Direct Formation Of Burkeite On The Geothermal Waters At Vranjska Banja, Serbia

There are no available data about direct burkeite formation on the geothermal waters pipelines in Europe. Data about accompanying minerals of burkeite are also scarce. This mineral has been found in the scale on pipelines of VG-2 and VG-3 boreholes at Vranjska Banja, Serbia. Geothermal waters from these boreholes have temperatures in the range of 103 - 105 oC which places them in the warmest waters in the continental Europe.Based on physicochemical and geochemical data, VG-2 and VG-3 geothermal waters can be classified as Na-Alk-SO4Cl type waters. According to their temperatures, total hardness and pH values, these waters belong to hyperthermal, very soft and moderately alkaline. Calculated LSI (0.4 and 1.7) and RSI (6.5 and 4.8) values indicate to their tendency to scale formation. Results of the spectrometric scale examination from the pipe deposits provide a clear qualitative and quantitative burkeite characterization with its accompanying minerals. FTIR analysis points to the presence of carbonate (1766, 1458, 877 and 705 cm-1) and sulphate bands (1139, 1116 and 617 cm-1). Apart from the qualitative analysis, XRD also shows the burkeite mass part in the scale. VG-2 geothermal water contains about 50 %, which is associated with trona mineral and smaller amount of halite of about 10 %. Burkeite is a dominant phase (>60 %) in VG-3 water, with the presence of a significant halite quantity (>35 %) and a minor calcite quantity (2 %).

Introduction

Geothermal waters represent a significant renewable source of energy of which greatly depends industrial and economic development of an area. The applications are certain by discharge volume and temperature of geothermal sources. While passing through the earth crust, geothermal waters partly dissolving the minerals on the way they are passing through, thus enables the cations and anions creation which cause scale formation. Therefore, geological processes are mostly the result of reactions which take place between minerals in the rock and the fluids. Based on this fact, it can be stated that geothermal waters can document the ancient geochemical records of the earth’s history. Due to high ion concentrations, rapid decrease in pressure and changes in temperature and pH values the formation of scales in the pipes might takes place, which creates a problem when using these waters (many authors). Due to the changes in physicochemical parameters, newly created scale does not surely contain the same minerals as the rocks of geothermal waters that have passed through.

There are two different types of sodium double anion salts of sulphate and carbonate. One of them is Burkeite. This is a sulphate-rich compound that contains approximately two moles of Na2SO4 and one mole of Na2CO3, thus the formula of this salt is Na6(SO4)2CO3. This mineral usually occurs as efflorescence’s in saline soils or in continental lacustrine evaporate deposits. For the first time, it was found in the geothermal waters at Copahue-Argentina. There are neither data about the origin of this mineral from hydrothermal fluids on the pipe deposits in Europe. The mineral thermodynamic field of stability depends on PCO2 and T. It increases with the increase of these parameters. This mineral is formingat the temperatures higher of 100 °C.

Vranjska Banja is located at the south of the Republic of Serbia, 10 km away from the Vranje town. Vranjska Banja is surrounded by rocks divided on the eastern brim of Vranje valley. Ingenuous rocks formed along the division are suitable ground for thermal springs appearance. Vranjska spa waters are used in various treatments: bathing, rinsing and inhalation in hydro-electro- and fango-therapy. Vranjska Banja geothermal waters properties are such that they offer many possibilities of its renewable energy sources at the south of Serbia. The temperature of hot springs in Vranjska Banja that goes up to 105 oC, makes them the hottest springs in the continental Europe. This spa has the highest geothermal potential in Serbia (>100 L s-1). There is a number of natural springs and artificial objects, 26 m to 1470 m deep. The two most important boreholes in Vranjska Banja are VG-2 and VG-3. In the VG-2 borehole granodiorite, shale and gneiss alternately appear. At570 m depth granodiorite is dominant, but it quantity decreases with the increase of the depth. The borehole is cemented in order to prevent the influence of shallow intervals. The most productive interval is drawn from gneissgranite at the depth of 864 to 890 m and temperature that goes up to 126 °C. Total drawn water quantity is 26.5 L s-1, the outflow temperature is 102 °C, and the well-head pressure is 9.5 bars. VG-3 borehole is located 120 m west from the VG-2 borehole.. This borehole is slant from the depth of 45 m, with the azimuth of 120, which makes the vertical depth of1470 m, and for the total boring length of 1604 m. In the borehole, shale and gneiss appear alternately while granodiorite are present up to 145 m together with gneiss and shale. Thermal water quantity from the VG-3borehole is 23.5 L s-1 at the temperature of 105 °C and the well-head pressure of 8.5 bars.Geological frameworkVranjska Banja spa is located in the Serbian Crystalline Core region (SCC). The SCC hydro-geological region principally belonging to the Serbo-Macedonian Massif geotectonic unit.High or medium size meta-igneus and sedimentary rocks, mostly comprising the region. Examples are amphibolite, amphibole–biotite schist,gneiss, micaschist, amphibole–pyroxene schist with marble as well asmigmatite sporadic occurrence.

