tlydrometallurgv, 26 ( 1991 ) 179-199 Elsevier Science Publishers B.V., A m s t e r d a m
Leaching of gold, silver and lead from plumbojarosite-containing hematite tailings in HC1-CaCIR media J. Vifials, C. Nufiez and J. Carrasco I)epartment q[('hemical Engineering and .lh'tallurgy, l ntver~ity qlBarcehma, ..t vda. Dtagona164 ", 08028-Barcelona. Spatn (Received March 29, 1990: accepted July 13, 19901
ABSTRACT Vifials, J. Nufiez, C. and Carrasco, J., 1991. Leaching of gold, silver and lead from plumbojarositecontaining hematite railings in HCI-CaCI2 media, ll.vdrometallurgy, 26:179-199. Plumbojarosite-containing hematite tailings t¥om the sulfating roasting of complex pyrite ores comprise a porous hematite matrix impregnated with plumbojarosite, anglcsile, ferric sulfate and elemental sulfur. About 70% of the silver content is in dilute solid solution in the plumbojarosite. Leaching of gold. silver and lead was studied in HCI-CaCI2 media. I-he extraction rates of silver and lead are controlled by the plumbojarosite leaching under conditions near equilibrium. The simultaneous leaching of gold was studied as a function of the solution potential. The extraction of 90-95% lead and noble metals can be achieved with a stoichiometric consumption of HCI with rcspcct to the plumbojarositc content (HCI, 0.3-0.5 M) and 0.6-4 kg Cl2/t. Under thcse conditions, hematite is onl,, slightly attacked.
Considerable amounts of oxidized, low-grade plumbojarosite-containing hematite tailings remain unprocessed at the present time. Little research was carried out on these materials until the eighties, when the development and application of pressure leach processes for complex sulfides highlighted the problem of the loss of silver and lead due to the formation of plumbojarositcargentojarosite ((Pbo.5,Ag)Fe3(SO4)2(OH)~,)in the leach residues. Dutrizac and co-workers [ I-7 ] studied the factors that determine the formation of these solid solutions. Other recent research has investigated different ways of recovering noble metals from these residues: sulfidization prior to cyanidation [ 8 ] ; hydrothermal sulfidization/chloride leach [9 ]; decomposition in hot alkaline media [ 10] and sulfidization/flotation [ l 1 ]. The present investigation concerns the mineralogical characterization of plumbojarosite-containing hematite railings generated during the second half 0304-386X/91/$03.50
(~) 1991 - - Elsevier Science Publishers B.V.
J. VlI~ ~LS ET .Xi.
of the 19th century in southwest Spain. At that time, large amounts of complex pyrites were treated by the process known as "artificial cementation" [ 12 ]. The ore was roasted slowly in the open air in large heaps under strongly sulfating conditions. The calcine was then leached by percolation to recover the copper by cementation with scrap iron. The noble metals remained in the residues from the preceding treatment, together with a large part of the silver in the form of argentian plumbojarosite, which was not recoverable by conventional cyanidation. The leaching of these materials in HCI-CaCI2 media has been studied in the present work as a part of the development of a hydrometallurgical process for the recovery of gold, silver and lead from these ores. EXPERIMENTAL
Materials from two different deposits in southwest Spain (samples 1 and 2 ) were used; the samples were supplied by the Tharsis Sulphur and Copper Company.
Characterization of materials The particle size distribution (Table 1 ) and the chemical composition of the untreated samples (Table 2) were determined. Phase identification was achieved by a combination of X-ray powder diffractometry, reflected-light microscopy and scanning electron microscopy (SEM) in conjunction with energy dispersive X-ray analysis. Microscopy was also used to determine the texture, grain sizes and leaching effects. The mineralogical composition was established, after phase identification, using selective solvents (Table 2 ). The chemical and diffractometric data of several components, such as plumbojarosite and ferric sulfate, were determined from small (0.05-0.3 g), "pure" samples which were obtained from the ore by mechanical separation. TABLE 1 Particle size distribution As received
Size ( m m )
Sample I (%)
Sample 2 (%)
Size ( ~ m )
Sample 1 (%)
Sample 2 (%)
+25 -25 +10 -10 + 4 - 4 +0.1 - 0.1
14 13 14 22 23
6 15 17 24 15
+200 -200+100 -I00+ 40 - 40
4 16 14 66
5 15 12 68
LE.\CHING OF -~g,atu .AND Pb FROM HEMATITE TAILINGS IN HCI-(TaCI~ MEI)I-~
TABLE 2 Chemical and approximate mineralogical composition Element
Fe Pb S,., _ S~,,,: S,,~ ('u Zn As Ag Au
Sample 1 (wt.%) 57.1 2.2 < 0.03 1.7 1.8 0.05 0.04 0.03 52 g / t 3.1 g / t
Sample 2 (wt.%) 42.8 2.8 < 0.03 3.0 3.9 0.08 0.09 0.36 136 g/t 1.3 g/t
Sample 1 (wt.%)
Sample 2 (wt.%)
anglesite' ferric sulfate' plumbojarosite-" hematite 3 quartz elemental sulfur
1.4 3.7 6.9 78 7 0.05
1.2 9.8 I1 54 20 0.9
' Determined by selective extraction with disodium tartrate. Calculated as l'bSO4 and F% ( SO~ ) ~"91120. 21)ctcrmined by difference between total lead and lead c o m b i n e d as anglesile. Calculated as Pbo sFe~(SO4)2(OH h,. •~Determined by difference between total iron and iron c o m b i n e d as ferric sulfate and plumbojarosite.
