In 2012, gray and ductile iron castings represented 68% of the total casting tonnage in the U.S., the world’s third largest producer of gray iron castings and second largest of ductile iron. The American metalcasting industry faces worldwide competition for low cost, high-grade iron and steel scrap amid ever tightening air quality standards. Technological innovation involving raw materials is essential, because they represent one of the largest production costs for many facilities.
Additionally, while conventional end-of-pipe emission controls can add substantially to capital investment and operating costs, innovations aimed at emission sources can reduce costs. These advancements also may reduce emissions of volatile organic compounds (VOCs) and enable compliance with increasingly stringent U.S. Environmental Protection Agency standards.
A team of university researchers, development entrepreneurs and metalcasting personnel has been pursuing sustainable technologies for the metalcasting industry, with a particular focus on cast iron. The researchers have developed three major innovations:
1. The replacement of phenolic urethane core binders with binders composed of hydrolyzed collagen, alkali silicates and other additives can cut VOC pollution by two-thirds.
2. The replacement of conventional metallurgical coke in cupolas with anthracite fines formed into bricks with hydrolyzed collagen, lignin and other additives burn as fast as coke while producing 35-40% more energy per volume.
3. An application of advanced oxidants and hydroacoustic cavitation to spent green sand and baghouse dust restores the binding activity of the clay and cleans the sand. This process reduces costs by decreasing clay, coal and sand consumption, while also diminishing air pollution from VOCs and hazardous air pollutants.
This study evaluates the flow of energy and materials in cast iron facilities to better understand the costs and environmental benefits of sustainable technologies currently under deployment in the industry and those nearing commercialization. The potential innovations involve advanced oxidation systems for recycling baghouse dust and green sand, binder technologies that reduce VOCs and replace coke, and binder systems developed from waste products in non-metalcasting processes to replace materials that produce large amounts of VOCs.
A Basis for Comparison
A baseline model is necessary to measure the use of resources, emission profiles and costs related to existing and prospective casting technologies. This control model is calibrated for an actual cupola-based iron casting facility in Wisconsin that uses metallurgical coke, conventional green sand molding, no recycling of baghouse dust or sand, and no innovations for releasing sands or clays from their carbon coating. This baseline also uses phenolic urethane cores cured with amine gas. The operation was used to estimate the impact of the various sustainable production technologies.
The life cycle boundaries include major upstream activities that supply inputs to iron casting facilities. The model accounts for energy, materials, environmental emissions, operating costs and capital costs. These metrics are tracked at each intermediate stage of production, including hot metal making, coremaking, green sand molding, pouring, cooling and shakeout. Downstream recycling of iron products and their recovery were not considered because the innovative technologies would not affect these processes.
Figure 1 illustrates the baseline input-output boundaries for the analysis. The major upstream activities-power production, coke making and sand mining-provide inputs of electricity, coke and sand, generate emissions and require fuel and raw materials. Cupola melting and the balance of foundry activities are the two major functional units on the right of Figure 1. These activities require melting inputs, such as ferrous metal alloys, scrap metal and natural gas. They also require process inputs, such as parts and supplies. This process chain produces finished iron castings and potential environmental discharges.
Core Changes: Collagen-Alkali Silicate Binders
Conventional metalcasting facilities commonly create cores via a coldbox process that uses a phenolic urethane binder cured with an amine gas that creates air pollution. Moreover, when subjected to molten metal in the mold, these core binders pyrolyze and release VOCs and hazardous air pollutants. Conventional core binders are the predominant source of VOCs and hazardous air pollutants from iron casting facilities.
Recent bench-scale and pilot-scale trials have shown a hybrid of collagen and alkali silicates will create a core binder that emits far less VOCs and hazardous air pollutants than phenolic urethane binders, while retaining the comparable strength and thermal resistance of conventional phenolic urethane. During the curing of the collagen-silicate hybrid, a proprietary system for heat-curing blown sand-binder systems warms the corebox by heating it with air, moisture or carbon dioxide under a vacuum. The system requires a modified core machine and proprietary instrumentation, but it does not require an amine scrubber and does not produce scrubber brine. The only emission from the core shop is water. Based on limited partner foundry experience, this coremaking process is more efficient, with lower cycle time, less corebox cleaning and less core scrap than conventional coremaking.
