Effect of Gangue Minerals on Hydrophobic Recovery of Gold

doi:10.4236/eng.2013.53043

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Engineering, 2013, 5, 316-321

http://dx.doi.org/10.4236/eng.2013.53043 Published Online March 2013 (http://www.scirp.org/journal/eng)

Effect of Gangue Mi nerals on Hydrophobic

Recovery of Gold

Anderson E. L. Mlaki, Jamidu H. Y. Katima, Henry T. Kimweri

Department of Chemical and Mining Engineering, College of Engineering and Technology,

University of Dar es Salaam, Dar es Salaam, Tanzania

Email: [email protected]

Received February 23, 2011; revised February 18, 2013; accepted February 26, 2013

ABSTRACT

In this paper, the effect of gangue minerals on the hydrophobic recovery of gold is being investigated using ores ob-

tained from the active small scale gold mining sites in Tanzania. Gold ores of different gangue contents were tested.

The effects of silica gangue and high sulphide gangue on gold attachment were examined including the effect of surface

activators (potassium amyl xanthate) and the possibility of depressing the effects of gangue using reagents. The results

were evaluated in terms of gold recovery, volumes and grade of concentrates formed. There was no change in gold re-

coveries when the amount of oxide gangue (quartz) in the ore was increased, indicating absence of competition between

gold and quartz gangue. High sulphide contents in the ore above 6% reduced gold recoveries considerably. It was noted

that potassium amyl xanthate surfactants increased the attachment of both gold and the sulphide gangue. Using lime at

pH 10 it was possible to depress the sulphide gangue which is mainly pyrite and hence increased gold recoveries con-

siderably.

Keywords: Aggl/Ore Ratio; Gangue Minerals; Hydrophobicity; Surfactants; Gangue Depressants; Oleophilicity

1. Introduction

The presence of gangue minerals in gold ores is among

the main reasons for the selection of appropriate gold

recovery method. The common gangue minerals which

are normally associated with gold ores include quartz,

fluorite, calcite, pyrites, chalcopyrites, galena and many

others in small amounts e.g. arsenopyrites, fluorites,

carbonates and chlorites [1,2].

The hydrophobic recovery of gold is dependent on

greater oleophilicity and hydrophobicity of gold com-

pared to many other minerals. However it is known that

some of the gangue minerals that are associated with

gold have oleophilic and hydrophobic properties and the

presence of such mineral gangue in the ore may be a

source of competition during the attachment to agglom-

erates. Such minerals include mainly the metallic sul-

phides gangue [3,4]. The presence of low amounts of

sulphides in the ore up to 5% chalcopyrite is reported to

have little effect on gold recoveries [5,6].

However, some of the analysed gold ore samples col-

lected from the active small scale gold mining sites in

Tanzania indicated the presence of high sulphides upto

12.9% as shown in Table 1. The presence of high sul-

phides in Tanzanian gold ores are also reported by [7].

This work is focused on the effects of high sulphides

and other gangue on gold recoveries using the hydropho-

bic recovery of gold. Potassium amyl xanthate was the

reagent used as surface activator in the tests. This work

was done as a contribution to the ongoing research on the

application of the hydrophobic recovery of gold as an

alternative to the hazardous use of mercury in small scale

gold mining in Tanzania [8].

Table 1. Gold ore mineralogy.

Mineral composition %

Mineral type Chunya

(Ore 1)

Geita

(Ore 2)

Buzwagi

(Ore 3)

Chalcopyrite CuFeS2 0.21 1.16 1.09

Pyrite FeS2 7.06 9.44 11.53

Arsenopyrite AsFeS2 0 0.01 0

Galena PbS 0 0.25 0.3

Quartz SiO2 89 79.5 75.64

Liberated gold 1 1 1

Other minerals 2.73 8.64 9.8

Total Mineral 100 100 99.36

A. E. L. MLAKI ET AL. 317

2. Experimental

2.1. Materials

a) Agglomerates

Agglomerate samples were prior formed using castor

oil as liquid hydrophobic phase and coal of particle size

–110 microns mixed at oil/coal ratio 0.3. The size of ag-

glomerates used in the tests was 100% below 2 mm.

b) Gold bearing materials

Three different gold ore samples from SSM areas in

Chunya (Ore 1), Geita Gold Mine (Ore 2) and Chocolate

reef-Buzwagi (Ore 3) were used.

Main gangue minerals in the three gold ore types were

determined as quartzite (SiO2), pyrite (FeS2), chalcopy-

rite (CuFeS2), Arsenopyrite (AsFeS2), and Galena (PbS).

