Selective dissolution of copper from copper-chromium spent catalyst by baking–leaching process

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Selective dissolution of copper from copper-chromium spent catalyst by baking–leaching process P.K. Parhi a,b,*, T.R. Sethy a, P.C. Rout a,c, K. Sarangi a,c a

CSIR-Institute of Minerals and Materials Technology (IMMT), Bhubaneswar, Odisha, India School of Applied Science and Center of Industrial Technology, KIIT University, Odisha, India c Academy of Scientific and Innovative Research, India b

A R T I C L E I N F O

Article history: Received 21 December 2013 Received in revised form 11 March 2014 Accepted 13 March 2014 Available online xxx Keywords: Copper Chromium H2SO4 Baking Leaching

A B S T R A C T

The selective leaching of copper from the spent Cu-Cr catalyst was carried out with H2SO4. The effect of different parameters such as acid concentration, pulp density, temperature and particle size on leaching was investigated. The maximum extraction of copper and chromium was 67.25 and 2.3%, respectively at particle size 45–53 mm, pulp density 2.5%, temperature 90 8C, time 180 min. Therefore, baking followed by leaching approach was adopted for dissolution of spent copper-chromium catalyst using H2SO4 to enhance the metal leaching efficiency. At the optimum baking–leaching condition i.e. baking time 2 h, baking temperature 300 8C, baking acid concentration 0.5 M, leaching temperature 35 8C, time 60 min, [H2SO4] 4%, P.D. 2.5%, the extraction of copper and chromium was 99.9% and 1.2%, respectively, ensuring the selective dissolution of copper. The XRD and Fe-SEM-Edax characterization analysis of typical samples (original, baked mass and typical residue) were compared and reported. The XRD and Fe-SEMEdax analysis of the baked mass indicated the complete sulfation of copper and chromium by H2SO4 yielding CuSO4 (H2O) and (Cr)2(SO4)3, respectively in solid phase. The absence of XRD peaks corresponding to CuSO4H2O in the final typical leach residue (obtained at optimum baking–leaching condition) confirmed the complete dissolution of copper from Cu-Cr catalyst. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction The rapid industrialization due to the growing demand of metals such as Cu, Co, Cr, and Ni in day to day life leads to the depletion of the primary sources. Therefore, reserve amount of primary sources bearing above metal values decreases [1,2]. The secondary resources such as spent catalysts, e-wastes, spent magnets, and spent batteries [1,3–6] which contain a considerable amount of various metals (Cu, Ni, Cd, Co, Li, Cr, Au, PGM group metals, rare and rare earth metals), are becoming the potential sources for above metals. The spent catalysts are of various kinds depending on their nature, composition and applications [2,6]. The catalyst namely copper-chromium composite oxides is widely used as a catalyst for hydrogenation, dehydrogenation, and alkylation processes [7–10]. These catalysts basically act as active

* Corresponding author at: CSIR-Institute of Minerals and materials Technology (IMMT), & KIIT University, Bhubaneswar, Odisha, India. Tel.: +91 8280042882. E-mail addresses: [email protected], [email protected] (P.K. Parhi).

catalysts for complete oxidation of carbon monoxide and hydrocarbons to carbon dioxide [11,12]. The mixed type of copperchromium oxide phases have good use in the simultaneous removal of nitric oxide and carbon monoxide from exhaust gases [11]. However, these catalysts gradually lose their activity after certain cycles of use due to coking, poisoning by metals, sulfur, or halides or loss of surface area due to sintering at high temperatures. These are then discarded and termed as spent catalysts [13]. These spent catalysts are hazardous in nature and their direct disposal to environment is highly restricted [14–16]. So much attention is being paid towards development of processes for extraction of metal values to minimize landfill space and to prevent pollution in land disposal [1,15,17,18]. The copperchromium spent catalyst has more economic importance since it contains 42% copper and 36% chromium [19] and a suitable technology is urged to recover and recycle the metal values from this catalyst. A considerable amount of work has been done by number of researchers on processing of spent catalyst [2,17–22] which involved either pyrometallurgical or hydrometallurgical process or combination of both the processes. The hydrometallurgical

