8_Kinetics of Copper Segregation_12-2dejahdjeddjehjdehjdehej

Kinetics of Copper Segregationby the Torco Process

SYNOPSIS

M. I. BRITTAN*, M.Sc.(Eng.)(Rand), M.S.(Yale),Ph.D.(Yale) (Associate Member.)

The key reactions of the copper segregation process have been studied with a view to establishing the kineticsof the system. The investigation was carried out using two refractory copper ores from the Zambian Copperbeltin a small-scale, fluidized bed, chloridization reactor. The rate of chloridization of the copper in the ores iscompared with the rate of volatilization of the cuprous chloride produced and with the rate of reduction of thecuprous chloride to form segregated metallic copper. In conjunction with available data on the rate of productionof hydrogen chloride in a segregation system and with due regard to the conditions applicable in a TORCOreactor, it is concluded that the chloridization reaction is the major rate limiting step of the process. Factorsinfluencing the rate of this extraction step and aspects of its mechanism are examined. The findings have confirmedthe validity of the existing theory of segregation and provide insight as to the viability of the system of reactionsinvolved in the copper segregation phenomenon. Experimental results which relate to observations made in thecourse of TORCO plant operation and segregation studies are discussed.

INTRODUCTION

Following the discovery of the copper segregationphenomenon in the 1920's33, numerous, though largelyunrewarding, attempts at its exploitation have beenreported46. It is only in recent times that the processreached fruition as a working metallurgical operation inthe form of the TORCO process of the Anglo AmericanCorporation group37. As described in the literature3, 25,26, 35, 37TORCO project development has advanced froma small-scale 10 tjday pilot plant through a 500 tjdaycommercial pilot stage culminating in an industrial-scale(4,000 short tjday crude copper ore) plant currentlyunder construction at Akjoujt in Mauretania.

The development involved has understandably beenconcerned mainly with improvements in the mechanicalaspects of plant construction and operation since techni-cal difficulties were in large measure responsible for theearlier failures, while the conditions necessary for segre-gation were reasonably well established. Although therehas been general acceptance of the basic mechanism ofthe segregation reactions and adequate demonstration ofthe applicability of the process to a wide range of ores8,10, 13, 26, 29, 31, 37, 38, 39, 41-46, 48, 55, 56, 57, the need forgreater knowledge of the chemistry of the segregationprocess has repeatedly been expressed, particularlyinsofar as the kinetics of the reactions involved are con-cerned26, 35, 36, 37, 48, 5.

Verification of the feasibility of the postulated reactionsystem and elucidation of its kinetics are desirable fromseveral viewpoints. Improvement in the efficiency ofsegregation can be brought about by concentrating onthe acceleration of the rate limiting step or, alternatively,by operating at a lower temperature without sacrifice ofreaction velocity. Furthermore, a mathematical model ofsegregation based on the process kinetics will assist inthe design of large-scale plants, predicting the behaviourof an ore, establishing the optimum operating para-meters and facilitating control measures to compensatefor variations in plant efficiency.

The experimentation described in this paper wasdesigned to test the viability of the current theory ofcopper segregation and to establish the kinetics of thesystem. The testwork was conducted in a small-scale,fluidized bed, chloridization reactor with accurate con-trol and measurement of temperatures, reaction times,gas flow rates and concentrations.

278 FEBRUARY 1970 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

OUTLINE OF THE SEGREGATIONPROCESS

In practice, the segregation process involves the heatingof a refractory oxide or mixed oxide-sulphide copper oreto temperatures of the order of 825C then adding smallamounts of sodium chloride and coke or coal. In theTORCO system the heating is carried out under oxidizingconditions in a fluidized bed roaster, the ore overflowinginto a semi-continuous tubular segregation chamberwhere the reagents are added. A complex series ofreactions occurs during which the copper is extractedfrom the ore and deposits as metallic particles on thecarbon surface. The copper in this form is readily recover-able by conventional flotation methods. The basicreactions considered to occur are:

(i) The salt is hydrolyzed by moisture in the presenceof silica, alumino silicates and other clay mineralsto form hydrogen chloride.

(ii) The hydrogen chloride reacts with the copperoxide in the ore to form volatile cuprous chloride.

(iii) The cuprous chloride is reduced by hydrogen inthe vicinity of the carbon particles to form thesegregated metallic copper at the same time regen-erating hydrogen chloride. Under optimum con-ditions, mild reducing conditions prevail in thesegregation chamber and thus the copper con-centrates in the stronger reducing environment ofthe carbonaceous reagents. Sources of adsorbedhydrogen are the carbonaceous volatiles and thereduction by the carbon of water vapour presentin the system.

The regeneration of hydrogen chloride is an interestingfeature of the system which is suggested by the fact thatthe quantity of sodium chloride involved is considerablyless than the stoichiometric equivalent quantity of copperpresent and is also essentially independent of the gradeof the ore3, 26, 37.

Confirmatory experimental evidence for the postulatedchemical reactions has accumulated in the literature overthe years8, 29,42-45,48.However, information on the via-bility of the scheme as a whole and the kinetics of thereactions involved is lacking.

.Anglo American Research Laboratory.

MASSF'LOW

METER

DRY N2 I ROTAMETERSF'LUSH I

HCI

CO

N2

CYLINDERS DRIERS FLpWCONTROLLERS

SCOPE OF THE PRESENT INVESTIGATIONIn essence, the investigation described here examined

the kinetics of the following more important segregationreactions and compared their rates.

(i) Chloridization of the copper in pre-oxidized orewith hydrogen chloride.

(ii) Volatilization of the cuprous chloride formed bythe chloridization.

(iii) Reduction of the cuprous chloride vapour on tocoke and coalchar. In addition, findings of anindependent study relevant to the initial produc-tion of hydrogen chloride are discussed to com-plete the picture.

Nchanga Banded Sandstone and Kansanshi ores fromthe Zambian Copperbelt were employed in the experi-mentation. While being refractory to conventional ex-traction methods, both these ores respond well to thesegregation process.

