Effects of Using Causticized Lignite as a Seacoal Replacement on Mold Gas Emissions

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1 Effects of Using Causticized Lignite as a Seacoal Replacement on Mold Gas Emissions G. Thiel S.R. Giese University of Northern Iowa, Cedar Falls, IA V. Losacco J. Darlington M. Van Leirsburg American Colloid Co., Arlington Heights, IL Copyright 2004 American Foundry Society ABSTRACT Emission testing was conducted to determine the effect of substituting causticized lignite for seacoal in a green sand molding system. During the testing, emissions were first collected from a typical baseline system sand containing seacoal. These results were then compared to the emissions resulting from increasing amounts of causticized lignite substituted for the seacoal in experimental green sand systems. In addition to the collection of CO/CO 2 samples, 63 different Hazardous Air Pollutants (HAPs) and Volatile Organic Compounds (VOCs) were also collected and evaluated. The results of the testing demonstrated that the total substitution of seacoal with causticized lignite reduced mold gas emissions of HAPs and VOCs by more than 45%. Proportional reductions in both HAPs and VOCs were observed with partial replacements of causticized lignite for seacoal in the different green sand mixtures. Other results of the testing included a decrease of 18% in bond requirement in the sand mixtures containing all caustisized lignite. INTRODUCTION Pollutants released into the atmosphere have always been a very important topic of concern to not only the foundry and foundry related industries, but also to public and environmental health as well. Recent and future legislation from the United States Environmental Protection Agency (USEPA) and many other government agencies relating to the 1990 Clean Air Act have prompted the foundry industry to reduce pollutant emissions, while continuing to produce quality products in an efficient and cost effective manner. The control of pollutant emissions, especially Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) is very important due to the role they play in chemical and atmospheric reactions that form ozone and other phytochemical oxidants, leading to increased environmental pollution. 1 The foundry industry is continuously looking for new materials and/or new processes that produce high quality castings in a cost efficient manner, while reducing or minimizing the amount of pollutant emissions from the casting processes. 2 In an effort to determine the environmental effects of replacing seacoal with causticized lignite as a carbonaceous additive in green sand systems, mold gas emission testing was conducted at the University of Northern Iowa Metal Casting Center. The sand was first matured by cycling pouring, cooling and shakeout while maintaining clay, moisture and combustible material levels. Emission testing was conducted by producing and pouring green sand molds with grey iron inside a collection hood. During the testing, emissions were first collected from a typical baseline system sand containing seacoal, and then were compared to increasing amounts of causticized lignite substituted for the seacoal in experimental green sand systems. In addition to the collection of carbon monoxide/carbon dioxide (CO/CO 2 ) samples, 63 different Hazardous Air Pollutants (HAPs) and Volatile Organic Compounds (VOCs) were also collected and evaluated. The results published in this report are specific to the materials and production methods used at the University of Northern Iowa Metal Casting Laboratory. The results are meant solely for the comparison of the relative emissions from alternate materials and are not suitable as general emission factors. Variables such as production rate, capture efficiency, sand to metal

