Study On Formation Environment Of Polyhalite And Extractin G Potassium From Z1 Polyhalite By In-situ Leaching In Kunteyi Salt Lake Qaidam Basin

Potash fertilizer is one of the key fertilizers for agricultural production.The lack of soluble potassium salt resources and the shortage of potash fertilizer restrict the development of agriculture in China.Currently the potassium-rich brine is the main raw material for the production of potash fertilizer in China.The brine resource is mainly distributed in the Qaidam Basin of Qinghai Province in the west of China and the Lop Nur salt lake in Xinjiang.According to research in recent years,the current liquid potassium resources that can be mined can only support the national demand for potash fertilizer for about 60 years.Under the background of the shortage of soluble potassium resources and the difficulty of prospecting,the development and utilization of the low-grade solid potassium resources are particularly important.Polyhalite is a poorly soluble potassium-rich mineral that is widely distributed in sulphate-type potash deposits.As one of the potential potassium resources,leaching the potassium in polyhalite for relieving the shortage of potassium resources has important practical significance.The polyhalite resource is widely distributed in the Qaidam Basin including the Qarhan Salt Lake,the Kunteyi Salt Lake,the Dalangtan Salt Lake,the Mahai Salt Lake,the Gasikule Salt Lake and the Chahansilatu Salt Lake.In this paper,we discuss the formation environments of polyhalite in the Qarhan Salt Lake and the Kunteyi Salt Lake in the Qaidam Basin.Studies have shown that polyhalite from two salt lakes have a similar formation environment:?1?potassium in the polyhalite of these two regions is all provided by granites outside the basin;?2?the formation and long-term evolution of the basins is good for potassium migration and enrichment;?3?both lakes have experienced similarly extremely dry climate conditions;?4?during the formation of polyhalite,both lakes are supplemented with calcium-rich water.Through the analysis and comparison of the formation environments of polyhalite in these two regions,the author believes that the polyhalite in the Qaidam basin may have similar origin.Therefore,the study on the leach test of polyhalite in the Kunteyi salt lake has important guiding significance on the exploitation of polyhalite in the whole Qaidam basin.We select the Kunteyi salt lake as the research area for the leach test,which has been accumulated the hydrogeological data and finished the general exploration of polyhalite.Relying on the project from the company,named“General exploration of solid polyhalite resource in the Kunteyi Salt Lake?ZI,ZII seams?”,combined with the existing data of drill core logging and mineral identification in the Kunteyi mining area,and based on the occurrence characteristics,distribution characteristics,and paragenesis regularity of the polyhalite occurred in the Upper Pleistocene and above stratum,finally the ZI polyhalite layer within 300m of the borehole ZK3108 located in the southern part of the mining area was selected as the research object,and the indoor and outdoor leaching experiments were respectively developed.The indoor experiment mainly includes two parts:the selection of the leaching agent and the preliminary study of the dissolution rule.In the first part of the study,9 inorganic salt leaching agents were set up,namely chloride-type leaching agent?5%NaCl?5%MgCl₂?and sulfate-type leaching agent?5%MgSO₄?5%Na₂SO₄?5%K₂SO₄?,carbonate-type leaching agent?5%NaHCO₃?5%Na₂CO₃?5%?NH₄?₂CO₃?,saturated calcium hydroxide leaching agent Ca?OH?₂,and using the ultrapure water as the experimental standard.The experimental results show that saturated calcium hydroxide leaching agent has the worst leaching effect on polyhalite,and the content of each ion in the final leaching solution is lower than that of water leached the mineral.Since the ultimate purpose of this experiment is to increase the potassium content in the brine,a comparative analysis of the concentration of K⁺ions under different leaching agents was performed.The results showed that the chloride-type leaching agent had the best effect.Comparing the leaching results of 5%NaCl with5%MgCl₂,we found that the 5%MgCl₂ leaching agent dissolved a large amount of K⁺ions and at the same time greatly increased the content of Mg²⁺ions in the brine.The high content of Mg²⁺has a negative effect on the late potash production process.Therefore,the NaCl solution was finally selected as the most suitable leaching agent.The study of the dissolution rule is based on the selected optimum leaching agent?5%NaCl?,and the leaching time and the pulp density are compared.The leaching time of each group of pulp density was studied.Samples were taken in different leaching periods to determine the concentration of K⁺in the solid and liquid phases.The experimental results show that the polyhalite in Kunteyi is a“slowly-soluble”type mineral.Under experimental conditions,the K⁺leaching rate reaches 87.3%when the slurry ratio is 1:4 and the leaching time is 90 h and the K⁺concentration in the solution is 12.20 g/L.Through the chemical analysis results of the solid samples before and after the leaching,it can be found that the K⁺content is significantly reduced after leaching,and K⁺is leached into the leaching solution.The results of XRD diffraction analysis of the solid polyhalite before and after leaching show that the content of polyhalite in the sample before leaching is about 30%and when the leaching is completed,no polyhalite is shown in the sample.The K⁺in the polyhalite can be leached,which provides a reliable theoretical basis for large-scale utilization of polyhalite resources in the salt lake.The main mineral occurred in the strata in Kunteyi Salt Lake is halite.In order to avoiding the large-scale collapse of the strata caused by dissolution and taking into account the factors such as the flow speed and the influence range,in the field in-situ leaching test,we finally select saturated NaCl solution as the leaching agent.The field in-situ leaching test is finished by the simulation of brine mining.10 observation drillings and 3 brine mining channels are set.The maximum distance between the drilling and the channels is 40 meters and the minimum distance is 3 meters.The average permeability coefficient of the stratum in this area is 0.7423 m/d,so the theoretical minimum time and maximum time needed when the leaching solution laterally flows to the drillings and the channels are 4 days and 54 days,respectively.Before and after the test,the concentration of K⁺ions in the leaching solvent increased from 0.02%to 0.96%.The X-ray diffraction results from solid samples at the same position showed that there were no polyhalite peak after leaching,indicating that the polyhalite was effectively dissolved.The success of the in-situ leaching test of polyhalite in the Kunteyi salt lake will have instructive significance for the exploration of polyhalite in the Qarhan Salt Lake,even in the entire Qaidam Basin.