Supercritical Carbonation for Stabilisation and Reuse of Hazardous Wastes

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.

Short Term Beneficiaries The most likely licensee will be the waste management consultancy Sustainable Resource Solutions (SRS). It is equally possible that it could be licensed directly to a waste management company such as WRG. Many of the Energy from Waste (EfW) incinerators in the UK are operated on behalf of local authorities. At least some of the reduction in operating costs is likely to be passed on to local authorities when contracts are re-negotiated. Central government will also benefit from this project as it will make two of its key environmental objectives more readily attainable - 1. The European Landfill Directive will force a reduction in the use of landfill over the next decade. By converting the APC residue into a useable or non-hazardous material the carbonation technology removes this potential barrier to the expansion of incinerator use. 2. The government's renewable energy strategy includes a target of producing 15% of the country's energy from renewable sources by 2020 [1]. Again, removing the barrier of having to deal with APC residue will assist with this aspiration. Encos, a University of Leeds spin-out company, will provide a market for the artificial aggregate formed when APC residue is pelletised and carbonated. The will use this as a filler in their innovative masonry blocks which manufactured entirely from waste. This project will impact positively on Encos by providing them with an inexpensive filler material. Environmental benefits include a reduction in the amount of hazardous waste sent to landfill. Even if the product of the carbonation is not useable, an environmental benefit will still be achieved by reducing the leachability of heavy metals and other contaminants present in the APC residue. Carbon dioxide released to the atmosphere during the preparation of lime for APC use will effectively be recombined with the lime by the process. Longer Term Beneficiaries Another industrial waste potentially treatable with this technology is 'red mud', a bi-product of aluminium extraction. It contains useful minerals which can be extracted, but its pH is extremely high and needs a large amount of acid to neutralise it first. This is often uneconomical. Carbon dioxide can be used for neutralisation of red mud [2] though the process takes many hours. Super-critical CO2 has the potential to significantly accelerate the neutralisation process making it much more economical. Companies handling this waste could do so more economically and produce saleable products from the treatment of the waste. Re-use of the waste also has obvious environmental benefits. Ensuring Impact is Achieved Licensing of the technology to SRS for exploitation in association with Encos, appears to be the approach most likely to result in achieving the broad range of impacts envisaged for the short term. By working with SRS and ENCOS throughout the project we will ensure that the project delivers a licensable technology. Other supporters such WRG, will assist with our understanding of the APC residue market and provide samples for testing. The results of the project will also be disseminated through articles in trade journals such as MRW - The Recycling and Waste Management Magazine, as well as through academic papers. Promotion at events such as the annual Recycling and Waste Management Exhibition at Birmingham's NEC and FutureSource 2011 will be used to promote the technology to potential customers. This will be arranged in collaboration with the licensee. References [1] The UK Renewable Energy Strategy 2009, UK Government. 2009. http://www.decc.gov.uk [2] S Khaitan, , DA Dzombak and GV Lowry. Mechanisms of Neutralization of Bauxite Residue by Carbon Dioxide. Journal of Environmental Engineering - ASCE, 2009, 135(6), 433-438