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PT N 001

Enero 2013

Geometalrgia,

Procesamiento de Oro

Sostenible al MedioAmbiente y diseo de

Planta


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Esther Rodriguez

B.Eng. (Chem), M.Env.Eng. Sc., PhD Minerals Sc.


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I. The Geo-Metallurgical Studywhat this means

and why is important.

II. Stages for the Development of a Gold Plantfrom the concept to construction.

III. The Design of an Environmental Sustainable GoldPlant.


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Geo-Metallurgy is the study of the drivers for the

metallurgical response of an ore deposit basedmainly on its mineralogy.

In other words; what is required to investigate torecover profitable a valuable metal from a specific

type of ore.


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Generally, the main topics to cover are

The several type of mineral species: valuable metal andgangue.

The size of grains.

The strength and abrasion of the ore to be processed.

The liberation of a valuable metal respect to the ore size.

The comminution test work.

The flotation and hydrometallurgy test works. The selection of the flow sheet.

Process simulation for optimisation of plant performance.

Models to characterize the ore variability throughout the

mine-life are not considered.


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Source: Metso Handbook


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Some common minerals found in mineral processing:

Native Gold, Au: > 75% of Au; S.G. 1619.3; deep yellow Electrum, (Au,Ag): 45 to 75 % of Au; S.G. 1316; pale yellow Calaverite, AuTe2: 39 to 43 % of Au; S.G. 9.2; white or creamy yellow

Sylvanite, (Au,Ag)2Te4: 24 to 30 % of Au; S.G. 8.2; creamy white Iron Sulphides: pyrite (FeS2), arsenopyrite (Fe,As,S), pyrrhotite (Fe1-

0.8S)

Copper Sulphides: chalcocite (Cu2S), chalcopyrite (CuFeS2)

Other Metallic Sulphides: sphalerite (ZnS), galena (PbS)

Quartz: SiO2 Silicates: feldspars XAl(1-2)Si(3-2)O8 Clays: kaolinite Al2(Si2O5)(OH)4

Carbonates: calcite (CaCO3), siderite (FeCO3)

Iron Oxides: goethite (FeO.OH), hematite (Fe2O3)


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SEM image of gold mineral grains: native gold associated with pyrite.

Grain # 7: liberated gold - 7 x 4 m.

Grain # 8: locked gold23 x 2 m88% Au.

The knowledge of the gold association and degree of liberation are importantto decide what techniques of extraction should be considered.


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The aim of the mineralogical investigations is to characterize the gold mineral

composition, gold grain size, its liberation and association.

Currently it is available automatic equipment that identify and quantify the

different mineral phases in an ore sample and also determine the degree ofliberation.

QEM*SEM (Quantitative Evaluation of Materials using Scanning Electron

Microscope) is one of these modern equipment. It is composed of a computer

controlled SEM fitted with backscattered electron (BSE) and X-ray detector.


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Type of gold ore bodies / deposits in relation to weathering.

Lateritic deposits: soft rocks and consists generally of SiO2 and iron oxides.Gold particles are less enclosed.

Supergene deposits: deeper down in the weathered profile, usually near thewater table. They form in arid areas where groundwater is very saline thatleach the weathered rock. Soluble gold chloride complexes are re-

precipitated at the water table by Fe2+.

Primary ore deposits: unweathered ore. Mainly igneous rocks are found in

gold deposits (granite, basalt, andesite).

Rock oxidation by weathering:

FeS2(s) + 15/4O2(g) + 7/2H2O(l) 2SO42-(aq) + Fe(OH)3(s) + 4H+(aq)

Feldspar Kaolinite


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Source: Gold Plant Operators Course, Western Australia School of Mines


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Source: Gold Plant Operators Course, Western Australia School of Mines


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The crushability/grindability is an important ore characteristic thatquantifies the difficulty to break down the ore by impact, abrasion and/orattrition forces. It is known as the WorkIndex, Wi.

Wi a key ore parameter that affect directly the energy requirements in the

comminution circuit.

More used Wi are: crushing work index (CWi), rod work index (RWi), ballwork index (BWi).

Generally, the RWi define an ore as:

Soft (less competent): 7 to 9 kWh/t

Medium: 9 to 14 kWh/t

Hard: 14 to 20 kWh/t

Very Hard (more competent): > 20 kWh/t

Each rock and mineral has their specific range of Wi (measured value).


