versión impresa ISSN 0327-0793
Lat. Am. appl. res. v.32 n.2 Bahía Blanca abr./jun. 2002
Regeneration and utilization of spent bleaching clay
E.L. Foletto, C.C.A. Alves, L.R. Sganzerla and L.M. Porto
Abstract A spent clay from the oil refining industry was recycled through an extraction process using organic solvents for removal of residual oil, followed by reactivation through heat treatment. The solvents used showed the following order in the efficiency of oil extraction: methylethylketone> acetone > petroleum ether @ hexane. The bleaching efficiency of regenerated clay samples for soybean oil was determined spectrophotometrically in the visible region. For comparison of the bleaching experiments, a sample of virgin commercial clay was used. Results showed that extraction process, using just solvent, is insufficient to recover the bleaching power of the spent clay, needing then a further high temperature treatment. The bleaching power of the regenerated clay is dependent of temperature and time of calcination. The regenerated clay samples presented a similar bleaching efficiency for soybean oil comparable to a virgin commercial clay utilized as reference.
Keywords Spent clay; Recycling; Regeneration; Oil recovery.
Crude vegetable oils are generally processed by degumming, alkali refining, bleaching and deodorizing. The step of vegetable oils bleaching with clays in industry has been reviewed by Norris (1964) and Kaufmann and Mukherjee (1967). Oils bleaching for edible purposes involves the removal of a variety of impurities by adsorption which include phosphatides, fatty acids, gums, metals trace, etc. followed by decolorization. This enables the production of a lightcolored and stable oil, acceptable to consumers. Both natural or acid activated clays are used as absorbents. Bleaching clay is composed mainly of smectite, an aluminosilicate mineral. It is well known that bentonites in their natural state have limited sorbing capacity. This ability is greatly enhanced by treatment with strong acids. When bentonites are acid-activated as a result of treatment with hot mineral acid solutions, hydrogen ions attack the aluminosilicate layers via the interlayer region (Taylor and Jenkins, 1987). This attack alters the structure, chemical composition and physical properties of the clay while increasing the adsorption capacity (Mokaya et al., 1993). At a dosage of 0.5-1.0% clay, the current world production of more than 60 million tons of oils is accompanied by the production of solid spent clay, containing 30-40% oil, estimated at 600,000 tons worldwide (Ng et al., 1997). This solid waste clay is currently disposed directly in landfills without treatment, causing severe water and air pollution problems (Svensson, 1976). However, recently dumping of spent clay in landfills or public disposal sites has been prohibited in most countries (Al-zahrani and Daous, 2000). Recovery of oil and the reuse of spent bleaching clay are the areas where great opportunity exists for cost saving in the oil processing industry. Patterson (1992) described different methods of oil recovery and some of the important factors affecting them. Feuge and Janssen (1951) have described solvent regeneration of spent clay. Organic solvents such as acids, alcohols, etheres, ketones, etc. were used. A low molecular weight ketone was found to be the most effective solvent. Kuck et al. (1962) have reported thermal regeneration of spent alumina used for cottonseed oil bleaching. Loven (1973) has described several methods for the regeneration of activated carbon including the economic aspects of regeneration. Regeneration of the spent clay by direct calcination with no prior treatment at different temperatures was studied by Al-zahrani and Alhamed (1996).
In the present study, spent clay from a brazilian vegetable oil processing industry has been regenerated by solvent extraction followed by calcination. The effects of temperature and time of calcination on the bleaching power for soybean oil of the regenerated clay samples were also investigated.
Spent clay obtained after bleaching of alkali-refined soybean oil was supplied by Santista-Ceval Alimentos, Gaspar-SC, Brazil. The virgin clay was also provided by the same company and it is a commercial acid-activated clay: designated as AX. The virgin clay sample was characterized by X-ray diffraction (XRD), chemical analysis and differential thermal analysis (DTA). X-ray diffractogram was obtained using a Philips 3020 diffractometer equipped with a PW3710 computerized control unit, operating at 40 kV and 20 mA, using radiation Cu-Kα (λ = 1.5405Å), at a scanning speed of 1°(2θ)/min. DTA was performed on a Netzsch STA 409 instrument at a heating rate of 10 ºC.min-1 under a flow of atmospheric air of 35 mL min-1, in the range 25- 1000ºC. Elemental composition of the sample was determined by X-ray fluorescence technique with a Philips PW 2400 spectrometer.
The experimental program consisted of three consecutive steps: extraction of oil from spent bleaching clay samples using one of the organic solvents, followed by calcination of the samples, and finally a bleaching test using these samples.
Extraction step: Different samples of equal weight of spent clay were extracted using one of the organic solvents, methylethylketone, acetone, petroleum ether or hexane, to recover the oil. The clay sample was added to the solvent in the proportion of 1/5 (g/mL) and the mixture was stirred mechanically for 10 min in a glass vessel, at room temperature. Upon completion of the extraction step, the extracted oil and solvent were separated using a Soxhlet apparatus. The percentage of extracted oil (PEO) for each sample was calculated by the following expression:
PEO (%) = (extracted oil / total oil content) x 100
The total oil content was determined in a separated experiment by burning 5 g of the spent clay sample at 1000ºC. The average of total weight loss of these samples was 43%. Loss on ignition for this clay sample (virgin) was found to be 12%. The average of moisture content was measured by drying 5 g of the spent clay in a drying oven at 110ºC for 24 h. The average of moisture content was found to be 6%. Therefore, the average of total oil content can be calculated as 25%.
Calcination step: The required number of 5 g of clay samples grouped according to the solvents used were loaded in alumina crucibles and placed in a preheated oven at the desired temperature. For each batch of samples at a given temperature, individual crucibles were then removed successively from the oven at a predetermined time interval. Each sample was allowed to cool until room temperature before it was ready to the bleaching test.
Bleaching test: The deoiled (before calcination) and calcinated clay samples produced in this work were tested for bleaching of the alkali-refined soybean oil, obtained from Santista-Ceval Alimentos, Gaspar-SC, Brazil. One hundred grams of alkali-refined oil were stirred and heated to 100ºC under vacuum of 450 mmHg. The clay sample (1.0 g) was then added to the heated oil, and the mixture was stirred mechanically for 30 min. A stream of N2 was kept in the oil batch throughout the experiment. The hot oil and clay mixture was filtered under vacuum in the end of the experiment just before measuring the absorbance. A schematic diagram of the apparatus used in the oil bleaching tests is shown in Figure 1. The bleaching efficiency of regenerated clays was then determined by measuring the color of the bleached oil using a UV-VIS spectrophotometer (Shimazdu 1240) at 450 nm. In this study, the bleaching efficiency (BE) is defined by the following expression:
where Aunbleached and Ableached are the absorbances of unbleached and bleached oil, respectively, at 450 nm.
The performance of the regenerated samples in the soybean oil bleaching was compared to that of a virgin clay (AX).
III. RESULTS AND DISCUSSION
The X-ray diffractogram of the virgin commercial sample is shown in Figure 1. XRD presents the 001 reflection (of smectites) with low intensity, and that happens due to the acid attack during the preparation of that material, that provokes the partial destruction of the smectite structure. The low intensity of peak 001 didn't imply that the virgin sample lost its bleaching power to soybean oil.
Figure 3 shows the DTA curve for the virgin sample. The first endothermic peak (at about 150°C) corresponds to the adsorbed water loss and the peaks at about 550°C correspond to the loss of structural hydroxyl groups. This confirms again the presence of beidelite in the smectite phase, and according to Grim and Kulbicki (1961), the second endothermic peak for beidelite appears between 550 and 600°C.
The chemical composition of the virgin clay is: 63.00% SiO2, 13.23% Al2O3, 4.78% Fe2O3, 3.60% MgO, 3.12% K2O, 1.70% CaO, 0.79% Na2O, 0.52% TiO2, 0.15% P2O5, 0.05% MnO, 9.06% H2O.
Table 1 presents a summary of the extraction conditions for the different solvents used and the results of PEO and BE. It is observed that the PEO and BE are maximum for the clay treated with methylethylketone, followed by acetone and minimum for petroleum ether and hexane. These two last solvents presented the same efficiency in the extraction of the residual oil as well as the BE. According to these results, we opted for submitting to the calcination only the samples that presented POE maximum and minimum: samples treated with methylethylketone and petroleum ether, respectively.
Table 1 - Summary of the extraction conditions for the different solvents used and the results of PEO and BE.
The results, upon calcination of the samples previously treated with methylethylketone and petroleum ether as solvents, are shown in Figures 4 and 5. It is observed that the BE increases with the calcination temperature. At a given calcination temperature, the BE increases with calcination time, reaching a maximum value in a short time for samples calcined in temperatures above 500°C. For temperatures below 500°C, the BE tends to take a longer time to remain unchanged. The BE of samples calcined at 600°C, after reaching a maximum at 15 minutes, decreases with time. That negative effect on the clay bleaching power is attributed to modification (sintering) or destruction of the clay structure which may occur at high temperatures (> 500°C) and it corroborates the results obtained byWang and Lin (2000). Comparing Figures 4 and 5, it is observed in the first 30 minutes that the samples treated with methylethylketone present a higher bleaching efficiency than those treated with petroleum ether, because the solvent methylethylketone removed a great amount of residual oil in the spent clay, leaving the clay more active than the samples treated with petroleum ether. However, after some time, the samples treated with both solvents and calcined at 500 and 600°C presented almost the same bleaching efficiency. A disadvantage of calcinating samples containing a lot of residual oil like the ones treated in this work with petroleum ether, hexane, and acetone is the contribution to air pollution caused by the burning of residual oil.
The clay samples treated with methylethylketone followed by calcination at 500 and 600°C (after 30 minutes) presented a BE of about 90%, and the virgin clay used as reference presented a BE of 89%, being then comparables in terms of bleaching power.
The bleaching efficiency of the clay samples just submitted to extraction with solvents (results shown in Table 1) was much below compared to that of the calcinated samples (results shown in Figures 4 and 5).
The percentage of extracted oil for the solvents used in this study can be written in the following order:
PEOmethylethylketone > PEOacetone > PEOpetroleum ether @ PEOhexane.
The extraction process using only solvents is insufficient to recover the bleaching power of the spent clay. The clay should be submitted to a thermal treatment at temperatures above 400oC.
The bleaching power of the regenerated clay for soybean oil is dependent of temperature and time of calcination.
The regenerated samples in this study presented similar bleaching efficiency for soybean oil comparable to the virgin clay used as reference.
The authors would like to thank Dr Cristina Volzone (CETMIC-Argentine) for the DTA analyses. The financial support of CAPES (scholarship provided to ELF) is also acknowledged.
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Received: December 3, 2001.
Accepted for publication: January 28, 2002.
Recommended by Subject Editor G. Meira.