FIELD OF THE INVENTION The present invention relates to a process for treating used oils and in particular to a process of removing the contaminants from used lubricating oils to obtain a reusable recyclable product suitable for use as a feed stock for various grades of lubricating oil. BACKGROUND OF THE INVENTION

In most industrialised countries, lubricating oils are used for many applications including lubrication of automotive, railway and marine engines, farm or industrial vehicles and equipment as well as being used in industrial applications such as the metal working, cutting and machining of metals. As a consequence, large volumes of used lubricating oils are produced each year requiring either purification or safe disposal.

Lubricating oils generally contain some additives such as dispersants, detergents, emulsifiers, corrosion inhibitors, oxidation inhibitors, viscosity index improvers, pour point depressants, anti-wear or extreme pressure additives or anti-foams or the like.

The contaminants in the used lubricating oils are generally suspended particulates such as carbonaceous particles and pieces of wear metal, lead compounds, light hydrocarbons, oil oxidation products, water and decomposition products from the additives themselves.

The additives are designed to increase the life and effectiveness of lubricating oils by maintaining the contaminants in suspension and thus by their very nature increase the difficulty of removing these contaminants particularly suspended particulates from the used oils.

OBJECT OF THE INVENTION

Accordingly it is an object of the present invention to provide an economic process for treating waste oils and particularly used lubricating oils.

SHORT SUMMARY OF THE INVENTION

According to the present invention there is disclosed a chemical precipitation process for removing the contaminants from used lubricating oils comprised of the steps of: a) introducing a batch of used lubricating oil into a vessel; b) dehydration of the used oil by vacuum stripping and flash evaporation or other means to reduce water content to 0.5% or less; c) mixing the dehydrated oil with a non-hygroscopic acid miscible inorganic solvent; d) adding to said oil and solvent mixture an inorganic acid of adequate concentration and strength to cause a proportion of said contaminants to precipitate from said oil and solvent mixture thereby forming in the vessel a contaminant rich subnatant layer and an oil rich supernatant layer; e) separating the contaminant rich subnatant layer from the oil rich supernatant layer; and f evaporating said solvent from said supernatant layer to recover a contaminant reduced oil.

According to a second aspect of the present invention there is disclosed an absorption process for removing the contaminants from used lubricating oils comprised of the steps of: a) introducing a batch of used lubricating oil into a vessel; b) mixing the used oil with an acid-activated absorbent material such as clay and heating to a temperature of between 300°C and 450°C whilst maintaining a substantially oxygen free atmosphere in the vessel; and c) cooling the oil and absorbent mixture so that the absorbent material loaded with contaminants may be removed from the mixture and purified oil thereby recovered. According to a third aspect of the present invention there is disclosed a multistage process for removing the contaminants from used lubricating oils comprised of the steps of: a) introducing a batch of used lubricating oil into a vessel; dehydration of the used oil by vacuum stripping with flash evaporation or other means to reduce water content to 0.5% or less; mixing the dehydrated oil with a non-hygroscopic acid miscible inorganic solvent; adding to said oil and solvent mixture an inorganic acid of adequate concentration and strength to cause a proportion of said contaminants to precipitate from said oil and solvent mixture thereby forming in the vessel a contaminant rich subnatant layer and an oil rich supernatant layer; separating the contaminant rich subnatant layer from the oil rich supernatant layer; evaporating said solvent from said supernatant layer to recover a contaminant reduced oil; introducing a batch of contaminant reduced oil into a vessel; mixing the used oil with an acid-activated absorbent material such as clay and heating to a temperature of between 300°C and 450°C whilst maintaining a substantially oxygen- free atmosphere in the vessel; cooling the oil and absorbent mixture so that the absorbent material loaded with contaminants may be removed from the mixture and purified oil thereby recovered. BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention is now described with reference to the accompanying drawings, in which:

Figure 1 is a flow diagram for the Chemical Precipitation process of the invention.

Figure 2 is a flow diagram of Final Absorption treatment process in accordance with the invention.

Figure 3 is a flow diagram of the heating circuit used to heat the high temperature reactor vessels in Final Absorption treatment process in accordance with the invention.

PREFERRED MODE OF CARRYING OUT THE INVENTION

According to the embodiment of the process of the invention as depicted in Figures 1 -3 hereof, the process is multistage with the two principal stages being referred to herein as the Chemical Precipitation (CP) phase and the Final Absorption (FA) phase. Chemical Precipitation Phase (CP

The Chemical Precipitation Phase of the described embodiment of process of the invention is illustrated in the Figure 1. The used oil for processing enters from source (O) and is contacted with a non-hygroscopic solvent (S) which is miscible with the used oil in vessel 2. The mixing ratio of the waste oil to solvent is in the range of about 1 :4 to about 2:3 by volume with the preferred mixture being 30% by volume oil and 70% by volume solvent. To the mixture of solvent (S) and waste oil (O) is added a strong inorganic acid frm source [A] in a sufficient quantity to cause a proportion of the particulate and contaminants in the solvent mixture to precipitate.

