remoción de cobre y niquel por electrorrefinamiento

Pergamon

0892--6875(97)00136-2

Minerals Engineering. Vol. 11. No. !, pp. 23-37, 1998 1997 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0892--6875/98 $19.00+0.00

THE REMOVAL OF NICKEL FROM COPPER ELECTROREFINING BLEED-OFF ELECTROLYTE

R.L. NYIRENDA and W.S. PHIRF

University of Zambia, School of Mines, Department of Metallurgy, P.O. Box 32379, Lusaka, Zambia. E-mail: [email protected] i" Zambia Consolidated Copper Mines Ltd., Ndola Copper Refinery,

P.O. Box 71065, Ndola, Zambia (Received 18 April 1997; accepted 1 October 1997)

ABSTRACT

When high nickel-containing copper concentrates from South Africa started to comprise part of the feed to a Zambian copper smelter at Mufulira, the level of Ni in the copper anodes produced increased. The high nickel, copper anodes started to pose a problem at the electrorefining stage as they led to a progressive increase in the Ni tenor of the electrolyte. In order to produce high quality copper cathodes with less than 1 ppm Ni, it became ~ecessary to bleed-off large volumes of foul electrolyte contaminated with nickel.

The study reported in this paper was part of the effort aimed at devising a less costly method figr the removal of nickel in the electrolyte. The investigation was carried out on a laboratory scale using contaminated electrolyte collected from the refinery. In thefirst of two nu,thods considered, it was established that aqueous ammonia is able to precipitate nickel from copper electrorefining bleed-off electrolyte, forming precipitates with 10-13% Ni. A rn~zjor drawback of this method, however, was found to be the cost of ammonia solution i'~sed for Ni precipitation relative to the value of acid retained in the electrolyte.

The other method considered, involved partial electrolyte evaporation with a view of crystallizing nickel sulphate from samples of bleed-off electrolyte issuing from a liberator circuit, h! has been demonstrated that evaporative crystallisation of nickel sulphate could be a very effective means of controlling nickel in the Mufulira tankhouse. At 66.5% and 80% electrolyte evaporation, 81% and 100% of the nickel was crystallized from foul electrolyte, respectively. Over 95% of the sulphuric acid in initial samples was retained in the purified electrolyte at a concentration of over I000 g/l. Initial estimates have indicated that the cost of evaporative crystallisation of nickel sulphate would be quite low compared to the value of sulphuric acid that would be present in the purified electrolyte. 1997 Elsevier Science Ltd. All rights reserve~

Keywon is

Hydrometallurgy; electrorefining

23

24 R.L. Nyirenda and W. S. Phiri

INTRODUCTION

The annual world production of copper is in excess of 8 million tonnes [1] and over 60% of this copper is used for electrical engineering applications and electronics where high electrical conductivity is the main requirement. For such high conductivity copper, it is necessary that the content of adverse impurity elements be as low as possible. Nickel is said [2] to be one of the worst elements in contributing to decreased copper electrical conductivity as it forms a solid solution with copper. Consequently, the maximum permissible level of nickel in copper for electrical uses is 7.0 ppm.

At Mufulira Copper Refinery (MCR) of the Zambia Consolidated Copper Mines Limited, very high grade copper cathodes are produced and the company has set 1.0 ppm as the maximum nickel content in its cathodes. In the past when MCR only electrorefined anodes arising from low nickel-containing Zambian concentrates (

Removal of nickel from copper electrorefining bleed-off electrolyte 25

high nickel in an(~les, the disposal of higher volumes of contaminated electrolyte is required. This resulted in increased electrolyte replacement cost and the cost of disposing the foul electrolyte.

The study reported in this paper was part of the effort aimed at devising, for MCR, a less costly method for the removal of nickel in the electrolyte so as to avoid its build-up to very high levels. The investigation was carded out on a laboratory scale using contaminated electrolyte collected from the refinery. Investigations performed at the smelter in an attempt to limit the level of nickel reporting into anodes produced will be reported at a later stage.

