Environ. Sci. Technol. 2008, 42, 7231–7235
A Green Process to Prepare Chromic Oxide Green Pigment P I N G L I , †,‡ H O N G - B I N X U , * ,† SHI-LI ZHENG,† YI ZHANG,† ZUO-HU LI,† AND YU-LAN BAI§ Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China, Graduate School of Chinese Academy of Sciences, Beijing 100039, P. R. China, and College of Science, Qingdao University of Agriculture, Qingdao 266109, P. R. China
Received June 30, 2008. Revised manuscript received July 30, 2008. Accepted August 5, 2008.
A hydrogen reduction and activated sintering process was proposed to prepare chromic oxide green pigment. Through ICP, XRD, SEM, FT-IR, UV, and CIE-L*a*b* colorimetric analysis, key factors and mechanism that influenced preparation of chromic oxide green pigment were studied. The results revealed that lower hydrogen reduction temperature, suitable addition of Al and Ba, were beneficial to obtaining the high quality chromic oxide green pigment. Typically, when the hydrogen reduction temperature was kept at 450-500 °C, physicochemical properties and color performance of the prepared chromic oxide green pigment doped with about 0.1-0.2 wt % Al and 0.2-0.5 wt % Ba conformed to commercial pigment standards. Additionally, characteristics of the green process were discussed. About 90 wt % KOH was reused directly and about 90 wt % Cr(VI) was conversed to Cr(III) directly from potassium chromate to chromic oxide green pigment. Integrating the proprietary green metallurgical process from chromite ore to potassium chromate of this laboratory, more than 99 wt % Cr(VI) could be conversed to Cr(III) compounds and about 99 wt % KOH could be recycled to use. The whole green process, ranging from chromite ore to chromic oxide green pigment, eventually not only provided the possibility for producing the high quality chromic oxide green pigment, but could reach comprehensive utilization of resources, inner recycle of KOH, and zero emission of Cr(VI).
1. Introduction Chromic oxide green pigment finds wide applications in coating, printing, gum elastic, plastic, and construction due to its excellent performance in green color, wear resistance, corrosion resistance, and chemical resistance (1). At present, the industrial production of chromic oxide green pigment generally employs two processes: one is produced by reducing an alkali metal chromate, preferably sodium dichromate (Na2Cr2O7), calcining reduction product, washing, drying, and grinding the resulting calcinate; byproduct is Cr(VI)-containing sodium sulfate(Na2SO4); the other is thermal decomposition of special Cr(VI)-compounds, for example CrO3, and a lot of Cr(VI)-containing solid wastes are * Corresponding author phone: +86-10-82627096; fax: +86-1082621022; e-mail:
[email protected]. † Key Laboratory of Green Process and Engineering. ‡ Graduate School of Chinese Academy of Sciences. § Qingdao University of Agriculture. 10.1021/es801724m CCC: $40.75
Published on Web 08/29/2008
2008 American Chemical Society
generated (1-3). For the former process, Cr(VI)-containing Na2SO4 needs to be properly resolved; for the latter process, environmental pollution resulting from the Cr(VI)-containing solid wastes is often serious. In recent years, various methods for preparing high quality chromium containing pigments have been developed with special consideration of environmental protection. Mun ˜ oz et al. (4) synthesized an environmentally friendly green pigment based on Cr2-xAlxO3 solid solution. Berry et al. (5) and Lazaˇu et al. (6) synthesized chromium-containing pigments from leather waste separately. A proprietary green metallurgical process of this laboratory for chromite ore has been proposed by Zheng (7); a key point of this process was a continuous liquid phase oxidation of chromite ore in the submolten salt medium(KOH) at 300 °C. The process could realize virtually complete utilization of chromite ore and zero emission of residue, which solved the serious environmental problem in the traditional production process of chromate. Reaction equation of this process from chromite ore to potassium chromate is as follows: FeCr2O4(chromite) + 7/4 O2 + 4 KOH f 1/2 Fe2O3+ 2 K2CrO4+2 H2O (1) In order to realize recycling use of the submolten salt medium (KOH) and produce high value chromium products such as high quality chromic oxide green pigment, a new hydrogen reduction and activated sintering process was proposed as the latter process of the green metallurgical process. An intermediate in the form of Cr(III) compounds, was first produced by the hydrogen reduction step at the low temperature of 300-800 °C, and was sintered to the chromic oxide green pigment at the activated sintering step. The reaction equation of this process from potassium chromate to chromic oxide green pigment is as follows: K2CrO4+1/2 H2 f Cr2O3+2 KOH + 1/2 H2O
(2)
Combining the green metallurgical process for chromite ore with the hydrogen reduction and activated sintering process, a reaction equation of the whole green process is as follows: Cr2O3(chromite) + H2+1/2 O2 f Cr2O3(product) + H2O (3) Besides chromite as raw materials, air as oxidant, hydrogen as reductant, nothing was exhausted in the whole green process. This study thus aimed at preparing high quality chromic oxide green pigment, and recycling submolten salt medium (KOH) through the hydrogen reduction and activated sintering process, so that zero emission of the whole green process could be realized. Key factors and mechanism that influenced preparation of the chromic oxide green pigment were studied and green process characteristics was discussed.
2. Experimental Section The K2CrO4 used in this work was of analytical grade manufactured by the green metallurgical process, and the purity of the hydrogen gas used was 99.99% v/v. Gas-solid reduction was carried out in a tube furnace with a programmable temperature controller. A nickel boat, loaded with 100-150 mesh K2CrO4 particles, was first put into the furnace tube. Hydrogen was then introduced at a constant flow rate into the tube, the reaction temperature was raised to and kept at different temperatures (450-650 °C) for 1 h depending on experiment requirements, and then the furnace was VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. SEM pattern of the Cr2O3 sample (S5). naturally cooled to ambient temperature. The reduction product was lixiviated by distilled water several times to completely removed soluble components. The intermediate was then dried at 105 °C for 12 h. The initial samples used for activated sintering were the above obtained intermediates doped with 0-2.0 wt% Al(NO3)3 and Ba(NO3)2, according to the following procedure. A required weight of Al(NO3)3 and Ba(NO3)2 as additive was first dissolved in a proper volume of pure water, and the intermediate was then mixed with and evenly dispersed in a given volume of the aqueous solution, that is, approximately 200 mL of the aqueous solution for 1000 g of the intermediate powder. Next, the wet mixtures of the intermediate powder and aqueous solution were sintered in air in an electric muffle furnace at 950 °C for 1.5 h and then quickly cooled. After sintering, the resulting Cr2O3 samples were lixiviated with distilled water several times, dried and sieved at 45 µm. The elemental composition of the reduction products, the intermediates and the Cr2O3 samples was carried out on an Optima 5300 DV Perkin-Elmer inductively coupled plasma (ICP-AES). The phases of the reduction products, the intermediates and the Cr2O3 samples were analyzed by XRD and FT-IR. The XRD analysis was carried out with a Rigaku diffractometer using CuKa radiation. The infrared spectra were recorded by a Spectra GX FT-IR Spectrometer (Perkin-Elmer, U.S.) in KBr pellets (0.002 g sample and 0.2 g KBr) with the scan number set to 8 and the resolution set to 4 cm-1. The morphology of the Cr2O3 samples was taken on a JSM 6700F NT. The color performance data were reported in the CIEL*a*b* colorimetric system. Values of CIE-L*a*b* color parameters were measured on the Cr2O3 samples and standard samples used in the industries with an SC-80C automatic differential colorimeter manufactured by Beijing KangGuang Instrument CO., LTD, China, with an illuminant D65 as required, and a measurement precision of (0.01. For each colorimetric parameter of the determined sample, three values were measured and the average value was chosen as the measurement result. According to the CIE-L*a*b* colorimetric system, the value of CIE-L* denotes the degree of lightness and darkness of the color in relation to the scale extending from white (L* ) 100) to black (L* ) 0), the value of CIE-a* denotes the scale extending from green (-a*) to red (+a*) axis, and the value of CIE-b* denotes the scale extending from blue (-b*) to yellow (+b*) axis. Capacity to absorb oil and sieve residue was measured based on theuniversal standard method. The UV spectra of the Cr2O3 samples were measured by employing a Perkin-Elmer spectrophotometer, with a scan 7232
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FIGURE 2. XRD pattern of the Cr2O3 sample (S5).
TABLE 1. Physicochemical Properties and Color Performance of Cr2O3 Samples and Commercial Pigmentsa chemical composition (wt%) capacity to absorb sieve oil residue (g/100 g) (wt%) Cr2O3 S1 S2 S3 S4 S5 S6 *P1 *P2
22.4 25.2 23.5 21.5 25.7 20.6
