Separation of Ions by Fractional Precipitation

In chemical analysis, it is sometimes desirable to remove one type of ion from solution by precipitation while leaving other ions in solution. For instance, the addition of sulfate ions to a solntion containing both potassium and barium ions causes BaS04 to precipitate out, thereby removing most of the Ba ions from the solution. The other product, K2SO4, is soluble and wiU remain in solution. The BaS04 precipitate can be separated from the solution by filtration. 

Example 16.11 describes the separation of only two ions (Cl and Br ), but the procedure can be applied to a solution containing more than two different types of ions if precipitates of differing solubility can be formed. 

A solution contains 0.020 M Cl ions and 0.020 M Br ions. To separate the CF ions from the Br ions, solid AgN03 is slowly added to the solution without changing the volume. What concentration of Ag ions (in mol/L) is needed to precipitate as much AgBr as possible without precipitating AgCl  

Strategy In solution, AgN03 dissociates into Ag and NOJ ions. The Ag ions then combine with the CF and Br ions to form AgCl and AgBr precipitates. Because AgBr is less soluble (it has a smaller than that of AgCl), it will precipitate first. Therefore, this is a fractional precipitation problem. Knowing the concentrations of CF and Br ions, we can calculate [Ag ] from the values. Keep in mind that refers to a saturated solution. To initiate precipitation, [Ag ] must exceed the concentration in the saturated solution in each case. 

Because [Br ] = 0.020 M, the concentration of Ag that must he exceeded to initiate the precipitation of AgBr is 

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Change in oxidation state can be used effectively because of the large solubility changes accompanying the changes in oxidation state. Examples of this method of separation are (i) separation of cerium by oxidation to Ce4+ and (ii) reduction of Eu, Sm and Yb to divalent state. Differences in solubility products of RE(OH)3 can be profitably used in their separation by fractional precipitation. For example, Ksp values of La(OH)3 and Lu(OH)3 are 10-19 and 10 24, respectively, and these values show that it is possible to preferentially precipitate the bulk of rare earths in the presence of ammonium ion leaving La3+ in solution. 

Cm is recovered from irradiated Pu/Al alloys and Am02(Pu02)/Al cermets by dissolution, extraction of plutonium with TBP in n-dodecane, extraction of americium and curium from the aqueous raffinate with 50 percent TBP in kerosene, purification of the americium and curium fraction by extraction with tertiary amines, and separation of americium by precipitation of the double carbonate K5 Am02 (003)3 A high-pressure ion-exchange system for the separation... 

We have described the precipitation reactions that occur when solid AgN03 is added slowly to a solution that is 0.0010 M in Cl", Br", and I". Silver iodide begins to precipitate first 99.955% of the l precipitates before any sohd AgBr is formed. Silver bromide begins to precipitate next 99.82% of the Br" and 99.999917% of the 1 precipitate before any solid AgCl forms. This shows that we can separate these ions very effectively by fractional precipitation. 

The percentage of Ba ion remaining in solution is qnite low, so most of the Ba ion has precipitated by the time SrCr04 begins to precipitate. Yon conclnde that Ba and Sr can indeed be separated by fractional precipitation. (Another application of fractional precipitation is shown in Figure 18.5.)... 

Carrageenans are a complex mixture of various polysaccharides. They can be separated by fractional precipitation with potassium ions. Table 4.20 compiles data on these fractions and their monosaccharide constituents. Two major fractions are >c (gelling and K+-insoluble fraction) and X (nongelling, K+-soluble). 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

The separation of solids from liquids forms an important part of almost all front-end and back-end operations in hydrometallurgy. This is due to several reasons, including removal of the gangue or unleached fraction from the leached liquor the need for clarified liquors for ion exchange, solvent extraction, precipitation or other appropriate processing and the post-precipitation or post-crystallization recovery of valuable solids. Solid-liquid separation is influenced by many factors such as the concentration of the suspended solids the particle size distribution the composition the strength and clarity of the leach liquor and the methods of precipitation used. Some important points of the common methods of solid-liquid separation have been dealt with in Chapter 2. 

The first fractionation of urinary ampholytes in this way was carried out by Boulanger et al. (BIO) in 1952 with the use of ion-exchange resins. They had designed this procedure previously for the fractionation of ampholytes in blood serum (B8). According to this method, deproteinized urine was subjected to a double initial procedure aiming at the separation of low-molecular weight substances from macro-molecular ones. One of the methods consisted of the fractionation of urinary constituents by means of dialysis, the second was based on the selective precipitation of urinary ampholytes with cadmium hydroxide, which, as had previously been demonstrated, permits separation of the bulk of amino acids from polypeptides precipitated under these circumstances. Three fractions, i.e., the undialyzable part of urine, the dialyzed fraction, and the so-called cadmium precipitate were analyzed subsequently. 

The further fractionation of this mixture by selective precipitation with Pb++ and Hg++ ions, undertaken by Bondzynski et al. (B7), proved unsuccessful, since no distinct separation of peptides was attained. 

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. 

The lanthanide group of elements (Table 11.7) is very difficult to separate by traditional methods because of their similar chemical properties. The techniques originally used, like the precious metals, included laborious multiple fractional recrystallizations and fractional precipitation, both of which required many recycle streams to achieve reasonably pure products. Such techniques were unable to cope with the demands for significant quantities of certain pure compounds required by the electronics industry hence, other separation methods were developed. Resin ion exchange was the first of these... 

Acid soluble rare earth salt solution after the removal of cerium may be subjected to ion exchange, fractional crystalhzation or solvent extraction processes to separate individual rare earths. Europium is obtained commercially from rare earths mixture by the McCoy process. Solution containing Eu3+ is treated with Zn in the presence of barium and sulfate ions. The triva-lent europium is reduced to divalent state whereby it coprecipitates as europium sulfate, EuS04 with isomorphous barium sulfate, BaS04. Mixed europium(ll) barium sulfate is treated with nitric acid or hydrogen peroxide to oxidize Eu(ll) to Eu(lll) salt which is soluble. This separates Eu3+ from barium. The process is repeated several times to concentrate and upgrade europium content to about 50% of the total rare earth oxides in the mixture. Treatment with concentrated hydrochloric acid precipitates europium(ll) chloride dihydrate, EuCb 2H2O with a yield over 99%. 

Uranium mineral first is digested with hot nitric acid. AH uranium and radium compounds dissolve in the acid. The solution is filtered to separate insoluble residues. The acid extract is then treated with sulfate ions to separate radium sulfate, which is co-precipitated with the sulfates of barium, strontium, calcium, and lead. The precipitate is boiled in an aqueous solution of sodium chloride or sodium hydroxide to form water-soluble salts. The solution is filtered and the residue containing radium is washed with boiling water. This residue also contains sulfates of other alkahne earth metals. The sohd sulfate mixture of radium and other alkahne earth metals is fused with sodium carbonate to convert these metals into carbonates. Treatment with hydrochloric acid converts radium and other carbonates into chlorides, all of which are water-soluble. Radium is separated from this solution as its chloride salt by fractional crystallization. Much of the barium, chemically similar to radium, is removed at this stage. Final separation is carried out by treating radium chloride with hydrobromic acid and isolating the bromide by fractional crystallization. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

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. 

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