California’s widely publicized water crisis and subsequent restrictions on usage has -- in some instances -- bumped up against some of the state’s stringent environmental regulations.
According to state rules, if there is a chance of dust emissions from substances such as slag that can potentially contain heavy metals, then the air quality regulations take precedent over water restrictions.
However, when Canada-based Tervita Corporation took over operations of a California slag recycling facility, it sought to both control dust and conserve water by integrating atomized mist technology with an innovative design that involved a tower mount and a standard shipping container.
A fugitive dust plan that protects neighbors
In 2012, the Gerdau Corporation, a global leader in the production of long steel, chose Tervita to take over operations of the slag processing plant in Rancho Cucamonga, California, servicing the company’s long steel mill located approximately 0.25 miles (0.4 k) away.
“As much as we run the machine, we’ve been impressed by how well it’s held up."
- Carson Swartz
Facing a transition that required updated equipment in a stringent regulatory environment, Tervita managers worked closely with the South Coast Air Quality Management District (SCAQMD) and local leaders to create an air quality management plan that made the operation compliant and more efficient.
An integral part of the plan was the installation of a new high-volume crusher and a DustBoss® 60 (DB-60) atomized mist dust suppression system.
Tervita is an environmental solutions provider that focuses on safety, efficiency, and regulatory compliance to minimize impact and maximize returns.
“Our goal was to create a sustainable operation that fulfills the needs of Gerdau, regulators, and the local community. Mitigating dust emissions and runoff is at the center of our modernization effort and the DustBoss has been an important part of the process,” said Carson Swartz, Operations Supervisor.
Generally molded into rebar used to reinforce concrete structures or shaped into support beams, the long steel process creates an inert, super-heated slag, which contains trace heavy metals.
Approximately 300 to 350 lbs (135 to 160 kg) of slag is produced for every ton (0.9 mt) of steel. Once separated, it is loaded into dump trucks for transport to the recycling facility.
“The unit runs 24/7 without much maintenance and no issues with runoff or low air quality.”
- Carson Swartz
Running 24 hours, five day per week, Tervita receives approximately 300 tons (272 mt) of material a day in ten to 14 “heats” (dump truck loads), which are offloaded into a 200-foot-deep by 100-foot-wide (61 meters x 30.5 meters) storage area separated into two sections.
Nearly a third of the temperature of the surface of the sun, newly delivered slag -- approximately 2500º to 3000ºF (1,370º to 1,650º C) -- is wetted, cooled, mixed and cured by a combination of water, front loader, and time.
This 30-day-long process is done in a controlled fashion to allow the minerals a stable transition so that a suitable microstructure is achieved. During cooling, a front loader moves the other section of cured material to the crusher to be processed, which also takes about 30 days. Once an area is cleared, it switches to become the offloading/cooling area and the other converts into the material processing area.
Fused by the intense heat, slag cools into various sized chalky masses similar to dense volcanic rock or thick glass, depending on its composition. Once loaded into the crusher, metal traces are removed and sent back to the mill, and the inert slag is crushed into aggregate grades based upon the intended use.
One and a half inch (38 mm) minus aggregate is used for backfill of roads and in some septic system applications. Other grades include half inch minus (13 mm), which is used mostly as a component in Portland Cement. “B-fines” and “C-fines” are different grades of grainy material, and can be used for applications from cement filler to absorbing oil.
Building dust control into the new process
“When we installed the new state-of-the-art crusher, we streamlined the recycling process into a faster operation that is dust free because of the bag house filtration system,” Swartz said. “But the storage and cooling area was a big issue. Whether offloading, churning or moving the slag to the crusher, it’s constantly being disrupted, causing a lot of dust.”
Fused by the intense heat, slag cools into various sized chalky masses similar to dense volcanic rock or thick glass, depending on its composition.
Prior to installing the DB-60, the company tried using the moveable sprinkler irrigation system left in place by the previous operators. Drawing from non-potable water provided by Gerdau, delivered to the site throughout the day in 2000-gallon tanker trucks, Tervita found that the sprinkler system only saturated the surface material, which caused large amounts of runoff and did not properly address the fugitive dust.
Add the occasional high winds of up to 50 mph (80 kph) blowing off the nearby San Gabriel Mountains and the company had to confront both potential compliance issues when seeking an SCAQMD permit and the risk of affecting air quality in the heavily regulated area around Los Angeles.
“The sprinkler system didn’t stop the dust,” Swartz said. “It just used a lot of water and caused a lot of runoff, so we really needed a better solution if we wanted to stay compliant.”
Creating a more efficient slag recycling process
Managers at Tervita heard from colleagues in the steel industry about the use of atomized mist dust control technology at another slag recycling operation.
A representative visited that site to see the machine in action and was impressed with how effectively the method pulled dust from the air and coated the surface of the storage piles, creating negligible runoff while using far less water than the irrigation sprinkler system currently in place at the Rancho Cucamonga facility.
