A new look at characterisation and oxidative ammonia leaching behaviour of multimetal sulphides

Minerals Engineering, Vol. 11, No. 11, pp. 1011-1024, 1998

Pergamon 0892-687$(98)00089--2

© 1998 Elseviex Science Ltd All rights reserved os92-6875/gs~ - - see front matter

A NEW LOOK AT CHARACTERISATION AND OXIDATIVE AMMONIA LEACHING BEHAVIOUR OF MULTIMETAL SULPHIDES

K. S A R V E S W A R A R A O and H.S. R A Y Regional Research Laboratory (C.S.I.R), Bhubaneswar 751 013, Orissa, India E-mail root @csrrlbhu.ren.nic.in (Received 22 December 1997; accepted 3 August 1998)

ABSTRACT

Systematic studies have been made to understand the oxidation behaviour and dissolution reaction mechanisms operative while pure copper, zinc and lead sulphide minerals and mixtures, and single concentrates~mixtures are treated by oxidative ammonia leaching. The information thus obtained is useful for characterisation of both the reactants and the products. During the leaching of bulk concentrate, it is possible to obtain high extraction rates ( > 95%) for copper from chalcopyrite, zinc from sphalerite, and lead from galena. Pyrite remains in the leach residue along with oxid~sed lead compounds and goethite is formed d~e to oxidation of iron from chalcopyrite. © 1998 Elsevier Science Ltd. All rights reserved Keywords Sulphide ores; hydrometallurgy; leaching; oxidation; particle size

INTRODUCTION The use of ammonia lixiviant, which provides mild oxidative leaching conditions, offers good scope for selective/total le~:hing of sulphide minerals. Selective leaching often results in dissolution of only the readily soluble metal species and not the sulphide sulphur. ,Accordingly, it may be possible to improve copper recoveries in the mill, and also produce high-quality copper concentrates for the smelter [1]. Any detailed investigation aimed at the dissolution of metal values from complex sulphide ores would be of importance theoretically and industrially. Presently, there is renewed emphasis in the search for newer reasoning to re-examine ammonia based processes in the light of advances made in various leaching studies. For example, there are some interesting questions about the Anaconda Arbiter Process [2] which have not been fully answered. These pertain to the sequence of oxidation reactions, the inertness of pyrite grains, nature of particle size distribution in leach residues due to likely in-situ grinding during leaching.

To provide reasonable answers to these aspects, oxidative ammonia leaching studies have been carded out with the following main objectives:

1011

1012

l) 2)

3)

K. Sarveswara Rao and H. S. Ray

To obtain leaching data on some pure sulphide minerals (CuS, FeS2, ZnS and PbS), single sulphide concentrates such as chalcopyrite (CuFeS2), sphalerite (ZnS), galena (PbS) and pyrite (FeS2), and bulk concentrates, and then to compare the data with those available in the literature. To obtain leaching data on binary, ternary and quaternary mixtures of the above sulphide minerals and to understand the possible mutual interference of reactions, if any, and the actual sequence of oxidation reactions. To use an interdisciplinary approach comprising chemical phase analysis, X-ray diffraction, thermal analysis, optical microscopy and surface area measurements to characterise partial leach residues and compare the data thus obtained with those from the original untreated bulk concentrate to identify mineral phases that disappear and/or the products that form during leaching.

