While the invention is described with reference to the production of porous clay granules, it will be appreciated that this invention has application in processes for the production of porous granules formed of any suitable material.

Background of the Invention There is considerable prior art which describe the mixing of clay and sawdust (or other particulate combustible material) and then firing the mixture in such a way that the sawdust is combusted to produce a porous calcined clay.

Porous calcined clay pellets have several applications particularly in horticulture where they may be used as a hydroponic growing medium, a propagation media, or a potting mix additive, for example.

Prior art methods of producing porous clay granules typically include initially mixing the clay with a combustible particulate material such as sawdust. The mixture of clay and sawdust may then be pelletised, before being dried, and calcined. It is during calcination of the pellets that the sawdust is combusted, resulting in porous clay granules.

German patent application No. 3823641 describes the manufacture of expanded, porous, ceramic granules from a mixture of lean bleaching earth and fatty bleaching earth. The energy produced in the process can be at least partially used to support the energy requirement of a parallel procedure in which rich bleaching clay is boiled.

Australian patent No. 583750 describes a method of treating combustible

waste materials. The method includes combining the waste material with a binder, forming pellets of the combined material, and passing the pellets successively through pyrolysis, oxidation, and vitrification zones. The volatile gases produced in the pyrolysis zone are used as fuel gas for combustion in the oxidation and/or vitrification zones.

It is an object of the present invention to provide an improved process of producing porous, calcined granules.

Summary of the Invention The present invention provides a method of producing porous, calcined granules which makes efficient use of the available energy. The present invention is based on the recognition that the process of manufacturing porous, calcined granules generates significant amounts of energy.

The present invention accordingly provides a method of producing porous calcined granules of a non-combustible material, the method including calcining agglomerates of said non-combustible material and a combustible material to effect combustion of said combustible material, wherein energy generated by the combustion of the combustible material is at least partially utilised for said calcining.

Advantageously, prior to forming agglomerates of the non-combustible and combustible material, the combustible material is dried. This drying step is particularly suitable when the combustible material is sawdust. The combustible material is preferably dried at 260°C or less until the water content of the combustible material is less than 30%, advantageously less than 10%. By reducing the water content of the sawdust it becomes more easily comminuted.

After drying, the combustible material may be comminuted using any suitable method, the comminuted combustible material then being combined with the non- combustible material to form agglomerates.

Preferably, the agglomerates of combustible material and non-combustible

material are formed as pellets or spheres in which the combustible material is incorporated interstitially in the non-combustible material.

Before calcining, the agglomerates, preferably in the form of pellets or spheres, may be at least partially dried.

Advantageously, the non-combustible material is clay, clay slurry, ceramic, or similar material with ceramic properties.

Preferably said combustible material is sawdust or the like.

In another aspect the invention provides a method of forming porous, calcined granules including forming a mixture of a particulate combustible material and a non-combustible material, forming agglomerates of said mixture, at least partially drying said agglomerates, and calcining the agglomerates in a furnace wherein energy generated by combustion of the combustible material is at least partially utilised for said calcining.

Advantageously, calcining is initiated by igniting the at least partially dried agglomerates, the calcining being sustained by the energy released by combustion of the combustible material.

Advantageously, the drying process involves subjecting the agglomerates to a rapid flow of hot air which partially includes exhaust heat from the calcining process.

In another aspect, the invention provides a method of calcining a non- combustible material in a furnace wherein the energy utilised for said calcination is at least in part derived from combustion of fuel incorporated interstitially in the material being calcined.

In this and other aspects, sufficient heat energy is provided by combustion of the combustible fuel contained within the pellets or spheres so as to partially fuse and calcine the non-combustible material. As a result, the calcining process

is essentially self-sustaining, in that no external energy input is required after initial ignition of the pellets or spheres.

Brief Description of the Drawings The invention will now be described with reference to the accompanying drawings in which: Figure 1 is a materials flow chart of an embodiment of the invention; and Figure 2 is a diagrammatic view of a furnace used with the invention; Description of Preferred Embodiments Referring to Figure 1, combustible material is provided in the form of sawdust or similar material at 10. Prior to mixing with non-combustible material 12, the sawdust 10 is preferably dried and comminuted. Where the combustible material is not sawdust, for example waste paper pulp, then this initial drying step may be excluded. The mixture is then mixed in mixer 14 before being formed into agglomerates in extruder 16, preferably into pellets or spheres. The agglomerates are then dried in dryer 18 and passed to the furnace 20. Calcination of the non- combustible material occurs in the furnace 20 and porous, calcined granules exit from the furnace 20.

Referring to the drying and comminution processes, which occur prior to mixing, combustible material in the form of sawdust is dried at 260°C or less to achieve a water content of less than 30% and ideally less than 10% in a rotary dryer. Residence time in the dryer is about 2 minutes when sawdust is dried at a temperature of 260°C.

