Precious Metals '15 Conference Diary

Precious Metals '15 was held at the St. Michael's Hotel, Falmouth, Cornwall from May 11-12 2015.

Precious metals have fallen on hard times recently, with low prices exacerbated by rising production costs, so it was always inevitable that Precious Metals '15, the 5th in the MEI series of highly focused conferences, would be small (see also posting of 11th May). This year's delegate list comprised 29 delegates, representing 12 countries, compared with 51 delegates at Precious Metals '12 in Cape Town, and 55 at the last Falmouth conference in 2010.

Gold and PGM prices over past 3 years
Source: metalprices.com

Those that did attend were treated to 2 days of high quality papers on the recovery of gold and PGMs from primary and secondary sources, and I thank ZEISS for recognising the importance of this small conference by providing corporate support, and to International Mining for their media support.

Monday May 11th


Jacques Eksteen (right) with Dave Dreisinger and Corby Anderson

I welcomed the delegates to Falmouth this morning before introducing the first session chairpersons, Al Cropp of sponsor ZEISS, and Marian Tredoux of South Africa's University of the Free State. The conference then got off to a fine start with the keynote lecture, given by Jacques Eksteen of Curtin University, Australia, who reviewed recent innovations in the processing of difficult and low grade gold ores (see posting of 30 June 2014), such as the need to reject valueless gangue early in the process by jigging or electronic sorting, which will be reviewed in more depth in next month's Physical Separation '15 conference.

There are of course intense social pressures on the use of traditional cyanide leaching and there is an urgent need to find viable alternative lixiviants. Potential alternatives are environmentally friendly amino-acids which Jacques discussed in some depth. He also mentioned the use of alkaline sulphide leaching, which was discussed more fully by Corby Anderson of Colorado School of Mines, the other CSM, his paper dealing with many of the fundamental concepts behind this selective gold leaching and recovery system.

Following coffee Akira Otsuki of Université de Lorraine, France, showed how coal-oil gold agglomeration could be used to assist flotation to recover gold from refractory ores.

Steve Flatman, of Maelgwyn Mineral Services, UK then described the development of the Aachen Reactor high shear mass transfer device from its original concept twenty years ago to its acceptance in 2014 as an important unit process in one of the world’s newest large gold processing plants, producing over 1 million ounces of gold per year, where nineteen Reactors are installed in various applications. It is now possibly one of the most significant advances in gold processing since the introduction of the carbon-in-pulp process in the late 1970’s.

Akira Otsuki, Marian Tredoux, Steve Flatman and Neil Snyders

Sustainable development is a major focus of worldwide research in the gold mining industry. An alternative gold processing method which is more environmentally friendly than cyanidation and amalgamation is, therefore, imperative. This is the thrust for the work described by John Ajayi of the Federal University of Technology, Nigeria, who showed that the alkaline salts of groundnut and palm kernel are potential flotation reagent substitutes for the conventional synthetic reagents.

With Corby Anderson and John Ajayi

Though the pronounced effect of the caustic cyanide pre-treatment step for the AARL gold elution process has been widely investigated, research into the reaction kinetics of this step are lacking and a set industry standard is yet to be determined with regards to pre-treatment time and reactor conditions. Neil Snyders of the University of Stellenbosch, South Africa presented results on a study of the effects of the pre-treatment temperature, contact time and agitation on the gold elution recovery and in light of these results, proposed pre-treatment mechanisms were reviewed and discussed.

Dave Dreisinger prepares for his talk

Dave Dreisinger of the University of British Columbia presented the first paper of the afternoon session, chaired by James Vaughan of the University of Queensland, on the dissolution of gold during the pyrite oxidation reaction. Although numerous papers discuss the mechanism of alkaline oxidation of pyrite there is limited information available describing the actual kinetics of the pyrite sulphide to thiosulphate reaction. A previous investigation in this series determined the rate of sulphide sulphur oxidation and thiosulphate yield in the reaction of pyrite with sodium hydroxide under various testing conditions. The goal of the current study was to validate these rates using two different gold-containing pyrite concentrates, a further objective being to investigate the simultaneous dissolution of gold with in situ formed thiosulphate during pyrite oxidation.

Jacques Eksteen of Curtin University, presented his second paper of the day, on gold extraction from a gold-copper concentrate in solutions containing caustic soda and low free cyanide. Copper-gold ores containing significant amounts of cyanide-soluble copper with gold can lead to high cyanide consumption with low gold extraction in the cyanidation process of gold. Cyanide should be destroyed before the discharge of tailings containing metal-cyanide complexes. This significantly increases the cost to the gold mining companies to cover both the cyanide consumption in the leaching stage and the additional cost of the required reagents for cyanide destruction. Results on the selective cyanide leaching process of gold over copper from a copper-gold concentrate were presented.

