Phase Equilibria Foraqueous Quinary Systems Focused On Pingluo Nderground Brine Enriched With Potassium, Rubidium, And Borate

Pingluo underground brine, located in the Sichuan Basin, is famous for its high ioncontent like potassium, rubidium, boron, and lithium. Although the resources ofPingluo underground brine are very valuable, nothing has been reported on theircomprehensive utilization because of a lack of data on the relative solubilities andphase diagrams.In accordance with the composition of the brine of Pingluo, the metastable phaseequilibria of the quaternary system Li+, K+, Rb+//Cl--H2O and its ternary subsystemKCl+RbCl+H2O at298K and323K were studied with the method of isothermalevaporation; the stable phase equilibria of the three quinary systems Li+, K+, Rb+//Cl-,borate-H2O, Li+, K+, Mg2+//Cl-, borate-H2O,and Li+, K+, Rb+, Mg2+//borate-H2O,six quaternary subsystems, and six ternary subsystems were studied with the method ofisothermal dissolution. The solubilitites and physicochemical properties includingrefractive indices and densities of the solution at equilibrium in the above systems werealso determined. Based on the experimental data, the phase diagrams and the diagramsof physicochemical properties (densities and refractive indices) vs composition in theternary systems were all plotted; the space diagrams, the planar projection diagrams,the diagrams of water content vs composition, and the physicochemical properties(densities and refractive indices) vs composition diagrams in the quaternary andquinary systems were all constructed.In the metastable phase equilibria of the quaternary system Li+, K+, Rb+//Cl--H2Oand ternary system KCl+RbCl+H2O at298K and323K, the solid solution [(K,Rb)Cl] was formed in these two systems, and the crystallization region of the solidsolution almost occupies all the phase region. Comparisons between the metastable andstable phase diagrams of ternary system at298K show that besides solid solution [(K,Rb)Cl], the single salts KCl and RbCl are simultaneously formed in the metastablephase diagram. Comparisons between the metastable phase diagrams of the ternarysystems, quaternary systems at298K~348K show that the crystallization zones ofthe single salts enlarged along with the increase in the temperature, whereas thecrystallization zone of solid solution [(K, Rb)Cl] decreased. Results show that theincrement of temperature is conducive to separate potassium chloride from thechloride solution.The stable phase diagrams of the six ternary systems at323K belong to an aquo-complex I type, without solid solution or double salt formed. Crystallographic formsof the solid phase were identified by Schreinemakers residue method and X-ray diffraction method. Comparisons between the stable phase diagrams at323K and348K show that the crystallographic form of the salts have not been changed, while thecrystallization zones have slight changes. Results show that Salt RbCl has salting outeffect on Rb2B4O7, MgCl2has salting in effect on MgB4O7, and K2B4O7has salting ineffect on the coexist alkalis (alkaline earth) borate in the solution.The reciprocal quaternary system Li+, Rb+//Cl-, borate-H2O is of a simpleeutectic type, without solid solution or double salt formed at323K. Salt LiCl hassalting out effect on Li2B4O7, which makes the solubility of Li2B4O7decreaseobviously, and the crystallization field enlarged to nearly65%of the generalcrystallization field. The reciprocal salt pair RbCl and Li2B4O7·3H2O is stable, thusthe Li2B4O7·3H2O can be obtained by cross-reaction between RbB5O6(OH)4·2H2Oand LiCl·H2O in the inorganic chemical production.The reciprocal quaternary system K+, Rb+//Cl-, borate-H2O is of a complex typewith the solid solution [(K, Rb)Cl] formed at323K. There is no solid solution ordouble salt formed between the borates of potassium and rubidium. The scope ofareas of crystallization of salts is