Further Results on the Effects of the Grinding Environment on the Flotation of Copper Sulphides

Abstract

Grinding conditions affect the flotation of copper sulphide minerals as changes in the properties of the grinding media and their interactions with the sulphide minerals, and between sulphide minerals themselves, affect the chemical environment in the flotation pulp. Galvanic interactions between steel grinding media and sulphide minerals, and between sulphide minerals, can lower the pulp potential, decrease the dissolved oxygen concentration in the mineral slurry, and lead to the dissolution of iron and copper from the media and the minerals. As a result, the formation of hydrophilic iron hydroxides and their adsorption on the copper sulphide minerals can be deleterious to copper flotation while pyrite (when present) can be activated to flotation by dissolved copper lowering the grade of the copper concentrate. Electrochemically less active grinding media (e.g., chrome alloy balls rather than mild steel media) can have beneficial effects on flotation performance due to the lower oxidation of the grinding media and consequently the lower production of oxidised iron species in the pulp. Copper activation of pyrite can be decreased by chemical additions to the pulp. In this paper, relevant experimental data published in the last 15 years are discussed.

1. Introduction

The major copper minerals of commercial significance are typically chalcopyrite (CuFeS₂), bornite (Cu₅FeS₄), covellite (CuS), and chalcocite (Cu₂S). In commercial operations, the beneficiation of copper sulphide ores generally utilises wet milling to liberate the sulphide minerals in a suitable size range for subsequent mineral separation by flotation. The flotation response of ground minerals can be influenced by the grinding conditions and so these conditions are important in achieving a satisfactory metal recovery and concentrate grade.

In our extensive review of the literature up to 2011 [1], the effects of the grinding environment on the flotation of copper sulphides were summarised. Different grinding methods, including ball and rod milling with a range of grinding media, as well as autogenous and semi-autogenous grinding were considered. Also considered were the generation of iron hydroxide species in the pulp, the gaseous atmosphere used in the mill, and the effects of reagents or chemicals added during grinding on the subsequent flotation response.

The previous review [1] included laboratory work carried out with sulphide mineral samples in solutions of varying compositions, and with ore samples, using batch laboratory grinding mills and small-scale flotation cells operating at low pulp density. It was noted that the media and laboratory conditions used in laboratory testing do not often match exactly with the conditions in modern sulphide concentrators or are not practical in such. Furthermore, evaluating grinding effects at pilot and plant scale often is costly due to the requirement for tests to be run over a long enough time to ensure the effects of any changes in the circuit are clearly seen. However, the development of the Magotteaux Mill^® [2] has allowed better simulation of laboratory and plant conditions by enabling the pH and pulp potential to be controlled during grinding and post-grinding conditioning.

Here, the earlier comprehensive review [1] is updated to consider experimental data since 2011 related to the effects of key factors such as grinding media composition, media-mineral interactions, properties of the copper minerals, electrochemical interactions between sulphide minerals, and reagent additions on the slurry properties. In particular, the effects of these interactions on the flotation of the copper sulphides resulting from changes in the redox potential, oxygen levels, and the formation of hydroxide species in the slurry are considered.

Mineral and ore samples may be ground dry or wet and differences between dry and wet grinding with respect to energy requirements, grinding media wear rates, particle size, and shape of the ground media have been discussed [1,3]. While it has been pointed out that the surface properties and floatability of the ground minerals differ depending on whether they were ground dry or wet, this current review generally is restricted to wet comminution in mills with a range of grinding media in air, since this is the most common approach used commercially.

2. Grinding Media

In the beneficiation of copper sulphide minerals, the comminution step is important since it can have significant effects on the efficiency of the subsequent flotation process. Also, comminution may account for as much as 30%–50% of typical operating costs with liner wear and media consumption accounting for around 50% of the cost [4,5]. Media wear arises as a consequence of complex interactions between a range of variables related to the processing conditions and the characteristics of the grinding media, ore, and slurry [4,6].

