GB2174381A - Manufacture of aggregate - Google Patents

Abstract

The manufacture of aggregate suitable for use in the building industry, particularly in the manufacture of lightweight Portland Cement bonded concrete products, is based on the use of sewage sludge which is the solid residue resulting from the processing and purification of domestic sewage and containing carbonaceous vegetable matter, fine siliceous particles, one or more alkaline oxides and small quantities of other chemical constituents which may include clay. The method of manufacturing aggregate comprises heating discrete pieces of sewage sludge to a temperature such that the fine siliceous particles and the oxide(s) and any clay present in the sludge react together to form a glassy phase material and the vegetable matter burns away to form voids in the pieces of aggregate so adding "lightness".

Description

SPECIFICATION Manufacture of aggregate Description of the Invention This invention relates to the manufacture of aggregate suitable for use in the building industry, particularly in the manufacture of Portland Cement bonded concrete products.

This invention is based on the use of sewage sludge which is the solid residue resulting from the processing and purification of domestic sewage and contains carbonaceous vegetable matter, fine siliceous particles, one or more alkaline oxides, usually sodium oxide and potassium oxide and small quantities of other chemical constituents which may include clay.

Seen from its broadest aspect the invention consists of a method of manufacturing aggregate which comprises heating discrete pieces of sewage sludge to cause constituents of each piece to react to form a hard mass.

Seen from another aspect the invention provides a method of manufacturing aggregate which comprises heating discrete pieces of sewage sludge to a temperature such that the fine siliceous particles and the oxide(s) and any clay present in the sludge react together to form a glassy phase material and the vegetable matter burns away to form voids in the pieces of aggregate.

The invention will now be described in more detail.

The processing and purification of domestic sewage produces a waste product, sewage sludge, which is the solids residue of the digested sewage and when dried resembles a soil-like material. This material is constituted of carbonaceous vegetable remains, an amount of clay and certain mineral and chemical substances.

When the sludge is processed by a suitable heat-treatment it can be caused to assume the form of a hard and strong somewhat glassy mass which contains a multiplicity of small voids. These voids are caused by the burning away of the carbonaceous vegetable remains during the heat-treatment process. The presence of the voids will act to reduce the volume weight of the material.

Suitably processed, the sludge can be converted to an aggregate which is suitable for use in the manufacture of Portland Cement bonded concrete products of value in the Building Construction industry.

While the analysis of sewage sludge may show minor variations from one source to another and, indeed, some slight variation from season to season from any one particular source, these variations are not of an order to bear with any importance upon the processing described hereunder, except perhaps to require a slight adjustment to the maximum temperature of heat-treatment. The vegetable residue components of the sludge are predominantly fibres but a proportion of the solids residue will include siliceous and chemical materials. The carbonaceous vegetable fibres and particles will burn away during the heattreatment process, leaving small voids in their place. The multiplicity of small voids in the mass of heat-treatment material will act to reduce its volume weight.

In addition to vegetable residues there will be an amount of clay material present in the sludge and this in combination with extremely fine siliceous particles and certain chemicals present will react together at a suitably elevated temperature. This reaction will generate an amount of glassy phase material which will bond the mass together to yield, subsequent to cooling, a hard and strong product, probably of a red-brown colour and containing the mass of small voids mentioned earlier.

If the heat-treatment process is suitably regulated so that some of the carbonaceous vegetable residues in the sludge are still burning with the evolution of gas, at the same time that the glassy phase material is caused to develop its lowest order of viscosity, then the pieces of sludge will expand and will come to possess a well developed vesicular structure. This reaction is known as 'bloating'.

This reaction will occur because the glassy phase material has attained a state rather similar to that of ordinary glass at the time when it is ready for 'blowing'. At this time the gas will be unable to escape from the pieces of sludge and its pressure will 'blow' bubbles in the pyroplastic (plasticity due to elevated temperature) mass forming small voids but of larger size than will be the case where the burning away of the carbonaceous material takes place before the glassy phase constituent attains low viscosity.

If, on the other hand, all the carbonaceous material is burnt away and the evolution of gas has ceased before the development of a low viscosity glassy phase, the resulting product will contain voids but only of a size directly related to the size of the pieces of carbonaceous material in the sludge which have burnt away. In such a case the temperature can be of a lower order during heat-treatment because there is no need to carry the glassy phase material to a sufficiently low viscosity to allow 'bloating' to occur. It is only necessary to develop the glassy phase to a point where it will bond the mass together with adequate mechanical strength, and this will be attained at a lower temperature than where the low viscosity glassy phase is necessary.

