Clarifying and Thickening: Neglected Unit Operations

Clarifying and Thickening: Neglected Unit Operations

Clarifiers and thickeners (the two duties hereafter combined in the single unit operation of “thickening” in this note) are ubiquitous in concentrators and extractive metallurgical plants.

Now in my fifth decade in the business I have seen a considerable number of them and the operation of many could only be described as “poor” to “appalling”. Depressingly it doesn’t appear to be getting any better with the passage of time.

Malfunctioning solid-liquid separation unit operations threaten the viability of hydrometallurgical process plants and recycled thickener overflows containing fine solids can materially affect flotation performance.

Consider the following thickener fundamentals.

  • In a thickener there are two flows; simplistically solid going downwards and liquid going upwards. Calculation of the rise velocity of the liquid going upwards gives a number that is usually higher than the settling velocity of the finest particles in the feed. Note that the settling rate of a 10 micron quartz sphere in water is 0.6 cm/min at 25ºC1. Hence the importance of adequate flocculation (and coagulation in water treatment) to create agglomerates with a higher settling velocity than the rise velocity.
  • Hydrometallurgical plants often have multiple thickener operations e.g. counter-current decantation. Taking a desired 92.5% operating time with a total of 5 thickeners in series in the flowsheet means that each thickener has to be performing as specified 98.5% of the time ignoring all other causes of downtime and poor operation!

In a mineral processing world of finer particle sizings due to the nature of ores treated leading to more difficult solid-liquid separation duties, what are some of the depressing realities of thickener operations.

  • Flocculant system malfunctioning with incorrect addition and subsequent dilution; flocs once formed are “maltreated” in their passage to and through the thickener feedwell.
  • Feedwell in poor condition leading to excessive turbulence damaging flocs, side feeding etc.
  • Thickeners acting as a further stage of cleaner flotation with voluminous froths on the surface exacerbated by upwelling air bubbles; technologies to de-aerate flotation concentrate froths have been available for over 10 years and should be standard for such duties.
  • Solid-laden concentrate thickener overflows reminding one of the Dutch farm adage “too wet to plough, too dry to drink”.
  • Thickener overflow launders choked with deposited solids.
  • Thickener underflow density reduced to cope with inadequate pumping systems: homicidal feelings have to be supressed when told by a process engineer that the downstream filtration stage “works better at lower solids density” when what he/she means is the underflow pumping system can’t deliver a high density slurry to the filters.

Too often the suggested remedy is to install additional instrumentation when the problems are caused by flawed process fundamentals.

Process engineers are encouraged to get “back to basics” (has anyone ever made a credible case for leaving them?) on thickening. This includes learning from the excellent work done by CSIRO in the AMIRA P266 Project “Improving Thickener Operations”.

1. Dorr, J.V.N. and Bosqui, F.L, 1950, Cyanidation and Concentration of Gold and Silver Ores, 2nd ed., McCraw-Hill, New York, USA, p. 23.

By Peter Munro

PDM Photo 01 31-07-2014Peter Munro graduated in Applied Chemistry from the University of Adelaide and worked in production and technical roles in both the Lead-Zinc Concentrator and Copper Concentrator of Mount Isa Mines Limited from 1970 to 1985. He was the Minerals Processing Research Manager from 1985 to 1987 and the Lead-Zinc Concentrator Manager from 1987 to 1993. From 1993 to 2000 Peter worked in the Brisbane office of M.I.M. Holdings Limited as an “internal consultant”. Since 2000 he has been with Mineralurgy Pty Ltd, a specialist processing consultancy to the minerals industry where he is a Senior Principal Consulting Engineer.