Gravity Concentration in Artisanal Gold Mining

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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

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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 on Hg thelost amalgamation:Auproduced process. Whole ore with amalgamating sluices ≥3 Amalgamation Method Hglost: Auproduced Whole ore in small ball mills >6 Whole ore with amalgamating sluices 3 Concentrates with no retort ≥ ~1 Whole ore in small ball mills >6 ConcentratesConcentrates with no with retort retorts ~1 <0.1 Concentrates with retorts <0.1 Minerals 2020, 10, x 3 of 51

For more than 40 years, researchers, non-governmental organizations (NGOs), international agencies, companies, etc. have tried to eradicate the use of mercury in AGM, usually demonstrating appropriate processing techniques to concentrate gold. This has been a great challenge, as most artisanal miners do not have the skills and financial resources to change their primitive practices. A better policy would be to reduce first the use of mercury and later eliminate its use. There are many methods of gold extraction—some, such as panning and sluices, have been used for thousandsMinerals of2020 years,, 10, 1026 many others are more recent developments. For instance, Grayson3 of(2007) 49 [22] identified 75 gold separation and leaching methods that do not use mercury or cyanide. This paperFor focuses more than on 40de years,scribing researchers, the most non-governmental common gravity organizations processes (NGOs),used or internationaltested by artisanal miners inagencies, various companies, countries, etc. discussing have tried to some eradicate of theuse limitations of mercury the in AGM, miners usually face demonstrating in adopting these techniques.appropriate processing techniques to concentrate gold. This has been a great challenge, as most artisanal miners do not have the skills and financial resources to change their primitive practices. 2. Gold ConcentrationA better policy would Used be by to reduceArtisanal first theGold use Mining of mercury (AGM) and later eliminate its use. There are many methods of gold extraction—some, such as panning and sluices, have been used for thousands of years, many others are more recent developments. For instance, Grayson (2007) [22] 2.1. Generalidentified Aspects 75 gold separation and leaching methods that do not use mercury or cyanide. Many gravityThis paper concentration focuses on methods describing used the most by arti commonsanal gravityminers processes are discon usedtinuous, or tested i.e., by miners artisanal miners in various countries, discussing some of the limitations the miners face in adopting stop the process to discharge the concentrate after some time of operation. Some miners have the these techniques. perception that when they see a yellow concentrate, the gold recovery is good, and the equipment works well.2. Gold This Concentration is a common Used mistake. by Artisanal High Gold gold Mining grade (AGM) in concentrates does not necessarily mean that the gold2.1. General recovery Aspects is high. Usually, in gravity concentration processes, the grade of gold in a concentrate is antagonistic to the gold recovery (Figure 2) (Veiga et al., 2014 [9]). An interesting Many gravity concentration methods used by artisanal miners are discontinuous, i.e., miners stop perceptionthe of process most toartisanal discharge miners the concentrate is that afterthe someonly timeway of to operation. produce Some more miners gold have is to the process perception more ore. They neverthat think when theyabout see aimproving yellow concentrate, gold recovery the gold recovery by using is good, better and concentration the equipment works methods. well. They generallyThis do not is a commoneven know mistake. how High to assess gold grade the gold in concentrates recovery does in their not necessarily rudimentary mean processes. that the gold Anotherrecovery important is high. Usually,point to in stress gravity is concentration that chemical processes, analyses the gradeof gravity of gold concentrates in a concentrate are is usually antagonistic to the gold recovery (Figure2) (Veiga et al., 2014 [ 9]). An interesting perception of most fraught with imprecision due to the “nugget effect” caused by some gold specks. Analyses of feed artisanal miners is that the only way to produce more gold is to process more ore. They never think and tailingsabout generally improving goldprovide recovery more by using accurate better concentrationgold recovery methods. results They than generally analyses do not evenof gravity concentrates.know how to assess the gold recovery in their rudimentary processes.

Less mass in the conc. = high grade, low recovery

Recovery

Concentrate Grade

Mass of concentrate More mass in the conc. = low grade, high recovery

Figure 2. FigureMore gold 2. More in goldthe concentrate, in the concentrate, less less Au Au recovery recovery had thethe process. process. Source: Source: Veiga Veiga et al. (2014)et al. [(2014)9]. [9]. Another important point to stress is that chemical analyses of gravity concentrates are usually Mostfraught gravity with concentration imprecision due toprocesses the “nugget consist eﬀect” causedof a single by some pass gold specks.of the Analyses feed material of feed and through equipmenttailings where generally gold is provide recovered more based accurate on gold the recovery gravitational results thanforce analyses (G) applied of gravity to the concentrates. particles. These methods oftenMost do gravitynot provide concentration high go processesld recoveries, consist particularly of a single pass with of therespect feed materialto fine gold through particles. equipment where gold is recovered based on the gravitational force (G) applied to the particles. For centuries, gravity methods were gold miners’ preferred separation method as there is a large These methods often do not provide high gold recoveries, particularly with respect to ﬁne gold particles. difference between the specific gravity (sg) of gold (sg ≈ 19.3) and common silicate gangue minerals (sg ≈ 2.7) such as quartz sand. The selection of a concentration equipment depends on several variables, such as the mineralogy of the ore, shape of gold particles, pulp density, pulp viscosity,

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For centuries, gravity methods were gold miners’ preferred separation method as there is a large difference between the specific gravity (sg) of gold (sg 19.3) and common silicate gangue minerals ≈ (sg 2.7) such as quartz sand. The selection of a concentration equipment depends on several variables, ≈ such as the mineralogy of the ore, shape of gold particles, pulp density, pulp viscosity, equipment cost, the availability of spare parts, and the grain size of the gold particles. Generally, the applicability of gravity concentrationMinerals 2020 equipment, 10, x for different gold ores is related to the grain size of4 of the 51 liberated gold particles. As shownequipment inFigure cost, the 3availability, some typesof spare ofparts, equipment and the grain are size muchof the gold more particles. e ffi Generally,cient to the concentrate fine applicability of gravity concentration equipment for different gold ores is related to the grain size of gold particles thanthe liberated others gold but particles. most As of shown them in areFigure not 3, some very types effi ofcient equipment for grainare much sizes more efficient finerthan 0.074 mm (200 Tyler mesh).to concentrate The exception fine gold particles is the centrifugalthan others but most concentrators, of them are not referredvery efficient to for in grain Figure sizes 3 as “Knelson Centrifuge”, whichfiner than are 0.074 capable mm (200 of Tyler recovering mesh). The fine exception and is coarse the centrifugal gold concentrators, particles. referred to in Figure 3 as “Knelson Centrifuge”, which are capable of recovering fine and coarse gold particles.

Figure 3. GrainFigure size of3. Grain gold size particle of gold versusparticle versus the type the type of of gravity gravity concentration equipment equipment (Adapted (Adapted from Knelson Concentrators’from Knelson catalog Concentrators’ (nolonger catalog (n available)o longer available) and and Sampaio Sampaio and and Tavares, Tavares, 2005 2005[23]). [23]). Gravity separation occurs primarily due to weight and size differences between different Gravity separationparticles. The occurs size of the primarily particles is an due important to weight parameter and as sizea large di grainfferences of a gangue between mineral (e.g., diff erent particles. The size of the particlesquartz) behaves is anlike importanta small grain of parameter gold in a classic as gravity a large separation grain process. of a gangue The rate atmineral which a (e.g., quartz) particle settles in a liquid depends on the particle weight and the viscosity of the liquid (in this case behaves like a smallthe pulp grain of ore).of As golda simplification, in a classic Gaudin gravity (1939) [24] separation established anprocess. equation to Thecalculate rate the atratio which a particle settles in a liquidof diameters depends of ona spherical the particle mineral particle weight to anothe andr, the both viscosity naturally sinking of the in a liquid fluid with (in a certain this case the pulp of density under the gravity influence: ore). As a simplification, Gaudin (1939) [24] established an equation to calculate the ratio of diameters 𝑑 𝜌 𝜌 of a spherical mineral particle to another, both naturally= sinking in a fluid with a certain density under 𝑑 𝜌 𝜌 the gravity influence:where: " #n da ρb ρ f 𝑑 and 𝑑 = diameter of mineral a (light) and= b (heavy)− 𝜌 and 𝜌= specific gravity of the mineralsdb a andρ ab ρ f 𝜌= density of the fluid (or suspension) − where: 𝑛 = coefficient ranging from 1 (Newtonian regime) to 0.5 (Stokesian regime), transitional regime between 0.5 and 1 da and db = diameterFor example, of mineral considering a (light) a Stokesian and regime b (heavy) (very diluted pulps with spherical particles flowing slowly. The drag forces around particles sinking, at low Reynolds number conditions, occur due to ρa and ρb = specific gravity of the minerals a and b the viscosity of the liquid (Sampaio and Tavares, 2005) [23]), the formula above reveals that a particle ρ f = density ofof the quartz fluid of 0.25 (or mm suspension) should behave in a classical gravity concertation process like a 0.074 mm particle n = coefficientof ranging gold. Stokesian from regime 1 (Newtonian considers spheres regime) falling in a tostationary, 0.5 (Stokesian non-turbulent regime),fluid (low Reynolds transitional regime between 0.5 and 1

For example, considering a Stokesian regime (very diluted pulps with spherical particles ﬂowing slowly. The drag forces around particles sinking, at low Reynolds number conditions, occur due to the viscosity of the liquid (Sampaio and Tavares, 2005) [23]), the formula above reveals that a particle of quartz of 0.25 mm should behave in a classical gravity concertation process like a 0.074 mm particle of gold. Stokesian regime considers spheres falling in a stationary, non-turbulent ﬂuid (low Minerals 2020, 10, 1026 5 of 49

Reynolds number, usually below 2000), in which a particle reaches the terminal velocity, vt (zero acceleration) when: 2 g d ρp ρ f vt = · − 18 µ · g = gravitationalMinerals 2020 acceleration, 10, x (9.81 m/s2) 5 of 51 d = particlenumber, diameter usually in below meter 2000), in which a particle reaches the terminal velocity, 𝑣 (zero acceleration) 3 ρp = densitywhen: of the particle (kg/m ), gold = 19,300 3 = density of the liquid (kg/m ), water = 1000𝑔. 𝑑 𝜌 𝜌 ρ f 𝑣 = 18. µ µ = viscosity of the liquid, for example, water at 20 C = 1.002 10 3(Pa s) ◦ × − · 𝑔 = gravitational acceleration (9.81 m/s2) While𝑑 =this particle is a diameter simplification in meter of a complex system, it provides us with a strong reason to believe 3 that size classification𝜌 = density of the (screening particle (kg/m or), hydrocyclone) gold = 19,300 of the particles is very important for any type of 3 gravity concentration.𝜌 = density of the Unfortunately, liquid (kg/m ), water artisanal = 1000 miners typically do not use size classification, frequently −3 leading toµ low = viscosity gold of recoveries the liquid, infor gravityexample, concentrationwater at 20 °C = 1.002 processes. × 10 (Pa·s) AnotherWhile important this is a factor simplification is that of gold a complex particles system, can it beprovides flaky us and with can a strong be lost reason easily to believe to the tailings in laminar concentrationthat size classification systems, (screening such as or sluiceshydrocyclone) or tables. of the In particles addition, is very gold important is naturally for any hydrophobic: type of thin, gravity concentration. Unfortunately, artisanal miners typically do not use size classification, flat particlesfrequently tend toleading “float” to low on gold the recoveries surface of in sluicesgravity concentration or tables (Wang processes. and Poling, 1983 [25]). As gold is very malleable,Another particles important can change factor is theirthat gold morphologies particles can be during flaky and the can comminution be lost easily to process the tailings thus affecting gravity concentration,in laminar concentration in particular systems, insuch a laminaras sluices or system tables. In (Ofori-Sarpong addition, gold is naturally and Amankwah, hydrophobic: 2011 [26]). Flaky particlesthin, flat can particles be characterized tend to “float” on by the their surface Corey of sluices shape or factortables (Wang (CSF) and= CPoling,/(A.B) 19831/2, [25]). for particles As with gold is very malleable, particles can change their morphologies during the comminution process thus dimensions: A, B, C such that A B C. Small CSFs indicate flattened grains (Figure4). Particles affecting gravity concentration,≥ in particular≥ in a laminar system (Ofori-Sarpong and Amankwah, with a CSF2011<0.15 [26]). are Flaky very particles hard tocan concentrate be characterized in laminar by their Corey regimes shape (Walsh factor and(CSF) Rao, = C/(A.B) 19881/2 [,27 for], Wang and Poling, 1983particles [25 ],with Gupta dimensions: and Yan, A, B, 2016C such [28 that]). A Giusti≥ B ≥ C. (1986)Small CSFs [29 ],indicate studying flattened placer grains deposits (Figure from the North Saskatchewan4). Particles with River, a CSF Canada,<0.15 are very observed hard to concentrate 0.125 mm in gold laminar flakes, regimes with (Walsh the and average Rao, 1988 CSF of 0.173, [27], Wang and Poling, 1983 [25], Gupta and Yan, 2016 [28]). Giusti (1986) [29], studying placer being lostdeposits from sluice from boxes.the North The Saskatchewan author found River, in Cana sedimentsda, observed from 0.125 this river,mm gold and flakes, in the with Athabasca the River as well, thataverage as gold CSF of particles 0.173, being become lost from flattened sluice boxes. their The sizes author increased. found in sediments Ofori-Sarpong from this and river, Amankwan (2011) [26]and highlighted in the Athabasca the importanceRiver as well, that of theas gold grinding particles process become flattened in the recovery their sizes ofincreased. gold as Ofori- some pieces of equipment,Sarpong such and as hammer Amankwan mills (2011) (very [26] popular highlighted in artisanalthe importance mining), of the produce grinding lessprocess flaky in goldthe particles recovery of gold as some pieces of equipment, such as hammer mills (very popular in artisanal (higher CSF)mining), than produce the ball less mill. flaky gold particles (higher CSF) than the ball mill.

Figure 4. Dimension of a particle to calculate Corey Shape Factor. Figure 4. Dimension of a particle to calculate Corey Shape Factor The dominantThe dominant idea held idea held by manyby many researchers researchers and and implementers implementers of solutions of solutions for artisanal for miners, artisanal miners, as well asas authorities, well as authorities, is that is that most most artisanal artisanal minersminers work work only only with with placer placer (alluvial) (alluvial) deposits. deposits.In fact, In fact, artisanal miners often process alluvial, colluvial and eluvial, or secondary, ore deposits until they artisanal miners often process alluvial, colluvial and eluvial, or secondary, ore deposits until they reach reach bedrock. However, based on the idea that gold in most alluvial ores is liberated, artisanal bedrock. However,miners rarely based invest on in theefficient idea comminution that gold in proc mostesses alluvial when processing ores is liberated, colluvial or artisanal eluvial ores. miners rarely invest in eTheyﬃcient often comminution use hydraulic monitors processes to create when a processingslurry and then colluvial pump the or ore eluvial pulp to ores. a gravity They often use hydraulicconcentration monitors to process create as awas slurry common and with then Californian pump thegold ore prospectors pulp to in a the gravity Gold Rush concentration from 1848 process to 1855 (Limbaugh, 1999) [30]. With no comminution, their gold recoveries for these secondary types as was common with Californian gold prospectors in the Gold Rush from 1848 to 1855 (Limbaugh, of ores are typically low, usually below 30%. However, the capital and operating costs for mining 1999) [30].(usually With nodredges comminution, and hydraulic their monitors) gold and recoveries processing for (usually these sluice secondary boxes) weathered types of ores are are typically low, usuallytypically below lower 30%. than However, those for primary the capital ores (Figure and 5). operating costs for mining (usually dredges and hydraulic monitors) and processing (usually sluice boxes) weathered ores are typically lower than those for primary ores (Figure5).

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(a)

(b)

Figure 5. (a) Characteristics of the ores processed by artisanal gold miners (Spiegel and Veiga, 2010) Figure 5. (a) Characteristics of the ores processed by artisanal gold miners (Spiegel and Veiga, 2010) [31]; [31]; (b) Photo of an old Californian miner using a hydraulic monitor and sluice box in a placer gold (b) Photo of an olddeposit Californian (Hardcore Miner, miner 2017 using[32]). a hydraulic monitor and sluice box in a placer gold deposit (Hardcore Miner, 2017 [32]). An interesting observation when analyzing tailings from artisanal miners is a usual U-shape distribution of gold with the grain size analysis (Figure 6) (Andrade-Lima et al., 2008 [33], Veiga et An interestingal., 2014 observation [9]). The gold in the when coarse analyzing fractions of the tailings tailings is unliberated, from artisanal associated miners with the coarse is a usual U-shape distribution of goldgrains of with gangue the minerals, grain due size to analysispoor comminution (Figure processes,6) (Andrade-Lima while the fine fractions et al., usually 2008 have [ 33 ], Veiga et al., fine gold particles (liberated or not) not recovered due to the rudimentary concentration methods 2014 [9]). The goldused by in the the artisanal coarse miners. fractions However, it of is theimportant tailings to highlight is unliberated, that in many cases associated gold does not with the coarse grains of gangueneed minerals, to be liberated due from to a poorgangue comminutionparticle to be concentrated processes, (Koppalkar, while 2009 the [34]). ﬁne For example, fractions a usually have quartz particle (sg = 2.65) with an inclusion of 15% of gold (sg = 19.3) by volume, generates a grain ﬁne gold particleswith (liberatedan sg of 5.15, which or not) may notbe recoverable recovered by gravity due toprocesses, the rudimentary especially where concentrationthere are multiple methods used by the artisanalpasses. miners. However, it is important to highlight that in many cases gold does not need to be liberated from aIt gangueis also important particle to stress to bethat concentrated gold is very malleable (Koppalkar, and usually liberates 2009 [ 34from]). the For gangue example, a quartz minerals by detachment, as demonstrated by Khalesi et al. (2011) [35]. Therefore, the comminution particle (sg = 2.65)and classification with an inclusion methods play of a 15% critical of role gold in the (sg total= 19.3) or partial by liberation volume, process generates of the gold a grain with an sg of 5.15, which maygrains be(Bode, recoverable 2020) [36]. by gravity processes, especially where there are multiple passes. Minerals 2020, 10, x 7 of 51

30 6 25 5 (g/t) Gold Grade 20 4 15 3

10 2 5 1

Gold Distribution (%) 0 0 601 357 274 194 126 89 58 Grain Size (µm)

Au Grade(g/t) %DAu

Figure 6. Grain size distribution of gold in artisanal mining tailing from Brazil (Adapted from Figure 6. Grain size distribution of gold in artisanal mining tailing from Brazil (Adapted from Andrade-Lima et al., 2008 data) [33]. Andrade-Lima et al., 2008 data) [33]. The main gravity concentration processes used by artisanal gold miners worldwide are: It is also important to stress that gold is very malleable and usually liberates from the gangue • Panning minerals by• detachment,Sluicing as demonstrated by Khalesi et al. (2011) [35]. Therefore, the comminution • Jigging • Tabling • Centrifuging

2.2. Panning Panning is the most ancient and traditional form of gravity concentration. The movement of water in the pan flushes out the light minerals allowing the heavy minerals to sink. A pan is the main companion of an artisanal miner, either if he/she is prospecting gold or using it as a final concentration tool. For the micro-miners, those processing less than 2 tonnes of ore per day (Veiga et al., 2018) [37], a pan can be the main equipment used for gravity concentration, especially working with alluvial ores. There are different types of gold pan in the market. In Africa, artisanal miners use gourds, kitchen bowls, or cooking pans to concentrate gold. Wooden and plastic pans have the advantage of buoyancy and of being lighter. In North America, flat bottom plastic or aluminum pans have been used for over 150 years to trap coarse gold specks. These types of pans are 380 to 460 mm in diameter and 50 to 60 mm in depth, with sides sloping 30–45° containing ridges to catch gold (Silva, 1986) [38]. The movement of the flat-bottom pans is usually back and forth, which is easy for experienced operators, (Figure 7). In South America and most Asian countries, the main popular panning equipment is the conical “batea”, also known as Chinese hat, with a diameter from 0.6 to 1.5 m and edges with an angle of 150° to 155°. Panning with a “batea” demands experience from the operator and the movements are circular, removing the lighter minerals by overflow. Minerals 2020, 10, 1026 7 of 49 and classiﬁcation methods play a critical role in the total or partial liberation process of the gold grains (Bode, 2020) [36]. The main gravity concentration processes used by artisanal gold miners worldwide are:

Panning • Sluicing • Jigging • Tabling • Centrifuging • 2.2. Panning Panning is the most ancient and traditional form of gravity concentration. The movement of water in the pan flushes out the light minerals allowing the heavy minerals to sink. A pan is the main companion of an artisanal miner, either if he/she is prospecting gold or using it as a final concentration tool. For the micro-miners, those processing less than 2 tonnes of ore per day (Veiga et al., 2018) [37], a pan can be the main equipment used for gravity concentration, especially working with alluvial ores. There are different types of gold pan in the market. In Africa, artisanal miners use gourds, kitchen bowls, or cooking pans to concentrate gold. Wooden and plastic pans have the advantage of buoyancy and of being lighter. In North America, flat bottom plastic or aluminum pans have been used for over 150 years to trap coarse gold specks. These types of pans are 380 to 460 mm in diameter and 50 to 60 mm in depth, with sides sloping 30–45◦ containing ridges to catch gold (Silva, 1986) [38]. The movement of the flat-bottomMinerals pans 2020, is 10,usually x back and forth, which is easy for experienced operators,8 of 51 (Figure 7).

