Rajiv Ranjan Srivastava

Simple recycling of copper by the synergistic exploitation of industrial wastes: a step towards sustainability

A “Green Recycling” process for copper recovery by the synergistic treatment of two different industrial wastes is presented. Firstly, the solid waste generated during the processing of epoxy modeling compounds was treated with the waste nitric acid solution produced in the sputter cleaning. A faster leaching rate of copper without any effect of temperature was noticed, which can be attributed to the intermediate-controlled kinetics along with the autocatalytic behaviour of Cu2+ present in the waste solution. An increase in pulp density above 25% adversely affected the solubilisation of copper because of the decreased lixiviant acidity corresponding to the metals being leached. Subsequently, the key experimental parameters for liquid–liquid separation of copper using LIX 84-I after Fe-removal with D2EHPA, viz. equilibrium pH, extractant concentration, temperature, and phase ratio (O/A), were optimized and correlated with complexation chemistry. The extraction equilibrium constant (log Kex = 1.67) under the studied conditions of nitrate media was determined by the log–log plot of D*[H+]2aqvs. ([HR]2)org Thermodynamic properties indicated a strong affinity of LIX 84-I towards Cu2+ due to the emergence of spontaneous inner-sphere coordination with disrupted hydration. A counter-current study for the separation and recovery of copper was performed under the optimized conditions, which has successfully validated the results of extraction and stripping isotherms exhibited by the McCabe-Thiele plots. The unfolded hydrometallurgical process described in this study has a potential for sustainable utilization of wastes to recover the metal of interest at a 46% lower energy cost than the processing of primary ores.

Efficient recycling of WC-Co hardmetal sludge by oxidation followed by alkali and sulfuric acid treatments

We present a process to recycle strategic metals, viz. tungsten and cobalt, from a WC-Co hardmetal sludge (WCHS) via oxidation followed by a two-step hydrometallurgical treatment with alkali and acid solutions. The oxidation of WCHS was investigated in the temperature range of 500 to 1000 °C and optimized at 600 °C to transform the maximum WC into an alkali-soluble WO3. The conditions for the selective dissolution of WO3 in stage-I were optimized as follows: 4.0 M NaOH, pulp density of 175 g/L, and temperature of 100 °C for 1 h, yielding maximum efficacy. Subsequently, in the second step, the optimal conditions for cobalt leaching from the alkali-treated residue were established as follows: 2.0 M H2SO4, 25 g/L pulp density, and 75 °C temperature for 30 min. Downstream processing of the obtained metal ions in solutions was also easier, as the only impurity of dicobaltite ions with the Na2WO4 solution was precipitated as Co(OH)3 under atmospheric O2; meanwhile, the CoSO4 solution obtained through the second step of processing can be treated via electrolysis to recover the metallic cobalt. The present process is simpler in operation, and the efficient use of eco-friendly lixiviants eliminates the previously reported disadvantage.

Reclamation of tungsten from carbide scraps and spent materials

This paper reviews the state-of-the-art recycling of tungsten from carbide (WC) scraps and other spent alloys generated by various production and application industries. With an aim of direct reuse or chemical recovery of tungsten, the reclamation of WC is commonly divided into three parts: (1) pyrometallurgy, (2) hydrometallurgy, and (3) a combined (pyro + hydro) metallurgical process. The pyrometallurgical process consists of a thermal treatment under an oxidizing, reducing, or carburizing condition and of breaking the structure of hardmetals by dissolving the binder metal in a molten bath to obtain WC from spent/scrap materials. The hydrometallurgical process, based on leaching in acid and/or alkali solutions, follows precipitation/solvent extraction/ion exchange/crystallization operations to concentrate and recover the salt/s of tungsten and associated metals. The combination of both processes is employed mainly to convert the carbide phase of WC (along with the binder and/or additive metals) to their oxide forms prior to leaching in the acid/alkali solution to enhance the extraction efficacy in the aqueous solution. A critical analysis with respect to the processing conditions for extracting tungsten with the binder metal cobalt from various scrap/spent materials is given. The present paper will be helpful in developing an overall understanding of tungsten reclamation from the WC and other alloys that can provide future research directions to obtain the sustainability of this strategically conflict element.

