MWH completed a feasibility study, prepared a conceptual design and detailed design to optimize the storage capacity of a neutralization pond for future disposal of tailings and wastes at a zinc processing plant in northeast Belgium.
The plant produces about 75,000 tons of dry Goethite tailings and 75,000 tons of dry neutralization sludges that have been stored in ponds along with other tailings since the early 1970s. A neutralization pond on the site used a wet-disposal technique that had become outdated and was nearing its capacity. The client wanted to optimize the total storage capacity of the neutralization pond for future disposal of tailings and wastes, including the Goethite, wastewater treatment plant residues and flotation tailings.The project consisted of the following:
- Pilot tests of dewatering of the sludges and evaluation of results
- Geotechnical investigation of in situ sludges and and dewatered material
- Stability and stelling study
- Design of scenarios of mechanical treatment of the different tailings
- Operational disposal plan
- Description of the improved environmental risks
- Evaluation of the current approach – hydraulic filling
- Cost-benefit analysis
- Conceptual design of a treatment plan and completed detailed engineering
The study developed a plan for simultaneous consolidation and final cover of part of the neutralization pond while continuing disposal of tailings in other parts of the pond. The plan included solutions that would ensure the stability of external dams of the neutralization pond and adjacent ponds during and after the works; ensure lifetime stability of the final rehabilitated neutralization pond; prevent leachate from seeping into the ground; create an environmentally sustainable site with vegetation; and prevent additional negative environmental impact.
The team prepared two scenario’s of mechanical treatment, one that would treat the Goethite and neutralized residues separately; the other would treat them together. The proposed operational disposal plan used the internal dikes to divide the pond into four cells so wet residues could be pumped to the treatment facility where dry residues can be stored in and empty cell. By reinforcing the internal dikes (making them higher and wider) a clear separation between wet and dry residues was created, allowing installation of the final cover in four phases to facilitate leachate control.
MWH then worked closely with the client to prepare conceptual design, completed detailed engineering of the plant, tender documents, bid evaluation and cost estimates. The treatment plant is composed of a dredging device to pump old residues from the neutralization pond to the treatment plant, residue conditioning, dewatering by filter presses, stockpiling and compaction of dewatered residues in the neutralization pond, filtrate management for reuse and treatment, and a management plan for stockpiling treated residues. Benefits:
- Environmental risks are significantly reduced by mechanical treatment.
- The lifetime of the existing pond increases from ± 6 years to ± 22 years.M
- Mechanical treatment is an economically feasible approach.
- Northeast Belguim
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