Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Patent
1998-05-19
1999-12-07
Marcantoni, Paul
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
106705, 106713, 106714, 106718, 106737, 106745, 106789, 106811, 106DIG1, 106900, 588256, 588257, 210751, 405128, 405266, C04B 1800
Patent
active
059976292
DESCRIPTION:
BRIEF SUMMARY
The United Kingdom produces approximately 4.5 million tonnes of hazardous wastes per annum, and it is estimated that over 80% of these are sent to landfill without pretreatment. There is, however, capacity to solidify about 0.5 million tonnes (approximately 10%) of these materials using cement-based solidification systems. Wastes most suitable for this solidification include industrial sludges and residues high in inorganic solids but low in organic constituents. The cost of treatment by this method depends upon the nature of the waste but may be high. Solidified products are either mono-disposed in dedicated landfill or co-disposed with domestic refuse after passing quality control criteria, which include development of specified strength and leach testing to establish the degree of waste component fixation.
In the USA, cement-based solidification systems are more widely employed for hazardous waste management and remediation of contaminated ground. The USEPA Site technology programme has examined a considerable number of trials involving this technology and cement-based solidification has proved to be one of the most popular remediation technologies. Although hitherto not used widely in Europe there is increasing interest in the potential application of this technology. There is also increasing concern over the suitability of this technique for managing certain hazardous wastes.
Solidification processes are generally based upon hydraulic binder `systems`. The two most popular binding agents currently used in the UK are Ordinary Portland Cement (OPC) and Pulverised Fuel Ash (PFA) although they may be blended with other materials such as cement kiln dust or lime during application.
OPC is a hydraulic binder and is composed of four main anhydrous phases, namely alite and belite, consisting of calcium silicates, and aluminate and ferrite, as calcium aluminate and aluminoferrite respectively. On addition of water the hydration reactions of OPC produce excess lime as Portlandite, whereas the pozzolanic reactions of PFA produce similar products but consume lime. PFA is therefore used as a cement replacement material on the grounds that it reduces process costs, has a limited sorbent function and facilitates the re-use of a problematic waste material.
The use of OPC and PFA as binder materials is known but it is alleged that the high pH environment of cementitious material is conducive to keeping heavy metals insolubilised and therefore stabilised. In addition subsequent setting and hardening by developing hydration products has been postulated to provide an interlocking framework to physically encapsulate waste particles and provide the product with strength and rigidity. In general, however, the claims made for these processes during industrial application are not supported by detailed independent research. Moreover, in certain cases there is increasing evidence to support the unsuitability of hydraulic binders for use with some wastes--for example, large deposits of `failed` solidified wastes have been recently identified in the English Midlands. These particular materials appear not to have set.
There is therefore a need for new processes and systems to treat difficult or environmentally hazardous wastes prior to landfill. At present solidification is used out of context as the science behind the process is poorly understood and not applied in practice. These serious doubts about the efficacy of solidification occur at a time when there is potential for a significant increase in its use.
In recognition of the lack of basic knowledge in this area a number of hazardous wastes considered suitable for cement-based solidification, including a commercially processed, neutralised and solidified waste, were examined by calorimetric and microstructural methods. It was found that both hydraulic and pozzolanic reactions could be indefinitely retarded or poisoned with as little as 3% (w/w) waste addition. All the wastes examined were typical of that solidified in the UK at present and all were capable of poisoning OPC h
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"Preliminary Investigation into the Effects of Carbonation on Cement Solidified Hazardous Wastes" Lange et al. Environ. Sci. Technology (1995), 30(1) pp. 25-30.
Imperial College of Science Technology and Medicine
Marcantoni Paul
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