GB2598764A - Aggregate and methods for producing aggregate - Google Patents

Abstract

An aggregate comprising a plurality of cellulose fibres; a mineral component, and a binding agent, wherein the cellulose fibres make up between 15% and 70% by weight of the aggregate, and wherein the mineral component makes up between 85% and 30% by weight of the aggregate. The mineral component may be a plastic, metal, ceramic, lignin or glass material present along with the cellulose fibres as a waste product from a paper recycling process. The binder may be Portland cement and be present at up to 10% by weight. The aggregate may be used to form construction products. Also disclosed are aggregates formed form cellulose fibres without a binder and the formation of an aggregate from cellulose fibres from which the mineral components have been removed.

Description

AGGREGATE AND METHODS FOR PRODUCING AGGREGATE

[0001] This invention relates to aggregate and methods of producing aggregate. BACKGROUND [0002] Concrete construction blocks are often heavy, weighing between 15kg to 20kg each, and require large quantifies of cement which has a negative impact on the environment. Furthermore, heavy concrete blocks require considerable energy to transport, which is also detrimental to the environment as transporting heavy concrete blocks generates large amounts of greenhouse gases.

[0003] Existing methods of producing more environmentally friendly construction products, such as concrete blocks, wall panels, bricks etc., have focussed on substituting some of the cement used in blocks to reduce the amount of cement used, as this reduces the cost of producing a block and also reduces the weight of the block. One type of substitute material is ash produced from coal power stations, straw, wood shavings or pulverised fuel ash (PFA or "fly-ash"). However, each of these has several drawbacks and disadvantages. In particular, the quantity of ash produced from each of the above sources is reducing, and given the desire to produce more environmentally friendly building materials, importing ash from abroad is undesirable. Furthermore, as straw is a seasonal crop that can vary in quality, this further impacts the quality of the resulting ash. In the case of wood shavings, the quality of the resulting ash is also unreliable. Additionally, due to environmental regulations, there is a limited quantity of wood available for producing pellets, and so it is undesirable to produce concrete blocks using such a limited resource.

[0004] Alternative substitute materials include pumice, clay and powdered glass. However, these materials are considerably more expensive and can be in limited supply, requiring additional transportation when importing the materials and transporting the materials to the construction site.

[0005] Currently, pellets are not used in the constructions industry, unless they are being burned as a fuel source to produce energy, for example, as fuel for wood burners or biomass fuelled steam plants. Aside from energy production, the other primary use for existing pelletisers is to produce pellets for animal food and wellbeing (e.g. animal bedding).

[0006] Prior art pressed pellets have a moisture content between 10% and 18% relative humidity, as above this range, existing devices that produce such pellets are not able to operate due to the high forces exerted on the pelleting die when compressing raw materials having a higher moisture content. Once a pellet is produced by compression, it falls into a machine which cools the compressed material which introduces cold air onto the pellet, referred to as a "cold-shock", which generates a sealed surface leaving the product ready for use.

[0007] Paper mills also produce large amount of waste, such as de-inked sludge, screen or ascot rejects, and primary and secondary effluent waste water streams. While the de-inked sludge can be used to improve the soil content due to the alkaline properties of the de-inked sludge, excess use of the de-inked sludge can result in the land becoming too alkaline for farming and in ink entering the food chain. In the case of screen rejects, this is typically disposed of in landfill.

[0008] The present invention seeks to address at least some of these problems.

BRIEF SUMMARY OF THE DISCLOSURE

[0009] According to one aspect, the present invention provides an aggregate comprising a plurality of cellulose fibres; a mineral component, and a binding agent, wherein the cellulose fibres make up between 15% and 70% by weight of the aggregate, and wherein 15 the mineral component makes up between 85% and 30% by weight of the aggregate.

[0010] Thus, pellets according to the present invention utilise one or more available waste streams that are not currently utilised to provide a suitably strong material suitable for producing pellets suitable for use in a variety of applications. A further advantageous effect of the present aggregate is to provide improved thermal insulation, as the cellulose fibres trap a plurality of air pockets within the pellets that make up the aggregate.

[0011] The aggregate may have a moisture content between 10% and 40% In some cases, the mineral component may make up between 85% and 40% by weight of the aggregate and the cellulose fibres may make up between 15% and 60% by weight of the aggregate. The length of the cellulose fibres may be less than 3mm.

[0012] The mineral component may comprise one or more of plastic, ceramic, metal, lignin and glass. The mineral component may comprise a plurality of fragments, and the plurality of fragments have a cross-sectional dimension between 0.1mm and 2mm.

[0013] Between 30% and 50% by weight of the aggregate may be made up of the mineral component. Up to 10% by weight of the aggregate may be made up of the binding agent. The binding agent may comprise particles distributed between the cellulose fibres.