NI In Cathodes (ppm) V, (mS/t Cu)

,2 t . A ~ 40 . , o,,.l o . , . / 1 ' '

J ~ 0 . 0 8

. o ,

0 0 -8 -6 -4 -2 0 2 4 6 6 10 12

NI in Electrolyte (|/I)

II 0

$

6

0 i I | I I I I I I

-$ -6 -4 -I! 0 tl 4 6 8 10 1:!

NI In Anodes (ppm) 11100

gee

600

soo

0 I I I I I I I I I -8 -6 -4 -2 0 I 4 6 8 10 12

Months before end a l ter commencing uee of h lghNI linodeo

Fig. 1 Selected plant data for MCR before and after use of high Ni anodes commenced. Bottom diagram: Ni in anodes; Middle diagram: Ni tenor of electrolyte; Top diagram: Ni content of cathodes and volume of electrolyte bled-off per tonne Cu cathode produced


Survey of Possible Methods for Nickel Removal from Foul Electrolyte

Besides controlling nickel at low levels in the electrolyte, a suitable nickel removal method should retain a significant amount of acid in the purified electrolyte for recycle to the tankhouse.

As a first step in this study, methods commonly used in hydrometallurgy for separation of species and solution purification were theoretically evaluated. Solvent extraction, ion exchange, and precipitation by hydrogen or hydrogen sulphide gas reduction were ruled out as they would have required electrolyte dilution from over 180 g/l H2SO 4 to pH levels of the order 1 to 14. This would have meant severe acid "loss".

Use of activated carbon was judged unsuitable as it works well for the removal of anionic species and organic molecules but not cations.

Nickel cementation on another metal was also excluded on theoretical grounds. The method would have introduced another impurity element, the cementing agent, into the electrolyte. The method would also have required the removal of the more electropositive Cu 2 cations from the electrolyte to very low levels before the commencement of nickel cementation.

Two methods - - chemical precipitation and evaporative crystallisation - - were thought to have good potential in removing nickel from the electrolyte and were consequently tested in the laboratory. Part I of this paper deals with the chemical precipitation of nickel from the foul electrolyte using aqueous ammonia as the precipitant. The work on evaporative crystallisation is covered in Part II of the paper.

PART I: NICKEL REMOVAL BY THE ADDITION OF AQUEOUS AMMONIA

THEORY

The precipitation of nickel as a crystalline nickel (II) ammonium sulphate, (NH4)2SO4.NiSO4.6H20 double salt, is used in the Sherritt-Gordon process [5] to produce Ni-free cobalt. Nickel is removed from solution before cobalt pentammine reduction by hydrogen gas to yield cobalt metal. Nickel is present in the cobalt pentammine solution as a diammine formed according to the reaction:

Ni2+(aq) + 2NH3(aq ) = Ni(NH3)22+(aq) (1)

The equilibrium constant, K 1, for the above reaction is:

K 1 "- aui~Mn')I' (2) 2

aNi2. * anti,

Nickel is precipitated from the ammoniacal solution by adding sulphuric acid which causes ammonium nickel sulphate to form. This greenish coloured double salt is precipitated according to the reaction:

Ni(NH3)22+(aq) + 2H(aq) + 2SO42-(aq) + 6H20 = (NH4)2SO4.NiSO4.6H20(s) (3)

From the above reaction, the solubility product, Ksp, of (NH4)2SO4.NiSO4.6H20 can be written as:

K p'- at~i~NH,) ?, * a2 u, * a2so :. (4)

Replacing in the above equation the activity of Ni(NH3)22+ with the value from Eqn. 2 results in Eqn. 5,

Removal of nickel from copper eleetrorefining bleed-off electrolyte 27

K-" K 1 * ajv,. * a2 * a2 u. * a2 (5) NH 3 SO,, z"

whose value is a constant at a given temperature and pressure.

For a given temperature and pressure, precipitation of ammonium nickel sulphate occurs when, in the solution, the ionic product as given on the right hand side of Eqn. 5 exceeds the Ksp value at that temperature and pressure in question.

Since, under steaty state conditions, the composition of the cobalt pentammine solution to be purified is more or less constant, the activities of Ni 2 and NH 3 may be assumed to be fairly constant. Under steady state operational conditions the temperature and pressure will also be fairly constant, thus only the activities of H and SO42- (:i.e. amount of H2SO 4 added) can be used to influence the extent of nickel precipitation from the ammoniacal solution.