The non-hygroscopic solvent is preferably an organic solvent containing a carbonyl group such as methyl ethyl ketone (ME ) or methyl iso-butyl ketone (MIBK). The preferred solvent is MIBK. The strong inorganic acid is preferably a mineral acid such as hydrochloric, sulphuric or even carbonic acid. The preferred acid is sulphuric acid which may be added in an amount of from 0.1% to 15% by volume, although proportional additions at the low end of the range up to about 5% are preferred. The sulphuric acid is preferably commercially available concentrated sulphuric acid which has a concentration of about 98 wt% H ₂ S0 ₄ .

The waste oil feed (O) mixed with solvent (S) preferably is not high in polychlorinated biphenyls (PCB's) and/or glycols and should have a low water content which is preferably less than 0.5 wt%. To reduce the water content to this level, the waste oil may be dehydrated by vacuum stripping with flash evaporation. In this way, oil feed stocks having a high water content can be handled by first dehydrating the feed stock to a water content of 0.5 wt% or less. Without limiting the invention by the theory of the reaction being carried out, the waste oil is considered to have a low residual acidity and it is thought that the addition of a small quantity of strong acid causes a polarity change in the mixture. This polarity change enables the carbon and contaminants to more easily precipitate from the oil/solvent solution. The acid does not appear to catalyse the precipitation in the sense that the acid initiates a reaction with the contaminants in the oil. However, since substantially all of the acid appears in the subnatant with the contaminant and particulate material, it appears that the acid does have a conditioning effect and undergoes some bonding on the particulate material. It is an essential element of the process that the acid is added after the oil and the solvent have been mixed in order for the polarity change to occur with such a small addition of strong acid. In general, the greater the proportion of oil in the oil solvent mixture, the greater the addition of acid required to bring about the required polarity change to enable the carbon and heavy metals to precipitate.

Once mixed in the mixing vessel 2 the oil/solvent/acid mixture is allowed to set for sufficient time to establish precipitation of the carbon and contaminants. Then the mixture is dumped into a buffer vessel 4 prior to being pumped to the coalescent plate precipitator shown as a coalescent plate separation vessel 6.

For economic reasons it is desirable that the subnatant layer passes to an evaporator where the solvent is recovered from the particulate material . The solvent recovered from the supernatant and subnatant layers may then be reused for contacting with the used oil to form a continually recycling solvent stream.

The coalescent plate separation filters have had the method of introducing the oil/solvent/acid plus the method of distributing the fluid within the filters designed specifically to ensure that there is no flow short tacking of supernatant or cross contamination of the incoming mixture and the precipitating subnatant, thus this design has eliminated the possibility of the remixing of the supernatant and the subnatant. The supernatant from the precipitator 6 flows into supernatant buffer vessel 10 before entering the solvent recovery stream via heat exchanger 12.

In the solvent recovery stream the supernatant is heated in heat exchanger 12 to a temperature of about 135°C at a pressure of 3.5 atma. The supernatant then passes through a throttle valve 14 to flash vessel 16 which is maintained at a pressure of about 0.2 atma.

The supernatant may then be repressurised and in a second heat exchanger 18, reheated to 135°C at 3.5 atma and passed through a second throttle valve 20 to a second flash vessel 22, maintained at a pressure of about 0.2 atma. After which the supernatant may then be repressurised and in a third heat exchanger 24, reheated to 135°C at 3.5 atma and passed through another throttle valve 26 to a finish vessel 28, maintained at a pressure of about 0.2 atma. The solvent vapour is collected from flash vessels 16, 22 and 28, passed to condenser 30 where the vapour is liquefied and collected in decant vessel 32. The solvent then returns to storage. The oil resulting from the vessels 16, 22 and 28, which are arranged in series, is referred to herein as CP oil and is then passed through another heat exchanger 34 to reduce the temperature of the oil to a safe level prior to being pumped to storage. CP oil may then be used as feed for the Final Absorption FA phase of the treatment of the present invention for further processing and purification to remove remaining carbon and oxidised colour pigments. This Final Absorption FA phase is described hereafter with reference to Figure 2. As is also represented in Figure 1 the subnatant from the precipitator 6 is collected and undergoes a pressurised heating and flash evaporation stage through heat exchangers 36 and flash vessel 38 to recover the solvent from the subnatant, in the same manner as previously described in relation to the supernatant solvent recovery. The subnatant is a substantially bituminous material having the heavy metals intimately bonded therein and may be used for asphalt and other road-making applications.

