CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997
Burning of hazardous industrial wastes in cement kilns has become a well accepted method for the disposal of hazardous wastes in France and a number of other European countries.
The principal processes employed in making cement clinker can be broadly classified as either "wet" or "dry" depending on the method used to prepare the kiln feed. In the wet process the feed material is slurried and fed directly into the kiln. In the dry process the kiln exhaust gases are used to dry raw meal (mixture of limestone and other raw materials) while it is being milled (Independent Panel on Intractable Wastes, 1992).
A cement kiln typically comprises a long cylinder of 50 to 150 metres, inclined slightly from the horizontal (3% to 4% gradient) which is rotated at about 1 to 4 revolutions per minute. The solid material passes down the kiln as a result of rolling and slipping as the kiln rotates. The material flows counter current to the combustion gases and fuel is fired at the lower (front) end of the kiln (Woodcroft, 1992).
Gases discharged from the kiln are normally cleaned of particulate matter by passing them through an electrostatic precipitator. Dust collected in the precipitator can be returned to the process.
Kiln fuel firing systems are designed to minimise energy consumption and to provide appropriate flame shape for clinkering of the raw materials.
Operating conditions within the kiln are maintained and controlled by monitoring numerous plant operating parameters throughout the system. These include feed material composition, gas temperatures and gas flow rates. These parameters are used for control of feed flow rates into the unit (for raw meal and fuel) and for controlling discharge gas flows from the unit.
Kilns are lined throughout with refractory bricks providing varying degrees of insulation to the steel shell. Brick compositions vary from around 25% to 30% alumina at the cooler back end, to 45% alumina in the calcining zone and rising to 70% alumina as the burning zone is approached. Dense magnesite or dolomite bricks are used in the burning zone. Heat transfer within the kiln system results from a complex interchange between the gas, inner kiln walls and feed surface.
The clinker manufacturing process starts by producing a fine powder containing strictly controlled proportions of:
When the powder is homogenised and heated to 1450C in the kiln, the lime molecules combine with all the silica, alumina and iron oxide molecules to form clinker.
The raw materials are transformed into clinker in several stages:
The material can only form at this temperature and consequently the material must reach this temperature to make clinker.
After reaching this temperature the clinker is rapidly cooled and finely ground, 3% to 5% gypsum is added to control the setting rate and other additives (slags, fly-ash, limestone filler, etc) may be introduced to form the final product (Schrieber, 1992).
A schematic diagram of a typical cement kiln is shown in Figure 7.1.
At the very high temperature of the cement kiln, and with the long residence times available, very high destruction efficiency is possible for scheduled wastes. The highly alkaline conditions in a cement kiln are ideal for decomposing chlorinated organic wastes. Chlorinated liquids, chlorine and sulphur are neutralised in the form of chlorides and sulphates. The quantities of the inorganic and mineral elements added in treating scheduled wastes are usually limited (and in general will be a small proportion of the large feed requirements of a commercial kiln), and should not adversely affect the quality of the clinker product. No liquid or solid residues requiring disposal are generated as all residues are bound within the product.
In some dry and some wet cement kiln processes there is a slight concentration of heavy metal in the by-pass dust waste produced when inorganic materials are included in the scheduled waste being treated. Dust waste can be utilised as fertiliser, for liming or is dumped.
7.1.1 Application of Cement Kilns to Scheduled Waste
The manufacture of cement requires the utilisation of large quantities of raw materials and utilises operating conditions consistent with the destruction of waste materials. As such, cement kilns are readily suitable for the destruction of scheduled wastes. To evaluate potential hazardous waste discharges from cement manufacturing, the chemical requirements of the process need to be considered from both a fuel and raw material standpoint.
Appropriate wastes for disposal in cement kilns are those which provide additional energy value as a substitute fuel, or material value as a substitute for portions of the raw material feed (eg calcium, silica, sulphur, alumina or iron). Contaminated soils and industrial sludge (and wastewater eg black liquor) may be substituted for the fuel, limestone, sulphur or silica used in the manufacturing process. Waste may contain constituents of calcium, alumina, silica, sulphur or iron, similar to the raw materials normally quarried for this use. The cement chemistry can easily be adjusted to best utilise these constituents.
The cement kiln provides a flame temperature of more than 20000C with long retention times for both gases and raw materials. The processing of large quantities of raw materials provides a high degree of thermal stability which is not easily affected by variations in raw material feed, or interruptions in fuel flow (Schrieber, 1992). Options for treatment of wastes are outlined as follows:
(a) Liquid Waste
Liquid waste or low ash wastes can be relatively easily burnt in cement kilns. The material can be fed in dry or in slurry form (especially with the 'wet' process), or as a fuel supplement into the burning zone of the kiln. In this zone, the temperature of 14500C is able to effect a high destruction efficiency as the gas passes though the kiln.
(b) Soils and Solid Wastes
With typical counter current process configuration soils and solid wastes cannot be fed into the firing end of the kiln, as they will discharge in the clinker without adequate treatment and they cannot be fed into the cool end of the kiln, as the wastes will volatilise and will not be adequately destroyed.
Two options are possible:
When operated properly, destruction of chlorinated compounds in cement kilns can be >99.0000% complete with no adverse effect on the quality of the exhaust gas (Benestad, 1989). The contribution of waste materials to the exhaust gases are relatively minor given that the wastes are only used as a minor supplement to the main energy or raw material stream. In addition, chlorine added to the kiln in the form of chlorinated wastes, is likely to combine with an excess of calcium from the limestone (Hansen, 1992). One example where a PCB waste was added to the feed stream of a cement kiln resulted in a PCB concentration in the stack gas of less than or equal to 0.1 µg/m3 and a dioxin concentration in the range of 0.0001 to 0.0002 µg/m3 (TE) (Karstensen, 1992). While the concentrations of waste constituents in the exit gases are relatively low, the total exhaust gas volumes are much greater than most other scheduled waste treatment systems.