Steel media are commonly used in laboratory and commercial operations for the grinding of copper sulphide minerals. The effects of the carbon content on the properties of the steel have been discussed [1], with the carbon levels determining the compositions of the phases present and so the strength of the media. Wear of the media during grinding releases iron into the pulp with the associated formation of iron hydroxides that may be detrimental to subsequent flotation [7]. Even so, in commercial operations, the choice of grinding media often is influenced by the economics of their use in a particular circuit on a particular ore, with little consideration to flotation performance.

Low carbon steels (with high wear rates) are the least expensive media and are used for primary grinding where metallic contamination generally is not a problem. To limit wear in commercial operations, steel media is alloyed with elements such as chromium, nickel, and molybdenum [4,6], although contamination of the copper minerals by the alloying elements is a possibility, especially when high chromium media are used. The wear detritus from comminution circuits and metal debris from upstream processing can affect pulp properties and hence also the subsequent flotation performance of copper sulphide ores [1]. Sampling from three Australian flotation operations has indicated that metal contaminants with a high surface area in the size range of 1–1000 μm were present in the flotation feed [8].

The use of stainless steel media is generally restricted to applications in laboratory studies due to their high cost. Also, in laboratory studies, ceramic and glass media have been used when contamination of the sulphide mineral surfaces with iron is to be prevented. Stirred milling technology with fully inert grinding media (ceramic media and mill), achieved better flotation of chalcopyrite compared with that obtained with mild steel media and mill under the same flotation conditions [9]. The higher recovery of copper was associated with the better flotation of the fine (−8 μm) chalcopyrite.

Mass losses in media during grinding can occur due to the mechanical actions of impact and abrasion and the chemical process of corrosion [4,6]. A schematic representation of the effects of these mechanisms on the grinding media has been presented [4]. The mechanical forces active during grinding can enhance the chemical activity of sulphide minerals, affecting the surface conductivity and subsequent electrochemical behaviours of the sulphide minerals [10]. Different grinding media produce different impacts and abrasions on sulphide ores due to their different densities and surface roughness, affecting the particle size distribution, surface morphology, and crystal structure of the ground sulphide ore. This can have a critical effect on the wettability and flotation behaviour of the minerals [11].

Experimental tests to assess the wear of grinding media have been summarised [6] while models have been developed to predict wear losses of the media during grinding that can be applied in specific situations [4,11].

3. Chemical Interactions Between Grinding Media and Sulphide Minerals

Galvanic interactions occur during the intimate contact between grinding media and sulphide minerals in the mill. A schematic representation of these interactions has been presented [10]. Steel grinding media (balls, rods, and liners) generally are more electrochemically active than the sulphide minerals, and so act as anodes being oxidised to produce iron species in the pulp. These interactions can not only increase the corrosive wear of the grinding media but also affect the surface properties and pulp chemistry of the ground minerals. On the other hand, the sulphide minerals usually act as cathodes and are sites for reduction and consequently develop higher rest potentials under most conditions. The resulting pulp potential, referenced to the standard hydrogen electrode (SHE), depends on these mixed potentials, and the potentials resulting from any other redox reactions occurring in the pulp, such as reactions of added collectors, although the contribution from flotation collectors can be expected to be small [1]. The reducing environment obtained when grinding with reactive steel media results in more negative pulp potentials and lower dissolved oxygen contents. These changes are associated with changes in the pH of the pulp [4,10].

Since most commercial sulphide milling is conducted in iron mills, reducing pulp potentials is generally seen whenever these measurements have been made in operating plants [12]. The floatability of copper sulphide minerals strongly depends on the pulp potential, and after grinding, it must be increased to a suitable value for a significant flotation of the copper minerals to be achieved. Also, as low dissolved oxygen concentrations can occur during grinding, they too must be raised to suitable levels to achieve satisfactory flotation. In commercial flotation cells, since air is the most common carrier gas used, more positive pulp potentials and increased oxygen concentrations are usually achieved automatically in the first few cells of a flotation bank or in a separate aeration stage between grinding and flotation. Once the pulp potential and oxygen concentration increase to the required levels, good flotation of copper sulphide minerals is generally achieved [1,10].

