Study on thiosalts formation in alkaline sulphidic ore slurries under anaerobic conditions and methods for minimizing treatment cost in the mining industry : A case study at Boliden Mineral AB

During mining and milling of sulphude-rich ores, sulphide is inherently generating intermediary sulphur compounds, collectively known as thiosalts. These partly oxidized sulphur species composes a threat to effective mine waste water treatment systems due to their natural oxidizing capability to eventually produce sulphuric acid. The acid can reduce the overall alkalinity within the treatment system and/or in recipients of nearby local aquatic systems. Mine corporations are in the process of identifying effective treatment systems with the potential to reduce late acidification specifically. Since their operations are affecting local environments, the acidification issue has attracted its attention to media and governmental organizations. In a case of 2007 at Boliden Mineral AB, it was estimated that 840 tonnes of potential sulphuric acid was released from the Gillervattnet tailings dam, and had cause late acidification in local aquatic systems near the Boliden Concentrator (where sulphur-rich ore is processed). Investigations concluded that thiosalts were able to continue oxidize even after its treatment process and hence, generate additional acidity in the effluent. As Boliden is in progress of commissioning its new tailings dam, Hötjärnsmagasinet, it is of great concern that an effective water treatment system is applied so that regulated standards are fulfilled in the final effluent. This study has investigated the chemical treatment process at Boliden Mineral AB, in which applies Fenton reagents of hydrogen peroxide and ferrous sulphate to oxidize thiosalts into stable sulphate. Another investigation was made on the microbial treatment that oxidizes thiosalts by means of bacteria. A “best-case scenario” of a potential thiosalt generation by adjusting alkalinity at discharge point at the Boliden Concentrator was set up by statistical tools. Today, the ore is normally monitored at about pH 12 at discharge point. Alkalinity in the process waters is necessarily reduced to pH 8-9 before the Fenton, which causes an alkalinity drop in the tailings pond. However, if alkalinity was reduced at discharge point, the overall thiosalt generation would possibly be reduced as well. A split-plot experimental design was conducted to investigate whether the salt production would decrease with lower alkalinity. One “hard-to-change” factor, temperature, and two “easy-to-change” factors, pH and pulp concentration, were used in the design. The split-plot experiment concluded that the alkalinity, or pH, and the interaction effect between pH and temperature had a significant effect on the hydrogen consumption logOH delta. The forecast provided two estimated linear models, one forecasting the theoretical amount of thiosalts, and one forecasting the measured amount by means of defining a pOH delta drop. These linear models became the major finding of this study. The models were able to conclude the overall reagents and energy costs for chemical and microbial treatment at the Boliden plant. According to results, Boliden Mineral AB would save approximately 36 MESK annually by reducing alkalinity to approximately two pH units (from pH 11.3 to pH 9). In case a microbial treatment process was installed, the costs difference to Fenton with same delta drop would reduce operative costs to about 27 MSEK annually. Validerat; 20111028 (anonymous