GB2272695A - Treating ash from burnt animal waste - Google Patents

Abstract

A particle containing ash from a burnt animal waste, notably burnt poultry litter ash, comprises a core particle or particles provided by the clinker ash and/or fine ash forms of such ash and an outer layer or layers provided by a self-adherent or cementitious form of such ash. The particle may be made by mixing the fine ash and clinker ash with water, then reducing the size of the clinker ash and causing the fine ash to adhere to the clinker particles. The particles produced are used as fertilizers, optionally after being coated with oil.

Description

TITLE: METHOD AND COMPOSITION The present invention relates to a method and composition, notably to a method for forming particles from a burnt animal waste and to particles produced by that method.

BACRGROUND TO THE INVENTION: Recently, it has been proposed to burn animal wastes, notably poultry litter, to generate steam which is then used, for example, to generate electrical power. A byproduct from such a process is ash which is produced in two distinct forms: finely divided fly ash which is recovered from the off-gases using electrostatic precipitators or other techniques, and coarse bottom ash or clinker which is recovered by the use of scrapers, moving chain grates or other techniques. Such ashes represent a waste product which must be disposed of.

However, the fly ash represents a dust and health hazard and is typically present in about 35% of the total ash produced and cannot readily be disposed of. It has been proposed to form solid pellets containing both the clinker and the fly ash by subjecting a mixture of the two ashes to high pressure using a roller or other compaction pelletizing machine. Such pellets find use as fertilizers or plant nutrients due to their potassium and phosphorus content.

However, the pellets readily disintegrate and release the fine ash as dust, which is unacceptable for most uses.

Surprisingly, we have found that the ash develops a cementitious or self-adherent property upon treatment with water. This property may arise in part from the formation of cementitious components by interaction between components of the ash in the presence of the water and/or may arise due to the formation of soluble salts and/or hydroxides by hydration of the alkali- or alkaline earth-metal oxides by at least part of the water, which soluble salts or hydroxides form a fluid binding phase in the ash mixture.

This property enables a mixture of the ashes to be formed into particles in which the fine particles of the ash are bound sufficiently firmly to resist break up of the particles during handling and storage to an acceptable degree.

SUMMARY OF THE INVENTION: Accordingly, the present invention provides a particle containing ash from a burnt animal waste, characterised in that the particle comprises a core particle or particles provided by the clinker ash and/or fine ash forms of such ash and an outer layer or layers provided by a self-adherent or cementitious form of such ash.

The term fine ash particles will be used herein to denote that fraction of the ash from the burning of the animal waste which is recovered from the off-gases of the burning process; and the term clinker ash will be used to denote that fraction of the ash which remains as a solid residue in the grate, hearth or other receptacle or carrier upon which the waste is burnt. Typically, the fine ash will have a particle size of less than 100 micrometres, notably about 50% less than 50 micrometres. Typically, the clinker will have a particle size greater than 0.5 mms, notably about 75% greater than 1 mm.

We believe that the self-adherent or cementitious property of the ash resides predominantly in the fine ash component and that the self-adherent or cementitious property is achieved by admixing the fine ash particles with water and allowing the water to be taken up by the particles.

Partially or wholly treated fine particles can then be agglomerated to form the particles of the invention.

The particles of the invention can be formed wholly from the fine ash particles by a conventional granulation or agglomeration process in which a particle having the desired size is progressively built up by causing the fine particles to adhere to one another, typically in the presence of or with the addition of sufficient water to cause the particles to become self-agglomerating. However, such a process does not make use of the clinker formof the ash, which then has to be disposed of separately.

It is therefore preferred that the particles comprise a core of crushed or comminuted particles of the clinker form of the ash having an outer layer formed from the self-adherent and/or cementitious form of the fine ash.

Accordingly, from another aspect, the present invention provides a particle containing ash from a burnt animal waste, characterised in that the particle comprises a core particle or particles provided by the clinker ash and/or fine ash forms of such ash and an outer layer or layers provided by a self-adherent or cementitious form of such ash.

Such composite particles are conveniently formed by a process in which the two forms of ash are admixed in an initial stage in the presence of water to cause the selfadherent and/or cementitious properties to develop; the mixture is then treated in a subsequent stage to reduce the size of the clinker form of the ash to within a desired range and the fine ash particles are caused to adhere to the exposed surface of the clinker particles, notably by causing further mixing of the moistened ash particles to take place, so that a composite particle is produced containing the clinker particles as the core particles and having their outer faces covered with or blinded by the self-adherent and/or cementitious fine ash particles.The outer layer need not form a continuous coating over the whole of the surface of the core particle(s) but may form a discontinuous coating in which the self-adherent and/or cementitious fine particles fill the irregularities or surface recesses of the core particle(s). For convenience, the invention will be described hereinafter in terms of the formation of such a composite particle.

