Process Mineralogy and Application in Mineral Processing and Extractive Metallurgy (Joe Zhou).pdf

5/19/2018 Process Mineralogy and Application in Mineral Processing and Extractive Metallurgy (Joe Zhou).pdf

1/13

1

PROCESS MIERALOGY AD APPLICATIO I MIERAL

PROCESSIG AD EXTRACTIVE METALLURGY

Joe Zhou

Joe Zhou Mineralogy Ltd., Canada

[email protected]

ABSTRACT

The ultimate goal of a mineral processing project is to recover as much of the target

mineral(s) as possible from the ore being treated to achieve the best economics.

Selection of the mineral processing techniques and development of the flowsheet are

the most important steps in achieving this goal. However, the recoveries of valuableminerals may be not satisfactory due to the complexity of the ore although the mineral

processing plant is well designed and built. In fact, extraction of most metals and

minerals is largely driven by mineralogical factors that often cause low recoveries,

low grade, high reagent consumption and other processing issues. Mineralogical

factors affecting ore processing and metal extraction may include mineral type,

metal/element deportment, grain size, liberation and association, surface chemistry,

and concentration and liberation characteristics of minerals detrimental to processing

(e.g., cyanide- and oxygen-consuming minerals, organic carbon, clays, etc.). Process

mineralogy helps address such issues related to ore processing and metal extraction. It

is widely used as a predictive and trouble-shooting tool in mineral processing and

extractive metallurgy, and provides useful information on process selection, flowsheetdevelopment, recovery improvement and reagent consumption optimization.

This paper presents an overview of process mineralogy and focuses on practical

processing problems encountered in ore processing. It also provides a comparison of

commonly used mineralogical techniques and methods for selecting the right tool for

the problem. Case studies covering gold, silver and base metals ores are provided

from a variety of processing options such as gravity separation, flotation, cyanidation

and pre-oxidation.

Keywords: process mineralogy, mineral processing, extractive metallurgy, prediction,

trouble-shooting

ITRODUCTIO

The minerals industry employs various processes to extract valuable metals and

minerals from different ores. Commonly used processes include crushing, grinding,

gravity concentration, flotation, leaching, magnetic and electrostatic separation, and

refining. Fine grinding and oxidative pretreatment are also commonly used in

processing of refractory ores. These processes can be used separately for different

ores, but more often they are employed collectively in many processing projects.

Presented at the First International Metallurgical Meeting Peru 2012, October 26t,

2012, Lima, Peru


2/13

2

Process selection is the most important step in a mineral processing project. Generally

speaking, the factors affecting process selection of a project include geological,

mineralogical, metallurgical, environmental, geographical, economical and political

situations (Marsden and House, 2006). Among these factors, mineralogical and

metallurgical factors have a direct impact on process selection as they determine the

response of the ore to mineral processing techniques. In many metallic ores, therecoveries of metals and minerals of interest are often driven by mineralogical factors

such as grain size, liberation, surface chemistry, association with other minerals,

coating and rimming, presence of cyanicides, oxygen consumers, preg-robbers and

clay minerals, and locking in other minerals as trace substitution. Among these

mineralogical factors, liberation, grain size and association (locking in other minerals

as fine-grained inclusions and/or trace substitution) are the most common factors

affecting gold ore processing and metal extraction. Processing of PGE ores faces the

same challenges as gold. Fine grain size and locking of PGM in sulphide and non-

sulphide minerals often cause PGM losses. Presence of talc in some PGE ores (such

as the Merensky reef and Stillwater Complex) can cause significant processing

difficulties (Cole, 2002). In the UG-2 ore located in the Bushveld Complex, the PGMare finely disseminated with the average grain size being about 10 microns, so that

grain size, liberation and association tend to dictate mineral floatability (Nel et al,

2004).

