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THE USE OF FLUIDIZED BED ASH AND CEMENT KILN DUST FOR ENVIROMENTAL RECLAMATION

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THE USE OF FLUIDIZED BED ASH AND CEMENT KILN DUST FOR ENVIROMENTAL RECLAMATION

ABSTRACT

 

The goal of this research is to determine the appropriate proportions for a grout composed of waste materials, fluidized bed combustion (FBC) fly ash and cement kiln dust (CKD), for use as a structural fill in land reclamation. The grout must comply with the laws and codes established by the Commonwealth of Pennsylvania’s Department of Environmental Protection (PADEP) to assure that the material is non-hazardous towards the environment after placement.

Two types of FBC ashes were evaluated in this research including mixed-fuel FBC ash, a by-product of a coal combustion process that uses a variety of different fuels (anthracite waste coal, petroleum coke, high carbon ash and paper processing residual) and tire derived fuels FBC ash, a by-product of a coal combustion process that uses tires and anthracite waste coal as fuels. The ashes used were a combination of the fly ash and bottom ash co-products of the combustion process in the Northampton Generating Plant at Northampton, PA.

The grout mixtures were tested for strength gain, Proctor density, permeability and leachate. No leaching test was performed on the mixed-fuels FBC ash grouts because at the stage where the tests began the Northampton Generating Plant had ceased the production of this ash in favor of the combustion process that would eventually produce the TDF FBC ash.

Results indicate that all the grouts were suitable for use as a structural fill in land reclamation. All grouts achieved strengths significantly higher than needed. The resulting grout was required to have a permeability value of 1×10-6 cm/s. None of the grouts achieved the required permeability value during the testing period but a tendency was established that the requirements would be met shortly after testing ended. Leachability tests showed that the TDF FBC ash/ CKD grout complies with PADEP MOD 25 limits.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE OF CONTENTS

 

LIST OF TABLES………………………………………………………………………………………………….. viii

LIST OF FIGURES………………………………………………………………………………………………….. ix

ACKNOWLEDGEMENTS…………………………………………………………………………………………. x

 

 

Chapter 1. INTRODUCTION …………………………………………………………………………………… 1

1.1 Problem Statement ……………………………………………………………………………………….. 1

1.2 Objectives …………………………………………………………………………………………………….. 4

1.2.1 General Goals ………………………………………………………………………………………….. 4

1.2.2 Methodology …………………………………………………………………………………………… 5

1.3 Fluidized Bed Combustion………………………………………………………………………………. 6

1.3.1 Introduction…………………………………………………………………………………………….. 6

1.3.2 Chemical Properties of FBC Ash …………………………………………………………………. 7

1.3.3 Tire Derived Fuel Ash ……………………………………………………………………………… 10

1.3.4 Mineralogy of FBC Ash ……………………………………………………………………………. 11 1.3.5 Reuse of FBC Ash ……………………………………………………………………………………. 13

 

1.4 Cement Kiln Dust (CKD) ………………………………………………………………………………… 14

1.4.1 Introduction…………………………………………………………………………………………… 14

1.4.2 Chemical Properties of CKD …………………………………………………………………….. 16

1.4.3 Reuse of CKD …………………………………………………………………………………………. 23

 

