Thesignificance ofthemineralogical andsurface ... ofthemineralogical andsurface characteristics...

The significance of the mineralogical and surfacecharacteristics of gold grains in the recoveryprocess

SYNOPSIS

by C. E. FEATHER B.Se. Hons. (Cape Town)* and G. M. KOEN D.Se. (Pretoria)*

A description is given of the nature and occurrence of gold at various stages in the process of its recovery fromWitwatersrand and related ores. Electron microprobe investigations show that the surface of gold grains in the oreis already coated. During metallurgical processing, further coatings, mainly hydrated iron oxides, accumulate on thegrains. Theories are advanced for the formation of these coatings, and the adverse effects on metallurgical operationsare discussed.

SINOPSIS

'n Beskrywing word gegee van die aard en voorkoms van goud op verskeie stadiums in die prosesse van herwinningvan Witwatersrand en verwante ertse. Elektronemikrobuis ondersoeke het aan die lig gebring dat die oppervlaktesvan goud greintjies in die erts alreeds aangepak is. Gedurende metallurgiese bewerking versamel meer aanpaksels.veral gehidreerde ysterhidroksiede. op die greintjies. Teoriee vir die ontwikkeling van hierdie aanpaksels word voor-gel!! en die ongunstige invloede op metallurgiese prosesse word bespreek.

INTRODUCTION

Several suites of plant productsand numerous individual sampleswere received from Anglo AmericanCorporation gold mines, notablyWestern Deep levels, the VaalReefs mines, Western Holdings, andFreddies. These samples were ex-amined in an attempt to find outmore about the degree of liberationof the gold, its mode of occurrenceand associations, size distribution,and other features that could in-fluence the efficiency of gold re-covery.

A number of reef samples werealso studied in detail to obtaininformation on the characteristics ofthe original gold particles beforetheir exposure to plant conditions.

Heavy concentrates were pre-pared from the various plant pro-ducts, and these, together with thelight fractions, were examined byore microscopy and other mineral-.ogical techniques supplementedwhere necessary by electron-micro-probe analysis of selected grains.The relative abundance ofthe variouskinds of gold-bearing grains presentin the samples was expressed interms of mass percentages, the massesbeing calculated from the sizes of thegold particles. It must be stressedthat, owing to the comparativelysmall number of gold grains exposedin most of the polished sections, theresults are approximate and give

..Anglo American Research Laboratories.Anglo American Corporation Ltd.

only a rough indication ofthe relativeamounts.

Gold particles that, although at-tached to or partially ~occluded bygangue constituents, are sufficientlyexposed to permit ready dissolution(or amalgamation) are, for the presentpurpose, classified as "free gold"-see, for example, photomicrograph I,Plate I.

RESIDUESIn an investigation of this nature

the most logical place to start is atthe tail-end of the plant - theresidue dumps. A knowledge of theappearance and mode of occurrenceof the gold still present in residuescould well provide important cluesto why this gold had not been re-covered in the plant.

We have studied residue samplesfrom several localities and havefound that certain features are com-mon to all.

One of the main gold carriers inall residue samples (in calcine leachresidues, frequently the only goldcarrier) is thucholite, that enigmatichydrocarbon loaded with occlusionsof uraninite and, usually, gold. Totalprecious metals assays on the thucho-lite concentrates we have preparedgave values ranging from a fewhundred to several thousand gramsper tonne.

Thucholite is not concentrated inthe plant, nor is much of its loadof occluded gold released on calcin-ing during sulphuric acid manu-facture, as practised in the Group. Inthucholite, the property of light-

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

ness is combined with extreme re-fractoriness, and it is not surprisingthat this constituent is such a com-mon cause of losses in gold-plantresid ues.

The problem of recovering andtreating thucholite is still beingexamined, and in this paper onlycasual reference will be made to thisimportant material.

Apart from thucholite, sulphidegrains occurring in residue sampleswere seen to occlude small specks orveinlets of gold. More rarely, goldocclusions were noted in quartz,in poorly preserved uraninite, skut-terudite, leucoxene, limonitic ma-terial, and a variety of other gangueconstituents.

An occasional grain of free goldwas seen in most of the residuesamples, and, in spite of its scarcity,this form of gold usually accounts foralmost the entire gold content of thesample (excluding gold associatedwith thucholite), as reference toTable I will show. These grains rangein size from less than 0,075 mm toover 0,075 mm, and the majorityhave the following features in com-mon:(i) The grains are enveloped in a

distinct coating of hydrated ironoxide.

(ii) The grai ns are freq uently flat-tened or otherwise distorted,and extraneous mineral mattermay have been pressed into thegrai ns.