Leaching tests Leaching experiments were performed in a reactor described previously in [ 13 ]. All tests were run with a solution volume of 800 ml using milled samples (Table 1 ). No liberation problems were detected for the - 2 0 0 / l m fractions. Solubility of Ag, Pb and Fe was established by sampling and atomic absorption analysis (A.A.). The particular experimental conditions are described together with the leaching results. In tests performed for the gold leaching study, the potential of the solution was measured continuously using a platinum electrode and a calomel reference electrode. The reference electrode was separated from the leach solution by a cooled salt bridge. All potentials are presented with respect to the standard hydrogen electrode (SHE) at the working temperature (90°C), taking+ 8 5 9 / t V / ° C as the temperature coefficient [ 14]. Since the most important parameters for the leaching of gold in HCI-CaCI2 systems are the chloride concentration and the solution potential, the experiments were designed to maintain both parameters at practically constant levels. Leaching tests were initiated by adding a 320 g sample to an 800 ml solution of known HC1-CaCI2 concentration and temperature. Immediately, sufficient sodium hypochlorite (4-12 ml solution of 0.044 g Cl2/ml) was added to give a solution of prefixed potential which was kept constant (usually to z 5 mV ) by small, periodic additions of dilute hypochlorite (0.004 g Cl2/ml). Samples of 25 ml of slurry were withdrawn at selected intervals, filtered immediately and analyzed for gold by atomic adsorption, following a
J. VINAI.S IF -X[_.
concentration process based on a mercury-collection method . Disturbances due to the sampling and hypochlorite additions were corrected (mass balance). RESULTS AND DISCUSSION
Characterization of materials The mineralogical study indicated that both samples had a similar qualitative composition. The material exhibited a typical "pyrite cinder" texture, which consists of a porous aggregate ( mean porosity of 0.4) made up of iron oxide grains 1-10pm in length. However, there were important differences between the mineralogy of thesc products and that of Iberian pyrite cinders [ 16-18 ] obtained by current processes which are based on multi-hearth or fluidized-bed roasting ( > 800:C). In present-day cinders the oxide matrix consists of fine intergrowths of hematite and spinels (magnetite and zinc and copper ferrites), whereas in the samples in the present work the matrix is exclusively hematite. This suggests that the roasting temperatures were lower than 800°C, at which temperatures the formation of spinels is kinetically [19-23] and thermodynamically [20,21,24,25] unfavourable. In addition, no residual iron sulfides (Fe~_,S) or intermediate phases of chalcopyrite roasting (CusFes_,.Ss_,-Cu2..,S) have been detected, although these are common in present-day cinders. Most likely these sulfides were eliminated by percolation leaching through processes of the type: MS+2Fe3+-----* M 2+ + 2 F e 2+ +S
Evidence to support the above postulate, is given by the fact that elemental sulfur was detected, especially in sample 2; the sulfur appeared as isolated crusts and also as an impregnation in some cinder grains. Since hematite can be considered to be virtually inert except during very strong acid leaching [26,27] the main mineralogical characteristic of these materials that conditions their hydrometallurgy is the presence of the more active sulfates: anglesite, plumbojarosite and ferric sulfate. According to the Fe-S-O system , Fe2(SO4)3 is only thermodynamically stable at temperatures below 550°C and under strong sulfating conditions (P~o2 0.1-1 atm). Even under these conditions, the oxidation mechanism produces mainly Fe203 , and this suggests that at least part of the ferric sulfate present in these materials comes from the recycling of fluids from the cementation step. The ferric sulfate consists of crystalline efflorescences that impregnate the cinders (Fig. IA). Its chemical composition (Table 3) is close to that ofcoquimbite (Fe2(SOa)3"9H20), although its X-ray
I . E . \ ( ' I I I N G ()F ,Xg. Au .AND Pb F R O M H E M A T I T E T A I I . I N G S IN HCI-CaCIz MEL)IA
Fig. IA. SecondaD' electron micrograph of an efflorescence of ferric sulfate (sample 1 ). B. Xray diffractogram of a ferric sulfate sample (CuK,). ('. Secondary electron micrograph of a peripheral area of" hematite (h) particles in which the original porosity has been filled by plumbojarosite microcrystals (PD: note the anglesite grains (a) surrounding the plumbojarosite aggregates (sample 1. cross section ). D. Detail of plumbojarosite aggregates (secondaq, electron micrograph ). TABI.E 3 Chemical analysis of ferric sulfate (Sample 2)
ferric sulfate coquimbite Fe.,(SO4) c 9H20
Fe203 ( wt.% )
SOs ( wt.% )
H20 ( wt.% )
The sample contains 0.56% Pb and 26 g/t Ag due to ~ 3% of admixed plumbojarosite. n.d = not determined.