In a closed mill, such as a standard batch laboratory mill, the oxidation of the media by reaction with oxygen continues until the available oxygen is consumed, often with the simultaneous lowering in the pH of the pulp. In an open mill system, the consumption of oxygen continues during the milling. With air commonly used as the carrier gas in flotation, the minerals in the pulp are normally subjected to uninhibited aeration during flotation and most sulphide flotation plants operate at “air set” potentials that are usually in the range +100 to +300 mV SHE. To operate at less positive (or negative) potentials requires the addition of reducing agents. This can be costly given that the reductants are often readily consumed leading to higher reagent consumptions [1].

During grinding and flotation at higher alkaline pulp pH values, the dissolved iron (ferrous iron) can oxidise to ferric ions and precipitate at the cathodic sulphide mineral sites as fine hydroxide species such as Fe(OH)₃ [13]. The finer copper sulphide mineral particles are reported to be more strongly oxidised and more susceptible to surface attraction of the iron oxidation species [7].

Hydrogen peroxide (H₂O₂) can be formed by the reaction of oxygen with sulphide minerals during grinding [14]. The presence of this oxidising agent can be expected to increase the oxidation of the sulphide minerals with a detrimental effect on subsequent flotation. However, the formation of H₂O₂ was decreased in the flotation of a Cu-Pb-Zn ore from Sweden by the addition of dextrin, sodium hydrogen sulphite (pyrite depressants), and zinc sulphate (zinc depressant) during grinding. The addition of the depressants and collectors (potassium amyl xanthate and dialkyl dithiophosphate mercaptobenzothiazole) increased H₂O₂ production [15].

The fine hydrophilic iron hydroxides can partially or completely cover the copper sulphide mineral surfaces as a slime coating that can reduce collector–mineral interactions and bubble adhesion on the surface, and so depress the floatability of the sulphide minerals [1,16]. However, with fully electrochemically inert grinding media (large diameter ceramic media), the formation of iron hydroxides was prevented, and the flotation of chalcopyrite was not inhibited [9]. Slime coatings may also be formed by clay minerals (e.g., kaolinite and bentonite) present in the ore [17,18]. Finely dispersed clays in the slurry can also affect its rheology and the stability of the bubbles in flotation, both leading to poorer flotation performance [19,20].

Results presented in our previous review [1] showed that the composition of the grinding media can change the properties of the pulp during grinding. Since then, results for a range of steel and inert grinding media have indicated that the electrochemical activity of the media decreased in the following order: cast iron > mild steel > alloy (Mn, Cr or Ni) steel > stainless steel > inert media (e.g., ceramic, zirconia, agate, glass) [10]. The less electrochemically active the grinding media, the higher the resulting pulp potential value was. Also, the dissolved oxygen content of the pulp was found to be significantly higher when using ceramic balls compared with cast iron balls, as was the subsequent copper flotation performance [21,22]. Other experimental data noted here, and summarised in Table 1, have confirmed these results.

Examples for the flotation of chalcopyrite indicated recovery was higher with stainless steel balls compared with high-carbon steel balls, while for bornite flotation, higher copper recovery was achieved when stainless steel media were used instead of carbon steel rods and balls [23,24]. It was considered that the galvanic interactions between sulphide minerals and the stainless steel media were weaker, by several orders of magnitude, than those between the sulphide minerals and mild steel media resulting in less reducing conditions.

Since 15, 21, and 31% chromium media are less electrochemically active than mild steel media, their use resulted in better bornite floatability as a result of lower oxidation of the grinding media and consequently lower production of oxidised iron species [25]. Also, the less electrically active grinding media (higher chromium content) was beneficial to the flotation of the fine (minus 10 μm) bornite. Similarly, for a chalcopyrite ore, a more oxidative pulp potential and better copper flotation was achieved with the less electrochemically active high chrome alloy grinding media when compared with forged steel media [26] and with stainless steel media [27].

The performance of low alloy steel balls and high carbon chromium steel balls were compared in the grinding of a chalcopyrite–pyrite ore under atmospheres of nitrogen, air, and oxygen [28]. Electrochemical measurements indicated that the high carbon chromium steel balls under nitrogen gas resulted in the lowest galvanic current, while the low alloy steel balls under oxygen gas had the highest galvanic current.