It is understandable that smallspecks of gold occluded in gangue

FEBRUARY 1973 223

PLATE I

Description of Photomicrographs

for ready dissolution. Freddies: Minus3/4" Dry crusher product. X 620.

2 Photomicrograph 2

Distorted gold grain (G). WesternHoldings: Feed to Cleaner Tables Nos.I and 2 banks. X 600.

Photomicrograph 4Partially amalgamated gold. The

porous zone of the grain is pale in ap-pearance, the colour gradually deepen-ing to normal golden yellow towardsthe solid, unamalgamated portions.Western Holdings: Johnson Concen.trator Tails No. 3 Bank. X 600.

Photomicrograph 5Gold grain enveloped in transparent,amorphous material. Western Hold-ings: Mine Sludge. X 600.Photomicrograph 6

Gold (white) encrusted by hydratediron oxide (grey). Sallies Dump. X 600.

--Photomicrographs --

3 4--

5 6-- Photomicrograph 3

Flattened gold grain (G). The flake isintersected at right angles by thesurface of the polished section.Freddies: Secondary cyclone underflow.X 620.

Photomicrograph IFractured aggregate of pyrite (grey)

and gold (white). In spite of the goldbeing attached to pyrite and partlyoccluded by it, it is sufficiently exposed

224 FEBRUARY 1973 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Precious metals assay Gold occurrence(Mass %) locked*

Gold Silver fLm Free

7,9 1,7 +15078}

I

Insufficient + 75 10Insufficient - 75 12

12,7 1,6 +15076 }

I

12,0 1,7 + 75 8Insufficient - 75 16

+15083 }

5,8 1,3 + 75 2- 75 15

3,8 0,7 +1509~ }19,7 2,2 + 75 <I

40,3 4,9 - 75

+150 53}

I

141.6 13,0 + 75 45 <I- 75 2

+15079 }14,7 2,0 + 75

- 75 20

+150 53

}161,9 13,5 + 75 46 <I- 75 I

+150 47}190,1 14,8 + 75 48 <I- 75 5

+150 62}18,1 2,4 + 75 31- 75 6

+150

92}0,5 0,6 + 75 8

- 75

+1500,4 0,4 + 75 insufficient

- 75

Crusher rake +600 82

I

Classifier +150 16Underflow -150 2

Crusher rake +600 <~~I

Classifier +150Overflow -150 94 j

Ball mill feed +600 85

I

+150 12-150 3

Ball mill +600 64}Cyclone +150 19Underflow -150 17

Ball mill +600 21

}Cyclone +150 27Overflow -150 52

Tube mill +6003~}Discharge +150

-150 60

Secondary cyclone +600 14}Underflow +150 39-150 47

Secondary cyclone +600

9!}Overflow +150

-150

Combined residue +600<~}Washed +150

-150 97

Freddies washed +600 <Ir~sidue (incompletely +150 4

-

constituents may find their way Intothe residue, but the loss of the freegrains is probably related to theheavy tarnish on the grains and tothe effects of excessive milling, whichchanges the properties of the goldgrains. In some of the residuesamples, gelatinous materials remini-scent of settling agents were seen.This material may envelop mineralgrains and could conceivably haveprevented the dissolution of someof the gold grains.

The question that now arises is,when and how did the grains of freegold acquire the properties thatprevented their recovery in theplant? Later in this paper the mech-anism of coating the grains and the

plant conditions that contribute tothe formation of a coating will beexamined in some detail.

Also, the fact that occluded gold(again excluding gold associated withthucholite) constitutes such a verysubordinate proportion of the goldin most of the residues examinedindicates that gold has been liberatedadequately in the plant - as a matterof fact, the flattened appearance of somany of the grains suggests grossover-milling.

As a next step we shall examinethe occurrence of gold in the variousplant products; but, before doing so,let. us consider the gold occurringin the mine sludge, before it evenreaches the plant.

TABLE IFREDDIES CONSOLIDATED MINES LIMITED

SUMMARY OF MINERALOGICAL DATA

MINE SLUDGi:The appearance of the sludge

differs from mine to mine, and de-pends largely on the particular pro-cedure adopted to separate thesludge from the mined ore. In themine sludge from Western Hold-ings, for example, the gold grains areconsiderably coarser grained thanthose in the Western Deep Levelssludges. (See Tables IIIand IV.) How-ever, there are certain significantfeatures in common with all thesludge samples examined. These are:

(i) Au riferous thucholite is abund-ant.