[0014] Viewed from a further independent aspect, the present invention provides an aggregate comprising a plurality of cellulose fibres, and a mineral component, wherein the plurality of cellulose fibres makes up between 35% and 70% by weight of the aggregate, wherein the mineral component makes up between 30% and 65% by weight of the aggregate, and wherein the aggregate has a moisture content between 30% and 65%. In some cases, the pulp of screen rejects 35 has a moisture content of 55% to 65%. In some cases, the crumble of de-inked sludge 45 as a moisture content of 30% to 40%.

[0015] The mineral component may be distributed between the cellulose fibres. The aggregate may further comprise at least one of a waterproof coating, an adhesive additive, a fire retardant and a colouring agent. The aggregate may have a density of between 150 to 1500 kg/m'.

[0016] Viewed from a further independent aspect, the present invention provides a concrete product comprising aggregate according to any of the appended claims. The concrete product may be any of a construction product, a tile, wall insulation, a construction panel for a wall or a concrete construction block.

[0017] Viewed from a further independent aspect, the present invention provides a method of producing an aggregate the method comprising providing a first cellulose fibre-containing material comprising a first mineral component, conditioning the first cellulose fibre-containing material to form a conditioned first cellulose fibre-containing material, mixing a binding agent with the conditioned first cellulose fibre-containing material to form a mixture, and forming an aggregate from the mixture, wherein a plurality of cellulose fibres make up between 15% and 70% by weight of the aggregate, and wherein the first mineral component makes up between 30% and 85% by weight of the aggregate.

[0018] The step of conditioning may comprise adding water to the first cellulose fibre-containing material to raise a moisture content of the first cellulose fibre-containing material to between 50% and 65%. The first cellulose fibre-containing material may have a moisture content of up to 40% before conditioning. The step of conditioning may include adding water to the first cellulose fibre-containing material. The step of conditioning may include blending the first cellulose fibre-containing material.

[0019] The first cellulose fibre-containing material may be provided as a pulp having a moisture content up to 65%. The step of conditioning may comprise blending the first cellulose fibre-containing material. The method may further comprise the step of providing a second cellulose fibre-containing material having a moisture content of between 10% and 40%. The second cellulose fibre-containing material may be provided before the step of mixing the binding agent. The second cellulose fibre-containing material may be provided as a crumble.

[0020] The method may comprise providing a second cellulose fibre-containing material having a moisture content between 50% and 65%. The second cellulose fibre-containing material may be provided before the first cellulose fibre-containing material is mixed with the binding agent. The first cellulose fibre-containing material may have a moisture content between 20% and 40%. The second cellulose fibre-containing material may be provided as a crumble.

[0021] The method may comprise the step of drying the mixture before forming an aggregate from the mixture, such that the aggregate has a moisture content between 10% and 40% before being formed as an aggregate. The aggregate may be formed by extrusion.

[0022] The second cellulose fibre-containing material may be derived from screen rejects and/or ascot rejects from a paper mill. The first cellulose fibre-containing material may be derived from one or more of a de-inking sludge from a paper mill and primary or secondary effluent sludge from wastewater from a paper mill. The method may include the step of drying the formed aggregate such that the formed aggregate has a strength of at least 0.2MPa.

[0023] Viewed from a further independent aspect, the present invention provides a method of producing an aggregate, the method comprising: providing a first cellulose fibre-containing material comprising a first mineral component making up between 45% and 65% by weight of the first cellulose fibre-containing material and plurality of cellulose fibres making up between 35% and 55% by weight of the first cellulose fibre-containing material, washing the first cellulose fibre-containing material to substantially remove any minerals from the first cellulose fibre-containing material, conditioning the first cellulose fibre-containing material such that a moisture content of the first material is reduced to between 50% and 65%, and forming an aggregate from the dried material.

[0024] The step of providing the first cellulose fibre-containing material may comprise the step of shredding recycled cardboard or paper and passing the shredded recycled cardboard or paper through an agitator to produce a slurry.

[0025] The aggregate may be formed by agglomeration, preferably in a centrifuge.

[0026] The method may further comprise the step of drying the formed aggregate such that the aggregate has a moisture content between 5 and 40%. The aggregate may be formed as pellets having a cross-sectional dimension between 4 and 15 mm.

[0027] The method may further comprise the step of adding at least one of a waterproof coating, an adhesive additive, a fire retardant and a colouring agent to the formed aggregate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 illustrates a process of recycling paper,

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[0029] Figure 2 illustrates a process of treating waste water from the recycling process, [0030] Figure 3 illustrates a cross-section of an exemplary pellet, [0031] Figure 4 illustrates a device suitable for producing aggregate according to the present method, [0032] Figure 5 illustrates a further exemplary pellet, [0033] Figure 6 illustrates a device for producing a construction product using aggregate according to the present method, [0034] Figure 7 illustrates a cross-sectional view of a device suitable for producing aggregate according to the present method.

io DETAILED DESCRIPTION

[0035] Throughout the description, the terms "pellet" and "aggregate" will be used to describe the product resulting from the described processes. The "aggregate" as described herein comprises multiple pellets and is used to refer to the bulk product.

