Copper recovery by the cementation method

hydrometallurgy ELSEVIER

Hydrometallurgy 47 (1997) 69-90

Copper recovery by the cementation method Tadeusz Stefanowicz *, Malgorzata Osifiska, Stefania Napieralska-Zagozda institute qf Chemistry and Technical Electrochemistry, Poznari Unkersi~ 60-965 Poznari, Poland

qf Technology, ul. Piotrowo .i.

Received 12 April 1996; accepted 2 April 1997

Abstract Cyanide-free, sulphate-based copper electroplating waste solutions and spent nitric acid solutions from the etching of copper were investigated for copper recovery by cementation. It was found that continuous rotation of pieces of iron together with waste copper sulphate solution or intense circulation of the solution alone through motionless iron pieces (e.g. scrap iron) with occasional shaking results in copper recovery of more than 99% within 15-30 min. In the case of copper-laden nitric acid solution before cementation it must be at least 2 fold diluted and sodium pyrosulfite should be added to overcome the oxidizing medium of nitric acid. When the pH of the solution is below 1, it is recommended to increase the pH to 1.5 (by adding NaOH prior to cementation). The postcementation solution treatment comprises pH correction to pH 9 followed by the removal of metal hydroxides by sedimentation or filtration. 0 1997 Elsevier Science 1S.V.

1. Introduction Waste solutions produced during copper electroplating in sulphate electrolytes or spent nitric acid-based copper etching solutions often contain significant concentrations of copper ions which exceed, many-fold, the levels permissible for dumping into rivers or into sewage systems. In Poland, for example, the permissible level for copper ions in waste solutions rejected into rivers is 0.05 mg/l [l] and, into sewage, it is limited to 0.2

mg/l [21. In the waste treatment processes for copper removal from cyanide-free electroplating waste solutions, NaOH solution is often added to adjust the pH above 7 and to coagulate the copper hydroxide followed by its removal in filter presses. Other technologies are

’ Corresponding author. Fax: +48-61.782571;

E-mail: [email protected]

0304-386X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI/ SO304-386X(97)00036-4

70

T. Stefanowicz et al. / Hydrometullurgy 47 119971 69-90

based on ion exchange methods using solid resins or on liquid/liquid extraction methods with oxime type extractants [3]. However, the ion exchange method using solid resins results in production of additional waste solutions laden with Na,SO, (formed during regeneration of ion exchange resin), while liquid/liquid extraction is followed by production of waste solutions contaminated additionally with organic solvents. The last method is normally utilized only in large-scale copper processing plants, e.g. in mining industry or in metallurgical plants [4], as it requires several-stage extraction and complex technology with unique equipment and is in no way suitable in moderate-scale electroplating waste treatment plants. The mixed sludge of copper and other metals hydroxides, produced by adding NaOH solution to cyanide-free waste solutions, has little or no effect, due to contaminants and high water content. This waste sludge usually contains large amounts of iron hydroxide from etching of steel material during the metal surface preparation stage and with nickel hydroxide from the first nickel underlayer electrodeposition stage. The waste cake even after dewatering on a filter press is not acceptable for metallurgy furnaces due to still high moisture (approximate water content is 70-80%). Moreover, the filter-cake is contaminated with sulphates. This waste product is sometimes utilized in a limited amount e.g. as colorizing admixture in building ceramics manufacture, or is dumped on hazardous waste landfills. It seems that the most reasonable and simple method for copper recovery is the method of its cementation using scrap iron to produce metallic copper sediments suitable for metallurgical processing. Due to different standard reduction potentials [5] Cu’+ + 2e = Cu Fe2++2e=Fe

(E” = +0.34 (I?=

V)

(1)

-0.44V)

(2)

copper ion becomes readily reduced on metal iron surface, while the related amount of iron is dissolved. In standard conditions the reaction CuSO, + Fe + FeSO, + Cu

(3)

is driven by standard emf= 0.78 V. Because industrial solutions besides copper also contain scrap iron is introduced, due to reduction potential Fe3+ + e = Fe*+ the reaction

( EO= +0.771

Fe’+

and Fe3+ ions, when

V).

(4)

[6]

Fe + Fe,(SO,),

-+ 3FeS0,

(5)

with standard emf = 1.211 V can be expected. The deposited copper due to Fe-” ions presence may undergo dissolving the reverse reaction [6] Cu + Fe,( SO,),

+ 2FeS0,

with standard emf = 0.431 V.

+ CuSO,

according

to

(6)

i? Srefanowicz et al./Hydrometallurgy

Because the waste copper solutions Fe + H,SO,

--) FeSO, + H,

47 CIYY7) 69-90

usually contain

H,SO,,

71

scrap iron etching (7)

should be also considered. Hence the mass relation between the amount of reduced copper and dissolved iron is hardly predictable and is naturally charged with deviations in both directions, not withstanding that the theoretical mass exchange coefficient Fe/Cu [g/g] related to reaction (3) is 0.879. The cementation process allows recovery of copper in a form of metal particles acceptable (when dry) in copper metallurgy. On the other hand, iron replaces copper in the final waste sludge and, consequently, it becomes less hazardous when disposed of on landfills. Such iron hydroxide-laden waste can also be utilized. after drying, as a coiorizing additive in building materials production. The cementation method, known for several centuries, is used in copper hydrometallurgy in some countries [7], but it is rather rarely used for copper electroplating waste solutions treatment and for copper recovery from them. Therefore, it is worth pointing out that the cementation method was utilized successfully in larger than bench scale in the Radio-Set plant ‘BANGA’ in Kaunas (Lithuania) for copper recovery from electroplating waste solutions. The copper recovery problem also arises when copper is etched in nitric acid solutions. This paper draws attention to the cementation method which can be an attractive way to recover copper and simultaneously to stimulate environmental protection. The cementation method was investigated for copper removal and recovery from sulphate-based electroplating waste solutions with a copper content up to 60 g/l, produced in the GALMAR plant in Poznan (Poland). To investigate the copper cementation from nitric acid medium the spent nitric acid solution formed during etching of copper in gold electrodeposit recovery (according to contract with industry) was used. The coppet cementation trials performed on the laboratory as well as bench scale disclose the parametric dependencies which should be considered when introducing the method on an industrial scale.