Due to the difference in volatility between the solvent and the oil and also economic efficiency it is of practical importance to recover as much solvent from the CP oil and the subnatant as possible and preferably have as little as less than 0.5 wt% of the solvent remaining in the solid subnatant materials or CP oil.

Final Absorption (FA)

Figure 2 depicts the Final Absorption (FA) process of the present invention.

In the FA phase a batch volume of the oil resulting from CP phase, suitable to the subsequent vessel ' s volume, is introduced to the preheat vessel 50 at ambient temperature and by continuous circulating the CP oil through heat exchanger 52 and back to the vessel 50 until the oil reaches 100°C +/- 1°C.

Preheating the oil aids the time sequencing of the two high temperature reactor vessels and facilitates better mixing of subsequent absorbent medium. The 100°C CP oil is then passed to one of two high temperature batch reactor vessels 54, 56. These vessels are operated in tandem so that as one vessel is being heated, the other is being filed or emptied. An absorbent, being acid activated clay in this embodiment, is added to the reactor vessel after it has been filled with CP oil, the oil/absorbent is kept turbulent with an agitator. This agitating facilitates better co-efficiency of heat transfer and increases the interface between the oil and the absorbent and maintains the absorbent in suspension. The ο ^' ή/absorbent is then heated to a temperature of between 300°C to 450° C for a period of up to 10 minutes, most preferably the oil/absorbent is heated to a temperature of between 390°C to 410°C for a period of 30 to 120 seconds.

Holding the oil/absorbent mixture at the elevated temperature beyond this time period is to no benefit and may not increase the removal of particulates and other contaminants from the oil and may result in higher percentages of light ends being produced.

The absorbent used may be any suitable commercially available absorbent and is preferably an acid activated clay such as bentonite. Preferably 6 to 15 wt% clay is added to the CP oil with a more preferred range of 7 to 11 wt%. The most preferred addition of bentonite to CP oil is about 9 wt .

Nitrogen is continuously supplied or bled to the reactor vessel to maintain an oxygen-free environment or blanket within the reactor vessel at a positive gauge pressure (0.2 bar gauge) to eliminate air from the vessels thus preventing oxidation of the oil whilst at high temperatures. It is important for safety and economic considerations.

Whilst still under the nitrogen blanket the oil/absorbent mixture is then cooled to about 120°C, before the nitrogen is turned off then the oil/absorbent mixture is passed to a clay separator system, such as a screw press (not shown) followed by a pressure leaf filter (not shown) to separate the absorbent from the product oil Further lower temperature contact and separation with an absorbent may also be preferred in order to produce a 'brighter' final product.

In this case the filtrate from the filters undergoes a further low temperature polishing step (not shown in the drawing). The filtrate/absorbent mixture is heated to a temperature of approximately 130°C to 140°C at a pressure of 0.3 Abs and then cooled to 120°C before passing through a polishing filter such as a pressure leaf filter, to remove the absorbent and give a bright luster to the refined oil. The refined oil from the second filter is then cooled before being pumped to storage as the finished refined product.

During the time the oil/absorbent mixture is being heated through temperature range 360°C to 410°C, 10% to 30% of the product oil may be cracked to a lighter fraction (Light Ends), these light ends in the form of vapour are passed from the reactor vessels to a condenser 58 where the vapour is condensed to liquid light ends and sub cooled. The light ends are then collected in a decant vessel 60 prior to being pumped to storage. The light ends decant vessel is vented via a pressure retention valve at 1.2 Bar Abs to an Odour Control Device. In this way an oxygen-free environment is maintained by a positive pressure nitrogen blanket within the reactor vessel, condenser and decant vessel. The temperature utilised in this phase becomes crucial in both end product and economic terms as the higher the temperature used the more light ends (and less re-processable oil) result. However, if the temperature is not sufficiently high, the re-processable oil produced may be of a quality where it cannot be used as a base to re-mix to various lubricating grades. It is difficult to increase the clay addition beyond 15% and operate effectively due to the loss in heat transfer coefficient resulting from such additions, the difficulty in maintaining the absorbent in suspension at this level, the increased capital cost of the filters and the disposal of the filter cake. The amount of absorbent material must be high enough to remove a substantial proportion of the remaining particulate material and contaminants from the oil . However if the amount of absorbent is substantially higher than that required, the duty on the subsequent apparatus used to separate the absorbent from the oil may affect the economics of the process adversely. Thus the above ranges will depend on the degree and to some extent the nature of the contaminants in the lubricating oils to be purified which may vary depending on the source of the waste oil and the government restrictions in the country where the oil is being sourced.

