A vibration spectroscopic study of the interaction between some sulphide minerals and O,O-diethyl dithiophosphate ions in aqueous solution

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 83 (1994) 221-236 0927-7757/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved.

221

A vibration spectroscopic study of the interaction between some sulphide minerals and 0, O-diethyl dithiophosphate ions in aqueous solution Mats Valli, Bo Malmensten,

Ingmar

Persson*

Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015, S-750 07 Uppsala, Sweden (Received

25 September

1992; accepted

1 November

1993)

Abstract The reactions between sodium O,O-diethyl dithiophosphate and the natural sulphide minerals acanthite, arsenopyrite, chalcocite, chalcopyrite, covellite, galena, marcasite, millerite, molybdenite, orpiment, pentlandite, pyrrhotite, pyrite, realgar, sphalerite and troilite have been studied in aqueous solution. Qualitative analysis of the species present on the surfaces before and after treatment with O,O-diethyl dithiophosphate ions has been performed by means of diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Nuclear magnetic resonance (NMR) has been used to identify some of the O,O-diethyl dithiophosphate species formed on the mineral surfaces, O,O-Diethyl dithiophosphate ions are sorbed to the surfaces of the studied minerals according to one of the following two reaction mechanisms: (1) formation of a solid metal O,O-diethyl dithiophosphate with low solubility in water, and (2) formation of bis(O,Odiethyl dithiophosphoryl) disulphide, the oxidation product of the 0,0-diethyl dithiophosphate ion in a redox reaction with highly oxidising species such as the S,Oi- and/or S,O:- ions. The results of this study have made it possible to predict the mechanism of how oxidised sulphide mineral surfaces interact with O,O-dialkyl dithiophosphate ions from the knowledge of the shortest SS distance in the sulphide mineral. When the SS distance is very short as in the disulphide ion, about 2.2 A, fairly large amounts of highly oxidising species are formed in the mineral surface during grinding in presence of air and these cause formation of large amounts of bis(O,O-diethyl dithiophosphoryl) disulphide during treatment with O,O-dialkyl dithiophosphate ions in aqueous solution. When the shortest S-S distance is in the range 3.1-3.4 A, which is significantly shorter than the S-S distances in close-packed structures containing only sulphide ions (greater than 3.6 A), small amounts of bis(O,O-diethyl dithiophosphoryl) disulphide are formed on the surfaces. When the shortest SS distance is longer than 3.6 A, no bis(O,O-diethyl dithiophosphoryl) disulphide is formed on the mineral surfaces and only precipitation of a solid metal O,O-dialkyl dithiophosphate on the surface can take place. No evidence for chemisorption of O,O-dialkyl dithiophosphate ions to sulphide minerals as surface complexes has been found. Key words: O,O-Diethyl

dithiophosphates;

Sulphide

minerals;

Vibration

Introduction The most commonly used collectors for sulphide minerals in the mining industry are the alkali alkylxanthates. However, they are not effective enough to separate all sulphide minerals from each other. They lack selectivity when copper and silver *Corresponding

author.

SSDI 0927-7757(93)02702-G

spectroscopy

sulphide minerals are to be separated from some other sulphide minerals, e.g. the separation of chalcopyrite from pyrite. Other collectors have therefore been developed to complement the alkali alkylxanthates and such a group is the alkali O,Odialkyl dithiophosphates. The chemical properties of the O,O-dialkyl dithiophosphate ions are very similar to those of the alkylxanthate ions. The O,O-dialkyl dithiophos-

M. Valli et al./Colloids

228

phate

ions are expected

eral surfaces according

to modify

sulphide

min-

to the same mechanisms

as

Surfaces A: Physicochem.

to the alkylxanthate

ions. The efficiency and recov-

ery of some O,O-dialkyl

the alkylxanthate ions do [l-3]. The alkylxanthate ions can be sorbed to a sulphide mineral surface according to one or two of the following three

have also been reported

reaction

found

mechanisms:

alkylxanthate the origin

(i) formation

with low solubility of the metal

of a solid metal in water,

ions is soluble

metal

where salts

Eng. Aspects 83 ( 1994 ) 227-236

dithiophosphate by Numata

collectors and Mamiya

C61. Solid copper(I) containing copyrite

O,O-diethyl

dithiophosphate

is

to be formed on the surfaces of the copperminerals during

covellite,

treatment

chalcocite with aqueous

and chalsodium

on the oxidised mineral surfaces [ 11; (ii) formation of dialkyl dixanthogen, which is the oxidation product of alkylxanthate ions in a redox reaction with highly oxidising ions, e.g. S20iP and/or S,O:-; these strong oxidising agents are present on the surfaces of oxidised sulphide minerals when the shortest SS distance in the crystal structure of the sulphide mineral is shorter than 3.4 A [a]; (iii) formation of chemisorbed surface complexes

