Abstract:
Mixing apparatus includes a batch mixing vessel with an inlet to receive constituent material, an agitation assembly for mixing the constituents and an outlet to convey batches of the mixed is provided. The apparatus includes a pressurizing device for pressurizing the vessel to assist in evacuation of the mix, and a conduit through which the mix is conveyed. A single pneumatic source is used both to pressurize the mixing vessel and to drive a jet pump including the batch loader. During the mixing cycle, the compressor used to discharge the finished mix is generally idle. This compressor can be used during these periods to drive a jet pump to load the mixing vessel. Therefore, manual loading is avoided and can be achieved such that the mixing cycle time is not interrupted.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to mixing apparatus, for mixing materials, such as aggregates used in the construction industry, and in particular to apparatus which not only mixes the materials, but then conveys the mixed material to the required point of discharge. Such apparatus is commonly known as mixer-placer apparatus. The invention also relates to a mixing vessel and to a suction conduit assembly for such apparatus. 
     Concrete floors, for example, may be laid and screed with mixer-placer apparatus. Although traditionally, this technology is used in the construction industry, the technology can be applied to other industries where mixing and displacement of materials is required. 
     Existing mixer-place machines comprise a diesel engine which drives an air compressor. The diesel-engined compressor is typically mounted upon a wheeled chassis, or road trailer, which also supports a cylindrical batch holding vessel. The holding vessel serves as a mixing vessel and thus includes a rotatable mixing paddle arrangement. The mixing paddle arrangement is powered either by a hydraulic pump and motor or a belt drive or gear box. The cylinder is mounted with its axis horizontal and the mixer paddle drive shaft is on the axis of the cylinder. 
     The holding/mixing vessel is provided with a large charging opening in the upper part of the cylindrical side wall, the opening being closable by a lid and seal arrangement, which provides an air-tight seal to the vessel on closure of lid. This allows the contents of the vessel to be pressurised. 
     A delivery hose is connected to an outlet provided in the lower side wall of one end of the mixing vessel, to deliver the mixed product to the required location once the mixing stage is completed. A pneumatic supply line from the compressor is connected to the mixing vessel to effect discharge of the mixture, once mixing of the constituents is completed. Therefore, the principal function of the compressor is to pressurise the mixing vessel for discharge. 
     In use, when the mixing apparatus is to be used for mixing a floor screed material, the mixing vessel must first be charged with the sand, cement and water, in the required proportions. The lid covering the charging opening is removed, and a measured quantity of sand is shovelled into the mixer vessel from an adjacent sand heap. A bag of cement is split and also emptied in, together with the required amount of water and any additives required. During this process, the paddles are rotated, and once loaded, the lid fastened down securely, whilst the mixing process is completed. 
     Once mixing is complete, compressed air is admitted to the mixing vessel via the air inlet. The paddles in the mixing vessel urge the material towards the discharge outlet. As the mixture covers the opening, the mixture discharges and is conveyed by the pneumatic pressure from the compressed air through the delivery hose. A hose tripod device may be employed to kill the energy in the concrete and allows it to fall in a heap beneath it whence it is spread and levelled. 
     It will be appreciated that the time spent filling the mixing vessel occupies a significant part of the cycle, and that this is a labour-intensive stage. 
     In addition, with current mixer-place machines, great care needs to be taken to clean the lid for the charging aperture, and its seal to ensure that the seal is effective. This step is time consuming, and adds to the overall cycle time for a batch of mixture. In addition, it has been found that the handling at this step can serve to accelerate the deterioration of the seal. 
     If the total cycle time could be reduced, and if the labourer&#39;s time could be better utilized, then the total operational efficiency of the process can be improved. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide mixing apparatus which attempts to address one or more of the disadvantages outlined above, and/or to provide improvements generally. 
     According to one aspect of the present invention there is provided mixing apparatus comprising a batch mixing vessel having an inlet to receive constituent material, an agitation assembly for mixing the constituents within the vessel and an outlet to convey batches of mixed material; the apparatus further comprising means for pressurising the mixing vessel to assist in evacuation of the mixed constituents from the outlet, and a batch loading means including a conduit through which the constituents are conveyed in use by fluid flow means; wherein a single pneumatic source is used both to pressurize the mixing vessel and to drive a jet pump comprising the batch loading means. 
