Field of the Invention

The present invention relates to a method and composition for suppressing dust emissions from dust containing materials. More particularly, the method and composition of the present invention is intended to suppress dust emissions while minimising water addition and consumption.

Background Art

Dust emissions are problematic to many industries, in particular the mining industry, due to environmental and occupational health and safety concerns. In Australia, for example, mining companies are required by legislation to monitor dust emissions and they may suffer heavy penalties if they exceed acceptable limits. Dust suppression becomes increasingly difficult for mine-sites that are required to handle, dust containing materials, for example ores, having a high content of fine and/or clay particles ("fines"). Fines can occur naturally in ores and earth materials, or they can be generated by mechanical processes, such as crushing.

As a result, a number of methods have been developed in order to attempt to control dust emissions. To date, these methods have focused on reducing the surface area of the ore material by applying crusting agents or additives to hold the fines in place.

A first method for suppressing dust is simply the process of adding water to the ore material at selected points in the ore handling process. Whilst arguably the most simple method, this has some significant drawbacks, including high evaporation rates resulting in high water consumption, and increased transport costs due to the increased weight of the "wet" ore material. Furthermore, moisture distribution is often irregular with some parts of the ore becoming quite wet while other parts remain dry. This in turn increases handling costs due to intensive mechanical mixing required to achieve some uniformity of the moisture content.

Another method includes the use of foams, which are sprayed onto the ore material to form a blanket. This method experiences lower evaporation rates than simply adding water. However, this method also involves the addition of potential contaminants which can interfere with downstream processes. Again, these foams also add significant weight to the ore material and therefore increase transport costs.

Finally, polymers may be employed to effectively bind fine particles together and reduce formation of dust. Again, this is often undesirable due to potential interference with downstream processing methods and the polymeric type suppressants can also be significantly more costly when compared to other methods.

All of these methods involve addition of substantial quantities of water to the ore material, which is also undesirable for ores that undergo treatment in furnaces (for example, iron ore). It is therefore desirable that such ores remain as dry as possible to reduce the time required for the ore material to be in the furnace.

One known process utilises de-sugared, concentrated molasses solids (a sticky waste residue from various molasses treatment methods) as a binder to bind fine particles to larger particles. This was found to have dust suppression effects without the need for the addition of polymers. This dust suppressant formulation was found to be effective at additions of between approximately 9kg to 45kg per ton of ore. The disadvantage of this method is that the binder results in the particulate material forming large particles and clusters, which can reduce the flow characteristics of particulate material and cause blockages in some plant designs. That is, the de-sugared molasses solids are acting as a physical binder to coagulate the particles.

The moisture content at which it is believed dust is no longer produced is termed the Dust Extinction Moisture ("DEM"). However, ores having a high fines content may have a high DEM moisture level that requires significant addition of moisture to the ore to reach DEM. For example, fine grade ore mined at 4% moisture may have a DEM of 9% requiring additional 5% of moisture weight added to reach DEM. The viability to add this volume of water in many cases is unobtainable. Financial penalties may be imposed by customers of target DEM are not met.

It is believed that the large surface areas of ores having high levels of fines, is a factor in the ability of water to reduce dust formation. Additionally, clay or fine particles tend to have a negative surface charge. Water's positive surface charge, together with surface tension, causes water droplets to hold onto a "clump" of fines, rather than resulting in the "wetting" of fine particles. This "clumping" also reduces the "flow characteristics" of ore material and can cause blockages in plants.

The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

Throughout the specification, it is to be understood that dust containing material in which at least 5% of particles have a Pso passing 75μm are considered to have a "high" fines content.

Throughout the specification and claims, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

In accordance with the present invention there is provided a dust suppressant composition comprising at least one organic acid, at least one alcohol and water. - A -

More preferably, the organic acid is a carboxylic acid, including lactic acid or citric acid, or a blend thereof.

Still further preferably, the at least one organic acid is a complex mix of organic acids produced in the fermentation of molasses.

Preferably, the acid content of the dust suppressant is about 80% (w/w).

Preferably, the acid contains salt cations, including magnesium, calcium or sodium, or a combination thereof.

