Correspondence- Scale Formation on Laboratory Evaporator

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  • 2938 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 41, No. 12

    sulfides and carbon disulfide). Such destruction was actually experienced in the firebox of a forerunner of the Conroy and Johnstone burner on a number of superduty high-alumina fire- brick as well as on silicon carbide [Kress, Otto et d , Paper Trade J . , 99, KO. 17, 48-51 (193411. But superrefractories made of electrically fused mullite grain with essentially a mullite bond met the requirements satisfactorily and gave good life a t 1430" to 1650" C. (W. W. Duecker, private communication, May 5, 1949).

    In view of the lack of thermodynamic data on the silicon sulfides and aluminum sulfide, free energy changes can be only approximated. Calculations based on entropies estimated by K. K. Kelley (private communication, July 11, 1949) and data in the literature suggest, however, that at the anticipated high furnace temperatures sulfur dioxide and sulfur gases should be harmless to silica, mullite, and alumina, but corrosive to silicon carbide though not a t 900" C. and lower. Monatomic sulfur gas would seem to be destructive to all four if present in appreciable concentrations. Alumina and pure mullite probably would be most resistant to the latter in the order named. The presence of free silica in either would be a potential source of weakness.

    Mullite, 3A120a 2Si02, is the chemically stable combination of alumina and silica up to 1810" C. where it dissociates to its two components-Le., melts incongruently to produce corundum and liquid [Bowen and Greig, J . A m . Cemm. SOC., 7 (4), 242 (1924)l. Crushed and formed into refractories, pure mullite shows a strik- ing chemical inertness, high strength, and resistance to spalling or cracking under the severest furnace conditions up to 1750' C. and even higher (Carborundum Co., "Super Refractories by Carborundum," 1945).

    Electric furnace mullite is made in an arc furnace from the correct proportions of the two purified raw materials. Thus it is practically free from contamination and so differs from the stock from which the usual bonded "mullite" bricks are made, in that the source of that mullite is usually kyanite or sillimanite, min- erals having the same formula, Al2O3.Si02. By the customary heat treatment which does not include fusion, both of these are convertcd in part to true mullite, with an excess of free silica, inasmuch as their alumina-silica ratio is 1 to 1 instead of 3 to 2. Although by addition of some alumina this free silica may be theoretically offset, in practice imperfect mixing, impurities, and absence of fusion of the charge permit nonuniformity and conse- quently all the advantages of bonded electric furnace mullite are not iealized in the converted kyanite or sillimanite refractories.

    -4t temperatures above that a t which mullite starts to revert to its constituents, alumina is undoubtedly the most resistant to sulfur attack of any of the commonly available refractories. This statement applies to 99% bonded alumina but not to the ordinary types that have 90% or less alumina, considerable free silica, and other impurities, The chief objections to the 99% alumina products are their cost (somewhat more than electrically fused bonded mullite) and their slight inferiority from the stand- point of cracking and spalling under sudden temperature changes. Where reasonably constant heat is maintained, the purified alumina should be given serious consideration up to about 1800" c.

    For a maximum of about 1900" C. a fused cast alumina of over 99% is available. This is unquestionably the most re- fractory of the materials so far mentioned, and the most inert to chemical attack because of its imperviousness resulting from prior fusion. However, as would be expected, it requires some- what careful handling to prevent cracking on heating and cooling. Fused cast mullite is likewise available, which is inert though much more subject to spalling than the bonded article discussed above.

    From a purely chemical standpoint, magnesite and chrome brick would seem to offer interesting possibilities. Although they are employed in open hearths up to about 1650" C. they begin to soften seriously above that region and exhibit spalling and cracking tendencies under fluctuating temperatures. Thev

    do offer the advantage of the lowest first cost of any of the materials under consideration.

    Carbon bricks, like silica and fireclay, are unsuitable because of the danger of chemical attack a t high temperatures.

    I t is hoped that in the not distant future the Bureau of Mines will be able to furnish accurate thermodynamic data on the silicon sulfides and aluminum sulfide and that satisfactory calculations can then be made on probable reactions of sulfur on refractories at high temperatures.


    SIR: Mr, Read's comments on refractory problems in high- capacity sulfur burners are greatly appreciated. Very little information seems to have been published on this subject, al- though it has been one of the major difficulties in sulfur burner design. In the spectrographic studies of sulfur flames mentioned in the article, emission bands of Sz have been discovered in a region previously thought to be blanked out by predissociation of the molecule. Some evidence of the existence of sulfur mon- oxide in the flame has also been found, but so far there has been no evidence of monatomic sulfur gas.

    I t is hoped that the spectrographic studies which are being continued, and other work on sulfur combustion involving a study of the effect of turbulence and mixing, u-ill indicate practical designs which may be used to protect furnace refractories.


    Scale Formation on Laboratory Evaporator

    SIR: A paper by Hildebrandt and m'arren [IND. ENG. CHEX, 41, 754 (19&9)] bears out some of the theories I have advanced in my preliminary work on the problem of scale formation.

    Hildebrandt and Warren have shown that by adding a defoamer to the liquid in an evaporator, the rate of scale formation is re- duced. However, they give no explanation for this. From funda- mental considerations I have shown that this effect should occur, although I have not yet verified it experimentally. An outline of my theory is as follows:

    The accepted theory of scale formation is that of E. P. Part- ridge [Ens. Research Bull., KO. 15 (1930)J. This theory state$ that the mechanism of scale formation is that of crystallization dl- rectly onto the heating surface. The crystals are deposited a t the periphery of the steam bubbles, in a superheated region a t the steam-iyater-metal interface. I t is not evident that, for a given rate of evaporation, the rate of scaling will depend on the size oi bubbles, since the total periphery of a large number of bubbles is greater than that of a small number of large bubbles. Thus it is inferred that the rate of scaling would be lower with Iarge bubbles than with small ones, In order to reduce the rate of scale forma- tion, it is therefore necessary to promote the formation of large bubbles.

    Jacoby and Bischman [IND. ENC-. CHEM., 40, 1360 (1948)l have shown by photographic means that the action of defoamers is to promote the formation of large bubbles, which are less stable than small bubbles. Hence a secondary action of defoamers should be the reduction of the rate of scaling.