The present invention relates to a method and apparatus according to the preambles of the independent claims presented later in this patent application, for producing a useful product formed of fine, separate particles obtained from digestion sludge.

Community waste, waste derived from livestock husbandry, and for example, waste derived from the food industry, as well as sewage sludge from waste water plants contain a great deal of useful organic and inorganic material which can be used in soil improvement. However, the waste is generally not suitable to be used as such. Previously, organic/biological waste was usually composted in order to convert it into a usable form. Composting takes a lot of time, and therefore also a lot of space.

It is also a known practice to treat the above-mentioned waste by decomposition in so-called bioreactors using anaerobic bacteria. The biogas formed in decomposition can be used in energy production. Digested sludge or digestion sludge left over as waste in decomposition contains large amounts of nitrogen, phosphorus and potassium, i.e. substances, which can be used as plant nutrients. Some sludges may, however, contain toxic substances, and are therefore unsuitable for this type of use.

The sludge formed in digestion is usually dried instantly to a dry matter content of 20 - 30%. Drying is typically conducted in screw driers, or by centrifuge.

The solid waste formed in digestion can be composted to be used as soil- improvement material, which, however, is time-consuming, and also requires much storage space. The consistency of the digested sludge poses a problem when it is being composted. Sludge is essentially in a sludge form - a gummy substance that sticks to everything. Therefore, a drying agent should be mixed into the sludge before composting, in order to make the sludge airy. The drying agent can be mixed, for example, by adding the sludge to a heap of drying agent and by mixing the heap using a bucket loader. The finished mixture is stored in pits which are turned over every now and then. Storage may take years. Mixing with a bucket loader is naturally not an optimal way to mix clumpy material, because this type of composting causes odour problems, as anaerobic reactions continue inside the clumps.

The purpose of the invention is to present an improved method and apparatus for producing a useful product from the digestion sludge.

The purpose is thus to cause the consistency of the sludge to change into the most suitable form according to each use.

The purpose of the invention is also to present a method and apparatus which stops the anaerobic decomposition in the sludge as efficiently as possible.

The above-mentioned objectives are achieved using a method and apparatus according to the invention, which are characterized by what is presented below in the characterizing parts of the independent patent claims.

A typical method for producing a useful product from the clumpy and dried sludge obtained using biological decomposition, the dry matter content of which sludge is typically 20 - 30%, comprises - comminuting (fragmenting) of clumpy sludge into small particles using a grinder, which operates on the principle of a double-action impact mill, - mixing of (dispersing) of the fragmented sludge with air, or with other corresponding oxygenous gas, in order to interrupt or stop altogether the anaerobic reactions taking place in the sludge, and the simultaneous - mixing (dispersing) of inorganic and organic solid material such as sand, ash or bark in the fragmented digestion sludge.

Digestion sludge clumps are fragmented in a grinder, typically into < 1 mm-sized particles, even powder-like particles, in which case the whole volume of digestion sludge becomes in contact with air to stop anaerobic activities. A necessary amount of air is usually automatically absorbed into the grinder, which operates on the principle of a double-action impact mill, through the sludge feed opening, which stops the anaerobic activity in the fragmented digestion sludge. More air or oxygenous gas can be fed into the device if necessary.

Inorganic substance, such as sand or fly ash, is added in the grinder simultaneously with the digestion sludge. Thus, a dryish material is obtained, the particles of which stay separate and do not reclump. Using this method, it is possible to obtain sludge which is easy to handle with regard to transport, storage and dosaging.

The amount of the inorganic substance to be fed depends on the desired consistency of the product. Also, an inorganic substance such as sand and ash, is capable of absorbing humidity. The drier the sludge, the less sand, ash or other additives are needed. The purpose of the useful product determines in part how much and what inorganic substance should and could be added to the digestion sludge.

Sand is fed to the digestion sludge typically in the proportion of Digestion sludge / sand in the range of 1 / 0.5 - 1 / 4

Correspondingly, ash can be typically fed into the digestion sludge in the proportion of Digestion sludge / ash in the range of 1 / 1 - 1 / 5.

Ash may even contain toxic substances in which case its use may be limited. Sand is pure, but relatively expensive, which restricts its use. Therefore, mixtures of sand and ash can also be used, if this ensures the most economic solution.

Some organic substance or drying agent such as bark, peat, wood chips, sticks, straw, or any other organic solid substance which will improve the airiness, can additionally be added to the sludge mixture if desired. In addition, it is possible to add additional substances, such as lime or other substance which influences the pH- value, to the sludge mixture.

Two or more double-action impact mill-type grinders in sequence can be used in sludge handling if desired.

Previously, it has been extremely difficult to handle sludge which is too wet, dry and clumpy, or too finely ground. Wet sludge is difficult to mix with anything. In partially dried clumpy sludge, the anaerobic reactions continue inside the clumps, which causes severe odour problems. On the other hand, very fine, ground sludge is difficult to handle as such.

