Transactions

474 TRANSACTIONS OF T H E A.S.M.E. AUGUST, 1941
on oxidation and reduction of slag particles under controlled
conditions support our viewpoint of 1934. Reduction of Fe2Oa
to FeO can occur relatively rapidly, as far as the change is due to
increase of temperature only, but less rapidly by the action of a
reducing gas or carbon. Reoxidation is relatively slow; the absorption
of oxygen must start at the surface of a particle of slag
and work inward; thus, the time required to oxidize will increase
rapidly with the size of the particles. We do not consider it
likely that the actions of reduction and reoxidation can occur in
the few seconds required for the passage of the ash through the
furnace.
The foregoing and Fig. 12, of reference (5) of this paper, explain
why the authors found ash having 90 ferric percentage in
the rear of the furnace, whereas, in our tests the ferric percentages
were lower for slags that reached equilibrium in air at higher
temperatures.
The paper raises a question on the dependability of the chemical
analyses for total iron and iron forms in slags. The question
was referred to W. A. Selvig, under whose direction our many
analyses were made; his report follows:
“No method for the determination of iron forms in materials
such as slags is entirely satisfactory. All methods are empirical
and subject to small errors, the extent of which it is difficult to
determine exactly. The method used by the Bureau is that of
the Bureau of Mines Technical Paper 8, Methods of Analyzing
Coal and Coke.
“By this method total iron can be determined accurately and
is calculated to Fe20 3.
“Metallic iron is determined by digesting the slag with mercuric-chloride
solution. Ferrous iron plus metallic iron are determined
by digesting a portion of slag with dilute sulphuric
acid under specified conditions, whereby these two forms are obtained
in solution, and this soluble iron determined. This iron
calculated to FeO minus the FeO equivalent of the metallic iron
represents the ferrous oxide in the slag.
“The ferric oxide in the slag is obtained by subtracting the
Fe2Oj equivalent of the ferrous oxide and the metallic iron from
the total equivalent Fe20 8 previously found.”
The authors’ definition of their term “per cent oxidation” is
the same as ours for “ferric percentage,” which we define as the
“ratio of ferric iron to total iron, expressed as a percentage.”
We question whether the term used by the authors has as definite
a meaning because both the ferrous and ferric states represent
per cent oxidation of the iron.
The authors refer to “stickiness” several times. This is an
interesting and also—we have found—a complex property. It
would be a blessing if slags would not stick to boiler tubes, but
independent of whether or not data will be of immediate use we
should have some understanding of factors involved in sticking.
A definition is required. Stickiness is more than “wetting;”
a rod dipped in water will be wetted but one would not call water
sticky. A rod pressed into tar, heated so that it is just soft, will
stick, but heat the tar enough, and it will be so liquid that it
would no longer be sticky. Thus, stickiness involves some measure
of “force to separate” and, in general, as a slag is heated
there will be a range of temperature over which it could be called
sticky. However, it is possible that measures of ranges of wetting
may also be required.
Two surfaces are always involved in sticking; so far our studies
have been limited to the sticking of slag to slag, both surfaces
being at the same temperature. We have records of the initial
temperature of sticking of over 400 slags, included in the report
of reference (4) of the paper. Most slags had one sticky range;
others would have two, that is, the stickiness disappeared as the
temperature was increased and then reappeared. Others seemed
to have no sticky temperature.
A more intensive study of a few slags and glasses showed that
the stickiness depended upon the liquid phase present in the slag,
its quantity, and its viscosity; also, the appearance of stickiness
was related to the rate of heating and cooling. The initial sticky
temperature of slag to slag tended to equal or be less than the
cone initial temperature of the premelted ash, but there was no
definite relationship to chemical composition.
Studies of the stickiness of slags to other material are included
in our plans for the future. We have recognized that deposits on
metal tubes may materially affect the sticking of slags. Condensation
of alkalies from their vapors can be one form of deposit
and, under special fuel-bed conditions, silica may be
deposited as the result of oxidation of silicon in the gaseous phase.
We have studied high-iron black deposits, such as the author
found on the probe. One sample had 53 per cent equivalent
Fe20 3, twice the CaO of the coal ash, and a somewhat higher
silica-alumina ratio; the ferric percentage was 78. A microscopic
examination showed that it was composed of particles of
fly ash, lightly fused together. The conclusion was that those
particles having high iron and lime stuck to the tube more readily
because they were stickier than the refractory particles.
The intensive studies on pulverized-coal furnaces which Mr.
Bailey has organized should be extended to other types of furnaces,
to producers, and to kilns, in the attempt to correlate the
life history and forms of the ash more definitely with its composition
and properties. A few complete studies should give patterns
for reference in each class of burning. The probe and other
innovations in methods of tests devised by the authors will be
valuable tools in such investigations.
However, before such studies are undertaken there must be
more complete knowledge of the absolute properties of the ash
than is given by even the three cone-fusion temperatures. The
cone values are at best related to arbitrarily fixed conditions of
test, and vary with changes in the conditions. Premelting the
ash and making up a cone can change the initial deformation
temperature as much as 300 F from that of the original ash.
Which is the correct value to use? Ashes have definite physical
properties, such as viscosity and surface tension, which are exactly,
or very closely, defined by the chemical composition. To
have definite meaning, the cone temperatures must be comparative
measures of the physical properties; therefore, it is necessary
to determine whether such relations exist and can be used, or
whether the values for the primary physical properties should
supplement the more easily obtained cone-fusion values.
E. B. P o w e l l . 16 The authors devote the greater part of their
comment to the influence of atmosphere on the properties of coal
ash as deposited in different parts of the furnace and on heatabsorbing
surfaces. In this they point out the effect of the
combustion stage of the individual particle in determining the
surrounding atmosphere and the importance of fineness of pulverization
as a factor in determining the combustion stage of the
particle in any part of the furnace or path of the combustion
products. These observations are of great value. Apparently,
however, the primary purpose of the paper is to urge further
study of the ash-fusion determination and the inclusion in the
determination of the effect of an oxidizing atmosphere. The
writer is in hearty agreement with the authors in this plea. He
would add, however, that in the further study special attention
should be given to definition of the atmosphere. The authors
themselves suggest as a possible explanation of differences obtained
in determinations made in their own and the Bureau of
Mines laboratories, differences in degree of reducing and oxidizing
properties of the respective furnace atmospheres. A further
16 Consulting Engineer, Stone & Webster Engineering Corporation,
Boston, Mass. Mem. A.S.M.E.