Vapour blasting

B) Vapour blast with garnet grit (200 grade and 400 grade) protect with oil, vapour degrease with trichloroethylene. 42.6 34.2 33.2... 

This continuous process is to be compared with a batch process, such as the Belgian retort process. In this, zinc oxide, free of lead or iron is reduced with carbon to produce zinc vapour, which is condensed in the cold section of the retort. The oxygen potential in this system is very much lower dran in the blast furnace, approximately at the C/CO equilibrium value. A vacuum-operated variant of dris level of reduction is caiTied out to produce zinc vapour which is subsequently converted to zinc oxide before condensation of the metal could take place. 

A similar logie is applieable to the eontrol of explosions involving gas or vapour, but other measures, e.g. dispersion by steam or eontainment by water eurtains, may be applieable to vapour elouds in the open air. Containment or diversion of a blast (e.g. by blast walls) and redueing its effeet by appropriate spaeing of equipment, buildings ete. are also applieable. 

These formerly involved the use of banks of externally heated, horizontal retorts, operated on a batch basis. They were replaced by continuously operated vertical retorts, in some cases electrically heated. Unfortunately none of these processes has the thermal efficiency of a blast furnace process (p. 1072) in which the combustion of the fuel for heating takes place in the same chamber as the reduction of the oxide. The inescapable problem posed by zinc is that the reduction of ZnO by carbon is not spontaneous below the boiling point of Zn (a problem not encountered in the smelting of Fe, Cu or Pb, for instance), and the subsequent cooling to condense the vapour is liable, in the presence of the combustion products, to result in the reoxidation of the metal ... 

An example of a physical explosion is the eruption of the Krakatoa volcano in 1883. During this eruption a large quantity of molten lava spilled into the ocean causing about 1 cubic mile of sea water to vapourize. This rapid vaporization created a blast wave which could by heard up to 3000 miles away. 

Interfacing the TEA to both a gas and a HPLC has been shown to be selective to nitro-based explosives (NG, PETN, EGDN, 2,4-DNT, TNT, RDX and HMX) determined in real world samples, such as pieces of explosives, post-blast debris, post-blast air samples, hand swabs and human blood, at picogram level sensitivity [14], The minimum detectable amount for most explosives reported was 4-5 pg injected into column. A pyrolyser temperature of 550°C for HPLC-TEA and 900°C for GC/TEA was selected. As the authors pointed out, GC uses differences in vapour pressure and solubility in the liquid phase of the column to separate compounds, whereas in HPLC polarity, physical size and shape characteristics determine the chromatographic selectivity. So, the authors reported that the use of parallel HPLC-TEA and GC-TEA techniques provides a novel self-confirmatory capability, and because of the selectivity of the technique, there was no need for sample clean-up before analysis. The detector proved to be linear over six orders of magnitude. In the determination of explosives dissolved in acetone and diluted in methanol to obtain a 10-ppm (weight/volume) solution, the authors reported that no extraneous peaks were observed even when the samples were not previously cleaned up. Neither were they observed in the analysis of post-blast debris. Controlled experiments with handswabs spiked with known amounts of explosives indicated a lower detection limit of about 10 pg injected into column. 

As a very essential requirement, there shall be absolutely no internal source of release of a combustible gas, vapour, mist or liquid. Even small volumes of gases with low LEL values (see Table 1.1 in Section 1.2), e.g. acetylene or benzene, can form an explosive mixture (with air) with a volume exceeding the volume of release by a factor 101 to 102 (as an order of magnitude). So, one mole of acetylene (=26g) can form a volume of more than 1 m3 explosive atmosphere. Or, as a second example a high voltage oil-blast circuit breaker with 200 MVA rated breaking capacity generates 70-80 litres... 

---