Mineralogical Study On The Formation Of Mg-K Sulfate Minerals In Salt Lake Brine System Of Magnesium Sulfate Subtype

External water supply plays a pivotal role in the evolution of brine in closed interior basin,leading to a great variety of salt lake brine types.The formation of magnesium sulfate subtype salt lake brines is critical for potassium salt deposit and potassium-rich sulfate minerals deposit.Mg-K sulfate minerals,both kainite and schoenite are present in salt deposits of magnesium sulfate subtype salt lakes,however,their formation mechanisms and deposition process remain to be elucidated.Furthermore,the presence of Mg-K sulfate minerals lays an obstacle for the production of potash from magnesium sulfate subtype salt lake brines,leading to lower product quality,decreased yield and failure of production at high temperatures,with the mechanisms still unclear.In the present dissertation,systematic investigation on the mechanism of the formation of Mg-K sulfate minerals was carried out with XRD,SEM,FTIR,and isotope D tracing techniques through chemical simulation process with a theoretical basis of phase chemistry,and the following conclusions were drawn:1.In the magnesium sulfate subtype salt lake system,MgSO₄ re-crystallizes with the inconsistent dissolution of carnallite minerals,as indicated by the dissolution and crystallization of epsomite minerals.Fine,rhombohedral intact crystal morphology was characterized by SEM,and a characteristic peak of D-O bond(2500 cm^-1)was indicated by FTIR.2.Kainite is formed by the conversion of minerals from carnallite,epsomite and halite in the presence of aqueous medium and the formation condition is sensitive to temperature.The formation is favored at a temperature higher than 20°C,while it is inhibited at a temperature lower than 10°C.The formation of kainite minerals is driven by the inconsistent dissolution of carnallite and the dissolution and crystallization of epsomite.The formation process is:?a?The decomposition of carnallite results in the formation of fine-grained sylvite and a MgCl₂-rich solution,?b?MgSO₄ re-crystallizes in the MgCl₂-rich solution.?c?The fine-grained sylvite and the re-crystallized epsomite have a high activity,and these two react and convert to kainite.Fine granular crystal morphology was characterized by SEM,and a characteristic peak of D-O bond(2500 cm^-1)was indicated by FTIR.3.Morphological and mineralogical analyses of natural solar-evaporation minerals from salt pan showed that dehydration of epsomite influences the water balance of the decomposition solution,resulting in errors in calculations of water amount in the process,which in turn results in a lower potassium yield in the decomposition process.Granularity difference between carnallite and epsomite is unfavorable for phase balance in the decomposition solution,resulting in the precipitation of impurity minerals?such as epsomite and halite?by salting out effect.These impurity minerals agglomerate with potassium chloride particles and adversely affects the quality of the potassium product.Mineralogical study on primary products from the factory revealed that:?a?during the decomposition of carnallite,fine-grained re-crystallized epsomite with high activity agglomerates with fine-grained potassium chloride,leading to increased or even ineffective use of flotation agents;?b?when the operating temperature is high?above 20°C?,the re-crystallized fine-grained epsomite interacts with sylvite and forms kainite,resulting in drastic decreases in both quality and yield of the potassium chloride product.4.According to the metastable phase diagram for the quinary system,Na⁺,K⁺,Mg²⁺//Cl^–,SO₄^2--H₂O at 25°C,the formation models of schoenite were established through the chemical simulation reaction process of carnallite and thenardite in an aqueous medium.In model I,carnallite minerals were converted to sylvite minerals under the presence of water medium;in model II,thenardite minerals were converted to epsomite minerals under the presence of Mg-rich solutions;in model III,sylvite minerals and epsomite minerals were converted to schoenite minerals under the presence of water fluids.5.A previous study on sedimentary profile of XY-1 in Dalangtan playa in Qinghai province shows that salt deposition is accompanied by detrital sedimentation,especially before the appearance of schoenite deposition and the sedimentation showed a rhythmic variation.In the present dissertation,based on the formation models of schoenite converted from carnallite and thenardite,the deposition process and formation mechanism of schoenite in Dalangtan playa were discussed.It was proposed that Mg-K sulfate was not formed directly from evaporation of brine,rather,it was formed by a series of mineral conversions with the presence of aqueous medium.The presence of a large amount of detrital sedimentation indicates external water supply,which led to dissolution of previously deposited carnallite and mirabilite as well as formation of sylvite and epsomite minerals.Part of the sylvite and epsomite were converted to schoenite in the presence of aqueous medium.In the geological background of the Holocene period,sylvite and epsomite were converted to kainite.This interpretation is consistent with the distribution of salt minerals in the XY-1sedimentary profile.In the present dissertation,the formation process of Mg-K sulfate minerals and the quantitative conversion relationship between mineral phases was studied for the first time based on the chemical simulation of water-salt system.Moreover,an in-depth analysis was carried out combining with a known geological profile.The results in the present dissertation shed significant light on further understanding of the formation mechanism of Mg-K sulfate minerals in evaporites of the Qaidam Basin and provides new research approaches for correlational studies on geochemical simulation processes of similar salt deposits.