(a) (b)

(c)

Figure 7. (a) North American flat bottom pan in Zimbabwe (b) Wooden “batea” in Venezuela (c) Steel Figure 7. (a) North American ﬂat bottom pan in Zimbabwe (b) Wooden “batea” in Venezuela (c) Steel “batea” in Indonesia. “batea” in Indonesia. 2.3. Sluicing In South AmericaSluice boxes and are most inclined, Asian flat-bottomed countries, troughs, the main with popularor without panningriffles on the equipment surface and is the conical “batea”, alsousually known lined as with Chinese a carpet hat,to trap with the gold a diameter particles together from with 0.6 other to 1.5 heavy m minerals. and edges Sluices with are an angle of usually used in alluvial, eluvial, and colluvial ores but many artisanal operations concentrate gold from primary ore ground in hammer mills or Chilean mills. An ore pulp usually with 10–20% (in mass) of solids is pumped to the sluices and gold and heavy minerals sink to the bottom of the box allowing the lighter particles to pass over and be carried and discharged off at the end of the sluice. Sluices are made in various sizes, from small hand-fed sluices to large sluices found on dredges (Veiga et al., 2006) [39]. With low capital and operating costs, worldwide the artisanal miners prefer the sluices to concentrate gold as they achieve very high ore to concentrate ratio, typically ranging from 10,000:1 to as high as 500,000:1 (Vieira, 2006) [40]. Michaud (2015) [41] described in detail how to make different types of sluice box and provided good tips on how to operate them. Teschner et al. (2017) [42] assessed the gold recovery of an artisanal operation mining alluvial ore in the Guiana region (site not identified) with an average grade of 0.852 Au g/t. After a metallurgical balance of nine samples and microscopic examination, the authors concluded that “more than 90% of the gold being captured by the sluice is sized between 0.075 and 0.5 mm”. Most of the gold particles were liberated. A well-designed sluice can provide high recoveries for relatively coarse gold particles. The rate of the gold particles settling in the sluice box depends on the gold particle weight, size, and shape.

Minerals 2020, 10, 1026 8 of 49

150◦ to 155◦. Panning with a “batea” demands experience from the operator and the movements are circular, removing the lighter minerals by overﬂow.

2.3. Sluicing Sluice boxes are inclined, flat-bottomed troughs, with or without riffles on the surface and usually lined with a carpet to trap the gold particles together with other heavy minerals. Sluices are usually used in alluvial, eluvial, and colluvial ores but many artisanal operations concentrate gold from primary ore ground in hammer mills or Chilean mills. An ore pulp usually with 10–20% (in mass) of solids is pumped to the sluices and gold and heavy minerals sink to the bottom of the box allowing the lighter particles to pass over and be carried and discharged off at the end of the sluice. Sluices are made in various sizes, from small hand-fed sluices to large sluices found on dredges (Veiga et al., 2006) [39]. With low capital and operating costs, worldwide the artisanal miners prefer the sluices to concentrate gold as they achieve very high ore to concentrate ratio, typically ranging from 10,000:1 to as high as 500,000:1 (Vieira, 2006) [40]. Michaud (2015) [41] described in detail how to make different types of sluice box and provided good tips on how to operate them. Teschner et al. (2017) [42] assessed the gold recovery of an artisanal operation mining alluvial ore in the Guiana region (site not identified) with an average grade of 0.852 Au g/t. After a metallurgical balance of nine samples and microscopic examination, the authors concluded that “more than 90% of the gold being captured by the sluice is sized between 0.075 and 0.5 mm”. Most of the gold particles were liberated. A well-designed sluice can provide high recoveries for relatively coarse gold particles. The rate of the gold particles settling in the sluice box depends on the gold particle weight, size, and shape. When the ore is naturally classified, which is usually the case with alluvial sand, gold recovery is much higher than unclassified feed, since coarse particles of gangue with low specific gravity can settle at the same velocity as finer gold particles.

2.3.1. Main Variable in Sluicing Some of the main variables affecting the concentration process in a sluice are listed and discussed below (Sivamohan and Forssberg, 1985 [43], Veiga et al., 2006 [39], Lehmann, 2020 [44]): Feed rate; • Water flow; • Particle size and shape; • Cleanup period (removal of concentrates); • Slope angle; • Riffles and/or carpets lining the sluice; • Width and length of sluice; • Pulp density; and, • Arrangement of the sluices. • The feed rate of material entering a sluice depends on the sluice width, water flow, and pulp density. Most sluices in artisanal mining operations are 1 to 2 m long, 30 to 50 cm wide, with walls 10 to 30 cm high. The relationship between the height of the sluice walls (H) and width (W) is usually H = 0.3 W (Lins and Farid, 1992) [45]. The use of screens on top of the sluices is critical to providing a basic size classification of the ore fed into the process. Most sluices in artisanal mining observed in alluvial operations in the Katingan District, Kalimantan, Indonesia, have a feed rate of 10 to 12 tonnes per hour (tph) operating with 15–20% of solids and producing between 3 to 6 g of gold after 10 h of operation (Veiga, 2003) [46]. The water flow, driving pulp density, is an important variable for efficient gold concentration. Silva (1986) [38] highlighted that too much water flow reduces the opportunities for small gold particles to settle and too little water creates a muddy sludge that does not allow the fine gold particles to settle. The adequate water flow for a 60 cm-wide sluice should be around 15 m3/min (Sampaio and Minerals 2020, 10, 1026 9 of 49

Tavares, 2005) [23]. McCraken (2013) [47] mentioned that in a sluice with riffles, it is not recommended to pass large amounts of water after ore has stopped being fed to the sluice. The author alleged that water flows can remove gold particles already concentrated behind the riffles. Water flow and velocity increase with the sluice slope. Mitchell et al. (1997a) [48] described an operation in Yukon using 3 riffled sluices with slopes of 7 to 12◦, a feed rate of 20 m /h of material, and the water flow of 40 L/s. This operation was designed to recover gold finer than 1 mm. Higher water flows, for example, 80 L/s, according to the authors, could be used to recover gold specks coarser than 1 mm. The particle size of gold that is concentrated in a sluice is usually coarser than 200 mesh (0.074 mm) but the efficiency of the concentration of fine gold particles does not depend only on the size but also on other variables herein discussed, in particular on the particle shape. As discussed above, flaky gold particles can be lost due to the hydrophobicity of the mineral, particularly with laminar flow. Cleanups to remove the concentrates are necessary, as sluices are batch-type equipment. The gold recovery increases with frequent cleanups, however this decreases the gold grades of the concentrates. The choice of the number of cleanups depends on the grain size of gold, the angle of the sluice (low angles accumulate more material), and the type of minerals associated with the gold. In some operations in Indonesia as well as in Colombia, Guyana, and Suriname, it was observed that miners clean the sluices every 8 h. In Brazil, some miners remove the concentrate from river dredges every 10–15 days. In Portovelo, Ecuador, concentrating fine gold, miners use to clean the low slope angle (5◦) wool-carpeted sluices every hour (Gonçalves et al., 2017 [49]). Lehmann (2020) [44] mentioned a placer gold ore study conducted by Russian researchers in which the gold recovery increased from 90% with cleanups every 12 h to 94% with cleanups every 2 h of operation. The best time of cleanup must be tested for each type of sluice and ore to determine the ideal gold recovery and mass of concentrate generated. The slope angle of a sluice depends on the type of gold ore being processed. The sluices are usually inclined with angles ranging from 5◦ to 15◦. The flow velocity of the pulp in a sluice is primarily controlled by the sluice slope. For efficient operation, the flow must be fast enough to assure that the sluice bed is not full and blocked and yet slow enough to allow as much fine gold as possible to settle to the bottom, where it can be trapped. Litvintsev et al. (2012) [50], in an experimental work with sluice boxes using a pulp density of 4% solids, concluded that the best gold recovery for their placer ore occurred when the slope was 7◦. In southern Ecuador, miners concentrate gold coming from Chilean mills in 7 m long cement sluices operating with an angle of 5◦. For an operation with an ore grade of 3 g/t Au, the concentrates obtained after one hour was around 17 g/t. At the end of the process, the Ecuadorian miners have a large mass of concentrate (200–225 kg/day) or 3% of the original feed (Velasquez et al., 2010) [51]. They then pan this large mass of concentrate, losing significant amounts of gold during panning, and then add mercury to extract gold from the final pan concentrates. Riffles retard the flow of heavy minerals that are settled on the sluice bed. There are many types of riffle. Hungarian ones promote fluidization of the bed as a “boiling” condition is created that expelled the light particles. Shallow riffles and milder water flow are recommended to trap small gold particles (McCracken, 2013) [47]. Riffles are good for coarse and medium-sized particles and carpets for fine gold. There are many manufacturers of riffled sluices. Keene Engineering (2020a) [52] offers a large selection of sluices with metallic frames, riffles, or expanded metal and carpets (Figure8). It is common to observe sluices in artisanal plants operating in a very turbulent fashion. This is caused by the presence of riffles on the sluice or by gravels and cobbles as the initial screening is not sufficient to remove coarse particles. Turbulence is one of the reasons for poor gold recoveries as the fine gold particles are often lost. Professor Herman Wotruba, from the University of Aachen, Germany, has demonstrated to the Brazilian artisanal miners the difference in turbulence between carpeted and a riffled sluice (Figure9). Most North American prospectors concentrating alluvial ores use expanded metal on top of the carpets to trap gold, as they create less turbulence and keep the carpet firmly set at the bottom of the sluice. Hamilton (1988) [53] also recommended expanded metals instead of riffles in conjunction with Nomad carpets to recover placer gold as fine as 150 Mesh (0.105 mm). 13

323 occurred when the slope was 7°. In southern Ecuador, miners concentrate gold coming from 324 Chilean mills in 7 m long cement sluices operating with an angle of 5º. For an operation with an ore 325 grade of 3 g/t Au, the concentrates obtained after one hour was around 17 g/t. At the end of the 326 process, the Ecuadorian miners have a large mass of concentrate (200-225 kg/day) or 3% of the 327 original feed (Velasquez et al., 2010). Then they pan this large mass of concentrate, losing significant 328 amounts of gold during panning, and then add mercury to extract gold from the final pan concentrates. Minerals 2020, 10, 1026 10 of 49

329 Water Flow 330

350 Hamilton (1988) also recommended expanded metals instead of riffles in conjunction with Nomad 351 carpets to recover placer gold as fine as 150 Mesh (0.105 mm).

352 Carpets are also used to retard the flow of heavy minerals in the sluice. When turbulence is low, the 353 settling process occurs smoothly. In a sluice, there are two types of movement of the particles: rolling 354 and sliding. The rolling velocity increases with the weight of the particle, but the sliding velocity 331 355 decreases thanks to the friction on the sluice surface. For e fficient separation, the friction force must Riffles must operate half full 332 356 be high. Carpets are ideal for increasing friction and are more suitable for fine gold particles than 333 334 357 riffles. In the 19 90s,(a) approximately 1 million artisanal miners were distributed in the (b)jungles of South 358 America producing 200 tonnes of gold per year. At that time, the approximately 400,000 Brazilian 335 Figure 7 - (a) Hungarian riffles (McCracken, 2013) 359Figure artisanal 8. (a) Hungarian gold miners riinffl thees Amazon (McCracken, were the 2013) world’s [47 largest]. (b) carpet Keene consumers sluice with per expandedcapita (Feijão metal and on top 336 (b) Keene sluice with expanded metal on top (Keene Engineering, 2020a). 360(Keene Pinto, Engineering, 1992, Veiga 2020a and Hinton, [52]). 2002). 337 Riffles retard the flow of heavy minerals that are settled on the sluice bed. There are many types of 338 riffles. Hungarian ones promote fluidization of the bed as a “boiling” condition is created that expelled 339 the light particles. Shallow riffles and milder water flow are recommended to trap small gold particles 340 (McCracken, 2013). Riffles are good for coarse & medium size particles and carpets for fine gold. 341 There are many manufacturers of riffled sluices. Keene Engineering (2020a) offers a large selection of 342 sluices with metallic frames, riffles, or expanded metal and carpets (Figure 7). It is common to observe 343 sluices in artisanal plants operating in a very turbulent fashion. This is caused by the presence of riffles 361 344 on the sluice or by gravels and cobbles as the initial screening is not sufficient to remove coarse 362 (a) (b) 345 particles363. Turbulence Figure is 8 –one (a) Sluice of boxthe with reasons riffles provokes for more poor turbulence gold than recoveries the one with carpet as the(photo fine H. Wotruba) gold particles are often 364Figure 9. (a) Sluice box with riffl(b)es Riffled provokes sluice with more high turbulence turbulence in Indonesia than the one with carpet (photo H. 346 lost. Professor Herman Wotruba, from the University of Aachen, Germany, has demonstrated to the 365Wotruba). Artisanal (b) Riminersffled use sluice a large with variety high of turbulence carpets to concentrate in Indonesia. gold. In Ecuador and Indonesia, miners 347 Brazilian366 artisanal use wool miners rags, but the the mostdifference popular ones in turbulence in the North of between South America carpeted are those and with grao riffledoves sluice (Figure 8). Carpets are also used to retard the flow of heavy minerals in the sluice. When turbulence is low, 367 (Figure 9 and Figure 10). Brazilian suppliers of equipment for artisanal gold miners sell these carpets 348 Mostthe North settling American process occursprospectors smoothly. concentrating In a sluice, alluvial there areores two use types expanded of movement metal on of top the of particles: the carpets 368 for prices ranging from US$ 50 to 70/m2. In laboratory tests with placer ores, Nomad carpets provided 349 to rollingtrap gold and, as sliding. they create The rolling less velocityturbulence increases and keep with the weightcarpet offirm thely particle, set at the but bottom the sliding of the velocity sluice. 369 excellent recoveries of gold particles as fine as 0.105 mm (Poling and Hamilton, 1986). The vinyl loop decreases thanks to the friction on the sluice surface. For efficient separation, the friction force must be 370 carpets, similar to the 3M NomadTM mats, have the same price range as the grooved carpets. These high. Carpets are ideal for increasing friction and are more suitable for fine gold particles than riffles. 371 mats, used in homes and offices to clean the shoes, are very efficient for trapping both coarse and fine In the 1990s, approximately 1 million artisanal miners were distributed in the jungles of South America 372 gold particles. One of the main problems with a typical vinyl loop mat is that the rubber on the back of producing 200 tonnes of gold per year. At that time, the approximately 400,000 Brazilian artisanal 373 the mat makes it difficult to wash out the gold particles entangled in the carpet. In Indonesia, miners gold miners in the Amazon were the world’s largest carpet consumers per capita (Feijão and Pinto, 374 were observed burning carpets to recover trapped gold. The Gold Hog (2012) has a series of good 1992 [54], Veiga and Hinton, 2002 [55]). Artisanal miners use a large variety of carpets to concentrate gold. In Ecuador and Indonesia, miners use wool rags, but the most popular ones in the North of South America are those with grooves (Figures 10 and 11). Brazilian suppliers of equipment for artisanal gold miners sell these carpets for prices ranging from US$ 50 to 70/m2. In laboratory tests with placer ores, Nomad carpets provided excellent recoveries of gold particles as fine as 0.105 mm (Poling and Hamilton, 1986 [56]). The vinyl loop carpets, similar to the 3M NomadTM mats, have the same price range as the grooved carpets. These mats, used in homes and offices to clean shoes, are very efficient for trapping both coarse and fine gold particles. One of the main problems with a typical vinyl loop mat is that the rubber on the back of the mat makes it difficult to wash out the gold particles entangled in the carpet. In Indonesia, miners were observed burning carpets to recover trapped gold. The Gold Hog (2012) [57] has a series of Minerals 2020, 10, x 11 of 51

particles than riffles. In the 1990s, approximately 1 million artisanal miners were distributed in the jungles of South America producing 200 tonnes of gold per year. At that time, the approximately 400,000 Brazilian artisanal gold miners in the Amazon were the world’s largest carpet consumers per capita (Feijão and Pinto, 1992 [54], Veiga and Hinton, 2002 [55]).

(a) (b)

Figure 9. (a) Sluice box with riffles provokes more turbulence than the one with carpet (photo H. Wotruba). (b) Riffled sluice with high turbulence in Indonesia.

Artisanal miners use a large variety of carpets to concentrate gold. In Ecuador and Indonesia, miners use wool rags, but the most popular ones in the North of South America are those with grooves (Figures 10 and 11). Brazilian suppliers of equipment for artisanal gold miners sell these carpets for prices ranging from US$ 50 to 70/m2. In laboratory tests with placer ores, Nomad carpets provided excellent recoveries of gold particles as fine as 0.105 mm (Poling and Hamilton, 1986 [56]). The vinyl loop carpets, similar to the 3M NomadTM mats, have the same price range as the grooved carpets. These mats, used in homes and offices to clean shoes, are very efficient for trapping both coarse and fine gold particles. One of the main problems with a typical vinyl loop mat is that the Minerals 2020, 10, 1026 11 of 49 rubber on the back of the mat makes it difficult to wash out the gold particles entangled in the carpet. In Indonesia, miners were observed burning carpets to recover trapped gold. The Gold Hog (2012) [57] has a series of good videos explaining the advantages of different types of carpet. The company15 good videos explaining the advantages of diﬀerent types of carpet. The company Gold Dog (2016) [58] recommendsGold Dog using(2016) unbacked[58] recommends Moss vinylusing loopunbacked carpet Moss (10–12 vinyl mm loop thick) carpet that (10–12 is setmm up thick) on topthat of is a felt set up on top of a felt or grooved rubber carpet (Figure 12). There is a variety of unbacked vinyl loop 375or groovedvideos explaining rubber carpet the advantages (Figure 12 of). different There istypes a variety of carpets. of unbacked The company vinyl Gold loop Dog carpets (2016) available for carpets available for artisanal miners but some of them are of poor quality and tear with little 376artisanal recommend minerss butusing some unbacked of them Moss are vinyl of poorloop carpet quality (10 and-12 mm tear thick) with that little is pressure.set up on top of a felt or pressure. 377 grooved rubber carpet (Figure 11). There is a variety of unbacked vinyl loop carpets available for 378 artisanal miners but some of them are of poor quality and tear with little pressure.

379 FigureFigure 10. Rubberized 10. carpetRubberized used in Braziliancarpet sluices used Source:in https:Brazilian//www.construvolts.com.br sluices Source:/calha- https://www.construvolts.com.br/calha-eclusa-galvanizada-50x185cm-desmontavel-e-mercurio- 380 eclusa-galvanizada-50x185cm-desmontavel-e-mercurio-azougue-100g-para-garimpo-p25277Minerals 2020, 10, x Figure 9 - Rubberized carpet used in Brazilian sluices 12 of .51 381 Source:azougue-100g-para-garimpo-p25277. https://www.construvolts.com.br/calha- eclusa-galvanizada-50x185cm-desmontavel-e-mercurio-azougue-100g-para- 382 garimpo-p25277 383

Figure 11. Rubber carpets with grooves. Sources: 384 Figure 11. Rubber carpets with grooves. Sources: https://www.youtube.com /watch? https://www.youtube.com/watch?v=WyswDAk6iMQ and 385 v=WyswDAk6iMQ and https://produto.mercadolivre.com.brFigure 10 – Rubber carpets with grooves/MLB-1202232696-carpete-feltro-pra- 386 https://produto.mercadolivre.coSources: https://www.youtube.com/watch?v=WyswDAk6iMQm.br/MLB-1202232696-carpete-feltro-pra-garimpo-ouro-fino-verde- 387 garimpo-ouro-ﬁno-verde-30cm-x-70cm-_JMhttps://produto.mercadolivre.com.br/MLB30cm-x-70cm-_JM. -1202232696. -carpete-feltro-pra-garimpo-ouro-fino-verde-30cm-x-70cm-_JM 388

389 390 Figure 11 – Gold Dog Vinyl loop unbacked MOSS carpet with a metal grid to hold the carpet 391 Figure 12. Gold Dog VinylSource loops: https://www.youtube.com/watch?v=lLmNY4mN5SQ unbacked MOSS carpet with a metal grid to hold the carpet 392 Sources: https://www.youtube.comhttps://www.youtube.com/watch?v=WyswDAk6iMQ/watch?v=lLmNY4mN5SQ, and https: //www.youtube.com/watch? v=WyswDAk6iMQ.