Stainless steel rods, mild steel rods, 21% chrome steel balls, and forged steel balls were used to grind a Cu-Ni base metal sulphide ore from South Africa [29]. Relatively inactive media such as chrome steel balls produced the highest dissolved oxygen levels while the lowest levels were produced by grinding with mild steel rods. The higher reactivity of the media was considered to have enhanced the galvanic interactions between grinding media and minerals resulting in greater reduction in the pulp oxygen content.

Table 1. Effect of grinding media on copper flotation.

Copper Mineral	Media Type Tested	Best Cu Flotation	References
Bornite	Carbon steel Stainless steel	Stainless steel	Gonçalves et al., 2003 [24]
Chalcopyrite	High carbon steel Stainless steel	Stainless steel	Ahn and Gebhardt, 1991 [23]
Chalcopyrite	Low alloy steel High carbon Cr steel	High carbon Cr steel	Azizi et al., 2013 [28]
Chalcopyrite	Stainless steel 20% Cr mild steel	20% Cr mild steel	Can and Basaran, 2023 [27]
Cu-Ni ore	Mild steel Forged steel Stainless steel 21% Cr alloy	21% Cr alloy	Corin et al., 2018 [29]
Bornite	Mild steel 15, 21, 31% Cr alloy	Cr alloy	Huang and Grano, 2008 [25]
Chalcopyrite	Forged steel High Cr alloy	High Cr alloy	Jaques et al., 2016 [26]
Chalcopyrite	Cast iron Ceramic	Ceramic	Zhang et al., 2020 [21]
Chalcopyrite	Cast iron Mild steel Alloyed steel Ceramic	Ceramic	Zhang et al., 2021 [10]

Table 1. Effect of grinding media on copper flotation.

Copper Mineral	Media Type Tested	Best Cu Flotation	References
Bornite	Carbon steel Stainless steel	Stainless steel	Gonçalves et al., 2003 [24]
Chalcopyrite	High carbon steel Stainless steel	Stainless steel	Ahn and Gebhardt, 1991 [23]
Chalcopyrite	Low alloy steel High carbon Cr steel	High carbon Cr steel	Azizi et al., 2013 [28]
Chalcopyrite	Stainless steel 20% Cr mild steel	20% Cr mild steel	Can and Basaran, 2023 [27]
Cu-Ni ore	Mild steel Forged steel Stainless steel 21% Cr alloy	21% Cr alloy	Corin et al., 2018 [29]
Bornite	Mild steel 15, 21, 31% Cr alloy	Cr alloy	Huang and Grano, 2008 [25]
Chalcopyrite	Forged steel High Cr alloy	High Cr alloy	Jaques et al., 2016 [26]
Chalcopyrite	Cast iron Ceramic	Ceramic	Zhang et al., 2020 [21]
Chalcopyrite	Cast iron Mild steel Alloyed steel Ceramic	Ceramic	Zhang et al., 2021 [10]

4. Galvanic Interactions Between Sulphide Minerals

A number of sulphide minerals are generally present in a base metal sulphide ore. As a result, in a differential flotation system, galvanic interactions between the different sulphide minerals and the grinding media, as well as between the sulphide minerals themselves, can play an important role in the separation efficiency of the flotation step. Galvanic interactions can occur when two sulphide minerals are brought into contact. The electrochemical activity of the two minerals determines which one acts as the cathode, and which one is the anode. The sulphide mineral with the higher rest potential will act as a cathode drawing electrons from the other mineral giving rise to a galvanic current. The galvanic interaction is stronger in the presence of dissolved oxygen, since oxygen can act as an electron acceptor to form hydroxyl ions.

