Supercritical Carbonation for Stabilisation and Reuse of Hazardous Wastes

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Supercritical carbonation has been identified as an innovative technique to treat air pollution control (APC) residues, a particularly troublesome class of hazardous wastes. Treatment enables them to be either safely disposed of or re-used as artificial aggregates in building materials. Contrary to other technologies available, the technique converts the powdered residues to a solid block whilst also immobilising heavy metals with minimal energy input. An additional benefit is that the process captures carbon dioxide in a geologically stable form. The purpose of this project is to develop a commercially viable method for the carbonation of APC residues, reducing the costs of disposal for waste management companies and generating a saleable product for the construction industry. Energy from waste (EfW) facilities are an increasingly favoured Waste Management option. However, trapping of the resultant emissions produces APC residues which are classed as hazardous materials. These environmental concerns make an acceptable waste management strategy imperative. However, to date, suitable treatment methods are limited. Accelerated carbonation has been proposed as a treatment method for APC residues, with carbonation of a powdered material being achieved in a matter of hours or days. The formation of geologically stable carbonates has been shown to immobilise many of the problematic heavy metals, including lead and zinc. However, accelerated carbonation still has drawbacks, namely incomplete carbonation, a powdered or granular product and slow reaction kinetics. Super-critical carbonation technology however will provide all of the benefits of accelerated, sub-critical, carbonation, whilst also overcoming its limitations. Carbonation will neutralise and solidify the powdered wastes, whilst immobilising many of the key components of environmental concern as stable carbonates. What's more, the process also immobilises carbon dioxide in a geologically stable chemical form within the stabilised residues. At 78.1 atm and 31oC carbon dioxide becomes supercritical, having the density of a liquid, but the permeability of a gas. The combination of high gas density and ability to permeate sub-micron sized pores ensures rapid carbonation. Furthermore, whilst with conventional carbonation both the starting and end products are powdered or granular, the ability of supercritical CO2 to penetrate small pores enables monolithic samples to be treated, overcoming problems associated with the handling of fine powders. By varying the solid-liquid ratio of a lime-based solid, i.e. APC residues, it is possible to control the solid's porosity, with high ratios giving greater porosity.Super-critical carbonation is a proven technology, having been shown to improve the technical properties of many calcium-rich materials, with reduced porosity and increased strength associated with the pore filling by carbonate species. The technology however has never previously been applied to the treatment of waste materials, in particular APC residues. The use of super-critical carbonation technology for the stabilisation and solidification of APC residues will provide a cost-effective means of treating these troublesome wastes. The benefits of this approach are;1. The conversion of an alkaline fine powder to a neutral solid block.2. More rapid and extensive carbonation than accelerated carbonation.3. Immobilisation of the heavy metals present in APC residues, particularly lead and zinc, as insoluble carbonates.4. Requires no additional material other than water and carbon dioxide (CO2).5. Does not require high temperatures and so is a relatively low energy approach.6. Chemically binds carbon dioxide within the stabilised blocks.We have identified two commercial routes to market; avoidance of landfill and the production of alternative construction materials. We are working with a number of companies to realise these aims.