Final Report: Sustainable Utilization of Coal Combustion Byproducts through the Production of High Grade Minerals and Cement-less Green ConcreteEPA Grant Number: SU836028
Title: Sustainable Utilization of Coal Combustion Byproducts through the Production of High Grade Minerals and Cement-less Green Concrete
Investigators: Mohanty, Manoj K. , Akbari, Hamid , Bhusal, Sudha , Culberth, Nick , Heller, Tom , Jha, Pravin , Kolay, Prabir , Kumar, Sanjeev , Rahman, Mohammad Wahid , Rimmer, Sue , Shin, Sanguok , Wiltowski, Tomasz , Yang, Xinbo , Zhang, Baojie
Institution: Southern Illinois University - Carbondale
EPA Project Officer: Lank, Gregory
Project Period: August 15, 2011 through August 14, 2012
Project Amount: $14,841
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2011) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Built Environment , P3 Challenge Area - Materials & Chemicals , P3 Awards , Sustainability
Coal has been and will continue to be the primary source of electricity in the US and most other parts of the world for many decades to come because of its abundant availability. Partly, because of coal's huge availability, coal-based electricity remains the lowest cost electricity with the natural gas based electricity the distant second. At the same time, coal is also referred as a "dirty fuel" because of the negative ways coal-based electricity generation process affects environment. Coal combustion process emits harmful gases like sulfur dioxide, various forms of nitrous oxides, mercury, fine particulate matters and many other hazardous air pollutants. However, over the years newer and better technologies have been developed to trap majority of these pollutants to an extremely low level before being emitted to the atmosphere. The major concerns against coal-based electricity generation process continue to be the carbon dioxide (known as a green house gas) emission and the solid residues resulting from the coal combustion process, which are referred as coal combustion byproducts (CCB).
More than 136 million tons of CCBs are being produced to meet the electricity needs of the US by combusting (burning) close to 1 billion tons of coal each year. Less than 45% of these CCBs is being successfully used in various applications and the remainder is being dumped as waste materials in the landfills and ash ponds at the utility sites. Many past studies indicate the presence of a variety of metal oxides, including iron oxide, titanium dioxide, aluminum oxide etc. in CCBs. Some of these metal oxides which are found in relatively higher proportion in coal combustion residues like fly ash and bottom ash produced from the high-sulfur coal typically found in the Mid Western US include iron oxide (magnetite) and also aluminous oxide (alumina). The proportions of iron oxides and aluminum oxides present in Illinois coal are generally in the range of 10% to 20%. However, not a single commercial installation exists that extract these valuable metal oxides from the combustion wastes and put them into much higher-value end use. Incidentally, these oxides are extracted from the mother-nature in the form of magnetite and alumina, in Iron ore and Bauxite mines, respectively to be used for Steel and Aluminum production. Understandably, the amount of mining activity and its associated negative environmental impact can be significantly reduced by extracting various minerals from the CCBs which are abundantly available in numerous ash ponds all over the US.
Developing beneficial use of CCBs have been a major focus of coal researchers for last several decades. However, the coal combustion residues, which are typical to the high-sulfur coal found in the Midwestern US, has not been looked into that thoroughly and that forms the basis for this research project. Near-term commercialization of metal oxide extraction from CCBs is an ultimate goal of this project effort. Another wide-scale application envisioned for the CCBs is cement-less geopolymer-based concrete. Usually, concrete is prepared using Portland cement which is manufactured utilizing a high CO2 producing chemical process and thus is responsible for about 5-8% of the total CO2 emissions as a green house gas. Water requirement for conventional cement-based concrete is also much higher than that of geopolymer-based concrete.
In light of the above discussions, the main objectives of the this Phase 1 study were:
- To develop a novel flowsheet for extracting valuable mineral oxides, such as Iron oxide and Titanium dioxide from the waste products of high sulfur coal combustion process utilized in electric utilities.
