FLAME OR EXPLOSION SUPPRESSANT FOR COMBUSTIBLE DUST

FIELD OF THE INVENTION

This invention relates to a hydrated polymer that is capable of suppressing or inhibiting the spread of flame through a combustible dust air mixture especially, but not exclusively, in a confined space for the purposes of preventing or limiting the extent of a dust explosion. An example of such a confined space is an underground coal mine, where the spread of the flames in an airborne coal dust can constitute an explosion of coal dust.

BACKGROUND OF THE INVENTION

Combustible dusts such as coal and metal dust, husks, flour and pharmaceuticals carry the risk of explosion. Particularly in the coal mining industry, the operation of mining machinery generates considerable quantities of coal dust. The coal dust tends to settle out of the airstream and accumulate on the floor, walls, roof and any other surfaces. If the accumulated coal dust is disturbed and lifted into the air, it can form a combustible dust cloud. If an ignition source is present, an explosion can result. Such an explosion is usually catastrophic as it is capable of propagating long distances, causing considerable asset damage and loss of life. Combustible dust is therefore a significant hazard anywhere in an underground coal mine, but particularly around working places where ignitable gas and sources of ignition might be present. The various techniques used to reduce the danger from coal dust explosions are broadly consistent around the word and involve a combination of ventilation; limiting ignition sources; dispersal of inert dust in the mine workings; and the displacement of inert dust or water from pre-placed shelves or suspended tubs (explosion barriers) upon explosions.

A commonly used inert dust is referred to as stone dust or rock dust; this consists of incombustible minerals from the carbonate group, such as limestone (calcium carbonate) and dolomite.

One method of suppressing or inhibiting coal dust explosions is through Broadcast Stone Dusting. A layer of stone dust is distributed in all roadways to cover coal dust accumulations. The top layer of the dust is therefore incombustible and will not support a coal dust explosion. If a sufficiently thick layer of stone dust is applied, the underlying coal dust can be prevented from becoming airborne, thus limiting the extent of flame propagation. Regular re-applications of the stone dust are required to cover new coal dust accumulations. A variation of this method involves the mixing of the stone dust with water to form a water-dust slurry. The stone or slurry dust can be sprayed onto the roadway surfaces using compressed air.

Another method of suppressing or inhibiting coal dust explosions is by Trickle Dusting. Trickle Dusting is the practice of blowing stone dust into the airstream carrying the combustible dust so that when it is deposited the resulting dust mix is non-combustible. To prevent dangerous accumulations of combustible dust, continuous re-application of the stone dust is required. Placement of repositories of either fine stone dust or water can be arranged in such a way that any explosion will disturb the placement system, causing it to quench the explosion by dispersing the stone dust or water. These repositories are commonly known as dust or water explosion barriers. These methods of using stone dust are applied in underground coal mines with varying degrees of effectiveness and difficulties in application. A number of disadvantages are associated with these existing methods. For example, a high proportion of stone dust must be present to prevent a coal dust explosion. The requisite proportion of stone dust can be as high as 85% of the total dust mixture mass. The mass ratio of coal dust and stone dust is 1:5.6. This mass ratio is referred to as the inerting ratio and is expressed in the form of 5.6kg/kg.

Further, stone dust has a higher density and usually larger particle size in comparison to coal dust. Because of this, stone dust tends to accumulate closer to the discharge point of the trickle duster. This results in an uneven distribution of stone dust relative to the coal dust, wherein the roadway surfaces further downwind are under-stonedusted. In order to ensure that a sufficient amount of stone dust reaches all points downwind of the duster, there has to be an overall over-application of stone dust.

Another disadvantage is that the application of stone dust disrupts mining operations. It is not possible for personnel to work immediately downwind of the area in which Broadcast or Trickle Stone Dusting is carried out as the dry fine power is carried a significant distance downwind by air movement. This has led to the development of various methods of wet stone dusting. Due to the large quantities of stone dust required, there are considerable transport costs both in the transportation of stone dust to and within the mine site.

