detection and control of calcium sulfate precipitationin ...

•

CHAPTER 29

DETECTION AND CONTROL OF CALCIUM SULFATE PRECIPITATIONIN THE LEAD

CIRCUIT OFTHE HILTON CONCENTRATOR OF MOUNT ISA MINES LIMITED, AUSTRALIA

S.R. GRANO', P.L.M. WONG2, W. SKJNNER\ N.W. JOHNSON,3 AND J. RALSTON'

'SCHOOL OF CHEMICAL TECHNOLOGY, THE UNIVERSITY OF SOUTH AUSTRALIA, THE LEVELS, ADELAIDE, 5095, AUSTRALIA;

2METALLURGY, SCHOOL OF MATERIALS ENGINEERING, THE UNIVERSITY OF NEW SOUTH WALES, SYDNEY, 2052, AUSTRALIA;

JMOUNT ISA MINES LIMITED, MT. ISA, 4825, AUSTRALIA.

ABSTRACT Surface analysis by X-Ray Photoelectron Spectroscopy (XPS) of samples taken from the galena flotation section of the Hilton Con­ centrator of Mount Isa Mines Limited are presented. Flotation rate and pulp chemical data taken during the sampling period are also presented and discussed. Surface analysis suggested that, under normal operating con­ ditions, an overlayer of precipitated calcium sulfate existed at the surface of the sulfide minerals. In particular, there was a decrease in the surface concentration of calcium sulfate for the concentrate samples relative to that occurring on the tailing samples. This sug­ gests that the flotation process was selective against minerals which have calcium sulfate predominating at the surface. Heterocoagulation of hydrophilic calcium sulfate on to the galena surface considerably reduced galena flotation rate. Methods to control calcium sulfate precipitation, which include sodium carbonate treatment of the pulp, are also discussed. Sur­ face analysis results of samples taken from the Hilton Concentra­ tor during sodium carbonate addition to the grinding circuit are also presented. This shows clear evidence for the removal of calcium sulfate from the mineral surfaces without concomitant surface pre­ cipitation of calcium carbonate. Reasons for the apparent dispersion of precipitated calcium carbonate are discussed. Galena flotation rate was increased by the addition of sodium carbonate apparently due to the removal of interfering calcium sulfate.

(Grano, et aI., 1994a). The initial galena flotation rate in the first five cells of flotation (approximately 20 minutes residence time) has his­ torically been poor when using ethyl xanthate collector exclusively. Typical galena recoveries of only 10%, with a total ethyl xanthate addition of 120 glt, have been measured in the first five cells of flo­ tation (Grano, et ai., 1994a). Paraliellaboratory flotation tests on samples of Hilton ore using demineralised water at a pulp tempera­ ture of 25 0 C demonstrated that ethyl xanthate was capable of ad­ equately recovering galena under laboratory conditions (Grano, et al.,1991). Addition of the collector, diisobutyldithiophosphinate (Cytec col­ lector, 3418A) became mandatory to achieve reasonable galena recovery rate and rougher concentrate lead grade. The poor perfor­ mance of ethyl xanthate as a galena colleCtor could not be linked to solution reactions involving metabisulfite (MBS, S2052·) depressant, which was used in the plant start up reagent suite (Grano, et aI., 1994b). Its use was discontinued after commissioning and survey data showed poor galena recovery rates even in the absence of this reagent (Grano, et ai., 1994a). In the tests described in this paper MBS has not been added at any stage. Reasons for Poor Galena Flotation in the Hilton Concentrator The following reasons have been cited to explain this phenom­ ena (Grano, et ai., 1994a): 1. Low initial Eh in the plant lead roughers lowering adsorption of ethyl xanthate collector on to galena (Prestidge et ai, 1993).

