WO2006027121A1

WO2006027121A1 - Treatment of oily sludges

Info

Publication number: WO2006027121A1
Application number: PCT/EP2005/009239
Authority: WO
Inventors: Ken Hambrecht; Steven Parsons; Jordi Ricart Parera
Original assignee: Ciba Specialty Chemicals Water Treatments Limited
Priority date: 2004-09-07
Filing date: 2005-08-26
Publication date: 2006-03-16
Also published as: GB0419815D0

Abstract

A process for dewatering a sludge containing hydrocarbon, comprising pretreating the sludge by adding to the sludge (a) a swellable clay, (b) an inorganic coagulant, (c) a polymeric flocculant and (d) a surface active agent and subjecting the pretreated sludge to mechanical dewatering. The process enables a sludge cake of improved dryness to be produced.

Description

Treatment of Oily Sludges

The present invention concerns a process for the dewatering of sludges containing hydrocarbon, generally known as oily sludges, in which the sludge is subjected to a chemical pretreatment followed by mechanical dewatering.

Oily sludges are formed in oil production, for instance in the oil refinery or even during oil recovery and also may form in tanker ballasts. Typically, the sludge is a liquid or semi solid mixture of water, particulate solids and hydrocarbons. It can in some instances be desirable to recover as much oil as possible from oily sludges and/or separate the solids from the water and oil of the sludge so that they form a dry cake suitable for disposal. It is also important in many circumstances to provide a process that rapidly releases water and in some cases rapidly releases water and oil in order to form a firmer more handleable cake.

Various methods have been proposed for the treatment of oily sludges. For instance in US 4834889 the solids are separated from oily sludge by treating it with lime and then with calcined perlite. The process permits the recovery of oil from the sludges and for the effective disposal of the solids.

JP-A-54054962 describes a way of solidifying oily sludges by mixing with an inorganic cement, sodium lignosulphonate, calcium chloride or sodium chloride and water to form slurries which are subjected to pressure filtration.

It is well known to dewater oily sludges by the use a combination of inorganic coagulant and polymer to pre-treat sludges.

GB-A-2095573 describes a process for separating oil sludges by adding a structure improving additive, for example flue dust to increase the shear strength of the pressed filter cake and to reduce resistance to filtration and then adding a polyelectrolyte and in some cases a metal salt. There is also disclosure of spraying on a ferric salt to the partially dewatered sludge.

J P-A-2000176499 describes a process in which oil sludges containing metal powders are dewatered by treating with a cationic or anionic polymeric coagulant and optionally an inorganic coagulant to agglomerate the floes. The sludge is dewatered on a screw press type dewatering machine.

JP-A-62156000 concerns dewatering oil containing sludges by mixing with inorganic coagulant and acrylic acid hydrazide polymers followed by mechanical dewatering.

EP-A-56090 describes treating mineral oil containing sludges by mixing with granulated ash, coal or sand and then flocculating the mixture with an anionic flocculant which is then gravity filtered and coagulated with aluminium or ferric salts followed by pressure filtration.

JP-A-61 ,025 689 describes a purification treatment of wastewater containing surfactant, oil and fluorine. Contaminants are removed from the wastewater by adding bentonite and a cationic high molecular weight flocculant, adding an aluminium salt and then adding an alkaline agent to regulate the pH. The bentonite and high molecular weight cationic polymer are said to be added to destabilize the emulsion and treated water is then introduced into a mixing vessel where aluminium salt and alkaline agent are added to carry out flocculation.

Japanese published patent application 2005125215 A2 (unpublished at the priority date for the present application) describes a method involving adding clay such as bentonite to the sludge, adding a cross-linking ionic polymer flocculant prepared from cationic monomers and vinyl monomers, and dewatering the mixture. There does not appear to be any disclosure of using coagulant or surfactant.

Various polymeric flocculants have been described for use in dewatering oily sludges, including anionic non-ionic and cationic polymers.

WO-A-2004041884 discloses the manufacture of cationic polymers by the continuous introduction of cross-linking agents useful as flocculants and dewatering aids in aqueous systems, such as oily sludges.

It is known to apply surfactants such as, a sodium alkyl sulfosuccinate, to oily sludges to aid dewatering.

JP-A-59112808 reveals the treatment of an oily sludge comprising a sodium dialkyl sulpho succinate and optionally an inorganic coagulant with a cationic surfactant. The stirred mixture is heated to form coagulated floes and a nonionic surfactant is added as a separating agent, followed by centrifugation to separate the mixture into sludge cake, oil and water.

