Monitoring ZnS Precipitation: Estimation, Error Analysis and Experiment Design

Abstract

A mass balance model of total soluble sulfide and free zinc with a second-order reaction term is theoretically able to reconstruct the zinc effluent concentration and the kinetic parameter (k). However, under real conditions this model predicts the zinc effluent concentration four orders of magnitude higher than the measured ones. The applied error analysis, based on linearization of the model followed by first-order variance propagation, showed that the accuracy of several of the input variables (flows and influent concentrations) jeopardized the estimation of the Zn concentration in the effluent, which is a phenomenon expected for every fast reaction with low product concentration. In order to overcome the inaccuracy issue, an ''apparent solubility product'' as a function of pS (11-20) was calculated from the experimental data, allowing for the subsequent determination of an ''apparent kinetic parameter'' (k A ), that excluding parallel reactions was between 1.7 Â 10 23 -6.2 Â 10 24 L=(mol Á h). This allowed for further tuning of the model such that the estimates of the Zn effluent concentration became of the same order of magnitude as the measured ones (10 À7 M Zn).

Determination of Zinc Sulfide Solubility to High Temperatures

“…This difference between the concentration of zinc and total sulfur could be due to the presence of byproducts in the ZnS or formation of other products during the solubilization process. It was mentioned by [23] that a soluble species Zn(HS)n could be present in the aqueous solution. They also observed an excess of total sulfur even though sulfur was added in stoichiometric quantities [23].…”

“…It was mentioned by [23] that a soluble species Zn(HS)n could be present in the aqueous solution. They also observed an excess of total sulfur even though sulfur was added in stoichiometric quantities [23]. The formation of complexes has been discussed by several authors; nonetheless the existence of those complexes cannot be easily proved [4,5,8,24].…”

Modelling and on-line estimation of zinc sulphide precipitation in a continuously stirred tank reactor

“…However, from other experimental data with much lower zinc concentrations, around 10 −5 g/l, less satisfactory results have been found. Further analysis of this phenomenon indicated that the accuracy of the measurements of flows and concentrations plays a crucial role in the estimation of the zinc concentration and reaction constant [16].…”