The oldest rocks on wide Vranjska Banja spa area are Proterozoic age (gneiss, crystalline schist and metamorphosed volcanite rocks) in which introduce Paleozoic, Mesozoic and Cretaceous-Paleogene granitoide rocks. Mica-gneiss (Gb) prevails over other the lithologic members. The rocks are grey, made of biotite, quartz plagioclase, microline and muskovite. Changing the content, feldspars mica-gneiss goes through to leptynolite and micaschist.Leptynolite and micaschist (Sm) are found as interlayer or lenses of different dimensions in fine-grained gneiss. They contain quartz, muscovite, biotite and varying amounts of albite.Amphibolite (A) appears like lens in gneiss, leptynolite and micaschist. They originated from volcano-sediment series and shallow basic intrusive. They consist of amphibole and plagioclase.

Quartzite (Q) also occurs in elongated lenses.In the mentioned metamorphic complexes there is Surdulica granodiorite (γδ), present in the southeastern side of the Vranjska Banja. Granodiorite is composed of quartz, plagioclase, orthoclase, biotite and amphibole with the accessory minerals apatite, sphene, magnetite, ortite and metallic minerals. In Surdulica granitoide pegmatites were not noticed, while aplites appear occasionally in thin strings in the middle of the massive. Granodiorite contains numerous dykes of quartz latite and dacite. Eocene sediments (E) are located northwest from the Spa and represent marls, alevrolites, conglomerates and sandstones. Tertiary effusive rocks with dacito-andesite (aq) and quartzlatite (xα), are present,at the wider Vranjska Banja area. The main components of these rocks are andesine, biotite and hornblenda, while the volcano type is determined with the quantity of quartz and sandine. Due to intense Tertiary volcanic activity the sedimentary rocks of Miocene to the Pliocene age have been formed.

Pyroclastic sediments (ωαq) are located north from the Vranjska Banja, while smaller massive are present north-east from the Spa. They are represented with tuff, agglomerates, volcanic breccia, sandstones and rarely tuffs.Sediments of Mio-Pliocene (M,Pl) are formed of clays, grey-greenish marls, sandstones, conglomerates and grey sands.Pliocene (Pl) is desribed as grey sands, clays and marls more than 220 m thick. Quartar is present in the form of alluvial sediments (al).

According to VG-2 and VG-3 geothermal waters’ temperatures, total hardness and pH, they belong to hyperthermal, very soft and alkaline waters. LSI of 0.4 for VG-2 indicates mild indication to corrosion and scaling, which is confirmed by RSI 6.5. LSI values of 1.7 for VG-3 water cause scale but not the corrosion, which is in accordance with RSI (4.8).

Degassing of the VG-2 and VG-3 geothermal waters contributes considerably to the scale formation composed of burkeite and admixtures. High temperatures of the examined geothermal waters as well as negative temperature coefficient of solubility of carbonate and sulphate minerals, create favorable conditions for this process.

The FTIR spectrum shows sharp band at 468 cm-1, which is possibly consequence of metal-O bonds. Carbonate bands at 1766, 1458, 877,850 and 705 cm-1 were assigned to the vibrational modes v1+v3, v3, v2 and v4, respectively. Intense bands at 1139 and 1116 cm-1 with the shoulder at 1088 cm-1 originate from sulfate (ν3) and (ν1) absorptions, respectively. The sulphate band at 617 cm-1 is due to ν4.The XRD qualitative and quantitative mineralogical analysis of the collected material from VG-2 borehole, showed that it has about 50 % of burkeite which is associated with trona mineral (about 40 % ) and smaller amount of halite (about 10 % ). Complete structural analysis of VG-3 also shows that in burkeite sample is predominant mineral phase (more than 60 %). However, in this sample the halite concentration is significantly higher than in the sample from the borehole VG-2 ((36 %) with small amount of calcite (about 2 %).