(ii) Almost all ofthe remainderofthegold is liberated, the commonpractice of drilling and blasting

Designation of sampleSize analysis

Dry crusher productMinus r from SymonsScreen

dissolved

*Excluding gold locked in thucholite

fLm

+600+150-150

Wt. %

9433

150 96

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY FEBRUARY 1973 225

Size analysis Precious metals assay Gold occurrenceDesignation of sample (Mass %) Locked *

fLm Wt. % Gold Silver fLm Free

Washing plant +150 8

I

22,0 1,6 +150

99}Overflow -150 92 24,4 1,7 + 75 I

- 75

Washing plant +150 91 31,8 0,8

I

+15078

}I

Underflow -150 9 131,8 6,5 + 75

I

IRake return - 75 21

Waste washing plant

I

+150 89}I

<IWaste sludge 18,3 -150 11

Waste washing plant +Imm 1,1 + 150-300 88} <IReef sludge and +300 1,9 -150 12stock pile fines +150 5,3

-150 6,9

Low grade +150 61 9,2 0,8 +150

95}Primary mill discharge -150 39 18,3 1,5 + 75 5

- 75

Low grade +150 55 9,9 0,8 +1506I}Primary cyclone -150 45 17,0 1,2 + 75 I

Overflow - 75 38

Low grade +150 84 12,4 1,0 +15069 }Pri mary cyclone -150 16 57,6 3,6 + 75 I

Underflow - 75 30

Low grade +150 56 23,0 2,7

I

+150 17}

I

Secondary tu be -150 44 171,7 9,3 + 75 68 <IMill discharge - 75 15

Low grade +150 18 3,7 0,2 +15080 }

I

Secondary cyclone -150 82 8,8 0,8 + 75 <IOverflow - 75 20

I I

---Low grade +150 81 20,S 1,9 +150

78 }Secondary cyclone -150 19 289,0 13,3 + 75 4Underflow - 75 18

+1507I~+ 75 <I

- 75 28 J+150+ 75 4- 75

+150+ 75 <I- 75

I

+150Br}+ 75 I

- 75 18

I

+15039 }+ 75 I

- 75 60

I

+150 66 13,7-150 34 22,3

--+'50 57 7,6-150 43 15,4

+150 81 14,5-150 19 55,2

+150 68 14,9-150 32 164,2

+150 15 3,1-150 85 11,8

+150 75 8,4-150 25 97,2

TABLE 11

VAAL REEFS WEST GOLD MINESUMMARY OF MINERALOGICAL DATA

Low gradePulp head orSlimes original

Low gradeUnwashedResidue

I

I

1--High gradePrimary millDischarge

High gradePrimary cycloneOverflow

+150+ 75- 75

+150+ 75- 75

1,11,8

0,71,2

High gradePrimary cycloneUnderflow

1,03,2

I

I

_0,8

I

5,8

1,28,3

+150+ 75- 75

+150+ 75+ 75

High gradeSecondary tubeMill discharge

High gradeSecondary cycloneOverflow

0,10,8

High gradeSecondary cycloneUnderflow

High gradePulp head orSlimes original

High gradeUnwashed residue

+150+ 75- 75

+150+ 75- 75

Flotation plant tails


~~}I

97}I

I

<I

3

80 }20

65 }33

<I

2

33 }625

8317

<I

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY226 FEBRUARY 1973

I

Size analysis Precious metals assay Gold occurrenceDesignation of sample (Mass %) Locked"

p.m Wt. % Gold Silver p.m Free

Sludge from I +15089 }No. 3 shaft Not determined + 75 I

I + 75 11

Sludge from I +150691No. 2 shaft Not determined + 75 I

I - 75 30 J

Low grade +2.4 mm 56 2 21Dry screening +Imm 18 13 2Minus 9" +400 1I 10 I

+200 7 10 I + 200----400 75}+150 3 12 2 +150 19 >1-150 5 19 2 -150 6

Low grade +2.4 mm 70 7 -Ball mill +Imm 12 14 IFeed +400 7 13 2

+200 6 15 2 + 200-400 77}+150 3 26 4 +150 20 >1-150 2 62 8 -150 3

Low grade +150 54} 11 I +150 66}Akins classifier -150 46 + 75 18 >1Overflow - 75 15