     According to a second aspect of the invention, there is provided a mixing vessel for mixing apparatus comprising a sealable charging inlet for loading constituents to be mixed, and an outlet for discharging the mixed constituents, and means for pressurising the vessel; wherein the mixing vessel is generally spherical. 
     In yet a further aspect, there is provided mixing apparatus comprising a mixing vessel according to the second aspect of the present invention. 
     According to the present invention, it has been identified that during the mixing cycle, the compressor, used to discharge the finished mixture is generally idle, and required only at the time that the mixture is to be discharged. By utilising the compressor during conventionally idle periods, it has been found that the same compressor can also be used to drive a jet pump. Such a jet pump can be used to create sufficient vacuum to load the mixing vessel with the mixture constituents, without the need for manual loading, and within a sufficient time frame which would still allow the required mixing cycle time to be achieved. 
     The compressor is sized to produce the required discharge of screen mix to the required distance and elevation, but is only needed for a short part of the cycle for that purpose. By making use of the existing power source and compressor to provide vacuum suction on the mixing vessel input, a reduced loading time, and thus batch time, is achieved, and the loading itself is far less labour intensive. 
     It has been possible to design a jet pump to use an air volume and pressure to generate a vacuum in the mixing vessel with the cycle time achievable by traditional means (men shoveling) which is identical to that necessary for the required discharge flow rates and pressure. 
     The batch loading means comprises a conduit. A further aspect of the invention relates to the suction conduit. In view of the stiffness of conduit hose required to load the constituent materials, handling such hose can be difficult. 
     In a further aspect of the invention, an inlet suction conduit assembly is provided which includes adjustment means, such that the effective length of the conduit assembly can be varied in use, in a controlled manner. Preferably, the adjustment means comprises a section which is capable of telescopic movement, and this may be achieved by hydraulic actuation. It is preferred that the suction conduit assembly is rotatably mounted. In a further embodiment, the suction conduit is articulated. In such an arrangement, the articulated pipe is provided with a gimbal joint at the connection to the vessel and a knuckle joint with the horizontal axis part way along its length. 
     In a further preferred embodiment, the suction conduit assembly is provided with means operable to vibrate the intake, to facilitate entry of the constituent material, such as sand, into the suction inlet. 
     It has been identified that in order to create a sufficient vacuum to load the mixture constituents in time required, an optimum diameter of suction pipe or hose of the suction conduit assembly can be determined by test guided by calculation. 
     It is preferred that the suction conduit assembly is provided with some balancing mechanism, and is also provided with the ability to swing horizontally and vertically so that its suction inlet could map any three-dimensional volume within its reach. In an embodiment, sensor means are provided to enable the apparatus to consume a heap of mixture constituents systematically and without human intervention. In an embodiment, such a sensor may include a timer, for timing the length of time that the compressor has been operating, the quantity of material collected being proportional to the length of time of operation. 
     In an embodiment there is provided a hydraulically operated telescopic suction pipe for the suction conduit assembly. The power assistance is provided to the telescoping action. 
     In an embodiment, a short gimbal support is provided, allowing swinging of the pipe within a solid angle of +/−30°. The hose is provided with open convolutions which are non-clogging. The pipe is partly supported by a gas spring, the degree of support depending upon the extension and amount of sand within it. The gimbal arrangement allows hydraulic power of both swinging and raising and lowering, using circuit design and components identical with those which are used for its extension and retraction. 
     The end of the inlet pipe may be provided with a whirling ball vibrator to motivate the sand, and an alternative would be to mount the vibrator elastically, with probes to motivate the sand. 
     In one aspect of the present invention it is preferred that the mixing vessel of the mixing apparatus is generally spherical. It has been found that by replacing the conventional cylindrical shape of mixing vessel with a generally spherical vessel has a number of technical advantages. 
     The spherical form has been found to eliminate bending stresses in the shell. This enables a thinner shell material to be used, which requires only a single fabrication weld. This can be compared with three or four weld required in the conventional cylindrical form. This results in a cost saving not only on materials, but also reduced costs in the welding stages, and in the requisite subsequent testing of such welds. 