Preferably, the alcohol includes ether alcohol.

More preferably, the alcohol is an alcohol derived from the fermentation of molasses or sugar.

The dust suppressant may contain a surfactant.

Preferably, the surfactant is any one or more of alcohol ethoxylate, alcohol alkoxylate, sodium lauryl ether sulphate and/or ether glycol.

The surfactant concentration of the dust suppressant is preferably within the range of about 5% to 40% (w/w).

More preferably, the surfactant concentration of the dust suppressant is within the range of about 10% to 20% (w/w).

Preferably, the dust suppressant has a pH of between about 3 and 4.8.

More preferably, the dust suppressant has a pH of between about 3.7 and 4.2.

In accordance with the present invention there is further provided a method for dust suppression of dust-containing material comprising the steps of adding a dust suppressant composition to a dust containing material wherein the dust suppressant contains a mixture of at least one organic acid, at least one alcohol and water

In accordance with the present invention there is provided a further method for suppressing dust comprising:

i) adding a dust suppressant composition having at least one organic acid, at least one alcohol and water to a dust-containing material prior to processing in a plant;

ii) adding a dust suppressant composition containing at least one organic acid, at least one alcohol, water and surfactant to a dust- containing material after processing in a plant

wherein the dust suppressant composition of step i) is substantially free of surfactant.

In accordance with the present invention there is provided a still further method for reducing the dust extinction moisture (DEM) level of a dust containing material, comprising the step of adding a dust suppressant composition to a dust containing material wherein the dust suppressant composition contains a mixture of at least one organic acid, at least one alcohol and water.

The amount of dust suppressant added is about 1 L dust suppressant to 5 ton of dust containing material.

More preferably, the amount of dust suppressant added is about 1 L of dust suppressant to 15 ton of dust containing material.

Prior to addition to the dust containing material, the dust suppressant is preferably mixed with water, to produce a diluted dust suppressant composition.

More preferably, the ratio of dust suppressant to water in the diluted dust suppressant composition is about 1 L dust suppressant to 1OL water. Altematively, where the moisture content of the dust containing material is at DEM or above DEM, the dust suppressant composition may be added without dilution.

Preferably, the dust containing material comprises one or more of an ore material or soil material.

More preferably, the ore material is iron ore.

The dust suppressant composition is preferably added to the dust containing material as early as possible after excavation and/or blasting. The dust suppressant composition may be added at multiple handling points of the dust containing material. In mining operations, the dust suppressant composition may be added at one or more points, including in the pit after blasting, in crushing plants, in transfer shoots, or any point at which the ore material is subjected to mechanical action such as lift, mix or fall.

The dust suppressant composition is most preferably added by spraying onto the dust containing material. Alternatively, it can be poured onto the dust containing material directly ("flooding").

In one form of the present invention, the method further comprises first determining the DEM of the dust containing material.

In accordance with the present invention, where the at least one organic acid in the dust suppressant composition is derived from the fermentation of molasses, the method for preparing the dust suppressant composition comprises the steps of:

i) forming a mixture of water, molasses, sugar, alkali metal salt and a yeast nutrient;

ii) adding yeast to the mixture of step i);

iϋ) adding at least one organic acid once the pH reaches a predetermined limit; and i) fortifying the fermented mixture of step iii) for a pre-determined residence time

Preferably, the alkali metal salt is in the form of a magnesium salt.

Magnesium salt is preferably in the form of magnesium sulphate.

The temperature of the water is preferably within the range of about 25 ⁰ C and 35 ⁰ C.

Preferably, the sugar is in the form of one or more of white sugar or raw sugar.

The yeast nutrient is preferably urea.

The pH of the mixture is preferably within the range of 3 and 4.8 when the at least one organic acid is added.

More preferably, the pH of the mixture is within the range of 3.7 and 4.2.

The at least one organic acid preferably includes but is not limited to any one of lactic acid or citric acid, or a mixture thereof.

Preferably, the residence time is within the range of about 1 and 30 days.

Where a surfactant is required to be added to the composition, the surfactant is preferably added after the completion of the residence time.