In relation to the use of stone dust or water barriers, these only present a final form of defence that is designed to protect the bulk of the mine. They can only be activated by a significant explosion, which has caused extensive loss before the stone dust or water barriers are displaced. The effectiveness of this method is largely dependent on the positioning of the stone dust or water barriers. The utilisation of stone dust still suffers from all the limitations previously discussed; and the equipment required in the utilisation of stone dust or water barriers is notoriously difficult to setup, maintain and disassemble. The largest problem is usually the mass of dust that needs to be transported to various places underground.

Whilst water is a better explosion suppressant than stone dust, its utilisation is limited by practicality of its application. Consequently, water cannot be effectively used as a substitute for stone dust in Broadcast or Trickle Stone Dusting. Moreover, whist it has been postulated in the prior art to employ superabsorbent polymers as stationary fire fighting compositions, nothing in the prior art suggests a polymer composition or the like that is capable of suppressing a coal dust explosion. A coal dust explosion is completely different to that of a stationary fire, and thus it should not be assumed that a polymer used in fire fighting could have an application in a coal dust explosion. OBJECT OF THE INVENTION As such, it is therefore an object of the invention to ameliorate some or all of the above disadvantages of the prior art by providing a novel and inventive type of inert dust in the form of a hydrated polymer.

STATEMENT OF INVENTION

According to a first aspect, the invention resides in a particulate or powdered polymer for suppressing a combustible dust explosion, comprising a polymer capable of absorbing significant amounts of water relative to its mass, wherein when hydrated the polymer is readily dispersible to suppress a coal dust explosion.

Preferably, the polymer is a polyacrylate, polyamide, or copolymers thereof.

Preferably, the polymer is selected from the group consisting of a cross-linked sodium polyacrylate and a cross-linked sodium poly(acrylate-co-acrylamide).

Preferably, when the polymer is hydrated it forms a hydrogel. More preferably the hydrated polymer has a particulate size of between 80 and 150 microns. . Preferably, the polymer comprises a suitable cross-linking agent. In some embodiments, the cross- linking agent is methylene bis-acrylamide.

Suitably, there may be added to the particulate flame retardant a surfactant to reduce the surface tension between the particles to facilitate distribution as powder or a dust.

In the most preferred embodiments, the combustible dust is a coal dust.

In yet another aspect of the present invention, a method of preventing a coal dust explosion in a coal mine is provided, the method comprising the steps of:

(a) providing a sufficient amount of a polymer capable of absorbing significant amounts of water relative to its mass;

(b) hydrating the polymer; and

(c) dispersing the hydrated polymer in the mine to thereby prevent a coal dust explosion from occurring in the mine.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the effect of acrylamide ratio on the swelling ratio of the polymeric hydrogel when preparing the polymer of the present invention;

Figure 2 shows the water absorption properties of the polymers of the present invention and the effect of varying acrylamide:acrylic acid ratios, as well as the replacement of acrylic acid with methacrylic acid;

Figure 3 shows the effect of cross linking agent concentration (MBA) in on swelling ratio; and Figure 4 shows the effect of imitator agent concentration ( PS) in on swelling ratio.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT There are a number of polymers that will absorb water in large proportions. These are currently used in the agriculture and health industries. However, none of these present the characteristics required for a stone dust substitute.

The present invention will incorporate chemical structures to prevent undesirable interactions between hydrated polymer particles as herein described so that when dispersed it will readily mix with coal dust and suppress a coal dust explosion. Preferably, when hydrated, the hydration ratio between the polymer and water will be in the order of 1:10-1:500. In other words, every mass unit of dry polymer will absorb approximately ten to five hundred mass units of water. More preferably, the hydration ratio between the polymer and water will between 1:100-1:500, preferably, between 1:200-1:500, and even more preferably, between 1:350-1:450.

