thery of ion

Upload: nkosikhona-hlabangna

Post on 08-Apr-2018

217 views

Category:

Documents


0 download

TRANSCRIPT

  • 8/7/2019 Thery of ion

    1/4

    T H E T H E O R Y O F C Y A N I D A T I O Nby F. Habashi

    Conclus ive ev idence i s g iven showing tha t thedissolut ion o f gold in cyanide solut ion fol lo ws main-ly the over-al l equat ion

    2 KOH + H 2 0 2A similar equat ion can a lso be wri t ten for the dis -so lu tion o f s il ver . Theoret ica l der iva t ion o f theveloci ty equat ion for this react ion has been obtained,which describes quant i tat ively the experimental facts .Th e eq u a t io n i s a s f o l lo ws :

    2 A D c N- DO , [ C N - ~O2 ]Rate = ~ ( D C N - [ C N - ]4 D o 2 [ 0 2 ] ]where[CN-1 and LO2] = the concen tra t ions ( in mo les /ml ) o fcyan ide and d i s so lve d oxygen , respect i ve ly .D cN - an d D o , = t he d i f f u s io n c o e f f i c i e n t s o f c ya n id eand d i s so lved oxygen; 1.83 x 1o - ~nd 2.76 x lo-'cm 2 s ec - ' , r e s p ec t i ve l y .A = the sur fac e area o f the meta l in con tact wi th theaqueous phase, in cm26 = the th ick ness o f the boundary layer ; var ies be-tween 2 and 9 x cm, depending on the speed andmethod o f ag i ta tion .Rate being expressed in g. equiv. sec- ' .

    INTRODUCTIOND uring recent years a number o f papers have b eenpublished on the kin et i cs o f dissolut ion of goldand s i lver in cyanide solut ion. T he authors agreetha t the ra te o f d i s so lu t ion i s d i f fus ion-con t ro ll ed ,but disagree regarding the mechanism o f the react ion.Some authors ' -3 support the Bodlander equa t ions:

    2 AU + 4 K C N + 0 , + 2 H 2 0 - 2 K A u (C N ) , +2 KOH + H 2 0 2 [ I 1

    2 Au + 4 KCN + H 2O2--t KA u (CN) + 2 KOH [2 ]Dr. F. HABASHI i s As socia te Professor, Dept. of

    Metallurgy, Montana Coll eg e of Mineral Scie nce & Tech-nology. Butte, Montana. TP65B244. Manuscript August 13,1965. Discussion of this paper submitted in duplicateprior to December 15, 1966 wi ll appear in SME Transac-tions March 1967 and AIME Transactions, 1967, vol. 238.

    othe rs4 are in favor o f E l sner 's equa t ion:~ A U + ~ K C N + O ~ + ~ H ~ O - C ~ K A U ( C N )OH

    131Th e apparen t ly con f l i c t ing resu l t s , when cr i ti ca ll y

    rev iew ed, are concordant and lead to th e conclusionthat the proper equation for the dissolution reactionshould be only Eq. 1 . Th i s co n c l u si o n i s b a s ed o nthe fol lowing facts:

    1 ) For every 2.2 eq uivalen ts o f metal dissolved onemole O 2 wa s co n su m ed .

    2) For every 1 equ iva len t o f metal d i s so lved , 2moles o f cyan ide were consumed . '

    3 ) Hydrogen peroxide i s formed during the dis solu -tion o f gold or silv er , and for every 2.2 e qui vale ntso f meta l d i s so l ved , one mole H 2 0 2 w as produced.

    T he fac t that Kudryk and el lo^^^ did not detectH 2 0 2 during the dis solu tion o f gold should not throwdoubt on the many af f irm ativ e reports o f other in-ves tiga to rs . ~ u n d ~as shown that depending on thecrys ta l l ine na ture o f the s i l ver sur f ace , (e .g . methodo f e t ch in g t h e s a m p l e ) H 2 0 2 may undergo catalyt icdecomposi t ion heterogeneously according to:

    Further Kudryk and el lo^^^ carried out their ex-periments in the presence o f 0.5% KC1 to achiev e agreater electrolyt ic conduct ivi ty, and ~atsudaira 'has shown that H 2 0 2 i s ca ta ly t i ca l ly decomposed(homogeneou s ly) in the presence o f C1- ion in a lka-l ine solut ion.

