Chapter 4 Gravity Concentration

Transcription

1 34 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners Chapter 4 Gravity Cncentratin Intrductin Gravity cncentratin is a prcess t cncentrate the mineral f interest (in this case gld) using the difference f specific gravity f gld and gangue minerals. The specific gravity f gld is 19.5 and the specific gravity f quartz (the cmmn gangue mineral assciated with gld) is 2.65 (i.e., gravity cncentratin wrks because gld is heavy, and quartz is light). Often gravity separatin methds are cnfused with size classificatin because large particles f light minerals can behave like a small particle f a heavy mineral. The mst effective gravity separatin prcesses ccur when applied t re particles f abut the same size. The mst imprtant factr fr a successful gravity separatin is liberatin f the gld particles frm the gangue minerals. It is nt easy t establish the degree f liberatin f lw-grade minerals such as gld. Classical micrscpy f screened fractins t establish mineral liberatin is unreliable with gld res. The mst recmmended methd t establish the ptimum gld liberatin size is grinding fr different times (r grain size distributins) and applying gravity cncentratin t the grund prducts. This is a classical and imprtant prcedure t recmmend any type f gravity cncentratin prcess. Because mst artisanal miners d nt classify (screen) the crushed/grund material (i.e. they wrk in pen circuit), their chances t imprve gld recvery are very limited. The main advantages f gravity cncentratrs ver gld cyanidatin are: relatively simple pieces f equipment (lw capital and perating csts) little r n reagent required wrks equally well with relatively carse particles and fine grained materials Sluices Sluices are inclined, flat-bttmed trughs that are lined n the bttm with a trapping mechanism that can capture particles f gld and ther heavy minerals. They can be used either fr alluvial r fr primary re (sluices are smetimes called strakes r blanket tables ). Ore is mixed with water and the pulp pured dwn the trugh. Sluice designs have utilized readily available materials fr thusands f years, such as animal furs as trapping mechanisms-- in the ancient Greek myths, the Argnaut her Jasn recvered the glden fleece, a sheepskin sluice lining, by killing the dragn that guarded it.

2 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 35 Sluices wrk n the principal that heavy particles tend t sink t the bttm f a stream f flwing water while the lighter particles tend t be carried dwnstream and discharged ff the end f the sluice. Sluices are used in varius sizes, frm small hand-fed sluices t large sluices fund n dredges r fed by trucks, frnt-end laders r bulldzers, which can prcess as much as 150m 3 f alluvial re per hur. Much like in the past, tday s hand-fed sluices are usually 1 t 2 meters lng, 30 t 50 cm wide, with walls 10 t 30 cm high. Sluices are usually inclined at a 5 t 15 degree angle. Many miners wrking alluvial depsits tday use large sluices when sufficient water and perating capital is available. In mnitr-gravel pump systems, slurry is pumped thrugh cm hses nt m wide by rughly 5 meter lng sluices, such as thse used in Guyana, Indnesia and Brazil. Used crrectly, sluices are efficient devices t separate gld frm gangue. While sluices are nt necessarily mre efficient than panning, they d allw miners t increase the amunt f re they prcess, thus bsting their incme cnsiderably. Unfrtunately, the resulting increase in the vlume f re prcessed can put large amunts f silt int streams, damaging reginal water supplies and thus harming peple, animals and aquatic life. Sluices can cause ther envirnmental prblems as well--thse lined with mercury cated cpper plates are especially destructive because the slurry slids scratch the mercury frm the cpper plate and carry it dwnstream, pisning fish and peple. Fig Hand fed sluice in Suth Africa Gd sluice design Fig Mnitr pump fed sluice in Indnesia. Nte the turbulent flw which limits recvery f fine gld Even thugh sluices have been used thrughut the wrld fr thusands f years, they are ften nt designed r perated crrectly. Limited knwledge f the basic perating principles, lack f capital and access t mre efficient

