GB2211836A

GB2211836A - Process for converting refuse into a material in pellet form

Info

Publication number: GB2211836A
Application number: GB8824980A
Authority: GB
Inventors: David Leonard Clenin
Original assignee: Catrel SA
Current assignee: Catrel SA
Priority date: 1987-11-02
Filing date: 1988-10-25
Publication date: 1989-07-12
Also published as: JO1546B1; KR890007802A; GR1000351B; FR2622483A1; JPH01151983A; IT1223844B; DE3832771A1; ZA888201B; PL275554A1; NO884866L; CN1034148A; AU2390488A; NO884866D0; PT88814A; ES2008610A6; IT8867841A0; SE8803844L; MA21420A1; FI885008A0; BR8805574A

Description

PROCESS FOR CONVERTING REFUSE INTO A MATERIAL IN PELLET FORM The present invention relates to a process for converting refuse into a material in the form of pellets. More particularly, the process enables refuse, particularly domestic refuse, to be converted into the form of hard pellets, which are rot-proof and insoluble in water, and capable of being used, for example, in the manufacture of building materials, such as bricks, or as an aggregate for the preparation of concrete.

As is known, the problems caused by the necessity of having to eliminate a constantly increasing quantity of refuse and garbage, while limiting the dangers of pollution and other nuisances, is becoming increasingly difficult to resolve.

With the object of providing an acceptable solution to these problems, it has been proposed to convert the garbage, crushed beforehand and freed from any metallic bodies which it may possibly contain, into solid bodies of various forms and dimensions, which are able to be used as a material for constructional purposes.

Thus, for example, French Patent No 2 098 777 describes the manufacture of hard and compact blocks, capable of being used as constructional or filling material, by incorporation of a metallic silicate into a mass of unsorted, disintegrated refuse, compacting the mass as thus treated in a press under high pressure to form blocks and finally hardening the blocks, either at ambient temperature, or by heating, for example, to a temperature from 80 to 200 C.

French Patent Application No 2 286 116 relates to a process for the manufacture, from dried and disintegrated garbage, after elimination of metallic elements, of aggregates of cylindrical form which are intended for the replacement of natural gravel in the preparation of concrete. According to one example of carrying this process into effect, the powder of disintegrated refuse is mixed with a protective agent against putrefaction, such as metallic silicates or furfural, and also with a binder such as a synthetic resin, a soluble pitch or quicklime, mineral ballasts and a binder-hardening catalyst, so as to form a thick paste from which are formed cylindrical compressed bodies by press moulding, these bodies being finally subjected to a heating to a temperature at most equal to 2000C, for the purpose of hardening them.

The processes according to the prior art, as illustrated by the aforementioned documents, are not entirely satisfactory, particularly because they do not permit the obtaining of solid bodies which have, at the same time, good mechanical properties, a high biological and chemical inertness and also practically zero solubility in water, such as would be desirable, in view of their use as a constructional material.

The present invention has for its object to convert a mass of garbage, which is unsorted and simply freed, in a manner known per se, from metallic elements, particularly ferrous metals, possibly contained in the refuse, then crushed, into hard pellets which are rot-proof and practically insoluble in water. To this end, the process according to the invention has the features which are set out in claim 1.

Thus, the process according to the invention is based on the combination of a heat treatment of compressed bodies obtained by granulation under pressure of a homogeneous mixture essentially formed of crushed refuse and mineral material containing calcium oxide, the grinding or crushing of these elements, so as to form a fine powder constituted of granular particles and fibres, and finally the agglomeration of this powder in pellet forms.

It has surprisingly been found that this novel combination of operations permits pellets to be obtained which have a high mechanical resistance and also a high chemical and biological inertia, associated with a practically zero solubility in water, it thus becoming possible for these pellets to be used for a large number of various useful purposes and particularly for the manufacture of constructional materials, without any risk of pollution and other nuisances.

The mineral matter containing calcium oxide in the free or combined state may, for example, be chosen from quicklime, fly ash, calcite, dolomite and argillaceous materials.