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leaching method has been extensively used for dissolution of metal values from several sources [1,2,18,21]. But in some cases, direct leaching may not be very effective for complete dissolution of metals due to the nature of the source materials. So some pretreatment steps such as surface activation of material [19,22,23], phase transformation prior to leaching are required to enhance the metal dissolution rate. The baking is one of such processes where the phase transformation takes place at the desired temperature and the baked mass gets easily dissolved in the leaching stage. The metal(s) exist in metal oxide, metal sulphide and matte are transformed to corresponding sulfate in the solid phase which can be easily leached out on to the liquid phase. Sulfuric acid baking–leaching process is a widely accepted methodology in hydrometallurgy and based on this process a number of successful technologies have been developed. But reported research investigations in this domain are limited [17,18,20]. In baking process, consumption of acid and time are also minimized and this enables to ease the leach operation and favors the process economy as well. The studies related to the recovery of metals from the spent CuCr catalyst are limited [15,19]. In a leaching study of Cu-Cr spent catalyst with H2SO4 [19] maximum 56% of copper and <2% of chromium were extracted and to improve the copper leaching efficiency mechanical surface activation of native material was carried out. By using above surface activation method the overall copper recovery was improved up to 80%. For quantitative recovery of chromium after extraction of copper, one additional second stage leaching (alkaline salt roasting leaching) was incorporated. The proposed baking–leaching process for the extraction of copper from the spent Cu-Cr catalyst is an innovative approach and no work has been reported relating to this investigation. Therefore, the present research work is aimed to describe the selective dissolution of copper from the spent Cu-Cr catalyst in H2SO4 medium. The direct leaching followed by the baking–leaching was carried out as the function of key leaching variables. The phase transformation during the baking and leaching of the solid phase was characterized and reported.

2. Experimental 2.1. Materials and reagents The spent Cu-Cr catalyst used in this investigation was crushed, ground and sieved to obtain different particle sizes. The particle size in the range of 75–105 mm was used in most of the experimental study. The elemental composition of the ground sample is given in Table 1. The chemicals and reagents used for the experimental work were of analytical reagent grade (Merck, India). 2.2. Baking of the materials The baking experiments were carried out by thoroughly mixing the fixed amount of definite size fraction (75–105 mm) of the spent catalyst and desired volume of conc. H2SO4 in a porcelain crucible. The mixture was then introduced to the pre-heated furnace (at the desired temperature). After the stipulated reaction time the crucible was taken out and kept for cooling inside desiccator. The activated materials were ground and leaching studies were

Table 1 Elemental assay of the Cu-Cr spent catalyst. Element

Cu

Cr

Mn

Ni

Co

Zn

%

42.3

36.2

2.45

0.78

0.085

0.13

carried out using predetermined amounts of H2SO4 for different time and temperature. 2.3. Leaching method The leaching experiments were carried out in a three necked 500 ml glass round bottom flask which was placed on a heating mantle. The solution was kept under agitation using an externally connected agitator. The lixiviant was heated with stirring at 600 rpm in most of the studies unless otherwise specified. After attainment of desired temperature, the spent catalyst sample/ baked spent catalyst of the appropriate particle size range was charged into the flask containing 250 ml of the H2SO4 solution. Periodically samples were collected for analyses. The possible chemical reaction associated during leaching and baking–leaching can be described as given in the following equations ((1)–(5)). 2CuCrO2 þ H2 SO4 ! CuSO4 þ Cu þ 4Cr2 ðSO4 Þ3 þ H2 O

(1)

CuO þ H2 SO4 ! CuSO4 þ H2 O

(2)

4CuCrO2 þ 4H2 SO4 þ O2 ! 4CuSO4 þ 2Cr2 O3 þ 4H2 O

(3)

CuCr2 O4 þ H2 SO4 ! CuSO4 þ Cr2 O3 þ H2 O

(4)

CuCr2 O4 þ 4H2 SO4 ! CuSO4 þ Cr2 ðSO4 Þ3 þ H2 O

(5)