Low grade +150 30} 41 6 +150

~}Tube mill -150 70 + 75 >1Discharge - 75

Low grade +150 49} 80 9 +150 83

}Secondary -150 51 + 75 15 >1Cyclone feed - 75 2

Low grade +1509~}

10 2 +150 65}Secondary cyclone -150 + 75 28 >1Overflow - 75 7

Low grade +150 54} 56 7 +150 51

}Secondary cyclone -150 46 + 75 40 >1Underflow - 75 9

Low grade +150 30} 45 6 +150 53

}Tertiary cyclone -150 70 + 75 43 >1Overflow - 75 4

Low grade +150 43} 80 10 +15062}Tertiary cyclone -150 57 + 75 33 >1

Underflow - 75 5

Low grade +150 17} 100 13 +150 23

}Johnson tails -150 83 + 75 64 <I

- 75 13

High grade +1509~}

119 15 +150 77

}Tube mill -150 + 75 17 <IDischarge - 75 6

High grade +150 32} 60 7 +15066}Secondary cyclone -150 68 + 75 28 <I

Feed - 75 6

High grade +150100 } 27 4 +150 55

}Secondary cyclone -150 + 75 28 <I,Overflow - 75 17

High grade +150 33} 143 18 +15032}Secondary cyclone -150 77 + 75 50 <I

Underflow - 75 17

High grade +1509:}

58 8 +150 38} ITertiary cyclone -150 + 75 35 I <IOverflow - 75 27

High grade +150 31} 240 34 +150 ~}Tertiary cyclone -150 69 + 75 IUnderflow - 75 16

High grade +150 29} 334 42 +150 57}I

Johnson Tails -150 71 + 75 31 <I- 75 12

TABLEIIIWESTERN DEEP LEVELS GOLD MINE



Designation of sample

Belt concentratorFeed

Belt concentrates

Belt tails

*Excluding gold locked in thucholite**Kg/tonne


Mine sludge

No's I and 2Shaft sludge

No. 3 shaft sludge

Composite rakeClassifier overflow

Composite rakeClassifier return

Composite washingPlant cycloneUnderflow

Composite dewateringCyclone underflow

Composite ballMill feed

Composite ball millCyclone overflow

Composite secondaryTube outlet

Composite secondaryCyclone underflow

228 FEBRUARY 1973

Size analysis

TABLE I11 (Continued)

Precious metals assay

p.m

+150-150

+150-150

+150-150

Wt. %

41 }59

25 }75

31 }69

Gold

2324

Silver

269

75**

1293 216

TABLE IVWESTERN HOLDINGS


p.m

+300+150+ 75- 75

+300+150+ 75- 75

+300+150+ 75- 75

+6mm+3mm+300+150+ 75- 75

+300+150+ 75- 75

+300+150+ 75- 75

+13mm+6mm+3mm+300

S

+150+ 75- 75

+300+150+ 75- 75

+300+150+ 75- 75

+300+150+ 75- 75

Size analysis

Wt. %

Not determined

2~

}5323

3~

J4124

21

}3437

1}~~

}167

~~

J3114

I~

}29

40

28

}241830

2~

}4532

193438

9


Gold Silver

Gold occurrence(Mass %)

p.m Free

9**

+150+ 75- 75

+150+ 75- 75

Locked*

~~}I

H}I

53

I

3512

<I

<I

18,0

9,0

41,8

41,2

63,0

350,3

21,3

17,7

50,2

52,1

+150+ 75- 75


p.m Free

+200+150+ 75- 75

+150+ 75- 75

+150+ 75- 75

+150+ 75- 75

+ 150-300+ 75- 75

+150+ 75- 75

+150+ 75- 75

+ 150-300+ 75- 75

+150+ 75- 75

+150+ 75- 75

+150+ 75- 75

Locked*

41

}2731

I

<I

IInadequateSample

II

Inadequate5ample

I64

}2510

80

}152

3

~~}I

I

3

48

}437

2

68

}282

2

66

}242

8

99 }

:!}I

<I


Size analysis

p,m Wt. %

+30011}+150 32

+ 75 35- 75 16

+300 ,:

f+150+ 75 39- 75 34+300

2~ f+150+ 75 45- 75 32

+30010}+150 35

+ 75 36- 75 19

+300':}+150

+ 75 33- 75 25

+300

~}+150+ 75 45- 75 23

+300,if+150

+ 75 46- 75 22

+300 ,:

f+150+ 75 49- 75 21

+300 7+150 27+ 75 39- 75 27

+150 47}+ 75 39- 75 13

+150 70}+ 75 2~- 75

+150 41

}+ 75 47- 75 7

+150 76}+ 75 22- 75 2

+150 24

}+ 75 67- 75 9

+150 58

}+ 75 26- 75 5

+150 77}+ 75 20- 75 3

+1509~ }+ 75

- 75

+150

~}+ 75- 75


Composite krebTertiary cycloneUnderflow

Composite tertiaryTube outlet

Composite tertiaryCyclone overflow

Composite JohnsonConcentrator tails No. I and 2Banks

Composite JohnsonConcentrator tails No. 3Bank

Composite feed toCleaner tables Nos.I and 2 banks

Composite feed to No. 3 cleaner bank

Composite cleanerTable tailings Nos.I and 2 banks

Composite cleanerTable tailings No. 3Bank

TABLE IV (Continued)

179,5

Silver


p,m FreeLocked *


Gold

426,4

349,8

40,6

212,5

179,5

2559,2

2 862,8

1244,9

<I

5

<I

<I

11

<I

<I


in the reef probably contributingto the ready liberation of themetal (see Tables III and IV). .