JP-A-01063099 describes blending a highly water absorbing resin and bentonite to form a mixture, which is said to be suitable for solidifying oil-containing sludge. However, in such a treatment the water is being absorbed and held in the solidified mass rather than separated from solids of the sludge. Although a solid product may result the total mass would be greater than if this water had been removed which would have a greater impact on disposal.

It is generally very hard to achieve satisfactory dewatering of oily sludges. Frequently the dewatered sludge cakes that form after mechanical dewatering are insufficiently dry. It would therefore be desirable to provide a treatment that results in more effective dewatering of oily sludges to provide a dry cake. According to the present invention we provide a process for dewatering a sludge containing hydrocarbon, comprising pre treating the sludge by adding to the sludge (a) a swellable clay, (b) an inorganic coagulant, (c) a polymeric flocculant and (d) a surface active agent and subjecting the pretreated sludge to mechanical dewatering.

We have found that this pretreatment before the mechanical dewatering enables a greater amount of water to the separated from the solids, resulting in a drier cake.

Preferably the components (a) a swellable clay, (b) an inorganic coagulant, (c) a polymeric flocculant and (d) a surface active agent are each added to the sludge sequentially.

We have found that (a) the swellable clay and (b) the inorganic coagulant may be added to the sludge sequentially and in either order or alternatively both components may be added simultaneously. Thus in one preferred form the swellable clay is added to the sludge and allowed to mix in prior to the addition of the inorganic coagulant. In some circumstances it may be desirable to add the inorganic coagulant and allow this to mix into the sludge prior to the addition of the swellable clay.

The preferred clays are swellable in water and include clays which are naturally water swellable or clays which can be modified, for instance by ion exchange to render them water swellable. Desirably, the swellable clay is a bentonite or bentonite type clay, preferably a smectite. The bentonite may be an alkaline earth bentonite such as calcium bentonite or magnesium bentonite, but preferably sufficient of the exchangeable cations are alkali metal such that the bentonite is rendered swellable in water and by preference will be a sodium bentonite. Suitable water swellable clays include but are not limited to clays often referred to as hectorite, montmorillonites, nontronites, saponite, sauconite, hormites, attapulgites and sepiolites.

Generally the swellable clay is added to the sludge in an amount of at least 1000 mg/l and in some cases it may be desirable to add it in an amount up to 20,000 mg/l. Preferably the clay will be added in an amount between 10,000 and 15,000 mg/l.

The inorganic coagulant is typically a metal salt traditionally used as a coagulant for the treatment of suspensions. Typically these will be ferric, ferrous or aluminium salts, preferably selected from the group consisting of ferric sulphate, ferric chloride, alum, polyaluminium chloride, aluminium chloride trihydrate and aluminochloro hydrate.

When the swellable clay is added first the inorganic coagulant may be added to the sludge substantially immediately after the addition of the swellable clay although usually the dewatering process will tend to be more effective if sufficient time and/or mixing is allowed to enable the swellable clay to be distributed throughout the sludge before the inorganic coagulant is added. The same would be true in cases where the inorganic coagulant is added first in that the swellable clay may be added substantially immediately after the addition of the inorganic coagulant. However, it may be more desirable to allow sufficient time and/or mixing for the inorganic coagulant to become distributed throughout the sludge before the swellable clay is added. Preference is given to the addition of the swellable clay first before the addition of the inorganic coagulant.

The inorganic coagulant will typically be added in an amount of at least 500 mg/l, usually no more than 5000 mg/l. The inorganic coagulant should be added in a sufficient amount to bring about coagulation of the sludge. A preferred dose of inorganic coagulant is normally between 2000 and 4000 mg/l. In a preferred form of this invention regardless of the order of addition of the swellable clay, inorganic coagulant and polymeric flocculant it is usually most desirable for the surfactant to be added last.

Typically following the addition of the inorganic coagulant and swellable clay the polymeric flocculant and surfactant would be introduced into sludge. Preferably the flocculant would normally be introduced to bring about flocculation before the addition of the surfactant. However, in some circumstances it may be desirable to add the surfactant either before the flocculant or simultaneously with the flocculant.

Preferably polymeric flocculant would be added after the swellable clay and the inorganic coagulant but before the surfactant. More preferably the swellable clay is added first followed by the inorganic coagulant and the polymeric flocculant is then added once the sludge has been coagulated by the inorganic coagulant.

