Minerals Engineering, Vol. 6, No. 2, pp. 207-210, 1993 0892-6875/93 $5.00+0.00 Printed in Great Britain Pergamon Press Lid

CORRESPONDENCE

Dear Sir,

I do not believe I have met Mr John Ralston, so cannot guess what lies behind his attack on my book (Flotation: theory, reagents and ore testing Book Reviews vol.6 number 1). Diatribes couched in terms of generalities are difficult to analyze, but a look at the very few specifics in the criticism may help the reader judge the validity and soundness of the reviewer's vaguer comments.

Taking citations in sequence, and ignoring the phrase taken out of context from pages 11 and 12, the next comment concerns figure 2, on p. 21, which is adapted, with permission, from Leja's Surface Chemistry of Froth Flotation (Figs. 7.1, 7.2, and 7.8). I agree with Leja's use of the "counter-ion" nomenclature, and question if Mr. Ralston's qualifications justify substituting "specifically adsorbed ions'.

As to comments on page 23, I fail to f ind the words "charge potential" or "thermodynamic work' . On p. 41, I f ind no mention of inorganic salts, and my only comment on frothers is that "preferred frothers used in flotation barely affect the liquid surface tension'. I quote Glembotskii, Klassen and Mokrousov, and Laskowski extensively to disqualify the use of contact angle measurements in the section headed "The nature of the bubble-particle bond' , which must have irritated the reviewer, considering his additional criticism of p. 41, in which he asserts the existence of stable contact angles at sub-monolayer coverage; should this be so, I would be grateful if he could provide me with references on contact angles measured at collector dosages of 5 to 15 g/t , on non-synthetic sulphide ore samples, or better, with the collector-frother complexes normally present on the mineral surface in a flotation cell.

The insinuation that I am unaware of the work of the most distinguished Australian researchers is negated by the first lines in Chapter 8, Flotation Modifiers, p. 101:

"The classical treatment of this subject is given by Sutherland and Wark (1955) in their book "Principles of Flotation'. By very elegant experiments based on contact angle measurements, they were able to show that flotation could occur only above a critical pH for a given mineral and reagent combination and dosage. Based on this concept, a rational explanation for the mechanism of activation and depression was developed which allows a systematic approach to the subject."

If his purpose is not to impugn me, then Mr. Ralston apparently thinks their work is more important to laboratory testing than to the modifier mechanisms. I do not agree.

His criticism of the selection of Case Histories is similarly subjective; I opted to pick detailed practical cases with real data, from the real world, that can be used either to illustrate points made in a lecture, or in classrooms and training programs to calculate out results and reach conclusions, or by a mill metallurgist who can browse through and see how others have approached a common problem.

The El Teniente case provides operating data on back-to-back acid and alkaline concentrators, and a set of duplicate acid and alkaline flotations tests made on drill core samples from a large sulphide ore body, which is unique. These are the only data I know that compare acid versus alkaline flotation using a broad range of ore samples. The Andina case, on the other hand, is classic for a sand/slimes separation; such detailed data has never been published. The San Manuel case is a snapshot of a traditional moly/copper separation, with a particularly good operator's review of depressants and modifiers.

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208 Correspondence

Mr. Ralston also demands size-by-size recovery tests, desirable if available, but in my opinion of very little general value because tests involving synthetic pulps are particularly tricky with sulphide ores - the sulphide mineral surfaces become oxidized within seconds whatever method is used to separate ore sizes, be it dry screening ground ore or wet screening pulp.

His must amusing suggestion is to f ind a firm willing to release millions of dollars worth of reports and data on an "in-depth study of liberation, particle size, reagent choice and addition, surface modification, grinding chemistry, the influence of circuit, etc. etc."; if Mr. Ralston has access to such, I certainly urge him to publish it.

Mr. Ralston's criticism is aimed mainly at the theoretical content, ignoring the audience targeted in the introduction: This text is primarily addressed to the mill metallurgist and to technical service and development engineers working for reagent suppliers, rather than to flotation researchers or mill" process designers.

His review ignores that another objective of the book is to provide guidance on what fundamental research can safely be extrapolated to industrial problems. My text, therefore, underlined ad nauseam that any conclusions based on contact angles cannot be extrapolated to mineral capture behaviour when the mechanism is electron transfer driven, but can be applied to oily flotation systems. In other words, what for an industrial minerals technologist is gospel, is nonsense to a sulphide flotation man, and vice versa. This dichotomy should be obvious even to a neophyte when the vastly different reagent surface coverage required to float the two ore types are compared: sulphide minerals require 5-45 g/ t of collector, while industrial minerals typically need dosages of 500-3000 g/t , and, just as important, sulphide ores always require a frother, while non-metallics frequently use only a collector.

I have left "littered with...grammatical errors..." to the last. This offensive comment can only be construed as malicious, and a gratuitous attack on Pergamon Press. The copy editor involved has handled over 50 of my English language publications and no publisher, on either side of the Atlantic, has ever complained.