Most of the gold particles al-ready show a thin iron oxidecoating, and on some of thegrains this coating is distinct. Ona few it may even be describedas heavy. Photomicrograph 4,Plate 1I depicts a grain of goldsurrounded by hydrated ironoxide, probably deposited froma suspension of colloidal ma-terials.

(iii) Particles of rusty tramp iron arecommon, and most probablyconstitute the main source ofthe secondary iron compoundscoating the gold. However, thebulk of the secondary hydratediron oxide in the sludge occursas fragments of massive, collo-form or banded deposits that do

not occlude other mineral par-ticles, and have evidently beenprecipitated from a comparative-ly clear, colloidal suspension. Inview of evidence of copioussupplies of secondary, hydratediron oxide, it appears anomalousthat mineral aggregates cement-ed together by iron oxide are sorare in mine sludge. In theplant, such aggregates can beseen to form very readily, fre-quently encrusting the walls of,for example, sumps and launders,and one would expect similardeposits underground in gullies,depressions, and channels. Theseferruginous agregates are fre-quently very rich in gold, and thepossibility of some of the metalbeing permanently lost under-ground is at present being ex-amined.


In the sludge samples examined,the presenceof materials resemblinggelatinous settling agents is verymuch in evidence. In photomicro-graph 5, Plate I, for example, a grainof gold enveloped in hardened,resinous material is illustrated. Suchmaterial must undoubtedly play aninhibiting role in the cyanidation ofthe gold. Mine sludges have alsobeen found to contain various un-expected constituents, usually of nomore than passing interest. Almostall of the sludges examined to datecontain, sometimes in fair amounts,pulverulent or scaly zincite (ZnO)probably derived from the decompo-sition of galvanized materials, suchas ventilation pipes.

We shall now consider the be-haviour of the gold during itspassagethrough the reduction plant.

FEBRUARY 1973 229

2

(white) incorporating a variety ofmineral fragments (black and grey).Sallies: Screening Plant Minus 1/2"Fraction. X 600.

Photomicrograph 5:Completely rusted grain of tramp

iron. Attached are grains of pyrite(white) and quartz (dark grey). West-ern Deep Levels: Belt feed. X 600.

Photomicrograph 6

Auriferous scale. The mineral grains(mainly pyrite, gold and quartz) arecemented together by hydrated ironoxide. The ratio of gold to pyrite (bothappear white on the photomicrograph)is about I :100. Western Deep Levels.X 35.

PLATE"--

Photomicrographs --3 4

5 6Photomicrograph 3

Newly formed limonitic nodule show-ing concentric build and typical shrink-age cracks. Sallies: Screening PlantMinus 1/2" Fraction. X 600.

Photomicrograph 4

Gold (white) encrusted by colloformhydrated iron oxide. Western Hold-ings: Mine Sludge. X 600. (Photomicrollraphs by M. Seda.)

--

--Photomicrograph I

Pyrite (white) partially altered tohydrated iron oxide (dark grey). Sal-lies: Screening Plant Minus 1/2" Fract-ion. X 600.

Photomicrograph 2Deposit of hydrated iron oxide

230 FEBRUARY 1973 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

GOLD RECOVERY PLANTReference to the tables summariz-

ing the mineralogical data revealsthat in most cases almost all of thegold in the minus 600 fLm fractions ofthe plant feed and the other pre-milling circuit materials is alreadyliberated. Furthermore, the gold isliberated preferentially: although theliberated gold occurs in a fractionconstituting only, say, 10 to 20 percent of the feed, it represents some50 per cent of the total gold content.These gold grains display their natural

0 Gold (e/t)

EB Tramp iron (vit p"r cent)

,,1-,

~es:',j

~1 2 ) 5

shapes, and very few show signs ofmechanical distortion. All of thegrains are tarnished.

After ball-milling, 90 per cent andmore of the gold is usually liberatedas free, tarnished grains frequentlyshowing evidence of flattening anddistortion. (See photomicrographs 2and 3, Plate I.) Even if part of thetarnish of an occasional gold particleis momentarily scraped off, it isimmediately replaced. Secondly,during ball-milling, copious quanti-ties of finely divided tramp iron are

6 8 97 10 11 12 1)

produced - sometimes as much asone kg per ton of ore milled, Le. anamount 50 times higher than theamount of gold in the ore. Fromhere onwards, the tramp iron playsits deleterious role in the circuit.