Preferably, the surfactant includes but is not limited to any one of alcohol ethoxylate, alcohol alkoxylate, sodium lauryl ether sulphate and/or ether glycol, or a combination thereof.

Best Mode(s) for Carrying Out the Invention In accordance with the present invention, it has been discovered that dust can be effectively suppressed using a composition comprising at least one organic acid, at least one alcohol and water.

The dust suppressant composition comprises at least about 80% organic acid. The at least one organic acid is in the form of one or more carboxylic acids, for example lactic acid or citric acid, or a combination thereof. The at least one organic acid can also be in the form of a complex mix of organic acids, including but not limited to, formic acid, acetic acid, oxalic acid, tartaric acid, lactic acid and citric acid. The complex mix of organic acids is derived from the fermentation of molasses.

The benefit of using acids generated from the fermentation of molasses is that molasses contains a variety of minerals, in addition to being rich in calcium, magnesium, potassium and iron. It is believed that these compounds result in the formation of a variety of organic acid salts, which in turn aid cation exchange. However, it is understood that any organic acid would also produce a favourable result.

The alcohol may be ether alcohol, or alcohols generated from the fermentation of sugar, for example, molasses.

The dust suppressant composition is best maintained at a pH within the range of about 3 to 4.8, for example, about 3.7 and 4.2. At pH below 3 the acid becomes too strong and tends to increase the friability of the soil. Above pH 4.8, cation exchange within the crystal lattice of the soil particles is inhibited, minimising the beneficial effect of the dust suppressant.

The dust suppressant composition can also contain a surfactant in concentrations of within about 5% and 40% (w/w), such as within the range of about 10% to 20% (w/w). Suitable surfactants include, but are not limited to, alcohol ethyloxylate, alcohol alkoxylate, sodium lauryl ether sulphate and either glycol, or combinations thereof. It is understood that surfactants may assist in stabilising the composition and assisting in altering the water surface tension, thereby effecting improved distribution throughout the dust-containing material. However, it is not essential to have a surfactant in the composition, and the Applicant has concluded that the inclusion of surfactant in the dust suppressant composition will be dependent on the point at which the dust suppressant composition is to be added to the dust containing material for reasons which are discussed later.

The dust suppressant composition of the present invention facilitates, in particular, the reduction of the dust extinction moisture (DEM) of a dust containing material, thereby substantially minimising the quantity of moisture which is required to be added to the dust containing material for effective dust suppression.

A dust containing material, for example a soil and/or an ore, or specifically, iron ore, has a DEM which can be determined by known methods. The DEM will change depending on the characteristics of the dust containing material. Iron ore in particular can contain a high level of fine and/or superfine particles, which when dry, can result in significant dust emissions.

Without wishing to be bound by theory, it is believed that the reduction of the DEM involves the organic acid working to release water bound in the crystal lattice of a soil particle.

As previously described, the organic acid present in the dust suppressant can be obtained from the fermentation of molasses. These organic acids will often be an ionised (salt) form due the presence of cations in solution (for example Na ⁺ , Mg ²⁺ , Ca ²⁺ ). Upon reaction with a soil particle these cations can react with hydroxyl groups that form part of the silicate crystal lattice of the soil particle. Once this occurs, protons present in solution can react with the hydroxyl group to form water. The cation (for example, Mg ²⁺ ) is released from the lattice leaving a negatively charged silicon- oxygen system. Protons then react with this to form a weak monobasic acid, which is ultimately released from the lattice as a silicate acid. This then leaves another hydroxyl group at the surface of the lattice for reaction with the organic acid. The formation of water from hydroxyl groups in the crystal lattice is understood to form a water layer around the surface of the soil particle. As this occurs with individual soil particles, the distribution of moisture to soil particles throughout the dust containing material is much more uniform. By releasing water already contained in the soil particles, into a form which is able to be effective for "wetting" the dust containing material, the amount of moisture which needs to be added to the dust containing material to dampen it and reduce dust formation, is reduced. That is, the DEM is reduced.