The importance of the width and length of sluices are poorly understood by artisanal miners. It is important to highlight to artisanal miners that the production capacity of a sluice box is given by the width of the sluice and not by the length. Nevertheless, there is a perception among the miners that the longer the sluice, the better it is at capturing ﬁne gold. Sluices with lengths up to 40 m were observed in manyFigure countries.12. Gold Dog Vinyl Some loop old unbacked Asian MOSS sluices, carpet with such a metal as the grid “Palong”,to hold the carpet were Sources: up to 300 m long (Sivamohan andhttps://www.youtube.com/wa Forssberg (1985) [43tch?v=lLmNY4mN5SQ,]. In Brazil, Indonesia, and Sudan artisanal minerand built sluices with lengths overhttps://www.youtube.com/ 10 m. A simple calculationwatch?v=WyswDAk6iMQ. can reveal the error of the perception that longer is better. For example, letThe usimportance calculate of the width ﬁnal and velocity length of sluices a sphere are poorly rolling unders downtood a by 3 artisanal m-long miners. sluice It box with a slope of 15is◦ importantwith friction, to highlight butnot to artisanal considering miners thethat dragthe production force of capacity the air of (or a sluice water): box is given by the width of the sluice and not by the length. Nevertheless, there is a perception among the miners that the longer the sluice, the better it is at capturing fine gold. Sluices with lengths up to 40 m were observed in many countries. Some old Asian sluices, such as the “Palong”, were up to 300 m long (Sivamohan and Forssberg (1985) [43]. In Brazil, Indonesia, and Sudan artisanal miner built sluices with lengths over 10 m. A simple calculation can reveal the error of the perception that longer is better. For example, let us calculate the final velocity of a sphere rolling down a 3 m-long sluice box with a slope of 15° with friction, but not considering the drag force of the air (or water): Considering the diagram (Figure 13):

𝐹 = gravitational force 𝐹 = frictional force N = Normal force

Minerals 2020, 10, x 13 of 51

𝑚 = mass of the particle 𝑑 = sluice length θ = slope of the sluice (θ = 15°) 𝑔 = gravitational acceleration (9.8 m/s2)

𝑣 = initial velocity of the sphere = zero 𝑣 = final velocity of the sphere 𝑟 = radius of the sphere Minerals 2020, 10, 1026 12 of 49 𝐼 = momentum of inertia of the sphere at the Center of Mass  = angular acceleration (rad/s2) Considering the diagram (Figure 13): 𝑎 = acceleration of the sphere at the Center of Mass 𝒯 = TorqueFg = gravitational force F f = frictional force Not consideringN = Normal the force Drag Force and considering the sphere only rolling: . 𝒯 = 𝑟. 𝐹 =𝐼.𝛼 then… 𝐹 = m= mass of the particle 𝑎 = 𝛼.𝑟d = sluicethen… length 𝛼= θ = slope of the sluice (θ = 15◦) Forces atg =thegravitational x-axis: acceleration (9.8 m/s2) 𝐹 =𝑚.𝑔.𝑠𝑖𝑛vi = initial velocity θ =𝐹 𝑚.𝑎 of the sphere = zero v f = ﬁnal velocity𝐼. 𝑎 of the sphere 𝑚. 𝑔. sin  = 𝑚.𝑎 r = radius of the𝑟 sphere 2 𝐼 𝑓𝑜𝑟I = momentum 𝑠𝑝ℎ𝑒𝑟𝑒 of = inertia.𝑚.𝑟 of the sphere at the Center of Mass = angular acceleration5 (rad/s2) 𝑎 =a .𝑔.𝑠𝑖𝑛= acceleration θ (rolling) of the sphere at the Center of Mass CM = Torque 𝑣 T= 𝑣 2 𝑎.𝑑

Figure Figure13. Diagram 13. Diagram of ofa asphere sphere of of gold gold rolling rolling down indown a sluice in box a with sluice carpet box (no dragwith force carpet considered). (no drag force considered).Not considering the Drag Force and considering the sphere only rolling:

I α = r F = I α then ... F = · ThenT the· velocityf · (𝑣 ) of fthe spherer with the distance is: aCM aCM = α r then . . . α = r · Forces at the x-axis: 10 𝑣 = ∙ 𝑔∙𝑑∙sin 7 Fg = m g sin θ = F f + m aCM · · I a · The speedm g sin of θthe= sphere· CM + m ina the first meter of the sluice is: 𝑣 (in the first 1 m) = 1.9 m/s, in the · · r2 · CM second meter,I ( f oris sphere2.7 m/s,) = 2andm r2 in the third meter is 3.3 m/s. Frictional forces are usually constant in 5 · · sluice boxesa = but5 theyg sin θ depend(rolling) on the type of sluice bed (carpet or wood). At a certain point, the sphere CM 7 · · will reach thev 2 terminal= v 2 + 2 avelocityd when the acceleration is zero. Now, if we think this sphere represents f i CM· the pulp flow, then the speed increases with the distance. Therefore, the fine particles of gold must Then the velocity (v f ) of the sphere with the distance is: be concentrated where the flow velocity is slowest, in the first few meters. 1 Burt (1984) [59] showed that the majority of10 the gold is 2recovered in the first meter of a sluice and v = ( g d sin θ ) for most of the coarse (−2 + 1 mm), mediumf (7−1· +· 0.1· mm), and fine gold particles (−0.1 mm) the cumulative Thegold speed recovery of the sphereessentiall in they flattens ﬁrst meter after of the lengths sluice is:ofv 2f (inm. theTesting ﬁrst 1 different m) = 1.9 m gold/s, in theplacer ores, Litvintsevsecond et al., meter, (2012) is 2.7 [50] m/s, also and in concluded the third meter that is approximately 3.3 m/s. Frictional forces88% of are the usually gold constant was recovered in sluice within the firstboxes meter but of they thedepend sluice, on 10% the in type the of second sluice bed meter, (carpet and or wood). 2% in At the a certainthird. point, the sphere will Thereach pulp the density terminal velocity(which whenis the the percentage acceleration solids is zero. in Now, the if pulp we think by thismass) sphere of representsthe slurry the should be low, in the range of 10% to 20% solids, and the thickness of the flow cannot be higher than 3 cm. This

Minerals 2020, 10, 1026 13 of 49

pulp flow, then the speed increases with the distance. Therefore, the fine particles of gold must be concentrated where the flow velocity is slowest, in the first few meters. Burt (1984) [59] showed that the majority of the gold is recovered in the first meter of a sluice and for most of the coarse ( 2 + 1 mm), medium ( 1 + 0.1 mm), and fine gold particles ( 0.1 mm) the − − − cumulative gold recovery essentially flattens after lengths of 2 m. Testing different gold placer ores, Litvintsev et al., (2012) [50] also concluded that approximately 88% of the gold was recovered within Minerals 2020the, first10, x meter of the sluice, 10% in the second meter, and 2% in the third. 14 of 51 The pulp density (which is the percentage solids in the pulp by mass) of the slurry should be low, allows goldin the particles range of 10%to sink to 20% to solids, the bottom and the of thickness the flo ofw the to flowbe trapped cannot be by higher the thansluice. 3 cm. If Thisa portable allows digital gold particles to sink to the bottom of the flow to be trapped by the sluice. If aortable digital scale is scale is available, weigh a top-cut Coca-Cola bottle with water and afterward weigh again with pulp available, weigh a top-cut Coca-Cola bottle with water and afterward weigh again with pulp coming coming fromfrom thethe sluice sluice box box (Figure (Figure 14). The 14). volume The volume of solids of plus solids the volume plus the of water volume will equalof water the volume will equal the volume ofof thethe bottle bottle (for (for example example 1.5 L). 1.5 The L). mass The of mass the bottle of the with bottle pulp will with be equalpulp to will the massbe equal of the to solids the mass of the solidsplus plus the the mass mass of the of water. the water.

FigureFigure 14. Pulp 14. Pulp with with solids solids collected collected atat thethe discharge discharge of the of sluicethe sluice box. box.

The speciﬁc gravity of the solids, sgs, can be determined by picnometry using the same bottle or a The specific gravity of the solids, sgs, can be determined by picnometry using the same bottle or vial or a beaker. The sg of solids can be calculated using the following formula (picnometry): a vial or a beaker. The sg of solids can be calculated using the following(M P) formula (picnometry): sgs = − (W + (M P) S) 𝑀 𝑃− − 𝑠𝑔 = sgs = speciﬁc gravity of the ore sample𝑊 𝑀𝑃 𝑆 P = weight of the empty picnometer (a vial or a beaker) sgs = specificM = weightgravity of of solid the sample ore sample+ picnometer P = weightW =ofweight the empty of pycnometer picnometer full of (a water vial (full or viala beaker) or beaker) M = weightS = weightof solid of sample + +pycnometer picnometer+ water (same volume as before) W = weightW + of(M pycnometerP) S = weight full ofof water water displaced (full vial by theor beaker) solid sample − − S = weight ofUsually, sample the + sgpycnometer of most ores processed+ water (same by sluices volume is around as before) 2.7, but it is always useful to obtain it W + (M −by P picnometry.) − S = weight Then of the water mass displaced of solids in theby pulpthe solid can be sample algebraically solved: Usually, the sg of most ores processed by sluices is around 2.7, but it is always useful to obtain VT (total volume of the bottle) = Vsolids + Vliquid it by picnometry. Then the mass of solids in the pulp can be algebraically solved: MT (total mass of the bottle with pulp) = Msolids + Mliquid

VT (total volume of the bottle) = Vsolids + Vliquid MT (total mass of the bottle with pulp) = Msolids + Mliquid The specific gravity of solids, sgs = Msolids/Vsolids or Vsolids= Msolids/sgs As the sgw (water specific gravity) is 1, Vliquid = Mliquid, then… VT − Vsolids = MT − Msolids Msolids = Vsolids + MT − VT = (Msolids/sgs) − MT − VT 1 Msolids 1- = MT − VT sgs

Total mass of bottle with pulp - Total volume of bottle Mass of solids = 1 1 sgs For example, in a 1.5 L bottle: • Mass of the full bottle with pulp = 2.00 kg (obtained in the scale); • Total volume = 1.50 L (measured); • Specific gravity of solids, sgs = 2.7 (determined by picnometry); • Mass of solids = 0.79 kg (calculated by equation above);

Minerals 2020, 10, 1026 14 of 49

The speciﬁc gravity of solids, sgs = Msolids/Vsolids or Vsolids= Msolids/sgs As the sgw (water speciﬁc gravity) is 1, V = M , then ... V V = M M liquid liquid T − solids T − solids Msolids = Vsolids + MT VT = (Msolids/sgs) MT VT − − − 1 Msolids 1 = MT VT − sgs −

Total mass of bottle with pulp Total volume of bottle Mass of solids = − 1 1 − sgs For example, in a 1.5 L bottle:

Mass of the full bottle with pulp = 2.00 kg (obtained in the scale); • Total volume = 1.50 L (measured); • Specific gravity of solids, sgs = 2.7 (determined by picnometry); • Mass of solids = 0.79 kg (calculated by equation above); • %S(%MassMinerals 2020, 10 of, x solids) = Mass of solids/Mass of the pulp = 0.79/2.00 = 40%; 15 of 51 • Then, this slurry has a pulp density of 40% solids by mass, too high for sluicing! • • %S (%Mass of solids) = Mass of solids/Mass of the pulp = 0.79/2.00 = 40%; • In theThen, market, this slurry there has are a scalespulp density to determine of 40% solids the pulp by mass, density too high of slurries for sluicing! that are very affordable and easyIn to the operate market, but there the are sgs scalesis still to necessary determine (Pulp the pu Densitylp density Scales, of slurries 2011) that [60 are]. very affordable andThe easyarrangement to operate ofbut the the sluice sgs is still is another necessary important (Pulp Density variable Scales, for 2011) efficient [60]. sluicing but is frequently overlookedThe by arrangement artisanal miners. of the Goldsluice ores is another usually important contain a mixturevariable offorcoarse efficient and sluicing fine-grained but is gold particles.frequently As fine overlooked gold can beby lostartisanal to the miners. tailings Gold when ores the usually pulp flowcontain speed a mixture is high, ofit coarse is always and fine- beneficial to havegrained multiple-stage gold particles. sluices. As fine As gold most can of be the lost gold to the is tailings recovered when in the the pulp first flow meter speed of a is sluice, high, ait wayis to always beneficial to have multiple-stage sluices. As most of the gold is recovered in the first meter of increase the gold recovery is by using a zigzag arrangement. In this case, the velocity of the flow will a sluice, a way to increase the gold recovery is by using a zigzag arrangement. In this case, the velocity be broken every time the direction of flow changes. Zigzag sluices also allow variable flow velocities of the flow will be broken every time the direction of flow changes. Zigzag sluices also allow variable changingflow velocities the slope changing and using the di slopefferent and carpets using diff in eacherent deck.carpets In in Suriname, each deck. zigzag In Suriname, sluices zigzag were used for thesluices concentration were used offor gold the concentration from both colluvial of gold fr andom alluvialboth colluvial ore. Local and alluvial miners ore. mentioned Local miners that they learnedmentioned how to that make they zigzag learned sluices how from to Brazilianmake zigz artisanalag sluices miners from (“garimpeiros”).Brazilian artisanal In miners fact, in the Brazilian(“garimpeiros”). Amazon, there In fact, are in sluices the Brazilian with up Amazon, to four th decksere are arranged sluices with in zigzags.up to four In decks Zaragoza, arranged Colombia, in artisanalzigzags. miners In Zaragoza, process materialColombia, from artisanal a dry miners alluvial process gold material deposit from using a backhoedry alluvial excavators gold deposit to feed a two-deckusing zigzagbackhoe sluice excavators (Figure to 15 feed). A Colombiana two-deck engineerzigzag sluice mentioned (Figure that,15). unfortunately,A Colombian engineer some miners add mercurymentioned into that, the unfortunately, lower deck some of the miners sluice add box. mercury into the lower deck of the sluice box.

(a) (b)

FigureFigure 15. (15.a) Zigzag(a) Zigzag sluice sluice operating operating dry dry alluvialalluvial gold gold ore ore in in Zaragoza, Zaragoza, Colombia Colombia (b) Zigzag (b) Zigzag sluice sluice in Suriname.in Suriname.

2.3.2. Dry Sluices A less popular sluice is the dry washers or waterless sluices. These sluices are very useful for regions where water is scarce, for example in the desert in Sudan (Figure 16), but they are not as efficient as the wet sluices. For example, in water, the weight difference between gold and quartz particles is around 11, however, in the air, this difference drops to 7 (911-Metallurgist, 2017) [61]. Dry washers work using pressured air that flows through a perforated bed expelling the light minerals from the sluice bed (Keene Engineering. 2020b) [62]. The main problem with this equipment is the amount of dust generated and most artisanal miners are reluctant to use masks.

Minerals 2020, 10, 1026 15 of 49

2.3.2. Dry Sluices A less popular sluice is the dry washers or waterless sluices. These sluices are very useful for regions where water is scarce, for example in the desert in Sudan (Figure 16), but they are not as efficient as the wet sluices. For example, in water, the weight difference between gold and quartz particles is around 11, however, in the air, this difference drops to 7 (911-Metallurgist, 2017) [61]. Dry washers work using pressured air that flows through a perforated bed expelling the light minerals from the sluice bed (Keene Engineering. 2020b) [62]. The main problem with this equipment is the amount of dust generated and most artisanal miners are reluctant to use masks. 20

497 498 Figure 14 – Artisanal miners in Sudan using a waterless sluice Figure 16. Artisanal miners in Sudan using a waterless sluice. 497 499 498Primitive Sluices Figure 14 – Artisanal miners in Sudan using a waterless sluice 2.3.3. Primitive Sluices 500 499Many Primitive primitive Sluicessluice boxes have been seen in different countries. In Indonesia, a wooden sluice with Many primitive sluice boxes have been seen in diﬀerent countries. In Indonesia, a wooden sluice 501 500no carpetMany primitiveand a low sluice slope boxes is usedhave beenby artisanal seen in differentminers countries.to concentrate In Indonesia, alluvial a cassiteritewooden sluice (SnO with2 ) and with no carpet and a low slope is used by artisanal miners to concentrate alluvial cassiterite (SnO2) 502 sometimes gold as well. The amount of sand accumulated in the sluice serves as a carpet to trap the and501 sometimes no carpet gold and a as low well. slope Theis used amount by artisanal of sandminers accumulated to concentrate alluvial in the cassiterite sluice serves(SnO2) and as a carpet to trap 503 502heavy sometimes mineral sgold. The as calculatedwell. The amount SnO2 of recovery sand accumulated in operations in the in sluice Bangka serves-Belitung as a carpet was to estimated trap the at the heavy minerals. The calculated SnO2 recovery in operations in Bangka-Belitung was estimated 503 heavy minerals. The calculated SnO recovery in operations in Bangka-Belitung was estimated at 504at approximatelyapproximately 12% (Maia et et al., al., 2019). 2019)2 Another [63]. Another rudimentary rudimentary sluice was observed sluice was in Uganda, observed where in Uganda, 504 approximately 12% (Maia et al., 2019). Another rudimentary sluice was observed in Uganda, where 505where artisanal artisanal miners miners make make sluice sluice boxes boxes carving carving a tree atrunk. tree Instead trunk. Insteadof carpets of to carpets trap gold to particles, trap gold those particles, 505 artisanal miners make sluice boxes carving a tree trunk. Instead of carpets to trap gold particles, those 506those micr micro-minerso-miners use use a bed a bed of grass of grass (Figure (Figure 15a). 17 Ina). Mozambique, In Mozambique, artisanal artisanal miners create miners a sluice create to atrap sluice to 506 micro-miners use a bed of grass (Figure 15a). In Mozambique, artisanal miners create a sluice to trap 507trap goldgold simply simply excavating excavating the ground the ground and expecting and expecting that the rough that the surfa roughce of the surface soil trap ofs the gold soil specks traps gold 507 gold simply excavating the ground and expecting that the rough surface of the soil traps gold specks specks from weathered ores. In South Africa, some miners use the same system as in Mozambique but 508 508from from weathered weathered ores. ores. In In South South Africa Africa,, somesome miners miners use use the the same same system system as in Mozambiqueas in Mozambique but the but the the improvised excavated sluice is lined with wool carpets (Figure 17b). 509 509improvised improvised excavated excavated sluice sluice is is linedlined withwith wool wool carpets carpets (Figure (Figure 15b ).15 b).

510 511 (a) (b) 510 512 Figure 15 – (a) Small sluice box in Uganda made of a tree trunk (photo: S. Siegel) 511 513Figure 17. ( a ) Small sluice (a) box (b) in Ground Uganda excavated made sluice of with a tree carpets trunk in South (photo: Africa. S. Siegel). (b) (b) Ground excavated 512 sluice with carpetsFigure in South 15 – (a) Africa. Small sluice box in Uganda made of a tree trunk (photo: S. Siegel) 513 (b) Ground excavated sluice with carpets in South Africa. 2.3.4. Amalgamating Sluice A common type of sluice used in AGM is the amalgamating sluice, observed in the ﬁeld in some countries such as Brazil, Chile, Colombia, Costa Rica, Ghana, Guyana, Honduras, Indonesia, Nicaragua, 21

Minerals5142020 ,Amalgamating10, 1026 Sluice 16 of 49

515 A common type of sluice used in AGM is the amalgamating sluice, observed in the field in some Suriname,516 countries Venezuela, such as and Brazil, Zimbabwe. Chile, Colombia, These Costa are Rica, copper Ghana, plates Guyana, covered Honduras, with Indonesia, mercury and set up as a 517 Nicaragua, Suriname, Venezuela, and Zimbabwe. These are copper plates covered with mercury and sluice after a grinding process, either by stamp or hammer or Chilean mills. The plates are not very 518 set up as a sluice after a grinding process, either by stamp or hammer or Chilean mills. The plates are efficient to trap unliberated gold when the ore is poorly ground, usually below 1 mm (Figure 18). 519 not very efficient to trap unliberated gold when the ore is poorly ground, usually below 1 mm (Figure The flow of pulp must be interrupted and the plates scratched with a blade of steel or plastic card to 520 16). The flow of pulp must be interrupted and the plates scratched with a blade of steel or plastic card remove the amalgam formed on the surface (Figure 19). The efficiency of the process is low due to 521 to remove the amalgam formed on the surface (Figure 17). The efficiency of the process is low due to the short time of contact between the liberated gold and mercury. These sluices are 1 or 2 m long and 522 the short time of contact between the liberated gold and mercury. These sluices are 1 or 2 m long and require about 0.3 m2 of copper plate to treat 1 tonne per day (tpd) of ore for pulps with 20% solids. 523 require about 0.3 m2 of copper plate to treat 1 tonne per day (tpd) of ore for pulps with 20% solids. The The friction betweenMinerals 2020, the10, x pulp and the plate surface removes mercury droplets that17 are of 51 lost with the 524 friction between the pulp and the plate surface removes mercury droplets that are lost with the tailings. tailings. In Venezuela, tailings were analyzed containing 60 to 100 ppm mercury (Veiga et al., 2005) [64]. 525 In Venezuelarequire about, tailings 0.3 m were2 of copper analyzed plate contain to treating 1 60tonne to 100 per ppm day (tpd)mercury of ore (Veiga for pulps et al., with 2005) 20%. As solids. shown As shown in Table1, the copper-amalgamating sluices are responsible for Hg lost: Auproduced ratios 526 in TableThe friction 1 the copper between-amalgamating the pulp and sluices the plate are surface responsible removes for mercury Hglost: Au dropletsproduced ratios that arearound lost with3. These the around 3. Thesetailings. sluices In Venezuela, are fabricated tailings were by analyzed the artisanal containing miners 60 to 100 who ppm buymercury copper (Veiga plateset al., 2005) (2–5 mm thick) 527 sluices are fabricated by the artisanal miners who buy copper plates (2 – 5 mm thick) and clean the and clean the[64]. plate As shown surface in Table with 1, acidsthe copper-amalgamating to remove copper sluices oxidation are responsible before for Hg addinglost: Auproduced a layer ratios of mercury. 528 platearound surface 3. Thesewith acids sluices to removeare fabricated copper by oxidation the artisanal before miners adding who a buylayer copper of mercury. plates (2–5 mm thick) and clean the plate surface with acids to remove copper oxidation before adding a layer of mercury.