Chapter 2. EXPERIMENTAL PROCEDURES …………………………………………………………….. 25

2.1 Analysis of Materials ……………………………………………………………………………………. 25

2.1.1 FBC Ash …………………………………………………………………………………………………. 26

2.1.2 CKD ………………………………………………………………………………………………………. 34

2.1.3 Instrumentation and Characterization ………………………………………………………. 34

2.1.3.1 Powder X-Ray Diffraction ……………………………………………………………….. 34

2.1.3.2 Specific Gravity …………………………………………………………………………….. 35

2.1.3.3 Moisture Content ………………………………………………………………………….. 35

2.1.3.4 Absorption Rate ……………………………………………………………………………. 35

2.1.3.5 Sieve Analysis ……………………………………………………………………………….. 36

2.1.3.6 Bulk Chemical Analysis …………………………………………………………………… 36

2.2 Parametric Experimental Design ……………………………………………………………………. 36

2.2.1 CKD/FBC Ash Ratios ………………………………………………………………………………… 37

2.2.2 Water/Cement Ratio ………………………………………………………………………………. 37

2.2.3 Summary of Parametric Experimental Design ……………………………………………. 38

2.2.4 TDF FBC Ash/CKD Mix Selection ……………………………………………………………….. 40

2.3 Mechanical Properties …………………………………………………………………………………. 44

2.3.1 Compressive Strength Development ………………………………………………………… 44

2.3.2 Proctor Compaction Test ………………………………………………………………………… 46

2.3.3 Permeability ………………………………………………………………………………………….. 48

2.4 Leach Testing………………………………………………………………………………………………… 50

Chapter 3. RESULTS AND DISCUSSION …………………………………………………………………. 53

3.1 Compressive Strength Development ……………………………………………………………… 53

3.2 Proctor Compaction Test ……………………………………………………………………………… 59

3.3 Permeability ……………………………………………………………………………………………….. 63

3.4 Mineralogical Analysis ………………………………………………………………………………….. 64

3.4.1 Cement Kiln Dust ……………………………………………………………………………………. 64

3.4.2 FBC Ashes ……………………………………………………………………………………………… 68

3.4.2.1 Mixed-Fuels FBC Ashes…………………………………………………………………………. 70

3.4.2.2 TDF FBC Ashes …………………………………………………………………………………….. 70 3.4.3 FBC Ash/CKD Grouts ……………………………………………………………………………….. 76

 

Chapter 4. CONCLUSION …………………………………………………………………………………….. 80

4.1 General Conclusion ……………………………………………………………………………………… 80

4.2 Future Work ……………………………………………………………………………………………….. 83

4.2.1 Field Structural Integrity ………………………………………………………………………….. 83

4.2.2 LWD and DCP Preliminary Tests ……………………………………………………………….. 86

 

BIBLIOGRAPHY …………………………………………………………………………………………………… 89

Chapter 1

INTRODUCTION

1.1 Problem Statement

In 2006, 49% of the electricity produced in the U.S. was by coal-fired units (Energy Information Administration, 2006). The burning of coal produces around 111 million metric tons of residues; among the residues produced are various types of fly ash which account for a total of 80 million metric tons (U.S. Geological Survey, 2006). Atmospheric fluidized bed combustion (FBC) systems have been developed in order to use low-grade or high-sulfur coals. This process produces large amounts of FBC fly ash that need to be disposed. Some of the applications given to the FBC ash are as construction material, for pollution control to neutralize acid waste, as an industrial raw material for cement production and carbon recovery and in agriculture as a soil amending agent, a lime substitute and a source of nutrient values (Berry, 1987). Most FBC ashes are moderately cementitious due to high-contents of CaO and CaSO4. FBC ash can have various limited purposes in different fields if an activator is added. But most of the waste FBC ash goes into coal mines as structural fill material for land reclamation.

Up to 20% percent of the raw materials that are used during the process of cement production are discarded for being too fine, highly alkaline, and volatile (Sri Ravindrarajah, 1992). This material is collected by using cyclones, bag houses, and electrostatic precipitators. Annually four million tons of Cement Kiln Dust (CKD) are produced in the United States. CKD is considered to be a health hazard and can also pose pollution problems when improperly managed (Baghdadi et al., 1995).  These reasons and the need to find more cost-effective materials have created some awareness towards the finding of new uses for this material. Among the researched alternatives for the disposal of CKD is its use as soil fertilizer, as stabilizer of waste water streams, as a partial replacement of soda in glass production, as an anti-stripping agent in asphalts and as a component of blended cements and masonry products among others (e.g. see Klemm, 1980 and Bhatty, 1995).

