On-line Trace Metal Monitoring in Zinc and Cadmium Solutions
Electrochemical Detection by Flow-Through Stripping Chronopotentiometry
Autors:
prof. Ernest Beinrohr
Institute of Analytical Chemistry, Slovak University of Technology, Bratislava, Slovakia
ISTRAN, Ltd., Bratislava, Slovakia
Ing. Pavol Beinrohr
ISTRAN, Ltd., Bratislava, Slovakia
Introduction
Zinc production by electrowinning relies on the electrolytic deposition of metallic zinc from concentrated zinc sulphate solutions. The quality of the final product and the efficiency of the process are significantly affected by the presence of impurities, including antimony (Sb), cobalt (Co), copper (Cu), cadmium (Cd), and thallium (Tl).
To maintain control over the process and product quality, it is essential to monitor these trace metals, preferably in real-time.
During the roasting of sulfidic zinc ores, cadmium vapours may form and convert into cadmium sulphate solutions. Cadmium is then recovered by zinc powder reduction in acidic media. This step is potentially hazardous, especially when arsenic (As) is present, as it may generate poisonous arsine gas. Therefore, on-line control of arsenic concentration is necessary for safety.
Methodology Overview
These monitoring challenges were addressed using flow-through stripping chronopotentiometry. The concentrated zinc sulphate samples, with zinc concentrations up to 160 g/L, were diluted tenfold on-line and analysed electrochemically.
Cadmium, cobalt, and copper were measured in sub-mg/L concentrations; thallium in the 0.1–50 mg/L range, and antimony in the 1–20 µg/L range.
To overcome the interference of cadmium in arsenic detection, cadmium was removed online using a strong cation-exchange column. After this removal, arsenic could be detected down to a few µg/L.
The same system was then used to quantify cadmium by eluting it from the column.
These procedures confirmed the suitability of electroanalytical techniques for monitoring trace metals in complex industrial matrices that are challenging for conventional methods.
Measurement Principle and System Design
The method involved constant potential deposition, followed by matrix removal, a quiescence step, and then stripping using a constant current. Measurements were performed in a flow-through electrochemical cell.
Electrodes used included RVC and Au types depending on the analyte. Reagents and parameters varied for each element to optimize sensitivity and specificity. For instance, Sb was measured with 8 M HCl and a stripping current of 50 µA, while Cd required 0.1 M HCl and 10 µA.
Experimental Results
Each element was evaluated individually, and data were recorded over multiple measurements:
Cadmium (Cd): Good repeatability (2.2%) and low detection limit (0.002 mg/L)
Cobalt (Co): More challenging, with higher variability (4.5%) and longer analysis time (18 min)
Copper (Cu): Detected down to 0.003 mg/L with reliable precision
Antimony (Sb): Detected at ultra-trace levels (0.001 mg/L), but with highest variability (6.5%)
Graphical data for each element demonstrated signal stability, response linearity, and effectiveness of matrix removal.
Application to Cadmium Solutions and Arsenic Monitoring
Cadmium sulphate solutions (up to 50 g/L) posed a major interference in arsenic detection. This was effectively resolved using cation-exchange resin, allowing separate quantification of both elements.
Conclusions
Stripping chronopotentiometry in a flow-through setup is a robust and reliable technique for trace metal monitoring in high-matrix industrial solutions. With low operational costs and automation capabilities, it is well-suited for continuous process control. The system is also adaptable for other metals such as Tl, In, Fe, and Hg.
Acknowledgements
This work was supported by the Competence Center for SMART Technologies for Electronics and Informatics Systems and Services (ITMS 26240220072), and the Slovak Grant Agency VEGA (project No. 1/0419/12).
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