O,O-diethyl dithiophosphate solutions [4,5,7-91. X-ray photoelectron spectroscopic measurements have shown that the copper in the surfaces of these minerals is present as copper(I), and that the copper dissolved from the mineral surfaces reacts with O,O-diethyl dithiophosphate to form copper(I) O,O-diethyl dithiophosphate as the only O,O-diethyl dithiophosphate species [lo]. The sorption mechanism on the chalcocite, covellite

where the alkylxanthate ions are coordinated to specific metal ion sites in the surfaces via interactions with a fairly high degree of covalency [3]. The mechanisms can be distinguished by identifying the products on the mineral surfaces before and after the treatment with a collector by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy since the possible products have significantly different vibrational spectra, and from the solution chemistry behaviour of the species formed on the mineral surfaces [l-3]. There are few reports on the reactions between alkali O,O-dialkyl dithiophosphate collectors and sulphide minerals compared with those on the corresponding reactions of the alkali alkylxanthates. A short summary of the most important results from studies on the sorption of alkali O,Odialkyl dithiophosphates on sulphide mineral surfaces will be given here. The effectiveness in terms of flotation rate, using the O,O-dialkyl dithiophosphate ions as collectors in comparison with the alkylxanthate ions, has been investigated by Ackerman et al. [4]. The sorption process of thiol collectors, including the O,O-dialkyl dithiophosphate ions, has been studied in terms of recovery and rest potential by Fuerstenau et al. [S]. They reported that the O,Odialkyl dithiophosphate ions react in a similar way

and chalcopyrite surfaces with O,O-diethyl dithiophosphate ions is therefore expected to be the same dissolution-precipitation mechanism as previously described for the corresponding alkylxanthate systems [ l,ll-131. This means that copper(I) species are dissolved during the slurrying of these minerals, and that a concentration gradient of copper(l) species is formed around the copper-containing mineral particles. When O,O-diethyl dithiophosphate ions are present in the aqueous phase during the dissolution, the solubility product of copper( 1) O,O-diethyl dithiophosphate is first exceeded in close vicinity to the desired mineral particles, and solid copper O,O-diethyl dithiophosphate precipitates on the particle surface. It has been shown that small islands of a hydrophobic metal collector salt are formed on the mineral surfaces [ 141. Such formation of islands of a highly hydrophobic species is most probably enough to make the mineral particles sufficiently hydrophobic to be attached to an air bubble, and thereby able to float. It can be assumed that the degree of coverage of hydrophobic species on the mineral surface necessary for a successful flotation increases with increasing particle size and weight. The reaction between oxidised pyrite or marcasite, both with the composition FeS,, and O,Odiethyl dithiophosphate ions in aqueous slurries

M. Vulli et al./Colloids Surfaces A: Physicochem. Eng. Aspects 83 ( 1994 ) 227-236

yields phide,

bis(O,O-diethyl dithiophosphoryl) disulan oxidation product of the O,O-diethyl

dithiophosphate reported that hydrophobated phosphate iron(II1)

ion [4,5,8,15]. A single study has pyrite and pyrrhotite, FeS,_,, are with iron(II1)

[ 151. O,O-diethyl

However,

O,O-diethyl the

dithiophosphate

dithio-

solubility

of

is estimated

to be fairly high [ 163 and it does not seem likely that this compound can precipitate at the O,Odiethyl dithiophosphate concentrations used in flotation processes. It has been possible to postulate very accurately the surface modification mechanisms of different sulphide minerals during treatment with alkylxanthate ions in aqueous slurries by comparing the species formed on the surfaces of sulphide minerals with the shortest SS distance in the sulphide mineral structure [ 171. The structures of sulphide minerals can be divided into three classes; minerals with (a) disulphide ions, Ss-, (b) clusters or sheets of sulphur atoms with S-S distances shorter than 3.4 A, and (c) close-packed sulphide ions with S-S distances longer than 3.6 A. When class (a) and (b) sulphide minerals are oxidised, e.g. during grinding in the presence of air, a powerful oxidising agent is formed on the surfaces of the minerals. This oxidising agent has not yet been characterised, but it is most probably S,Oiand/or S& ions. These ions are both possible side products in the oxidation of the disulphide ion, and they are both known to be able to dialkyl dixanthogen x ~6 do not have potential (in aqueous

oxidise alkylxanthate ions to [lS]. The S20zP ions with a sufficiently high oxidation solution) to oxidise alkylxan-