    4 ) E xp er im e nt s carried o ut by ~ o o n s t r a ' and ~ u n dshowed that the disso lutio n o f gold and s ilve r re-spect i ve ly i n KCN + H 2 0 2 i n t h e a b s en ce o f o xyg enwas a s low process. Th us the react ion

    H 2 0 2 + 2 Au + KCN-2 KAu (C N )2+ 2 KOH [21i .e. the reduct ion s tep

    H 2 0 2 + 2 e-- 2 OH-tak es place to a minor exte nt only.Further kinet ic s tudies showed that:

    1 ) Th e ra te o f d is so lu t ion depends on the sur facearea o f the metal in contact wi th the l iquid phase,thus ind ica ting tha t the process o f d i s so lu t ion i s aheterogeneous process .

    2 ) Th e rate o f d i sso lu t ion depends on the speed o f~ t i r r i n ~ , ~ ' ~hus indicat ing that the process i s con-trolled by a physical phenomenon.

    236 - SEPTEMBER 1966 TRANSACTIONS

  • 8/7/2019 Thery of ion

    2/4

    3) T h e r a t e of d i s s o lu t i o n i s o n ly s l i g h t l y a f f e c t e dby the increase of temperature , and the activatione n er gy i s 2 t o 5 Kca l /mole , which is typical of ad i f fus ion-cont ro l led process .

    MECHANISMT h e d i s s o lu t i o n p r o c e s s c a n b e c o n s id e r e d a s a n

    e lec t rochemica l p rocess , in which oxygen takes up

    Fig. 1 - Schematic Representation o f the Dissolution ofGold in Cy anide Solution.

    A 1 /Cathoaic Area///o2 + a 2 0 + 2e'

    + 0 + 20H-2 2 ,/

    aI

    1 I I7.48 ATM.

    0

    4

    3.40 ATM.

    Electrons

    CN -

    AqueouePhaee

    Nernst sBoundaryLayer 6

    + 4

    Fig. 2 - Rate of Dissolut ion of Si lver a t Dif ferentO2Pressur es and Different NaCN Concentrations at 2 4 ' ~ .Reprinted from Deitz and Halpem. 3

    Dissolved02

    e lec t rons a t one pa r t of the m e ta l l ic sur face ( theca thodic zone ) , whi le the m e ta l g ives them up a ta n o th er ( t h e a n o d i c z o n e) a s s h o w n in F ig . 1. T h eoxida t ion s t ep in volves the re fore the format ion of theauro- or argentocyanide ion, e .g.

    AU + 2 CN----c Au (cN); + e - [61and the r educ t ion s tep i s :

    0 , + 2 H 2 0 + 2 e m - H 2 0 2 + 2 OH- [71By apply ing Nerns t ' s a ssumpt ion of a s tagnant laye r

    of so lu t ion a t the su r face of the m e ta l th rough whichth e r e a c t i n g s u b s t a n c e s d i f fu s e ( F ig . I ), t h e n a c-cord ing to F ick ' s law:

    where r e spec t ive lyd ( CN -) /dt a n d d ( 0 2 ) / d t = the ra tes of dif fusion of

    CN- ion and 0 2 , n moles /sec .*DcN - and Do, = the d i f fus ion coe f f ic ien ts ofc y a n id e a n d d i s s o lv e d o x y g e n , c m2 s e c - ' .

    [CN-1 and [o,] = the concentra t ion of CN- and 0,in the bu lk of the s o lu t ion in m oles /ml .[ CN - I s a n d [ 0 , I s = the concentra t ion of CN- and0, a t th e sur f ace of the me ta l in moles /ml .

    A l a n d A 2 = t h e s u r f a c e a r e a a t w h ic h t h e c a th o d i ca n d a n o d i c re a c t i o n s t a k e p l a c e , i n c m2 .