diffraction pattern (Fig. 1B) differs markedly from that species, and probably corresponds to the compound FeI403(SO4)Is"nH20 (ASTM 16-897 [281). Anglesite occurs as 5-20 pm long grains ( Fig. 1C ), but is a minor, residual phase. Most of the lead sulfate produced in the roasting step reacted with ferric sulfate to produce plumbojarosite through the following process:
J. Vl,qat.s ET AL.
PbSO4 + 3Fe2 (SO4) 3 + 12H20--" 2Pb05 Fe3 (SO4)2 (OH)6 + 6H2 SO4
Plumbojarosite is found as aggregates of loose crystals (0.3-6/zm) scattered throughout the cinder pores (Figs. I C and D). The chemical analysis (Table 4) and X-ray diffraction characterization indicate a composition close to the hydronium jarosite-plumbojarosite binary series [ 6,29 ]. The qualitative analysis by X-ray fluorescence indicates only traces of Na, K, Cu and Zn. A feature of importance is that about 70% of the silver present in these materials is bound to the plumbojarosite. The silver concentration is relatively variable, but, despite some high silver concentrations, energy dispersive Xray (EDS) analysis detected no argentojarosite or other silver-rich species in the plumbojarosite aggregates, which indicates that these concentrations are due to substitution. In spite of exhaustive microscopic examination, no gold was detected even at high SEM resolutions, and this suggests that the gold contained in these materials is present as particles of sub-micron size.
Selection of the leaching treatment According to the characterization studies, it could be expected that the use of the conventional cyanidation process would present the following limitations: ( I ) The presence of considerable amounts of ferric sulfate would result in a high lime consumption to achieve the correct pH if direct cyanidation was employed. (2) The occurrence of ferric sulfate scattered throughout the cinder pores would make it difficult to obtain high, stable pH values inside the particles, while at the same time the precipitation of iron hydroxides in these spaces may hinder the diffusion of reagents. TABI_E 4
Chemical composition of plumbojarosite (wt.%)
Pb() Fe_~O~ SO~ Ag ( g / l )
Sample 2 (average)
18.4 43.3 28.7 4802
19.0 42.0 28.8 ~ 920-"
19.72 42.44 28.29 -
'Single analysis. ~('alculated from the Ag/Pb ratio in cinders after the removal ofanglesite and Na('l-soluble silver by treatment with 2 M NaCl.
I.EACHING OF Ag, Au AND Pb FROM HEMATITE TAILINGS IN HCI-CaCI, MEDIA
(3) The binding of 70% of the silver in plumbojarosite would result in low silver recoveries. Leaching in a HC1-CaCI2 system appears to be an alternative route for a more effective recovery of the metals. High silver and lead extractions can be achieved by selective leaching of anglesite and plumbojarosite, and gold recovery is possible by adjusting the oxidation potential. The election of CaCI2 as the chloridizing agent is supported by the following: ( 1 ) It increases the hydrochloric acid activity noticeably. This effect is more pronounced in this salt than in NaCl-type monovalent chlorides [ 30 ]. (2) It regulates the level of sulfates in solution due to the low solubility of calcium sulfate which, together with the stabilization of Fe and Pb chlorocomplexes, shifts the equilibrium from plumbojarosite dissolution to regions of low free acidity. (3) It is generated in the same process if the iron is removed from the solulion by precipitation with CaCO3 in a stage subsequent to leaching.