In ammonia leaching, the oxidation of sulphur in sulphide minerals is rather complicated. For various reasons, the order of oxidation as estimated from the reaction potential does not agree with those determined experimentally. This is more true for leaching a bulk concentrate wherein many sulphide minerals are associated as mechanical mixtures. Majima and Peters [3] have studied oxidation rates and compared the oxidation order of single sulphide minerals in terms of decreasing order of oxidisability in ammonia at elevated temperatures as: Cu2S > CuS > CusFeS4 > CuFeS2 > Sb2S3 > PbS > FeS = FeS2 = ZnS. Subsequently, Tozawa et al. [4] made attempts to describe the order of oxidation reaction for complex sulphide bulk concentrate. Without referring to pyrite and galena minerals, they have indicated the order to be CuS > Cu2S > CuFeS2 > ZnS > NiS > Ag2S. There appears to be some ambiguity about the oxidation of pyrite in ammoniacal medium. The oxidation rate of pyrite is stated to be lower than for other minerals initially but subsequently it perhaps reacts at the same rate as pyrrhotite (FeS) and sphalerite [3]. The oxidation rate in terms of mineral consumed rather than oxygen consumed could be as low as half that of pyrrhotite and sphalerite. All these factors combined would lead to an overall observation that pyrite is one of the least attacked minerals during ammonia leaching [3]. However, it is also reported that the pyrite does become leached to some extent [4], or dissolves significantly in the presence of Cu2S [5]. The present authors [6] have examined the role of galvanic interaction during ammonia leaching of multimetal sulphides whereby pyrite (Py) enhances the dissolution of chalcopyrite (Cp), sphalerite (Sp), and galena (Gn) minerals and is itself nearly inert [6-8]. For ammonia-ammonium sulphate leaching, the oxidation of minerals in bulk concentrate follows an order: Gn > Sp > Cp. As mentioned earlier, Py is inert. This order may be compared to that reported for sulphuric acid leaching of multimetal sulphides [9,10] which is given as: FeS > PbS > ZnS > CuFeS2 > FeS2 > Cu2S > CuS > Ag2S. This information, as available from the literature may still not be sufficient to explain the oxidation behaviour of a bulk concentrate. This is because different mineral samples of the same species may oxidise at different rates during leaching, due to the presence of minor oxidation compounds, and complex sulphide solutions/compounds that remain to some extent in the bulk concentrates even after beneficiation. Moreover, multimetal sulphides having semiconductor properties may give raise to galvanic interactions during leaching and/or flotation as evident from the literature. One should also be aware that several factors can lead to unexpected uncertainties in the leaching data generated. Ray [11] has discussed the sources of some factors that lead to uncertainties in kinetic studies in metallurgical systems and concluded that many kinetic rate expressions have only limited theoretical significance. Even if they represent the kinetic data extremely well, very often these are basically only empirical. On similar lines, Presser [12] has extensively reviewed the literature and identified some possible sources of uncertainties involved in the collection and i n - - r a t i o n of leaching data, and the variables and phenomena that may affect the rate of a mineral leaching process. It has been correctly pointed out by him that all the variables are not expected to influence each system. Yet, at present there is no way to forecast which will. Commonly effects of only about 5 variables have been deliberately studied. Recently, the present authors [13] have discussed the strategy of experimentation to generate meaningful kinetic data during leaching of sulphide minerals from a complex sulphide ore; where, ranges were fixed for eight experimental variables (temperature, agitation, time, ammonia concentration, ammonium sulphate

Characterisation and oxidative ammonia leaching behaviour

1013

addition, pH measurements, oxygen partial pressure, pulp density and particle size). There is a possibility that particles may disintegrate during leaching. Particle splitting may result in exposing greater surface area for further leaching. A thorough investigation is needed on the interrelationship between mineral liberation and leaching behaviour. For these complex materials, one can adopt a multidisciplinary approach and generate useful information as described in our earlier study of the mineralogical characteristics and reactivity of a Cu-Zn-Pb bulk concentrate during roasting, and during oxidative ammonia leaching [7]. The combined application of chemical analysis, X-ray diffractometry, thermal analysis and optical microscopy was more useful for identification of the various mineral phases of the leach residues and for qualification of the leaching reaction mechanism. The present study aims at, a). supplementing Prosser's observations made on leaching of single sulphides, concerning external area (particle size and shape), particle disintegration, galvanic effects, etc., and, then, b). providing additional information pertaining to multimetal sulphides. EXPERIMENTAL Samples used in l~lis study were CuS, ZnS and FeS2 of reagent grade, PbS (pure galena mineral) and natural sulphide concentrates of chalcopyrite, sphalerite and galena. The chalcopyrite and sphalerite concentrates contained 20-30% of pyrite. One bulk concentrate sample (labelled as B) of complex sulphide ore from Ambaji (Gujarat, India) origin and provided by Gujarat Mineral Development Corporation, was used as received without size selection to carry out characterisation and leaching studies. The chemical compositions of reagent grade sulphides, single and bulk concentrates are shown in Table 1. XRD data of these sulphide minerals have been presented in our earlier publication [8]. TABLE 1 Chemical composition of synthetic and natural sulphide minerals Serial No.

Smnple

Source

1.

CulS

Fluka AG

--

2.

Fe~;2

ICN

.

Size (Inn)

.

Cu (%)

Zn (%)

Pb (%)

Fe (%)

S (%)

64.46

--

--

--

32.54

46.08

52.92

.

.

Acid insolubles

(%)

Pharma 3.

ZnlS

Reidel . De Haan

4.

Chalcopyrite

HCL,

concentrate,

Ghatsila

.

.

.

65.75

--

--

32.25

--

-63+45

24.30

--

0.30

34.50

35.92

4.98

-63+45

0.20

49.76

1.13

10.86

37.00

0.10

cp 5.

Sphalerite

HZL,

concentrate,

Udaipur

Sp 6.

Galena pure mineral, Cap

Saintala, Orissa

-63+27

0.03

1.01

84.64

0.26

13.62

0.20

7.

