The absorption of gold cyanide onto activated carbon. 11. Application of the kinetic model to multistage absorption circuits

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1 J. S. AIr. Inst. Min. Metal/., vol. 84, no. 3. Mar pp! The absorption of gold yanide onto ativated arbon. 11. Appliation of the kineti model to multistage absorption iruits by M. J. NICOL*, C. A. FLEMINGt, and G. CROMBERGE:j: SYNOPSIS A relatively simple rate equation for the kinetis of the absorption of gold from yanide solutions onto ativated arbon was li5ed as the basis in the development of a model for multistage arbon-in-pulp and arbon-in-ieah absorption iruits. The model was used in the predition of both the steady-state proess, and the results were found to be in reasonable agreement and the transient behaviour of the with those observed in an extended absorption pilot-plant ampaign. A number of pratial impliations of the model are disussed, and an assessment is made of the eonomi onsequenes of the various proess options available for the design of a arbon-in-pulp plant. SAMEVATTING 'n Betreklik eenvoudige tempovergelyking vir die kinetika van die absorpsie van goud uit sianiedoplossings op geaktiveerde koolstof is gebruik as grondslag vir die ontwikkeling van 'n model vir veeltrapkoolstof-in-pulp- en koolstof- in-ioogabsorps iekri nge. Die model is vir die voorspelling van sowel die vastetoestand- as die oorgangsgedrag van die absorpsieproses gebruik en daar is gevind dat die resultate redelik ooreenstem met die wat in 'n uitgebreide proefaanlegkampanje waargeneem is. 'n Aantal praktiese implikasies van die model word bespreek en die ekonomiese gevolge van die verskillende prosesopsies wat vir die ontwerp van 'n koolstof-in-pulpaanleg beskikbaar is, word geevalueer. Introdution A previous paperl desribes a relatively simple kineti model for the absorption of gold from yanide solutions and pulps onto ativated arbon, and illustrates the use of the rate equation in prediting the performane of single-stage ontinuous and bath absorption tests. The appliation of this kineti model to the desription, design, and optimization of multistage arbon-in-pulp (CIP) absorption iruits is developed in this paper. As suggested in the previous paper, the rate at whih ativated arbon absorbs gold is the most important property of the arbon used in this appliation beause this fator determines the operating effiieny of a CIP plant, in whih true equilibrium between the arbon and the solution is seldom ahieved in any stage. The wellknown MCabe-Thiele approah to the desription of multi stage ounterurrent extration iruits annot be expeted to yield adequate results when applied to CIP iruits beause it is based on an equilibrium desription of the system. For this reason, workers at the United States Bureau of Mines, in the only published work on this topi2, adopted a modified MCabe-Thiele approah in whih pore diffusion was oupled to a Freudlih isotherm to desribe the equilibrium between silver yanide and ativated arbon. * Diretor t Assistant Diretor :j: Senior Sientist All of the Mineral and Proess Chemistry Division, Counil for Mineral Tehnology (Mintek), Private Bag X315, Randburg, 2125 The South Afrian Institute of Mining and Metallurgy, SA ISSN X/S Continuous Counterurrent Extration In the simplest approah to the modelling of a CIP iruit, the assumption is made that the flow of pulp and arbon is ontinuous but ounterurrent, and that a steady-state ondition is reahed in whih the onentrations of gold in the solution and on the arbon are onstant in eah stage of the absorption iromt. It is further assumed that eah ontator an be treated as a perfetly mixed reator with a onstant residene time for both the pulp and the arbon. In the previous paper it was shown that, under onditions of onstant gold onentration in solution, the following equation an be used to desribe the rate of absorption of gold onto arbon: d[au]jdt = k(k[au]s - [A'u] ), (1) where [Au] is the onentration of gold on the arbon at any time t, [Au]s is the onentration of gold in solution, k is a rate onstant, and K an equilibrium onstant. Both k and K an be obtained from bath absorption tests as desribed in the previous paper. This rate equation an be ombined with mass-balane equations to yield an overall model for a iruit onsisting ofn stages (shown shematially in Fig. 1) in whih pulp (at an equivalent solution flowrate of Vs) enters the first stage with a gold onentration of [Au] and arbon (at a flowrate of V ) enters the last stage with a gold onentration of [Au]. As shown, the onentrations of gold in solution and on the arbon in any stage i are represented by [Au] and [AU]-l respetively. The overall mass balane of gold aross the iruit requires that Vs ([Au] - [Au] = V([Au] - [Au] ) (2) 7 MARCH 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

