Environ. Sci. Technol. 2006, 40, 3095-3101
Effects of Advanced Oxidation on Green Sand Properties via Iron Casting into Green Sand Molds YUJUE WANG, FRED S. CANNON,* ROBERT C. VOIGT, AND SRIDHAR KOMARNENI The Pennsylvania State University, University Park, Pennsylvania 16802 J. C. FURNESS Furness-Newburge, Inc., Versailles, Kentucky 40383
The effects of advanced oxidation (AO) processing on the properties of green sand were studied via pouring cast iron into green sand molds. Upon cooling, the green sand molds were autopsied at various distances from the metalsand interface. Autopsy green sand samples collected from a mold that incorporated AO water were characterized and compared to controlled samples collected from a similar autopsied mold made with conventional tap water (TAP). It was found that the AO processing removed a coating of coal pyrolysis products from the clay surface that typically accumulated on the clay surface. As a result, the AO-conditioned green sand retained 10-15% more active clay as measured by the standard ultrasonic methylene blue titration than did the TAP-conditioned green sand. The AO processing also nearly doubled the generation of activated carbon from the normalized amount of coal composition of the green sand during the casting process. The AO-enhanced activated carbon generation and the AOincurred clay surface cleaning provided the AO-conditioned green sand with higher normalized pore volume, and thus higher normalized m-xylene adsorption capacity, i.e., relative to before-metal-pouring conditions. Furthermore, mathematical analysis indicated that the AO-conditioned green sand better retained its important properties after pouring than did the TAP-conditioned green sand. Effectively, this meant after metal pouring, the AO-conditioned sample offered about the same net properties as the TAPconditioned sample, even though the AO-conditioned sample contained less clay and coal before metal pouring. These results conformed to the full-scale foundry empirical finding that when AO is used, foundries need less makeup clay and coal addition through each casting cycle, and they release less air emissions.
Introduction Solid waste and air pollutant emissions are the two main pollution sources from green sand foundries. As a response to the ever more stringent environmental regulations, foundries have been seeking new production process modifications that will reduce their pollutants while also diminishing operation costs. The recent development of advanced oxidation (AO) technologies offers these foundries great * Corresponding author e-mail:
[email protected]. 10.1021/es060013y CCC: $33.50 Published on Web 03/30/2006
2006 American Chemical Society
opportunity to achieve both pollution prevention and material conservation. An AO system (Sonoperoxone by Furness-Newburge) (see Supporting Information) has been incorporated into the production process in several foundries; it has achieved encouraging results by reducing air pollutant emission by 20-75% (1-4). At the same time, it diminishes clay and coal consumption by 20-35% in these foundries (1, 5-8). This in turn has decreased the amount of solid waste that needs to be landfilled. The AO systems have been operating well in full-scale foundries for several years; and the authors have sought to better understand the fundamental mechanisms by which the AO process diminished the air pollutants and raw material consumptions. In a previous study, we employed a thermogravimetric analyzer (TGA) to simulate thermal conditions that green sand experiences during iron casting. Tests showed that AOwater-wetted green sand releases 15-20% less mass and volatile organic compound (VOC) emissions than does TAPwater-wetted green sand (7). We also found that when starting from the same green sand source, AO-conditioned green sand possesses higher pore volume, and thus higher VOC adsorption capacity, than does TAP-conditioned green sand after multiple casting cycles (7). All of these results conformed with full-scale foundry experience, wherein AO has decreased air emissions (1-3). However, the reasons for the different pore volume of AO- and TAP-conditioned green sand were not identified in this previous TGA test. Others have established preoxidation as a helpful step for producing activated carbon from bituminous coal (9-11). Based on that, we hypothesized that the coal composition of the green sand would be more activated during the casting process when the green sand was AO conditioned. We were particularly interested in how AO processing affects the degradation of green sand when the green sand is exposed to the intense and abrupt molten iron heat during the casting process. This phenomenon could not be simulated by the slow TGA heating (7). Full-scale foundries have repeatedly observed that when AO is employed, the green sand molding and casting performance can be maintained with less clay and coal. Moreover, the AO-conditioned green sand systems have required less clay and coal makeup addition after each casting cycle (5, 6). We hypothesized that this was partly because AO processing caused less degradation of green sand under the intense and abrupt temperatures that are imparted by iron pouring. In light of the slow heating limitations of TGA, the authors sought to discern AO effects within a green sand mold while the green sand experienced the temperature of actual metal pouring. To this end, metal pours were conducted at a pilotscale foundry at Penn State. Specifically, the objectives of this study were to: 1. Identify the reasons why AO-conditioned green sand possesses higher normalized pore volume than TAPconditioned green sand by monitoring how green sand properties change as a function of distance from the metalsand interface. 2. Study the role of AO conditioning in the degradation of green sand during the casting process by comparing AO conditioning of an AO green sand through yet one more cycle of moisturizing, molding, and metal pouring, versus TAP water conditioning of a TAP green sand through yet one more cycle. VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Properties of the TAP and AO As-Received and As-Modified by Thermal Treatment Following Iron Pouring in Mold 0-8 cm composite after poura