     It has also been found that the spherical cavity allows more complete filling (the spherical shape provides a better shaped air space above the mixture to reduce interference with the incoming constituents), more complete discharge (the shape also provides a positive low-point) and easier cleaning. The generally spherical shape also provides a more space-efficient structural form, such that the apparatus occupies less space. 
     In an embodiment, the mixing vessel is provided with a protective liner. The protective liner comprises a plurality of segments. The use of “orange-peel” segments to provide a protective liner for the mixing vessel avoids the use of castings, which would be heavier, more costly and more liable to fracture. The double curvature facilitates manufacture of the lining segments by simple rolling. It has been identified that in use, most of the wear takes place towards the lower end of the “segments”. In view of their shape, the liner segments are reversible, and can simply be rotated through 180° and then re-used. 
     In an embodiment, the mixing vessel is provided with a level sensor. The level sensor preferably operates on the principle of the oscillating-vane level sensor. When the vessel is under vacuum, a pressure difference exists which can be used to energise the incoming water to form a jet. This jet may be directed against any parts which would benefit from washing, such as a screen through which the incoming solids have to pass (to filter out stones and fragments of cement bags etc.), the level sensor, and the suction inlet control valve. The washing of the seal of the lid is advantageous to reduce its deterioration, in view of the abrasive materials being mixed. 
     In an embodiment, there is provided an hydraulic drive for the mixing paddles. This is considered advantageous over other drive means, because it frees the mixture vessel from constraints of location relative to the engine for the compressor. 
     In an embodiment, clamping means are provided to prevent accidental lifting of the lid/cover of the mixing vessel in any condition of internal pressure of vacuum. The clamp is in an over-centre arrangement, such that high pressure in the vessel prevents operation because an unrealisable operator force is required. At a low vessel pressure, operation of the clamps may be prevented by the additional release mechanism. Accidental operation of release mechanism is also prevented, and venting of any escaping jet is controlled. The edge of the lid is formed with a downward deflector provided by the profile of the edge of the lid, in such a way as to prevent injury to the operator and at the same time to provide necessary structural integrity. Forcing the release lever would vent the pressure safely and stop the engine. 
     Other modifications which are envisaged within the scope of the present invention is the use of one or more additional smaller jet pumps which can be used to load additives such as fibres to the mixing vessel. These would be driven from the same air supply as the main jet pump but operated only as long as necessary. In this way, the provision of an additional or secondary self-loading facility is achieved at almost no additional cost. The use of additional small jet pumps prevents interaction of loading pipes or hoses and possible unsolicited variation of mix. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the various aspects of the present invention will now be described, by way of example only, with reference to the following illustrative drawings in which: 
         FIG. 1  is a side elevation of a suction conduit assembly in accordance to one aspect of the present invention, attached to a holding vessel; 
         FIG. 2  is a plan view of the embodiment shown in  FIG. 1  which is in part in cross section; 
         FIG. 3  is an enlarged view of  FIG. 2 ; and 
         FIG. 3A  is an enlarged view of the area circled in  FIG. 3 ; 
         FIG. 4  is a schematic diagram of a mixer-placer machine according to a second aspect of the invention; 
         FIG. 5  is a detailed view of a mixing vessel according to a further aspect of the invention; 
         FIG. 6   a  is a partial cross section of the lid/seal arrangement for the mixing vessel of  FIG. 5  taken along line  6   a - 6   a  of  FIG. 6   b , and  FIG. 6   b  is a plan view of  FIG. 6   a  viewed along arrow  6   b  of  FIG. 6   a;    
         FIG. 7   a  is a plan view of the lid housing for the mixing vessel, and  FIG. 7   b  is a sectional view on line  7   b - 7   b  of  FIG. 7   a  showing the inlet screen; 
         FIG. 8   a  is a plan view of the mixing vessel lid and  FIGS. 8   b,    8   c,    8   d,    8   e,    8   f,    8   g  and  8   h  are further cross-sectional views of the lid locking assembly; 
         FIG. 9  is a cross section view of a pressure release valve assembly for part of the lid closure mechanism of  FIGS. 8   a  to  8   h;    
         FIG. 10   a  depicts a modification to a standard ball valve for the suction pipe  160 , and  FIG. 10   b  is a sectional view of  FIG. 10   a  along line  10   b - 10   b;    
         FIG. 11  illustrates the valve assembly provided in the discharge hose; 
         FIG. 12  is a diagrammatic view of the drive shaft assembly of the apparatus; 
         FIG. 13  is a side elevation of a suction conduit assembly according to a further embodiment of the invention; and 
         FIG. 14  is a partial plan view of the embodiment of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a suction conduit assembly  1  through which material is conveyed comprises a first pipe section  3 , a second pipe section  4  and an intake  5 . The bore of the first pipe section  3  is dimensioned such that the second pipe  4  is capable of telescopic movement within said first pipe section  3 , so that the effective length of the suction conduit can be varied. 