    6 = the th ick ness o f the boundary laye r , in cm.If we a ssum e tha t the chemica l r eac t ions a t the

    meta l in te r f ace a r e ve ry r ap id a s compared wi th ther a t e s a t w h ic h t h e c y a n id e i o n a n d 0, diffuse throughth e s t a g n a n t l a y e r , t h e n t h e s e w i l l be c o n su me d a ssoon a s they r each the sur fa ce of the m e ta l, i . e.

    [o,], = O and ~ C N - I s OThere fore

    d ( 0 2 ) D o ,- -- A , t o , ]d t 6Tab le I. Estimated Values of Diffusion Coefficients

    famp. KCN D~~~ Do, -02 lnvesti-OC % sq.cm/sec. rq.cm/sec. D K C N gator18 - 1.72 x lo-' 2.54 x 16' 1.48 white925 0.03 2.01 3.54 1.76 ~arneda '27 0.0175 1.75 2.20 1.26 Kudryk and

    Kellogg4Average 1.5

    * ( ) represents the number of moles of th e substance while [ Irepresents mole/ l .

    Society o f Mining Engineers SEPTEMBER 1966 - 237

  • 8/7/2019 Thery of ion

    3/4

    Table II . Limiting Rate of Dissolution of Gold a n d Silver a t D i f f e r e n t Cyanide a n d O x y g e n ConcentrationsLo, ITemp. i n Sol ut io n [cN - I I c N - I / [ o ~ IMetal Oc a t m . mole/l mole/l l im . rate Investigator

    Gold 2 52 535

    Silver 2 42 4352 53525

    4.69 Kakovskii and4.86 Kholmanskikh64.625.85 Deitz and Halpern35.757.40 Kakovskii and7.35 Kholmanskikhs7.357.40

    d ( C N - ) D c N -and - - A, [CN-Idt 6Since the r a te of meta l d i s so lu t ion i s tw ice the r a teof oxygen consumption and '/2 the r a te o f cyan ideconsumption (Eq. I ) , therefore

    Ra te of d i s so lu t ion

    I t fol lows f rom Eq s. 12 and 13 t h a t a t t h e s t ead ys t a t e

    But , s in ce A , the to ta l su r f ace area o f meta l in con-t ac t w i th t h e aq u eo u s p h a s e = A , + A,, therefore:

    2 A D cN - Do, [CN-I LO2]R a t e = [I516 IDcN- [CN-1 + 4 Do, [o,] 1From Eq . 15 i t fo l lows tha t a t low cya n ide concen-trat ion, the f irs t term in the denominator may ben eg l ec t ed i n co m p ari s on w i t h t h e s eco n d , s o t h a t t h eequat ion s impl i f ies to :

    A D ~ ~ -R a t e = '/2- CN-1 = k [CN-1 [ I616T h i s co i n c i d es w i t h t h e ex p e r i m en t a l f ac t s ( F i g . 2 )tha t a t low cyan ide concen t r a t ion the r a te o f d i s so lu -t ion depends on ly on the cyan ide concen t r a t ion .In the sam e manner , i t f o l lows a l so f rom Eq . 1 5 tha ta t h igh cyan ide concen t r a t ion , the seco nd term in thedenominator may be neglected in compar ison with the

    f i r s t , and the equat ion s im pl i f ies to:A Do,R a t e = 2-LO,] = k, [o,]

    T h i s co i n c i d es a l s o w i th t h e ex p e r im en t a l f ac t s( F i g . 2 ) t h a t a t h i g h cy an i d e co n cen t r a t i o n , t h era te o f d i s so lu t ion depe nds on ly on the oxygenconcen t r a t ion .

    I t can a l so be de duced f rom Eq . 15 , tha t , when

    thenR a t e = fl&pTT A [O ,I' [CN- 1%2 6 [ I91

    T h i s m ean s t h a t a t t h e s e co n cen t r a t i o n s of cy an i d eand oxygen , the r a te o f d i s so lu t ion chan ges i t s de-pendence from one to the o ther which are the b re aksshown in Fig. 2 .

