Back to Library

Gossman Consulting, Inc.

GCI TECH NOTES©


Volume 3, Number 06                  A Gossman Consulting, Inc. Publication                    June, 1997

Why All Cement Kilns Should Test Their CKD

David L. Constans

GCI routinely assists our clients in addressing concerns regarding the metals or organic content of waste CKD generated by cement kilns that utilize hazardous waste fuels. A number of opponents of hazardous waste fuel burning kilns have focused on the concentration of certain substances in waste CKD as a way of illustrating that burning waste in cement kilns is a flawed concept. Repeatedly, GCI has demonstrated that compliance with the BIF regulations regarding metals input limits and other features of the BIF regulation are more than sufficient to protect human health and the environment, plus help keep CKD environmentally manageable.

What GCI finds is that the CKD produced from cement kilns utilizing hazardous waste fuels is not greatly different than the CKD produced before the utilization of hazardous waste fuels. In some cases, CKD from HWF burning kilns has lower metal or organic content. In nearly every case, analysis provides valuable information about that kiln's CKD simply because no one had previously analyzed the CKD. Most BIF regulated kilns know much more now about the source and fate of the metals and organics entering their kilns than before BIF regulation.

The hazardous waste fuel does not contain any metal that is not already contained in the coal or raw feed. In fact for certain metals such as beryllium, silver, and thallium, the input into the kiln from these natural sources is often the only source of these metals. Consequently, it was often a surprise to new BIF units, or kilns outside of the U.S., utilizing alternative fuels for the first time, to discover that they had higher than expected levels of metals such as arsenic, lead, cadmium or thallium in their CKD before utilizing hazardous waste fuels. This is especially the case for kilns that have managed to eliminate the wasting of CKD.

There are a number of reasons for this surprising discovery. 1) CKD had generally not been analyzed before, especially not with the modern sample digestion and analytical methods currently available. For CKD that had been analyzed, a "non-detect" previously meant zero, simply because detection limits frequently were not even included in the analytical report. 2) Raw materials such as limestone had not been analyzed with modern techniques, and sometimes the concentrations are surprisingly high. 3) "Yes we knew these metals were there, but not in sufficient concentrations to create problems". This would be correct except for the nature of the clinkering process that forces the more volatile metals into the CKD. If not purged, as occurs in a kiln that does not waste CKD, the metal accumulates in increasing concentrations in the CKD.

As consultants that have seen a lot of CKD analytical data, and a lot of data on the concentrations of toxic metals in coal, limestone, shale, sand and a number of other kiln inputs, GCI encourages non-hazardous waste burning cement kilns to perform an analysis of their CKD. We base this on instances where we have seen higher than expected levels of a number of metals in CKD in kilns not utilizing alternate fuels.

Organics have also been found in CKD as well. The opponents of cement kilns attribute this to "poor destruction efficiency of the hazardous waste fuel" or to "favorable formation conditions in the air pollution control device". Properly executed destruction and removal efficiency tests have shown cement kilns to be awesome destroyers of the organics that enter as fuel into the combustion zone. However, there are naturally occurring constituents that enter the kiln with the raw feed and these constituents can be vaporized and can end up adsorbed onto the CKD. These naturally occurring organic constituents may include polychlorinated dibenzodioxins or furans (PCDD/PCDFs). GCI is careful to note that none of these organics compounds have been found to be present at concentrations that pose a hazard to human health or the environment.

Clearly then, cement kilns that do not utilize hazardous waste fuels may exhibit higher than expected levels of these metals or organics in their CKD due to the concentrations of metals or organics in their raw feed or the concentration of metals in coal, and the manner in which the systems are operated.

As noted earlier, the organics constituents have never been found to exceed safe levels. Certain metals in CKD from non-hazardous waste burning kilns have reached concentrations that have caused concern, particularly thallium levels in kilns that do not waste CKD.

Metal Probable Reason Concentration was Higher than expected in the CKD.
Arsenic Unusually high levels in feed sources
Barium Unusually high levels in feed sources
Cadmium High levels in feed sources or coal and/or low or no wastage of CKD
Chromium Unusually high levels in feed sources
Lead High levels in feed sources or coal and/or low or no wastage of CKD
Thallium Measurable (>1 ppm) levels in feed sources (especially boiler fly ash) and/or low or no wastage of CKD

GCI encourages all cement kilns to analyze their CKD. CKD regulation is definately on the way. It is better to be prepared than not. Wasting as little as 20 tons/week of CKD can provide an effective "bleed" on the system and prevent the build-up of "problem" metals.