A significant advantage of the cement kiln process is that no chemical residues or solids requiring disposal are generated as these are incorporated into the clinker. However, if a large quantity of solid waste is to be introduced, it may be necessary to review the cement chemistry to ensure that the quality of the clinker is maintained.
The application of cement kilns to the treatment of mixed wastes such as OCP/iron and OCP/arsenic mixtures requires careful consideration. Low volatility metals such as iron are likely to be effectively immobilised in the cement product. However a significant proportion of the volatile metals such as arsenic are likely to escape in the gaseous emissions. If such wastes are slowly bled into the cement kiln the gaseous emissions are unlikely to pose a significant health or ecological risk, however cement kilns could not be considered as an effective treatment process for arsenic containing wastes.
Treatment costs are not reported in the literature and it is difficult to estimate the cost of local treatment at this stage. The costs will vary depending on the characteristics of the waste to be treated, and whether modifications to the kiln are necessary, but it is expected that they will be much less than a dedicated rotary kiln incineration system which involves high capital cost, high energy consumption and a significant cost for disposal of the solid residues produced. The operating and maintenance costs of mobile rotary kiln incinerators range from $550 to $950 per tonne, excluding the costs incurred in transport, setting up equipment, excavating soil and disposal of treated material and residue.
There are apparent advantages associated with the treatment of wastes in cement kilns, and advice from the local Cement Industry Federation indicates current cement kiln operators in Australia could be interested in burning scheduled waste materials in the foreseeable future depending on economic, political, environmental and production considerations (Cement Industry Federation, 1997). In practice, the willingness to consider treatment of scheduled wastes may vary from cement kiln operator to cement kiln operator, reflecting the commercial considerations, the attitude of the cement kiln operator, and the acceptability of the proposals to the surrounding community.
Burning of non-scheduled waste materials such as disused car tyres and waste oil is a relatively new practice in Australia and a significant amount of work is currently under way to assess potential fuel usage reduction (Riley, 1996). Some cement kiln operators consider that at present it is not in their best interests to seek to destroy chlorinated wastes, because of the ready availability of non-chlorinated waste materials for use as fuel, whereas others suggest each proposal would be judged on its merits. Part of the reluctance to treat scheduled wastes is due to the potential adverse public reaction from the handling and burning of scheduled wastes (in particular the risks associated with air emissions and transport of the wastes), and from the wider public in terms of perceived adverse effects on the quality and safety of finished concrete materials (Riley, 1996).
Rhone-Poulenc has expressed interest in facilitating cement kiln waste treatment in Australia, based on its European experience, by formulation of waste derived fuels (Bowles, 1997). However, activity of this kind would depend on cement kiln operators proceeding with scheduled waste treatment.
On this basis, while from a technical perspective cement kilns are quite capable of being used for the destruction of scheduled waste materials, it is uncertain whether this will occur on a commercial scale in Australia in the foreseeable future.
(a) Proponents (in Australia)
Various (represented by The Cement & Concrete Industry Association in Australia). Rhone-Poulenc has expressed interest in treating scheduled wastes in cement kilns in Australia.
(b) Wastes Applicable
PCB liquids are commonly destroyed using cement kilns overseas. Liquid scheduled wastes can be destroyed in a cement kiln with appropriate limitations on the proportion of the waste in the feed stream. Solid wastes cannot be treated unless special mid-kiln feeding systems are provided, or the wastes are pre-treated. Ferrous materials and equipment items can be treated in cement kilns. Large quantities of soil cannot be disposed of directly to cement kilns, although pre-treatment followed by treatment of the off gas is possible.
(c) Contaminants Applicable
All scheduled compounds.
Cement kilns (both 'wet' and 'dry') are currently available. Because of the availability of non-chlorinated waste materials for use as fuels and the potential adverse public reaction to the burning of scheduled wastes, some cement kiln operators do not consider it to be in their best interests at present to burn scheduled wastes. In order for scheduled wastes to be burnt in kilns, trials would need to be initiated and approvals sought. Changes in off-gas processing may be required, although as hazardous wastes would only represent a very small portion of the total feed, contaminant concentrations in the off gas are unlikely to alter significantly.
(e) Timing for Commercialisation in Australia
Commercialisation is not expected in the immediate future due to non-technical impediments (see Section 7.3 (h)). However, some kiln operators have stated a willingness to consider specific proposals on their merits.
(f) Cost (example only)
Unknown. Expected to be lower than for most other treatment methods.
(g) Safety/Environmental Risk
Treatment of wastes in cement kilns can be regarded as relatively safe if feed systems are properly designed and appropriately qualified personnel are available for operation. The high thermal mass of the system and the low proportion of waste in the feed, limits the potential release of incompletely combusted material in the event of a failure of the fuel feed system. Acid gases formed in the combustion of chlorinated wastes react with the alkaline cement materials and reformation of dioxins is not expected.
The burning of scheduled wastes is unlikely to significantly affect the quality of air emissions unless a high proportion of waste is used as a feed or fuel.
(h) Non-technical Impediments
Advice indicates that the cement kiln operators in Australia could be interested in burning scheduled wastes pending resolution of related economic, political and environmental issues. These include potential adverse reaction from the public and the availability of other non-chlorinated waste feedstocks. Support from relevant government agencies would be required before a cement kiln operator is likely to take the risk of moving to toward treatment of scheduled wastes.
(i) Preferred Mode of Implementation
Soil in large quantities is not able to be treated. Operators are primarily interested in treating high calorific non-hazardous wastes and, in general, are not interested in applying the technology to scheduled wastes at this point in time.
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