EP0748882A1 - Process for cleaning oil contaminated components - Google Patents

Abstract

Removal of oil or oil-water emulsion from the surface of components consists of evacuating the furnace to remove residual air followed by heating with an inert gas which has a sub-atmospheric pressure above the vapour pressure curve of the oil. The gas is passed in a continuous manner over a heating source and the component. When the heating has been carried out the pressure is reduced to below the vapour pressure curve of the oil and the oil is vaporised, drawn off and condensed. The component is then ready to pass to a subsequent work operation.

Description

The invention relates to a method for cleaning oil-wetted components in a vacuum oven, which is first evacuated to a predetermined first pressure by a vacuum pump to remove the residual air as far as possible, and in the following to accelerate the heating of the components, an inert gas until a second is reached is introduced under atmospheric pressure, which is above the first pressure, the inert gas is circulated through the components and a heat source and the pressure is reduced to evaporate the oils to a value which is below the same vapor pressure curve, so that the oils evaporate and be condensed in a condenser.

Components wetted with oil often occur in the intermediate stages of manufacturing processes. The oils or oil-containing liquids (emulsions) in question are, for example, cooling liquids that are used in machining and grinding processes or hardening or quenching oils. In any case, these liquids have to be removed again, since they are not only annoying in subsequent processing processes, but also cause disposal problems. The release of vapors in downstream production systems, such as in hardening or tempering furnaces, is particularly disruptive. This can not only contaminate these ovens come, but environmental toxins can also be formed by the temperature treatment.

It is known to carry out intermediate cleaning with alkaline cleaning agents or with solvents from the group of chlorinated hydrocarbons, chlorofluorocarbons, TRI and PER. In all cases, contaminated cleaning agents remain after prolonged use, which have to be disposed of in a costly manner. In addition, the oils removed from the components by the cleaning process are lost.

It is also known to remove oil residues from oil-wetted or oil-soaked solids by vacuum processes. For this purpose, the solids in question are introduced into a heatable vacuum chamber and are gradually freed from the oils and fats as the pressures and temperatures rise, whereby individual fractions of the condensates may also occur. This so-called vacuum distillation is time-consuming because it is difficult to bring the oil to be de-oiled or degreased sufficiently quickly to an adequate evaporation temperature. The time may still be acceptable when it comes to the de-oiling or degreasing of wastes that are also of relatively low density, e.g. Oil filters and tin cans.

However, the low heating-up speed caused by the vacuum proves to be extremely disruptive in production processes. For example, it is extremely time-consuming to heat up fillings or packs of gearwheels, tools, etc. sufficiently quickly to a sufficiently high temperature at which evaporation of the adhering oils is economically possible.

A method of the type described in the introduction is known from US Pat. No. 4,141,373 C, but only for the de-oiling of scrap Improvement of the heat transfer from an internal heat source to the components during the heating and evaporation period continuously supplied with inert gas and circulated via a condenser and a vacuum pump and / or pumped to the atmosphere. A range from 65 ° C to 593 ° C and a pressure range from 564 mbar to 691 mbar (during heating) and up to a minimum of 173 mbar (during the main phase of oil evaporation) is specified for this. From the beginning, oil evaporates in an amount that increases with temperature.

This procedure leads to a high consumption of inert gas if it is continuously discharged to the atmosphere, but also for energy in the circuit due to constant heating of the inert gas, since this is continuously cooled down with the oil vapors in the condenser. This is also due to the fact that the connection between the vacuum furnace and the condenser cannot be interrupted, so that there is an ongoing energy gradient to the condenser. A mixture of oil and inert gas can only be de-oiled continuously with a very large heat exchanger or condensation surface. A very large amount of energy is wasted in both the flow and the cycle.

This should also be the reason why the heating-up period is 5.5 hours and the final temperature is only 371 ° C or 343 ° C. After the heating-up period and at the beginning of the main phase of oil evaporation, the supply of inert gas is halved, but not completely interrupted. The open process is necessary because otherwise at the high final temperatures for most of the oils in question, their vapor pressure significantly exceeds atmospheric pressure. At these high final temperatures, thermal damage to most oils is inevitable and their reuse is impossible.