As discussed above, the invention resides in providing a chemical polymer that is capable of absorbing significant proportions of water relative to its own mass, which when hydrated and dispersed suppresses a coal dust explosion. Suitable polymers included within the scope of the present invention can include, but not limited to, a polymer selected from the group consisting of polyacrylates, polyamides, or copolymers thereof. In preferred embodiments, the polymer is selected from the group consisting of a cross-linked sodium polyacrylate and a cross-linked sodium poly(acrylate-co-acrylamide) . While any known method can potentially be used to fabricate the polymer of the present invention, it has been found that the process known as inversion dispersion polymerization (e.g. as described in Mayoux et at. (1999), "Inverse suspension polymerization of sodium acrylate: synthesis and characterisation", which us herein incorporated by reference in its entirety) is one of the most advantageous. In particular, using this technique it has been found that a resultant polymer has the following beneficial characteristics: a. Glassy polymer/hard hydrogel that can be ground to size; and b. Increased water uptake while at the same time having the ability to maintain a rigid structure once hydrated (a slimy gel will be problematic for application usage).

During the production of the polymer of the present invention, a suitable cross-linking agent can be added to enable a well-structure gel to be produced. In particular, when using a cross-linking agent, the cross-linked properties of the resultant polymer prevent the rapid loss of water from the hydrated polymer when exposed to the atmosphere in the mining site.

Those skilled in the art would readily understand that the amount of cross-linking agent and conditions for the reaction can be routinely selected and optimized. In particular, it is understood that in absence of a cross-linking agent, a polymer gel resembling a viscous liquid would be achieved, while in contrast, too much cross-linking agent would result in a polymer that would have significantly decreased water uptake properties. The process of "cross-linking" is generally achieved by chemical reactions that are initiated by heat, pressure, change in pH, or radiation. For instance, mixing of an un-polymerized or partially polymerized product with specific chemicals, commonly referred to as a cross-linking agent or reagent, results in a chemical reaction that forms cross-links. Suitable cross-linking agents can be routinely selected by those skilled in the art, but by way of example, can include but not be limited to methylene bis-acrylamide (MBA) and can be used in a concentration of approximately 3-5mM.

Suitable initiator systems can also be used during the fabrication of the polymers of the present invention when desired, which can include but not be limited to those selected from the group consisting of potassium persulfate.-sodium metabisulfite and benyl peroxide:potassium persulfate. It has been found by the present inventors that the concentration of the initiator system or agent used in the fabrication of the polymer of the present invention is not overly critical, and thus can be routinely optimized to achieve a polymer within the scope of the present invention. Those skilled in the art would understand that the process of initiation is generally the first step of the polymerization process, where an active centre is created by which a polymer chain can be rapidly generated. As it is commonly understood that not all monomers are susceptible to all types of initiator systems, a suitable system can be routinely selected by those skilled in the art for the purposes of polymerization. .

It should be understood that the particle size of the hydrated polymer can be tailored to provide a better aerodynamic match with coal dust and such that the polymer will be non-toxic. In the most preferred embodiments, it is envisaged that the particle size of the polymer in a dry state (i.e. before hydration) would be between about 50-200μηι, more preferably between about 50-100μπ), and more preferably between about 50-80μηι. In the most preferred embodiments, the particle size of the polymer in the hydrated state is between about 80-250μΓη, more preferably about 10Ο-180μΓϊΐ, and more preferably about 110-150μτη. It is anticipated that there will be a significant reduction in the amount of the polymer of the present invention required to prevent propagation of a coal dust explosion when compared to water and stone dust. It is expected that the requisite inerting ratio will be reduced from 5.6kg kg to around lkg/kg. That is, the requisite mass percentage of inert dust in the dust mixture will be significantly reduced from 85% to less than 50% if stone dust is replaced by the hydrated polymer. This reduction in the requisite mass of the inert dust per unit mass of coal dust increases the capability to achieve a ^• satisfactory inerting ratio. Moreover, as the polymer of the present invention has excellent water absorption and retention properties, it consists almost exclusively of water, and thus will possess the properties at least comparable to that of water. It is anticipated that there will be a significant reduction in surface transportation requirements as the dry polymer alone will be transported to the mine site for hydration.

It is anticipated that there will be significant reduction in the underground transportation requirements as it may be possible to delay hydration of the polymer until placement or dispersal.