2.4. Analytical method and equipment The slurry samples of 5 ml were withdrawn at regular time intervals, filtered, diluted and analyzed for metal content by an AAnalyst-200, Perkin-Elmer, (USA) Atomic Absorption Spectrophotometer (AAS). At the end of the leaching tests, the slurry was filtered using filter paper (Whatmann-42) followed by in situ washing of the residues with de-ionized water. The washed residues were dried overnight in an oven at 100 8C, weighed and analyzed for their metal content after digestion. The extent of leaching was calculated on the basis of both solution and solid assays. Some of the leaching tests were repeated in order to ascertain the reproducibility of the experimental results. The typical samples were characterized by XRD and FE-SEM-EDAX study (JOEL JSM-6510 model) analysis. The XRD patterns of the sample were obtained using a Phillips Powder Diffractometer (Model PAN ANALYTICAL PW 1830) in the range of 5–408 (2u) at a scanning rate of 28/min with molybdenum target. 3. Results and discussions The effect of different parameters such as (i) pulp density, (ii) agitation speed, (iii) acid strength, (iv) temperature and (v) particle size range for leaching of copper was investigated where one of the above parameters was changed in the desired range while other parameters were kept constant in most of the leaching studies (unless otherwise specified). The experimental conditions and the results of the maximum leaching of copper and chromium for direct leaching of metals from spent Cu-Cr catalyst was summarized and presented in Table 2. 3.1. Direct sulfuric acid leaching of copper (function of time, agitation speed, H2SO4 concentration, particle size, temperature and pulp density) The extent of copper and chromium dissolution was investigated within the time period of 300 min. Other leaching parameters for the study were: 0.75 M H2SO4, particle size 75– 105 mm, agitation speed 600 rpm, temperature 35 8C, pulp density

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Table 2 Summary of conditions and results for direct leaching of spent Cu-Cr catalyst using H2SO4 (the values of leaching variable (s) highlighted in bold indicting the optimum condition, at which metal leaching efficiency was maximized). Leaching variables

Optimum metal leaching efficiency, %

Time, min

Agitation speed, rpm

[H2SO4], M

Temperatur, 8C

Pulp density, %

Particle size range, mm

Cu

Cr

10, 30, 60, 120, 180, 240, 300 180

600

0.75

35

2.5

105–75

54.28

2.27

0.75

35

2.5

105–75

54.28

2.24

180

200, 400, 600, 800, 1000 600

35

2.5

105–75

54.28

2.94

180 180 180

600 600 600

0.25, 0.5, 0.75, 1.0, 2.0 0.75 0.75 0.75

35, 50, 75, 90 35 35

2.5 2.5, 510, 15 2.5

105–75 105–75 147–105,105–75, 75–53, 53–45

56.35 54.28 67.25

3.84 2.21 2.3

2.5%. From the results of time effect it was found that within 10 min of initial leaching time, 45.87% of copper was extracted and thereafter, leaching of copper was steadily increased and reached 54.28% in 180 min (Table 2). Beyond 180 min, the percentage leaching remained constant. Therefore, leaching time of 180 min was kept constant for further experimental studies. The effect of agitation speed (200–1000 rpm) on leaching was carried out while keeping other parameters such as H2SO4 concentration, particle size, leaching time, temperature, pulp density constant at 0.75 M, 75–105 mm, 180 min, 35 8C and 2.5%, respectively. From this study it was seen that the dissolution of cooper was maximum of 54.28% (Table 2) and it remained constant throughout the studied agitation speed range. The result suggested that the leaching reaction was chemically controlled. However, to ensure the substantial leaching of copper, agitation speed of 600 rpm was kept fixed for further studies. The influence of sulfuric acid concentration on leaching of copper was investigated within the range of 0.25–2 M. Other parameters such as particle size 75–105 mm, leaching time 180 min, agitation speed 600, temperature 35 8C, and pulp density 2.5% were kept constant. Copper extraction was increased from 30.11% to 54.28% while increasing the H2SO4 concentration from 0.25 M to 0.75 M and thereafter a plateau value was observed. The chromium leaching efficiency was increased from 1.16% to 2.94% with increasing acid concentration from 0.25 M to 2.0 M. The maximum metal extraction obtained at the optimum condition is provided in Table 2. The leaching of copper mainly depends on H2SO4 concentration, but in this case copper recovery was not very high even at high acid concentration. The low copper dissolution was may be due to the nature of the original Cu-Cr catalyst in which, part of the copper exists as CuCrO2 in Cu oxidation state of +1 and remaining as +2 in CuO. The resulted dissolved copper obtained during H2SO4 leaching was from CuO. The low dissolution of Cr was due to occurrences of major content of stable Cr in its low oxidation state of +3 in the CuCr2O4 of spent catalyst [19,24]. The temperature of the solution was varied from 35 8C to 90 8C to know about the leaching behavior of the spent catalyst where other parameters such as particle size 75–105 mm, agitation speed 600 rpm, pulp density 2.5%, 0.75 M H2SO4 concentration were kept constant. The copper leaching efficiency did not increase significantly and chromium leaching was varied within 2.21– 3.84% at the studied temperature range of 35–90 8C. The results of this study were summarized and the optimum condition and maximum extraction of metals are presented in Table 2. The experiments were carried out to study the dissolution of spent catalyst of different particle size ranges i.e. 45–53 mm to 106–150 mm at 35 8C in the solution containing 0.75 M H2SO4. The leaching of copper showed an increasing trend i.e. from 48.25% to

