Copper and Cyanide Recovery in Cyanidation Effluents

doi:10.4236/aces.2011.14028

Paper Menu >>

Journal Menu >>

Advances in Chemical Engi neering and Science , 2011, 1, 191-197

doi:10.4236/aces.2011.14028 Published Online October 2011 (http://www.SciRP.org/journal/aces)

Copper and Cyanide Recovery in Cyanidation Effluents

José R. Parga1, Jesús L. Valenzuela2, Héctor Moreno3, Jaime E. Pérez1

1Department of Metall ur gy an d Mat eri al s Science, Institute Technology of Saltillo, Saltillo, México

2Departament of Chemistry and Metallurgy, University of Sonora, Hermosillo, México

3Chemistry Department, Institute Technology of Laguna, Torreón, México

E-mail: [email protected]

Received August 26, 2011; revised Septembe r 13, 2011; accepted Se pt em b e r 23, 2011

Abstract

Cyanidation is the main process for gold and silver recovery from its ores. In this study, a process is pro-

posed to recover copper and cyanide from barren solutions from the Merrill-Crowe cementation process with

zinc dust. This technology is based on inducing nucleated precipitation of copper and silver in a serpentine

reactor, using sodium sulfide as the precipitator, and sulfuric acid for pH control. Results show that pH value

has a significant effect on copper cyanide removal efficiency, and it was determined the optimal pH range to

be 2.5 - 3. At this pH value, the copper cyanide removal efficiency achieved was up to 97% and 99%, when

copper concentration in the influent was 636 and 900 ppm. respectively. In this process (sulphidiza-

tion-acidification-thickening-HCN recycling), the cyanide associated with copper cyanide complexes, is re-

leased as HCN gas under weakly acidic conditions, allowing it to be recycled back to the cyanidation process

as free cyanide. Cyanide recovery was 90%. Finally, this procedure was successfully run at Minera William

in México.

Keywords: Precipitation, Cyanide Removal, Copper Recovery, Cyanidation

1. Introduction

Actually, the most common process for gold and silver

recovery from ores is cyanidation, due to the selectivity

of free cyanide for both metals, and the stability of the

cyanide complex (2

Au , k = 2 × 1038) [1 ]. Chemical

recovery of gold from Merrill-Crowe process cyanide

solution, involves two different operations: 1) gold dis-

solution, where it is oxidized and dissolved to form Au (I)

ion and cyanide complex 2, and 2) precipitation

by reduction of metallic gold . In the cyanidation pro cess,

free cyanide ions in solution can only be provided at a

pH of 9.0. The pH of the pulp can be increased adding

alkalis (e.g. Ca(OH)2, NaOH, etc.), known as protective

alkalis. It is accepted that gold dissolution in cyanide

solutions occurs as a sequence of two reactions, as

shown in Equations (1) and (2). These reactions apply to

silver as well.

( CN)

Au(C N)



22 2

2Au4NaCNO2H O2NaAuCN

2NaOHHO









(1)



22 2

2Au4NaCNH O2NaAuCN2NaOH



 



(2)

Elsner’s Equation (1) shows that oxygen is critical for

the dissolution of gold. Stoichiometry of the process

shows that 4 moles of cyanide are needed for each mole

of oxygen present in solution. At room temperature and

standard atmospheric pressure, approximately 8.2 mg of

oxygen are present in one liter of water. This corre-

sponds to 0.27 × 10–3 mol/L. The corresponding sodium

cyanide concentration for a complete reaction (molecular

weight of NaCN = 49) sh ould be equa l to 4 × 0.27 × 10–3

× 49 = 0.05 g/L or approximately 0.01%. This was con-

firmed in practice at room temperature by a very dilute

solution of NaCN of 0.01% - 0.5% for ores, and 0.5% -

5% for gold and silver concentrates [2]. Gold dissolution

is an electrochemical reaction in which oxygen takes up

electrons at one section of the metallic surface [cathodic

zone], while the metal gives them up in another section

[anodic zone]. Details of this electrochemical reaction

have received considerable attention and under certain

circumstances the reaction is limited by the coupled dif-

fusion of CN– and O2 to the gold surface. Dissolution

rate is normally mass-transport controlled in cyanide

solutions and the activation energy is 8 - 20 kJ/mol [3].

