Materials Characterization Paper

In Support of the

Advanced Notice of Proposed Rulemaking –

Identification of Nonhazardous Materials That Are Solid Waste

Silica Fume - Used as ingredient in clinker manufacture

December 16, 2008

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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., stainless 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
glassy, spherical particles are extremely small, measuring less than one
micrometer (μm) in diameter, with an average diameter of about 0.1 μm.
Silica fume particles are composed primarily of silicon dioxide (usually
more than 85 percent).

2.	Annual Quantities Generated and Used

Sectors that generate silica fume: 

Silica fume is generated by NAICS sector 331112, Electrometallurgical
Ferroalloy Product Manufacturing. This includes facilities that
manufacture stainless steel and cast iron.

Quantities and prices of silica fume generated:   

The Silica Fume Association estimates that silica fume production in the
United States in 2004 was between 110,000 and 132,000 short tons.

In 2006, one major silica fume supplier reported the bulk price of
silica fume, freight on board (FOB) (i.e., not including shipping
costs), at $205/ton.  It is unclear if this is representative of the
price of silica fume from other suppliers.

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 ferroalloys and of silica fume as a
byproduct are likely to remain constant or increase in the next several
years.

3.	Uses of Material/Commodity

Ingredient uses of silica fume:  

Silica fume can be used in clinker manufacture as a partial replacement
for silica-containing 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 occurs (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).

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 clinker in the
kiln) either by premixing silica fume with finished portland cement to
produce a blended cement (during the cement production process) or by
adding the silica fume to portland cement concrete as a mineral
admixture. In addition, silica fume is frequently used in consumer
products such as toothpaste (Guerra 2008). Use of silica fume in
non-combustion uses in 2004 totaled approximately 70,000 tons (EPA
2008).

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 the commodity
value of silica fume, it is not clear why any is landfilled; possible
reasons include contamination of the silica fume or lack of market
access due to location of the furnaces.

Quantities of silica fume stockpiled/stored:

Data regarding stockpiling or storage of silica fume are not readily
available, but demand for the material and the relatively modest
quantities produced suggest that little, if any, silica fume is
collected for long-term storage.  

Exhibit 1:  Overview of Generation and Use of Silica Fume in 2004

Commodity	Annual Quantity Generated 	Annual Quantity Used as Ingredient
Annual Quantity Landfilled	Annual Quantity in Other Uses	Total Quantity
Stockpiled



Cement Kilns	Other





---------------------------------- 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 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.

4.	Management and Combustion processes

Types of combustion units using silica fume:

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

Sourcing of silica fume:

Silica fume is marketed by a variety of admixture suppliers in the U.S.
who obtain the material from the generators and prepare it for resale. 
There are five domestic silica fume producers listed in the Silica 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).

Processing of silica fume: 

Prior to use as a supplementary cementitious material, silica fume may
be densified by tumbling the material in a silo, which causes surface
charges to build up. The charges draw the particles together to form
weak agglomerates. Because of the increased bulk density, the material
is more economical for truck transport (Silica Fume Association 2005).

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 activities and
regulations.

5.	Commodity Composition and Impacts

Composition of silica fume:

Silica fume consists of 85 percent amorphous (non-crystalline) silicon
dioxide (SiO2).  There may be additional materials in silica fume
depending on the metal being produced in the smelter from which the
silica fume was collected (Silica Fume Association 2005).

Impacts of silica fume use as ingredient:

If used in clinker manufacture, silica fume partially offsets natural
sources of silica, such as clay, shale, sand, and limestone. Thus, using
silica fume in the cement kiln would avoid the energy use and
environmental releases associated with the extraction and processing of
these virgin materials.

The specific lifecycle impacts of silica fume use as a raw material in
clinker production are not evaluated here because it appears silica fume
has limited use in this application, and because of uncertainties in
lifecycle scenario development. For example, it is difficult to
determine the replacement ratio between silica fume and other raw feed
materials in clinker production. 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 well as other secondary materials
(e.g., blast furnace slag, CCPs, foundry sand, cement kiln dust, etc.);
the choice of material often depends on location-specific factors such
as the proximity of material sources to the cement kiln and relative
availability of different materials. Avoided upstream impacts depend
heavily on the specific material being displaced in the lifecycle
scenario.



References

Personal communication with Erika Guerra, Holcim Support Group Ltd.,
Corporate Industrial Ecology, on August 26, 2008.

Robl, Thomas and John Groppo, University of Kentucky, 2007, Advanced
Multi-Product Coal Utilization By-Product Processing Plant, accessed at:
  HYPERLINK
"http://www.netl.doe.gov/technologies/coalpower/cctc/ccpi/pubs/KY/UKRF%2
0Apr%202007%20FinalRep.pdf" 
http://www.netl.doe.gov/technologies/coalpower/cctc/ccpi/pubs/KY/UKRF%20
Apr%202007%20FinalRep.pdf . 

Silica Fume Association, 2005, Silica Fume User’s Manual, accessed at:
  HYPERLINK "http://www.silicafume.org/pdf/silicafume-users-manual.pdf" 
http://www.silicafume.org/pdf/silicafume-users-manual.pdf . 

United States Census Bureau, 2002 Economic Census, Industry Snapshots
for Cement Manufacturing, accessed on August 18, 2008 at:   HYPERLINK
"http://quarterhorse.dsd.census.gov/TheDataWeb_HotReport/servlet/HotRepo
rtEngineServlet?emailname=bh@boc&filename=mfg1.hrml&20071204152004.Var.N
AICS2002=327310&forward=20071204152004.Var.NAICS2002" 
http://quarterhorse.dsd.census.gov/TheDataWeb_HotReport/servlet/HotRepor
tEngineServlet?emailname=bh@boc&filename=mfg1.hrml&20071204152004.Var.NA
ICS2002=327310&forward=20071204152004.Var.NAICS2002 .

United States Environmental Protection Agency (EPA). April 28, 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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