Brooklyln Center police arrested seven protesters at a conference on silica sand mining Wednesday, after they climbed on top of a bus that was scheduled to carry conference participants to a tour of silica sand mines in Wisconsin.
The protesters unfurled banners that said "Stop frac sand mining" and "No right way to do something so wrong."
The seven were charged with unlawful assembly and released.
Western Wisconsin and southeastern Minnesota are rich in silica sand deposits. The sand is used in the hydraulic fracturing process to produce oil and natural gas.
Critics say mining the sand can produce harmful dust, increased truck traffic, and lower property values.
The conference was organized by the Society for Mining, Metallurgy and Exploration and the University of Minnesota Duluth.