Comminution of the dried sawdust may be performed using existing milling methods and equipment. Comminution of the dried sawdust serves to reduce the sawdust to smaller fragments-the size of the fragments of sawdust determining the size of the pores in the porous, calcined granules finally produced. Prior to

comminution, the size of the sawdust particles may be in the range up to 3mm.

After comminution, the particle size distributions typically 5%: 300 microns to 1 mm, 90%: 30 microns to 300 microns, and 5%: less than 30 microns.

The clay and comminuted combustible material are then mixed into a dough and extruded to form pellets. The mixing process can also be used to break down the combustible material. If spherical shape is desired then this can be achieved by rolling the pellets in a rotating drum after they are extruded.

The pellets are then dried and calcined. In this invention these two stages are preferably integrally linked and make use of the heat generated by combustion of the sawdust during calcining.

In general, the pellets can be dried in any conventional manner, for example in a horizontal rotating cylinder through which hot air is blown.

Alternatively, the wet granules are allowed to continuously pass under gravity through a vertical tank or drum through which hot air from the calcining furnace is blown, falling out through an exit hole in the bottom directly into the calcining furnace. They are dried by rapid flow of hot air which is achieved through combination of exhaust heat from the furnace and input of cold air (the temperature being sufficient to rapidly dry the spheres but not to cause ignition of the comminuted sawdust). The drying process is very rapid and takes approximately 2 to 3 minutes depending on granule size, using a temperature of about 300°C.

Furnace 20 consists of a static, vertical, stainless steel cylinder 21 which is a closed system. The axis of the cylinder is aligned vertically. A rotary valve 22 is located at the top end of the cylinder 21, offset from the axis of the cylinder, through which the pre-dried granules are fed directly from the dryer 18. Exhaust gases from the furnace are fed upwardly into the dryer 18 by ducts 28,30 at the top of the furnace. A heating element (not shown) is located on the air inlet to the furnace 20 so that heated air is introduced into the furnace 20 to initially ignite the material. The method of heating the air can either be electrical, gas or solid fuel.

Uncalcined material is introduced into the furnace preferably at a rate not exceeding about 1 cm per minute to avoid smoke production and overheating and fusion of the granules due to too high a fuel loading. The flow rates of uncalcined granules into the furnace 20 and calcined granules out of the furnace are controlled in order to maintain a predetermined level of the granules in the furnace. This level can be anywhere between air sparge 34 and the top of the furnace. Preferably a space of at least 300 mm is left free above the burning granules as the combustion zone of the gases. This is essential to prevent smoke from being generated.

Referring to Figure 2, as the granules enter the furnace 20, they are spread by a series of paddles 32 at the top of the cylinder 21, which are attached to a central rotating pipe 26. The central pipe 26 is attached to a variable speed drive (not shown). There are preferably four evenly spaced, radially extending paddles 32 which rotate with the central pipe 26. The paddles 32 ensure that the material is spread evenly across the cylinder 21 as it begins to diffuse downwardly through the furnace 20. As the material diffuses downwardly it passes successively through pre-heating 35, hot 36, and cooling 38 zones before exiting the furnace 20 at the lower end thereof.

In the pre-heating zone 35, hot gases from the hot zone 36 of the furnace flow upwardly through the material enabling the incoming material to be fully snap dried and partially combusted as it is being evenly spread by the paddles 32.

The granules continue to diffuse downwards and calcining of the granules takes place spontaneously in the hot zone 36 which is formed at a controlled optimum height within the cylinder 21. As the granules fall on the hot material they quickly turn black due to carbonisation of the fuel and burn with a flame. As they pass down through the furnace 20 they start to glow red hot about 1 to 2 cm beneath the surface and continue to glow for a further 2 or 3 cm as the carbon near the surface of the granules is burnt. The temperature of this red hot zone is preferably maintained at 700°C to 900°C by controlling air flow. At temperatures above 900°C the granules tend to fuse together. At temperatures below 700°C not all the carbon residues will have burnt away and the granules will be weaker in

strength due to inadequate fusing of the clay colloidal particles.

As the red hot granules pass further down the furnace 20 they turn black as they slowly run out of fuel. This black zone will extend for about a further 10cm when most of the fuel has burned away. At this stage they will slowly cool although low levels of carbon residues will be slowly oxidised at the centre of the granules. This cooling zone 38 is about a further 300 mm in depth. Cooling is achieved by air flow into the furnace 20 through air inlet 24 which is blown downwards into the cooling zone 28 by rotational air sparge 34. The rotating air sparge 34 is connected to the central rotating pipe 26 at the lower end thereof.

The sparge consists of six radially extending pipes 34 which are preferably welded to and rotate with the central pipe 26 at the same speed as the paddles 32.

The granules diffuse downwards through the cooling zone 38 and when cooled exit through a rotational valve 40. The granules can then be bulk stored or loaded or can be bagged.

