Field of the invention

The present invention relates to a process for the purifica¬ tion of spent process oils, i.e. process oils contaminated with solid or dissolved impurities, which have been accumu¬ lated in the oil. The present invention further relates to the use of various substances for the purification of spent process oil.

Background of the invention

All patent citations are expressly incorporated by reference in their entirety.

There are different kinds of process oils used within for instance the steel industry. When used as lubricating agents they become contaminated by impurities, which accumulate in the oil. So far no industrially useful process is available for the purification of these oils.

The term process oil as used herein and in the claims gener¬ ally relates to oil used in various industrial processes. Non-limiting examples of process oils are rolling oil, honing oil, engine lubricating oil, mineral oil, paraffin oil, and paraffin oil containing chlorine. A common method for the destruction of process oils is destruction by burning but such destruction causes environmental problems; this is par- ticularly true for chlorine-containing oils of the type chloroparaffinic oils.

At present many processes within the steel industry do not have environmentally friendly alternatives to chloroparaf- finic oils for efficient lubrication at high load (heavy duty- operations) .

Previously it is known that oils (based on vegetable or min- eral oil) may be purified from contaminants with phase chemi¬ cal methods based on organic polymers and/or polymer mixtures comprising a charged control polymer (WO95/14752) . The main disadvantage of this process according to the prior art is due to the fact that the distribution between the target fluid and the separation additive mainly occurs between the target fluid and the surface between the target fluid and the separation additive. The background for this is the chemical interactions at the surface and that the density difference between the polymer/polymer mixtures and the target fluid is considerable.

Further there is an earlier described process for the separa¬ tion of particular impurities in mineral oils based on floc- culation with dicarboxylic acids dissolved in a suitable or- ganic solvent (SE464306) .

Accordingly there is a continuous need for a process by means of which contaminated process oils in general and environmen¬ tally non-friendly contaminated process oils such as chloroparaffinic oils, in particular, can be purified to such an extent as to enable the recycling thereof.

Separation with several phases involved may be employed for applications where solid dispersed impurities or dissolved substances can not be separated from a fluid with other con¬ ventional techniques such as filtration or centrifugal sepa¬ ration. Such separations are in general based on surface- chemical phenomena and comprise choosing a suitable separa- tion additive. Said separation additive is under given condi¬ tions substantially insoluble in the fluid to be purified. The aim of the chemical separation process is to separate the target fluid to be processed from at least one impurity.

The process is called a chemical phase separation when a sub¬ stance in the fluid to be purified, depending on its interac¬ tions with the surrounding molecules migrates to the separa¬ tion additive. Chemical phase separation may constitute steps comprising more than two phases. This may be accomplished for instance by using several different separation media, which are not miscible.

Chemical phase separation for the purification of chloropar- affinic oils has not been reported earlier in literature. This is in particular true for chloroparaffinic oils used at "heavy duty operations" within steel industry and other manu¬ facturing processes regarding machining of steel and iron and with the type of impurities (lubricants and particular impu- rities) that are thereby accumulated in the oil.

Methods to improve the separation of impurities consisting of particular and/or dissolved inorganic or organic agents from process oils of the type rolling oil or engine lubricating oil using phase chemical methods have also not been described previously.

Brief description of the invention

The present invention solves the problems of unnecessary de¬ struction of contaminated process oils by admitting recycling of contaminated process oils after purification, which can not be achieved by any known technology. -A -

According to the invention there is provided a process which solves the problem to purify process oils from solid and dis¬ solved impurities by means of a combined surface chemical and mechanical separation process. The process according to the invention for the purification of contaminated process oil thus comprises in a first step the addition to the contami¬ nated oil to be purified of a specific separation additive, which by chemical interactions absorbs contaminating solid, or dissolved impurities in the process oil, and in a second step, separation of said separation additive and absorbed impurities using a method selected from the group consisting of static settling, centrifugal separation, vacuum filtra¬ tion, press filtration, pre-coat filtration and centrifugal filtration.