G~Jena

HZL,

-63+45

0.50

4.44

64.84

4.69

17.61

7.35

co ncentrate,

Zawar -106+22.5

3.60

19.96

12.50

13.57

26.20

20.90

-63+45

5.07

29.53

15,50

14.15

32.41

2.73

Gn 8.

9.

BLdk

GMDC,

concentrate B

Oujarat

Mechanical

Cp, Sp and Gn

n~ure

(w,~logue of Sl, No.S)

[

1014

K. SarveswaraRan and H. S. Ray

In the present study the leaching behaviours of pure sulphide minerals, single and bulk concentrates are compared using the uniformly maintained values or ranges of variables, hereafter known as standard leaching conditions, fixed on the basis of earlier reported work of the present authors [6-8,13-16];

a) b) c) d) e) f)

g) h) i)

temperature - - ambient (25 °) to 135°C, agitation, rpm - - 1080 min -~, ammonia concentration - - 3.34 mole/l, ammonium sulphate when added - - 0.34 mole/l, pH - - 11.2 with liquor ammonia, c) as above, 10.1 with d) as above, solid concentration - - 1% for synthetic sulphides & 10% for natural sulphide minerals, and bulk concentrate B, particle size - - as indicated in Table 1, oxygen partial pressure - - 150 kPa, and leaching time ~ 0 to 2 h.

The leaching experiments were carded out in a two litre capacity stainless steel autoclave with controls for temperature and agitation. More details of the equipment are available elsewhere [6--8,13-16]. The leach slurry samples were collected at regular intervals through a sampling tube, filtered; residues were dried and then subjected to XRD and TA studies. The required volume of leach liquor sample was acidified, diluted, and analysed for Cu and Zn by atomic absorption spectrophotometer (AAS, Perkin Elmer Model 372). To determine the products of oxidation of sulphide minerals present in leach residues, the samples were analysed by chemical phase analysis as recommended by Vogel [17] and Steger [18], XRD [8,19] and optical microscopy [7,16]. Diffractograms were obtained using an Automatic Powder Diffractometer (Philips Model PW 1710) in a 20 range of 10-75 ° at a scanning rate of 2°/minute using a copper target. The exposure conditions of the unit and a methodology for interpretation of highly reproducible XRD data for the minerals present in the complex sulphide raw material and in the ammonia leach residues were described in our earlier publications [7,8]. The surface area of powdered or solid porous materials (single sulphide concentrates, their mixtures, bulk concentrate, various leach residues) has been measured with the high speed surface area analyser (Model 2200, Micromeritics, USA). The principle involved in all these measurements is to determine the quantity of gas that is necessary to form a single layer of gas molecules on a representative sample. Testing was accomplished using nitrogen gas at the temperature of liquid nitrogen, as under these conditions the gas molecules are strongly adsorbed on the solid surface and the space occupied by each adsorbed molecule is known within relatively narrow limits. Surface morphology data were also generated on the starting material and some partial leach residues with SEM (Model JSM 35 CF, JEOL, Japan) [15].

RESULTS AND DISCUSSION Characterisation and Leaching of Metal Sulphides and Mixtures Leaching of binary mixture, CuS-FeS 2 (1:4) An attempt was made to study the oxidation of a binary mixture of CuS and FeS 2in the ratio 1:4, analogous to the composition of CuFeS 2, which is the main copper mineral present in bulk concentrate B. This was to study the effect of FeS 2 on CuS dissolution. The oxidation reactions of CuS, and FeS 2 during oxidative ammonia leaching are written as follows [4]. CuS + 4 NH 3 + 2 0 2 = Cu(NH3)4SO 4

(1)

2 FeS 2 + (15/2) 02 + 8 Nil 3 + (4+n) H20 = Fe203 .nH20 + 4 (NI-I4)2SO4

(2)

Figure la shows that more than 90% copper dissolves within 20 minutes and then dissolution is complete by 2 h. There is no dissolution of iron as shown by solution analysis of leach liquor samples. However, the XRD data of the leach residue of CuS ~and FeS 2 mixture collected after 2 h indicates the presence of

Characterisationand oxidativeammonialeachingbehaviour

1015

hematite (Fe203) and traces of elemental sulphur and totalabsence of CuS & FeS 2, (Figure 2a). This shows that FeS2 oxidatic,n,which forms a solid reaction product, Fe20 ~,starts only afar CuS dissolution. The cathodic nature offFeS2 appears to contribute to dissolution of CuS in the initialstages.

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¢x--tplols for (a) copper, and (b) zinc dissolution from synthetic sulphide minerals in oxidative ammonia medium.

