Recovery of slag in steel mills and creation of mitigation hills

The steel company had a major and urgent problem to manage: approximately 500,000 tons of waste were stored in a yard next to the plant. It was necessary to eliminate this enormous pile of material and find an alternative solution to their disposal in landfill, and to promote their recovery, an activity that the steelworks already had in its DNA.

Furthermore, the plant needed to complete the construction of the environmental mitigation embankment with virgin inert material along the south-west border of the site so that it would reduce the landscape and acoustic impact of the plant on the nearby town.

They turned to us for our long experience in waste treatment and for our ability to address the problem with a targeted technical and scientific approach. Thanks to our analysis laboratory and specialized professionals, we were able to identify the most appropriate recovery treatment and reuse the material for the construction of the “environmental” mitigation work planned on site PTI (Infra-regional Territorial Plan).

Steel slag and its treatment

Steel slag is currently managed as by-products generated during steel production pursuant to Article 184-bis of Legislative Decree 152/06 and subsequent amendments.

In the production of steel from an electric arc furnace, slag is formed above the melting bath as a result of the oxidation of the scrap and the compounds generated by the additives used. These can be divided into white slag and black slag.

Black slag, obtained from the production of carbon steel in EAF electric arc furnaces, mainly contains iron, calcium and silicon oxides and is used in bituminous conglomerates, concrete and cemented mixtures.

White slag, on the other hand, originates in the steel refining phase in the ladle and has a high residual calcium oxide content. They are typically used in the production of cement and in soil stabilization activities.

In this case, the steel plant needed to identify an effective method for managing the waste accumulated in the area to the west of the site. This waste had already been previously classified as waste with code EER 10 09 03 and, therefore, could not be managed as by-products. The goal was to find a solution that would allow this waste to be treated appropriately in accordance with the current regulatory framework and minimizing the environmental impact.

We addressed the problem with a complex and innovative approach, starting from a complex authorization process and supporting our client in all phases of management and solution of the problem.

The authorization process

In order to carry out the project, it was necessary to follow a specific authorization process that included the following steps:

• submission to the competent bodies of the authorization request for the project for the recovery of steelworks slag following stabilization through the construction of an embankment pursuant to art. 208 of Legislative Decree 152/06 and subsequent amendments;
• approval of the project by the Competent Bodies;
• presentation of data and information relating to the effectiveness of the stabilization process;
• certification by the regional ARPA (Regional Agency for Environmental Protection) of the efficiency of the proposed treatment and indication of the management and control protocol for the slag stabilization and solidification process (process validation).

We therefore took care of all these activities starting with the first chemical and geotechnical analyses.

Preparatory activities

The objective of the preliminary analyses was to verify that the stabilization-solidification treatment process identified was capable of transforming the waste into a material with the desired qualitative characteristics and the necessary structural integrity.

The selected chemical-physical process allows the pollutants to be immobilized in an inert matrix with the use of binders based on inorganic reagents, such as silicates, cement and alkaline sulphides with the aim of reducing the release of contaminating elements so as to make them suitable for the subsequent recovery phases.

In the preliminary phase, using our specialized technicians and our accredited internal analysis laboratory, we performed the following analysis activities on the waste to be treated:

• collection of slag samples;
• preparation of samples;
• characterization and release tests;
• definition of the proportions of the materials constituting the mixture to be used for stabilization/solidification.

Installation of the material treatment plants

Only after the authorization clearance, the technical/analytical checks and the definition of the environmental monitoring plan, we were able to deal with the creation of the construction site area, located within the site, and the installation of the material treatment plant that we designed specifically for the steelworks.

Below are the main machines and operating means used on the construction site:

• n. 1 mobile crushing plant of the jaw type with iron separator;
• n. 1 complete screw mixing plant;
• n. 1 blade mixing plant for the preparation of the stabilized mix;
• n. 3 cement containment silos complete with fluidification and dust collector systems, and dosing system complete with screw;
• n. 4 fiberglass tanks for the containment of chemicals complete with dosing system with volumetric pumps;
• n. 1 cryogenic tank for the storage of CO2;
• n. 1 command and control system for the supervision of the plant;
• n. 3 tracked excavators;
• n. 3 wheel loaders;
• n. 5 trucks with quarry operation;
• n. 2 equipped hook lift vans to serve the construction site areas;
• n. 1 compactor roller.

Several Gesteco professionals were involved and worked on this construction site, including chemists, operations managers, environmental engineers, process engineers, surveyors, drivers, and specialized plant and vehicle operators.