Detailed description of the invention

According to one aspect of the present invention there is provided a process for the purification of spent process oil, which process comprises,

mixing the process oil with an additive comprising a liquid mixture of

a) at least one alkanolamine, and b) at least one member selected from the group consisting of alkanoic acids, and alkenoic acids having from 6 to 18 carbon atoms and the melting point/points such that the mix- ture of a) and b) is liquid at the tempera¬ ture at which the process is carried out,

with a pH-value of said mixture of a) and b) in the range of 5 to 8, preferably 6 to 7, and most prefera- bly 6.3, and said mixture being substantially insolu¬ ble in the process oil, forming a two phase mixture upon mixing, and

subsequent separation of the phase with process oil from the phase with said mixture of a) and b) with absorbed contaminants.

Different preferred embodiments of the present invention are defined in the dependent claims. In another aspect the pre¬ sent invention thus comprises a process, wherein a) is se¬ lected from the group consisting of mono-, di-, and trietha- nolamine.

In a further aspect the present invention comprises a proc¬ ess, wherein a) is triethanolamine.

In another aspect the present invention comprises a process, wherein b) is selected from the group consisting of lauric acid, oleic acid, pentadecanoic acid, decenoic acid, 2- ethylhexanoic acid, and caprylic acid.

In another aspect the present invention comprises a process, wherein b) is selected from the group consisting of 2- ethylhexanoic acid and caprylic acid.

In another aspect the present invention comprises a process, wherein the said separation of the two phases is carried out by one method or a combination of methods selected from the group consisting of static settling, centrifugal separation, vacuum filtration, press filtration, pre-coat filtration and centrifugal filtration. In another aspect the present invention comprises a process, wherein the mixture of a) and b) is added in an amount of 0.0001 to 10% by weight, calculated on the weight of the spent process oil.

In another aspect the present invention comprises a process, wherein said additive in addition to components a) and b) further comprises c) at least one agent selected from the group consisting of monoethylene glycol, dipropylene mono- ethylether, glycerol, and propylene glycol.

In another aspect the present invention comprises a process, which is carried out at a temperature above 100C and prefera¬ bly at ambient temperature.

In still another aspect the present invention comprises use of at least one member selected from the group consisting of alkanoic acids, and alkenoic acids having from 6 to 18 carbon atoms in the preparation of a mixture for the purification of spent process oil.

In yet another aspect the present invention comprises use of at least one alkanolamine in the preparation of a mixture for the purification of spent process oil.

In another aspect the present invention comprises use of a liquid mixture of

a) at least one alkanolamine, and b) at least one member selected from the group consisting of alkanoic acids, and alkenoic acids having from 6 to 18 carbon atoms and the melting point/points such that the mix- ture of a) and b) is liquid at the tempera¬ ture at which the process is carried out,

for the purification of spent process oil.

In yet another aspect the present invention comprises a use where the spent process oil is selected from the group con¬ sisting of mineral oil, rolling oil, honing oil, drawing oil, engine lubricating oil.

In another aspect the present invention comprises a use where the temperature is above 100C and preferably ambient tempera¬ ture.

In the first process step a separation additive is mixed with the target fluid under vigorous stirring. The separation ad¬ ditive is not soluble in the target fluid because of its po¬ lar properties and thus colloids consisting of small droplets of separation additive are formed under the stirring, which through the chemical interactions (hydrophilic, hydrophobic, and charge interactions) may absorb the unwanted solid or the dissolved impurities in the target fluid. Because the separa¬ tion additive has a higher density than most mineral oils (except chloroparaffinic oils and certain other derivatised oils) the separation additive will at a gravity separation form a lower phase together with the solid or dissolved impu¬ rities and in the cases were the separation additive has a lower density than the target fluid, it will form a lighter phase.

The mixture to be added to the oil to be purified is called the separation additive. A fatty acid with added alkanolamine is preferably used as a separation additive. The separation additive may be in liquid form at room temperature. Fatty acids of animal as well as vegetable origin are suitable can¬ didates. The fatty acid is at least one selected from the group consisting of alkanoic acids and alkenoic acids having a melting point so that the mixture is liquid at the tempera¬ ture at which the process is carried out. The temperature at which the process is carried out is generally above 10°C and preferably at ambient temperature. The process temperature is preferably below the boiling point of the separation additive and the oil to be purified and any other component of the mixture. Preferably the separation additive is liquid at room temperature.

