Evaporation of acid mine water with mechanical vapor recompression
1University of Oulu, Faculty of Technology, Environmental Engineering
|Online Access:||PDF Full Text (PDF, 1.8 MB)|
|Persistent link:|| http://urn.fi/URN:NBN:fi:oulu-201603161318
|Publish Date:|| 2016-03-16
|Thesis type:||Master's thesis (tech)
Acid mine water formation is a wide environmental problem that occurs especially in mines, but also anywhere else where the bedrock minerals produce acid when exposed to oxygenated waters due to excavation. Iron containing sulfide minerals are particularly problematic due to their efficient ability to produce acid. In a favorable case, acid mine drainage and process waters from mining industry can also be valuable sources of metals or other raw materials. Potential methods for the utilization of mine waters were searched from literature for this thesis. Examples of found alternatives were the selective separation of metals by precipitation and refining the components from mine water into construction materials, water purification chemicals or pigments.
The recovery of metals from acid mine water may require concentration of metals, which can be achieved by utilizing the evaporation process described in this thesis. The mechanical vapor recompression (MVR) evaporation process is an energy-efficient alternative method for neutralizing and precipitation of the metals, which are most often performed by chemical neutralization and precipitation. The applicability of the MVR-evaporation process was studied with pilot evaporation experiments for acid mine waters from mines X and Y. The process water from mine X was suitable for evaporation in the pilot plant, whereas mine drainage from mine Y caused remarkable corrosion in the equipment. Hence, based on the results of this study, the evaporation of the mine drainage from mine Y is not feasible with the pilot plant used. The reason for this is the steel grade used to construct the pilot plant equipment. In addition, mine drainage from mine Y formed lots of dregs into the evaporator, which is harmful for heat transfer and requires difficult and time-consuming washing sequences.
Corrosion experiments were conducted with the concentrate that was produced at the pilot evaporation experiment for the process water from mine X. Corrosion rate was defined with two electrochemical methods on the basis of applied currents and potentials. With the concentrate from mine X, the corrosion rate was observed to increase exponentially with temperature. The increase of the concentrate density increased the corrosion rate less than the temperature. In addition to the pilot experiments, the behavior and energy consumption of the evaporation process was evaluated by simulating the evaporation of water-sulfuric acid mixture in conditions corresponding to the pilot process experiments. The evaporation rate was observed to slow down after the sulfuric acid concentration reached of 25 mass-%. The MVR fan and the pumps in the pilot plant were found to be oversized in relation to the evaporator size.
As a conclusion, the applicability of an MVR-evaporation process for acid mine water treatment depends on the properties of the mine water. Pilot-scale evaporation experiments and laboratory-scale corrosion experiments are needed to study the applicability of mine waters for evaporation. The energy efficiency of the evaporation process can be enhanced by optimizing and sizing the equipment appropriately. By investigating reuse and recycling methods for mine water concentrate, the profitability of the evaporation process can be further improved.
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