3 36 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners mdern lining carpets greatly reduces gld recvery, especially recvery f fine gld. Particles suspended in a slurry stream settle when the intensity f the turbulence cannt supprt them. A well-designed sluice insures that the maximum amunt gld can settle near the bttm f the slurry stream where it can be caught by trapping mechanisms such as carpets r riffles. Trapping mechanisms shelter gld particles frm being lifted back int the current by turbulent frces, hlding the gld frm being washed ff the end f the sluice. Gravity causes gld t settle in water faster than silica and ther gangue minerals. The rate f settling depends n particle density, size and shape: Large, dense, spherical grains settle quickly, whereas small, less dense and flatter particles settle much mre slwly. Carser grained lw-density particles can settle at the same rates as finer high-density particles. In sluices where turbulence is lw, the difference in settling rate between heavy and light particles tends t separate the slurry int lsely stratified znes. As the slurry stream flws dwn a sluice, the densest and largest particles accumulate in a zne clse t the bttm where they can becme trapped within the lining carpet s pile r weave and sheltered frm the current, while the smaller, lighter particles tend t stay in suspensin near the tp f the stream and be carried ff the end f the sluice. The rate f flw influences hw gld and gangue particles in the feed stream settle t the bttm f the sluice, and hw they becme re-suspended. Flw velcities are cntrlled by the amunt f feed pulp, and by the sluice bx s inclinatin, width, and length. At lw flw velcities, the densest and largest particles settle t the bttm, while the less dense and smaller particles remain suspended in the feed stream. On the bttm f the sluice, sediments in the surface layer mve slwly dwn the sluice by rlling and sliding. Increased flw velcities can cause these sediments t be lifted and suspended r bunced dwnstream: High flw velcities cause turbulent currents that, if strng enugh, can fully re-suspend the bttm bed lad and carry it all dwnstream. Fr efficient peratin, the slurry flw velcity must be adjusted fit bth the range f gld particle sizes in the feed, as well as the trapping mechanism used. Flw shuld be fast enugh t insure that the trapping spaces created by the riffles r carpet liner are nt filled and blcked with gangue (i.e., the carpet must be kept frm sanding up ), yet slw enugh t allw as much fine gld as pssible t settle t the bttm where it can be trapped.

4 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 37 Increasing the angle f the sluice causes the flw velcity t increase; increasing the slurry depth by narrwing the width (r by increasing the input) als causes the flw velcity t increase; lengthening the sluice als increases the flw velcity as the slurry mves dwn the sluice because the fluid accelerates with distance. Fr a given feed rate and sluice width, the ptimum flw velcity is empirically determined by incrementally increasing the angle f inclinatin until the trapping mechanism is clear f silica and ther light gangue minerals (r the ther way rund, reducing the angle f inclinatin until the sluice starts sanding ut, and increasing it again slightly). The flw rate shuld be cnstant. The highly variable, discntinuus feed rates in hand-fed sluices are nt efficient because the bttm carpet quickly becmes clgged with gangue, blcking the trapping spaces. In hand-fed sluices, water is pured nt the sluice ne bucket at a time, but even at the peakflw f each bucket pur, the velcity is usually t lw t lift much f the gangue and keep the trapping mechanism pen. Even thugh sme gld particles can becme entrained within the surface Fig Stream cntaminated with silt in Mzambique sediments as they rll and slide dwn the sluice, the trapping efficiency f these surface sediments is much lwer than that f a carpet with expsed fibers. Cntinuus gravity flw frm a diesel barrel filled with water is better than puring ne bucket f water at a time int hand-fed sluices. Gld res typically cntain a mixture f carse and fine-grained gld particles. Because fine gld settles much mre slwly than curse gld, it is ften best t use multiple stage sluices capture the carse gld using riffles, carse expanded metal r/and vinyl lp carpets in a relatively steeply inclined first stage (faster flw velcity); then screen the carse material ff by using an inclined grizzly screen at the end f the first sluice, and feed the passing fine material (plus the water) nt a mre shallw angled, perhaps wider sluice, where the remaining fine gld is recvered n a mre tightly wven r pile carpet. This secnd sluice Fig Feed bx fr hand fed sluice: mixing re with water