The invention also provides a method for forming a particle of the invention which comprises treating particles of the ash with water so as to develop a self-adherent and/or cementitious property; and forming the treated particles into particles by tumbling or agitating the wetted particles. Preferably, the two forms of the ash are admixed in an initial stage in the presence of water to cause the self-adherent and/or cementitious property to develop; the mixture is subsequently treated to reduce the size of the clinker component of the ash to within a desired range; and the fine ash particles are caused to adhere to the exposed surface of the clinker particles, notably by causing further mixing of the moistened ash particles to take place, so that a composite particle is produced containing the clinker particles as the core particles and having their exposed faces covered with or blinded by the self-adherent and/or cementitious fine ash particles.

We have found that the fine ash particles interact with water at ambient temperature to form the self-adherent and/or cementitious forms of the ash. Such treatment may involve an exothermic reaction and it is preferred to carry out such treatment as an initial stage prior to the particle formation stage, whereas in conventional granulation art the agglomeration is achieved by addition of water or fluid phase during the agglomeration process. The pre-treatment of the ash is conveniently carried out by mixing the required amount of water with a mixture of the clinker and fine ash particles to form a substantially uniformly wetted mixture in which the self-adherent and/or cementitious properties develop during and subsequent to the application of water to the ash.

The amount of water required to achieve the desired mixture may vary according to the nature of the material from which the ash is derived, and the conditions under which the ash has been stored prior to the particle formation process.

Thus, with burnt poultry litter the optimum water content of the mixture will usually lie in the range 5 to 15 by dry weight of the ash. However, since the ash is usually hygroscopic, this figure represents the total amount of water to be added and the water taken up by the ash by absorbtion from the atmosphere, typically as much as 5%, during transport and storage needs to be taken into account when determining the optimum amount to be added directly to the ash mixture for present use.

The water can be added to the fine ash to form a wetted mixture thereof which is subsequently admixed with the clinker particles prior to the particle formation stage of the process of the invention. However, it is preferred to admix the fine and clinker forms of the ash to form a mixture to which the water is added, optionally during the mixing process. The development of the self-adherent and/or cementitious properties in the ash mixture will typically occur within minutes of treatment with the water. It is therefore not usually necessary to allow the mixture to stand to complete development of the desired property and the wetted ash mixture may be used directly in the subsequent particle formation stage. However, if desired, the treated ash may be allowed to stand to dissipate any excessive heat which may develop during the treatment with the water.

It is preferred to mix the fine and clinker forms of the ash in an approximate 1:2 weight proportion so as to achieve a mixture which has the two forms of ash in approximately the same weight proportions as are produced in the burning process, since we have found that such a mixture produces particles having suitable physical and fertilizer properties. However, the proportions of the two forms of ash can be varied according to the nature of the particle desired. Thus, a greater proportion of the fine ash may be used if it is desired to produce a harder particle, and the weight proportions of fine ash to clinker can be in the range 1:5 to 5:1, notably from 1:1 to 1:3. The optimum proportion for any given case can readily be determined by simple trial and error tests.For convenience, the invention will be described hereinafter in terms of a 1:2 weight mixture of the fine ash to clinker, since this conveniently consumes the two forms of ash in the proportions in which they are produced in the burning process.

The two forms of ash are conveniently physically admixed in the desired proportions in the form and particle size in which they are produced during the burning process.

However, we have found that the clinker form of the ash is friable and is readily broken up during transport and handling or readily weathers to a finer particle material if exposed to the weather during storage prior to use in the method of the invention. The invention can be applied to either freshly burnt ash or to ash which has been stored prior to use and is at least partially wetted. For convenience, the invention will be described hereinafter in terms of the use of ash which has undergone little weathering or storage.

The two forms of ash in the desired proportions are preferably mixed by simple physical mixing, for example turning of a heap of the ashes using a bulk handling bucket or the like, as the water is applied to the heap. During the addition of the water an initial exothermic reaction may take place. It is preferred to allow this reaction to proceed to virtual completion prior to the particle formation stage of the method of the invention. Thus, it will usually be preferred to carry out the addition of water in several aliquots with intermediate mixing until the total desired amount of water has been added and uniformly admixed through the ashes. If desired, mixing may be carried out in a rotary drum or the like. However, we have found that simple turning of a heap of the ash is sufficient.

The mixed wetted ash is then subjected to the particle formation stage of the method of the invention. As indicated above, the coarse clinker ash can have been comminuted to a fine particle size and the mixture of the fine ash and comminuted clinker then subjected to a conventional granulation process in which the particles are caused to agglomerate to form larger particles of the desired size. However, we have surprisingly found that this initial comminution of the coarse particles to a size below that desired in the final product is not required and that the desired final sized particles can be produced by a mixture of particle comminution of the coarse clinker particles to a size only slightly below that desired for the final product, and accretion of the self-adherent and/or cementitious fine particles upon the surface of the coarse particles.

