The manufacture of asbestos-cement sheet and pipe represents the largest single use of asbestos. As mentioned previously, most asbestos-cement uses chrysotile. Crocidolite or amosite are used for pipe manufacture in only a limited number cases. The processes for sheet and pipe are similar and consist of a number of operations in sequence, i.e.:
1) fibre debagging,
2) fibre preparation,
3) asbestos-cement slurry preparation [some cement is replaced by silica when autoclaving is part of the process, usually for pipe manufacture only,
4) sheet or pipe forming,
5) curing,
6) finishing, and
7) recycling or disposing of solid waste material or effluent water.
(In some areas, paper or textile fibre are added to the asbestos-cement. This does not constitute a health hazard to the worker and, therefore, is not covered in this report).
In preparation for the first process step, the asbestos bags must be transported to the debagging station either from the warehouse or from the storage area of the plant. This is usually done by forklift truck or hand-drawn wagon. Under normal circumstances, i.e. when the bags are not damaged, this presents no problem from a dust control point of view. However, in many cases the bags that arrive at the debagging station are cut or broken for a variety of reasons. If this situation exists in a plant, it must be corrected immediately. The damaged bags must be repaired or rebagged if necessary. This should be done by a crew trained and equipped for this purpose. Failure to do so will expose nearly every worker in the plant to high dust levels.
It is often recommended to clean-up spilled fibre by applying a spray of water, followed by sweeping. Obviously, the water is meant to wet the fibre and suppress dust during sweeping. However, wet sweeping involves a number of potential problems that the worker performing this task must keep in mind. For example, if the spray of water is too strong, dust is raised together with airborne water droplets. This will settle in another area and become a problem after the water has dried out. Also, wet sweeping may not achieve proper cleaning for acceptable dust control. If not enough water is used, a great deal of dust can still be generated. When too much water is used, a layer of wet fibre remains on the floor and becomes a source of dust after drying. Similarly, unless the floor is flat and smooth, some wetted asbestos will remain behind after sweeping and become a problem after the water has evaporated.
For this reason, it is recommended to replace wet sweeping with vacuum cleaning (HEPA vacuum cleaner or hose connected to the bag house) except for areas that are constantly kept wet, or can actually be washed, such as the areas around the sheet or pipe machine.
A bag house is an industrial vacuum system consisting of several cloth filters (bags) mounted in an enclosure (house), a fan and appropriate ducting. The fan draws dusty air through the ducts and through the cloth filters, where the dust is collected. It is an important aspect of the bag house operation that the air being exhausted is clean and, therefore, does not contaminate the environment outside the plant.
Fibre Debagging
The first processing step involves the debagging of the fibre and transfer to the fiberizing equipment. If for any reason this is done improperly or if the equipment is not suitable, debagging becomes a major source of dust and, therefore, a major hazard for the worker.
The best arrangement for debagging is obviously equipment that does so without involvement of manual steps. There are a number of automatic debaggers available in different sizes suitable for small, medium or large operations. This equipment, when operating properly, provides excellent protection for the worker. Unfortunately, the equipment can malfunction or be improperly operated. If this happens, manual steps are necessary to correct the situation and the risks to the worker may be severe.
For example, automatic debaggers that are open for constant access because of problems with the bag cutting and fibre removal mechanism of the machine may result in the release of large quantities of fibre. This is an improper operation and steps should be taken immediately to correct the situation. Also, an automatic debagger is designed to shred the plastic bags and deposit the shredded material in a plastic sleeve or bag. If this is not done properly, intervention by the worker is required which can result in hazardous exposure.
Automatic debaggers are usually found only in modern plants that are properly designed to operate with little generation of dust. In the majority of cases, debagging is carried out by hand. Properly designed manual debagging stations used for this purpose, consist of a table covered by a hood. The hood should cover both sides and the back of the space above the table, leaving only an adequate opening at the front. These hoods require exhaust ventilation, i.e. they must be connected to a bag house (cloth filter) with a capacity of approximately 3,000 cubic feet per minute. This is necessary to achieve the required velocity for the air being drawn into the hood opening, which should be about 200 feet per minute.
During the manual debagging operation, a bag of asbestos is placed underneath the hood of the station, the plastic bag is removed by cutting and the asbestos is pushed into a discharge port. The plastic bag is then deposited into a disposal bag or shredder directly attached to the hood of the debagger. This means that the plastic bag is not removed from the underneath of the hood in order to avoid spreading dust adhering to the bag. It is most inappropriate to remove bags from the hood and to manipulate them (shaking, flattening, compressing, etc.) or to stack them outside the hood. This type of handling will result in the formation of a great deal of dust and constitutes a severe hazard for the worker. The proper procedure is to place bags into a plastic sleeve, bag or container. This should be followed by burning or safe disposal (i.e. burial) at an approved landfill site. Used asbestos bags must not be reused for other purposes.
