Mining is not possible without consideration of the component water. This is coupled to a variety of questions. These include, e. g., dewatering in underground and open-cast/pit mining, the post-mining flooding of open-cast/pit residual holes, the remediation of acidic post-mining lakes, general management and treatment of such acid waters (Acid Mine Drainage – AMD) etc. In relation to the above-mentioned aspects, but also for the re-mining of secondary mining bodies (tailings), it is essential to gain a detailed process and structural understanding of the respective site.
Uncontrolled water influxes into open pit mines impair the efficiency of ore extraction and production conditions. Therefore, it is important to understand the inflow system, to detect the sources of these water influxes and so to initiate targeted and effective dewatering measures. The detailed hydrogeochemical investigations of the water inflows and of the potential “source terms” showed that the system is significantly more complex, than toughed in before.
Furthermore, investigation results of Chilean mining sites are presented. In recent years, secondary mining has become more and more important. The residual content of valuable metals contained in such secondary mining bodies is often on the same level (or above) of primary deposit bodies mined today. The investigations focused on the structural understanding of the tailings body. So in detail the main question is, if there are layer-like enrichment zones. This is important in relation to selective re-mining. For another heap leaching site it is shown that, beneath the leaching of copper, there is also a relevant mobilisation of e. g. cobalt and rare earth elements (REE). That seems to be related to the acidic attack on alumo silicate phases.
1 Mine water management and dewatering technology in teaching and research
The aspect water often plays a decisive role in public perception only after mining. With detailed knowledge, it is of course clear that mining is not possible at all without the consideration of the water component. In this respect, the working group “Mine Water Management” contributes knowledge for the understanding of hydraulic and hydrogeochemical processes in mining to the students education. Since 2011, the working group has been part of the Department of Mining and Special Civil Engineering of TU Bergakademie Freiberg and also works extensively on research projects. The findings from these projects are then fed back into the education of the students.
Thus, the modules “Mine Water Management” and “Dewatering Technology” are integrated into the education of the diploma study course “Geotechnics and Mining”. The Master of Science programs “Groundwater Management” and “Sustainable Mining and Remediation Management” contain modules on “Groundwater chemistry/Hydrogeology for GW-Management” and “Mine water”.
The above-mentioned work on research projects plays an essential role. The following range of research fields is covered by the working group “Mine Water Management” and is worked on in joint projects with national and international project partners:
- hydrogeochemical processes in old mining bodies
(dumps, tailing bodies etc.); - acid mine water formation – technological countermeasures (also sulphate reduction);
- understanding of hydraulic inflow and dewatering systems;
- hydraulic/hydrogeochemical processes for geotechnical events in old dumps/tailings; and
- re-mining of tailings bodies/extraction of valuable elements from mine waters.
In this regard, a variety of questions have to be answered in mining: Dewatering in underground and opencast mining, post-mining flooding of residual opencast/pits, the remediation of acidic post-mining lakes, general management and treatment of such acid mine waters (Acid Mine Drainage – AMD). In relation to the above-mentioned, but also for the deconstruction of secondary mining bodies (tailings), it is essential to gain an understanding of the process and details of the respective site.
Here, detailed hydrogeochemical investigations are often of crucial importance. These are used to obtain a detailed understanding of the process in order to develop well-founded, problem-oriented and differentiated technical solutions for the respective site. This will be illustrated by the following examples.
2 Example – deepening of an open pit ore mine – understanding the water inflow system
The ore mine under consideration is one of the deepest in Europe. In recent years, this open pit mine has expanded significantly in terms of surface area and, above all, depth. This naturally increases the hydraulic potential. Increasing water inflow into the open pit mine is potentially relevant, even if the climatic conditions, partly permafrost, have to be considered. Water influx and its influence on the production conditions impair the efficiency of ore extraction. Thus there is great interest to understand the local hydraulic inflow conditions in order to initiate targeted and effective dewatering measures.
The basis is to gain a deeper understanding of the geohydraulic system of the open pit mine. In relation to that, hydrogeochemical investigations were carried out in order to verify the initially prevailing idea that the inflowing waters of the different areas are all more or less of the same origin. In this respect, the different water inflow areas of the open pit mine, as well as potentially conceivable “areas of origin” of these waters were investigated. In addition to the complete “main chemistry” of the waters, investigations of inorganic trace elements and isotope measurements were also carried out on the waters.
Four different types of water inflow could be distinguished in total:
- Lateral inflows from the rock in place – related to fault and fissure zones.
- The question of inflows from the “unknown sealed” general drainage ditch.
