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The recovery of gold from ammoniacal thiosulfate solutions containing copper using ion exchange resin columns

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The recovery of gold from ammoniacal thiosulfate solutions containing copper using ion exchange resin columns Powered By Docstoc
					                                               Hydrometallurgy 72 (2004) 225 – 234
                                                                                                      www.elsevier.com/locate/hydromet




      The recovery of gold from ammoniacal thiosulfate solutions
         containing copper using ion exchange resin columns
                                  Hongguang Zhang 1, David B. Dreisinger *
Department of Metals and Materials Engineering, The University of British Columbia, 309-6350 Stores Road, Vancouver, BC, Canada V6T 1Z4

                         Received 7 October 2002; received in revised form 8 April 2003; accepted 2 July 2003



Abstract

    The loading of gold and copper, both individually and simultaneously, from thiosulfate solutions onto ion exchange resin
columns and the subsequent elution of these species have been investigated. In the presence of copper, effective loading with
good selectivity for gold can be achieved at pH 11, which balances the stability of the solution and minimizes the formation of
poisoning polythionates. The loaded metals can be eluted with high efficiencies using a number of composite eluant solutions.
These include sodium tetrathionate stabilized with sodium sulfite and ammonium thiosulfate (ATS), sodium sulfite coupled with
ammonia, and sodium chloride with the addition of ATS. The addition of thiosulfate to the eluant seems to have a negative
effect on the gold elution efficiency and should be avoided wherever it is possible. Of all the three eluants investigated, the
sodium sulfite/ammonia combination is found to be the most efficient, but the sodium chloride/ATS combination is likely the
most economical.
D 2004 Elsevier B.V. All rights reserved.

Keywords: Gold recovery; Copper; Thiosulfate; Ion exchange




1. Introduction                                                            leaching reaction has been proposed as (Li et al.,
                                                                           1996):
   The leaching of gold from its ores using thiosulfate
solutions has been extensively studied in the last two                     Au þ CuðNH3 Þ2þ þ 4S2 O2À
                                                                                        4         3
decades (Zipperian and Raghavan, 1988; Tao et al.,
1993; Abbruzzese et al., 1995; Li et al., 1996). It is                       ! AuðS2 O3 Þ3À þ CuðS2 O3 Þ3À þ 4NH3
                                                                                         2              2                          ð1Þ
found that the dissolution of gold in a thiosulfate
solution is usually very slow, unless catalyzed by
                                                                           CuðS2 O3 Þ3À þ O2 þ 2H2 O þ 16NH3
                                                                                     2
copper and ammonia. A possible mechanism for the
                                                                             ! 4CuðNH3 Þ2þ þ 8S2 O2À þ 4OHÀ
                                                                                        4         3                                ð2Þ

   * Corresponding author. Fax: +1-604-822-3619.
   E-mail address: drei@interchange.ubc.ca (D.B. Dreisinger).              In most studies, ammonium thiosulfate (ATS), which
   1
     Now at A.J. Parker Cooperative Research Centre for Hydro-             is primarily used as a fertilizer, is used as the leaching
metallurgy, Murdoch University, Murdoch, WA 6150, Australia.               agent as it is inexpensive and already contains the

0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/S0304-386X(03)00183-X
226                             H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234

