GB2319246A - Separating oil-in-water emulsion - Google Patents

Abstract

A process for separating an oil-in-water emulsion contained in a waste-stream comprises centrifuging 11 the stream so as to separate it into components consisting of dirty water which contains the oil-in-water emulsion, floating oil and settled solids, acidifying the dirty water by addition of CO 2 12a and mineral acid 12b and then centrifuging 13 the acidified mixture to separate the dirty water into oil, water and solid components. Preferably the acidification breaks the emulsion and precipitates minerals. Saturation of the dirty water with CO 2 may be accelerated by pumping the CO 2 into a number of diffusion chambers containing baffles (Fig 2), the CO 2 being pumped against the flow of the stream through these chambers. The mineral acid may be HCl, the HCl being pumped into a second set of diffusion chambers with baffles. The final product may be further cleaned using absorbent charcoal 14a,14b.

Description

SEPARATION PROCESS This invention relates to separation processes, and concerns in particular processes for the separation of oil and water mixtures, especially those wherein the oil is emulsified (as a colloidal suspension) in the water, and the water contains dissolved therein various metal cat ions.

There are many occasions when, as the result of some industrial process, there is produced an oil-inwater emulsion with metal ions dissolved in the water.

Indeed, waste streams of this type arise globally and are produced in huge volumes by a number of industries including oil refining, oil re-refining and shipping. A typical such oily, aqueous waste stream contains both 'free' and emulsified hydrocarbons, as well as heavy metals, sulphur, and limited amounts of other toxic substances; it incorporates around 98% water and 2% oil and contaminants (though some can contain as much as 10% oil and other contaminants).

Current methods used for the removal of the oils and other contaminants from the water in such waste streams are varied, and well documented. Their prime purpose is simply to reduce the level of hydrocarbons and other toxic materials such that the resultant product is acceptable to a waste contractor for ultimate disposal, usually to landfill after extensive and expensive further treatment. They all have several serious drawbacks.

For example, the processing required is extremely slow. The two commonly used methods involve either heavy chemical dosing followed by settlement, taking around a week to achieve reasonable separation, or the use of bacteria, which takes some 4 to 6 weeks.

Moreover, they all utilise batch processing techniques. Because of the nature of the treatment, very large containment vessels are required to store the waste prior to and during treatment; a typical installation would comprise many 100,000 gallon tanks in a tank farm, with each tank being at a different stage of processing but with each vessel fully utilised until processing is considered complete.

The long processing times and the huge areas of space required for such tank farms severely limit the rate at which the industry producing the waste can operate, and often inhibits growth in that industry.

This may have very serious cost implications.

There are, however, yet further problems with the presently-available techniques. Thus, they are 'total loss processes', in that none of the separated fractions, with the exception of any free floating oil, are recovered or recycled. Moreover, as part of the chemical dosing required large amounts of chemicals are added to the waste stream, and these eventually end up forming a large part of the effluent produced by the process. In the case of microbiological degradation, large amounts of biologically-active but toxic slurry are formed as well as contaminated water flows. Such methods give rise both to enormous quantities of stillcontaminated water and also to large volumes of toxic slurry; this has serious additional cost implications to the producer.

Thus the present separation methods are costly in time and space, have very minimal recycling benefits, and end up producing similar amounts of toxicity (albeit in a different form) to those that existed before treatment began. They also, and without exception, achieve an incomplete separation, leaving each separated fraction contaminated to some degree by the others.

The present invention proposes a novel separation method that, it is suggested, does not suffer from these problems, or at the very least suffers significantly less from them. More specifically the novel separation procedure here put forward is a continuous rather than a batch process, involves only the simplest dosing of the waste stream (with chemicals in themselves of no, or very little, toxicity), and permits the recovery of almost all the ingredients of the waste stream, as well as the production of an aqueous output that is almost either re-usable or returnable to the environment as it is, even without further treatment.

