Materials Characterization Paper

In Support of the

Advanced Notice of Proposed Rulemaking –

Identification of Nonhazardous Materials That Are Solid Waste

Cement Kiln Dust (CKD)

December 16, 2008

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1.	Definition of CKD

Cement kiln dust (CKD) is a fine-grained, solid, highly alkaline
material removed from the cement kiln exhaust gas by scrubbers
(filtration baghouses and /or electrostatic precipitators).  The
composition of CKD varies among plants and over time at a single plant. 
Much of the material comprising CKD is incompletely reacted raw
material, including a raw mix at various stages of burning, and
particles of clinker. 

2.	Annual Quantities of CKD Generated and Used

Sectors that generate CKD: 

CKD is generated by NAICS industry sector 327310, Cement Manufacturing
(U.S. Census Bureau, 2002).  This sector included 247 facilities in 2002
according to the U.S. Census of Manufacturers.  EPA reports that this
sector included 144 facilities in 2006 (EPA 2006).

Quantities of CKD generated:   

An estimated 13 to 17 million short tons of CKD are generated per year
(EPA 2008).

Trends in generation of CKD:  

Generation of CKD is directly connected to the production of cement
clinker (the initial product of a cement kiln before processing). 
Significant increases in U.S. clinker capacity are expected to
materialize during 2008 to 2012, anticipating an increase in production.
The U.S. cement industry has announced plans to increase clinker
capacity by nearly 25 million metric tons between 2007 and 2012. The
capacity expansion reflects a $5.9 billion investment, which will
increase capacity 27 percent compared to 2006 U.S. clinker capacity
(Cement Americas 2008).  Assuming that capacity utilization is constant,
a CKD production increase of 27 percent is also anticipated.

The U.S. cement industry has adopted a year 2020 voluntary target of a
60 percent reduction (from a 1990 baseline) in the amount of cement kiln
dust disposed per ton of clinker produced (EPA 2007). Through CKD
reduction efforts by Portland Cement Association (PCA) member companies,
this goal was accomplished in 2004. The PCA is currently developing a
new CKD reduction goal (IEEE-IAS 2008).

3.	Uses of CKD

Ingredient uses of CKD:  

Because of the high percentage of raw mix and clinker in CKD, CKD can be
directly reused in the cement kiln for clinker manufacture (EPA 2008).
The cement industry recycles more than 75 percent of its CKD, nearly
eight million tons each year (EPA 2006).

CKD partially substitutes for virgin raw materials in kiln feed. Kiln
feed is generally comprised of about 80 percent carbonate of lime and
about 20 percent silica with much lower quantities of alumina and iron. 
Limestone is the primary source of calcium for nearly all cement plants.
Shale, clay, and sand are the primary materials fed as sources of silica
and alumina (EPA 1995). CKD can substitute for any or all of these
materials, depending on the composition of the clinker from which it was
generated.

Non-combustion uses of CKD:

CKD not returned to the production process can be sold for various types
of commercial applications, including agricultural soil enhancement,
base stabilizing for pavements, wastewater treatment, waste remediation,
low-strength backfill and municipal landfill cover. These applications
depend primarily on the chemical and physical characteristics of the
CKD.  The major parameters that determine CKD characteristics are the
raw feed material, type of kiln operation, dust collection systems, and
fuel type. Since the properties of CKD can be significantly affected by
the design, operation and materials used in a cement kiln, the chemical
and physical characteristics of CKD must be evaluated on an individual
plant basis (IEEE-IAS 2008).

Quantities of CKD landfilled: 

The majority of CKD is recycled directly back to the cement kiln, thus
avoiding disposal; however, CKD must periodically be removed from the
system due to increasing concentrations of various contaminants that may
compromise the quality of the clinker. The Portland Cement Association
estimates that 6.4 million tons of CKD were landfilled in 2003 (EPA
2006).

Quantities of CKD stockpiled/stored:

CKD that cannot be recycled back into the cement kiln (because of
excessive alkali content, as well as other operational factors) is
removed from the system and often collected onsite in piles or monofills
(EPA 1995).  Current estimates of the stockpiled quantity of CKD are not
available, however. 

A recent trend at some cement manufacturing facilities is the removal or
“mining” of CKD placed in landfills or other long-term management
units. From the PCA CKD surveys of member companies, it was estimated
that the amount of CKD removed from onsite landfills has grown from just
over 13,400 metric tons in 1998 to more than 261,000 metric tons in
2006. Because CKD is very similar to the raw materials entering the kiln
system and may contain partially processed feed or final product, it
requires less processing prior to use in the kiln and thus, less energy.
Consequently, CKD is becoming more valuable as manufacturing costs rise
(IEEE-IAS 2008).

Exhibit 1:  Overview of Generation and Use of CKD 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
------------------------------

Cement kiln dust	13.23–16.53 million	0.36 milliona	0	6.4 million	1.23
million	Not Available

 

Sources:

Unless otherwise noted, data is from U.S. EPA, 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, April 28, 2008. 

Notes:

Based on PCA data and discussions with industry personnel, actual rate
of reuse in cement kilns appears to be underreported.  Subsequently, the
estimated quantity landfilled is likely overstated.



4.	Management and Combustion processes for CKD

Types of units using CKD:

Cement kiln dust is returned to cement kilns during the clinker
manufacturing process.

Sourcing of CKD:

A closed-loop process captures CKD from exhaust gas scrubbers at cement
plants and returns it directly to the kiln.

