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Vol. 8. Issue 5.
Pages 5004-5011 (September - October 2019)
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Vol. 8. Issue 5.
Pages 5004-5011 (September - October 2019)
Review Article
DOI: 10.1016/j.jmrt.2019.07.053
Open Access
A comparative study on the effects of dry and wet grinding on mineral flotation separation–a review
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S. Chehreh Chelgani
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saeed.chelgani@ltu.se

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, M. Parian, P. Semsari Parapari, Y. Ghorbani, J. Rosenkranz
Minerals and Metallurgical Engineering, Dept. of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-971 87, Luleå, Sweden
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Table 1. A comparison between the effects of different grinding environments on size reduction ratio [5].
Table 2. Effects of various grinding media and environment on PGM size reduction [11].
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Abstract

Water scarcity dictates to limit the use of water in ore processing plants particularly in arid regions. Since wet grinding is the most common method for particle size reduction and mineral liberation, there is a lack of understanding about the effects of dry grinding on downstream separation processes such as flotation. This manuscript compiles various effects of dry grinding on flotation and compares them with wet grinding. Dry grinding consumes higher energy and produces wider particle size distributions compared with wet grinding. It significantly decreases the rate of media consumption and liner wear; thus, the contamination of pulp for flotation separation is lower after dry grinding. Surface roughness, particle agglomeration, and surface oxidation are higher in dry grinding than wet grinding, which all these effects on the flotation process. Moreover, dry ground samples in the pulp phase correlate with higher Eh and dissolved oxygen concentration. Therefore, dry grinding can alter the floatability of minerals. This review thoroughly assesses various approaches for flotation separation of different minerals, which have been drily ground, and provides perspectives for further future investigations.

Keywords:
Flotation
Energy consumption
Grinding media type
HPGR
Dry grinding
Wet grinding
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1Introduction

Water scarcity, environmental protection, and water treatment costs are some of the critical challenges in the current mining and ore processing. Thus, dry mineral beneficiation techniques can be considered as an alternative solution. It is well-understood that grinding for particle size reduction and liberation is the essential step prior to ore concentration [1–4]. In mineral processing plants, the choice between wet or dry grinding is one of the critical issues, particularly in dry climate countries. In arid areas such as North Western Australia, the Andes and the Sahara Desert, and Africa, the shortage of water dictates the use of dry grinding methods. However, in the industry, wet grinding is the common method for particle size reduction, except in some cases such as industrial minerals where the material has to be drily treated and ground [1,4].

To select dry or wet grinding, the differences between their process conditions should be taken into consideration. Transfer and motion of particles by air in case of dry grinding or water may significantly affect power draw and energy consumption of grinding circuits [5]. Particularly, these energy differences become significant when considering that around 3% of the world-generated electricity is consumed in mineral comminution [6]. Furthermore, due to the better flow properties of a pulp in comparison with dry material in the air, the throughput of a continuous mill feed in a wet process is higher than in a dry environment [2]. On the other hand, the rate of ball and liner corrosion in the wet grinding is extensively higher than in the dry case [1,2]; thus, in processes where there are distinct limits with regard to contamination, the decision for having a wet or dry grinding is significantly challenging. These differences in the grinding process also change the properties of grinding products, which in turn can affect the efficiency of downstream separation processes such as froth flotation.

Froth flotation is the main ore concentration method for recovery of fine particles, i.e. particles −100 µm [7,8]. Particle size distribution, mineral liberation, particle surface properties, and pulp chemistry are some of the decisive factors for flotation that are controlled by the grinding environment [9–13]. However, limited studies have been conducted to compare these effects on mineral flotation. This study aims to comprehensively review the effects of dry grinding on flotation efficiency and compare it with an entire wet processing environment. This work thoroughly discusses how different grinding environments can affect pulp chemistry, surface properties of minerals, size distribution, liberation, and finally flotation efficiency.