INTRODUCTION The Hilton mine is situated 20 km north of Mt. Isa in north-west Queensland, Australia. The concentrator produces separate prod­ ucts of lead, zinc and a combined lead/zinc concentrate (LGM con­ centrate). The Hilton Concentrator was commissioned in January, 1990. In the Hilton ore, lead and zinc occur as galena and sphalerite, as for Mt. Isa ore. Pyrrhotite is the predominant iron sulfide mineral in the hanging wall with typical feed assays of between 15 and 30% for this ore type (Rohner, 1993). Pyrite is the predominant iron sul­ fide mineral in the footwall ore with typical feed assays of 5 to 15'% for this ore type. Dolomite is a significant component of the non-sulfide gangue and, potentially, can give rise to a high concentration of dissolved calcium in both the mine and circuit waters (Blanchard and Hall, 1942). The significance of water quality on flotation performance is discussed in more detail. Important reagent changes which have occurred in the Hilton Concentrator since commissioning have recently been reviewed

2. Elevated plant pulp temperatures which may retard the flota­ tion of galena with ethyl xanthate collector. 3. Decomposition of ethyl xanthate in the solution phase reduc­ ing ethyl xanthate availability for adsorption on to galena. 4. EX1ensive oxidation of galena prior to flotation modifying ad­ sorption of xanthate on to its surface. 5. The presence of precipitated overlayers which may impede adsorption of ethyl xanthate collector on to galena. The latter possibility appears to be the most likely reason for the inhibition of galena flotation with ethyl xanthate collector (Grano, et aI., 1994a) and results from gypsum precipitation on mineral sur­ faces under normal processing conditions (ie. in the absence of re­ agents added specifically to control gypsum formation). Evidence for gypsum precipitation was provided by a number of measure­ ments: (1) The product of the calcium and sulfate concentrations ex­

172

PROCEEDINGS OF THE XIX IMPC

Figure 1. Mount Isa Mines Limited Hilton Concentrator Lead Circuit Flowsheet, Head Grades and Reagent Additions. Feoo Composition (%} Po Zn Cu Pyn!1otrte Pyrite

SAG Mill

5.8 9.7 0.'

7.6

Total Non lIOn Sullide Sulfide Gangue

9.6

,7.2

58.S

Ball

Gypsum slime particles have been shown to strongly heterocoagulate on to galena surfaces (Sun, 1943). This was as­ cribed to the gypsum slime having a low zeta-potential (-18 mV) relative to that of the galena particles (-17 mV), which did not pre­ vent gypsum slime adsorbing on to galena (Sun, 1943). It was con­ sidered by Sun (1943) that slime coating was prevented if the ab­ solute zeta-potential of the slime was greater than 40 mV and was the same sign as the particle of interest.

Mill/ Cyclone To Zinc Circuli.

Lead Final

Concentrate. X

»

XP$ Sa.rq:lkt Point

To Low Grade Middlings Circuit

ceeded the solubility product for gypsum precipitation in all lead cir­ cuit pulps and circuit water samples (Grano, et aI., 1994a). (2) XPS analysis of pulp samples taken from the Hilton Concen­ trator showed that up to 40% of the surface of the minerals in the lead flotation feed was covered by calcium sulfate (Grano, et aI., 1994a). This paper describes further the results of XPS analysis of samples taken from the Hilton Concentrator with and without soda ash (Na2C03) addition. • . Calcium in the circuit water (internal recycle), generally, is de­ rived from the following sources: (i) Dissolution of the dolomitic component of the are. Blanchard and Hall (1942) have discussed the mechanism for the dissolution of dolomite as a result of its interaction with the oxi­ dation products of pyrrhotite. The overall reaction shown by Blanchard and Hall (1942) is: Fe7Sa + 1202 + 12H20 + 12CaC03 + 6Fe(OH)2 + 6Ca(HC03h + 6CaS04

-t FeS2

[1J

In effect, the dolomitic (basic) component acts to neutralise the acidiC oxidation products of pyrrhotite. (ii) Addition of lime for pH control during sphalerite flotation. Other studies in the Hilton Concentrator (Lauder, et aI., 1994) have shown very high concentrations of calcium dissolved in the pulps of the zinc circuit (3.2x1 0-2 mol dm-3) due to lime addition to this part of the circuit. This latter source of calcium ion contributes significantly to the calcium dissolved in the circuit water via the tail­ !ng, zinc and LGM product thickener overflows. This substantially Increases the concentration of calcium in the circuit water relative to that of the new water. REVIEW Problems Associated with Galena Flotation at High Calcium Con­ centration

