Patent Description:
The disclosure is particularly applicable in the field of alumina production. In this field, it is common to obtain alumina from bauxite by grinding the bauxite, mixing this with caustic liquor to form a slurry, and heating the slurry. This leads to the alumina trapped in the bauxite dissolving to form a supersaturated solution of alumina in the caustic liquor with a suspension of the residue from the bauxite. Using known methods, alumina can be recovered from such a solution.

Slurry is, for example, heated in a tube heat exchanger that comprises several kilometres of piping having a diameter of approximately <NUM> to <NUM> inches (<NUM> centimetres to <NUM> centimetres). In view of the great length of the piping, this is typically arranged in a compact form as a plurality of linear pipes in parallel joined by bends. The piping is heated to increase the temperature of the slurry as it passes through the piping. The slurry is forced through the piping using a high-pressure pump.

An undesirable result of this process is that minerals from the slurry can crystallise on the inner surface of the piping. Amongst the deposits, it is common to find quartz, sodalite, vishnevite, boehmite, calcium titanate, cancrinite, natrodavyne, hematite, magnetite and other components. As the deposits build up in thickness on the inner surface of the piping, the heat transfer to the slurry is hindered, the pressure loss increases and the efficiency of the overall process is reduced.

Accordingly, it is known to periodically clean the piping in order to maintain good heat transfer to the slurry.

In the field of alumina extraction, this is typically done by passing sulphuric acid down the piping to remove sodalite, and by high pressure jet washing to remove the harder deposits, such as quartz, iron titanate, boehmite, or calcium titanate. Whilst these processes have had some success, unless carried out for prolonged periods, they cannot guarantee that after cleaning the piping will have sufficient diameter. In addition the process of high pressure jet washing is time consuming and can present dangers to the operator - in particular during the loading and unloading of the jet washing apparatus from the piping and furthermore presents danger to the integrity of the tubes of the heat exchanger.

In unrelated fields, it is known to carry out a process called "pigging", using a device commonly referred to as a "pig". An example of a prior art pig is described in <CIT>. This pig is a device that is forced down a pipe under the action of water pressure. It has three spring plates, arranged to fit tightly within a pipe so that water forced into the pipe pushes the device along. Ahead of the spring plates is an interchangeable cutting head having a series of cutting wheels for cutting relatively soft encrustations on the inner surface of a pipe. Such pigs are designed to remove all of the deposits within a pipe and thereby to completely uncover the inner wall of the pipe. The pig of <CIT> has been found to be unsuitable for use in removing hard deposits, such as those encountered during alumina production, because the design of the cutting wheels is insufficiently robust.

<CIT> discloses a pipeline cleaning tool with an elongated support mechanism having a rigid cutting unit mounted at one end and a folding propulsion assembly mounted at the other end.

<CIT> discloses a pipe cleaning apparatus arranged for passage along a pipe during use.

A method of removing a layer of encrustation from an inner surface of a pipe is provided according to claim <NUM>.

A method of operating an alumina extraction plant is provided according to claim <NUM>.

Further embodiments of the invention are provided by the dependent claims.

The inventors have discovered that a pipe cleaning device may be applied in an alumina extraction system such as that described above to obtain surprisingly good results by repeated operations. Pigging is not normally carried out on deposits as hard as those found in alumina extraction systems. Furthermore, the inventors have realised that the use of a pipe cleaning device allows a controlled amount of deposit to be left on the inner surface of a pipe to act as a protective layer preventing or hindering corrosion of the pipe.

Advantageously, pipe cleaning devices may be used in multiple pipes simultaneously, whereas prior art water-jetting cleaning approaches cannot.

For a better understanding of the disclosure and to show how the same may be put into effect, reference is now made, by way of example only, to the accompanying drawings in which:.

The figures show a pipe cleaning device <NUM> for removing deposits from the inner surface of a pipe <NUM>.

The pipe cleaning device <NUM> comprises a propelling piston <NUM>. The propelling piston <NUM> substantially blocks the pipe <NUM> so that fluid pressure can force the pipe cleaning device <NUM> along the pipe. The propelling piston <NUM> preferably comprises apertures to allow a portion of the fluid driving the pipe cleaning device <NUM> along the pipe <NUM> to pass the propelling piston <NUM> and thereby dislodge any loose matter ahead of the pipe cleaning device <NUM>.