5 38 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners can be riented either perpendicular t, r underneath the first sluice in a zigzag cnfiguratin. Differently angled zigzag sluices allw variable flw velcities, while reducing acceleratin f the slurry stream by shrtening the length f the bed. The feed bx fr zigzag sluices usually has t be higher than straight sluices, s they wrk best when the feed can be pumped t the sluice, and when the discharge can be in the ppsite directin as the feed. Zigzag sluices are used ften in large alluvial mining peratins. The American cmpany Keene Engineering ffers a large variety f riffled sluice bxes made f aluminum with rubber ribbed matting and vinyl carpets. The A52 Keene 10 x 51 (25 x 129 cm) may be an interesting alternative t demnstrate t ASM. The cst in the USA f this sluice is arund US$ 100. This small prtable sluice (weighing 5kg) has the capacity f prcessing up t 5 tnnes/h f re. Keene sluices were very ppular in Africa sme years ag. A cpy f the Keene sluice was widely prmted in Zimbabwe as the Bambaznke. The cmpany als prvides pumps (8 t 20 centimeters) and a large variety f accessries. Fig Clgged trapping mechanism (blanket) f hand-fed sluice Fig. 4.6 Grund re frm a Chilean pan feeding alternating sluice bxes with carpet (Ecuadr)

6 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 39 Feed preparatin The re shuld first be screened s that the particle size is as unifrm as pssible and the carse barren material is eliminated. Under ideal cnditins, the feed shuld nt be carser than the largest pssible gld particle. Large rcks n the sluice create eddies and turbulence that keeps the fine gld in suspensin; the high flw velcities required t mve rcks ff the sluice als leads t lss f gld. In alluvial sluices, a grizzly (inclined parallel bars spaced abut cm apart) can be used t screen the feed and make sure larger rcks are kept ut f the sluice. Grizzlies als remve clumps f clay that can rll dwn the sluice bed, sticking t gld particles and carrying them int the tailings. D nt ver-grind primary re. Grinding t much can make smaller and flatter gld grains that tend t stay in suspensin and ultimately be washed ff the end f the sluice. Gld particles are very difficult t be cncentrated when the material is a slurry f fine-grinding minerals. The same happens when the re is rich in clayminerals. This frms a muddy-viscus pulp that must be adequately dispersed with caustic sda r dispersants t create cnditins fr gld t be cncentrated by gravity prcesses. The mre fine particles in the re, the less % f slids must be used in the cncentratin prcess. The ideal feed cntains between 5 t 15% slids. A high percentage f slids makes the slurry t viscus - dense particles are buyed upwards by less dense particles, limiting the ability t the slurry t stratify accrding t density. If very little water is available, and the gld is nt t fine, carse gangue particles can carefully be raked ut f the sluice. Fig Pump-fed zigzag sluice in Suriname Fig Cleaning up Hessian carpets using a series f buckets

