Granite and quartz countertops are very popular in Québec. More than 200 companies are involved in manufacturing them, and the process can generate dust, including crystalline silica, to which the workers, whose job it is to polish them, are then exposed.

Options exist and are available to workers. A guide, published by the CNESST and the IRSST in 2010, presents the basic principles for reducing workers’ exposure to crystalline silica in the processing of granite and other materials containing quartz. However, additional knowledge on the characterization of means of controlling crystalline silica dust during dry and wet polishing operations was necessary. An IRSST research team therefore studied the issue.

The risks of exposure to crystalline silica are well documented. The International Agency for Research on Cancer (IARC) considers it to be carcinogenic for humans. It is also responsible for silicosis, a disease that leads to progressive and irreversible reduction of lung capacity. In light of these findings, further research is justified to limit the exposure of workers to dust containing crystalline silica. Ali Bahloul, IRSST researcher, developed methods to reduce the dust emitted during granite processing by evaluating various preventive engineering measures to reduce exposure in the laboratory. With his team, he first worked to understand how the dust is generated and dispersed in order to propose solutions that take into account the realities of the sector that processes these materials.

A Question of Speed

In order to polish stone effectively, the abrasive tool must rotate at high speeds. However, high speeds generate more dust, which are difficult to capture. How can workers’ health be reconciled with a satisfactory surface finish? After testing several speeds, 1500 rotations per minute was found to be the best compromise. “With an electron microscope, we are able to observe that above that speed, cracks develop on the granite surface. High polishing speeds can therefore affect the surface finish in addition to generating a lot of dust,” notes Bahloul.

Among the other parameters studied, the research team also looked at wet polishing. More and more industries are using this technique to force the dust to the ground and prevent it from dispersing. Laboratory trials have effectively demonstrated that the airborne concentration of large-sized particles and some of the fine particles is decreased under the impact of jets of water. However, this process has little effect on the ultrafine particles, which are the most harmful to health. “Wet polishing solves only part of the problem. It must therefore be combined with a solution to deal with the ultrafine particles,” the researcher remarks.

Three Concepts Assessed

In the search to reduce particulate emissions at source, three concepts were identified. To avoid exposure to dust, tests were carried out in the laboratory with a machine tool that simulates the polishing performed by a worker. This technique made it possible to observe the effectiveness of the concepts on a flat granite surface, but not on corners or sides. Field trials will provide a clearer picture. Nevertheless, all three concepts resulted in an emission reduction of more than 95%.

The first concept, a push-pull system, combines an exhaust hood with a process that blows the contaminant towards it. It proved to be ineffective at low flow rates, but above a certain flow rate its efficacy suddenly increased. The two other concepts, a dust shroud and suction slots built into the disc, operate by means of systems incorporated into the tool. In these cases, efficiency increased as the suction flow increased. Afterward, two of the concepts were tested by means of digital simulation. The comparison of the experimental and virtual models showed that they were in line with each other. The development of this software thus provides the interesting possibility of modifying parameters and predicting behaviour, without experimentation.

This study also provided the opportunity to verify the effectiveness of using tracer gases to simulate the behaviour of the airborne particles. In the laboratory, the findings were comparable with those of particles that were subjected to high rotation speeds. These gases could therefore be used instead of particles to test the effectiveness of the system in the field. To date, the researchers have performed their experiments to understand the dispersion process of dust and to propose the appropriate tools only in the laboratory. These concepts will be applied to the workplace in order to determine which are best suited to the industry. As Bahloul reiterates, “Before we issue recommendations resulting from our research to the granite processing sector, we must confirm our observations in the field and ensure that the solutions are appropriate.”