2. Experimental

details

Copper, iron and nickel concentrations were generally determined by atomic absorp tion spectrometry (AAS lN, Carl Zeiss Jena, Germany), while concentration of iron in etching trials was determined spectrophotometrically by the o-phenanthroline method (at pH 3.4) [S] with use of spectrophotometer VSU 2P, Carl Zeiss Jena, at wavelength of 513 nm. The pH value of the solutions was measured by pH-meter type NS 17 Mcra-Elwro (Poland) with combined glass-calomel electrode type ERH-I I Hydromet (Poland). For copper cementation on an industrial scale, the use of scrap iron was intended, but for laboratory trials, iron plates of defined surface area were used to enable the

T. Stefanowicz et al./Hydrometallurgy

12

47 (1997169-90

quantitative evaluation of its influence (iron chips and rods were also used in some trials). For technological reasons three alternatives were considered: (a) the introduction of copper solution into a container with an iron plate (or plates) for a particular time without moving the solution or the iron plate (variant ‘without stirring’), (b) intense rotational stirring of the copper solution together with the iron plate (or plates) to realize an continuous refreshing of the iron surface by friction of the solid providing good access of the solution to the greatest possible area of iron surface (variant ‘with stirring’), and (c) intense circulation of the copper solution around a motionless stationary fixed iron plate (variant ‘with circulation’). Iron plates of defined dimensions were prepared by cutting them with punching dies using sheet metal of thickness of 0.07 cm. Two sets of iron plates were prepared having the dimensions: (1) 5.41 X 2.04 cm with total surface of 23.1 cm* and (2) 4.97 X 6.07 cm with total surface of 61.9 cm’. In some trials, for cementation, iron rods of diameter 4.7 mm were used. Their length was 30 mm or 250 mm depending on reaction vessel dimensions (beaker or drum-scale trials). Consequently the surface of the single rod surface was approximately 4.8 cm2 or 37.2 cm* respectively. The dimensions of the beakers were: h = 65 mm, diameter 36 mm (for 50 ml of the solution), (2) h = 85 mm, diameter 44 mm (for 100 ml of the solution). Prior to etching or cementation, plates and rods were subjected to anode cleaning in a degreasing electrolyte with subsequent rinsing in running tap water. 2.1. Copper recovery from sulphate-based

electroplating

waste solutions

The main parameters of four waste copper solutions from the GALMAR plant which were used during trials are presented in Table 1. As the waste solutions are acidic, it is obvious that iron plates when immersed for cementation are subjected to dissolution not only due to cementation but also due to the etching of iron in acidic solutions. Hence, it was considered reasonable to investigate whether it is more advantageous to proceed with the cementation in strongly acidic solution or better neutralize part of the acid prior to cementation limiting the etching process share.

Table 1 The main parameters cementation trials Designation the solution I II III IV

of

of copper

sulphate

waste

solutions

Concentration

Concentration

of Cu [g/l1

of Fe

14.0 32.5 51.5 43.0

0.375 1.275 3.075 2.300

WI

from

GALMAR

plant

which

were

Concentration of Ni [g/l]

PH

0.022 0.074 0.134 0.086

1.30 0.87 0.29 1.45

used

in

T. Stefanowicz et al. / Hydrometallurgy 47 (I 9971 69-90

73

Table 2 The etching trials of single iron plates having a surface of 23.1 cm* immersed in 50 ml of diluted sulfurrc acid solutions for 90 min without stirring Trial No.

H2SO,

I

36.6 1 1.o 4.0 2.6

2. 3. 1.

[g/l]

pH value

The mass of etched iron [mg]

before etching

after etching

Fe(II)

Fe(III)

Fe(tota1)

0.29 0.87 1.30 1.45

0.30 0.89 1.30 1.46

6.33 3.06 1.62 1.15

0.59 0.3 I 1.02 0.50

6.92 3.31 2.64

I .65

Therefore, comparative trials were performed with iron plates immersed in sulfuric acid solutions and in copper waste solutions with the same pH, followed by the determination of the iron concentration obtained after particular time period in both solution samples. For etching trials the sulfuric acid solutions were prepared with pH values identical to those of waste solution, i.e. with pH 0.29, 0.87, 1.30 and 1.45. Four iron plates with a surface of 23.1 cm* were immersed vertically into 50 ml volumes of each solution (in tail beakers) and left without motion during 90 min. Then the iron plates were removed and the dissolved iron was determined by spectrophotometric method with ophenanthroline, with absorbance measurement at a wavelength of 512 nm according to the calibration curve and considering the dilution factor [8]. The etching results of the plates are shown in Table 2. The iron plate of 23.1 cm’ surface etched in the sulfuric acid solution of concentration 36.6 g/l (pH 0.29) for 90 min resulted in dissolution of only 6.92 mg of metal. The other higher pH Hz SO, solutions dissolved lesser amounts of iron. The etching of plates (23.1 cm2 surface) in the sulfuric acid solution of concentration 4.0 g/l (pH 1.3) was performed as a function of time without stirring (Fig. 1). The iron etching speed decreased with time which can be assigned to the lack of stirring as the pH value in all trials remained the same (pH 1.3). Consequently, besides pH, also

the mass of etched iron lrng]

0

10

20

30

etching

40 time

50

60

70

[mini

Fig. 1. The course of iron plate etching (plate surface 23.1 cm*) in 50 ml of sulfuric acid solution of pH (HZ SO4 concentration 4.0 g/l) as a function of time.

I .3

14

T. Stefanowicz et al. / Hydrometallurgy

47 CIY97/ 69-90

stirring and time should be evaluated as the essential factors in etching process. During etching mainly Fe *+ ions are formed, because the standard reduction potential, Fe’++3e=Fe

(E”=

-O.O36V),

(8)

is higher than that of reaction (2) and consequently the Fe-” ions concentration is much lower. The cementation trials were mainly conducted according to two process conditions, e.g. ‘with stirring’ and ‘without stirring’. The variant ‘with circulation’ was performed in the beakers, where copper solutions were stirred by magnetic stirrer MM6 (PM-Poland) with bar rotated at 3000 min-’ under the stationary iron plate suspended in solution. In the studies without stirring particular plates were placed vertically into a beaker filled with 50 ml of copper solution and left still for a defined time, while in the variant with stirring, 10 or 15 plates were put into a similar beaker filled with the copper solution and the whole set of iron plates was intensively moved with a glass rod around the periphery of the beaker. In this second case, the particular plates prior to assembling into the set were irregularly bent in various ways so that when they were put together and stirred, the contact between adjacent surfaces was minimized to provide good solution access to each plate. In addition the mechanical friction between plates resulted in scraping of the layer of copper deposited on the plates. This renewal of plate surfaces appeared to favor a continuous cementation route. The copper cementation trials with a solution of concentration 14 g/l of Cu and pH 1.3 (solution I> without stirring (variant a) and with stirring (variant b) at various process times are shown in Table 3. Trials l-5 were performed with single iron plates (with a surface of 23.1 cm’) immersed into copper solution which remained without stirring during a certain time from 10 to 90 min. After cementation for 90 min (trial 51, the content of copper in solution decreased from an initial concentration of 14 g/l to 1750 mg/l (87.5% of the copper was precipitated). Simultaneously with reduction of the copper ion after 90 min of cementation, the concentration of dissolved iron increased from 375 mg/l Fe to 11750 mg/l (the coefficient of mass exchange Fe/Cu [g/g] was 0.929). Trial 6 comprised the cementation with a single iron plate with a greater surface (6 1.9 cm21 immersed into copper solution of pH 1.3 for 90 min without stirring. The greater surface of the iron plate resulted in a higher percentage of removal of dissolved copper (95.9%) compared with trial 5. Compared with etching, when the iron plate was immersed into the sulfuric acid solution (Fig. l), the amount of iron dissolved due to cementation highly exceeded the share of reaction (7) which, in practice, can be neglected. Trials 7- 10 (Table 3) were performed with one plate of iron in the beaker, where solution and plate were intensively stirred for 3 to 15 min. The copper concentration decreased much faster; the cementation with stirring for 3 min gave better results than cementation without stirring for 20 min (trial 21. Similarly, cementation with stirring for 15 min resulted in removing a greater amount of copper than cementation without stirring for 90 min. After 15 min 96.5% of copper was removed. Trials II- 14 were performed with 10 iron plates, while trials 15- 19 were made with

T. Stefanowicz et al. /Hydrometallurgy

47 (1997) 69-90

:5

Table 3 The copper cementation course in 50 ml of copper sulphate waste solution of copper concentration 14.0 g/l at pH I .30 (the initial concentration of iron is 0.375 g/l). The single iron plate surface is 23.1 cm’ or 61 .Y cm2 Trial No.

