THICKENING & FILTRATION
Five flat CCD thickeners at Comika supplied by ANDRITZ
multiple thickeners in a train , dewater this PLS . Norinco was looking for the most cost-effective thickener solution incorporating the latest separation technology and knew that it had to include counter-current decantation ( CCD ). As the name implies , CCD works by sending water in the opposite direction to the solids .
Just two months after the first meeting , ANDRITZ presented its solution : a unique CCD circuit consisting of five flat thickeners paired with an external dilution system , capable of handling flow rates of between 2,800 and 15,000 m 3 per hour , as well as a paste thickener . In-house software Metris Digital Twin ( powered by IDEAS ) was used to model the technology and prove that it could achieve the required recovery rates . The new and improved ANDRITZ inter-stage mixing and dilution system for CCD circuits was the ideal fit as it helps to reduce the footprint , consequently cutting CAPEX while maintaining low operating costs .
The innovative paste-thickener design facilitates highly clarified liquid recovery as well as allowing maximum underflow densities . At the DRC Comika plant , the system has eliminated the need for needle tanks and interstage transfer pumps as it provides high levels of closed-circuit dilution to counteract the negative effects of surging underflow pumps . What ’ s more , gravity feeding of the wash liquor and the pumped underflow into the inter-stage mixing tank ensure highly effective washing of the PLS .
“ The technical know-how was never in question as ANDRITZ has references in oxide ore applications along with ample data
demonstrating the effectiveness of the CCD thickeners . And yet the actual results since production of the first batch of copper cathode in July 2020 are still impressive . Civil costs have been slashed by around 20 % without any additional power consumption thanks to the unique ANDRITZ flat CCD design and superior technology . Five transfer pumps have become redundant , estimated at around 75 kW each .”
The hydrometallurgy plant ANDRITZ says is achieving outstanding recovery rates of more than 95 % of the PLS due in part to the newly improved feedwell that reduces turbulence by using guide fins for enhanced flocculation . The robust technology promises less downtime and 95 % availability based on a detailed lifecycle calculation of all mechanical components that guarantees 100,000 hours of operation or longer if necessary .
WesTech ’ s proven thickener retrofit experience
There is a lot of new equipment out there but what about improving what is already operating ? Modernising and improving thickener performance may be considered a priority in the industry . Recent improvements have drawn attention to thickener retrofits as an inexpensive way to improve thickener performance and save water . For example , improvements in feedwell design , such as the WesTech EvenFlo ® feedwell , have addressed performance issues like short circuiting and inefficient flocculation . Mechanism improvements have also been studied , some of which have confirmed the effectiveness of original designs that WesTech has used for decades .
Jerold Johnson at WesTech told IM that it has conducted audits of several paste-type
thickeners and identified that some important basic design criteria are neither universally used nor understood in the industry , resulting in lower than targeted underflow density and rheology .
“ One published study showed that truss rake arms inhibit production of non-Newtonian underflows by allowing solids to deposit within the truss . This restricts the flow through the truss and causes solids to become stagnant . The low-profile rake arm with dewatering pickets , WesTech ’ s standard design , was identified as the best design . The compaction of the solids is dependent on the rate at which water percolates up out of the bed , and properly designed dewatering pickets greatly improve this dewatering rate .”
Another important design feature that is not well understood in the industry is raking capacity in paste-type thickeners . Many suppliers use rake designs that are similar to those of high-rate thickeners . While the underflow from high-rate thickeners flows easily , their blade designs have a low raking capacity ( defined as the fraction of the underflow volume that is supplied by the blades ) and count on gravity to provide the needed transport inside the thickener . This design becomes ineffective when the material has yield stress , which is the objective of paste-type thickeners . The resistance to movement inherent to yield stress can negate the gravity transport , developing a rat-hole . Without adequate transport from the outer volume of the thickener , the rat-hole develops with the central area providing most of the volume for the underflow discharge .
Johnson adds : “ The industry is also lacking in steady state operation . Achieving steady state operation requires proper control of the thickener discharge based on a reliable , consistent measurement of the bed level . In 2015 , WesTech introduced the MudMax™ , a bed level sensor mounted on the rake mechanism that provides the accurate measurements needed to maintain a consistent bed level . Monitoring and then controlling the bed level gives a steady solids residence time that produces consistent underflow density or rheology . Without the ability to consistently monitor the bed level , the bed level will vary . This results in varying underflow density and yield stress , giving way to problems that plague the industry , such as water loss and upsets in downstream operations .”
WesTech says it has provided many retrofits for paste and high-rate thickeners , modernising the feedwell and mechanisms and installing the MudMax for control . These retrofits provide improved overflow clarity while decreasing flocculant consumption . They also provide increased water recovery and steady
18 International Mining | OCTOBER 2021