Minerals Engineering 53 (2013) 91–100

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Minerals Engineering

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A review of the effect of water quality on flotation

0892-6875/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.mineng.2013.07.011

⇑ Corresponding author. Tel.: +61 7 3346 4027; fax: +61 7 3346 4045.E-mail address: wy.liu@uq.edu.au (W. Liu).

Wenying Liu ⇑, C.J. Moran, Sue VinkCentre for Water in the Minerals Industry, Sustainable Minerals Institute, The University of Queensland, Brisbane, Queensland 4072, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 May 2013Accepted 16 July 2013

Keywords:Water qualityFlotationAbiotic water constituentBiotic water constituents

As water resources become scarcer and society’s demands to reduce freshwater extraction haveincreased, mine sites have been increasing water reuse and accessing multiple water sources for mineralprocessing to save freshwater, particularly in froth flotation. Implementation of either strategy may leadto water quality variation that may impact flotation efficiency. A large number of studies have been car-ried out to enhance the understanding of water quality variation in flotation. However, these studies tendto be performed on a case by case basis. There is a lack of a framework to put together these existing stud-ies, which makes it difficult to understand the topic comprehensively and therefore difficult to identifygaps and directions for future research. This would eventually hinder the ongoing implementation ofwater conservation practices and thus lead to more pressure being placed on freshwater. In this paper,a review of the existing studies on water quality variation in flotation is given in three aspects: causesof water quality variation, consequences of water quality variation and solutions for problems causedby water quality variation. Based on the three aspects, a framework was developed, with which thesestudies were categorized and structured. Organizing literature in this way makes it possible to identifygaps in current research and future research directions.

� 2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 912. Existing research on water quality variation in flotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

2.1. Causes of water quality variation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

2.1.1. Internal factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 922.1.2. External factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

2.2. Consequences of water quality variation and associated pathways. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

2.2.1. Abiotic water constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 942.2.2. Biotic water constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

2.3. Solutions for water quality variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

2.3.1. Internal solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 962.3.2. External solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

3. Identifying gaps and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

1. Introduction

The minerals industry is being driven to save freshwater andminimize mine water discharge by a combination of water chal-lenges, such as limited freshwater availability (Peters and Mey-beck, 2000; Ridoutt and Pfister, 2010), environmental pollutionfrom mine water discharge (Carlson et al., 2002; Johnson et al.,2002), increasing competition for water among multiple users

(Boulay et al., 2011; Rijsberman, 2006), community concerns overwater security and cultural or spiritual issues regarding water (Jen-kins and Yakovleva, 2006; Kapelus, 2002), and corporate sustain-ability policies and goals (Amezaga et al., 2010; Moran, 2006).Two important strategies being implemented to improve waterefficiency are increasing water reuse and accessing alternativesto freshwater for mineral processing, particularly in flotation.Implementation of either strategy has been shown to increasethe tendency for water quality to change, which, in turn, may affectflotation efficiency. In general, flotation is most effectively under-taken with clean water. As a second preference, metallurgists seek

Fig. 1. A conceptual view of a site water system showing the internal and external factors causing water quality variation in a flotation plant.

92 W. Liu et al. / Minerals Engineering 53 (2013) 91–100

a consistent water quality so that reagent regimes for flotation canbe developed and applied consistently. Variation in water quality isundesirable because it could complicate operating conditions andcompromise flotation performance (Broman, 1980; Hoover, 1980;Levay et al., 2001; Malysiak et al., 2003; Rao and Finch, 1989).

A significant body of research seeks to understand issues asso-ciated with water quality in flotation. These studies focus mainlyon three aspects: (1) understanding the reasons for water qualityvariation in flotation, (2) quantifying the effect of water qualityvariation on flotation efficiency and investigating the underlyingmechanisms to explain the effect, and (3) testing different solu-tions to deal with the effect. However, one common limitation ofthese studies is that they approach the multifaceted topic on a caseby case basis. Lacking of a method to organize these existing stud-ies hinders the understanding of this topic in a holistic way andtherefore makes it difficult to identify gaps in this area and possibledirections for future research. Given the limitation stated above,after reviewing the existing research on water quality variationin flotation, a framework was developed to organize these studies.This framework can enhance the comprehensive understanding ofthis topic by showing the emphasis of current research. This helpsidentify gaps in current research and directions for future research.

2. Existing research on water quality variation in flotation

Water quality for flotation on a mine site can vary significantlyover time (Levay and Schumann, 2006; Luukkanen et al., 2003;Stén et al., 2003). The composition of flotation water depends onthe ore being processed, the reagent suite, the water source andthe way the site water system is managed. The entire water systemis also affected by climate. Fig. 1 shows a conceptual view of a sitewater system which puts the mineral concentrator in perspectivewith the main water system components. The system consists ofraw water stores, worked water stores, tailings facilities, other sitewater tasks (operational uses of water), the concentrator itself anda blender. The blender is a virtual representation of site infrastruc-ture used to demonstrate that worked water might need to be di-luted with raw water or treated water prior to its use in a specificwater task. Raw water is water that has not been previously usedfor any purpose within the site (Cote and Moran, 2009). In contrast,worked water is water that has been used for a purpose on site andis returned for potential future use (Cote and Moran, 2009). A de-tailed explanation of Fig. 1 is given in Section 2.1.

