Metallic ions, fractional precipitation

Overflow at the rate of 2700 m (713,000 gal) per day from a zinc-concentrate thickener is treated by ion flotation, precipitate flotation, and untrafine-particle flotation [Nagahama, Can. Min. Metall. Bull., 67, 79 (1974)]. In precipitate flotation only the surface of the particles need be coated with collector. Therefore, in principle less collector is required than for the equivalent removal of ions by foam fractionation or ion flotation. 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

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. 

FA can interact with clay minerals and are known to form stable complexes with metal ions and hydrous oxides [59,61]. The operational technique for isolation of HA involves a pH-induced precipitation and it is likely that accessory minerals may be associated with the precipitation process. Complexes of HA and clay minerals are also formed, the increased ash content of HA suggesting that amorphous silica, iron hydroxides, and clay may aggregate with the HA fraction [58,60,61]. 

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... 

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. 

In different compartments of the environment, sediments accumulate mainly heavy metals and persistent organic pollutants (POPs), which reach aquatic ecosystems from different sources. Metal ions do not remain dissolved in water for long they are liable to be precipitated as a result of oxygenation, the formation of different compounds (carbonates and sulfates), or sorption on mineral surfaces and the organic fraction of sediments.83... 

Fractional crystallization and precipitation are classical methods of separation of rare earth metal ions. Complex forming agents may be used to give better separations than simple or double salts. Some of the complexing agents used in fractionation separation are given below. 

The effect of complex formation is to increase the solubility in proportion to the square root of the decreasing fraction of metal ion in the uncomplexed form. Similar considerations may be applied to more involved types of precipitates. 

The precipitation of cadmium and lead in hydroxyde form in basic solution interrupted further examination at pH > 6. A comparison of the elimination efficiency of these metal ions by modified corn-stick powder may be drawn. When the mercury could be removed in about 95% at the optimal condition, the fractional elimination of cadmium and lead was only 21% and 10% respectively. 

Calculations of metal ion speciation from literature data support the conclusions reached in the europium and uranyl systems. Literature data for the carbonate, hydroxide, and phosphate complexes of europium and uranium were used (5-72), with correction from infinite dilution to 0.1 M ionic strength via the Davies equation where necessary. The calculated fraction of metal ion precipitated as EUPO4... 

Figure 3. Model calculations of the expected fraction of metal ions precipitated as (a) EUPO4 or (b) (U02)3(P04)2 with 10" M (—), 10 M (- -), and 10 M (—) total phosphate under the experimental conditions of the study, and (c) for europium in the presence of 0.1 mg/L Lake Bradford humic acid at p[H] = 7.0 and 5 X 10" M total carbonate Eu(humate) (—), EUPO4 (-—).

Two primary resolving agents for metal complexes are D-tartrate and antimonyl D-tartrate ions. Optical resolution is achieved using either chromatography (e.g., ion exchange with D-tartrate salt as the eluent) or by fractional precipitation. We will use this latter technique for resolutions. When a racemic mixture of a metal complex is combined with an optically... 

A solution is 0.015 M in Pb + and 0.015M in Ag+. As CE is introduced to the solution by the addition of solid NaCl, determine (a) which substance will precipitate first, AgCl or PbCl2, and (b) the fraction of the metal ion in the first precipitate that remains in solution at the moment the precipitation of the second compound begins. 

Given the relatively unsophisticated nature of the techniques available to them, it is not surprising that early workers experienced so much difficulty in coming to terms with humic substances. Indeed, it is perhaps surprising that they made as much progress as they did. Inevitably the fractionation procedures adopted by these early workers were based on solubility properties and utilized precipitation techniques, particularly those based on adjustment of pH or the addition of metal ions. This early work has been extensively reviewed by Kononova (1966) and more recently has been concisely and astutely summarized by Stevenson (1982). 

Perhaps because of the ease of precipitating humic acid by the adjustment of pH, the use of metal ions for fractionation has not received great atten-... 

Classical fractionation procedures usually involve precipitation by adjustment of pH, adjustment of salt concentration, addition of organic solvents, or addition of metal ions. The fractionations produced are rather crude, but are generally quick and easy, and the manipulation of pH still remains a popular method today. 

Acidification of aqueous concentrates and extracts to pH near 1 is the standard procedure to precipitate humic from fulvic acid, and this procedure also has been applied to aquatic humic substances (Thurman and Malcolm, 1981). Aquatic humic substances that interact significantly with metal ions can be precipitated from water by addition of lead(Il) nitrate (Klocking and Mucke, 1969). Co-precipitation of aquatic humic materials with aluminum, copper, iron, and magnesium hydroxides has been used to recover aquatic humic substances from various types of water (Jeffrey and Hood, 1958 Williams and Zirino, 1964 Zeichmann, 1976). Humic acids can also be precipitated from an unconcentrated water sample by adding acetic acid and isoamyl alcohol to a sample contained in a separatory funnel, and after shaking, humic acid precipitates at the alcohol-water interface (Martin and Pierce, 1971). Precipitation methods are among the crudest of fractionation methods... 

The metals have the tendency to form compounds of low solubility with the major divalent cations (Pb, Cd being found in natural water. Hydroxide, carbonate, sulfide, and, more rarely, sulfate may act as solubility controls in precipitating metal ions from water. A significant fraction of lead and, to a greater extent, cadmium carried by river water is expected to be in an undissolved form. This can consist of colloidal particles or larger undissolved particles of lead carbonate, lead oxide, lead hydroxide, or other lead compounds incorporated in other components of surface particulate matter from runoff. The ratio of lead in suspended solids to lead in dissolved form has been found to vary from 4 1 in rural streams to 27 1 in urban streams. The US Environmental Protection Agency (USEPA) has reported Maximum Contaminant Levels in water that are permissible to be 0.005 m L for cadmium and 0.015 mg/L of lead. ... 

Complexation of drugs in the GI tract can occur with the luminal content. Any non-metallic atom, whether free or contained in a neutral molecule, or an ionic compound that can donate an electron pair, may serve as a donor. The acceptor is frequently a metal ion. In general, complexes can be divided into two classes, depending on whether the acceptor component is a metal ion or an organic molecule (Dakas and Takla 1991, Horter and Dressman 1997). Complex formation with components of food, such as milk, can give precipitation of the drug compound and reduce the bioavailability and fraction of the dose absorbed. Complex formation of peptide-like compounds and enzymes in the GI tract lumen has also recently been reported (Sjostrom et al. 1999) and a reduced bioavailability was observed. 

There are no additional parameters required for implementation of the equations above if all of the sorbent material is able to participate in the formation of the solid solution. However, an additional parameter is required if only a fraction of the sorbent phase can participate in the solid solution. In the case of a crystalline solid material, this parameter is expected to be a small fraction of the total solid added to the system, accounting for the fact that only the surface and some of the first few layers of the sorbent will participate in the formation of a solid solution with the co-precipitating metal ion. The value chosen for this parameter is likely to depend on the dissolution properties of the sorbent and can be considered a fitting parameter. 

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