The presence of PGE in pentlandite and other sulphide minerals as solid

solution is another issue that causes processing difficulties. Compared to gold and

PGE, silver mineralogy is more complex. In Pb-Zn-Ag ore, silver often occurs as

inclusions in and attachment to primary lead and zinc minerals (commonly galena and

sphalerite) and can be lost with these minerals during flotation. In silver ore

containing significant amounts of pyrargyrite, proustite and stephanite, silver recovery

by cyanidation can be very low because these minerals dissolve slowly in cyanide

solution. Silver can also occur in other minerals as submicroscopic silver as well,

making silver extraction more difficult (Zhou, 2010). Base metal ores are mainly

processed by flotation. In processing of base metal ores, high content of pyrite, fine

grain size of galena, intergrowth of chalcopyrite with sphalerite and presence of clay

minerals can cause serious processing problems. Surface chemistry may affect

extraction of base metals as well. In some base metal ores containing precious metals,

precious metal-bearing minerals (such as electrum and kstelite) may misreport to

zinc concentrate rather than copper concentrate because of the surface coating (Zhou

et al., 2005). As high-grade nickel sulphide ores are being depleted and processing

laterite ores continue to pose challenges, the future of nickel extraction lies in low-

grade ultramafic ores. The main challenge in processing low-grade ultramafic ores is

the presence of MgO-containing silicate minerals. The impact of MgO content on Nirecovery is very pronounced and high MgO requires the effective rejection of MgO

containing minerals. A study conducted on fibrous minerals in ultramafic nickel

sulphide ores indicated the significant depressing effect of MgO on pentlandite

flotation. As the impact of fibrous minerals on slurry viscosity, grinding, and flotation

becomes understood, nickel recovery will increase, making the mining and low-grade

ultramafic deposits viable (Xu et al, 2012). In iron ore processing, elevated sulphur

content of the concentrate is a common issue in some operations. To be able to reduce

the sulphur content, the sulphide mineralogy including mineral speciation, grain size,

liberation and association with iron minerals must be well understood and determined.

This paper introduces the scope of process mineralogy and mineralogical factors that

may affect mineral processing and extractive metallurgy of various ores, anddiscusses the application of process mineralogy through case studies.


3/13

3

OBJECTIVES AD ROLES OF PROCESS MIERALOGY

Process mineralogy is an inter-discipline in the fields of mineralogy and metallurgy. It

uses the theories, principles, methods and tools of mineralogy to study all

mineralogical characteristics of an ore and potential problems related to mineral

processing and extractive metallurgy with prediction and trouble-shooting being twomajor objectives and roles. Process mineralogy provides useful information on

process selection, flowsheet development, recovery improvement and reagent

consumption optimization. The information acquired from a process mineralogical

study can be used as a guide for a metallurgical testwork program for process design

or optimization. The scope of a process mineralogy program may include but not

limited to the following:

Prediction:

1.

Response of a new ore to various processes and most likely processing options

2. Estimated recovery of valuable minerals and grade of concentrate

3.

Potential mineralogical factors affecting ore processing and metal extraction

Trouble-shooting:

1.

Deportment of valuable minerals in tailings and deleterious elements in

concentrates

2. Cause for valuable losses and opportunity for recovery improvement

3. Cause for high reagent consumption and opportunity for reagent optimization

Figure 1 is a good example of using mineralogy as a predictive tool. It illustrates some

common forms and carriers of gold in different gold ores and indicates the impact of

gold deportment on extractive metallurgy. The large gold grain in Figure 1(1) is

coarse (approximately 600 x 1000 m) and completely liberated, and can be

recovered by gravity, flotation or cyanidation. High gold recovery (>90%) is expected,

and can generally be achieved when gold occurs as coarse grains like this one.

However, gold leaching by cyanidation is a slow, diffusion-controlled reaction, and

large gold particles can take many days to fully dissolve. Therefore, gold can be lost

to tailings owing to insufficient residence time during cyanide leaching. To avoid gold

losses, it is better recover the coarse gold by gravity prior to cyanide leaching. The

gold grains in Figure 1(2) are medium-grained and entirely locked in pyrite, and are

not recoverable by direct cyanidation. These gold grains can only be recovered by fine

grinding followed by cyanidation, with or without flotation, or by pre-oxidation of the

pyrite followed by cyanidation. Gold grains in Figure 1(3) are extremely fine, andlocked in arsenopyrite. This type of gold may be recovered by ultra-fine grinding

followed by cyanidation of the sulphide concentrate, although this is in fact seldom

beneficial, as the gold grains are usually finer than the finest practical grind size that

can be achieved in commercial mills. Therefore, the only alternative is to oxidize the

arsenopyrite to liberate the gold, and then leach with cyanide. Gold in Figure 1(4)

occurs as sub-microscopic gold in disseminated pyrite, making the gold extremely

refractory. Pre-concentration of the gold in these types of ores by gravity or flotation

is usually inefficient, and therefore, to recover gold in this type of ore, pre-oxidation

processing (autoclaving, roasting and biological oxidation) of the whole ore may be

required to liberate the gold and achieve high recovery efficiency. Based on the gold

deportment shown in Figure 1 and other results, the mineralogist is able to balance thevarious types of gold in an ore and to comment on most likely processing options.