Figure 3 illustrates the heating circuit for the dual (FA) reactor vessels.

To heat the high temperature reactor vessels 54, 56, crack-resistant high temperature thermal oil is circulated through heating coils in these vessels. The thermal oil is heated in a separate thermal oil heater 66 as shown. Since high temperature thermal oils have a shortened operating life when used at temperatures in excess of 400°C for any extended period of time due to oxidation, it is necessary for the heater to operate at two or more temperature settings. THERMOL® oil is suitable for this stage.

When the oil/absorbent mixture is first added to one of the two depicted reactor vessels 54, 56, the thermal oil circulates between the two reactors to retrieve heat from the vessel being cooled and then, after leaving that vessel and entering the second vessel, transfers the heat to increase the temperature in the vessel being heated to round 240°C (whilst the thermal oil heater is at idle). Then the heater circulates thermal oil at 380° C in the vessel being heated until the oil/absorbent mixture passing through the vessel being heated reaches a temperature of about 350°C to 36Q°C. The heater then switches to a high temperature mode to circulate the thermal oil through vessel being heated to high temperature at 425°C for time sufficient for the oil/absorbent mixture to be maintained at a temperature in excess of 00°C for the prescribed period preferred.

The vessel being cooled, having shared its high temperature (indexes of 400°C) with the other vessel being heated, is now further cooled to around 120°C by circulating thermal oil between it and heat exchanger 68. The above mentioned phases of heating and cooling the tandem reactor vessels 54 and 56 is achieved by a bank of valves indicated collectively as 70 and two pumps 72, 74 in the thermal oil system. Test Results

The process of the present invention has been trialed and has given the test results appearing in the following tables. Table 1 shows the results after the CP treatment process (stage 1) and Table 2 the cumulative result after the waste oil had undergone both the CP and FA stages of the process in accordance with the present inventive process as above described.

Waste oil having a water content of < 0.5 wt% and having the analysis shown in Table 1 was mixed with MIBK and subjected to acid treatment with 98 wt% concentrated sulphuric acid. The acid was added to the mixture in an acid/oil ratio of about 1 :300. The proportions of solvent, oil and acid on the mixture were 70 vol% MIBK, 29.9 vol% waste oil and 0.1 vol% H ₂ S0 ₄ . After Stage 1 of the purification treatment in accordance with the invention, the oil was analysed and the results are shown immediately below in Table 1.

Table 1

Note:★ = Not Determined

The above example shows that with only a small addition of concentrated sulphuric acid, a large reduction in the colour and ash content of the waste oil can be achieved in this CP process. Additionally, the amount of suspended metals and other contaminants is greatly reduced.

The substantially MIBK-free supernatant was then subjected to high temperature absorption treatment in accordance with FA in which acid-activated bentonite was added to create a mixture containing 9 wt% clay. Bentonite was also used in a low temperature absorption treatment to produce refined oil which was analysed and tabulated in Table 2. Table 2

Sample FP 1 FP 2 FP 3

Appearance Clear dark red/ Clear dark red/ Clear dark red/ brown brown brown

KV 40°C 81.6 84.6 79.0 V 100°C 10.2 12.1 9.8

Sulphate Ash <0.01 <0.01 <0.01 Sample FP 1 FP 2 FP 3

TBN <0.01 0.14 0.19

Metals (ppm)

Fe 11 2 2

Cr 0 0 0

Cu 0 0 0

Pb 0 0 0

Al 2 1 0

Sn 0 0 0

Si 4 2 2

Na 0 0 0

Zn 0 0 0

Ca 10 0 0

Mg 6 6 6 By subjecting the treated oil to a further treatment in accordance with FA, the colour, carbon and contamination content of the refined oil can be reduced to a level which allows the refined oil to be recycled as a base feed stock for lubricant oil.

Thus depending on the level of contamination, subjecting waste oil to CP treatment with subsequent FA treatment, enables a potential pollutant to be recycled and used as a valuable resource and supply of the heavier hydrocarbons which are used as lubricants.

Since modifications within the spirit and scope of the invention may be readily effected by persons skilled in the art it is to be understood that the invention is not limited to the particular embodiment described, by way of example, hereinabove.

It is to be understood that the word "comprising" as used throughout the specification is to be interpreted in its inclusive form, ie. use of the word "comprising" does not exclude the addition of other elements.