- To develop a suitable process to utilize majority of the coal combustion residues as a useful product in the form of a geopolymer-based concrete without the use of any Portland cement.
A part of the above listed objective 1 was achieved, when the Faculty Advisor and former Student Team Leader of this group completed a project (Mohanty et al., 2008) a few years ago by developing a low-cost proprietary process flowsheet to extract a specific type of iron oxide, i.e., Fe3O4, at high grade (>95%) and reasonably high recovery (~75%) from Illinois coal fly ash.
Two coal fly ash samples were collected from two Illinois high sulfur coal burning utilities (identified as CWLP and SIPC for this proposal) and brought to the Coal Development Park in Carterville, IL. The Davis Tube tests showed that the magnetite (a specific type of iron oxide) content of CWLP and SIPC fly ash samples are about 17% and 10%, respectively. Our tests conducted in a previous study (Mohanty, 2008) showed for the CWLP sample that the wet magnetic separator could produce a product with 82% magnetite and 22.3% recovery and the dry magnetic separator could upgrade the magnetite content of the fly ash up to 34.5% grade with a recovery of about 90%.
With the initiation of the P3-Phase I study, it was also desired to test if the pre-concentration of the fly ash magnetite used in the previous study would be successful for the new SIPC fly ash sample, collected for this project. A 6-inch diameter classifying cyclone was used for this preconcentration step. The results indicate that the fly ash magnetite could be enriched from 10% in the feed to 26.7% in the underflow stream with 92.7% recovery. We hope to enrich the magnetite content in Step 1 to about 35% level by using another process equipment, known as water-only-cyclone, which provides a density-based separation. Step 2 will be conducted using a wet magnetic separator.
Preliminary leaching tests were conducted on SIPC fly ash sample for extracting Al, Ti and the remaining Fe, from the non-magnetite tailing obtained from Step 1. The results gave an Al, Ti and Fe extraction efficiency of 82.7 %, 86.2% and 89.6%, respectively. These results showed that the ability of leaching method in extracting the Al, Ti and remaining part of Fe is very high and more tests and investigations are needed to optimize the process parameters.
Illinois coal fly ash is known to be a Si-Al rich material making it a very suitable feed material for geopolymerization process and production of geopolymer concrete. In this Phase 1 study, tests were done to make geopolymer concrete using both SIPC and CWLP fly ash samples. The results of a fractional factorial design experiment showed that alkali concentration and type, curing temperature and H2O:Na2O ratio have the most significant effects on the compressive strength of the geopolymer concrete samples. Using these results, a new concrete design having both fine and coarse aggregates for making geopolymer and Portland cement concrete samples were selected. The results of compressive strength tests indicated that the geopolymer concrete samples made from the SIPC fly ash has the highest compressive strength, even more than the Portland cement concrete samples. This preliminary finding support our original hypothesis and suggests the feasibility of using IL coal ash as a suitable feed stock for the geopolymer concrete, that may replace conventional cement-based concrete some day in future.
- This study so far has envisioned a suitable processing scheme to extract high-grade (> 96%) magnetite ( a specific type of iron oxide) from fly ash generated from burning high sulfur coal. The process flowsheet is being validated through actual lab experiments during this Phase 1 study.
- The exploratory leaching tests done on the non-magnetite part of SIPC fly ash samples showed high efficiency achievable for Al, Ti and Fe extraction. In the leaching tests, the extraction efficiencies of Al, Ti and Fe were 82.7 %, 86.2% and 89.6%, respectively. These results show that the leaching process can effectively extract Al, Ti and Fe from the non-magnetite part of Illinois fly ash. It may also be possible to leach and recover other valuable elements like Germanium, Gallium and Vanadium as probable secondary products during the alumina extraction process.
- The results obtained from this study exhibited that the geopolymer concrete samples made from the SIPC fly ash sample showed a higher compressive strength than the Portland cement concrete. This preliminary finding supports our original hypothesis of suitability of Il coal fly ash to serve as a good raw material for its wide-scale in making geopolymer-based concrete.