Recent studies of low-emission hybrid core binders are awaiting publication so this data is offered only as potential improvement, but collagen-alkali silicate-based binders for green sand molds can potentially reduce total operating costs by 1.3% and emissions of VOCs by 35% (Table 1).
Because cupolas are charged with limestone, they can accommodate scrap iron and carbon sources with higher proportions of impurities. Batch electric induction furnaces use electricity for melting and require more expensive (i.e., purer) iron and carbon sources for alloying. Overall, an electric induction furnace requires 207% more fossil energy and 207% more non-fossil energy to melt iron (see Table 2). This energy demand is profoundly higher because of the relative inefficiencies in transmitting electrical power and in converting heat energy to electrical energy and then back to heat energy. The induction furnace’s actual cost of energy, however, is only 9.7% higher than the cupola because the metallurgical coke in cupolas costs considerably more than electricity produced in coal-fired power plants.
Cupolas impact the local environment via air emissions, while electric induction furnaces generate fewer emissions. Several facilities have replaced cupolas with electric induction furnaces in response to local air quality rules that limit emissions, but this is not favorable in regard to overall life cycle. Replacing a cupola with an electric induction furnace merely transfers emissions upstream to the electricity-producing sector, as illustrated in Table 2. While emissions of particulates are lower for the electric induction furnace, life cycle emissions of criteria air pollutants increase 150% when electric induction furnaces are used. Greenhouse gas emissions are 58% higher, and VOCs are 88% higher. This is a classic example of how local air pollution standards can have indirect and deleterious effects on national emissions. Operating costs for batch induction furnaces increase 1.7% relative to the cupola furnace. (Note: These findings reflect Wisconsin’s electrical generation system and could differ in locations with lower power sector emissions and/or other non-fossil fuel energy sources.)
Emission Check: Replacing Coke With Anthracite Fines
To produce coke, bituminous coal must be heated to 1,652-1,832F (900-1,000C) for 28-30 hours, consuming 15-20% of the raw coal’s energy and releasing an equivalent amount of its carbon as greenhouse gases and VOCs. Technological advances may reduce or eliminate such emissions by either partially or completely replacing coke. A second proposed technology capitalizes on the exhausted cupola heat to yield a lignite-based activated carbon. This lignite can absorb VOC emissions, and the loaded lignite can be used in green sand molds in place of bituminous seacoal and as feedstock for a coke replacement.
The first of these options is to use waste anthracite fines formed into bricks to partially replace coke and ferrosilicon (see Figure 2). These bricks use binder materials made from collagen, silicon/silicate and other biomaterials to match the strength and energy value of coke. The anthracite fines and biomaterials used in these bricks have limited value as low-grade fuel or are otherwise waste. Also, the bricks include silicon, which is charged into the cupola to provide silicon to the cast iron and control the cupola’s redox level. These bricks include 85% anthracite fines, 10% biomaterials and 5% silicon/silicate. The binders become thermally conditioned within the cupola to provide strength from ambient temperature to iron melting temperature. These bricks have 35-40% higher BTU content per volume than coke and burn as quickly.
These formed anthracite bricks were used in a full-scale cupola facility in Pennsylvania. This trial employed four tons of bricks formed with biomaterials, with 25% substitution of the bricks for coke for a half day. The bricks remained intact during rough handling when charged into the cupola and were still intact as they descended to the tuyere windows, where temperatures reached 3,000F (1,550C). During this brick substitution, the total carbon charged into the furnace (i.e., carbon in the coke plus bricks) decreased 6%, while maintaining a constant melt temperature, more favorable CO/CO2 ratio and a favorable olive-green slag color that indicated suitably reduced conditions for metallic iron formation. The carbon content of the iron product remained constant, while the iron maintained acceptable levels of silicon, sulfur and other trace metals. The demonstrations maintained the iron product quality.
The more effective energy release could diminish natural gas requirements in metalcasting facilities that inject supplemental natural gas into the cupola. The anthracite bricks must be dried at 248F (120C), which requires considerably less energy than coking coke at 1,652-1,832F (900-1,000C) for 26-30 hours.
