Utilization of Solar Energy on Exploitation of Salt Lake Resources in Tibetan Plateau

Transcription

1 Utilization of Solar Energy on Exploitation of Salt Lake Resources in Tibetan Plateau Wu Qian 1,2 1 Institute of Mineral Resources, Chinese Academy of Geological Sciences 2 Key Laboratory of Saline Lake Resources and Environment, Ministry of Land and Resources Beijing , China wuqian@cags.ac.cn Huang Weinong 4 4 Tibet Zabuye High-Tech Lithium Industry Company Ltd. Lhasa , China Liu Malin 3 3 Institute of nuclear and new energy technology, Tsinghua University Beijing , China Zheng Mianping 5,6 5 Institute of Mineral Resources, Chinese Academy of Geological Sciences 6 Key Laboratory of Saline Lake Resources and Environment, Ministry of Land and Resources Beijing , China Abstract Across Tibetan Plateau, there is bountiful solar energy and salt lake resource. However, the lack of the conventional energy for the mineral exploitation of the salt lake is a touchy issue. The solar energy utilization and the salt lake exploitation can be combined to bridge the gap between the energy demand and the economic development. Solar pond technique is a special way for the exploitation of the salt lake mineral resources by using the solar energy. In this paper, it was illustrated that Tibetan Plateau is an ideal place for the construction and operation of solar pond. Also, this paper described the significant superiority of solar pond crystallization compared with the natural evaporation crystallization in the mineral resource exploitation of the salt lake in Tibet. It was pointed out that the crystallization rate of the high added-value saline minerals, especially the lithium carbonate, can be speeded up in the solar pond, thereby increasing the production efficiency. In addition, the grade of lithium carbonate in the mixed salt can also be improved significantly, the salt product obtained from the solar pond can be used for the final fine processing directly. The research and development of the solar pond technology, which has very important significance in the utilization of the solar energy resource and the exploitation of the salt lake resource in Tibetan Plateau, should be paid more attention in the future. Keywords-solar energy; solar pond; salt lake exploitation; Tibeten Plateau; Li 2 CO 3 I. INTRODUCTION At present, the contradiction between the development of economy and the energy poverty has become severe increasingly, and it is imperative to develop and utilize the new energy. As a clean, durable and safe new energy, solar energy has been paid more and more attention. It is very important to find a simple and efficient way of the solar energy utilization technology, especially for the energy-thirsty region such as Tibetan Plateau. The average elevation of Tibetan Plateau is about 4000 meters and the solar radiation can reach up to 6000~8000 MJ/m 2. It has long hours of sunshine, and the average annual sunshine hours is around 3000 h [1]. Tibetan Plateau has rich solar energy resource that ranks first in China, and also is one of the most abundant regions in the world. Although the solar energy has been widely used in many aspects including the solar cookers and solar water heaters, biogas and power generation in Tibet, but it remains to be explored further how to change the advantages of the solar energy resource into the driving force for national economic development. It is wellknown that there are numerous resource-rich salt lakes with large reserves of lithium, borate, potassium and other salt mineral resources in Tibet. However, due to the harsh geographical environment and the traffic inconvenience, especially the particular lack of coal, oil, electricity and other conventional energy sources, the development intensity for the salt lakes of Tibet is very weak. If the utilization of the rich solar energy resource can be combined with the exploitation of the salt lake resources, it will surely contribute to the economic development of Tibet. The traditional method of solar energy utilization in the salt lake resources exploitation is similar to the productive technology of sea salt that is to use the sunlight for the natural evaporation of the salt lake brine directly. Comparing with the seawater, the composition of the salt lake brine is more complex and its productions are more diverse, not limited in the sodium chloride as the sea salt production. Therefore, if the direct utilization of solar natural evaporation is adopted, the high added-value mineral materials such as high-grade lithium carbonate cannot be obtained, only the mixed salt can be obtained. It will result in many technical difficulties in the further purification step and the increasing capital investment. So it is necessary to find an efficient technology of solar energy utilization for the exploitation of salt lake resources. Supported by National Natural Science Foundation of China (No ) U.S. Government work not protected by U.S. copyright