ammonia required to support the copper catalysis of              has been shown (O’Malley and Nicol, 2001; Zhang
gold leaching. Copper is usually added to the leach              and Dreisinger, 2002a) that tetrathionate (S4O6 À) and
                                                                                                                  2
                                                                                                            2À
solution as cupric sulfate. In many cases, sufficient            its degradation product, trithionate (S3O6 ), strongly
copper may be naturally leached from the ore. The                inhibit the adsorption of gold and copper onto the
efficiency of the leaching depends on many factors,              resins. These results are consistent with the findings of
including pH, Eh, temperature, ammonia-to-thiosul-               polythionates being poisonous to the resins used in the
fate ratio, solution stability, etc. However, satisfactory       recovery of uranium (Nugent, 1956; Merritt, 1971).
gold recovery can be achieved under well-maintained              Fortunately, it has also been shown that tetrathionate
conditions. Because of the advantages of low toxicity            can be effectively removed by alkaline decomposition
and low reagent costs, thiosulfate leaching is proba-            at ambient or slightly elevated temperatures (Zhang
bly, at the present time, the most promising method to           and Dreisinger, 2002b)2. The experimental conditions
replace the conventional cyanidation process for gold            for the current work could then be established.
extraction (Ritchie et al., 2001).
    However, one of the problems associated with
thiosulfate leaching is the difficulty in recovering gold         2. Experimental
from pregnant solutions. For example, it has been
shown that the gold – thiosulfate complex does not                   Commercially available strongly basic anion ex-
effectively adsorb onto activated carbon (Gallagher et           change resins were obtained in their wet forms and
al., 1990) and hence the carbon-in-pulp technology is            used without treatment. The resins involved in this
not applicable to the thiosulfate system. Cementation            work, including Dowex G51, Dowex 21K and Amber-
with zinc or copper (Berezosky and Sefton, 1979;                 lite IRA-410, were selected from a group of resins that
Perez and Galaviz, 1987; Guerra, 1997) has also been             had been investigated previously (Zhang and Drei-
studied but this method suffers from high zinc con-              singer, 2002a). These resins are all of gel type, with a
sumption or possibly passivation by cuprous sulfide              polystyrene divinylbenzene matrix and quaternary
formation on the copper cement surface. Recent                   ammonium functional groups. Other properties are as
publications and patents (O’Malley and Nicol, 2001;              given in Table 1. The ion exchange column was
Zhang and Dreisinger, 2002a; Fleming et al., 2002)               constructed with a 25-mL burette with small glass
have shown the possibility of using ion exchange                 wool plugs both at the bottom and on the top of the
resins to recover gold from thiosulfate solutions and            resin bed. The volume of the bed was approximately
leach pulps. Therefore the loading and elution behav-            taken as 1.5 mL/g of wet resin. The solution was
ior of gold and copper has been investigated in the              pumped in through a rubber stopper on the top of the
current work using ion exchange resin columns and                burette using a peristaltic pump. The effluent was
the results are now presented.                                   collected from the tip at the bottom.
    Another problem associated with the use of thio-                 Synthetic solutions prepared with deionized water
sulfate is its oxidation by dissolved oxygen, which is           were employed for the study. Ammonium thiosulfate
also strongly catalyzed by copper (Li et al., 1996).             (ATS) obtained from Aldrich was 99% pure. All other
Given the high redox potential, the Cu(II) species can           chemicals were of analytical grade. Gold was intro-
                                                                                                                    À
oxidize thiosulfate to tetrathionate.                            duced to thiosulfate solution in the form of AuCl4 , as
                                                                 the commercial reference solution (1000 F 1 ppm Au
                                                                 with 10% HCl), which was immediately reduced by
2CuðNH3 Þ2þ þ 6S2 O2À ! S4 O2À þ 2CuðS2 O3 Þ3À
         4         3        6               2                    thiosulfate to form Au(S2O3)2 À. Copper was added as
                                                                                               3

                             þ 8NH3                    ð3Þ       CuSO4 5H2O. All loading experiments were carried
                                                                 out at the ambient temperature (23 – 25 jC) and pH 11.
The cupric species is regenerated by dissolved oxygen
through Eq. (2). The undesirable oxidation of thiosul-
fate not only remarkably increases reagent consump-                  2
                                                                       In initial laboratory experiments conducted at the University
tion and hence operating cost, but also undermines the           of British Columbia, the destruction of trithionate appears to be
subsequent process for gold recovery. For example, it            . more difficult than the destruction of tetrathionate.
                                                                 much
                                     H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234                               227

Table 1                                                               arises due to the small change in copper concentration
Physical and chemical properties of the resins used                   between the feed and product solution. The change in
Resins               Ionic pH   Moisture Bead          Capacity       copper concentration is small relative to the precision
                     form range (%)      size          (meq/mL)
                                                                      of the analysis procedure, introducing the possibility
                                         (mm)
                                                                      of poor balances.
Dowex G51         ClÀ       0 – 14 43 – 48    0.3 – 0.9 1.4
Dowex 21K         ClÀ       0 – 14 43 – 48    0.6 – 1.2 1.2
Amberlite IRA-410 ClÀ       0 – 14 42         0.48      1.35
                                                                       3. Results and discussion

