Thickener upgrade technologies improve recoveries and downstream processing

Froth on thickeners - common issue

Froth on concentrate thickeners is a common issue faced by many sites. For every 2200 ppm and 75 m3/hr of overflow, approximately 4 tonnes per day of concentrate is either lost or recycled back into the operation. The recycled flotation concentrate is less likely to be recovered each time round, reducing overall recovery.

Scope

In June 2014, one of our customers in Australia decided to modernise its 20m concentrate thickener. One of two concentrate thickeners at site, this thickener was not performing optimally. It had significantly aerated froth on the thickener surface, resulting in high solids content in the thickener overflow. Additionally, site was also soon to transition to a new orebody, with thickener throughput to increase from the original design of 26 tph to 50 tph.

Outotec was awarded the project on an EPC basis, with full responsibility for the engineering, procurement and construction of the thickener upgrade.

Testwork

Following an initial site visit, Outotec conducted a full testwork programme to optimise the concentrate thickener. Representative thickener feed material from site helped determine a benchmark for improvement, more consistent thickener performance, and the design requirements to achieve the higher throughput which was due to come on line. Dynamic testwork was carried out using an Outotec 99mm bench scale test rig. Following this an engineering study was completed to detail the solution, Vane FeedwellTM technology and a Deaeration tank were proposed as part of Outotec’s upgrade solution.

Vane Feedwell

Feedwells are essential in generating optimal thickener performance. Outotec’s Vane Feedwell incorporates seven patents and is now the standard on all its thickeners globally. One of the key design features is the interconnected upper and lower zones.

The upper zone, with ‘vanes’, is where feed, dilution water, and flocculent are added, and incoming slurry is mixed and deaerated, maximizing flocculant dispersion, and therefore minimizing coarse/fine segregation. The lower zone is concentrated around promoting gentle mixing through aggregate growth, with the option of secondary flocculant addition. 

This patented design delivers benefits including higher solids throughput, increased solids density, optimized operability, lower flocculant costs and minimized downtime, amongst others.

Deaeration tank

The Deaeration tank was developed by Outotec to remove aeration and excessive froth from the feed before it reaches the thickener feedwell. The Deaeration tank is mounted on the thickener bridge. Part of this patented design incorporates tangential feed inlet and outlet pipes. The internals of the tank are designed so that the feed is received tangentially in the upper section of the tank and passes through an orifice plate to the lower section before exiting to the feedwell.

Installation

Working closely with the customer and meticulously planning the installation, the focus on site was remaining operational during the upgrade. All disruptive work was therefore completed during shutdowns, ensuring minimal interruption to production. This type of upgrade typically occurs over one shutdown but shutdown schedules at site were such that the best option was to implement the upgrade in two phases.

Some modifications occurred during one shutdown, in February 2015, with the actual installation of the feedwell and  Deaeration tank during a subsequent shutdown in May 2015. The final installation was a smooth process, with all equipment bolting together without modification first time round.

With this EPC partnership approach, procurement was also carried out on a collaborative basis to deliver the most cost-effective and timely result for site. Outotec utilized, where possible, the customer’s local labour and contractors for the turnkey upgrade. The customer carried out the commissioning of the upgrade, partnered and supported by Outotec. The upgrade was on time and on budget and, most importantly, was incident-free.

Results

The thickener upgrade has delivered multiple benefits. Following commissioning, visible froth was significantly reduced from the surface of the thickener. Lost product in the overflow was decreased by 91% post installation. This performance improvement had a payback period of approximately nine months. Flocculant consumption was also halved, with the additional benefit of less overload in the flotation circuit from recirculating load.

Downstream processing

Additionally, following improved mixing in the feedwell and lower flocculant addition, improvement in the underflow slurry with respect to density and rheological characteristics have been observed. This has positively affected further downstream processing, such as overland pumping costs, eliminating the additional water required for pumping the concentrate to its filtration plant located away from the concentrator (and then removing this water again during filtration). This facilitated an additional 10 tph capacity for the filter, so the cost per filter cycle has been lowered, which is not costed in the payback.

*This case study has been written prior to the merger of Metso and Outotec.