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Extraction of Cu and Cr from a spent Cu–Cr catalyst: Recovery enhancement through mechanical activation
Hydrometallurgy 136 (2013) 8–14
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Extraction of Cu and Cr from a spent Cu–Cr catalyst: Recovery enhancement through mechanical activation Siksha Swaroopa, Malay Kumar Ghosh ⁎, Kali Sanjay, Barada Kanta Mishra CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India
a r t i c l e
i n f o
Article history: Received 17 August 2012 Received in revised form 11 February 2013 Accepted 2 March 2013 Available online 14 March 2013 Keywords: Spent catalyst Chromium Leaching Mechanical activation Roasting
a b s t r a c t Treatment process for a spent Cu–Cr catalyst (Cu 42.4% and 36.2% Cr) was described. The catalyst contained cuprous chromite (CuCrO2), cupric chromite (CuCr2O4) and copper sub-oxide (Cu8O) phases. High energy ball milling reduced the mean particle size from 9.4 to 1.7 μm along with simultaneous increase in surface area by about 63%. Cuprous chromite phase is selectively mechanically activated. Sulfuric acid leaching and alkali roasting followed by water leaching techniques were employed for Cu and Cr recovery respectively. Un-milled catalyst showed maximum 67% Cu extraction by varying H2SO4 concentration (0.94 M–3.75 M) and leaching temperature (ambient - 150 °C) whereas a 5 h milled sample resulted in 90% Cu extraction under the conditions of 1.31 M H2SO4, 20 g/L S:L ratio, 80 °C. Cr recovery was examined by varying NaOH amount, roasting temperature and time. More than 90% Cr extraction was achieved when a milled sample was roasted with 100 wt% NaOH at 850 °C for 2 h. Cr recovery from the Cu-leached residue was comparatively more than that obtained from raw milled sample. The leach residue after Cr recovery was nearly pure CuO which can be recycled for Cu recovery. A conceptual flow-sheet for the recovery of both metals was described. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The Cu–Cr composite oxides are widely used as catalyst for hydrogenation, dehydrogenation, alkylation etc. (Barnicki et al., 2009; Huang et al., 2005; Valdes-Solis et al., 2006). These catalysts are generally composed of equi-molar combination of cupric chromite and cupric oxide and not merely mechanical mixture of these compounds (Vogel, 1956). These composite oxides are also used for complete oxidation of carbon monoxide and hydrocarbons to carbon dioxide and for the simultaneous removal of nitric oxide and carbon monoxide from exhaust gas (Prasad and Singh, 2012). Besides the above commercial applications they find a great promising role in the composite solid propellants to enhance the burning rate of the propellant (Li and Cheng, 2007). Like any other catalyst it also gradually loses its catalytic activity due to various reasons such as coking, poisoning by sulfur, metals etc. or loss of surface area due to sintering effects at high process temperature. Catalyst regeneration is possible up to certain degree and cycles depending upon the degree of irreversible deactivation. However, when regeneration is not economically feasible catalysts are discarded and termed as spent catalysts (Trimm, 2001). Due to hazardous nature and restricted disposal option recovery of metal values from these catalysts is a suitable alternative considering economic and environmental points of view. So far as
⁎ Corresponding author. Tel.: +91 674 2379374; fax: +91 674 2581637. E-mail address: [email protected] (M.K. Ghosh). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.03.001
Cu–Cr catalysts are concerned past attention has been focused on the regeneration only (Cheng, 1995; Corrigan et al., 1985; Habermann, 1972) and no attempt has been made so far to recover the metal values. Silva et al. (1994) reported an electrochemical treatment process using Ce(IV) as oxidizer for complete dissolution of copper chromite spent catalyst with an objective to use the solution for fresh catalyst preparation. It was also mentioned that processing of spent copper chromite catalyst through any conventional recovery method is not economical due to insoluble nature of the catalyst even in concentrated acid or alkali. Considering the spent Cu–Cr catalysts as potential secondary resource copper and chromium values can be recovered through a hydrometallurgical and/or pyrometallurgical route. Traditionally chromium is extracted from the chromite ore by soda ash roasting at 1200 °C by converting trivalent chromium oxide to water soluble hexavalent chromate (Antony et al., 2006). A novel alternative process was developed by Zhang et al. (1999) which comprises of continuous oxidation of chromite in molten NaOH, one-way separation in highly concentrated alkali solution, meta-stable phase separation and carbonate recycle conversion of sodium chromate to sodium dichromate or chromic acid. The advantage of the process being almost zero waste and waste is free from toxic Cr(VI) unlike the conventional processing. Mechanical activation is now a widely accepted methodology to increase the leaching rate of minerals (Balaz, 2003). Mechanical activation is generally carried out through high energy ball milling process in which the particles go through repeated fracturing and cold welding during collisions either between the balls or the ball and the container inner wall. Several past studies indicate that mechanical activation significantly accelerates
S. Swaroopa et al. / Hydrometallurgy 136 (2013) 8–14
the dissolution rate of solid minerals like ilmenite, tantalite, pyrrhotite, complex sulfide, copper sulfide etc. by increasing their chemical reactivity (Ficeriova et al., 2002; Hashemzadehfini et al., 2011; Sasikumar et al., 2004; Vafaeian et al., 2011; Wei et al., 2009; Welham, 2001; Welham and Llewellyn, 1998; Zhao et al., 2009). Yarkadaş and Yildiz (2009) studied the effect of mechanical activation on soda roasting of chromite and could improve the conversion from 18% (non-activated chromite) to 74% (120 min activated chromite) while roasting at 800 °C. Zhang et al. (2010) reported the alkali leaching of mechanically activated chromite and it was observed that leaching efficiency of non-activated chromite was only 34% while a 10 min mechanically activated sample showed 97% Cr extraction. One of the main reasons for the increased reactivity after mechanical activation is the combination of new surface area and formation of crystalline disorder in minerals. The literature survey reveals that although spent Cu–Cr catalysts contain high amount of copper surprisingly not much attention has been directed in the past towards metal recovery from this source. In the present investigation preliminary assessment has been made to recover both the metals - copper through H2SO4 leaching and chromium through NaOH roasting followed by water leaching. Mechanical activation technique has been employed for better metal recovery and material characterization through different instrumental techniques has been carried out also for both unactivated and mechanically activated samples. Effects of different leaching parameters and roasting conditions were investigated. Spent catalysts and leach residues were characterized through X-ray diffractometry (XRD) to corroborate the leaching results. Ultimately a tentative flow sheet is proposed for total metal recovery from the spent catalyst.
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Acid leaching experiments were carried out in a 250 mL doublewall cylindrical glass reactor. The reactor lid had a provision for reflux condenser and sampling port. Hot water from a constant temperature water bath was circulated between the walls of the reactor to achieve the required temperature in the leaching medium. The agitation was accomplished using a magnetic paddle by keeping the glass reactor over a magnetic stirrer. At the required temperature catalyst powders were added and leaching continued for 2 h. At different intervals slurry sample was collected, filtered and analyzed for Cu and Cr. Leaching at higher temperature (150 °C) was performed in Amar® 2 L capacity Ti-autoclave. Copper concentration in the leach liquor was estimated volumetrically by standard iodometric method (Vogel, 1961) using starch as indicator. Chromium in leach liquor was analyzed by Perkin-Elmer-2380 Atomic Absorption Spectrophotometer. 2.4. Characterization The XRD of the un-milled, milled and leach residue samples were performed using a Phillips Powder Diffractometer (Model PAN ANALYTICAL PW 1830) in the range of 5–70° (2θ) at a scanning rate of 2°/min with Cu - Kα radiation. Surface areas of the un-milled and milled catalyst powders were measured with Quantachrome® surface area analyzer (Model – Autosorb-iQ) using liquid nitrogen. Particle size measurements of the ground and milled catalyst powders were carried out using CILAS® 1064 Particle Size Analyzer. 3. Results and discussion 3.1. Characterization of the catalyst
2. Experimental 2.1. Material The copper chromium spent catalyst used in this study was crushed, ground and screened to provide material with a particle size of b100 μm. The elemental composition of the ground sample is given in Table 1. All reagents used in the study were in the analytical reagent grade. 2.2. Mechanical activation For mechanical activation work a twin-bowl INSMART planetary ball mill (Model PBM-07) of overall capacity 500 mL and useful capacity 250 mL was used. Milling was carried out at 300 rpm using 10 mm diameter stainless steel balls keeping the ball to powder weight ratio at 10:1. Toluene was used as process control agent to avoid any reaction of the milled powder with open atmosphere and also to minimize the cold welding of powders to vial and balls.