Suitable fatty acids are at least one selected from the group consisting of lauric acid, oleic acid, pentadecanoic acid, decenoic acid, 2-ethylhexanoic acid, and caprylic acid. Pre¬ ferred fatty acids are 2-ethylhexanoic acid and caprylic acid, with the latter one being particularly preferred.

To the fatty acid or the fatty acids at least one alkanola- mine is added, so that the pH value of the liquid mixture is in the range of 5 to 8, preferably 6 to 7, with 6.3 as a par¬ ticularly preferred value. With a pH range of for instance 6 to 7 the inventor means all pH value within this range such as 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0. A person skilled in the art realizes that also other pH val¬ ues, although not optimal, outside the interval 6 to 7 may be used, such as 5.9, 5,8, 5.7, 5.6, 5.5, 5.4, 5.3, 5.2, 5.1, 5.0 and so on, or 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0 and so on.

For the pH adjusting process at least one alkanolamine se¬ lected from the group consisting of mono-, di-, and trietha- nolamine is used. Monoethanolamine and triethanolamine are preferred. Triethanolamine is particularly preferred.

The properties of the mixture of at least one fatty acid and at least one alkanolamine should be such that the mixture is substantially insoluble in the oil to be purified.

The mixture to be added to the process oil may further com¬ prise at least one substance selected from the group consist- ing of monoethylene glycol, dipropylene glycol monoethyl ether, propylene glycol and glycerol.

The separation additive comprising the liquid mixture de¬ scribed above is added to the oil to be purified in an amount from 0.0001 to 10% by weight of the oil to be purified. For chloroparaffinic oils with a high amount of solids the puri¬ fication mixture is for instance added in an amount of about 3% to about 5% or even up to 10% by weight of the oil to be purified. Whereas for the continuous purification of rolling and honing oil for instance only about 0.0001 to 0.0002% by weight of the oil to be purified is required. For other ap¬ plications any amounts between these extremes are used.

The separation additive is separated from the oil to be puri- fied with one method or a combination of methods selected from the group consisting of static settling, centrifugal separation, vacuum filtration, press filtration, pre-coat filtration and centrifugal filtration.

The following examples serve the purpose to further illus¬ trate the invention and are not intended to be limiting in any way. Examples

Example 1: Purification of solid and dissolved impurities in rolling oils.

3 m3 rolling oil (valsolja-20, Statoil) which has been used during a normal cycle (12 weeks) without any kind of purifi¬ cation and which thus would be destroyed, was pumped to a pilot plant built for this purpose. The plant consisted of a storage tank with a volume of 3 m3 to which a centrifugal separator was connected (MAB 309 Alfa Laval) equipped as a purifier. The rolling oil was mainly contaminated with par¬ ticular carbon particles (90% with a particle diameter less than 2 μm) and dissolved metal stearates, which are lubri- cants from earlier process steps. The centrifugal separator, which was connected for by-pass purification was of the type "solid wall machine", which is a separator only intended to continuously separate two non-miscible fluid phases from each other.

When the experiment was started a liquid wall was filled in the centrifugal separator, comprising the separation addi¬ tive. Then the corresponding separation additive was mixed into the feed to the separator prior to a static mixer. The dosing was approximately 0.1%, The separation additive con¬ sisted of caprylic acid with added triethanolamine with a resulting pH value of 6.3. 70% by weight of the fatty acid with triethanolamine was mixed with 30% by weight of ethylene glycol to bring down the viscosity of the product prior to dosing. The system was operated with circulation during 6 hours with a flow of 900 1/h. The particle reduction for the rolling oil was measured with a turbidity meter (HACH) by sampling from the separator outlet with and without dosing of product. The reduction of dissolved stearate-soaps was ana¬ lysed with atom emission analysis.

Results

Table 1 Quantitative FTIR analysis of the additives present in the tested rolling oil. Sample No. 1: untreated rolling oil, rolling oil used for 12 weeks (spent rolling oil) , and oil purified according to example 1.