Preliminary leaching studies For purposes of interpreting the results obtained in the simultaneous leaching of anglesite, plumbojarosite and ferric sulfate in HCI-CaCI2 media, some leaching characteristics of the individual phases were first examined.
Ferric sulfate As shown in Fig. 2A, the water leaching of ferric sulfate from these materials ( 1.3% Fe,ot in sample 1 and 4.8% Fem~ in sample 2) is very fast and no significant temperature effect was observed up to 40°C. The chemistry of the solution is, however, complex. According to the Fe~+-SO ] - - w a t e r system , Fe 3+ and FeSOg~ are the predominant solution species at 25°C under the leaching conditions (pH 2, Fe(IIl) 10 -1 M, SO2,~, 10 -1 M), but the pH drops at temperatures > 40 ° C, possibly as a consequence of the formation of mixed hydroxyl-sulfate complexes, and the solubilized iron precipitates gradually as hydroxysulfates. Similar results were observed by heating the solutions obtained from water leaching at 20°C and separating the cinder by filtration. The characterization of the precipitates obtained at 90 °C indicates the formation of hydronium jarosite through the following overall process: 2Fe(SO4)3 + 14H20
,2 (H30)Fe3(SO4)2 (OH)6 + 5H2SO4
The presence of ferric sulfate may cause additional acid consumption during the treatment of these materials in hot HCI media due to these precipitation phenomena. However, this problem can be minimized by using high concentrations of CaCI2. Figure 2B gives the results obtained at 90°C for different CaCle concentrations. The leaching of ferric sulfate under these conditions is also a very fast process, which is accompanied by the simultaneous precipi-
j. VII~-XLS; ET a.l..
. ' . 0 - - .'.0 ' S
l Ill"--"~& ~
x .T,~. v
g " ~x\
X~ X ~
X~ . ~ . ,
,-/ ~ x .--._.. 1
Fig. 2A. Temperature effect on the water leaching of ferric sulfate. B. Effect of the CaCI2 concentration on the leaching of ferric suffate at 9 0 : C (sample 2. solid weight:liquid ~olume = 1:5: 300 rain ').
tation of calcium sulfate (gypsum). Nevertheless, concentrations of CaCI, of over 100 g/1 are required so that the activity of Fe 3+ (due to the formation of chloro-complexes) is sufficiently low to minimize hydrolyzation.
A nglesite The anglesite contained in these materials (43% Pb,o, in sample 1 and 29% Pbto, in sample 2 ) is also rapidly dissolved in concentrated CaCI2 solutions as lead chloro-complexes. Figure 3A gives the results obtained at 90°C. In the case of CaCI2> 100 g/I, the iron from the ferric sulfate may co-exist in the solution with the lead without the precipitation of plumbojarosite under the conditions: 90~C; pH 1; F e ( I I I ) = 1 0 -1 M; P b ( I I ) = 1 0 -2 M; S O ] - = 1 0 -2 M. However, at lower concentrations (CaC12= 50 g/l curve), hydronium jarosite-plumbojarosite is detected among the iron precipitation products. Simultaneously with the leaching of anglesite and ferric sulfate in CaCI2 solutions, approximately 30% of the silver contained in these materials is dissolved; this is the silver not in solid solution in plumbojarosite. Figure 3B shows the results obtained for different CaCI2 concentrations. The dissolution is practically instantaneous, and the silver becomes stabilized in solution even at concentrations of 25 g/l CaCI2, which indicates that silver is more strongly complexed than lead in these media [31 ]. This is also consistent with Dutrizac's results [2 ] which state that the formation of argentojarosite is not favored in chloride media.
