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# I Chromium Emissionsfrom

# Wool Fiberglass Melting Furnaces
Denis A. Brosnan, Ph.D., PE

Bishop Chaired Professor Materials Science & 

### Engineering Clemson\University, Clemson, SC February 1, 

### 2012 Executive Summary 

### The scope of this work included review of a letter from the North American Insulation Manufactures Association (NAIMA) sent by Mr. Angus E. Crane to Ms. Susan Fairchild ofthe U.S. Environmental Protection Agency (EPA) of September 14, 2011. In addition, numerous documents supporting the positions in this letter were reviewed, and an independent literature review was performed. Further, the author used his education and experience in chrome refractory technology to formulate independent opinions on the hexavalent chromium emissions from woolfiberglass furnaces. Theoretical evidence from thermodynamics and kinetic observations confirm that chromium emissions result from volatilization from the surface of chrome alumina refractories used at or above the glass line in the melting fUrFIaces, and the theory also confirms that the most significant variable with respect to quantity of chromium volatilized and to the presence of hexavalent chromium in volatilized matter is the flame temperature. However, the stability of chrome alumina refractories inherently limits chromium emissions to temperatures much greaterthan those for pure chromic oxide; i.e. the aluminum oxide component of the chrome alumina refractories limits volatilization of chromium due to the thermodynamic stability of aluminum oxide in chromealumina solid solutions. Chromium emissions from refractories below the glass line are not thermodynamically favored, as any chromium released by glass corrosion ofrefractories entersthe glass batch. Furnaces with oxy-fuel combustion have a significantly higher potential for generation of hexavalent chromium emissions as compared to air-gas or other types of furnaces, to include cold top melters and steel shell furnaces when chromium refractories are used in furnace linings. The observations supportthe concept that mineral wool furnaces should be subcategorized by the heating method used to produce the mineral fibers.

1Purpose ofThis Document The primary purpose ofthis review is dealing with the science involved with chromium emissions from mineral fiber furnaces. A secondary purpose of this document is tutorial information for non-engineers or non-scientists. The overall goal is placing what is scientifically known in perspectiveto aid the regulatory process. Background on Glass Melting- A General Discussion Glass melting technology is at a mature level of development with innovations in the last decade aimed primarily in pollution minimization. The basics of glass melting are wellknown, with fluxes acting on silicon dioxide or Si02 to achieve a melted state that forms an amorphous "network" of atoms of oxygen and silicon with "fluxing" metals resulting in rigid solids at room temperature1

. The fluxing species are typically alkali or alkaline earth oxides or oxides of boron, which terminate chains and sheets of silicon and oxygen tetrahedrons. This termination forms large molecules (macromolecules) which are kinetically unable to form crystals at lowtemperature, so glasses are polymeric in nature. Chromium is known as a constituent of glasses used for coloring purposes. Chromium enters the silicate network structure ofthe glass as a "modifier" ofthe network, as it cannot form glass on its own due to thermodynamic constraints. As a modifier, any chromium in the glass is rigidly held in the silicate structure in interstices in the atomic network. Infrared spectroscopy now suggests the chromium is present in coordinated complexes with oxygen 2• The result is that chromium is notvolatile from the glass at normal glass meltingtemperatures3• Chromium which enters glass in mineral wool furnaces below the glass line, i.e. goes into rsolution, does not have potential for volatilization. Therefore, it is only potential volatilization from refractories at orabove the glass line in furnaces that is ofconcern in this report. In order to produce glass, silica sand is mixed with the fluxes in an operation called "batching" so that the desired chemistry of the glass is obtained on melting. Batch fluxing ingredients include salt cake (alkali oxides), borax, limestone, and waste glass, the latter known as "cullet". The premixed ingredients are charged into a melting area or "zone" in a furnace where energy must be supplied to melt the glass composition (the melting process is endothermic). The energy is supplied by combustion processes or by dielectric heating of the glass batch. Batch constituents are lower in density than glass, and the batch typically floats on the molten glass surface as the constituents react toform glass.