The XRD patterns of the un-milled and milled spent catalysts (Fig. 1) contain six diffraction peaks at (006), (012), (104), (018) and (110) planes corresponding to the typical diffraction peaks of cuprous chromite (CuCrO2, ICCD 39–0247), a delafossite group compound where Cu is in +1 oxidation state and Cr in +3 oxidation state. The peaks corresponding to (101) and (211) planes are characteristic of cupric chromite (CuCr2O4) peaks (ICCD 88–0110), a spinel structure. Besides the above two phases a sub-oxide of copper (Cu8O) phase is also observed corresponding to the planes of (004) and (220) respectively (ICCD 78–1588). Depending upon the Cu:Cr ratio and calcination temperature adopted during the catalyst synthesis procedure constituent phases may vary but generally for equi-molar ratio of CuO and Cr2O3 cupric chromite (CuCr2O4) is only observed (Li and Cheng, 2007; Li and Flytzani-Stephanopoulos, 1997). Since a Cu–Cr catalyst undergoes
2.3. Roasting and leaching procedure Required amounts of spent catalyst powders and sodium hydroxide pellets were mixed properly in a quartz crucible and the crucible was put inside a muffle furnace. The sample was heated to the required temperature and roasting was carried out for a prefixed time followed by stepwise cooling. The roasted mass was then transferred from the crucible to a glass reactor along with measured amount of distilled water followed by room temperature leaching for about 30 min.
Table 1 Elemental composition of the Cu–Cr spent catalyst. Cu
Cr
Mn
Ni
Co
Zn
42.4%
36.2%
2.4%
0.77%
0.08%
0.12%
Fig. 1. XRD patterns of (a) un-milled (b) 3 h milled (c) 5 h milled samples.
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through high process temperature coupled with oxidizing/reducing environment hence phase changes may occur and constituent phases of the spent catalyst may be different from the virgin catalyst. XRD patterns of 3 h and 5 h milled samples (Fig. 1b and c respectively) with comparatively wider and weaker diffraction peaks are indicative of finer particle size caused due to milling. The crystal lattice strain (ε) and the crystallite size (D) can be calculated from the line broadening of the peaks using Scherrer's formula (Eq. (1)): 2
Bt ¼
h
i2
0:9λ D cosθ
2
2
þ ½4ε tan θ þ B0
ð1Þ
Where, Bt is the full width at half maximum intensity of the peak, λ is the wavelength of the radiation used, D is the average crystallite size, ε is the strain, θ is the diffraction angle and B0 is the instrumental line broadening. Table 2 summarizes the milling effect on crystallite size (D), lattice strain (ɛ), surface area and mean particle size (d50). Cystallite size and lattice strains were calculated using X'pert HighScore Plus® (Ver. 3.0d) software. It is observed from Table 2 that with increase in milling time average crystallite size of CuCrO2 decreases and lattice strain increases significantly compared to CuCr2O4 phase indicating CuCrO2 phase is selectively activated. Mean particle size (d50) decreased from 9.4 μm to 1.7 μm by milling for 5 h. About 63% increase in surface area due to milling is observed which may be due to generation of new surfaces. 3.2. Extraction of Cu
Fig. 2. Leaching response of non-activated catalyst. (a) Effect of acid concentration at ambient temperature. (b) Effect of temperature at 3.75 M H2SO4.
3.2.1. Leaching of un-milled sample Sulfuric acid is the most widely used acid for base metals recovery (Silva et al., 2005) because it is comparatively cheap amongst the mineral acids and is easily regenerated during the electrolysis step. In the present study also H2SO4 was used as leaching agent for recovering copper values. Concentration of sulfuric acid was varied at ambient temperature from 0.94 M to 3.75 M keeping the S:L ratio constant at 50 g/L. It is observed from Fig. 2a that there is no significant effect of acid concentration on overall copper extraction beyond 1.88 M. Higher initial acid concentration yields higher copper recovery in the initial period. It is also observed that time has no significant effect on copper extraction beyond 30 min after which increase in extraction is minimal. Maximum recovery of copper is about 55% after 2 h leaching period with H2SO4 concentration of 1.88 M and above. Cr extraction varied from 1.15 to 3.44% over the ranges of acid concentration studied. In order to study the effect of leaching temperature on copper extraction temperature was varied in the range of 60–150 °C while keeping the acid concentration and S:L ratio constant at 3.75 M and 50 g/L. It is observed that increase in temperature also has not increased the overall Cu extraction appreciably (Fig. 2b). Cu extraction at ambient temperature is 56% which increases to 59% and 67% by increasing the temperature to 80 °C and 150 °C respectively. Maximum Cr recovery of 19.2% was observed at 150 °C. Low recovery of Cr was observed by many investigators (Geveci et al., 2002; Vardar et al., 1994) during sulfuric acid leaching of chromite (FeO.Cr2O3) without
any perchloric acid addition. Possible reason as explained was the insufficient attack by sulfuric acid on the chromite lattice and inability to oxidize part of the Cr(III) to Cr(VI). Following are the possible reactions during sulfuric acid leaching of Cu–Cr catalyst sample: 2CuCrO2 þ H2 SO4 ¼ CuSO4 þ Cu þ Cr2 O3 þ H2 O
ð1Þ
4CuCrO2 þ 4H2 SO4 þ O2 ¼ 4CuSO4 þ 2Cr2 O3 þ 4H2 O
ð2Þ
CuCr2 O4 þ H2 SO4 ¼ CuSO4 þ Cr2 O3 þ H2 O
ð3Þ
CuCr2 O4 þ 4H2 SO4 ¼ CuSO4 þ Cr2 ðSO4 Þ3 þ 4H2 O
ð4Þ
Since oxidation state of Cu in CuCrO2 is + 1 it can dissolve in two ways: (i) in absence of oxygen copper will be distributed partly as Cu metal and partly as CuSO4. This behavior is similar to leaching of Cu2O (Majima et al., 1989; Wadsworth and Wadia, 1955). Although oxygen gas was not separately introduced in the present study reaction (2) may proceed as long as oxygen availability is there from air. Reaction (4) indicates complete dissolution of solid material which is more probable at high acid concentration and very high reaction temperature because of the spinel structure similar to chromite. Hence
Table 2 Effect of milling time on crystallite size, lattice strain, particle size and surface area. Milling time
0h 3h 5h
Average crystallite size, nm
Average lattice strain, %
CuCr2O4
CuCrO2
CuCr2O4
CuCrO2
93 92.5 90
176 111.7 72.5
1.9 1.9 2.1
0.8 1.2 1.9
Particle size, μm
BET surface area, m2/g
9.4 2.6 1.7
31.0 32.0 50.5
S. Swaroopa et al. / Hydrometallurgy 136 (2013) 8–14
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Fig. 3. Effect of mechanical activation on Cu extraction. [Conditions: 1.31 M H2SO4, 80 °C, S:L ratio 20 g/L]. Fig. 5. Effect of leaching temperature on copper extraction from 5 h milled sample. [Conditions: 1.31 M H2SO4, S:L ratio 20 g/L].