67.25% while changing the particle size range from 105–150 mm to 45–53 mm and chromium extraction was almost remains unchanged at the varied particle size ranges. The maximum copper and chromium extracted at the optimum condition is as given in Table 2. The degree of increasing dissolution rate of copper was due to increase in fineness of the particles with decrease in the particle size and the above result was consistent and supported by reported literatures [17,19]. The effect of pulp density on leaching was carried out in the range of 2.5–15% (w/v) at H2SO4 concentration of 0.75 M, particle size 75–105 mm, in ambient temperature. The copper extraction was maximized (54.28%) at the low pulp density i.e. Pd 2.5% (Table 2) and then significantly decreased from 54.28% to 19.35% and at the same time chromium leaching percentage was also decreased from 2.21% to 1.02% with increase in the pulp density values from 2.5% to 15%, which may be due to depletion of acid in the solution mixture. Above study on leaching showed maximum copper extraction of 67% with very less amount of chromium extraction. The lowering of chromium extraction can be attributed to the insufficient attack by sulfuric acid on the chromite lattice and inability to oxidize the part of Cr (III) to Cr (VI) [19]. Similar kind of results has been reported by many researchers [25,26] while leaching the chromite (FeOCr2O3) in sulfuric acid medium. 3.2. Baking study (temperature, time and H2SO4 concentration) In this study the original material was baked at the desired temperature and the respective activated mass was leached with dilute sulfuric acid (1% H2SO4). This acid concentration was kept constant in most of the baking–leaching study unless otherwise specified. The baking as well as baking–leaching results for each study was determined based on the metal leaching efficiency. The activation of the Cu-Cr catalyst was carried out at varying baking temperature within the range of 200–500 8C using 0.75 M H2SO4 (baking acid concentration) and the leaching of copper was carried out after baking. Other parameters such as baking time, leaching time, pulp density, leaching acid (H2SO4) concentration were kept constant at 2 h, 180 min, 2.5% and 1% (v/v), respectively. As showed in Fig. 1, the leaching efficiency of copper increased from 39.54 to 95.74% with increase of baking temperature from 200 8C to 300 8C followed by decrease of leaching efficiency on further increase of baking temperature. From this result it was ensured that baking temperature of 300 8C was suitable for effective leaching of copper. The lowering of copper leaching efficiency beyond 300 8C was due to the substantial loss of H2SO4 (H2SO4 boiling point is 337.3 8C) leading to the incomplete sulfidation of CuCrO2 and CuCr2O4. The above finding was further supported by the literatures [18,20].

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4

100

Copper Extraction, %

80 60 40 20 0 100

200

300

400

500

600

Baking temperature, oC Fig. 1. Effect of baking temperature (baking–leaching).

Fig. 2. Effect of baking H2SO4 concentration (baking–leaching).

H2SO4 used became insufficient for dissolution of both the metals effectively. From the above observation, it was ensured that 35 8C was the optimum temperature for the present study and higher than that lead to substantial loss of copper and increase in coextraction of chromium. The leaching of chromium was further favored with increase in temperature as well as leaching time. Therefore, temperature of 35 8C was maintained in all most all the experimental study. The objective of this investigation was for