7 40 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners Sluice design and cnstructin Miners shuld design sluices t accmmdate the anticipated feed rate by adjusting the width (increasing width decreases depth and flw velcity). Nte that adjusting the width strngly influences the flw velcity - width is cnsidered by sme researchers t be the best cntrl f flw velcity. Flw rates can be fine-tuned by adjusting the slpe. When pssible, miners shuld design sluice features (e.g., angle, width, etc.) s that they can be changed t insure ptimum recvery. Wide sluices need t have carefully designed feed bxes t insure even slurry distributin ver the whle width f the sluice. Flw accelerates with distance, making it harder fr the trapping mechanism t capture small gld particles. Research has shwn that 90% f gld is recvered in the first 1/3 rd f the sluice, 9% in the 2 nd 1/3 rd, and nly 1% in the last 1/3 rd. Mst gld is caught in the first 0.5 meter f the sluice, s keep the sluice length shrt (less than 2 m fr hand-fed sluices). Zigzag cnfiguratins break flw velcity and help t increase recvery; three 2m zigzag sluices are usually better than ne single 6m sluice. The ptimal slpe is usually between 10 and 15 degrees, but can be as lw as 5% fr fine grained primary feed. Use multiple stage sluices t capture carse and fine gld in different passes. Capture the carse gld first, then the fine gld. The turbulence frm faster flws needed t capture carse gld in riffles can be calmed by placing a shrt smth sectin (a slick plate ) befre the next stage. Clean-up time is a critical activity. Trapping mechanisms shuld be easily remvable and cleaned. Cmplex assemblies reduce the likelihd f cleaning. Trapping efficiency can be mnitred by checking sluice tailings cnstantly by panning. Clean-up time can be as ften as nce an hur t prevent blcking f the carpet, especially fr primary re with high sulfide cntent. T enable cntinuus peratin, parallel sluices shuld be installed (ne sluice in peratin, ne in cleanup and preparatin). T imprve recvery and prevent theft, the tp sectins f alluvial sluices shuld be washed at least nce per day. Secndary sluices can be used t re-cncentrate the cncentrate recvered by the primary sluice, therefre reducing the mass f material t be amalgamated. Secndary sluice tailings shuld be recycled t the primary sluice.

8 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 41 Trapping mechanisms Bed linings shuld be firmly fixed t the bttm f the sluice, especially when nt backed, t prevent captured gld frm migrating dwn the sluice underneath the lining and being lst ff the end f the sluice. Riffles a) Crss riffles made frm railrad rails, angle irn, wd r split bamb are ften used t trap gld particles >1mm. The simplest riffles are stnes, but these can cause turbulence likely t cause gld lss. Carpet and/r expanded metal shuld be used underneath the riffles. b) Rudimentary riffles d nt necessarily imprve recvery--turbulence can break up stratificatin, and cause the lss f fine gld. While catching sme f the carse gld, riffles ften nly leave the impressin that recveries have imprved. c) Riffles prtect the carpet lining frm wear and keep it firmly n the bed f the sluice. d) Ore with a range f carse particle sizes may need t utilize several kinds f riffles (e.g., large and small expanded metal riffles). e) Select riffle size and spacing, then select the flw rate that keeps the sheltering spaces behind the riffles clear f sand. f) 25 mm angle irn riffles are cmmnly used with cm gaps, canted uphill at abut 15 degrees. There shuld be very little sand between the riffles. If there is t much sand, the flw is either t slw, r the riffles are t high. g) Expanded metal grating (see picture, belw) frms shallw riffles which cause a lcal turbulence that keeps the sand mving dwnstream while prviding effective shelter fr gld grains less than 0.1 mm. Wider sluices need a heaver gauge metal t hld the liner flat. Fig. 4.9 Expanded metal and crss riffles in a Keene sluice