LISACIIIN(i OF .',g. Au .AN/) Pb FR()M HEMATITE I A I I . I N G S IN H(/I-('a('lz MEDIA
CaCI 2 (g/I) • 300 ~, 2 0 0
0100 v 50
0 100 V 50
- - ~
- - ~
_ f A, A
60 Time train)
Fig. 3.4. Effect of the CaCIe concentration on anglcsite leaching. B. Effect of the ( ' a ( ' l 2 c o n c e n on the leaching of "soluble silver" (sample 2, solid weight:liquid v o l u m e = 1: 5, 300 rain - 1). tration
Plumbojarosite The kinetics of CaCI2-HCI leaching of the plumbojarosite contained in these materials (54% Pbto,, 3.6% Fe,,~ in sample 1 and 71% Pb~ot. 7.7% Fe,,L in sample 2) have been previously studied for diluted pulps in the absence of anglesite and ferric sulfate. The results obtained, which have already been published [ 13 ], indicate that plumbojarosite can be leached with insignificant dissolution of the hematite matrix for a-m~ < 1.5. Under these conditions, the leaching is slow with stirring speed and cinder particle size having negligible effects. The chemical reaction at the surface of the plumbojarosite crystals is the rate-limiting step, and a first-order reaction with respect to the hydrochloric acid activity, and activation energy of 96 kJ/mol, were found under the conditions: 60-90°C; 0.175-1 M HC1 and 0-200 g/1 CaCI> Nevertheless, the acid leaching of plumbojarosite in concentrated pulps and in the presence of the reaction products of anglesite and ferric sulfate can be limited by the kinetics and also by thermodynamic factors, since the leaching reaction is relatively unfavored [ 13 ]: Pb,.5 Fe~ (SO4)_~ (OH)6 + 6 H + ~0.5Pb2+ + 3Fe3+ + 2SO4 - + 6 H 2 0
/JG~7~=+ 155 kJ mol-I In HCI-CaC12 media, however, the activity of S O l - , Pb 2+ and Fe 3+ ions can be substantially reduced as a result of calcium sulfate precipitation and the formation of HSO4 and different iron and lead chloro-complexes [ 13,31,32 ]. Thus, the determination ofthe equilibrium position ofeq. (4) is possible in principle on the basis of the distribution of the concentrations and the activity coefficients of the species involved. However, the requisite data
J. \"I.~ALS ET a.l.
for the activity coefficients are not presently available for the conditions of interest in this study, i.e., high temperatures and high ionic strength. For this reason, the conditions suitable for the leaching of these materials were studied experimentally, to determine the optimal temperature, and hydrochloric acid and calcium chloride concentrations, in addition to the reaction products and pulp density effects.
Leaching in HCI-CaCI2 solutions Temperature effect The temperature effect was studied at 100 g/l CaCI2, using an initial HCI concentration about three times the stoichiometric amount required for plumbojarosite leaching. As shown in Fig. 4, there are two distinct regions on the leaching curves: an area of very rapid solubilization of Pb, Ag and Fe, which corresponds to the solubilization of anglesite, ferric sulfate and "soluble silver" and a second region that reflects the plumbojarosite leaching. Temperature is essentially a kinetic factor that in practice only affects plumbojarosite leaching. Under conditions of low acidity, temperatures > 80°C are necessary for the reaction to proceed at acceptable rates. From the data in Fig. 4 it can also be observed that when nearly all the plumbojarosite has been leached (Ag and Pb extraction 95%), the iron solubility stops at 5% Fe,o, in sample 1 and 12% Fe,ot in sample 2. These values are approximately the sums of the iron contents present as ferric sulfate and plumbojarosite, thus confirming that the solubility of the hematites is negligible.
Effect of hydrochloric acid and calcium chloride concentrations The effect of the hydrochloric acid concentration was studied at 90~C for different calcium chloride concentrations. Since plumbojarosite leaching releases silver, lead and iron according to their composition (Fig. 4), for the sake of simplicity only data on silver are given in Fig. 5. It will be noted that the amount of acid needed for complete leaching is highly dependent on the calcium chloride concentration and that, when using stoichiometric amounts of HC1, the process stops at extraction levels which increase as the CaCI~ concentration rises, which suggests that the plumbojarosite leaching is limited by eq. (4) under these conditions.
Equilibrium position The equilibrium constant for the reaction given in eq. (4) can be written:
K=a3..e~' apb2+ ~ a,~o~" i 6+ -,.16H20/alt in which:
I.E.X.f'HING O F *g. Au -~ND Pb FROM t l E M A T r r E TAILIN(iS IN H('I-CaCI, MEDIA
f ' "
f • -
.. _. ~ ; ,
.,.- ~ v
T i m e (rain)
Fig. 4. Temperature effect (solid weight:liquid v o l u m e : 1:5; CaCI2 = 100 g/l: 300 m i n - ' : sample 1 HC1,=0.38 M: sample 2 HC1,=0.75 M).
an + = mH + YH+
T h e a c t i v i t i e s o f Fe 3+ a n d Pb 2 ~ can be related to the m o l a l i t i e s o f trivalent iron, mv,.(]H), a n d d i v a l e n t lead, mpb(H), by m a s s b a l a n c e a n d the f o r m a t i o n c o n s t a n t s o f the different c h l o r o - c o m p l e x e s as: (-/F e3 + ---- m F e ( l l l a p h - ' + ---~ m p b (
) FFe(III ii ) F p b ( 11 )
.I \'IN.~,L.",; ET At..