Examination of polished sectionsreveals that much of the tramp ironis heavily oxidized, the grains be-coming surrounded by a thick crustof hydrated oxide. Tramp iron thathas been circulating in the plant forsome time may be completely rustedaway, or only a small remnant of the

~ ~ ~

SAMPLES

FIGURE I

+

14 15 16 17 20 21le 19

Sympathetic relationship between the abundance of gold and tramp iron in a suite of samples fromWestern Holdings Gold Mine.

DESCRIPTION OF SAMPLES

SampleNo.

Designation

I. Nos. I and 2 shaft sludge2. No. 3 shaft sludge3. Rake classifier a/flow4. Rake classifier return5. Washing plant cyclone a/flow6. Washing plant U/flow

7. Dewatering cyclone U/flow8. Ball mill feed9. Ball mill cyclone a/flow

10. Secondary tube outlet11. Secondary cyclone U/flow12. Kreb tertiary cyclone U/flow13. Tertiary tube outlet14. Tertiary cyclone a/flow15. Spargo tertiary cyclone U/flow


16. Johnson concentrator tails, nos.I and 2 banks

17. Johnson concentrator tails, no. 3bank

18. Feed to cleaner tables, nos. I and2 banks

19. Feed to cleaner table, no. 3 bank20. Cleaner table tailings, nos. I and

2 banks21. Cleaner table tailings, no. 3 bank

FEBRUARY 1973 231

original metallic iron may remain inthe core of the pseudo morph ofalteration products. (See photomicro-graph 5, Plate 11.)

Unfortunately, the particles of freegold and tramp iron have by nowacquired very similar properties:both are covered by hydrated ironoxide, and both are of high specificgravity. It is not surprising, there-fore, that the free gold and trampiron show a similar concentrationbehaviour during passage throughthe plant. This relationship is il-lustrated in Fig. I, which revealsthat, by and large, the relativeabundance of gold and tramp ironfluctuates sympathetically fromsample to sample.

One unfortunate aspect of thesimilar concentration behaviour ofthe gold and tramp iron is the veryreal possibility that a proportion ofthe gold may literally be "crowdedout" of a concentration circuit by thesuperabundant tramp iron. A secondresult of the tendency of these twoconstituents to concentrate togetheris that the iron, which is in a state ofrapid oxidation, is strategically placedto contribute to the coating of thegold particles, and to cause the for-mation of firmly cemented aggre-gates of gold and other heavy-mineral constituents. The commonnature of the coatings on gold andtramp iron most probably assists sub-stantially in joining the particlestogether.

Examples of mineral aggregatescemented by hydrated iron oxideare common. Possibly the mostsignificant is the scale deposited onthe concrete walls of sumps andlaunders, and on the walls of cyclonesand, presumably, other equipment.In photomicrograph 6, Plate 11, across section of gold-bearing scalefrom a sump at Western DeepLevels is shown. Strangely, goldseems to be preferentially attractedto such scales, which may assayseveral kg per tonne oftotal preciousmetals.

It is also interesting that theaggregates cemented with hydratediron oxide must be capable of form-ing very rapidly, as evidenced by thebuild-up of a thick deposit of gold-rich scale on the walls of the cycloneused to fill the amalgam barrel atWestern Deep Levels. The gold andother mineral grains show a distinct

232 FEBRUARY 1973

preferred orientation, and are ar-ranged in cyclic layers, thus indi-cating formation of the scale duringoperation of the cyclone.

At this juncture, mention shouldalso be made of the deleteriouseffects of stockpiling ore for anylength of time. As one could imagine,the coating on the gold grains soonbecomes so pronounced that priortreatment is needed before efficientgold recovery is possible. Bleedingthe untreated material into the re-covery circuit invariably leads tonoticeable increases in the residuevalues.

The same problem, only in anintensified form, applies to the re-covery of gold from old dumps. Insuch dumps, the sulphides are in aprocess of decomposition (see photo-micrograph I, Plate 11), thus pro-

viding copious additional quantitiesof secondary iron colloids, firmlycementing aggregates of mineralfragments (including gold), andheavily encrusting free gold (seephotomicrographs 4 and 6, Plate 11).Photomicrographs 2 and 3, Plate 11illustrate the deposits of nodular,hydrated iron oxide formed typicallyin this environment.