2 ratio, casting surface area, percentage of combustibles, and production equipment all have a significant effect on the actual emission levels. For these reasons the results are not reported in pounds of HAPs per ton of metal poured. COMPARISON OF CARBONACEOUS ADDITIVES Comparisons of the different physical properties between seacoal and causticized lignite have been well documented through various papers and technical reports. The general consensus has been that causticized lignite has higher ph value, higher combustible volatile material at 1800 F (982 C), lower fixed carbon, and higher ash when compared to seacoal (bituminous coal). These properties are shown in Table 1. Material ph Volatile Combustible 1800 F / 982 C (%) Fixed Carbon (%) Ash Content (%) Seacoal % Max. Causticized Lignite Table 1: Typical Properties of Seacoal and Causticized Lignite 3,4 The base material source for causticized lignite, commonly referred to as lignite, Leonardite shale, coal humate, or humic acid, varies greatly according to the chemical structure of the material and the source of where the material is mined. The lignite with the highest grade of active ingredients and the most consistent in chemical and physical structure in North America has been found to come from the state of North Dakota (USA). Lignite contains mostly large amounts of humic acids, along with other naturally occurring organic acids. The exact nature of humic acids is unknown. Typically, lignite from North Dakota contains a humic acid content of approximately 80%. The lignite used to produce the causticized lignite product used for this emission study was mined from the North Dakota area. 5,6,7. Causticized lignite is manufactured by taking lignite and adding a caustic solution (typically NaOH) to bring the ph of the lignite material from the acidic range to a ph range between 6 and 8. The resulting product is dried and ground, and is typically provided as a free flowing powder to the foundry industry. TESTING METHODOLOGY All of the test mixtures of system sand were prepared using a Wedron 520 silica sand prepared in a Simpson model 1-1/2 F Muller. The green sand mixtures were prepared using 8.0 % sodium bentonite clay based on a final green sand batch size of 1,100 lbs. The carbonaceous additives were introduced at 20% addition levels based on the amount of the dry clay used. The mulling cycle consisted of adding clay (bond), carbonaceous materials, and temper water to the sand, and mulling the green sand for five (5) minutes. The green sand was then tested for a target compactability of 39%. Additional temper water was added (if needed), and the green sand was mulled for an additional two (2) minutes if an increase in compactability was required. Total mull time on all sand cycles was held between 7 to 10 minutes. TEST PATTERN The pattern used in the experiments consisted of a 3 wide wooden step block with a thickness ranging between 3 and 0.5. The pattern was located equally in the cope and drag and mounted on ½ plywood plates. The gating system was nonpressurized. MOLD PREPARATION After the desired compactability was achieved, the molds were then prepared for each green sand batch. Three (3) molds of each green sand mixture were prepared. Each mold consisted of two step patterns (2 per mold) with a metal pour weight of 67 lbs., and used approximately 300 lbs of green sand. A petroleum based parting agent was applied to the molds to emulate actual foundry practice. The parting agent was brushed on the cope and drag pattern directly before making the molds to quantify the amount of parting agent used. Approximately 3-4 grams of parting agent was applied to each mold. An example of the molds with the parting spray applied is shown below in Figure 1.

3 Fig. 1: Cope Mold Section Molds were produced on a Herman high pressure molding machine using an aluminum 19 x 25 x 6 /6 pop-off flask. Squeeze pressure was controlled at 400 psi. which typically produced a mold with a B-scale mold harness between The sand to metal ratio was approximately 5:1. MELTING PROCEDURE AND METAL CHEMISTRY The composition of the metal used in the trials was consistent with the chemistry used to produce a standard class 30 gray iron with a nominal chemistry of 3.0% Carbon and 2.10% Silicon. Charge materials used in the heats included pig iron, steel, foundry returns, carbon raisers, and ferro silicon. The chemistry for the grey iron used is shown in Table 2. Required Chemistry Range (%) Target (%) Carbon Equivalence (CE) Carbon Silicon Sulfur Manganese Phosphorous Table 2: Grey Iron Chemistry

4 The metal was melted in a 300 lb. high frequency coreless induction furnace utilizing a neutral refractory lining. After meltdown, the slag was removed, a thermal analysis sample was taken, and the temperature of the molten metal was raised to approximately 2750 F (1510 C). The heats were tapped into a 300 lb. heated monolithic ladle, and post inoculated with ferro silicon, as shown in Figure 2. The metal was then poured into the mold in the emission collection hood, and into the two molds on the pouring line using a target pouring temperature of 2600 F (1427 C). An approximate total target pour time of 25 to 35 seconds was used, with each mold requiring approximately 67 lbs. of iron. EMISSION COLLECTION EQUIPMENT Fig. 2: Tapping of Grey Iron into Ladle with Inoculation The emission collection hood and emission probe sampling system was manufactured by the university for the emission testing (Figure 3). Emission sampling was completed in accordance with EPA methods 1, 2, and 3. Fig. 3: Emission Collection Hood