2 (Aul [Au Slage 1 Siege 2 [AuC 1 [Auf. r-- """"""""""" IAuJ: [AUJ:'-1 1i [AuJ:'-' SIge (Aul [AuJ:' Fig. I-Conentrations of gold in the solution and on the arbon N N In general, [Au]a < < [Au]a an d [Au]e < < [Au]e so that this equation an be simplified and rearranged to give V e = Va[Au]j[Au], (3) whih speifies the rate at whih arbon must be moved for the desired loading, [Au]. Experiene has shown that a value of 15 for the ratio [Au]j[Au] is a reasonable expetation. This an be seen from the data for atual operating plants shown in Fig. 2. It is interesting to note that the results obtained from pilot-plant investigations invariably predit better performane than is generally ahieved during full-sale operation. The only variable left that an be adjusted if the desired performane is to be ahieved is the mass of arbon (M e) in eah stage. This determinesthe average residene time of arbon per stage (te = M ejv e). The following iterative proedure has been found useful for the derivation of the required amount of arbon. Consideration of the mass balane and reation aording to equation (1) ourring in Stage 1 yields the following well-known form of equation for the performane of a stirred- tank reator:...,.. [Au ] O = [Au] + e K[Au] kte (4) l+kte It should be pointed out that, beause of the form of equation (1), it is idential whether homogeneous or segregated reation is assumed in the stage. (This an be shown quite simply by substitution of the integrated form of equation (1), whih applies to a bath reator, into the expression for a stirred-tank reator, whih inorporates the exit age distribution funtion for the segregated phase, i.e. the arbon). An initial estimate (say 1 hours) is made for te, whih is used in the alulation of [Au] and [Au] from equation (4) and the mass balane aross the first stage (equation (2) with N = 1). 'fhe values of [Au] and [Au] an then be used in the alulation of the orresponding quantities for the seond stage, and so on until the values of [Au];<and [Au] are obtained. If [Au] thus obtained does not math the desired barren solution, te is adjusted aordingly and the iterative proedure is repeated until the alulated value of [Au] onverges to the desired value. The use of this rate equation in simulating the performane of a typial CIP iruit is illustrated in the following example, whih is based on kineti data and operating parameters pertaining to the operation of the Mintek pilot plant at Western Areas Gold Mine under onditions of low (4 Ijh) and high (8 Ijh) pulp flowrate. The following parameters are ommon to both tests: No. of stages 5 Gold on arbon to GIP gjt Rate onstant (k),27 h-l Equilibrium onstant (K) 185. From Table I it an be seen that, although the predited and observed solution and arbon profiles were similar, the amounts of arbon alulated as neessary for the observed performane were onsiderably less than those atually employed. One of the reasons for this lak of agreement is the fat that gold from the ore is dissolved or desorbed during GIP absorption. This an, in ertain ases, signifiantly influene the performane of a GIP plant, and this aspet will be disussed later. Another possible reason is sensitivity to small hanges in the barren solution. Thus, a derease in the gold onentration of the barren solution from,3 to,2 gjt in Test 2 resulted in an inrease in the predited amount of arbon required from 6,4 to 8,3 kg per stage Cl 12 t:i is III e III CJ B Pilot-plant data Full-sale-plant data B Q;) Pregnant solution, g/t Fig.2-bserved performane of full. and pilot-sale CIP plants B B B 12 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH

3 TABLE PREDICTED AND OBSERVED OPERATION OF PILOT PLANT AT WESTERN AREAS GOLD MINE I Stage Feed Test 1, 4 ljh Test 2, 8 ljh Gold in solution, gjt Gold on arbon, gjt Gold in solution, gjt Gold on arbon, gjt Observed Predited Observed Predited Observed Predited Observed Predited 2,2 2,2 2,26 2,26,76, ,12 1, ,24, ,39, ,39, ,15, ,15,9 4 49,8, ,2,2 18 1,3, Carbon per stage: Observed 1 kg Predited 6,6 kg I Observed Predited 1 kg 6,4 kg Periodi Transfer of Carbon In pratie, arbon is not transferred ontinuously ounterurrent to the flow of the pulp. Instead, a fration of the arbon inventory in eah stage is removed at fixed time intervals and transferred to the preeding stage by means of airlifts or pumps. In the extreme ase, all the arbon in anyone stage is transferred during a yle. If it is assumed that this transfer is aomplished in a very short time ompared with the time tha.t the arbon spends in eah ontator between transfers, then the following simple analysis enables the periodi performane of an absorption setion to be modelled. The rate of loading of gold onto a bath of arbon (mass M ) transferred into a ontator (ontaining a mass Ms of solution) through whih pulp ontaining gold at a onentration of [Au]nis passed at a flowrate Vs an be derived if the expression for the mass balane aross the ontator is ombined with a suitable rate equation. Thus, for the mass balane, M d[] = Vs([Au]n- [Au]s) + M s d[au]s dt (5) As shown in the previous paperl, the onentration of gold in solution inreases with time, but the term Ms d[au]s/dt is small in relation to the others for reason. able onentrations of arbon. This equation an then be simplified to d[] = ;: ([Au]n- [Au]s). (6) Equation (1) an be ombined with this equation to yield the following expression for [AuJs: [A Us ] = Substitution M k[au] + Vs[Au]n in (6) yields M kk + Vs. d[au] - k ( K [A ] In [A ] ) - A us- U, where ka = kvs/(mkk + Vs) (7) (8) For the first stage of a multistage iruit with a onstant feed onentration [Au]n= [Au], equation (8) an be integrated to yield the following expression for the onentration of gold on the arbon as a funtion of time in the first stage: [Au]= K[Au] - exp( -kat)(k[au] - [Au]),. (9) where [Au] is the onentration of gold on the arbon added to the ontator. Substitution of equation (9) into equation (7) yields the following expression for the onentration of gold in solution in the first stage as a funtion of time: 1 [Au]s kam COl = [Au]s - exp( -ht)(k[au]s-[au]). (1) As shown in the previous paper, these expressions an be simplified if the absorption reation is far from equilibrium. The resulting equations predit a linear dependene of both [Au] and [Au] with time in the first stage after arbon has been transferred into the ontator. This proedure annot be followed for the seond or subsequent stages beause the feed to eah stage is not onstant and the equation (written for the seond stage) d[au] = ka (K[Au]l- [Au]) (ll) dt """ annot be simply integrated. The simplest approah has been found to be the following. (i) [Au] and [Au] are obtained as a funtion of time by the proedure previously outlined. (ii) The values of [Au] thus obtained are used in a numerial integration of equation (ll) to give [Au]. (iii) Values for [Au]: are then obtained by use of equation (7). (iv) This proedure an be repeatedfor eah subsequent stage. Simulation of the start-up and approah to the steadystate operation of a iruit an be made if it is assumed that fresh arbon is added to eah stage at the start and all the arbon is transferred at fixed time intervals. This alulation proedure an be arried out after eah transfer, and Fig. 3 shows the approah to steady-state onditions predited for the first and fourth stages of the Mintek pilot plant as detailed in Table MARCH 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

4 1,,9 :u,8 :;.." :;,7,6 [Au):' is steady-state,5 2 3 No. of transfers onentration Fig. 3-Approah to steady-state operation 4 5 Atual plant data are shown in Fig. 4 for the gold on the arbon over four yles in the first stage, and the solid lines were alulated aording to the above proedure. The agreement between observed and alulated performane is seen to be reasonably good and, if the inauraies assoiated with the sampling and analysis of both the solution and the arbon are taken into onsideration, the results demonstrate that the model an adequately desribe the performane of a CIP iruit operated in this periodi-transfer mode. The alulated periodi variation in the onentration of gold in solution is also shown in Fig. 4. 