WO1995011865A1

WO1995011865A1 - Ceramic materials and method of manufacturing such materials

Info

Publication number: WO1995011865A1
Application number: PCT/GB1994/002352
Authority: WO
Inventors: Michael Anderson; Glynn Skerratt; Colin Birchall
Original assignee: British Technology Group Limited
Priority date: 1993-10-26
Filing date: 1994-10-26
Publication date: 1995-05-04
Also published as: AU7998094A; GB9322067D0

Abstract

The invention relates to a method of mixing incinerated sewage sludge ash (ISSA) and pulverised fuel ash (PFA) so as to produce a raw material suitable for producing building articles such as bricks, tiles or bloated aggregates. A problem with the incineration of sewage sludge to reduce the bulk prior to disposal, is that on incineration of the sludge, heavy metals normally present in raw sewage, are able to survive combustion and may subsequently be concentrated in the ash formed as a combustion residue. This has been undesirable from both an environmental and a health position. The present invention overcomes this problem by utilising a technique of entrapping such toxic species within the fired product, thereby minimising their release. Furthermore, because the heavy metals are entrapped within the bulk of the brick, slab or aggregate, they do not leach from the matrix of the building article and are therefore able to be used in a wider range of applications.

Description

CERAMIC MATERIALS AND METHOD OF MANUFACTURING SUCH MATERIALS

This invention relates to ceramic material and a method of manufacturing same, and is particularly concerned with ceramic articles, such as paving tiles, wall tiles and bricks, and also to a method of manufacturing such articles.

The treatment of waste water, and in particular the treatment of sewage, generally involves screening the sewage and then directing this to storage/settlement tanks where sewage sludge settles out. After any necessary thickening, the sewage sludge may be fed to filter presses, which form it into sludge cake.

Conventionally sewage sludge has been disposed of in a number of ways. In the U.K. approximately 53% of the annual tonnage produced has been used in agriculture, where it is used as a fertiliser or soil conditioner, 25% is dumped at sea, 18% is used as landfill and 4% is incinerated to produce an ash.

Over the last decade, this disposal pattern has not changed significantly. However as a result of new legislation, changes in this disposal pattern will have to take place in the near future. Perhaps the most immediate change will be as a result of the required phasing out and eventual termination of sea dumping in the next few years to comply with new European Community legislation. The disposal of this extra 25%, of material on land by increased agricultural application and/or by use as landfill is likely to prove extremely difficult.

Accordingly faced with this disposal problem, the water treatment companies are considering how to change the disposal pattern. The most attractive option is to increase the amount of sewage sludge which is incinerated, and several of the water treatment companies are concentrating their future disposal planning on incineration. Nevertheless, while incineration reduces the sludge to approximately a tenth of its original bulk volume, widespread adoption of this process creates its own potential disposal problem in that heavy metals normally present in the raw sewage survive combustion and are subsequently further concentrated in the ash formed as the combustion residue. Accordingly whilst the heavy metals content of such incinerated sewage sludge ash, particularly from industrial areas, may in most cases be too low for extractive reclamation, it may be of a sufficiently high level as to require special precautions when disposed of as landfill. This could be undesirable financially in the future by adding considerably to the costs involved.

In GB 12, 623 of 1889 there is disclosed the use of sewage sludge with common clay to produce a composition which can then be fired to provide articles such as bricks and slabs, suitable for building.

The use of sewage sludge in bricks was investigated in the U.S.A., for example by the University of Maryland, in the 1970's, and presently a brick manufacturing factory is in operation in South Africa using sewage sludge as part of the composition of the brick material. The drawback to the use of sewage sludge, however, is the smell which will inevitably occur during its use at the brickworks. An additional drawback is that it is potentially hazardous to health. Moreover transport and storage facilities for raw sewage delivered to a typical brickworks would require local authority approval, which under increasing environmental pressures, might prove difficult to obtain.

Japanese water companies have in the 1990's been working on the introduction of small proportions of sewage sludge ash into conventional ceramic products, such as clay wall and floor tiles, purely as a means oi disposing of the ash and not as a fuel, i.e. not to replace existing fuel added to this sort of ware - such as coal dust, sawdust etc.

In GB-B-2061241 there is disclosed the non-conventional manufacture of bricks from a mixture containing pulverised fuel ash (P.F.A.) and clay, the manufacturing method including, in one embodiment, mixing with the P.F.A. and the clay, a predetermined quantity of fuel. One form of a suitable low grade fuel mentioned is raw sewage sludge.