The concentration of cyanide used to dissolve gold in

ores is typically higher than the stoichiometric ratio, due

192 J. R. PARGA ET AL.

to the solubility o f other minerals. Free cyanide produces

complexes with several metallic species, especially tran-

sition metals, which show a broad variation in both sta-

bility and solub ility [4]:



2yy

MCN MCN





 (3)

Many common copper minerals are soluble in the di-

lute cyanide solutio n, typical of leach conditio ns found in

the gold cyanidation process. Minerals such as azurite

and malaquite, are fast leached and soluble in dilute cya-

nide solutions. Enargite and chalcopyrite leach more

slowly but are sufficient soluble to cause excessive cya-

nide loss and contamination of leach solutions with arse-

nic [5]. For example, in the cyanidation of malachite and

azurite minerals, the copper carbonate component lea-

ches as follows:

Malachite and Azurite (Leaching rate = Fast):









3223

2CuCO8NaCN2Na CuCN2Na COCN 

(4)

Then:



CN2NaOHNaCNONaCNH O (5)

Some others possible reactions are shown below:

Cuprite (Leaching rate = Fast):



222

Cu O6NaCNH O2NaCuCN2NaOH  (6)

Tenorite (Leaching rate = Fast):



2CuO7NaCNH O2NaCuCN

2NaOH NaCNO



 (7)

Chalcocite (Leaching rate = Fast):



2222

Cu S7NaCNOH O2NaCuCN

2NaOH NaCNS





(8)

Covellite (Leaching rate = Fast):



2223

2CuS8NaCNOH O2Na CuCN

2NaOH 2NaCNS





(9)

The cyanidation of Cu (II) minerals with the conse-

quent formation of cynogen, (CN)2 results in the loss of

cyanide in the proportion of 0.5 mol of cyanide per mole

of Cu (II) leach ed, i.e., 0.39 kg NaCN/kg Cu (II). Cupr ic

cyanogen complexes are first formed and then they are

broken down to the cuprous form liberating cyanogen,

which in turn reacts with alkali to form cyanide and cy-

anate [6].

1.1. Copper Removal after the Merrill-Crowe

Process

The presence of cyanide-soluble copper affects gold and

silver recovery from the cyanide solutions. In the

Merrill-Crowe process, the copper is precipitated along

with gold and silver, resulting in a higher consumption of

zinc dust, fluxes in the smelting of the precipitate and

shorter life for crucibles. For these reasons, copper must

be separated from the precious metals by digesting the

silver/gold/zinc precipitated in sulfuric acid prior to

smelting. This is a common practice in William Mining

Co., which produ ces a copper sulfate acidic solution that

goes to the zinc and arsenopyrite froth flotation circuit or

to an iron cementation process before disposal. Cementa-

tion of copper using scrap iron is practiced when the

quantity of copper makes recovery worthwhile. Also, to

increase the recovery of silver and gold, with less copper,

it is necessary to use conditions which result in the for-

mation of 2

Cu(CN)



and . High pH values

and high free-cyanide concentrations stabilize copper in

solution resulting in lower levels of copper. Increment of

copper in the barren solution poses serious metallurgical

problems in the cyanidation circuit and it is necessary to

include a process to strip the copper, prior to the gold

and silver leaching step. Failure to do so will result in

lower dissolution of precious metals and production of

high-copper-silver/gol d bullion.

Cu(CN) 

This research fo cuses on the removal of copper before

smelting the gold and silver precipitate and in prevention

and/or minimization of the impact of copper in the barren

solution after the filter press in the Merrill-Crowe proc-

ess. Treatment of high-copper silver/gold leach solutions,

before or after precious metals recovery, is focused on

precipitation of copper as chalcocite (Cu2S) and cyanide

recovery.

1.2. Cyanide and Copper Recovery Processes

There is a growing interest for the recovery of both cop-

per and cyanide from silver and gold barren solutions

due to high cyanide consumption costs. William Mining

Co. is also interested in reducing costs in this way. The

cost of recovering and recycling cyanide from the barren

leach solution will be lower than the cost of purchasing

new cyanide. It has been almost a century since the

Mills-Crowe process for cyanide regeneration was de-

veloped by the Mining Company Beneficiadora de

Pachuca in México (England Patent No. 241669, 3.9.24)

[7] and until today no significant changes to the process

have been made. The simplest process for cyanide recy-

cling involves acidifying the barren clarified solution

(pH between 2 and 5). During acidification, free cyanide

J. R. PARGA ET AL.

193

and relatively weakly complexed cyanide (Ag, Cu, Zn,

Fe) are converted into HCN gas, which is then volatil-

ized by passing a stream of air bu bbles through the solu-

tion. The air/HCN gas stream is scrubbed in a caustic

solution in a second tower reactor to convert the HCN

back into free cyanide ions for recycling [8]. In this

process copper and silver are not recovered for resale.