The theory presented above would also be applicable to the precipitation of nickel from copper refinery electrolyte by the addition of ammonia solution. It should be noted, however, that in this case ammonia solution would have to be added to the foul electrolyte to cause nickel precipitation. From the discussion presented above for the Sherritt-Gordon process, it can be deduced that the parameters which would have an influence on Ni precipitation from refinery electrolyte would be the amount of ammonia solution added, the temperature, and pressure. In the experiments conducted, precipitation was carried out at atmospheric pressure and the effect of the other two parameters was studied.

Depending on the level of acid retained in the purified electrolyte, the cost of ammonia solution utilized, and the extent of nickel precipitation, the method of Ni removal from foul electrolyte by precipitation with aqueous ammonia was evaluated.

EXPERIMENTAL

A bulk sample of foul electrolyte about to enter the liberator circuit, analyzing 47.0 g/1 Cu, 11.0 g/l Ni, and 190 g/1 H2SO 4, was collected and used in the experiments conducted. Due to the violent reaction caused by excessive heat generation when ammonia solution contacted the electrolyte or vice-versa, Ni precipitation with ammonia solution could not be done on foul electrolyte issuing from the liberator circuit which had 330 g/1 HISO 4.

20% ammonia solution was used as the precipitant and precipitation was carried out on 500 cm s electrolyte samples in a 1 litre glass vessel that was placed in a thermostatted water bath. The lid of the precipitation vessel had an allowance for a stirrer to be inserted to provide gentle agitation. The lid had two other ports, one for a thermometer and the other for sample and precipitant addition. The effect of precipitant amount, precipitation time, and temperature was investigated.

At the required time, precipitates formed from solution were quickly recovered on a ceramic filter. The precipitates were then dried at 98C, weighed, and analyzed for nickel via a re-dissolution method. The volume of each filtrate collected at room temperature was noted before taking a sample for analysis of H2SO 4 acid and nickel content. Nickel analysis was performed by atomic absorption spectrophotometry while acid tenor was determined by acid-base titration. Filtrates collected at a higher temperature, it later became apparent, could not be analyzed and such filtrates were immediately used for producing secondary precipitates.


RESULTS AND DISCUSSION

Nickel Precipitation at 500C and at Room Temperature

Initial interest for the removal of nickel by the addition of ammonia stemmed from the thought that, with this method, nickel could be precipitated at a high temperature. If this could be accomplished, then under plant conditions the purified electrolyte would be recycled with minimal expenditure on heating it up to the tankhouse electrolyte temperature of 60-65C.

Precipitation experiments carried out at 50C for up to 36 hours with a constant ammonia solution addition of 222 cm 3 revealed, however, that only a low level of nickel (31.6%) was precipitated at this temperature, and providing that the precipitation temperature of 50C was maintained. Figure 2 shows the results which were obtained in this set of experiments. It was observed during these experiments that as the filtrate cooled after filtering of nickel precipitates, more "secondary" precipitation of nickel occurred. This observation implied that the double salt (NH4)2SO4.NiSO4.6H20 is more soluble in aqueous media at higher temperature. A check of published data [6] indicated that this was so, as the solubility of (NH4)2SO4.NiSO4.6H20 in 100 cm 3 of water is given as 10.4 g at 20C and 30 g at 80C.

Fig.2

NI Preoipltated (%) 1001-

] Temperature [ x

.ol y

0 0 4 8 12 16 20 24

Tlme (Hour)

r:

o

I ! I

28 32 36

Ni precipitation from MCR bleed-off electrolyte at 50C and at room temperature. (Electrolyte volume=500 cm3; volume 20% NH 3 solution=222 cm 3)

When precipitation experiments with 222 cm 3 ammonia solution were performed at room temperature, much higher nickel precipitation (86.6%) was achieved, as would be expected if ammonium nickel sulphate is less soluble at lower temperature. The results of the room temperature experiments are also shown in Figure 2.