4/13

4

Figure 1: Gold deportment and impact on gold extractive metallurgy(1) Liberated coarse gold grain (Au, 6001000m); (2) gold grains (inside red circles) lockedin pyrite (Py); (3) fine-grained gold particles (inside red circles) locked in arsenopyrite (Apy);(4) submicroscopic gold locked in fine-grained gold-bearing pyrite (yellowish white grains)that is disseminated in quartz (Q) (Zhou and Fleming, 2007)

Gravity concentration and flotation processes are very effective at recovering large

and fully liberated gold grains, and are used in many gold plants. Gravity

concentration is also an environmentally friendly process because it doesnt use

chemicals, and should be considered when an ore contains certain amount of liberatedgold. However, liberated gold can be lost from a gravity circuit if the grain size is

small or if the gold grain is attached to a low density mineral. The majority of gravity

equipment can only recover gold grains greater than 50m, and some equipment is

effective at sizes down to about 10m (Marsden and House, 2006). A GRG (gravity-

recoverable gold) study led by the late professor Andre Laplante proved that most

gold particles recovered in commercial gravity recovery plants are coarser than 37 m.

This is because gravity circuits are always installed in mills, and the flow to the

gravity circuit is regulated by installing cyclones, with only the cyclone underflow

reporting to the gravity equipment. GRG finer than 37m tends to report to the

cyclone overflow much more readily than its coarser counterpart, and therefore by-

passes the gravity equipment (Laplante and Stauton, 2005). In a typical golddeportment study, the mineralogist will determine the size distribution of all observed

gold grains and the size distribution of liberated gold grains can be used in assisting

the equipment selection and for predicting the gold recovery by various processes.

Figure 2 shows the size distribution of liberated gold in a copper-gold ore. Gold

balance indicated that liberated gold in this ore accounts for approx. 65% of the head

assay with a size range from 100m. This part of gold can be recovered

by cyanidation, and also by flotation at slightly lower recovery. The recovery by

gravity will be much lower than cyanidation and flotation (approx. 33%). Flotation is

a common process that is used in many base metals and precious metals ore

processing plants. It is very effective at recovering both liberated valuable andsulphide minerals over 10m. In flotation of gold ores, liberated gold grains less than

3

Au

1 2 4

More free-milling More refractory

Liberated,

coarse-grained

Locked,

medium-grained

Locked,

fine-grained

Locked,

submicroscopic

Gravity,

Flotation,

Cyanidation

Fine grinding

Cyanidation

Flotation &

Preoxidation

Flotation,

Fine grinding

Preoxidation &

Cyanidation

Pre-oxidation

& Cyanidation

Flotation

Py

QApy


5/13

5

10m can also be recovered by flotation, but the efficiency is much lower. In addition

to unliberated gold, gold losses to gravity and flotation tailings include very fine

liberated gold grains as well as coarse liberated gold grains whose surface is coated or

rimmed by non-floatable material (e.g., iron oxides or hydroxides, silicates) and

hydrophobic gold grains whose surface is coated with fine hydrocarbon or sulphurous

material (Zhou, Jago and Martin, 2004; Marsden and House, 2006).

Figure 2: Prediction of gold recovery by common processes

In addition to being a predictive tool in process flowsheet development, process

mineralogy also plays an important role in process optimization and plant operation,

and is often used as a trouble-shooting tool by operating mines. Due to the complexity

of the ore, the recoveries of valuable minerals may be not satisfactory although the

processing flowsheet is well developed and implemented. In other words, minerals of

interest can be lost to the tailings for mineralogical reasons. When recovery and

concentrate grade drop or fluctuate, or acceptable recovery is achieved only with the

use of significantly more reagents or more complex pretreatment processes, it usually

indicates the change of feed mineralogy. Once this happens, a detailed mineralogical

study program should be considered to find out what has caused the processing

issue(s) and how to improve the plant performance. The information acquired from

such a mineralogy program can be used as a guide for process optimization.