A common gangue mineral present in copper sulphide ores is pyrite and since pyrite has the highest rest potential of the common sulphide minerals [30] it acts as the cathode and the copper sulphides present (e.g., chalcopyrite or chalcocite) act as the anode. Consequently, the oxidation and dissolution of chalcopyrite is enhanced [31,32], resulting in the production of hydroxide species on the chalcopyrite surface, inhibiting its flotation, and copper sulphide species on the pyrite surface activating it to flotation. Increasing amounts of pyrite decreased chalcopyrite flotation and increased pyrite flotation [33,34,35]. An optimum amount of aeration (i.e., concentration of oxygen) was found to be required for maximum chalcopyrite flotation and that the amount needed to be increased with increasing pyrite content of the feed to facilitate xanthate adsorption, and its oxidation to dixanthogen, on the chalcopyrite surface during flotation [36]. However, chalcopyrite flotation could not be fully restored by simply increasing the extent of aeration. This was proposed to be due to increasing galvanic interactions between the chalcopyrite and pyrite resulting in an increase in hydroxide species on the surface of the chalcopyrite with a resulting decrease in hydrophobicity and possibly a lower adsorption of the flotation collector. Compared with fresh water, flotation in saline water made the separation of chalcopyrite from pyrite more difficult due to increased copper activation on the pyrite surface [37]. The difficulties, opportunities, and industrial experiences of copper flotation in sea water have been reviewed [38].

The effects of increasing pyrite additions on the flotation of chalcopyrite and pyrite from a chalcopyrite–pyrite–quartz mixture have been investigated [39]. The copper concentrate grade from the flotation of chalcopyrite decreased when 5% pyrite was added and decreased significantly more with the addition of 25% pyrite despite the copper feed grade remaining the same. The increasing pyrite proportion was considered to increase the cathodic surface area in the chalcopyrite–pyrite couple resulting in a stronger galvanic current with greater pyrite activation and so pyrite flotation. Chalcopyrite oxidation was proposed to result in a reduced amount of hydrophobic species (CuFe_1−xS₂, CuS and S), and an increased amount of hydrophilic species (Fe(OH)₃, SO₄²⁻ and S₂O₃²⁻), on the chalcopyrite surface leading to poorer flotation [39]. The resulting lower copper concentrate grade often results in operators in commercial flotation plants having to sacrifice copper recovery to achieve the required copper grade [35]. However, with 25% pyrite in the mixture, the flotation of chalcopyrite was not affected even though the high pyrite proportion promoted a strong galvanic interaction between the chalcopyrite and the pyrite. It was proposed that the strong chalcopyrite oxidation resulted in the formation of Cu(OH)₂ on the chalcopyrite surface resulting in efficient adsorption of the collector on the surface, and so flotation of the chalcopyrite [39]. Chalcopyrite flotation decreased as the pyrite content in mixtures of chalcopyrite and pyrite was increased up to 80% pyrite [33]. Surface analyses indicated that the pyrite was copper activated by copper ions dissolved from the chalcopyrite, while iron hydroxide species on the chalcopyrite surface increased as the galvanic interactions between the two sulphide minerals increased with a higher pyrite content in the mixture.

The effects of pyrite content on the flotation of chalcocite have been determined [40]. Chalcocite, being more electrochemically active, is oxidised faster than chalcopyrite [18]. Electrochemical studies showed that chalcocite acted as a cathode in a galvanic system in the absence of pyrite. However, with the addition of pyrite of a similar surface area to the chalcocite, chalcocite was the anode. For flotation tests, a fixed amount of chalcocite was combined with 0 to 25% pyrite, with quartz making the balance of the mix. The mixture was ground in a stainless steel mill with forged steel grinding rods. An increase in pyrite surface area increased the galvanic interactions between the cathodic pyrite and the anodic grinding media and chalcocite. The resulting more reducing pulp conditions increased chalcocite oxidation and copper activation of pyrite, leading to enhanced pyrite flotation and a decrease in the copper grade of the flotation concentrate, as was found with chalcopyrite [39]. Also, copper flotation recovery increased with an increase in pyrite feed grade in line with the increased chalcocite oxidation [40]. The generation of copper hydroxide, and its adsorption on chalcocite increased the efficiency of the collector for chalcocite, similar to what was observed for chalcopyrite [39]. However, more pyrite was floated with an increase in pyrite feed grade due to the promotion of copper activation of pyrite and its subsequent flotation. A schematic diagram of the galvanic interactions in this three-phase system has been given [40].