What also became apparent from the results presented in Figure 2 was that Ni precipitation using ammonia occurred slowly. Consequently, further precipitation experiments were performed for 24 hours.

Ni Precipitation at Different Temperatures and Total Precipitation when Cooled to Room Temperature

The lower Ni precipitation at a higher temperature was confirmed in other experiments, in which precipitates were recovered in two stages. To the foul electrolyte at the required precipitation temperature, 222 cm 3 of


ammonia solution was added and the temperature maintained for 24 hours. Filtration quickly followed after this to recover a "primary "precipitate. The filtrate was allowed to cod in air to room temperature and a second, the "secondary" precipitate which formed, was then collected. Figure 3 shows the results of this set of experiments.

In addition to showing a decrease of nickel primary precipitation at a higher temperature, Figure 3 shows that in the range studied, and at a constant ammonia solution addition of 222 cm 3, the total Ni precipitated was more or less constant and independent of the temperature for primary precipitation. Figure 3 allows a conclusion to be raade that if nickel was to be precipitated from foul electrolyte by the addition of ammonia solution, the final temperature to which the solution is cooled would have a very strong influence on the level of nickel removed.

Fig.3

NI Precipitated ('It)

80 ~ 0 o ._ . . -~

60

0 Total 20

-a-- Primary

O I i i i i i i i

100

25 $0 38 40 48 60 65 eO e5 70 Temperature ('(3)

Ni primary precipitation at different temperatures and the corresponding total Ni precipitated. (Electrolyte volume=500 cm3; volume 20% NH 3 solution=222 cm 3)

Primary and Secondary Precipitation at Different Additions of Ammonia Solution

By first maintaining a precipitation temperature of 50C and then cooling the filtrate to room temperature in a manner similar to the previous experiments, the degree of primary and secondary precipitation of Ni was determined at different levels of ammonia solution addition. Figure 4 summarizes the results which were obtained.

The difference in Ni removal after primary and secondary precipitation was found to reduce as the amount of ammonia solution added increased. Thus it appeared at this stage that, at the plant level, if the hot foul electrolyte proceexling to the liberator could be maintained at a high temperature in a lagged vessel, significant Ni removal could be accomplished with a suitably high amount of ammonia solution without the need of coding file electrolyte.

Acid Retained in the Purified Electrolyte and Its Value Relative to the Cost of Ammonia Used for Ni Precipitation

When the acid remaining in the electrolyte after different ammonia additions was determined (Figure 5), it became clear that very little acid was left in the electrolyte with high ammonia usage. With 310 cm 3 precipitant added, only 3.4% of the initial acid was retained in the purified electrolyte.


The unsuitability of high ammonia usage could be seen more prominently when the cost of ammonia solution used for Ni precipitation was computed to the value of acid that remained in the purified electrolyte. This comparison is presented as a ratio of "ammonia cost" relative to the "acid value" and this information is also shown in Figure 5. For 148 cm 3 ammonia solution added, the "ammonia cost"/"acid value" ratio was 6, but this ratio shot up to 317 with 310 cm 3 precipitant used. In the calculations made, the cost of 20% ammonia solution per tonne ammonia contained was taken as 3 times the cost of 98.6% sulphuric acid per tonne H2SO 4. This cost ratio remains almost the same on the local market although the actual prices of ammonia solution and acid are hardly stable due to frequent currency devaluation.

Fig.4

I00

80

60

40

20

Ni Precipitated (~.)

-ig-- Primary

0 I I I

120 170 220 270 320

Volume Ammonia Solution Added (cm s )

Ni primary precipitation at 50C using different precipitant amounts and the corresponding total Ni precipitated. (Electrolyte volume=500 cm 3)

Fig.5

tO0

80

60

40

20

NI Precipitated (%) Acid Retained (?.) Ammonia Cost /Ac id Value (Ratio)

0 0 120

. . . . . . . . ~ . . . . . . . . . . . . . e . . . . . . . . -E l ' / /

""* X i --13--

170 220 270 320

Volume Ammonia So lut ion Added (cmS)

320

240

160

80

Ni precipitated and acid retained (at room T) with different precipitant amounts used. Plotted alongside the ratio "ammonia cost"/"Acid value". (Electrolyte volume=500 cm 3)


The massive increase in the cost of ammonia solution relative to the value of acid retained at high ammonia solution usage is an indication of the possible reaction of excess ammonia with free acid according to the reaction;

NH3(aq ) + H+(aq) --~ NH4+(aq) (6)

Such acid consuraption would be in addition to the acid consumed by the Ni precipitation reaction as represented by Eqn. [3].