Gawen Jenkin (left) with James Vaughan

Gawen Jenkin of the University of Leicester, UK, presented an interesting paper showing how ionometallurgy could offer a new set of environmentally benign tools for metallurgists to augment existing techniques. Ionic liquids are anhydrous salts that are liquid at low temperature. They are powerful solvents and electrolytes with potential for high selectivity in both dissolution and recovery. Those pioneered at Leicester are deep eutectic solvents (DES), which are environmentally benign, chemically stable and the components are already produced in bulk at low cost. Gold (electrum), tellurium, galena and chalcopyrite, as well as tellurobismuthite (Bi2Te3), are soluble in DES by oxidation at 45-50°C at very favourable rates compared to bio-oxidation or high-temperature hydrometallurgy. Gold can be recovered from solution by electrodeposition. Pyrite is notably insoluble under the same conditions, but pyrite, arsenopyrite, and indeed any sulphide, can be selectively dissolved by electrolytic reduction in a DES, thus suggesting a protocol whereby gold inclusions can be liberated by reduction and then dissolved by oxidation.

The final paper of the day was presented by Seija Kurki, of Outotec, Finland. She introduced the new Outotec® Hydrometallurgical Precious Metals Process, which enables cost-efficient fully hydrometallurgical precious metals recovery from copper electrorefining anode slimes. The process is based on leaching of copper, selenium, silver and tellurium in sequential H2SO4 leaching steps, ensuring high recovery rate and high purity of the precious metals and saleable side products. H2SO4 solutions can also partly be recycled back to electrolysis or electrolytic solution purification.

Seija Kurki and Kristian Lillkung of Outotec with James Vaughan (left)

The majority of the delegates joined me in the late afternoon for the usual MEI conference walk along the coast to old Falmouth, and a chance for everyone to get to know each other over drinks at the 17th century Chain Locker pub by the inner harbour (see more photos on the 12th May posting).

At the 16th century Pendennis Castle

Beers at the Chain Locker

Tuesday May 12th
A later start than scheduled due to a speaker withdrawing at the very last hour, which was disappointing as two of the co-authors are based at the nearby Camborne School of Mines. The morning session was chaired by two of yesterday's speakers, Corby Anderson and Dave Dreisinger.

Neil Snyders, of the University of Stellenbosch, South Africa, showed how several individual gold cyanidation processing techniques were combined into different flowsheets to recover platinum group metals (PGMs) from dilute cyanide leach solutions. Four flowsheets were developed, consisting of a SART (sulphidization, acidification, recycling, and thickening) process, precious metal recovery by either adsorption onto activated carbon or ion exchange resins, followed by elution, and final metal recovery through thermal hydrolysis and/or electrowinning. These flowsheets were simulated in combination with economic analyses to determine the option that yields high PGM recoveries with maximum net present value (NPV). Ion exchange resin technology was found to be best, with a NPV significantly higher than for the carbon-based flowsheets. The best flowsheet option utilized the strong-base Amberlite PWA 5 ion exchange resin and elution with zinc cyanide. The proposed final metal recovery was by means of thermal hydrolysis and electrowinning. This option requires low capital and operating costs, while achieving high PGM recoveries.

There has been a steady increase in the amount of Precious Metals (PM) and Platinum Group Metals (PGM) being recovered from secondary sources in recent years, driven in large part by high commodity prices and a rapid growth in automotive catalyst recycling. The increasing costs and complexities associated with the primary extraction of PM/PGM and the lower costs and environmental impacts of recycling-based extraction have provided added incentives for this growing trend. As a result, the last few years have also seen a substantial uptake in PM/PGM recovery using Tetronics’ DC plasma smelting technology, particularly for the recovery of PM and PGM from automotive catalysts, chemical catalysts and electronics waste as a result of the ability of Tetronics’ plasma smelting technology to achieve high recovery rates from a range of materials. Saeed Ismail, of Tetronics International, UK discussed Tetronics’ plasma smelting technology and gave details of plant performance in the recovery of PM/PGM from catalysts and other materials. The range of metals applicable to this technology were also discussed and results from a number of secondary sources provided.

Ancuta Musina and Saeed Ismail prepare their presentations

The ZEISS Mineralogic Mining was showcased at Process Mineralogy '14 . The release of ZEISS Mineralogic Mining has provided an extra dimension to the world of Automated Mineralogy (AM) through the utilization of a fully quantitative Energy Dispersive Spectroscopy (EDS) mineral classification system and advanced image analysis capabilities. Unlike the current AM techniques, the mineralogy is classified by using the wt% contribution of the elements present, and thus, the minerals stoichiometry. This method provides the operator a more accurate, flexible and efficient method for generating mineral classifications. ZEISS have developed the capability to correlate Light Microscope (LM), Scanning Electron Microscope (SEM) and AM coloured mineral maps for more efficient and seamless analysis between these techniques in order to make the most of the information each technique provides. Shaun Graham of ZEISS presented an example of correlating these techniques for locating and identifying Base Metal Sulphides (BMS) and Platinum Group Minerals (PGM) from PGE-bearing chromitite from the Berit ophiolite, Turkey, showing how ZEISS is able to provide a fully automated and correlated analysis across LM - SEM - AM techniques.

