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Effect of Particle Size and Grinding Time on Gold Dissolution in Cyanide Solution

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Effect of Particle Size and Grinding Time on Gold Dissolution in Cyanide Solution minerals Article Effect of Particle Size and Grinding Time on Gold Dissolution in Cyanide Solution Jessica Egan 1, Claude Bazin 2,* and Daniel Hodouin 2 1 Produits Chimiques Magnus, Boucherville, QC J4B 5Z5, Canada; [email protected] 2 Department of Mining, Metallurgical and Materials Engineering, Laval University, Quebec, QC G1V 0A6, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-418-656-5914 Academic Editor: William Skinner Received: 15 May 2016; Accepted: 30 June 2016; Published: 7 July 2016 Abstract: The recovery of gold by ore leaching is influenced by the size of the particles and the chemical environment. The effect of particle size on the dissolution of gold is usually studied using mono-size particles as the gold in solution comes from the ore of a unique leached particle size. This paper proposes a method to estimate the gold dissolution as a function of particle size using a bulk ore sample, i.e., with the dissolved gold coming from the various sizes of particles carried by the ore. The results are consistent with the fact that gold dissolution increases with the decreasing particle size but results also indicate that gold dissolution of the ore within a size interval is not significantly affected by the grinding time used for the ore size reduction. Results also show a good dissolution of the gold contained in the fine-size fractions without oxidation and lead nitrate pre-treatment for an ore that is known to require such pre-treatment. Keywords: gold ore; leaching; cyanide; particle size; laboratory testing 1. Introduction More than 75% of the gold produced in the world is extracted by the cyanide leaching of gold ores. The ore is ground to expose the surface of the gold and the ground material is reacted with cyanide in oxygen-enriched slurry. Under these conditions gold is dissolved and recovered from the pregnant solution by adsorption on carbon or cementation on zinc powder [1]. The barren solids are disposed in tailings ponds or returned as paste fill into underground mines. The effect of ore particle size on gold recovery is usually studied using mono-size particles and experimental results indicate that gold recovery increases with the increasing fineness of the grind [2,3]. Other studies report similar results from leaching bulk coarse ore samples without referring to the individual ore size fractions [4]. The first objective of the paper is to evaluate the dissolution of gold from various particle size intervals using bulk ground ore samples rather than mono-size particles. The second objective of the paper is to determine whether the gold contained within a given size interval responds differently if it belongs to a coarsely or a finely ground ore. 2. Objective and Experimental Procedure The objective of the test work is first discussed, followed by a description of the ore characteristics and of the experimental procedure. 2.1. Objective of the Test Work The objective of the test work is to assess if gold dissolution as a function of particle size can be assessed from a natural ground ore rather than by using single-size ore particles [2,5]. The second Minerals 2016, 6, 68; doi:10.3390/min6030068 www.mdpi.com/journal/minerals Minerals 2016, 6, 68 2 of 9 Minerals 2016, 6, 68 2 of 9 objective is to determine if gold dissolution from the ore within a given size interval is affected by the history of the production of that size interval, i.e., for instance, if the gold within a size fraction, say history of the production of that size interval, i.e., for instance, if the gold within a size fraction, say 0.150/0.106 mm in size, dissolves differently when the size fraction belongs to a coarsely or a finely 0.150/0.106 mm in size, dissolves differently when the size fraction belongs to a coarsely or a finely ground ore. Such independence may facilitate the development of an integrated grinding-leaching ground ore. Such independence may facilitate the development of an integrated grinding-leaching simulator [6]. simulator [6]. 