The method the invention proposes is one wherein, following a conventional continuous centrifugation, which mechanically separates the feedstock waste stream into disposable floating oil, disposable settled solids, and "dirty water", that "dirty water", which is basically an oil-in-water emulsion with minerals dissolved in the water, is further treated by a cascade-like acidification stage, both to "break" the emulsion and to precipitate most of the minerals, first by dissolving therein CO2 (using a diffuser) to start the two processes off, and then by mineral acid acidification to complete the two processes, whereafter the "mixture" is again centrifuged, and the oil, water and solids components are again mechanically separated off. At this point the water component should contain little except tiny quantities of hydrocarbons (often in colloidal form), and is suitable for final purification by being passed through absorbent charcoal, whereafter it should be almost drinkably clean.

In one aspect, therefore, the invention provides a continuous process for the separation of an oil-in-water emulsion waste-stream feedstock material, in which process: the feedstock is subjected to a continuous centrifugation, which mechanically separates the feedstock into disposable floating oil, disposable settled solids, and "dirty water"; that "dirty water", which is an oil-in-water emulsion with minerals dissolved in the water, is further treated by a two-stage acidification, both to "break" the emulsion and to precipitate most of the minerals, first, by dissolving therein CO2, and second, by mineral acid acidification; whereafter the resultant mixture is again centrifuged, and the oil, water and solids components are again mechanically separated off.

As explained further hereinafter, the thus-formed water component, which commonly contains small amounts of hydrocarbons (probably in colloidal solution), may then be further treated by being passed through absorbent charcoal to remove (and recover) those hydrocarbons. The water from this further stage is then clean enough to be disposed of directly (into the sewers), or possibly even to be passed back into the original industrial system.

A fuller appreciation of the significance of the process of the invention, and of the advantages and benefits conferred thereby, may be gained from the following discussion of the principles lying behind the inventive process.

a) Solutions A solution is defined as a homogeneous mixture of pure substances in which no settling occurs. True solutions consist of a solvent and one or more so lutes whose proportions vary from one solution to another (by contrast, a pure substance has fixed composition). The solvent is the medium in which the so lutes are dissolved. The fundamental units of solutes are usually ions or molecules.

Solutions may involve many different combinations in which a solid, liquid or gas can be either solvent or solute. The most common kinds are those in which the solvent is a liquid. For instance, sea water is an aqueous solution of many salts and some gases (such as carbon dioxide (CO2) and oxygen (02). Carbonated water is a saturated solution of C02 in water.

Examples of solutions in which the solvent is not a liquid are also quite common. Air is a solution of gases (with variable composition). Dental fillings are solid amalgams or solutions of liquid mercury dissolved in solid metals. Alloys are solid solutions of solid metals dissolved in one another.

It is usually obvious which of the components of a solution is the solvent and which is (are) the solute(s). The solvent is usually the most abundant species present. In a cup of instant coffee, the coffee and any added sugar are considered solutes, and the hot water is the solvent. If 10g of alcohol is mixed with 90g of water, the alcohol is the solute. If 10g of water is mixed with 90g of alcohol, the water is the solute. Where equal quantities are mixed together the decision as to which is the solute and which solvent is arbitrary and unimportant.

b) Colloids A solution is a homogeneous mixture in which no settling occurs and in which solute particles are at the molecular or ionic state of subdivision. This represents one extreme of mixtures. The other extreme would be a suspension of a clearly heterogeneous mixture in which a solvent-like phase, such a situation results when a handful of sand is stirred into water.

Colloids (colloidal sols, suspensions or dispersions) represent an intermediate kind of mixture in which the solute-like particles or dispersed phases, are suspended in the solvent-like phase, or dispersing medium. The particles of the dispersed phase are small enough for settling to be negligible. However, they are large enough to make the mixture appear cloudy (and in many cases opaque) because light is scattered as it passes through the colloid. The oily wastes with which the present invention is concerned are instances of such an intermediate mixture.

c) The absorption Phenomenon Since colloidal particles are so finely divided, they have enormous surface areas in relation to their volumes. It is not surprising therefore, that an understanding of colloidal behaviour requires an understanding of surface phenomena.

Atoms on the surface of a colloidal particle are bonded to other atoms of the particle in only two dimensions. Since these atoms, like all others, are capable of bonding in three dimensions, they have a tendency to interact with whatever comes into contact with the surface.