Preparation of the material to be treated

The operating cycle began in the area where the steelworks slag was stored. The slag collection operation proceeded in horizontal layers, in successive layers from top to bottom and was preparatory to the subsequent grinding. The crawler excavator directly fed the hopper of the crushing plant.

The excavation, grinding and dumping activities also had the purpose of intercepting any foreign materials. Furthermore, thanks to the magnetic separator of the shredder positioned on the conveyor belt, all materials of ferrous origin were removed.

Already from this phase, the material was divided into batches of 2,000 tons that were uniquely identified. Once the analyses defined within the Operating Protocol with the regional ARPA had been carried out, to establish the conformity of the individual batches, the dumped materials were transferred by trucks to the treatment area.

Stabilization and solidification process

The batch of compliant, ground and deferrized slag was transported to the stabilization and solidification plant to be subjected to the actual treatment process that made the material compatible for use in the construction of the environmental mitigation work.

The operations can be summarized as follows:

• loading of the slag into the hopper;
• weighing;
• starting the mixer;
• dosing of the reagents based on the weight and composition identified. The main chemicals used were: sodium sulfide, cement, silicate, water;
• mixing;
• unloading of the material;
• maturation of the stabilized material to complete the solidification phase;
• analytical control;
• transportation of the stabilized and solidified material to the embankment formation area.

Compliance checks

For each batch leaving the plant, two samples were prepared to be subjected to the analytical and geotechnical checks defined in the operational protocol established with the regional ARPA to guarantee the effectiveness of the treatment.

All data relating to the release tests, the composition of the mixture and the geotechnical tests were kept available to the control bodies for a period of 1 year from the conclusion of the project activities.

Construction of the mitigation hills

The stabilized and solidified material exiting the treatment plant was transported by trucks to the areas where the embankment was formed.

Each of the 265 batches of material installed was identified and georeferenced to ensure complete traceability.

At the end of the installation activities, for each individual batch, density and load tests were carried out on plates, reproducing the load-bearing and compaction tests conducted in the laboratory on the reference specimens.

Once the levels envisaged by the project were reached, the material was covered with topsoil and drainage works for surface rainwater were carried out.

To complete the work, native tree and shrub species, both evergreen and deciduous, were planted.

Various stages of construction

The environmental monitoring plan

To support the project, in agreement with the control bodies, Gesteco planned and executed a systematic environmental monitoring plan. This had the purpose of:

• maintaining continuous and comprehensive knowledge of the evolution of the main environmental parameters;
• forming the information base for the verification and compliance action with current legislation.

The environmental matrices investigated monthly during the execution of the project were:

Air quality (analyses also carried out in collaboration with the Dynamic Olfactometry Laboratory (LOD)

Environmental acoustics

Underground waters​

Monitoring was also carried out both in the ante operam phase and in the post operam phase.

The periodic verification of significant environmental matrices allowed optimal management of the activities, ensuring the timely detection of potential unexpected environmental emergencies that did not occur in the specific case, thus allowing for appropriate measures to be taken if necessary.

Testing and certification of the work

At the end of the works, the control bodies tested the project and the mitigation hills.

The activity carried out is today a concrete example of reuse of steel waste and environmental redevelopment.

After having identified, in agreement with the authorities, the most suitable operational practice to manage the material, we stabilized, solidified and reused the entire pile of steelworks waste, then created the environmental mitigation hills with this material.

The embankment created led to a significant improvement in the landscape, helping to enhance the surrounding environment by improving the usability of the area. Furthermore, the phonometric campaigns carried out attested a general decrease in the noise contribution of the plant at the receptors investigated. In particular, in the southern area a decrease of approximately 4 dB(A) was found between the ante operam condition and the post operam condition.

The implementation of the complex project had positive outcomes thanks to the synergy between the different skills of Gesteco professionals. This integrated approach made it possible to bring together and make different specializations and technologies communicate, optimizing each phase.

The multidisciplinary collaboration implemented also made it possible to reduce construction times and ensure a high standard of quality in every phase of the project.

Through the stabilization and solidification treatment of the slag, the work was enriched with multiple benefits, as it made it possible to:

• avoid the disposal of the slag with the consequent accumulation of material in landfill;
• avoid all possible impacts on the various environmental matrices deriving from the transport of the slag to the landfill and the collection and transport on site of inert material for the construction of the embankment;
• save natural raw materials for the construction of the work.