The propelling piston <NUM> may be formed of one or more flexible diaphragms <NUM>. The flexible diaphragms <NUM> may, for example, comprise a plurality of flexible plates forming sectors of a circle that are spaced apart to provide apertures through which fluid may flow. The plates may be leather, rubber, or some other material. The leather, rubber, or otherwise flexible plates will be flexible enough to deform where it meets the inner wall of the pipe and/or encrustations to substantially seal the pipe but for the apertures. The apertures can be configured to allow a small amount of fluid to pass ahead of the pipe cleaning device <NUM>, whilst still enabling fluid to force the propelling piston <NUM> along the pipe <NUM>. The flexible plates may be reinforced by providing metal plates adjacent one or both faces of the flexible plates.

One or more cleaning tools <NUM> may be attached to the propelling piston <NUM>. Three are shown in <FIG>.

Each cleaning tool <NUM> comprises a main body <NUM> and a plurality of circumferentially-spaced arms <NUM>. The arms <NUM> may be arranged in pairs on opposing sides of the main body <NUM>. However, in the example shown in the Figures, the cleaning tools <NUM> each have three circumferentially-spaced arms <NUM>. In general, it is preferable to provide as many arms <NUM> as possible in view of the space requirements of the particular application. The arms <NUM> are pivotably attached to the main body and urged by an urging force to pivot away from a longitudinal axis of the main body <NUM>. When in use, the longitudinal axis of the main body <NUM> coincides with the direction of travel of the pipe cleaning device <NUM> along the pipe <NUM>. Thus, the longitudinal axis defines a front end and a rear end of the main body <NUM>.

The arms <NUM> are prevented from rotating beyond a predetermined angle by the provision of a stop member <NUM>, which is shown in <FIG> and <FIG> as a collar surrounding the body <NUM>. The arms <NUM> have a working surface arranged to engage encrustations on the inner surface of the pipe <NUM>. When the arms <NUM> abut the stop member <NUM>, the working surfaces of the arms <NUM> define the maximum outer diameter of the cleaning tool <NUM>.

An urging means <NUM> is provided for each arm <NUM>. The urging means <NUM> is arranged to pivotally bias the arm <NUM> away from the main body <NUM>. When the pipe cleaning device <NUM> contacts an encrustation, the momentum of the pipe cleaning device <NUM> moving along the pipe <NUM> causes the working surfaces to strike and/or scrape against the encrustations. Since the arms <NUM> are able to pivot towards the main body <NUM> against the bias, encrustations that are too hard to be immediately removed will cause the arms <NUM> to deflect inwardly, allowing the pipe cleaning device <NUM> to pass the obstacle. Whilst such a contact may not immediately completely remove the encrustation, the repeated percussive action of potentially plural working surfaces on each subsequent pass of the pipe cleaning device <NUM> along that region of the pipe <NUM> can gradually weaken the encrustation until it is removed entirely or substantially.

An example of a suitable urging means <NUM> is a piston in a bore formed in the main body <NUM>. The piston may be driven outwardly by pneumatic and/or hydraulic pressure (for example, in the range <NUM> to <NUM> bar) to generate the urging force. In such an embodiment, it is preferable that the bores of the urging means <NUM> in each cleaning tool <NUM> are in communication such that the pistons are driven outwardly by the same force. The pressure within the pistons may be controlled to determine the urging force.

Alternatively, the urging means <NUM> may comprise a resilient member, such as a spring. The resilient member may extend between a cavity in the main body <NUM> and a cavity in the arm <NUM>. The resilient member may be replaceable with a different resilient member of different stiffness and/or length in order to change the urging force.

The cleaning tools <NUM> may be connected together via connectors <NUM>, which can be connected to a connection point <NUM> at the front end of the body <NUM> and a connection point <NUM> at the rear end of the body <NUM>.

The propelling piston <NUM> may comprise a connection point <NUM> at its rear end, and so the connection point <NUM> at the front end of a cleaning tool <NUM> may be connected to the propelling piston <NUM> via a connector <NUM>.

The connectors <NUM> may be an elongate member with a bore at each end, wherein the bores extend perpendicular to the direction of elongation of the connector <NUM> and perpendicular to each other. The connector may therefore be pivotally attached to a connection point <NUM>, <NUM>, <NUM> at each end such that it defines a universal joint between adjacent cleaning tools <NUM>, or between a cleaning tool <NUM> and the propelling piston <NUM>.

