L. Frame
ARCPROCESS, INC.

S. Hickel
PHOENIX Solutions Co.

D. H. Davis, K. J. Hewlett, D. M. Bennert
Envitco, Inc.

A production scale, mobile waste treatment system has been constructed and demonstrated on asbestos material by ARCPROCESS, Inc. The system has been deployed to treat several million pounds of excess South African amosite asbestos currently being held in inventory by the U.S. Government. The materials are being vitrified without additives to produce a glass wasteform that can be recycled to a practical use or landfilled.

The heart of the system is a 500 kW plasma arc torch supplied by PHOENIX Solutions. The torch is configured to start non-transferred and shift on-the-fly from non-transferred to transferred. The torch is mounted with a ball seal which seals the furnace and allows pitch and extension action. The vitrification reactor is a highly insulated tank designed and constructed by Envitco, Inc. allowing for either continuous or batch operation. The vitrification unit (torch and tank) is integrated with a semi-automatic feed system and off-gas treatment train, complete with HEPA filtration. The melter, torch system, cooling system, off-gas treatment system, air compressor and power supplies are all mounted on a single trailer to facilitate moving and set-up. The treatment train is designed for continuous operation at a rate of approximately 1000 lb/hr.

Although being demonstrated on obsolete inventory asbestos, the system can be applied to ashes, concrete and other asbestos-containing wastes including construction debris, insulation and D&D wastes.

This paper will discuss the melter design approach and plasma torch system. Results will also be presented on the treatment program, including waste data, throughput rates, glass quality and overall performance data of the melter system. Future treatment programs will also be discussed.

Asbestos fibers have long been used in insulation of various items in the defense industry, including ships, storage tanks and buildings.

The DLA (Defense Logistics Agency), an agency of the DOD (Department of Defense), has millions of pounds of raw asbestos fiber stockpiled at multiple sites in the U.S. This material was acquired years ago to maintain strategic positions. Substantial tonnages of asbestos-containing materials (ACM) have also accumulated since asbestos has been regulated and are stored at other DOD sites around the country. In some cases, the asbestos or ACM is bagged or, in other cases, stored in bulk in a tank or silo.

Asbestos and ACM is hazardous because of the characteristics of the individual fiber. While asbestos can legally be landfilled at the present time if properly contained, such disposal carries a severe potential future liability. However, when asbestos is melted, the fibrous nature of the material is destroyed and the resulting material becomes non-fibrous, non-hazardous and, indeed, non-asbestos.

This vitrified non-asbestos is not only no longer fibrous, but passes TCLP leaching requirements and can be considered non-hazardous for disposal in a municipal landfill. In fact, the huge reduction in volume from vitrification is significant by itself. This vitreous material can also be safely recycled for a variety of applications but, to date, the total tonnage does not merit developing these uses.

Other means of melting asbestos have been used employing either fossil fuel or Joule-heated electric melting. These processes operate at significantly lower temperatures than does the plasma arc process, and their maximum operating temperatures are only slightly higher than the asbestos melting point. As a result, the melt rate is much lower, the equipment much larger, and the specific energy requirement higher. In the case of either fossil fuel or Joule-heated melting, the asbestos is normally combined with additional non-waste fluxing materials in order to improve the melting rate of the asbestos materials. As a result, this makes the energy usage higher yet, in addition to greater volatile losses and more complex air pollution control systems. All of the above makes such treatment facilities larger, more costly and unlikely to be transportable.

Plasma arc melting systems can be packaged to be simple to set up and tear down, and easy to transplant from one site to another. They are efficient, high-throughput systems well-suited to melting materials such as asbestos.

The ArcProcess Plasma Arc Treatment System is currently deployed to treat several million pounds of excess South African amosite asbestos currently being held in inventory by the U.S. Government. This form of asbestos is basically a mixture of iron and silica with about 40% iron. Amosite melts at approximately 2550 ° F (1400 ° C), which implies that the melt should be at 2700° F or more to flow easily from the tap.

The raw asbestos currently being treated is stored with a broad size range, from extremely small fibers to sizes resembling sticks perhaps 3/4" diameter by 6" to 8" long. This size range, together with composition, makes it very difficult to feed with traditional feed mechanisms. A sample of this raw amosite is shown in Figure 1 in addition to a sample of the vitreous, fiber-free product resulting from the treatment.

A photograph of the processing system deployed on a DLA Asbestos storage site is shown in Figure 2.

After observing disappointing results from a batch process melting system, ARCPROCESS made the decision to produce a system that would be capable of continuous tapping. The design of a continuous flow melter is a balance between the corrosion of the refractories by the liquid slag, the surface temperature of the ceramics, the radiant heat from the plasma arc, the temperatures required for good flowability and the feed rate of the raw asbestos. The state-of-the-art in melting asbestos is still in its youthful phase, if not its infancy. As such, the continuous system must start up in a batch mode as the melter, the emission system, and the variables of the melting process are drawn into harmony.

