More than 95% of the world's zinc comes from zinc blende (ZnS) [1]. Before obtaining metallic zinc, which can be done using hydrometallurgical or pyrometallurgical techniques, the sulfur in the concentrate must be removed.

This is accomplished by roasting or sintering at high temperatures (>900 °C), causing zinc sulfide (ZnS) to change into the more reactive zinc oxide (ZnO) (Equation 1). The acquired sulfur dioxide is converted into sulfuric acid within a nearby plant connected to the smelter.

If there is any iron sulfide in the ore, it will be transformed into iron(III) oxide (Fe₂O₃), which then reacts with zinc oxide (ZnO) to produce zinc ferrite (ZnO·Fe₂O₃). This zinc compound is challenging to recover, making ores with low iron content more desirable.

In the leaching step (hot acid leach) shown in Figure 1, zinc oxide is isolated from the other calcines formed in the roasting process. The isolation is done by using sulfuric acid (spent electrolyte) to create zinc sulfate (ZnSO₄) and water (Equation 2).

Zinc dissolves and iron precipitates, while other metals like lead and silver remain undissolved. However, the resulting solution also contains impurities, like trace metals, which must be removed to produce high-purity zinc.

First- and second-stage purification prior to electrolysis is carried out by zinc dust precipitation or cementation. The resulting purified neutral zinc sulfate solution is then subjected to electrolysis in the cell room to create zinc metal.

In the complex process of zinc refining, efficient and accurate analysis of key parameters such as zinc, sulfuric acid, and iron is crucial to maintaining optimal process conditions and ensuring high-quality zinc production. Traditional laboratory analysis methods have long been used to determine these parameters, but they are often time-consuming, labor-intensive, and prone to human error. This is where online analysis systems come into play, revolutionizing the way these critical measurements are made in the zinc refining industry.

In several stages of the zinc refining process, online process analyzers are used to determine the acid, zinc, and ferric concentrations for completion rate monitoring. Metrohm Process Analytics offers a multi-parameter process analyzer solution for the simultaneous analysis of such analytes over a wide concentration range: the 2060 TI Process Analyzer (Figure 2). Other combinations of measurements (e.g., pH), as well as measurement points taken from multiple streams, can be realized with this process analyzer.

The acid concentration is measured by a straightforward acid/base titration while the zinc concentration is analyzed by a complexometric titration. The ferric iron is analyzed by redox titration. Another common impurity, cobalt, can be measured with photometry, and other trace metals can be measured by voltammetry (Table 1).

Besides the chemical analysis, sample preconditioning is a crucial factor for the success of online analysis. Figure 2 shows a 2060 TI Process Analyzer with a customized preconditioning panel able to handle this type of slurry sample. Due to the acidic environment and high sample temperature, all parts are made (or coated) with perfluoroalkoxy (PFA) or polytetrafluoroethylene (PTFE).

It is necessary to also measure impurities like nickel, cobalt, copper, cadmium, antimony, and germanium for product quality control, process efficiency, environmental compliance, health, and safety reasons. Additionally, measurement of these impurities is crucial for troubleshooting process issues and potentially recovering valuable resources.

These impurities can be monitored with a dedicated voltammetric process analyzer in the purification filtrates and reactor trains (Figure 1). This process analyzer can also be applied to monitor trace metals in the effluent of the zinc plant for environmental purposes.

1. Production - Zinc.Org India. http://zinc.org.in/why_zinc/production/