Treatment and recovery of nickel rich precipitate from plating plant waste/Nikeliavimo gamyklu atlieku apdorojimas ir nikelio isgryninimas.
The treatment of processed effluents is the most serious environmental issue faced by the processing industries. Wastewater streams may contain heavy metals, organic waste, and oils, including the waste liquids generated by metal finishing or the mineral processing industries. Toxic metals, probably existing in high concentrations, must be effectively treated / removed from the wastewaters (Barnes et al. 1981). However, the disposal process can bring about some difficulties. Besides the environmental sensitivity, the companies are also suffering from an increased disposal cost. This has lead to focus on their ability to clean up this waste and return or recycle a significant proportion. The corrosion resistance of ferrous metal products is commonly rendered by the application of electroplated coatings of non-ferrous metals such as zinc, nickel, copper, cadmium and chromium. Liquid effluents from plating plants, mainly consisting of the mixture of washing water and hazardous plating chemicals, must be treated prior to the release of the wastewater into the environment. The treatment of these effluents is typically performed by the adjustment of the pH of the wastewater from 8 to 11 to precipitate the metallic cations as the corresponding hydroxide, which is then filtered to yield toxic heavy metal sludge. The pre-treated sludge is not reusable in plating baths since it contains a substantial amount of harmful chemicals, iron and water hardness factors (Frankard 1987; Rivoallan et al. 1994). Therefore, it must be shipped to environmentally-secure landfill sites or incineration plants. The cost of such safe permanent disposal is very high, and can equal or exceed the value of the chemicals used in the plating process. In some countries the incapability of the incineration plants to treat the hazardous waste also leads to unlawful disposal of this waste into the soil. Therefore, there are very stringent national waste framework directives based on the "Basel Convention" forcing producers to employ the treatment or recovery methods in situ and recycle the valuable metals or chemicals in process. Furthermore, this waste, which is generally regarded as a hazardous, could be considered as a rich source of a secondary metal such as nickel and chromium.
In this study, a commercial PPFW was treated by hydrometallurgical method in order to find out the recovery of valuable metals as a metal reaches precipitate, i.e. nickel rich precipitate and the safe disposal of filtered solid product after the leaching process. The [H.sub.2]S[O.sub.4] leaching was employed to extract the valuable metals from waste sludge. The effects of the test parameters such as reagent concentration, time and temperature on extraction process were investigated. A special emphasis was also given to the kinetics of nickel dissolution. After leaching, the solution was subjected to two-stage controlled precipitation leading to selective separation of dissolved metallic species as hydroxide forms. XRD, XRF, IR and SEM techniques were also employed to characterize the resulting precipitates.
Background of the recovery method
The ability to recover metals econo ically after end-of-life is largely a function of their che ical reactivity and ho they are initially used in econo y. In a general route, a scrap etal in a solid for is subjected to the pyro etallurgical recycling route hile those- arising in liquid form, such as spent electrolytes fro electroche ical processing of metals, are recycled by hydro etallurgical methods (Moore 1993). A typical processing of nickel from Ni-ore consists of the extraction of Ni, Co and other metals such as Mn, Mg and Fe from the ore into the aqueous phase and treatment of leach solution to produce a precipitate containing Ni and Co and further treatment of the precipitate to separately recover Ni and Co at a satisfactory level of purity. he last stage ay involve further leaching to extract Ni and Co, follo ed by S (Solvent extraction) to separate the Ni selectively (White 2002). Operating experiences and a number of research studies perfor ed to separate nickel fro other dissolved etallic species sho ed that the precipitation of nickel as nickel hydroxide (Ni[(OH).sub.2]) by using alkaline reagent has so e processing difficulties such as selectivity, settling time, purity etc (Oustadakis et al. 2006). For this reason the precipitation condition should be optiized to reduce the i purity uptake and i prove the settling characteristics. For exa ple, Sist and e opoulos (2003) reported that the use of the stepwise neutralization (controlled precipitation) at relatively higher precipitation temperature (>50[degrees]C) aids to the generating of well-grown crystalline particles. Recently Jones (2001) also discloses a precipitation process for selective separation of nickel and cobalt ions from other dissolved ions such as copper, zinc, iron, magnesium and manganese, when the leach solution does not contain sufficient amount of zinc and copper for solvent extraction.
