Flotation behavior of nickel arsenide

Flotation behavior of nickel arsenide

International Journal o[ Mineral Processing, 18 (1986) 191-202 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands 191 Flotat...

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International Journal o[ Mineral Processing, 18 (1986) 191-202 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands


Flotation B e h a v i o r of N i c k e l A r s e n i d e H. NAKAZAWA and I. IWASAKI

Mineral Resources Research Center, University of Minnesota, Minneapolis, M N 55455, U.S.A. {Received November 14, 1985; revised and accepted April 15, 1986)

ABSTRACT Nakazawa, H. and Iwasaki, I., 1986. Flotation behavior of nickel arsenide. Int. J. Miner. Process., 18: 191-202.

Platinum group minerals are closelyassociated with nickel arsenides in copper-nickel sulfide bearing Duluth gabbro. To improve their flotation recoveries,the flotationbehavior of nickel arsenide was investigated.Nickel arsenide floated well with xanthate and mercaptan. However, nickel arsenide was extremely susceptible to oxidation and its floatabilitydecreased markedly. X P S indicated that surface coating containing nickel hydroxide and arsenic trioxidewas formed on oxidized nickel arsenide. Its floatabilitycould be improved by using inert atmosphere and copper sulfateactivation.Flotation behavior of cuprous arsenide, X P S and S E M suggested the formation of cuprous arsenide at the surface.


In a previous article, a cursory attempt was made to study the flotation behavior of nickel arsenide in order to improve its flotation recovery since platinum group metals were found to be closely associated with this mineral in copper-nickel bearing Duluth gabbro (Iwasaki et al., 1987). It was noted that nickel arsenide oxidized rapidly and lowered its floatability. The flotation recovery, however, could be improved by using an inert atmosphere and copper activation. In this article attempts were made to establish the effect of oxidation on the surface properties of nickel arsenide through rest potential, solubility and zeta potential measurements, and flotation tests. Since naturally occurring arsenide mineral grains were too small to be isolated for use in the experiment, a synthetic nickel arsenide was used. X-ray photoelectron spectroscopy and scanning electron microscopy were used to study the surface conditions of nickel arsenide.


© 1986 Elsevier Science Publishers B.V.


A synthetic nickel arsenide used in this investigation was obtained from Gallard-Schlesinger Chemical Mfg. Corp., Carle Place, New York. The nickel arsenide was reported to analyze 49.93% Ni and to consist of Nil~Ass as the major component with a minor amount of Ni.~As2 through X-ray diffraction analysis. For Hallimond-tube flotation tests, these samples were ground to 200/325 mesh in a porcelain mortar and pestle in air immediately before use. One gram of sample was conditioned with a collector solution in a 100-mL pyrex flask by tumbling for five minutes and floated for five minutes. The collectors employed in the study were potassium ethyl xanthate (KEX), potassium n-butyl xanthate (KBX), both prepared in the usual manner (Foster, 1928). Since mercaptan is reported to form insoluble nickel mercaptide (Gaudin, 1957), 1-dodecane thiol (DDT), obtained from Eastman Kodak Company, was also used. For rest-potential measurements, a nickel arsenide electrode was prepared by mounting a sample in a 6.25-mm ID lucite tubing with about 20 mm 2 of the area exposed. The electrode surface was cleaned by polishing with 400 and 600 grit abrasive papers and ~hen on a metallurgical polishing wheel with 0.5/tm alumina powder as the abrasive. The measurements were made with respect to a saturated calomel electrode (SCE) either in distilled water or in 10 :~mol/L sodium sulfate solution using an EG&G model 350A corrosion measurement console. Surface analysis techniques were used to study the surface condition of nickel arsenide. A Physical Electronics 555 general purpose surface analysis instrument was used for X-ray photoelectron spectroscopy (XPS). The C (ls) peak at 284.6 eV was used for charge referencing. A Hitachi S-450 scanning electron spectroscope (SEM) was used to examine the morphology of the surface. Samples for surface analyses were prepared in the following manner: (a) fresh NiAs denotes nickel arsenide ground to 200/325 mesh; (b) NiAs-Air indicates the particles held in aerated water for a given time period; (c) NiAs-Air-HC1 denotes the particles oxidized in aerated water for 2 hours and then conditioned with hydrochloric acid of pH 4 for 5 rain; (d) Ni(OH)2 represents the precipitate obtained by adjusting the pH of nickel sulfate solution; (e) Ni(OH)z-As(III) and Ni(OH),~-As(V) denote nickel hydroxide conditioned with sodium arsenite or sodium arsenate solution (As concentrations 200 ppm); and (f) Ni(II)-As(III) denotes the precipitate obtained by adjusting pH of a solution that contained 200 ppm each of Ni(II) and As(III) ions. The precipitate was washed with distilled water several times by centrifugation-decantation. All the samples except for fresh nickel arsenide were dried in a freeze drier after freezing with liquid nitrogen and kept in a vacuum desiccator until analysis. A Pen Kern Lazer Zee meter, Model 501, was used for electrophoresis measurements, In this experiment, - 325 mesh fraction of nickel arsenide (0.05%