[0036] Figure 1 illustrates a process of recycling paper. The paper recycling process 5 is typically begins with a paper collection step 10, for example from homes, businesses or recycling centres and transporting the materials to a recycling facility. Once the paper arrives at a recycling facility, it is sorted, graded and baled 20 based on the type and quality of the materials. The bales of paper are then mixed 30 with water in a large container in order to make a pulp. The pulp is then screened 32 to remove any plastics, glass, ceramics or glue from the pulp. The screened waste is typically referred to as "screen rejects" 35 and is a waste stream in the paper recycling process 5, as it is not used to produce recycled paper. However, while the screen rejects 35 does not contain a sufficiently high proportion of cellulose fibres or sufficiently long cellulose fibres to be used in recycled paper production, the screen reject 35 pulp does still contain a large amount of cellulose fibres. The waste water from the mixing step 30 will be discussed further below and in relation to Figure 2. The recycling process 5 continues from mixing 30 and screening 32 to a de-inking step 40, where air is passed through the pulp to produce a foam which removes a large proportion of any inks present on the original paper product that remain in the pulp. Chemicals can also be used to separate ink from the pulp which are then washed away. The foam on top of the waste water is drained off as a "de-inked sludge" 45 and is a further waste stream in the recycling process 5. After the pulp has been de-inked, it is pressed and ironed 50 into a sheet to straighten the pulp, before being arranged 60 into a large roll of recycled paper for subsequent distribution or cutting to a desired specification. The screen rejects 35 and the de-inked sludge 45 are exemplary raw materials that are suitable for the present methods and pellets. Whilst recycling process 5 has been described here, it would be apparent that this process was merely an example of a paper recycling process 5 and that similar processes producing similar waste streams may be suitable for use with the present method and pellets.

[0037] Figure 2 illustrates a process for treating water used in the mixing step 30 in the paper recycling process 5 described above. After the bales of paper have been mixed 30 with water to produce pulp, the excess water is drained off and a pre-treatment step 70 is performed. One example of a pre-treatment step 70 is the provision of a settlement tank that allows cellulose fibres within the water to separate out from the remaining water and fall to the bottom of the tank. The cellulose waste that settles at the base of the tank is typically referred to as a "primary sludge" 75, and is often sent to a digestion tank 110 for treatment, which typically generates methane and carbon dioxide and further waste sludge 115. As cellulose fibres settle in the settlement tank, water from the pre-treatment step 70 can be passed to an anaerobic treatment tank 80 for further treatment to clean the waste water. The anaerobic tank 80 typically produces further waste sludge 85 and methane and carbon dioxide. Any remaining waste from the anaerobic treatment tank 80 can be sent to an aeration basin 90 and secondary clarifier 100 for further cleaning, before being discharged as effluent 105 or activated sludge 95. The activated sludge 95 can also be sent to the digestion tank 110 for treatment in a similar manner to the primary sludge 75. The primary sludge 75 and activated sludge 95 are further examples of raw materials that are suitable for the present methods and pellets. Further raw materials that are suitable for the present methods and pellets include primary and second effluent discharged from the water treatment process described above. Similarly, paper rejects from paper mills are a suitable raw material for use in the present methods.

[0038] Some paper mills produce de-inked sludge 45 and a small amount of effluent sludge from an effluent plant. The de-inked sludge 45 and the effluent sludge are then combined before dewatering and sending to land. However, the combined material sent to land are undesirably odorous. In some cases, the effluent sludge can be treated in an anaerobic digestion plant to remove bacteria within the effluent sludge which causes the odour, which is advantageous.

[0039] Table 1 below provides one example of the composition of the screen reject 35 and the de-inked sludge 45 initially after receipt from a paper mill ("Fresh"), and subsequently after drying ("Dried").

[0040] Table 1 [0041] Screen reject De-inked sludge Dried [mg/kg] Fresh [mg/L] Dried [mg/kg] Fresh [mg/L] Water Soluble Sulphate [1:5] 226 29 94.9 27.1 Total Phosphorus 187 <50 195 56 Total Potassium 170 <50 180 51 Total Magnesium 881 113 2337 667 Total Copper 28.3 3.6 74.2 21.2 Total Sulphur 1012 130 578 165 Total Calcium 38674 4956 236121 67353 Total Iron 1861 239 567 162 Total Manganese 45.5 5.8 82.2 23.4 Total Sodium 665 85 385 110 Total Boron 14.4 <5 <5 <5 Total Zinc 62.9 8.1 24.3 6.9 Total Lead 12.4 1.6 3.59 1.03 Total Cadmium 0.13 <0.1 0.12 <0.1 Total Mercury <0.1 <0.1 <0.1 <0.1 Total Nickel 7.26 <1 2.57 <1 Total Chromium 13.9 <2 4.72 <2 Total Antimony 1.2 0.2 Total Beryllium <0.1 <0.1 Total Lithium 1.3 1.8 Total Molybdenum 1.34 1.3 Chloride 451 190 Total Aluminium 4406 1340 Fluoride [100:1 H2504 Soluble] 14.5 29.6 Total Arsenic <0.5 <0.5 Total Selenium <0.1 <0.1 Vanadium 2.31 2.13 Total Strontium 68.2 573 Total Barium 65.1 18.6 Total Silver 0.89 <0.01 Tin 1.13 <1 Total Cobalt 1.26 <1 Thallium <0.1 <0.1 Table 2 below illustrates further parameters of the screen reject 35 and de-inked sludge 45.

