WO2000051787A1

WO2000051787A1 - Abrasive blasting apparatus

Info

Publication number: WO2000051787A1
Application number: PCT/GB2000/000759
Authority: WO
Inventors: Bruce Nicholson
Original assignee: Gagemark Limited
Priority date: 1999-03-04
Filing date: 2000-03-03
Publication date: 2000-09-08
Also published as: EP1075352A1; EP1075352B1; GB9904872D0; AU2925400A

Abstract

A blasting apparatus for wet abrasive blasting comprises a vessel (1) with an inlet (4) and a bottom outlet (6) into which a particulate blasting medium and a blasting liquid can be introduced (1, 6). The flow of pressurized fluid along a delivery line (17) entrains the contents of the vessel (1) for dispersal into the fluid flow in the delivery line for blasting. A pressurizing means (15, 16) directs pressurized fluid into the top of the vessel (1). An injection means (31) is also provided for injecting pressurized fluid into the lower portion (7) of the vessel (1) to agitate its contents. A control means (43) controls the pressure within the vessel (1) and ensures that during blasting the pressure of the pressurized fluid entering the vessel (1) through the injection means (31) is always maintained at a higher level than the pressure of the pressurized fluid directed into the vessel (1) by the pressurizing means (15, 16).

Description

ABRASIVE BLASTING APPARATUS

The present invention relates to an abrasive blasting apparatus and in particular to a wet abrasive blasting apparatus .

In wet abrasive blasting, an abrasive, such as sand, is entrained in a pressurized liquid flow or gaseous-entrained liquid flow and is directed against the surface to be treated by a controllable nozzle.

In a conventional apparatus, a dry abrasive is entrained into a gaseous stream and liquid is then added to wet the abrasive prior to its egress from the nozzle. In this way, dust generation at the work area is reduced. Such an arrangement, however, requires a separate pump to be used to deliver the liquid into the abrasive and gaseous stream, which makes the cost of the blasting apparatus more expensive than is desirable and renders the blasting apparatus more prone to breakdown.

In WO 88/07916 is described an apparatus which overcomes the aforementioned problems by entraining separately contained liquid and an abrasive/liquid mixture in a flow of pressurized fluid whilst simultaneously applying the pressurized fluid to the receptacles containing the liquid and the abrasive/liquid medium in order to balance the fluid pressure between the receptacles themselves and the receptacles and the delivery line. This has the advantage of requiring only a single pressurized fluid source and therefore of reducing the cost of the apparatus.

These conventional apparatus were primarily designed for blasting silica sand, which has several disadvantages. First and foremost, when silica sand is blasted against a hard surface free silica is released, which is now considered environmentally damaging. Second, sand is too abrasive for use when removing soiling, rust, oxidations, efflorescences and the like where it is not wished to distort or scratch the underlying substrate. Sand may also become embedded in the substrate.

More modern blasting media comprise fine particles of calcium carbonate, calcium magnesium carbonate, calcium oxide, calcium bicarbonate, calcium magnesium carbonate, magnesium oxide, magnesium sulphate and soda ash which overcome the aforementioned disadvantages. However, when these fine particles are mixed with a blasting liquid such as water, they tend to produce a lumpy slurry or sludge which will not flow and which is difficult to entrain in a flow of pressurized fluid for even blasting. They are therefore difficult if not impossible to use with many conventional forms of blasting apparatus.

The object of the present invention is therefore to provide a versatile wet abrasive blasting apparatus which overcomes the aforementioned problem and which can be used to blast the aforementioned fine particles in addition to conventional abrasive materials such as various sands and grit, when required.

According to the present invention there is provided a blasting apparatus comprising a vessel adapted to contain a particulate blasting medium and a liquid and including an inlet and a bottom outlet; a delivery line along which an entraining pressurized fluid can be fed from a source and with which the interior of the vessel communicates through the outlet whereby a flow of pressurized fluid along the delivery line can entrain the contents of the vessel for dispersal into the fluid flow in the delivery line for blasting; a pressurizing means to direct pressurized fluid from the source into the vessel behind its contents in terms of its contents' outflow from the vessel through the bottom outlet; and characterised in that an injection means is provided for injecting pressurized fluid into the lower portion of the vessel into the contents of the vessel to agitate the contents; and a control means is provided to control the pressure within the vessel and to ensure that during blasting the pressure of the pressurized fluid entering the vessel through the injection means is always maintained at a higher level than the pressure of the pressurized fluid directed into the vessel by the pressurizing means.