2 Solar pond technique is such a good way to couple the solar energy utilization and the exploitation of salt lake resources. With its advantages of simple structure and low cost, much more heat from the solar energy can be absorbed and stored by the solar pond for a long time. The pre-evaporation brine can be heated up quickly by using the solar pond, resulting in the precipitation of the high-grade lithium carbonate minerals which is a kind of scarce product in the market. In this paper, the structure of the solar pond and the technology principle is illustrated, and its utilization on Zabuye and Dangxiongcuo salt lakes in Tibet will be given as examples. It can be found that Tibet has its own advantage in constructing and operating the solar pond. Also, this paper describes the great superiority of solar pond comparing with the solar natural evaporation in the exploitation of the salt lake mineral resources. Through the analysis of the onthespot experimental results, it is pointed out that the crystallization rate of the saline minerals, especially the lithium carbonate, can be speeded up in the solar pond, thereby increasing the production efficiency. In addition, the grade of lithium carbonate in the mixed salt can also be improved significantly. II. SOLAR POND TECHNOLOGY The basic structure of the solar pond is shown in Figure 1. It has three regions. The upper layer is called the upper convective zone (UCZ), which is generally composed by the fresh water or light salt brine. Its temperature is close to ambient, and has the function of heat insulation and preventing the lower solution from disturbance. The bottom region is the lower convective zone (LCZ). It consists of the saturated brine with almost homogeneous salinity and density, mainly storing and absorbing heat. The maximum temperature can reach up to about 100. The intermediate region, which is the key component of the solar pond, is called the nonconvective zone (NCZ), and also called the salinity gradient zone. In the NCZ, the brine salinity and density increase gradually with depth, effectively preventing the natural vertical convection generated by the higher water temperature of LCZ. The heat cannot be transferred by convection in this layer, with the higher temperature of LCZ than UCZ, so as to achieve the purpose of collecting and storing the solar energy. the absorption of solar energy by water and the heat-insulation function of the upper water layer with low-density. The temperature is not homogeneous and increases gradually with depth to form a temperature gradient. Figure 2 shows the salinity and temperature of brine changing with the depth of the solar pond in Tibet. A solar pond is an effective way of capturing and storing solar energy [2]. The stability and efficiency were focused on by many researchers [3-5]. It has been indicated that the efficiency and the lifetime of a solar pond depends mainly on the behavior of the gradient NCZ [4]. Some improved methods of heat extraction from the nonconvective zone are also investigated in order to enhance the solar pond performance [3]. It has been used in many ways, such as electric power generation [6], distillation for freshwater production [7] and so on. It has been indicated that solar pond is the most convenient and least expensive option for heat storage for daily and seasonal cycles without dependence upon supplementary sources [8], especially in border areas where the infrastructure is underdeveloped. The solar pond technology has also been successfully used in the production of lithium carbonate from the Zabuye salt lake [9, 10], here we will elaborate further on the advantages of solar pond technology in application of salt lake exploitation in Tibetan Plateau. Figure 2. Salinity and temperature profile: An example of solar pond in Tibet. Figure 1. Sketch map of solar pond. The general feature of the solar pond is that the water temperature of the pond bottom is much higher than the pond surface, and the temperature difference can reach up to 20~40 C. The accumulated heat of the pond bottom comes from III. UTILIZATION OF SOLAR POND TECHNOLOGY ON SALT LAKE EXPLOITATION IN TIBETEN PLATEAU In Tibetan Plateau, there are some natural salt-lake solar ponds due to its unique geographical and climatic conditions. Cuoni lake is one of the typical representatives [11]. According to the scientific investigation, the salinity difference between the surface and the bottom of salt lake is about 12% in winter. Meantime, the temperature difference is as high as 33 C, which confirmed the effect of solar pond. Some factors, such as the solar energy and salt lake resources, the climatic and geological conditions, can mainly affect the construction and operation of solar pond [2]. In Tibet, the general climatic features are windy, dry and little humidity. It has the highest solar radiation intensity, coupled with abundant salt lake and freshwater resources, providing the excellent natural conditions for the establishment of solar pond.