529 530 Figure 16 - Amalgamating sluices in Venezuela after hammer mills and after stamp mills in Zimbabwe Figure 18. AmalgamatingFigure 18. Amalgamating sluices sluices in Venezuela in Venezuela after after hamme hammerr mills mills and after and stamp after mills stamp in Zimbabwe. mills in Zimbabwe.

Figure 19. Scratching the surface of the amalgamating plate to remove the amalgam (Brazil and Figure 19. Venezuela).Scratching the surface of the amalgamating plate to remove the amalgam (Brazil and Venezuela). Amalgamation is a simple and effective way to trap liberated gold particles, usually coarser than Amalgamation0.074 mm. When is a simplethe gold particles and eff areective too small, way the tosurface trap tension liberated of the mercury gold particles,makes it hard usually for coarser the fine gold particles to form an amalgam. The three main types of gold amalgam are AuHg2, Au2Hg, than 0.074 mm.and Au When3Hg (Taggart, the gold 1945) particles[65]. The amalgams are too vary small, in composition the surface depending tension on how of much the force mercury makes it hard for theis used fine to gold squeeze particles off the toliquid form mercury an amalgam. from the amalgam The three during main the filtration types process. of gold An amalgam are amalgam manually squeezed usually contains 40 to 60% mercury. If a filter is made with a polyvinyl AuHg2, Au2Hg, and Au3Hg (Taggart, 1945) [65]. The amalgams vary in composition depending on chloride (PVC) tube and a piece of cloth, a Knelson centrifuge can be used, as observed in Venezuela, how much forceto remove is used the excess tosqueeze of mercury ofromff the the liquidamalgam. mercury The final amalgam from the is less amalgam than 20% mercury. during An the filtration process. An amalgamimprovised centrifuge manually (or squeezed even a bicycle usually wheel) can contains also be locally 40 to manufactured 60% mercury. for this If end. a filter (Figure is made with a polyvinyl chloride20) (Veiga (PVC) et al., 2006) tube [39]. and a piece of cloth, a Knelson centrifuge can be used, as observed in Venezuela, to remove the excess of mercury from the amalgam. The final amalgam is less than 20% mercury. An improvised centrifuge (or even a bicycle wheel) can also be locally manufactured for this end. (Figure 20) (Veiga et al., 2006) [39].

Minerals 2020, 10, 1026 17 of 49

Minerals 2020, 10, x 18 of 51

FigureFigure 20. 20. ImprovisedImprovised centrifuge centrifuge to remove to remove excess me excessrcury from mercury amalgams from (devised amalgams by Edmundo (devised by EdmundoVeiga). Veiga).

A majorA major reason reasonFigure for 20. mercuryfor Improvised mercurylosses centrifuge losses to tothe removeto tailingsthe excess tail meings duringrcury during from the amalgams amalgamationthe amalgamation(devised by Edmundo process process is the is oxidationthe oxidation of the Veiga).copper plate beneath the mercury and oxidation of the mercury layer itself. Some of the copper plate beneath the mercury and oxidation of the mercury layer itself. Some miners, miners, when they observe yellow spots on the surface of the plates, stop the operation and remove when they observeA yellow major reason spots for on mercury the surface losses to of the the tail plates,ings during stop the the amalgamation operation process and remove is the the oxidation the oxidationoxidation with a oftablet the copper of sodium plate beneath cyanide the mercuryor a cyanide and oxidation solution of the as mercuryobserved layer in itself. Zimbabwe Some (Veiga withet a al., tablet 2014 of[12],miners, sodium Green when cyanideet they al., observe2019 or [66]) ayellow cyanide and spots Nicaragua. on solution the surface Miners as of observedthe plates,do not stop take in the Zimbabwe any operation precautions and (Veiga remove in et handling al., 2014 [12], Greentablets etal., of cyanide 2019the oxidation [66 in]) neutral and with a Nicaragua. tablet pH ofwhen sodium HCN Minerscyanide (gas) or doa iscyanide notformed solution take (Figure any as observed precautions 21). Inin Zimbabwe Chile, in artisanal (Veiga handling miners tablets of et al., 2014 [12], Green et al., 2019 [66]) and Nicaragua. Miners do not take any precautions in handling cyanideeliminate in neutral thetablets yellow pH of cyanide whenspots byin HCN neutral rubbing (gas) pH whenurine is formed HCN on the (gas) (Figurecopper is formed plates, 21 (Figure). In as 21). Chile, observed In Chile, artisanal artisanalin Andacollo. minersminers eliminate the yellow spots byeliminate rubbing the yellow urine spots on theby rubbing copper urine plates, on the copper as observed plates, as observed in Andacollo. in Andacollo.

(a) (b)

Figure 21. (a) Copper-amalgamating plate showing oxidation yellow spots (Brazil) (b) Miner using a Figure 21. (a) Copper-amalgamating plate showing oxidation yellow spots (Brazil) (b) Miner using a tablet of sodium(a )cyanide to clean the amalgamating sluice (Zimbabwe). (b) tablet of sodium cyanide to clean the amalgamating sluice (Zimbabwe). Figure 21.2.3.5. (a) EnvironmentalCopper-amalgamating Impacts of Sluicing plate showing oxidation yellow spots (Brazil) (b) Miner using a 2.3.5. Environmentaltablet of sodiumAs sluice Impactscyanide boxes to of areclean Sluicingthe the main amalgamating equipment used sluice by artisanal(Zimbabwe). gold miners, the environmental impacts are remarkable, as many operations are conducted near the water streams. In 1987, the 2.3.5.As sluice Environmental boxesBrundtland are Impacts Report the main mentioned of Sluicing equipment that poverty used is one by of artisanal the main causes gold of miners, environmental theenvironmental problems impacts are remarkable,(WCED, as many 1987) operations[67]. Artisanal miners are conducted often are not concerned near the about water their streams.impacts on the In land 1987, and the Brundtland As sluicewater boxes streams are that the provide main their equipment food nor on theirused own by health. artisanal The survival gold instinct miners, is the the main, environmental and Report mentioned that poverty is one of the main causes of environmental problems (WCED, 1987) [67]. impacts are sometimesremarkable, the sole, as drivingmany force operations for their work. are Most conducted miners, for near example, the do water not believe streams. in mercury In 1987, the Artisanal minerspollution often and are intoxication, not concerned and they see about all wa theirrnings impactsand awareness on thecampaigns land as and efforts water by streams that Brundtland Reportgovernments mentioned to stop their that activities poverty (Veiga, is 2009) one [68]. of the main causes of environmental problems provide(WCED, their 1987) food [67]. nor Artisanal on their miners own health.often are Thenot concerned survival instinctabout their is the impacts main, on and the sometimesland and the sole,water driving streams force that for provide their work. their food Most nor miners, on their for own example, health. The do notsurvival believe instinct in mercury is the main, pollution and and intoxication,sometimes and the sole, they driving see all force warnings for their and work. awareness Most miners, campaigns for example, as eﬀ doorts not by believe governments in mercury to stop theirpollution activities and (Veiga, intoxication, 2009) [68 and]. they see all warnings and awareness campaigns as efforts by governmentsIn Ghana, a to truck stop their was observedactivities (Veiga, with two 2009) sluice [68]. boxes set up after two hammer mills. The truck follows the ore deposits that feed the mills using water from a local creek. The system does not use any dam to retain the tailings, which are simply dumped into the rivers or spread on the ground (Figure 22). Minerals 2020, 10, x 19 of 51

In Ghana, a truck was observed with two sluice boxes set up after two hammer mills. The truck Minerals 2020, 10, 1026follows the ore deposits that feed the mills using water from a local creek. The system does not use 18 of 49 any dam to retain the tailings, which are simply dumped into the rivers or spread on the ground (Figure 22).

(a) (b) Figure 22. (a) SluiceFigure boxes22. (a) Sluice on a boxes truck on in a truck Ghana, in Ghana, (b) River (b) River impacted impacted by by artisanal artisanal mining mining in French in French Guiana. Guiana.

The use of amalgamatingThe use of amalgamating copper-plate copper-plate sluices sluices has has obvious obvious chemic chemicalal impacts onimpacts the operators, on the operators, due to constantdue evaporation to constant evaporation of mercury, of mercury, as wellas well as as on the the environment environment as droplets as of droplets metallic mercury of metallic mercury are carried with the tailings. But another invisible impact occurs when miners add mercury into the are carried withriffles the of tailings. the sluice. The But friction another of the invisiblepulp with the impact liquid metal occurs results whenin high levels miners of mercury add mercuryin into the riffles of the sluice.the tailings. The Some friction miners of throw the mercury pulp with in an excavated the liquid holemetal with water results and colluvial in high or alluvial levels of mercury in ore believing that mercury has a “magic” property to go after the gold. When pumping the ore pulp the tailings. Someto the miners sluices, free throw gold particles mercury meet in the an mercury excavated trapped on hole the withriffles, generating water and the impression colluvial or alluvial ore believing that mercurythat mercury has found a the “magic” gold specks property on the ground to go. Artisanal after theminers gold. in Suriname, When Venezuela, pumping and the ore pulp to some places in Indonesia are still using this primitive method. The environmental pollution is the sluices, freenoteworthy gold particles as not all meetmercury the spread mercury out on the trapped ground is onrecovered the ri whenffles, the generating pulp is processed the in impression that mercury foundthe the sluice gold boxes. specks on the ground. Artisanal miners in Suriname, Venezuela, and some places in IndonesiaThe are systematic still using releases this of tailings, primitive sometimes method. with mercury, The environmental into the local rivers in pollution the Brazilian is noteworthy as Amazon have been creating problems for the communities downstream that have fish as their main not all mercurystaple spread food. outFor example, on the from ground 3 to 6 million is recovered tpa of tailings when from thesluices pulp dumped is processed into an affluent in of the sluice boxes. The systematicthe Tapajós releases River, the of second tailings, most sometimesimportant riverwith in the mercury,Amazon region, into the the pulp local of tailings rivers flow in the Brazilian over 150 km downstream (Figure 23). Telmer et al. (2006) [69] highlighted that “mining-induced Amazon have beensediment creating plumes have problems been a source of for sediment the to communities the Tapajós River system downstream for decades”. that have fish as their main staple food. For example, from 3 to 6 million tpa of tailings from sluices dumped into an affluent of the Tapajós River, the second most important river in the Amazon region, the pulp of tailings flow over 150 km downstream (Figure 23). Telmer et al. (2006) [69] highlighted that “mining-induced sediment plumes have been a source of sediment to the Tapajós River system for decades”. Minerals 2020, 10, x 20 of 51

Figure 23. TailingsFigure 23. from Tailings the from Crepori the Crepori River River pass pass to to the the Tapajós Tapaj Riverós and River goes andmore goesthan 150 more km than 150 km downstream. (Imagedownstream. obtained (Image obtained from Googlefrom Google Earth, Earth, 2020). The GEF/UNDP/UNIDO Global Mercury Project assessed the volume of 20 abandoned open pits The GEF/UNDPthat ranged/UNIDO from 10,000 Global m3 to 50,000 Mercury m3. Every Project year up to assessed 600 new pits the are volumeopened in the of Tapajós 20 abandoned open pits that rangedregion. from When 10,000 miners mreach3 to depths 50,000 of 10 m m,3 .they Every move their year equipment up to 600(pumps new and pitssluices) are to opened in the another pit. The project used these old pits to receive sluice tailings that were dumped into the Crepori River. A straw barrier was used to “filter” the pulp recirculating the water. After the pits were filled, organic soil was generated planting legumes on the top of the reclaimed pits (Figure 24) (Sousa and Veiga, 2009) [70].

Minerals 2020, 10, 1026 19 of 49

Tapajós region. When miners reach depths of 10 m, they move their equipment (pumps and sluices) to another pit. The project used these old pits to receive sluice tailings that were dumped into the Crepori River. A straw barrier was used to “ﬁlter” the pulp recirculating the water. After the pits were ﬁlled, organic soil was generated planting legumes on the top of the reclaimed pits (Figure 24) (Sousa and Veiga, 2009) [70]. Minerals 2020, 10, x 21 of 51

Figure 24. Instead of dumping tailings from sluices into the Crepori River, the tailings were conducted Figure 24. Instead of dumping tailings from sluices into the Crepori River, the tailings were conducted toto openopen pits,pits, ﬁlteredfiltered withwith aa barrierbarrier ofof straw,straw, and and reclaimed reclaimed (Sousa (Sousa and and Veiga, Veiga, 2009) 2009) [70 [70].].

2.3.6. Sluicing—Summary Sluice boxes are a very adequate technology for artisanal miners and can be very efficient if well designed and operated. The main points related to sluicing are listed as follows:

• Large amounts of water are needed: 10–15 m3/min for a typical 60-cm wide sluice; the water is rarely recycled; • Artisanal miners do not use an efficient size classification of the sluice box feed. They only remove the large cobbles with a grizzly screen. This provokes turbulence removing the fine gold particles from the bed, reducing the recovery; • Riffles are not good for fine gold particles, only for large gold specks, as high riffles can create turbulence and fine gold particles are lost;

Minerals 2020, 10, 1026 20 of 49

2.3.6. Sluicing—Summary Sluice boxes are a very adequate technology for artisanal miners and can be very eﬃcient if well designed and operated. The main points related to sluicing are listed as follows:

Large amounts of water are needed: 10–15 m3/min for a typical 60-cm wide sluice; the water is • rarely recycled; Artisanal miners do not use an efficient size classification of the sluice box feed. They only remove • the large cobbles with a grizzly screen. This provokes turbulence removing the fine gold particles from the bed, reducing the recovery; Riffles are not good for fine gold particles, only for large gold specks, as high riffles can create • turbulence and fine gold particles are lost; Miners often work with high amounts of solids in the pulp (e.g., 30–40% in mass). This makes it • difficult for gold to sink in the sluice to be concentrated; 10–20% of solids in the pulp would be more adequate; Miners use long sluices hoping to catch fine gold but this is not effective. Most gold is concentrated • within the first 2 m of the sluice; Fine gold is concentrated where the pulp flow speed is slower, i.e., at the beginning of the sluice. • Zigzag sluicing, with multiple decks, is a good way to break the flow velocity and capture more fine gold than a turbulent single-deck sluice; Wide sluices can process more material than narrow sluices; • The carpets cannot be allowed to clog as this stops further gold particles’ deposition. In sluices • with low angles, the material settles faster. The usual angle of a sluice box is 10–15◦. Some miners in Ecuador use 5◦ to increase the gold recovery, but they have to clean the carpets every hour, and consequently, the concentrate grade is poor; There are a variety of carpets available in the market. The vinyl loop carpets are very efficient, • but they must be unbacked, i.e., no rubber at the back, otherwise the gold particles become entrained in the carpet and are hard to dislodge at the end of the operation; Flat particles of gold can float on the sluices and are lost; and, • Spirals or other types of sluice such as pinched sluices or Reichert cones, have not been observed • in use in AGM operations. These would bring relevant improvements since the operations do not need to be interrupted to discharge the concentrates. The main reasons for not using these pieces of equipment in AGM are: (1) high price, (2) no local availability, (3) high mass of concentrates to be treated after concentration, and (4) lack of knowledge of the artisanal miners about these technologies.

2.4. Jigging Gravity concentration by jigging has been used in conventional mining operations for more than 100 years (Abols and Grady, 2006 [71]), but they are not common in AGM operations. Jigs have been used for gold concentration since the Old Egyptian times (Sampaio and Tavares, 2005) [23]. Jigging is a continuous gravity concentration process that separates heavy from lighter particles by a pulsating flow of water caused by a piston. This pumps and sucks water, creating a flow that throws the light particles upwards. This process fluidizes the particles, and the heavy particles sink into a bed of steel balls (or pebbles) called “ragging”. The ragging is held by a steel screen but during the pulsation, it also moves up and down opening room for the heavy particles to pass through it to be discharged at the bottom of the jig. Wills and Finch (2016) [72] explained the mechanisms involved in a jig, showing that the action of pulsing and suction are responsible for the stratification and, therefore, the concentration of the heavy particles (Figure 25). The smaller and lighter particles are expelled from the jig and the heavier particles, by density or by size, settle down faster. Minerals 2020, 10, 1026 21 of 49 Minerals 2020, 10, x 23 of 51

Figure 25. 25. SnapshotSnapshot of ofa jig a jigcycle. cycle. Water Water pulses pulses and concentration and concentration (adapted (adapted from Wills from and Wills Finch, and 2016)Finch, [72]. 2016) [72]. Minerals 2020, 10, x 23 of 51 InPriester most etColombian al. (1993) [operations73] showed visited, a jig used the bydischarge artisanal of miners a ball withmill awas manual classified system in thata trommel creates witha pulsating a 1 to 2 emmffect screen on a bed attached of minerals to the toball concentrate mill (Figure gold 28). specks The coarse at the particles bottom of (+1 the mm) jig. are Jigs returned are very tosensitive the ball to mill particle manually size and with shape a shovel. and usually The tailings are eff ectivefrom tothe separate jig are processed particles of in gold a shaking coarser table than without100 mesh any (0.15 further mm). classification Mitchell et al. (Figure (1997a) 29). [48 As] reinforce the jig concentrates the idea that are jigs not should very rich process in gold separately due to largescreened amounts fractions of sulfides and highlighted and coarse that particles recovery of gangue, falls to some less than miners 50% amalgamate for gold finer the thanconcentrates 0.1 mm. inJigs small accept ball large mills amounts or leach ofwith ore cyanide and can after process re-gri upnding. to 1000 This tph configuration of ore (Wills was and an Finch, initiative 2016) of [the72]. governmentPulp density that is usually hired a 10–30% company solids. to implement this design in over 70 formalized operations, without takingIn into Antioquia, consideration Colombia, the mineralogical many artisanal differences operations among use 8 them. inch 12 inch jigs with the capacity × to process 0.3–1.5Figure tph, 25. but Snapshot in many of a jig cycle. cases Water processing pulses and concentration less than (adapted 0.07 from tph. Wills and These Finch, jigs have 0.75 kW 2016) [72]. electric motors kW (1 HP), use 2.4 L/min of water, and are located at the discharge of the ball mills (Figures 26 and 27).In Themost pulpColombian entering operations these visited, small the discharge jigs, after of athe ball additionmill was classified of water, in a trommel averaged 25% solids. with a 1 to 2 mm screen attached to the ball mill (Figure 28). The coarse particles (+1 mm) are returned The Colombianto jigs the ball use mill alateral manually plunger with a shovel. which The tailings is an adaptationfrom the jig areof processed the Harz in a shaking jig. The table plunger, located outside the jig,without moves any vertically further classification and presses (Figure 29). down As the on jig theconcentrates water are in not the very jig rich tank in gold in due small, to but frequent, large amounts of sulfides and coarse particles of gangue, some miners amalgamate the concentrates pulses (aroundin 100 small pulses ball mills/min). or leach This with cyanide moves after the re-gri waternding. flow This configuration against the was ragging. an initiative New of the jigs are sold in Colombia for US$government 2000–5000 that hired/unit a company depending to implement on whetherthis design in the over jig 70 formalized is locally operations, made orwithout imported. taking into consideration the mineralogical differences among them.

Figure 26. Jig in Colombia uses a bed of small steel balls.

