Investigation of wet-route carbonation as a CCUS strategy for alkaline residues valorization in construction applications

Industrial alkaline residues and carbon dioxide are the largest and most relevant by-products from thermal and industrial processes. Among these processes, steelmaking plants and thermal power plants are the most significant, being responsible for the generation of large amounts of steel slags and combustion ash, respectively. Generally, the majority of these residues are still landfilled as a primary and common route, because of their physical and chemical properties, which could hinder their utilization, but this management strategy is unsustainable. Hence, in the last years, alternative ways of managing these materials were investigated with the aim of obtaining potential products from these residues. The present work investigated innovative treatments based on accelerated carbonation of alkaline solid residues from different thermal processes, finalized at their valorization for civil applications and at the utilization and storage of carbon dioxide. Namely, this work focused on the wet-route carbonation of alkaline residues applied in two different ways: 1) a combined carbonation-granulation process, aimed at producing aggregates from steel slags (from Basic Oxygen Furnace, BOF) or Circulating Fluidized Bed Combustion (CFBC) fly ash, and 2) curing by accelerated carbonation of compacts manufactured with BOF or EAF (Electric Arc Furnace) slags. The carbonation-granulation experiments were carried out both at lab-scale and larger-scale in a rotary kiln reactor to evaluate the feasibility of the scaling-up of the wet-route carbonation process at demonstrative or full scale. In particular, for CFBC ash, the combined carbonation-granulation process was tested by using two types of binder, i.e. deionized water or an alkaline activator. In addition, a gas mixture made up by 40% CO2 and 60% N2 was employed in order to test direct CO2 storage/utilization, avoiding the energy-intensive CO2 capture pre-treatment. For the BOF slags, the carbonation process was investigated with the aim to develop an operating procedure to increase the automation of the process in view of its scale-up, wetting the residue directly inside a rotary kiln reactor by flowing steam to obtain the target value of the liquid to solid ratio. The results of the tests performed in the rotary kiln reactor showed that the wet-route carbonation of alkaline industrial residues is a feasible option for scaling up the process, also using diluted CO2 sources, such as syngas from coal gasification after the water gas shift or biogas from anaerobic digestion treatments. Anyhow, significant technological breakthroughs in reactor design and system optimization are needed before a full-scale PAOLA LIBRANDI 11 deployment can be considered. The experiments performed on steel slag compacts were carried out at different pressures (1.3-10 bar) and reaction times (15-240 min), setting the temperature at 50 °C in order to assess the evolution of the carbonation reaction of the material and its effects on the properties of the product. In addition, in view of assessing the feasibility of using CO2 diluted sources, experiments were also carried out on BOF slag at different CO2 partial pressures, from 0.13 bar to 10 bar, varying the CO2 concentration between 10% and 100%. The fundamental study conducted on the carbonated compacts, produced from BOF or EAF slags, allowed to gain a deeper insight on the carbonation mechanism. The main reacting phases for both slags were identified and their contribution to the development of the mechanical and environmental properties of the carbonated compacts was assessed. Besides, correlations between CO2 uptake, mechanical strength and changes in the mineralogy of the material were identified. In particular, the development of the mechanical properties of the product was associated to the changes of the microstructure induced by carbonation. Moreover, the carbonated compacts produced from BOF slags showed to achieve acceptable CO2 uptake values and mechanical properties also using CO2 diluted sources (with a CO2 content of least 20%) for CO2 partial pressures above 1 bar. The leaching properties of the carbonated products tested employing the different setups showed to be affected by the treatment, exhibiting a reduction in pH and mobility of Ba and sulphates. However, in some cases, depending on the type of treatment and operating conditions employed, the leaching of Cr, Mo and V and oxyanions increased. This should be taken into consideration when assessing the product’s valorisation options.