If the components are workpieces for mechanical engineering, such high final temperatures are also detrimental to most workpieces, since the temperatures are, for example, above the normal tempering temperature of hardened workpieces, especially if the workpieces are surface-hardened.

Due to the specified pressure and temperature curve, the high pressure inevitably leads to the area below the respective vapor pressure curve, so that any oil accumulation on or in the components leads to the boiling of the oils and thus to the formation of heat sinks and uneven temperature zones, which only take a long time Process duration can be compensated again.

From the same US Pat. No. 4,141,373 C it is also known to dispense with the use of inert gas when the scrap is heated by electrical contact heaters. However, this measure is completely useless for the heating of workpiece packs because it results in considerable temperature irregularities which can only be tolerated when scrap oil is removed from oil.

From EP 0 541 892 A2 it is known to heat components for mechanical engineering to 200 ° C. and to lower the pressure to 10 hPa (10 mbar) in order to continuously supply oil and degreasing. Flooding with inert gas to shorten the heating-up times is not disclosed.

Due to the unpublished DE 44 15 093 it belongs to the state of the art to heat scrap with proportions of organic substances, including waste oils, to temperatures up to 450 ° C and to lower the pressure to 10 ^-3 mbar in order to openly close closed scrap parts, for example and de-oil. Flooding with inert gas to shorten the heating-up times is not disclosed.

From JP 5-78875 it is known to de-oil parts first in a nitrogen atmosphere and then by high pressure superheated steam. The process is complex and there is a corresponding amount of condensed water which has to be separated from the likewise condensed oil.

The invention is therefore based on the object of specifying a vacuum method of the type described at the outset, by means of which temperature-sensitive workpieces, in particular surface-hardened workpieces, with a relatively large density can be heated as uniformly as possible to a temperature in the shortest possible time, at which deoiling by a greater reduction in pressure is possible and which is already in the range of temperatures for a subsequent machining operation.

• a) der zweite Druck oberhalb der Dampfdruckkurve des benetzenden Öles gewählt und durch Fluten des Vakuumofens eingestellt wird,
• b) die Inertgaszufuhr und die Evakuierung nach dem Fluten unterbrochen und das Inertgas und die sich bildenden Öldämpfe innerhalb einer Aufheizperiode ausschließlich im Innern des Vakuumofens über die Bauteile geleitet werden, bis eine vorgegebene Endtemperatur der Bauteile erreicht ist, und daß
• c) nach Beendigung der Aufheizperiode eine Verbindung vom Vakuumofen zum Kondensator und zur Vakuumpumpe hergestellt und der Druck auf einen Wert abgesenkt wird, der unterhalb der Dampfdruckkurve liegt, und die hierbei verdampften Öle abgezogen und kondensiert werden.

According to the invention, the object is achieved in the method specified at the outset in that

• a) the second pressure above the vapor pressure curve of the wetting oil is selected and set by flooding the vacuum furnace,
• b) the inert gas supply and the evacuation after the flooding are interrupted and the inert gas and the oil vapors which form are passed over the components inside the vacuum furnace only within a heating period until a predetermined final temperature of the components is reached, and that
• c) after the end of the heating-up period, a connection is established from the vacuum furnace to the condenser and to the vacuum pump and the pressure is reduced to a value which is below the vapor pressure curve, and the oils evaporated in the process are drawn off and condensed.

"Flooding" means a one-time filling of the vacuum furnace with the relevant gas and no ongoing gas supply from the outside. After flooding and until the evacuation again, it is a closed atmosphere with an internal cycle.

By flooding the vacuum furnace with inert gas to a pressure that is significantly higher than the initial pressure in the vacuum furnace and, for example, between 500 and 1000 mbar, and by circulating inert gas within the vacuum furnace, which is sealed off from the outside, via the components and a heat source in the closed Circulation enables the components to be heated up extremely quickly, with a significantly steeper temperature rise than in the generic method. However, the circulation of the appropriately heated inert gas with increasing oil proportions due to evaporation not only enables faster heating of the components, the so-called charge, but also the inner surfaces of the vacuum furnace, on which the released vapors could otherwise condense. During the heating phase under inert gas, there is initially no evaporation of the oils by boiling. This evaporation begins at the moment when the pressure is reduced to a value which is below the relevant vapor pressure curve. At this moment, the evaporation of the oil begins, which condenses and can be collected and recovered practically quantitatively.