2.1. Causes of water quality variation

Factors that cause water quality variation in flotation can begrouped into two categories by reference to the concentrator:internal and external (Fig. 1). Internal factors include the ore beingprocessed by the concentrator, reagents added into the concentra-tor and water internal reuse, where water is recovered from theconcentrator with the help of different kinds of concentrate/tail-ings thickeners, local storage tanks, etc. and used again by the con-centrator. External factors are divided into ‘‘external toconcentrator’’ and ‘‘external to site’’. ‘‘External to concentrator’’factors refer to factors that are within the site boundary, which in-clude the raw water stream and water external reuse, where wateris recovered from different site water tasks (tailings storage facili-ties and other site water tasks), stored in an appropriate place andused again in the concentrator. Raw water and worked waterstores are open systems that interact with the surrounding envi-ronment and local climate. These interactions result in mass andenergy transfer processes taking place across the site boundary.These processes are defined as ‘‘external to site’’ factors, whichare shown as ‘‘inputs’’ and ‘‘outputs’’ in Fig. 1. The possible inputsand outputs of raw water and worked water stores are summa-rized in Table 1 (Cote and Moran, 2009).

Different water constituents associated with both the internaland external factors can be introduced into flotation water byeither simple mixing or complex physical and biophysicochemicalwater–mineral interactions during mineral processing. Further,chemical reactions may occur among constituents from differentwater streams when coexisting in flotation water. The processesof different constituents being introduced into flotation water aregiven below.

2.1.1. Internal factors2.1.1.1. Ore oxidation and dissolution. The chemical composition ofthe ore being processed by a mine can be very complex and usuallyincludes a variety of minerals. Ore oxidation and dissolution duringmineral beneficiation processes can introduce various substancesinto flotation water, which may alter the chemistry of the systemand thereby influence flotation efficiency. The exact nature of thereaction products depends on reaction conditions, such as Eh andpH.

There are examples in the literature showing the dissolution ofore and its potential implications for flotation. For example, sur-face oxidation of copper ore (chalcopyrite, enargite and tennan-tite) leads to the dissolution of copper (II) to different extents

Table 1Inputs and outputs of raw water and worked water associated with ‘external to site’ factors.

Inputs Outputs

Raw water The inputs include multiple sources of raw water supplies:

� Surface water rainfall/runoff reservoirs, lake and rivers)� Ground water (aquifers, water entrained in materials to be

processed)� Sea water (estuaries and oceans)� Third-party water (an entity in an arrangement with site to supply

water)

The outputs can be a combination of raw water and worked water:

� Seepage (water leaks through the base or sides of water stores)� Evaporation (water loss to the atmosphere)� Supply to third-party (water supplied to an entity in an entanglement

with site)� Entrainment (water contained in the rock or coal after being processed)� Discharge worked water release to the surrounding environment)� Discharge of raw water is unlikely� Miscellaneous loss (losses that cannot be attributed to the above

processes)

Workedwater

No exteriia1 to site’’ inputs for worked water

W. Liu et al. / Minerals Engineering 53 (2013) 91–100 93

at different pH values (Sasaki et al., 2010). Thiosalts and calciumions are released into flotation water from copper ore, which mayhave beneficial effects on flotation by depressing pyrite (Liu et al.,1993), and improve the selectivity of nickel–copper separation(Kirjavainen et al., 2002a,b). At lower pH and in a stronger oxida-tive environment, chalcocite oxidation produces copper (II) andsoluble sulfur species (Chander and Fuerstenau, 1983), whichmay affect the subsequent adsorption of polymeric dispersantonto chalcocite particles (He et al., 2011). The rate and extentof galena dissolution depends on pretreatment conditions, withair treatment having the highest dissolution rate and extent, fol-lowed by oxygen and nitrogen (Fornasiero et al., 1994). The pro-duction of lead ions by dissolution leads to the subsequentformation of lead hydroxide precipitates onto galena surfaces,which could affect galena flotation (Chernyshova, 2003). Dissolu-tion of galena also produces dissolved sulfur species, with sulfateand sulfide being the major species with and without air, respec-tively (Hsieh and Huang, 1989). Another example is molybdeniteoxidation, which produces molybdate ions and sulfur (Chanderand Fuerstenau, 1983).

The ore itself contributes substances to flotation water that arenormally minimal in quantity due to their low solubility and lowavailability from limited oxidation of valuable and gangue miner-als (Johnson, 2003). However, these constituents can build up inflotation water through water reuse, and eventually affect flotationperformance, e.g., the accumulation of dissolved copper leading toinadvertent activation of sphalerite (Slatter et al., 2009).

The grinding media employed might also affect flotation waterquality if they are not inert materials (Huang and Grano, 2006;Peng and Grano, 2010). For example, Eh and dissolved oxygenof the slurry has been found to decrease immediately after grind-ing upon application of reducing steel grinding media (Granoet al., 1990). Conversely, iron hydroxide is produced due to theoxidation of electrochemically reactive steel media (Adam et al.,1984; Grano, 2009). As a result, flotation performance can be af-fected because of the strong dependence of thiol collector adsorp-tion on Eh and the presence of iron hydroxide (Freeman et al.,2000; Grano, 2009).