This study estimates the impacts of two variations of coke replacement: 20% and 50% replacement of coke with anthracite bricks. In light of the energy consumed when making coke, the coal-related life cycle energy potentially diminishes 0.6% when using 20% coke substitution and 1.5% when using 50% brick substitution (see Table 3). Criteria air emissions fell and emissions of greenhouse gases and VOCs decreased significantly.
Another potential method of replacing coke uses waste heat from the cupola to start pyrolysis of granular bituminous coal that can be made into briquettes using the same binder materials mentioned previously. The byproducts of this process could be sold and/or used as fuel, though direct and indirect labor is needed because the process takes place on site. Emission reductions are based on the reduced energy required to complete this process. VOCs are condensed as a byproduct or burned directly in the cupola. This process has been successfully demonstrated in a full-scale trial and also can produce material suitable to replace metallurgical coke.
The process generates potential energy savings of 3% (Table 3), not including the energy available in the sold byproducts, and greenhouse gases are reduced 2.8%. Criteria pollutant and VOC emissions also improve dramatically due to the closed nature of the process. Overall costs are reduced 20.6%, with the increase in labor costs offset by reduced material and energy costs. Income from the sale of the byproduct materials (shown in “other costs”) is substantial, making the net payback approximately two years.
Tune-Up: Advanced Oxidation With Hydroacoustics
A U.S.-based company has devised an advanced oxidation system that applies ozone, hydrogen peroxide and sonication to a water slurry known as “blackwater” to reprocess baghouse waste dust. The baghouse dust slurry is treated with advanced oxidation and replaces the conventional water source for the green sand molds. Research indicates the process, currently in use at 50 iron casting facilities, uses 27-60% less clay and coal, 20-37% less silica sand and produces 19-70% less VOCs during pouring, cooling and shakeout.
An upgrade of this process includes hydroacoustics, cavitation, recirculation and virtual cyclone (AO-HAC), which has been installed in 10 U.S. facilities. This process acoustically dislodges the hydrophobic carbonaceous coating that forms when volatiles are pyrolyzed from the coal and binders near molten iron. The VOCs then migrate into the cooler green sand mold and recondense. Advanced oxidation dislodges the condensed volatiles from the clay and sand surfaces, restores the hydrophilic binding propensity of the clay, and prevents them from re-volatilizing as VOCs during molding, pouring, cooling and shakeout.
In pilot-scale production, reclaimed sand was clean enough to use as core sand. When used in a full-scale demonstration, reclaimed sand cores produced castings of equal or better quality than when using cores from pristine commercial sand. The AO-HAC system can reclaim 85% of normally disposed green sand. Based on this performance data and the synergistic effects of AO-HAC relative to an advanced oxidation-blackwater clarifier system, the life cycle and cost comparisons in Table 4 show that the AO-HAC system, even when used for baghouse dust only, can significantly reduce VOC production.
The most significant cost, energy and material savings occur when metalcasting facilities adopt a combination of these innovative technologies. One such strategy combines 50% coke replacement with advanced oxidation-hydroacoustic cavitation and a collagen-alkali silicate binder for core production.
Some of these options are symbiotic. For example, when a water-based low-emission collagen-alkali silicate binder replaces a phenolic urethane binder, VOCs and hazardous air pollutants will be diminished during the binder’s first exposure to molten iron and during subsequent passes as the advanced oxidation process cleans the sand and clay grains. Additionally, the water-based system potentially improves the cleaning of the water-based binder residuals off core sand when compared to residual phenolic urethane binders off core sand in the green sand system at shakeout.
Table 5 shows the life cycle and cost analysis for these three technologies. This “Foundry of the Future” could decrease life cycle energy costs by 15%, new sand consumption by 85%, VOCs by 57%, home iron scrap by 9% and clay, coal and coke use by 50%. Overall, this could translate to a 6.6% reduction in total costs.
Advanced oxidation systems that recycle baghouse dust and sand offer clear cost savings and environmental benefits, primary reasons why 60 facilities in the U.S. have adopted some form of the technology. The operating costs savings from adopting these sustainable technologies are significant, and they yield fast payback periods of 0.2-1.4 years. Given dwindling supplies and higher prices for high-grade metallurgical grade coal for coke making, coke replacement could provide significant cost savings. ■
This article is based on a paper (14-062) that was presented at the 2014 AFS Metalcasting Congress in Schaumburg, Ill.