9 42 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners h) When the re cntains sme magnetite, Cleangld (see belw) sluices can frm a magnetite bed that can trap fine gld. Carpets a) Type f carpet lining is usually determined by what is available. b) Fibrus r hairy fabrics like sacking, sisal, blankets, r ld carpets have hairs that can trap fine gld particles and prevent them frm being lifted back up int the current by turbulence. Animal hides are usually nt a gd ptin, because they tend t fwl. c) In general, the best carpets have pen fibrus structures that let gld particles settle deeply in the lining. d) If rubber backed carpets are nt available, use a tighter weave clth backing underneath the carpet t prevent lss f gld. e) Wash carpets in a series f buckets, in barrels cut lengthwise int trughs, r in tubs. It is imprtant t demnstrate different types f sisal fabrics and carpets. The best carpet used in ASM peratins is the 3M Nmad Dirt Scraper Matting, especially the type 6050 (medium traffic with backing) which cnsists f a ciled vinyl structure; type 8100 (an un-backed versin) can als be used. Nmad is usually recmmended fr relatively carse gld, but can als trap fine gld efficiently. The price f Nmad carpets in ASM sites can reach up t US$ 150/m²; alternately, a Nmad type vinyl lp carpet is available frm manufacturers in China fr a fractin f this price. The Brazilian cmpany Smmer (subsidiary f the German cmpany Tarkett Smmer) sells 2 types f carpets widely used by Brazilian ASM: Multiur Tariscad (which is gd fr gld f medium sized rice grains) and Multiur Lis (which is gd fr mm gld). These carpets can cst arund US$ 10 t 15/m². Nne f these carpets are easily accessible t ASM in Africa. Sisal fabrics hwever, can cst as little as US$ 3/m², and are available in mst African cuntries; depending n the type, they can be used fr carse, medium and fine gld recvery. It is a matter f trying different types. Raffia mats have been used in Zimbabwe fr fine gld cncentratin. This definitely must be further investigated and cllabrative tests can be dne with the miners t establish the ideal type f sisal clth. Anther interesting sluice liner is the ne manufactured by Cleangld, a cmpany based in Lincln City, Oregn. The Cleangld sluice uses plymeric

10 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 43 magnetic sheets similar t the material used t make magnetic advertisements, with the magnetic ples aligned perpendicular t the directin f the slurry flw, glued t the bttm f a simple aluminum sluice bx. Magnetite, a mineral cmmnly fund in gld-re depsits, is held by the magnetic sheeting, and frms a fine-grained crdury-like trapping mechanism n the sluice flr. These inserts can be available in any size, but they are relatively expensive. Fr example, Cleangld can manufacture a 60 x 50 cm insert fr arund US$ 165 (in USA). The main advantage f this sluice is the high cncentratin rati. Gld becmes trapped in a magnetite layer which can be easily scrapped and washed int a pan. Using a magnet, the magnetite is remved and a high grade f gld cncentrate is btained. In many cases the use f mercury t amalgamate the cncentrate is nt necessary. Hwever, as the magnetic separatin isn t always 100% efficient, and the cncentrate tailings can carry sme gld, s amalgamatin r even leaching f the cncentrates is recmmended. In ne test cmparing the Cleangld sluice with a centrifugal cncentratr, the sluice btained slightly better gld recveries than the centrifuge. In a recent field test in Venezuela cnducted by UNIDO, tailings frm hammer mills and Cu-amalgamating plates were passed withut regrinding thrugh a 60 cm lng Cleangld sluice bx. Abut 11% f gld in the tailings was recvered and the cncentrate analyzed 2850 ppm Au. The Cleangld sluice can als take advantage f the fine pieces f steel released frm hammer and ball mills. They are als cncentrated n the magnetic sluice and play the rle f magnetite. MINTEK (Suth Africa) has devised interesting sluice bxes (strakes) with rubber mat glued t it. Black ribbed vinyl mats are als useful t recver gld; they are easy t clean and cst in USA abut US $15/m². Optimal slurry flw velcity Different trapping mechanisms require different flw velcities. Adjust the width and/r slpe t cntrl flw velcity t ptimize the perfrmance f the varius riffles and carpets used. Fig Cleangld magnetic sluice Carse gld recvery needs faster flw velcity (narrwer and/r steeper sectins); finer gld recvery requires slwer speeds (brader and/r less steep sectins).