80

60 % Metal Extraction

The baking time of spent Cu-Cr catalyst was varied from 0.5 h to 4.0 h followed by leaching of the resulted activated mass using 1% H2SO4 solution. Other parameters such as baking acid concentration of 0.75 M, PD 2.5%, leaching time 180 min, agitation speed 600 rpm, were kept constant. The results obtained were given in Table 3. It was observed that baking time of 2 h was sufficient for effective dissolution of copper. So further experiments were carried out with baking time of 2 h. To study the baking acid concentration effect on leaching, sulfuric acid concentration was varied from 0.25 M to 1.0 M, while the baking temperature and duration were kept constant at 300 8C and 2 h, respectively. After baking the Cu-Cr catalyst with different acid concentration as mentioned above, the activated samples were leached at 35 8C using 1% H2SO4 solution. The results were plotted in Fig. 2 which showed that at 180 min of leaching time the dissolution of copper increased from 46.48% to 95.74% with increasing acid concentration from 0.25 to 0.75 M and after that it remained almost same. The complete dissolution of copper could not achieved even by increasing more than two times of the stoichiometric amount of H2SO4 during baking. This could be due to low acidity of leach solution to dissolve the activated mass during leaching. The leaching of chromium was remained same (1.5%) in the studied range of baking acid concentration.

35 oC Cu 35 oC Cr 50 oC Cu 50 oC Cr 75 oC Cu 75 oC Cr 90 oC Cu 90 oC Cr

40

20

0

3.3. Baking–leaching study

0

3.3.1. Effect of temperature on leaching The dissolution of copper and chromium from the activated sample (baking condition: baking temperature 300 8C, H2SO4 concentration 0.5 M, baking time 2 h) was carried out with 1% H2SO4 within the temperature range of 35–90 8C. The results were shown in Fig. 3. At 35 8C and 50 8C, copper dissolution increased with increase of time from 10 to 240 min and then remained unaltered. At 75 8C and 90 8C, copper dissolution increased up to 60 min and after that it remained constant. Up to 50 8C, Cr extraction was within 1.2%, but it increased to 12.5% and 30.25% with increase of temperature to 75 8C and 90 8C, respectively. The dissolution of copper was decreased marginally from 70.12 to 63.12% after 240 min of leaching time at higher temperature (75 8C and 90 8C). This decrease in copper leaching may be due to simultaneous partial dissolution of chromium. The amount of

50

100

150

200

250

300

Fig. 3. Effect of temperature on extraction of Cu and Cr (baking–leaching).

Table 3 Results of copper leaching at different baking time. Time, min

30

60

120

180

240

Copper extraction, %

84.59

92.68

95.74

95.75

95.75

350

Time,min.

Fig. 4. Effect of leaching acid concentration (baking–leaching).

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Fig. 5. Effect of pulp density (S/L) (baking–leaching).

selective dissolution of copper which seems to be consistent with the above results and it was also well compromised from the results of leaching studies carried out at an ambient condition. 3.3.2. Effect of H2SO4 concentration on leaching The effect of H2SO4 concentration within the range of 1% (v/v) to 5% (v/v) on leaching of activated mass (baking condition: baking temperature 300 8C, H2SO4 concentration 0.5 M, baking time 2 h)

Fig. 6. XRD analysis of (a) original Cu-Cr spent catalyst, (b) baked sample (baking condition: time 2 h, [H2SO4]: 0.5 M, temperature 300 8C). (c) Typical residue sample (resulted at optimum baking–leaching condition: baking time 2 h, baking temperature 300 8C, baking acid concentration 0.5 M, leaching temperature 35 8C, leaching time – 60 min, [H2SO4] – 4%, P.D. 2.5%).

Fig. 7. Fe-SEM analysis of (a) original Cu-Cr spent catalyst, (b) baked sample (baking condition: time 2 h, [H2SO4]: 0.5 M, temperature 300 8C) and (c) typical residue sample (resulted at optimum baking–leaching condition: baking time 2 h, baking temperature 300 8C, baking acid concentration 0.5 M, leaching temperature 35 8C, leaching time – 60 min, [H2SO4] – 4%, P.D. 2.5%).

