WO2006030224A1

WO2006030224A1 - Processing of pulverised fuel ash

Info

Publication number: WO2006030224A1
Application number: PCT/GB2005/003567
Authority: WO
Inventors: Robert A. Carroll
Original assignee: Hanson Building Products Ltd
Current assignee: Hanson Packed Products Ltd
Priority date: 2004-09-15
Filing date: 2005-09-15
Publication date: 2006-03-23
Anticipated expiration: 2007-03-15
Also published as: GB0706943D0; GB2436024B; GB0420530D0; GB2436024A; GB2418201A

Abstract

There is disclosed a method of making autoclaved aerated concrete, comprising drying and grinding stockpile or lagoon pulverised fuel ash and mixing the resultant material with other materials to make autoclaved aerated concrete. The drying and grinding are carried out substantially simultaneously in a mill.

Description

Processing of pulverised fuel ash

The invention relates to a method of processing stockpile or lagoon pulverised fuel ash (PFA) and to the use of processed stockpile or lagoon PFA to make autoclaved aerated concrete (AAC). It also relates to processed stockpile or lagoon PFA and to autoclaved aerated concrete made from processed stockpile or lagoon PFA. .

Pulverised fuel ash (PFA), also known as fly ash, is a stable by-product of coal-burning power stations. For many years this recycled material has been used in the manufacture of autoclaved aerated concrete blocks. The blocks have a micro - cellular structure, featuring millions of tiny pockets of trapped air, which gives the autoclaved aerated concrete blocks features including a high strength/weight ratio, lightness for handling, high thermal insulation and excellent moisture resistance. They are also very workable, being easily cut, sawn and chased accurately with ordinary hand tools. The autoclaved aerated concrete blocks thus provide a lightweight building product used in the construction of domestic dwellings and commercial buildings.

Autoclaved aerated concrete is made by mixing PFA with sand, cement, lime, aluminium powder, additives and water. These materials are mixed to form a low-viscosity water-based slurry which is discharged from the mixer into oiled steel moulds. The aluminium powder reacts with the alkaline mix to produce hydrogen gas, so that after casting numerous small bubbles of hydrogen form within the slurry, causing the cast "cake" to expand and form a micro-cellular structure. The cake stiffens in the mould and once sufficient green strength has been achieved demoulding occurs and the cake is moved on to a cutting stage. The cut uncured material is loaded into an autoclave, where it is cured in a saturated steam environment at elevated pressures. The autoclaving step ensures sufficient strength gain for the structural strengths required. Further details of the manufacturing process for making autoclaved aerated concrete blocks from PFA are given in the book "Properties and use of coal fly ash", compiled and edited by Lindon K. A. Sear, published 2001, at Chapter 8. The PFA used in the mix provides a siliceous raw material. In many countries autoclaved aerated concrete is manufactured using quartz sand as the siliceous raw material, rather than PFA. There are however environmental benefits in using a by-product raw material as an alternative to a primary aggregate such as sand. When PFA is used for the manufacture of autoclaved aerated concrete it is "run of station" ash, i.e. the PFA is fresh from the power station.

The use of PFA to make autoclaved aerated concrete is only one use of this waste material. PFA is widely used for fill purposes, for example for road construction projects or filling in quarries. It is used in the manufacture of concrete or for making grout, which may for example be used to fill in old mine workings. Much PFA has also simply been disposed of in above-ground schemes where a landscaped mound is created.

Power stations tend to make by-product raw materials at a steady rate, whereas in-fill and other projects may be intermittent. It has therefore been necessary to store the PFA, either in a stockpile or in a lagoon. PFA is stored in stockpiles or lagoons for periods ranging from months to several years.

In order to handle and transport PFA to a stockpile it is normally conditioned by adding a controlled amount of water. In particular, this alleviates environmental concerns by preventing dust blow. In addition an appropriate moisture content ensures that the PFA can be adequately compacted when it is tipped to form the stockpile. The conditioned PFA is then transported from the power station by road vehicle or conveyor to the stockpile.