The proportions of the mixture of crushed refuse and mineral matter containing calcium oxide are advantageously from 90 to 95% by weight of refuse for 5 to 10% by weight of this mineral matter.

Moreover, the mixture of crushed or ground refuse and mineral matter containing calcium oxide may also have incorporated therein various fillers and especially a mineral material containing calcium carbonate and magnesium carbonate, in the free state or combined, for example, dolomite.

The different operations for carrying into effect the process according to the invention may be carried out in any suitable manner, particularly using the known techniques and the conventional equipment for treating refuse.

In particular, for obtaining refuse which contains practically no metallic elements, it is possible to use refuse or garbage originating from the removal thereof to the collection depots and subjected to the usual operations of sorting and screening, in order to separate out therefrom the objects which may possibly be recovered and/or recycled, such as particularly, metallic items of large dimensions.

Thereafter, the refuse may be brought to the form of a homogeneous powder-like mixture, which is practically free from metallic materials and especially iron. For this purpose, the refuse may be subjected to crushing and mixing operations conducted simultaneously or separately and possibly combined with one or more sorting operations intended for separating out lumps of metal and more particularlry iron-based pieces. For example, the sorted refuse may be subjected to a first rough crushing, down to an average lump size of about 50 mm, this crushing being carried out before or after a magnetic sorting for the removal of iron particles and other ferromagnetic materials, and being followed by a second crushing, e.g.

using a rotary hammer, mill or shredder, so as to reduce the refuse to particles, in the form of granules and fibres, having maximum dimensions which are smaller than 10 to 20 mm.

The operation of mixing the mass of crushed refuse with the mineral material containing calcium oxide may also be effected by using any suitable technique and equipment, particularly by means of proportioning devices and automatic mixers which function intermittently or continously.

For the preparation of compressed bodies from the mixture of crushed refuse and mineral materials containing calcium oxide, it is possible with advantage to use a rotary granulating apparatus having rollers and dies, such as an apparatus of the type which is used for the production of granulated animal feeding stuffs, permitting compressed bodies of cylindrical form to be obtained, having for example a length of 5 to 60 mm and a diameter of 2 to 20 mm, by granulation under a pressure of the order of 150 to 900 bars.

The heat treatment of the compressed bodies may advantageously be effected by being passed axially through a rotary tubular kiln with a horizontal axis of rotation, the said kiln being heated by hot air and having an internal helical wall assuring the the circulation of the said bodies. It is particularly possible to use a kiln or oven traversed by an air current heated to a temperature in the region of 250-400 by means of a burner and flowing in the same direction as the compressed bodies.

The effect of this treatment is to reduce the water content of the bodies, for example, to a value 5% by weight, and to increase their hardness, because of the carbonation effect due to the transformation of the calcium oxide into calcium carbonate. The crushing of the compressed bodies, after the drying and hardening thereof, may be effected in any suitable manner, particularly by using a hammer mill with a detachable grid for the passage of the crushed material.

Hereby obtained is a divided material having a certain proportion of fibrous materials, originating for example from the textiles, plastics and cellulosic materials contained in the initial refuse, as well as a certain proportion of granular particles resulting from the crushing of mineral substances, such as glass, limestone, etc., or organic substances contained in this refuse. The length of the fibres is so advantageously regulated that the majority of them have a length which is within a given range, preferably between 0.1 and 30 mm, either by acting on the crushing or grinding conditions of the compressed bodies or by carrying out a subsequent operation for regulating the length of the fibres, for example, a screening operation.For example, it is possible to regulate the length of the fibres and, in a general matter, the grain size of the divided material, when crushing the compressed bodies, by suitable choice of the fineness of the grid of the grinding mill, for example, from 1 to 4 mm.

For carrying out the agglomeration treatment to form pellets from the divided material obtained by crushing or grinding the compressed bodies, any suitable means may be used, for example, one selected from the drum-type and the pan-type agglomerating devices, as well as the combinations of these two kinds of apparatus.

Prior to the agglomeration treatment, it is advantageous to add to the divided material at least one binder and at least one mineral filler.