as-received property
TAP
AO
loss on ignition, 1800 °F (%) AFS ultrasonic MB clay (%) boiling, ultrasonic MB clay (%) pH-dependent charge (meq/g) pore volume < 500 Å (µL/g) BET surface area (m2/g) m-xylene sorbed at P/Po ) 0.52 (mg/g green sand)
4.13 7.40 8.84 0.065 3.53 4.51 3.05
3.42 5.89 7.58 0.076 2.71 1.72 1.84
∆
%b
-17.2 -20.4 -14.3 -14.5 -23.2 -61.9 -39.7
TAP
AO
3.86 6.14 7.96 0.077 2.56 2.33 1.79
3.28 5.48 6.83 0.077 2.17 1.85 1.33
∆
%b
-15.0 -10.7 -14.2 0 -15.2 -20.6 -25.7
0-6 cm composite after poura TAP
AO
∆ %b
3.85 5.51 7.34 0.075 1.57 1.05 1.05
3.19 5.02 6.30 0.075 1.90 1.26 0.91
-17.1 -8.9 -14.2 0 +21.0 +20.0 -13.3
a Composite values mathematically derived from increments normalized by the fraction of the mold the increments represent. b ∆ % ) 100 × (AO - TAP)/TAP.
Materials and Methods As-Received Green Sand Samples. Two green sands were sampled from the Neenah Foundry (Neenah, WI), which processes cast iron. The first green sand sample (termed herein as TAP as-received) originated from a production line that does not employ advanced oxidation, whereas the second green sand sample (termed herein as AO as-received) originated from a production line that does employ advanced oxidation. Before the sampling, the TAP and AO as-received green sands had experienced multiple casting cycles at the foundry that employed TAP or AO processing, respectively. Green sand additions in the TAP and AO production lines are presented in Table S1 (see Supporting Information). Sodium-bentonite clay was used as binding material in both production lines. TAP and AO Water. The TAP water used in the test was the Pennsylvania State University tap water that has a nearly neutral pH. The hardness and alkalinity were low and the iron concentration was about 0.01-0.03 ppm (12). The AO water was generated from a pilot Sonoperoxone machine that employed hydrogen peroxide (200 ppm), ozone (to saturation), ultrasonic (20 kHz), and underwater plasma to condition the TAP water for 5 min. After the treatment, the AO water contained various advanced oxidants, e.g., approximately 170 ppm hydrogen peroxide, 0.1 ppm ozone, 0.3 ppm hydroxyl radicals, and other radicals (12). Metal Pouring and Autopsy Sample Collection. The metal pours were conducted in a pilot-scale foundry at Penn State. In one set of the pours, the TAP as-received green sand was used to make the mold; while in the other set of pours the AO as-received green sand was used. Moisture content in the molds was set at 3.5% for both sets of pours: for the molds that were made from the TAP as-received green sand, the TAP water was used, while for the molds that were made from the AO as-received green sand, the AO water was used. After the moisture was added, the green sand was mulled in a lab muller for 3 min, then the molds were prepared by a jolt/squeeze-molding machine. The casting was a 7.5 kg gray iron plate (22.9 cm × 17.8 cm × 1.9 cm). The green sand mold (35.6 cm × 30.5 cm × 20.3 cm) included a top and bottom part that together weighed 29.5 kg. The metal pouring was conducted at 1440 °C and the pouring took about 10 s. A 3 cm diameter sprue hole allowed entry of molten metal down into the mold cavity. After the mold cooled for several hours, green sand samples at an array of distances (0-8 cm) from the metal-sand interface in the molds were autopsied. All samples came from the bottom side of the mold, where there was no sprue hole. The samples were termed herein according to the raw green sand type (and water type) and its distance from the metal-sand interface, e.g., AO 0-1 cm means that the sample was collected from the 0-1 cm zone from the metal-sand interface in the mold that was made from the AO as-received green sand and wetted with AO water. 3096
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Sample Characterizations. The green sand samples were characterized by X-ray diffraction, loss on ignition, methylene blue titration, pH-dependent charge titration, pore structure analysis, and m-xylene adsorption test. We then compared these properties to discern whether the relative changes conformed to the full-scale empirical finding, namely that when AO is used foundries need less clay and coal makeup addition through each casting cycle, and they release less air emissions. Loss on ignition (LOI) of the green sand samples was measured by placing a dried green sand sample in a furnace with air exposure at 982 °C (1800 °F) for 2 h and measuring the percent mass loss (13). Methylene blue (MB) titration was conducted to determine the active clay contents in the green sand, where “active clay” is defined as clay that can sorb MB and can thus participate in bonding the green sand mold together. Two MB titration protocols were used herein. The first employed the standard American Foundry Society (AFS) ultrasonic method (13). Five grams of a dried (110 °C) green sand was mixed with 50 mL of 2% tetrasodium pyrophosphate (Na4P2O7) solution, then the mixture was ultrasonically scrubbed for 7 min, followed by the MB titration. The second method employed the protocol presented by Odom (14): the mixture of green sand (5 g) and 2% Na4P2O7 (50 mL) was gently boiled for 10 min before the 7 min ultrasonic scrubbing and MB titration. The two protocols have been referred to herein as the standard AFS ultrasonictitration method and the boiling-ultrasonic-titration method, respectively. pH-dependent charge of the green sand samples was measured with classic potentiometric titration (15) (see Supporting Information). Pore structure of the green sand samples were analyzed by argon adsorption, following the protocol of Moore (16) (see Supporting Information). The VOC adsorption capacities were gravimetrically measured by m-xylene adsorption in a TGA at 25 °C, following the procedures of Paulsen (17) (see Supporting Information).
