GB2026551A - Cleaning oily surfaces - Google Patents

Abstract

Surfaces which are contaminated with oil are cleaned by contacting the surfaces with a cleaner comprising an organic solvent and an aqueous solution of a substance which decreases the surface tension and/or increases the alkalinity of the solution, and in which subsequent to the cleaning operation the contaminated cleaner is separated from the bodies being cleaned and on standing separates into organic and aqueous phases. The preferred cleaner comprises an alcohol, ketone or hydrocarbon solvent and an aqueous solution of an alkali, phosphate or silicate. Metals, glass, ceramic or plastics may be cleaned.

Description

SPECIFICATION Cleaning oily surfaces The present invention relates to the cleaning of surfaces which are contaminated with oil and in particular to the cleaning of oily metal surfaces.

There are numerous procedures currently used in manufacturing industries in which metallic bodies such as castings and the like are subjected to machining operations giving rise to a particulate oily metallic material known as swarf, in the form of chips or turnings of metal contaminated with lubricating fluids from the machining operation. It is highly desirable to separate the oil and metal components of this mixture so that both may be recycled separately.

After centrifuging however, this mixture still contains up to 5% by weight of oil and under current practice the residual oil is burnt off either prior to or when the metal is introduced into a blast furnace for remelting. Not only is such practice wasteful, a significant and continuing amount of expensive lubricating oil being lost together with loss of oxidised metal fines, but it also gives rise to undesirable atmospheric pollution.

A method has now been found by which surfaces which are contaminated with oil, such as oily metal surfaces, can be cleaned, for instance to effect substantial separation of the oil and metal components.

According to the present invention a method for the cleaning of surfaces which are contaminated with oil comprises contacting the oily surfaces with a cleaner comprising an organic solvent and an aqueous solution of a substance which decreases the surface tension and/or increases the alkalinity of the solution, and in which subsequent to the cleaning operation the contaminated cleaner is separated from the bodies being cleaned and on standing separates into organic and aqueous phases.

Thus typically the cieaner of the present invention is a bi-phase cleaner the organic solvent being either immiscible or only partially miscible with water; though water-miscible organic solvents which become water-immiscible on uptake of oil from the contaminated surfaces are included within the scope of the present invention.

The surfaces which may be cleaned by the method of the present invention are surfaces of solid bodies which may be regular or irregular in shape. The surfaces may be extensive for instance when the bodies are in the form of tubes, sheets and the like; though advantageously the method may be applied to particulate solid materials in both coarsely and finely divided form. The solid materials which may be cleaned by this method may include glass, ceramic or plastic materials but the method is particularly suited to the cleaning of oily metal surfaces especially where the metal is in a particulate form, such as swarf and other by-products of machining operations.

The cleaner may comprise any suitable surface tension lowering agent and in one mode this may be an organic chemical surfactant or other organic chemical having detersive properties, for example soaps, alkylarenesulphonates or alkyl sulphates. Preferably, however, the surface tension lowering agent is an inorganic chemical such as an alkali, a phosphate or silicate and such surface tension lowering agents are particularly desirable from the point of view of economy when compared with synthetic organic detergents. In addition inorganic alkalis are desirable in view of the increase in pH of the aqueous component of the cleaner which they cause, which appears to be advantageous for the purposes of the present invention.Particularly preferred surface tension lowering agents are alkalis such as alkali or alkaline earth metal hydroxides, carbonates, bicarbonates, sesquicarbonates and borates. In practice the use of sodium hydroxide has been found to be highly satisfactory.

The amount of surface tension lowering agent present in the aqueous solution may be varied widely in accordance with the detersive properties desired for the cleaner. In the case when the surface tension lowering agent is sodium hydroxide it has been found that a solution of concentration at least about 1 or 2% w/v (i.e. 1 or 2 gm/100 ml) is required in most cases, and excellent results have been obtained using sodium hydroxide solutions of concentration of at least about 5% w/v up to about 10% w/v.