Leaching of binary mixture, ZnS-FeS 2 (3:2) There is good lit:rature on ammonia leaching of ZnS [7,14,20]. In the present work leaching behaviour of ZnS in the presence of PeS 2 has been studied. The well known dissolution reaction of ZnS in ammoniacal medium is given by the following equation [4]. ZnS + 4 Nil 3 + 12 02 -- Zn(NH3),SO 4

(3)

Figure lb shows a - t plots for dissolution of ZnS and FeS2 binary mixture. It is seen that about 80% zinc is solubilised within 100 minutes, and in presence of CuS the recovery is around 90%. The leach liquor samples are free of iron. XRD data of the leach residue obtained after 2 h indicates that FeS 2 partially dissolves to give Fe~O3 (Eq.2), and while ZnS is still present as wurtzite (o.--ZnS), elemental sulphur is present in minor amounts (Figure 2b). Surprisingly, the XRD dam of the final leach residue as shown in Figure 2c indicate FeS 2as a major phase. This clearly confirms that pyrite is the least attacked mineral in a mixture conmhfing CuS, ZnS and FeS2. Such an observation has not been reported in the literature before. Due to the cathcdic nature of FeS2,it contributes to dissolution of CuS and ZnS which are anodic.

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XRD patterns of leach residues obtained after oxidative ammonia leaching of synthetic sulphide minerals, for 2 hours. (a) CuS-FeS 2, (b) ZnS-FeS 2, and (c) CuS-ZnS-FeS2. (Symbols: H- hematite, Py- pyrite, W- wurtzite, S- sulphur, and, C- covellite)

Leaching of binary mixture, PbS-FeS 2 The characterisation of the oxidative ammonia leaching behaviour of pure galena mineral, Saintala (Orissa, India) origin, has been recently described [19]. This mineral also contains some minor amounts (0.5%) of FeS2. It is possible to recover about 94% of lead as oxidated compounds within 100 rain. During progressive leaching, the pH value changes from 11.4 to 10.39 in the induction period (25-125°C), and to 10.17 at the end of leaching. XRD and chemical phase analysis data indicated PbSO4 as the major oxidation product (82%), with PbO.PbSO4 (11%), PbS (6%) and FeS2 as minor phases. The initial dissolution data (20 min) of single sulphides of CuS, ZnS, and PbS in the presence of FeS2 indicates the solution metal recoveries at Cu 92%, Zn 45% ~Lndrpb20~ i respectively. This shows the order in terms of decreasing reactivity to be CuS > ZnS > WoS similar to that given in the literature [4]. But, when final recoveries are taken into account, the sequence alters to CuS > PbS > ZnS which matches that reported elsewhere [3]. On the other hand, the sequence Of sulphide minm'al dissolutionfrom a mixture of CuS, ZnS, PbS, and FeS2"follows an order: PbS, CuS, ZnS, and ~ S 2 reminds inert [6]. This explains the ambiguity concerning the oxidation of pyrite in ammoniacal medium.

Characterisationand oxidativeammonialeachingbehaviour

1017

Leaching of Single Sulphide Concentrates and Mixtures Leaching of ehaleopyrite concentrate The well known chemical reaction for copper dissolution from chalcopyrite in oxidative ammonia medium is described as follows [4]. CuFeS 2 + (17/4) 02 + 6 NH 3+ (l+n) H20 = Cu(NH3)4SO4+ (NI-I4)2SO4*J- (1/2) F~O 3 .nH20

(4)

