GB2178434A - Manufacture of building components - Google Patents

Abstract

A method of manufacturing a building component comprises mixing together sand washings and sewage sludge and heating the mixture to cause constituents of the materials to react together to form a hard mass.

Description

SPECIFICATION Manufacture of building components Description of the invention This invention relates to the manufacture of building components such, for example, as building bricks and blocks.

This invention is based on the use of two industrial waste materials, by-products of industrial process and of no value individually, which are mixed together and processed to provide a range of useful products for the building construction industry. These materials are sand washings, being the solid residue resulting from the industrial process of washing naturally occuring sand in order to render it suitable for other uses in the building industry and sewage sludge being the solid residue from the processing and purification of domestic sewage.

Seen from its broadest aspect the invention consists of a method of manufacturing a building component comprising mixing together sand washings and sewage sludge and heating the mixture to cause constituents of the materials to react together to form a hard mass.

Sand washings comprise particles of sand and usually a small amount of clay. Sewage sludge comprises 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 another aspect the invention provides a method of manufacturing a building component comprising mixing together sand washings and sewage sludge to form a plastic mass, forming the mass into a desired shape and then heating it so that the fine siliceous particles, the oxide(s) and any clay present in the mixture react together to form a glassy phase material bonding larger particles of sand together and the vegetable matter burns away to form voids in the component.

The invention will now be explained in more detail.

Sand washings In order to render the sand from most natural deposits suitable for inclusion as a fine aggregate component in Portland Cement bonded concrete products it is necessary to remove certain deleterious materials from the sand by means of a washing process. The water coming from the washing process carries certain solid materials and these are separated from the water to constitute a silt-like mass containing a major proportion of extremely fine particles of sand, a minor proportion of clay and a small amount of vegetable remains in the form of fibres.

Where the sand washings contain sufficient clay, which is rare, it is possible to manufacture building products, such as bricks, from the material. Where the clay content of the sand washings is too low, which is more normal, the washings possess no industrial value.

Sewage sludge The purifiction of domestic sewage results in the production of a solid soil-like material. This is composed of vegetable fibres and other vegetable remains of a carbonaceous nature. There will be present, also, some very fine particles of siliceous material of vegetable origin and other chemical substances of which sodium oxide and potassium oxide are important. A certain amount of clay is generally present as well.

When suitably heat-treated the sewage sludge will yield a hard glassy mass, usually of a red-brown colour. This mass will include a multiplicity of small voids resulting from the burning away of vegetable fibres and other vegetable remains of a carbonaceous nature. The inclusion of voids within the mass will act to reduce its volume weight and to improve its thermal insulation value.

The use of sewage sludge as an additive to sand washings Because of the relatively small amount of clay substance present in sand washings and the high proportion of extremely fine particles of sand, it is difficult ot even impossible on technical or economic grounds, to process the material by forming it into a coherent mass, drying the mass and subjecting it to heat-treatment so as to yield a product of value to the building industry.

This is so because the fine particles of sand are sufficiently non-cohesive to render it impossible to form a product, such as a building brick or block, which will possess sufficient mechanical strength to pass through manufacturing processes, subsequent to product forming, without damage.

The heat-treatment process, which develops the final mechanical strength in the end product, must generate a sufficient amount of glassy phase material in the sand washings to bond, by reacting with the fine sand particles, the mass together with adequate mechanical strength. This reaction must take place at a temperature which will be industrially economic to apply. The amount of material present in sand washings which is of a nature to yield the glassy phase material is generally of too small a presence.

