THICKENERS
Feedwell fine tuning
Paul Moore reviews market trends , fundamentals and technology from four of the global leaders in the supply of thickeners in mining
The main industry players in thickening
technology offer a range of options in concentrate and tailings thickeners that are suitable for a variety of commodities , showing that no single solution fits all situations . As an example , the WesTech Deep Bed™ Paste Thickener can thicken a tailings stream to nonsettling paste at solids concentrations approaching that of a filter cake . Design characteristics of the Deep Bed result in maximum water recovery , maximum underflow density , and paste with minimal slump . Meanwhile , WesTech ’ s HiDensity™ Paste Thickener produces non-settling thickened tailings with lower viscosities and underflow concentrations than what is produced in the Deep Bed Paste Thickener . The HiDensity Thickener also has a lower height-to-diameter ratio , which allows for on-the-ground construction and much larger diameters . Another option from WesTech , HiFlo™ high-rate thickeners , are used in many types of process circuits to separate liquids and solids at very high rates .
But relative demand is market driven . WesTech Senior Process Engineer , Jerold Johnson told IM : “ The thickener market is certainly evolving – mainly driven by the scrutiny on tailings management and disposal . Paste and filtered tailings approaches include differing technologies , and both have their advantages . But there is no doubt that things in recent years have shifted more to filtration . There have definitely been some underperforming paste thickeners which have affected the level of interest in paste technology . Part of this problem has been operator inexperience and improper thickener operation , but there have also been problems with some fundamental designs . On delivery , they have not provided the expected performance in capacity and in yield stress . You have variations in material coming out of thickeners which can cause problems in disposal .”
On the flip side , in Indian iron ore , several plants decided to opt for a filtered tailings approach , but are now looking at paste due to the very fine tailings they are dealing with . That said , filters can handle variations in the material coming from a thickener , whereas with paste you have to live with what it produced . The weight % will depend on the particle size , and this can vary significantly at many processing sites . So , you can be producing the right yield stress but the weight % can shift around .
Johnson added that for thickener size , paste and high-density thickening have quite defined limits due to the necessity of keeping the large beds active and dewatered . “ The economics of supplying a drive to supply sufficient torque is an issue ; as an example , a paste thickener can have 10 or even 15 times the drive torque of a high rate thickener . And operational flexibility means that installing several thickeners is often preferred to one big one . WesTech has a design for a 60 m
paste thickener but has delivered them up to 45 m . It is worth noting also that an important market for paste thickeners is for CCD circuits in the alumina industry .”
For reducing flocculant costs , the feedwell is at the heart of the issue . The functions of converting the horizontal momentum of the feed to a vertical flow and promoting flocculation of the suspended solids are the same for any thickener . The effectiveness of the feedwell design to accomplish these functions has generally been poor in the industry . “ Short circuiting flows carry the solids into the thickener prematurely , before all the suspended solids have been captured . Also , these flows generate such force that solids are carried to the overflow launder before settling can occur . Paste thickeners are oftentimes used
as the final dewatering stage of the tailings and
The patented WesTech Evenflo is a design that uses an inner feedwell that communicates with an outer feedwell through radial ports
the main source for process water return . Clarity of the process water is often regulated if discharged , and poor clarity recirculates solids that cause a burden on the system .”
WesTech ’ s patented Evenflo ® feedwell brings significant improvements to overflow clarity and overall thickener operation . It uses an inner feedwell that communicates with an outer feedwell through ports that direct the feed flow radially out toward the outer feedwell wall . This simple addition to an open-with-shelf feedwell has shown to provide great benefits in correcting the described shortcomings . This two-stage feedwell design produces a very consistent mix and produces uniform floccule size over a wide range of feed conditions . The optimised mixing provided by the Evenflo two-stage feedwell allows the plant to optimise the flocculant dosage for its thickener .
Challenges and opportunities
FLSmidth ’ s Todd Wisdom , Director of Tailings Solutions , says that bigger thickener diameters do remain of interest . “ Greater than 100 m or even 120 m is possible and these have been supplied to big tonnage open pit mines , especially in South American copper . But these giants are what we would call high rate thickeners . Once you start talking about high density , meaning you are getting a little bit more underflow out of it and higher densities , you start to limit the size as you run into limitations on the torque with the drives .”
Wisdom says that high density thickeners therefore run up to about 80 m in diameter . Then with paste thickeners where you really start to get high underflow solids , with higher and deeper sidewalls to give more residence time , and even more powerful drives , “ then you are looking from FLSmidth ’ s offering , at a maximum of about 55 m in diameter depending on what underflow strength we are designing for . We currently have three 45 m diameter paste thickeners being installed at a project , which will be some of the largest ever commissioned . In terms of going bigger , we are working on more powerful drives to handle higher underflow solids in high density and paste thickeners . I am not sure there will be any demand for high rate thickeners above 120 m as those customers are now interested in high density thickeners in many cases .”
Like Johnson , Wisdom agreed that there are certainly challenges with big paste thickeners – once you start getting into thick material , you start to run this unit operation at its edge so it requires very good operator and process control to get right . “ You may have to deal with
OCTOBER 2020 | International Mining 55