such that RbCl <KCl <[(K, Rb)Cl]<K2B4O5(OH)4·2H2O <RbB5O6(OH)4·2H2O.The reciprocal quaternary system Li+, Mg2+//Cl-, borate-H2O is of a complextype with the double salt lithium carnallite (LiCl·MgCl2·7H2O) formed at323K. Thecrystallographic form of the lithium carnallite in the solid phase was identified bycontrast with the lattice parameter, cell parameter, and crystalline interplanar spacing.The crystallization fields of the salts decrease in the order of MgB4O5(OH)4·7H2O,Li2B4O7·3H2O, MgCl2·6H2O, LiCl·MgCl2·7H2O, LiCl·H2O. With a view to thecrystallization zones, the salt MgB4O5(OH)4·7H2O has the largest crystallizationregion and it contains almost90%of the general crystallization field.The reciprocal quaternary system K+, Mg2+//Cl-, borate-H2O is of a complex typewith the double salt potassium carnallite (KCl·MgCl2·6H2O) formed at323K. SaltMgCl2·6H2O has the smallest crystallization region, while salt MgB4O5(OH)4·7H2Ohas the largest crystallization region and it contains nearly75%of the generalcrystallization field. Comparisons between the stable phase diagrams at288K and323K show that the crystallization form of the salts have not been changed, whilecrystallization zones of salts K2B4O5(OH)4·2H2O and KCl increase obviously, saltMgB4O5(OH)4·7H2O decrease obviously and salt KCl·MgCl2·6H2O and MgCl2·6H2Ohave slight changes at323K.The quinary system Li+, K+, Rb+, Mg2+//borate-H2O and its two quaternarysubsystems Li+, K+(Mg2+), Rb+//borate-H2O are all of simple eutectic type at323K. The crystallographic form of the alkalis (alkaline earth) borates found in thequaternary and quinary system are RbB5O6(OH)4·2H2O, K2B4O5(OH)4·2H2O,Li2B4O7·3H2O, and MgB4O5(OH)4·7H2O. Results show that in the planar projectiondiagram (saturated with Li2B4O7) of the quinary system, the crystallization zone of salt K2B4O5(OH)4·2H2O is the smallest, while salt MgB4O5(OH)4·7H2O has thelargest field and it contains nearly90%of the general crystallization field.The reciprocal quinary system Li+, K+, Rb+//Cl-, borate-H2O is of a complex typewith solid solution [(K, Rb)Cl] formed at323K. The space diagram consists of fiveinvariant points, fifteen univariant curves, and seven crystallization fieldscorresponding to single salts Li2B4O7·3H2O, LiCl·H2O, RbCl, K2B4O5(OH)4·2H2O,KCl, RbB5O6(OH)4·2H2O and solid solution [(K, Rb)Cl]. The crystallization zone ofsalt LiCl·H2O is the smallest, while salt RbB5O6(OH)4·2H2O has the largest field.The reciprocal quinary system Li+, K+, Mg2+//Cl-, borate-H2O is of a complextype with two carnallite type double salts lithium carnallite (LiCl·MgCl2·7H2O) andpotassium carnallite (KCl·MgCl2·6H2O) formed at323K. The space diagram consistsof five invariant points, sixteen univariant curves, and eight crystallization fields.Under the condition of MgB4O7saturated, the scope of areas of crystallization of saltsis such that Li2B4O7·3H2O> K2B4O5(OH)4·2H2O> KCl> KCl·MgCl2·6H2O>MgCl2·6H2O> LiCl·MgCl2·7H2O> LiCl·H2O.Results show that when the cations of inorganic salt were same, the solubility ofborate is lower than that of chloride salt. The borate type salts MgB4O5(OH)4·7H2Oand RbB5O6(OH)4·2H2O have larger crystallization fields, chloride type saltsLiCl·H2O, MgCl2·6H2O, and RbCl have smaller crystallization regions. Thecrystallization zones of the carnallite type double salts are smaller and the solubility ofKCl·MgCl2·6H2O is lower then that of LiCl·MgCl2·7H2O. Crystallographic form ofborates are tetraborates and pentaborate, no double salt or solid solution formed in theborate system. The crystallographic form of inorganic salts and the crystallizationrules of salts obtained form the phase equilibria research can provide the theoreticaldirection to establish the best chemical production process and technology for thecomprehensive utilization of Pingluo underground brine, especially the preparation ofpotassium, rubidium, and borate from the brine.