At this point it should be observed that the sand washings do not contain sufficient constituents to facilitate product forming or to provide adequate strength in the formed product, nor does it contain a sufficient quantity of glassy phase forming constituents to bond the mass together strongly during the heat-treatment process. In view of the foregoing it was considered desirable to discover a low cost material which is readily available and which, when mixed with the sand washings would provide four main characteristics, namely: 1. Render the mixture sufficiently workable or plastic to facilitate forming the mixture in to a coherent mass, a brick or a block for example. Sewage sludge contains an amount of clay and vegetable remains of a fibrous nature which are of extremely fine particle size and exhibit plastic characteristics in the presence of an adequate amount of water.The addition of the sludge to the sand washings thus acts to improve the plasticity and formability of the mixture. While clay might be added in order to achieve the same reaction the use of clay would be more costly than the use of sludge. Further, additional equipment would be needed in the production line to proportion and mix the clay with the sand washings.

In this regard the use of sludge is novel in that it provides improved plasticity and workability at low cost and renders the mixture capable of being formed into the desired products.

2. Provide sufficent mechanical strength to enable the formed mass to pass safely through subseqent manufacturing processes without any loss or damage.

The addition of sludge to the sand washings promotes mechanical strength in the mixture by virtue of its plastic bonding characteristics while in the wet state and because the sludge acts as a cementing agent after the formed product has been dried. The mass of vegetable fibres in the sludge behave as a reinforcement in the formed mass, promoting strength.

In this context the use of sludge is novel in that it provides two essential characteristics, namely (i) improved plasticity and formability and (ii) improved mechanical strength to both wet and dry formed products.

3. Provide an adequate presence in the mixture of materials which will develop a glassy phase during economic heat-treatment, thus reacting with the fine sand particles of the washings to bond the mass together strongly and provide the essential mechanical strength neeeded in the end product, a building brick or block example. The sand washings do not contain the components essential to the development of an adequate amount of glassy phase material. The sludge contains very fine siliceous particles of both mineral and vegetable origin and certain chemical substances, sodium oxide, potassium oxide and frequently calcium oxide for example, which at a suitably elevated temperature will react together and with the sand particles of the washings to bind the mass together with considerable strength.

In this regard the use of the sludge as an additive to the sand washings is novel in that it provides the essential heat-treatment bonding of the mass and yields the adequate mechanical strength in the end product.

4. Because the sand washings are constituted predominantly of fine sand particles the unit weight of an end product constructed of sand washings only would be very high, a characteristic undesirable from several points of view. It is thus considered desirable that the material added to the sand washings should, in addition, act to reduce the unit weight of the fired product. This would require that the additive material should contain an amount of constituents which will burn away during the heat treatment process, leaving a multiplicity of small voids in the mass, thus reducing volume weight. Where the building products are to used in the construction of walls it is desirable that they should exhibit low thermal conductivity, that is that they should possess good insulating properties.

The use of sludge as the material added to the sand washings will, as described previously, result in there being small voids in the fired end products. The presence of these voids will act to promote low thermal conductivity in the products. In this context the use of sludge is novel in that it will act to lower the thermal conductivity of the end products.

It is to be noted that the addition to clay or a mixture of ceramic materials of a material which will burn away during the heat-treatment or firing operation in order to yield a light-wright product is not new, nor is the use of the same technology in the case of refractory products for the purpose or reducing thermal conductivity. Wood sawdust has been used for this purpose.

In such a case, however, the ceramic components of the mixture possess sufficient plasticity, in the presence of an adequate amount of water, to carry the non-plastic sawdust and yet provide formability and formed strength to the products, the latter in regard to both strength prior to heat-treatment and subsequent to heat treatment.

The addition of a completely non-plastic material, such as wood sawdust, to sand washings would render the mixture quite unworkable and of absolutely no formed strength. Further no glassy phase bonding constituent would be developed during the heat-treatment operation. This dicovery of the use of sewage sludge as the additive material is thus novel in that it provides other essential characteristics in the mixture, along with the important characteristics of light weigth and low thermal conductivity.

Consideration of the characteristics of candidate materials as additives to the sand washings, and bearing in mind technical requirements and availabiiity at low cost, leads to the selection of sewage sludge.