I 2 3 4 5 6 7 8 9 I(, II

12 I .i IJ 15 16 17 18 I9 --

Iron plate surface [cm’] 23. I 23.1 23.1 23.1 23.1 61.9 23. I 23.1 23.1 23. I 23 I .2 231.2 231.2 231.2 346.8 346.8 346.8 346.8 346.8

Immersion time [min]

10

b

20 30 60 90 90

h h h h h

3’ 5‘ 10 c I5 ‘

3‘ 5’ IO r I5 r 3’ 5‘ IO c I5 ‘ I5 ‘

Final concentr. of copper

Concentr. of total iron

hs/ll

[mg/ll

12250 10500 7750 3500 1750 575 9300 4600 2200 500 500 225 13.25 26 II60 50 5.5 4.25 0.003

1420 3575 7215 10872 11750 10530 2750 5000 8750 12750 9100 9500 9600 13000 7500 8000 9000 12750 15200

’ The theoretical mass exchange h Without stirring. With intense stirring.

ratio Fe/Cu

Mass exchange

-

in 50 ml

copper removed

iron dissolved

coefficient of mass exchange

[mgl

[msl

Fe/Cu [g/g1

87.5 175 312.5 525 612.5 671 235 470 590 675 675 688.75 699.33 698.7 642 697.5 699.72 699.78 700

52.25 I60 342 525 569 507 II8 231 419 619 436.2 456.25 451.2 631 356 38 I .2 431.2 381.2 741

0.597 0.9 I ,4 I .09.1 I.000 0.92’3 0.75.5 0.502 0.49 I 0.711) 0.9 1’7 0.646 0.662 0.64.5 0.903 0.555 0.547 0.6 I b 0.545 1.050

li

Degree of coppet cementation

[al 12.5 25.0 44.6 75.0 87.5 Y5.9 33.6 67. I x4.3 96.3 96.1 Y8.4 09.‘) 99.x 01.7 Y9.6 - 100 - IO0 - IO0

[g/g] = 0.879.

IS plates placed into beakers with 50 ml of copper solution. Both series were performed with intense stirring with a glass rod for 3 to 1.5 min. In conclusion, by increasing the iron plate surface together with intense stirring. the decrease of copper concentration proceeded much faster and copper removal of nearly 100% was gained within 10 to 15 min. The calculation of the mass exchange between copper and iron (when considering the initial concentration of iron) showed that the mass of both exchanged metals was 01 similar order, nevertheless it differed from the theoretical approach (reaction (311, where 63.6 mg of copper should be balanced with 55.9 mg of iron (the theoretical mass exchange coefficient Fe/Cu being 0.879). The results obtained demonstrate that in practice deviation occurs in both directions and they prove that the cementation balance is influenced by the accompanying reactions (5-7). The cementation trials in solution of initial copper concentration 32.5 g/l and pH 0.87 (solution II) are presented in Table 4. Trials were performed for 0.5, 1.5 and 48 h without stirring with either two plates or 15 plates with 15 min stirring. The cementation results confirmed the favorable influence of larger iron plate surfaces and stirring and additionally showed that during prolonged processing time

T. Stefanowicz et al. /Hydrometallurgy

16

47 (1997) 69-90

Table 4 The copper cementation course in copper sulphate waste solution II (32.5 g/l of Cu and pH 0.87). The initial concentration of iron 1.275 g/l. Solution volume is 50 ml, the single iron plate surface is 61.9 cm2 or 23.1 cm2 a Trial No.

Plate surface

Time

Solution after cementation

[hl

reaction

concentration

PH

cu

Fe

0.86 0.88 0.87 0.88 0.79 0.88 0.85

11.75 6.00 1.375 1.000 0.04 0.05 0.0095

26.0 31.5 37.5 36.0 48.0 52.0 37.5

[cm21 I 2 3 4 5 6 7”

61.9 123.8 61.9 123.8 61.9 123.8 346.8

b b b b b b ’

0.5 0.5 1.5 1.5 48 48 0.25

g/l

Mass exchange in 50 ml [g] Cu

Fe

I.038 I.325 1.556 I.575 1.623 1.622 I.625

1.236 1.511 1.811 I.736 2.336 2.536 1.811

Coefficient of mass exchange

Degree of cu cementat

Fe/Cu

[%I

1.191 1.140 1.164 1.102

1.439 1.563 1.114

[g/g1

63.8 81.5 95.8 96.9 - 100 - 100 -100

b Without stirring. ’ With stirring.

(trials 5 and 6) the amount of dissolved iron increases. Therefore the intense stirring combined with large iron plate surface is preferred because it shortens the processing time and limits dissolution of iron. For large surfaces and intense stirring (trial 7) nearly all the copper was removed in 15 min, and the final solution contained only 9.5 mg/l of Cu to compare with initially 32.5 g/l. In this case, the mass exchange of copper and iron also seemed to be of the same order, however the mass exchange coefficient Fe/Cu was lower than in long-time trials 5 and 6. Similar trials were performed (Table 5) with solution of copper concentration 57.5 g/l and pH 0.29 (solution III> with the difference that in some trials, instead of stirring, intense circulation of the copper solution was applied around the motionless iron plate suspended stationary above the vessel bottom (variant ‘with circulation’). The copper removal in these trials was more effective compared to trials performed without stirring, especially at shorter times (0.5 and 1.5 h). It is worth noticing that in the case of circulation the increase of the iron plate surface was not as effective as it was found to be in the case of no stirring. Also in these series the longer processing time (trials 7-9) resulted in greater dissolution of the iron. Additionally it was noticed that during cementation in the copper solution of pH 0.29, Cu was deposited on iron plates more as metallic layer than spongy sludge formed in less acidic waste solutions. Compared to the ‘with stirring’ variant with an iron plate surface of 346.8 cm2, where during 15 min nearly 100% of copper was removed (trial lo), the cementation performed according to the ‘circulation’ variant even with a single plate having a surface of 61.9 cm* for 30 min (trial 2) resulted in 99.8% copper removal and after 1.5 h (trial 51 N 100% removal. Continuous stirring of both solution and scrap iron is technically more burdensome than circulating the solution alone, so the circulation method seems to be more attractive. Probably the intense circulation of the solution through suspended

T. Stefanowicz et al./ Hydrometallurgv 47 (1997) 69-W

77

Table 5 The copper cementation course in copper sulphate waste solution III (57.5 g/l of Cu and pH 0.29) performed rn the trials l-10, and in copper waste solution IV (43 g/1 of Cu and pH 1.45) performed in trial I I, The solution volume is 50 ml, the single iron plate surface is 61.9 cm2 or 23.1 cm7 ’ Trial Yo.