In applying the solution according to the invention, it is possible to grind sludge clumps into a very fine material and mix them with air so that anaerobic reactions stop altogether. By mixing sand, ash or other corresponding substance into the ground sludge, it is possible to obtain different types of sludge mixtures which are suitable for various useful applications. Sand and ash make the mixture refined and easy to handle, and thus easy to use. Mixtures of inorganic substance and digestion sludge become mineral-rich and nutrient-rich, and therefore they are well-suited for soil improvement, landscaping, road-side constructions, or the like. An ash-rich sludge mixture is well-suited as a foundational layer, for example, for waste disposal sites or roads, or for covering waste disposal sites.

An apparatus according to the invention is especially well-suited for use when highly different substances, such as sludges, sand or ash and air must be fed or mixed together simultaneously.

The present invention will now be described in more detail by way of reference to the accompanying figures, presented only as examples, in which

Figure Ia shows schematically an apparatus for producing a useful product containing digestion sludge; Figure Ib shows schematically another apparatus according to the invention for producing a useful product containing digestion sludge;

Figure 2 shows schematically a vertical cross-section of an apparatus applied in an embodiment of the invention, equipped with five rings, and operating on the principle of a multi-ring double-action impact mill, taken from the cross-section AA of the apparatus presented in Figure 3;

Figure 3 shows schematically a horizontal cross-section of an apparatus applied in an embodiment of the invention, and operating on the principle of a multi-ring double-action impact mill, taken from the cross-section BB of the apparatus presented in Figure 2;

Figure 4 shows a top-side view of one ring of the rotor presented in Figures 2 - 3;

Figure 5 shows schematically a cross-section of an apparatus according to Figure 2, applied in an embodiment of the invention in which a rotor pair has an open outermost ring;

Figure 6 shows the clumpy digestion sludge obtained in digestion and

Figure 7 shows the useful product produced according to the invention.

Figure 1 shows schematically an apparatus 10, which is used to produce a useful product containing digestion sludge. The apparatus 10 is a known effective grinder, operating on the principle of a double-action impact mill, which has been previously presented in Finnish patent publications FI-105699 B and FI-105112 B and PCT publication WO-96/18454.

Dried digested sludge 12, which is dried to the 20 - 30% dry matter content, and which is typically a clumpy mixture, is fed into the apparatus 10. The digestion sludge is ground in the apparatus into a fine powder-like substance, which exits from the apparatus through the discharge opening 14. Air 16 is mixed with the sludge in the apparatus 10. Air flows into the apparatus automatically, unless the apparatus is closed and air flow is blocked. The air ensures that the anaerobic reactions in the digestion sludge stop. In this apparatus, different inorganic substances, such as sand or ash, can be mixed in the digestion sludge through the connection pipe 18. The sludge mixture discharged from the apparatus is suitable for several different uses. The consistency of the mixture is such that the mixture is easy to handle and transport. It is no longer sticky or clumpy.

Figure Ib presents an alternative solution according to the invention for producing a useful product. This solution uses two grinders/mixers operating on the principle of a double-action impact mill. The first apparatus 10 operates, according to the apparatus presented in Figure 1, as a grinder of the clumpy digested sludge. The sludge mixture 14, discharged from the first apparatus 10, is fed to another apparatus 10', operating on the principle of a double-action impact mill, which can primarily operate, for example, as a mixer, in which more inorganic substance 18, such as sand or ash, or other additional substances 18', or more digestion sludge, according to the need, can be added to the ground digestion sludge mixture.

It is possible to run digestion sludge twice or several times through the same grinder if desired, as indicated by the arrow 14' in Figure Ib.

Figures 2 and 3 present a typical apparatus 10 applied in an embodiment of the invention, by which dried, clumpy digested sludge is fragmented (ground) into small particles which can be mixed (dispersed) with air so that the anaerobic reactions which are still on-going in the sludge clumps stop. The apparatus is a centrifugal mixer operating on the principle of a double-action impact mill or pin mill.

The apparatus shown in Figures 2 and 3 comprises two coaxial rotors 20 and 22 which rotate in opposite directions. One of the rotors could be a stator, or it could rotate in the same direction as the first rotor, but clearly at a different speed. It is very important that the speed difference between the rotors be relatively great.

In the first rotor 20, blades 26, 26' and 26" have been mounted on three rings 24, 24' and 24". In the other, opposite rotor 22, blades 30 and 30' have been mounted on rings 28 and 28' in the same way. The rotors may have more or fewer rings or blades than presented in the figures, depending on the case. The blades 26, 26' and 26" of the rings of the first rotor 20 are advantageously supported not only on the frame 32 of the rotor 20, but also on the lower part of the ring-shaped support elements 34', as presented in the example in Figure 4. The support element of each ring is radially essentially only the same size as the blade. Figure 4 shows a section of the rotor 20 at the ring 24'.