The leaching data obtained in the present study for copper is shown as o,--t plot in Figure 3a. It was possible to solubilise the copper completely within 100 min of leaching. The same Figure also shows the variation of pH in the leaching medium due to the oxidation of sulphur to form ammonium sulphate. Here, the initial pH value is 10.53 which stabilises at 9.48 after 2h with the formation of (NH4)2SO4. Chalcopyrite is reported to be the most dit!ficult to leach of all the copper minerals owing to the formation of a tenacious hematite reaction product under certain conditions. However, in this study, the XRD data [7,8,16] has indicated that goethite [cz-FeO(OH)] formation is observed up to 40 min. and, subsequently, hematite is also present as a major phase. ~[his indicates that the iron present in chalcopyrite is mostly converted to goethite that remains in the residue. Microscopic examination has also confirmed this observation [7,8,16]. Further, XRD and TA studies show that chalcopyfite dissolution precedes ithe pyrite dissolution and hematite formation [21]. This study (:learly shows that iron is released from chalcopyrite and pyrite separately. The sigmoidal type of dissolution curve obtained for chalcopyrite (Figure 3a) may be ascribed to cupric ion effect or copper ~s Cu(OH)2 reporting to leach residue due to the high initial pH value, and also to the tendency for formation of goethite instead of hematite (Figures 1 and 2), where the copper dissolution behaviour is different. Leaching of sphalerite concentrate Figure 3b shows the dissolution behaviour of sphalerite with about 80% zinc extraction within 2 h. After 40 rain of leaching, the zinc solubilisation appears to be slow and followed by a decrease in pH value. This concentrate contains an appreciable amount (20-30%) of pyrite. It appears that pyrite initially contributes to zinc dissolutio:n and subsequently starts dissolving to form hematite, and (NH4)2SO4 due to oxidation of sulphur. This arr~rnonium sulphate contributes to a decrease in pH value. Leaching of gal(,,na concentrate Lead does not form the ammine complex similar to copper and zinc, and after oxidation it remains insoluble in the residue. Galena oxidation is slow until all the sphalerite present (Table 1) is oxidised, then, the oxidation behaviour of galena concentrate is similar to that of pure insoluble mineral with a final recovery of 95%. The pH value fell from 11.28 to 9.87 at the end of the leaching reaction. This observation is in agreement with the suggested formation of H, SO4 or (NH4)2SO4during oxidation of galena. Leaching of clmleopyrite-sphalerite mixture (1:3) The oc-t values (~btained for copper and zinc metals, together with the variation of pH in the leaching medium, are indicated in Figure 3c. Here, the amount of zinc extracted initially is significantly more than copper. The pH i:s stabilised at a value of 9.55 from 80 min onwards indicating that pyrite does not dissolve to form ammonitun sulphate or hematite. This observation is similar to that made with pure CuS, ZnS and FeS2 system (Figure 2c). It is also supported by the total absence of iron in the leach liquor. Leaching of copper from chaleopyrite-galena mixture (9:11) Figure 3d shows that the initial pH of this system is the same as that of chalcopyrite and pyrite, and, up to 60 min of leaching, the dissolution behaviour of copper appears to be similar to that in the presence of pyrite (Figure 3a,). It is noted that the final pH value is stabilised at 9.73.

K. SarveswaraRao and H. S. Ray

1018

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Fig.3 Oxidative ammonia leaching of sulphide concentrates and mixtures at 135°C. (a) chalcopyrite (Cp), (b) sphalerite (Sp), (c) Cp-Sp, (d) Cp--Gn, (e) Sp-Gn, and (f) analogue of concentrate B. Leaching of sphalerite-galena mixture (13:7) The dissolution behaviour of sphalerite in the presence of galena is shown in Figure 3e. This looks similar to the oct plot obtained for sphalerite in the presence of chalcopyrite, shown in Figure 3c, The pH plot is also similar up to 40 minutes in both the systems. It indicates that the initial sphalerite dissolution is the

Characterisation and oxidative ammonia leaching behaviour

1019

same in the presence of either chalcopyrite or galena. The only difference observed is regards the final pH value which stabi]'Jsed at 9.95. Leaching of mechanieal mixture of sulphide concentrates (Cp--Sp--Gn, in the ratio 20:57:23) as analogous to bulk concentrate B The data for the oxidative ammonia leaching of a mechanical mixture with its composition analogous to bulk concentrate ]~ are shown in Figure 3f. The pyrite present here within chalcopyrite and sphalerite is comparable to the amount present in the bulk concentrate. Sphalerite dissolution is comparable to that obtained in the presence of chalcopyrite or galena. The final pH is stabilised at 9.75 indicating pyrite is not attacked.

Characterisation of oxidation behaviour during leaching of bulk concentrate Figure 4 shows the dissolution behaviour of copper from chalcopyrite and concentrate B. It is seen from the same Figure flint sphalerite addition changes the copper dissolution behaviour. Addition of galena also enhances the dissolution of chalcopyrite up to an a value of 0.65. From these observations, it is likely that copper is forming Cu(OH) 2 only in the initial stages, as described earlier. The dissolution of the mechanical mixture (analogue), except in the initial stages, is very similar to dissolution of chalcopyrite present in bulk concentrate B. It iis also seen that copper recovery from bulk concentrate B is about 95% in 100 rain and similar to that va]Lueobtained in ammonia-ammonium sulphate medium. This indicates that sulphate ion addition does not contribute to any beneficial effect at 135°C.

Standard l e o c h i ~ conditions as in text Conc.B- ammonia Cone.B- ammonia + emma. sulphate Analogue of cone. B - ammonia Cp 4- Gn - ommoniQ Sp + Cp - a m m o n i a

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DissolufiLonof copper from chalcopyrite in oxidative ammonia medium, as single concentrate, and when present in combination with other minerals. •

.

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.

.