Results and Discussion As-Received Samples. The properties of the TAP and AO as-received green sand samples are presented in Table 1. The TAP as-received green sand had higher loss on ignition (LOI) and methylene blue (MB) clay than did the AO asreceived green sand. The reason for the differences was mainly due to the fact that foundries have found that when they employ AO, they can use less coal in the green sand while still getting a desirable casting surface (2, 3); also, they can lower the clay content in the green sand while still maintaining the necessary green compressive strength of the mold (5, 6). This has diminished the consumption of coal and clay, while also diminishing air emissions from coal pyrolysis during the casting process (2-8).
TABLE 2. Temperature Regimes and Possible Coal and Clay Reactions in the Sampling Zones distance (cm) 0-1
temp. (°C)
fraction of mold (%)
1440-800
5.0
possible coal and clay reactions/alteration (14, 26-29) coal clay
1-2
800-500
6.8
coal clay
2-4
500-250
19.5
coal clay
4-6
250-150
28.8
coal
6-8
150-80
39.9
clay coal clay
For as-received samples, MB clay measured by the boilingultrasonic-titration method was 20-30% higher than the value measured by the ultrasonic-titration method for a given green sand sample. An increase, according to Odom (14), would be mainly due to the further dispersion of clay platelets during the boiling. However, the boiling-incurred increase was usually less than 10% when virgin clean sodiumbentonite clay was used in others’ tests (14, 18). To account for the 20-30% increase observed herein, we further hypothesized that the boiling increased the MB clay via a surface cleaning mechanism. Our previous study has shown that when coal and core binder undergo pyrolysis reaction during the casting process, their pyrolysis products, e.g., tar and condensed volatiles, could form a carbon coating on the clay surface that diminish the clay’s MB adsorption capacity; and AO water washing could remove the coating and restore the clay’s MB adsorption capacity (8). Similarly, we anticipated that boiling could also remove a portion of the carbon coating from the clay surface (19, 20). Thus, the portion of active clay that was masked by the carbon coating during the ultrasonic-titration test could be exposed during the boilingultrasonic-titration test. Mold Heating. Following metal pouring, the peak temperatures that the green sand experienced as a function of the distance from the metal-sand interface in the mold were simulated using computer software (21) (see Supporting Information Figure S1). The peak temperatures reached at the various distances decreased approximately exponentially as the distance increased (Table 2). Besides the peak temperatures, the temperature ramping rates at the different locations varied dramatically, from over thousands °C/s at the metal-sand interface to several °C/min at the outer part of the mold (22-24). The huge differences in the heating conditions could profoundly affect green sand properties. Silica sand, with its fusion point at 1700 °C, will not appreciably change its properties during this heating (25). In contrast, coal and clay compositions could undergo several different transformations that are dictated by their proximity to the metal-sand interface (see Table 2) (14, 26-29). Autopsied Green Sand Following Metal Pouring. The X-ray diffraction patterns of the clay contents in the green sand samples showed no apparent difference between AO and TAP samples (see Supporting Information Figure S2).