- Seepage water from tailings bodies that likely enters the pit through near surface quaternary sediments. Pre-mining “valley structures” with zones of greater hydraulic permeability play an important role here.
- Inflows of deeper formation waters via the pit bottom, which will increase as the mine depth increase.
As a first big step, a pragmatic understanding in the sense of a water balance is necessary. The detailed hydrogeochemical investigation showed that there were clear differences in the composition of the inflowing waters. This is to be understood with regard to different “areas of origin” for these waters and therefore requires different countermeasures. It is therefore important to understand how the total lifted waters are distributed with regard to the different “areas of origin”.
Thus, the different areas of inflowing waters in the open pit mine itself, as well as the potential “areas of origin” were sampled and hydrogeochemically investigated (Figure 1). All water samples taken were examined for their hydrochemical milieu parameters – pH value, electric conductivity (EC), redox potential, alkalinity and acidity. Furthermore main and minor elements and a wide range of trace element contents were measured.
Fig. 1. Ore open pit mine – Overview of main influx areas with sampling points. // Bild 1. Erztagebau – Übersicht der Hauptzutrittsbereiche mit entnommenen Proben. Source/Quelle: TUBAF
It was found out that especially the contents for aluminium, rare earths elements (REE) and copper are of decisive importance for the understanding of the inflowing waters and their relation to potential water inflow areas.
In open pit mining, water inflow is mainly concentrated in two areas – named here A and B.
2.1 Water influxes in area A of the open pit
In area A there are significant water influxes. The assumption prior to the investigations, that water from the unsealed general drainage ditch percolates in large scale into the rock body and that nearby water treatment basins (clarification ponds) also play such a role could be clearly rejected by the hydrogeochemical investigations. The budgets potentially provided for sealing measures could thus be used elsewhere. The samples of the general drainage ditch and the clarification ponds show REE contents (lanthanum, cerium, neodynium) up to approximately 1 mg/l and high aluminium and copper contents. This is opposed by the incoming water in which the elements mentioned are mostly < LOD. Further hydrochemical measurement results, in combination with the isostope measurements, show a different inflow regime for area A to be relevant.
2.2 Water influxes in area B of the open pit
On the basis of the data obtained, there is a connection for area B between the water inflows in the upper part of the open-pit and the water from the general drainage ditch. The more precise interpretation of the data suggested, however, that it is rather a matter of water being formed in the tailings bodies. The similarity to the ditch waters is rather due to the fact that the ditch is largely fed by these dump waters. It is also discussed that the inflowing waters of the upper part in area B are possibly a mixture of both – dump waters via quaternary flow paths, which are under the dump bodies, and partly from the insufficiently sealed drainage ditches.
This relationship is not the case for the inflows in the lower part of area B. The waters of this lower part of area B did not show a higher content of aluminium, copper and REE. Thus, the origin of these inflowing waters is different from that of the upper part of area B. This means that if one wants to meet these two water access areas, different technical measures have to be considered.
3 Example – Re-Mining of tailings bodies – Importance of hydrogeochemical investigations
Mining and above all copper mining plays an important role in Chile and in South America in general. In this context, secondary mining is becoming more and more important. The residual contents of valuable metals contained therein are often at the level of primary deposit bodies mined today. Based on investigations and the resulting basic understanding of tailings bodies, which is carried out jointly for a long period together with the German Federal Institute for Geosciences and Natural Resources (BGR) in Hanover, one main question is whether these bodies contain layer-like enrichment zones, that should be selectively re-mined. One can name and understand these zones as a kind of “anthropogenic seams”. Therefore it is important to get a better understanding of the structure of these tailings bodies in general and for the enrichment zones in detail, with site-specific investigations.
In the South American context of copper production, the mining of “porphyry copper“ deposits often results in very large tonnages, also for such older tailings bodies. These bodies can thus contain more than 50 Mt of material that was flushed out after the flotation process.
In order to be able to characterize the tailings bodies more closely, various investigation methods were applied. Profiles were taken from test excavations to characterise the general structure. After the main and partial horizons had been defined, the element contents were immediately measured directly at the surface using a hand-held X-ray fluorescence analysis (hXRF). Furthermore, field elutions – determination of the hydrochemical milieu parameters pH-value, ELF and redox potential – for all subsamples of these profiles were carried out immediately in the field. With these hydrogeochemical investigations, the differences of the materials are immediately characterised in the field, in relation to their weathering status and thus also the mobilisation behaviour (water solubility of elements).