The reactor feed gas comprised mixtures of 5. 5 percent or 10,7 per cent HCl in nitrogen admixed withcarbon monoxide which varied in concentration between0 per cent and 20 per cent. This encompassed the rangeof reducing conditions which prevail in a TORCOsegregation chamber in practice. The total feed gas flowrate was 930 cc/min (measured at 25C and 630 mm Hg)for all runs except those in which the flow rate was varied.This corresponded to a space velocity of 7, 2 cm/sec at825C, this being well in excess of the minimum fluidizingvelocity associated with the small reactor and ores used.The ores were observed to be well stirred and thefluidized condition promoted temperature and concen-tration uniformity. Fluidized bed depths at 825C wereapproximately 5, 5 cm in the case of Nchanga BandedSandstone (an ore of low bulk density) and 3.2 cm forKansanshi.

Reaction temperatures studied were 750C, 825C and900C. The pressure in the reaction zone varied slightly

I

REC~.D

head sample assay values were 3, 54 per cent Cu and4. 63 per cent Fe. Thermogravimetric analysis of the pre-roasted sample (including the 5 min fluidized warm-upperiod) showed only a O.25 per cent loss of weightstarting above 800c. Above 900C the weight loss canagain be attributed to the reduction of cupric to cuprousoxide.

Minus 60 plus 100 mesh ore fractions (unroasted ore)were used. This enabled effective fluidization to beachieved on the small scale involved and eliminated dustproblems. The bulk ore samples were split down to therequired 20 g reactor charges by means of a riffler tominimize head value variations.Carbonaceous reductant

For the majority of the reduction tests, minus 35 plus65 mesh Johannesburg coke was used. After thoroughdrying at 115C, thermogravimetric analysis indicated a4. 2 per cent volatile content. Total bound hydrogenamounted to O.96 per cent. In a few tests, where greaterreducing power was required to counteract strongchloridization conditions, a higher volatile coalchar wasemployed.Gases

High purity nitrogen was used as an inert fluidizingcarrier gas. Technical grade hydrogen chloride andcarbon monoxide both of purity >99 per cent wereemployed as chloridizing and reducing gases respectively.

EXPERIMENTALExperimental apparatus

Fig. 1 is a schematic flow diagram of the experimentalapparatus, while details of the fluidized bed reactor areshown in Fig. 2.

The reactor consists of a 3, 1 cm ID silica tube fittedwith a sintered silica disc to support the 20g ore chargeand to act as a gas distributor. The concentric insert tubearrangement shown in Fig. 2 permitted the ground silicajoint to be placed outside of the hot zone (thereby elimin-ating seizing of the joint), minimized the dead volumeabove the ore charge and minimized the time lag of gaselements between the reaction zone and the detector. Asilica wool plug at the base of the insert tube acted as afilter for any dust from the fluidized bed and, whennecessary, supported a fixed bed of coke or coalcharparticles. As has been noted elsewhere9, silica is anextremely suitable constructional material for reactionstudies of this nature and the reactor performed satis-factorily for several hundred runs with temperatures upto 1,000c.

The reactor was heated by means of a three-zone tubefurnace precisely controlled to provide a uniform tem-perature zone (within 1C) over about 38 cm of length,thus ensuring that relevant reaction zones were at thesame temperature. The furnace was mounted on a trolleypermitting vertical movement. This facilitated rapidheating and cooling of the stationary reactor, there beingno necessity to disturb its inlet and outlet connections.A thermocouple in the reaction zone coupled to arecorder provided a continuous record of reactiontemperature.

The feed gas entered at the top of the furnace, travelleddown alongside the reactor and entered just below thesilica disc as shown in Figs. 1 and 2. The gas stream wasthereby preheated and, in addition, this design, whereinboth inlet and outlet connections are at the top of thefurnace, aided the heating and quenching of the reactorby movement of the furnace.

280

THERMOCOUPLE IN THERMOWELL

'"VENT

t~EATlNG

/.TA~E..

. . . . . . . .

~CI,CON2 . . _ABSORPTION

.. VESSEL

.'"CUPROUS CHLORIDEFILTER

VERTICALLYMOVABLE

TUBE FURNACE

1 CARBON BED

SINTERED DISC

SILICA WOOL PLUG

FLUIDIZED BED

Fig. 2-Chloridization reactor

As indicated in Fig. 1, the nitrogen and carbon mon-oxide were dried by passage through driers containingmolecular sieves and silica gel and were metered bymeans of micrometering flow controllers and rotameters.The hydrogen chloride flow was controlled by a 316stainless steel flow controller. A three-way tap arrange-ment enabled the hydrogen chloride and carbon mon-oxide to be added to or diverted from the fluidizingnitrogen stream as shown in Figs. 1 and 2.

The detection system consisted essentially of anabsorption vessel charged with water, and an automatictitrator and it served to monitor the hydrogen chloridein the reactor effluent. This first passed through a heatedline (to prevent water condensation) containing a cuprouschloride filter trap before entering an absorption vesselvigorously agitated by means of a Vibro Mixer. Thehydrogen chloride was absorbed into the solution whichwas titrated continuously by an automatic titration unitusing a calomel-glass electrode and standard sodiumhydroxide as titrant. The titrant addition necessary tomaintain a solution pH of 3 was recorded automatically(a pH of 3 avoided interference by dissolved carbondioxide).Experimental procedure

Having charged the reactor with ore and the insertwith carbonaceous granules (if required) the reactor wasflushed with nitrogen while the hydrogen chloride andcarbon monoxide streams were diverted to a fume cham-ber. The furnace was then raised over the reactor. During

FEBRUARY 1970 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

the few minutes required for the reactor to attain tem-perature, the solution in the absorption vessel wasadjusted to pH 3. At time zero, the hydrogen chloride andcarbon monoxide were introduced to the reactor and thetitrator recorder chart started. At the end of a run, thereactant gases were diverted from the reactor (the nitro-gen fluidizing gas being retained), the furnace loweredand the reactor cooled in a stream of compressed air.

To provide the extraction-time data, the reaction wasstopped after various times and a portion of the residueground and analyzed for total copper and iron. Theunground remainder was used for metallic copper assaysince grinding was found to promote oxidation.

A series of tests was conducted with Nchanga BandedSandstone at 750C, 825C and 900C using shortreaction times (2 min) followed by rapid cooling inwhich the fluidizing space rate and charge weight werevaried in such a way as to maintain their ratio constant.This series was designed to test the reaction rate for fluidfilm diffusional resistance and also to reveal if, perhaps,the volatilization of cuprous chloride was a rate limitingstep of the process.