Preferably, the pressurizing means comprises a control valve defining a first outlet port through which the pressurized fluid can flow into the delivery line and a second outlet port through which the pressurized fluid can flow into the vessel to pressurize the interior of the vessel, the first and second outlet ports being connected by a conduit whereby during blasting when the pressure within the vessel exceeds the pressure of the pressurizing fluid flowing through the second outlet port, the flow of pressurizing fluid can reverse and pass from the interior of the vessel, through the conduit and into the flow of pressurized fluid egressing from the first outlet port into the delivery line.

Preferably also, the control means comprises a manostat which is connected to the control valve and to the injection means and which enables a predetermined pressure difference to be maintained between the pressure of the pressurized fluid entering the vessel through the injection means and the pressure of the pressurized fluid directed into the vessel by the pressurizing means. Preferably also, the delivery line terminates in a blasting nozzle with a control handle which is supplied with pressurized fluid from the source and which is connected to the control means and to the control valve whereby an operative can open the control valve and thereafter control the flow of pressurized fluid through the nozzle by varying the flow of pressurized fluid through the control handle.

Preferably also, the inlet in the vessel is provided with an air lock valve which is closed by the application of pressurized fluid thereto from within the vessel by the pressurizing means; and the outlet in the vessel is provided with a valve means which is connected to the air lock valve such that closure of the air lock valve causes the valve means at the outlet of the vessel to open and vice versa.

Preferably also, the outlet from the vessel comprises an orifice and the valve means for the outlet comprises a spigot which is inserted into the orifice, the tip of the spigot remaining within the orifice when the valve means is opened to permit outflow of the contents of the vessel.

Preferably also, the valve means for the outlet is connected to the air lock valve by a flexible coupling whereby, when the valve means is open, agitation of the contents of the vessel causes agitation of the tip of the spigot within the orifice.

Preferably also, the injection means comprises a nonreturn valve connected to an egress means located within the vessel in proximity to the outlet of the vessel.

Preferably also, the egress means comprises a perforated coil looped around the outlet of the vessel. Preferably also, the delivery line is connected to a liquid supply upstream of the outlet from the vessel but downstream of the pressurizing means whereby liquid can be injected into the flow of entraining fluid passing down the delivery line.

Also according to the present invention there is provided the use of the blasting apparatus to blast a mixture of a particulate blasting medium and a liquid by entraining the mixture in a flow of pressurized fluid in the delivery line whilst simultaneously pressurizing the interior of the vessel by directing a flow of the pressurized fluid from the source into the vessel behind its contents in terms of its contents' outflow from the vessel; and characterised in that a flow of pressurized fluid is injected into the contents of the vessel to agitate the contents; and in that the pressure within the vessel is controlled by ensuring that during blasting the pressure of the pressurized fluid injected into the contents of the vessel is always maintained at a higher level than the pressure of the pressurized fluid directed into the vessel behind its contents.

The present invention will now be described by way of example with reference to the accompanying drawings, in which :-

Fig. 1 is a part sectional elevation of an apparatus for wet abrasive blasting according to the present invention;

Fig. 2 shows schematically a pressurized fluid control circuit for the apparatus;

Fig. 3 is a cross sectional plan view of the lower portion of the vessel along the line III-III in Fig. 1; and Fig. 4 is a part section view, to an enlarged scale, of a flow control device which can be employed at the outlet of the vessel.

The apparatus comprises a pressure vessel 1 which is adapted to contain up to a level 2 a blasting mixture 3 of a particulate material and a liquid, typically water or a water based blasting solution, such as a rust inhibiting solution. The vessel 1 is provided with an inlet 4, which is defined centrally at the base of a filling hopper 5 located at the top of the vessel 1, and with an outlet 6, which is disposed at the central portion of the bottom of the vessel 1. In order to assist in the egress of the blasting mixture 3 out of the vessel through the outlet 6 and to discourage the particulate material from settling out of the liquid, the wall of the lower portion 7 of the vessel 1 can either be dished, as shown in Fig. 1, or made conical dependent on the particulate material which is to be used therewith. A dished shape is preferable when using fine particulate materials whereas a conical shape is better for use with coarser or denser media such as sand and grit.