Figure 3 shows the distribution of gold in a cyanidation tail. The sample, assaying

8.3g/t Au and 92g/t Ag, originates from a bulk flotation concentrate subjected to

regrinding to approximately 33 microns, followed by gravity concentration and 24

hour cyanide leaching of the gravity tailing. The leach residue was filtered and

washed and then dried at low temperature for a complete gold and silver deportment

study. The results showed that liberated gold in the sample was effectively recovered

by gravity and subsequent cyanide leaching. The gold left in leach residue is all

locked in other minerals as micron-size inclusions (0.5-13m) and submicroscopicgold with a total of 74% in pyrite, 7.5% in arsenopyrite and 18.5% in non-sulfide

0

5

10

15

20

25

30

0-10 10-20 20-40 40-60 60-80 80-100 100-120

Distribution(%

)

Grain Size (m)

Size Distribution of Liberated Gold

Flotation recoverable gold

Cyanide recoverable gold

Gravity recoverable gold


6/13

6

minerals. Visible gold inclusions (approx. 40%) locked in other minerals are not

recoverable at current grinding fineness unless the sample is ground much finer. To

extract submicroscopic gold (approx. 60%) locked in sulphide minerals, oxidative

pretreatment is required to create a porous structure through which the cyanide leach

solution can penetrate to reach the minute gold particles. The study also showed that

the silver lost in the leach residue occurs mainly as liberated freibergite, acanthite andstephanite (up to 85m) with minor amounts of native silver and other silver minerals.

It is understandable that those coarse and liberated silver minerals were not recovered

by gravity due to their lower specific gravity, but why they did not dissolve during

cyanide leaching? Mineralogical studies suggested that the concentration of cyanide

in leach solution was too low for silver leaching although all liberated gold was

dissolved to completion. The slow-leaching kinetics of silver minerals is considered to

be another main cause for silver loss. Based on the mineralogical findings, the process

was optimized by incorporating an oxidative pretreatment for gold extraction. High

cyanide concentration and extended leach process has substantially improved the

silver recovery.

Figure 3: Distribution of gold in a cyanidation tail

MIERALOGICAL FACTORS AFFECTIG MIERAL PROCESSIG

There are a number of mineralogical factors that may affect mineral processing and

extractive metallurgy of various ores. Some common factors are listed in Table 1.Among these factors, liberation, grain size and association are the most important

ones. The impact of these mineralogical factors on mineral processing and extractive

metallurgy is discussed below. More information can be found in the reference (Zhou,

Jago and Martin, 2004; Zhou and Fleming, 2007; Zhou, 2010; Zhou, 2011).

1. LiberationMost valuable minerals, if not liberated, will not be recovered by conventional

processes, or it can be recovered only at low recovery and low concentrate grade. This

makes liberation extremely important in many (if not all) mineral processing projects.

Prior to the recovery process, valuable minerals need to be liberated from non-

valuable minerals with those they are associated. Poor liberation will result in low

recovery and grade for most valuable minerals. In the case of precious and base metal


7/13

7

ores (such as Au, Ag, PGM, Cu, Pb, Zn and Ni), encapsulation in sulphide and

silicate minerals is a common and major cause for valuable mineral losses. In the UG-

2 ore, 25% of PGM were locked in silicates and finer grinding was required in a

secondary milling step to increase the PGM recoveries (Nel et al., 2004). In gold

mining industry, as the free-milling gold ores are being depleted and more refractory

ores are discovered and processed, liberation becomes even more important in such aproject. In addition to physical liberation (such as fine grinding and ultrafine grinding),

several chemical liberation processes (such as pressure oxidation) have been

developed and implemented. In processing of refractory gold ores, particularly the

Carlin-type gold ores in Nevada, the sulphide host minerals must first be oxidized, to

create a porous structure through which the cyanide leach solution can penetrate to

reach the minute gold particles. This oxidation is traditionally achieved by roasting,

autoclaving or bacterial oxidation.

