minerals Review Gravity Concentration in Artisanal Gold Mining Marcello M. Veiga * and Aaron J. Gunson Norman B. Keevil Institute of Mining Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; [email protected] * Correspondence: [email protected] Received: 21 September 2020; Accepted: 13 November 2020; Published: 18 November 2020 Abstract: Worldwide there are over 43 million artisanal miners in virtually all developing countries extracting at least 30 different minerals. Gold, due to its increasing value, is the main mineral extracted by at least half of these miners. The large majority use amalgamation either as the final process to extract gold from gravity concentrates or from the whole ore. This latter method has been causing large losses of mercury to the environment and the most relevant world’s mercury pollution. For years, international agencies and researchers have been promoting gravity concentration methods as a way to eventually avoid the use of mercury or to reduce the mass of material to be amalgamated. This article reviews typical gravity concentration methods used by artisanal miners in developing countries, based on numerous field trips of the authors to more than 35 countries where artisanal gold mining is common. Keywords: artisanal mining; gold; gravity concentration 1. Introduction Worldwide, there are more than 43 million micro, small, medium, and large artisanal miners extracting at least 30 different minerals in rural regions of developing countries (IGF, 2017) [1]. Approximately 20 million people in more than 70 countries are directly involved in artisanal gold mining (AGM), with an estimated gold production between 380 and 450 tonnes per annum (tpa) (Seccatore et al., 2014 [2], Thomas et al., 2019 [3], Stocklin-Weinberg et al., 2019 [4], UNEP, 2020 [5]). One of the mistakes commonly used by many researchers and authorities is to categorize “artisanal” and “small” miners in the same group. The term “artisanal” indicates only the elementary mining and processing practices and not the size of the operation (Veiga, 1997) [6]. Often in developing countries impacted by artisanal mining, mining regulations create barriers to formalizing artisanal miners due to the lack of a clear definition of artisanal mining. Frequently, laws categorize miners into awkward subdivisions such as “traditional, subsistence, panners, informal, formal, etc.” with different regulations for each category; however, the distinctions between the types of miner are often unclear (Veiga and Marshall, 2019) [7]. The opposite of artisanal mining is “conventional” mining, which refers to the activities of organized companies. Conventional mining operations can be large, medium, or small but typically use the most appropriate technical, legal, and environmental practices to mine and process ores—some small conventional operations responsibly mine less than 20,000 tpa of raw material. On the other hand, some artisanal mines can be large—artisanal operations processing more than 2 million tpa of secondary (weathered or placer) ores are not uncommon. Typically, an artisanal miner works based on instinct, the need to feed his/her family, and pay bills. There are no “classical” geological exploration activities such as drilling and reserve definition or engineering studies undertaken. The concept of survival is constantly the driving force for those miners. However, they are excellent gold prospectors. One of the main concerns about AGM is the use of mercury to extract gold. AGM is the largest world’s anthropogenic mercury pollution with approximately 2000 tpa of mercury lost to the Minerals 2020, 10, 1026; doi:10.3390/min10111026 www.mdpi.com/journal/minerals Minerals 2020, 10, 1026 2 of 49 Minerals 2020, 10, x 2 of 51 environment (UNEP, 2020 [5], ArtisanalMining.org, 2018 [8]). Mercury is used as it forms an amalgam withenvironment gold (and most (UNEP, other 2020 metals, [5], ArtisanalMining.org, except iron and 2018 platinum), [8]). Mercury allowing is used the as it gold forms to an be amalgam more easily with gold (and most other metals, except iron and platinum), allowing the gold to be more easily concentrated and recovered (Veiga et al., 2014 [9]). According to Pryor (1965) [10] amalgamation, concentrated and recovered (Veiga et al., 2014 [9]). According to Pryor (1965) [10] amalgamation, or or “the wetting of gold by mercury is not alloying, but a phenomenon of moderately deep sorption, involving “the wetting of gold by mercury is not alloying, but a phenomenon of moderately deep sorption, involving some someinterpenetration of ofthe the two two elements” elements. In”. aIn large a large number number of countries, of countries, artisanal artisanal miners minersamalgamate amalgamate the the wholewhole ore either