thate and O,O-diethyl dithiophosphate ions [ 2,181. This reaction overrules all other possible reactions between alkylxanthate or O,O-diethyl dithiophosphate ions with species on or in the surface. Among the minerals in this study, pyrite, marcasite and covellite belong to class (a), and millerite, molybdenite, pyrrhotite, pentlandite, realgar, orpiment and arsenopyrite belong to class (b). Sulphide minerals with isolated sulphide ions, class (c) sulphide minerals, may undergo two kinds of reaction during the mineral processing: (a) the

sulphide

229

mineral

may

be oxidised

to relatively

soluble metal sulphate and/or thiosulphate, e.g. during grinding in the presence of air, and a subsequent precipitation of a metal collector salt can take place if this compound has a sufficiently low solubility (the minerals galena, chalcocite, acanthite

and chalcopyrite

belong

to this group of

sulphide minerals); (b) chemisorbed alkylxanthate surface complexes may be formed during treatment with

an

aqueous

alkali

alkylxanthate

solution,

when the metal ions in the mineral surface are able to form a specific bond with a high degree of covalency to the ligand. Formation of a chemisorbed surface complex has been shown for the sphalerite-alkylxanthate and cadmium sulphidealkylxanthate systems under conditions similar to those in practical flotation [3]. The precipitation reaction overrules in most cases the formation of chemisorbed surface complexes. The aim of this study is to increase the basic knowledge on the interaction between O,O-dialkyl dithiophosphate collectors and sulphide mineral surfaces. An understanding of the reaction mechanisms of the surface modification of sulphide minerals with collectors such as the alkylxanthate and O,O-dialkyl dithiophosphate ions is essential for the development of more efficient collectors and flotation processes. This study is part of a larger investigation concerning the sorption of sulphurcontaining collectors on sulphide mineral surfaces [l-3,11-13,17,19]. This study is focused on the reactions between O,O-diethyl dithiophosphate ions and 16 sulphide minerals. The O,O-diethyl dithiophosphate species formed have been identified by means of DRIFT and nuclear magnetic resonance (NMR) spectroscopy. Comparisons with the corresponding reactions with alkylxanthate ions are made. Experimental Materials Sodium O,O-diethyl pared from O,O-diethyl

dithiophosphate dithiophosphoric

was preacid (syn-

M. Valli et al./Colloids

230

thetic grade, carbonate

> 98%, Merck, Germany) (analytical

grade,

Merck)

and sodium in ethanol

(99.5%). The excess of water and remaining O,Odiethyl dithiophosphoric acid were evaporated off and

the

end-product

was

dried

under

reduced

pressure over phosphorus pentoxide at room temperature. The melting point was 188.8-189.7”C. Copper(I),

iron(III),

lead(I1)

and zinc(I1)

O,O-

diethyl dithiophosphates were prepared by mixing aqueous solutions of Cu(CH,CN),ClO, [20], FeCl, (analytical grade, Riedel-de Ha&n, Germany), Pb( NO,), (analytical grade, Merck) and Zn( NO,), (analytical grade, Mallinckrodt, UK), respectively, and an aqueous solution of sodium O,O-diethyl dithiophosphate. The products were washed with several portions of ethanol. The melting points were 74.5575.7”C and 77.0-78.3”C for lead(I1) O,O-diethyl dithiophosphate and zinc(I1) O,Odiethyl dithiophosphate respectively. Iron( III) and copper(I) O,O-diethyl dithiophosphates both seem to decompose when heated. Bis(O,O-diethyl dithiophosphoryl) disulphide, was prepared by oxidising an ((C,H,G),PS,),> aqueous solution of sodium O,O-diethyl dithiophosphate with a slightly alkaline iodine-potassium iodide solution. Bis(O,O-diethyl dithiophosphoryl) disulphide was extracted from the reaction mixture with diethyl ether [21-241. The product was identified by ‘H and 13C NMR spectroscopy. The refractive index ni” was found to be 1.5600, which is in agreement with earlier reports [25,26]. Mineral powders were prepared large natural crystals of acanthite, chalcocite,

chalcopyrite,

covellite,

by crushing arsenopyrite, galena,

marca-

site, orpiment, pyrite, realgar, sphalerite and troilite (Wards Natural Science, USA) and millerite (National Museum of Natural Science, Stockholm, Sweden) into smaller pieces and the pieces free from visible impurities were selected and ground manually in a porcelain mortar. The obtained powders were ground dry for 5 min with a Retsch Spectra Mill in a 2 ml agate mortar. The powders were sifted with a Retsch Sieving Machine and the fraction 32245 urn (32-63 urn for covellite) was used in the experiments. The sifted material was

Surfaces

stored

A: Physicochem.

under

Eng. Aspects 83 ( 1994 ) 227-236

reduced

room temperature.

pressure

ders were freshly prepared ment in order to minimise Pentlandite pared mixture

oxidation

[1,3].

powders

large crystals,

were pre-

containing

in the same way as described was ground

at

pow-

before every measure-

and pyrrhotite

by crushing

minerals,

in a desiccator

The galena and sphalerite

to a fraction

both

above. The

less than 250 urn

and the two minerals

were separated

by the use of

a magnet;

is ferromagnetic

while pent-

landite

pyrrhotite

is diamagnetic

[27].

