Eliminating Negative Effects

Wendel Rodrigues, Wagner Silva, Pierre Fernandes, Pedro Gonzaga, and Ronaldo Fonseca, Clariant, describe how using a new reagent suite can reduce the negative effects of aluminosilicate minerals on gold fl otation.

As a result of declining sulfide orebody quality, low grade and complex ores that were previously not economically feasible, such as transitional ores and former tailings, are now being considered for processing. The beneficiation of these ores is hindered by the presence of aluminosilicates – such as: kaolinite, chlorite, biotite, amphiboles, and montmorillonite. The various deleterious eff ects of aluminosilicates on flotation froth have been reported by numerous research studies, which show the aluminosilicate content influences on mining, operations, and processing of ores.1,2,3,4,5,6,7,8,9

Aluminosilicates aff ect mineral processing in grinding, froth flotation, thickening, dewatering, and in the final disposal stages. The presence of aluminosilicates results in changes in slurry rheology. In flotation, aluminosilicates trigger a wide variety of problems, such as: increased reagent consumption by fine particles; low quality concentrate, due to silicate gangue entrainment; and recovery losses, possibly as a result of the formation of slime coating on air-bubbles or on mineral surfaces. In addition, aluminosilicate particles cause the flocculation phenomenon in the froth zone of flotation cells.4,10,11

On the other hand, gold and sulfide particles frequently occur as fine-grained inclusions (< 5 µm) in silicates, which do not present a satisfactory flotation performance with sulfhydryl collectors – such as xanthate, dithiophosphate,

and thionocarbamate – due to low particle/bubble attachment eff iciency.12,13,14,15,16

Several mechanisms have been proposed for the surface charge generation of various systems. For oxide minerals – such as hematite, quartz, and alumina – the origin of the electrical charge at the oxide surface/aqueous phase can be ascribed to protonation/deprotonation of the surface

hydroxyls:17,18,19,20,21,22

Experiments

Ore samples

MOH(surf) – MO(surf) + H+ (aq) MOH(surf) + H+ (aq) – MOH2 + (surf)

The mechanism of flotation of silicate and sulfide minerals are dependent on the electrical properties and the solubility of the mineral, the charge and chain length of the collector, and the stability of the salt metal-collector.

In addition, the depressant and dispersant adsorptions are also related to surface mineral characteristics, such as: the chemical composition; the electrical charge distribution and solubility; the potential determining ions content in slurry; and the chemical and structural composition of the modifier.18,19,23,24,25,26 In this study, a new approach was taken to improve the flotation behaviour of gold ores with high silicate content. This involved conducting studies using new reagents, in addition to the collectors and frothers conventionally used in sulfide flotation.

Table 1. Chemical analysis of gold ore sample

Component Assay Gold 0.4 g/t Sulfur 3.44% Carbon 0.15% Aluminium 6.78% Magnesium 0.19% Iron 5.22% Silica 28.82% The gold ore tested was comprised of predominantly gold bearing pyrite and arsenopyrite from the state of Minas Gerais in Southeast Brazil. X-ray diff raction (XRD) analysis showed that the ore has 6.1% sulfides (5.9% pyrite, 0.1% arsenopyrite, and 0.1% between chalcopyrite and pyrrhotite), 43.2% quartz, 30% muscovite, 5.6% feldspar (K-feldspar and albite), 2.8% other silicates (pyroxenes, amphiboles, and clays), 1.3% iron oxides, and 10.8% carbonates. The ore was first crushed to -3.36 mm in size prior to the grinding. Aft er the crushing, the ore was ground to the particle size of 80% passing 0.12 mm. This ore sample was homogenised and then quartered to produce 1500 g fractions for the flotation experiments. Table 1 shows analytical results obtained on the head samples for the gold ore sample. Analysis of gold was carried out by fire assay and atomic absorption spectrometry (AAS), while the other elements were determined by inductively coupled plasma optical emission spectrometry (ICP-OES). The amount of total sulfur and carbon was determined using the technique of direct combustion and infrared detection (carbon and dual range sulfur analyser [LECO]).

Reagents

Flotation tests of gold ore used Clariant FLOTIGAM® 7381 (alkyl etheramine) as a collector in addition to the collectors widely used in sulfide flotation, potassium amyl xanthate (PAX), and potassium diethyl dithiophosphate (DTP). These tests also used Clariant FLOTANOL® M28 (aliphatic alcohols and non-ionic reagent) or methyl isobutyl carbinol (MIBC) as frothers, commercial sodium silicate (SiO2/Na2O = 3.3) as a depressant, and lime to adjust the pH.

Figure 1. Clariant flotation conditions for gold ore with silicate presence.

Figure 2. Recovery of gold and sulfur with PAX, DTP, FLOTIGAM 7381 (F7381), sodium silicate (SS), and FLOTANOL M28 (M28). Experimental conditions and procedures

Rougher flotation experiments were performed using a Denver laboratory machine with a 2 l flotation cell, in which 1500 g of gold ore samples and tap water were added to achieve the required pulp density. The impeller speed was 1200 rpm, the airflow rate was set at 5 l/min., and pH was adjusted with the addition of lime. The experimental conditions are shown in Figure 1, including reagent dosages (collectors, frothers, and depressant) and total flotation time. A total of four concentrates of gold ore were skimmed off at 1, 2, 4 and 9 minutes, in order to determine the flotation recovery as function of time.

