N.O. Lotter’s scientific contributions

The formulation and use of mixed collector suites in sulphide flotation can be traced back as far as 1957 with the work of Glembottskii and his co-workers in Moscow. The subject was reviewed by Lotter and Bradshaw in 2010, and proposed a synergy between ideally-selected different reagent types to deliver improved performance, in particular the in-situ catalysis of a xanthate to dixanthogen by a dithiocarbamate. Correctly formulated for a particular application, an optimally arranged mixed collector suite delivers the following improvements to the flotation process: • An increase in the flotation kinetics • Improvement in coarse, or middling, particle recovery • Reduction in total dosage requirement • Best results are found from testing the ratios of the collectors. This is of critical importance. In all an optimally formulated mixed collector system usually delivers an increase in paymetals recovery of 1-4% absolute, with a gain in concentrate grade in the range 10-20% relative. In recent years, the addition of semi-conductor theory and associated electrochemistry has contributed significantly to this toolbox by way of identifying optimum domains of Eh and pH for the flotation of individual minerals. In the case of mixtures of minerals, this is where the mixed collector system delivers significant value by way of presenting a mixed set of collector radicals and dimers in a common Eh and pH domain for overall performance. This calls on a fundamental understanding of organic chemistry, mineralogy, semiconductor theory, and electrochemistry. Whereas the selection of grinding media type effectively influences the Eh, the pH can be easily controlled by pH modifiers. The theory of this system, and some industrial case studies, are discussed.

The powerful modern toolbox of hybrid Process Mineralogy for flowsheet development uses best practice sampling as one of its tools. In this paper, the three key components of best practice sampling are reviewed with case studies. These three components are:. 1.Minimum sample mass.2.Rules of unbiassed sampled extraction.3.The safety line.These excellent models and rules are not commonly taught in undergraduate programmes. In this review paper, which is intended as an introductory reference for those practitioners in Process Mineralogy who have not had exposure to the sampling models, simple and practical explanations are presented for reference. It is shown that finer particle size distributions lead to smaller minimum sample mass requirements.While sampling theory allows us to estimate the error involved obtaining a mass of sample for mineralogical analysis it is also useful to account for errors in the process mineralogy measurements themselves. Examples of the confidence intervals on liberation measurements made on high- and low-grade samples are provided to illustrate the importance of sample size-specifically measuring sufficient numbers of particles-in these analyses.

Modern Process Mineralogy has been making significant advances in methodology and data interpretation since it was assembled in the mid-1980s as a multi-disciplined team approach to obtaining mineralogical information from drill core and plant samples so as to infer the metallurgical processing requirements of that ore. This hybrid discipline consists of teams that include geologists, mineralogists, samplers, mineral processors and often others, working together. The degree of cross-training, communication and trust dictates the potential capacity of the team and it is possible to develop technical capabilities that surpass those of conventional teams. A pivotal tool for technically efficient and plant-oriented process mineralogy is, of course, the use of modern, automated laboratory technology. In these cases, process mineralogy, though associated with some capital investment, is a valuable risk reduction tool and an operations optimization tool for any mining company, not only in terms of finances but also in terms of human and intellectual capital. However, if the teams are dysfunctional and information is not interpreted correctly due to limited experience in the team or less than best practice, or it is not implemented or used, much of the value can be lost. Process Mineralogy can then be regarded as ‘time consuming and expensive’. In this paper, the business value of best practice Process Mineralogy is outlined and discussed. Case studies that include ‘green fields’ new design applications and ‘brown fields’ interventions to mature operations have been selected to demonstrate the tremendous financial value that can be achieved are presented, along with those where costly disasters could have been averted. The list is not intended to be exhaustive or complete, and the reader is referred to the extensive literature available. Examples are selected for this publication specifically to illustrate the delicate balance between generating additional business value through potentially expensive mineralogical analyses and the lost opportunities of underperforming flowsheets, unanticipated losses due to high feed variance, inadequate liberation or deleterious minerals, over-reagentised circuits, or extra costs of unnecessary or underutilised equipment.

Recoverable economic copper sulphide minerals such as chalcopyrite, bornite, chalcocite and covellite often occur together in varying proportions in the major copper-bearing ores, and have individual flotation requirements and characteristics. Pyrite also occurs in these ores to varying extents as the sulphide gangue, and is problematic because of its natural tendency to float quickly and easily. In a bulk sulphide float, selectivity against pyrite is desirable, particularly if it does not host other paymetals such as gold or silver. At the same time it is a requirement to float all of the copper sulphides despite their electrochemical differences. The electrochemistry and semiconductor properties of these minerals are reviewed, together with implications for flotation with and without collector addition. Mixed collector systems for the improved flotation of these sulphides are proposed as a solution. The use of xanthate and dithiophosphate in the collector suite allows the co-existence of dixanthogen and free dithiophosphate radical because the latter has a higher redox potential requirement than xanthate to oxidize to the dithiolate. Because some of these minerals require dixanthogen, and others, free thiolate, to generate surface hydrophobicity, a bulk flotation of all the species becomes possible in the overlapping area of Eh and pH between the two dithiolate equilibrium lines on the Pourbaix Diagram. The arsenic-signature copper minerals are added to the study, since many copper operations encounter arsenic as a penalty element in the saleable concentrate. It is shown that the addition of arsenic to the copper and iron sulphides alters the semiconductor and electrochemistry properties, and in turn, its flotation characteristics. The degree of mineral association and liberation between these minerals can be a complicating factor due to textural associations, and should also be considered in the process as a next step.

The Kamoa resource, located in the Democratic Republic of the Congo, contains an array of copper sulphide minerals which are present as small grains, averaging 10–27 μm. An initial flowsheet was developed in 2011/12 for the prefeasibility study that was robust enough to handle flotation of all the copper sulphide minerals. Copper recoveries of the flowsheet were 85.4% for the hypogene ore and 83.4% for the supergene ore. Further work on the flowsheet required reduction of the SiO2 grade of the concentrate, which at 19.1% negatively affected the downstream smelter processing, and also required improvement to copper grades and recoveries given the high grade of the ore. When new sample material became available as part of the Phase 6 drilling program, a fundamental reassessment of the ore and its flotation behaviour was conducted. Although mineralogical characterisation of the ore and liberation of the sulphides was quantified in previous phases of work, there was little understanding of the kinetics of each of the copper sulphide minerals and how they performed in the flowsheet. Comprehensive flotation kinetic tests at various primary grind sizes were performed. The corresponding timed concentrates of the three best performing grinds were characterised by QEMSCAN on a size-by-size basis to fully understand the flotation kinetics and liberation characteristics of the various copper sulphides. A simple and practical recovery model using minerals, particle size and liberation and association was developed from these data, and various flowsheet configurations were simulated. These simulations led to some robust process implications completely rearranging the flowsheet from the previous iteration into a more simple and economic configuration with better performance. The modelled data was confirmed with practically achieved data, extending the use of process mineralogy as a valid, predictive tool in process design. Additionally, the simulations using mineralogical, reduced empirical flotation testing needed to develop the new flowsheet.