Table 1: Common mineralogical factors

# Mineralogical Factors Associated Ores & Processes1 Liberation/locking Various ores & processes

2 Grain size Various ores & processes

3 Association Various ores & processes

4 Coating & rimming Mainly flotation, also gravity and leaching

5 Surface chemistry Mainly flotation, also gravity and leaching

6 Cyanicides & oxygen consumers Mainly cyanide leaching

7 Preg-robbing Cyanide leaching of gold and silver ores

8 RefractorinessMainly cyanide leaching, also gravity andflotation of precious metals ores

9 Slow-dissolving kinetics Cyanide leaching of gold and silver ores

10Deleterious minerals/toxicelements (As, Hg, asbestos)

Mainly leaching and flotation of base andprecious metals ores, but can also affect otherore processing

11Gangue mineralogy (clays,sulfide gangue and more)

Flotation and leaching of base and preciousmetals ores, flotation and magnetic separation

of iron ores, tailings disposal and more

2. Grain Size

Grain size is one of the most important mineralogical factors that affect processing ofmany ores. For example, gold is mainly recovered from ores using gravity, flotation,

cyanidation or a combination of these processes. Gold grain size can be a significant

factor driving the efficiency of gold recovery processes.As discussed above, liberated

gold can be recovered by gravity concentration but it can be lost from a gravity circuit

if the grain size is fine. The majority of gravity equipment can only effectively

recover gold grains greater than 50m. Fine gold, which is too small to be recovered

by the installed equipment (Marsden and House, 2006):


8/13

8

As mentioned above, flotation is a common process that is used in many gold ore

processing plants. It is very effective at recovering both liberated gold (which tends to

be naturally floatable), and gold-bearing sulphide minerals. Mineralogical

examinations conducted on metallurgical products have shown that gold grains over

10m can be effectively recovered by flotation. Gold grains less than 10m can also

be recovered by flotation, but the efficiency is much lower. Gold losses to flotationtailings include very fine gold grains as well as coarse gold grains whose surface is

coated or rimmed by non-floatable material (e.g., iron oxides or hydroxides, silicates)

(Zhou, Jago and Martin, 2004; Marsden and House, 2006).

Cyanidation is the most efficient and widely applied process for extracting gold from

various ores, and high recoveries (>80%) are usually expected and commonly

achieved. The gold left in the leaching tailings is often encapsulated in sulphide, oxide

and/or silicate minerals that are impervious to the cyanide leach solution. However,

liberated gold can also be lost to cyanidation tailings. This generally occurs when

some of the gold in the ore is too big or too slow-leaching to dissolve to completion in

the time allocated to the leaching process. Studies have indicated that a spherical goldparticle with a diameter of 44 m will take approximately 13 hours to dissolve under

normal cyanide leach conditions, while particles with a diameter of 150 m will take

48 hours or more (Fleming, 1992). Generally, when an ore contains large liberated

gold particles, the flowsheet will incorporate a gravity circuit to recover these

particles, so it is not necessary to design for excessively long leach times. An average

gold plant would have a 24 hours leach residence time (Fleming, 1998). Deschenes

and his team at CANMET have made a breakthrough in extracting gold and silver in

high-grade Au-Ag ores (Deschnes et al, 2009; Rajala and Deschnes, 2009;

Deschnes and Fulton, 2011).

In the processing of base metal ores, grain size also has a significant impact. Galena,

sphalerite and chalcopyrite are often associated with each other. Galena is often fine-

grained and sphalerite is relatively coarse. Chalcopyrite often occurs as inclusions in

sphalerite (so-called chalcopyrite disease) in addition to being discrete particles. To

recover these minerals of interest, the Cu-Pb-Zn ore often needs to be ground to such

a degree that these minerals are liberated from other minerals and also separated from

each other. Both over-grinding and under-grinding may cause processing issues. To

maximize the recovery of valuable minerals, grain size distribution of each mineral

needs to be determined through a mineralogy program.

3.

AssociationAssociation is also an important mineralogical factor that may impact ore processing.

In processing of copper-gold ores (such as porphyry Cu-Au ores), precious metals

(Au and Ag) are often recovered into copper concentrate by flotation with or without

gravity concentration. When gold is intimately associated with copper minerals, it will

report to copper concentrate. However, if gold is associated with pyrite, iron oxide

and silicate minerals, the recovery of gold might be lower. In some plants, a separate

sulfide flotation circuit may be required to produce an Au-bearing sulfide concentrate.