"'--" CaCI 2
Ca CI 2
1 0 0 q./
:': ~ . _ _
~.~/" ~ -
f1" 1 , .
_> LaC: 2
2 0 0 ~,':
I ., _ _
i./ • o "5 M (St(~hlc~trlcl
'.' O 3 ~ M ,. O~,'3M •. ~75v
Fig. 5. Effect of the hydrochloric acid and calcium chloridc concentrations (solid weight :liquid volume= 1:5: 90:C: 300 min -~).
I.F,.(,U ~= [ l _ _ + K ] a ( t : + LTve3+
+ fl-3a3`'- -t-f14a4.~ . .f12a2'..... 7Fc¢, +._ "/I.~¢13 ;'V~(,Z
I],,(,,,=[I K,a(.,_+[32a~.,-, fl3a3,` +~4a~I-.
Substituting cqs. (6), (7) and (8) in (5) and rearranging, we get: mv~,~m) -
7h+ ~,~ill >--l>b(n) ( a s ( ) ~ - ) pil6
i T b l ( / ] >)
I [! .\( " t l I N G t )F .~.g..'xu A N D Pb F R O M I I E M A T I I E T A I I . I N G S IN I R . ' I - C a ( ' I , M E D I ,~.
During the leaching of these materials in concentrated CaCI2 solutions, it can be assumed that ac~-, a¢:a2÷ and aH,o remain practically constant for a given CaCI2 concentration. Under these conditions, by virtue of the precipitation equilibrium of calcium sulfate, the a~,d remains virtually constant. Likewise, as the ionic strength is practically fixed at a given CaCI2 concentration. it can also be assumed that y,,+ and the activity coefficients of the chlorocomplexes are also constant. Furthermore, although the total lead concentration varies from 4 to 9 g/l, depending on the extent of plumbojarosite leaching and the pulp density, m b-b'/6 is only slightly sensitive to this variation. This suggests that, for a given CaCI2 concentration, a plot of log mv, cIll, against log m.+ should be approximately linear with 2 as the slope if mh.~ .~, and m~* are the molalities determined at the point of equilibrium. Some additional experiments were performed to determine the final concentrations under conditions in which the leaching stopped at extraction levels < 90%. Figure 6 shows that the results obtained are consistent with these equilibrium considerations and that the shift of the lines to areas of lower acidity when the CaCI_~ concentration rises is also consistent with the fact that all the magnitudes of the denominator in parentheses in eq. ( 11 ) decrease under these conditions. The correlation between the values of rn~.~¢m ~/m ~+ and the concentration of CaCI~ is given in Fig. 7, in which the equilibrium position in the range considered can be described by the following empirical relation at 90 ° C: mr,.~ lll~ = 10 ~ o.5+o.s,,,,.~,.,:)
( 12 )
E/]ect of the reaction products The effect of the addition or removal of the reaction products (Fig. 8 ) reflects the corresponding shift in the equilibrium shown in eq. (4). As opposed to the pronounced effect of the addition of Fe (III), adding moderate amounts of Pb (II) ( 10 g/1 ) has no significant effect (eq. ( 11 ) ). Furthermore, while CaCI2 concentrations of 300 g/l are necessary to achieve extraction rates of 90% with a stoichiometric acid consumption with respect to the plumbojarosite content, the removal of ferric sulfate by prior washing makes it possible to work at 200 g/I CaC12.
E(fect of the pulp density The effect of the pulp density was studied at 200 g/1 CaCI2 using samples from which the ferric sulfate had already been removed and with a stoichiometric acid: solid ratio with respect to the plumbojarosite content. No significant effect was observed on the silver extraction by varying the pulp density from 1 : 5 to I : 1.5. The silver concentrations obtained under these conditions ( 30-90 mg/l ) were much lower than the saturation levels in these media (940
A Data frolxl Sample 1 ~7 Data from Sample 2
/ A, ~-~-
(1) i1 E -1.2 ol
/ ~7 ~
CaCI250g/' z~/ /
CaCl 2 2 5 g / I
Fig. 6. C o r r e l a t i o n b e t w e e n log ,.I,l~c(m) a n d log m u + d e t e r m i n e d for different CaCI2 c o n c e n t r a tions w h e n leaching s t o p s at e x t r a c t i o n s < 90%.
