Suppression of iron powder oxidation

Abstract

A method for supressing and preventing fires involving iron powder or granules in an iron storage facility, said method comprising positioning of sealed storage units containing carbon dioxide saturated zeolite powder, whose release is triggered by exposure to small amounts of heat within said iron storage facility.

Description

[0001] Suppression of iron powder oxidation

[0002] Technical Field

[0003] As the world becomes increasingly concerned about human carbon dioxide emissions, focus on steelmaking has become intense world-wide. The use of green hydrogen has been proposed as a more suitable method for iron ore reduction. However, the high cost of green hydrogen has made this process less viable. Other gaseous reductants have also been proposed, for example carbon monoxide as the basis for the direct reduction of iron (DRI).

[0004] Most DRI processes will be based on the direct reduction of finely particulate iron ore through gas / solid interfacial reactions, and this will lead to production of fine iron particles, either individually or combined in a larger, porous structure. The high surface area of this type of product presents significant problems for both storage and transport because of its susceptibility for uncontrolled chemical oxidation or spontaneous burning when exposed to normal atmospheric conditions. The present invention aims to address the problem of safely storing and transporting of fine iron particles.

[0005] Background

[0006] Almost all fire-retardant additives work by imposing a layer of less flammable materials onto a flammable surface. For example, some commercially available fire-retardant coatings will swell under heat to form a multicellular charred layer, which acts as an insulating barrier and slows heat and mass transfer between the condensed and vapor phases. Fire retardants stop the fire by interacting with the fire cycle in the gaseous phase and stop the chemical chain reaction. Fire retardant coatings look like architectural paints and are mainly available in solvent form and are applied by conventional methods, brush, roller, and spray. Although protection of materials against fire by the use of coatings for indefinite periods is impossible, it can delay the spread of fire or keep a structure intact against fire, thereby allowing sufficient time for safety measures to be taken.

[0007] Current industrial steel making is based on the high temperature (> 1200 °C) combustion of coal which reduces iron ore and produces huge amounts of carbon dioxide. And iron ore sintering is also required to produce a sinter cake for use in a blast furnace. This process is a further major source of carbon dioxide emissions, which the use of DRI technologies may reduce. However, the production of particulate iron poses serious issues associated with oxidation (i.e. burning) when exposed to the atmosphere.

[0008] The most common Class D metal fires are caused by aluminium, magnesium, and titanium. Therefore, these metals in dust form are extremely hazardous in dust collection systems. However, most metals in a fine dust form can ignite and produce dangerous situations in confined dust collector systems. The finer the dust, the more likely it is to burn and water is the worst thing to put on metal fires (for example, see: https; / / www^

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[0010]

[0011] Preventing the combustion of metal powders is crucial because many metals, especially in powdered form, can be highly reactive and combustible when exposed to air or other oxidizers. Some key strategies to prevent metal powder combustion include control of oxidising atmosphere, moisture control, storage in sealed containers, surface passivation and use of fire suppression systems.

[0012] However, none of the currently available methods are cost effective in supporting the large scale manufacture of iron using DRI processes based on the direct reduction of finely particulate iron ore through gas / solid interfacial reactions that lead to the production of fine iron particles. The present invention is mainly directed at a novel and efficient method for suppressing combustion of fine iron particles produced by such processes.

[0013] Summary of the invention

[0014] Iron dust fires occur when fine particles of iron, or iron dust, are dispersed into the air and ignite, often causing explosions. This is because the surface area of the iron particles is greatly increased, allowing them to react more quickly with oxygen in the air. For iron to combust or oxidise rapidly requires both oxygen and water, with the latter acting as a catalyst for the oxidation reaction. Hence, it is important to avoid any use of water in preventing fires involving iron particles.

[0015] The present invention has identified that zeolites can play a key role in preventing or controlling iron powder fires due to their unique structure and properties. Zeolites, being chemically stable and non-flammable, can act as an inert medium when mixed with iron powder. This reduces the overall flammability of the material mixture and could prevent the spread of fire in case of ignition. Zeolites are porous, crystalline materials that are widely used for adsorption and catalysis due to their unique structure. They have uniform pores that can selectively trap or adsorb specific molecules, including gases like carbon dioxide.

[0016] WO 2021 / 253090 discloses a method of using carbon dioxide saturated zeolite in, granular forms (see Figure 1), as a novel mechanism for using as a fire suppressant for bushfires. Surprisingly, the present invention has identified that zeolite 5A, saturated with carbon dioxide is an ideal fire suppressant for preventing combustion and oxidation of newly formed iron granules or particles.