2Dry versus wet grinding2.1Grinding conditions and mill product properties

In terms of the energy consumption (EC) for a certain particle size distribution, several investigations reveal that in dry grinding the EC is around 15–50% higher than in a wet environment [2,10,14,15]. Ogonowski et al. (2018) conducted an investigation to compare wet and dry grinding processes in an electromagnetic mill. They reported that for the same products, dry grinding needs 3 kW of power while the wet condition requires 1.6 kW. They also showed that for the same energy levels, the size reduction ratio of samples during dry grinding is less than wet grinding (Table 1). These differences can be due to the fact that wet grinding can produce finer particles, while in dry grinding the agglomeration rate is high. Furthermore, they noted that the maintenance and control system for dry grinding is more expensive than for the wet process [5]. Kotake et al. (2011) reported that the median product particle size of a dry ball mill was four times coarser than the wet grinding products [16]. In general, the size distribution of particles from dry grinding is considerably broader than particles ground under wet conditions [10,16–18]. In dry grinding, part of this higher EC may dissipate in the form of defects in the structure of minerals. This may lead to mechanical activation (MA) of minerals. However, in wet grinding, new surfaces and slight structural deformation can be anticipated [14]. The higher EC in the dry grinding can lead to structural defects and consequently alter flotation kinetics in comparison with wet grinding, which means lower EC during flotation (higher flotation kinetic for dry ground samples) [14].

Table 1.

A comparison between the effects of different grinding environments on size reduction ratio [5].

Time (s)  Energy consumption (kWh)  Dry (dF80/dP80Wet (dF80/dP80
0.025  5.46  3.62 
10  0.050  7.98  9.08 
15  0.075  9.78  10.10 

Several investigations indicated that variations of the surface energy of solids and the degree of dispersion are the main factor when dry and wet grinding systems are compared [19–21]. It was documented that in a wet grinding system by increasing the percentage of solids, part of the energy is converting to the surface energy, and dry ground samples have higher surface energy than wet ground ones [14]. Keeping the energy input constant, and comparing the probability of fracture for particles in wet and dry grinding showed that the crack length on the surface of particles is larger in water than in the air [15]. Ozkan et al. (2009) indicated that, although the primary breakage (B) distribution values in the dry and wet grinding were approximately the same, the specific rates of breakage (S) were higher for the dry conditions [18]. The S ratio between dry and wet grinding for various materials varies from 1.1 to 2.0 [22].

In the inter-particle breakage mechanism, particles are broken by compression. Particles in a high-pressure grinding roll (HPGR) are crushed by inter-particle breakage mechanism. HPGR as a dry comminution method was commercialized since the 1980s. HPGR can process a wide range of coarse crushing to very fine grinding [23,24]. Using HPGR has several advantages such as energy savings (i.e. 10–50% compared to tumbling mills) compared to traditional comminution units [25–27]. HPGR can be fed with ores with a top size of 70–75 mm (based on machine size) and produce particles as fine as P80 of 25–15 µm [28,29]. Micro-cracks that occur along mineral boundaries during grinding by HPGR may lead to preferential liberation [25,30–32]. HPGR may be used in open or closed circuits by using screens or air classification [4]. It was reported that dry grinding by using a closed circuit HPGR could improve the throughput by 25%, reduce the specific energy consumption and increase the circuit efficiency [33]. In a feasibility study, dry grinding of magnetite ore for particle production from 50 mm to 90 µm was performed by using HPGR. Results showed that two stages HPGR followed by dry grinding in a closed circuit with 7 and 1 mm screens could reduce energy consumption by 46% in comparison with an open circuit [4].