Typically the amount of polymeric flocculant required will be at least 100 mg/l (calculated on active polymer content) and maybe as much as 800 mg/l. Preferred amounts will be between 300 and 600 mg/l.

The polymeric flocculant can be a natural polymer but preferably is a synthetic polymer, particularly polymers formed from ethylenically unsaturated monomers. In the process the polymeric flocculant may be non-ionic or ionic, for instance anionic or amphoteric, but preferably it is cationic.

Typically, the polymer is water-soluble or water swellable and may be derived from any water soluble monomer or monomer blend. By water soluble we mean that the monomer has a solubility in water of at least 5g/100cc at 25⁰C. When the polymer is non-ionic it will be generally formed from one or more non-ionic ethylenically unsaturated monomers, such as acrylamide. Suitable anionic polymers may be formed from at least one anionic ethylenically unsaturated monomer optionally with one or more non-ionic monomers. An example of a suitable anionic polymer is the copolymer of acrylamide with acrylic acid or sodium acrylate. The amphoteric polymers may be prepared by the copolymerisation of at least one anionic ethylenically unsaturated monomer with at least one cationic ethylenically unsaturated monomer, optionally with acrylamide. This may be for instance, the polymer of acrylamide with sodium acrylate and dimethyl amino ethyl acrylate, methyl chloride quaternary ammonium salt.

The preferred cationic polymers are generally prepared from at least one ethylenically unsaturated cationic or potentially cationic monomers, optionally with a non-ionic ethylenically unsaturated monomer. Suitably this will include polymers of cationic monomers selected from the group consisting of diallyl dialkyl ammonium chlorides, acid addition salts or quaternary ammonium salts of either dialkyl amino alkyl (meth) acrylate or dialkyl amino alkyl (meth) acrylamides. Preferably, the polymeric flocculant is a copolymer of acrylamide with dialkyl amino alkyl (meth) acrylate either as the freebase amine, acid addition salt or quaternary ammonium salt. Particularly preferred cationic polymers include polymers of methyl chloride quaternary ammonium salts of dimethylaminoethyl acrylate or methacrylate, especially copolymers of these monomers with acrylamide.

Alternatively the cationic polymer may be rendered cationic by a post-treatment. A polyacrylamide can be treated with the addition product of formaldehyde with dimethyl amine to produce a cationic Mannich polyacrylamide. This may for instance be prepared as in aqueous solution or alternatively as a microemulsion.

The polymeric flocculant may be water-soluble in that it substantially dissolves in water. The polymer may be linear or branched but preferably is cross-linked. Branched and cross-linked polymers would normally be prepared by including a cross-linking agent or branching agent in the monomer, for example as in EP-A- 202780.

Generally the polymeric flocculant will be a high molecular weight polymer, normally having a molecular weight of at least one million and usually several million, for instance up to 20 or 30 million.

Typically the intrinsic viscosity will be at least 3 dl/g or 4 dl/g, preferably at least 7 dl/g and often it can be as high as 20 or 30 dl/g but more preferably will be between 7 and 10 dl/g.

Intrinsic viscosity of polymers may be determined by preparing an aqueous solution of the polymer (0.5-1 % w/w) based on the active content of the polymer. 2 g of this 0.5-1% polymer solution is diluted to 100 ml in a volumetric flask with 50 ml of 2M sodium chloride solution that is buffered to pH 7.0 (using 1.56 g sodium dihydrogen phosphate and 32.26 g disodium hydrogen phosphate per litre of deionised water) and the whole is diluted to the 100 ml mark with deionised water. The intrinsic viscosity of the polymers is measured using a Number 1 suspended level viscometer at 25⁰C in 1M buffered salt solution.

Generally the polymeric flocculant will be prepared by the polymerisation of water-soluble ethylenically unsaturated monomer or monomer blend. Typically an aqueous solution of water soluble monomer may be polymerised by solution polymerisation to provide an aqueous gel or by reverse phase polymerisation in which an aqueous solution of monomer is suspended in a water immiscible liquid and polymerised to form polymeric beads or alternatively by emulsifying aqueous monomer into an organic liquid and then effecting emulsion polymerisation. Examples of reverse phase polymerisation are given in EP-A- 150933, EP-A-102760 or EP-A-126528. The process has been found to be particularly effective when the polymeric flocculant is as described in EP-A-202780. Preferably, the polymeric flocculant comprises particles of cross-linked polymer having a weight average particle size diameter of below 10 microns, preferably below 2 microns and more preferably between 500 nanometres and 1 micron.

