Separation and Purification Technology 55 (2007) 232–237

Flotation of chromite and serpentine

G.P. Gallios ∗ , E.A. Deliyanni, E.N. Peleka, K.A. Matis
Section of Chemical Technology & Industrial Chemistry, School of Chemistry, Aristotle University (Box 116),
GR-54124 Thessaloniki, Greece
Received 27 June 2006; received in revised form 7 December 2006; accepted 13 December 2006

Abstract
Chromite is an important strategic mineral usually associated with other gangue minerals, mainly silicates. A good knowledge of the flotation
behaviour of chromite and the establishment of conditions for its selective separation from the gangue minerals might help the future exploitation
of chromite deposits. In this paper, the floatability of chromite and serpentine fine particles by sodium oleate was investigated in-depth, aiming
to separation during mineral processing, i.e. to float chromite from serpentine. Process parameters investigated were the effect of collector
concentration, the pH value, the type and concentration of various modifiers on the flotation behaviour of chromite. The conditions investigate
where selective flotation from gangue minerals (serpentine) would be possible.
© 2007 Elsevier B.V. All rights reserved.

Keywords: Chromite; Flotation; Modifier; Separation; Serpentine

1. Introduction Selective flocculation is the aggregation of particles of a

desired material from a mixture of materials; the main mech-
The world supply and reserves of chromite or chrome ore anisms for aggregation are polymeric bridging by using high
(FeCr2 O4 ) have been dominated mainly by South Africa and molecular weight polymers or selective charge neutralization.
the ex-USSR, which made chromite a strategic material in many Recent developments for recovering very fine particles are shear
Western countries [1]. Bhappu highlighted the need for more flocculation, carrier flotation, column flotation, oil agglom-
research into the processing of low grade refractory ores of eration and electroflotation [5]. Gravity concentration based
strategic metals such as chromium [2]. It is also known that on differences between the specific gravities of chromite and
the Alpine belt, which extends in an arc through the former gangue minerals has been primarily used for concentrating
Yugoslavia and Albania to Greece, is notably rich in chromite. chromite [6]. For economic reasons, gravity methods are usually
Furthermore, some of the chromite ores are platiniferous [3]. applied for the concentration of chromium ores, but they gen-
The relationship between chromium, the platinum group ele- erally fail to recover fine size fractions; the latter can be used,
ments (PGE) and the type of PGE minerals in these deposits for example, in the manufacture of refractories. The fines below
depends largely on the geological environment. Therefore, the approximately 100 ␮m are generally discarded as gangue from
processing characteristics of these ore types are quite variable. gravity concentration processes. Thus, flotation may be a pro-
According to the flotation characteristics and difficulties in ben- cess necessary to separate the finest fractions or possibly, when
eficiation, these deposits can be classified into two preliminary the difference in the specific gravity of the minerals is small.
sub-groups [4]: podiform deposits, in which the PGE occur as Much work has been done on the fundamentals of chromite
alloys enriched in osmium, ruthenium and iridium; and strata flotation [7]. The importance of charged metal hydroxy-species
chromium deposits in which a large portion of the PGE are in in such flotation has been well demonstrated [8–10]. These
the form of minerals (laurite and alloys), a portion of which are researchers have considered oleate, saturated carboxylic acids
associated with chromium. and alkyl sulphates as collectors for the mineral. It is well-
known that flotation of chromite and serpentine is very difficult,
especially in oleate flotation, due to similar surface properties
∗ Corresponding author. of both minerals and to the ions such as Mg2+ deriving from
E-mail address: gallios@chem.auth.gr (G.P. Gallios). the ore and depressing the adsorption of RCOO− ions on the

1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.seppur.2006.12.015
 G.P. Gallios et al. / Separation and Purification Technology 55 (2007) 232–237 233