The inlet 4 of the vessel 1 can be closed by an air lock valve 8, for example a mushroom-type valve, which is closed on the application thereto of pressurized air. The outlet 6 is also provided with a valve 9 located outside the vessel 1.

A liquid inlet 10 located above the level 2, is provided in the vessel 1 and is closable by a valve 11, and a liquid outlet 12, located below the level 2 and closable by a valve 13, is also provided.

The apparatus is provided with pressurized air as its operational or motive fluid from a pressurized air source, such as an air compressor (not shown) , via a control circuit, which is shown in detail in Fig. 2. Air under pressure is delivered from the source to a single air inlet 14 for the system and thence to an inlet port of a control valve 15. The control valve 15 comprises two outlet ports, the outflows from which are connected by a conduit 15a, which may comprise an external connection, as shown in Fig. 1 in hidden detail, or be within the body of the valve 15. When the valve 15 is open, the pressurized air from the source can flow through a first outlet port via a pipe 16 into the interior of the vessel 1 and through a second outlet port into a delivery line 17. The pipe 16 delivers the pressurized air into the vessel 1 immediately below the air lock valve 8 whereby the valve 8 is closed and the vessel 1 is then pressurized by the air flow. It will be appreciated that the pipe 16 delivers the pressurized air into the vessel above the level 2 of the blasting mixture 3, which is behind the mixture 3 in terms of its outflow from the vessel 1 through the bottom outlet 6.

The delivery line 17 comprises a further valve 18 and is connected to an abrasive blasting hose 19 and nozzle 20 by a bayonet joint connection as indicated at 21. The outlet 6 of the vessel 1 communicates with the delivery line 17 via the valve 9 whereby when the valve 9 is open and pressurized air is passing along the delivery line 17, the blasting mixture 3 within the vessel 1 is entrained by the air flowing along the line 17 and dispersed into the fluid flow for blasting through the nozzle 20.

If necessary during a blasting operation, further liquid can be injected from a supply pipe 22 into the delivery line 17 through a valve 23 connected to an injection port 24 located slightly upstream of the outlet valve 6. The port 24 projects forward into the delivery line 17 so that a degree of suction is created to prevent back-flow down the pipe 22. This means that a mains water supply could be connected to the pipe 22 without danger of contamination occurring to the supply. A dead-man's control handle 25 is preferably connected to the delivery nozzle 20, but could be located elsewhere such as attached to an operative's body, and is separately supplied with pressurized air from the air inlet 14. The control handle 25 is operationally linked to the control valve 15 so that release of the handle 25 closes the control valve 15 to halt the blasting operation whereas squeezing the handle signals the control valve 15 to open, as is described below. With particular reference to Fig. 2, it can be seen that the control handle 25 is supplied with pressurized air via the inlet 14 from the source along a supply line 26 via a water filter 27, a control valve 28, which comprises the main on/off air control for the apparatus, a flow regulator 29 and a pressure regulator 30.

Pressurized air is also injected into the lower portion of the vessel 1 to agitate the blasting mixture 2 by an injection means 31 comprising a non-return valve 32 and an egress means such as an agitation coil 33, as shown in Figs. 1 and 3, or a nozzle. A metering flow valve (not shown) could also be included between the non-return valve 32 and the egress means.

The purpose of the injection means 31 is to agitate the blasting mixture 3 to keep the particulate material in suspension in the liquid. However, it is important that this agitation takes place without creating turbulence in the vessel 1 which would inhibit the smooth flow of blasting mixture 3 out of the outlet 6 and into the entraining fluid in the delivery line 17. Hence, the agitation coil 33 or nozzle can be specifically adapted according to the type of particulate material to be used. If fine particulate materials with diameters from 150 microns down to 5 microns are used then an agitation coil 33 similar to the one shown in Figs. 1 and 3 can be used. The coil 33 is looped around the outlet 6 at the base of the vessel 1 and is provided with a series of spaced output ports 34 through which the pressurized air supplied to the coil 33 is injected into the vessel 1. The ports 34 can be provided with air outlet nozzles (not shown) .

