Method of processing moist fibre

Abstract

A method of processing moist fibre, the method comprising the steps of: A) passing the fibre through a pellet mill such that the fibre is formed into pellets; B) at least partially breaking up the pel

Description

Field of the Invention The present invention concerns processing of fibre. More particularly, but not exclusively, this invention concerns a method of processing moist fibre. The invention also concerns a pellet mill for use in processing moist fibre. Background of the Invention It is known to treat, dry and then pelletize some waste products. For example, residual mixed municipal wastes can be treated, then dried and pelletized to produce refuse derived fuels (RDF) or solid recovered fuels (SRF), and sewage sludge from sewage treatment works can be treated, dried and pelletized to produce compost or SRF. Treatment is typically to turn a waste product into a sanitary or otherwise more useful product. Many waste materials, including treated post-consumer absorbent hygiene product wastes from the method discussed in GB2581214, have a relatively high moisture content when collected. In the method described in GB2581214, used absorbent hygiene products are recycled to produce a “pulp”, which is pressed to reduce the moisture content from about 90% wt to about 45% wt. Thus the pulp leaving the method having been pressed is moist recycled absorbent hygiene product fibre, also called a moist fibre, having a moisture content around 45% wt. GB2581214 discusses that the pressed pulp may be further processed, for example further dehydrated (dried) prior to being burnt as a fuel. In most cases this moisture content needs to be reduced if those materials are to be recovered for recycling into new products or for efficient combustion in a waste to energy facility or industrial process. Drying of a waste material typically occurs after it has been treated, for example on treated waste, or on the pulp output described in GB2581214. Drying may be carried out prior to a pelletizing process and typically utilises hot air blown across or through the treated waste material. These systems include technologies such as; fluid bed, rotary kiln and moving belt dryers. The heat is usually sourced from the combustion of fossil fuels such as oil and gas or biomass e.g. wood. Pelletizing forms a material into a pellet, which helps to enable efficient handling, storage and use of the material. Materials in pellet form can be used as a feedstock in forming other products, and / or can be burned as a fuel, depending on the properties of the material. If a material is formed of strands / pieces which are too large, the ability of the material to be formed into pellets may be compromised. Thus it is desirable to have a material which has greater “formability” by having a reduced size fraction. Drying is an energy intensive process, which typically involves the use of fossil fuels / wood for heat. It is desired, for economic and environmental reasons, to reduce the energy requirements of this process and / or to enable more environmentally friendly energy sources to be used for drying. The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved method of method of processing moist fibre. Summary of the Invention According to a first aspect of the invention, there is provided a method of processing moist fibre, the method comprising the steps of: A) passing the fibre through a pellet mill such that the fibre is formed into pellets; B) at least partially breaking up the pellets, thereby releasing moisture from the pellets, C) repeating steps A and B at least once; and D) passing the fibre through a pellet mill a final time, such that pellets of the processed fibre are formed. When the fibre is passed through a pellet mill, heat energy is transferred to the fibre from friction generated during the pelletizing process. Therefore, the fibre, and moisture present in the fibre, is hotter once it has passed through the pellet mill than before the fibre has passed through the pellet mill. At least partially breaking up the pellets increases the exposed surface area of the pellets, and increases the amount of water that evaporates from the fibre as compared to an intact pellet. By repeating the steps of passing the fibre through a pellet mill, and at least partially breaking up the resultant pellets, the method utilises an inherent property of the pellet making process in an unexpected way to improve the efficiency of the drying process. This is contrary to conventional methods of processing moist fibre, which can comprise i) a single step of passing the fibre through a pellet mill, and then transferring the resulting pellets to an industrial dryer to release moisture from the pellets, or ii) drying the fibre in an industrial dryer and then pelletising the fibre in a pellet mill to form a pellet. The use of an industrial dryer requires a significant amount of energy to increase the heat of the pellets for an extended period of time to allow for the moisture of the pellets to be reduced to an acceptable level. The present invention provides an alternative method of processing fibre which may result in significant energy savings. Additionally, capital expenditure can be reduced, making the process cheaper, and the simplicity of the process can be increased, because no additional dryer required. Moreover, the carbon footprint for process can be reduced in comparison to preceding methods. Moreover, the repetition of the passing of fibre through a pellet mill and then at least partially breaking up the resultant pellets may improve the quality of the final product. The formability of the fibre / fibre pellets may be improved through the repeated pelletizing of the fibre into fibre pellets. The process of repeating the passing of fibre through a pellet mill and then at least partially breaking up the resultant pellets may reduce the average fibre particle size, which may improve the formability of the final fibre pellet obtained by the method. The step A) passing the fibre through a pellet mill such that the fibre is formed into pellets may be followed by a step of transferring the pellets into a moisture separator. The step B) at least partially breaking up the pellets, thereby releasing moisture from the pellets may be undertaken in a moisture separator. The method may comprise the step of hot air created in the pellet mill during the pelletizing process being passed into the moisture separator along with the pellets. Such an arrangement may advantageously utilise what would otherwise be waste heat to further increase the drying effect of step B. The pellet mill may be insulated to prevent heat loss to the surrounding environment. Such an arrangement may increase the temperature inside the pellet mill during the pelletizing process, with a knock-on increase in temperature of the pellets created by the pellet mill, and / or an increase in temperature of hot air which is passed into the moisture separator with the pellets. Such an arrangement may improve the drying effect of step B, without requiring any additional energy input into the system. Such an arrangement is also contrary to conventional arrangements, where it is more typical to provide cooling systems to reduce the temperature of pellet mills during operation. The pellet