Such a pH was found to be optimal for the following                    3.1. Loading of gold or copper only
reasons. Firstly, the Cu – ATS solution tends to decom-
pose at lower pHs, forming copper sulfides, while at                      The results for gold loading from 0.1 M ATS
higher pHs the chance for copper hydroxide precipi-                   solutions onto the columns of Dowex G51 and Dowex
tation increases. Secondly, the resins can be prevented               21K resins are shown in Fig. 1. The effluent was
from poisoning at pH 11 since tetrathionate, which                    collected in fractions at different times in order to
may be formed through Eq. (3) prior to and during the                 demonstrate the kinetics of the loading process. On
loading, is not stable in strongly alkaline solutions and             G51, 300 bed volume (BV) loading solution contain-
quickly decomposes to thiosulfate and sulfite (Zhang                  ing 20 ppm Au was pumped through the column at a
and Dreisinger, 2002b). The loading solution was                      rate of 13 BV/h and virtually no gold was detected in
prepared in such a way that the ATS was dissolved                     the effluent. This indicates that the ion exchange
first with the addition of a suitable amount of NaOH to               reaction was fast since the retention time of the
raise the pH to slightly above 11 before CuSO4 was                    solution in the resin bed was less than 5 min. On
added. The solution was then adjusted to pH 11 by                     21K, 630 BV solution was used and almost exactly
NaOH or H2SO4 and used immediately for loading                        the same loading as on G51 was obtained for the first
experiments. The elution of gold and copper from                      300 BV, but with the effluent gold concentration being
loaded resins was carried out using different eluant                  slightly higher. As more solution passed through the
solutions with their compositions described in the text.              column, gold loading increased almost linearly, reach-
    An automatic fraction collector was used to take                  ing about 18 kg/t, in correspondence to 0.7 ppm gold
solution samples during either loading or elution if the              in the effluent. Both columns were not saturated.
kinetics were to be followed. In other experiments, the               These results agree well with those obtained previ-
loading or elution effluents were collected as a whole
only for the final results. Gold and copper in solution
were analyzed by commercial analytical laboratories
using fire assay and ICP, respectively. The loading Q,
presented as the amount of a metal on unit mass of
wet resin, as well as the elution efficiency, could then
be calculated. Yet, more reliable loading values could
be determined from elution, i.e. from the total amount
of metals eluted and remained, the latter being found
out by resin assay. Accordingly, the elution efficien-
cies could be calculated on the same basis. As found
both previously (Zhang and Dreisinger, 2002a) and in
the current work, the mass balance results for gold
loading and elution from both calculation procedures
are generally well consistent. However, the results for
copper calculated from solution analysis are not                      Fig. 1. Loading of gold onto columns of Dowex G51 (flow rate 14
reliable or consistent with the loading and elution                   BV/h) and Dowex 21K (flow rate 13 BV/h) from 0.1 M ATS
values calculated by resin assay. This inconsistency                  containing 20 ppm Au at pH 11.
228                                H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234

