Mining wastewater treatment by slag filters

The two objectives of this project were to propose a slag filters treatment system for the leachate of the Joplin mine, Missouri, and propose a preliminary model for phosphorus removal mechanisms in slag filters. Three experimental phases were realized: individual batch tests, sequential batch tests and column tests. Individual batch tests were used to compare the treatment performance of 10 slags and 1 apatite and to select the best materials from this list. Sequential batch tests were used to compare the treatment performance of several filter sequences. Finally, column tests were used to characterize the removal performance and longevity of controlled filters. During column tests, the effect of influent composition, void hydraulic retention time and filter sequences was measured. Some column tests were operated especially for the study of phosphorus removal mechanisms.;The proposed model is expressed by the following steps: 1) the rate limiting dissolution of slag is represented by the dissolution of CaO, 2) a high pH in the slag filter results in phosphorus precipitation and crystal growth, 3) crystal retention takes place by filtration, settling and growth densification, 4) the decrease in available reaction volume is caused by crystal and other particulate matter accumulation (and an associated decrease in available reaction time), and 5) the pH decreases in the filter over time if the reaction time is too short (which results in a reduced removal efficiency). A new approach to define filter performance is proposed. Saturation retention capacity is expressed in units of mg P/ mL voids.;The proposed system for the treatment of the Joplin mine leachate is composed of two successive Fort Smith EAF slag filters operated at a total void hydraulic retention time of 34 hours. After 179 days of operation, the treatment performance of this system was 99.9 %, 85.3 %, 98.0 % et 99.3 % for phosphorus, fluoride, manganese and zinc respectively.