"Iron Spherules: Another curious phenomenon thought to be linked to the structural steel is creation of tiny spheres of steel or iron, found in the dust after collapse. Several researchers report this, including Lowers and Meeker who documented a few examples of particles found to be nearly pure iron and quite spherical, approximately 7 microns in diameter; and the RJ Lee Group, who identified small, round iron particles as evidence of high temperatures. The significance of these spheres is still debated, along the following lines:
As discussed previously, there is no evidence at all for large amounts of melted steel. If the spheres are formed by melting steel, it must be surface melting or some other highly localized process.
It is also not known when the iron spheres were produced. The RJ Lee Group report considers samples taken several months after the collapses, and it is certain that torch-cutting of steel beams as part of the cleanup process contributed some, if not all, of the spherules seen in these samples.
There appear to be several plausible candidate sources of the iron spherules in office materials or other building contents. Perhaps the most obvious is the flyash itself used in structural concrete, a residue of combusted coal, which contains iron spheres in a variety of sizes that would have been liberated as the concrete was destroyed. Another example is magnetic printer toner, used to print financial instruments, that could have been present in printer cartridges or found in a large volume of paper documents. This candidate has the advantage of matching the size, shape, uniformity, and elemental composition of the observed spherules from one report. We also cannot discount their origin in building contents, rather than building structure, without much more careful study.
The quantity of these spherules is unknown, but thought to be very small – the iron-rich content of all dust samples was between 0.1 and 1.3%, most of which was not in the form of spherules. A large quantity would suggest melting of steel on large scales, but a small quantity suggests otherwise.
Small quantities of structural steel or other iron-rich objects could be partially melted through sheer friction, originating in the aircraft impact or the collapses.
Much like the sulfidized samples, it is impossible to tell whether these spherules were created prior to collapse, after collapse, or both. After collapse, it is plausible for the debris to have reached much higher temperatures.
As mentioned above, there is potential site contamination from salvage operations, in which numerous steel pieces were cut, involving nontrivial amounts of melted steel. It is also possible for the spherules to have been left over from the buildings’ original construction.
Iron that appears to have melted may have merely oxidized, and surface chemistry effects of merely heated iron may give rise to tiny amounts of melting even at moderate temperatures.
Chemical factors, combined with heat, could lead to eutectic mixtures of iron with other elements (such as sulfur) melting and dissociating at relatively low temperatures, potentially creating the iron spherules.
For purposes of this discussion, we will focus on the latter two inferences, and speculate that the spherules may be a result of a chemical process, catalyzed by moderate heat but below the actual melting temperature of steel. It is, therefore, possible (but unproven) that the spherules and the sulfidized steel are related.To further understand sulfidization, we should begin by attempting to understand the source of the sulfur. Sulfur is an abundant element, with numerous possible sources. The following is a brief list of some possible origins of sulfur:
Diesel fuel, found in emergency generators and in vehicles in the WTC parking garages, contained a fairly high concentration of organosulfuric compounds, providing a possible source of sulfur in an energetically favorable form. WTC 7, where all but one of the sulfidized samples came from, had exceptionally large stores of diesel fuel to power emergency command and control equipment.
Large banks of batteries existed in a few locations, as backup for computers involved in the financial services, and could plausibly have provided a significant quantity of sulfuric acid.
Acid rain could have potentially exposed some surfaces to low concentrations of sulfuric acid over many years.
Ocean water, bearing sulfate salts, was pumped onto the burning debris piles as part of the firefighting effort.
Gypsum wallboard, omnipresent in large buildings, is almost entirely composed of sulfur-bearing minerals. However, this sulfur is not in an energetically favorable form, and some other chemical process would be required to react with steel structural members.
The Worcester Polytechnic Institute is continuing to experiment with sulfur compounds in an effort to recreate the reactions seen in the recovered steel. Given the complexity of the debris fires and the many chemicals present, it appears plausible that sulfidization could have occurred after collapse. Whether or not this could occur prior to collapse remains an open question, and if true, could be a factor in future building fires.
A related possibility, voiced by Dr. Greening, is that of burning plastics or other chemicals giving rise to other caustic compounds, such as creation of hydrogen chloride (which in contact with water forms hydrochloric acid) from burning PVC (polyvinyl chloride). This is relevant because large quantities of PVC, along with other plastics, are found in modern offices. Chemicals such as this could potentially catalyze sulfur reactions, and also lead to a chemical weakening of steel structural elements, an additional hazard. A historical example of this is the Plastimet Fire in Hamilton, Ontario, in July of 1997. In this fire, roughly 200 tons of PVC and other plastics burned over a period of a few days. Among the fire’s effects were reports of localized metal corrosion, linked to the creation of HCl gas which was measured at 53 to 930 micrograms per cubic meter.
The volume of PVC burned in this fire was comparable to the amount of plastics in the WTC fire floors, and it is also conceivable that caustic chemicals would be trapped within the structure, raising their concentrations to this level or possibly much higher.
However, the use of PVC in construction is not new, and there have been numerous studies on its effects in fires. Industry sources question its ability to weaken a structure through chemical means:
Burning PVC has resulted in corrosion damage to electrical equipment in the vicinity. This has led to suggestions that PVC should not be used in construction applications. Against this should be set other factors. PVC components can be formulated to combine a good technical performance and high resistance to ignition and flame-spread. Formulations can also be designed to reduce the quantity of hydrogen chloride emitted. There have been suggestions that hydrogen chloride from burning PVC may damage steel reinforcement in concrete, or significantly weaken unprotected steel structures. The UK Fire Research Station has shown that reinforcement is not normally affected. It has also been confirmed that unprotected steel structures are distorted and weakened by heat rather than by hydrogen chloride.
For applications with very high fire risks, for example oil rigs and nuclear installations, more expensive, high performance insulating materials are preferred to PVC. The cost of post-fire clean-up operations must be included in assessing the total cost of fire damage. Just as soot can be removed from affected equipment, so chloride corroded parts can be reconditioned. This is well recognized by fire salvage consultants and by insurance companies.