Parsonage (1985) has studied the effect of gypsum slimes on galena flotation using zeta-potential and flotation techniques. The Hallimond tube flotation of galena with ethyl xanthate collector at pH 8 was only slightly affected by the presence of 3.5 gm dm-3 gypsum slimes. According to Parsonage (1985) gypsum slime coatings were expected to occur on the basis of zeta-potential measurements which showed that gypsum had a low potential energy barrier for heterocoagulation on to galena. This discrepancy between the ex­ pected and actual flotation behaviour was attributed by Parsonage (1985) to a chemical effect of unknown origin. In larger scale flota­ tion experiments, Parsonage (1985) found galena depression by 5.3 gm drn-3 gypsum slimes but ascribed this phenomena to froth destabilisation by the slimes. In addition to problems associated with gypsum formation, the high dissolved calcium concentration in the pulp liquor may promote unselective flocculation of the minerals in the pulp due to calcium ion adsorption on to sulfide minerals. This may increase their zeta­ potential to less negative values. Calcium (II) adsorption on to sphalerite can occur in the pH range 7.5 to 10.5 (Moignard, James and Healy, 1977). Unlike many other cations, calcium ion hydroly­ sis is not requisite for its adsorption on to sphalerite. Adsorption can result in sphalerite zeta-potential reversal at pH 7.5 to 8.0 and cal­ cium concentration of 5x10- 4 mol dm·3, well below the pH required for calcium ion hydrolysis (Moignard, J~mes and Healy, 1977). Potential Solutions to Problems Associated with Gypsum Precipita­ tion and High Calcium Concentration In general. solutions to the problems associated with gypsum precipitation and high calcium concentration can be grouped in the following manner:

0) Treatment of the process water in isolation of the pulps. This approach has the advantage that the waste material produced in the treatment (eg. calcite or calcium oxalate) may be removed prior to process water addition to the circuit. (ii) Treatment of the pulps in-situ. This approach has the advan­ tage that soluble calcium generated during processing itself can be addressed by reagent addition to the pulp. Also, if the reagent is carefully selected, extra benefits can be accrued. apart from simple removal of the interfering ion. In the case of carbonate treatment, adsorption of carbonate ion can substantially increase pulp disper­ sion (Sun, 1943).

The use of oxalic acid for treatment of circuit water at the Hilton Concentrator was discounted on the basis of reagent cost and its ineffectiveness at alkaline pH values (Espinosa-Gomez, Finch and Laplante,1987).Chelating reagents such as ethylenediami­ netetraacetic acid disodium salt (EDTA), while demonstrating in­ creased galena flotation rates in the presence of ethyl xanthate col­ lector, were discounted on the basis of reagent cost and high addi­ tions required. Soda ash treatment of the pulp was considered the

J

DETECTION AND CONTROL OF CALCIUM SULFATE PRECIPITATION IN LEAD CIRCUIT most viable option. Mechanisms in the Regulation of Galena Flotation by Carbonate Ion

It was realised that increased carbonate concentration via soda ash addition would have the following effects (depending on the carbonate concentration) : (i) Precipitation of soluble calcium as calcite. (ii) Dissolution of precipitated gypsum and concomitant calcite formation. (iii) Possible modification of the galena surface by carbonate ion which may alter its interaction with xanthate (Palsson and Forssberg, 1988). Parsonage (1985) found that the Hallimond tube flotation of galena was significahtly depressed in the presence of 3.5 gm dm·3 calcite slimes at pH 8 which was attributed to its heterocoagulation on to galena. The heterocoagulation of calcite (pH iep = 8.2, Espinosa- Gomez, Finch and Laplante, 1987) on to galena is how­ ever expected to be highly dependent on pH and carbonate concen­ tration as shown by Sun (1943). Thus, while calcite slimes adsorbed on to galena at low carbonate concentration, there was highly effec­ tive dispersion of calcite from galena at high carbonate concentra­ tions of 5x1 0.3 mol dm-3 (Sun, 1943). At high carbonate ion concen­ tration, the zeta-potential of galena was ·120 mV which was suffi­ ciently negative to prevent calcite slime (-30 mV) adsorption (Sun, 1943).

