15.01.2009 | Editor: Marcel Dröttboom
Rheinberg, Germany – The continuous and reliable transport of the raw meal to the cement kiln and the cement clinker from the kiln cooler to the silo and onward from the silo to the
Rheinberg, Germany – The continuous and reliable transport of the raw meal to the cement kiln and the cement clinker from the kiln cooler to the silo and onward from the silo to the mill is fundamental to the efficiency of cement production in every plant regardless of size or production capacity. Illustrated above this single Aumund type BWG vertical belt-bucket elevator raises the raw meal from blending silos below to the pre-heater tower where the material is pre-calcined before being passed into the rotary kiln for final calcination and formation of the cement clinker.
In a green-field site at Cementos Balboa in Spain the client chose to standardise on all Aumund transport equipment both for the clinker and finished cement, including a type BWG vertical belt-bucket elevator (Fig. 1), that raises the raw meal from blending silos to the pre-heater tower where the material is pre-calcined before being passed into the rotary kiln for final calcination and formation of the cement clinker. For larger plants, such as the recently delivered new 10000 ton per day lines for Saudi Cement at Hofuf, built by Sinoma of China but supplied with Aumund transport equipment, these elevators are installed with a single vertical lift in excess of 150 m.
At the same plant but with a reduced vertical lift the Aumund BWG elevators handle the raw meal with a design handling capacity of 1000 t/h, in this case with a bucket width of 1600 mm, which is presently the largest size offered, capable of extending the capacity range up to 1600 t/h. Unique to the Aumund system the type BWG elevator uses a single or double rope strand cross stabilised steel cord conveyor belt with close pitch cords to maximise the belt strength with a reduced belt thickness allowing selection of smaller pulley diameters to minimise gearing size. In addition compact close pitch buckets are employed to reduce the fixing stress and, combined with the special fixing design and close cord pitch, bucket detachment in service is virtually eliminated. To accommodate the high puncture strength of the cross stabilised close pitch steel cord belts Aumund have developed a special punching machine allowing the belts to be easily and reliable prepared before shipment (Fig. 2).
With this equipment the bucket fixing holes are punched using a hydraulic press and the belt automatically advanced in accordance with the design bucket pitch ensuring excellent repeatability and easy bucket fixing in situ. For the steep angle BZB “Bucket-Conveyor” design the material is conveyed in fabricated buckets with overlapping front and back plates to prevent spillage (Fig. 3).
Both the KZB and BZB conveyors share the same range of conveyor chains and supporting rollers both of which have seen recent upgrades to obtain enhanced performance and reliability. Cast iron rollers are machined with a single flange designed to run on standard rail sections and include two ball bearings and a multi-pass labyrinth seal to prevent material ingress for maximum life.
Aumund have recently unveiled a completely new range of conveyor chains formed in precision laser cut fine grain steels with an extended range and increased rupture strength allowing longer single flight conveyors. Often the traditional intermediate transfer tower may be completely eliminated and the clinker transferred direct from cooler to silo if the plant geometry permits. In this manner not only is the cost of the transfer tower eliminated but so is one drive and one transfer point. Thereby dust extraction costs are reduced as are electrical control requirements making the total package much more competitive. Where vertical elevation is required then for handling hot materials such as cement clinker and other raw materials or additions the central chain bucket elevator type BWZ is the favoured solution in any critical application (Fig. 4).
Suitable for handling materials up to 400 °C. and with vertical lift height to over 70 m these machines are extremely robust and reliable, proven in continuous duty applications running 24/7. Available with either a single bucket strand and suitable for handling rates to 800 t/h or with a double strand to 1600 t/h the range of options covers most applications including raw material intake to storage and mill recirculation. The central chain system is based on a forged chain design with extended pins which carry loose bucket brackets mounted back to back and thus effectively retaining the bucket to the chain without a rigid bolted fixing.
This loose bucket fixing is an Aumund patent and eliminates transmission of vibration and thus stress cracking of the bucket mounting is almost eliminated with a corresponding increase in bucket life. For the larger chain sizes the forged side plates have an integral labyrinth seal and the bolt and bush are pre-lubricated on assembly. The seal substantially prevents ingress of material to the chain bush/bolt interface and the pre-lubrication promotes the development of a high quality surface finish to the bearing areas in service reducing the wear rate and consequently extending the chain life. Under test conditions these chains gave over double the average chain life compared to the standard un-sealed alternative.
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