Another use that has been given to fluidized bed combustion ashes and CKD has been as a structural fill material for the reclamation of limestone quarries and coal mines (Lolcama, 2003). Many problems arise from the use of CKD as landfill material. Among these problems is the ability of CKD to penetrate the karst limestone terrain and filter into the groundwater in smaller quantities that over time may cause serious contamination (Lolcama, 2003). The Environmental Protection Agency (EPA) has implemented regulations for the management of CKD (Office of Solid Waste U.S. EPA, 1998). The regulations call for the study of a karst limestone site before it is used to deposit the CKD. The study is required to maintain extensive monitoring of ground water and landfill conditions.

The accumulation of these materials in landfills is so great that by 1983 an estimated 100 million tons of CKD were present in disposal sites across the United States (Collins and Emery 1983). Researchers have started to look for uses on the CKD currently present in the landfills to provide some relieve to these sites (Sreekrishnavilasam, 2006).

Most of the studies that have combined these two materials have concentrated themselves in their usage in mine reclamation and their benefits to abate acid-mine drainage. Currently no study has been concentrated in the combination of these two materials, produced by two different industries, in order to develop a more engineered approach for their placement in limestone quarries. The combination of these two materials has the prospect of providing a material with enough strength to be used in other engineering applications. Since both materials have cementitious properties, the combination of both along with water will cause the activation of these properties due to the alkali present in the CKD. When these cementitious properties are activated the resultant material can provide a low-strength concrete that would be capable of handling mass loading. With the activation of the cementitious properties a structural fill will be created making its placement in limestone quarries safer and environmentally friendly. Among other applications where this material may be useful are:

  • The material might serve has a structural fill for different types of engineering projects were a fill material is needed and the resources are limited.
  • The mix may be found useful has a sub-grade material in the construction of roads and such.
  • The mining industry can find use in the material for the reclamation of mines while taking out the risk of collapse due to poor compaction.

In 2006, the Keystone Cement Company sponsored a project that intended to study the interaction between fluidized bed combustion ashes and cement kiln dust in order to create a material strong and environmentally safe enough to be used has a structural fill in the reclamation of a limestone quarry. The study was to be divided in two phases; a laboratory testing phase and field application phase. The first phase of the research intended to find the optimum mix design and subsequently study the properties of the material as a concrete for structural use. The second phase is to take the results of the first phase and apply them to the field as a structural fill for the reclamation of a limestone quarry located in Bath, Pennsylvania and proceed to study the site, with the new material on it, the durability of the material and its impact on the environment. The work done in the first phase is summarized in this thesis.

 

1.2 Objectives

The goal of this study was to optimize a mix of FBC ash and Keystone CKD into the proportions that would provide the most efficient mix in terms of strength and manageability that would yield a low strength grout which could be utilized in the reclamation of a limestone quarry while maintaining properties that would make it structurally and environmentally safe.

 

1.2.1 General Goals

The grout resulting from the mix between FBC ash and CKD will have the following properties:

  • Minimum low strength of 4000 psf
  • Low-permeability (less than 1 x 106 cm/s)
  • Environmentally safe

1.2.2 Methodology

A CKD provided by the Keystone Cement Company from Bath, Pennsylvania was investigated in this study. Two types of ashes were combined with the CKD, FBC ash from the burning of mixed-fuels (anthracite waste coal, petroleum coke, high carbon ash and paper processing residual) and FBC ash from the burning of tires also known has tire derived fuels (TDF) FBC ash. Both ashes were obtained from the Northampton

Generating Plant, a 112-megawatt electric generation facility in the Borough of Northampton, PA. The CKD was combined separately with the two types of FBC ashes and two mixes were optimized by taking multiple samples with different ratios of quantity and then tested for strength and manageability. Proctor tests were performed on the different mix ratios to determine their densities. The mineralogy of the CKD and both FBC ashes was determined using x-ray diffraction. After the optimized mixes were determined, x-ray diffraction characterized both mixes to obtain their mineralogy. The toxicity of the mixes was measured using the synthetic precipitation leach procedure (SPLP) following the Environmental Protection Agency method 1312 and then compared with the Pennsylvania Department of Environmental Protection (PA DEP) thresholds established in Module 25 release limits to ensure that there are no elemental releases that may harm the environment. Permeability tests were performed on the optimized mix designs using the falling head method. The results obtained are expected to provide information that will determine if the optimized mix can be used has a structural fill in limestone quarries.