6'.. Some Pratial Impliations A number of onlusions regarding the design and operation of an absorption iruit an be made as a result of the appliation of this model. Transfer of Carbon A omparison of the data alulated from the predited urves in Fig. 4 with those in Table I shows that there is very little differene in the expeted performane of an absorption iruit with ontinuous transfer of arbon, or with periodi omplete transfer of all the arbon, from stage to stage. A full analysis of the relative performane for the transfer of various frations of the arbon inventory of eah stage during eah yle produed the values given in Table H. It should be emphasized that these values are for an ideal ase and do not allow for possible 'poisoning' of the arbon or other similar effets. Amount of Carbon per Stage It has often been suggested, without explanation, that more arbon should be added to the latter stages of an absorption iruit than to the early stages. This is sup- TABLE II ANALYSIS OF RELATIVE PERFORMANCE FOR THE TRANSFER OF VARIOUS FRACTIONS Fration per transferred yle 1,,5,25,1 Continuous. Observed [Au) Predited [Au) - -- Predited [Au)a Relative performane 1,,95,92,9,88,9 4 -Q U -::::a C- 2 t t Transfers of arbon 1 Time, h Fig. 4--Cyll behaviour t of the CIP pilot plant 2,7 -Q ;, -::::a C-,5,3 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH

5 posed to result in a more effetive operation with redued onentrations of gold in the barren solution. The predited effets of suh a non-uniform distribution of arbon in the Mintek pilot plant at Western Areas are shown in Fig. 5. It an be seen that, if the arbon is distributed uniformly throughout the plant, the gold in solution dereases exponentially with the number of stages, i.e. eah stage extrats the same fration of gold as that whih enters the stage. It should be noted that a nonuniform distribution of the arbon in the diretion of a greater fration in either the first or last two ontators results in a higher onentration of gold in the barren solution. The pratial impliation of this finding is obvious in that, for a given performane, the minimum arbon inventory is realized when the arbon is distributed uniformly throughout the absorption iruit. It is interesting to note, however, that the amount of gold 'loked up' in the arbon in a plant has a bearing on the optimum distribution of arbon. This an be seen from Table Ill. The amount of arbon required and the gold lok-up in the four-stage plant were alulated from the model for the following possibilities: (a) a uniform distribution of arbon throughout the plant, (b) three times as muh arbon in stages 1 and 2 as in stages 3 and 4, and () three times as muh arbon in stages 3 and 4 as in stages 1 and 2. i S' S 1, 1,,1,1 1 2 Stag. (!)1kg In.ah stag. 5 kg In stages 1 and 2 15kg In stag.s 3 and 4. 15kg In stag.s 1 and 2 5kg In stage. 3 and 4 Fig. 5-Effet of arbon distribution on CIP performane 3 4 TABLE EFFECT OF NON-UNIFORM DISTRIBUTION OF CARBON Example (a) Uniform (b) Stages 1,2 > Stages 3, 4 () Stages 1, 2 < Stages 3, 4 Stage Effiieny Stage Gold in solution Stage effiieny g/t % Feed 6,14 1 4, , , ,4 33 5, , ,15 68 III Relative arbon inventory 1 1,19 1,19 Relative gold lok-up 1 1,68,77 The stage effiieny, i.e. the perentage of gold extrated in eah stage, is usually onstant if the onentration of arbon in eah stage is the same. This has generally been found to be the ase on both pilot and full-sale plants. However, there are exeptions, as shown by Table IV. TABLE IV SOLUTION PROFILE FOR PLANT TREATING A CALCINE AT THE PRESI- DENT BRAND GOLD MINE The dereasing stage effiieny as the arbon progresses through the plant is indiative of a umulative 'poisoning' effet, and an investigation with a sanning eletron mirosope revealed the presene of a layer of fine hematite that had built up on the arbon partiles and was hindering the mass transport of gold to the arbon surfae. Carbon Inventory As in most multistage proesses, the total amount of absorbent material (in this instane arbon) dereases as the number of stages inreases. Calulations similar to those for Table I for onditions pertaining to the pilot plant at Western Areas Gold Mine produed the values that were plotted in Fig. 6. Here, the arbon inventory and gold lok-up were normalized to the ase in whih an infinite number of stages are used. It is apparent from the urves that there is a signifiant advantage to be gained in terms of redued inventories of both arbon and gold (on the arbon) by the use of a large number of stages. This is obviously offset by the higher apital and operating osts assoiated with eah additional stage, and it an be expeted that there will therefore be an optimum number of stages for eah individual appliation. This aspet is treated later in more detail. Appliation to Carbon-in-Leah The addition of arbon diret to a gold-leahing iruit is an attrative alternative to a separate arbon-in-pulp 74 MARCH 1984 JOURNAL OF THESOuni AFRICAN INSTITUTE OF MINING AND METALLURGY