    Eq. 18 can be transformed in t he form:

    As shown from Tab le I , the avera ge r a t io Do , /DcN- =1.5, therefore

    Actual ly the exper imenta l va lue a s g iven in Ta b le 2,ranges f rom 4 .6 t o 7 .4 w h ich s h o u l d b e co n s i d e r ed a sa good agreement.

    The value o f 6 w a s c a l cu l a t ed f ro m t h e ex p e r i m en t a ldata o f var ious inves t iga to r s us in g Eq . 15 af ter sub-s t i t u t i n g t h e v a l u es o f d i ff u si o n co e f f i c i en t s . I t w asfound that 6 v a r i e s b e t w een 2 x an d 9 x cm ,which i s a typ ica l t h ick nes s fo r a boundary layer ina d i f fus ion-con t ro l led p roces s , depend ing on thespe ed an d method o f s t i r r ing .

    238 - SEPTEMBER 19 66 TRANSACTIONS

  • 8/7/2019 Thery of ion

    4/4

    REFERENCES'M. Kameda: Funda menta l S tud ies on So lu t ion o f Gold inC yanide So lu t ions . 11. On Equat i ons of R eac t i ons and Ef f ec t sof C ya n ide S t r eng th and Other Var i ab les on Dis s o lu t ion R ate .Sci . Report Resea rch Ins t . Tohoku Univ. , la pe n, Ser . A, 1,223-230 (1949).

    'v. Lund: The Corros ion of Si lver by Potass ium Cyan ideSolut ions and Oxygen. Acta Chim. Scend. 5 , 555-567 (1951).

    3 ~ .. Dei t z and J. Halpern: Rea ct ion of Si lver with AqueousSolut ions of Cya nide and Oxygen. I. M e t a l s 5, 1109-16 (1953).

    4 ~ .udryk and H. H. Kellogg: Mechanism and Rate-Control l ingFac to r s in the Dis s o lu t ion o f Gold in C yan ide So lu t ions .I . M e t a l s 6, 541-8 (1954).

    'I. A. Kakovski i and Yu. B. Kholmanskikh: Th e Kinet ics of theSllver Cyanlding Proce ss . Izves t . Aked. Neuk SSR, Otdel .Tekh. Nauk, Met . i Toplfvo 5 , 97-106 (1959).

    6 ~ . . Kako vski i an d Yu. B. Kholmanskikh: Inves t igat io n of theKin et i cs of Cyaniding Copper and Gold. Izves t . Akad. NeukSSSR, Otde l, Tekh . Nauk, Met. i Topliv o 5 , 207-18 (1960).

    7 ~ . atsudaira: T he Cata lyt ic A ct ivi ty of Sea Water . TohokuI . Agr. Research 1 , 177-198 (1950).

    '6. B oons tr a : Uber d i e Los ungs ges chwind igke i t von Gold inKaliumcyanidlosungen . Korros ion u. Metel lech utz 1 9 .146-15 1 (1943).

    'H. A. White: Th e Phy sic s of Gold-Solut ion. I . Chem. Metall.Min. Soc. S. Afric a 35. 1-1 1 (1934).'OG. Barsky, S. J . Swainson and N. Hediey: Dissolut ion of Gold

    and Si lver in Cya nid e Solution. Trans . Am. Ins t . Min. Metel l .Eng. 112, 660-677 (1934).

    Note: A detailed review of the cyanida tion proce ss (83 refere nces) is scheduled for publication,a s a bulletin, by th e Montana Bureau of Mines and Geology in Butt e, Montana [F . Habashi: Kinetics andMechanism of Gold and Silver Dissolutio n in Cy anide solutions].