3 A. Li 2 CO 3 Extraction Principle Using Solar Pond Normally, it is traditional to use the solar energy directly to evaporate water, in order to extract the salt from the seawater/salt lake brine in the open evaporation pond. The brine temperature is close to the ambient temperature because the evaporation pond is usually shallow. The brine is easy to cool down by the heat loss of water convection Water can only be continued evaporating under the condition with the higher concentration of salt solution and the dry climate. Therefore, the salts are difficult to precipitate using this traditional method. Moreover, this way is limited by the climatic conditions, so that the available time for the salt production has been shortened. Salt lake brine includes a lot of salt mineral resources, such as Li 2 CO 3, KCl, NaCl, Na 2 CO 3, Na 2 B 4 O 7 and Na 2 SO 4, in which Li 2 CO 3 is one of the highest added-value salt minerals. However, the mixed salt with high-grade Li 2 CO 3 cannot be obtained only by the natural evaporation pond. As shown in Figure 3, it is clear that the solubility of Li 2 CO 3 and Na 2 CO 3 decrease while the solubility of KCl, NaCl and other salt minerals increase with the increasing temperature. And the decreasing amplitude of Li 2 CO 3 is much larger than Na 2 CO 3. Therefore, the difference between the solubility variation trends of Li 2 CO 3 and other minerals can be used to separate Li 2 CO 3. The salt lake brine can be evaporated and concentrated to be the lithium-rich brine in the evaporation pond by solar radiation. And then the pre-evaporation brine can be poured into the artificial solar pond, making the Li 2 CO 3 precipitate in great quantities under the high temperature condition, while other salt minerals such as the halite and thermonatrite are still dissolved in the brine. Ultimately, the mixed salt with high-grade Li 2 CO 3 can be obtained. is extraordinarily rich. The relevant meteorological data is listed in Table 1, and the chemical composition of brine and the reserves of salt mineral resource are shown in Table 2 and Table 3 respectively. The traditional method adopted by Zabuye salt lake is to use the static state evaporation. That is to say, the salt lake brine is pumped into the insolation natural evaporation pond once at first. After the concentration of Li + in the brine rises up to the saturation point under the solution temperature through a period of time for freezing and solar evaporation, the preevaporation brine is pumped again into the open crystallization pond until evaporating to full crystallization. The mixed salt with lower grade of Li 2 CO 3 (generally less than 5%) can be acquired by using this traditional way. Meantime, the annual production has also been reduced because the available time for production is limited. It is estimated that the annual output of Li 2 CO 3 is about 15 t and the grade of Li 2 CO 3 in the mixed salt is only 3% in the open evaporation pond with the total area of 2500 m 2. TABLE I. Figure 4. Panorama of Zabuye salt lake in Tibet. METEOROLOGICAL DATA OF ZABUYE SALT LAKE Evaporation Precipitation Sunshine hours (h/y) Solar radiation ( 10 6 kj/m 2 y) TABLE II. CHEMICAL COMPOSITION OF ZABUYE SALT LAKE BRINE (g/l, #: mg/l) Li + Na + K + Rb +# Cs +# Br -# CO 3 Cl - - HCO 3 B 2O 3 SO 4 Mineralization TABLE III. SALT MINERAL RESOURCE RESERVES OF ZABUYE SALT LAKE ( 10 4 t, #: t ) Figure 3. Solubility of different salt minerals changing with temperature. B. Zabuye Salt Lake Zabuye salt lake, which is located in Xigaze, the western of Tibet, is a successful example of Li 2 CO 3 production by using the solar pond. The panoramic view of Zabuye salt lake is shown in Figure 4. Zabuye salt lake has the total area of km 2, and the ph value of brine is 9.7 or so. It has great prospect of industrial development because there are many salt minerals such as Li 2 CO 3, KCl, NaCl, Na 2 SO 4, Na 2 B 4 O 7 and Na 2 CO 3 in the brine, especially the reserve of lithium resource Li 2CO 3 Na 2SO 4 KCl B 2O 3 NaCl Rb # Cs # Br # After the solar pond technology has been used, the product quality will be optimized greatly. In Zabuye salt lake, the area of single solar pond is about 2500 m 2, with the depth of 3~4 m (Figure 5). During the operating process, the pre-evaporation brine is irrigated to the depth of 2~3 m, and then the freshwater is poured up to 0.5 m. The height of gradient layer is about 0.6 m, and the temperature of brine at the pond bottom can reach up to 60 C nearly.