The organic solvent is typically a solvent for the oily contaminant of the surfaces to be cleaned, and is generally a water-immiscible organic liquid. The solvent may be an alcohol or ketone though more usually is a hydrocarbon solvent including halogenated and other substituted hydrocarbon solvents, and may be either saturated or unsaturated and either aliphatic or aromatic. Preferred solvents are petroleum fractions, amongst which the petroleum fraction known as kerosene or white spirit has been found to be highly satisfactory, and outstanding results have also been obtained using xylene.

The relative proportions of aqueous solution and organic solvent, usually in the form of separate aqueous and organic phases, which go to make up the cleaner may be varied widely. Generally the aqueous component is the major component of the cleaner, and generally the organic component is present to the extent of at least 2% v/v preferably at least 5% v/v and especially at least about 10% v/v. For example, satisfactory results have been obtained using a cleaner consisting of a 5% w/v aqueous solution of NaOH together with 10% v/v of white spirit. It will be appreciated that some cleaner compositions are novel and theseperse are included within the scope of the present invention.

Advantageously the cleaning operations involve agitation, for instance agitation of the oil contaminated bodies when in the cleaner or agitation of the cleaner bath as a whole. For example, the oil contaminated bodies together with cleaner are maintained in a fluidised bed or passed through a transported bed contactor. Alternatively, in a particularly preferred method, the oil contaminated bodies are maintained in a rotating centrifugal spinner such as a centrifuge e.g. a basket-type centrifuge, and cleaner is introduced to the bodies, for instance by means of a spray. Feed and removal of contaminated or cleaned bodies and fresh or contaminated cleaner to and from the centrifugal spinner may be either batchwise or continuous.Such centrifugal apparatus is included within the scope of the inven tion and essentially comprises a centrifugal con tainer adapted to hold oil contaminated bodies when the container is rotated, means for rotating the con tainer, inlet means for introducing a cleaner as hereinbefore defined to the oil contaminated bodies held in the container, and outlet means for the removal of oil contaminated cleaner from the con tainer. The apparatus may also include means for introduction of contaminated bodies and means for removal of cleaned bodies, for instance means for continuous introduction and removal of bodies. In a further application the method may be used for the cleaning of oil contaminated tubes or pipes e.g.

pipelines, in which case for instance the cleaner e.g.

in emulsion form, is passed down the contaminated pipe.

Generally the conditions employed during cleaning operations may be chosen having regard to well known solvent and "detergent" properties. For example, the use of cleaner at elevated temperatures usually gives rise to improved cleaning efficiency as compared with ambient temperature; temperatures of around 60"C having been found to give excellent cleaning efficiencies.

Subsequent to the cleaning operation the now contaminated cleaner is separated from the bodies being cleaned and on standing separates into organic and aqueous phases, and typically the organic component contains the major proportion of the oil which has been taken up by the cleaner. The organic and aqueous phases may then be separated, for instance by simple decantation, and the oil separated from the organic solvent, e.g. by distillation. In this way substantial separation of oil from the previously contaminated bodies e.g. the swarf, may be achieved, giving preferably a cleaned product having an oil content of about 1% by weight or less. For example, it has been found that the oil content of mild steel swarf can be reduced to about 0.1% by weight, and the oil content of machining waste from cast iron to about0.5% by weight.

Most surprisingly, it has been found, according to the present invention, that in many cases the contaminated cleaner on standing separates to give a further discrete phase of intermediate density between the organic and aqueous phases. On analysis this phase has been shown to contain oil, solvent and aqueous solution usually together with fine particles of metal and other inorganic substances such as insoluble calcium and magnesium salts from the water. This appears to a metastable phase in the form of an unstable emulsion which may have a separating time of the order of from a few minutes up to several days. Without prejudice, it is believed that the formation of this phase may depend, among other factors, upon the nature of the oily contaminant and the various components of the cleaner system employed.Thus in general, cleaners based on hydrocarbon solvents such as white spirit, kerosene and the like and aqueous solutions of material having only lowdetersive activity such as inorganic alkalis e.g. NaOH, Na2CO3, NaHCO3, may give rise to such metastable emulsion phases. Also the lubricat ing fluids may initially be in the form of oil in water emulsions, the oil containing for example an emul sifying agent which may render it only partially misc ible with the organic phase, and such lubricating fluids may also contribute to the formation of an unstable emulsion phase in the contaminated cleaner. Furthermore the presence of metal fines, which may be either added to the cleaner or derived from the material being cleaned, may assist in the temporary stabilisation of the emulsion phase when formed.