173

Ralston, 1994) are negatively charged at pH values greater than 6.0, suggesting that if a lead carbonate species is formed its disper­ sion from the galena surface is likely at alkaline pH values. The dra­ matic decrease in the zeta-potential of galena in the presence of carbonate ion as noted by Sun (1943) is likely to be due to hydrocerussite formation. Significantly, Fuerstenau, et al. (1987) also showed that the surface of anglesite transforms into lead carbonate in the presence of 1x10-3 mol dm-3 carbonate causing the zeta-potential of angles­ ite (positive in the pH range 7 to 10) to decrease to negative values for pH values greater than 5. Fornasiero, et al. (1994) has shown that galena oxidised in air saturated water develops lead hydroxycarbonate at its surface, which given the lower pHiep of this compound, may show improved dispersion from the galena surface at alkaline pH values. Thus, while increased carbonate concentra­ tion may decrease the stability of certain lead-xanthate compounds the improved dispersion of galena oxidation products and calcium compounds may have a dominating influence. Methodology of Investigations and Analytical Techniques This paper focuses on two plant surveys in the Hilton Concen­ trator investigating the effect of soda ash addition to the SAG mill feed on the flotation rate of the minerals in the galena roughing and scavenging section. Pulp chemical changes in the flotation feed are determined. XPS analysiS was carried out on plant samples taken for each survey from selected points in the lead Circuit. EXPERIMENTAL Plant Surveys

Palsson and Forssberg (1988) showed, through thermodynamic calculations, that increased carbonate concentration (total H2 C03 Conc. = 2.16x1 0. 3 mol dm-3 ) decreased the stability of the mixed lead hydroxo xanthate species, Pb(OH)EX (s), in favour of hydrocerussite, Pb 3 (OHb(C0 3b (s), for pH values greater than 9.0 (Eh +177 mV). Palsson and Forssberg (1988) considered Pb(OH)EX (s) to be an important surface specie occurring on ga­ lena at alkaline pH values. At high carbonate concentrations the most stable xanthate specie was the bulk lead xanthate complex, PbEX 2 (s), under these conditions (Palsson and Forssberg, 1988). Palsson and Forssberg (1988) considered that at high carbonate concentrations the stability of hydrocerussite would decrease ga­ lena hydrophobicity, in spite of Pb(EX)2 (s) stability, because the latter specie is not necessarily a surface related compound. How­ ever, slow formation kinetics of the ternary compound, hydrocerussite, may have a dominating influence, allowing Pb(OH)EX (s) to exist at high carbonate concentrations. Palsson and Forssberg (1988) showed that the stability of lead sulfate (pH 90% of the signal from < 8 nm. After the initial scan the surface is than etched to approximately 2.5 nm depth using an accelerated Ar+ beam at 3 kV. A second XPS spectra is recorded and atomic concentrations measured. The sec­ ond spectra gives an indication of the depth of surface layers exist­ ing on samples. The powdered nature of the samples dictates that some proportion of the surface will not be etched due to shadowing effects. Only spectra for the surface prior to etching are shown.

water quality. Traditionally. initial galena retardation results in an unusually shaped lead grade/recovery relationship in the rougher section (Grano. et aI., 1994a). For the current surveys the lead grade/recovery relationship (not shown) has a normal shape sug­ gesting that initial galena retardation was less pronounced. Figure 2. Mineral Fraction Remaining Versus Residence Time for the Plant Lead Rougher and Scavenger Sections for Surveys 1 and 2. (a) Galena, (b) Sphalerite and Water.

