LECA production process features
The raw material for LECA production is mostly sedimentary clay materials, less frequently – metamorphic rocks (argillite, clay slate and other stone-like materials).
Their composition is complex and they include both clay minerals (hydrous micas, kaolinite, etc.) and field spar, quartz, carbonates, organic and iron admixtures.
So what is the raw material for production of this construction material? Clays containing no more than 30% of quartz are the best material for production of LECA.
Special research is conducted to determine if a particular material is suitable for production. The main requirements for the raw materials:
1. ability to swell up during baking;
2. high fusibility;
3. a specific swelling-up interval.
Sometimes special additives are introduced into raw materials to increase the swelling-up coefficient. Organic (diesel fuel, fuel oil residue, etc.), ferruginous substances, geological materials (perlite, alunite) or artificial substances can be used as additives.
The product of the raw material processing is raw granules of a specific size and composition. Such granules are subjected to heat treatment: they are dried, baked and cooled. At the net stage the produced material is sorted in terms of density, and is chipped into smaller particles. After sorting LECA is ready for storage or shipment to the point of use.
The main LECA production stages:
• delivery of the raw material;
• clay drying;
• raw material cooling;
• cooling of granules;
• transportation and storage.
Methods of LECA granule production
There are four main LECA production methods which are different in terms of their technology:
3. powder and plastic;
The choice of a particular method depends on the raw material quality.
Dry LECA production method
This is the simplest method. It is used for stone-like raw material. First it is chipped and then charged into the rotary furnace. Such a method is most efficient if the rock is uniform, without harmful inclusions. It is characterised by modest costs and low power consumption.
The process flow diagram of a typical LECA gravel production shop with the capacity of 100 thous. m3 per year consists of three rooms:
• preparatory forming room;
• furnace room;
• finished product storage area
The process consists of the following stages:
• feeding, preparation and forming of the raw material;
• clay drying;
• clay baking;
• LECA cooling;
• feeding of the finished product to the storage area.
LECA granule cooling
After baking LECA should be cooled. The cooling begins back in the rotary furnace due to the incoming air. Then it continues on airslide conveyors and in drum coolers, etc.
The material strength depends on the granule cooling rate. If cooling was too fast, granules may crack. In case of slow cooling the material quality may decrease as a result of softening and crumpling of granules.
That is why LECA is cooled to 800 to 900 degrees immediately after swelling up. Then it is cooled to 600 to 700 degrees for 20 minutes.
This is required so that the hardening proceeds without significant thermal stress and that crystal minerals form, which increase the strength properties of the material. Ffurther cooling may be performed relatively quickly.
LECA fractionation and transportation
Fractionation is performed using drum screens of a cylindrical or multifaceted shape.
LECA is transported within the plant using compressed air or a belt conveyor. The conveyor method is more popular as transportation via pipes using air flow results in damage of the sheath and crushing of granules.
The finished material is stored in silo- or bin-type storage. Before storage LECA is preliminarily packaged into special moisture-proof sacks of varying volume.
LECA production equipment
The LECA production line consists of:
• ripping devices;
• perforated rollers;
• clay mixer;
• drying drums;
• baking furnace;
• air-slide conveyors;
• belt- or other types of conveyors;
• gravel sorters, etc.
A spacial crusher is used for primary processing of the raw material. It consists of a chipping unit which comprises such components as
1. bladed counterrotating shafts;
2. rigid frame;
3. gear drive;
4. receiving bin.
When the shafts start to rotate, the raw material is chipped to the necessary size. A mixer is used for the inclusion of necessary additives.