As binder, it is preferred to use a substance selected from the hydraulic cements, the calcination ashes of cleaning sludges and ferrosilicon in powder form. As mineral filler, it is, for example, possible to use calcium carbonate, powdered limestone, hydraulic lime, clay, the dust originating from installations for recovery of constructional materials and also dusts originating from other industrial installations.

For causing the agglomeration of the divided material or the mixture of divided materal and binder and also the eventual filler, it is advantageous to add a certain quantity of water to this material or mixture, bevore or during the agglomeration treatment.

It is, for example, possible to effect the addition of binder, possible mineral filler and water to the divided material, formed of granular particles and fibres, in such a way as to form a homogeneous mixture, in a mixer situatated ahead of the device used for carrying out the agglomeration treatment. It is also possible for this mixture to be formed actually in this latter device. As an alternative, it is possible for the binder and eventual mineral filler to be added to the divided material formed of granular particles and fibres, so as to form a first homogeneous mixture, in a mixer situated ahead of the means for carrying out the agglomeration treatment and to effect the addition of the quantity of water which is sufficient to cause the agglomeration of this mixture in this last device.

In accordance with another modification which is advantageous for carrying the process into effect, the binder and the possible mineral filler are mixed with the said quantity of water in a mixer situated upstream of the granulating device, and the mixture thus obtained is introduced into this device at the same time as the divided material formed of granular particles and fibres. According to yet another modification of the operating procedure of the process, a homogeneous mixture of divided material, formed of granular particles and fibres, with the binder and a mineral filler, is prepared in a drum-type granulating device also serving the purpose of a mixer, and then there is added the quantity of water permitting the agglomeration of this mixture in the form of pellets.The drum is caused to operate in such manner as to form the pellets and then the pellets as thus obtained are transferred into a plate-type granulating device, in which the agglomeration treatment is terminated. For example, it is possible to supply the granulating device with a homogeneous mixture of binder and mineral filler, or even binder, mineral filler and divided material formed of particles in granule form and fibres, so as to cause the enlargement of the initial pellets and to form pellets constituted of a central part coated with a shell having a composition different from that of the central part. This particular arrangement makes it possible to obtain pellets which have a peripheral part which is harder than the central part.

It is advantageously possible to subject the pellets obtained by an agglomeration treatment to a subsequent hardening treatment. For example, this hardening treatment, of which the conditions are adapted to the nature of the materials which constitute the pellets and principally to that of the binder being used, may comprise at least one operation which consists in subjecting the pellets to the action of a high pressure, heat or steam, or even to a combination of at least two of these three factors. It is obviously possible for the aforementioned operations to be combined with one another, so as to obtain pellets which have any desired structure and, in particular, a composite structure.

Example 1 A first homogenous mixture of 952 parts by weight of crushed or pulverized household garbage is prepared, said mixture having a humidity content of the order of 30% by weight (resulting from the removal of 48 parts by weight of scrap metal from a quantity of 1000 parts by weight of household garbage resulting from the collection), 50 parts by weight of quick lime and 5 parts by weight of calcium carbonate. This mixture is compressed, under a pressure of the order of 150 bars, in a press comprising rollers and dies, in such manner as to form cylindrical compressed bodies with a diameter of 8 mm and a variable length, which -is between 10 and 20 mm. These said bodies are subjected to a drying and hardening treatment by passing through a rotatable tubular furnace or kiln with a horizontal axis, which is traversed by a stream of hot air having a temperature of the order of 2500C, the travel time of the bodies in the kiln being of the order of 30 minutes.

Obtained in this way are 720 parts by weight of said bodies, having a humidity content slightly below 1% by weight.

These bodies are reduced to fine powder, by means of a hammer mill equipped with a grid of 2 mm diameter. A divided material is obtained, which is formed of granular particles and fibres with varying fibre lengths which are between 0.1 and 30 mm.