The dissolution bohaviour of zinc from various sulphide minerals m ammomacal medium is shown in Figure 5. ZnS, present as sphalerite, dissolves very slowly when compared to ZnS present in combination with other sulphide n~nerals. The dissolution behaviour of galena during oxidative ammonia leaching of bulk concentrates follows a complex mechanism. The formation of intermediate products depends mainly on the

1020

K. Sarveswara Rao and H. S. Ray

composition of the raw materials and the experimental conditions used. The oxidation mechanism is also influenced by galvanic interactions arising due to the presence of other sulphides which have a semiconductor nature. There are several reactions that need to be considered to understanding the oxidation of Cu-Zn-Pb bulk concentrate (containing more pyrrhotite) at 100°C [22].

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I 120

Dissolution of zinc from sphalerite in oxidative ammonia medium, as single concentrate, and when present in combination with other minerals.

XRD studies have shown [8] that the bulk concentrate did not produce jarosite. PbSO4 and PbO.PbSO4 formed instead and these formed rapidly during initiation of leaching even in the absence of oxygen. XRD data have also indicated [8] that there is near total extraction of lead in about 20 min at 135°C. Surface area measurements during leaching of sulphide minends

The surface areas of all the starting feed materials are taken as negligible since the values are approximately one m2/g only, in each case. Table 2 lists the data obtained from particle size dis~bution of leach residues of single sulphide concentrates, mixtures and bulk concentrate. Tlie ~*indicate an increase in surface area of the leach residues of single sulphide concentrates and mixtures when chalcopyrite is associated as a single mineral or in combination with other minerals like sphalerite and

Characterisation and oxidativeammonialeac~ng behaviour

TABLE 2

1021

Size distribution of final leach residues of various sulphide concentrates* and their s~urface area values as recorded by B. E. T. apparatus, particle weight distribution of untreated feed mterials shown in parenthesis St.

~

I.Jeds

Pit~.k ~ . d b ~ (S)

(-d3÷4P tim}

1-4S+381a)

(-38÷~J t~m)

I.

~

I~

19.'7 (IGO)

26.7

:L

SiP

135

15.~ 1100)

13.1

:J.

Ga

135

- (100)

,I.

op

13.5

5,

Cp..Sp (1".3)

t35

17.6 (IQ0~

Sw~m

M e ~ mco~y I*~ J

53.6

15.23

tO0

71,7

Nil

leo

Nil

95.3.5

Tide Is * 100 mq~

. (1~o)

lOO

N~]

93.61

1'i~ b it IO0 mbJm romp*,

16.1

M.3

6,27

A ~ S4S nmm am=m~ v,~Jtm b e • mf~8 site 8.t.50

~4,10

Meat fdm, tMnO robe

~,GO

S~

~wC'p

~ (9:.11)

13.5

4.6 (100)

?'6

8?.0

ap.Oe (13:'/)

135

~ Oe.u untoS utd e.,...

t~

6"6 (1~0~

6.3

87.1

18.~

88.'m

92.99

Ld

~.,~

81,40

n,d tekaaed ~Cpb mmela mnf~

to No.I 7.

at.

6.7 (too)

8.4

8~.9 k,t m

SS)

0.

•dk Com.

~

28.2 (M.~

16.1}(t3..5)

54.9 (41,~

8.d.

33.10

54A0

t,.d

tO.

Bulk

~

]6.7

N~

19.7

oct.

24.3O

43,5O

s.d.

11.

Ikdk Caw.

IW

34.0

31.8

34.2

n.d.

27.70

74.60

e,d.

J2.

Bulk r,..,.

115

40,6

35.1

34,4

n,d.

45.83

17.00

m.d.

13.

•elk

13P

35.4

40"6

~.0

aid.

7?.00

1O0

it.d.

14.

•talk

135

29.7

18,6

$1,7

40.10

100

I00

lO0

tom Cp e m m :,bed rod. m amp,Jet w SI.No. 6, 1~ lmapo n,mb, sdomed to a.,,. f m ~ m

Morner

tM am tm mkals fmaim 15,

Bulk Caw.

95

~0,9 (46..~)

M.4 (i2,S)

32.? (41.0)

n~d,

~,40

63.30

n,d,

16.

Ikdk Cml,

IIS

14.?

I1,$

73J

n,d.

gO.30

iGO

n.d.

17,

~ r,,..,.