 

1.3 Fluidized Bed Combustion (FBC) ash

This section will discuss the origins and composition of both FBC ashes used in the study.

1.3.1 Introduction

In 1991 the Clean Air Act was amended by congress, mandating that all electric utilities to retrofit their combustion systems to meet with new air emissions standards. Long before the act was amended, the fluidized bed combustion (FBC) system was in development and arose as the most promising and economically viable option to comply with the new air emission standards (Martinez et al. 1991; Nelkin and Dellefield 1990; Makanski and Schweiger 1987; Smock 1987; Abelson et al. 1978a).

Fluidized beds allow for the use of coal, biomass and other materials has fuel to fire the combustion process. The Northampton Generating Plant uses anthracite waste coal, petroleum coke, high carbon ash and paper processing residual as fuel. The fuels are burned in a bed of noncombustible material suspended by injecting air from below the bed. The ash used in this study originates from an FBC system that employs limestone as the noncombustible material. The limestone serves as a sorbent to extract the sulfur pollutants allowing for the emissions of sulfur into the air to be reduced. The amount of limestone added is proportional to the amount of sulfur-dioxide present in the fuels (Loop et al. 2005, Schueck et al. 2001, Behr-Andres et al. 1994). To account for the variations on the sulfur content present in the fuels, the amount of limestone is kept in excess.

1.3.2 Chemical Properties of FBC Ash

During the combustion process most of this limestone is spent for absorption purposes, but there’s always some material that’s left unspent. Due to the unspent limestone, the resulting ash contains high amounts of calcium that give the FBC ash cementitious properties. Typically the chemical processes that influence the ash formation and its mineralogy are:

 

CaCO3  CaO  +  CO2                     lime                                                                      (1.1)

FeS2 (Pyrite) [and marcasite]   Fe2O3  + SO2                                                        (1.2)

2CaO  +  2SO2  + O2    2CaSO4            anhydrite                                                  (1.3)

Clay   dehydroxylated clay + H2O                                                                       (1.4)

 

The residues of these processes may also contain quartz, iron compounds and carbon, in the form of char (Schueck et al. 2001, Zhao 1995). Any quartz present on the combustion process will remain unaffected. If there is any CaS present, research has shown that it will not have any adverse effect on the FBC ash performance (Pera, 1989). FBC systems burn fuel at a much lower temperature than regular electric-utility systems, combustion in FBC systems occur around 815 oC to 870 oC and around 1500 oC to 1600 oC in electric-utility systems. The difference in temperatures allow for the most optimum conditions for the capture of sulfur in FBC systems. This difference in temperature combined with the high amounts of limestone results in very different ashes being formed in the combustion process. These differences are significant when compared in a more in depth analysis (Table 1.1). The concentrations of aluminum, barium, chromium, potassium, silicon, sodium, strontium, titanium, and zinc are smaller in FBC ash because high concentrations of calcium help dilute these other elements. Because  FBC ash contains high concentrations of calcium, the processes shown in equations (1.1) and (1.2) are achieved and when the ash is hydrated the CaO and the CaSO4 react exothermically to form:

 

CaO  + H2O   Ca(OH)2 slaked lime         (1.5)  CaSO4  + 2H2O   CaSO4.2H2O gypsum        (1.6)

 

The majority of the exothermicity when water and FBC ash are combined is due to the unspent CaO and to reaction (1.5). FBC ash can also interact with atmospheric carbon dioxide, but most of the chemical reactivity and exothermicity of the ash is associated with processes involving water (Bulewicz, 2000).

 

CaO  +  CO2    CaCO3              calcite                                                                   (1.7)

 

When ash is combined with water under strong alkaline conditions, at pH over 11, at

ambient temperatures and at normal consistency, the Ca2+ and SO42- ions interact with the aluminum to form ettringite (Bulewicz 2000, Pera 1989).