     Referring to  FIGS. 3 and 3A , attached to the inner surface  12  of the distal end  7  of second pipe section  4  is a sealing skirt  6 . The sealing skirt  6  is of substantially frusto-conical shape and forms a substantial seal with the inner surface  10  of the first pipe section  3 . 
     Similarly, a sealing skirt  9  is provided on the outer surface  13  of the distal end  8  of the first pipe section  3 . The sealing skirt  9  is of substantially frusto-conical shape and forms a substantial seal with the outer surface  11  of the second pipe section  4 . The distal end  7  and the distal end  8  are flared and tapered respectively to ensure sealing contact of the skirts with the respective surfaces. 
     The suction conduit assembly  1  further comprises pistons  14   a  and  14   b  and associated hydraulic cylinders  15   a  and  15   b.  The pistons  14   a  and  14   b  are secured to the second pipe section  4  in the following manner. Two pairs of plates  16  and  17  (one plate of each pair is shown in  FIGS. 2 and 3 ) which are rigidly attached to the second pipe, are spaced so as to accommodate bored portions  18   a  and  18   b.  Bores  19   a  and  19   b  are aligned with holes in the plate pairs  16  and  17  so that securing pins (not shown) can pass there through. The bored portions  18   a  and  18   b  are attached to the respective pistons by ball joints (not shown). The pistons  14   a  and  14   b  are actuated by respective hydraulic lines  24 , the hydraulic lines being in communication with opposite ends of each cylinder  15   a  and  15   b.  The pistons  14   a  and  14   b  may conveniently be powered from the hydraulic system associated with the holding vessel  45 , if one is so provided. 
     Each of the hydraulic cylinders  15   a  and  15   b  is attached at two locations on the first pipe section  3 . The hydraulic cylinders are secured in the first instance by securing pins (not shown) through plate pairs  20  and  21  (one plate of each pair is shown in  FIGS. 2 and 3 ) and bored portions  22   a  and  22   b  are attached to the respective cylinders by ball joints (not shown). 
     A gimbal  44  comprising two double-limbed components  26  and  27  and frame  32  is provided between the first pipe section  3  and pipe  37 , the double-limbed components  26  and  27  partially surrounding a length of flexible hosing  25 . The double-limbed component  26  is attached to frame  32  for rotation of the pipe sections  3  and  4  about an axis Y-Y. The double-limbed component  27  is attached to the frame  32  in a plane perpendicular to the axis Y-Y for rotation of the pipe sections  3  and  4  about an axis X-X. The pipe  27  is clamped to pipe  38  by a Bauer clamp  39 , and the pipe  38  leads up to a shut-off valve  40  and then into a holding vessel  45 . The holding vessel  45  may house agitators for mixing materials therein. 
     A vibrator  47  is provided on the second pipe section  4  and comprises a circular track  48  around which a ball bearing is circulated by means of a compressed air supply (not shown). A support rod  60  is provided between the underside of the suction conduit assembly  1  and the holding vessel  45 . 
     In use, the assembly operate as follows. Evacuation means  58  comprises an air outlet  61 , an evacuation pump  62  and an isolation valve  63 . In an embodiment, the evacuation pump  62  is a jet pump, as it is not practicable to protect the pump with certainty from solid particles. The jet pump is preferably of the type sold under the trade mark GENFLO™. If the holding vessel  45  is part of the apparatus for pneumatically conveyed batches of mixed material, then the jet pump may conveniently be driven from the air compressor used therefor. 