DATA

The raw experimental results were basically of twotypes:

(I) Gangue copper concentration as a function oftime. Extractions were calculated based on boththe measured residue weight and that calculatedby accounting for the copper, iron

*and oxygen

removed. In all cases the measured and calculatedweights agreed extremely well.

(2) Titrant consumption, equivalent to the moles (andhence the partial pressure) of hydrogen chlorideleaving the reaction zone as a function of time.(Account was taken of the few seconds' lag be-tween a gas element leaving the reaction zone andits detection downstream.)

TABLE ITHERMODYNAMIC DATA 8,17,'4

Reaction L',GO 8()()OC

2CuO(s) + 2HCI(g)~'/3Cu3C13(g) + H.O(g) +1/.0.(g)Cu.O(S) + 2HCI( g)~'/3CU3CI3( g) + H.O( g)Cu3CI3(g) +3/.H.(g)~3Cu(s) +3HCI(g)Cu3CI3(g) +3/.C(S)~3/.CCI.( g) +3Cu(s)Cu3C13( g) +3/.CO(g)~"!.COCI.( g) +3Cu(s)COCI.(g)~CO(g)+CI.(g)COCI.(g) +H.O(g)~CO(g) + 2HCI(g) +1/.0.(g)2CuO(s)+H.(g)~Cu.O(s)+H.O(g)2CuO(s) +CO(g)~Cu.O(S) +CO.( g)Cu.O(s) + H.(g)~2Cu(s) + H.O(g)Cu.O(s) +CO(g)~2Cu(s) +Co.( g)2CO(g)~C(S)+CO.(g)C(s)+H.O(g)~CO(g)+H.(g)CO(g)+H.O(g)~CO.(g)+H.(g)C(S)+O.(g)~CO.(g)CO(g)+1/.0.(g)~CO.(g)H.(g)+1/.0.(g)~H.O(g)ySiO.(s)+xNaCI(1)+x/.H.O(g)~x/.Na.O. ySiO.(gl)

+xHCI

Minus 6,5Minus 13,5Minus 14.6

90,467,3

Minus 6.3Minus 9,6Minus 37. 65Minus 37, 94Minus 23.35Minus 23.64

3,95Minus 4, 13Minus O' 3Minus 94.42Minus 45.24Minus 44, 94

~26 (x=2)

Several of the tests were carried out in duplicate toexamine the reproducibility of the data. This was foundto be satisfactory and is reflected in the reasonableconsistency of the experimental data reported here. Noattempt was made to monitor the carbon monoxide inthe effluent gas since this was not essential for the pur-poses of the investigation and, in any case, it is unlikelythat downstream detection could provide an accurateestimate of the carbon monoxide concentration in thereaction zone.

Thermodynamic data at 800C for relevant reactionsare given in Table I.

DISCUSSION OF RESULTS

CHLORIDIZATION OF ORES WITH HYDROGENCHLORIDE

Extraction-time behaviour and influence of reductantExtraction-time data for Nchanga Banded Sandstone

at 825C with varying quantities of carbon monoxidein the reactor feed are shown in Fig. 3. It is apparent thatthe rate of reaction is initially extremely rapid, the ratethen declining markedly as high extractions are ap-proached. A noteworthy feature of the data is the im-provement in the rate of extraction wrought by increasingquantities of carbon monoxide in the feed gas up toabout 4. 5 per cent CO, showing that mild reducingconditions are an aid to the chloridization reaction.Whereas, for example, it requires 18 min to attain 83per cent extraction in the absence of carbon monoxide,with the gas present in sufficient quantities the timerequired is reduced to 4 min. Increase in the carbonmonoxide concentration up to 20 per cent had no signi-ficant effect on the extraction. It is thus established thatthe optimum extraction rate is uninfluenced by thereductant over a wide range of its concentration.

100

80

-!-c 600....

uCl'-....)(11 40

"UTEMPERATURE 825

'CHCI IN FEED 1D-7'"

0 0", coX 2", co

4.5'" co10", co

20

00 8 12

time (mln)16 204

Fig, 3-Extraction-time data, Nchanga Banded Sandstone

*The iron extraction was usually extremely Iow except for a fewoccasions where conditions were favourable for its chloridization(viz. high temperature and stronger reducing conditions) andextractions of up to 40 per cent were recorded.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY FEBRUARY 1970 281

Corresponding extraction data for Kansanshi ore areshown in Fig. 4. By comparison with the data of Fig. 3it is apparent that the tesponses of the two ores tochloridization aregenerally~ similar. The influence ofcarbon monoxide on the extraction is, however, less pro-nounced than with Nchanga Banded Sandstone. This isno doubt accounted for by differences in mineralogicalcomposition, in addition to which, the chloridizationbehaviour is influenced to some extent by the fact thatthe hydrogen chloride feed rate was not increased inrelation to the higher head grade of Kansanshi ore. Itmay further be noted that slight amounts of segregatedcopper were observed in the gangue after chloridizationat the higher carbon monoxide concentrations. Whilethis represents a small amount of copper extracted fromthe ore, it is not included in the calculated extractionsshown in Fig. 4, which are therefore slightly conservative.

100

::JU

80

0..

~60

reaction rate is, as noted above, controlled by the hydro-gen chloride feed rate. Were it not for this fact, theinitial reaction rate would be still greater. During theperiod of rapid reaction, the partial pressure of hydrogenchloride in the reaction zone is extremely low. It thenclimbs rapidly as the reaction rate declines. Conse-quently, substantial chloridization can take place at arapid rate with a low hydrogen chloride partial pressureprovided that the rate of supply of the gas is adequate.lnjluence o~ten1perature

The influence of temperature on the rate of copperextraction from Nchanga Banded Sandstone is shown inFig. 7 for a feed gas consisting of 10. 7 per cent HCl and4. 5 per cent CO. As expected, increase in temperaturefavours the extraction. Mechanistic complexities aside,quantitative correlation of the temperature effect interms of an activation energy is rendered difficult byvirtue of the constraint placed on the initial reaction rateby the hydrogen chloride feed rate and by the variationin partial pressure of the gas. This is more the provinceof an extensive kinetic analysis still to be carried out onthe system.