Table 2

Parameter Screen reject De-inked sludge Dried [mg/kg] Fresh [mg/L] Dried [mg/kg] Fresh [mg/L] Bulk density [g/L] 401 460 Oven dry matter [°/0] 32.0 62.0 Moisture Pid 68.0 38.0 [0042] It would be apparent that the values provided above in Tables 1 and 2 are merely exemplary and that the precise mineral composition of cellulose-fibre containing materials derived from waste streams of paper mills will vary between paper mills and between batches of recycled paper collected from a single paper mill. Furthermore, different paper mills produce different waste outputs and so not all waste streams will be available from all paper mills.

[0043] While a range of minerals are present in both the de-inked sludge 45 and the screen rejects 35, these account for different proportions of the overall content of the respective materials. The screen rejects 35 can contain up to 95% cellulose fibres, as plastic contaminants and minerals and lignin make up approximately 2% and 3% by weight respectively of the bulk material. It should be noted that the term "minerals" or "mineral component" will be used inter-changeably throughout this description to refer to the plastic contaminants, the lignin and other organic polymer waste, minerals and metal contaminants. Contrary to this, a significant proportion of the de-inked sludge 45 is made up of minerals, such as polystyrene fragments approximately 2mm across. Typically the de-inked sludge 45 comprises 70% or more by weight of minerals and up to 30% by weight cellulose fibres. For example, a de-inked sludge 45 comprising 14% cellulose fibre and 86% mineral content has been found suitable for the methods described herein. The screen rejects 35 typically have a moisture content of 55-65% and the crumble of de-inked sludge 45 typically has a moisture content of between 30-40%. It should also be noted that because the pulp of screen rejects 35 and the crumble of de-inked sludge 45 are typically stored outside, they are exposed to the environment, and therefore, the moisture content of the received materials may vary from those described herein. Where carboard rejects are used, this will typically comprise 45-65% cellulose by weight, with the remainder of the cardboard reject comprising minerals. Where paper rejects are used, this will typically comprise between 70% to 99% cellulose, with the remainder of the paper reject comprising minerals. Where primary or secondary effluent is used, this will typically comprise 90% water and 10% cellulose fibre. Although specific values are provided here, it would be apparent these values will vary depending on the manufacturing and recycling processes used in specific paper mills.

[0044] The de-inked sludge 45 typically contains cellulose fibres with a length of less than 3mm, which are considered too short for the recycled paper process 5. However, it would be apparent that in some cases, the length of the cellulose fibres in the de-inked sludge 45 may be longer than 3mm. It should be noted that screen rejects are typically treated as contaminants in the recycled paper process 5 and are typically discarded. Thus, an important aspect of the present methods and pellets, are the use of two waste streams in the recycled paper process as sources of raw material for the production of pellets.

[0045] Reference will be made to Figure 3 when describing the steps of an exemplary method of producing an aggregate ("Aggregate 1"): Step A) Provide a crumble of de-inked sludge 45 with a moisture content of up to 40%, Step B) Add water to the crumble to increase the moisture content of the de-inked sludge 45 to 65%, Step C) Add Ordinary Portland Cement (OPC), preferably in particulate form, to provide a ratio of 90% hydrated de-inked sludge pulp to up to 10% OPC, Step D) Blend using a high shearing blender until a homogenous blend is achieved, Step E) Form pellets of between 6-15mm diameter and approximately spherical in shape using a planetary mixer, Step F) Dry at ambient temperature until the pellets have a strength of at least 1-5MPa.

[0046] The exemplary pellet 200 illustrated in Figure 3 comprises cellulose fibres 205, plastic shards 210, metal shards 212 and OPC particles 215. The mineral component provides a matrix onto which the cellulose fibres 205 may adhere and the OPC particles 215 can thus bind the cellulose fibres 205 and the mineral component together to form the resulting pellet 200. The cellulose fibres 205, mineral component 210, 212 and OPC particles 215 illustrated in Figure 3 are merely exemplary and are not necessarily drawn to scale. It would also be apparent that more or fewer of each of the elements illustrated in Figure 3 may be present in a pellet 200. Furthermore, it has been found that pellets of sufficient strength for some applications can be formed using only 2-3% OPC, thus requiring less binding agent, as 97-98% of the pellets 200 are made of the de-inked sludge 45. The present method has been found to produce pellets having a density of between 300-600kg/m3. The present method has been found to produce pellets having a strength of 3MPa.