The dust suppressant can be used in its most concentrated form. Alternatively, it can be diluted further in water prior to addition to the dust containing material. If an ore is very dry (natural moisture content of 3% or less, the dust suppressant is best diluted). Significant reductions in dust emissions can be achieved by adding very small quantities of the dust suppressant. As little as 1 L in 15 tons of dust containing material can provide good results, although as much as 1L per 5 tons of dust containing material can be used. The amount of dust suppressant added will depend on how significantly downstream processes are likely to be affected.

The benefits of the dust suppression addition have been known to last from 2 to 6 weeks, without the requirement for further additions, when exposed to climatic conditions. In sealed canisters, the benefits have been known to last up to 18 months.

The method for suppressing dust in a dust containing material involves the initial step of determining the DEM of the dust containing material. Once the DEM for the dust containing material has been determined, the dust suppressant can be added, for example by spraying, or it can be added by flooding and allowing the handling, such as digging and dumping, of the dust containing material to facilitate distribution. The dust suppressant can be added at the point of excavation and/or blasting, or it may be added in crushing plants, transfer shoots or at any point at which the ore material is subjected to mechanical action (lift, mix or fall).

The dust suppressant is added to the dust containing material as early as possible after excavation and/or blasting. The dust suppressant can be added to the dust containing material at multiple handling points. For example, in mining operations, the dust suppressant may be added at one or more points including, in the pit after blasting, in crushing plants, in transfer shoots, any point at which the ore material is subjected to mechanical action (lift, mix or fall).

A further method for dust suppression in accordance with the present invention involves as a first step, the addition of a dust suppressant composition comprising at least one organic acid as described above, at least one alcohol and water, to a dust containing material, for example an ore material such as iron ore, prior to processing in a plant. Importantly, the dust suppressant composition used in the first step is substantially free of surfactant, for example, contains no surfactant. A second step in the method for suppressing dust, involves a further addition of a dust suppression composition after plant processing, for example, prior to stacking. The dust suppressant composition used in the second step can contain surfactant.

It is believed that this method is beneficial as surfactants can interfere with plant processing. Furthermore, the addition of surfactants can easily lead to "over- wetting" of the ore which results in clumping of the ore material and blockages in the plant. That is, the Applicant has discovered that the best flow characteristics for the ore in the plant are achieved in the absence of surfactant.

Another benefit of this approach is that surfactants are cost intensive and therefore limiting the use of them, whilst maintaining desirable results, has clear benefits.

The dust suppressant composition can be produced cost effectively and can even be formulated on-site as required. As described above, the organic acid/s and also the alcohol in the dust suppressant composition can be derived from the fermentation of molasses. The dust suppressant composition can therefore be formulated by forming a mixture of molasses, sugar, for example white sugar, raw sugar or a blend thereof, a metal salt, for example magnesium sulphate and a yeast nutrient, for example urea, together with warm water having a temperature within the range of about 25 ⁰ C and 35 ⁰ C. Yeast is added to the mixture to promotθ fermentation and then the progress of fermentation is monitored by monitoring the pH.

The fermentation reaction is stopped, by adding acid, for example organic acid such as lactic or citric acid or a blend thereof, once the pH of the mixture falls within the range of 3.7 and 4.2. Other organic acids can also be used to stop the fermentation reaction. The mixture is then left to fortify (that is, to allow the sugars to fortify producing alcohol and ethers), for a pre-determined residence time, for example within the range of about 1 to 30 days. If a surfactant is added to the composition, it can be added after the mixture has been fortified for the desirable residence time.

It is envisaged that whilst the dust suppressant composition can be formulated from off-the shelf-organic acids and alcohols, the dust suppressant composition derived from the fermentation of molasses is beneficial because the conditions also support bacterial life. Without wishing to be bound by theory, it is thought that bacteria and/or microbes present in the dust containing material are believed to assist in the leaching of aluminates and silicates from, for example, iron ore. This effectively assists in removing these impurities from the final product.