mill may be insulated with one or more insulating layers. The insulating layer may have a thermal conductivity of less than 1 W m'1 K’1. The step B) at least partially breaking up the pellets, thereby releasing moisture from the pellets may comprise at least partially breaking up the pellets with friction. The friction may be created due to one or more of the fibre pellets coming into contact with each other, and / or walls of the moisture separator, and / or fixed or moving obstructions placed such that the pellets make contact with the obstructions. The step of transferring the pellets into a moisture separator may comprise passing the pellets through air-blown ducting. Passing the pellets through air-blown ducting may comprise the use of a fan to blow the pellets through the air-blown ducting. The ducting may be insulated to prevent heat loss to the surrounding environment. Such an arrangement may increase the temperature inside the ducting during the process, with a knock-on increase in temperature of the pellets and / or an increase in temperature of hot air which is passed into the moisture separator with the pellets in comparison to a non-insulated ducting. Such an arrangement may improve the drying effect of step B, without requiring any additional energy input into the system. The moisture separator may comprise a cyclone. Passing the pellets through a cyclone may cause collisions between the fibre pellets, thereby creating friction, with a resultant increase in temperature of the pellets, and / or facilitating the at least partial break up of the pellets. The at least partial break up of the pellets facilitates moisture release from the pellets. The cyclone separates the input stream comprising pellets and hot air into a light output stream leaving the top of the cyclone and a heavy output stream leaving the bottom of the cyclone. The light output leaving the cyclone may comprise water vapour and light fibre particles and strands. The light output stream may be passed to a separator wherein the fibre particles and strand may be separated from the water vapour. The separated fibre particles and strands may be recycled into the process as a feed to a pellet mill. This may be as an independent feed or combined with another feed entering a pellet mill. The heavy output leaving the cyclone may be fibre and broken-down pellets. The cyclone may be insulated to prevent heat loss to the surrounding environment. Such an arrangement may increase the temperature inside the cyclone during the process, with a knock-on increase in temperature of the pellets / fibre in comparison to a non-insulated ducting. Such an arrangement may improve the drying effect of step B, without requiring any additional energy input into the system. The fibre may be recycled absorbent hygiene product fibre. The fibre may be recycled absorbent hygiene product fibre output from a recycling process, such as that described in GB2581214. Steps A and B may be repeated twice, three times, four times, five times, six times, seven times, eight times, nine times or ten times, or however many times is necessary to reduce the moisture content of the fibre to a desired level, and / or to reduce the size fraction of the fibre within the pellets to the desired level. Steps A and B may be repeated between two and ten times. Fibre leaving the moisture separator may be transferred to a pellet mill suitable for use in step A. The moisture content of the moist fibre before the process may be between 40-50% by weight. The moisture content of the moist fibre may be reduced to between 5 and 20 % by weight, after at least two repetitions of method steps A and B. The moisture content of the moist fibre may be reduced to between 5 and 20 % by weight, after six repetitions of method steps A and B. The moisture content of the fibre may be monitored at different points within the method. The moisture content of the fibre may be monitored after the moisture separation stage and / or before being fed into a pellet mill. By repeating steps A and B as necessary, the moisture content of the fibre can be precisely controlled. For example, the steps A and B may be repeated any number of times until the moisture content of the fibre reaches a desired value. Depending on the initial moisture content of the fibre and potentially the ambient conditions in which the method occurs, this may be more or less for different batches of fibre. Monitoring the moisture content of the fibre during the process may eliminate the unnecessary repetition of steps A and B when the moisture content of the fibre reduces to the desired level more quickly than usual. After the fibre has been dried to a specified moisture content, the fibre may be passed to the final pelletising step, as in step D, from which pellets with a desired moisture content is obtained. After the final pelletising step, the pellets may be cooled with a pellet cooler using ambient airflow. After the final pelletising step, the pellets may be further dried by an industrial drying process, for example a drying bed or a hot air dryer such as fluid bed, rotary kiln and / or moving belt dryers. After the final pelletising step, the pellets may be cooled and then dried by an industrial drying process. As the moisture content of the pellets is already lower than that common in the prior art, the energy expended by the drying bed or hot air drier may be less than in prior art arrangements. The method according to the first aspect may comprise an initial step of passing the moist fibre through a granulator to granulate the moist fibre before it is passed into the pellet mill. Granulating the moist fibre comprises shredding the fibre into small shredded pieces suitable for milling. The granulated moist fibre may be fed as an input to the pellet mill. The moisture separator may separate the feed of pellets and warm air into output streams of i) moist air together with fine fibre particles; and ii) fibre. The method may comprise the step of passing the moist air and fine fibre particles through a filter system to obtain: i) condensate; and ii) fine fibre particles as separate output streams. The fine particles may be transferred to and fed into the granulator or a pellet mill. There may be multiple pellet mills, for example a separate pellet mill for use in each repetition of step A. The repetition of step A within the method may comprise feeding the separated fibre from the moisture separator to the same pellet mill as has previously been used for pelletizing said fibre in a previous stage. The method may comprise a continuous process, whereby a plurality of pellet mills are used and the fibre passes from one pellet mill to the next. Such a method may comprise the step of continuously feeding new fibre material into the process, and a continuous output of dried pellets. The method may comprise a batch process, whereby the fibre is passed through the same pellet mill at least twice, and the output of dried pellets is performed in batches. According to a second aspect there is provided a pellet mill for use in the method of any preceding claim. The pellet mill may comprise an insulation layer to reduce heat loss to the environment. The pellet mill may be configured such that hot exhaust air and fibre pellets can be transferred away from the pellet mill