100x

80

HCI IN FEED 10.7./.CO 4.5./.

0 750.C. 825.C

X 900.C

7-e60.2....

uCL......

x11>40:J

U

20

00 4 16 208 12

time (min)Fig. 7-Effect of temperature on extraction, Nchanga Banded

Sandstone

Mechanisn1 o~ chloridizationIt is instructive to examine the chloridization reaction

in the absence of carbon monoxide where the copper inthe pre-oxidized ore is in the form of Cu2+. A 1:1 stoich-iometric equivalence was found to exist between thecopper reacted and the hydrogen chloride consumed.This is in full accord with the findings of Hadjiat andRichardson15 who similarly observed that cupric oxideand hydrogen chloride reacted with 1:1 stoichiometry.This corresponds, in the case of the basic mineraltenorite*, to a reaction equation of the form

2CuO + 2HCl--+ 2CuCl(1) + H2O + 1/202 . . . .(1)The presence of oxygen in the reactor effluent, as pre-

dicted by equation (1), was confirmed by chromato-graphic analysis. Since the oxidation of hydrogenchloride to chlorine1 is favoured kinetically by highertemperatures, and seeing that copper chloride is a catalystfor this reaction, the presence and participation ofchlorine in the reaction zone in the absence of a reducing

agent is likely. Alternatively the chloridization coulddirectly form some cupric chloride which, due to its in-stability 21, 28, 29, 42, 45, 52, would immediately break downto cuprous chloride and chlorine. The latter would thenreact with further copper oxide to yield the overall re-action stoichiometry of equation (1). Tests showed thatno chlorine could be detected in the reactor effluent, a re-sult in keeping with the expectation that any chlorineescaping from the reaction zone would combine with con-densed cuprous chloride in the cooler regions. Analysis infact verified the presence of some cupric chloride down-stream as observed by other investigators15, though thequantity involved must have been small, since otherwisethe 1:1 stoichiometry between Cu2+ and hydrogenchloride would not have been found.

With sufficient carbon monoxide present, formation ofcupric chloride downstream ceased and only cuprouschloride was found. The carbon monoxide could removethe oxygen of reaction (1) thereby rendering the equili-brium conversion more favourable, but it also createsthe following additional reaction path:

2CuO + CO --+ CU20 + CO2"""""", .(2)Cu2O + 2HCl--+ 2CuCl(1) + H2O.. .. ..,. .. . .(3)It is unlikely that the enhancement of reaction rate in

the presence of carbon monoxide is due to an equilibriumconsideration. Although this possibility is not ruled outduring the very early stages of reaction, the form of theextraction curves and the hydrogen chloride-time data arenot compatible with an equilibrium conversion limitation.The increase in reaction rate is more probably the resultof pre-reduction of the oxide by removal of latticeoxygen, the lower oxide form being more reactive thanthe higher. This was confirmed by tests in which the orewas first reduced by carbon monoxide for short periodsof time and then chloridized with hydrogen chloride inthe absence of carbon monoxide. The extractions' weresignificantly better than those obtained without thecarbon monoxide pretreatment and were nearly com-parable to the extractions achieved with the hydrogenchloride and carbon monoxide present simultaneously.

The more favourable kinetics associated with the loweroxide is remarkable in view of the probable presence ofchlorine (generally a more powerful chloridizing agentthan hydrogen chloride) during chloridization of thehigher oxide. It may be noted, however, that the greateraffinitity of cuprous oxide towards hydrogen chloride atlower temperatures is well known.

The increase in rate of chloridization with increase inthe carbon monoxide concentration at lower concen-trations is probably a result of an increase in the rate ofremoval of the more labile lattice oxygen. Beyond thepoint where insignificant quantities of the more refractoryhigher oxide remain at any instant, further increase in thereductant concentration will not improve the chloridiza-tion rate. In this regard it must be pointed out that thelability of the oxygen of multivalent metal oxides such ascupric oxide is well recognized as a factor in promotingthe oxidation of carbon monoxide5.

*This is a simplification to facilitate formulation of the necessaryequations. Representation of the actual copper minerals involvedwould incur considerable complexity and is not warranted. Invokingthe basic minerals tenorite (CuO) and cuprite (CU20) as descriptivespecies satisfactorily illustrates the reactions occurring and does notaffect the reasoning involved. It may also be noted that oxides ofthis type are rarely stoichiometric. and thus representation by adefinite stoichiometric formula is in itself a simplification.


x -

r"J( x x

-

0 0o~

e e0

Further reduction of oxide to reduced-in situ metalliccopper

Cu2O+ CO-+2Cu+ CO2 (4)must be a slow reaction under the mild reducing con-ditions employed. This was confirmed by independentreduction tests. Since metallic copper is more refractorythan its oxides towards hydrogen chloride, the copperoxides are apparently chloridized with hydrogen chloridein preference to being reduced by carbon monoxide(indeed a remarkable phenomenon) enabling the rapidand substantial extractions to be achieved. The data ofFigs. 3 to 7 could not have been obtained had metalliccopper been an intermediate in the chloridization reac-tion. Residue analyses confirmed that, in all cases, themetallic copper formed in the gangue was insignificant,even with high carbon monoxide concentrations in thefeed gas.

The use of a reductant in chlorine metallurgy to pro-mote the chlorination of metal oxides is well known.With carbon, for instance, it has been suggested that themechanism involves the removal by the carbon of theoxygen produced by the chlorination reactionso. In thecase of segregation involving hydrogen chloride, thecarbonaceous reagent can render the equilibria ofreactions (1) and (3) more favourable by removal ofsome of the reaction products. However, the promotereffect is considered to be more a kinetic phenomenonwhich is a consequence of the variable oxygen capacityof the copper, involving rapid reduction to a lower oxideform of greater reactivity. Similar behaviour has beenobserved in the chloridization with hydrogen chloride ofoxides of other metals of variable valency, notably ironoxides6, where reduction to lower oxides can occur. Asin the present study, increase in the reducing power of thegas by admixing carbon monoxide and also hydrogen6was found to enhance the rate of extraction. There isevidence to suggest that cobalt oxides behave in a similarfashion21.

