This invention relates to a storage cabinet for fumes and toxic vapour producing materials. The invention finds particular application in the prevention of mercury vapour contamination within dental surgeries. However, the invention is not limited to such an application, and it is ' envisaged that it may be used for storing- a range of fume producing materials. in dental surgeries, for example, considerable amounts of mercury are both stored and manipulated in the production of dental amalgams. Liquid mercury has a vapour pressure, thus some toxic mercury vapour tends to escape into the surgery atmosphere creating a health hazard for dentist, dental staff and patient alike.

It has been known to store mercury and locate devices such as amalgam mixing machines within conventional fume cabinets. However, such cabinets suffer from various disadvantages. An exhaust system as is required in a conventional fume cabinet to exhaust vapours to the atmosphere is expensive to install. Such an exhaust system can also be expensive to operate and furthermore is often unsightly.

When such a conventional fume ^' cabinet is opened to permit access to the cabinet interior, there- is a tendency for some small amount of the vapour within the fume cabinet interior to escape into the surgery atmosphere via an access opening..

Experiments were conducted with cabinets exhausting to the ambient atmosphere through a filter. However these were unsatisfactory due to the inability of the-filters to absorb all the mercury vapour passing through the filter.

The object of the present invention is to overcome, or. substantially ameliorate the mentioned disadvantages and to ' provide an improved storage cabinet.

According to the invention there is provided a storage cabinet comprising a body of the cabinet; an aperture in the cabinet body for access to the interior of the cabinet? a door movably mounted on the body of the cabinet for closing the access aperture; characterised in that the cabinet further comprises means for circulating air within the storage cabinet and a fume and/or toxic vapour absorbing filter through which air within the cabinet is circulated for absorption of toxic vapour. Normally the body of the cabinet is substantially airtight and the door is sealed to the body when closed. However the degree of airtightness need not be such that all fitting apertures in the body are totally hermetically sealed. The door itself is preferably curved and pivoted about a point eccentric from the centre of the curve whereby in closing, sealing elements are closed onto as opposed to being merely slid along. Elastomeric sealing elements may be provided on the door or on the cabinet's body.

The air circulation means is preferably arranged to induce a swirling action of the air in the cabinet to effect thorough mixing of the air whereby no stagnation pockets form in which toxic vapours can accumulate.

Further, the air circulation means is preferably provided with an elongate exit slot, or series of aligned exit apertures in the form of holes or short slots, for directing filtered air across the aperture in a so-called "air curtain".

A pump included in the air circulating means may be provided either within the cabinet body or outside the body. In the preferred embodiment, the pump's motor is provided outside the body with an impeller provided within the body. Although other arrangements are possible, in the preferred arrangement the impeller, the pump, associated controls and a portion of a wall of the cabinet comprise a replaceable unit. The filter also is preferably a replaceable unit at

the input to the air circulating means.

Conveniently the motor is provided with a speed control, or is alternatively a two speed motor, whereby when the door is closed, the motor runs at slow speed causing a slow circulation of air in the cabinet and when the door is open the motor runs at a fast speed causing the "air curtain" to be more intense. This has dual advantages in that should vapour from the cabinet escape it will be in the first place predominantly filtered air and secondly it will be fast moving and hence quickly dispersed. The second advantage is that during operations in the cabinet releasing toxic vapour, such as the dispensing of mercury, the vapour will be rapidly drawn into the filter. The motor is slowed- when the door is closed to avoid the noise, power consumption and wear on the circulating means associated with the high speed.

The switching from high to low speed is preferably achieved by means of a cam provided on the door and co¬ operating with a switch during opening of the door. To help understanding of the invention, a specific embodiment thereof will now be described with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a storage cabinet of the invention; Figure 2 is a front view of the cabinet when closed; Figure 3 is a cross-sectional front view on the line

III - III in Figure 5 of the cabinet when open;

Figure 4 is a cross-sectional end view on the line IV -

IV in Figure 2 of the cabinet when closed; Figure 5 is a cross-sectional end view on the line V -

V in Figure 3 of the cabinet when open;

Figure 6 is a perspective view of the air circulating means in the cabinet;

Figure 7 is an end view of the cabinet in the direction of the arrow VII in Figure 2;

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Figure 8 is a view similar to Figure 7 with a motor housing removed;

Figure 9 is a plan view of the replaceable pump unit; Figure 10 is a scrap view in the direction of the arrow X in Figure 5;

Figure 11 is a scrap view similar to Figure 10 of a modified embodiment; and

Figure 12 is a cross-sectional view of a filter unit for the cabinet. Referring first to Figures 1 to .5, the illustrated storage cabinet has a body 11 fabricated from 5mm Acrylic sheet. A door 12 is hinged at end walls 13, 14 of the cabinet by means of nylon bolts and bushes 15 passing through the end walls and wings 16 bonded to a transparent front panel 17 of the door. The front panel 17 is cir¬ cularly curved with a centre of curvature 18, see Figures 4 & 5. When the door which is provided with a knob 19 is closed elastomeriσ sealing strips 20, 21, 22 along the inside end and rear edges of the front door panel 17 abut the top edges 23, 24 of the end walls 13, 14 and the top side 25 of the front edge of a top wall of the cabinet. A further sealing strip 26 on the front edge of the door panel abuts a lip 27 provided at the top front edge of a front wall 28 of the cabinet body. The bolts 15 are provided above and behind the centre of curvature 18 whereby they are eccentric from the curve of the front panel. This ar¬ rangement causes the sealing elements to lift away from their abutment surfaces on the cabinet body when the door is opened, thus avoiding wear of the elements which would result from pure sliding movement of the sealing elements over their abutment surfaces. The opened position of the door is shown in Figure 4 wherein the door rests on stops 29 at the back of the cabinet. The sealing elements could alternatively be provided on the body at the abutment surfaces. For complete airtightness of the cabinet, the

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nylon bolts 15 could be sealed to the end walls 13, 14. However in practice this is found not to be necessary.