Tower mounted dust suppression system installed on modified shipping container
“When we installed the new state-of-the-art crusher, we streamlined the recycling process into a faster operation that is dust free because of the bag house filtration system.
But the storage and cooling area was a big issue.
Whether offloading, churning or moving the slag to the crusher, it’s constantly being disrupted, causing a lot of dust.”
- Carson Swartz
Tervita had an innovative plan to cover the whole area in mist and secure the equipment in case of high winds.
The company strategically placed a 20-foot-long by 8-foot-wide by 8.5-foot-tall (6 m x 2.4 m x 2.5 m) shipping container in the storage and cooling area. Working with DustBoss engineers, the container was reinforced with a heavy-duty steel frame and modified to mount a 12-foot (3.65 meters) tall steel tower.
The tower was topped by the DB-60 equipped with a 359º oscillation system, making the total height from the ground approximately 28 feet (8.5 meters). Inside the container, a touch screen panel is mounted on the wall that allows operators to control the elevation, oscillation arc, booster pump pressure, fan output, and water volume. Many of these functions can also be modified outside the container by remote control.
“DustBoss was involved in the process every step of the way,” Swartz said. “Operation of the equipment is straightforward and the maintenance training was simple.”
Using non-potable water for dust suppression
Since the water used by Tervita for dust suppression is non-potable, it is first pumped through an in-line 30 mesh, 595-micron filter before being delivered to the booster pump, where the water pressure is raised from 10 PSI (0.68 BAR) up to 160 PSI (11.03 BAR), with a potential maximum of 250 PSI (17.23 BAR) depending on the need.
Pumped through a 1.5-inch (38.10 mm) hose to a circular stainless steel manifold, the water is forced through 30 atomizing spray nozzles, which fractures it into millions of tiny droplets. The mist is then propelled by a powerful 25 HP electric fan that produces 30,000 CFM (849.50 CMM) of airflow through a specialized cylindrical barrel design.
Atomized droplets are launched in a 200-foot-long (60 meter) cone at an adjustable 0 to 50° angle, covering a total area of 125,000 square feet (11,613 square meters) when oscillating the full 359º rotation.
Matching droplet to dust
To remain airborne, dust particles typically must be 200 microns or less in diameter and 100 microns or smaller to be inhalable. Industrial irrigation sprinklers produce droplets ranging in size between 200 and 1000 microns in diameter, whereas the atomized mist from the DB-60 has been engineered to create droplets between 50 and 200 microns.
“The sprinkler system didn’t stop the dust.
It just used a lot of water and caused a lot of runoff, so we really needed a better solution if we wanted to stay compliant.”
- Carson Swartz
The science behind it is fairly straightforward.
As they fall through the air, large droplets create a slipstream effect, in which air travels rapidly around them, causing smaller dust particles to get caught within the air stream and be deflected away from the droplet, rather than being absorbed.
Atomized droplets -- being roughly the same size as fugitive dust particles -- travel with the dust and initiate the maximum number of collisions with the particles. Once absorbed, the collective weight drives both to the ground.
Atomization introduces more droplets into the atmosphere, using a fraction of the water volume required by large sprinklers.
Because the mist is propelled by a fan rather than water pressure, the DB-60 uses only about 25 GPM (94.5 lpm) to cover the area, as opposed to an industrial irrigation system that can require up to 500 GPM (1,893 lpm) pumped at 60-100 PSI (4.1-6.9 BAR) to adequately service the same area.
Not only do the atomized droplets capture airborne particles, once they land, they also quench the storage piles, offering surface protection with far less runoff and product loss.
“The unit runs 24/7 without much maintenance and no issues with runoff or low air quality,” Swartz said.
Compliance and corporate values converge
Due to the use of atomized mist, the facility has dramatically reduced the volume of water needed for dust suppression. That makes water more available for other parts of production in the plant, thus promoting sustainable overall water usage for the entire facility.
With fugitive dust levels compliant to SCAQMD regulations, Tervita achieved the goals set for the project and successfully applied a new technology that could be used in other locations and applications.
“As much as we run the machine, we’ve been impressed by how well it’s held up,” Swartz said. “Since the installation, the couple of times we’ve called them, they were very responsive and even came out to visit just to see the setup. The service matches the quality of the equipment.”
Rancho Cucamonga, California (US)
Cooling and controlling dust at the storage and cooling area for a slag recycling facility for Gerdau Corporation's Rancho Cucamonga long steel mill
Prevent dust from escaping from where slag is dumped, stored, and processed while conserving water for the drought-stricken area.
One DustBoss 60 (DB-60) mounted on a tower, which itself is mounted onto a reinforced shipping container that protects controls for the dust suppression machine. The dust control unit must run 24/7 using non-potable water.
Regulations are being met, avoiding facility shutdowns. Tervita claims the mist is more effective than a previous sprinkler dust control system and the water now saved on dust suppression can be redirected for other functions of the plant. Officials are amazed at the durability of the unit, which runs constantly.
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