Dynamic Modelling and Process Control of ZnS Precipitation

Abstract

This paper presents the dynamic modelling and design of a control strategy for the ZnS precipitation process. During lab-scale experiments, the sulfide concentration in a precipitator was controlled at a prespecified pS value by manipulating the flow from a buffer vessel. Batch tests showed that the optimal condition for zinc sulfide precipitation is at a sulfide concentration of 10 215 mole/l (pS 15). Experiments with the precipitator showed that the sulfide concentration highly deviates from a given setpoint when proportional (P) control is used, but this deviation can be decreased using a Proportional Integral (PI) controller. Moreover, the PI controller was able to handle sudden disturbances in the process conditions (pH, influent flow rate, or zinc and sulfide concentration). Additional precipitation experiments were conducted using effluent from a sulfate reducing gas-lift reactor to determine if the compounds present in the effluent influence the control process. With the gas-lift reactor effluent and a PI

Selective precipitation of Cu from Zn in a pS controlled continuously stirred tank reactor

“…König et al [19] obtained concentrations below 0.1 ppm when titrating zinc with Na 2 S at pH 6. Therefore, zinc titrations indicate that zinc is not efficiently removed at low pH (3 and 4) ( Table 2).…”

“…For the first two, commercially available devices were used, while for pS a proportional integral (PI) feedback control strategy was developed. In this feedback controller, the process variable pS is measured in the reactor and the manipulated variable Q sulf (sulfide flow) is adjusted using the deviation (ε(t)) between the measured pS and a predefined pS set-point [19]. In general, the feedback controller parameters should be related to the process parameters.…”

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

“…Consequently, there should be some excess of sulphide ions, which requires an appropriate control strategy for the H 2 S or Na 2 S supply to the metal rich wastewater. König et al [8] found that a relatively simple PI control strategy using a pS electrode suffices. For other metals, i.e.…”

“…The sulphide process realizes an almost zero-emission of heavy metals at low costs. Furthermore, selective precipitation of individual heavy metals can be realized by controlling the pH and pS [3,4,8]. This results in pure precipitates of metal sulphides that can be reused as a raw starting material in the metal industry.…”

Pollution control and metal resource recovery for acid mine drainage

“…Supersaturation, achieved by adding more sulphide, could speed up the precipitation process (Veeken et al, 2003). Furthermore, the feeding of the Na 2 S might affect the precipitation process, as König et al (2006) tested, a precipitator with a titration mode may be more appropriate for lowering the value of MOR, although this is not suitable for large-scale application.…”