To effectively depress pyrite during copper flotation, the use of high-chromium steel grinding media in place of forged steel media was proposed since high-chromium media are more resistant to wear and corrosion and so produce less iron contamination [35]. Consequently, chalcopyrite flotation was improved. Also, the higher pulp potential created by the high-chromium steel medium decreased copper activation on the pyrite surface [41]. Thus, high-chromium media can give improved copper sulphide flotation with depression of pyrite and so produce a higher copper grade.

In the flotation of chalcopyrite from pyrite in seawater, it was found that the addition of sodium metabisulphite could lead to depression of the pyrite [42]. When forged steel grinding media were used, a high dosage of sodium metabisulphite was required to depress the pyrite, but with high chrome steel grinding media a low dosage of sodium metabisulphite could almost completely depress the pyrite. Electrochemical measurements indicated that the presence of cupric ions and oxygen were necessary for the formation of the SO₅^•− radical species. This species deactivated the pyrite surface. The more inert high chrome grinding media generated a higher pulp potential, compared with forged steel media, resulting in higher concentrations of cupric ions and oxygen in the pulp, promoting the generation of higher concentrations of the radical species and the deactivation of the pyrite.

In sea water, potassium ferrate (K₂FeO₄) was used as an oxidizing agent to strongly depress pyrite while insignificantly affecting chalcopyrite flotation [43]. It was suggested that the potassium ferrate oxidised the pyrite to form hydrophilic species such as Fe₂O₃, Fe(OH)₃, FeOOH, and Fe₂(SO₄)₃ on the surface of the pyrite that prevented the adsorption of the collector and so depressed pyrite flotation. Only small amounts of hydrophilic species were observed on the chalcopyrite surfaces.

Numerous inorganic compounds (e.g., cyanide, sulphite, hydroxide, lime, and oxygen) have been used as depressants for pyrite flotation [1]; however, these reagents often have environmental or operating problems [44]. Organic compounds, such as starch, dextrin, carboxymethyl cellulose, and polyacrylamide polymers, have also been used to depress pyrite but they are not selective, depressing all minerals at high doses [45]. When three modified lignosulfonate biopolymers were tested to reject pyrite in chalcopyrite flotation, the flotation of chalcopyrite decreased in the absence of pyrite with increasing additions of the biopolymers [45]. However, chalcopyrite flotation was less depressed in the presence of pyrite, while the pyrite was selectively depressed. It was shown that pyrite weakened the interaction of the biopolymers with chalcopyrite due to preferential adsorption of the biopolymers on copper-activated pyrite [45]. When kaolinite was present in the ore, the biopolymers dispersed the clay, but while copper recovery was increased, the copper grade decreased due to modification of the froth by the polymers resulting in enhanced mechanical entrainment of the clay [46].

5. Effects of Re-Grinding

The earlier review of the effects of the grinding environment on copper sulphide flotation [1] considered the different types of re-grind mills used, including tower mills and stirred mills, where very fine re-ground products are required for cleaning circuits. Only a handful of new papers on this topic have emerged recently and these are discussed below.

In many copper sulphide flotation concentrators, an initial rougher float is used to maximise copper recovery from the ore [47]. Modelling to optimise the rougher flotation circuit in an Australian concentrator found that a Gaussian process regression gave the most precise optimisation results for the models evaluated [48].

Considerable amounts of iron sulphides often also float along with the copper sulphides, resulting in a low copper grade for the rougher concentrate [49]. Consequently, rougher concentrates, and rougher scavenger concentrates, are re-ground and re-floated to obtain acceptable final copper concentrate grades. During the re-grind, the particle sizes of the minerals are reduced and may result in better liberation of the copper sulphides in the coarser particle. However, after re-grinding, the copper activation of pyrite, and its flotation, can also be increased [33].