2.1.1.2. Reagent addition. Reagent addition can introduce variousinorganic or organic substances into flotation water in the formof residue reagents, reaction by-products, and impurities (Schu-mann et al., 2009). For example, lime is added in flotation ofmolybdenite–copper ore to maintain an alkaline pH for pyritedepression, which causes the release of calcium ions into solu-tion (Raghavan and Hsu, 1984). Water for reverse flotation ofiron ore contains a significant concentration of amine flotationreagents used to remove silica particles (Batisteli and Peres,2008; Stapelfeldt and Lima, 2001). Copper cyanide complexesare often found in water for galena–sphalerite flotation because

of the addition of sodium cyanide in the milling circuit (Sekeand Pistorius, 2006).

Similar to ore dissolution, flotation reagents themselves maycontribute a small portion of water constituents (Slatter et al.,2009), but these constituents can be accumulated in water inan altered or unaltered state during water reuse and eventuallyreach a concentration high enough to impact flotation perfor-mance. For example, flotation selectivity of copper–lead–zincore is compromised by the accumulation of the decompositionproducts of a xanthate copper collector Z-200 due to water reuseand its subsequent non-selective adsorption (Ozkan and Acar,2004).

2.1.1.3. Water internal reuse. Water internal reuse is a typical watermanagement strategy targeting a concentrator. For example, theRosh Pinah mine in Namibia recovers water from the lead roughertailings thickener and lead concentrate thickener, and uses therecovered water for milling and lead flotation (Seke and Pistorius,2006). Water recovered from concentrate filtration/drying is usedagain in the ore grinding circuit in Olympic Dam (Torris and Trotta,2009). Water internal reuse may cause issues related with process-ing, e.g., ore dissolution and reagent addition, to later becomewater quality issues due to the recirculation of these substancesin processing plants. However, this strategy could, to some extent,simplify water chemistry because the water is generally re-circu-lated rapidly without experiencing significant alterations (Raoand Finch, 1989).

2.1.2. External factors2.1.2.1. Multiple sources of raw water supplies. Water quality varia-tion in the raw water stream is defined as one of the ‘‘external toconcentrator’’ factors that can contribute to water quality variationin the concentrator. The reason why water quality varies in rawwater stores can be tracked down to ‘‘external to site’’ factors, be-cause raw water stores receive water inputs from multiple sourcesof different quantity and quality from outside of the site boundary.These sources, as shown in Table 1, include surface water, ground-water, sea water and third-party water.

The use of multiple sources of raw water supplies of differentquality is very common for many mine sites. For example, primarywater supplies containing high levels of salinity including calcium,magnesium and iron salts are being used in several remote areas(Levay et al., 2001). Humic acids, abundant in natural waters asso-ciated with dead vegetation decomposition, have been found to ex-ist in flotation water, which could depress molybdenite flotation(Lai et al., 1984), and coal flotation (Arnold and Aplan, 1986). Tan-nic acid, commonly found in ground water, is a potential depres-sant which may be introduced into flotation water from rawwater streams (Levay and Schumann, 2006). Some mine sites areusing treated effluent (third-party water) as a make-up water

94 W. Liu et al. / Minerals Engineering 53 (2013) 91–100

supply, e.g., Mt Arthur coal mine (Brereton et al., 2007), Cadia Val-ley gold–copper mine (Schumann et al., 2003), and Mogalakwenaplatinum mine (Slatter et al., 2009). This water source may containhigh levels of total organic carbon (TOC) and microbes, which mayaffect flotation performance (Levay et al., 2001). Apart from theseinputs, a variety of outputs can also contribute to water qualityvariation in the raw water stream (Table 1). For example, waterconstituents can be concentrated due to evaporation and diluteddue to rainfall/runoff which may vary seasonally (Rao and Finch,1989; Vink et al., 2009).

2.1.2.2. Water external reuse. A common external reuse strategy isthat water is recovered from tailings storage facilities and othersite water tasks, stored in the worked water stores and used againin the concentrator (Department of Resources Energy and Tourism,2008). Besides the raw water stream, this worked water stream isdefined as the other ‘‘external to concentrator’’ factor that cancause water quality variation in flotation. Similarly, this variationcan be tracked down to ‘‘external to site’’ factors. As opposed tothe raw water stream, there are no inputs but only outputs forthe worked water stream (Table 1).

One of the outputs is associated with the local climate, whichmay raise significant issues regarding water quantity and quality.For example, evaporation can alter concentrations of dissolvedand colloidal constituents at different times of the year (Levayet al., 2001). Changes in temperature due to diurnal (day/night)or seasonal (summer/winter) cycles may influence water composi-tion, either directly or indirectly, through chemical, physico-chem-ical and biological processes, e.g., mineral dissolution andoxidation, solubility of metal precipitates, all of which are affectedby temperature (Lin, 1989). These processes may change waterconstituents in concentration and form. For example, physico-chemical and biological processes mainly related to degradationof flotation reagents could take place in the tailings storage facili-ties (Chen et al., 2011; Nedved and Jansz, 2006). Tailings particlescan contain a high proportion of fine clay and other colloidal mate-rials, which are difficult to separate from water (Ofori et al., 2011).These changes in the tailings storage facilities can be propagated tothe concentrator through water external reuse. Similar to waterinternal reuse, water external reuse may cause initial processing is-sues to eventually become water quality issues.

2.2. Consequences of water quality variation and associated pathways

Water quality variation is among the factors that can influenceflotation efficiency. The consequences of water quality variation inflotation can be grouped into two broad categories based on thetype of water constituents: abiotic, meaning not alive (e.g., inor-ganic metal ions), and biotic, meaning of or related to life (e.g.,microorganisms). For each category, the consequences can beeither negative or positive effects on flotation efficiency, whichcan be quantified by different variables, such as recovery, gradeand floatability of valuable minerals, and selectivity between valu-able minerals and gangue.