Particularly suitable polymers comprise cross-linked or at least partially cross- linked polymers and may comprise a blend of substantially linear water-soluble polymer and swellable cross-linked polymer.

The preferred way of making the aqueous composition is by mixing into water particles of polymeric material having a weight average dry particle size diameter of below 10 microns, preferably below 2 microns and more preferably between 500 nanometres and 1 micron, and which have been made by reverse phase emulsion or suspension polymerisation of one or more monoethylenically unsaturated monomers. The polymer may be soluble but is preferably insoluble as a result of a controlled addition of cross-linking agent to the monomer or monomer blend, which is preferably water soluble.

The monoethylenically unsaturated material may be contaminated with a small amount of cross-linking agent and the amount of additional cross-linking agent that is added will therefore be selected having regard to this fact. Preferably the monoethylenically unsaturated material is as free of cross-linking agent as is commercially possible, for instance containing cross-linking agent in an amount that gives cross linking or chain branching less than is given by ppm MBA (1 part methylene bis acrylamide per million parts monomer). The amount of MBA that is added is generally at least 0.1 or 0.2 ppm and below 100 ppm (based on monomer), generally 1 to 50 ppm. The precise amount will depend upon the polymerisation and other processing conditions. Instead of using MBA, cross- linking my be by equally effective amounts of other diethylenically unsaturated compounds such as ethylene glycol d-acrylate, diacrylamide, cyanomethylacrlate, vinyloxyethylacrylate or methacrylate and other means of cross linking, e.g., formaldehyde or glyoxal or metal salt addition. Preferably a water-soluble cross-linking agent is used.

The degree of non-linearity can additionally be controlled by the inclusion of chain transfer agents in the polymerisation mixture. Their use, in combination with cross-linking agent, will tend to promote chain branching rather than cross linking. Amounts may vary widely. For instance 1 ,000 to 5,000 ppm (based on monomer) of a moderate chain transfer agent such as isopropyl alcohol may be suitable whilst much lower amounts, typically 100 to 500 ppm, of more effective chain branching agents such as mercaptoethanol are useful. Often, however, adequate results are obtained by conducting polymerisation under conventional conditons, without deliberate addition of chain transfer agent, using commercially pure monoethylenically unsaturated monomer together with the specified amount of MBA or other cross-linking agent.

When the polymeric material is cross linked and cationic, and in particular when it is a copolymer of acrylamide with at least 5%, and preferably at least 10%, by weight dialkylamino alkyl acrylate (generally as acid addition or quaternary ammonium salt), the degree of non-linearity is preferably such that the polymer has an ionic regain (IR) of at least 15%. IR is calculated as (x-y)/x.times.100 where x is the ionicity measured after applying standard shear and y is the ionicity of the polymer before applying standard shear.

These values are best determined by forming a 1% composition of the polymer is deionised water, allowing this to age for 2 hours and then further diluting it to 0.1% active polymer. The ionicity of the polymer y is measured by Colloid Titration as described by Koch-Light Laboratories Limited in their publication 4/77 KLCD-1. (Alternatively the method described in GB 1 ,579,007 could possibly be used to determine y.) The ionicity after shear, x is determined by measuring by the same technique the tonicity of the solution after subjecting it to standard shear.

The shear is best applied to 200 ml of the solution in a substantially cylindrical pot having a diameter of about 8 cm and provided in its base with a rotatable blade about 6 cm in diameter, one arm of the blade pointing upwards by about 45 degrees and the other downwards by about 45 degrees. The blade is about 1 mm thick and is rotated at 16,500 rpm in the base of the pot for 10 minutes. These conditions are best provided by the use of a Moulinex homogeniser but other satisfactory conditions can be provided using kitchen blenders such as Kenwood, Hamilton Beach, lona or Osterizer blenders or a Waring Blendor.

In practice the precise conditions of shear are relatively unimportant since, provided the degree of shear is of the same order of magnitude as specified, it will be found that IR is not greatly affected by quite large changes in the amount, for instance the duration, of shear, whereas at lower amounts of shear (for instance 1 minute at 16,500 rpm) IR is greatly affected by small changes in shear. Conveniently, therefore, the value of x is determined at the time when, with a high speed blade, further shear provides little or no further change in ionicity. This generally requires shearing for 10 minutes, but sometimes longer periods, e.g., up to 30 minutes with cooling, may be desired.

