VPA filter refurbishment and upgrade increased availability by more than 30% for Hecla

Hecla’s Greens Creek Mine in southeast Alaska, USA is one of the largest and lowest-cost primary silver mines in the world. The mine is allowed to operate inside a federally protected natural monument and Greens Creek has a safety and environmental record that’s among the best in the world. The Greens Creek ore body contains silver, zinc, gold and lead, and it lies within the Admiralty Island National Monument. The Greens Creek property includes 17 patented lode claims and one patented mill site claim.

The entire project area is accessed by boat. It consists of the mine, an ore concentrating mill, a dry stacked tailings facility, a ship-loading facility, camp facilities and a ferry dock.

The mine's remote location on Admiralty Island adds complexity, with limited access to infrastructure, services, transportation, power and skilled labor.  All supplies and equipment must be first delivered to Seattle and then transported by barge to the site taking 5-7 days to arrive via the Inside Passage.

Increased availability with filter refurbishment

In 1987, Hecla Greens Creek decided to acquire and deploy the newly designed first-generation VPA15 pressure filters from Svedala (acquired by Metso in 2001). During the summer of 1988, the first two concentrate (zinc, lead & bulk) VPA1530-16 filters and two tailings VPA 1530-32 filters were installed, commencing production in the early fall of the same year. Subsequently, in 1990, Hecla installed a third VPA1530-32 tailings filter. As of 2024, the two concentrate VPA1530-16 filters remain operational, consistently producing zinc, lead, and bulk materials for Hecla Greens Creek.

In 2019, Hecla approached Metso regarding their VPA 1530-32 tailings filters. The mine faced misalignment and poor availability of approximately 60-70% of their filters, initially installed in 1988. Rectifying misalignment was done using a crane to reposition the head, which could take anywhere from five hours to two days, depending on the extent of damage and size of the misalignment.

In 2020, Metso was assigned to conduct a major refurbishment of one of the filters. The refurbishment process presented an additional challenge as all equipment had to be lifted piece by piece through a limited space opening in the floor. Storage constraints necessitated the gradual construction of the new parts over 22 days. To secure the filter in position, old bolts were removed by excavating the surrounding concrete, allowing for the welding of new bolts.

The refurbishment included new hydraulic cylinders to handle plate compression, cake discharge, plate opening and closing, and pressure control. These functions are crucial for achieving efficient solid-liquid separation and cake removal and ensuring continuous and reliable filter operation.

Real-time monitoring of each cylinder was implemented to detect misalignment and prevent breakdowns by proactively stopping the filter. Since installation, no misalignment alarms have been triggered, resulting in increased availability from 60-70% to over 95%.

In 2022, a second filter retrofit and upgrade were conducted with the same scope as the previous one, allowing for interchangeable parts among all the filters. The first installation was done in collaboration with the customer, whereas the second retrofit was conducted by Metso on an advisory basis based on all the mine operator’s experience and training gained from the first retrofit.

Operator training was also provided to familiarize the customer with the functioning principles of the VPA filter. Maintenance practices have been adjusted due to the improved wear life of the parts and reduced maintenance requirements. 

More functionality and automation with filter upgrade

The filters were also upgraded with modern technology for additional functionality and automation. A Programmable Logic Controller (PLC) was installed to ensure precise control, reliable operation and safety by monitoring, controlling and automating various filter functions.

Another addition to the filter was a new generation Hydraulic Power Unit (HPU) which is responsible for generating hydraulic power, maintaining pressure control, integrating with the filter's control system, optimizing energy efficiency, monitoring performance and ensuring safety. The new HPU requires half the amount of oil than the older units, making it more environmentally friendly. It plays a vital role in enabling efficient and reliable operation of the VPA filter for effective solid-liquid separation.

Position sensors were also incorporated to monitor pressure, contribute to automation and control and provide valuable data for analysis and performance evaluation. These functions help improve the efficiency, productivity and reliability of the VPA filter.

The retrofit and upgrade project was completed with zero harm according to the highest safety standards.

Full equipment scope of supply

• Support with stainless steel load cells and weighing system for extended life and accuracy.
• OEM frame design.
• Pressure arrangement with new stainless steel piston rods in hydraulic cylinders. Cylinders equipped with transducers for measuring the length and ensuring exact alignment between cylinders.
• New filter and membrane plate pack according to Metso OEM design.
• Pressure transmitter added to existing slurry feed pipe for additional control functions.
• Innovative cake drying air system for more efficient air drying – less air consumption and reduced excessive wear on plates and cloths.
• New air supply system for the filter membrane.
• New generation High Flow Hydraulic Power Unit for more efficient and reliable operation.
• New control system to operate the filter and optimize the dewatering process.
• New, additional sensors, valves and switches.

What is next?

The third tailings filter will be retrofitted and upgraded according to the same scope in 2024 on an advisory basis. Metso and Hecla are currently looking into refurbishing and upgrading the two remaining VPA 1530-16 concentrate filters which would take place in 2025. These filters are used for processing lead, nickel, copper and silver. The fourth and fifth filters will present a new challenge for the installation crew as the filters need to be assembled and leveled on the outside of the mill building and then hoisted through the roof to minimize downtime.