Location: Manatí, Puerto Rico
OEM: Dynatec Systems Inc.
Application: Direct filtration for recovery of valuable products
Capacity: 272 m3/day (50 gpm)
Membrane: 8 mm, PVDF, backwashable
DuPont Electronics facility in Manatí (Puerto Rico) manufactures microcircuit materials. In their process, high purity silver is produced. As a result, the wastewater stream contains dissolved silver. These compounds are highly valuable and DuPont had strong interest on recovering them from the waste stream.
The wastewater treatment plant at DuPont Electronics had an existing membrane system to recover the silver from the wastewater. However, this system was not running as expected and DuPont had to deal with several underperformance issues. Moreover, the maintenance requirements were excessive and the chemical cleaning costs were high.
When the plant expanded its operations, DuPont performed a detailed evaluation for assessing the best approach for the silver recovery system. The choice was between expanding the existing membrane system or, alternatively, installing a new system based on tubular UF modules. The main drivers for this second option were the simple automatic operation and the lower operating costs.
Dynatec was finally selected to provide the complete silver recovery solution for treating approximately 11 m3/h (50 gpm). Tubular UF membranes provided the robustness, stability and reliability required for this application. Additionally, the implementation of one stand-by skid and the possibility for future expansion were also key requirements from DuPont.
In order to recover the silver with maximum efficiency, Dynatec designed a tubular UF system coupled to a chemical precipitation process. The installation includes transfer pumps, two reaction tanks, three ultrafiltration skids designed for 5.7 m3/h each (25 gpm each) with room for expansion, a sludge conditioning system, and a filter press. Figure 2 shows the Process Flow Diagram of the solution.
Wastewater from the process is stored in an equalization tank. Transfer pumps deliver the equalized wastewater to the first reaction tank. Here, the pH is adjusted to approximately pH = 10 to optimize the precipitation of the silver compounds. The water then enters another reaction tank where DTC (dithiocarbamate) is added to react with the aqueous silver compounds and form insoluble metal salts.
The precipitated silver compounds are then separated from the treated water in the tubular UF membranes. Three UF units were installed: two of them are in operation and the third one is kept as a standby unit. The UF coupled to chemical precipitation allows the recovery of more than 90 % of the silver, with permeate average concentration of 0.1 mg/L. Additionally, more than 96 % turbidity reduction is achieved, with average permeate values of 0.4 NTU.
The ultrafiltration system, based on tubular PVDF membranes, was designed for automatic operation. Each unit comprises four modules of 8 inch diameter and 3 meters length. The permeate from the ultrafiltration is discharged and the concentrated silver stream is directed to a sludge conditioning tank, where a high molecular weight cationic polymer is added to improve the filterability of the sludge. Finally, the sludge is sent to a filter press and the cake is processed in the silver reclamation facility.
In this project, the main focus was not on the permeate but on the valuable silver components retained by the membranes and obtained in the reject stream. Implementing tubular UF after the precipitation stage, instead of the conventional settlers and multimedia filters, entails major benefits:
The installation of the new system based on tubular UF was highly successful. The system has been running for several years and the operators are fully satisfied with its performance:
are the key features highlighted by the end-user.
This case study shows how direct filtration with tubular ultrafiltration membranes enables efficient recovery of valuable metals from industrial wastewater while optimizing energy use and process reliability.
This case study shows how tubular UF technology enhances wastewater treatment and enables efficient recovery of valuable metals in the microelectronics industry.