ously from batch studies (Zhang and Dreisinger,                     saturated. Accordingly the copper loading reached a
2002a) and indicate that gold can be removed from                   maximum. In the last stage, Cu concentration in the
pure solutions rapidly and loaded on resins to very                 effluent increased slowly exceeding the initial level
high concentrations.                                                and Cu loading declined slightly, indicating that some
   In real leach solutions, however, copper is almost               of the loaded copper returned to the solution. This
always present, usually in much higher concentrations               may be attributed to the presence of a small amount of
than gold, since it is added as a catalyst for gold                 tetrathionate or trithionate in the original loading
dissolution. Therefore, the interaction between copper              solution, which may have not completely decomposed
and the resins was investigated. Typical kinetic curves             under the given conditions. The deleterious species
for the loading of copper on Dowex 21K resin from                   accumulated on the resin as more solution passed
0.1 M ATS containing 500 ppm Cu are presented in                    through the column, replacing some of the already-
Fig. 2. As can be seen, the change in effluent Cu                   loaded copper. Similar curves were recorded with
concentration underwent three stages as a total of 125              Dowex G51 resin, but with somewhat higher loading
BV solution passed through the column. In the first                 capacity. The maximum Cu loading obtained on 21K
stage (0– 20 BV), Cu concentration was maintained at                was about 23 kg/t while it was about 30 kg/t on G51.
about 120 ppm. This probably reflected the presence
of some Cu(II) in the original loading solution since                3.2. Simultaneous loading of Au and Cu
the cationic Cu(II) – ammine complex would not be
expected to load with the anion exchange resin.                        The kinetics for the co-adsorption of gold and
However, this level varied in different experimental                copper on Dowex 21K resin from a solution contain-
runs possibly because the ratio of Cu(II)/Cu(I) was not             ing both metals is demonstrated in Fig. 3. Different
steady due to the reaction in Eq. (3). It was believed              flow rates were used but a similar trend to the single
that virtually all the anionic Cu(I) – thiosulfate com-             metal tests was observed. It can be seen from Fig. 3a
plex in the original solution had been adsorbed by the              that the kinetic curves for gold loading in the presence
resin in the first stage. As a result, the amount of                of copper are closely similar to those obtained using
copper loaded on the resin increased rapidly with                   pure solutions for the initial 200 BV. After this point,
increasing solution volume. In the second stage                     gold broke through the column with its concentration
(20 – 80 BV), the breakthrough of Cu(I) occurred                    in the effluent increasing sharply, indicating the resin
and the total Cu concentration in the effluent rose                 was near saturation. Apparently, the capacity for gold
gradually to the same level as in the initial loading               on the resin was much lower as a result of competitive
solution (dashed line), suggesting the column was                   adsorption of copper. It can also be seen that the flow
                                                                    rate at which the solution passed the column affected
                                                                    gold loading appreciably. With the higher flow rate,
                                                                    the breakthrough occurred earlier but the saturation
                                                                    was achieved later. This is because at the higher flow
                                                                    rate, the solution retention time was shorter, the ion
                                                                    exchange reaction was less complete and so more
                                                                    solution was required to saturate the column.
                                                                       The kinetic curves for copper loading in the pres-
                                                                    ence of gold are shown in Fig. 3b. As it is illustrated,
                                                                    copper loading increased very rapidly to the saturation
                                                                    level within about 50 BVand then dropped gradually as
                                                                    more solution was provided. No effect of flow rate was
                                                                    seen, probably because the kinetics of the process was
                                                                    so fast that the change from 13.3 to 6.9 BV/h did not
Fig. 2. Loading of copper onto a column of Dowex 21K resin from     make an appreciable difference in copper loading. A
0.1 M ATS containing 500 ppm Cu at a flow rate of 15 BV/h and       complete picture for the co-loading of gold and copper
pH 11.                                                              was then obtained by combining Fig. 3a and b. At the
                                     H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234                                 229

                                                                      due to the fact that the difference between the Cu
                                                                      concentrations in the initial and final solutions was not
                                                                      sufficiently large compared with the error in copper
                                                                      analysis. For comparison, a sample of loaded resin
                                                                      was analyzed giving gold and copper loadings to be
                                                                      9.17 and 9.75 kg/t, respectively. The copper loading
                                                                      from resin analysis was then used as a reference in the
                                                                      elution studies.
                                                                          The concentration of gold in practical leach solu-
                                                                      tions may well be below 20 ppm, the level used in the
                                                                      above experiments. Therefore, it is necessary to dem-
                                                                      onstrate that gold can be loaded effectively on resins
                                                                      from solutions containing very low levels of gold. Fig.
                                                                      4 shows the results of gold loading on Dowex 21K
                                                                      resin from solutions comprising of 0.1 M ATS, 1 ppm
                                                                      Au and various amounts of Cu. As can be seen, gold
                                                                      loading is very much dependent on the copper level in
                                                                      solution. In the case of 100 ppm Cu, high gold loading
                                                                      (5.5 kg/t) was achieved when sufficient amount of
                                                                      solution was pumped through the column. As the Cu
                                                                      concentration increased to 200 and 500 ppm, the
                                                                      maximum gold loading decreased to 3.3 and 1.6 kg/
                                                                      t, respectively. However, copper loading was found by
                                                                      resin assay to remain similar to the previous values,
Fig. 3. Co-loading of gold (a) and copper (b) onto Dowex 21K resin    regardless of the feed Cu concentration in solution.
columns at different flow rates from 0.1 M ATS containing 20 ppm
Au and 500 ppm Cu (pH 11).                                             3.3. Elution

beginning, the column was quickly saturated with                         In the elution experiments, gold and copper were
copper due to its high concentration in the loading                   pre-loaded onto the resin columns from a solution
solution. As more solution passed, however, gold                      containing 0.1 M ATS, 20 ppm Au and 500 ppm Cu
accumulated on the resin, taking the place of copper.
Therefore the amount of copper on the resin decreased
as that of gold increased. Considering the fact that the
effluent gold concentration was much lower than
copper, it can be concluded that the adsorption of gold
on the resin is much stronger than copper. This result
favors the selective loading of gold over copper.
   The co-loading of gold and copper was repeated a
number of times under the same conditions as de-
scribed in Fig. 3. However, no fraction samples were
taken but, instead, the total amount of effluent was
collected over the entire period of the experiments.
Reproducible final gold loading values ranging from
8.70 to 9.21 kg/t were obtained. These numbers were
calculated from solution analysis. On the other hand,                 Fig. 4. Loading of gold from 0.1 M ATS solutions containing 1 ppm
the calculated numbers for copper loading were found                  Au and various concentrations of Cu at a flow rate of 30 BV/h and
to vary significantly between 5 and 25 kg/t. This was                 pH 11.
230                             H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234