The mineral percentages were determined as per major

element analysis which was done with the assistance of

the Southern and Eastern Africa Mineral Center labora-

tory (SEAMIC) as presented in Table 1. Gold contents in

each sample were upgraded to 1% Au, to easy recovery

calculations during experimentation. (The ore mineral-

ogy in Table 1 pertain to samples collected for experi-

mentation and do not necessarily represent the particular

deposit).

c) Other mater i al s

Gold powder from small scale mining areas in Chunya.

Silica sand (–150 microns) originating from mineral

quartz obtained from the defunct Pugu kaolin plant lo-

cated 25 kilometers from Dar es Salaam. Potassium Amyl

Xanthate (KAX) water soluble, HCl, HNO3, and Lime.

Pyrite (FeS2) was used to provide incremental sulphide

contents in the ores during experimentation. The pyrite

was obtained as complement from Geology Department

UDSM.

2.2. Methods

2.2.1. Sampl e Pr eparation

The ore samples were ground to 100%, –150 microns.

The gold contents in each ore sample were determined by

fire assay then upgraded by addition of free gold (minus

150 microns) to 1% Au. The overall mineral percentage

in the ore is shown in Table 1.

2.2.2. Gold Attachment Tests

The gold attachment tests were carried out in a 1000 ml

beaker equipped with two baffles 180˚ apart. The con-

tents were agitated at 700 rpm by a four-bladed stainless

steel vertical impeller at 20% solids and aggl/ore ratio

0.4. The tests were carried out in the following proce-

dures: (i) Wetting 100 g of gold bearing material with

500 ml de-ionized water then surfactants such as potas-

sium amyl xanthate was added and/or depressants such

as lime was added then agitated for 3 - 5 minutes fol-

lowed by (ii) addition of the agglomerate sample and

stirring for a given time; (iii) Screening the mixture on a

0.325 mm sieve in water to separate the gold-loaded ag-

glomerates from the pulp; (v) Drying agglomerates in

open air for 6 hrs and weighing them then assessing the

recovery of gold by analyzing the amount of gold at-

tached to agglomerates.

2.2.3. Analysis of Gold Recovered in Agglomerates

The gold content in the dry agglomerates was analyzed

by roasting the dry agglomerates to ash at 500˚C in a

muffle furnace and treating the ash with aquaregia to

dissolve gold followed by analysis of the diluted gold

solution by atomic absorption spectrometry.

3. Results and Discussions

3.1. Effect of Silica Gangue on Gold Attachment

Four different sets of samples of increasing silica content

were made by addition of silica sand to Ore 3 (lowest

silica content). Each of the samples was treated for gold

recovery at 60 minute and the recoveries attained did not

show any particular relationship with the changes in sil-

ica content as shown in Table 2.

According to the results, silica content in the ore did

not interfere with gold attachment and hence does not

affect gold recovery. This result seemed to agree with the

theory that quartz has a strong covalent surface bonding

which exhibit high free energy values at the polar surface

[9] and therefore during agitation the polar surface of the

quartz could have reacted with the water dipoles and

making them hydrophilic thus preventing attachment of

quartz to agglomerates.

3.2. Effect of High Sulphide Ores on Gold

Attachment

The effect of high sulphide contents in the ore, was

shown by using the three natural ores Ore 1 (7.27% S),

Ore 2 (10.6% S) and Ore 3 (12.9% S) with two additional

samples prepared to give a wide variation of sulphide

contents from 6% to 15% S as shown in Figure 1. The

contact time was varied from 30, 60 and 90 minutes. The

gold recoveries determined after each test showed a sig-

nificant decrease with increase in sulphide content. The

highest recovery of 85.1% was attained by the lower

sulphide sample (6% S) at 90 minutes while the high

Table 2. Recoveries attained at different amount of quartz

content in ore.

Description Sample 1Sample 2 Sample 3Sample 4

Quartz content %76 79 81 89

Recoveries % 75.4 77 75.8 76

A. E. L. MLAKI ET AL.

318

sulphide sample (15% S) which had a low recovery of

only 72.5%. The same recovery trend (high sulphides

content—low gold recovery) was also noted at lower

contact time as shown in Figure 1.

The lower recoveries attained due to high sulphides

were therefore attributed to natural hydrophobicity and

oleophilicity of sulphides which enabled attachment to

agglomerate thus creating competition between gold and

gangue.