Q)

iii o en

g C1

c:

Supporting Laboratory Flotation Tests

'c '(ij E Q)

Laboratory flotation tests were also carried out on Hilton Con­ centrator flotation feed pulp. The flotation experiments investigated the effect of soda ash addition on galena and sphalerite flotation kinetics.

0::

c: .2

~

u.

At the flotation feed sample pOint no reagent had been added in the circuit. After sampling the pulp into individual samples suitable for flotation tests, the pulp was transferred into a 3 dm3 Agitair flo­ tation machine. At this point the pH was 8.0. After conditioning for 5 minutes with soda ash (O to 4.0 kg/t) a small pulp sample was taken from the flotation cell and immediately centrifuged. The super­ natant was then analysed for C032- !'Ind HC03- by standard meth­ ods. The pH at the highest addition of soda ash was 9.8.

(b) Sphalerite and Water

o The pulp was then conditioned with 80 g/t of ethyl xanthate for a further 5 minutes prior to flotation at 0.5, 2.0 and 8.0 cumulative flotation times. The data are displayed as galena and sphalerite flo­ tation rate constants, assuming first order flotation kinetics of a single floatable species, as a function of carbonate concentration.

A • ...

SUf'IIey 1. Sphalerite

SUf'IIey 2, Sphalerite

SUf'IIey 1, Water

SUf'IIey 2, Water

Residence Time Minutes)

.1--\-t-+-+-+-i-+--+-+-+-t--+-;-+-+-t-+++-+-j-+.....-t......,f-+-++-+-!

o

10

20

30

40

50

60

Thermodynamic Calculations and Assumptions The hydrolysis and complexation constants compiled by Palsson and Forssberg (1988) for the Pb-Ca-S-H2C03-EX system were used in all calculations. The program used was SOLGASWATER {Eriksson, 1979}. Sulfur species considered were elemental sulfur, thiosulfate and sulfate. No adjustment to the equilibrium formation constants was made as was carried out by Palsson and Forssberg (1988) in an attempt to simulate pulp sulfate concentrations. In this study, the high sulfate concentration was considered to be a separate compo­ nent in the pulp, not necessarily derived from oxidation of the sul­ fides during processing. RESULTS AND DISCUSSION Flotation Kinetics of Minerals in the Lead Roughing and Scaveng­ ing Section Survey 1 exhibited moderate galena recovery in the first five cells of the rougher bank (Fig. 2a) where ethyl xanthate has been the only collector added at that stage. This is an improvement over that historically observed (Grano, et aI., 1994a) and is generally ascribed to the use of soda ash in the zinc circuit improving circuit

Survey 2 demonstrated increased galena recovery, due to soda ash addition to the SAG mill, which perSisted to the scavengers (Fig. 2a). However this was offset by increased flotation rate of sphaler­ ite which lowered selectivity against sphalerite (Fig. 2b). Very simi­ lar water recoveries (Fig. 2b) are apparent for both surveys indicat­ ing that differences in flotation kinetics between the surveys was not related to physical variables which influence water recovery and subsequently entrainment. Pulp Chemical Measurements The flotation feed sample in Survey 1 exhibited low carbonate and bicarbonate concentrations at pH 8.5 (Table 1). Carbonate spe­ cies would be derived from dissolution of CO2 from air and also carbonate gangue via reaction [1]. At pH 8.5, the dissolution of CO2 from air would result in approximately 1x1 0-4 and 3x10-3 mol dm-3 of carbonate and bicarbonate respectively. The low carbonate con­ centration in Survey 1 does not control the calcium concentration which would be derived from Ca (II) and CaS04 (aq). Given the high calcium and sulfate concentrations, CaS04 (s) precipitation would be expected. This was confirmed by both XPS analysis and specia­ tion modelling_