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was studied keeping agitation speed 600 rpm, Pd 2.5%, temperature 35 8C constant. As shown in Fig. 4, the copper extraction was increased from 65.28 to 99.99% with increase of H2SO4 concentration from 1 to 4% (at 1 h of leaching time) and thereafter it remained constant. As can be seen from the results, extraction rate of copper was significantly enhanced with 4% H2SO4 and the optimum leaching time was reduced to 1 h. The extraction of chromium was very less (1%) within the studied H2SO4 acid concentration range. Same observation was also obtained by other researchers [19,25] while leaching chromite and Cu-Cr spent catalyst in sulfuric acid medium. 3.3.3. Effect of pulp density The variation in the extraction of metal values with H2SO4 (H2SO4 varied stoichiometrically with respect to 2.5% PD) at different pulp densities within 1.0% to 15% (w/v) was studied while keeping other leaching conditions constant was shown in Fig. 5. Almost complete dissolution (99.99%) of copper was observed up to 2.5% Pd and there is a steady decrease of copper leaching efficiency (98.7%) up to 5% Pd. However, with further increase in pulp density from 5% to 15%, the dissolution rate was significantly decreased. This may be due to the increase in viscosity of the activated mass in the lixiviants [18]. From this study it was observed that 2.5% pulp density is a suitable optimum condition for selective and effective leaching of copper from spent catalyst. The leach liquor resulted at optimum leaching condition; Pd: 2.5%, Time: 60 min, temp: 35 8C, agitation speed: 600 rpm, reads to contain copper and chromium concentration as 10.57 g/l and 0.108 g/l, respectively. The final leach solution obtained can further be purified in the subsequent purification stage for recovery of high pure copper metal or its compound.

4. Conclusions Direct sulfuric acid leaching of Cu-Cr catalyst, carried out as the function of different leaching variables resulted low dissolution of metals (67.25% of copper and 2.3% chromium). Sulfuric acid baking followed by leaching route was observed to consume much less acid as compared to direct sulfuric acid leaching of the same sample and that lead to generate lesser amount of effluents than the direct sulfuric acid leaching during overall processing of the spent catalyst. In addition, this route has the advantage of its operation at lower temperature than the other practiced alkali/salt fusion processes. The optimum metal leaching of Cu 99.99% and Cr 1.2% was obtained at the condition; baking time 2 h, baking temperature 300 8C, baking acid concentration 0.5 M, leaching temperature 35 8C, time – 60 min, [H2SO4] – 4%, Pd-2.5%. This investigation revealed selective copper dissolution from spent CuCr catalyst by baking–leaching process at the ambient temperature condition. The above observations were consistent and further ensured by subsequent XRD and FE-SEM-Edax analysis results of original, baked mass and residue, samples, respectively. Acknowledgments The authors wish to thank Prof. B.K. Mishra, Director, CSIRIMMT, Bhubaneswar and Dr. I.N. Bhattacharyya HOD, H & EM Department for their kind permission to publish this paper. Author P.K. Parhi is highly grateful and would like to acknowledge Department of Science and Technology (DST), New Delhi, India for the financial assistance and support provided under DST-INSPIRE Faculty scheme for the investigation of this work. Co-author T.R. Sethy also wishes to thank DST, New Delhi, India, for partially funding the work.

3.4. Characterization of original and typical samples The XRD patterns and surface morphology of original, activated sample (2 h baking time, 0.5 M H2SO4 and baking temperature 300 8C) and final typical residue sample obtained at optimum leaching condition (leaching time 30 min, 4% H2SO4 and temperature 35 8C, PD 2.5%) was compared. The results were shown in Fig. 6. The XRD peak pattern of original sample showed major peaks corresponding to CuCrO2, CuO, CuCr2O4, indicating the presence of major constituent of Cu and Cr in the oxidation state of +1, +2 and +3, respectively. On the other hand, the XRD patterns of baked sample at 300 8C ensured the complete phase transformation from metal oxide to metal sulfate phase (CuSO4 (H2O) and Cr2 (SO4)3). The final residue sample showed the XRD patterns of Cr2 (SO4)3. The absence of peak corresponding to CuSO4 (H2O) in the residual phase indicated the complete dissolution of copper from the baking sample. The SEM microphotograph in Fig. 7(a) showed that the copper-chromium oxide and copper oxide particles were distributed in large number of platelet shaped particles. The phase transformation during baking to form hydrated CuSO4 and Cr (SO4)3 crystals ensured from the FeSEM-Edax as showed in Fig. 7(b). Nevertheless, the regular flower shaped crystal patterns of both copper and chromium sulfate were further revealed in the microphotograph Fig. 7(b). Fig. 7(c) revealed unique corn shaped crystal structure of Cr2 (SO4)3 of the residue sample and this composition was again evident from the Fe-SEMEdax result. Fe-SEM-Edax analysis result of typical residue did not show any peak due to copper, confirming leaching of all copper values from the activated mass.

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