Gold with silicate is a common association in many ores. When gold is finely

disseminated in silicates, the extraction can be challenging because the ore has to be

ground finer to make the gold accessible to the cyanide solution. It has been found

that a substantial amount of gold (up to 50%) in some ores can occur as tiny


9/13

9

Category Technique Technique Application MDL

Qualitative/Semi-Quant OM Optical MicroscopyMineral ID & qualitative/semi-quant

mineral analysis of bulk samplesHigh (%)

ADIS Automated Digital Image System High (%)

XRD X-ray Diffracton High (%)

SEM Scanning Electron MicroscopyMineral ID & qualitative/semi-quant

elemental analysis of individual particlesHigh (%)

MLA Mineral Liberation Analyser Low (%)

QEMSCAN

Quantitative Evaluation of Materials

by Scanning Electron Microscopy Low (%)

EPMA Electron Probe Microanalysis Low (ppm)

PIXE Proton-induced X-ray Emission Low (ppm)

D-SIMSDynamic Secondary Ion Mass

SpectrometryLow (ppm-ppb)

LAM-ICP-MS

Laser Ablation Microprobe

Inductively Coupled Plasma Mass

Spectrometry

Low (ppm-ppb)

Synchrotron Synchrotron Radiation Light Source Low (ppm-ppb)

TOF-LIMSTime of Flight Laser Ion Mass

SpectrometryLow (ppm)

TOF-SIMSTime of Flight Secondary Ion Mass

SpectrometryLow (ppm)

XPS X-ray Photon Spectrometry Surface analysis of bulk material Low

Surface Analysis

Surface analysis of bulk material &

individual particle

Mineral ID & qualitative/semi-quant

mineral analysis of bulk samplesSemi-Quant

QuantitativeQuantitative mineral analysis of bulk

samples and individual particles

QuantitativeQuantitaive elemental analysis of

individual particles

inclusions (a few microns) in quartz or other minerals, which makes gold extraction

extremely difficult and uneconomic.

Same as the porphyry copper-gold ores, flotation is the principal process for

recovering gold minerals and copper sulphides in iron copper gold (IOCG) ores. Gold

in IOCG ores is mainly associated with copper sulphides and, to a lesser extent, ironoxide minerals (mainly as inclusions or submicroscopic gold in hematite). Liberated

gold and gold associated with copper sulphides can be recovered by flotation into a

copper concentrate and high gold recovery (over 80%) can be achieved. However,

gold can be lost to flotation tails if it is fine-grained or if it is associated with hematite.

To recover the fine-grained gold particles (often below 20 m) and gold that is

associated (exposed) with hematite or other gangue, cyanide leaching may be

considered. Gold locked in hematite or other gangue as tiny inclusions will not be

recovered unless the ore is ground finer. Submicroscopic gold in hematite is not

recoverable by conventional techniques. Precious metals associated with zinc

minerals (mainly sphalerite) in Au-Ag ore and Cu-Pb-Zn-Au-Ag ore is another

example of unfavourable association (Zhou, 2005).

Table 2 lists a number of techniques commonly used in process mineralogy. These

techniques can be used separately, but a typical mineralogy program often uses a

comprehensive mineralogical and analytical approach including several techniques.

The advantage of using such a comprehensive approach is that a large sample can be

studied to get better statistics and each mineralogical issue will be addressed properly

using specific techniques.

It should be noted that each technique is designed for certain purposes and has its

advantages & limitations. They should be selected wisely and used properly.

Table 2: Techniques commonly used in process mineralogy


10/13

10

APPLICATIO OF PROCESS MIERLOGY I MIERAL PROCESSIG

AD EXTRACTIVE METALLURGY

Case 1: Process mineralogy of a refractory sulphide gold ore

Gold in refractory sulphide gold ores is predominantly associated with sulphideminerals and occurs as both fine-grained gold particles and/or submicroscopic gold.

Microscopic gold (visible gold) usually accounts only for an insignificant portion,

with the majority of gold being contained in sulphides. To extract gold from

refractory ores, the ore needs to be finely ground and pre-oxidation is required.

Sulphide host minerals must first be oxidized to create a porous structure through

which the cyanide leach solution can penetrate to reach the minute gold particles.