DETECTION AND CONTROL OF CALCIUM SULFATE PRECIPITATION IN LEAD CIRCUIT The addition of soda ash in Survey 2 dramatically lowered the calcium concentration as expected (Table 1). Under these condi­ tions complete dissolution of CaS04(S) would be expected as was evident from the increase in sulfate concentration and confirmed by XPS analysis and speciation modelling. There was also a significant decrease in Mg (II) concentration due to its hydrolysis and possible adsorption at the elevated pH (8.8). Table 1. Concentration of Soluble Species in Plant Flotation Feed for Surveys 1 and 2. All Concentrations in mol dm·3 . Soda Ash (Na2CQ3) Addition to the SAG Mill Feed in Survey 2. Survey

1

2

Difference (2-1)

Na2C03 (kglt)

0

4

4

0

2.04e-2

2.04&-2

1.4ge-2

6.99&-4

-1.426-2

[C03:l-]

Added ICa

2+

[Mg

I

2+

l S04

2­

[C03

2­

-3.266-3

4.00e-3

]

]

I

[ HC0 3 ] [ C0 3"-]

2.92e-2

3.51e-2

5.90e-3

3.33e-4

1.27e-3

9.378-4

9.51e-4

9.848-4

3.3Oe-5

1.286-3

2.256-3

9.7Qe..4

+ [HCOa ] pH

8.5

0.3

X-Ray Photoelectron Spectroscopy Table 2 (Surveys 1 and 2) shows the surface atomic concentrations of the elements. The XPS results of Survey 1 will be discussed in isolation of Survey 2 with focus on the surface chemical changes occurring through each stage. Survey 1 The SAG mill discharge shows low exposure of the metals, lead and zinc, due to a relatively large coverage of oxygen which is present as sulfate (highest contribution), carbonate (minor), silica (major) and alumina (minor). The C 1s spectra shows considerable charge shifting of 4.0 eV indicating a predominantly insulating surface (Fig. 3A). For un­ charged surfaces, the C 1s Signal of carbon bound only to other carbon or hydrogen atoms occurs at 284.5 eV (Briggs and Seah, 1983). Carbonate is indicated by a peak at 293.5 eV for this charge shifted sample. The sulfur emission region shows a S 2P3/2 and S 2P1/2 doublet with the former occurring at 161.1 eV for uncharged, unreacted py­ rite (Buckley and Woods, 1987). Sulfate lies at a binding energy of 169 eV for uncharged substrates (Buckley and Woods, 1987). The major contribution of sulfate, taking into account charge shifting of the spectra, to the total sulfur present is apparent from Figure 4A