Chapman et al. (2013) compared the effects of using different circuits HPGR (dry) -rod mill (wet) and cone crusher (dry) -rod mill (wet) on the flotation of base metal sulfides. They indicated that apart from the grinding environment, HPGR could produce finer particles than a cone crusher. This difference was higher in the fully dry system. Moreover, HPGR in the dry environment liberated more PGM than the wet processing circuit. Flotation of products from different grinding HPGR-rod mill systems showed that, on average, the flotation efficiency of sphalerite was higher in the dry system than for the wet method (Fig. 1). For PGMs, flotation of HPGR-rod wet ground products showed higher recovery than the dry process (Fig. 2). In the case of PGMs, slimes coating the grinding media during dry grinding played the main role and decreased the flotation recovery. This issue was not significant for sphalerite which indicates grinding environments had different effects on the flotation of various minerals [11].

Fig. 1.

Grade and recovery of sphalerite in different HPGR-rod mill grinding environments [11].

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Fig. 2.

Grade and recovery of PGM in different HPGR-rod mill grinding environments [11].

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2.2Mechanical activation

Juhász and Opoczky (2003) divided the grinding kinetics of a dry batch ball mill into three different stages. In the initial stage of grinding (called Rittinger section), the size of the particles is decreased, and the grinding time is mostly dedicated to developing new specific surfaces (Fig. 3a). As the grinding process continues, interactions between particle-particle, particle-grinding media and particle-liners are increased which is called the aggregation section (Fig. 3b). After a long period of grinding, the dispersion starts to decrease and agglomeration of fine particles occurs (Fig. 3c). Mechanical transformation/mechanical activation (MA) or structural alteration of particles mainly occurs in this section [34]. The transition from aggregation to MA section can be due to the fact that by decreasing particle sizes, the defects within the particle structure and grinding resistance are enhanced (in the intensive grinding procedure by using the wet grinding, particularly by stirred media mills, MA of minerals occurs as well). Subsequently, the mode of mechanical energy within particles changes from breakage to plastic flow, which causes dislocations and/or distortion of the crystal structure of the particles. This can significantly affect the surface chemistry of particles [35] (Fig. 3).

Fig. 3.

Degree of dispersity as a function of grinding time [34].

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As dry grinding takes more time and energy than wet grinding for the production of the same particle size distribution, portions of this demanded higher energy may be consumed in the form of defects. In one hand, these defects can include excess enthalpy content even more than the surface energy. In other words, the excess enthalpy in the dry ground sample was higher than in wet ground ones. On the other hands, defects can make relatively rough particle surfaces. Topographical studies by scanning electron and atomic force microscopy showed a higher surface roughness on dry ground particles in comparison with wet ground samples. Thus, defects can highly activate or deactivate the surfaces of particles and change their behavior in the downstream process [14]. Li and Hitch (2017) investigated the dry MA of mine waste rock by using a planetary mill. They reported that after 120 min of dry grinding, serpentine partially converted to olivine. They concluded that for carbonation purposes the dry MA is preferable to wet grinding [36]. However, one of the unwanted phenomena of mineral defects during dry grinding is the increasing preg-robbing of gold by silicates. Mohammadnejad et al. (2013) indicated that after 30 min dry grinding of quartz samples, various types of physicochemical alteration took place such as amorphization, lattice deformation, agglomeration, and increment of the surface area. When the surface chemistry of ground particles changes, low valence silicon and non-bridging oxygen centers are generated which play the main role in the gold preg-robbing of fine quartz particles [35]. In a similar manner, MA of K-feldspar may enhance the leachability of potassium. High intensive dry grinding of K-feldspar increases reactivity, surface area, and deforms its lattice structure, thus results in potassium recovery enhancement [37].

3Effects on mineral flotation3.1Pulp and surface chemistry

It is well documented that grinding environments have significant effects on particle surfaces and downstream pulp chemistry. As mentioned above, dry grinding shows significantly lower media wear than wet grinding [2,38]. However, this can lead to surface and pulp chemistry variation. A comparison between the Fe concentrations in the pulp after dry and wet grinding illustrated that the amount of Fe ions in the solution was approximately 4 times lower after dry grinding [13,17]. Regarding the mineral surfaces, it was proposed that wet grinding of sulfides mainly deals with the chemical surface reaction of products while dry fine grinding mostly affects the oxidation of mineral surfaces [14]. Feng and Aldrich (2000) also observed that the dissolution of Cu, Ni, and Pd from a platinum-bearing sulfide ore was higher after dry grinding compared with the wet process. They indicated the dry ground particles showed more stable, higher loaded froths than wet ones [14].