combustion with residual oxygen; complete devolatilzation to form coke (above 1100 °C); pyrolysis to form activated carbon (800-1100 °C). melting (above 1000 °C); recrystallization; irreversible loss of structural -OH (dehydroxylation) and becoming dead clay. pyrolysis to release VOCs; conversion to activated carbon (700-800 °C); substantial breakdown of the coal structure. recrystallization at 600-800 °C; Na-bentonite dehydroxylation above 600 °C. pyrolysis to release VOCs; breakdown of the coal structure above 350-400 °C; rearrangement of the coal structure at 250-350 °C. Ca-bentonite dehydroxylation above 400 °C; dehydration below 400 °C. slight rearrangement of the coal structure; Activated coal sorbs VOCs dehydration; clay sorbs VOCs slight rearrangement of the coal structure; Activated coal sorbs VOCs dehydration; Clay sorbs VOCs
The loss on ignition (LOI) and methylene blue (MB) clay contents measured by the two titration protocols are depicted in Figures 1 and 2, respectively. The pH-dependent charge titration result is shown in Supporting Information Figure S3. To account for the changes of these properties relative to their original counterparts, the results were normalized relative to the values of the TAP as-received for all the TAP autopsy samples, and to the values of AO as-received for all the AO autopsy samples. The LOI measured after the metal pouring reflected the burnable fraction of the green sand (mostly coal, plus a small fraction for phenolic urethane and hydration waters of the clay) that had not yet been driven off the green sand during the casting process. We herein propose that the LOI could be taken as a rough estimate for the thermal degradation extent of the green sand: foundry personnel have found that they must maintain a certain LOI level by adding makeup coal (and clay) after each casting cycle in order to maintain casting performance. As shown in Figure 1, no statistically different trends in the LOI between the AO versus TAP samples were detected. Thus, AO processing did not affect the green sand thermal degradation when compared to TAP processing. The MB clay measured by the standard AFS ultrasonictitration method is shown in Figure 2a. Generally, as the peak temperatures increased with closer proximity to the metal-sand interface, the remaining active clay decreased.
FIGURE 1. Loss on ignition of the AO and TAP autopsy green sand samples. All values are normalized relative to as-received counterpart (see Table 1). VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Methylene blue clay in the green sand samples measured by (a) the AFS standard ultrasonic-titration method, and (b) the boiling-ultrasonic-titration method. All values are normalized relative to as-received counterpart value measured by the same method (see Table 1).
FIGURE 3. Differential pore volume distribution of sea coal, bentonite, and green sand. Also bituminous-coal-based activated carbon (right y-axis). Significantly, AO processing maintained active clay better than TAP processing at every distance interval. These results are consistent with full-scale foundry experience, where it has been shown that AO processing helps to maintain the standard MB clay value while adding less makeup clay (5, 6). We also monitored MB sorption after the green sands had been boiled in water for 10 min before employing the ultrasonic-titration method. This boiling could remove some fraction of the carbonaceous coating (19, 20). As shown in Figure 2b, at every distance interval, the normalized boiledMB values were the same for the AO samples as for the TAP samples, with the exception of 0-1 cm where activated carbon created from coal sorbed MB (30) (see Supporting Information). Because the boiling eliminated the influences of the carbon coating on the MB adsorption capacities of the clay, we proposed that this boiling step allowed us to measure the actual amount of the clay that had not been thermally degraded (dehydroxylated) to “dead clay” in the green sand after the casting process; and this method reflected the real thermal degradation rates of the clay. In comparison, the ultrasonic-titration method (without boiling) measured the amount of the clay that could behave like the virgin clean active clay (i.e., adsorb MB, participate in bonding, etc.) without being fettered by a carbonaceous “raincoat”; and this method reflected the apparent degradation rates of the clay in a realistic foundry situation. 3098
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Thus, it could be seen that the actual thermal degradation (dehydroxylation) of the clay in the TAP and AO green sand molds was the same (Figure 2b), but the clay in the AO autopsy green sand samples was much cleaner (i.e., without carbon coating) than that in the TAP autopsy samples (Figure 2a). As green sand is recycled through multiple times of iron casting and reuse in foundries, each cycle’s surface cleaning by AO helps the clay to maintain its properties, whereas when TAP water is used, the clay becomes yet further contaminated by the coal pyrolysis products after each casting cycle, and loses its foundry functions quickly (refer to Wang et al., 8). The higher apparent degradation rates resulted in the higher makeup clay requirement (as well as coal, which is usually added proportionally with clay in foundries) when TAP water was used. The AO processing achieves raw material conservation by preserving their properties better. The pH-dependent charge titration results generally followed the same trend as the MB curve in Figure 2a (see Supporting Information Figure S3), and further confirmed the role of AO processing in cleaning carbon coating from the clay particles. Figure 3 shows the differential pore volume distributions of green sand (TAP as-received), and its component materials: bituminous coal (unprocessed), bentonite clay (400 °C calcined for 30 min), and bituminous coal-based activated carbon. We also monitored the pores of silica sand, but found no pores in the testing range for this material. It was found
FIGURE 4. Differential pore volume distribution of the AO and TAP green sand samples (4-100 Å).
TABLE 3. Cumulative Pore Volume of the Green Sand Samples in the Different Pore-Width Ranges small micropores (w