As a result, not only the consumption of inert gas but also the energy consumption decrease compared to the generic state of the art, since the inert gas does not have to be continuously heated again from the temperature in the condenser to a temperature suitable for workpiece heating.

In addition, the inert gas is briefly sucked off at the start of evaporation and condensation and not constantly in the circuit passed through the condenser. As a result, much smaller condensation areas are required, for example only one tenth of the condensation areas in the generic method.

The evaporation process is particularly intensive when the pressure after the end of the heating period for evaporating the oils is reduced to a value below 100 mbar, preferably below 10 mbar.

To protect the components, it is particularly advantageous if the heating-up period is ended at a temperature of at most 350 ° C., preferably at most 300 ° C.

Such a vacuum cleaning can be easily integrated into a manufacturing process. For example, metal workpieces are completely freed from quenching oils and cooling lubricants. Neither aqueous alkaline solutions are required for cleaning, nor solvents of the type specified above. Costly preparation processes are superfluous, and only extremely small amounts of gas reach the ambient air via the vacuum pumps. However, a condenser is installed upstream of these vacuum pumps in which the oil vapors released are condensed.

A particularly advantageous procedure is characterized in the course of a further embodiment of the invention, that for starting components wetted by a quenching oil, the vacuum furnace is flooded during heating and cleaning of the components to a pressure which is above the evaporation pressure of the quenching oil concerned at the following applied tempering temperature of the component material, and that after reaching this tempering temperature, the total pressure is lowered and kept down until the quenching oil is at least largely evaporated and the tempering process is complete.

This process allows the quenching oil to be cleaned and the starting process to be carried out immediately in succession and without interruption in the same vacuum. The de-oiling and the starting process merge practically into one another, which results in a huge saving of time within a manufacturing process. In this case, upstream or intermediate cleaning units are not required.

• a) die anfängliche Druckabsenkung zur weitgehenden Beseitigung der Restluft auf einem Wert erfolgt, der oberhalb der Dampfdruckkurve des Wasser liegt,
• b) in einem folgenden Verfahrensschritt zur Beschleunigung des Aufheizens der Bauteile der Vakuumofen mit einem Inertgas bis auf einen Druck geflutet wird, der oberhalb der Dampfdruckkurve von Wasser liegt, wenn das Inertgas hierbei umgewälzt wird, und wenn zum Verdampfen des Wassers der Gesamtdruck auf einen Wert abgesenkt wird, der unterhalb der Dampfdruckkurve des Wassers, aber oberhalb der Dampfdruckkurve des Öls liegt, und wenn
• c) in einem weiteren Verfahrensschritt zur weiteren Beschleunigung des Aufheizens der Bauteile der Vakuumofen erneut mit einem Inertgas bis zu einem Druck geflutet wird, der oberhalb der Dampfdruckkurve des Öls liegt, wenn das Inertgas hierbei umgewälzt wird, und wenn zum Verdampfen des Öls der Gesamtdruck auf einen Wert abgesenkt wird, der unterhalb der Dampfdruckkurve des Öls liegt.

When cleaning components that are wetted with oil-water emulsions, it is particularly advantageous if

• a) the initial pressure drop to largely remove the residual air takes place at a value which lies above the vapor pressure curve of the water,
• b) in a subsequent process step to accelerate the heating of the components of the vacuum furnace with an inert gas to a pressure which is above the vapor pressure curve of water, if the inert gas is circulated here, and if the total pressure to evaporate the water to a value which is below the vapor pressure curve of the water but above the vapor pressure curve of the oil, and if
• c) in a further process step to further accelerate the heating of the components of the vacuum furnace is flooded again with an inert gas to a pressure which is above the vapor pressure curve of the oil, if the inert gas is circulated, and if the total pressure is to evaporate the oil a value is lowered which is below the vapor pressure curve of the oil.