chromite surface [5]. Modifying agents may be used to increase in a disc pulverizer, the −75 + 45 ␮m size fraction was taken by
selectivity. Another work [11] considered the effect of Al3+ sieving. The samples were analyzed using a scanning electron
on chromite flotation in detail. Collectors studied included microscope (SEM) and their micrographs are presented in Fig. 1.
laurate, dodecyl amine and several amphoteric surfactants. Their chemical analysis showed that chromite was very pure; so
The application of fluoride in the process was patented a was serpentine (42.5% SiO2 , 42% MgO and 1% FeO). Iron was
long time now [12]. The effects of pretreatment and aging on the admixture in the latter case. Scanning electron microscopy
chromite flotation were studied with sodium dodecyl sulphate was used for inspecting the morphology of samples. The instru-
and dodecyl ammonium chloride as collectors [13]. ment used was a JEOL JSM 6300 equipped with an Oxford ISIS
The main interest in the flotation properties of serpentine 2000 energy dispersive system (EDS).
(i.e. a hydrated magnesium silicate) arises from the fact that it A modified-type of Hallimond tube was the flotation micro-
is a common accessory mineral of chromite; serpentine (antig- cell used for the batch experiments [18]; each time with 2 g of
orite) belongs to the sheet silicates [14]. In the latter handbook, mineral in 220 mL solution of deionized water. Pure sodium
the importance of cations in solution when attempting to sep- oleate (denoted hereafter as SO) was the flotation collector
arate these two minerals was also stressed. A beneficiation applied in this work. The solution pH was carefully controlled
process of chromite ore was published, for the flotation of olivine by aqueous solutions of sodium hydroxide or sulphuric acid.
(i.e. magnesium iron orthosilicate) by dodecylamine [15]; with As modifying reagents carboxymethyl cellulose (CMC), fluo-
this, however, the chromite concentrate appeared to have a rosilic acid (FSA), ethylene diamine-tetraacetic acid (EDTA) as
low grade due to serpentine, which was following chromite the sodium salt, dextrin and cyclodextrin has been tried. The
and did not float. Some studies were also reported by IGME contact and mixing time with the surfactant was kept constant
(in Greece) [16]; cationic flotation gave poor results, while at 600 s, as found appropriate, plus a contact of another 600 s,
anionic flotation was successful using fatty acids to alkaline when a modifier was also added, while the flotation time was
media. 120 s. The obtained results in the concentrate were expressed as
The chromite deposits of Greece occur exclusively in ophi- flotation recovery (R, %), in the normal manner.
olite complexes [17]. The objective of current work is to study
the effect of pH, collector concentration, type and concentration 3. Results and discussion
of modifiers on the flotation behaviour of chromite and establish
the conditions where selective flotation from gangue minerals 3.1. Anionic flotation
(i.e. serpentine) would be possible. Preliminary results of this
work were presented at the “Froth flotation/Dissolved-air flota- The stable surface structure of chromite should theoretically
tion: Bridging the gap” Conference organised by UEF, at Tahoe favour the physical adsorption of the collector. Depending on
City California (20–25 May 2001). the ionic nature of the collector (anionic or cationic) and on the
surface charge of individual minerals, the collector is attracted
2. Experimental or repelled from the surfaces. The potential determining ions of
chromite and associated gangue are H+ and OH− . Their ratio, or
Samples of naturally occurring chromite, with theoretical pH, determines the potential at the Stern-layer (zeta-potential).
composition FeO·Cr2 O3 and serpentine, Mg3 [Si2 O5 ](OH)4 , The collector ions act as counter ions and are attracted to the sur-
were tested. Both samples came from the Skoumtsa area, near face if the surface is oppositely charged or near to the isoelectric
Kozani (Northern Greece), following careful hand collection. point (zero Stern-potential). Good selectivity is achieved in the
After crushing in a jaw crusher in the Laboratory and grinding region where collector is adsorbed only on chromite surfaces.

Fig. 1. SEM images of (a) chromite and (b) serpentine.

