The Geochemical Simulation Of The Formation And Evolution Of Salt Lakes In Qaidam Basin Based On Pitzer Model

Qaidam Basin is one of large-scale interior basins in China.There are many salt lakes in the basin and the salt mineral resources and elements that can be utilized are very abundant. The major hydrochemical types of the salt lakes are MgSO₄ sub-type and chloride type. In 1973, Pitzer put forward an interaction model that can be used to calculate the activity coefficients of major ions in electrolyte solution. The model can be used to study the formation and evolution of different salt lakes, the composition of brine and the technological processes of a salt pan.It is the first time to use Pitzer model to study the salt lakes in China.Using Pitzer model as a basis, simulant calculation as a major means, simulation results compared with actual geological materials and synthetic analysis as a method, we studied the formation and evolution of salt lakes, the laws of sulphate minerals related to K separating from brine of MgSO₄ sub-type salt lakes and the feasibility that produces K₂SO₄ though sulphate minerals related to K separating from brine of MgSO₄ sub-type salt lakes in Qaidam Basin.About the formation of salt lakes in Qaidam Basin, We respectively simulated and studied the relation between the formation of salt lakes and the origin of waters in Chaka Salt Lake distributed in the Small basin on the south of Qilan mountain and all salt lakes distributed in Big basin. The results indicated that it is impossible to form the salt lakes that distributed in the Small and Big Basin only by the evaporation of surface waters and their formation is closely related to the mixture of CaCl₂ water originating from the deep. In Chaka Salt Lake,the comparison between the evaporation sequence of the salts simulated and that of the actual salts indicates that the average formation temperature of the salt lake is not high and the temperature is about 0-5 ℃ .We systematically simulated the evaporation sequence and path of the salts of all salt lakes that have different hydrochemical compositions in Qaidam Basin.The stimulation results are very identically to the results of the phase figure that is used to study the evolution of the salt lake.Basing on the evolution simulation of all salt lakes,we are aimed at the MgSO₄ sub-type salt lakes that will be developed in the near future to study the laws of salts and sulphate minerals related to K that separate from brines.There are four paths of salts separating from MgSO₄ sub-type salt lakes in Qaidam Basin,but the mineral related to K still is sylvite or carnallite in many salt lakes and the sulphate minerals related to K can be separated by evaporation only in East Taijinaier and West Taijinaier Salt Lake. The ratio of K/Mg is an important index to judge whether the sulphate minerals related to K can be separated from MgSO₄ sub-type salt lakes or not. Compared with the Great Salt Lake in America, there is a very low ratio of K/Mg in many MgSO₄ sub-type salt lakes of Qaidam Basin. In order to obtain sulphate minerals related to K, carnallite can be added to increase the ratio of K/Mg of MgSO₄ sub-type salt lakes which have a very low ratio of K/Mg. Our discussion to the method indicated that, in theory, it is not feasible to Chaka, KeKe, Chaikai and Gasikule Salt Lake and is feasible to Big and Small Chaidan, Big Bieletan and Dezunmahai Salt Lake, especially to Dezunmahai Salt Lake.In China, our research is a beginning to use Pitzer model to make geochemical calculations in salt lakes with high concentrations. Aimed at salt lakes in Qaidam, the qualitative and quantitative simulationwe made can further enrich and develop the theory of mixture about the formation and evolution of salt lakes. Some results we obtained can be theoretically used to choose and establish technological processes of a salt pan for exploiting K resource of MgSO4 sub-type salt lakes in Qaidam Basin.