The evolution of the calomel process

Even well-run metallurgical sulfuric acid plants suffer from mercury contamination, and there is more to it than simply equipment corrosion. Mercury abatement is a matter of environmental regulatory compliance, and product integrity. Even trace amounts of mercury that end up in the product acid are hazardous, and it is the plant’s responsibility to prevent this.

Over the past few decades, with increasingly stringent regulations and public scrutiny, the industry has been re-examining how to approach mercury removal. What used to be ‘good enough’ is not anymore. This article will take a close look at how methods to control mercury have improved through trial, breakthrough, and a keen understanding of engineering realities. This article will analyse those challenges, examine successes and failures from the site, and showcase the kinds of technologies that are pushing the industry forward.

Understanding the problem of mercury contamination in metallurgical acid plants

Mercury is poisonous to the environment and removing it from metallurgical gas streams presents special challenges. In smelter-based sulfuric acid operations, mercury impurities can be present within the process in both vapour and particulate forms. Unless intercepted properly, this gets absorbed into the product acid, which can have detrimental effects on people’s health, and make it difficult to comply with environmental regulatory requirements.

The traditional calomel process – relying on chevrons for mist elimination – used to be sufficient for sulfuric acid plants to meet regulatory requirements. However, increasing throughput specifications, more stringent emission limits, and greater process complexities have made meeting the target more challenging. For these reasons, new, innovative hybrid scrubber technologies and high-efficiency mesh-pad-style mist eliminators are becoming more common.

Technical spotlight: the chemistry behind the calomel process

The calomel process is a well-established process for removing mercury from the feed gas in sulfuric acid plants. The chemical pathway involves converting elemental mercury (Hg₂) into a solid, filterable form. The process is based on the oxidation of mercury vapour by mercuric chloride (HgCl₂) to form mercurous chloride (HgCl₂), also known as calomel.

1. Hg⁰ (gas) + HgCl₂ (solution) → Hg₂Cl₂ (solid)
2. Hg₂Cl₂ + Cl₂ → 2 HgCl₂ (regeneration step)

This reaction scheme allows for continuous cycling of the scrubbing solution. The solid calomel is separated from the circulating liquid in a settler or decanter, while chlorine gas is used to regenerate the oxidant. The efficiency of this process depends on several parameters, including the gas temperature, chlorine concentration, and liquid contact time.

One of the challenges in practical applications is managing the multi-phase nature of the system, as mercury in the gas stream may exist in the gas-phase as well as in aerosol and particulate-bound forms. Because of this, effective removal relies on both chemical reaction kinetics and physical separation techniques. This is where modern mist elimination and structured media scrubbers provide critical support since they capture droplets and aerosols that would otherwise escape the scrubbing tower.

Temperature control is also crucial. As temperature increases, the vapour pressure of mercury increases, reducing absorption efficiency. Operating within the optimal range (typically 30 - 40°C) helps ensure that mercury remains in solution long enough for the conversion and precipitation reactions to occur.

Mercury removal: design best practices

A well-designed mercury control system begins with a thorough understanding of gas composition, flow characteristics, and phase interactions within the scrubber. Engineers must consider not only the chemical reaction kinetics of mercury-containing species, but also the physical behaviour of fine aerosols and liquid entrainment. This is why mist elimination should not be treated as just a final polishing step. It is a vital design element that can determine the overall success or failure of the system.

Key design considerations, such as mist eliminator placement, mesh pad composition, and irrigation strategy, must be carefully coordinated. It is important to never make the mist eliminator a better scrubber than the scrubber. Relying on the mist pad to compensate for weaknesses earlier in the process will ultimately result in reduced system performance, such as fouling and increased pressure drop.

Kimre’s multi-stage B-GON^® Mist Eliminators, with dedicated stages for scrubbing and entrainment removal, deliver consistent and reliable results. Proper pad sizing and placement to avoid bypass, spray nozzles specification and arrangement, and materials of construction (MOC) selection are all essential for long-term performance. Properly designed support structures and systems that allow for easy access and cleaning of the B-GON Media also help maintain system efficiency over time. To summarise, successful mercury control comes down to thoughtful engineering practices to design solutions that adapt to that sight’s particular process.

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