The negative or positive effects of abiotic and biotic water con-stituents can occur through different pathways. Firstly, water con-stituents could interact with and therefore change the properties ofany of the three phases involved in flotation: mineral particle, airbubbles and aqueous solution. These changes could then affectthe efficiency of the three sub-processes occurring sequentially ina flotation cell. These sub-processes include collision of mineralparticles with bubbles, attachment of mineral particles to bubbles,and formation of stable particle–bubble aggregates which then riseto the surface of the flotation cell forming the froth phase (Daiet al., 1999). Only mineral particles with certain degrees of hydro-phobicity, either naturally occurring or artificially rendered by

adding certain flotation reagents, can attach to bubble surfaces.The efficiency of the three sub-process are defined as the fractionof particles colliding with a bubble, the fraction of colliding parti-cles which actually attach to the bubble surface, and the fractionof attached particles which are successfully transported acrossthe pulp–froth interface, respectively (Dai et al., 1999; Seamanet al., 2006). Finally, these changes are reflected in the change ofthe final flotation efficiency. In addition, water constituents canalso react with reagents added to flotation circuits, thus changingreagent properties.

The pathways by which a particular water constituent affectsflotation performance may be different for each mineral and re-agent combination. These possible pathways to explain the nega-tive and positive effects of different abiotic and biotic waterconstituents are given below.

2.2.1. Abiotic water constituents2.2.1.1. Negative effects.2.2.1.1.1. Reduction in particle surface hydrophobicity by metal ions.Metal ions are important abiotic water constituents which can im-pact flotation performance. Metal ions hydrolyze in alkaline pHsolutions and precipitate as hydrophilic metal hydroxides, sulfatesor carbonates if their concentrations are above their respective sol-ubility limits (Fuerstenau et al., 1985). Formation of metal hydrox-ides is influenced by aqueous pH (Font et al., 1999). Precipitation ofthese hydrophilic metal hydroxides on mineral surfaces has beengenerally described as indiscriminate (Kitchener, 1984), resultingin formation of a hydrophilic barrier to collector adsorption onmineral surfaces (Fornasiero and Ralston, 2006; Senior and Trahar,1991).

Calcium, iron and aluminum ions are the cations most cited inthe literature as precipitated species with a detrimental effect onmineral recovery and grade (Hoover, 1980). The effect of thesemultivalent ions on coal flotation has been investigated, the resultof which shows that multivalent electrolytes depress coal flotationin the pH region of metal hydroxide precipitation (Celik andSomasundaran, 1986; Rao and Finch, 1989; Somasundaran et al.,2000). Sphalerite recovery has been found to be reduced by thepresence of zinc ions (200 ppm) due to the formation of colloidalhydroxide on the mineral surface (Williams and Phelan, 1985).Copper (II) acts as the activator at a threshhold concentration buta depressant for sphalerite at high concentrations (Fornasieroand Ralston, 2006). A critical concentration of copper sulfate existswhere sphalerite gains the maximum recovery. Above that criticalconcentration, sphalerite recovery decreases as a result of excesscopper hydroxide on the sphalerite surfaces (Boulton et al.,2005). Coarse particles are the first particles to be affected by a re-duced surface hydrophobicity as they are more easily detachedfrom bubbles in the high turbulent regions of a flotation cell (Ral-ston et al., 2007).

The reduction in mineral surface hydrophobicity due to the pre-cipitation of hydrophilic metal hydroxides could compromise theefficiency of the particle–bubble attachment sub-process (Kohet al., 2009; Schwarz and Grano, 2005). It can also cause lower con-tents of mineral particles entering the froth, which might compro-mise the efficiency of the sub-process of formation of stableparticle–bubble aggregates (Ali et al., 2000; Ata, 2012; Johanssonand Pugh, 1992; Moolman et al., 1996). The presence of dissolvedions in water can also change the stability of particle–bubbleaggregates in the froth phase (Bıçak et al., 2012; Farrokhpay andZanin, 2012).

2.2.1.1.2. Change in particle surface charge by metal ions. Metalions in flotation water can alter the surface charge of particlesand consequently affect interactions between particles and wastegangue or between particles and collectors. This could affect parti-cle–bubble attachment and also formation of stable particle–bub-

W. Liu et al. / Minerals Engineering 53 (2013) 91–100 95

ble aggregates (Ali et al., 2000; Ata, 2012; Johansson and Pugh,1992; Moolman et al., 1996).

Investigations have been carried out on the role of calcium ionsin modulating the surface properties of molybdenite and in con-trolling the interaction between molybdenite and the most pre-dominant gangue mineral, namely quartz, in copper porphyries.The results show that the floatability of fine molybdenite particlesis significantly reduced when calcium ions and silica coexist in theflotation pulp. This is because the adsorption of calcium ions onmolybdenite and quartz reduces the magnitude of negative surfacecharges and thus causes heterocoagulation of molybdenite andquartz (Raghavan and Hsu, 1984). The adsorption of calcium andother metal ions that exist in flotation water leads to a reductionin the negative surface charges and xanthate adsorption on galena,which may have detrimental effects on galena flotation (Ikumapayiet al., 2012). Experimental results suggest that generally, metalliccations present in electrolytes hinder the flotation of pyrochlore,an oxide mineral. This is because the negative surface charge onpyrochlore is reduced by the adsorption of cationic species, whichhinders the adsorption of cationic amine collector on pyrochloresurfaces (Espinosa-Gomez et al., 1987; Rao et al., 1988).

