The project commissioning team. From left to right: Marty Martinez, Scott Glenn, Brent Lewis, Pat Haye and Kenneth Marta.
A $40 million project at Barrick’s Goldstrike mine in Nevada created a new approach to utilizing the operation’s existing autoclaves. A significant portion of the cost is associated with a new air emissions control system that captures more than 99 percent of the mercury emitted by the autoclaves.
The Goldstrike POX Off-Gas Abatement Project extended the life of the autoclaves, preserving the jobs of the 125 workers who maintain and operate them. “If this project wasn’t successful, the autoclaves would have had to close,” says Tony Carroll, a Project Manager at Goldstrike who headed the Off-Gas Abatement Project. “Nobody wanted to see that happen.”
Autoclaves are large, cylindrical vessels used to process ore that is high in sulphides and/or carbon. In an autoclave, ore is subjected to high-pressure and high-temperature conditions, using steam, water and oxygen to help liberate gold particles embedded in the sulphides or carbon.
Temperatures reach up to 420 degrees Fahrenheit, so the heat and pressure must be reduced periodically in a process known as off-gassing. “When you vent the autoclaves, a mixture of gaseous substances is released,” says Simon Hille, Barrick’s Senior Manager for Metallurgy and Process, and a key player in the implementation of the project.
Historically, the autoclaves at Goldstrike were operated in “acidic” mode. In the acidic operating environment, the ore processed in the Goldstrike autoclaves emitted only very low levels of mercury during off-gassing. However, by 2009, the ore type amenable to the acidic processing had largely been depleted, and preparations were underway to convert the autoclaves to an “alkaline” mode of operations, which better suited remaining ores at Goldstrike.
Before the alkaline mode of processing could begin, testing was required to determine if it would increase mercury emissions during off-gassing. Those tests, in fact, did reveal the potential for an increase in mercury emissions, so the transition was delayed until a way to reduce emissions could be found. In the interim, the autoclaves continued to process ore in acidic mode.
Mercury is a naturally occurring element found in air, water and soil, including ore mined by gold companies. When heated, it is a colorless, odorless gas that can remain in the air for long periods and travel thousands of miles before being deposited on the earth’s surface. The most common way people in the U.S. are exposed to mercury is by eating fish containing mercury in a form known as methylmercury.
In the U.S., the Environmental Protection Agency has promulgated strict regulations that limit the amount of mercury that businesses can emit. In fact, most of the EPA’s environmental regulations and programs are implemented by the states, which are often permitted to adopt more stringent laws and regulations than federal requirements, according to information on the EPA’s website.
In Nevada, the Nevada Mercury Air Emissions Control program requires precious-metal mines that emit mercury to install state-of-the-art technology that limits emissions to strictly defined targets. Failure to comply will result in the denial or revocation of a mercury operating permit.
Barrick’s operations in Nevada comply with all state and federal laws regarding mercury emissions. The company communicates regularly with regulators and works closely with them to ensure compliance and to keep informed of new developments around mercury abatement technology. In 2009, for example, after the initial tests revealed the increased off-gas mercury emissions from the Goldstrike autoclaves, the company worked closely with the Nevada Division of Environmental Protection to resolve the problem, Carroll says. “After those discussions, we began working on a study to determine technologies that might be available to handle the issue,” Carroll says. “That study involved close collaboration with state regulators.”
The study evolved into the Off-Gas Abatement Project that resulted in the new air emission control system, which went operational in July 2011. The system has four stages, each of which captures varying amounts of mercury, Hille says. In the initial stage, off-gas is funnelled into a venturi scrubber that removes water and particulates from the off-gas, including large, liquid mercury particles. The first stage captures 15 percent of the mercury emitted during off-gassing.
In the second stage, the off-gas is cooled to 70 degrees Fahrenheit in a cooling tower, causing the mercury to coalesce into large particulates, making them easier to capture. In the third stage, the temperature of the off-gas is brought down to 40 degrees Fahrenheit, causing further condensation of gaseous mercury into its liquid, or elemental, form. The two stages capture a combined 25 percent of the total mercury emitted during off-gassing, and serve the additional purpose of reducing the volume of off-gas that requires further treatment. This ensures that the mercury that remains in the off-gas is more manageable, making it easier to capture during the final phase of the process.
In that final phase, the off-gas is passed through a two-stage, 20-ton carbon filter. Because mercury has a high affinity for carbon, once inside the filter, it bonds to the carbon. The carbon filter captures an additional 59.99 percent of the total mercury emitted during off-gassing, meaning that the combined capture rate of the four stages of the new control system is 99.99 percent. The target capture rate of the new system, set in cooperation with state regulators, was 98 percent, Hille says. “We have far exceeded that target.”
While the technology used in the control system is not new, it has never been used in such a large-scale application in this particular configuration, Carroll says. The success at Goldstrike means that the system can be built at other Barrick operations that use autoclaves that have a similar potential to emit mercury, he says. “It’s been a real win for us.”