This material satisfies all the following requirements: a. Improves workability of the mixture and facilitates forming of the products.

b. Provides improved mechanical strength in the formed products thereby facilitating safe move ment through subsequent manufacturing processes.

c Provides an adequate proportion of materials which will form a glassy phase component in the mixture during the heat-treatment process, thereby bonding the mass together with adequate mechancial strength. This reaction should occur between the temperatures of 1050 and 1 200 C.

d. Contain a useful proportion of material which will burn away during the heat-treatment process, leaving a multiplicity of small voids in the mass, which will act to reduce the unit weight of the products.

e. Reduces the thermal conductivity of the fired product by virtue of the multiplicity of small voids generated in the mass.

It will be appreciated that both sand washings and sewage sludge will show minor variations of composition from one source to another. This is not of particular significance and provided that the sludge satisfies the above enumerated characteristics then adjustments to the proportion of sludge to sand washings can be made in order to yield the desired characteristics in the end products. Experimental work has indicated that generally a proportion of 60% of sand washings to 40% of sludge, both amounts calulated on a dry basis, will provide satisfactory results. However, it is to be understood that the proportions indicated are not essential and any proportion that will yield the desired characteristics for manufacture and in the end product are considered to fall within the scope of the invention.

Manufacturing methods There are two main methods of product forming applicable to the material of this invention, that is a mixture of sewage sludge and sand washings. These two forming methods are extrusion and pressing.

However, it is to be understood that other methods of product forming are not exlcuded from the scope of the invention.

Extrusion This technique is preferred because of its high productivity and because the mixture of sand washings and sludge would need to be prepared for extrusion with a free water content probably in the range of from 17% to 20%.

In view of the fact that both sand washings and sludge are likely to be delivered to the processing plant with some water in them it is advantageous to employ a forming technique that does not require the water content of the materials to be reduced.

In the extrusion maching the materials will receive some additional mixing and will pass through a de-airing chamber where the air will be removed from the material, increasing its plasticity and density. The material will then pass through an extrusion die which is designed to form the material in to the product shape, extruding it in a continuous column. As the formed column issues from the machine it will pass through an automatic cutter which will cut the column into suitable product lengths. The cut length is adjustable.

The extrusion machine and cutter used would be of the type normally employed in the heavy-clay side of the ceramic industry.

Pressing Pressing is not favoured because it is less productive than extrusion and because changes of product shape or size are more difficult and costly. However, less free water is needed in the mixture for pressing than for extrusion and a water content of probably around 5% to 8% would be adequate. Where the two raw materials are received at the production plant with a greater water content than that mentioned above, then water removal would be necessary before the materials could be used. This does not preclude their use but it does increase operational costs. This may or may not be significant.

In the event that the order of plasticity needed for extrusion could not be developed then it might be economic and technically viable to opt for the pressing technique.

Manufacturing apparatus Figure 1 of the accompanying drawings is a diagram matic illustration of one form of production flow-line which may be used in the extrusion method. The incoming raw materals are delivered to two storage areas 10, 11 and from there they are picked up by a front-end loader and delivered to two box feeders 12 and 13, one for sludge and the other for sand washings. Each box feeder is a hopper with a slat or belt type conveyor bottom. The rate of movement of the conveyor is adjustable so that the rate of material discharge can be carefully controlled.

Each box feeder is adjusted to discharge its material at a rate proportional to the proportions of sand and sludge required in the mixture. The two materials come together on a belt conveyor 14 which delivers them to a double shaft mixer 15. This is a trough-shaped machine with two shafts revolving in the trough. Each shaft is fitted with a number of paddles which chop and mix the material, feeding it forward towards the discharge end of the machine. Provision exists on the machine for the addition of water in the form of a fine spray. From the double shaft mixer the material passes, via a belt conveyor 16, to a storage-silo-feeder 17.

This piece of equipment provides a reservoir of prepared material in the production flow-line and includes facilities for discharging the material on to a conveyor 18 which carries it to an extrusion machine 19 at the desired rate. The discharge rate from the silo-feeder is variable.

The continuous column of product issuing from the extrusion machine passes directly to an automatic cutter 20 which cuts the column in to the required product length. The length of the cut product is variable.