Colloidal particles often absorb ions or other charged particles, as well as gases and liquids. The process of absorption involves adhesion of any such specifies onto the surfaces of particles. In this case, the oil has heavy metals which come from the materials of construction of an engine, i.e., lead, tin, antimony, silicon, iron, aluminium. These metal ions act as emulsifying agents which are described below under the heading of hydrophobic colloids.

Colloids are classified as hydrophilic (water loving) or hydrophobic (water hating) based on the surface characteristics of the dispersed particles.

d) Hydrophilic colloids Proteins such as the oxygen-carrier haemoglobin form hydrophilic solutions when they are suspended in saline aqueous body fluids such as blood plasma. Such proteins are macromolecules (giant molecules) that fold and twist in an aqueous environment so that polar groups are exposed to the fluid, while non polar groups are encased.

Protoplasm and human cells are examples of gels, which are special types of solutions in which the solid particles (in this case mainly proteins and carbohydrates) join together in a semi-rigid network structure that enclosed the dispersing medium. Other examples of gels are gelatin and jellies and gelatinous precipitates such as aluminium hydroxide and ferric hydroxide.

e) Hydrophobic colloids Hydrophobic colloids cannot exist in polar solvents without the presence of emulsifying agents, or emulsifiers. These agents coat particles of the dispersed phase to prevent their coagulation into a separate phase. Milk and mayonnaise are examples of hydrophobic colloids (milk fat in milk, vegetable oil in mayonnaise) that stay suspended with the aid of emulsifying agents (casein in milk and egg yolk in mayonnaise).

In the mixture resulting from vigorous shaking of oil (a non-polar material) and water (a polar material), droplets of hydrophobic oil are temporarily suspended in the water. However, in a short time, the polar water molecules, which attract each other strongly, squeeze out the non-polar oil molecules. The oil then coalesces, and floats to the top.

f) Oil-in-water emulsions Some of the oil in the waste feedstocks relevant to the present invention is derived from car engines, and thus is contaminated with products which act as emulsifying agents. These products include heavy metals from the materials of construction of the engine itself; these atoms of these metals are bonded to the oil molecules, but also ionise in water. The oil also includes water (even oil that is apparently water-free contains water absorbed from the atmosphere), along with other substances such as C02, which readily dissolves in oil. CO2 is a non-polar gas which also dissolves in water to produce carbonic acid, H2C03, which does ionise in water; the water, the metals and the CO2 all act as emulsifiers, and cause there to be formed an oil-inwater emulsion.

The invention provides a continuous process for the separation of an oil-in-water emulsion waste-stream feedstock material. The process is continuous in that the feedstock is continuously fed into the first centrifuge (which is naturally of the type that permits this), the three products thereof are continuously removed therefrom, the "dirty water" product is continuously fed into and through the CO2 diffuser, and then into and through the HC1 mixing stage, the material output from the latter is continuously fed into the second centrifuge (again, of the type that permits this), and the output thereof is continuously extracted.

Thereafter the "clean" water produced may be further treated in any way thought appropriate - which way may itself be continuous, or not, as relevant. Passing it through absorbent charcoal is one such further treatment, and is discussed hereinafter.

The process of the invention separates out the three main components - solids, oil and water - of an oil-in-water emulsion waste-stream feedstock material.