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There is a relationship between the area being extracted, the ore

mineralogy and the ore competence. Data below are test results of samples

from the same gold deposit.

Orebody Zone Ore Type RWi BWi

Lateritic Oxide 5 46

Supergene Sulphide + Oxide 14 74


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Rock/Mineral BWi, kWh/t

Andesite 20

Basalt 19Granite 17

Quartz 15

Limestone 14

Hematite 14Magnetite 11

Pyrrhotite ore 11

Pyrite ore 10

Clay 7


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The BWi is defined as the energy required to reduce the ore from infinitivesize to a P80 of 100m.

The test involves a series of consecutive batch grinds in a standard ball millfed with 10 kg of minus 3.5 mm ore sample. After each grind, thedischarge is screened to remove the undersize that is replenished with an

equal mass of a new feed. Testing continues until the weight of theundersize becomes constant.

The Wi (rod and ball) is used with the Bonds Third Theory ofComminution to calculate the specific energy required for a mill (E, kWh/t)to reduce a tonne of feed of which 80% passes size F80 microns to a

product of which 80% passes P80 microns.

E = Wi (10/P 10/F)

F = size at which 80% of the feed passes, m

P = size at which 80% of the product passes, m


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The power required for the mill at the pinion (P, kW) is calculated asfollowed:

P = E.T

T = new feed throughput, t/h

The energy requirement calculated with the Bond method work well for ball

mills charged with rod mill product and for diameter up to 4.9 m.

The Bond method also requires that the feed and product size distributions to

be approximately parallel lines when plotted cumulative percentage vs. Log

size.

Larger diameter ball mills require correction factor to the Bond equation.

The estimation of the energy requirement for AG and SAG mills needs a

larger set of correction factors and/or a pilot scale test.


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The Abrasion Index quantifies the capacity of the ore to wear

equipment/tools when motion is applied.

Ai is a key ore parameter because it allows the estimation of the wearing

rates.

Correlations have been developed to estimate wearing rates in kg of metal

wearing/ kWh used in comminution circuit. They are:

Crusher liners (jaw, gyratory, cone): (Ai + 0.22)/ 24.3

Roll shell of the roll crusher: 0.45(0.1Ai)0.667

Balls of the wet ball mill: 0.159(Ai0.015)0.33

Liners of the wet ball mill: 0.0118(Ai0.015)0.3


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The abrasion test is carried out in a rotating drum in which dry ore is placedover a hardened steel paddle rotating concentrically but in opposite direction

for 1 h. Every 15 minutes, the ore is removed and replaced. The Ai is

determined from the weight loss of the paddle.

Each rock and mineral has their specific range of Ai.

The operational cost increases when processing an ore of bigger Ai.

Rock/Mineral Average, Ai

Granite 0.400.55

Basalt 0.20 - 0.45

Quartz 0.69 - 0.75

Feldspar 0.19

Clay 0.04

Limestone 0.001 - 0.05

Hematite 0.37 - 0.50Magnetite 0.20 - 0.48


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Apart of the Wi and Ai, additional testworks are sometimes required for a

complete characterisation of the ore and definition of the comminution

circuit. They are briefly described below.

Unconfined Compressive Strength (UCS): the max. compressive stress that

a sample can withstand before failure is measured. It is used for crusher

selection.

JK Tech Drop Weight Test: It assesses the impact breakage characteristics

of an ore five sized fractions over the range of 13 to 60mm. A steel weight

falls under gravity to crush a single particle. The test generates the rockbreakage parameters A and b Axb indicates the strength of the ore.Higher value indicates lower strength. A and b are used in model

calculations for ball, SAG, AG mills and crushers.


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As shown in this figure, bigger particles have lower strength, for soft andhard ores.

SAG Mill Comminution Test (SMC): this is a shorter version of the JKdrop weight test, and only one size fraction is tested. It also generates theA and b parameters for use in simulation, though with less accuracy.


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Once the ore mineralogy and the comminution parameters have beenobtained, the crushing and milling circuits can be determined.

This involves the selection of the type of crushers and mills that have thecapacity to produce the size and degree of liberation required downstream

the process at the minimum operational cost (energy, balls consumption).

For example, for a medium competent and non-abrasive ore, and aproduct size of P80 < 120 m, it is suggested:

Primary Crusher + Single Stage SAG Mill

Three Stage Crushing + a Ball Mill For a medium competent and abrasive ore, and a product size of P80

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