GB2292940A - Lightweight aggregate - Google Patents

Abstract

A lightweight aggregate is produced by mixing a bloatable argillaceous material with 20 to 80% pfa having 15 to 85% retained at 45 mu m, and firing the mixture at a temperature controlled to produce an aggregate having a particle density in the range 700 to 1900 kg/m3. Instead of pfa an equivalent pulverised inherently non-bloatable argillaceous material may be utilised. The pfa or equivalent material contains 3 to 15% coke, or this amount of coke or other fuel is added as a supplementary material.

Description

Title Lightweight Aggregate and Method of Production thereof Field of the invention This invention relates to a lightweight aggregate and to a method of production thereof.

Background to the invention In many countries, including the United Kingdom, there exist no known naturally occurring reserves of commercially exploitable lightweight aggregates. Lower density naturally occurring aggregates such as pumice, scoria etc., are imported, for example for use in the manufacture of lightweight masonry blocks for building, in order to improve thermal efficiency. The high costs of manufactured imported lightweight aggregates make the local manufacture of low density aggregates a commercial proposition. Lightweight aggregates the manufacture of which is based on bloatable clays, shales, slates, mudstones or other natural or manufactured argillaceous based rocks/materials, together with pelletized slag and/or sintered pulverised-fuel ashes are already manufactured in the U.K.

As far as the present invention is concerned, the following information is relevant to existing techniques for the commercial production of lightweight aggregates.

In relation to the use of clays, shales, slates, mudstones or other natural or manufactured argillaceous rocks/minerals/materials, for convenience collectively referred to herein as argillaceous materials, it is known that certain geological sources of argillaceous materials expand when they become pyroplastic. Most expandable argillaceous material, when it reaches the optimum temperature for bloating, expands rapidly from two to five times its original volume in about five minutes. For this purpose, the bloating temperature employed is normally in excess of 950 degrees C and lower than 1250 degrees C.

Bloating is normally carried out in a kiln or sinter strand, and control of the kiln or sinter strand with respect to speed, volume of product, and temperature are critical to successful bloating, because for a particular argillaceous material if the optimum temperature for bloating is exceeded by 25 degrees C, the expanded product can change into a thick liquid, both adhering to the equipment and causing serious conglomeration.

Lightweight low density pellets manufactured by expanding clay (for example pellets known under the Trade Marks LECA and FIBO) are more suitable for lightweight concrete masonry than are dense and stronger pellets which are more costly per unit volume to produce. Some of these aggregates are used in structural grades of concrete, but due to their low particle density, the strengths of concrete obtainable are usually restricted to less than 25 N/mm2 at 28 days.

In relation to other lightweight aggregates, apart from those based on pulverisedfuel ash (pfa), which is a type fly ash, these have been produced during the past 80 years and have used shale (known by Trade Marks such as Aglite, Brag and Russlite) or slate (known by the Trade Mark Solite). However, U.K. production has ceased at the present time.

When pfa is used, it can be pelletized with small amounts of water and/or coal or coal shale and pyro-sintered on a sinter strand to form granules (for example as known by the Trade Mark LYTAG). This aggregate is conventionally produced with a particle density of about 1800 to 2000 kg/m2, as no bloating is possible.

However, the grading and granulometry of the pfa is critical to successful formation of the green pellets. While coarse pfas (+ 50% retained at 45 pm) have the lowest particle density, they are difficult to pelletize.

Moreover, BS 3892 Part 1: 1993 requires that for pfa to be used to form a cementitious component, the amount retained at 45 pm shall not exceed 12 %.

On the other hand, "run of station" pfa from power stations typically has a sieve residue of 20 to 50% retained at 45 pm size, so that an elutriator is commonly used as a means of classifying "run of station" pfa into "fines", having less than 10% retained at 45 jjm, and "coarse", having more than 50% retained at 45 jim.

Although in "run of station" pfa the coarse fraction is the major proportion of the split, there has to date been very limited industrial application for this grade of material other than in bricks, and it is usually disposed of by dumping.

Pfa, which is basically an in itself non-bloatable argillaceous material, has been found to possess some unusual physical properties:i) As both coarseness and unburnt fuel increases the particle density decreases; ii) As the exposure temperature in the power station furnace increases, the shape of the particle becomes increasingly spherical and less dense; and iii) Coarse pfa from bituminous and other hard coals invariably contain a greater proportion (0.25 to 1.0% by mass) of carbonaceous material (coke) and grits from furnace slag than finer pfa.