234 G.P. Gallios et al. / Separation and Purification Technology 55 (2007) 232–237

In the above conditions, the electrokinetic (zeta) potential is an

indication of flotation or non-flotation [7].
The point of zero charge (PZC) of chromite was around
3.0–3.5, which was slightly lower than the PZC values reported
in literature [19–20,13]. The reported PZC values generally
varied from 4.2 to 7.7, dependent on the mineralogical com-
position of the mineral. The presence of sodium oleate caused
the zeta potential to shift to more negative values, even when the
zeta potential was initially above the mineral’s PZC. Therefore,
adsorption of collector on mineral surfaces was not encouraged
by electrostatic repulsions. Chemisorption of oleate is well doc-
umented in flotation practice of oxides and silicates [10], or
salt-type minerals [21]. It was further claimed by Fuerstenau
and Palmer [9] that flotation of a negatively charged mineral
using an anionic collector can take place if a hydroxy complex
of a heavy metal is present over a certain range of concentration Fig. 3. Effect of sodium oleate dose on the flotation of chromite.
of the complex; the latter is a function of the pH for a given
total concentration of that metal. Oxidation of Fe(II) to Fe(III) tion of chromite (at pH values 4–6 the flotation was depressed).
on the mineral’s surface can be also expected. One can assume, As a consequence, the chromite fines were not sufficiently
for example, that the flotation obtained may be due to activa- hydrophobic to be floated. Surfactant is used as adsorbate
tion of the mineral via the charged iron hydroxy complexes, for at the solid–liquid interface in order to control the surface
samples aged in air for long periods of time; the floatability of charge and/or hydrophobic/hydrophilic character of the surface.
chromite started at lower pH values than that of serpentine. The Adsorption occurs by either electrostatic interactions between
metal generally could be derived from the mineral to be floated the surfactant head group and oppositely charged surface or by
itself or from other minerals present in the ore (or can be added chemical interaction between surfactant and mineral. The first
intentionally). From the various impurities, the divalent cations type is known as physical adsorption or physisorption and the
would be expected to dissolve more readily than the trivalent second type is referred to chemical adsorption or chemisorption
cations. The mechanism of hydroxy complex adsorption was [22]. Moreover, as it is evident from Fig. 2, the use of SDS and
extensively discussed [14]. CTMAB as flotation collectors, is strongly dependent on pH, a
Recovery of the floated chromite using sodium oleate, sodium disadvantage for practical operation.
dodecyl sulphate (SDS) (literature results) and cetyl trimethyl Fig. 3 shows that chromite floated quantitatively with
ammonium bromide (CTMAB) – literature results for compari- 20 mg L−1 sodium oleate (or more) at pH 8.0, so there is no
son reasons, although different material [20] – as function of point of increasing the collector concentration. Lower collector
pH at constant collector concentration, are shown in Fig. 2. doses decreased the flotation recovery probably meant that not
Chromite floated quantitatively with 20 mg L−1 SO at pH values enough surfactant was available to form a monomolecular layer
8–12, while acidic pH values affected significantly the flota- on particle surfaces.
The effect of pretreatment on chromite flotation was com-
mented [13]. For instance, aging the ore in a stockpile will
probably change its flotation characteristics. The found decrease
in anionic flotation with increased conditioning in collector solu-
tion was related to increased dissolution of surface metal species
[23]. It was elsewhere reported by Manser [14] that serpentine
is capable of activating other minerals when using anionic col-
lectors, as it releases magnesium ions and is readily attacked by
acids. Surfactants, such as for instance the fatty acids and their
soaps, certainly hydrolyse in aqueous solutions.

3.2. Influence of the modifier

In another paper [24], carboxymethyl cellulose was added as

modifier in flotation of magnesium carbonates; it was reported
that dolomite was depressed in the alkaline pH range, while
magnesite was not affected. This modifying action during pro-
cessing was attributed to carboxymethyl cellulose preferential
Fig. 2. Flotation recovery of chromite (using SO, SDS and CTMAB as col- adsorption on sites where calcium existed (i.e. dolomite, in that
lectors) as a function of pH. The comparative results were reprinted with kind case). At alkaline pH, CMC is known to be ionized completely
permission; copyright Elsevier, 2004 [20]. and possesses a high negative charge.
 G.P. Gallios et al. / Separation and Purification Technology 55 (2007) 232–237 235

action in pH values 4–6. EDTA (Na salt) has a double action and
increases chromite’s flotation significantly at pH values 5–7.5,
while decreases recovery at pH values 11–12. It is also noted
that no action is observed at pH values 8–10. Dextrin has a
strong depressing effect in the alkaline pH range depressing
chromite’s flotation by 55% at pH 10–12, while cyclodextrin
activates chromite’s flotation by around 30% at pH values 5–7.5