1 W. David Kingery, H. Bowen, and D. Uhlmann, Introduction to Ceramics (2nd Edition), Wiley (1976).
2 C. Nelson, Transition MetalIons in Glasses: NetworkModifiers orQuasi-MolecularComplexes, Mat. Res. Bull. 18 (1983) 959-966. 3 Chromium volatilization is only reported in the non-equilibrium meltingofglasses at plasma processingtemperatures, i.e. with flame temperaturestypically reported as above 7000°C 

### (>12,OOOOF). 2The softening temperature of glass compositions used in producing wool fiberglass typically exceeds 13000F(700oC)4, but substantial additional heating above the softening point is required to allow for melting of the batch and to allow the glass to reach a low enough viscosity to be fiberized. The batch temperature in wool fiberglass furnaces may reach or exceed 28000F(1537°C). The molten glass is contained within a refractory lined enclosure or furnace. In modern technology, refractories are used to contain the molten glass forming astructure called a "glass tank". For chrome-alumina refractories, the chromic oxide content (Cr203) ofthese refractories can reach 30-50% by weight with the balance as highly refractory aluminum oxide (Ab03), the latter also known by its mineral name corundum. Corundum refractories (no chromium) are used in many industrial furnaces where there is no corrosion potential from basic slags or high lime content glasses. Aluminum oxide exhibits a melting point of3713°F or2045°C. The oxide exhibits aGibbs free energy offormation of about 850 kilojoules per mole (KJ/mole) in the temperature range 1500-1600°C. By contrast, pure chromic oxide exhibits a free energy offormation of less than 600 KJ/mole in the same ,temperature ranges. Oxides with higherfree energies offormation are known as more stable (much more difficult to decompose, dissolve, or destroy). Therefore, chrome alumina compositions are much more stable than chrome oxide alone. The fundamental reason is that aluminum has a greater affinity for oxygen than chromium; but when they are in joint compounds (solid solutions), such as "(AI,CrhO{, the composition is more thermodynamically stable situation than the lesserstable end member (Cr203)' Chromic oxide (Cr203) and aluminum oxide (Ah03) form a continuous "solid solution" (see the phase equilibrium diagram below), meaning that the oxides exhibit more stability on heatingthan aluminum oxide alone,Le. 

### thesolidso/ution;sthermodynamically favored.

4 The softening point is atemperature in glass terminology defined in terms ofthe viscosity of the glass. This temperature relates to the potential for deformation of a glass object, but the temperature is insufficientforcasting orfiberizing processes. S See Ellingham diagrams, forexample, at http://en.wikipedia.org!wiki/Ellingham_diagram. 

### 340 60 

### Mol % FIG, 309.-5ystem

### a-AlzOrCfltOa • E. N. Bunting, Bur', Standards 1.

### Research, 6 [61 948 (1931); 

### 1<.. P.31'1.
The very low volatility of aluminum from aluminum oxide at elevated temperatures in air atmospheres6 is well-known, and it is only of interest in aerospace science that alumina volatility is of interese. Thus, the thermodynamics and the science strongly imply that any significant chromium volatilization from chrome alumina refractories (as solid solutions) occurs attemperatures above 1500°C (2732°F). This is consistent with the author's direct experimental work at the German Environmental Institute (Munich).
KEY POINT: In the solid solution. the chrome alumina material is less prone to chrome volatilization than chromic oxide since the chromium is "tightly held" within the solid solution, Solid solutions of chromic oxide and aluminum oxide are also 

### much more corrosion resistant to metallurgical slags and glasses than aluminum oxide compositions. The fundamental reason for the stability of alumina-chrome products is, again, the very high thermodynamic energy of formation. Corrosive agents of most types found in industrial furnaces have a profoundly lower rate of corrosion for chrome alumina refractories than for alumina refractories. This means that chrome alumina brick are useful making production of wool fiberglass as economically viable in specific furnaces; however, some furnaces are economicallyviablewith use ofchromium-free refractories.