13,5

2?.0

"~t

~0.2

41 ,GO

94,40

I00

IOO

i m C~'s

s m s m l s - e m m ~ ~ p l ~ s ~ c ~ s , tot 2 h m , n.d.. m dsw~d

1022

K. Sarveswara Rao and H. S. Ray

galena. The fine fraction (-38+26.5~ra) present in Cp leach residue is only 54% when compared to Cp-Sp mixture (66%) and around 87% present in leach residue samples of both Cp-Gn, and mechanical mixture analogous to bulk concentrate B. Silica is mostly associated with the chalcopyrite concentrate only because the sphalerite and galena contain hardly any silica (Table 1). This implies that enhanced release of fine gangue, which remains in the leach residue of the Cp-(3n mixture, contributes to a higher surface area of about 27.5 m2/g as compared to 15 m2/g of Cp leach residue. The feed materials are of narrow particle size range of -63+45pm excepting bulk concentrate B. The particle size distribution of bulk concentrate B in this range is 46.5% which also includes 28.8% as oversize particles. The oxidation products of galena remain in the solid residue along with the oxidised iron part of chalcopyrite. Silica can be present as free-silica or in association with chalcopyrite during leaching. There are two possibilities that can be ascribed to the increase in surface area of chalcopyrite leach residues. In case the free-silica contributes to an increase in surface area, its composition vis-a-vis increase in surface area has to match. It is not so when the quartz content in Cp is compared to that present in bulk concentrate B. The other possibility is that the iron released from chalcopyrite forms goethite of a porous nature thereby contributing to a rise in surface area. The overall increase in the product fines generated suggests that a certain degree of in-situ grinding takes place during oxidative ammonia leaching of multimetal sulphides. It is possible that the gangue which is uniformly distributed in the chalcopyrite mineral particles, when released during leaching, contributes to an increase in the total surface area of particles as measured by the B.E.T. apparatus. The microscopic examination of thin sections [23] appears to support the observation that there is no segregation of gangue. It is noted that the bulk concentrate after leaching shows a significant increase in the fines fraction which is presumably the particles of silicate minerals released from the matrix. In order to further understand the increase in the surface area phenomenon, both the starting materials, i.e. single sulphide concentrates, mixtures and bulk concentrates, and then the partial leach residues were subjected to SEM study [15]. The original sulphide particles before leaching were found to be mostly massive with uniform surface features. However, some characteristic features such as triangular pits, microchannels, spongy texture and micropores have developed on the surface of partial leach residues [15]. The overall observations indicate that the increase in surface area, as measured by B.E.T. apparatus, is not due to external area (particle size and shape), but attributed to the effect of internal area (cracks and pores) of reaction products, viz. hematite, goethite and silica formed during dissolution of chalcopyrite mineral present in the bulk concentrate B.

CONCLUSIONS Leaching of a binary mixture of CuS and FeS2, as analogous to the chalcopyrite present in a bulk concentrate, indicates that about 90% of the copper dissolves within 20 minutes and the balance within 2 h. FeS2 is totally oxidised to Fe203. However, in a ternary mixture of CuS, ZnS, and FeS2, copper totally solubilises within 60 rain., as compared to about 90% for zinc in 2 h. XRD data of this leach residue indicate FeS2 as a major phase, o~--ZnS (wurtzite), Fe203 and S as minor phases, and CuS as traces. The observed reaction sequence is CuS, ZnS, and then minor amounts of FeS2. The sequence of sulphide mineral dissolution from a quaternary mixture of CuS, ZnS, PbS and FeSe follows the order: PbS, CuS, and then ZnS. FeS2 does not react. Copper dissolution from an analogue of concentrate B is very similar to dissolution of chalcopyrite present in concentrate B, except in the initial stages (cupric ion effect and Cu(OH)2 formation). Sphalerite addition changes the copper dissolution behaviour. ZnS present as sphalerite dissolves very slowly when compared to ZnS present in combination with other sulphide minerals. The solid products formed during the leaching reaction, such as goethite (from oxidation of iron present in chalcopyrite) and oxidised lead compounds, viz. PbSO+ and PbO.PbSO+ (due to galena oxidation) remain insoluble in the leach residue along with the unreacted pyrite.

Charactefisation and oxidative ammonia leaching behaviour

1023

It is shown that more than 95% metal values can be recovered from a Cu-Zn-Pb bulk concentrate of wide range particle size distribution. Increased fines present in the leach residue indicates that there is a certain degree of in-situ grinding taking place during leaching.

REFERENCES °

2. 3. 4. 5.

.

7. 8.

9.

10.

I1. 12. 13.

14. 15.

16. 17. 18. 19.

20.