Thus in a preferred embodiment the present invention comprises a method for the cleaning of surfaces which are contaminated with oil in which a cleaner, comprising an aqueous solution of a substance which lowers the surface tension and/or increases the alkalinity of the solution together with an organic solvent, is contacted with the oily surfaces, the composition of the cleaner and nature of the oily contaminant being such that contaminated cleaner on standing separates into discrete organic and aqueous phases having in between a metastable emulsion phase of intermediate density.

Advantageously the concentrations of the oil present in the organic and emulsion phases differs, and preferably the oil concentration is higher in the emulsion phase. This is particularly desirable from the oil regeneration point of view, for preferably only the phase of highest oil concentration e.g. the emulsion phase, need be separated from the contaminated cleaner for the purpose of oil regeneration.

For example, a cleaner consisting of a 5% w/v aqueous solution of NaOH, together with 10% v/v of white spirit, on standing, after contact with oily swarf, gives rise to a three phase system, an upper pale yellow organic phase, a cloudy aqueous phase at the bottom and in between a dark brown or black emulsion phase. The middle emulsion phase is the metastable phase and initially is about 1/10th of the volume of the top organic phase. Under normal conditions this emulsion phase remains stable for a period of from about 12 to about 24 hours. Also this middle phase has a higher oil concentration than the top phase and typical valuesforthe distribution co-efficient of oil between the two phases are found to be in the range from about2:1 to about 3:1.This middle fraction has also been found to contain small amounts of metal fines and other solids such as insoluble materials fromthe water e.g. calcium and magnesium salts. The middle phase may be decanted and the oil regenerated by distillation, and the distilled "clean" solvent may be returned to "top up" the cleaner. Generally, after removal of the middle phase, the cleaner is recycled, and it may be periodically topper up with fresh solvent.

The invention is further illustrated in the following non-limiting examples which described the cleaning of oily cast iron "swarf" by centrifuge and transported bed techniques.

Example 7 A cleaner according to the present invention is prepared as follows: 200g of NaOH, flake or pellet, is dissolved in 10 1. of "soft" water to give a 2 wt% NaOH solution which is heated to 50"C and 1 I. of white spirit, kerosene, orturps substitute is added.

The mixture is kept well agitated so that the phases do not separate.

100 g of cast iron machining "swarf" containing about 5% by weight of lubricating contamination is placed in a basket centrifuge provided with appropriate means for delivery and removal of cleaner. The basket centrifuge is rotated at a speed of about 2,000 r.p.m. and the liquid cleaner mixture is pumped through the cast iron at a flow rate of approximately 3 1. minute for a period of from 1 to 3 minutes.

The cleaned cast iron is removed and is now found to contain 0.5% by weight of oil contamination. The contaminated cleaner is passed to a suitable cylindrical column reciever provided with outlet taps at various heights along its length and allowed to settle for 30 minutes. The cleaner separates into phases giving a cloudy aqueous phase at the bottom occupying approximately 90 /O of the total volume, a yellow organic solvent phase on top and in between a dark oily emulsion phase occupying about 1/10th of the volume of the organic phase.

The bottom tap of the column receiver is then opened to release liquid so that the level of the emulsion layer falls until it is aligned with one of the intermediate taps and the bottom tap is closed. The intermediate tap is then opened and the whole of the emulsion layer is withdrawn. The oil may be recovered by evaporation or distillation of the solvent, and the upper and lower organic and aqueous layers may be returned to the cleaner reservoir for re-use.

Example 2 A large volume ( > 200 1.) of 10:1 vN cleaner comprising 2% w/v aqueous NaOH solution together with white spirit is prepared, as in Example 1, and heated to 500C. The mixture is kept well-agitated to prevent separation of phases.