Table 1.1 – Average Oxide Concentrations of Coal Ash Types (Ainsworth and Rai 1987, Perri et al. 1988)

FBC-Ash

Bottom      Anthracite

Fly Ash

Oxide               %                 Ash                %

%

SiO2   44.71   22.20   58
Al2O3   42.70   38.17   20.4
Fe2O3   21.73   30.02   5.7
CaO   8.68   7.28   4.1
MgO   1.96   1.61   0.62
Na2O   2.45   1.67   0.59
K2O   3.45   2.89   2.56
SO3       1.1
             

 

 

 

 

 

 

 

 

 

 

 

 

6Ca2+  +  2Al(OH)4-  +  3SO42-  +  4OH  +  32H2O   Ca6[Al(OH)6]2(SO4)2 . 26H2O            (1.8)

 

A more simple form of this reaction can be written:

 

3Ca + 3 gypsum + 26 water   ettringite                                                                            (1.9)

 

Ettringite starts forming from the moment water makes contact with the ash and the process can continue for a year since there are no competing reactions (Mulder et al. 1995). Ettringite can also be responsible for the cracking and splitting of the resulting product from the mix of water and ash due to its crystallization.

 

1.3.3 Tire Derived Fuel Ash

When pieces of scrap tires are added as fuel in the combustion process of an FBC system the resultant residue ash is known has TDF-FBC ash (Tire Derived Fuel). Burning of TDF serves to add high heating value fuel to the coal blend, especially when low-rank coals are part of the fuel blend, and it also helps to alleviate the excess of scrap tires that need to be disposed (Hower and Robertson, 2003). Emissions for SO2 and HCl are increased due to the higher content of sulfur in the tires, it is the SO2 formed in combustion that converts the NaCl present in the hog salt into HCl, meanwhile NOx emissions are reduced (Duo, 2007). In a study performed by Hower and Robertson (2003), it was found that zinc, carbon and copper concentrations were higher in the residue ash due to the addition of scrap tires. It was also determined that lead and arsenic concentrations were higher than normal but this could have been due to the coal has well as the tire derived fuels. Duo (2007), performed an analysis (Table 1.2) on TDF ash and found similar results regarding zinc and carbon concentrations but no mention of copper was made. Duo (2007), also found an increase in iron present in the ash. The source of the zinc can be traced to ZnO used in white wall tires and the iron can be traced to residual amounts of steel remaining in the tire scraps.

 

1.3.4 Mineralogy of FBC Ash

The mineralogical composition of electric-utility ash is very similar to the composition of FBC ash containing phases of mullite, quartz and iron oxides like hematite. The difference between the two is due to the higher concentrations of calcium and sulfate minerals (anhydrite, lime, calcite and portlandite) present in the FBC ash (Behr-Andres and Hutzler, 1994). This is consistent with the large amounts of limestone that is usually used as sorbent in the coal combustion process in a FBC system.

Tire derived fuel FBC ash mineralogical composition distinguish itself from mixed-fuels FBC ash in that it has higher concentrations of hematite making it the dominant mineral and the presence of ZnO as the mineral with the second higher concentrations in the ash. This behavior is expected due to the addition of the scrap tires that have high concentrations of iron and zinc. In descending order the major constituents in the mineralogy of TDF ash consists of: hematite, zincite, quartz, alumina, lime and sulfur trioxide (Duo, 2007)

 

12

 

Table 1.2  – TDF Ash Metal Analysis (Duo et al. 2007)

 

 

 

Unit

 

BC

Limit

  TDF addition rate, by weight  
0% 2.50%

Fly Ash

5% 0% 2.50% 5%
  Bottom Ash  
No. of Tests —- —- 3 4 4 3 3 1
Cadmium ppmw —- 1.0 0.7 0.9 n/da n/da n/da
Mercury ppmw —- 0.19 0.175 0.21 n/da n/da n/da
Chromium ppmw —- 53 53 48 28 29 18
Nickel ppmw —- 27 29 25 20 21 15
Lead ppmw —- 55 40 57 n/da n/da n/da
Iron % —- 2.43 3.73 4.03 1.74 2.76 2.53
Zinc % —- 0.04 0.26 0.64 0.02 0.13 0.29
Leachate Test        mg/L Zn     500           4.6             20.8             104              0.38             8.1             12.9