An alternative method of removal of PFA from a power station is to slurry it with water and to pump it to lagoons, where it is allowed to settle. The lagoon deposited PFA may be later recovered by draining the lagoon and digging out the PFA.

Lagoon and stockpiled material may be mixed to obtain an optimum moisture content required for use as a fill material. When the material is stored for months or years agglomeration of ash particles occurs, so that when it is to be used in concrete or grout, it will normally have to be screened to remove agglomerates. We have experimented with the use of stockpile or lagoon PFA ("aged" or "weathered" PFA) for the manufacture of autoclaved aerated concrete, but have found that there are considerable difficulties.

These problems arise because of the chemical and physical changes that occur to ash particles exposed to water for periods of months to several years. The presence of water and compaction leads to physical agglomeration of ash particles. Chemical reactions further encourage agglomeration of the discrete ash particles. These processes have the most profound effect on the "ageing" of the PFA and severely limit its use as a raw material for autoclaved aerated concrete. The first effect of moisture is the rapid dissolution of alkalis and soluble salts from the surfaces of ash particles. Calcium oxide (CaO) (quicklime) hydrates to calcium hydroxide (Ca(OH)₂) (lime) which can further react with atmospheric CO₂ to form calcite (CaCO₃). Anhydrite (CaSO₄) converts to gypsum (CaSO₄. 2H₂O). Ettringite, a calcium aluminate sulphate hydroxide hydrate (3CaO.Al₂O₃.3CaSO₄.32H₂O)_; is also likely to form relatively quickly after exposure to water. Prolonged weathering leads to some dissolution of the aluminosilicate glass core, which constitutes many of the ash particles. An amorphous clay-like material is deposited on the surfaces. The formation of these various reaction products binds discrete ash particles into agglomerates. An impermeable layer forms on the ash particles. This impedes dissolution of silicate ions from the aluminosilicate glass during the autoclave stage of autoclaved aerated concrete manufacture. Critically, the formation of calcium silicate hydrates, which bind the intercellular matrix together, depends on the availability of silicate ions. The practical consequence of this reduction in the "hydrothermal reactivity" of the PFA is the manufacture of autoclaved aerated concrete with low compressive strength.

Agglomeration of ash gives rise to lumps and "gritty" particles. This makes handling the raw material troublesome. It is difficult to disperse the PFA effectively and low quality mixes are achieved. An unsatisfactory cellular structure forms during initial setting and this leads to unstable mixes, which are prone to collapse or "slumping". We have now discovered that with appropriate processing of stockpile or lagoon PFA, it may be used to make autoclaved aerated concrete of surprisingly high quality.

Viewed from one aspect, the invention provides a method of making autoclaved aerated concrete, comprising drying and grinding stockpile or lagoon pulverised fuel ash and mixing the resultant material with other materials to make autoclaved aerated concrete.

The invention also provides autoclaved aerated concrete made by the method. Thus the invention further provides autoclaved aerated concrete made from a mixture comprising stockpile or lagoon pulverised fuel ash which has been dried and ground.

Viewed from another aspect the invention provides a method of processing stockpile or lagoon pulverised fuel ash, comprising drying and grinding the stockpile or lagoon pulverised fuel ash. The invention also provides stockpile or lagoon pulverised fuel ash, which has been processed by the processing method. Thus the invention further provides stockpile or lagoon pulverised fuel ash which has been processed by drying and grinding. After such processing, the stockpile or lagoon PFA is suitable for use in making autoclaved aerated concrete. An unsatisfactory raw material, which is difficult to process, can be transformed into an ash that is compatible with the existing dry handling and mixing equipment installed at many autoclaved aerated concrete factories.