[0017] Use of carbon dioxide is an efficient way of putting out and suppressing a fire. The present invention provides an efficient delivery mechanism for extinguishing or preventing fires involving iron particles. Also, the fire suppressant material disclosed in the present invention works at low temperatures, which contrasts with other fire suppressant materials in the prior art. Furthermore, this fire suppressant material is non-toxic, is readily available and is highly efficient at supressing a fire involving particles of iron in a short period of time.

[0018] In accordance with the present invention, the fire suppressant material for controlling or extinguishing a combustion process involving iron particles comprises zeolite particles with an internal porous structure, wherein molecules of a fire extinguishing or prevention substance are contained within the internal porous structure of the zeolite material. The fire suppressant material comprises zeolite 5A particles. The fire extinguishing or prevention substance used in the present invention is carbon dioxide. The fire suppressant material of the present invention releases significant amounts of carbon dioxide gas when even moderately heated. The fire extinguishing substance is released in the form of a gas thus reducing oxygen levels and extinguishing, or substantially extinguishing, or at least supressing the fire locally. The zeolite will also reduce water vapour levels which also inhibits the iron oxidation as water is an important catalyst for this chemical reaction.

[0019] According to the novel features of the present invention, fires involving iron powder or granules are prevented by using appropriately positioned sealed storage units containing carbon dioxide saturated zeolite powder, whose release is triggered by even moderately raised temperatures, within an iron storage facility. Because the zeolite needs to be stored for long periods of time, it is appropriate to use a metal box type storage with tightly sealed gaskets to prevent gas leakage, of the 1 atm CO2, before heat activation of the release cover, as illustrated in Figure 2. The released zeolite can be in the form of powder, large granules or even granules contained in a mesh net for easy re-collection, removal and re-activation.

[0020] Even powdered zeolite could be dispersed directly onto the stored iron powder, if required, because the large difference in density between iron and zeolite, of about 10 x, would facilitate simple density separation of the two materials.

[0021] In accordance with the present invention, appropriate zeolite powder equilibrated with 1 atm of carbon dioxide when released produces a carbon dioxide volume typically in the range of 100-200 times the volume of the zeolite powder, depending on temperature in the storage unit. Note that using pressurized, thick metal cylinders of gas would require large numbers at 150 - 200 atm pressurized containers, which would be expensive and heavy. In addition, the Joule-Thomson cooling effect produced by releasing high pressure CO2 requires continuous heating to prevent valves freezing CO2 release. The use of zeolite gas delivery offers a much simpler process.

[0022] The desorption of carbon dioxide from the zeolite is an endothermic process, which will also act to suppress the fire, but in a much more controllable cooling effect compared with the Joule-Thomson effect. Exclusion of available oxygen by this release and the absorption of water vapor by the zeolite will both act to suppress iron oxidation and prevent the formation of high temperature hot spots. Cylinders of high pressure CO2 would also not aid in the removal of water vapor.

[0023] Detailed description of the invention

[0024] Zeolites are common materials which are microporous, aluminosilicate crystalline minerals widely used as commercial adsorbents and catalysts, which have many different structural forms, and occur both naturally and through manufacturing. A wide range of zeolites are available, and some zeolites strongly absorb molecular carbon dioxide into their nanopores which when released by heat can produce greater volumes of carbon dioxide gas which will locally extinguish fire.

[0025] These materials all offer good insulation properties: such as low thermal conductivity of zeolite depends on temperature and pressure, adsorbed gases, and the saturation percentage. At P =1 bar, saturated with carbon dioxide, the zeolite thermal conductivity = 0.145 [W / (mK)]. This means that the average thermal conductivity of zeolite 5A is 2.75 to 4.8 (3.78 average) times less than the thermal conductivity of insulation bricks.

[0026] In 1998 N ASA developed and studied the properties of a common zeolite (5 A) to absorb CO2 from the cabin of hum an -manned spacecraft at normal ambient conditions and then heated to say 70 °C to emit the absorbed CO2 and so regenerate the absorbent. (Ref: N ASA / TM-1998-208752. ' Carbon Dioxide Adsorption on a 5A Zeolite Designed for CO2 Removal in Spacecraft Cabins.”) Typical data for different pressures and temperatures are shown in Figure 3.