5 EMISSION COLLECTION Carbon monoxide (CO) and carbon dioxide (CO 2 ) emissions were sampled at pouring, cooling, and shakeout on the first five cycles for each green sand mixture (heats #1 through #5 of each green sand mixture). The first five cycles were determined to be the minimum number of cycles needed to normalize and condition each green sand system and coat the sand grains a minimum of 85%. The CO, CO 2, HAPs, and VOCs, emissions were then collected for the next two cycles (heats #6 and #7) at pouring, cooling, and shakeout. The HAP and VOC analysis included 63 compounds that are considered hazardous resulting from the decomposition of foundry materials. After collection, the samples were sent to an independent environmental laboratory for analysis. CO and CO 2 were analyzed using a modified ASTM D-1946 method. HAP and VOC analysis were conducted using a modified TO-15 method. Baseline samples were taken prior to the actual emission collection runs to check for background compounds that might be obtained from the ambient atmosphere. No molding, core making, or pouring was allowed two days prior to emission testing to minimize background emissions. For each emission collection test, the mold was first placed on the shakeout table in the collector hood. The iron was then poured into the mold under the hood, while the emission collection was then initiated. The hood to the emission collector was closed, and the pouring emissions sample was collected over 5 minutes, starting 30 seconds before the iron was poured. Immediately after the collection of the first sample, the mold was allowed to cool for 60 minutes while the cooling emission sample was collected. The third sample was taken during the shakeout of the mold in the collection hood for 5 minutes, with emissions being collected for the entire period. A summary of the emission collection times is shown in Table 3. The spent sand was then conveyed back to the Muller for use in the next cycle. After return sand from shakeout was added to the Muller, enough water was added to keep the sand from segregating while mulling. Emission Collection During Casting Process Minutes of Emission Collection Pouring of Casting 0 5 Cooling of Casting 5 65 Shake Out of Casting Table 3: Emission Collection GREEN SAND TESTING CRITERIA Samples of green sand were collected at the molding station during the production of the molds used for emission testing. To ensure consistency between green sand mixtures, the physical properties, including Methylene Blue clay and compactability, were held constant throughout all the testing. Complete sand testing included Methylene Blue clay testing (both on the green sand and the shakeout sand), compactability (both at the molding station and on the system sand sample taken), green compression strength, wet tensile strength, permeability, moisture, sample weight, dry compression strength, volatile combustible material (VCM), and loss on ignition (LOI). The physical properties of each green sand mixture are shown in Tables 4 through 7. RESULTS AND DISCUSION Figure 4 shows that the total substitution of seacoal with causticized lignite reduced the overall mold HAP and VOC emissions by more than 45%. This reduction in emissions was also observed by reduced smoke emitted from the molds after pouring. Partial seacoal replacements with caustisized lignite were also effective in reducing the mold gas emissions. Figure 5 shows the total benzene emissions from the testing. Benzene comprised approximately 40% of the gas emissions from all the test molds as shown in figure 7 and 8. The benzene emissions were reduced by over 42% with the total substitution of seacoal by the causticized lignite. As we can see from Figure 5 the majority of the benzene is emitted during the shakeout of the molds. It is generally thought that benzene is produced at the mold/metal interface where temperatures

6 exceed 900 F (482 C) and trapped in the mold during cooling. The benzene is then released during shakeout after the mold loses integrity. Toluene emissions comprised approximately 25% of the total mold gas emissions. The toluene emissions were reduced by over 54% with the total substitution of seacoal by the causticized lignite as shown by Figure 6. The toluene emission profile was similar to benzene in that the greatest amounts were released during the shakeout of the molds. Benzene and toluene emissions were generally higher during the shakeout of the molds. This was true throughout all of the test mixtures. Figures 7 and 8 illustrate the breakdown of compounds found in the emissions from 100% seacoal and 100% causticized lignite green sand test mixtures. The charts show the percentage of each of the compounds found in the total analysis of the green sand mixtures. It was noted that both carbon disulphide and 2-propanol were observed in the emissions from the causticized lignite green sand mixes. These two compounds were not detected in the 100% seacoal mixtures. Also noted was the decrease in the proportion of acetone found in the 100% causticized lignite mixes when compared to the 100% seacoal mixtures. Figure 9 shows a comparison of mold gas emissions from three green sand mixtures. The first is a mixture that contained 100% of the carbonaceous additive in the form of causticized lignite. The second is the mixture that contained 100% of the carbonaceous additive in the form of seacoal. Although not the baseline for the test series, the third mixture contained no carbonaceous additive. As the chart shows, the mixture that contained no carbonaceous additives still resulted in the emission of HAPs and VOCs. This was thought to be the contribution of the casting process to the HAP and VOC emissions. This might be considered as the lowest possible emission level one could obtain with the use of green sands without changes to the casting process. Figure 10 and 11 illustrate the effect of caustisized lignite additions on CO/CO 2 over the series of the tests. The amount of CO/CO 2 emissions for the mold does not appear to be related to the amount of caustisized lignite substituted for seacoal in the sand system. Samples for the baseline CO/CO 2 emissions were taken after a heat had been melted and the ladle held in front of the emission cabinet to simulate pouring of the test molds. The amount of CO/CO 2 observed in the baseline analysis was approximately one third (1/3) of the total observed in the pouring, cooling and shakeout of the actual test castings analyses. Although it is felt that lower carbonaceous material addition levels will lead to lower CO/CO 2 levels, it was not shown to be true in this test series. This may be a result of the combination of an abundance of carbonaceous material in relation to the oxygen content of the mold atmosphere and the amount of CO/CO 2 emitted from the molten iron in the pouring ladle. Figure 12 and 13 show the amount of volatile and combustible material in the sand mixes used for the testing. The mixtures containing the caustisized lignite material show a slight decrease in the amount of total combustible material. The sand mixture containing only caustisized lignite required approximately 18% less bentonite to reach the same M.B. clay level as the mixtures containing only seacoal. The lower clay additions resulted in lower carbonaceous material additions. The mixes containing caustisized lignite additions had volatile contents approximately 20% lower than the 100% seacoal levels which matched the reduced bond requirements. It should be noted that although a lower percentage of combustible material was present, the castings produced using the caustisized lignite material demonstrated a similar peel or release of adhering sand after cooling and shakeout when compared to the 100% seacoal mixes. This has been previously supported by other researchers investigating causticized lignite as a seacoal replacement. 8 CONCLUSIONS AND RECOMENDATIONS Based on the results of the present study the following conclusions can be made: 1. The total amount of HAPs and VOCs emitted from green sand molds during pouring, cooling, and shakeout can be reduced substantially by substituting causticized lignite for seacoal. The research work also illustrated blending of causticized lignite with seacoal has a beneficial effect in reducing HAPs and VOCs. 2. Benzene and toluene emissions from foundry green sand molds can be reduced by substituting causticized lignite for seacoal. The extent of substitution of caustisized lignite for seacoal will be dependent on individual foundry operations. 3. The research demonstrated that causticized lignite can be substituted for seacoal in green sand molding of gray iron castings in a limited production operation.