INFLUENCE OF DIFFUSION ANDVOLATILIZATION OF CUPROUS CHLORIDE

Fluid film diffusionThe tests in which the fluid velocity and charge weight

were varied were conducted with a 2 min reaction timefollowed by immediate cooling. In view of the rapidinitial chloridization reaction it could be expected that asubstantial amount of cuprous chloride is produced inthe first 2 min. If this volatilized slowly, or, in general, ifresistance to transport of the reactants and products toand from the solid surface were appreciable, it is reason-able to assume that the quantity of copper removed fromthe charge should be a function of the gas velocity, par-ticularly in the light of the high rate of cuprous chlorideformation.

The results for Nchanga Banded Sandstone at 750C,825C and 900C with 10, 7 per cent HCI and 4. 5 percent CO are shown in Fig. 8. It is apparent that the datafollow the classic trend wherein the reaction rate isindependent of the space velocity above a certain mini-mum value. Below this space rate (where, in fact, the bedwas defluidized) there is a decline in the rate of extractioncaused by an increase in resistance to fluid film diffusion.As expected, this minimum appears to occur at higherspace rates as the temperature is raised, in accordancewith the fact that molecular diffusion has a weak tem-perature dependence whereas the reaction rate increasesexponentially with temperature. The results show posi-tively that under the experimental conditions employed,

284

corresponding to space rates at the extreme upper limitof the ranges shown in Fig, 8, the reaction is not diffusionlimited,

100 I I

80

;!-c 60.2....

uc

'-....

~40-

:>U

20

REACTION TIME 2 mm

HC' IN FEED 10.7",CO 4.5".

0 750'C825

'CX gOO'C

00

I

2 4 6 8superficial space velocity (cm/sec)

10

Fig. a-Effect of flow rate on extraction, Nchanga BandedSandstone

Kinetics of volatilizationIt was further apparent that the volatilization of cup-

rous chloride is extremely rapid since even at low spacerates, where a diffusional limitation exists, residueanalysis showed that the chloride remaining in the chargewas negligible. If the liquid cuprous chloride evaporatedslowly, any chloride remaining after quenching thereactor should have appeared in the analysis, Conse-quently, even when the reaction rate is hampered bydiffusion, this is not caused by sluggishness on the partof cuprous chloride volatilization as this must evaporatemore rapidly than it is produced by chloridization.

Nature of the cuprous chlorideIn view of the extremely low vapour pressure of

monomeric cuprous chloride-approximately 0.05 mmHg at 825CS4-the above conclusion as to rapid eva-poration would be untenable unless the cuprous chlorideevaporated as some more volatile species. It has in factbeen shown that at the temperatures in question, cuprouschloride exists as the trimer, CusCIs4 12,16, 51, withvapour pressures at 750C and 825C of approximately17 mm Hg and 40 mm Hg respectively12, 58,Assumingthat the copper is transported away from the fluidizedbed as CusCIs, calculation using the highest reaction rate(the initial rate) shows that at 750C the gas wouldinitially be saturated with cuprous chloride whereas at825C the gas would only be about 57 per cent saturated.Transport at this rate would be impossible if monomericcuprous chloride were the species involved. Existence ofa dimeric species postulated in earlier studies has notbeen substantiated in more recent work.

It is thus concluded that the volatilization of cuprouschloride occurs at an extremely rapid rate according to

3CuCI(1)-+CusCI3(g) ..".., ,.(5)


REDUCTION OF CUPROUS CHLORIDEKinetics of reduction

Curve D of Fig. 5 represents a chloridization testidentical to that of curve A except for a bed of 2g cokeplaced in series with the fluidized ore charge (Fig. 2). Thecuprous chloride produced in the fluidized stage isreduced on the coke to metallic copper while regeneratinghydrogen chloride* which leaves in the reactor effluent.The shape of curve D and its position relative to curve Aindicate whether the reduction reaction is faster or slowerthan the chloridization or is of comparable velocity.Curve D is, in fact, essentially linear and is virtually aduplicate of the hydrogen chloride feed curve, showingthat the hydrogen chloride is more rapidly regeneratedby the reduction reaction than it is consumed in thechloridization reaction. Thus, all the hydrogen chlorideentering the reactor leaves in the effluent. This is the caseeven at low reaction times where the chloridization stepis extremely rapid. Consequently, the reduction of cup-rous chloride with hydrogen over coke

Cu3CI3(g)+3/2H2-+3Cu+3HC1 (6)is virtually an instantaneous reaction. This conclusion isborne out by the experimental observation that, in spiteof a retention time of gas elements in the carbon bed ofonly a fraction of a second, all the copper was depositedon the leading edge of the bed.

Curve E of Fig. 5 represents a repeat of run B (i.e. with4, 5 per cent CO in the feed gas) with a bed of coalchardownstream of the fluidized bed. Again, it is evidentthat the curve is essentially similar to that for the hydro-gen chloride feed, showing that even with the fasterchloridization rate in the presence of carbon monoxide,the reduction of cuprous chloride is still more rapid.Small deviations, characterized by a slight decline in theslope of curve D, did become evident at higher conver-sions than shown in Fig. 5. This was apparently causedby a promotion of iron chloridization by the reducingcondition and build up of the hydrogencl1loride partialpressure. Since ferrous chloride resists reduction byhydrogen under the experimental conditions employed2, 6, 53, no hydrogen chloride regeneration takes placeand thus less leaves in the effluent.

Available data on the reduction of cuprous chlorideare in full agreement with the conclusion reached here.The equilibrium represented by reaction (6) is stronglyon the side of copper metal and hydrogen chloride attemperatures of the order of 825C2, 4, 45,60. This wasborne out by the experimental observation that, evenwith a hydrogen chloride partial pressure of 36 mm Hg,little or no chloridization of the precipitated copperoccurred. With higher hydrogen chloride partial pres-sures, however, it was necessary to use the higher volatilecoal char to prevent chloridization. Kinetically, theestablishment of the reduction equilibrium has beenfound to be rapid from the left but slow from the right2,19.

Hydrogen vs. carbon monoxideWhile hydrogen is now recognized as the reductant of

cuprous chloride in a segregation system3, 8, 29, 35, 37,42,45, 48, possible reduction by carbon monoxide and par-ticipation of carbonyl chloride have been postulated insome earlier segregation studies. Experimental evidenceon this question obtained in the investigation and otherpoints of relevance are considered below.

