Dynamic Characteristics Of Brine And Its Indicative Effect On Solid-Liquid Transformation Process In The Qarhan Salt Lake

Potassium mineral is the key strategic mineral resource of our country,which has great strategic significance in relation to our food security and agricultural security.China’s potassium resource is up to 50%dependent on foreign countries,and a large amount of potassium salt needs to be imported every year.The sustainable supply of potassium resource is an important bottleneck restricting the development of potassium fertilizer industry and the construction of world-class salt lake industrial base.Modern salt lakes in our country are rich in potassium resources,such as Qarhan Salt Lake and Lop Nur Salt Lake.Qarhan Salt Lake more than 60 years of high intensity development,resulting in the rapid reduction of the total amount of salt lake brine resources,brine grade declining,liquefied mining of low grade solid potassium salt lake has become one of the most important methods to ensure the sustainable supply of high quality brine.Although predecessors have carried out a lot of relevant work,there are still the following 3 problems:（1）The theoretical basis of solid potassium salt liquefaction mining is weak;（2）the evaluation of the quality and efficiency of liquefaction mining is not unified;（3）the environmental geological problems associated with the liquefaction mining process remain to be solved.And the root cause of the above problems lies in the lack of a comprehensive and profound understanding of the mechanism and law of solid-liquid conversion.It is necessary to continuously consolidate and improve the research level of solid-liquid transformation mechanism,improve the efficiency of liquefied mining of potassium resources in salt lake,and ensure the sustainable supply of potassium resources.Therefore,in this study,the water replenishment demonstration area on the north bank of Dabson Lake,which has the largest carnallite deposition scale in Qarhan Salt Lake,is taken as the research area,and the superficial brine as the research object（15hydrological observation holes,1 recharge canal,1 mining canal,3 brines in test pits and the lake water）,to carry out brine collection and monitoring with a cycle of 1 year from August 2021 to August 2022.The sampling interval was 10 days,and the hydrologic observation holes were sampled at different depths.A total of 1224 brine samples were collected.Test and analyze the main components of brine(K⁺,Na⁺,Ca²⁺,Mg²⁺,SO₄^2-,Cl^-,CO₃^2-,HCO₃^-,Li⁺,Sr²⁺,B³⁺,Br^-)and physical and chemical parameters（temperature,p H,specific gravity,Eh）.The hydrodynamic and hydrochemical dynamic change characteristics and influencing factors of shallow brine in the process of potassium salt liquefaction mining were determined,the range and degree of water replenishment were delineated,the quality of liquefaction mining was evaluated,and the geochemical index of brine potassium solubility was established to indicate the solid-liquid transformation process,so as to deepen the understanding of solid-liquid transformation mechanism under complex solution mining conditions,improve the research level of solid-liquid transformation and the efficiency of liquefaction mining.Main conclusions are as follows:（1）In the process of liquefaction mining,water bodies with low TDS are supplied to the study area,and the chemical characteristics of brine water will change dynamically under the influence of recharge degree.When the influence of recharge water is strong,the hydrochemical type is magnesium sulfate subtype,with low TDS and specific gravity,high Eh,low K⁺and Mg²⁺,and high Na⁺.As supplemental water supply weakens,Na⁺decreases,and Mg²⁺increases,K⁺will peak.When the influence of recharge water is weak,the hydrochemical type is chloride transition type,with high TDS and specific gravity,low Eh,low K⁺and Na⁺,and high Mg²⁺.（2）The research shows that the occurrence state of ions in underground brine is mainly restricted by the dissolution and precipitation of three minerals:halite,sylvite,and carnallite.And the brine is divided into three types:1)saturated halite and unsaturated sylvite;2)saturated sylvite and unsaturated carnallite;3)carnallite saturated.The influencing factors are the retention time of brine in the brine reservoir,the permeability difference of the brine reservoir,the mineral composition and distribution characteristics of the brine reservoir,and the brine temperature,with the different of permeability being the most important factor.（3）The evolution trend lines of two types of brine and delineates the maximum range of water replenishment.The magnesium sulfate subtype water recharge weakening trend line is K⁺,SO₄^2-ratio increasing,and Mg²⁺ratio decreasing.And The chloride transition type water concentration strengthening trend line is the ratio of K⁺and SO₄^2-decreased,and an increase of Mg²⁺ratio.（4）The shows that the brine has the highest K⁺concentration with the ratio of Na⁺to Mg²⁺milligram equivalent concentration is close to 0.7721.Moreover,brine has the capacity of dissolving potassium when the ratio of Na⁺to Mg²⁺milligram equivalent concentration is greater than 0.7721,while conversely,it does not have the capacity of dissolving potassium.And the geochemical index of potassium solubility in brine is established.According to the difference of potassium solubility in brine,it is suggested to study the mining cost and efficiency of primary and secondary driven mining modes,and select the best liquefied mining mode.Based on the testing and analysis of a large number of brine samples,this paper clarified that the mechanism restricting the dynamic change characteristics and rules of brine in the process of liquefied mining is the dissolution/precipitation state of main salt minerals in brine.It deeply studied the dynamic change characteristics of brine and the solid-liquid transformation process,which enriched the monitoring methods and means of the liquefied mining process.At the same time,it also provided certain scientific guidance for the water replenishment work in the liquefied mining process,which is of great significance for reducing the mining cost,improving the mining efficiency and ensuring the sustainable supply of potassium resources.