In the gold recovery plant, theoverflow and underflow figures re-lating to the various cyclones (seeTables I to IV) reveal the interestingfact that the grain size of the gold inthe overflow fraction is consistentlyvery similar to that in the underflow,and, which is more important, re-veal that in both fractions the pro-portion of locked gold is almostnegligible. The comparatively coarsegrain size of the gold in the overflowfraction is most probably due to theflattish shape of the grains. Thucho-lite also collects in the overflow, andis sent from here, via the cyanidationplant, to the residue dump. Re-cycling any of the cyclone productsfor remilling seems to serve nouseful purpose, but could result infurther deformation of the goldparticles and their contaminationby mineral matter pressed intothem. These mutilated grains re-porting in the tertiary cyclone under-flow eventually collect in the con-centrator tailings and are almostinvariably returned for yet anotherdose of milling. The circulating loadthus created can only result in theloss of gold particles that have lost

ali of their orlginai properties. Veryobviously, it would be beneficialrather to remove the gold from thecircuit as soon as it is liberated.

It testifies to the efficiency of thecurrent cyanidation and amalgama-tion processes that such a highproportion of the tarnished, over-milled gold grains is still being re-covered. The tarnish is probably of aporous nature. In a successful at-tempt at increasing the recoveryefficiencies, concentrates were treat-ed with hydrochloric acid to reducethe effect of the iron oxide coatings.This treatment has, for example,contributed to the remarkable amal-gamation efficiencies (99,46 per cent)achieved by Western Deep Levels.

The gold in amalgam-barrel resi-dues consists essentially of free,tarnished or incompletely amalga-mated grains (see photomicrograph4, Plate I), and the current practiceat some mines of recycling these tothe milling circuit can only result ingold losses.

The locked gold in amalgam-barrel residues constitutes a minorproportion of the total gold. Most ofthis gold occurs as minute specks(less than 10 microns in size) andveinlets in uraninite. Gold occlusionsalso occur in sulphide, skutterudite,silicate minerals, hydrated iron ox-ides, titanium compounds, and avariety of other minerals. In all ofthese, the gold occlusions are sominute that no amount of additionalmilling will ensure their completeliberation. We shall now considerthe mechanism of the developmentof hydrated iron oxide tarnish on thegold grains.

THE NATURE AND ORIGIN OFTHE COATING ON GOLD

By means of the electron micro-probe, sectioned Witwatersrand goldgrains in unprocessed Carbon Leader,Basal and Vaal Reefs were foundto contain a more or less constantfew per cent of silver, together withtraces of copper and nickel.

The samples of ore were crushedto fragments approximately one inchin diameter, thereby providing freshexposures of gold grains for exami-nation. The surface of fragmentsselected for examination were clean-ed in an ultrasonic cleaner bathusing acetone, alcohol, and methylethyl ketone successively. The speci-


Electron Microprobe CRr ImagesFigure I

Secondary electron image of a goldgrain in Basal Reef, showing delicatesurface textures giving rise to a largesurface area. The grain is coated witha very thin layer of iron sulphide.

PLATE III

--2

Figures --3 4

--5

~.

Figure 3Distribution of gold.

Figure 4Distribution of iron, in the surface

coating and in iron oxide crystalsattached to the surface of the gold grain.

Figure 5Distribution of sulphur (in the surface

coating).

-- Figure 2Compositional back-scattered elec-

tron image of the same are.aLinear magnification: X 600Scale: 10 microns = 6 mm.


I'nens were thereafter flash-coatedwith a conductive layer of purecarbon and examined in the electronmicroprobe.

Without exception, the outersurfaces of the gold grains examinedwere found to possess a thin layerof iron sulphide or iron oxide (PlateIll) or both. Where the sulphidecoating was thicker than average, theiron and sulphur were found to bepresent in constant proportions,probably as pyrrhotite or pyrite.

On some grains the alloy was seento be coated so thickly with ironoxide as to be almost completelyconcealed from detection by themicroprobe. It is likely that manygrains were so thickly coated as tohave escaped detection altogether.

Where gold-alloy grains werefound in the proximity of zincsulphide (sphalerite), the surfacecoating on the alloy grains was foundto contain zinc in addition to sulphurand iron. Lead sulphide may coatthe alloy grains when galena isabundant and in close proximity tothe alloy grains.

Laboratory experiments werecarried out with pure gold, puresilver, and gold/silver alloy con-centrated from the WitwatersrandReefs. The metals were treated withFeH, S2-, H+, and S042- ions indilute aqueous solution at 40°c.

After twelve hours the silver washeavily coated with a layer of silversulphide containing minor concen-trations of iron, the pure gold hadnot reacted at all, and a readilydetectable sulphide coating formedon the gold/silver alloy.

It thus appears that the silver insolution in the alloy becomes avail-able for reaction with sulphide ionsin solution, thereby forming a silversulphide surface deposit.

