Appendix A Extended abstract for Stahre (2012)
Stahre N (2012) Grönlutslam och flygaska som tätskiktsmaterial för sluttäckning av sulfidhaltig anrikningssand – Utvärdering efter 5 års användning. MSc thesis, Luleå University of Technology, Sweden. (In Swedish with abstract in English)
EXTENDED ABSTRACT
Sweden has a large mining industry compared to the rest of Europe that results in large amounts of mine waste that must be taken care of in an environmentally and economicallysustainable way. The mine waste consist of either of waste rock, that isunmineralized/uneconomic that have to be mined for practical reasons to get access to the ore,or tailings, which is a waste product from the enrichment process. The most common oretype to be mined is metal sulfide ore, out of 13 active metal ore mines in the end of 2009 sulfide ore was mined in 11 mines. The sulfide ore often have a metal content of a few percent thatresults in large amounts of tailings. The tailings are stored in large deposits at the mine siteand when the tailings are exposed to oxygen in the air the sulfides in the tailings oxidize. Theoxidation of sulfide minerals creates acid mine drainage with high metal content and isconsidered to be harmful to health and environment. In combination with Sweden’s more andmore strict environmental legislation the demands have increased on the mining industry tofind good and sustainable solutions to prevent oxidation of sulfide rich tailings. At the same time the heavy industry has become affected by increased costs for landfilling of nonrecyclable wastes and has therefore started to search for new alternatives for reusing the waste instead of landfilling it. Two types of wastes that have become interesting to use as cover materials for landfills and tailing deposits are green liquor dregs and fly ashes.
To evaluate how green liquor dregs and fly ash ages when used as cover materials in landfillsthe Rönnskär processing plant and smelter placed two covers on their tailings deposits as pilotstudy in 2006. One of the covers consisted of green liquor dregs from Smurfit KappaKraftliner AB and the other cover consisted of fly ashes from Skellefteå Kraft AB. This studyis an evaluation of the two covers after being in use for 5 years.Sampling was conducted on 27-28 Oct 2011. Five pits were dug with an excavator. Samples were taken from two fly ash covered pits and another two green liquor dregs covered pits. In addition, samples were taken from a reference pit where the tailings was not covered with sealing layer but covered with a thin layer of coarse gravel. Upon sampling, a number of parameters such as dry matter content, compact density have been analyzed. CRS test, water retention capacity, elemental characterization was performed.
Results show that samples of the green liquor dregs from the field contained primarily calcite, elemental S and pirssonite. The fly ash contained quartz, calcite and ettringite. The chemical composition of an array of elements such as Si, Al, Ca, Fe, As and Hg were different from the previousresults for the green liquor dregs (Lindgren 2005), suggesting that the cause of the variations was due to the dregs have aged and less heterogeneity.
Due to a high water saturation degree both of the covers can be considered to decrease oxygendiffusion to the tailings via air. This has resulted in an oxygen level in the covers to be lowaccording to the mobility of the elements interpreted from the chemical analysis. The coversalso have a chemical impact on the tailings where the most noticeable change is the pH. ThepH in the tailings under the covers have increased from ca pH 2 to ca pH 3-6 compared to thetailings without cover. The covers also have a negative impact on the tailings where thehighest impact is that Zn is leaching from the covers and accumulated in the tailings below. Seen from a technical point of view the green liquor dregs and the fly ashes is not clearlysuitable as covers since there are some qualities, for example permeability, that can beimproved. But the materials does have several qualities that makes them suitable, for examplewater saturation degree, buffer capacity etc., and these qualities in combination with how thematerials reacts to ageing gives room for additional studies for the purpose of making thematerials more suitable to be used as covers.