Atomisation

We have seen that in a metal the atoms are close-packed, i.e. each metal atom is surrounded by a large number of similar atoms (often 12, or 8). The heat required to break up 1 mole of a metal into its constituent atoms is the heat of atomisation or heat of sublimation. Values of this enthalpy vary between about 80 and 800 kJ. for metals in their standard states these values indicate that the bonds between metal atoms can vary from weak to very strong. There is a rough proportionality between the m.p. of a metal and its heat of atomisation. so that the m.p. gives an approximate measure of bond strength. 

A/i the dissociation or bond energy of hydrogen (it is also, by definition, twice the enthalpy of atomisation two gram atoms being produced). 

A/i, the enthalpy of atomisation of chlorine, which is also half the bond dissociation enthalpy. 

Ah second ionisation energy for sodium (additional) +4561 A/13 enthalpy of atomisation of chlorine, x 2 (since two... 

The enthalpy changes AH involved in this equilibrium are (a) the heat of atomisation of the metal, (b) the ionisation energy of the metal and (c) the hydration enthalpy of the metal ion (Chapter 3). 

Heat of atomisation Sum of 1st and 2nd ionisation energies Hydration enthalpy AH... 

All the cations of Group I produce a characteristic colour in a flame (lithium, red sodium, yellow potassium, violet rubidium, dark red caesium, blue). The test may be applied quantitatively by atomising an aqueous solution containing Group I cations into a flame and determining the intensities of emission over the visible spectrum with a spectrophotometer Jlame photometry). 

Table 11.1 and Table 11.2 (p. 314) give some of the physical properties of the common halogens. Figure 11.1 shows graphically some of the properties given in Table 11.1, together with enthalpies of atomisation. 

Table 14.2 shows that all three elements have remarkably low melting points and boiling points—an indication of the weak metallic bonding, especially notable in mercury. The low heat of atomisation of the latter element compensates to some extent its higher ionisation energies, so that, in practice, all the elements of this group can form cations in aqueous solution or in hydrated salts anhydrous mercuryfll) compounds are generally covalent. 

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

Viscosity. For optimum performance of diesel engine injector pumps, the fuel should have the proper viscosity. Too low viscosity results in excessive injector wear and leakage. Viscosity that is too high may cause poor atomisation of the fuel upon injection into the cylinders. 

J ct Spra.y, The mechanism that controls the breakup of a Hquid jet has been analy2ed by many researchers (22,23). These studies indicate that Hquid jet atomisation can be attributed to various effects such as Hquid—gas aerodynamic interaction, gas- and Hquid-phase turbulence, capillary pinching, gas pressure fluctuation, and disturbances initiated inside the atomiser. In spite of different theories and experimental observations, there is agreement that capillary pinching is the dominant mechanism for low velocity jets. As jet velocity increases, there is some uncertainty as to which effect is most important in causing breakup. 

A. H. Lefehvte, Atomisation and Sprays, Hemisphere Publishing Corp., New York, 1989. 

Proceedings of International Conferences on Eiquid Atomisation and Spray Systems, ICLASS-1978, ICLASS-1982, ICLASS-1985, ICLASS-1988, ICLASS-1991, and ICLASS-1994, BegeU House, Inc., New York. 

Spinning-cup atomisers are used ia some plants to provide finer atomisation, allowiag smaller burner chambers and easier turndown, but with the burden of added rotating equipment. Rotary kiln burners were once popular to bum lower quaHty sulfur, but few are stiU ia operation. Spray burners can be operated intermittently and used at higher rates than rotary burners. 

Low temperature viscosities have an important influence on fuel atomisation and they affect engine starting. Cycloparaffinic and aromatic fuels reach unacceptably high viscosities at low temperatures. A kinematic viscosity of 35 mm /s (=cSt) represents the practical upper limit for pumps on aircraft, whereas much higher limits are acceptable for ground iastaHations. 