Despite some gold concentration, the Colombian jigs seem to work merely as a size classification device for the shaking tables, since the coarse particles of gangue are reported to the jigs’ concentrates together with theMinerals heavy 2020, 10 , xminerals. Figure Most 26. Jig local in Colombia artisanal uses a bed miners of small steel criticize balls. this arrangement24 of 51 as it is not Figure 26. Jig in Colombia uses a bed of small steel balls. very efficient for gold concentration and the jig provides a low concentration ratio. Most Colombian Despite some gold concentration, the Colombian jigs seem to work merely as a size classification artisanal plantsdevice abandoned for the shaking this tables, configuration since the coarse toparticles use ofa gangue1 mm are trommel reported to atthe thejigs’ concentratesball mill discharge and only the shakingtogether tables. with the heavy minerals. Most local artisanal miners criticize this arrangement as it is not very efficient for gold concentration and the jig provides a low concentration ratio. Most Colombian artisanal plants abandoned this configuration to use a 1 mm trommel at the ball mill discharge and only the shaking tables.

FigureFigure 27. Plunger27. Plunger jig used used in Colombia. in Colombia.

Figure 28. The ground material is screened: the −1 mm goes to the jig and +1 mm returns to the ball mill.

Figure 29. Tailings from the jig are processed by a shaking table (Colombia).

Jigging—Summary The main points related to jigs in AGM are as follows:

677 Despite some gold concentration, the Colombian jigs seem to work 678 merely as a size classification device for the shaking tables, since the Minerals 2020, 10, 1026 679 coarse particles of gangue are reported to the jigs’ concentrates22 of 49 680 together with the heavy minerals. Most local artisanal miners criticize In most Colombian681 operations this arrangement visited, the dischargeas it is not of very a ball efficient mill wasfor gold classified concentration in a trommel and with a 1 to 2 mm screen attached682 the to thejig prov ballide mills a (Figure low concentration 28). The coarse ratio. particles Most Colombian (+1 mm) are artisanal returned to the ball mill manually with a shovel. The tailings from the jig are processed in a shaking table 683 plants abandoned this configuration to use a 1 mm trommel at the ball without any further classification (Figure 29). As the jig concentrates are not very rich in gold due to large amounts of sulfides684 and coarsemill discharge particles ofand gangue, only the some shaking miners tables. amalgamate the concentrates in small ball mills or leach with cyanide after re-grinding. This configuration was an initiative of the government that hired a685 company to implement this design in over 70 formalized operations, without 686 Figuretaking 25 into– Plunger consideration jig used in Colombia the mineralogical differences among them.

732 (Figure 29). Some AGM operators use another shaking table to upgrade the concentrates. Gemeni 733 tables operate with no lateral slope, unlike traditional shaking tables, and with grooves, instead of 687 734 rifles, following the trapezoidal shape of the table. Gemeni tables have low capacity and can process Figure 28. The ground material is screened: the 1 mm goes to the jig and +1 mm returns to the 688 Figure 26 - The ground material is screened: the -1 mm− goes to the jig and +1 mm returns to the ball mill 735 from ball27 kg/h mill. to 455 kg/h of material (MD Mineral Technologies, 2005). 689 Jigging - Summary

690 The main points related to jigs in AGM are as follows:

691  Jigs operate much better with feed classified into narrow particle size ranges. Jigs are sensitive 692 to wide ranges of particle size fractions and without size classification are not efficient as the 693 coarse particles of gangue minerals concentrate together with the heavy minerals; 694  Jigs are not efficient for fine gold particles; 695  Jigs require large amounts of water; 696  For primary ores, particles ground and screened through 1 mm sieves rarely contain liberated 697 gold, therefore the gold grade of the concentrates is not high; and 698736  In Colombian AGM operations, jigs work as size classifiers, basically removing the coarse

699737 particles beforeFigureFigure the 29. 27 Tailingsshaking –Tailings from table from the theconcentration jig jig are areprocessed processed. by a shaking table table (Colombia). (Colombia). Despite some gold concentration, the Colombian jigs seem to work merely as a size classification device for the shaking tables, since the coarse particles of gangue are reported to the jigs’ concentrates together with the heavy minerals. Most local artisanal miners criticize this arrangement as it is not very efficient for gold concentration and the jig provides a low concentration ratio. Most Colombian artisanal plants abandoned this configuration to use a 1 mm trommel at the ball mill discharge and only the shaking tables.

Jigging—Summary The main points related to jigs in AGM are as follows: Jigs operate much better with feed classified into narrow particle size ranges. Jigs are sensitive to • wide ranges of particle size fractions and without size classification are not efficient as the coarse 738 particles of gangue minerals concentrate together with the heavy minerals; 739 Figure 28 –Shaking tales in Colombian artisanal plants in Antioquia.

740 Minerals 2020, 10, 1026 23 of 49

Jigs are not efficient for fine gold particles; • Jigs require large amounts of water; • For primary ores, particles ground and screened through 1 mm sieves rarely contain liberated • gold, therefore the gold grade of the concentrates is not high; and In Colombian AGM operations, jigs work as size classifiers, basically removing the coarse particles • before the shaking table concentration.

2.5. Tabling A shaking table is a riffled inclined deck that shakes horizontally. The most popular is the Wilfley table developed in 1896 (Gupta and Yan, 2016) [28]. The height of the riffles depends on the grain size of the ground ore but is usually 3 to 5 mm. Shaking tables operate in the conventional gold mining sector to obtain the final product for gold smelting after a previous concentration in a centrifuge, for example. The tables are efficient to recover gold grains as fine as 200 mesh (0.074 mm) but the efficiency depends on the particle size distribution range. The narrower the grain size fraction, the more efficient is the operation. Therefore, classification by screening can improve the gold recovery, which is also applicable for all gravity concentration processes. Shaking tables can process up to 2 tonnes per hour (tph) of ore ground to 1 to 1.5 mm and 1 tph for ore ground to 0.10 mm (150 mesh) operating with a pulp of 20–25% solids by mass. The length of the horizontal movement of the table has a great influence on the gold recovery. Fast speed is useful for fine grain sizes of gold (Wills and Finch, 2016) [72]. Industrial tables usually have decks 2 to 4.5 m long made of rubber or fiberglass or wood (Silva, 1986) [38]. The operation of shaking tables is not complicated but the two table slopes, ranging from 0 to 6◦ (higher in some cases as mentioned by Mitchell et al., 1997b [74]), must be adjusted according to the type of ore and its grain size (Sampaio and Tavares, 2005) [23]. Higher angles of the lateral slope of the table allow more ore to be processed. Lower angles of the frontal slope allow the retention of finer and lighter materials on the table. Heavy particles (coarse and/or dense) are moved along the table’s riffles by the throw/stroke (horizontal movement), while small/light particles are washed across the riffles, down into the tailing discharge. The table also discharges the “middling” fraction (with intermediate specific gravity) between the concentrate and the tailings, which is usually composed of unliberated heavy minerals such as iron oxides, sulfides, and gold. Most artisanal miners do not recirculate the middlings to the grinding circuit and submit concentrates plus middlings to amalgamation. As a result, the mercury traps only the liberated gold leaving behind a rich gold and mercury-contaminated tailing. Operators must find the adequate table slope for the type of ore and the grain size they are processing. Shaking tables are usually inclined in two directions. There are tables specifically designed to process very fine particles of ground ore, also known as “slimes table” (Silva, 1986) [38]. Shaking tables are becoming popular in AGM operations in Latin America. In many operations in Colombia, usually with processing capacity below 30 tpd, miners use the shaking table to process tailings from the jig (Figure 29) or just at the discharge of the ball or Chilean mills (Figure 30). Some Colombian plants use a spiral gold panning machine or a homemade elutriator to upgrade the table concentrates. These machines are also used to separate the amalgam from the heavy minerals (Figure 31). Some AGM operators use another shaking table to upgrade the concentrates. Gemeni tables operate with no lateral slope, unlike traditional shaking tables, and with grooves, instead of rifles, following the trapezoidal shape of the table. Gemeni tables have low capacity and can process from 27 kg/h to 455 kg/h of material (MD Mineral Technologies, 2005) [75]. 32

736 Minerals 2020, 10, 1026 24 of 49 737 Figure 27 –Tailings from the jig are processed by a shaking table (Colombia).

738 33 739 FigureFigure 30. 28Shaking –Shaking tales tales in in Colombian Colombian artisanal plants plants in inAntioquia Antioquia..

740

741 742 FigureFigure 29 – 31.GemeniGemeni table, spiral table, gold spiral wheel, gold and wheel, elutriator and used elutriator by Colombian used artisanal by Colombian miners to upgrade artisanal table miners concentrates to upgrade table concentrates. 743 As using one shaking table often does not provide high gold recoveries, several Colombian miners use 744 twoAs or usingthree tables one shaking to re-process table often tailings. does There not provide are many high different gold recoveries, arrangements several of these Colombian tables miners 745 useworking two or in three parallel tables or in to series, re-process and then tailings. treating There the concentrates are many di throughfferent arrangementsamalgamation and of these treating tables 746 workingthe tailings in parallel with cyanidation. or in series, An and example then treatingof an AGM the plant concentrates processing through sulfide amalgamation-rich gold ores and and treating the tailings with cyanidation. An example of a Colombian AGM plant processing sulfide-rich gold 747 suggestion of improvements using three tables as rougher, cleaner, and scavengers is shown in ores using three tables as rougher, cleaner, and scavengers is shown in Figure 32a. The process of 748 re-grindingFigure 30. concentratesThe process of before re-grinding using concentrates amalgamation before or even using conducting amalgamation amalgamation or even conducting in small ball 749 mills,amalgamation which is popular in small in ball many mills, AGM which sites, is popular is a result in many of poor AGM size sites, classification is a result inof thepoor primary size grinding 750 circuit.classificat Theion amalgamation in the primary of grinding the concentrates circuit. The and amalgamation middlings resultsof the concentrates in tailings rich and in middlings mercury that in 751 manyresults cases in tailings are leached rich in with mercury cyanide that in exacerbating many cases the are toxicity leached of with the cyanide final tailings. exacerbating The amalgamation the toxicity is completely unnecessary if the miners upgrade the concentrate from the shaking table 3a in a Gemeni 752 of the final tailings. The amalgamation is completely unnecessary if the miners upgrade the table smelting the concentrate and leaching the middlings and tailings with cyanide. Even with the 753 concentrate from the shaking table 3(a) in a Gemeni table smelting the concentrate and leaching the suggested improvement shown in Figure 32b, the final tailing from the scavenger shaking table 3b, 754 mightmiddlings still have and atailings highamount with cyanide of fine. Even unliberated with the goldsuggested since theimprovement trommel screen shown of in1 Figure mm at 30 theb, ballthe mill 755 dischargefinal tailing is toofrom coarse. the scavenger If the concentrate table 3(b), frommight tablestill have 2b has a high gold amount specks of above fine unliberated 0.1 mm, it gold is suggested since 756 tothe remove trommel them screen in a of Gemeni 1 mm at table, the ball upgrading mill discharge it for is a too final coarse. smelting If the process concentrate with f borax.rom table Therefore, 2b more efficient classification methods should be used at the discharge of the ball or Chilean mills. 757 has gold specks above 0.1 mm, it is suggested to remove them in a Gemeni table, upgrading it for a 758 final smelting process with borax. Therefore, more efficient classification methods should be used at 759 the discharge of the ball or Chilean mills. Minerals 2020, 10, 1026 25 of 49 34

760 761 (a)

762 763 (b)

764 FigureFigure 30 – 32. (a)(a Example) Example of of a agold gold concentration concentration plant plant in in Colombia Colombia and (b) (b) Suggested suggested simple simple improvements improvements in the configuration of the shaking tables. 765 Homemade Shaking Tables 2.5.1. Homemade Shaking Tables 766 The CICAN (Colleges and Institutes Canada) Project in Colombia (Veiga et al., 2018), was a project The CICAN (Colleges and Institutes Canada) Project in Colombia (Veiga et al., 2018) [37], was a 767 sponsoredproject by sponsored Global Affairs by Global Canada Affairs Canadato provide to provide technical technical support support to artisanal to artisanal miners miners in in Colombia. The 768 project,The led project, by the led CÉGEP by the C ÉfromGEP froml'Abitibi l’Abitibi-T-Témiscamingueémiscamingue,, Rouyn Rouyn-Noranda,-Noranda, Québec Québec,, decidedto to use use inexpensive locally found materials to teach artisanal gold miners and their trainers how to make 769 inexpensivetheir own locally processing found equipment.materials to This teach approach artisanal is extremely gold miners noble, and valuable, their trainers and very how much to in make line their 770 own processingwith what Schumacherequipment. This (1973) approach [76] described is extremely as “appropriate noble, valuable technology”, and in very his best-sellermuch in line “Small with what is Beautiful”. This, according to the author of this book, is the basis for the local sustainability of a 771 Schumachercommunity. (1973) The described project built as a “appropriate simple ore impact technology” mill using in a his steel best chain-seller (Figure “Small 33) using is Beautiful” the same. This, principle as a hammer mill. The pieces of equipment do not have high capacity and are sufficient for micro-miners producing less than 2 tpd. Beyond different types of grinder, the project also developed Minerals 2020, 10, x 28 of 51 areMinerals sufficient 2020, 10 for, x micro-miners producing less than 2 tpd. Beyond different types of grinder,28 of 51 the projectMinerals also2020 developed, 10, 1026 homemade shaking tables and flotation cells and transferred the blueprints26 of 49 to are sufficient for micro-miners producing less than 2 tpd. Beyond different types of grinder, the local mechanical shops to make these pieces of equipment at the artisanal mining sites in Colombia. project also developed homemade shaking tables and flotation cells and transferred the blueprints to Small shaking tables were built using plywood, plastic buckets, and iron bars. The shaking tables can localhomemade mechanical shaking shops tables to make and flotation these pieces cells andof eq transferreduipment at the the blueprints artisanal mining to local sites mechanical in Colombia. shops process 100–200 kg/h of ground ore and work either with a 1 HP electric motor to generate the Smallto make shaking these piecestables ofwere equipment built using at the plywood, artisanal plastic mining buckets, sites in and Colombia. iron bars. Small The shaking shaking tables tables were can horizontal stroke or by a pedal that shakes the table and provides energy for a pump that recirculates processbuilt using 100–200 plywood, kg/h plasticof ground buckets, ore and ironwork bars. either The with shaking a 1 HP tables electric can process motor 100–200to generate kg/ hthe of thehorizontalground water. ore A stroke andbucket work or with by either a pedalwater with that is a filled 1shakes HP electricon the the table top motor andof tothe provides generate table andenergy the the horizontal for ground a pump stroke ore that is or recirculatesfed by manually a pedal (Figurethethat water. shakes 34). TheA the bucket cost table of with and building provides water thisis energyfilled shaking on for the atable pump top canof that the range recirculatestable from and US$the the ground 100 water. to 200 Aore bucket dependingis fed with manually water on the accessibility(Figureis filled on34). theof The materials top cost of theof buildingand table wel and ders.this the shaking ground Tests, conducted oretable is can fed manuallyrange on a fromgold (Figure US$ore from100 34). to TheQuebec, 200 cost depending of using building theon thisthechain millaccessibilityshaking grinding table belowof can materials range 0.5 to from and 1 mm US$wel followedders. 100 toTests, 200 by dependingconducted the shaking onon thetable,a gold accessibility resulted ore from in ofQuebec, a materials gold usingrecovery and the welders. chainof 88%. ProjectsmillTests, grinding conductedin Colombia below on are a 0.5 gold demonstrating to ore1 mm from followed Quebec, these by using techni the theshakingques chain to table, millartisanal grindingresulted miners belowin a (Veiga gold 0.5 torecovery and 1 mm Correa, followed of 88%. 2019) [77].Projectsby the shaking in Colombia table, resultedare demonstrating in a gold recovery these techni of 88%.ques Projects to artisanal in Colombia miners (Veiga are demonstrating and Correa, 2019) these [77].techniques to artisanal miners (Veiga and Correa, 2019) [77].

Figure 33. Two different chain mills from the CICAN (Colleges and Institutes Canada) project. FigureFigure 33. 33. TwoTwo different diﬀerent chain mills from the CICAN (Colleges(Colleges and Institutes Canada) project.

Figure 34. Shaking table operated by a foot-operated lever. (CICAN project). FigureFigure 34. 34. ShakingShaking table table operated operated by a foot-operated lever. lever. (CICAN (CICAN project). project). 2.5.2. Tabling—Summary 2.5.2.2.5.2. Tabling—Summary Tabling—Summary The main aspects related to shaking tables are as follows: TheThe main main aspects aspects related related to to shaking shaking tables are asas follows:follows: • Shaking tables, when used as primary gravity concentrators, do not provide high production • Shaking tables, when used as primary gravity concentrators, do not provide high production • Shakingrates, with tables, production when used usually as primarybetween gravity20 to 40 tpdconcentrators, of ores; do not provide high production rates, with production usually between 20 to 40 tpd of ores; • rates,Ball withand Chilean production mills usuallyusually betweenflatten gold 20 particles,to 40 tpd which of ores; can lead to the particles floating over • Ballthe and shaking Chilean table mills riffles usually to beflatten lost goldwith particles, tailings. whichGold iscan naturally lead to thehydrophobic particles floating and this over the shaking table riffles to be lost with tailings. Gold is naturally hydrophobic and this

Minerals 2020, 10, 1026 27 of 49

Minerals 2020, 10, x 29 of 51 Ball and Chilean mills usually flatten gold particles, which can lead to the particles floating over the • phenomenonshaking table riofffl “floatinges to be lost flaky with gold” tailings. is commonly Gold is naturally observed hydrophobic in laminar and gravity this phenomenon separation methods;of “floating flaky gold” is commonly observed in laminar gravity separation methods; • TheThe classification classification of of the the ground ground material material (grinding (grinding in in a a closed closed circuit) circuit) is is important important for for an an efficient efficient • goldgold concentration. TheThe culture culture of of the the artisanal artisanal miners miners in all in continents all continents is to introduce is to introduce re-grinding re- grindingcircuits rather circuits than rather using than closed using grinding closed grinding circuits. circuits. It is interesting It is interest thating artisanal that artisanal miners miners usually usuallyneglect neglect the classification the classification processes processes after milling after andmilling do notand believe do not in believe any benefit in any of benefit creating of a creatingcirculating a circulating load in the load grinding in the circuit. grinding The circui returnt. ofThe 100–400% return of of 100–400% the ore mass of the to theore mill mass typically to the millreduces typically the retention reduces time the ofretention the material time inside of the the material mill increasing inside the the mill grinding increasing capacity, the improving grinding capacity,the energy improving efficiency, the resulting energy in efficiency, a more homogenous resulting in ground a more product homogenous (Jankovic ground et al., 2013 product)[78]. (JankovicArtisanal et miners al., 2013) frequently [78]. Artisanal grind in miners open circuits frequently with grind the gravity in open separation circuits with equipment. the gravity It is separationalso worth equipment. noting that It putting is also gravityworth noting recovery that equipment putting gravity in a circuit recovery with equipment a circulating in a load circuit can withimprove a circulating gold recovery load can given improve the inherently gold recovery low single-pass given the inherently efficiency of low gravity single-pass recovery efficiency devices; of gravity recovery devices; Over 50 plants in Colombia were observed using shaking tables as their main gravity concentration •• Over 50 plants in Colombia were observed using shaking tables as their main gravity step, with gold recoveries rarely above 60%. Gold is mainly lost in coarse grains (unliberated concentration step, with gold recoveries rarely above 60%. Gold is mainly lost in coarse grains gold) and the fines (not trapped by the table). For ores with fine-sized gold particles and/or high (unliberated gold) and the fines (not trapped by the table). For ores with fine-sized gold particles concentration of sulfides, AGM plants combining gravity separation with flotation can obtain and/or high concentration of sulfides, AGM plants combining gravity separation with flotation gold recoveries above 85%; can obtain gold recoveries above 85%; The adjustment of the length of the stroke and the slope of the shaking tables are critical for •• The adjustment of the length of the stroke and the slope of the shaking tables are critical for efficient separation of fine gold particles; and, efficient separation of fine gold particles; and, Homemade shaking tables have been proven an affordable solution for micro-miners processing •• Homemade shaking tables have been proven an affordable solution for micro-miners processing less than 2 tpd. less than 2 tpd. 2.6. Centrifuging 2.6. Centrifuging Centrifugal concentrators are the most efficient pieces of equipment for gravity separation. Centrifugal concentrators are the most efficient pieces of equipment for gravity separation. The The main initial application of the centrifugal concentrators was to recover fine gold (Sampaio and main initial application of the centrifugal concentrators was to recover fine gold (Sampaio and Tavares, 2005) [23]. The principle of gold concentration in centrifugal equipment is the separation Tavares, 2005) [23]. The principle of gold concentration in centrifugal equipment is the separation of of gold from gangue mineral particles based on the difference in the settling velocity (Chen et al., gold from gangue mineral particles based on the difference in the settling velocity (Chen et al., 2020) 2020) [79]. When the particles are subjected to the normal gravitational force (G-force = 1), like in a jig, [79]. When the particles are subjected to the normal gravitational force (G-force = 1), like in a jig, a a gold particle with a diameter of 0.02 mm falling in water reaches the same terminal velocity as a gold particle with a diameter of 0.02 mm falling in water reaches the same terminal velocity as a quartz particle of 0.07 mm. If a very narrow grain size classification is not previously employed, there is quartz particle of 0.07 mm. If a very narrow grain size classification is not previously employed, there poor selectivity in the separation. Considering that the acceleration of both particles in a centrifuge is is poor selectivity in the separation. Considering that the acceleration of both particles in a centrifuge 60 or more times higher than the gravitational acceleration, the difference in terminal velocity between is 60 or more times higher than the gravitational acceleration, the difference in terminal velocity the two particles increases, therefore the difference between the sizes of the particles to be separated between the two particles increases, therefore the difference between the sizes of the particles to be also increases proportionally. The acceleration of a centrifugal concentrator is expressed as “G”, separated also increases proportionally. The acceleration of a centrifugal concentrator is expressed as i.e., how many times the centrifugal acceleration acting on a particle is higher than the gravitational “G”, i.e., how many times the centrifugal acceleration acting on a particle is higher than the acceleration. The “G” of a centrifugal concentrator is expressed as dividing the centrifugal acceleration gravitational acceleration. The “G” of a centrifugal concentrator is expressed as dividing the (ω2 r) by the gravitational acceleration (g) (Sampaio and Tavares, 2005) [23]: centrifugal acceleration (ω2 r) by the gravitational acceleration (g) (Sampaio and Tavares, 2005) [23]: 2 ωωrr GG== gg where:where: ωω == angularangular velocity velocity (rad/s), (rad/s), r r== radiusradius of of the the centrifuge, centrifuge, and and g g= =gravitationalgravitational acceleration acceleration