In this way, the pipe cleaning device <NUM> may be formed of a series of a propelling piston <NUM> and a selectable number of cleaning tools <NUM>. By the provision of the universal joint between each component, the pipe cleaning device <NUM> can follow a pipe <NUM> having a number of bends, irrespective of the length of the pipe cleaning device <NUM>. Preferably, the cleaning tools <NUM> are arranged such that the arms <NUM> of each cleaning tool <NUM> may be set at a desired relative rotational alignment relative to the arms of another cleaning tool <NUM>. In this way, the arms <NUM> of the plurality of cleaning tools <NUM> forming the pipe cleaning device <NUM> may be arranged to contact a different region of the circumference of the inner surface of the pipe <NUM> (and/or the encrustations).

The pipe cleaning device <NUM> may advantageously be provided as a kit of parts including a propelling piston <NUM> and a set of cleaning tools <NUM>. The operator may select an appropriate number of cleaning tools for each pass of the pipe cleaning device <NUM> along the pipe <NUM>.

Each arm <NUM> has an insert <NUM> on its distal end for engaging deposits on the inner surface of the pipe <NUM>. The insert <NUM> comprises the working surface, preferably formed by one or more teeth <NUM> of the insert <NUM>.

Each insert <NUM> may be arranged to pivot about an axis <NUM> relative to the arm <NUM>. As can be seen in <FIG>, the insert <NUM> may be pivotably held within an aperture <NUM> in the arm <NUM>. The insert <NUM> is prevented from rotating continually by its abutment with the sides of the aperture <NUM>. In general, it is preferable for the insert to be able to rotate such that it is aligned with the wall of the pipe <NUM> when the arm <NUM> is extended outwardly fully. In some preferred embodiments, this will mean that each insert <NUM> is arranged to pivot relative to the arm <NUM> from a nominal position in each direction by an angle of up to <NUM> degrees.

Preferably, the working surface of the insert <NUM> comprises two or more rows of teeth <NUM>. Preferably, each row is spaced apart in the direction of the longitudinal axis of the main body <NUM>.

The inserts <NUM> are preferably interchangeable, which allows them to be replaced when worn. Furthermore, as can be seen in <FIG> and <FIG>, each tooth <NUM> of an insert <NUM> may be individually replaceable. The number of rows of teeth may therefore be adjusted. As can be seen in <FIG> and <FIG>, the teeth <NUM> may be held in place using a grub screw <NUM>.

As can be seen from <FIG>, each of the teeth <NUM> that define the working surface on the distal end of each arm <NUM> have a height h1 on a rear face of the tooth <NUM> (in the longitudinal direction of the main body <NUM>) that is greater than the height h2 on a front face of the tooth <NUM>. The teeth <NUM> thus have a sloped surface as can be seen in <FIG>, which enables the teeth <NUM> to run across the inner surface of the pipe <NUM> without engaging or scratching the pipe <NUM> when the encrustations have been completely or substantially removed.

A front edge at the distal end of the front face of the teeth <NUM> forms a cutting edge <NUM>. Whilst the teeth <NUM> may be rectangular in cross-section, with a front edge <NUM> and a rear edge <NUM>, it has been found to be advantageous to provide teeth <NUM> with a triangular cross-section, with a front edge <NUM> and a rear point <NUM> - as shown in the figures.

The rear <NUM> of each of the teeth <NUM> is located at a distal end of the rearmost face or edge. When encrustations are present on the inner surface of the pipe, the arm <NUM> will be deflected inwardly as shown in <FIG> which will orientate the cutting edge <NUM> so that it engages the encrustation with a cutting, percussive or hammering action.

The cutting edge <NUM> will be spaced from the inner pipe surface when the surface is free or substantially free of encrustations, as can be seen in <FIG>, due to the greater height h1 of the rear <NUM> which causes the rear <NUM> to engage the inner pipe surface when the arm <NUM> is pivoted outwardly. Advantageously, when a pivotable insert <NUM> is provided, the provision of multiple rows of teeth <NUM> will tend to maintain a rotation of the insert about pivot <NUM> that is aligned with the inner surface of the pipe <NUM> and so will ensure that the cutting edge <NUM> is spaced therefrom.

<FIG> depicts an arm <NUM> deflecting toward the main body <NUM> during a collision with an encrustation. As noted above, this deflection will preferably bring the cutting edge <NUM> of a first tooth <NUM> into contact with the encrustation. The cutting edge <NUM> may remove the encrustation or, if not, then as the first tooth <NUM> passes the encrustation the next tooth <NUM> will be driven into the encrustation percussively by the urging means <NUM> as the arm <NUM> is biased outwardly.

