Background of the invention The quenching technique in the heat treatment of metals has been recently addressed towards the search for oil compositions and products which are characterized by good biodegradability, waste disposal with low environmental impact, low toxicity and easy procedural handling with secure process conditions. Another important aspect is certainly linked to the recovery conditions regarding not only the oil recycling in subsequent quenching processes but also the quantity of obtained treated material unaffected by structural deficiencies due to thermal stress.

A development in this direction has been pursued by using tempering bath oils with mineral and vegetal compositions intended to suit the above mentioned features. Selection of a quenching agent is primarily governed by the processing specifications, the required physical properties, and the required microstructure.

Due to its versatile quenching performance, oil is the most widely used quenching medium, next only to water.

The worldwide requirement for quenching oil today is estimated at between 50 million and 100 million gallons per year.

Among the various quenching media, oil continues to be favored because its quenching mechanism and cooling curves are well suited to the TTT (time, temperature, and transformation) and CCT (continuous cooling transformation) diagrams of many types of steel.

Quenching of steel in liquid medium consists of three distinct stages of cooling: the vapor phase, nucleate boiling, and the convective stage. In the first stage, a vapor blanket is formed immediately upon quenching. This blanket has an insulating effect, and heat transfer in this stage is slow since it is mostly through radiation.

As the temperature drops, the vapor blanket becomes unstable and collapses, initiating the nucleate boiling stage.

Heat removal is the fastest in this stage, due to the heat of vaporization, and continues until the surface temperature drops below the boiling point of the quenching medium. Further cooling takes place mostly through convection and some conduction.

During the quenching process, there are two sorts of stresses involved: thermal stresses due to rapid cooling, and transformation stresses due to the increase in volume from austenite to martensite microstructure. Those stresses can cause excessive distortion or even cracks.

However, oil has a unique desirable cooling response in minimizing those effects. Consequently, oil will continue to be used for quenching as long as it is affordable.

For the application in heat baths there are several types of quenching oils suitable for steels with low to high hardenability. Thanks to the properties of these oils, it is possible to quench also into the martensitic temperature range-i. e. in a range between 160 and 250°C - with minimum distortion, while still obtaining the desired properties in metal parts.

Besides hardenability, selection of an oil formulation depends on part geometry and thickness, and the degree of distortion that can be tolerated. For example, hot oil is required for smaller parts with high hardenability to achieve the desired mechanical properties with minimum distortion.

Quenching oils are available with flash points ranging from 130°C to 290°C. The operating temperature of the oil in an open quench tank is normally at least 65°C below its flash point. When the quench tank is operated under a protective atmosphere, oil can be used at as high as 10°C below the flash point. The operating range of a heat bath quenching oils is normally from 10°C to 230°C.

A lower operating temperature is in any case helpful in minimizing thermal degradation of the oil.

Originally, oil was used without any additives. It was slow in cooling and susceptible to oxidation. Research was carried out to overcome these shortcomings by adding certain chemical additives to the oil. In addition, the objective was to make oil quenching more reliable and uniform, and to control the vapor phase by starting the nucleate boiling stage sooner. Consequently, the term "fast oil"is applied to oil with such additives. Some oils also have additives that extend the nucleate boiling stage to achieve deeper hardening for some steel.

Specially formulated oils also are available for vacuum heat-treating operations.

The use of mineral oils mixtures for quenching purposes is described for instance in the patents US 3 853 638, US 4 465 523 and US 6 239 082.

Those patents disclose different compositions for quenching processes comprising mineral oils and additive substances which should achieve the stabilization of the chemical and technological properties of the mixtures.

Nevertheless the use of oil baths-even of vegetal origin-in the tempering process still shows limits in terms of life-time and stability of the oils, due to the continuous thermal shocks involved.

This kind of degeneration exhibits its effects approximately one year after the first use of the quenching composition and is manifested by exhalations of vapors containing, among other components, alkenes and Cg-Cis Aldheydes which, besides the disadvantages related to the composition integrity, confer a typical bed smell to the oil.

In the prior art this problem is solved in the most cases by pumping the degeneration vapors into a thermal burner which neutralizes them, or by filtering the oil composition by means of complex apparatuses, like the cases disclosed in the patents US 5,630, 912 and US 5,824, 211. This assuages the aspect of the environmental impact of those compounds but leaves unsolved the problems connected to the unavoidable lowering of the so called"oil flash point", whose value is established by formal norms. Furthermore, the necessity of employing a regeneration plant leads to the logistical and economical inconvenient of collecting and transporting the used oil to the plant site, thus influencing and conditioning negatively the productivity of the quenching process.