Copper waste solution

Number of iron plates

Plate surface

61.9 61.9 123.8 61.9 61.9 123.8 61.9 61.9 123.8 346.8 346.8

I

111

Ih

7

III III III III 111 111 III III III IV

Id 2d lh Id

-i

5 h 7 x 0

IO II --

2 d

lh Id 2d 15 SC 15 I.C

Time [h]

[cm* 1

0.5 0.5 0.5 1.5 1.5 1.5 72 72 72 0.25 0.25

Solution after cementat. concentr.

PH

cu

Fe

Fe/Cu

0.13 0.30 0.11 0.30 0.31 0.28 0.46 0.36 0.58 0.55

11.0

40.0 67.5 45.0 42.0 67.5 44.0 70.0 80.0 75.0 63.0 47.0

0.794 1.122 0.766 0.705 1.120 0.722 1.164 1.338 1.251 I.042 1.040

1.62

0.105 2.75 2.25 0.003 0.8 0.004 0.002 0.004 0.0122 0.004

[g/l]

Coefficient of mass exchange

reaction

[g/g1

Degree of Cu cement

lg 1

-

80.9 09.8 95.2 96. I Itlo 9.x.6 I(10 Itlo I(10 itro It!0 .-

” Without stirring.

’ With intense stirring both: iron plates and solution. ’ With intense circulation

only solution under the suspended

plates.

motionless scrap iron with occasional mechanical shaking applied to scrap iron would be a reasonable compromise. When cementation is performed in rotating drums filled with scrap iron, in order to limit noise the continuous rotation probably should be replaced by short rotary strokes with breaks, with continuous and intense circulation of the solution. To result in better rasping of the iron surface, together with scrap iron pieces of coke can be added to the drum. Trial 11 (Table 5) represents the cementation in the copper solution with a concentration of 43 g/l at pH 1.45 (waste solution IV), prepared by partial neutralization of acid in waste solution III by adding an adequate amount of NaOH solution. Cementation was performed with 15 plates with intense stirring for 15 min and the results obtained show that the mass exchange coefficient was the same as in trial 10 with removal of - 100% of copper. In this case the copper deposit was spongy and it was rather loosely adhered to the iron surface so is more easily removable than in the case of the solution at pH 0.29. Hence the additional conclusion can be drawn that for strongly acidic copper solutions, for easier renewing of scrap iron surfaces it is worth adjusting the pH before cementation to values near pH 1.5. In these cases during cementation the pH still remains below pH 2 preventing iron hydrolysis and iron hydroxide precipitation. The cementation results obtained with 100 ml of copper sulphate solution (copper concentration 14 g/l) which was intensively circulated around the suspended iron plate (surface 24 cm2 and 64.8 cm’) were used to calculate the cementation rate equations. For the particular iron plate surfaces the reaction rates were calculated by the least

T. Stefanowicz et al. / Hydrometallurgy 47 (1997) 69-90

78

squares method using a quadratic polynomial. obtained: (a) For iron plate surface of 24 cm* kc, = 4.4.

10-4.

The

following

rate equations

were

(9)

[CU]O’~~(M s-‘)

(c) For iron plate surface of 64.8 cm2 k,,

=

4.9.

10-4.

(10)

[CU]~‘~” (M s-‘)

The influence of iron plate surface is undoubted equations, one common equation can be used k,,

=

1.16. 10-j.

[CU]~?S.~~

and if introduced

into reaction

rate

(11)

(MS-‘),

where the copper concentration [Cu] is in mol dm-’ and the iron plate surface S in dm2. It is obvious that the iron plate surface parameter S automatically comprises the applied intensity of the solution circulation as well as the space parameter connected with reactor scale and shape. Consequently the rate equation should be considered only as an illustration of complexity of the reaction system to prompt further investigations of the system. The reaction rate values calculated by particular reaction rate equations are presented in Table 6. In order to evaluate the ecological aspect of the method with copper removal by cementation, the investigations were performed with a posttreatment solution to determine the possibility of further removal of iron by increasing alkalinity and the precipitation iron hydroxide with copper and nickel remainders. The trials presented in Table 7 demonstrate that iron as well as nickel and copper residues in postcementation solutions, after pH correction above pH 9 (by adding NaOH), readily precipitate as hydroxides which can effectively been removed by

Table 6 Copper cementation reaction rate calculated: determined cementation reaction rate equation Trial No.

Iron plate surface

Cementation time [min]

(a) by the least squares method (LSM) and (b) with use of constants (Eqs. (91-t 11))

Copper concentration

WI

Reaction rate calculated by LSM [M s-‘1

Reaction rates [M s-l]

Reaction rates [M s-‘1

Wm*l 1 2 3

0.24 0.24 0.24

0 15 30

14.0 3.0 0.136

2.208E - 04 I .258E - 04 3.085E - 05

2.295E - 04 ’ l.l84E-04” 3.130E-05 a

I .745E - 04 ’ l.l16E-04 ’ 4.5508 - 05 ’

I 2

0.648 0.648

0 15

14.0 0.05 1

3.651E-04 I .222E - 04

3.26lE-04 l.178E-04b

4.805E - 04 ‘ 9.430E - 05 ’

’ Calculated b Calculated ’ Calculated

by Eq. (9). by Eq. (10). by Eq. (11).

b

l‘able 7 The postcementation solution treatment results. The solution pH was adjusted by adding NaOH solution. The precipitated sediments were removed by filtration -_ Waste solution designation pH after adjustment Concentration [mg/lI after treatment

I

II III IC

12.86 9.50 9.06 9.02

cu

Ni

Fe

< 0.4 < 0.4 < 0.4

0.4 I.2

< 0.25 0.50 0.75

I .o

I .I? 1.o

I .50

filtration. After removal of the metal hydroxide sludge, the pH can be corrected below 9. e.g. by adding sulfuric acid. In practice even better cleaning of final waste solutions is usually achieved by using sedimentation with flocculants followed by filtration on filter presses, so this stage of postcementation solution treatment can probably be improved [91. For the technological approach, the iron scrap pieces were put into a stainless steel drum rotated at 30 rpm with the copper solution (IV) circulation in it (Fig. 2). The rotating drum was 330 mm long having a diameter of 105 mm. The volume of the solution was 2.5 1 and the surface of the iron rods was 1636 cm’. The results obtained on this facility confirmed the conclusions drawn from beaker-scale trials. During 15 min of rotation, more than 99% of copper precipitated in the case of continuous rotation. With occasional shaking of the scrap iron with intense circulation of the solution provided by pump operation, a similar result was achieved after 30 min. The results of investigations seem to be satisfactory because: (a> copper can be practically recovered in a metal form (powder-type sludge with foil particles) which after drying is suitable for metallurgical utilization,