6 Reduction ofaluminum oxide, i.e. removal of oxygen, is possible as "carbothermal reduction" in extremely reducing atmospheres at elevated temperature. Such atmospheres are not attainable in fuel fired furnaces. 7 E. OpHa and D. Myers, 

### Alumina volatility in water vapor in elevated temperatures, J. Am. Ceram. Society87 (2004) 1701-5. 4

### SUMMARY

• Glass melting bath temperatures typically can exceed 28000

### Fduetothe requirements of heating the batch raw materials to the point of melting and to reduce the viscosity of theglass toallowforformingofthe intended product.

• Alumina-chrome compositions are inherently more stable toward corrosion than the composition's end members
- either aluminum oxide orchromic oxide
- because ofthe high thermodynamics free energy offormation ofaluminum oxide.

• Volatilization of chromium from chrome alumina refractories is not thermodynamically favored until extremely high temperatures of 15000

### Cor greater are reached. This fact agrees withtheauthorsdirectexperimental evidence.

• Below the glass line in mineral wool furnaces, chromium from refractory corrosion enters the network structure ofthe molten glass where it is held to the extentthat it is not volatile at the flame temperatures or batch temperatures within these furnaces. Therefore volatilization from chromium refractories within mineral fiberglass furnaces originates at or above the glass lime in the furnaces from the exposed refractory surfaces. Glazing of these surfaces with batch constituents retards chromium volatilization.

5Volatilization ofChromiumfrom Pure Chromic Oxide
Volatilization of chromium from oxide species is well-known8• In theory, chromic oxide as trivalent chromium vaporizes below 1200°C (2200°F) in an air atmosphere via a process such as:

It is also possible for hexavalent chromium to be present in volatilized species. In an incineration atmosphere where gaseous hydrogen chloride and hydrogen fluoride are present, an alternatevolatilization process is given by Ebbinghaus10 (Figure below) as:

Fig, 3, Plot of log p{er species) vcn;us I fur the chromium olides (ero.},**hyd~.** id¢.1l aM**QX}'h)~deJ;** <CrO.,<OfO,), chlo· rides andtlXYclOOrides (CtO"C1pJ.ftlltll'ides &ld usyftooride:s (QO"F

..). oxyd1lO1'{l!1y' drolides**tCrO~CJiomq.** oxy1luorohy­ (QO"F

..

lOH)"t oxyfiuaroehJl). ride$ {CfO.F

..C1,,). WId**o~rod!loro** hydfOxidc$ (CrO.F

..0.l'(OH)l1) where P{Oz) '" 0.16 atm.**p(}1~m** '" 0.10 11tm. P(HU)**~** IJ.i,110 atm, and p(HF> {lOO32 Atilt.

8 D. A. Brosnan, Kinetics of the Reaction between MgO and Cr2031 Master's Thesis, Clemson University (1968). 9 Some references state that the species oftrivalent chromium that is volatilized is a sub-oxide of chromium, i.e. an oxygen deficient species with electrical neutrality compensation by hydroxide molecules. 10 B. Ebbinghaus, 

### Thermodynamics ofGas Phase Chromium Species: the Chromium Chlorides, Oxychlorides, Fluorides, Oxyfluorides, Hydroxides, Mixed Oxyfluorochlorohydroxides, and Volatility Calculations in Waste Incineration Processes, Combustion and Flame 101, The Combustion Institute, Elsevier (1995). 

### 6In general, the volatility of pure chromic oxide as Cr203 gas increases rapidly above \ 24000F (132S°C), particularly is the oxygen content of the atmosphere is increased (toward a partial pressure of 1 atm. for pure oxygen)l1.