6 5 4 i 3 >.: of! III 2. e Gi ex: No. of stages Fig. 6-lnfluene of the number of stages on arbon inventory and gold lok-up absorption iruit. This approah has obvious advantages from the point of view of apital osts, but at the same time presents a number of operational problems. These are not disussed here, but the appliation of the CIP model for the absorption proess to a typial arbon-inleah (CIL) appliation is useful in an assessment of the tehnial feasibility of suh an option and the identifiation of some of the more important design and operating parameters. Suh a model should also be of value in an estimation of the effets of the additional gold leahing that an our during absorption in a onventional CIP iruit. The derivation of suh a model requires that a suitable rate equation should be obtained for the leahing of gold. This equation an then be ombined with the rate expressions for absorption and the relevant massbalane equations. Kinetis of Leahing The rate of gold leahing from ores suh as those enountered in the present example is a omplex kineti problem involving a number of hemial, mass-transport, and mineralogial fators. However, a relatively simple expression has evolved from experiene at Mintek with the leahing of a number of loal ores. This rate expression has the following form: d[au]p t - A A 2 dt ' = kp ([ u]p,t - [ u]p,e),.. (12) 8 6 whih data for the leahing of a high-grade ore (7 gjt) and a low-grade residue (,8 gjt) are plotted on a logarithmi sale. The lines were alulated from the integrated form of equation (12), and best-fit values for the parameters Cl. [Au]p,e and kp were derived from a non-linear least- Z squares treatment of the data. The agreement an be seen to be good, and it is interesting that suh an analysis of!! some six different ores yielded values for the rate onstant & kp that vary in the rather limited range,4 to 1, h-l. This suggests that most ores are leahed at roughly the 2 4 ]! same rate, and that an average of,7 h-l is a reasonable estimate for use in the absene of atual values. The finer grind that is neessary for CIL probably results in somewhat faster leahing, and the suggested value of,7 h-l is probably a lower limit. Model for the Multistage OIL Proess The methods outlined earlier for a multistage ontinuous CIP plant operating at steady state an be simply extended by the inlusion of the solids in the mass balane for eah stage after the onentration profile for the solids has been alulated from equation (12) with the average residene time for the pulp in eah stage; that is, the assumption is made that the rate of leahing is independent of the presene of ati vated arbon. Bath tests at Mintek have shown this to be the ase. The appliation of this proedure to a typial low-grade ore (suh as that used in the Mintek pilot plant at Western Areas Gold Mine) yields the following performane for a simulated CIL plant designed to ahieve the same performane as the CIP plant operated under the onditions of Test I in Table I, viz a pulp flowrate of 4 Ijh with a barren solution of,2 gjt gold and a arbon loading of approximately 3 gjt from a head grade of 2 gjt. The CIL plant has 8 stages with a pulp residene time of 3 hours per stage, and the alulations were made on the assumption that ativated arbon is present in all the stages or only the last seven, six, five, or four stages of.. :;' et :5 2 The lines were alulated from the leahing-rate equation (12).\. H;gh d _... -2,. b. """"- "',AA ALA"'b. - Low-grade residue where [Au]p,t is the onentration of gold in the ore at time t and [Au]p,e is the orresponding quantity at infinite time (Le. the minimum ahievable residue grade), while kp is a rate onstant. Examples of the use of this expression to desribe the kinetis during bath leahing are given in Fig. 7, in Time,h Fig. 7-Leahing rates of two ores from different loalities JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH

7 leahing, i.e. with varying numbers ofpreleahing stages. The profiles alulated for the solution and solids (ore) in the example in whih the first three stages do not ontain arbon are shown in Fig. 8. It an be seen that almost 9 per ent of the gold is leahed in the first stage, implying that arbon ould be added to the first stage without affeting the ahievable loading on the arbon while possibly reduing the gold in the barren solution. However, the alulations show that there is an optimum point for the addition of arbon. Thus, the amounts of arbon required to ahieve the same performane and the gold loked.up in the plant (relative to the example where all the stages ontain arbon) are plotted in Fig. 9, from whih it is apparent that two stages of preleahing would be optimum. The point of addition of arbon will vary with head grade and the desired barren-solution value. However, only in very exeptional ases are more than 5 or 6 hours of preleahing required. Thus, alulations in whih the desired barren solution in this applia. tion is inreased to,1 gjt results in an optimum situation of one preleahing stage, and an inrease of the head-grade to 8 gjt results in the predition that arbon should be added to all the stages. This model permits an assessment of the effet of so-alled 'extra gold dissolution' during CIF absorption. Observations on both pilot-and full-sale plants have shown that the washed residues after CIF are invariably lower than those obtained by sampling of the feed to 2,4 2,2 2, 1,8 1,6 1,4 -'at :; 1,2 1,,8,6 Carbon added 1,2 1,1 ī 1, >.5 CD IQ,9 'ii a:,8, No. of pre-ieahing stages Fig. 9-Effet of number of pre-ieahing stages on the relative performane of a CIL plant CIF. This differene an in some instanes amount to an additional gold reovery of as muh as 1 to 2 per ent. However, ontinued slow leahing of the gold during CIF absorption an lead to apparent poor performane of the iruit, espeially if low (less than,1 gjt) barren values are desired. This an be seen from the following fairly typial example in whih the pulp produed by the leahing in 8 stages of 3 hours eah as desribed above is subjeted to CIP absorption. In the absene of additional leahing, five-stages of CIF would result in the predieted results shown in Table I (Test 1), in whih 6,6 kg of arbon per stage would be required to produe a barren solution ontaining,2 g/t of gold. An allowane for ontinued gold leahing results in the predited profiles shown in Table V. TABLE V PREDICTED EFFECT OF ADDITIONAL LEACIDNG OF GOLD ON CIP ABSORPTION, Conditions: The same as for Test I in Table I Stage Solids, g/t Solution, g/t Carbon, g/t Feed,26 2,2 1,25, ,25, ,24, ,23,6 44 5,23,2 12,4,2 Fig. 8-Calulated Stage solids and solution profiles for a CIL plant 8 Table V an be ompared with Table I and the results predited for a CIL plant (Table VI). It is apparent that the dissolution ourring during absorption an have a signifiant effet on the amount of arbon required, and this results in a higher arbon (and gold) inventory for a CIL plant than for a CIP plant. This would need to be taken into aount in any eonomi evaluation of the alternative iruits. 76 MARCH 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

8 TABLE Carbon Gold Leahing Head CIP Residue inventory lok-up Plant stages g/t stages g/t kg kg Leah-CIP 8(3) 2,3 5(1),26 32,9,27 *Leah-CIP 8(3) 2,3 5(1),23 55,2,44 CIL 8(3) 2,3,26 68,5,55 VI COMPARISON OF CIP AND CIL * The figures in parentheses represent pulp residene times Allowane was made for leahing during CIP absorption. Appliation to the Eonomi Optimization of Design It should be apparent from these onsiderations that the design of a CIP plant involves deisions on a number of proess options, the most important of whih are the desired extration (i.e. the maximum onentration of gold in the barren pulp), the loading of gold on the ar. bon, and the number of stages to be employed. Having a knowledge of the kinetis of extration and using this model, one an show that the orret deisions on these options an lead to eonomi optimization of the plant design. It should be emphasized that the approah suggested is a simplifiation of a omplex eonomioptimization proedure. However, it serves to iliustrate how the models developed in this paper an be inorporated into suh a ost-minimization proedure. A major onsideration is the value of the arbon and the gold absorbed on the arbon in the absorption iruit. As a first approximation, the value of the arbon and its assoiated gold was onsidered to be part of the apital ost of the plant. The following example was based on the operating harateristis of the Mintek pilot plant at Western Areas Gold Mine, whih was fully desribed earlier. The alulations were made for a hypothetial plant to treat pulp ontaining 2 kt of solution per month, and the following assumptions were made: R Value of ativated arbon per ton 3 Value of gold per kilogram 15 Cost of elution: and reativation