In practice, slaked lime (Ca[(OH).sub.2]) is preferred as a precipitation reagent due to its relatively low cost. However, it should be considered that precipitation by using Ca[(OH).sub.2] can result in formation of insoluble calcium sulfate- precipitate which causes contamination of the nickel product. The use of MgO as an alternative precipitation reagent has also been suggested by several researchers (Kay 2002; Oustadakis et al. 2006). Based on such studies, it can be concluded that relatively purer and ore sufficient a mount of precipitated Ni[(OH).sub.2] is obtained under a stringent pH control and relatively higher precipitation temperature (~55[degrees]C).
[FIGURE 1 OMITTED]
The PPFW used in laboratory experiments was supplied by a co ercial electro-plating plant, Eskisehir-Turkey. This plant disposes nearly 70 tons waste of sludge annually. The chemical composition (determined by the XRF, X-Ray Fluorescence, Philips PW-2404) of a representative waste material is given in Table 1.
The crystalline ineral phases of PPF were determined by using X-Ray-Diffraction (XRD), model S5000 diffractometer. In regard to the XRD analysis, it was discovered that PPFW mainly consisted of hannebachite, CaS[O.sub.3]x[H.sub.2]O (HA), calcium silicate (CS), portlantite (P), little a mount of nickel chromium oxide (NCO), calcium chromium oxide(CCO) and aluminium chromium silicate (ACS) and quartz (Q) (Fig. 1).
The bench type leaching experiments were performed to provide the necessary information about the re-usability of the metallic compounds in PPFW instead of their disposal. The dissolution tests were carried out in a 1L three-necked round-bottom flask reactor placed on a temperature controlled magnetic stirrer. In each test a 10 g of sample was added into 0.5L [H.sub.2]S[O.sub.4] solution and then stirred at a fixed stirring speed of 450 rpm. 10 mL of solution were taken at predetermined time intervals to evaluate the effect of reaction time, acid concentration and leaching temperature on metal dissolution. After leaching, the solution was filtered and subjected to two-stage controlled precipitation process in order to selectively separate the dissolved metals from the leached solution. The precipitation tests were performed by drop- wise addition of 1 Mol [L.sup-1] NaOH or MgO (10% by weight) into 100 mL of the leached solution (1N [H.sub.2]S[O.sub.4] at 24[degrees]C). For the firt stage of precipitation, the pH of the solution was slowly raised to about 4-5.5 to produce a solid containing non nickel and then filtered solution was subjected again to the second stage precipitation at a pH of about 8 to separate the dissolved Ni from the leached solution. The temperature as kept at 55[degrees]C during the precipitation tests. The resulting precipitates were filtered, ashed ith distilled after and dried in an oven at 100[degrees]C. All precipitates were characterized by a partical size analyzer Malvern, Mastersizer 2000, powder X-ray diffractometry, IR spectroscopy (Shimadzu HMV 2000-L FTIR spectrometer, [cm.sup.-1] resolution, KBr pellets) and scanning electron microscopy (Zeiss Evo 50 VP). The chemical analysis of dissolved species was performed by Atomic Absorption Spectrometer (AAS).
3. Results and discussion
3.1. Effect of leaching time and [H.sub.2]S[O.sub.4] concentration
The effect of leaching time on the dissolution amount of nickel and other main etals in PPFW, leached with 1N [H.sub.2]S[O.sub.4] at 24[degrees]C, is shown in Fig. 2. As seen from Fig. 2, the a ount of dissolved etals increased extending the leaching time. However, the extraction process remained constant after 1h. The extraction percentage of Ni, Cr, Zn and Cu was calculated as 84%, 35%, 79% and 86% respectively. A lower extraction percentage of Cr compared to other metallic compounds in PPFW can be attributed to the high resistance or insolubility of the chromiumaluminum silicate in diluted [H.sub.2]S[O.sub.4] solution.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Fig. 3 also indicates that the increase in [H.sub.2]S[O.sub.4] concentration had a positive effect on the dissolution process. The total extraction percentage of Ni was incresed from 84% for 0.5 N to 87% for 1 N; and 95.5% for 1.5 N acid concentration respectively.
