Precipitated extenders

Electrostatic precipitators have been used in various gas-cleaning applications almost for a century. During the past decades, a large number of modifi cations to electrostatic precipitators have been developed, the nn.ist common being duct and pipe types. The utilization of electrostatic precipitation extends from small household air cleaners up to huge industrial gas-cleaning systems. 

A 500 ml 3-necked round bottom flask is equipped with a reflux condenser (preferably double-surface), an efficient Hg-sealed variable-speed mechanical stirrer and a wide inlet tube (to avoid possible clogging by the solid precipitate) extending almost to the bottom of the vessel. 

Precipitated Extenders. Precipitated extenders such as calcium carbonate (calcium carbonicum praecipitatum), barium sulfate (blanc fixe), and silica have been known for a fairly long time. When natural minerals are micronized, a minimum particle size limit is encountered. Lower particle sizes (< 1 pm) can only be obtained with considerable technical effort and expense. Extenders with finer particle sizes are produced synthetically by precipitation. The structure of one and the same extender can be modified by controlling the precipitation conditions. Particularly pure and thus bright extenders can be produced by a suitable choice of starting substances [4.12]. 

The release of metallic cations from weathering processes may be greater than the input of hydrogen ions therefore, there may be situations where the increased output of metallic ions is greater than would be expected from the increased acidity of the precipitation. Extended over very long periods of time, this could lead to serious depletion of the soil nutrient supply. 

Common teniiinology used to characterize impurities and defects in semiconductors includes point and line defects, complexes, precipitates and extended defects. These teniis are somewhat loosely defined, and examples follow. 

If tlie level(s) associated witli tlie defect are deep, tliey become electron-hole recombination centres. The result is a (sometimes dramatic) reduction in carrier lifetimes. Such an effect is often associated witli tlie presence of transition metal impurities or certain extended defects in tlie material. For example, substitutional Au is used to make fast switches in Si. Many point defects have deep levels in tlie gap, such as vacancies or transition metals. In addition, complexes, precipitates and extended defects are often associated witli recombination centres. The presence of grain boundaries, dislocation tangles and metallic precipitates in poly-Si photovoltaic devices are major factors which reduce tlieir efficiency. 

Equip a 1-litre three-necked flask with a powerful mechanical stirrer, a separatory funnel with stem extending to the bottom of the flask, and a thermometer. Cool the flask in a mixture of ice and salt. Place a solution of 95 g. of A.R. sodium nitrite in 375 ml. of water in the flask and stir. When the temperature has fallen to 0° (or slightly below) introduce slowly from the separatory funnel a mixture of 25 ml. of water, 62 5 g. (34 ml.) of concentrated sulphuric acid and 110 g. (135 ml.) of n-amyl alcohol, which has previously been cooled to 0°. The rate of addition must be controlled so that the temperature is maintained at 1° the addition takes 45-60 minutes. AUow the mixture to stand for 1 5 hours and then filter from the precipitated sodium sulphate (1). Separate the upper yellow n-amyl nitrite layer, wash it with a solution containing 1 g. of sodium bicarbonate and 12 5 g. of sodium chloride in 50 ml. of water, and dry it with 5-7 g. of anhydrous magnesium sulphate. The resulting crude n-amyl nitrite (107 g.) is satisfactory for many purposes (2). Upon distillation, it passes over largely at 104° with negligible decomposition. The b.p. under reduced pressure is 29°/40 mm. 

Solubility Considerations An accurate precipitation gravimetric method requires that the precipitate s solubility be minimal. Many total analysis techniques can routinely be performed with an accuracy of better than 0.1%. To obtain this level of accuracy, the isolated precipitate must account for at least 99.9% of the analyte. By extending this requirement to 99.99% we ensure that accuracy is not limited by the precipitate s solubility. 

Precipitate particles grow in size because of the electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution. Ions common to the precipitate are chemically adsorbed, extending the crystal lattice. Other ions may be physically adsorbed and, unless displaced, are incorporated into the crystal lattice as a coprecipitated impurity. Physically adsorbed ions are less strongly attracted to the surface and can be displaced by chemically adsorbed ions. 

Occlusions are minimized by maintaining the precipitate in equilibrium with its supernatant solution for an extended time. This process is called digestion and may be carried out at room temperature or at an elevated temperature. During digestion, the dynamic nature of the solubility-precipitation equilibrium, in which the precipitate dissolves and re-forms, ensures that occluded material is eventually exposed to the supernatant solution. Since the rate of dissolution and reprecipitation are slow, the chance of forming new occlusions is minimal. 

Precision The relative precision of precipitation gravimetry depends on the amount of sample and precipitate involved. For smaller amounts of sample or precipitate, relative precisions of 1-2 ppt are routinely obtained. When working with larger amounts of sample or precipitate, the relative precision can be extended to several parts per million. Few quantitative techniques can achieve this level of precision. 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving precipitation titrations are similar to those described earlier in the chapter for other titrimetric methods. Precipitation titrations also can be extended to the analysis of mixtures, provided that there is a significant difference in the solubilities of the precipitates. Figure 9.43 shows an example of the titration curve for a mixture of % and Ch using Ag+ as a titrant. 

Transparent white pigments (extenders) commonly used in inks, in order of decreasing transparency, ate alumina hydrate, magnesium carbonate, calcium carbonate, blanc fixe (precipitated barium sulfate), talc, and clay. Extenders ate sometimes used to reduce the color strength and change the theology of inks. 

Whereas many of these technologies are not really new, they have never had the regulatory and economic justification for their use in metallizing. Each of these general methods has many variants. Some may be directed to waste treatment, some to recycle, and some to reclaim. An example is filtration, used to prevent release to air of zinc particles from flame spraying, microfiltration of cleaners to extend hfe, in combination with chemical precipitation to remove metal particles from wastewater, and many other uses. 

Aragonite. Calcium carbonate is a common deposit in shallow tropical waters as a constituent of muds, or in the upper part of coral reefs where it precipitates from carbon dioxide-rich waters supersaturated with carbonate from intense biological photosynthesis and solar heating. Deposits of ooHtic aragonite, CaCO, extending over 250,000 km in water less than 5 m deep ate mined for industrial purposes in the Bahamas for export to the United States (19). 

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. 

Lakes are either dry toner pigments that are extended with a soHd diluent, or an organic pigment obtained by precipitation of a water-soluble dye, frequendy a sulfonic acid, by an inorganic cation or an inorganic substrate such as aluminum hydrate. 

The resulting solutions contain high dissolved soHds content in the range of 30 wt % or more. Special surfactant technology (25) is sometimes used to avoid precipitation of Al(OH)3 or at least to extend the shelf life of the caustic Hquor. 

Calcium carbonate is also used in industrial finishes and powder coatings. These paints typically include finer products the primary purpose is rheological and gloss control. Calcium carbonate is also used in paints to extend and enhance the use of titanium dioxide. This is accompHshed by using the finest of natural ground products or precipitated grades. 

---