Using a mixer, there is prepared a homogeneous mixture of this divided material with hydraulic cement and powdered chalk, in the following proportions (in parts by weight) garbage-based divided material : 2 calcareous rock powder : 1 hydraulic cement : 1 This mixture then has added thereto, always in the same mixer, formed by a granulation drum, a quantity of water corresponding to a proportion of 70 parts by weight of water to 100 parts by weight of the said mixture, so as to form a paste. The drum mixer is then caused to function as a granulating drum so as to bring the paste into the form of spheroidal pellets. The first pellets as thus obtained are then transferred into a granulating pan.While causing the granulating pan to turn, there is then added a dry homogeneous mixture of cement and calcareous powder, in equal proportions by weight, using about 40 parts by weight of this mixture to 100 parts by weight of divided material in the initial mixture. In this way, pellets are obtained of which the diameter progressively increases, the pellets having the desired final diameter falling to the outside of the plate. The pellets finally obtained are allowed to dry, by being stored for several days in the open air at the ambient temperature and under normal hygrometric conditions. Obtained in this way are pellets having a diameter which is between 2 and 15 mm and of which the 3 desity in bulk is 750 kg/m3 for a humidity content of the order of 30% by weight.Each of the pellets has a peripheral part or shell which is harder and less soluble in water than the central part of the pellet.

Example 2 Domestic garbage is transformed into an inert and insoluble divided material, constituted of granular particles and fibres, in the manner as indicated in the first part of Example 1. A material in the form of spheroidal pellets is then prepared from this divided ' material, by being granulated by means of a granulating pan, the procedure being similar to that which is described in Example 1, but using, instead of a granulating pan with a single rim, one which has a double rim, that is to say, comprising an annular zone surrounding the central part.

The composition of the initial mixture introduced into the central part of the pan is as follows: (in parts by weight): garbage-based divided material : 10 hydraulic cement : 7 This mixture is poured on to the granulating pan and water is added in a proportion identical with that specified in Example 1, by spraying this mixture into the granulating pan. The pellets which fall into the annular granulation zone, by passing over the rim of the central circular granulation part, have a diameter which is variable and which is between 7 and 11 mm.In the annular granulation zone, the pellets are sprinkled with a homogeneous mixture of Portland cement and calcareous powder having the following composition (in part by weight): Portland cement : 1 Calcareous powder : 1 The proportions of this latter mixture being used is 50 parts by weight to 100 parts by weight of divided material, formed of granular particles and fibres with a garbage base. At the same time as this last mixture, water is added, by sprinkling, in order to permit the agglomeration.

The pellets which are finally obtained and of which the diameter is between 8 and 12 mm, pass over the outside rim of the annular granulation zone and are exposed to the ambient temperature, under normal hygrometric conditions, in order to obtain the final hardening thereof.

In this way, pellets are obtained which have a peripheral shell harder than the central part. The density in bulk of the material is in the region of 700 kg/m3.

Example 3 The pellets obtained according to Example 1 are used as aggregate for the preparation of concrete. For this purpose, the pellets are mixed with a thin mortar of cement and water corresponding to a dosage of 400 kg of cement per cubic meter of mixture of pellets and cement, with a water/cement weight ratio equal to 0.57.

After a hardening period of 36 days, a concrete block as thus obtained has a density of 1500 kg/m3 and has the following mechanical characteristics: Tractive force under flexion N/mm2 : 1,78 Resistance to compression (on cube) (N/mm2 : 7.17 Its thermal insulation coefficient (W/m.K) is 0.55.

Example 4 The pellets obtained according to Example 1 are used as aggregate for the manufacture of bricks, by mixing these pellets with cement, adding water and compacting the mixture in moulds with vibration. The proportion of cement used corresponds to a weight of 300 kg of cement per cubic meter of mixture. After drying and hardening at ambient temperature for 32 days, which brings the water content to about 20% by weight relatively to the weight of the cement, the bricks as thus obtained have a specific gravity of 1650 kg/m3 and a resistance to compression of 4.3 N/mm2. The thermal insulation coefficient t thereof is 0.45 W/m.K, this permitting the bricks to be used for the construction of walls having very good thermal insulation properties.

