Effect of Furnace Atmosphere

Perhaps the most widely studied instrument variable has been the effect of furnace atmosphere on the TG curve of a sample. The effect of the atmosphere on the mass-change curve depends upon (I) the type of reaction, (2) the nature of the decomposition products, and (3) the type of atmosphere employed. For (1), three types of reactions may be studied, either reversible... 

Numerous other studies on the effect of furnace atmosphere on the TG curves have been reported (12,21, 32-41 -... 

New batch compositions can be studied to see the effects on foaming, melting-in fining behaviour and to optimize a batch for fining. The use of alternative raw materials can be explored, as can the effect of furnace atmosphere on fining onset (temperature) or foaming. The equipment is well-suited to determine foam decay rates, or the fining onset temperature (for... 

Indirectly fired furnaces find applications in metallurgy, paint enameling, the pharmaceutical industry, and other situations where it is necessary to control the chemistry of the furnace atmosphere [186]. An important aspect of the heat treatment of metals, for example, is the effect of the furnace atmosphere on the stock being heated. In most cases, the need is to minimize or eliminate completely the undesirable effects of furnace gases, such as oxidation or decarbonization. Both directly [187,188] and indirectly [189,190], natural-gas-fired furnaces have been analyzed, but the details cannot be included here. 

Visualizing furnace pressure requires measuring it by an inclined manometer with one leg connected to a tap through the wall to the furnace interior and the other manometer tap simply receiving pressure from the atmosphere just outside the furnace. To control the effects of furnace pressure, one must determine the elevation within the furnace of the zero pressure level (i.e., zero AP inside to outside the furnace) and understand how it affects interior furnace gas flows. (See pp. 58-69 of reference 52.)... 

The enamel mill addition, degree of firing and furnace atmosphere all affect acid resistance. An increase in clay and alkaline electrolyte detracts from this property and underfiring also has an adverse effect. The use of organic suspending agents is thus preferable to clays, from this aspect, but... 

Recovery of inorganic chemicals is crucial to the cost effectiveness of the Kraft process. The black liquor which is obtained from delignification is rich in solubilised lignin and carbohydrate degradation products and, after concentration, is combusted in a recovery furnace. The Carbon dioxide which is produced during combustion converts unused sodium hydroxide into sodium carbonate. In addition, the sodium sulfate is converted, under the reducing atmosphere of the furnace, to sodium sulfide. 

Such special atmosphere is often necessary in decomposition and rehydration studies of hydroxides, hydrates etc. Furthermore also the catalytic effect of water vapor on certain reactions is of interest. For such studies the gas is saturated with water, or other vapors e.g. D20, alcohol, CS2, etc. When higher water vapor concentrations are required special furnaces are available (see Sect. 2.4). 

Rats received intermittent exposure to the atmosphere in the furnace room of a phosphorus factory for 1-4 months (Ruzuddinov and Rys-Uly 1986). Histology of rats killed monthly revealed progressive morphological degeneration of the tongue and oral mucosa of the cheek, gum, and hard palate. It is likely that the effects of white phosphorus in the oral cavity are local rather than systemic resulting from direct contact of white phosphorus-containing atmosphere with tissues in the mouth. For this reason this study is also discussed in Section 2.2.2.2. 

The above distinction between non-reactive and reactive systems does not take into account the possible effect on reactivity and wettability of the furnace atmosphere. The concentration of dissolved oxygen in the liquid close to the S/L/V triple line will lie between two limits. The first of these is that for congruent dissolution of the oxide. This can be calculated using equations (6.8) by assuming that local equilibrium is established between the liquid metal and the oxide at the interface. This limit will be identified as Xq where the superscript I denotes its relevance to the interface. The second limit, Xq(Po2) at the liquid surface, is determined by equilibration with the furnace atmosphere in which the oxygen partial pressure is Pq2 (see Figure 6.3). Note that even if Xq and Xq can be calculated, it is impossible in practice to calculate the actual value of XQ at the... 

If material can be lost by volatilization, as with phases containing alkalis, fluoride, or sulphate, it may be necessary to use a sealed platinum container and to test its effectiveness by chemically analysing the product. Control of the furnace atmosphere, e.g. by employing mixtures of CO and CO2 to buffer the oxygen pressure, may be needed if variable oxidation states are possible, as with iron compounds. Special methods may be needed to make single crystals of sufficient size for X-ray structure deterniinations or other purposes. These are described in papers on X-ray structure determination. Single crystals of C3S can be obtained from CaClj melts (NIO). 

In practice AT vs furnace temperature is plotted givinga thermogram of the type illustrated in figure 11.8. Figure 11.9 shows a curve for the DTA examination of calcium oxalate monohydrate exemplifying also the effect of changing the atmosphere from nitrogen to air. 

---