193 TABLE 1 Rest potential of nickel arsenide and pyrrhotite Bubbling gas

NiAs (mV vs SCE)

Pyrrhotite (mV vs SCE)

Distilled water

N2 Air 02

- 72 - 70 - 44

+ 31 + 40 + 73

10- 3M Na2S04

N2 Air 02

-93 -91 - 88

+17 +32 + 63

solids by weight) was conditioned in a given solution for five minutes. After conditioning, the coarse particles were allowed to settle and the supernatant containing the fine particles was transferred into the electrophoresis cell. 10- ~ mol/L KC1 was used as a supporting electrolyte. Hydrochloric acid and potassium hydroxide were used for p H adjustments. RESULTS

Rest-potential measurements: The rest potentials of nickel arsenide and pyrrhotite in distilled water and in 10 -3 m o l / L sodium sulfate solution aerated with nitrogen, air or oxygen are given in Table 1. Nickel arsenide was appreciably more active electrochemically than pyrrhotite. The rest potentials of nickel arsenide were higher in the oxygenated solution than in the deoxygenated solution. In fact, in line with the above observations of their electrochemical activities, it was determined that nickel arsenide oxidized more readily than pyrrhotite by dissolved oxygen in distilled water (Iwasaki et al., 1987).

Solubility measurements: To investigate the extent of oxidation, a test was carried out by placing nickel arsenide in aerated distilled water as follows: one gram of 200/325 mesh particles of nickel arsenide was placed in a 150-mL beaker together with 100 mL of distilled water aerated with air at a flowrate of 200 mL/min. After aeration for a given time period, the solution was filtered and the filtrate was analyzed for nickel and arsenic with atomic absorption spectroscopy. The results are shown in Table 2. The concentrations of nickel and arsenic increased to 60 min. Hamels et al. (1973) investigated the mechanism of dissolution of nickel arsenide in acidic solutions and reported that the dissolution occurred sto-


TABLE 2 Dissolution of nickel arsenide in aerated distilled water Aeration time (min)

As (mol/L)

Ni (tool/L)

Ni cone. obtained from K. of Ni(OH):,


30 60 120

5.00x10' 5.86x10 ' 3.63x10 :~

4.74x10 ~' 5.31~10 :' 2.93/10 '

6.37>(10' 2.54y10 i.01xl0 "

8.2 8.4 8.6

ichiometrically and that arsenic existed in the solution as arsenite. However, their concentrations decreased after aeration fbr 120 min. Since nickel concentrations, calculated from the solubility product of Ni(OH)2 (K~p= 10 ,s s), are close to the experimental values after aeration of 120 min, nickel ion might be precipitating as nickel hydroxide. Arsenic ion concentration must have decreased by adsorption on the nickel hydroxide precipitate. It is well known that arsenic ion adsorbs strongly on the hydroxide precipitates of iron, magnesium or aluminum (Mellor, 1929; Miyamoto et al., 1973). Flotation tests

Flotation tests were carried out using KEX, KBX and D D T at natural pH of 7.5 - - 9. The results are shown in Fig. 1 as a function of the collector concentration. Although the recoveries increased as the collector concentration increased, KBX and D D T were superior to K E X for nickel arsenide. Figure 2(a) shows the flotation recoveries of nickel arsenide after 5 and 60 min of agitation in aerated water at different pH followed by conditioning with 7 X 10 4 mol/L KBX. - lOO [ -



- -



oo°t ':

o~- ..... 10-6

~f,,, lO-5

. . . . . .

J lO-4

, ~ .......

I lO,3


, ,~,,~


Fig. 1. Flotation recoveries of nickel arsenide as functions of potassium ethyl xanthate (KEX), potassium butyl xanthate (KBX) and dodecane thiol (DDT) concentrations.