øø ͷ G=G = 182182 where:where: ø ø == centrifugecentrifuge diameter diameter (m) (m) and and ͷ = =rotationrotation frequency frequency (rpm) (rpm) AA centrifugal centrifugal concentrator concentrator operates operates when when a ground a ground ore ore pulp pulp is introduced is introduced at the at bottom the bottom of the of equipmentthe equipment and particles and particles in the in slurry the slurry flow upward ﬂow upward and outward and outward due to duethe centrifugal to the centrifugal force. When force. theWhen particles the particles are trapped are trapped in a V-shape in a V-shape bowl with bowl ring withs of rings riffles, of ri theseﬄes, theseparticles particles can or can cannot or cannot find counter-flow water that fluidizes the bed, expelling the light particles and retaining the heavy ones. It is important to stress that a centrifugal concentrator works in an exchange process where a light trapped particle leaves the bed to open room for a heavy one. In this process, the volume of

Minerals 2020, 10, 1026 28 of 49

find counter-flow water that fluidizes the bed, expelling the light particles and retaining the heavy ones. It is important to stress that a centrifugal concentrator works in an exchange process where a light trapped particle leaves the bed to open room for a heavy one. In this process, the volume of concentrate (which is the volume of the space between rings) remains constant. The counter-flow water pressure helps in this process. The discharge of the trapped concentrate can occur intermittently or continuously through an automatic diaphragm. More water pressure is needed when the centrifuge operates with coarse or high specific gravity particles. The centrifuge with fluidizing water was first patented by McNicol in Australia in 1935 (Sampaio and Tavares, 2005) [23]. As centrifuges operate at high G-Forces, they are less sensitive to gold particle shape than other gravity concentration equipment and accept larger variations of particle size than other concentrators. The maximum feed grain size is 6 mm but it is recommended to only feed particles smaller than 2 mm. Some centrifuges can operate with extremely small particles, as fine as 3 µm (Sepro, 2019) [80]. Unliberated particles can also be concentrated depending on the proportion of gold in the gangue particle. However, centrifugal concentration is sensitive to the proportion of clay minerals in the pulp as clay minerals lead to an increase in pulp viscosity, thus, decreasing gold recovery (Agorhom and Owusu, 2020) [81]. Conventional mines typically place centrifuges in closed-loop grinding circuits, either at the discharge of the mills or the underflow of cyclones to remove coarser gold grains from the circuit before the ore enters the flotation and/or cyanidation circuits. The percentage of solids in the feed pulp is usually from 20% to 40% and for fine materials, this ranges from 10% to 20% solids. In a discontinuous centrifuging system, which is the one used by some artisanal miners, after a certain time of operation the centrifuge stops and the concentrate is washed out, usually manually, from the rings. According to Figure2, large amounts of concentrate (which is obtained when the discharging cycles are shorter) produce high gold recoveries but low gold grades. The operators must assess the concentrate discharging cycle for each type of ore. In many cases, the rinse cycle occurs between 20 and 120 min of operation. The mass of concentrate in a centrifuge is typically much less than 0.1% of the original feed. It was observed in the field, in an artisanal operation in Ecuador, that by feeding 2 tph of ore with 4 g Au/t in a centrifuge, in 30 min of operation, 1 kg of concentrate analyzed 3000 gAu/t. The sizes of commercial centrifugal concentrators vary considerably and can process up to 1000 tph of ore. There are many different types of centrifugal concentrators in the market. Some are from Chinese, South African, Brazilian, etc. manufacturers, but the most popular ones are the Knelson, by FLSmidth, and the Falcon, by Sepro, both developed separately in British Columbia, Canada. They produce the main centrifuges used in conventional mining operations. The main differences between a Knelson concentrator and a Sepro centrifuge are the design of the bowls and the “G” (G = 60–150 for Knelson and G is variable up to 200 for Sepro) (FLSmidth, 2013 [82], Falcon Concentrator, 2019 [83]). The bowl of a Knelson, from bottom to top, has rings with riffles whereas a Sepro centrifuge has a smooth surface where the light minerals move up faster than the heavy minerals. At the top of the bowl, the centrifuge is similar to the Knelson concentrator, with rings and fluidizing water assisting the particles’ exchange process. These centrifuges are becoming popular in artisanal mining plants. The lack of fluidization of the rings that retain concentrates, as seen in the old original centrifuges, resulted in compaction of the riffles and gold losses. Many models of centrifuges without fluidizing water are still available in the market, such as the Knudsen and GoldKacha (Figure 35). They are popular in Africa and have been observed in some artisanal gold operations in Colombia, Ecuador, and Peru. They rotate much slower than the Canadian centrifuges and have a metal scraper that continuously scratches the bed of concentrate trapped in the rings to allow the particles’ exchange. To obtain reasonable gold recoveries, the discharge processes must be more frequent than those used by the Canadian centrifuges. They are less expensive than the Canadian centrifuges, simpler, and provide reasonable gold concentration, possibly better than jigs. Minerals 2020, 10, 1026 29 of 49 Minerals 2020, 10, x 31 of 51

Knelson Concentrator Falcon Concentrator

GoldKacha Concentrator Knudsen Concentrator

FigureFigure 35. 35. CentrifugalCentrifugal concentrators concentrators with with fluidized fluidized bed ( bedon top (on) and top )without and without counter-flow counter-flow water (waterdown).( down). Sources: http://Sources:www.flsmidth.com /en-US/About+http://www.flsmidth.com/en-FLSmidth/Our+History/Our+ US/About+FLSmidth/Our+History/OuProduct+Brands/Knelson, http://www.concentrators.netr+Product+Brands/Knelson/ and http://aptprocessing.com/modular-and http://www.concentrators.net/.mining-equipment/gravity-concentration http://aptproc/gold-concentrator-goldkachaessing.com/modular-mining-equipment/gravity-/. concentration/gold-concentrator-goldkacha/. Koppalkar (2009) [34] mentioned that it is impractical to mimic the performance of full-scale centrifugesKoppalkar in a (2009) laboratory [34] mentioned unit. The authorthat it describedis impractical the gravityto mimic recoverable the performance gold (GRG) of full-scale process centrifugesdeveloped in by a Dr. laboratory André Laplante unit. The in author the early described 1990s as the a useful gravity way recoverable to obtain angold assessment (GRG) process of the developedrecoverable by gold Dr. (liberated André Laplante or not) usingin the a early laboratory 1990s centrifuge.as a useful Theway procedure to obtain consistsan assessment of a sequential of the recoverablegrinding and gold processing (liberated of a 30or tonot) 150 using kg sample a labo (dependingratory centrifuge. on the grade) The in procedure a 3 in laboratory consists Knelson of a sequentialcentrifuge. grinding In the first and stage, processing the entire of sample a 30 to is ground150 kg belowsample 0.85 (depending mm and concentrated.on the grade) The in tailingsa 3 in laboratoryfrom the first Knelson stage centrifuge. are then split In andthe typicallyfirst stage, 27 the kg isentire then sample ground is at ground 45 to 50% below passing 0.85 0.075mm and mm, concentrated.before being processedThe tailings through from the the first laboratory stage are unit. then Insplit the and third typically step, the 27 tailingskg is then from ground the second at 45 tostage 50% are passing ground 0.075 to 80%mm, below before 0.075 being mm processed and processed through again. the laboratory Once the unit. adequate In the size third is obtained,step, the tailingsa whole from sample the second of the orestage is groundare ground at the to adequate80% below size 0.075 and mm processed and processed to confirm again. the Once findings the adequate(Laplante, size 2000 is [obtained,84], Laplante a whole and sample Dunne, of 2002 the [ 85ore], is Laplante ground andat th Clarke,e adequate 2006 size [86]). and processed to confirmAt the Universityfindings (Laplante, of British 2000 Columbia [84], Laplante (UBC), aand similar Dunne, GRG 2002 process [85], wasLaplante also applied and Clarke, to find 2006 the [86]).adequate particle size to grind an ore before concentrating in a centrifuge. A pulp of ore is ground withAt 70% the solidsUniversity for 5 of min British in a laboratoryColumbia (UBC), ball mill a similar and submitted GRG process to centrifugal was also applied concentration to find inthe a adequate particle size to grind an ore before concentrating in a centrifuge. A pulp of ore is ground with 70% solids for 5 min in a laboratory ball mill and submitted to centrifugal concentration in a

Minerals 2020, 10, x 32 of 51

laboratory centrifuge.Minerals 2020, 10 The, 1026 tailings from the first concentration step are drained and ground30 of 49 with 70% solids for a further 5 min to be concentrated again. The procedure is repeated two or more times. In the example laboratoryshown in centrifuge. Figure The36, tailings the ore from ground the ﬁrst concentrationfor 15 min step resulted are drained in 92.4% and ground of gold with 70% extraction and a grain size solidsanalysis for a furtherrevealed 5 min toa beP80 concentrated (80% of again. ma Thess procedurebelow a is repeatedcertain two size) or more of times.0.136 In mm. the A similar procedure hasexample been shown described in Figure 36by, theZhang ore ground et al. for 15(202 min0) resulted [87]. in Re-grinding 92.4% of gold extraction the tailings, and a grain these authors size analysis revealed a P80 (80% of mass below a certain size) of 0.136 mm. A similar procedure has determined thatbeen the described studied by Zhang ore yielded et al. (2020) a [gold87]. Re-grinding recovery the of tailings, 64.7% thesewhen authors 45% determined of the ore that particles were ground belowthe 200 studied mesh. ore yielded a gold recovery of 64.7% when 45% of the ore particles were ground below 200 mesh.

(a)

(b)

Figure 36. (a)Figure A simple 36. (a) testing A simple procedure testing procedure to determine to determine gold gold recovery recovery vs. particlevs. particle size in asize centrifuge in a centrifuge(b) (b) Cumulative gold recovery of three steps of grinding and processing in a laboratory centrifuge. Cumulative gold recovery of three steps of grinding and processing in a laboratory centrifuge Sepro developed two types of small centrifuge: the Icon 150 (Figure 37), which can process up to Sepro developed2 tph of ore, andtwo the types Icon 350 of withsmall a capacity centrifuge: of 15 tph. th Thesee Icon have 150 been (Figure used in many37), countrieswhich can by process up artisanal and small-scale miners. The main characteristics of an Icon 150 are (Sepro, 2020) [80]: to 2 tph of ore, and the Icon 350 with a capacity of 15 tph. These have been used in many countries by artisanal and small-scale miners. The main characteristics of an Icon 150 are (Sepro, 2020) [80]: • Price in Canada

Minerals 2020, 10, 1026 31 of 49

Price in Canada

(a) (b) (c)

FigureFigure 37. 37.( a(a)) Scheme Scheme ofof anan Icon 150 Centrifuge, (b (b) )Operation Operation in in Brazil Brazil and and (c) ( cin) inColombia. Colombia.

KnelsonA single Concentrators pass of ore in alsoa batch off erscentrifugal a small concen versiontrator of a can centrifuge, provide therelati KC-MDvely good 7.5, recoveries which is an adequatefor some size ores. for While many conventional artisanal miners. mining This companies centrifuge typically can process use the 0.68 centrifuge tph of ore in with closed amax grinding particle sizecircuits, of 6 mm. AGM It operations uses 0.7 L rarely/minof use water this type and of generates configuration. 1 to 1.4 In kghard of rock concentrate AGM operations, per batch. miners It has a 0.75typically HP motor grind and in usesan open 48 to circuit 68 L/ minand ofthe clean concentr counter-flowator is located fluidizing at the water discharge (Knelson, of the 2020) grinding [88]. equipment.A single passTo increase of ore ingold a batch recovery, centrifugal some artisanal concentrator miners can use provide two or relatively three centrifuges good recoveries in series for someinstead ores. of While using conventional a closed circuit. mining To demonstrate companies typically this approach, use the a centrifuge laboratory in Knelson closed grindingConcentrator circuits, unit was used to concentrate ore from a group of Colombian artisanal miners. After 15 min of AGM operations rarely use this type of configuration. In hard rock AGM operations, miners typically grinding in a rod mill, 60.6% gold recovery was achieved with a single pass. When the tailings of the grind in an open circuit and the concentrator is located at the discharge of the grinding equipment. first pass were reprocessed in the same centrifuge without grinding, the gold recovery increased to To increase gold recovery, some artisanal miners use two or three centrifuges in series instead of using 76% and 85% in the second and third passes, respectively (Figure 38). In closed circuits, when the a closed circuit. To demonstrate this approach, a laboratory Knelson Concentrator unit was used centrifuge tailings are returned to a ball mill, placing centrifugal concentrators in series is not topracticed. concentrate It is ore hard from to compare a group data of Colombian from labora artisanaltory centrifuges miners. testing After a 15few min kilograms of grinding of ore inwith a rod mill,a field 60.6% centrifuge gold recovery processing was tonnes achieved of ore with per hour. a single Sometimes pass. When the results the tailings are not compatible. of the first passA pilot were reprocessedtest is always in therecommended same centrifuge to confirm without whet grinding,her scavenger the gold centrifuges recovery are increased justifiable. to 76% and 85% in the second and third passes, respectively (Figure 38). In closed circuits, when the centrifuge tailings are returned to a ball mill, placing centrifugal concentrators in series is not practiced. It is hard to compare data from laboratory centrifuges testing a few kilograms of ore with a field centrifuge processing tonnes of ore per hour. Sometimes the results are not compatible. A pilot test is always recommended to confirm whether scavenger centrifuges are justifiable.

Minerals 2020, 10, 1026 32 of 49 Minerals 2020, 10, x 34 of 51

(a) 100 88.98 90.88 85.19 76.07 80 60.62 60

CUMULATIVE AU RECOVERY (%) 0

(b)

Figure 38.38. ((aa)) Laboratory tests tests with a Colombian ore usin usingg ﬁvefive sequential passes in a KnelsonKnelson Concentrator. (b)) ResultsResults ofof goldgold recoveryrecovery inin 55 passespasses withoutwithout re-grindingre-grinding

Centrifuging—Summary The mainmain pointspoints toto highlighthighlight forfor centrifugalcentrifugal concentratorsconcentrators inin AGMAGM are:are: • CentrifugesCentrifuges areare muchmuch betterbetter gravitygravity concentratorsconcentrators forfor aa feedfeed withwith aa widewide particle size range than • otherother gravitygravity concentratorsconcentrators andand theythey cancan concentrateconcentrate veryvery finefine gold;gold; • CentrifugesCentrifuges areare lessless sensitivesensitive toto goldgold particleparticle shapes,shapes, suchsuch asas flakes;flakes; • • Counter-flowCounter-flow fluidizing fluidizing water water must must be be clear. clear. Of Often,ten, operators operators use use muddy muddy water, water, which which clogs clogs the • thecentrifuge; centrifuge; • TheThe counter-flowcounter-flow pressurepressure andand thethe rotatingrotating speedspeed areare twotwo parametersparameters thatthat mustmust bebe investigatedinvestigated • forfor aa betterbetter performanceperformance ofof thethe equipment.equipment. ArtisanalArtisanal minersminers frequentlyfrequently neglectneglect this;this; • HighHigh counter-pressurecounter-pressure waterwater increasesincreases thethe gradegrade ofof thethe concentrate,concentrate, eliminatingeliminating moremore ganguegangue • minerals,minerals, butbut thisthis cancan reducereduce thethe recoveryrecovery ofof finefine goldgold particles;particles; • CentrifugesCentrifuges areare generally generally best best operated operated in in a a closed closed circuit circuit with with a balla ball mill. mill. The The use use of of centrifuges centrifuges in • seriesin series (scavengers), (scavengers), when when grinding grinding in an in open an circuitopen circuit may increase may increase gold recovery gold recovery but this involvesbut this moreinvolves capital more and capital operating and operating costs and costs it is not and worthwhile it is not worthwhile for all types forof all ore. types of ore. • Operators must find out the adequate discharge time of concentrates. This requires chemical analyses of the products (feed and tailings) and this is not often locally available. Visual assessment by panning the concentrate and tailings can provide useful information; and,

Minerals 2020, 10, 1026 33 of 49

Operators must find out the adequate discharge time of concentrates. This requires chemical • analyses of the products (feed and tailings) and this is not often locally available. Visual assessment by panning the concentrate and tailings can provide useful information; and, Skill is needed to properly operate a centrifuge using fluidizing water. Many AGM operators • prefer non-fluidized bed centrifuges and, unfortunately, in some cases, they add mercury inside the bowls of the centrifuge, as was observed in Zimbabwe.

3. Extracting Gold from Gravity Concentrates Once the gold is concentrated into a small mass, the challenge is to extract gold from these concentrates. Four methods are discussed below:

1. Direct smelting 2. Amalgamation 3. Leaching 4. Coal-oil agglomeration

3.1. Direct Smelting The concentrates from gravity concentration can be directly melted with borax but this is not a trivial process since the concentrates must have high grades of gold. Borax (Na B O 10H O) has 2 4 7· 2 been used by conventional and artisanal miners for centuries as a flux to melt concentrates and pour gold bars. Appel and Na-Oy, (2011) [89] have been publicizing a “new Borax Method” as a panacea to extract gold from gravity concentrates without the use of mercury or any other leaching processes. Borax reduces the melting point of metals and silicates in the gravity concentrates. However, to bring all gold down to the bottom of a crucible, a large amount of gold must be in the concentrate as the gold particles must coalesce to create enough weight to sink in the molten bath, otherwise, the gold will stay in the viscous slag. These authors have suggested that gold grades in a concentrate must be higher than 30,000 g Au/t (or 3% Au) to avoid large losses of gold to the slag. As shown in Figure2, if miners wish to obtain by gravity concentration a very rich (yellow) concentrate, the gold recovery, in most cases, would be poor. The other inconvenience of the direct smelting of concentrates is the presence of sulfides. In tests with a sulfide-rich concentrate, from a centrifuge, with 3300 g Au/t and 1170 g Ag/t from Ecuadorian miners, most gold was lost to the slag (Veiga et al., 2014) [9]. When a mixture of KNO3, SiO2, and NaHCO3 was used to oxidize the sulfides, gold losses were reduced, but 16% of the gold still remained in the slag, contained 840 g Au/t. The process is not useful for most concentrates from primary ores, however, miners working with alluvial ores have been using borax to pour gold bars from concentrates for many decades. With alluvial ores, the gold is usually liberated, and no sulfides are normally present. An option for artisanal miners to melt concentrates with gold grades below 30,000 ppm is to add more gold or silver to the crucible to collect disperse particles of gold bringing them to the bottom of the melted mixture. This option seems to be hard for the miners to accept, as they want to sell their gold as soon as possible.