11 44 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners Keep feed rate and pulp density cnstant. Increasing the flw can increase turbulence and make it mre difficult fr gld particles, especially the fine gld grains which tend t stay in suspensin, t cntact and be trapped at the bttm f the sluice. Slwing r stpping the flw fills the trapping mechanism with gangue. Avid turbulent flw, especially when trying t capture fine gld. Higher flw velcities can be necessary t keep the gangue frm clgging riffles and carpets, but high speed tends t push fine gld ff the end f the sluice. Lwer flw velcities can yield higher recvery (fine gld is recvered in additin t the carse gld), but if t slw, can lead t clgging f the trapping mechanism. Distribute the flw evenly ver the sluice bttm by making sure the sluice bttm is flat avid twisting and sagging. When water supply is shrt, use narrwer sluices t insure adequate flw velcity t keep trapping mechanism clear. Assess the efficiency f recvery by panning the tailings, r by passing them ver a shrt test-sluice. Centrifugal cncentratrs Centrifugal cncentratrs were riginally develped t imprve gld recvery frm alluvial sands. Since abut 1990, they have been used increasingly fr hard rck mines, which is nw the largest area f applicatin. Centrifugal cncentratrs cnsist f a vertical rtatin bwl with a series f cncentric rings that trap the gld. A centrifugal frce is applied n the re particles, in such a way that this frce is 60 (in the case f Knelsn) t 300 (in the case f Falcn) times higher than the gravitatinal frce. The rtr is accelerated and feed slurry is intrduced t the cncentrating cne thrugh a statinary feed tube. Upn reaching the deflectr pad at the bttm f the cne, the slurry is driven utward t the cne wall by the centrifugal acceleratin. As slurry flws up the cne wall, the slids fill each ring t capacity creating the cncentrating bed. The tailings prduct verflws the bwl and the gld becmes trapped in the rings. Sme centrifuges have a smth wall at the bttm f the bwl where stratificatin takes place. The high density gld is cncentrated at the wall frcing and displacing lwer density particles away frm the wall. Cmpactin f the bed in the rings can be prevented by intrducing pressurized fluidizatin water frm behind the rings. This helps the high-density gld particles displace the lwer density gangue particles causing the gld grade t increase in the cncentrating rings with time. After a perid f time, the feed is stpped and

12 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 45 the rtr is shut ff. The cncentrate is flushed frm the cne int the cncentrate launder and can be upgraded further by panning. The tw main manufacturers f centrifugal cncentratrs are: Knelsn and Falcn, bth frm British Clumbia, Canada. Bth cncentratrs have a ribbed rtating cne int which the pulp f 20 t 40% slids is fed and the cncentrate is accumulated in the riffles. Cmpactin f the cncentrate layer is avided by injectin f water in cunter flw. This water fluidizes the cncentrate bed and allws fine gld particles t penetrate int the cncentrate layer. The main prblems f these centrifuges in ASM peratins are: high cst lack f skilled peratrs lack f clean water and cntrlled pressure fr cunter flw Fr hard rck depsits, the re must be grund befre feeding t the centrifuge. The feed shuld als be screened t belw abut 1 mm t remve carse material, which can be grund further and fed back t the separatr. These centrifuges recver between 15% and 60% f the gld, depending n the mineralgy f the re. In large scale mines the tailings are usually prcessed by cyanide leaching. Fig Centrifuges perating in Brazil There are many mdels f centrifugal cncentratrs with capacity up t 100 tnnes f re/h. Fr a small-scale prductin, a centrifuge with 1-2 tnnes/h capacity is mre than enugh. This uses a mtr with 1.5 hp and liters/min f water. Many ther types f centrifuge are available in artisanal mining sites. In Brazil there are at least 4 manufacturers f cheap centrifuges. The bwls f these machines are nt made f plyethylene like the nes f Knelsn cncentratr but f carbn steel. In Pcné, Brazil, these cheap centrifuges wrk fr 8 hurs with nminal capacity f 24 tnnes/h resulting in a cncentratin rati f 1000 t 1 r higher. It is cmmn t bserve cncentrates with mre than 1000 g/t f gld. The vlume f cncentrates is fixed, limited by the vlume f the riffles s the weight f the cncentrate recvered each cycle is almst cnstant.

