Optimizing Water Treatment at a Zinc Smelter

In operation since the early 1900s, this zinc smelter and refinery currently produces over 200,000 tonnes of zinc and over 100,000 tonnes of lead per year. The existing wastewater treatment plant at the site was installed several decades ago to treat a blend of streams generated by smelter off-gas cleaning and conversion of the SOx contained in the off gas to ammonium sulphate and sulphuric acid. With a hydraulic throughput of approximately 30 m3/h, the water treatment plant utilizes sulphidization and neutralization to treat the waste streams. By-products of the plant are two types of solid waste requiring additional handling and disposal that adds to the operating costs.

One of the strategic operational initiatives of the smelter operation is the drive to minimize water consumption by promoting and increasing water re-use. At issue is the inability of the existing water treatment plant to meet the water quality requirements for water re-use. Specifically the concentrations of zinc, arsenic, cadmium, selenium and lead in the treated effluent are all above their respective water quality targets. Furthermore, while no limit exists for sulphate, there is interest in reducing its concentration to broaden the range of potential water re-use applications. This led the site owner to contract BQE Water to conduct a technical review and assessment of options to help the existing plant meet the water quality targets with minimal capital expenditure.

After an initial technical visit, we analyzed the site specific conditions, issues and objectives, and generated a series of recommendations to optimize and improve the water treatment plant design and operation. These recommendations were then pilot tested on-site to verify performance, establish operating conditions, and confirm what new equipment is needed and their specifications.

Results from piloting confirmed that the implementation of our recommendations will result in the treated effluent to meet water quality targets, reduce the mass of solid waste residue generated and reduce reagent costs. The improvements we recommended to optimize the existing water treatment plant are outlined below.

Reduce number of precipitation stages
The water treatment plant currently uses two stages of precipitation to remove the various metals from the feed, sulphidization and neutralization. During site testing, we confirmed the optimum pH and ORP set points for the water treatment plant and demonstrated that discharge limits for the multiple metals could be achieved using a single-stage sulphidization circuit. This also has the benefit of simplifying the neutralization stage to only being required to adjust the pH of the final effluent. And with the new process configuration, solid waste residue production is reduced and any residue generated is recycled back to the smelter to completely eliminate off-site disposal requirements and costs.

Lower SO2 concentration in the plant feed
High levels of SO2 in the feed interfere with key process controls, causing higher reagent consumption and increasing sulphate and dissolved solids concentrations in the treated water. When we tested SO2 removal upstream of the water treatment plant, we were able to reduce reagent use and the level of sulphate and dissolved solids. This also had the positive effect of reducing the volume of sludge produced in the sulphidization stage by up to 73%.

Controlled reagent addition
At present, reagent dosing is a task operators perform manually by setting the speed and stroke of a metering pump manually in the field. We know from experience that controlled reagent addition can maximize metals precipitation to reach water quality targets while reducing reagent consumption. These efficiencies can be achieved with an automated control system that has been programmed with key process metrics and set points to manage reagent addition and that automatically adjusts dosing based on fluctuations in the feed composition and flow.

Recirculate solids
Solids recirculation allows for the solids to act as “seeds” to help with particle growth. Larger solid particles offer improved settling and filtration characteristics which increases the removal efficiency of target constituents. Implementation of solids recirculation systems in both the sulphidization and neutralization stages will improve the effluent water quality and minimize unwanted scaling of equipment in the neutralization stage.

Upon completion of the pilot, the project was advanced to the next stage and we have recently completed the detailed process engineering design of these recommendations to the existing water treatment plant. Implementation of these changes are now in progress.

Written by
Alain Consigny, PEng & Farzad Mohamm, MSc, PhD