2.2.1.1.3. Inadvertent activation of unwanted minerals by metalions. Some metal ions can also inadvertently activate unwantedminerals, thus affecting flotation selectivity in varying degrees.For example, metal ions such as lead, silver and iron are presentin flotation water as impurities and can inadvertently activatesphalerite surfaces (Chandra and Gerson, 2009; Finkelstein,1997). The undesirable activation by metal ions, copper (II), iron(II) and calcium (II), causes pyrite to float together with sphaleritein sphalerite flotation circuits, leading to poor selectivity (Boultonet al., 2003; Zhang et al., 1997). Inadvertent activation of sphaleriteand pyrite by copper ions in water leads to a low copper grade incopper flotation circuits (Broman, 1980; Rao and Finch, 1989). Cop-per (I) cyanide could activate sphalerite in flotation of lead–zincsulfide ore reducing lead grade (Seke and Pistorius, 2006).

2.2.1.1.4. Slime coatings on mineral surfaces. The formation of aslime coating on valuable mineral surfaces can lead to depressionof flotation of valuable minerals. For example, positively chargedserpentine gangue minerals, such as chrysotile and lizardite, hasbeen found to severely reduce the flotation of negatively charged(unoxidized) pentlandite by forming a hydrophilic slime coatingon pentlandite surfaces (Edwards et al., 1980). The coverage of col-loidal iron oxide (hematite) slimes originating from the steel grind-ing media, iron sulfide minerals and non-sulfide gangue, on galenasurfaces can reduce the mineral surface hydrophobicity and there-fore have a significant depressing effect on the flotation of galenaparticles (Bandini et al., 2001). The slime coating of montmorillon-ite clay on coal is detrimental to coal flotation (Xu et al., 2003).

2.2.1.1.5. Interactions with flotation reagents. Like metal cations,anions may also have a negative effect on flotation performance byinteracting with flotation reagents. For example, anions in treatedeffluent have been identified as having a negative effects on copperand molybdenum recoveries (Fisher and Rudy, 1976). Sulfide ionshave been found to decompose xanthate collector in the presenceof oxygen (Shen et al., 2001). Under solution conditions which fa-vor rapid xanthate decomposition by sulfite, xanthate adsorptiononto galena is significantly reduced and galena flotation stronglydepressed (Grano et al., 1997a,b). Salts in water are particularly lia-ble to react with fatty acid reagents and form insoluble complexes(Ozkan and Acar, 2004). Calcium ions react with the collector usedin apatite flotation, thus reducing its concentration for flotation,leading to a decline in apatite recovery (dos Santos et al., 2010).

Variations in pH and Eh usually affect the chemistry of flotationreagents and species present in flotation water. For example, pulppH can alter the form of some frothers and thus influence theireffectiveness. Cresol, with the optimum performance in the molec-

ular form, has been found to be converted into ionized form at highpH, which then does not act as a frother. Quinoline exists in ionizedform in acid pH, andconsequently has poor frothing properties(Bulatovic, 2007). Dissolved oxygen is also important in controllingthe composition of the mineral surface of some minerals and theeffect of the reagents on the minerals in flotation systems (Gaudin,1974).

2.2.1.2. Positive effects.2.2.1.2.1. Compression of electrical double layer. The presence ofelectrolytes can improve particle–bubble attachment efficiencythrough compressing the electrical double layer and thus reducingthe electrostatic repulsion between particles and bubbles (Kurnia-wan et al., 2011). Investigation of oil agglomeration of coal in inor-ganic salt solutions shows that coal recovery increases markedly assalt concentration (NaCl) is raised (Yang et al., 1988). Coal recoveryis improved by using saline water (Ofori et al., 2005; Wang andPeng, 2013). Flotation of methylated quartz is improved withincreasing KCl concentration (Laskowski and Kitchener, 1970).The efficiency of the attachment of methylated quartz particlesto nitrogen bubbles has been found to increase with the increasein KCl electrolyte concentration (Dai et al., 1999). Flotationimprovement in the presence of electrolytes is explained by thecompression of the electrical double layer by electrolytes, thusreducing electrical repulsion and subsequently facilitating the par-ticle–bubble attachment process. The reduction in electrostaticrepulsion between particles or between particle and bubbles mayin turn decrease the adsorption of positively charged hydrophilicslimes such as magnesium silicates and thus increase bubble–par-ticle attachment (Bremmell et al., 2005; Hewitt et al., 1994; Morriset al., 1995).

2.2.1.2.2. Formation of smaller bubbles. Electrolytes are favor-able to the formation of smaller stable bubbles due to the influenceof the electrolytes on surface tension and gas solubility (Pugh et al.,1997). Smaller bubbles increase the particle–bubble collision prob-ability (Bournival et al., 2012; Pugh et al., 1997), and also improveparticle–bubble attachment efficiencies (Hewitt et al., 1994). Finergas bubbles in high salt concentrations may result in reduced re-agent consumption (Quinn et al., 2007). However, it is noteworthyto mention that along with the benefits discussed above, an in-crease in ionic strength can cause a negative effect by enhancingfrothability and therefore increasing gangue recovery (Manonoet al., 2012, 2013).