It is therefore preferred that the wetted mixture of clinker ash, which is of larger size than required in the final product, and the fine ash is passed through a stage in which the coarse clinker particles are reduced in size to approximately the desired size, for example to a size between 2 and 6 mms. These particles are subjected to further mixing or agitation with the fine particles to allow the accretion of the fine particles upon the exposed surfaces of the coarse particles to produce composite particles with a size less than 6 mms. Surprisingly we have found that the wetted mixture is capable of forming product sized particles readily and that further tumbling and mixing or the particles as occurs on a conveyor belt or other transport means is often sufficient to produce product sized particles from the fine particles within the mixture.

Thus, in a preferred embodiment of the method of the invention, the wetted mixture of clinker and fine ash particles is fed to a conveyor belt or other transport means which transports the wetted mixture to a separation device, for example a vibrating screen or series of screens, at which oversize particles are separated from those having a desired product size. During this transport, some product sized particles are formed and are separated from the oversize particles in the separation device.Oversize material from the separation device is preferably separated into a very large particle fraction, typically greater than 20 mms, and this is fed to a crusher, for example a jaw crusherr or other size reducing means in which these large particles are reduced to a particle size which corresponds approximately to the desired final particle size, for example less than 6 mms. The crushed particles are then returned to the conveyor feeding the separation device. The other fraction of the oversize material, typically of a particle size between the desired size and the very coarse material, for example between 6 and 20 mms, is fed to a second comminution device adapted to reduce the particle size to below the desired particle size, for example to below 6 mms and the material is then returned to the separation device.The crushed intermediate material is preferably recycled directly to the separation device in admixture with the crushed very large particles.

Alternatively, it can be returned to the initial mixing stage to form part of the fine material being admixed with the incoming clinker ash to form the wetted mixture of particles. However, it is preferred to recycle all the crushed oversize particles to the separation device, preferably in admixture with incoming wetted particles so as to maintain the overall composition of the mixture in the particle formation stage of the method of the invention.

In a particularly preferred embodiment, the initial wetted mixture of clinker and fine ash is passed to a vibrating screen having an upper deck set at 20 mms and a lower deck set at 6 mms so as to produce a coarse fraction greater than 20 mms which is fed to the jaw crusher, an intermediate fraction which is fed to the roller crusher, and a product sized fraction which is discharged from the particle formation process. The crushed material from the jaw crusher is mixed with the crushed material from the roller crusher on a conveyor and then returned to the conveyor feeding the incoming wetted mixture to the vibrating screens.

Due to the intensive mixing action which takes place during the transport, comminution and recycle of the material, fine particles adhere to and/or accrete upon the exposed surfaces of the coarse particle so as to form a particle of the desired size having a core of the clinker particle having its surface irregularities filled or blinded by the selfadherent and/or cementitious fine particles. This process continues until the particle escapes from the recycling mixture as a product size particle.

Surprisingly, we have found that such a process involving intensive and prolonged mixing of the fine ash and comminuted clinker causes formation of the desired final particles without the need for further water or specific agglomeration techniques. However, if desired some further water may be added during the comminution and/or recycle stages and the mixture may be subjected to tumbling in a drum or similar device during its recycle.

The above particle formation process is simple and requires no additives apart from water or specialist equipment and readily lends its self to operation in a simple equipment, for example coal washing and grading apparatus. However, if desired more sophisticated equipment may be used and other additives, for example other fertilizer ingredients, may be incorporated in the mixture of ashes and water in the particle formation stage of the method of the invention.

The product from the process of the invention is a flowable, particulate product which can be used directly as a fertilizer. If desired, the product can be subjected to a drying or other treatment stage, for example an oil coating process as is conventional with granulated fertilizers.

However, we have found the process of the invention produces physically stable particles at a moisture content of from 3 to 7%, as determined by simple oven drying of the product, and that no further treatment or drying of the product is required.

The invention provides a simple and effective method for producing a useful product from the ashes from burning a wide range of animal wastes, notably poultry litter, and overcomes the problems of producing a substantially dust free particulate material in a simple and effective process without the need for additives or complex equipment.

DESCRIPTION OF THE DRAWING: A preferred embodiment of the invention will now be described in the following examples and with respect to the accompanying drawing which is a diagrammatic block diagram of the stages of the method of the invention. In the examples all parts and percentages are given by weight unless stated otherwise.

Example 1: A series of samples of a 2:1 weight mixture of the fine and clinker ashes produced by burning chicken litter from battery rearing of chickens at a temperature of from 550 to 8500C, and having a particle size of less than 6 mms were wetted up with varying amounts of water. The wetted mixtures were then subjected to mixing to allow the exothermic reaction between the water and the ashes to take place. The ability of the mixture to form pellets in the size range 2 to 4 mms was visually assessed.