Scope of the invention Scope of the present invention is therefore to find a convenient way of regenerating and deodorizing used quenching oils of both mineral and vegetal origin, which allows to neutralize the compounds resulting from the degeneration and degradation process of such oil compositions without influencing their flash point value.

A further scope of the invention is to provide a regeneration apparatus and process which facilitate the regeneration procedure and which does not affects the heat treatment performance.

Description of the invention A solution to the above cited problems is given by an apparatus as defined in claim 1 and by the process defined in claim 11. Further advantages are listed in the dependent claims. The regeneration apparatus according to the present invention comprises: - a device for extracting fractions of said compositions from a treatment tank, - an evaporation circuit comprising a return conduct, a heater and a flash reactor, - a heat exchanger for condensing the waste vapors - a waste collector which collects the vapors condensed in the heat exchanger.

This apparatus, allows to extract effectively the undesired degradation compounds from the oil composition, by submitting every fraction of used oil to different extraction cycles.

This is made possible by the return conduct, which by means of a return pump can bring back the purified oil, which has already undergone an extraction cycle, to the heater and then from here to the flash reactor of the evaporation circuit for a new waste separation. The purification process can be therefore repeated as many times as necessary until the desired number of cycles and thus the desired purity level have been reached. The treated oil can then exit the return conduct and reach the initial treatment tank at a flow rate which is controlled by the use of oil level-check devices.

At this point, a new fraction of quenching oil composition to be regenerated can be extracted from the tempering tank by means of the extracting device and then introduced in the return conduct of the evaporation circuit. The extracting device can be every sort of volumetric sampler, like e. g. a pump.

The undesired waste compounds are extracted from the oil composition by heating each oil fraction and then introducing it into the flash generator. The heating is accomplished by a heater which works at a temperature comprised between 150 and 250°C and which brings the heated oil fraction to the flash reactor. Here, by means of inlet nozzles positioned at the inflowing side of the heated oil, the waste products contained in the oil fraction are vaporized.

In order to optimize the vaporization process, the flash reactor works under vacuum by means of a high vacuum pump. Through this simple and quick evaporation process, the flash point value of the treated oil composition is not affected and contemporary the undesired oil components are selectively removed.

The vaporized substances are then conducted to the heat exchanger, which is cooled by a chiller at a temperature comprised between-5 and 5°C. By lowering the temperature of the vapors, the effects of avoiding volatile exhalations and condensing the separated fractions with high recovery are contemporary achieved. The so obtained condensed product is then collected in the waste collecting unit connected to the heat exchanger.

The entire oil regeneration process is controlled and run by means of a central control unit, with which every single procedural parameter can be modified and suited to the specific operating conditions by every cycle and in every case.

The duration of the process and thus the number of cycles depends on the oil quantity introduced in the evaporation circuit by the extracting device. The present apparatus allows to reach a treated oil quantity up to 400 Kg/h, which corresponds to 50 cycles circa.

With this method it is therefore possible to obtain a high purity final product, where the degeneration compounds have been completely eliminated and which shows a purity level allowing it to be immediately employed in a new quenching procedure.

A further advantageous aspect of the present invention is given by fact that the whole apparatus is assembled on a mobile base made of different materials, like e. g. steel but also other non-metals materials, which can be moved and maneuvered by a single operator.

This allows to avoid the used quenching oil being transported to the regeneration plant-with consequent logistic and economic disadvantages-and brings instead the regeneration apparatus directly to the tempering tank, where the process can be immediately applied. This shortens extremely the dead-time of the quenching procedure and contemporary does not affect the purification process itself.

Description of the drawings Fig. 1 shows a scheme of a preferred embodiment of the apparatus according to the present invention. The components are represented without any limitation regarding the application of the inventive concept and represent more a simplified view which can ease the explanation of the essential features of the apparatus.

The quenching oil composition which has to be treated is extracted from the tempering tank 1 by means of the extracting device 2, which in the specific case is a volumetric pump, which brings the collected oil fraction to the return conduct 3. From here, the oil fraction reaches the heater 3c and from there the flash reactor 4 where the undesired fractions are volatilized by passing through the inlet nozzles 4a. A high vacuum pump 5a facilitates the vapors extraction in the flash reactor 4.

The oil purified after the first cycle is then reintroduced in the return conduct 3, where by means of the valve 3b can either re-enter the heater 3c for another separation process or return to the tank 1 if the desired purity level has already been reached.

The vapors of the separated substances are cooled by the heat exchanger 5, which is connected to the chiller 5b.

The undesired waste compounds are then condensed into the collecting unit 6.