Processing of CKD:  

CKD captured from kiln exhaust gasses is directly reinjected back into
the kiln, and does not require processing.  CKD mined from stockpiles
may require processing prior to injection into the cement kiln.  

State status of CKD use as ingredient:  

According to state responses to a 2006 survey by the Association of
State and Territorial Solid Waste Management Officials (ASTSWMO),
Florida and Iowa have pre-approved the use of CKD in the production of
cement as a beneficial use, exempting it from the case-by-case approval
process for designation of beneficial use (ASTSWMO 2007, p.B-9).

5.	CKD Composition and Impacts

Composition of CKD:

CKD is comprised of thermally unchanged raw materials, dehydrated clay,
decarbonated (calcined) limestone, ash from fuel, and newly formed
minerals corresponding to all stages of processing up through the
formation of the clinker. An unusual feature of CKD is that, unlike
typical by-product materials that are substantially different than the
product, CKD is essentially cement clinker that does not quite meet
commercial specifications (EPA 1995). 

The primary constituents in CKD are silicates, calcium oxide, carbonates
(expressed as loss of CO2 and H2O on ignition), potassium oxide,
sulfates, chlorides, various metal oxides, and sodium oxide (EPA 1995).

CKD contains insignificant amounts of trace metals and therefore metal
concentrations are not usually a concern for most applications. A
comprehensive study by the Portland Cement Association evaluated the
presence of trace metals in CKD from 79 plants in the United States and
10 plants in Canada using both conventional and other fuels. Each CKD
sample was tested for the eight RCRA metals: arsenic, barium, cadmium,
chromium, lead, mercury, selenium, and silver. The samples were also
analyzed for antimony, beryllium, thallium and nickel. Results showed
that the average level of trace metals found in the CKD were
significantly below the regulation limits (IEEE-IAS 2008).

The predominant trace metals in CKD include antimony, barium, lead,
manganese, strontium, thallium, and zinc, and the minor trace metals
include beryllium, copper, hexavalent chromium, mercury, nickel, silver,
and thallium (EPA 1995).  The concentration of these metals in CKD
varies significantly by cement plant depending on the mix of raw feed
materials used in clinker production and the type of fuel used in the
cement kiln (May 1999).

Impacts of CKD use:

In clinker manufacture, CKD partially offsets the need for virgin
material feed, such as limestone and natural constituents (rock). Thus,
returning CKD to the cement kiln can reduce the unit consumption of
virgin feed stock in the kiln, which may then reduce the costs of raw
materials and emissions from extraction and processing of virgin
materials, as CKD requires less processing (and thus less energy) for
use in cement than virgin raw materials. However, using CKD in place of
virgin materials may change the emissions from the kiln to the extent
CKD has a different emissions profile than the alternative virgin
materials when burned in the kiln.

Some of the metal constituents in CKD (e.g., mercury and lead) may be
volatilized when the material is heated to high temperatures in the
cement kiln.  

The specific lifecycle impacts of CKD use as a raw material in clinker
production are not evaluated here because of uncertainties in lifecycle
scenario development. For example, it is difficult to determine the
replacement ratio between CKD and other raw feed materials in clinker
production. Thus, the correct quantity of material to be modeled is
unclear. In addition, CKD may substitute for a variety of virgin raw
materials as well as other secondary materials (e.g., blast furnace
slag, CCPs, foundry sand, 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

Association of State and Territorial Solid Waste Management Officials
(ASTSWMO). 2007, 2006 Beneficial Use Study Report, published by
Association of State and Territorial Solid Waste Management Officials,
November 2007.

Cement Americas, January 1, 2008, Cement Americas Forecast 2008,
accessed at:   HYPERLINK
"http://cementamericas.com/mag/cement_forecast_0101/index.html" 
http://cementamericas.com/mag/cement_forecast_0101/index.html . 

IEEE-Industry Applications Society (IEEE-IAS). 2008, Beneficial Uses of
Cement Kiln Dust, presented at the 2008 IEEE/PCA 50th Cement Industry
Technical Conference., Miami, FL, May 19-22, 2008

May, Mallory. May 1, 1999, A Study on Trace Metals  in Portland Cement
Kiln Dust, accessed at:   HYPERLINK
"http://cementamericas.com/mag/cement_study_trace_metals/" 
http://cementamericas.com/mag/cement_study_trace_metals/ . 

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). 1995, Report to
Congress on Cement Kiln Dust, accessed at:   HYPERLINK
"http://www.epa.gov/epaoswer/other/ckd/cement2.htm" 
http://www.epa.gov/epaoswer/other/ckd/cement2.htm . 

United States Environmental Protection Agency (EPA). 2006, Sector
Strategies Performance Report: Cement, accessed at:    HYPERLINK
"http://www.epa.gov/ispd/pdf/2006/cement.pdf" 
http://www.epa.gov/ispd/pdf/2006/cement.pdf . 

United States Environmental Protection Agency (EPA). March 2007, Energy
Trends in Selected Manufacturing Sectors: Opportunities and Challenges
for Environmentally Preferable Energy Outcomes, accessed at:   
HYPERLINK "http://www.epa.gov/ispd/pdf/energy/report.pdf" 
http://www.epa.gov/ispd/pdf/energy/report.pdf . 

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.

 Because of the high rate of direct reuse, CKD generation rates are not
routinely measured, and limited data are available. U.S. cement kiln
industry personnel estimate that CKD generation (including material
returned to the kiln) is equivalent to approximately 15% to 20% (by
weight) of total annual clinker production (EPA 2008).

  The sources consulted for this document do not provide detailed
information on the actual processing steps.

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Cement Kiln Dust (CKD)