more probable reactions in the present investigation are reactions (1), (2) and (3) considering low Cr recovery observed during acid leaching. 3.2.2. Leaching of mechanically activated sample 3.2.2.1. Influence of milling. Fig. 3 shows the results obtained from mechanically activated catalyst powders. It is found that by increasing the milling time copper extraction increases proportionately. Under the leaching conditions of 1.31 M H2SO4, 20 g/L S:L ratio and 80 °C a 3 h milled sample yields 65% Cu extraction in 2 h which is increased to 90% with increase in milling time to 5 h. On the contrary under the same conditions only 47% Cu extraction is feasible from the un-milled sample. Unlike un-milled sample, Cu extraction from milled sample increases with increase in leaching time beyond 30 min. The increased leaching efficiency with the milled sample is possibly due to the surface modification and increase in surface area. Cr extractions from 3 h and 5 h milled samples were 5.2% and 6.3% respectively.
3.2.2.3. Effect of temperature. The effect of leaching temperature was studied at 1.31 M H2SO4 and S:L ratio of 20 g/L. It is apparent from Fig. 5 that temperature has more pronounced effect in the leaching of milled catalyst in contrast to the un-milled samples. By increasing the temperature from ambient to 80 °C Cu recovery increases from 54% to nearly 90% whereas in the case of un-milled sample recovery increase was from 57 to 60% (Fig. 2b) even after keeping the acid to catalyst amount slightly more for the latter (6.4 and 7.3 for milled and un-milled respectively).
3.2.2.2. Effect of H2SO4 concentration. In order to study this effect sulfuric acid concentration was varied from 0.56 M to 1.31 M while maintaining the leaching temperature at 80 °C and solid to liquid ratio 20 g/L. The results obtained are shown in Fig. 4. It is evident that mechanical activation has significantly improved overall copper recovery. By increasing acid concentration from 0.56 M to 1.31 M copper recovery increases from 80% to nearly 90% after 2 h. By varying acid concentration from 0.56 to 1.31 M chromium extractions varied from 3.65% to 6.3%.
3.2.2.4. Leach residue characterization. XRD patterns of two typical leach residues with different extent of Cu extraction are shown in Fig. 6. It is evident from Fig. 6a that XRD patterns of un-milled sample leach residue are nearly same as the original un-milled sample (Fig. 1a) except some reduction in peak intensity of the corresponding phases which is quite obvious due to low Cu extraction (~47%). Whereas the XRD patterns of 5 h milled sample leach residue (Fig. 6b) indicate nearly complete disappearance of CuCrO2 peaks besides appearance of Cr2O3 and elemental Cu peaks. More reactivity of CuCrO2 phase (Fig. 6b) supports the observation (Sec. 3.1) that this particular phase is preferentially mechanically activated. Although un-milled sample leach residue also indicates the presence of Cr2O3 peaks but peak intensity is comparatively low due to dominance of other two phases CuCr2O4 and CuCrO2.
Fig. 4. Effect of acid concentration on copper extraction milled sample. [Conditions: (a) 80 °C temperature, 2% pulp density, 5 h milled sample].
Fig. 6. XRD patterns of leach residues. (a) Un-milled sample (47% Cu recovery). (b) 5 h milled sample (84% Cu recovery).
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90
100 95
Milled (5h)
90
Milled (3h)
70
% Cr extraction
% Cr extraction
80
Unmilled 60 50 40
85 80 75 70 65
850°C
60
650°C
30 55 20 20
40
60
80
50 30
100
Amount of NaOH (wt%)
3.3. Chromium extraction It has been mentioned in the previous sections that during sulfuric acid leaching of Cu–Cr catalyst Cr extraction is negligible. It implies that chromium has to be converted to some soluble form through pretreatment method. Sodium chromate (Na2CrO4) which is the starting material for all chrome chemicals are produced through high temperature roasting of chromite in the presence of soda ash/alkali (Arslan and Orhan, 1997; Tathavadkar et al., 2003). Oxidation state of chromium in the chromite ore and the Cu–Cr catalyst is same i.e. both contain Cr(III) as oxide. In the present study alkali roasting was adopted as pretreatment method and during NaOH roasting of the Cu–Cr catalyst following probable reactions will occur: 1CuCr2O4 + 4NaOH + 3/2 O2 → 2H2O + 2Na2CrO4 + CuO
(5)
CuCrO2 + 2NaOH + O2 → H2O + Na2CrO4 + CuO
(6)
During the above reactions chromium associated with the catalyst is converted to sodium chromate which is highly soluble in water even at room temperature. On the contrary copper is converted to oxide form which is not soluble in water unless acidified. 3.3.1. Effect of NaOH amount
100 unmilled Milled (3h)
100 wt %NaOH 60 wt %NaOH
Crextraction, %
Milled (5h) 80
90
120
150
180
210
240
270
Time, minute
Fig. 7. Effect of NaOH amount on Cr extraction. [Roasting conditions: 750 °C, 2 h].
90
60
70 60 50
Fig. 9. Effect of roasting time on Cr extraction. NaOH 100 wt%, 5 h milled.
Fig. 7 illustrates the Cr recovery results obtained under varying amount of NaOH while roasting the un-milled and milled catalysts at 750 °C for 2 h. By increasing the NaOH amount from 30 wt% to 60 wt% calculated on the basis of weight of NaOH (in g) per 100 g catalyst chromium extraction from a 5 h milled sample increased from 24% to 57%. Further increase in NaOH amount to 100% resulted in nearly 80% chromium extraction. On the contrary results obtained with alkali roasting of un-milled sample or 3 h milled samples are not very encouraging. By increasing NaOH amount from 40 wt% to 100 wt% Cr recovery increased from 28% to merely 34% and 32% to 54% in case of un-milled and 3 h milled sample respectively. 3.3.2. Effect of roasting temperature Roasting temperature variation was examined at two levels of NaOH amount and the results obtained are depicted in Fig. 8. Roasting time of 2 h was kept constant during these tests. Results indicate strong influence of roasting temperature on Cr recovery. Maximum extraction of Cr with 100 wt % NaOH at 850 °C was 92.3% while roasting at 650 °C yielded merely 71.4% Cr extraction. With 60 wt% NaOH maximum extraction at 850 °C was only 76.7%. Roasting behavior with respect to temperature and NaOH amount shows the same trend for both un-milled and milled samples. Low Cr extraction (45% at 850 °C) in case of un-milled sample establishes the importance of milling for maximum chromium recovery. 3.3.3. Effect of roasting time Effect of roasting time on Cr extraction was carried out with 100 wt% NaOH at two temperatures 650 °C and 850 °C. Fig. 9 indicates that roasting time has more pronounced effect at lower roasting temperature. Chromium recovery at 650 °C increased from 65 to 85% by increasing roasting time from 1 to 4 h while at 850 °C the recovery increased from 87 to 100%. Fig. 10 illustrates the XRD patterns of typical leach residues after Cr extraction through roast–leach process. The un-milled sample leach residue (Fig. 10 a) contains unreacted CuCr2O4 phase besides CuO. Due to maximum 45% Cr recovery most of the CuCr2O4 phase remains unattacked. XRD patterns of milled sample leach residue shown in Fig. 10 b match well with CuO (Tenorite) phase [ICCD 45–0934] which supports the experimental observation of total Cr extraction.
40 30 600
650
700
750
800
850
Roasting temperature,
900
950
0C
Fig. 8. Effect of roasting temperature on Cr extraction at two different NaOH amounts.