2.2. Ore Ore Characteristics The oreore samplesample used used for for the the tests tests comes comes from from a gold a gold mine mine located located 80 km 80 north km ofnorth Amos of in Amos Québec, in Québec,Canada. Canada. The ore The composition ore composition is given is in given Table in1. Tabl Thee mine1. The extracts mine extracts a gold-rich a gold-rich vein-type vein-type ore body. ore body.The main The ganguemain gangue minerals minerals are silicates are silicates and carbonates. and carbonates. Because Because of the presence of the presence of sulfide of minerals, sulfide minerals,particularly particularly pyrrhotite, pyrrhotite, the plant the uses plant an aerationuses an aeration and lead and nitrate lead pre-treatmentnitrate pre-treatment of the oreof the before ore beforecyanidation cyanidation in order in to order achieve to achieve gold recovery gold recovery above 90%above [7 ].90% [7]. Table 1. Gold ore composition [[7].7]. Elements Minerals Au Elements10.4 g/t Quartz, albite, Minerals phogopite 58.5% Ag Au 10.510.4 g/t g/t Quartz,Calcite, albite, chlinochlore phogopite 58.5%31.1% Si Ag26.0% 10.5 g/t Calcite, chlinochlorePyrite 31.1%3.2% Fe Si8.0% 26.0% PyritePyrrhotite 3.2% 0.8% Fe 8.0% Pyrrhotite 0.8% Al Al6.0% 6.0% ChalcopyriteChalcopyrite 0.4% 0.4% Ca Ca4.4% 4.4% SphaleriteSphalerite 0.1% 0.1% 2.3. Experimental Experimental Procedure A 100 kg ore sample was collected from from the rod mi millll feed conveyor of of the the concentrator, concentrator, crushed down to to <2.0 <2.0 mm mm and and split split into into 4.8 4.8 kg kg samples. samples. Four Four <2 <2mm mm samples samples are areindividually individually dry-ground dry-ground for 15for min; 15 min; four four other other ones ones are are ground ground for for 35 35min min an andd four four other other samples samples for for 65 65 min min as as shown shown in Figure1 1.. TheThe ground ground samples samples are are kept kept in in a freezera freezer to minimizeto minimize the the oxidation oxidation of the of sulphides.the sulphides. The sizeThe sizedistributions distributions of the of ground the ground products products are given are ingiven Table in2 .Table A coarse 2. A grind coarse is usedgrind here is used to have here sufficient to have sufficientmaterial for material measuring for measuring the gold content the gold of thecontent ore within of the theore conventionalwithin the conventional sieves used tosieves characterize used to characterizethe ore size distribution.the ore size distribution. The plant uses The a finerplant grinduses a of finer the ore.grind of the ore. 100 kg ore sample Rotary splitter 4.8 kg 4.8 kg 4.8 kg 4.8 kg batch batch batch batch Production of 4 batches Production of 4 batches of 4.8 kg: Same testing as of 4.8 kg: Same testing as for 15 minute grinding for 15 minute grinding Grinding Grinding Grinding Grinding but with 35 minute but with 65 minute 15 minutes 15 minutes 15 minutes 15 minutes grinding grinding Characterization Leaching 6 Leaching Leaching of the feed hours 12 hours 24 hours Solution for Solution for Solution for gold assaying gold assaying gold assaying Characterization Characterization Characterization of the solid of the solid of the solid reject reject reject FigureFigure 1. 1. TestTest procedure for the leaching tests. One sample of the four batches ground for a given time is used to characterize the size distribution of gold in the feed while the three other samples are leached for three different periods of time, as illustrated in Figure 1. The solids samples from the fresh feed and leached residues are processed the same way to obtain the chemical composition and the size distribution of the ore as Minerals 2016, 6, 68 3 of 9 Table 2. Size distributions of the ground samples. % <0.037 mm % <0.106 mm % <0.212 mm Grinding Time (min) D80 (mm) Minerals 2016, 6, 68 (´400 mesh) (´150 mesh) (´65 mesh) 3 of 9 15 0.850 21 26 40 well as the gold content35 of the size intervals. 0.318 The procedure 25 is shown 33 in Figure 2. Each 51 ore sample is divided using a conventional65 splitter 0.106 to extract a 3530 g and a 200 49 g sample. The 76 30 g sample is fire-assayed for gold. The 200 g sample is washed onto the 400 mesh (0.038 mm) screen. The +400 Onemesh sample is dry-sieved of the four on a batches column ground using forthe a14, given 28, 48, time 100 is and used 400 to characterizemesh sieves. the The size ore distribution from each sizeof gold interval in the is feedfire-assayed while the for threegold. otherThe −400 samples mesh arefrom leached dry sieving for three is combined different with periods the washed of time, −as400 illustrated mesh and in Figurethe composite1. The solids material samples is sampled from the and fresh assayed feed and for leachedgold. Finally residues the are−400 processed mesh is wet-sievedthe same way on to the obtain 635 themesh chemical (0.020 composition mm) screen and to theproduce size distribution the 0.037/0.020 of the oremm as and well <0.020 as the goldmm fractionscontent of that the are size also intervals. assayed The for proceduregold. The cost is shown of assaying in Figure all 2the.
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