When using cross-linked polymeric material, polymers having IR of 15% have a relatively low degree of non-linearity whilst those having IR 90% have a high degree of non-linearity. It is generally preferred for IR to be below 80%; preferably below 70%, and usually below 60%. If IR is too low, the invention may give inadequate benefit compared to conventional polymers and preferably IR is above 20%. Best results are generally obtained at above 25%, preferably 30 to 60%. A particularly preferred polymeric flocculant is a cross-linked polymer has been prepared by reverse phase emulsion polymerisation of acrylamide with dialkyl amino alkyl (meth) acrylate either as the freebase amine, acid addition salt or quaternary ammonium salt in the presence of a polyethylenically unsaturated cross-linking agent, e.g. MBA.

The surface active agent used in the process is desirably a demulsifying agent, preferably with wetting and detergent properties. Typically it can be a water- soluble surface active agent, preferably ionic and more preferably anionic. The surface active agent particularly preferably comprises at least one salt selected from the group consisting of salts of alkyl sulphates, alkyl sulphonates, alkyl aryl sulphonates, alkyl ether sulphates, alpha olefin sulphonates, alkyl aryl ether sulphates, sulfated alcohols and ethoxylated sulphated alcohols, taurates, petroleum sulphonates, alkyl naphthalene sulphonates, alkyl sarcosinates and alkyl sulphosuccinates in which the alkyl group contains from about 8 to about 22 carbon atoms and the aryl group is phenyl or naphthyl. Sodium alkyl sulfosuccinate, especially sodium dioctyl sulphosuccinate, are considered particularly suitable as surface active agents for this process. The surface active agent will generally be added to the flocculated sludge in an amount of at least 100 mg/l, although the dose may be as high as 800 mg/l. Preferably, the treatment comprises a dose of between 300 and 600 mg/l.

The sludge used in the process is generally in the form of a liquid or semi solid mixture of water, particulate solids and hydrocarbon. The hydrocarbon component can be a single hydrocarbon although in practice it may often be a blend of hydrocarbons. Typically this may be one or more C8 to C12 hydrocarbons, for instance octanes and/or decanes. Suitably the oily sludge is typical of that frequently encountered in the petrochemical industry.

The mechanical dewatering step may be by any conventional means, and for instance may be centrifuge, belt press but preferably chamber filter press. Suitably the sludge is separated into a dewatered solid cake and a centrate or filtrate containing the water and hydrocarbon. The water and hydrocarbon may then be separated in order to allow maximum recovery of the hydrocarbon and suitable disposal or reuse of the water component. Alternatively it may be more desirable to separate the sludge into a solid cake containing hydrocarbon and a centrate or filtrate containing the water with essentially no hydrocarbon or relatively low levels of hydrocarbon, for instance the below 5% by weight, preferably below 1%. Typically, the water removed may be returned to the watercourses if it a contains essentially no hydrocarbon. On the other hand it may be necessary to treat the water further to remove any traces of hydrocarbon.

The process may be automated using a suitable feedback control mechanism for controlling the dose of ingredients such as high molecular weight flocculant and/or coagulant. A suitable process is described in WO 96/31265 or in WO 85/02836.

A particularly preferred treatment of the oily sludge involves the sequential addition of: (a) a sodium bentonite clay in an amount between 10,000 and

15,000 mg/l,

(b) ferric sulphate or ferric chloride in an amount between 2000 and 4000 mg/l,

(c) cross-linked cationic copolymer of acrylamide and dimethylaminomethacrylate quaternized with methyl chloride, with IV 9.5 dl/g and IR 39.2%, prepared in the form of a 50% active liquid dispersion added in an amount between 300 and 600 mg/l, and

(d) sodium dioctyl sulpho succinate added in an amount of between 300 and 600 mg/l. The following example is intended to illustrate the invention without in anyway limiting it.

Example 1

An oily lagoon waste sludge is treated using different chemical treatments followed by filter pressing to produce a dewatered cake. .

1 , Sludge aliquots of 200 ml are taken for each test.

2, Bentonite microparticle, being the sodium form of montmorillonite swelling clay is added as 5% slurry under moderate shear mixing of 500rev/min for 5 seconds

3, Inorganic coagulant Ferric Sulphate is added as a 40% solution under moderate shear mixing of 500rev/min for 5 seconds

4, Polymeric flocculant: cross-linked cationic copolymer of acrylamide and dimethylaminomethacrylate quatemized with methyl chloride, with IV 9.5 dl/g and IR 39.2%, prepared in the form of a 50% active liquid dispersion is diluted to 0.2% and added under moderate shear mixing of 500rev/min, maintained until optimum flocculation was achieved. The flocculated sludge is visually assessed for floe size and dewaterability.