Claims

1. Process for converting refuse into a material of pellet form, characterized in that a homogeneous mixture is prepared from crushed refuse, practically free from metallic bodies, with at least one powdered mineral material containing calcium oxide in the free or combined state, that this mixture is brought to the form of compressed bodies, that these bodies are heated so as to dry them and increase their hardness, that the compressed bodies thus dried and hardened are crushed so as to obtain a finely divided material, formed of a mixture of granular particles and fibres, and that this material is subjected to an agglomeration treatment, so as to bring it to the form of pellets.

2. Process according to claim 1, characterized in that the fibre length of the said divided material is regulated, during the crushing of the said compressed bodies and/or by an operation subsequent to the crushing.

3. Process according to claim 2, characterized in that the said operation of regulating the length of the fibres subsequent to the crushing is a screening.

4. Process according to one of claims 1 to 3, characterized in that crushing of the compressed bodies and/or the subsequent operation of regulating the length of the fibres are so effected that the fibres have lengths between 0.1 and 30 mm.

5. Process according to one of claims 1 to 4, characterized in that the said agglomeration treatment is effected by means of a device selected from the drum-type and the pan-type agglomeration devices, as well as the combinations of these two types of devices.

6. Process according to one of claims 1 to 5, characterized in that at least one binder is added to the said divided material, so as to form a homogeneous mixture before carrying out the said agglomeration treatment.

7. Process according to claim 6, characterized in that the binder is selected from the hydraulic cements, the calcination ashes from purification sludges and ferrosilicon in powder form.

8. Process according to claim 6, characterized in that also at least one mineral filler in powder form is added to the divided material.

9. Process according to claim 8, characterized in that the mineral filler is selected from calcium carbonate, powdered calcareous rock, hydraulic lime, clay and the dust which originates from installations for the recovery of construction materials.

10. Process according to one of claims 6 to 9, characterized in that the total proportion of binder and mineral filler added to the divided material formed of granular particles and fibres is such that it corresponds to a weight of 300 to 1800 kg per cubic metre of final material, in the dry state.

11. Process according to one of claims 1 to 10, characterized in that the said divided material, or the said mixture of divided material, binder and possible mineral filler agent, has added thereto a quantity of water sufficient for permitting the agglomeration of this material or of this mixture.

12. Process according to one of claims 1 to 11, characterized in that the addition of binder, mineral filler and water to the divided inert material is effected in a mixer situated upstream of the device used for effecting the agglomeration treatment.

13. Process according to one of claims 1 to 11, characterized in that the addition of binder and water, and also of the possible mineral filler, to the divided material, is effected in the device used for carrying into effect the agglomeration treatment.

14. Process according to one of claims 1 to 12, characterized in that the addition of binder and mineral filler to the divided material formed of granular particles and fibres, in such manner as to form a first dry homogeneous mixture, is conducted in a mixer upstream of the device which is used for carrying out the agglomeration treatment, and that the addition of the said quantity of water to this first mixture is effected in this latter device.

15. Process according to one of claims 1 to 12, characterized in that a mixture of the said divided material, binder and possible mineral filler is prepared and that this mixture is then brought into pellet forms, these two operations being carried out in a same mixer-granulator device, then the pellets thus obtained are placed on a granulation pan and that the binder in powder form or a powdered mixture of binder and mineral filler is added thereto so as to form pellets comprising a central part surrounded by a peripheral part having a composition different from that of the central part.

16. Process according to one of claims 1 to 12, characterized in that the binder and mineral filler are mixed with the said quantity of water in a mixer situated upstream of the device which is used for carrying out the agglomeration treatment and that, into this latter, are introduced said divided material and the mixture obtained in this mixer, so as to cause the agglomeration of the whole in the form of pellets constituted of a central part enclosed by a shell having a composition different from that of the central part.

17. Process according to one of claims 1 to 16, characterized in that the pellets which are obtained by the said agglomeration treatment are subjected to a hardening treatment.

18. Process according to claim 16, characterized in that the said hardening treatment comprises at least one operation which consists in subjecting the pellets to the action of a high pressure, heat or steam, or to a combination of at least two of these three factors.

19. A process for converting refuse into a material of Pellet form substantially as herein described and/or exemplified in any Example.