Waste Treatment - Turning pollutants back to rock05 April 2005

By treating waste containing calcium compounds (lime) with carbon dioxide, it possible to quickly and inexpensively convert materials to substances closely resembling limestones or other natural rocks.

This technique can be used to deal with waste that contains chemicals which it is desirable to fix and render inert for safe disposal as landfill or for use in construction. The calcium-containing material may either be the waste itself, or some feedstock that is added to it such as cement kiln flue dust.

This approach appears to work well with a large number of feedstocks, including some whose radioactive isotopes are a major problem for the nuclear industry, and can be made to employ carbon dioxide from flue stacks, reducing their effects on global warming. The process is presently at pilot plant and trail stage with a West Midlands contractor.

Carbonation as a process has been around for thousands of years. The hardening of traditional lime-based mortars depends on their reaction with the small amounts of carbon dioxide in the atmosphere. The idea of curing concrete in a carbon dioxide-rich environment originated in the mid-1800s. The new technology has the same basis - carbon dioxide plus lime in the presence of moisture produces calcium carbonate - but is specifically tweaked towards waste stabilisation. The tweaks are the subjects of patents held and applications made by the University of Greenwich and Forkers Ltd, a long-established civil and ground engineering company based in West Bromwich.

The technology is outlined to Plant Engineer on a visit to Chatham to meet Dr Paula Carey and Dr Colin Hills, the inventor or the process, in the University of Greenwich's Centre for Contaminated Land Remediation. The idea started in 1993, recounts Dr Hills, when he was faced with stabilising mixtures of cement and waste that remained "gloopy" for up to a year or more. First experiments were conducted with the aid of a small cement mixer, followed by use of a Hobart mixer in the laboratory. This has progressed to use of a 4m3 Winkworth mixer at Forkers. A proposal is underway to obtain funding to develop 25-50 ton/hr pre-industrial scale equipment. As the process has been scaled up, it has so far been found to work better at each stage, says Dr Hills.

Field trials to address a real environmental problem have been undertaken on the site of the former Astra Pyrotechnics factory in Dartford. Production processes there involved the use of a wide range of heavy metal-based chemicals. Taking soil, blending it with cement, and then exposing it to carbon dioxide in an enclosed, rotating carbonation chamber has resulted in the immobilisation of more than 99% of the heavy metals present in the soils. In order to verify the longevity of the effects of this treatment, the natural leach stability of the treated soil has been being monitored since 2000 in 10m x 5m x 0.5m test cells. So far, despite the heavy metals still being in the soil, water drawn from the site is of drinking water quality.

The project is of particular relevance to government plans to greatly expand housing and industrial development in the Thames Gateway, of which Dartford forms a part. North Kent is full of brownfield sites previously used for industrial and/or military purposes, with few records existing of exactly what was done there. Conventional dumping sites in Kent are all expected to be full by the end of this year.

The possibilities for the new technology are endless but the team is currently engaged in investigating the feasibility of several specific projects. One involves working with stabilising paper mill ash - resulting from a new process - which has a material of pH12, more alkaline than previously. This is at a time when new regulations have put an upper limit of pH 9.5 on landfill material.

The other hot prospect is using cement kiln dust - which is largely cement, but not of high enough quality to sell - as the basis of a binder to prevent the leaching out of other substances. Some of the cement kiln dust is being taken from dumps, where it remains active, even if crusted over on top.

The team is also looking at stabilising municipal incinerator ash, both bottom ash and fly ash. Carbonation is particularly effective at stabilising lead, zinc and copper, all of which are found as water insoluble carbonates in nature. More difficult, according to Dr Hills, are oxyanions. Stabilisation of mixtures containing metals such as arsenic, chromium-6, and cadmium thus require additional process steps. A sample of ground stainless steel slag (full of chromium), shown to Plant Engineer, is said to have a compressive strength of 6 to 10 MPa after being only flushed with carbon dioxide.

Another project is to turn fines from local quarries producing Kentish Ragstone (a sandy limestone found in Kent) into a construction material. So far the team has produced pellets up to 20mm across, with strengths in some cases "stronger than concrete blocks".

Although there are various ways in which the process can be enhanced, by controlling temperature and pressure, the team is concentrating on working under conditions of least cost, that is, near atmospheric pressure and around ambient temperatures. Typical process times are of the order of 15 minutes.

The basic reaction is exothermic. One problem, Dr Carey explains, is that the process tends to be too efficient. The carbon dioxide has to dissolve in the water to form the bicarbonate ions which react with the calcium-containing phases in the mixture. Too much heat, however, drives off the water and stops the reaction. Too much water, on the other hand, leads to an ultimately weak material. What is wanted are strong, dense materials that remain stable if left in the ground, are not leached by rainwater and are capable of being used for construction.

Ultimate strength in some cases can be aided by compaction. If this is undertaken, there is again an optimum because too much compaction prevents carbon dioxide penetrating fully to complete the carbonation process.

One of the most important potential applications is in the prevention of the leaching of dumped radioactive waste. With pressures to resurrect nuclear power in an effort to reduce fossil fuel use because of global climate change, there needs to be a major effort to solve the waste disposal problem. US interests are working hard to promote their technology to stabilise such wastes in the form of glasses, but there are considerable concerns as to what happens to the glasses when they devitrify. All synthetic glasses are unstable and devitrify eventually, but those containing radioactive substances may devitrify more quickly. Turning such substances into materials that are physically and chemically similar to those that have held them in nature for millions of years could provide a much more effective long-term solution. Dr Hills reports that the process looks "very promising" for caesium, to take one example, but the laboratory has neither the equipment (nor license) to work with radioactive isotopes, only the non-radioactive ones.

SOE

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