7 The authors recommend continued research to assess the potential for the total substitution of causticized lignite for seacoal. Research issues that should be assessed are as follows: 1. Presently, no information exists in literature supporting the use of 100% causticized lignite green sand systems. Preliminary data presented in the paper demonstrates the possibility of a total causticized lignite system and has shown beneficial advantages in reducing emissions. Continued research is recommended to assess the total performance and durability of a 100% causticized lignite system for use in a production environment. 2. With total replacement of seacoal with caustisized lignite, an 18% reduction in the bentonite addition requirement was observed in order to maintain the system at a minimum of 8% Methylene Blue (M.B.) Clay content. The amount of clay added to the system was decreased to account for the increase in M.B. clay observed in the tests. Although no specific relationship was developed, the green strengths observed during the cycle 3 tests seem to support the M.B. test results. Continued research is recommended in this area to further quantify the reduction in bentonite obtained with the use of caustisized lignite as a seacoal replacement and to determine the extent and reason for this effect. 3. The M.B. test procedures used were based on standard AFS sand testing guidelines and are valid for typical green sand system mixtures. Specialized guidelines or testing procedures do not exist for the M.B. clay determination accounting for the possible influence of additives, water hardness, ozone, and shakeout residues on the starting ion exchange potential of the test. The researchers recommend investigation into the possible influence of new additives and system technologies on the standard M.B. clay testing methodology. ACKNOWLEDGEMENTS The authors would like to thank the American Colloid Company and the University of Northern Iowa Metal Casting Center for their invaluable assistance, support, and materials throughout the testing. We would also like to thank the staff at Air Toxics Ltd. for their assistance in sampling and analysis techniques.

8 Cycle No Compactability at the Molding Station (%) System Sand Sample Compactability (%) Green Compression Strength (psi) Wet Tensile Strength (N/cm^2, 6 sec, 310C) Permeability Moisture (%, 110C) x2 AFS Sample Weight (g) Dry Compression Strength (psi) Volatile Combustible Matter (%, VCM) Loss on Ignition (%, LOI) System Sand Active Clay (MB, %) Shakeout Sand Active Clay (MB, %) Table 4: Cycle 1: 20% Seacoal (based on dry clay addition); No Causticized Lignite Cycle No Compactability at the Molding Station (%) System Sand Sample Compactability (%) Green Compression Strength (psi) Wet Tensile Strength (N/cm^2, 6 sec, 310C) Permeability Moisture (%, 110C) x2 AFS Sample Weight (g) Dry Compression Strength (psi) Volatile Combustible Matter (%, VCM) Loss on Ignition (%, LOI) System Sand Active Clay (MB, %) Shakeout Sand Active Clay (MB, %) Table 5: Cycle 2: 15% Seacoal (based on dry clay addition); 5% Causticized Lignite (based on dry clay addition)