It is also anticipated that there will be a significant reduction in the application rate if hydrated polymer is used as the inertant in Trickle Dusting and a reduction in the reapplication frequency is the hydrated polymer is used as the inertant in Broadcast Dusting This invention will also reduce disruption to mining operations as personnel may be permitted to work downwind whilst the polymer is distributed for the purposes of Broadcast Stone Dusting.

The hydrated polymer will also be able to substitute stone dust or water in explosion barriers possibly decreasing the overall loading of the inert material. This could also significantly increase the effectiveness in suppressing a coal dust explosion.

The commercial advantage of the current invention is that it reduces the overall cost of inert material, transportation and placement associated with current explosion suppression procedures. This is because there will be a lower overall cost per mass unit of requisite inert dust and the - requisite frequency of inert dust re-application is reduced.

In particular, there will be a reduction in operating costs by reducing or eliminating production stoppages associated with Broadcast Dusting or distribution of prior art particulate flame retardants. Further, there will be a reduction in downtime by the eliminating of production stoppages required to remedy inadequate prior art stone dust application in underground roadways.

EXAMPLES

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the polymers described and claimed herein can potentially be made and evaluated, and are intended to be purely exemplary and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. There are numerous variations and combinations of reaction conditions, e.g., component concentrations, desired solvents, solvent mixtures, temperatures, pressures and other reaction ranges and conditions that can be used to optimize the products hydration characteristics from the described process. Only reasonable and routine experimentation will be required to optimize such process conditions.

Example 1: Investigation of Hydrogel Polymerisation Conditions and Parameters:

In order to carry out an investigation as to the hydrogel polymerization conditions, solution polymerization was carried out. During the investigation the monomer ratios were adjusted from 30- 50%, the cross-linker concentration was adjusted from 3-5mM, and also adjusting the initiator concentrations. In addition, methacrylic acid was tested as a replacement for acrylic acid. The initial investigation was carried out at room temperature and using the Mettler-Toledo ultimax ART1250 platform. The goal of the study was to prepare a water-swollen hydrogel that would be suitable for the replacement of the stone dust commonly used to suppress coal dust explosions, the hydrogel preferably having the following properties: a. Glassy polymer/ hard hydrogel that can be ground to size

b. Minimum water uptake of 50g of water/g of polymer - preferably above 100 g of water/g of polymer

c. Must maintain a rigid structure once hydrated (a slimy gel will be problematic for application usage)

-Effect of Acrylamide Content:

Balancing the ratio of acrvlamide to acid will likely alter the structure of the gel and how it can handle both acidic and basic water supplies. In this regard, the results shown in Figure 1 show the effect of the acrvlamide ratio in the hydrogels on the swelling ratio.

As was observed from the water absorption curves for the samples shown displayed in Figure 1, it is suggested that within the first two days, the samples are within 70% of their final weight. While this study was carried out on large particulate sized polymer samples, it is anticipated that with the grinding of the samples to a micron sized powder that this reported time to maximum absorption should be significantly quicker due to an increase in interfacial surface area. As is further observed from Figure 2, as the acrvlamide content was increased, the equilibrium water absorption is also increased, but is was also visually observed that with an increased acrvlamide content the hydrogel lost its rigid structure and become a gel-like substance. A gel-like substance would not be overly desirable from the point of handling.

-Effect of Acrylic Acid vs. Methacrylic Acid:

Methacrylic acid is known to have a high glass transition temperature, and thus was postulated to increase the structural integrity of the resultant hydrogel structure. However, as it can be also observed from Figure 1 it was found that the amount of weight reached over a week is only half of the initial target. Visually, however, the samples that contained methacrylic acid had a better structural integrity and were less likely to break-down over time, as was evidenced by the large error bars for the acrylic acid system compared to the methacrylic acid. That said, by varying the ratio or methacrylic acid to acrylic acid, obtaining a equilibrium water absorption above the target and obtaining a more rigid hydrogel sample could be obtained.

-Effect of the Concentration of the Cross-Linking Agent:

The function of the cross-linking agent was used to enable the hydrogel sample to form a well- structured gel. In fact, it should be understood that without the cross-linker, the inventors postulate that the sample would likely resemble a viscous liquid. On the other hand, having too much cross- linking agent would lead to a decrease in water uptake, as when there are a greater presence cross- linked points in a gel, it cannot swell as much before the network breaks down. In any event, as can be observed from Figure 3, which shows the effect of changing the cross-linker concentration on the swelling ratio, there a trend between O.023mol% up to 0.032mol with a decrease in swelling ratio with increased cross-linker appeared.