4 TABLE V. CHEMICAL COMPOSITION OF DANGXIONGCUO SALT LAKE BRINE IN TIBET (g/l, #: mg/l) Li + Na + K + Rb +# Cs +# Br -# CO 3 Cl - - HCO 3 B 2O 3 SO 4 Mineralization TABLE VI. MINERAL RESOURCE RESERVES OF DANGXIONGCUO SALT LAKE IN TIBET ( 10 4 t, #: t) Figure 5. Solar pond in operation of Zabuye salt lake. According to the experimental study, the production process can be divided into two stages for Zabuye salt lake. The concentration of Li + in the brine can reach to 2.5 g/l, 2.2 g/l and 1.7 g/l after pre-evaporation in winter (from Dec. to Mar.), spring and autumn (Nov., Apr. and May.) and summer (from Jun. to Oct.) respectively. It is estimated that there are four times for production if every two months as a production cycle from Nov. to Jun and the total output of Li 2 CO 3 is up to about 12 t in these 8 months. But from Jul. to Oct., a production cycle can be shortened to one month or a little more, due to the lower concentration of Li + in the brine. That means each solar pond can produce Li 2 CO 3 about 3 t, and the total output of three production cycles is 9 t. To sum up, the salt production can be operated seven times per year and one single solar pond can produce 21 t Li 2 CO 3 with the high-grade up to 65~80%. At present, the annual output of the lithium-full mixed salt in Zabuye salt lake has reached up to 6000 t using many solar ponds, and the good growth trend is presenting. C. Dangxiongcuo Salt Lake Dangxiongcuo salt lake, which is located in Nyima county of Naqu district in the northern Tibetan Plateau, has the total area of 54.5 km 2 and the ph value of brine is 9.2~9.4 (Figure 6). The salt mineral of brine is mainly composed by the Zabuyelite, sylvinite, halite, Glauber's salt and borax. The meteorological data of Dangxiongcuo salt lake is shown in Table 4, and the chemical composition and the mineral resource reserves are shown in Table 5 and Table 6 respectively. Li 2CO 3 Na 2SO 4 KCl B 2O 3 NaCl Rb # Br # It can be found that the chemical composition of brine and the climate condition of Dangxiongcuo salt lake are very similar to Zabuye salt lake. For this reason, the brine of Dangxiongcuo salt lake is also suitable for the exploitation of Li 2 CO 3 by using the solar pond technology. The solar pond experiment showed that the grade of Li 2 CO 3 can also be as high as 85%, as shown in Figure 7. The whole production process is proposed and can be described as follow: brine preevaporation Li 2 CO 3 extraction by using the salt-gradient solar pond to accumulate the brine temperature the mixed salt with high grade of Li 2 CO 3 scrubbed by the fresh water Li 2 CO 3 concentration and purification Li 2 CO 3 product harvested. Figure 7. Li 2CO 3 precipitated at the bottom of solar pond. TABLE IV. Evaporation Figure 6. Panorama of Dangxiongcuo salt lake in Tibet. METEOROLOGICAL DATA OF DANGXIONGCUO SALT LAKE Precipitation Sunshine hours (h/y) Solar radiation ( 10 6 kj/m 2 y) D. Comparision Between Insolation Natural Evaporation and Solar Pond Technology The comparison of Li 2 CO 3 grade in the mixed salt between the traditional crystallization method which uses the insolation natural evaporation pond and the solar pond technology is shown in Figure 8. It can be seen that the grade of Li 2 CO 3 in the mixed salt is significantly higher with solar pond technology than natural evaporation, indicating the product quality is greatly improved. In addition, using the solar pond technology can not only reduce the production cost, as described above, but also increase the annual production of salts and the effective utilization rate of the per unit area. Therefore, the research and development of solar pond technology, which has important significance on the utilization of solar energy resource and the large-scale exploitation of salt lake resources in Tibetan Plateau, should be paid more attention in the future.