 

 

an/d = nondetectable

BC Limit  – establishes the limits imposed by the Special Waste Regulations of British Columbia, Canada

 

 

1.3.5 Reuse of FBC ash

FBC systems generate more ash than regular coal combustion systems due to the extra material present on the fuel (non-combustible fuel), subsequently increasing the waste material that needs to be disposed or recycled. Numerous reuse options have been researched for ash generated by electric-utilities. The major and more successful applications can be divided in four categories: construction products and structural fill materials, agricultural applications, waste-management applications, and industrialmaterial recovery (Behr-Andres et al. 1994). The mineralogical composition of the FBC ash is similar to electric-utility ash but FBC ash differentiates itself because it contains high concentrations of calcium and sulfate minerals: anhydrite, lime, calcite, and portlandite (Behr-Andres et al. 1994). The differences in the mineralogical composition of between electric utility and FBC ashes suggest that the behavior of FBC ashes might vary in the applications previously named has alternative for reuse of electric-utility ash. The chemical and physical characteristics of the FBC ash must be investigated and their reuse in other applications must be reevaluated. Technical issues like conditioning and handling characteristics, permeability and leachate generation and stability and post closure uses need to be addressed.

Due to the high concentrations of CaO and CaSO4 some environmental concerns were raised due to high alkaline (pH = 12) leachate that may be generated when the ash is in contact with water (Ross, 1989). But new regulations by the Department of Environmental Protection have eliminated these concerns. When the ash is contacted with water the reaction may be accompanied by swelling which may impair the physical integrity of the disposal site, leading to low compressive strength and high permeability (Zhao, 1995). Recent studies have successfully investigated the use of FBC ash for: the control of acid-mine drainage and subsidence in underground coal mines, the replacement of a percentage of portland cement with FBC ash in cast-concrete, as roadway fill or structural fill, and has a stabilizing agent for aggregates among others (Khoury and Zaman 2007; Naik et al. 2005; Siriwardane et al. 2003; Deschamps 1998;).

 

1.4 Cement Kiln Dust (CKD)

This section will discuss the origins and composition of cement kiln dust (CKD).

 

1.4.1 Introduction

In 2007, the cement industry across the world produced and estimated 2,600 million metric tons of cement. The top producer was China with 1,300 million metric tons, followed by India with 160 and in third the United States with a production of 95 million metric tons (Environmental Protection Agency, 2008). With nine cement plants, the state of Pennsylvania was the third largest producer in the United States manufacturing approximately 6.3 million metric tons. The production of portland cement is accompanied by the generation of large amounts of waste material known has cement kiln dust (CKD). There is no established way to determine how much CKD is produced in the portland cement manufacturing process because it is dependent on many variables and configurations of the individual cement plants. The generation of CKD is affected by (Sreekrishnavilasam, 2006):

  • The raw materials used in the cement kiln
  • The design of gas velocities in the kiln
  • The type of process involving an intermittent bypass operation or a continuous bypass operation
  • Kiln performance and operation (wet kiln, long dry kiln, preheater kiln and suspension heater kiln)
  • Dust collection system

Cement Kiln Dust is produced during the third stage of cement manufacturing when clinker is formed. In a wet process, some of the CKD is removed from the kiln as waste dust. In a dry process, dust is collected from the kiln in precipitators. CKD removed from the clinker cooler at the end of the kiln is re-circulated in the cyclone and pre-heaters (dry process). Dust collected from the upstream portion of the kiln is removed from the system as by-pass dust. The by-pass dust is removed as a precaution against materials (heavy metals, chlorides, and alkalis) that may cause the clinker to be out of

specification limits (Williams, 2005).