per ton of arbon 1 Cost of absorption vessel 15 As a result of the interdependene of the various parameters, an iterative proedure was employed in whih, for example, the number of stages and the desired onentration of gold in the barren solution are fixed and an optimum gold loading on the arbon is alulated. This value an then be used in the alulation of the optimum number of stages, and in the estimation of the barren-solution value, whih an then be used in another yle of alulations. In this partiular instane, the assumption was made that 6 stages are required to produe a barren solution ontaining,1 gjt of gold. These values were then substituted into the model together with various possible gold loadings, and' the neessary amounts of arbon and the gold loked up were alulated. The results are shown in Fig. 1, in whih it an be seen that the optimum loading is 4 gjt. The existene of this optimum is attributed to the oppo. sing effets of lower operating osts assoiated with a higher gold loading (i.e. a slower movement of arbon through the plant) and the inrease in the amount of arbon and gold in the absorption vessels. The interest on these values was alulated for a rate of 15 per ent per annum. It has already been demonstrated that the amount of arbon required to ahieve a desired extration and the amount of assoiated gold derease with inreasing numbers of stages (Fig. 6). In this example, the derease in apital assoiated with the arbon and loaded gold must be balaned against the ost of extra absorption vessels. The use of the optimum loading of 4 gjt and a desired barren solution of,1 gjt in the model for various numbers of stages produed the results iliustrated in Fig. ll, from whih it an be seen that the previous estimated optimum of 6 stages is orret. Finally, these values of 4 gjt loading and 6 stages an be used in a similar alulation to give an optimum value for the gold in the barren solution. In this ase, inreased reovery is offset by the need for the employ- 3 ;:- 1 It) 2 It) S G) ;: > G' 1 «I I '" x 'iij 'a Cl C «I 'iii!; as -1 ii Z -2 (141) (13) Figures in parentheses are the alulated arbon flow. rates in kg/h ' Gold on loaded arbon, g/t Fig. IO-Optimization of the arbon flowrate and the gold loading in a six-stage CIP plant JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH

9 a;,...:. x 6 Cii' CD g) as -1/1 5 - CD > 4 ::: ;;- as 3 'ij g) "C C as iii - a:- '. 2 as u iii "- CD > Number of stages Fig, I I-Optimization of the number of stages in a CIP plant Cl,.. S 3.5! : 2 :::.!!. )(!,.. 1/1 as 'g! U «I.5 S 1 ii C «I Gi z,5,1,2,4,6,1 Gold in barren solution, g/t Fig. 12-ptimization of the extration effiieny in a sixstage CIP plant ment of more arbon with its assoiated gold, The results of these alulations are shown in Fig. 12, from whih it an be seen that the plant should be designed to produe barren values below,2 g/t. A omparison of the relative revenues in Figs. 1 and 12 shows that the loss of gold in the barren pulp is a more important onsideration than the amount of gold loked up in the plant. Conlusions A relatively simple rate expression for the absorption of gold onto ativated arbon was used as the basis for the development of a model for a multi stage CIP absorption iruit, When the model was used in the predition of both the steady-state and the transient behaviour of the absorption proess, the results were in reasonable agreement with those observed in an extended pilotplant ampaign. Extension of the model to the desription of a CIL proess was made possible by the inlusion, in the overall CIF model, of a suitable rate equation for leahing. This enabled the optimum point of addition of arbon to the leahing iruit to be identified, and also allowed for an assessment of the effets of gold dissolution during a nor. mal CIF absorption. Finally, a brief assessment was made of the eonomi onsequenes of the various proess options available for the design of a CIF plant. Aknowledgement This paper is published by permission of the Counil for Mineral Tehnology. 1. Referenes NrOL, M. J., FLEMING, C. A., and CROMBERGE,G. The absorption of gold yanide onto ativated arbon. I. The kinetis of absorption from pulps. J. S. Afr. Inst. Min. Metall., vo!. 84, no. 2. Feb pp HUSSEY, S. J., SALISBURY,H. B., and POTTER, G. M. Carbonin-pulp silver absorption from yanide leah slurries of a silver ore. U.S. Bur. Mines, Report oj Investigation no. 8268, MARCH 1984 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY