Does Anyone Know A Way To Solve Cr2O3 + 6 Hcl ↠ 2 Crcl3 + 3 H2O

In this study the viability of utilising ashes with high chromium oxide content, obtained by thermal treatment of footwear leather waste, in the production of low-carbon ferrochromium alloy (Fe-Cr-LC) by aluminothermic reduction was investigated. The following key-factors were selected for process modelling: the quantity of aluminium (Al) employed in the reaction, the iron amount added, the iron compound (Fe and/or Fe2O3) used, and the chromic acid addition. The process was investigated using a 2(4) full factorial design where the percentage of Cr2O3 reduced was used as the response. Variance analysis was employed to determine the significant effects and to validate the obtained model. The model was useful for finding the optimal operating conditions, including the maximisation of chromium conversion and the gross margin. Both resulted in similar process conditions, with 76.8±12.3% of chromium being reduced to the metallic phase, and 1.65±0.52 USD (kg ash)-1 as the gross margin. The qualities of some alloys obtained were investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy analysis (SEM/EDS). The results showed that the main problem for these alloys in a standard specification was the P and S content, suggesting that a pre-treatment is required.

Đang xem: Cr2o3 + 6 hcl → 2 crcl3 + 3 h2o

Footwear leather waste; Chromium oxide; Aluminothermic reduction; Low-carbon ferrochromium; Statistical experimental design; Optimisation

ENVIRONMENTAL ENGINEERING

Aluminothermic reduction of Cr2O3 contained in the ash of thermally treated leather waste

B. M. WenzelI, II, ** E-mail: bruno.wenzel
uffs.edu.br** E-mail: nilson
enq.ufrgs.br; T. H. ZimmerI; C. S. FernandezI; N. R. MarcilioI, *** E-mail: bruno.wenzel
uffs.edu.br** E-mail: nilson
enq.ufrgs.br; M. GodinhoIII

ILaboratory of Waste Treatment, (LPR), Department of Chemical Engineering, Phone: + (55) (55) 3359-3979, Federal University of Rio Grande do Sul, (UFRGS), Eng. Luiz Englert str. s/n°, 90040-040, Porto Alegre – RS, Brazil

IIFederal University of Fronteira Sul, (UFFS), Major Antônio Cardoso str. 590, 97900-000, Cerro Largo – RS, Brazil

IIIDepartment of Chemical Engineering, University of Caxias do Sul, (UCS), Francisco Getúlio Vargas str. 130, 95070-560, Caxias do Sul – RS, Brazil

ABSTRACT

In this study the viability of utilising ashes with high chromium oxide content, obtained by thermal treatment of footwear leather waste, in the production of low-carbon ferrochromium alloy (Fe-Cr-LC) by aluminothermic reduction was investigated. The following key-factors were selected for process modelling: the quantity of aluminium (Al) employed in the reaction, the iron amount added, the iron compound (Fe and/or Fe2O3) used, and the chromic acid addition. The process was investigated using a 24 full factorial design where the percentage of Cr2O3 reduced was used as the response. Variance analysis was employed to determine the significant effects and to validate the obtained model. The model was useful for finding the optimal operating conditions, including the maximisation of chromium conversion and the gross margin. Both resulted in similar process conditions, with 76.8±12.3% of chromium being reduced to the metallic phase, and 1.65±0.52 USD (kg ash)-1 as the gross margin. The qualities of some alloys obtained were investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy analysis (SEM/EDS). The results showed that the main problem for these alloys in a standard specification was the P and S content, suggesting that a pre-treatment is required.

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Keywords: Footwear leather waste; Chromium oxide; Aluminothermic reduction; Low-carbon ferrochromium; Statistical experimental design; Optimisation.

INTRODUCTION

According to Brazilian governmental institutions (State Foundation of Environmental Protection – FEPAM), 118 thousand tonnes/year of hazardous solid wastes are generated from the leather industries in Rio Grande do Sul State, corresponding to 62% of the total hazardous wastes produced in this State (FEPAM, 2003). This solid waste is hazardous because it contains trivalent chromium derived from the salts used in tannery. Chromium is considered to be very harmful for living cells, and may cause cancer (Langard, 1990; Tsou et al., 1997) and cell death (Blankenship et al., 1994). The majority of these wastes are disposed in landfills, and only about 3% are recycled (FEPAM, 2003). In specific environmental conditions, chromium (III) can be converted to chromium (VI) (Fathima et al., 2001; Milacic and Stupar, 1995), thus causing environmental impacts.

As an alternative to the actual practice, thermal treatment can be applied as a suitable technology to mitigate this problem, producing energy (waste-to-energy process) as a by-product from the heat which is generated in the combustion process. Recently, a semi-pilot unit (350 kWth) projected by the Laboratory of Waste Treatment was built to study leather residue gasification and combustion (described by Godinho et al. (2007; 2009; 2010)). The thermal capacity of this unit will be increased in the near future, reaching a higher level of energy recovery (600 kWth).

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About 5.8 wt% (dry basis) of the leather waste is composed of ash (Godinho et al., 2007). The ash resulting from this process contains 50-62 wt% of chromium oxide (Cr2O3) and less than 5 ppmw hexavalent chromium (Cr+6) (Godinho, 2006; Godinho et al., 2007; Vieira, 2004; Wenzel, 2008), and might be utilised as a raw material, potentially replacing chromic ore (chromite). Some researchers approach the chromium (VI) route to obtain chromium compounds from ashes (Dettmer et al., 2010a; Dettmer et al., 2010b).

Low carbon ferrochromium alloy (Fe-Cr-LC) is an important input in the production of stainless steel. Chromium is one metal used as an alloying element in steel. Its use provides high corrosion resistance, by forming a thin oxide layer on the metal surface, called the passive layer, which protects against the action of the environment (Chiaverini, 2005). In this context, this study investigates the production of Fe-Cr-LC employing chromium-rich leather waste ashes via non-isothermal aluminothermic reduction.

The aluminothermic reduction process was patented primarily in Germany by Goldschmit, and was named the thermite reaction. Pure metals and ferro-alloys, including chromium, were commercially produced by variations of the original patent. The process is also being used to recover metals (Shibata et al., 2002). During the reaction, the temperature must exceed 2,100 ºC to permit the adequate separation of the product metal and slag (Nelson, 1996). According to Nelson (1996), the energy balance of the process is a key factor that can lead to poor metal yield when energy is insufficient or can cause excessive fumes and explosions when too much energy is injected into the system. Therefore, new sources of raw material must be evaluated in small scale tests. The reaction of chromium oxide is highly exothermic (= -272.8 kJ (mol Al)-1), but is insufficient to be autothermic, requiring the input of additional energy. An autothermic process requires the addition of boosters or reactant preheating in some cases. One of the chemical boosters, also known as an exothermical reactant, is chromic acid (CrO3). The aluminothermic reduction of CrO3 (-548.5 kJ (mol Al)-1) is indicated as an energy source and to initiate and sustain the aluminothermic process (Nelson, 1996). However, such compounds present the inconvenience of carcinogenic effects. Another compound that could be used is hematite (Fe2O3; =-851.4 kJ (mol Al)-1) (Bodaghi et al., 2009).

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The present study was conducted using an experimental design technique (full factorial design) with four independent variables (factors) across two levels. The simultaneous effects of Al amount, iron amount, the iron compound used (oxide or metal), and chromic acid addition were selected as factors to be investigated. The quantity of the reduction agent (Al) was tested in the range between stoichiometric levels and an excess of 30%, considering the reactions that occur in the system. Chromic acid was used as a chemical booster, to provide additional energy to the system. Iron was used in two forms: metallic iron and iron (III) oxide. The oxide (hematite) is known to supply more energy to the reaction system (acting as booster), but consumes more Al. A representative model that involves these factors was validated, and optimal conditions were obtained. Due to the relatively high price of raw materials involved in this process, a simple economic optimisation was performed (gross margin maximisation). The alloy quality was investigated using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Alloy samples obtained from two experimental conditions were used for comparison with the standard specifications of Fe-Cr-LC.

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EXPERIMENTAL

Ash from Leather Wastes and Other Materials

The ash used in the experiments was collected in the gasification reactor of the pilot plant for leather waste thermal treatment (described by Godinho et al. (2010; 2007; 2009)). A representative ash sample was prepared through successive manual quartering and comminution in a ball mill until particle sizes of 2O3 was performed according to the standard D2807 (ASTM, 2009), and the resulting deviation was less than 1%. Carbon, hydrogen and nitrogen contents were determined by D5373 (ASTM, 2008), and presented as a sum of these elements (CHN). Table 1 summarises the results of chemical characterisation. According to Godinho et al. (2007), the hexavalent chromium content in the ash is less than 5 ppmw.

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