Owing to the high Na/V molar ratio of Na 3 VO 4 solution, traditional precipitation of vanadium will consume more H 2 SO 4 and discharge ammonia–nitrogen wastewater with more Na 2 SO 4 . In present work, a new process was proposed, in which CO 2 was employed to carbonate Na 3 VO 4 solution with simultaneous recovery of NaHCO 3 , followed by NH 4 VO 3 precipitation and solution recycling. Carbonation kinetics of Na 3 VO 4 solution under both atmospheric and high pressures were studied. Under atmospheric pressure, the solution pH first decreased rapidly, slowly, rapidly again with carbonation time, and finally stabilized at pH 7.3; the formation of CO 3 2− and its conversion to HCO 3 − mainly occurred and the CO 2 absorption rates of different vanadium concentrations were almost the same (about 0.026 mol·m −3 ·s −1 ) at pH of 9.0‒9.6. Under high pressure, the reaction rates at early stage increased with CO 2 pressure and vanadium concentration, and stirring speed had the most significant effect on reaction rate with the highest CO 2 absorption rate of 1.73 mol·m −3 ·s −1 at 550 rpm. The average CO 2 utilization efficiency was ~ 90% with NaHCO 3 precipitation of 42.3% at 70 g·L −1 V under 1 MPa of CO 2 pressure, while the average CO 2 utilization efficiency was ~ 35% under atmospheric pressure. Finally, the reaction rate equation of carbonation under high-pressure was deduced. Graphical Abstract
It is of importance to reclaim single-salt of potash from Li + -type brines of low concentration. In this work the potassium salts KHCO 3 was efficiently separated from the lithium carbonate-type brines with the aid of CO 2 carbonation. The phase field was provided in which the precipitation of KHCO 3 occurs and single-salt of potassium can be obtained. The low concentration of [Li + ] was also successfully concentrated to a high level (8.1 g/L), and this concentration can be used as sources for Li + -salts production using pre-developed strategies, which looks forward wider applications of the lithium resources of natural brine. This state-of-art was traced via isotherm evaporation on the quaternary system of Li + –K + –CO 3 2− –H 2 O at 298.15 K in which the carbonation steps were performed by CO 2 . Metastable phase diagram was found to consist of three invariant points, seven univariant curves, and four crystallization fields corresponding to Li 2 CO 3 , KHCO 3 , potassium carbonate sesquihydrate (K 2 CO 3 ⋅ 3/2H 2 O), and a potassium carbonate and potassium bicarbonate double salt (K 2 CO 3 ⋅ 2 KHCO 3 ⋅ 1.5H 2 O). There was no crystallization field corresponding to LiHCO 3 . The pH-composition diagram and density-composition diagram were also plotted. This work was carried out in aim of extracting/separating single salts of alkali metals from carbonate-type brines of west China.
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