Accordingly it can be seen that there is a problem with the disposal of sewage sludge ash, and it is an object of the present invention to at least partly overcome this by providing ceramic material and a method of manufacturing same.

According to the present invention, there is provided a method of manufacturing ceramic material comprising mixing together pulverised fuel ash (P.F.A.), sewage sludge ash and carbon containing material, and firing the mixture so that combustion gases from the combustible carbon containing material are trapped within the fired mixture.

According to a further aspect of the present invention, there is provided ceramic material formed by firing a mixture of P.F.A., sewage sludge ash and combustible carbon containing material.

According to another aspect of the present invention, there is provided a method of manufacturing a ceramic article comprising mixing together pulverised fuel ash (P.F.A.) and sewage sludge ash, shaping the mixed material into the form of the ceramic article to be produced, and firing the shaped mixture to produce vitrification.

Preferably the mixture is shaped by pressing.

According to a still further aspect of the present invention, there is provided a ceramic article formed by firing a mixture of P.F.A. and sewage sludge ash in the shape of the article.

Desirably said ceramic article is a paving tile for use as flooring for concourses and walkways, and said tile may have a non-slip surface. Alternatively said article is a brick or a wall tile. It has been found by the inventors that finely milled sewage sludge clinker ash from multiple hearth incinerators or the fine ash from fluidised bed incinerators fuses over a similar temperature range to many brickmaking clays. However the ash has also been found to display unsatisfactory firing properties if used alone. Most seriously, it has been found to possess a narrow firing range and exhibit excessive shrinkage.

Accordingly when investigating the possible use of sewage sludge ash in the production of ceramic articles, the inventors found that, for example, pressed discs made of 100% sewage sludge ash were unsatisfactory, for example having a volume shrinkage in excess of 55% with firing at 1 100°C.

However, in accordance with the invention, the inventors found that discs of sewage sludge ash intermixed with pulverised fuel ash (P.F.A.), when fired over the same temperature range, produced acceptable shrinkage, attractively coloured, durable products. In particular, the glassy melt phase which develops during firing can encapsulate, and thus immobilise, the heavy metals present in the sewage sludge ash, thereby preventing their subsequent release, by leaching of the heavy metals, which may occur with standard leaching tests on the sewage sludge itself.

It is now known that carbon is present in P.F.A. in amounts which vary with the quality of coal burned and the efficiency of combustion. This variability of the carbon content makes it difficult to provide appropriate firing conditions in any instance where P.F.A. is the predominant ingredient in a ceramic product. If the firing conditions are not correct the properties of the fired article will be variable and there will be local over or under firing. Often the majority of the carbon content occurs in the form of relatively large particles and as a consequence air- classification or, less preferably, sieving, can be used to homogenise the normal day to day variability of normal coal fired power station ash. Such size classification has been found to have a dominating influence on the maturing temperature of P.F.A., and thus normally a suitably sized fraction/grade of the P.F.A. is used for combination with the sewage sludge ash in the mixture to be fired. However as explained hereinafter, unclassified P.F.A. can in some instances be used in the mixture.

Influent sewage to sewage works comprises both domestic and industrial effluent as well as some infiltration water, and undergoes both physical settlement and biological oxidation during treatment, and produces two products. One of these, liquid effluent is discharged to a watercourse or to the sea. Approximately 2% of the influent volume, corresponding to the 'pollution' removed, remains within the treatment plant as sludge. The composition of this sludge will depend upon the precise treatment processes and on the nature of the sewage being treated, particularly with respect to the input ratio of domestic to industrial sewage.

Table 1 shows a typical metal analysis of raw sewage sludge. Raw sewage sludge is usually made up of approximately 2-8% dry solid matter and 92-98% water. Of this dry solid matter approximately 20- 35% is mineral, 65-80% organic and volatile matter, 3-5% nitrogen and 2-3% phosphates. Sewage sludge from some highly industrialised areas may well have much greater heavy metal concentrations.