The minerals

were

further ground after the separation, and the fraction 32-45 urn was used in the experiments. Molybdenite

powder was prepared

sheets of molybdenite ground

manually

by separating

from each other. These were

to a fraction

less than

250 urn,

which was used in the experiments. Potassium

bromide

(spectroscopic

was used in the DRIFT

grade, Merck)

spectroscopic

studies. The

potassium bromide was ground and sifted, and the fraction 32245 urn was used. The potassium bromide vacuum

was stored

in a desiccator

in order to minimise

The organic

solvents

de Ha&n and purification.

Merck)

Deionised

at 120°C

uptake

(analytical

in

of water. grade,

Riedel-

were used without

further

water was used throughout

this study.

DRIFTspectroscopy

The diffuse reflectance a Perkin-Elmer equipped Scientific detector

spectra were obtained

1760X FTIR

spectrometer.

with a diffuse reflectance Corp., (E.G.

compartment Complete

bromide

range

USA).

MCT

The sample

was purged with air, which had been

dried and cleaned in a CO, removal and

unit (Harrick

USA) and a narrow & E. Judson,

on

It was

Protection

Ltd,

dryer (Carrier

UK).

was used as non-absorbing

Potassium matrix

and

background. The DRIFT spectra were obtained from a mixture of 25 mg of the sample and 500 mg potassium averaging

bromide. All spectra were recorded 128 scans at a resolution of 4 cm-‘.

by

M. Valli et al./Colloids Surfaces A: Physicochem. Eng. Aspects 83 (1994 ) 227-236

231

Nuclear magnetic resonance NMR

was used to identify

sodium

O,O-diethyl

diethiophosphate and bis(O,O-diethyl dithiophosphoryl) disulphide. ‘H and i3C spectra were recorded NMR

on a Varian XL 400 MHz high resolution

spectrometer.

The measurements methylsilane

CDCl,

was used as solvent.

were made relative to the tetra-

(TMS)

peak.

Reactions between O,O-dialkyl dithiophosphate and sulphide minerals in aqueous slurries

ions

diethyl dithiophosphate was increased to 10.0 mM for sphalerite. The adsorbed species were identified by comparison with spectra of synthetically prepared reference samples. Results and discussion Sodium O,O-diethyl dithiophosphate was oxidised by an aqueous iodineepotassium iodide solubis-(O,O-diethyl dithiophosphoryl) tion to disulphide: R-O

S

\

\//

2

//” “\ /“-” +

A R-O

G=====+ S’

/p\ R-O

2 e-

/p\ S-S

I

I

I,

1,

4

The mineral powders were treated with an aqueous solution of sodium O,O-diethyl dithiophosphate ( 1.0 mM) for 15 min and the mineral was then filtered off. The concentration of sodium O,O-

R-O

I

O-R

The product was identified by means of ‘H and 13C NMR spectroscopy. The presence of an AA’XX’X”X”‘YY’Y”Y”’ system as reported by Komber et al. [22] was confirmed (see Fig. 1). The ‘H NMR shifts and the coupling constants are given in Table 1. The two -CH,protons in the ethyl groups are diastereotopic, and they have a coupling constant of 10.20 Hz. They couple not only with each other but also with the CH, protons in the ethyl groups and with the phosphorus. Since they have different shifts the peak splitting is quite complex. However, the complexity is somewhat reduced because the vicinal coupling constant and

I

I,

I,

3

I

I,

I

2

I

I

-A I,

I,,

1

,

,

0

Fig. 1. ‘H NMR spectrum of bis(O,O-diethyl dithiophosphoryl) disulphide in CDCl,. The spin system is AA’XX’X”X”‘YY’Y”Y”‘,