13 .n 12 o -;:; II 

### e '6 10**~~~** __ L­

__**~!~~~~** 0.4 0.6 0.& 1.0 p;f4 • otm.3/4

2 Fig. 3-D<Jperidence of v<)latllity of Cr,o, on the preellUl'1l ofolCYll'en nt aeveral temperatures plotted in aCCOl'dance with Eq. 12J.
The volatilization data from these references is summarized below at like atmospheric conditions (same oxygen partial pressure and temperature). A 1324°C, the data is as follows: Temperature Partial Pressure

### of Partial Pressure

### of Partial Pressure of Cr03 (Cr+6), atm. Cr203 (Cr+3), atm. Oxygen. atm. 1324°Cor**~1600oK** 0.8 X 10-5 0.3 X 10-5 0.1 (241S0F)
The data indicates thatthe presence of hexavalent chromium dominates the percentage of volatilized chromium, i.e. Cr+3 /Cr+6 :::: 0.4 at 1324°C (pure oxide data). The data reflects the well-known fact that thermal oxidation oftrivalent chromium to form hexavalent chromium is significant when temperature increases especially above lS00-1600o C.
­ Furnace atmospheres are much more complicated than most people imagine. In additionto products ofcombustion and molecularfragments ofaircomponents (ifair is used in combustion), the atmospheres contain a multiplicity of hydroxide species. In glass melting, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide are present due to

11 Y-W Kim and G. Belton, The Thermodynamics of Volatilization 

## ifChromic Oxide: Part I, The Species Cr02 andCr02 OH, Metallurgical Transactions 5 (1974) 1811-1816.


### 7the volatility of the alkalis::'. Further, particulate matter entrained in the furnace exhaust can react with volatilized species priorto capture in an air pollution control device. Given sufficient residence time in the exhaust flues, conversion of chromium from trivalent to hexavalent is possible but probably not significant13• In other words, a dynamic process of reactions is present in aglass furnace atmosphere.**--~"""** 

### Yet, itis clearthattemperatureis thestrongestinfluentialvariable on (1) 

### thequantityaj. chromium valatilized in glass melting and (2) 

### in the speciatian afchromium. Ellinghaus' data (Reference 8) shows**aJogarithmic~atiQn~p** between the Qartial pressure of chromium and temperature (Figure below).

Fig" PlOt 

### or 11)<$p(<'Y**~)** ,,_ T iorchfOlfllu1I1**m~ tJyll~**  iltld OX)'.**~,_;di""** "",I**~"'"** tI

.........**~·~ ~,.d** **~** • a

### nmtl:ion T. *reptHt1

..lUI), IHllt {l,fli

## n. 0.0032,. (UlOW, aru!**o.~2** iftm,**OIt{l~ ~&lorolr)~** **M~** 4Wl pNJQDt butnm**~,** KEYRESULT

• A logarithmic relationship meansthatvolatilization 

### increases very rapidly as 

## ctemperature is increased. • Ellinghaus' data atthe lowest hydrogenfluoride content, as is applicableto wool fiberglass melting, provides a relationshipfor partial pressure ofchromium as: 

### alogP c .**a~speaes:::::** 

### 1.2Slogatm/K

• The relationship between chromium pressure and temperature is then:

aP Crspecies 

### atm 101.25

- 

# = 18

### atm/K 

## aT K

12 D. A. Brosnan, observations on float glass furnaces usingthe simulation software MT Data, proprietary research.
13 Formation ofsodium chromates in the presence of lime is reported in "Hexavalent Chromium in Cement Manufacturing: Literature Review", PCA R&D Serial No. 2983 (2007). 

### 8The data from Ellinghaus thus indicates for every hundred degrees 

### (0C) temperature \ increase above 1500-16000C. the chromium partial pressure of hexavalent species increases by 1800times. In terms oftemperature changes in degrees Fahrenheit,the partial pressure of hexavalent chromium species changes by 1000 times as temperature is increased above 

### 2730- 2912°F. There is no literature on volatilization of chromium from chrome alumina refractories. The scientific data quoted in this section reflects experiments where pigment grade (green) chromic oxide (trivalent chromium) was heated undervarying atmospheric conditions or using thermodynamic models ofchromic oxide' ofunitthermodynamic activity (pure Cr203). As stated earlier, the actual stability of chrome alumina solid solutions, particularly in refractory bricks, is much greater than in pure chromic oxide. 