Thus, although able to remove Ni from foul electrolyte, ammonia solution was found unsuitable on account of the low residual acid in the remaining electrolyte. Furthermore, the method would be costly if it had to achieve substantial Ni removal. The concentration of residual acid in the purified electrolyte after Ni precipitation couhJ perhaps be improved upon by the use of ammonia gas instead of ammonia solution, but this was not investigated. It would appear, however, that even with ammonia gas, a low amount of acid would remain at high levels of Ni precipitation. This is because if the Ni became rather low in the electrolyte any ingoing ammonia gas would more readily encounter and react with free acid than with the relatively sparse Ni cations.

In all the experiments conducted, copper was not precipitated as evidenced by its presence in only trace amounts in the precipitates. The copper must have remained in solution as Cu 2+ cations. Even though copper was not precipitated, which is desirable, another important consideration regarding Ni precipitation with ammonia solution or gas would be the effect of ammonium cations resulting from Eqn. 6 on refinery operations, if the purified electrolyte was to be recycled. This would have to be investigated.

Nature of Precipitates formed in Experiments Conducted

Although examination by X-ray diffraction was not undertaken, all precipitates analyzed, whether primary or secondary, had a nickel content in the range 10.2 - 13.2% Ni which compares favourably with 14.86% for pure (NH4)2SO4.NiSO4.6H20. Within the given Ni content range, it was observed that secondary precipitates always had less Ni than primary precipitates. Another observation was that the higher the amount of ammonia solution used, the lower the Ni content of the precipitates formed. Neither observations were, however, pursued further.

PART II: NICKEL REMOVAL BY EVAPORATIVE CRYSTALLISATION

THEORY

Evaporative crystallisation, as used for Ni control in copper tankhouses, involves the evaporation of some of the water from the bled-off foul electrolyte so that the solubility limit of nickel sulphate in the remaining acid-enriched liquor is exceeded.

The crystallisation of nickel sulphate occurs according to the reaction:

Ni:+(aq) + SO42-(aq) = 6H20 ~ NiSO4.6H20(s ) (7)

Figure 6 shows tile effect of temperature on the solubility in water of nickel sulphate and a few other selected metal sulphates, based on data from Perry []7. The general trend exhibited in the diagram is that the solubility of metal sulphates decreases with decreasing temperature. Therefore by cooling the hot electrolyte, the excess nickel sulphate soluble at high temperature would erystallise, thus increasing the amount of Ni removed from solution. Thus in the experiments carried out, evaporation at high temperature was followed by cooling, before recovering the solid nickel sulphate formed.

The effectiveness of evaporative crystallisation for Ni control has to be gauged not only on the amount of nickel removed from the foul electrolyte and acid retained in the purified electrolyte, but also on the cost of evaporation. HIIE II-I-B


8air dlnolved. 9

'1 I eo

40

2(] ~ ~ ~-I~ICUS04

i I I i i I i I i

0 10 20 80 40 60 60 70 80 80 100

Temperature , O

Fig.6 Effect of temperature on the solubility of selected metal sulphates per 100 g water [7]

EXPERIMENTAL

A bulk sample of electrolyte issuing from the liberator was collected and used in the experiments. The electrolyte analyzed 2.0 g/l Cu, 0.59 g/l Ca, 11.0 g/l Ni, 2.3 g/l Fe and 330 g/1 H2SO 4.

In the experiments conducted, 400 cm 3 samples of the liberator electrolyte were placed in beakers. These samples were heated on a hot plate set at 400C for different durations, so as to attain different degrees of electrolyte evaporation. The hot plate temperature used achieved rapid evaporation without sputtering the beaker contents.

