Saturday, 10 September 2011

Challenges in copper flotation

How flotation continues to, and needs to, evolve is perhaps no better illustrated than by the case of copper, the commodity where it finds its most important use, and produces the greatest annual tonnage. Total world tonnage is around 15 million tonnes per year, and with developing countries demanding more each year, this is bound to increase.

The development of froth flotation had an enormous impact on copper mining, enabling chalcopyrite and other sulphides to be efficiently separated from ores of relatively low grade and fine grain size. Until 1907 practically all the copper mined in the USA was from underground vein deposits, averaging 2.5% Cu, whereas flotation allowed the mining of porphyry deposits of only 0.6% grade.

Nevertheless, the flotation of copper ores still presents challenges to the minerals engineer. Copper is characterised by having a number of economic ore minerals, many of which may occur in the same deposit, and in various proportions according to depth. Chalcopyrite is the dominant primary mineral, and floats readily with anionic collectors such as xanthates, as do other economic sulphides such as chalcocite, covellite and bornite. However many other minerals respond poorly to flotation, and may even impede the flotation of readily floatable minerals, as at Palabora in South Africa, where the ore contains small amounts of valleriite, a copper-iron sulphide containing Mg and Al groups in the crystal lattice, and occurring intergrown with other sulphide minerals. Valleriite has a low flotation response, and it is a very soft mineral. During comminution breakage occurs along the soft and friable valleriite, leaving grains of other copper sulphides with a valleriite coating, preventing these grains from floating.

But perhaps the biggest challenge in copper flotation is that of copper oxides which have very poor floatabilities and are usually treated by hydrometallurgical methods, such as vat and heap leaching, sometimes in association with solvent extraction. Oxidised minerals such as malachite and azurite can to some extent be upgraded by flotation after sulphidisation, but minerals such as cuprite and chrysocolla, respond very poorly.

There are a number of papers on copper flotation scheduled for Flotation ’11 but two of particular interest to those involved with oxidised and oxide copper minerals will be given by workers from Australia, who have found encouraging flotation response of malachite and chrysocolla using hydroxamate reagents.

Also of interest will be papers on the depression of the arsenic-containing copper sulphides, tennantite and enargite. Arsenic is a toxic, volatile element that has little commercial use. This is causing some concern to copper smelters as they are obliged to dispose of arsenic materials produced as a by-product to the smelting process in accordance with ever tightening environmental guidelines. The onus is moving back to concentrate producers to remove toxic elements, such as arsenic, earlier in the concentrate supply chain.

To complicate matters, the common copper-arsenic bearing minerals in copper ores, enargite (Cu3AsS4) and tennantite (Cu12As4S13), also contain significant amounts of copper; 48.4% and 51.6% respectively. Removal of these minerals from the concentrate removes valuable metal, hence income. There is a dearth of literature concerning the selective removal of enargite and tennantite from sulphide ores, but what there is reports some success using either chemical oxidation or potential control. This has applications in mines such as Rosebery in Australia, where arsenic levels in concentrate are becoming prohibitive.

A paper by workers from MMG Rosebery, the JKMRC and the University of Queensland, will present a review of the literature with regard to the selective removal of the arsenic bearing minerals from copper ores during flotation, summarizing the pros and cons of the various options, and will recommend a strategy for evaluation, while a paper from Japan will also consider the selective flotation of an arsenic rich sulfide copper ore.

8 comments:

  1. The challenges in copper flotation that we came across during testing may be grouped as internal, species, and associated ganuge. Still we have not fully understood and each case is unique. Every project/program is a learning curve for me.
    1] Internal: Due to mixed copper ore minerals like oxides, carbonates, chrysocolla associated with primary-secondary copper sulphides like chacocite, covellite, chacopyrite, bornite. Single flotation step may not yield results
    2] Species: Due to a group/ species of base metal sulphides with or without precious metal bearing eg Cu, Zn, Pb, Fe sulphides
    3] Associated Gangue: ore minerals associated with carbonates, quartz/feldspars, ironoxides, mica schists besides with clay, talc and carbonaceous material.
    In addition to the above due to labile sulphides galvanic interaction with grinding media and tarnished surfaces[ transition zone of oxide- sulphide lode] the selectivity suffers besides consuming more steel, lime and surfactants.
    Besides above we have to look into environmental issues like AMD, reducing the CAPEX costs for sustainability as running small mills during economic recession is a big challenge.
    It is duty of mineral processing professionals to compliment the concerned for taking up this topic for discussion.
    B P Ravi, Indian Bureau of Mines

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  2. Barry,

    Thanks for asking. Some that come immediately to mind include...