3.3. Selective flotation

Chromite–serpentine separation was performed applying the

Fig. 4. Effect of type of modifier and pH value on chromite flotation.
froth flotation method, in which chromite was concentrated in
the concentrate and serpentine was enriched in the tailings.
In preliminary tests, it was found that the modifier should
The modifying action of carboxymethyl cellulose, fluorosilic
be first added and the pulp conditioned, followed by the col-
acid, ethylene diamine-tetraacetic acid, dextrin and cyclodex-
lector addition and new conditioning; the obtained difference
trin during chromite flotation by SO is presented in Fig. 4. As
in results with the two modes was significant. From collec-
it results from this figure CMC has a strong depressing effect
tor abstraction experiments (unpublished results) also, it was
on chromite’s flotation at alkaline pH values, depressing flota-
observed that the presence of modifier in the pulp was reduc-
tion by 50% at pH 11, while it does not have any significant
ing the adsorption of SO collector on the mineral surface. It
effect at pH values 4–7. FSA increases chromite flotation at
was reported [23], during similar studies on other chromite
pH values 5–7. The best increase (around 40%) is achieved at
gangue minerals (olivine, diopside, bronzite and hedenbergite),
pH 6, while it does not affect chromite’s flotation at pH values
that adsorption only very roughly corresponded to flotation
8.5–12. With 30 mg L−1 collector FSA has a strong activating
results.

Fig. 5. Floatability of the two minerals in presence of carboxymethyl cellulose Fig. 6. Floatability of the two minerals in presence of fluorosilic acid as function
as a function of: (a) pH and (b) collector (SO) concentration. of: (a) pH and (b) collector (SO) concentration.
236 G.P. Gallios et al. / Separation and Purification Technology 55 (2007) 232–237

ies of minerals differ significantly at pH values 6–10 with the

best difference around pH 8. At the peak value of pH 8 it seems
possible to concentrate chromite from serpentine gangue. The
effect of collector concentration on minerals flotation at pH 8,
is performed in Fig. 5b. The results of the figure indicate that
using 100 mg L−1 CMC there are differences of more than 60%
in flotation recoveries of both minerals, while the best condi-
tions for separation are 100 mg L−1 CMC and 30–40 mg L−1
collector.
Following the same rationale, the modifying action of flu-
orosilic acid as function of pH, as well as the effect of its
concentration during chromite and serpentine flotation using
sodium oleate as collector are presented in Fig. 6, respectively.
FSA seems to be an effective medium for selective separation
of chromite from serpentine. At pH values 6–10 the flotation
recoveries differ by around 65% and chromite floated almost
quantitatively. The best conditions for selective separation of
chromite from chromite–serpentine mixture were achieved at
pH 8.0 using 20–30 mg L−1 collector (around 70% difference in
recoveries). At pH 9.0 with 30 mg L−1 FSA there are differences
of around 60% in flotation recoveries with 15–25 mg L−1 collec-
tor, while increasing the concentration of FSA a small depression
is observed on chromite, while serpentine is not affected at all.
Similar experiments were realized using the sodium salt of
ethylene diamine-tetraacetic acid as modifier. From Fig. 7a it
is evident that EDTA is also an effective reagent for minerals
selective separation. The flotation recoveries of the two minerals
differ significantly at all the studied pH range. At pH 8.0 using
30 mg L−1 EDTA the conditions seem to be very promising for
Fig. 7. Floatability of the two minerals in presence of EDTA as function of: (a) selective separation and the difference in recoveries is around
pH and (b) collector (SO) concentration. 80% (Fig. 7b). Selective separation seems to be possible also
at pH 9.0; however the differences in flotation recoveries are
The modifying action of carboxymethyl cellulose during smaller.
minerals (chromite and serpentine) flotation by sodium oleate As far as the depressing effect of dextrin on chromite selec-
is presented in Fig. 5a. CMC adsorption on the minerals gave tive separation, it was found that is possible at pH values 7.0–8.0
a more negative charge to the surfaces (unpublished data). The (Fig. 8). At this pH range the flotation recoveries difference of
influence was more noticeable in the case of serpentine. The minerals is about 60%, thus their separation is feasible. More-
simultaneous existence of the collector (with the modifier in over, cyclodextrin can also be used for selective separation of
the pulp) resulted in almost similar values. Flotation recover-

Fig. 9. Floatability of the two minerals in presence of cyclodextrin as function

Fig. 8. Floatability of the two minerals in presence of dextrin as function of pH. of pH.
 G.P. Gallios et al. / Separation and Purification Technology 55 (2007) 232–237 237

minerals as the differences in flotation recoveries are around References

65% at pH value 7.0 (Fig. 9).
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