The case in favour of hydrogen is strong in view of theregeneration of hydrogen chloride which inevitablyoccurs in the course of the segregation cycle, and in thelight of the thermodynamics of the respective reactions.

Adequate quantities of hydrogen should be availableunder normal circumstances from the reaction of waterwith carbon and carbon monoxide, and also from thecarbonaceous volatiles. Experiments have shown furtherthat when the hydrogen chemisorbed on a carbonaceoussurface is depleted, the reduction of cuprous chlorideceases15. Reduction by carbon monoxide is thermo-dynamically very unlikely8 even considering the slightdisplacement of the equilibrium due to the instabilityof the phosgene product7, 8, 54,and its ready hydrolysisto produce hydrogen chloride8, 39,44,45. This wouldrender chloridization by phosgene unlikely. Reduction ofcuprous chloride by carbon itself is thermodynamicallyextremely unfavourable8, 23 and, under stringently dryconditions, graphite failed to reduce cuprous chloride toany extent at 800C8. In the presence of a small quantityof water, however, precipitation of the copper on thegraphite occurred.

Referring again to Fig. 5, with appreciable carbonmonoxide present, resemblance of the titration curve Eto curve C rather than B might be construed as evidencein support of hydrogen reduction of cuprous chloriderather than reduction by carbon monoxide. However,with hydrogen present on the carbon, regeneration ofhydrogen chloride could occur, albeit indirectly. Con-sequently, carbon monoxide cannot definitely be ex-cluded as a primary reductant in this case.

More convincing evidence is provided by the observa-tion that no reduction of cuprous chloride to metalliccopper was apparent in the charge, or on the reactorwalls, except in the case of tests with substantial carbonmonoxide concentrations where traces of metallic copperwere evident in the gangue (particularly with Kansanshi).Whether this was due to direct reduction of cuprouschloride or via carbon deposition from the dispropor-tionation of carbon monoxide2 was not established. Withhydrogen, on the other hand, reduction of cuprouschloride would be unavoidable, as observed by Rey48(hence the use of carbon monoxide rather than hydrogenas the reducing gas). Consequently, in the light of theabove observations and thermodynamic considerations,there can be little doubt that hydrogen is the activereductant of cuprous chloride.Adsorption effects

It may be noted from Fig. 5 that following the start ofreaction at t = 0, a lag of several seconds occurs beforeelements of hydrogen chloride appear in the outlet. In thecase of a straight chloridization reaction, withoutcarbonaceous material in the system, this is due to theconsumption by reaction of all the entering hydrogenchloride, the rate of reaction being limited by the rate ofsupply of the gas. In the case where the carbon bed ispresent, this situation would not apply, since hydrogenchloride would be regenerated by reduction of the cup-rous chloride produced at the start of reaction. It is,however, well known that halogens are strongly held bycarbons4 and thus it is likely that the hydrogen chlorideproduced by this reaction does not desorb until thecarbonaceous material is saturated with hydrogenchloride.Production of hydrogen chloride

An intensive investigation of copper extraction withhydrogen chloride would, of course, be unwarranted inthe absence of evidence on the kinetics and equilibrium

*Aside from the hydrogen chemisorbed on the coke surface,

hydrogen is also available from the reaction of the coke with waterproduced by the chloridization reaction or liberated from the orelattice.


of hydrogen chloride production under segregation con-ditions. Such evidence is available from an independentstudy in this laboratory on the hydrolysis of sodiumchloride in the presence of silica and ore constituents17.

The results showed that the rate of hydrogen chlorideproduction by hydrolysis of sodium chloride is so rapidthat the equilibrium partial pressure of the gas is estab-lished in a fraction of a second, even below 800Dc.While the equilibrium yield with silica alone is limited, itwas found that a mixture of the preroasted NchangaBanded Sandstone with 2 per cent sodium chlorideimmediately yielded more than 20 mm Hg of hydrogenchloride at 825DC in the absence of added moisture,caused presumably, by residual bound moisture. Thiswas so in spite of rapid scavenging of the hydrogenchloride produced initially by reaction with the copperin the ore, and thus the hydrogen chloride is producedmore rapidly than it is consumed in chloridizing thecopper, The hydrogen chloride, in fact, begins to appearat temperatures as low as 500DC17,29. Consequently,under segregation conditions, where substantial quan-tities of water exist42,45,48,it can be expected that rela-tively copious amounts of hydrogen chloride will beproduced at a rapid rate, Since chloridization can beeffected at low hydrogen chloride partial pressures, pro-vided the rate of supply of the gas is adequate, thereshould, under optimum segregation conditions, be nolimitation caused by hydrogen chloride starvation, par-ticularly in view of the rapid regeneration of the gas byreduction of cuprous chloride.

EXPERIMENTALCONCLUSIONS(1) The rate of production of hydrogen chloride by the

hydrolysis of sodium chloride is almost instantane-ous at reaction temperatures, and in the presenceof ore constituents an appreciable equilibriumconcentration of hydrogen chloride can be gener-ated.

(2) The rate of chloridization of the copper in the orestested is initially rapid but the rate declinesmarkedly as high extractions are approached,

(3) Mild reducing conditions increase the rate ofchloridization, apparently, by reducing the copperin the pre-oxidized ore to a more reactive loweroxide species.

(4) The chloridization proceeds to a substantial degreeat a rapid rate even with low partial pressures ofhydrogen chloride, provided the rate of supply ofgas is adequate,

(5) The undesirable reduction of cuprous oxide toform reduced-in situ copper metal is a sluggishreaction under optimum segregation conditions.

(6) The rate of volatilization of the cuprous chlorideproduct of chloridization is extremely fast and ismore rapid than the rate of chloridization. Theexperimental data are commensurate with volatili-zation of the cuprous chloride as the trimer.

(7) The rate of reduction of cuprous chloride byhydrogen over coke or coalchar is also extremelyfast and favourable, and is more rapid than therate of chloridization. This reaction regenerateshydrogen chloride almost instantaneously andensures that the gas is always available for furtherchloridization of the copper oxides.

(8) For the system of reactions considered above, whichconstitute the segregation sequence, the chloridiza-tion, being the slowest reaction, is the rate limitingstep.

