Application Study Of Hot Water Solar Pond In Lithium Extraction From Damxung Co Salt Lake Of Tibet

The Damxung Co salt lake is located on the southwest of the northern Tibetplateau hinterland, the water of which is a moderate carbonate type and its Mg/Li<0.1.The water is rich in elements with great economic value, like Na、K、Br、B、Li、Rb. After insolating and evaporating brine, the concentration of K、B、Li、Rb increasessignificantly and meets the criteria of industrial development. This paper takes theDamxung Co salt lake of a low ratio of Mg/Li carbonate type as research object. Theexperiments of Li extraction from lithium-rich brine are taken based on K extractionby using hot water solar pond （HWSP） technology for the first time.The HWSP technology combines the geothermal energy and the salt gradientsolar pond （SGSP） for the first time. The heat exchanger is installed at the bottom onthe basis of SGSP and the energy is transferred from the heat exchanger to the brineby using hot water circulation so that the temperature of the brine is raised rapidly andeventually obtained the high grade lithium carbonate. The HWSP technology cansolve the problems we have encountered in the process of lithium extraction fromplateau salt lakes, for instance the long production cycle, the lower brine temperatureand the dunaliella salina problems. Most of all, lithium carbonate can be extractedfrom brine without adding any chemical reagent.The walls of hot water solar pond （HWSP） are made of reinforced concrete andbricks. The angle between sidewall and horizontal plane is exactly the same as thesolar altitude of Damxung Co area at winter solstice, thus the brine in HWSP canreceive more solar radiation within a day. The bottom of HWSP has a shape ofrectangular, which is2meters long and1meter wide. The internal walls of HWSP arecovered with10centimeters thick insulation material to keep brine warm in the following experiments. The brine in the hot water solar pond can be divided into threeparts of Lower Convective Zone （LCZ, concentrated brine layer）, NonconvectiveZone （NCZ, salt gradient layer） and Upper Convective Zone （UCZ, fresh water layer）.The LCZ is composed of lithium-rich brine and the high salinity brine can absorb andstore the heat from the heat exchanger and solar radiation; the salinity of LCZincreases exponentially from top to bottom, which contributes to prevent the heatfrom penetrating, thus it has the insulation effect; the UCZ is composed of freshwaterand is used to protect the underlying brine from destroyed.This research is mainly revolve round the HWSP, of which the performance ischaracterized through the study of heat and salinity diffusion, lithium extractingsimulation and the heat exchanger. The HWSP technology is used in lithiumextracting experiments on Damxung Co salt lake.The experiment of lithium extraction from the rich lithium brine inspected theinfluence of the heat exchanger in the different LCZ position on the distribution oftemperature. When the heat exchanger is on LCZ and5cm distant from the bottom ofthe pool, the maximum temperature difference is only10℃and the highest averagetemperature could reach55℃. On the other hand, the experiment shows that theLCZ temperature distribution also affects the crystallization of lithium carbonate. Thebest crystallization temperature of lithium carbonate is above45℃in the brine.With continuously heating, the temperature distribution of the brine will change andform high temperature zone and low temperature zone.The salt gradient layer makes an important role on HWSP. The experimentsinspected and analyzed the brine dynamical change after the heat exchanger isinstalled on the brine pool. The experiment shows that without the salt gradient layer,blending can speed up the brine warming, but at the same time due to the waterevaporation, sodium chloride, sulfate and alkali mineral in great quantities areseparated in advance for supersaturate precipitation, eventually the content of lithiumcarbonate mineral drops. The stability of salt gradient layer would raise the averagetemperature of the LCZ and change the grade of lithium carbonate mineral. Theexperiment also found that the thickness of the salt gradient layer takes great effect onheat preservation. The thicker the salt gradient layer is, the better effect of heatpreservation and higher the average temperature of brine on LCZ will be.The general similarity and difference of lithium extraction on the fieldexperiment is researched and analyzed by comparing with50℃simulated the solarpond experiment. The experiments analyzed the amount of ion of brine in which lithium is extracted and found except Na⁺, the amount of all ions drop, especially Li⁺and CO₃^2-and discussed the irregular change of the amount of Na⁺. Throughcomparing mineral identification of two experiments, the amount of carbonate oncrystal mineral precipitation by the field experiment is higher than that in simulatedexperiment.The heat exchanger is the core component of HWSP. The surface type of heatexchanger is conducive to carry out the further experiments for HWSP. Therefore,according to the structural characteristics of the surface type heat exchanger,orthogonal experiment of L4（32） was used to examine and analyze the three factors ofimpacting the heat exchange efficiency. The three factors are the materials of heatexchangers, the shape of the heat exchange tube and the hot water flow rate in thepipeline. The experimental results show that the properties of thermal conductivity ofstainless steel are better than PERT material. The heating efficiency of spiral-typeheater exchanger is better than that of the U-type, but the difference is not significant.The flow rate of hot water on the heating tube also affects the efficiency of the heatexchanger. The quicker the flow rate is, the higher the heating efficiency will be. Atthe same time by the comparison and analysis of extreme value R in the orthogonalexperiment, the sequence of three factors impacting the heat exchanging efficiency is:the material factors> flow rate> the tube shape.