Around 8 million tons of CKD or approximately 75% of the produced waste is recycled back into the cement creation process has a raw material (Portland Cement Association, 2006). The reuse of the CKD reduces the amount of limestone needed as well has other raw materials and also helps conserve energy. The dust that is not returned to the cement production process is removed from the process due to the high concentrations of alkalis and heavy metals. The great majority of the remaining 25% goes into landfills although a small percentage is used for agricultural soil benefaction and for soil stabilization (Portland Cement Association, 2006). In 1990 the Portland Cement Association (PCA) started tracking the amount of CKD being disposed in landfills across the United States. The results were compared with the amount of CKD produced by the clinkers and the comparison shows that the amount of CKD that goes into landfills has been reduced dramatically (Figure 1.1) as well has the amount of CKD produced (Figure

1.2).

 

1.4.2 Chemical Properties of CKD

The chemical composition of CKD is dependent on the raw materials used, the kiln design and on the fuel used to fire the kiln. The alkalis (Na2O and K2O) and the loss of ignition tend to be higher on CKD than in portland cement, while the lime (CaO) content is higher in portland cement. Although CaO concentrations are lower in CKD, it still is a major constituent of CKD along with SiO2. Dusts from gas or oil-fired kilns have been reported to contain higher proportions of soluble alkalis as compared to those from coal fired kilns (Klemm, 1980). Significant differences can be observed between total and separated dust collected, with the finer dust particles usually having a higher concentration of sulfates and alkalis and a lower free lime content (Collin and Emery,

1983).

 

 

 

90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06

Year

 

Figure 1.1 – Cement Kiln Dust Sent to Landfills (PCA, 2007)

 

 

 

 

 

 

90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06

                                                       Year                                                                    

Figure 1.2 – CKD per Unit of Clinker Produced (PCA, 2007)

 

 

 

 

The chemical composition has been documented by many researchers in the past 25 years via characterization of various samples from different cement plants. To assess the variability in the chemical composition of CKD, Sreekrishnavilasam (2006) compiled and summarized oxide data for 63 different CKDs (58 plants located in the US). The average percentages for the main oxides, loss of ignition, and free lime are summarized for all 63

CKD samples in Table 1.3. Overall, the data examined indicate that there exists no “average” cement kiln dust, and that each CKD source should be considered as having its own unique properties. The variability in the composition of CKD, and in particular the large range in variation in free lime content, highlight the importance of fully characterizing a particular CKD before recommending it for use as a construction material (Sreekrishnavilasam, 2006).

Research has been carried out in order to develop parameters that are capable of predicting the reactivity of CKD based on its oxide composition. Two empirical parameters have been proposed, Hydration Modulus (Kamon and Nontananandh, 1991) and the Total Reactive Oxide content (Collins and Emery, 1993). The Hydration Modulus is defined by:

 

HM = CaO/(SiO2+Al2O3+Fe2O3)                                                                             (1.10)

 

where CaO, SiO2,Al2O3,Fe2O3 are expressed in percentage values. Using alite  (HM~1.7) and belite (HM~2.4) as references Kamon and Nontananandh (1991) suggested that a

CKD within this range can be characterized has reactive. Alite and belite where used has references because alite, also known as Ca3SiO5, is the major mineral constituent of portland cement that causes setting and strength development in the first few weeks, and belite, Ca2SiO4, another mineral constituent in portland cement responsible for strength gain after a week. An average HM and TRO are also included in Table 1.3 for the 63 CKDs compiled by Sreekrishnavilasam (2006). The Hydration Modulus has been proved to be a questionable parameter since comparisons between free lime, the likeliest source of reactivity in CKD, doesn’t correlate very well with the Hydration Modulus.

Collins and Emery (1983) introduced the parameter known has the total reactive oxide content (TRO) as an indicator of the reactivity of CKD. TRO is defined as:

 

TRO = (CaO+MgO-LOI)-(K2O+Na2O)                                                                 (1.11)

 

where CaO, MgO, K2O, Na2O, and LOI are expressed as percentage values. The authors have recommended not using the equation if the total alkali content is higher than 6%. The higher the resulting value obtained from the equation the greater the potential of the CKD to contribute to higher compressive strength when mixed. In contrast to the HM parameter, TRO does correlate well with free lime supporting the authors hypothesis that a higher total reactive oxide signifies a CKD with more reactive phases. Haynes and Kramer (1982) performed an extensive study to determine the mineralogy of CKD analyzing 113 samples from 102 cement plants in the United States. Their results are summarized in Table 1.4. It was determined that the major constituent

