Chapter 19
Supercritical Fluid Extraction of Grinding and Metal Cutting Waste Contaminated with Oils
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N. Dahmen, J. Schön, H. Schmieder, and K. Ebert Institut für Technische Chemie, ITC-CPV, Forschungszentrum Karlsruhe GmbH, P.O.B. 3640, D-76021 Karlsruhe, Germany The wellknown technique of supercritical fluid extraction with compressed carbon dioxide is used for an environmental application. The development of a process is described, in which residues with high oil contentsfrommetal machining operations such as grinding or cutting can be separated in two fractions. One consists of the oil which can be recovered from the grinds in such a quality that direct reuse as grinding oil is possible; the metalfractioncan be recycled by common processes. The results obtained in laboratory scale equipment were scaled up and successfully demonstrated in a pilot plant. In Germany oil-containing residues from metal and glass working are classified as hazardous wastes. Disposal of hazardous waste requires special and costly monitoring procedures. These costs, depending on the disposal path, vary between U.S.$ 400 and 1200 per ton of grinds. According to an estimate by the German Federal Environmental Office, 150,000 to 200,000 tons of these residues are produced in Germany each year, up to 50% comingfrommachining with pure oils; often with oil contents up to 50%. The majority of this material arise in machining of medium and high alloyed steels or non-ferrous materials using pure grinding and cutting oils as cooling lubricants immiscible with water. In contrast to metal chips and metal powders this material consists offinemetal chips, abrasives, metal workingfluidsand various impurities. At present, the following processes are available for treatment of these wastes: • centrifugation and briquetting as pretreatment to lower the oil content to 10-20 % • for materials with an oil content of more than 4%, thermal treatment as aggregate (additional iron) in the concrete production • disposal by combustion with the oil acting as fuel • for materials with an oil content of less than 4%, storing at a hazardous waste site 270
© 1997 American Chemical Society
In Supercritical Fluids; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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SFE of Waste Contaminated with OUs
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A direct recycling in metallurgical or smelting plants is not possible because the high organic contents cause off-gas problems. Here oil contents of less than 1 % are required. In Germany, several techniques are under development and testing. For example, oil separation through vacuum distillation or extraction using different organic solvents or water emulsions. The quality of the end products should permit direct recycling, since with respect to the diverse laws of waste management recycling has to be preferred to disposal. The recovered oils has to be reusable as grinding and cutting oils and the metal matrix in the respective steel and alloys producing processes. In this paper, an extraction process in which supercritical carbon dioxide is used as solvent is described. The process was originally developed for treating glass grinds with high oil and lead contents arising in optical glass machining (i), but it has proved to be suitable also for treating metal grinds (2). The results obtained in laboratory scale equipment were scaled up and demonstrated in a pilot plant. Metal grinding sludge, also as compacts were treated successfully with supercritical carbon dioxide as extractant. The recovered oils were suitable for reuse in the respective metal machining processes.
Solubility measurements of technical oils
The oils used in metal grinding and cutting can be divided into mineral and synthetic hydrocarbon oils of different composition of their hydrocarbon components. In addition they contain a variety of additives, e.g. aging inhibitors, anticorrosives, detergents, pourpoint depressants, foam inhibitors and others. Increasingly, lubricants are used which are based on native oils like rape oil. The phase behavior of these complex mixtures was measured applying the synthetic method at temperaturesfrom40 to 80°C and at pressuresfrom10 MPa up to 40 MPa. The experiments were carried out in a commercial phase equilibrium apparatus (Sitec PH 251-500K). The cell has a variable volume of 12 to 25 cm , controlled by a nitrogen-driven piston; the position of the piston is monitored by inductive measurement. Temperature in the cell was maintained constant using an electrical heating jacket. To start an experiment, a known amount of oil was filled into the cell. By adding C 0 a homogeneous mixture was obtained and to speed up equilibration a magnetic stirrer was used. The amount of carbon dioxide was calculated from the cell volume and the density for given temperature and pressure and was determined analytically after the experiment. Then the oil was adsorbed on a charcoal filter (Supelco EnviCarb) and was determined gravimetrically, C O 2 was collected and weighed and in a gas-flask. While sampling, the pressure was kept constant by means of the piston. To determine the phase boundary, the pressure was lowered until phase separation occurred, which was observed visually by a camera system. The pressure rates were kept constant by using a computer controlled back pressure regulator (Tescom ER2000). In contrast to pure substances, phase separation of mixtures is a rather slow process, because the components of different molecular mass or chemical nature separate successively from the mixture. To obtain an objective measure for the phase separation, the clouding of the mixture was monitored by a device containing a 3