     After the shut-off valve has been opened and the evacuation pump has been started, an operator then directs the intake  5  of the second pipe section  4  at a heap of material (not shown) to be loaded into the holding vessel  45 . The operator then, via suitable controls, extends the second pipe section  4  to penetrate the heap of material, and in so doing, material is taken into the conduit assembly  1  and then into the holding vessel  45 . 
     Once the second pipe section  4  has been fully extended, said second pipe  4  section is then retracted out of the heap after which the second pipe  4  is again extended into the heap to load further material into the holding vessel  45 . This repeated extension and retraction of the second pipe section  4  thus produces a reciprocating action, the rate of which is determined by the characteristics of the material to be loaded. The vibrator  47  produces an orbital motion of the intake  5 , thus ensuring that the intake  5  does not clog, and that any clumps of material around the intake  5  in the heap are loosened. 
     If the holding apparatus  45  is part of apparatus to convey batches of mixed material pneumatically, then once the required amount of material has been loaded into the vessel then valves  40  and  63  can be turned off and an outlet valve (not shown) opened. The holding vessel can then be pressurised to force the mixed material out of the vessel. 
     Handles  30  mounted on the pipe sections  3  and  4  ensure that once the suction conduit assembly is detached by the clamp  39 , it can be carried with a reduced risk of causing damage. 
     In a modification of the assembly  1 , the gimbal  44  may be adapted to be used with or replaced with appropriate service means so as to control the rotational movement of the suction conduit assembly. 
     In another modification of this aspect of the invention, the second pipe section  4  may be provided with an attachment to facilitate the loading process of a given material. The attachment may, for example, comprise an actuated scoop to capture material in the heap to be sucked away through the intake  5 . 
       FIG. 4  is a schematic diagram of a mixer-placer machine according to a second aspect of the invention. As can be seen, the apparatus generally designated  100  comprises a diesel engine  110  which drives an air compressor  120 . The diesel engine  110 /compressor  120  is mounted upon a wheeled road trailer  130 , which also supports a generally spherical batch holding, or mixing, vessel  140 . The mixing vessel  140  includes a rotatable paddle assembly which is mounted on a drive shaft powered by an hydraulic pump and motor assembly  150  which is driven by the diesel engine  110 . 
     The top of the mixing vessel  140  is provided with a closeable charging inlet into which the materials to be mixed are loaded. The charging inlet is sealed with a lid and seal arrangement, which provides an air tight seal to the mixing vessel  140  on closure of the lid, to enable the vessel to be pressurised. 
     In the illustrated embodiment, the loading of at least some of the constituent materials (in this case sand, water and cement to make concrete), is achieved automatically without manual loading using a suction conduit assembly  160  substantially as described above in relation to  FIGS. 1 to 3A  (equivalent parts are designated by the same reference numerals). Additional loading means include a cement hopper  180  and a water supply  190  which are also in communication with the mixing vessel  140  via the charging inlet. 
     A discharge outlet  200  is provided in the base of the mixing chamber  140 , to which is connected a delivery hose  210 . The delivery hose  210  transports the discharged mixture and conveys it by pneumatic pressure, supplied via supply line  220  from the compressor  120 , when the discharge valve is opened. 
     In use, the engine  110  is started, and the controls are set to operate the jet pump  62 . The engine also drives the mixing paddles in the mixing vessel  140 , which rotate to agitate and mix the added constituents. 
     The mixing vessel  140  is evacuated, and the sand is added to the mixing vessel via the suction conduit assembly  160 . At the same time, the valve to the cement hopper  180  is opened to admit a pre-determined amount of cement to the mixing vessel, and similarly a volume of water from the water supply  190  is also emptied into the mixing vessel. The valves to the cement hopper  180  and water supply  190  are closed once the required amount has been discharged into the vessel  140 . 
     Once the constituents are sufficiently mixed to the required consistency, the mixing vessel is pressurised using air from the compressor  120  along supply line  220 . It can be seen that advantageously, the same air supply may be used to supply both the jet pump  62  and to pressurise the mixing vessel  140 , as each requires the air supply at a different stage in the process. The valve in the discharge hose  210  is opened and the mixture is discharged via the discharge outlet  200  and is conveyed along discharge hose  210  to its required location. A conventional tripod arrangement  220  may be provided to kill the energy in the mixture (concrete) and allow it to fall in a heap to be spread. 
