Output Australia | December 2010 | Page 12

Current economic and environmental constraints compound the need for highly efficient thickenerperformance to yield superior process results. With greater throughputs, less environmental impact andoptimized energy efficiency in mind, a new technology, Shear Enhanced Thickening (SET), has beendeveloped. This article will outline the thickening sedimentation process, the theory of SET and, finally,validation of SET by results from site testwork.

How does sedimentation work?

Authors: Chad Loan & Ian ArbuthnotAuthors: Chad Loan & Ian ArbuthnotAuthors: Chad Loan & Ian ArbuthnotAuthors: Chad Loan & Ian Arbuthnot

Get SET go!

Clarification ZoneOverflow

Cla

rify

ing

Th

ickenin

g

Networked compacted pulp bed

Hindered Settling Zone

Free Settling Zone

Flocculant adsorbs

onto discrete particlesGrow and loosely bind

into highly porous

aggregates

Consolidation and

compression under

self-weight forces

Clarification ZoneOverflow

Cla

rify

ing

Th

ickenin

g

Networked compacted pulp bed

Hindered Settling Zone

Free Settling Zone

Flocculant adsorbs

onto discrete particlesGrow and loosely bind

into highly porous

aggregates

Consolidation and

compression under

self-weight forces

Underflow

Aggregates become crowded,

impeding each others settling

self-weight forces

Underflow

Aggregates become crowded,

impeding each others settling

self-weight forces

Within the body of the thickener tank, several zones or layers of varying aggregate composition andsuspension density exist. It is conventionally considered that to achieve an appropriate underflowdensity, the free settling, hindered and networked zones should be largely undisturbed. As the pulp bedincreases in depth, it becomes increasingly difficult for released liquid to permeate through the pulpbed. Dewatering pickets mounted to the rake arms aid removal of such liquid and pickets are typicallyarranged at equally spaced intervals to produce dewatering channels. It has also been found that therotation of the rake assembly with pickets increases the possibility of pulp bed rotation, which is alsoknown as “donutting” or “islands”.

The theory of SET

Following previous studies of applying shear to the hindered settling zone to increase sedimentationrate and compressive yield stress of the pulp bed, Outotec carried out further rigorous modelling andinvestigative testwork, following which the Shear Enhanced Thickening (SET) technology was developed.

Figure 1: Settling zones and processes within a thickener

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The shear rate applied to aggregates by a moving shearing element is generally expressed by:

�= k.ul / ξ ……..(1)

where � is the shear rate in s-1,ul is the linear velocity of the shearing element in ms-1,ξ is the distance between the shearing element and the aggregate in meters, andk is a constant, which is a function of the material properties of the pulp.

ul = 2pv.ℓ ………(2)

where v is the rotational speed of the shaft in s-1; andℓ is the distance from the center in meters.

Equations (1) and (2) indicate distance from the axis of rotation increases, the linear velocity of theshearing element increases proportionally. Angling the proportionally spaced shearing elements to thevertical plane will result in a substantially uniform cumulative shear being applied to the aggregates bythe time they exit the shear zone.

Validation of SET

Many months of dynamic bench scale thickener testwork was used to validate the application of uniformcumulative shear and to determine the resultant impacts on thickener performance and design.Outotec’s testwork units (94mm, 190mm and 1m pilot scale) were modified to incorporate a SETmechanism to provide uniform cumulative shear to the hindered settling zone. The following outlinesthe results from two series of testwork programs (further testwork series available on request) and isthen followed by details on SET installations.then followed by details on SET installations.

1. Gold Tailings testwork

The aim of this series of tests was to evaluate the association between flux rate, uniform cumulativeshear and underflow density.

The tests were conducted in the 94mm dynamic thickener under “standard high rate” and “shearenhanced” thickener conditions. Tests ranged in flux rates from 0.5 to 2.0 t/m2h and a rake shaftrotational speed from 1 rpm, under both standard and sheared conditions. Both systems ran with aconstant bed height of 500mm.

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Following this testwork, it was clear that a higher underflow density for the same thickener size could beachieved. A 40m SET could be used to produce a paste-like underflow of 62%w/w, density far superiorto the 52%w/w from a 57m high rate thickener. For example, if the gold sample tested here required afull-scale underflow density of 52%w/w, a flux rate of 0.5 t/m2h would need to be employed using a highrate thickener. If the proposed plant was to treat 1500 t/h of tailings material, a 62m diameter high ratethickener would be required. However, if the proposed SET technology were to be employed, a flux rateof 2.0 t/m2h or higher could be selected and would result in a 31m diameter SET thickener. If the datawere extrapolated and assuming a linear drop in density, it could be implied that to obtain an underflowdensity of 52%w/w with SET a flux rate of 3.0 t/m2h could be employed. At this flux rate, a SET thickenerof only 25m diameter would be required, versus the 62m high rate thickener.