using a flow rate of about 15 BV/h. The elution was
conducted immediately after the completion of the
loading.
    A number of eluant solutions were used to strip the
gold and copper from the resins. First of all, tetrathi-
onate was selected as an eluant since it strongly
competes with the metals to adsorb onto the resins
(Zhang and Dreisinger, 2002a). Ammonium thiosul-
fate was added to the stripping solution to stabilize the
eluted gold and copper. A low concentration of
tetrathionate (0.02 M) was initially employed but the
elution was not complete. An attempt to raise the
tetrathionate concentration, however, led to the for-
mation of a white precipitate. This is most likely due           Fig. 5. Elution of gold from columns of Dowex G51 (8.92 kg/t Au)
to the reaction between tetrathionate and thiosulfate to         and Dowex 21K (18.83 kg/t Au) with an eluant composed of 0.25
form pentathionate (S5O6 À), which soon decomposes
                          2                                      M Na2S4O6, 0.5 M ATS and 0.25 M Na2SO3 with the pH adjusted
                                                                 to 9 (flow rate 2 BV/h).
to precipitate elemental sulfur (Lyons and Nickless,
1968):
                                                                 shown in Fig. 6. It is clear from the comparison with
S4 O2À þ S2 O2À WS5 O2À þ SO2À
    6        3       6      3                          ð4Þ
                                                                 Fig. 5 that the elution of copper was faster than that of
                                                                 gold, with the maximum concentration in the eluate
To prevent the formation of sulfur, sulfite (SO3 À) was
                                               2
                                                                 appearing at about 5 BV. However, the elution curve
added according to the above reaction, and this proved
                                                                 also shows a tail similar to those in Fig. 5, indicating
effective. However, the following reaction is also
                                                                 the slow kinetics for the removal of the last 20%
likely to occur upon the addition of sulfite (Lyons
                                                                 copper from the resin. A copper recovery of 96.5%
and Nickless, 1968) to form trithionate (S3O6 À):
                                               2
                                                                 was obtained at about 30 BV.
S4 O2À þ SO2À WS3 O2À þ S2 O2À
    6      3       6        3                          ð5Þ           Fig. 7 shows the elution of gold and copper
                                                                 together with the tetrathionate eluant from a resin
Therefore the tetrathionate eluant solution may well             loaded with both metals. The results agree very well
be a mixture of tetrathionate, trithionate, thiosulfate          with those from Figs. 5 and 6 in that copper was
and sulfite. In terms of elution, trithionate is expected        removed from the column faster than gold in the
to be as effective as tetrathionate, since it is also a          initial stage of elution. In the later stage (about 20%
poison to the resins (O’Malley and Nicol, 2001). The             metal remaining), the elution of both gold and copper
mixture is referred to as the tetrathionate eluant in this       became much slower. Nearly complete elution of both
paper usually with a composition of 0.25 M Na2S4O6,              metals was achieved at about 50 BV. Consistent
0.25 M Na2SO3 and 0.5 M ATS though other compo-                  elution efficiencies (99.7% Au and 99.3% Cu) were
sitions may also be used.                                        obtained from separate experiments where the effluent
    Fig. 5 shows the kinetic curves for gold elution             was collected as a whole with all 50 BV eluant,
from resins loaded with gold only using the tetrathi-            generating an eluate solution of 120 ppm Au and
onate eluant. As can be seen, the initial elution rate           130 ppm Cu. It should be mentioned that similar
was fast with the maxima gold concentration in the               elution results could be achieved using the tetrathio-
eluate reached at 10 – 15 BV. In the later stage,                nate eluant with a different composition, e.g. 0.1 M
however, the elution rate slowed down as indicated               Na2S4O6, 1 M Na2SO3 and 0.1 M ATS.
by a long ‘‘tail’’ following the maxima on the kinetic               Sodium sulfite alone was also found to be very
curves. The total elution efficiencies at about 55 BV            effective for gold and copper elution. However, cop-
were 95.3% (G51) and 98.7% (21K), respectively.                  per hydroxide precipitated in the eluate solution and
    The kinetic curve for the elution of copper from a           this was prevented by the addition of ammonia. A
copper loaded column using the tetrathionate eluant is           separate experiment showed that ammonia itself was
                                     H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234                                 231