195 100

~" k~ 8 0 ' °w

60 ~



Agitation "-0-


o 20

[ 2

5 ~0 I

I 4



I 8


-401 (b)

_,op o~- .... ,o t 2O 1 0


I 2

6 pH

I 8

I 10

I 12

Fig. 2. (a) Flotationrecoveriesof nickelarsenideas a functionofpH. (b) Zetapotentialsof nickel arsenideas a functionof pH. In Fig. 2(b) the zeta potentials of nickel arsenide corresponding to the flotation tests are shown. Nickel arsenide had an isoelectric point (IEP) of about pH 3. After 5 min of agitation the addition of KBX made the zeta potentials more negative. Above pH 10, the curves in the presence of KBX tended to join the curve in the absence of KBX implying that the collector did not adsorb effectively. These zeta potential behaviors parallel with the flotation behaviors shown in Fig. 2(a). Such an observation resembles that of Moignard et al. (1977), who reported that at high pH the zeta potentials of NiS approached those of Ni(OH)2 formed on the surface by oxidation. After 60 rain of agitation, the zeta potentials were somewhat less negative than before and the difference between the zeta potentials in the absence and in the presence of the collector was very small above pH 8, again in agreement with the flotation behavior in that pH region. To investigate the effect of oxidation, the flotation tests were carried out as a function of aeration time using KBX and DDT as collectors. The results are shown in Fig. 3. The floatability decreased as the aeration time increased. Since nickel arsenide liberated nickel ion and arsenic ion by oxidation, a flotation test was carried out by removing the supernatant solution by decantation and replacing with distilled water prior to conditioning with KBX. The results are

196 1C:,


, MAs KBX • NiA~- KBX [AerGled woter de(:on~e~ E] NiAs- DDT A Cu

BO, \ >.:

o~6oi W


~_ 40 < [ o ;_ d

, oJ 0

i ~




' ~ ~ - ~ - - J 60 AERATION TIME, Min.


Fig. 3. F l o t a t i o n recoveries o f nickel a r s e n i d e a n d c u p r o u s a r s e n i d e as a f u n c t i o n of a e r a t i o n t i m e ( K B X 7 × 10 4 tool/L; D D T 1 × 10 ~ tool/L).

included in Fig. 3. The floatability of nickel arsenide improved only slightly indicating that dissolved species had minimal effect. To explore if the floatability of nickel arsenide oxidized in aerated water for one hour may be improved by increasing the KBX concentration, a series of flotation tests was carried out. The flotation recovery of oxidized nickel arsenide increased as the KBX concentration increased and reached 95% at 5 × 10 -:~ mol/L KBX. The recovery of the nickel arsenide conditioned with 5>( 10 -3 mol/L KBX, however, decreased to 35% after one hour of bubbling with air. Such an observation suggests that nickel arsenide will not stay floatable under oxidizing conditions even if the collector addition is increased and, therefore, its floatability will be lost in the cleaner flotation step. Formanek and Lauvernier (1963) reported that copper activation followed by sulfidizing was effective on the flotation of cobalt arsenide containing over 3 - - 4% iron. A possibility of activating nickel arsenide, whose floatability was lowered by aeration for one hour, was examined by conditioning with different concentrations of copper sulfate for 5 min and then by floating as before. The results, depicted in Fig. 4, indicate that copper sulfate activation improved the flotation recovery, which reached 85% at 10 -4 mol/L. However, a further increase in copper sulfate concentration decreased the flotation recovery due to the precipitatioa of the collector by excess copper ion in solution. Replacement of the supernatant solution after activation with distilled water improved the flotation recovery.









0 Residual Cu++Included

~>- 8O





/ / ~ ) ~

~r uJ > o 6o


o 4o


20 0

[,~[ 10.6


n , ninth[ 10-5

a [ in[nHI 10"4


I 4 x 10-4

Cu++CONCENTRATION, r n o l / L

Fig. 4. Effect of copper sulfate concentration on notation recovery of nickel arsenide (conditioning time l 0 rain; K B X 7 X 10 4 moVL).

Surface analysis Since nickel hydroxide might be formed on the surface of oxidized nickel arsenide, nickel hydroxide precipitates prepared in different ways were analyzed by XPS. The results are shown in Fig. 5. All Ni(2p) spectra of precipitates showed intense peaks at the binding energies characteristic of nickel hydroxide (B.E. = 856.6 eV and 862.4 eV [Kim and Winogrand, 1974]). The As(3d) spectra of the precipitates showed that arsenic ion was adsorbed on nickel hydroxide and existed as arsenic pentoxide or arsenic trioxide (As203, B.E. = 44.4 eV, As20~, B.E. =46.1 eV [Bahl et al., 1976]).