With Ben Tordoff, Shaun Graham and Al Cropp of ZEISS

Ancuta Musina of Magpie Polymers, France, presented results on the recovery of PGMs from industrial effluents by an innovative process involving sorption on a functional polymer, MP-101, containing aminophosphine groups. Sorption performances and mechanisms onto MP-101 were investigated and compared to other commercial PGM sorbents with a targeted metal, the Pd. The results showed higher sorption capacities of MP-101 because of the high phosphine groups content, giving it potential applications in refining and mining industries. Separation of metals loaded MP-101 from effluents was achieved by flocculation-decantation processes and PGMs were recovered in metallic form by incineration. This process combining sorption and decantation was successfully applied on two industrial solutions from mining and recycling industries, highlighting the selectivity of MP-101 toward precious metals.

Ancuta Musina (right) with Marian Tredoux

The Bushveld Igneous Complex (BIC) situated in Northern South Africa is the largest deposit of platinum group elements (PGE) in the world. Froth flotation is used to beneficiate these PGE ores in the BIC due to their close association with the base metal sulphides. The performance of the froth flotation process is largely dependent on the chemical additives used in the process. Consequently, chemicals, especially collectors, have been used in mixtures to combine the individual properties of the respective chemicals. Westhein Maree of the Cape Peninsula University of Technology, South Africa, reported on the concentrate grades and recoveries achieved in batch flotation tests using various molar mixture ratios of thiol collectors on a Merensky Reef platinum ore. Collector mixtures of a xanthate (SIBX) with a dithiophosphate and a dithiocarbamate at 20, 40, 60 and 80 mole% ratios were considered.

The final session of the conference, chaired by Saeed Ismail of Tetronics International, commenced with a presentation by Ryo Kasuya of the National Institute of Advanced Industrial Science and Technology, Japan, who discussed the dissolution of platinum in catalyst materials. Currently PGMs are recovered after dissolution in strong acids that contain toxic oxidizing agents such as aqua regia. To avoid the use of such toxic agents a route to dissolve PGMs in hydrochloric acid via complex oxides was proposed. The Pt complex oxides were prepared by calcining mixtures of Pt/Al2O3 and alkali metal salts at 600-800°C in air and they were then dissolved in HCl. The results showed that Pt in the calcined samples easily dissolved in HCl, and the Pt solubility recorded over 80% after dissolution at 80°C, so the process is probably applicable to the Pt catalysts after refining.

Processing of platinum-group element (PGE) ores is based on the assumption that the primary association of these elements is with sulphide, i.e. covalent bonding between PGE cations and S2- anions. In order to explain the tenor of the PGE in economically viable ores, the PGE must favour such bonds 105 times over either remaining in solution in the magma or entering co-precipitating silicates (olivine, pyroxene, plagioclase) and oxides (chromite). Experimental work presented by Marian Tredoux of the University of the Free State, South Africa, has not confirmed this, and led the authors of the paper to speculate that a more primary association might be involved, i.e. that the formation of PGE minerals are preceded by nano-entities. The presentation reported on an experiment which proved that nano-association of Pt and As does occur at magmatic temperatures long before sperrylite can stabilise. The results could have profound influence on the design of future PGE beneficiation plants.

Direct electrowinning (EMEW) is an emerging technology within the precious metal extraction sector with a number of applications including silver recovery. In the final presentation of the conference Arun Vathavooran, of Tetratech Minining & Minerals, UK, discussed the testing and scale up of the EMEW technology for a high grade, low throughput silver deposit in Yukatia. The testwork and the analysis has indicated that the EMEW technology can be effectively utilized for selective silver recovery from polymetalic ores with potential operating cost savings for silver head grades as low as 150g/t.

Ryo Kasuya with Arun Vathavooran

Amanda closed the conference and invited delegates to attend Precious Metals '17 in Falmouth in two years time, when hopefully the market situation will have improved. We have thoroughly enjoyed the two days of Precious Metals '15 with a great group of people, who all seemed to appreciate the small focussed conference format.

The papers from the conference, in unrefereed form, are available from MEI, and authors have been invited to submit their final papers for peer-review, for possible publication in a special issue of Minerals Engineering.

High resolution image of all photos in the blog can be obtained by clicking on the image.