Thus LYTAG (Trade Mark), the sintered pfa aggregate currently commercially available in the U.K., is used principally for structural grades of concrete because strengths in excess of 70 N/mm2 can be obtained at 28 days. Although the density of the concrete approaches 2000 kg/m3, this is nevertheless a density reduction of about 15% on natural aggregate concrete.

Due to pellet forming requirements, LYTAG (Trade Mark) is conventionally manufactured on a sinter strand and not in a rotary kiln, which would be a more thermally efficient method of manufacture. For enabling the manufacture of an expanded pfa aggregate on a sinter strand, it is proposed in published U.K. Patent Application No. 2 273 095 that it should be homogenized with a minimum proportion, say about 15 to 40% by mass, of suitable prepared expandable argillaceous material. The resultant aggregate can have a particle density of less than 2000 kg/m3, so that the applications for this aggregate would be increased.

It is also relevant that the brick making techniques often, to improve the burnability of bricks during firing, use so-called "grog", which is a pre-burnt vesicular lightweight porous material containing some carbonaceous material.

This is blended with the brick clay to enhance combustion and enable escape of combustion products during firing, without damage to the fusion structure of the brick. The technique also increases brick production.

The invention According to one aspect of the present invention, there is provided a method of manufacturing a lightweight aggregate according to which a bloatable argillaceous material is mixed with pfa or equivalent pulverised argillaceous material containing 3 to 15% coke by mass and having 15 to 85% retained at 45 m, the bloatable argillaceous mineral in the mixture being in the range of 20 to 80% by mass and the pfa or equivalent material 80 to 20% by mass, and the mixture is fired in a rotary kiln, sinter strand or other pyro-technique equipment at a firing temperature controlled to produce a lightweight aggregate having a particle density in the range 700 to 1900 kg/m3.

The invention, in a second aspect, relates to a lightweight aggregate produced by the afore-defined method.

The method in accordance with the invention is believed to achieve the production of the specified lightweight aggregate from bloatable argillaceous minerals (as distinct from brick making clays), because, during bloating, the particles of pfa or equivalent argillaceous material are ceramically bonded together, which on cooling provides a continuous film of vitrified product that not only increases strength but also reduces water permeability without reducing porosity.

The use of pfa not only ventilates the internal structure of the composite particulate material but also heating across its section is more effective, while the presence of an amount of coke in the pfa, between 3 and 15%, is an effective method of dispersing fuel which adds to the controlled combustibility of the composite particle during firing. If the required amount of coke is not inherently present in the pfa, extra coke can be added up to the required amount to improve firing.

Description of embodlment A preferred method in accordance with the invention has three stages for the manufacture of the expanded aggregate with a particle density in the range 700 to 1900 kg/m3. First, either appropriate de-stoned bloatable clay or argillaceous material is brought to a suitable consistency with water in a primary pug mill, before adding the selected proportion (80 to 20% by mass) of pfa or equivalent argillaceous material which has 15 to 8556 by mass retained at 45 pm. This mixture may then pass for homogenization to a second pug mill prior to extrusion through die casts of approximately S to 15 mm diameter, which extrusions are cut into pellet lengths of about 5 to 15 mm.

Alternatively the appropriate de-stoned bloatable clay or natural or manufactured argillaceous material may be dried sufficiently for it then to be commuted to a powder, with not more than about 50% retained at 90 jim by appropriate crushing and/or grinding. After homogenization the prepared argillaceous material is added to and mixed with a selected proportion of between 80 to 20% by mass of pfa, prior to preparation into the appropriately sized pellets. The pellets can be prepared by spraying the homogenized mixture of powders with a fine mist of water on a rotary dish pelletizer, before other treatment which may be required such as compaction by rolling and dusting with calcium carbonate powder.

The actual pellet size before firing depends on the required particle size of the finished product. This in turn depends upon the amount of expansion required and the strength of the pellet as a result of firing.

The second stage is performed in a rotary dryer and/or pre-heater. This is conveniently a revolving, inclined, semi-lined large diameter long cylinder which, as it revolves, rounds the damp elongated cylindrical pellets into small spheres 5 to 15 mm diameter and/or dries the already spherical dish pelletized particles to a pre-heating temperature of 400 to 500 degrees C. At this temperature the organic materials associated with bloating are not burnt out. Alternatively the pre-drying stage can be omitted and the green pellets may be prepared in such a way as to make them suitable for immediate fusing in a rotary furnace or kiln or on a sinter strand or in any other pyrbtechnique furnace for which no pre-heating is required.