The feedstock may be almost any having such components; it may, for example, be old engine oil, oil flushed out of (ship's) tanks, and oil/water mixtures from oil refineries. Typical ionic material (and potential solids) in the feedstock is one or more of Zinc, Copper, Cadmium, Nickel, Chromium, Lead, Iron, Manganese, Calcium, Magnesium, Aluminium, Tin, Silicon, Boron, Vanadium, Phosphorus, Sulphur, Arsenic, Selenium, Antimony, and Mercury. An actual feedstock might match the following: Test Description Result Detection Limit Units pH 6.6 Acidity to pH 4.0 nr 1000 mg/kg as CaC03 Acidity to pH 8.0 nr 1000 mg/kg as CaC03 Alkalinity to pH 8.0 nr 1000 mg/kg as CaCO3 Alkalinity to pH 4.0 nr 1000 mg/kg as CaCO3 Relative density 1.05 g/ml Cyanide - Total bdl 1 mg/kg Suspended Solids 3040 1000 mg/kg DoE 40 -ve Oil - Free 10000 1000 mg/kg Oil - Total us 1000 mg/kg Metals by ICP Zinc 72 10 mg/kg Copper bdl 10 mg/kg Cadmium bdl 10 mg/kg Nickel bdl 10 mg/kg Chromium bdl 10 mg/kg Lead bdl 10 mg/kg Iron 49 10 mg/kg Manganese bdl 10 mg/kg Calcium 966 10 mg/kg Magnesium 1000 10 mg/kg Aluminium 11 10 mg/kg Tin bdl 10 mg/kg Silicon 56 10 mg/kg Boron 253 10 mg/kg Vanadium bdl 10 mg/kg Phosphorus 344 10 mg/kg Sulphur 1383 10 mg/kg Metals by hydride ICP Arsenic bdl 1.0 mg/kg Selenium bdl 1.0 mg/kg Antimony bdl 1.0 mg/kg Mercury bdl 1.0 mg/kg bdl = Below Detection Level In the invention's process the feedstock is subjected to a continuous centrifugation which mechanically separates the feedstock into recyclable floating oil, disposable settled solids, and "dirty water". Centrifuges that can do this are commercially available. A typical example is that known as a TRICANTOR and supplied by Krauss Maf fey; such a centrifuge is suitable for use in the invention.

A centrifuge separates not only solids from liquids but can also separate two liquids of different specific gravities. The TRICANTOR can in fact simultaneously separate both solids and liquids and also two liquids.

It thus gives rise to three separate streams from the single original feedstock, namely: a slurry of solid materials (this is those solids that were actually mixed with or suspended in the feedstock); a concentrated oil/hydrocarbon fraction (this is the oily material that was floating as large droplets in the feedstock); and a somewhat purified aqueous fraction (this is the rest, which will contain both dissolved minerals and emulsified oil).

The centrifuge will of course be operated at whatever speed is appropriate. In practice the most suitable speed for the TRICANTOR (on this type of feedstock) is equivalent to 9,000 G.

The process is continuous, with the capacity of the centrifuge matched to the required rate of flow of feedstock. The TRICANTOR is available in sizes up to 300m3 (300x103 litre, or about 6x104 Imp.gallons) per hour capacity The resultant fractions taken from the centrifuge are then dealt with as appropriate. Thus, the solids (as a slurry) are fed to a containment vessel to await disposal, the oil/hydrocarbons are fed to another containment vessel, and thence to re-refining, and the remaining oil-in-water emulsion is sent off to the next stage of the process. Thus, once the feedstock has been separated into disposable floating oil, disposable settled solids, and "dirty water", the "dirty water", which is an oil-in-water emulsion with minerals dissolved in the water, is further treated by a twostage acidification, both to "break" the emulsion and to precipitate most of the minerals. In the first stage there is dissolved therein C02, and in the second there is added a by mineral acid.

The second part of the process of the invention is designed to remove the oil emulsion from its 'carrier' water, and it is this part of the process that is believed to be unique in this application, for as yet no-one seems to have appreciated the advantages of utilising a two-stage, cascade procedure here.

In the first stage of this cascade procedure carbon dioxide gas is pumped into the "dirty water". This saturation of the liquid with carbon dioxide has the twin effect of both 'destabilising' the oil held in emulsion and also lowering the pH of the stream to around 6.0. At this value, and in such liquids, a 'cascade' effect is initiated whereby with an excess of CO2 not only is the oil 'squeezed' out of the water but moreover heavy metal contaminants start to precipitate out from solution.

The CO2 is most preferably pumped into the liquid against the flow of the stream, and very advantageously utilising a specific diffusion manifold into specially designed diffusion chambers. Although not actually crucial, nevertheless the design of both the diffuser manifold and the chamber itself are important to the success of this part of the process. In practice it has been discovered that a particularly efficient mixing chamber/diffuser is one of those static in-line mixers manufactured in titanium by Statiflow. Such diffusers are available to cope with various rates of flow and chamber size, and have been found to achieve the very rapid saturation of the solution with the COz gas which is desirable to initiate the 'cascade' especially when arranged in a specific configuration (the optimum design achieved is as shown in the Figures of the accompanying Drawings described hereinafter). The design of the diffusion chamber itself is also important, and it has been found that the design shown in the aforementioned Figures is a very effective configuration.