the coupling constant to the phosphorus are of the same magnitude (see Table 1). Solid copper(I) O,O-diethyl dithiophosphate is the sole O,O-diethyl dithiophosphate species found on the surfaces of the copper-containing minerals chalcocite (Cu,S), chalcopyrite (CuFeS,) and covellite (CuS) during treatment with sodium O,Odiethyl dithiophosphate in aqueous slurries (see Fig. 2). This is in accordance with the previously reported results [4-61 and with the results obtained during treatment of these coppercontaining sulphide minerals with alkylxanthate ions in aqueous slurries [ ll-13,281. This shows that these mineral surfaces are hydrophobated by solid copper(I) O,O-dialkyl dithiophosphate according to the same reaction mechanism as previously described for these minerals with alkylxanthate ions [ 1 l-131. No traces of iron(II1) O,Odiethyl dithiophosphate have been observed on chalcopyrite surfaces as determined by DRIFT spectroscopy (see Fig. 2). The reactions of the iron-containing sulphide minerals pyrite (FeS,) and marcasite (Fe&) with O,O-dialkyl dithiophosphate ions in aqueous slurries yield bis(O,O-diethyl dithiophosphoryl) disulphide as the sole O,O-dialkyl dithiophosphate species whereas the minerals arsenopyrite (FeAsS), pyrrhotite (Fe& ox, pentlandite ((Fe,Ni),S,) and troilite (FeS) yield no detectable reaction product (see Fig. 3 and Table 2). The formation of bis(O,Odiethyl dithiophosphoryl) disulphide is certainly

M. Vulli et al./Colloids

232 Table 1 ‘H NMR

shifts (ppm) and coupling Shift

Proton

(Hz) of bis(O,O-diethyl

Coupling

1.375 4.200 4.312

CH,-CI&Ht, CH,-C&H, CH,-C&H,

constants

Surfaces A: Physicochem.

dithio-phosphoryl)

Eng. Aspects 83 ( 1994) 227-236

disulphide

constant

CH,-C&H,

CH,-CH,H,

CH,-CH,H,-OP

7.10

7.10 10.20

0.65 10.20 10.20

_

-

L C

d

e

I

I

1200

I

I

1100

,,,,I,

loo0

!wo

800

V/cm-’ Fig. 2. DRIFT spectra of covellite (curve a), chalcocite (curve b) and chalcopyrite (curve c) treated with a 1.0 mM aqueous solution of sodium O,O-diethyl dithiophosphate after substraction of the spectrum of the untreated mineral. The DRIFT spectrum of solid copper(I) O,O-diethyl dithiophosphate (curve d) is given as reference. The ordinate scale is in log(R,,r/R sample)units and is arbitrary.

I

1300

I

1200

I,,,,,,,

1100

1lXlO

590

800

V/cm-’

due to the fact that some highly oxidising species are formed on the surfaces, e.g. during grinding in air, when the shortest S-S distance in the mineral is less than 3.6 A. It seems that the distribution of oxidation products of sulphur in sulphide minerals, mainly sulphate ions, and small amounts of other

Fig. 3. DRIFT spectra of pyrite (curve a), marcasite (curve b), pentlandite (curve c). pyrrhotite (curve d) and troilite (curve e) treated with a 1.0 mM aqueous solution of sodium O,O-diethyl dithiophosphate after subtraction of the spectrum of the untreated mineral. The DRIFT spectrum of bis(O,O-diethyl dithiophosphoryl) disulphide (curve f) is given as reference. The ordinate scale is in log(R,,,/R sample)units and is arbitrary.

M. Vulli et al./Colloids

Surfaces A: Physicochem.

Eng. Aspects 83 (1994)

227-236

233

Table 2 Sulphur-sulphur distances (A) in some sulphide minerals, and the product expected and obtained during minerals with aqueous solutions of potassium alkylxanthate and sodium O,O-diethyl dithiophosphate Mineral

Formula

Acanthite Arsenopyrite Chalcocite Chalcopyrite Covellite Galena Marcasite Millerite Molybdenite Orpiment Pentlandite Pyrrhotite Pyrite Realgar Sphalerite Troilite aNo product

Ag,S FeAsS cu,s CuFeS, cus PbS Fe& NiS MoS, As& (Fe,Ni)$s FeS,_, FeS, ASS ZnS FeS observed.

Shortest SS distance

4.135 3.197 3.710 3.685 2.084, 3.757 4.194 2.223 3.244 3.154 3.242 3.362 3.390 2.177 3.295 3.821 3.348