 

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Table 1.3 – Statistics and Composition of Fresh CKD Based in a Compilation of 63 CKDs (Sreekrishnavilasam, 2006)

  CaO SiO2 Al2O3 Fe2O3 MgO SO3 Na2O K2O LOI Free  CaO Total Alkali TRO HM
Mean 43.99 15.05 4.43 2.23 1.64 6.02 0.69 4 21.57 6.75 3.32 21.49 2.33
SD 8.01 4.74 1.82 1.04 0.68 3.93 1.02 3.01 8.5 7.83 2.44 12.97 1.61
COV (%) 18 31 41 47 41 65 147 75 39 116 74 60 69
Max 61.28 34.3 10.5 6 3.5 17.4 6.25 15.3 42.39 27.18 11.42 56.08 13.91
          Min           19.4      2.16      1.09      0.24     0.54    0.02       0       0.11      4.2         0        0.14      1.86        0.53

 

Oxide values expressed in % by mass of cement

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Table 1.4 – Average Mineralogical Composition of Cement Kiln Dust (Haynes and Kramer, 1982)

Constituent    CaCO3    SiO2    CaO    K2SO4     Fe2O3      KCl MgO      Na2SO4     CaSO4    Al2O3    KF                            Others

% by

55.5      13.6     8.1       5.9         2.1       1.4     1.3         1.3           5.2         4.5    0.4                                                 0.7

weight

 

 

present in CKD is calcite, and in smaller but significant concentrations lime, anhydrate, quartz and dolomite.

 

1.4.3 Reuse of CKD

The Portland Cement Association (PCA) established the cement manufacturing sustainability program (CMS) to balance society’s need for cement products with stewardship in the air, land, and water, conservation of energy and natural resources, and maintenance of safe work places and communities. Environmental performance measures were put in place as part of this program in order to set a long-term reduction target for CKD, energy consumption, SO2 emissions, NOx emissions among others. The self-imposed reduction target by the cement industry called for a reduction in the amount of CKD disposed per ton of clinker produced by 60% using a 1990 baseline until 2020. The program can be considered a success; by 2006 a reduction of 75% had been achieved, mainly due to new developments in cement manufacturing technology that allows reusing most of the CKD back into the cement manufacturing process. A small percentage of the reduction can be attributed to other reuse options that have been researched and are currently in development.

 

Reuse options given to CKD throughout the years include soil benefaction with agricultural purposes, soil stabilization and stabilization and solidification of waste materials (PCA, 2008 and ASTM D 5050). Some other engineering applications are currently being researched in order to find bigger applications that could lead to more effective and widespread methods of recycling. Due to the large amounts of materials involved in highway construction there’s been great interest in researching the use of CKD in highway engineering. Research has been focused in sub-grade stabilization/modification (e.g. McCoy and Kriner, 1971; Zaman and Sayah, 1992; Baghdadi et al., 1995; Bhatty et al., 1996; Miller and Azad, 2000; Miller et al., 2003), pavement filler (Zhu et al. 1999). Other research has focused itself on small percentage substitution of cement with CKD (Wang et al., 2002; Shoaib et al., 2000 and Kurdi et al.,

1996) and in the use of CKD to create Controlled Low-Strength Materials (Williams, 2005).

Research has shown that the addition of CKD to any type of concrete will

significantly decrease its compressive strength, flexural strength and modulus of rupture

(Udoeyo and Hyee, 2002; Shoaib et al., 2000; Al-Harthy et al., 2003; Ozyildirim and Lane, 1995). For CKD to be used without significant adverse effects, the percentage in the mix should be kept under 10% when used for reinforced concrete (Al Harthy et al., 2003; Shoaib et al. 2000).

THE USE OF FLUIDIZED BED ASH AND CEMENT KILN DUST FOR ENVIROMENTAL RECLAMATION

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