     As can be seen in  FIG. 5 , the mixing vessel  140  is generally of spherical configuration. The pressure shell  141  forming the outer part of the mixing vessel  140  is formed from a pair of hemispheres produced by pressing or spinning. The shell is provided with  12  lining segments  142 . These sufficiently duplicate the form of the pressure shell  141 , which they protect from abrasive wear of the constituents to be mixed within the vessel. Eight of the lining segments  142  are reversible, top to bottom. The charging inlet  143  is provided at the top of the vessel  140 , and the discharge outlet  200  is provided slightly offset from the bottom of the vessel  140 . 
     As can be seen in  FIGS. 6   a  and  6   b,  the charging inlet of the mixing vessel  140  is provided with a lid  210  to seal the vessel. Water is admitted by a branch formed in the lid housing  211 . It passes through a hole in a flat seal  212  and thence to a nozzle formed in the seal retaining place  213 . A non-return valve (not shown) prevents reverse flow through the branch when the vessel  140  is pressurised. The seal plate  213  and the housing  211  are formed to dish the seal. 
     The shape of the seal plate  213  and housing  211  forms the seal shape, thus allowing a low cost flat seal to be used. The dished form of the seal  212  provides a simple means of giving a low-torque sealing action when under vacuum and a tight seal when under pressure. 
     As can be seen in  FIGS. 7   a  and  7   b,  the water provided to the mixing vessel  140  can be used to form a washing jet. The energy to drive the washing jet is provided by the vacuum generated in the mixing vessel  140  by the jet pump  62 . 
     The washing jet may be used to provide a directional water injection to clean an internal screen area. All constituent material enters the mixing vessel  140  via a circular screen  215 , which is located beneath the lid  210 . The screen  215  is crowned, with radial and circumferential vanes, and serves to trap material such as fragments of bag and stones which should not be present in the sand, or stones, but which have been sucked up from beneath the sand heap. A gully surrounds the screen. Water is injected into the screen area through a nozzle  216  which is adjustable in orientation, for example the nozzle may be rotatable to direct the water spray as required. 
     The conical screen form is largely self-cleaning, with stones which have been sucked up falling to the gully surrounding the screen. Cleaning is assisted by the water jets. The adjustability of the nozzle  216  allows it to be set effectively. Advantageously, the washing water forms part of the concrete mix. 
     As can be seen in  FIGS. 8   a  to  8   h,  there are two identical securing latching assemblies  217  for the lid assembly  210  for the mixing vessel  140 . When the mixing vessel  140  is pressurised, opening is prevented by the internal pressure acting through over-centre links, lever  218  and handle extension  219 , and a cam, part of shaft  220 , arrangement working through a small lever, part of shaft  220  shown by Section  8   f - 8   f  ( FIG. 8   f ). 
     The travel of this lever is controlled by the flat bar, part of shaft  220 , shown on  8   h - 8   h  ( FIG. 8   h ) and two roll pins  221 . The shaft  220  is held in the closed position by a torsion spring  222 . Additional resistance to rotation is introduced by valve  223  shown in  FIG. 9 . This is held closed by the pressure in the vessel  140  plus the spring load of item  220  required to prevent the valve  223  being opened by a vacuum while the vessel  140  is filling. 
     Adjustment of the valve  223  is by means of a screw  224  prevented from loosening by a self-locking helical insert  225 . Valves must be left open before starting the engine  110  to prevent overload of the compressor  120 . The engine  110  cannot be started until both latch levers  218  have secured the lid  210 . Up to this point the limit switch  226  on Section  8   f - 8   f  de-energises the solenoid shut-down valve on the fuel pump. The limit switch  226  under the lid  210  shown on Section  8   e - 8   e  ( FIG. 8   e ) also activates the solenoid shut-down valve. 