The digested sludge 12 is fed into the apparatus 10 through the feed opening 36 located in the rotor hub. The rotors 20 and 22, rotating in different directions, transport the sludge by the rotor force and by centrifugal force in a zigzag path to the outermost ring of the rotors, from where the fragmented sludge exits tangentially through the discharge opening 38 as shown in Figures 2 - 3.

Blades 26, 26' and 26" have impact surfaces a, a' and a" on the front side of their direction of motion, indicated by the arrows. Correspondingly, blades 30 and 30' have impact surfaces b, b' on the front side of their direction of motion. The digestion sludge fed into the apparatus collides first with the impact surfaces a of blades 24, which fling the sludge diagonally outward so that the sludge encounters the impact surfaces b of the blades 30, rotating in opposite directions, and drifts diagonally outward in the other direction. Thus, by the force of the impact surfaces, the sludge is flung from ring to ring, towards the outermost ring 40 of the rotors, from where the sludge exits through the discharge opening 38.

Thus, the sludge and other optional substance collide with the impact surfaces in question with high kinetic energy when travelling through each ring. The rotors rotating in opposite directions at high speed, 1500 - 3000 1/min, typically at 1600 1/min, generate a powerful centrifugal force that, in addition to efficiently fragmenting, maintain an efficient flow in the apparatus.

The space 44 between the outermost ring 24" and the housing wall 42 forms an open ring-shaped or cylindrical chute which opens up towards the rotors and along which the treated material, which has travelled through the rings, is chanelled to the discharge opening of the housing. It is possible to connect a motor-driven discharge screw tangentially to the discharge opening, for further transport of the treated material.

The cross-section profile of the blades 26, 26', 26", 30, 30' in Figures 2 - 4 is rectangular, and turned so that the impact surfaces of the blades are radially oriented. The impact surface of one or more blades may deviate from the radial orientation. The cross-section profile of the blades may naturally differ from the rectangular shape, for example, they may be triangular.

The rings formed by the impact surfaces are located at a slight horizontal distance L from each other. This distance may be the same between all adjacent rings, typically about 2 - 4 mm. On the other hand, the distance L between the rings may decrease in the direction from the center of the device outwards, for example, the distance may be > 3 mm between the inner rings, and < 1 mm between the outer rings.

The space between the blades, a, a', a" or b, b', the distance S, is advantageously much smaller at the outer rings 24", 30' than at the inner rings 24, 30. The distance S may also be the same.

A great distance between the blades on the inner rings makes it possible to feed material flows into the apparatus which contain relatively large, even coarse clumps, agglomerates. The small distance S at the outer rings enables efficient grinding of clumps, agglomerates. A substantial advantage is achieved when the number of blades and the distances between the rings, i.e. the so-called tightness, can be selected according to the need. The distance between both the rings and the blades can be arranged to decrease towards the outer ring, whereupon the clumps etc. being fragmented are packed into an increasingly tight space as they flow towards the outer ring, which makes fragmentation more effective.

Figure 5 presents a variation of the apparatus shown in Figures 2 - 3. Figure 5 illustrates an apparatus in which the housing walls 42' are located at a distance from the outermost ring 24" of the rotors, creating a wide space 48 between the rotors and the housing walls, into which the sludge is flung from the impact surfaces a" of the blades of the outermost ring. Some of the sludge can freely fall down to the funnel- shaped bottom 50 of the housing, while some is flung against the housing walls 42 and flows down along the walls towards the discharge opening 52 located in the funnel-shaped bottom.

By "a useful product produced using a method and apparatus according to the invention" is specifically meant a final product which can be used without composting, as soil-improvement material, plant beds, road beds, base or cover material for waste disposal sites or as fuel in a cement kiln. Digestion sludge, to which ash has been added, is well-suited for use as fuel in a cement kiln, because it still contains large amounts of burning material. The possible toxic substances of ash disintegrate at the kiln's high temperature of 1450 0C. Possible remaining toxic substances do not cause any harm in cement or concrete, from where they cannot escape. Substances other than ash and sand can be used in the sludge as an inorganic, mineral binding agent. For example, waste plaster or stone dust or other industrial dust, obtained, for example, from purification of combustion gases, can be mixed into the sludge.

Figure 6 presents an image of untreated, partly dried digestion sludge. Correspondingly, Figure 7 shows an image of a loose and easy-to-use useful product, the particle size of which is less than 1 mm, produced from the digestion sludge and ash according to the invention.

The invention is not intended to be limited to the example of a solution according to the invention presented above; on the contrary, it is intended that the invention be broadly adapted within the scope of the claims presented below.