treatment

of oxidised sulphide

Product Expected

Observed

AgX X, cux cux cux, x, PbX,

AgX 8.b

X, X* X* X* X, X, X, X, X, X,

X, X, X, _a.b _a.b

cux cux cux PbX,

X, X, a.b _a.b _a.b

bSee text for comment,

oxosulphur compounds including the highly oxidising ions S,O$- and/or S,O:-, is a function of the length of the shortest S-S distance in the sulphide mineral. The fraction of the sulphur oxidised to SzOg- and/or S@ions seems to be significant when the S-S distance is very short as in the disulphide ions, while the fraction is considerably less when the shortest S-S distance is in the range 3.1-3.4 A. When the shortest S-S distance is longer than 3.6 A, as in sulphide minerals with close-packed sulphide ions, the oxidation products of sulphur are oxosulphur ions that cannot oxidise O,O-dialkyl dithiophosphate ions. The sorption mechanism of pyrite with O,O-diethyl dithiophosphate ions will in principle be the same as that described for the sorption of alkylxanthate ions on this mineral surface [2,4,5,8,15]. The sorption mechanism of pyrite with alkyl-xanthate ions can be summarised as follows [2]. (i) The disulphide ions in pyrite are oxidised to oxosulphur anions with high oxidation potentials, most probably SzOg- and/or S,O:-. This oxidising agent is not dissolved from the surface before the reaction with alkylxanthate ions takes place. (ii) The alkylxanthate ions are oxidised to dialkyl dixanthogen by the oxidising agent on the mineral surface.

(iii) The highly hydrophobic dialkyl dixanthogen formed is physisorbed on the mineral surface in aqueous slurries. No reaction seems to occur between troilite and O,O-dialkyl dithiophosphate ions in aqueous slurries as no O,O-dialkyl dithiophosphate species is present on the troilite surfaces (see Fig. 3); similarly alkylxanthate species were not found on the troilite surfaces after a corresponding treatment with aqueous solution of potassium alkylxanthate [ 121. The amounts of iron dissolved are too small to allow precipitation of solid iron(II1) O,O-diethyl dithiophosphate on the troilite surfaces. None or very small amounts of S& and/or S20G- ions seem to be present on the troilite surfaces as no bis(O,Odiethyl dithiophosphoryl) disulphide has been detected. Consequently the troilite surfaces will not be modified by alkylxanthate and O,O-diethyl dithiophosphate ions. The shortest S-S distances in the minerals arsenopyrite (FeAsS), millerite (NiS), molybdenite (MO&), realgar (ASS) and orpiment (As,&) are in the range 3.1-3.4 A, and small amounts of bis(O,Odiethyl dithiophosphoryl) disulphide are expected to be formed on the surfaces of these minerals during treatment with sodium O,O-diethyl dithio-

234

M. Valli et al./Colloids

phosphate in aqueous slurries. Small amounts of bis(O,O-diethyl dithiophosphoryl) disulphide are indeed formed on the surfaces of molybdenite, while no O,O-diethyl dithiophosphate species was detected on the surfaces of millerite, arsenopyrite, realgar and orpiment when treated with O,O-dialkyl dithiophosphate ions (see Fig. 4). The amounts of bis(O,O-diethyl dithiophosphoryl) disulphide expected to be formed on these mineral surfaces are small, and obviously too small to exceed the solubility in aqueous solution. It should probably be possible to detect bis(O,O-dialkyl dithiophosphoryl) disulphide with longer alkyl chains. In the studies on the same minerals with alkylxanthate ions, didecyl dixanthogen was detected on these

Surfaces A: Physicochem.

Eng. Aspects 83 (1994)

227-236

mineral surfaces while diethyl dixanthogen was not [ 131. This is certainly because the solubility of dialkyl dixanthogens in water decreases with increasing number of carbon atoms in the alkyl chain [29]. Solid lead(I1) O,O-diethyl dithiophosphate is formed on the surfaces when galena, natural PbS, is treated with an aqueous solution of sodium O,Odiethyl dithiophosphate (see Fig. 5). No O,O-dialkyl dithiophosphate species were detected on the galena surfaces after treatment with O,O-diethyl dithiophosphate ions in acetone solution. This implies that no O,O-dialkyl dithiophosphate surface complexes are formed. This is in contrast to the properties of alkylxanthate ions, which do form chemisorbed surface complexes on galena surfaces in acetone solution [ 161. This means that galena displays the very same reactions and thereby also

a

b

I

1 1500

1400

1300

12rnl

1100

loo0

9OLl

I

I

1200

I

I,,,,,,

1100

loo0

900

800

800

_ v I cm-’

Fig. 4. DRIFT spectra of millerite (curve a), molybdenite (curve b), arsenopyrite (curve c) and orpiment (curve d) treated with a 1.0 mM aqueous solution of sodium O,O-diethyl dithiophosphate after subtraction of the spectrum of the untreated mineral. The DRIFT spectrum of bis(O,O-diethyl dithiophosphoryl) disulphide (curve e) is given as reference. The ordinate scale is in log(R,,JR,,,,& units and is arbitrary.