The products coming from the cutter are charged on to a belt conveyor 21 which carries them to a station where they are transferred to a dryer 22. This equipment might be of one of several designs but whatever the specific design used the products will pass through the dryer, exiting therefrom in the dry state and ready to go to the heat-treatment equipment, namely a kiln 23.

Products leaving the exit end of the dryer are set upon refractory topped cars that will carry them through the kiln. The cars travel in a continuous path from a loading station L, through the kiln, past an unloading station U where the fired products are removed, and thence back to the loading station. The kiln is of a form generally used in the ceramic industry and probably, most advantageously, a continuous tunnel kiln through which the products will pass and the subjected to a gradually increasing temperature followed by a period of steady temperature and, finally, a gradual decline of temperature or cooling period.

Because of the specific nature of the product to be heat-treated it is considered very probable that a firing cycle of short duration will be practical. In such an event it is likely that the kiln will possess certain special features designed to process the particular material of the present invention. However, any specific design of kiln that can provide the type of firing cycle necessary for this product can be used.

It is to be understood that the form of production flow-line illustrated in Figure 1 is typical only and that any arrangement of machines that will process the materials and products in the same manner would be suitable.

Figure 2 of the accompanying drawings show diagrammatically one form of production flow-line for the pressing method. The incoming raw materials are delivered to two storage areas 30,31 from which they are picked up by a front-end loader and delivered to two box feeders 32,33, one for each material. The rate of discharge from the box feeders is so adjusted that the discharged materials come together on a belt conveyor 34 in the deisred percentage proportions for the mixture of materials. The materials are delivered by the belt conveyor to a continuous pan mill 34 which thoroughly mixes and kneads them. Should the materials contain too little free water for the pressing process then water would be added in the pan mill to bring the water content of the mixture to the correct figure for the pressing operation.Should, however, the mixture of materials contain too great an amount of water for the pressing operation, then the water content would need to be suitably reduced.

From the pan mill the mixed materials pass to a disintegrator 36. Here the material is reduced in size, probably into pieces of between 1 mm and 3mm in size, to make it suitable for the pressing operation.

The disintegrator can be provided with a hot air generator 37, or can use waste hot air from the kiln.

In the event that the water content of the mixture is too great for the pressing operation then hot air would be charged to the disintegrator and the water content of the material issuing from the machine would be made suitable for the pressing operation.

As for the extrusion line the prepared material is charged to a silo-feeder 38 and the discharge from that equipment controlled to suit the feed rate of the press or presses 39. Upon ejection from the presses the formed product passes, as shown in Figure 1, to a conveyor 40 which conveys it to a dryer and from there to a kiln.

In this instance the products going the dryer contain less water than products from the extrusion machine.

The drying operation is simplified and probably quicker than for extruded products.

It is to be understood that the form of production flow-line illustrated in Figure 2, is typical only and that any machine or arrangement of machines that will perform the same processing functions can be used.

Claims (7)

1. A method of manufacturing a building component comprising mixing together sand washings and sewage sludge and heating the mixture to cause constituents of the materials to react together to form a hard mass.

2. A method of manufacturing a building component comprising mixing together sand washings and sewage sludge to form a plastic mass, forming the mass into a desired shape and then heating it so that the fine siliceous particles, the oxide(s) and any clay present in the mixture react together to form a glassy phase material bonding larger particles of sand together and the vegetable matter burns away to form voids in the component.

3. A method as claimed in 1 or Claim 2 wherein the mixture is heated to a temperature in the range of 1 050 -1 250"C.

4. A method as claimed in Claim 1, 2 or 3 wherein the sand washings and the sewage sludge are mixed together in proportions in the range of 55-65% sand washings (in a dry state) and 55-45% of sewage sludge (in a dry state).

5. Methods of manufacturing building components substantially as herein described.

6. A method of manufacturing building components substantially as herein described with reference to the accompanying drawings.

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