Fig. 7. Correlation between log rove(m)/m2+, determined when leaching stops at extractions < 90%, and the molality of CaCI2. mg/I Ag at CAC12=200 g/l; F e ( I l l ) = 15 g/l; P b ( I I ) = 14.5 g/l; free acidity = 0.1 M; 9 0 ° C ) . Nevertheless, the saturation concentration for lead ( 14.5 g/I) does not permit the use of pulps with a density greater than I: 2 under these conditions.
I . E A C H I N G O F Ag, Au A N D Pb F R O M H E M A T I T E "FAILINGS IN HCI-CaCI2 M E D I A
Z'a A .------'------
m r , 7 - -
add. l O g / I
'~ add. l O g / I
~. ferric sulfate removed
Fig. 8. Effect of the reaction products (sample 2. solid weight:liquid v o l u m e = 1:5" HCI, = 0.25 M; CaCI2 = 200 g/I; 90°C; 300 m i n - ~).
Leaching of gold Leaching without addition of hypochlorite During the treatment of the samples in hot HCI-CaCI2 media, small but significant amounts of gold are also leached (Fig. 9). The potentials observed are relatively constant for a given CaCI2 concentration, although they tend to decrease slightly with time, as a result of the increase in the Fe z+ concentration that probably takes place due to attack of trace residual sulfides. These measured potentials can be related to the equilibrium potential of the Fe3+/Fe 2+ pair. If we assume that, for a given chloride concentration, there is a predominant ferric chlorocomplex, FeCI~3-")+ , and a predominant ferrous chloro-complex, FeCI~ 2- ") +, the potential can be written as: o7" ~.(3-,.)+ , RT, fl,, , RT. E = E(ve3+/Fc'-+) o -+" F'~'ln+t~FeClm a~C-l.~) t---ff-m~, . ~ - ( n - -
re)In a o -
From the thermodynamic data [31,32], the predominant Fe(II) species was FeCI2 under all the leaching conditions used, while the predominant Fe(III) species was FeCIJ- at 100 g/l CaClz and FeCI3 at higher concentrations. Therefore, ( n - m ) in eq. (13) is negative at high chloride concentrations and, consequently, the potential decreases when a o - increases (Fig. 9 ). Although Fe(III) is less oxidized when a(.l_ increases, gold is also more reduced under these conditions (eq. (14) ). It has been observed from experiments that gold extraction increases as the CI - concentration rises (Fig. 9 ). Nonetheless, the leaching rates are very slow under these conditions, resulting
:-e : .)023 M CaCI2
78(] / ' k E I1
14I' A L
/ ' ~ .u ~' -e., ~' q....
e ( , . ; ' , ~ ; , ( ) ~ : x,, t
(D O L~
CaCI2 ( g i l l
A O V
300 2OO 100
Fig. 9. Potential evolution and gold extraction as a function of time in HCI-CaCI2 leaching ( s a m p l e 1. solid weight : liquid v o l u m e = 1 : 2.5: HCI, 0.3 M: 9 0 ; C: 300 r a i n - t ). TABLE 5
Comparison between expected and experimental potential (sample I. 90~'C. 4 h ) CaCI2 (g/l) 100 200 300
-l(-rfree (mol/kg) 1.4 3.4 5.5
mA,ct, ( mol/kg ) 3 . 3 × 10 -¢' 4.7 × 10 -6 5.9X 10 -~
Potential ( mV )
807 793 780
814 780 763
in incomplete extractions even with long residence times. This suggests that the solution potentials are close to the equilibrium potential of the AuCI4 / Au pair: RT
E=k, •o(.,xo(14/,,xo) + ~ : l n a_~Lqz a4L_
Substituting the standard potential at 90°C  (0.959 V) in cq. (14) and taking the molalities for the activities, these potentials can be evaluated and compared with the experimental solution potentials. Table 5 shows that
Lt!-~( I I I N ( i ()1- .\g, ,%u &ND Pb FROM H E M A T I T E T M L I N ( ; S IN Hl'l-('aCl~ .MEI)I-~
the results obtained differ by less than 20 mV. Since it is reasonable to expect that the leaching of gold in these media is electrochemical in nature, the low rates observed under these conditions are not surprising. In this way, Law ghans and Lei [ 33 ] demonstrated that the anodic dissolution of gold in CaCI_, solutions occurs with significant current densities only at potentials approximately 100 mV higher than the reduction potentials. Therefore, the logical route for increasing the leaching rate is to work under more strongly oxidizing conditions.