This has prompted interest to also recover copper by se-

lective metal sulphide precipitation. The copper sulphide

precipitate is then recovered by conventional clarifica-

tion and filtration to produce a filter cake (45% to 60%

Cu) which can be shipped to a copper smelter.

Among the chemical precipitation methods, precipita-

tion of metal hydroxides is the most conventional, but it

suffers from shortcoming, such as high solubilities for

some metals. Sulphide precipitation of metals is a viable

alternative process for copper recovery from the barren

cyanide solutions because of the possible high degree of

metal removal over a broad pH range. However; hydro-

gen sulfide is odorous and highly toxic. It tends to accu-

mulate in poorly ventilated spaces because it is heavier

than air. Exposure to low level concentrations of this gas

can result in eye irritation, sore throat and cough, short-

ness of breath, and fluid in the lungs [9]. Sulphide pre-

cipitation of metals has several advantages over hydrox-

ide precipitation, such as low solubility, high stability of

metal sulphides, fast reaction rates, better settling prop-

erties and potential for re-use of sulphide precipitates by

smelting. The thermodynamic equilibrium involved in

metal sulphide precipitation has been proposed as [10]:

Kp1

HSHS H







p1 2

HS H

KHS





 



 

, 1

K6.99



(10)

Kp2 2p2 2

HSSH ,K,pK17.4











 





(11)



22 s

MS MS



 (12)



MHSMSH



 



(13)

These equations show that concentration of sulphur

species is a strong function of pH. The pK2 value is cur-

rently the most reliable value.

The use of sulphide precipitation process for copper

and cyanide recovering after cyanidation has a key ad-

vantage, the ability to operate in the barren solution to

first recover copper/silver and after that establish acidic

conditions in the solution. This results in rapid release of

free cyanide (HCNgas) that is easily recoverable by vola-

tilization at lowered pH value.

If cyanide ions are present in the barren solution after

precipitation from the Merrill Crowe process as free cya-

nide (pKa = 9.4), it is possible to convert 99% of the

cyanide into HCN gas by lowering the pH value of the

solution to about 6:



CN HHCN



 (14)

On the other hand, if cyanide ions are present as me-

tal-cyanide complexes, pH must be lowered to more aci-

dic values to break down the complex and produce HCN

gas. As an example, the weak zinc-cyanide complex (log

B4 = 17.4) breaks down completely at a pH close to 5,

producing zinc sulfate as an aqueous soluble species,

plus HCN gas:



 

24 4

4s g

ZnCN2H SOZnSO4HCNSO



  (15)

The copper cyanide complex does not break down

completely, even in strong acid solution, unless there is

an oxidant present in the solution. In the absence of an

oxidant, the copper tricyanide (which is the most stable

copper complex under normal cyanidation conditions:

log B3 = 28) decomposes to form a CuCN precipitate,

plus HCN gas (Equation (16)), at pH values lower th an 3.

Hence, 33% of potentially recoverable cyanide is lost to

the precipitate:



 

Cu CN2HCuCN2HCN



 g

(16)

Barren solution in the William Mining Co. process

also contains ferrocyanide and cuprous cyanide that, a

pH = 4, produces doub le metal cyanide precip itates such

as Cu2Fe(CN)6 and Cu4Fe(CN)6:

 



4CuCNFe CN12HCuFe CN

12HCN









(17)

From the stoichiometry, it can be seen that the ferro-

cyanide molecule releases the third molecule of CN from

the copper tricyanide complex. Therefore, the presence

of ferrocyanide results in increased recovery of cyanide

from the copper-cyanide species.

