J~'.fl r .::... 

-(/vI i j 

~~J. f 

'\ -I- W/""'C/( If IJ (Mfj 

~,lWl~ . 1'l.fM- ~ ~I- ...)~-h ~vL I--+- ~''- /}S~ .
 ";) IJ<:>AvI\ Technology Review (UI'\..Jl /1 7,. ) 011 J!V"" r) I 

Chromium Emissions from 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 of the 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 wool fiberglass 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 greater than 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 of refractories enters the 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 

support the concept that mineral wool furnaces should be subcategorized by the 

heating method used to produce the mineral fibers. 

1 

Purpose of This Document 

The primary purpose of this 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 perspective to 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 low temperature, so glasses are polymeric in nature. 

Chromium is known as a constituent of glasses used for coloring purposes. Chromium 

enters the silicate network structure of the glass as a "modifier" of the 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 oxygen2

• 

The result is that chromium is not volatile from the glass at normal glass melting temperatures3

• 

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 or above the glass line in furnaces that is of concern 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 to form glass. 

1 W. David Kingery, H. Bowen, and D. Uhlmann, Introduction to Ceramics (2nd Edition), Wiley 


(1976). 


2 C. Nelson, Transition Metal Ions in Glasses: Network Modifiers or Quasi-Molecular Complexes, 


Mat. Res. Bull. 18 (1983) 959-966. 


3 Chromium volatilization is only reported in the non-equilibrium melting of glasses at plasma 


processing temperatures, i.e. with flame temperatures typically reported as above 7000°C 


(>12,OOOOF). 

2 
The softening temperature of glass compositions used in producing wool fiberglass 

typically exceeds 13000 F (700o

C)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 28000 F (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 a structure 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 of 3713°F or 2045°C. The oxide exhibits a Gibbs 

free energy of formation of about 850 kilojoules per mole (KJ/mole) in the temperature range 

1500-1600°C. By contrast, pure chromic oxide exhibits a free energy of formation of less than 

600 KJ/mole in the same ,temperature ranges. Oxides with higher free energies of formation 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 lesser stable 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 

heating than aluminum oxide alone,Le. the solid so/ution;s thermodynamically favored. 

4 The softening point is a temperature in glass terminology defined in terms of the viscosity of 


the glass. This temperature relates to the potential for deformation of a glass object, but the 


temperature is insufficient for casting or fiberizing processes. 


S See Ellingham diagrams, for example, at http://en.wikipedia.org!wiki/Ellingham_diagram. 


3 

40 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 

at temperatures 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 

economically viable with use of chromium-free refractories. 

6 Reduction of aluminum 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. Society 87 (2004) 1701-5. 


4 

SUMMARY 


• 	 Glass melting bath temperatures typically can exceed 28000F due to the requirements of 

heating the batch raw materials to the point of melting and to reduce the viscosity of 

the glass to allow for forming of the intended product. 

• 	 Alumina-chrome compositions are inherently more stable toward corrosion than the 

composition's end members - either aluminum oxide or chromic oxide - because of the 

high thermodynamics free energy of formation of aluminum oxide. 

• 	 Volatilization of chromium from chrome alumina refractories is not thermodynamically 

favored until extremely high temperatures of 15000

C or greater are reached. This fact 

agrees with the authors direct experimental evidence. 

• 	 Below the glass line in mineral wool furnaces, chromium from refractory corrosion 

enters the network structure of the molten glass where it is held to the extent that 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. 

5 

Volatilization of Chromium from 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 alternate volatilization 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 and tlXYclOOrides (CtO"C1p J. 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!lorohydfOxidc$ (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 of trivalent 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 of Gas 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). 

6 

In general, the volatility of pure chromic oxide as Cr203 gas increases rapidly above \ 

24000 F (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 

of olCYll'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 Cr203 (Cr+ ), atm. 3 Oxygen. atm. ), atm.

1324°C or ~1600oK 0.8 X 10-5 0.3 X 10-5 0.1 

(241S0 F) 

The data indicates that the 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 of trivalent chromium to form hexavalent chromium is 

significant when temperature increases especially above lS00-1600oC. ­

Furnace atmospheres are much more complicated than most people imagine. In 

addition to products of combustion and molecular fragments of air components (if air 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 if Chromic Oxide: Part I, The 

Species Cr02 and Cr02 OH, Metallurgical Transactions 5 (1974) 1811-1816. 

7 

the volatility of the alkalis::'. Further, particulate matter entrained in the furnace exhaust can 

react with volatilized species prior to 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 a glass furnace atmosphere. --~"""

Yet, it is clear that temperature is the strongest influential variable on (1) the quantity aj. 

chromium valatilized in glass melting and (2) in the speciatian af chromium. 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. *re ptHt1 .. lUI), 

IHllt {l,fli n. 0.0032,. (UlOW, aru! o.~2 

iftm, OIt{l~ ~&lorolr)~ M~ 

4Wl pNJQDt but nm ~, 

KEY RESULT 

• 	 A logarithmic relationship means that volatilization increases very rapidly as ctemperature is increased. 

• 	 Ellinghaus' data at the lowest hydrogen fluoride content, as is applicable to wool 


fiberglass melting, provides a relationship for partial pressure of chromium as: 


alogPc . a~speaes::::: 1.2Slogatm/K 

• 	 The relationship between chromium pressure and temperature is then: 

aPCr species atm 

101.25 - =18 atm/K aT 	 K 

12 D. A. Brosnan, observations on float glass furnaces using the simulation software MT Data, 


proprietary research. 


13 Formation of sodium chromates in the presence of lime is reported in "Hexavalent Chromium 


in Cement Manufacturing: Literature Review", PCA R&D Serial No. 2983 (2007). 


8 

The data from Ellinghaus thus indicates for every hundred degrees (0C) temperature \ 

increase above 1500-16000 C. the chromium partial pressure of hexavalent species increases 

by 1800 times. In terms of temperature 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 under varying atmospheric conditions or using 

thermodynamic models of chromic oxide' of unit thermodynamic 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 amount of volatilized material from chrome alumina refractory 

is reasonably anticipated as 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 of Refractories as Affected by Combustion 

The theoretical flame temperature of methane combusted in oxygen is S03SoF when the 

oxygen used in combustion is 60o

F

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 air for combustion. 

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 chromium species 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 the furnace. 

Radiation heat transfer is described by the Stefan-Boltzmann Law where the driving 

force for heat transfer is the fourth power of the heat source minus the fourth power of the heat 

receiver, i.e. 

where Ts and Tr are the source and receiver temperatures respectively in degrees Kelvin. 

14 North American Combustion Handbook, 3rd Edition (2007). 

9 
To 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 Source Temperature or Ts 

en) 5035°F or 5308 K 6.42 X1014 

3562°F or 3835 K 1.61 X1014 

KEY POINT: This viewpoint indicates that heat transfer by radiation within the furnace is four 

times as intense when oxy-fuel is employed as compared to air-gas (6.42/1.61). 

The consequences, then, of oxy-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 heat transfer toward volatilized species within 

the furnaces result in greater presence of hexavalent chromium in oxy-fuel furnaces 

when chromium refractories are used in furnace linings. 

Furnace Type 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 apply to other furnaces such as steel shell melters. 

10 

Conclusions 

The science of chromium 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 of the 

very significant flame temperature differences between oxy-fuel and air-gas furnaces (S03S0 F 

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, or faculty or staff. 

11 
12