Leaching with the addition of hypochlorite A group of experiments was performed to establish the dependence of the leaching rate on the solution potential by means of hypochlorite addition. It can be seen in Figs. 10 and 11 that, for potential < 900 mV, the leaching rate increases when the solution potential rises and, for a given potential increases when the C1- concentration rises. These results are consistent with an electrochemical dependence . However, at potentials of 900 mV or higher, the leaching response and particularly the time required for complete leaching of the accessible gold (about 10 min for 90% extraction), was found to be virtually independent of both parameters. Possibly, under these conditions the oxidation of gold is very fast and the kinetic response may be limited by pore diffusion phenomena. O p t i m u m leaching conditions are achieved at about 900 mV, at which po100 f~
- - E t - - -
/ 40 ,,7
Potential vs SHE ImVl A 1090"- 7 0 940"' 5 • 897z3 0 863-*5 ~7 787*-5
Fig. 10. Gold extraction as a function of time for different potentials at 300 g/1 CaCI2 (sample 1. I lCI, = 0.3 M, solid weight:liquid v o l u m e = 1:2.5; 90: (': 300 min--').
J. V I N A L S I-'1 A[..
A 0~/0 /O/l ~
CaCI 2 (g/I)
& 0 •
300 200 tO0 50
Fig. 11. Gold extraction as a function of time for different calcium chloride conccntrations ( s a m p l e 1, solid wcight :liquid volumc = I : 2.5: ttCI, 0.3 .'tI: 9 0 : C: 3 0 0 m i n - t ). T.,XBI.E 6
Chlorine consumption f'or extracting gold at different potentials Sample
Potential ( m V )
g CI2/kg cinder
I I 1 2 2
897 940 1.090 897 1.086
0.55 0.86 3.20 4.20 48.00
_+ 3 + 5 ~ 7 + 3 _+_10
tential the gold extraction was fast and chlorine consumption low (Table 6). This occurs because, at 900 mV, the oxidation of elemental sulfur occurs slowly. On the contrary, maintaining potentials > 1000 mV results in high chlorine consumption, especially for sample 2, according to the following process: S+3C1~ + 4 H , O
,SO2- +6C1- + 8 H +
( 1 ) Plumbojarosite-containing hematite tailings can be described as an impregnation of ferric sulfate, plumbojarosite and minor amounts of anglesite and elemental sulfur in a porous hematite matrix. About 70% of the silver present in these materials is in dilute solid solution in the plumbojarosite.
LEACHING OF Ag. Au -~ND Pb FROM H E M A T I T E T A I L I N G S IN HCI-CaCI, MEDIA
(2) Extraction rates in HCI-CaCI2 media show the simultaneous leaching of ferric sulfate, anglesite and plumbojarosite. In the range considered (HC1, 0.15-0.75 -14; CaC12 25-300 g/l; 20-90°C) the hematite remains virtually unchanged. ( 3 ) The extraction rates are controlled by plumbojarosite leaching, which in concentrated pulps depends on both kinetic and thermodynamic factors. Temperatures > 80°C are required for the reaction to proceed at acceptable rates. (4) The equilibrium position at 90°C can be described approximately, under the conditions studied, as: tT/I:c ( II1 ) - - 1 0 I+ 0.5 + O.SmCaCl., ) -
t n It '
Minimum acid consumption (stoichiometric) thus requires working at high CaCI~ concentrations (300 g/I). ( 5 ) The potentials achieved during HCI-CaCIe treatment (about 800 mV versus SHE) permit the leaching of significant amounts of gold. The extraction rates increase as the CaC12 concentration rises and on raising the potential to 900 mV. At higher potentials the rates are practically independent of both parameters. Optimum conditions were found at 900 mV, at which potential the oxidation of elemental sulphur occurs slowly. ( 6 ) The extraction of 90-95% of gold, silver and lead is achieved under the following conditions: CaCI2, 300 g/l; 90°C; 900 mV; pulp density ratio, 1:2.5; 1 h. Reagent consumption is: HCI, stoichiometric with respect to the plumbojarosite content (HCI, 0.3-0.5 M): C12, 0.6-4 kg/t depending on the elemental sulfur content. ACKNOWLEDGEMENTS
The authors wish to thank Mr. K.G. Gray of the Tharsis Sulphur and Copper Company for supplying the ore samples. Thanks are also due to Mrs. E. Vilalta for her assistance in reflected light microscopy and to Mrs. M. Marsal, Department of Materials, ETSII, Barcelona, for her valuable co-operation in thc SEM and EDS studies.
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9 1() 1I
14 15 1(~ 17 18
19 2(I 21
j. VINAI+S E l .\1+
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LEA('ItlN(iOF Ag. ,Xu AND Pb FROM HEMATITE TAILINGS IN HCI-CaCI, MEDIA
25 26 27 28 29 30
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