Formatted Title

Analysis of Alternative Techniques for the Rehabilitation of Areas Impacted by Acid Sludges Generated at a Brazilian Steel Mill

Background/Objectives

This work was conducted at a steel mill located in the state of São Paulo, Brazil. The activities of this steel mill began in the 1960s when raw materials and processes involved the transformation of metallurgical coke, iron ore, and sinter to produce pig iron. In this process, volatile organic compounds were extracted and sent to the carbochemical unit, a plant that treated these volatile compounds, generating coke oven gas, tar, and light oils. The process of obtaining and separating light oils involved a sequence of distillations and acid washing. From 1988 onwards, the acid washing process was replaced, ceasing the generation of acid sludges. However, it was decided to dispose of the environmental liability generated in the previous years in a prepared masonry structure with dimensions of 200 m² and up to 8 meters deep. For the remediation of this site where the acid sludge was stored, several interventions were developed, including the excavation of the residue in the shallower portions of the masonry box (from the surface to approximately 6 meters deep), the operation of pumping and treatment wells (P&T) to neutralize the pH and maintain hydraulic control inside the old underground masonry structure. In the recent supplementary investigation study (2023), concentrations of petroleum hydrocarbons were still identified at deeper levels inside the box, advancing to the external area, indicating that there is transmissivity of the dissolved-phase plume to the area outside the masonry box. The objective of this work was to evaluate intervention alternatives for the environmental rehabilitation of the area in question, based on pre-established criteria such as operational safety, future occupation, environmental sustainability, economic feasibility, and time for completion.

Approach/Activities

The intervention approaches evaluated for the area's rehabilitation were excavation, pumping, and geotechnical capping. A decision matrix was established to select the best intervention alternative for the case, with pre-established criteria applicable to the Brazilian market. The criteria evaluated were: availability of technologies, sustainability, performance in the impacted environment, ease of application, consequences of its application, cost, historical use of the technique, and the time required for the rehabilitation of the impacted area. Of the evaluated alternatives, considering the stability of contaminant concentrations (Mann Kendall statistical analysis), control of plume advancement, as observed in the flow and transport modeling study (Visual Modflow 2014), and a scenario of controlled risks (no concentrations of vapors in the soil and ambient air), and taking into account the results of the selection matrix established, geotechnical capping was the best alternative for the area's rehabilitation.

Results/Lessons Learned

After selecting remediation techniques, the most suitable and viable technology for the area is geotechnical capping, as it allows for a quick reduction of vapor emissions from the soil into open environments and consequent inhalation. It also prevents the entry of rainwater into the masonry box. In addition, it does not require operating costs, only implementation costs. Finally, it should be noted that the choice of this technique involves less critical safety factors for execution, as the other evaluated techniques (pumping and/or excavation) carry the risk of contact with the contaminant (acid sludge).