The agitation caused in the blasting mixture 2 by the air egressing from the ports 34 in the coil 33 produces an even mix of the particulate material with the liquid wherein the particles form a suspension in the liquid and each individual particle is enveloped in a coating of liquid. Preferably, the agitation created is sufficient for the blasting mixture 3 to be fluidized so that it flows smoothly through the outlet 6 and the outlet valve 9 into the entraining flow in the delivery line 17. An even mix is also important to provide a smooth and even delivery of the blasting material to the work surface, which is important from the point of view of achieving an even finish.

In order to prevent the particulate material from being deposited around the outlet 6 and clogging it, which can happen when a blasting operation stops, the outlet 6 is provided with an automatic valve arrangement, as indicated generally at 35 in Fig. 1. The valve arrangement 35 comprises an upper first rod 36 which is connected at its upper end to the air lock valve 8 and at its lower end to a flexible connector 37, such as a piece of rubber or the like. A lower second rod 38 is connected at its upper end to the connector 37 and at its lower end to a bush 39 which forms a valve member that can obturate the outlet 6. The outlet 6 itself comprises a seating 40, which may be made of plastics or of metal, against which the bush 39 can seat, and which defines a cylindrical orifice 41 therethrough that communicates with the valve 9. Attached to the bush 39 is a spigot 42, which may comprise the tip of the rod 36 and which penetrates the orifice 41 without closing it. When the vessel 1 is pressurized by the passage of air along the pipe 16, the air lock valve 8 is raised to close the inlet 4. Raising of the valve 8 also causes the upper rod 36 to be raised and thereby the lower rod 38 to be raised, lifting the bush 39 off the seating 40 to open the outlet 6. However, the tip of the spigot 42 remains within the inner end of the orifice 41. Thereafter, when pressurized air is injected into the blasting mixture through the agitation coil 33, the agitation of the mixture causes vibration of the lower rod 38, which can move from side to side independently of the upper rod 36 by virtue of the flexible connector 37. The spigot 42 thereby also vibrates within the orifice 41 and this prevents the particulate material from being deposited out of the liquid around and in the orifice 41 and ensures that it stays well mixed with the liquid as it egresses from the outlet 6. In addition, on depressurization of the vessel 1, the air lock valve 8 falls, opening the inlet 4 and lowering the rods 36 and 38 so that the bush 39 contacts the seating 40, closing the outlet 6. This then prevents the orifice 41 from becoming plugged with particulate material.

In an alternative arrangement, the tip of the lower rod 38 can be tapered so that in the lower position of the valve arrangement 35 it acts as a stopper in the orifice 41, the bush 39 being omitted from the arrangement 35, and in the upper position of the arrangement 35 just penetrates the orifice 41 without closing it.

Air is supplied to the non-return valve 32 of the injection means 31 from the source via the inlet 14 through the water filter 27 and a bias controller or manostat 43. As is described below, the function of the manostat 43 is most important as it must ensure that the pressure of the pressurized fluid entering the vessel 1 through the injection means 31 is always maintained at a predetermined higher level than the pressure of the pressurized fluid directed into the vessel 1 through the pipe 16. To this end, the manostat 43 is connected via a sensing line 44 to the control valve 15 and to the delivery line 26 for the control handle 25 and can adjust the pressure of the air output to the pipe 16 and the delivery line 17 by the control valve 15.

In use, prior to commencing a blasting operation, the on/off valve 28 and the valves 15, 18 and 23 are closed and the liquid inlet and outlet valves 11 and 13 are opened. A water or another liquid supply is connected to the inlet 10 and the vessel 1 is then filled with liquid until liquid emanates from the outlet 12. The valves 11 and 13 are then closed. The source of pressurized fluid, typically air, is then connected to the air inlet 14 of the apparatus. This air will pass through the water filter 27 and the manostat 43 into the injection means 31 and thence into the base of the vessel where it will emanate into the liquid therein through the agitation coil 33. A predetermined quantity of particulate blasting medium is then introduced into the vessel 1 via the hopper 5 through the inlet 4. The level of the blasting mixture 3 within the vessel 1 is then raised typically to the level 2, which is below the bottom of the air lock valve 8. As air is being injected into the liquid through the agitation coil 33 whilst the particulate medium is introduced into the vessel 1, the particulate material is well mixed with the liquid and the resulting mixture is constantly agitated by the inflowing air.