When thiocyanate is present, as it is often the case

when leaching sulphide-bearing ores, insoluble CuSCN

may also be responsible for copper precipitation and

HCN gas formation in acid conditions, the following

reaction show this behavior:



 

Cu CNSCN3HCuSCN3HCN



 (18)

Addition of sulphide ions (Na2S) to the acidified cya-

nide solution results in the precipitation of cuprous sul-

phide (chalcocite), which is favored because of its ex-

tremely low solubility (Ksp = 2.3 × 10–48) [11]. The fol-

J. R. PARGA ET AL.

194

lowing reaction takes place:



  

24 22

2CuCN2H SOH SCu S6HCN

2SO







(19)

Stoichiometric rate of sulfide is approximately 0.25

grams S2– per gram of copper, 0.44 grams NaHS per

gram of copper or 0.61 grams Na2S per gram of copper.

However, the actual sulfide dosage required for near-

complete copper precipitation is normally in excess of

200% due to additional ions in the barren solution. In

precipitating copper, sulfide addition also results in the

near-complete precipitation of silver, as shown in the

following reaction:



  

2AgCNH SAgS4HCN

  (20)

Based upon reactions 15 - 20, acid conditions may

cause the dissociation of the complexes, due to the for-

mation of some copper precipitate and subsequent libera-

tion of HCN by volatilization, considering these reac-

tions, up to 99% of copper could be recovered and HCN

gas could be stripped from the barren solutions and ad-

sorbed in an alkali solution of NaOH. The simplified

chemistry of the process is presented in the following

reaction:



HCNNaOHNaCNH O (21)

The precipitate is a sellable copper product on its own,

or can be blended with the arsenopirite flotation concen-

trate from the flotation sulphide plant.

2. Materials and Methods

Experiments were carried out on barren cyanide solution

after the filter press on the Merrill Crowe process. The

pregnant solution came from the cyanidation leach plant

(500 ton/day), where the ores, from the Minera William

mines, are a mixture of oxides and sulfides, with the

copper ranging from 0.04% to 0.25% as the norm an

average sample contains: 1.7 g/ton Au, 100 g/ton Ag,

0.6% Pb, 0.61% Zn, 0.12% Cu, 2.3 % Fe and 2% of As.

A wet screen analysis of the plant sample indicated that

the granulometry was 80%—74 m. The leaching practice

in the plant was: leach pulps containing 40% solids over

a period of 72 hours leaching at Ph = 11.0, O2 = 5 ppm

and 2 kg/ton NaCN; leached residue: 0.20 g/ton Au and

0.18 g/ton Ag. Then, copper precipitation and cyanide

regeneration experiments were performed to determine

the effect of different process conditions on the solids of

copper/silver sulphide produced by sulphide precipitation.

Precipitation experiments were carried out in a 1 liter

round-bottomed reaction vessel with ports for an over-

head stirred, a gas sparger and a pH electrode. The pH

meter is VWR 8005 Scientific and stirring motor with a

glass impeller driven BDC 1850 CAFRAMO and cone

size settler (1000 ml). The barren solutions used had

copper, silver, zinc and iron ions of varying concentra-

tion. The pH of the barren was adjusted to the required

level with sulfuric acid and then a mixture of Na2S/water

was added. All experimental samples of the liquor and

solid were taken at known times, solutions and solids

from the process were separated by filtration through

cellulose filter paper. The sludge from the precipitation

was dried either in an oven or under vacuum at room

temperature. Analysis of copper, silver, zinc, iron, and

arsenic were performed by digestion of the precipitate

and subsequent ICP/Atomic Emission Spectrometry de-

termination and free cyanide content was determined

directly via titration, whereas the total cyanide was

measured by means of titration after distillation. At the

end of the experiment, HCN volatilization reached effi-

ciencies above 97% and the capture of cyanide gas by

NaOH (1 M) solution was almost 95%.

3. Results and Discussion

The experimental results of the copper, silver, zinc and

iron precipitation as well as CN removal (%) at different

pH values are presented in Table 1.

Results show that pH has a significant effect on copper

cyanide removal efficiency, and it was determined the

optimal pH range to be 2.5 - 3. With these pH values,

when influ en t copp er co n c entr atio n was 636 pp m, co pp er

cyanide removal efficiency was 99%. Some black pre-

cipitates were observed in the solution of experiments 2

to 6; which suggested the presence of copper, silver, ar-

senic, zinc and iron as sulphides. The presence of these

sulphides was confirmed in Figure 1. The measured

sample, which was collected from experiments of pH 2,

3 and 4 (see Table 2), gives rise to peaks corresponding

to covellite, esfalerite and pyrite. The size, EDAX and

morphology of the solids are also shown in Figure 1 by

SEM micrograph and EDAX analysis. The solids in the

precipitate are spherical and approximately 100 nm in

diameter.