Fig. 6. Elution of copper from a column of Dowex G51 (24.7 kg/t Cu)   Fig. 8. Elution of gold and copper from a column of Dowex 21K
using the same eluant as for Fig. 5 (flow rate 1.3 BV/h).             (8.87 kg/t Au and 7.66 kg/t Cu) using an eluant composed of 2 M
                                                                      Na2SO3 and 1 M NH3 with the pH adjusted to 11 (flow rate 2
                                                                      BV/h).

not able to strip the metals. Interestingly, gold was
stable in the eluate solution and may have been                       copper as compared to the tetrathionate eluant. As a
present as Au(SO3)2 À, which is known to exist and
                       3                                              result, complete elution of gold and copper required
has found ample applications in gold plating baths                    only about 25 BV of the eluant solution.
(Zur and Ariel, 1982). Alternately, the gold complex                      Further experimental results given in Table 2 con-
in solution may have been stabilized by the small                     firm that sodium sulfite is more efficient than the
amount of thiosulfate present. Fig. 8 shows the kinetic               tetrathionate eluant for the elution of gold and copper.
curves for the elution of gold and copper using the                   With 33 and 37 BV eluant comprising of 2 M Na2SO3
sulfite eluant. It is evident from Fig. 8 that both gold              and 1 M NH3, gold and copper were completely
and copper were stripped rapidly from the resin                       removed from the resin, generating eluate solutions
column, with the maximum concentrations achieved                      with at least 170 ppm Au. Table 2 also shows that the
within 10 BV. More significantly, no obvious tails                    addition of 0.1 M ATS to the eluant solution greatly
were observed on the elution curves for both gold and                 reduces the elution efficiency, particularly for gold.
                                                                      With a total of 53 BV eluant containing ATS, only
                                                                      85.5% Au was stripped. This result may explain the
                                                                      ‘‘tail’’ effect of the tetrathionate eluant that contains
                                                                      ATS. The presence of thiosulfate might have hindered
                                                                      the elution of gold, and possibly copper as well, in
                                                                      some way which is not currently understood.
                                                                          Another effective eluant studied was sodium chlo-
                                                                      ride. The addition of not only ammonia, but also

                                                                      Table 2
                                                                      Elution of Au and Cu from Dowex 21K resin with the sodium
                                                                      sulfite eluant added with ammonia and ATS
                                                                      Eluant composition             Total   Au elution    Cu elution
                                                                                                     BV      (%)           (%)
                                                                      2 M Na2SO3, 1 M NH3            33      99.9           98.0
                                                                      2 M Na2SO3, 1 M NH3            37      99.9          100.0
Fig. 7. Elution of gold and copper from a column of Dowex 21K
                                                                      2 M Na2SO3, 1 M NH3,           38      74.4           98.6
(9.12 kg/t Au and 8.19 kg/t Cu) using the same eluant as for Fig. 5
                                                                        0.1 M ATS                    53      85.8           98.7
(flow rate 2 BV/h).
232                             H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234

