Use of a flux

Normally a flux is used to remove oxides from surfaces when brazing or soldering, and a gas-oxide torch also requires the use of a flux. However, if the flame is oxygen-rich, as the flux is boiled or burnt off, the excess oxygen from the torch will oxidize the metal s surface before the solder can wet the surface. In a reducing flame, as the flux is boiled or burnt off, the reducing flame will maintain the pure metal until the solder can flow over and wet the surface. 

Additional filler metal can be applied separately as rod or wire. The argon shield enables aluminium, magnesium alloys and a wide range of ferrous metals to be welded without the use of a flux. 

In a comprehensive study, Lassner (32) comes to the following conclusions the combustion of niobium, tantalum, molybdenum and tungsten in a stream of oxygen is easy, but to analyse carbon accurately the use of a flux is necessary. In the case of molybdenum it suppresses the sublimation of molybdenum trioxide, in the case of niobium and tantalum it lowers the reaction velocity. Larger pieces of these metals burn so quickly that the boats fuse and can thus not be used even with a flux. Tungsten, finally, burns slowly at a temperature below 1300°C, but so quickly above this temperature that for a short time very high temperatures are reached which cause melting on of the formed oxide. As it solidifies very quickly and encloses the rest of the metal, it causes low carbon results. 

To obtain the necessary cleanliness, degreasing and mechanical abrasion may be required followed by the use of a flux to remove any remaining oxide film and to ensure that no tarnish film develops on subsequent heating. 

Ammonium chloride is used as a flux ia the melting furnace because the large surface of the cathodes favors the formation of dross, ie, oxide-coated globules of ziac. The dross is separated by Hquation or air-swept milling iato metal and oxide fractions. In the latter, the oxide fraction is swept out of the mill and can be returned to roasting for the elimination of chloride. Metallic ziac is recycled. Overall melting efficiency is 96—98%. 

Potassium cyanide [151 -50-8] KCN, a white crystalline, deUquescent soHd, was initially used as a flux, andlater for electroplating, which is the single greatest use in the 1990s. The demand for potassium cyanide was met by the ferrocyanide process until the latter part of the nineteenth century, when the extraordinary demands of the gold mining industry for alkah cyanide resulted in the development of direct synthesis processes. When cheaper sodium cyanide became available, potassium cyanide was displaced in many uses. With the decline in the use of alkah cyanides for plating the demand for potassium cyanide continues to decline. The total world production in 1990 was estimated at about 4500 t, down from 7300 t in 1976. 

To determine the value of p, the saturation or the B-H curve of the silicon steel being used must be available. Assuming a normal flux density for such a core to be 1 1 wb/ m (see also Section 1.9) and making use of a normal B-H curve as shown in Figure 28.28, the corresponding value of H for a value of B as 1.1 wb/m can be read as 200 A/m,... 

The large variability in elemental ion yields which is typical of the single-laser LIMS technique, has motivated the development of alternative techniques, that are collectively labeled post-ablation ionization (PAI) techniques. These variants of LIMS are characterized by the use of a second laser to ionize the neutral species removed (ablated) from the sample surface by the primary (ablating) laser. One PAI technique uses a high-power, frequency-quadrupled Nd-YAG laser (A, = 266 nm) to produce elemental ions from the ablated neutrals, through nonresonant multiphoton ionization (NRMPI). Because of the high photon flux available, 100% ionization efflciency can be achieved for most elements, and this reduces the differences in elemental ion yields that are typical of single-laser LIMS. A typical analytical application is discussed below. 

NaF for water fluoridation, wood preservatives, the formulation of insecticides and fungicides, and use as a fluxing agent. It is also used to remove HF from gaseous Fz in the manufacture and purification of Fz-SnFz in toothpastes to prevent dental caries,... 

A new process for the partial oxidation of n-butane to maleic anhydride was developed by DuPont. The important feature of this process is the use of a circulating fluidized bed-reactor. Solids flux in the rizer-reactor is high and the superficial gas velocities are also high, which encounters short residence times usually in seconds. The developed catalyst for this process is based on vanadium phosphorous oxides... 

An aqueous solution of zinc chloride, acidified with hydrochloric acid, may be used as a flux. A light flux cover is used on the tinning bath and this is normally produced and maintained by the carry-in of flux solution on the work. The bath temperature should be 250-260°C. Bright annealed copper wire may be fluxed in stannous chloride and tinned at 300° C excess tin is wiped off by pulling the emerging wire between lightly clamped rubber blocks. 

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