     To prevent accidental shut-down, the pressure in the mixing vessel  140  must be released by means of a gate valve (not shown) before it is possible to operate the small lever, part of shaft  220 , in turn operating the limit switch  226  and venting any residual pressure through branch  227 , part of the pressure vessel  140  and release valve  223 . The main lever  218  cannot be rotated until the cam, part of shaft  220 , on Section  8   g - 8   g  ( FIG. 8   g ) is rotated by means of the small lever, part of shaft  220 , shown on Section  8   f - 8   f  ( FIG. 8   f ). 
     If at any time the lid  210  is raised, perhaps through failure of part of the mechanism described, the jet emerging from the gap between lid  210  and vessel  140  is deflected downwards by the profile of the edge of the lid. It is unlikely that failures of both latch mechanisms will fail together. 
     As can be seen in  FIGS. 10   a  and  10   b,  a modification to standard ball valve for the suction pipe  160  is provided. A ball valve will admit solids into the cavity between the seals, if it is operated when not completely clean. A standard ball valve will then quickly jam, and is thus not usable in the mixer-placer environment. In the present invention, the body of a conventional ball valve  228  is drilled through on a diameter. A steel split cover  229  is formed with branches to correspond with these ports when clamped into position. A seal  230  is formed in-situ by a two-part sealing compound. Once every operating cycle, when the vessel  140  is under vacuum, water is flushed through the valve via the branches in the cover  229 . 
     This modification makes a standard, low-cost ball valve suitable for vacuum solids transmission. The energy to drive the sluicing water is provided by the vacuum generated in the vessel  140  by the jet pump  62 . Advantageously, the sluicing water forms part of the concrete mix. In an alternative embodiment (not shown) the ball valve is replaced with a pinch valve. 
       FIG. 11  illustrates the valve assembly provided in the discharge hose  210 . It has been found advantageous to inject air into the flow discharged from the mixing vessel  140  in order to achieve discharge to the required distance and elevation from the available pressure. There is provided a duckbill chopper valve  239  (so called because it chops up what would otherwise be a solid column of concrete mix into alternating slugs of air and mix) through which air is admitted. The valve element  231  is formed from reinforced rubber. A branch is provided in the housing  232  for water cleaning if necessary. 
     The lined duckbill form needs no maintenance. The greater the pressure, the better the seal from a thick, soft natural rubber lining. The form is also strictly one-way and concrete cannot be forced into the air line. 
     As can be seen in  FIG. 12 , there is provided a safety mechanism, in the event that one of the mixing paddles in the mixing chamber  140  should jam. In this event, a torque limiter prevents damage to the gearbox. The torque limiter operates by shearing a sacrificial shear pin  233 . When the pin  233  shears, a spring  234  pushes the shear pin carrier  235 , located in a fork extension on the drive shaft  236 , away from the drive shaft  236  and operates a limit switch  237  which in turn shuts the engine  110  down via the fuel pump solenoid. The ends of the sheared pin would tend to fly out, but are retained by the screws attaching shear pin housing  238 . The waist of the shear pin is located in an annular gap between shear pin carrier  235  and shear pin housing  238 . 
     In this way, damage to the gear box and remainder of paddle drive train caused by jamming is prevented. The engine  110  is shut down immediately the shear pin fails, and the broken shear pins cannot escape until released. The annular gap prevents internal damage which might be caused by the ends of the shear pin fragments. 
       FIGS. 13 and 14  illustrated a suction conduit assembly according to a further embodiment of the invention. In this embodiment, the suction conduit  260  is in the form of an articulated arm assembly. Other than set out below, the functioning of the suction conduit is the same as hereinbefore described. 
     The articulated suction inlet pipe  260  is part balanced by gas springs  262 . A spring clip  264  locates in grooves in hose  266  extension tube carrying a seal to prevent loss of vacuum. The suction pipe is articulated at the centre about a horizontal axis, in gimbal arrangement. The mean weight of the pipe and contents is balanced by gas springs. The end of the suction pipe carries a vibrator  268 . 
     It has been found that with this arrangement, the assembly of the suction pipe to the pressure vessel  141  is quicker. The articulation of the suction pipe allows access to a large volume of sand without moving the machine. The vibrator  268  secures continuous collapse of the sand heap, securing a constant feed to the pressure vessel and minimum cycle time.