TABLE 1

TYPICAI PUB! ISHFD VAI UFS ASSOCIATFD WITH THF ANAI YSIS OF SFWAGF SI UDGF

DFTERMINAND CONCFNTRATION fas mg/Kg drv sol^'cs'

DOMESTIC. DOMESTIC/INDUSTRI^ήL

As 6 12

Cd 10 21

Cr 107 712

Cu 505 881

Hg 4 6

Mo 4 10 Ni 62 144 Pb 431 731 Se 3 5 Zn 1031 1807

Just as there is a described variability with P.F.A., there is also variability with sewage sludge ash, whether produced as grate material or by electrostatic precipitation, after incineration. It is thus necessary to examine separately ash produced in both forms and also to examine a proportional mixture of both in its current disposable form. Monitoring of the level of variability of the ash on a day to day basis will help establish the commercial viability of the ceramic products under development, with electron and optical microscopy being used to examine collected samples. The flow diagram below shows a typical physical and chemical analysis of an I.S.S.A. (incinerated sewage sludge ash) sample.

Soluble Salts Determination of I.S.S.A. Material /

JMetal Analysis of I.S.S.A. Material via Pyrosulphate Fusion /

Crystalline Phases Investigation by Scanning Electron Microscope |

j LOI Carbon Leco Analysis

Wet Size Screening of I.S.S.A. Material j

Investigation of Possible Metal Enrichment Bands /

As previously mentioned herein, the firing of the mixture immobilises the metals, primarily the heavy metals, therein, and thus it is the metal concentration of the sludge, and the ash, that is of prime concern in relation to the present invention. The phosphorus content may also be of interest, however, with regard to fluxing properties. Before the sewage sludge is incinerated, it is dewatered to decrease the water content from about 95% to approximately 70-75%. This increases the dry solids concentration and produces 'sludge cake'. This is then burnt and loses the organic matter present (approximately 75% of the mass of the sludge cake) leaving the mineral ash (approximately 25% of the mass). The composition of the ash will clearly depend on the composition of the initial sludge. Table 2 shows some general data on an analysis of typical ash from sewage sludge with a relatively high metals level.

TABLE 2

TYPICAI PUBI.ISHFP VALUES ASSOCIATED WITH THF ANALYSIS OF SFWAGF SLUDGE INCINFRATOR ASH

DETERMINAND CONCENTRATION fas % of drv solids)

Sϊ0₂ 54.9

Al₂0₃ 18.4

P₂0₅ 6.91

Fe₂0₃ 5.83

CaO 5.43 κ₂o 1.86

MgO 1.27

Ti0₂ 1.06

Na₂0 0.93 so,²- 0.46 cr 0.20

BaO 0.18

Cr₂O_s 0.1 1 so₃ 0.09

SeO 0.03

C 6.4

N 0.3

S 0.69

F 0.108 8

CONCENTRATION (as mg kg drv sol irk)

Al 15413

As 22.6

Cd 134

Cr 2223

Cu 3579

Fe 4508

Hg <0.1

Mg 8304

Ni 361

Pb 1439

Zn 1 1383

The main ceramic product will, at least initially, be a paving tile for use as flooring for concourses and walkways. This type of product was selected primarily to minimise any customer resistance to products with "sewage origin connotations", it being felt that this would be at a minimum with a product used for walking on, rather than, for example, a wall tile. Of course other ceramic articles, such as bricks, can be produced instead.

As previously mentioned there may be some instances where size classification of P.F.A. may not be necessary. However this may only be practical on a large commercial scale on a day to day basis if a suitable batch-blending system is used. One such system is in the form of a large capacity silo with air-jets inside. In use the jets create a convection action which blend the ash charged into the silo into a homogeneous mixture, which can then be used in the mixture to produce the tile or other ceramic article.

In use, the P.F.A., which may or may not have been size classified, is mixed with the sewage sludge ash in correct proportions, along with an appropriate amount of binder material, e.g. clay, and water. The mixture is then formed into the required shape by pressing, this being more suitable than extrusion or roller bat due to the non-plastic nature of the body. The forming, drying (if applicable) and firing properties can be monitored and evaluated to determine the most suitable mix-design route to be followed.

Drying may or may not constitute a stage in the manufacturing process, depending on the method of firing. If the tiles are fired one-high in a slot kiln where they move forward on a conveyor, they could be taken straight off the production machine and dry in the early zone of the kiln as they are conveyed to the firing zone. If alternatively they are to be fired in a kiln which requires them to be stacked-up in piles, they would have to be dry to support the loading weight. Thus drying is an optional stage of production dependent on the firing method to be employed.