3.3.4. Cr recovery from 1st stage leach residue Leach residue obtained after extraction of Cu through acid leaching was tested for Cr extraction through roast–leach method. Acid leaching carried out with 5 h milled sample under the conditions of 1.31 M H2SO4 and 80 °C generated a residue containing about 46.5% Cr and 11.7% Cu. Table 3 summarizes the Cu recovery results obtained from
S. Swaroopa et al. / Hydrometallurgy 136 (2013) 8–14
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Fig. 10. XRD patterns of (a) un-milled roast–leach residue (b) 5 h milled roast–leach residue. Roasting conditions: 850 °C, NaOH 100 wt% and time 4 h.
Table 3 Cr extraction from milled sample vis-à-vis Cu leach residue. Roasting time 2 h. Roasting conditions
might be easier than CuCrO2 or CuCr2O4 to react with NaOH during the roasting, resulting in higher Cr extraction. These results facilitate the integration of Cu and Cr recovery stages.
% Cr recovery
Temperature (°C)
NaOH (wt%)
Milled sample
Cu leach residue
850 850 750 750
60 100 60 100
76.7 92.3 57.5 79.3
86.7 98.3 67.7 87.2
this residue under various roasting conditions. Results indicate that Cr extraction from leach residue is more compared to milled sample under all roasting conditions. This is due to the formation of Cr2O3 during the acid leaching step [as shown in Eqs. (1) to (3)] and the freshly formed Cr2O3
3.4. Conceptual flow-sheet Fig. 11 depicts the conceptual flow-sheet for the treatment of spent Cu–Cr catalyst. The acid leach liquor generated in the 1st stage leaching can be recycled for increasing the Cu concentration in the leach liquor to the required level. After the roast–leach step the residue generated is nearly pure CuO which can be recycled to the 1st stage leaching. A preliminary test on the electrolysis of a typical acid leach liquor (Cu 25 g/L and Cr 0.9 g/L) carried out under the conditions: Current density 200 A/m2, Current 0.66 A, 2 h resulted in a smooth
Fig. 11. Conceptual flow-sheet for Cu and Cr extraction from Cu–Cr spent catalyst.
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S. Swaroopa et al. / Hydrometallurgy 136 (2013) 8–14
deposit of Cu metal of purity 99.1%. Estimated current efficiency was 100% and energy consumption 0.9 kWh/kg. From the 2nd stage leach liquor sodium chromate can be recovered by standard techniques such as evaporation and crystallization (Siemens et al., 1986). However no attempt was made in this study to optimize the conditions for this step. 4. Conclusions Due to high metal content spent Cu–Cr catalyst can be a potential source for Cu and Cr extraction. These catalysts mainly consist of cupric chromite (CuCr2O4) and cuprous chromite (CuCrO2). Sulfuric acid leaching is feasible at 80 °C for selective Cu extraction but recovery is comparatively low (about 60%) without mechanical activation. Mechanical activation through high energy ball milling selectively activates CuCrO2 phase resulting in enhanced Cu recovery. Milled catalyst samples can yield 90% Cu recovery in 2 h at 80 °C using 1.31 M H2SO4. Chromium extraction as soluble chromate is feasible by employing NaOH roasting followed by water leaching. A 5 h milled sample roasted with 100 wt% NaOH at 850 °C for 2 h yields 92% Cr extraction whereas for un-milled sample Cr recovery is only 37%, which indicates the significance of mechanical activation for Cr recovery also. Chromium recovery from leach residue (after Cu recovery) is more in comparison to from milled sample as such. Acknowledgements Authors are thankful to Prof. B.K. Mishra, Director, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar for his kind permission to publish this paper. Authors are also thankful to Shri K. Jaysankar, Scientist, IMMT for his help in the mechanical activation work and to Dr. S. Anwar, Scientist, for helpful discussion regarding XRD results. Financial assistance for this work received from Council of Scientific and Industrial Research (CSIR), New Delhi in the form of Major Laboratory Project (MLP-17) is gratefully acknowledged. References Antony, M.P., Jha, A., Tathavadkar, V.D., 2006. Alkali roasting of Indian chromite ores: thermodynamic and kinetic consideration. Miner. Process. Ext. Metall. 115 (2), 71–80. Arslan, C., Orhan, G., 1997. Investigation of chrome (VI) oxide production from chromite concentrate by alkali fusion. Int. J. Miner. Process. 50, 87–96. Balaz, P., 2003. Mechanical activation in hydrometallurgy. Int. J. Miner. Process. 72, 341–354. Barnicki, S.D., Gustafson, B.L., Liu, Z., Perri, S.T., Worsham, P.R., 2009. Palladium-copper chromite hydrogenation catalyst. US Patent 7538060 B2. Cheng, W.H., 1995. Deactivation and regeneration of Cu/Cr based methanol decomposition catalysts. Appl. Catal. B Environ. 7 (1), 127–136. Corrigan, P.J., King, R.M., Vandiest, S.A., 1985. Regeneration of copper chromite hydrogenation catalyst. US Patent 4, 533–648. Ficeriova, J., Balaz, P., Boldizarova, E., 2002. Thiosulfate leaching of gold from a mechanically activated CuPbZn concentrate. Hydrometallurgy 67, 37–43.
Geveci, A., Topkaya, Y., Ayhan, E., 2002. Sulfuric acid leaching of Turkish chromite concentrate. Miner. Eng. 15, 885–888. Habermann, C.E., 1972. Regeneration of copper oxide and copper chromite catalysts. US Patent 3, 645–913. Hashemzadehfini, M., Ficeriova, J., Abkhoshk, E., Shahraki, B.K., 2011. Effect of mechanical activation on thiosulphate leaching of gold from complex sulfide concentrate. Trans. Nonferrous Met. Soc. China 21, 2744–2751. Huang, X., Cant, N.W., Wainwright, M.S., Ma, L., 2005. The dehydrogenation of methanol to methyl formate - Part I: Kinetic studies using copper-based catalysts. Chem. Eng. Process. 44 (3), 393–402. Li, W., Cheng, H., 2007. Cu-Cr-O nanocomposites: synthesis and characterization as catalysts for solid state propellants. Solid State Sci. 9 (8), 750–755. Li, Z., Flytzani-Stephanopoulos, M., 1997. Cu-Cr-O and Cu-Ce-O regenerable oxide sorbents for hot gas desulfurization. Ind. Eng. Chem. Res. 36, 187–196. Majima, H., Awakura, Y., Enami, K., Ueshima, H., Hirato, T., 1989. Kinetic study of the dissolution of cuprite in oxyacid solutions. Metall. and Materi. Trans. B. 20 (5), 573–580. Prasad, R., Singh, P., 2012. A review on CO oxidation over copper chromite catalyst. Catal. Rev. Sci. Eng. 54, 224–279. Sasikumar, C., Rao, D.S., Srikanth, S., Ravikumar, B., Mukhopadhyay, N.K., Mehrotra, S.P., 2004. Effect of mechanical activation on the kinetics of sulfuric acid leaching of beach sand ilmenite from Orissa, India. Hydrometallurgy 75, 189–204. Siemens, R.E., Jong, B.W., Russel, J.H., 1986. Potential of spent catalysts as a source of critical metals. Conserv. Recycl. 9 (2), 189–196. Silva, L.J., Bray, L.A., Frye, J.G., Buehler, M.F., 1994. Spent catalyst processing with electrochemistry. Presented at The Eighth International Forum on Electrolysis in the Chemical Industry (Nov. 13–17, Florida, USA). Silva, J.E., Soares, D., Paiva, A.P., Labrincha, J.A., Castro, F., 2005. Leaching behaviour of a galvanic sludge in sulphuric acid and ammoniacal media. J. Hazard. Mater. 121 (1–3), 195–202. Tathavadkar, V.D., Antony, M.P., Jha, A., 2003. The effect of salt-phase composition on the rate of soda-ash roasting of chromite ores. Metall. Mater. Trans. 34B, 555–563. Trimm, D.L., 2001. The regeneration or disposal of deactivated heterogeneous catalysts. Appl. Catal. Gen. 212, 153–160. Vafaeian, S., Ahmadian, M., Rezaei, B., 2011. Sulphuric acid leaching of mechanically activated copper sulphidic concentrate. Miner. Eng. 24, 1713–1716. Valdes-Solis, T., Marban, G., Fuertes, A.B., 2006. Nanosized catalysts for the production of hydrogen by methanol steam reforming. Catal. Today 116, 354–360. Vardar, E., Eric, R.H., Letowski, F.K., 1994. Acid leaching of chromite. Miner. Eng. 7, 605–617. Vogel, A.I., 1956. A Text Book of Practical Organic Chemistry, 3rd ed. Longman, London. Vogel, A.I., 1961. A Text-Book of Quantitative Inorganic Analysis, 3rd ed. Longman, London. Wadsworth, M.E., Wadia, D.R., 1955. Reaction rate study of the dissolution of cuprite in sulfuric acid. J. Met. 7 (6), 755–759. Wei, L., Hu, H., Chen, Q., Tan, J., 2009. Effects of mechanical activation on the HCl leaching behavior of plagioclase, ilmenite and their mixtures. Hydrometallurgy 99, 39–44. Welham, N.J., 2001. Enhanced dissolution of tantalite/columbite following milling. Int. J. Miner. Process. 61, 145–154. Welham, N.J., Llewellyn, D.J., 1998. Mechanical enhancement of the dissolution of ilmenite. Miner. Eng. 11 (9), 827–841. Yarkadaş, G., Yildiz, K., 2009. Effects of mechanical activation on soda roasting of chromite. Can. Metall. Q. 48 (1), 69–72. Zhang, Y., Li, Z.h, Qi, T., Wang, Z.K., Zheng, S.L., 1999. Green chemistry of chromate cleaner production. Chin. J. Chem. 17 (3), 258–266. Zhang, Y., Zheng, S., Du, H., Xu, H., Zhang, Y., 2010. Effect of mechanical activation on alkali leaching of chromite ore. Trans. Nonferrous Met. Soc. China 20, 888–891. Zhao, Z., Zhang, Y., Chen, X., Chen, A., Huo, G., 2009. Effect of mechanical activation on the leaching kinetics of pyrrhotite. Hydrometallurgy 99, 105–108.