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In Supercritical Fluids; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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photocell recording the transmission of the light from the opticalfiberof the camera system. From the resulting clouding curve a starting point was chosen to characterize the beginning of the separation. In Figure 1 the solubility of different metal working oils is presented as a function of pressure at 50°C. Depending on the composition of the oils, different solubility behaviors are observed. The lowest solubility is found for the ester-basal native oil A. Large solubility differences were obtained for the two synthetic oils C and D. C is an oil of synthetic hydrocarbons and esters, containing some additives, and D is a synthetic mixture of hydrocarbons comparable to pharmaceutical white oils. For comparison, the solubility of squalane (3) (as oil representative hydrocarbon) is presented in Figure 1, showing a similar behavior as the oils Β and C: an increase of pressurefrom15 to 35 MPa leads to an increase of the solubility by a factor 2 to 4. Extraction Experiments Analysis. For analysis of the residues sum parameters were considered to be sufficient. In most cases the samples were characterized by total carbon (TC), a widely used method in environmental analysis. For TC determination 10-50 mg of material were burned within 3-5 min in an incinerator (Strôhlein C-mat 550) in an oxygen stream at 700-900°C; the C O 2 content in the off-gas stream is continuously measured with an IR detector. The time integral of the output signal is directly proportional to the carbon content of the sample. The reproducibility of the TC-values was not better than 0.25 wt%, because of the heterogeneity of the sample material. Finally the TC had to be corrected by the carbon content originating within the various steels. Some of the samples contained considerable amounts of organics in addition to the oil, i.e.filteringaids. For these samples another method of analysis was required to prove that the carbon content remaining in a sample after extraction was not oil. Here an EPA method no. 3560 was applied using a Dionex SFE/SFC Series 600 device modified for this task. This method describes the determination of total recoverable petroleum hydrocarbons by SFE. The extract was trapped in trichlorotrifluoroethane (C2CI3F3); by IR-spectroscopy then the concentration of oil was determined. To calibrate the quantitative analysis, squalane was used as a representative standard oil. Laboratory scale experiments. The extraction parameters pressure, temperature, spécifie C0 -throughput (g C O 2 / g sample material) and flow rate were studied. The laboratory scale experiments were performed in two analytical SFE devices: in a Suprex MPS 225, and an equipment using a piston metering pump with automatic pressure control (Gilson M 308 and M 821). Samples of 2-4 g of grind were treated in a 3 ml extraction cell with supercritical C O 2 flowing continuously through the cell (dynamic extraction). Frequently the extraction was interrupted and the weight reduction of the sample was determined. When constant weight was achieved, the TC value of the remaining residue was determined. After extraction the loaded C 0 stream was depressurized through a restrictor into a solvent reservoir which trapped the precipitated oil. Depending on the solubility behavior of the extracts, precipitation or even plugging in the restrictor sometimes occurred during depressurizing. In this experiments a variable restrictor (type Suprex Duraflow) in which the cross-section of 2
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In Supercritical Fluids; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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the flow could be adapted to the requirements, proved to be less troublesome than the integral fused silica restrictors, which had been used initially. Since the oil solubility was determined using a static method and the extraction experiments were performed in a dynamic mode, the influence of the flow rate on the extraction efficiency was examined. Extraction experiments were conducted at 20 MPa and 50°C at flow rates of 0.5 to 4 g cm" min . Within the experimental error, no significant influence of the flow rate on the extraction yield could be detected. The pressure dependence of the extraction behavior was determined in the same way as described above. The weight reduction in wt% residual oil vs. specific mass flow rate is shown in Figure 2 for pressures of 15 to 30 MPa at a constant flow rate of 0.5 g cm" min" and 50°C (material no.50, Table I). In this pressure range the solvent density changes from 0.701 to 0.872 g cm" . At 30 MPa, after a specific throughput of about 13 g C 0 /g material, more than 99.9 wt% of the oil was extracted. With the same amount of carbon dioxide at 20 MPa a value of about 9 wt% and at 15 MPa even about 27 wt% of the oil remained in the sample. The TC content achieved at constant weight was about 0.9 to 1.5 wt%. A similar extraction behavior was found for all materials tested. The specific C 0 mass flow rate required for complete deoiling (