together. According to a third aspect there is provided a system for processing moist fibre, the system comprising: a pellet mill; a moisture separator; a duct connecting the pellet mill to the moisture separator; and a means for transferring pellets from the pellet mill to the moisture separator. The duct and / or the moisture separator may comprise an insulation layer to reduce heat loss to the environment. The pellet mill may be arranged to form fibre into pellets. The pellet mill may be in accordance with the second aspect. The moisture separator is arranged to separate moisture from fibre. The moisture separator may have an outlet for i) moisture and fine fibre particles; and ii) an outlet for fibre; and a duct connecting the pellet mill to the moisture separator and a means for transferring pellets from the pellet mill to the moisture separator. The moisture separator may be a cyclone. The system may comprise a duct connecting the outlet for fibre to the inlet of a pellet mill. The system may comprise one, two, three, four, five, six, seven or eight pellet mills, and may comprise the same number of cyclones. It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a block diagram of the method according to a first aspect of the invention; Figure 2 shows a diagram of a pellet mill according to a second aspect of the invention; and Figure 3 shows a system diagram of the system for processing moist fibre according to a third aspect of the invention. Detailed Description Figure 1 shows a block diagram of a method 100 of processing moist recycled absorbent hygiene product fibre 102. The method 100 comprises the step of passing 103 the moist fibre 102 through a pellet mill. The pellet mill forms pellets, which fall under gravity into a chamber of the pellet mill, and the operation of the pellet mill produces heat which results in hot air leaving the pellet mill together with the pellets 104. The method then comprises at least partially breaking up the pellets 105. The pellets which are produced by the pellet mill are transferred, along with warm air from the pellet mill, through ducting by blown air and into a moisture separator. Within the ducting and the moisture separator the pellets are at least partially broken down back into fibre 106. The method then comprises repeating 107 the steps of passing 103 the fibre 106 through a pellet mill and then at least partially breaking up the pellets 105. Optionally, the steps are repeated 107 more than twice, for example up to eight times. Repetition 107 of these steps further dries the fibre and improves its formability by reducing the size fraction of the fibre. The method 100 then comprises passing 109 the partially dried fibre 110 through a pellet mill for a final time, so as to obtain pellets 112 of processed fibre. The method 100 reduces the moisture content of the moist fibre entering the process. The method 100 optionally has the step of monitoring 108 the moisture content of the fibre and deciding based on the monitored moisture content how many times to repeat 107 the method steps of passing 103 the moist fibre through a pellet mill and then at least partially breaking up the pellets 105. Monitoring the moisture content may be performed by any method or using any known moisture monitoring means. The method 100 optionally has the steps of cooling 111 the pellets and drying 113 the pellets. Cooling 111 the pellets is done by allowing the pellets to rest in a cooler environment than that present in the earlier steps of the method. The step of drying 113 the pellets is done using a hot air dryer or other suitable drying equipment. The method 100 optionally begins with the step of passing the moist fibre through a granulator 101. Passing the moist fibre through the granulator 101 shreds the moist fibre into fine pieces 102 which are easily processed by the pellet mill. The breaking down of the pellets may be at least partially done in a cyclone separator. The cyclone has two outputs: i) a stream of water and fine fibre particles 114; and ii) the stream of fibre 106. The method 100 optionally comprises the step of separating 115 the water and fine fibre particle stream into water 117 and fine fibre particles 116 using a separator. The fine fibre particles 116 may be added to a stream 102 entering a pellet mill. Figure 2 shows a diagram of a pellet mill 200. The pellet mill 200 has a roller 201 and a die 203 and an area for receiving fibre 205 above the die and below the roller 201. The roller 201 is arranged to force fibre 205 through the die holes. Fibre is compressed in the die holes and cut by a knife at the exit of the die hole so that a pellet 211 is shed below the die 203. The pellet mill 200 has a casing 207 and an insulating layer 209 arranged around the casing to reduce heat loss from the pellet mill to the environment. The pellet mill 200 of the second aspect is suitable for use in the method of the first aspect and as part of the system of the third aspect. Figure 3 shows a system diagram of the system for processing moist fibre. The system 300 comprises a pellet mill 200 and a cyclone 400. The pellet mill 200 receives fibre and 102 and forms it into pellets 104. The pellets 104 are at least partially broken apart as they come into contact with walls of ducting 250, which connects the pellet mill 200 and the cyclone 400. The cyclone is configured to separate water vapour from the broken down fibre pellets and output fibre in a separate stream to the separated water. Some light fibre particles also leave the cycle in the water stream. The system optionally further comprises a separator 500, configured to receive light fibre particles and water from the cyclone, and separate these into water and light fibre particles. The system can have a plurality of pellet mills 200, and / or cyclones 400. In the case where there are a plurality of pellet mills, the at least partially broken down pellets exiting a first cyclone may enter the second pellet mill in a series. In this case, the pellets leaving the second pellet mill may enter the second cyclone 400 in a series of cyclones. The at least partially broken down pellets exiting the second cyclone may be transferred to the third pellet mill in the series. This can continue in such a way for a series of up to ten pellet mills and cyclones. Alternatively, the at least partially broken down fibre leaving a cyclone may be transferred into the feed of a pellet mill which has already milled that fibre. Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. For example, variations in the number of repetitions of the steps of forming the fibre into pellets and then passing them through a cyclone are disclosed herein. Alternative drying equipment may be used instead of the hot air dryer for drying the pellets after the final step. 5 Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that 10 integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A method of processing moist fibre, the method comprising the steps of:A) passing the fibre through a pellet mill such that the fibre is formed into pellets;B) at least partially breaking up the pellets, thereby releasing moisture from the pellets,C) repeating steps A and B at least once; andD) passing the fibre through a pellet mill a final time, such that pellets of the processed fibre are formed.