### Nevertheless, the trends in chromium volatility are \ likely similar

- even though the amountofvolatilizedmaterialfrom chrome alumina refractory is reasonablyanticipatedas much less.
KEY POINT: In mineral fiberglass furnaces, the local temperature at the surface of chrome

# I

alumina refractories at or above the glass line is the single most influential variable influencing hexavalent chromium emissions. Surface Temperature ofRefractories as Affected by Combustion The theoretical flame temperature ofmethane combusted in oxygen is 

### S03SoFwhen the oxygen used in combustion is 

### 60oF 14

, and this magnitude of temperature is appropriate for consideration of oxy-fuel furnaces. By contrast, air-gas flames exhibit flame temperatures of 

### 3S62°F using 

### 60°F airforcombustion. Heat transfer within a furnace is through convection and radiation. Radiation is particularly important with respect to speciation of volatilized chromium. In other words, volatilized chromiumspecies from the refractory surface must pass through the freeboard in the furnace (distance above the melt and up to the furnace crown) as the gases travel to the exhaust port in thefurnace. Radiation heat transfer is described by the Stefan-Boltzmann Law where the driving force for heattransfer is 

### thefourthpowerofthe heatsource minusthe

### fourth poweroftheheat receiver, i.e.

where 

### Ts and 

### Tr are the source and receivertemperatures respectively in degrees Kelvin.

14 NorthAmerican Combustion Handbook, 

### 3 rd Edition 

### (2007). 9To compare the situation in a straightforward manner for 

### a wool fiberglass furnace, consider the source temperature is either the mey-fuel or air gas flame temperature and that the bath temperature is 2800°F (1911°K). The relative heattransfer rates are shown below: Situation SourceTemperature or

### Ts en) 5035°For5308 K 6.42 X1014 3562°For3835 K 1.61

### X1014


### KEYPOINT: Thisviewpoint indicatesthat heattransfer byradiation withinthefurnace is four timesas intensewhen oxy-fuel is employed as compared to air-gas (6.42/1.61). The 

### consequences, then, ofoxy-fuel firing are two-fold:

• Higher refractory surface temperatures result from oxy-fuel firing as compared to airgas firing explaining fundamentally why the quantity of chromium volatilized is greater in oxy-fuel firing when chromium refractories are used in furnace linings.

• Higher surface temperatures and greater heattransfertoward volatilized species within the furnaces result in greater presence of hexavalent chromium in oxy-fuel furnaces when chromium refractories are used in furnace linings.

FurnaceType and Chromium Emissions The discussion above has shown that air-gas combustion in wool fiberglass furnaces employing chromium refractories has significantly less potential for chromium vaporization than for oxy fuel combustion. In cold top electric melting, there is no radiation heat transfer above the melt line as the melted glass is insulated or covered with the batch, i.e. the furnace has much less potential for chromium volatilization than the case of air-gas furnaces. Similar considerations applyto otherfurnaces such as steel shell melters.

10

### Conclusions
The science ofchromium emissions clearly indicates that flame temperature is the most important variable in furnace operation influencing the quantity and speciation of chromium emissions from wool fiberglass furnaces employing chromium refractory linings. Because ofthe very significant flame temperature differences between oxy-fuel and air-gas furnaces (S03S0F versus 3S62°F respectively), there is engineering rationale to differentiate or subcategorize the furnaces by combustion type from a standpoint of emissions. Chromium emissions originate from chrome-alumina refractory surfaces above the glass line exposed to radiation heat fluxes. Other furnaces, such as cold top melters and steel shell melters, should be in any lower emissions subcategory.

Denis A. Brosnan, Ph.D., PE SC Registration No. 13888

Notice: The opinions of Dr. Brosnan are based on his education, experience, and independent research. These opinions do not reflect any ideas or conclusions of Clemson University, its President, Trustees, orfaculty orstaff.

11

### 12