Asbestos in the content of the roofing sheet could help to increase the strength of the sheet and make it more durable. The Kraft powdered paper, which is bought or got right from cement packing material grinded, could help to fill in the hole caused by cement grains, eliminate air bubble and help to asbestos and cement combine well and evenly to make products more durable.
The hazardous factors in asbestos roofing sheets production line consist of asbestos dust, cement dust, powdered paper, high humidity, manual high work load, posture and some other factors which may cause working accidents like working at height, floor and working platform usually wet, etc. The process stages potentially causing hazardous factors adverse to workers’ health are material incoming, debagging, asbestos dry grinding, asbestos bags carrying after grinding to mixing machine.
Asbestos: packed in PP bags when sent to the factories and going on the conveyor or by manual delivery to the debagging stations then to the grinder. There are a number of plants having automatic debuggers and bag destroyer machine; asbestos being separated from bags to pour into silos, bags are milled and mixed into silo using as material.
Cement: Cement could be sent to factories in the forms of bags or bulk-load. Cement bags sent to the store, bulk-load cement being pumped directly into the silos. There are a number of factories using clinker miller for being more independent in their production and reducing input costs.
For the process line using bagged asbestos and cement, this stage is very dusty because the bags may be torn during transportation. In addition, this stage requires a high workload because the workers must carry the 40-50 kg bags. For the process line using clinker grinder, the workers who discharge clinker from grinder into bags is exposed to very high noise level, concentration of dust and work load. The workers in this process stage may have asbestosis, silicosis, respiratory disease, and dermatitis, ophthalmic and muscle-skeletal diseases.
In many asbestos sheet production lines in Vietnam, asbestos bags are quantified and opened manually then poured into the grinder through an uncovered conveyor. If asbestos packed in paper-bags, it could be put into grinder by whole bags. This is done using various types of equipment, using either dry fiber, wetted fiber or aqueous slurry. Here, a small quantity of Kraft powdered paper (about 5% of raw material) could be added. The treated fiber may be transported by conveyor or by bags (in dry grinding process) to mixing tanks.
Cement bags could be transported by conveyor or manually to debagging station, manually quantifying, then being transported to mixing tanks where cement is mixed with asbestos and water. Bulk-load cement from silos could be automatically quantified and going on the screw conveyor to go to the mixing tanks.
For the process line without automatic debugger, this process stage is the most polluted in asbestos roofing sheet plants, especially if there is lack of the exhaust systems or existing exhaust systems do not work properly. The dust is emitted mostly during debagging, pouring asbestos and cement into conveyor or directly into grinder or mixer, shaking the empty bags and their putting in certain place. In addition, the workers’ posture presents many un-ergonomic features: they usually work in standing posture; they must bend to catch the bags, to carry them to pouring point, to lift them and to pour into conveyor or grinder. So the workers at this process stage may also endure the same risks as that in previous stage.
For the process line using asbestos dry grinder, the dust may emit from grinders themselves if they are uncovered and badly exhausted, from asbestos discharging point to conveyor or into bags; during bags transfer to next stage. Similarly of two previous stages, the workers must work heavy in postures unfitted to ergonomic requirements and dust polluted environment. The dust released from this stage may affect to the workers not only at this stage, but also at other stages in plant.
Asbestos-cement sheet formation
Asbestos-cement mixture in aqueous form is pumped to mixed material tanks or to combine stirring machine, and then is pumped to the conventional Hatschek machine. It consists of a series of tanks (up to five, usually three), each fitted with a rotating sieve cylinder. 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 belt of the machine. By running over suction boxes, the amount of water (about 75%) 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 at required dimension (generally, a sheet has the area of about 1.36m2). The machine then restarts and repeats the process.
After forming, the uncured (green) sheets may be automatically or manually transferred to stacking area where they are stacked between steel sheets or they may be corrugated by a separate process prior to stacking. The sheets are allowed to cure for a limited period of time before destacking and further curing. The waste edge from sheets cutting and the breakdown sheets may be transferred to asbestos grinder for reuse. The wastewater from Hatcheck machine and endless belt is returned to sedimentation tanks and then should be recycled.
At this stage, the asbestos sheets are still wet and it is unlikely that workers in this area will be exposed to dust. However, likely at fiber preparation stage, good housekeeping is required to remove broken pieces of asbestos sheets, etc., before they can dry out and become a source of dust. The working platform is usually wet at certain height, which facilitates slipping or falling. The unguarded machines may cause the minor working accidents.
The curing of asbestos-cement sheet is by definition involves the use of water, steam or a moist atmosphere. The asbestos-cement sheets are usually cured by storage under humid conditions produced by water spray in open area during about 28 days, and then directly selling to the buyers. In order to reduce the curing time, the sheets could be pre-cured during about 2 hours in curing tunnels using saturated steam provided by boilers. After natural cooling, the sheets are usually submerged in water tanks for a period of time that varies according to the temperature of the water (generally about 5 minutes) and then they are stacked and constantly sprayed with water. After quality checking, the sheets are transferred to storage area for delivery.
Generally, the workers in this area are not exposed to health risk. However, in process line using steam tunnel kiln for procuring, workers may be exposed to heat, especially to hot steam during opening of kiln door, to dangerous factors of unsafely steam pipes.