The Minnesota Pollution Control Agency has fined a frac-sand processing plant in North Branch for the second time in two years, this time for repeated air pollution and noise violations.

Tiller Corp., a sizable player in the region's frac sand industry, agreed to pay a penalty of $85,000 and increased its noise mitigation practices to resolve findings that the facility produced excessive noise at night and emitted more particulate matter, carbon monoxide, volatile organic compounds and sulfur dioxide than allowed under its air quality permit.

Mike Caron, the company's director of land use affairs, said Tiller made changes almost a year ago to address the state's concerns. It stopped operating certain equipment at night and eliminated its use of problematic fuels that caused emissions violations, he said.

"Our plant is probably the cleanest plant in the nation," Caron said.

The agency said in a news release that Tiller's North Branch plant was in "continued noncompliance" in November 2013 over emission limits for carbon monoxide and volatile organic compounds. One of the primary emissions sources was a dryer used to treat washed frac sand.

Caron disputed the term "continued noncompliance." He said the plant violated its air pollution limits while burning diesel fuel instead of natural gas in a backup generator while testing the equipment. Another violation occurred when the plant was testing fuel oil as a backup energy source for the dryer, he said. He said the dryer meets emission standards when it operates on natural gas.

Tiller first ran afoul of state and federal pollution laws when it started to build its North Branch frac sand facility in late 2011. The agency reported that the Maple Grove-based company failed to obtain required air-quality permits before starting construction. To resolve the conflict, Tiller paid a $5,000 fine and agreed to invest $14,800 in supplementary air monitoring. The agreement was announced in January 2013.

Over the past year, Tiller's North Branch operation has been the subject of speculation about possible expansion plans. But Caron said Thursday that the company has no plans to expand the facility.

Tiller mines silica sand in western Wisconsin and transports it to North Branch for processing and shipping via rail. The region's highly specialized sand is used by drilling companies in other states in the oil- and gas-extraction process known as hydraulic fracturing, or fracking.