V/cm-t Fig. 5. DRIFT spectra of galena (curve a) and sphalerite (curve b) treated with 1.0 mM and 10.0 mM aqueous solutions of sodium O,O-diethyl dithiophosphate, respectively, after subtraction of the spectrum of the untreated mineral. The DRIFT spectra of solid lead(I1) O,O-diethyl dithiophosphate (curve c) and solid zinc(I1) O,O-diethyl dithiophosphate (curve d) are given as references. The ordinate scale is in log(&dR sample)units and is arbitrary.

M. Valli et al./Colloids

Surfaces A: Physicochem.

Eng. Aspects 83 ( 1994 ) 227-236

the same sorption mechanism with O,O-dialkyl dithiophosphate ions as previously reported for alkylxanthate ions in aqueous media. Sphalerite, natural ZnS, does not seem to react with the O,O-diethyl dithiophosphate ions at all. No O,O-diethyl dithiophosphate species were found on the sphalerite surface after treatment with an aqueous solution of 1.0 mM sodium O,Odiethyl dithiophosphate (see Fig. 5). The experiment was repeated at an increased concentration (10.0 mM) but the sphalerite surfaces remained free from O,O-diethyl dithiophosphate species. The ability of a ligand to form surface complexes depends mainly on its electron donor properties, and thus its ability to form strong covalent interactions. The electron donor properties of bidentate dithio ligands have been found to increase in the order O,O-dialkyl dithiophosphate (( RO),-PS;) < NJ-dialkyl dithiocarbamate (R,N-CS;) < alkyl thioxanthate (R-S-CS;) < alkylxanthate (R-OCS;) < dithiocarboxylate (R-CS;) [ 301. A preliminary study of the HgBr,($COC,H,), and HgBr,(S,P(OC,H,),), complexes in the solid state shows that the ethylxanthate ions are markedly more strongly coordinated to the soft acceptor mercury(I1) than the O,O-dialkyl dithiophosphate ions. The vi (Hg-Br) stretching vibrations are found at 164 cm-’ and 177 cm-’ respectively (cf. results in Refs. 31 and 32). The O,O-dialkyl dithiophosphate ions are the weakest electron donors of the dithio ligands, and they will form weaker covalent interactions and thereby also weaker surface complexes than the alkylxanthate ions. The alkylxanthate surface complexes on sphalerite and synthetic zinc and cadmium sulphides are fairly weak [3], and it is therefore not surprising that no O,O-dialkyl dithiophosphate complexes are formed.

235

ing species

such

of oxidised present

minerals

ions,

the shortest

S-S dis-

is less than 3.4 A. Copper

minerals

rite and covellite.

S@-

on the surfaces is

in the surfaces of the oxidised

copper-containing

chalcocite,

Copper(I)

and a concentration

chalcopy-

species are dissolved

gradient

of copper(I)

species

around the copper-containing sulphide mineral particles is formed during slurrying in aqueous solutions

of sodium

O,O-diethyl

dithiophosphate,

and solid copper(I) O,O-diethyl dithiophosphate precipitates on the surface. Galena surfaces are modified according to the same reaction mechanism by solid lead(I1) O,O-diethyl dithiophosphate. Troilite

and sphalerite

O,O-diethyl

do not seem to react with

dithiophosphate

ries. Both bis(O,O-diethyl phide

and

dithiophosphates of even relatively species

on

mineral

particles

the

metal

of

slurdisul-

O,O-diethyl

are hydrophobic and formation small amounts of some of these mineral

surfaces

will

make

the

hydrophobic.

Like the alkylxanthate nism

ions in aqueous dithiophosphoryl)

solid

O,O-diethyl

ions, the reaction dithiophosphate

mechaions

is

strongly dependent on the structure of the mineral. If the mineral contains disulphide ions, large amounts

of bis(O,O-diethyl

sulphide

are formed,

amounts

are formed

tance

is in the range

bis(O,O-diethyl formed when mineral

dithiophosphoryl)

while when

substantially the shortest

structure

dismaller

S-S

dis-

3.1-3.4 A (see Table 2). No

dithiophosphoryl) the shortest S-S

disulphide is distance in the

is 3.6 A or longer. If metal species

from the mineral

ried in an aqueous

This investigation has shown that the chemical properties of the O,O-diethyl dithiophosphate ions are very similar to those of the alkylxanthate ions. They are oxidised to bis(O,O-diethyl dithiophosphoryl) disulphide and dialkyl dixanthogen, respectively, in these pure systems by highly oxidis-

where

as copper(I)

and

are present

tance in the structure

are dissolved Conclusions

as the SzOi-

which most probably

solution

surfaces when slur-

containing

O,O-diethyl

dithiophosphate ions, solid metal O,O-dialkyl dithiophosphate salts can be formed on the surfaces if these salts have a sufficiently low solubility in water.

