Heating – Processes of heating or heater operation – Including passing – treating or conveying gas into or through...
Patent
1994-05-23
1996-01-09
Bennet, Henry A.
Heating
Processes of heating or heater operation
Including passing, treating or conveying gas into or through...
432103, 432105, 432106, 432111, 432179, 432180, 432181, F27B 1500
Patent
active
054824583
DESCRIPTION:
BRIEF SUMMARY
There is considerable demand for lightweight aggregate for use by the construction industry in lightweight aggregate concretes. Such concretes are used in the form of building blocks, precast components such as panels, beams and roofing slabs, and in many other applications where lightness coupled with sufficient strength and low thermal conductivity are important. There are also applications where the aggregates can be used for insulation alone for example as space infill below suspended ground floors, fillings for hollow cavities and so on. Them are yet other uses, in horticulture for instance, where lightweight aggregates are used in composts and as a growing media in hydroponic cultivation.
It is well known that lightweight aggregates can be produced by subjecting certain suitable natural materials, such as some clays, shales and slates, to heat treatment at elevated temperatures to bring about their expansion. Where such expansion is possible, the material to be expanded usually has to be in particulate form as slate or shale chippings, or as pellets made from finely ground clay, slate or shale.
In this invention much consideration has been given to the physical/chemical mechanism whereby materials are expanded and its aim is to provide as far as practicable conditions whereby maximum expansion can be achieved. For expansion to take place the material has to be heated to the point of incipient fusion. At these high temperatures the material becomes plastic and impervious and the gases that are then produced within the material are prevented from escaping. This results in the formation of a myriad of small gas pockets which cause the plastic material to expand and on cooling the porous structure remains within the solidified material. Some gases are evolved within the material at temperatures substantially lower than incipient fusion and a prolonged period of heating to incipient fusion allows these gases to escape uninhibited. It is a teaching of this invention that the material to be expanded should be heated rapidly to incipient fusion, and that it should only be held at this expansion temperature for a time sufficient for full expansion to take place as unnecessary exposure at an elevated temperature can lead to deterioration and a weakened structure.
A proportion of expansion gases, mainly steam from the water of crystallisation of the mineral constituents are evolved at comparatively low temperatures, below 500.degree. C. Thus preheating the material up to say 500.degree. C. prior to the rapid heating process will detract to a degree from the expansion obtained, but this may be desireable in the interests of fuel economy where such preheating can be effected by using waste heat which would otherwise pass out of the system.
Existing commercial processes for producing expanded lightweight aggregate from natural materials have centred principally on two methods of heat treatment, namely using a sinter strand or travelling grate, or using a rotary kiln. Both methods involve heating large quantities of material to elevated temperatures comparatively slowly.
To put this invention into perspective it is necessary to consider existing rotary kiln practice. The rotary kiln consists of a tubular vessel, lined with refractory and insulating material, which can rotate about its longitudinal axis. The kiln is inclined to the horizontal, e.g. by up to 10 degrees, so that when it is rotating any particulate or other material fed into it at its upper end will gradually progress down the kiln until it is discharged at its lower end. Heat is supplied to the discharge end by burners firing directly up the kiln or perhaps by glancing the flames on to the walls or the roof of the kiln. Transit time for material to pass through the kiln is governed by the speed of rotation and the angle of inclination. Material travels in countercurrent against the direction of flow of combustion gases. There is a substantial temperature gradient along the length of the kiln such that at the discharge end the temperature attained is t
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Bennet Henry A.
Ohri Siddharth
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