A comparison between surface properties of boron particles ground under dry and wet conditions showed that dry ground particles have higher rough surfaces and are more agglomerated than wet ground ones. Based on surface analyses, it was observed that products from dry grinding actively react with oxygen. The rate of reaction increased when the particle size was decreasing. In general, energy-dispersive X-ray spectroscopy (EDS) results demonstrated a higher amount of oxygen on the surface of dry ground boron particles [17]. Chapman et al. (2013) reported that after dry grinding of sphalerite, the particles had a higher concentration of passivated ions (Ca, Al, Mg, and Si) on their surfaces compared with wet ground particles in the pulp. When dry ground particles are subject to flotation, these ions from the process water are attracted to active centers and may generate passive layers on the particle surfaces. Moreover, time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis of flotation feed showed a higher level of oxygen species on the surface of dry ground samples in comparison with the wet ground [11]. Seke and Pistorius (2006) conducted various batch flotation tests and compared the pulp redox potential of a lead–zinc sulfide composite ore that was ground in different wet and dry environments. They indicated that the pulp that was made from dry ground ores had a higher Eh (more positive pulp potential) and dissolved oxygen than the one prepared from wet ground ore (Fig. 4). This Eh difference between dry and wet grinding environment diminished at the end of the flotation stage. Thus, they concluded that the grinding environment is effective on the pulp chemistry prior to conditioning while the aeration mainly controls the dissolved oxygen and pulp potential of the rougher stage [9]. Exploring the effect of wet and dry grinding environments on the flotation of a Cu-Zn sulfide ore showed similar results, i.e. the dry ground samples had a more positive potential than the wet ground (approximately 250 mV-SHE higher).

Fig. 4.

Variation of pulp potential and dissolved oxygen through flotation of Rosh Pinah ores ground with various methods [9].

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Ikumapayi et al (2012) reported that the generation of hydrogen peroxide (H2O2) as a strong oxidizing agent (stronger than oxygen) in the pulp affects the flotation of sulfides [39]. Nooshabadi and Rao (2014) indicated that H2O2 generation in sulfides may have the following order pyrite > chalcopyrite > sphalerite > galena [40]. It was noted that at the natural pH (6–7.5), after dry grinding for 50 min in a laboratory ball mill when pyrite particles were placed into water, generated more H2O2 concentration than wet ground particles (Fig. 5) [41]. Liu et al., 2007 examined the effect of dry and wet grindings on coal flotation separation for coarse and fine particles (−600 and −75 µm). Their results showed lower flotation efficiency for dry ground samples in comparison with wet ground ones. The differences between flotation efficiency for dry versus wet ground feeds were greater for finer size fraction (−75 µm). They indicated that higher oxidation of dry ground samples can be due to the high concentration of H2O2, which diminished the coal floatability (Fig. 6) [13].

Fig. 5.

H2O2 concentration at natural pH after dry and wet grinding of pyrite [41].

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Fig. 6.

Recovery of coal samples ground in the various environments (−75 µm) [13].

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3.2Media types

Besides of different grinding environments (dry or wet), the type of grinding media (such as stainless and mild steel) can also affect the surface properties of particles subject to flotation separation [11]. During grinding of sulfides, galvanic interactions between minerals and media occur. These interactions change several factors throughout the flotation. For instance, the amount of dissolved oxygen decreases, the corrosive wear of media enhances and thus the concentration of iron ions increases in the pulp [42]. Liu et al. (2018) reported that using chromium medium in a wet grinding system can enhance selective flotation of chalcopyrite from pyrite in comparison with mild steel medium [13]. Chapman et al., (2013) showed that dry grinding of a PGM ore by using mild steel media could liberate a higher percentage of PGM particles than wet grinding by mild steel and stainless media (Table 2). However, wet grinding by mild steel resulted in the highest PGM flotation efficiency [11].