[0027] How the nano-pores in the zeolite will suppress fires, can be illustrated using this data. For example, the cool zeolite (at a density of about 0.7 g / mL) will absorb about 15 g of CO? at 1 atm per 100 g of zeolite, or 140 ml of zeolite. The volume of this released CO2 will be roughly 100 - 200 times the volume of the zeolite, especially as the local temperature increases further. This is equivalent to a heavy metal gas cylinder containing compressed CO2 at between 150 - 200 atm, which would be much more costly and less practical.

[0028] The advantages of using CO2 saturated zeolite materials are the potential for heat driven release and expansion of CO2 to create a barrier for flame propagation and produce fire suppression. The ability to release large amounts of CO2 in a short amount of time and at relatively low temperatures not only could retard fire propagation but also extinguish it. In addition, the exhausted material has the ability to reabsorb CO2 back from the surrounding air, as it returns to lower ambient temperatures.

[0029] The advantages of the present invention are many, for example; the method completely avoids the use of water to extinguish a fire (water is not particularly efficient at extinguishing fire and is specifically not recommended for metal powder fires); because no water is used, there can be no water catalysis or water damage; also the fire suppressant material of the invention works at low temperatures, which is in contrast to other fire suppressant materials in the prior art. Furthermore, this fire suppressant material of the invention is non-toxic, is readily available, completely reusable and can be used in a number of applications, and is highly efficient at supressing a fire in a short period of time.

[0030] The application proposed here is based on the use of CO2 saturated zeolite to suppress hot spots of oxidation in stored, particulate iron, via the exclusion of oxygen combined with the cooling effect of CO2 emission and the absorption of water vapour. Iron can only oxidise in the presence of oxygen and with water as a catalyst. The release of the zeolite material as the temperature starts to increase, to even modest levels, say around 100 C, will suppress iron oxidation.

[0031] This protection from oxidation will be of greater importance for most DRI iron, which will generally have higher surface area products with lower particle sizes, such as from nanometres to particles typically in the range of 0.05 mm to 0.15 mm, compared with much larger pig iron ingots of between 3 kg and 50 kg, and even granulated pig iron, with sizes of 5-25 mm.

[0032] Disclosed is a fire suppressant material for controlling or extinguishing a combustion process associated with particulate iron exposed to atmospheric gases, the fire suppressant material comprising zeolite particles with an internal porous structure, wherein molecules of CO2 are released at modest temperatures to extinguish iron particle fires. The fire suppressant material comprises zeolite 5A particles, and, if required, mixed with some components of other zeolites specifically for water vapour absorption, to improve water vapour removal from the stored iron. The pores of the internal porous structures of the zeolite particles are substantially consistent in size and shape. This ensures that the selectivity of the pores is maintained to the desired molecules of the fire extinguishing substance CO2 and for efficient water vapour absorption.

[0033] The fire suppressant material may be configured or adapted to activate and release the molecules of the fire extinguishing substance upon absorption or exposure to the heat generated in the combustion process. This heat absorption process can deprive the combustion process of the heat required to sustain itself, effectively acting as a heat sink.

[0034] The long-term storage of CO2 equilibrated at about 1 atm with zeolite 5 A, at up to about 17 % CO2 by weight offers a huge reservoir storage system in a useful product, which could store, almost permanently, many megatons of CO2. After the fire is extinguished the material can be easily collected, leaving little or no damage, and the material can be easily recycled. The material is safe and environmentally acceptable. For example, it has been used to filter breathable air in the International Space Station. There are also environmentally acceptable synthesis routes to produce this zeolite. This fire suppressant material may be configured or adapted such that its activation upon absorption of heat results in a release of a volume of CO2 gas that is 100 - 200 times greater than the volume of the fire suppressant material (i.e., zeolite). The release of such a large volume of fire extinguishing substance (in the form of fire extinguishing gas) on absorption of heat has multiple effects in absorbing heat and blocking access to oxygen which acts to extinguish a fire locally.

[0035] Activation of the fire suppressant material to release the fire extinguishing substance can occur at a relatively low temperature range of up to 80 °C. In some forms, that activation may occur at any temperature above 80 °C, and in some forms, activation may occur at or below 200 or 300 °C. It will be appreciated that the fire extinguishing substance can be delivered or substantially delivered at a predetermined temperature, or temperature range, and the fire suppressant it delivers. This method offers the possibility of delivering a fire extinguishing substance in a staged extinguishing method, whereby a specific amount of fire extinguishing substance can be released from the fire suppressant material at a predetermined temperature (or temperature range), and a second or third etc release of the substance at different predetermined temperatures (or temperature range) which can be used to suppress iron fires for both storage and transport.