KINETICS OF THE TORCO PROCESSIn view of differences between the experimental system

used and a system in which copper segregation per se iseffected, it is necessary to relate the experimental findingsto conditions appropriate to TORCO segregation. Thisis considered below, along with general conclusionsregarding segregation kinetics.

Viability of the reaction systemThe reaction kinetics observed in this investigation do,

in fact, support the basic outline of segregation chemistryas postulated previously. The key to the process is theextremely rapid production and recycling of hydrogenchloride which ensures that adequate supplies of the gasare available for chloridization, particularly during therapid initial rate period. Coupled with the rapid volatili-zation of the cuprous chloride, this enables the systemof reactions to proceed viably. It has also been shown thatadequate partial pressures of hydrogen chloride aregenerated from sodium chloride and that the chloridiza-tion of the copper oxide will proceed rapidly to a highconversion even with low hydrogen chloride pressures.The speed of the process is aided by the promotion ofthe rate limiting chloridization reaction by the reducingagents present,

Rate determining stepThe chloridization reaction is the important rate

determining step of the process. Confirmatory evidencefor this is provided by segregation studies32, and resultsfor Kansanshi ore at 825DCare shown in Fig. 9. Reagentconcentrations used were O.5 per cent sodium chlorideand 2 per cent coalchar. The data give the degree ofsegregation (determined by screening off the segregatedconcentrate) as a function of time after mixing in thereagents at temperature. The results show that the con-version to segregated copper varies with time in a mannerwhich is remarkably similar to the extraction-time dataof Fig. 4. Thus, it would appear certain that the chlori-dization reaction controls the overall rate of segregation.This may not be the case for the first few seconds ofreaction where the chloridization reaction is extremelyrapid and may possibly be of comparable velocity toother reactions in the sequence. This, however, is of littleconsequence.

100

~

';;60"0l-t

0

~ 60c:.2~

00>

"I-~40on

--'U

TEMPER"URE 625'CNQCI D.5",

COALCHAR 2 ',.0 SEGREGATIONRECOVERYX CONCENTRATE GRACE

20

00 12

time Cmin)20 24164 6

Fig. 9-Segregation-time data, Kansanshi ore

286 FEBRUARY 1970 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Residence timeSince the chloridization stage is the rate limiting step

of the segregation process, it might be expected from thedata presented in this study that segregation of copper inexcess of 75 per cent can be achieved in a few minutes at825c. This conforms to the low residence times reportedas being satisfactory in plant operation3, 26,37.The rapidinitial reaction is, nevertheless, followed by a muchslower rate of extraction. Consequently, the residencetime in the TORCO segregation chamber will be deter-mined, for the most part, by the degree to which it iseconomically desirable to achieve recoveries closer to100 per cent. Ever-increasing residence times will berequired to reduce the loss of copper in the tailings.Above about 90 per cent extraction, the rate of reactionbecomes extremely slow. While the rate could be acceler-ated somewhat by higher partial pressures of hydrogenchloride, the resistance to extraction of the residualcopper would suggest that the ores contain a smallamount of highly refractory copper, either inherent, orpossibly produced in the course of the treatment.

Influence of waterIn view of the potential reversal of reactions (1) and (3)

by excess water (i.e. the hydrolysis of cuprous chloride),it is necessary to examine the possible effects of water onthe segregation system. Water is, of course, essential forthe initial production of hydrogen chloride and thus abalance between the water and hydrogen chloride partialpressures is indicated. There are several sources of waterin a TORCO system (roaster combustion gases, oxidationof carbonaceous volatiles, residual water bound in theore) and investigators have found substantial quantitiesof water in the gas phase over a segregating mixture42, 45,48. From an equilibrium viewpoint, the hydrolysis ofcuprous chloride is not favourable8, 15,45,48and thus thehigh water vapour partial pressures apparent in practicecan be counteracted by low hydrogen chloride pressuresand are thereby tolerated45. Chloridization tests in thepresence of 20 per cent steam verified that the copper ex-traction was not hindered. Under segregation conditions,competition for reaction with the cuprous chloride willexist between the water and hydrogen. Since the reductionby hydrogen to form segregated copper is a fast andfavourable reaction, the hydrolysis reaction is renderedunlikely. Furthermore, by removal of the cuprouschloride product, the chloridization equilibrium isassisted.Influence of reducing agent

The reducing conditions which prevail during segrega-tion (emanating from reactions and volatiles associatedwith the carbonaceous reagent) play an interesting rolein the reaction scheme. They are, of course, involved inthe reduction of cuprous chloride to form segregatedcopper. In this regard it may be noted that at sufficientlylow temperatures where conditions would no longer beoptimum, the reduction by hydrogen is less favourable,both thermodynamically and kinetically and thus couldpossibly impose a rate limitation on the overall segrega-tion scheme.

The results of this study have brought to light the addi-tional influence of the reductant in accelerating thechloridization reaction. The results suggest that an opti-mum reductant concentration exists below which thereducing power is too mild to aid the chlorization and toreduce effectively the cuprous chloride, and above whichthe reducing environment is strong enough to producereduced-in situ copper which resists segregation. Such anoptimum carbon concentration yielding maximum

segregation has, in fact, been observed in plant operationand segregation studies. Illustrative data are Presentedby Rey43,44,45.Although the optimum is a complicatedfunction of carbonaceous type, size fraction and volatilecontent, the present results have indicated that controlof the reductant concentration is not as critical as mightat first sight be expected, since a fair latitude in gaseousreducing power can be tolerated.

The results are also in accord with the observed colourchanges of several ores during segregation37. A light greycolour of the calcine (indicative of mild reducing con-ditions) is generally associated with good segregation.This was certainly the case in the chloridization testswhere efficient extractions were accompanied by a lightgrey gangue. It is indeed fortunate that under optimumsegregation conditions the kinetics of the various reac-tions influenced by the reducing conditions are such thatthe desired reactions (the chloridization step and thereduction of cuprous chloride) are assisted, while thedetrimental reduction to reduced-in situ copper is notfavourable. It may be noted that the efficacy of mildreducing conditions has also been observed in non-TORCO type segregation systemsll.

