be enhanced by the decreasing basicity of the rare earth ions. The gas chromatographic separation of rare earth complexes with 2,2,6,6-tetramethyl-3,5-heptanedione has already been mentioned. The acetylacetonate complexes of the rare earths are reported to exist as either anhydrous [420, 421], mono- [422], di- [422] or trihy-drates [422, 423], Stites et al. [424] have studied the pH of the precipitation of several rare earth acetylacetonates and reported the melting points of the complexes. The europium acetylacetonate precipitated at pH 6.5, and melted at 144—45° C. The existence of monomers and dimers for these complexes in nonaqueous solvents has been proposed [421, 425-427],... 

The solvent extraction of rare-earth nitrates into solutions of TBP has been used commercially for the production of high-purity oxides of yttrium, lanthanum, praseodymium and neodymium from various mineral concentrates,39 as well as for the recovery of mixed rare-earth oxides as a byproduct in the manufacture of phosphoric acid from apatite ores.272 273 In both instances, extraction is carried out from concentrated nitrate solutions, and the loaded organic phases are stripped with water. The rare-earth metals are precipitated from the strip liquors in the form of hydroxides or oxalates, both of which can be calcined to the oxides. Since the distribution coefficients (D) for adjacent rare earths are closely similar, mixer—settler assemblies with 50 or more stages operated under conditions of total reflux are necessary to yield products of adequate purity.39... 

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