2.2.2. Biotic water constituents2.2.2.1. Negative effects. Biotic water constituents in flotationwater, including organics and microorganisms, may be surface ac-tive, or act as dispersants or flocculants, which can interfere withflotation process (Levay et al., 2001; Rao and Finch, 1989). Residuereagents such as xanthate and their decomposition products in flo-tation water could absorb non-selectively on most sulfides and re-duce flotation selectivity (Seke and Pistorius, 2006).

There are cases in literature showing the effect of organic spe-cies on flotation efficiency through different pathways. For exam-ple, humic acids, abundant in some natural waters, have beenfound to readily adsorb on molybdenite surfaces and result in de-creased hydrophobicity and floatability of molybdenite (Lai et al.,1984). The presence of a small amount of strongly hydrophiliclignosulfonates is sufficient to render molybdenite surfaces hydro-philic, which hinders the attachment and spreading of an oily col-lector over the molybdenite surface (Ansari and Pawlik, 2007a,b).The presence of organic species, the exact composition of whichis not known, has a negative effect on pyrochlore flotation selectiv-ity by promoting the flotation of silicate minerals along withpyrochlore (Espinosa-Gomez et al., 1987; Rao et al., 1988). Theaccidental incorporation of oils and lubricants into the mill feed

96 W. Liu et al. / Minerals Engineering 53 (2013) 91–100

at a copper–gold concentrator has been shown to promote libratedgangue particles, leading to uncontrollable frothing associatedwith a drop in selectivity (Bos and Quast, 2000). Copper and goldflotation has been found to be negatively affected by total organiccarbon (TOC) in flotation water (Schumann et al., 2003).

A variety of microorganisms at meaningful concentrations canbe introduced into flotation water due to the use of alternativewater sources that contain microorganisms, such as treated efflu-ent. Furthermore, flotation circuits can provide conditions condu-cive to microbial growth, given that there is potential nutritionfrom reagent addition, appropriate oxygen levels and suitable tem-perature. For example, the total bacterial count in the plant flota-tion pulp on a mine site can be as high as 109 cfu/mL (Levayet al., 2001), where cfu stands for colony-forming units. Unlikedirect microscopic counts (cell/ml) where all cells (dead and via-ble) are counted, cfu only estimates viable cells. Thus the concen-tration of dead plus viable bacterial cells in the flotation pulp maybe higher than 108 cell/ml. For comparison, E. coli concentration intypical raw sewage (i.e., untreated municipal wastewater) is at amagnitude of 105 cell/mL (Carr et al., 2004). The presence of bacte-ria in flotation water could pose potential risks to flotation perfor-mance (Levay et al., 2001; Slatter et al., 2009). For example,bacteria (c-Proteobacteria) have been shown to reduce the float-ability of apatite due to their interactions with calcium-containingminerals and strong flocculation (Evdokimova et al., 2012). E. colihas been found to have a negative effect on copper flotation mainlydue to its attachment onto chalcopyrite surfaces and the resultingreduction in chalcopyrite surface hydrophobicity (Liu et al., 2013a;Liu et al., 2013c).

2.2.2.2.. Positive effects. In some cases, biotic water constituentsmay bring positive effects on flotation efficiency. Two types of bio-tic water constituents can exemplify the positive effects: residuereagents and bacteria. Residue reagents brought to flotation cir-cuits through water external reuse allows retention of some re-agents and therefore lowers reagent consumption. For example,the dosage of amine collector in the reverse flotation of iron orescan be reduced by up to 50% through water external reuse (Batist-eli and Peres, 2008; Stapelfeldt and Lima, 2001). Bacteria could beused to reduce the recovery of one mineral over that of anotherin situations where a selective flotation is required (Elmahdyet al., 2011; Evdokimova et al., 2012; Pecina-Trevino et al., 2012).Further research will advance and refine the search for suitablesolutions to take advantage of the positive effects of biotic waterconstituents.

2.3. Solutions for water quality variation

As flotation efficiency can be affected by water quality change,different strategies are required to manage water quality changeto avoid compromising flotation efficiency. Solutions that can beapplied to deal with flotation problems caused by water qualitycan be divided into two categories by reference to the concentra-tor: internal and external (‘‘external to concentrator’’ and ‘‘externalto site’’).

2.3.1. Internal solutionsGenerally, flotation problems from water quality are dealt with

by internal solutions, which focus on the flotation operation itself.One conventional internal solution to deal with the problems is toadjust the reagent regime.

There are examples of reagents added to control flotation prob-lems caused by water quality in literature. Zinc hydroxide precip-itates can be dissolved by acids, thus improving sphalerite–galenaseparation (Levay et al., 2001). Removal of calcium ions by oxalicacid improves pyrochlore recovery (Rao et al., 1988). The addition