2. A method according to claim 1, wherein step A) is followed by a step of transferring the pellets into a moisture separator.

3. A method according to claim 1 or claim 2, wherein step B) is undertaken in a moisture separator.

4. A method according to claim 3, wherein the moisture separator comprises a cyclone.

5. A method according to any preceding claim, wherein hot air created in the pellet mill during step A) is passed into the moisture separator along with the pellets.

6. A method according to any preceding claim, wherein the pellet mill is insulated with one or more insulating layers.

7. A method according to claim 2, wherein the step of transferring the pellets into a moisture separator comprises passing the pellets through air-blown ducting.

8. A method according to any preceding claim, wherein the fibre is recycled absorbent hygiene product fibre.

9. A method according to any preceding claim, wherein after step D) the pellets are cooled and / or dried.

10. A method according to any preceding claim, wherein the method comprises an initial step of passing the moist fibre through a granulator to granulate the moist fibre before it is passed through the pellet mill in step A.5 11. A pellet mill for use in the method of any preceding claim.

12. A pellet mill according to claim 11, wherein the pellet mill comprises an insulation layer.10 13. A system for processing moist fibre, the system comprising:at least one pellet mill according to claim 11 or 12;at least one moisture separator;ducting connecting the pellet mill to the moisture separator; anda means for transferring pellets from the pellet mill to the moisture separator.

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