Mineral Precipitation Sequence During Evaporation of Lake Katwe Brine Joseph Ddumba Lwanyaga1,2, Hillary Kasedde2, John Baptist Kirabira2
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Mineral Precipitation Sequence During Evaporation of Lake Katwe Brine Joseph Ddumba Lwanyaga1,2, Hillary Kasedde2, John Baptist Kirabira2 1Department of Mining and Water Resources Engineering, Faculty of Engineering, Busitema University, P.O. Box, 236, Tororo, Uganda. 2School of Engineering, College of Engineering, Design, Art, and Technology, Makerere University, P.O.Box, 7062, Kampala, Uganda. Abstract Traditional salt mining techniques are the dominant salt extraction methods at Lake Katwe in Uganda. In this work, the effect of temperature on the mineral precipitation sequence during evaporation of lake brine was studied to foster the design of a commercial salt extraction process. Isothermal evaporation experiments at different temperatures were undertaken. PHREEQC was used to predict the precipitation sequence. The precipitates were characterised by the XRD and SEM techniques. Halite is the most abundant mineral, with Thenardite, Trona and Glaserite following, respectively. Halite will be best extracted at higher temperatures whereas the lower temepratures will favour sulphate based minerals. Keywords: Lake Katwe, Evaporation Temperature, PHREEQC, Precipitation, Brine Introduction A salt plant was established for industrial In Uganda today, salt for both domestic processing of the Lake Katwe brine. The plant and industrial use is mainly imported from never perfomed to its design expectations neighouring countries. The salt import bill is and the components were heavily corroded steadily increasing as the country develops (Driver and Tukahirwa 1990). This led to its which is increasingly straining the country’s abandonment without ever producing any merger resource envelope. To avert this, the meaningful products. Plans for commercial salt deposit at Lake Katwe need to be fully exploitation of the brine commenced with the exploited. This salt lake has been a source characterisation of brine and phase chemistry of salt for a long time. However, the salt experiments. The brines are highly alkaline + + - 2- 2- produced from the lake does not meet the and rich in Na , K , Cl , SO4 , CO3 , and required purity standards for table salt and HCO3- showing an intermediate transition the chlor-alkali industry(Kirabira et al. 2013; between Na-Cl and Na-HCO3 water types Kasedde et al. 2014). (Arad and Morton 1969; Kasedde et al. 2014). Currently at the lake, salt is recovered An isothermal laboratory evaporation by traditional means; brine is channelled experiment at 30°C revealed precipitation from the lake into salt pans. In the salt pan, of commercial salts such as Thenardite the salt crystallizes due to solar evaporation (Na2SO4), Anhydrite (CaSO4), Mirabilite producing two salt grades of different (Na2SO4·10H2O), Burkeite (Na2CO3·2Na2SO4), qualities. Grade I which is averagely about 93% Hanksite (9Na2SO4·2Na2CO3·KCl), Gypsum, Halite (NaCl) is the best these rudimentally Trona (Na2CO3·NaHCO3·2H2O), Halite, techniques can produce. Grade II is harvested Nahcolite (NaHCO3), Soda ash (Na2CO3), after Grade I with an average composition of and Thermonatrite (Na2CO3·H2O) from the about 65% Halite. The third grade is obtained brine of Lake Katwe. Moreover, the mineral by hacking the lake bed with iron bars; this salts crystallize in the order following the has a composition of about 50% halite. Due sequence starting with sulfates, followed to their composition, these three salts attract by chlorides and carbonates, respectively low prices as the demand for them is low (Kasedde et al. 2013) with a rich source (Kasedde et al. 2014). of mineral salts. The present work aims at Wolkersdorfer, Ch.; Khayrulina, E.; Polyakova, S.; Bogush, A. (Editors) 515 IMWA 2019 “Mine Water: Technological and Ecological Challenges” evaluating possibilities of future salt ex- membrane. The filtrate was then poured traction from the lake deposit. An isothermal into a 50 ml beaker which was then placed evaporation experiment was conducted on in a Labcon thermostatic water bath at 30°C. the lake brines. The precipitated salts were The formed precipitates were filtered off characterized by X-ray diffraction (XRD. using a 5 µm membrane and then left to dry Fractional crsytallisation is often times overnight in open air on an aluminium foil. used to extract most of the aforementioned This procedure was repeated for the other salts from brine. This, therefore necessitates study temperatures (40, 50, 60 & 70°C). investigating the precipitation sequences of minerals from Katwe brine at different Characterization of the precipitates temparatures in order to facilitate the design To determine the mineralogy of the a commercial salt extraction process. precipitates, the