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Extraction of Cu and Cr from a spent Cu–Cr catalyst: Recovery enhancement through mechanical activation Siksha Swaroopa, Malay Kumar Ghosh ⁎, Kali Sanjay, Barada Kanta Mishra CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India
a r t i c l e
i n f o
Article history: Received 17 August 2012 Received in revised form 11 February 2013 Accepted 2 March 2013 Available online 14 March 2013 Keywords: Spent catalyst Chromium Leaching Mechanical activation Roasting
a b s t r a c t Treatment process for a spent Cu–Cr catalyst (Cu 42.4% and 36.2% Cr) was described. The catalyst contained cuprous chromite (CuCrO2), cupric chromite (CuCr2O4) and copper sub-oxide (Cu8O) phases. High energy ball milling reduced the mean particle size from 9.4 to 1.7 μm along with simultaneous increase in surface area by about 63%. Cuprous chromite phase is selectively mechanically activated. Sulfuric acid leaching and alkali roasting followed by water leaching techniques were employed for Cu and Cr recovery respectively. Un-milled catalyst showed maximum 67% Cu extraction by varying H2SO4 concentration (0.94 M–3.75 M) and leaching temperature (ambient - 150 °C) whereas a 5 h milled sample resulted in 90% Cu extraction under the conditions of 1.31 M H2SO4, 20 g/L S:L ratio, 80 °C. Cr recovery was examined by varying NaOH amount, roasting temperature and time. More than 90% Cr extraction was achieved when a milled sample was roasted with 100 wt% NaOH at 850 °C for 2 h. Cr recovery from the Cu-leached residue was comparatively more than that obtained from raw milled sample. The leach residue after Cr recovery was nearly pure CuO which can be recycled for Cu recovery. A conceptual flow-sheet for the recovery of both metals was described. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The Cu–Cr composite oxides are widely used as catalyst for hydrogenation, dehydrogenation, alkylation etc. (Barnicki et al., 2009; Huang et al., 2005; Valdes-Solis et al., 2006). These catalysts are generally composed of equi-molar combination of cupric chromite and cupric oxide and not merely mechanical mixture of these compounds (Vogel, 1956). These composite oxides are also used for complete oxidation of carbon monoxide and hydrocarbons to carbon dioxide and for the simultaneous removal of nitric oxide and carbon monoxide from exhaust gas (Prasad and Singh, 2012). Besides the above commercial applications they find a great promising role in the composite solid propellants to enhance the burning rate of the propellant (Li and Cheng, 2007). Like any other catalyst it also gradually loses its catalytic activity due to various reasons such as coking, poisoning by sulfur, metals etc. or loss of surface area due to sintering effects at high process temperature. Catalyst regeneration is possible up to certain degree and cycles depending upon the degree of irreversible deactivation. However, when regeneration is not economically feasible catalysts are discarded and termed as spent catalysts (Trimm, 2001). Due to hazardous nature and restricted disposal option recovery of metal values from these catalysts is a suitable alternative considering economic and environmental points of view. So far as
⁎ Corresponding author. Tel.: +91 674 2379374; fax: +91 674 2581637. E-mail address: [email protected] (M.K. Ghosh). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.03.001
Cu–Cr catalysts are concerned past attention has been focused on the regeneration only (Cheng, 1995; Corrigan et al., 1985; Habermann, 1972) and no attempt has been made so far to recover the metal values. Silva et al. (1994) reported an electrochemical treatment process using Ce(IV) as oxidizer for complete dissolution of copper chromite spent catalyst with an objective to use the solution for fresh catalyst preparation. It was also mentioned that processing of spent copper chromite catalyst through any conventional recovery method is not economical due to insoluble nature of the catalyst even in concentrated acid or alkali. Considering the spent Cu–Cr catalysts as potential secondary resource copper and chromium values can be recovered through a hydrometallurgical and/or pyrometallurgical route. Traditionally chromium is extracted from the chromite ore by soda ash roasting at 1200 °C by converting trivalent chromium oxide to water soluble hexavalent chromate (Antony et al., 2006). A novel alternative process was developed by Zhang et al. (1999) which comprises of continuous oxidation of chromite in molten NaOH, one-way separation in highly concentrated alkali solution, meta-stable phase separation and carbonate recycle conversion of sodium chromate to sodium dichromate or chromic acid. The advantage of the process being almost zero waste and waste is free from toxic Cr(VI) unlike the conventional processing. Mechanical activation is now a widely accepted methodology to increase the leaching rate of minerals (Balaz, 2003). Mechanical activation is generally carried out through high energy ball milling process in which the particles go through repeated fracturing and cold welding during collisions either between the balls or the ball and the container inner wall. Several past studies indicate that mechanical activation significantly accelerates
S. Swaroopa et al. / Hydrometallurgy 136 (2013) 8–14
the dissolution rate of solid minerals like ilmenite, tantalite, pyrrhotite, complex sulfide, copper sulfide etc. by increasing their chemical reactivity (Ficeriova et al., 2002; Hashemzadehfini et al., 2011; Sasikumar et al., 2004; Vafaeian et al., 2011; Wei et al., 2009; Welham, 2001; Welham and Llewellyn, 1998; Zhao et al., 2009). Yarkadaş and Yildiz (2009) studied the effect of mechanical activation on soda roasting of chromite and could improve the conversion from 18% (non-activated chromite) to 74% (120 min activated chromite) while roasting at 800 °C. Zhang et al. (2010) reported the alkali leaching of mechanically activated chromite and it was observed that leaching efficiency of non-activated chromite was only 34% while a 10 min mechanically activated sample showed 97% Cr extraction. One of the main reasons for the increased reactivity after mechanical activation is the combination of new surface area and formation of crystalline disorder in minerals. The literature survey reveals that although spent Cu–Cr catalysts contain high amount of copper surprisingly not much attention has been directed in the past towards metal recovery from this source. In the present investigation preliminary assessment has been made to recover both the metals - copper through H2SO4 leaching and chromium through NaOH roasting followed by water leaching. Mechanical activation technique has been employed for better metal recovery and material characterization through different instrumental techniques has been carried out also for both unactivated and mechanically activated samples. Effects of different leaching parameters and roasting conditions were investigated. Spent catalysts and leach residues were characterized through X-ray diffractometry (XRD) to corroborate the leaching results. Ultimately a tentative flow sheet is proposed for total metal recovery from the spent catalyst.