[0047] It would be apparent that not all of the steps listed above were essential. For example, a crumble having the necessary moisture content of Step B may be provided in the first instance, thus combining steps A and B. In step B, it is not essential for water to be added to the pulp in order to achieve the desired moisture content. As an alternative, the crumble may be provided having a moisture content greater than 65% and the initial crumble could be dehydrated to achieve the target moisture level. In step C, OPC is merely provided as an exemplary binding agent suitable for binding the cellulose-fibre containing material together. It would be apparent that other binding agents, in particulate form or otherwise, would be suitable for use in the present methods. In step D, blending for three to four minutes has been found sufficient, but it would be apparent this will vary depending on the particular blender used. In step E, it would be apparent that pellets could be formed using standard techniques known in the art, including the procedure of mixing and granulation by agglomeration. Furthermore, it would be apparent that the formed pellets may have different diameters, and that the diameter of a given pellet may be greater than 15mm or less than 6mm. The formed pellets may be formed into spheres, rods or ellipsoids, or any other shape depending on the requirements of the pellet. In step F, it would be apparent that the drying process may take more or less time depending on the ambient conditions. Furthermore, it would be apparent that the drying process may be is controlled using an apparatus, such as a dehumidifier, configured to control the temperature, humidity and other parameters in order to provide pellets having a suitable strength. Whilst a specific ratio of constituent components de-inked sludge 45 and OPC have been provided here, it would be apparent these ratios could be adjusted depending on the requirements of the pellet.

[0048] An exemplary agglomeration process includes the steps of: 1) transferring the blended material to a disk mixer 2) rotating the disk mixer while adding water and saw dust ash to form pellets, 3) spraying water in the disk mixer as pellets are formed, 4) drying the mixture at ambient temperature or using a furnace, 5) cooling the pellets, 6) screening pellets of the correct size, and 7) coating the correctly-sized pellets.

[0049] Additional steps may be included in the agglomeration process depending on the requirements of the pellets. For example, sodium silica may be added to the blended material. Additionally or alternatively, the formed pellets may be sieved and heated at 90°C for 40 mins. Introducing the correctly-sized pellets into a cast round device or similar can be used to increase the surface tension of the pellets, further increasing their strength. Such a device can, for example, introduce cold air to the surface of the formed pellets to increase the hardness of the pellet surface. Whilst strengthened pellets are particularly suited for use in making concrete construction blocks, such pellets are not essential in the production of concrete construction blocks, and where formed pellets do not need such high strengths, it will be apparent this step would be entirely optional. While this agglomeration process has been described in relation to Aggregate 1, it would be apparent that a similar process was suitable for use in the production of the other aggregates described herein, and will therefore not be repeated below.

[0050] Reference will be made to Figure 3 when describing the steps of an exemplary method of producing an alternative aggregate ("Aggregate 2"): Step A) Provide a pulp of screen rejects 35 with a moisture content of up to 65%, Step B) Blend the pulp of screen rejects 35, Step C) Add a crumble of de-inked sludge 45 with a moisture content of up to 40% whilst the screen rejects are being blended until both the de-inked sludge 45 and screen rejects 35 are evenly blended and the de-inked sludge 45 is no longer a crumble, Step D) Add OPC to provide a ratio of 45% de-inked sludge pulp to 45% screen reject pulp to 10% OPC and mix until well distributed, Step E) Form pellets between 6-15mm diameter and roughly spherical in shape, Step F) Dry at ambient temperature.

[0051] It would be apparent that not all of the steps listed above were essential. For example, a mixture of blended crumbles may be provided in the first instance, thus combining steps A to C. In steps B and C, blending the screen reject pulp for three to four minutes has been found to be sufficient. However, it would be apparent this is merely provided as an example and the blending duration will depend on the specific equipment used and the composition of the screen rejects 35 and de-inked sludge 45. In step D, OPC is merely provided as an exemplary binding agent suitable for binding the cellulose-fibre containing material together. Furthermore, it has been found that only 2-3% OPC is sufficient to produce pellets according to the method described above, thus requiring less binding agent, as up to 98% of the pellets are made of the de-inked sludge 45 and the screen rejects 35. OPC can be added to a mixture comprising between 40-60% de-inked sludge 45 and between 40-60% screen rejects 35. In step E, it would be apparent that pellets could be formed using standard techniques known in the art, including the procedure of mixing and granulation by agglomeration. Furthermore, it would be apparent that the formed pellets may have different diameters, and that the diameter of a given pellet may be greater than 15mm or less than 6mm. The formed pellets may be formed into spheres, rods or ellipsoids, or any other shape depending on the requirements of the pellet. Such pellets are suitable for use in packaging, and so pellets formed into different shapes will be better-suited to providing adequate protection for any items being transported. Pellets according to the method described above have a strength of between 0.2 -1 MPa. Pellets according to the method described above have a density of between 100-300Kg/m3. In step F, it would be apparent that the drying process may take more or less time depending on the ambient conditions, and that the drying process may be controlled using an apparatus, such as a dehumidifier, to control the temperature, humidity and other parameters in order to provide pellets having a suitable strength. Whilst a specific ratio of constituent components has been provided here, it would be apparent these ratios could be adjusted depending on the requirements of the pellet. Pellets formed according to the method described above may comprise cellulose fibres, minerals and OPC as illustrated in Figure 3.