Hydrometallurgical recycling of surface-coated metals from automobile-discarded ABS plastic waste

The ammoniacal leaching of surface-coated metals from automobile-discarded ABS plastics followed by their recovery through solvent extraction has been investigated. The leaching of ABS (typically containing 4.1% Cu, 1.3% Ni, and 0.03% Cr) could efficiently dissolve the ammine complexes of Cu and Ni, leaving Cr unleached as fine particles. The optimization studies for achieving the maximum efficiency revealed that the leaching of metal ions in different ammoniacal solutions follows the order CO32- > Cl- > SO42-. The leaching carried out in a carbonate medium by maintaining the total NH3 concentration 5.0 M at a NH4OH/(NH4)2CO3 ratio of 4:1, pulp density of 200 g/L, agitation speed of 400 rpm, temperature of 20 °C, and time of 120 min yielded the optimum efficiency of >99% Cu and Ni (i.e., 8.14 g/L and 2.57 g/L, respectively, in the leach liquor). Subsequently, the solvent extraction of metals from ammoniacal leach liquor as a function of extractant (LIX 84-I) concentration and organic-to-aqueous (O:A) phase ratio was examined. Based on the extraction data, a three-stage counter-current extraction at O:A = 1:1 was validated using 0.8 M LIX 84-I, yielding the quantitative extraction of both metals into the organic phase. Thereafter, the stripping of metals in acid solutions indicated that 0.5 M H2SO4 could quantitatively strip Ni from the loaded organic phase; however, ∼27% Cu was also co-stripped. The rest of Cu from the Ni-depleted organic phase was separately stripped with 1.0 M H2SO4 that can be directly sent to the electrowinning process. On the other hand, the co-stripped metals from the acidic solution can be easily separated, again using LIX 84-I as the extractant, by adopting the pH-swing method. Finally, a process has been proposed for the hydrometallurgical recovery of surface-coated metals from waste ABS plastics; that does not affect the physicochemical characteristics of the polymer substances for their reuse.

Extraction of the Surface-Coated Metals from Waste Acrylonitrile Butadiene Styrene Plastics in an Ammoniacal Solution

Leaching of the surface-coated metals from waste acrylonitrile butadiene styrene plastics (WABSP) has been investigated in the ammoniacal solution. The ammoniacal leaching of WABSP (typically containing 4.1 wt% Cu, 1.3 wt% Ni, 0.03 wt% Cr) efficiently dissolved copper and nickel, leaving unleached chromium as fine particles. The results indicate that the anions of different buffer media for metals leaching follow the order: CO32− > Cl− > SO42−. A declined leaching with high NH4OH–(NH4)2CO3 ratio (8:1) and a steep rise in extraction with high pulp in solution revealed the influence of anion (CO32−) and cation (Cu2+) present in the system. The leaching performed in carbonate solution by maintaining 5.0 M total NH3 concentration as NH4OH–(NH4)2CO3 ratio, 4:1; pulp density, 200 g/L; agitation speed, 400 rpm; temperature, 20 °C and time, 120 min yielded the maximum extraction (>99%) of copper and nickel. The study reveals a plausible recovery of the surface-coated metals from the WABSP without altering the properties of polymer material that can be recycled separately.

Hydrometallurgical Extraction of Lead in Brine Solution from a TSL Processed Zinc Plant Residue

Brine leaching of a TSL processed zinc-plant residue (ZPR), containing 83.3% anglesite, has been investigated for lead recovery . A prior treatment of ZPR in acid solution did not show any significant effect of acid concentration in zinc dissolution. The study reveals that the solubility limits of PbCl2 is as an important factor with respect to the brine concentration and pulp density in lixiviant. The dissociated sulfate ions hinder the formation of PbCl2 on prolong leaching (above 60 min), while exhibiting the reverse solubility of lead as PbSO4. A 3-step leaching process could yield >92% leaching efficiency when the parameters were maintained as: 10% pulp density in a 250 g/L NaCl solution, 80 °C temperature, and 60 min time. This leaves the insoluble ferrite, sulfides and oxy-sulfates of zinc and lead in the final residue as revealed by the XRD characterization.