The SEM micrograph confirms the excellent crystal-

linity of synthetic covellite (CuS) formed during the sul-

phide precipitation process. The EDAX chemical analy-

sis pattern of the precipitate at different pH values is

shown in Table 2.

Results of Table 2, indicate that pH = 2 to 3 is the best

condition for the sulphide precipitation of copper, be-

cause the high recoveries > 99% of Cu and excellent

quality.

J. R. PARGA ET AL.

195

Table 1. Results of copper, silver, zinc and iron sulphide precipitates and CN removal at different pH values.

Ag Zn Cu Fe Na2S (gr s) pH CN Removal (%)

Feed Barren Solution(ppm) 0.1 184 636 4 0 10.95

Solution 1 (ppm) 0 73 389 2 1.0 6 68

Precipitate 1 (%) 124 17.60 44.90 1.2- -

Solution 2 (ppm) 0 93 420 2 0.5 6.0 63

Precipitate 2 (%) 121 21.40 40.70 1.0- -

Solution 3 (ppm) 0 22 31 2 1.0 5.5 75

Precipitate 3 (%) 114 11.90 51.70 1.0- -

Solution 4 (ppm) 0 8 0 0 1.0 5.0 80

Precipitate 4 (%) 119 13.4 51.27 1.0- -

Solution 5 (ppm) 0 32 0 0 1.0 4.5 95

Precipitate 5 (%) 138 9.92 56.34 0.9- -

Solution 6 (ppm) 0 54 0 0 1..0 4 96

Precipitate 6 (%) 118 1.49 62.68 1.1- -

Solution 7 (ppm) 0 40 0 0 1.0 3.0 99

Precipitate 7 (%) 129 9.53 62.24 1.1- -

Solution 8 (ppm) 0 134 0 0 1.0 2.5 99

Precipitate 8 (%) 106 11.24 60.5 0.9- -

Table 2. EDAX analysis of solids precipitates at different pH values.

pH = 2 pH = 3 pH = 4

Element Weight% Atomic% Weight% Atomic% Weight% Atomic%

O K 7.69 20.37 9.64 24.35 7.73 20.03

Na K 2.51 4.52

S K 27.85 36.80 29.02 36.57 26.44 34.18

Ca K 0.60 0.61 0.45 0.46

Fe K 2.06 1.56 1.96 1.42 1.90 1.41

Cu K 43.46 28.98 41.19 26.19 39.94 26.06

Zn K 18.94 12.28 17.58 10.87 21.04 13.34

Totals 100.00 100.00 100.00

Figure 1. SEM micrograph (×5000) and Chemical analysis of the powder as determined by EDAX, shows the presence of

copper, sulphur, zinc and ir on in a sulphide particle.

3.1. Industrial Application  A feed pump located in the precipitation area.

 A line carrying barren solution at a rate of 10 li-

ters/second, with 1500 ppm of cyanide, 600 to 900

ppm of copper complexes and 0.1 ppm of silver and

at a pH of 11. At this flow rate precipitation of cal-

cium sulphate (scale) would not occur.

Based on the experimental evidence, obtained with the

sulphide precipitation study for copper and cyanide

removal from the barren solution after the Merrill-

Crowe process, this process was installed on a mine

site at full scale.  A Serpentine. Barren solution is currently feed

along with Na2S solution and sulphuric acid, to a 4

inch plastic pipe section in the shape of a SER-

PENTINE, (with inside tripack rings mixers as tur-

bulence promoters).

A simplified process flow diagram, which uses so-

dium sulphide to precipitate copper/silver, and to con-

vert cyanide to HCN gas, under acid conditions (pH 2

to 3) is shown in Figure 2.  Three enclosed vacuum vessels of various sizes/ The system consists of:

J. R. PARGA ET AL.

196

Figure 2. A schematic diagram, showing the SERPENTINE process for Cu, Ag precipitation and cyanide recovering.

shapes meant to be sulfide precipitate collectors.

Formulation of poly-electrolyte conditioners that

effectively flocculate the fine metal sulfide particles

has eliminated the difficulty in separating the pre-

cipitate from the discharge and has resulted in

sludges that are easily dewatered.