Rotary atomisation produces the most uniform atomisation of any of the aforementioned techniques, and produces the smallest maximum particle sise. It is almost always used with electrostatics and at lower rotational speeds the electrostatics assist the atomisation. At higher rotational speeds the atomisation is principally mechanical in nature and does not depend on the electrical properties of the coating material. If the viscosity of a coating material is sufficiendy low that it can be deUvered to a rotary atomiser, the material can generally be atomised. The prime mover is usually an ak-driven turbine and, provided that the turbine has the requked power to accelerate the material to the angular velocity, Hquid-dow rates of up to 1000 cm /min can be atomised using an 8-cm diameter beU. 

Rotary atomisation produces an excellent surface finish. The spray has low velocity, which allows the electrostatic forces attracting the paint particles to the ground workpiece to dominate, and results in transfer efficiencies of 85—99%. The pattern is very large and partially controlled and dkected by shaping ak jets. The spray when using a metallic cup has relatively poor penetration into recessed areas. Excessive material deposited on the edges of the workpiece can also be a problem. 

Increasing use is now being made of alternative processing routes. In powder metallurgy the liquid metal is atomised into small droplets which solidify to a fine powder. The powder is then hot pressed to shape (as we shall see in Chapter 19, hot-pressing is... 

In conventional spraying paint is forced under pressure to the spray gun, where it mixes with air and, forced through a small orifice, atomises. Airless spray is created by forcing paint at extremely high pressures through an accurately designed small hole. Rapid expansion as it leaves the gun produces an extremely fine and very even spray pattern. No air is mixed with the paint before it leaves the gun, so avoiding dry spray . A wetter, heavier... 

These are processes in which the paint is used once only and the excess material is not returned to the main bulk. A typical example is the normal spray system in which the paint is fed to the spray gun, atomised by air jets and applied to the article as a stream of small droplets. The excess paint and overspray are deposited on the walls of the booth and are collected by various methods depending on the type of spray booth used . There are many modifications of the conventional spray system which include the following. 

Air assisted airless spray This concept is a combination of air spray and airless methods. Paint can be atomised with full spray patterns at low pressures. Turbulence is reduced significantly and overspray is minimised. 

Airless Spraying the process of atomisation of paint by forcing it through an orifice at high pressure. This effect is often aided by the vaporisation of the solvents especially if the paint has been previously heated. The term is not generally applied to those electrostatic spraying processes which do not use air for atomisation. 

The most widely used accelerated tests are based on salt spray, and are covered by several Government Specifications. BS 1391 1952 (recently withdrawn) gives details of a hand-atomiser salt-spray test which employs synthetic sea-water and also of a sulphur-dioxide corrosion test. A continuous salt-spray test is described in ASTM B 117-61 and BS AU 148 Part 2(1969). Phosphate coatings are occasionally tested by continuous salt spray without a sealing oil film and are expected to withstand one or two hours spray without showing signs of rust the value of such a test in cases where sealing is normally undertaken is extremely doubtful. 

Metal Spraying application of a metal coating to a surface (metallic or non-metallic) by means of a spray of metal particles. The metal particles may be produced by atomising a metal wire in a flame-gun or by introducing metal powder into a similar gun. 

Atomic absorption spectroscopy involves atomising the specimen, often by spraying a solution of the sample into a flame, and then studying the absorption of radiation from an electric lamp producing the spectrum of the element to be determined. 

This chapter describes the basic principles and practice of emission spectroscopy using non-flame atomisation sources. [Details on flame emission spectroscopy (FES) are to be found in Chapter 21.] The first part of this chapter (Sections 20.2-20.6) is devoted to emission spectroscopy based on electric arc and electric spark sources and is often described as emission spectrography. The final part of the chapter (Sections 20.7-20.11) deals with emission spectroscopy based on plasma sources. 

The use of a plasma as an atomisation source for emission spectroscopy has been developed largely in the last 20 years. As a result, the scope of atomic emission spectroscopy has been considerably enhanced by the application of plasma techniques. 

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