Impurity metal ions

Most base-metal sulfides are very insoluble compounds and, in principle, processes based on the differences in their solubilities can be used in the selective precipitation of sulfides. For example, the strong affinity of copper ions for sulfide ion is used to good effect in the removal of traces of copper from leach liquors in the recovery of nickel,406 cobalt407 and manganese.408 For manganese, other impurity metal ions such as cobalt, nickel and zinc are also selectively precipitated by the use of sulfide ions. 

Active sites might be impure metal ions located near the surface, which can reversibly bind O2, + Oi(g) as a peroxy or superoxy compound. 

The presence of impurity metal ions in slurry brought by raw materials will have a direct impact on the slurry performance such as static etch rate and removal rate. For example, as shown in Fig. 7.6, using the same formulation with various sources of hydrogen peroxide, the static etch and polishing rates are quite different [12]. 

Liquid micronutrient compositions are made by dissolving metal salts such as CUSO4 and MnCl2 in phosphoric acid, and then neutralising with anunonia. A small amount of a phosphonate of type (12.13a) or (12.13b) may be added to complex and prevent any precipitation of the metallic salts. Commercial superphosphoric acid, when used for superphosphate manufacture, functions as a source of micronutrient elements. The add itself can be used for liquid fertilisers, since the small quantities of polyphosphates which are present will sequester the impurity metal ions present. 

In comparison to a conventional extractirMi system, where PC-88A was dissolved in n-dodecane as an extractant, the DOEXjAA BL system showed a better extraction selectivity for the rare-earth metal ions. PC-88A in n-dodecane system, the separation from impurity metal ions in the leach solution such as Fe, Zn, Al, Ti, etc., seemed to be difficult, whereas DODGAA dissolved in the [C4mim][Tf2N] significantly enhanced the extraction efficiency even for light rare earths and the selectivity to other common metal ions. Only rare-earth metal ions could be successfully recovered from the first and second leaching solutions, the pH of which was adjusted for the extraction operation, as shown in Fig. 4.6. 

PFSA membranes are stable against 30% even at SO C in the absence of impurity metal ions. Laconti et al. posmlated that the formed could react with minor impurities, forming hydroxyl ( OH) and hydroperoxy ( OOH) radicals that could attack the membrane (LaConti 1982 LaConti et al. 1977, 2003). 

Finally, micellar systems are useful in separation methods. Micelles may bind heavy-metal ions, or, through solubilization, organic impurities. Ultrafiltration, chromatography, or solvent extraction may then be used to separate out such contaminants [220-222]. 

Viable glass fibers for optical communication are made from glass of an extremely high purity as well as a precise refractive index stmcture. The first fibers produced for this purpose in the 1960s attempted to improve on the quahty of traditional optical glasses, which at that time exhibited losses on the order of 1000 dB/km. To achieve optical transmission over sufficient distance to be competitive with existing systems, the optical losses had to be reduced to below 20 dB/km. It was realized that impurities such as transition-metal ion contamination in this glass must be reduced to unprecedented levels (see Fig. 

In natural water, the half-hves fall between these extremes. For example, the half-life of Lake Zbrich water (pH 8, 1.5 meq/LHCO ) is 10 min (27). The decomposition in natural water also can be initiated by trace metal ions, eg, Fe , promoted by impurities such as organic matter, and inhibited by HO radical scavengers, eg, HCO3, COg , HPO (25,28). 

With most transition metals, eg, Cu, Co, and Mn, both valence states react with hydroperoxides via one electron transfer (eqs. 11 andl2). Thus, a small amount of transition-metal ion can decompose a large amount of hydroperoxide and, consequendy, inadvertent contamination of hydroperoxides with traces of transition-metal impurities should be avoided. 

Calcium Pyrophosphates. As is typical of the pyrophosphate salts of multiple-charged or heavy-metal ions, the calcium pyrophosphates are extremely insoluble ia water. Calcium pyrophosphate exists ia three polymorphic modifications, each of which is metastable at room temperature. These are formed progressively upon thermal dehydration of calcium hydrogen phosphate dihydrate as shown below. Conversion temperatures indicated are those obtained from thermal analyses (22,23). The presence of impurities and actual processing conditions can change these values considerably, as is tme of commercial manufacture. 

