Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iva metal
Reexamination Certificate
1998-11-18
2001-11-20
Griffin, Steven P (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iva metal
C423S092000, C423S094000, C423S095000, C423S098000, C423S155000, C423S178000, C423S197000, C075S961000, C106SDIG001
Reexamination Certificate
active
06319482
ABSTRACT:
TECHNICAL FIELD
The present invention relates to air pollution control (APC) technologies. More particularly the present invention relates to a process for treating the fly ash/air pollution control process residues obtained from air pollution control processes that utilize dry/semi-dry lime injection, such that the ash residues may be disposed of as non-hazardous materials.
BACKGROUND
During the late 1970's and early 1980's, concern over the emissions from the practice of municipal solid waste (MSW) incineration resulted in the development of more efficient incinerator operating conditions and improved air pollution control technologies. In turn, this enhanced ability to minimise emissions has resulted in the capture of greater volumes of contaminated residues in modern air pollution control (APC) systems. It is generally acknowledged that incineration typically results in a 90% reduction in the overall volume of material for disposal, and a 60 to 75% reduction in weight of municipal solid waste (MSW).
The incineration of municipal wastes is generally recognized to produce the following products: bottom ash and fly ash. Between 70 and 95% of the total ash generated by municipal waste incineration is characterized as bottom ash. Bottom ash typically contains less than 2% combustible material. Due to the extremely high temperatures at which the incineration of municipal solid wastes occur (typically about 1000° C.), there is virtually complete dissociation of organic compounds and volatilization of some metal species, leaving the bottom ash relatively heat stable and chemically inert. Typically bottom ash is relatively insoluble, with only approximately 5% of the total mass thereof being soluble in water.
The remainder of the waste incineration product is classified as fly ash. Fly ash is very soluble in water. For example, up to 30% of heat recovery system ash is soluble in water. The most common species measured in the leachates from fly ash are salts and other flue gas reaction products, specifically chloride and sulphate compounds. Chlorides alone can account for almost 40% of the weight of the soluble fraction of some fly ash. Accordingly, fly ash poses a contamination risk if it is disposed of in such a way that it may come into contact with ground water. Fly ash is considered to be a hazardous waste, necessitating “storage” in underground abandoned mines, or in specialized cells in landfill sites. Alternatively, fly ash must be treated prior to disposal if it is to be disposed of as a non-hazardous material.
Wide spread acceptance and proliferation of municipal solid waste (MSW) or energy-from-waste (EFW) facilities have been tempered by public and government concerns regarding the lack of environmentally acceptable means to dispose of classified residues from the incinerator operations. In most countries around the world, the residues generated within the air pollution control (APC) unit operations of an EFW incinerator are considered classified due to the high concentrations of readily soluble salts and potentially soluble trace metals. As a consequence of these concerns, disposal of the APC residues is one of the major issues limiting the acceptance of new EFW incinerator facilities.
To comply with increasingly stringent air emission regulations, modern air pollution control (APC) systems are designed to cool and chemically condition incinerator flue gases. The hydrogen chloride (HCl) and sulphur dioxide (SO
2
) gases that are formed during the combustion of MSW are humidified to reduce the gas temperature to below 150° C. Cooling the incinerator off-gases promotes condensation of vaporous contaminants and enhances certain chemical reactions. This chemical conditioning is generally facilitated by the injection of powdered hydrated lime, or some other form of caustic solution, into the flue gas stream to act as an acid gas sorbent and provide reactive surfaces for condensation of volatile compounds. There are two major types of APC systems: i) wet lime injection and ii) dry or semi-dry lime injection. Acid gas neutralization reactions in dry and semi-dry lime injection APC systems result in the significant production of a predominantly calcium chloride (CaCl
2
) and calcium sulphate (CaSO
4
) salt waste residue stream. The CaCl
2
and excess lime present in the APC residues are not solubility limited and therefore are released from the solid matrix quickly upon contact with water. In fact, up to 85% of some APC ash residues are water soluble. Furthermore, since the stoichiometric ratio of the lime addition during chemical conditioning is greater than 1, the APC ash residues are highly alkaline, and the potential to solubilise amphoteric metal compounds, such as certain lead compounds, is greatly increased. Consequently, leachates from these residues may contain high concentrations of salts and trace metals including aluminum, lead, chromium and zinc.
Today, disposal of hazardous APC residues is one of the major issues limiting the acceptance and proliferation of new MSW incinerators. It has been argued that the benefits derived from landfill solid waste diversion and energy recovery from MSW incineration are negatived by the precautions which must be taken in the disposal of hazardous APC residues. The risk of leachate contamination of soil and ground water gives rise to long term environmental concerns. In response to these concerns, legislators around the world have generally responded by drafting guidelines that recommend pre-treatment of APC residues prior to disposal. Regulatory guidelines are based on the different characteristics of the two ash streams generated from municipal incinerators. Typically, incinerator design, operation and the type of air pollution control (APC) system each impact upon the residue characteristics. In particular, these factors are responsible for the partitioning of trace metals between the bottom ash and fly ash residues, and the overall solubility of the fly ash residues (see Sawell, S. E. and T. W. Constable.
The National Incinerator Testing and Evaluation Program: A Summary of the Characterization and Treatment Studies on Residues from Municipal Waste Incinerators
. Environment Canada Report, 1993). The fly ash/APC residue mixture is highly soluble (up to 65% soluble) By contrast, bottom ash is only about 5% soluble. Therefore, bottom ash is generally classified as acceptable for disposal in a sanitary landfill sites, whereas fly ash/APC residue is considered to be a classified hazardous waste due to the high concentrations of readily soluble salts and potentially soluble trace metals. There are considerable costs and significant future liability considerations associated with the generation (and storage/disposal) of classified hazardous wastes, as compared with the storage or disposal of non-classified wastes. In Canada, the Canadian Council of Ministers of the Environment (CCME) have published guidelines that specify that APC system residues should be collected and processed separately from the rest of the incinerator ash streams in order to allow for treatment prior to disposal. (see
Operating and Emission Guidelines for Municipal Solid Waste Incinerators
Report CCME-TS/WM-TRE003, June 1989.)
After disposal, solid wastes may come into contact with a leachant, such as rainwater, open surface water, or groundwater. Although attempts are made to isolate waste materials to prevent them from coming into contact with water, it is rarely possible to completely prevent contact with water. Accordingly, some of the constituents in the disposed of waste may dissolve into the leachant. Water is continually cycling between the atmosphere, surface water, and groundwater, such that the leaching of solid wastes and the subsequent transport of dissolved waste constituents can have far-reaching environmental implications. The kinds of solid wastes which are of greatest concern for leaching are incinerator, fly and other combustion ashes; sludges and cakes from physical and chemical wastewater treatment operations; contaminated soil
Hetherington Stephen A.
Sawell Steven E.
Apex Residue Recovery Inc.
Griffin Steven P
Hofbauer Patrick J.
Ildebrando Christina
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