The first stage in the destabilisation of the oil emulsion is initiated by the diffusion thereinto of CO2, and the cascade effect commences. This is then enhanced by the acidification of the liquid with dilute mineral acid (using the preferred apparatus of the invention, as shown in the accompanying Drawings, this is effected by injecting the acid into a second mixing chamber in series with the first). The acid may be any suitable acid, especially a mineral acid such as hydrochloric acid, and it has typically been found that a solution of 2M hydrochloric acid is particularly effective.

The method of addition and the mixing of the acid into the waste stream is of importance, and is essentially encompassed in the preferred equipment design (as is discussed hereinafter with reference to the accompanying Drawings).

In both the first and second stages of the inventive process' acidification part the rate of dosing is conveniently monitored by sensors operatively connected to a suitable control system in order to keep the relevant parameters within appropriate limits.

By the time the waste stream exits the second stage of the acidification part the pH has fallen further.

All the oil is freed from its emulsified state, and the heavy metals along with other contaminants are out of solution and amenable to subsequent removal from the waste stream.

The flow exiting the diffusion vessel is then pumped to a further centrifuge; in this case this can be of a more conventional design as the separation required is that of liquid and solid only. In practice a centrifuge of the DECANTOR type provided by Krauss Maffey has proved suitable; such centrifuges are available in a variety of sizes from 3m3 to 300m3 per hour capacity. This stage of the process is quite conventional and need not be further described.

The second centrifugation gives rise to two streams, the first being the now substantially 'cleaned' water flow, which contains no solids to speak of and is clear but still a pale brown in colour, and a slurry of solid wastes extracted from the water. The slurry is piped to a container for ultimate disposal, whilst the water stream is pumped to whatever next stage of processing is thought desirable.

For example, in one preferred embodiment of this next stage the water from the second centrifugation is pumped under pressure into one of a pair of identical stainless steel vessels each containing a bed of charcoal. Many types of this product are available, but an efficient product seems to be that available under the designation YAO 14/40 and supplied in the UK by Eurocarb.

The size of vessel and charcoal bed are not critical, save that very preferably the minimum size required is equivalent to 400 cm2 surface area of charcoal per m3 of flow per hour. Thus, a useful ruleof-thumb ratio for this application would be Im3 of charcoal bed per 2,000m3 treated. This ratio gives a reasonable operational life between 'regenerations'.

Charcoal beds work by a process of adsorption, and their ability to remove contaminants from a waste stream is finite, whereupon they must be regenerated. Sensors on the exit pipework detect the output of undesirable contaminants (and thus when the adsorptive capacity of the bed begins to fail), and when this occurs the relevant control system switches the flow to the next vessel (a system known in engineering as a duty/standby arrangement), thus maintaining the quality of the exiting water stream.

In most such systems the exhausted charcoal bed is then replaced with new charcoal whilst the contaminated charcoal is sent away to a specialist for regenerating, but in the case of the present invention and in view of the value (to a refiner) of the adsorbed 'hydrocarbons' the charcoal containment vessels may advantageously be linked to a high pressure steam supply, so that regeneration of the charcoal bed is effected in situ.

Steam at a pressure of 15 bar and temperature of 120"C is forced via injectors into the charcoal bed for a period of approximately 5 minutes. The resultant relatively small volume of condensate is collected in a further vessel for subsequent re-refining. This process like all others can be monitored by sensors, and so can be completely automatic.