of citric acid has been found to eliminate the detrimental effects ofcalcium ions and to restore the selectivity of chalcopyrite–galenaseparation with dextrin (Liu and Zhang, 2000). Chemical disper-sants, e.g., polyphosphate, can be used to disperse precipitated col-loidal species from mineral surfaces (Levay et al., 2001). Additionof complexing agents, such as amine complexes, can help to revertinadvertent activation effects of metal ions on the selective flota-tion of pentlandite–pyroxene (Shackleton et al., 2003). The selec-tivity of sphalerite and pyrite can be increased by the use ofsulfoxy depressants against pyrite that has been inadvertently acti-vated by metal ions (Chandra and Gerson, 2009). Dithionite hasbeen used as a reducing reagent to control electrochemical reac-tions in flotation, notably to prevent oxidation of sulfide minerals(Sui et al., 2000). Activation of sphalerite by copper species duringthe selective flotation of galena and sphalerite can be eliminated byremoving copper with sodium cyanide (Seke and Pistorius, 2006).Soda ash is added to precipitate calcium and other interfering cat-ions from the pulp for flotation of phosphate ore (Nanthakumaret al., 2009). Changing the slime surface charge by chemical addi-tives, such as carboxy methyl cellulose, has been shown to beeffective in reducing the adverse effect of the slime on pentlanditeflotation (Edwards et al., 1980). Coal flotation recovery increasessignificantly by removing the fine fraction from the feed using ahydrocyclone (Oats et al., 2010).

In some cases, the internal solution of adjusting reagent regimeis an efficient way to deal with flotation problems associated withboth internal and external factors. However, this solution requiresmuch time, energy and attention to be spent on the selection of re-agents and control of reagent additions to ensure proper concen-trations and the most effective separation. In addition,operational efficiency cannot be guaranteed. For example, despitecontinuous process improvement, the problem of pyrite activationby metal ions and its misreporting to sphalerite concentrates stillremains (Boulton et al., 2003; Zhang et al., 1997). Chemical side-reactions between the reagents added and flotation water constit-uents, metal ions for example, may also affect mineral surfaceproperties and thus their floatability (Sui et al., 2000).

Water treatment by conventional technologies and operationaltechniques to meet flotation requirements is also considered as aninternal solution. A wide variety of technologies and techniquesare available for the minerals industry. The main types of watertreatment and their application domains in mining operationscan be found in The Water Management Handbook (Departmentof Resources Energy and Tourism, 2008). A few examples of watertreatments employed to meet mineral processing requirements aregiven here. Metal ions can be removed using ion exchange andmembrane technologies. For example, the adverse effect of zincions on sphalerite flotation is eliminated by treating flotation waterto remove zinc ions before the water enters the mill (Williams andPhelan, 1985). The use of low-cost natural iron ore has been shownto be effective in removing arsenic from water (Aredes et al., 2012).Laboratory experimental results show that a combination of acti-vated carbon and ion exchange to remove Ca2+, Mg2+ and organicspecies appears to be an effective means for improving selectiveflotation of pyrochlore (Espinosa-Gomez et al., 1987; Rao et al.,1988). Pretreatment of water with aeration or with activated car-bon has been found to significantly improve molybdenite recoveryin the presence of humic acid (Lai et al., 1984). Removal of organicspecies from flotation water by activated carbon can produce a sta-tistically significant improvement in nickel grade and recovery (Le-vay et al., 2001).

Each water treatment technique has its own advantages andlimitations. Therefore, the optimum treatment approach shouldbe selected according to the water quality requirements for a spe-cific water task. For example, treatments for most mineral process-ing applications would not require potable grade water. A

W. Liu et al. / Minerals Engineering 53 (2013) 91–100 97

combination of different approaches may be used to deal with dif-ficult or complex water quality issues. The selection of water treat-ment approaches, which match the water quality requirement forspecific water tasks, can be challenging. The cost of water treat-ment and the undesirable additional energy incurred may pose achallenge for maintaining an environmentally and economicallysustainable mining operation.

Fig. 3. Emphasis of current research on water quality management in flotation(darker color equals more work).

2.3.2. External solutionsAlthough they may efficiently deal with flotation problems from

water quality, the technologies and processes implemented byinternal solutions have not considered the connectivity of the sitewater system, leading to a constrained view of opportunities todeal with the problems. In contrast, external solutions attempt toconsider opportunities across the entire water system to deal withflotation problems caused by water quality. External solutionshave generally received less attention compared to internal solu-tions. Therefore, there is a lack of literature on external solutionsthat could be applied to manage water quality problems.

Flotation water quality is dynamic because of the influence bythe internal and external factors discussed in Section 2.1. Thewater system on a mine site acts as a complex system with feed-backs and interactions between the local climate and the engi-neered reticulation (Vink et al., 2009). Therefore, solving flotationproblems caused by water quality requires an understanding ofthe properties of different water streams and their connectivity.If this information is known, in some cases relatively simple exter-nal water management practices can be applied to help stabilizeplant operations without overreliance on chemical additions. Forexample, taking water from a location in the water store that pro-vided water of consistent quality was proposed as a possible‘‘external to concentrator’’ solution to control the risk posed bywater salinity variation to a coal washing plant (Liu et al., 2011).Using tailings as a potential adsorbent to bacteria was consideredas a possible ‘‘external to concentrator’’ solution to mitigate therisk posed by water-borne bacteria to copper and gold flotation(Liu et al., 2013b). In this case study, one possible ‘‘external to site’’solution might be to treat the bacteria-containing water stream be-fore it enters the mine.

It is understandable that more attention has been paid to inter-nal solutions simply because it is simpler to do so. It takes more ef-fort to think about external factors and interconnectivities of thesystem. Looking at external solutions may require a change in par-adigm or change in the way of thinking.

Fig. 2. Framework for organizing the literature on water quality management inflotation.