either in in small small ball ball mills mills or passing or passing the ground the ground ore on sluices ore on made sluices of copper made plates of copper covered plates coveredwith with mercury mercury (Figure (Figure 1). These1). These procedures procedures are the are main the causes main causesof large of losses large of losses mercury of mercury to the to the environment,environment, but it is hard to to change change miners’ miners’ be behaviorhavior as asthey they believe believe that that gravity gravity concentration concentration methodsmethods are are ine ffiinefficientcient to to extract extract fine fine particlesparticles of of gold. gold. However, However, the theamalga amalgamationmation of whole of wholeore is ore is generallygenerally not not a a good good solution solution toto capture fine fine gold gold as as amalgamation amalgamation is typically is typically inefficient inefficient when when mercurymercury is oxidized is oxidized and and gold gold particles particles are are smaller smaller than 0.05 0.05 mm mm (Marsden (Marsden and and House, House, 2006 2006 [11]). [ 11]). (a) (b) FigureFigure 1. Whole 1. Whole ore ore amalgamation amalgamation in in ( a(a))small small ball mills mills in in Colombia Colombia and and (b) (inb )copper-amalgamating in copper-amalgamating sluicessluices in Nicaragua. in Nicaragua. WhenWhen gravity gravity concentrates concentrates are are amalgamated,amalgamated, mercury mercury losses losses are are drastically drastically reduced reduced but but amalgamation-tailings, still with mercury, cannot be discharged into a water stream or recirculated amalgamation-tailings, still with mercury, cannot be discharged into a water stream or recirculated to to the grinding circuit or leached with cyanide. In reality, the largest mercury polluters are the the grindingprocessing circuit centers or leachedthat offer with amalgamation cyanide. In servic reality,es theto artisanal largest mercuryminers and polluters retain arethe themercury- processing centerscontaminated that offer amalgamationtailings to leach with services cyanide to artisanal(Veiga et al., miners 2014 and[12]). retainIn this theprocess, mercury-contaminated mercury forms tailingscyanide to leach complexes with cyanide that become (Veiga retained et al., 2014 in [the12]). final In thistailings. process, These mercury mercury-cyanide forms cyanide complexes complexes that becomepollute much retained more inaquatic the finalbiota tailings.than metallic These mercury mercury-cyanide alone (Drace et complexesal., 2016 [13], pollute Marshall much et al., more aquatic2020 biota [14], thanSeney metallic et al., 2020 mercury [15]). An alone additional (Drace problem et al., 2016 is that [13 miners], Marshall rarely et use al., retorts 2020 [to14 condense], Seney et al., 2020mercury [15]). An vapors additional generated problem when they is thatburn minersamalgams rarely in an useopen retorts pan with to a condense torch. The mercuryvapors have vapors generatedbeen intoxicating when they burnmillions amalgams of individuals in an opendirectly pan involved with a torch.in the Thegold-processing vapors have steps, been such intoxicating as millionsminers of and individuals gold buyers, directly as well involvedas community in themembers gold-processing in mining towns steps, (Malm, such 1998 as [16], miners Hentschel and gold et al., 2002 [17], Hilson et al., 2007 [18], Esdaile and Chalker, 2018 [19], Zolnikov and Ramirez, 2018 buyers, as well as community members in mining towns (Malm, 1998 [16], Hentschel et al., 2002 [17], [20]). Hilson et al., 2007 [18], Esdaile and Chalker, 2018 [19], Zolnikov and Ramirez, 2018 [20]). The ratio Hglost:Auproduced has been used to assess the efficiency of the amalgamation process and Themercury ratio losses Hglost can:Au be producedvery lowhas when been only used gravity to concentrates assess the e arefficiency amalgamated of the amalgamationand retorts are used process and mercury(Table 1). losses This ratio can bemust very below carefully when used, only gravityas it can concentrates cause confusion are amalgamatedwhen just a little and gold retorts is are usedproduced. (Table1). For This a national ratio must or regional be carefully assessment used, of as mercury it can cause losses, confusion it is a good when indicator just (Veiga a little and gold is produced.Baker, 2004) For a [21]. national or regional assessment of mercury losses, it is a good indicator (Veiga and Baker, 2004) [21]. Table 1. Approximate mercury losses depending on the amalgamation process. Table 1. ApproximateAmalgamation mercury losses Method depending onHg thelost amalgamation:Auproduced process. Whole ore with amalgamating sluices ≥3 Amalgamation Method Hglost: Auproduced Whole ore in small