In the third stage, either the pre-heated pellets pass either via an insulated holding silo or the green pellets pass directly into the rotary furnace, sinter strand kiln or other pyro-technique furnace for expansion by bloating. The rotary kiln is preferably similar to a conventional refractory lined kiln for producing Portland cement clinker. The principle requirement is that the refractory lining is sustained at the temperature required for bloating, i.e. typically between 900 to 1200 degrees C by firing pulverized coal (the ash being insufflated into the product), or fuel oil or gas, or mixtures of fuel.Alternatively green pellets, containing a predetermined and intennixed amount of fuel, may be fired on a sinter strand where they are fed by spreading to form an open textured and permeable bed to the width and depth of the continuously moving grate that comprises a typical sinter strand. Once prepared on the bed the strand immediately carries the green pellets under an ignition hood that fires the intermixed fuel. Once ignited at about 900 degrees C and as the bed moves forward air for combustion raises the temperature sufficiently to cause bloating of the argillaceous mineral. The speed of the sinter strand and the amount of combustion air is controlled to vary the amount of expansion and thus the particle density.However, as firing and temperature control is critical for 100% argillaceous minerals the products and heating times are limited, because pyroplastic argillaceous minerals, if overheated, change into viscous and sticky liquids. The temperature range over which the pellets start to expand to the point of pyroplasticity is usually limited to within +/25 degrees C of the critical pyroplastic temperature, and the pellets having reached that critical optimum temperature are then frozen quickly into a solid state by cooling.

The present invention uses pfa (or equivalent argillaceous material) to permit the expansion of suitable bloatable argillaceous materials to be controlled over a much wider range of temperature, as "bloating" commences gradually from about 900 degrees C continuously to 1200 degrees C, avoiding the critical optimum temperature of bloating. The invention thus permits manufacture of a range of controlled particle density aggregates, which has not previously been possible with the different types and ranges of argillaceous materials available and pfa separately.

Claims (8)

Claims

1. A method of manufacturing a lightweight aggregate according to which a bloatable argillaceous material (as hereinbefore defined), which has been ground or comminuted, is mixed with pfa or equivalent pulverised inherently non-bloatable argillaceous material containing 3 to 15% coke by mass and having 15 to 85% retained at 45 ym, the bloatable argillaceous material in the mixture being in the range 20 to 80% by mass and the pfa 80 to 20% by mass, and the mixture is fired in a rotary kiln, on a sinter strand or in another suitable pyro-technique furnace, at a firing temperature controlled to produce a lightweight aggregate having a particle density in the range 700 to 1900 kg/m3.

2. A method as claimed in claim 1, according to which, if coke is not inherently present in the range 3 to 15% by mass in the pfa used as a stng material, supplementary coke or other fuel up to at least the minimum required amount is added.

3. A method as claimed in claim 1 or claim 2, according to which, in a first manufacturing stage, the bloatable argillaceous material is destoned and either ground to a predetermined consistency with water, or comminuted by drying and pulverizing to a powder, before adding a preselected proportion of pfa (or equivalent material) thereto.

4. A method as claimed in claim 3, according to which, in the first manufacturing stage, after mixing the prepared bloatable argillaceous material and pfa (or equivalent material), the mixture is homogenized prior to forming into pellets.

5. A method as claimed in claim 3 or claim 4, according to which, in a second manufacturing stage, the pellets are shaped into either particulate spheres or irregular particles of 5 to 15 mm size, simultaneously being dried to a pre-heating temperature in the range 400 to 550 degrees C.

6. A method as claimed in claim 3 or claim 4, according to which green pellets are shaped either into particulate spheres or irregular particles of 5 to 15 mm size which are suitable for firing without drying or pre-heating.

7. A method as claimed in claim 5 or claim 6, according to which, in a third manufacturing stage, particulate material either pre-heated or in a green pellet state is bloated at a temperature in the range 900 to 1200 degrees C, either in a kiln, on a sinter strand or in other suitable pyro-technique equipment, to produce the lightweight aggregate.

8. A lightweight aggregate produced by the method of any of claims 1 to 7.