After heating, the samples were removed from the hot plate and cooled overnight to room temperature. The crystals formed were removed on a ceramic filter and the filtrate collected, The volume of filtrate collected at room temperature was noted before taking a sample for analysis of H2SO 4 acid, nickel, copper, calcium, and iron. The acid content was determined by acid-base titration while all the other analyses were done by atomic absorption spectrophotometry. The crystals formed were not analyzed, as all the information required could be calculated from filtrate analyses.

RESULTS AND DISCUSSION

Nickel Precipitated and Acid Retained at Different Degrees of Electrolyte Evaporation

Figure 7 shows that virtually all the nickel was removed from solution at high levels of electrolyte evaporation. At the same time, as can be seen in Figure 8, very little acid was lost by evaporation or entrapment in precipitates. It would be possible to recover the small amount of acid evaporated in a condenser if such a unit could be coupled to the evaporator, but in the experiments conducted, this was not done. The condenser would also allow the distilled water, which would be free of metal salts, to be recovered for recycle to the tankhouse. The low acid loss resulted in very high acid concentrations of purified electrolytes which exceeded 1000 g/l H2SO 4 when electrolyte evaporation was over 66.5%.


Fig.7

% Crystalll=d

l . . . . . . . . . . . . . . . . . . . . . . - . . . . . . . . .

8 0 ~ N ~ ~ N~- 6O

40!

o Iron

--a- Calcium & Copper

I t 0 60 60 70 80

Volume % Eleotrolyte Evaporated Percentage of Ni, Fe, Ca, and Cu removed from the foul electrolyte at different degrees of electrolyl:e evaporation

Fig.8

% Acld Retained Acld Concentration (g/l) 1 t 8o. 40 ' 1900

20f ~ Acid Conoentratlon ]600

0 c , T "800 50 60 70 80

Volume % Electrolyte Evaporated

Percentage H2SO 4 acid retained and its concentration in the purified electrolyte at different degrees of foul e;(ectrolyte evaporation

Calcium, copper,, and Iron Precipitated at Different Degrees of Electrolyte Evaporation

In view of the excellent results recorded above, it was decided to analyze purified electrolytes for iron, calcium, and copper in order to evaluate the behaviour of these metals in the evaporative method. Figure 7


includes the results for the removal of these metals from solution. In all the tests done, and these were at 50% electrolyte evaporation and above, all the calcium and copper and most of the iron must have formed sulphates which were filtered from the purified electrolyte. Although the sulphates of these metals would contaminate the nickel sulphate formed, this disadvantage is regarded as minor compared to the greater goal of purifying the electrolyte. The same can be said regarding the loss of the initial 2 g/l copper which would not be available for recycle to the tankhouse.

The results presented above demonstrated how effective evaporative crystallisation could be for controlling the level of Ni at MCR, but the cost of energy required for the process had to be considered. A related aspect which also had to be considered was the volume of liberator electrolyte which would have to be bled-off for evaporation in order to maintain suitable levels of Ni in the electrorefining tankhouse.

Electrolyte to be Bled-off for Evaporation to Control Nickel at Specified Levels

Figure 9 shows the volume of electrolyte that would need to be withdrawn from the liberator and fed to an evaporator for Ni removal so as maintain the level of Ni in the tankhouse at a specified level. The volume bled-off, V, is expressed in cubic metres per tonne of cathode copper produced. As the amount of electrolyte to be withdrawn would also depend on the extent of vaporization, Figure 9 shows the bleed-off volume if the evaporator operated at an electrolyte evaporation, E, of 50%, 60%, 66.5%, and 74%.

