Mobile Plant

Conquering dust in limestone crushing

Trinidad Cement Limited (TCL) has installed a high-efficiency mobile dust suppression unit to reduce the potential for health hazards, environmental effects or nuisance dust from its limestone crushing operation. With the significant reduction in airborne dust, TCL is helping to ensure a healthy workplace and safeguard the surrounding community. The move also minimises the opportunity for abrasive dust to enter air intakes on heavy equipment, helping to extend air filter life and reduce unnecessary wear.

TCL has been a leading supplier to the Caribbean region for more than 50 years, currently averaging about 1.1 million tonnes annually from its Claxton Bay production facility. Limestone supply comes from the firm?s own quarry at Mayo, in central Trinidad. Containing 120 hectares of yellow limestone deposits, the reserve has been forecast to sustain anticipated production rates for as long as 200 years.

Like most quarries, the company faces some difficult challenges in its attempts to control dust during extraction and processing. Operations such as quarrying, loading, transportation and mobile crushing typically do not lend themselves to integrated suppression techniques, yet all of these add to the potential for releasing vast amounts of dust.

?In the past, our crews had done their best to control dust particles by hand spraying, but we found that watering with hoses or sprinklers offered limited benefit in containing surface dust,? said Amarchandra Maharaj, TCL?s health, safety and environmental manager. ?Once the dust particles became airborne, the water droplets from these methods were far too large to have a meaningful effect on migration.?

The methods also had a tendency to saturate the material surface and contributed to standing water, both undesirable side effects. Even with dust screens and vegetation to help create a containment barrier, company officials believed there had to be a way to better suppress airborne particles and contain surface dust, without the saturation problems and manpower requirements that typically accompany hand spraying techniques.

The primary target for improving dust suppression at TCL was the mobile crushing unit. The company uses one of the largest tractor rippers in the Caribbean to extract raw limestone, the Caterpillar D11R, and the quarried stone is trucked to a Hazemag impactor with a capacity of 530 tonnes per hour, which crushes the rock to an average size of 25mm.

To control the airborne dust generated from crushing and transporting, TCL selected a DustBoss DB-60, a heavyweight design from Illinois-based Dust Control Technology (DCT). The DB-60 uses a series of 30 specially-designed brass nozzles to atomise water droplets to the optimum size for maximum particle attraction and coverage area, then launches them with a powerful 18kW fan. In contrast to most suppression equipment suppliers, the company offers a wide range of nozzles for different applications. Customers can select the optimum spray pattern and droplet size to suit the specific type of dust.

The high-powered, ducted fan delivers 850m3 of air flow per minute, giving it a throw of more than 60m. It can also be equipped with a 7kW booster pump to increase water pressure as much as 150 PSI from a 5cm supply hose. The DB-60 has an adjustable elevation from zero to 50 degrees, and the oscillating head delivers 40 degrees of motion. The workhorse design can cover nearly 0.2 hectares from a single location, and the wheeled carriage-mounted mobile unit can be easily located wherever it is needed most on a given day.

The unit?s suppression efficiency derives from its ability to balance a number of critical properties that tend to work against one another, such as droplet size, spray pattern, air flow, range and velocity. ?If spray nozzles create large droplets, they can be thrown a longer distance than small ones, giving a unit more range,? explained DCT?s CEO Edwin Peterson.  ?Unfortunately, large drops have a lower capture rate and more space in between, making them less efficient overall.

?The optimum condition is one in which water droplets are approximately the same size as airborne particulates, producing the greatest attraction between the two,? said Peterson. Given the average dust particle sizes involved in most quarrying and crushing operations, testing and experience have shown that droplets 50 to 200 microns in size typically deliver the highest efficiency in most situations.

?Similarly, a high velocity air flow allows an atomised spray to be thrown farther, so a powerful fan delivers an advantage. But the higher the velocity, the smaller the drops tend to become, and even though small drops produce good capture and surface wetting, there is a point at which suppression efficiency falls off in the attempt to reach greater air speed and distance,? he added. Maximum particle control is the result of considering all the goals and trade-offs, ultimately producing a design that balances each component in relation to the others.

The versatility of the dust control design allows TCL?s crews to quickly adjust positioning, throw angle and oscillation to accommodate specific work sites and weather conditions. Crews run the DB-60 whenever the mobile crusher is operating, blanketing material piles, storage areas or truck paths to best suit the day?s workload and wind conditions.

As the regulatory landscape in Trinidad continues to evolve, with new and emerging legislation likely to dictate a need for improved dust control in many industries, effective suppression has become an important element of TCL?s process. The Draft Air Pollution Rules of Trinidad and Tobago as well as the region?s Occupational Health and Safety Act already have applicable sections, and TCL is working to stay ahead of minimum requirements and demonstrate environmental leadership. The company is certified as fully compliant with ISO 14001: 2004, the most widely recognised set of environmental management standards in the world.

Source: Dust Control Technology

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