Some I.S.S.A. may be size classified before mixing it with the P.F.A. This is more likely to be the case with multiple hearth ash, which needs prior grinding to a powder before mixing with the P.F.A., than with fluidised - ash. This latter ash has individual ash particles which appear very friable and possibly prone to break-down during mixing with the P.F.A. Accordingly they may well break-down in the rough air-turbulence occurring inside an air classifier. However it is considered that the behaviour of I.S.S.A. is unlikely to be size-dependent in the way that P.F.A. is. Instead, any difference in melting behaviour will be dependent on composition variations, i.e. the presence of more or less of the recognised ceramic fluxing oxides, such as iron, nitrogen and phosphorus. Thus air classification of the I.S.S.A. may or may not be required, and indeed may not be possible with some sources of supply, so that as a consequence it may be necessary to size-cut the P.F.A. around any particular I.S.S.A.'s fusion behaviour.

As the proposed P.F.AJI.S.S.A. body is non-plastic, there will be little or no shrinkage during drying, and consequently the internal stresses that occur in a normal clay body as it shrinks, thus dictating a safe-drying schedule, will not exist. Accordingly a very much faster drying schedule is possible, resulting in a lower energy input/tonne of ware compared with conventional clavware. The sewage sludge is incinerated at a low temperature, compared to the final firing temperature of the P.F.A/I.S.S.A. mixture, and this kills pathogens and sterilises the material into an odourless, easily handleable residue with no storage problems at the tile/brick factory. This incineration also sinters together the non-combustible residue in the raw sewage into a 'char-like' form. In this condition it has only limited mechanical cohesion and is consequently still reasonably friable, but it is readily transportable and mixable in suitable tile/brickmaking equipment with the P.F.A. and binder.

The P.F.AVl.S.S.A. mixture ready for firing would be shaped, preferably in a hydraulic press. As used herein, the terms 'shape', 'shaping' and 'shaped' in relation to the mixture include in their respective scopes the extrusion and soft-mud making methods also used for making conventional bricks and tiles.

Physical tests on the product can include bulk density, shrinkage, strength, water absorption and textural character,its microstructure being examined by an optical microscope and a scanning electron microscope. A study of the firing behaviour can involve dilatometry, Differential Thermal Analysis Thermo Gravimetric Analysis and thermal shock determination. The product will be tested for compliance with any relevant British Standard, e.g. BS 1286: 1974, covering clay tiles. A key area of product testing is in relation to the immobilisation within the ceramic product of the heavy metals contained in the sludge ash. As mentioned previously, these can leach from the ash itself but are encapsulated in the ceramic product. Chemical leaching studies can be carried out to determine the degree of heavy metal encapsulation, and thus the ability of the product to effect such immobilisation. Increased dosages of appropriate heavy metals can be added to the tile mix to investigate further the efficiency of the process as an encapsulation technique. Tests can be based upon the EPA Extraction Procedure Toxicity Test and the EPA Toxicity Characteristics Leaching Procedure, and the DEV - S4 procedure for example. The sewage sludge ash is preferably produced by using a fluidised bed incinerator, this giving an ash of fine quality. However the ash can also be produced by a multi-hearth incinerator. The clinker produced can be reduced to a fine powder of the same size as that produced from a fluidised bed incinerator by milling. The firing of the mixture containing P.F.A. and sludge ash is carried out in a conventional tile/brick kiln and at conventional temperatures, for example 1000-1 100°C, this killing any residual pathogens, and encapsulating the heavy metals in a glass matrix. The firing can take place in a conventional tunnel or intermittent kiln or a clamp. The proven fast drying/firing cycle of P.F.A. based ceramics should permit significantly lower energy costs compared with traditional clay based tiles.

A typical mixture ready for firing has the following percentage by-weight composition:

Raw Materials (dry basis)

Pulverised Fuel Ash 44% [Coarse to Medium Grade Ash]

Incinerated Sewage Sludge Ash 44%

Sodium Silicate 2%

Water 10%

The example of the mixture given above would be most suitable for a coarse to medium size grading of P.F.A., e.g. between 250 and 45 microns. With a fine grade of P.F.A., e.g. between 45 and 0.5 microns, it may be advantageous to add Furnace Bottom Ash (F.B.A.) to the mixture. F.B.A. is composed of approximately 20% of ash which does not escape a power station boiler, but falls to the floor and is quenched with water into a mainly glassy slag. The F.B.A. performs useful work as a 'grog' or filler in ceramic products that are of very fine particle composition, as it serves to open the body up for rapid oxidation. A typical mixture incorporating F.B.A. could be as below, with the percentage by-weight being indicated: 12