4

Energy Cost/Aoid Value, R, (ratio)

-i

O

8 E

>-

~ 2 0 I

"0 Q Q

0 0

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . _ _ _ _R_ _ _a_t_ _E_ _-_ _ _r_4_ . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

f R at E 60%

, 1 R at E - 60%

~ i.-a.- V at E 80%

,,~'~ o VatE -60%

+ v at E - 88.5

I

2 4 6 8 10 Desired Ni Tenor in Tankhouse (ppm)

0.066

0.048

0.04

0.032

0.024

0.016

0.008

0 12

Fig.9 Electrolyte bleed-off volume, V, to maintain specified Ni levels in the tankhouse if the evaporator achieves the indicated % evaporation, E. Plotted alongside the ratio, R="energy cost"/"acid value". (Ni in anodes--0.068%)

Figure 9 assumes steady state operation of one DC unit of the electrorefining tanldaouse, comprising 12 sections of 40 cells each, in which 164.7 tonnes of anodes averaging 0.068% Ni dissolve per day. Appendix 1 sets out the method which was used to generate the data for plotting Figure 9. A computer program in GwBASIC was written to facilitate calculation of bleed-off volumes at different levels of Ni in anodes and extent of vaporization in the evaporator. Hence the data plotted in Figure 9 is merely an example of the results which could be obtained from the computer program and the diagram is drawn to illustrate the overall conclusions which were arrived at.


It can be seen in Figure 9 that there is an exponential increase in the volume of electrolyte to be bled-off if it is desired to :maintain a very low level of Ni in the tankhouse electrolyte. At the same time, for a particular percentage of electrolyte evaporation, a low bleed-off would not sufficiently decrease Ni in the tankhouse. It is also apparent that the electrolyte to be bled-off to maintain a particular Ni level decreases at higher levels of electrolyte evaporation, although this decrease is minimal above about 66.5% evaporation.

Energy for Evaporation to Control Nickel at Specified Levels

As discussed abow~, to maintain a specified level of Ni in the tankhouse, a volume of electrolyte would have to be bled-off for partial evaporation. Besides the desired Ni level in the tankhouse, the volume of electrolyte bled-off would also depend on the degree of vaporization to be achieved in the evaporator. Thermal energy would have to be expended on the bled-off electrolyte for vaporization to occur, thus causing Ni to precipitate. After removal of the Ni precipitated, the remaining electrolyte would be available for recycle to the ltankhouse as it would be enriched in H2SO 4 acid.

The ratio, R, of "energy cost" for electrolyte evaporation to the "acid value" recovered is included in Figure 9. In the diagram, each curve for the electrolyte volume bled-off and vaporized has a corresponding line showing the value of the ratio, R.

The energy cost u:;ed in computing R is a very approximate one, as heat losses from the evaporator were arbitrarily taken a,,; 50% of the heat required for vaporization. Furthermore, the specific heat capacity of water, 4.186 kJ/kgeK [8], was used for estimating the heat required to raise the temperature of the foul electrolyte from an assumed initial temperature of 50C to the evaporation temperature of 100C. At 100C, only water was assumed to be vaporized after the application of thermal energy equivalent to its latent heat of vaporization of 2260.1 kJ/kg [8]. The evaporator was assumed to be electrically heated and the local cost of a kWh of electrical energy was taken as 0.022 times the local cost of a kg of 98.6% H2SO 4 acid.

Even though caution has to be exercised in view of the simplistic assumptions made in calculating the heat needed for electrolyte evaporation, what is most striking about evaporative crystallisation for Ni control is its low cost relative to the value of acid that would be made available for recycle. From Figure 9, it can be seen, for example, that the computed ratio of "energy cost"/"acid value" was 0,034 and 0.052 at 50% and 74% evaporation, respectively.

Figure 9 also shows that the ratio, "energy cost"/"acid value" would be independent of the volume of electrolyte bled-off but would depend on the percentage of the bled-off electrolyte that would have to be evaporated. The higher the percentage of electrolyte evaporated, the higher would be the cost of evaporation relative to the vailue of acid made available. In spite of the cost, the advantage of high electrolyte evaporation would be that less electrolyte would have to be bled-off to maintain a specified Ni level in the tankhouse. Bleeding off low amounts of electrolyte would augur well for more stable operations of the tankhouse.