The speed of combustion will depend on granule size and smaller granules will burn out faster than larger ones since the fuel is more available. The combustion depth described is for granules of about 10mm in diameter. Granules of 1mm or less will burn much faster and the depth of the combustion zone is adjusted accordingly.

Cycle time may be regulated by adjustment of furnace temperature and/or the rate at which material is fed into the furnace. A short residence time can help ensure that nutrients and/or cation exchange capacity of the clay are not diminished by the calcining process. This would be important for horticultural applications of the product.

The interior temperature of the furnace can be controlled by varying one or more of the following: (a) air regulation and distribution which is achieved by varying the rotational speed of the sparge and the pressure of the air flowing

through the sparge.

(b) regulation of the speed of input of material through the rotary valve at the top of the unit.

(c) regulation of the speed of output of material through the rotary valve at the bottom of the unit.

Exhaust gases from the unit are directed via an intercooler and are then used for the pre-drying of the granules.

It will be appreciated that the furnace is a significant energy source. Surplus energy contained within the exhaust gases can be used for roasting of the sawdust, generation of electricity, or distillation of water from clay slurry.

It will also be appreciated that some waste clay slurries have a high salt content, as high as 6000ppm which has been reduced to about 100ppm using the process of the invention. Allowing a longer residence time in the furnace assists in reducing the salt content in the calcined product which is desirable for horticultural applications of the product.

The cation exchange capacity and/or nutrient content of the calcined clay can be enhanced by adding particular clays such as montmorillinite and illite during the mixing process.

The low cost, porous calcined clay pellets, spheres or granules produced with this invention will have several applications.

Within horticulture the water absorption and water availability, air porosity, nutrient availability, and cation exchange capacity enable them to be used as: 1. hydroponic growing medium 2. a propagation media, used in the nursery industry to propagate plant cuttings

3. a wetting agent carrier, added to composted pine bark and other potting mixes to give the potting mix improved wettability 4. a potting mix additive, added to composted pine bark to give the potting mix improved buffering and cation exchange capacity as well as improved water holding capacity. The porous clay granules can be a substitute for perlite, vermiculite, peat, coir fibre etc. presently used. In addition the replacement of iarger pine bark particles (more than 5mm) with porous clay granules would improve water absorption and water availability of the mix.

5. potting mix growing media in their own right, with differing optimum levels of air porosity and water availability being achieved for different plants by varying the particle size distribution of the sawdust.

A growing medium with particle size range from 0.5 mm to 3 mm was manufactured using the process and was found to have a porosity of 22 volume percent and water absorption of 45 volume percent (most of which would have been readily available water). Very low cost production of the porous calcined clay granules can open the possibility of using them as an engineered soil to replace deteriorated and/or eroded agricultural soils.

Other possible applications for the low cost porous calcined clay granules, pellets or spheres include industrial oil absorbent, pet litter, or filtering material.

Example 1 Granules 10mm in diameter were introduced into the continuous calciner of 1 m diameter at a rate of 7 litres per minute. The flow rate out of the furnace was controlled so that the level of granules was about 300 mm from the top of the furnace. The exit flow rate at this point was about 5 litres per minute due to the shrinkage of the material in the furnace.

The combustion was viewed through a clear side window and it noticed that the granules blackened after about 45 seconds. The red hot zone extended a further 100 mm below this when the granules then started to extinguish upon cooling. The temperature of the red hot zone was 850°C as was the combustion zone above it.

The air flow was controlled using a leak off system and this was set at about 7 m3/min to avoid over heating and smoke production.

These flow rates were established in order to achieve a furnace temperature of 850°C. The granules were a terracotta colour and were extremely hard. They were found to absorb 34 wt% of water (wt of water/wet wt of granules). The pore size was found to be: 90% 30 to 300 microns 5% 300 to 500 microns 5% Less than 30 microns This proved to be an excellent potting mix when granulated to give a particle size range of 33% 0.1 to 0.5 mm, 33% 0.5 to 2mm and 33% 2 to 5 mm, giving a uniform water release over suctions from 1 Kpa to 10Kpa the level defined as'readily available water'to plants.

Example 2 Example 1 was repeated but with an air flow increased to 10m3/min.

The combustion temperature rose to 950°C and some of the granules were observed to stick together. When cold they were white in colour and were found to have a water absorption of only 24 wt% due to excess shrinkage during the calcining.

It will be appreciated that the present invention recognises that the energy

released by the combustion of the sawdust is not only sufficient to fuel the calcining and drying process, but also produces surplus energy which can be utilised for other purposes. The energy produced may be up to four times that required for calcining.

Effective harnessing and utilisation of this combustion energy can open the possibility of producing calcined porous clay granules at relatively low cost. This is achieved with the present invention which consists of a continuous and integrated process for the production of porous calcined clay. Use of waste clay and waste sawdust or other organic wastes would enable this process to be low cost.