The cabinet body has a rectangular base 30. An air duct 31 extends in the corner between the base and both the end wall 13 and the front wall 28. The air duct has up¬ right walls 32, 33, parallel respectively with the end wall 13 and the front wall 28, and tops 34, 35. The top 34 has a front lip and defines between the end wall 13 and a con¬ tinuation of the upright wall 32 a toxic material container storage, i.e. a mercury container storage where the cabinet is for use by dentists. The top 35 has an upturned rear rim 36 and defines an instrument storage. An air curtain slot 37 described in more detail below with reference to Figure 10 is defined between the rim 36 and both the upper edge of the wall 33, which is turned backwards and lies under the rim 36, and the front edge of the top 34. At the rear of the cabinet, the rear edge of the top 34 is turned up and the upright wall 32 is continued up and turned over to define a space for an impeller 39 of an air circulating pump. The turned-over top of the wall 32 supports an electric socket 38 for apparatus such as an amalgamator (not shown) stored on the base 30. A replace¬ able filter 40 described in more detail below with reference to Figure 12 is supported by three screws 41 at an aperture 42 in the wall 32.

In the impeller space a scroll 43 of formed styrene is fitted with an inlet coincident with the aperture 42 and an outlet aligned with the air duct 31. Referring now to Figures 8 and 9, the impeller 39 is mounted on the shaft of a motor 44 itself mounted on a moulded-styrene replaceable wall portion 46 bolted across an aperture 47 in the end wall 13. Sealing of the aperture 47 is effected by close fit¬ ting of the moulded wall portion 46, although an O-ring seal could be provided. An O-ring seal, not shown, is provided between the shaft 44 and the moulded portion 46. An elec-

tronic speed control unit 48 is mounted on the moulded portion 46 as is a switch 49. A pivot arm 50 extends beyond the switch for actuation thereof by a cam 51 on the door wing 16, the arrangement being such that when the door is open the speed control is switched for the motor to run at its full speed and when the door is closed the speed control is switched for the motor to run at a slow speed. The actual slow speed can be set with a rheostat in the speed control. The moulded portion 46, impeller 39, motor 44, speed control 48 and switch 49 are all replaceable as a single unit. A housing 52 covers the motor and its associated components.

Air circulation is through the filter 40, the impeller 39, along the duct 31 and out of the slot 37. The slot is oriented to direct an air curtain across an access aperture 53 in the body of the cabinet which is normally closed by the door 12. As may be seen in Figure 10, the slot tapers and is narrower at the end thereof adjacent the end wall 14; since this end is remote from the impeller 39 the static pressure head in the air duct at this end is highest. In an alternative arrangement shown in Figure 11, a number, three as shown, of partition members 54 in the air duct defining subsidiary passageways lead air to a number of aligned air outlets 55. In this alternative there is no outlet directed towards the container storage at top 34. The generally circular filter 40 as shown in cross- section in Figure 12 comprises a pad 56 of activated carbon in a foam structure for absorbing mercury vapour, or other fumes or toxic vapours, sandwiched between on either side an inner scrim layer 57 and an outer open dust filter 58 of non-woven fabric. The sandwich structure is retained together by a pair of formed styrene rings 59, 60 which are clipped or glued together. Each ring has an inwardly directed flange 61, 62 between which the edge of the sand-

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wich is located. The outer ring 59 has a rim 63 having three cut outs allowing passage of the heads of the screws 41 on fitting of the filter. It is secured by twisting to bring the heads over the rim. When secured the inner ring 5 61 bears against the end wall 13 with a small rim 64 en¬ gaging in the aperture 42.

In normal operation of the cabinet with lid closed, the air therein is filtered by means of repeated passes through the filter 40. The efficiency of the filter 40 is not 0 100%, however by the time the air in the cabinet has passed repeatedly through the filter, the level of fumes and toxic vapours in the atmosphere of the cabinet interior is sub¬ stantially reduced to zero or immeasurably small concen¬ tration levels. 5 In the arrangement of the preferred embodiment, at its low speed of operation the motor and impeller is capable of passing the entire air volume of the cabinet interior through the filter 40 three times per minute. An extremely severe test of the cabinet was carried out by placing 500 mg. (a o massive amount compared to day-to-day spillages or droplets) of liquid mercury in an open dish placed on the base 30. The internal temperature of the cabinet was 23°C whilst vapourisation of the mercury proceeded. The resulting level of mercury vapour contamination vastly exceeded the 5 maximum reading 2.00 mg.Hg/cubic meter of the available mercury vapour detector. With the door 12 closed, the measured level of mercury vapour contamination was reduced as follows:-

Elapsed Time Measured Level 0 20 minutes 0.05 mg.Hg/cubic metre

27 minutes 0.04 mg.Hg/cubic metre

75 minutes 0.00 mg.Hg/cubic metre

Such performance levels are sustainable in practice with regular replacement of the filter at monthly intervals. When the door is opened, the air curtain indicated by

the arrows 65 in Figures 1 and 5, is intensified. If an operator releases mercury vapour, either by spillage or mere pouring of mercury, the high speed of filtration quickly absorbs the vapour. Further, the high speed air curtain tends to disperse quickly any vapour which does escape possibly where the operator's hands pass through the air curtain.

Thus it may be seen that the cabinet provides a safe environment for working with toxic vapour producing sub- stances such as mercury. Spillages in the cabinet are contained. The vapour is filtered. Possibly contaminated instruments and the like can be stored.

The invention is not intended to be restricted to the details of the above described embodiment.

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