13 46 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners The ABJ Bwl, in effect is a cpy f the Knudsen cncentratr ut f Califrnia, is extensively used in Zimbabwe. The cnic centrifuge des nt have cunter-flw water. The centrifuge has 3 vertical pieces f steel that prmtes turbulence f the slurry flwing up the riffles n the bwl, facilitating the ability f the gld t replace ther less dense minerals trapped in the riffles. When the cncentrate bed in the riffles is thus scratched, sites n the bed are pened fr gld cncentratin. Abut 30 t 33 kg f cncentrate are btained during a given run. The main specificatins f the ABJ centrifuge used by artisanal miners in Zimbabwe are: Diameter f the bwl: 0.78 m Operatin: unfluidized centrifuge, ribbed cne Cne Material: butyl rubber Operating Speed: 102 rpm Feed Capacity: up t 3 tnnes/h in slurry at 30% slids Feed Size: -4mm max Shipping Weight: 130kg Extent f Mechanizatin: partially mechanized; batch discharge f cncentrates Mde f Operatin: batch Discharge: frm bttm Drive Bevel gear and V-Belt Installed Pwer: 0.7 kw Price: abut US$ 2000 Fig ABJ centrifuge perating in Zimbabwe Fig Rubber cne with ribs (ABJ centrifuge) The cmpany Small Mining Supplies frm Zimbabwe recently intrduced a new centrifuge called GldKacha Cncentratr. The specificatins f this centrifuge are: High capacity 3-4 tnnes/hur

14 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 47 Feed size -3mm N fluidizatin water N drive belts Replaceable plyurethane cne Slurry distributr Lw pwer requirement: 0.5 kw Weight 108 kg Cst apprximately $US 2200, but variable depending n currency fluctuatins One f the main prblems bserved in Zimbabwe ASM peratins is the use f mercury in the ABJ cncentratrs. Mercury flurs in the prcess, and it is lst t the tailings tgether with fine gld. Very little has been dne t change this bad practice. Fig Small Mining Supplies (Harare) new centrifuge, the GldKacha Panning Panning is the mst ancient frm f gravity cncentratin. Pictures f gld pans appear n Egyptian mnuments as ld as 2900 BC. The circular r back-and-frth shaking f re and water in a pan causes the re t stratify-- the heavy minerals settle t the bttm f the pan while the lighter gangue can be washed ff the tp. Panning is the basic means f recvering gld frm alluvial and high-grade primary re. In Zimbabwe, fr example, miners rarely use the stamp milling plants fr their high-grade reef res. They rely n punding this re with mrtar and pestle and use pans fr cncentratin. They later wash these panning cncentrates thrugh a clay/sand sluice lined with blanket r rubber matting. Assays f these tailings are always high, suggesting that either gld liberatin is pr, r panning efficiency is lw. Primary crushing uses a 4-pund hammer t prduce affrdable sizes fr grinding in a mrtar using a steel bar pestle. The grund up material is then classified thrugh a sieve t cllect the fine undersize material. In mst cases in Zimbabwe, the carser versize is thrwn away and nt regrund fr better liberatin. All the undersize is then panned and the gld recvered. A magnet can be used t remve the black magnetite. The magnet is applied t the bttm side f the pan and mved in a circular

15 48 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners mtin with the pan slightly tilted. Ths rich gld cncentrate des nt need amalgamatin, as it can be smelted directly. Different kinds f gld pans While artisanal miners are usually expert panners and are able t achieve incredible results, even with the mst rudimentary pans, they are smetimes unaware f the advantages f the range f shapes used by their fellw miners arund the wrld. Panners cmmnly use gurds, kitchen bwls r cking pans t pan fr gld. They als use specially designed gld pans made f wd, metal and plastic. Gld pans are usually rund, but can smetimes be rectangular, as in Vietnam. Wden pans have the advantage f having a slightly rugh surface than can hld the gld a bit, but steel pans can be rughened by rusting with acid (r lemn juice), and new plastic pans can be given a tth by rubbing with sand. Wden pans als have the advantage f buyancy, making it easier t supprt a pan full f re in water. Pans can be cut frm steel sheeting and riveted, r punded int shape frm steel il r cyanide drum tps using aut-bdy repair techniques. Steel and aluminum pans are smetimes mass-prduced by metal spinning prcesses. Cast aluminum pans are als available in sme places. In Nrth America, flat bttm pans have been used fr ver 150 years t capture carse gld. Typically these types f pans are made f steel, with sides abut 35 degrees frm hrizntal. Tday these pans are als frmed f plastic and have 2 r 3 small riffle-like ridges circling the sides. Fig Miners can capture fine gld even in a very wrn ut pans (Tanzania). Fig Aluminum kitchen bwl used as a gld pan (Sudan)