Alternatively, and perhaps of more interest to many sites and applications, a higher underflow densityfor the same thickener size could be achieved. That is to say, a 44m SET thickener could be used toproduce a paste-like underflow of 62%w/w, density far superior to the 52%w/w from a 62m high ratethickener.

2. Mineral Sands Slimes testwork

A mineral sands slimes sample was studied on site at an industrial operation. The tests were conductedin a 1m diameter Pilot thickener, at a bed depth of 0.5m. Testing focused on the current operatingconditions at the site, a flux rate of 0.25 t/m2h and an increased flux rate of 0.75 t/m2h.

The graphic outlines the impacts of SETtechnology under pilot scale operation.Standard high rate thickening producedan underflow density of 24%w/w at aflux rate of 0.25 t/m2h, whereas SETflux rate of 0.25 t/m2h, whereas SETproduced a density of 35%w/w at 3times the flux rate of 0.75 t/m2h.

As was evident with the previoustestwork, the impact of the results onthickener sizing and design areremarkable. The results indicate thatthe existing thickener could be fittedwith SET technology and produce anunderflow density of 11%w/w higherand this is at a flux rate of 3 times theexisting operating conditions. Theachieved density with SET takes theunderflow into the paste realm fromonly a bed depth of 0.5m, whenconventional understanding is that highbed depths 3-10m are required toachieve paste like underflowsFigure 2: Pilot thickener results

Theory becomes reality – full scale installations

With such compelling results from testwork, the next step in validation of SET technology is full scaleinstallations. Although still early days, results are extremely promising. At one particular site, Outotecretrofitted a SET mechanism to an existing 8m CCD thickener application. The existing high ratethickener produced an underflow density of 45%w/w with a flux rate of 1.33 t/m2h. When the SETmechanism was installed, the underflow increased to 54% w/w and the flux rate was increased 20% to1.61t/m2h. The increase in flux rate was limited to what could be supplied by the existing pipework/feedarrangement and process constraints.

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Chad Loan is a thickening specialist with Outotec’s Global Thickener Technology group. Chad has spentthe past nine years working in the minerals processing industry, with particular emphasis on thickening

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Outotec have also supplied a 24m SET thickener to replace an original order of 2 x 24m paste thickeners. This greenfield installation will be commissioned in 2011. Furthermore, Outotec is also carrying out testwork for other sites, with additional installations in the pipeline.

Conclusions

Experimental investigations of the theories have resulted in some remarkable outcomes. In all workpresented, standard high rate thickening testwork shows a drop in underflow density with increasing fluxrates. When uniform cumulative shear is applied, the underflow densities remain and also, in somecircumstances, increase significantly at up to 3 times the flux rate.

In the series of testwork on mineral sands and gold tailings outlined previously, the SET underflowdensities are all significantly higher than those produced under normal testwork conditions. Theimplications on thickener design sizes are highly significant. As an example, if the data were extrapolatedand assuming a linear drop in density, it could be implied that to obtain an underflow density of 52%w/wwith SET thickening a flux rate of 3.0 t/m2h could be employed. At this flux rate a SET thickener of only25m diameter would be required, versus the 62m high rate thickener. Alternatively, paste-like underflowcould be produced at higher flux rates with Shear SET technology, without the need for deep sidewallpaste thickeners.

If you would like more information, click here to contact

chad.loan@outotec.com

the past nine years working in the minerals processing industry, with particular emphasis on thickeningand flocculation involving a wide range of minerals and applications. He holds First Class Honors inApplied Science from Curtin University, a global patent in thickening and has authored numerouspapers.

Ian Arbuthnot has a wide range of experience in process engineering, acquired over 35 years. Hiscurrent position is Director - Special Projects and Ian is based in Perth. Previous roles have includedgeneral management, sales and marketing, project management and process engineering design. Hehas specialised experience in solid/liquid separation including filtration and thickening and has beenresponsible for a number of design patents in this area. Ian has been involved in industries includingmining and minerals processing, water and wastewater treatment and chemical/industrial.

OUTPUT AUSTRALIA EDITOR

Laura White – Marketing Manager, Minerals Processing

Tel: +61 2 9984 2500

Email: laura.white@outotec.com