Activity coefficients, phase diagram and separation process design for NaNO3 and Pb(NO3)2 of waste liquor in the production of lead–chromium pigments

Abstract

Lead contamination is one of the common soil heavy metal pollution elements, and the waste solution discharged from chromium–lead pigment plants is one of the potential sources of lead contamination. In order to separate and recover sodium nitrate and lead nitrate from waste liquid of the production of lead–chromium pigments, it is necessary to make systematical studies on thermodynamic properties, and thermodynamic phase diagram of the salt–water system containing NaNO3 and Pb(NO3)2. Accordingly, in this work, the cell potential method were chosen to measure the thermodynamic activity coefficients of mixed solutions NaNO3– Pb(NO3)2–H2O. The thermodynamic properties (mean activity coefficient, osmotic coefficient, water activity, excess Gibbs free energy) of NaNO3–Pb(NO3)2–H2O mixed solutions at 308.15 K were also determined by Pitzer model. Furthermore, the phase equilibria of the NaNO3–Pb(NO3)2–H2O system was investigated by the isothermal dissolution equilibrium method, and its isothermal equilibrium phase diagram was drawn in detail. The solubility data were used to obtain the Pitzer mixing ion parameters, and the solubilities of salts in the ternary system were predicted by Pitzer model. Based on the plotted phase diagrams, as a result, the process to separate NaNO3 and enrich the recycling of Pb(NO3)2 was designed based on the calculated phase diagram. Among them, the recovery of NaNO3 reached 70.82% and the enrichment of Pb(NO3)2 reached about 15 times. The designed separation process in this work has the advantages of green, low energy consumption and sustainability, which is of positive practical significance for the full utilization of resources and the prevention of environmental pollution.