Thus, two types of aggregate material may be produced from the sludge. The characteristics of each type of aggregate can be discussed, as follows: (a) The aggregate produced at the lower temperature, and on a somewhat different time-temperature cycle, during heat-treatment than aggregate which contains voids resulting from the burning away of the vegetable carbonaceous material, will be of a less glassy nature than the 'bloated' aggregate and probably of slightly greater volume weight. The surface of the pieces of aggregate will be rough and irregular and will present an ability to bond well to Portland Cement.

(b) The aggregate produced by developing the galssy phase to a point where 'bloating' can occur will present a more glassy and smooth surface and will be less suitable to bond with Portland Cement. The voids within each piece of aggregate will be larger than in the case of the type of aggregate mentioned under (a) above. The pieces in this case of somewhat spherical form.

Both of the above types of aggregate can serve a particular purpose in the production of Portland Cement bonded concrete products.

The accompanying drawing is a diagrammatic illustration of one form of processing plant which may be used for the method of the invention. The sludge received at the processing plant is delivered to a covered storage area 10. The material as picked up from the storage area by a front-end loading shovel and delivered to a box feeder 11. This box feeder consists of a box shaped hopper, the bottom of which is a slat type conveyor. The rate of movement of the slat conveyor can be varied over a wide range of speeds so that the rate of material discharge from the feeder can be regulated to the required amount of material per unit of time.

The material coming from the box feeder is carried by a belt type conveyor 12 to an extrusion machine 13. In this machine the material is thoroughly mixed and water can be added should this be desirable. The screw type augers of the machine feed the material towards the discharge end of the machine where it passes through a shredding device 14, chopping the material into small pieces of the size required.

As the pieces of material are discharged from the extrusion machine they are carried by a belt conveyor 15 to a rotary kiln 16. This type of kiln consists of a refractory lined cylinder which is mounted at a slight inclination toward its discharge end and so that it can be revolved slowly. The speed of revolution is variable so that the rate at which the material will pass through the kiln may be varied. At the discharge end of the kiln is positioned a burner system 17, which can be oil or gas fired. This system projects its flame along inside the kiln toward the product charging end, that is counter flow to the movement of the product through the kiln.

The high temperature end of the kiln is near to the burner system and the low temperature end is near to the material charging end. It will be seen that the material charged to the kiln will enter at the cool end and will move progressively through increasing temperature toward the discharge end. For the particular heat-treatment required by the invention the hot zone of the kiln will be in the region of 1 0500C to 1250"C, according to the particular temperature treatment required of a specific sewage sludge or according to which of the previously mentioned types of aggregate is to be produced.

Upon discharge of the heat-treated aggregate from the kiln it is cooled in a cooler 18 provided with an air blower 19. The aggregate will contain a considerable amount of heat energy as it comes from the kiln and it may thus be of economic advantage to use the air from the cooling to dry the pieces of sludge before they enter the kiln. This could be made to provide dry or almost dry material to the charging end of the -rotary kiln, with some saving in fuel utilisation during the heat-treatment operation. Thus, the air from the cooler may be conveyed to a dryer 20 through which the pieces of sludge are arranged to pass in their travel from the shredding device 14 to the kiln 16.

On leaving the discharge end 21 of the cooler the aggregate is ready for sale and amy be charged to storage silo. It may be desirable to crush some of the material to smaller sizes and to screen it into a number of different sizes. The particular treatment given to the fired aggregate must depend upon the particular use to be made of it.

It is to be understood that the specific form of production line illustrated in the drawing is only a typical one and many different arrangements could be employed to produce the same end product.

Claims (6)

1. A method of manufacturing aggregate which comprises heating discrete pieces of sewage sludge to cause constituents of each piece to react to form a hard mass

2. A method of manufacturing aggregate which comprises heating discrete pieces of sewage sludge to a temperature such that the fine siliceous particles and the oxide(s) and any clay present in the sludge react together to form a glassy phase material and the vegetable matter burns away to form voids in the resulting pieces of aggregate.

3. A method as claimed in Claim 1 or Claim 2 wherein the pieces are heated to a temperature in the range of 1050 to 12500C.

4. Methods of manufacturing aggregate substantially as herein described.

5. A method of manufacturing aggregate substantially as herein described with reference to the accompanying drawing.

6. Any novel feature or novel combination of features hereinbefore described.