A hood of better design resembles a glove box, that is, its front opening is covered by glass or a plastic sheet fitted with long sleeve gloves. In this case the bags are placed onto the table through a side door than can be completely closed after this step. The operator can reach the bag by using the gloves to cut the bag and push the asbestos cake through the discharge port. The empty bags can be deposited into a collection bag or into a shredder. This arrangement has the advantage that the chances of exposure to dust for the worker are greatly reduced. Also, considerably less air capacity is needed which reduces the cost of this operation.
For a more automated version of this type of debagging station, mechanical bag cutters are used to replace the hand operation inside the hood.
Manual debagging stations can provide adequate protection if they are of correct design incorporating a hood and exhaust ventilation. However, this is often not the case and all sorts of poorly constructed debagging stations exist. Some of these lack proper hoods and have inadequate exhaust ventilation. With this type of arrangement, dust is created in a number of ways. Firstly, fibre is usually spilled when the bags are cut by hand and when the fibre cake is broken up to be pushed into the discharge port. Secondly, more dust is generated when the empty bags are handled and stored adjacent to the hood. Obviously, these debagging stations provide no protection for the worker even when the separation of plastic bag from the fibre cake takes place under the hood.
Frequently there is no debagging station at all and the fibre is debagged by hand and placed directly into a blender or fiberizer through the side or from the top. Feeding from the top is done either by conveyor belt or through a port from the floor above. This forces the worker to cut and empty the bag outside the entrance to the conveyor, blender, fiberizer, etc., often on the floor rather than a table. Needless to say, these operations are dusty and place the worker in a hazardous position, and should not be condoned.
Some improvement of this procedure has been achieved by supplying asbestos in paper bags. These bags can be fed into the fiberizer without opening them. Therefore, there is no dust and the worker is protected. This procedure is used primarily for the manufacture of asbestos-cement sheet because paper fibre are normally used to improve the impact resistance of the sheet. A hydropulper is particularly suitable for this method of fiberizing.
As an interim measure, it is possible to provide adequate protection to the worker by introducing water between the inside of the plastic bag and the outside of the fibre cake. This can be done prior to opening the bag by injection through a hole cut into the plastic, or immediately after removing the sewing string. Wetted fibre will not release dust and therefore, the worker is protected. However, the amount of water added to the bag must be controlled, otherwise the weight of the bag is increased too much for easy handling.
Fibre Preparation (Fiberizing)
The next step of the process after debagging is fibre treatment usually referred to as fiberizing or opening of the fibre. It typically starts with the mechanical or pneumatic conveying of the fibre to a fiberizer, sometimes with prior blending. The purpose of the fiberizing process is to reduce the diameter of the fibre bundles which increases their ability to act as reinforcing agent for cement. This is done using various types of equipment, using either dry fibre, wetted fibre or an aqueous slurry.
The most common of these are the kollergang (dry or wetted fibre), the hollander (aqueous slurry), the hydropulper (aqueous slurry), the disintegrator (dry fibre) and the willow (dry fibre). These can be used alone or in combination, depending upon the perceived technical advantages.
The kollergang may be operated with dry fibre or with fibre to which about 30 % water has been added. Its use is usually followed by the hollander or a hydropulper. However, both the hollander and the hydropulper may be used alone. With dry kollergang treatment, one or two passes through a disintegrator are often used ahead of the hollander. For wet kollergang treatment, the disintegrator may be bypassed. There are also completely dry fiberizing procedures in use which rely entirely on disintegrator or other devices, such as the willow.
The fibre treated by the kollergang or one of the dry fiberizers may be stored in a silo prior to the next step. This depends on the size of the operation, the number of fibre grades used, etc.
Dust free operation of the kollergang requires that it is enclosed, even when water is added to the fibre. In addition, the enclosure or hood must be connected to an exhaust ventilation system consisting of a bag house (cloth filter) and a suitable fan. This places the space within the kollergang under negative pressure. Therefore, dust is not released when the door of the hood is opened to inspect the internal parts of the kollergang, etc. Consequently, workers are not exposed to dust under these circumstances. Unfortunately, kollergangs are quite often operated without a cover which results in a severe exposure of workers to dust. This misuse cannot be condoned.