-Effect of the Concentration of Initiator:

The inventors postulated that the concentration of the initiator used in the polymerization process of the final polymer would have little effect on its swelling properties. The results of the effect of the initiator concentration can be observed in Figure 4, where it was confirmed that there is no real effect on the initiator concentration and thus not deemed to be overly critical. Example 2: Scale Up Usins Solution Polymerization or Inverse Dispersion/Suspension

Polymerization Techniques

2.1. Solution Polymerization:

Subsequent to carrying out the investigation described in Example 1, a study was then carried out to determine optimized reaction conditions to scale-up so that a larger sample size could be obtained as the initial studies were carried out using only 8g of polymer. In the present investigation and based on the parameters tested in Example 1, the following reaction conditions were used and scale-up was performed on the basis of giving suitable equilibrium water swelling and a rigid gel: a. 1:1 wt:wt acrylic acid.methacrylic acid, 45 mol% acrylamide, 4mM KPS (Potassium Persulfote), 7mM MBA (Methyl bis-acrylamide) solution polymerization in water where monomer content is 40wt%; and

b. 45 mol% acrylamide, 4mM KPS, 7mM MBA solution polymerization in water where monomer content is 40wt%.

Using a 4-neck 500mL glass reactor with an overhead stirrer and a semi-circle-shaped impellor the reaction was carried out at room temperature. The reactor was charged with all the ingredients excluding the acid-based monomers and the initiator solutions. A 50:50 aqueous: acid solution was then prepared and the acrylic acid and methacrylic acid were partially neutralized (pH=4.5) with concentrated NaOH over an ice-bath. The resulting monomer solution was charged to the reactor and the initiator was then added, where the reaction proceeded under a nitrogen purge and the stirrer speed was kept at 300rpm. The reaction was left overnight. The following morning, the resultant samples were gels that occupied the entirety of the reactor. The sample was removed from the reactor and then sliced into a smaller size (l-2cm sub-samples) and dried in a vacuum oven at 50°C until a constant weight was obtained. Ideally the goal of this study was to obtain a sample that was in a powder form. ft was found that after vacuum drying the samples resembled small hard rocks that could not be appropriately handled in most milling applications. The sample was then broken down to size roughly with a hammer after being cryogenically frozen. The small lumps were then milled under the following conditions: i) dried in a ball-mill (spex mill) using 1mm zirconia media - the result was a charred material; ii) dispersed in ethanol and ball-milled (spex mill) using 1mm zirconia media - the result was a small reduction in size after several hours (~5 hrs) of milling. However, only a small amount of sample (lg per batch) could be run per batch; iii) using a hammer-type mill - a reduction in size was obtained, however a charred material was also obtained.

iv) using a freezer mill - no charring of the material occurred by neither did any appreciable size reduction.

Following the above protocols, the milled samples were then hydrated to check its water absorption properties and suitability for the intended application. However, following this step, it was found that the sample tended to clump together severely and took the form of the polymer before milling. This lack of success in milling and the product achieved during scale-up led the present inventors to carry out an alternative method of producing the hydrogel, that is, by using inverse dispersion or suspension polymerization as described below in further detail. 2.2 Inverse Dispersion or Suspension Polymerization:

Inverse dispersion or suspension polymerization was then carried out, where the monomer, initiator and cross-linker (as highlighted above in 2.1) were dispersed in water and then added on a dropwise manner to a solution of dispersant in an immiscible solvent, such as cyclohexane or n-heptane. An oil-soluble dispersant was added to promote emulsion stability and this dispersant requires to have a

HLB (hydrophilic-lipophilic balance) in the range of 3-9. The benefit of using this polymerization technique is that the hydrogel was polymerized as discrete particles, which have some steric repulsion due to the dispersant present and thus fewer tendencies to clump. This led to less milling or effort is required to form a fine dust. The basic reaction parameters that were used in carrying out Inverse dispersion or suspension polymerization were as follows:

Ingredient

n~fwpbme 27.2ο.

scfbitan manostearate (span 60) 0.32g

aoySc acitf (neutralized to pH -5) 3.2g

water 9.6g

pota¾¾ium pcrwjeotc D.032g

BIS 0.032g

ifivfeiyfuei i2&rw og

total 40.384g

O W ratio 2.83

(inuxlur % 1%

Bt5% 1«

TaM« 2 - F^xmuLnon at mmrt wsptraion pol mcrizjlbn readion. Mote: aR pM en-tges «m based on monomer contert.

NOTE: All Percentages are based on monomer content.

Following carrying out Inverse dispersion polymerization, it was found that the resultant material showed considerable improvements in terms of its ability to reduce the particle size with milling or grinding. As such, to further optimize the process of inverse dispersion or suspension polymerization a further investigation was carried out based on the protocols described in Mayoux et at. (1999), "Inverse suspension polymerization of sodium acrylate: synthesis and characterisation". In this regard, two formulations were prepared, one of such being cross-linked sodium polyacrylate (SP-BIS) and the other cross-linked sodium poly(acrylate-co-acrylamide) (SP-A-BIS). The quantities of reagents were chosen as follows and obtained from Sigma Aldrich:

The acrylic acid, with or without acrylamide, was mixed with water to a final volume of 10ml. The pH was adjusted to 4.5 with NaOH and the cross-linker and initiator were added. The organic phase was prepared by dissolving 2 g of the dispersant Span-80 in 100 ml of π-heptane and oxygen was removed by nitrogen gaseous exchange. The resulting solution was then heated to 65 °C.

The polymer solution was added dropwise to the organic phase while stirring at lOOOrpm under a nitrogen atmosphere. The reaction proceeded for 1-2 hours, after which time a biphasic solution was observed. The top layer contained the organic phase and a viscous layer containing the hydrogel beads was underneath. Both phases of the reaction mixture were added slowly to a beaker containing approximately two volumes of methanol to remove the water from the polymer beads. A fine white powder was achieved, where the powder was filtered and washed with methanol and dried in a vacuum oven overnight. Example 3: Characterization of Particle Size of the Polymer Obtained by Inverse Dispersion or Suspension Polymerization:

The resultant polymers of formulations SP-BIS and SP-A-BIS prepared by the methods described in Example 2.2 above, were then investigated in relation to their particle sizes by way of using a Malvern Mastersizer 2000. The particles were measured in propan-2-ol to characterise their dehydrated size and in water to assess their hydrated size.

The dry SP-BIS sample showed a diameter of the single particles to be around 70-80μιη, which was concluded due to a main peak on the data points from the Malvern Mastersizer 2000. In the hydrated state only a single peak was observed, which indicated that the diameter of the particles increased to 138μπι in the hydrated state. Following the examination of the results obtained for SP-A-BIS, it was found that the addition of acrylamide results in a reduction of the particle size. In fact, in contrast to SP-BIS, in the dry state the SP-A-BIS had a diameter of approximately 56μιη and in the hydrated state they had a diameter of approximately 119μτη.

Example 4: Water Uptake Investigation of SP-BIS and SP-A-BIS:

Following on from the above investigations, a study in relation to the water uptake of the polymers SP-BIS and SP-A-BIS prepared in accordance with the findings of Example 2.2 was performed by adding a known amount of water, 30mg, to a constant amount of each polymer. It was found that both SP-BIS and SP-A-BIS could take up at least 300 to 400 times their weight in water before free water could be observed. This was a dramatic increase when compared to a water uptake study in relation to a commercially available sodium polyacrylate power, which only retained 190 times their weight in water before reaching a saturation point. Example 5: Coal Dust Explosion Suppression Testing

5.1 - Protocols:

Two polymeric products SP-BIS and SP-A-BIS obtained by the optimized Inverse suspension polymerization protocols explained above in relation Example 2.2 were used to carry out a test to determine their effectiveness in suppressing a coal dust explosion. Each of the polymer compositions were hydrated to saturation point (i.e. about 400:1 hydration).