3.2. Effect of temperature on nickel dissolution and leaching kinetics
The temperature (ranging from 24 to 55[degrees]C) test results, obtained by the leaching of PPFW at constant solution concentration of 1N [H.sub.2]S[O.sub.4] are presented in Fig. 4. It was found out that a rapid dissolution of Ni occurred at the beginning period (within one minite) of the leaching proces for all temperature values. Although the dissolution processes beyond this stage, continued up to a certain value for each temperature, the dissolution rate of Ni decreased with the time progression. This suggests that there's a series of steps in which more than one rate controlling process is involved in the observed kinetics. The rapid reaction of Ni at the beginning of leaching (1st stage) is probably due to the dissolution of surface metallic hydroxides. This change appears to be a separate process (chemical reaction). On the other hand, the dissolution rate of Ni beyond the plateau value (Anp), the end point of first stage, is considerably lower than that of the first stage for all temperature. The extracted amounts of Ni within the 60 minutes of leaching time were calculated approximately 84%, 92% and 99% for 24, 45 and 55[degrees]C, respectively. Approximately 81% of the total extractable amount of Ni was extracted at the initial period for 24[degrees]C.
[FIGURE 4 OMITTED]
By increasing the temperature from 24[degrees]C to 55[degrees]C, the dissolution percentage of Ni increased from 84% to 99% in which 87% of Ni was extracted in the 1st stage. A rapid and higher dissolution percentage of Ni within the first minute even at room temperature suggests that the usage of this waste as a source of Ni has an advantage compared to Ni recovery from natural ores. The XRD diffractogram of the leach residue obtained from the leached of PPFW at 55[degrees]C shows that Gypsum (CaS[O.sub.4]) is a main environmentally inert solid product that formed after the metals leaching process (Fig. 5). Hence the slower reaction rate observed in 2nd stage could be attributed to the formation of the product layer on the reacting surface and diffusion of reactant ([H.sub.2]S[O.sub.4]) through this product.
A normalized extraction curve, obtained by using a well known equation of 1-2/3 [alpha]-[(1-[alpha]).sup.2/3] = kt (Sohn and Wadsworth 1979), and the slope of each line [K.sub.p] values (parabolic rate constant) are given in Fig. 6 and Table 2 respectively ([alpha] = fractional amount of element leached, k = rate constant, t = time). The reacted amounts for each temperature were calculated by interpolating the concentration / time data after one minute of the zero time and zero concentration. The experimental activation energy for the diffusion process was calculated as 12 Kcal /mol.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
The precipitation tests showed that the solution pH and metal concentration strongly affect the precipitation of the dissolved ions, especially in the first stage of precipitation (Fig. 7). The dissolved amounts of metal ions such as Cu, Zn and Cr decreased from 4.50 to 5.50 for both types of reagents with the pH increase. A relatively lower amount of dissolved ion concentrations in leach solution after the 1st precipitation by using MgO reagent, may thought to be a slow release of hydroxyl ion by MgO compared to NaOH that resulted in lower metal ion concentration in solution due to the beneficial effect on the growing of hydroxide particles, as previously reported by Sist and Demopoulos (2003) for Ni precipitation. The particle size analyses of precipitates by using NaOH (P-Na) and MgO (P-Mg) showed that a type of the considered reagent did not have a significant effect on the particle size of the final precipitate. The d50 and d90 values of the P-Na were 22.58 and 53.16 [micro]m, whereas by using MgO pulp instead of NaOH, the [d.sub.50] and [d.sub.90] values of the precipitate were determined as 21.51 and 60.93 [micro]m respectively. This can be attributed to a higher precipitation temperature (55[degrees]C) than that of normally applied (24[degrees]C). The precipitation and the crystallinity increased with the rise of temperature. The un-detectable Ni(OH)2 peaks (XRD analyses) of the final precipitates, which performed at room temperature, confirmed this finding. The chemical analysis' results of the final solution showed that the separation of dissolved metallic species from leach solution was achieved effectively at the end of the precipitation stages. Nevertheless, the selective separation of nickel was not obtained at the 2nd stage of precipitation. The final precipitates still contained a little amount of zinc. This can be attributed to the closeness of the precipitation pH of the dissolved metals such as Zn and Ni with the existing ion concentrations in solution after leaching.
