Neutralization and solidification of waste acid using on-site materials

Ponds used to evaporate sulfuric acid raffinate solution from uranium-tailing heap leaching contained saturated solution, sulfate salt and a buoyant gel-like material. Unsuccessful approaches to dewatering the ponds for reclamation included pumping of free liquid, skimming off foam material, mucking out salts, and application of limestone. Shale in an isolated hill on site had substantial CaCO3 content and non-smectitic clays. A mixture of two parts raffinate to one part shale produced a solid that was neutral, immobilized sulfate and heavy metals, absorbed some of the free liquid and allowed the remainder to evaporate. Crushed shale was dumped into the ponds and mixed by dozer and track loader. Placement and mixing techniques were refined to produce a heavy mud that, after chemical reaction and drying, could be excavated, compacted and covered for final pond closure. This experience shows that calcargillites can be used to neutralize and solidify acidic mine wastes.