|Name of Project||Arsenic removal plant for west African mine|
|Start Date||February 2020|
|Completion Date||Estimated Completion February 2021|
|Project Engineer||Nicholas Burman, Process Engineer|
Description of project
WEC Projects was responsible for the full process, mechanical, control and instrumentation design, procurement, manufacture of some key components, full plant assembly, packaging of the plant for delivery to the mine, as well as supervising the installation and commissioning on site. Multotec was the principle contractor on this project, and provided great support for the various installations. The project involves the removal of arsenic, a very toxic substance that is situated within the dam. This project has now been completed as of February 2021.
The goal of the project was to create sufficient drinking water for the people working at the mine. Previously, clean drinking water was only accessible via the delivery of water tankers, which meant many logistical complexities.
Various challenges faced in this project
Removal of concentrated arsenic from the mine water dams is a challenge. Additionally, the client requested the plant to be modular and movable to a different site during future expansion of the mine.
There have been various challenges that WEC faced in this project. The short time frame to manufacture, procure and complete 16 steel manufactured tanks posed a serious challenge during the COVID-19 pandemic, such as delays for delivery of procured items from suppliers due to the impact on supply lines during this time.
Various solutions were considered; however, the following solutions were provided by our team of experts:
- We designed a process with a multistage removal system to meet the discharge specifications.
- Additionally, we engineered the system to be easily dismantled for future relocation of the plant to a different portion of the mine.
- Nine WEC proprietary lamella clarifiers were also used.
The lamella clarifiers, custom designed by WEC, were assembled at the company’s workshops in Fourways, South Africa.
The process to complete this project which was jointly developed by the client and WEC Projects, consists of:
- Primary clarification
This is then followed by a two-stage chemical precipitation and clarification step to remove arsenic. Arsenic is highly toxic, with long-term exposure to arsenic from drinking-water and food causing cancer and skin lesions, amongst other serious ailments. The products as well as the steps mentioned below provide a solid solution for the removal of arsenic.
Basic process steps for the treatment plant
- Inlet Works: Raw water is pumped from the return water dam by submersible pumps and screened before entering the water supply tank. This water is then pumped with pumps on variable frequency drives (VFD’s) to allow the raw water flow rate to be controlled to achieve a normal flow rate of 150 m3/hr.
- Primary Clarification: Incoming water is split into three equal streams (50m3/hr each under normal operation). The streams flow into three lamella clarifiers which provide primary clarification through settling of suspended solids. Clarified water exits the top of the clarifiers and will be combined into one stream that flows to Stage 1 arsenic removal. Sludge is removed from the bottom of the clarifiers daily according to a de-sludge sequence, and flows by gravity to the sludge storage tank.
- Stage 1 Arsenic Removal: Clarified water from primary clarification flows into the stage 1 mixing and flocculation tank. Chemicals responsible for the precipitation of arsenic as well as a polymer coagulant are dosed inline before the mixing zone, which provides rapid mixing for chemical dispersion and initial floc formation. Mixed water flows into the flocculation zone where gentle mixing aids in the formation of larger flocs. Water is split into three equal streams which flow into three lamella clarifiers. These provide arsenic removal through settling of precipitated arsenic. Clarified water exits the top of the clarifiers and iscombined into one stream that flows to Stage 2 arsenic removal. Sludge is removed from the bottom of the clarifiers similar to the primary clarifiers.
- Stage 2 Arsenic Removal: Water from the stage 1 clarifiers flows into the stage 2 removal mixing and flocculation tank. This has the same design as the stage 1 mixing and flocculation tank and is followed by three more clarifiers similar to the primary clarification and stage 1 clarifiers. The incorporation of two stages results in a significantly lower final arsenic concentration with minimal chemical consumption. Water from the stage 2 clarifiers flows to the treated water storage tanks.
- Treated Water and Sludge Storage: Lastly, treated water from the Stage 2 clarifiers flows into the treated water tanks. Sludge from the Primary Clarifier, Stage 1 Clarifiers, and Stage 2 Clarifiers is stored in the sludge tank which is equipped with a sludge agitator to prevent the settling of solids. Sludge is pumped from the sludge storage tank to the filter press to be discarded.
This process can be installed anywhere in the world, thereby reducing the environmental impact of discharging arsenic into the environment.
The result and benefits of the project
- Removal of arsenic – Reuse the effluent water for numerous purposes with further treatment without arsenic present.
- Discharge / disposal of effluent water – within limits and with reduced arsenic.
- Standard drinking water – by treating the effluent further with a different process through reverse osmosis and nanofiltration.