minerals Article Optimisation of a Multi-Gravity Separator with Novel Modifications for the Recovery of Ferberite Robert Fitzpatrick 1,*, Patrick Hegarty 1, Keith Fergusson 1, Gavyn Rollinson 1, Weiguo Xie 1 and Treve Mildren 2 1 Camborne School of Mines, University of Exeter, Penryn Campus, Exeter TR10 9FE, UK; [email protected] (P.H.); [email protected] (K.F.); [email protected] (G.R.); [email protected] (W.X.) 2 Gravity Mining Ltd., Glencarrow, New Road, Stithians TR3 7BL, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-(0)1326-254-141 Received: 15 February 2018; Accepted: 20 April 2018; Published: 2 May 2018 Abstract: Tungsten is considered by the European Union as a critical raw material for future development due to its expected demand and scarcity of resource within Europe. It is therefore, critical to optimize European tungsten operations and maximise recoveries. The role of enhanced gravity/centrifugal concentrators in recovering tungsten from ultra-fine fractions should form an important part of this aim. Reported herein are the results of investigations to improve efficiency of Wolf Minerals’ Draklends mine, a major European tungsten mine, by recovering saleable material from a magnetic waste stream of a low-intensity magnetic separator using an enhanced gravity concentrator. The mine hosts wolframite and ferberite as the main tungsten bearing mineral species. A Mozley multi-gravity separator (MGS) C-900 was selected as it is suited to exploiting small variations in mineral density to affect a separation. Working with a current manufacturer, a novel scraping blade system was tested. To assess the MGS in a statistically valid manner, a response surface methodology was followed to determine optimal test conditions. The test programme showed that the most important parameters were drum speed and wash water rate. Under optimal conditions the model predicted that 40% of the tungsten could be recovered above the required grade of 43% WO3. Keywords: centrifugal gravity separation; tungsten-bearing minerals; quantitative mineralogy; response surface method; central composite rotatable design 1. Introduction 1.1. Background Tungsten is recognised by the European Union as one of 27 critical raw materials for future economic growth and prosperity. Tungsten has a wide range of properties due to its unique physical properties which include extremely high melting point and hardness when alloyed with carbon. The principle application of tungsten is as a cemented carbide followed by alloys with various other metals. This economic importance combined with a high reliance on imports into the EU, have led to the classification as a critical metal, see Figure1. The European Union imported 15,000 tonnes of tungsten concentrate in 2012, relying on Russia for 98% of the total. World supply of tungsten is dominated by China which in 2012 produced 85% of global supply and consumed 51% [2]. To help reduce the reliance on import of tungsten the European Union has providing funding for the OptimOre project (Horizon 2020 research and innovation programme grant 642201). One objective of this project is to optimise the processing of tungsten ores to help diversify global supply and make European deposits more competitive. To support this objective Minerals 2018, 8, 191; doi:10.3390/min8050191 www.mdpi.com/journal/minerals Minerals 2018, 8, 191 2 of 18 this article reports the results of using an enhanced gravity concentrator to improve the recovery of tungsten at the Drakelands mine, a major European mine site operated by Wolf Minerals Ltd. Specifically, investigations were undertaken to optimise recovery of tungsten-bearing minerals using a Mozley MGS with novel modifications. The objective of the work was to assess the feasibility of producing a saleable product from this stream and to compare the novel modifications to the MGS withMinerals the 2018 conventional, 8, x FOR PEER system. REVIEW 2 of 17 Non-critical raw materials 5.0 LREEs Critical raw materials HREEs 4.5 Antimony Magnesium 4.0 Phosphorus Bismuth 3.5 Niobium 3.0 Borate Natural graphite Scandium 2.5 PGMs Indium Beryllium Supply Risk Supply 2.0 Bauxite Germanium Tungsten Helium Vanadium Cobalt Baryte 1.5 Sapele wood Hafnium Gallium Phosphat e rock Natural cork Coking coal Silicon Fluorspar Natural Rubber Rhenium Manganese 1.0 Lithium metal Tantalum Natural Teak… Tellurium Tin Molybdenum Chromium Feldspar Magnesite Iron ore Gypsum Sulphur Potash 0.5 Talc Silver Perlite Kaolin Titanium Selenium Aluminium Silica sand Nickel Gold Aggregates Diatomite Zinc Copper 0.0 Bentoni… Limestone Lead 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Economic Importance FigureFigure 1.1. European Commission criticality assessment for raw materials withwith criticalcritical materialsmaterials highlightedhighlighted inin red,red, tungstentungsten hashas beenbeen furtherfurther highlightedhighlighted byby thethe authorsauthors [[1]1].. The European Union imported 15,000 tonnes of tungsten concentrate in 2012, relying