 A HCN gas collection system, located over all ves-

sels with a gas adsorption tower, with sodium hy-

droxide as the absorbant.

 A pump in the treated barren solution line to feed

the filter press.

In five continuous working days the treated solution

exited the circuit at a pH value of 4, carrying about 0 to

10 ppm of copper and 200 ppm cyanide and was

pumped to two neutralizing (pH 7) tanks.

4. Conclusions

The SERP ENTINE system is a viab le technology for th e

recovery of copper, silver and subsequent recovery of

HCN gas by scrubbing in NaOH.

Advantages of the SERPENTIN include: an odor free

hermetic process and compact treatment facility, high

precipitation rate of copper and silver (99%) and rela-

tively low operation cost, and also the precipitate is a

sellable copper/silver product. However; the main ad-

vantages of using the SERPENTIN system are: low en-

ergy consumption, production of high grade copper sul-

phide precipitate in the range of 40% to 55% of Cu with

130 gr/ton Ag, and recoveries of cyanide of 90%.

5. Acknowledgements

The authors acknowledge the support of this project to

Minera William in México, the National Council of Sci-

ence and Technology (CONACYT) and to the Dirección

General de Educación Superior Tecnológica (DGEST)

from México.

6. References

[1] F. Habashi, “One Hundred Years of Cyanidation Histori-

cal Note,” CIM Bulletin, Vol. 80, No. 905, 1987, pp. 108-

114.

[2] J. R. Parga, J. L. Valenzuela and F. Cepeda, “Pressure

Cyanide Leaching for Precious Metals Recovery,” Jour-

nal of Metals, Vol. 59, No. 10, 2007, pp. 43-47.

doi:10.1007/s11837-007-0130-4

[3] F. Habashi, “Kinetics a nd Mechanism of Gold and Silver

Dissolution in Cyanide Solution,” Bulletin No. 59, Bure-

au of Mines and Geology, State of Montana, 1967.

[4] J. R. Parga, H. M. Casillas, V. Vazquez and J. L. Valen-

zuela, “Cyanide Detoxification of Mining Wastewaters

with TiO2 Nanoparticles and Its Recovery by Electroco-

agulation,” Chemical Engineering and Technology, Vol.

32, No. 12, 2009, pp. 1901-1908.

doi:10.1002/ceat.200900177

[5] D. M. Muir, “A Review of the Selective Leaching of

Gold from Oxidised Copper-Gold Ores with Ammonia-

cyanide and New Insights for Plant Control and Opera-

tion,” Minerals Engineering, Vol. 24, No. 6, 2011, pp.

576-582. doi:10.1016/j.mineng.2010.08.022

[6] C. A. Fleming, “Cyanide Recovery,” Developments in

Mineral Processing, Vol. 15, 2005, pp. 703-727.

doi:10.1016/S0167-4528(05)15029-7

[7] C. W. Lawr, “Cyanide Regenera tion as Practiced by Com-

pañia Beneficiadora de Pachuca,” Mexico Technical Pub-

lication AIME, Vol. 6, No. 208, 1929, pp. 1-37.

[8] J. R. Parga and D. L. Cocke, “Enhance Cyanide Recovery

by Using Air-Sparged Hydrocyclone,” Chemical Engi-

J. R. PARGA ET AL.

197

neering and Technology, Vol. 26, No. 4, 2003, pp. 503-

507. doi:10.1002/ceat.200390075

[9] Z. M. Shareefdeen, W. Ahmed and A. Aidan, “Kinetics

and Modeling of H2S Removal in a Novel Biofilter,” Ad-

vances in Chemical Engineering and Science, Vol. 1, No.

2, 2011, pp. 72-76. doi:10.4236/aces.2011.12012

[10] A. E. Lewis, “Review of Metal Sulphide Precipitation,”

Hydrometallurgy, Vol. 104, No. 2, 2010, pp. 222-234.

doi:10.1016/j.hydromet.2010.06.010

[11] M. Adams, R. Lawrence and M. Bratty, “Biogenic Sul-

phide for Cyanide Recycle and Copper Recovery in

Gold-Copper Ore Processing,” Minerals Engineering,

Vol. 21, No. 6, 2008, pp. 509-517.

doi:10.1016/j.mineng.2008.02.001