[0052] Advantageously, and as noted in Table 1 above, the screen rejects 35 include plastics such as polystyrene, glass, ceramics and metals. These different minerals are present as fragments in the screen reject 35 which fill any voids between the cellulose fibres. This causes the cellulose fibres to become entangled as an irregular fibre matrix. Thus, the formed pellets have a high mineral content, due to the minerals within the screen reject 35, which provides a matrix onto which the cellulose fibres can be bonded to using the OPC. This advantageously increases the strength of the formed pellets.

[0053] Reference will be made to Figures 4 and 5 when describing the steps of an exemplary method of producing an alternative aggregate ("Aggregate 3"): Step A) Providing recycled cardboard in a crumble form, Step B) Milling the recycled cardboard using a hammer mill, Step C) Adding the milled recycled cardboard to a container of water and blending the milled recycled cardboard to form a pulp, Step D) Pressing the blended pulp using a screw press to reduce the moisture content to between 60% and 65%, Step E) Forming pellets having a maximum dimension of between 4mm and 8mm and a moisture content of 40%.

[0054] Figure 4 illustrates a device 300 suitable for producing pellets 340 according to the method described above. As shown, the device 300 comprises a housing 305 having an inlet 310 for receiving the pressed blended pulp and an outlet 315 for dispensing formed pellets 340. The device 300 also includes a rotating shaft 320 extending through the housing 305 and driven by a motor 330. The shaft 320 has a plurality of fingers 325 extending perpendicularly therefrom and configured to break up the pressed blended pulp and form the pellets 340 and drive the material from the inlet 310 to the outlet 315. As the shaft 320 rotates, this exerts a centrifugal force on the material within the housing which causes the material to agglomerate in the desired manner. Pellets 340 are dispensed from the outlet 315 and collected in a hopper 345 as an aggregate 350 that can be packaged for dispatching or use. While the fingers 325 are shown extending in a helical manner around a longitudinal axis of the shaft 320, it would be apparent this was not essential. Other arrangements of fingers 325 would be equally suitable for use in the present method.

[0055] Recycled cardboard and printed paper are examples of cellulose fibre containing material suitable for use in the present methods and pellets. The cardboard is shredded, milled and finally passed through an agitator, where the cellulose fibres and minerals are separated by the action of water and mechanical agitation. Typically this will result in a 1:2 ratio of solid to water components. In some cases, the moisture content of the pellets can be reduced to between 35-40% and the water exuded from the blended pulp can be re-introduced into the agitator to reduce the amount of water needed for the described method.

[0056] After step C, the pulp is typically transfer to horizontal shaft spiral centrifuge rotator machine for forming pellets. Such a device is advantageous, as the rotational speed of the centrifuge can be controlled. By controlling the speed at which the centrifuge rotates, it is possible to control the size of the pellet that is formed. The rotational speed necessary for a desired pellet size may also depend on the moisture content of the blended pulp. As the rotational speed and/or the moisture content of the blended pulp increases, this will result in the formation of smaller pellets. Conversely, lower rotational speeds and/or moisture content results in the formation of larger pellets. During the pressing step D, the raw material is typically compressed to approximately one third of its initially volume to increase the density, and therefore the strength, of the resulting pellets. It should be noted that in each of the aggregates and associated pellets described herein, there is a balance between the density of the formed pellet and the strength of the final pellets. Contrary to prior art pellets, which must be produced with a low moisture content, the present method utilises a much higher moisture content to allow the centrifuge to form the pellet. The formed pellets can be used immediately after step E, or they can be left to dry naturally or put it into a dryer to accelerate the drying process prior to subsequent use. Pellets having a moisture content of approximately 5% are particularly suited for use as animal bedding, or as an oil or solvent absorbing material.

[0057] Depending on the raw materials available, some or all of steps B and C may be omitted. For example, where only a crumble of de-inked sludge 45 is used, and is provided in fine enough form, it is possible omit steps B and C, and proceed directly to step D, and press the crumble to reduce the moisture content to the target level. If only screen rejects 35 are used, then step B can be omitted, and the screen rejects can be added to water directly to clean the screen rejects and blended to form a pulp.

[0058] Depending on the requirements of the final pellet, further steps can be included in any of the methods described above to produce Aggregates 1, 2 or 3. For example, where the pellet will be used as a construction material to form a solid concrete block, a binding agent such as OPC can be added to the formed pellet. Where the pellet is to be used as a packaging material, a biodegradable glue can be added to the formed pellet. Where the pellet is to be used as a insulation product, a biodegradable glue can be added to the formed pellet in addition to a waterproof coating and ceramic base fire retardant after sealing the surface of the pellet to close any pores present in the surface of the pellet. If the pellets are to be used in the health and safety sector, a colourant may be added to the pellets. This provides coloured pellets that, for example, can be added to a ditch containing different pipes to indicate what particular pipes are buried there. Suitable waterproof coatings are known in the art and need not be repeated here.