Flotation concentrates from 1:1 mixtures (by weight) of chalcopyrite and pyrite, and chalcocite and pyrite, were produced with feed that had been ground to a P₈₀ of 75 µm. These concentrates were re-ground to a P₈₀ of 20 µm and re-floated. With the chalcopyrite-pyrite mixture, a high chalcopyrite recovery was achieved in the cleaner stage, with the pyrite strongly depressed. Pyrite flotation could not be restored even at a high collector addition [50]. By comparison, with the chalcocite–pyrite mixture, flotation of both the chalcocite and pyrite was decreased after regrinding, but the recovery of both minerals could be partly restored by an additional collector. The differences were proposed to be due to chalcocite being more electrochemically active than chalcopyrite. A stronger galvanic interaction between chalcocite and pyrite resulted in greater oxidation of chalcocite compared with chalcopyrite. Consequently, increased amounts of hydrophilic oxidation species adsorbed on the chalcocite depressed its flotation, and an increased copper activation of pyrite resulted in greater flotation of pyrite [50].

Pebbles (−20 + 10 mm) from a semi-autogenous mill were used as grinding media in a vertical stirred mill to re-grind a rougher concentrate from a Chinese copper sulphide ore. When compared with steel balls, the pebbles produced less corrosion and were more selective with higher recovery of the valuable mineral [51].

While balls are the media of choice in most commercial plants, rod milling can produce a narrower size range product but often with lower efficiency. Recent laboratory results [52] indicated that a greater degree of liberation and better copper flotation recoveries were achieved for a chalcopyrite ore when short cylinders were used as the grinding media instead of balls. The authors attributed the effects to the cylinder media comminuting the ore in point and line contact modes which produced more active chalcopyrite surfaces with a higher collector adsorption capacity. These results highlighted the effect of the grinding media shape on the nature of the particles formed after grinding.

6. Effects of Particle Shape

The shape properties of particles are largely determined by the milling method and grinding media used and the effects of particle shape on flotation behaviour has been well studied [53], with general consensus that angular shaped particles float more efficiently than round particles over a wide size range [54]. Also, the effects of particle shape on entrainment specifically have been investigated with some evidence in a chalcopyrite flotation system that angular, elongated particles show higher entrainment than round particles [55]. Controlling the shape of ground particles during milling is another matter. In a study on shape modification during milling [56], it was proposed that the particle shape produced by grinding was a function of the nature of the material being ground, the type of mill used, and the time spent in the mill. The study concluded that the products of individual breakage events are typically angular or irregular in shape but continued grinding leads to the rounding of the particles. However, there has been little advance in the development of new or revised comminution devices which can produce a particular or preferred particle shape for subsequent flotation.

7. Effect of Added Reagents

The earlier review of the effects of the grinding environment on the flotation of copper sulphide minerals [1] noted that, while there is extensive literature on the effects of reagents added during flotation on the flotation of copper sulphides, there is little hard evidence in the literature that reagent addition during grinding has any significant effects on the subsequent floatability of copper sulphide particles. Typically, the common reagents added to grinding mills in copper sulphide concentrators are collectors, lime, cyanide, and sometimes sodium sulphide. In the present review, we note there are no significant papers on this topic since the last review.

8. Summary

This review, an update on a previous one [1], has provided further examples of the effects of the grinding environment on the flotation performance of copper sulphide minerals.

Galvanic interactions between sulphide minerals and steel media, and between sulphide minerals, can lower the pulp potential and dissolved oxygen levels in the pulp resulting in increased corrosion (dissolution) of the grinding media and sulphide minerals. This results in an increase in the formation of iron hydroxide species that can adsorb on the surface of the copper sulphide minerals inhibiting their flotation. Also, froth stability may be adversely affected during flotation.

The increased corrosion of the media and minerals due to galvanic interactions can result in an increase in the amount of copper ions in the solution. This increase in concentration can result in copper activation of pyrite, usually the major sulphide gangue mineral in the ore, enhancing its flotation and so decreasing the grade of the copper concentrate. Numerous chemical additions have been evaluated to limit the extent to which the pyrite is activated to improve the copper recovery and concentrate grade.

The use of more inert chromium alloy balls, compared with steel balls, can limit the corrosion of the grinding media and sulphide minerals giving improvements in flotation performance. However, commercially, the choice of grinding media is often influenced by the economics of their use in a particular circuit on a particular ore, with little consideration of flotation performance.