9 Cycle No Compactability at the Molding Station (%) System Sand Sample Compactability (%) Green Compression Strength (psi) Wet Tensile Strength (N/cm^2, 6 sec, 310C) Permeability Moisture (%, 110C) x2 AFS Sample Weight (g) Dry Compression Strength (psi) Volatile Combustible Matter (%, VCM) Loss on Ignition (%, LOI) System Sand Active Clay (MB, %) Shakeout Sand Active Clay (MB, %) #N/A Table 6: Cycle 3: No Seacoal; 20% Causticized Lignite (based on dry clay addition) Cycle No Compactability at the Molding Station (%) System Sand Sample Compactability (%) Green Compression Strength (psi) Wet Tensile Strength (N/cm^2, 6 sec, 310C) Permeability Moisture (%, 110C) x2 AFS Sample Weight (g) Dry Compression Strength (psi) Volatile Combustible Matter (%, VCM) Loss on Ignition (%, LOI) System Sand Active Clay (MB, %) Shakeout Sand Active Clay (MB, %) Table 7: Cycle 4: 10% Seacoal (based on dry clay addition); 10% Causticized Lignite (based on dry clay addition)

10 HAPs and VOCs (micrograms/m^3) Percentage of Causticized Lignite in System Sand Fig. 4: Reduction of HAP and VOC Emissions as a Function of the Seacoal: Causticized Lignite Ratio Benzene (micrograms/meter^3) During Shake-out During Cooling During Pouring :0 75:25 50:50 0:100 Seacoal : Causticized Lignite Ratio Fig. 5: Total Benzene Emissions as a Function of the Casting Process (µg/m 3 )

11 Toluene (micrograms/meter^3) During Shake-out During Cooling During Pouring :0 75:25 50:50 0:100 Seacoal : Causticized Lignite Ratio Fig. 6: Total Toluene Emissions as a Function of the Casting Process (µg/m 3 ) Hexane 3% Heptane 2% Acetone 9% Benzene Toluene 1,3-Butadiene 8% o-xylene 2% m,p-xylene 6% Ethyl Benzene 2% Benzene 39% Ethyl Benzene m,p-xylene o-xylene 1,3-Butadiene Hexane Heptane Toluene 29% Acetone Fig. 7: Proportions of Emissions from 100% Seacoal Green Sand Mixes

12 Carbon Disulfide 17% Acetone 2% 2-Propanol 2% Benzene Toluene Ethyl Benzene Heptane 1% Hexane 4% 1,3-Butadiene 2% o-xylene 2% Benzene 40% m,p-xylene o-xylene 1,3-Butadiene Hexane Heptane Acetone m,p-xylene 5% Ethyl Benzene 1% Toluene 24% Carbon Disulfide 2-Propanol Fig. 8: Proportions of Emissions from 100% Causticized Lignite Green Sand Mixes Carbon Disulfide Acetone Heptane HAP and VOC Compounds Hexane 1,3-Butadiene 1,2,4-Trimethylbenzene o-xylene m,p-xylene 100% Causticized Lignite 100% Seacoal No Carbonaceous Additive Ethyl Benzene Toluene Benzene Concentration (micrograms/m^3) Fig. 9: Comparison of emissions from Green Sand test Mixes (µg/m 3 )

13 CO Emissions (%) % Seacoal 75% Seacoal, 25% Causticized Lignite 50% Seacoal, 50% Causticized Lignite 100% Causticized Lignite Metal Heat Fig. 10: Carbon Monoxide (CO) Measurement Through All Cycles CO2 Emissions (%) Metal Heat 100% Seacoal 75% Seacoal, 25% Causticized Lignite 50% Seacoal, 50% Causticized Lignite 100% Causticized Lignite Fig. 11: Carbon Dioxide (CO 2 ) Measurement Through All Cycles

14 Percent Volatile Combustible Material (%, VCM) Metal Heat 100% Seacoal) 25% Causticized Lignite, 75% Seacoal 50% Causticized Lignite, 50% Seacoal 100% Causticized Lignite, No Seacoal Fig. 12: Volatile Combustible Material (VCM) Results of Lab Testing % Seacoal Percent Loss on Ignition (%, LOI) % Causticized Lignite, 75% Seacoal 50% Causticized Lignite, 50% Seacoal % Causticized Lignite Metal Heat Fig. 13: Loss on Ignition (LOI) Results of Lab Testing

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