[0059] Figure 5 illustrates a pellet 400 according to the method described above. Pellet 400 comprises cellulose fibres 405 and minerals 410, 412 and no binding agent. As pellet 400 can be used in different applications, some of which do not require a binding agent, the pellet 400 illustrated in Figure 5 has no binding agent. However, where the pellet 400 is to be used as a construction product, such as a concrete block, aggregate in the form of a plurality of pellets 400, can be mixed with a binding agent 415, such as OPC, and the resulting mixture compressed in a block maker 450 to form the concrete block (not shown).

[0060] Reference will be made to Figure 7 when describing the steps of an exemplary method of producing an alternative aggregate ("Aggregate 4"): Step A) Provide a crumble of de-inked sludge 45 having a moisture content between 20-40%, and a pulp of screen rejects 35 having a moisture content between 50-65%, Step B) Add OPC to provide a ratio of 45% de-inked sludge pulp to 45% screen reject pulp to 10% OPC and mix until well distributed, Step C) Dry the pellets until they have a moisture content of between 10-40%, Step D) Form pellets having a diameter between 6mm and 12mm and a length of between 10mm -20mm.

[0061] It should also be noted that, whilst polystyrene is currently considered as a contaminant in the waste streams of paper recycling, in the present methods and pellets, the presence of polystyrene provides a more robust matrix from which the OPC can attach to. These pellets will typically be formed by agglomeration as is known in the art and described above, and achieve a mean average density of between of between 600kg/m3 and 900kg/rn3 and a strength of approximately 6MPa to 10MPa, such as 8MPa. In some cases, the mean average density of the formed pellets may be 700kg/m3. Such pellets are particularly suited for use in the production of construction products, such as concrete construction blocks, wall panels or wall insulation. It is possible for the pellets to be formed by compression, which can homogenise the irregular arrangement of fibres and further improve the strength of the pellets. Furthermore, it has been found that only 2-10% OPC is needed to produce pellets according to the method described above, thus requiring less binding agent, as up to 98% of the pellets are made of the de-inked sludge 45 and the screen rejects 35. OPC can be added to a mixture comprising between 40-60% de-inked is sludge 45 and between 40-60% screen rejects 35. In some cases, it is desirable for the pellet to comprise 5% by weight of OPC, with between 40-60% de-inked sludge 45 and between 40-60% screen rejects 35 [0062] One exemplary method of producing Aggregate 4 is to use a standard biomass pellet die that is used to make biomass pellet products. A device 500 comprising a die plate 505 having a plurality of holes 510 extending through the thickness of the die plate 505 is shown in Figure 7. One on side of the die plate 505, there is provided a roller 520 mounted for rotation about a longitudinal axis 525 and configured to press the mixture 515 formed from Step B through the holes 510, as the roller 520 rotates about the axis 525. On an opposed side of the die plate 505, there is provided a cutting blade 530 mounted for rotation about the longitudinal axis 525 and configured to shear material that has protruded through the holes 510, thus providing the resulting pellets (not shown). A die plate 505 having a thickness of 35mm and a hole diameter of 6mm has been found to be suitable for use with the present method. A compression ratio of between 4:1 to 6:1 has also found to be suitable for use in the present method. However, it would be apparent that this was merely an example, and any of the compression ratio, the hole diameter or the die plate thickness may be adapted depending on the particular composition of the raw materials and the desired strength of the resulting pellets. Similarly, while a flat die plate is illustrated and described, it would be apparent that other die plates, for example tubular die plates with material extruded from the interior of the tube to the exterior of the tube may be suitable for use with the present method.

[0063] One of the parameters which determines the strength of the formed pellets is the dispersion of cellulose fibres in the pellets. Fibre dispersion of between 150-300kg/m3 has been found to be suitable for producing pellets according to the present methods.

[0064] VVhilst specific dimensions of the formed pellet have been provided above, it would be apparent these were merely exemplary, and that the pellet may have a diameter of 8mm or lOmm or less than 6mm or more than 12mm The formed pellets may also have a length of 15mm, or a length less than lOmm or greater than 20mm.