3. Identifying gaps and future directions

A review of the literature shows that a significant amount of re-search has been carried out on each of the three aspects of waterquality variation in flotation (reasons, effects and solutions). Thereasons why water quality varies are explained by internal andexternal factors. The consequences of water quality variation areeither negative or positive effects on flotation efficiency, whichcan be quantified by different variables, such as recovery, gradeand floatability of valuable minerals, and selectivity between valu-able minerals and gangue. Solutions to deal with the effect are di-vided into internal and external solutions.

Given the absence of a method to organize these case-by-casestudies, we developed a three-component framework to classifythe existing literature. This three-component framework consistsof causes of water quality variation, consequences of water qualityvariation and solutions for problems caused by water quality var-iation (Fig. 2). The framework structures the literature in such away that makes it possible to identify gaps in each componentand associated research directions. Fig. 3 shows the emphasis ofcurrent research on each of the three components by different lev-els of color. Darker color equals more research work. Accordingly,research gaps were identified in three broad areas.

The first gap lies in the focus of current research on the twotypes of water constituents. Compared to abiotic water constitu-ents, there is a lack of an understanding on the impacts, processesand solutions associated with biotic water constituents (Rao et al.,2010). This could be a barrier for mine sites sourcing alternativewater sources that contain biotic water constituents of concentra-tions sufficiently high to affect flotation efficiency, such as treatedeffluent, when attempting to reduce freshwater withdrawal. Con-sequently, more pressure would be put on the limited freshwaterresources and the people and ecosystems that rely upon it. There-fore, further research is needed to understand how biotic waterconstituents affect flotation and the optimal solutions that can beadopted to deal with the effect. It is important for mine sites tobe aware that the effect of biotic water constituents may be site-specific. Different mine sites may have different external watersources with different biotic constituents. Some mines are locatednear populated areas with an option of treated effluent and mayneed to study the impact of fecal bacteria, etc. Other mines are inforested areas, which may have a water source containing differenttypes of bacteria, algae or tannic acids (from plant decomposition)that may affect flotation.

98 W. Liu et al. / Minerals Engineering 53 (2013) 91–100

The second gap is about the solutions applied to deal with waterquality problems in flotation. Currently, water quality problems inflotation are generally dealt with by internal solutions that focuson the flotation operation itself more or less haphazardly, that is,without a diagnostic and proactive understanding of the causesfor water quality variation. This may lead to a constrained viewof opportunities to deal with the problems and thus hinders theongoing implementation of good water management practices thatconserve freshwater. Therefore, further research work is requiredto explore multiple external solutions that consider opportunitiesacross the entire water system as opposed to the flotation opera-tion itself. The ultimate solution might be the best combinationof internal and external solutions. Finding such a solution requiresmine sites to have a good understanding of the connectivity of thesite water system and the properties of different water streams.This leads to the third gap.

The third gap is associated with understanding the reasonswhy water quality varies. This relies on water quality monitoringinformation and an understanding of the connectivity of the sitewater system. Even though water quality monitoring is conductedon mine sites, in some cases, the monitoring tends to be per-formed without clear objectives and consideration of the connec-tivity of different water streams, and not at the desired locationsor at the desired frequency. This poses a potential challenge forsubsequent utilization of these data to analyze water quality var-iation. Therefore, there is an essential need to develop a methodwhich could help set up clear objectives and therefore targetthe monitoring at the desired water streams. The way the infor-mation on water quality variation in flotation is organized, i.e.,from the perspective of internal and external factors, could be astarting point for developing such a method. After understandingthe internal and external factors that cause water quality varia-tions, mine sites can design a monitoring program that measuresthe chosen components at the right locations and for a suitablelong time period.

The applicability of this three-component framework may notbe limited to organizing research on water quality problems in flo-tation but extended to broader ‘‘external to concentrator’’ and‘‘external to site’’ areas involving water quality change, such asthe impact of water quality variation on heap leaching within thesite boundary and on the local ecosystem outside mine sites. Fur-ther research is required to test the applicability of this framework.

From the perspective of water management, integrated watermanagement requires water issues be treated as a risk manage-ment exercise. This is because water is one of the risk factors thatincrease the likelihood of mining operations having accidentswhich could injure or kill people, damage the environment andcause serious loss of production and profits (Byrne et al., 2012; Cat-alan et al., 2000; Cote and Moran, 2008; Saleh and Cummings,2011). As for water quality management in flotation, a previouslypublished risk-based approach was suggested as a tool to assistmine sites in managing the risk of water quality variation in flota-tion in a consistent and structured manner (Liu et al., 2011). Thisapproach integrates the three components stated above to quantifythe risk posed by water quality variation to flotation and evaluatedifferent scenarios for risk mitigation. The potential of the ap-proach as such a management tool that can be put in place hasbeen tested using two flotation case studies. The first is to managethe risk of saline water to coal flotation (Liu et al., 2011). The sec-ond deals with the risk posed by water-borne bacteria to copperand gold flotation (Liu et al., 2013a). The two case studies demon-strate the potential of the risk-based approach to deal with com-plexity and allow specific situations to be considered in a genericmanner. They also show that water quality problems can be dealtwith more effectively by considering opportunities from the entire

water system rather than only focusing on the local area where theproblems occur.

Acknowledgements

Funding for this study was provided by Australian ResearchCouncil linkage project (LP0883872) ‘‘Impact of recycled and lowquality process water on sustainable mineral beneficiation prac-tices’’, which is part of AMIRA project P260E.

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