Figure 4 shows the gold-bearing arsenopyrite (4A) and pyrite (4B) in a sulphide gold

ore. The ore is a highly refractory ore with only about 2% of gold being fine-grained

"free gold" (not locked in sulphide grains), making gold extraction difficult because

the gold is mainly locked inside the two main sulphide minerals arsenopyrite (75%Au) and arsenic-rich pyrite (23% Au). The ore is treated through grinding, flotation,

pressure oxidation and carbon-in-leach circuits. Gold recovery is ~83%.

Figure 4: Gold in a refractory sulphide oreA: Micron-size gold inclusions in arsenopyrite (Apy); B: Submicroscopic gold-bearing pyrite (Py)

Case 2: Process mineralogy of a Pb-Zn-Ag ore

In Pb-Zn-Ag ores, silver is often associated with sulfides and is recovered by flotation

and concentrate smelting with or without leaching to recover the base metals as wellas the silver values. As discussed above, a number of mineralogical factors, such as

liberation, association, grain size, and rimming and coating, can impact flotation and

leaching of silver minerals. In a Pb-Zn-Ag operation processing high-grade silver ore

(~400g/t Ag), the silver recovery by flotation was approx. 85% for a number of years.

The mill endeavors to recover additional silver to the lead concentrate and minimize

the placement of silver to the zinc concentrate. To explore the opportunity for further

silver recovery improvement, a silver deportment study was initiated to identify the

cause for silver loss and to determine the possible recovery increase. The conclusions

are listed below (Zhou, 2010).

1.

Silver occurred mainly as freibergite ((Ag, Cu, Fe)12

(Sb, As)4S

13), dyscrasite

(Ag3Sb), pyrargyrite (Ag3SbS3) with a moderate to minor amounts of acanthite

(Ag2S), native silver and other minerals.

B51.9ppm Au

Py


11/13

11

2. Silver also occurs in galena as inclusions and submicroscopic silver.

3.

Lead Tail (Zinc Feed) contained 60 g/t Ag (accounting for ~14% of the head

assay). Approximately 15% of the lost silver was liberated and associated with

galena, and can be recovered inlead circuit without further grinding, which means

the current silver recovery in lead circuit can be increased by 2%.

4.

Zinc Tail (i.e. Final Tail) contained 48 g/t Ag (accounting for 12% of the headassay). Approximately 15% of the lost silver (with a maximum of 20%) was

locked in galena, sphalerite and other sulphide minerals and was recoverable by

flotation. This indicates that 2% of the silver lost in the Final Tail can be

recovered in zinc circuit.

5. Theoretically, a total improvement of 4% recovery can be expected.

6.

The main causes for silver loss are unfloated liberated galena and silver minerals

and association of silver with sulphide gangue and non-sulphide gangue.

Figure 5 presents some lost silver minerals in the flotation tail. Liberated galena with

dyscrasite inclusions (Figure 5-1) and minor amounts of liberated silver minerals (5-2)

can be recovered in the lead circuit without finer grinding. The collector chemistryneeds to be reviewed to maximize the recovery of these liberated silver minerals. To

recover silver minerals and galena associated with non-sulphide gangue (5-3) and

sulphide gangue (5-4), the ore needs to be ground finer to liberate the silver minerals.

Composite particles shown in Figure 5-3 and 5-4 can be recovered into lead or zinc

concentrate without finer grinding, but they will lower the concentrate grade.

Figure 5: Silver minerals lost in flotation tail1: Liberated galena with dyscrasite inclusions (inside red circles); 2: Liberated dyscrasitegrain (Dy); 3: Freibergite (Fr) associated with galena (Gn) and silicate (Si); 4: Dyscrasite

(inside red circle) locked in arsenopyrite (Apy)

3

2

Dy

4

Apy


12/13

12

DISCUSSIO

The minerals industry is continuing its growth as the world economy keeps growing.

A number of large projects are being developed and will be commissioned in the next

few years. At the same time, the minerals industry is facing challenges in processing

difficult ores with low grade and complex mineralogy. To help determine the miningmethod, extraction process requirements, and in particular, the performance and

optimization of all processes involved, process mineralogy is becoming increasingly

important in a mineral processing project. It includes ore characterization during pre-

feasibility and feasibility study stages and subsequent periodic mineralogical analyses

of feed, intermediate process streams and final products during plant operation. As a

predictive and trouble-shooting tool, process mineralogy helps address all issues and

problems related to mineral processing and extractive metallurgy and provides useful

information on process selection, flowsheet development, recovery improvement and

cost reduction.