3.2. Amalgamation As discussed above, the amalgamation of gravity concentrates, as opposed to whole ore amalgamation, can bring a substantial reduction of mercury use and losses to the tailings. However, this process cannot amalgamate unliberated gold particles. When amalgamation is conducted in small ball mills, mercury pulverizes (“flouring”), forming droplets that do not amalgamate the gold grains. In this process, mercury also oxidizes faster, losing coalescence, i.e., the capacity of the droplets to agglomerate. This is popularly known as the “mercury sickening” effect. Sulfides also contribute to sickening mercury (Beard, 1987) [90]. Around the world, artisanal miners use different reagents to mix with mercury during the amalgamation process to reduce flouring, such as lemon juice, bicarbonate, quicklime, guava leaves, Coca-Cola, brown sugar, toothpaste, urea, detergents, etc. Minerals 2020, 10, 1026 34 of 49

The actual effect of these reagents on increasing mercury coalescence is not well understood, but the miners, anecdotally, confirm that they are useful to improve gold amalgamation and avoid droplets formation. Amalgamation must be a gentle process of mixing metallic mercury with the ore to contact Mineralsit with 2020 free, 10 gold, x particles. This can be done in a pan or a mixing barrel without grinding media.36 of 51 Pantoja and Alvarez (2000) [91] developed an electrolytic process to avoid mercury flouring by Pantoja and Alvarez (2000) [91] developed an electrolytic process to avoid mercury flouring by forming sodium-amalgam that has better gold amalgamation properties. This simple process consists forming sodium-amalgam that has better gold amalgamation properties. This simple process consists of making a solution of 10% of NaCl (a tablespoon of salt in a glass of water) and then contacting of making a solution of 10% of NaCl (a tablespoon of salt in a glass of water) and then contacting the the metallic mercury at the bottom of the glass to the negative pole of a 12 V motorbike battery or a metallic mercury at the bottom of the glass to the negative pole of a 12 V motorbike battery or a radio radio battery. A rod of graphite (extracted from the core of a radio battery) is connected to the positive battery. A rod of graphite (extracted from the core of a radio battery) is connected to the positive pole pole of the battery and stays in the solution for 15 min (Figure 39). In this process, sodium ions are of the battery and stays in the solution for 15 min (Figure 39). In this process, sodium ions are attracted attracted to the cathode (negative pole) forming a sodium-amalgam that has better gold amalgamation to the cathode (negative pole) forming a sodium-amalgam that has better gold amalgamation properties than pure mercury. The sodium retained by the mercury slowly reacts with water and forms properties than pure mercury. The sodium retained by the mercury slowly reacts with water and soluble NaOH. However, the activated mercury (sodium-amalgam) does not last long, likely less than forms soluble NaOH. However, the activated mercury (sodium-amalgam) does not last long, likely 2 h. Chloride ions are attracted to the anode and form hypochlorite anions. Using potassium chloride less than 2 h. Chloride ions are attracted to the anode and form hypochlorite anions. Using potassium to activate the mercury, a potassium-amalgam is formed—interestingly this can also amalgamate chloride to activate the mercury, a potassium-amalgam is formed—interestingly this can also platinum, as observed at a placer mining operation in Colombia. The sodium-mercury is brighter, amalgamate platinum, as observed at a placer mining operation in Colombia. The sodium-mercury has better coalescence, and amalgamates gold faster than pure mercury. is brighter, has better coalescence, and amalgamates gold faster than pure mercury.

Battery - + 12 V wire

Graphite rod Water with NaCl (or KCl) (10%) Mercury

(a) (b) (c)

Figure 39. Pantoja’s method being used in Brazil and Mozambique to activate mercury before Figureamalgamation. 39. Pantoja’s (a) Schememethod of being the mercury used in activation Brazil an process;d Mozambique (b) Pantoja’s to activate method mercury in use generating before amalgamation.hypochlorite gas(a) Scheme in the anode; of the ( cmercury) Use of aactivation radio battery process; to activate (b)Pantoja’s the mercury. method in use generating hypochlorite gas in the anode;(c) Use of a radio battery to activate the mercury. Once the amalgamation process ends, the heavy mass of the amalgam with excess mercury can easilyOnce be the separated amalgamation from the process rest of theends, heavy the heavy minerals, mass by of panning the amalgam or using with a spiralexcess or mercury an elutriator. can easilyThe excessbe separated mercury from is separatedthe rest of the from heavy the solidminera (paste)ls, by gold-silver-amalgampanning or using a spiral by filtration or an elutriator. which is Thenormally excess conductedmercury is by separated manual squeezingfrom the solid with a(p pieceaste) ofgold-silver-amalgam fabric or by using a by centrifuge filtration as which described is normallyabove in conducted session 2.3.4 by (Veiga manual et al.,squeezing 2006) [39 with]. The a piece final methodof fabric to or separate by using mercury a centrifuge from theas described gold-silver abovealloy in is session a chemical 2.3.4 decomposition (Veiga et al., 2006) of the [39]. amalgam The final that method can be doneto separate by leaching mercury the from mercury the goldwith a silvernitric alloy acid is solution a chemical (30% decompositionv/v) or by heating of thethe amalgamamalgam atthat temperatures can be done above by leaching 400 ◦C. Asthe observed mercury in withBolivia a nitric and acid Guyana, solution the dissolved(30% v/v) mercury,or by heating when the HNO amalgam3 is used, at cantemperatures be recovered above by 400 cementation °C. As with a bar of aluminum. Frequently, miners do not want to recover mercury and dump the acidic observed in Bolivia and Guyana, the dissolved mercury, when HNO3 is used, can be recovered by cementationsolution into with a river.a bar of When aluminum. decomposing Frequently, thermally miners the do amalgam,not want to unfortunately, recover mercury not and all artisanal dump theminers acidic use solution retorts into to condense a river. When the mercury decompos fumes.ing Theythermally often the prefer amalgam, to burn amalgamsunfortunately, in open not pansall artisanalor a shovel, miners even use if retorts simple to solutions, condense such the asmercur home-madey fumes. retorts, They haveoften alreadyprefer to been burn demonstrated amalgams in to openthem pans (Veiga or eta al.,shovel, 2014b) even [39 ].if Thesimple final solutions, product is such a gold asdor home-madeé with 2% toretorts, 5% mercury, have already depending been on demonstratedthe temperature to them of the (Veiga evaporation et al., process2014b) (Veiga[39]. The and final Hinton, product 2002 [is56 ]).a gold If the doré heating with temperatures 2% to 5% mercury,are low ordepending the heating on timethe temperature is insufficient of to the evaporate evaporation all mercury, process for (Veiga example and whenHinton, it is 2002 conducted [56]). If in thea bonfire,heating thetemperatures final doré can are have low as or much the heating as 20% mercurytime is insufficient (Veiga et al., to 2006) evaporate [39]. Even all mercury, a well-burned for example when it is conducted in a bonfire, the final doré can have as much as 20% mercury (Veiga et al., 2006) [39]. Even a well-burned amalgam still carries 2% to 5% of mercury, which is then usually evaporated in gold shops when the doré is melted, contaminating the operators and neighbors (Moody et al., 2020) [92].

Minerals 2020, 10, 1026 35 of 49 amalgam still carries 2% to 5% of mercury, which is then usually evaporated in gold shops when the doré is melted, contaminating the operators and neighbors (Moody et al., 2020) [92].

3.3. Leaching Minerals 2020, 10, x 37 of 51 Most conventional gold mining companies use cyanide to extract gold from concentrates or whole ore.3.3. Leaching Cyanide has been used and misused by artisanal mining in many countries. Since the implementationMost conventional of processing gold centers mining incompanies the early use 1990s, cyanide the to operatorsextract gold of from the concentrates centers have or offered amalgamationwhole ore. for Cyanide their clients, has been the used miners, and andmisused then by kept artisanal the mercury-contaminated mining in many countries. tailings Since tothe leach the residualimplementation gold with cyanide. of processing This, centers as mentioned in the earl above,y 1990s, can the cause operators significant of the centers environmental have offered problems amalgamation for their clients, the miners, and then kept the mercury-contaminated tailings to leach due to the residual formation gold of with mercury-cyanide cyanide. This, complexesas mentioned and above, poor can management cause significant of the environmental cyanidation tailings. Artisanalproblems cyanidationdue to the formation operations of mercury-cy use vat leaching,anide complexes usually and for poor gravity management concentration of the tailings, or agitatedcyanidation tanks tailings. for concentrates or tailings as well. To remove gold from the cyanide solution, some minersArtisanal use activated cyanidation carbon, operations but ause large vat majorityleaching, usually precipitate for gravity gold concentration with zinc in tailings, an adaptation or of the Merrill-Croweagitated tanks process. for concentrates These plants or tailings leach as ores,well. To concentrates, remove gold or from amalgamation the cyanide solution, tailings some for 8 to 12 h, miners use activated carbon, but a large majority precipitate gold with zinc in an adaptation of the then stop the agitation and wait for about 3 to 6 h for the material to settle. Then, the gold-rich solution Merrill-Crowe process. These plants leach ores, concentrates, or amalgamation tailings for 8 to 12 h, is siphonedthen stop to a the series agitation of 10 and cm-diameter wait for about PVC 3 to 6 columnsh for the material filled withto settle. zinc Then, shavings the gold-rich where solution the gold (and eventualis siphoned part ofthe to a mercury)series of 10 iscm-diameter precipitated, PVC and colu themns solutionfilled with is zinc recirculated shavings where to the the agitation gold (and tanks to be leachedeventual again. part Thisof the cycle mercury) lasts is 4precipitated, to 6 days. and After the precipitation,solution is recirculated the gold-loaded to the agitation zinc tanks shavings to are transferredbe leached to a gas again. furnace This cycle and lasts then 4 theto 6 zincdays. is After evaporated precipitation, in a metallicthe gold-loaded bucket zinc at temperaturesshavings are above transferred to a gas furnace and then the zinc is evaporated in a metallic bucket at temperatures above 950 ◦C, contaminating with vapors of zinc, and other associated metals, operators and neighbors of the 950 °C, contaminating with vapors of zinc, and other associated metals, operators and neighbors of facility (Figure 40). Condensers or filters when evaporating shavings are rarely used. the facility (Figure 40). Condensers or filters when evaporating shavings are rarely used.

Figure 40. The decanted gold-loaded cyanide solution passed a column of polyvinyl chloride (PVC) Figure 40. The decanted gold-loaded cyanide solution passed a column of polyvinyl chloride (PVC) tubes with zinc shavings to precipitate the gold. The zinc is later recklessly evaporated to leave behind tubes withgold. zinc shavings to precipitate the gold. The zinc is later recklessly evaporated to leave behind gold. The precipitation of gold with zinc (see equations below) should be conducted in a vacuum to Theavoid precipitation the re-dissolution of gold of with gold. zincHowever, (see equations miners typically below) run should the gold-rich be conducted solutionsin through a vacuum PVC to avoid the re-dissolutioncolumns at atmosphere of gold. However, and lose around miners 6% typically of the gold run in the the gold-richdischarged solutions solution (Velasquez through PVCet al., columns at atmosphere2011) [93]. and lose around 6% of the gold in the discharged solution (Velasquez et al., 2011) [93]. - 4Au + 8CN + O2 + 2H2O = 4AuCN2 + 4OH‾ (gold leaching with cyanide) 4Au + 8CN− + O2 + 2H2O = 4Au(CN)2− + 4OH− (gold leaching with cyanide) 2- 2AuCN2 + Zn = ZnCN4 + Au (gold precipitation with zinc) 2 Hedley2Au and( CNTabachnick)2− + Zn (1968)= Zn [94](CN listed)4− some+ Au of( goldthe factors precipitation affecting withcyanidation, zinc) such as: dissolved oxygen, pH, the stability of the cyanide solution, cyanide concentration, gold particle size Hedley and Tabachnick (1968) [94] listed some of the factors a ecting cyanidation, such as: and shape, accessibility of cyanide to gold (exposure), temperature, presenceff of minerals consuming dissolvedoxygen, oxygen, presence pH, of the cyanicides, stability etc. of As the all cyanide these factors solution, also increase cyanide the cyanide concentration, consumption, gold AGM particle size and shape,operations accessibility typically ofuse cyanide uncontrollable, to gold and (exposure), in many cases temperature, unnecessary, presencelevels of cyanide of minerals in solution, consuming oxygen,2 to presence 6 g NaCN/L, ofcyanicides, to leach tailings. etc. Cyanidation As all these is quite factors slow alsofor relatively increase coarse the gold cyanide particles. consumption, In AGM operationslaboratory tests, typically it was use observed uncontrollable, that a 0.15 and0 mm in many(100 mesh) cases particle unnecessary, of pure levels gold ofneeds cyanide in approximately 44 h to be dissolved in normal cyanidation conditions. This is a reason why most solution, 2 to 6 g NaCN/L, to leach tailings. Cyanidation is quite slow for relatively coarse gold companies remove the coarse gold specks from the ores with gravity concentration before particles.cyanidation In laboratory (Hedley and tests, Tabachnick, it was observed 1968 [94], Marsden that a 0.150and House, mm 2006 (100 [11], mesh) Egan particleet al., 2016 of [95], pure gold needs approximatelyChen et al., 2020 [79]). 44 h to be dissolved in normal cyanidation conditions. This is a reason why most companies remove the coarse gold specks from the ores with gravity concentration before Minerals 2020, 10, 1026 36 of 49 cyanidation (Hedley and Tabachnick, 1968 [94], Marsden and House, 2006 [11], Egan et al., 2016 [95], Chen et al., 2020 [79]). Cyanide is usually supplied as sodium cyanide (NaCN) or potassium cyanide (KCN). In water, + + these salts dissociate forming free cyanide and Na or K . Free cyanide is a term that refers to CN− or HCN (aq), in which the predominance of each one of these species depends on the pH of the solution. Cyanidation is normally conducted at pH 10.5–11 using, in most cases, Ca(OH)2 to adjust the pH. Lower pH levels favor the formation of HCN (aq), which reduces the cyanidation kinetics, and it is unstable, generating toxic gaseous HCN. Higher pH levels decrease the gold leaching reaction kinetics. The main variable to increase the gold dissolution reaction speed is the oxidant that in most cases is oxygen from the air. Excessive oxygen can oxidize the free cyanides and this is not desirable (Marsden and House, 2006 [11]). The level of dissolved oxygen increases when the temperature of the water decreases. For example, at 25 ◦C, the theoretical concentration of dissolved oxygen in water is 8.6 mg/L whereas at 5 ◦C is 12.8 mg/L (Higgins, 2013 [96]). Recently some commercial intensive cyanidation processes have been used by conventional mining companies to leach coarse gold particles using high levels of oxygen in the process. However, when too much oxygen is added, the cyanidation reaction is accelerated but cyanide in solution is also oxidized to cyanate (CNO−). Then, more NaCN must be added to balance the losses. Cyanide concentrations in these processes are usually around 5 to 20 g/L of NaCN. Veiga et al. (2009) [68] proposed a method to extract gold from gravity concentrates by adding an oxidizing agent into small ball mills, using the same mills used for whole ore amalgamation. Oxidizing agents such as hydrogen peroxide, or Vanish or Oxiclean, common cloth detergents that have around 16–21% of H2O2 in 50–66% of sodium percarbonate, are suitable to be used in AGM. After about 12 h of leaching gravity concentrates with 5–10 g NaCN/L, the balls are removed and a perforated PVC tube with a porous nylon bag filled with activated charcoal is introduced into the mill for gold adsorption. In tests in Brazil, Colombia, Ecuador, and Indonesia AGM sites, this mill-leaching process extracted more than 92% of gold from concentrates in less than 24 h (Sousa et al., 2010 [97], Veiga et al., 2014) [9]. A test was conducted in Portovelo, Ecuador, to convince the local miners that this technique is better than amalgamation to produce more gold. A 240 kg of a gravity concentrate with 17.3 g/t of gold obtained from a Chilean mill and sluice box was split into two subsamples. A 160 kg portion was given to the miners to conduct amalgamation as they used to do manually in a batea. A smaller 80 kg portion was used for the mill-leaching technique with the conditions described above. After 8 h of amalgamation, miners burned the amalgam and obtained a small gold bar with a calculated gold recovery of 26%. The mill-leaching process of the concentrates with 6 g NaCN/L after 8 h achieved a calculated gold recovery of 95%. The gold was eluted from the activated charcoal using a hot alkaline solution with alcohol and then smelted with borax. Miners could see that the mill-leaching process produced a heavier gold bar from less concentrate processed. Several other alternatives to cyanidation have been used to leach gold from gravity concentrates, such as the use of sodium hypochlorite and hydrochloric acid developed by MINTEK, in South Africa. The gold in solution is then precipitated with sodium metabisulphate (Mahlatsi and Guest, 2003) [98]. The iGoli process was demonstrated in South Africa, Tanzania, Mozambique, and Peru to artisanal miners, but after training the miners, this mercury-free method was never widely used. Styles et al. (2010) [99] classified the process as technically complicated and expensive. An additional problem is the storage of liquid and unstable solutions of sodium hypochlorite in hot climates (Veiga et al., 2014) [9]. Many methods have been described to leach gold replacing mercury and cyanide (Hilson and Monhemius, 2006 [100], Gökelma et al., 2016 [101]). One example is the American Haber method using undisclosed reagents (Grayson, 2007) [22]. Another example is the patented iodide process promoted by the Canadian company EnviroLeach (2020) [102], which has been used to leach gold from e-waste and concentrates from small gold mines. The sophistication and proprietary characteristics of these technologies are some of the hurdles for the dissemination of these processes in AGM. The use of cyanogenic plants, such as cassava, is also an interesting solution for leaching gold Minerals 2020, 10, 1026 37 of 49 from concentrates. On a laboratory scale, this has been studied at the University of British Columbia. In preliminary tests with a Brazilian bitter cassava extract, which generated 300 mg/L of free CN when hydrolyzed, 50% of gold was recovered from an artisanal mining ore with 48 g/t of gold in less than 24 h (Torkaman et al., 2020) [103]. However, any clean and responsible leaching process recommended for AGM should consider higher capital and operating costs as well as the need for skilled operators.

3.4. Coal-Oil Agglomeration The coal-gold agglomeration (CGA) process has its merits for very fine liberated gold particles (below 25 µm) (Sen et al., 2010) [104], as commonly found in alluvial ores. A comparative study in South Africa using synthetic gold ore, where gold particles were mixed with silicates, concluded that the gold recoveries with the CGA process are compatible with amalgamation (Koltze and Petersen, 2000) [105]. This study also mentioned that the presence of sulfides reduces the oleophilicity of gold, thereby reducing its recovery. The CGA process was tested 26 years ago to replace amalgamation in Brazil by Marciano et al. (1994) [106]. Using a synthetic ore with pure gold particles added and a concentrate from an artisanal mine, the authors tested different types of oil and types of coal. The results showed that diesel oil produced better agglomerates than vegetable oils and kerosene. Like other authors (Mehrotra et al., 1983 [107]; Calvez et al., 1998) [108], this Brazilian study suggested that gold recovery increases with more oil in the coal agglomeration process and that the quality of the coal (e.g., metallurgical coal) has an influence on the quality of the agglomerates. This technique, being recently brought to the artisanal miners’ attention as an environmentally friendly solution to replace mercury and cyanide, seems to be less efficient in recovering gold from concentrates when the quality of the coal is low, the concentrate has high amount of sulfides, the concentrate has clay minerals, and/or the pulp is viscous. The technique works well to recover fine, liberated gold particles (Hilson and Monhemius, 2006 [100]; Otsuki and Yue, 2017 [109]). However, it is important to keep in mind that alluvial deposits have low gold grades and high amounts of ore must be daily processed to pay back the investment in equipment. Other technical and operating difficulties to prepare the coal agglomerates and extract them from the pulp are also hurdles for the broad use of this process. These limitations, in addition to the capital cost required to acquire pelletizers to make the agglomerates and flotation cells, reduce drastically, like the direct smelting process, the number of cases in which this method is applicable in artisanal gold mines.