    - Recovery of liberated + 150 micron copper minerals.
    - Consistently high recovery of 100 X 150 micron copper minerals.
    - REcovery of these coarser particles is particularly critical as we push throughput in concentrators to maximize overall metal production, but move to coarsesr grinds.
    - High Moly recovery from selective Cu-Mo separation.
    - Copper mineral / pyrite separation particularly with high pyrite / active pyrite porphyry ores and skarn ores.
    - Dealing with partially oxidized ores

    R. Seitz, Principal Advisor- Processing, Rio Tinto, USA

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  3. In the spirit of this question, it's interesting to revisit the paper, "Malghan, S.G., Treatment methods for difficult-to-float copper porphyry ores, Mining Eng., Sept., 1986, 905-910."

    "Summary

    Flotation of difficult-to-float copper porphyry ores indicates that the primary reasons for the problems are mineralogical diversity, metallurgical factors, and process limitations. A detailed analysis of these factors indicates that, most often, difficulties in the flotation of copper porphyry ores arise due to mineralogy and metallurgy interrelated factors. Identification of the exact nature of the problem is often the most difficult task, requiring a detailed knowledge of the mineralogy and assay of concentrates, tailings, and middlings of the ore in question. Recently developed X-ray-based mineralogical analysis
    systems and surface analysis systems such as ESCA and SEM-microprobe have allowed metallurgists to develop detailed knowledge of minerals. Technical developments in the ore processing practice - metallurgical and process optimization - have further advanced our chances of success in treating difficult-to-float porphyry copper ores."

    It's interesting to reflect on how much we have progressed in the 25 years since this paper was published in 1986. While our theoretical understanding may have progressed considerably, what are thoughts on industrial practice?
    Robert Seitz, Rio Tinto, USA

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  4. I think it will be important to employ high chromium content steel ball. This kind of balls help to treat copper-zinc ores with variable content of pyrite. I mean the selectivity is enhanced. In this case, flotation operators must learn to understand the information provided by ORP sensors. In my opinion this is valuable information, which is easily overlooked.

    Other importnat point is related to the treatment of copper deposits characterized by the presence de copper minerals that contain arsenic. Arsenic-bearing sulfide minerals like tennantite are difficult to separate from other non-arsenic sulphide copperminerals in conventional copper flotation circuits. The most successful approaches to effecting separation appear to be based on selective oxidation techniques and the use of pulp potential. The situation can be more complex if the ore body has enargite, which is an arsenic-bearing sulphide mineral similar to tennantite, from other sulphide copper minerals based on the flotation response of various copper minerals and enargite to changes in pulp potential. Then, the flotation must be capable to produce copper concentrates of low and high arsenic content. The second one must be treated by an alternative process.

    The treatment of copper ores with some degree of oxidation is other problem. In this case, the mine usually tries to send to the flotation process a material easy to treat. Nevertheless, the presence of oxide copper minerals is a problem. Then, the manufacturers of flotation reagents must try to develop effective collectors to float this kind of altered material.
    Jorge Ganoza, Bear Creek Mining Company, Peru

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  5. Jorge, be careful thinking high chrome balls are a cure all, it depends on the mineralogy. I have seen cases where using high chrome balls decrease selectivity also, so it is ore body and plant design specific. I agree with your comment on the effect of ORP.
    Roger Strickland, Ok Tedi Mining Ltd, Australia

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  6. There are many more comments on LinkedIn at: http://www.linkedin.com/groups/What-are-main-challenges-in-2809754.S.70087244?qid=14cf8a90-fb46-4355-a855-88789c7f6e34&trk=group_most_popular-0-b-ttl&goback=%2Egmp_2809754

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  7. The papers from Flotation ’11 have now been published in Minerals Engineering, and can be found at ScienceDirect

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  8. is Na2S better or NaHS for sulfidation of copper oxide ore? kindly suggest which is good & y?

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