The type of carbonaceous material employed, that is,the quantity and rate of liberation of the volatiles, theparticle size, surface area etc., is also of importance sincethe relative strength of the reducing environment can alsoaffect the form of the segregated copper3, 37,41,47,48andthereby the subsequent flotation concentration.

Chloridizing reagentAn optimum sodium chloride reagent addition has also

been observed in segregation testwork. At extremely lowconcentrations it is likely that the supply of hydrogenchloride is inadequate to satisfy the needs of the initialchloridizing reaction rate in the face of possible absorp-tion of hydrogen chloride by gangue constituents, orlosses from the system. With too high a sodium chlorideconcentration, the decline in segregation efficiency isthought to be connected with the solubility of cuprouschloride in sodium chloridel4.Influence of diffusion

The chloridization tests of this study were conductedusing a fluidized system which differs, of course, from thesemi-continuous tubular reactor arrangement of theTORCO segregation chamber. The data of Fig. 8 showthat when the chloridization reaction is very rapid, andwhen the space rates are low, a decline in initialchloridization efficiency occurs due to the influence offluid film diffusional resistance on the rate of extraction.It could thus be expected that in a static segregationsystem, the overall rate of reaction during the rapidinitial chloridization stage would be controlled bydiffusional considerations and that agitation to minimizethe diffusional resistance will prove beneficial in speedingup the rate of segregation. It is now accepted that farfrom being a packed bed, the material in a TORCOsegregation chamber is in a fluid state, presumably as aconsequence of the liberation of reaction gases. This isparticularly so at the top of the segregation chamberwhere the material spilling over from the roaster is in afluidized state, and where liberation of most of thereagent carbon volatiles and reaction gases takes place.It is precisely in this region that the segregation reactionis most rapid and would therefore be most susceptibleto a diffusionallimitation. Consequently, under conven-tional plant conditions where the ore is in an agitatedstate, there should be little or no inhibition of the rate ofsegregation due to fluid film diffusional resistance. The


apparent improvement in segregation of a moving oragitated charge compared with that of a static chargeobserved by earlier workers46, 47,49is probably, at leastin part, associated with the reduction in diffusionalresistance occasioned by movement of the ore during therapid initial chloridization stage. This point has pre-viously been noted by Rey47,49.

SUMMARY OF IMPORTANT REACTIONSAND RATES

A summary of the more significant reactions of thesegregation process and their relative rates under opti-mum conditions is presented here for completeness.Reactions other than those listed undoubtedly occur, butthese do not affect the basic conclusions reached. Forconvenience, only the simplest copper minerals are con-sidered in formulating the equations (see footnote p. 283)although the scheme will generally be applicable to morecomplex oxide copper minerals where the copper existsin different oxidation states. Some variability in the rateof chloridization of different copper minerals can beexpected.

The reaction cycle is initiated with the production ofhydrogen chloride by hydrolysis of sodium chloride inthe presence of certain gangue minerals:

ySi02(i) xNaCI + x/2H2O + zA12O3' 2SiO2

etc.x/2Nap . ySi02

---+x/2Na2O. zA12~ . 2SiO2 + xHCIetc.

This reaction is extremely fast and can yield adequatequantities of hydrogen chloride in the presence of boundor vapour phase water.

The hydrogen chloride can attack the copper in itspre-oxidized form in a relatively slow step.

(ii) 2CuO + 2HCI---+ 2CuCI(l) + H2O + 1/202 slowReduction of cupric copper by removal of labile

lattice oxygen to form a lower oxide (represented forconvenience as cuprous oxide) can occur rapidly, asfollows, provided sufficient reducing agent is present.

(iii) 2CuO + H2---+CU20 + H2O rapid(iv) 2CuO + CO ---+Cup + CO2 rapidChloridization of the lower oxide will occur at a more

rapid rate than step (ii), and is thus the preferred pathcommensurate with optimum kinetics. The reaction isnevertheless relatively slow.

(v) CU20 + 2HCl ---+2CuCI(I) + H2O slowWhile the chloridization steps (ii) and (v) are initially

rapid, they are nevertheless rate limiting since, overall,the other reactions leading to segregated copper are verymuch more rapid. During the rapid initial rate period,agitation of the charge could enhance the overallreaction rate by minimizing fluid film diffusional resis-tance.

Volatilization of the cuprous chloride to form a gas-eous trimer is a rapid process.

(vi) 3CuCI(l) ---+ Cu3C13( g)Reduction of cuprous chloride to form

copper is a fast and thermodynamicallyreaction.

(vii) Cu3CI3(g)+ 3/2H2---+3Cu + 3HCI rapidThe reduction is effected by hydrogen in preference to

carbon monoxide. This reaction regenerates hydrogenchloride almost instantaneously for further chloridiza-tion.

rapid

rapidsegregatedfavourable

288

The following undesirable side reactions can producereduced-in situ copper which resists segregation:

(viii)Cu20 + H2 ---+ Cu + H2O slow(ix) CU20 + CO ---+ 2Cu + CO2 slowIn addition, ancillary reactions take place involving

C, CO, CO2, H2, H2O and O2 (see Table I). The kineticsof these reactions may be influenced, inter alia, by thetype of carbonaceous reagent used, its particle size andby the nature of its surface 59. While it is likely that theancillary reactions are probably faster than the ratelimiting chloridization reactions at segregation tem-peratures, the relative rates and equilibria of thesereactions will determine the atmospheric composition.This, in turn, can influence the overall rate or course ofthe reaction sequence, although a fair degree of com-position latitude can apparently be tolerated withoutadversely affecting the segregation.

It is apparent that under optimum segregation con-ditions, a remarkable balance exists between the kineticsand equilibria of the various reactions which take place.This is such as to drive the reaction sequence through itsmost favourable course while undesired side reactionsare minimized. The overall rate is limited principally bythe rate of extraction of the copper with hydrogenchloride. By virtue of the phenomenon wherein thehydrogen chloride rapidly recycles, the viability of thesystem is facilitated.

ACKNOWLEDGEMENTThe author is indebted to Mr. R. R. Liebenberg for

painstaking and invaluable experimental assistance inproducing the data of the investigation. Discussions withMr. N. W. Hanf are acknowledged and thanks are dueto Mr R. de L. Murray-Smith and Miss J. A. Snegg forthe chemical and mineralogical analyses.

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