Table 2.

Effects of various grinding media and environment on PGM size reduction [11].

Properties  Mild steel (wet)  Stainless (wet)  Mild steel (dry) 
d50(µm)  39  38  27 
PGM grain size (µm)  7.2  6.9  4.5 

Moreover, it was documented that mild steel can produce higher H2O2 concentration than stainless media [12,41]. As it was stated, this phenomenon can influence the flotation of sulfides. For example, the recovery of wet ground galena after grinding with mild steel media is lower than for samples ground by stainless media. Similar results were observed when sample dry ground.

3.3Mineral types3.3.1Sulfides

There are several studies reporting that the flotation efficiency of dry ground sphalerite ores is higher than for wet ground [9–13]. Seke and Pistorius (2006) suggested that this higher flotation efficiency originates from MA and a higher positive pulp potential, which is provided by the dry ground sphalerite [9]. Moreover, the higher efficiency of dry ground sphalerite can be explained by an enhanced sphalerite floatability under stronger oxidizing conditions [9,11,40]. The difference between recoveries of sphalerite flotation after dry and wet grinding can be even more than 10% [9].

There is also a positive correlation between the concentration of dissolved oxygen in the pulp and chalcopyrite recovery [43]. Lepetic (1974) claimed that in laboratory and pilot scale flotation, the efficiency of chalcopyrite flotation increased after dry grinding relative to the wet environment [44]. Koleini et al. (2012) showed that the flotation recovery of chalcopyrite from a complex Cu-Zn sulfide ore was higher for dry ground feed versus wet ground ones. Dry ground pyrite can generate higher H2O2 in the pulp compared to other mineral sulfides [10]. It is understood that pyrite depression is enhanced under high oxidizing conditions. Thus, the flotation efficiency of dry ground pyrite is lower than for wet ground samples [9,10,12]. The same scenario is reported for galena and PGMs and their recoveries decrease after dry grinding [11,13,45,47]. Although there is a difference between floatability of dry ground galena and sphalerite, the selective flotation separation of galena from sphalerite is recommended through wet grinding [9].

In case of PGMs, scanning electron microscope (SEM) and atomic force microscope (AFM) analyses of the PGMs after dry and wet grinding illustrated that dry ground samples have rougher surfaces (activated centers) than wet ground PGMs (i.e. the concentration of microstructural defects is high in the dry ground samples). Due to these high MA through dry grinding, the leaching from the particle surfaces and the kinetic of collector adsorption were higher than for wet ground samples (Fig. 7). This can be interpreted as a less selective flotation separation [14]. Moreover, it was proposed that as a result of dry grinding, agglomeration of ultrafine liberated PGMs may expose Pt compounds on the surface of other coarser particles and decrease the process efficiency [48]. Intense conditioning and removing ultrafine particles from dry ground ores prior to flotation potentially can be considered for improving selective separation [14].

Fig. 7.

Sodium isobutyl xanthate (SIBX: collector) adsorption on the surface of sulfide particles in the different grinding environments [14].

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3.3.2Oxides

Dry grinding of CuO with sulfur in a ball mill was conducted to increase floatability of the copper oxide. It was noted that increasing grinding time was the main effective parameter on CuO flotation efficiency. X-ray photoelectron analyses of dry ground samples showed that there are SO and S–Cu bondings on the surface of CuO samples (i.e. mechanochemical adsorption of sulfur on CuO surfaces). Zeta potential measurement demonstrated that the surface properties of CuO were changed as a result of dry grinding where the isoelectric point of CuO sample was shifted from pH 4 to 7. Thus, the zeta potential of CuO samples was more positive even in the neutral pH range after dry grinding. In other words, the MA by dry grinding in the presence of sulfur could enhance floatability of CuO samples [45,46]. Rosenberg and Cliff (1980) recognized that dry grinding can distort the structure of pyrophyllite crystals and that disordering can change surface properties of pyrophyllite [49]. Erdemoglu and Sarıkaya (2002) explored the effects of dry grinding on pyrophyllite flotation. Fourier-transform infrared (FTIR) spectroscopy analyses of pyrophyllite samples before and after dry grinding indicated that extensive grinding increased Si–O stretching vibrations and lead to structural alteration. This structural deformation showed negative impacts on the efficiency of pyrophyllite flotation [50].