With the absence of binders, particles do not form large "clumps" and instead take on a more granular texture, which significantly improves the flow characteristics of the dust containing material. That is, the dust suppressant composition does not attempt to minimise the dust by tackifying the dust containing material to cause particle to stick together. Rather, without wishing to be bound by theory, it is believed that the dust suppressant composition causes a change in water behaviour on the surface particle through cation exchange. This in turn alters electrical charges of the particles resulting in granulation and effecting a change in particle terminal velocity, thus reducing dust lift off. Furthermore, as the dust containing material does not become saturated, the fine material does not stick together to form a mud-like composition. In, for example, iron ore processing, this is of particular advantage, as granular material is preferred for the sintering process. The primary advantage during sintering is the efficiency of the heating process. A granular material allows heated air to pass around the sinter bed material better than tightly packed fine ore material that is not granular. Thus less energy is required to make the sinter cake material.

Furthermore, the ability to reduce DEM of a dust containing material and significantly reducing dust emissions with minimal addition of water is highly desirable, especially on mine sites where water is often scarce.

The present invention is further illustrated by way of the following non-limiting example:

Example

DEM Tests

DEM tests were conducted on samples of iron ore material, using a drum tumbler test. This involves placing an approximately 2kg sample of ore into a drum which revolves at approximately 30 rpm, and is fitted with internal lifters. The rotating drum lifts the sample which then falls through an air stream. Fine particles are captured in the air stream and collected for weighing.

A number of samples made up to a specific moisture content are measured this way. The weight information obtained from the drum tumbler test is plotted against the moisture content to provide a dust-moisture relationship. The slope of the curve provides an indication of how "dusty" the ore material is with respect to the moisture content.

A 'dust number' is calculated by the mass of particles recovered in the drum tumbler test, divided by the feed mass, and multiplied by a factor of 100,000. A dust number of 10 corresponds to an acceptable dust emission level.

The DEM is therefore the moisture level in the ore material which corresponds to the dust number of 10 when all of the data is graphed as described above.

DEM tests were conducted on an iron ore sample which underwent dust suppression treatment with water only, and compared with DEM tests conducted on the same ore using the dust suppressant of the present invention. The resulting moisture content and DEM values are provided in Table 1.

Table 1

Water = sample treated with water to reduce dust DS = sample treated with dust suppressant

As can be seen from the above example, the DEM of an dust containing material can be reduced using a dust suppressant comprising acid and water, together with surfactants as required. The DEM has been shown to be reduced by approximately 1%. Lowering the DEM therefore reduces the amount of moisture which is required to be added to the dust containing material in order to suppress dust. This can correspond to significant cost savings for transport of iron ore when taking into consideration the tonnage of iron ore required to be transported per annum. Problems associated with furnace treatment, transport costs and DEM targets are substantially reduced. Durham Cone Test

Durham cone tests provide an indication of the flow characteristics of an ore material. The test involves placing a sample of ore material in a truncated cone being wider at the top than at the base. The base of the truncated cone is fitted with a trap door which can be closed when the cone is loaded with ore, and then opened to allow ore to flow through the bottom of the cone. The time taken for the cone to be emptied entirely of the ore material is measured. Times in excess of 120 seconds are recorded as "no flow" and are assigned a Flow Index of 1.0

(Flow Index of below 1.0, or closer to 0 is desirable and indicates good flow).

Tests conducted on an iron ore sample having a DEM of 7% showed a Flow Index of 1.0 at 7% moisture. That is, the ore sample, with a moisture content at DEM or greater would cause significant problems within a plant. The same ore treated with the dust suppressant composition of the present invention resulted in a Flow Index of 0.6 at the same moisture content of 7%, indicating a significant improvement of the flow characteristics of the ore material.

Clearly, if the DEM is lowered, requiring an even lower moisture content, then flow characteristics are improved even more significantly.

Using the method of the present invention, the Applicant has surprisingly discovered that it is possible to lower the DEM of various dust containing materials, such as ore and/or soil. This method does not require large quantities of water to be added to the ore, and more specifically, results in significant reduction in dust emissions whilst maintaining low moisture content within the dust containing material.

The flow characteristics of the ore were also significantly improved, substantially overcoming the problems associated with the use of binders to "stick" particles together to reduce dust formation. It is also observed that the dust suppressant of the present invention facilitates substantially uniform distribution of moisture throughout the ore to improve flow characteristics.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.