Skelly and Loy have reported on a case study in the northeast of Bedford County, Pennsylvania, US. http://www.skellyloy.com/Pressroom/MiningPortal/mp/Volume%20VII,%20Issue%204%20Portal.pdf The costs involved with ARD treatment continue to increase. While passive treatment has been a common treatment, it is not applicable for all sources, depending on water quality and flow rates. Limestone has been the primary alkaline source for passive treatment, but there is significant interest to find additional alkalinity-generating alternatives to avoid the need for active chemical treatment and the associated increase in operating and maintenance needs. Steel slag looks promising.
With the potential to expand the applicability of passive treatment and develop a beneficial use of a waste material, Broad Top Township successfully applied for an innovative ex-situ treatment technology grant through the Pennsylvania Department of Environmental Protection Bureau of Abandoned Mine Reclamation to evaluate steel slag from an existing waste pile in Northeastern Bedford County, Pennsylvania, for ARD treatment.
Throughout the Appalachian region, studies have been performed and passive treatment systems have been
constructed using steel slag as an alkaline material for ARD treatment. Steel slag is a byproduct of the smelting process in steel mills and is formed from numerous iron, calcium, and aluminum oxides.
Steel slag wastes near ARD areas may be a valuable source of alkalinity. However, past studies have produced mixed results in steel slag’s ability to effectively treat acidic, metal-laden water. While the alkaline characteristics of steel slag may be favourable for passive treatment, the physical characteristics and reactivity of steel slag create unique design challenges for use in ARD treatment. The ARD source must be properly evaluated with the target steel slag material being considered for use in treatment in order to ensure predictable results.
The historic Riddlesburg Steel Mill, operated in Broad Top Township, generated significant volumes of steel slag from coke ovens used as part of a steel-making operation. The surrounding area is part of the Broad Top coal fields that were mined with many abandoned sites, resulting in numerous acidic and metal-laden discharges throughout the area’s watersheds. With the knowledge that this supply of alkaline material existed within one of the three area watersheds, grant funding was sought to evaluate the use of steel slag alkaline material as a potential alternative for passive treatment of ARD and general use in adding alkalinity to the area streams to improve water chemistry and create biological habitat.
Previous studies indicated steel slag materials can generate high levels of alkalinity (1,500-2,000 ppm) in water within a relatively short period of time. However, steel slag materials vary in both composition and ability to generate alkalinity. Commercially available steel slag from active steelmaking operations is processed for use in water treatment, which adds to the cost of the material as well as the freight costs, based on geographic availability. The grain size of the steel slag plays a key role in the ability of the material to maintain porosity as a treatment bed, but the material also has cementing properties that may cause clogging. Most reported successes that use steel slag as an alkaline treatment have been with clean or slightly acidic water containing very low levels of dissolved metals.
Through the use of a relatively clean water source as the carrier for the alkalinity, the steel slag reacts with the water within a treatment bed, imparting alkalinity to the water, which is then either mixed with ARD or an acidic stream, but avoids problems associated with precipitated metals within the treatment material.
The project study focused on characterising the Judy Hollow steel slag material, evaluating treatment options for use of the steel slag through bench scale tests, and addressing potential environmental concerns with steel slag. The results of the study conducted in 2008 and 2009 indicate that the Judy Hollow steel slag contains two mineral forms that are the primary alkalinity generation components: calcite (limestone) and akermanite (calcium silicate), with iron as the primary elemental constituent. Before and after use in bench-scale tests, the steel slag was analysed for Toxicity Characteristic Leaching Procedure (TCLP), ASTM extractions, and leachate analyses, which revealed the low leaching potential of the material with all parameters below the Resource Conservation and Recovery Act (RCRA) limits.
Bench-scale tests were conducted, both in the field and at Skelly and Loy’s laboratory facility, using the steel slag in contact with rain water, headwater stream flow not impacted by ARD, and ARD sources with high and low dissolved carbon dioxide content. When contacted with the rain water and headwater stream flow, the steel slag material rapidly raised the pH and generated excess alkalinity within one hour. The tests using ARD sources in contact with the steel slag required a few hours to increase the pH and generate excess alkalinity. Because the steel slag material contains calcite, testing also evaluated the use of open vs. closed test systems to evaluate potential benefits of retaining carbon dioxide to increase the dissolution rates.
Three steel slag-size fractions were used in contact tests, including: raw (up to 76-102 mm); 13-51 mm screened; and < 13 mm screened material. All three sizes exhibited reasonable hydraulic conductivity that was comparable to the size of limestone commonly used in AMD treatment beds (e.g., AASHTO #57). However, when considering steel slag as a water treatment alkaline material, attention must be given to the gradation of the steel slag for use, type of water to be treated, depth of the steel slag bed, and potential to flush the steel slag bed to maintain porosity. Therefore, with minimal processing other than excavation, sizing, and hauling, the Judy Hollow steel slag may be an economical alternative to limestone as an alkaline source for water treatment in the region.
The field tests that used two ARD sources, one net-acidic and another net-alkaline, were revealing with respect to the alkalinity-generating capacity of the calcium minerals in the steel slag. In either a closed or open system, the net alkaline ARD reacted with the steel slag and generated high levels of excess alkalinity over 100 mg/litre within four hours. A closed system worked best for contacting the steel slag with the net-acidic ARD. Within one hour, the pH of the ARD was increased from 3.0 to 7.4, and the net acidity was decreased from 160 to -70 mg/litre.
Conclusions
The Judy Hollow steel slag has existed in large piles in Broad Top Township for many decades adjacent to the historic Riddlesburg coke ovens. Information obtained from the evaluation project indicates that the steel slag is an industrial byproduct containing thousands of tonnes of alkalinity that could be beneficially used in restoring the regional watersheds affected by ARD discharges. With today’s environmental movement towards resource recovery, this steel slag offers an opportunity improve the watersheds with limited financial obligation, conduct recycling and re-use of a byproduct of steel making, and reclaim a steel slag pile.
Three primary methods are suggested for additional investigation of the steel slag as an alkaline material in water treatment. The first is the use of rain barrels in the community containing the steel slag material and conveying rain water from roofed areas into the barrels for alkalinity addition to the watershed. The second is by using steel slag in a controlled manner to add alkalinity to headwater streams and springs within the ARD impacted watersheds. Finally, the steel slag would be a potential alternative for direct treatment of certain ARD discharges within a treatment bed as a means of passively treating the low to moderately contaminated waters.
Mark A. Williams – [email protected]
Terry W. Schmidt, P.E. – [email protected]
Bradley R. Shultz – [email protected]