At 2% added water some, but inadequate, particle formation occurred; at 5% and 10% addition of water, the particle formation was acceptable.

Example 2: A mixture 1 of 1 tonne of the fine ash (FA - particle size 100% less than 100 micrometres) and 2 tonnes of the clinker ash (CA - 100% greater than 0.5 mm) was mixed on a covered hard standing 2 using a tractor fitted with a front end bucket loader. Water was applied as a spray at the rate of 44 Kgs per tonne to the mixture by overhead spray nozzles 3 as the heap 1 was being turned and spread to achieve a substantially uniformly wetted mixture, as assessed by the general colour of the mixture.

This wetted mixture was fed into the input hopper 10 of a machine comprising a main conveyor 11 feeding material to the top deck 12 of a two deck vibrating screen separator 13.

The tod deck had a mesh aperture size of 20 mms and the second deck 14 had a mesh size of 6 mms. Material passing through both screens was discharged as product by conveyor 5. The very coarse material from the upper deck 12 was fed to a jaw crusher 16 where its particle size was reduced to below 20 mms, typically to from 6 to 10 mms. The material from the second deck 14 was fed to a roller crusher 17, whose nip was set to about 3 mms. The crushed materials from the jaw crusher 16 and the roller crusher 17 was fed to a conveyor 18 where further mixing of the particles took place. This mixture was returned by an elevator 19 to the conveyor 11 feeding the vibrating screens 13.

Product sized particles were discharged from the vibrating screen separator 13 at the rate of about 40 tonnes per hour at a water content of about 5% as assessed by drying the product in an oven at 1050C. The product was a composite particle having a core of the clinker particle with an outer partial coating of accreted fine ash.

The weight ratio of material being recycled to the product was about 67% so that material passed through the circuit of the screens, the jaw crusher, the roller crusher and the various conveyors about three times before it issued in the form of product particles. Due to the repeated mixing and tumbling action during this process, the wetted fine ash particles built up a hard accreted layer on the exposed surfaces of the clinker particles without the need for other treatments or heating the material.

The product particles were substantially dust free and sufficiently strong to resist excessive break up and dust formation during packaging, transport and storage and were suitable for spreading over the land using a conventional spinning disc type of spreader. Alternatively, the particles can be applied to peat based or other growing media in glass houses and the like in commercial or amateur horticulture.

Claims (12)

WHAT WE CLAIM IS:

1. A particle containing ash from a burnt animal waste, characterised in that the particle comprises a core particle or particles provided by the clinker ash and/or fine ash forms of such ash and an outer layer or layers provided by a self-adherent or cementitious form of such ash.

2. A particle as claimed in claim 1, wherein the particles comprise a core of crushed or comminuted particles of the clinker form of the ash having an outer layer formed from the self-adherent and/or cementitious form of the fine ash.

3. A particle as claimed in either of claims 1 or 2, wherein the ash is that obtained by burning poultry litter.

4. A particle as claimed in either of claims 2 or 3, wherein the particles contain the fine ash and clinker ash in weight proportions of from 1:1 to 1:3.

5. A method for making a particle as claimed in claim 1, which method comprises treating particles of the ash with water so as to develop a self-adherent and/or cementitious property; and forming the treated particles into particles of a desired size by tumbling or agitating the wetted particles.

6. A method for making a particle as claimed in claim 1, which method comprises an initial step of mixing the fine and clinker forms of the ash from the burning of animal waste, the mixing being carried out in the presence of added water to cause a self-adherent and/or cementitious property of the ash to develop; and a subsequent step of forming the wetted material into particles by treating the wetted mixture so as to reduce the size of the clinker form of the ash to within a desired range and causing the fine ash particles to adhere to the exposed surface of the clinker particles so that a composite particle is produced containing the clinker particles as the core particles and having their exposed faces at least in part covered with or blinded by the self-adherent and/or cementitious fine ash particles.

7. A method as claimed in either of claims 5 or 6, wherein from 5 to 15% total water is added to the ash on a dry weight basis.

8. A method as claimed in any one of claims 5 to 7, wherein the ash comprises a mixture of fine and clinker forms of the ash and these are present in weight proportions of from 1:1 to 1:3.

9 A method as claimed in either of claims 5 or 6, wherein the clinker form of the ash is reduced in size by passing the wetted mixture through a jaw crusher.

10. A method as claimed in claim 1 substantially as hereinbefore described.

11. A particle produced by a method as claimed in any one of claims 5 to 10.

12. A method for applying plant nutrients to the soil or other growing medium, which method comprises applying a particle as claimed in either of claims 1 or 11 thereto.