The melting process is manually controlled by regulating the feed rate, plasma arc power and plasma arc position. This control is instituted by the operator based on observation of the furnace interior and melt process via a video camera directed at the tap outlet (drain). The operator observes the build-up of asbestos in the melt zone and regulates the torch power, the torch position and the asbestos feed rate to achieve the desired condition. Figure 3, a frame from the video monitor in the control room, shows the asbestos in the melt zone. The coarse, vitreous, water-quenched, granular product resulting from treatment (non-asbestos) is pictured in Figure 1.

Environmental requirements on the system include that there be no "sneakage" from the system, i.e. no untreated asbestos fiber leaving the system. The slag product is evaluated using TEM (Transmission Electron Microscopy) for residual fibers. To eliminate airborne "sneakage", the furnace is maintained at a slight negative pressure, and exhaust gases pass through a heat exchanger followed by a baghouse operating at a minimum of 500 ° F. The selected bags are rated at 99.99% efficient on particles of one micron and larger.

The energy to melt the asbestos is supplied by a 500 kW input D.C. plasma arc torch built by PHOENIX Solutions Co. This torch is combined with the power supply, the plasma gas supply, the torch cooling system, the starter, the integrator box, the central control console, and the torch positioning system with a ball seal. These components are integrated to make up the complete plasma heating system.

The system is set up in the furnace to provide for vertical translation and single-plane pitch actuation of the torch. The ball seal is the pivot point for pitch actuation, and seals the torch penetration into the furnace to allow a slight vacuum to be maintained in the furnace. Pitch actuation of the torch is used to direct the arc toward regions of the melt where the raw feed may be accumulating or, as necessary, toward the tap to add heat to that area. The PHOENIX torch may also be installed so that it can be actuated in two planes (X and Y), although these actuators are not installed in the current asbestos treatment system. Where appropriate, the torch with the ball seal may be programmed to provide a stirring motion by using both X and Y actuators.

The PHOENIX plasma heating system is a robust system, and, in this application, is typically used in 24 hour per day campaigns twelve (12) days in length. This schedule is the norm so that inspection, adjustment and maintenance of the entire processing system may be accomplished approximately every 300 hours. The system is provided with interchangeable torches so that electrodes and other high-wear parts may be replaced, if necessary, with only 10 - 15 minutes of down-time. The twelve (12) day schedule also provides for operator down-time every other weekend.

The torch can be started in either the non-transferred or transferred mode. In the transferred mode, the arc uses the molten raw material as one of the electrodes. The molten bath is grounded via a bottom electrode connection. Therefore, in a transferred arc application, heating occurs by a combination of convection, radiation, and some degree of electrical Joule-heating. When started in the non-transferred mode, the torch may be changed to transferred mode on-the-fly. After the system has been cold between campaigns, the surface or all of the melt may be solid and non-conductive. In this condition, non-transferred operation may be necessary for 15 minutes to 1-1/2 hours to melt the slag layer before changing on-the-fly to the transferred mode. One of the benefits of the well-insulated furnace being used in this application is that the temperature droop per unit time is much less than with a water-cooled furnace. This provides for faster restarts following maintenance procedures.

Torch power is typically modulated according to the feed characteristics of the raw fibers. The raw fiber does not flow easily and the feed rate is not uniform. In normal operation, the feeding consists of asbestos entering the furnace in 10 to 20 pound batches at some times and, at other times, fairly continuously. Torch power also needs to be reduced at times when it is necessary to hold the liquid bath without feed. The PHOENIX Solutions central console and proprietary software provide for these changes to be made easily. When power needs to be reduced, the arc is moved close to the bath and vice versa, as applied power is influenced by arc length between the torch and the grounded bath.

The Envitco - Phoenix Solutions team was able to merge their unique experiences and perspectives to design a furnace for useful operation in spite of sometimes conflicting requirements. Envitco then completed the final design and supervised the construction of the furnace.

At present, the furnace is configured with a solid roof of castable high-alumina refractory with the outlet penetrations cast into the shape. The melter bowl and sidewalls were constructed from refractory bricks and ram refractory, with a high temperature sealing material between the top of the sidewall and the roof. A propane-air torch is used to add local heat to the slag in the tap area. Figure 4 shows an elevation view of the refractory furnace.

The furnace in which the melting process is being carried out is the second generation design as of October, 1997. The details of design continue to evolve as processing operations continue. A third generation design is currently in progress. The furnace must provide the appropriate relationship of torch, material input, slag outflow, off-gas outlet and viewports. The construction of the furnace and insulation must consider the required melt temperature, the radiant characteristics of the torch, the corrosive nature of the slag, the localized cooling required, the prevention of loss to the environment ("sneakage"), and the structural requirement of the ceramic insulating system.

The offgas treatment system is a simple filtration system, consisting of heat exchangers and a baghouse followed by a HEPA filter, a blower, and exhaust stack. The offgas is generated by:

1. the plasma gas
2. the propane torch exhaust products
3. water vapor from water in and on the asbestos
4. ambient air sucked in through the feed auger and other points due to maintenance of a negative pressure in the melter system.

The construction permit from the U.S.E.P.A. is for an asbestos conversion process under 40 CFR 61.155. The Ohio EPA, through its northwest region, is the air emission regulator. The exhaust from the stack is very clean and free of asbestos fibers.

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