Fig. 2. The rotating drum with scrap iron pieces inside, used for copper recovery waste solutions. The rotating speed is approx. 30 rpm (bench scale).

by cementation

from coppet-

80

T. Stefanowicz et al. / Hydrometallurgy

47 (I 997) 69-90

(b) iron together with remaining heavy metals can be easily removed from the waste postcementation solutions with the same methods as usually applied, i.e. through alkalization and sedimentation of metal hydroxide sludge, which can be partially dewatered on a filter-press and utilized as pigment admixture in the production of building materials or to manufacture ceramics for the building materials industry (e.g. tiles, aggregates) [lo-131.

2.2. Copper recovery from spent nitric-acid

based etching solutions

When rejected gold-electroplated contacts are subjected to gold recovery by dissolving the base metal (usually copper) in nitric acid solutions, highly concentrated waste solutions are produced. Similar solutions are formed when copper foil is removed from printing board wastes. By adding base (e.g. NaOH) copper hydroxide can be precipitated and the resulting sludge separated from the solution on a filter press. After washing (NaNO, removal), the filter-cake can be dissolved in sulfuric acid or in cyanides and subjected to electrowinning. Alternatively after wash and dewatering on the press filter, the cake can be dried and utilized in metallurgy processes. However, the above method is a costly operation due to high water content in the cake. The investigations intended to determine conditions for copper recovery by cementation directly from spent nitric acid solutions instead of treating them according to these methods. The copper cementation trials from sulfuric acid solutions showed the substantial dependencies which should be considered also when performing copper cementation from a nitric acid solution. Due to the strongly oxidizing character of nitric acid, its medium is unfavorable for cementation because it acts more towards copper dissolution than towards its precipitation. Therefore, it is conceivable that in these specific conditions modification of the cementation method is unavoidable. Two stock solutions were used in trials, obtained by etching copper in a nitric acid solution during gold recovery (the original etching solution was prepared by diluting nitric acid with water at a ratio of 1:l). Analysis of the waste solution showed, besides copper and nitric acid, also some iron. The reaction of both solutions was below pH 0. The main components of the stock waste solutions are presented in Table 8. To evaluate the area influence of the iron, plates or rods of defined dimensions were used. The dimensions of plates were the same as in previous investigations performed in

Table 8 The main components Designation

‘A’ ‘B’

of stock waste solutions

of the solution

Concentration

[g/l]

Cu

Fe

190 620

0.01 8.75

T. Stefanowicz et al. / Hydrometallurgy

47 (19971 69--W

XI

sulfuric acid solutions (part I). In some trials much larger surfaces (iron chips) were used to approach supposed technical conditions. The initial copper cementation trials were performed with the ‘A’ solution. where iron plates were immersed vertically into samples of 50 ml and after defined time they were removed and solutions were analyzed for copper and iron concentrations. Fig. 3 shows the results of the trials performed with the undiluted ‘A’ solution. with and without stirring, as well as with cooling of the solution. As is shown in the figure. without stirring the copper recovery (12.7%) was poor compared with trials in the sulfuric acid medium. A higher cementation degree (22.6%) was achieved doubling the iron plate surface. A significantly better result (50.5%) was obtained when cementation was performed at a temperature below 0°C. A similar percentage (52.6%) was obtained after correction of the pH to 0.85 (by adding NaOH). Correction of pH to 1.5 resulted in an increase to 62.1%. When stirring was applied and the iron plale surface was increased to 23 1.2 cm2, the cementation rose to 70% after 2.5 h. The above results show that in the case of the nitric acid medium, cemenlation proceeds much slower and with less output. The positive influence of cooling as well as of partial neutralization of the acid can be explained by slower redissolution of cemented copper in a nitric acid solution. A cementation degree of 78.9% was obtained when 15 g of iron chips (approximate iron surface 285 cm21 was used for 24 h with continuous vibration of iron chips due to a magnetic stirrer bar oscillation. An additional trial was performed with the solution cooled to below 0°C and with a

70 60 50 40 30 20 10 0

II’S

1

2

3

(,I.‘)

I2J.X

123.H

4

5

6

I23.tl

123.x

231.2

pll

4

4

41

ll.Xr?

1.50

4

Stirring

N

N

N

I\

h

I’ “(’

illll b.

;~lllll.

CO

illllb.

Qllll~. -__-.

\ ;llllll.

_. -

Fig. 3. Copper cementation from spent copper nitrate waste solution of Cu concentration plate surface (cm2), trials l-5 time 24 h, trial 6 time 2.5 h.

190 g/l.

IPS iron

T. Stefanowicz et al./ Hydrometallurgy

82

47 (1997) 69-90

bundle of 21 iron rods, having a total surface of 100 cm2, stirred occasionally for 48 h. The solution with iron rods was stored in a refrigerator at approximately - 5°C and occasionally stirred resulting in the copper removal of 65.8%. After another 24 h the percentage increased only to 88.4%. The above results show that the concentration of the nitric acid in the original spent etching solution is too high to start cementation; the solution must be initially diluted. Without this dilution the cementation degrees remain below the values obtained in sulphate-based copper solutions. The next series of copper cementation trials were performed with stock solution ‘A’ diluted to various concentrations and with solutions prepared separately. The results of these trials have been collected in Table 9. The maximum cementation degree (99.4%) was obtained in trial 2 with 15 g of iron chips (with a surface of 285 cm*) with continuous vibration provided by a magnetic stirrer for 24 h. The trials collected in Table 9 have long processing times. The degree of copper cementation in this series was high (above 90%) compared with trials shown in Fig. 3. They confirm the positive influence of initial dilution. The favorable influence of dilution on the cementation degree seems to be specific in the case of nitric acid medium. The positive influence of stirring during such long cementation times was not clearly shown and additional trials were performed at much shorter periods using a rotating drum (Fig. 2) with 22 free iron rods having a total surface of 8 18 cm’. The rotating speed of the drum was 30 rpm. In these trials the concentrated stock solution ‘B’ (Cu 620 g/l) was used without dilution as well as after 5 fold dilution. The results obtained, presented in Fig. 4, confirm that the initial dilution of the post-etching solution increases the degree of copper cementation. The noise generated by the rods falling inside the rotating drum undoubtedly is an

Table 9 The effect of long-time copper cementation trials from copper nitrate waste solution ‘A’ initially diluted to various concentrations either from solutions prepared separately or obtained in earlier cementation trials. The solution volume is 50 ml Trial No.

Concentration after dilution [g/l1

1. 2. 3. 4. 5. 6.

