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In accordance with EPA guidance, metal compounds should be selected to be consistent with the form which closely resembles the form of the metal in the Hazardous Waste Fuel (HWF). HWF is a blend of both organic and aqueous wastes coupled with a high suspended solids content. Metal sources in HWF include suspended inorganic paint pigment solids, organometallic catalysts or viscosity modifiers, inorganic and organic metal containing rinse solutions and organic based residual tank bottoms to mention a few examples. Obviously, both organic and inorganic sources of metals are common in HWF.
The introduction of metered quantities of metals for the purpose of documenting precise feedrates is best accomplished by the direct pumping of organic or inorganic metal solutions into the HWF feedline. This technique assures greater accuracy than blending of the metals with the bulk HWF prior to pumping.
The selection process can be simplified to four criteria important in evaluating a metal compound:
1. Readily available commercially at a reasonable cost and in the quantities required for the test.
2. The selected material must be "manageable". That is, it must not be extremely corrosive, reactive, or flammable. Preferably the compound should not be significantly more of a health hazard than the metal alone.
3. Readily soluble in simple solvents such as mineral spirits, fuel oil or water.
4. If permitted by items 1,2 and 3 above, the compound should preferably have the following properties:
a) Organic based material.
b) If inorganic the material should have the ability to go into solution at a concentration that allows easy handling.
c) Does not contain anions, in sufficient quantity, that might interfere with or complicate the performance test protocol.
It is our experience that Pb, Cr and Sb can be readily obtained as organometallic compounds. As, Be and Cd are generally prepared in aqueous solutions. This approach provides a conservative indication of metal emissions since these solutions insure that the metals input will be atomized in the flame. Metal oxide suspensions are not recommended since they are much less volatile than other species and atomization in the flame is not assured. A conservative permit writer could conceivably restrict the form of metals in waste feeds if metal oxides were used for spiking.
Chromium Spiking for BIF Compliance Testing
EPA guidance released in late March of 1992 indicates that a facility electing to measure hexavalent chromium should spike with hexavalent chromium. The EPA guidance also indicates that if an owner/operator deviates from this guidance that documentation should be provided to show why it is impossible or impractical to follow the guidelines. There are a number of technical and operational reasons why the use of hexavalent chromium for spiking cannot be done and/or is not necessary when performing a BIF Trial Burn/Compliance Test on cement kilns.
1. For BIF Trial Burn/Compliance Tests performed in the April-June, 1992 timeframe, the release of the EPA guidance was too late. Test plans and arrangements for spiking metals had already been completed or were well underway.
2. EPA guidance is internally contradictory. It indicates that to the maximum extent possible, organic wastes should be spiked with organic soluble metal compounds. Clearly this presents the greatest potential for emissions since the metal is assured to reach the vapor state. This is in direct conflict with guidance regarding hexavalent chromium since there are no organometallic or organic soluble hexavalent chromium compounds that would not react and reduce the chromium to a trivalent state when mixed with the waste (organics).
3. EPA guidance also suggests that the Cr+6 concentration in the waste should be determined. No methodology was provided, nor does any exist that we are aware of.
4. EPA guidance says "Based on available data, emissions of Cr+6 result from the feeding of Cr+6 in feed streams to the combustion system. Very little, if any, Cr+3 is converted to Cr+6 in the feed streams to the combustion systems (16)."1
The noted reference (16) is apparently an internal unpublished EPA document with neither an EPA publication number nor an NTIS number. Discussions indicate that the "available data" was a single test in an incinerator operating at less than 2,000F. Chromium was apparently fed in a non-organic matrix and/or in a non-soluble form. The extension of this single test in an incinerator to guidance for cement kilns in the face of significant contradicting data in the published literature is technically unjustified.
The following information was readily obtained from the scientific literature with less than one day's effort.
1. Chromium (VI) oxide (CrO3) has a melting point of 197C. Above this point it is unstable and decomposes to Cr2O3 and oxygen.2
2. At 1200C (2,200F) or greater, trivalent chromium (Cr2O3) in the presence of oxygen is converted to CrO3 according to the following reaction:
2Cr2O3(s) + 3 O2 (g) 4CrO3(g)3
This reaction typically reverses itself upon cooling.
3. The only known hexavalent chromium species found in hazardous waste fuel and not chemically incompatible with organic compounds is lead chromate (PbCrO4) a relatively common but less frequently used paint pigment. At temperatures above 844C (1,551F), this compound is a chemically unstable liquid.4 It decomposes according to the following reaction:
This information is supported by reference as well as common knowledge among cement chemists that 30-40 ppm of hexavalent chromium is not uncommon in Portland cement.5 Since it is unlikely that hexavalent chromium would be found in the raw materials, trivalent chromium must to some significant extent be converted to hexavalent in the cement kiln. For these reasons as well as the risk factors introduced by handling highly concentrated hexavalent chromium for spiking it is our conclusion that use of a trivalent chromium organometallic compound is the most sound choice in resolving the conflicting guidance currently available.
1. Technical Implementation Document for EPA's Boiler and Industrial Furnace Regulations, PB92-154 947, Washington, DC, US EPA Government Printing Office, March, 1992, page 5-10.
2. F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry 5th Edition, New York, John Wiley & Sons, 1988, page 683.
3. H.C. Graham and H.H. Davis, "Oxidation/Vaporization Kinetics of Cr2O3", J. Am. Cer. Soc., 54(2) 89-93 (1871).
4. Norbert Adolph Lange Ph.D., Lange's Handbook of Chemistry 13th Edition, ed. by John A. Dean, New York, McGraw-Hill Book Company, 1985, page 4-67.
5. F.M. Lea, The Chemistry of Cement and Concrete 3rd Edition, New York, Chemical Publishing Co. Inc., 1971, page 555.
6. Harlan U. Anderson, "Precise TGA Methods as Used on Chromites",28th Annual Symposium on Refractories, St. Louis, MO, March 27, 1992.
7. D. Caplan and M. Cohen, "Volatilization of Chromium Oxide", J. Electrochem. Soc., 108(5) 438-42 (1961).
8. R.T. Grimley, R.P. Burns, and Mark G. Inghram, "Thermodynamics of the Vaporization of Cr2O3: Dissociation Energies of CrO, CrO2, and CrO3", J. Chem. Phys., 34(2) 664-67 (1961).
In our March 1992 issue of HWF, which summarized the AWMA BIF conference, there was an incorrect reference to a benzene source mentioned from a paper by Dr. Michael Von Seeback, et. al. The source of the benzene should have been from raw materials, not "from the combustion process itself". Our thanks to Dr. Greg Miller for bringing this to our attention.