Materials Characterization Paper In Support of the Proposed Rulemaking:

Identification of Nonhazardous Seco ndary Materials That Are Solid Waste Silica Fume (used as ingredient in clinker manufacture)

March 18, 2010

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1. Definition of Silica Fume

Silica fume is a fine, dust-like material generated during silicon metal and ferrosilicon and related ferroalloys (e.g., stainles s steel) production. Specifically, it is produced by the reduction of high purity quartz with coal or coke and wood chips in an electric arc furnace during silicon metal or ferrosilicon alloys production. The gla ssy, spherical particles are extremely small, measuring less than one micrometer ( μ m) in diameter, with an average diameter of about 0.1 μ m. Silica fume particles are composed primarily of silicon dioxide (usually more than 85 percent).

2. Annual Quantities Generated and Used

(1) Sectors that generate silica fume:

• Silica fume is generated by NAICS sector 331112, Electrometallurgical Ferroalloy Product Manufacturi ng. This includes facilities that manufacture

stainless steel and cast iron.

(2) Quantities and prices of silica fume generated:

• The Silica Fume Association estimates th at silica fume pr oduction in the United States in 2004 was between 110,000 and 132,000 short tons (EPA 2008).

• In 2006, one major silica fume supplier re ported the bulk pric e of silica fume, freight on board (FOB) (i.e., not includi ng shipping costs), at $205/ton (Robl and Groppo 2007). It is unclear if this is representative of the price of silica fume from other suppliers.

(3) Trends in generation of silica fume:

• Growing U.S. customer needs for alloy and stainless steel is expected to continue to have a strong positive influence on ferroalloy demand (USGS 2006). As a result, production of ferroal loys and of silica fume as a byproduct ar e likely to

remain constant or increase in the next several years.Silica Fume

3. Uses of Material/Commodity

(1) Ingredient uses of silica fume:

• Silica fume can be used in clinker manufacture as a partial replacement for silicacontaining raw materials (e.g., clay, shale, sand, and limestone). Because of the high price of this material, however , and the relative abundance of other secondary materials and virgin materials, use of silica fume as a raw material for clinker is less common, though it still o ccurs (Guerra 2008). In 2004, 20,000 tons of silica fume were used in cement kilns (EPA 2008).

• Cement kilns are represented by NAICS industry sector 327310, Cement Manufacturing, which included 247 facilities in 2002 (U.S. Census Bureau 2002).

(2) Non-combustion uses of silica fume:

• Silica fume can be also be used at cement plants as a supplementary cementitious material (i.e., after the production of cli nker in the kiln) either by premixing silica fume with finished portland cement to produce a blended cement (during the cement production process) or by addi ng the silica fume to portland cement

concrete as a mineral admixture. In add ition, silica fume is frequently used in consumer products such as toothpaste (G uerra 2008). Use of silica fume in noncombustion uses in 2004 totaled approximately 70,000 tons (EPA 2008).

(3) Quantities of silica fume landfilled:

• The Silica Fume Association estimates that about 28,000 short tons of silica fume were landfilled in 2004 (EPA 2008). Given th e commodity value of silica fume, it is not clear why any is la ndfilled; possible reasons incl ude contamination of the

silica fume or lack of market acces s due to location of the furnaces.

(4) Quantities of silica fume stockpiled/stored:

• Data regarding stockpiling or storage of si lica fume are not readily available, but

demand for the material and the relative ly modest quantities produced suggest that little, if any, silica fume is collected for long-term storage.

Exhibit 1: Overview of Genera tion and Use of Silica Fume in 2004

Annual Quantity Used as Ingredient Annual Quantity

Generated Cement

Kilns Other Annual Quantity

Landfilled Annual Quantity in

Other Uses Total Quantity Stockpiled Commodity

---------------------------------- Short Tons -----------------------------

Silica fume 0.11-0.13

million 0.02 million 0 0.03 million 0.07 million Undetermined; not likely to be

significant

Sources : Unless otherwise noted, data is from U.S. EPA, April 28, 2008, Study on Increasing th e Usage of Recovered Mineral Components in Federally Funded Projects Involving Procurement of Cement or Concrete to Address the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users

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4. Management and Combustion processes

(1) Types of combustion units using silica fume:

• Silica fume is used in cement kilns during the clinker manufacture process.