Cu

Fe

95 38 5.2 12 95 38

0.005 0.002 62.5 61.5 0.005 0.002

Iron plate surface [cm’]

PH corrected to

123.8 285 il 123.8 123.8 123.8 72.0 b

1.5 0.85 -

Stirring + or -

Time [hl

Final cont.

Coefficient of mass exchange

[g/l1

(-) t+) t-1 (-1 t-1 (&l

(+ 1 cementation with stirring; ( -1 cementation a Iron chips. ’ Solution with iron rods cooled below 0°C.

24 24 48 24 24 24

Cu

Fe

Fe/Cu

5.2 0.22 0.312 5.4 4.1 0.48

62.5 21.5 75 150 60 26.25

0.696 0.728 2.557 1.239 0.66 0.70

without stirring; ( f 1 with occasional

Degree of cementation [%I

[g/&l 94.5 99.4 94.0 92.5 95.7 98.7 stirring

T. Stefanowicz et al. / Hydrometallurgy 47 (1997) 69-90 Dcgrcc

orcopper ccriicnl:ltion

I’l/nl

DO

60 “Y” 40

20

0

Fig. 4. Degree of copper cementation from copper nitrate waste solution of copper concentration 620 g/l ( 0 1 and from 5 fold diluted solution I24 g/l (0). Trials were performed at intense stirring in rotating drum with tree iron rods of total surface approximately 820 cm’. The rotating speed of the drum 30 rpm.

important drawback for this technical application (to limit this noise small pieces of coke, loaded together with scrap iron, occurred to be effective). The following trials were performed with a stationary iron plate suspended above the bottom of the beaker leaving sufficient space for a magnetic stirrer to rotate the stirring bar beneath the plate and to provide intense circulation of the solution alone. The initial trial was performed with the undiluted ‘A’ solution (copper concentration 190 g/l) with an iron plate having a surface of 61.9 cm* immersed in 50 ml solution. After 4 h of intense stirring, the dissolved copper removal was 12.610, while in the case of the diluted ‘A’-stock solutions after 15 and 30 min much higher cementation values were obtained (Table 10). The degree of copper cementation from these solutions rose subsequently as a function of time in all cases with concentrations of the copper solutions of 95.0, 63.3, 47.5 and 19.0 g/l. After 30 min of intense circulation of the solution less than half the amount of copper was removed. After a pH correction to 3.4 this amount increased to 70% (trial 1 l), however, after this pH correction some precipitation of iron hydroxide together with cemented copper particles occurred. The characteristic feature was that the cementation of copper happens most effectively in the beginning and then slows down significantly (Fig. 5). This series of trials confirmed that specific to the nitric acid medium higher degrees of cementation are achieved when the solution is initially diluted and cementation is performed during a prolonged time. The circulation of the solution alone (without stirring of iron plates) results in a smaller percentage of copper removal but it allows the elimination of noise. Thus it is concluded that to provide effective cementation of copper using the ‘circulation’ method, a longer processing time is required. The influence of time on the cementation degree during the first 12 min is shown in Fig. 6, when trials were performed as a function of time with 5 fold diluted ‘A’ solution (Cu 38 g/l, Fe 0.002 g/l, pH 1.03) using a stationary mounted iron plate. Due to a pH

T. Stefanowicz et al. / Hydrometallurgy 47 (1997) 69-90

84

Table 10 The degree of copper cementation from diluted ‘A’-series copper stationary mounted iron plate of surface 61.9 cm* in 50 ml solution Trial No.

I

2 3 4 5 6 I 8 9 10 11

Initial

Cu concentration

PH

k/l1

0.2 0.2 1.0 I.0 1.0

I .o 1.5 1.5 1.5 1.5 3.4 a

a pH was corrected

Time [min]

before

after

95.0 95.0 63.3 63.3 47.5 41.5 19.0 19.0 19.0 19.0 19.0

74.0 52.0 32.0 30.0 34.0 28.0 12.0 12.5 13.3 14.8 5.1

nitrate

solutions

Coefficient of mass exchange

circulated

around

the

Degree of copper cementation

[%I 15 30 1s 30 15 30 15 30 30 30 30

1.143 0.977 0.958 1.039 2.044 1.538 0.77 I 0.954 1.017 1.476 1.068

22.1 45.3 49.4 52.6 28.4 41.1 36.8 34.2 30.0 22.1 70.0

with NaOH

increase during cementation to 1.8-1.9, some iron hydroxide was formed. As a consequence the cemented copper was contaminated with iron hydroxide which certainly has to be removed from the final sedimented copper material (e.g. by dissolution in a diluted sulfuric acid solution). To compare with copper cementation in sulphate solutions, where 15 min of intense stirring resulted in nearly total removal of copper, the cementation from nitric acid solution required besides initial dilution much longer processing time and in addition the final product usually was contaminated with iron hydroxide slime.

Degree of coppet cemenlalion I?/;)]

ot/. 0

15

30

45

60

75

90

Fig. 5. The effect of copper cementation from diluted ‘A’ series copper nitrate waste solution of Cu (concentration 38 g/l) as a function of time with solution circulated around the stationary mounted iron plate of surface 61.9 cm* (solution volume is 50 ml).

T. Stefanowicz et ul./Hydrometallurgy dcoppcr

Dcgrcc

ccmcnlnlion

47 (19971 69--90

1’%1

0

I

0

1

2

3

4

5

6

7

6

9

10

11

12

min

Fig. 6. The effect of copper cementation from 5 fold diluted copper nitrate waste solution as a function of time. The solution of initial copper concentration 38 g/l was circulated around the iron plate (surface 61.9 cm’. solution volume SO ml).

The copper cementation rate equations for copper nitrate solution were calculated using 100 ml of 5 fold diluted ‘B’ solution (copper concentration 38 g/l and pH 1.03) and iron plate surfaces of 25.6 and 65.9 cm*. The copper nitrate solution was intensively circulated under stationary suspended iron plate with a magnetic stirrer. The reaction rates were calculated by the least squares method (LSM) using quadratic polynomials. In the first approach, the reaction rate equation constants for particular iron plate surface were determined considering that the reaction rate depends only on copper concentration Cu [mol/l]. In such a case the following reaction rate equations were obtained: (a) in the case of iron plate surface 25.6 cm2 kc, = 3.79.

10-4.

(‘2)

[CU]“‘~ (M s-l)

(b) in the case of iron plate surface 65.9 cm2 kc, = 9.05 . 1O-4 . [CU]“*~ (M s- ‘). By introducing the iron plate surface parameter the following equation (after rounding of constant values) was obtained k,,

=

8.48.

10-4.

[Cu]‘.08 .So,37 (M s-r),

(13) common

reaction

rate

(14)

where [Cul is in mol dm-” and S in dm2. The experimental reaction rates (determined by the least squares method) and values calculated by using the above cementation reaction rate equations are collected in Table 11. As follows from the table, the cementation rates calculated by the particular equations are of a similar order, however all equations can be considered only as illustrative examples to show the complexity of the heterogeneous reaction course, where besides iron plate surface, other parameters, such as stirring intensity, its volume, shape of the vessel and the scale of reactor are important parameters which influence the cementation course and should be considered when formulating the general conclusions.