thiosulfate, was necessary to stabilize the eluted               Table 4
metals in the eluate solution. Table 3 shows the results         Results of loading/elution cycling experiments from Dowex G51
                                                                 resin (2.00 g) using Eluant A
of gold and copper elution with sodium chloride
                                                                 Cycle       Metals loaded      Metals eluted     Recovery
solutions. With 2 M NaCl and 1 M NH3 (27 BV)
gold elution was only 16.4%, while copper was                                Au/mg     Cu/mg    Au/mg    Cu/mg    Au (%)     Cu (%)
completely removed. In accordance with the low gold              1           19.66      18.96   17.87    16.24     90.9       85.6
elution, heavy gold deposit was observed on the resin            2           19.30      41.80   19.23    18.12     99.7       43.3
                                                                 3           19.48      20.00   19.20    18.02     98.6       90.1
surface. Obviously, gold was first stripped from the
                                                                 4           19.16      25.16   18.91    17.68     98.7       70.3
resin and then decomposed due to the lack of com-                5           19.10       9.00   19.13    18.60    100.2      206.7
plexing agent. The addition of 0.1 M ATS prevented               Overall     96.68     114.9    94.32    88.66     98.5a      99.0a
gold deposition. However, gold elution was not satis-                a
                                                                         Total metals remained on resin: Au 1.43 mg and Cu 0.86 mg.
factory using 2 M NaCl, with gold recoveries being
only 71.8% and 82.1% at 37 and 52 BV, respectively.              eluant solution at 2 BV/h. The compositions of the
Copper elution was also somewhat reduced. These                  eluants used were:
results are comparable to those obtained with the
sodium sulfite eluant containing ATS (Table 2). Again,           (A) 0.25 M Na2S4O6, 0.5 M ATS and 0.25 M Na2S2O3
the presence of thiosulfate may also be responsible for              (pH 9.0);
the low elution efficiencies. In further experiments, the        (B) 2 M Na2SO3 and 1 M NH3 (pH 11.0); and
concentration of sodium chloride was increased to 4 M            (C) 4 M NaCl and 0.1 M ATS (pH 9.2).
to compensate the negative effect of thiosulfate. Am-
monia was no longer necessary because ATS was used.                 Two different resins were tested for each of the
As can be seen from Table 3, with the eluant contain-            eluants. After elution with Eluant A, the column,
ing 4 M NaCl and 0.1 M ATS, copper was completely                being saturated with tetrathionate (or trithionate as it
removed at 30 BV and gold at 42 BV.                              might appear in the solution), was reconditioned
                                                                 before it was reused for loading in the next cycle.
3.4. Loading/elution cycling                                     This was done by washing the column with 10 BV
                                                                 0.05 M NaOH at 5 BV/h, followed by a wash with
   In practical operations, it is required that the col-         deionized water. In the case of Eluant B and Eluant C,
umns be used repeatedly. Therefore the regeneration              however, no column reconditioning was necessary.
behavior of different resins was tested by repeating the            The concentrations of Au and Cu in the initial
loading and elution for five cycles. In each loading             loading solutions, loading effluents and eluates were
cycle, a total volume of 1 L (333 BV) standard loading           analyzed so that the loadings of the metals and the
solution, composed of 0.1 M ATS, 20 ppm Au and 500               elution efficiencies could be determined for the indi-
ppm Cu, was passed through the column at 14– 16 BV/              vidual cycles. At the end of the experiments, the resins
h. The loaded column, after washing with 3– 5 BV
deionized water, was then eluted with 40 –50 BV of an
                                                                 Table 5
                                                                 Results of loading/elution cycling experiments from Dowex 21K
Table 3                                                          resin (2.00 g) using Eluant B
Elution of Au and Cu from Dowex 21K resin with the sodium        Cycle       Metals loaded      Metals eluted     Recovery
chloride eluant added with ammonia and ATS                                   Au/mg     Cu/mg    Au/mg    Cu/mg    Au (%)     Cu (%)
Eluant composition      Total    Au elution      Cu elution
                                                                 1           18.80     21.00    18.81    20.00    100.0       95.2
                        BV       (%)             (%)
                                                                 2           18.56     18.26    18.48    19.80     99.6      108.5
2 M NaCl, 1 M NH3       27       16.4             99.9           3           18.50     13.56    18.36    16.76     99.3      123.7
2 M NaCl, 1 M NH3,      37       71.8             95.7           4           18.46     13.50    18.55    17.02    100.5      126.1
  0.1 M ATS             52       82.1             95.8           5           18.60     20.36    18.39    17.58     98.9       86.4
4 M NaCl,               30       94.5             99.6           Overall     92.90     86.66    92.58    91.16    100.0a     100.0a
  0.1 M ATS             42       99.5            100.0               a
                                                                         Total metals remained on resin: Au 0.01 mg and Cu 0.02 mg.
                                      H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234                          233