The fibre treated in the kollergang, must be conveyed to the next piece of equipment or the silo using an enclosed conveyor or another safe device. It is improper procedure to discharge the treated fibre onto the floor to be transported manually to the next piece of equipment. This procedure releases a great deal of dust into air and is hazardous not only to the worker performing this task, but also to other workers in the plant.
Both the hollander and the hydropulper are generally used to blend asbestos fibre and cement at the end of the fiberizing stage. This type of blending is also done by other devices, such as a pump running in a conical tank, etc. This fibre cement slurry is further blended with water, usually in a slurry vat. The slurry prepared in this manner is fed to the asbestos cement machine. In some instances a small hydropulper is used following the slurry vat to assure that the slurry is sufficiently homogeneous and does not damage the felt of the asbestos-cement machine.
Since both the hollander and the hydropulper are using an asbestos or asbestos-cement slurry, there is no need for a cover other than to prevent material from splashing. Even if this occurs, splashed material can be removed with water and there should be no exposure to dust. It should, however, be noted that the slurries can dry out and dust can then become airborne due to foot or vehicle traffic. A similar comment can be made for the slurry vat and the small hydropulper which usually present no problem.
The Formation of Asbestos-Cement Sheet and Pipe
The formation of sheet and pipe is obviously the heart of the process. For sheet manufacture, the conventional Hatschek machine is used. It consists of a series of tanks (up to five, usually three), each fitted with a rotating sieve cylinder and each filled with the asbestos-cement slurry prepared during the previous process stage. As the sieve cylinder rotates in the slurry, a thin layer of asbestos-cement is continuously screened from the slurry and transferred to the endless felt of the machine. By running over suction boxes, the amount of water in the asbestos-cement layer is reduced further and the layer is transferred to a cylinder drum.
When the layers of asbestos-cement being wrapped around the cylinder drum have reached the required thickness, the machine stops automatically and the layer is cut parallel to the axis of the cylinder to form a sheet. The machine then restarts and repeats the process.
After forming, the uncured (green) sheets may be cut to the desired dimension on the discharge belt of the machine or moved to another location to be cut. This avoids the necessity of cutting sheets after curing and drying, which is not a recommended procedure because it tends to create a great deal of dust. The material cut from the uncured sheet is returned to the wet end of the machine to be dispersed in water and returned to the process. Process water removed by the sieve cylinders and the suction boxes is reused for the process after passing through settling tanks.
The uncured sheets are stacked between steel sheets and they may be corrugated by a separate process prior to stacking. Both types of sheet may be pressed. The sheets are allowed to cure for a limited period of time before destacking (removal of steel sheets) and further curing. At this stage, the asbestos-cement sheets are still wet and it is unlikely that workers in this area will be exposed to dust. However, good housekeeping is required to remove broken pieces of asbestos-cement sheet, etc., before they can dry out and become a source of dust. Considering that the formation of sheet is entirely a wet process, it is not surprising that dust is created only in exceptional circumstances. However, some care is required to avoid or control spills around the machine, particularly from return waste water. When spills occur, they can be cleaned-up relatively easily by washing with water. In fact, that is the method recommended to prevent creation of dust around the machine.
Flat sheet can be removed from the process prior to stacking and used for the production of hand moulded pieces, such as special roofing parts, flower pots, etc. This operation does not create dust and therefore, does not constitute a health risk for the workers as long as moulding and cutting is done before the sheets dry out.
The manufacture of asbestos-cement pipe is in many respects similar to the manufacture of sheet. In fact, up to the slurry vat, the two processes are identical. The actual pipe making machine has only one or two sieve cylinder vats and the felt has a different configuration. Alternatively, two felts are used. Also, the cylinder drum is replaced by a mandrel onto which the asbestos-cement layer is wound. Different diameter mandrels are used to produce different diameter pipes as well as pipe couplings.
After forming the pipe on the mandrel, both are removed from the machine, replaced by a new mandrel and the entire process is repeated. The pipe is then removed from the mandrel by various methods and transferred to a low temperature curing tunnel. Because the process is wet, no dust is created and, therefore, this part of the process does not present a risk to the worker.
It appears that even in a well operated plant, the transfer points for various materials are a distinct weakness from a dust control point of view. Almost invariably, spills and losses of material occur at these points requiring constant clean-up. If clean-up is not carried out promptly and efficiently, these situations can place workers in a potentially hazardous position. Mechanical solutions, such as enclosures, that prevent spills at transfer points should be established rather than relying on clean-up or housekeeping.
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