[0036] In one example, a long, hollow, thin metal cylinder sealed at both ends as shown in figure 2 is fixed along the inside roof of a sealed container. The cylinder opens completely along the bottom half but is joined using a metal spring clip and tightly sealed using a rubber seal (which can hold gas for very long periods, e.g. car tyres). The spring clip opens by expansion of the metal spring clip at a certain temperature, say 100 °C.

[0037] The cylinder is filled with, say, zeolite beads saturated with CO2 (15%) at a pressure of 1.5 bar (so that any gas leakage can be readily detected). The cylinder can be fitted with a pressure gauge and a one-way valve - so that it can be checked regularly and, if needed, repressurised.

[0038] In summary, a method is disclosed aimed at preventing an exothermic oxidative reaction which can occur by exposure of finely particulate iron to both oxygen and water vapour. Zeolite infused with carbon dioxide is passively triggered to release the gas by any source of modest heat, even at a low temperature of 100 °C. The aim of the low temperature release of the zeolite is to prevent iron oxidation from generating high temperatures and even local hot spots. No further automation is required, other than the thermally controlled release of clouds of carbon dioxide to suppress a fire. Released zeolite produces excess CO2 and also absorbs water vapour to limit and suppress an iron fire, since it both reduces O2 exposure and also reduces access to the catalytic effect of water.

[0039] Release of large volumes of CO2 on supplying even modest heat to the CO2 saturated zeolite 5A, i.e. as the Fe starts to oxidise, will displace O2 but also the empty zeolite pores will absorb water vapour, removing the catalyst.

[0040] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not by way of limitation. It will be apparent to a person skilled in the relevant art that various changes in form and details can be made therein to suit different situations without departing from the spirit and scope of the present invention. Thus, the present invention should not be limited by any of the abovedescribed exemplary embodiments.

Claims

Claims defining the invention1. A method for supressing and preventing fires involving iron powder or granules in an iron storage facility, said method comprising positioning of sealed storage units containing carbon dioxide saturated zeolite powder, whose release is triggered by exposure to small amounts of heat within said iron storage facility.

2. A method as defined in claim 1 wherein the saturated zeolite powder is made of zeolite 5 A.

3. A method as defined in claim 2 wherein zeolite powder is equilibrated with one atmosphere (or more) of carbon dioxide and when released produces a carbon dioxide volume in the range of 100-200 times the volume of the zeolite powder.

4. A method as defined in claim 3 wherein the carbon dioxide saturated zeolite 5 A is configured to activate and release large volumes of carbon dioxide on exposure to small amounts of heat generated as the iron particles start to oxidise, continuously replacing oxygen in the immediate environment and allowing empty zeolite pores to absorb water vapour thereby removing the catalytic water vapour in the environment and preventing any further oxidation of iron particles.

5. A method as defined in claim 4 wherein the iron granules or powder is formed by the direct reduction of iron ore powder, even down to nanometres in size, which gives high surface area products, often in the particle size range of 0.05 mm to 0.15 mm.

6. A method as defined in claim 5 where in the said storage unit containing carbon dioxide saturated zeolite powder is a long hollow thin metal cylinder sealed at both ends which is fixed along the inside roof of the iron powder storage facility, in which the metal cylinder opens completely along the bottom half but is joined using a metal spring clip and tightly sealed using a rubber seal which can hold gas for very long periods where the spring clip opens by expansion of the metal spring clip on exposure to temperatures of 80°C to 100°C, or higher.

7. A method as defined in claim 6 wherein the metal cylinder is filled with zeolite beads saturated with CO2 at about 15% by weight at a pressure of, say, 1.5 bar and the said metal cylinder is fitted with a pressure gauge and a one-way valve.

8. An apparatus for supressing and preventing fires involving iron powder or granules in an iron storage facility, said apparatus comprising long hollow thin metal cylinder, filled with zeolite beads saturated with carbon dioxide, sealed at both ends which is fixed along the inside roof of the said iron powder storage facility, in which the metal cylinder opens completely along the bottom half but is joined using a metal spring clip and tightly sealed using a rubber seal which can hold gas for very long periods where the spring clip opens by expansion of the metal spring clip on exposure to temperature of 80 °C to 100 °C or higher.

9. An apparatus as defined in claim 8 wherein the zeolite beads are saturated with CO2 at about 15% by weight at a pressure of about 1.5 bar and the said metal cylinder is fitted with a pressure gauge and a one-way valve.

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