Solid

copper(I)

and

lead(I1)

O,O-diethyl

dithiophosphate are formed on copper-containing minerals and galena respectively. No indications of the formation of surface complexes with O,Odialkyl dithiophosphate ions have been found on

M. Valli et al./Colloids

236

the surfaces of the minerals regardless of solvent used.

examined

in this study

10 11 12

Acknowledgements

13

The financial support from the Swedish National Board of Technical development is gratefully acknowledged. Dr. Bengt Lindqvist at the National Museum of Natural Science is gratefully acknowledged

for

providing

the

millerite

sample.

The

Skandinaviska Enskilda Bankens Foundation for Economic and Technical Research is acknowledged for its support of the FTIR instrument.

References 1 2 3 4 5 6 7 8 9

P. Persson and I. Persson, Colloids Surfaces, 58 (1991) 161. M. Valli, P. Persson and I. Persson, Colloids Surfaces, 59 (1991) 293. P. Persson, B. Mahnensten and I. Persson, J. Chem. Sot., Faraday Trans., 87 (1991) 2769. P.K. Ackerman, G.H. Harris, R.R. Klimpel and F.F. Aplan, Int. J. Mineral. Process, 21 (1991) 105. M.C. Fuerstenau, J.L. Huiatt and M.C. Kuhn, Trans. Sot. Min. Eng. AIME, 250 (1971) 227. Y. Numata and M. Mamiya, Kozangakubu Kenkyu Hokoku (Akita Daigaku), 3 (1982) 9. J.O. Leppinen, Valt. Tek. Tutkimuskeskus, Report 726, 1991, 23 pp. L.A. Goold and N.P. Finkelstein, National Institute of Metallurgy, Johannesburg, Rep. No. 1439, 1972, 9 pp. L.Y. Shubov, LB. Zalesnik, S.I. Mitrofanov and T.I. Moiseeva, Tsvetn Metall., (1974) 64.

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Surfaces A: Physicochem.

Eng. Aspects 83 (1994)

227-236

R.K. Clifford, K.L. Purdy and J.D. Miller, AIChE Symp. Ser., 71 (1975) 138. P. Persson and B. Malmensten, Colloids Surfaces, 59 (1991) 279. M. Valli and I. Persson, Colloids Surfaces A: Physicochem. Eng. Aspects, 83 (1994) 199. M. Valli and I. Persson. Colloids Surfaces A: Physicochem. Eng. Aspects, 83 (1994) 207. I.N. Plaksin, C.V. Bessonov and V.I. Tiurnikova, Bull. Acad. Sci. SSSR, Tech. Sci. Sect., No. 3 (1957). M.C. Fuerstenau, US Bureau of Mines Information Circular, 8818 (1980) 7. E. Stamboliadis and T. Salman, Trans. Sot. Min. Eng. AIME, 260 (1976) 250. M. Valli, B. Malmensten and I. Persson, Colloids Surfaces, in press. LG. Farbenindustrie, German Patent 678 732, 1936; Deutsches Reichspatent Org. Chem., 6 (1936) 1468. P. Persson and I. Persson, J. Chem. Sot., Faraday Trans., 87 (1991) 2779. P. Hemmerich and C. Sigwart, Experientia, 19 (1963) 488. A.J. Bridgewater, J.R. Dever and M.D. Sexton, J. Chem. Sot., Perkin Trans. 2, (1980) 1006. H. Komber, G. Grossman and A. Kretschmer, Phosphorus Sulfur, 35 (1988) 335. A.N. Shishkov, N.K. Nikolov and A.I. Busev, Nauchni Tr. Plovdivski Univ., Mat., Fiz., Khim., Biol., 10 (1972) 117. A.N. Shishkov and N.K. Nikolov, Nauchni Tr. Plovdivski Univ., Mat.. Fiz., Khim., Biol., 10 (1972) 87. P.F. Hu and W.Y. Cheng, Acta Chim. Sin., 22 (1956) 215. L. Almasi and A. Hantz, Monatsh. Chem., 100 (1960) 798. C. Klein and C.S. Hurlbut, Jr., Manual of Mineralogy, 20th edn., Wiley, New York, 1985, pp. 278 and 280. E. Baumgarten, Chem.-Ing.-Tech., 44 (1972) 613. I.C. Hamilton and R. Woods, Aust. J. Chem., 32 (1979) 217. R.D. Bereman, M.L. Good, B.J. Kalbacher and J. Buttone, Inorg. Chem., 15 (1976) 618. I. Persson, M. Sandstrom and P.L. Goggin, Inorg. Chim. Acta, 129 (1987) 183. M. Sandstrom, I. Persson and P. Persson, Acta Chem. Stand., 44 (1990) 653.