9
Acid leaching experiments were carried out in a 250 mL doublewall cylindrical glass reactor. The reactor lid had a provision for reflux condenser and sampling port. Hot water from a constant temperature water bath was circulated between the walls of the reactor to achieve the required temperature in the leaching medium. The agitation was accomplished using a magnetic paddle by keeping the glass reactor over a magnetic stirrer. At the required temperature catalyst powders were added and leaching continued for 2 h. At different intervals slurry sample was collected, filtered and analyzed for Cu and Cr. Leaching at higher temperature (150 °C) was performed in Amar® 2 L capacity Ti-autoclave. Copper concentration in the leach liquor was estimated volumetrically by standard iodometric method (Vogel, 1961) using starch as indicator. Chromium in leach liquor was analyzed by Perkin-Elmer-2380 Atomic Absorption Spectrophotometer. 2.4. Characterization The XRD of the un-milled, milled and leach residue samples were performed using a Phillips Powder Diffractometer (Model PAN ANALYTICAL PW 1830) in the range of 5–70° (2θ) at a scanning rate of 2°/min with Cu - Kα radiation. Surface areas of the un-milled and milled catalyst powders were measured with Quantachrome® surface area analyzer (Model – Autosorb-iQ) using liquid nitrogen. Particle size measurements of the ground and milled catalyst powders were carried out using CILAS® 1064 Particle Size Analyzer. 3. Results and discussion 3.1. Characterization of the catalyst
2. Experimental 2.1. Material The copper chromium spent catalyst used in this study was crushed, ground and screened to provide material with a particle size of b100 μm. The elemental composition of the ground sample is given in Table 1. All reagents used in the study were in the analytical reagent grade. 2.2. Mechanical activation For mechanical activation work a twin-bowl INSMART planetary ball mill (Model PBM-07) of overall capacity 500 mL and useful capacity 250 mL was used. Milling was carried out at 300 rpm using 10 mm diameter stainless steel balls keeping the ball to powder weight ratio at 10:1. Toluene was used as process control agent to avoid any reaction of the milled powder with open atmosphere and also to minimize the cold welding of powders to vial and balls.
The XRD patterns of the un-milled and milled spent catalysts (Fig. 1) contain six diffraction peaks at (006), (012), (104), (018) and (110) planes corresponding to the typical diffraction peaks of cuprous chromite (CuCrO2, ICCD 39–0247), a delafossite group compound where Cu is in +1 oxidation state and Cr in +3 oxidation state. The peaks corresponding to (101) and (211) planes are characteristic of cupric chromite (CuCr2O4) peaks (ICCD 88–0110), a spinel structure. Besides the above two phases a sub-oxide of copper (Cu8O) phase is also observed corresponding to the planes of (004) and (220) respectively (ICCD 78–1588). Depending upon the Cu:Cr ratio and calcination temperature adopted during the catalyst synthesis procedure constituent phases may vary but generally for equi-molar ratio of CuO and Cr2O3 cupric chromite (CuCr2O4) is only observed (Li and Cheng, 2007; Li and Flytzani-Stephanopoulos, 1997). Since a Cu–Cr catalyst undergoes
2.3. Roasting and leaching procedure Required amounts of spent catalyst powders and sodium hydroxide pellets were mixed properly in a quartz crucible and the crucible was put inside a muffle furnace. The sample was heated to the required temperature and roasting was carried out for a prefixed time followed by stepwise cooling. The roasted mass was then transferred from the crucible to a glass reactor along with measured amount of distilled water followed by room temperature leaching for about 30 min.
Table 1 Elemental composition of the Cu–Cr spent catalyst. Cu
Cr
Mn
Ni
Co
Zn
42.4%
36.2%
2.4%
0.77%
0.08%
0.12%
Fig. 1. XRD patterns of (a) un-milled (b) 3 h milled (c) 5 h milled samples.
10
S. Swaroopa et al. / Hydrometallurgy 136 (2013) 8–14
through high process temperature coupled with oxidizing/reducing environment hence phase changes may occur and constituent phases of the spent catalyst may be different from the virgin catalyst. XRD patterns of 3 h and 5 h milled samples (Fig. 1b and c respectively) with comparatively wider and weaker diffraction peaks are indicative of finer particle size caused due to milling. The crystal lattice strain (ε) and the crystallite size (D) can be calculated from the line broadening of the peaks using Scherrer's formula (Eq. (1)): 2
Bt ¼
h
i2
0:9λ D cosθ
2
2
þ ½4ε tan θ þ B0
ð1Þ
Where, Bt is the full width at half maximum intensity of the peak, λ is the wavelength of the radiation used, D is the average crystallite size, ε is the strain, θ is the diffraction angle and B0 is the instrumental line broadening. Table 2 summarizes the milling effect on crystallite size (D), lattice strain (ɛ), surface area and mean particle size (d50). Cystallite size and lattice strains were calculated using X'pert HighScore Plus® (Ver. 3.0d) software. It is observed from Table 2 that with increase in milling time average crystallite size of CuCrO2 decreases and lattice strain increases significantly compared to CuCr2O4 phase indicating CuCrO2 phase is selectively activated. Mean particle size (d50) decreased from 9.4 μm to 1.7 μm by milling for 5 h. About 63% increase in surface area due to milling is observed which may be due to generation of new surfaces. 3.2. Extraction of Cu
Fig. 2. Leaching response of non-activated catalyst. (a) Effect of acid concentration at ambient temperature. (b) Effect of temperature at 3.75 M H2SO4.