16 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 49 Fig Using a gurd ( calabace ) as a gld pan (Guinea) Fig Wden bateas in La PDR Fig Riveted shallw batea made f sheet metal (Brazil). Fig Nrth American plastic pan with riffles in the side (Mzambique). Fig Using a steel batea (Brazil) Fig Small rubber cated cneshaped batea fr testing re (Peru).

17 50 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners Fig Hand carved wden bwl and gurds (Sudan). Fig Steep-walled plastic kitchen bwl (Mzambique). Fig Child in La PDR panning gld with a wden batea

18 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners 51 In Brazil, Indnesia and elsewhere, 150 t 155 cnical bateas capture carse gld relatively quickly, and are als gd t capture fine gld. In Peru, miners use small 15 cm diameter bateas cated with rubber t evaluate recvery by testing sluice tailings. In Africa, miners ften use steep-walled plastic pans, but these are less efficient than bateas. Sudanese panners use hand-carve ellipsidal wden bwls and gurds. Basic knwledge fr miners Gld pans, sluices and centrifuges are all methds f gravity cncentratin Gravity cncentratin wrks because gld settles faster than ther minerals (e.g., sand) in water Gravity cncentratin wrks best when the particles in the feed are clse t the same size screen the re befre cncentrating, and regrind the versize Using mercury in sluices and centrifuges des nt imprve recvery indeed, gld is ften lst alng with the mercury. These practices release much mercury t the envirnment and shuld be banned in all cuntries. Panning Sluices Panning is an efficient and very lw cst methd f gravity cncentratin unfrtunately, miners can nly prcess small amunts f re in a day. There are many shapes and sizes f gld pans that are used by miners in the wrld Sluices are as efficient as panning, but a miner can prcess much mre re per day with a sluice Sluices wrk best when the water/slurry is fed at a cnstant rate Use an ld diesel drum t feed the water at a cnstant rate Miners shuld experiment with different kinds f carpets and riffles Cntrl the slurry flw velcity by adjusting the slpe angle t be just steeper than the angle where the sand clgs the carpet Keep the sand ut f the spaces in the carpet fibers in rder t trap the gld Adjust the sluice width and slurry depth t fit the feed rate, kind f re and grain size, the slurry density and trapping

19 52 GMP - Manual fr Training Artisanal and Small-Scale Gld Miners Centrifuges mechanism (e.g., abut 1 cm depth is gd fr ne meter wide Nmad carpet sluices at stamp mills grinding quartz re in Zimbabwe) Mst gld is captured in the first 1/3 rd f the sluice Use shrt sectins f sluice (cnsider using a zigzag cnfiguratin if pssible) t limit acceleratin f slurry If the slurry flwing ver a sluice is deep, the pulp can speed up t much and lead t lss f gld; shallw slurry (e.g., 1 cm deep n a Nmad carpet) des nt accelerate as much as deep slurry Use different trapping mechanisms and/r slpe angles t capture carse and fine gld Cpper amalgamatin plates are a frm f sluice that shuld be banned in all cuntries Centrifuges (e.g. Knudsen type-abj, GldKacha) can be simple and relatively inexpensive; these centrifuges recver abut the same amunt f gld as a cpper amalgamating plate Mre sphisticated centrifuges (Knelsn and Falcn) are mre expensive and require a clean cnstant water supply t perate, hwever these centrifuges can recver mre gld than ther frms f gravity cncentratin equipment Putting mercury in centrifuges leads t the lss f fine gld with the flured mercury