4Summary

In general, wet grinding based on energy consumption and particle motion through a processing system is preferred to dry grinding prior to mineral separation. However, water shortage specifically in arid areas dictates dry grinding to liberate valuable minerals. Various assessments indicated that regardless of the mill type, dry environment consumes 15–50% higher energy than wet grinding to produce the same size distribution. Nevertheless, at the same energy, dry grinding can produce coarser particles than the wet process, and the size distribution of particles after wet grinding is much narrower than the dry environment. Dry grinding has significantly lower media and liner wear than wet grinding; thus, the Fe concentration in the pulp of downstream process for the same mineral is relatively lower after dry grinding.

Dry grinding produces rougher surfaces (mechanically activated centers) and more agglomerated particles than wet grinding. On the other hand, the greater energy in dry grinding can deform the structure of minerals and mechanically activate their surfaces. These activated centers adsorb passivated ions and generate passive layers on the surfaces or enhance collector adsorption and flotation kinetics, reduce energy consumption during flotation and as results improve flotation separation. Surface analyses indicated that dry ground samples have a higher portion of oxygen component on the surface than wet ground ones. In other words, the pulp after dry grinding has higher Eh and dissolved oxygen than wet ground samples. This higher dissolved oxygen in the pulp can cause higher H2O2 concentration (strong oxidizing agent) on the surface of dry ground samples in comparison with the wet ground ones. The existence of high oxygen species on the mineral surfaces may diminish or improve their floatability based on the type of minerals.

Sphalerite showed high flotation efficiency after dry grinding. This can be the result of mechanical activation of the sphalerite surface which enhances the H2O2 concentration on the surface and leads to increased floatability due to stronger oxidizing conditions. Chalcopyrite follows the same procedure as sphalerite and its floatability improve after dry grinding. However, depression of pyrite, PGMs, and galena increased under high oxidizing conditions and their flotation efficiency is lower after dry grinding in comparison with wet grinding. Furthermore, agglomeration of liberated particles of these minerals on the surface of other associated minerals during dry grinding can decrease their flotation separation efficiency. For silicates, dry grinding can make structural deformation and weaken their floatability. Therefore, some measures such as using intense conditioning after dry grinding of sulfides in a close HPGR circuit and prior to flotation can be applied to save energy, remove ultrafine particles from the surface of coarse particles and improve selective flotation separation.

Acknowledgement

This project is part of KO1030 SEESIMA, a Kolarctic CBC (Cross-Border Collaboration). This publication was produced with the financial support of the European Union, Russia, Norway, Finland and Sweden. Its contents are the sole responsibility of the authors at the Lulea university of technology, and do not necessarily reflect the views of the European Union or the participating countries. This publication also was produced with the financial support of CAMM - Center of Advanced Mining and Metallurgy as a center of excellence at LTU.

Conflicts of interest

The authors declare no conflicts of interest.

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As an associate professor at the Lulea University of Technology, Prof. Chelgani developed various investigations in process modeling, flotation, leaching and coal processing. Since 2016, he has been an editorial board member of various journals Minerals, Materials, etc. and certified as an outstanding journal reviewer by various journals. He was adjunct prof. at University of Michigan between 2015 till 2018. He has been in several industrial and academia project and outcomes of those projects have been successfully published in high ranked journals (more than 90 articles and h-index: 25). Moreover, he has been awarded the most prestigious scholarships in Canada, and USA (OGS, NSERC, and outstanding researcher).

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