In addition to a large number of profiles (Figure 2), drillings with drill cores were subsequently also examined.
Fig. 2. Field investigation and sampling of a Chilean tailings body. // Bild 2. Felduntersuchung und Probenahme an einem chilenischen Tailingskörper. Source/Quelle: TUBAF
Using the hXRF data, it was possible to underpin the assumed layer characteristics for the tailings body with enrichment zones, which are decisive for the recovery aspect. The comparison of the hXRF-data collected in the field with the data obtained later in the laboratory under ideal sample preparation conditions showed that the less ideal field measurements, however lead to reliable statements. The general basic understanding of the essential characteristics of the tailings body is immediately available. Accordingly, the layer aspect can already be investigated in the field with the help of hXRF. However, it is important to note that the hXRF-device has to be calibrated on the general material itself by means of its own preliminary work. This is the only way to obtain reliable measurements. Especially with regard to secondary and trace elements and above all with regard to “light elements” (lower atomic masses), it is absolutely necessary to collect values based only on one’s own calibration and to critically question the standard contents output by the device.
In addition to the field investigations and the subsequent laboratory measurements, a large number of other investigations were carried out. The focus was also on characterizing the different contents of valuable elements – not only copper but also other economic strategic elements – in relation to different grain size fractions with a view to later processing. A granulometric analysis was carried out for a large number of samples. Depending on the type of grain size analysis (dry sieving, wet sieving, laser granulometric examination with ultrasonic), there were clear differences in the results. The tailings material has, understandably due to the earlier flotation process, a high proportion of fine grains. This leads to the aggregation of fine grain fractions to larger conglomerates. Figure 3 shows the results of a sample with the determined grain size distributions exemplarily.
Fig. 3. Exemplary sieve passage of a tailings sample in comparison with dry sieving (pink), wet sieving (yellow) and laser granulometry with ultrasonic (blue). // Bild 3. Beispielhafter Siebdurchgang einer Tailingsprobe im Vergleich bei Trocken- (rosa), Nasssiebung (gelb) und Lasergranulometrie mit Ultraschall (blau). Source/Quelle: TUBAF
First, it becomes clear that there are significant differences in the grain size characteristics. The increasing energy input, during the individual examination methods – wet sieving by water or laser granulometry additionally by ultrasonic – leads to a disaggregation of the conglomerated grains. This means that laser granulometry with ultrasonic is best suited to describe the real grain size distribution. However, it is also important to consider the later processing technology. Which later procedure will be applied in practice – laser granulometry with ultrasonic is of course not practically relevant, but dry sieving might become relevant.
The separated grain fractions were then examined for their element contents. It became clear that especially the fractions < 80 µm have relevant high valuable element contents. This can be attributed to the primary unfloated material. Furthermore, there are higher copper contents in the fraction > 150 or 250 µm – these are not non liberated grains. These results underline the great importance of deaggregation with regard to the potentially applicable extraction and processing technology.
4 Example – Mobility of elements on heap leaching material after the active production time of approximately 60 days
When ores are processed by heap leaching, the materials are subjected to a leaching process after crushing either with microbial generated or added acid. The aim of the investigations was to characterise the mobility of other elements than copper at the leaching residues, i. e. after active leaching. Beside copper the interest was also in relation to REE, cobalt, molybdenum etc.
In this process the easily leachable/readily available copper is extracted. This leaching process typically takes place over a period of 60 days during which the material is treated with acid (Figure 4).
Fig. 4. Ore processing by heap leaching – sampling of the residues. // Bild 4. Erzaufbereitung mittels Haufenlaugung (Heap-Leaching) – Probenahme an den Rückständen. Source/Quelle: TUBAF
During the leaching process about 80 % of the copper is recovered. Subsequently, the leaching residues are washed and dumped afterwards. The carried out investigations show that the acid attack also leads to the mobilization of other elements.
The dumped residual materials are still in a strongly acidic state. The pH level of water eluates of these materials are in the range of pH 2 to 3. The materials show clear mobilizations of different elements, this is mainly due to the acid attack on the alumo-silicate phases. So in addition to copper, the investigated materials also release cobalt and REE, e. g., lanthanum, cerium, yttrium, gadolinium, dysprosium, scandium. The partly very large differences in the mobilized elements and their concentrations from the leached materials are due to the heterogeneous initial materials. Furthermore, the attac of other mineral phases may also play a role.
5 Summary
In summary, the presented examples show how essential hydrogeochemical investigations are for the characterisation of inflow systems to open pit ore mines and the recovery of valuable elements from tailings bodies and mine waters.