3.2.1. Leaching of un-milled sample Sulfuric acid is the most widely used acid for base metals recovery (Silva et al., 2005) because it is comparatively cheap amongst the mineral acids and is easily regenerated during the electrolysis step. In the present study also H2SO4 was used as leaching agent for recovering copper values. Concentration of sulfuric acid was varied at ambient temperature from 0.94 M to 3.75 M keeping the S:L ratio constant at 50 g/L. It is observed from Fig. 2a that there is no significant effect of acid concentration on overall copper extraction beyond 1.88 M. Higher initial acid concentration yields higher copper recovery in the initial period. It is also observed that time has no significant effect on copper extraction beyond 30 min after which increase in extraction is minimal. Maximum recovery of copper is about 55% after 2 h leaching period with H2SO4 concentration of 1.88 M and above. Cr extraction varied from 1.15 to 3.44% over the ranges of acid concentration studied. In order to study the effect of leaching temperature on copper extraction temperature was varied in the range of 60–150 °C while keeping the acid concentration and S:L ratio constant at 3.75 M and 50 g/L. It is observed that increase in temperature also has not increased the overall Cu extraction appreciably (Fig. 2b). Cu extraction at ambient temperature is 56% which increases to 59% and 67% by increasing the temperature to 80 °C and 150 °C respectively. Maximum Cr recovery of 19.2% was observed at 150 °C. Low recovery of Cr was observed by many investigators (Geveci et al., 2002; Vardar et al., 1994) during sulfuric acid leaching of chromite (FeO.Cr2O3) without
any perchloric acid addition. Possible reason as explained was the insufficient attack by sulfuric acid on the chromite lattice and inability to oxidize part of the Cr(III) to Cr(VI). Following are the possible reactions during sulfuric acid leaching of Cu–Cr catalyst sample: 2CuCrO2 þ H2 SO4 ¼ CuSO4 þ Cu þ Cr2 O3 þ H2 O
ð1Þ
4CuCrO2 þ 4H2 SO4 þ O2 ¼ 4CuSO4 þ 2Cr2 O3 þ 4H2 O
ð2Þ
CuCr2 O4 þ H2 SO4 ¼ CuSO4 þ Cr2 O3 þ H2 O
ð3Þ
CuCr2 O4 þ 4H2 SO4 ¼ CuSO4 þ Cr2 ðSO4 Þ3 þ 4H2 O
ð4Þ
Since oxidation state of Cu in CuCrO2 is + 1 it can dissolve in two ways: (i) in absence of oxygen copper will be distributed partly as Cu metal and partly as CuSO4. This behavior is similar to leaching of Cu2O (Majima et al., 1989; Wadsworth and Wadia, 1955). Although oxygen gas was not separately introduced in the present study reaction (2) may proceed as long as oxygen availability is there from air. Reaction (4) indicates complete dissolution of solid material which is more probable at high acid concentration and very high reaction temperature because of the spinel structure similar to chromite. Hence
Table 2 Effect of milling time on crystallite size, lattice strain, particle size and surface area. Milling time
0h 3h 5h
Average crystallite size, nm
Average lattice strain, %
CuCr2O4
CuCrO2
CuCr2O4
CuCrO2
93 92.5 90
176 111.7 72.5
1.9 1.9 2.1
0.8 1.2 1.9
Particle size, μm
BET surface area, m2/g
9.4 2.6 1.7
31.0 32.0 50.5
S. Swaroopa et al. / Hydrometallurgy 136 (2013) 8–14
11
Fig. 3. Effect of mechanical activation on Cu extraction. [Conditions: 1.31 M H2SO4, 80 °C, S:L ratio 20 g/L]. Fig. 5. Effect of leaching temperature on copper extraction from 5 h milled sample. [Conditions: 1.31 M H2SO4, S:L ratio 20 g/L].
more probable reactions in the present investigation are reactions (1), (2) and (3) considering low Cr recovery observed during acid leaching. 3.2.2. Leaching of mechanically activated sample 3.2.2.1. Influence of milling. Fig. 3 shows the results obtained from mechanically activated catalyst powders. It is found that by increasing the milling time copper extraction increases proportionately. Under the leaching conditions of 1.31 M H2SO4, 20 g/L S:L ratio and 80 °C a 3 h milled sample yields 65% Cu extraction in 2 h which is increased to 90% with increase in milling time to 5 h. On the contrary under the same conditions only 47% Cu extraction is feasible from the un-milled sample. Unlike un-milled sample, Cu extraction from milled sample increases with increase in leaching time beyond 30 min. The increased leaching efficiency with the milled sample is possibly due to the surface modification and increase in surface area. Cr extractions from 3 h and 5 h milled samples were 5.2% and 6.3% respectively.
3.2.2.3. Effect of temperature. The effect of leaching temperature was studied at 1.31 M H2SO4 and S:L ratio of 20 g/L. It is apparent from Fig. 5 that temperature has more pronounced effect in the leaching of milled catalyst in contrast to the un-milled samples. By increasing the temperature from ambient to 80 °C Cu recovery increases from 54% to nearly 90% whereas in the case of un-milled sample recovery increase was from 57 to 60% (Fig. 2b) even after keeping the acid to catalyst amount slightly more for the latter (6.4 and 7.3 for milled and un-milled respectively).
3.2.2.2. Effect of H2SO4 concentration. In order to study this effect sulfuric acid concentration was varied from 0.56 M to 1.31 M while maintaining the leaching temperature at 80 °C and solid to liquid ratio 20 g/L. The results obtained are shown in Fig. 4. It is evident that mechanical activation has significantly improved overall copper recovery. By increasing acid concentration from 0.56 M to 1.31 M copper recovery increases from 80% to nearly 90% after 2 h. By varying acid concentration from 0.56 to 1.31 M chromium extractions varied from 3.65% to 6.3%.
3.2.2.4. Leach residue characterization. XRD patterns of two typical leach residues with different extent of Cu extraction are shown in Fig. 6. It is evident from Fig. 6a that XRD patterns of un-milled sample leach residue are nearly same as the original un-milled sample (Fig. 1a) except some reduction in peak intensity of the corresponding phases which is quite obvious due to low Cu extraction (~47%). Whereas the XRD patterns of 5 h milled sample leach residue (Fig. 6b) indicate nearly complete disappearance of CuCrO2 peaks besides appearance of Cr2O3 and elemental Cu peaks. More reactivity of CuCrO2 phase (Fig. 6b) supports the observation (Sec. 3.1) that this particular phase is preferentially mechanically activated. Although un-milled sample leach residue also indicates the presence of Cr2O3 peaks but peak intensity is comparatively low due to dominance of other two phases CuCr2O4 and CuCrO2.
Fig. 4. Effect of acid concentration on copper extraction milled sample. [Conditions: (a) 80 °C temperature, 2% pulp density, 5 h milled sample].
Fig. 6. XRD patterns of leach residues. (a) Un-milled sample (47% Cu recovery). (b) 5 h milled sample (84% Cu recovery).
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S. Swaroopa et al. / Hydrometallurgy 136 (2013) 8–14
90
100 95
Milled (5h)
90
Milled (3h)
70
% Cr extraction
% Cr extraction
80
Unmilled 60 50 40
85 80 75 70 65
850°C
60
650°C
30 55 20 20
40
60
80
50 30
100
Amount of NaOH (wt%)
3.3. Chromium extraction It has been mentioned in the previous sections that during sulfuric acid leaching of Cu–Cr catalyst Cr extraction is negligible. It implies that chromium has to be converted to some soluble form through pretreatment method. Sodium chromate (Na2CrO4) which is the starting material for all chrome chemicals are produced through high temperature roasting of chromite in the presence of soda ash/alkali (Arslan and Orhan, 1997; Tathavadkar et al., 2003). Oxidation state of chromium in the chromite ore and the Cu–Cr catalyst is same i.e. both contain Cr(III) as oxide. In the present study alkali roasting was adopted as pretreatment method and during NaOH roasting of the Cu–Cr catalyst following probable reactions will occur: 1CuCr2O4 + 4NaOH + 3/2 O2 → 2H2O + 2Na2CrO4 + CuO
(5)
CuCrO2 + 2NaOH + O2 → H2O + Na2CrO4 + CuO
(6)
During the above reactions chromium associated with the catalyst is converted to sodium chromate which is highly soluble in water even at room temperature. On the contrary copper is converted to oxide form which is not soluble in water unless acidified. 3.3.1. Effect of NaOH amount
100 unmilled Milled (3h)
100 wt %NaOH 60 wt %NaOH
Crextraction, %
Milled (5h) 80
90
120
150
180
210
240
270
Time, minute
Fig. 7. Effect of NaOH amount on Cr extraction. [Roasting conditions: 750 °C, 2 h].