A pipe cleaning device <NUM> of the type described above is suitable for use in the following embodiment of a method in accordance with the disclosure.

A method of removing a layer of encrustation from an inner surface of a pipe <NUM> forming part of a heat exchanger or other tubes, such as tubes connecting parts of the heat exchanger together, comprises a first phase in which a pipe cleaning device <NUM> is inserted into the pipe <NUM>, fluid is forced into the pipe <NUM> to drive the pipe cleaning device <NUM> along the pipe <NUM>, thereby removing a layer or amount of encrustation, and removing the pipe cleaning device <NUM> from the pipe <NUM>. A second phase and subsequent phases may be provided, each repeating these steps of the first phase.

Whereas the prior art methods attempt to remove all of the encrustations from the inner surface of the pipe <NUM>, in the disclosure these steps are preferably repeated only until the thickness of the layer of encrustation has reduced to a required thickness.

The working surfaces are urged outwardly, but not beyond an outer diameter of the cleaning tool <NUM>. It is preferable that the cleaning tool <NUM> is selected to have an outer diameter that is substantially equal to the inner diameter of the pipe <NUM>. However, in some cases, the pipe <NUM> may include intentional protrusions such as welds, the pipe diameter may vary along its length within pipe manufacturing tolerances, and tolerances introduced by any bends in the pipe. Accordingly, the available diameter for the pipe cleaning device <NUM> may be smaller than the nominal inner diameter of the pipe <NUM>. In which case the outer diameter of the cleaning tool <NUM> may be smaller than the nominal inner diameter of the pipe <NUM> by an amount suitable to avoid such obstacles. For example, the maximum diameter of the pipe cleaning device <NUM> (i.e. when the arms <NUM> extend fully outward) may be from <NUM>% to <NUM>% of the nominal diameter of the pipe <NUM>. By configuring the pipe cleaning device in this way, the force urging the working surfaces outwardly will not be applied to the pipe <NUM>, but will simply act to centre the pipe cleaning device <NUM> within the pipe <NUM>.

Once the thickness of the layer of encrustation has reduced to a required thickness, the cleaning process is stopped. In this way a desirable amount of encrustation may be retained. Preferably, a layer of encrustation having a thickness in the range <NUM> to <NUM> is retained. Such a retained layer of encrustation has been found to protect the pipe <NUM> from corrosion and erosion by slurry.

As described above, the pipe cleaning device <NUM> may comprise a propelling piston <NUM> and multiple cleaning tools <NUM>. It is preferable to use a pipe cleaning device <NUM> for which the cleaning tools <NUM> are attachable to and detachable from the propelling piston <NUM> and each other. Such a pipe cleaning device <NUM> can be used in the first phase without a cleaning tool <NUM>, or with only a small number of cleaning tools <NUM>. This is useful, because in the first phase there can be large amounts of encrustation that would hinder the passage of the pipe cleaning device <NUM> along the pipe <NUM>. It is for this reason that pigs are not used in the prior art with encrustations as hard as those found in bauxite processing (these can include one or more of: iron titanate; boehmite; calcium titanate; quartz; cancrinite; vishnevite; natrodavyne; sodalite; alumosilicates; hematite; magnetite; and/or other components of bauxite or compounds formed from Bayer liquor during the process of digestion). The inventors have found that by carrying out the first phase without a cleaning tool <NUM>, or with only a small number of cleaning tools <NUM>, the first phase can be completed successfully without the pipe cleaning device <NUM> becoming lodged in the pipe. If the first phase is carried out with a large number of cleaning tools <NUM>, then the passage of the pipe cleaning device <NUM> down the pipe <NUM> will be greatly resisted by the engagement of many working surfaces with the encrustations.

Similarly, it has been found beneficial to gradually increase the amount of encrustation removed in each phase of the method by restricting the maximum diameter of the pipe cleaning device <NUM> in the early phases and allowing this to increase in subsequent phases (up to the maximum diameter discussed above).

The maximum pipe cleaning device <NUM> diameter may be adjusted by varying the axial position of the stop <NUM>. Alternatively, or additionally, the length of the teeth <NUM> can be varied.