The water stream exiting the charcoal vessels is a colourless odourless liquid, a typical analysis of which would be as follows: Test Description Result Detection Limit Units pH 6.5 - 7.0 Acidity to pH 4.0 nr 1000 mg/kg as CaCO3 Acidity to pH 8.0 nr 1000 mg/kg as CaC03 Alkalinity to pH 8.0 nr 1000 mg/kg as CaC03 Alkalinity to pH 4.0 nr 1000 mg/kg as CaC03 Relative density 1.02 g/ml Cyanide - Total bdl 1 mg/kg Suspended Solids bdl 1000 mg/kg DoE 40 N/A Oil - Free bdl 1000 mg/kg Oil - Total bdl 1000 mg/kg Metals by ICP Zinc 5 10 mg/kg Copper bdl 10 mg/kg Cadmium bdl 10 mg/kg Nickel bdl 10 mg/kg Chromium bdl 10 mg/kg Lead bdl 10 mg/kg Iron 6 10 mg/kg Manganese bdl 10 mg/kg Calcium 763 10 mg/kg Magnesium 822 10 mg/kg Aluminium 4 10 mg/kg Tin bdl 10 mg/kg Silicon 7 10 mg/kg Boron bdl 10 mg/kg Vanadium bdl 10 mg/kg Phosphorus bdl 10 mg/kg Sulphur 3 10 mg/kg Metals by hydride ICP Arsenic bdl 1.0 mg/kg Selenium bdl 1.0 mg/kg Antimony bdl 1.0 mg/kg Mercury bdl 1.0 mg/kg bdl = Below Detection Level Such water is ideally suited for reuse or for further purification by conventional means for demanding situations such as boiler feed, or is acceptable for direct discharge without penalty.

The whole of the system described may be controlled and run automatically by a central computer, linked to remote sensors throughout the plant. The plant requires minimal maintenance and operator input.

As will be apparent, the method of the invention presented here involves the use of several existing technologies arranged in a specific order and with the addition of the diffusion of gaseous CO2 into the waste stream. This has a 'destabilising' effect on oil emulsions which facilitates their removal from the water.

The process described exhibits distinct advantages over presently available methods.

1. The process described is one of 'continuous flow'. The new process enables the waste stream to be treated continuously at a pre-designated but adjustable rate, removing the necessity for the huge storage facilities required with other methods.

2. The new process is to all intents and purposes instantaneous, with a total processing time of around ten minutes, and the capacity of the plant can be readily expanded without requiring large areas of space thus negating the 'growth' constraints imposed by other methods.

3. The new process allows almost all of the water (95) to be recycled or discharged without penalty.

The water produced by the process is of a high quality, and being free from toxic and particulate matter is ideally suited to further additional (conventional) treatment to provide large volumes of water to any required specification.

Most industries producing this kind of waste also consume large quantities of water in their refining etc. and the water produced in the new process is ideal for reuse.

4. The hydrocarbon fraction is separated from the waste stream at three different points; at two of these stages the hydrocarbon is collected, and is available for recycling. In particular, the final steam condensate produced by the new system is not extracted in conventional processing. The contents of this 'fraction' are especially valuable.

5. The new process, by virtue of the effect of the C02 gas being used as a destabiliser, consumes considerably less 'chemicals' than present techniques. The chemicals commonly used are in themselves 'toxic' to some extent; the new process requires much less use of such substances.

6. The CO2 gas consumed in the new process is an industrial by-product generally considered to be harmful - a 'greenhouse gas'. It is converted during the process to harmless carbonic acid radicals. Any excess gas not consumed in the process can be collected and recycled continuously into the waste stream. CO2 gas is readily and cheaply available.

7. Because the new process requires less chemical dosing, much smaller amounts of 'effluent' are produced than in conventional systems. This ensures much lower processing and disposal costs than at present.

8. The 'effluent' produced by the new process is both low in water content (and because of its low volume therefore more cheaply dealt with, and also contains much smaller amounts of hydrocarbons than conventionally produced.

9. The nature and costs associated with the new process are such as will enable relatively small producers of such waste to purchase the 'system', thereby the treatment of such waste can be accomplished on site. alleviating the toxic waste 'haulage hazards' presently associated with these industries.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying Drawings in which: Figure 1 shows a flow diagram of a preferred embodiment of the process of the invention; Figure 2 shows a sequence of diffuser/mixer units employed in effecting the process of the invention; Figure 3 shows one of the diffuser/mixer units of Figure 2 in more detail; and Figure 4 shows a single diffuser/mixer in the units of Figure 2 in even more detail.