CONCLUSIONS

Bench scale tests have shown that aqueous ammonia is able to precipitate nickel from copper electrorefining bleed-off electrolyte, forming precipitates with 10-13% Ni. The precipitation reaction is however slow and needed some 24 hours to reach an appreciable extent. The precipitates formed were soluble in the hot mother liquor so that the final temperature to which the solution was cooled had a very strong influence on the amount of nickel precipitated. Using 20% aqueous ammonia, it is possible to precipitate about 80% of the nickel in elecl~orefining bleed-off electrolyte containing 11 g/l Ni and 190 g/1 H2SO 4 acid whilst retaining about 60% of the initial acid in the purified electrolyte. The cost of ammonia solution used would, however, be at least six times the value of acid that would be retained in the purified electrolyte. Thus, even if other concerns about the method could be addressed, the cost would make the method of nickel precipitation using aqueous ammonia unsuitable.

In other laboratory experiments conducted it has been demonstrated that evaporative crystallisation of nickel


sulphate from copper electrorefining bleed-off electrolyte would be a very effective method for nickel control in the MCR tankhouse. In addition, the method would also achieve the removal of calcium and a significant amount of iron from the bled-off foul electrolyte. The purified electrolyte would have a sulphuric acid concentration in excess of 1000 g/1 if 66.5-80% of the bled-off foul electrolyte had to be evaporated. At the same time, over 95% of the acid initially present in the foul electrolyte would be retained in the purified electrolyte, which would be available for recycle to the tankhouse. Initial estimates have indicated that the cost of evaporative crystallisation of nickel sulphate would be quite low compared to the value of sulphuric acid that would be present in the purified electrolyte.

.

2.

3. 4.

5. 6.

7.

8.

REFERENCES

Anon., Mining Annual Review. Mining Journal Ltd., London, (1996). Biswas, A.k., & Davenport, W.G., Extractive Metallurgy of Copper, 2nd Edn., Pergamon Press, Oxford, 357 (1980). Blair, I.S. & Verney, I,R., Copper Metallurgy, R.P. Ehrlich (EA.), A.I.M.E., New York, 275 (1970). Tsedler, a.A., Metallurgy of Copper and Nickel, 2nd FAn., Israel Program for Scientific Translations, Jerusalem, 155 (1970). Kunda. W., Worner, J.P., & Mackiw, V.N., Trans. Can. Instn. Min. Metall., 65, 21 (1962). Weast, R.C. (Ed.), Handbook of Chemistry and Physics, 24th Edn. CRC Press, Boca Raton, Florida, (1983). Perry, R.H., & Green, D., Perry's Chemical Engineers' Handbook, 6th Edn., McGraw-Hill Book Company, New York, (1984). Kubaschewski, O. & Alcock, C.B., Metallurgical Thermochemistry, 5th Edn., Pergamon Press, Oxford, (1979).

APPENDIX 1

Calculation of Electrolyte Volume to be Bled-off for Evaporation to Control Nickel at Specified Levels

If M kg is the mass of anodes, containing an average of x% Ni, which corrode into the electrolyte per day, Mass Ni entering tankhouse via anodes = O.01Mx kg/day.

If Ni in the tankhouse electrolyte is to be kept at c ppm, then the bled-off electrolyte, V m3/day, also has a Ni content of e ppm.

Therefore, mass Ni in bleed-off = 10 -3 *Vc kg/day.

Assuming the evaporator to which the bled-off electrolyte is fed is set to evaporate E% of the electrolyte, and if the fraction of Ni precipitated from the bleed-off = x (Read off Figure 7), then fraction of Ni in bled-off volume which remains in the purified electrolyte recycled = 1 - x.

Therefore, mass Ni remaining in purified electrolyte that is recycled = 10 -3 *Vc* (1 - o~) kg

Assuming steady state conditions, Ni mass balance in the tankhouse is:

Ni entering via anodes + Ni contained in recycled purified electrolyte = Ni leaving in bleed-off electrolyte i.e. 0.01Mx + 10 -3 *Ve*(1 - x) = 10 -3 *Vc.

Rearranging the above equation and assuming anodes have 99.7% Cu and that all the anode Cu which dissolves plates out at the cathode, the volume of electrolyte bled-off is:


V : 10x m3/t Cu cathode (6) 0.997ca

For bleed-off volumes plotted in Figure 9, x was taken as 0.068 while a which was experimentally determined was read off Figure 7. The concentration of Ni to be maintained in the tankhouse, c, was varied from 0.5 to 11.

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