HYPOTHESIS FOR THE FOR-MATION OF REFRACTORYCOATED GOLD IN WIT-

WATERSRAND GOLD ORE

Studies of the conglomerate reefsof the Witwatersrand have indi-cated that the age of the system is inthe region of 2600 million years.There is abundant evidence that,during the vast expanse of timefollowing its formation, it has beensubjected to metamorphic processes;and it may be concluded that, at

234 FEBRUARY 1973

least during the metamorphic historyof the reef, free sulphide ions wereavailable, and sulphides were formedreadily.

In the laboratory experimentsdescribed earlier, it was demon-strated that silver-bearing gold iseasily attacked by sulphide ions,whereas pure gold is inert. It isconcluded that similar reactions havetaken place in the reefs.

The hypothesis advanced is that,in nature, silver atoms in the peri-pheral regions of gold/silver alloygrains are able to combine with theavailable sulphur. Once sulphur atomshave been bonded to the alloysurface by this means, the alloygrains become excellent seeds forthe growth of yet more sulphide,especially iron sulphides. To atleast some extent such coatings havebeen observed on all the Wit-watersrand gold grains studied.

By subsequent oxidation, the ironsulphide coatings have becomecapable of seeding further growth ofiron oxides. In the course of miningand plant operations, the processinitiated in nature can be continuedwith extraordinary rapidity owing tothe abundance of introduced metalliciron and the prevailing highly oxi-dizing conditions.

The chemical and galvanic de-composition of tramp iron and theprecipitation of ferric hydroxides inthe alkaline milling and processingcircuits are readily observable. In asimilar manner the introduction ofcalcium into the surface coatings, bythe addition of calcium hydroxide tothe pulp, has also been demon-strated.

It is interesting also to note thatdilute nitric acid is found to bebetter than dilute hydrochloric acidfor removing the coating from thegold/silver alloy prior to mercuryamalgamation on the gold plants.Silver in the surface of the alloygrains will react with the acids, andwith hydrochloric acid only, forms avery thin coating of insoluble silverchloride on the surface of the alloygrains. With nitric acid, no tarnishforms.

Coated gold, and its adverse effectsin plant operations, was known in theU.S.A. as early as 19341. Head2states, "These coatings may encasethe gold particles entirely, or they

may exist as films or tarnishes whosepresence is rendered perceptibleonly by comparison with normal,clean gold". Using microchemicaltests, he established that iron wasthe essential constituent of thecoatings, but offered no explanationfor its presence. He observed thatcoated gold was prevalent in pyriticdeposits, and noted that the coatingrenders the gold refractory to amal-gamation and other recovery pro-cesses.

As early as 1889, reference wasmade to "rebellious gold" that re-sists recovery3, but this property wasnot specifically correlated with thepresence of an iron oxide coating.Even earlier, Stetefeldt4 came veryclose to the truth when he spoke ofrefractory gold as "rusty gold".

CONCLUSIONSGreat benefit should result from

removing the gold from the circuitas soon as it is liberated. Not only isunnecessary milling a wasteful opera-tion, but it contributes substantiallyto gold losses.

Tramp iron is a most deleteriousconstituent in the plant circuit. Itresults in the formation of copiousquantities of colloidal iron com-pounds, which add to the hydratediron oxide coating on individual goldgrains, and contributes to the for-mation of rusty mineral aggregates.As a result of the iron oxide coat-ings and encrustations, gold re-covery becomes more difficult andoccasionally, impossible.

Removal of tramp iron from thecircuit (and separate treatment torecover entrained gold) should clear-ly have a beneficial effect on goldrecoveries in general.

ACKNOWLEDGEMENTSIt is a pleasure to express our

appreciation for the assistance andco-operation received from the HeadOffice Metallurgical Department, thevarious Group mines concerned, andour colleagues at Anglo-AmericanResearch Laboratories.

REFERENCESI. LEAVER, E. s., WOOLF, J. A. and HEAD, R. E.

(1934). Progress Report-Metallurgical DivisionReport of Investigations 3226, United StatesBureau of Mines.

2. HEAD, R. E. (1935). Progress Report- Metal-

lurgical Division. Report on Investigations 3275,United States Bureau of Mines.

3. LOCK, C. G. W. (1889). "Practical Gold Mining".Chapter VII: "Treating rebellious auriferous oresfor recovery of their gold and other valuable con-tents". E. and F. M. Spon, London and New York.

4. STETEFELDT, C. A. (1885). "The Amalgamation ofGold Ores, and Loss of Gold in ChloridisingRoasting". Trons. Amer. Instit. Min. fngs.


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