Stockpile or lagoon PFA is "reactivated" by drying and grinding. This is attributed to de-agglomeration of the ash into discrete particles, an increase in surface area, breaking of impermeable reaction layers and/or the creation of new fracture surfaces. More amorphous glass is exposed to the intercellular matrix during autoclaving and this increases the rate at which silica is solubilised.

The process is environmentally beneficial. PFA stored within stockpiles and lagoons is a large potential silica source, and by using this source there will be a reduced reliance on obtaining silica from primary aggregates such as quartz sand or gravel deposits. The stockpile or lagoon PFA may be dried and then subsequently ground. The PFA may for example be oven-dried and then ball-milled. Preferably, however the drying and grinding of the stockpile or lagoon pulverised fuel ash are carried out substantially simultaneously. This is advantageous in that it is a single stage process. Equipment exists for carrying this process out on a large scale, for example an Atritor (trade mark) dryer -pulveriser, which is known for heavy mineral grinding and drying duties.

Oven or other drying techniques may be used. Preferably the drying is effected by a flow of hot air. This can achieve flash drying. Grinding is preferably effected in a mill. The Atritor (trade mark) dryer -pulveriser is a mill which dries material using a flow of hot air whilst simultaneously grinding the material. The flow of hot air carries the material through the mill.

In order to improve the efficiency of the drying and grinding, the stockpile or lagoon PFA is preferably sieved prior to drying and grinding. This removes the major agglomerates but allows smaller ones to pass to the next stage. A vibrating mechanical sieve may be used, serving to break up the bulk mass.

The methods of processing stockpile or lagoon PFA disclosed herein make it suitable for the manufacture of autoclaved aerated concrete. The moist (for example a moisture content of more than 15%) and cohesive feed ash is transformed from its coarse aggregated state into a finely divided, free-flowing material. Such processed ash can be used with the existing silo and handling equipment, designed for the dry "run of station" PFA currently used.

The stockpile or lagoon PFA may for example have a median particle size of at least 40μm or 45 μm or 50μm, depending on the original source and the nature and period of ageing. Most commonly, stockpile or lagoon PFA would have a median particle size of 50μm or greater.

In a preferred method for determining median particle size, the stockpile or lagoon PFA is first passed through a vibrating mechanical sieve with a mesh having 5mm diameter openings and a sample of the sieved material is collected. A suitable instrument used for determining median particle size is a Coulter LS230 Particle

Sizer. The sample is dispersed in excess water, passed through a measuring cell and a full particle size distribution undertaken by laser diffractometry. The median is then determined from the particle size distribution.

A preferred embodiment of the invention will now be described by way of example and with reference to the accompanying Figure, which shows schematically the steps in a preferred embodiment of processing stockpile PFA.

At step (a) a mechanical shovel 2 collects PFA from a stockpile 4. At step (b) the mechanical shovel 2 deposits the PFA onto a conveyor (not shown) which lifts the material (at arrow 5) onto a vibrating mechanical sieve 6 with two outlets, a first outlet 7 feeding a conveyor 10 with coarse material and a second outlet 12 feeding a conveyor 14 with fine material. The sieve has a mesh with 5mm diameter openings. Respective piles 16 and 18 of coarse and fine material collect below the outlets of the conveyors 10 and 14.

At step (c) a mechanical shovel 2 collects the fine material, which has passed through the sieve openings, and deposits it in a tipper truck 20. The tipper truck is covered and transports the material to a hopper 21, shown at step (d), where the material is deposited and conveyed by a belt conveyor 22 to a dryer-pulveriser unit 23, shown at step (e). The material is fed from conveyor 22 onto a conveyor 24 of the dryer-pulveriser unit (at arrow 25). From there it is metered into an inlet 26 of a mill 28 of the unit. The mill 28 is also supplied with a horizontal feed of air from a heater, the air inlet being shown at 30. The hot air, which may be heated to temperatures up to 600°C, dries the feed material. At the same time, the material is ground by a combination of radially arranged hammers which rotate on a main rotor, in conjunction with static pins which project in a direction parallel to the rotation axis in close proximity to the rotating hammers. The air carries the dried and ground product via an outlet 32 to a cyclone and/or bag filter 34, from which the product is discharged at outlet 36 and is then available for use in making autoclaved aerated concrete. The cleaned air is discharged via a fan 38 via an outlet 40 to atmosphere.