ACKOWLEDGEMETS

The author wishes to acknowledge the conference organizers for opportunity to

present this paper at the First International Metallurgical Meeting Peru 2012. Mining

companies that have contributed indirectly to this paper by providing projects and

financial support are also acknowledged.

REFERECES

Cole, S., A review of PGM metallurgy highlighting recent process innovations. In:Proc 34thAnnual Meeting of Canadian Mineral Processors. CMP, Ottawa, 2002, pp

445-467.

Deschnes, G., Rajala, J., Guo, H., Fulton, M. and Mortazavi, S., Leaching of Gold

and Silver from Kupol Samples: Part I: Preliminary Investigation. In: Proc 41st

Annual Meeting of Canadian Mineral Processors, CMP, Ottawa, Canada, 2009, 409-

426.

Deschnes, G. and Fulton, M., The CELP Technology for Leaching Gold and Silver

from High Grdae Ores: Preliminary Investigation for Two New Potential Applications.

In: Proc 43rd

Annual Meeting of Canadian Mineral Processors, CMP, Ottawa,Canada, 2011, 467-476.

Fleming, C.A., 1992, Hydrometallurgy of precious metals recovery. Hydrometallurgy

30, pp 127-162.Elsevier Science PublishersB.V., Amsterdam.

Fleming, C.A., 1998. Cyanidation of gold and silver, in Gold Symposium, pp 1-49

(Royal Melbourne Institute of Technology:Melbourne).

Laplante, A.R. and Staunton, W.P. 2005 Gravity recovery of gold An overview of

recent developments. Treatment of Gold Ores, First International Symposium, 44th

Annual Conference of Metallurgists of CIM, Calgary, Alberta, Canada (edited by G.Deschenes, D. Hodouin and L. Lorenzen), pp 49-63.


13/13

13

Marsden, J. and House, I. 2006. The chemistry of gold extraction, second edition (ed:

E Horwood), pp 651 (The Society for Mining, Metallurgy and Exploration Inc:

Littleton).

Nel, E., Theron, J., Martin, C. and Raabe, H., 2004. PGM ore processing at ImpalasUG-2 concentrator in Rustenburg, South Africa. In:Proceedings 36thAnnual Meeting

of Canadian Mineral Processors, pp 121-139.

Rajala, J. and Deschnes, G., Extraction of Gold and Silver at the Kupol Mill using

CELP. In: World Gold 2009, The Southern African Institute of Mining and

Metallurgy, Johannesburg, South Africa, 2009, 35-42.

Xu, M., Dai, Z., Dong, J., Ford, F., and Lee, A. W., 2012: Fibrous minerals in

ultramafic nickel sulphide ores. CIM Journal, Vol. 3, No. 1, 2012, 1-8.

Zhou, J., Jago, B. and Martin, C., 2004. Establishing the process mineralogy of goldores. In:Proceedings 36thAnnual Meeting of Canadian Mineral Processors, pp 199-

226.

Zhou, J., Martin, C., Blatter, P., Boss, P.-A. and Robitaille, 2005. The process

mineralogy of precious metals in LaRonde flotation products and its effect on process

optimization. In:Proceedings Treatment of Gold Ores,First International Symposium,

44th Annual Conference of Metallurgists of CIM, Calgary, (eds: G Deschenes, D

Hodouin and L Lorenzen), pp 93-109.

Zhou, J. and Fleming, C.A., 2007. Gold in tailings mineralogical characterisation

and metallurgical applications. In: Proceedings World Gold 2007. Cairns, (eds: J

Avraamides, G Deschnes and D Tucker), pp. 311-317.

Zhou, J., 2010. Process mineralogy of silver ores and applications in flowsheet design

and plant optimization. In: Proceedings of the 42nd Annual Meeting of Canadian

Mineral Processors, CMP, Ottawa, Canada, 2010, 143-161.

Zhou, J., The mineralogy and predictive metallurgy of major types of gold ores. In:

World Gold 2011, Proceedings of the 50thAnnual Conference of Metallurgists of CIM,

Montreal, QC, Canada, 1-15.

Process Mineralogy and Application in Mineral Processing and Extractive Metallurgy (Joe Zhou).pdf

Documents

Transcript of Process Mineralogy and Application in Mineral Processing and Extractive Metallurgy (Joe Zhou).pdf