Once the vessel 1 has been filled with water and the particulate medium, the main on/off valve 28 is opened by being switched to the "on" position. This allows pressurized air to move through the flow regulator 29, the pressure regulator 30 and then along the supply line 26 to exit from the operative's control handle 25. Valves 9 and 18 are then opened. When the operative wishes to commence blasting, he closes or partially closes the control handle 25 and as a result increases the air pressure in the sensing line 44. This causes the control valve 15 to open thus pressurizing the pipe 16 and the main delivery line 17 through valve 18. Air is now being blasted through the nozzle 20. The air pressure in the pipe 16 causes the air lock valve 8 to close to that it seals the inlet 4. The interior of the vessel 1 then becomes pressurized by the air flowing out of the pipe 16. At the same time, the bush 39 is raised from its seating 40 to open the outlet 6 and permit the blasting mixture to flow out of the vessel 1 and be entrained in the air flow along the hose 19.

The increased air pressure in the sensing line 44 is also detected by the manostat 43 which controls the air pressures to ensure that air entering the vessel 1 via the non-return valve 32 of the injection means 31 is always kept at a higher pressure than the air entering the vessel 1 through the pressurizing pipe 16. This ensures that pressurized air continues to enter the vessel 1 through the agitation coil 33 via the non-return valve 32 to keep the blasting mixture 3 in constant agitation. This air bubbles up through the blasting mixture and collects at the top of the vessel 1. However, because the pressure of this air is greater than that of the air in the pipe 16, the air entering the vessel 1 in this way will then travel down the pipe 16, through the conduit 15a and into the delivery line 17, thus reversing the initial flow of air through the pipe 16.

Hence, the constant flow of pressurized air into the vessel is disposed of in a highly efficient and cost effective way without the need for pressure-reducing or blow- off valves to be employed. It will be appreciated that the greater the pressure difference between the air injected into the vessel 1 by the agitation coil 33 and the air in the pipe 16, the greater the agitation of the blasting mixture that can be achieved. However, despite this it is desirable to keep the pressure difference as low as possible as the upper of the two pressures cannot be greater than the air pressure received from the source and every bar of difference reduces the working pressure of the blasting process itself. The manostat 43 is therefore set to maintain a fairly low pressure difference of approximately 0.3 bar (approximately 34.5 x 103 pascals or 5 psi), or lower, if possible.

During the blasting operation, the effect of the air emanating from the agitation coil 33, which mixes the blasting mixture 3 under pressure, is to coat each particle of the particulate medium with a film of liquid. This in itself has a beneficial effect on the blasting process as the momentum of each coated particle blasted from the nozzle 20 is thereby increased and results in a more efficient cleaning action. With fine particulate materials, such as the calcium carbonate particles and the like as mentioned above, it is believed that the film of water coating each particle enables the particle to roll over the surface being blasted to create a light scouring action. The surface deposits are therefore lifted from the substrate before breaking up and being washed away by the liquid in the blast stream. Such particles typically have a rounded appearance, which aids the surface action, and a hardness of between 2.5 and 3.5 mohs in order to prevent damage to the underlying substrate. For comparison, silica sand has a hardness of 6 mohs.

However, if abrasion of the substrate surface during blasting is required, for example when cleaning steel surfaces it is often necessary to produce a suitable anchor pattern for subsequent paint coatings, then particulates which produce more of a cutting action than a scouring action are required. Grit-like materials such as special dolomites, garnets, or olivine sands, which do not - release free silica on blasting, can be employed. Again, the effectiveness has been found to be increased by the coating of water that they receive by the apparatus of the present invention.

In both of the above cases, the increased efficiency of the blasting process owing to the coating of the particulate material with a water film means that far less particulate material is required than would be the case in a conventional blasting process. For example it is estimated that with grit-like materials, a reduction of between 12% and 48% with the norm in the upper half of the range, i.e. between 30% and 45%, can be achieved. This means that a blasting operation can be carried out costing less than would otherwise be the case and also means that afterwards less clearing up of the used particulate material is necessary, again saving money.