were also assayed for the remaining metals and the                     is probably one of the disadvantages of Eluant A. The
overall elution efficiency for a particular metal was                  same problem is unlikely to occur with Eluant B and
calculated based on the total amount of the metal                      Eluant C because no column reconditioning is neces-
eluted and remained.                                                   sary. Even if incomplete elution happened, the
    Some of the results from the cycling experiments                   remaining gold would not be expected to precipitate
are presented in Tables 4 – 6. As can be seen, regard-                 and therefore could be eluted in the following cycles.
less of the resins, gold loading was 9– 10 kg/t, which
is closely consistent with the results obtained from the
single-run experiments. Gold elution efficiency for                     4. Conclusions
individual cycles was close to 100% based on the
solution analysis, which agrees well with the overall                      In the absence of copper, gold can be loaded onto
elution efficiency. Resin assay showed that almost no                  strongly basic ion exchange resin from thiosulfate
gold was left on the resin after the final elution,                    solutions rapidly and to high loading concentrations.
particularly when Eluant B and Eluant C were used.                     In the presence of copper, effective ion exchange
Similar results were obtained for the loading and                      operation can only be performed under limited con-
elution of copper. There are large errors in copper                    ditions due to the instability of the thiosulfate solution
loadings and elution efficiencies determined from                      and the possible formation of poisoning polythionates.
solution analysis, as pointed out in Section 2. How-                   For a typical solution of 0.1 M ATS with 500 ppm
ever, the loading values estimated from elution were                   copper, the optimum operation pH is about 11 and
around 9 kg/t. This is, again, consistent with previous                there is not much allowance for changes. Under such
results under the same conditions. The overall elution                 conditions, the adsorption of gold onto the resins is
efficiencies indicate that Cu elution was always com-                  much stronger than copper. With a large excess of
plete at the end of the experiments. For all the resins,               copper in the loading solution, the resin column is
almost identical results were obtained for each indi-                  usually first loaded with copper, some of which is
vidual cycle and no appreciable deterioration in load-                 then gradually replaced by gold.
ing and elution behavior was observed.                                     Effective elution of the loaded gold and copper can
    It is perhaps worth noting that, with Eluant A                     be achieved using a number of composite eluant
(Table 4), the amount of gold finally remaining on                     solutions. The eluant containing tetrathionate is dis-
the resin was slightly higher than with the other two                  advantageous due to its instability and the need of
eluants. This is likely to result from Cycle 1 during                  resin reconditioning before returning the resin to
which gold was incompletely eluted for some reason.                    loading. Sodium sulfite is the best eluant in terms of
The remaining gold might have precipitated when the                    efficiency, while sodium chloride is likely the most
column was treated with NaOH solution. The precip-                     economical. The addition of thiosulfate reduces the
itated gold was then locked in the resin and could not                 elution efficiency of the sulfite eluant. The mechanism
be removed in the following column operations. This                    of thiosulfate inhibition of elution requires further
                                                                       study.
                                                                           The loading/elution cycling experiments demon-
Table 6
Results of loading/elution cycling experiments from Amberlite IRA-
                                                                       strate that the resin can be used repeatedly with-
410 resin (2.00 g) using Eluant C                                      out deterioration in the resin loading or elution
Cycle      Metals loaded      Metals eluted     Recovery
                                                                       performances.
           Au/mg     Cu/mg    Au/mg    Cu/mg    Au (%)     Cu (%)
1          18.56      17.00   19.22     22.80   103.6      134.1       Acknowledgements
2          19.08      21.56   19.31     20.80   101.2       96.5
3          19.58      21.80   19.17     21.00    98.0       96.3
4          19.58      29.16   19.39     20.20    99.1       69.3          The authors wish to acknowledge the financial
5          20.04      26.10   19.65     20.80    98.1       79.7       support of Anglogold, Barrick Gold, Kinross Gold,
Overall    96.82     115.6    96.75    105.6     99.9a     100.0a      Newcrest Mining, Normandy, Placer Dome and Teck
   a
       Total metals remained on resin: Au 0.06 mg and Cu 0.02 mg.      Cominco for this work.
234                                   H. Zhang, D.B. Dreisinger / Hydrometallurgy 72 (2004) 225–234

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Description: The loading of gold and copper, both individually and simultaneously, from thiosulfate solutions onto ion exchange resin columns and the subsequent elution of these species have been investigated. In the presence of copper, effective loading with good selectivity for gold can be achieved at pH 11, which balances the stability of the solution and minimizes the formation of poisoning polythionates. The loaded metals can be eluted with high efficiencies using a number of composite eluant solutions. These include sodium tetrathionate stabilized with sodium sulfite and ammonium thiosulfate (ATS), sodium sulfite coupled with ammonia, and sodium chloride with the addition of ATS. The addition of thiosulfate to the eluant seems to have a negative effect on the gold elution efficiency and should be avoided wherever it is possible. Of all the three eluants investigated, the sodium sulfite/ammonia combination is found to be the most efficient, but the sodium chloride/ATS combination is likely the most economical.