Sheet and Pipe Curing
The curing of asbestos-cement sheet and pipe is a relatively simple process and by definition involves the use of water, steam or a moist atmosphere. Therefore, it is unlikely that dust is generated during this process as long as normal precautions are taken.
After initial curing in a stack, asbestos-cement sheet is usually cured by storage under humid conditions produced by water spray or steam. Asbestos-cement sheet can also be cured by autoclaving, but this procedure is not often used.
Asbestos-cement pipes are normally pre-cured in curing tunnels which are heated to accelerate the hardening process. For this purpose, the pipes are moved through the tunnel on a multi-stage conveyor belt. The pipes are rotated on the belt to prevent deformation. Pipes with a large diameter may be fitted with wooden mandrels or end plugs for the same reason.
After pre-curing, the pipes are usually submerged in water tanks for a period of time which varies according to the temperature of the water. Alternatively, the pipes are stacked and constantly sprayed with water. Also, pipes are often cured by autoclaving which requires special equipment but is accomplished in a much shorter period of time. Furthermore, autoclaved pipe is considered to be more resistant to attack by aggressive soil because calcium hydroxide (free lime) has been removed by reaction with silica. Hence, silica is only used in pipe manufacture when the pipe is subsequently autoclaved.
Cutting and Finishing of Sheet and Pipe
As mentioned above, asbestos-cement sheet should be cut before curing. However, in some plants, cured asbestos-cement sheet (hard sheet) is trimmed to size, or small pieces are cut from larger ones, using circular saws. This is usually an extremely dusty operation even when the sheets are wetted, and should take place in a separate location with special equipment. If hard sheet is cut by saws, properly designed hoods and exhaust ventilation should be provided. Alternatively, water can be used for dust suppression by directing a stream of water or fog directly onto the saw blade or cutting tool. Obviously, provisions for collecting and discarding the contaminated water must be available.
Even under these circumstances, asbestos-cement dust and debris can collect around the equipment. Clean-up of this material should be carried out on a frequent basis using wet sweeping, HEPA vacuum cleaners or hoses attached to the bag house. Clean-up crews must wear approved HEPA respirators when performing vacuuming operations.
It must also be kept in mind that exhaust ventilation has the purpose of removing and collecting large quantities of asbestos-cement dust. This dust must be collected and disposed of by proper procedures. If this point is neglected, large amounts of dust will be redistributed and the workers in the particular area can be exposed to an unacceptable hazard.
Under normal circumstances, the ends of asbestos-cement pipes are cut by saws in one stage and finished on a lathe in another. Pipes are also cut into smaller pieces and finished on a separate lathe to produce pipe couplings. There are several modifications that can be introduced to reduce or eliminate the dust created during these operations. For example, the pipes can be used in a wet condition because wet asbestos-cement generates less dust. Also, the end cutting and lathing operations can be performed in one stage using the lathe only. The lathe produces chips and shavings instead of dust particularly when cutting wet material. In addition, water may be sprayed onto the cutting tool to further reduce the chance of dust formation.
Finally, both saws and lathes can be fitted with proper hoods and exhaust ventilation. This applies as well to the cutting, finishing and drilling of couplings. Automatic and semi-automatic equipment is particularly suited for this purpose.
Care must be taken when performing maintenance and repair work on equipment. The crew involved must wear protective clothing (disposable overalls or overalls than can be laundered) as well as approved HEPA personal respirators.
Of particular concern is the material collected in bag houses which are often located outside the plant building and tend to escape attention. If the material collected by the bag house is allowed to spill onto the ground, a new source of dust is created than can be hazardous to the workers as well as personnel around the plant. Bag houses must have properly designed collecting devices (containers) that allow removal of asbestos-cement dust without creating a problem.
Disposal of Solid Waste and Effluent Water
In many situations, it is possible to grind all the solid asbestos-cement waste generated in the plant and return it to the sheet or pipe process. In some cases it is neither possible nor desirable. In such cases, this material should be wetted, collected and disposed of. The recommended practice calls for disposal and burial in a landfill site approved by regulatory agencies. It is essential that the crew performing this task be trained to recognize hazards due to high dust levels and to introduce appropriate corrective measures.
Most process water can be collected in silos and reused after a simple settling process. However, the solids settling in silos and settling tanks present a problem. If this material cannot be returned to the process, steps must be taken to dispose of it in a proper manner. This means correct disposal in landfill sites.
The handling of this material can present a problem to the worker, particularly when it is allowed to dry out. Therefore, the crew removing this material must receive proper training, equipment and protective clothing, i.e. disposable overalls or overalls that will be laundered. As in all cases where dusty material must be handled, showers and facilities for the cleaning, laundering or disposal of protective clothing should be made available.
If some of the process water is discarded, care must be taken that it does not contaminate land and water around the plant. A recommended procedure is to lagoon this water and remove the material settled out to a landfill site. This task should be performed by trained personnel capable of handling potentially hazardous situations.
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