Glen that is interesting. I worked for a POCO in OK for the first 10 years of my career, and they have a coal plant in Oologah OK burning Wyoming coal though. I have never heard of using Lignite as back-fill before. My only question would be the corrosive effect using it with copper as sulfur does not play well with copper. Anyway Bentonite Clay is cheap and plentiful, especially in TX, OK or any state with Oil and Gas production. Heck I bet with a case a beer and a visit to any drilling rig, the Roughnecks would load your pick truck up.