According to the chemical compositions of the final precipitate, P-Na and P-Mg, the purer precipitate can be obtained by using NaOH (Table 3) but these precipitates can only be evaluated as pre-concentrates for further selective separation processes such as ion exchange or solvent extraction.
[FIGURE 7 OMITTED]
3.4. Characterization of the precipitate
In order to characterize precipitates a series of structural and microstructural analyses were also performed. Regarding the X-ray diffraction data of the precipitates, obtained at the 2nd stage (Fig. 8a, b), it was discovered that the P-Na and P-Mg mainly consisted of Ni[(OH).sub.2] having different polymorphism and Brucite (Mg[(OH).sub.2]) phases. It was established as well that nickel hydroxide mainly exists in two polymorphic forms, [alpha] and [beta] The [beta]-(Ni[(OH).sub.2]) is isostructural with brucite and consists of an ordered stacking of well oriented Ni[(OH).sub.2] slabs ([beta] form). This anhydrous phase exhibits hexagonal platelet morphology, and OH groups of the adjacent layers are not hydrogen bonded. However, [alpha]-Nickel hydroxide phase is not only simple hydrated hydroxides but also hydroxyl deficient, including a variety of anions in the interlayer region along with water olecules (Kamath and Subbanna 1992).
[FIGURE 8 OMITTED]
Except the well known polymorphisms, different forms of nickel hydroxide phases such as [[beta].sub.bc], (badly crystallized), a short-range structure of nickel hydroxide phases having both [alpha] and [beta] phases, [alpha]-LDH (layered double hydroxide) and a novel nickel hydroxide phases (neither [alpha] nor [beta]) were reported in literature (Jayashree and a ath 2001; Kamath et al. 1997; Rajamathi et al. 1997). In Fig. 8a, detected peaks at 7.62, 3.84, 2.64 and 2.33 [Angstrom] indicate the existing of [alpha]-for Ni[OH).sub.], hile, other peaks, hich appeared at 4.65, 2.69 and 2.33 [Angstrom], may also assign the minor amount of p-(Ni[(OH).sub.2]) phases in P-Na.
Thus, it can be concluded that the use of NaOH as a precipitation reagent has ainly resulted in the occurrence of a disordered [alpha]-form Ni[(OH).sub.2] or a nickel hydroxide having features of both [alpha] and [beta] phases. On the other hand, the XRD pattern of the P-Mg (Fig. 8b) shows that P-Mg consists of mainly Mg[OH).sub.] and/or [beta]-(Ni[(OH).sub.2]) phases and [alpha]-(Ni[(OH).sub.2]) phases.
[FIGURE 9 OMITTED]
The IR spectrum of both P-N[alpha] and [beta]-Mg is given in Fig. 9. According to Fig. 9, P-Na could be characterized as a disordered [alpha]-Ni(OH) or as a blend of [alpha] and [beta]Ni[(OH).sub.2]. The well detected broad band centred around 3400 [cm.sup.-1], characteristic of [alpha]-Ni[(OH).sub.2], is due to the v(OH) mode of the hydroxide groups. This is involved in hydrogen bonding both from Ni[(OH).sub.2] sheets and the adsorbed [H.sub.2]O, indicating the existence of [alpha]-Ni[(OH).sub.2] as described by earlier research report (Deabate et al. 1999). In addition to this characteristic band, three visible bands were also detected; the first band, which is around 1630 [cm.sup.-1] was due to the interaction of water molecules with Ni[(OH).sub.2] by hydrogen bonding. The second band centred around 1100 [cm.sup.-1] was, due to the mixture of single-and double-bonded S[O.sub.4.sup.-2] ions absorption, and finally, the third absorption band around 640 [cm.sup.-1] show strong absorption which is normally expected for [alpha]-phase. The presence of the characteristic sharp peak at 3650-3694 [cm.sup.-1], due to non-hydrogen bonded OH groups stretching vibration, can refer to the P-Mg consisting of mainly Mg[OH).sub.] together with [beta]-Ni(OH)-21. Although the observed v(OH) band centred at 3400 [cm.sup.-1] may be attributed to the existence of [alpha]-Ni[(OH).sub.2].