T. Stefanowicz et al./Hydrometailurgy

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47 (19971 69-W

Table 11 Copper cementation reaction rate in nitrate solution of pH 1.03, calculated: (a) by the least squares method (LSM) and (b) by using the determined cementation reaction rate Eqa. (12)-(14) Trial No.

[g/l1

Reaction rates calculated by LSM [M s-‘1

Reaction rates [M s-‘1

Reaction rates [M s-‘1

0 15 45

38.0 15.1 12.5

2.29lE-04 1.743E - 04 6.795E - 04

2.46OE - 04 a l.l33E-04 a 9.667E - 05 ‘I

2.938E-04’ l.O84E-04’ 8.843E - 05 ’

0 15 30

38.0 16.1 11.1

4.227E - 04 2.47OE - 04 7.132E - 05

4.856E - 04 h 1.718E-04 h l.O95E-04 h

4.169E-04‘ I .649E - 04 ’ l.104E-04c

Iron plate surface [dm’]

Cementation time [min]

1 2 3

0.256 0.256 0.256

1 2 3

0.659 0.659 0.659

’ Calculated ’ Calculated ’ Calculated

Copper concentration

by Eq. (12). by Eq. (13). by Eq. (14).

To find more favorable cementation conditions and especially to shorten the processing time, some modifications were performed to check if cementation could be supported e.g. by connecting the iron plate to the negative pole of a dc current source to increase its affinity to positively charged copper ions. Reaction which proceeds on the cathode during electrolysis is the same as (11, so it could be expected that electrolysis would support the copper cementation. Trials with a dc source (Table 12, trials l-8) were performed with graphite rods used for anodes at both sides of the cell, with an iron plate having a surface of 61.9 cm2 connected as cathode. As electrolyte 50 ml of the concentrated as well as diluted ‘A’ solutions of were used with intense circulation of the solution by a magnetic stirrer (3000 mini ’1. The electric charge was measured by Ah-counter type UMG-05-06 ASPAN (Poland). In the case of the concentrated ‘A’ solution (190 g/l) with current of 0.05 A (charge 0.025 A h) after 30 min the copper removal was 5.3% (trial 1). The higher current of 0.5 A for 30 min (0.25 A h) increased the result to 10.5% (trial 2). It is worth noticing that without current for the undiluted ‘A’ solution a similar value (12.6%) was obtained after processing for 4 h (trial 3). However, in the case of a 2 fold diluted ‘A’ solution the cementation degree without current after 15 min of intense stirring was 22.1% and after 30 min 45.3% (Table 10; trials 1 and 2), while with current the above value was only 2.1% at 0.025 A h and 12.6% at 0.25 A h (Table 12; trials 4 and 5). Poor cementation results with current-supported trials in the concentrated solutions are assigned to intense hydrogen ion reduction on the cathode at high acidity in the case of the ‘A’ solution (pH below 0) as well as in the 2 fold diluted ‘A’ solution (pH 0.19). It can be supposed that intense evolution of hydrogen on the cathode hindered the copper ion access and reduction on iron plates surface. In the case of the 5 fold diluted ‘A’ solution (initial copper concentration 38 g/l), the cementation performed for 1 h without current resulted in copper cementation of 7 1.8%

T. Stefanowic: et al. / Hydrometallurgv 47 (19971 69--W

x7

Table 12 The degree of copper cementation from ‘A’ series copper nitrate solutions (concentrated and diluted) with applying electrolysis (current of 0.5 or 0.05 A) or after introducing sodium pyrosulfite batches into 50 ml I olume of the solution samples. Solution was circulated around the stationary mounted iron plate of ~rface 01.9 cm’ ‘Trial NO.

Initial concentration of cu [g/l]

190 190 190 95 95 38 38 38 I90 I90 190 95 95 38 3x 38 I9 I9

Sodium pyrosulfite batch [g]

n.a. na. n.a. n.a. n.a. n.a. n.a. ,,.a. 0.8 0.8 1.2 3.0 3.5 I.5 2.0 2.0 0.5 I .o

Electrolysis [A h]

Time [min]

pH before

after

Final concentration

k/II

0.025 0.25 n.a. 0.025 0.25 n.a. 0.25 0.05 n.a. na. na. n.a. n.a. n.a. n.a. n.a. n.a. na.

30 30 240 30 30 60 30 60 15 30 I5 30 30 30 30 30 30 30

< 0 < 0 < 0 0.2 0.2 I .o

I .o 1.o < 0 < 0 1.2 0.7 1.2 I .5 I .6 I .6

I .7 2.0

<0 < 0 < 0 0.15 < 0 1.2 1.x 1.7 0. IO 0.07 0.09 4.0 3.0 3.9 3.6 3.2 5.0 4.6

cu

Fe

I 80 I70 166’ 93 83 10.7 5.4 6.5 153 166 I60 0. I28

s.0 10.0 3.7 6.8 I I.9 4.5 32 25.4 80.0 57.5 50.0 IO8 122 32.0 50 20 17.6 23.0

I .04 0.032 3.4 3.5 0.005 0.003

Degree of cu cementation

.5.:1

IO.! 12.11 2.1

12.6 71.r 85.8 X2.‘> I’).’ 12.6 15.x 99.9 9x.9 - 100 91.1 90.8 - 100 - IO0

1r.a.: not applied

(Table 12; trial 6), while the same process with current of 0.5 A for 30 min (0.25 A hl resulted in 85.8% (trial 7). In the second trial performed at lower current value of 0.05 A for I h (0.05 A h) the degree of copper removal was 82.9% (trial 8). Consequently some positive influence of current seems to be confirmed in the case of more than 2 fold diluted solutions (pH of the 5 fold diluted solution was 1.0 and hydrogen evolution on cathode was insignificant). To limit the adverse influence of the nitric acid in the post-etching solution. trials were also performed adding sodium pyrosulfite batches prior to cementation expecting to suppress the oxidizing properties of nitric acid medium according to reaction [ 131 by nitric acid reduction to nitrogen dioxide 4HN0, + Na,S,O, --) 2NaHS0, + 4N02 + H,O. (15) The stationary mounted iron plates (surface of 61.9 cm’> were immersed into the samples with intense circulation of the solution by the magnetic stirrer (Table 12. trials 9 - 18). Sodium pyrosulfite batches were ineffective when added to the concentrated ‘A’ solution (trials 9-l l), but they seemed to be very helpful when added to the diluted solutions (trials 12-18). The supporting influence of sodium pyrosulphite is convincing when these trials are compared with results presented in Table 10.

88

T. Stefanowicz et al. /Hydrometallurgy

47 (19971 69-90

Table 13 The degree of copper cementation from diluted ‘A’-series copper nitrate solutions with adding various sodium pyrosulfite batches into 50 ml volume samples of the solution prior to cementation. Solution was circulated around the stationary mounted iron plate (surface of 61.9 cm’) Trial No.