3.5. Extracting Gold from Concentrates—Summary The main points to highlight are:

Direct smelting of gold concentrates with borax is not a definitive solution for all types of ore, • as the gold grade of the concentrate must be very high (>30,000 g/t Au), otherwise high amounts of gold stay in the slag, leading to a substantial reduction of gold recovered. Direct smelting can be useful for alluvial ores or for recovering free gold from a primary ore before cyanidation; however, sulfides in a concentrate must be previously oxidized before smelting; The amalgamation of the concentrates is the quickest, easiest, and cheapest method for artisanal • miners to extract gold from concentrates. However, this works only for free gold particles. Oxidation contributes to the reduction of mercury coalescence leading to the formation of fine droplets that lowers the effectiveness of amalgamation. Methods to activate mercury or reduce its surface tension can improve amalgamation, but they will not be useful in trapping unliberated gold particles. Amalgamation is an ancient poisonous process banned by all conventional gold mining companies and it must be also avoided in AGM operations; Leaching processes can dissolve unliberated but exposed gold particles. Cyanide is widely used • by conventional and artisanal gold miners. The cyanidation of mercury-contaminated tailings Minerals 2020, 10, 1026 38 of 49

exacerbates the pollution producing toxic mercury-cyanide complexes that can be bioaccumulated. Although alternative lixiviants exist to replace cyanide, they are not easily available or affordable at most AGM sites; The coal-oil agglomeration process is limited to fine, liberated gold particles and the feed • concentrate must have low sulfides and clays. Minerals 2020, 10, x 40 of 51 In many cases, the best option for artisanal miners is to sell their concentrates (or even their ores) • to companies• In many with cases, legal the best and option responsible for artisanal systems miners is to of sell cyanidation. their concentrates The (or buyers even their should ores) pay the miners basedto companies on the goldwith legal content and responsible in the concentrates systems of cyanidation. (Veiga and The Fadina, buyers2020) should [ 110pay]. the miners based on the gold content in the concentrates (Veiga and Fadina, 2020) [110]. 4. Comparing Gravity Concentration Equipment 4. Comparing Gravity Concentration Equipment New piecesNew of pieces gravity of gravity concentration concentration equipmentequipment regularly regularly come comefrequently frequently to the market to and the are market and are promotedpromoted to gold to gold miners miners andand organizations organizations working working with AG withM. Most AGM. manufacturers Most manufacturers of the new of the new equipmentequipment claim claim that that theythey provide provide better better goldgold recoveries recoveries than any than other anyequipment. other However, equipment. gold However, recovery is not the only variable to be observed. There are other important parameters to be gold recoverycompared, is not such the as: only grade variable of the concentrate, to be observed. enrichment Therefactor (grade are otherof the concentrate important divided parameters by to be compared,the such grade as: of the grade feed), of prices, the concentrate, local availability enrichment of spare parts, factor energy (grade consumption, of the water concentrate use (quality divided by the grade ofand the quantity), feed), the prices, durability local of availability the equipment, of environmental spare parts, impacts, energy etc. consumption, water use (quality and quantity),To the illustrate durability the challenge, of the equipment, the following environmental example describes impacts, how to etc.evaluate new types of equipment. A type of hypothetical equipment is tested, using the same ore, and compared with some To illustrateof the traditional the challenge, concentrators the discussed following above: example sluice with describes carpets, sluice how with to riffles, evaluate batea, new dry types of equipment.sluice, A type jig, shaking of hypothetical table, and centrifuge. equipment Only is based tested, on usingthe gold the recovery same of ore, 97% and (Figure compared 41), there with is some of the traditionalan impression concentrators that the new discussed equipment above: had the sluice best performance. with carpets, However, sluice if with the enrichment riffles, batea, factor dry sluice, jig, shakingis used table, for and evaluation, centrifuge. it becomes Only clear based that th one new the equipment gold recovery produces of concentrates 97% (Figure with 41 low), there is an grades and a large mass. impression thatGravity the newconcentration equipment processes had can the also best be performance.compared through However, a graph of if gold the recovery enrichment and factor is used for evaluation,gold grade. Using it becomes data from clear the previous that the example, new equipment it is clear that produces the best gravity concentrates equipment with is the low grades and a largecentrifuge, mass. which generated both high gold recovery and a high gold grade in the concentrate (Figure 42).

100 90 80 70 60 50 40 30 20 10 0 Gold Recovery (%) Gold Recovery

(a)

300 250 200 150 100 50 0 Enrichement Factor Enrichement

(b)

Figure 41. Comparison of performance of gravity concentration equipment with a hypothetical type of

new equipment. (a) Gold recovery using diﬀerent hypothetical pieces of equipment (b) Enrichment factors using the same pieces of equipment. Minerals 2020Minerals, 10,2020 x , 10, 1026 39 of 49 41 of 51

Minerals 2020, 10, x 41 of 51 Figure 41. Comparison of performance of gravity concentration equipment with a hypothetical type Gravity concentration processes can also be compared through a graph of gold recovery and gold of newgrade.Figure equipment. Using41. Comparison data (a from) Gold the of recovery previousperformance example,using of gravitydifferent it is clear concen hy thatpotheticaltration the best equipment gravitypieces equipmentof with equipment a hypothetical is the (b centrifuge,) Enrichment type factorswhichof new using generated equipment. the same both (a pieces) highGoldgold recoveryof equipment. recovery using and different a high hy goldpothetical grade inpieces the concentrate of equipment (Figure (b) Enrichment 42). factors using the same pieces of equipment. 120 120 100 100 New equipmentNew Centrifuge equipment Centrifuge 80 80 Sluice Riffles Sluice Riffles Sluice Carpet 6060 Sluice Carpet ShakingShaking TableTable 4040 BateaBatea JigJig 2020 Gold Recovery (%) Gold Recovery Gold Recovery (%) Gold Recovery DryDry sluice sluice 0 0 00 1000 1000 2000 2000 3000 3000 4000 4000 Gold Grade (ppm) Gold Grade (ppm)

Figure 42. Gold recovery versus gold grade for different types of equipment. FigureFigure 42. Gold 42. Gold recovery recovery versus versus gold gold grade for for di ffdifferenterent types types of equipment. of equipment. 5. Economic5. Economic Assessment Assessment 5. Economic Assessment The Theevolution evolution of AGM of AGM is isonly only possible possible with with technicaltechnical capacity capacity (and (and technical technical assistance) assistance) for the for the artisanalTheartisanal evolution miners miners ofand AGM and more more is investment. only investment. possible The The with type type tec ofhnical gravitygravity capacity concentration concentration (and technology technical technology availableassistance) available to forto the artisanalartisanalartisanal miners miners miners and depends more depends oninvestment. on the the technical technical The knowledg knowledge type ofe ofgravityof thethe operators operators concentration and and capital. capital. technology An An exercise exercise wasavailable was to conducted to find out the costs of a complete plant with gravity concentration + flotation (for the fine artisanalconducted miners to finddepends out the on costs the of technical a complete knowledg plant withe of gravity the operators concentration and +capital. flotation An (for exercise the fine was gold) + cyanidation of concentrates as depicted in Figures 43 and 44. conductedgold) + cyanidationto find out theof concentrates costs of a complete as depicted plant in Figures with gravity 43 and concentration44. + flotation (for the fine gold) + cyanidation of concentrates as depicted in Figures 43 and 44.

FigureFigure 43. 43. ExampleExample of of a aflowsheet ﬂowsheet of aa smallsmall plant plant to to concentrate concentrate gold. gold.

Figure 43. Example of a flowsheet of a small plant to concentrate gold.

Minerals 2020, 10, 1026 40 of 49

Minerals 2020, 10, x 42 of 51

Figure 44. ExampleFigure 44. Example of the of stepsthe steps to to leach leach flot ﬂotationation concentrates concentrates with cyanide. with cyanide.

This exercise evaluated all pieces of equipment to process from 2 to 200 tpd (tonnes per day) of This exerciseore. It evaluated also considered all all pieces ancillary of equipment, equipment as well to as processwater and power from distribution 2 to 200 systems, tpd (tonnes per day) of ore. It also consideredtailing dams, infrastructure, all ancillary labor, equipment, reagents, etc., asusing well actual as price water quotations and powerfrom suppliers distribution in systems, tailing dams, infrastructure,Colombia. Based on labor,these data, reagents, it was possible etc., to using obtain actualthe CAPEX price (capital quotations cost) and OPEX from suppliers in (operating expenses) (Table 2 and Figure 45). The CAPEX and OPEX per tonne of material being Colombia. Basedprocessed on these and data,the US$/tpa it was (tonnes possible per annum) to obtain revenue the becomes CAPEX more (capital advantageous cost) andwith OPEXa (operating expenses) (Tableproduction2 and Figure rate above 45 50). tpd. The A CAPEX 200 tpd plant and operat OPEXing only per with tonne centrifuge of material + cyanidation being will cost processed and the around US$ 3.5 million with an operating cost of 64 US$/tpa, considering 300 days per year of US$/tpa (tonnesoperation. per annum) If flotation revenue is introduced becomes to recover more gold advantageousfrom the gravity circuit with tailings, a production the investment rate above 50 tpd. A 200 tpd plantrequired operating rises to only close to with US$ 4 centrifuge million with an+ opcyanidationerating cost of 66 will US$/tpa. cost The around highest capital US$ costs 3.5 million with an are related to comminution equipment. The capital cost can be drastically reduced using second hand operating cost ofand/or 64 locally US$/ tpa,made consideringequipment. 300 days per year of operation. If ﬂotation is introduced to recover gold from the gravity circuit tailings, the investment required rises to close to US$ 4 million Table 2. CAPEX (capital cost) and OPEX (operating expenses) for small gold processing plants. with an operating cost of 66 US$/tpa. The highest capital costs are related to comminution equipment. The capital cost canProduction be drastically Rate (tpd) reduced CAPEX, US$ using OPEX, second US$/a hand CAPEX and(US$/tpa)/or locally OPEX (US$/tpa) made equipment. 200 3,946,266 855,091 66 14 100 2,603,565 564,150 87 19 Table 2. CAPEX (capital50 cost) and1,717,712 OPEX (operating372,200 expenses)115 for small gold 25 processing plants. 25 1,133,267 245,560 151 33 Production Rate10 (tpd) CAPEX,653,986 US$ OPEX,141,708 US$ /a CAPEX218 (US$ /tpa) OPEX 47 (US$/tpa) 5 431,470 93,492 288 62 200 3,946,266 855,091 66 14 2 248,993 53,953 415 90 100 2,603,565 564,150 87 19 50Note: 1,717,712 tpa = tonnes per annum 372,200 of 300 days of operation. 115 25 25 1,133,267 245,560 151 33 The10 other economic factor 653,986 that must be 141,708taken into consideration 218 is the minimum ore 47 grade required 5to operate these plants 431,470 and to pay back 93,492 the investment, i.e., to 288 provide a positive net 62present value (NPV).2 The break-even 248,993 point characterizes 53,953 the minimum ore 415 gold grade required to 90 avoid economic losses. For example, according to Figure 46, a processing plant operating with a production rate of 5 tpd and Note:80% gold tpa recovery= tonnes should per annum process of ores 300 with days no of less operation. than 5.21 g/t of gold to be profitable.Minerals 2020 , 10, x 43 of 51

100 600 90 OPEX/tpa CAPEX/tpa 500 80 CAPEX (US$/tpa) 70 400 60

50 300 40 OPEX (US$/tpa) 200 30 20 100 10 0 0 2 5 10 25 50 100 200

Production rate (tpd)

Figure 45. CAPEX and OPEX per tpa of complete small-scale gold plants. Figure 45. CAPEX and OPEX per tpa of complete small-scale gold plants.

The other economic factor that must be taken into consideration is the minimum ore grade required to operate these plants and to pay back the investment, i.e., to provide a positive net present value

Figure 46. Gold grades that provide break-even point (80% Au recovery).

For micro-miners producing and processing less than 2 tpd of ore, the economic assessment of homemade pieces of equipment is more favorable than what is offered by the processing centers using amalgamation. The CAPEX and OPEX in Table 3 indicate different levels of production using the chain mills and shaking tables developed by the CICAN project as described above, with a 5-year operation. The OPEX does not contemplate salaries for employees—all miners work in partnership splitting equally the results. The CAPEX calculation assumes that miners will not build their own equipment but will acquire them from a local metal shop. The same occurs for other accessories such as pumps, bins, screens, etc. Based on these assumptions, if the plant has gold recoveries above 50%, they are more profitable at any level of gold grade and production rate than the amalgamation of the whole ore in processing centers recovering 30% of the gold. However, if they do not have skills or

Minerals 2020, 10, x 43 of 51

100 600 90 OPEX/tpa CAPEX/tpa 500 80 CAPEX (US$/tpa) 70 400 60 50 300 40 OPEX (US$/tpa) 200 30 20 100 10 0 0 Minerals 2020, 10, 1026 2 5 10 25 50 100 200 41 of 49

Production rate (tpd) (NPV). The break-even point characterizes the minimum ore gold grade required to avoid economic losses. For example, according to Figure 46, a processing plant operating with a production rate of 5 tpd and 80% gold recovery should process ores with no less than 5.21 g/t of gold to be proﬁtable. Figure 45. CAPEX and OPEX per tpa of complete small-scale gold plants.

FigureFigure 46. 46. GoldGold grades grades that that provide provide break- break-eveneven point point (80% (80% Au Au recovery). recovery).

ForFor micro-miners micro-miners producing producing and and processing processing less less than than 2 2 tpd tpd of of ore, the economic assessment of of homemadehomemade piecespieces ofof equipmentequipment is is more more favorable favorable than than what what is o ffisered offered by the by processingthe processing centers centers using usingamalgamation. amalgamation. The CAPEXThe CAPEX and OPEXand OPEX in Table in Table3 indicate 3 indicate di fferent different levels levels of production of production using using the thechain chain mills mills and and shaking shaking tables tables developed developed by by the the CICAN CICAN project project as as described described above,above, with a 5-year operation.operation. The The OPEX OPEX does does not not contemplate contemplate salaries salaries for for employees—all employees—all miners miners work work in in partnership splittingsplitting equally equally the the results. The The CAPEX CAPEX calculatio calculationn assumes assumes that that miners miners will will not not build build their their own equipmentequipment but will will acquire acquire them from a local metal shop. The The same same occurs occurs for for other other accessories accessories such such asas pumps, pumps, bins, bins, screens, screens, etc. etc. Based Based on on these these assump assumptions,tions, if if the the plant plant has has gold gold recoveries recoveries above above 50%, 50%, theythey are are more more profitable profitable at at any any level level of of gold gold grade grade and and production production rate rate than than the the amalgamation amalgamation of of the the whole ore in processing centers recovering 30% 30% of of the gold. However, However, if if they they do do not not have have skills skills or or funds to set up a simple cyanidation plant to extract the gold from the concentrates, they must sell the concentrates at a price above 60% of the gold content, which is the break-even point to pay back the CAPEX and OPEX at any ore grade. If their gold recovery increases to 85%, with a production rate of 1 tpd, the miners can make more money than using processing centers if they sell concentrates for more than 35% of the gold content (Veiga et al., 2018) [37].

Table 3. CAPEX and OPEX for homemade processing plants. (Source: Veiga et al., 2018 [37])

Production Rate (tpd) CAPEX, US$ OPEX, US$/a OPEX/tpa 0.2 2600 4500 75 0.5 4500 6259 42 1 5500 8032 27 2 10,500 10,309 17

6. Conclusions Gravity separation equipment can concentrate liberated and unliberated gold particles. However, it has been observed that the main losses of gold in AGM occur in the coarse grain size fractions, Minerals 2020, 10, 1026 42 of 49 as small gold grains are trapped inside the gangue minerals, and in the fine fractions. Fine gold particles (below 200 mesh = 0.074 mm), when liberated, are not concentrated by most typical types of gravity equipment seen at AGM operations. Efficient closed-circuit grinding, size classification, and flotation can increase the gold recoveries to above 80%, for the large majority of AGM operations. Many manuals have been published, and are widely available, on how artisanal miners can improve their methods and gold recovery. The pioneers of this educational approach were Priester et al. (1993) [73] with their excellent review of the pros and cons of equipment and techniques used in AGM. Another example is the guide produced by the United Nations Environment Programme (UNEP, 2012) [111] in conjunction with the Artisanal Gold Council. Other manuals have been published by UNIDO (Veiga et al., 2006) [39] and the COMUNICA Project, a Canadian project to demonstrate to artisanal miners cleaner techniques to concentrate gold (Veiga and Correa, 2019) [77]. Despite the merit of bringing simple solutions to eliminate mercury and improve gold recovery, the effectiveness of these toolkits, pamphlets, guides, and manuals, is questionable since miners need permanent technical assistance and capital to sustain the changes in their techniques and behaviors. This is discussed well by Jønsson et al. (2009) [112] and Shandro et al. (2009) [113] when they assess, after some years, the efforts to implement simple retorts to remove mercury safely from amalgams. Rarely did artisanal miners continue using the techniques they learned. The solutions to improve the efficiency and reduce the pollution from AGM operation are not solely related to technology, but also require better participation from governments to simplify laws and mining companies to provide a range of assistance to the miners (Veiga and Marshall, 2019) [7]. There is a common perception from those implementing solutions in AGM that one technique to concentrate and extract gold from concentrates fits all, however, not all ores have the same mineralogical characteristics. This is the most usual mistake observed in the field. It is common to observe agencies looking for “the technical solution” for AGM pollution, focusing on identifying a specific type of equipment to do so. As a result, no sustainable solution has been implemented as miners sooner or later, without technical assistance, return to their rudimentary practices. The idea of demonstrating gold concentration equipment to miners overlooks the following key aspects: (1) the lack of mineralogical information about the ores, (2) the lack of permanent technical assistance, (3) the lack of understanding of the level of miners’ skills, (4) the lack of consultation with the miners, (5) the lack of understanding the miners’ motivations to change procedures, and, (6) the lack of miners’ financial resources to implement and sustain the suggested solutions. This is not an exhaustive list of problems, but just the tip of an iceberg of problems to change the behavior of artisanal miners. A complete small gravity + flotation (for fine particles) + cyanidation, including all ancillaries and infrastructure as well as tailing dams, can involve a capital cost (CAPEX) from US$ 60/tpa (tonnes per annum) for a 200 tpd (tonnes per day) plant, to US$ 415/tpa for a 2 tpd. The operating costs (OPEX) 1 are approximately 4 of the CAPEX, varying from US$ 14/tpa to US$ 90/tpa respectively. Homemade solutions can be useful for micro-miners processing less than 2 tonnes of primary ore per day (tpd). The cost for this small plant can be approximately US$ 11,000 including chain impact mills, gravity concentration, flotation equipment, and cyanidation in small ball mills (Veiga et al., 2018) [37]. From the technical viewpoint, some simple measures can be brought to the attention of the miners to improve their operations. Sluice boxes in a zigzag arrangement operating with good carpets can be very efficient in trapping coarse and fine gold particles. Classifying feed into narrow size fractions would improve substantially the gold recovery with any concentrator, but rarely do miners understand the importance of size classification. Artisanal miners frequently are keen to invest in increasing their production by increasing the mass of ore mined with excavators and trucks instead of improving gold recovery. This increases environmental pollution, as tailing dams are rarely properly designed and managed by artisanal miners. Artisanal gold miners who achieve initial success often later become bankrupted by over-investing in expensive mining equipment, such as excavators and big trucks, rather than investing in improving the efficiency of their concentration plants. Minerals 2020, 10, 1026 43 of 49

In the large majority of AGM operations, gold from gravity concentrates or high-grade ores is amalgamated. In the past, many researchers and international agencies promoted the amalgamation of only gravity concentrates, instead of whole ore amalgamation, as the best way to reduce mercury losses. Even without success of reducing mercury use, nowadays all efforts from projects aim at the complete elimination of mercury in AGM by using only gravity concentration; however, this is not practical for the miners if they do not have an affordable and simple method to extract gold from the concentrates. Amalgamating gravity concentrations can reduce drastically the use and loss of mercury, but mercury use cannot be eliminated unless an alternative, such as a leaching process, is used to extract gold from concentrates. The direct smelting of concentrates is typically only practical for some alluvial ores where the gold is mostly liberated from the gangue minerals, allowing the generation of a rich concentrate. If direct smelting is used on low-grade gold concentrates, most gold will end up in the slag. However, if the low-grade concentrate is further concentrated to produce a high gold grade to be directly smelted, a large portion of gold will be lost during the upgrading process. This paradox cannot be easily resolved. Gravity concentration followed by the cyanidation of the concentrates involves capital and skilled operators. It seems that the best practical solution for the AGM operations is to sell their ores for a fair price instead of using processing centers which often exploit the miners, allowing them to only extract the free gold by amalgamation while the centers retain the tailings for processing with cyanide (Veiga and Fadina, 2020) [110]. Artisanal miners frequently use sluice boxes, shaking tables, and jigs for gold concentration, and a few plants are now using centrifuges. Unfortunately, many miners do not have the financial resources to acquire more capital-intensive equipment or do not have the skills to operate more sophisticated equipment efficiently. Without technical assistance, most operators abandon new pieces of equipment when they become frustrated with the additional complexity of the operation, then returning to their previous methods. Worldwide, the presence of tens of millions of micro-miners panning gold from sediments on the river margins is a problem of poverty and subsistence in rural communities. Micro-miners typically use very small pieces of equipment for gold concentration, such as pans or small sluice boxes, followed by the amalgamation of the concentrate. Bringing more efficient methods to concentrate gold to micro-gold miners will help them produce more gold and temporarily alleviate their poverty problem, but it will not reduce drastically the mercury pollution from this sector. Processing centers recklessly processing tens or hundreds of tonnes of ore daily using mercury and cyanide are the main villains in this scenario of chemical and physical pollution. Therefore, the efforts of governments, NGOs, academics, and international sponsoring agencies must focus on the larger artisanal processing operations, increasing enforcement, educating, and facilitating financial support for the transformation of these polluters into responsible and formalized small gold producers.

Author Contributions: Conceptualization, methodology and writing the original draft were conducted as a team work by M.M.V. and A.J.G. based on their experiences in working with mineral processing and artisanal mining in over 35 countries. All authors have read and agreed to the published version of the manuscript. Funding: The financial supports from Global Affairs Canada and the Canadian Embassy in Bogotá, as well as the NSERC Discovery Grant, are acknowledged. Acknowledgments: The authors also recognize the support of the economist Gabriel Jimenez in the economic assessment of this article and the collaboration of the manuscript reviewers to improve the quality of this article. Conflicts of Interest: The authors declare no conflict of interest.

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