5 ACKNOWLEDGMENT Thanks go to Niezhen and Bulingzhong with helpful discussion on the analysis of the experimental results. Figure 8. Comparison of mineral salts grade with or without solar pond. IV. CONCLUSION From the discussions above, the following conclusions can be drawn out: 1, Tibetan plateau has abundant solar energy resource and salt lake brine resources. It can bring huge economic benefits to the local development if combining these two kinds of resources together. 2, Solar pond technique is a good way coupling the solar energy utilization and the salt lake resources exploitation. The unique geographical and climatic conditions of Tibet are very suitable to establish and operate the solar pond. 3, Application of solar pond can not only increase the production efficiency of salt minerals especially Li 2 CO 3, but also improve the Li 2 CO 3 grade, consequently bringing significant economic benefits. It is a new technology worthy of promotion and development in the future. REFERENCES [1] Gong Y F, Duan T Y, Chen L X, He J H. The variation characteristics of radiation budget components of the western Tibetan Plateau in 1997/1998[J]. Acta Meteorologica Sinica, 2005, 63: [2] Akbarzadeh A, Andrews J, Golding P. Solar pond technologies: a review and future directions[j]. Advances in Solar Energy, 2005, 16: [3] Dah M M O, Ouni M, Guizani A, Belghith A. The influence of the heat extraction mode on the performance and stability of a mini solar pond[j]. Applied Energy, 2010, 87(10): [4] Karim C, Jomaa S M, Akbarzadeh A. A laboratory experimental study of mixing the solar pond gradient zone[j]. Solar Energy, 2010, 85(2): [5] Karim C, Slim Z, Kais C, Jomaa S M, Akbarzadeh A. Experimental study of the salt gradient solar pond stability[j]. Solar Energy, 2009, 84(1): [6] Singh R, Tundee S, Akbarzadeh A. Electric power generation from solar pond using combined thermosyphon and thermoelectric modules[j]. Solar Energy, 2010, 85(2): [7] Suarez F, Tyler S W, Childress A E. A theoretical study of a direct contact membrane distillation system coupled to a salt-gradient solar pond for terminal lakes reclamation[j]. Water Research, 2010, 44(15): [8] Saleh A, Qudeiri J A, Al-Nimr M A. Performance investigation of a salt gradient solar pond coupled with desalination facility near the Dead Sea[J]. Energy, 2010, 36(2): [9] Nie Z, Bu L, Zheng M, Huang W. Experimental study of natural brine solar ponds in Tibet[J]. Solar Energy, 2011, 85(7): [10] Huang W N, Sun Z N, Wang X K, Nie Z, Bu L Z. Progress in industrialization for lithium extraction from salt lake[j]. Modern Chemical Industry, 2008, 28: [11] Zheng M. "Brine solar pond effect" first appeared in the salt lake of northern Tibet.[J]. China Mining News, 2002, 5(16): 1.