5, Surface active agent: 60% active solution of sodium dioctylsulfosuccinate is added as a neat product under moderate shear mixing of 500rev/min, for 5 seconds. 6, The conditioned samples were then subjected to an increase in pressure to a maximum of 120psi during a 30-minute press cycle. Cake volume is then calculated.

The results are shown in Table 1.

Table 1

The results in table 1 show that the combination of swellable clay with inorganic coagulant, polymeric flocculant and surface active agent provides improved dewatering and improved cake volume over the same process but in the absence of swellable clay.

Claims

1. A process for dewatering a sludge containing hydrocarbon, comprising pre treating the sludge by adding to the sludge (a) a swellable clay, (b) an inorganic coagulant, (c) a polymeric flocculant and (d) a surface active agent and subjecting the pretreated sludge to mechanical dewatering.

2. A process according to claim 1 in which the swellable clay, inorganic coagulant, polymeric flocculant and surfactant are added sequentially.

3. A process according to claim 1 or claim 2 in which the surfactant is the last component added to the sludge.

4. A process according to any of claims 1 to 3 in which the swellable clay is a bentonite or bentonite type clay, preferably a smectite.

5. A process according to any of claims 1 to 4 in which the swellable clay is selected from the group consisting of hectorite, montmorillonites, nontronites, saponite, sauconite, hormites, attapulgites and sepiolites.

6. A process according to any of claims 1 to 5 in which the inorganic coagulant is selected from ferric sulphate, ferric chloride, alum, polyaluminium chloride, aluminium chloride trihydrate and aluminochloro hydrate.

7. A process according to any of claims 1 to 6 in which the polymeric flocculant is non-ionic or ionic, preferably cationic.

8. A process according to any of claims 1 to 7 in which the polymeric flocculant is formed from ethylenically unsaturated cationic or potentially cationic monomers, optionally with a non-ionic ethylenically unsaturated monomer.

9. A process according to any of claims 1 to 8 in which the polymeric flocculant is a copolymer of acrylamide with dialkyl amino alkyl (meth) acrylate either as the freebase amine, acid addition salt or quaternary ammonium salt.

10. A process according to any of claims 1 to 9 in which the polymeric flocculant is cross-linked.

11. A process according to any of claims 1 to 10 in which the polymeric flocculant exhibits an intrinsic viscosity of at least 3 dl/g, preferably at least 7 dl/g.

12. A process according to any of claims 1 to 11 in which the polymeric flocculant comprises particles of cross-linked polymer having a weight average particle size diameter of below 10 microns, preferably below 2 microns and more preferably between 500 nanometres and 1 micron.

13. A process according to any of claims 1 to 12 in which the polymeric flocculant is a cross-linked polymer exhibiting an ionic regain of at least 15%, preferably between 30 and 60%.

14. A process according to any of claims 1 to 13 in which the polymeric flocculant is a cross-linked polymer has been prepared by reverse phase emulsion polymerisation of acrylamide with dialkyl amino alkyl (meth) acrylate either as the freebase amine, acid addition salt or quaternary ammonium salt in the presence of a polyethylenically unsaturated cross-linking agent.

15. A process according to any of claims 1 to 14 in which the surface active agent comprises at least one salt selected from the group consisting of salts of alkyl sulphates, alkyl sulphonates, alkyl aryl sulphonates, alkyl ether sulphates, alpha olefin sulphonates, alkyl aryl ether sulphates, sulfated alcohols and ethoxylated sulphated alcohols, taurates, petroleum sulphonates, alkyl naphthalene sulphonates, alkyl sarcosinates and alkyl sulphosuccinates in which the alkyl group contains from about 8 to about 22 carbon atoms and the aryl group is phenyl or naphthyl.

16 A process according to any of claims 1 to 15 in which the surface active agent is a sodium alkyl sulfosuccinate, preferably sodium dioctyl sulphosuccinate.

17. A process according to any of claims 1 to 16 in which the sludge is a liquid or semi solid mixture of water, particulate solids and hydrocarbon.

18. A process according to any of claims 1 to 17 in which the mechanical dewatering is by centrifuge, belt press and preferably chamber filter press.

19. A process according to any of claims 1 to 18 in which the sludge is separated into a dewatered solid cake and a centrate or filtrate containing the water and hydrocarbon.

20. A process according to any of claims 1 to 18 in which the sludge is separated into a dewatered solid cake containing hydrocarbon and a centrate or filtrate.