Selective extraction of potassium chloride from schoenite end liquor employing tartaric acid as safe, benign and recyclable extractant

摘要

Although U S patent 8,182,784 teaches the recovery of potassium chloride from schoenite end liquor (SEL) using dipicrylamine as extractant, and consequently simplifies the recovery of sulphate of potash (SOP) from kainite mixed salt employing the scheme disclosed in U S patent 7,041,268, the hazards associated with this extractant have thwarted practical utilization of the invention. Many other extractants for potash recovery have been disclosed in the prior art but none has been found suitable so far for practical exploitation. It is disclosed herein that the bitartrate ion, and particularly L-bitartrate, precipitates out potassium bitartrate very efficiently from SEL with ca. 90% utilization of the extractant. In contrast, recovery of potassium bi-tartrate from sea bittern directly is relatively much lower. It is further disclosed that this precipitate can be treated with magnesium hydroxide and magnesium chloride to throw out magnesium tartrate with ca. 90% recovery while yielding a nearly saturated solution of potassium chloride which can be utilized for the reaction with schoenite to obtain SOP. It is further demonstrated that the magnesium tartrate can be treated with an appropriate amount of aqueous HCl and added into a subsequent batch of SEL to throw out potassium bitartrate once again which demonstrates the recyclability of the extractant. The overall loss of tartrate over a cycle was ca. 20% but the dissolved tartrate remaining in the K-depleted SEL and KCl solutions can be precipitated out as calcium tartrate from which tartaric acid can be recovered by known methods, curtailing thereby the loss of tartaric acid per kg of KCl to <5 g. It is also demonstrated that through a similar approach, seaweed sap containing ca. 4% KCl can be concentrated to 20-22% KCl, with excellent utilization efficiency of tartaric acid, and this solution can similarly be utilized for SOP preparation. Potassium salts bearing other anions such as sulphate, nitrate, phosphate and carbonate can also be prepared from the isolated potassium bitartrate.