90
60
70 60 50
Fig. 9. Effect of roasting time on Cr extraction. NaOH 100 wt%, 5 h milled.
Fig. 7 illustrates the Cr recovery results obtained under varying amount of NaOH while roasting the un-milled and milled catalysts at 750 °C for 2 h. By increasing the NaOH amount from 30 wt% to 60 wt% calculated on the basis of weight of NaOH (in g) per 100 g catalyst chromium extraction from a 5 h milled sample increased from 24% to 57%. Further increase in NaOH amount to 100% resulted in nearly 80% chromium extraction. On the contrary results obtained with alkali roasting of un-milled sample or 3 h milled samples are not very encouraging. By increasing NaOH amount from 40 wt% to 100 wt% Cr recovery increased from 28% to merely 34% and 32% to 54% in case of un-milled and 3 h milled sample respectively. 3.3.2. Effect of roasting temperature Roasting temperature variation was examined at two levels of NaOH amount and the results obtained are depicted in Fig. 8. Roasting time of 2 h was kept constant during these tests. Results indicate strong influence of roasting temperature on Cr recovery. Maximum extraction of Cr with 100 wt % NaOH at 850 °C was 92.3% while roasting at 650 °C yielded merely 71.4% Cr extraction. With 60 wt% NaOH maximum extraction at 850 °C was only 76.7%. Roasting behavior with respect to temperature and NaOH amount shows the same trend for both un-milled and milled samples. Low Cr extraction (45% at 850 °C) in case of un-milled sample establishes the importance of milling for maximum chromium recovery. 3.3.3. Effect of roasting time Effect of roasting time on Cr extraction was carried out with 100 wt% NaOH at two temperatures 650 °C and 850 °C. Fig. 9 indicates that roasting time has more pronounced effect at lower roasting temperature. Chromium recovery at 650 °C increased from 65 to 85% by increasing roasting time from 1 to 4 h while at 850 °C the recovery increased from 87 to 100%. Fig. 10 illustrates the XRD patterns of typical leach residues after Cr extraction through roast–leach process. The un-milled sample leach residue (Fig. 10 a) contains unreacted CuCr2O4 phase besides CuO. Due to maximum 45% Cr recovery most of the CuCr2O4 phase remains unattacked. XRD patterns of milled sample leach residue shown in Fig. 10 b match well with CuO (Tenorite) phase [ICCD 45–0934] which supports the experimental observation of total Cr extraction.
40 30 600
650
700
750
800
850
Roasting temperature,
900
950
0C
Fig. 8. Effect of roasting temperature on Cr extraction at two different NaOH amounts.
3.3.4. Cr recovery from 1st stage leach residue Leach residue obtained after extraction of Cu through acid leaching was tested for Cr extraction through roast–leach method. Acid leaching carried out with 5 h milled sample under the conditions of 1.31 M H2SO4 and 80 °C generated a residue containing about 46.5% Cr and 11.7% Cu. Table 3 summarizes the Cu recovery results obtained from
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Fig. 10. XRD patterns of (a) un-milled roast–leach residue (b) 5 h milled roast–leach residue. Roasting conditions: 850 °C, NaOH 100 wt% and time 4 h.
Table 3 Cr extraction from milled sample vis-à-vis Cu leach residue. Roasting time 2 h. Roasting conditions
might be easier than CuCrO2 or CuCr2O4 to react with NaOH during the roasting, resulting in higher Cr extraction. These results facilitate the integration of Cu and Cr recovery stages.
% Cr recovery
Temperature (°C)
NaOH (wt%)
Milled sample
Cu leach residue
850 850 750 750
60 100 60 100
76.7 92.3 57.5 79.3
86.7 98.3 67.7 87.2
this residue under various roasting conditions. Results indicate that Cr extraction from leach residue is more compared to milled sample under all roasting conditions. This is due to the formation of Cr2O3 during the acid leaching step [as shown in Eqs. (1) to (3)] and the freshly formed Cr2O3
3.4. Conceptual flow-sheet Fig. 11 depicts the conceptual flow-sheet for the treatment of spent Cu–Cr catalyst. The acid leach liquor generated in the 1st stage leaching can be recycled for increasing the Cu concentration in the leach liquor to the required level. After the roast–leach step the residue generated is nearly pure CuO which can be recycled to the 1st stage leaching. A preliminary test on the electrolysis of a typical acid leach liquor (Cu 25 g/L and Cr 0.9 g/L) carried out under the conditions: Current density 200 A/m2, Current 0.66 A, 2 h resulted in a smooth
Fig. 11. Conceptual flow-sheet for Cu and Cr extraction from Cu–Cr spent catalyst.
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deposit of Cu metal of purity 99.1%. Estimated current efficiency was 100% and energy consumption 0.9 kWh/kg. From the 2nd stage leach liquor sodium chromate can be recovered by standard techniques such as evaporation and crystallization (Siemens et al., 1986). However no attempt was made in this study to optimize the conditions for this step. 4. Conclusions Due to high metal content spent Cu–Cr catalyst can be a potential source for Cu and Cr extraction. These catalysts mainly consist of cupric chromite (CuCr2O4) and cuprous chromite (CuCrO2). Sulfuric acid leaching is feasible at 80 °C for selective Cu extraction but recovery is comparatively low (about 60%) without mechanical activation. Mechanical activation through high energy ball milling selectively activates CuCrO2 phase resulting in enhanced Cu recovery. Milled catalyst samples can yield 90% Cu recovery in 2 h at 80 °C using 1.31 M H2SO4. Chromium extraction as soluble chromate is feasible by employing NaOH roasting followed by water leaching. A 5 h milled sample roasted with 100 wt% NaOH at 850 °C for 2 h yields 92% Cr extraction whereas for un-milled sample Cr recovery is only 37%, which indicates the significance of mechanical activation for Cr recovery also. Chromium recovery from leach residue (after Cu recovery) is more in comparison to from milled sample as such. Acknowledgements Authors are thankful to Prof. B.K. Mishra, Director, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar for his kind permission to publish this paper. Authors are also thankful to Shri K. Jaysankar, Scientist, IMMT for his help in the mechanical activation work and to Dr. S. Anwar, Scientist, for helpful discussion regarding XRD results. Financial assistance for this work received from Council of Scientific and Industrial Research (CSIR), New Delhi in the form of Major Laboratory Project (MLP-17) is gratefully acknowledged. References Antony, M.P., Jha, A., Tathavadkar, V.D., 2006. Alkali roasting of Indian chromite ores: thermodynamic and kinetic consideration. Miner. Process. Ext. Metall. 115 (2), 71–80. Arslan, C., Orhan, G., 1997. Investigation of chrome (VI) oxide production from chromite concentrate by alkali fusion. Int. J. Miner. Process. 50, 87–96. Balaz, P., 2003. Mechanical activation in hydrometallurgy. Int. J. Miner. Process. 72, 341–354. Barnicki, S.D., Gustafson, B.L., Liu, Z., Perri, S.T., Worsham, P.R., 2009. Palladium-copper chromite hydrogenation catalyst. US Patent 7538060 B2. Cheng, W.H., 1995. Deactivation and regeneration of Cu/Cr based methanol decomposition catalysts. Appl. Catal. B Environ. 7 (1), 127–136. Corrigan, P.J., King, R.M., Vandiest, S.A., 1985. Regeneration of copper chromite hydrogenation catalyst. US Patent 4, 533–648. Ficeriova, J., Balaz, P., Boldizarova, E., 2002. Thiosulfate leaching of gold from a mechanically activated CuPbZn concentrate. Hydrometallurgy 67, 37–43.
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