Table 2 Particle reduction over time by means of turbidity measure¬ ment of a rolling oil subjected to purification according to the invention.

Table 3. Analysis of untreated and treated oil with respect to the presence of trace elements.

Example 2. Purification of honing oil.

Honing oil (Castrol Honilo 971) which has been used in the manufacturing of connecting rods was purified in a bypass process according to the process described above. To a stor¬ age tank with a volume of 1000 litre a solid wall separator was connected (MAB 204, Alfa Laval) equipped as a purifier. A liquid wall of separation additive was supplied after which the system was by pass purified with a flow of 180 - 240 li¬ tres per hour. To the flow of oil to the separator the sepa¬ ration additive was dosed with a dosing pump (IVAKI) and sub¬ sequently the oil together with separation additive was passed through a centrifugal pump and a static mixer before the inlet to the separator. The dosing rate was approximately 300 rαl/h. The separation additive was based on caprylic acid with added triethanolamine with a resulting pH value of 6.3. 70% by weight of the caprylic acid with triethanolamine was mixed with 30% by weight of ethylene glycol to bring down the viscosity of the product prior to dosing. The particle con¬ centration in the oil was measured by means of turbidity (HACH) .

Results The results are shown in table 4.

Table 4 Summary and analysis in example 2.

Example 3. Purification of lubricating oil.

25 litres of marine lubricating oil (Argina x-40) with a duty time of 12 000 hours was purified by separation with a solid wall separator (Emmie, Alfa Laval) equipped as a purifier. The oil was heavily contaminated with soot particles and in¬ organic particles. Prior to separation the oil was heated to 950C. The separator was filled with a liquid wall of separa¬ tion additive and subsequently 250 ml (1% weight/volume) separation additive was mixed into the lubricating oil with a mechanical mixer. The mixture used as separation additive was caprylic acid with added triethanolamine with a pH of 6.3. 70% by weight of the caprylic acid with triethanolamine was mixed with 30% by weight of ethylene glycol to bring down the viscosity of the product prior to dosing. The oil was puri¬ fied by circulation through the separator until no heavy- phase could be noted in the heavy phase outlet, thus all fatty acid added to the lubricating oil had been removed. The amount of impurities insoluble in n-pentan was measured in untreated and treated oil. Any influence on the additives in the oil was analysed by comparing IR-spectra for purified oil, unpurified oil and fresh oil.

Results

As can be seen in table 5, the amount of impurities insoluble in n-pentan decreased by 77% when subjecting the oil to ex¬ traction and separation utilizing the separation additive according to the invention. IR-spectra of untreated oil, treated oil and new oil were identical in the wavelength interval 6500 - 3577 cm"1. In the wavelength interval 3577 - 3070 cm"1 a small difference in the spectra can be observed between untreated and treated oil. This difference is most likely caused by the presence of water in the untreated oil. In the rest of the wavelength interval no major differences in spectra could be noticed.

Table 5. Rest amount of impurities insoluble in n-pentan in untreated oil and oil treated according to the invention.

Example 4. Purification of chloroparaffinic oil.

Chloroparaffinic oil (Castrol 5051) from a steel industry was purified in laboratory scale. The oil which had been in use for about one year was heavily contaminated with carbon par- tides, lime fillers, and lubricating fats (approximately 20% impurities by weight) . Initially the viscosity was measured for the unpurified oil with a Brookfield viscosimeter. 5% by weight of the separation additive was added to 1 kg of the contaminated oil. The product used was caprylic acid with added monoethanolamine with a resulting pH of 6.3. The mix¬ ture was allowed to stand in a warm hood in a separation fun¬ nel at 400C during 18 hours after which the heavy phase (the chloroparaffinic oil phase) was separated. When the oil had reached room temperature the viscosity was measured. As a comparison the viscosity for new chloroparaffinic oil was also measured. To study whether any residues of separation additive remained in the oil an IR analysis was conducted where the oil was compared to unpurified oil.

Results

Table 6. Viscosity measurement of new, contaminated and treated oil respectively in example 4.