Claims (1)

1. CLAIMS1. An aggregate comprising: a plurality of cellulose fibres; a mineral component, and a binding agent, wherein the cellulose fibres make up between 15% and 70% by weight of the aggregate, and wherein the mineral component makes up between 85% and 30% by weight of the aggregate.113 2. An aggregate according to claim 1, wherein the aggregate has a moisture content between 10% and 40% 3. An aggregate according to claim 1 or 2, wherein the length of the cellulose fibres is less than 3mm 4. An aggregate according to any preceding claim, wherein the mineral component comprises one or more of plastic, ceramic, metal, lignin and glass.5. An aggregate according to any preceding claim, wherein the mineral component comprises a plurality of fragments, and wherein the plurality of fragments have a cross-sectional dimension between 0.1mm and 2mm.6. An aggregate according to any preceding claim, wherein between 30% and 50% by 20 weight of the aggregate is made up of the mineral component, and wherein between 50% and 70% of the aggregate is made up of the cellulose fibres.7. An aggregate according to any of claims 1 to 5, wherein between 70% and 85% by weight of the aggregate is made up of the mineral component, and wherein between 15% and 30% by weight of the aggregate is made up of cellulose fibres.8. An aggregate according to any preceding claim, wherein up to 10% by weight of the aggregate is made up of the binding agent.9. An aggregate according to any preceding claim, wherein the binding agent comprises particles distributed between the cellulose fibres.10. An aggregate comprising: a plurality of cellulose fibres, and a mineral component, wherein the plurality of cellulose fibres makes up between 35% and 70% by weight of the aggregate, wherein the mineral component makes up between 30% and 65% by weight of the aggregate, and wherein the aggregate has a moisture content between 30% and 65%.11. An aggregate according to any preceding claim, wherein the mineral component is distributed between the cellulose fibres.12. An aggregate according to any preceding claim, further comprising at least one of a waterproof coating, an adhesive additive, a fire retardant and a colouring agent.13. An aggregate according to any preceding claim, wherein the aggregate has a density of between 150 to 1500 kg/m'.14. A construction product comprising aggregate according to any preceding claim.15. A method of producing an aggregate the method comprising: providing a first cellulose fibre-containing material comprising a first mineral component, conditioning the first cellulose fibre-containing material to form a conditioned first cellulose fibre-containing material, mixing a binding agent with the conditioned first cellulose fibre-containing material to form a mixture, and forming an aggregate from the mixture, wherein a plurality of cellulose fibres make up between 15% and 70% by weight of the aggregate, and wherein the first mineral component makes up between 30% and 85% by weight of the aggregate.16. A method according to claim 15, wherein the step of conditioning comprises adding water to the first cellulose fibre-containing material to raise a moisture content of the first cellulose fibre-containing material to between 50% and 65%.17. A method according to claim 15 or 16, wherein the first cellulose fibre-containing material has a moisture content of up to 40% before conditioning.18. A method according to claim 15, wherein the first cellulose fibre-containing material is provided as a pulp having a moisture content up to 65%, and wherein the step of conditioning comprises blending the first cellulose fibre-containing material, wherein the method further comprises the step of providing a second cellulose fibre-containing material having a moisture content of between 10% and 40%, wherein the second cellulose fibre-containing material is provided before the step of mixing the binding agent.19. A method according to claim 15 comprising the step of providing a second cellulose fibre-containing material having a moisture content between 50% and 65%, wherein the second cellulose fibre-containing material is provided before the first cellulose fibre-containing material is mixed with the binding agent, wherein the first cellulose fibre-containing material has a moisture content between 20% and 40%.20. A method according to claim 19 comprising the step of drying the mixture before forming an aggregate from the mixture, such that the aggregate has a moisture content between 10% and 40% before being formed as an aggregate.21. A method according to claim 19 01 20 wherein the aggregate is formed by extrusion.22. A method according to any of claims 18 to 21, wherein the second cellulose fibre-containing material is derived from screen rejects or ascot rejects from a paper mill.23. A method according to any of claims 15 to 22, wherein the first cellulose fibre-containing material is derived from one or more of a de-inking sludge from a paper mill and primary or secondary effluent sludge from wastewater from a paper mill.24. A method according to any of claim 15 to 23, comprising the step of drying the formed aggregate such that the formed aggregate has a strength of at least 0.2MPa.25. A method of producing an aggregate, the method comprising: providing a first cellulose fibre-containing material comprising a first mineral component making up between 45% and 65% by weight of the first cellulose fibre-containing material and plurality of cellulose fibres making up between 35% and 55% by weight of the first cellulose fibre-containing material, washing the first cellulose fibre-containing material to substantially remove any minerals from the first cellulose fibre-containing material, conditioning the first cellulose fibre-containing material such that a moisture content of the first material is reduced to between 50% and 65%, and forming an aggregate from the dried material.26. A method according to claim 25, wherein the step of providing the first cellulose fibre-containing material comprises the step of shredding recycled cardboard or paper and passing the shredded recycled cardboard or paper through an agitator to produce a slurry.27. A method according to any of claims 16 to 18 or any claim dependent thereon, wherein the aggregate is formed by agglomeration, preferably in a centrifuge.28. A method according to any of claims 15 to 27 further comprising the step of drying the formed aggregate such that the aggregate has a moisture content between 5 and 40%.29. A method according to any of claims 15 to 28, wherein the aggregate is formed as pellets having a cross-sectional dimension between 4 and 15 mm.30. A method according to any of claims 15 to 29 further comprising the step of adding at least one of a waterproof coating, an adhesive additive, a fire retardant and a colouring agent to the formed aggregate.