The coal dust explosion suppression testing was undertaken using the 20-litre explosion chamber known as the SIWEK Chamber, which is readily available. Results obtained from the SIWEK Chamber were then correlated to the values, which would be obtained if the same tests were done in the ISO Standard lm ³ chamber. Each test was carried out using the recommended test method by the manufacturer of the SIWEK Chamber, including the use of two chemical igniters as an ignition source, with a combined energy of lOkJ.

Using the SIWEK Chamber and associated control equipment allowed a pre-weighed amount of dust to be injected, via a dispersion nozzle, and ensured that ignition of the dust cloud occurred at the optimum condition. Partial evacuation of the chamber prior to ignition of dust ensured that ignition occurred at. atmospheric pressure. Two pressure transducers fitted to the side of the chamber monitored the pressure produced during combustion, with the signal from each transducer routed through a charge amplifier to the computer. The pressure time curve for each test was recorded and stored in memory, analysed to provide information on maximum pressure and rate of pressure rise. Critical operating parameters for the proper operation of the chamber were also measured and displayed for each test. The testing in the 20-litre SIWEK Chamber was conducted by spraying the inert product onto the coal dust and then intimately mixing it by shaking such in a plastic container prior to placing it in the dispersing chamber prior to injection into the sphere. The amount of coal dust used in each test was Sg, which was intended to be sufficient to achieve a strong explosion at 250gm ³ during each ignition. The corresponding quantity of inert product required is shown in the following table.

Coal Coal 20% 22.5% 25% 27.5% 30% 32.5% 35% 37.5% 40% 45% 50% g/m3 (9) (g) (9) (9) (g) (g) (g) (g) (g) (g) (g) (g)

250 5 1.25 1.45 1.67 1.90 2.14 2.41 2.69 3.00 3.33 4.09 5.00

5.2 - Results:

Table 1: Results of Coal Dust Explosion Suppression Test (SP-A-BIS):

(series) Cone. Pm dP/dt tl tv eff Inertant Inertant . Comment Test [bar] [bar/s] [ms] [ms] [g] [%]

(1) 2 250 .0 0 79 60 1.5 23 Non Ignition

(1) 3 250 .1 29 105 60 1.5 23 Non Ignition

(1) 4 250 .0 0 98 60 1.6 24.24 Non Ignition

(1) 5 250 5.0 44 130 60 1.6 24.24 Ignition

(1) 6 250 .0 0 84 60 2.2 30.56 Non Ignition

(1) 7 250 .0 0 92 60 2.3 31.5 Non Ignition

(1) 8 250 .1 0 131 60 2.3 31.5 Non Ignition

Table 2: Results of Coal Dust Explosion Suppression Test (SP-BIS):

(series) Cone. Pm dP/dt tl tv eff Inertant Inertant Comment

Test Ig/m3] [bar] [bar/s] rmsl tms] lg) [%)

(2 ) 3 250 6 . 8 139 71 61 .5 9 - 1 Ignition

( 2 ) 4 250 . 0 0 87 60 1 . 2 19 . 48 Non Ignition

(2 ) 5 250 . 0 0 134 60 1 - 3. 20. 76 Non Ignition

(2) 6 250 . 0 0 113 60 1 . 6 24 . 35 Non Ignition

Based on the results achieved in relation to the coal dust explosion suppression tests for SP-B-BIS shown in Table 1 (above) it can be observed that a 250g/m ³ coal dust explosion can be significantly suppressed with the addition of 31.5% of the inertant {i.e. SP-B-BIS). On the other hand, in the case of SP-BIS, the coal dust explosion was suppressed with the addition of 24.35% inert product. These results show that the present invention has effects that are clearly superior to the results achieved with stone dust commonly used to suppress these events, which generally requires around an 85% proportion of the total dust mixture mass to be effective when tested in the same manner.

It will of course be realised that while the foregoing has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is - herein set forth. Moreover, throughout the description and claims in this specification the word "comprise" and variations of that word such as "comprises" and "comprising", are not intended to exclude other additives, components, integers or steps.