Duyvesteyn, W.P.C. & Sabacky, B.J., Ammonia leaching process for Escondida copper concentra.tes. Trans. Inst. Min. Met., 1995, 104, C125-C140. Kuhn, MC., Arbiter, N. & Kling, H., Anaconda's Arbiter Process for Copper. CIM Bulletin, Feb. 1974, 62--73. Majima, H. & Peters, E., Trans. Metall. Soc., AIME, 1966, 236, 1409-1413. Tozawa, K. Umetsu, Y. & Sato, K., Extractive Metallurgy of Copper, vol.H, Edt. Yannopoulos, J.C. and Agarwal, J.C., Met. Soc. AIME, 1976, pp.706-721. Williams~, R.D. & Light, S.D., Copper concentrate dissolution chemistry and kinetics in an ammonia-oxygen environment, in Fundamental aspects of hydrometallurgical processes, AIChE Sym. Series, 1978, 74(174), p.21. Sarveswara Rao, K, Pararnguru, R.K., Das, R.P. & Ray, H.S., The role of galvanic interaction during ararnonia leaching of multimetal sulphides, Min. Pro. Ext. Met. Rev., 1992, 11, 21-37. Sarveswara Rao, K., Das, R.P. & Ray, H.S., Study of leaching of multimetal sulphides through an interdisciplinary approach. Min. Pro. Ext. Met. Rev., 1991, 7, 209-234. Sarveswara Rao, K., Das, R.P., Mukunda, P.G. & Ray, H.S., Use of X-ray diffraction in a study of ammonia leaching of multimetal sulphides, Metall. Trans., 1993, 24B, 937-945. Forward, F.A. & Veltman, H., A process for direct leaching zinc sulphide concentrates with sulphuric acid and oxygen under pressure, in Physical Chemistry of Extractive Metallurgy, Part 2, Interscience Publishers, New York, 1961, p.1275. Umetsu, Y., Tozawa, K. & Sasaki K., Studies on behaviour of copper, zinc, iron and sulphur in oxygen pressure leaching of complex sulphide concentrates, Bull. Res. Min. Dress. Min., Tohoku Univ., 1973, 29, p.62. Ray, H.S., Some factors that lead to uncertainties in kinetic studies in metallurgy, J. Thermal Analysis, 1990, 36, 743-764. Prosser P. Alan., Review of uncertainty in the collection and interpretation of leaching data, Hydrometallurgy, 1996, 41, 119-153. Sarveswara Rao, K. & Ray, H.S., Selection of experimental conditions for leaching studies: a case study on oxidative ammonia leaching of multimetal sulphides. Met. Mater. Sci., Instn. Engrs. (India), 1997, 78, 70-78. Sarveswara Rao, K., Anand, S., Das, R.P. & Ray, H.S., Kinetics of ammonia leaching of multime~d sulphides, Min. Proc. Extr. Met. Rev., 1992, 10, 11-27. Sarvesw~xa Rao, K. & Ray, H.S., Leaching kinetics of Cu-Zn-Pb bulk concentrate--surface area measure~aents. In Recent Trends in 8iotechnology. Proc. Ninth National Convention of Chemical Engineering Division of The Institution of Engineers (India), and International Symposium on Importance of Biotechnology in the Coming Decades, June 5-7, 1993, Visakhapatnam, Tata McGraw-Hill Pub. Co. Ltd., New Delhi, 1993, pp.218-222. Sarvesw~xa Rao, K. & Ray, H.S., Effect of roasting and ammonia leaching on the mineralogy of multimetal sulphides. Metals Materials and Processes, 1997, 9(1), 33-44. Vogel, A.I., Quantitative Inorganic Analysis including Elementary Industrial Analysis, 3rd edn., The English Language Book Society and Longman Green and Co. 1962. Steger, E[.F.,Chemical phase-analysis of ores and rocks, Talanta, 1976, 23, 81-87. Sarvesw~a Rao, K., Muralidhar, J. & Ray, H. S., Characterisation of oxidative ammonia leaching behaviour of galena by chemical phase analysis. Metals Materials and Processes, 1997, 9(1), 25-32. Sarvesw~xa Rao, K., Anand, S., Srinivasa Rao, K., Das, S.C., Subbaiah, T. & Das, R.P., Process development for extraction of zinc, copper and lead from complex sulphide ore/concentrates of AmbamataDPart I: An overview of the process. Trans. Indian Inst. Met., 1984, 37(1), 49-53.

1024

21. 22. 23.

K. SarveswaraRao and H. S. Ray Sarveswara Rao, K. & Ray, H.S., Use of thermal analysis for study of characterisation of multimetal sulphides and their oxidation behaviour, Min. Pro. Ext. Met. Rev., 1997, 16, 261-278. Anand, S., Sarveswara Rao, K. & Das, R.P., Part HI: Lead recovery from leach residue. Trans. Indian Inst. Met., 1985, 38(2), 101-106. Reference 20. Sarveswara Rao, K., Unpublished work.

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