Ronald D. Crozier 4 Daisy Lane Ridgefield CT 06877 USA

Dear Sir,

The article by M.T. I tyokumbul ("A new modelling approach to flotation design N Minerals Engineering 5 (6):685-693, 1992) immediately caught my attention (as any challenge to the status quo should) but disappointed me when I read it. I make the following points on issues in the sequence in which they appear in the document. Pn /m refers to page n, paragraph m.

. (P685/1). The first paragraph bodes ill for the rest of the paper: The division of the collection zone into a washing zone (between the froth/slurry interface and the feed point) and a recovery zone (feed point to the base of column) propagates a fallacious view of processes occurring in industrial columns. The mixing in the collection zone (correctly defined as the slurry phase above the base of the bubbler) is generally so intense that recovery of valuable particles occurs throughout both the author's zones, and minimal washing (presumably caused by the bias flux which is generally much smaller than the feed flux) occurs above the feed point. These distinctions can be made in pilot columns of small diameter only, where minimal axial mixing occurs.

Correspondence 209

. (P686/3). The observation concerning the relative sizes of the particles and liquid dispersion coefficients (as measured by Yianatos and Bergh, Proc. Int. Conf. on Column Flotation Sudbury:221, 1991) are valid and valuable. I hope these authors comment at some stage.

. (P687/2). The correlations for particle settling velocity do not take into account the presence of other particles, so must be inferior to the more sophisticated treatment of Masliyah (Chem. Eng. Sci. 34:1166-1168, 1979).

. (P687/4). The attack on the first order reaction rate analogy commences here, and would be welcomed if it produced an improvement. The result however is a model where the number of parameters to be determined by experiment is not reduced, and which produces essentially the same equations, so little is gained. In paragraph 5 the lack of dependence of k on temperature is used as an argument against the reaction analogy: this misses the point - we are using an analogy only, and since k for a particle of given size, surface properties and for given bubble properties is affected only by fluid viscosity (which has a relatively weak dependence on temperature), no- one expects a high "activation energy" for this process.

. (P688/2). The availability of bubble surface area is often a limiting step in column flotation (particularly for coal flotation with which the author presumably has a lot of involvement).

. (P689/1). The comment re the need to "consider the particles as starting with zero terminal velocity, which increases to V~,in a finite height" is a red herring; particles of typical flotation size reach their terfninal settling velocity within 5 mm and 0.04 S.

. (P689). The model developed on this page assumes plug flow behaviour of the particles which has been shown convincingly to be completely inaccurate. In fact most large diameter columns are almost perfectly mixed.

. (P690/2). The argument given here to simplify equation (14) is incorrect, kj does not change in the column, while the xj for floatable components "seen" by a bubble will change by an order of magnitude. Equations (15)-(17) must therefore be hopelessly inaccurate.

. (P691/1). The conclusion that we need a height for columns of 1.5 m is based on the following incorrect premises: (i) the column is operating under plug flow conditions, and (ii) that columns will be designed to produce 90% recovery of floatable components. A more realistic assumption of perfect mixing and 95% recovery produces a column height of 13 m - very similar to those used in the industry. Since the column will not be perfectly mixed, a height somewhat less than this (e.g. 10 m) will probably be sufficient. The axial mixing model would be used to obtain more accurate estimates.

Notwithstanding the above comments, the paper by Ounpuu and Tremblay (Proc. Int. Conf. on Column Flotation, Sudbury: 313, 1991) (where the need for tall columns is challenged) certainly calls our understanding of column design into question. We still have a lot to learn!

The second word ( 'unique') in the abstract of the paper by Yalcin ( 'Determination of flotation rate constants from semi-batch flotation data', Minerals Engineering 5 (6): 695- 706, 1992) indicates the Achilles heel of the paper: why produce a "unique" method for solution of a standard engineering problem, i.e. the estimation of parameters in a model from experimental data? Numerous software packages exist using standard methods based on well-defined statistics. These can easily be adapted to the problem, providing not only estimates but also confidence intervals for the estimates of the parameters based on several hypotheses for the underlying mechanisms applicable to the ore being tested. For example, the following hypotheses can be made:

210 Correspondence

Hypothesis HI: All particles have the same rate constant H2: Some particles float with kl, some don't float H3: Particles float with k 1 or k 2 H4: Particles float with k 1 or kz, some don't float

etc.

Parameters kl kl Q1

k 1 k 2

While H4 is seldom encountered, it should be possible to consider it; the published method cannot do so. Nor does the method allow one to test whether H1 or H2 are statistically more sound than H3 (the hypothesis on which the method is based).

My last paragraph is not a comment on Yalcin's paper. I would like again to raise the question: what is the point of obtaining parameter estimates on such data? My devil's advocate view is that it quantifies an empirical model relevant only to the performance of the ore in the batch cell and under the specific conditions tested, and tells us virtually nothing reliable about the true nature of the particles being tested, nor does it help in predicting the performance of the slurry in other conditions (e.g. in a continuous plant). Used with caution, it can for example be helpful in comparing reagent dosage regimes on a lab scale. Only when we have reliable models for froth phase behaviour will we move beyond the above limitations.

Michael H. Moys Department of Chemical Engineering University of the Witwatersrand Private Bag 3 WITS 2050 South Africa

The Editor invites correspondence on all matters related to papers and reviews published in Minerals Engineering.