    A N A L O G C O M P U T E R S I M U L A T I O N O F A W A L K I N G D R A G L I N Eby P. N. N ik i f o r u k and M. C. Zoerb

    An ana log compute r mode l h as been deve loped of al a rge, wa lk ing drag l ine . T his mode l pe rmi ts chang esin the conf igura t ion of the drag l ine , o r chang es in i t sd i g g i ng c y c l e , t o b e r e a d i l y i n v e s t i g a t e d o n a com-p u te r. A s a r e s u l t , i t is p o s s ib l e t o d e te r min e w a y sof improving the dragline performance without havingto ca r ry ou t many expens ive expe r imen ts on thea c t u a l ma c h in e . T h i s mod el c a n a l s o be u s e d a s asimulator for dragline operator tra ining.

    INTRODUCTIONT he movement of la rge volumes of mater ia l is a l -w a y s a c o s t l y o p e r a ti o n a n d t h e i n c r e a s in g t re n dtowards the use of la rge capa c i ty equipment does no tnecessa r i ly produce the economy des i r ed . This p rob-lem i s of pa r t icu la r conce rn to th e min ing indus t ry ini t s o p e n - p it o p e r a t i o n s w h e r e l a r g e d r a g l i n e s a r e u s e dto r emove th e ove rburden . S ince t h i s ope ra t ion of tenforms a very la rge par t of the tot a l c os t of open-pitmin ing i t is par t icu la r ly impor tant tha t i t be kep t a se c o n o mic al a s p o s s ib l e . T h i s r e q u i re s t h a t t h e d r ag -l ine be ope ra ted a t i t s maximum poss ib le produc tiv i ty .

    In orde r to op t im ize the ope ra t ion of a d rag l ine i t isDr. P. N. NIKIFORUK is Chairman, Div. of Control

    Engineering, University of Saskatchewan, Saskatoon, Can-ad a and M. C. Z OE RB is System s Engineer, ComputingDev ices of Canad a, Ltd., Ottawa, Canada. TP65K42.Manuscript December 13, 1964. Discus sio n of thi s papersubmitted in dupl icate prior to December 15, 1966 willappea r in SME Tra nsa ctio ns, March 19 67 and AIMETransac tions, 1967, vol. 238.

    des i rab l e tha t a su i tab l e model of the drag l ine bea v a i l a b l e f o r s t u d y . O n c e s u c h a mo d e l i s a v a i l a b l eth e effec ts of cha ng es in the configuration of thed r a g li n e , o r c h a n g e s i n i t s d ig g in g c y c l e , c o u l d ber ead i ly inves t ig a ted on a compute r and me thods ofimproving i t s pe r formance cou ld be determined with-out hav ing to ca r ry ou t expens ive exp e r iments on theact ual machine. Such models would be of us e not onlyto drag l ine use r s , but a l s o to the manufac ture rs ofdrag l ines for des ign purposes . P resumably s tud ie s ofth i s type could eve ntua l ly lead t o compute r -cont ro lleds t r i p p in g o p e r a t i o n s .

    A s f a r a s c a n b e d e t er mine d v e ry l i t t l e h a sbeen done on the der ivation of dragline models. Suchmo dels must b e der ived before work of the nature in-d ica ted in the prev ious pa ragraph c an be ca r r ied ou t .As a sm a l l cont r ibu t ion to th is f ie ld th is pape rdes c r ibe s work tha t h as been ca r r ied ou t by thesen ior au thor dur ing the pa s t f ew ye a r s on th e de r iva-t ion of one such mode l. T his mode l desc r ib es thedrag , h o is t and s wing ope ra t io ns of a 35 cu yd, com-merc ial ly ava i la b le , wa lk ing drag l ine . T his mode lwas de r ived for use wi th an an a log computer and ca nbe used e i the r for op t imizat ion s tud ie s , o r fo r thet r a in ing of d rag l ine ope ra tor s .

    DESCRIPTION OF DRAGLINEGeneral: Th e pa r t icu la r d rag l ine for which an an a logcomputer model was developed is owned by the GreatWest Coa l Company of Es tevan , Saska tchewan. I t i sa w a lk in g d r a g li n e a n d i t s f u n ct i on i s t o r e mo ve 40

    Society of Mining Engineers SEPTEMBER 1966 - 23 9