on Russia The feed to the LIMS separator is gravity concentrate which has been roasted under reducing for 98% of the total. World supply of tungsten is dominated by China which in 2012 produced 85% conditions to convert hematite (Fe O ) to magnetite (Fe O ). The LIMS magnetic stream is currently of global supply and consumed 251%3 [2]. To help reduce3 4 the reliance on import of tungsten the treated as waste but has a high tungsten grade (over 20%) and so represents a measurable loss within European Union has providing funding for the OptimOre project (Horizon 2020 research and the operation. If saleable material can be recovered from this stream in an efficient manner it will innovation programme grant 642201). One objective of this project is to optimise the processing of increase profitability at the mine site and help to decrease the energy per tonne of concentrate produced. tungsten ores to help diversify global supply and make European deposits more competitive. To Both of these outcomes are important to help ensure the competitiveness of European based mines in a support this objective this article reports the results of using an enhanced gravity concentrator to global market dominated by a small number of countries. improve the recovery of tungsten at the Drakelands mine, a major European mine site operated by 1.2.Wolf Multi-Gravity Minerals Ltd Separator. Specifically, Device investigations were undertaken to optimise recovery of tungsten- bearing minerals using a Mozley MGS with novel modifications. The objective of the work was to assessThe the multi-gravity feasibility of separatorproducing (MGS) a saleable also knownproduct as from the enhanced-gravitythis stream and to separator compare (EGS) the novel was designedmodifications in the to late the 1980’sMGS with to early the 1990’sconventional for the system. concentration of fine and ultra-fine heavy minerals, withThe early feed applications to the LIMS focusing separator on cassiterite,is gravity concentrate chromite, celestite which has and been magnetite roasted [3 under]. The reducing machine operates on similar principles to a shaking table but utilises rotational motion to generate centrifugal conditions to convert hematite (Fe2O3) to magnetite (Fe3O4). The LIMS magnetic stream is currently forcestreated greater as waste than but those has a of high gravity. tungsten This grade increase (over in force20%) allowsand so forrepresents more efficient a measurable separation loss atwithin finer the operation. If saleable material can be recovered from this stream in an efficient manner it will increase profitability at the mine site and help to decrease the energy per tonne of concentrate produced. Both of these outcomes are important to help ensure the competitiveness of European based mines in a global market dominated by a small number of countries. Minerals 2018, 8, x FOR PEER REVIEW 3 of 17 1.2. Multi-Gravity Separator Device The multi-gravity separator (MGS) also known as the enhanced-gravity separator (EGS) was designed in the late 1980’s to early 1990’s for the concentration of fine and ultra-fine heavy minerals, with early applications focusing on cassiterite, chromite, celestite and magnetite [3]. The machine Minerals 2018, 8, 191 3 of 18 operates on similar principles to a shaking table but utilises rotational motion to generate centrifugal forces greater than those of gravity. This increase in force allows for more efficient separation at finer sizes. The main main design and operating variables of the MGS MGS are are summarised in Figure 2 2 alongsidealongside thethe principle forces acting on a particle. Figure 2. MGS operating and design (denoted with *) variables and principle forces (Force Diagram modifiedmodified after [[4]).4]). Due to the large number of variables and interactions, most modelling work for the MGS has consisted of determining optimal conditions using regression analysis. A A review review of of this research literature reveals that the rotational velocity of the drum has been identifiedidentified as the most important factor on grade and recovery [[55–88].]. Both [[5,7]5,7] concluded that rotational velocity and pulp density were the most important factors with regardsregards toto optimisation. optimisation. In In [6 ][6] the the conclusion conclusion was was that rotationalthat rotational velocity velocity had a significanthad a significant impact whereasimpact whereas inclination inclination and wash and water wash flowrate water flowrate had a trivial had effecta trivial on effect performance. on performance. In [8] it was In [8] observed it was observedthat wash that water wash had anwater important had an interaction important withinteraction performance with performance at high wash at water high flowrates wash water and flowrateshigh rotational and high velocity. rotational The authors velocity. of [5 ]The also authors observed of an[5] important also observed interaction an important between washinteraction water betweenand pulp wash density.