Examples

Five-year old stockpile PFA, obtained from a coal-burning power station in the United Kingdom, was used in experiments. Three processing routes were assessed. Examples 1 and 2 relate to laboratory scale experiments, and Example 3 relates to a large scale experiment.

Examples 1 and 2

In Example 1, laboratory specimens of autoclaved aerated concrete (250 mm cubes) were produced using stockpile PFA, which was oven dried at 105°C overnight and ball-milled in 5 kg batches for 90 minutes. In Example 2, a quantity of stockpile ash was processed through a pilot-plant Atritor™ dryer-pulveriser, Model 8 A/B Test unit at Atritor Limited, Coventry.

Other specimens were produced using ash obtained from the same power station. Stockpile ash used "as received", without drying or grinding, gave considerable process problems because of the unstable cellular structure formed. As a control raw material, dry "run of station" ash, obtained from the same power station was used (Control Experiment 1). The physical properties of the ashes used in the experimental work are given in Table 1.

In Example 1, drying and subsequent ball-milling the stockpile PFA achieved a free-flowing powder which had similar process properties to that achieved in Example 2, with ash processed through the pilot -plant Atritor™ unit. Test specimens of autoclaved aerated concrete were produced by autoclaving for 6 hours at 10 bar gauge (184°C). The test specimens produced with the ball- milled stockpile ash (Example 1), had comparable strengths to that achieved with material processed through the pilot-plant Atritor™ unit (Example 2) (see Table 2). This autoclaved aerated concrete had a compressive strength which was approximately 90% of that achieved with the control ash (2.06 N/mm² for the ball-milled stockpile ash or 2.08 N/mm² for the pilot-plant Atritor™ unit, compared to 2.23 N/mm² for the control ash). In contrast, the strength achieved with the "as received" stockpile ash was only 30% of the control material (0.65 N/mm² compared to 2.23 N/mm²) .

Table 1 Physical properties of ashes

Table 2

Properties of autoclaved aerated concrete laboratory specimens

* Compressive strengt s a measure n accor ance w t t e same a oratory method

Example 3

An Atritor (trade mark) pulveriser-dryer was used to dry and grind the stockpile PFA. This industrial mill combined a strong grinding action with the ability to remove water rapidly by flash drying, thereby providing a single stage process. A description of the processing scheme is given below, referring to the drawings and relating to a large-scale experiment. Approximately 50 tonnes of stockpile PFA was extracted and processed. The ash was used to produce autoclaved aerated concrete blocks.

Ash was extracted from a stockpile 4 by a mechanical shovel 2 and passed through a 5mm "piano wire" sieve to remove major agglomerates. Two loads of feed ash were transported to an industrial dryer pulveriser unit by covered tipper trucks 20. The unit in this case was an Atritor (trade mark) dryer-pulveriser, Model 17 A.

The moist stockpile PFA was transferred by belt conveyor 22, from a reception hopper 21, onto a conveyor 24 of the dryer pulveriser unit. From there it was supplied to the inlet 26 of the mill 28 of the unit. Hot air from a gas-fired burner was blown via horizontal feed into inlet 30 of the main body of the unit. Within the dryer-pulveriser unit a high-shear zone was created by the hammers of the steel rotor, which revolved within stator pins. Attrition between ash particles and impact with the hammers, the stator pins and the shell of the device caused de- agglomeration to occur. Also, fragmentation of discrete particles took place, creating new fracture surfaces. The high volume of turbulent hot air, relative to the feed ash, ensured rapid moisture loss.

