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Chemicals management
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Salt slag, also known as black dross or salt cake, is a hazardous waste produced in a rotary furnace where aluminium waste is melted under a layer of salt. If the aluminium waste is an industrial waste, the salt slag belongs to category Y18, residues arising from industrial waste disposal operations, in Annex I of the Basel Convention on the Control of the Transboundary Movements of Hazardous Wastes and their Disposal ("the Basel Convention") and is controlled under that Convention. However, if the aluminium waste is in the form of a non-industrial waste, such as used beverage containers, the salt slag has the same hazardous characteristics but does not appear to belong to any category in Annex I of the Basel Convention. This means that, unlike other salt slag, it is not controlled under that Convention.
| Aluminium dross: | A thick liquid or solid phase that forms at the surface of molten aluminium. |
| Aluminium scrap: | A secondary raw material which arises through the recycling of aluminium-containing products, or during production and processing of aluminium. |
| Aluminium skims: | An aluminium dross with a higher aluminium content, a mixture of aluminium metal and aluminium oxide. |
| Salt slag: | A residue produced in a rotary furnace where aluminium waste is melted under a layer of salt. |
Aluminium melts at 720°C and burns at 800°C. The narrow gap means that aluminium can be recovered from waste by melting it in a furnace but it is difficult to melt all of it without burning some of it: other materials are typically present in waste and form hot spots. Most recovery facilities avoid burning the aluminium by adding salt. This floats on top of the aluminium and prevents air from reaching the metal.
Most commonly the layer of salt consists of a mixture of approximately 30% KCl (potash) and 70% NaCl (rock salt) which may also contain small amounts of CaF2 (fluorspar). This salt layer fulfils a variety of tasks. It enhances heat transfer to the metal, it prevents oxidation of the metal, and it takes up contaminants such as oxides, carbides and others contained in the waste or produced by reactions during the melting process.
The amount of salt used for the melting process depends on the waste characteristics and varies from 300 to 400 kg/t aluminium waste. First, the salt is melted down in the rotary furnace. Next the aluminium waste is charged into the salt bath. Once the metal is molten it is tapped and cast into steel moulds where it cools down.
The advantage of the rotary furnace is that even highly contaminated waste can be handled. There are two disadvantages. One is the high energy demand because in addition to the metal the salt has to be melted. The other is the cost of processing the salt slag. Depending on the waste mix the amount of salt slag produced per tonne of secondary aluminium varies from 400 to 700 kg.
Salt slag contains contaminants like oxides, carbides and sulphides as well as metallic aluminium which is entrapped in the slag and traces of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD and PCDF).
Salt slag exhibits two hazardous characteristics listed in Annex III of the Basel Convention. It exhibits hazardous characteristic H4.3 because in contact with water it emits flammable gases such as acetylene. It also exhibits hazardous characteristic H10 because in contact with water it is liable to give off toxic gases, such as ammonia, in dangerous quantities.
It is uncertain whether salt slag contains hazardous concentrations of PCDD and PCDF. These chemicals are unintentionally formed and released from thermal processes involving organic matter and chlorine as a result of incomplete combustion or chemical reactions. Secondary aluminium production is listed, in Annex C to the Stockholm Convention on Persistent Organic Pollutants, as an industrial source that has the potential for comparatively high formation and release of these chemicals to the environment.
Salt slag is produced in the recovery of aluminium from aluminium waste. This falls within the scope of disposal operation R4, Recycling/reclamation of metals and metal compounds, in Annex IV to the Basel Convention. The Basel Convention defines "disposal" as any operation specified in Annex IV. When the aluminium waste is in the form of an industrial waste such as aluminium dross or scrap, including coated scrap, the salt slag belongs to category Y18 in Annex I to the Basel Convention, Residues arising from industrial waste disposal operations.
Salt slag of this kind is a hazardous waste covered by paragraph 1(a) of Article I of the Basel Convention. Movement from one country to another is controlled under the Basel Convention. Export of salt slag of this kind from Australia without a permit under the Act would contravene section 40 of the Act, which provides for fines in excess of one million dollars and imprisonment for not more than five years.
However, if the aluminium waste is in the form of a non-industrial waste such as used beverage containers, the salt slag does not belong to any category in Annex I to the Basel Convention. Salt slag of this kind is a waste that possesses hazardous characteristics, but is not a hazardous waste covered by paragraph 1(a) of Article I of the Basel Convention. Movement from one country to another is not controlled under the Basel Convention. Export of salt slag of this kind from Australia without a permit under the Act would not contravene section 40 of the Act.
The objective is to ensure that all exports and imports of salt slag, whatever their source, are regulated under the Hazardous Waste (Regulation of Exports and Import) Act 1989 ("the Act").
The most feasible option is to amend the Hazardous Waste (Regulation of Exports and Imports) Regulations 1996 so that all forms of salt slag produced in the recovery of aluminium from aluminium waste, or secondary aluminium production, are regulated in accordance with paragraph (a) of the definition of hazardous waste in section 4 of the Act.
There are understood to be five facilities that generate salt slag in Australia. In the event that any of these facilities processes non-industrial wastes only, that is, that no industrial waste of any kind enters the furnace, there is a risk that the resulting salt slag could be exported without being approved under the Act.
The proposed amendment would ensure that all persons exporting or importing salt slag from or to Australia must obtain a permit under the Act to ensure that the hazardous waste is managed in an environmentally sound manner so that human beings and the environment, both within and outside Australia, are protected from the harmful effects of the waste.
The purpose of consultation is to focus on how the proposal is expected to affect business and stakeholders, including changes to regulation, the effects of extended coverage for salt slag from non-industrial waste, and what it may mean for business costs, environmental outcomes and the like.
Information and comments on this paper, including answers to the following questions, are invited:
Comments will be received up until close-of-business on Friday 4 June 2004, and must be sent to:
The Manager
Hazardous Waste Section
Department of the Environment, Water, Heritage and the Arts
GPO Box 787
Canberra ACT 2601
Fax: 02 6274 1164
Email: hwa@environment.gov.au
For further information on this paper, contact Andrew Inglis by phone 02 6274 1609, facsimile 02 6274 1164, or e-mail andrew.inglis@environment.gov.au. Further information about the Department of the Environment, Water, Heritage and the Arts can be found on the web site: http://www.environment.gov.au.