This measure makes it possible to at least largely separate oil and water from one another and also in the form of separate ones To collect condensates. Only in process step b), depending on the partial pressure ratios, small amounts of oil vapors already pass into the condensate with the water vapor.

Two exemplary embodiments of the subject matter of the invention and a device for carrying out methods according to the invention are explained in more detail below with reference to FIGS. 1 to 4.

Figur 1

ein Diagramm über den zeitlichen Ablauf eines kombinierten Reinigungs-Anlaßverfahrens,

Figur 2

ein Diagramm eines zeitlichen Ablaufs des Verdampfens von Wasser und Öl als Vorstufe für einen Härteprozeß,

Figur 3

einen Vertikalschnitt durch einen Vakuumofen in Verbindung mit einem Fließschema für die Erzeugung der verschiedenen Prozeßparameter und

Figur 4

ein Diagramm mit den Dampfdruckkurven für Wasser sowie für Abschrecköl.

Show it:

Figure 1

a diagram of the timing of a combined cleaning-starting process,

Figure 2

2 shows a diagram of a time course of the evaporation of water and oil as a preliminary stage for a hardening process,

Figure 3

a vertical section through a vacuum furnace in connection with a flow diagram for the generation of the various process parameters and

Figure 4

a diagram with the vapor pressure curves for water and for quenching oil.

In Figure 1, the time is plotted on the abscissa; on the ordinate pressure and temperature. The pressure curve is characterized by solid lines, the temperature curve by a dashed line. The individual process parameters result from Example 1 described below. It can be seen that within the time period t ₁ to t ₂ of 45 minutes the tempering temperature of 180 ° C. was reached with a heating power of 90 kW. After this heating-up period, the pressure was rapidly reduced at time t ₂ . The resulting flow of oil vapor is symbolically indicated by the dotted field and the arrows. At time t ₃ , that is after one After a further 120 minutes, both the cleaning process and the tempering process had ended.

FIG. 2 shows the process flow according to example 2 in diagram form in an analogous representation as in FIG. 1. Reference is made to this example 2 with regard to the individual process parameters. At time t ₁ , the vacuum furnace was evacuated to a pressure of 25 mbar, and was immediately flooded to 950 mbar by letting in nitrogen. The diagram shows that in this time span t ₁ to t ₂ the workpiece temperature rose rapidly to a value of 80 ° C. In this operating phase, there is rapid heating, but no significant evaporation of water takes place. By reducing the pressure in the operating phase t ₂ to t ₃ , a pressure of 120 mbar was initially achieved, at which the water is evaporated very quickly at the specified workpiece temperature of 80 ° C. This process is symbolically indicated by the arrows and the dot field. The low temperature drop is due to the withdrawal of the heat of vaporization. When the vacuum pumps are running, the end of the evaporation of water is pushed down to a value of approximately 1 mbar by a steep pressure drop. From this point on, it was necessary to further heat the workpieces or components in order to also evaporate the oil remaining from the emulsion. For this purpose, the vacuum furnace was again flooded with nitrogen up to a pressure of 700 mbar, within the period from t ₃ to t ₄ . It can be seen that during this operating phase, in which the nitrogen is passed over a heating device by means of the blower, the steep temperature rise within the batch shown in dashed lines resulted. Likewise, there was no noticeable evaporation of oil during the period between t ₃ and t ₄ . The evaporation of oil only started abruptly when the pressure in the vacuum furnace was rapidly reduced from 700 mbar to 0.1 mbar at time t ₄ . The flow of the oil vapor is symbolized by the arrows and the dot field. The oil evaporation ended shortly before the time t ₅ , so the workpieces were lying in a clean, dry form and could now be subjected to a hardening process by being heated and quenched with quenching oil. The cleaning of the components hardened in this way could now again be carried out in accordance with the operating sequence according to FIG.