Initial concentration of cu

1 2 3 4 5 6 7 8 9 IO II 12 13 14 15 16

[g/II

9s 95 95 63.3 63.3 63.3 63.3 45 4s 45 4s 38 38 38 19 19

Sodium pyrosulfite batch [g]

03

2.0 2.0 0.3 0.7 0.3 0.7 0.3 0.6 0.3 0.6 0.2 0.2 0.2 0.5

1

Time [min]

pH before

after

Final concentration

Degree of cu cementation

[g/l1

15 15 30 15 I5 30 30

0.3 0.6 0.5 1.0 I.0 I.0 1.o

IS

I.1 1.2

15 30 30 15 30 30 30 30

1.1

1.2 1.3 1.3 1.3 1.8 I .9

0.8 2.2 3.7 1.7 2.0 2.6 3. I 1.8 2.4 2.2 3.0 2.8 4.4 4.4 4.4 5.0

cu

Fe

[aI

28 21 5.9 28 17,s 6.3 5.4 41.0 9.3 14.8 4.2 3.0 2.1 0.002 0.710 0.008

S8 86 1 IO 41 36 50 58 25.4 42.0 28.0 50.0 2.5.0 46.8 46.7 13.4 24.4

70.5 17.9 93.8 55.8 72.4 90.0 91.5 x.9 79.3 67.1 90.7 92.1 94.5 99.5 96.3 100

The further trials (Table 13) demonstrate that sodium pyrosulfite batches increase the copper cementation degree and allow, significantly, shortening of the processing time. Such a positive result was achieved when the solution was circulated around the stationary suspended iron plate. During the same processing time, larger sodium pyrosulfite batches resulted in better copper cementation effect and vice versa, for the same addition of sodium pyrosulfite batches the longer processing time resulted in a higher percentage of copper removal. As follows from Tables 12 and 13 copper can be effectively recovered from spent the nitric acid etching solution by at least 2 fold dilution, by adding sufficient batches of sodium pyrosulfite and by applying intense circulation of the solution around the stationary suspended iron plate surface. Processing time not shorter than half an hour (preferably one hour) seems to be effective.

3. Conclusions

The investigations showed that copper ions can be effectively removed from sulphate type copper electroplating waste solutions by using scrap iron. If such waste solution is strongly acidic (pH below l), then for easier peeling and renewing of the iron surface,

, 90

T Stefanowicz

et al. / Hydrometuliurg~ 47 (1997) 69-W

References [I] Dziennik Ustaw, No. I16 z dnia 16.12.1991, poz. 503, ZaF4cznik No. I. [2] Dziennik Ustaw, No. 42 z dnia 31.12.1987, poz. 248, Za&znik No. 2. [3] T.C. Lo, M.H.I. Baird, C. Hanson, Handbook of Solvent Extraction. John Wiley and Sons. New York, 1983. [4] L. Twidwell, Metal value recovery from metal hydroxide sludges, Montana College of Mineral Science and Technology, MT, Draft Report, November 1984. [5] D.R. Lide, CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 1995. [6] K. Othmer. Encyclopedia of Chemical Technology, vol. 6. John Wiley and Sons. New York. 1965, p. 169. [7] F. Letowski, Podstawy Hydrometalurgii, WNT, Warszawa 1975, p. 302. [8] J. Minczewski, Z. Marczenko, Chemia Analityczna, PWN Warszawa, 1965. p. 600. [9] S.A.K. Palmer, M.A. Breton, T.J. Nunno, D.M. Sullivan, N.F. Surprenant, Metal/Cyanide Containing Wastes Treatment Technologies, Pollution Technology Review No. 158. Noyes Data Corp.. Park Ridge. NY, 1988, p. 141. [IO] A. Roy, H.C. Eaton, F.K. Cartledge, M.E. Tittlebaum, Environ. Sci. Technol. 26 (7) ( 1992) 1339. [I 1] J. Budilovsky, Novaja sistiema ochistki promyshlennyh stokov, ili opyt vniedrienija “najholieje dobttjimoj tiehnologii”, Narodnoje Hoziajstvo Litvy 2 (1990) 47. [ 121 J. Budilovsky, Non-Waste production in the region, The Economist of Lithuania, 3(6 I ) ( I99 1). [ 131 B.A. Johnson. Ch.B. Rubenstein, R.J. Martin, J.O. Leckie, Heavy metal waste conversion by thermallydrrven chemical bonding. in: Proc. of Metal Waste Management Alternatives Symp.. Pasadona, San Jo\e. CA, September, 1989. [14] R. Chang. General Chemistry, Random House, New York, 1986, p. 496.

T. Stefanowicz et al./Hydrometallurgy

47 (1997) 69-90

89

prior to cementation the pH should be adjusted to approximately 1.5. Stirring scrap iron together with the solution or circulating the solution alone through motionless suspended scrap iron results in a greater cementation effect than increasing the iron surface if no stirring is applied. Stirring or circulation shortens the cementation time and favors the decrease of iron etching. Copper cementation can be realized effectively using either of the two methods tested: (1) An intense and continuous overturning of scrap iron pieces together with the solution in a rotating drum type reactor to provide the maximum contact of the solution with the surface of scrap iron. This intense motion renews the iron surface through mechanical friction, while copper sludge pours out from the drum together with circulating solution through the hole at the outlet of the drum and is withdrawn from the reactor bottom. The copper removal exceeded 99% and was achieved by this method within 15 min. (2) An intense circulation of the solution through a motionless drum with scrap iron inside, with occasional short rotary strokes of the drum followed by 4-5 min breaks, resulting in a similar percentage of removed copper in 30 min, longer than the previous method. The second method seems to be more convenient because of the noise produced by the first method. Scrap iron can be used and copper can be recovered in a form suitable for utilization in copper metallurgy, while the postcementation solution from this process can be effectively treated to remove heavy metals through hydrolysis followed by metal hydroxide precipitation and filtration. The filter-press cake can be utilized in the production of ceramics for building materials. In the case of spent nitric acid-based copper solutions some additional steps should be applied before performing the cementation process: (1) The post-etching solution should be at least 2 fold diluted and sufficient amount of sodium pyrosulfite is to be added to overcome the oxidizing medium of nitric acid (the batch should be determined experimentally, as it depends on initial dilution). (2) Instead of using sodium pyrosulfite, the copper cementation in nitric acid medium can be supported by electrolysis, where scrap iron is used for cathode and graphite rod for anode. In such a case the post-etching solution also should be diluted more than 2 fold to increase the pH to at least 1. (3) During copper cementation the intense circulation of the solution is required similar to the case of sulfuric acid medium. (4) Cooling of the solution is impractical, nevertheless it was found to be advantageous, probably due to slower redissolving of cemented copper in the nitric acid medium.

Acknowledgements This work was supported by the Committee of Scientific according to Statutory Program, item No. 31-473/95.

Investigations

(Poland)