Seeding agents

To produce the mtile titanium dioxide pigment, hydrolysis of the mother Hquor has to be carried out in the presence of a specially prepared hydrosol as a seeding agent. This hydrosol is made by the neutralization of a portion of the mother Hquor in the presence of hydrochloric or some other monohydric acid. Because of the large amount of the hydrosol that must be added to the mixture (about 6% concentration), the hydrolysis reaction takes only about 1 hr. 

Nevertheless, metastable forms of substances often can be synthesized under much milder conditions. In these cases the thermodynamically unstable phase is (co)precipitated or (co)deposited from a solution or gas phase under conditions when the formation of the target phase results from chemical transformations of deliberately chosen reactants. Usually, at least two conditions must exist (1) the initial chemical potential of the initial reaction group (the precursor of the phase to be synthesized) must meet conditions (5.1)-(5.5), and (2) the seeds in the target phase must be in the system to favor, on the (co)precipitation or (co)deposition step, the formation of the phases replicating the seed phase structure. In epitaxial growth of films, the substrate with the deliberately chosen structure may be the seeding agent. 

M lead acetate and 2 M sodium azide solutions to the reactor which contains a quantity of sodium carbonate solution. The lead acetate is started a little ahead of the sodium azide so that some lead carbonate is formed which serves as a seeding agent for lead azide crystals. It contains 98.1% Pb(N )j, crystals are large (cb. 55 jjin), density under 15,000 kg/cm is 3.31. Its temperature of explosion is 350 C with an induction period of 5 sec. It is non-hydroscopic, contrary to the dextrinated lead azide which is slightly hygroscopic due to the presence of dextrin. 

To produce the rutile form of titanium dioxide, the clarified Uquor is hydrolyzed in the presence of a rutile seeding agent, obtained by neutralizing a small portion of the mother liquor in the presence of hydrochloric acid. The resultant crystals are filtered, washed, and calcined at temperatures between 900 and 930 °C. 

The need for an organic seeding agent may explain why apatite crystallites are not formed in soft tissues. The only difficulty is that the epitactic grouping, forming the nucleation centre on the organic matrix of mineralizing tissues, has never been identified with certainty. 

Tamarind kernel powder is insoluble in cold water, but upon heating forms thick viscous coUoidal dispersions at relatively low concentrations (2—3%). Upon drying, elastic films are formed. Tamarind seed polysaccharide is used as a low cost textile sizing agent in India. 

The softened seawater is fed with dry or slaked lime (dolime) to a reactor. After precipitation in the reactor, a flocculating agent is added and the slurry is pumped to a thickener where the precipitate settles. The spent seawater overflows the thickener and is returned to the sea. A portion of the thickener underflow is recirculated to the reactor to seed crystal growth and improve settling and filtering characteristics of the precipitate. The remainder of the thickener underflow is pumped to a countercurrent washing system. In this system the slurry is washed with freshwater to remove the soluble salts. The washed slurry is vacuum-filtered to produce a filter cake that contains about 50% Mg(OH)2. Typical dimensions for equipment used in the seawater process may be found in the Hterature (75). 

Opium is the dried, powdered sap of the unripe seed pod of Papaver somniferum, a poppy plant indigenous to Asia minor. Theophrastus described its medical properties in the third century BC, but the Sumerians, ca BC 4000, probably perceived its utility. Arab physicians knew of the dmg, and Arab traders carried it to the Orient where it was used as a treatment for dysentery. Paracelsus is credited with repopularizing the dmg in western Europe in the early sixteenth century by formulating opium into "laudanum", which is still in use. More than 20 different alkaloids (qv) of two different classes comprise 25% of the weight of dry opium. The benzylisoquinolines, characterized by papaverine [58-74-2] (1.0%), a smooth muscle relaxant, and noscapine [128-62-1] (6.0%), an antitussive agent, do not have any analgesic effects. The phenanthrenes, the second group, are the more common and include 10% morphine (1, = R = H), 0.5% codeine [76-57-3], C gH2 N03, (1, R = H, R = CH3), and 0.2 thebaine [115-37-7], C 2H2 N03, (2). 

Methyl bromide finds use as a methylating agent in the syntheses of agricultural and dmg chemicals. It is also used in ionization chambers, for degreasing wool, and for extracting oil from nuts, seeds, and flowers. 

Fig. 9.2. The excellent crystallographic matching between silver iodide and ice makes silver iodide a very potent nucleating agent for ice crystals. When clouds at sub-zero temperatures are seeded with Agl dust, spectacular rainfall occurs.

In addition to the nucleating agents discussed in Section 18.4, many other materials have been found to be effective. Whilst the nylons may be self-nucleating, partieularly if there is some unmelted crystal structure, seeding with higher melting point polymers can be effective. Thus nylon 66 and poly(ethylene terephthalate) are reported to be especially attractive for nylon 6. 

---