Liesbeth Horckmans

accelerated carbonation of steel slag compacts: Development of high-strength construction Materials

Mineral carbonation involves the capture and storage of carbon dioxide in carbonate minerals. Mineral carbonation presents opportunities for the recycling of steel slags and other alkaline residues that are currently landfilled. The Carbstone process was initially developed to transform non-hydraulic steel slags (stainless steel slag and basic oxygen furnace slags) in high quality construction materials. The process makes use of accelerated mineral carbonation by treating different types of steel slags with CO2 at elevated pressure (up to 2 MPa) and temperatures (20 to 140°C). For stainless steel slags raising the temperature from 20 to 140°C had a positive effect on the CO2 uptake, strength development and the environmental properties (i.e. leaching of Cr and Mo) of the carbonated slag compacts. For BOF slags raising the temperature was not beneficial for the carbonation process. Elevated CO2 pressure and CO2 concentration of the feed gas had a positive effect on the CO2 uptake and strength development for both types of steel slags. In addition also the compaction force had a positive effect on the strength development. The carbonates that are produced in-situ during the carbonation reaction act as a binder, cementing the slag particles together. The carbonated compacts (Carbstones) have technical properties that are equivalent to conventional concrete products. An additional advantage is that the carbonated materials sequester 100 to 150 g CO2/kg slag. The technology was developed on lab scale by optimisation of process parameters with regard to compressive strength development, CO2 uptake and environmental properties of the carbonated construction materials. The Carbstone technology was validated using (semi-)industrial equipment and process conditions.

Closed-loop solvometallurgical process for recovery of lead from iron-rich secondary lead smelter residues

A solvometallurgical process based on the use of concentrated acetic acid as lixiviant is proposed as an alternative for conventional hydrometallurgical processes to recover lead from iron-rich industrial residues generated by recycling of spent lead-acid batteries in a secondary lead smelter. Under the optimal conditions, a high selectivity for lead was obtained: more than 90% of the lead content could be dissolved, while only a small amount of iron (<6%) was codissolved. Lead was quantitatively recovered from the acetic acid leachate by addition of a stoichiometric amount of sulphuric acid. Acetic acid was recycled by distillation and reused in the leaching step, so that a closed-loop process was obtained. The process was optimised for iron-rich residue (matte), but also a proof-of-principle is given for lead recovery from another lead-containing residue (slag). The main advantages of this solvometallurgical process are the low power consumption (room-temperature process), the low consumption of chemicals (only sulphuric acid is consumed), full recycling of the acetic acid and the limitation of waste water formation.

Concrete with Flash-Calcined Dredging Sediments as a Novel Supplementary Cementitious Material

Maintenance dredging in the port of Antwerp annually generates about 450.000 tons of dry matter sediment, for which suitable disposal solutions or applications are required. Mechanical dewatering of the sediments results in filter cakes, comprising clays (2:1 clay minerals and kaolinite), quartz, calcite and an amorphous phase as major mineral phases. Flash calcination of these filter cakes reduces the total organic carbon fraction and results in a dehydroxylation of the clay minerals. Isothermal conduction calorimetry tests demonstrated the pozzolanic reactivity of the calcined material, being superior to that of a siliceous fly ash. As a result of the pozzolanic reactions, replacing 20, 30 or 40 wt% of cement by calcined dredging sediments leads to a strength development equivalent to a reference mix with Portland cement up to 28 days, despite low strength at early age. This paper presents material characteristics and pozzolanic reactivity of the flash-calcined dredging sediments, as well as their effect on setting time, fresh concrete properties and mechanical characteristics. The initial results clearly show that the flash-calcined clay-rich dredging sediments have great potential to be used as a novel pozzolanic supplementary cementitious material, for the production of sustainable, low-CO2 blended cements and concrete.

BIND-AMOR: Reuse of dredged sediments as supplementary cementitious materials

In the Port of Antwerp, 500.000 tonnes dry matter base (DM) of sediments need to be dredged each year to ensure the navigability of the waterways. Traditionally, these maintenance dredged sediments were disposed in settling ponds or underwater cells. However, due to near exhaustion of the existing storage capacity as well as the limited availability of new storage sites, an alternative, sustainable solution needed to be developed. Between 2008 and 2011, the state-of-the-art treatment and storage facility AMORAS was realised by the Flemish Government represented by the Department of Mobility and Public Works (MOW), division Maritime Access. Since 2011, the AMORAS-installation treats up to 600.000 ton DM of sediments annually by mechanical dewatering with membrane filter presses. Pre-treatment steps include sand separation (63 µm cut-off hydrocyclones) and lime addition to enhance flocculation. The produced fine grained (< 63 µm) filter cakes have a dry matter content of minimally 60 %, resulting in a significant volume decrease of the sediments as well as physical properties suitable for storage in an onsite landfill. To increase the operational lifetime of the storage facility, reuse of the non-contaminated filter cakes was considered. A strict separation between highly and marginally contaminated sediments is maintained throughout the process. Furthermore, the sediments are homogenized in different steps, resulting in a continuous supply of homogeneous filter cakes with a good environmental quality. As such, the fine fraction filter cakes of AMORAS are uniquely suited for valorisation.