Thickener sizing & the role of testwork – Part two

By Andrew McIntosh*

To read the first part of this report click here.

3. Dynamic thickener testwork
This is the method through which the most accurate thickener sizing and prediction of process performance is achieved.
Most reputable and experienced thickener suppliers have developed a number of dynamic testwork units of various sizes and configurations. These dynamic test rigs are essentially scale versions of commercial thickeners, with feedwells, flocculant addition, underflow pumps, and rotating rake mechanisms to duplicate the full-scale thickening process.
For example, Outotec carry out most dynamic testwork in a bench-scale 99mm internal diameter test rig. Other tests rigs include a 190mm internal diameter with extended wall height to simulate the additional compression in paste thickeners, and 1m diameter test units of various sidewall depths. The 190mm and larger tests rigs are used for paste thickening applications and other larger scale test campaigns.
Generally, due to the amount of sample required, paste thickening and larger scale testwork campaigns are conducted on-site. This is also the case for bench-scale testwork for feed types expected to age significantly during transport or where the water quality characteristics will affect results of testwork.

Dynamic testwork presents a number of benefits including:
. Ability to accurately ascertain the relationship between feed solids loading rate and underflow density and overflow clarity, and flocculation requirements.
. Any requirements for feed dilution or solids re-circulation to achieve optimal conditions for flocculation can be determined.
. The safety factors necessary with less accurate sizing methods are not required. This leads to lower costs in most cases as smaller thickeners can be specified.
. Process guarantees are available on thickeners sized using this method, thus fulfilling any requirement for guaranteed thickener process performance.

Ensuring that the most accurate testwork possible is utilised can mean the difference between achieving your desired thickener performance and ‘almost’ achieving it.  ‘Almost’ achieving process targets can lead to problems such as: Ability to accurately ascertain the relationship between feed solids loading rate and underflow density and overflow clarity, and flocculation requirements.
. Unnecessarily high operating costs due to non-optimised flocculant consumption.
. Unacceptable thickener overflow clarity, meaning possible inability to recycle plant process water.
. Downstream effects on other equipment in the circuit such as, for example, filter operation.
. Complications with underflow pumping or other intended transportation methods.
. Problems with tailings disposal, such as unstable tailings deposition, tailings disposal volume consumed at a higher rate than envisioned, additional rehabilitation costs.
. Inadequate washing – CCD circuit.

In the early stages of a project, rudimentary rule of thumb sizing can fulfil basic requirements. However, the temptation to scrimp on the costs and effort associated with testwork can have lasting ramifications.
Comprehensive dynamic testwork performed on indicative feed samples ensures truly optimised thickener performance, achieving process targets and minimising operational costs. It can also avoid costly conservative oversizing, which can mean the difference between whether your project proceeds or not. As tends to be the case in life, investing a little bit of expense and effort up front can save substantial costs and avoid disappointment further down the track.

* Andrew McIntosh is currently Senior Proposals Engineer, thickening, for Outotec in Sydney, Australia.

For more information email: laura.white@outotec.com