200 kg of machining swarf, containing about 6% by weight of lubricating oil, is loaded into a basket centrifuge (capacity about 0.2m3). The centrifuge is rotated using a 10 HP motor at a speed of about 1000 r.p.m. to develop a force of 340G, and cleaner liquid, as prepared above, is sprayed into the top of the rotating basket at a rate of about 100 I./min for 3 minutes. The contaminated cleaner from the centrifuge is recycled via the cleaner reservoir.

After completion of the cleaning cycle the centrifuge is stopped and the cleaner allowed to stand and separate into phases over a period of about 5 minutes. Three phases separate: an upper white spirit layer containing about 5% bywt. of oil, a cloudy aqueous layer at the bottom, and in between an emulsion phase of about 1/1 ooth the volume of the upper white spirit layer and containing about 15% by weight of oil. On examination the cleaned swarf is found to contain less than .5% by weight of oily contamination.

The volumes of the three layers are in the ratio 10:1:100 from top to bottom. The top and bottom layers may be used again without any treatment for further cleaning cycles, and the middle layer may be discarded or subjected to further processing e.g. distillation, for recovery of oil.

Example 3 In an alternative technique oil contaminated machining swarf is cleaned in a transported bed contactor.

200g of contaminated cast iron swarf is loaded into the bottom of a contactor in the form of a 1" I.D.

vertical glass tube, 60 cm long. Cleaner mixture, as prepared in previous examples is pumped through the tube in an upward direction at a rate of about 20-40 1. per minute per a period of about 10-20 seconds. The swarf is transported upward through the contactor exiting into a screened receiver by means of which the metal is separated from the liquid mixture.

The swarf initially contained about 6% by wt. of oil and after cleaning is found to contain about 1% by wt. of oil. The contaminated cleaner is allowed to stand in a separating column in which it separates into three phases after about 30 minutes. The three phase system is similar to that obtained in previous examples, an emulsion layer containing about 15% w/v of oil which may be regenerated by distillation forming between the upper white spirit and lower aqueous NaOH layer.

Claims (11)

1. A method for the cleaning of surfaces which are contaminated with oil comprising contacting the oily surfaces with a cleaner comprising an organic solvent and an aqueous solution of a substance which decreases the surface tension and/or increases the alkalinity of the solution, and in which subsequent to the cleaning operation the contaminated cleaner is separated from the bodies being cleaned and on standing separates into organic and aqueous phases.

2. A method according to Claim 1 in which the cleaner is a bi-phase cleaner and the contaminated surfaces are oily metal surfaces, e.g. swarf.

3. A method according to Claim 1 or 2, in which the substance which decreases the surface tension and/or increases the alkalinity of the solution is an inorganic alkali, phosphate or silicate.

4. A method according to any of the preceding claims in which the organic solvent component of the cleaner comprises an alcohol, a ketone or preferably a hydrocarbon solvent.

5. A method according to any of the preceding claims, in which on standing the contaminated cleaner separates to give a further discrete metastable phase between the organic and aqueous phases.

6. A method according to any of the preceding claims in which the cleaner comprises a from about 1 up to about 10% w/v aqueous solution of an alkali metal hydroxide and at least 2% v/v white spirit or similar petroleum fraction.

7. A method according to Claim 5 or 6, in which the concentration of oil in the metastable intermediate phase is higher than in the organic phase.

8. A method according to any of the preceding claims in which after separation of phases on stand ing the metastable intermediate phase or organic phase is separated from the remainder of the cleaner.

9. A method according to Claim 8, in which oil is separated and/or recovered from the organic solvent, e.g. by distillation, after separation of the organic or metastable phase from the cleaner.

10. A method according to any of the preceding claims, in which the oil contaminated bodies are maintained in a rotating centrifugal spinner during cleaning, and cleaner is introduced to the bodies e.g.

by means of a spray.

11. A method according to any of the preceding claims, substantially as hereinbefore described with particular reference to Example 1 or 2.