Ni (2p) "~/~ Ni(OH) 2 - - Ni(OH)2+ As(~/) - - - Ni(OH)z+As(lll) ....... Ni(ll)+ As (Ill) -



~ ,


'~;o' sVo'sLo's~o


.'" i

.sI d,

/ / !Ii



" "'~.i




48 44 ENERGY, eV


Fig. 5. X P S spectra for nickel and arsenic of different nickel precipitates.

198 Aeration o

Iotation r~------7 Recovery_ 9~__2

50 - - ~ 56





120 91


o) Ni(2p) Nihs









850 860


b) As(3d) NiAS



-~ --~, ~ 40




40 ~-44




414 J ~ 4 0


Fig. 6. XPS spectra for nickel and arsenic of nickel arsenide aerated for different periods of tirae,

Ni(2p) and As(3d) spectra of nickel arsenide prepared by placing in aerated water for different periods of time are shown in Fig. 6 together with respective flotation recovery data using 7 × 10 -4 mol/L KBX. In the Ni(2p) spectrum of fresh nickel arsenide, an intense peak was noted at about 852 eV {characteristic of NiAs). When nickel arsenide was exposed to aerated water, other intense peaks at the binding energies characteristic of nickel hydroxide appeared, and the intensity of these peaks increased whereas that of nickel arsenide decreased as the exposure time increased. In the As(3d) spectrum of fresh nickel arsenide, an intense peak was noted at about 42 eV (characteristic of nickel arsenide). The spectra of nickel arsenides that were exposed to aerated water showed a peak characteristic of As203, and the intensity of the peak increased as the exposure time increased. Argon sputtering of the surface decreased the peaks of nickel hydroxide and of arsenic trioxide simultaneously. It is evident that flotation recoverie ~ decreased as the intensity of the peaks representing nickel hydroxide and arsenic trioxide increased. Such an observation of increased formation of nickel hydroxide coating also agrees well with the zeta potential data of Fig. 2 (b). When nickel arsenide aerated for two hours was conditioned


col2 . . . . . . .



- NiA,.-Ai, CuSO,'-,,., / I --" Cu3As ~"'-'J L 9?0





As(3d) '~ NiAs-Air - CuSO4 ~ # - - ' - NiAs ]r~











416 [ 42


BINDINGENERGY,eV Fig. 7. XPS spectra for nickel and arsellic of cuprous arsenide and nitRe| &rsenide activated with copper sulfate.

with hydrochloric acid solution of pH 4 for 5 min, the peaks of nickel hydroxide and of arsenic trioxide decreased. Acid conditioning of oxidized nickel arsenide must have removed the reaction product layer and restored its floatability. Figure 7 shows the As(3d) and Cu(2p) spectra of nickel arsenide aerated for one hour and then activated with 10- 4 mol/L copper sulfate solution are shown together with those of fresh nickel arsenide and synthetic cuprous arsenide (Atomergic Chemetals Corp., New York). In the As(3d) spectrum of activated nickel arsenide, an intense peak appeared at a binding energy between those of nickel arsenide and cuprous arsenide, and a shoulder at about 46 eV, characteristic of As2Os, was noted. In the Cu(2p) spectra of activated nickel arsenide, two intense peaks appeared at the same binding energies as the peaks of cuprous arsenide. Frost et al. (1972) analyzed 46 copper compounds and showed that Cu(II) compounds had Cu(2p3n) binding energy in the range of 934.6 - 939.2 eV. Hence, copper may be present mainly as Cu(I) on the activated nickel arsenide. Copper peaks were observed even after argon sputtering of 30 s at an estimated etching rate of 0.2 nm/min. The SEM photomicrographs, shown in Fig. 8, indicate that fresh mineral surface had some minute angular particles of the mineral. Nickel arsenide after two hours in aerated water showed many minute precipitates. The morphology of the precipitates on the surface of copper-activated nickel arsenide was quite different from that on the surface of nickel arsenide exposed to aerated water, and the population of cubic precipitates increased.