The agitation produced in the blasting mixture 3 in the vessel 1 also causes the lower rod 38 of the automatic valve arrangement 35 to vibrate. As a result, the spigot 42 which remains partially in the orifice 41 also vibrates and assists with the mixing of the liquid and the particulate medium at this point. This has the benefits of preventing de- watering or settling of the particulate medium inside the orifice 41 and of assisting with a smooth flow of the blasting mixture 3 down into the entraining fluid.

During blasting, further blasting liquid, such as water, can be introduced into the blasting operation by opening valve 23, as determined by the operative.

When it is desired to stop blasting, the operative releases the handle 25, which allows pressurized air to escape along the line 26 through the control handle 25, reduces the pressure in .the sensing line 44 and thereby causes the control valve 15 to shut off the main air flow from the source 14. The pressurized air within the vessel 1 is then vented through the valve 15 and the air lock valve 8 falls, at the same time shutting off the flow of blasting mixture 3 through the outlet 6 by means of the valve arrangement 35. A wash-down operation can now be initiated by closing valve 9, opening valve 23 and then closing the control handle 25. This causes the control valve 15 to allow pressurized air again to pass through into the delivery line 17, which sucks water in through the valve 23 and blasts it out through the nozzle 20. The object of the blasting operation is thereby cleaned of any remaining particulate material and blasting liquid, if not water, and the delivery line beneath the vessel 1 and the blasting nozzle 20 are flushed out.

Once the wash-down operation has been completed, the handle 25 can again be released, which will cause valve 15 to close to shut of the air flow through the nozzle 20 once more and the whole apparatus can then be closed down by switching the on/off valve 28 to the "off" position and closing the valve 23.

It will be appreciated that various modifications can be made to the apparatus. In particular, the pressurized fluid control circuit shown in Fig. 2. can be varied by, for example, replacing the single line feed 26 to the control handle 25 by a twin line with a bleed-off point to line 44 to allow correct sensing.

Various factors such as the length of the line 26, will affect the efficiency of the operation of the control circuit and each circuit will operate most successfully when it has been tuned to the actual operating parameters. This ensures that maximum air pressure is available for the blasting process whilst at the same time allowing the vessel 1 to depressurize when the handle 25 is released.

Tuning the control circuit is accomplished as follows. First, the air regulator 29 should be fully opened to allow excess air into the line 26. The control handle 25 should then be fully opened to simulate a "close down" situation. The vessel 1 will not exhaust at this point because the control handle 25 will not allow sufficient air to escape so as to reduce the pressure in the line 44 to activate the control valve 15. However, by then gradually closing the air flow regulator 29 until the quantity of air passing through is reduced to a level at which it is all dissipated through the control handle 25, the pressure in the line 44 is reduced. This is sensed by the control valve 15, which in turn causes the vessel 1 to be exhausted.

Claims

1. A blasting apparatus comprising a vessel (1) adapted to contain a particulate blasting medium and a liquid and including an inlet (4) and a bottom outlet (6) ; a delivery line (17) along which an entraining pressurized fluid can be fed from a source and with which the interior of the vessel (1) communicates through the outlet (6) whereby a flow of pressurized fluid along the delivery line (17) can entrain the contents (3) of the vessel (1) for dispersal into the fluid flow in the delivery line (17) for blasting; a pressurizing means (15, 16) to direct pressurized fluid from the source into the vessel (1) behind its contents (3) in terms of its contents' outflow from the vessel (1) through the bottom outlet (6); and characterised in that an injection means (31, 32, 33, 34) is provided for injecting pressurized fluid into the lower portion (7) of the vessel (1) into the contents (3) of the vessel (1) to agitate the contents (3) ; and a control means (43) is provided to control the pressure within the vessel (1) and to ensure that during blasting the pressure of the pressurized fluid entering the vessel (1) through the injection means (31, 32, 33, 34) is always maintained at a higher level than the pressure of the pressurized fluid directed into the vessel (1) by the pressurizing means (15, 16) .