Results and discussion

The gold flotation recoveries in the presence of FLOTIGAM 7381, sodium silicate, PAX, and DTP are shown in Figure 2. The results indicate a sharp increase in gold and sulfur recoveries when using FLOTIGAM 7381 and sodium silicate. The gold recovery exceeded 80%, and the sulfur recovery rose from 60.9% to 84%.

Figure 2 shows that thio collectors (DTP+PAX) or FLOTIGAM 7381 alone did not yield the best results. However, when thio collectors, FLOTIGAM 7381, sodium silicate, and FLOTANOL M28 were added together at pH 10.5, the gold and sulfur recovery both improved. These results reinforce the idea that Figure 3. Flotation recovery of gold as a function of time. both fine liberated sulfide particles and sulfide minerals associated with aluminosilicates and quartz, which can float with etheramines, must be floated in order to maximise gold recovery. The sulfide bearing mineral in this gold ore sample was predominantly pyrite, but only 30% was fully liberated, while 34% was associated with silicates (quartz, muscovite, K-felspar, and others) and 36% was associated with other minerals, such as: phosphates, metal oxides, and carbonates. Despite the low arsenopyrite presence, gold particles were identified in this mineral, which presents 43% of particles fully liberated and 57% associated with other minerals (phosphates, oxides, and Figure 4. Species distribution of FLOTIGAM 7381 as a function of pH carbonates). (initial concentration of etheramine = 1.10-4 mol/L).

Figure 3 compares rates of gold flotation when only thio collectors (PAX and DTP) are used with the surface product on gold and iron-bearing species, providing addition of FLOTIGAM 7381, FLOTANOL M28, and sodium the hydrophobic character to these sulfide particles. silicate in pH 10.5 as well as PAX and DTP. It can be On the other hand, the amount of thio collectors observed that the addition of FLOTIGAM 7381, adsorbed on the surface of sulfide-silicate mineral FLOTANOL M28, and sodium silicate strongly improved the associations and locked gold particles in silicate minerals gold flotation performance compared to the addition of may not be suff icient to overcome the detachment forces only thio collectors (PAX and DTP). The new reagent inside the flotation cell. Therefore, the addition of scheme reached gold recoveries greater than 80% at FLOTIGAM 7381 rendered the formation of a more packed 4 minutes, while the gold recovery with only thio collectors layer of collector molecules on the surface of sulfide-silicate did not exceed 40%. mineral associations, which enhances their hydrophobicity,

Indeed, once there is a thermodynamically favourable and thus results in flotation performance improvement. The environment for flotation of sulfide liberated particles and species distribution diagrams of FLOTIGAM 7381 as a those associated with aluminosilicate and quartz, this ore function of pH at the bulk total concentration of 1.10-4 mol/l type can be selectively recovered. Firstly, xanthate and are shown in Figure 4. The values of the thermodynamic dithiophosphate were adsorbed on the sulfide surface, via a equilibrium constants of the etheramine were determined strong electrochemical mechanism. In fact, the using titration curves, in order to calculate the respective chemisorption results in the formation of a hydrophobic ionic and molecular species concentrations of the collector

at various pH values. FLOTIGAM 7381 showed pKa1 and pKa2 of 4.7 and 8.9, respectively, and pKsol of approximately 4.9.

Based on Figures 3 and 4, it appears that an ion-molecular species complex of etheramine is responsible for the flotation performance improvement of the sulfide-silicate associations and locked gold particles in silicates, because the adsorption of its ionic and molecular species on the silicate surfaces causes an increase in the attachment eff iciency for the gold ore sample.

Moreover, sodium silicate was able to depress aluminosilicate and quartz particles, fully liberated or with an insignificant amount of sulfides or gold, due to the adsorption of silicate species (Si(OH)4 and SiO(OH)3 -), which are predominant at flotation pH > 10. These silicate species can penetrate the innermost water layer and react with surface sites of silicate minerals, which improves the flotation selectivity.

Conclusions

From this study, one may conclude: Xanthate (PAX) and dithiophosphate (DTP) can be used to hydrophobise sulfide minerals, such as pyrite and arsenopyrite. However, the thio collectors are not able to provide a sufficient degree of hydrophobic coverage on

the sulfide particles associated with silicates and locked gold particles. The addition of FLOTIGAM 7381 leads to higher gold recovery, due to the adsorption of the etheramine on the silicate surface. Thus, the ionic and molecular species presence of FLOTIGAM 7381 can enhance the attachment forces, through the concentration increase of collector species on the surface of sulfide-silicate mineral associations and silicate minerals with locked gold. Despite the promising recovery, the selectivity of the flotation process may be jeopardised by the significant increase of collector species onto the flotation cell.

However, the sodium silicate reduces the flotation rate of fully liberated silicate particles and those with a small amount of sulfide minerals.

Gold ores that contain silicates minerals – such as quartz, muscovite, clays, chlorite, and amphiboles – can suff er significant losses of recovery. This is due to low hydrophobicity levels in the sulfide-silicate mineral associations and locked gold particles in the silicates. On the other hand, these recovery drops can be overcome by the addition of FLOTIGAM 7381 and sodium silicate, which constitutes a new concentration route for this kind of ore.

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