Most of the heavy-metal impurities present in 2inc salt solutions must be removed before the precipitation reaction, or these form insoluble colored sulfides that reduce the whiteness of the 2inc sulfide pigment. This end is usually achieved by the addition of 2inc metal which reduces most heavy-metal ions to their metallic form. The brightness of 2inc sulfide can be improved by the addition of a small amount of cobalt salts (ca 0.04% on a Co/Zn basis) (20). Barium sulfate [7727-43-7] formed in the first step is isolated and can be used as an extender. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

The ultraviolet cutoff or the absorption edge for pure vitreous siUca is 8.1 eV or 153 nm (171). This uv cutoff is influenced by the impurity level and stoichiometry of the material. Several impurities, such as the transition metals (Fe, Cu, Ti, etc) and alkaU metal ions (Na, Li, K), degrade the ultraviolet performance, shifting the uv cutoff to longer wavelengths. Ferric ions (Fe " ) cause absorption or result in network defects under reducing conditions. This contaminant at only a few ppm can be detected as an absorption at 230 nm and below (176). 

Impurities such as chloride ion or other reducing agents generate chlorine dioxide when the chloric acid solution is heated. Transition-metal ions do not affect the stabiUty of pure chloric acid at room temperature. Thirty-five percent solutions of HCIO have been shown to be stable for 20 days at room temperature containing up to 1000 ppm Ni ", 800 ppm Zn ", 700 ppm Fe ", or 600 ppm Cr " (2). The solubiUty of chloric acid in water is shown in Figure 1. 

Color from Transition-Metal Compounds and Impurities. The energy levels of the excited states of the unpaked electrons of transition-metal ions in crystals are controlled by the field of the surrounding cations or cationic groups. Erom a purely ionic point of view, this is explained by the electrostatic interactions of crystal field theory ligand field theory is a more advanced approach also incorporating molecular orbital concepts. 

Greater selectivity in purification can often be achieved by making use of differences in chemical properties between the substance to be purified and the contaminants. Unwanted metal ions may be removed by precipitation in the presence of a collector (see p. 54). Sodium borohydride and other metal hydrides transform organic peroxides and carbonyl-containing impurities such as aldehydes and ketones in alcohols and ethers. Many classes of organic chemicals can be purified by conversion into suitable derivatives, followed by regeneration. This chapter describes relevant procedures. 

Because of their zwitterionic nature, amino acids are generally soluble in water. Their solubility in organic solvents rises as the fat-soluble portion of the molecule increases. The likeliest impurities are traces of salts, heavy metal ions, proteins and other amino acids. Purification of these is usually easy, by recrystallisation from water or ethanol/water mixtures. The amino acid is dissolved in the boiling solvent, decolorised if necessary by boiling with Ig of acid-washed charcoal/lOOg amino acid, then filtered hot, chilled, and set aside for several hours to crystallise. The crystals are filtered off, washed with ethanol, then ether, and dried. 

However, under anhydrous conditions and in the absence of catalytic impurities such as transition metal ions, solutions can be stored for several days with only a few per cent decomposition. Some reductions occur without bond cleavage as in the formation of alkali metal superoxides and peroxide (p. 84). 

Mass-transfer deposits can lead to blockages in non-isothermal circulating systems, cis in the case of liquid-metal corrosion. In fused salts, the effect can be reduced by keeping contamination of the melt by metal ions to a minimum e.g. by eliminating oxidising impurities or by maintaining reducing conditions over the melt . 

Vitreous silica produced by this route contains small amounts of impurities such as Fe, Cr, A1 and Ca. To achieve metal ion impurities < 10 % the synthetic hydrolysed silane process is used. Organic silica compounds or SiCl4 are hydrolysed in a flame to produce fine molten droplets of SiOj which is deposited on a cold base. 

Impurities, such as heavy metal ions, accelerate the decompn and may cause an expln to occur at a lower temp. Open Cup flash pt is 105°F (Ref 12). Since the material is an extremely powerful oxid agent it must be handled with extreme caution and kept away from combustible mats (Ref 12)... 

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