In Figure 3, a vacuum furnace 1 is shown, which consists of an oven chamber 2 and a door 3, both of which are each surrounded by thermal insulation 4. The vacuum furnace has an inner surface 5. The furnace atmosphere can be circulated by a fan 6, which consists of a fan wheel 7 and a drive motor 8. The heating device, via which the furnace atmosphere is circulated, is not shown for the sake of simplicity. It is arranged as a heating resistor between the heat insulation 4 and the furnace chamber 2, the inner surface 5 of which thereby becomes the heat exchange surface. Temperature sensors T ₁ and T ₂ serve to control and, if necessary, regulate the wall temperature of the vacuum furnace and the batch; the pressure of the furnace atmosphere is detected by a pressure sensor P and regulated if necessary.

A suction line 9, in which a shut-off valve 10 is arranged, leads to a condenser 11, to which two vacuum pumps 12 and 13 are connected. The condenser 11 is connected to a coolant circuit, of which only the two lines 14 and 15 are shown here, to which the vacuum furnace 1 and the motor 8 are also connected. A template 16 is provided for collecting the condensate or condensates. The individual associated shut-off valves are not numbered for the sake of simplicity.

The shut-off valve 10 is an important prerequisite for the rapid heating of the components: it is decided after the respective evacuation and before flooding through the inert gas source N ₂ (nitrogen) and remains closed during the individual heating periods, so that during this time (s) there can be no pressure and temperature drop to the condenser 11. It is only opened again for a sudden drop in pressure to pressures below the respective vapor pressure curve (s), so that the flooded and limited amount of inert gas can be sucked off briefly and then the evaporation of the condensable components can be brought to an end by boiling without supplying inert gas. There is no external circuit for the continuous return of the inert gas to the furnace chamber 2. The condenser 11 and the amount of heat dissipated therein can thereby be kept very small.

FIG. 4 shows a diagram in which the temperature in ° C. is plotted on the abscissa and the pressure in mbar on the ordinate. Curve 17 indicates the thermodynamic data of water, while curve 18 indicates the thermodynamic data of a possible quenching oil. There is no noticeable evaporation of the liquid in question in the fields to the left or at the top of the curves; The parameters for evaporation of the liquid in question are located in the fields on the right and below. Figure 4 is used in particular to illustrate the operating conditions in the claims and in the examples.

Example 1:

Gearwheels hardened by oil quenching made of the alloy 16MnCr5 with a total weight of 400 kg and room temperature, after the quenching oil had dripped off, were placed in a basket for tempering in the plant shown in FIG. 3, the vacuum chamber furnace of which had an internal volume of 2.4 m ³ . The furnace was first evacuated to a vacuum of 4 mbar without the addition of nitrogen. The shut-off valve 10 was then closed and the furnace was immediately flooded with nitrogen to a pressure of 700 mbar. The nitrogen supply was then shut off. The nitrogen in the Passed inside the furnace chamber with the shut-off valve 10 in the circuit via the heating device of the furnace and via the gear wheels. The heating power was 90 kW. The tempering temperature of 180 ° C. was reached after about 45 minutes. At this temperature, the vapor pressure of the quenching oil was 1 mbar, ie the nitrogen pressure was significantly above this vapor pressure, so that no significant evaporation of the quenching oil by boiling was observed. Due to the circulation of the closed furnace atmosphere with small but increasing proportions of oil, an extremely uniform batch temperature was achieved. The shut-off valve 10 was now opened and the furnace was now evacuated to a pressure of 0.1 mbar, which was below the said vapor pressure of the quenching oil at the gear wheel temperature, so that the oil began to evaporate by boiling. After a period of 120 minutes, the heating was ended, the furnace was flushed with nitrogen and the gearwheels were cooled. The gears were dry and 9800 grams of quenching oil had accumulated as recyclable condensate.

Example 2:

Gearwheels made of the alloy 16MnCr5 with a total weight of 400 kg and room temperature wetted with a water-oil emulsion as a coolant were introduced after the emulsion had drained off in a basket into the system shown in FIG. 3, the vacuum chamber furnace of which had an internal volume of 2. 4 m ³ . The furnace was first evacuated to a vacuum of 25 mbar and the shut-off valve 10 was closed. The furnace was immediately flooded with nitrogen to a pressure of 950 mbar, and then the nitrogen supply was shut off. The nitrogen in the inner circuit was passed through the blower over the heater of the furnace and over the gearwheels with a heating power of 90 kW and a temperature of 80 ° C was reached. At this temperature, the vapor pressure of the water was 473 mbar, ie the nitrogen pressure was above this vapor pressure, so that no significant evaporation of the water was observed. The shut-off valve 10 was opened again and the furnace was now evacuated to a pressure of 120 mbar, which was below the said vapor pressure of the water at the gear temperature, so that the evaporation of the water but not the oil of the emulsion began. By withdrawing the heat of vaporization of the water, the temperature of the gears dropped slightly. A total of 1800 g of water was collected as condensate within 10 minutes. The furnace was then evacuated to a pressure of 1 mbar in order to remove all water vapor from the furnace.