2. An apparatus as claimed in Claim 1, characterised in that the pressurizing means (15, 16) comprises a control valve (15) defining a first outlet port through which the pressurized fluid can flow into the delivery line (17) and a second outlet port through which the pressurized fluid can flow into the vessel (1) to pressurize the interior of the vessel (1), the first and second outlet ports being connected by a conduit (15a) whereby during blasting when the pressure within the vessel (1) exceeds the-- pressure of the pressurizing fluid flowing through the second outlet port, the flow of pressurizing fluid can reverse and pass from the interior of the vessel (1), through the conduit (15a) and into the flow of pressurized fluid egressing from the first outlet port into the delivery line (17) .

3. An apparatus as claimed in Claim 2, characterised in that the control means (43) comprises a manostat which is connected to the control valve (15) and to the injection means (31, 32, 33, 34) and which enables a predetermined pressure difference to be maintained between the pressure of the pressurized fluid entering the vessel (1) through the injection means (31, 32, 33, 34) and the pressure of the pressurized fluid directed into the vessel (1) by the pressurizing means (15, 16) .

4. An apparatus as claimed in Claim 2 or Claim 3, characterised in that the delivery line (17) terminates in a blasting nozzle (20) with a control handle (25) which is supplied with pressurized fluid from the source and which is connected to the control means (43) and to the control valve (15) whereby an operative can open the control valve (15) and thereafter control the flow of pressurized fluid through the nozzle (20) by varying the flow of pressurized fluid through the control handle (25) .

5. An apparatus as claimed in any one of Claims 1 to , wherein the inlet (4) in the vessel (1) is provided with an air lock valve (8) which is closed by the application of pressurized fluid thereto from within the vessel (1) by the pressurizing means (15, 16) and is characterised in that the outlet (6) in the vessel (1) is provided with a valve means (35) which is connected to the air lock valve (8) such that closure of the air lock valve (8) causes the valve means (35) at the outlet of the vessel. (1) to open and vice versa.

6. An apparatus as claimed in Claim 5, characterised in that the outlet (6) from the vessel (1) comprises an orifice

(41) and the valve means (35) for the outlet (6) comprises a spigot (42) which is inserted into the orifice (41), the tip of the spigot (42) remaining within the orifice (41) when the valve means (35) is opened to permit outflow of the contents (3) of the vessel (1) .

7. An apparatus as claimed in Claim 5 or Claim 6, characterised in that the valve means (35) for the outlet (6) is connected to the air lock valve (8) by a flexible coupling (37) whereby, when the valve means (35) is open, agitation of the contents (3) of the vessel (1) causes agitation of the tip of the spigot (42) within the orifice (41).

8. An apparatus as claimed in any one of Claims 1 to 7, characterised in that the injection means (31, 32, 33, 34) comprises a non-return valve (32) connected to an egress means (33, 34) located within the vessel (1) in proximity to the outlet (6) of the vessel (1).

9. An apparatus as claimed in Claim 8, characterised in that the egress means (33, 34) comprises a perforated coil looped around the outlet (6) of the vessel (1) .

10. An apparatus as claimed in any one of Claims 1 to 9, characterised in that the delivery line (17) is connected to a liquid supply (22) upstream of the outlet (6) from the vessel but downstream of the pressurizing means (15, 16) whereby liquid can be injected into the flow of entraining fluid passing down the delivery line (17) .

11. The use of a blasting apparatus as claimed in Claim 1 to blast a mixture of a particulate blasting medium and a liquid by entraining the mixture in a flow of pressurized fluid in the delivery line (17) whilst simultaneously pressurizing the interior of the vessel (1) by directing a flow of the pressurized fluid from the source into the vessel (1) behind its contents (3) in terms of its contents' outflow from the vessel (1); and characterised in that a flow of pressurized fluid is injected into the contents (3) of the vessel (1) to agitate the contents (3) ; and in that the pressure within the vessel (1) is controlled by ensuring that during blasting the pressure of the pressurized fluid injected into the contents (3) of the vessel (1) is always maintained at a higher level than the pressure of the pressurized fluid directed into the vessel (1) behind its contents (3) .

12. The use of a blasting apparatus as claimed in Claim 11, characterised in that the particulate medium comprises a medium which substantially comprises at least one of calcium carbonate, calcium magnesium carbonate, calcium oxide, calcium bicarbonate, calcium magnesium carbonate, magnesium oxide and magnesium sulphate, or comprises soda ash, an olivine sand or garnet.