The flow diagram in Figure 1 shows the process of the invention, in which an oil-in-water emulsion wastestream feedstock material is separated, as a continuous process, into its several portions. In the first part of the process the feedstock F is fed via pH, flow rate and conductivity sensor valves V1 into a centrifuge (11) and there subjected to a continuous centrifugation which mechanically separates the feedstock into disposable floating oil, disposable settled solids, and "di emulsion with minerals dissolved in the water; this initiates the "breaking" of the emulsion and the precipitation of most of the minerals dissolved in the water.

The output of the first stage 12 is then passed via another set of valves V3 to the second stage (12b), where HC1 is added to the liquid to acidify it further.

This second acidification completes the process of "breaking" the emulsion and precipitating most of the minerals dissolved in the water.

The output of this second stage acidification is then fed via yet another set of valves V4 to a second centrifuge (13), where the mixture is separated into a solids/slurry output stream and a water stream. These are extracted from the centrifuge along the relevant pipelines, and the water output is fed on to one or other of a pair of charcoal bed vessels (14a, ), from which effectively "pure" water is taken. Each bed can be regenerated with steam, and the steam distillate contains the various hydrocarbons that the bed has absorbed during its use.

The valve sets V mentioned above allow a determination of pH and turbidity, and their outputs can be fed back to the plant control unit (not shown) where alterations to the flow rate of the feedstock, or to the CO2 gas flow, can be initiated as may be necessary due to variations in the levels of contamination present.

Details of the diffuser/mixer units, and of the individual diffusers, can be seen from Figures 2, 3 and 4. It should be understood that in Figures 2 and 3 each diffuser/mixer unit is made up of five identical diffusers/mixers, and that in practice these are arranged in a bundle of five side by side but are here shown purely for convenience and clarity as a line of five.

As will be most apparent from Figure 4, each diffuser/mixer is a long hollow tube (41) filled with baffles (as 42), to ensure rapid and thorough mixing, and with flanged ends (43) by which it is fitted sealingly to an apertured manifold (44) out through which its contents flow. Adjacent the upstream end (the bottom end as viewed) of the tube 41 there is a lateral input port (45) through which CO2 gas or HCl, as appropriate, may be fed into the tube.

For each of the CO2 diffuser and HCl mixer five of these diffuser/mixer tubes 41 (1-5) are arranged in a cluster (see Figure 3) between common input and output manifolds (44i, o), and the cluster is associated with both input and output valves V each of which has an appropriate sensor - input pH , conductivity and flow are measured by sensors A, B and C respectively, while output pH, conductivity and flow are measured by sensors E, D and C respectively.

Claims (8)

1. A continuous process for the separation of an oilin-water emulsion waste-stream feedstock material, in which process: the feedstock is subjected to a continuous centrifugation, which mechanically separates the feedstock into disposable floating oil, disposable settled solids, and a dirty water liquid; that dirty water liquid, which is an oil-in-water emulsion with minerals dissolved in the water, is further treated by a two-stage acidification, both to break the emulsion and to precipitate most of the minerals, first, by dissolving therein CO2, and second, by mineral acid acidification; whereafter the resultant mixture is again centrifuged, and the oil, water and solids components are again mechanically separated off.

2. A process as claimed in Claim 1, in which, in the second part's the first stage, sufficient carbon dioxide gas is pumped into, and dissolves in, the dirty water liquid to lower the pH thereof to around 6.0.

3. A process as claimed in either of the preceding Claims, in which the carbon dioxide is pumped into the dirty water liquid against the flow of the stream.

4. A process as claimed in Claim 3, in which, to achieve a rapid saturation of the dirty water liquid with the carbon dioxide gas, the liquid is fed by way of a common manifold into a multiplicity of parallelydisposed diffusion chambers each containing appropriate baffles, into which chambers the carbon dioxide gas is injected.

5. A process as claimed in any of the preceding Claims, in which in the second part's second stage the mineral acid is 2M hydrochloric acid.

6. A process as claimed in any of the preceding Claims, in which in the second part's second stage the carbon-dioxide-treated dirty water liquid is fed by way of a common manifold into a multiplicity of parallelydisposed diffusion chambers each containing appropriate baffles, into which chambers the mineral acid is injected.

7. A process as claimed in any of the preceding Claims, in which the waste water output from the second centrifugation is further cleaned by being passed through absorbent charcoal.

8. A process as claimed in any of the preceding Claims and substantially as described hereinbefore.