Raw Materials (dry basis)

Pulverised Fuel Ash 39% [Fine Grade Ash]

Incinerated Sewage Sludge Ash 39%

Furnace Bottom Ash 10% [Milled to less than 0.5mm grading]

Sodium Silicate 2%

Water 10%

Binders other than sodium silicate could be used, for example clay or starch, or possibly phosphoric acid, lignosulphonate etc.

It will appreciated that it is advantageous if the plant manufacturing the ceramic articles is adjacent to a suitably located sewage sludge incinerator, using P.F.A. imported from a nearby power station. A number of sites already exist where the geographical positioning of sewage sludge incinerators are in close proximity to existing coal fired power stations. Proposed sitings of future incinerators by the water companies may offer similar geographical "couplings".

Instead of producing ceramic material in the form of a ceramic article, such as a brick or a tile or the like, the invention is also applicable to the production of ceramic material in the form of lightweight aggregate comprising P.F.A. and I.S.S.A. The method of manufacture would be similar to that used in the Lytag (Registered Trade Mark) process where pelletised P.F.A. is 'flash-fired' into a lightweight ceramic aggregate. All the considerations mentioned above in relation to the production of a ceramic article can apply to the production of this aggregate, including sizing of ash. However the aggregate can probably be manufactured using P.F.A. which is subject to neither size classification nor batch- blending. This is because the pellets are formed by rolling and accretion, building up coating after coating of ash around a moist 'seed' pellet, and not pressed to a close dimension as with the ceramic article mixture, which requires the grading of the mixture to be consistent to produce products of invariable dimensions. In this aspect of the present invention, the P.F.AJI.S.S.A. mixture has a small quantity of pulverised coal or other combustible carbon containing material added to it. The mixture is then pelletised in the same way as for producing Lytag conveyed on a sinter-strand. Before they reach a gas firing hood their temperature is well below the combustion temperature of the carbon within them. Their sudden arrival under the high temperature gas hood, where the temperature is 1200 - 1300°C, results in an almost immediate ignition of the pellet bed and sintering of their outer surfaces at a time when most of the carbon is still within their interiors and commences to burn. Whilst a ceramic article produced from P.F.A. and I.S.S.A. is a vitrified, non-'bloated' solid, the aggregate pellet is 'bloated' by the generation of such combustion gases, produced from the coal or other carbon providing material in the mixture, which gases are trapped by the formation of a surface skin due to fusion, which causes the body to 'bloat' as the pressure of the combustion gases expands the 'plastic' enclosing skin, producing an open-textured or porous interior sealed within a semi-vitreous outer shell.

As far as a typical mixture for aggregate production is concerned, one component is P.F.A. and water, with the P.F.A. ideally containing up to 8% unburned carbon for the sintering process. Any diffϊcieπcy can be corrected by adding the appropriate amount of pulversised coal/breeze at the mixing stage. The P.F.AJI.S.S.A. 'Lytag-like' product would be made up of P.F.A. and I.S.S.A. in a 50:50 ratio, instead of P.F.A. alone.

Claims

1. A method of manufacturing a ceramic article comprising mixing together pulverised

fuel ash (PFA) and sewage sludge ash, shaping the mixed material into the form of a

ceramic article to be produced, and firing the shaped mixture to produce vitrification.

2. A method of manufacturing ceramic material comprising mixing together pulverised

fuel ash (PFA), sewage sludge ash and carbon containing material, and firing the mixture

so that combustion gases from the combustible carbon containing material are trapped

within the fired mixture.

3. A method according to Claim 1 or 2 wherein a binding material is introduced into

the mixture.

4. A method according to any preceding claim wherein the ceramic material is a

lightweight ceramic aggregate.

5. A method according to any preceding claim wherein the mixture is shaped by

pressing.

6. A ceramic material formed by firing a mixture of pulverised fuel ash. sewage sludge

ash and combustible carbon containing material.

7. A ceramic material formed by firing a mixture of pulverised fuel ash and sewage

sludge ash in the shape of the article.

8. A ceramic material according to Claim 6 or 7 having a non slip surface.