After the water had been evaporated and the shut-off valve 10 had been closed, the furnace was again flooded with nitrogen to a pressure of 700 mbar. The nitrogen supply was then switched off and the nitrogen was passed in a closed, internal circuit by means of the blower while the heating was continued, with the same output, over the gearwheels until after 30 minutes the temperature had reached 180.degree. The vapor pressure of the oil was 1 mbar below the nitrogen pressure, so that no significant oil evaporation by boiling was observed. After this temperature had been reached, the shut-off valve 10 was opened again and the furnace was evacuated to a pressure of 0.1 mbar, which was below the vapor pressure of the oil. Oil evaporation started immediately. After a period of 120 minutes, the heating was ended, the furnace was flushed with nitrogen and the gearwheels were cooled. The gears were dry and 160 g of oil had accumulated as recyclable condensate. The gearwheels dried in this way were then heated to the hardening temperature, quenched with a quenching oil and freed from the quenching oil and tempered in accordance with the method according to Example 1.

Claims (5)

Process for cleaning oil-wetted components in a vacuum oven (1), which is first evacuated to a predetermined first pressure by a vacuum pump (12, 13) in order to remove the residual air as far as possible, and in which an inert gas is subsequently used to accelerate the heating of the components is introduced to reach a second subatmospheric pressure which is above the first pressure, the inert gas being circulated through the components and a heat source and the pressure being reduced to evaporate the oils to a value which is below the same vapor pressure curve so that the oils are evaporated and condensed in a condenser, characterized in that a) the second pressure above the vapor pressure curve of the wetting oil is selected and set by flooding the vacuum furnace (1), b) the inert gas supply and the evacuation after flooding are interrupted and the inert gas and the oil vapors which form are passed over the components within a heating period exclusively inside the vacuum furnace (1) until a predetermined final temperature of the components is reached, and that c) after the end of the heating period, a connection from the vacuum furnace (1) to the condenser (11) and to the vacuum pump (12) is established and the pressure is reduced to a value which is below the vapor pressure curve, and the oils which have been evaporated are drawn off and condensed. A method according to claim 1, characterized in that the pressure after the heating period for evaporating the oils is reduced to a value below 100 mbar, preferably below 10 mbar. A method according to claim 1, characterized in that the heating period is ended at a temperature of at most 350 ° C, preferably at most 300 ° C. A method according to claim 1, characterized in that for tempering components wetted by a quenching oil, the vacuum furnace (1) is flooded to a pressure during the heating and cleaning of the components which is above the evaporation pressure of the quenching oil concerned at the subsequently used tempering temperature of the component material lies, and that after this tempering temperature is reached, the total pressure is again lowered and kept lowered until the quenching oil has at least largely evaporated and the starting process has ended. A method according to claim 1, characterized in that for cleaning components which are wetted with oil-water emulsions, a) the initial pressure drop to largely remove the residual air takes place to a value which lies above the vapor pressure curve of the water, b) in a subsequent process step to accelerate the heating of the components of the vacuum oven with an inert gas to a pressure which is above the vapor pressure curve of water, that the inert gas is circulated, and that to evaporate the water, the total pressure to a value is lowered below the Vapor pressure curve of the water, but above the vapor pressure curve of the oil, and c) in a further process step to further accelerate the heating of the components of the vacuum furnace is flooded again with an inert gas to a pressure which lies above the vapor pressure curve of the oil, that the inert gas is circulated here, and that the total pressure is applied to evaporate the oil a value is lowered which is below the vapor pressure curve of the oil.

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