175

and is estimated to be five times higher than the sulfide contribution. The sulfate is likely to occur as calcium sulfate rather than as metal sulfates since the metal atomic concentrations are low but calcium occurs at relatively high concentration. For uncharged surfaces, the Fe 2p signal of FeS2 lies at a bind­ ing energy near 707.4 eV (Buckley and Woods, 1987), whilst oxidised iron in ferrous and ferric species lie at 708 eV (Briggs and Seah, 1983) and 711 eV (Buckley and Woods, 1987) respectively. Due to substantial overlap of these latter signals as well as the vari­ able charge shifting between the samples, it is difficult to differenti­ ate the oxidised components. All the exposed iron occurs in the hydroxide form for the SAG mill discharge (not shown in figures). The Ilotation leed shows very similar characteristics to the SAG milt discharge with the same surface coverage of calcium sulfate. Etching demonstrates the coverage to be relatively thick. The main differences between this sample and the SAG mill discharge is the increase in exposure of iron after etching. This is most likely to be iron oxyhydroxide species emanating from the grinding media used in the ball mill (Grano, 1990). The lead rougher tail and lead scavenger tail show very similar atomic concentrations with similar chemical lorms present at the surface, ie., the surface is dominated by Calcium sulfate. The scav­ enger tail shows reduced exposure of lead and zinc compared to the rougher tail. The charge shifting which occurs on these samples (4.5 eV, Fig. 3A) is slightly greater than that on the flotation feed sample indicating a more insulating surface. The rougher concentrate shows increased exposure of lead and to lesser extent zinc and copper. Now the major sulfur form is sul­ fide (Fig. 4A) though a significant quantity still occurs as sulfate and, possibly, thiosulfate. The lead now occurs predominantly in the sul­ fide form (galena, lead-xanthate complex; Laajalehto, et aI., 1991). There is less oxygen at the surface due to lower aluminium, silicon and sulfate with a commensurate increase in carbon. Unlike both tailings samples, etching does not reduce the surface concentration of carbon dramatically. The pOSition of the carbon is characteristic of the alkyl chain group in xanthate (saturated hydrocarbon, Fig. 3A). Etching, does however, dramatically increase the exposure of all metals especially iron. The occurrence of iron was in the sulfide form as well as hydroxide (not shown in figures). The C 1s spectra (Fig. 3A) shows much less charge shifting than the feed indicating the surface is more conducting which is consistent with the in­ creased exposure of metal sulfides at the surface. The final concentrate shows increased carbon and decreased oxygen surface concentration relative to that occurring on the rougher concentrate (Table 2). There is increased exposure of all metals, especially lead. However the total sulfur on the surface is reduced due to a lower contribution from sulfate (Fig. 4A). Calcium is also reduced at the surface of the final concentrate indicating that the flotation process is selective against minerals which have cal­ cium sulfate predominating at the surface. A feature is the presence of exposed iron in the sulfide form in­ dicating the probable collector-induced recovery of pyrite into the final concentrate. It should be noted that no inorganic depressant was being added in either of these surveys. The C 1s spectra shows similar charge shifting as on the rougher concentrate indicating simi­ lar conducting properties (Fig. 3A). The lead retreat tail shows reduced carbon and increased oxy­

PROCEEDINGS OF THE XIX IMPC

176 Table 2. XPS Results Inla if I Surface Only

Survey 1

Survey 2

Sample Point SAG Mill DIC Flotation Feed Rougher Tall PbScav. Tall Rougher Con. FInal Cone. Retreat Tall SAG Mill DIC Flotation Feed Rougher Tall PbScav. Tall Rougher Con. FInal Cone. Retreat Tall