(2) Sourcing of silica fume:

• Silica fume is marketed by a variety of admixture supplie rs in the U.S. who obtain the material from the generators and prepar e it for resale. There are five domestic silica fume producers listed in the Silic a Fume Association, all of whom are

located in the Southeastern United States . The product is typically sold packaged in 25 pound bags and in bulk (Robl and Groppo 2007).

(3) Processing of silica fume:

• Prior to use as a supplementary cemen titious material, silica fume may be densified by tumbling the material in a silo, which causes surface charges to build up. The charges draw the part icles together to form w eak agglomerates. Because

of the increased bulk densit y, the material is more ec onomical for truck transport (Silica Fume Association 2005).

(4) State status of silica fume use as ingredient:

• At this stage, we have not identified any states that have specifically granted beneficial use designation to the use of silica fume in clinker manufacture, or that prohibit use of silica fume in clinker, but we have not performed an exhaustive investigation of state ac tivities and regulations.

5. Commodity Composition and Impacts

(1) Composition of silica fume:

• Silica fume consists of 85 percent am orphous (non-crystalline) silicon dioxide (SiO2). There may be additional materi als in silica fume depending on the metal being produced in the smelter from whic h the silica fume was collected (Silica Fume Association 2005).

(2) Impacts of silica fume use as ingredient:

• If used in clinker manufact ure, silica fume partially offsets natural sources of silica, such as clay, shale, sand, and lim estone. Thus, using si lica fume in the cement kiln would avoid the energy use and environmental releases associated with the extraction and processi ng of these virgin materials.

• The specific lifecycle impacts of silica fume use as a raw mate rial in clinker production are not evaluated here because it appears silica fume has limited use in this application, and because of uncertain ties in lifecycle scenario development. For example, it is difficult to determine the replacement ratio between silica fume

and other raw feed material s in clinker product ion. Thus, the correct quantity of

material to be modeled is unclear. In addition, silica fume may substitute for a variety of virgin raw materials, as we ll as other secondary materials (e.g., blast

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furnace slag, CCPs, foundry sand, cement kiln dust, etc.); the choice of material often depends on location-specific factor s such as the proximity of material

sources to the cement kiln and relative avai lability of different materials. Avoided upstream impacts depend heavily on the specific material being displaced in the lifecycle scenario.

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References

Guerra, Erika, 2008, Holcim Support Group Ltd., Corporate Industrial Ecology, personal communication, August 26, 2008.

Robl, Thomas and John Groppo, University of Kentucky, 2007, Advanced Multi-Product Coal Utilization ByProduct Processing Plant , accessed at: http://www.netl.doe.gov/technologi es/coalpower/cctc/ccpi/pubs/KY/UKRF%20Apr%202007%20FinalRep. pdf

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Silica Fume Asso ciation, 2005, Silica Fume User’s Manual , accessed at: http://www.silicafume. org/pdf/silicafumeusers-manual.pdf

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United States Census Bureau, 2002 Economic Census , Industry Snapshots for Cement Manufacturing , accessed on August 18, 2008 at: http://quarterhorse.dsd.census.gov/TheDataWeb_HotReport/servlet/HotReportEngineServlet?emailname=b h@boc&filename=mfg1.hrml&20071204152004.Var.NAICS2002=327310&forward=20071204152004.V

ar.NAICS2002

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United States Environmental Protectio n Agency (EPA). June 3, 2008, Study on Increasing the Usage of Recovered Mineral Components in Federally Funded Projects Involving Procurement of Cement or Concrete to Address the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users

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United States Geological Survey (USGS). 2006, Minerals Yearbook: Ferroalloys , accessed on August 18, 2008 at: http://minerals.usgs.gov/minerals/pubs/commodity/ferroalloys/myb1-2006-feall.pdf

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