Moist, cohesive stockpile PFA was transformed into a dry, free-flowing material by the dryer-pulveriser. The feed to the unit had a moisture content of about 17%, compared with about 0.7% for the processed ash (Table 3). Significant size reduction was achieved, from a median particle diameter of about 50 to about 20μm. Consequently, the specific surface area increased from about 4,500 to about 10,000 cm²/cm³. As expected, no changes in the bulk chemical analysis were observed (Table 5).

Physical properties of control "run of station" PFA from another power station are also shown in Table 3, and the bulk chemical analysis of this material is shown in Table 5 (Control Experiment T).

The processed stockpile ash was used successfully to produce about 57 m³ of autoclaved aerated concrete blocks. Satisfactory flow properties were observed through the silo and powder handling equipment, comparable to that using the control "run of station" PFA. Homogenous and cohesive mixes were produced. These had a satisfactory cellular structure and were stable. After steam autoclaving for 10 hours at 10 bar gauge (184° C), a mean compressive strength of 3.9 N/mm² was obtained, compared with 5.1 N/mm² for blocks made with the control "run of station" PFA (Table 4). Hence 76% of the strength of a normal production was achieved. Although the autoclaved aerated concrete of Example 3 is not as strong as that of Control Experiment 2, it is considerably stronger than that which was made in the laboratory scale experiment with the unprocessed, "as received" stockpile ash. Higher strengths may also be achieved by blending unaged PFA with processed stockpile ash. Table 3

Physical properties of ashes in large-scale experiment

Table 4

Autoclaved aerated concrete block properties

Compressive strengths measure n accordance with British Stan ard 6073

Table 5

Claims

1. A method of making autoclaved aerated concrete, comprising drying and grinding stockpile or lagoon pulverised fuel ash and mixing the resultant material with other materials to make autoclaved aerated concrete.

2. A method as claimed in claim 1, wherein the drying and grinding of the stockpile or lagoon pulverised fuel ash are carried out substantially simultaneously.

3. A method as claimed in claim 1 or 2, wherein the drying is effected by a flow of hot air.

4. A method as claimed in claim 1, 2 or 3, wherein the grinding is effected in a mill.

5. A method as claimed in any preceding claim, wherein the stockpile or lagoon pulverised fuel ash is sieved prior to drying and grinding.

6. Autoclaved aerated concrete made by a method as claimed in any preceding claim.

7. Autoclaved aerated concrete made from a mixture comprising stockpile or lagoon pulverised fuel ash which has been dried and ground.

8. A method of processing stockpile or lagoon pulverised fuel ash, comprising drying and grinding the stockpile or lagoon pulverised fuel ash.

9. A method as claimed in claim 8, wherein the drying and grinding of the stockpile or lagoon pulverised fuel ash are carried out substantially simultaneously.

10. A method as claimed in claim 8 or 9, wherein the drying is effected by a flow of hot air.

11. A method as claimed in claim 8, 9 or 10, wherein the grinding is effected in a mill.

12. A method as claimed in any of claims 8 to 11, wherein the stockpile or lagoon pulverised fuel ash is sieved prior to drying and grinding.

13. Stockpile or lagoon pulverised fuel ash, which has been processed by a method as claimed in any of claims 8 to 12.

14. Stockpile or lagoon pulverised fuel ash, which has been processed by a method comprising drying and grinding.

15. A method of making autoclaved aerated concrete, substantially as hereinbefore described with reference to the accompanying Figure or Example 1 or Example 2 or Example 3.

16. Autoclaved aerated concrete made by a method as claimed in claim 15.

17. A method of processing stockpile or lagoon pulverised fuel ash, substantially as hereinbefore described with reference to the accompanying Figure or Example 1 or Example 2 or Example 3.

18. Stockpile or lagoon pulverised fuel ash, which has been processed by a method as claimed in claim 17.