Fig. 8. Scanning electron micrographs of nickel arsenide surfhces: (a) Freshly ground: (bi tw,, hours in aerated water: (c) copper activation fifth)wing one hour in aerate(| water. DISCUSSION

The flotation responses of arsenide minerals have not been well documented. Only Formanek and Lauvernier (1963) reported that cobalt arsenides were quite sensitive to conditioning and the presence of a few percent iron necessitated copper activation in flotation with amyl xanthate as a collector. Nickel arsenide floated well with xanthate and mercaptan so long as its surface was unoxidized. The floatability of nickel arsenide depended on the length of hydrocarbon chain of xanthate; KBX was superior to KEX as a collector. Pilipenko et al. (1957) gave the solubility products of nickel ethyl xanthate and nickel butyl xanthate as 1.37 x: 10 12and 4.51 × 10 is, respectively. The results of zeta-potential measurements showed that zeta potentials of nickel arsenide became more negative as the concentration of KBX increased, indicating increased adsorption of xanthate on nickel arsenide. According to Critchley and Straker (1981), zeta potentials of nickel xanthate were negative in the pH range of 3 - - 9 and in the presence of excess xanthate of 6 X 10 4 mol/L, and charge reversal occurred at higher pH through the hydrolysis of nickel xanthate to nickel hydroxide. The results of dissolved oxygen uptake by nickel arsenide and of dissolution experiment showed that nickel arsenide was susceptible to oxidation and liberated nickel ion and arsenic ion upon oxidation. Shishkin et al. (1963) investigated the oxidation of arsenides in distilled water and noted that rammelsbergite (NiA%) and smaltite-chloanthite (Co, Ni, Fe) A%3 oxidized rapidly whereas safflorite (Co, Fe) A% and niccolite (NiAs) oxidized slowly. Formanek and Lauvernier (1963) noted that cobalt arsenides oxidized readily and that mine water contained arsenic. In this study it was shown that oxidation decreased the floatability of nickel arsenide markedly. Surface analysis using XPS showed that surface coating consisting of nickel hydroxide and arsenic trioxide was formed on oxidized nickel arsenide, and that the intensi-

201 ties of nickel hydroxide and arsenic trioxide peaks increased whereas that of nickel arsenide decreased as aeration time increased. These results suggest that the surface coating interferes with collector adsorption and thereby requires much more collector to render the nickel arsenide surface hydrophobic. It has been reported that copper activation was effective on cobalt arsenide and arsenopyrite (Formanek and Lauvernier, 1963; Abeidu and Almahdy, 1979). In this study, copper activation was shown to improve the flotation recovery of oxidized nickel arsenide. But the mechanism of activation by copper sulfate is not clear. Abeidu and Almahdy (1979) pointed out the ability of both Cu(I) and Cu(II) to precipitate As(III) in the form of insoluble copper arsenides, such as Cu3As and Cu3As2. Surface analysis using XPS showed that in the Cu(2p) spectrum of activated nickel arsenide, intense peaks appeared at the same binding energies as the peaks of cuprous arsenide. The binding energy of Cu20 (B.E. =932.4 eV [McIntyre et al., 1976]) is close to that of cuprous arsenide, and, therefore, it is difficult to distinguish them. Apparently, Cu(I) in the spectrum is inherent to the activated nickel arsenide and is not formed through the photo-reduction of Cu(II) during the analysis. Folkesson et al. (1983) and Perera et al. (1980) showed that no beam reduction for Cu(II) salts had been observed under average operating conditions. The As(3d) spectrum of activated nickel arsenide indicated that arsenic trioxide, arsenic pentoxide and cuprous arsenide might be present on the surface. Since cuprous arsenide floated well with KBX regardless of aeration time (Fig. 4), the cuprous arsenide formation might be improving the floatability of oxidized nickel arsenide. Thermodynamically, arsenite (HAs02) should be oxidized to arsenate (H3As04) in the presence of air, but arsenite is reported to be the stable species over a wide pH range (Tozawa and Nishimura, 1967). Cu(II) is known to catalyze the oxidation of arsenite. Reinders and Vles (1925) noted that the oxidation rate increased as the alkalinity of solution increased though it was immeasurably slow in acid and neutral solutions in the presence of Cu(II), and that the catalysis of Cu salts was caused by slow reduction of Cu(II) in cupric arsenite complex to Cu(I) by arsenite. Ponomareva et al. (1971) reported that CuO catalyzed the oxidation of arsenite resulting in the reduction of CuO to Cu20 in sodium hydroxide solutions. Woods et al. (1965) investigated the kinetics of this oxidation based on an assumption that As (IV) is an intermediate and that Cu(II) was reduced to Cu(I). These investigations suggest that Cu(II) might be reduced to some extent to Cu(I) through the oxidation of arsenite to arsenate. Although copper arsenate (Cu3(AsO4)2) is insoluble (K~p=10 -~5"29) (Lur'e, 1965), there was no direct evidence of its formation on the nickel arsenide surface. ACKNOWLEDGMENT This work was performed as a part of a project funded by the Legislative Commission on Minnesota Resources, the State of Minnesota.

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