Furthermore, during the first phase it is preferable that the pump, which drives the fluid down the pipe <NUM>, does not provide its maximum output pressure unless the pipe cleaning device <NUM> becomes stuck. If the pipe cleaning device <NUM> is stuck in the pipe <NUM>, there is then the ability to increase the pressure to dislodge the pipe cleaning device <NUM>.

For the second phase, and/or for later phases, further cleaning tools <NUM> may be attached to the pipe cleaning device <NUM> to form a larger pipe cleaning device <NUM> with a greater capacity for removing the encrustations. Since, by this stage, the encrustations that protruded the greatest distance into the pipe <NUM> will have mostly been removed, it is now much safer to use a larger number of cleaning tools <NUM>.

For the second phase, and/or for later phases it can also be advantageous to operate the pump to provide a larger output pressure, owing to the increased amount of engagement of the pipe cleaning device <NUM> with encrustations.

In preferable embodiments, the pressure of the fluid is controlled to drive the pipe cleaning device <NUM> at a speed in the range <NUM>/s to <NUM>/s.

As described above, the propelling piston <NUM> preferably comprises apertures. This can allow a portion of the fluid driving the pipe cleaning device <NUM> along the pipe <NUM> to pass the propelling piston <NUM> and thereby dislodge any loose matter ahead of the pipe cleaning device <NUM>.

Using the adjustable pipe cleaning device <NUM> described above (whether it be pneumatic and/or hydraulic or sprung), the force urging the working surfaces outwardly can be modified for a particular task. For example, low pneumatic or hydraulic pressures and/or low spring forces can be used if the encrustations are likely to trap the pipe cleaning device <NUM> in the pipe <NUM>. In such a situation, a low urging force will allow the arms to deflect more easily, permitting passage of the pipe cleaning device in the pipe. Higher pneumatic or hydraulic pressures and/or higher spring forces can be used to increase the cutting effect of the pipe cleaning device <NUM>.

The method set out above can be used to clean the pipes <NUM> of a heat exchanger or other tubes in, for example, an alumina extraction plant. The method of operating such a plant may include mixing ground bauxite with caustic liquor to form a slurry, and forcing the slurry along a heated pipe <NUM> thereby forming a solution of alumina in the caustic liquor along with a suspension of residue. An unwanted result of the operation involves the crystallisation or deposition of material on the inside surface of the pipe <NUM> forming the encrustations. The method of operating the plant will periodically comprise removing some of the encrustations using the method discussed above.

This method can be aided by washing the inner surface of the pipe <NUM> with acid before utilising the pipe cleaning device <NUM>.

It has been found that the method of cleaning described above can form grooves in the encrustation. The grooves form discontinuities between regions of the encrustation around the circumference of the pipe <NUM>. Surprisingly, this has been found to reduce the tolerance of the encrustation to heat shock. Heating the pipe <NUM> following the pigging steps has been found to cause fracture of the discontinuous regions of encrustation leading to greater removal of encrustation. The pipe is heated to produce a difference in temperature across the thickness of the pipe (i.e. in the radial direction) in the range <NUM> to <NUM>. The pipe <NUM> is substantially empty or contains a flow of cold liquor (or at least is not receiving a flow of hot slurry) during this heating step, which is thus different from the normal heating of the pipe <NUM> during the processing of slurry.

Claim 1:
A method of removing a layer of encrustation from an inner surface of a pipe (<NUM>), comprising the following steps in order:
a. providing a pipe cleaning device (<NUM>) having a propelling piston (<NUM>) to which at least one cleaning tool (<NUM>) is attachable that comprises circumferentially spaced moveable working surfaces for removing deposits from the inner surface of the pipe (<NUM>), wherein the working surfaces are urged apart;
b. inserting the propelling piston (<NUM>) into the pipe (<NUM>) without a cleaning tool (<NUM>); driving the propelling piston (<NUM>) along the pipe (<NUM>); and removing the pipe cleaning device (<NUM>) from the pipe (<NUM>);
c. attaching the cleaning tool (<NUM>) and inserting the pipe cleaning device (<NUM>) into the pipe (<NUM>);
d. driving the pipe cleaning device (<NUM>) along the pipe (<NUM>) and thereby removing a layer of encrustation;
e. removing the pipe cleaning device (<NUM>) from the pipe (<NUM>);
f. repeating steps c. to e. until the thickness of the layer of encrustation has reduced to a required thickness; and
g. retaining the required thickness of encrustation as a protective layer.