powder X-ray diffraction method was used. A Phillips diffractometer Methods and Materials PW3050 with a X-ray source of Cu Kα Labarotory experiments radiation (λ = 1.540598 Å) was used. The The rainy season surface brine was sampled scan step size was 0.013°., the collection from Lake Katwe and stored in 1.5 L time 1s, and in the range 2θ CuKα from 1° plastic bottle at room temperature prior to 90.5°. The X-ray tube voltage and current to the evaporation study. Using 10 ml of were fixed at 45 kV and 40 mA respectively. the original brine from Lake Katwe, the The Crystallography Open Database (COD) polythermal evaporation experiment was powder diffraction database was searched run at five different temperatures (30, 40, 50, by QUALX2.0 search–match identification 60 & 70°C). These were chosen because they software for phase identification (Altomare et are easily achievable on a commercial scale al. 2015). The morphology of the precipitates with less material selection problems and was determined by a Zeiss Supra 55 VP Field the evaporation process is faster compared Emission Scanning Electron Microscope to lower temperatures. Before evaporation (FEG-SEM); all samples were carbon coated was started, both the biological and organic before SEM analysis in order to make them materials were filtered off using a 5 µm electrically conductive. Table 1 Brine Composition pH Brine Density EC Major ions (g/L) Temp (g/ml) (us/m) Na K Mg Ca Cl Br SO HCO CO (°C) 4 3 3 9.72 25.2 1.15 14109 69.6 11.6 0.0519 0.0048 44.2 0.461 32.7 18.2 38.8 Figure 1 XRD Pattern of mineral precipitates (Gl-Glaserite, Th-Thenardite, Ha-Halite, Tr-Trona, B-Burkeite, Na-Natron, Tn-Thermonatrite, H-Hanksite). 516 Wolkersdorfer, Ch.; Khayrulina, E.; Polyakova, S.; Bogush, A. (Editors) IMWA 2019 “Mine Water: Technological and Ecological Challenges” Table 2 The mineral composition of the precipitates at the different study temperatures Mineral Phases Temp Sample 1 Sample 2 Sample 3 Sample 4 °C 30 Calcite, Magnesite, Tychite, Calcite, Calcite, Anhydrite, Anhydrite, Dolomite, Halite Glaserite Thermonatrite, Magnesium Sulphate, Thenardite, Burkeite, Burkeite, Thenardite, Burkeite, Thermonatrite, Hanksite, Trona, Natrite, Halte Halite, Glaserite, Sylvite Halite, Glaserite, Sylvite 40 Calcite, Anhydrite, Calcite, Magnesite, Magnesite, Dolomite, Burkeite, Hanksite, Halite, Magnesite, Magnesium Halite Thenardite, Burkeite, Glauberite, Sylvite Sulphate, Thenardite, Trona, Thermonatrite, Halite, Natron, Halite, Glaserite, Glauberite, Glaserite, Sylvite Arcanite, Sylvite 50 Anhydrite, Magnesite, Calcite, Magnesite, Calcite, Magnesite, Calcite, Anhydrite, Dolomite, Dolomite, Trona, Thenardite, Trona, Dolomite, Gaylussite, Thenardite, Burkeite, Thermonatrite, Halite Halite Burkeite, Hanksite, Trona, Hanksite, Nahcolite, Thermonatrite, Natrite, Thermonatrite, Halite, Halite, Glauberite Glaserite, Sylvite 60 Calcite, Magnesite, Halite, Magnesite, Dolomite, Calcite, Magnesite, Anhydrite, Dolomite, sylvite Halite Dolomite, Trona, Natrite Burkeite, Hanksite, Natron, Halite, Sylvite 70 Calcite, Magnesite, Calcite, Magnesite, Calcite, Anhydrite, Anhydrite, Magnesite, Magnesium Sulphate, Dolomite, Trona, Magnesite, Dolomite, Halite, Dolomite, Gaylussite, Dolomite, Thenardite, Halite, Halite, Sylvite Sylvite Thenardite, Natrite, Halite, Thermonatrite, Glaserite, Glauberite, Sylvite Sylvite Thermodynamic modelling the carbonates and sulphates of calcium and To model the precipitation sequence during magnesium (Calcite, Tychite, and Dolomite) the evaporation of the Lake Katwe brine, in the first precipitates as predicted by Arad PHREEQC code (Parkhurst and Appelo and Morton (Arad and Morton 1969). This is 2013) version 3.4.0 was used. Due to the high attributed to their low solubilities within the concentration and ionic strength of brine, study temperature range. the geochemical calculations were done Table 2 shows the mineral composition using the Pitzer ion interaction approach of the precipitates at the different study with a modified version of the pitzer.dat temperatures. Four samples are analysed at thermodynamic database distributed with each temperature and these were collected the PHREEQC code. Evaporation was as soon as crystals appeared. At all the study simulated in steps, 51 moles of water in temperatures, Halite emerges as the most 102 steps was removed (92% of the initial abundant mineral as the phase is observed water was evaporated). The sequence of the in almost all the samples. This is also evident modelled mineral precipitation at the study by the dominance of its microstructures in temperatures during the evaporation was Fig.2. noted. The major ions in Lake Katwe brine At 30°C, apart from Halite, Trona is as shown in Table 1 were used as PHREEQC the most abundant mineral followed by input (Kasedde et al. 2014). Thenardite and then Natrite. Halite,