Lal Bahadur Nagar, India
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Nageswararao K.,Taijasa Consultants | Medronho R.A.,Federal University of Rio de Janeiro
International Journal of Mineral Processing | Year: 2014

Two distinctly different types of fish hooks are reported in literature, of which, the most common shape shows a decrease in efficiency to a minimum followed by a monotonic increase with size. The other shape exhibits an increase in efficiency with size till a critical point is reached; the efficiency then decreases till a minimum is reached, followed by a monotonic increase with size. Early theories to explain fish hook phenomenon through variable bypass are shown to be simple mathematical transformations. The 'mechanistic model' attributes occurrence of fish hook to a sharp fall in settling velocities in a centrifugal field with change in flow from Stokesian to transient regime. This explanation is shown to be dubious and not in conformity with known principles of physics. Also, the boundary layer model and the entrainment model require considerable refinement as mechanics of fluid flow around irregular particles is not well developed. It is shown that CFD simulations show efficiency curves with or without fish hook effect depending upon the assumptions for simulation. The method of sizing analyses in detection of fish hook effect is critically discussed. Most of the occurrences of fish hook are reported when sizing analyses are carried out by laser diffractometry using Fraunhofer approximation for data interpretation. When alternate sizing techniques, such as Andreasen pipette, disc centrifuge, Coulter counter, Dynamic Light Scattering etc. are used or when Mie theory is applied in Laser techniques, fish hook is not reported. Fish hook effect appears to be repeatable where it was studied. However, the conditions of reproducibility are to be specified by the proponents, if the phenomenon is to find general acceptance. It is likely that fish hook would continue to be regarded as a placebo. Its exclusion in simulation models does not appear to affect cyclone performance prediction. © 2014 Elsevier B.V. All rights reserved.

This note deals with reports of fish hook effect in classifier efficiency curves in recent publications in Powder Technology. Particle size characterisation methods followed in all of them are critically examined. It is shown that where the particle size distributions are determined by laser diffractometry, there is a distinct possibility that correct optical parameters of the test materials may not have been used. In one occurrence, where sieving is the method for measurement of particle size, sufficient details are not available. In another study, the method of size analysis is not disclosed. That is, in all the cases, the accuracy in estimating the efficiency and hence the quality of the data cannot be assessed. Consequentially, the results of all these reports on fish hook should be treated with caution. © 2015 Elsevier B.V.

Nageswararao K.,Taijasa Consultants
Powder Technology | Year: 2016

In early 1950s, Kelsall visualised the classification process in which a fraction of feed solids reaches underflow without undergoing classification in a hydrocyclone. We bring to light that this notion of 'bypass' is beyond experimental reach and is purely imaginary. This implies that the corrected efficiency, which is a key performance characteristic in all the successfully used hydrocyclone models and for which bypass is the foundation is also hypothetical.We discuss the reasons for the continued acceptance of a physical meaning to 'bypass' among the hydrocyclone practitioners. Following a discussion on why the classification process as visualised by Kelsall is purely notional, we bring to light the lack of an experimental basis for 'bypass' and why no physical meaning can be attributed to it. A mathematical interpretation of the method suggested by Kelsall for calculating corrected efficiency is also presented.A general method for normalising any function y = f(x) where x and y vary between xmin to xmax and ymin to ymax respectively into Yn(x) where the range of Yn is from 0 to 1, is then proposed. We show that this normalisation can be done in an infinite number of ways by choosing user defined normalising functions and demonstrate our method with a numerical example. Taking into consideration that classifications function is a special case of y = f(x), we show that it too can be normalised in an infinite number of ways. While illustrating our method with examples, we show that the procedure suggested by Kelsall is the simplest method to normalise actual efficiency curves. The similarity between bypass and normalising functions is illustrated with examples; it is shown that both of them are purely notional and have no physical significance.We bring to light the possibility of development of improved hydrocyclone models using a normalised cut size other than Kelsall cut size. Finally, we propose that the corrected efficiency be denoted as Kelsall efficiency and the corrected classification size as Kelsall cut size, as a tribute to the landmark contribution of Kelsall towards modelling of hydrocyclones. © 2016 Elsevier B.V.

Nageswararao K.,Taijasa Consultants | Karri B.,Indian Institute of Technology Hyderabad
Separation and Purification Technology | Year: 2016

Fish hook in classifier efficiency curves has been receiving attention in the last three decades, more so with the advent of laser diffractometry. In the first part of this paper, we analyse two occurrences of fish hook reported recently in Separation and Purification Technology. It is shown that in both the cases, inaccuracies in measured particle size distributions could be the likely cause of the observed fish hook. In the second part, we re-examine the present state of knowledge on fish hook including the limitations of experimental observations reported so far and the drawbacks of theoretical explanations. Finally, we provide a basis on why it is to be considered nothing more than a scientifically insignificant placebo. © 2015 Elsevier B.V. All rights reserved.

Recent report on fish hook effect by Zhu and Liow [Chemical Engineering Science 111(2014) 94-105] involves size analyses using laser diffractometry. Since their test materials contain a significant amount of submicron particles, the refractive index and extinction coefficient of the particles have a major impact on the size analyses results. As such, if correct optical parameters are not used systematic errors in size analyses are inevitable. The data of Zhu and Liow are subject to this uncertainty. As a result, while their size analyses are repeatable, the accuracy is indeterminate. The reproducibility of their data is also questionable. Consequently, their results and conclusions on fish hook are to be treated with caution. © 2014 Elsevier Ltd.

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