The scans of electron micrographs of samples P-Na and P-Mg are shown in Fig. 10 and 11, respectively. The P-Na sample mainly consists of irregularly shaped chunky particles, generally observed for [alpha]-Ni[(OH).sub.2], and also a little amount of relatively small granular particles. Though a different particle morphology was observed for P-Mg. This can be seen from Fig. 11b, where the precipitate consists of many tiny agglomerated spherical particles. These particles sometimes occurred as smaller granular particles sticking to their faces, as reported earlier by Jayashree and Kamath (2001). These micrographs indicate the effect of precipitation reagent on the structure orphology of the resultant precipitate and confirm the above finding determined by XRD and FTIR characterizations.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
According to the experimental test results, the following conclusions can be drawn;
1. Dissolution of PPFW in [H.sub.2]S[O.sub.4] solution increases ith the increase of acid concentration and leaching te perature. Based on the findings of the kinetics study, it could be suggested that there is a series of steps in hich ore than one rate controlling process is involved in the observed kinetics. Approximately 81% of the extractable amount of Ni (84%) is extracted within the first minute of leaching process (1st stage) for 24[degrees]C leaching te perature.
2. In the precipitation part, it as found out that the separation of dissolved etallic species fro leach solution as effectively achieved. However, the selective separation of nickel, at the second stage of precipitation, was not obtained. This can be attributed to the resemblance of the precipitation pH of the dissolved metals such as Zn and Ni to the existing ion concentrations in solution after leaching.
3. The XRD and IR analysis of the final precipitate showed that P-Na could be characterized as a disordered [alpha]-Ni(OH) or as a blend of [alpha] and [beta]-Ni[(OH).sub.2], while P-Mg composed of mainly [beta]-(Ni[(OH).sub.2]) phase and/or a (Ni[(OH).sub.2]) phase.
The present study, hich as directed to test the treatment and usability of electroplating plant waste as a secondary metal extraction, showed that the potential environmental impact of toxic metals in PPF can be reduced by employing hydrometallurgial treatment method. However, a further benefication process such as solvent extraction has to be performed to selectively separate nickel from Ni- reach pre-concentrate.
Submitted 5 June 2008; accepted 22 Nov 2008
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Haldun KURAMA. Dr, Assoc. Prof. Department of Mining Engineering, Eskis.ehir Osmangazi University. Doctor of Science (Mining Engineering), Osmangazi University, 1994. Publications: author of more than 10 research papers. Research interests: mineral processing, waste treatment and recycling, water and wastewater treatment.
Eskisehir Osmangazi University, Mining Engineering Department, Eskisehir-Turkey. E-mail: firstname.lastname@example.org
Table 1. Chemical analysis results of PPFW Oxide Weight % MgO 4.57 [Al.sub.2][O.sub.3] 8.85 Si[O.sub.2] 2.91 [P.sub.2][O.sub.5] 2.13 S[O.sub.3] 15.2 CaO 23.0 [Cr.sub.2][O.sub.3] 8.88 NiO 9.46 CuO 1.71 ZnO 1.44 LOI * 20.1 * Loss of ignition Table 2. Calculated [K.sub.p] rate constant Temperature, [degrees]C Kp [min.sup.-1] 24 6.[10.sub.-4] 45 1,6.[10.sub.-3] 55 5,1.[10.sub.-3] Table 3. Chemical analysis of final precipitates Oxide P-Na (wt. %) P-Mg (wt. %) MgO 2.91 53.63 [Al.sub.2][O.sub.3] 0.07 0.06 Si[O.sub.2] 4.82 2.84 CaO 2.58 0.92 [Cr.sub.2][O.sub.3] 0.07 0.10 NiO 58.74 33.70 CuO 1.43 0.55 ZnO 7.01 3.28
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|Publication:||Journal of Environmental Engineering and Landscape Management|
|Date:||Dec 1, 2009|
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