Surveys 1 and 2 Surface Atomic Concentrations (mol %). C

0

Pb

Zn

Fe

S

AI

SI

Ca

Mg

16.3

56.4

0.2

0.5

1.4

6.1

2.5

4.5

4.1

2.5

16.6

56.7

0.3

0.4

1.3

6.1

2.1

4.7

3.9

2.5

18.4

56.1

0.3

0.6

1.8

4.4

2.3

4.9

3.8

2.8

15.4

56.1

0.2

0.4

0.9

7.6

1.3

3.5

4.9

1.8

26.3

44.5

2.4

1.2

2.5

10.2

1.6

2.5

3.6

1.5

2.2

2.2 1.2

31.4

40.1

2.9

1.6

2.7

8.3

1.6

3.6

18.6

53.9

0.1

0.7

0.7

9.3

0.9

2.1

6.4

20.7

55.2

0.4

0.5

1.4

2.9

1.9

6.2

1.5

1.9

3.6

2.6

6.4

1.6

3.2

5.5

1.4

1.9

17.1

53.6

0.5

0.8

2.1

15.9

54.5

0.3

0.6

1.3

5.1

2.6

14.7

55.6

0.5

1.1

2.1

3.8

2.5

6.2

1.9

3.2

35.7

38.5

1.9

1.1

1.8

7.7

0.9

2.7

1.9

3.1

33.8

35.2

3.8

1.7

2.6

9.4

1.3

.2.9

1.3

5.1

21.5

52.1

0.4

1.4

1.3

6.2

1.4

4.1

3.2

2.7

gen surface concentration relative to that occurring on the final con­ centrate. Despite having a higher bulk lead analysis than the scav­ enger tail, its exposure of lead at the 'Surface occurs at a similar level to the scavenger tail. The exposure of sulfur at the surface is less than on the final concentrate but is now dominated by sulfate forms (Fig. 4A), again indicating that calcium sulfate dominated minerals report to the tails. There is no iron sulfide apparent in the spectra (not shown in figures) of the retreat tail which indicates that the ex­ posed pyrite in the rougher concentrate all reports to the final con­ centrate. The cleaner/retreat section in this survey demonstrated poor selectivity against pyrite which apparently undergoes collector­ induced flotation into the final concentrate. The Cis spectra (Fig. 3A) shows intermediate charge shifting of the samples surface between that occuning on the final concentrate and the rougher/scavenger tailings. Survey 2 The XPS results of Survey 2 will be discussed in light of Survey 1 with focus on differences between each survey. The most striking alteration of the SAG mill discharge surface for this survey is the dramatic reduction in both calcium and sulfur exposures. The calcium is now reduced to very low levels with the sulfur occurring in equal atomic concentrations of sulfate and sulfide (Fig. 48). Apparently the soda ash addition in this survey is suc­ cessful in removing the overlayer of calcium sulfate which was dominant in Survey 1. There is an increased exposure of sodium and carbon at the surface. The increase in carbon occurs mainly due to the increased proportion of carbonate at the surface (Fig. 38). It is impossible to ascribe the chemical association of this car­ bonate. Etching also shows more carbonate on the surface of this sample than in Survey 1. There is also increased exposure of lead at the surface relative to that occurring in Survey 1. The extent of charge shifting is also lower for this sample relative to Survey 1, suggesting a more conducting surface as a result of the increased exposure of metal sulfides (Fig. 38). The flotation feed sample again shows much lower levels of calcium sulfate than the equivalent sample in Survey 1 . As a result

there are increased exposures of lead, zinc and iron. The increase in zinc exposure is greater than that occurring for lead which may explain the lower selectivity against sphalerite apparent for Survey 2. The extent of charge shifting is again lower than that for the equivalent sample in Survey 1 (Fig. 38). 80th the lead rougher tail and lead scavenger tail show reduced exposure of calcium than the equivalent samples in Survey 1. Sul­ fate is the dominant form of sulfur on the surface though its relative and absolute atomic concentration is much lower than in Survey 1 (Fig. 48). There is increased exposure of lead and zinc on these surfaces with a similar range of chemical environments apparent for lead as occurred in Survey 1. The rougher concentrate shows reduced exposure of calcium than the equivalent sample in Survey 1. The major surface alter­ ation is the greatly increased exposure of carbon which is nearly all as hydrocarbon suggesting improved adsorption of xanthate collec­ tor occurs under these conditions (Fig. 38). The final concentrate, in a similar pattern to the other samples in this survey, showed reduced exposure of calcium though calcium was also low on this sample in Survey 1. No sulfate is present on the surface of the final concentrate with all sulfur occurring in the sulfide form (Fig. 48). The total sulfur at the surface is actually in­ creased in this survey. The increase in sulfide is attributable to the increased exposure of lead for the final concentrate whose bulk analysis also indicated a higher lead grade. The lead retreat tail, in a similar pattern to the other samples in this survey, showed reduced exposures of calcium. The most sig­ nificant change that occurred on this sample was the occurrence of pyrite forms of iron at the surface (not shown in figures). The met­ allurgical results also show improved rejection of pyrite in the cleaner/retreat section for Survey 2. Some general comments regarding surface alterations in Sur­

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DETECTION AND CONTROL OF CALCIUM SULFATE PRECIPITATION IN LEAD CIRCUIT C1s

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300 298 296 294 292 290 288 286 284 282 280

Binding Energy, eV

Binding Energy, eV

Figure 3B. SURVEY 2. CARBON 1s SPECTRA

Figure 3A. SURVEY 1. CARBON 15 SPECTRA

(a) SAG MILL DISCHARGE. (b) FLOTATION FEED. (e) LEAD ROUGHER TAIL, (d) LEAD SCAVENGER TAIL, (e) LEAD COMBINED ROUGHER CONCENTRATE, (1) LEAD FINAL CONCENTRATE. (g) LEAD RETREAT TAIL.

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