Colemanite is a calcium borate mineral from which boric acid and then borates are obtained. These products are useful for different applications. Colemanite leaching by sulphuric acid leads to production of boric acid. Arsenic minerals, mainly realgar and orpiment, which are often associated together with colemanite, depending on mineral paragenesis, are not leached by the acid. Arsenic components. contaminate the tailings resulting from the chemical process, mainly constituted by gypsum. Sometimes the high arsenic pollution allows characterising the tail as toxic, implying very high disposal costs. In order to avoid this drawback, As content in gypsum was reduced from a level up to 2000 ppm to a value less than 500 ppm after Na2S leaching. Reagents consumption, pH and pulp density were adjusted in order to obtain a material that does not need to be disposed of in an expensive landfill as it is suitable for use as a cement additive and plaster. (C) 2003 Elsevier Science Ltd. All rights reserved.