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The VitroArc® Process Description
Overview
VitroArc is a vitrification process for stabilising and detoxifying fly ash from waste incineration plants. The process converts the dusty materials into a stable non-leaching mineral/slag.
The fly ash enters the process in the flash melting zone where all toxic organic materials are gasified and decomposed. Volatile inorganic material like zinc and lead evaporate while the rest of the inorganic material melts. A plasma generator and natural gas provide for energy to the process in the flash melting zone.
The melted inorganic material is collected at the bottom of the reactor and continuously tapped through a siphon type tapping device.
Oxides of zinc and lead can be recovered in the gas leaning system of the process.
Background
Fly ash from waste incineration is a growing environmental problem. High contents of chlorines and sulphates together with various heavy metals and organic compounds make this residue highly leachable and toxic.
There are a large number of waste incineration plants in the world today. Although incineration emits limited pollutants to the atmosphere, many hazardous components of the waste concentrate in the solid residues from the incineration. These residues can represent up to 20 percent of the waste treated and are divided into a bottom slag and fly ash. In particular, the fly ash contains large quantities of hazardous compounds such as dioxins and heavy metals. To prevent these toxic and hazardous materials from polluting the environment, many countries have passed regulations that prohibit landfill of untreated fly ash. Stabilisation of the fly ash with cement has not been very successful and is normally not accepted by environmental agencies as a final solution. The regulations, while laying down conditions for the disposal of residues, also call for the possibility of utilizing the residue, provided the polluting potential is within specified requirements.
The VitroArc process converts the fly ash into a vitrified black, glassy slag that is stable and leach resistant. The hazardous metals evaporated in the process are recovered while the glassy slag is usable for various applications. The process has been tested in a pilot plant with an hourly capacity of 1 ton. More than 400 tons have been treated.
Vitrification of fly ash
There are many vitrification processes potentially suitable for pulverized products. These processes use electricity to generate high fusion temperatures to form a usable slag and to recover a heavy metal fraction.
The challenge for all vitrification processes is that fly ash, compared to traditional pulverized products, has a smaller particle size, lighter specific weight, exhibits a higher diffusivity into air and has a higher water absorption capability. In addition, the properties and composition of the fly ash depends very much on the type of waste incinerated and whether wet or dry flue-gas cleaning is used.
The VitroArc® process
The VitroArc process represents a solution where any harmful organic compounds of the fly ash are completely decomposed and removed.
The VitroArc process shown in fig 1 is intended for a stand alone facility and consists of the following main elements:
- Material handling including storage and injection system.
- Thermal treatment unit including the unique plasma generator system.
- Air pollution control including gas cleaning and water purification.
Fly ash from the storage tank is pneumatically fed into the plasma heated vitrification reactor. The volatile part of the fly ash (chlorine, sulphur and possibly zinc and lead) is evaporated. The residue is vitrified to a black glassy slag, which is stable and leach resistant.
Electric energy to the plasma generator and partial combustion of a fuel provide for energy to the melting of the fly ash and for the gasification reactions.
The fly ash is treated in a reactor by the unique plasma generator technology, which has been utilised for more than fifteen years in the metallurgical industry.
Several reactions take place in front of the plasma generator (the flash melting zone) at a temperature of about 1400 C:
- Decomposition and evaporation of chlorines, sulphates, carbonates and part of the alkaline metals
- Reduction and evaporation of zinc and lead
- Decomposition and partial combustion of organic compounds.
- Vitrification of the remaining part of the fly ash
The retention time in the flash melting zone is about one second. Gas and slag are separated after the flash melting zone. The vitrified slag leaves the reactor in a continuous flow through a siphon type, tapping device.
Most of the heavy metals are vaporised when the fly ash is heated. The chemical composition and the leachability of the remaining slag depend on the ratio between SiO2 and CaO as well as on the Al2O3 content. The leachability also depends on whether the vitrification takes place under reducing or oxidising conditions. Al2O3 modifies the property of the glassy slag and reinforces the stability, in particular resistance to chemical agents. Likewise, oxides as Na2O, K2O, CaO and MgO act as modifiers and reduce the vitrification temperature.
Most fly ashes contain the main elements, silica and aluminium, in amounts favourable to vitrification.
In order to evaporate most of the zinc and lead from the fly ash and suppress the NOx content in the gas to below 50 ppm, only about 20 -30 % of the CO is oxidised to CO2 in the flash melting zone.
Air is injected into the reactor to fully oxides the gas i.e. CO to CO2 and H2 to H2O before the gas leaves the reactor.
The gas from the reactor is quenched with water at the outlet and is then cleaned from particles and acid gas components in a venturi scrubber and a conditioner. The produced fuel gas contains approximately equal amounts of CO, CO2 and H2.
The major part of the impurities is extracted from the gas in the water quench. The sludge from the quench and the remaining gas cleaning system is rich in zinc and lead. The sludge can be sold to refining companies for recovery of the zinc and lead. The soluble alkali salts may be dried or concentrated for reuse in the industry.
Products
The products from the vitrification process are:
- About 600 kg of a vitrified, stable and leach resistant glassy slag per tonne of fly ash. The content of lead and zinc in the vitrified slag can be kept below 0.04% and 0.4% respectively and depends on the degree of oxidation during operation.
- A sludge containing mainly metal hydroxides. The content of zinc and lead is in the order of 50% and consequently the sludge is a good basis for recovery of those metals.
- A cleaned solution of water-soluble salts, mainly NaCl and KCl.
Test results
To verify and test the process described above, a pilot plant, with a treatment capacity of one ton of fly ash per hour, was built at the ScanArc facility in Sweden. An extensive test programme was carried out and more than 400 tons of fly ash was treated.
The energy consumption in the vitrification is about 1000 kWh per ton of dry fly ash in addition to about 40 kg coal powder or hydrocarbons used to control the NOx and zinc as described below.
A significant amount of testing has been done to optimise the separation of zinc from the slag. The results show that the zinc may be separated from the slag by controlling the combustion ratio shown in fig. 3. Simultaneously the NOx formation is also controlled as shown in fig. 4.
Leachability
The produced slag has been tested for leachability with the most stringent test method available.
Slag samples have been ground and repeatedly mixed with sulphuric acid. Analysis of the lechate shows that the VitroArc slag fulfils the demands for unrestricted use, se table below.
RESULT OF LEACHING TEST, CEN TC298
Metals (mg/kg) leached from different materials compared to Dutch threshold limits for non restricted use as construction material (building). Numbers below the limits are green while numbers above is red.
MSWI: Municipal Solid Waste Incinerator
System Description
The VitroArc plant for vitrification of fly ash may be built either as an integrated system in an existing incineration plant or as a stand alone unit comprising full gas cleaning and water cleaning facilities as shown on Figure 1.
The plant consists of three main areas:
- Storage and feeding system
- VitroArc vitrification reactor
- Gas and water cleaning system
By integration into an existing incineration plant, considerable savings and simplifications may be obtained in particular in the gas- and water cleaning system.
If the gas is used as additional fuel in the incinerator only a quencher and a simple scrubber is necessary in the VitroArc plant. The final gas cleaning takes place in the gas cleaning system of the incineration plant.
Further, if the existing plant has a wet gas cleaning system with separation of soluble salts only a one step precipitation and filtration unit is required in the VitroArc plant for removal of the zinc concentrate. The remaining water treatment may take place in the existing water treatment plant.
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