3.3. Effect of Potassium Amyl Xanthate as

Surface Activator on Gold and Gangue

Recovery

Generally there were higher recoveries attained when

surfactants were added. As compared to the recoveries

attained in the previous Section 3.2, at 6% sulphide, gold

recoveries increased from the previous 85.1% to 94.0%

at 90 minutes contact as shown in Figure 2.

However, although the recoveries were higher due to

the effect of surfactant, they continued to proportionally

decrease with increase in sulphide content in the ore. The

results indicated that, the action of surfactant increased

gold recoveries, but did not affect the attachment compe-

tition between gold and sulphide gangue.

3.4. (i) Effect of Surfactant on Gangue Recovery

Measured by Concentrate Weights

The results of two sample groups which were treated

with and without surfactant (Figure 3) and (Figure 4)

showed that samples treated with surfactant had higher

concentrate weights. The increase in concentrate weight

was attributed to increased attachment of both gold and

sulphide gangue due to the action of surfactant. The re-

sults agreed with previous results that, the action of xan-

thate surfactant could not decrease attachment compete-

tion between gold and gangue but only promoted at-

tachment of both gold and sulphide gangue as presented

in Figures 3 and 4. Higher concentrate weights also

showed a decrease in grade as discussed in Section 3.4

(ii).

3.4. (ii) Effect of Concentrate Weight on Grade

The concentrate weights were plotted against grades as

shown in Figure 5. The grade decreased with increase in

concentrate weight indicating high gangue recovery, thus

confirming high competition between gold and sulphides.

It was therefore necessary to carryout further tests as an

attempt to successfully depress the effect of sulphide

gangue and reduce competition on gold recoveries as

presented in Section 3.5.

3.5. Depressing the Effects of Gangue on

Recovery Using Reagents

Due to reported effective performance of lime in selec-

tive depression of pyrite among other sulphides in copper

flotation [4,9,10], slaked lime solution Ca(OH)2 was

Figure 1. Gold recoveries from high sulphide ores without surfactants.

A. E. L. MLAKI ET AL. 319

Figure 2. Gold recoveries from high sulphide ores with addition of surfactants.

Figure 3. Concentrate weights attained due to gangue recovery (without surfactant).

A. E. L. MLAKI ET AL.

320

Figure 4. Concentrate weights attained due to gangue recovery (with surfactant).

Figure 5. Weights of concentrates formed versus concentrate grade.

prepared and tested as the reagent of choice for gangue

depression in the experiments.

The results showed a significant increase in gold re-

coveries after lime addition to the gold bearing slurries.

According to Figure 6, lime showed effectiveness as

gangue depressant for all ore samples which contained

high pyrite above 6%. This was shown by the changes in

recovery between corresponding points on the two curves

in Figure 6 when sulphide concentrations were above

6%. It was noted that pyrite concentrations below 6%

had no effect on gold recoveries and hence the effect of

lime could not be noticed in the region. This results

agrees with a report by Calvez et al. [6] who reported

that sulphide contents higher than 5% in the ore may

have detrimental effects to gold recovery. According to

the results it was therefore possible to depress the effect

of sulphide gangue (above 6%) on gold recoveries by

usg lime at pH 10 as shown in Figure 6. in

A. E. L. MLAKI ET AL. 321

Figure 6. Curves showing gold recoveries before and after pyrite depression using lime at 90 minute contact time.

The depression of sulphide gangue (pyrite) could be

explained as due to possible reaction between pyrite sur-

face and lime which changed its natural hydrophobic and

oleophilic properties to hydrophilic, reducing its attach-

ment competition.

It is known that the oleophilicity of unoxidised sul-

phides is decreased due to formation of hydroxides on

the surface in alkaline conditions [10,11].

It was therefore necessary to do the attachment tests at

pH 10 which was considered alkaline enough for hy-

droxide formation on the pyrite surface.

4. Conclusions

1) The results have shown that the presence of certain

types of gangue in the ore may have detrimental effect to

gold recoveries in the hydrophobic recovery of gold, e.g.

high sulphides contents above 6%.

2) The use of surfactants such as potassium amyl xan-

thate increased the attachment of both gold and sulphide

gangue thus increasing attachment competition.

3) Using lime at pH 10 it was possible to depress the

sulphide gangue thus reducing competition on gold at-

tachment to agglomerates and raising gold recoveries

considerably.

4) There was no change in gold recoveries when the

amount of quartz gangue in the ore was increased, sug-

gesting the existence of quartz particle hydrophilicity due

to reaction between the polar surface and the water di-

poles.

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